Role of alpha 1- and alpha 2-adrenoceptors in the modulation of the baroreflex vagal bradycardia.

Yohimbine (100 micrograms/kg), an alpha 2-adrenoceptor blocking agent when injected into the vertebral artery of anaesthetized dogs decreased the vagally mediated bradycardia induced by carotid sinus nerve stimulation. Intracisternal administration of phenylephrine (30 micrograms/kg) an alpha 1-adrenoceptor agonist decreased, whereas AR-C 239 (5 micrograms/kg) and prazosin (5 micrograms/kg) two potent alpha 1-adrenoceptor antagonists injected into the vertebral artery, potentiated the bradycardiac response. These results suggest the presence of two types of alpha-adrenoceptors to modulate the baroreceptor pathway: alpha 1-adrenoceptors inhibit and alpha 2-adrenoceptors facilitate the transmission of baroreceptor impulses.