Role of α1-and α2-adrenoceptors in the modulation of the baroreflex vagal bradycardia

Yohimbine (100 μg/kg), an α₂-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 μg/kg) an α₁-adrenpceptor agonist decreased, whereas AR-C 239 (5 μg/kg) and prazosin (5 μg/kg) two potent α₁-adrenoceptor antagonists injected into the vertebral artery, potentiated the bradycardic response. These results suggest the presence of two types of α-adrenoceptors to modulate the baroreceptor pathway: α₁-adrenoceptors inhibit and α₂-adrenoceptors facilitate the transmission of baroreceptor impulses.