Figure 3. Voltage gated Ca²⁺ currents.

A, current responses to 25 ms voltage steps of 10 mV increments from a holding potential of −70 mV, in a whole cell voltage clamp recording applied at ∼1 Hz. The inset shows the voltage pulse protocol. The patch pipette contained ∼140 mm Cs⁺ and the bath contained 5.6 mm Cs⁺ and 20 mm TEA⁺ to block K⁺ currents and 0.3 μm tetrodotoxin to block Na⁺ currents. B, current–voltage relationship of the steady state currents obtained using the experimental setup shown in A (□, n = 5). Similar currents were recorded when extracellular Ca²⁺ was replaced by Ba²⁺ as the charge carrier (◊, n = 15). C, current responses to 500 ms depolarizing voltage steps of 10 mV increments from a holding potential of −70 mV, followed by a 200 ms depolarization to 0 mV (the voltage protocol is given in the inset and was applied at 0.033 Hz) to assess Ca²⁺- and/or voltage-dependent inactivation of Ca²⁺ currents. Solutions as in A; the response to −40 mV is shown in grey. D, the peak currents obtained in response to the 200 ms test pulse to 0 mV (as in C, n = 9) were normalized to the maximum peak current, and plotted against the holding potential applied during the conditioning pulse. The data were fitted with a Boltzmann equation (eqn (1): V_0.5 =−25 ± 3 mV, k = 8.5 ± 1.0 mV, A₂ = 0.5 ± 0.1). E, current responses to 200 ms voltage steps to −10 mV from a holding potential of −70 mV (voltage pulse protocol is shown in the inset) in the presence of Ba²⁺ as charge carrier and the additional presence of 5 μm nifedipine, 1 μmω-conotoxin GVIA or 5 mm Co²⁺ as indicated. F, mean steady state current before (unfilled bars), during (filled bars) and after (unfilled bars) addition of Ca²⁺-channel inhibitors nifedipine (5 μm), ω-conotoxin GVIA (1 μm) and CoCl₂ (5 mm). The number of cells is given below the bar. Error bars represent 1 s.e.m. and significance was tested comparing the current during application of the inhibitor with the mean current before and after the inhibitor addition using a paired t test. ***P < 0.001, *P < 0.05.