Expression cloning and characterization of a renal electrogenic Na⁺/HCO₃^- cotransporter

Bicarbonate transporters are the principal regulators of pH in animal cells, and play a vital role in acid-base movement in the stomach, pancreas, intestine, kidney, reproductive system and central nervous system. The functional family of HCO₃^-transporters includes Cl^--HCO₃^- exchangers, three Na⁺/HCO₃^- cotransporters, a K⁺/HCO₃^- cotransporter, and a Na⁺- driven Cl^--HCO₃^- exchanger. Molecular information is sparse on HCO₃^- transporters, apart from Cl^--HCO₃^- exchangers ('anion exchangers'), whose complementary DNAs were cloned several years ago. Attempts to done other HCO₃^- transporters, based on binding of inhibitors, protein purification or homology with anion exchangers, have so far been unsuccessful. Here we monitor the intracellular pH and membrane voltage in Xenopus oocytes to follow the expression of the most electrogenic transporter known: the renal 1:3 electrogenic Na⁺/HCO₃^- cotransporter from the salamander Ambystoma tigrinum. We now report the successful cloning and characterization of a cDNA encoding a cation-coupled HCO₃^- transporter. The encoded protein is 1,035 amino acids long with several potential membrane-spanning domains. We show that when it is expressed in Xenopus oocytes, this protein is electrogenic, Na⁺ and HCO₃^- dependent, and blocked by the anion-transport inhibitor DIDS, and conclude that it is the renal electrogenic sodium bicarbonate cotransporter (NBC).