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Na+/K+-ATPase; voltage dependence; Xenopus laevis; oocyte; current-voltage relationship
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.
