Effects of clonidine on baroreceptor function in anesthetized dogs

The effects of clonidine (15-30 mug/kg i.v.) on carotid sinus and other baroreceptors were investigated in anesthetized dogs. In 14 control dogs, right carotid sinus pressure was controlled by retrograde perfusion through the common carotid artery at constant flow with femoral arterial blood. Graded reductions in heart rate and blood pressure induced by graded increases in carotid sinus pressure were prevented, whereas reflex bradycardias associated with norepinephrine pressor activity were potentiated by clonidine. Norepinephrine-induced bradycardia, although reduced, still persisted after chronic bilateral sinusectomy and these responses were also potentiated by clonidine. In contrast, clonidine did not potentiate reflex bradycardia in dogs 20 days after aortic stripping. In intact dogs, clonidine inhibited the response to bilateral carotid artery occlusion and to carotid sinus nerve stimulation. These studies suggest that clonidine can inhibit carotid sinus baroreceptor function and simultaneously potentiate other, presumably aortic, baroreceptor activity.