Heterogeneity of NH+4 transport in mouse inner medullary collecting duct cells

S. M. Wall, H. N. Trinh and K. E. Woodward Division of Nephrology, University of Texas Medical School at Houston 77030, USA. Previous studies from our laboratory have demonstrated that NH+4 substitutes for K+ on the Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) in rat terminal inner medullary collecting duct cells (tIMCD). To examine other NH+4 transport pathways, a transgenic mouse cell line, mIMCD-3, was employed. 86Rb+ was used as a K+ congener to explore NH+4/Rb+ (K+) competition on the extracellular K+ binding site of the Na(+)-K(+)-2Cl- cotransporter and the Na(+)-K(+)-ATPase. Addition of K+ or NH+4 reduced both bumetanide- and ouabain-sensitive Rb+ uptake. This reduction in Rb+ uptake with NH+4 addition was not due to intracellular pH-mediated changes in transporter activity. K+ and NH+4 are competitive inhibitors on both transporters. On the Na(+)-K(+)-2Cl- cotransporter, the Michaelis constant (Km) for K+ was 4.6 +/- 0.5 mM with an inhibitory constant (Ki) for NH+4 of 2.8 mM. In contrast, on the Na(+)-K(+)-ATPase, the apparent affinity for K+ was greater than for NH+4. To test Na(+)-K(+)-2Cl- cotransport-mediated NH+4 flux, bumetanide-sensitive NH+4/Rb+ exchange was measured. Bumetanide-sensitive Rb+ efflux was greater with extracellular K+ or NH+4 present relative to efflux with extracellular N-methyl-D-glucamine. This demonstrates both K+/Rb+ and NH+4/Rb+ countertransport by the Na(+)-K(+)-2Cl- cotransporter. In conclusion, NH+4 is transported in a bumetanide-sensitive Na(+)-NH+4-Cl- mode, and both NH+4 and Rb+ (K+) are competitive inhibitors for the extracellular K+ binding site. However, the kinetics of Na(+)-K(+)-2Cl(-)-mediated NH+4 transport differ from other K+ transport-mediated NH+4 pathways, such as the Na(+)-K(+)-ATPase.