CONTRACTURES ELICITED BY TETRAETHYLAMMONIUM IN AVIAN MUSCLE TREATED WITH METHOHEXITONE

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Abstract
1 The chick biventer cervicis muscle immersed in methohexitone (8.8 × 10−5 m) responded to tetraethylammonium with contractures which were dose-related. The ED50 for tetraethylammonium was 2.1 × 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 × 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 × 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.