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

Alper SL, Stewart AK, Vandorpe DH, Clark JS, Horack RZ, Simpson JE, Walker NM, Clarke LL. Native and recombinant Slc26a3 (downregulated in adenoma, Dra) do not exhibit properties of 2Cl(-)/1HCO(3)(-) exchange. Am J Physiol Cell Physiol 300: C276-C286, 2011. First published November 10, 2010; doi: 10.1152/ajpcell.00366.2010.-The recent proposal that Dra/Slc26a3 mediates electrogenic 2Cl(-)/1HCO(3)(-) 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 (V-a) 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 Cl-36(-) 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 V-a 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 V-a 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 Cl-36(-) 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(-)/1HCO(3)(-) 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.