Responses of Skeletal Musculature and its Vasculature During “Diving” in the Duck: Peculiarities of the Adrenergic Vasoconstrictor Innervation

The vasoconstrictor response to “diving” was simulated in pentobarbital‐anesthetized ducks by administration of 20 % CO 2 combined with vagotomy and controlled bleeding. Muscle blood flow, 30–50 ml/min ×. 100 g, at rest, could by reflex vasoconstriction be almost stopped and was then barely increased by exercise; in the absence of vasoconstriction, however, exercise increased flow to about 150 ml. In cats exercise produced considerable flow increases also during vasoconstriction; further, maximal flow reached only some 50 ml. Simultaneous recordings of venous outflow and pressures in the femoral artery and a small (0.3–0.5 mm) extramuscular artery revealed that large extramuscular arteries constrict far more in ducks than in cats and then fail to manifest ascending dilatation. The femoral artery has a far richer supply of adrenergic nerve terminals in ducks than in cats. In all the above respects, turkeys were more similar to cats than to ducks. In duck muscles maximal contractile force increased during CO 2 breathing due to augmentation via the sympathetic innervation (Orbeli effect), an effect mimicked by intra‐arterial catechol amines. The ability of diving animals to maintain muscle vasoconstriction during exercise while diving may well reside in the differently balanced vascular innervation with intense constrictions of extramuscular vessels. The sympathetic augmentation of contractility may partly compensate for the effect of flow reduction. The larger maximal flow capacity and capillary area in duck muscles facilitates rapid post‐dive restitution.