title: A negative slope in the current-voltage relationship of the Na+/K+ pump in Xenopus oocytes produced by reduction of external [K+].
abstract: 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.
authors.affiliation: Max-Planck-Institut für Biophysik, Frankfurt am Main, Federal Republic of Germany.
authors.forename: R F
authors.initials: RF
authors.lastname: Rakowski
authors.forename: L A
authors.initials: LA
authors.lastname: Vasilets
authors.forename: J
authors.initials: J
authors.lastname: LaTona
authors.forename: W
authors.initials: W
authors.lastname: Schwarz
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism; D004594:Electrophysiology; D005260:Female; D066298:In Vitro Techniques; D007474:Ion Exchange; D007700:Kinetics; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism*; D010042:Ouabain / Q000031:analogs & derivatives / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: C000977:dihydroouabain; D010042:Ouabain; D012964:Sodium; D011188:Potassium; D002118:Calcium