Planès Carole, Blot-Chabaud Marcel, Matthay Michael A, Couette Sylviane, Uchida Tokujiro, Clerici Christine
Department of Physiology, INSERM U426, Université Paris 7, 75018 Paris, France.
J Biol Chem. 2002 Dec 6;277(49):47318-24. doi: 10.1074/jbc.M209158200. Epub 2002 Oct 7.
Alveolar hypoxia may impair sodium-dependent alveolar fluid transport and induce pulmonary edema in rat and human lung, an effect that can be prevented by the inhalation of beta(2)-agonists. To investigate the mechanism of beta(2)-agonist-mediated stimulation of sodium transport under conditions of moderate hypoxia, we examined the effect of terbutaline on epithelial sodium channel (ENaC) expression and activity in cultured rat alveolar epithelial type II cells exposed to 3% O(2) for 24 h. Hypoxia reduced transepithelial sodium current and amiloride-sensitive sodium channel activity without decreasing ENaC subunit mRNA or protein levels. The functional decrease was associated with reduced abundance of ENaC subunits (especially beta and gamma) in the apical membrane of hypoxic cells, as quantified by biotinylation. cAMP stimulation with terbutaline reversed the hypoxia-induced decrease in transepithelial sodium transport by stimulating sodium channel activity and markedly increased the abundance of beta-and gamma-ENaC in the plasma membrane of hypoxic cells. The effect of terbutaline was prevented by brefeldin A, a blocker of anterograde transport. These novel results establish that hypoxia-induced inhibition of amiloride-sensitive sodium channel activity is mediated by decreased apical expression of ENaC subunits and that beta(2)-agonists reverse this effect by enhancing the insertion of ENaC subunits into the membrane of hypoxic alveolar epithelial cells.
肺泡低氧可能损害钠依赖的肺泡液体转运，并在大鼠和人肺中诱发肺水肿，吸入β₂受体激动剂可预防这种效应。为了研究在中度低氧条件下β₂受体激动剂介导的钠转运刺激机制，我们检测了特布他林对暴露于3%氧气24小时的培养大鼠肺泡II型上皮细胞中上皮钠通道（ENaC）表达和活性的影响。低氧降低了跨上皮钠电流和氨氯地平敏感的钠通道活性，而没有降低ENaC亚基的mRNA或蛋白质水平。功能下降与低氧细胞顶端膜中ENaC亚基（尤其是β和γ）丰度降低有关，这通过生物素化定量。特布他林刺激cAMP通过刺激钠通道活性逆转了低氧诱导的跨上皮钠转运下降，并显著增加了低氧细胞质膜中β-和γ-ENaC的丰度。特布他林的作用被布雷菲德菌素A（一种顺行转运阻滞剂）所阻断。这些新结果表明，低氧诱导的氨氯地平敏感钠通道活性抑制是由ENaC亚基顶端表达降低介导的，而β₂受体激动剂通过增强ENaC亚基插入低氧肺泡上皮细胞膜来逆转这种效应。
