Voltage-dependent stimulation of Na+/K(+)-pump current by external cations: selectivity of different K+ congeners.
Currents generated by the endogenous Na+/K+ pump in the oocytes of Xenopus laevis were determined under voltage-clamp as currents activated by different K+ congeners. The voltage dependence of the pump current reflects voltage-dependent steps in the reaction cycle. The decrease of K(+)-activated pump current at positive potentials has been attributed to voltage-dependent stimulation by the external K+ (Rakowski, Vasilets, LaTona and Schwarz (1991) J. Membr. Biol. 121, 177-187). In Na(+)-free solution, activation of the pump by external cations seems to be the dominating voltage-dependent and rate-determining step in the reaction cycle. Under these conditions, the voltage dependence of apparent Km values for pump activation can be analyzed. The dependence suggests voltage-dependent binding of extracellular cations assuming that an effective charge of about 0.4 of an elementary charge is moved in the electrical field during a step associated with the cation binding. The apparent Km values at 0 mV differ for various cations that stimulate pump activity. The values are in mM: 0.10 for Tl+, 0.63 for K+, 0.71 for Rb+, 9.3 for NH4+, and 12.9 for Cs+. The corresponding apparent affinities follow the same sequence as the cation permeability of the K(+)-selective delayed rectifier channel of nerve cells. The results are compatible with the interpretation that the cations have to pass an ion-selective access channel to reach their binding sites in the pump molecule.
Omay
H S
HS
Max-Planck-Institut für Biophysik, Frankfurt/Main, Germany.
Schwarz
W
W
D000818:Animals; D002412:Cations / Q000378:metabolism; D005260:Female; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009865:Oocytes / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000502:physiology*; D014982:Xenopus laevis
D002412:Cations; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium