The cardioprotective effects of R56865 were studied in isolated rabbit hearts, blood-perfused with a support rabbit system. The effect on ischemic injury was evaluated by comparing myocardial contracture and contents of ATP catabolites and of lactate during 60 min of normothermic ischemia in untreated hearts (group I) and in hearts treated with 0.63 mg/kg of R56865 starting 20 min before ischemia (group II; n = 5 in each group). R56865 delayed the onset, and decreased the extent of ischemic contracture, but had no effect on the myocardial content of ATP, of its catabolites of lactate. The effect on reperfusion injury was studied by monitoring left ventricular function during 80-min reperfusion after the 60-min ischemia in three groups (n = 6 in each): an untreated group (group I) and two groups treated with R56865 given either before (group II) or after ischemia (group III). Ultrastructural changes and cellular calcium distribution after reperfusion were also studied. R56865 improved the recovery of function and prevented contracture during reperfusion. Left ventricular end-diastolic pressure was 13.2 +/- 2.8 mmHg in group II and 31.3 +/- 8.1 mmHg in group III vs 45.0 +/- 2.6 mmHg in group I (P < 0.0001 for II vs I; P > 0.05 for III vs I). Left ventricular developed pressure, maximum dP/dt and minimum dP/dt recovered to 71.0 +/- 5.4%, 98.9 +/- 6.1%, 85.3 +/- 4.8% of baseline values, respectively, in group II, to 64.5 +/- 3.0% (P > 0.05), 76.8 +/- 3.0%, 70.2 +/- 4.0% in group III, vs 52.0 +/- 6.5%, 58.9 +/- 6.9% and 53.6 +/- 5.8% in untreated hearts (P < 0.05 for II or III vs I). Coronary flow was 24.5 +/- 2.2 ml/min and 19.8 +/- 1.8 ml/min in groups II and III vs 14.8 +/- 0.7 ml/min (P < 0.05) in the untreated group. On histology the myocardium in hearts treated either before or after ischemia was well protected and calcium distribution was almost normal after reperfusion, while in untreated hearts, most of the myocardium displayed irreversible damage accompanied by massive intracellular calcium accumulation. We conclude that R56865 could attenuate Ca(2+)-overload, thereby reducing myocardial ischemia-reperfusion injury after an extended period of ischemia.