Carotid and aortic baroreceptor control of heart rate in conscious monkey

This study looks at the difference between the aortic and the carotid baroreceptors in controlling the heart rate of conscious monkeys in the cage vs. the laboratory. Macaca mulatta and M. radiata underwent sequential baroreceptor denervation by first stripping the aortic arch, then one carotid sinus, and finally the second carotid sinus [sinoaortic denervation (SAD)]. Four monkeys had the carotid sinuses denervated before denervation of the aortic arch. Mean arterial blood pressure (MABP)-heart rate (HR) curves were determined by infusing nitroprusside and phenylephrine hydrochloride. The peak sensitivity (PS) was significantly less for all animals (control and denervated) in the laboratory than in the cage (P less than 0.05) with the exception of the SAD animals, which showed no difference. There was no significant difference in sensitivity between control and aortic-denervated animals in the cage. The PS decreased significantly after denervation of aortic arch and one carotid sinus. Monkeys with only the aortic baroreceptors intact had gains for the baroreflex curves that were similar to that of the SAD animals. After complete SAD the PS were essentially zero. We concluded that 1) the maximum slope of the MABP-HR reflex is greater when the monkey is in the cage than it is in the laboratory, probably resulting from an increase in the sympathetic tone; and 2) the carotid baroreceptors exert a greater influence on the HR component of the baroreflex in the conscious monkey than do the aortic baroreceptors.