Effects of acute and chronic baroreceptor denervation on diving responses in ducks

To investigate the effects of barodenervation on the cardiovascular responses to forced submersion, we implanted snares around the aortic nerves innervating the baroreceptors in anesthetized (pentobarbital sodium, 35-45 mg/kg) ducks (Anas platyrhynchos). Snares were withdrawn 1 wk after surgery to effect barodenervation in the absence of complications from surgical trauma or anesthesia. After barodenervation, resting heart rate (HR), hind limb vascular resistance (HLVR), and mean arterial pressure (MAP) all increased, with different time courses, over the 16-day observation period. Bradycardia developed during forced submersion in ducks before and after barodenervation, although this response was reduced for 1-3 days after nerve section. The HR response to diving intensified 4-5 days after denervation, and the level of dive bradycardia at 16 days was similar to that before denervation. End-dive MAP fell further, as a proportion of predive MAP, with time after baroreceptor loss. The absolute level of HLVR reached during diving was unaffected by barodenervation. These results show that diving responses develop in the absence of baroreceptors, but MAP in the dive is not maintained as well in barodenervates as in baroreceptor-intact animals.