ANTAGONISM OF ACETYLCHOLINE BY ADRENALINE ANTAGONISTS

Abstract

Phenoxybenzamine antagonized the inhibitory action of acetylcholine on the guineapig isolated atrium. The antagonism was slow in onset, very slowly reversible, and could be overcome by increased concentrations of acetylcholine. In contrast, atropine inhibited the action of acetylcholine quickly, and the effect disappeared soon after withdrawal. The pA10 of phenoxybenzamine (2 hr of contact) was 6.8, and that of atropine (30 min of contact) was 8.4. In the presence of atropine phenoxybenzamine did not exert a slowly reversible antagonism, and the dose‐ratio of acetylcholine returned to normal soon after withdrawal of both drugs. Phenoxybenzamine also antagonized acetylcholine in the guinea‐pig isolated ileum, but with higher concentrations acetylcholine did not overcome the antagonism. The pA10 (60 min of contact) was 6.6. The pA10 of chlorpromazine in the atrium (2 hr of contact) and ileum (60 min of contact) was 5.9. Phentolamine, 2‐diethylaminomethylbenzo‐1,4‐dioxan hydrochloride (883 F), and yohimbine antagonized acetylcholine in the atrium and ileum but required higher concentrations than chlorpromazine. 1963 British Pharmacological Society