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Figure 2 (A) Dose-response curves are shown for isoproterenol (left panel) and norepinephrine (right panel) for APD₉₀ [an index for the magnitude of early afterdepolarization (EAD) formation] prolongation (EADs) at 1 Hz (closed circles) and for the first beat (post-pause) after a premature beat (open circles). (B) Isometric force is shown during 1.0-Hz pacing for control, norepinephrine, and norepinephrine + acetylcholine. (C) Action potentials are shown for the first post pacing beat (1.5-s pause) for control, acetylcholine, and acetylcholine + norepinephrine. Addition of norepinephrine resulted in a triggered extrasystole.