HORMONAL-CONTROL OF THERMOGENESIS IN PERFUSED MUSCLE OF MUSCOVY DUCKLINGS

Endocrine stimulation of muscle nonshivering thermogenesis (NST) in du cklings was investigated in vitro using a perfused hindlimb preparatio n maintained at: 25 degrees C. Effects of flow rate. norepinephrine (N E), epinephrine, and glucagon on perfused muscle oxygen consumption (M O2) and perfusion pressure were studied. Control ducklings (Cairina mo schata, 5 wk old) reared at thermoneutrality (25 degrees C, TN) were c ompared with two age-matched groups exhibiting muscle NST in vivo: col d-acclimated ducklings (4 degrees C, 4 wk, CA) and glucagon-treated du cklings (103 nmol/kg twice daily, intraperitoneally, GT). Basal MO2 wa s higher in CA than in TN or GT ducklings and increased in all groups with elevated flow rates. Catecholamines increased both MO2 and perfus ion pressure. The maximal effect on MO2 was higher in CA (+36%) and GT ducklings (+43%) than in controls (+31%), but was associated with red uced vasoconstriction. Flow rate did not consistently potentiate the N E response. At high doses, catecholamines became inhibitory on MO2 whi le a monotonous increase of pressure was still observed. Glucagon, by contrast, slightly decreased both Mo-2 and pressure. This vasodilatory effect was greater Fn CA ducklings than controls in preconstricted pr eparations. In vivo, low-dose epinephrine induced a modest thermogenic effect (+10%) in CA ducklings. These findings showed that duckling mu scle thermogenesis is directly stimulated in vitro by catecholamines b ut not by glucagon. Higher in vitro thermogenic effects of NE in duckl ings that were expected to exhibit muscle NST in vivo suggests catecho lamine involvement in muscle NST in vivo. Potential vascular control o f avian muscle NST is discussed.