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GK Wang and GR Strichartz
Several different toxins having specific effects on the kinetics of sodium channels have been isolated from the venoms of two scorpions. A combination of two steps of ion-exchange chromatography has been used to purify these toxins, whose sizes and purities have been assayed by gel filtration, urea/sodium dodecyl sulfate/polyacrylamide gel electrophoresis, and isoelectric focusing. The actions of the toxins and their relative potencies have been determined by studying the modifications they produce in action potential shape, using the sucrose- gap method, and in ionic current kinetics, measured under voltage- clamp, both assays performed on myelinated axons of frogs and toads. The venom of Leiurus quinquestriatus scorpions yielded two active neurotoxins; the major neurotoxin has a mass of approximately 7000 daltons. This major toxin affected the sodium channel inactivation process exclusively, slowing the rates of inactivation as well as preventing complete inactivation from occurring in some of the channels. Such slowed and incomplete sodium inactivation resulted in action potentials that were prolonged, from their usual duration of 5-8 msec to hundreds of milliseconds or even seconds. Five toxins were isolated from the venom of Centruroides sculpturatus Ewing scorpions, all of which also had masses of approximately 7000-7500 daltons. Four of these toxins acted primarily on the activation process of sodium channels, producing a novel increase in sodium permeability upon repolarization of the nerve membrane following a depolarizing pulse, as previously described for the crude venom [Cahalan, M. D. J. Physiol. (Lond.) 244:511-534 (1975)]. These toxins also caused repetitive firing of action potentials in single axons in response to one stimulating pulse, as well as spontaneous impulse firing. A fifth neurotoxin from C. sculpturatus venom had effects similar to those of the L. quinquestriatus toxins, slowing and preventing complete sodium inactivation. The effect of this toxin was slowly removed during external perfusion by Ringer's solution.
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