EXPRESSION AND LOCALIZATION OF EPITHELIAL SODIUM-CHANNEL IN MAMMALIANURINARY-BLADDER

The mammalian urinary bladder exhibits transepithelial Na+ absorption that contributes to Na+ gradients established by the kidney. Electroph ysiological studies have demonstrated that electrogenic Na+ absorption across the urinary bladder is mediated in part by amiloride-sensitive Na+ channels situated within the apical membrane of the bladder epith elium. We have used a combination of in situ hybridization, Northern b lot analysis, and immunocytochemistry to examine whether the recently cloned epithelial Na+ channel (ENaC) is expressed in the rat urinary b ladder. In situ hybridization and Northern blot analyses indicate that alpha-, beta-, and gamma-rat ENaC (rENaC) are expressed in rat urinar y bladder epithelial cells. Quantitation of the levels of alpha-, beta -, and gamma-rENaC mRNA expression in rat urinary bladder, relative to p-actin mRNA expression, indicates that, although comparable levels o f alpha- and beta-rENaC subunits are expressed in the urinary bladder of rats maintained on standard chow, the level of gamma-rENaC mRNA exp ression is 5- to 10-fold lower than alpha- or beta-rENaC mRNA. Immunoc ytochemistry, using an antibody directed against alpha-rENaC, revealed that ENaCs are predominantly localized to the luminal membrane of the bladder epithelium. Together, these data demonstrate that ENaC is exp ressed in the mammalian urinary bladder and suggest that amiloride-sen sitive Na+ transport across the apical membrane of the mammalian urina ry bladder epithelium is mediated primarily by ENaC.