Hilgemann D W, Collins A, Matsuoka S
Department of Physiology, University of Texas Southwestern Medical, Dallas 75235.
J Gen Physiol. 1992 Dec;100(6):933-61. doi: 10.1085/jgp.100.6.933.
Dynamic responses of cardiac sodium-calcium exchange current to changes of cytoplasmic calcium and MgATP were monitored and analyzed in giant membrane patches excised from guinea pig myocytes. Secondary dependencies of exchange current on cytoplasmic calcium are accounted for in terms of two mechanisms: (a) The sodium-dependent inactivation process, termed I1 modulation, is itself strongly modulated by cytoplasmic calcium. Recovery from the I1 inactivated state is accelerated by increasing cytoplasmic calcium, and the calculated rate of entrance into I1 inactivation is slowed. (b) A second modulation process, termed I2 modulation, is not sodium dependent. As with I1 modulation, the entrance into I2 inactivation takes place over seconds in the absence of cytoplasmic calcium. The recovery from I2 inactivation is a calcium-dependent transition and is rapid (< 200 ms) in the presence of micromolar free calcium. I1 and I2 modulation can be treated as linear, independent processes to account for most exchange modulation patterns observed: (a) When cytoplasmic calcium is increased or decreased in the presence of high cytoplasmic sodium, outward exchange current turns on or off, respectively, on a time scale of multiple seconds. (b) When sodium is applied in the absence of cytoplasmic calcium, no outward current is activated. However, the full outward current is activated within solution switch time when cytoplasmic calcium is applied together with sodium. (c) The calcium dependence of peak outward current attained upon application of cytoplasmic sodium is shifted by approximately 1 log unit to lower concentrations from the calcium dependence of steady-state exchange current. (d) The time course of outward current decay upon decreasing cytoplasmic calcium becomes more rapid as calcium is reduced into the submicromolar range. (e) Under nearly all conditions, the time courses of current decay during application of cytoplasmic sodium and/or removal of cytoplasmic calcium are well fit by single exponentials. Both of the modulation processes are evidently affected by MgATP. Similar to the effects of cytoplasmic calcium, MgATP slows the entrance into I1 inactivation and accelerates the recovery from inactivation. MgATP additionally slows the decay of outward exchange current upon removal of cytoplasmic calcium by 2-10-fold, indicative of an effect on I2 inactivation. Finally, the effects of cytoplasmic calcium on sodium-calcium exchange current are reconstructed in simulations of the I1 and I2 modulation processes as independent reactions.
在从豚鼠心肌细胞分离出的巨大膜片中监测并分析了心脏钠钙交换电流对细胞质钙和MgATP变化的动态响应。交换电流对细胞质钙的二级依赖性可通过两种机制来解释：(a) 钠依赖性失活过程，称为I1调制，其本身受到细胞质钙的强烈调制。增加细胞质钙可加速从I1失活状态的恢复，并且计算得出的进入I1失活的速率减慢。(b) 第二个调制过程，称为I2调制，不依赖于钠。与I1调制一样，在没有细胞质钙的情况下，进入I2失活需要数秒时间。从I2失活的恢复是一个钙依赖性转变，在存在微摩尔游离钙的情况下很快（<200毫秒）。I1和I2调制可被视为线性、独立的过程，以解释观察到的大多数交换调制模式：(a) 当在高细胞质钠存在的情况下增加或减少细胞质钙时，外向交换电流分别在数秒的时间尺度上开启或关闭。(b) 当在没有细胞质钙的情况下施加钠时，不会激活外向电流。然而，当细胞质钙与钠一起施加时，在溶液切换时间内会激活完整的外向电流。(c) 施加细胞质钠时达到的峰值外向电流的钙依赖性相对于稳态交换电流的钙依赖性向较低浓度偏移约1个对数单位。(d) 随着钙降低到亚微摩尔范围，细胞质钙降低时外向电流衰减的时间进程变得更快。(e) 在几乎所有条件下，施加细胞质钠和/或去除细胞质钙期间电流衰减的时间进程都能很好地拟合为单指数形式。这两个调制过程显然都受到MgATP的影响。与细胞质钙的作用类似，MgATP减慢进入I1失活的速度并加速从失活状态的恢复。MgATP还使去除细胞质钙时外向交换电流的衰减减慢2至10倍，表明对I2失活有影响。最后，在I1和I2调制过程的模拟中，将细胞质钙对钠钙交换电流的影响重建为独立反应。
