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

<jats:p> The recent proposal that Dra/Slc26a3 mediates electrogenic 2Cl<jats:sup>−</jats:sup>/1HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> exchange suggests a required revision of classical concepts of electroneutral Cl<jats:sup>−</jats:sup> transport across epithelia such as the intestine. We investigated 1) the effect of endogenous Dra Cl<jats:sup>−</jats:sup>/HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> activity on apical membrane potential ( V<jats:sub>a</jats:sub>) of the cecal surface epithelium using wild-type (WT) and knockout (KO) mice; and 2) the electrical properties of Cl<jats:sup>−</jats:sup>/(OH<jats:sup>−</jats:sup>)HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> exchange by mouse and human orthologs of Dra expressed in Xenopus oocytes. Ex vivo <jats:sup>36</jats:sup>Cl<jats:sup>−</jats:sup> fluxes and microfluorometry revealed that cecal Cl<jats:sup>−</jats:sup>/HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> exchange was abolished in the Dra KO without concordant changes in short-circuit current. In microelectrode studies, baseline V<jats:sub>a</jats:sub> of Dra KO surface epithelium was slightly hyperpolarized relative to WT but depolarized to the same extent as WT during luminal Cl<jats:sup>−</jats:sup> substitution. Subsequent studies indicated that Cl<jats:sup>−</jats:sup>-dependent V<jats:sub>a</jats:sub> depolarization requires the anion channel Cftr. Oocyte studies demonstrated that Dra-mediated exchange of intracellular Cl<jats:sup>−</jats:sup> for extracellular HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> is accompanied by slow hyperpolarization and a modest outward current, but that the steady-state current-voltage relationship is unaffected by Cl<jats:sup>−</jats:sup> removal or pharmacological blockade. Further, Dra-dependent <jats:sup>36</jats:sup>Cl<jats:sup>−</jats:sup> 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<jats:sup>−</jats:sup>/1HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> exchange; and 2) acute induction of Dra Cl<jats:sup>−</jats:sup>/HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> exchange is associated with secondary membrane potential changes representing homeostatic responses. Thus, participation of Dra in coupled NaCl absorption and in uncoupled HCO<jats:sub>3</jats:sub><jats:sup>−</jats:sup> secretion remains compatible with electroneutrality of these processes, and with the utility of electroneutral transport models for predicting epithelial responses in health and disease. </jats:p>