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Figure 3 (a) Effect of bisoprolol on action potential duration (APD) in the right ventricular papillary muscle of rabbits in the beta group. Representative action potentials at a pacing cycle length of 2,000 ms, obtained from beta group rabbits before and after administration of 1 µmol/l of bisoprolol, have been superimposed. In the inset, values of APD at 90% repolarization (APD₉₀), measured before and after administration of bisoprolol, were averaged. Bisoprolol did not affect the prolonged APD in the beta group. Representative E-4031 (100 nmol/l) effects on the action potentials in the control (b) and beta groups (c) are shown. Slight prolongation in APD was observed in the control group, but it was not early after depolarization. On the other hand, E-4031 induced significant APD prolongation in the beta group and finally caused pacing failure at 20 min after administration. C = control; P = pacing artifact.