Evidence for endocytotic uptake of cobra neurotoxin in mouse skeletal muscle

An isolated 3H-labelled neurotoxin from Naja naja siamensis binds irreversibly to the extensor digitorum longus muscle of the mouse in vitro. The binding consists of an adsorption to cholinergic receptors and of a slower temperature sensitive binding mechanism. The slow binding is markedly stimulated by the presence of cationic proteins (protamine, histone and polylysine) and is blocked at low temperature (+4 degrees C). Vinblastine and colchicine inhibit the stimulatory effect of protamine on the slow binding. Unlabelled neurotoxin blocks the adsorption binding but fails to affect the slow binding. The results suggest that the slow binding of neurotoxin is not associated with the presence of cholinergic receptors but is the result of endocytotic uptake into the muscle cell. Unless properly recognized this uptake will give a considerable overestimate of the number of cholinergic receptors present in the muscle.