We investigated a role of p38 MAPK in the regulation of transepithelial Na+ reabsorption by chronic application (20-24 h) of hypotonicity (hypotonic stress) in renal epithelial A6 cells. Pretreatment with a specific p38 MAPK inhibitor (SB202190) significantly reduced the chronic hypotonicity-stimulated transepithelial Na+ reabsorption by diminishing the Na+ entry through epithelial Na+ channel (ENaC) in the apical membrane and the Na+ extrusion via the Na+/K+ ATPase (pump), although the rate limiting step was still the Na+ entry step. We further examined whether the inhibitory effects of SB202190 on the transepithelial Na+ reabsorption is caused through suppression of mRNA expression of ENaC participating in the transepithelial Na+ reabsorption as the Na+ entry pathway. The chronic hypotonicity increased the mRNA expression of alpha-, beta-, and gamma-subunits of ENaC. Moreover, we found that inhibition of p38 MAPK by SB202190 diminished the mRNA expression of beta- and gamma-ENaC but not alpha-ENaC. Based on these observations, it is suggested that the chronic hypotonicity stimulates the renal transepithelial Na+ reabsorption by upregulating the mRNA expression of beta- and gamma-ENaC via a p38 MAPK-dependent pathway. (c) 2007 Elsevier Inc. All rights reserved.