Relationship between cold-induced thermoregulation and spontaneous rapid body weight loss of aging F344 rats

We previously showed that, although cold-induced thermoregulation is attenuated in 26-mo-old male Fischer 344 (F344) rats, not all rats this age exhibit the same degree of cold-exposed hypothermia or diminished brown adipose tissue nonshivering thermogenic capacity. Examination of this heterogeneity suggested the hypothesis that it was associated with a difference in the physiological state between aged rats that were maintaining stable body weight versus those showing the rapid weight loss often occurring near the end of the rat's natural life span. To test this, we acutely exposed male F344 rats to cold (4 h at 6°C) beginning at 24 mo of age. This exposure was weekly for the first 2 wk and then on alternate weeks as long as the rat's body weight was stable. If body weight progressively declined for 3-5 consecutive days, the rat's response to the acute cold exposure was again measured, as was that of two additional rats not displaying this rapid loss in body weight. If body temperature decreased during the cold exposure to intraperitoneal temperatures ≤32.5°C, the rat was killed with pentobarbital sodium and interscapular brown adipose tissue was removed. One of the age- matched controls was also killed at this time. The age at which body weight showed a spontaneous rapid decline ranged from 24.5 to 29 mos. All eight rats displaying spontaneous rapid weight loss had significant hypothermia during the acute cold exposure, whereas none of the eight weight-stable rats did. The development of hypothermia in the spontaneous rapid weight loss group was not, in general, observed before their weight loss. The weight loss and hypothermia were associated with lower levels of brown fat uncoupling protein and significant changes in body fat and protein. These data suggest that the development of senescence-related hypothermia occurs rapidly and is not a simple function of chronological age or the median life span of the animals. Furthermore, these data imply that the rate of aging in terms of maintenance of thermoregulatory homeostasis has both a gradual and rapid component, the latter being associated with a different physiological state than the former.