Block of non-L-, non-N-type Ca2+ channels in rat insulinoma RINm5F cells by ?-agatoxin IVA and ?-conotoxin MVIIC

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Abstract
The high-voltage-activated (HVA) Ba2+ currents of rat insulinoma RINm5F cells insensitive to dihydropyridines (DHP) and ω-conotoxin GVIA (ω-CTx-GVIA) have been studied for their sensitivity to ω-agatoxin-IVA (ω-Aga-IVA) and ω-CTx-MVIIC. Blockade of HVA currents by ω-Aga-IVA was partial (mean 24%), reversible and saturated around 350 nM (half block ≈ 60 nM). Blockade by ω-CTx-MVIIC was more potent (mean 45%), partly irreversible and saturated above 3 μM. The effects of both toxins were additive with that of nifedipine (5 μM) and were more pronounced at positive potentials. ω-Aga-IVA action was additive with that of ω-CTx-GVIA (3 μM) but was largely prevented by cell pre-treatment with ω-CTx-MVIIC (3 μM). In contrast, ω-CTx-MVIIC block was attenuated by ω-CTx-GVIA treatment (≈ 15%), suggesting that ω-CTx-MVIIC blocks the N-type (≈ 15%) and the non-L-, non-N-type channel sensitive to ω-Aga-IVA (≈ 30%). Consistent with this, cells deprived of most non-L-type channels by pre-incubation with ω-CTx-GVIA and ω-CTx-MVIIC exhibited predominant L-type currents that activated at more negative potentials than in normal cells (-30 mV in 5 mM Ba2+) and were effectively depressed by nifedipine (maximal block of 95% from -30mV to +40 mV). Our results suggest that, besides L- and N-type channels, insulin-secreting RINm5F cells possess also a non-L-, non-N-type channel that contributes significantly to the total current (≈ 30%). Although the pharmacology of this channel is similar to Q-type and α 1 class A channels, its range of activation (>-20 mV) and its slow inactivation time course resemble more that of N- and P-type channels. The channel is therefore referred to as “Q-like”.