Low-affinity spermine block mediating outward currents through Kir2.1 and Kir2.2 inward rectifier potassium channels
Low-affinity spermine block mediating outward currents through Kir2.1 and Kir2.2 inward rectifier potassium channels
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DOI：
10.1113/jphysiol.2007.136028
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发表时间：
2007-09-15
影响因子：
5.5
通讯作者：
Yan, Ding-Hong
中科院分区：
文献类型：
--
作者：
Ishihara, Keiko;Yan, Ding-Hong
强内向整流钾电流(I-KIR)的外向分量在可兴奋和不可兴奋细胞的膜极化中起着关键作用，并受内部阳离子电压依赖性通道阻滞剂的调节。使用Kir2.1通道，我们以前证明了只对低亲和力模式的阻断敏感的一小部分电导可能携带很大一部分外向电流。为了进一步研究低亲和力阻断与外向I-KIR的相关性并探索其分子机制，我们研究了主要的Kir2通道阻滞剂精胺对Kir2.1和Kir2.2通道的阻断作用。外向Kir2.2电流的电流-电压关系在10、1和0.1mU精胺存在时分别出现一个峰、一个平台峰和两个峰，这是由于存在两个对精胺阻断敏感性不同的导体所致。当电流-电压关系呈现单峰时，如天然的I-Kir，外向Kir2.2电流主要由对低亲和力阻断敏感的电导所介导。它们的流动范围也比相应的Kir2.1电流更窄，因为对低亲和力阻断的敏感性比Kir2.1大3到4倍。减少外部[K+]使Kir2.1的高亲和力和低亲和力区块的电压依赖性与反转电位的变化平行，证实了低亲和力区块在调节外向I-KIR中的重要性。当检测到对内部阻滞剂敏感性降低的Kir2.1突变体时，跨膜孔区的D172N突变使几乎所有的电导只对低亲和力阻断敏感，而细胞质孔区的E224G突变降低了对低亲和力阻断的敏感性，而对高亲和力阻断或高/低电导比没有明显改变。这些突变的影响支持这样的假设，即Kir2通道存在于两种状态，对内部阳离子阻滞剂的易感性不同。
The outward component of the strong inward rectifier K+ current (I-Kir) plays a pivotal role in polarizing the membranes of excitable and non-excitable cells and is regulated by voltage-dependent channel block by internal cations. Using the Kir2.1 channel, we previously showed that a small fraction of the conductance susceptible only to a low-affinity mode of block likely carries a large portion of the outward current. To further examine the relevance of the low-affinity block to outward I-Kir and to explore its molecular mechanism, we studied the block of the Kir2.1 and Kir2.2 channels by spermine, which is the principal Kir2 channel blocker. Current-voltage relations of outward Kir2.2 currents showed a peak, a plateau and two peaks in the presence of 10, 1 and 0.1 mu M spermine, respectively, which was explained by the presence of two conductances that differ in their susceptibility to spermine block. When the current-voltage relations showed one peak, like those of native I-Kir, outward Kir2.2 currents were mediated mostly by the conductance susceptible to the low-affinity block. They also flowed in a narrower range than the corresponding Kir2.1 currents, because of 3- to 4-fold greater susceptibility to the low-affinity block than in Kir2.1. Reducing external [K+] shifted the voltage dependences of both the high- and low-affinity block of Kir2.1 in parallel with the shift in the reversal potential, confirming the importance of the low-affinity block in mediating outward I-Kir. When Kir2.1 mutants known to have reduced sensitivity to internal blockers were examined, the D172N mutation in the transmembrane pore region made almost all of the conductance susceptible only to low-affinity block, while the E224G mutation in the cytoplasmic pore region reduced the sensitivity to low-affinity block without markedly altering that to the high-affinity block or the high/low conductance ratio. The effects of these mutations support the hypothesis that Kir2 channels exist in two states having different susceptibilities to internal cationic blockers.