We investigated the antiischemic and antiarrhythmic effects of
R 56865 in
pentobarbital-anesthetized, open-chest rabbits subjected to 10 min regional
myocardial ischemia and 20 min reperfusion, using two experimental protocols. In the first,
R 56865 (0.02-0.16 mg/kg) was administered as a bolus intravenous (i.v.) injection 5 min before
ligation of a branch of the left circumflex coronary artery (LCX); in the second, the
drug, at the highest dose studied (0.16 mg/kg), was injected by the same route during
ischemia, 5 min after coronary artery
ligation.
Ischemia-induced ST segment increase and reperfusion-induced ventricular arrhythmias were determined in lead II of the four-limb ECG. Mean carotid arterial pressure and heart rate (HR) were also measured. When given before
ischemia,
R 56865 dose-dependently prevented
ischemia-induced ST segment increase and reperfusion arrhythmias. The antiischemic and antiarrhythmic dose-response curves were superimposable, suggesting a common mechanism of action.
R 56865 (0.16 mg/kg) fully attenuated
ischemia-induced ST segment shift and ventricular arrhythmias on reperfusion. These protective effects were not associated with systemic
hypotension or
bradycardia. When high-dose
R 56865 (0.16 mg/kg) was given during
ischemia, ST segment shift and ventricular arrhythmias on reperfusion were not attenuated. The results strongly suggest that
R 56865 affords protection against the deleterious effects of moderate
ischemia by mechanisms not associated with an indirect reduction of cardiac work.
R 56865 may elicit cardioprotection directly in ischemic tissue.