[Indication of myocardial "memory". Gradual, persisting modulation of myocardial decrease in excitation following change in the activation sequence].

It is well established that regional differences in electrical repolarization are responsible for the electrocardiographic T-wave. Regional differences in ventricular repolarization traditionally have been attributed to variance of action potential duration (APD), believed to be an intrinsic property of individual cardiac cells (17, 21, 24). To evaluate whether the excitation sequence influences ventricular repolarization we studied the effect of an altered excitation sequence on the APD distribution in isolated rabbit hearts. We found that APD is not primarily a function of the cell's location in the ventricle but rather is modulated by the sequence with which the ventricles are electrically excited. This excitation sequence-dependent modulation of APD occurs with a very slow time-course, requiring hours to develop and hours to dissipate after resumption of the original excitation sequence. Our findings suggest a new, as of yet unrecognized form of intercellular communication by which the myocardial cells adjust their repolarization sequence to a change in excitation sequence.