Characterization of human SLC4A10 as an electroneutral Na/HCO₃ cotransporter (NBCn2) with Cl^- self-exchange activity

The SLC4A10 gene product, commonly known as NCBE, is highly expressed in rodent brain and has been characterized by others as a Na⁺-driven Cl-HCO₃ exchanger. However, some of the earlier data are not consistent with Na⁺-driven Cl-HCO₃ exchange activity. In the present study, northern blot analysis showed that, also in humans, NCBE transcripts are predominantly expressed in brain. In some human NCBE transcripts, splice cassettes A and/or B, originally reported in rats and mice, are spliced out. In brain cDNA, we found evidence of a unique partial splice of cassette B that is predicted to produce an NCBE protein with a novel C terminus containing a protein kinase C phosphorylation site. We used pH-sensitive microelectrodes to study the molecular physiology of human NCBE expressed in Xenopus oocytes. In agreement with others we found that NCBE mediates the 4,4′-diisothiocyanato-stilbene-2,2′-disulfonic acid-sensitive, Na⁺-dependent transport of HCO₃^-. For the first time, we demonstrated that this transport process is electroneutral. Using Cl^--sensitive microelectrodes positioned at the oocyte surface, we found that, unlike both human and squid Na⁺-driven Cl-HCO₃ exchangers, human NCBE does not normally couple the net influx of HCO ₃^- to a net efflux of Cl^-. Moreover we found that that the 36Cl efflux from NCBE-expressing oocytes, interpreted by others to be coupled to the influx of Na⁺ and HCO₃^-, actually represents a CO₂/HCO₃^--stimulated Cl^- self-exchange not coupled to either Na⁺ or net HCO₃^- transport. We propose to rename NCBE as the second electroneutral Na/HCO₃ cotransporter, NBCn2.