Modification of biophysical properties of lung epithelial Na+ channels by dexamethasone

There is considerable interest in identifying the basic mechanisms by which dexamethasone alters ion transport across the adult alveolar epithelium. H erein, we incubated synchronized A549 cells, a human alveolar epithelial ce ll line, with dexamethasone (1 mu M) for 24-48 h. When normalized to HPRT ( a housekeeping gene), A549 beta- and gamma-subunit mRNA levels for the huma n amiloride-sensitive epithelial sodium channel (hENaC), assessed by RT-PCR , increased by 1.6- and 17-fold respectively, compared with control values (P < 0.05). These changes were abolished by actinomycin D, indicating trans criptional regulation. Western blotting studies revealed that dexamethasone also increased expression of beta- and gamma-hENaC protein levels. In cont rast, alpha-hENaC mRNA increased by onefold (P > 0.05) and alpha-hENaC prot ein level was unchanged. Incubation of A549 cells with dexamethasone increa sed their whole cell amiloride-sensitive sodium currents twofold and decrea sed the K-0.5 for amiloride from 833 +/- 69 to 22 +/- 5.4 nM (mean +/- SE; P < 0.01). Single channel recordings in the cell-attached mode showed that dexamethasone treatment increased single channel open time and open probabi lity threefold and decreased channel conductance from 8.63 +/- 0.036 to 4.4 +/- 0.027 pS (mean +/- SE; P < 0.01). We concluded that dexamethasone modu lates the amiloride-sensitive Na+ channels by differentially regulating the expression of beta- and gamma-subunits at the mRNA and protein levels in t he human A549 cell line, with little effect on alpha-hENaC subunit.