Department of Physiology, School of Medical Sciences, University of New South Wales, Sydney, NSW, 2052, Australia.
Pflugers Arch. 2010 Jun;460(1):131-52. doi: 10.1007/s00424-010-0792-6. Epub 2010 Mar 3.
The functional role of ligand-gated ion channels in the central nervous system depends on their relative anion-cation permeability. Using standard whole-cell patch clamp measurements and NaCl dilution potential measurements, we explored the effect of external divalent ions on anion-cation selectivity in alpha1-homomeric wild-type glycine receptor channels. We show that increasing external Ca(2+) from 0 to 4 mM resulted in a sigmoidal increase in anion-cation permeability by 37%, reaching a maximum above about 2 mM. Our accurate quantification of this effect required rigorous correction for liquid junction potentials (LJPs) using ion activities, and allowing for an initial offset potential. Failure to do this results in a considerable overestimation of the Ca(2+)-induced increase in anion-cation permeability by almost three-fold at 4 mM external Ca(2+). Calculations of LJPs (using activities)_ were validated by precise agreement with direct experimental measurements. External SO (4) (2-) was found to decrease anion-cation permeability. Single-channel conductance measurements indicated that external Ca(2+) both decreased Na(+) permeability and increased Cl(-) permeability. There was no evidence of Ca(2+) changing channel pore diameter. Theoretical modeling indicates that the effect is not surface charge related. Rather, we propose that, under dilution conditions, the presence of an impermeant Ca(2+) ion in the channel pore region just external to the selectivity filter tends to electrostatically retard outward movement of Na(+) ions and to enhance movement of Cl(-) ions down their energy gradients.
配体门控离子通道在中枢神经系统中的功能作用取决于其相对阴离子-阳离子通透性。本研究采用标准全细胞膜片钳测量和 NaCl 稀释电位测量，探索了外源性二价离子对α1-同型野生型甘氨酸受体通道阴离子-阳离子选择性的影响。结果表明，将外源性 Ca(2+)从 0 增加到 4 mM 时，阴离子-阳离子通透性呈 S 型增加 37%，在大约 2 mM 以上达到最大值。我们对这种效应的精确定量需要使用离子活度严格校正液接电位 (LJP)，并允许初始偏移电位。如果不这样做，会导致在 4 mM 外 Ca(2+)时对 Ca(2+)诱导的阴离子-阳离子通透性增加的估计值大大高估，几乎增加了三倍。使用活度计算的 LJP（使用活度）通过与直接实验测量的精确一致性得到验证。外源性 SO(4)(2-)被发现降低了阴离子-阳离子通透性。单通道电导测量表明，外 Ca(2+)既降低了 Na(+)通透性，又增加了 Cl(-)通透性。没有证据表明 Ca(2+)改变了通道孔径。理论模型表明，这种效应与表面电荷无关。相反，我们提出，在稀释条件下，选择性滤器外通道孔区域中不可渗透的 Ca(2+)离子倾向于静电阻碍 Na(+)离子的外向运动，并增强 Cl(-)离子沿其能量梯度的运动。
