Comparison of hypoxic effects induced by chemical and physical hypoxia on cardiomyocytes

The degree and duration of chemical hypoxia induced by sodium dithionite (Na₂S₂O₄) have not been reported. It is not yet clear how much reduction in the O₂ concentration (physical hypoxia) can lead to hypoxia in cultured cardiomyocytes. In this study, oxygen microelectrodes were used to measure changes in the O₂ concentration in media containing different concentrations of Na₂S₂O₄. Then, hypoxic effects of 0.8, 1.0, and 2.0 mM Na₂S₂O₄ or 1%, 3%, and 5% O₂ in cultured cardiomyocytes from neonatal rats were observed and compared. The results showed that the O₂ concentration failed to remain constant by Na₂S₂O₄ treatment during the 180-minute observation period. Only the 2.0 mM Na₂S₂O₄ group significantly increased the expression of hypoxia-inducible factor 1α (HIF-1α) and hypoxic responses. Notably, 3% O₂ only significantly increased the expression of HIF-1α in cardiomyocytes, while 1% O₂ not only increased the expression of HIF-1α but also increased the apoptotic rate in cardiomyocytes. These results suggest that Na₂S₂O₄ is not suitable for establishing a hypoxic model in cultured neonatal rat cardiomyocytes, and neonatal rat cardiomyocytes cultured at or below 1% O₂ induced significant hypoxic effects, which can be used as a starting O₂ concentration for establishing a hypoxic cell model.