Thermogenic responses of younger and older rats to cold exposure: comparison of two strains.

Oxygen consumption and colonic temperatures at rest and during 6 hours at 6 degrees C, body composition, adipose tissue cellularity, and the binding of guanosine 5'-diphosphate (GDP) to isolated brown fat mitochondria were measured in younger (12 months) and older (24 months) male Fischer 344 (F344) and Osborne-Mendel (O-M) rats. The rates of resting mass-independent oxygen consumption, the total cold-induced oxygen consumption expressed as body mass-independent (L.kg body mass-.67) or as lean body mass-independent (L.kg lean body mass-.67) did not differ among the four groups. Although the absolute (g) and the percentage of carcass mass as lean body tissue did not differ within the strains, these measures were significantly greater in the younger and older O-M rats compared to the F344 counterparts. In contrast, cold-exposed O-M rats showed significantly greater total specific GDP binding to isolated brown fat mitochondria than did the O-M animals (an index of brown fat thermogenic capacity); GDP binding did not differ between 12 and 24 months in either strain except when expressed in terms of body mass to the 0.67 power. In general, the results presented here showed no significant age-related loss in oxygen consumption in either strain. However, there did appear to be strain-related differences in brown fat mitochondrial GDP binding, and body composition.