An Analysis of the Variations in Potency of Grayanotoxin Analogs in Modifying Frog Sodium Channels of Differing Subtypes

Abstract

Responses of tetrodotoxin-sensitive (TTX-s) and insensitive (TTX-i) Na⁺ channels, in frog dorsal root ganglion (DRG) cells and frog heart Na⁺ channels, to two grayanotoxin (GTX) analogs, GTX-I and α-dihydro-GTX-II, were examined using the patch clamp method. GTX-evoked modification occurred only when repetitive depolarizing pulses preceded a single test depolarization; modification, during the test pulse, was manifested by a decrease in peak Na⁺ current accompanied by a sustained Na⁺current. GTX-evoked modification of whole-cell Na⁺ currents was quantified by normalizing the conductance for sustained currents through GTX-modified Na⁺ channels to that for the peak current through unmodified Na⁺ channels. The dose-response relation for GTX-modified Na⁺ channels was constructed by plotting the normalized slope conductance against GTX concentration. With respect to DRG TTX-i Na⁺ channels, the EC₅₀ and maximal normalized slope conductance were estimated to be 31 μM and 0.23, respectively, for GTX-I, and 54 μM and 0.37, respectively, for α-dihydro-GTX-II. By contrast, TTX-s Na⁺ channels in DRG cells and Na⁺ channels in ventricular myocytes were found to have a much lower sensitivity to both GTX analogs. In single-channel recording on DRG cells and ventricular myocytes, Na⁺ channels modified by the two GTX analogs (both at 100 μM), had similar relative conductances (range, 0.25–0.42) and open channel probabilities (range, 0.5–0.71). From these observations, we conclude that the differences in responsiveness of DRG TTX-i, and ventricular whole cell Na⁺ currents to the GTX analogs studied are related to the number of Na⁺channels modified.