Relief of the CPA block examined by the combination of fast solution exchange and voltage jump. Experiments in both the top and bottom panels were made in excised inside-out patches with 140 mM Cl⁻ in the pipette. The voltage of the patch was first clamped at −160 mV in 0 [Cl⁻]_i and 1 mM CPA. The voltage was then changed to V_c (indicated by the vertical line below the traces), followed by a rapid change of the intracellular solution at the time indicated by the vertical line shown on top. Starting from 40 ms after the fast solution exchange, the membrane voltage was stepped to −160 mV at various time points to monitor the CPA-dissociation process (see inset for the voltage protocol). Red dots are the instantaneous current upon the voltage jump to −160 mV. Green, dashed curves are the single-exponential fit of these red dots with the indicated time constant. Notice that the dissociation of CPA at V_c can also be directly observed from the current relaxation after the solution exchange. This current relaxation is more obvious in the bottom panel because V_c in this experiment (−80 mV) is away from the Cl⁻ reversal potential. The current relaxation course upon stepping the voltage to −160 mV, however, reflects the CPA-dissociation rate at −160 mV.