Carotid sinus compliance and baroreflex responses in hypertensive dogs

The hypothesis that changes in baroreflex function seen in hypertension could be explained by a decreased vascular compliance in the carotid sinus region itself was tested. Six dogs were made chronically hypertensive (MAP = 146.0 +/- 3.3 mm Hg) using a bilateral renal wrap technique, while six other dogs were sham operated and served as normotensive controls (MAP = 125.8 +/- 4.7 mm Hg). Six weeks after the procedure, compliance of the carotid sinus region was measured, and carotid baroreflex control of arterial pressure and heart rate was assessed acutely. Dogs were anesthetized with sodium pentobarbital and the carotid sinus was isolated and perfused at controlled pressures. Vagotomy was performed to eliminate aortic and cardiopulmonary reflex buffering. The carotid sinus pressure (CSP) was changed from 25 to 250 mm Hg in a stepwise fashion, and the corresponding arterial pressure, heart rate and volume changes were recorded. Compliance was determined as the change in volume infused divided by the changes in pressure achieved. Significant differences between the normotensive and hypertensive groups were found in the reflex responses of arterial pressure and heart rate to changes in CSP. Carotid sinus compliance decreased with increasing CSP, but was not different in the two groups. Changes in baroreflex responses seen in mild hypertension occur without significant changes in carotid sinus compliance, and cannot be explained solely by a decreased compliance in the receptor wall.