THERMOREGULATION WITH AGE - ROLE OF THERMOGENESIS AND UNCOUPLING PROTEIN EXPRESSION IN BROWN ADIPOSE-TISSUE

To investigate whether attenuation of thermogenesis in interscapular b rown adipose tissue (IBAT) may account for the loss of thermoregulatio n with age, we examined two indices of thermogenesis after two types o f cold exposure: one in which the senescent rats maintained homeotherm y and the other in which the senescent rats became hypothermic. To thi s end, we assessed body temperature, guanosine 5'diphosphate (GDP) bin ding to the IBAT mitochondrial uncoupling protein (UCP) and the induct ion of UCP mRNA after both 1-hr and 48-hr mild cold exposures at 8 deg rees C and after a more severe, 1-hr cold exposure at 4 degrees C in 3 - and 24-month-old F-344 rats. Thermoneutrality was determined to occu r at an ambient temperature of 26 degrees C in rats of both ages. In t he 1-hr mild cold-exposed rats, there was no significant increase in G DP binding to IBAT UCP. However, after 48 hr of mild cold exposure, th ere was a 3-fold increase in GDP binding and a 5 fold increase in the expression of UCP mRNA despite no hypothermia in either the young or o ld rats. During the more severe cold exposure, the senescent rats, but not the young rats, became hypothermic. GDP binding to UCP increased 75% following cold exposure and, surprisingly was the same in young an d old rats. UCP transcripts did not increase during the 1-hr cold expo sure. These data, coupled with our previous findings of diminished bet a(3)-agonist-stimulated IBAT thermogenesis, suggest that (i) IBAT ther mogenesis, at least in the senescent rats, may be mediated by other th an beta(3)-adrenergic receptors, and (ii) that altered heat dissipatio n or impaired thermogenesis at some site other than Interscapular BAT is responsible for the observed hypothermia.