In order to compare and clarify the effects of various antiarrhythmic drugs when given as monotherapy, we reevaluated our previous data on antiarrhythmic drugs and recalculated antiarrhythmic plasma concentrations of drugs for several canine arrhythmia models. We used three spontaneously occurring arrhythmias: a) digitalis-, b) two-stage coronary ligation-, and c) adrenaline-induced arrhythmias. All antiarrhythmic drugs of class I suppressed digitalis arrhythmia, and, except for lidocaine, also suppressed coronary ligation arrhythmia. Class II antiarrhythmic drugs, beta blockers, and class IV antiarrhythmic drugs, Ca antagonists, had common features of effectiveness and suppressed adrenaline arrhythmia in relatively low concentrations. Class III drugs were not effective on these three arrhythmias. Differences among the antiarrhythmic effects of class I drugs could not be explained by their subclassification based either on action potential duration or kinetic properties of dissociation or association with Na channels. New triggered arrhythmia models in vivo and in vitro canine hearts were developed, and drug effects were not the same as those on the three spontaneously occurring arrhythmia models.

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Antiarrhythmic and electrophysiological effects of SD-3212, a novel antiarrhythmic agent, which has both Na+ channel and Ca++ channel blocking activities, were compared with those of its (+)-stereoisomer, SD-3211, which has only a Ca++ channel blocking activity, and bepridil, a known Ca++ channel blocker with additional Na+ channel blocking activity, using the two-stage coronary ligation induced arrhythmia (24 h after the ligation of the left anterior descending coronary artery) and 7 day-old myocardial infarcted hearts in anaesthetized dogs. SD-3212 showed a dose-dependent antiarrhythmic effect on the two-stage coronary ligation induced arrhythmia. SD-3212 at a dose of 3 mg/kg reduced the arrhythmic ratio, i.e. ectopic beats per min divided by the sum of ectopic beats and sinus beats per min, significantly from 1 up to 12 min after the administration. Neither bepridil (1-6 mg/kg) nor SD-3211 (1 mg/kg) had an antiarrhythmic effect. SD-3212 (0.3-3 mg/kg) prolonged both the conduction time in the normal myocardium and the delayed potential in the infarcted myocardium in the 7 day-old myocardial infarcted hearts in anaesthetized dogs in a dose-dependent manner. This effect of SD-3212 was shown at coupling intervals of 150-1000 ms increasing with decreasing interval. In this respect, SD-3212 is similar to drugs which show fast recovery of Vmax from use-dependent block such as lidocaine. Bepidril (1-6 mg/kg) also prolonged these parameters in a dose-dependent manner, however, the prolongation induced by bedripil was limited to shorter coupling intervals as compared with that induced by SD-3212. SD-3212 (0.1-1 mg/kg) did not show this prolonging effect.(ABSTRACT TRUNCATED AT 250 WORDS)

, 4-benzothiazine hydrogen fumarate, were investigated using canine models of ventricular arrhythmias, i.e. spontaneously occurring digitalis-, two-stage coronary ligation-and adrenaline-induced arrhythmias. SD-3212 suppressed adrenaline-induced arrhythmia and showed some antiarrhythmic effect on digitalis-and 48hr coronary ligation-arrhythmias. These results indicate that SD-3212 has antiarrhythmic effects common among class IV antiarrhythmic drugs and also has additional efficacy common among class I antiarrhythmic drugs, thus when considering the level of experimental arrhythmias it somewhat resembles propafenone. It may therefore become a clinically useful antiarrhythmic drug among typical class I or class IV antiarrhythmic drugs.

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