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 Na⁺ 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⁺],) or indo-1 (to index [Ca²⁺]_i) and exposed to glucose-free hypoxia (Po₂ <0.02 mm Hg). Both [Na⁺]; and [Ca²⁺]_i increased during hypoxia when cells became inexcitable following ATP-depletion contracture. The hypoxic rise in [Na⁺]; 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⁺]; 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 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 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.