Impaired absorption of sodium (Na⁺) and water is a major factor in the pathogenesis of diarrhoea in ulcerative colitis (UC). Electrogenic Na⁺ absorption, present mainly in human distal colon and rectum, is defective in UC, but the molecular basis for this is unclear. The effect of UC on the expression of apical Na⁺ channels (ENaC) and basolateral Na⁺, K⁺‐ATPase, the critical determinants of electrogenic Na⁺ transport, was therefore investigated in this study. Sigmoid colonic and/or proximal rectal mucosal biopsies were obtained from patients with mild to moderate UC, and patients with functional abdominal pain (controls). ENaC subunit expression was studied by immunohistochemistry, western blot analysis, and in situ hybridization, and Na⁺, K⁺‐ATPase isoform expression was studied by immunohistochemistry, western blotting, and northern blot analysis. UC was associated with substantial decreases in the expression of the ENaC β‐ and γ‐subunit proteins and mRNAs, whereas the decrease in ENaC α‐subunit protein detected by immunolocalization was less marked. The levels of expression of Na⁺, K⁺‐ATPase α₁‐ and β₁‐isoform proteins were also lower in UC patients than in controls, although there were no differences in Na⁺, K⁺‐ATPase α₁‐ and β₁‐isoform mRNA levels between the two groups. Taken together, these results show that UC results mainly in decreased expression of the apical ENaC β‐ and γ‐subunits, as well as the basolateral Na⁺, K⁺‐ATPase α₁‐ and β₁‐isoforms. In conclusion, these changes provide a basis for the low/negligible levels of electrogenic Na⁺ absorption seen in the distal colon and rectum of UC patients, which contribute to the pathogenesis of diarrhoea in this disease. Copyright © 2004 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

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