Endocrine stimulation of muscle nonshivering thermogenesis (NST) in ducklings was investigated in vitro using a perfused hindlimb preparation maintained at 25°C. Effects of flow rate, norepinephrine (NE), epinephrine, and glucagon on perfused muscle oxygen consumption (M˙o 2) and perfusion pressure were studied. Control ducklings ( Cairina moschata, 5 wk old) reared at thermoneutrality (25°C, TN) were compared with two age-matched groups exhibiting muscle NST in vivo: cold-acclimated ducklings (4°C, 4 wk, CA) and glucagon-treated ducklings (103 nmol/kg twice daily, intraperitoneally, GT). BasalM˙o 2was higher in CA than in TN or GT ducklings and increased in all groups with elevated flow rates. Catecholamines increased bothM˙o 2and perfusion pressure. The maximal effect onM˙o 2was higher in CA (+36%) and GT ducklings (+43%) than in controls (+31%), but was associated with reduced vasoconstriction. Flow rate did not consistently potentiate the NE response. At high doses, catecholamines became inhibitory onM˙o 2while a monotonous increase of pressure was still observed. Glucagon, by contrast, slightly decreased bothM˙o 2and pressure. This vasodilatory effect was greater in CA ducklings than controls in preconstricted preparations. In vivo, low-dose epinephrine induced a modest thermogenic effect (+10%) in CA ducklings. These findings showed that duckling muscle thermogenesis is directly stimulated in vitro by catecholamines but not by glucagon. Higher in vitro thermogenic effects of NE in ducklings that were expected to exhibit muscle NST in vivo suggests catecholamine involvement in muscle NST in vivo. Potential vascular control of avian muscle NST is discussed.

The characteristics and modulation of glucose transport were investigated in skeletal muscles of 5-wk-old Muscovy ducklings (Cairina moschata). Glucose uptake by sarcolemmal vesicles isolated from gastrocnemius muscle followed typical Michaelis-Menten kinetics with a K(m) value (17 mM) similar to that described in equivalent mammalian preparations. Western blot analysis of duckling sarcolemma using antibodies directed against rat GLUT-4 transporter revealed an immunoreactive protein of similar molecular mass (45 kDa) to that present in rats. When ducklings were killed in the postabsorptive state, GLUT-4 homologous protein was located predominantly (80%) in intracellular membranes. Insulin stimulation of a perfused leg muscle preparation in vitro led to the translocation of GLUT-4 homologous proteins from intracellular pools to the sarcolemma, with a subsequent increase in glucose uptake by sarcolemmal vesicles and perfused muscles. Glucose transport was positively controlled by the metabolic needs of skeletal muscle as reflected by the increased glucose uptake of sarcolemmal vesicles isolated from cold-acclimated ducklings. Present results, therefore, demonstrate, for the first time in an avian species, the existence in skeletal muscle of a glucose transporter showing molecular and functional homologies with the mammalian GLUT-4 transporter.