SKELETAL-MUSCLE AS THE MAJOR SITE OF NONSHIVERING THERMOGENESIS IN COLD-ACCLIMATED DUCKLINGS

Despite their lack of brown adipose tissue, 6-wk-old cold-acclimated m uscovy ducklings (4-degrees-C; CA) exhibit nonshivering thermogenesis (NST) in the cold. To determine the site of this NST, the regional dis tribution of blood flow was measured by the microsphere method in the thermoneutral zone (25-degrees-C) and during acute exposure to cold (8 -degrees-C). Ducklings reared at thermal neutrality (TN), which use sh ivering to produce extra heat in the cold, were compared with CA duckl ings, which substitute NST for shivering. Further, the contribution of skeletal muscle thermogenesis to the increased heat production in the cold was estimated by measuring leg muscle blood flow and arterioveno us difference in oxygen content [(a-v)O2] across the leg, enabling an estimation of muscle O2 consumption. During cold exposure, a similar i ncrease in total leg muscle blood flow occurred in TN and CA ducklings (+127 and +130% respectively), while hepatic arterial blood flow incr eased less (+56 to +37%, respectively). This rise in blood flow was ac counted for by an increase in cardiac output, which was smaller in CA than in TN ducklings, and in both groups by a redistribution of blood flow to the most thermogenic organs (skeletal muscles and liver). The (a-v)O2 across the leg was not changed by cold exposure, indicating th at the increase in leg muscle O2 consumption resulted mainly from the increase in blood flow. From the estimation of leg muscle O2 consumpti on in the cold, an extrapolation to total skeletal muscle suggests tha t in TN ducklings 84% of the cold-induced thermogenesis of the whole b ody in vivo could be attributed to shivering skeletal muscle. On the o ther hand, in CA ducklings, in the absence of shivering, an estimated 70% of the total cold-induced NST may be attributed to the same tissue . These results indicate a major role for skeletal muscle in avian NST .