Chloride and bicarbonate have similar affinities to the intestinal anion exchanger DRA (down regulated in adenoma)

Abstract

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.

Intestinal Anion Exchanger Down-regulated in Adenoma (DRA) Is Inhibited by Intracellular Calcium

“…These values are then used for the statistical analysis. Using this analytical approach, we have previously obtained evidence that DRA is activated by alkaline pH i (18), a finding that was recently corroborated (39). Regulation of transport activity of DRA may principally occur through changes in V max (by changing the number of DRA molecules in the plasma membrane or by effects on the membrane resident transporter molecules) or by changes of the pH i dependence of the transporter (the so-called set point).…”

“…18, calculates for each individual tracing the initial slope and the maximal change in pH i after change from chloride-containing to chloride-free buffers. These values are then used for the statistical analysis.…”

“…The pH i traces from the transfected HEK cells were analyzed as described previously (18). This nonlinear curve-fitting algorithm allows the estimation of the initial slope after the change of the buffer solutions as well as the estimation of the maximal pH i change from base line after removal of extracellular chloride.…”

Functional coupling of apical Cl−/HCO3− exchange with CFTR in stimulated HCO3− secretion by guinea pig interlobular pancreatic duct

“…H 2 DIDS similarly inhibited duct cell HCO 3 Ϫ efflux under basal conditions and in response to stimulation by secretin (12). H 2 DIDS inhibited both basal and secretin-stimulated fluid secretion into Cl Ϫ -rich luminal fluid (17 (35), but Slc26a3-mediated anion exchange with properties of V m sensitivity or electrogenicity was not observed by other laboratories (4,24,28). Moreover, Slc26a3-mediated Cl Ϫ /HCO 3 Ϫ exchange across the apical enterocyte membrane of intact mouse ileum does not exhibit properties of electrogenicity (42).…”

SLC26 anion exchangers of guinea pig pancreatic duct: molecular cloning and functional characterization

“…2) (Fig. 2) would favor this model, but Lamprecht et al (27) reported that the presence of even 24 mM luminal HCO 3 Ϫ may increase the K 0.5 of human SLC26A3 for Cl o Ϫ by ϳ10-fold (27). Moreover, the axial distribution of Slc26a3 along the length of the pancreatic duct in any species remains undefined.…”