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Figure 2 4-Aminopyridine (4-AP) enhances calcium transient [Ca²⁺]_i in cardiomyocytes. Cardiomyocytes were pretreated with Fluo-4 and treated with 4-AP for the indicated times. [Ca²⁺]_i was recorded as described in Methods. (A) Representative tracings of [Ca²⁺]_i. The tracings were recorded from the same cells. The cells were treated with 1 mmol/l of 4-AP for 10 min. (B) The time course of [Ca²⁺]_i. One mmol/l of 4-AP increased [Ca²⁺]_i by 52% at 10 min. The experiments were repeated three times, and the data are presented as the mean value ± SE obtained from 15 different cells. *p < 0.01 and **p < 0.05 vs. control cells.