Induction of endogenous tissue antioxidant enzyme activity attenuates myocardial reperfusion injury

Efforts to reduce reperfusion injury have focused on exogenous therapies; however, endogenous attenuation of reperfusion injury can be induced by a single sublethal dose of endotoxin (ETX) prior to ischemia. The purposes of this study were to investigate (i) the early neutrophilendothelial (PMN-EC) adherence, (ii) the associated myocardial oxidant stress, (iii) the relationship of oxidant stress to antioxidant enzyme activity, and (iv) the correlation of increased antioxidant enzyme activity to myocardial recovery following ischemia/reperfusion (I-R) injury at 36 hr. Rats were administered a sublethal dose (2% of LD₅₀) of endotoxin (500 μg/kg, ip, Salmonella typhimurium). At 6 hr, myocardial neutrophil accumulation (histology), hydrogen peroxide (H₂O₂) levels, and myocardial tissue glutathione (glutathione and oxidized glutathione) levels were determined. At 24 hr myocardial tissue glutathione levels and catalase (CAT) activity were assayed. At 36 hr, myocardial tissue superoxide dismutase, glutathione peroxidase, glutathione reductase, catalase, and glucose-6-phosphate dehydrogenase (G-6-PD) were assayed. At 36 hr, hearts were subjected to a standard (20 min, global, 37°C) ischemic insult followed by reperfusion. At 40 min of reperfusion, ventricular function was assessed (ventricular balloon; ventricular developed pressure +dP/dt, and t-dP/dt). Pretreatment with ETX resulted in (i) increased myocardial PMN accumulation at 6 hr (P < 0.05), (ii) increased tissue H₂O₂ levels at 6 hr (P < 0.05) and increased tissue glutathione oxidation at 24 hr (P < 0.05), (iii) increased myocardial CAT activity (U/mg protein) at 24 and 36 hr and increased G-6-PD activity (U/mg protein) at 36 hr (P < 0.05), and (iv) improved recovery of myocardial function following I-R injury at 36 hr (P < 0.05). We conclude that a single sublethal dose of endotoxin results in (i) early myocardial accumulation of neutrophils and associated increased H₂O₂ levels, (ii) a low-grade oxidant stress, (iii) subsequent increases in myocardial antioxidant enzyme activity, and (iv) enhanced myocardial functional recovery following ischemia-reperfusion injury.