Spencer C Ian, Sham James S K
Division of Pulmonary and Critical Care Medicine, Johns Hopkins Medical Institutions, Baltimore, MD 21224, USA.
Am J Physiol Heart Circ Physiol. 2003 Dec;285(6):H2552-62. doi: 10.1152/ajpheart.00274.2003. Epub 2003 Aug 21.
In cardiac cells, evoked Ca2+ releases or spontaneous Ca2+ waves activate the inward Na+/Ca2+ exchange current (INaCa), which may modulate membrane excitability and arrhythmogenesis. In this study, we examined changes in membrane potential due to INaCa elicited by sarcoplasmic reticulum (SR) Ca2+ release in guinea pig ventricular myocytes using whole cell current clamp, fluorescence, and confocal microscopy. Inhibition of INaCa by Na+-free, Li+-containing Tyrode solution reversibly abbreviated the action potential duration at 90% repolarization (APD90) by 50% and caused SR Ca2+ overload. APD90 was similarly abbreviated in myocytes exposed to the Na+/Ca2+ exchange inhibitor KB-R7943 (5 microM) or after inhibition of SR Ca2+ release with ryanodine (20 microM). In the absence of extracellular Na+, spontaneous SR Ca2+ releases caused minimal changes in resting membrane potential. After the myocytes were returned to Na+-containing solution, the potentiated intracellular Ca2+ concentration ([Ca2+]i) transients dramatically prolonged APD90 and [Ca2+]i oscillations caused delayed and early afterdepolarizations (DADs and EADs). Laser-flash photolysis of caged Ca2+ mimicked the effects of spontaneous [Ca2+]i oscillations, confirming that APD prolongation, DADs, and EADs could be ascribed to intracellular Ca2+ release. These results suggest that Na+/Ca2+ exchange is a major physiological determinant of APD and that INaCa activation by spontaneous SR Ca2+ release/oscillations, depending on the timing, can account for both DADs and EADs during SR Ca2+ overload.
在心肌细胞中，诱发的Ca2+释放或自发性Ca2+波激活内向Na+/Ca2+交换电流（INaCa），这可能会调节膜兴奋性和心律失常的发生。在本研究中，我们使用全细胞电流钳、荧光和共聚焦显微镜，研究了豚鼠心室肌细胞中肌浆网（SR）Ca2+释放引起的INaCa所导致的膜电位变化。用无钠、含锂的台氏液抑制INaCa可使90%复极化时的动作电位时程（APD90）可逆性缩短50%，并导致SR Ca2+过载。暴露于Na+/Ca2+交换抑制剂KB-R7943（5 microM）的心肌细胞或用ryanodine（20 microM）抑制SR Ca2+释放后，APD90也有类似程度的缩短。在没有细胞外钠的情况下，自发性SR Ca2+释放引起静息膜电位的微小变化。当心肌细胞恢复到含钠溶液后，增强的细胞内Ca2+浓度（[Ca2+]i）瞬变显著延长了APD90，[Ca2+]i振荡引起延迟后去极化和早期后去极化（DADs和EADs）。笼锁Ca2+的激光闪光光解模拟了自发性[Ca2+]i振荡的作用，证实APD延长、DADs和EADs可归因于细胞内Ca2+释放。这些结果表明，Na+/Ca2+交换是APD的主要生理决定因素，并且自发性SR Ca2+释放/振荡对INaCa的激活，取决于其发生时间，可解释SR Ca2+过载期间的DADs和EADs。
