Diving responses in ducks after acute barodenervation

The contribution of systemic arterial baroreceptors to the cardiovascular adjustments to diving has been investigated in unanesthetized ducks after acute baroreceptor denervation. Both intact and denervated ducks exhibited bradycardia on diving, although denervated ducks showed a lesser and more variable fall in heart rate. Hindlimb vascular resistance rose significantly in both intact and denervated ducks. Continuous stimulation of one depressor nerve, through miniature electrodes implanted on the cut central end, resulted in a diving bradycardia intermediate between that recorded in intact and denervated animals. Intermittent stimulation of the depressor nerve, for 20-s periods, at intensities high enough to cause a large fall in mean arterial pressure (MAP) predive caused a smaller reduction in MAP as a dive was prolonged, due to a large decline in the ability of the baroreceptor reflex to affect peripheral resistance. There was no change in the effect of stimulation on cardiac control before or during diving. The present experiments indicate that a barostatic reflex, which exerts its effects primarily through cardiac control and not control of total peripheral resistance, is active through the dive but that the majority of the diving response in ducks is independent of baroreceptor integrity.