The protective effect of a branched chain amino acid (BCAA) formulation on the ischemic myocardium was determined in four groups of passively perfused normothermic (37 C) isolated rat hearts perfused with Krebs-Henseleit (KH) or KH + BCAA. Time to ischemic contracture was significantly increased by the infusion of BCAA compared with KH (19.4 +/- 2.3 vs. 15.9 +/- 1.7 min). In a second set of experiments, rat hearts were perfused with KH or KH + BCAA solution followed by 2 min of glucose-free KH buffer. After 17 min of ischemia, both groups were reperfused for another 15 min. Percent of pressure recovery was significantly better with KH + BCAA (90.1 +/- 8.4) than with KH (56.4 +/- 40.0). Percent +dP/dt (time derivative of pressure at contraction) recovery was also significantly improved with KH + BCAA (96.5 +/- 26.9 vs. 58.1 +/- 44.0). Furthermore, arrhythmias were minimized by BCAA. There was no difference in change of coronary flow between these two groups. Thus, BCAAs appear to protect the heart against myocardial ischemic injury and to delay the time to ischemic contracture, particularly in the depleted ischemic heart. BCAAs enhance postischemic pressure recovery and improve postischemic systolic and diastolic myocardial function.