School of Medicine, Department of Physiology, National Autonomous University of Mexico (Unam), México City, México.
Channels (Austin). 2021 Dec;15(1):648-665. doi: 10.1080/19336950.2021.1993037.
K ions exert a structural effect that brings stability to K selective pores. Thus, upon bathing Shab channels in 0 K solutions the ion conductance, G, irreversibly collapses. Related to this, studies with isolated KcsA channels have suggested that there is a transition [K] around which the pore takes one of two conformations, either the low (non-conducting) or high K (conducting) crystal structures. We examined this premise by looking at the K-dependency of G stability of Shab channels within the cell membrane environment. We found that: K effect on G stability is highly asymmetrical, and that as internal K is replaced by Na G drops in a way that suggests a transition internal [K]. Additionally, we found that external permeant ions inhibit G drop with a potency that differs from the global selectivity-sequence of K pores; the non-permeant TEA inhibited G drop in a K-dependent manner. Upon lowering internal [K] we observed an influx of Na at negative potentials. Na influx was halted by physiological external [K], which also restored G stability. Hyperpolarized potentials afforded G stability but, as expected, do not restore G selectivity. For completeness, Na interaction with Shab was also assessed at depolarized potentials by looking at Na block followed by permeation (pore unblock) at positive potentials, in solutions approaching the 0 K limit. The stabilizing effect of negative potentials along with the non-parallel variation of Na permeability and conductance-stability herein reported, show that pore stability and selectivity, although related, are not strictly coupled.
钾离子产生结构效应，使钾选择性孔稳定。因此，当用 0K 溶液浴 Shab 通道时，离子电导 G 不可逆地崩溃。与此相关，用分离的 KcsA 通道进行的研究表明，存在一个转变[K]，孔在这个转变点采取两种构象之一，要么是低（非传导），要么是高 K（传导）晶体结构。我们通过观察细胞膜环境中 Shab 通道的 G 稳定性对 K 的依赖性来检验这一前提。我们发现：K 对 G 稳定性的影响高度不对称，并且随着内部 K 被 Na 取代，G 以一种表明内部[K]转变的方式下降。此外，我们发现外部可渗透离子以一种不同于 K 孔整体选择性序列的效力抑制 G 下降；非渗透 TEA 以 K 依赖性方式抑制 G 下降。当内部[K]降低时，我们观察到负电位处 Na 的内流。生理上的外部[K]可以阻止 Na 内流，同时恢复 G 的稳定性。超极化电位赋予 G 稳定性，但正如预期的那样，不会恢复 G 的选择性。为了完整性，还在去极化电位下评估了 Na 与 Shab 的相互作用，方法是观察 Na 阻断后在正电位下的渗透（孔再阻断），在接近 0K 极限的溶液中进行。负电位的稳定作用以及此处报告的 Na 通透性和电导稳定性的非平行变化表明，尽管孔稳定性和选择性相关，但它们不是严格耦合的。
