Measurement of Na,K-ATPase pump current in isolated mouse FDB fibers. Na,K-ATPase pump current is identified as the outward current activated by extracellular K⁺ and inhibited by ouabain. (A) Outward currents are elicited upon changing the extracellular K⁺ concentration from 0 to 20 mM. A return to 0K-EXT elicits a decrease in current, as expected if the Na,K-ATPase is stopped as a result of removal of the transported ion. Complete exchange of solutions was achieved in 2–5 s for this and subsequent figures. //, elapsed time of ∼2 min. (B) The addition of 10 µM ouabain after a subsequent change to 20K-EXT completely inhibits Ip. (C) The Na,K-ATPase contributes outward current at resting extracellular K⁺. A change from 4K- to 0K-EXT elicits a decrease in current. A change to 20K-EXT elicits an outward current that disappears upon return to 0K-EXT. The application of ouabain after Ip is stopped in 0K-EXT elicits a smaller, slowly activating outward current that is attributed to a residual ouabain-insensitive potassium current. The holding potential was −90 mV, and the temperature was 22°C for all panels. The dashed lines represent the zero-level current in the presence of 0K-EXT.