In distal segments of colon and kidney tubules, Na+ absorption is mediated by the epithelial Na+ channel ENaC. We wondered if there is a mechanism for reducing paracellular Na+ back-leak during induction of ENaC. Human colon epithelia or HT-29/B6- GR colon cells were mounted in Ussing chambers for measuring transepithelial resistance (Rt), electrogenic Na+ transport (JNa ), and paracellular flux of 22 Na. After incubation Western blots, confocal microscopy, and quantitative PCR were performed.

Results: Aldosterone (3 nM) stimulated JNa and induced ENaC in the apical membrane. After 8 h of incubation a decrease of Rt was observed in both, controls and aldosterone-incubated tissues. After amiloride, Rt in aldosterone-treated tissues was higher than in controls. Decreased 22 Na fluxes revealed a selective sealing of the barrier. Aldosterone caused occludin and claudin-8 expression to increase. This effect was based on transcriptional regulation. In HT-29/B6-GR cells, which express a functional glucocorticoid receptor, the effect was reproduced with dexamethasone (1 mM), indicating a general mechanism.

Conclusion: As demonstrated in human colon as well as in cell cultures, induction of ENaC is accompanied by tightening the paracellular Na+ back-leak pathway. This novel mechanism is mediated by claudin-8 and increases the maximum concentration gradient against which Na+ can be absorbed.