PAT-1 (Slc26a6) is the predominant apical membrane Cl-/HCO-3 exchanger in the upper villous epithelium of the murine duodenum

Basal HCO3- secretion across the duodenum has been shown in several species to principally involve the activity of apical membrane Cl-/HCO3(-) exchanger(s). To investigate the identity of relevant anion exchanger( s), experiments were performed using wild-type (WT) mice and mice with gene-targeted deletion of the following Cl-/HCO3- exchangers localized to the apical membrane of murine duodenal villi: Slc26a3 [ down-regulated in adenoma (DRA)], Slc26a6 [ putative anion transporter 1 (PAT-1)], and Slc4a9 [ anion exchanger 4 (AE4)]. RT-PCR of the isolated villous epithelium demonstrated PAT-1, DRA, and AE4 mRNA expression. Using the pH-sensitive dye BCECF, anion exchange rates were measured across the apical membrane of epithelial cells in the upper villus of the intact duodenal mucosa. Under basal conditions, Cl-/HCO3- exchange activity was reduced by 65 - 80% in the PAT-1( -) duodenum, 30-40% in the DRA( -) duodenum, and < 5% in the AE4( -) duodenum compared with the WT duodenum. SO42-/ HCO3- exchange was eliminated in the PAT-1(-) duodenum but was not affected in the DRA( -) and AE4( -) duodenum relative to the WT duodenum. Intracellular pH (pH(i)) was reduced in the PAT-1( -) villous epithelium but increased to WT levels in the absence of CO2/HCO3- or during methazolamide treatment. Further experiments under physiological conditions indicated active pH(i) compensation in the PAT-1(-) villous epithelium by combined activities of Na+/H+ exchanger 1 and Cl--dependent transport processes at the basolateral membrane. We conclude that 1) PAT-1 is the major contributor to basal Cl-/HCO3- and SO42-/ HCO3- exchange across the apical membrane and 2) PAT-1 plays a role in pH(i) regulation in the upper villous epithelium of the murine duodenum.