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Am J Physiol Lung Cell Mol Physiol (April 16, 2004). doi:10.1152/ajplung.00407.2003
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Submitted on November 25, 2003
Accepted on April 10, 2004

A Regulated apical Na+ conductance in dexamethasone-treated H441 airway epithelial cells

Sarah J. Ramminger1, Kerry Richard1, Sarah K. Inglis1, Stephan C. Land1, Richard E. Olver1, and Stuart M. Wilson1*

1 Lung Membrane Transport Group, Division of Maternal and Child Health Sciences, University of Dundee, Dundee, Scotland, United Kingdom

* To whom correspondence should be addressed. E-mail: S.M.Wilson{at}dundee.ac.uk.

Treating H441 cells with dexamethasone raised the abundance of mRNA encoding the epithelial Na+ channel (ENaC) {alpha} and {beta} subunits and increased transepithelial ion transport (measured as short circuit current, ISC) from < 4 µA cm-2 to 10 - 20 µA cm-2. This dexamethasone-stimulated ion transport was blocked by amiloride analogues with a rank order of potency of benzamil >= amiloride > ethyl-isopropyl amiloride and can thus be attributed to active Na+ absorption. Studies of apically permeabilised cells showed that this increased transport activity did not reflect a rise in Na+ pump capacity whilst studies of basolateral permeabilised cells demonstrated that dexamethasone increased apical Na+ conductance (GNa) from a negligible value to 100 - 200 µS cm-2. Experiments that explored the ionic selectivity of this dexamethasone-induced conductance showed that it was equally permeable to Na+ and Li+ and that the permeability to these cations was ~4 fold greater than to K+. There was also a small permeability to N-methyl-D-glucammonium, a nominally impermeant cation. Forskolin, an agent that increases cellular cAMP content, caused ~60% increase in ISC, and measurements made after these cells had been basolaterally permeabilised demonstrated that this response was associated with a rise in GNa. This cAMP-dependent control over GNa was disrupted by brefeldin-A, an inhibitor of vesicular trafficking. Dexamethasone thus stimulates Na+ transport in H441 cells by evoking expression of an amiloride-sensitive apical conductance that displays moderate ionic selectivity and is subject to acute control via a cAMP-dependent pathway.




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