Fig. 4. Infarct size in  in vitro rat hearts. Rats were subjected to 30 min of regional ischemia followed by 2 h of reperfusion. Postconditioning (Post-C) and morphine (Mor, 1 μm) reduced infarct size. The infarct-sparing effect of morphine was reversed by both  N -nitro-l-arginine methyl ester (l-NAME, 20 μm) and 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ, 10 μm) . *P < 0.05  versus control . # P < 0.05  versus morphine .