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

• Published in: The Journal of membrane biology Vol. 121; no. 2; p. 177
• Main Authors: Rakowski, R F, Vasilets, L A, LaTona, J, Schwarz, W
• Format: Journal Article
• Language: English
• Published: United States 01.04.1991
• Subjects: Animals; Biological Transport, Active; Calcium - metabolism; Electrophysiology; Female; In Vitro Techniques; Ion Exchange; Kinetics; Myocardium - cytology; Myocardium - metabolism; Oocytes - drug effects; Oocytes - metabolism; Ouabain - analogs & derivatives; Ouabain - pharmacology; Potassium - metabolism; Sodium - metabolism; Xenopus laevis
• ISSN: 0022-2631
• DOI: 10.1007/BF01870531

To investigate the voltage dependence of the Na+/K+ pump, current-voltage relations were determined in prophase-arrested oocytes of Xenopus laevis. All solutions contained 5 mM Ba2+ and 20 mM 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 microM ouabain or dihydro-ouabain or by K+ removal, therefore, primarily represent current generated by the Na+/K+ pump. In Na(+)-free solution containing 5 mM K+, Na+/K+ pump current is relatively voltage independent over the potential range from -160 to +40 mV. If external [K+] is reduced below 0.5 mM, negative slopes are observed over this entire voltage range. Similar results are seen in Na(+)- and Ca(2+)-free solutions in the presence of 2 mM 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 90 mM Na+, 5 mM 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 1.3 mM, 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.