Dept of Physiology, School of Medical Sciences, University of New South Wales, Sydney, Australia.
Channels (Austin). 2010 May-Jun;4(3):142-9. doi: 10.4161/chan.4.3.11020.
The functional role of ion channels, which allow counterion permeation, depends critically on their relative anion-cation relative selectivity. From whole-cell patch clamp reversal potential measurements under dilution potential conditions, we have already shown that anion-cation permeabilities of anion-selective wild-type (WT) and mutant (with larger pore diameter) glycine receptor (GlyR) channels in the presence of Li(+), Na(+) and Cs(+) counterions, were inversely correlated with the equivalent hydration diameter of the counterion, with chloride-cation permeability increasing as counterion equivalent hydration diameter increased with respect to the channel minimum pore diameter. Corrected for liquid junction potentials (LJPs; using ion activities), the previous chloride-cation permeabilities for the alkali cations were 23.4 (Li(+)), 10.9 (Na(+)) and 5.0 (Cs(+)) for the smaller WT channel. Further analysis to incorporate an initial offset potential correction, to fully allow for slight differences between internal cell composition and external control salt solution, changed the above permeability ratios to 30.6 (Li(+)), 11.8 (Na(+)) and 5.0 (Cs(+)), adding enhanced support for the inverse correlation between anion-to-counterion permeability ratio and equivalent hydrated counterion diameter relative to channel pore diameter (erroneously ignoring LJPs reduces each permeability ratio to about 4). Also, new direct measurements of LJPs (for NaCl and LiCl salt dilutions) using a 3M KCl-agar reference salt bridge (with freshly-cut end for each solution composition change) have shown excellent agreement with calculated LJPs (using ion activities), validating calculated LJP values. We continue to suggest that counterion cations permeate with chloride ions as neutral pairs.
离子通道的功能作用是允许反离子渗透，这取决于它们对阴阳离子的相对选择性。根据全细胞膜片钳反转电位测量，在稀释电位条件下，我们已经表明，在存在 Li(+)、Na(+) 和 Cs(+) 抗衡离子的情况下，阴离子选择性野生型 (WT) 和突变型 (具有更大孔径) 甘氨酸受体 (GlyR) 通道的阴离子-阳离子渗透性与抗衡离子的等效水合直径呈反比，氯离子-阳离子渗透性随着抗衡离子的等效水合直径相对于通道最小孔径的增加而增加。校正了液接电位 (LJP; 使用离子活度)，以前对于碱金属阳离子的氯离子-阳离子渗透性为 23.4 (Li(+))、10.9 (Na(+)) 和 5.0 (Cs(+))，对于较小的 WT 通道。进一步分析包括初始偏移电位校正，以充分考虑细胞内组成和外部控制盐溶液之间的微小差异，将上述渗透性比率改变为 30.6 (Li(+))、11.8 (Na(+)) 和 5.0 (Cs(+))，为阴离子与抗衡离子渗透性比率与通道孔径相对于等效水合抗衡离子直径之间的反比关系提供了更多支持（错误地忽略 LJP 将每个渗透性比率降低到约 4）。此外，使用 3M KCl-琼脂参考盐桥（每个溶液组成变化都有新鲜切口）直接测量 LJP（对于 NaCl 和 LiCl 盐稀释）的新方法与使用离子活度计算的 LJP 非常吻合，验证了计算的 LJP 值。我们继续建议抗衡离子阳离子与氯离子一起以中性对的形式渗透。
