LIPID METABOLISM IN WARM-ACCLIMATED AND COLD-ACCLIMATED RATS EXPOSED TO COLD

Abstract

The turnover of triglycerides and phospholipids of various tissues was studied in warm-acclimated and cold-acclimated rats, exposed to either a cold or a warm environment, by injecting glucose-U-C¹⁴intraperitoneally and measuring its incorporation into glycerol, glycerophosphate, and fatty acid portions of triglycerides and phospholipids at various times after the injection. The incorporation of glucose carbon into brown (interscapular) adipose tissue lipids (principally into glyceride glycerol) was 10 to 50 times greater in cold-acclimated rats in the cold than in warm-acclimated rats in the warm. The results are interpreted to indicate a great acceleration of the triglyceride cycle (triglyceride hydrolysis and resynthesis with concomitant ADP formation, the latter occasioning an increase in the activity of the electron transport system) in brown adipose tissue of cold-acclimated rats in the cold. Other possible interpretations are discussed. This acceleration was not observed in cold-acclimated rats in the warm or in warm-acclimated rats in the cold. No evidence was obtained for acceleration of triglyceride cycle in liver, heart, skeletal muscle, or white (epididymal) adipose tissue. The hypothesis is advanced that acceleration of the triglyceride cycle of brown adipose tissue is a major mechanism of nonshivering thermogenesis in cold-acclimated rats; such a mechanism is known to be under the control of the sympathetic nervous system and is consistent with the requirement that non-shivering thermogenesis can be switched on and off in response to changes in the environmental temperature.