Clay J R
Laboratory of Neurophysiology, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, MD 20892, USA.
J Membr Biol. 1996 Oct;153(3):195-201. doi: 10.1007/s002329900122.
An increase in extracellular potassium ion concentration, Ko, significantly slows the potassium channel deactivation rate in squid giant axons, as previously shown. Surprisingly, the effect does not occur in all preparations which, coupled with the voltage independence of this result in preparations in which it does occur, suggests that it is mediated at a site outside of the electric field of the channel, and that this site is accessible to potassium ions in some preparations, but not in others. In other words, the effect does not appear to be related to occupancy of the channel by potassium ions. This conclusion is supported by a four-barrier, three-binding site model of single file diffusion through the channel in which one site, at most, is unoccupied by a potassium ion (single-vacancy model). The model is consistent with current-voltage relations with various levels of Ko, and, by definition, with multiple occupancy by K+. The model predicts that occupancy of any given site is essentially independent of Ko (or Ki). The effects of extracellular Rb+ and Cs+ on gating are strongly voltage dependent, and they were observed in all preparations investigated. Consequently, the mechanism underlying these results would appear to be different from that which underlies the effect of K+ on gating. In particular, the effect of Rb+ on gating is reduced by strong hyperpolarization, which in the context of the occupancy hypothesis, is consistent with the voltage dependence of the current-voltage relation in the presence of Rb+. The primary, novel, finding in this study is that the effects of Cs+ are counterintuitive in this regard. Specifically, the slowing of channel deactivation rate by Cs+ is also reduced by hyperpolarization, similar to the Rb+ results, whereas blockade is enhanced, which is seemingly inconsistent with the concept that occupancy of the channel by Cs+ underlies the effect of this ion on gating. This result is further elucidated by barrier modeling of the current-voltage relation in the presence of Cs+.
如先前所示，细胞外钾离子浓度（Ko）的增加会显著减慢鱿鱼巨轴突中钾通道的失活速率。令人惊讶的是，并非所有标本都会出现这种效应，而且在出现这种效应的标本中，该结果与电压无关，这表明它是在通道电场之外的位点介导的，并且在某些标本中该位点可被钾离子接近，而在其他标本中则不然。换句话说，这种效应似乎与钾离子占据通道无关。这一结论得到了一个四屏障、三结合位点的单通道扩散模型的支持，在该模型中，通道中最多只有一个位点未被钾离子占据（单空位模型）。该模型与不同Ko水平下的电流 - 电压关系一致，并且根据定义，与K + 的多重占据一致。该模型预测任何给定位点的占据基本上与Ko（或Ki）无关。细胞外Rb + 和Cs + 对门控的影响强烈依赖于电压，并且在所有研究的标本中都观察到了这种现象。因此，这些结果背后的机制似乎与K + 对门控的影响机制不同。特别是，Rb + 对门控的影响会因强超极化而减弱，在占据假说的背景下，这与存在Rb + 时电流 - 电压关系的电压依赖性一致。本研究的主要新发现是，Cs + 的效应在这方面违反直觉。具体而言，Cs + 导致的通道失活速率减慢也会因超极化而降低，这与Rb + 的结果类似，而阻断作用增强，这似乎与Cs + 占据通道是该离子对门控影响的基础这一概念不一致。在存在Cs + 的情况下对电流 - 电压关系进行屏障建模进一步阐明了这一结果。
