Recovery of SK channels in the absence of Ca²⁺. (A) Time course of the average current level at −80 mV with a patch from an oocyte coexpressing SK channels with CaM (E1Q). Solid line is a single-exponential fit with a τ = 39.1 min. (B) After patch was excised from an oocyte expressing SK channel and CaM (E1Q), current ran down to 60 pA before the patch was subjected to 20 µM CaM (WT) in Ca²⁺-free solution for 60 min. Then, recording pipette was quickly moved into a laminar flow of solution containing 7.7 µM Ca²⁺ to measure the time course of activation. Current was continuously recorded at −80 mV during the movement and shown in the top panel. Later in the experiment, pipette was moved back to Ca²⁺-free solution and then to 7.7 µM Ca²⁺ again (middle). Solid lines are fits of data with a single-exponential time course (top, τ = 46 ms; middle, τ = 38 ms). The bottom panel compares the time constants for the first activation after recovery to the average values for later activations. Results were from six similar experiments. (C) Similar experiment as in B, except that CaM (WT) was replaced by 20 µM CaM (E1Q) plus 100 µM NS309, which were also present during the measurement of the activation time course. Before the application of Ca²⁺-free solution containing CaM (E1Q) and NS309, there was 18 pA of residual current after rundown. Solid lines are fits of data with a single-exponential time course (top, τ = 70 ms; middle, τ = 66 ms). Data from five similar experiments are shown in the bottom panel as in B.