Na+ transport by the (Na+)-stimulated adenosine triphosphatase

We have previously shown that the (Na+ and K+)-stimulated adenosine triphosphatase from canine kidney reconstituted into phospholipid vesicles is capable of K+-independent transport of Na+ against a concentration gradient (Forgac, M., and Chin, G. (1981) J. Biol. Chem. 256, 3645-3646). The Na+ dependence of the stoichiometry of this process has been investigated and suggests that Na+ ions can be bound and transported by the sites normally occupied by K+. Measurement of the membrane potential generated during active Na+ uptake by [3H]triphenylmethylphosphonium ion distribution gave a value of 50 mV (positive inside). The electrogenicity of Na+ transport is sufficient to explain the observed uptake of an approximately equivalent amount of Cl-. We have also measured Na+ transport into reconstituted vesicles containing neither Na+ nor K+. Na+ uptake was half-maximal at an external Na+ concentration of 1 mM and was accompanied by a less than equivalent amount of Cl- uptake. In order to determine whether protons might also be moving in this system, the intravesicular pH was measured with the pH-sensitive fluorescent probe 1-hydroxypyrene-3,6,8-trisulfonic acid. Active Na+ uptake was accompanied by proton efflux, the amount of proton movement depending on the counterion present.