Haigney M C, Lakatta E G, Stern M D, Silverman H S
Laboratory of Cardiovascular Science, National Institute on Aging, Baltimore, MD.
Circulation. 1994 Jul;90(1):391-9. doi: 10.1161/01.cir.90.1.391.
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.
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+]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 mumol/L R 56865. Tetrodotoxin (60 mumol/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 mumol/L lidocaine, 42% in 50 mumol/L lidocaine, and 93% in 25 mumol/L lidocaine. The Na(+)-H+ exchange blocker, ethylisopropylamiloride (10 mumol/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 mumol/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.
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.
研究表明，缺氧心肌细胞内离子钙[Ca2+]i升高是由钠离子[Na+]i增加所驱动，但钠离子的来源尚不清楚。
使用电压门控钠离子通道抑制剂来研究钠离子通道阻断对分离的成年大鼠心肌细胞缺氧时钠离子内流、钠依赖性钙离子内流以及复氧后超收缩的影响。对单个电刺激（0.2 Hz）的细胞加载SBFI（用于指示[Na+]i）或indo-1（用于指示[Ca2+]i），并使其暴露于无糖缺氧环境（PO2 < 0.02 mmHg）。在缺氧期间，当细胞因ATP耗竭性收缩而失去兴奋性时，[Na+]i和[Ca2+]i均升高。1 μmol/L的R 56865可显著减弱缺氧时[Na+]i和[Ca2+]i的升高。河豚毒素（60 μmol/L），一种选择性钠离子通道阻滞剂，也显著降低了缺氧和复氧期间[Ca2+]i的升高。复氧诱导的细胞超收缩从对照条件下的83%（54个细胞中的45个）在R 56865存在时降至12%（32个细胞中的4个）（P < 0.05）。利多卡因剂量依赖性地降低超收缩，在100 μmol/L利多卡因中13%的细胞超收缩，在50 μmol/L利多卡因中42%的细胞超收缩，在25 μmol/L利多卡因中93%的细胞超收缩。钠离子-氢离子交换阻滞剂乙基异丙基氨氯吡咪（10 μmol/L）也有效，将超收缩限制在12%。R 56865、利多卡因和乙基异丙基氨氯吡咪在预防由75 μmol/L藜芦碱诱导的常氧心肌细胞超收缩方面也有效，藜芦碱是一种损害钠离子通道失活的药物。乙基异丙基氨氯吡咪可预防藜芦碱诱导引起的[Ca2+]i升高，而不影响钠-钙交换，这表明氨氯吡咪衍生物可通过阻断钠离子经钠离子通道进入细胞来减少钙离子内流，这一作用可能部分解释了它们预防缺氧时钠离子和钙离子内流的能力。
通过电压门控钠离子通道的钠离子内流是分离的大鼠心肌细胞缺氧时钠离子内流、钠依赖性钙离子内流以及复氧后超收缩的重要途径。重要的是，在心肌细胞失去兴奋性后，钠离子通道似乎成为缺氧时钠离子内流的一条途径。
