STUDIES OF THE NEUROTOXIN FRACTION IX FROM BUNGARUS FASCIATUS VENOM

A postsynaptic neurotoxin—Fraction Ⅸ was Separated from the venom of Bungarus fasciatus. Fraction Ⅸ was different from other postsynaptic neurotoxins in its blocking manner. Fraction Ⅸ non-competitively inhibited the concentration-response curve of acetylcholine on frog abdominal rectus muscle; neostigmine could not completely antagonize the effects of Fraction Ⅸ, On the contrary, under higher agonist concentration range, the muscle relaxed progressively with increasing agonist concentration. Under the experimental conditions, Fraction Ⅸ did not modify the binding of ¹²⁵Ⅰ-cobrotoxin to the membrane preparation from the electric organ of Narcine maculata which is rich in nicotinic acetylcholine receptors. Under the effect of Fraction Ⅸ, the endplate current (EPC) of frog sartorius muscle was inhibited and disappeared eventually. Fraction Ⅸ showed some effect on the time course of EPC: the EPC half decay time (T_1/2)was shortened, EPC decay rate constant (α) was increased. However, the EPC equilibrium potential was not changed by Fraction Ⅸ. Based on these data, it was suggested that Fraction Ⅸ may act on endplate ion channels and result in neuronluscular blockage.