Increased [Mg2+]o reduces Ca2+ influx and disruption of mitochondrial membrane potential during reoxygenation

Increase in extracellular Mg2+ concentration ([Mg2+](o)) reduces Ca2+ accumulation during reoxygenation of hypoxic cardiomyocytes and exerts protective effects. The aims of the present study were to investigate the effect of increased [Mg2+](o) on Ca2+ influx and efflux, free cytosolic Ca2+ ([Ca2+](o)) and Mg2+ concentrations ([Mg2+](i)), Ca-2+ accumulation in the presence of inhibitors of mitochondrial or sarcoplasmatic reticulum Ca2+ transport, and finally mitochondrial membrane potential (Delta psi (m)). Isolated adult rat cardiomyocytes were exposed to 1h of hypoxia and subsequent reoxygenation. Cell Ca2+ was determined by Ca-45(2+) uptake, and the levels of [Mg2+](i) and [Ca2+](i) were determined by flow cytometry as the fluorescence of magnesium green and fluo 3, respectively. Ca2+ influx rate was significantly reduced by similar to 40%, whereas Ca2+ efflux was not affected by increased [Mg2+](o) (5 mM) during reoxygenation. [Ca2+](i) and [Mg2+](i) were increased at the end of hypoxia, fell after reoxygenation, and were unaffected by increased [Mg2+](o). Clonazepam, a selective mitochondrial Na+/Ca2+ exchange inhibitor (100 muM), significantly reduced Ca2+ accumulation by 70% and in combination with increased [Mg2+](o) by 90%. Increased [Mg2+](o), clonazepam, and the combination of both attenuated the hypoxia-reoxygenation-induced reduction in Delta psi (m), determined with the cationic dye JC-1 by flow cytometry. A significant inverse correlation was observed between Delta psi (m) and cell Ca2+ in reoxygenated cells treated with increased [Mg2+](o) and clonazepam. In conclusion, increased [Mg2+](o) (5 mM) inhibits Ca2+ accumulation by reducing Ca2+ influx and preserves Delta psi (m) without affecting [Ca2+](i) and [Mg2+](i) during reoxygenation. Preservation of mitochondria may be an important effect whereby increased [Mg2+](o) protects the postischemic heart.