Bicarbonate and chloride compete with comparable affinities at the extracellular transport site of the intestinal anion exchanger DRA (down regulated in adenoma)

DRA is the intestinal Cl/HCO3 exchanger mediating NaCl absorption in the ileum and proximal colon. It is also involved in duodenal and pancreatic bicarbonate secretion, but it is controversial whether it is activated or inhibited upon stimulation of pancreatic secretion. Previously we have determined a Cl affinity between 23 and 36 mM in DRA transfected HEK cells in the presence of 25 mM HCO3. This differs from published values (between 3.5 mM and 5.8 mM), that were measured at low extracellular HCO3 (0.285 mM, pH 5.5) or as 36Cl/Cl exchange. Aim: To determine if extracellular HCO3 competes with extracellular Cl…