Coupling Modes and Stoichiometry of Cl−/HCO3− Exchange by slc26a3 and slc26a6

Abstract

The SLC26 transporters are a family of mostly luminal Cl− and HCO3 − transporters. The transport mechanism and the Cl−/HCO3 − stoichiometry are not known for any member of the family. To address these questions, we simultaneously measured the HCO3 − and Cl− 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 2Cl−/1HCO3 − exchanger. The membrane potential modulated the apparent affinity for extracellular Cl− of Cl−/HCO3 − exchange by slc26a3. Interestingly, the replacement of Cl− with NO3 − or SCN− uncoupled the transport, with large NO3 − and SCN− currents and low HCO3 − transport. An apparent uncoupled current was also developed during the incubation of slc26a3-expressing oocytes in HCO3 −-buffered Cl−-free media. These findings were used to develop a turnover cycle for Cl− and HCO3 − transport by slc26a3. Cl− and HCO3 − flux measurements revealed that slc26a6 mediates a 1Cl−/2HCO3 − exchange. Accordingly, holding the membrane potential at 40 and −100 mV accelerated and inhibited, respectively, Cl−-mediated HCO3 − influx, and holding the membrane potential at −100 mV increased HCO3 −-mediated Cl− influx. These findings indicate that slc26a6 functions as a coupled 1Cl−/2HCO3 − exchanger. The significance of isoform-specific Cl− and HCO3 − transport stoichiometry by slc26a3 and slc26a6 is discussed in the context of diseases of epithelial Cl− absorption and HCO3 − secretion.

SLC26 anion exchangers of guinea pig pancreatic duct: molecular cloning and functional characterization

“…3D shows nonsignificant increases in Cl Ϫ /HCO 3 Ϫ exchange-associated outward current recorded at holding potentials of Ϫ30 and Ϫ90 mV for human SLC26A3 and gpSlc26a3. These data together suggest that gpSlc26a3, like human SLC26A3 (8,17,26) and mouse Slc26a3 (1,46), mediates electroneutral Cl Ϫ /HCO 3 Ϫ exchange, in contrast to reports of electrogenic Cl Ϫ /HCO 3 Ϫ exchange by mouse Slc26a3 (33,44).…”

“…Mouse and human Slc26a3-mediated Cl Ϫ /HCO 3 Ϫ exchange have been reported to be electrogenic (33,44) or electroneutral (1,8,31,59). We therefore measured electrical parameters of gpSlc26a3 expression.…”

“…Murine Slc26a3 and Slc26a6 have been proposed to mediate electrogenic Cl Ϫ /HCO 3 Ϫ exchange of opposing stoichiometries, 2Cl Ϫ /1HCO 3 Ϫ and 1Cl Ϫ /2HCO 3 Ϫ , respectively. These electrogenic apical Cl Ϫ /HCO 3 Ϫ exchange activities have been proposed to be important for stimulated human and guinea pig pancreatic duct secretion of HCO 3 Ϫ -rich fluid (25,44). However, the axial expression pattern of Slc26a3 and Slc26a6 along the pancreatic duct remains unknown.…”

The distribution and expression of CFTR restricts electrogenic anion secretion to the ileum of the brushtail possum, Trichosurus vulpecula

“…Similarly, the minimal increase in I sc stimulated by PGE 2 or forskolin suggests that any HCO 3 -secretion stimulated by these secretagogues is also electroneutral. Both processes may involve SLC26A3 or, as has been suggested for eutherian epithelia (Shcheynikov et al, 2006), simultaneous activities of equal magnitude of SLC26A3 and SLC26A6. Because of their opposite stoichiometries, this would result in electroneutral transport.…”

“…However, SLC26A6 contributes little to HCO 3 -secretion (Wang et al, 2005) whereas SLC26A3 is responsible for ~60% of the spontaneous HCO 3 -secretion and ~50% of the cAMP-stimulated secretion, CFTR and a small paracellular leak accounting for the remainder (Walker et al, 2009). It is generally accepted that SLC26A6 is electrogenic (Jiang et al, 2002;Ko et al, 2002;Xie et al, 2002) with an apparent stoichiometry of 1Cl -:2HCO 3 - (Shcheynikov et al, 2006), and early evidence suggested that SLC26A3 was also electrogenic, but with a stoichiometry of 2Cl -:1HCO 3 -, opposite that of SLC26A6 (Ko et al, 2002;Shcheynikov et al, 2006). However, more recent evidence indicates that SLC26A3 is electroneutral Lamprecht et al, 2005;Melvin et al, 1999).…”