Figure 7.

Ca²⁺ currents in WT-MCCs and KO-MCCs during an action potential clamp recorded in current-clamp conditions from a spontaneously firing cell. K⁺ and Na⁺ currents were blocked by 135 mm TEA and 0.3 μm TTX in the bath solution. In a is illustrated the voltage-clamp command consisting of a train of three APs separated by different interpulse intervals. The cell was initially held at −70 mV and then clamped at the AP waveform (see Materials and Methods). In b are shown the overlapped Ca²⁺ currents recorded from a WT-MCC before, during, and after application of 3 μm nifedipine (nife). Right inset, Ca²⁺ current traces corresponding to the second AP (dashed rectangle) on a more expanded timescale. c, Same as in b except that the recording was from a nonfiring KO-MCC. The cell possessed a small prespike Ca²⁺ current that was potentiated by BayK-8644 (Bay K; n = 8). Notice the large prespike L-type current increase and how the mean amplitude at −45 mV with BayK-8644 increases to nearly the same size of WT-MCC control current (gray box in b). Left inset, Mean amplitudes of the prespike Ca²⁺ currents measured from WT-MCCs and KO-MCCs (n = 16) at the time when the interpulse potential reached −45 mV, before the second AP (arrow in a). Notice the strong current reduction induced by nifedipine in both WT-MCCs and KO-MCCs. **p < 0.01, ***p < 0.001 vs control using Student's paired t test.