Figure 3. Nimodipine blocks the calcium current in a voltage-dependent manner.

A, calcium currents elicited by depolarizing the cell to −14 mV from holding potentials of −84 mV or −64 mV in the presence and absence of 5 μm nimodipine for a high-frequency cell. No difference in the control response is seen, but nimodipine block was greater at a −64 mV holding potential. B, I–V curves generated from cells using holding potentials of −84 or −64 mV in the absence (−84 mV, □; −64 mV, ○) and presence (−84 mV, ▪; −64 mV, •) of 5 μm nimodipine. C and D, normalized I–V curves for data obtained in the absence (□) and presence (▵) of 5 μm nimodipine and the difference (nimodipine-sensitive, •), from a holding potential of −84 mV (C) and −64 mV (D). There was a slight shift in the V_1/2 from −40 ± 0.3 to −37 ± 0.3 mV for the difference plot at −84 mV, but no differences in slope (5.2 ± 0.3 mV⁻¹). No difference in I–V plots was observed for the holding potential of −64 mV, where the V_1/2 was −39 ± 1 mV and the slope 4.2 ± 0.8 mV⁻¹. E, dose-response curves with their corresponding fits to the Hill equation for holding potentials of −84 mV (□) and −64 mV (▵). The number of cells tested is given in parenthesis as (−84 mV, −64 mV, respectively). Values for B_max, the half-blocking dose and the Hill coefficient were 69 ± 4 %, 2.2 ± 0.2 μm and 2.1 ± 0.4, respectively, for a holding potential of −84 mV and 90 ± 4 %, 1.8 ± 0.2 μm and 1.7 ± 0.2, respectively, for the holding potential of −64 mV (r² values of 0.989 and 0.99, respectively, were obtained for the two conditions). F, plots normalized to the maximal blocking dose for holding potentials of −84 mV (□) and −64 mV (▵) with the corresponding fit to the Hill equation. Here, values of 1.0, 2.0 ± 0.1 μm and 1.9 ± 0.2 were obtained for B_max, half-blocking dose and the Hill coefficient, respectively (r²= 0.99). G, the voltage dependence of nimodipine block was investigated further by plotting the ratio of current obtained in the presence of 5 μm nimodipine to control current at different test potentials against the test potential. A single exponential (Y = Y₀+Aexp(−x/δx)) best fitted this plot with values of 0.3 ± 0.01, 0.003 ± 0.001 and 8.8 ± 1 mV for Y₀, A and δx, respectively (r²= 0.98). H–J, use dependence of nimodipine block was investigated by depolarizing a cell to −14 mV for 20 ms at a frequency of 1 Hz. Examples of the first and last response from a holding potential of −84 mV (H) or −64 mV (J) are given with the thicker lines representing the first and the thin lines the last. Time zero represents the start of the 1 Hz stimulus protocol that begins 10 min after drug application. Traces at the top are in the absence and the bottom in the presence of 5 μm nimodipine (Nim.). I, plot of the peak current against time where the symbols correspond to their respective condition (H, J). The continuous lines reflect the initial value. From a holding potential of −84 mV a further decrease of 9 % was observed while from a holding potential of −64 mV a further reduction of 32 % was observed for this cell.