Ae4 (Slc4a9) is an electroneutral monovalent cation-dependent Cl/HCO exchanger

Ae4 (Slc4a9) belongs to the Slc4a family of Cl/HCO exchangers and Na-HCO 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/HCO 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 HCO (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-HCO cotransport-like activity under Cl-free conditions. However, whole cell recordings show that this apparent Na-HCO cotransport activity is in fact electroneutral HCO/Na-HCO 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 HCO 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/HCO exchange. Indeed, we find that Ae4 mediates Cl/HCO 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-HCO exchanger. We postulate that the physiological role of Ae4 in secretory cells is to promote Cl influx in exchange for K(Na) and HCO ions.