Arterial baroreceptor reflex control of heart rate in two species of turtle

Attempts were made to stimulate an arterial baroreceptor reflex in anesthetized and conscious pond turtles. In turtles anesthetized with either alpha-chloralose or pentobarbital, occlusion of the ascending or descending aortas produced no reflex heart rate (HR) changes. In pentobarbital-anesthetized turtles, direct electrical or mechanical stimulation of potential baroreceptor sites along the central aortic arches and carotid arteries produced no significant changes in either HR or blood pressure (BP). Occlusion of the common carotid arteries also produced no HR or BP changes. Intravenously administered nitroglycerin lowered BP but caused no reflex tachycardia in anesthetized turtles. Phenylephrine and angiotensin elevated BP in the anesthetized turtle but caused no reflex bradycardia. In conscious turtles phenylephrine increased BP and nitroglycerin decreased BP. Neither response produced a statistically significant HR change, although HR tended to decrease transiently with phenylephrine and increase with nitroglycerin. These HR changes were abolished by atropine. Rapid intra-arterial infusion of 6% dextran transiently raised BP but caused no reflex bradycardia. These experiments suggest that cardiovascular regulation in the turtle is accomplished without a major contribution from arterial baroreceptor reflexes.