Neural and humoral effects on hindlimb vascular resistance of ducks during forced submergence

Circulating catecholamines increase by three orders of magnitude in forced submerged ducks; their role in promoting vascular resistance was studied using vascular perfusion of hindlimbs. Perfusion pressure and blood flow were recorded simultaneously in hindlimbs of ducks during forced submergence. One leg was perfused with blood mixtures devoid of catecholamines (test leg) and compared with the other autoperfused leg. Perfusion pressures in test legs perfused with hyperoxic and hypocapnic blood or with hypoxic and hypocapnic blood were not significantly different from those in the matching autoperfused legs. However, when test legs were perfused with hypercapnic blood, regardless of whether blood was hypoxic or hyperoxic, perfusion pressures were significantly lower than in autoperfused legs. Adding catecholamines to test legs perfused with hypoxic and hypercapnic blood, after 3 min submergence, significantly enhanced vasoconstriction. Hypercapnia acts directly on the peripheral vascular beds and impairs the neurally mediated vasoconstriction during submergence, while circulating catecholamines restore peripheral vasoconstriction. Thus circulating catecholamines are needed to compensate for the depressant action of hypercapnia on neurally mediated vasoconstriction.