Class III antiarrhythmic drugs share the common mechanism of widening the cardiac action potential without affecting conduction velocity. This thesis reports on the actions of newly developed putative Class III antiarrhythmic drugs, tedisamil, KC 8851, RP 62719, UK 68798, and risotilide, as well as an ATP-sensitive K⁺ channel blocker, glibenclamide. Studies were performed to examine the actions of these drugs in acute myocardial ischaemia and possible mechanisms responsible for these actions. The hypothesis tested was that drug treatment prevented arrhythmias induced by acute myocardial ischaemia. Species dependent actions of these drugs on ECG and blood pressure were examined in rats, guinea pigs, pigs and primates. The five putative class III drugs listed above were assessed for antiarrhythmic activity in a conscious rat model of myocardial ischaemia. It was found that only tedisamil and KC 8851, which widened the Q-T[formula omitted] interval of the ECG (by up to 65%) , were effective at suppressing fibrillation in this species. None of the drug treatments decreased the incidence of ventricular premature beats. Tedisamil, but not glibenclamide, prevented tachycardias in a rat model of myocardial ischaemia- and reperfusion-induced arrhythmias. In an anaesthetized pig model of acute myocardial ischaemia, tedisamil and UK 68,798 were shown to mildly prolong the Q-T[formula omitted] interval by less than 20%, but protection against arrhythmias was equivocal. In further studies, tedisamil and UK 68,798 were compared to each other for effects on ventricular epicardial action potential morphology using intracellular recording in vivo, and effects on ventricular effective refractory period using electrical stimulation in vivo in both rats and guinea pigs. Tedisamil (4 mg/kg, i.v.) prolonged rat ventricular epicardial action potential duration fourfold in vivo, while UK68,798 (up to 1 mg/kg, i.v.) was ineffective in this species. Tedisamil (4 mg/kg, i.v.) widened guinea pig ventricular epicardial potentials by 80%, while UK 68,798 (25 μg/kg, i.v.) increased these by 30%. Action potential widening paralleled increases in ventricular refractoriness to electrical induction of premature beats. It was found that the species selective actions of these drugs was most likely related to differences in selectivity for K⁺ channels which contribute to repolarization in myocardium. / Medicine, Faculty of / Anesthesiology, Pharmacology and Therapeutics, Department of / Graduate