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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 collecting duct is the primary site of urinary ammonia secretion; the current study determines whether apical ammonia transport in the mouse collecting duct, mIMCD-3, cell occurs via non-ionic diffusion or a transporter-mediated process, and, if the latter, the characteristics of this apical ammonia transport. We used confluent cells on permeable support membranes and examined apical uptake of the ammonia analogue, 14C-methylammonia ([14C]MA). mIMCD-3 cells exhibited both diffusive and saturable, transporter-mediated, non-diffusive apical [14C]MA transport. Transporter-mediated [14C]MA uptake had a Km of 7.0 ± 1.5 mM and was competitively inhibited by ammonia with a Ki of 4.3 ± 2.0 mM. Transport activity was stimulated by both intracellular acidification and extracellular alkalinization, and it was unaltered by changes in membrane voltage, thereby functionally identifying an apical, electroneutral NH4 +/H+ exchange activity. Transport was bidirectional, consistent with a role in ammonia secretion. In addition, transport was not altered by Na+ or K+ removal, was not inhibited by luminal K+ and was not mediated by apical H+-K+-ATPase, Na+-K+-ATPase or Na+/H+ exchange. Finally, mIMCD-3 cells express the recently identified ammonia transporter family member, Rh C Glycoprotein (RhCG), at its apical membrane. These studies indicate that the renal collecting duct, mIMCD-3 cell, has a novel, apical electroneutral Na+- and K+-independent NH4 +/H+ exchange activity, possibly mediated by RhCG, that is likely to mediate important components of collecting duct ammonia secretion.
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