title: Increased epithelial permeability is the primary cause for bicarbonate loss in inflamed murine colon.
abstract: 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 α overexpressing mouse model (TNF(ΔARE)). Luminal alkalization, (3)H-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 (3)H-mannitol fluxes were increased in TNF(+/ΔARE) proximal colon, whereas forskolin-stimulated I(sc) 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(3)(-) loss may occur through leaky tight junctions rather than through HCO(3)(-) secreting ion transporters.
authors.affiliation: Department of Gastroenterology, Hepatology and Endocrinology, Hannover Medical School, Hannover, Germany.
authors.forename: Marina
authors.initials: M
authors.lastname: Juric
authors.forename: Fang
authors.initials: F
authors.lastname: Xiao
authors.forename: Salah
authors.initials: S
authors.lastname: Amasheh
authors.forename: Oliver
authors.initials: O
authors.lastname: May
authors.forename: Kristin
authors.initials: K
authors.lastname: Wahl
authors.forename: Heike
authors.initials: H
authors.lastname: Bantel
authors.forename: Michael P
authors.initials: MP
authors.lastname: Manns
authors.forename: Ursula
authors.initials: U
authors.lastname: Seidler
authors.forename: Oliver
authors.initials: O
authors.lastname: Bachmann
mesh_terms: D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D015153:Blotting, Western; D015820:Cadherins / Q000378:metabolism; D002463:Cell Membrane Permeability / Q000502:physiology*; D057167:Claudins / Q000378:metabolism; D003106:Colon / Q000276:immunology* / Q000378:metabolism / Q000473:pathology; D016207:Cytokines / Q000378:metabolism; D005260:Female; D007249:Inflammation / Q000503:physiopathology*; D007413:Intestinal Mucosa / Q000276:immunology* / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D019108:Tight Junctions / Q000502:physiology; D014409:Tumor Necrosis Factor-alpha / Q000502:physiology*; D062826:Zonula Occludens-1 Protein / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D015820:Cadherins; D057167:Claudins; D016207:Cytokines; D014409:Tumor Necrosis Factor-alpha; D062826:Zonula Occludens-1 Protein; C502011:claudin 8