Centruroides toxin, a selective blocker of surface Na+ channels in skeletal muscle: voltage-clamp analysis and biochemical characterization of the receptor.

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Abstract
The effects of a toxin from the scorpion C. suffusus suffusus on frog skeletal muscle were described. Centruroides toxin (CssII) blocked the early phase of the inward Na current in the muscle that arose from influx via Na+ channels in the surface membrane, but it did not affect the late phase of the inward current that represented flux through Na+ channels in the T-tubule membranes, CssII, in marked contrast to tetrodotoxin, did not affect contraction of the muscle. Measurements of the binding of 125I-CssII to a partially purified membrane preparation from the muscle indicated that the Kd of the CssII-receptor complex was .apprxeq. 0.4 nM. The half-life for the dissociation of this complex was 3 min at 22.degree. C and 16 min at 2.degree. C. Binding of the radiolabeled toxin varied markedly with pH and became insignificant at pH > 8.5. Proteolytic digestion of the membrane preparation decreased its ability to bind CssII, suggesting that the receptor was a protein. The number of binding sites for a radiolabeled derivative of tetrodotoxin on the membrane preparation was similar to that for CssII. Neither tetrodotoxin or any of 7 other neurotoxins and some local anesthetics that alter the functioning of the Na+ channel had any effect on the binding of CssII to the muscle membrane. CssII belonged to a different class of neurotoxins that had a different receptor on the Na+ channel.