AJP - Renal Physiology, Vol 267, Issue 4 660-F670, Copyright © 1994 by American Physiological Society

ARTICLES

NH+4 transport mediated by Na(+)-K(+)-ATPase in rat inner medullary collecting duct

S. M. Wall and L. M. Koger
Division of Nephrology, University of Texas Medical School at Houston 77030.

The interaction of K+ and NH+4 on the basolateral membrane of cultured rat inner medullary collecting duct cells (IMCD) was explored using 86Rb+ as a K+ congener. Ouabain addition decreased Rb+ uptake to 22.0 +/- 2.6% of control. Possible ouabain-sensitive NH+4 transport was therefore explored. Replacement of N-methyl-D-glucamine chloride with NH4Cl or KCl decreased both total and ouabain-sensitive Rb+ uptake. This inhibition of Rb+ uptake by NH+4 was not due to changes in intracellular pH or differences in cell viability. We conclude that NH+4 competes with K+ for binding on the Na(+)-K(+)-adenosinetriphosphatase (ATPase). An inhibitory constant (Ki) for NH+4 of 11.0 +/- 1.6 and a Michaelis constant (Km) for K+ of 1.9 +/- 0.7 mM were measured. To extend these observations, NH+4 was tested as a substrate for ouabain-sensitive ATPase activity in native permeabilized IMCD cells. With 20 mM K+, activity was 202 +/- 73 nmol ATP hydrolyzed.min-1.mg protein-1, and with 20 mM NH+4, activity was 259 +/- 81 nmol ATP hydrolyzed.min-1.mg protein-1. Thus NH+4 substitution for K+ on the Na(+)-K(+)-ATPase is a likely mechanism for NH+4 uptake by the IMCD.

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