We studied the effects of the aminosteroid U-74389G (21-[4-(2, 6-di-1-pyrrolidinyl-4-pyrimidinyl)-1-piperazinyl]-pregna-1,4,9(11)- triene-3,20-dione(2)-2-butenenedionate), a putative inhibitor of lipid peroxidation, which protects the rat myocardium after ischemia and reperfusion. Pentobarbital-anesthetized (50 mg/kg) rats were subjected to 60 min of occlusion of the left main coronary artery followed by 60 min of reperfusion. Myocardial ischemia/reperfusion produced a large cardiac necrosis (81 +/- 8.6% of the area at risk and 65 +/- 14.8% of the total left ventricle), polymorphonuclear infiltration in the jeopardized tissue (myeloperoxidase activity = 4.2 +/- 2.1 U X 10(-3)/g tissue in the area at risk and 7.0 +/- 3.6 U X 10(-3)/g tissue in the necrotic area), hydroxyl radical (OH.) formation (0.55 +/- 0.16 nmol/ml), increased plasma malonylaldehyde (40.2 +/- 3.9 nmol/ml) and lactate dehydrogenase (431 +/- 30 mIU/ml) and caused a decrease in the survival rate. Treatment with U-74389G (15 and 30 mg/kg i.v.) at the onset of reperfusion caused a reduction of necrotic area expressed as a percentage of either the area at risk (76 +/-7.4% with 15 mg/kg and 69 +/- 13.5% with 30 mg/kg; P < .05) or the total left ventricle (53 +/- 13.6% with 15 mg/kg and 46 +/- 16.8% with 30 mg/kg; P < .05). Treatment U-74389G reduced the myeloperoxidase activity, evaluated as an index of neutrophil infiltration, both in the area at risk (2.7 +/- 1.1 and 2.2 +/- 1.7 U X 10(-3)/g tissue with the doses of 15 and 30 mg/kg, respectively; P < .05) and in the necrotic area (4.3 +/- 2.4 and 3.8 +/- 2.9 U X 10(-3)/g tissue with 15 and 30 mg/kg, respectively; P < .05); decreased OH. formation (measured indirectly by the administration of the trapping agent salicylic acid); and analyzing the hydroxylation product 2,5-dihydroxybenzoic acid during reperfusion (0.35 +/- 0.12 and 0.32 +/- 0.15 nmol/ml with the doses of 15 and 30 mg/kg, respectively; P < .005). Treatment inhibited lipid peroxidation by blunting plasma malonylaldehyde (26.7 +/- 3.1 and 20.8 +/- 3.3 with the doses of 15 and 30 mg/kg, respectively; P < .001), prevented cellular disruption by reducing the increase of plasma lactate dehydrogenase (288.6 +/- 28 and 201.3 +/- 16 mIU/ml with the doses of 15 and 30 mg/kg, respectively; P < .001). Finally, U-74389G enhanced the survival rate evaluated at the end of the experiment (from 40 to 87%). These outcomes suggest that the drug may have potential for cardioprotective use in acute myocardial infarction.