Abstract. The effects of K + -channel blockers on the action potential duration of the myocardium were examined in isolated right ventricles from the 7 -10-day-old, 11 -13-day-old, and 14 -20-day-old embryo and 1 -7-day-old hatched chicks. E-4031 significantly prolonged action potential duration at all developmental stages examined; the prolongation was largest in the 11 -13-day-old embryo and was accompanied by early after-depolarizations. Chromanol 293B showed smaller prolongation at all stages examined. Terfenadine prolonged action potential duration in the 11 -13-day-old embryo, but not in other stages. Thus, the chick ventricular myocardium changes its repolarization properties during development.

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