Cardiac electrical restitution properties in response to dynamic stimulation. A) Action potential duration restitution curves at APD30, APD50, and APD70 in neonatal (n = 8), younger adult (n = 6–7), and older adult hearts (n = 18–21). B) Calcium transient duration restitution curves at CaD30, CaD50, and CaD70 in neonatal (n = 9), younger adult (n = 10), and older adult hearts (n = 12). C) Maximum restitution slopes; individual replicates shown. Ventricles were dynamically paced (S1–S1) at the apex; the PCL was decremented from 220 to 120 ms at 20 ms intervals. D) APD50 (top), CaD50 (middle), and heart rate (bottom) measured during NSR in the excised hearts. Values reported as mean ± SEM. Comparisons by two-way ANOVA with multiple comparisons; *P < 0.01, **P < 0.01, ****P < 0.001 compared with neonate (red), young adult (blue), or old adult (green). NSR, normal sinus rhythm; PCL, pacing cycle length.