Studies have shown that the rise in intracellular ionized calcium, [Ca2+]i, in hypoxic myocardium is driven by an increase in sodium, [Na+]i, but the source of Na+is not known.Methods and ResultsInhibitors of the voltage-gated Na+channel were used to investigate the effect of Na+channel blockade on hypoxic Na+loading, Na+dependent Cal2+loading, and reoxygenation hypercontracture in isolated adult rat cardiac myocytes. Single electrically stimulated (0.2 Hz) cells were loaded with either SBFI (to index [Na+]i) or indo-1 (to index [Ca2+]i) and exposed to glucose-free hypoxia (PO2< 0.02 mm Hg). Both [Na+]iand [Ca2+]iincreased during hypoxia when cells became inexcitable following ATP-depletion contracture. The hypoxic rise in [Na+]iand [Ca2+]iwas significantly attenuated by 1 μmol/L R 56865. Tetrodotoxin (60 μmol/L), a selective Na+-channel blocker, also markedly reduced the rise in [Ca2+]iduring hypoxia and reoxygenation. Reoxygenation-induced cellular hypercontracture was re-duced from 83% (45 of 54 cells) under control conditions to 12% (4 of 32) in the presence of R 56865 (P< .05). Lidocaine reduced hypercontracture dose dependently with 13% of cells hypercontracting in 100, μmol/L lidocaine, 42% in 50 μmol/L lidocaine, and 93% in 25 μmol/L lidocaine. The Na+-H+exchange blocker, ethylisopropylamiloride (10 μmol/L) was also effective, limiting hypercontracture to 12%. R 56865, lidocaine, and ethylisopropylamiloride were also effective in preventing hypercontracture in normoxic myocytes induced by 75 μmol/L veratridine, an agent that impairs Na+channel inactivation. Ethylisopropylamiloride prevented the veratri-dine- induced rise in [Ca2+]iwithout affecting Na+-Ca2+exchange, suggesting that amiloride derivatives can reduce Ca2+loading by blocking Na+entry through Na+channels, an action that may in part underlie their ability to prevent hypoxic Na+and Ca2+loading.ConclusionsNa+influx through the voltage-gated Na+channel is an important route of hypoxic Na+loading, Na+−dependent Ca2+loading, and reoxygenation hypercontracture in isolated rat cardiac myocytes. Importantly, the Na+channel appears to serve as a route for hypoxic Na+influx after myocytes become inexcitable.