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

Treating H441 cells with dexamethasone raised the abundance of mRNA encoding the epithelial Na+ channel a- and -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 analogs with a rank order of potency of benzamil = amiloride > EIPA and can thus be attributed to active Na+ absorption. Studies of apically permeabilized cells showed that this increased transport activity did not reflect a rise in Na+ pump capacity, whereas studies of basolateral permeabilized 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 approximately fourfold 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 an ~60% increase in Isc, and measurements made after these cells had been basolaterally permeabilized 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.