A study on the molecular basis of the cardiac inward rectifier potassium channels

1.通过心脏强内向整流钾（K^+）通道的电流振幅已知是由细胞内多胺和Mg^<2+>对通道的电压依赖性阻断调节的。然而，封锁的详细机制是复杂的，并没有很好地理解。我们在人胚胎肾细胞293T中表达了Kir2.1，这是心脏向内流K^+通道的主要亚基，并使用电压钳法研究了多胺和Mg^<2+>对Kir2.1通道电流的阻断。我们发现，在存在不同浓度的细胞质多胺和Mg^<2+>时记录的Kir2.1电流都可以很好地描述为流经对多胺和Mg^<2+>阻塞具有不同敏感性的两个Kir2.1通道群体的电流。两个种群的相对大小由于多胺与胞内通道的相互作用而略有改变。我们的结果表明，通过心脏内向整流K^+通道的外向电流主要通过对细胞质阻滞剂敏感性较低的一小部分通道（~ 10%）。细胞内Mg^<2+>的生理浓度阻断了对细胞质阻滞剂表现出更高敏感性的主要通道（~ 90%），从而在多胺存在时增加了向外电流的振幅。我们研究了心房和心室肌细胞中强内向整流K^+电流差异的机制。我们的研究结果表明，组成通道的亚基（Kir2.1、Kir2.2或Kir2.3）的差异不能解释这种差异，但细胞内高浓度的多胺可以改变流过Kir2.1通道的电流，从“心室型”转变为“心房型”。测定的总多胺和游离多胺浓度在豚鼠心脏心房组织中确实较高。少

1.The amplitude of the currents through the cardiac strong inward rectifier potassium (K^+) channels is known to be regulated by a voltage-dependent blockade of the channels by intracellular polyamines and Mg^<2+>. However, the detailed mechanism of the blockade is complicated, and is not well understood. We expressed Kir2.1,which is the major subunit of the cardiac inward rectifier K^+ channels, in human embryonic kidney cells 293T and studied the polyamine and Mg^<2+> blockades of the Kir2.1 channel currents recorded from inside-out patch membranes using the voltage-clamp method. We found that the Kir2.1 currents recorded in the presence of various concentrations of cytoplasmic polyamines and Mg^<2+> can be all well described as those flowing through two populations of Kir2.1 channels having different sensitivities to the blockades by polyamines and Mg^<2+>. The relative size of the two populations was slightly altered by an interaction of polyamines with the channel at an intracellu … More lar regulatory site. Our results suggested that the outward currents through the cardiac inward rectifier K^+ channels flow mostly through a small population (〜10%) of the channels having lower sensitivity to cytoplasmic blockers. Physiological concentrations of intracellular Mg^<2+> blocks a major population (〜90%) of the channels showing higher sensitivity to cytoplasmic blockers and thereby increases the amplitude of the outward currents in the presence of polyamines.2.We studied the mechanisms underlying the difference between the strong inward rectifier K^+ currents in the cardiac atrial and ventricular myocytes. Our results indicated that the difference in the subunits (Kir2.1,Kir2.2,or Kir2.3) composing the channel cannot explain the difference, but a higher concentration of intracellular polyamines can change the currents flowing through the Kir2.1 channels from a ‘ventricular type' to an ‘atrial type'. The total and free polyamine concentrations measured were indeed higher in the atrial tissues of the guinea-pig hearts. Less