We have investigated the action of SNX482, a toxin isolated from the venom of the tarantula Hysterocrates gigas, on voltage-dependent calcium channels expressed in tsa-201 cells. Upon application of 200 nM SNX482, R-type α1E calcium channels underwent rapid and complete inhibition, which was only poorly reversible upon washout. However, upon application of strong membrane depolarizations, rapid and complete recovery from inhibition was obtained. Tail current analysis revealed that SNX482 mediated an ∼70 mV depolarizing shift in half-activation potential, suggesting that the toxin inhibits α1E calcium channels by preventing their activation. Experiments involving chimeric channels combining structural features of α1E and α1C subunits indicated that the presence of the domain III and IV of α1E is a prerequisite for a strong gating inhibition. In contrast, L-type α1C channels underwent incomplete inhibition at saturating concentrations of SNX482 that was paralleled by a small shift in half-activation potential and which could be rapidly reversed, suggesting a less pronounced effect of the toxin on L-type calcium channel gating. We conclude that SNX482 does not exhibit unequivocal specificity for R-type channels, but highly effectively antagonizes their activation.
