Caffeine induces Ca2+ release by reducing the threshold for luminal Ca2+ activation of the ryanodine receptor.
Caffeine induces Ca2+ release by reducing the threshold for luminal Ca2+ activation of the ryanodine receptor.
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DOI：
10.1042/bj20080489
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发表时间：
2008-09-15
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影响因子：
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通讯作者：
Chen SR
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文献类型：
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作者：
Kong H;Jones PP;Koop A;Zhang L;Duff HJ;Chen SR
长期以来，咖啡因一直被用作研究RyR受体介导的Ca2+释放和心律失常的药理学探针。然而，咖啡因激活ryr的确切机制尚不清楚。在这里，我们研究了咖啡因对自发Ca2+释放的影响，以及对单心脏RyR （RyR2）通道对腔内或细胞质Ca2+的反应的影响。我们发现表达RyR2的HEK293细胞在重复添加亚最大浓度的咖啡因时表现出部分或“定量”的Ca2+释放。这种量Ca2+释放被瑞诺定消除。监测内质网腔内Ca2+显示，咖啡因降低腔内Ca2+阈值，在自发Ca2+释放发生。有趣的是，在10毫米咖啡因的存在下，自发的Ca2+释放以Ca2+振荡的形式持续存在，并且被ryanodine减少，这表明与ryanodine不同，咖啡因，即使在高浓度下，也不能保持通道开放。在单通道水平上，咖啡因显著降低了腔内Ca2+激活阈值，但对胞质Ca2+激活阈值影响不大，表明咖啡因的主要作用是降低腔内Ca2+激活阈值，而不是胞质Ca2+激活阈值。此外，与咖啡因一样，临床相关的、促心律失常的甲基黄嘌呤、氨茶碱和茶碱增强了RyR2的腔内Ca2+激活，并增加了自发Ca2+释放的倾向，模拟了与疾病相关的RyR2突变的影响。总的来说，我们的研究结果表明，咖啡因通过降低RyR2腔内Ca2+激活的阈值来触发Ca2+释放，并表明与疾病相关的RyR2突变和RyR2相互作用的促心律失常药物可能具有相同的心律失常机制。
Caffeine has long been used as a pharmacological probe for studying ryanodine receptor (RyR)-mediated Ca2+ release and cardiac arrhythmias. However, the precise mechanism by which caffeine activates RyRs is elusive. Here we investigated the effects of caffeine on spontaneous Ca2+ release and on the response of single cardiac RyR (RyR2) channels to luminal or cytosolic Ca2+. We found that HEK293 cells expressing RyR2 displayed partial or “quantal” Ca2+ release in response to repetitive additions of submaximal concentrations of caffeine. This quantal Ca2+ release was abolished by ryanodine. Monitoring of endoplasmic reticulum luminal Ca2+ revealed that caffeine reduced the luminal Ca2+ threshold at which spontaneous Ca2+ release occurs. Interestingly, spontaneous Ca2+ release in the form of Ca2+ oscillations persisted in the presence of 10 mM caffeine, and was diminished by ryanodine, demonstrating that unlike ryanodine, caffeine, even at high concentrations, does not hold the channel open. At the single channel level, caffeine markedly reduced the threshold for luminal Ca2+ activation, but had little effect on the threshold for cytosolic Ca2+ activation, indicating that the major action of caffeine is to reduce the luminal, but not the cytosolic, Ca2+ activation threshold. Furthermore, as with caffeine, the clinically relevant, pro-arrhythmic methylxanthines aminophylline and theophylline potentiated luminal Ca2+ activation of RyR2, and increased the propensity for spontaneous Ca2+ release, mimicking the effects of diseased-linked RyR2 mutations. Collectively, our results demonstrate that caffeine triggers Ca2+ release by reducing the threshold for luminal Ca2+ activation of RyR2, and suggest that disease-linked RyR2 mutations and RyR2-interacting pro-arrhythmic agents may share the same arrhythmogenic mechanism.