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 sou ght in control and cold-acclimated (CA) ducklings. Twenty-four 6-wk-ol d muscovy ducklings were reared either at thermoneutrality (TN) (25-de grees-C) or in the cold (4-degrees-C) from the age of 1 wk. Glucagon-i nduced calorigenesis (GIC) was estimated at 25-degrees-C after a perit oneal glucagon injection (103 nmol/kg). Glucagon induced a higher incr ease in animal heat production (indirect calorimetry) and body tempera ture 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 cle arance method) and temperature of gastrocnemius increased (+130% and 1.0-degrees-C) shortly after glucagon, whereas tissue oxygenation (pol arography) decreased (-34%). There was no significant effect of glucag on in TN controls. These changes, which peaked 45-60 min after glucago n injection, indicated a prolonged increase of muscle O2 consumption i n CA ducklings. Leg muscle blood flow (radioactive microspheres) measu red 45 min after glucagon was slightly increased in controls (+20%; P < 0.05), while in CA ducklings, the increase was larger (+76%; P < 0.0 5). The arteriovenous difference in O2 content was not markedly affect ed by glucagon in both groups. These parameters indicated an increase in leg muscle O2 uptake in response to glucagon of +29% in controls an d +76% in CA ducklings. In controls, 28% of the GIC measured in vivo c ould be attributed to whole body skeletal muscles, compared with 53% i n CA ducklings. The remaining part might be accounted for mostly by th e liver. In CA ducklings, the increase over control ducklings of GIC i n muscles can account for all their extra whole body GIC, showing that skeletal muscle is a major site of the GIC in CA ducklings.