Non-Lethal Levels of Oxidative Stress in Response to Short-Term Intermittent Hypoxia Enhance Ca2+ Handling in Neonatal Rat Cardiomyocytes

Background/Aims: Intermittent hypoxia (IH) may exert pre-conditioning-like cardioprotective effects and alter Ca2+ regulation; however, the exact mechanism of these effects remains unclear. Thus, we examined Ca2+-handling mechanisms induced by IH in rat neonatal cardiomyocytes. Methods: Cardiomyocytes were exposed to repetitive hypoxia-re-oxygenation cycles for 1-4 days. Mitochondrial reactive oxygen species (ROS) generation was determined by flow cytometry, and intracellular Ca2+ concentrations were measured using a live-cell fluorescence imaging system. Protein kinase C (PKC) isoforms and Ca2+-handling proteins were analysed using immunofluorescence and western blotting. Results: After IH exposure for 4 days, the rate of Ca2+ extrusion from the cytosol to the extracellular milieu during 40-mM KCl-induced Ca2+ mobilization increased significantly, whereas ROS levels increased mildly. IH activated PKC isoforms, which translocated to the membrane from the cytosol, and Na+/Ca2+ exchanger-1, leading to enhanced Ca2+ efflux capacity. Simultaneously, IH increased sarcoplasmic reticulum (SR) Ca2+-ATPase and ryanodine receptor 2 (RyR-2) activities and RyR-2 expression, resulting in improved Ca2+ uptake and release capacity of SR in cardiomyocytes. Conclusions: IH-induced mild elevations in ROS generation can enhance Ca2+ efflux from the cytosol to the extracellular milieu and Ca2+-mediated SR regulation in cardiomyocytes, resulting in enhanced Ca2+-handling ability.