ALPHA-ADRENERGIC AND BETA-ADRENERGIC MECHANISMS IN THE CONTROL OF VASCULAR CAPACITANCE BY THE CAROTID-SINUS BAROREFLEX SYSTEM

We examined the active and passive contributions of the alpha- and bet a-adrenergic receptor mechanisms to the changes in systemic vascular c apacitance caused by the carotid sinus baroreflex system in anesthetiz ed, vagotomized dogs. The carotid sinuses were isolated from the syste mic circulation and perfused with controlled pressures. To determine t he changes in vascular capacitance, a constant flow, constant venous p ressure cardiopulmonary bypass was used. The changes in unstressed vas cular volume were calculated when carotid sinus pressure was reduced f rom 200 to 50 mmHg without any adrenergic receptor antagonist, with ei ther an alpha- (phentolamine) or a beta- (propranolol) antagonist and then with both. The reflex change in unstressed vascular volume in the systemic circulation (22.6 +/- 9.0 ml/kg without any antagonist) was reduced by 72% with phentolamine, by 35% with propranolol, and by 73% with both antagonists. Our results suggest that the alpha-adrenergic m echanisms contribute significantly to active changes in systemic venou s capacity. In addition, the beta-adrenergic system has very little ef fect on active changes in venous vessels but does contribute to the ov erall capacity changes by dilating the hepatic outflow resistance when the carotid sinus baroreflex system is activated.