Background & Aims
Electroneutral NaCl absorption across small intestine contributes importantly to systemic fluid balance. Disturbances in this process occur in both obstructive and diarrheal diseases, eg, cystic fibrosis, secretory diarrhea. NaCl absorption involves coupling of Cl−/HCO3− exchanger(s) primarily with Na+/H+ exchanger 3 (Nhe3) at the apical membrane of intestinal epithelia. Identity of the coupling Cl−/HCO3− exchanger(s) was investigated using mice with gene-targeted knockout (KO) of Cl−/HCO3− exchangers: Slc26a3, down-regulated in adenoma (Dra) or Slc26a6, putative anion transporter-1 (Pat-1).
Intracellular pH (pHi) of intact jejunal villous epithelium was measured by ratiometric microfluoroscopy. Ussing chambers were used to measure transepithelial 22Na36Cl flux across murine jejunum, a site of electroneutral NaCl absorption. Expression was estimated using immunofluorescence and quantitative polymerase chain reaction.
Basal pHi of DraKO epithelium, but not Pat-1KO epithelium, was alkaline, whereas pHi in the Nhe3KO was acidic relative to wild-type. Altered pHi was associated with robust Na+/H+ and Cl−/HCO3− exchange activity in the DraKO and Nhe3KO villous epithelium, respectively. Contrary to genetic ablation, pharmacologic inhibition of Nhe3 in wild-type did not alter pHi but coordinately inhibited Dra. Flux studies revealed that Cl− absorption was essentially abolished (>80%) in the DraKO and little changed (<20%) in the Pat-1KO jejunum. Net Na+ absorption was unaffected. Immunofluorescence demonstrated modest Dra expression in the jejunum relative to large intestine. Functional and expression studies did not indicate compensatory changes in relevant transporters.
These studies provide functional evidence that Dra is the major Cl−/HCO3− exchanger coupled with Nhe3 for electroneutral NaCl absorption across mammalian small intestine.