Intermittent hypoxia-generated ROS contributes to intracellular zinc regulation that limits ischemia/reperfusion injury in adult rat cardiomyocyte

Intermittent hypoxia (IH) has been shown to exert cardioprotective effects against ischemia/reperfusion (I/R) injury through the preservation of ion homeostasis. I/R dramatically elevated cytosolic Zn²⁺ and caused cardiomyocyte death. However, the role of IH exposure in the relationship between Zn²⁺ regulation and cardioprotection is still unclear. The aim of the present study was to study whether IH exposure could help in intracellular Zn²⁺ regulation, hence contributing to cardioprotection against I/R injury. Adult rat cardiomyocytes were exposed to IH (5% O₂, 5% CO₂ and balanced N₂) for 30 min followed by 30 min of normoxia (21% O₂, 5% CO₂ and balanced N₂). Changes in intracellular Zn²⁺ concentration were determined using a Zn²⁺-specific fluorescent dye, FluoZin-3 or RhodZin-3. Fluorescence was monitored under an inverted fluorescent or confocal microscope. The results demonstrated that I/R or 2,2′-dithiodipyridine (DTDP), a reactive disulphide compound, induced Zn²⁺ release from metallothioneins (MTs), subsequently causing cytosolic Zn²⁺ overload, which in turn increased intracellular Zn²⁺ entry into the mitochondria via a Ca²⁺ uniporter, hence inducing mitochondrial membrane potential loss, and eventually led to cell death. However, the cytosolic Zn²⁺ overload and cell death caused by I/R or DTDP was significantly reduced by treatment of cardiomyocytes with IH. The findings from this study suggest that IH might exert its cardioprotective effect through reducing the I/R-induced cytosolic Zn²⁺ overload and cell death in cardiomyocytes.