Molecular and functional evidence for electrogenic and electroneutral Na+-HCO3- cotransporters in murine duodenum

Inward Na+-HCO3- cotransport has previously been demonstrated in acidified duodenal epithelial cells, but the identity and localization of the mRNAs a nd proteins involved have not been determined. The molecular expression and localization of Na+-HCO3- cotransporters (NBCs) were studied by RT-PCR, se quence analysis, and immunohistochemistry. By fluorescence spectroscopy, th e intracellular pH (pH(i)) was recorded in suspensions of isolated murine d uodenal epithelial cells loaded with 2',7'-bis(2-carboxyethyl)-5(6)-carboxy fluorescein. Proximal duodenal epithelial cells expressed mRNA encoding two electrogenic NBC1 isoforms and the electroneutral NBCn1. Both NBC1 and NBC n1 were localized to the basolateral membrane of proximal duodenal villus c ells, whereas the crypt cells did not label with the anti-NBC antibodies. D IDS or removal of extracellular Cl- increased pH(i), whereas an acidificati on was observed on removal of Na+ or both Na+ and Cl-. The effects of inhib itors and ionic dependence of acid/base transporters were consistent with b oth inward and outward Na+-HCO3- cotransport. Hence, we propose that NBCs a re involved in both basolateral electroneutral HCO3- transport as well as b asolateral electrogenic HCO3- transport in proximal duodenal villus cells.