PREJUNCTIONAL ALPHA(2)-ADRENOCEPTORS INHIBIT ACETYLCHOLINE-RELEASE FROM CHOLINERGIC NERVES IN EQUINE AIRWAYS

To determine the presence and function of alpha2-adrenoceptors on chol inergic nerves innervating horse airway smooth muscle, the effects of some alpha2-adrenoceptor agents on contractions of and acetylcholine ( ACh) release from equine airway smooth muscle preparations were studie d. Muscle contractions were elicited by either electrical field stimul ation (EFS) or exogenous ACh. ACh release was induced by EFS and measu red by high-pressure liquid chromatography and electrochemical detecti on. The alpha2-adrenoceptor agonists clonidine (10(-7) to 10(-5) M) an d UK-14,304 (10(-8) to 10(-6) M) concentration dependently inhibited A Ch release and the contractile response to EFS but not the response to exogenous ACh. This inhibition was attenuated by the alpha2-adrenocep tor antagonists yohimbine and idazoxan but not by the alpha1-adrenocep tor antagonist prazosin. These results indicate that alpha2-adrenocept ors exist on cholinergic nerves innervating equine airway smooth muscl e, and activation of these receptors inhibits cholinergic neurotransmi ssion. The observation that yohimbine alone had little effect on the c ontractile response to EFS suggests that, under these experimental con ditions, endogenous norepinephrine had no influence on tracheal cholin ergic neurotransmission via prejunctional alpha2 adrenoceptors.