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In a physiological setting, mitochondria increase oxidative phosphorylation during periods of stress to meet increased metabolic demand. This in part is mediated via enhanced mitochondrial Ca 2+  uptake, an important regulator of cellular ATP homeostasis. In a pathophysiological setting pharmacological modulation of mitochondrial Ca 2+  uptake or retention has been suggested as a therapeutic strategy to improve metabolic homeostasis or attenuate Ca 2+ -dependent arrhythmias in cardiac disease states. To explore the consequences of mitochondrial Ca 2+  accumulation, we tested the effects of kaempferol, an activator of mitochondrial Ca 2+  uniporter (MCU), CGP-37157, an inhibitor of mitochondrial Na + /Ca 2+  exchanger, and MCU inhibitor Ru360 in rat ventricular myocytes (VMs) from control rats and rats with hypertrophy induced by thoracic aortic banding (TAB). In periodically paced VMs under β-adrenergic stimulation, treatment with kaempferol (10 μmol/L) or CGP-37157 (1 μmol/L) enhanced mitochondrial Ca 2+  accumulation monitored by mitochondrial-targeted Ca 2+  biosensor mtRCamp1h. Experiments with mitochondrial membrane potential-sensitive dye TMRM revealed this was accompanied by depolarization of the mitochondrial matrix. Using redox-sensitive OMM-HyPer and ERroGFP_iE biosensors, we found treatment with kaempferol or CGP-37157 increased the levels of reactive oxygen species (ROS) in mitochondria and the sarcoplasmic reticulum (SR), respectively. Confocal Ca 2+  imaging showed that accelerated Ca 2+  accumulation reduced Ca 2+  transient amplitude and promoted generation of spontaneous Ca 2+  waves in VMs paced under ISO, suggestive of abnormally high activity of the SR Ca 2+  release channel ryanodine receptor (RyR). Western blot analyses showed increased RyR oxidation after treatment with kaempferol or CGP-37157 vs. controls. Furthermore, in freshly isolated TAB VMs, confocal Ca 2+  imaging demonstrated that enhancement of mitochondrial Ca 2+  accumulation further perturbed global Ca 2+  handling, increasing the number of cells exhibiting spontaneous Ca 2+  waves, shortening RyR refractoriness and decreasing SR Ca 2+  content. In  ex vivo  optically mapped TAB hearts, kaempferol exacerbated proarrhythmic phenotype. On the contrary, incubation of cells with MCU inhibitor Ru360 (2 μmol/L, 30 min) normalized RyR oxidation state, improved intracellular Ca 2+  homeostasis and reduced triggered activity in  ex vivo  TAB hearts. These findings suggest facilitation of mitochondrial Ca 2+  uptake in cardiac disease can exacerbate proarrhythmic disturbances in Ca 2+  homeostasis via ROS and enhanced activity of oxidized RyRs, while strategies to reduce mitochondrial Ca 2+  accumulation can be protective.

frontiers in physiology

Pharmacological Modulation of Mitochondrial Ca2+ Content Regulates Sarcoplasmic Reticulum Ca2+ Release via Oxidation of the Ryanodine Receptor by Mitochondria-Derived Reactive Oxygen Species