Isolation of the Ca²⁺-activated K⁺ currents in the rod bipolar cells. (A-B) Selection of the rod bipolar cell in the retinal slice. The cell body of the rod bipolar cell was placed at the top of the inner nuclear layer (asterisk), and its axon terminal was detected in the innermost layer of the inner plexiform layer (circle) through the intracellular diffusion of the Alexa 488 dye. (C-E) Effects of the BAPTA on the sustained components. Currents were recorded by step-voltage stimulation from −70 mV to 30 mV. In the control, the currents were minimally changed 5 min after initial stimulation (C), while a definite change of the sustained components was recorded by intracellular perfusion of the BAPTA (D). The differences of the fast components and sustained components were normalized by initial amplitudes and presented by a bar-graph (E). Only sustained components were significantly reduced by BAPTA (n = 5, *P < 0.05, Student’s t-test). (F-I) Current traces induced by step-voltage protocol, from −70 mV-holding potential to −50 ∼ 30 mV-target voltage by 10 mV-intervals for 2000 ms, in the control (F), mibefradil (G), nifedipine (H), and mibefradil + nifedipine (I) treated rod bipolar cells. The concentration of mibefradil (T-type VGCC inhibitor) and nifedipine (L-type VGCC inhibitor) was 15 μm, and both were pre-applied for 3 min before recording. J-K. Current-voltage (I-V) relationship of K⁺ current with the VGCC inhibitors. Fast components were measured at the peak amplitudes at 200 ms from the initiation of the step-voltage, and sustained components were measured at 25 ms before the end of the stimulation. In the I-V curve of the fast components (J), outward currents were similarly reduced by VGCC inhibitors (n = 7, P > 0.05, ANOVA). In the I-V curve of the sustained components (K), outward currents were strongly reduced by mibefradil or mibefradil + nifedipine than by nifedipine (n = 7, P < 0.05 at 30 mV, ANOVA).