IK_V records in osmotically treated fibers and isradipine effects. (A) IK_V records elicited by 500-ms depolarizations from V_H to membrane potentials spanning from −30 to 90 mV every 20 mV. The inset shows the onset of the currents in an expanded timescale. (B) Effects of 1 µM isradipine on IK_V. Same fiber and protocol as in A. The onset of the currents is shown in an expanded timescale in the inset. (C) Peak IK_V (closed circles) and peak gK_V (open circles) determined from the data in A (black circles, control conditions) and B (red circles, isradipine). IK_V data points are connected by straight lines. The solid black lines are double Boltzmann fits to the control gK_V data with the following parameters: A = 3.2 mS/cm²; ε = 0.21; V1 = −10 mV; K1 = 6 mV; V2 = 31 mV; K2 = 19 mV. The red solid lines are double Boltzmann fits to the isradipine gK_V data with the following parameters: A =2.4 mS/cm²; ε =0.2; V1 = −2 mV; K1 = 10 mV; V2 = 31 mV; K2 = 19 mV. (D) Peak gK_V plots for a population of fibers maintained in control conditions (black circles; 12 experiments, 11 fibers) and a subpopulation of fibers (n = 4) exposed to 1 µM isradipine (red circles). Both datasets were fitted with double Boltzmann equations. Parameters for control data (black line) were (mean ± SD) A = 2.93 ± 0.5 µA/cm²; ε = 0.29 ± 0.06; V1 = −11.8 ± 3.8 mV; K1 = 5.8 ± 0.7 mV; V2 = 26.5 ± 6.2 mV; K2 = 16.7 ± 3 mV. For this dataset, the fit with a double Boltzmann is >8 × 10⁷ times better than with a single Boltzmann function. Parameters for isradipine data (red line) were A = 2.17 ± 0.7 mS/cm² (*); ε = 0.22 ± 0.07; V1 = −10.4 ± 2.5 mV; K1 = 7 ± 1 mV (*); V2 = 25.6 ± 4.3 mV; K2 = 16 ± 1.7 mV. The * indicates statistical significance with respect to the control (P < 0.05). The AIC test indicates that the fit to the isradipine data with a double Boltzmann is only 52-fold better than with a single Boltzmann function. Error bars represent SEM.