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Figure 1 4-Aminopyridine (4-AP) prolonged the action potential duration (APD) in cardiomyocytes. Action potentials of the cardiomyocytes were recorded as described in the Methods section, using a conventional glass micropippete, and the cells were treated with 4-AP (0.1 and 1 mmol/l) for 5 min. (A) Representative tracings of the action potentials. These tracings were obtained from the same cells. (B) Effect of 4-AP on the APD₉₀. The experiments were repeated three times, and the data are presented as the mean value ± SE obtained from 15 different cells. 4-Aminopyridine increased the APD by 37% at 1 mmol/l. *p < 0.01 vs. control cells. NS = not significant.