We have previously demonstrated that the venom of the scorpion Leiurus quinquestriatus blocks small-conductance Cl- channels, derived from epithelial cells, when applied to the cytoplasmic surface. We have now purified to near homogeneity, and characterized, the component responsible for this blocking activity. It is a small basic peptide of 4,070 Da. The primary amino acid structure shows considerable homology to a class of previously described putative short insectotoxins. A brief characterization of the kinetics of Cl- channel block as well as a demonstration of toxicity to arthropods is also presented.

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We have characterized voltage-dependent sodium channels in growth cones (GCPs) isolated from fetal rat brain using saxitoxin and TTX binding as well as recordings from channels reconstituted into lipid bilayer membranes. Both high- and low-affinity binding sites are present in GCP membranes. However, the two binding sites are segregated largely or completely, with the high-affinity binding sites in the plasmalemma, and the low-affinity sites in an internal membrane compartment. Plasmalemmal insertion of these internal sites can be triggered by high-potassium depolarization and depends on a metalloendoprotease-requiring mechanism. These observations indicate that a precursor-product relationship exists between the internal and external sodium channels of the growth cone, and therefore suggest that channel externalization causes conversion of low-affinity to high-affinity saxitoxin receptors. This conversion may represent a step of channel capacitation.