Time-dependent responses of brown fat in cold-exposed rats

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Abstract
Brown fat of rats exposed to cold for 1 hr. to 60 +days has been examined for time-dependent changes in mass, composition, qO2 ([mu]1 02/mg N per hr.) and estimated caloric output. In both homogenate and mitochondrial (Mw) qo2 was depressed in rats exposed to cold for 3 hr. In the Mw this returned to control values by 6 hr. and in homogenates by 12-24 hr. Near maximum qo2''s were reached in all systems by 8 days. Brown fat mass increased on exposure to cold and reached near maximum levels by 8 days, whereas the content (mg N/g) continued to increase throughout the period of exposure. Estimated heat production of brown fat from 60-day cold-acclimated rats was some 6 to 8 times higher than the control levels. The coincidence between the development of the increased potential for heat production in brown adipose tissue and the onset of non-shivering thermogenesis in the cold-exposed rat is discussed.