The intravenous electrophysiological effects of a new antifibrillatory agent, Org 7797, were studied in closed chest anaesthetized dogs. Effects of fast sodium and slow calcium-mediated action potentials were also examined in guinea-pig isolated papillary muscle. 2. The major effects of a known antifibrillatory dose of Org 7797 (0.5 mg kg-1) were a protracted slowing of AV nodal conduction (for at least 20 min) and prolongation of the AV nodal functional refractory period. Conduction in the atria and His-Purkinje system (reflected by the St-A and HV intervals) were not significantly modified whilst ventricular conduction (reflected by the QRS interval) and the ventricular functional refractory period were only transiently prolonged. No other electrophysiological changes were seen. 3. A higher dose of Org 7797 (1.5 mg kg-1) slowed conduction at all levels of the myocardium (as evidenced by increases in the St-A, AH, HV and QRS intervals), slightly shortened cardiac repolarization (as assessed from JTc) and decreased Wenckebach rate. Atrial refractory periods were increased whereas effects on ventricular refractory periods were modest. 4. Neither heart rate nor sinus node recovery time were modified by either dose of Org 7797. 5. Org 7797, at a concentration (20 microM) which reduced Vmax of fast sodium-mediated action potentials in isolated papillary muscle by 83%, did not modify Vmax of slow calcium-mediated action potentials. It prolonged duration of the latter but did not modify that of the former. However, the plateau phase of both the 'fast' and especially the 'slow' action potentials was prolonged. 6. It is concluded that the main electrophysiological effects of a known antifibrillatory dose of Org 7797 in dogs with normal cardiac function are seen at the level of the AV node, actions which are unlikely to be explained by calcium channel block. Higher doses display a class Ic profile. This preferential action on the AV node may contribute to the control of ventricular rate during atrial fibrillation in the absence of infra-nodal conduction disturbances.7. These results contrast with those previously obtained in infarcted dogs and might further suggest that myocardial infarction enhances the Class I action of Org 7797.
