Contribution of ouabain-insensitive currents to measurements of Ip. (A) Example of Ip illustrating a pronounced current decay during exposure to a 40K-EXT solution (left), as seen in some experiments. The record also demonstrates an initial inward tail current upon the return to 0K-EXT. The addition of 10 µM ouabain reveals a slowly activating inward current in response to the same [K⁺] changes as in A (right). //, elapsed time of ∼2 min. (B) Ouabain-insensitive currents elicited by step changes in extracellular K⁺ concentration from 0 to 40 mM in the absence of Ba²⁺, and (C) in the presence of 1 mM Ba²⁺. Notice the differences in current scales. (D) Maximum ouabain-insensitive currents in B and C plotted as a function of the K⁺ concentration. Stars and circles represent ouabain-insensitive current measured in the absence of Ba²⁺ (left axis; circles and dashed lines) or in the presence of 1 mM Ba²⁺ (right axis; stars and solid lines), respectively. Note different y-axes scales. The ouabain-insensitive currents are inward at all K⁺ concentrations above 4 mM. (E) A change to 0K-EXT solution, in which E_K is outward, elicits a slowly activating outward current both in the absence (4K- to 0K-EXT change) and presence (20K- to 0K-EXT change) of 10 µM ouabain. The holding potential was −90 mV, and the temperature was 22°C for all panels. Dashed lines in all panels indicate the zero-current level in 0K-EXT. Data in A–C and E were obtained from three different fibers.