Endotoxin induces a dose-dependent myocardial cross-tolerance to ischemia-reperfusion injury

Objective: To test whether or not endotoxin induces a dose-dependent reduction of myocardial contractile dysfunction after a standardized period of myocardial ischemia and reperfusion and whether nitric oxide is involved in this form of myocardial protection, Design: Prospective, randomized, controlled animal study. Setting: University research laboratory. Subjects: Twenty-five male Sprague-Dawley rats. Interventions: After anesthesia, the left carotid artery was cannulated under sterile conditions and animals were allowed to recover from surgery for 12 hrs. Sterile saline or increasing doses (2.5, 5, or 10 mg/kg body weight) of endotoxin (Escherichia coli 026:B6; Sigma, Mississauga, Ontario, Canada) were given intravenously (1 mL over 5 mins). In some rats, diaspirin-crosslinked hemoglobin (200 mg/kg) was infused 6 hrs and 60 min before endotoxin infusion (10 mg/kg). Hearts were rapidly excised for retrograde perfusion through the ascending aorta (Langendorff apparatus)6 hrs later. After baseline data collection, hearts were subjected to global ischemia (30 mins, 37 degrees C [98.6 degrees F]), followed by 30 mins of reperfusion. Measurements and Main Results:Physiologic variables were recorded 6 hrs after saline and endotoxin infusion. Baseline myocardial systolic contractility and diastolic compliance were assessed, respectively, by left ventricular developed pressure (LVDP) and left ventricular (LV) volume-preload relationships. After 30 min of reperfusion, LVDP recovery and left ventricular end-diastolic pressure were measured. Endotoxin induced LV systolic contractile depression, irrespective of the dose of endotoxin administered, LV diastolic dysfunction varied between different doses of endotoxin administered. On reperfusion, endotoxin produced a dose-dependent improvement of postischemic LVDP recovery: 30 +/- 6% in sham, 78 +/- 9% in 2.5 mg/kg, 93 +/- 8% in 5 mg/kg, and 107 +/- 10% in 10 mg/kg endotoxin heart. In rats treated with 10 mg/kg endotoxin, diaspirin-crosslinked hemoglobin pretreatment abrogated endotoxin-induced postischemic LVDP recovery improvement (105 +/- 10% vs. 43 +/- 7%, p = .01). Conclusion: Sublethal doses of endotoxin induce in a dose-dependent manner a delayed form of myocardial protection against ischemia. Although free-cell hemoglobin solution abrogates this endotoxin-induced cross-tolerance, we propose that possible mechanisms involved in this form of myocardial protection include nitric oxide pathway activation.