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Figure 5 Left panels. Increase in APD90 induced by E-4031 10–7 M (open symbols) and 10–6 M (solid symbols) as a function of PCL in (A) endomyocardial biopsies of unaffected (n = 6; circle) and affected dogs (n = 7; triangle) and (B) Purkinje fibers of unaffected (n = 9; circle) and affected dogs (n = 5; triangle). At PCL = 4,000 ms, the number of experimental data decreased to 4 under E-4031 10–7 and 10–6 M in the unaffected group and to 4 under E-4031 10–7 M and 2 under 10–6 M in the affected group because of the occurrence of early afterdepolarizations or triggered activity. Right panels. Typical action potentials recorded in (C) endomyocardial biopsies and (D) Purkinje fibers of affected dogs (top) and unaffected dogs (bottom) under control conditions (C) and in the presence of E-4031 10–7 M (10–7 for Purkinje fibers only) and 10–6 M (10–6) at PCL = 1,000 ms. Vertical bar, 50 mV; horizontal bars, 100 ms in (C) and 200 ms in (D). APD = action potential duration; PCL = pacing cycle length.