Biophysical and molecular properties of amiloride-inhibitable na+
channels in alveolar epithelial cells.
Matalon, Sadis, Dale J. Benos, and Robert M. Jackson.
Departments of Anesthesiology, Physiology and Biophysics, Medicine
and Pediatrics, University of Alabama at Birmingham, Birmingham, AL
35233-6810 and Veterans Administration Medical Center, Birmingham, AL
35233
APStracts 3:0056L, 1996.
The recent immunopurification and cloning of various lung Na+ channel
proteins, have 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
[delta]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 cAMP 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 PKA and ATP, and was decreased to the same extent
by addition 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 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.
Received 28 December 1995; accepted in final form 11 April 1996.
APS Manuscript Number L385-5.
Article publication pending Am. J. Physiol. (Lung Cell. Mol.
Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 23 April 96