Effects of nifedipine on triggered activity in 1-day-old myocardial infarction in dogs.

Triggered activity arising from a delayed afterdepolarization occurs in canine subendocardial Purkinje fibers 1 day after myocardial infarction (MI). Standard microelectrode techniques were used to study small preparations (20 to 48 mm2) in vitro. Nifedipine, 1 mg/liter, reversibly suppressed triggered activity by reducing maximum diastolic potential, action potential amplitude and the rate of depolarization of the delayed afterdepolarization. Complete quiescence or exit block resulted. The effects of nifedipine were antagonized by elevating extracellular calcium ion concentration. These results suggest that spontaneous ectopic rhythms 1 day after MI that are the result of triggered activity are dependent on transmembrane calcium ion movement, which nifedipine can directly antagonize.