BIOPHYSICAL AND MOLECULAR-PROPERTIES OF AMILORIDE-INHIBITABLE NA-CELLS( CHANNELS IN ALVEOLAR EPITHELIAL)

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 macro scopic measurements of Na+ transport have shown that Na+ ions enter th e 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 stud ies have demonstrated the existence of various types of amiloride-inhi bitable 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 the se channels, leading to increased Na+ transport across the alveolar ep ithelium in vivo. Immunopurification of a putative channel protein fro m adult ATII cells showed that it contains an amiloride-binding subuni t with a molecular mass of 150 kDa. When this protein was reconstitute d in planar lipid bilayers, it exhibited single channels with a conduc tance of 25 pS, which were moderately selective for Na+ over K+. The o pen probability of these channels was increased by the addition of pro tein kinase A (PKA) and ATP, and was decreased to the same extent by a ddition of [N-ethyl-N-isopropyl]-2'-4'-amiloride (EIPA) and amiloride (1 mu M each) in the apical side of the bilayer, in agreement with the results of patch-clamp studies in ATII cells. Exposure of rats to sub lethal hyperoxia increased alpha-rENaC mRNA and the functional express ion of Na+ channels in alveolar epithelial cells and limited alveolar edema. These findings indicate that alveolar epithelial channels conta in at least one family of amiloride-sensitive Na+ channel proteins, wh ich displays a number of unique properties, including sensitivity to E IPA.