Basolateral ammonium transport by the mouse inner medullary collecting duct cell (mIMCD-3)

The renal collecting duct is the primary site for the ammonia secretion necessary for acid-base 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 cell (mIMCD-3). 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 analog [(14)C] methylammonia ([(14)C] MA). mIMCD-3 cell basolateral MA transport exhibited both diffusive and transporter-mediated components. Transporter-mediated uptake exhibited a K(m) for MA of 4.6+/-0.2 mM, exceeded diffusive uptake at MA concentrations below 7.0+/-1.8 mM, and was competitively inhibited by ammonia with a K(i) 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-MA transport mechanism. Increasing the outwardly directed transmembrane H(+) gradient increased transport activity by increasing V(max). 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 NH(4)(+)/H(+) exchange activity that may be mediated by RhBG.