Baroreflex control of arterial blood pressure during involuntary diving in ducks (Anas platyrhynchos var.)

The dynamic role of arterial baroreceptors in control of mean arterial blood pressure (MAP), heart rate (HR), cardiac output (CO), hindlimb vascular (HLVR) and total peripheral (TPR) resistance responses to forced dives was investigated in acutely and chronically barodenervated ducks. To activate the baroreflex, the proximal end of one aortic nerve was stimulated electrically with bipolar electrodes that had been implanted under pentobarbital sodium anesthesia. Predive nerve stimulation caused CO to fall (by reducing HR; stroke volume remained constant), producing a decrease in MAP to half the prestimulation level. During diving (for 2.5-min periods) nerve stimulation did not affect HR and MAP after the first minute of submersion. Neither HLVR nor TPR contributed to the fall in MAP during aortic nerve stimulation before or during diving. The effects of nerve stimulation on HR and MAP were maintained to the end of dives in animals given 100% O₂ to breathe before diving. In separate experiments, increasing arterial chemoreceptor input by perfusing one vascularly isolated carotid body with venous blood caused a reduction in the effects of aortic nerve stimulation on MAP. Arterial baroreceptors may thus act on HR to alter MAP early in the dive, but as the dive progresses the baroreflex is attenuated by an increase in peripheral chemoreceptor drive.