Differential baroreflex control of heart rate and vascular resistance in rabbits. Relative role of carotid, aortic, and cardiopulmonary baroreceptors.

We assessed the relative roles of aortic (ABR), carotid sinus (CBR), and vagal cardiopulmonary baroreceptors in the reflex control of heart rate and vascular resistance during changes in arterial blood pressure. Injections of phenylephrine (PE) and nitroglycerin (NG) were given intravenously to anesthetized rabbits (chloralose-urethane). Reflex, heart rate responses were impaired significantly by denervation (X) of either CBR or ABR. In contrast, reflex vascular responses in the hindlimb (perfused at constant blood flow) were preserved except for a slight impairment of reflex vasoconstriction after ABRX. Vagotomy with intact CBR and ABR impaired only the reflex bradycardia. After vagotomy, neither CBRX nor ABRX altered significantly the reflex heart rate or vascular responses except, again, for an impairment of reflex vasoconstriction after ABRX. Combined CBRX and ABRX eliminated all reflex responses except for a small bradycardia and a biphasic change in perfusion pressure (constrictor-dilator) during PE. Vagotomy eliminated the bradycardia and the dilator phase; the constrictor phase persisted and was abolished by lumbar sympathectomy. The results indicate that (1) reflex control of heart rate may be impaired when reflex control of hindlimb resistance is preserved; thus reflex changes in heart rate may not be used as a reliable index of the integrity of arterial baroreceptor control of the total circulation; (2) one set of arterial baroreceptors does not compensate for the absence of the other with respect to activation of vagal neurons; in contrast, one set of baroreceptors compensates fully for the absence of the other with respect to inhibition of sympathetic neurons; (3) cardiopulmonary and other baroreceptors contribute minimally to reflex responses only during large PE-induced increases in arterial pressure.