An Analysis of the Variations in Potency of Grayanotoxin Analogues in Modifying Frog Sodium Channels of Differing Subtype

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Responses of TTX-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) analogues, GTX-I and alpha-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 control peak current through unmodified Na^+ channels. The dose-response relation for GTX-modified Na^+ channels was constructed by plotting the normalized conductance against GTX concentration. With respect to TTX-i Na^+ channels found in DRG cells, the EC_<50> (helf-maximal concentration) and maximal normalized conductance were estimated to be 35 mu M and 0.23 for GTX-I, and 54 mu M and 0.37 for alpha-dihydro-GTX-II.By contrast, TTX-s Na^+ channels in DRG, and Na^+ channels in ventricular cells were found to have a much lower sensitivity to both GTX analogues. Currents through single Na^+ channels were recorded from cell-attached membrane patches to elucidate, the factors responsible for the marked differences in potency of the GTX analogues in modifying DRO TTX-i and ventricular Na^+ channels. In DRG cells and ventricular myocytes, single Na^+ channels modified by the two GTX analogues (both at 100 mu M), 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 analogues studied are related to the number of Na^+ channels modified.