Charge translocation by the Na+/K+ pump under Na+/Na+ exchange conditions: intracellular Na+ dependence.
The effect of intracellular (i) and extracellular (o) Na+ on pre-steady-state transient current associated with Na+/Na+ exchange by the Na+/K+ pump was investigated in the vegetal pole of Xenopus oocytes. Current records in response to 40-ms voltage pulses from -180 to +100 mV in the absence of external Na+ were subtracted from current records obtained under Na+/Na+ exchange conditions. Na+-sensitive transient current and dihydroouabain-sensitive current were equivalent. The quantity of charge moved (Q) and the relaxation rate coefficient (ktot) of the slow component of the Nao+-sensitive transient current were measured for steps to various voltages (V). The data were analyzed using a four-state kinetic model describing the Na+ binding, occlusion, conformational change, and release steps of the transport cycle. The apparent valence of the Q vs. V relationship was near 1.0 for all experimental conditions. When extracellular Na+ was halved, the midpoint voltage of the charge distribution (Vq) shifted -25.3+/-0.4 mV, which can be accounted for by the presence of an extracellular ion-well having a dielectric distance delta=0.69+/-0.01. The effect of changes of Nai+ on Nao+-sensitive transient current was investigated. The midpoint voltage (Vq) of the charge distribution curve was not affected over the Nao+ concentration range 3.13-50 mM. As Nai+ was decreased, the amount of charge measured and its relaxation rate coefficient decreased with an apparent Km of 3.2+/-0.2 mM. The effects of lowering Nai+ on pre-steady-state transient current can be accounted for by decreasing the charge available to participate in the fast extracellular Na+ release steps, by a slowly equilibrating (phosphorylation/occlusion) step intervening between intracellular Na+ binding and extracellular Na+ release.
Holmgren
Miguel
M
National Institutes of Health, National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, USA.
Rakowski
Robert F
RF
D000818:Animals; D002478:Cells, Cultured; D003198:Computer Simulation; D007424:Intracellular Fluid / Q000378:metabolism*; D015640:Ion Channel Gating / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology*; D008954:Models, Biological; D009865:Oocytes / Q000201:enzymology*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D055672:Static Electricity; D014982:Xenopus laevis
D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase