Watanabe S, Narahashi T
J Gen Physiol. 1979 Nov;74(5):615-28. doi: 10.1085/jgp.74.5.615.
Ionic selectivity of the acetylcholine-activated ionic channel of frog endplate membranes to various organic cations has been studied. The ratio of test cation permeability (PX) to sodium permeability (PNa) was estimated by two methods, one based on the measurements in test cation solutions of the amplitude of transient depolarization induced by iontophoretic application of acetylcholine, and the other on the measurements of the reversal potential for the membrane current induced by iontophoretic application of acetylcholine under voltage-clamp conditions. The endplate channel is relatively nonselective to various test cations. The permeabilities relative to Na are ammonium (1.71), formamidine (1.49), methylamine (1.39), hydrazine (1.35), and Li (0.76), as measured from the reversal potential for acetylcholine currents, and guanidine (0.74), aminoguanidine (0.20), methylguanidine (0), and choline (0) as measured from the amplitude of acetylcholine potential. Methylguanidine and aminoguanidine block the endplate channel with the apparent dissociation constants of 0.5 and 15 mM, respectively. Based on these data, the dimensions of selectivity filter of acetylcholine-activated channel appear to be slightly larger than those of the sodium channel of frog nodes and smaller than those of the epithelial membrane of gallbladder of frogs and rabbits.
对蛙终板膜乙酰胆碱激活离子通道对各种有机阳离子的离子选择性进行了研究。通过两种方法估算测试阳离子通透性（PX）与钠通透性（PNa）的比值，一种方法基于在测试阳离子溶液中测量离子电泳施加乙酰胆碱引起的瞬时去极化幅度，另一种方法基于在电压钳制条件下测量离子电泳施加乙酰胆碱引起的膜电流的反转电位。终板通道对各种测试阳离子相对不具有选择性。从乙酰胆碱电流的反转电位测量得出，相对于Na的通透性为铵（1.71）、甲脒（1.49）、甲胺（1.39）、肼（1.35）和Li（0.76），从乙酰胆碱电位幅度测量得出，胍（0.74）、氨基胍（0.20）、甲基胍（0）和胆碱（0）。甲基胍和氨基胍分别以0.5和15 mM的表观解离常数阻断终板通道。基于这些数据，乙酰胆碱激活通道的选择性过滤器尺寸似乎略大于蛙神经节钠通道，小于蛙和兔胆囊上皮膜钠通道。
