Adams D J, Nonner W, Dwyer T M, Hille B
J Gen Physiol. 1981 Dec;78(6):593-615. doi: 10.1085/jgp.78.6.593.
Motor endplates of frog semitendinosus muscles were studied under voltage clamp. Current fluctuations induced by iontophoretic application of acetylcholine were analyzed to give the elementary conductance, gamma , and mean open time, tau , of endplate channels. Total replacement of the external Na+ ion by several other metal ions and by many permeant organic cations changed both gamma and tau . Except with NH4+ ions, the gamma values with foreign test ions were all smaller than expected from the independence relation and their previously measured permeability ratios. The more hydrophobic ions gave the smallest gamma values. Foreign permeant cations also depress gamma when mixed with Na+ ions. These effects could be interpreted in terms of binding of ions to a saturable site within the endplate channel as they pass through. The site for organic ions would have a hydrophobic component. Similar evidence is given for a metal ion binding site on the cytoplasmic end of the channel accessible to internal ions. Most foreign cations also shortened tau when applied externally. The changes of gating did not seem to be correlated with changes in gamma . Thus there is no evidence for control of tau by ions bound within the pore.
在电压钳制条件下研究了青蛙半腱肌的运动终板。分析了离子电泳施加乙酰胆碱所诱发的电流波动，以得出终板通道的基本电导γ和平均开放时间τ。用其他几种金属离子和许多可通透的有机阳离子完全替代外部Na⁺离子，会改变γ和τ。除了NH₄⁺离子外，与外来测试离子相关的γ值均小于独立关系及其先前测量的渗透率比值所预期的值。疏水性越强的离子，γ值越小。当外来可通透阳离子与Na⁺离子混合时，也会降低γ。这些效应可以通过离子在穿过终板通道时与通道内一个可饱和位点的结合来解释。有机离子的结合位点会有一个疏水成分。对于通道胞质端可被内部离子接近的金属离子结合位点，也给出了类似的证据。大多数外来阳离子在外部施加时也会缩短τ。门控的变化似乎与γ的变化无关。因此，没有证据表明孔内结合的离子会控制τ。
