Recording and analysis of an experiment to determine the K_0.5 for K⁺_o. (A) Current at −50 mV of a Na⁺-loaded oocyte overexpressing the α1β1 pump, bathed in Na⁺_o solution. Substituting Na⁺_o with K⁺_o stimulated outward current in a [K⁺]-dependent manner. Application of 0.5 mM ouabain for 2 min blocked currents in subsequent K⁺ applications. Vertical deflections along the recording indicate pulses to voltages between −100 and +40 mV. Lowercase letters indicate currents used to obtain ouabain-sensitive currents in B. (B) I-V plot of ouabain-sensitive current (current before ouabain application − current after ouabain application in the same condition) measured during the last 5 ms of 100-ms-long voltage pulses. (C) [K⁺_o] dependence of ouabain-sensitive current at −100 and 0 mV fitted with Hill equations (line plots). (D) Voltage dependence of K_0.5 for K⁺_o.