Abstract
Serum- and glucocorticoid-inducible kinase 1 (SGK1) plays an important role in the body NaCl homeostasis by fine tuning of renal NaCl reabsorption in the distal nephron via activity modulation, transcription, and trafficking of epithelial Na+ channel (ENaC). Although the SGK1 induction at mRNA and protein levels is involved in the stimulatory action of hypotonicity on Na+ transport, it is unknown how hypotonicity regulates the SGK1 expression. In the present study, we studied if the Ca2+ signal is involved in the hypotonic action on SGK1 expression and Na+ transport. Using QRT-PCR and western blotting, we observed that BAPTA/AM (an intracellular Ca2+ chelator) and W7 (a calmodulin antagonist) diminished the hypotonic induction of SGK1 mRNA and protein. Ionomycin (a Ca2+ ionophore) stimulated SGK1 transcription under an isotonic condition. In short-circuit current measurement, BAPTA/AM showed an inhibitory effect on Na+ current 60 min after hypotonic challenge, the time course of which correlated with that of hypotonic induction of SGK1 protein, suggesting the contribution of SGK1 to the hypotonicity-provoked Na+ transport. Regarding ENaC expression, a possible correlation was found between SGK1 activity and expression of αENaC, but not β or γENaC. Taken together, we conclude that hypotonic stress has genomic action on SGK1 mRNA expression in a Ca2+/calmodulin-dependent manner, stimulating Na+ transport and αENaC expression. JSPS17590191, 17390057 [J Physiol Sci. 2007;57 Suppl:S121]
