Molecular Pharmacology, Vol 19, 78-86, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Ion Flux Studies of Voltage-Sensitive Sodium Channels in Synaptic Nerve-Ending Particles

¹ Department of Pharmacology, University of Washington School of Medicine, Seattle, Washington 98195

Neurotoxins which activate voltage-sensitive sodium channels cause a 10-fold increase in the initial rate of ²²Na⁺ uptake in synaptic nerve-ending particles (synaptosomes). Batrachotoxin is a full agonist in activating synaptosomal sodium channels, whereas veratridine and aconitine are partial agonists acting at the same receptor site. The ²²Na⁺ uptake induced by alkaloid toxins is completely blocked by tetrodotoxin and saxitoxin. The K_i for saxitoxin (6 nM) is identical with the K_d for ³H-labeled saxitoxin binding to sodium channels in synaptosomes. The polypeptides scorpion toxin, Anemonia sulcata toxin II, and Anthopleurin A enhance activation of sodium channels by alkaloid toxins, reducing the K_0.5 for all alkaloid toxins and increasing the fraction of sodium channels activated by the partial agonists. Scorpion toxin and A. sulcata toxin II are full agonists in this respect, whereas Anthopleurin A is a partial agonist. The three polypeptide toxins bind to a common receptor site, and the concentration dependence of binding and activation of ²²Na⁺ influx are closely correlated. Alkaloid toxins markedly enhance ¹²⁵I-labeled scorpion toxin binding to its receptor site, demonstrating bidirectional allosteric coupling between the receptor sites for polypeptide and alkaloid toxins. The concentration-effect curves for alkaloid toxin enhancement of ¹²⁵I-labeled scorpion toxin binding and ²²Na⁺ uptake are closely correlated. Depolarization enhances the effect of alkaloid toxins on ¹²⁵I-labeled scorpion toxin binding but does not alter the effect of saturating concentrations of scorpion toxin on batrachotoxin activation. In depolarized synaptosomes, this allosteric coupling between the alkaloid toxin and polypeptide toxin receptor sites is quantitatively fit by a model which assumes that both the alkaloid and polypeptide toxins act by binding selectively to active states of sodium channels.

Note:
ACKNOWLEDGMENTS We thank Dr. T. R. Norton for providing Anthopleurin A, Dr. L. Beress for providing sea anemone toxin II, and Drs. J. Daly and B. Witkop for providing batrachotoxin.

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