Ae4 (Slc4a9) is an electroneutral monovalent cation-dependent Cl-/HCO3- exchanger.
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.
Secretory Mechanisms and Dysfunction Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892.
Gaspar
G
Peña-Münzenmayer
Secretory Mechanisms and Dysfunction Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892.
Alvin T
AT
George
Department of Molecular Genetics, Biochemistry, and Microbiology, University of Cincinnati College of Medicine, Cincinnati, OH 45267.
Gary E
GE
Shull
Secretory Mechanisms and Dysfunction Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892.
James E
JE
Melvin
Secretory Mechanisms and Dysfunction Section, National Institute of Dental and Craniofacial Research, National Institutes of Health, Bethesda, MD 20892 marcelo.catalan@nih.gov.
Marcelo A
MA
Catalán
D061354:Acinar Cells / Q000378:metabolism / Q000502:physiology; D000200:Action Potentials; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism; D016466:CHO Cells; D002478:Cells, Cultured; D027963:Chloride-Bicarbonate Antiporters / Q000235:genetics / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism; D006224:Cricetinae; D003412:Cricetulus; D017136:Ion Transport; D051379:Mice
D001639:Bicarbonates; D027963:Chloride-Bicarbonate Antiporters; D002712:Chlorides; C480025:Slc4a9 protein, mouse