This study was designed to determine the effect of sodium 6-(2-[1-(1H)-imidazolyl]methyl-4,5-dihydrobenzo[b] thiophene)carboxylate (RS-5186), a new thromboxane A2 (TXA2) synthetase inhibitor, on mitochondrial function and lysosomal integrity in ischemic myocardium. 17 anesthetized mongrel dogs were divided into 2 groups. In the control group (n = 11), the left anterior descending arteries (LAD) of the dogs were occluded for 2 h and physiological saline was infused until the end of the experiment. In the RS-5186 treated group (n = 6), 25 min prior to LAD occlusion, RS-5186, 10 mg/kg, was injected for 10 min. 2 h after occlusion, mitochondria were prepared from both ischemic and non-ischemic areas, which were confirmed by Evans' blue dye, and mitochondrial function (respiratory control index: RCI, and the rate of oxygen consumption in state III respiration: St.III O2) was measured polarographically with succinate as substrate. Fractionation of myocardial tissue from both ischemic and non-ischemic areas was also performed, and the activities of lysosomal enzymes (N-acetyl-beta-glucosaminidase: NAG, beta-glucuronidase: beta-gluc) of each fraction were measured. 2-h LAD occlusion induced a significant greater decrease in mitochondrial function from the ischemic area of the control group (RCI: 2.80 +/- 0.45, St.III O2: 133.5 +/- 35.6 natoms/mg protein/min) compared with those from the non-ischemic area (RCI: 4.49 +/- 0.46, St.III O2: 344.0 +/- 31.9).(ABSTRACT TRUNCATED AT 250 WORDS)