Presynaptic inhibitory effects of catecholamines on cholinergic transmission in the smooth muscle of the chick stomach

Abstract

In the vagus nerve-smooth muscle preparation isolated from the chick proventriculus, adrenaline, clonidine (10 −8-2.5 × 10 −7 M), noradrenaline (10 −7-2.5 × 10 −6 M) and dopamine (10 −5-10 −4 M) inhibited the contraction induced by low frequency (0.5 Hz) stimulation of the vagus nerve, but they did not indibit the contraction elicited by acetylcholine (5 × 10 −8-5 × 10 −7 M). The concentration producing 50% inhibition was 10 −7 M for adrenaline and clonidine, 10 −6 M for noradrenaline, and 5 × 10 −5 M for dopamine. Isoproterenol (5 × 10 −8-5 × 10 −7 M) inhibited the responses induced by both stimulation of the vagus nerve and acetylcholine. The inhibitory effects of the catecholamine and clonidine were blocked by phentolamine (2.7 × 10 −6 M) but not by 5-(3-tert-Butylamino-2-hydroxy)-propoxy-3, 4-dihydrocarbostyril hydrochloride (OPC 1085) which blocked the effect of isoproterenol. It is suggested at presynaptic α-receptors are present in the myenteric plexus of the chick proventiculus, and that the catecholamines and clonidine exert their inhibitory effects on cholinergic transmission via these receptors.