Department of Cell and Molecular Physiology, Loyola University Chicago, Stritch School of Medicine, Maywood, IL 60153, USA.
J Physiol. 2010 Dec 1;588(Pt 23):4743-57. doi: 10.1113/jphysiol.2010.197913. Epub 2010 Oct 20.
Sarcoplasmic reticulum (SR) Ca²(+) leak is an important component of cardiac Ca²(+) signalling. Together with the SR Ca²(+)-ATPase (SERCA)-mediated Ca²(+) uptake, diastolic Ca²(+) leak determines SR Ca²(+) load and, therefore, the amplitude of Ca²(+) transients that initiate contraction. Spontaneous Ca²(+) sparks are thought to play a major role in SR Ca²(+) leak. In this study, we determined the quantitative contribution of sparks to SR Ca²(+) leak and tested the hypothesis that non-spark mediated Ca²(+) release also contributes to SR Ca²(+) leak. We simultaneously measured spark properties and intra-SR free Ca²(+) (Ca²(+)) after complete inhibition of SERCA with thapsigargin in permeabilized rabbit ventricular myocytes. When Ca²(+) declined to 279 ± 10 μm, spark activity ceased completely; however SR Ca²(+) leak continued, albeit at a slower rate. Analysis of sparks and Ca²(+) revealed, that SR Ca²(+) leak increased as a function of Ca²(+), with a particularly steep increase at higher Ca²(+) ( >600 μm) where sparks become a major pathway of SR Ca²(+) leak. At low Ca²(+) (< 300 μm), however, Ca²(+) leak occurred mostly as non-spark-mediated leak. Sensitization of ryanodine receptors (RyRs) with low doses of caffeine increased spark frequency and SR Ca²(+) leak. Complete inhibition of RyR abolished sparks and significantly decreased SR Ca²(+) leak, but did not prevent it entirely, suggesting the existence of RyR-independent Ca²(+) leak. Finally, we found that RyR-mediated Ca²(+) leak was enhanced in myocytes from failing rabbit hearts. These results show that RyRs are the main, but not sole contributor to SR Ca²(+) leak. RyR-mediated leak occurs in part as Ca²(+) sparks, but there is clearly RyR-mediated but Ca²(+) sparks independent leak.
肌浆网（SR）Ca²⁺渗漏是心脏 Ca²⁺信号的重要组成部分。与 SR Ca²⁺-ATP 酶（SERCA）介导的 Ca²⁺摄取一起，舒张期 Ca²⁺渗漏决定了 SR Ca²⁺负荷，从而决定了引发收缩的 Ca²⁺瞬变的幅度。自发 Ca²⁺火花被认为在 SR Ca²⁺渗漏中起主要作用。在这项研究中，我们确定了火花对 SR Ca²⁺渗漏的定量贡献，并检验了以下假设：非火花介导的 Ca²⁺释放也有助于 SR Ca²⁺渗漏。我们在兔心室肌细胞透化后用 thapsigargin 完全抑制 SERCA 的情况下，同时测量火花特性和 intra-SR 游离 Ca²⁺(Ca²⁺)。当 Ca²⁺下降到 279 ± 10 µm 时，火花活动完全停止；然而，SR Ca²⁺渗漏仍在继续，尽管速度较慢。对火花和 Ca²⁺的分析表明，SR Ca²⁺渗漏随 Ca²⁺的增加而增加，在较高的 Ca²⁺（>600 µm）时增加特别陡峭，此时火花成为 SR Ca²⁺渗漏的主要途径。然而，在较低的 Ca²⁺（<300 µm）时，Ca²⁺渗漏主要发生在非火花介导的渗漏中。用低剂量咖啡因敏化肌浆网 Ca²⁺释放通道（RyRs）会增加火花频率和 SR Ca²⁺渗漏。完全抑制 RyR 会消除火花并显著降低 SR Ca²⁺渗漏，但不能完全阻止它，这表明存在 RyR 独立的 Ca²⁺渗漏。最后，我们发现衰竭兔心心肌细胞中的 RyR 介导的 Ca²⁺渗漏增加。这些结果表明，RyRs 是 SR Ca²⁺渗漏的主要但不是唯一贡献者。RyR 介导的渗漏部分是 Ca²⁺火花，但显然也有 RyR 介导但 Ca²⁺火花独立的渗漏。
