Functional expression of the rat colonic H+-K+-ATPase was obtained by coexp
ressing its catalytic alpha-subunit and the beta(1)-subunit of the Na+-K+-A
TPase in Xenopus laevis oocytes. We observed that, in oocytes expressing th
e rat colonic H+-K+-ATPase but not in control oocytes (expressing beta(1) a
lone), NH4Cl induced a decrease in Rb-86 uptake and the initial rate of int
racellular acidification induced by extracellular NH4Cl was enhanced, consi
stent with NH4+ influx via the colonic H+-K+-ATPase. In the absence of extr
acellular 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 N
H4+ acts as a competitive inhibitor of the pump. Taken together, these resu
lts 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.