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1 Division of Nephrology, Hypertension and Transplantation, University of Florida College of Medicine, Gainesville, FL, USA
2 Department of Pathology and Laboratory Medicine, University of Pennsylvania, Philadelphia, PA, USA; American Red Cross, Philadelphia, PA, USA
3 Renal Section, North Florida/South Georgia Veterans Health System, Gainesville, FL, USA; Division of Nephrology, Hypertension and Transplantation, University of Florida College of Medicine, Gainesville, FL, USA
* To whom correspondence should be addressed. E-mail: weineid{at}ufl.edu.
The renal collecting duct is the primary site for the ammonia secretion necessary for acidbase homeostasis. Recent studies have identified the presence of putative ammonia transporters in the collecting duct, but whether the collecting duct has transporter-mediated ammonia transport is unknown. The purpose of this study was to examine basolateral ammonia transport in the mouse collecting duct, mIMCD-3, cell. To examine mIMCD-3 basolateral ammonia transport, we used cells grown to confluence on permeable support membranes and quantified basolateral uptake of the radiolabeled ammonia analogue, 14C-methylammonia (14C-MA). mIMCD-3 cell basolateral MA transport exhibited both diffusive and transporter-mediated components. Transporter-mediated uptake exhibited a Km for methylammonia of 4.6 ± 0.2 mM, exceeded diffusive uptake at methylammonia concentrations below 7.0 ± 1.8 mM, and was competitively inhibited by ammonia with a Ki of 2.1 ± 0.6 mM. Transporter-mediated uptake was not altered by inhibitors of Na+-K+- ATPase, Na+-K+-2Cl- cotransporter, K+ channels or KCC proteins, by excess potassium, by extracellular sodium or potassium removal or by varying membrane potential, suggesting the presence of a novel, electroneutral ammonia-methylammonia transport mechanism. Increasing the outwardly directed transmembrane H+ gradient increased transport activity by increasing Vmax. Finally, mIMCD-3 cells express mRNA and protein for the putative ammonia transporter, Rh B Glycoprotein (RhBG), and they exhibit basolateral RhBG immunoreactivity. We conclude that mIMCD-3 cells express a basolateral electroneutral NH4+/H+ exchange activity that may be mediated by RhBG.
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