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The inability to maintain body temperature and a selective pattern of changes in the regulation of cell activities were revealed by briefly exposing ageing C57B1/6J male mice to cold (10°C). The induction of liver tyrosine aminotransferase (TAT) during exposure to cold (a gene-dependent process) was markedly delayed in senescent mice (26 months old) as compared with younger mice (3–16 months old); after the delay, the rate of increase of TAT was similar to that prevailing in younger mice. Direct challenge of the liver with injections of corticosterone or insulin elicited the induction of TAT on an identical time course in young and senescent mice. These experiments provide an example of an age change in a gene-dependent cell process (the delayed induction of TAT in senescent mice during exposure to cold) which is not due to a change in the potential of the genome for responding when exogenous stimulae are supplied (injection of hormones). In contrast to the age-related change in liver cell activities, no significant changes were found in the secretion of corticosterone during exposure to cold. Although the seat of these selective age-related changes in the regulation of cell activities remains unclear, it is argued that generalized damage to the genome of cells throughout the body is not involved. The results of this and other studies showing the selective effect of age on cell activities are considered in terms of the concept that many cellular age changes represent the response of cells to primary age-related changes in humoral factors in the internal environment of the body.
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