Cardiac alternans, defined as beat-to-beat alternations in action potential duration, cytosolic Ca transient (CaT) amplitude and cardiac contraction, is associated with atrial fibrillation (AF) and sudden cardiac death. At the cellular level, cardiac alternans is linked to abnormal intracellular calcium handling during excitation-contraction coupling. We investigated how pharmacological activation or inhibition of cytosolic Ca sequestration via mitochondrial Ca uptake and mitochondrial Ca retention affects the occurrence of pacing-induced CaT alternans in isolated rabbit atrial myocytes.Cytosolic CaTs were recorded using Fluo-4 fluorescence microscopy. Alternans was quantified as the alternans ratio (AR=1-CaT_Small/CaT_Large; CaT_Smalland CaT_Largeare the amplitudes of the small and large CaTs of a pair of alternating CaTs). Inhibition of mitochondrial Ca sequestration via mitochondrial Ca uniporter complex (MCUC) with Ru360 enhanced the severity of CaT alternans (AR increase) and lowered the pacing frequency threshold for alternans. In contrast, stimulation of MCUC mediated mitochondrial Ca uptake with spermine rescued alternans (AR decrease) and increased the alternans pacing threshold. Direct measurement of mitochondrial [Ca] in membrane permeabilized myocytes with Fluo-4 loaded mitochondria revealed that spermine enhanced and accelerated mitochondrial Ca uptake. Stimulation of mitochondrial Ca retention by preventing mitochondrial Ca efflux through the mitochondrial permeability transition pore with cyclosporin A also protected from alternans and increased the alternans pacing threshold. Pharmacological manipulation of MCUC activity did not affect sarcoplasmic reticulum Ca load. Our results suggest that activation of Ca sequestration by mitochondria protects from CaT alternans and could be a potential therapeutic target for cardiac alternans and AF prevention.

中文翻译：

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线粒体钙单向转运蛋白复合物激活可防止心房肌细胞中的钙交替。

心脏交替，定义为动作电位持续时间、细胞溶质钙瞬变 (CaT) 振幅和心脏收缩的逐搏交替，与心房颤动 (AF) 和心源性猝死有关。在细胞水平上，心脏交替与兴奋-收缩耦合期间异常的细胞内钙处理有关。我们研究了通过线粒体 Ca 摄取和线粒体 Ca 保留对细胞溶质 Ca 螯合的药理学激活或抑制如何影响离体兔心房肌细胞中起搏诱导的 CaT 交替的发生。使用 Fluo-4 荧光显微镜记录细胞溶质 CaT。交替糖被量化为交替糖比（AR = 1-CaT_小/CaT_大；CaT_小和 CaT_大是一对交替 CaT 的小和大 CaT 的振幅）。通过含有 Ru360 的线粒体 Ca 单向转运蛋白复合体 (MCUC) 抑制线粒体 Ca 螯合可增强 CaT 交替的严重程度（AR 增加）并降低交替的起搏频率阈值。相比之下，用精胺刺激 MCUC 介导的线粒体 Ca 摄取拯救了交替糖（AR 减少）并增加了交替糖起搏阈值。用加载 Fluo-4 的线粒体直接测量膜透化肌细胞中的线粒体 [Ca] 显示精胺增强并加速了线粒体 Ca 摄取。通过使用环孢菌素 A 通过线粒体通透性转换孔阻止线粒体 Ca 外流来刺激线粒体 Ca 保留，也保护免受交替，并增加了交替起搏阈值。MCUC 活性的药理学操作不影响肌浆网 Ca 负荷。我们的研究结果表明，线粒体对 Ca 螯合的激活可以防止 CaT 交替，并且可能是心脏交替和 AF 预防的潜在治疗靶点。