Abstract

Ae4 (Slc4a9) belongs to the Slc4a family of Cl-/HCO3- exchangers and Na+-HCO3- cotransporters, but its ion transport cycle is poorly understood. In this study, we find that native Ae4 activity in mouse salivary gland acinar cells supports Na+-dependent Cl-/HCO3- exchange that is comparable with that obtained upon heterologous expression of mouse Ae4 and human AE4 in CHO-K1 cells. Additionally, whole cell recordings and ion concentration measurements demonstrate that Na+ is transported by Ae4 in the same direction as HCO3- (and opposite to that of Cl-) and that ion transport is not associated with changes in membrane potential. We also find that Ae4 can mediate Na+-HCO3- cotransport-like activity under Cl--free conditions. However, whole cell recordings show that this apparent Na+-HCO3- cotransport activity is in fact electroneutral HCO3-/Na+-HCO3- exchange. Although the Ae4 anion exchanger is thought to regulate intracellular Cl-concentration in exocrine gland acinar cells, our thermodynamic calculations predict that the intracellular Na+, Cl-, and HCO3- concentrations required for Ae4-mediated Cl-influx differ markedly from those reported for acinar secretory cells at rest or under sustained stimulation. Given that K+ ions share many properties with Na+ ions and reach intracellular concentrations of 140-150 mM (essentially the same as extracellular [Na+]), we hypothesize that Ae4 could mediate K+-dependent Cl-/HCO3- exchange. Indeed, we find that Ae4 mediates Cl-/HCO3- exchange activity in the presence of K+ as well as Cs+, Li+, and Rb+. In summary, our results strongly suggest that Ae4 is an electroneutral Cl-/nonselective cation-HCO3- exchanger. We postulate that the physiological role of Ae4 in secretory cells is to promote Cl- influx in exchange for K+(Na+) and HCO3- ions.