Pharmacologic properties of voltage-sensitive sodium channels in chick muscle fibers developing in vitro

Abstract

The alkaloids veratridine and batrachotoxin cause persistent activation of voltage-sensitive sodium channels in cultured chick muscle cells. The concentration dependence of activation follows a Langmuir isotherm with K 0.5 (veratridine) = 10 μM and K 0.5 (batrachotoxin) = 1.5 μM. Veratridine is a partial agonist in activating sodium channels while batrachotoxin is a full agonist. The polypeptides scorpion toxin and sea anemone toxin II enhance activation of sodium channels by veratridine and batrachotoxin, but do not cause persistent activation themselves. Scorpion toxin ( K 0.5 = 30 n M) acts at lower concentration than sea anemone toxin ( K 0.5 = 70 n M). The polypeptide toxins both reduce K 0.5 and increase the fraction of sodium channels activated by the partial agonist veratridine, whereas they reduce K 0.5 without effect on the fraction of sodium channels activated by batrachotoxin. These results are consistent with an allosteric model of toxin action in which the alkaloid toxins bind tightly to active states of sodium channels and shift an existing equilibrium in favor of the activated states while the polypeptide toxins reduce the energy required to cause persistent activation. The action of scorpion toxin is inhibited by depolarization in the range of −55 mV to 0 mV. Sodium channels activated by toxin treatment are inhibited by tetrodotoxin and saxitoxin ( K i = 6 n M). A similar high affinity for tetrodotoxin was observed in newly formed myotubes as soon as toxin-activated sodium channels could be detected. Toxin-activated, tetrodotoxin-insensitive sodium channels were not detected. The pharmacologic properties of sodium channels in cultured chick muscle cells resemble those described in previous studies with neuroblastoma cells in all respects studied, except that scorpion toxin action requires higher concentrations and is more voltage dependent.