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      Biophysical and molecular properties of amiloride-inhibitable Na+ channels in alveolar epithelial cells.

      The American journal of physiology

      Amiloride, pharmacology, Animals, Biophysical Phenomena, Biophysics, Epithelial Cells, Epithelium, metabolism, Humans, Pulmonary Alveoli, cytology, Sodium Channel Blockers, Sodium Channels, physiology, genetics

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          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.

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