Role of Down-Regulated in Adenoma Anion Exchanger in HCO3- Secretion Across Murine Duodenum

Background & Aims: The current model of duodenal HCO3- secretion proposes that basal secretion results from Cl-/HCO3- exchange, whereas cyclic adenosine monophosphate (cAMP)-stimulated secretion depends on a cystic fibrosis transmembrane conductance regulator channel (Cftr)-mediated HCO3- conductance. However, discrepancies in applying the model suggest that Cl-/HCO3- exchange also contributes to cAMP-stimulated secretion. Of 2 candidate Cl-/HCO3- exchangers, studies of putative anion transporter-1 knockout (KO) mice find little contribution of putative anion transporter-1 to basal or cAMP-stimulated secretion. Therefore, the role of down-regulated in adenoma (Dra) in duodenal HCO3- secretion was investigated using DraKO mice. Methods: Duodenal HCO3- secretion was measured by pH stat in Ussing chambers. Apical membrane Cl-/ HCO3- exchange was measured by microfluorometry of intracellular pH in intact villous epithelium. Dra expression was assessed by immunofluorescence. Results: Basal HCO3- secretion was reduced similar to 55%-60% in the DraKO duodenum. cAMP-stimulated HCO3- secretion was reduced similar to 50%, but short-circuit current was unchanged, indicating normal Cftr activity. Microfluorimetry of villi demonstrated that Dra is the dominant Cl-/HCO3- exchanger in the lower villous epithelium. Dra expression increased from villous tip to crypt. DraKO and wild-type villi also demonstrated regulation of apical Na+/H+ exchange by Cftr-dependent cell shrinkage during luminal Cl- substitution. Conclusions: In murine duodenum, Dra Cl-/HCO3- exchange is concentrated in the lower crypt-villus axis where it is subject to Cftr regulation. Dra activity contributes most basal HCO3- secretion and similar to 50% of cAMP-stimulated HCO3- secretion. Dra Cl-/HCO3- exchange should be considered in efforts to normalize HCO3- secretion in duodenal disorders such as ulcer disease and cystic fibrosis.