Inward-directed current generated by the Na+,K+ pump in Na(+)- and K(+)-free medium.

In Na(+)- and K(+)-free solution, an inward-directed current can be detected in Xenopus oocytes, which is inhibited by cardiac glycosides and activated by ATP. Therefore, it is assumed to be generated by the Na+,K+ pump. At negative membrane potentials, the pump current increases with more negative potentials and with increasing [H+] in the external medium. This current is not observed when Mg2+ instead of Ba2+ is the only divalent cation present in the bath medium, and it does not depend on whether Na+ or K+ is present internally. At 5 to 10 mM Na+ externally, maximum pump-generated current is obtained while no current can be detected in presence of physiological [Na+]. It is suggested that in low-Na+ and K(+)-free medium the Na+,K+ pump molecule can either form a conductive pathway that is permeable to Ba2+ or protons or operate in its conventional transport mode accepting Ba2+ as a K+ congener. A reversed pump mode or an electrogenic uncoupled Na(+)-efflux mode is excluded.