Persistent sodium current and Na+/H+ exchange contributes to the augmentation of the reverse Na+/Ca2+ exchange during hypoxia or acute ischemia in ventricular myocytes

The increases in persistent sodium currents (I(Na.P)) and Na(+)/H(+) exchange (NHE) causes intracellular Ca(2+) overload. The objective of this study was to determine the contribution of I(Na.P) and NHE on the hypoxia- or acute ischemia-induced increase in the reverse Na(+)/Ca(2+) exchange current (HIR- or AIR-I(NCX)). I(Na.P) and I(NCX) in rabbit ventricular myocytes were recorded during hypoxia or acute ischemia, combination of acidosis (pH values were 6.0 intracellularly and 6.8 extracellularly) and hypoxia, using whole-cell patch-clamp techniques. The results indicate that (1) under hypoxic condition, the augmentation of both HIR-I(NCX) and I(Na.P) was inhibited by TTX (2 to 8 μM) in a concentration-dependent manner. The inhibitions of I(Na,P) and HIR-I(NCX) reached maximum in the presence of either 4 μM TTX or 10 μM KR-32568 (a NHE inhibitor), respectively. The maximal inhibitions of HIR-I(NCX) by 4 μM TTX and 10 μM KR-32568 were 72.54% and 16.89%, respectively. (2) Administration of 2 μM TTX and 10 μM KR-32568 in either order in the same cells decreased HIR-I(NCX) by 64.83% and 16.94%, respectively. (3) I(Na.P) and the reverse I(NCX) were augmented during acute ischemia. TTX (4 μM) and KR-32568 (10 μM) reduced AIR-I(NCX) by 73.39% and 24.13%, respectively. (4) Under normoxic condition, veratridine (20 μM) significantly increased I(Na.P) and the reverse I(NCX), which was reversed by 4 μM TTX. In conclusion, during hypoxia or acute ischemia, both increased I(Na.P) and NHE contribute to the HIR- or AIR-I(NCX) with the former playing a major role comparing with the latter.