Activation of electrogenic Rb+ transport of (Na,K)-ATPase by an electric field.
Previous study shows that human erythrocytes when exposed, in an isotonic suspension, to an electric field that generated 6-15 mV of transmembrane potential induced a Rb+ uptake that was sensitive to ouabain, a potent inhibitor of (Na,K)-ATPase ( Serpersu , E. H., and Tsong , T. Y. (1983) J. Membr . Biol. 74, 191-201). Here we present evidence that this uptake indeed involved the activity of (Na,K)-ATPase. Transport of Rb+, K+, and Na+ were carefully monitored during the voltage stimulation. It is shown that the electric field stimulated only the ouabain-sensitive influx of Rb+, and this uptake was against a chemical concentration gradient. The rate of the stimulated Rb+ uptake was measured under different intracellular Na+ and extracellular Rb+ concentrations. The Km for the stimulated Rb+ uptake was, respectively, 7 mM for the intracellular Na+ and 1.7 mM for the extracellular Rb+, consistent with the values for the red cell (Na,K)-ATPase. Yet, the voltage-sensitive Rb+ uptake did not depend on the intracellular ATP level. Neither did the voltage stimulation cause an elevation of ATP concentration in the red blood cells as was observed in mitochondrial and chloroplast ATP synthetase systems under higher electric field conditions. Since only Rb+ uptake was stimulated by the voltage, it follows then that the Na+ and the K+ pumping activities of the (Na,K)-ATPase could be decoupled, and the K+ pumping activity may derive from the electrogenic component of the enzyme action. In the present case, the applied electric field could polarize the membrane to provide membrane potential required for the electrogenic transport of Rb+. Data also show that vanadate at 180 microM completely inhibited the ATP-dependent Na+ and Rb+ pumping activities of the enzyme, but only reduced the voltage-stimulated Rb+ uptake to 50% level. This represents the first systematic study of the activation of a transport ATPase by an externally applied electric field.
D001693:Biological Transport, Active / Q000187:drug effects; D004558:Electric Stimulation; D004912:Erythrocytes / Q000187:drug effects / Q000502:physiology*; D006801:Humans; D007700:Kinetics; D008564:Membrane Potentials; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000097:blood; D012413:Rubidium / Q000097:blood*; D012964:Sodium / Q000097:blood; D000254:Sodium-Potassium-Exchanging ATPase / Q000097:blood*
D010042:Ouabain; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium; D011188:Potassium