SNX482 selectively blocks P/Q Ca2+ channels and delays the inactivation of Na+ channels of chromaffin cells

The effects of the toxin SXN482 on Ca2+ channel currents (ICa), Na+ currents (INa), and K+ currents (IK) have been studied in bovine adrenal medullary chromaffin cells voltage-clamped at -80 mV. Currents were elicited by depolarising pulses to 0-10 mV (ICa and INa) or to +60 mV (IK). SNX482 blocked ICa in a concentration-dependent manner. The inhibition curve exhibited two phases. The first high-affinity phase comprised 28% of the whole-cell current and exhibited an IC50 of 30.2 nM. The second low-affinity phase comprised over 70% of ICa and had an IC50 of 758.6 nM. Blockade was rapid and fully reversible upon washout of the toxin. Occlusion experiments showed additivity of blockade exerted by nifedipine plus SNX482 (0.3 microM) and by omega-conotoxin GVIA plus SNX482. In contrast, blockade exerted by combined omega-agatoxin IVA plus SNX482 (about 50% of the whole cell) did not show additivity. At 0.3 microM and higher concentrations, SNX482 delayed the inactivation of INa. The time constant (tau) for inactivation of INa in control conditions doubled in the presence of 0.5 microM SNX482. At 0.3 microM, SNX482 did not affect IK. Our data demonstrate that: (i) SNX482 selectively blocks P/Q Ca2+ channels at submicromolar concentrations; (ii) the toxin partially blocks Na+ channels; (iii) SNX482 delays the inactivation of Na+ channels. These results reveal novel properties of SNX482 and cast doubts on the claimed selectivity and specificity of the toxin to block the R-type Ca2+ channel.