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Abstract-In order to search into the underlying mechanisms of ECG changes sug gestive of ischemia observed in humans and in rabbits after administration of 5-fluorouracil (5-FU), experiments were performed in anesthetized open-chest guinea pigs. The substance produced similar ECG changes in this species as well, after a rather long latent period of around 3 hours after intravenous administration. The incidence of ECG abnormality in animals given 60 mg/kg was 7/7, while that in animals given 30 mg/kg was 4/9. With 10-20 mg/kg, ECG changes were not observed during an experimental period as long as 5 hours . Associated with these ECG changes, a depletion of the high-energy phosphate compounds of the ventricular myocardium was observed. Analysis of tricarboxylic acid cycle (TCA cycle) intermediates revealed an accumulation of citrate within the myocardium, suggesting a malfunction of TCA cycle resulting from an inhibition of aconitase by fluorocitrate, as a cause of depletion of the high-energy phosphates.

ABSTRACT-The antiarrhythmic effects of MS-551, which prolongs cardiac action potential duration without affecting the maximum upstroke velocity of the action potential, were assessed in three different canine ventricular arrhythmia models: 1) ventricular tachycardia (VT) induced by electrical stimuli 3-5 days after myocardial infarction, 2) spontaneous ventricular tachyarrhythmias 24 48 hr after two-stage coronary ligation and 3) ventricular tachyarrhythmias induced by digitalis. Intravenous administration of MS-551 (0.1-1 mg/kg) decreased the susceptibility in 10 dogs out of 13 to VT or ventricular fibrillation evoked by programmed electrical stimulation (PES) delivered to the ventricular septum 3 5 days after myocardial infarction. Oral administration of MS-551 (3 mg/kg) also decreased the susceptibility to VT evoked by PES in 7 out of 10 conscious postinfarction dogs. Concurrently, intravenous (0.1-1 mg/kg) or oral (3 mg/kg) administration of MS-551 produced increases in the ven tricular effective refractory periods (ERP) by 7 ± 1%-17 ± 3% or 13 ± 2%, respec tively. Similarly, d-sotalol (0.3-3 mg/kg, i.v. and 10 mg/kg, p.o.) decreased the sus ceptibility to VT with increased ERP. However, MS-551 (1 and 10 mg/kg, i.v.) failed to inhibit both canine two-stage coronary ligation arrhythmia and digitalis arrhythmia. These results suggest that MS-551 is a pure class III antiarrhythmic drug which may be effective in the treatment of life-threatening reentrant tachyarrhythmias, but not in automaticity arrhythmias.

A series of 1-methyl-7-(4-pyridyl)-5,6,7,8-tetrahydro-3(2H)-isoquinolinones and related compounds were synthesized and evaluated for positive inotropic activity. Most members of this series exerted a dose-dependent increase in myocardial contractility in the dog acute heart failure model, whereas they caused only slight changes in heart rate and blood pressure. Several derivatives, especially those with cyano, acetyl, and ethyl substituents at the 4-position, were more potent than milrinone, which was used as a reference. 4-Acetyl-1-methyl-7-(4-pyridyl)-5,6,7,8-tetrahydro-3(2H)-isoquinolinone (MS-857) is one of the most potent positive inotropic agents in this series.

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