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A negative slope in the current-voltage relationship of the Na+/K+ pump inXenopus oocytes produced by reduction of external [K+]

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Vasilets,  Larisa A.
Department of Cell Physiology, Max Planck Institute of Biophysics, Max Planck Society;
Institute of Chemical Physics, Chernogolovka 143 432, Moscow Region, U.S.S.R.;

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Schwarz,  Wolfgang
Department of Cell Physiology, Max Planck Institute of Biophysics, Max Planck Society;

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Citation

Rakowski, R., Vasilets, L. A., LaTona, J., & Schwarz, W. (1991). A negative slope in the current-voltage relationship of the Na+/K+ pump inXenopus oocytes produced by reduction of external [K+]. Journal of Membrane Biology, 121(2), 177-187. doi:10.1007/BF01870531.


Cite as: https://hdl.handle.net/21.11116/0000-0008-6DFE-1
Abstract
To investigate the voltage dependence of the Na/K pump, current-voltage relations were determined in prophasearrested oocytes ofXenopus laevis. All solutions contained 5mm Ba2− and 20mm tetraethylammonium (TEA) to block K channels. If. in addition, the Na+/K+ pump is blocked by ouabain, K+-sensitive currents no larger than 50 nA/cm2 remain. Reductions in steady-state current (on the order of 700 nA/cm2) produced by 50 μm ouabain or dihydro-ouabain or by K+ removal, therefore, primarily represent current generated by the Na/K pump. In Na-free solution containing 5mm K+, Na+/K+ pump current is relatively voltage independent over the potential range from −160 to +40 mV. If external [K+] is reduced below 0.5mm, negative slopes are observed over this entire voltage range. Similar results are seen in Na+- and Ca2+-free solutions in the presence of 2mm Ni2+, an experimental condition designed to prevent Na+/Ca2+ exchange. The occurrence of a negative slope can be explained by the voltage dependence of the apparent affinity for activation of the Na+/K+ pump by external K+, consistent with the existence of an external ion well for K binding. In 90mm Na+, 5mm K+ solution, Na+/K+ pump current-voltage curves at negative membrane potentials have a positive slope and can be described by a monotonically increasing sigmoidal function. At an extracellular [K+] of 1.3mm, a negative slope was observed at positive potentials. These findings suggest that in addition to a voltage-dependent step associated with Na+ translocation, a second voltage-dependent step that is dependent on external [K+], possibly external K+ binding, participates in the overall reaction mechanism of the Na+/K+ pump.