RAPID CHANGES IN METABOLIC COLD DEFENSE AND GDP BINDING TO BROWN ADIPOSE-TISSUE MITOCHONDRIA OF RAT PUPS

To determine developmental changes of brown adipose tissue (BAT) therm ogenic activity at defined circadian and thermal states, we evaluated the time course of cold-induced increases of in vitro guanosine 5'-dip hosphate (GDP) binding in parallel with whole body metabolism (oxygen consumption, VO2) and core temperature (T(c)) in 1- to 11-day-old rat pups. During the maximum phase of the juvenile diurnal cycle, T(c) of littermates was recorded continuously and VO2 alternately until 2 min before animals were killed for removal of interscapular BAT. GDP bindi ng after 1.5 h at thermoneutrality and its increase during physiologic ally comparable cold loads were significantly lower in 1-day-old pups than in 5- and 11-day-old pups. Cold defense was activated more rapidl y in the older pups, but GDP binding in even the 1-day-old pups was si gnificantly increased during the second 10-min period of cold exposure . We conclude that rapid changes in thermogenic activity, in connectio n with the known developmental changes in the dependence of the suckli ng rat's metabolic cold defense on maternal and sibling contact and ci rcadian phase, will distort longitudinal studies of any fast-changing BAT parameter when the conditions immediately before tissue removal ar e not thoroughly controlled.