Sodium Channel Blockade Reduces Hypoxic Sodium Loading and Sodium-Dependent Calcium Loading

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BackgroundStudies 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+]i and [Ca2+]i increased during hypoxia when cells became inexcitable following ATP-depletion contracture. The hypoxic rise in [Na+]i and [Ca2+]i was significantly attenuated by 1 μmol/L R 56865. Tetrodotoxin (60 μmol/L), a selective Na+ -channel blocker, also markedly reduced the rise in [Ca2+]i during 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+]i without 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.

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