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Figure 4 (A) Superimposed action potentials are shown for the first post-pause beat in normal Tyrode solution (NTyr), immediately after the resumption of normal Tyrode solution after superfusion for 15 min with 30 mM LiCl substituting for 30 mM NaCl, and immediately after a transient increase in [Ca2+]o from 1.35 to 5 mM. (B) Superimposed action potentials and force development are shown for 32°C to 38°C. With an increase in isometric force magnitude and duration, terminal repolarization is prolonged. (C) Action potentials are shown under control conditions and after norepinephrine, both before and after ryanodine. Recordings are shown both during pacing at 1 Hz (left panel) and for the first paced beats observed 1 s after a 10-beat pacing train (5 Hz) (right panel).