Ammonium as a substrate for Na+-K+-ATPase in rabbit proximal tubules

The role of the ammonium ion (NH+4) as a substrate for Na+-K+-ATPase was determined in intact rabbit proximal tubules. Since ouabain-sensitive oxygen consumption and Na+-K+-ATPase transport activity are tightly coupled in the proximal tubule with a stoichiometry of 12 K+ pumped/oxygen consumed, we used the ouabain-sensitive oxygen consumption of rabbit proximal tubule suspensions as an assay of Na+-K+-ATPase pump activity. The addition of NH+4 to K+-depleted tubules in nominally K+-free media resulted in a dose-dependent increase in oxygen consumption with an apparent affinity (Km) of 0.4 mM NH+4. Oxygen consumption was increased by 39.3 +/- 3% over control (n = 7) by 5 mM NH4Cl. This stimulation was completely inhibited by the addition of 5 X 10(-4) M ouabain. Under the same conditions, the addition of 5 mM KCl stimulated oxygen consumption by 52.4 +/- 2.9% (n = 7) with a Km of 0.5 mM. This stimulation was also completely inhibited by ouabain. Ouabain was also found to decrease the initial rate of NH+4 uptake into the proximal tubule cells. K+ and NH+4 competed with each other for active uptake into tubule cells. These results demonstrate that NH+4 can substitute for K+ on the Na+-K+-ATPase of the rabbit proximal tubule cell. Based on these data we have developed a kinetic model that predicts that the competition between NH+4 and K+ for transport on the Na+-K+-ATPase is not significant in the cortical labyrinth but potentially very significant in the inner medulla.