Figure 10.

Time course and contribution of K⁺ and Ca²⁺ currents to the AP firing is markedly different in KO-MCCs. Test solutions and protocols were similar to those used for WT-MCCs (see Fig. 8). a, Overlapped current traces recorded in control conditions, during nifedipine application (3 μm; nife; red trace), and with 135 mm TEA (blue trace) in a KO-MCC. The prespike Ca²⁺ current was particularly small compared with the postspike current. Left inset, K⁺ and Ca²⁺ currents relative to the dashed rectangle on a more expanded timescale. Nifedipine had a marked action on the inward currents but had only minor blocking effects on the outward K⁺ current, suggesting a small I_KL component. Middle inset, Time course of I_Ktot, I_KCa, and I_Kv, showing the larger contribution of I_Kv. I_Ktot and I_KCa were obtained as described in Results. Right inset, Mean values of I_Ktot, I_KL, I_Knon-L, and I_Kv (n = 19). Notice the small contribution of I_KL compared with WT-MCCs. b, K⁺ and Ca²⁺ currents recorded from a KO-MCC in which the BK currents (I_KCa) were particularly small. Despite the near full block of the sizeable Cav1.2 current, nifedipine (nife; 3 μm) caused a paradoxical increase of the outward K⁺ current. As shown to the right, I_Kv was responsible for most of the outward K⁺ current.