Abstract
N. nigricollis, N. naja, N. nivea and N. melanoleuca produced contraction of the guinea-pig ileum which was not blocked by hexamethonium tetrodotoxin, or mepyramine but markedly reduced by atropine. In small doses the venoms increased the tone and twitch height of the electrically stimulated guinea-pig ileum. However, in large doses they produced a blockade of the twitches. All Naja venoms studied significantly increased the resting output of acetylcholine from the longitudinal muscle strip. This increase in acetylcholine was not prevented by hexamethonium, tetrodotoxin, mepyramine or by desensitization of 5-HT receptors, but was blocked by adrenaline, calcium lack and magnesium excess. It is concluded that Naja venoms increase the spontaneous release of acetylcholine from the longitudinal muscle strip of the guinea-pig ileum through inhibition of (Na +K +Mg 2+) activated ATP-ase in the nerve terminal membrane.
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