Identification of an apical [Cl.sup.-]/HC[O.sup.-.sub.3] exchanger in the small intestine

HC[O.sup.-.sub.3] secretion is the most important defense mechanism against acid injury in the duodenum. However, the identity of the transporter(s) mediating apical HC[O.sup.-.sub.3] secretion in the duodenum remains unknown. A family of anion exchangers, which include downregulated in adenoma (DRA or SLC26A3), pendrin (PDS or SLC26A4), and the putative anion transporter (PAT1 or SLC26A6) has recently been identified. DRA and pendrin mediate [Cl.sup.-]/base exchange; however, the functional identity and distribution of PAT1 (SLC26A6) is not known. In these studies, we investigated the functional identity, tissue distribution, and membrane localization of PAT1. Expression studies in Xenopus oocytes demonstrated that PAT1 functions in [Cl.sup.-]/HC[O.sup.-.sub.3] exchange mode. Tissue distribution studies indicated that the expression of PAT1 is highly abundant in the small intestine but is low in the colon, a pattern opposite that of DRA. PAT1 was also abundantly detected in stomach and heart. Immunoblot analysis studies identified PAT1 as a ~90 kDa protein in the duodenum. Immunohistochemical studies localized PAT1 to the brush border membranes of the villus cells of the duodenum. We propose that PAT1 is an apical [Cl.sup.-]/HC[O.sup.-.sub.3] exchanger in the small intestine. duodenum; bicarbonate secretion; apical [Cl.sup.-]/HC[O.sup.-.sub.3] exchange; putative anion transporter; downregulated in adenoma; chloride/formate exchanger