This report contains the collective views of an international group of
    experts and does not necessarily represent the decisions or the stated
    policy of the United Nations Environment Programme, the International
    Labour Organisation, or the World Health Organization.

    Published under the joint sponsorship of
    the United Nations Environment Programme,
    the International Labour Organisation,
    and the World Health Organization

    World Health Orgnization
    Geneva, 1993

         The International Programme on Chemical Safety (IPCS) is a
    joint venture of the United Nations Environment Programme, the
    International Labour Organisation, and the World Health
    Organization. The main objective of the IPCS is to carry out and
    disseminate evaluations of the effects of chemicals on human health
    and the quality of the environment. Supporting activities include
    the development of epidemiological, experimental laboratory, and
    risk-assessment methods that could produce internationally
    comparable results, and the development of manpower in the field of
    toxicology. Other activities carried out by the IPCS include the
    development of know-how for coping with chemical accidents,
    coordination of laboratory testing and epidemiological studies, and
    promotion of research on the mechanisms of the biological action of

    WHO Library Cataloguing in Publication Data

    Principles for evaluating chemical effects on the aged population.

        (Environmental health criteria ; 144)

        1.Aged 2.Aging 3.Environmental exposure 4.Environmental
        pollutants - adverse effects 5.Hazardous substances - adverse

        ISBN 92 4 157144 6        (NLM Classification: WT 104)
        ISSN 0250-863X

         The World Health Organization welcomes requests for permission
    to reproduce or translate its publications, in part or in full.
    Applications and enquiries should be addressed to the Office of
    Publications, World Health Organization, Geneva, Switzerland, which
    will be glad to provide the latest information on any changes made
    to the text, plans for new editions, and reprints and translations
    already available.

    (c) World Health Organization 1993

         Publications of the World Health Organization enjoy copyright
    protection in accordance with the provisions of Protocol 2 of the
    Universal Copyright Convention. All rights reserved.

         The designations employed and the presentation of the material
    in this publication do not imply the expression of any opinion
    whatsoever on the part of the Secretariat of the World Health
    Organization concerning the legal status of any country, territory,
    city or area or of its authorities, or concerning the delimitation
    of its frontiers or boundaries.

         The mention of specific companies or of certain manufacturers'
    products does not imply that they are endorsed or recommended by the
    World Health Organization in preference to others of a similar
    nature that are not mentioned. Errors and omissions excepted, the
    names of proprietary products are distinguished by initial capital





         1.1. Objectives
         1.2. Definitions
              1.2.1. Aging versus senescing
              1.2.2. Aging of individuals and populations
              1.2.3. Chemicals of concern
              1.2.4. Time and dose of exposure
         1.3. Chemical exposure
         1.4. Aged population
              1.4.1. Demographic consideration
              1.4.2. Life expectancy
              1.4.3. Life-style in aged populations
         1.5. Theories of aging


         2.1. Changes in gene structure and function in aging
              2.1.1. Chromatin structure
              2.1.2. DNA repair
              2.1.3. Transcription
              2.1.4. Translation
         2.2. Changes in tissues, organs and systems in aging
              2.2.1. Nervous system
                 Structural changes
                 Biochemical changes
                 Functional changes
              2.2.2. Sensory organs
                 Somatic sensations
              2.2.3. Endocrine system
                 The pituitary-thyroid axis and the
                                  basal metabolism
                 The pituitary-adrenal axis
                 The endocrine pancreas and
                                  carbohydrate metabolism
              2.2.4. Reproductive system
                 Female aging
                 Male aging

              2.2.5. Immune system
                 Aging of lymphoid organs
                 Aging of cellular constituents
              2.2.6. Cardiovascular system
                 Blood vessels
                 Characteristics of atherosclerotic
                 Theories of atherosclerosis
              2.2.7. Respiratory function
                 Gas-exchange organs
                 Erythropoietic activity
              2.2.8. Kidney and body fluid distribution
                 Renal function
                 Lower urinary tract
              2.2.9. Gastrointestinal function
                 Gastrointestinal tract
              2.2.10. Musculo-skeletal system
                Skeletal muscles
              2.2.11. Skin


         3.1. Pharmacokinetics
              3.1.1. Absorption
              3.1.2. Distribution
              3.1.3. Metabolism
              3.1.4. Excretion
         3.2. Pharmacodynamics
              3.2.1. Central nervous system
              3.2.2. Endocrine system
                 Changes in hormonal availability
                                  with age
                 Changes with age in the reception
                                  of the signal by the target cells
                 Changes in the nature of the
                                  hormonal message with age
              3.2.3. Kidney
              3.2.4. Immune system
              3.2.5. Other tissues and systems
         3.3. Modifying factors
              3.3.1. Nutrition
              3.3.2. Alcohol intake
              3.3.3. Smoking

         3.4. Interactions of chemicals and diseases
              3.4.1. Cancer
              3.4.2. Other diseases


         4.1. Experimental approaches
              4.1.1. Principles for testing chemicals in
                        the aged population
              4.1.2. Animal models
                 Animal species
                 Animal strain
                 Animal sex
                 Selection of age groups for
                 Underlying pathology of animals of
                                  different ages
                 Transgenic animals
                 Animal husbandry
              4.1.3. Chemical exposure
                 Dose level
                 Route of administration
                 Duration of exposure
              4.1.4. Non-mammalian models
              4.1.5. In vitro studies
              4.1.6. Statistical considerations
              4.1.7. Extrapolation of animal data to humans
         4.2. Epidemiological and clinical approaches
              4.2.1. Disease pattern of aged population
              4.2.2. Assessment of effects of environmental
                        chemicals in the elderly population
              4.2.3. Acute episodes
              4.2.4. Concerns for the aged population
         4.3. Biomarkers of aging




         APPENDIX 1


    Dr   V.N. Anisimov, N.N. Petrov Institute of Oncology, Ministry of
         Health, St Petersburg, Russian Federationa,b,c,d

    Dr   L.S. Birnbaum, US Environmental Protection Agency, Research
         Triangle Park, North Carolina, USAa,b,d

    Dr   G. Butenko, Institute of Gerontology, Kiev, Ukraineb

    Dr   R.L. Cooper, US Environmental Protection Agency, Research
         Triangle Park, North Carolina, USAa,b,d

    Dr   V.M. Dilman, N.N. Petrov Research Institute of Oncology,
         Ministry of Health, St Petersburg, Russian Federationa,c,d

    Dr   N. Fabris, Italian National Research Centre on Aging, Ancona,

    Dr   N.S. Gradetskaya, Research Institute of Industrial Hygiene  
         and Occupational Diseases, Academy of Medical Sciences,     
         Moscow, Russian Federationa

    Dr   K. Kitani, Tokyo Metropolitan Institute of Gerontology, Tokyo,

    Dr   J. Leaky, National Center for Toxicological Research,
         Jefferson, Arkansas, USAb

    Dr   A.Y. Likhachev, N.N. Petrov Institute of Oncology, Ministry of
         Health, St Petersburg, Russian Federationa,d

    Dr   S. Li, Chinese Academy of Preventive Medicine, Department of
         Scientific Information, Beijing, Chinaa,b,d

    Dr   G.M. Martin, University of Washington, Department of Pathology,
         Seattle, Washington, USAa,b*,c,d

    Dr   E. Masoro, The Texas University at San Antonio, San    
         Antonio, Texas, USAb

    Dr   N.P. Napalkov, N.N. Petrov Research Institute of Oncology,
         Ministry of Health, St Petersburg, Russian Federationa

    Dr   G.I. Paramonova, Institute of Gerontology, Academy of Medical
         Sciences, Kiev, Ukrainea

    Dr   J. Parizek, Czechoslovakia Academy of Sciences, Institute of
         Nuclear Biology and Radiochemistry, Prague, Czechoslovakiaa

    Dr   P.K. Ray, Industrial Toxicology Research Centre, Council of
         Scientific and Industrial Research, Lucknow, Indiaa,b*,d

    Dr   A. Richardson, Illinois State University, Normal, Illinois, 

    Dr   G.S. Roth, National Institute of Health, National Institute on
         Aging, Baltimore, Maryland, USAb

    Dr   G.J.A. Speijers, National Institute for Public Health and
         Environmental Protection (RIVM), Bilthoven, The

    Dr   K.T. Suzuki, National Institute for Environmental Studies,  
         Ibaraka, Japana,c

    Dr   J. Vijg, Medscand Ingeny, Leiden, The Netherlandsb

    Dr   J.R. Zhu, Zhong Shan Hospital, Shanghai Medical University,
         Shanghai, Chinab,d


    Dr   E.I. Komarov, Central Research Institute of Roentgenology and
         Radiology, Ministry of Health, St Petersburg, Russian


    Dr   G.C. Becking, International Programme on Chemical Safety,
         Interregional Research Unit, World Health Organization,
         Research Triangle Park, North Carolina, USA (Secretary for the
         Planning Meeting)a

    Dr   B.H. Chen, International Programme on Chemical Safety,      
         World Health Organization, Geneva, Switzerland (Secretary   
         for the Task Group Meeting)b

    Dr   Z.P. Grigorevskaya, Centre for International Projects,      
         Moscow, Russian Federationa

    Dr   M.I. Gounar, Centre for International Projects, Moscow,     
         Russian Federationa

    Dr   H. Hermanova, Regional Office for Europe, World Health      
         Organization, Copenhagen, Denmarka*

    Dr   P.G. Jenkins, International Programme on Chemical Safety, World
         Health Organization, Geneva, Switzerlandb

    Dr   M. Mercier, International Programme on Chemical Safety,     
         World Health Organization, Geneva, Switzerlandb


    a    Participant in Planning Meeting, St Petersburg, Russian
         Federation, 5-9 September, 1988

    b    Participant in Task Group Meeting, Geneva, Switzerland,     
         9-13 December, 1991

    c    Submitted background information for planning meeting

    d    Prepared background paper for the preparation of the first

    *    Invited but unable to attend


         Every effort has been made to present information in the
    criteria monographs as accurately as possible without unduly
    delaying their publication. In the interest of all users of the
    Environmental Health Criteria monographs, readers are kindly
    requested to communicate any errors that may have occurred to the
    Director of the International Programme on Chemical Safety, World
    Health Organization, Geneva, Switzerland, in order that they may be
    included in corrigenda.


         The aged population and the number of chemicals in the
    environment have been increasing and will undoubtedly continue to
    increase. It is estimated that there will be 612 millon people aged
    60 years and over by the year 2000, and of these 61% will live in
    developing countries. The numerous physiological and biochemical
    changes occurring during aging can modify the pharmacokinetics and
    pharmacodynamics of chemicals in the elderly, resulting in either
    higher or lower levels of toxicity. It is expected that the adverse
    effects of chemical exposure on the elderly will increase in
    importance as a health care issue. IPCS has been active in the
    development and validation of methodology for the assessment of
    risks from exposure to chemicals. One area of concern has been the
    evaluation of methodology appropriate for the assessment of risks in
    "high-risk" groups. The fifth meeting of the IPCS Programme Advisory
    Committee endorsed the need for an Environmental Health Criteria
    monograph dealing with the effects of chemicals on the aged
    population and the aging processes. This monograph integrates
    relevant studies of toxicology and gerontology; toxicology examines
    the potential health effects of exposure to chemicals, while
    gerontology focuses on the scientific explanations for the phenomena
    and mechanism of aging.

         A planning meeting was held in St Petersburg from 5 to 9
    September 1988 and was organized locally by the N.N. Petrov Research
    Institute of Oncology, Ministry of Health, Russian Federation.
    Financial support through the UNEP Country Projects was provided by
    the Centre for International Projects (CIP), State Committee for the
    Protection of the Environment, Moscow, Russian Federation. Dr M.I.
    Gounar, CIP, formally opened the meeting, and Dr V. Anisimov, on
    behalf of Dr N.P. Napalkov, former Director of the Petrov Research
    Institute of Oncology, welcomed the participants. Dr G.C. Becking
    welcomed the participants on behalf of the Executive Heads of the
    three IPCS cooperating organizations (UNEP/ILO/WHO). Dr V. Anisimov
    and Dr L. Birnbaum were Joint Chairmen and Dr P.K. Ray and Dr A.
    Likhachev were Joint Rapporteurs.

         After discussing the scientific issues relevant to both the
    aged population and aging processes, the committee considered that
    there was sufficient epidemiological, clinical and experimental data
    to support the preparation of an Environmental Health Criteria
    monograph to evaluate chemical effects on the aged population.
    However, the differing views on the mechanisms of aging and how
    chemical exposure might alter such mechanisms preclude at present
    the preparation of an evaluation of the chemical effects on the
    aging process. It was decided to prepare a monograph on principles
    for evaluating chemical effects on the aged population, with only a
    brief discussion of the present concept of aging. An outline of the
    monograph together with a list of possible authors was produced.

         Drs L. Birnbaum and V. Anisimov prepared the first draft of
    this monograph based on 13 background papers written by various
    authors (Appendix 1). Dr V. Anisimov prepared the second draft
    incorporating comments received following the circulation of the
    first draft to IPCS Contact Points for Environmental Health Criteria
    monographs and to IPCS Participating Institutions. Dr L. Birnbaum
    made a considerable contribution to the preparation of the final

         A WHO Task Group Meeting met from 9 to 13 December 1991 in
    Geneva. Dr B.H. Chen, IPCS, opened the meeting and welcomed the
    participants on behalf of the Director, IPCS, and the three IPCS
    cooperating organizations. Dr J. Vijg and Dr K. Kitani were Chairman
    and Vice-Chairman, respectively, and Drs L. Birnbaum and V. Anisimov
    were Joint Rapporteurs.

         The Task Group considered it likely that the aged population is
    more susceptible to the harmful effects of environmental chemicals.
    However, very few environmental chemicals have been tested for
    toxicity in the elderly. Some age-associated diseases may lead to an
    increased susceptibility to the harmful action of specific
    environmental chemicals. The effects of environmental chemicals on
    the process of aging remain to be evaluated. It was suggested that a
    special scientific workshop be devoted to this topic.

         Drs B.H. Chen (IPCS Central Unit) and G.C. Becking
    (Interregional Research Unit) were responsible for the overall
    scientific content, and Dr P.G. Jenkins (IPCS Central Unit) was
    responsible for the technical editing.

         The efforts of all who helped in the preparation and
    finalization of the monograph are gratefully acknowledged.

    ACTH           adrenocorticotrophic hormone

    BMAA           beta- N-methylamino-1-alanine

    BOAA           beta- N-oxalylamino-L-alanine

    cDNA           complementary DNA

    CNS            central nervous system

    GABA           gamma-aminobutyric acid

    GH             growth hormone

    GI             gastrointestinal

    HDL            high density lipoprotein

    hnRNA          heterogeneous nuclear RNA

    LDL            low density lipoprotein

    LH             luteinizing hormone

    mRNA           messenger RNA

    SDAT           senile dementia of Alzheimer type

    T3             triiodothyronine

    T4             thyroxine

    TSH            thyroid-stimulating hormone

    UDP            uridine diphosphate

    UDPGA          UDP-glucuronic acid


    1.1  Objectives

         The main objective of the Group involved in the preparation of
    this report was to review present knowledge concerning the effects
    of environmental chemicals on the aged population and to evaluate
    available models for the assessment of these effects and the
    consequent risk to human health in the aged population.

         About ten million natural and synthetic chemicals have been
    identified by the Chemical Abstract Service Registry and some eighty
    to one hundred thousand have been identified as important to
    commerce. The restricted knowledge of the toxicological  properties
    of the natural substances makes it difficult to give clear evidence
    of whether the elderly population is at risk for this category of
    compounds, but the impact of these natural toxins might be even
    larger than that of most man-made toxicants. As the requirements for
    more toxicological data on natural toxicants become more important
    internationally, the possible effects on the elderly population
    should also be included in the assessment.

         The following relationships need to be considered: a) the
    special response of the aged as compared to that of the young
    following exposure to environmental chemicals; and b) the impact of
    exposure to environmental chemicals on the processes of aging.  This
    report focuses on the first relationship, i.e. the elderly as a
    population at special risk. The elderly are heterogeneous with
    respect to aging processes, life-style and diseases.  Indeed, in
    most instances the deficit in the majority of the elderly relates
    more to life-style and diseases than to the aging processes  per se.
    The Group recommended that the evaluation of the effects of
    environmental chemicals on the process(es) of aging should be the
    focus of a separate scientific workshop. This monograph will focus
    on environmental chemicals as opposed to pharmaceuticals and food
    additives, although information on these latter chemicals will be
    used when necessary to support the issues.

         The study of the effects of chemicals on the aged population
    requires the integration of two disciplines, toxicology and
    gerontology. Toxicology examines the potential health effects of
    exposure to chemicals, while gerontology focuses on the scientific
    explanations for the phenomena and mechanisms of aging. The lack of
    a unified theory of aging, together with the inability at present to
    distinguish intrinsic aging from natural disease and toxic response,
    creates difficulties which make the objective of the Group only
    partially attainable.

    1.2  Definitions

    1.2.1  Aging versus senescing

         Plant biologists often sharply differentiate between these
    terms (Leopold, 1975). They may use the word aging to refer to all
    of the changes in structure and function in an organism throughout
    the life course, including the period of development. They reserve
    the term senescence for the deteriorative alterations in structure
    and function that are the immediate precursors of tissue and
    organismal death. Mammalian gerontologists, however, typically use
    the terms aging and senescence (or, more properly, senescing)
    interchangeably to describe the constellation of changes that occur
    after the attainment of sexual maturity and the young adult stage of
    life. This is not to deny the critical importance of developmental
    events in setting the stage for subsequent patterns of senescence.
    For example, a specific chemical, physical or infectious agent,
    acting during a crucial period of ontogeny, could conceivably
    deplete, but not ablate, a subset of stem cells or their partially
    differentiated progeny without any phenotypic consequences until
    additional depletion, related to some normative aging process,
    reaches a clinically significant threshold.

    1.2.2  Aging of individuals and populations

         At the organismal level, endogenously and exogenously induced
    injuries are more likely to occur as the organism ages. There is a
    decreasing probability, as a function of chronological time, that
    the organism will survive. Thus, there is an exponential increase in
    the death rate over time. Although subject to important
    environmental influences, the ages at which such exponential
    increments of death rates begin and the kinetics of their
    progression are subject to strong genetic influences in that they
    are species-specific. The basic observations have been summarized by
    the following equation (Gompertz, 1825):

         Rm = R0.ealpha t

    where Rm indicates the mortality rate at time t, R0 is a
    parameter empirically determined by extrapolating an exponential
    curve back to zero time (sometimes referred to as the "initial
    vulnerability"), e is the natural logarithm, t is time and alpha is
    a slope constant.  Better fits to empirical data are obtained if a
    second constant is added to the right hand side of the above
    equation (the Gompertz-Makeham equation).

         The Gompertz-Makeham equation is a satisfactory approximation
    to the kinetics of specific mortality in human populations in the
    age range 20-80 years. Correspondingly, the value of alpha
    characterizes the rate of aging only within this interval. Although

    some deviations of alpha within this interval in human populations
    have been noted (Pakin & Hrisanov, 1984), the analysis of the
    parameters of the Gompertz-Makeham equation permit one to make
    objective estimations of the changes in the mortality in populations
    (Sacher, 1977; Hirsch, 1982). It is important to note that the use of
    this method for measuring the rate of aging in populations of
    experimental animals is especially reliable when the external
    conditions (e.g., the housing of animals) remain constant throughout
    the whole period of a study.

    1.2.3  Chemicals of concern

         The first class of agents of concern would be those with a
    special potential to injure elderly subjects because of their
    unusual susceptibility. This sensitivity might be the result of
    intrinsic biological aging, chronic exposure to deleterious
    environmental agents, a high prevalence of various age-related
    diseases, or a combination of all of these. A rational approach to
    this problem requires detailed knowledge of the altered physiology,
    biochemistry and special pathologies of older people (those over age
    65) and, especially, of the very old (those over age 85). Examples
    include the special vulnerability of many elderly subjects to air
    pollutants, to certain pharmaceuticals and combinations of
    pharmaceuticals, and even to injury from methane gas explosions. The
    latter results from a high prevalence of atrophic change in the
    olfactory network, with consequent marked reduction in the ability
    of many older people to detect the low concentration of sulfide
    contaminants that are deliberately added to household gas to warn of
    leakage. It is apparent that, with respect to such classes of
    agents, public health actions can be of immediate benefit to older

         The second class of chemicals of concern would be those that
    might modulate the processes of aging. These could either accelerate
    ("gerontogens") or retard ("geroprotectors") the aging processes.

    1.2.4  Time and dose of exposure

         As indicated above, chemical agents that have the potential to
    accelerate aspects of aging could act at any time during the life
    course, from before birth to death. Strictly speaking, however,
    agents that are most likely to closely mimic natural aging processes
    are slow, insidious and progressive. Moreover, since the phenotypic
    consequences of aging are often subtle and, for the human species,
    develop over a period of decades, it would be difficult indeed to
    establish a minimum effective dose for such putative chemicals. The
    task is somewhat less difficult in the case of agents to which the
    elderly have some special vulnerability, since acute and sub-acute
    end-points are often involved. For example, the prevalence of
    cardiopulmonary morbidity can be related to ambient concentrations
    of specific urban pollutants.

    1.3  Chemical exposure

         The world is irrevocably dependent on man-made chemicals,
    modern technology bringing a dramatic increase in their production
    and consumption. More than 750 000 chemicals are known to be in our
    environment and between 1000 and 2000 new ones enter the market each
    year. A major proportion of these chemicals find use as components
    of various consumer products, or they enter the environment as
    industrial waste, posing health risks as well as benefits.

         In present-day society, we use chemicals to boost our food
    production, make our lives easier and protect our health. Many of
    these chemicals are hazardous and great care must be taken during
    their usage, storage and disposal. Their releases into the
    environment, whether intentional or not, can have severe

         Billions of tons of hazardous industrial waste materials,
    produced every year, may enter the environment through complex and
    interrelated pathways (air, water, food, etc.), and could affect
    humans. Pesticides, fertilizers and herbicides enter the environment
    as a result of direct application; nitrogen oxides, sulfur oxides
    and polycyclic aromatic hydrocarbons result from combustion
    processes. Many manufacturing processes liberate unwanted
    by-products and waterborne and airborne wastes, which are sometimes
    more toxic than the raw materials. Incidents such as the
    contamination of water by mercury, the widespread distribution of
    industrial oils (e.g., polychlorinated biphenyls), and the
    destruction of the ozone layer in the stratosphere due to the
    release of aerosol propellants (chlorofluorocarbons) have made the
    public aware of the ability of some chemicals to cause unexpected
    results at some point far removed from where they were originally
    introduced. Chemicals undergo transformation once they enter the
    environment, and a relatively harmless chemical may become a toxic
    by-product. It may further enter the food chain and accumulate in
    living organisms, eventually reaching humans.

         In both developed and developing countries there has been a
    great impact of life-styles upon the quality of life and upon the
    life span of the population concerned. Of particular interest are
    the aged individuals, who, having lived longer in an environment
    containing toxic agents, may suffer from their cumulative effects
    even when exposure levels are relatively low. The multiple
    life-long, though low-level, human exposure to chemicals is
    difficult to assess adequately in terms of associated health risks
    (Pines et al., 1987). It has been reported that some chronic low
    level exposures to chemical or physical stressors have beneficial
    effects on longevity (hormesis) (Sacher, 1977; Neafsey, 1990).

         Among the chronic health effects of chemicals, cancer is of
    major concern. Many substances have found in recent years to be
    carcinogenic in one or more species of laboratory animals (WHO,
    1983). In humans, cancer is seldom manifest until 10-40 years after
    exposure to the carcinogenic agent (IARC, 1990). Thus, cancers
    caused by chemicals are most often observed in the aged population.
    However, it is not easy to identify the hazards unless past exposure
    is known. Similar comments may be made about atherosclerosis, which
    may also be related to chemical exposure (Penn et al., 1981).

         In many cases, especially with respect to long-term effects,
    the response to a chemical may vary, quantitatively or
    qualitatively, in different groups of individuals depending on
    predisposing conditions, such as nutritional status, disease status,
    current infection, climatic extremes, and genetic features, sex and
    age of the individuals. Understanding the response of such specific
    risk groups is an important area of toxicology research today.

         There is no biological basis for classifying substances
    according to their environmental source (e.g., industry or 
    agricultural use), or use patterns (e.g., food additives). Chemical
    substances with neurotoxic potential, for example, are found as
    natural metabolites (e.g., quinolinate), biological poisons in
    plants (e.g., gossypol) and animals (e.g., batrachotoxin), natural
    components of food (e.g., beta-oxalylamino-L-alanine (BOAA)) and
    beverages (e.g., ethanol), food contaminants (e.g., ergot),
    synthetic food additives (e.g., aspartame), flavours and fragrances
    (e.g., dinitromethoxybutyl toluene), pollutants of air (e.g., lead),
    water (e.g., zinc pyridinethione) and industrial processes (e.g.,
    carbon disulfide), and therapeutic drugs (e.g., phenothiazines). In
    addition, numerous chemicals, if they have damaging actions, may
    contribute to the aging phenotype.

         Chemicals influencing the processes of aging and/or affecting
    the aged population may be classified into several groups according
    to their chemical properties and metabolic behaviours. Chemicals
    that are poorly metabolized fall into two groups. The first are
    absorbed and distributed into certain tissues according to their
    partitioning behaviour, based on their physical/chemical properties.
    For example, organochlorine compounds concentrate in adipose tissue.
    During fasting, adipose tissue is mobilized and accumulated
    chemicals are liberated into body fluids. Organo-chlorine compounds
    are detectable in adipose tissue, blood and breast milk long after
    cessation of exposure. The second group comprises chemicals that are
    poorly excreted and accumulate in the body. Some of these chemicals
    are detoxified by binding to specific proteins, resulting in
    long-term storage. For example, cadmium, lead and mercury induce
    specific proteins such as metallothionein which help in the
    detoxification of heavy metals (Oh et al., 1978; Onasaka & Cherian,

    1981). The appearance of cadmium toxicity among the population over
    50 years of age may be related to the decreased capacity of
    metallothionein synthesis with advancing age (Hunziker & Kagi,

         Chemically and biologically active chemicals are readily
    metabolized. Thus, they do not accumulate in the body.  Continuous
    exposure to these chemicals is of concern because these may be
    metabolized to reactive intermediates that can interact and damage
    cellular macromolecules. Such damage may be cumulative, resulting in
    the aged population being more vulnerable. Other types of chemicals
    which belong to this group (e.g., NOx, SO2) can also cause more
    severe adverse effects on the aged whose defensive mechanisms are
    weakened. It should be stressed that several chemicals can enter the
    body at the same time, causing a more complex problem.

    1.4  Aged population

    1.4.1  Demographic consideration

         The United Nations has defined people of 60 years and over as
    the aged. In 1988, it was estimated that there were about 488
    million people in the world fitting this criterion. The number is
    expected to rise to 612 million by the year 2000, 61% of whom (i.e.
    376 million) will be living in developing countries (Fig. 1) (WHO,

         Many countries are, however, using 65 years and over as the
    definition of the elderly. The corresponding numbers for this age
    group are 327 million in the world in 1990 and 423 million by the
    year 2000, of whom 250 million will be in developing countries (WHO,
    1990). The increase in the elderly population will be particularly
    marked in Asia, primarily as a result of the rapid growth expected
    in the numbers of the aged in China and India. This trend is
    illustrated in Fig. 2, which indicates the 20 countries with the
    largest aged population in 1980 and the expected growth of the aged
    population. By the year 2020, there will be an increase of 270
    million elderly citizens in China and India. The size of the aged
    population is expected to rise by more than 20 million in both
    Brazil and Indonesia, and by roughly half that number in Mexico,
    Nigeria and Pakistan (WHO, 1989).

         On the other hand, a much smaller absolute increase in the
    elderly population is anticipated for the European countries, where
    population aging began much earlier. As a result, the developing
    countries will gradually account for the largest elderly population
    in the world. Indonesia, for example, is expected to move from tenth
    place in 1980 to fifth in 2020 (Fig. 2), and Mexico is expected to
    have the eighth largest elderly population, ahead of Italy, France,
    and the United Kingdom (WHO, 1989).

         The elderly population of the USA is growing much more rapidly
    than the population as a whole. In the 1970s, the population aged 65
    and over increased by 28% and the population aged 85 and over
    increased by 59%, whereas the total population increased only by
    11%. The population aged 85 and over is expected to triple between
    1980 and 2020 and is the fastest growing of the four older age
    groups (55-64, 65-74, 75-84, and 85 and over). Census projections
    for 2050 indicate that the proportion of the population aged 65 and
    over (22%) will be almost twice as great as it is today (12%). In
    the last two decades alone, the 65-plus population has grown by 54%
    while the under-65 population has increased by only 24%. At the
    beginning of this century, less than one in eight Americans was age
    55 and over. The increase in the numbers of elderly people is
    expected to occur in two stages. Until the year 2000, the proportion
    of the population age 55 and over is expected to remain relatively
    stable at 22%. By 2010, because of the maturation of the post World
    War II baby boom, more than a quarter of the total population of the
    USA is expected to be at least 55 years old, and one in seven of the
    population will be at least 65 years old. By 2050, one in three
    persons is expected to be 55 years or older and one in five will be
    over 65 (US Senate Special Committee on Aging, 1986).

    FIGURE 1

    FIGURE 2

         It is commonly assumed that today's large percentage of elderly
    people in the population is a result of increased longevity and
    decreased birth rate. For example, in Japan the proportion of people
    aged 65 or more had increased to 10.3% in 1985. At the same time,
    the values were 15.1%, 14.5%, 12.4% and 16.9% in the United Kingdom,
    Federal Republic of Germany, France and Sweden, respectively. Japan
    is a new "aged-type" country with the greatest rate of increase in
    the elderly in the world. By the year 2025, the proportion of the
    Japanese population aged 65-plus will rise to 23.4% (Hosomi, 1990).

         In China, the proportion of the population aged 60 and over was
    7.3% of the total population in 1953. By 1984 it had increased to 9%
    of the total population, and by the year 2000 the proportion of the
    population 60 and over will be as high as 10.6%. The proportion is
    predicted to increase to 26.2% by 2025 (Xiong, 1990).

         At present, the aged population is growing more rapidly in
    China than in countries of Europe and North America. According to
    data from the US Bureau of the Census, it took 115 years (1865-1980)
    for the proportion of people aged 65 or more in France to increase
    from 7% to 14%, 85 years (1890-1975) in Sweden, 66 years (1944-2010)
    in the USA, 45 years (1930-1975) in the United Kingdom, and 26 years
    (1970-1996) in Japan. For China, the pattern of the growing number
    of elderly is similar to Japan, i.e. the proportion of people aged
    65 or more will be 7.4% in 2000, and by 2025 it will have increased
    to 12.8% (Hosomi, 1990; Yao, 1990; Xiong, 1990).

    1.4.2  Life expectancy

         Life expectancy at birth is a statistical index calculated by
    the use of a life table from the age-specific death rates of the
    population. This illustrates the overall level of health in a
    country or a region. Throughout this century, it has been evident
    that, as a result of improvements in many aspects of health status,
    an individual can expect to live longer. From 1960 to 1990, life
    expectancy at birth for the total population had increased by 13.5
    years. The life expectancy at birth for the period 1985-1990 was
    estimated to be 63.9 years for the world as a whole, 74.0 years for
    the more developed regions, and 61.4 years for the less developed
    regions. The longest life expectancies are in Japan (78.3), Iceland
    (77.5), Sweden (77.1), Switzerland (77.1), and the Netherlands
    (76.9) (World Population Prospects, 1991).

         The trend for life expectancy at birth in the USA showed an
    increase from 1900 (46.4 for males and 49.4 for females) to 1950
    (65.6 and 71, respectively) and in the year 2000 is expected to be
    72.1 and 79.5 (for males and females). By 2050, it may have
    increased to 73.6 for males and 81 for females (US Senate Special
    Committee on Aging, 1986).

         In China, the life expectancy at birth before 1949 was about 35
    years of age, being one of the lowest in the world at that time. By
    1957 it had increased to 57 years, from 1973 to 1975 it was 63.6
    years for males and 66.3 years for females, and in 1981 it was 68
    years. It is expected that the continued increase in the average
    life expectancy of the people of China will be slow due to a high
    death rate from cardiovascular diseases (Gu, 1986).

    1.4.3  Life-style in aged populations

         A basic issue in planning for the consequences of demographic
    aging is whether elderly people should be considered a specific
    target group for the development of services, or whether their needs
    should be catered for within the context of planning for the
    population as a whole. One approach to a rational policy for this
    issue is to consider the nature of human aging. For this it is
    necessary to view the physical, psychological and sociological
    dimensions of aging as a whole.

         Life-style influences the effects of chemicals on human health,
    including that of the elderly, both quantitatively and
    qualitatively. Environmental chemicals and their uses are diverse. 
    Specialized nutritional elements of the diet have become popular,
    while in certain countries many people prefer predominantly
    vegetarian diet. Others take supplements and additives which contain
    pure preparations of vitamins, minerals, amino-acids and other
    substances. Whether such substances have either adverse or
    beneficial effects on the elderly and aging processes, has not
    generally been fully evaluated. Another important source of human
    exposure to chemicals comes from the intake of different kinds of
    cosmetic agents and fragrances, such as shampoos, creams, perfumes,
    oral deodorants, sunscreen and suntan lotions, and insect
    repellents. These are often chemical mixtures whose components have
    not been evaluated or tested beyond acute toxic potential.

         Occupational status, indoor air quality, recreational
    activities, exercise, eating and drinking habits, alcohol
    consumption, and tobacco smoking can all affect the elderly and
    aging processes to a certain degree. Elements of life-style can
    strengthen or reduce the risk of developing aged-related
    degenerative diseases. They can also accelerate or delay
    physiological and anatomical changes. Typical examples are the
    various age-related diseases caused by toxic chemicals in tobacco
    smoke and the reduction of the risk of cardiovascular diseases
    produced by regular exercise (Committee on Chemical Toxicity and
    Aging, 1987).

         As far as the possible influence of life-style factors on the
    manifestations of aging is concerned, many studies have shown that
    loneliness and physical and intellectual inactivity are common among

    the elderly, especially widowed people. Several studies have
    revealed that living conditions have an influence on health and
    well-being, resulting in an increase in the demand for social care
    and medical service. Marital status and living arrangements have
    important significance for the unique life-style of the elderly. 
    There are striking differences between the proportions of elderly
    males and females who are married: in many countries the proportion
    of married males is twice that of married females. In general, the
    proportion of widows is very high and that of widowers relatively
    low (WHO, 1984).

         Migration is also one of the life-style variables of the
    elderly. In rural areas of Asia, many older women move to cities to
    join their children after they have been widowed. Another common
    type of move is the migration of the recently widowed or chronically
    ill elderly from urban areas to their home towns or villages. For
    many countries in Africa and Asia, the urban-rural migration is most
    apparent among males, who return from urban to rural areas when they
    are old. Worldwide, only a minority of elderly people live in urban
    areas (WHO, 1984).

    1.5  Theories of aging

         During the last century, more than 100 various hypotheses
    concerning the origin and mechanism of aging have been put forward.
    All of them could be grouped generally into two broad categories:
    those that invoke deterministic, or "programmed", alterations in
    gene expression or gene structure; and those that invoke a variety
    of stochastic, or "random", alterations in the structure and
    function of macromolecules, cells, and organ systems. This
    distinction, however, has some limitations, because stochastic
    alterations in individual cells can lead to predictable phenomena in
    the large populations of cells. The use of terminal differentiation
    to explain the limited replicative life span of somatic cells
    (Martin et al., 1974) could be an example of the blurring of the
    stochastic and non-stochastic categories. For each individual cell,
    differentiation is a random event; however, for a population of
    cells, the process appears deterministic.

         The mechanisms of aging are likely to be coupled to the
    reproductive strategy of the organism. One example is the
    synchronous, rapid physiological declines and mortalities that are
    characteristic of species with single massive episodes of
    reproduction (e.g., migrating Pacific salmon or soybean plants). 
    Placental mammals, however, have ample opportunity for a variety of
    stochastic processes to take place during their long reproductive
    and postreproductive phases. The associated patterns of structural
    and functional decline can vary substantially, both qualitatively
    and quantitatively, among individuals within a species and among
    different related species. Evolutionary biologists in fact present
    compelling arguments that aging did not evolve because of any

    adaptive value to the individual or to the species, as would be
    assumed by strictly programmed theories (reviewed in Rose, 1991). 
    Aging is thought to occur simply because of the decline in the force
    of natural selection for gene action that is postreproductive. Such
    gene action could be related to accumulations of late-acting
    mutations in the constitutional genome or to selection for forms of
    genes that have positive effects on reproductive fitness early in
    the lifespan, but whose effects may be negative late in the lifespan
    (the "antagonistic pleiotropy" theory of aging) (Rose, 1991).

         It is beyond the scope of this monograph, however, to consider
    the potentially large numbers of specific mechanisms that may be
    modulated by such accumulated constitutional mutations or
    pleiotropic genes. The reader is referred to recent reviews of the
    many postulated theories of aging (Warner et al., 1987; Committee on
    Chemical Toxicity and Aging, 1987; Finch, 1991; Cutler, 1991). 
    These can be classified in a variety of ways (Dilman, 1987;
    Medvedev, 1990).


    2.1  Changes in gene structure and function in aging

         Changes in gene expression are of critical importance to an
    organism. Aging can potentially alter not only the structure of
    genes, but the way in which they function. Changes in the DNA are
    often thought to be integral to aging. It is clear that not only
    mutations, but chromosomal rearrangements accumulate with age (Vijg,
    1990). Repetitive sequence families may play a crucial role in the
    processes of aging. In addition, the organization of DNA and protein
    in chromatin is important structurally and functionally. Therefore,
    changes in chromatin could play a major role in the age-related
    change in the regulation of gene expression (Richardson et al.,
    1983; Medvedev, 1984; Thakur, 1984; Richardson et al., 1985).

    2.1.1  Chromatin structure

         Chromatin changes may involve either proteins that interact
    with DNA or the chemical structure of the DNA molecule itself. 
    Although no change in the stoichiometry of the major histones has
    been observed with increasing age (Richardson et al., 1983;
    Medvedev, 1984), several investigators have reported changes in the
    subspecies of histone H1 (Medvedev, 1984; Mitsui et al., 1980;
    Niedzwiecki et al., 1985). The acetylation of histones, which has
    been proposed to alter histone-DNA interactions thereby making DNA
    more accessible, decreases by 30% to 70% with increasing age
    (O'Meara & Pochron, 1979).

         With respect to age-related changes in DNA chemical structure,
    there is now conclusive evidence for the "spontaneous" induction of
    a variety of DNA lesions in different organs and tissues of both
    humans and experimental animals (for a review, see Mullaart et al.,
    1990). Most of these lesions seem to be repaired (see below), but
    not all. For example, Cathcart et al. (1984) and Fraga et al. (1990)
    estimated that in rats about 105 oxidative DNA lesions occur per
    cell per day. Since the rate of repair does not entirely equal the
    rate of induction of damage, there is a net increase of spontaneous
    DNA lesions with age. Fraga et al. (1990) calculated for one
    specific lesion, 8-hydroxy-deoxyguanosine, that about 80 residues
    accumulate per rat cell per day. 

         Although some DNA lesions are repaired quickly, this is not the 
    case for all lesions. Indeed, after treating rats with low doses of 
    2-acetyl-aminofluorene (AAF), Mullaart et al. (1989) were still able 
    to detect about 30% of the major lesions induced as late as 21 days 
    after treatment. Such incomplete repair could be responsible for 
    accumulation of DNA lesions during continuous or frequent exposure to 
    genotoxic agents.

    2.1.2  DNA repair

         To preserve the DNA chemical structure, cells are equipped with
    a battery of repair systems to remove damage. As yet the various
    mechanisms of action of these DNA repair systems and their
    interrelationships are incompletely understood (for a recent review,
    see Lehmann et al., 1992). In general, repair systems can be divided
    into three categories, i.e. direct repair, excision repair and
    post-replication repair. In direct repair, the lesion itself is
    removed without any further (transient) changes in the DNA
    structure. Direct repair includes the enzymatic photo-reactivation
    of UV-induced pyrimidine dimers and the removal of O6-alkyl
    adducts by specific alkyl transferases.

         DNA excision repair is brought about by a complex multi-enzyme
    system, the components of which are involved in the various steps in
    this repair process (Vijg & Knook, 1987). The third type of repair,
    post-replication repair, does not actually remove the damage but
    allows the replication system to bypass the damage. It is this
    latter process especially that is considered to be associated with
    nucleotide misincorporation (mutation).

         Accurate assessment of an organism's capacity to repair
    specific lesions is difficult and subject to error. In general, the
    most reliable data can be obtained when the induction and
    disappearance of the relevant lesions themselves are monitored in
    the different organs and tissues of an experimental animal. 
    Unfortunately, in most studies on the possible existence of a
    decline in DNA repair activities with age, assays were used which
    measured the DNA synthesis phase of excision repair. The general
    conclusion from these data, mostly obtained with cultured cells, is
    that there is no age-related decline in the efficiency of DNA repair
    systems (Tice & Setlow, 1985; Likhachev, 1985; Hanawalt, 1987). It
    cannot be ruled out, however, that during aging DNA repair systems
    become more error prone, leading to an accelerated induction of
    mutations (Vijg & Knook, 1987). In any case, a certain degree of
    imperfection is a general characteristic of DNA repair systems as
    indicated by the actual accumulation of both DNA lesions and DNA
    sequence changes (see above). The question that should be addressed
    is what type of DNA alterations occur, how many exist, and at what
    rate do they accumulate with age. Finally, their relevance in terms
    of actual physiological decrements or the initiation of disease
    should be assessed.

    2.1.3  Transcription

         Several review articles have been published in the past decade
    that discuss the effect of age on transcription (Rothstein &
    Seifert, 1981; Richardson et al., 1983; Richardson et al., 1985;
    Richardson & Semsei, 1987; Slagboom & Vijg, 1989). A major problem

    in this area has been the difficulty in accurately measuring the
    rates of synthesis of specific RNA species and their intracellular
    levels. With the major advances in recombinant DNA technology, this
    problem has now been virtually eliminated and our knowledge of how
    aging affects the expression of specific genes is rapidly growing.

         At present, it appears that the overall transcriptional
    activity of a cell declines as an organism ages. However, the level
    of total RNA tends to remain constant suggesting a decline in the
    rate of RNA turnover (Horbach et al., 1986).

         The levels of some specific mRNA species using cDNA probes for
    specific genes have been measured recently (Richardson & Semsei,
    1987). In general, no consistent trend has emerged. The levels of
    some mRNA species decrease with age; however, other mRNA species do
    not change with age, and others actually increase (Slagboom & Vijg,

         In most of the studies, a good correlation has been found
    between the age-related changes in the level of an mRNA species and
    the level of protein (or enzyme activity) specified by the mRNA
    species. This has been demonstrated in rat liver for albumin
    (Horbach et al., 1984), apha2u-globulin (Richardson et al., 1987),
    and superoxide dismutase and catalase (Semsei et al., 1989), and in
    rat kidney and small intestine for calbindin-D (Armbrecht et al.,
    1989). The age-related decline in mitogen-induction of interleukin 2
    (IL-2) (Wu et al., 1986; Nagel et al., 1988; Pahlavani et al., 1988)
    and IL-3 (Li et al., 1988) mRNA in lymphocytes from rodents and
    humans corresponded to the age-related decline in the biological
    activities of these two interleukins. In contrast, Strong et al.
    (1990) reported an uncoupling of tyrosine hydroxylase transcription
    and translation in the adrenal glands of old rats.

         Investigators usually assume that age-related changes in the
    levels of a particular mRNA species arise from a change in
    transcription. However, only a few studies have actually measured
    the transcription of a specific gene as a function of age using
    nuclear run-off assays. While an age-related decrease occurs in the
    nuclear transcription of the alpha2u-globulin (Richardson et al.,
    1987; Murty et al., 1988a), cytochrome P450(b+e) (Rath & Kanungo,
    1989), and superoxide dismutase and catalase (Semsei et al., 1989)
    genes, the nuclear transcription of tyrosine amino-transferase and
    tryptophan oxygenase (Wellinger & Guigoz, 1986), albumin (Horbach
    et al., 1988b) and the c-myc (Buckler et al., 1988) genes was
    similar in young and old rodents. Studies are now underway to
    explore in more detail age-changes in specified mRNA species in
    terms of the transcription factors involved (Post et al. 1991).

         One exciting development in the area of transcription and aging
    has been the observation that dietary restriction, which enhances
    the longevity of rodents, alters the expression of some genes at the
    level of transcription (Richardson et al., 1987; Semsei et al.,
    1989). However, the expression of all genes is not affected by
    dietary restriction (Waggoner et al., 1990).

         In addition to nuclear synthesis, post-transcriptional
    processing of hnRNA plays an important role in the regulation of
    gene expression. Müller et al. (1989) recently discussed various
    views of how the post-transcriptional processing of hnRNA might
    alter with age. At present, there is little evidence that major
    changes occur with age in the size of the poly(A)-segment of mRNA
    (Birchenall-Sparks et al., 1985). Interestingly, in the many studies
    in which mRNA species have been analysed by Northern blot analysis,
    there has been not a single report of a significant change in the
    size of the mRNA species examined with increasing age (Richardson &
    Semsei, 1987). Thus, there is very little direct evidence at present
    to support the view that the processing and/or nuclear transport of
    hnRNA is altered with age.

    2.1.4  Translation

         Increasing age generally results in a decrease in total protein
    synthesis in plants, invertebrates, rodents and cultured cells
    (Richardson & Birchenall-Sparks, 1983; Ward & Richardson, 1991).
    Recent studies have focused on the influence of age on the
    translation of mRNA into specific proteins and on the ability to
    modulate age changes in protein synthesis. There is no evidence that
    a decrease in the fidelity of protein synthesis occurs with
    advancing age but technical limitations do not permit a definitive
    conclusion (Rosenberger & Kirkwood, 1986). The influence of age on
    protein synthesis differs from protein to protein and much more work
    must be done in assessing the effect on key individual proteins.
    Attempts to modulate protein synthesis have recently begun. The rate
    of protein synthesis in the liver is higher after maturity for
    dietary restricted than for  ad libitum fed rats (Ward, 1988). In
    an  in vitro system, growth hormone increases protein synthesis in
    muscles of old rats to the level found in muscles of young rats
    (Sonntag et al., 1985). Much more study is required, focusing on
    individual proteins, different tissues and different organisms.

    2.2  Changes in tissues, organs and systems in aging

         The progressive modification of body functions with age
    involves alterations not only at the genetic, molecular and cellular
    levels, but at the level of the tissues, organs, systems and entire
    organism. It is important to attempt to differentiate between
    age-related pathology and true physiological aging. This is often
    difficult because the majority of age-related changes increase the
    vulnerability of the aging organism to disease and ultimately death.

         In the following, each organ or system will be discussed in
    reference to age-related changes in its structure which might
    predispose to alterations in function, not only inherently as part
    of aging, but in response to environmental agents. The focus will be
    on the healthy aged as opposed to the diseased.

    2.2.1  Nervous system

         The brain may undergo a progressive deterioration with age at
    all levels of organization - structural, biochemical and functional. 
    CNS disorders, including Parkinson's and Alzheimer's diseases, are
    common in the elderly.  Structural changes

         Brain weight decreases slightly with aging. This is due to
    atrophy of both grey and white matter (Creasey & Rapoport,1985).  At
    the cellular level, the major age-associated modification is in the
    number of neurons, which are significantly diminished in discrete
    areas of the brain (Brizzee, 1985), particularly in the basal
    ganglia, cerebellum (probably related to decreased motor control),
    locus ceruleus (associated with alterations in sleep patterns),
    nucleus basalis of Meynert (associated with senile dementia of
    Alzheimer type (SDAT) (Bondareff, 1986), and the spinal cord. 
    Neuronal loss, which is associated with an increase in the number of
    glial cells, is relatively mild in the healthy aged, but is much
    more severe in SDAT, Parkinson's disease and in the early aging
    associated with Down's syndrome.

         In addition to a reduced number of neurons, the aged brain is
    characterized by a reduction in the number of dendrites and
    dendritic spines, probably due to a slowing renewal process
    (Scheibel & Tomiyasu, 1978). Synapse density declines in discrete
    areas of the brain, but this is partially compensated by enlargement
    of the remaining synapses (Bertoni-Freddari et al., 1990).
    Intracellular changes include dilation and fragmentation of the
    Golgi apparatus (Mervis, 1981), distortion of membranes and the
    nucleus, and accumulation of lipofuscin, in both neurons and glial
    cells within discrete brain areas. With advancing age, there is an
    increase in neurofibrillary tangles (intracellular tangled masses of
    paired helical filaments) (Terry, 1963), extracellular neuritic
    plaques (a core of amyloid surrounded by material derived from
    dystrophic neurites), and reactive glial and microglial cell
    accumulation (Master et al., 1985). Again, these changes occur in
    normal aging at a moderate level, but are much more frequent in SDAT
    (Iqbal et al., 1982) and other dementias.

         There are also age-related changes in the morphology of the
    peripheral and autonomic nervous systems. These include reductions
    in the number of sensory and motor neurons, increases in
    demyelination, increases in connective tissue, and a mild loss of

    myelinated fibres (Tomlinson & Irving, 1977; Spencer & Ochoa, 1981).
    The central processes of dorsal root ganglion cells typically
    undergo distal dystrophic and degenerative changes. Regressive
    changes have been reported in the terminals of motor axons.  Biochemical changes

         Besides the pathological changes, there are many age-related
    alterations in brain chemistry required for cell-to-cell
    communications (Rogers & Bloom, 1985; Finch, 1991). These include
    changes in the concentration and/or turnover of the amines (e.g.,
    acetylcholine, norepinephrine, epinephrine, dopamine, serotonin),
    amino acids (e.g., glycine, glutamate and GABA) and peptides (e.g.,
    enkephalin, substance P, thyrotropin-releasing hormone,
    cholecystokinin, somatostatin). There are numerous studies showing
    impairments of adrenergic, dopaminergic and serotonergic activity in
    the senescent animal (Zhou et al., 1984; Roth & Joseph, 1988;
    Telford et al., 1988).  One of the underlying causes of these
    alterations seems to be an overall loss of receptors (Weiss et al.,
    1984; Roth & Joseph, 1988).

         Synapses may utilize one or more neuromodulator (e.g.,
    norepinephrine and neuropeptide). The multiple levels of control and
    the regional diversification of different synapses in discrete brain
    regions make it difficult to define the age-related alterations in
    neurotransmitter/neuropeptide function. In fact, rather than a
    uniform drop in the level of a specific neurotransmitter throughout
    the nervous system, a "desynchronization" of signals may occur. For
    example, while the brain content of norepinephrine and dopamine are
    decreased in old age, that of serotonin is unchanged or even
    increased, depending on specific brain areas. In some cases, the
    greater the concentration of a neurotransmitter in a discrete brain
    region, the higher the decrement with aging and vice versa (Timiras
    et al., 1984). In fact, age-dependent alterations in different
    neurotransmitter/neuropeptide concentrations do not always occur
    simultaneously. Each neurotransmitter has its own timetable:
    dopamine levels decrease in the cerebral hemispheres of rats from
    the age of one year, whereas in the same areas serotonin levels
    remain unaffected until three years of age (Timiras et al., 1984).
    Aged-related changes in neurotransmitter receptor number and
    function have also been reported (Greenberg & Weiss, 1983; Roth &
    Joseph, 1988). Changes in binding affinity have not been frequently
    detected. Beta-adrenergic receptor responsiveness is decreased in
    the elderly (Vestal et al., 1979; Lakatta, 1980). This appears to be
    due to uncoupling of the beta-receptor from the adenylate cyclase
    complex which transmits the signal (Wood, 1985). In the rat pineal
    gland, corpus striatum and cerebellum, a reduced responsiveness to
    catecholamines is present due to a decrease in the affinity of

    beta-adrenergic receptors to their ligand. However, there is no
    change in receptor number (Greenberg & Weiss, 1978). This may be due
    in part to a reduced ability to increase the number of
    beta-adrenergic receptors after decreased noradrenergic input
    (Greenberg & Weiss, 1979).

         Changes in general biochemical properties of the cells occur in
    the nervous system as they do elsewhere in the aging organism. 
    Lipid composition may change, resulting in altered membrane
    viscosity. Protein synthesis decreases in discrete brain regions. 
    Lipofuscin accumulates, although the functional significance is
    unclear, and there are alterations in electrolytes and trace
    elements (Brizzee, 1985). For example, aluminium levels may increase
    sharply in elderly people (Bjorksten et al., 1989). Decreases in
    zinc may be important in the light of the zinc requirement of
    various enzymes and growth factors, including nerve growth factor
    (NGF) (Dunn et al., 1980). In addition, aging is accompanied by a
    decreased brain water content (Meisami, 1988). Alterations in
    vascular flow have also been reported (Katzman & Terry, 1983).  Functional changes

         Despite the morphological and biochemical changes observed in
    the aging brain, the functional efficiency of the nervous system
    seems to be well maintained in most elderly people. However, CNS
    disorders do occur in some individuals, though it may be difficult
    to discriminate age-related pathology from physiological aging
    phenomena. Perhaps the most ubiquitous and significant change
    observed in the older organism is slowness of behaviour (Birren et
    al., 1979). The slowing of behaviour with age not only appears in
    motor responses and perceptual processing, but is also apparent for
    the more complex processing of information associated with
    short-term memory (Smith et al., 1980). Related cross-sectional
    studies using global measures of intellectual function such as the
    Wechsler Adult Intelligence Scale (WAIS) show evidence that some
    performance abilities decline by the late 60s and early 70s, while
    others (e.g., verbal abilities) appear to be maintained throughout
    life in healthy individuals (Gallagher et al., 1980). The slowing of
    reaction time may be associated with the age-associated slowing and
    loss of coordination in motor tasks, such as those involved in
    handwriting and other purposeful movements.

         The age-related modification of biorhythms is exemplified by
    the alterations of the sleep/wakefulness cycle, which is largely
    dependent on the reticular system. Alterations of sleep patterns
    with aging are qualitative rather than quantitative (Dement et al.,
    1985) and affect primarily the "deep sleep" phases, as confirmed by
    the alterations observed in the brain electrical activity (Müller &
    Schwartz, 1978). Among neurotransmitters, serotonin seems to be

         Alterations in posture and locomotion in the elderly (Klawans &
    Tanner, 1984) also depend on CNS impairment. Peripheral
    modifications such as decreased nerve conduction velocity, reduced
    muscle mass and increased rigidity occur. Autonomic system
    dysfunction is also implicated in many pathophysiological  changes
    of age including hypotension, thermoregulation, gastrointestinal
    function and urinary incontinence (Finch & Landfield, 1985). Other
    changes in the autonomic system include changes in vascular and
    cardiac reflexes, galvanic skin responses, and potency (Katzman &
    Terry, 1983). Sympathetic hyperactivity is commonly present in the
    aged and could interfere with cognitive functioning.

    2.2.2  Sensory organs

         All of the sensory organs are affected by aging, both those in
    which the cells are continuously renewed (such as cutaneous sense
    tissues) and those in which the cells are terminally differentiated
    early in life (vision and hearing).  Vision

         Both neural (retina) and optical (cornea, lens, pupil, aqueous
    and vitreous humours) components of vision are affected by age. The
    changes in the optical compartment are probably the primary cause of
    visual impairment in the elderly (Sekuler et al., 1982). The most
    common alterations are in the lens with increased hardness and
    decreased transparency (Graham, 1985). The former results in reduced
    refractive power (Marsh, 1980). The loss of transparency relates to
    the following chemical changes in the lens: protein oxidation,
    racemization, glycation, aggregation, polymerization and
    precipitation (Taylor, 1989). These alterations are associated with
    presbyopia and cataracts, respectively.

         In the neuronal compartment, the retina undergoes progressive
    loss of rods, while cones may be augmented. Morphometric analysis of
    the retina demonstrates an increase in electron-dense plaques and a
    decrease in the ground substance during aging. Such retinopathies
    result in decreased light sensitivity and reduced colour vision
    (Marsh, 1980).

         Vision declines as a function of age (Weale, 1986) and can be
    measured in several tests, such as the Humphrey Field Analyser
    (Iwase et al., 1988), and retinal potentials (Trick, 1987). Visual
    acuity is substantially decreased. The ability to detect light
    gradually decreases (Sample et al., 1988) and light adaptation
    declines (Katz & Robinson, 1987).  Hearing

         Decrements in hearing are frequently observed in the elderly. 
    There is also a progressive loss of hearing in animals with age
    (Willott, 1986). Both auditory structures and neuronal components
    are involved. While the outer and middle ear show few modifications,
    degenerative changes occur in the hair cells, which are the auditory
    receptors, and in the mechano-electrical transducing organs
    resulting in otosclerosis. This accounts for the preferential loss
    of hearing of high frequency sounds (presbycusis) (Marsh, 1980). The
    degree of hearing loss may affect the two ears differentially, thus
    causing defects in sound localization. Presbycusis has a great
    impact upon speech perception, since consonants, which make speech
    intelligible, are generated by high frequency sounds, whereas
    vowels, responsible for audibility, are produced by low frequency

         Hearing defects may also result from changes in the neural
    components (Allison et al., 1984) of hearing, and in particular in
    the nerves connecting the cochlea with the auditory centres in the
    brain, specifically in the superior temporal gyrus.  Olfaction

         The age-related alteration in the sense of smell is generally
    underestimated. The reduction in olfactory sensitivity is mainly due
    to the progressive loss of olfactory neurons, which protrude through
    cilia from the superior nasal cavity and represent the receptor
    sites for odour and the chemo-electrical transducing mechanism
    (Naessen, 1971). Loss of neurons have also been demonstrated in the
    olfactory bulbs of the brain (Bhatnagar et al., 1987).  Taste

         Taste thresholds are known to increase with age. The taste of
    salt is preferentially altered in the elderly. The loss appears due
    both to a decline in the number of taste buds and papillae (Bradley,
    1979) in the tongue, as well as to the loss of neurons in the
    cerebral centers of the gustatory system.  Somatic sensations

         The somatic sensory system (touch, pressure, vibration,
    proprioception, heat, cold and pain) is variably affected by age. 
    Tactoperceptual ability and vibrotactile sensations are decreased in
    the elderly due to the loss of Meissner end-organs and Pacinian
    corpuscles present in the skin (Bruce, 1980). For more complex
    somatesthetic abilities (stereognosis, body part recognition) as
    well as for pain and thermal sensitivity, the biological causes of
    their alterations with age involve not only the sensory end-organs,
    but also affective and cognitive factors (Marsh, 1980).

    2.2.3  Endocrine system

         Hormones play an important, often critical, role in the
    regulation of a large number of physiological and behavioural
    processes, and their influence can be demonstrated throughout the
    lifespan. Some hormones have a role in differentiation in that their
    presence or absence during certain developmental periods will affect
    the way in which physiological and behavioural processes proceed or
    are expressed in adulthood. Throughout each period of the life span,
    the maintenance of an appropriate endocrine milieu is essential to
    the numerous homeostatic processes required for survival. With
    advancing age, there are several, well-documented changes in the
    ability of the organism to synthesize and secrete a number of
    hormones. It is, therefore, likely that the typical age-related
    change in an organism's endocrine balance would result in, or at
    least contribute to, the impairment of homeostasis frequently
    observed in the elderly. Such impairments can be noted in the
    decreased rate of recovery of the elderly from the insults of injury
    or disease.

         Hormones may also play a significant role in the aging process. 
    For example, age-related changes in several physiological functions
    appear to be closely linked to the level and pattern of hormonal
    stimulation present during adulthood. As such, different patterns of
    exposure to a hormonal environment may alter the "rate of aging"
    within a specific neuroendocrine system and, in turn, affect the
    susceptibility of the organism to environmental insults at different
    segments of the life span. There are a number of different ways in
    which endocrine systems and the hormonal signalling operations that
    they use may undergo alterations with age and toxicant exposure.
    These can be categorized as changes in: (a) the availability of
    hormones for binding to the target tissues, (b) the reception of the
    pertinent transmitter or hormonal signal by the target cells, and
    (c) the nature of the hormonal message.

         At any point in time, the concentration of a hormone in the
    blood is a consequence of both its metabolism and secretion. Such
    changes in the size of the available signal pool may have
    corresponding effects on the magnitude of the response by the target
    tissue. Other changes may reflect declines with age in the
    homeostatic controls, which rely heavily on endocrine feedback
    relationships within organ systems.

         Serum hormonal levels, as a rule, are not maintained at
    constant levels. They tend to fluctuate, sometimes markedly,
    throughout a 24-h period. In the young adult man, peak morning
    testosterone values can fall by one-third to an early evening nadir,
    before rising again through the late evening and early morning hours
    (Bremner et al., 1983). A similar circadian rhythm in circulating
    levels of testosterone is prevalent in the rat (e.g., Kinon & Liu,

    1973; Ellis & Desjardins, 1982). Human cortisol (Bilchert-Toft,
    1978) and rat corticosterone (Moberg et al., 1975; Kato et al.,
    1980) concentrations also exhibit well-known rhythmic fluctuations,
    as do those of thyrotropin (Vanhaelst et al., 1972; Leppaluoto
    et al., 1974) and growth hormone (Millard et al., 1985). Reported
    attenuations with age in the rhythms of human and rat serum
    testosterone (Bremner et al., 1983; Steiner et al., 1984),
    luteinizing hormone (LH) (Vermeulen et al., 1989), and growth
    hormone (Sonntag et al., 1980; Prinz et al., 1983), among other
    hormones, can present differences in young-versus-old comparisons,
    depending on when such sampling is performed.

         While observable changes in hormonal rhythms or significant
    differences in circulating hormone concentrations may reflect
    disturbances in the overall functional integrity of the associated
    organ system, the absence of such changes should not be necessarily
    assumed to indicate a corresponding absence of a functional
    alteration. The notion of a "system at risk" presupposes an increase
    in the susceptibility to disruption of the homeostatic controls. An
    aging system that may be undergoing a subtle erosion in its
    endocrine balance could be more likely to exhibit alterations in its
    response to a stressor or toxic insult. In this respect the
    stimulation of growth hormone release by clonidine, L-dopa and
    insulin is substantially depressed (Riegel & Miller, 1981), while
    arginine-stimulated growth hormone (GH) secretion after arginine
    infusion is preserved (Aschoff, 1979).  Secretion stimulated by GHRH
    (GH releasing hormone) is only partially reduced (Coiro et al.,

         Regardless of these alterations, it remains established that
    the 24 h production of GH is significantly reduced in elderly humans
    (Prinz et al., 1983), whereas that of prolactin is increased
    (McGinty et al., 1988; Blackman, 1987). These data have been
    confirmed in animals (Ceda et al., 1986; Sonntag & Gough, 1988),
    although measurements of hormonal profiles may have involved
    different procedures in animals and man, thus giving rise to
    slightly different interpretations.

         Similar difficulties are encountered in studies of age-related
    alterations in pineal hormone secretion, including melatonin, whose
    circadian rhythmicity is certainly changed with age (Reiter, 1986;
    Anisimov & Reiter, 1990).

         In order to illustrate age-related alterations in hormone
    control, it is useful to focus on the integrated systems which
    involve more than one gland or hormone. Although three such systems
    are reviewed below, this discussion is by no means intended to be
    comprehensive. One theme common to studies of age-related changes in

    endocrine function is that such alterations are often hormone and
    species specific. Finally, the extent to which any of these changes
    relate to potential adverse health outcomes in the older organism
    remains to be demonstrated.  The pituitary-thyroid axis and the basal metabolism

         Thyroid hormones are required during development for growth and
    in adult life for regulating oxygen consumption. Maintenance of
    thyroid function is generally assured even in old age, although
    following repeated stress and demands the reserve function may
    become exhausted and a dysthyroid state may follow (Ingbar, 1978).

         Changes with aging in the levels of both thyroid stimulating
    hormone (TSH) and thyroid hormones (thyroxine (T4) and
    triiodothyronine (T3)) are controversial, because concomitant health
    disturbances may cause significant fluctuations in the levels of
    these hormones (Gregerman & Solomon, 1967; Utiger, 1980). Both hypo-
    and hyperthyroidism are not uncommon in the elderly. In general, the
    size of the thyroid decreases with age (Gambert & Tsitouras, 1985).

         Older people show a normal response to decreased thyroid
    function by increased secretion of TSH (Eden, 1987). TSH levels
    undergo few changes (Miller, 1989), suggesting that the hypothalamic
    control of TSH release has not been altered. However, structural
    modifications of TSH have been reported (Klug & Adelman, 1977). T4
    levels remain unchanged with age, even though the rate of synthesis
    is reduced. However, the blood levels of T3 are reduced with
    advanced age (Chopra et al., 1978), while levels of reverse T3 are
    unchanged. It should be noted that severe and chronic illnesses, not
    directly involving the thyroid, can lower the levels of T3 and

         The alterations observed in thyroid hormone levels are
    inadequate to explain the age-associated decline in various
    functions that are dependent on thyroid hormone. One possible
    explanation is that peripheral sensitivity to thyroid hormone action
    is modified by aging. However, with advancing age, the basal
    metabolic rate remains unchanged if based on lean body mass, but
    decreases if expressed based on body surface area (Masoro, 1985).  The pituitary-adrenal axis

         The major function of this axis, which is largely based on
    pituitary hormones (ACTH) and adrenal hormones (corticosteroids), is
    to provide an adaptive response to environmental stress (Selye,
    1950; Sapolsky et al., 1986). Any harmful agent, in addition to
    inducing a specific reaction in the body (anaesthesia, emotion,
    fever, etc.), activates a specific and common response, the
    so-called "General Adaptation Syndrome" (Selye, 1950), characterized

    by increased adrenocortical secretion, thymic involution,
    lymphopenia and eosinopenia. With advancing age this axis may
    undergo desynchronization, thus resulting in a failure of
    homeostasis and adaptation (Anisimov & Reiter, 1990).

         ACTH secretion, which shows a circadian rhythm based on
    melatonin fluctuations, is generally preserved in advanced age
    (Halberg, 1982), although minor modifications of blood levels may
    occur due to variations in renal clearance or alterations in sleep
    patterns. However, there is evidence of diminished sensitivity of
    the hypothalamic/pituitary axis feedback inhibition by
    glucocorticoids (Greden et al., 1986; Blackman, 1987; Dilman, 1987;
    Sapolsky et al., 1987). The elderly suffering from Alzheimer's
    disease are extremely resistant to glucocorticoid negative feedback
    (Sapolsky et al., 1986).

         Finally, certain cell populations (e.g., CA3 neurons in the
    hippocampus) are particularly susceptible to glucocorticoids. 
    Long-term stress may result in their dysfunction and death (Sapolsky
    et al., 1987).  The endocrine pancreas and carbohydrate metabolism

         It is well documented that with advancing age the ability to
    maintain glucose homeostasis is impaired, but the underlying
    mechanisms are still not well defined. Several hormones contribute
    to the regulation of glucose homeostasis: above all, insulin and
    glucagon, secreted by the endocrine pancreas, and somatostatins and
    the pancreatic polypeptide, which modulate the secretion of insulin
    and glucagon, respectively. In addition, glucose metabolism may be
    affected by other hormones, including T3 and T4, growth hormone,
    glucocorticoids and epinephrine (Minaker et al., 1985).

         Only modest morphological alterations are observed in the
    endocrine pancreas with advancing age. In spite of this fact, blood
    sugar levels after fasting are elevated and glucose tolerance is
    lowered in the elderly (Magal et al., 1986; Eden, 1987; Ammon
    et al., 1987; Wang et al., 1988; Groop, 1989). Plasma insulin
    concentration increases and insulin sensitivity decreases.
    Alterations in insulin turnover are detectable in the elderly after
    glucose load, such as reduced insulin secretion and increased
    secretion of the inactive prohormone, proinsulin (Marx, 1987), but
    these changes are too modest to account for the observed glucose
    intolerance. One alternative explanation is an increase in
    peripheral resistance to insulin. In fact, peripheral uptake of
    glucose is indeed reduced in the elderly, due to a reduction in
    insulin receptors (Pagano et al., 1981) as well as alterations in
    the post-receptor signalling process (Rowe et al., 1983). No
    evidence exists regarding the possible involvement of age-related
    changes in glucagon affecting glucose intolerance in the elderly.

         Other factors, however, may contribute to glucose intolerance. 
    These include: (a) reduced liver sensitivity to insulin, resulting
    in reduced glycogenesis; (b) changes in diet and physical exercise;
    and c) increased body fat with reduced muscle mass. This last point
    seems to merit particular consideration in view of the observation
    that insulin resistance is certainly increased in obese humans
    (Runcie, 1985). In fact, intracellular fat accumulation leads to a
    reduced concentration of insulin receptors (Bolinder et al., 1983).

    2.2.4  Reproductive system

         The age-related modifications of the reproductive system are
    primarily based on alterations in the central nervous system,
    pituitary gland and gonads. While menopause is a time-fixed event
    involving cessation of ovarian function, the decline of testicular
    function is a slow and gradual process, involving limited hormonal
    alterations. Older persons show the normal response to deficient
    gonadal function by increased synthesis of gonadotropins (Piva
    et al., 1987). This occurs in both sexes. In fact, alterations in
    serum levels of both luteinizing hormone (LH) and follicle
    stimulating hormone (FSH) have been reported (Blackman, 1987). The
    reduced presence of sex steroids in women may have an influence on
    the function of other endocrine glands. Estrogens have
    well-documented effects on salt and water balance and on plasma
    proteins, which in turn have effects on the level of thyroid
    hormones through a suppression of TSH secretion. Estrogen also
    stimulates the production of growth hormone and prolactin. Thus, the
    decline in gonadal function during age could have far-reaching
    consequences on the individual's physiological function.  Female aging

         In females, cessation of ovarian function consists of the
    transfer from regular menstrual cycles to amenorrhoea, usually
    preceded by a period of cycle irregularity. The initial changes have
    been reported to occur in hypothalamic-pituitary control of the
    ovaries. For example, the age-related decline in reproductive
    function is associated with a decreased sensitivity of the
    hypothalamic-pituitary complex to feed-back regulation by estrogens
    (Dilman, 1971, 1987). This leads to an age-related enhancement of
    pituitary gonadotropins (FSH, LH) (Chakravarty et al., 1976),
    leading in turn to hyperstimulation of the ovaries. However, despite
    the compensatory increase in ovarian hormone production, the level
    of estrogens is insufficient to induce ovulation because of
    hypothalamic insensitivity, possibly due to an age-related decrease
    in the level of biogenic amines and/or peptide hormone receptors
    (Dilman & Anisimov, 1979). In addition, the progressive loss of
    oocytes plays an important role in the decline in reproductive
    function since the reduction of maturating oocytes may induce
    desynchronization of pituitary-ovary hormonal interactions
    (Aschheim, 1976).

         The most common consequences of menopause include imbalances of
    the autonomic nervous system, psychological modifications, and
    physiological alterations of target organs due to metabolic changes.
    Alterations of estrogen target organs are among the most evident
    effects of menopause. Vulvar skin and vaginal epithelium may undergo
    atrophy. Glycogen content is generally reduced, with a consequent
    decrease of lactobacilli, rise of vaginal pH, and increased growth
    of pathogenic microbes. The uterus and oviducts atrophy due to the
    decreases in estrogen levels. In ovaries, follicular cysts and
    atresia result in response to the altered hormonal status.
    Hyperplasia of the theca cells occurs. Fibrosis also occurs in these
    tissues but, in addition, can affect the bladder and urethra,
    resulting in an increased incidence of cystitis, dysuria and
    non-infectious urethritis. The reduced thickness of the skin is also
    a result of the decrease in estrogen (Schiff & Wilson, 1979).

         Menopause has major health consequences for the cardiovascular
    and skeletal systems. The reduction in estrogen secretion removes
    the protection offered by these hormones against coronary heart
    disease, development of atherosclerosis, and accompanying
    alterations of lipid metabolism.  Osteoporosis, resulting from
    increased bone reabsorption relative to bone formation, is a common
    problem in postmenopausal women (Riggs, 1987). Two types of
    osteoporosis may be identified. Type I is associated with estrogen
    withdrawal and may begin in middle age (Riggs & Melton, 1983). The
    biological effects are linked to disruption of the complex
    relationship between calcium intake and loss, and the secretion of
    calcitonin, parathyroid hormone and 1,25-dihydroxy-vitamin D.
    Estrogens prevent the transfer of calcium from bone to blood and its
    loss through urine. This induces parathyroid hormone secretion,
    which stimulates the formation of 1,25-dihydroxycholecalciferol, the
    active metabolite of vitamin D. The function of the parathyroid is
    affected by increasing age (Eden, 1987). The relevance of estrogen
    for bone loss is further supported by the effectiveness of estrogen
    therapy in delaying the osteoporotic process in post-menopausal
    women (Edman, 1983). With advancing age type II osteoporosis (senile
    osteoporosis) may occur, which is probably due to the poor
    intestinal absorption of calcium (Riggs, 1987).  Male aging

         The reproductive system is less affected by aging in males than
    in females. It is generally accepted that testosterone levels are
    maintained within the physiological range throughout life, although
    a decrease in testosterone production in response to gonadotropin
    action may occur in old age due to a reduction in Leydig cell number
    and function (Harman et al., 1982). Testis and accessory sex organs
    do not show substantial modifications with age, and sperm is found
    in the ejaculate of very elderly men. The volume of seminal fluid is
    generally decreased.

         While the prostate undergoes involution in the majority of old
    men, in about one-third of males it undergoes hypertrophy with
    consequent obstruction of the urethra and urinary flow from the
    bladder. The cause of the hypertrophy is still unclear (Mawhinney,
    1985). The prostatic enlargement results in compensatory hypertrophy
    of the bladder. When such  compensation is no longer sufficient,
    retrograde filling of the renal pelvis and ureters may occur,
    resulting in hydronephrosis and eventually renal failure.

    2.2.5  Immune system

         With advancing age a progressive increase occurs in the
    incidence of various infectious diseases, autoimmune processes and
    tumours. These may be in part based on age-related defects in the
    immune system. The association of so many age-related pathologies
    with defects in the immune system has led to the suggestion that
    aging of the immune system may be rate limiting for life span
    (Walford, 1969). However, while there are numerous experimental and
    clinical studies demonstrating an age-related deterioration in
    immune efficiency, this decline is not sufficient to account for all
    manifestations of aging.

         There are several recent reviews on aging and the immune system
    (Revskoy et al., 1985; Lipschitz, 1987; Segre et al., 1989; Miller,
    1991). However, it is still difficult to draw a comprehensive
    picture, because of the many cellular and humoral components
    involved in immune reactions and the many modulating
    extra-immunological factors which may also be compromised in the
    elderly. The immune and haematopoietic systems are intimately
    related, being derived from a common pluripotent stem cell. Both
    play central roles in host defense, prevention of neoplasia, and
    response to infectious agents (Lipschitz, 1987). However, basal
    haematopoiesis in both animal models and man seems to be either
    unchanged or minimally altered with age (Dybkder et al., 1981;
    Lipschitz, 1987). The reserve capacity may be reduced resulting in a
    decreased ability to respond to stress.  Aging of lymphoid organs

         Peripheral lymphoid organs, such as the spleen and lymph nodes
    do not show consistent modifications in size with aging. Bone marrow
    is not consistently affected by age. Stem cell production is
    generally well preserved in old age (Harrison et al., 1978),
    although a slight change in the replication rate of stem cells has
    been reported by some authors (Schneider et al., 1979). Thymic
    involution has been considered to account for the major age-related
    changes in the immune system, beginning at puberty. Such an
    involution consists of a progressive loss of cellularity with
    lymphoid cell depletion in the cortical areas and cystic changes in
    the epithelial cells. These are the source of various peptides

    involved in differentiating thymic lymphocytes (T-cells) from
    lymphoid cells of earlier lineage. The export of newly
    differentiated T-cells is reduced with advancing age (Globerson
    et al., 1989). The synthesis and the secretion of polypeptide thymic
    hormones, such as thymosin (McClure et al., 1982), thymopoietin
    (Lewis et al., 1978) and thymulin (Bach et al., 1972), are
    progressively diminished. In all cases, the reduction of thymic
    endocrine activity seems to have a pathogenic role in age-related
    immune dysfunctions, since replacement by exogenous administration
    of the hormones is capable of restoring various immune functions in
    old age (Zatz & Goldstein, 1985). The turnover of zinc, which is
    essential for immunocompetence (Iwata et al., 1979; Chandra, 1985),
    decreases in old age. Zinc supplementation can restore immune
    functions (Fabris et al., 1990).  Aging of cellular constituents

         Mature T-cells, bone marrow lymphocytes (B-cells) and natural
    killer cells (NK-cells) can be detected in blood and in lymphoid
    organs by specific monoclonal antibodies. With this type of
    analysis, no major modifications in the proportion of the various
    lymphoid cell subpopulations have been observed in humans. However,
    the major alteration in the immune system appears to arise in the
    functioning of T-cells (Thompson et al., 1987). While the total
    number of T-cells in the peripheral blood does not change
    appreciably with age, there are clear-cut differences in the
    relative proportion of T-cell subtypes (Wagner et al., 1983;
    Fernandes, 1984; Revskoy et al., 1985; Thompson et al., 1987;
    Lipschitz, 1987).

         The number of immature lymphocytes of the T-lineage increases
    with age, as does the percentage of apparently activated
    T-lymphocytes bearing immature thymic phenotypic markers. There is a
    relative increase in cytotoxic/suppressor T-cells, and a decrease in
    the number of helper/inducer T-cells (Lipschitz, 1987; Thompson et
    al., 1987). Correlated with the decrease in the helper/inducer
    population, is a functional defect in cell-mediated immunity
    (Lipschitz, 1987; Thompson et al., 1987). Cells from aged humans or
    experimental animals are less capable of responding to allogeneic
    lymphocytes, phytohaemagglutinin, concanavalin A and soluble
    antigens. Lymphocytes from older mice are less able to elicit
    graft-versus-host reactions than those from younger mice of the same
    inbred strains (Thompson et al., 1987). Fifty percent of healthy
    people over age 50 have impaired cutaneous hypersensitivity
    (Lipschitz, 1987; Dilman, 1987).  Accompanying the decrease in
    helper/inducer T-cells and cell-mediated immune functions is a rise
    in autoantibodies and autoimmunity (Thompson et al., 1987).

         Changes in humoral immunity (B-cell function) with aging are
    more subtle (Lipschitz, 1987; Senda et al., 1989). Studies on the
    effects of age on antibody production have yielded conflicting
    results, perhaps because of the wide range of experimental values
    generally observed in older individuals. It has, however, been well
    established that aging is significantly associated with the presence
    of various autoantibodies, in particular, antibodies against nuclear
    antigens. There is also evidence that aging effects the rate of
    antibody production by activated B-cells (Lipschitz, 1987).

         From a functional point of view, defects have been observed at
    various levels. Firstly, the proliferative capacity of T-cells from
    old individuals is generally reduced, regardless of the stimuli used
    (antigens, mitogens), and the defect consists both in a reduced
    number of cells responding to stimulus and in a precocious
    exhaustion of the cloning capacity (Fabris et al., 1983) of
    responding cells. Secondly, the response to interleukins, which
    physiologically mediate the modulation of the proliferative
    reaction, is depressed and this phenomenon has been documented not
    only for T-cells but also for NK cells, which are less sensitive in
    old age to the boosting action of IL-2 or interferons (Provinciali &
    Fabris, 1990).

         With respect to accessory cells (phagocytic cells,
    macrophages), their number and function are not altered by age, and,
    in certain circumstances, their activity seems to be enhanced.  Neuroendocrine-immune interactions

         The immune system, although regulated to a large extent by
    intrinsic cellular and humoral events, is also sensitive to signals
    generated from the nervous and endocrine systems. Communication
    between nervous and immune networks is mediated by hormones and
    neurotransmitters which reach lymphoid organs and cells via blood or
    direct autonomic nervous system connections (Bullock, 1985; Felten
    et al., 1985). The neuroendocrine immune interactions are mediated
    by circulating humoral factors from the
    pineal-hypothalamic-pituitary axis, either directly via
    neuropeptides and hormones, or indirectly by the effects of this
    axis on the hormonal secretion of peripheral endocrine glands, which
    also exert immunomodulating actions (for review, see Fabris, 1991).

         The nervous and neuroendocrine systems not only act as
    modulators of the immune network, but also as targets for signals
    generated within the immune system, such as those exerted by thymic
    factors (Hall et al., 1989) and interleukins, (Besedovsky et al.,
    1985), and by pituitary-like factors (ACTH, TSH, GH, PRL,
    gonadotropins, endorphins), which are produced by mature lymphocytes
    upon antigenic stimulation (Weigent et al., 1990).

         The sharing of humoral signals, as well as of the specific
    receptors between neuroendocrine and immune cells (for reviews, see
    Fabris & Provinciali, 1989; Weigent et al., 1990), implies that
    biological response modifiers of neuroendocrine-immune origin might
    be developed in the near future for therapeutical purposes. On the
    other hand, potentially harmful agents for one of these homeostatic
    systems may also cause alterations in others. 

         From an experimental point of view, it has been demonstrated that 
    treatments of old animals with thyroid hormones (Fabris et al., 1989), 
    GH (Kelley et al., 1986) and analogues of LH releasing hormone 
    (Greenstein et al., 1987) are able to induce regrowth of the thymus 
    and reacquisition of its endocrine activity (for review, see Fabris
    1991). Analogous treatments, such as with melatonin (Pierpaoli
    et al., 1991), GH (Davila et al., 1987), TSH and thyroid hormones
    (Provinciali & Fabris, 1990), are also able to recover various
    age-related peripheral immune deficiencies, such as T-cell
    functioning and NK cytotoxicity. In humans, little work has been
    done in this area, although indirect evidence, obtained primarily
    from studies on endocrinopathies in the elderly (Fabris et al.,
    1989; Travaglini et al., 1990), suggest that recovery of both thymic
    and peripheral immune function can be achieved by a neurohumoral

         Little is known on the potential effect of thymosins,
    interleukins and lymphocyte-derived pituitary-like cytokines on
    age-related alterations of the nervous and of the neuroendocrine
    system. Experimental information from old animals showing recovery
    of the hormonal and metabolic profile following immune manipulation
    (for review, see Fabris et al., 1988) is undoubtedly opening a new
    research approach for human investigations.

         Receptor sites for many hormones are present on the membrane of
    lymphoid cells (Fabris & Provinciali, 1989). The number of
    glucocorticoid receptors in spleen cells decreases in old animals
    (Roth, 1979a). Hormones that modify the turnover of cyclic
    nucleotides result in consequent activation or inhibition of immune
    functions (Hadden, 1983). Hormones influence the production of
    several lymphokines and monokines (Kelso & Munck, 1984).

         The neuro-endocrine system seems to act not only as a modulator
    of the immune network but also as a target for signals generated
    within the immune system. Examples of such interactions are the
    alterations that can be induced in the neuroendocrine balance,
    either by removal of relevant lymphoid organs such as the thymus or
    by dysfunction of the immune system itself as a result of reactions
    to immunogenic or tolerogenic doses of antigen (Besedovsky et al.,
    1975). In addition, mature lymphoid cells, when stimulated by
    antigens, produce humoral factors similar, if not identical, to
    classical hormones and neurotransmitters (such as ACTH, TSH, GH,

    PRL, gamma-endorphins) (Blalock et al., 1985). These reciprocal
    influences between the neuroendocrine and the immune systems
    (Fabris, 1981; Fabris et al., 1988) occur throughout life, but have
    particular relevance during aging (Fabris & Piantanelli, 1982).

    2.2.6  Cardiovascular system

         The frequency of cardiovascular diseases, which are the major
    cause of death in industrialized countries, increases with age.
    Diseases such as hypertension and atherosclerosis occur most
    commonly in the elderly. In addition, degenerative changes of the
    cardiovascular system, involving the myocardial cells as well as
    cells of the pacing-conduction system, that arise during the aging
    process lead to impaired cardiac function and arrhythmia even in
    people without any clinical evidence of hypertension or coronary
    artery diseases. Inadequate function of the cardiovascular system
    induces effects in peripheral tissues and organs. Changes in
    peripheral organs resulting in hyperlipidaemia,
    hypercholesterolaemia, and hypo- and hyperglycaemia can also effect
    the cardiovascular system in the elderly.  Heart

         The heart itself can be considered to be made up of two parts:
    a) the conduction system responsible for electrically controlling
    the heart rhythm; and b) the myocardium performing the contractile
    function of the heart and composed of a system of trabeculae.

         The biophysical and biochemical mechanisms that govern cardiac
    muscle change with age, resulting in characteristic alterations in
    muscle function (Lakatta, 1987a,b). Many of the steps in the
    excitation-contraction system in cardiac muscle are altered by
    aging. In an isometric contraction, the transmembrane action
    potential (TAP) excites the cell and the contractions that ensue are
    longer in duration. The magnitude of the prolongation of
    depolarization of the TAP in senescent muscle is striking (i.e. 
    about twofold). The action potential amplitude is also greater in
    senescent than in adult muscle in both high and low calcium-loading
    conditions. These deficits of the senescent muscle may be related in
    part to the diminished Ca2+ pumping rate by sarcoplasmic
    reticulum. The duration of the elevated myoplasmic Ca2+ level is
    prolonged in senescent muscle.

         Sagiv et al. (1988) found that left ventricular contractility
    increases less on stimulation in elderly subjects than in younger
    people. Although the aging process is associated with normal resting
    contractile function, diastolic properties are altered, resulting in
    reduced and delayed early left ventricular filling and enhanced
    atrial contribution to diastolic volume. Exercise cardiac output is

    maintained in healthy elderly individuals, but there is a shift from
    reliance on an increase in heart rate and a decrease in end systolic
    volume to use of the Frank-Starling mechanism to increase stroke
    volume. This age difference in the cardiovascular response to
    exercise is probably mediated by an age-associated decreased
    responsiveness to beta-adrenergic stimulation.

         In muscle tissue of the aging heart, some morphological changes
    are observed both in animal and human studies (Koobs et al., 1978;
    Speijers, 1983). The most common change in the aging heart is
    hypertrophy (Lakatta, 1985). Other alterations consist of the
    appearance of slight focal necrosis and fibrosis in the myocardium,
    amyloidosis (Finch & Hayflick, 1977) and the appearance of
    lipofuscin (Hendley et al., 1963; Koobs et al., 1978). Peroxidative
    damage to the myocardium is cumulative and irreversible (Koobs
    et al., 1978).  Blood vessels

         Both physiological and morphological changes are observed in
    the vascular system, especially in the small and large arteries. The
    morphological changes in the arteries seen in the elderly vary
    considerably both in appearance and in localization (Goyal, 1982;
    Hazzard, 1985). The thickness of the aorta increases significantly
    with age, while the number of nuclei in the cells of the arterial
    media decreases in humans as well as in mice. The majority of these
    changes in humans are categorized as atherosclerotic. These changes
    can progress and result in complicated coronary atherosclerosis and
    ischemic heart disease, but other factors may also cause clinical
    effects such as angina pectoris, arterial spasms, and myocardial
    infarcts (Speijers, 1989a).

         Morphological changes in the veins are less pronounced than in
    the arteries.

         The physiological changes observed with aging are often a
    result of changes both in heart function and in the arteries. These
    changes are reflected in haemodynamic parameters such as an increase
    in diastolic and systolic blood pressure, mean arterial blood
    pressure and vascular resistance, and a decrease in responsiveness,
    and in contraction and relaxation responses (Lakatta, 1986, 1987b;
    Duckles, 1987; Mazzeo & Horvath, 1987;  Zemel & Sowers, 1988;
    O'Malley et al., 1988; Cleroux et al., 1988).

         Aging is often accompanied by increases in the incidence and
    prevalence of hypertension. Geriatric hypertension is generally of a
    salt-sensitive nature with a disproportionate frequency of isolated
    systolic hypertension. The age-related increase in salt sensitivity
    is due to a decline in renal function (Zemel & Sowers, 1988) and

    deregulation of vascular tonus. Age-associated declines in the
    activity of membrane sodium/potassium-ATPase may also contribute to
    geriatric hypertension because this results in increased
    intracellular sodium loading, causing reduced sodium/calcium
    exchange and thus increased intracellular calcium and vascular

         It is commonly accepted that atherosclerotic changes take place
    to a certain extent in every individual. Multiple factors determine
    the extent and velocity of the atherosclerotic process (Hazzard,
    1985). The incidence of clinically observed atherosclerotic effects
    is higher in elderly subjects than in younger individuals. 
    Atherosclerotic damages result in impaired cardiovascular function.

         Atherosclerosis is defined as a multifactorial disease with
    variable effects in the intima followed by changes in the media of
    arteries. These changes consist of focal accumulation of lipids,
    proliferation of smooth muscle cells, and accumulation of complex
    carbohydrates (i.e. glycosaminoglycans, proteoglycans), blood and
    blood products, collagen and calcium compounds (Campbell &
    Chamley-Campbell, 1981; Velican, 1981; Speijers, 1989a). The
    resultant modifications of arterial wall integrity can lead to the
    following: erosion of the wall with consequent reduced resistance to
    blood pressure, rupture, and haemorrhage; progressive thickening of
    the wall due to reactive proliferation of tissues with consequent
    reduction of blood flow; and clotting of the blood at the level of
    the injured wall with consequent sudden obstruction. The basic
    lesion seems to develop in the first decade of life (Lee, 1985).

         In the etiology of the lesion two localized cofactors should be
    taken into account: the blood supply of the arterial wall; and blood
    turbulence, since lesions are more frequently found around the
    orifices of arteries branching off major arteries or at bifurcations
    (Patel & Vaishnaw, 1980). Hypertension, diabetes, autoimmunity  and
    stress are also risk factors contributing to atherosclerosis.  Characteristics of atherosclerotic lesions

         The first event in the formation of the lesion is still
    debated. Both an initial thickening of intima, due to an
    accumulation of blood-born amorphous material (lipids, protein and
    sulfated proteoglycans) and a proliferation of muscle cells (due to
    still undefined stimuli), with consequent degeneration and reactive
    macrophage and connective tissue accumulation, have been proposed as
    major initiating phenomena (Benditt, 1977; Ross, 1981). The
    subsequent phase of the lesion involves repair mechanisms that cause
    further thickening of the intima. Following this, lipid increases
    both in the cells and in the intercellular spaces. Lipid
    accumulation is progressive, leading to an increased number of foam
    (lipid-containing) cells which disintegrate and form the gruel-like

    substance that has given the name of atheroma to the lesion. The
    accumulation of such material acts as an irritant, inducing a
    proliferative reaction (encapsulation) which leads to the
    development of a plaque. Calcification follows, making the arterial
    wall more rigid. Alternatively, when the capsule breaks, an ulcer
    can occur, leading to loss of tissue in the arterial wall, blood
    clots, haemorrhage, and consequent thrombosis and/or rupture of the
    arterial wall.  Theories of atherosclerosis

         Atherosclerosis is undoubtedly a multifactorial process, which
    is reflected by the many theories proposed (Baker & Rogul, 1987).
    The lipid accumulation theory is based on the progressive
    accumulation of oxidized lipids (mainly low density lipoproteins)
    not only in smooth muscle cells but also in migrating monocytes
    (Avogaro et al., 1983). The link between lipid deposition and
    consequent alterations remains unclear (McCaffrey et al., 1988).

         Theories on monoclonal proliferation of smooth muscles cells
    are based on a mutagenic event in these cells, due to a
    physico-chemical or viral insult (Benditt, 1977; McCaffrey et al.,
    1988), leading to the formation of a kind of benign tumour. The
    thrombogenic theory is based on the early adherence of platelets to
    small alterations in the endothelium, leading to thrombus formation
    and release of growth factors by platelets which induce smooth
    muscle cell proliferation (Ross, 1981; Bang et al., 1982). Other
    risk factors that should be taken into consideration are
    hypertension, cigarette smoking, increased body weight, high serum
    uric acids and consumption of saturated fats.

         The immune system in the elderly is weakened. Consequently
    there is increased susceptibility to chronic infections,
    autoimmunity, and elevation of circulating immune complexes. New
    data in the literature indicate that some viral agents may cause
    cardiac and arterial cell lesions and subsequent inflammation. Thus,
    viruses and altered immune cells may cooperate and play a role in
    arterial wall lipid accumulation, possibly acting as initiating
    factors for atherosclerosis (Butenko, 1985).

    2.2.7  Respiratory function

         Respiratory function is based on gas exchange (oxygen
    absorption and carbon dioxide elimination), gas transport (red blood
    cells), and on internal metabolic processes that utilize oxygen at
    the cellular level. Aging may affect all of these processes (Masoro,
    1981), but it is the first that is usually most compromised in
    advanced age. The decline in the gas-exchange system may involve the
    lungs, the thoracic cage, the respiratory muscles and the

    respiratory centres in the CNS. The deterioration of the lungs is
    largely dependent on environmental factors, in particular on the
    contamination of air with toxic substances, dust and microbiological
    agents. Therefore, age-related lung alterations may vary according
    to life styles (smoking, physical exercise), environmental
    conditions (urban/rural), and intercurrent diseases (infections,
    work-related diseases) (Davies, 1985).  Gas-exchange organs

         The most evident lung alterations with advancing age are
    represented by enlargement of alveolar ducts with flattening of
    alveoli and loss of septal tissue, reduction of elastic fibres, and
    increased fibrosis of the capillary system (Liebow, 1964). 
    Functional consequences are a reduction in the surface area for gas
    exchange with an increase in the physiological dead space, reduction
    of the ventilatory flow rate, and irregular distribution of blood
    flow (Mauderly, 1978). Age-related alterations also occur in the
    chest as a consequence of calcification of costal cartilage,
    increased stiffness of costovertebral and vertebral joints, and
    general rigidity of the chest. Both lung and chest alterations
    contribute to the changes in lung volume and pressure: vital
    capacity decreases, residual volume increases, and flow rates 
    (particularly expiratory flow rate) decline (Morris et al., 1971). 
    The alteration in the gas-exchange capacity causes reductions in
    oxygen uptake and pressure and lower arterial pO2, whereas pCO2
    remains constant even in very old age (Morris et al., 1971). The
    control of ventilation by brain centres is also altered in old age. 
    It is still unknown whether such alterations are due to intrinsic
    damage of the neural component or to a reduced responsiveness of
    neuromuscular activity in the chest (Peterson et al., 1981).

         All these alterations, while not necessarily life-threatening
    for the elderly, may favour pathologies such as chronic bronchitis,
    pneumonia and emphysema (Peterson et al., 1981). The concomitant
    hypoxia (low oxygen levels) may cause increased production of red
    blood cells with consequent polycythaemia. This may contribute to
    hypertension and cardiac failure.  Erythropoietic activity

         Although specific age-related alterations in the life cycle of
    erythrocytes have been reported (Danon, 1969), the overall function
    of the erythropoietic system seems to be well preserved. The
    regenerative potential following hypoxia seems nearly inexhaustible.
    In addition, haemoglobin turnover does not seem to be affected by
    age (Lipschitz, 1987).

    2.2.8.  Kidney and body fluid distribution

         The urinary tract is affected by aging both in its renal
    functions of excretion and ionic control of body fluids, and in its
    control of bladder and urethral activity.  Renal function

         Kidney function decreases both due to anatomical and
    physiological alterations with age (Wesson, 1969; Kaysen & Myers,
    1985; Brown et al., 1986; Corman & Michel, 1986; Owen & Heywood,
    1986; Meyer & Bellucci, 1986; Anderson & Brenner, 1986, 1987;
    Goldstein et al., 1988; Euans, 1988; Rudman, 1988). These
    alterations have been observed in both experimental animals and in

         The weight and volume of the kidney decrease by 20 to 30%
    between the ages of 30 and 90 years. The atrophy is primarily
    cortical and seems to be related to intrarenal vascular changes. The
    number of surviving nephrons is reduced and these remaining nephrons
    tend to be enlarged (Kaysen & Myers, 1985; Brown et al., 1986;
    Lindeman, 1986; Rudman, 1988). The number of glomeruli decreases by
    30 to 50%, and there is an increasing percentage of sclerotic and/or
    abnormal glomeruli. The glomerular filtration rate (GFR) decreases
    with age resulting in an adaptive increase in glomerular perfusion
    pressure (Kaysen & Myers, 1985; Lindeman, 1986; Meyer & Bellucci,
    1986; Anderson & Brenner, 1986, 1987; Blum et al., 1989). This
    decline in GFR is due in large part to the progressive reduction in
    blood flow to the kidneys (Brown et al. 1986; Lindeman, 1986).
    Glomerular mesangial volume increases by 50% and 1 out of 10
    glomeruli is sclerotic at the age of 80 years compared with 1 out of
    100 in the young adult (Brown et al., 1986). The renal tubules
    decrease in number, proximal tubule volume and length decrease, and
    distal tubules develop increased diverticula. The renal arterioles
    develop intimal thickening, reduplication of the lamina elastica
    interna, and mild hyalinization (Brown et al., 1986; Lindeman, 1986;
    Rudman, 1988).

         An age-related reduction in secretory and resorptive capacity
    is seen in the tubules, which is explained by a progressive loss of
    functioning nephrons (Lindeman, 1986). Tubular function, which
    regulates water and salt balance, is also affected. A decrease in
    the ability to concentrate urine with age has been well documented
    in humans. This appears to result from a decreased medullary
    tonicity caused mainly by an inability to respond normally to
    antidiuretic hormones (ADH) (Kaysen & Myers, 1985; Brown et al.,
    1986; Meyer & Bellucci, 1986; Lindeman, 1986; Euans, 1988). Despite
    age-related decreased renal function, the blood pH, partial pressure
    of carbon dioxide, and serum hydrogen carbonate concentration of the
    geriatric population without renal disease do not differ
    significantly from those of the young under basal conditions

    (Lindeman, 1986). Both the ability to maximally dilute the urine and
    to maximally concentrate it, are controlled by serum ADH and by the
    action of that hormone on the collecting ducts (Kaysen & Myers,
    1985; Os et al., 1987). Increased arginine-vasopressin (AVP)
    secretion per unit of plasma reflects a decrease in collecting
    tubule sensitivity to AVP. This change in sensitivity is not
    completely offset by increased ADH release (Davis & Davis, 1987).
    The suppression of ADH secretion is not maximal when serum
    osmolality is reduced.

         The renin-angiotensin-aldosterone system is also poorly
    responsive to volume depletion in aging subjects. As a result, the
    elderly cannot maximally retain sodium under conditions of plasma
    volume contraction (Kaysen & Myers, 1985; Lindeman, 1986; Euans,
    1988). The activity of the renin-angiotensin system is progressively
    reduced with age. It has been suggested that angiotensin II does not
    play an important role in the maintenance of blood pressure and
    kidney hemodynamics in normal senescence (Corman & Michel, 1986).
    The mean blood pressure is correlated with age and the decline in
    renal function (Lindeman et al., 1987).

         The kidney is also the site of vitamin D1 hydroxylation, which
    is dramatically reduced during aging (Kaysen & Myers, 1985).  Lower urinary tract

         The most frequent age-related alterations in the lower urinary
    tract result in urine incontinence in both sexes and urine retention
    in males. Incontinence occurs at high incidence although it is not
    an inevitable consequence of aging. The high number of physiological
    requirements for urinary continence may account for such frequent
    failure (Williams & Pannill, 1982). In women, the reduction of
    estrogen levels after menopause may decrease the tone of the smooth
    muscle around the pelvic floor and the bladder outlet, thus
    favouring urinary incontinence. In men, hypertrophy of the prostate
    represents the major cause for involuntary loss of urine, because of
    the associated frequent instability of the detrusor muscle. Other
    causes for urinary incontinence are represented by delirium,
    infections, restricted mobility and polyuria.

    2.2.9  Gastrointestinal function

         Changes in the gastrointestinal tract during aging consist
    mainly of a reduced cell turnover, leading to mucosal hypoplasia. 
    Accessory organs, such as the exocrine pancreas and the liver, are
    affected by aging independently.  Gastrointestinal tract

         In contrast to the cardiovascular or excretory system, the
    gastrointestinal (GI) tract does not exhibit marked structural or
    functional changes with age (Penzes, 1984). Aging may affect all
    regions of the GI tube. The first signs of aging are generally
    observed in the mouth. Teeth undergo discoloration, pulp recedes
    from the crown, dentine is often poorly renewed, and the gingiva
    frequently recede (Walker, 1985). These alterations favour bacterial
    growth which can lead to chronic periodontal inflammation and tooth

         In the stomach, reduced secretion of hydrochloric acid and
    pepsin is often found in the elderly. These changes result from
    alterations in enzyme-secreting cells or from altered hormonal and
    neural regulation. However, in some cases, increased acid secretion
    may occur with advancing age, leading to gastritis, erosions and,
    ultimately, ulcers (Kumpuris, 1983).

         No major morphological alterations are observed in the
    intestine. The relatively mild modification of villi, the increased
    collagen content and the reduced mucosal cell proliferation
    (Webster, 1985) cannot account for the impaired absorption of
    nutrients, including minerals (calcium, iron and zinc) and vitamins
    often found in the elderly. Other factors such as reduced motility
    and inadequate intestinal blood supply may play a more important
    role than the slightly altered anatomical integrity.

         In the lower GI tract the rectal tone is generally decreased in
    the elderly and the sphincter is weakened, leading to incontinence. 
    Neurological alterations (dementia), muscle atrophy, diarrhoea and
    constipation can all contribute to anal incontinence.  Pancreas

         Pancreatic function is not seriously compromised during normal
    aging. Structural changes mainly involve a reduction in the number
    of secreting cells, with a consequent decrease in size of the
    pancreas and a moderate increase in collagen content. Reduction in
    the levels of trypsin, hydrogen carbonate, amylase and lipase in the
    pancreatic juice occurs commonly in the elderly (Vellas et al.,
    1988). Overall, however, the functional efficiency of the pancreas
    is not lost during aging.  Liver

         Structural changes in the liver with age are relatively minor. 
    However, the most significant change in human liver is a decrease in
    volume (Wynne et al., 1989). The life-span of the hepatocyte is
    long, with cells only dividing once or twice during the lifetime in

    the absence of a growth stimulus (Popper, 1986). Whether the
    function of the aging hepatocyte is impaired is unclear. Kupffer
    cell function may be impaired as demonstrated by a reduction in
    phagocytic activity. Endocytosis is reduced in Kupffer, but not
    endothelial, cells. An increase in collagen also characterizes aging

         The function of aging liver also undergoes a number of
    alterations. In humans, cholesterol synthesis is reduced in the
    elderly, while biliary secretion is increased (Popper, 1986). 
    Hepatic blood flow is reduced by as much as 50% (Sherlock et al.,
    1955). Age-related changes in liver enzyme expression have been
    studied in more detail in rodents (Fishbein, 1991). The efficiency
    of carbohydrate and intermediary metabolism is decreased in
    senescent rats or mice of both sexes, due in part to decreased
    insulin-binding and the impaired regulation of enzymes such as
    pyruvate kinase. Hepatic protein synthesis also decreases with age,
    and the sex-specific expression of drug and steroid-metabolizing
    enzymes in male rats appears to be feminized in senescence (Kitani,

    2.2.10  Musculo-skeletal system

         Aging dramatically affects bone, joints and skeletal muscles. 
    The basic phenomena responsible for the aging of the
    musculo-skeletal system are not completely understood because of the
    involvement of many factors, e.g., hormones, nutrition and physical
    exercise, in addition to specific age-related alterations at the
    tissue level, resulting in general deterioration of the system.  Bones

         Bone should not be considered as metabolically quiescent since
    it is constantly remodelled according to mechanical demand and
    continuous turnover due to new bone formation (by osteoblasts) and
    bone resorption (by osteoclasts) (Exton-Smith, 1985).

         With aging, the balance between bone formation and bone
    resorption is altered in favour of resorption, resulting in a
    reduction of bone mass. This starts from the medullary region with
    enlargement of the cavity and moves towards the epiphysis of the
    bone and then to the outer surface with a final reduction of cortex
    thickness. As a consequence, the bone strength is reduced (Smith
    et al., 1981). The prolonged period where bone resorption exceeds
    bone formation may result in osteoporosis (Dawson-Hughes et al.,
    1987; Eastell & Riggs, 1987; Riggs, 1987; Croucher et al., 1989).

         The factors responsible for bone resorption and remodelling are
    not well known. More knowledge is available concerning two other
    factors which profoundly affect bone metabolism: calcium turnover
    and hormonal profile. Calcium is absorbed through the intestinal

    mucosa and is excreted from the kidney, the blood level remaining
    remarkably constant throughout life. Calcium is required for many
    essential functions in the body, such as cell division, cell
    intermediary metabolism, and cell functions (excitability, secretion
    and movement). In the case of a negative balance, calcium is
    mobilized from the bone with a consequent reduction in bone mass.

         Calcium metabolism is under hormonal control. Parathyroid
    hormone increases the plasma calcium level by promoting bone
    resorption and mobilization, whereas calcitonin lowers blood calcium
    by inhibiting bone resorption. Calcitriol (the active derivative of
    vitamin D) increases intestinal absorption and decreases excretion
    of calcium, thus acting to increase the body burden of calcium. The
    age-associated decrease in intestinal calcium absorption lowers the
    plasma calcium level, leading to an increase in parathyroid hormone.
    Calcitriol levels may also be deficient in the elderly, as a result,
    perhaps in part, of the decrease in vitamin D synthesis in old skin
    and alterations in renal metabolism.

         Bone metabolism also depends on other hormonal factors. 
    Estrogens have a strong positive effect on bone density. After
    menopause, bone resorption exceeds bone formation resulting in bone
    mass loss. However, the occurrence of osteoporosis greatly depends
    on the adult bone peak mass, physical activity, and calcium intake
    (Eastell & Riggs, 1987; Riggs, 1987; Bornor et al., 1988; Stevenson
    et al., 1988; Lindsay, 1989).

         Glucocorticoids lower plasma calcium levels, which can increase
    bone resorption by stimulating parathormone secretion. Thyroid
    hormones may also cause bone loss, although the mechanism is still
    unclear. Growth hormone, somatomedins and insulin all promote bone
    formation, and therefore defective secretion (as in diabetes) may
    cause osteoporosis.  Joints

         Articular disorders are practically universal in elderly
    people, and are associated with pain and disability. The major
    alteration consists of the loss of the smooth surface of cartilage,
    which becomes thicker and less elastic. The cartilage develops rough
    surfaces and mechanical irregularities, which represent the early
    steps in osteoarthritis (Evens & Hawkins, 1984).  Skeletal muscles

         With advancing age muscles become smaller in size, due to
    reduction in the number of fibres, and less elastic, due to an
    increase in collagen content. The fat content of muscle tissue also
    increases with age. The causes for such defects are still
    controversial: alteration of sarcoplasmic reticulum, decrease of
    contractile protein synthesis and a reduction in the number of
    mitochondria are all found in aged muscle cells.

         In addition to muscle cell alterations, the neuromuscular
    junction is modified in aged muscle. The acetylcholine content of
    nerve terminals is consistently reduced and acetylcholine receptors
    are irregularly distributed. Motor nerve conduction velocity is also
    impaired. All these defects contribute to the progressive failure of
    muscular efficiency, although some muscles, such as the diaphragm,
    do not seem to suffer age-related effects (Finch & Hayflick, 1977).
    Thus, the additive results of the loss of bone mass, disorder of
    joints, decline in skeletal muscle power and nervous system
    discoordination may lead to loss of body stability, falls and bone
    fracture in old people.

    2.2.11  Skin

         The age-related modification of the skin is the most dramatic,
    common and constant event in life, so that it may constitute one of
    the best external markers of aging. Only recently has it been
    possible to distinguish between intrinsic aging of the skin and
    photo-aging (Gilchrest, 1984). Cutaneous neoplasia is one of the
    best documented interactions between aging of the organism and
    environmental effects. A variety of environmental factors, the most
    notable being ultraviolet radiation, has been shown to cause cancer
    of the skin in humans and animal models (Rogers &  Gilchrest, 1990).

         The structure of skin changes throughout life (Behl et al.,
    1987), although many of the alterations are due to exposure to
    sunlight rather than intrinsic aging (Gilchrest, 1984). Skin
    thickness increases during maturation and then gradually declines
    (Vogel, 1983). However, recent studies in mice and rats by
    Monteiro-Riviere et al. (1991) have not reported any significant
    changes in skin thickness or blood flow from maturity throughout the
    life span. Hydration, which can affect chemical solubility,
    decreases while keratinization increases.

         Skin aging is linked to alterations affecting both the
    epidermal and the dermal components. The epidermis suffers from a
    reduction in the turnover of epidermal cells, due both to a
    reduction of basal cell renewal and to a slower maturation toward
    the stratum corneum. The dermo-epidermal interface is flattened so
    that the total external body surface area is reduced (Selmanovitz
    et al., 1977). The reduction in epidermal cell turnover is

    responsible for the slowing of wound healing and possibly for the
    dryness and/or roughness of aged skin. The dermis is reduced in
    thickness in the elderly and is characterized by a reduced collagen
    content with a biochemical modification of the collagen itself,
    which makes it more strong and less elastic. Vascularization in
    humans is also reduced.

         Aging also results in modifications in the skin appendages. 
    Sweat gland function is decreased resulting in impaired
    thermoregulation. Sebaceous glands produce less oil and wax (dryness
    of skin), and hair usually loses pigment. The numbers of skin
    sensory organs (Pacinian and Meissner's corpuscles) decrease with
    age and sensation is consequently modified.

         Collagen aging occurs in the skin, as in other tissues, leading
    to increased rigidity. Collagen is composed of several related
    proteins, which are produced by fibroblasts and extruded into the
    extracellular space. Here they can be chemically deposited in a
    nearly pure form, as in the tendons, or immersed in an extracellular
    matrix, also produced by fibroblasts, as in the skin. The collagen
    fibres undergo maturation in the extracellular ground substance by
    parallel arrangements of tropocollagen fibres which became assembled
    together through cross-linking. This phenomenon is an index of
    maturational change. With age, however, the cross-linking increases
    (Houck et al., 1967), leading to a reduction of tensile strength and
    plasticity (Verzar, 1968). The extracellular matrix also shows
    age-related changes, since alterations in its physicochemical
    composition lead to increased density, reduced permeability and
    impaired transport of nutrients (Imayama & Braverman, 1989).


         The perception of altered sensitivity of the elderly to
    environmental insults is based in large part on the enhanced
    incidence of adverse or idiosyncratic drug reactions in this segment
    of the population (Vestal et al., 1985). The clinical pharmacology
    of the aged has been extensively reviewed in the last two decades
    (Triggs & Nation, 1975; Crooks et al., 1976; Reidenberg, 1980;
    Vestal et al., 1985), and evidence indicates that the response to
    drugs changes with age, as does the frequency of adverse drug
    reactions (Krupka & Vener, 1979). This may be in part related to
    issues of polypharmacy and compliance (Weber & Griffin, 1986).
    Morbidity and malnutrition, which are often associated with aging,
    could also contribute to altered pharmacokinetics in the elderly
    (Kitani, 1988). However, it is clear that age-related differences in
    drug/chemical disposition (pharmacokinetics/toxicokinetics) or
    sensitivity (pharmacodynamics/toxicodynamics) also play a role in
    altered responses to chemicals in the elderly. For sake of
    simplicity, the terms "pharmacokinetics" and "pharmacodynamics" will
    be used for both drugs and environmental chemicals.

    3.1  Pharmacokinetics

         Pharmacokinetics describes the processes of the absorption,
    distribution, metabolism and excretion of drugs or other chemicals
    in the body. The numerous physiological and biochemical changes that
    occur during aging can modulate any of these processes of
    disposition. Changes in chemical disposition (pharmacokinetics being
    the mathematical description of these processes) can lead to an
    altered dose or dose-rate to the target tissue resulting in changes
    in response. This topic has been the subject of several recent
    reviews (Sellers et al., 1983; Van Bezooijen, 1984; Stevenson &
    Hosie, 1985; Blumberg, 1985; Birnbaum, 1987, 1989, 1991; Ritschel,
    1988; Loi & Vestal, 1988; McMahon & Birnbaum, 1990a).

         The highlights in this field will be covered as well as the
    more recent data, especially that focusing on age-related
    alterations in the pharmacokinetic behaviour of xenobiotics, as
    opposed to drugs.

    3.1.1  Absorption

         The absorption of drugs and environmental chemicals, defined as
    uptake into the blood, occurs primarily via the skin, lungs and
    gastrointestinal (GI) tract. Alterations in the structure and
    function of these three organs clearly occur with age. In contrast
    to the skin and GI tract, little is known about the effects of aging
    on pulmonary absorption of xenobiotics. Few age-related differences
    have been reported in lung structure (Stiles & Tyler, 1988),

    although the lung volume is greater in old rats. An age-related
    decrease in respiratory function has been reported in several
    species (Mauderly, 1979a,b, 1982). In addition, there appears to be
    a decrease in the rate of alveolar-capillary gas exchange in older
    organisms (Mauderly, 1979b).

         Dermal exposure represents a major portal of entry for
    environmental chemicals. Several studies have indicated a decline in
    human skin permeability with aging (Christophers & Kligman, 1965).
    In experimental animals, the scarce data suggest that percutaneous
    absorption is decreased in senescent rodents as compared to young
    adults. The chemical absorption of
    2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) and related chemicals
    is greatest at weaning (Jackson et al, 1990), decreases at maturity
    and then undergoes a further decline during aging (Banks et al.,

         Age-related alterations in GI absorption have been studied in
    more depth. A decrease in gastric acid secretion is commonly seen in
    the elderly (Bender, 1968). The resulting increase in pH can alter
    the ionization of compounds, enhancing or retarding their ability to
    diffuse passively across cellular membranes. Decreases in gastric
    motility (Lin & Hayton, 1983) can prolong the transit time of
    chemicals in the gut, thus enhancing their potential for absorption.
    In rats, splanchnic blood flow declines in the first year of life
    but stays unchanged in the second year (Yates & Hiley, 1979; Kitani,
    1988). In contrast, splanchnic blood flow declines progressively
    with age in humans (Sherlock et al., 1955). An increase with age in
    mucosal weight (Holt et al., 1984; Hebert & Birnbaum, 1987), as well
    as intestinal epithelial cell proliferation (Holt et al., 1988), has
    been reported in rats.

         GI absorption can be active, passive or involve phagocytosis. 
    Xenobiotics are primarily absorbed by passive diffusion. Age-related
    changes in the oral absorption of a variety of drugs in people have
    not been observed (Castleden et al., 1977b; Stevenson et al., 1979;
    Greenblatt et al., 1988). The passive absorption of TCDD in the
    small intestine does not change with age in rodents (Hebert &
    Birnbaum, 1987), nor does that of many small endogenous molecules,
    such as glucose (Eastin & Birnbaum, 1987), vitamin A (Hollander
    et al., 1986), vitamin D, vitamin B12 and niacin (Fleming &
    Barrows, 1982a,b) and many amino acids (Penzes, 1974).

         Active transport appears to decrease with age (Eastin &
    Birnbaum, 1987). Doubek & Armbrecht (1987) have demonstrated that
    the decrease in the carrier-mediated component of glucose transport
    in rats occurs in the brush-border membrane of the small intestine.
    Their suggestion that the decrease is due to a reduction in the
    number of sodium-linked glucose carriers has been supported by
    recent studies in human intestinal tissue (Vincenzini et al., 1989).

    The active transport of other small molecules such as calcium and
    phosphorus (Armbrecht, 1986), and galactose and iron (Reidenberg,
    1980) has also been reported to decrease with age. As with the
    active transport of glucose, the marked decrease in calcium active
    transport of calcium occurs between young adulthood and middle age
    (Mooradian & Song, 1989) and results from changes in the number of
    calcium transporters in the intestinal basal lateral membranes
    (Armbrecht et al., 1988).

    3.1.2  Distribution

         The distribution of chemicals throughout the body is governed
    by the fact that the physicochemical properties of the compound
    affect its transport within the body and localization to various
    tissues. Lipophilic molecules readily pass across cellular membranes
    and accumulate in lipid-rich tissues. Binding to proteins also
    modulates distribution, since few compounds are transported free in
    the blood. While there is no evidence of alterations in relative
    blood volume with age, changes in body composition, blood flow and
    macromolecular binding have all been documented.

         The decrease in lean body mass in both animals (Lesser et al.,
    1973) and humans (Novak, 1972) has been well documented. Loss of
    body water with age (Edelman & Leibman, 1959) results in a decrease
    in the volume of distribution for water-soluble compounds, leading
    to enhanced toxicity of ethanol (York, 1982)  and ethylenediamine
    (Yang et al., 1984). Body fat can account for approximately 15-40%
    of the total body weight in humans (Ritschel, 1983) and rats
    (Bertrand et al., 1980; Birnbaum, 1983). The increased size of the
    fat compartment in older, sedentary animals would be expected to
    increase the body burden of lipid-soluble substances and reduce the
    overall rate of elimination from older animals. Following an
    inhalation exposure of old rats to methylchloroform (Schumann
    et al., 1982a,b), a better fit to a physiologically based
    pharmacokinetic (PbPk) model was obtained by increasing the volume
    of the fat compartment from 7 to 18% of the body weight for rats and
    from 4 to 18% for mice (Reitz et al., 1988). Similar improvements
    were noted by Lutz et al. (1977) in their PbPk model of
    polybrominated biphenyl compounds in rats.

         Not only does cardiac output decrease with age (Bender, 1965),
    but regional blood flow can change differentially (Yates & Hiley,
    1979). Since adipose tissue volume increases but blood flow
    decreases with age, lipophilic compounds tend to show greater
    retention in the elderly. This has been shown for polychlorinated
    biphenyls (Birnbaum, 1983) and halogenated solvents (Schumann
    et al., 1982a,b). A clinical pharmacokinetic study (Klotz et al.,
    1975) which demonstrated a 5-fold increase in the distribution
    volume of diazepam in the elderly is in agreement with the data on
    experimental animals.

         Binding to blood components can also change with age. Although
    the total plasma protein content does not change dramatically with
    age, there is a small but significant reduction in albumin in both
    animals (Rodgers & Gass, 1983) and humans (Bender et al., 1975). For
    drugs or xenobiotics that can be bound, such a decrease in albumin
    enables a higher concentration of free drug to reach the target
    site. A decrease in binding of drugs to red blood cells has also
    been reported to occur during aging (Chan et al., 1975), again
    leading to a higher level of free drug.

    3.1.3  Metabolism

         As with absorption and distribution, there are physiological
    changes that occur during aging which can influence
    biotransformation reactions, both in the liver and in extrahepatic
    tissues. Although the liver is the main site for the metabolism of
    drugs and environmental chemicals, significant metabolic reactions
    occur in all the portals of entry tissues (skin, respiratory tract,
    GI tract) as well as the kidneys, gonads, adrenals, etc. Metabolism
    is often considered to be divided into two types of reactions: phase
    I (functionalization reactions) and phase II (conjugation
    reactions). Phase I reactions tend to increase the polarity of
    chemicals, and the corresponding enzymes include the
    cytochrome-P450- and FAD-containing monooxygenases as well as the
    alcohol and aldehyde dehydrogenases, monoamine oxidases, nitro- and
    azo-reductases, esterases and amidases. Phase II reactions involve
    the conjugation of functional group (either already present on the
    chemical or as a result of phase I activity) with an endogenous
    cofactor such as an amino acid (glycine, glutamate), peptide
    (glutathione), sugar (glucuronic acid) or a small molecule such as a
    sulfate, acetate or methyl group. These reactions may occur on
    cellular membranes or in the cytosol, and are regulated in a
    tissue-specific manner.

         The only generalization that can be made concerning age-related
    changes in biotransformation is that few patterns exist. Metabolic
    changes with aging appear to be substrate, sex, strain and species
    dependent (Schmucker & Wang, 1989; Kitani, 1988; Birnbaum, 1989;
    McMahon & Birnbaum, 1990b). In addition to intrinsic changes related
    to altered physiology, the study of the influence of age on
    metabolism is further complicated by the effects of diet, alcohol,
    drugs and pollutants, which can induce or inhibit enzyme activities
    (Ritschel, 1988).

         Most of the research on the effects of aging on metabolism has
    focused on the microsomal mixed-function oxidases. The protein
    components of this system have been reported to decline or remain
    unchanged with age. In general, the decline in total cytochrome P450
    levels reported in rats appears to be due to the age-related
    decrease in the amount of the male-specific forms (Kamataki et al.,
    1985a,b; Sun et al., 1986). This results from the age-related

    decrease in circulating testosterone levels (Kitani, 1985) or to
    alterations in the secretory profiles of growth hormone (Fishbein,
    1991). This pattern is most pronounced in rats, few changes being
    observed in other rodents or sub-human primates (Birnbaum, 1987).
    However, recent human studies have suggested that some degree of
    sexual dimorphism does exist in hepatic drug metabolism. Plasma
    antipyrine half-lives were prolonged in elderly males, but not
    elderly females, as compared to young adults (Greenblatt et al.,
    1988). This was caused by an age-related reduction in clearance,
    which has been interpreted as being the result of an impairment of
    antipyrine metabolism in elderly men. Similar gender-specific
    results were observed for the kinetics of chlordiazepoxide, another
    low-clearance oxidatively metabolized drug (Loi & Vestal, 1988).

         A study by Chengelis (1988a) has supported the earlier report
    of Rikans (1984) that measuring components of the monooxygenase
    system cannot lead to predictions of toxicity. Sex differences in
    rats were only significant up to one year of age. However, Rikans
    (1989a) has recently reported that metabolism of specific substrates
    (aniline, benzphetamine and nitroanisole) does decline with age in
    the female as well as in the male rat, but the magnitude of change
    in the female is smaller. This agrees with a study by Bitar &
    Shapiro (1987), who suggested that an age-related increase in haem
    degradation plays a role in the decreased metabolism of hexobarbital
    and aniline. This observation, that microsomal drug metabolism
    activities do not have to be sexually dimorphic to be altered in old
    age, implies that factors other than loss of the male-specific
    cytochromes P450 contribute to the age-associated alterations in
    some rat strains. Decreases in NADPH cytochrome P450 reductase in
    rat liver with age have been reported by Blanco et al. (1987),
    Chengelis (1988a) and Rikans (1989a).

         Despite a large body of data obtained from experimental animal
    studies, there is no clear evidence that monooxygenase activities
    decline in the livers of healthy elderly humans. All the data
    derived so far from human liver biopsy specimens have shown no
    correlation between enzyme activities expressed per mg protein and
    age of subjects (Boobis & Davies, 1984; Schmucker et al., 1990). The
    often reported decreases in clearance values of drugs metabolized
    primarily by the liver in the elderly may in large part be accounted
    for by the decrease in liver volume with age (Kitani, 1988). Further
    evidence is needed to determine whether hepatic drug-metabolizing
    enzyme activities in humans decline with age, as is observed in some
    rodents, but the extent is unlikely to be as drastic as that
    observed in male rat liver.

         A change in the composition of cytochrome P450 enzymes,
    suggested by differential changes in enzyme activity, has been
    demonstrated in male rats (Kamataki et al., 1985a; Sun & Strobel,
    1986). Leakey et al. (1989a) showed that dietary restriction could
    also alter the profile of cytochrome P450 isoforms, possibly by

    delaying the age-related demasculinization of the liver. Friedman
    et al. (1989) showed that aging affected the composition of
    testosterone-binding cytochromes P450 in the liver of male rats.
    Changes in P450 isoforms in old rats were also reported by
    Paramonova & Dovgi (1987). Studies with humans have demonstrated
    that the effect of age on metabolism even varies for different
    metabolites of the same parent compound (Posner et al., 1987). This
    supports the observation of changes in cytochrome P450 isoform
    composition with age, as has been reported for male rat liver.

         Hepatic xenobiotic metabolism can be modulated by many factors.
    In humans, smoking often confounds studies of age-related changes in
    pharmacokinetics. However, the age-related decrease in the
    metabolism of antipyrine (Loft et al., 1988), theophylline and
    cortisol (Crowley et al., 1988) may be independent of smoking
    status. In fact, the inductive properties of smoking on hepatic
    metabolism do not appear to diminish with age. Phenytoin also
    increased theophylline metabolism to an equal extent in both young
    and old healthy men (Crowley et al., 1988).  In contrast, Rath &
    Kanungo (1989) demonstrated that the rate of transcription of the
    phenobarbital-specific isozymes was nearly two-fold higher in young
    rat liver than in old. However, these differential effects may
    reflect the specific isoforms involved, since smoking and
    phenobarbital preferentially induce different forms. The recent
    studies of Rikans (1989b), which demonstrate that induction of
    hepatic microsomal drug metabolism by ethanol or acetone is
    unaffected by the aging process, lend support to this concept.

         Age-related changes in phase I hepatic drug-metabolizing
    enzymes other than the mixed-function oxidases have been subjected
    to much less examination. Rikans & Moore (1987) demonstrated an
    age-related increase in liver alcohol dehydrogenase in male rats.
    However, no age differences have been observed in the activity of
    this enzyme in female rats. Hydrolytic reactions may decrease with
    age. For example, liver esterase activity, using diethylhexyl
    phthalate as a substrate, declines in old rats (Gollamudi et al.,
    1983). Using aspirin as a substrate, no correlation was found
    between age and esterase activity in human liver (Yelland et al.,
    1991). These results provide further evidence that age is not a
    major determinant of hepatic drug metabolism in the elderly
    (Schmucker et al., 1990).

         Alterations with age in extrahepatic phase I metabolism also
    appear to be substrate, sex, strain and species specific. Pulmonary
    metabolism of benzo[a]pyrene was found to increase in old rats (Sun
    & Strobel, 1986) in agreement with earlier studies of Rabovsky
    et al. (1984). In contrast, oxidation of 2-aminofluorene, which is
    catalysed by a different isoform of cytochrome P450, decreased
    (Robertson & Birnbaum, 1982). Renal metabolism of acetaminophen has
    been reported to decrease with age (Beierschmitt & Weiner, 1986),
    while salicylate oxidation by kidney extracts did not change in rats

    (Kyle & Kocsis, 1985). Age-related alterations in intestinal Phase I
    metabolism appear to be site specific. Sun & Strobel (1986) reported
    that oxidation of benzo [a]pyrene in the colon increased throughout
    the life of rats, while McMahon et al. (1987) observed no
    age-related change in the metabolism of this substrate in the small
    intestine. Newaz et al. (1983) observed higher metabolic rates of
    dimethylhydrazine in colonic tissue from aging humans as compared to
    younger individuals. McMahon et al. (1989) suggested that plasma
    esterase hydrolysis of benzyl acetate may decline with age in both
    rats and mice. Alcohol dehydrogenase activity was found to remain
    unchanged in the aging colon (McMahon et al., 1987).

         Changes in phase II enzymes also appear greatly variable in
    rodent liver, although Loi & Vestal (1988) conclude that age has
    little effect on phase II reactions in humans. The major conjugation
    reactions involve sulfation, glucuronidation or reaction with
    glutathione leading to mercapturic acid formation. All these
    reactions are catalysed by multiple isoforms showing varying degrees
    of substrate and sex specificity, as is the case for the phase I
    enzymes. Iwasaki et al. (1986) demonstrated differential age effects
    on two distinct sulfotransferases using male and female rats and
    various alcohols and amines. Sulfation of phenolic substrates
    appears to decline with age (Galinsky et al., 1986; Sweeny & Weiner,
    1986) while conjugation of bile salts increases in old male rats
    (Galinsky et al., 1986). In contrast, changes in female rats were
    not seen (Galinsky et al., 1990). Chengelis (1988b) observed no
    significant changes in sulfotransferase activity with age in either
    male or female rats using beta-naphthol as the substrate. Similar
    results were seen by Leakey et al. (1989b) for the conjugation of
    both estrone and naphthol, whereas the sulfation of androsterone and
    corticosterone increased with age in male rats.

         Hepatic glucuronidation also shows substrate specificity for
    alterations with age. Depending on the substrate, increases with
    estrone and acetaminophen (Galinsky et al., 1986), decreases with
    the rubber antioxidant 4,4'-thiobis-(6- t-butyl- m-cresol)
    (Borghoff et al., 1988), or no effect with naphthol,
     p-nitrophenol, morphine and testosterone have been observed
    (Galinsky et al., 1986; Sweeny & Weiner, 1986) in male rats. In
    contrast, Chengelis (1988b) observed a marked decrease in senescent
    male and female rats using both  p-nitrophenol and chloramphenicol.
    To complicate matters further, Leakey et al. (1989b) also saw a
    decrease in naphthol, testosterone, androsterone and
    tetrahydrocortisone conjugation with glucuronic acid, but observed
    no effects of aging on conjugation with 2-aminophenol,
    5-hydroxytryptamine, bilirubin or estrone. Tarloff et al. (1989b)
    saw no change in acetaminophen glucuronidation with advancing age.
    Although the activities of the UDP-glucuronosyltransferases appear
    highly variable, an age-related decrease in hepatic levels of the
    cofactor, UDP-glucuronic acid (UDPGA) (Borghoff et al., 1988),
    suggests that glucuronidation could be limited in older animals.

         The concentration of hepatic glutathione, the cofactor involved
    in the third major class of conjugation reactions, has been reported
    to increase (Borghoff & Birnbaum, 1986), decrease (Stohs et al.,
    1982) or remain unchanged (Chengelis, 1988b; Rikans & Moore, 1988)
    in old rodents. Variability in age effects has also been reported in
    the activity of the glutathione- S-transferases, which exist as a
    family of dimeric proteins having broad and overlapping substrate
    specificity. The isozymes are composed of two subunits from at least
    six different peptides, including both homo- and heterodimers. Thus
    reports of an increase (Leakey et al., 1989b), decrease (Fujita
    et al., 1985; Blanco et al., 1987; Leakey et al., 1989b), or no
    change (Birnbaum & Baird, 1979; Sweeny & Weiner, 1985; Borghoff &
    Birnbaum, 1986; Chengelis, 1988b; Leakey et al., 1989b; Carrillo
    et al., 1991) in hepatic glutathione-S-transferase activity may, as
    in the cases of cytochrome P450 mixed-function oxidases,
    sulfotransferases and UDP-glucuronosyltransferases, reflect
    age-dependant changes in isozyme ratios (Spearman & Leibman, 1984;
    Carrillo et al., 1991). Restriction in dietary protein decreases the
    activity of the glutathione-S-transferases more in old than young
    rodents (Carrillo et al., 1989, 1990).

         Much less has been reported on the effects of aging on other
    phase II metabolic reactions in the liver. Hydrolysis of expoxides
    to form diols or dihydrodiols is catalysed by the epoxide
    hydrolases. Epoxide hydrolase activity has been reported both to
    increase (Birnbaum & Baird, 1979) and decrease (Ali et al., 1985;
    Kaur & Gill, 1985; Leakey et al., 1989b) with age. Again, this may
    reflect differential age effects on multiple enzymatic species. 
    However, some of the difference may be due to the ages of animals
    used for comparison, since Chengelis (1988b) reported a gradual
    increase in epoxide hydrolase activity throughout much of the life
    span, followed by an abrupt decrease in senescence.

         Human studies have suggested that the alterations observed in
    salicylate pharmacokinetics (Cuny et al., 1979) with age might be
    due to a decrease in glycine conjugation (Kyle & Kocsis, 1985).
    However, elevated levels of salicyluric acid, the glycine conjugate
    of salicylate, have been observed in elderly humans undergoing
    chronic salicylate treatment (Montgomery & Sitar, 1981). Recent
    studies in rodents have not demonstrated any age-related decline in
    glycine conjugation with non-nephrotoxic doses of salicylate
    (McMahon et al., l990a) or in the formation of hippuric acid from
    benzoate (McMahon et al., 1989). Acetylation, however, has been
    reported to decline both in man (Bauer et al., 1989) and rats
    (Leakey et al., 1989b).

         Changes in extrahepatic phase II reactions have been
    investigated in even less depth than extrahepatic phase I reactions.
    In the lung and small intestine, glucuronidation of  p-nitrophenol
    appeared not to change with age (Borghoff & Birnbaum, 1985), whereas
    in the colon an age-related decrease was reported by McMahon et al.

    (1987). In contrast, colonic glucuronidation of
    4-methylumbelliferone rose significantly in older rats (McMahon
    et al., l990b). A decrease in UDP-glucuronosyl transferase activity
    in the kidney (Borghoff & Birnbaum, 1985; Tarloff et al., 1989a) was
    accompanied by a decrease in the renal concentration of UDPGA
    (Borghoff et al., 1988).

         Glutathione content has been examined in a variety of tissues
    (lung, kidney, brain, testes and blood) and, except for an
    age-related decrease in the lens, has been found to remain constant
    (Rikans & Moore, 1988). Unchanged glutathione levels in the colon
    occurred although the activity of glutathione- S-transferase
    decreased in this tissue (McMahon et al., 1987). Spearman & Leibman
    (1983, 1984) observed differential age- and sex-related changes in
    this activity in the lung depending on the substrate examined. The
    renal activity of glutathione- S-transferase declines significantly
    in old rats (Beierschmitt & Weiner, 1986). In contrast, elevated
    gluthathione- S-transferase levels were observed in the brain of
    old rats (Blanco et al., 1987), while no changes were observed in
    the heart.

         Epoxide hydrolase activity has been examined in the lungs,
    small intestine and kidneys by Kaur & Gill (1985). They observed a
    decrease in lung and small intestine activity in old rats which was
    substrate dependent. In the rat kidney, epoxide hydrolase activity
    decreased using trans-stilbene oxide as the substrate but did not
    change with cis-stilbene oxide, supporting again the differential
    effects of aging on different isozymic forms of drug-metabolizing
    enzymes. In addition, deacetylation of acetaminophen in the kidney
    appears to be either unchanged with age (Beierschmitt & Weiner,
    1986) or slightly decreased (Tarloff et al., 1989b). Acetylation in
    the kidney also decreases with age (Wabner & Chen, 1984).

         One additional enzyme which can be considered to fall into the
    phase II class is beta-glucuronidase. This enzyme hydrolyses
    glucuronic acid from conjugated xenobiotics. The activity of this
    enzyme has been reported to increase with age in rat liver
    (Schmucker & Wang, 1979; Van Manen et al., 1983) and kidney
    (Borghoff & Birnbaum, 1985), but remain unchanged in the colon
    (McMahon et al., 1987). However, beta-glucuronidase was found to
    decrease with age in fecal contents (McMahon, 1988). The balance
    between glucuronidation and deglucuronidation reactions may play a
    role in determining the level of reactive compounds in the organism.

         Depending on the chemical, aging can result in either an
    increase or a decrease in the metabolizing capacity of different
    organs and tissues (liver, kidney, gastrointestinal tract, lungs and
    skin). The elevation or reduction in metabolism can both lead to
    higher and lower toxicity, depending on the relative reactivity of
    the metabolic intermediates and end-products. Thus studies on the
    effect of aging on metabolism should be considered case by case.

    3.1.4  Excretion

         Excretion leads to the elimination of a chemical and/or its
    metabolites from the body. The kidney is the major excretory organ,
    with the liver and lung also playing important roles in the
    elimination process. In addition, sweat, saliva and sex-linked
    processes such as lactation can serve as routes of excretion.

         The effects of age on renal function appear to play a major
    role in altered pharmacokinetics in the elderly (Vestal, 1978;
    Koch-Weser et al., 1982). The fact that changes in the physiology of
    the kidney occur has been known for many years (Schmucker, 1979).
    Renal blood flow decreases with age leading to a decrease in
    glomerular filtration rate. Tubular secretion and resorption are
    also reduced in the elderly. The number of functional nephrons
    declines to a similar extent as the decline in glomerular filtration
    rate and active secretion, suggesting that the nephron loses its
    function as a unit (Friedman et al., 1972). Decreases in renal
    function can result in a decreased rate of renal clearance, leading
    to a greater potential for elevated and/or persistent levels of
    chemicals in the body which could lead to toxicity (Sellers et al.,
    1983). In humans, decreased renal clearance in the elderly has been
    demonstrated for many drugs, including the aminoglycosides,
    tetracyclines, lithium, digoxin, procainamide, methotrexate, and
    phenobarbital (Kampmann & Hansen, 1979).

         Aging rodents are extremely susceptible to chronic
    glomerulonephropathy (Goldstein et al., 1988), much of which may be
    attributable to diet (Masoro & Yu, 1989). Altered glomerular
    morphology, characterized by thickening of the basement membrane and
    sclerosis, is progressive and increases in severity with advancing
    age, eventually resulting in scarring and loss. Renal tubules are
    also subject to degenerative changes, which are accompanied by
    proteinuria, especially albuminuria (Neuhaus & Flory, 1978). This
    appears to result from increases in glomerular permeability and a
    loss of fixed glomerular polyanion (Baylis et al., 1988). The high
    percentage of urinary protein represented by albumin in the aging
    rat may be due to non-selective protein leakage into the urine
    resulting from increases in glomerular permeability to large
    proteins, since the protein percentage in urine approaches that in
    the plasma of senescent rats (Horbach et al., 1988a). Similar
    albuminuria has been observed in aging mice (Yumura et al., 1989),
    and was correlated with glomerular sclerosis. Such changes, however,
    should not be simply extrapolated to humans, since there is no
    evidence for an increase in protein loss in the urine of the healthy

         Additional tubular changes occur in the aging kidney, resulting
    in hyperplastic and degenerative changes. Some of these changes
    resemble responses to specific environmental chemicals (Konishi &

    Ward, 1989). A decrease in renal transport of organic acids has been
    observed (Wabner & Chen, 1984). Aging appears to diminish the
    turnover of sodium/potassium ATPase in the proximal tubules (Marin
    et al., 1985), which could play a role in the observed age-related
    decrease in tubular secretion.

         Hepatic elimination may also be compromised by aging. Kitani
    (1985) has suggested that the excretory capacity of the liver
    decreases with age due to some functional alteration in the
    hepatocytes. In contrast to rats, where blood flow does not change
    after maturity (Kitani, 1988), blood flow to the liver declines in
    humans (Sherlock et al., 1955). Bile flow rate has been reported to
    be reduced (Borghoff et al., 1988) or remain unchanged (Kitani
    et al., 1985a) in rats. Biliary transport declines, especially in
    the case of polar compounds (Kitani et al., 1985a). The elimination
    of sulfobromophthalein, a model compound for the study of biliary
    excretion of organic anions, is also decreased in old rats (Kanai
    et al., 1985). Sato et al. (1987) demonstrated that the biliary
    excretion of the neutral glycoside ouabain decreases with age in
    both males and females. This may be due to an age-related decrease
    in hepatic uptake, resulting in less biliary elimination (Ohta
    et al., 1988). In addition, the biliary canalicular transport system
    declines steadily during aging (Kanai et al., 1988). The decreases
    in both hepatic uptake and biliary excretion may reflect changes in
    the hepatocyte plasma membrane (Zs-Nagy et al., 1986).

    3.2  Pharmacodynamics

         Age-related changes in chemical sensitivity cannot all be
    explained on the basis of altered pharmacokinetics in the elderly. 
    Pharmacodynamic changes occur at the target site and may involve
    changes in cell populations, cellular receptors, cellular
    responsiveness or in the regulation of the amount or activity of
    drugs, including the cardiac glycosides, benzodiazepines, tricyclic
    anti-depressants, and the non-steroidal anti-inflammatory agents
    (Bender, 1979), which have demonstrated altered receptor sensitivity
    in the aged (Wilson & Hanson, 1980).

    3.2.1  Central nervous system

         Numerous studies have shown that, with advancing age, there is
    a decrease in the ability of the nervous system to synthesize and/or
    release neurotransmitters and neuropeptides (for review, see Rogers
    & Bloom, 1985). This decline may reflect the fact that there are
    fewer neurons present to synthesize the chemical messenger or that
    the enzymes involved in their synthesis are altered. This would
    imply that xenobiotics which lead to neuronal death or interfere
    with neurotransmitter or neuropeptide synthesis and release could
    have a greater adverse effect on the elderly than on the young
    organism. An example of such an interaction has been noted within

    the dopaminergic extrapyramidal circuits controlling motor movements
    (nigrostriatal pathway). This pathway has been the subject of many
    experimental and clinical studies and provides one of the best
    functional units in which to study neurotoxicity in the aged.
    Nigrostriatal neurons are lost as a normal correlate of aging, and
    while these losses may not be expressed in the majority of
    individuals as movement disorders, there is a clear reduction in the
    functional reserve of this dopaminergic system, making it more
    vulnerable to neurotoxicants. This has been demonstrated by the
    observed acceleration or simulation of "age-associated" movement
    disorders by neurotoxicants such as
    1-methyl-4-phenyl-1,2,5,6-tetrahydro-pyridine (MPTP).

         The reduced capacity to synthesize neurotransmitters that
    occurs in the aged organism may also potentiate the effect of toxic
    substances. Carbon disulfide is an organic solvent with a variety of
    industrial applications that produces neurobehavioural dysfunctions
    (Wood, 1981). The mechanism of CS2 toxicity seems to be inhibition
    of dopamine-beta-hydroxylase, the enzyme that converts dopamine to
    norepinephrine (McKenna & DiStefano, 1977). Since norepinephrine
    metabolism declines with age, the spectrum of physiological effects
    regulated by this catecholamine would be expected to suffer greater
    disturbance, following exposure to CS2 or to pesticides that
    reduce brain catecholamines such as methyl bromide (Honma et al.,
    1987), in old rats compared to young ones.

         Toxic compounds that serve as neurotransmitter receptor
    blockers could have their effects on behaviour accentuated, making
    neuroendocrine control of homeostasis more difficult in the elderly.
    For example, the formamidine pesticides, amitraz and chlordimeform,
    have been shown to block alpha-noradrenergic receptors (Costa &
    Murphy, 1987; Costa et al., 1988) and induce a variety of
    behavioural disorders (Boyes & Dyer, 1984; Hsu & Kakuk, 1984;
    Landauer et al., 1984) in rats. Since there is an age-related
    decline in noradrenergic receptors in several species (Rogers &
    Bloom, 1985), exposure to these compounds may have a more pronounced
    effect on the older organism.

         Certain movement disorders associated with senescence may be
    related to age-related impairments in the brain dopaminergic systems
    (Marshall & Berrios, 1979). Waddington et al. (1985) observed a
    decrease in the density of brain dopamine receptors in old rats,
    with no changes in affinity. In contrast to young animals, the
    receptors of old animals were not able to respond effectively to
    long-term treatment. The decrease in dopamine receptor density was
    selective for the D-2 receptor subtype, although the coupling
    between D-1 receptors and adenylate cyclase appears to be affected
    by aging. A similar loss of D-2 receptors in the elderly has also

    been measured using positron tomography in the living human brain
    (Wong et al., 1984). An age-related decrease in binding to the
    serotonin receptor may relate to cerebral dysfunction in the elderly
    (Shih & Young, 1978).

         Studies with rats have indicated that the increased sensitivity
    of older rats to diazepam is due to pharmacodynamic differences
    (Guthrie et al., 1987). Pedigo et al. (1981) suggested this could
    relate to changes in the benzodiazepine/GABA/chloride ionophore
    complex. Human studies have also indicated that the site of
    increased sensitivity to the benzodiazepines lies distal to the
    receptor (Swift, 1985), possibly involving changes in the chloride

         Alterations in endogenous opioid systems may play a role in
    some of the behavioural changes observed in the elderly. Binding of
    dihydromorphine to the opiate receptor decreases in specific areas
    of the brain in aged rats as compared to young ones (Messing et al.,
    1980). This reduction is due to a decrease in receptor number, with
    no change in affinity. Despite a relatively large body of evidence
    that some receptors and neurotransmitters are at least qualitatively
    altered with aging, it is still not clear whether and how these
    changes are causally related to increased sensitivity of the CNS to
    certain drugs with aging, as described below.

         The brain appears to exhibit increased sensitivity to
    phenobarbital (Kitani et al., 1985b; Van Bezooijen et al., 1989) in
    both mice and rats. Such enhanced sensitivity with age to an
    anticonvulsant supports earlier studies with phenytoin (Kitani
    et al., 1984). It is possible that the aging brain of both
    experimental animals and humans has increased sensitivity to all CNS
    depressants. Enhanced brain sensitivity has also been reported for
    hexobarbital in rats (Van Bezooijen et al., 1989) and oxazepam in
    mice (Kitani et al., 1986). The aging human brain appears to be more
    sensitive to nitrazepam (Castleden et al., 1977a) and other
    benzodiazepines (Reidenberg, 1980). Recently, zonisamide, a new drug
    whose anticonvulsant properties are distinct from the other drugs,
    was demonstrated to have an increased anticonvulsant effect in aging
    mice (Kitani et al., 1987). Taken together, these results suggest
    that these pharmacodynamic changes with age may reflect a decreased
    response capability for seizures in the elderly, rather than a
    specific age effect on all the distinct receptors (Kitani et al.,
    1986). In fact, the lethal threshold for pentylenetetrazole in mice
    has been shown to decrease with age (Nokubo & Kitani, 1988), coupled
    to an increase in the threshold for maximal seizure.

         The action of other environmental compounds on CNS function may
    be more diffuse. Recent studies have demonstrated enhanced
    sensitivity of old mice to cyanide intoxication (McMahon & Birnbaum,
    1990b). Exposure to various metals has been shown to alter a variety

    of CNS functions. There has been a particular interest in a
    potential link between Alzheimer's disease and aluminium toxicity.
    It was initially reported that the autopsied brains of Alzheimer's
    patients showed elevated aluminium levels (e.g., Perl & Brody,
    1980). However, others have found no such correlation. Marksberry
    et al. (1981) compared the brains of Alzheimer's patients with older
    adult controls and found no correlation between the density of
    aluminium content and the density of neurofibrillary tangles and
    neuritic plaques. However, a more recent study showed that the brain
    aluminum levels were significantly increased as a function of age in
    both control and Alzheimer patient populations (Bjorksten et al.,

         Manganese is a metal that causes age-type neuropathy. However,
    unlike aluminium, it is associated with extrapyramidal disorders,
    characterized by intention tremor (Donaldson, 1987).  In monkeys,
    the neurological symptoms of choreo-athetoid movement, rigidity and
    tremor occurred after 18 months of manganese exposure. These
    clinical signs, in association with severe lesions of the globus
    pallidus and subthalamic nucleus, resembled Parkinson's disease and
    suggested a possible link between environmental exposure and
    occurrence of the disease in aging individuals.

         The neurotoxic effects of other metals have been widely
    studied, particularly those of lead, which has been implicated in
    reduced intellectual abilities in children (Needleman et al., 1979),
    slowed reaction time (Hunter et al., 1985), and impairment of other
    cognitive abilities (Winneke et al., 1982; Hansen et al., 1985). 
    While psychophysiological parameters appear to be relatively
    insensitive to low levels of lead exposure, several studies have
    demonstrated both cognitive and emotional effects due to long-term
    exposure to lead (Hogstedt et al., 1983; Mantere et al., 1984). 
    Long-term exposure to mercury vapour has been reported to interfere
    with verbal intelligence and memory performance (Piikivi et al.,
    1984). Hanninen (1982) suggested that abnormalities due to mercury
    exposure affect the motor system and result in intellectual
    impairment, a gradual and progressive deterioration of memory
    function, and emotional disability. Other metals including copper,
    iron and manganese have also been reported to cause CNS dysfunction
    (Grandjean, 1983). However, there are no available data on the role
    of exposure to these metals in dysfunction of the CNS in the aged.

         Examples of naturally occurring neurotoxic agents that
    stimulate nervous system senescence have been reported in certain
    island populations. For example, the Chamorro peoples of the
    Marianna Island, specifically Guam and Rota, exhibited a high
    incidence of amyotrophic lateral sclerosis, Parkinsonism and
    Alzheimer's-like dementia that was recently linked to their diet. 
    The Chamorro's diet consists in part of a flour made from the seeds

    of  Cycas circinalis. When one of the components of these seeds,
    beta- N-methylamino-l-alanine (BMAA), a compound similar in
    structure to excitotoxic amino acids, was fed to macaques, they
    exhibited signs of motor neuron, extrapyramidal and behavioural
    dysfunction (Spencer et al., 1987).

         It seems that several motor neuron disorders are associated
    with the appearance of endogenous excitotoxic glutamate agonist-type
    molecules. One such molecule is 2,3-pyridine dicarboxylic acid
    (quinolinic acid), which has been isolated from the brains of
    humans, rabbits and laboratory rodents (Moroni et al., 1984) and has
    been shown to excite CNS neurons when applied iontophoretically
    (Perkins & Stone, 1983). An interesting correlate is the fact that
    brain concentrations of quinolinic acid increase with age (Moroni
    et al., 1984), suggesting that its presence may result in the
    spontaneous onset of neurode-generative conditions that are mimicked
    by environmental neurotoxicants such as BMAA and
    beta- N-oxalylamino-L-alanine (BOAA).

    3.2.2  Endocrine system

         There are several different ways in which the endocrine system
    and the hormonal signalling operations involved may undergo
    alterations with age and toxicant exposure. These can be categorized
    as changes in: (a) the availability of hormones for binding to the
    target tissues; (b) the reception of the pertinent transmitter or
    hormonal signal by the target cells; and (c) the nature of the
    hormonal message.  Changes in hormonal availability with age

         Age-related or toxicant-induced shifts in synthesis, rate of
    clearance and rate of secretion will all function to alter hormonal
    concentrations. Such changes in the size of the available signal
    pool may have corresponding effects on the magnitude of the response
    by the target tissue. These changes may reflect a decline with age
    in the homeostatic controls, which rely heavily on endocrine
    feedback relationships.

         Several toxicants have also been observed to cause changes in
    circulating hormonal levels (Cooper et al., 1986). Significant
    reductions in serum testosterone, for example, have been seen
    following short-term exposure of rats to the plasticizer
    dinitrobenzene (Rehnberg et al., 1988b) and the pesticide
    chlordimeform, the latter also causing marked reductions in serum
    LH, thyroid-stimulating hormone (TSH), T4 and T3 levels. The
    effects, moreover, may be remarkably specific. Following three days
    of exposure in male rats, the pesticide linuron, for instance, was
    reported to decrease the serum T4 level in a dose-related manner,
    while leaving T3 and the pituitary and gonadal hormones unaffected
    (Rehnberg et al., 1988a).

         There is also a growing body of evidence for a hormonal
    influence on toxicant metabolism. A sizeable number of xenobiotics,
    including both drugs and environmental toxicants, are metabolized by
    the hepatic cytochrome P450 monooxygenase system (Nebert & Gonzalez,
    1987). Components of this system have been found to be influenced by
    glucocorticoids (Schuetz et al., 1984; Simmons et al., 1987) and
    markedly affected by sex steroids (Kamataki et al., 1985b) and
    growth hormone (Yamazoe et al., 1987; Zaphiropouos et al., 1989).
    Consequently, persistent shifts in the circulating levels of such
    hormones, as have been reported for the aging animal, could affect
    the manner in which xenobiotics are metabolized following exposure.

         Reported attenuations with age in the rhythms of human and rat
    serum testosterone (Bremner et al., 1983; Steiner et al., 1984;
    Tenover et al., 1988), LH (Vermeulen et al., 1989) and GH (Sonntag
    et al., 1980), among other hormones, can present differences in
    young-versus-old comparisons, depending on when such sampling is
    performed. Comparable effects on hormonal rhythms have been reported
    to occur in response to toxicant exposure. For example, single
    injections of 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD)
    resulted in some evidence of alterations in prolactin and
    corticosterone rhythms in rats (Jones et al., 1987). It may be that
    an aging system, while still exhibiting rhythmic hormonal changes,
    may be increasingly sensitive to their disruption by low toxicant
    levels.  Changes with age in the reception of the signal by the
             target cells

         A general decline in the transmitter regulation of hormonal
    function may also place an aging animal at increased risk for
    toxicant exposure (Govoni et al., 1988), given that various
    environmental toxicants (including, for example, the solvents
    vinyltoluene, ethylbenzene and styrene, the halogenated hydrocarbon
    TCDD, and certain heavy metal cations) have been reported to
    interact with catecholaminergic systems (Govoni et al., 1979; Lucchi
    et al., 1981; Arfini et al., 1987; Mutti et al., 1988; Russell
    et al., 1988).

         These changes have been observed for both neurotransmitter and
    hormone receptors. There is some evidence of a decreased
    responsiveness of target tissues to steroid hormones during
    senescence. For example, age-related declines in the concentration
    of the estrogen receptor may reflect a decline in the circulating
    estrogen levels (Thakur, 1988). Impaired responsiveness could also
    be due to reduced receptor translocation (Belisle & Lehoux, 1983) or
    other steps in steroid action which occur after hormone binding. In
    fact, age-related alterations in glucocorticoid responsiveness are

    due to both receptor and post-receptor events (Kalimi, 1982).
    Testicular luteinizing hormone receptor concentration and total
    content also decrease with age (Amador et al., 1985). In general,
    receptors for steroids, insulin, glucagon, catecholamines and
    prolactin appear to decrease in concentration with increasing age in
    rodents, dogs and humans (Roth, 1979b).

         An additional consideration of alterations with age or toxicant
    exposure in the reception of a hormonal (or transmitter) signal by
    target cells concerns not only effects on the receptor itself, but
    changes in the cell's membranes. In the aging rat brain, there is
    evidence for a progressive decrease in membrane fluidity
    (Hershkowitz, 1983; Nagy et al., 1983). This is at least partially
    attributable to alterations in the lipid composition. For example,
    it has been reported that aging is associated with elevations in
    cholesterol, sphingolipids and saturated fatty acid chains, all
    leading to increases in rigidification (Rouser et al., 1972). Since
    protein activity in the membrane is influenced by the fluidity of
    the lipid micro-environment, any alterations with age in membrane
    viscosity may affect not only receptor functions but also enzymatic
    activity.  Changes in the nature of the hormonal message with age

         The antigenic site(s) on a hormone recognized by antibodies can
    be quite distinct from those regions that bind to the receptors and
    trigger a physiological response in the target tissue. This
    distinction may have an added importance for studies in aging, since
    alterations with age in peptide hormone structure have been reported
    (Conn et al., 1980) that reflect changes in post-translational
    processes. These effects, moreover, may be influenced by shifts in
    the steroid hormonal milieu in the older animal (Ulloa-Aguirre
    et al., 1988). A number of hormones are glycosylated to varying
    degrees and such differences in their carbohydrate residues may
    alter biological activity (Ulloa-Aguirre & Chappel, 1982; Warner
    et al., 1985) and/or plasma half-life (Morell et al., 1971).
    Consequently, hormonal measures based solely on immunoreactivity
     per se potentially offer a somewhat inaccurate picture of
    endocrine alterations with age.

    3.2.3  Kidney

         The aging kidney appears to be more susceptible than the young
    one to drug-induced nephrotoxicity as well as to renal ischaemia. In
    fact, cortical tubules of senescent rats seem more sensitive to
    oxygen deprivation than do those of young rats (Miura et al., 1987).
    Thus, increased susceptibility of the aging kidney can be
    independent of pharmacokinetic effects.

         Age-related susceptibility to nephrotoxicity has been
    demonstrated for numerous drugs including salicylate, acetaminophen,
    cephaloridine and doxorubicin. Kyle & Kocsis (1985) showed that
    kidneys from older rats develop nephrotoxicity to a greater extent
    than do those of young rats following an equal dose of salicylate on
    a body weight basis. However, the dose to the kidney could be
    greater in the older rats due to a decrease in the volume of
    distribution. In addition, McMahon et al. (l990a) has recently
    demonstrated that at toxic doses old rats produce more reactive
    metabolites of salicylate than do young ones. The age-related
    enhancement in cephaloridine nephrotoxicity may also be due to
    pharmacokinetic changes (Goldstein et al., 1986), although renal
    cortical slices from old rats demonstrated alterations in active
    uptake of organic anions and cations following exposure to the
    antibiotic, which were not observed in young rats. Doxorubicin
    treatment resulted in greater toxicity in old rats (Colombo et al.,
    1989). The onset of the delayed nephrotoxicity was noticeably faster
    in the old rats; this may have been related to higher drug retention
    in the kidneys of old rats.

         Age-related increased susceptibility to acetaminophen
    nephrotoxicity has been investigated more than that of any other
    chemical in rats. Tarloff et al. (1989b) stressed that while
    enhanced sensitivity to acetaminophen nephrotoxicity clearly occurs
    with advancing age, great care must be taken in choosing the ages
    for comparison. Beierschmitt et al. (1986a) demonstrated by both
    functional and histological criteria that susceptibility to
    acetaminophen-induced acute tubular nephrotoxicity increases with
    age in rats. This may reflect altered susceptibility rather than an
    alteration in pharmacokinetic parameters, since the generation of
    reactive metabolites from acetaminophen decreases with age
    (Beierschmitt & Weiner, 1986). Increasing delivery of acetaminophen
    to functioning nephrons, whose numbers are decreased in the aged
    kidney, does not appear to be responsible for the age-related
    increase in susceptibility (Beierschmitt et al., 1986b).

         Alterations with advancing age do not have to result in
    increasing sensitivity. Studies by Murty et al. (1988b) indicated a
    decrease in hydrocarbon-induced hyaline droplet nephropathy in male
    rats during senescence. This male-specific nephropathy is associated
    with the presence of alpha2u-globulin, whose synthesis is strongly
    age dependent (Roy et al., 1983). Although gasoline causes extensive
    nephrotoxicity in young rats, old rats are resistant. In fact,
    gasoline failed to alter hyaline droplet numbers in aged male rats,
    which also demonstrated an altered lysosomal response as compared to
    young rats. This lack of response of aged rats to
    hydrocarbon-induced hyaline droplet nephropathy suggests that these
    rats would also be resistant to hydrocarbon-induced renal neoplasia.

    3.2.4  Immune system

         With advancing age a progressive decline in the concentration
    of glucocorticoid receptors occurs in the spleen (Roth, 1979a). This
    alteration may depend either on an age-related modification of the
    ratio among various subsets of lymphoid cells carrying different
    receptor densities or on an intrinsic failure of aged cells to
    maintain an adequate turnover of receptor molecules. With advancing
    age, membrane receptors for hormones of low relative molecular mass
    on lymphoid cells also show alterations, although these are more
    related to impaired coupling to signal transduction systems (Feldman
    et al., 1984; Roth, 1988). It has been shown that the number of
    receptor molecules decreases with advancing age whereas the affinity
    does not seem to change. In the aged population with altered
    function of the immune system, it is possible that immunomodulatory
    agents present in the food potentiate immune dysfunction, making the
    elderly more vulnerable to age-related diseases.

         The effects of immunosuppressive compounds can be identified
    from toxicological experiments with young adult rodents. These
    chemicals include organotin compounds, some pesticides, halogenated
    aromatic hydrocarbons (e.g., hexachlorobenzene and dioxins) and
    cyclosporin (Poland & Knutson, 1982; Vos & Penninks, 1987; Vos &
    Luster, 1989; Schuurman et al., 1990; Vos et al., 1990), and they
    can also potentiate autoimmunity, which is found more frequently in
    the elderly.

    3.2.5  Other tissues and systems

         There are few data on pharmacodynamic changes in other tissues
    and systems during aging. An increase in the sensitivity of the
    aging human myocardium to digoxin (Chavaz et al., 1974), as well as
    to intravenous anaesthetics (Dundee, 1979), has been suggested. The
    stomach, kidney and bone marrow are more susceptible to the adverse
    effects of non-steroidal anti-inflammatory agents in the elderly
    than in the young, and cerebral side effects are more common
    (Huskisson, 1983). The anticoagulation properties of warfarin are
    also changed in the elderly, with the aged demonstrating increased
    sensitivity (Shepherd et al., 1979).

         Although in most toxicological studies the cardiovascular
    system is not well examined, there are several chemicals known which
    can cause cardiovascular toxic or atherosclerotic effects. Such
    compounds in food include erucic acid in combination with
    omega-3-linolenic acid, cetolenic acid, brominated vegetable oils,
    cobalt salts in combination with ethanol, lead, nitrates and high
    amounts of caffeine (Speijers, 1983). On the other hand, compounds
    such as saturated fatty acids, odd-numbered cyclopropenoid fatty
    acids (sterculia acid), ergot alkaloids, halogenated aromatic

    hydrocarbons and cadmium induce toxic effects on the vascular system
    and possible potentiate atherosclerotic effect (Speijers, 1989a,b).
    Conrad & Bressler (1982) investigated the cardiovascular effects of
    caffeine in elderly men. Caffeine, in doses equal to those contained
    in 2 to 3 cups of coffee, produces an increase in blood pressure,
    but has no positive inotropic effect in healthy elderly men.

         Kim & Kaminsky (1988) suggested that age plays an important
    role in modifying susceptibility to the toxic metabolite of
    fluroxene, 2,2,2-trifluoroethanol. They noted greater effects on the
    stomach, liver, testicles, brain and kidneys of aged rats as
    compared to younger animals. Enhanced sensitivity to ethanol
    intoxication has also been noted in old rats (Guthrie et al., 1987),
    suggesting altered tissue sensitivity. Acute ethanol hepatotoxicity
    may be due to enhanced liver susceptibility to the toxin (Rikans &
    Snowden, 1989).

    3.3  Modifying factors

    3.3.1  Nutrition

         Nutrition has been shown to influence the aging processes in
    rodents and the occurrence and progression of age-associated
    diseases in rodents and humans. Moreover the age-associated changes
    in physiological processes affect the nutrition of the mammalian
    organism. However, there is little information on how aging
    influences the nutritional requirements of humans, a subject that
    urgently requires scientific study.

         Nitrates in foods can be reduced to nitrites in the oral
    cavity, GI tract and urinary bladder (Tannenbaum et al., 1978).
    Nitrites react with amines in the stomach forming  N-nitroso
    compounds, many of which have been shown to be carcinogenic in
    animal studies, and it is difficult to deny their hazard to man
    (Searle, 1976; Bartsch, 1991).

         During commercial processing and domestic preparation, foods
    may become contaminated with toxic chemicals. For example, smoked
    and grilled foods contain small amounts of polycyclic aromatic
    hydrocarbons and a wide variety of phenols and other organic
    compounds derived from smoke (IARC, 1990). Canned foods can become
    contaminated with tin or lead.

         In a study by Spagnoli et al. (1991), both 4-month-old and
    4-year-old New Zealand white rabbits were fed an atherogenic diet. 
    Increased incidence and degree of atherosclerotic lesions were seen
    in the 4-year-old rabbits. These results showed an increased
    susceptibility of the older arterial wall to hypercholesterolaemia. 
    Although 4-year-old rabbits are still relatively young, considering
    the life span of this species (10-12 years), this study suggests
    that aging arteries might be vulnerable to atherogenic compounds.

         Malnutrition associated with inadequate intake or uptake of
    nutrients in the elderly is caused by several factors. These include
    socio-economic conditions (Munro, 1984) such as a) ignorance of the
    need for a balanced diet, b) poverty, c) social isolation, d)
    physical dependence (disability) and physical inactivity, e) mental
    disorders, and f) changes in habits, e.g., retirement (Munro, 1984;
    American Dietetic Association, 1984; Ferro-Luzzi et al., 1988).
    Other conditions resulting in malnutrition include malabsorption due
    to a variety of intestinal conditions, alcoholism, and the use of
    therapeutic drugs that interfere with nutrient utilization and
    therefore with the toxicity of chemicals (Krupka & Vener, 1979;
    Kohrs, 1981; Munro, 1984; Chen et al., 1985; Robertson et al, 1988).
    Malnutrition in much of the aged population can enhance the
    vulnerability of the elderly to the effects of the toxic chemicals
    in food. It has also been reported that malnutrition alters
    pharmacokinetics in different ways depending on the drugs examined
    (Roe, 1983; Cusack & Denham, 1984). This can be another factor in
    the altered sensitivity of the malnourished elderly to toxicants.
    Major deficits or marginal nutrient status occur for proteins,
    calcium, vitamins A, C and B (thiamin, riboflavin, niacin, B6 and
    B12) and zinc (American Dietetic Association, 1984; Ferro-Luzzi
    et al., 1988).

         In aged women, a deficiency in calcium is especially associated
    with osteoporosis (American Dietetic Association, 1984; Munro, 1984;
    Caraceni et al., 1988; Cauley et al., 1988). The deficiency in iron
    and zinc might play a role in haematological status and immune
    function, while a higher intake of aluminium might have neurological
    sequelae (American Dietetic Association (ADA), 1984). An iron
    deficiency might make individuals more vulnerable to compounds toxic
    for the haematopoietic system, e.g., hexachlorobenzene and lead.

         Data showing that older individuals have a reduced need for
    energy, due to slower metabolism and decreased activity, has an
    important impact on the intake of both macro and micronutrients,
    because the composition of the diet is based on the average caloric
    or energy intake (American Dietetic Association (ADA), 1984;
    Ferro-Luzzi et al., 1988). Considering this reduced need for energy,
    the elderly often reduce their intake of nutrients.

         Dietary manipulation can alter life span in laboratory animals
    (Barrows & Kokkonen, 1978; Young 1979). Restricting food intake in
    laboratory rats produces a significant extension of life span (McCay
    et al., 1935; Ross, 1961). Similar effects have been observed in
    mice (Weindruch & Walford, 1988; Kubos et al., 1984), as well as in
    more primitive organisms including fruit flies (Harman, 1981), and
    nematodes and Neurospora (Harman, 1982). Other studies have
    indicated that the reduction in caloric intake that accompanies
    protein restriction, and not the protein restriction  per se, is
    responsible for the increased longevity (Leto et al., 1976; Davis

    et al., 1983; Schneider & Reed, 1985). Dietary restriction has been
    shown to be effective when started as late as middle age (Cheney
    et al., 1983; Kubos et al., 1984; Masoro, 1988). Food restriction
    appears to act either by influencing primary aging processes or by a
    general protective mechanism, rather than directly modulating
    multiple specific pathogenic processes underlying specific diseases
    (Masoro et al., 1991).

         Chronic nephropathy in rats has been reported to be retarded by
    food restriction (Saxton & Kimball, 1941). Rats fed  ad libitum a
    semisynthetic diet of 21% casein as the protein source exhibited a
    marked age-associated progression of chronic nephropathy, whereas
    rats fed 60% of the  ad libitum intake developed almost no
    age-related progression of this disease process (Maeda et al.,
    1985). Caloric restriction appears to be more important than protein
    restriction in retarding nephropathy (Maeda et al., 1985). Fat or
    mineral restriction was found to have no influence on longevity
    (Iwasaki et al., 1988).

         Dietary restriction delays the age-dependent loss of adipocyte
    responsiveness to hormones, prevents the decline in serum free fatty
    acid levels, delays the increase in serum cholesterol, and reduces
    the increasing triglyceride level observed in aging rats (Cooper
    et al., 1977; Liepa et al., 1980; Masoro et al., 1980; Yu et al.,

         Dietary restriction from weaning has been shown to delay the
    onset and reduced the severity of chronic nephrosis, periarteritis,
    myocardial degeneration and muscular dystrophy in very old animals
    (Berg, 1976). Chronic restriction has also been shown to inhibit
    certain types of tumours, decrease the incidence of neoplasms, and
    increase tumour latency (Ross, 1976; Weindruch et al., 1982;
    Weindruch & Walford 1988). Early work using a chemically induced
    mammary carcinoma model suggested that the primary effect of caloric
    restriction was on tumour promotion (Weindruch & Walford, 1988).
    More recent data (Fishbein, 1991) have demonstrated that the
    initiation of chemical carcinogenesis can also be decreased by
    caloric restriction. For example, exposure to aflatoxin B1
    resulted in less DNA-adduct formation in liver from young
    calorically restricted male rats than from controls fed  ad libitum.
    Such changes are most probably due to changes in hepatic cytochrome
    P-450 expression (Fishbein, 1991).

         There is ample evidence of better maintenance of
    T-cell-dependent immunological responses in aging mice chronically
    restricted from weaning (Weindruch & Walford, 1988). However, it has
    been reported that restriction initiated at an adult age can also
    delay the age-specific decrease in immune function (Fernandes
    et al., 1977; Friend et al., 1978; Weindruch & Walford, 1988).

         Although caloric restriction extends life span in invertebrate
    and lower vertebrate species as well as rodents (Weindruch &
    Walford, 1988), as yet there is no definitive evidence whether or
    not caloric restriction will increase longevity, or decrease
    neoplastic and degenerative diseases, in higher mammals or in man.
    There is some evidence that reduced caloric intake in man reduces
    urinary output of thymidine glycol and 8-hydroxy-guanosine, which
    implies reduced free-radical-mediated DNA damage (Fishbein, 1991).
    However, until the mechanisms by which caloric restriction evokes
    its effect are fully understood, the only way that it can be
    conclusively proved whether or not caloric restriction does prolong
    life in higher mammals, is to perform longevity studies in these
    species. Such experiments, using non-human primates, are underway in
    the USA (Fishbein, 1991), but it will be some time before definitive
    data are available.

         Chronic disease in the elderly is accompanied by caloric
    deficit, which in turn causes breakdown of body proteins and
    negative nitrogen balance as well as the utilization of fat stored
    in adipose tissues. Dietary protein appears to promote
    age-associated renal disease in both humans and rats (Brenner
    et al., 1982).

         When illness occurs, nutritional deficiencies frequently become
    clinically manifest (Rudman, 1987). For example, trauma or a fall
    causing fractures leads to immobilization resulting in rapid loss of
    body stores of nitrogen and calcium. This slows down mending of the
    fracture. Similarly, surgical procedures in the elderly often result
    in delayed recovery and risks far exceeding those of younger
    persons. Heart failure and malignancies can lead to cachexia with
    loss of weight, muscle mass and nutrient reserves. Infection may
    produce similar changes and intervene to become the terminal event.

    3.3.2  Alcohol intake

         Ethanol is one of the chemicals most commonly ingested by
    humans. Its effects on the body are numerous and varied. Loneliness
    and isolation would seem to foster consumption of alcohol beverages
    among older persons. The decrease in lean body mass and body water
    that occurs with aging is responsible for higher blood levels of
    alcohol in elderly people than in younger adults consuming the same
    quantities (Vestal et al., 1977).

         Elderly individuals have a decreased tolerance for alcohol, due
    to an increased sensitivity of the CNS to the depressant effect of
    ethanol. The metabolic effects of ethanol are the result of either
    the increase in the NADH/NAD+ ratio occurring due to ethanol
    metabolism or to direct toxic effects of ethanol or its metabolite,
    acetaldehyde. An increase in serum uric acid level (hyperuricaemia)
    is common during heavy alcohol ingestion, thus inducing acute gouty

    arthritis in patients with known gout. Hypoglycaemia and
    hyperlipidaemia occur in patients who are not eating adequately or
    who are ingesting a high-fat diet with ethanol, respectively.
    Thrombocytopenia is caused by alcoholism in patients with advanced
    alcoholic liver disease (IARC, 1988).

         The use of drugs increases with aging. Acceleration or
    inhibition of drug metabolism by ethanol depends on the duration of
    ethanol ingestion and the presence or absence of ethanol in the body
    at the time the drug is ingested (Mezey, 1981). The presence of
    alcohol in the body causes a decrease in the metabolism of certain
    drugs, such as antipyrine, meprobamate, pentobarbital and
    benzodiazepines, resulting in increased bioavailability of the drugs
    to the CNS and thereby contributing to unwanted side-effects. In
    contrast, ethanol can increase the metabolism and metabolic
    activation of certain xenobiotics, such as the known human
    leukaemogen benzene, thus potentially leading to enhanced toxicity
    (IARC, 1988).

         Chronic ethanol ingestion increases tolerance to CNS
    depressants in young individuals, but its effect in the elderly is
    unknown. The concomitant administration of ethanol and barbiturates
    results in an enhanced depressant effect of these drugs on the CNS
    and can result in coma or even death. All other sedative-hypnotic
    drugs tested have either synergistic or additive effects with
    ethanol. Intellectual deterioration and dementia are common
    complications of chronic alcoholism. Alcoholic patients show more
    signs of mental aging at every chronological age (Gaitz & Baer,

         Chronic excessive alcohol ingestion is associated with
    increased mortality from cancer, cirrhosis, non-malignant
    respiratory diseases such as emphysema, and accidents (Klatsky
    et al, 1981; IARC, 1988). However, a decrease in coronary artery
    disease and mortality is associated with moderate alcohol ingestion.
    This may be due to increases in plasma HDL-cholesterol and decreases
    in LDL-cholesterol that occur during ingestion of alcohol (Mezey,
    1981). Ethanol consumption is also associated with hypertension and
    an increased mortality from cerebrovascular accidents (Kozararevic
    et al., 1980; Blackwelder et al., 1980).

    3.3.3  Smoking

         Smoking clearly plays an etiological role and produces an
    acceleration of a wide spectrum of age-associated disease. Smoking
    contributes to an increased mortality rate (Gupta et al., 1980). It
    also provides an excellent example of problems faced in the
    consideration of the environmental impact on aging.

         Scientists recognize that smoking presents a complex toxic
    insult through inhalation. Increased pathology in aged mice has been
    reported after exposure to cigarette smoke (Matulionis, 1984). The
    immune response in aged mice exposed to cigarette smoke has been
    shown to be decreased at some ages but not at others (Keast & Ayre,
    1981). The role of smoking in coronary heart disease has been
    reviewed by Kannel (1981), who observed that the relative effect of
    cigarette smoking decreases in old age and proposed that this could
    be due to the selection of a more resistant population.
    Estrogen-related diseases have also been associated with smoking
    (Baron, 1984). Reif (1981) has examined susceptibility to lung
    cancer and concluded that the shape of the susceptibility
    distribution is determined by the effects of all environmental
    carcinogens (both known and unknown) to which the population has
    been exposed, as well as by differences in genetic susceptibility
    among members of the population. Smokers and non-smokers both get
    lung cancer during the same age range. Some of these studies suggest
    that aged individuals are more susceptible to the effects of
    smoking, whereas others suggest that duration of smoking appears to
    be the critical factor (IARC, 1990). The smoker is exposed to
    multiple toxic agents simultaneously, the effects of which are more
    pronounced in aged individuals. Major aspects of metabolism and
    pharmacokinetics are altered in aged individuals. Therefore, the
    effective dose of any chemical reaching the systemic target tissues
    in aged humans would be dependent on these perturbations.

    3.4  Interactions of chemicals and diseases

    3.4.1  Cancer

         Cancer morbidity is expected to rise with age and with an
    increasing percentage of elderly people living in industrialized
    countries (Magnus, 1982). There is no consensus on the causes of the
    age-related increase in tumour incidence. Various arguments support
    the concept that an age-related accumulation of total dose of all
    carcinogens accounts for tumour induction as a function of age in
    sensitive individuals (Peto et al., 1975, 1985). One viewpoint is
    that the sensitivity to carcinogens is stable and independent of
    age, whereas another is that changes in the internal milieu of the
    organism, such as the metabolic and immunological shifts of natural
    aging, provide favourable conditions for tumour development with
    increasing age (Burnet, 1970; Dilman, 1971).

         Comparison of human epidemiological data with  in vivo and
     in vitro animal experimental results is difficult but does allow
    some limited conclusions. It seems that environmental carcinogenic
    factors as well as endogenous carcinogens are important causes of
    increased tumour incidence in old people (IARC, 1990). This
    conclusion is supported by the increasing incidence of occupational
    cancer with increased exposure time to carcinogenic agents and by

    the correlation of lung cancer incidence with the number of
    cigarettes smoked (Doll & Peto, 1981; Peto, 1986). Humans, in
    general, have an age-related increase in the incidence of epithelial
    neoplasms (Doll, 1978), but the relationship of age to incidence of
    other cancers in different organs varies (Doll, 1973; Moolgavkar &
    Venzon, 1979; Anisimov, 1987; Dix, 1989). Some tumours appear most
    frequently in childhood, some increase exponentially with age, and
    others reach a peak at a certain age and then decline.

         The data on cancer incidence among the atomic bomb survivors in
    Hiroshima and Nagasaki, Japan, have been very informative concerning
    radiation-related solid tumours as well as leukaemia. Age at time of
    exposure appears to be a strong determinant of leukaemia risk; the
    greatest absolute risk was experienced by those who were exposed at
    ages 0-9 or 50 years and over (Beebe, 1979). Most of the excess
    cancer deaths from solid tumours among the atomic bomb survivors
    have occurred among those who were over 35 at the time of the blast.

         Analysis of data on the positive correlation between the aging
    rates of different species with their cancer rates and the
    observation that these two processes, aging and carcinogenesis, may
    be initiated and promoted by impairments of gene regulation led
    Cutler & Semsei (1989) to conclude that both cancer and aging may
    arise from a common set of genetic alterations. The analysis of the
    interrelationship between aging and carcinogenesis should be based
    on epidemiologically and experimentally confirmed data. 
    Epidemiological data, analysed in terms of a multi-stage model
    (Kaldor & Day, 1987), can estimate the importance of age at onset,
    duration of carcinogen exposure, and latency in a population. On the
    level of the organism, carcinogenic agents influence not only the
    cell, causing genomic damage that leads to neoplastic
    transformation, but also create in the cell a microenvironment that
    facilitates proliferation and clonal selection (Anisimov, 1987,
    1989). Multi-stage carcinogenesis is accompanied by various
    disturbances in tissue homeostasis and systems of anti-tumour
    resistance that, in turn, are under the influence of systemic
    (nervous, hormonal and metabolic) factors. How long it takes for
    frank neoplasia to develop depends on the state of those systems at
    the moment of exposure to a carcinogen or tumour promoter and the

         According to the multi-stage model of carcinogenesis, the
    carcinogen whose effect increases in proportion to age at exposure
    affects the partially transformed cell. In this case the tumour
    incidence would increase and latency would decrease, as compared to
    a population exposed to the same effective dose of carcinogen at a
    young age. For example, application of 7,12-
    dimethylbenz [a]anthracene in small doses or
    12-0-tetradecanoylphorbol-13-acetate to the skin of mice of
    different ages caused neoplasms more frequently in older animals

    (Stenbäck et al., 1981; Ebbesen, 1985). Exposure of mice and rats of
    various ages to phenobarbital resulted in hepatocarcinogenesis only
    in old animals (Ward, 1983; Ward et al., 1988). The number of events
    necessary for complete malignant transformation in 15-month-old rats
    under the influence of  N-nitrosomethylurea is lower than in
    3-month-old rats (Anisimov, 1988). In every tissue, the number of
    events occurring in the stem cell before its complete transformation
    is variable and depends on many factors, in particular the rate of
    aging of the target tissue and of the regulatory system(s) of the
    tissue (Anisimov, 1987, 1989). This model is consistent with the
    analysis of age-related distribution of tumour incidence in
    different sites in humans and experimental animals (Doll, 1978; Dix,
    1989; Anisimov, 1987).

         Epidemiological observations have shown that exposure to some
    carcinogenic agents leads to a rise in cancer incidence independent
    of age at the start of exposure (e.g., smoking), while other agents
    induce more tumours when the exposure begins in the elderly (e.g.,
    lung cancer following asbestos exposure) (Kaldor & Day, 1987; IARC,

    3.4.2  Other diseases

         Aging affects the functional capacity and structural integrity
    of many organ systems. Environmental chemicals can also affect
    several target organs. The combination of the influence of aging and
    the toxic effects of chemicals in the environment might potentiate
    the risk for elderly persons. An inherent problem common to all
    research in chronic disease is the dissection of the respective
    roles of time per se from those of primary aging. Hazzard (1985)
    stated that an essential feature of human aging is the change in
    physiological competence across the life span, as reflected in
    homeostatic reserve. Homeostatic reserve declines at an accelerating
    rate with age, normally producing death in old age from
    multifunctional etiologies.

         The relationship between aging and atherosclerosis is a prime
    example of this conundrum. It seems most likely that the changing
    picture of atherogenesis in western society has led to a large
    number of people who survive into old age with not only a degree of
    clinical atherosclerosis, but also with other chronic progressive
    diseases, such as chronic obstructive pulmonary disease, immobility
    from osteoarthritis and/or osteoporosis, and mental incompetence
    from Alzheimer's disease, multi-infarct dementia or other
    age-related dementing processes.

         These diseases could significantly modify the response of the
    organism to various environmental chemicals by decreasing or
    increasing their susceptibility, followed by an acceleration of
    these diseases or induction of new ones.


    4.1  Experimental approaches

    4.1.1  Principles for testing chemicals in the aged

         There are two principal approaches to the study of age-related
    changes of any functional, morphological and/or biochemical
    parameter, i.e. "longitudinal" and "cross-sectional". Longitudinal
    studies consist of repeated estimations of any parameter in the same
    animal in different periods of life. Cross-sectional studies involve
    separate groups of animals of different ages who are examined at a
    given point in time. Results obtained using the longitudinal
    approach may significantly differ from the results from experiments
    carried out using cross-sectional approaches. In studies on the
    toxicity of chemicals, it is obligatory that identical conditions be
    provided and maintained for all the animals. Problems related to the
    choice of animal species, strain, sex, age, life stage and chemical
    treatment (both route and dose selection) will be considered below.

    4.1.2  Animal models

         An extensive discussion of the choice and use of animal models
    in research has been recently published (Rogers et al., 1991).  Animal species

         The selection of suitable species obviously involves both
    practical and economic factors. Animals with a short life span are
    preferred. However, good life-table information is not available for
    all species. A knowledge of species-related differences in metabolic
    pathways and inherent sensitivities is also important in choosing
    animal species for study. The choice of species should depend on the
    experimental question as well as homology of response. For example,
    while closely related isoforms of drug-metabolizing enzymes may
    exist in different species, their tissue distribution and substrate
    specificity may vary greatly (Nebert et al., 1991). Such differences
    in isoform expression, together with reported differences in DNA
    repair efficiency, could be responsible for species-related
    differences in an organism's susceptibility to chemical carcinogens
    (Daniel et al., 1983; Mehta et al., 1984; Anisimov, 1987; 1989).

         Regardless of such metabolic differences, mice and rats are 
    most often used in aging research because of a short life span,
    relative ease of maintenance under defined conditions, wide use in
    biological research, and suitability for a variety of molecular and
    genetic analyses. There are extensive life-table information for
    some strains of mice and rats as well as a data base on their
    spontaneous pathology. If old animals are purchased from a supplier

    rather than being maintained in an investigator's own laboratory, it
    is necessary to obtain information on the lifetime environment of
    the animals, including housing conditions and dietary history. It
    may be preferable to keep animals from weaning until the desired age
    for investigation under the same controlled conditions.

         Mammalian species other than rodents have only been used
    sporadically in aging research, owing to their long life span, lack
    of life-table information, genetic heterogeneity, restricted
    availability and high cost. However, it is critical that such larger
    species be used to answer certain questions. For example, studies
    are currently underway to determine if dietary restriction can
    prolong the lifespan of two different monkey species (Ingram et al.,
    1990).  Animal strain

         The choice of adequate rodent strain for experiments on aged
    animals is critical. The male Fischer-344 rats is a popular model in
    aging studies because of its size and growth characteristics. 
    However, testicular interstitial cell tumours begin to appear at the
    age of 18 months in rats fed ad libitum, and by the age of 2 years
    the tumour incidence approaches 100% (Fishbein, 1991).  Recently, a
    highly significant positive trend with time has been observed for
    the increasing prevalence of leukaemia, anterior pituitary tumours
    and thyroid c-cell tumours in both sexes, adrenal pheochromocytomas
    in males, and mammary tumours and endometrial stromal polyps in
    female F-344 rats (Rao et al., 1990).  Some mouse strains suffer
    from a single major disease process (e.g., tumours of the mammary
    gland or liver, or chronic nephropathy), and the presence of this
    disease in most animals could modify the response to chemical
    exposure and complicate the interpretation of the results.

         In the USA and Japan, Fischer-344 and Sprague-Dawley rats and
    B6C3F1, Swiss, and CD-1 mice are most frequently used, whereas
    in European countries Wistar-derived and Brown Norway rats and NMRI
    and Swiss mice are the most popular strains. Inbred animals have the
    advantages of greater stability and predictability of response.
    However, heterogeneity of outbred animals more closely resembles the
    heterogeneity of human populations. The choice of a certain animal
    strain also depends on previous experience with these animals, the
    final choice of an appropriate species and/or strain being dependant
    on the scientific hypothesis under investigation. Each strain has
    its own pattern of background pathology, which can be greatly
    influenced by animal husbandry.  Animal sex

         Sex-differences in response to chemicals are well known. Thus,
    the use of both sexes is often necessary in toxicity testing.
    However, the large gender differences in chemical toxicity and
    pharmacokinetics that occur in rats become less apparent in old age
    due to the decreased expression of sex-specific isoforms of the
    hepatic drug-metabolizing enzymes (Kitani, 1991).

         Ovarian status may significantly influence the sensitivity to
    some chemical agents, modifying the biological response, as been
    demonstrated for chemical carcinogens (Anisimov, 1971, 1987). It is
    noteworthy that, when using females of the post-reproductive period,
    the investigator must be aware that a) the ovaries in females of
    some species (i.e. rats and mice) are not atrophied and may continue
    to secrete steroid hormones, and b) some animals may be in
    persistent estrus, while others may be in anestrus or
    pseudopregnancy status (Aschheim, 1976). Furthermore, an animal's
    reproductive history could influence its response to a chemical in
    later life.  Selection of age groups for comparison

         The problem of appropriately characterizing the animal's age
    within the context of its life span is of particular importance in
    experimental gerontology. Since the aging process causes significant
    changes in various systems of the organism, there is a need for the
    definition of some reference points for adequate comparison of the
    results of different experiments. The importance of these points is
    particularly significant in cross-sectional studies. One of the
    confounding factors in such comparisons between experiments is the
    need for frequent comparisons of results from different animal
    species with different life spans (long-lived and short-lived
    species and/or strains). Many authors have used two kinds of age
    groups: "mature" or "adult" animals; and "old" animals. However, the
    true age of "mature" rats in the reports from different
    investigators fluctuates from 2 to 14 months and the age of "old"
    animals from 12 to 37 months. The life cycle of experimental animals
    can be divided into four periods: a) prior to weaning
    (developmental); b) sexual maturation (maturational);
    c) reproductive; d) pronounced age-related changes (senescent
    period) (Zapadnyuk, 1971). All studies should compare animals from
    multiple age periods.  Underlying pathology of animals of different ages

         As mentioned previously, the background pattern of pathology
    must be taken into account in the selection of animal species and
    strains. A species-, strain- and sex-specific incidence of
    pathology, whether neoplastic or not, is observed to increase
    rapidly in the second half of the life span, even for those animals

    maintained under specific pathogen-free conditions (Burek, 1978;
    Anisimov, 1987; Frith & Ward, 1988; Fishbein, 1991). Among
    non-neoplastic diseases in rats, the most frequent are chronic
    glomerulonephropathy, cardiomyopathy, amyloidosis, and peripheral
    nerve degeneration. For example, the incidence of spontaneous
    leukaemia in F-344 rats frequently used in long-term studies may
    exceed 30%, and the frequency of intertitial cell tumours of the
    testis may reach 100% in old males. This pathology may significantly
    modulate the host response to xenobiotics.  Transgenic animals

         During the last decade it has become possible to add new
    genetic information to the germ line of experimental animals
    (Jaenisch, 1988). More recently successful attempts have even been
    made to specifically alter gene sequences in the mouse germ line,
    thereby ablating or correcting specific gene functions (Capecchi,
    1989). In the study of the effects of chemicals on the aged
    population, transgenic animal models have at least two contributions
    to make. Firstly, the influence of specific genes on the age-related
    susceptibility to environmental chemicals can be assessed in the
     in vivo situation (Vijg & Papaconstantinou, 1990). Secondly, by
    using transgenic animals harbouring a shuttle vector with one or
    more mutational target genes, mutagenic effects can be studied in
    different organs and tissues of animals as a function of age (Gossen
    et al., 1989).  Animal husbandry

         In experimental research on the sensitivity of aged animals and
    the aging process, the husbandry aspects should be defined. The
    quantitative and qualitative results might depend greatly on the
    dietary conditions. The dietary composition should meet the minimal
    requirements for nutrients, minerals, vitamins and (raw) fibre for
    adult animals according to established and published standards for
    laboratory animal diets. Another important dietary factor is the
    quantity to which the test animals have access. They may either
    receive a restricted diet containing adequate level of nutrients or
    be fed  ad libitum. The choice of diet depends on the questions to
    be asked.

         In addition to the dietary status of the animals, their
    microbiological status should be defined. In some cases interaction
    of the chemical with the gut microflora might modify the outcome of
    the study. Specific-pathogen-free (SPF) animals are preferred. The
    direct environment of the test animal, including relative humidity,
    temperature, light and dark cycles, seasonal influence and stress
    can all influence the final outcome of a study and therefore should
    be defined carefully (Masoro, 1991).

         Differences in animal husbandry may cause large variations in
    biochemical and pathological effects. This is clearly illustrated by
    alterations in the background data among different laboratories.

    4.1.3  Chemical exposure  Dose level

         The selection of dose is a difficult issue. The usual
    requirement is to have a minimum number of test groups (3) plus one
    control (vehicle) group. This permits the development of a
    "dose-response" curve and allows for appropriate statistical
    evaluation of the results. The highest dosage should induce minimal
    signs of toxicity, bearing in mind that the maximum tolerated dose
    for young animals could in some cases be toxic for old ones, and
    that high doses may alter the toxicokinetics of the chemical.

         Considering that the body weight of young and old animals could
    be significantly different, a correct dosage calculation of test
    substances in comparison groups is an important problem, i.e.
    whether it is better to normalize per unit of body weight or body
    surface, or to some other parameter such as lean body mass (Travis
    et al., 1990). The available data indicate similarity of the results
    when calculation of dose is performed per unit of body weight or
    body surface. It should be taken into consideration that the growth
    and development of various organs may have different rates, and that
    relative organ weight (and consequently, the effective target dose
    of the substance) may not be the same in animals of different ages.
    When the substance is administered in food or water, the consumed
    quantities should be taken into account, because they may differ in
    animals of various ages. When the oral or dermal route of
    administration is used, age-related changes in the extent and rate
    of absorption may be important. Age-related changes in lung
    ventilation capacity may also alter the internal dose of a chemical
    when the inhalation route of administration is used. When looking
    for portal-of-entry effects, a constant concentration of the agent
    may be used in all age groups.  Route of administration

         There is general agreement that the test substance should be
    administered by the route that corresponds most closely to human
    exposure. For humans, the main exposure routes are oral, dermal and
    inhalation, while in animal experiments the oral route is most
    frequently used. However, when pharmacokinetic studies show that
    other routes of administration result in equivalent target tissue
    levels, such alternative routes can be used. The use of injections
    (subcutaneous, intraperitoneal, intramuscular or intravenous) may be
    expedient under certain circumstances.  Duration of exposure

         The question being addressed should guide the design of the
    study. If the potential risk involves acute exposure of the elderly,
    then an acute experimental scenario is required. If human exposure
    is ongoing, long-term studies may be necessary. Exposure to the test
    substance in chronic studies should start shortly after weaning and
    continue for the major portion of the animal's life (at least
    through the mean life span). IARC (1986) recommended 24 months of
    exposure for rats and mice, and 18 to 20 months for hamsters when
    studying the carcinogenic potential of chemicals. Choice of exposure
    duration based on life-table characteristics would be optimal.
    However, these guidelines do not apply to experiments when animals
    of different ages are used at the start of the study. In this case
    the dose and duration of exposure could differ in groups of young
    and old animals. However, it is preferable to use identical exposure
    durations whenever possible.

    4.1.4  Non-mammalian models

         Many species can be used as models in the study of age-related
    sensitivity to environmental toxicants. These include fungi
     (Neurospora crassa and  Podospora anserina), protozoa
     (Paramecium tetraurelia and  Tetrahymena pyriformis), rotifers,
    nematodes  (Caenorhabditis elegans, C. briggsae and  Turbatrix
     aceti), and insects  (Drosphila melanogaster, Musca domestica and
     Tribolium confusum) (Committee on Chemical Toxicity and Aging,
    1987). Non-mammalian species such as fish and salamanders have been
    used in aging research (Weindruch & Walford, 1988) and are
    potentially available for toxicology studies. Several of these
    models have already been used to examine the effects of chemicals on
    aging. Because of their short life span, ease of use and relatively
    low cost, non-mammalian organisms could be important in the initial
    phases of a test system to identify environmental chemicals that
    might affect aging.

    4.1.5  In vitro studies

         For the purposes of screening xenobiotics, the use of  in vitro
    models can result in substantial economy and efficiency. Stock cells
    and, in some cases, tissue explants can be cryopreserved in large
    amounts. This permits repeated assays with comparable materials and
    the sharing of common stocks by numerous laboratories. Moreover,
    such stocks can be used to investigate cell-to-cell interactions,
    such as metabolic cooperation and metabolic transformation. Finally,
    tissue culture approaches can substantially reduce the numbers of
    animals required for experimentation. Such methods cannot, however,
    be expected to substitute for the intact animal experiment.

         There are three general categories of  in vitro methods: organ
    culture, tissue explants and cell culture. Organ culture involves
    the short-term maintenance of variable intact segments of tissue,
    for example, the full thickness of a segment of aorta. In tissue
    explants, the early migration and proliferation of epithelial and
    fibroblast cell types can be observed. Cell cultures are of four
    general types:

    (a)  primary cell cultures and mass cultures of cells taken directly
         from the animal, usually after enzymatic dispersion of biopsied

    (b)  established, serially passaged cultures with relatively
         reproducible cycles of growth in early phases, but with limited
         replicative life span, and with a genetic make-up reflecting
         that of the donor age;

    (c)  "transformed" cell cultures with indefinite replicative
         potential and generally with altered genetic makeup;

    (d)  established or transformed cell lines into which specific DNA
         sequences ("transgenes") have been transfected.

         Each of these types of models could prove useful for studies of
    the effects of environmental agents on the elderly. One general
    approach would be to explore the toxic effect of an agent as a
    function of donor age, so as to detect unusual susceptibilities of
    the cells and tissues of aged subjects. Another general approach
    would be to culture the cells  in vitro after  in vivo treatments.
    If a set of behaviours or phenotypes was observed with tissue from
    young, treated subjects that proved to be comparable to that
    observed with tissue from old untreated animals, an effect of the
     in vivo treatments on the aging process could be inferred.

         An entirely different experimental paradigm could be based on
    the hypothesis that established cultures with finite replicative
    life spans recapitulate the natural history of comparable cell types
     in vivo - the well-known " in vitro model of cellular aging"
    first developed by Hayflick & Moorhead (1961). The appropriateness
    of such models for the study of aging is controversial, since it has
    been proposed that the attenuation of growth observed  in vitro
    corresponds to terminal differentiation  in vivo (Norwood & Smith,
    1985; Bayreuther et al., 1988).

         Of special interest would be the evaluation of agents that
    exhibit unusual toxicity to putative stem cells. An excessive
    depletion of stem cells could seriously compromise the regenerative
    potential of tissues in aging subjects. In such studies, it would be
    important to investigate a variety of cell types. Most research to

    date has concentrated on  in vitro aging of cultures of
    fibroblastoid cells established from the fetal lung or from the
    dermis of individual subjects of various ages. The precise origin of
    such cells is not clear. Thus, with such cells it would be difficult
    to compare age-related changes  in vivo with those observed
     in vitro.

         A final experimental paradigm would be to use postreplicative
    terminally differentiated cells in culture in order to investigate
    agents for their potential to accelerate age-related alterations
    observed  in vivo. Such cell types might be derived through
     in vitro terminal differentiation, or from normal or transformed
    embryonic neuroblasts or myoblasts. A major concern, however, would
    be the extent to which the experimental milieu reflected  in vivo

    4.1.6  Statistical considerations

         For statistical treatment of the results of short-term testing,
    the statistical methods that are generally available may be used. 
    However, for statistical analysis of the results of long-term
    testing, in particular carcinogenicity testing, the comparison of
    results from treatment groups with different survival rates is a
    major issue. In these cases the recommendation of IARC (Peto et al.,
    1980; Gart et al., 1986) could be applied. All the tumours found at
    necropsy must be evaluated as "fatal" or "incidental". This approach
    permits one to conduct a comparison between young and old animals
    despite the very different patterns of survival from those expected.
    A crude analysis, ignoring the fact that young animals survive
    longer than old ones, will overestimate the ratio of tumour
    incidence in young and old animals. Conversely, a "death-rate"
    analysis, treating all tumours as if they were fatal, will
    over-correct for the effects of differences in survival on the
    incidence of tumours discovered at the autopsy of animals that died
    of unrelated causes. Bias can be eliminated only if tumours that
    were discovered in an incidental context are analysed by the
    prevalence method, while tumours discovered in a fatal context are
    analysed by the death-rate method (Peto et al., 1980; Gart et al.,

    4.1.7  Extrapolation of animal data to humans

         Extrapolation of animal data to humans may be either
    qualitative or quantitative. On the basis of experimental results,
    the weight-of-evidence approach can help predict whether a given
    substance may be considered dangerous for humans. Such information
    includes data on the pharmacokinetics of a chemical, its
    cytotoxicity and other toxic properties, as well as the data
    obtained in  in vitro and short-term tests. Of primary importance
    for quantitative extrapolation, a dose-response relationship must be
    detected in experimental animals. An important stage of quantitative

    evaluation is extrapolation of the data obtained from exposure to
    rather high doses to lower doses to which humans may be exposed in
    the environment. It is necessary to take into account biological
    differences, both in pharmacokinetics and pharmacodynamics, between
    the test species and humans. For example, a dose taken per unit of
    the body surface area, or its concentration in the daily ration,
    includes a correction factor for species sensitivity and allows
    extrapolation of experimental doses to man (Mantel & Schneiderman,
    1975; Turusov & Parfenov, 1986; Travis et al., 1990). Of course,
    species sensitivity to the action of some chemicals may also vary

         Various physiological and mathematical models have been
    proposed for extrapolation of toxicological effects. Most of these
    models refer to carcinogenesis (Turusov & Parfenov, 1986; Swenberg
    et al., 1987; Clayson, 1988). The predictive nature of these models
    for non-carcinogenic end-points remains to be determined.

    4.2  Epidemiological and clinical approaches

    4.2.1  Disease pattern of aged population

         In order to study the pattern of disease occurring in the
    elderly, the first question is "What illnesses are most common and
    important among the elderly?" The answer can be most easily provided
    in terms of diseases that cause death, hospitalization or visits to
    a doctor's surgery.

         The assessment of health status among the elderly on an
    international basis is essentially limited to the use of mortality
    data, since these are the only comprehensive data available. In the
    developed countries, roughly 50% of all deaths occurring between the
    ages of 65 and 74 years are attributable to cardiovascular diseases.
    Among males in this age group, ischaemic heart disease accounts for
    25% of deaths, while 11% are due to cerebrovascular diseases. For
    women, 20% of deaths are attributed to ischaemic heart disease and
    about 15% to stroke. Cancer accounts for another 25% of deaths among
    men and women aged 65-74, lung cancer being the cause of 10% of all
    deaths among elderly males. Roughly 7% of deaths are due to
    respiratory diseases and 3% to external causes (WHO, 1989).

         The trends in the four leading causes of death, i.e. malignant
    neoplasms, heart diseases, cerebrovascular diseases and respiratory
    diseases, within the age range 65-74 showed that, in selected
    developed countries between 1950-1954 and 1980-1984, death rates
    from all malignant neoplasms rose slightly for men but remained
    essentially unchanged for women except in France. A more marked
    decline in mortality is apparent for heart disease in the USA and
    Australia where death rates have fallen by 25-30% since the late
    1960s. Male mortality has fallen in France and Japan, but has risen

    in Hungary. A similar pattern of change is also apparent for
    females. Mortality from stroke has also declined in most countries,
    although the timing and extent of the duration has varied from
    country to country. For example, the decline in Japan occurred ten
    years earlier than that in Australia. In the United Kingdom, France
    and USA, rates have been falling since the 1950s. The trend in
    mortality from respiratory diseases is less clear, there being
    little evidence of sustained and comprehensive decline. However, it
    is apparent that there has been some progress in reducing mortality
    from these diseases in Australia and the United Kingdom over the
    last decade (WHO, 1989).

         It must be recognized from the outset that mortality data do
    not always accurately reflect the underlying morbidity and are
    particularly inappropriate in the case of many conditions for which
    the fatality rate is low, yet which are important causes of
    morbidity among the elderly. Data on causes of death are also less
    reliable for the elderly than for other age groups owing to the
    multiple pathological conditions often present at the time of death. 
    Nevertheless, with few exceptions, comprehensive morbidity data are
    not available for the elderly.

         Data from the USA shows that cardiovascular diseases cause the
    largest proportion of hospitalization in the elderly. As a group,
    diseases of the digestive system account for the second most common
    diagnostic category while neoplastic diseases (90% malignant) are
    the third most common cause of hospitalization. The remaining causes
    are diseases of the respiratory system, injury or poisoning, and
    diseases of the genitourinary or musculoskeletal systems (White,
    1989). In a study from Shanghai, China, the common diseases of
    hospitalized patients over 65 years of age in the 1950s, when
    arranged in order of frequency, were hypertension, coronary artery
    disease, chronic bronchitis, prostate hypertrophy, femur fracture,
    pulmonary tuberculosis, diabetes mellitus, cholelithiasis and
    tumours of various organs. Coronary artery disease, pneumonia,
    hypertension and chronic bronchitis became the leading causes of
    hospitalization in the 1970s (Zhu et al., 1982). In the 1980s, data
    from other parts of China (Jiangxi, Liaoning, Xinjiang) showed that
    chronic bronchitis or pneumonia was the most frequent cause of
    hospitalization for elderly patients, followed by hypertension and
    coronary artery disease (Xu et al., 1986; Shen et al., 1987; Xiong,

         Data from the USA shows that diagnoses arising from visits to
    the doctors surgery by people aged 75 or older, when arranged in
    order of frequency, are hypertension (17.6%), chronic ischaemic
    heart disease (9.5%), diabetes mellitus (6.7%), osteoarthritis (6%),
    cataracts (5.1%), heart failure (4.4%), cardiac arrhythmia (3.6%),
    arthropathies (3.6%), glaucoma (2.8%), hypertensive heart disease
    (2.6%), angina pectoris (2.3%), chronic airway obstruction (2%) and
    neoplasms (1.4%) (White, 1989).

    4.2.2  Assessment of effects of environmental chemicals in the
           elderly population

         There are more limitations in clinical studies than in animal
    toxicology studies. Firstly, the conditions in the study are not
    easily controlled. Secondly, presumably harmful effects to the
    subjects under study need to be avoided in designing the protocol. 
    The following approach is suggested.

         Comprehensive, multidimensional functional assessment of the
    elderly should be carried out. This involves analysis of the
    physical, psychological, social, environmental and other aspects of
    functioning (Zarit et al., 1985; Kane, 1987). Measurements of the
    ability to perform the activities of daily life should be obtained. 
    Physical functioning can be measured by a combination of diagnosis,
    symptom description, health reporting, days in bed or hospital
    during a specific period, and reported pain or discomfort. The
    subject's orientation with respect to time, place and person, as
    well as short-term and long-term memory, should be assessed. 
    Psychological measures are used to evaluate depression, anxiety,
    loneliness and sense of mental well-being, and a psychiatric history
    should be recorded.

         Clinical assessment involves a complete physical examination
    and selective laboratory or instrumental examinations. As most
    environmental chemicals affect certain parts or organ systems of the
    body, laboratory or instrumental examination of these particular
    parts or organ systems should be performed. The purpose of clinical
    assessment is to find any structural or functional abnormalities
    that may occur during the course of exposure to the environmental

         Measures of environmental chemicals in the human body, such as
    the determination of their concentration or that of their
    metabolites in blood, body fluids or tissues (including nails,
    hairs, etc.), faeces, urine and expired air, should be conducted. 
    Biomarkers of exposure, such as haemoglobin adducts, may be used
    when available.

         The results of the above-mentioned assessments may be
    correlated and analysed to arrive at a conclusion as to whether the
    environmental chemicals have harmful effects on the subjects

    4.2.3  Acute episodes

         There are few epidemiological reports on the effects of
    exposure to environmental toxicants on the aged population. Several
    acute air pollution incidents resulted in marked increases in
    illness and death, mostly among the elderly. One such incident
    occurred in the Meuse River Valley in Belgium in 1930 when

    accumulating air contaminants trapped by an inversion caused the
    death of 63 people and illness in 6000 residents. An incident in
    1948 in Donora, Pennsylvania, USA, resulted from a similar inversion
    that covered a wide area. Of the population of 14 000, 20 died
    (compared with the expected two deaths for the same period) and 43%
    fell ill. Again, the elderly were the most seriously affected. In
    London, 4000 excess deaths were attributed to smoke and sulfur
    dioxide in the fog of 1952, and an incident in 1962 caused 400
    deaths above the normal value for the period. Similar episodes of
    fog-induced mortality in various places have been described (Amdur,
    1986). In all these episodes, the most vulnerable people were the
    elderly, and the victims were usually suffering from acute
    bronchitis and pneumonia, resulting in acute respiratory failure.
    However, detailed and systematic study of the effects of
    environmental chemicals on the elderly is still lacking (Amdur,

    4.2.4  Concerns for the aged population

         The health conditions of the elderly are quite different from
    those in the early or middle age of human life. Many chronic
    diseases that begin at an early age extend into advanced age. Some
    pathological conditions occurring in middle age may become
    symptomatic in the elderly. Certain medical problems are clearly
    more prominent among the elderly, e.g., cancer of the prostate,
    temporal arteritis and osteoarthritis. Illness in older people often
    involves multiple organ systems that are interrelated by symptoms,
    physical findings, functional capacity and treatment. The emotional
    and social consequences of the physical conditions are also related.
    Furthermore, the manifestations of aging processes that usually
    comprise degeneration, both structural and functional, may sometimes
    be difficult to differentiate from those of diseases. Aging
    processes and existing diseases, as well as social and environmental
    factors, act together to make the assessment and care of the elderly
    complex and difficult.

         As far as the effects of chemicals upon human health are
    concerned, the elderly are subjected to long-term exposure to
    environmental chemicals. Some exposures occur daily. The source may
    be polluted air, water, food or products made of inorganic or
    organic chemicals with which people frequently come into contact. 
    Some exposures occur during occupational activities in which people
    are engaged for many years. The cumulative effect of continuous or
    repeated exposure to chemicals may result in pathological changes
    and clinical manifestations found in later stages of life. In
    addition, the structural and functional changes of aging, usually
    degenerative in nature, make the elderly more vulnerable to adverse
    effects of environmental chemicals.

         As mentioned above, the multiple disease status in the elderly
    is usually accompanied by polypharmacy. In developed countries, the
    consumption of drugs by people over 65 years of age accounts for
    25-50% of total drug consumption (Vestal, 1978). The most commonly
    used are neuropsychiatric and cardiovascular drugs,
    anti-inflammatory analgesics and diuretics. Since these drugs are
    used for the relief of symptoms rather then the cure of diseases,
    repeated or sustained prescribing is the rule. Under such
    conditions, interactions between environmental chemicals and drugs
    should be carefully considered.

    4.3  Biomarkers of aging

         The term "biomarker of aging" arose in conferences sponsored by
    the Fund for Integrative Biomedical Research (Regelson, 1983) and
    the National Institute on Aging (Reff & Schneider, 1982). The
    Committee on Chemical Toxicity and Aging (1987) defined a biomarker
    of aging as "a biological event or measurement of a biological
    sample that is considered to be an estimate or prediction of one or
    more of the aging processes". Baker & Sprott (1988) offered the
    following specific features for a biomarker of aging: (a) nonlethal;
    (b) highly reproducible within and across species; (c) reflects
    physiological age or some basic biological process of aging or
    metabolism; (d) displays significant alterations during relatively
    short time periods; (e) is crucial to the effective maintenance of
    health and prevention of disease; and (f) reflects a measurable
    parameter that can be predicted at a later age. Development of a
    panel of biomarkers would assist in assessing the effects of
    environmental chemicals that modulate aging processes in laboratory
    animals and man.

         There is currently a significant research effort, especially
    with rodent species, to establish batteries of biological markers of
    aging (Allaben et al., 1990). But how can one differentiate
    alterations that are simple functions of chronological time from
    those that are the results of intrinsic aging or of intrinsic aging
    coupled with the effects of chronological time? One approach has
    been that of comparative gerontology. Starting with a group of
    taxonomically related species that vary substantially in their
    maximum life-span potentials, one simply asks if the rate of change
    of the putative biomarker reflects the maximum life-span potential
    of the species.

         From a practical point of view, various biomarkers may be used
    for the measurement of aging. For different levels of integration of
    an organism, different parameters may be used. At the subcellular
    and cellular levels, measurements of macromolecular lesions, the
    degree of collagen cross-linking, the level of lipofuscin
    accumulation, specific enzyme activities, the  number of particular

    hormone receptors, numbers of cell divisions, etc. may be
    investigated as biomarkers. Tissue and organ weights, cellularity,
    growth or some functional activity (muscle strength, visual acuity,
    secretion rate) may be considered.  The functional activity of
    motor, cardiovascular, nervous, endocrine, immune, haematopoietic
    and other systems and subsystems may be considered at the systemic
    level as potential biomarkers of aging. In addition, at the level of
    the whole organism, the functions of the main integrative systems
    should be addressed.

         Test batteries that attempt to measure functional age (Webster
    & Logie, 1976), physiological age (Hollingsworth et al., 1965) and
    biological age (Furukawa et al., 1975; Nakamura et al., 1982;
    Voitenko & Tokar, 1983; Reis & Poethig, 1984; Dubina et al., 1984;
    Hochschild, 1989) have similar conceptual underpinnings in that they
    utilize the great variability in performance that emerges among
    adults of the same chronological age in standardized, age-sensitive
    tests. Studies using this approach have been conducted on human
    populations, and similar measures have been employed in a few rodent
    studies (Hofecker et al., 1980; Dubina et al., 1983). This approach
    can be viewed as the performance variability model of biological
    age. It has considerable intuitive appeal and, in many cases,
    statistical elegance. An individual may be judged biologically
    younger if he is performing better than expected for his
    chronological age. However, in their extensive and critical reviews,
    Costa & McCrae (1980, 1985) revealed many conceptual and
    methodological flaws in this approach.

         Biomarkers of aging, which are markers of susceptibility, may
    have their greatest utility in the study of the effects of
    environmental chemicals on the processes of aging. The effects of
    environmental chemicals on the elderly would be assessed most
    efficiently by using biomarkers of effect (NAS, 1989).


    a)   With increasing age, humans become more vulnerable to
         environmental challenges due to the deterioration of
         physiological and psychological processes. Therefore, it is
         likely that the elderly will be more susceptible to the harmful
         effects of environmental chemicals.

    b)   The elderly are heterogeneous with respect to the extent of
         deterioration of physiological and psychological processes.

    c)   Few, if any, of the hundreds of thousands of environmental
         chemicals have been tested for increased toxicity in the

    d)   Some of the many age-associated diseases may cause an increased
         susceptibility to the harmful action of specific environmental

    e)   The use of animal models for aging research requires special
         considerations, such as housing conditions, diets, monitoring
         for infectious agents, and specifically defined pathologies. It
         is also necessary to assess several age ranges based on
         life-table information in order to distinguish the senescent
         from the mature and developing animal.

    f)   The selection of the animal model should be based on the
         likelihood of providing information of relevance to specific
         problems in humans.

    g)   The characteristics of the elderly result from intrinsic aging
         processes, environmental factors, and other life-span events.  
         Therefore, the study of the elderly may not provide 
         information on basic aging processes.

    h)   Both the size of the aged population and the number of 
         chemicals in the environment will undoubtedly increase over 
         the next few decades. It is therefore expected that the adverse 
         effects of chemical exposure on the elderly will increase in 
         importance as a health care issue.


    a)   Experimental, epidemiological and clinical data should be
         obtained on:

         *    the toxicity, mutagenicity and carcinogenicity of
              environmental chemicals in old individuals as compared to
              young adults;

         *    the effect of age on the pharmacokinetics and
              pharmaco-dynamics of environmental chemicals;

         *    the long-term effect of environmental chemicals on 
              molecular, cellular and physiological parameters as 
              potential biomarkers in the elderly.

    b)   New models should be developed for assessing the susceptibility
         of the elderly to environmental chemicals as compared to young
         adults. Such models should be suitable for assessment of
         particular consequences and relevant to humans, and could

         *    non-mammalian and mammalian models (e.g., Drosophila, 
              fish, rabbits, mini-pigs);

         *    transgenic animal models;

         *    cell lines with specific genetic characteristics.

    c)   For each study the appropriate applicability and use of
         experimental models and techniques must be validated:

         *    animal models should be well-defined in terms of survival,
              pathology and husbandry;

         *    human subjects should be examined carefully for subtle 
              disease status, medical history, life-style and
              socio-economic status.

         *    standard procedures for measuring molecular, cellular and
              physiological parameters should be defined (and developed
              if necessary) in order to prevent misinterpretation.

    d)   The effects of environmental chemicals on the processes of
         aging remain to be evaluated. A special scientific workshop
         should be devoted to this topic.


    American Dietetic Association (ADA) (1984) ADA takes proactive
    stance, testifies on older Americans act reauthorization. J Am Diet
    Assoc, 84(7): 822-835.

    Ali B, Walford RL, & Imamura T (1985) Influence of aging and poly IC
    treatment on xenobiotic metabolism in mice. Life Sci, 37: 1387-1393.

    Allaben WT, Chou MW, Pegram RA, Leakey J, Feues RJ, Duffy PH,
    Turturro A, & Hart RW (1990) Modulation of toxicity and
    carcinogenesis by caloric restriction. Korean J Toxicol, 6: 167-182.

    Allison T, Hume AL, Wood CC, & Goff WR (1984) Developmental and
    aging changes in somatosensory, auditory and visual evoked
    potentials. Electroencephalogr Clin Neurophysiol, 58: 14-24.

    Amador A, Steger RW, Bartke A, Johns A, Siler-Khodr TM, Parker R Jr,
    & Shepherd AM (1985) Testicular LH receptors during aging in Fischer
    344 rats. J Androl, 6: 61-64.

    Amdur MO (1986) Air pollutants. In: Klassen CD, Amdur UO, & Doull J
    ed. Casarett and Doull's toxicology: The basic science of poisons,
    3rd ed. New York, Macmillan Publishing Co., pp 801-824.

    Ammon HPT, Fahmy A, Mark M, Wahl MA, & Youssif N (1987) The effect
    of glucose on insulin-release and ion movement in isolated
    pancreatic islets of rats in old age. J Physiol, 384: 347-354.

    Anderson S & Brenner BM (1986) The aging kidney : Structure,
    function, mechanisms and therapeutic implications. J Am Geriatr Soc,
    35(6): 590-593.

    Anderson S & Brenner BM (1987) Effects of aging on the renal
    glomerulus. Am J Med, 80(3): 435-442.

    Anisimov VN (1971) [Blastomogenesis in persistent estrous rats.]
    Vopr Onkol, 17(8): 67-75 (in Russian).

    Anisimov VN (1987) Carcinogenesis and aging. Boca Raton, Florida,
    CRC Press, vol 1 & 2.

    Anisimov VN (1988) Effect of age on dose-response relationship in
    carcinogenesis induced by single administration of
     N-nitroso-methylurea in female rats. J Cancer Res Clin Oncol, 114:

    Anisimov VN (1989) Age-related mechanisms of susceptibility to
    carcinogenesis. Semin Oncol, 16: 10-19.

    Anisimov VN & Reiter RJ (1990) [Pineal function in cancer and
    aging.] Vopr Onkol, 37: 259-268 (in Russian).

    Arfini G, Mutti A, Vescovi P, Ferroni C, Ferrari M, Giaroli C,
    Passeri M, & Franchini I (1987) Impaired dopaminergic modulation of
    pituitary secretion in workers occupationally exposed to styrene:
    Further evidence from PRL response to TRH stimulation. J Occup Med,
    29: 826-830.

    Armbrecht HJ (1986) Age-related changes in calcium and phosphorus
    uptake by rat small intestines. Biochim Biophys Acta, 882: 281-286.

    Armbrecht HJ, Doubek WG, & Porter SB (1988) Calcium transport by
    basal lateral membrane vesicles from rat small intestine decreases
    with age. Biochim Biophys Acta, 944: 367-373.

    Armbrecht HJ, Boltz M, Strong R, Richardson A, Bruns MEH, &
    Christakos S (1989)  Expression of calbindin-D decreases with age in
    intestine and kidney. Endocrinology, 125: 2950-2956.

    Aschheim P (1976) Aging in the hypothalamic-hypophyseal-ovarian axis
    in the rat. In: Everitt AV & Burgess JA ed. Hypothalamus, pituitary
    and aging. Springfield, Illinois, Charles C. Thomas, pp 376-418.

    Aschoff J (1979) Circadian rhythms: general features and
    endocrinological aspects. In: Krieger DT ed. Comprehensive
    endocrinology. New York, Raven Press, pp 1-61.

    Avogaro P, Bittolo-Bon G, Cazzolato G, Belussi F, & Pontoglio E
    (1983) Lipids and proteins of lipoproteins in human atherosclerosis.
    Przegl Lek, 40: 713-718.

    Bach JF, Darcenne M, Papiernik M, Barvis A, Lavasseur P, & Lebrand H
    (1972) Evidence for a serum-factor secreted by the human thymus.
    Lancet, ii: 1056-1058.

    Baker SR & Rogul M ed. (1987) Environmental toxicity and the aging
    processes: Proceedings of a workshop held in Columbia, Maryland, 1-2
    October 1985. New York, Alan R. Liss, Inc., 139 pp (Progress in
    Clinical and Biological Research, Volume 228).

    Baker GT & Sprott RL (1988) Biomarkers of aging. Exp Gerontol, 23:

    Bang NU, Tessler SU, Heidenreich RO, Marks CA, & Mattler LE (1982)
    Thrombosis and atherosclerosis. Chicago, Year Book Medical

    Banks YB, Brewster DW, & Birnbaum LS (1990) Age-related changes in
    dermal absorption of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and
    2,3,4,7,8-pentachlorodibenzofuran (4-PeCDF). Fundam Appl Toxicol,
    15: 163-173.

    Baron JA (1984) Smoking and estrogen-related disease: A review. Am J
    Epidemiol, 119: 9-22.

    Barrows CH & Kokkonen GC (1978) Diet and life extension in animal
    model systems. Age, 1: 131-143.

    Bartsch H (1991) N-Nitrosocompounds and human cancer: where do we
    stand? In: Relevance to human cancer of N-nitroso compounds, tobacco
    smoke and mycotoxins. Lyon, International Agency for Research on
    Cancer, pp 1-110 (IARC Scientific Publications No. 105).

    Bauer LA, Black D, Gensler A, & Sprinkle J (1989) Influence of age,
    renal function, and heart failure on procainamide clearance and
    n-acetylprocainamide serum concentrations. Int J Clin Pharm Ther
    Toxicol, 27: 213-216.

    Baylis C, Fredericks M, Leypolat J, Frigon R, Wilson C, & Henderson
    L (1988) The mechanisms of proteinuria in aging rats. Mech Ageing
    Dev, 45: 111-126.

    Bayreuther K, Rodemann KP, Francy PI, & Maier K (1988)
    Differentiation of fibroblast stem cells. J Cell Sci, 10(suppl):

    Beebe GW (1979) Reflections on the work of the Atomic Bomb Casuality
    Commission in Japan. Epidemiol Rev, 1: 184-210.

    Behl CR, Bellantone NH, & Flynn GL (1987) Influences of age on
    percutaneous absorption of drug substances. In: Kydonieuks AF &
    Berrer B ed. Transdermal delivery of drugs. Boca Raton, Florida, CRC
    Press, vol 2, pp 109-132.

    Beierschmitt WP & Weiner M (1986) Age-related changes in renal
    metabolism of acetaminophen in male Fischer 344 rats. Age, 9: 7-13.

    Beierschmitt WP, Keenan KP, & Weiner M (1986a) Age-related increased
    susceptibility of male Fischer 344 rats to acetaminophen
    nephrotoxicity. Life Sci, 39: 2335-2342.

    Beierschmitt WP, Keenan KP, & Weiner M (1986b) The development of
    acetaminophen-induced nephrotoxicity in male Fischer 344 rats of
    different ages. Arch Toxicol, 59: 206-210.

    Belisle S & Lehoux J-G (1983) Endocrine aging in C57BL Mice. II.
    Dynamics of estrogen receptors in the hypothalamic pituitary axis. J
    Steroid Biochem, 18: 737-743.

    Bender AD (1965) The effect of increasing age on the distribution of
    peripheral blood flow in man. J Am Geriatr Soc, 13: 192-198.

    Bender AD (1968) Effect of age on intestinal absorption:
    Implications for drug absorption in the elderly. J Am Geriatr Soc,
    16: 1331-1339.

    Bender AD (1979) Drug sensitivity in the elderly. In: Crooks J &
    Stevenson IH ed. Drugs and the elderly. Baltimore, Maryland,
    University Park Press, pp 147-153.

    Bender A, Post A, Meier J, Higson J, & Reichard G (1975) Plasma
    protein binding of drugs as a function of age in adult human
    subjects. J Pharm Sci, 64: 1711-1713.

    Benditt EP (1977) The origin of atherosclerosis. Sci Am, 236: 74-85.

    Berg BN (1976) Pathology and aging. In: Everitt AV & Burgess JA ed.
    Hypothalamus, pituitary and aging. Springfield, Illinois, Charles C.
    Thomas, pp 43-67.

    Bertoni-Freddari C, Fattoretti P, Casoli T, Meier-Rouge W, & Ulrich
    J (1990) The effect of ethanol on neuronal membrane permeability and
    synaptic ultrastructure in adult and old rats. In: Bergamini E ed.
    Protein metabolism and aging. New York, Wiley-Liss, Inc., p.

    Bertrand HA, Lynd FT, Masoro EJ, & Yu BP (1980) Changes in adipose
    mass and cellularity through the adult life of rats fed ad libitum
    or a life-prolonging restricted diet. J Gerontol, 35(6): 827-835.

    Besedovsky H, Sorkin E, Keller M, & Müller J (1975) Changes in blood
    hormone levels during the immune response (39057). Proc Soc Exp Biol
    Med, 150: 466-470.

    Besedovsky HO, Del Rey AE, & Sorkin E (1985) Immune-neuroendocrine
    interactions. J Immunol, 135: 750-754.

    Bhatnagar KP, Kennedy RC, Baron G, & Greenberg RA (1987) Number of
    mitral cells and the bulb volume in the aging human olfactory bulb:
    a quantitative morphological study. Anat Rec, 218: 73-87.

    Bilchert-Toft M (1978) The adrenal glands in old age. In: Greenblatt
    RB ed. Geriatric endocrinology. Vol 5: Aging. New York, Raven Press,
    pp 81-102.

    Birchenall-Sparks M, Richardson A, Roberts M, & Utherford M (1985)
    The effect of aging on the structure and function of liver messenger
    RNA. Mech Ageing Dev, 32: 99-111.

    Birnbaum LS (1983) Distribution and excretion of 2,3,6,2',3',6'-and
    2,4,5,2',4',5'-hexachlorobiphenyl in senescent rats. Toxicol Appl
    Pharmacol, 70: 262-272.

    Birnbaum LS (1987) Age-related changes in carcinogen metabolism. J
    Am Geriatr Soc, 35: 51-60.

    Birnbaum LS (1989) Age-related changes in drug disposition. In:
    Zenser TV & Coe RM ed. Cancer and aging. Berlin, Heidelberg, New
    York, Springer-Verlag, pp 25-138.

    Birnbaum LS (1991) Pharmacokinetic basis of age-related changes in
    sensitivity to toxicants. Annu Rev Pharmacol Toxicol, 31: 101-128.

    Birnbaum LS & Baird MB (1979) Senescent changes in rodent hepatic
    epoxide metabolism. Chem-Biol Interact, 26: 245-256.

    Birren JE, Woods AM, & Williams MV (1979) Speed of behavior as an
    indicator of age changes and the integrity of the nervous system.
    In: Hoffmeister F & Muller C ed. Brain function in old age. Berlin,
    Heidelberg, New York, Springer-Verlag, p 10.

    Bitar MS & Shapiro BH (1987) Aberration of heme and hemoprotein in
    aged female rats. Mech Ageing Dev, 38: 189-197.

    Bjorksten J, Yaeger LL, & Wallace T (1989) Control of aluminium
    ingestion and its relation to longevity. Int J Vitam Nutr Res, 58:

    Blackman MR (1987) Pituitary hormones and aging. Endocrinol Aging,
    16(4): 981-994.

    Blackwelder WC, Yano K, Rhoads GG, Kagan A, Gordon T, & Palesch Y
    (1980) Alcohol and mortality: The Honolulu heart study. Am J Med,
    68: 164-169.

    Blalock JE, Harbour-McMenamin D, & Smith EM (1985) Peptide hormones
    shared by the neuroendocrine and immunologic systems. J Immunol,
    135(2): 858/5-861/5.

    Blanco P, Machado A, & Satrustegui J (1987) Variations due to
    hypoxia and ageing in the activities of glutathione-S-transferase
    and NADPH-Cytochrome C reductase. Mech Ageing Dev, 39: 11-19.

    Blum M, Averbuch M, Wolman Y, & Aviram A (1989) Protein intake and
    kidney function in humans: its effect on 'normal aging'. Arch Inter
    Med, 149(1): 211-212.

    Blumberg JB (1985) A discussion of drug metabolism and actions in
    the aged. Drug-Nutrient Interact, 4: 99-106.

    Bolinder J, Kager L, Ostamn J, & Arner P (1983) Differences at the
    receptor and postreceptor levels between human omental and
    subcutaneous adipose tissue in the action of insulin on lipolysis.
    Diabetes, 32: 117-123.

    Bondareff W (1986) Neuropathology of nucleus basalis and locus
    ceruleus in Alzheimer's disease. In: Scheibel AB, Wechsler AF, &
    Brazier MAB ed. The biological substrates of Alzheimer's disease.
    New York, London, San Francisco, Academic Press, pp 103-113.

    Boobis AR & Davies DS (1984) Human cytochromes P-450. Xenobiotica,
    14: 151-185.

    Borghoff SJ & Birnbaum LS (1985) Age-related changes in
    glucuronidation and deglucuronidation in liver, small intestine,
    lung and kidney of male Fischer rats. Drug Metab Dispos, 13: 62-67.

    Borghoff SJ & Birnbaum LS (1986) Age-related changes in the
    metabolism and excretion of allyl isothiocyanate: A model compound
    for glutathione conjugation. Drug Metab Dispos, 14: 417-422.

    Borghoff SJ, Stefanski SA, & Birnbaum LS (1988) The effect of age on
    the glucuronidation and toxicity of
    4,4'-thiobis(6-t-butyl-m-cresol). Toxicol Appl Pharmacol, 92:

    Bornor JA, Dilworth BB, & Sullivan KM (1988) Exercise and
    osteroporosis: A critique of the literature. Physiother Can, 40(3):

    Boyes WK & Dyer RS (1984) Chlordimeform produces profound,
    selective, and transient changes in visual evoked potentials of
    hooded rats. Exp Neurol, 86: 434-447.

    Bradley RM (1979) The effect of aging on the olfactory sense. In:
    Special senses in aging. Ann Arbor, Michigan, University of
    Michigan, Institute of Gerontology, pp 3-8.

    Bremner WJ, Vitiello MV, & Prinz PN (1983) Loss of circadian
    rhythmicity in blood testosterone with aging in normal men. J Clin
    Endocrinol Metab, 56: 1278-1281.

    Brenner BM, Meyer TW, & Hostetter TH (1982) Dietary protein intake
    and the progressive nature of kidney diseases. New Engl J Med, 307:

    Brizzee KR (1985) Neuron aging and neuron pathology. In: Johnson AH
    ed. Relations between normal aging and disease. New York, Raven
    Press, pp 191-224.

    Brown WW, Davis BB, & Spry LA (1986) Aging and the kidney. Arch
    Intern Med, 146: 1790-1796.

    Bruce MF (1980) The relation of tactile thresholds to histology in
    the fingers of the elderly. J Neurol Neurosurg Psychiatry, 43:

    Buckler AJ, Vie H, Sonenshein GE, & Miller RA (1988) Defective T
    lymphocytes in old mice: Diminished production of mature c-myc RNA
    after mitogen exposure not attributable to alterations in
    transcription or RNA stability. J Immunol, 140: 2442-2446.

    Bullock K (1985) Neuroanatomy of lymphoid tissue. In: Guillelmin R
    ed. Neural modulation of immunity. New York, Raven Press, pp

    Burek JD (1978) Pathology of aging rats. Boca Raton, Florida, CRC

    Burnet FM (1970) The concept of immunological surveillance. Prog Exp
    Tumor Res, 13: 1-27.

    Butenko GM (1985) Ageing of the immune system and diseases. In:
    Likhachev A, Anisimov V, & Montesano R ed. Age-related factors in
    carcinogenesis. Lyon, International Agency for Research on Cancer,
    pp 71-83 (IARC Scientific Publications No. 58).

    Campbell GR & Chamley-Campbell JH (1981) Invited review: The
    cellular pathobiology of atherosclerosis. Pathology, 13: 423-440.

    Capecchi MR (1989) Altering the genome by homologous recombination.
    Science, 244: 1288-1292.

    Caraceni MP, Corghi E, Ortolani S, Dindell M, Candotti G, & Ferrari
    A (1988) Prevention of early postmenopausal bone loss. The effect of
    calcitonin (Staporos). Clin Trials J, 25(6): 401-405.

    Carrillo MC, Kitani K, Kanai S, Sato Y, Nokubo M, Ohta M, & Otsubo K
    (1989) Differences in the influence of diet on hepatic glutathione
    S-transferase activity and glutathione content between young and old
    C56 black female mice. Mech Ageing Dev, 40: 1-15.

    Carrillo MC, Nokubo M, Sato Y, Kanai S, Ohta M, & Kitani K (1990)
    Effect of protein-free diet on activities and subunits of
    glutathione S-transferase in livers of young and aged female rats.
    Mech Ageing Dev, 56: 230-251.

    Carrillo MC, Nokubo M, Kitani K, Kimihiko S, & Sato K (1991)
    Age-related alterations of enzyme activities and subunits of hepatic
    glutathione S-transferases in male and female Fischer-344 rats.
    Biochim Biophys Acta, 1077: 325-331.

    Castleden CM, George CF, Marcer D, & Hallet C (1977a) Increased
    sensitivity to nitrazepam in old age. Br Med J, 1: 10-12.

    Castleden CM, Volans CN, & Raymond K (1977b) The effect of aging on
    drug absorption from the gut. Age Ageing, 6: 138-143.

    Cathcart R, Schwiers E, Saul RL, & Ames BN (1984) Thymine glycol and
    thymidine glycol in human and rat urine: a possible assay of
    oxidative DNA damage. Proc Natl Acad Sci (USA), 81: 5633-5637.

    Cauley JA, Gutai JP, Kuller LH, Ledonne D, Sandler RB, Sashin D, &
    Powell JG (1988) Endogenous estrogen levels and calcium intakes in
    postmenopausal women. Relationships with cortical bone measures. J
    Am Med Assoc, 260(21): 3150-3155.

    Ceda GP, Valenti G, Butturini U, & Hoffman AR (1986) Diminished
    pituitary responsiveness to growth hormone-releasing factor in aging
    male rats. Endocrinology, 118(5): 2109-2114.

    Chakravarty S, Collins WP, Forecast JS, Newton JR, Oram DH, & Strudd
    JWW (1976) Hormonal profiles after the menopause. Br Med J, 2:

    Chan K, Kerdall MJ, Mitchard M, Wells WOE, & Vickers MD (1975) The
    effect of aging on plasma pethidine concentration. Br J Clin
    Pharmacol, 2: 297-302.

    Chandra RK (1985) Trace element regulation of immunity and
    infection. J Am Coll Nutr, 4: 5-16.

    Chavaz A, Baleurt L, Simonin P, & Fabre J (1974) Influence de l'âge
    sur la digoxinémie et la digitalisation. Schweiz Med Wochenschr,
    104: 1823-1825.

    Chen LH, Liu S, Cook Newell ME, & Barnes K (1985) Survey of drug use
    by the elderly and possible impact of drugs on nutritional status.
    Drug-Nutrient Interact, 3: 73-86.

    Cheney KE, Liu RK, Smith GS, Merdith PJ, Mickey MR, & Walford RL
    (1983) The effect of dietary restriction of varying duration on
    survival, tumour patterns, immune fraction and body temperture in
    BIOC3F1 female mice. J Gerontol, 38: 420-430.

    Chengelis CP (1988a) Age- and sex-related changes in the components
    of the hepatic microsomal mixed function oxidase system in
    Sprague-Dawley rats. Xenobiotica, 18: 1211-1224.

    Chengelis CP (1988b) Age- and sex-related changes in epoxide
    hydrolase, UDP-glucuronyl transferase, and PAPS-sulfotransferase in
    Sprague-Dawley rats. Xenobiotica, 18: 1225-1237.

    Chopra IJ, Solomon DH, Chopra U, Wu SY, Fisher DA, & Nakamura Y
    (1978) Pathways or metabolism of thyroid hormones. Recent Prog Horm
    Res, 34: 521-567.

    Christophers E & Kligman AM (1965) Percutaneous absorption in aged
    skin. In: Montagna W ed. Advances in the biology of the skin. Vol 6:
    Ageing. Oxford, New York, Pergamon Press, pp 163-175.

    Clayson DB (1988) Needs for biological risk assessment in
    interspecies extrapolation. Environ Health Perspect, 77: 93-97.

    Cleroux J, Giannattasio C, Grassi G, Seravalle G, Sampieri L,
    Cuspidi C, Bolla G, Valsecchi M, Mazzola C, & Mancia G (1988)
    Effects of aging on the cardiopulmonary receptor reflex in
    normotensive humans. J Hypertension, 6(suppl 4): 141-144.

    Coiro V, Volpi R, Cavazzini V, Bertoni P, Corradi A, Bianconi L,
    Davoli C, Rossi G, & Chiodera P (1991) Restoration of normal growth
    hormone responsiveness to GHRH in normal aged men by infusion of low
    amounts of theophylline. J Gerontol Med Sci, 46(5): M155-M158.

    Colombo T, Donelli MG, Urso R, Dallara S, Bartosek I, & Guaitani A
    (1989) Doxorubicin toxicity and pharmacokinetics in old and young
    rats. Exp Gerontol, 24: 159-171.

    Committee on Chemical Toxicity and Aging (1987) Aging in today's
    environment. Washington, DC, National Academy Press.

    Conn PM, Cooper RL, McNamara MC, Rogers DC, & Shoenhardt L (1980)
    Qualitative change in gonadotropin during normal aging in the male
    rat. Endocrinology, 106: 1549-1553.

    Conrad K & Bressler R (1982) Drug therapy for the elderly. St Louis,
    Missouri, C.V. Mosby & Co.

    Cooper B, Weinblatt F, & Gregerman R (1977) Enhanced activity of
    hormone sensitive adenylated cyclase during dietary restriction in
    the rat. J Clin Invest, 39: 467-474.

    Cooper RL, Goldman JM, & Rehnberg GL (1986) Pituitary function
    following treatment with reproductive toxins. Environ Health
    Perspect, 70: 177-184.

    Corman B & Michel JB (1986) Renin-angiotensin system,
    converting-enzyme inhibition, and kidney function in aging female
    rats. Am J Physiol, 251: 450-455.

    Costa PT Jr & McCrae RR (1980) Functional age: A conceptual and
    empirical critique. In: Haynes SG & Feinleib M ed. Epidemiology of
    aging. Washington, DC, National Institutes of Health, National
    Institute of Aging, National Heart, Lung, and Blood Institute, pp
    23-50 (NIH Publication No. 80-969).

    Costa PT & McCrae RR (1985) Concepts of functional or biological
    age: A critical view. In: Andres R, Bierman EL, & Hazzard WR ed.
    Principles of geriatric medicine. New York, McGraw-Hill, pp 30-37.

    Costa LG & Murphy SD (1987) Interaction of the pesticide
    chlordimeform with adrenergic receptors in mouse brain: An
     in vitro study. Arch Toxicol, 59: 323-327.

    Costa LG, Olibet G, & Murphy SD (1988) Alpha2-adrenoreceptors as a
    target for formamidine pesticides:  in vitro and  in vivo studies
    in mice. Toxicol Appl Pharmacol, 93: 319-328.

    Creasey H & Rapoport SI (1985) The aging of human brain. Ann Neurol,
    17(1): 2-10. Crooks J, O'Malley K, & Stevenson IH (1976)
    Pharmacokinetics in the elderly. Clin Pharmacokinet, 1: 280-296.

    Croucher PI, Mellish RWE, Vedi S, Garrahan NJ, & Compston JE (1989)
    The relationship between resorption depth and mean interstitial bone
    thickness: Age-related changes in man. Calcif Tissue Int, 45(1):

    Crowley JJ, Cusack BJ, Jue SG, Koup JR, Park BK, & Vestal RE (1988)
    Aging and drug interactions. II. Effects of phenytoin and smoking on
    the oxidation of theophylline and cortisol in healthy men. J
    Pharmacol Exp Ther, 245: 513-523.

    Cuny G, Royer RJ, Mur JM, Serot JM, Faure G, Netter P, Maillard A, &
    Penin F. (1979) Pharmacokinetics of salicylates in elderly.
    Gerontology, 25: 49-55.

    Cusack B & Denham MJ (1984) Nutritional status and drug disposition
    in the elderly. In: Roe DA ed. Drugs and nutrition in the geriatric
    patients. New York, Churchill Livingstone, pp 47-70.

    Cutler RG (1991) Human longevity and aging: possible role of
    reactive oxygen species. Ann NY Acad Sci, 621: 1-28.

    Cutler RG & Semsei I (1989) Development, cancer and aging: possible
    common mechanisms of action and regulation. J Gerontol, 44(6):

    Daniel FB, Schut HA, Sandwiesh DW, Schenck KM, Hoffmann CO, Patrick
    JR, & Stoner GD (1983) Interspecies comparison of benzo(a)pyrene
    metabolism and DNA-adduct formation in cultured human and animal
    bladder and tracheobronchial tissues. Cancer Res, 43: 4723-4729.

    Danon D (1969) Biophysical aspects of red cell ageing. Bibl
    Haematol, 15: 275-287.

    Davies BH (1985) The respiratory system. In: Pathy MSJ ed.
    Principles and practice of geriatric medicine. New York, Chichester,
    Brisbane, Toronto, John Wiley and Sons.

    Davila DR, Brief S, Simon J, Hammer RE, Brinster RL, & Kelley KW
    (1987) Role of growth hormone in regulating T-dependent immune
    events in aged, nude and transgenic rodents. J Neurosci Res, 18:

    Davis PJ & Davis FB (1987) Water excretion in the elderly.
    Endocrinol Aging, 16(4): 867-875.

    Davis TA, Bales CW, & Beauchene RE (1983) Differential effects of
    dietary, caloric, and protein restriction in the aging rat. Exp
    Gerontol, 18: 427-435.

    Dawson-Hughes B, Shipp C, Sadowski L, & Dallal G (1987) Bone density
    of the redius, spine, and hip in relation to percent of ideal body
    weight in postmenopausal women. Calcif Tissue Int, 40(6): 310-314.

    Dement W, Richardson G, Prinz P, Carskadon M, Kripke D, & Czeisler C
    (1985) Changes of sleep and wakefulness with age. In: Finch CE &
    Schneider EL ed. Handbook of the biology of aging, 2nd ed. New York,
    Van Nostrand Reinhold Company, pp 692-717. Department of
    International Economic and Social Affairs (1991) World population
    prospects 1990, New York, United Nations, pp 192-215 (Population
    Studies No. 120).

    Dilman VM (1971) Age associated elevation of hypothalamic threshold
    to feedback control and its role in development, aging and disease.
    Lancet, i: 1211-1219.

    Dilman VM (1987) [Four models of medicine.] Leningrad, Meditsina (in

    Dilman VM & Anisimov VN (1979) Hypothalmic mechanism of ageing and
    of specific age pathology. I. Sensitivity threshold of
    hypothalamo-pituitary complex to homeostatic stimuli in the
    reproductive system. Exp Gerontol, 14: 161-174.

    Dix D (1989) The role of aging in cancer incidence: an
    epidemiological study. J Gerontol, 44(6): 10-18.

    Doll R (1973) Age. In: Doll R & Vodopija I ed. Host environment
    interactions in the etiology of cancer in man. Lyon, International
    Agency for Research on Cancer, pp 39-48 (IARC Scientific
    Publications No. 7).

    Doll R (1978) An epidemiological perspective of the biology of
    cancer. Cancer Res, 38: 3573-3583.

    Doll R & Peto R (1981) The causes of cancer. J Natl Cancer Inst, 66:

    Donaldson J (1987) The physiopathologic significance of manganese in
    brain: Its relation to schizophrenia and neurodegenerative
    disorders. Neurotoxicology, 8: 451-462.

    Doubek WG & Armbrecht HJ (1987) Changes in intestinal glucose
    transport over the lifespan of the rat. Mech Ageing Dev, 39: 91-102.

    Dubina TL, Dyundikova VA, & Zhuk EV (1983) Biological age and its
    estimation. II. Assessment of biological age of albino rats by
    multiple regression analysis. Exp Gerontol, 18: 5-18.

    Dubina TL, Mints A, & Zhuk EV (1984) Biological age and its
    estimation. III. Introduction of a correction to the multiple
    regression model of biological age and assessment of biological age
    in cross-sectional and longitudinal studies. Exp Gerontol, 19:

    Duckles SP (1987) Influence of age on vascular adrenergic
    responsiveness. Blood Vessels, 24(3): 113-116.

    Dundee JW (1979) Response to anaesthetic drugs in the elderly. In:
    Crooks J & Stevenson IH ed. Drugs and the elderly. Baltimore,
    Maryland, University Park Press, pp 179-187.

    Dunn MF, Pattison SE, Storm MC, & Quiel E (1980) Comparison of the
    zinc binding domains in the 7S nerve growth factor and the
    zinc-insulin Hexamer. Biochemistry, 19: 718-725.

    Dybkaer R, Lauritzen DM, & Krakauer R (1981) Relative reference
    values for clinical chemical and haematological quantities in
    "healthy" elderly people. Acta Med Scand, 209: 1-9.

    Eastell R & Riggs BL (1987) Calcium homeostasis and osteoporosis.
    Endocrinol Metab Clin, 16(4): 829-843.

    Eastin WC Jr & Birnbaum LS (1987) Intestinal absorption of two
    glucose analogues in rats of different ages. Exp Gerontol, 22:

    Ebbesen P (1985) Papilloma development on young and senescent mouse
    skin treated with 12-0-tetradecanoylphorbol-13-acetate. In:
    Likhachev A, Anisimov V, & Montesano R ed. Age-related factors in
    carcinogenesis. Lyon, International Agency for Research on Cancer,
    pp 167-170 (IARC Scientific Publications No. 58).

    Edelman IS & Leibman J (1959) Anatomy of body water and
    electrolytes. Am J Med, 27: 256-277.

    Eden S (1987) Endocrinology in older people. Acta Obstet Gynaecol
    Scand, 140(suppl): 9-22.

    Edman CD (1983) Estrogen replacement therapy. In: Buchsbaum HJ ed.
    The menopause. Berlin, Heidelberg, New York, Springer-Verlag.

    Ellis GB & Desjardins C (1982) Male rats secrete luteinizing hormone
    and testosterone episodically. Endocrinology, 110: 1618-1627.

    Euans DW (1988) Renal function in the elderly. Am Fam Physician,
    38(3): 147-150.

    Evens RP & Hawkins DW (1984) Bone and joint disorders. In: Covington
    TR & Ingram Walker J ed. Current geriatric therapy. Philadelphia,
    Pennsylvania, W.B. Saunders Co., pp 277-306.

    Exton-Smith AN (1985) Mineral metabolism. In: Finch CE & Schneider
    EL ed. Handbook of the biology of aging. New York, Van Nostrand
    Reinhold Company, pp 511-539.

    Fabris N (1981) Ontogenetic and phylogenetic aspects of
    neuroendocrine-immune network. Dev Comp Immunol, 5(suppl 1): 49-60.

    Fabris N (1991) Neuroendocrine-immune interactions: a theoretical
    approach of aging. Arch Gerontol Geriatry, 12: 219-230.

    Fabris N & Piantanelli L (1982) Thymus-neuroendocrine interactions
    during development and aging. In: Adelman RC & Roth GS ed. Hormones
    and aging. Boca Raton, Florida, CRC Press, pp 167-181.

    Fabris N & Provinciali M (1989) Hormones. In: Nelson D ed. Natural
    immunity. New York, London, San Francisco, Academic Press, pp

    Fabris N, Giunta S, & Muzzioli M (1983) Decline of T-cell potential
    in diabetic and aged men. J Gerontol, 38(5): 548-554.

    Fabris N, Mocchegiani E, Muzzioli M, & Provinciali M (1988)
    Neuroendocrine-thymus interactions: perspectives for intervention in
    aging. Ann NY Acad Sci, 521: 72-87.

    Fabris N, Mocchegiani E, Marriotti S, Pacini F, & Pinchera A (1989)
    Thyroid-thymus interactions during development and aging. Horm Res,
    31: 85-98.

    Fabris N, Mocchegiani E, Muzzioli M, & Provinciali M (1990) Zinc,
    immunity and aging. In: Goldstein AL ed. Biomedical advances in
    aging. New York, London, Plenum Press, pp 271-281.

    Fando JL, Salinas M, & Wasterlain CG (1980) Age-dependent changes in
    brain protein synthesis in the rat. Neurochem Res, 5: 373-378.

    Feldman RD, Limbird LE, Nadeau J, Robertson D, & Wood AJ (1984)
    Alterations in leukocyte ß-receptor affinity with aging. New Engl J
    Med, 310: 815-819.

    Felten DL, Felten SY, Carlsan SL, Olschowka JA, & Livnat S (1985) 
    Neuroadrenergic and peptidergic innervation of lymphoid tissue. J
    Immunol, 135: 755.

    Fernandes G (1984) Nutrional factors: Modulating effects on immune
    function and aging. Pharmacol Rev, 36(2): 123-129.

    Fernandes G, Friend P, & Yunis EJ (1977) Influence of calorie
    restriction on autoimmune disease. Fed Proc, 6: 1313.

    Ferro-Luzzi A, Maiani G, Scaccini C, D'Amicis A, Borgioni G, Polito
    A, Ronaldi L, Sett S, Branca F, Azzini E, Arena A, Raguzzini A,
    Castata G, & Dente MG (1988) In: Lintas C & Spadoni MA ed.
    Nutritional vulnerability of the Italian elderly: Facts and figures.
    Rome, National Council for Research, pp 275-294 (Monografia No. 28).

    Finch CE (1991) Longevity, senescence, and the genome. Chicago,
    University of Chicago Press.

    Finch CE & Hayflick L ed. (1977) Handbook of the biology of aging.
    New York, Van Nostrand Reinhold Company.

    Finch CE & Landfield PW (1985) Neuroendocrine and autonomic
    functions in aging mammals. In: Finch CE & Schneider EL ed. Handbook
    of the biology of aging, 2nd ed. New York, Van Nostrand Reinhold
    Company, pp 645-691.

    Fishbein L (1991) Biological effects of dietary restriction. Berlin,
    Heidelberg, New York, Springer-Verlag.

    Fleming BB & Barrows CH Jr (1982a) The influence of aging on
    intestinal absorption of vitamins A + D by the rat. Exp Gerontol,
    17: 115-120.

    Fleming BB & Barrows CH Jr (1982b) The influence of aging on
    intestinal absorption of vitamin B12 and niacin in rats. Exp
    Gerontol, 17: 121-126.

    Fraga CG, Shigenaga MK, Park JW, Degan P, & Ames BN (1990) Oxidative
    damage to DNA during aging: 8-hydroxy-2-deoxyguanosine in rat organ
    DNA and urine. Proc Natl Acad Sci (USA), 87: 4533-4537.

    Friedman SA, Raizner RE, Rosen H, Solomon NA, & Wilfredo SY (1972)
    Functional defects in the aging kidney. Ann Intern Med, 76: 41-45.

    Friedman FK, Robinson RC, & Rifkind J (1989) Age-related changes in
    the iron spin state of testosterone-binding rat liver microsomal
    cytochromes P450. Biochem Biophys Res Commun, 158: 480-484.

    Friend PS, Fernandes G, Good RA, Michael AF, & Yunis EJ (1978)
    Dietary restrictiions early and late. Effects on nephropathy of the
    NZB and NZW mouse. Lab Invest, 38: 629-632.

    Frith CH & Ward JM (1988) Colour atlas of neoplastic and
    non-neoplastic lesions in aging mice. Amsterdam, Oxford, New York,
    Elsevier Science Publishers.

    Fujita S, Katagawa H, Ishizawa H, Suzuki T, & Kitani K (1985)
    Age-associated alterations in hepatic glutathione-s-transferase
    activities. Biochem Pharmacol, 34: 3891-3894.

    Furukawa T, Inoue M, Kajiya F, Inada H, Takasugi S, Fukui S, Takeda
    H, & Abe H (1975) Assessment of biological age by multiple
    regression analysis. J Gerontol, 30: 422-434.

    Gaitz CM & Baer PE (1971) Characteristics of elderly patients with
    alcoholism. Arch Gen Psychiatry, 24: 372.

    Galinsky RE, Kane RE, & Franklin MR (1986) Effect of aging on
    drug-metabolizing enzymes important in acetaminophen elimination. J
    Pharmacol Exp Ther, 237: 107-113.

    Galinsky RE, Johnson DH, Kane RE, & Franklin MR (1990) Effect of
    aging on hepatic biotransformation in female Fischer 344 rats:
    changes in sulfotransferase activities are consistent with known
    gender-related changes in pituitary growth hormones secretions in
    aging animals. J Pharmacol Exp Ther, 255: 577-583.

    Gallagher D, Thompson LW, & Levy SM (1980) Clinical psychological
    assessment of older adults. In: Poon LW ed. Aging in the 1980s:
    Psychological issues. Washington, DC, American Psychological Press,
    pp 19-40.

    Gambert SR & Tsitouras PD (1985) Effect of age on thyroid hormone
    physiology and function. J Am Geriatr Soc, 33(5): 360-365.

    Gart JJ, Krewski D, Lee PN, Tarone RE, & Wahrendorf J (1986)
    Statistical methods in cancer research. Vol III: The design and
    analysis of long-term animal experiments. Lyon, International Agency
    for Research on Cancer, 219 pp (IARC Scientific Publications No.

    Gilchrest BA (1984) Skin and aging processes. Boca Raton, Florida,
    CRC Press, pp 1-120.

    Globerson AR, Abel EI, & Ren-Menahem D (1989) Developmental aspects
    of T-lymphocytes in aging. New York, London, Plenum Press, pp

    Goldstein RS, Pasino DA, & Hook JB (1986) Cephaloridine
    nephrotoxicity in aging male Fischer 344 rats. Toxicology, 38:

    Goldstein RS, Tarloff JB, & Hook JB (1988) Age-related nephropathy
    in laboratory rats. FASEB J, 2: 2241-2251.

    Gollamudi HR, Prasanna R, Rao RH, Lawrence WH, & Autian J (1983)
    Impaired metabolism of di (2-ethylhexyl)phthalate (DEHP) in old rats
    - an  in vitro study. J Toxicol Environ Health, 12: 623-632.

    Gompertz B (1825) On the nature of the function expressive of the
    law of human mortality and on a new mode of determining life
    contingencies. Phil Trans R Soc London, II: 513-585.

    Gossen JA, De Leeuw WJF, Tan CHT, Zwarthoff EC, Berends F, Lohman
    PHM, Knook DL, & Vijg J (1989) Efficient rescue of integrated shutle
    vectors from transgenic mice: a model for studying mutations in
    vivo. Proc Natl Acad Sci (USA), 86: 7971-7975.

    Govoni S, Memo M, Spano PF, & Trabucchi M (1979) Chronic lead
    treatment differentially affects dopamine synthesis in various rat
    brain areas. Toxicology, 12: 343-349.

    Govoni S, Rius RA, Battaini F, Magnoni MS, Lucchi L, & Trabucchi M
    (1988) The central dopaminergic system: Susceptibility to risk
    factors for accelerated aging. Gerontology, 34: 29-34.

    Goyal KK (1982) Changes with age in the aorta of man and mouse. Exp
    Gerontol, 17: 127-132.

    Graham, P (1985) The eye. In: Pathy MSJ ed. Principles and practice
    of geriatric medicine. New York, Chichester, Brisbane, Toronto, John
    Wiley and Sons, pp 833-844.

    Grandjean P (1983) Behavioral toxicology of heavy metals. In:
    Zbinden G, Cuomo V, Recagni G, & Weiss B ed. Application of
    behavioral pharmacology in toxicology. New York, Raven Press, vol 2,
    pp 331-339.

    Greden JF, Flegel P, Haskett R, Dilsaver S, Carrol BJ, Grunhaus L, &
    Genero N (1986) Age effects in serial hypothalamic-pituitary-adrenal
    monitoring. Psychoneuroendocrinology, 11(2): 195-204.

    Greenberg LH & Weiss B (1978) beta-Adrenergic receptors in aged rat
    brain: reduced number and capacity of pineal gland to develop
    supersensitivity. Science, 201: 61-63.

    Greenberg LH & Weiss B (1979) Ability of aged rats to alter
    beta-adrenergic receptors of brain in response to repeated
    administration of reserpine and desmethylimipramine. J Pharmacol Exp
    Ther, 211: 309-315.

    Greenberg LH & Weiss B (1983) Neuroendocrine control of
    catecholaminergic receptors in aging brain. In: Agnoli A, Crepaldi
    G, Spano PF, & Trabucchi M ed. Aging brain and ergot alkaloids. New
    York, Raven Press, pp 37-52.

    Greenblatt DJ, Divoll MK, Harmatz JS, & Shader RI (1988) Antipyrine
    absorption and disposition in the elderly. Pharmacology, 36:

    Greenstein BD, Fitzpatrick FT, Kendall MD, & Wheeler MJ (1987)
    Regeneration of the thymus in old male rats treated with a stable
    analogue of LHRH. J Endocrinol, 11: 345-350.

    Gregerman RI & Solomon N (1967) Acceleration of thyroxine and
    triiodothyronine turnover during bacterial pulmonary infections and
    fever: implications for the functional state of the thyroid during
    stress and in senescence. J Clin Endocrinol Metab, 27: 93-105.

    Groop PH (1989) The influence of body weight, age and glucose
    tolerance on the relationship between GIP secretion and beta-cell
    function in man. Scand J Clin Lab Invest, 49(4): 367-379.

    Gu X (1986) The life expectancy of the people of China. In: Cui Y
    ed. Public health in the People's Republic of China. Beijing,
    People's Medical Publishing House, pp 87-91.

    Gupta SP, Mehta FS, & Irani RR (1980) Comparison of mortality rates
    among bidi smokers and tobacco chewers. Ind J Cancer, 17: 149-152.

    Guthrie S, Cooper RL, Thurman R, & Linnoila M (1987)
    Pharmacodynamics and pharmacokinetics of ethanol, diazepam, and
    pentobarbital in young and aged rats. Pharmacol Toxicol, 61:

    Hadden JW (1983) Cyclic nucleotides and related mechanisms in immune
    regulation: A mini review. In: Fabris N, Garaci E, Hadden J, &
    Mitchison NA ed. Immunoregulation. New York, London, Plenum Press,
    pp 201-230.

    Halberg F (1982) Biological rhythms, hormones and aging. In:
    Vernandakis A ed. Hormones in development and aging. New York,
    Spectrum Publications, pp 451-476.

    Hall HRS, O'Grady MP, & Farrah JM Jr (1989) The
    hypothalamic-pituitary-adrenal axis by thymic peptides. In: Hadden
    JW, Massek K, & Nistico G ed. Interactions among central nervous
    system, neuroendocrine and immune system. Rome, Pytagora Press, pp

    Hanawalt PC (1987) On the role of DNA damage and repair processes in
    aging: evidence for and against. In: Warner HR, Butler RN, Sprott
    RN, & Schneider EL ed. Modern biological theories of aging. New
    York, Raven Press, pp 183-198.

    Hanninen H (1982) Behavioral effects of occupational exposure to
    mercury and lead. Acta Neurol Scand, 66(suppl 92): 167-175.

    Hansen ON, Trillingsgaard A, Beese I, Lyngbye T, & Grandjean P
    (1985) Neurobehavioral methods in assessment of children with
    low-level lead exposure. In: Neurobehavioral methods in occupational
    and environmental health (Document 3). Copenhagen, World Health
    Organization, Regional Office for Europe, pp 183-187. Harman D
    (1981) The aging process. Proc Natl Acad Sci (USA), 78: 7124-7128.

    Harman D (1982) Nutritional implications of the radical theory of
    aging. J Am Coll Nutr, 1: 27-34.

    Harman SM, Tsitouras PD, Costa PT, & Blackman MR (1982) Reproductive
    hormones in aging men. II. Basal pituitary gonadotropins and
    gonadotropin responses to luteinizing hormone releasing hormone. J
    Clin Endocrinol Metab, 54: 547-551.

    Harrison DE, Astle CM, & Delaittre JA (1978) Loss of proliferative
    capacity in immunohemopoietic stem cells caused by serial
    transplantation rather than ageing. J Exp Med, 147(5): 1526-1531.

    Hayflick L & Moorhead PS (1961) The serial cultivation of human
    diploid cell strains. Exp Cell Res, 25: 585-621.

    Hazzard WR (1985) Aging and atherosclerosis. Teasing out the
    contributions of time, secondary aging, and primary aging. Clin
    Geriatr Med, 1(1): 251-284.

    Hebert CD & Birnbaum LS (1987) The influence of aging on intestinal
    absorption of TCDD in rats. Toxicol Lett, 37: 47-55.

    Hendley DD, Mildvan AS, Reporter MC, & Strehler BL (1963) The
    properties of isolated human cardiac age pigment. II Chemical and
    enzymatic properties. J Gerontol, 18: 250-259.

    Hershkowitz M (1983) Mechanisms of brain aging - The role of
    membrane fluidity. In: Gispen WH & Traber J ed. Aging of the brain.
    Amsterdam, Oxford, New York, Elsevier Science Publishers, pp 85-98.

    Hirsch HR (1982) Evolution of senescence: natural increase of
    population dispaying Gompertz or power-law death rates and constant
    of age-dependent maternity rates. J Theor Biol, 98: 321-346.

    Hochschild R (1989) Improving the precision of biological age
    determinations. Part I. A new approach to calculating biological
    age. Exp Gerontol, 24: 289-300.

    Hofecker G, Skalicky M, Kment A, & Niedermuller H (1980) Models of
    the biological age of the rat. I. A factor model of age parameters.
    Mech Ageing Dev, 14: 345-359.

    Hogstedt C, Hane M, Agrell A, & Bodin L (1983) Neuropsychological
    test results and symptoms among workers with well-defined long-term
    exposure to lead. Br J Ind Med, 40: 99-105.

    Hollander D, Dadufalz V, Weindruch R, & Walford RL (1986) Influence
    of life-prolonging dietary restriction on intestinal vitamin A
    absorption in mice. Age, 9: 57-60.

    Hollingsworth JW, Hashizume A, & Jablon S (1965) Correlations
    between tests of aging in Hiroshima subjects - an attempt to define
    "physiological age." Yale J Biol Med, 38: 11-26.

    Holt AR, Pascal RR, & Kotler DP (1984) Effect of aging upon small
    intestinal structure in the Fischer rat. J Gerontol, 39: 642-647.

    Holt PR, Yeh KY, & Kotler DP (1988) Altered controls of
    proliferation in proxismal small intestine in the senescent rat.
    Proc Natl Acad Sci (USA), 85: 2771-2775.

    Honma T, Miyagawa M, & Sato M (1987) Methyl bromide alters
    catecholamine and metabolite concentrations in rat brain.
    Neurotoxicol Teratol, 9: 369-375.

    Horbach GJM, Princen HMG, Van der Kroef M, Van Bezooijen CFA, & Yap
    SH (1984)  Changes in the sequence content of albumin mRNA and in
    its translational activity in the rat liver with age. Biochim
    Biophys Acta, 83: 60-66.

    Horbach GJM, Van der Boom H, Van Bezooijen CFA, & Yap SH (1986)
    Molecular aspects of age-related changes in albumin synthesis in
    rats. In: Van Bezooijen CFA, Miglio F, & Knook DL ed. Liver drugs
    and aging. Rijswijk, The Netherlangs, Eurage, pp 121-126 (Topics in
    Aging Research in Europe, Volume 7).

    Horbach GJM, Durham SK, Yap SH, & Van Bezooijen CFA (1988a) Albumin
    elimination in female WAG/Rij rats with age: A longitudinal study.
    Mech Ageing Dev, 43: 137-152.

    Horbach GJM, Van der Boom H, Yap SH, & Van Bezooijen CFA (1988b)
    Molecular aspects of age-related changes in albumin synthesis in
    female WAG/Rij rats. Life Sci, 43: 1707-1714.

    Hosomi T (1990) Japanese economy and society. In: Institute of
    Japanese Problems, Chinese Academy of Social Sciences and Japanese
    Vital Insurance Company ed. Proceeding of a Symposium on Sino-Japan
    Social Countermeasure for Aging Problems. Beijing, Dong-Fang
    Publishing House pp 7-30.

    Houck JC, Dehesse C, & Jacob R (1967) The effect of aging upon
    collagen catabolism. Symp Soc Exp Biol, 21: 403-425.

    Hsu WH & Kakuk TJ (1984) Effect of amitraz and chlordimeform on
    heart rate and pupil diameter in rats: Mediated by alpha
    2-adrenoreceptors. Toxicol Appl Pharmacol, 73: 411-415.

    Hunter J, Urbanowicz MA, Yule W, & Landsdown R (1985) Automated
    testing of reaction time and its association with lead in children.
    Int Arch Occup Environ Health, 57: 27-34.

    Hunziker PE & Kagi JHR (1985) Metallothionein. In: Harrison PM ed.
    Metalloproteins: Part II. London, MacMillan Press Ltd.

    Huskisson EC (1983) NSAIDS: A topical review. Geriatr Med, 12:

    IARC (1982) Chemicals, industrial processes and industries
    associated with cancer in humans. IARC Monographs, Volumes 1 to 29.
    Lyon, International Agency for Research on Cancer, 292 pp (IARC
    Monographs on the Evaluation of the Carcinogenic Risk of Chemicals
    to Humans, Supplement 4).

    IARC (1986) In: Montesano R, Bartsch H, Vainio H, Wilbourne J, &
    Yamasaki H ed. Long-term and short-term assays for carcinogens: A
    critical appraisal. Lyon, International Agency for Research on
    Cancer, 564 pp (IARC Scientific Publications No. 83).

    IARC (1988) Alcohol drinking. Lyon, International Agency for
    Research on Cancer, 416 pp (IARC Monographs on the Evaluation of
    Carcinogenic Risks to Humans, Volume 44).

    IARC (1990) In: Tomatis L ed. Cancer: causes, occurrence and
    control. Lyon, International Agency for Research on Cancer, 352 pp
    (IARC Scientific Publications No. 100).

    Imayama S & Braverman IM (1989) A hypothetical explanation for the
    aging of skin. Chronologic alterations of the three-dimensional
    arrangement of collagen and elastic fibres in connective tissue. To
    provide a morphologic basis for a better understanding of the
    "aging" of human. Am J Pathol, 134(5): 1019-1025.

    Ingbar SH (1978) The influence of aging on the human thyroid hormone
    economy. In: Greennblatt RB ed. Geriatric endocrinology. New York,
    Raven Press, pp 13-31.

    Ingram DK, Cutler RG, Weindruch R, Renquist DM, Knapka JJ, April M,
    Belcher CT, Clark MA, Hatcherson CD, Marriott M, & Roth GS (1990)
    Dietary restriction and aging: the initiation of a primate study. J
    Gerontol, 45(5): 148-163.

    Iqbal K, Grundke-Iqbal I, Merz PA, & Wisniewski HM (1982)
    Age-associated neurofibrillary changes. In: Giacobini E, Filogamo G,
    Giacobini G, & Vernadakis A ed. The aging brain: cellular and
    molecular mechanisms of aging in the nervous system. New York, Raven
    Press, pp 247-257.

    Iwasaki K, Chirago T, Tada K, Noda K, & Noguchi H (1986) Age- and
    sex-related changes in amine sulphoconjugations in Sprague-Dawley
    strain rats. Comparison with phenol and alcohol sulphoconjugations.
    Xenobiotica, 16: 717-723.

    Iwasaki K, Gleiser CA, Masoro EJ, McMahan CA, Seo E, & Yu BP (1988)
    Influence of the restriction of individual dietary components on
    longevity and age-related disease of Fisher rats: The fat component
    and the mineral component. J Gerontol, 43: B13-B21.

    Iwase A, Kitazawa Y, & Ohno Y (1988) On age-releated norms of the
    visual field. Jpn J Ophthalmol, 34(4): 429-437.

    Iwata T, Incefy GS, Tanaka T, Fernandes G, Menendez-Botel CI, Pih K,
    & Good RA (1979) Circulating thymic hormone levels in zinc
    deficiency. Cell Immunol, 47: 100-105.

    Jackson J, Banks YB, & Birnbaum LS (1990) Maximal dermal absorption
    of TCDD occurs in weanling rats. Toxicologist, 10: 309.

    Jaenisch R (1988) Transgenic animals. Science, 240: 1468-1474.

    Jones MK, Weisenburger WP, Sipes IG, & Russell DH (1987) Circadian
    alterations in prolactin, corticosterone, and thyroid hormone levels
    and down-regulation of prolactin receptor activity by
    2,3,7,8-tetrachlorodibenzo-p-dioxin. Toxicol Appl Pharmacol, 87:

    Kaldor JM & Day NE (1987) Interpretation of epidemiological studies
    in the context of multistage model of carcinogenesis. In: Barrett JC
    ed. Mechanisms of environmental carcinogenesis. Boca Raton, Florida,
    CRC Press, vol 2, pp 21-57.

    Kalimi M (1982) Comparison of glucocorticoid receptors in various
    tissues of adult and senescent rats. Gerontology, 28: 371-376.

    Kamataki T, Maeda K, Shimado M, Kitani K, Nagai T, & Kato R (1985a)
    Age-related alteration in the activities of drug-metabolizing
    enzymes and contents of sex-specific forms of cytochrome P450 in
    liver microsomes from male and female rats. J Pharmacol Exp Ther,
    233: 222-228.

    Kamataki T, Shimada M, Maeda K, & Kato R (1985b) Pituitary
    regulation of sex-specific forms of cytochrome P-450 in liver
    microsomes of rats. Biochem Biophys Res Commun, 130: 1247-1253.

    Kampmann JP & Hansen JEM (1979) Renal excretion of drugs. In: Crooks
    J & Stevenson IH ed. Drugs and the elderly. Baltimore, Maryland,
    University Park Press, pp 77-87.

    Kanai S, Kitani K, Fujita S, & Kitagawa H (1985) The hepatic
    handling of sulfobrompthalein in aging Fischer-344 rats:  in vivo
    and  in vitro studies. Arch Gerontol Geriatr, 4: 73-85.

    Kanai S, Kitani K, Sato Y, & Nokubo M (1988) The age-dependent
    decline in the biliary transport maximum of conjugated
    sulfobromophthalein in the rat. Arch Gerontol Geriatr, 7: 1-8.

    Kane R (1987) Comprehensive assessment. In: Maddox GL ed. The
    encyclopedia of aging. Berlin, Heidelberg, New York,
    Springer-Verlag, pp 137-139.

    Kannel WB (1981) Update on the role of cigarette smoking in coronary
    artery disease (review). Am Heart J, 101: 319-328.

    Kato H, Saito M, & Suda M (1980) Effect of starvation on the
    circadian adrenocortical rhythm in rats. Endocrinology, 106:

    Katz ML & Robinson WG Jr (1987) Light and aging effects on vitamin E
    in the retinal pigment epithelium. Vision Res, 27(11): 1875-1879.

    Katzman R & Terry RD (1983) Normal aging in the nervous system. In:
    Katzman R & Terry RD ed. The neurology of aging. Philadelphia,
    Pennsylvania, F.A. Davis, pp 15-50.

    Kaur S & Gill SS (1985) Age-related changes in the activities of
    epoxide hydrolyases in different tissues of mice. Drug Metab Dispos,
    13: 711-715.

    Kaysen GA & Myers BD (1985) The aging kidney. Clin Geriatr Med,
    1(1): 207-222.

    Keast D & Ayre DJ (1981) Effects of chronic tabocco smoke exposure
    on immune responses in aged mice. Arch Environ Health, 36: 201-207.

    Kelley WW, Brief S, Westly HJ, Novakofski J, Bechtel PJ, Simon J, &
    Walker EB (1986) GH3 pituitary adenoma cells can reverse thymic
    aging in rats. Proc Natl Acad Sci (USA), 83: 5663-5667.

    Kelso A & Munck A (1984) Glucocorticoid inhibition of lymphokine
    secretion by alloreactive T lymphocyte clones. J Immunol, 133(2):

    Kim JCS & Kaminsky LS (1988) 2,2,2-Trifluoroethanol toxicity in aged
    rats. Toxicol Pathol, 16: 35-45.

    Kinon GA & Liu C (1973) Diurnal variation in plasma testosterone of
    the male laboratory rat. Horm Metab Res, 5: 233-234.

    Kitani K (1985) The role of the liver in pharmacokinetic and
    pharmacodynamic alterations in the elderly. In: O'Malley K &
    Waddington J ed. Therapeutics in the elderly. Amsterdam, Oxford, New
    York, Elsevier Science Publishers, pp 19-34.

    Kitani K (1988) Drugs and the ageing liver. Life Chem Rep, 6:

    Kitani K (1991) Aging of the liver: facts and theories. Arch
    Gerontol Geriatr, 12: 133-154.

    Kitani K, Masuda Y, Sato Y, Kanai S, & Nokubo M (1984) Increased
    anti-convulsant activity in aging BDF1 mice. J Pharmacol Exp Ther,
    229: 231-236.

    Kitani K, Ohta M, Kanai S, & Sato Y (1985a) Sex difference in the
    biliary excretion of digoxin and its metabolites in aging Wistar
    rats. Arch Gerontol Geriatr, 4: 1-12.

    Kitani K, Sato Y, Kanai S, Nokubo M, Ohta M, & Masuda Y (1985b)
    Increased anticonvulsant effect of phenobarbital with age in mice -
    a possible pharmacologic index for brain aging. Life Sci, 37:

    Kitani K, Sato Y, Kanai S, Nokubo M, Ohta M, Masuda Y, & Klotz U
    (1986) Altered response to convulsants and anticonvulsants in aging
    BDF1 mice. In: Kitani K ed. Liver and aging 1986: Liver and brain.
    Amsterdam, Oxford, New York, Elsever Science Publishers, pp 293-307.

    Kitani K, Sato Y, Kanai S, Nokubo M, Ohta M, & Masuda Y (1987)
    Increasing anticonvulsant effect of AD-810 (Zonisamide) in aging
    BDF1 mice. Life Sci, 41: 1339-1344.

    Klatsky AL, Friedman GD, & Siegelaub AB (1981) Alcohol and
    mortality. A ten year Kaiser-Permanente experience. Ann Intern Med,
    95: 139-145.

    Klawans HL & Tanner CM (1984) Movement disorders in the elderly. In:
    Albert M ed. Clinical neurology of aging. Oxford, Oxford University
    Press, pp 387-403.

    Klotz U, Avant GR, Hoyumpa A, Schenker S, & Wilkinson GR (1975) The
    effects of age and liver disease on the disposition and elimination
    of diazepam in adult man. J Clin Invest, 55: 347.

    Klug TL & Adelman RC (1977) Evidence for a large thyrotropin and its
    accumulation during aging in rats. Biochem Biophys Res Commun, 77:

    Koch-Weser J, Greenblatt DJ, Seller EM, & Shoder RI (1982) Drug
    disposition in old age. New Engl J Med, 306: 1081-1086.

    Kohrs MB (1981) Nutritional needs of the aged. Nutrition, 7: 39-46.

    Konishi N & Ward JM (1989) Increased levels of DNA synthesis in
    hyperplastic renal tubules of aging nephropathy in female F344/NCR
    rats. Vet Pathol, 26: 6-10

    Koobs DH, Schultz RL, & Jutzy RV (1978) The origin of lipofuscin and
    possible consequences to the myocardium. Arch Pathol Lab Med, 102:

    Kozararevic DJ, McGee D, Vojvodic N, Racic Z, Dawber T, Gordon T, &
    Zukel W  (1980) Frequency of alcohol consumption and morbidity and
    mortality: The Yugoslavia cardiovascular disease study. Lancet, i:

    Krupka L & Vener A (1979) Hazards of drug use among the elderly.
    Gerontologist, 19: 90-95.

    Kubos C, Day NK, & Good RA (1984) Influence of early or late dietary
    restirction on life span and immunological parameters in
    MRL/MP-Ipr/Ipr mice. Proc Natl Acad Sci (USA), 81: 5831-5835.

    Kumpuris D (1983) Gastrointestinal bleeding in the older patient.
    In: Texter EC ed. The aging gut. New York, Masson Publishing Co., pp

    Kyle ME & Kocsis JJ (1985) The effect of age on salicylate-induced
    nephrotoxicity in male rats. Toxicol Appl Pharmacol, 81: 337-347.

    Lakatta EG (1980) Age-related alteration in the cardiovascular
    response to adrenergic mediated stress. Fed Proc, 39: 3173-3177.

    Lakatta EG (1985) Heart and circulation. In: Finch CE & Schneider EL
    ed. Handbook of the biology of aging, 2nd ed. New York, Van Nostrand
    Reinhold Company, pp 377-413.

    Lakatta EG (1986) Diminished beta-adrenergic modulation of
    cardiovascular function in advanced age. Cardiol Clin, 4(2):

    Lakatta EG (1987a) Cardiac muscle change in senescence. Annu Rev
    Physiol, 49: 519-531.

    Lakatta EG (1987b) Catecholamines and cardiovascular function in
    aging. Endocrinol Metab Clin North Am, 16(4): 877-891.

    Landauer MR, Tomlinson WT, Balster RL, & MacPhail RC (1984) Some
    effects of the formamidine pesticide on the behavior of mice.
    Neurotoxicology, 5: 91-100.

    Leakey JEA, Cunny HC, Bazare J Jr, Webb PJ, Feuers RJ, Duffy PH, &
    Hart RW (1989a) Effects of aging and caloric restriction on hepatic
    drug metabolizing enzymes in the Fischer 344 rat. I: The cytochrome
    P-450 dependent monooxygenase system. Mech Ageing Dev, 48: 145-155.

    Leakey JEA, Cunny HC, Bazare J Jr, Webb PJ, Lipscomb JC, Siikkes W
    Jr, Feuers RJ, Duffy PH, & Hart RW (1989b) Effects of aging and
    caloric restriction on hepatic drug metabolizing enzymes in the
    Fischer 344 rat. II. Effects on conjugating enzymes. Mech Ageing
    Dev, 48: 157-166.

    Lee KT ed. (1985) Atherosclerosis. Ann NY Acad Sci, 454: 1-327.

    Lehmann AR, Hoeijmakers JHJ, Van Zealand AA, Backendorf CMP, Bridges
    BA, Collins A, Fuchs RPD, Margison GP, Montesano R, Moustacchi E,
    Natarajan AT, Radman M, Sarasin A, Seeberg E, Smith CA, Stefanini M,
    Thompson LH, Van der Schans GP, Weber CA, & Zdzienicka MZ (1992)
    Workshop on DNA repair. Mutat Res, 273: 1-28.

    Leopold AC (1975) The biological significance of death in plants.
    In: Benke JA, Finch CE, & Moment GB ed. The biology of aging. New
    York, London, Plenum Press, pp 101-114.

    Leppaluoto J, Ranta T, & Tuomisto J (1974) Diurnal variations of
    serum immunoassayable thyrotropin (TSH) concentration in the rat.
    Acta Physiol Scand, 90: 699-702.

    Lesser GT, Deutsch S, & Markofsky J (1973) Aging in the rat:
    Longitudinal and cross-sectional studies of body composition. Am J
    Physiol, 225: 1472-1478.

    Leto S, Kokkahen GC, & Barrows CH Jr (1976) Dietary protein, life
    span and physiological variables in female mice. J Gerontol, 31:

    Levy ML (1987) Tous les pays du monde (1987). Bull Mens Inf Démogr
    Econ Soc, 216:1-6.

    Lewis VM, Twomey JJ, Bealmar P, Goldstein G, & Good RA (1978) Age,
    thymic involution, and circulating thymic hormone activity. J Clin
    Endocrinol Metab, 47: 145-150.

    Li DD, Chien YK, Gu MZ, Richardson A, & Cheung HT (1988) The
    age-related decline in interleukin-3 expression in mice. Life Sci,
    43: 1215-1222.

    Liebow AA (1964) Biochemical and structural changes in the aging
    lung. Summary. In: Cander L & Moyer JH ed. Aging of the lung. New
    York, London, Grune and Stratton, pp 97-104.

    Liepa GU, Masoro EJ, Bertrand HA, & Yu BP (1980) Food restriction as
    a modulation of age-related changes in serum lipids. Am J Physiol,
    238: E253-E257.

    Likhachev AJ (1985) Effect of age on DNA repair in carcinogenesis
    due to alkylating agents. In: Likhachev A, Anisimov V, & Montesano R
    ed. Age-related factors in carcinogenesis. Lyon, International
    Agency for Research on Cancer, pp 239-246 (IARC Scientific
    Publications No. 58).

    Lin CF & Hayton WL (1983) GI motility and subepithelial blood flow
    in mature and senescent rats. Age, 6: 46-51.

    Lindeman RD (1986) The aging kidney. Compr Ther, 12(3): 43-49.

    Lindeman RD, Tobin JD, & Shock NW (1987) Hypertension and the
    kidney. Nephron, 47(1): 62-67

    Lindsay R (1989) Osteoporosis: An updated approach to prevention and
    management. Geriatrics, 44: 45-54.

    Lipschitz DA (1987) Nutrition, aging and the immunohematopoietic
    system. Clin Geriatr Med, 3(2): 319-328.

    Loft S, Dossing M, & Poulsen HE (1988) Influence of age and
    consumption of tobacco, alcohol, and caffeine on antipyrine
    clearance. Hum Toxicol, 7: 277-280.

    Loi CM & Vestal RE (1988) Drug metabolism in the elderly. Pharmacol
    Ther, 36: 131-149.

    Lucchi L, Memo M, Airaghi ML, Spano PF, & Trabucchi M (1981) Chronic
    lead treatment induces in rat a specific and differential effect on
    dopamine receptors in different brain areas. Brain Res, 213:

    Lutz RJ, Dedrick RL, Matthews HB, Elling TE, & Anderson MW (1977) A
    preliminary pharmacokinetic model for several chlorinated biphenyls
    in the rat. Drug Metab Dispos, 5: 386-396.

    McCaffrey TA, Nicholson AG, Szabo PE, & Wexsler BB (1988) Aging and
    atherosclerosis. The increased proliferation of arterial smooth
    muscle cells isolated from old rats is associated with increased
    platelet-derived growth factor-like activity. J Exp Med, 167:

    McCay CM, Crowell MM, & Mayrar LA (1935) The effect of reduced
    growth upon the length of the life span and upon ultimate body size.
    J Nutr, 10: 63-79.

    McClure JE, Lameris N, Wara DW, & Goldstein AL (1982) Immunochemical
    studies on thymosin: radioimmunoassay of thymosin alpha-1. J
    Immunol, 128(1): 368-375.

    McGinty D, Stern N, & Akshoomoff N (1988) Circadian and
    sleep-related modulation of hormone levels changes with aging. In:
    Sowers JR & Felicett JV ed. The endocrinology of aging. New York,
    Raven Press, pp 75-111.

    McKenna MJ & DiStefano V (1977) Carbon disulphide. II. A proposed
    mechanism for the action of carbon disulfide on
    dopamine-beta-hydroxylase. J Pharmacol Exp. Ther, 202: 253-266.

    McMahon TF (1988) Age-related changes in colonic phase I and phase
    II biotransformation: possible relationship to colon carcinogenesis.
    Baltimore, University of Maryland, School of Pharmacy

    McMahon TF & Birnbaum LS (1990a) Age-related changes in the
    disposition and metabolism of xenobiotics. In: Cooper RL, Goldman
    JM, & Harbin TJ ed. Aging and environmental toxicology. Baltimore,
    Maryland, Johns Hopkins University Press, pp 7-30.

    McMahon TF & Birnbaum LS (1990b) Age related change on toxicity and
    biotransformation of potassium cyanide in male C57BL/6N mice.
    Toxicol Appl Pharmacol, 105: 305-314.

    McMahon TF, Beierschmitt WP, & Weiner M (1987) Changes in phase I
    and phase II biotransformation pathways with age in male Fischer 344
    rat colon: relationship to colon carcinogenesis. Cancer Lett, 36:

    McMahon TF, Diliberto JJ, & Birnbaum LS (1989) Age-related changes
    in the disposition of benzyl acetate: A model compound for glycine
    conjugation. Drug Metab Dispos, 17: 506-512.

    McMahon TF, Diliberto JJ, & Birnbaum LS (l990a) Effects of age on
    metabolism and disposition of salicylic acid (SAL) in male Fischer
    344 rats. Drug Metab Dispos, 18: 494-503.

    McMahon TF, Peggins JO, Centra MM, & Weiner M (l990b) Age-related
    changes in biotransformation of azoxymethane and methylazoxymethanol
     in vitro. Xenobiotica, 20: 501-513.

    Maeda H, Gleiser CA, Masoro EJ, Murata I, McMohan CA, & Yu BP (1985)
    Nutritional influences on aging of Fisher 344 rats. II. Pathology. J
    Gerontol, 40: 671-688.

    Magal E, Chaudhuri M, & Adelman RC (1986) The capability for
    regulation of insulin secretion by somatostatin in purified
    pancreatic islet B cells during aging. Mech Aging Dev, 33(2):

    Magnus K (1982) Trends in cancer incidence: Causes and practical
    implications. Washington, DC, Hemisphere.

    Mantel N & Schneiderman MA (1975) Estimation "safe" levels, a
    hazardous undertaking. Cancer Res, 35: 1379-1386.

    Mantere P, Hanninen H, Hernberg S, & Luukkonen R (1984). A
    prospective follow-up study on psychological effects in workers
    exposed to low levels of lead. Scand J Work Environ Health, 10:

    Marin R, Proverbio T, & Proverbio F (1985) Characterization of the
    Na+, K+ -ATPase activity of basolateral membranes of kidney proximal
    tubular cells from young and old rats. Biochem Pharmacol, 34:

    Marksberry WR, Ehmann WD, Hossain IM, Alauddin M, & Goodin DT (1981) 
    Instrumental neutron activation analysis of brain aluminum in
    Alzheimer's disease and aging. Ann Neurol, 10: 511-516.

    Marsh G (1980) Perceptual changes with aging. In: Busse EW & Blazer
    DG ed. Handbook of geriatric psychiatry. New York, Van Nostrand
    Reinhold Company, Chapter 7, pp 147-168.

    Marshall JF & Berrios N (1979) Movement disorders of aged rats:
    Reversal by dopamine receptor stimulation. Science, 206: 477-479.

    Martin GM, Sprague CA, Norwood TH, & Pendergrass WR (1974) Clonal
    selection, attenuation and differentiation in an  in vitro model of
    hyperplasia. Am J Pathol, 74: 137-154.

    Marx JL (1987) A new wave of enzymes for cleaving pro-hormones.
    Science, 235: 285-286.

    Masoro EJ ed. (1981) CRC handbook of physiology in aging. Boca
    Raton, Florida, CRC Press.

    Masoro EJ (1985) Metabolism. In: Finch CE & Schneider EL ed.
    Handbook of the biology of aging. New York, Van Nostrand Reinhold
    Company, pp 540-563.

    Masoro EJ (1988) Food restrictions in rabbits: an evolution of its
    role in the study of aging. J Gerontol, 43: B59-B64.

    Masoro EJ (1991) Use of rodents as models for the study of "normal
    aging": Conceptual and practical issues. Neurobiol Aging, 12:

    Masoro EJ & Yu BP (1989) Diet and nephropathy. Lab Invest, 60:

    Masoro EJ, Yu BP, Bertrand HR, & Lynd FT (1980) Nutritional probe of
    the aging process. Fed Proc, 39: 3178-3182.

    Masoro EJ, Shimokawa I, & Yu BP (1991) Retardation of the aging
    processes in rats by food restriction. Ann NY Acad Sci, 621:

    Master CL, Multhaup G, Simms G, Pottgieser J, Martins RN, &
    Beyreuther K (1985)  Neuronal origin of a cerebral amyloid:
    Neurofibrillary tangles of Alzheimer's disease contain the same
    protein as the amyloid of plaque cores and blood vessels. EMBO J, 4:

    Matulionis DH (1984) Chronic cigarette smoke inhalation and aging in
    mice: Morphologic and functional lung abnormalities. Exp Lung Res,
    7: 237-256.

    Mauderly JL (1978) Effect of age on pulmonary structure and function
    of immature and adult animals and man. Fed Proc, 38: 173-177.

    Mauderly JL (1979a) Ventilation, lung volumes, and lung mechanics of
    young adult and old Syrian hamsters. Exp Aging Res, 5: 497-508.

    Mauderly JL (1979b) Effect of aging on pulmonary structure and
    function of immature and adult animals and man. Fed Proc, 38:

    Mauderly JL (1982) The effect of age on respiratory function of
    Fischer 344 rats. Exp Aging Res, 8: 31-36.

    Mawhinney MG (1985) Etiological considerations for the growth of
    stroma in benign prostatic hyperplasia. Fed Proc, 45: 2615-2617.

    Mazzeo RS & Horvath SM (1987) A decline in myocardial and hepatic
    norepinephrine turnover with age in Fischer 344 rats. Am J Physiol,
    252: E762-E763.

    Medvedev ZA (1984) Age changes of chromatin: A review. Mech Ageing
    Dev, 28: 139-154.

    Medvedev ZA (1990) An attempt at a rational classification of
    theories of aging. Biol Rev, 65: 375-398.

    Mehta T, Labuc GE, Urbanski SJ, & Archer MC (1984) Organ specificity
    in the microsomal activation and toxicity of
    N-nitrosomethylbenzylamine in various species. Cancer Res, 44:

    Meisami E (1988) Aging of the nervous system structural and
    biochemical changes. In: Timiras PS ed. Physiological basis of aging
    and geriatrics. New York, Macmillan Publishing Co, pp 123-143.

    Mervis R (1981) Cytomorphological alterations in the aging animal
    brain with emphasis on Golgi studies. In: Johnson JE ed. Aging and
    cell structure. New York, London, Plenum Press, Chapter 4, pp

    Messing RB, Vasquez BJ, Spiehler VR, Martinez JL Jr, Jensen RA,
    Rigter H, & McCaugh JL (1980) 3H-Dihydromorphine binding in brain
    regions of young and aged rats. Life Sci, 26: 921-927.

    Meyer BR & Bellucci A (1986) Renal function in the elderly. Cardiol
    Clin, 4(2): 227-234.

    Mezey E (1981) Alcohol and drug interactions in injury to the
    digestive tract. Clin Gastroenterol, 10: 485-495.

    Millard WJ, O'Sullivan DM, Fox TO, & Martin JB (1985) Sexually
    dimorphic patterns of growth hormone secretion in rats. In: Crowley
    WF Jr & Hofler JG ed. The episodic secretion of hormones. New York,
    Chichester, Brisbane, Toronto, John Wiley and Sons, pp 287-304.

    Miller SM (1989) Changes in endocrine function with aging. Clin Lab
    Sci, 2(1): 19-20.

    Miller R (1991) Aging and immune function. Int Rev Cytol, 124:

    Minaker KL, Meneilly GS, & Rowe JW (1985) Endocrine systems. In:
    Finch CE &  Schneider EL ed. Handbook of the biology of aging. New
    York, Van Nostrand Reinhold Company, pp 433-456.

    Mitsui Y, Sakagami H, Murota SI, & Yamada MA (1980) Age related
    decline in histone H1 fraction in human diploid fibroblast cultures.
    Exp Cell Res, 126: 289-298.

    Miura K, Goldstein RS, Morgan DG, Pasino DA, Hewitt WR, & Hook JB
    (1987) Age-related differences in susceptibility to renal ischemia
    in rats. Toxicol Appl Pharmacol, 87: 284-296.

    Moberg GP, Bellinger LL, & Mendel VE (1975) Effect of meal feeding
    daily rhythms of plasma corticosterone and growth hormone in rat.
    Neuroendocrinology, 19: 160-169.

    Monteiro-Riviere NA, Banks YB, & Birnbaum LS (1991) Laser doppler
    measurements of cutaneous blood flow in ageing mice and rats.
    Toxicol Lett, 57: 329-338.

    Montgomery PR & Sitar DS (1981) Increased serum salicylate
    metabolites with age in patients receiving chronic acetylsalicylic
    acid therapy. Gerontology, 27: 329-333.

    Moolgavkar SH & Venzon DJ (1979) Two-event models for
    carcinogenesis; incidence curves for childhood and adult tumors.
    Math Biosci, 47: 55-77.

    Mooradian AD & Song MK (1989) Age-related alterations in duodenal
    calcium transport rate in rats. Mech Ageing Dev, 47: 221-227.

    Moroni F, Lombardi G, Moneti G, & Aldinio C (1984) The excitotoxin
    quinolinic acid is present in the brain of several mammals and its
    cortical content increases during the aging process. Neurosci Lett,
    47: 51-55.

    Morell AG, Gregoriadis G, Scheinberg IH, Hickman J, & Ashwell G
    (1971) The role of sialic acid in determining the survival of
    glycoproteins in the circulation. J Biol Chem, 246: 1461-1467.

    Morris JF, Mosk A, & Johnson LC (1971) Spirometric standards for
    healthy non-smoking adults. Am Rev Respir Dis, 103: 57-67.

    Mullaart E, Boerrigter METI, Lohman PHM, & Vijg J (1989) Age-related
    induction and disappearance of carcinogen-DNA-adducts in livers of
    rats exposed to low levels of 2-acetylaminofluorene. Chem-Biol
    Interact, 69: 373-384.

    Mullaart E, Lohman PHM, Berends F, & Vijk J (1990) Review DNA damage
    metabolism and aging. Mutat Res, 237: 189-210.

    Müller HF & Schwartz G (1978) Electroencephalograms and autopsy
    findings in geropsychiatry. J Gerontol, 33: 504-513.

    Muller WEG, Wenger R, Bachmann M, Ugarkovic D, Courtis NC, &
    Schroder HC (1989) Poly(A) metabolism and aging: a current view.
    Arch Gerontol Geriatr, 9: 31-250.

    Munro HN (1984) Nutrition-related problems of middle age. Proc Nutr
    Soc, 43: 281-288.

    Murty CVR, Mancini MA, Chatterjee B, & Roy AK (1988a) Changes in
    transcriptional activity and matrix association of alpha-2u-globulin
    gene family in the rat liver during maturation and aging. Biochim
    Biophys Acta, 949: 7-34.

    Murty CVR, Olson MJ, Garg BD, & Roy AK (1988b) Hydrocarbon-induced
    hyaline droplet nephropathy in male rats during senescence. Toxicol
    Appl Pharmacol, 96: 380-392.

    Mutti A, Falzoi M, Romanelli A, Bocchi MC, Ferroni C, & Franchini I
    (1988) Brain dopamine as a target for solvent toxicity: Effects of
    some monocyclic aromatic hydrocarbons. Toxicology, 49: 77-82.

    Naessen R (1971) An enquiry on the morphological characteristics and
    possible changes with age in the olfactory region of man. Acta
    Otorhinol, 71: 49-62.

    Nagel JE, Chopra RK, Chrest FJ, McCoy MT, Schneider EL, Holbrook NJ,
    & Adler WH (1988) Decreased proliferation, interleukin 2 synthesis,
    and interleukin 2 receptor expression are accompanied by decreased
    mRNA expression in phytohemagglutinin-stimulated cells from elderly
    donors. J Clin Invest, 81: 1096-1102.

    Nagy K, ZS-Nagy V, Bertoni-Freddari C, & ZS-Nagy I (1983)
    Alterations of the synaptosomal membrane 'microviscosity' in the
    brain cortex of rats during aging and centrophenoxine treatment.
    Arch Gerontol Geriatr, 2: 23-29.

    Nakamura E, Kimura M, Nagata H, Miyao K, & Ozeki T (1982) Evaluation
    of the progress of aging based on specific biological age as
    estimated by various physiological functions. Jpn J Hyg, 36:

    NAS (1989) Biological markers in reproductive toxicology.
    Washington, DC, National Academy of Sciences, National Academy
    Press, pp 2-3.

    Neafsey PJ (1990) Longevity hormesis. A review. Mech Ageing Dev, 51:

    Nebert DW & Gonzalez FJ (1987) P450 Genes - Structure, evolution and
    regulation. Annu Rev Biochem, 56: 945-993.

    Nebert DW, Nelson DR, Minor JC, Estabrook RW, Feyereisen R,
    Fujii-Kuriyama Y, Gonzalez FJ, Guengerich FP, Gunsalus IC, Johnson
    EF, Loper JC, Sato R, Waterman MR, & Waxman DR (1991) The P450
    superfamily: Uptake on new sequences, gene mapping, and recommended
    nomenclature. DNA Cell Biol, 10(1): 1-14.

    Needleman HL, Gunnoe C, Leviton A, Reed R, Peresie H, Maher C, &
    Barrett P (1979) Deficits in psychologic and classroom performance
    of children with elevated dentine lead levels. New Engl J Med, 300:

    Neuhaus OW & Flory W (1978) Age-dependent changes in the excretion
    of urinary proteins by the rat. Nephron, 22: 570-576.

    Newaz SN, Fang W, & Strobel HW (1983) Metabolism of the carcinogen
    1,2-dimethylhydrazine by isolated human colon microsomes and human
    colon tumor cells in culture. Cancer, 52: 794-798.

    Niedzwiecki A, Lewis PN, & Cinader B (1985) Changes of histone H1
    subtypes with aging in strains of mice that possess different
    immunological characteristics. J Gerontol, 40: 695-699.

    Nokubo M & Kitani K (1988) Age-dependent decrease in the lethal
    threshold of pentylenetetrazole in mice. Life Sci, 43: 41-47.

    Norwood TH & Smith JR (1985) The cultured fibroblast-like cells as a
    model for the study of aging. In: Fincht CE & Schneider EL ed. The
    biology of aging, 2nd ed. New York, Van Nostrand Reinhold Company,
    pp 291-321.

    Novak LP (1972) Aging, total potassium, fat free mass and cell mass
    in males and females between the ages of 18 and 85 years. J
    Gerontol, 27: 438-443.

    Oh SH, Deagen JT, Whanger PD, & Wesnig PH (1978) Biological function
    of metallothionein. V. Its induction in rats by various stresses. Am
    J Physiol, 234: E282-E285.

    Ohta M, Kanai S, Sato Y, & Kitani K (1988) Age-dependent decrease in
    the hepatic uptake and biliary excretion of ouabain in rats. Biochem
    Pharmacol, 37: 935-942.

    O'Malley K, Docherty JR, & Kelly GJ (1988) Adrenoceptor status and
    cardiovascular function in ageing. J Hypertension, 6(suppl 10):

    O'Meara AR & Pochron SF (1979) Age-related effects on the
    incorporation of acetate into rat liver histones. Biochim Biophys
    Acta, 586: 391-401.

    Onasaka S & Cherian MG (1981) The induced synthesis of
    metallothionein in various tissues of rats in response to metals. I.
    Effect of repeated injection of cadmium salts. Toxicology, 22:

    Os I, Kjeldsen SE, & Westheim A (1987) Aging and urinary vasopressin
    excretion in healthy men. Scand J Urol Nephrol, 21(3): 235-239.

    OECD (1986) Existing chemicals systematic investigation, priority
    setting and chemical reviews. Paris, Organisation for Economic
    Co-operation and Development.

    Owen RA & Heywood R (1986) Age-related variations in renal structure
    and function in Sprague-Dawley rats. Toxicol Pathol, 14(2): 158-167.

    Pagano GN, Casadar N, Diana A, Psue E, Bozzo C, Ferrero F, & Lenti G
    (1981) Insulin resistance in the aged: the role of peripheral
    insulin receptors. Metabolism, 30: 46-49.

    Pahlavani MA, Cheung HT, Cai NS, & Richardson A (1988) Influence of
    dietary restriction and aging on gene expression in the immune
    system of rats. In: Biomedical advances in aging. New York, London,
    Plenum Press, pp 259-270.

    Pakin YV & Hrisanov SM (1984) Critical analysis of the applicability
    of the Compertz-Makeham law in human population. Gerontology, 30:

    Paramonova GI & Dovgi AI (1987) Induction of multiple forms of
    cytochrome P450 of rat liver during aging. In: Abstracts of the All
    Union Conference on Cytochrome P450 and the Environment,
    Novosibirsk, 27-31 July 1987. Novosibirsk, USSR Academy of Medical
    Sciences, Siberian Division, pp 100-101.

    Patel DJ & Vaishnaw RN (1980) Basic hemodynamics and its role in
    disease process. Baltimore, Maryland, University Park Press.

    Pedigo NW, Schoemaker H, Morellia M, McDougal JN, Malisck JB, Burks
    TF, & Yamamura TF (1981) Benzodiazepine receptor binding in young,
    mature, and senescent rat brain and kidney. Neurobiol Aging, 2:

    Penn A, Batastini G, Soloman J, Burns F, & Albert R. (1981)
    Dose-dependent size increases of aortic lesions following chronic
    exposure to 7,12-dimethylbenz(a)-anthracene. Cancer Res, 71:

    Penzes L (1974) Intestinal absorption of glycine, L-alanine, and
    L-leucine in the old rat. Exp Gerontol, 8: 245-252.

    Penzes, L.S. (1984) Intestinal response in aging: changes in reserve
    capacity. Acta Med Hung, 41(4): 263-277.

    Perkins MN & Stone TW (1983) Pharmacology and regional variations of
    quinolinic acid-evoked excitations in the rat central nervous
    system. J Pharmacol Exp Ther, 226(2): 551-557.

    Perl DP & Brody AR (1980) Alzheimer's disease: X-ray spectrometric
    evidence of aluminum accumulation in neurofibrillary tangle-bearing
    neurons. Science, 208: 297-299.

    Peterson DD, Pack AI, Silage DA, & Fishman AP (1981) Effects of
    aging on ventilatory and occlusion pressure responses to hypoxia and
    hypercapnia. Am Rev Respir Dis, 124: 387-391.

    Peto R (1986) Influence of dose and duration of smoking on lung
    cancer rates. In: Zaridze D & Peto R ed. Tobacco: A major
    international health hazard. Lyon, International Agency for Research
    on Cancer, pp 23-33 (IARC Scientific Publications No. 74).

    Peto R, Parish SE, & Gray RG (1975) Cancer and aging in mice and
    men. Br J Cancer, 32: 411-426.

    Peto R, Pike MC, Day NE, Gray RE, Lee PN, Parish S, Poto J, Richard
    S, & Wahrendorf J (1980) Guidelines for simple, sensitive
    significance tests for carcinogenic effects in long-term animal
    experiments. In: Long-term and short-term screening assays for
    carcinogens: A critical appraisal. Lyon, International Agency for
    Research on Cancer, pp 311-426 (IARC Monographs on the Evaluation of
    the Carcinogenic Risk of Chemicals to Humans, Supplement 2).

    Peto R, Parish SE, & Gray RG (1985) There is no such thing as aging,
    and cancer is not related to it. In: Likhachev AJ, Anisimov V, &
    Montesano R ed. Age-related factors in carcinogenesis. Lyon,
    International Agency for Research on Cancer, pp 43-53 (IARC
    Scientific Publications No. 58).

    Pierpaoli W, Dall'Ara A, Pedrinis E, & Regelson W (1991) The pineal
    control of aging. The effects of melatonin and pineal grafting on
    the survival of older mice. Ann NY Acad Sci, 621: 291-313.

    Piikivi L, Hanninen H, Martelin T, & Mantere P (1984) Psychological
    performance and long-term exposure to mercury vapors. Scand J Work
    Environ Health, 10: 35-41.

    Pines A, Cucos B, Ever-Hadani P, Ron M, & Lemesch C (1987) Changes
    in pattern of organochlorine residues in blood of general Israeli
    population, 1975-1986. Sci Total Environ, 66: 115-125.

    Piva F, Limonta P, Maggi R, Dondi D, Messi E, Yanisi M, & Motta M
    (1987) Aging and the hypothylamo-pituitery-testiculas axis in the
    rat. J Steroid Biochem, 27(4-6): 707-712.

    Poland A & Knutson JC (1982) 2,3,7,8,-Tetrachlorodibenzo-p-dioxin
    and related halogenated aromatic hydrocarbons: examinations of the
    mechanism of toxicity. Annu Rev Pharmacol Toxicol, 22: 517-554.

    Popper H (1986) Aging in the liver. In: Popper H ed. Progress in
    liver diseases. New York, London, Grune & Stratton, vol VIII, pp

    Posner J, Danhof M, Teunissen MWE, Breimer DD, & Whiteman PD (1987)
    The disposition of antipyrine and its metabolites in young and
    elderly healthy volunteers. Br J Clin Pharmacol, 24: 51-55.

    Post DJ, Carter K, & Papaconstantinou J (1991) The effect of aging
    on constitutive mRNA levels and lipopolysaccharide inducibility of
    acute phase genes. Ann NY Acad Sci, 621: 66-77.

    Prinz PN, Weitzman ED, Cunningham GR, & Karacan I (1983) Plasma
    growth hormone during sleep in young and aged men. J Gerontol, 38:

    Provinciali M & Fabris H (1990) Role of pituitary-thyroid axis on
    basal and lymphokine-induced NK cell activity in aging. Int J
    Neurosci, 51: 373-375.

    Rabovsky, J, Marion KJ, Groseclose RD, Lewis TR, & Peterson M (1984)
    Low dose chronic inhalation of diesel exhaust and/or coal dust by
    rats: effect of age and exposure on lung and liver cytochrome P450.
    Xenobiotica, 14: 595-598.

    Rao GN, Haseman JK, Grumbein S, Crawford DD, & Eustis SL (1990)
    Growth, body weight, survival, and tumor trends in F344/n rats
    during an eleven-year period. Toxicol Pathol, 18: 61-70.

    Rath PC & Kanungo MS (1989) Age-related changes in the expression of
    cytochrome p-450 (b+e) gene in the rat after phenobarbitone
    administration. Biochem Biophys Res Commun, 157: 1403-1409.

    Reff ME & Schneider EL ed. (1982) Biological markers of aging.
    Washington, DC, US Government Printing Office, 252 pp (NIH
    Publication No. 82-2221).

    Regelson W (1983) Biomarkers in aging. In: Regelson W & Marott Sinex
    F ed. Intervention in the aging process. New York, Alan R. Liss,
    Inc., pp 3-98.

    Rehnberg GL, Goldman JM, Cooper RL, Hein JF, McElroy WK, Booth KC, &
    Gray LE (1988a) Effect of linuron on the brain-pituitary-testicular
    reproductive axis in the rat. Toxicologist, 8: 121.

    Rehnberg GL, Linder R, Goldman JM, Hein JF, McElroy WK, & Cooper RL
    (1988b) Changes in testicular and serum hormone concentrations in
    the male rat following treatment with m-dinitrobenzene. Toxicol Appl
    Pharmacol, 95: 255-264.

    Reidenberg MM (1980) Drugs in the elderly. Bull NY Acad Med, 56:
    703-714. Reif AE (1981) Effect of cigarette smoking on
    susceptibility to lung cancer. Oncology, 38: 76-85.

    Reis W & Poethig D (1984) Chronological and biological age. Exp
    Gerontol, 19: 211-216.

    Reiter RJ (1986) The pineal gland: an important link to the
    environment. News Physiol Sci, 1: 202-205.

    Reitz RH, McDougal JN, Himmelstein MW, Nolan RJ, & Schumann AM
    (1988) Physiologically based pharmacokinetics modeling with methyl
    chloroform: implications for interspecies, high dose/low dose, and
    dose route extrapolations. Toxicol Appl Pharmacol, 95: 185-199.

    Revskoy SY, Poroshina TE, Kovaleva IG, Berstein LM, Ostroumova MN, &
    Dilman VM (1985) Age-dependent metabolic immunodepression and
    cancer. In: Likhachev A, Anisimov V, & Montesano R ed. Age-related
    factors in carcinogenesis. Lyon, International Agency for Research
    on Cancer, pp 253-259 (IARC Scientific Publications No. 58).

    Richardson A & Birchenall-Sparks MC (1983) Age-related changes in
    protein synthesis. Rev Biol Res Aging, 1: 255-273.

    Richardson A & Semsei I (1987) Effect of aging on translation and
    transcription. In: Review of biological research in aging. New York,
    Alan R. Liss, Inc., vol 3, pp 467-483.

    Richardson A, Birchenall-Sparks MC, & Staecker JL (1983) Aging and
    transcription. In: Review of biological research in aging. New York,
    Alan R. Liss, Inc., vol 2, pp 275-294.

    Richardson A, Butler JA, Rutherford MS, Semsei I, Gu MZ, Fernandes
    G, & Chiang WH (1987) Effect of age and dietary restriction on the
    expression of alpha-2u-globulin. J Biol Chem, 262: 12821-12825.

    Richardson A, Roberts M, & Rutherford M (1985) Aging and gene
    expression. In: Review of biological research in aging. New York,
    Alan R. Liss, Inc., vol 2, pp 395-419.

    Riegel GD & Miller AE (1981) Aging effect on hormone concentrations
    and actions. In: Florini JR ed. Handbook of biochemistry in aging.
    Boca Raton, Florida, CRC Press, pp 231-256.

    Riggs BL (1987) Pathogenesis of osteoporosis. Am J Obstet Gynecol,
    156: 1342-1346.

    Riggs BL & Melton LJ III (1983) Evidence for two distinct syndromes
    of involutional osteoporosis. Am J Med, 75: 899-901.

    Rikans LE (1984) Influence of aging on the susceptibility of rats to
    hepatotoxic injury. Toxicol Appl Pharmacol, 73: 243-249.

    Rikans LE (1989a) Hepatic drug metabolism in female Fischer rats as
    a function of age. Drug Metab Dispos, 17: 114-116.

    Rikans L (1989b) Effects of ethanol on microsomal drug metabolism in
    aging female rats. I. Induction. Mech Ageing Dev, 48: 267-280.

    Rikans LE & Moore DR (1987) Effect of age and sex on allyl alcohol
    hepatotoxicity in rats: role of liver alcohol and aldehyde
    dehydrogenase activities. J Pharmacol Exp Ther, 243: 20-26.

    Rikans LE & Moore DR (1988) Effect of aging on aqueous phase
    antioxidents in tissues of male Fischer rats. Biochem Biophys Acta,
    966: 269-275.

    Rikans LE & Snowden CD (1989) Influence of aging on acute ethanol
    hepatotoxicity in female rats. Pharmacologist, 31: 177.

    Ritschel WA (1983) Pharmacokinetics in the aged. In: Pagliaro LA &
    Pagliaro AM ed. Pharmacologic aspects of aging. St. Louis, Missouri,
    C.V. Mosby & Co., pp 219-256.

    Ritschel WA (1988) Gerontokinetics: The pharmacokinetics of drugs in
    the elderly. Caldwell, New Jersey, Telford Press, 114 pp.

    Robertson IGC & Birnbaum LS (1982) Age-related changes in mutagen
    activation by rat tissues. Chem-Biol Interact, 38: 243-252.

    Robertson DRC, Waller DG, Renwick AG, & George CF (1988) Age-related
    changes in the pharmacokinetics and pharmacodynamic of nifedipine.
    Br J Clin Pharmacol, 25: 297-305.

    Rodgers J & Gass G (1983) The effect of age on serum proteins in
    mice. Exp Gerontol, 14: 169-173.

    Roe DA (1983) Drugs and nutrition in the elderly. In: Geriatric
    nutrition, 2nd ed. Englewood, New Jersey, Prentice-Hall, Inc., pp

    Rogers J & Bloom FE (1985) Neurotransmitter metabolism and function
    in the aging central nervous system. In: Finch CE & Schneider EL ed.
    Handbook of the biology of aging, 2nd ed. New York, Van Nostrand
    Reinhold Company, pp 645-691.

    Rogers GS & Gilchrest BA (1990) The senile epidermis: environmental
    influences on skin ageing and cutaneous carcinogenesis. Br J
    Dermatol, 122(suppl 35): 55-60.

    Rogers J, Magistretti PJ, & Bolis LC (1991) Animal models for aging
    research. Neurobiol Aging, 12(spec issue): 625-701.

    Rose MR (1991) Evolutionary biology of aging. New York, London,
    Oxford University Press.

    Rosenberger RF & Kirkwood TBL (1986) Errors and the integrity of
    genetic information transfer. In: Kirkwood TBL, Rosenberger RF, &
    Galas DJ, ed. Accuracy in molecular processes. London, Chapman and
    Hall, pp 17-35.

    Ross MH (1961) Length of life and nutrition in rat. J Nutr, 75:

    Ross MH (1976) Nutrition and longevity in experimental animals. In:
    Winick M ed. Nutrition and aging. New York, Chichester, Brisbane,
    Toronto, John Wiley and Sons, pp 23-41.

    Ross R (1981) Atherosclerosis: A problem of the biology of arterial
    wall cells and their interactions with blood components.
    Arteriosclerosis, 1: 293-311.

    Roth GS (1979a) Hormone action during aging: alterations and
    mechanisms. Mech Ageing Dev, 9: 497-514.

    Roth GS (1979b) Hormone receptor changes during adulthood and
    senscence: significance for aging research. Fed Proc, 38: 1910-1914.

    Roth GS (1988) Receptor and postreceptor changes during lymphocyte
    aging. In: Platt D ed. Blood cells, rheology, and aging. Berlin,
    Heidelberg, New York, Springer-Verlag, pp 150-154.

    Roth GS & Joseph JA (1988) Peculiarities of the effect of hormones
    and transmitters during aging: Modulation of changes in dopaminergic
    action. Gerontology, 34: 22-28.

    Rothstein M & Seifert SC (1981) RNA synthesis. In: Handbook of
    biochemistry in aging. Boca Raton, Florida, CRC Press, pp 51-63.

    Rouser G, Kritchevsky G, & Yamamoto A (1972) Lipids in the nervous
    system of different species as a function of age: Brain, spinal
    cord, peripheral nerve, purified whole cell preparations and
    sub-cellular particulates: Regulatory mechanisms and membrane
    structure. Adv Lipid Res, 10: 261-360.

    Rowe JW, Minaker KL, Paollotta J, & Fliers JS (1983)
    Characterization of the insulin resistance of aging. J Clin Invest,
    71: 1581-1587.

    Roy AK, Nath TS, Motwani NM, & Chatterjee B (1983) Age dependent
    regulation of the polymorphic forms of alpha2u-globulin. J Biol
    Chem, 258: 10123-10127.

    Rudman D (1987) Assessment of nutritional status. In: Braunwald E,
    Isselbacher KJ, Petersdorf RG, Wilson JD, Martin JB, & Fauci AS ed.
    Harrison's principles of internal medicine, 11th ed. New York,
    McGraw-Hill, Chapter 71, pp 309-392.

    Rudman D (1988) Kidney senescence: A model for aging. Nutr Rev,
    46(6): 209-214.

    Runcie J (1985) Obesity. In: Pathy MWSJ ed. Principles and practice
    of geriatric medicine. New York, Chichester, Brisbane, Toronto, John
    Wiley and Sons, Chapter 22.1, pp 277-284.

    Russell DH, Buckley AR, Shah GN, Sipes IG, Blask DE, & Benson B
    (1988) Hypothalamic site of action of
    2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Toxicol Appl Pharmacol,
    94: 496-502.

    Sacher GA (1977) Life table modification and life prolongation. In:
    Finch CE & Hayflick L ed. Handbook of the biology of aging. New
    York, Van Nostrand Reinhold Company, pp 582-638.

    Sagiv M, Goldhammer E, Abinader EG, & Rudoy J (1988) Aging and the
    effect of increased after-load on left ventricular contractile
    state. Med Sci Sports Exercise, 20(3): 281-284.

    Sample PA, Esterson FD, Weinreb RN, & Boynton RM (1988) The aging
    lens:  in vivo assessement of light absorption in 84 human eyes.
    Invest Ophtalmol Visual Sci, 29(8): 1306-1311.

    Sapolsky R, Armanini M, Packan D, & Tornbaugn G (1987) Stress and
    glucocorticoids in aging. Endocrinol Metab Clin, 16(4): 965-979.

    Sapolsky RM, Krey LC, & McEwen BS (1986) The adrenocortical axis in
    the aged rat: Impaired sensitivity to both fast and delayed feedback
    inhibition. Neurobiol Aging, 7: 331-335.

    Sato Y, Kanai S, & Kitani K (1987) Biliary excretion of ouabain in
    aging male and female F344 rats. Arch Gerontol Geriatr, 6: 141-152.

    Saxton JA Jr & Kimball GC (1941) Relation to nephrosis and other
    diseases of albino rat to age and to modifications of diet. Arch
    Pathol, 32: 951-965.

    Scheibel AB & Tomiyasu U (1978) Dendritic sprouting in Alzheimer
    presenile dementia. Exp Neurol, 60: 1-8.

    Schiff I & Wilson E (1979) Clinical aspects of aging of the female
    reproductive system. Aging, 4: 10-28.

    Schmucker D (1979) Age-related changes in drug disposition. Pharm
    Rev, 30: 445-456.

    Schmucker DL & Wang RK (1979) Rat lysosomal enzymes: effects of
    animal age and phenobarbital. Age Aging, 2: 93-96.

    Schmucker DL & Wang RK (1989) Dietary restriction postpones the
    age-dependent compromise of male rat liver microsomal
    monooxygenases. Prog Clin Biol Res, 287: 283-288.

    Schmucker DL, Woodhouse KW, Wang RK, Wynne H, James QF, McManus M, &
    Kremers P (1990) Effects of age and gender on  in vitro properties
    of human liver microsomal monoxygenases. Clin Pharmacol Ther, 48:

    Schneider EL (1978) The aging reproductive system. New York, Raven
    Press, vol 4.

    Schneider EL & Reed JD (1985) Life extension. New Engl J Med, 312:

    Schneider EL, Sternberg H, Tice RR, Senula GC, Kra D, Smith JR, &
    Bynum G (1979) Cellular replication and aging. Mech Ageing Dev, 9:

    Schuetz EG, Wrighton SA, Barwick JL, & Guzelian PS (1984) Induction
    of cytochrome P-450 by glucocorticoids in rat liver. I. Evidence
    that glucocorticoids and pregnenolone 16 alpha-carbonitrile regulate
    de novo synthesis of a common form of cytochrome P-450 in cultures
    of adult rat hepatocytes and in the liver in vivo. J Biol Chem, 259:

    Schumann AM, Fox TR, & Watanabe PG (1982a) A comparison of the fate
    of inhaled methyl chloroform (1,1,1-trichloroethane) following
    single or repeated exposure in rats and mice. Fundam Appl Toxicol,
    2: 27-32.

    Schumann AM, Fox TR, & Watanabe PG (1982b) Methylchloroform
    (1,1,1-trichloroethane): Pharmacokinetics in rats and mice following
    inhalation exposure. Toxicol Appl Pharmacol, 62: 390-401.

    Schuurman HJ, Krajnc-Franken MAM, Kuper CF, Van Loveren H, & Vos JG
    (1990) Immune system. In: Haschek-Hoch WM & Rousseaux CG ed.
    Fundamentals of toxicologic pathology. New York, London, San
    Francisco, Academic Press, pp 1-103.

    Searle CE (1976) Chemical carcinogens. Washington, DC, American
    Chemical Society (ACS Monograph No. 173)

    Segre D, Miller RA, Abraham GN, Weigle WO, & Warner HR (1989) Aging
    and the immune system. J Gerontol, 44(6): 8164-8168.

    Sekuler R, Kline D, & Dismuskes K (1982) Aging and human visual
    functions. New York, Alan R. Liss, Inc.

    Sellers EM, Frecker RC, & Romach MK (1983) Drug metabolism in the
    elderly: confounding of age, smoking, and ethanol effects. Drug
    Metab Rev, 14: 225-250.

    Selmanowitz VJ, Rizer RL, & Orentreich N (1977) Aging of the skin
    and its appendages. In: Finch CE & Hayflick L ed. Handbook of the
    biology of aging. New York, Van Nostrand Reinhold Company, pp

    Selye H (1950) Stress: The physiology and pathology of exposure to
    stress. Montreal, Acta Inc.

    Semsei I, Rao G, & Richardson A (1989) Changes in the expression of
    superoxide dismutase and catalase as a function of age and dietary
    restriction. Biochem Biophys Res Commun, 164: 620-625.

    Senda S, Cheng E, & Kawanishi H (1989) IgG in murine intestinal
    secretions. Aging effect and possible physiological role. Scand J
    Immunol, 29(1): 41-47.

    Shen DX, Ren R, Zhao YR, & Ma XZ (1987) [Disease pattern analysis in
    hospitalized elderly patients in Jingzhou.] Chin J Geriatr, 6: 44
    (in Chinese).

    Shepherd AMM, Wilson N, & Stevenson IH (1979) Warfarin sensitivity
    in the elderly. In: Crooks J & Stevenson IH ed. Drugs and the
    elderly. Baltimore, Maryland, University Park Press, pp 199-209.

    Sherlock S, Bearn AG, Billing BH, & Paterson JCS (1955) Splanchnic
    blood flow of peripheral plasma bromosulphthalein method: The
    relation of peripheral bromosulphthalein level to the calculated
    flow. J Lab Clin Med, 35: 923-932.

    Shih JC & Young H (1978) The alteration of serotonin binding sites
    in aged human brain. Life Sci, 23: 1441-1448.

    Simmons DL, McQuiddy P, & Kasper CB (1987) Induction of the hepatic
    mixed-function oxidase system by synthetic glucocorticoids -
    Transcriptional and post-transcriptional regulation. J Biol Chem,
    262: 326-332.

    Slagboom PE & Vijg J (1989) Genetic instability and aging: theories,
    facts, and future perspectives. Genome, 31: 373-385.

    Smith DBD, Thompson LW, & Michalewski HJ (1980) Average evoked
    potential research in adult aging - status and prospects. In: Aging
    in the 1980s. Washington, DC, American Psychological Association, pp

    Smith EL, Sempos CT, & Purvis RW (1981) Bone mass and strengh
    decline with age. In: Smith EL & Serfass RC ed. Exercise and aging:
    the scientific basis. Hillside, New Jersey, Enslow Publishers, pp.

    Sonntag WE & Gough ME (1988) Growth hormone releasing hormone
    induced release on growth hormone in aging male rats: dependence on
    pharmacological manipulation and endogenous somatostatin release.
    Neuroendocrinology, 47: 482-488.

    Sonntag WE, Steger RW, Forman LJ, & Meites J (1980) Decreased
    pulsatile release of growth hormone in old male rats. Endocrinology,
    107: 1875-1879.

    Sonntag WE, Hylka V, & Meites J (1985) Growth hormone restores
    protein synthesis in skeletal muscle of old rats. J Gerontol, 40:

    Spagnoli LG, Orlandi A, Manuello A, Santeusanio G, Angelis C,
    Lucreziotti R, & Ramacci MT (1991) Aging and atherosclerosis in the
    rabbit 1. Distribution, prevalence and morphology of atherosclerotic
    lesions. Atherosclerosis, 89: 11-24.

    Spearman ME & Leibman KC (1983) Hepatic and pulmonary cytosolic
    metabolism of epoxides: effects of aging on conjugation with
    glutathione. Life Sci, 33: 2615-2625.

    Spearman ME & Leibman KC (1984) Aging selectivity alters
    glutathione-S-transferase isozyme concentrations in liver and lung
    cytosol. Drug Metab Dispos, 12: 661-671.

    Speijers GJA (1983) [Toxic effects of rapeseed oil, erucic acid and
    linolenic acid in the rat.] Utrecht, The Netherlands, University of
    Utrecht, pp 1-143 (in Dutch).

    Speijers GJA (1989a) [Literature review on experimental
    atherosclerosis research.] Bilthoven, The Netherlands, National
    Institute Public Health and Environmental Protection (in Dutch).

    Speijers GJA (1989b) [Toxicological studies of ergot alkaloids.] De
    Ware(n) Chem, 19(3): 189-194 (in Dutch).

    Spencer PS & Ochoa J (1981) The mammalian peripheral nervous system
    in old age. In: Johnson GJ Jr ed. Aging and cell structure. New
    York, London, Plenum Press, pp 35-103.

    Spencer PS, Nunn PB, Hugon J, Ludolph AC, Ross SM, Roy DN, &
    Robertson RC (1987). Guam amyotrophic lateral
    sclerosis-Parkinsonism-Dementia linked to a plant excitant
    neurotoxin. Science, 237: 465-466.

    Steiner RA, Bremner WJ, Clifton DK, & Dorsa DM (1984) Reduced
    pulsatile luteinizing hormone and testosterone secretion in the male
    rat. Biol Reprod, 31: 251-258.

    Stenbäck F, Peto R, & Shubik P (1981) Initiation and promotion at 
    different age and doses in 2200 mice: III. Linear extrapolation from
    high doses may underestimate low-dose tumor risk. Br J Cancer, 44:

    Stevenson IH & Hosie J (1985) Pharmacokinetics in the elderly. In:
    O'Malley K & Waddington JL ed. Therapeutics in the elderly.
    Amsterdam, Oxford, New York, Elsevier Science Publishers, pp 35-41.

    Stevenson IH, Salem SAM, & Shepherd AMM (1979) Studies on drug
    absorption and metabolism in the elderly. In: Crooks J & Stevenson
    IH ed. Drugs and the elderly. Baltimore, Maryland, University Park
    Press, pp 51-63.

    Stevenson JC, Whitehead MI, Padwick M, Endacott JA, Suttonm C, Banks
    LM, Freemantle C, Spinks TJ, & Hesp R (1988) Dietary intake of
    calcium and postmenopausal bone loss. Br Med J, 297: 15-17.

    Stiles J & Tyler WS (1988) Age-related morphometric differences in
    responses of rat lungs to ozone. Toxicol Appl Pharmacol, 92:

    Stohs SJ, Al-Turk WA, & Angle CR (1982) Glutathione-S-transferase
    and glutathione reductase activities in hepatic and extra-hepatic
    tissues of female mice as a function of age. Biochem Pharmacol, 31:

    Strong R, Moore MA, Hale C, Wessels-Reiker M, Armbrecht HJ, &
    Richardson A (1990) Modulation of tyrosine hydroxylase gene
    expression in the rat adrenal gland by age and reserpine. Brain Res,
    525(1): 126-132.

    Sun J & Strobel HW (1986) Aging affects the drug metabolism systems
    of rat liver, kidney, colon, and lung in a differential fashion. Exp
    Gerontol, 21: 523-534.

    Sun J, Lau PO, & Strobel HW (1986) Aging modifies the expression of
    hepatic microsomal cytochrome P450 after pretreatment of rats with
    b-naphoflavone or phenobarbital. Exp Gerontol, 21: 65-73.

    Sweeny DJ & Weiner M (1985) Metabolism of acetaminophen in
    hepatocytes isolated from mice and rats of various ages. Drug Metab
    Dispos, 13: 377-379.

    Sweeny DJ & Weiner M (1986) Effect of aging on the metabolism of
    p-nitroanisole and p-nitrophenyl in isolated rat hepatocytes. Age,
    9: 95-98.

    Swenberg GJA, Richardson FC, Boucheron JA, Deal FH, Belinsky SA,
    Charbonneau M, & Short BG (1987) High- to low-dose extrapolation:
    critical determinants involved in the dose response of carcinogenic
    substances. Environ Health Perspect, 76: 57-63.

    Swift CG (1985) Benzodiazepine pharmacodynamics in the elderly. In:
    O'Malley K & Stevenson IL ed. Therapeutics in the elderly.
    Amsterdam, Oxford, New York, Elsevier Science Publishers, pp 77-90.

    Tannenbaum SR, Fett D, Young VR, Land PD, & Bruce WR (1978) Nitrite
    and nitrate are formed by endogenous synthesis in the human
    intestine. Science, 200: 1478-1489.

    Tarloff JB, Goldstein RS, Milo BA, & Hook JB (1989a) Role of
    pharmacokinetics and metabolism in the enhanced susceptibility of
    middle-aged Sprague-Dawley rats to acetaminophen nephrotoxicity.
    Drug Metab Dispos, 17: 139-146.

    Tarloff JB, Goldstein RS, Morgan DG, & Hook JB (1989b) Acetaminophen
    and p-aminophenol nephrotoxicity in aging male Sprague-Dawley and
    Fischer 344 rats. Fundam Appl Toxicol, 12: 78-91.

    Taylor A (1989) Associations between nutrition and cataract. Nutr
    Res, 47: 225-234.

    Telford N, Mobbs CV, Osterburg HH, & Finch CE (1988) Alterations in
    hypothalamic serotonergic-catecholaminergic relationships in aging
    C57BL/6J female mice. Exp Gerontol, 23: 481-489.

    Tenover JS, Matsumoto AM, Clifton DK, & Bremner WJ (1988)
    Age-related alterations in the circadian rhythms of pulsatile
    luteinizing hormone and testosterone secretion in healthy men. J
    Gerontol, 43: M163-M169.

    Terry RD (1963) The fine structure of neurofibrillary tangles in
    Alzheimer's disease. J Neuropathol Exp Neurol, 22: 269-292.

    Thakur MD (1984) Age-related changes in the structure and function
    of chromatin: A review. Mech Ageing Dev, 27: 263-286.

    Thakur MK (1988) Molecular mechanism of steroid hormone action
    during aging. A review. Mech Ageing Dev, 45: 93-110.

    Thompson JS, Robbins J, & Cooper JK (1987) Nutrition and immune
    function in the geriatric population. Clin Geriatr Med, 3(2):

    Tice RR & Setlow RB (1985) DNA repair and replication in aging
    organisms and cells. In: Finch CE & Schneider EL ed. Handbook of the
    biology of aging, 2nd ed. New York, Van Nostrand Reinhold Company,
    pp 173-224.

    Timiras PS, Hudson DB, & Segall PE (1984) Lifetime brain serotonin:
    Regional effects of age and precursor availability. Neurobiol Aging,
    5: 235-242.

    Tomlinson BE & Irving D (1977) The numbers of limb motor neurons in
    the human lumbosacral cord throughout life. J Neurol Sci, 34:

    Travaglini P, Mocchegiani E, Togni E, Muratori M, Re T, Bazzani N, &
    Fabris N (1990) Thymulin and zinc circulating level in patient with
    GH and PRL secreting pituitary adenomas. Int J Neurosci, 51:

    Travis CC, White RC, & Ward RC (1990) Interspecies extrapolation of
    pharmacokinetics. J Theor Biol, 142: 285-307.

    Trick LR (1987) Age-related alterations in retinal function. Doc
    Ophthalmol, 65(1): 35-43.

    Triggs EJ & Nation RL (1975) Pharmacokinetics in the aged: A review.
    J Pharmacokinet Biopharm, 3: 387-418.

    Turusov VS & Parfenov YuD (1986) [Methods of revealing and risk
    assessment of chemical carcinogens.] Moscow, Meditsina, pp 134-136
    (in Russian).

    Ulloa-Aguirre A & Chappel SC (1982) Multiple species of
    follicle-stimulating hormone exist within the anterior pituitary of
    male golden hamsters. J Endocrinol, 95: 257-266.

    Ulloa-Aguirre A, Espinoza R, Damian-Matsumura P, & Chappel SC (1988)
    Immunological and biological potencies of the different molecular
    species of gonadotrophins. Hum Reprod, 3: 491-501.

    US Senate Special Committee on Aging (1986) Aging America: Trends
    and projections, 1985-86 edition. Washington, DC, US Government
    Printing Office, pp 14-17.

    Utiger RD (1980) Decreased extrathyroidal triiodothyronine
    production in monthyroidal ilness: benefit or harm? Am J Med, 69:

    Van Bezooijen CFA (1984) Influence of age-related changes in rodent
    liver morphology and physiology on drug metabolism - A review. Mech
    Ageing Dev, 25: 1-22.

    Van Bezooijen CFA, Groen C, Stijnen AM, & Danhof M (1989) The effect
    of age on the pharmacokinetics and pharmacodynamics of drugs in
    rats. Age, 12: 112.

    Vanhaelst L, Van Cauter E, Degaute JP, & Goldstein J (1972)
    Circadian variations of serum thyrotropin levels in man. J Clin
    Endocrinol Metab, 35: 489-492.

    Van Manen R, De Priester W, & Knook DL (1983) Lysosomal activities
    in aging rat liver. I. Variation in enzyme activity within the liver
    lobule. Mech Ageing Dev, 22: 159-165.

    Velican C (1981) Are animal models suitable for the study of
    ischaemic heart disease. Rev Roum Méd Méd Interne, 19(1): 5-19.

    Vellas B, Balas D, Guidet M, Moreau J, Senegas F, Bouisson M,
    Albarede JL, & Ribet A (1988) Vieillissement du pancréas. Son
    implication dans les états de dénutrition chez les personnes agées.
    Presse Méd, 17: 2067-2069.

    Vermeulen A, Deslypere JP, & Kaufman JM (1989) Influence of
    antiopioids on luteinizing hormone pulsatility in aging men. J Clin
    Endocrinol Metab, 68: 68-72.

    Verzar F (1968) Intrinsic and extrinsic factors of molecular aging.
    Exp Gerontol, 3: 69-75.

    Vestal RE (1978) Drug use in the elderly: a review of problems and
    special considerations. Drugs, 16: 358-382.

    Vestal RE, McGuire EA, Tobin JD, Andres R, Norris AH, & Ulzey E
    (1977) Aging and ethanol metabolism. Clin Pharmacol Ther, 21:

    Vestal RE, Wood AJJ, & Shand DG (1979) Reduced b-adrenoceptor
    sensitivity in the elderly. Clin Pharmacol Ther, 26: 181-186.

    Vestal RE, Jue SG, & Cusack BJ (1985) Increased risk of adverse drug
    reactions in the elderly: Fact or myth. In: O'Malley K & Waddington
    JL ed. Therapeutics in the elderly. Amsterdam, Oxford, New York,
    Elsevier Science Publishers, pp 97-104.

    Vijg J (1990) DNA sequence changes in aging: How frequent, how
    important? Aging, 2: 105-123.

    Vijg J & Knook DL (1987) DNA Repair in relation to the aging
    process. J Am Geriatr Soc, 35(6): 532-541.

    Vijg J & Papaconstantinou J (1990) Aging and longevity genes:
    strategies for identifying DNA sequences controlling life span. J
    Gerontol, 45(5): B179-B182.

    Vincenzini MT, Iantomasi T, Stio M, Favelli F, Vanni P, Tonelli F, &
    Treves C (1989) Glucose transport during ageing by human intestinal
    brush-border membrane vesicles. Mech Ageing Dev, 48: 33-41.

    Vogel HB (1983) Effects of age on the biomechanical and biochemical
    properties of rat and human skin. J Soc Cosmet Chem, 34: 453-463.

    Voitenko VP & Tokar AV (1983) The assessment of biological age and
    sex differences of human aging. Exp Aging Res, 9: 239-244.

    Vos JG & Luster MI (1989) Immune alterations. In: Kimbrough RD &
    Jensen AA ed. Halogenated biphenyls, terphenyls, naphtalenes,
    dibenzodioxins and related products. Amsterdam, Oxford, New York,
    Elsevier Science Publishers, pp 295-322.

    Vos JG & Penninks AH (1987) Dioxin and organotin compounds as model
    immunotoxic chemicals. In: De Matteis F & Lock EA ed. Selectively
    and molecular mechanism of toxicity. London, MacMillan Press Ltd, pp

    Vos JG, De Klerk A, Krajnc EI, Van Loveren H, & Rosing J (1990)
    Immunotoxicity of Bis (tri-n-butylin) oxide in the rat; effects on
    thymus-dependent immunity and on non specific resistance following
    long-term exposure in young versus aged rats (1990). Toxicol Appl
    Pharmacol, 105: 144-155.

    Wabner CL & Chen TS (1984) Renal transport of organic acids in the
    aging. Pharmacologist, 26: 206.

    Waddington JL, O'Boyle KM, Molloy AG, Youssef HA, King DJ, & Cooper
    SJ (1985) Neurotransmitter receptors and ageing:
    Dopamine/neuroleptic receptors, involuntary movements and the
    disease process of schizophrenia. In: O'Malley K & Stevenson IL ed.
    Therapeutics in the elderly. Amsterdam, Oxford, New York, Elsevier
    Science Publishers, pp 63-76.

    Waggoner S, Gu MZ, Chiang WH, & Richardson A (1990) The effect of
    dietary restriction on the expression of genes. In: Genetic effects
    of aging, II. Caldwell, New Jersey, Telford Press, pp 255-273.

    Wagner PA, Jernigan JA, Bailey LB, Nickens C, & Brazzi GA (1983)
    Zinc nutriture and cell-mediated immunity in the aged. Int J Vitam
    Nutr Res, 53: 94-101. Walford RL (1969) The immunological theory of
    aging. Copenhagen, Munksgaard.

    Walker DM (1985) Oral disease. In: Pathy MSJ ed. Principles and
    practice of geriatric medicine. New York, Chichester, Brisbane,
    Toronto, John Wiley and Sons.

    Wang SV, Halban PA, & Rowe JW (1988) Effects of aging on insulin
    synthesis and secretion. Differential effects on preproinsulin
    messenger RNA levels, proinsulin biosynthesis, and secretion of
    newly made and preformed insulin in the rat. J Clin Invest, 81(1):

    Ward JM (1983) Increased susceptibility of livers of aged F344/NCr
    rats to the effects of phenobarbital on the incidence, morphology,
    and histochemistry of hepatocellular foci and neoplasms. J Natl
    Cancer Inst, 71: 815-823.

    Ward WF (1988) Enhancement by food restriction of liver protein
    synthesis in the aging Fischer 344 rat. J Gerontol, 43: B50-B53.

    Ward W & Richardson A (1991) Effect of age on liver protein
    synthesis and degradation. Hepatology, 14: 935-948.

    Ward JM, Lynch P, & Riggs C (1988) Rapid development of
    hepatocellular  neoplasms in aging male C3H/HeNCr mice given
    phenobarbital. Cancer Lett, 39: 9-18.

    Warner BA, Dufau ML, & Santen RJ (1985) Effects of aging and illness
    on the pituitary testicular axis in men: Qualitative as well as
    quantitative changes in luteinizing hormone. J Clin Endocrinol
    Metab, 60: 263-268.

    Warner HR, Butler RN, Spratt RW, & Schneider EL (1987) Modern
    biological theories of aging. New York, Raven Press.

    Weale RA (1986) Aging and vision. Vision Res, 26(9): 1507-1512.

    Weber JCP & Griffin JP (1986) Prescriptions, adverse reactions, and
    the elderly. Lancet, i: 1220.

    Webster SGP (1985) Absorption of nutrients in old age. In: Pathy MSJ
    ed. Principles and practice of geriatric medicine. New York,
    Chichester, Brisbane, Toronto, John Wiley and Sons, Chapter 11.2, pp

    Webster IW & Logie AR (1976) A relationship between function and
    health status in female subjects. J Gerontol, 31: 546-550.

    Weigent DA, Carr DJ, & Blalock JE (1990) Bidirectional communication
    between the neuroendocrine and immune system. Common hormones and
    hormone receptors. Ann NY Acad Sci, 579: 17-27.

    Weindruch R & Walford RL (1988). The retardation of aging and
    dietary restriction. Springfield, Illinois, Charles C. Thomas.

    Weindruch R, Gottesman SR, & Walford RL (1982) Modification of
    age-related immune decline in mice dietarily restricted from or
    after mid-adulthood. Proc Natl Acad Sci (USA), 79: 898-902.

    Weiss B, Clark MB, & Greenberg LH (1984) Modulation of
    catecholaminergic receptors during development and aging. In: Lajtha
    A ed. Handbook of neurochemistry. Vol 6:- Receptors in the nervous
    system. New York, London, Plenum Press, pp 595-627.

    Wellinger R & Guigoz Y (1986) The effect of age on the induction of
    tyrosine aminotransferase and tryptophan oxygenase genes by
    physiological stress. Mech Ageing Dev, 34: 203-217.

    Wesson LG Jr (1969) Renal hemodynamics in physiological states. In:
    Physiology of the human kidney. New York, London, Grune and
    Stratton, pp 98-100.

    White L (1989) Epidemiology. In: Maddox GL ed. The encyclopedia of
    aging. Berlin, Heidelberg, New York, Springer-Verlag, pp 216-223.

    WHO (1983) Environmental Health Criteria 27: Guidelines on studies
    in environmental epidemiology. Geneva, World Health Organization,
    351 pp.

    WHO (1984) The uses of epidemiology in the study of the elderly.
    Report of a WHO Scientific Group on the Epidemiology of Aging.
    Geneva, World Health Organization, pp. 23-25 (WHO Technical Report
    Series 706).

    WHO (1987) World health statistics annual. Geneva, World Health

    WHO (1989) Health of the elderly. Report of a WHO Expert Committee.
    Geneva, World Health Organization, pp 14-30 (WHO Technical Report
    Series 779).

    WHO (1990) Global estimates for health situation assessment and
    projections, 1990. Geneva, World Health Organization, Division of
    Epidemiological Surveillance and Health Situation and Trend
    Assessment, p 3.

    Williams ME & Pannill FC (1982) Urinary incontinence in the elderly.
    Ann Intern Med, 97: 895-907.

    Willott JF (1986) Effects of aging, hearing loss, and anatomical
    location on thresholds of inferior colliculus neurons in C57BL/6 and
    CBA mice. J Neurophysiol, 56(2): 391-408.

    Wilson K & Hanson J (1980) The effects of extremes of age on drug
    action. Methods Findings Exp Clin Pharmacol, 2: 303-312.

    Winneke G, Hrdina KG, & Brockhouse A (1982) Neuropsychological
    studies in children with elevated tooth-lead concentrations. A pilot
    study. Int Arch Occup Environ Health, 51: 169-183.

    Wong DF, Wagner HN, & Dannals RF (1984) Effects of age on dopamine
    and seratonin receptors pressured by positron tomography in the
    living human brain. Science, 226: 1393-1396.

    Wood RW (1981) Neurobehavioral toxicity of carbon disulfide.
    Neurobehav Toxicol Teratol, 3: 397-405.

    Wood AJJ (1985) Beta adrenoceptors and aging. In: O'Malley K &
    Stevenson IL ed. Therapeutics in the elderly. Amsterdam, Oxford, New
    York, Elsevier Science Publishers, pp 43-49.

    Wu W, Pahlavani M, Cheung HT, & Richardson A (1986) The effect of
    aging on the expression of interleukin 2 messenger ribonucleic acid.
    Cell Immunol, 100: 224-231.

    Wynne HA, Cope LH, Mutch E, Rawlins MD, Woodhouse KW, & James OF
    (1989) The effect of age upon liver volume and apparent liver blood
    flow on healthy man. Hepatology, 9: 927-301.

    Xiong ZM (1989) [Common diseases and major causes of death in
    elderly patients in Jiangxi.] Chin J Geriatr, 8: 135 (in Chinese).

    Xiong BZ (1990) Technology progress, aging process and reappoint to
    a position.] In: Institute of Japanese Problems, Chinese Academy of
    Social Sciences ed. [Proceeding of Symposium on Sino-Japan Social
    Countermeasure for Aging Problem.] Beijing, East Publishing House,
    pp 186-196 (in Chinese).

    Xu XF, You K, Liu D, He L, Cheng S, & Wu JS (1986) [Disease pattern
    analysis in hospitalized elderly patients in Uromqi.] Chin J
    Geriatr, 5: 15 (in Chinese).

    Yamazoe Y, Shimada M, Murayama N, & Kato R (1987) Suppression of
    levels of phenobarbital-inducible rat-liver cytochrome P-450 by
    pituitary hormone. J Biol Chem, 262: 7423-7428.

    Yang RSH, Tallant MJ, & McKelvey JA (1984) Age-dependent
    pharmacokinetic changes of ethylenediamine in Fischer 344 rats
    parallel to a two-year chronic toxicity study. Fundam Appl Toxicol,
    4: 663-670.

    Yao SB (1990) [The trend and characters of Chinese aged population.]
    In: Institute of Japanese Problems, Chinese Academy of Social
    Sciences ed. [Proceedings of Symposium on Sino-Japan Social
    Countermeasure for Aging Problems.] Beijing, East Publishing House,
    pp 64-74 (in Chinese).

    Yates MS & Hiley CR (1979) The effect of age and cardiac output and
    its distribution in the rat. Experientia (Basel), 35: 78-79.

    Yelland C, Summerbell J, Nicholson E, Herd B, Wynne H, & Woodhouse
    KW, (1991) The association of age with aspirin esterase activity in
    human liver. Age Ageing, 20(1): 16-8.

    York JL (1982) Body water content, ethanol pharmacokinetics, and the
    responsiveness to ethanol in young and old rats. Dev Pharmacol Ther,
    4: 106-116.

    Young VR (1979) Diet as modulator of aging and longevity. Fed Proc,
    38: 1994-2000.

    Yu BP, Bertrand HR, & Masoro EJ (1980) Nutrition-aging influence of
    catecholamine-promoted lipolysis. Metabolism, 29: 438-444.

    Yumura W, Sugino N, Nagasawa R, Kubo S, Hirokawa K, & Maruyama N
    (1989) Age-associated changes in renal glomeruli of mice. Exp
    Gerontol, 24: 237-249.

    Zapadnyuk VI (1971) [On an age-periodisation of laboratory animals.]
    In: [Gerontology and geriatrics. 1970-1971 Annual book: Aging of the
    cell.] Kiev, Research Institute of Gerontology, pp 433-438 (in

    Zarit SH, Eiler J, & Hassinger M (1985) Clinical assessment. In:
    Birren JE & Schaie KW ed. Handbook of the psychology of aging, 2nd
    ed. New York, Van Nostrand Reinhold Company, pp 725-754.

    Zaphiropouos PG, Mode A, Norstedt G, & Gustavsson JA (1989)
    Regulation of sexual differentiation in drug and steroid metabolism.
    Trends Pharmacol Sci, 10: 149-153.

    Zatz MM & Goldstein AL (1985) Thymosins, lymphokines and the
    immunology of aging. Gerontology, 31: 263-277.

    Zemel MB & Sowers JR (1988) Salt sensitivity and systemic
    hypertension in the elderly. Am J Cardiol, 61: 7H-12H.

    Zhou L-W, Weiss B, Freilich JS, & Greenberg LH (1984) Impaired
    recovery of alpha - and alpha2-adrenergic receptors in brain tissue
    of aged rats. J Gerontol, 39: 538-546.

    Zhu JR, Li WX, & Wang YY (1982) [Common diseases and major causes of
    death in the elderly with analysis of 8947 cases over 65 years of
    age.] Chin J Geriatr, 1: 110-118 (in Chinese).

    Zs-Nagy I, Kitani K, Ohta M, & Imahori K (1986) Age-dependent
    decrease in lateral diffusion constant of proteins in the plasma
    membrane of hepatocytes as revealed by fluorescence recovery after
    photobleaching in tissue smears. Arch Gerontol Geriatr, 5: 131-146.

    APPENDIX 1.  Background Papers

    Anisimov, V.N., Approaches to examine the effects of chemicals on
    the aged population: experimental approaches.

    Birnbaum, L.S., Basis of altered sensitivity of the elderly to
    chemicals - pharmacokinetics and pharmacodynamics.

    Cooper, R.L., & Goldman, J.M., Alterations in susceptibility to
    toxic compounds in the aged central nervous system and endocrine

    Dilman, V.M., Theories and mechanics of aging.

    Fabris, N., Systemic biology of aging.

    Ingram, D.K., Evaluating effects of chemical exposure on aging:
    development of conceptual models.

    Li, S., Aged population: demographic, life expectancy, and

    Likhachev, A.J., Age related peculiarities of repair of DNA damage
    with carcinogens.

    Martin, G.M., Definitions on aging.

    Ray, P.K., Jaffery, F.N., & Viswathan, P.N.,

         1.   Chemical exposure of the elderly
         2.   Modifying factors: nutrition, state of health, life style,
              alcohol intake, smoking and ionizing radiation.

    Richardson, A., The molecular biology of aging: translation,
    transcription and chromatin structure.

    Speijers, G.J.A., Groups at risk and their chemical exposure as well
    as nutrition as a source of chemicals and as a confounding factor.

    Zhu, J.R., Approaches to examine the effects of chemicals on the
    aged population: epidemiological and clinical approaches.

    See Also:
       Toxicological Abbreviations