Oxidation of plasma free fatty acids in nonshivering thermogenesis of rats treated chronically with norepinephrine and thyroxine.

In rats treated chronically with norepinephrine and thyroxine, enhanced nonshivering thermogenesis was observed, with a concomitant increase in the utilization of plasma free fatty acids (FFA). The increased utilization of plasma FFA was characterized by a more preferential increment in the oxidation of FFA in tissues rather than that in the uptake of FFA by tissues. These biochemical findings were accompanied by an increased mitochondrial mass in red fibers of skeletal muscle. Both biochemical and morphological changes of a similar nature were found in cold-acclimated rats.