Effect of Altered Repolarization Course Induced by Antiarrhythmic Drugs and Constant Current Pulses on Duration of Premature Action Potentials in Canine Cardiac Purkinje Fibers

Abstract

The purpose of this study was to elucidate the mechanism of the upward shift of the electrical restitution curve, i.e., the lengthening of premature action potential duration (APDt) expressed as percentage of basic APD, induced by class I antiarrhythmic drugs in dog Purkinje fibers. In this study, six class I antiarrhythmic drugs lengthened APDt at a diastolic interval of 20 ms by 2.5–14.1%. The drugs also decreased the ratio of APD at 50% to APD at 90% of repolarization from 70.8 ± 1.8% (n = 60) to 47.4 – 60.8%. The relation between the decrease in the ratio of APD50 to APD90 of the basic AP and lengthening of the normalized APDt was linear (r = 0.92; p < 0.01). We attributed the lengthening of normalized APDt to the decreased ratio of APD50 to APD90, and applied repolarizing current pulses in short (≤2 mm) fibers to simulate the drug-induced decrease in the ratio of APD50 to APD90. The altered repolarization course of basic AP by the current pulse during late plateau and early phase 3 caused APDt lengthening. The relation between the current-induced decrease in the ratio of APD50 to APD90 of the basic AP and the lengthening of normalized APDt was linear (r = 0.91; p < 0.01). The slope of regression line describing this relation was similar to that in the presence of drugs. These results suggest that lengthening of the normalized APDt by class I antiarrhythmic drugs results from a more rapid repolarization during phase 2 of the preceding basic AP, possibly due to lesser influence of the delayed outward rectifying current. The lengthening of APDt by class I drugs may contribute to their antiarrhythmic action.