Department of Physiology, University of Toronto, Heart and Stroke/Richard Lewar Centre of Excellence, Fitzgerald Building, 150 College Street, Room 68, Toronto, ON M5S 3E2, Canada.
Basic Res Cardiol. 2011 Nov;106(6):967-77. doi: 10.1007/s00395-011-0202-z. Epub 2011 Jul 21.
Excitation-contraction coupling (ECC) in cardiac myocytes involves triggering of Ca(2+) release from the sarcoplasmic reticulum (SR) by L-type Ca channels, whose activity is strongly influenced by action potential (AP) profile. The contribution of Ca(2+) entry via the Na(+)/Ca(2+) exchanger (NCX) to trigger SR Ca(2+) release during ECC in response to an AP remains uncertain. To isolate the contribution of NCX to SR Ca(2+) release, independent of effects on SR Ca(2+) load, Ca(2+) release was determined by recording Ca(2+) spikes using confocal microscopy on patch-clamped rat ventricular myocytes with Ca(2+) fixed at 150 nmol/L. In response to AP clamps, normalized Ca(2+) spike amplitudes (ΔF/F (0)) increased sigmoidally and doubled as Na(+) was elevated from 0 to 20 mmol/L with an EC(50) of ~10 mmol/L. This Na(+)-dependence was independent of I (Na) as well as SR Ca(2+) load, which was unchanged under our experimental conditions. However, NCX inhibition using either KB-R7943 or XIP reduced ΔF/F (0) amplitude in myocytes with 20 mmol/L Na(+), but not with 5 mmol/L Na(+). SR Ca(2+) release was complete before the membrane repolarized to -15 mV, indicating Ca(2+) entry into the dyad (not reduced extrusion) underlies Na(+)-dependent enhancement of ECC. Because I (Ca,L) inhibition with 50 mmol/L Cd(2+) abolished Ca(2+) spikes, our results demonstrate that during cardiac APs, NCX enhances SR Ca(2+) release by synergistically increasing the efficiency of I (Ca,L)-mediated ECC.
兴奋-收缩偶联（ECC）在心肌细胞中涉及 L 型钙通道触发肌浆网（SR）中的 Ca2+释放，其活性受动作电位（AP）形态的强烈影响。AP 期间，通过 Na+/Ca2+交换体（NCX）进入的 Ca2+对触发 SR Ca2+释放的贡献仍不确定。为了分离 NCX 对 SR Ca2+释放的贡献，使其独立于对 SR Ca2+负荷的影响，我们使用共聚焦显微镜在经膜片钳钳制的大鼠心室肌细胞上记录 Ca2+峰来确定 Ca2+释放，[Ca2+]i 固定在 150 nmol/L。响应 AP 钳制，标准化 Ca2+峰幅度（ΔF/F（0））呈 S 型增加，当 [Na+]i 从 0 增加到 20 mmol/L 时增加一倍，EC50 约为 10 mmol/L。这种[Na+]i-依赖性与 I（Na）以及 SR Ca2+负荷无关，在我们的实验条件下，SR Ca2+负荷保持不变。然而，使用 KB-R7943 或 XIP 抑制 NCX 会降低 20 mmol/L [Na+]i 下心肌细胞的ΔF/F（0）幅度，但对 5 mmol/L [Na+]i 下的细胞无影响。SR Ca2+释放在膜复极化到-15 mV 之前完成，表明 ECC 依赖[Na+]的增强是由 Ca2+进入二联体（而不是减少的外排）引起的。由于用 50 mmol/L Cd2+抑制 I（Ca，L）消除了 Ca2+峰，因此我们的结果表明，在心脏 AP 期间，NCX 通过协同增加 I（Ca，L）介导的 ECC 的效率来增强 SR Ca2+释放。
