EFFECTS OF AGE AND GENDER ON BROWN FAT AND SKELETAL-MUSCLE METABOLIC RESPONSES TO COLD IN F344 RATS

Older male Fischer 344 (F344) rats do not maintain core temperature as well as do older females during cold exposure. To elucidate factors c ontributing to the decreased thermoregulatory ability of older males, the metabolic potentials of interscapular brown adipose tissue (IBAT) and skeletal muscle were evaluated at rest (26 degrees C) and during 4 h of cold (6 degrees C) in male and female F344 rats, aged 6, 12, and 26 mo. Compared with 26-mo-old females, cold-exposed 26-mo-old males exhibited a greater drop in core temperature and lower amounts of IBAT mitochondrial uncoupling protein (UCP) and IBAT thyroxine 5'-deiodina se (T5'D) activity. Unlike females, 26-mo-old males showed no cold-ind uced increase in total IBAT UCP or T5'D activity. In contrast, plasma norepinephrine was higher in cold-exposed 26-mo-old males vs. females, whereas plasma insulin and thyroxine did not differ with gender. Skel etal muscle oxidative capacity (measured by citrate synthase activity) and carbohydrate availability (measured by muscle glycogen and plasma glucose levels) did not differ between the 26-mo-old males and female s. Our data suggest that altered regulation of IBAT UCP levels during cold exposure of aged rats, due at least in part to attenuated cold-in duced IBAT T5'D activity, contributes to the gender difference in ther moregulatory ability of older males vs. females.