INCREASED ROLE OF SKELETAL-MUSCLE IN THE CALORIGENIC RESPONSE TO GLUCAGON OF COLD-ACCLIMATED DUCKLINGS

The site of the calorigenesis observed in birds after glucagon was sought in control and cold-acclimated (CA) ducklings. Twenty-four 6-wk-old muscovy ducklings were reared either at thermoneutrality (TN) (25-degrees-C) or in the cold (4-degrees-C) from the age of 1 wk. Glucagon-induced calorigenesis (GIC) was estimated at 25-degrees-C after a peritoneal glucagon injection (103 nmol/kg). Glucagon induced a higher increase in animal heat production (indirect calorimetry) and body temperature in CA (+45 % and + 1.1-degrees-C) than in control ducklings (+30% and +0.4-degrees-C). In CA ducklings, the perfusion rate (thermal clearance method) and temperature of gastrocnemius increased (+130% and +1.0-degrees-C) shortly after glucagon, whereas tissue oxygenation (polarography) decreased (-34%). There was no significant effect of glucagon in TN controls. These changes, which peaked 45-60 min after glucagon injection, indicated a prolonged increase of muscle O2 consumption in CA ducklings. Leg muscle blood flow (radioactive microspheres) measured 45 min after glucagon was slightly increased in controls (+20%; P < 0.05), while in CA ducklings, the increase was larger (+76%; P < 0.05). The arteriovenous difference in O2 content was not markedly affected by glucagon in both groups. These parameters indicated an increase in leg muscle O2 uptake in response to glucagon of +29% in controls and +76% in CA ducklings. In controls, 28% of the GIC measured in vivo could be attributed to whole body skeletal muscles, compared with 53% in CA ducklings. The remaining part might be accounted for mostly by the liver. In CA ducklings, the increase over control ducklings of GIC in muscles can account for all their extra whole body GIC, showing that skeletal muscle is a major site of the GIC in CA ducklings.