Dexamethasone prevents transport inhibition by hypoxia in rat lung and alveolar epithelial cells by stimulating activity and expression of Na⁺-K⁺-ATPase and epithelial Na⁺ channels

Abstract

Hypoxia inhibits Na and lung fluid reabsorption, which contributes to the formation of pulmonary edema. We tested whether dexamethasone prevents hypoxia-induced inhibition of reabsorption by stimulation of alveolar Na transport. Fluid reabsorption, transport activity, and expression of Na transporters were measured in hypoxia-exposed rats and in primary alveolar type II (ATII) cells. Rats were treated with dexamethasone (DEX; 2 mg/kg) on 3 consecutive days and exposed to 10% O(2) on the 2nd and 3rd day of treatment to measure hypoxia effects on reabsorption of fluid instilled into lungs. ATII cells were treated with DEX (1 muM) for 3 days before exposure to hypoxia (1.5% O(2)). In normoxic rats, DEX induced a twofold increase in alveolar fluid clearance. Hypoxia decreased reabsorption (-30%) by decreasing its amiloride-sensitive component; pretreatment with DEX prevented the hypoxia-induced inhibition. DEX increased short-circuit currents (ISC) of ATII monolayers in normoxia and blunted hypoxic transport inhibition by increasing the capacity of Na(+)-K(+)-ATPase and epithelial Na(+) channels (ENaC) and amiloride-sensitive ISC. DEX slightly increased the mRNA of alpha- and gamma-ENaC in whole rat lung. In ATII cells from DEX-treated rats, mRNA of alpha(1)-Na(+)-K(+)-ATPase and alpha-ENaC increased in normoxia and hypoxia, and gamma-ENaC was increased in normoxia only. DEX stimulated the mRNA expression of alpha(1)-Na(+)-K(+)-ATPase and alpha-, beta-, and gamma-ENaC of A549 cells in normoxia and hypoxia (1.5% O(2)) when DEX treatment was begun before or during hypoxic exposure. These results indicate that DEX prevents inhibition of alveolar reabsorption by hypoxia and stimulates the expression of Na transporters even when it is applied in hypoxia.

Hydrogen sulfide decreases β-adrenergic agonist-stimulated lung liquid clearance by inhibiting ENaC-mediated transepithelial sodium absorption

“…We have shown that sodiumabsorbing epithelia have the capacity to produce H 2 S in concentrations that reflect the range of the observed inhibition of ENaC. H 2 S might, thus, contribute to hypoxic inhibition of sodium transport in pulmonary epithelial cells (35,47,65,77,82). 2) In low concentrations, H 2 S exerts anti-inflammatory, anti-oxidant, and vasoactive properties and is currently being evaluated as a potential therapeutic option in animal models of various lung diseases (15,49,81), including acute lung injury (ALI) (25-27, 29, 64).…”

Oxygen regulation of the epithelial Na channel in the collecting duct

“…Instead, O 2 alters expression of HO-1 and Glut1, genes that are minimally altered by steroids. Given these differences, the previously described prevention of hypoxiainduced ENaC inhibition by glucocorticoids in lung epithelia (25) probably represents a counterregulation of ENaC via a parallel pathway rather than prevention of the hypoxic effects (Fig. 1).…”

Mechanisms of ENaC Regulation and Clinical Implications

“…125,126 Hypoxia also has direct effects on expression of ENaC subunits and activity in rodent airway epithelial cells. 127,128 Further clinical correlations of these phenomena are warranted to ascertain their physiologic or pathophysiologic importance. A schematic working model of the regulation of ENaC by various signaling pathways is shown in Figure 2.…”