Background: Bicarbonate loss into the lumen occurs during intestinal inflammation in different species. However, candidate pathways like CFTR or DRA are inhibited in the inflamed gut. This study addressed the question whether and how inflammation-associated increased intestinal permeability may result in epithelial HCO-3 loss. Methods: Murine proximal colon was studied because it does not express functional DRA but is inflamed in the tumor necrosis factor a overexpressing mouse model (TNFδARE). Luminal alkalization, 3H-mannitol fluxes, impedance spectroscopy, and dilution potentials were measured in Ussing chambers, whereas expression and localization of tight junction-associated proteins were analyzed by Western blots and immunohistochemistry. Results: Luminal alkalization rates and 3H-mannitol fluxes were increased in TNF+/δARE proximal colon, whereas forskolin-stimulated Isc was not altered. Epithelial resistance was reduced, but subepithelial resistance increased. The epithelial lining was intact, and enterocyte apoptosis rate was not increased despite massively increased Th1 cytokine levels and lymphoplasmacellular infiltration. Measurement of dilution potentials suggested a loss of cation selectivity with increased anion permeability. Western analysis revealed a downregulation of occludin expression and an upregulation of both claudin-2 and claudin-5, with no change in ZO-1, E-cadherin, claudin-4, and claudin-8. Immunohistochemistry suggested correct occludin localization but reduced tight junction density in TNF +/δARE surface epithelium. Conclusions: Inflammation during TNF-α overexpression leads to increased epithelial permeability in murine proximal colon, decreased tight junctional cation selectivity, and increased HCO-3 loss into the lumen. Inflammation-associated colonic HCO-3loss may occur through leaky tight junctions rather than through HCO23 secreting ion transporters. Copyright © 2013 Crohn's & Colitis Foundation of America, Inc.