Biophysical and molecular properties of amiloride-inhibitable Na+ channels in alveolar epithelial cells.
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.
Matalon
S
S
Department of Anesthesiology, University of Alabama at Birmingham 35233-6810, USA. Sadis.Matalon@ccc.uab.edu
Benos
D J
DJ
Jackson
R M
RM
D000584:Amiloride / Q000494:pharmacology*; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D004847:Epithelial Cells; D004848:Epithelium / Q000378:metabolism; D006801:Humans; D011650:Pulmonary Alveoli / Q000166:cytology / Q000378:metabolism*; D026941:Sodium Channel Blockers; D015222:Sodium Channels / Q000235:genetics / Q000502:physiology*
D026941:Sodium Channel Blockers; D015222:Sodium Channels; D000584:Amiloride