Characterization of Na+/HCO3- cotransporter isoform NBC-3

Na+-HCO3-, cotransporters mediate the transport of HCO, into or out of the cell. Two Nai-HCO, cotransporters (NBC) have been identified previously, wh ich are referred to as NBC-1 and NBC-2. A cDNA library from uninduced human NT-2 cells was screened with an NBC-2 cDNA probe. Several clones were iden tified and isolated. Sequence analysis of these clones identified a partial coding region (2 kb) of a novel NBC (called here NBC-S), which showed 53% and 72% identity with NBC-1 and NBC-8, respectively. Northern blot analysis revealed that NBC-S encodes a 4.4-kb mRNA with a tissue distribution patte rn distinct from NBC-1 and NBC-S. NBC-3 is highly expressed in brain and sp inal column, with moderate levels in trachea, thyroid, and kidney. In contr ast with NBC-1, NBC-3 shows low levels of expression in pancreas and kidney cortex. In the kidney, NBC-3 expression is predominantly limited to the me dulla. Cultured mouse inner medullary collecting duct (mIMCD-3) cells showe d high levels of NBC-1 and low levels of NBC-3 mRNA expression. Subjecting the mutagenized mIMCD-3 cells to sublethal acid stress decreased the mRNA e xpression of NBC-1 by similar to 90% but increased the Na+-dependent HCO3- cotransport activity by similar to 7-fold (as assayed by DIDS-sensitive, Na +-dependent, HCO3--mediated intracellular pH recovery). This increase was a ssociated with similar to 5.5fold enhancement of NBC-8 mRNA levels. NBC sho wed significant affinity for Li+ in the mutant but not the parent mIMCD-3 c ells. On the basis of the widespread distribution of NBC-3, we propose that this isoform is likely involved in cell pH regulation by transporting HCO3 - from blood to the cell. We further propose that enhanced expression of NB C-3 in severe acid stress could play an important role in cell survival by mediating the influx of HCO3- into the cells.