INTRACELLULAR CA-TRANSIENTS IN RAT CARDIAC MYOCYTES - ROLE OF NA-CA EXCHANGE IN EXCITATION-CONTRACTION COUPLING
INTRACELLULAR CA-TRANSIENTS IN RAT CARDIAC MYOCYTES - ROLE OF NA-CA EXCHANGE IN EXCITATION-CONTRACTION COUPLING
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DOI：
10.1152/ajpcell.1990.258.5.c944
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发表时间：
1990-05-01
影响因子：
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通讯作者：
BERLIN, JR
中科院分区：
文献类型：
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作者：
BERS, DM;LEDERER, WJ;BERLIN, JR
在电压钳控制下，记录大鼠心肌细胞膜电流和细胞内钙离子浓度([Ca~(2+)]_i)的瞬变。用含钙荧光指示剂Indo-1和0.5 mM Na的贴片移液管溶液透析细胞。在这些实验条件下，即使在没有细胞外Na的情况下，也不能观察到通过Na-Ca交换进入细胞内的钙。电压钳脉冲持续时间从5ms增加到80ms时，[Ca]i瞬变幅度增大，而峰值钙电流不变。在较负的测试电位(例如，-30至-30 mV)下，[Ca]i瞬变的这种持续时间依赖性最明显，并且不能被无钠溶液定性地改变。后者的结果表明，当膜在非常短暂的去极化后被重新极化时，通过Na-Ca交换的Ca挤出不是观察到的较小的[Ca]i瞬变的原因。虽然峰值钙电流不随脉冲宽度的增加而改变，但积分钙电流增加。这些观察结果与心肌兴奋-收缩偶联的钙释放机制相一致：1)钙释放过程受膜电位的调节；2)通过钙通道进入细胞的钙优先进入控制肌浆网钙释放的部位(S)。本文还探讨了Na-Ca交换在松弛时[Ca]i下降中的作用。去细胞外钠(NaO)使松弛过程中[Ca]i的下降速度减慢20%。由此，我们得出结论，Na-Ca交换与SR竞争从细胞质中去除Ca，在我们的控制条件下，交换器可能占这种下降的20%。膜电位越正，对Na_0的依赖程度越低，而肌浆网钙负荷越大，对Na_0的依赖性越强。
Membrane current and intracellular Ca concentration ([Ca2+]i) transients were recorded from isolated rat ventricular myocytes under voltage-clamp control. The cells were dialyzed by the patch pipette solution, which contained the flurorescent Ca indicator indo-1 and 0.5 mM Na. Under these experimental conditions, Ca entry via Na-Ca exchange did not appear to be appreciable even in the absence of extracellular Na. Increasing the duration of voltage-clamp pulses from 5 to 80 ms produced [Ca]i transients of increasing amplitude, while the peak Ca current was not changed. This duration dependence of the [Ca]i transient was most demonstrable at more negative test potentials (e.g., -30 to -30 mV) and was not qualitatively modified by Na-free solutions. This latter result indicates that Ca extrusion by Na-Ca exchange is not responsible for the smaller [Ca]i transients observed when the membrane is repolarized after very brief depolarizations. Although the peak Ca current was not changed by increasing pulse duration, the integrated Ca current was increased. These observations are consistent with a Ca-release mechanism in cardiac excitation-contraction coupling in which 1) the Ca-release process can be modulated by membrane potential or 2) the Ca entering the cell via Ca channels has a preferential access [compared with Ca from the sarcoplasmic reticulum (SR)] to the site(s) that control SR ca release. The role of Na-Ca exchange in the decline of [Ca]i during relaxation was also explored. Removal of extracellular Na (Nao) resulted in 20% slowing of the decline in [Ca]i during relaxation. From this, we conclude that the Na-Ca exchange competes with SR to remove Ca from the cytoplasm and that under our control conditions the exchanger may account for 20% of this decline. The Na0 dependence of relaxation was reduced at more positive membrane potentials and increased by SR Ca loading.