In vivo demonstration of alpha-2-adrenoceptor-mediated inhibition of the excitatory non-cholinergic neurotransmission in guinea pig airways.

In anaesthetized guinea pigs, vagal nerve stimulation caused an atropine- and hexamethonium-resistant increase in the respiratory insufflation pressure. Clonidine (0.05 mg/kg i.v.) inhibited the increase in insufflation pressure caused by vagal stimulation. This effect of clonidine was antagonized by the alpha-2-antagonist yohimbine (0.5 mg/kg i.v.), but not by the alpha-1-antagonist prazosin (0.03 mg/kg i.v.). In conclusion, the increase in the respiratory insufflation pressure, caused by stimulation of noncholinergic nerves, seemed to be controlled by inhibitory alpha-2-adrenoceptors in guinea pig airways.