Essential role of inducible nitric oxide synthase in monophosphoryl lipid A-induced late cardioprotection - Evidence from pharmacological inhibition and gene knockout mice

Background-Monophosphoryl lipid A (MLA), a nontoxic analogue of endotoxin, is a pharmacological agent that is known to have anti-ischemic effects. Mec hanisms involved with the cardioprotection are still unclear. A role for in ducible nitric oxide synthase (iNOS) was recently proposed, We tested this hypothesis using S-methylisothiourea (SMT), one of the specific pharmacolog ical inhibitors of iNOS, as well as iNOS gene knockout mice, Methods ann Results-Adult male ICR or B6,129 mice were pretreated with eith er MLA 35 or 350 mu g/kg IP (MLA35 or MLA350) or vehicle 24 hours before gl obal ischemia/reperfusion, which was carried out in a Langendorff isolated perfused heart model (n=8 to 9 per group). Another group of MLA350 mice rec eived SMT 3 mg/kg IP 30 minutes before heart perfusion. Ventricular contrac tile function and heart rate were not different between the groups during t he preischemia and reperfusion periods (P>0.05). Preischemic basal coronary flow was significantly increased in all MLA350 but not MLA35 mice. Myocard ial infarct size was reduced significantly, from 26.9+/-2.9% of risk area i n vehicle-treated mice to 13.5+/-2.4% in the MLA350 group (mean+/-SEM, P<0. 05). This reduction in infarct size was accompanied by augmented nitrite/ni trate accumulation, from 0.23+/-0.05 nmol/mg protein in the vehicle group t o 0.97+/-0.27 nmol/mg protein in MLA350 mice (P<0.01). Infarct size increas ed significantly, to 22.2+/-2.8% after treatment with SMT in the MLA350 gro up. Furthermore, MLA350 failed to reduce infarct size in iNOS knockout mice (25.5 +/- 3.6%). Conclusions-These results demonstrate a direct association of infarct size reduction with increased NO production with MLA350. An obligatory role for iNOS in mediating the cardioprotective effect induced by MLA was confirmed with the pharmacological inhibition and gene knockout mice.