Cloning and characterization of a human electrogenic Na+-HCO3- cotransporter isoform (hhNBC)

group recently cloned the electrogenic Na+-HCO3- cotransporter (NBC) from salamander kidney and later from mammalian kidney. Here we report cloning an NBC isoform (hhNBC) from a human heart cDNA library. hhNBC is identical to human renal NBC (hkNBC), except for the amino terminus, where the first 85 amino acids in hhNBC replace the first 41 amino acids of hkNBC. About 50% of the amino acid residues in this unique amino terminus are charged, compared with similar to 22% for the corresponding 41 residues in hkNBC. Northern blot analysis, with the use of the unique 5' fragment of hhNBC as a probe, shows strong expression in pancreas and expression in heart and brain, although at much lower levels. In Xenopus oocytes expressing hhNBC, adding 1.5% CO2/10 mM HCO3- hyperpolarizes the membrane and causes a rapid fall in intracellular pH (pH(i)), followed by a pH(i) recovery. Subsequent removal of Na+ causes a depolarization and a reduced rate of pH(i) recovery. Removal of Cl- from the bath does not affect the pH(i) recovery. The stilbene derivative DIDS (200 mu M) greatly reduces the hyperpolarization caused by adding CO2/HCO3-. In oocytes expressing hkNBC, the effects of adding CO2/HCO3- and then removing Na+ were similar to those observed in oocytes expressing hhNBC. We conclude that hhNBC is an electrogenic Na+-HCO3- cotransporter and that hhNBC is also electrogenic.