Almers W, McCleskey E W
J Physiol. 1984 Aug;353:585-608. doi: 10.1113/jphysiol.1984.sp015352.
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 was found to have similar time and voltage dependence. The amplitudes of ICa and Ins varied widely, but Ins was always large in fibres with large ICa, and small in fibres 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 microM for ICa and 0.7 microM for Ins). ICa and Ins were also equally sensitive to block by diltiazem (KD = 80 microM). 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 mole-fraction behaviour, 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. They suggest that in ion mixtures, Ca2+ channels transport Ca2+ in preference to Na+ and Ba2+. Hence Ca2+ channels are selective for Ca2+, even though current amplitudes suggest that the Na+ or Ba2+ permeabilities in the absence of Ca2+ are as high as, or higher than, the Ca2+ permeability. We conclude that the selective permeability of Ca2+ channels depends on the presence of Ca2+. In model calculations, our observations are explained as a consequence of Ca2+ channels being single-file pores. It is proposed that Ca2+ channels derive much of their ion selectivity from high-affinity Ca2+ binding sites located in an otherwise unselective aqueous pore.
进行了电压钳研究，以比较通过Ca2+通道的电流（ICa）与通过被微摩尔浓度的细胞外Ca2+阻断的非选择性阳离子电导的Na+电流（Ins）。发现这两种电流的门控具有相似的时间和电压依赖性。ICa和Ins的幅度变化很大，但在ICa大的纤维中Ins总是很大，而在ICa小的纤维中Ins很小。ICa和Ins都被特异性Ca2+通道阻滞剂硝苯地平阻断，其半阻断浓度几乎相同（ICa的KD = 0.9 microM，Ins的KD = 0.7 microM）。ICa和Ins对地尔硫卓的阻断也同样敏感（KD = 80 microM）。如果Ins通过Ca2+通道流动，那么Ins和ICa之间的这些相似之处最容易解释。显然，Ca2+通道带有高亲和力的Ca2+结合位点，并且在没有Ca2+时对单价阳离子具有高度通透性。在含有Ba2+和Ca2+混合物的细胞外溶液中测量了Ba2+电流（IBa）和ICa。IBa和Ins一样被Ca2+阻断。向Ca2+中添加Ba2+只会使电流产生很小的增加或没有增加，就好像当存在Ca2+时Ba2+只是少量通透一样。Ba2+/Ca2+混合物中的膜电流表现出异常的摩尔分数行为，这表明Ca2+通道是单排多离子孔。在Na+/Ca2+和Ba2+/Ca2+混合物中维持去极化时观察到复杂的电流瞬变。它们表明在离子混合物中，Ca2+通道优先转运Ca2+而不是Na+和Ba2+。因此，即使电流幅度表明在没有Ca2+时Na+或Ba2+的通透性与Ca2+的通透性一样高或更高，Ca2+通道对Ca2+也是有选择性的。我们得出结论，Ca2+通道的选择性通透性取决于Ca2+的存在。在模型计算中，我们的观察结果被解释为Ca2+通道是单排孔的结果。有人提出，Ca2+通道的许多离子选择性来自位于原本无选择性的水性孔中的高亲和力Ca2+结合位点。
