The facilitatory actions of aminopyridines and tetraethylammonium on neuromuscular transmission and muscle contractility in avian muscle

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Abstract
The actions of 3,4-diaminopyridine, 4-aminopyridine and tetraethylammonium were studied on the chick biventer cervicis muscle preparation. All three compounds produced a greater augmentation of indirectly elicited twitches than of directly elicited twitches. The compounds did not restore transmission in OmMCa2+ solutions but rather produced contractures that were inhibited by acetylcholine receptor antagonists. The compounds restored twitch height in one-tenth normal Ca2+ solutions and induced spontaneous muscle twitching. The compounds reversed dantrolene-induced block of directly elicited twitches. Interactions between tetraethylammonium and 3,4-diaminopyridine were also studied. In indirectly stimulated preparations, the combined effects of the two compounds were more than additive at one concentration level only. In directly stimulated preparations, the effects of 3,4-diaminopyridine were greatly enhanced by tetraethylammonium pretreatment. 3,4-Diaminopyridine pretreatment produced less synergism than tetraethylammonium pretreatment. It is concluded that the actions of the aminopyridines and tetraethylammonium on transmitter release and muscle contractility are essentially similar. These actions are postulated to arise from an inhibitory action on potassium conductance and on ability to release calcium from nerve and muscle membranes. On the basis of the interaction studies, it is suggested that the compounds possess different binding capacities for two different sites on the potassium conducting channel.