TISSUE DISTRIBUTION OF COLD-INDUCED THERMOGENESIS IN CONSCIOUS WARM-ACCLIMATED COLD-ACCLIMATED RATS RE-EVALUATED FROM CHANGES IN TISSUE BLOOD-FLOW - DOMINANT ROLE OF BROWN ADIPOSE-TISSUE IN THE REPLACEMENT OF SHIVERING BY NONSHIVERING THERMOGENESIS

Radioactive microspheres (12-16 μm) were used to measure cardiac output (CO), its fractional distribution, and hence tissue blood flow in conscious, warm acclimated (WA) or cold acclimated (CA) white rats exposed to temperatures of 25, 21, 6, -6, and -19°C, the objective being to assess the tissue distribution of cold induced thermogenesis. Total oxygen consumption was also measured. CA rats at 25°C (CA25) had elevated arteriovenous shunting and other signs of heat stress. CA21 proved more suitable controls for the CA group. The cold induced changes in blood fow to total skeletal muscle not involved in respiratory movements (M) and to the major masses of brown adipose tissue (BAT) were quantitatively very different in the two acclimation groups: in WA25 and CA21 flows to M were 31 (0.24 CO) and 27 (0.17 CO) mL/min, respectively, while flows to BAT were 2.1 and 9.7 mL/min; in WA(-19) and CA(-19) flows to M were 62 0.32 CO) and 35 (0.16 CO) mL/min, respectively, while flows to BAT were 25 and 56 mL/min. In contrast, the effects of cold exposure on flows to other tissues and organs were remarkably alike in the two acclimation groups: e.g., flows to heart, ribcage, and diaphragm increased about three times being 25 and -19°C, flow to the skin fell about 50%, and flows to the hepatosplanchnic region and kidneys were little or not at all affected by cold exposure. Estimates of the contributions of different tissues and organs to cold induced thermogenesis were made on the basis of the relative changes in blood flow. It is concluded that BAT is by far the dominant anatomical site of the increased heat production of cold exposed CA rats, and that nonshivering thermogenesis in BAT supplements considerably the shivering thermogenesis of cold exposed WA rats.