Contractures elicited by tetraethylammonium in avian muscle treated with methohexitone.

1 The chick biventer cervicis muscle immersed in methohexitone (8.8 x 10(-5) M) responded to tetraethylammonium with contractures which were dose-related. The ED50 for tetraethylammonium was 2.1 x 10(-3) M. 2 In the absence of methohexitone, tetraethylammonium produced contractures only at much higher concentrations: these contractures were accompanied by fasciculations and neuromuscular block of the twitch fibres. 3 The contractures produced by tetraethylammonium in the presence of methohexitone were not reduced by exposure to botulinum toxin which eliminated all response of the muscle to indirect stimulation. 4 Tubocurarine (1.2 x 10(-6) M) displaced the dose-response curve for tetraethylammonium-methohexitone-induced contractures to the right. The dose-ratio was 15.63 +/- 1.98. 5 Physostigmine (1.8 x 10(-6) M) potentiated the activity of tetraethylammonium-methohexitone 3.26 or 3.84 fold, depending on the method of calculation used. 6 Physostigmine potentiated contractures elicited by indirect repetitive stimulation 4.8 to 6.0 fold more than it potentiated contractures due to tetraethylammonium-methohexitone. 7 It is concluded that in the presence of methohexitone, tetraethylammonium produces contractures of the chick muscle by releasing acetylcholine but also by a direct agonist action on the cholinoceptor.