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Apical ammonia transport by the mouse inner medullary collecting duct cell (mIMCD-3)

¹Division of Nephrology, Hypertension, and Transplantation, University of Florida College of Medicine, and ²Renal Section, North Florida/South Georgia Veterans Health System, Gainesville, Florida; ³Department of Pathology and Laboratory Medicine, University of Pennsylvania, and ⁴American Red Cross, Philadelphia, Pennsylvania

Submitted 8 July 2004 ; accepted in final form 22 March 2005

The collecting duct is the primary site of urinary ammonia secretion; the current study determines whether apical ammonia transport in the mouse inner medullary collecting duct cell (mIMCD-3) occurs via nonionic diffusion or a transporter-mediated process and, if the latter, presents the characteristics of this apical ammonia transport. We used confluent cells on permeable support membranes and examined apical uptake of the ammonia analog [¹⁴C]methylammonia ([¹⁴C]MA). mIMCD-3 cells exhibited both diffusive and saturable, transporter-mediated, nondiffusive apical [¹⁴C]MA transport. Transporter-mediated [¹⁴C]MA uptake had a K_m of 7.0 ± 1.5 mM and was competitively inhibited by ammonia with a K_i 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, not inhibited by luminal K⁺, and 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 cell mIMCD-3 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.

Rh C glycoprotein

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