Cloning and characterization of a human electrogenic Na⁺- HCO 3 − cotransporter isoform (hhNBC)

Our group recently cloned the electrogenic Na⁺-${HCO}_3^{-}$ 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 ∼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. InXenopus oocytes expressing hhNBC, adding 1.5% CO₂/10 mM${HCO}_3^{-}$ 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 μM) greatly reduces the hyperpolarization caused by adding CO₂/${HCO}_3^{-}$ . In oocytes expressing hkNBC, the effects of adding CO₂/${HCO}_3^{-}$ and then removing Na⁺ were similar to those observed in oocytes expressing hhNBC. We conclude that hhNBC is an electrogenic Na⁺-${HCO}_3^{-}$ cotransporter and that hkNBC is also electrogenic.