Coupling modes and stoichiometry of [Cl.sup.-]/HC[O.sub.3.sup.-] exchange by slc26a3 and slc26a6

The SLC26 transporters are a family of mostly luminal [Cl.sup.-] and HC[O.sub.3] transporters. The transport mechanism and the [Cl.sup.-]/HC[O.sub.3.sup.-] stoichiometry are not known for any member of the family. To address these questions, we simultaneously measured the HC[Osub.3] and [Cl.sup.-] fluxes and the current or membrane potential of slc26a3 and slc26a6 expressed in Xenopus laevis oocytes and the current of the transporters expressed in human embryonic kidney 293 cells, slc26a3 mediates a coupled 2[Cl.sup-]/1HC[O.sub.3.sup.-] exchanger. The membrane potential modulated the apparent affinity for extracellular [Cl.sup.-] of [Cl.sup.-]/HC[O.sub.3] exchange by slc26a3. Interestingly, the replacement of [Cl.sup.-] with N[O.sub.3.sup.-] or SC[N.sup.-] uncoupled the transport, with large N[O.sub.3.sup.-] and SC[N.sub.-] currents and low HC[O.sub.3.sup.-] transport. An apparent uncoupled current was also developed during the incubation of slc26a3-expressing oocytes in HC[O.sub.3.sup.-]-buffered [Cl.sup.-]-free media. These findings were used to develop a turnover cycle for [Cl.sup.-] and HC[O.sub.3.sup.-] transport by slc26a3. [Cl.sup.-] and HC[O.sub.3.sup.-] flux measurements revealed that slc26a6 mediates a 1[Cl.sup.-]/2HC[O.sub.3.sup.-] exchange. Accordingly, holding the membrane potential at 40 and -100 mV accelerated and inhibited, respectively, [Cl.sup.-]-mediated HC[O.sub.3.sup.-] influx, and holding the membrane potential at -100 mV increased HC[O.sub.3.sup.-]-mediated [Cl.sup.-] influx. These findings indicate that slc26a6 functions as a coupled 1[Cl.sup.-]/2HC[O.sub.3.sup.-] exchanger. The significance of isoform-specific [Cl.sup.-] and HC[O.sub.3.sup.-] transport stoichiometry by slc26a3 and slc26a6 is discussed in the context of diseases of epithelial [Cl.sup.-] absorption and HC[O.sub.3.sup.-] secretion.