Native and recombinant Slc26a3 (downregulated in adenoma, Dra) do not exhibit properties of 2Cl−/1HCO3− exchange

Abstract

The recent proposal that Dra/Slc26a3 mediates electrogenic 2Cl−/1HCO3− exchange suggests a required revision of classical concepts of electroneutral Cl− transport across epithelia such as the intestine. We investigated 1) the effect of endogenous Dra Cl−/HCO3− activity on apical membrane potential ( Va) of the cecal surface epithelium using wild-type (WT) and knockout (KO) mice; and 2) the electrical properties of Cl−/(OH−)HCO3− exchange by mouse and human orthologs of Dra expressed in Xenopus oocytes. Ex vivo 36Cl− fluxes and microfluorometry revealed that cecal Cl−/HCO3− exchange was abolished in the Dra KO without concordant changes in short-circuit current. In microelectrode studies, baseline Va of Dra KO surface epithelium was slightly hyperpolarized relative to WT but depolarized to the same extent as WT during luminal Cl− substitution. Subsequent studies indicated that Cl−-dependent Va depolarization requires the anion channel Cftr. Oocyte studies demonstrated that Dra-mediated exchange of intracellular Cl− for extracellular HCO3− is accompanied by slow hyperpolarization and a modest outward current, but that the steady-state current-voltage relationship is unaffected by Cl− removal or pharmacological blockade. Further, Dra-dependent 36Cl− efflux was voltage-insensitive in oocytes coexpressing the cation channels ENaC or ROMK. We conclude that 1) endogenous Dra and recombinant human/mouse Dra orthologs do not exhibit electrogenic 2Cl−/1HCO3− exchange; and 2) acute induction of Dra Cl−/HCO3− exchange is associated with secondary membrane potential changes representing homeostatic responses. Thus, participation of Dra in coupled NaCl absorption and in uncoupled HCO3− secretion remains compatible with electroneutrality of these processes, and with the utility of electroneutral transport models for predicting epithelial responses in health and disease.