Effect of hypoxia on vascular responses to the carotid baroreflex

The effect of systemic hypoxia on the vascular responses to the carotid baroreflex was studied in anesthetized, vagotomized, artificially ventilated dogs. One hindlimb, kidney, gracilis muscle, and paw were perfused at constant flow, and neurograms were obtained from renal sympathetic fibers. Bilateral carotid occlusions were performed while the animal was breathing a mixture of air and O2 (mean arterial PO2 = 106 mmHg) and again during ventilation with 10% O2 (PO2 = 40 mmHg). With occlusion, the average increase in mean aortic pressure was 36 mmHg greater during hypoxia than during normoxia and the increase in renal perfusion pressure was 87 mmHg greater; the increase in hindlimb perfusion pressure was identical in both situations. Hypoxia did not change the reflex response of the paw to carotid occlusion and increased that of the muscle vessels by only 10%; the increase in renal sympathetic activity averaged 56 plus or minus 10% more with hypoxia than with normoxia. When the carotid chemoreceptors were destroyed, the greater increase in aortic and renal pressure response to carotid occlusion during hypoxia as compared to normoxia was abolished. Thus systemic hypoxia markedly potentiates the reflex renal constriction caused by the baroreflex, and this effect is due to the carotid chemoreceptor afferent input.