Facilitation of Chemoreceptor-induced reflex vasoconstriction by intravertebral arterial administration of clonidine

The influence of clonidine on the reflex vascular responses to stimulation of carotid body chemoreceptors and bilateral carotid occlusion was studied in morphine, chloralose-urethane anesthetized dogs. Bilateral carotid occlusion and intracarotid injection of nicotine (30 and 100 microgram) or sodium cyanide (200 and 500 microgram) elicited reflex vasoconstriction in the perfused gracilis muscle vascular bed. Infusion of clonidine (2-4 microgram/kg) into the vertebral artery significantly lowered blood pressure. Reflex vasoconstrictor responses to chemoreceptor stimulation were significantly enhanced after clonidine administration whereas reflex vasoconstrictor responses to carotid occlusion were markedly reduced. The facilitation of chemoreceptor reflex responses by clonidine was observed in dogs with intact or sectioned vagi and in animals in which the carotid arteries were perfused at constant blood flow. Inhibition of carotid occlusion responses by clonidine was observed in dogs with intact or sectioned vagi. Infusion of clonidine directly into the carotid arteries did not significantly alter responses to chemoreceptor stimulation. These experiments demonstrate that clonidine antagonizes the reflex vasoconstriction caused by carotid occlusion while potentiating the vasoconstriction elicited by chemoreceptor stimulation. The data suggest that clonidine exerts central actions which result in a facilitation of the chemoreceptor reflex and a simultaneously occuring hypotension which is probably due to an action on baroreceptor pathways.