Yang T, Snyders D J, Roden D M
Vanderbilt University School of Medicine, Department of Medicine, Nashville, TN 37232-6602, USA.
Circ Res. 1997 Jun;80(6):782-9. doi: 10.1161/01.res.80.6.782.
Two characteristic features of the rapid component of the cardiac delayed rectifier current (IKr) are prominent inward rectification and an unexpected reduction in activating current with decreased [K+]o. Similar features are observed with heterologous expression of HERG, the gene thought to encode the channel carrying IKr, moreover, recent studies indicate that the mechanism underlying rectification of HERG current is the inactivation that channels rapidly undergo during depolarizing pulses. The present studies were designed to determine the mechanism of IKr rectification and [K+]o sensitivity in the mouse atrial myocyte cell line, AT-1 cells. Reducing [Mg2+]i to 0, which reverses inward rectification of some K+ channels, did not alter IKr current-voltage relationships, although it did decrease sensitivity to the IKr blockers dofetilide and quinidine 2- to 5-fold. To determine the presence and extent of fast inactivation of IKr in AT-1 cells, a brief hyperpolarizing pulse (20 ms to -120 mV) was applied during long depolarizations. Immediately after this pulse, a very large outward current that decayed rapidly to the previous activating current baseline was observed. This outward current component was blocked by the IKr-specific inhibitor dofetilide, indicating that it represented recovery from fast inactivation during the hyperpolarizing step, with fast reinactivation during the return to depolarized potential. With removal of inactivation using this approach, current-voltage relationships for IKr ([K+]o, 1 to 20 mmol/L) were linar and reversed close to the predicted Nernst potential for K+. In addition, decreased [K+]o decreased the time constants for open-->inactivated and inactivated-->open transitions. Thus, in these cardiac myocytes, as with heterologously expressed HERG, IKr undergoes fast inactivation that determines its characteristic inward rectification. These studies demonstrate that the mechanism underlying decreased activating current observed at low [K+]o is more extensive fast inactivation.
心脏延迟整流电流（IKr）快速成分的两个特征是显著的内向整流以及随着细胞外[K+]浓度降低激活电流意外减小。在异源表达HERG（被认为编码携带IKr的通道的基因）时也观察到类似特征，此外，最近的研究表明HERG电流整流的潜在机制是通道在去极化脉冲期间迅速发生的失活。本研究旨在确定小鼠心房肌细胞系AT-1细胞中IKr整流和对[K+]o敏感性的机制。将细胞内[Mg2+]降至0（这可逆转某些K+通道的内向整流），虽使对IKr阻滞剂多非利特和奎尼丁的敏感性降低2至5倍，但并未改变IKr的电流-电压关系。为确定AT-1细胞中IKr快速失活的存在及程度，在长时去极化期间施加一个短暂的超极化脉冲（20毫秒至-120毫伏）。在此脉冲后立即观察到一个非常大的外向电流，该电流迅速衰减至先前的激活电流基线。这个外向电流成分被IKr特异性抑制剂多非利特阻断，表明它代表超极化步骤期间从快速失活的恢复，以及回到去极化电位期间的快速再失活。采用这种方法去除失活后，IKr（[K+]o为1至20毫摩尔/升）的电流-电压关系呈线性，且在接近预测的K+能斯特电位处反转。此外，降低[K+]o会减小开放→失活和失活→开放转变的时间常数。因此，在这些心肌细胞中，与异源表达的HERG一样，IKr经历快速失活，这决定了其特征性的内向整流。这些研究表明，在低[K+]o时观察到的激活电流降低的潜在机制是更广泛的快速失活。
