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

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 [C-14] methylammonia ([C-14] MA). mIMCD-3 cells exhibited both diffusive and saturable, transporter-mediated, nondiffusive apical [C-14] MA transport. Transporter-mediated [C-14] MA uptake had a Km 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.