In distal colon, the limiting factor for Na+ absorption is represented by the epithelial sodium channel (ENaC). During absorption, high transepithelial Na+ gradients are observed. In human colon and in HT-29/136-GR cells, we investigated whether Na+ back-leakage is prevented by paracellular sealing. Tissues and cells were incubated with corticosteroids. Barrier properties were analyzed in electrophysiological experiments. Subsequently, analysis of ENaC and tight junction protein expression, localization, and regulation was performed. In colon, nanomolar aldosterone induced sodium absorption via ENaC. Concomitantly, paracellular Na-22(+) permeability was reduced by half and claudin-8 within the tight junction complex was nearly doubled. Real-time PCR validated an increase of claudin-8 transcripts. Two-path impedance spectroscopy following ENaC induction in HT-29/136-GR revealed a specific increase of paracellular resistance. These results represent an important physiological implication: Na+ absorption is paralleled by claudin-8-mediated sealing of the paracellular barrier to prevent Na+ back-leakage, supporting steep Na+ gradients in distal colon. (C) 2008 Elsevier Inc. All rights reserved.