Non‐selective conductance in calcium channels of frog muscle: calcium selectivity in a single‐file pore.

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Abstract
Voltage-clamp studies were carried out to compare currents through Ca2+ channels (ICa) with Na+ currents (Ins) through a non-selective cation conductance blocked by micromolar concentrations of external Ca2+. The gating of both currents had similar time and voltage dependence. The amplitudes of ICa and Ins varied widely, but Ins was always large in fibers with large ICa, and small in fibers with small ICa. Both ICa and Ins were blocked by the specific Ca2+ channel blocker nifedipine, with half-blockage concentrations that were virtually identical (KD = 0.9 .mu.M for ICa and 0.7 .mu.M for Ins). ICa and Ins were also equally sensitive to block by diltiazem (KD = 80 .mu.M). These parallels between Ins and ICa are most easily explained if Ins flows through Ca2+ channels. Apparently, Ca2+ channels bear high-affinity Ca2+-binding sites, and are highly permeable to monovalent cations when Ca2+ is absent. Ba2+ currents (IBa) and ICa were measured in external solutions containing mixtures of Ba2+ and Ca2+. IBa is blocked by Ca2+, as is Ins. Adding Ba2+ to Ca2+ produces only small or no increases in current, as if Ba2+ is only sparingly permeant when Ca2+ is present. Membrane currents in Ba2+/Ca2+ mixtures show anomalous mol-fraction behavior, suggesting that Ca2+ channels are single-file, multi-ion pores. Complex current transients are observed under maintained depolarizations in Na+/Ca2+ and Ba2+/Ca2+ mixtures. In ion mixtures, Ca2+ channels apparently transport Ca2+ in preference to Na+ and Ba2+. Hence Ca2+ channels are selective for Ca2+, even though current amplitudes suggest that the Na+ of Ba2+ permeabilities in the absence of Ca2+ are as high as, or higher than, the Ca2+ permeability. The selective permeability of Ca2+ channels depends on the presence of Ca2+. In model calculations, the observations are explained as a consequence of Ca2+ channels being single-file pores. Ca2+ channels apparently derive much of their ion selectivity from high-affinity Ca2+ binding sites located in an otherwise unselective aqueous pore.