Expression and localization of epithelial sodium channel in mammalian urinary bladder

The mammalian urinary bladder exhibits transepithelial Na+ absorption that contributes to Na+ gradients established by the kidney. Electrophysiological 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 epithelium. We have used a combination of in situ hybridization, Northern blot analysis, and immunocytochemistry to examine whether the recently cloned epithelial Na+ channel (ENaC) is expressed in the rat urinary bladder. In situ hybridization and Northern blot analyses indicate that alpha-, beta-, and gamma-rat ENaC (rENaC) are expressed in rat urinary bladder epithelial cells. Quantitation of the levels of alpha-, beta-, and gamma-rENaC mRNA expression in rat urinary bladder, relative to beta-actin mRNA expression, indicates that, although comparable levels of alpha- and beta-rENaC subunits are expressed in the urinary bladder of rats maintained on standard chow, the level of gamma-rENaC mRNA expression is 5- to 10-fold lower than alpha- or beta-rENaC mRNA. Immunocytochemistry, 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 expressed in the mammalian urinary bladder and suggest that amiloride-sensitive Na+ transport across the apical membrane of the mammalian urinary bladder epithelium is mediated primarily by ENaC.