Ammonium transport by the colonic H+-K+-ATPase expressed in Xenopus oocytes

Functional expression of the rat colonic H+-K+-ATPase was obtained by coexpressing its catalytic alpha-subunit and the beta(1)-subunit of the Na+-K+-ATPase in Xenopus laevis oocytes. We observed that, in oocytes expressing the rat colonic H+-K+-ATPase but not in control oocytes (expressing beta(1) alone), NH4Cl induced a decrease in Rb-86 uptake and the initial rate of intracellular acidification induced by extracellular NH4Cl was enhanced, consistent with NH4+ influx via the colonic H+-K+-ATPase. In the absence of extracellular K+, only oocytes expressing the colonic H+-K+-ATPase were able to acidify an extracellular medium supplemented with NH4Cl. In the absence of extracellular K+ and in the presence of extracellular NH4+, intracellular Na+ activity in oocytes expressing the colonic H+-K+-ATPase was lower than that in control oocytes. A kinetic analysis of Rb-86 uptake suggests that NH4+ acts as a competitive inhibitor of the pump. Taken together, these results are consistent with NH4+ competition for K+ on the external site of the colonic H+-K+-ATPase and with NH4+ transport mediated by this pump.