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Figure 2 Superimposed representative action potentials recorded from the right ventricular papillary muscle of rabbits in the control group (a) and beta group (b) at various pacing cycle lengths (PCLs). Retardation of repolarization around membrane potentials of –10 to –20 mV and prolongation of action potential duration (APD), especially at the lower PCL, were observed in 46% of the beta group. (c) Action potential durations measured at 90% repolarization (APD₉₀) were averaged and plotted against the PCL on a semi-log scale. In the control group, APD₉₀ shortened as a function of a shorter PCL (<1,000 ms), whereas at >2,000 ms, it shortened as a function of a longer PCL. The APD₉₀ of the beta group of rabbits was significantly longer at all PCLs, compared to the control group rabbits, and the degree of the prolongation was marked at longer PCLs. (d) The action potential of one of the beta group rabbits showed early afterdepolarization (*), which was more prominent at slower PCLs. C = control.