Department of Pharmacology, University of California Davis, Davis, CA 95616, USA.
J Physiol. 2013 Apr 15;591(8):2067-86. doi: 10.1113/jphysiol.2013.252080. Epub 2013 Feb 11.
Cardiac Na(+)-Ca(2+) exchange (NCX) activity is regulated by [Ca(2+)]i. The physiological role and dynamics of this process in intact cardiomyocytes are largely unknown. We examined NCX Ca(2+) activation in intact rabbit and mouse cardiomyocytes at 37°C. Sarcoplasmic reticulum (SR) function was blocked, and cells were bathed in 2 mm Ca(2+). We probed Ca(2+) activation without voltage clamp by applying Na(+)-free (0 Na(+)) solution for 5 s bouts, repeated each 10 s, which should evoke [Ca(2+)]i transients due to Ca(2+) influx via NCX. In rested rabbit myocytes, Ca(2+) influx was undetectable even after 0 Na(+) applications were repeated for 2-5 min or more, suggesting that NCX was inactive. After external electric field stimulation pulses were applied, to admit Ca(2+) via L-type Ca(2+) channels, 0 Na(+) bouts activated Ca(2+) influx efficaciously, indicating that NCX had become active. Calcium activation increased with more field pulses, reaching a maximum typically after 15-20 pulses (1 Hz). At rest, NCX deactivated with a time constant typically of 20-40 s. An increase in [Na(+)]i, either in rabbit cardiomyocytes as a result of inhibition of Na(+)-K(+) pumping, or in mouse cardiomyocytes where normal [Na(+)]i is higher vs. rabbit, sensitized NCX to self-activation by 0 Na(+) bouts. In experiments with the SR functional but initially empty, the activation time course was slowed. It is possible that the SR initially accumulated Ca(2+) that would otherwise cause activation. We modelled Ca(2+) activation as a fourth-order highly co-operative process ([Ca]i required for half-activation K0.5act = 375 nm), with dynamics severalfold slower than the cardiac cycle. We incorporated this NCX model into an established ventricular myocyte model, which allowed us to predict responses to twitch stimulation in physiological conditions with the SR intact. Model NCX fractional activation increased from 0.1 to 1.0 as the frequency was increased from 0.2 to 2 Hz. By adjusting Ca(2+) activation on a multibeat time scale, NCX might better maintain a stable long-term Ca(2+) balance while contributing to the ability of myocytes to produce Ca(2+) transients over a wide range of intensity.
心脏钠钙交换（NCX）活性受 [Ca2+]i 调节。这一过程在完整心肌细胞中的生理作用和动力学仍知之甚少。我们在 37°C 下检查了完整的兔和鼠心肌细胞中的 NCX Ca2+激活。肌浆网（SR）功能被阻断，细胞在 2 mM Ca2+中孵育。我们通过施加 5 秒的无 Na+（0 Na+）溶液脉冲来探测 Ca2+激活，每 10 秒重复一次，这应该会由于 NCX 介导的 Ca2+内流而引起 [Ca2+]i 瞬变。在休息的兔心肌细胞中，即使在 0 Na+应用重复 2-5 分钟或更长时间后，也无法检测到 Ca2+内流，这表明 NCX 处于无活性状态。施加外部电场刺激脉冲后，通过 L 型 Ca2+通道允许 Ca2+进入，0 Na+脉冲有效地激活 Ca2+内流，表明 NCX 已变得活跃。钙激活随电场脉冲的增加而增加，通常在 15-20 个脉冲（1 Hz）后达到最大值。在休息状态下，NCX 的失活时间常数通常为 20-40 秒。兔心肌细胞中 Na+-K+泵抑制导致 [Na+]i 增加，或正常 [Na+]i 高于兔的鼠心肌细胞中 [Na+]i 增加，均可使 0 Na+脉冲对 NCX 的自激活敏感。在 SR 功能正常但最初为空的实验中，激活时间过程变慢。这可能是因为 SR 最初积累了 Ca2+，否则会导致激活。我们将 Ca2+激活建模为四阶高度协同的过程（半激活所需的 [Ca]i K0.5act = 375nm），动力学比心脏周期慢几倍。我们将这个 NCX 模型纳入到一个已建立的心室肌细胞模型中，这使我们能够在保留 SR 的生理条件下预测对抽搐刺激的反应。在从 0.2 到 2 Hz 的频率增加时，模型 NCX 的分数激活从 0.1 增加到 1.0。通过在多拍时间尺度上调整 Ca2+激活，NCX 可以更好地在广泛的强度范围内保持稳定的长期 Ca2+平衡，同时有助于心肌细胞产生 Ca2+瞬变的能力。
