Figure 5. Forced Ca²⁺ influx under extreme conditions.

SR release was blocked and the Na⁺ current was activated repetitively in the presence of 1 μm isoproterenol. A, a Ca²⁺ influx signal could be elicited by a train of 50 depolarizing pulses at 33 Hz. The Ca²⁺ influx signal was eliminated by 5 mm Ni²⁺. B, an analogous experiment was performed in the absence of isoproterenol but with 100 μm cAMP in the pipette filling solution. Sodium currents were activated repetitively 100 times at 33 Hz.