Properties of toxin‐resistant sodium channels produced by chemical modification in frog skeletal muscle.

Abstract

Single skeletal muscle fibers from the frog Rana pipiens were treated with the carboxyl group modifying reagent trimethyloxonium ion (TMO) and voltage clamped. TMO treatment reduced current through Na channels to 0.33 .+-. 0.03 that before treatment, but only 45 .+-. 3% of this remaining current was blocked by 1 .mu.M tetrodotoxin (TTX) and only 37 .+-. 5% by 100 nM-saxitoxin (STX). This toxin resistance persisted in 90 .mu.M-TTX, was not due to inactivation of toxin or to components of the reaction solution other than TMO, but was prevented by the presence of 100 nM-STX during treatment with TMO. TMO-modified Na channels can be blocked by the local anaesthetic lidocaine. The permeabilities of TMO-modified channels to hyroxylammonium, ammonium, guanidinium, aminoguanidinium, methylammonium and tetramethylammonium ions relative to Na were not significantly different from the permeabilities of untreated Na channels. H+ blocked TMO-modified Na channels, but the apparent pKa for block at +38 mV of 5.07 was significantly less than the corresponding value of 5.32 in untreated Na channels. TMO apparently produces toxin resistance by esterifying an ionized carboxyl group which is an essential part of the toxin binding site. Such esterification would electrostatically reduce the local cation concentration, thus reducing the apparent pKa of H+ block and the single-channel conductance. The Na channel apparently contains a second acid group, near but distinct from an acid group previously hypothesized to be part of the selectivity filter and H+ ion binding site.