Slow fluorescence return upon hyperpolarization correlates with deactivation of ionic current. (A) Kv1.2 A291C currents (top) and fluorescence (bottom) traces at −120, −60, 0, and 60 mV from a holding potential of −120 mV. The right panels show an enlarged view of the tail currents and off-fluorescence emissions. (B) Overlay of ionic tail current (black lines) and slow off-fluorescence quenching (gray) for Kv1.2 A291C at the three potentials labeled after a depolarization to 20 mV. Scale bars are as shown, left to right, for the corresponding holding potentials. (C) Deactivation and slow off-fluorescence time constants for Kv1.2 A291C at the three holding potentials shown in B as a function of prepulse potential (n = 3–5). Mean data are shown every 20 mV for clarity. (D) Representative current and fluorescence records from a dual pulse (P1–P2) protocol with varying interpulse recovery time. Data are shown for 100-ms pulses from −80 to 60 mV with interpulse intervals of 6.25, 25, 100, and 250 ms. For clarity, data are only shown up to the end of the second depolarizing pulse of intermediate records. The gray arrow shows the instantaneous level of ionic current at P1; the gray dashed line is an inverted fit of the slow component of off-fluorescence from P1. (E) Overlay of the slow off-fluorescence component in D with the normalized initial P2 current amplitudes (black diamonds). The black line is a single exponential fit to the ionic current data. Mean time constants for the fits to individual datasets and the off-fluorescence component are shown in the panel (n = 5).