Background & aims: Duodenal bicarbonate secretion is in part mediated by an apical Cl-/HCO3- exchanger of unknown molecular nature. The recently discovered dra (down-regulated in adenoma) gene encodes a transport protein (DRA) for SO4(2-), Cl-, and HCO3-. The aim of this study was to investigate whether DRA may be the duodenal apical Cl-/HCO3- exchanger.
Methods: DRA, Na+/H+ exchanger (NHE) isoform 3, and anion exchanger isoform (AE) 2 messenger RNA expression levels were studied in rat, rabbit, and human gastrointestinal tract by semiquantitative reverse-transcription polymerase chain reaction and in situ hybridization (DRA in human intestine). The subcellular localization of DRA was determined by Western analysis and immunohistochemistry. Using rabbit and rat duodenal brush border membrane vesicles, anion exchange characteristics were investigated.
Results: DRA expression was high in duodenum and colon of all species, whereas NHE3 messenger RNA expression was low in duodenum and high in colon. Western analysis and immunohistochemistry showed an apical localization for DRA. Rabbit and rat duodenal brush border membrane vesicles showed 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive Cl-/Cl-, HCO3-/Cl-, SO4(2-)/Cl-, and Cl-/SO4(2-) exchange, with evidence for one major brush border membrane Cl-/anion exchanger, an affinity for Cl- > HCO3-, and a much higher affinity for SO4(2-) in rat than rabbit. The strong predominance of DRA over NHE3 and NHE2 expression in duodenum was paralleled by much higher Cl-/HCO3- than Na+/H+ exchange rates in brush border membrane vesicles and likely explains the high duodenal HCO3- secretory rates.
Conclusions: These data suggest that DRA is the major apical anion exchanger in the duodenum as well as the colon and the likely transport protein for duodenal electroneutral HCO3- secretion.