Blockade of Clonidine Potentiation of Baroreceptor Responses by Carotid Occlusion in the Anesthetized Rat

Experiments were conducted in anesthetized rats to determine the effect of unilateral carotid artery occlusion on the reflex bradycardia caused by elevated arterial pressure (AP) and on clonidine\u27s ability to potentiate that bradycardia. AP was recorded by catheterizing the carotid (CA-cath) or the iliac (IA-cath) artery. Elevated AP was obtained by rapid intravenous injection of methoxamine. Rats with CA-cath had higher APs and heart rates (HRs) than rats with IA-cath. The maximum increase in AP and reflex decrease in HR caused by methoxamine did not differ between sites of recording. Clonidine markedly increased the reflex bradycardia in rats with IA-cath. The reflex decrease in HR caused by methoxamine was not potentiated by clonidine in rats with either CA-cath or IA-cath with the CA clamped or following chemical denervation of the carotid sinus area with phenol. It is concluded that while unilateral carotid artery occlusion does not alter the reflex response to elevations in AP, it significantly interferes with clonidine\u27s ability to potentiate baroreceptor responses