Role of Na+HCO3- cotransporter NBC1, Na+/H+ exchanger NHE1, and carbonic anhydrase in rabbit duodenal bicarbonate secretion

Background & Aims: HCO3- supply to the enterocyte is rate limiting for duodenal HCO3- secretion (J(HCO3-)). This study defines the molecular nature of the major HCO3- uptake pathways in rabbit duodenocytes and investigates their physiologic significance and regulation during basal and stimulated J(HCO3-). Methods & Results: pH gradient-driven Na-22(+) uptake into duodenal basolateral membrane vesicles was partly HCO3- dependent, stilbene sensitive, and therefore mediated by Na+HCO3- cotransport, and partly HCO3- independent, Hoechst 642 sensitive, and therefore mediated by the Na+/H+ exchanger isoform NHE1. Semiquantitative polymerase chain reaction (PCR) revealed high duodenal expression revels for the NBC1 isoform of the Na+HCO3- cotransporter gene family and NHE1. Cloning and comparison of full-length rabbit with human gastrointestinal and kidney NBC1 subtype revealed a conserved protein kinase A consensus sequence in the cytoplasmic N-terminus of the gastrointestinal NBC1, Inhibition of either Na+HCO3- cotransport or carbonic anhydrase reduced ouabain-sensitive J(HCO3-) in in vitro rabbit duodenal mucosae by approximately 50%, but did not affect 8-Br-cAMP-induced Delta J(HCO3-), suggesting cAMP-mediated up-regulation of the alternative pathway. However, inhibition of both Na+HCO3- cotransport and either carbonic anhydrase or NHE1 strongly reduced Delta J(HCO3-). Conclusions NBC1 and NHE1 are the major base importers in rabbit duodenocytes, Na+HCO3- cotransport and CO2 hydration/Na+/H+ exchange are equally important pathways for duodenal HCO3- supply and are upregulated during cAMP-mediated stimulation.