Ammonium transport by the colonic H1-K1-ATPase expressed in Xenopus oocytes

Cougnon, Marc, Patrice Bouyer, Frédéric Jaisser, Aleksander Edelman, and Gabrielle Planelles. Ammonium transport by the colonic H1-K1-ATPase expressed in Xenopus oocytes. Am. J. Physiol. 277 (Cell Physiol. 46): C280–C287, 1999.—Functional expression of the rat colonic H1-K1-ATPase was obtained by coexpressing its catalytic a-subunit and the b1-subunit of the Na1-K1-ATPase in Xenopus laevis oocytes. We observed that, in oocytes expressing the rat colonic H1-K1-ATPase but not in control oocytes (expressing b1 alone), NH4Cl induced a decrease in 86Rb uptake and the initial rate of intracellular acidification induced by extracellular NH4Cl was enhanced, consistent with NH4 1 influx via the colonic H1-K1-ATPase. In the absence of extracellular K1, only oocytes expressing the colonic H1-K1-ATPase were able to acidify an extracellular medium supplemented with NH4Cl. In the absence of extracellular K1 and in the presence of extracellular NH4 , intracellular Na1 activity in oocytes expressing the colonic H1-K1ATPase was lower than that in control oocytes. A kinetic analysis of 86Rb uptake suggests that NH4 1 acts as a competitive inhibitor of the pump. Taken together, these results are consistent with NH4 1 competition for K1 on the external site of the colonic H1-K1-ATPase and with NH4 1 transport mediated by this pump.