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.
Amlal
H
H
Department of Medicine, University of Cincinnati School of Medicine, Cincinnati, Ohio 45267-0585, USA.
Wang
Z
Z
Burnham
C
C
Soleimani
M
M
D001639:Bicarbonates / Q000378:metabolism; D002352:Carrier Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D003001:Cloning, Molecular; D018076:DNA, Complementary; D006246:Harmaline / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport; D007668:Kidney / Q000378:metabolism*; D008094:Lithium / Q000378:metabolism; D011994:Recombinant Proteins / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D027982:Sodium-Bicarbonate Symporters
D001639:Bicarbonates; D002352:Carrier Proteins; D018076:DNA, Complementary; D011994:Recombinant Proteins; D027982:Sodium-Bicarbonate Symporters; D008094:Lithium; D012964:Sodium; D006246:Harmaline