SODIUM-CHANNEL BLOCKADE REDUCES HYPOXIC SODIUM LOADING AND SODIUM-DEPENDENT CALCIUM LOADING

Background 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 Results Inhibitors of the voltage-gated Na+ channel were used to investigate the effect of Naf channel blockade on hypoxic Na+ loading, Na+-dependent Ca2+ 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 (Po-2 < 0.02 mm Hg). Both [Na+](i) and [Ca](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 mu mol/L R 56865. Tetrodotoxin (60 mu mol/L), a selective Na+-channel blocker, also markedly reduced the rise in [Ca2+](i) during hypoxia and reoxygenation. Reoxygenation-induced cellular hypercontracture was reduced 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 mu mol/L lidocaine, 42% in 50 mu mol/L lidocaine, and 93% in 25 mu mol/L lidocaine. The Na+-K+ exchange blocker, ethylisopropylamiloride (10 mu mol/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 mu mol/L veratridine, an agent that impairs Na+ channel inactivation. Ethylisopropylamiloride prevented the veratridine-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. Conclusions Na+ 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.