Pharmacological properties of axonal sodium channels in the cockroach Periplaneta americana L. I. Selective block by synthetic saxitoxin.

Voltage-clamp experiments on isolated giant axons of the cockroach Periplaneta americana L. show that chemically synthesized saxitoxin specifically and reversibly blocks the transient inward sodium current without affecting the steady-state outward potassium current. From the concentration depending of sodium current suppression it is concluded that individual sodium channels are blocked by single molecules of synthetic saxitoxin which bind reversibly to part of the channel with a dissociation constant of 3.0 x 10(-9) M. Synthetic saxitoxin blocks sodium channels in cockroach axons at a lower concentration than tetrodotoxin. Sodium channel block by synthetic saxitoxin is more readily reversed than tetrodotoxin-induced block.