Chloride and bicarbonate have similar affinities to the intestinal anion exchanger DRA (down regulated in adenoma).
DRA (down regulated in adenoma, SLC26A3) is an anion exchanger that mediates electroneutral NaCl absorption in the ileum and proximal colon together with NHE3 (Na/H exchanger isoform 3), and that is involved in duodenal and possibly pancreatic bicarbonate secretion. Thus, its chloride and bicarbonate affinities are important for both processes. [Cl]i and pHi transients were measured using MQAE and BCECF. HEK293 cells stably expressing DRA were exposed to 0 mM Cl at various [HCO3] (9 to 51 mM, at 5% CO2 or 15 to 57 mM, at pH 7.5) to determine the HCO3 affinity. After intracellular Cl depletion, 10, 30, and 90 mM Cl were readded at various [HCO3]s to determine the relative Cl and HCO3 affinities. The k0.5 for extracellular HCO3 is between 18.5 and 32.8 mM. Cl and HCO3 compete with similar affinities for transport by DRA. DRA activity is independent of pHo between 7.0 and 7.75. DRA is activated by alkaline pHi. Competition of Cl and HCO3 does not significantly impair NaCl absorption, because in the ileum and colon, luminal Cl is comparably high. Activation at alkaline pHi supports functional coupling of DRA and NHE3 by the subapical pHi. In the distal pancreatic ductal system, luminal HCO3 is high compared to luminal Cl. Under these conditions, competition of Cl and HCO3 is difficult to reconcile with a role of DRA in Cl reabsorption in exchange for HCO3. Our data, thus, provide indirect evidence against a role of DRA in pancreatic HCO3 secretion.