Role of inducible nitric oxide synthase in pharmacological "preconditioning" with monophosphoryl lipid A

Pretreatment with monophosphoryl lipid A (MLA) can pharmacologically mimic the second window of ischemic preconditioning (SWOP) to protect the heart from prolonged ischemia and reperfusion injury. Based on the delayed time course for development of MLA associated cardioprotection, this study was designed to test if MLA's cardioprotective effect is mediated by signalling through production of inducible nitric oxide synthase (iNOS), a proposed effector of SWOP. Rabbits were assigned to one of four groups: (1) vehicle control; (2) MLA: (3) vehicle+aminoguanidine (AMG) control; or (4) MLA+AMG. Monophosphoryl lipid A (35 micrograms/kg) or vehicle was given intravenously 24 h before ischemia. The selective iNOS inhibitor AMG (300 mg/ kg) was injected subcutaneously 1 h before ischemia. All rabbits experienced 30 min coronary artery occlusion followed by 3 h of reperfusion. Infarct size was measured by triphenyltetrazolium chloride (TTC) staining. followed by 3 h of reperfusion. Infarct size was measured by triphenyltetrazolium chloride (TTC) staining. Myeloperoxidase activity, an index of neutrophil infiltration, was also quantified in heart tissue collected from the post-ischemic viable border zone surrounding the infarct area. MLA pretreatment significantly reduced infarct size and neutrophil infiltration in rabbit hearts compared to control (P < 0.05). Inhibition of iNOS activity by AMG abolished the infarct size reductive effect of MLA. Aminoguanidine also blocked the ability of MLA to significantly reduce neutrophil infiltration. Although measurement of iNOS activity did not show induction of the enzyme in normal myocardial tissue 24 h after MLA pretreatment, an increase in iNOS activity in ischemic tissue relative to non-ischemic tissue was found after either 15 or 30 min of coronary occlusion in MLA treated rabbits. These results suggest that MLA pretreatment may enhance iNOS enzyme activity by MLA during ischemia which may be responsible for the observed cardioprotection.