The role of insulin in nonshivering thermogenesis

The regulation of nonshivering thermogenesis by insulin was studied in cold-acclimated rats (2 weeks at 5 degrees C) made diabetic after injection with streptozotocin (75 mg/kg, i.p.) and maintained in the warm (25 degrees C) for 2-7 days. To investigate whether thermogenesis was activated in brown adipose tissue (BAT) of diabetic rats under physiological conditions, conscious rats were briefly exposed to cold (2 h at 5 degrees C) and the temperature of interscapular BAT (Tbat) was compared with the colonic temperature (Tcol). It was found that Tbat, Tcol, and Tbat-Tcol (an index of thermogenesis activation in BAT) were significantly reduced in 7-day diabetic rats (P less than 0.01) but not in 2-day diabetic animals, suggesting that diabetes progressively decreases BAT thermogenic capacity. To further assess whether the maximal capacity of BAT for nonshivering thermogenesis was affected by the lack of insulin, the calorigenic response to noradrenaline (0.4 mg/kg, i.m.) was determined at 25 degrees C in anesthetized animals using an open circuit respirometer. The results showed that the calorigenic response to noradrenaline was inhibited by 50 and 70% in 2- and 7-day diabetic rats, respectively. Significantly, tissue weight, protein content, and cytochrome oxidase activity of interscapular BAT were also decreased by 30-40% and 50-70% in the same animals. Insulin treatment of 3-day diabetic rats for 4 days (6.5 U/day per rat) restored their calorigenic response to noradrenaline to control levels.(ABSTRACT TRUNCATED AT 250 WORDS)