Age Changes in Rhythms of Energy Metabolism, Activity, and Body Temperature in Mus and Peromyscus
The decrease in resting metabolic rate with age for experimental animals and man is, as someone has remarked, the most over-confirmed fact in gerontology (Pettegrew and Ewing, 1971; Grad, 1953; Kleiber et al., 1956; Schock and Yiengst, 1955; Keys et al., 1963). However, if one looks at the information available on age changes in the metabolism of activity, in respect to either its amount or its temporal organization within the diel cycle, the scene is much more sparsely populated. The qualitative registration of motor activity, sufficient to distinguish between the active and inactive phases of the circadian cycle, has, of course, been extensively used for the measurement of the circadian periods of experimental animals, and one investigation has appeared on the change in length of the circadian period with age in Peromyscus (Pittendrigh and Daan, 1974). However, those studies are not closely related to the objective of the present paper, which is to examine the modulation of energy metabolism, motor activity, and body temperature over the diel cycle as a function of age. For this purpose we need quantitative, closely spaced measurements of at least three variables: (a) the kinetic energy of motor activity; (b) rate of energy expenditure, measured indirectly by rate of oxygen consumption or carbon dioxide production; and (c) deep body temperature, measured by radiotelemetry. Two additional variables would be valuable but cannot yet be conveniently measured on the unconstrained animal: skin temperature and rate of heat loss. Monitoring of diel cycles of body temperature and/or oxygen consumption is widely employed for ecological energy budgeting, analysis of thermoregulation and hibernation studies (Heusner et al., 1971; Kayser and Hildwein, 1969; Heusner, 1957), and for research on circadian rhythm (Ehret et al., this volume), but we have no information about the diel cycles for any of the aforementioned variables in relation to aging.
KeywordsBody Temperature Metabolic Rate Motor Activity Metabolism Module Diel Cycle
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