Josephson I R, Cui Y
Department of Molecular and Cellular Physiology, University of Cincinnati, College of Medicine, Cincinnati, OH 45267-0576, USA.
Pflugers Arch. 1995 Sep;430(5):682-9. doi: 10.1007/BF00386162.
The nonlinear charge movements which occur during membrane depolarization of cardiac ventricular myocytes (QON) have been previously identified and separated, by kinetic and steady-state criteria, into constituent components arising from the gating of Na channels and Ca channels. In contrast, the nature and time course of the OFF charge movements (QOFF), which follow membrane repolarization have not been as clearly established. In order to address this question cardiac QOFF was studied using small-diameter, 17-day-old embryonic chick ventricular myocytes that can be rapidly and uniformly voltage-clamped. The application of brief (5.4 ms) depolarizing steps were employed to produce Na channel inactivation but little Ca channel inactivation. Following the return of the membrane potential to -100 mV QOFF, measured as the gating current termed IgOFF, displayed two kinetic components. Double exponential fits to IgOFF yielded time constants of a few tenths of a millisecond for the fast component (IgOFFfast) and of 1-2 ms for the slower component (IgOFFslow). The time course and voltage dependence for the slower component suggested that it might be linked to the inactivation, and the recovery from inactivation, of Na channels. In order to identify these kinetic components double-pulse protocols were employed in which the duration of the prepulse and the interval separating the prepulse and test pulse were varied. The time course for the decay of IgOFFslow following a brief inactivating prepulse was similar to the time course for the recovery of the Na channel QON (QNaRecov). Both IgOFFslow and QNaRecov preceded the recovery of the Na channel (ionic) current. The recovery from inactivation of both the Na current and QNa displayed a similar voltage dependence. These experiments have helped to identify the two components of cardiac IgOFF and, therefore, will facilitate the interpretation of further biophysical and pharmacological studies concerning cardiac Na channel and Ca channel gating charge movements.
在心室肌细胞去极化过程中发生的非线性电荷移动（QON），先前已根据动力学和稳态标准，被识别并分离为由钠通道和钙通道门控产生的组成成分。相比之下，膜复极化后出现的关断电荷移动（QOFF）的性质和时间进程尚未明确确立。为了解决这个问题，我们使用可快速且均匀电压钳制的17日龄小直径鸡胚心室肌细胞对心脏QOFF进行了研究。应用短暂（5.4毫秒）的去极化步骤来使钠通道失活，但很少使钙通道失活。在膜电位回到-100 mV后，作为门控电流测量的QOFF（称为IgOFF）显示出两个动力学成分。对IgOFF进行双指数拟合得到快速成分（IgOFFfast）的时间常数为几十分之一毫秒，较慢成分（IgOFFslow）的时间常数为1 - 2毫秒。较慢成分的时间进程和电压依赖性表明它可能与钠通道的失活及失活恢复有关。为了识别这些动力学成分，采用了双脉冲方案，其中预脉冲的持续时间以及预脉冲和测试脉冲之间的间隔是变化的。短暂失活预脉冲后IgOFFslow的衰减时间进程与钠通道QON（QNaRecov）的恢复时间进程相似。IgOFFslow和QNaRecov都先于钠通道（离子）电流的恢复。钠电流和QNa的失活恢复都表现出相似的电压依赖性。这些实验有助于识别心脏IgOFF的两个成分，因此将有助于解释关于心脏钠通道和钙通道门控电荷移动的进一步生物物理和药理学研究。
