Heterogeneity of nh4+ transport in mouse inner medullary collecting
duct cells.
Wall, Susan M., Hoang N. Trinh, and Karen E. Woodward.
Division of Nephrology, University of Texas Medical School at
Houston, Houston, Texas 77030
APStracts 2:0092F, 1995.
Previous studies from our laboratory have demonstrated that NH4+
substitutes for K+ on the Na+-K+-ATPase in rat terminal inner
medullary collecting duct cells (tIMCD). To examine other NH4+
transport pathways, a transgenic mouse cell line, mIMCD-3, was
employed. 86Rb+ was used as a K+ congener to explore NH4+/Rb+ (K+)
competition on the extracellular K+ binding site of the Na+-K+-2Cl-
cotransporter and the Na+-K+-ATPase. Addition of K+ or NH4+ reduced
both bumetanide- and ouabain-sensitive Rb+ uptake. This reduction in
Rb+ uptake with NH4+ addition was not due to pHi mediated changes in
transporter activity. K+ and NH4+ are competitive inhibitors on both
transporters. On the Na+-K+-2Cl- cotransporter the Km for K+ was 4.6
+ 0.5 mM with a Ki for NH4+ of 2.8 mM. In contrast, on the Na+-K+
-ATPase the apparent affinity for K+ was greater than for NH4+. To
test Na+-K+-2Cl- cotransport-mediated NH4+ flux, bumetanide-sensitive
NH4+/Rb+ exchange was measured. Bumetanide-sensitive Rb+ efflux was
greater with extracellular K+ or NH4+ present relative to efflux with
extracellular NMDG+. This demonstrates both K+/Rb+ and NH4+/Rb+
countertransport by the Na+-K+-2Cl- cotransporter. In conclusion,
NH4+ is transported in a bumetanide-sensitive Na+-NH4+-Cl- mode and
that both NH4+ and Rb+ (K+) are competitive inhibitors for the
extracellular K+ binding site. However, the kinetics of Na+-K+-2Cl-
mediated NH4+ transport differs from other K+ transport-mediated NH4+
pathways, such as the Na+-K+-ATPase.
Received 9 February 1995; accepted in final form 25 April 1995.
APS Manuscript Number F48-5.
Article publication pending Am. J. Physiol. (Renal Fluid Electrolyte
Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 8 June 1995.