Centruroides Toxin, a Selective Blocker of Surface Na+Channels in Skeletal Muscle: Voltage-Clamp Analysis and Biochemical Characterization of the Receptor

This paper describes the effects of a toxin from the scorpion Centruroides suffusus suffusus on frog skeletal muscle. The main findings are the following. (i) Centruroides toxin (CssII) blocks the early phase of the inward sodium current in the muscle that arises from influx via Na+channels in the surface membrane, but it does not affect the late phase of the inward current that represents flux through Na+channels in the T-tubule membranes. (ii) CssII, in marked contrast to tetrodotoxin, does not affect contraction of the muscle. (iii) Measurements of the binding of125I-labeled CssIIto a partially purified membrane preparation from the muscle indicate that the Kdof the CssII-receptor complex is ≈ 0.4 nM. The half-life for the dissociation of this complex is 3 min at 22 degrees C and 16 min at 2 degrees C. Binding of the radiolabeled toxin varies markedly with pH and becomes insignificant at pH >8.5. Proteolytic digestion of the membrane preparation decreases its ability to bind CssII, suggesting that the receptor is a protein. (iv) The number of binding sites for a radiolabeled derivative of tetrodotoxin on the membrane preparation was similar to that for CssII. However, neither tetrodotoxin nor any of seven other neurotoxins and some local anesthetics that alter the functioning of the Na+channel have any effect on the binding of CssIIto the muscle membrane. These results therefore indicate that CssIIbelongs to a different class of neurotoxins that has a different receptor on the Na+channel.