Elam T R, Lansman J B
Department of Physiology, University of California School of Medicine, San Francisco 94143-0450, USA.
J Gen Physiol. 1995 Apr;105(4):463-84. doi: 10.1085/jgp.105.4.463.
We have studied the role of Mg2+ in the inactivation of inwardly rectifying K+ channels in vascular endothelial cells. Inactivation was largely eliminated in Mg(2+)-free external solutions and the extent of inactivation was increased by raising Mg2+o. The dose-response relation for the reduction of channel open probability showed that Mg2+o binds to a site (KD = approximately 25 microM at -160 mV) that senses approximately 38% of the potential drop from the external membrane surface. Analysis of the single-channel kinetics showed that Mg2+ produced a class of long-lived closures that separated bursts of openings. Raising Mg2+o reduced the burst duration, but less than expected for an open-channel blocking mechanism. The effects of Mg2+o are antagonized by K+o in manner which suggests that K+ competes with Mg2+ for the inactivation site. Mg2+o also reduced the amplitude of the single-channel current at millimolar concentrations by a rapid block of the open channel. A mechanism is proposed in which Mg2+ binds to the closed channel during hyperpolarization and prevents it from opening until it is occupied by K+.
我们研究了Mg2+在血管内皮细胞内向整流钾通道失活过程中的作用。在无Mg2+的细胞外溶液中，失活作用基本消除，提高细胞外Mg2+浓度（Mg2+o）可增强失活程度。通道开放概率降低的剂量反应关系表明，Mg2+o与一个位点结合（在-160 mV时KD约为25 microM），该位点感知到从细胞外膜表面起约38%的电位降。单通道动力学分析表明，Mg2+产生了一类长时程关闭事件，将开放簇分隔开。提高Mg2+o可缩短开放簇持续时间，但短于开放通道阻断机制预期的程度。细胞外Mg2+（Mg2+o）的作用被细胞外K+（K+o）拮抗，这表明K+与Mg2+竞争失活位点。在毫摩尔浓度下，Mg2+o还通过快速阻断开放通道降低单通道电流幅度。我们提出了一种机制，即在超极化过程中Mg2+与关闭通道结合，阻止其开放，直到被K+占据。
