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IS THE CONTRACTILE RESPONSE TO EXOGENOUS ACETYLCHOLINE DUE TO A PRESYNAPTIC EFFECT?

1 Whether the contractile response induced by exogenous acetylcholine (ACh) chiefly involved the pre‐or post‐synaptic junctional site of the motor endplate was studied by using the cat gastrocnemius nerve muscle preparation poisoned with β‐bungarotoxin (β‐BuTX), a toxin isolated from the venom of Bungarus multicinctus which acts presynaptically. 2 After neuromuscular transmission was completely blocked by β‐BuTX, the dose‐response curve of the contractile response induced by close intra‐arterial injection of ACh, was compared with that of the control. No appreciable difference was observed. 3 In contrast, the response to ACh was completely abolished when neuromuscular transmission was blocked by α‐bungarotoxin, a toxin isolated from the same venom which acts postsynaptically. 4 It is concluded that postjunctional site of the motor end‐plate is chiefly involved in the contractile response produced by exogenous ACh.