Characterization of an endogenous Na+/H+ antiporter in Xenopus laevis oocytes.

The amiloride-sensitive Na+/H+ antiporter in defolliculated oocytes of Xenopus laevis was characterized by measurements of 22Na+ influx and apparent H+ efflux. Uptake of 22Na+ was linear over a 90-min incubation period and was inhibited approximately 80% with 5 x 10(-4) mol l-1 amiloride. Amiloride-sensitive sodium uptake was reduced following collagenase treatment or oocyte aging. K0.5 for amiloride inhibition was 4.13 x 10(-6) +/- 1.33 x 10(-6)mol l-1 and the Km for Na+ was 4.25 x 10(-3) mol l-1. Hill analysis of the kinetic data for Na+ revealed an nH value of 1.14, indicating an absence of interacting binding sites for Na+. Parallel measurements of amiloride-sensitive Na+ uptake and H+ efflux indicated a Na+/H+ exchange ratio of 0.88:1. Our conclusion is that the Na+/H+ antiporter of Xenopus oocytes exhibits a nominal 1:1 Na+/H+ exchange stoichiometry and is similar in its properties to the antiporter of other vertebrate cells.