Neuromuscular effects of fractions isolated from the venom of the Thailand cobra, Naja naja siamensis

Four fractions (α, β, γ and δ) were isolated from the venom of the Thailand cobra, Naja naja siamensis by ion-exchange chromatography. Each fraction was tested for activity on the isolated chick biventer cervicis nerve-muscle preparation. Fractions α, β and γ inhibited neuromuscular transmission by blocking acetylcholine receptors, but had no direct action on muscle contractility. Fraction α produced neuromuscular blockade in about 30 min when added at a concentration of 1 μg/ml, and was about 100 times more potent than fractions β and γ. The blockade of indirect stimulation was apparently irreversible, although the responses to acetylcholine and carbachol recovered to about 50% after 4 hr. Fraction δ produced contracture of the muscle followed by blockade of responses to both direct and indirect stimulation. The effects of fraction δ could be inhibited by raising the calcium concentration of the bathing solution; on return to normal calcium concentration the effects of fraction δ also returned. It is concluded that fractions α, β and γ are postjunctional "neurotoxins" and that fraction δ contains a "cardiotoxin" which may act by displacing calcium from membranes.