FUNCTIONAL-CHARACTERIZATION OF A CLONED HUMAN KIDNEY NA-HCO3- COTRANSPORTER()

Functional properties of a cloned human kidney Naf: HCO3- cotransporte r (NBC-1) were studied in cultured HEK-293 cells that were transiently transfected with NBC-1 cDNA. The Na+:HCO3-, cotransporter activity wa s assayed as the Na+ and HCO3- dependent pH(i) recovery from intracell ular acidosis with the use of the pH sensitive dye 2',7'-bis(2-carboxy ethyl)-5(6)-carboxyfluorescein In acid-loaded cells and in the presenc e of amiloride (to block Na+/H+ exchange), switching to a Na+ containi ng solution (115 mM) resulted in rapid pH(i), recovery only in the pre sence of HCO3-. This recovery was completely abolished by 300 mu m 4,4 '-diisothiocyanostilbene-2,2'-disulfonic acid. Replacing the Na+ with Li+ (115 mM) caused significant HCO3- dependent, DIDS-sensitive pHi re covery from intracellular acidosis, with Li+ showing lower affinity th an Na+, Potassium (K+) had no affinity for the Na+:HCO3-, cotransporte r. The Na+-dependent HCO3- cotransport was abolished in the presence o f 0.2 mM harmaline. The Na+:HCO3- cotransporter could also function in Na+:OH- cotransport mode, although only at high external pH (7.8), Ba sed on functional similarities with the mammalian kidney experiments, we propose that NBC-1 is the proximal tubule Na+:HCO3- cotransporter.