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-HCO(3) exchanger. However, some of the earlier data are not consistent with Na(+)-driven Cl-HCO(3) 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 HCO(3)(-). 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-HCO(3) 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 (36)Cl efflux from NCBE-expressing oocytes, interpreted by others to be coupled to the influx of Na(+) and HCO(3)(-), actually represents a CO(2)/HCO(3)(-)-stimulated Cl(-) self-exchange not coupled to either Na(+) or net HCO(3)(-) transport. We propose to rename NCBE as the second electroneutral Na/HCO(3) cotransporter, NBCn2.
Parker
Mark D
MD
Department of Cellular and Molecular Physiology, Yale University, New Haven, Connecticut 06520, USA.
Musa-Aziz
Raif
R
Rojas
Jose D
JD
Choi
Inyeong
I
Daly
Christopher M
CM
Boron
Walter F
WF
D000818:Animals; D001639:Bicarbonates / Q000737:chemistry / Q000378:metabolism*; D001692:Biological Transport; D002712:Chlorides / Q000737:chemistry / Q000378:metabolism*; D003001:Cloning, Molecular; D004594:Electrophysiology; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008969:Molecular Sequence Data; D009474:Neurons / Q000378:metabolism*; D009865:Oocytes; D018408:Patch-Clamp Techniques; D012333:RNA, Messenger / Q000235:genetics; D012964:Sodium / Q000737:chemistry / Q000378:metabolism*; D027982:Sodium-Bicarbonate Symporters / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis
D001639:Bicarbonates; D002712:Chlorides; D012333:RNA, Messenger; C529475:SLC4A10 protein, human; D027982:Sodium-Bicarbonate Symporters; D012964:Sodium