Matalon S, Benos D J, Jackson R M
Department of Anesthesiology, University of Alabama at Birmingham 35233-6810, USA.
Am J Physiol. 1996 Jul;271(1 Pt 1):L1-22. doi: 10.1152/ajplung.1996.271.1.L1.
The recent immunopurification and cloning of various lung Na+ channel proteins has provided the necessary tools to study Na+ transport at a fundamental level across a number of epithelial tissues. Various macroscopic measurements of Na+ transport have shown that Na+ ions enter the cytoplasm of alveolar cells mainly through amiloride-inhibitable Na+ channels. Molecular biology studies have shown the existence of three Na+ channel subunit mRNAs (alpha-, beta-, and gamma-rENaC) in mature fetal (FDLE) and adult alveolar type II (ATII) cells. Patch-clamp studies have demonstrated the existence of various types of amiloride-inhibitable Na+ channels, located in the apical membranes of FDLE and ATII cells. beta-Agonists and agents that enhance intracellular adenosine 3',5'-cyclic monophosphate levels increase the open probability of these channels, leading to increased Na+ transport across the alveolar epithelium in vivo. Immunopurification of a putative channel protein from adult ATII cells showed that it contains an amiloride-binding subunit with a molecular mass of 150 kDa. When this protein was reconstituted in planar lipid bilayers, it exhibited single channels with a conductance of 25 pS, which were moderately selective for Na+ over K+. The open probability of these channels was increased by the addition of protein kinase A (PKA) and ATP, and was decreased to the same extent by addition of [N-ethyl-N-isopropyl]-2'-4'-amiloride (EIPA) and amiloride (1 microM each) in the apical side of the bilayer, in agreement with the results of patch-clamp studies in ATII cells. Exposure of rats to sublethal hyperoxia increased alpha-rENaC mRNA and the functional expression of Na+ channels in alveolar epithelial cells and limited alveolar edema. These findings indicate that alveolar epithelial channels contain at least one family of amiloride-sensitive Na+ channel proteins, which displays a number of unique properties, including sensitivity to EIPA.
近期对多种肺钠通道蛋白的免疫纯化和克隆，为在基础水平上研究钠在多种上皮组织中的转运提供了必要工具。对钠转运的各种宏观测量表明，钠离子主要通过氨氯地平可抑制的钠通道进入肺泡细胞的细胞质。分子生物学研究表明，在成熟胎儿（FDLE）和成体肺泡Ⅱ型（ATII）细胞中存在三种钠通道亚基mRNA（α-、β-和γ-rENaC）。膜片钳研究证明，在FDLE和ATII细胞的顶端膜中存在多种类型的氨氯地平可抑制的钠通道。β-激动剂和提高细胞内3',5'-环磷酸腺苷水平的药物会增加这些通道的开放概率，导致体内钠跨肺泡上皮的转运增加。从成体ATII细胞中免疫纯化出一种假定的通道蛋白，结果显示它含有一个分子量为150 kDa的氨氯地平结合亚基。当这种蛋白重构于平面脂质双分子层中时，它表现出电导为25 pS的单通道，对钠离子的选择性略高于钾离子。加入蛋白激酶A（PKA）和ATP可增加这些通道的开放概率，而在双分子层顶端侧加入[N-乙基-N-异丙基]-2'-4'-氨氯地平（EIPA）和氨氯地平（各1 microM）可使其开放概率同等程度降低，这与ATII细胞的膜片钳研究结果一致。将大鼠暴露于亚致死性高氧环境会增加α-rENaC mRNA以及肺泡上皮细胞中钠通道的功能表达，并减轻肺泡水肿。这些发现表明，肺泡上皮通道至少包含一个对氨氯地平敏感的钠通道蛋白家族，该家族具有许多独特特性，包括对EIPA敏感。
