THE MG-2+ BLOCK AND INTRINSIC GATING UNDERLYING INWARD RECTIFICATION OF THE K+ CURRENT IN GUINEA-PIG CARDIAC MYOCYTES
THE MG-2+ BLOCK AND INTRINSIC GATING UNDERLYING INWARD RECTIFICATION OF THE K+ CURRENT IN GUINEA-PIG CARDIAC MYOCYTES
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DOI：
10.1113/jphysiol.1989.sp017874
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发表时间：
1989-12-01
影响因子：
5.5
通讯作者：
TAKANO, M
中科院分区：
文献类型：
--
作者：
ISHIHARA, K;MITSUIYE, T;TAKANO, M
用油隙电压钳方法研究了内向整流钾电流的内向整流钾电流的内向整流性基础--镁离子的阻断和通道的本征门控作用。内向整流性K+电流是在细胞外K+浓度([K+]o)为0 mM时记录的跨间隙电流减去在细胞内一定浓度([Mg2+]i)下14 mM[K+]o时记录的跨间隙电流而得到的。差值电流的反转电位(V0)接近K+的平衡电位(EK)。在跨EK的复极化时，内向整流钾电流呈现迅速的指数增长。时间常数随超极化程度的增加而减小，但不受[Mg~(2+)]i和前一次去极化的影响。当预脉冲电位在V0-20和V0+30 mV之间逐渐变正时，在任何[Mg2+]i处，时间相关分量的幅度变大，前一电流跳跃减小。当预脉冲比V0+40 mV更正时，时间相关分量几乎从2µm[Mg2+]i的零电流水平开始。然而，在较高的[Mg2+]i(350,500和3000µm)时，随着预脉冲电位变得比V0+40 mV更正，时间依赖分量变小。当膜在2µm[Mg~(2+)]i时从完全激活的电势去极化时，外向电流的初始跳跃是欧姆的，随后是指数衰减。内向电流的时间相关分量，在增加前一个去极化持续时间后记录在复极化上，随着外向电流的衰减而发展。这两个过程的时间常数很好地吻合。在500µm[Mg~(2+)]_i时，去极化的外向电流瞬间被整流。记录在复极化上的含时分量随前脉冲的延长而发展，其时间进程比2µm[Mg~(2+)]i慢。包络时间进程随着去极化电位的增加而变慢。将温度从23度降至15度。C使随时间变化的电流变慢，在Vo时的表观Q10约为3.5。根据实验数据，估算了能较好地模拟K+电流内向整流的镁离子阻塞模型的动力学参数。结果表明，去极化的瞬时内向整流是由于生理[Mg~(2+)]i处的镁离子阻断所致。在维持去极化的过程中，通道逐渐从阻断状态向闭合状态转变。通过复极化解除镁离子块几乎是瞬间的，并且内向电流的随时间变化的分量反映了门控机构从闭合状态到打开状态的转变。
The blockade by Mg2+ and intrinsic gating of the channel, which underlie the rectification of the inward rectifier K+ current, was investigated using the oil-gap voltage clamp method in isolated guinea-pig ventricular cells. The inward rectifier K+ current was isolated by subtracting trans-gap currents recorded at an extracellular K+ concentration ([K+]o) of 0 mM from those obtained at 14 mM [K+]o in the presence of a given concentration of intracellular Mg2+ ([Mg2+]i). The reversal potential (V0) of the difference current was near the equilibrium potential for K+ (EK). On repolarization across EK, the inward rectifier K+ current showed a rapid exponential increase. The time constant decreased with increasing hyperpolarization, but it was independent of both [Mg2+]i and the preceding depolarization. When the pre-pulse potential was made progressively positive between V0-20 and V0 + 30 mV, the amplitude of the time-dependent component became larger and the preceding current jump decreased at any [Mg2+]i. With pre-pulses more positive than V0 + 40 mV, the time-dependent component started from almost the zero current level at 2 .mu.M [Mg2+]i. At higher [Mg2+]i (350, 500 and 3000 .mu.M), however, the time-dependent component became smaller as the pre-pulse potential was made more positive than V0 + 40 mV. When the membrane was depolarized from a potential of full activation at 2 .mu.M [Mg2+]i, the initial jump in the outward current was ohmic and was followed by an exponential decay. The time-dependent component of the inward current, recorded on repolarization after increasing durations of the preceding depolarization, developed as the outward current decayed. The time constants of both processes were in good agreement. At 500 .mu.M [Mg2+]i, the outward current on depolarization was instantaneously rectified. The time-dependent component recorded on repolarization developed with prolongation of the pre-pulse with a time course slower than at 2 .mu.M [Mg2+]i. The envelope time course became slower as the potential of the depolarization became more positive. Lowering the temperature from 23 to 15.degree. C slowed the time-dependent current with an apparent Q10 of about 3.5 At Vo. Based on the experimental data, kinetic parameters were estimated for a model of Mg2+ block, which well simulated the inward-going rectification of the K+ current. It is concluded that the instantaneous inward rectification on depolarization is due to the Mg2+ block at physiological [Mg2+]i. During maintained depolarization, the channels gradually transit from the blocked state to the closed state. The relief of the Mg2+ block by repolarization is virtually instaneously and the time-dependent component of the inward current reflects transition from the closed state to the open state of a gating mechanism.