Tetrodotoxin-insensitive sodium channels. Ion flux studies of neurotoxin action in a clonal rat muscle cell line.

The action of neurotoxins on tetrodotoxin-insensitive sodium channels in cultured rat muscle cells has been studied by ion flux methods. The alkaloid neurotoxins batrachotoxin, veratridine, and aconitine act at a common receptor site to cause persistent activation of sodium channels. Batrachotoxin is a full agonist, while veratridine and aconitine are partial agonists activating 8% and 1% of sodium channels, respectively. The activation of sodium channels is inhibited noncompetitively by tetrodotoxin but a 200-fold higher concentration of toxin (KI = 1 p ~ ) is required than for inhibition of tetrodotoxin-sensitive sodium channels. The polypeptides scorpion toxin and sea anemone toxin I1 enhance activation of these sodium channels by alkaloid neurotoxins. The polypeptides lower K0.S for activation by the full agonist batrachotoxin without altering V,, whereas they both lower and increase V, with the partial agonist veratridine. The concentration dependence of the scorpion toxin effect is hyperbolic consistent with a single class of sites with K0.6 of 60 m. The concentration dependence of sea anemone toxin I1 action is shallower than hyperbolic consistent with two or more sites of toxin action. Half-maximal effects are observed with 15 n~ sea anemone toxin 11. Depolarization of the muscle cells to 0 mV with K+ increases for scorpion toxin 30-fold and for sea anemone toxin 11 5-fold. Taken together, our results show that tetrodotoxin-insensitive sodium channels in cultured rat muscle cells have the same three neurotoxin receptor sites as sodium channels in nerve and that the mechanism of toxin action at these sites is the same. However, the affinity and binding specificity at the tetrodotoxin/saxitoxin receptor site and the scorpion toxin/sea anemone toxin receptor site are markedly altered. The functional similarity we have observed between tetrodotoxin-sensitive and -insensitive sodium channels suggests substantial structural homology in these two different ion channels.