Arrhythmogenic effect of forskolin in the isolated perfused rat heart: influence of nifedipine or reduction of external calcium [corrected].

1. This study investigated first the effects of forskolin on cardiac rhythm, and second the roles of calcium in cardiac arrhythmogenesis by cAMP. 2. Two series of experiments were performed. In the first series, forskolin was administered into the isolated perfused rat heart. In the second series, forskolin administration was preceded by administration of nifedipine, a calcium channel blocker, or infusion of a low concentration calcium solution. In both experiments, the myocardial cAMP level and electrocardiogram were determined. 3. It was found that forskolin increased cAMP level as well as inducing arrhythmia. Pretreatment with nifedipine or a reduction of external calcium, that either maintained or further enhanced the forskolin-induced increase in the cAMP level, abolished the forskolin-induced arrhythmia. 4. The results of the present study support the hypothesis that myocardial cAMP mediates cardiac arrhythmia, and provide evidence that calcium is essential in arrhythmia mediated by cAMP.