Functional characterization of a cloned human kidney Na+:HCO3- cotransporter

Functional properties of a cloned human kidney Na+:HCO3- cotransporter (NBC-1) were studied in cultured HEK-293 cells that were transiently transfected with NBC-1 cDNA. The Na+:HCO3- cotransporter activity was assayed as the Na+ and HCO3-dependent pHi recovery from intracellular acidosis with the use of the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein. In acid-loaded cells and in the presence of amiloride (to block Na+/H+ exchange), switching to a Na+-containing solution (115 mM) resulted in rapid pHi recovery only in the presence of HCO3-. This recovery was completely abolished by 300 microM 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid. Replacing the Na+ with Li+ (115 mM) caused significant HCO3--dependent, DIDS-sensitive pHi recovery from intracellular acidosis, with Li+ showing lower affinity than Na+. Potassium (K+) had no affinity for the Na+:HCO3- cotransporter. The Na+-dependent HCO3- cotransport was abolished in the presence of 0.2 mM harmaline. The Na+:HCO3- cotransporter could also function in Na+:OH- cotransport mode, although only at high external pH (7.8). Based on functional similarities with the mammalian kidney experiments, we propose that NBC-1 is the proximal tubule Na+:HCO3- cotransporter.