Down-regulated in adenoma mediates apical Cl-/HCO3- exchange in rabbit, rat, and human duodenum.
OBJECTIVE
Duodenal bicarbonate secretion is in part mediated by an apical Cl-/HCO3- exchanger of unknown molecular nature. The recently discovered dra (down-regulated in adenoma) gene encodes a transport protein (DRA) for SO4(2-), Cl-, and HCO3-. The aim of this study was to investigate whether DRA may be the duodenal apical Cl-/HCO3- exchanger.


METHODS
DRA, Na+/H+ exchanger (NHE) isoform 3, and anion exchanger isoform (AE) 2 messenger RNA expression levels were studied in rat, rabbit, and human gastrointestinal tract by semiquantitative reverse-transcription polymerase chain reaction and in situ hybridization (DRA in human intestine). The subcellular localization of DRA was determined by Western analysis and immunohistochemistry. Using rabbit and rat duodenal brush border membrane vesicles, anion exchange characteristics were investigated.


RESULTS
DRA expression was high in duodenum and colon of all species, whereas NHE3 messenger RNA expression was low in duodenum and high in colon. Western analysis and immunohistochemistry showed an apical localization for DRA. Rabbit and rat duodenal brush border membrane vesicles showed 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive Cl-/Cl-, HCO3-/Cl-, SO4(2-)/Cl-, and Cl-/SO4(2-) exchange, with evidence for one major brush border membrane Cl-/anion exchanger, an affinity for Cl- > HCO3-, and a much higher affinity for SO4(2-) in rat than rabbit. The strong predominance of DRA over NHE3 and NHE2 expression in duodenum was paralleled by much higher Cl-/HCO3- than Na+/H+ exchange rates in brush border membrane vesicles and likely explains the high duodenal HCO3- secretory rates.


CONCLUSIONS
These data suggest that DRA is the major apical anion exchanger in the duodenum as well as the colon and the likely transport protein for duodenal electroneutral HCO3- secretion.
Jacob
Petra
P
Department of Medicine, University of Tübingen, Tübingen, Germany.
Rossmann
Heidi
H
Lamprecht
Georg
G
Kretz
Alexandra
A
Neff
Christina
C
Lin-Wu
Elena
E
Gregor
Michael
M
Groneberg
David A
DA
Kere
Juha
J
Seidler
Ursula
U
D000236:Adenoma / Q000378:metabolism; D000818:Animals; D027321:Anion Transport Proteins; D017920:Antiporters; D001639:Bicarbonates / Q000378:metabolism*; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism*; D016764:Cell Polarity; D027963:Chloride-Bicarbonate Antiporters / Q000235:genetics / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D003001:Cloning, Molecular; D018076:DNA, Complementary; D015536:Down-Regulation; D004386:Duodenum / Q000378:metabolism*; D015870:Gene Expression / Q000502:physiology; D006801:Humans; D007413:Intestinal Mucosa / Q000378:metabolism; D008565:Membrane Proteins / Q000235:genetics* / Q000378:metabolism*; D008969:Molecular Sequence Data; D011522:Protons; D011817:Rabbits; D051381:Rats; D064606:SLC4A Proteins; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics; D013045:Species Specificity; D000076988:Sulfate Transporters; D013431:Sulfates / Q000378:metabolism
D027321:Anion Transport Proteins; D017920:Antiporters; D001639:Bicarbonates; D002352:Carrier Proteins; D027963:Chloride-Bicarbonate Antiporters; D002712:Chlorides; D018076:DNA, Complementary; D008565:Membrane Proteins; D011522:Protons; C081011:SLC26A3 protein, human; D064606:SLC4A Proteins; C000613385:SLC9A3 protein, human; C489861:Slc26a3 protein, rat; C000613387:Slc9a3 protein, rat; D000074083:Sodium-Hydrogen Exchanger 3; D017923:Sodium-Hydrogen Exchangers; D000076988:Sulfate Transporters; D013431:Sulfates; D012964:Sodium