Curare-like effect of propranolol on rat extraocular muscles

1 The effects were studied of the beta-adrenoceptor blocking drug, (+/-)-propranolol and its (+)- and (-)-isomers on contractility of rat isolated diaphragm and inferior rectus muscles. 2 Propranolol (10(-5) M) did not modify the resting tension nor the electrically-induced twitch contraction in diaphragm and extraocular muscles, nor the tonic tension evoked in the latter by high K concentrations. 3 In inferior rectus preparations (+/-)-propranolol (5 x 10(-6) M) reduced significantly tensions evoked by succinylcholine (SCh). Cumulative dose-response curves to SCh were shifted to the right in a nearly parallel manner by (+)- and (-)-propranolol which were about equiactive. 4 It is concluded that the depressant action of propranolol is independent of its adrenoceptor blocking potency and it is not due to an alteration of the contraction mechanism. Propranolol appears to depress SCh-evoked tensions through a curare-like effect on cholinoceptors.