PRECONDITIONING OF RAT-HEART WITH MONOPHOSPHORYL LIPID-A - A ROLE FORNITRIC-OXIDE

Preconditioning with monophosphoryl lipid A (MLA) protects rabbit hear ts from prolonged ischemic reperfusion injury by a mechanism involving inducible nitric oxide synthase (iNOS) activation. This study was und ertaken to determine whether MLA also could precondition rat hearts in a similar manner. Rats were injected with two different doses of MLA (300 mu g/kg or 450 mu g/kg i.v.) or vehicle (control), and after 24 h r the animals were sacrificed for preparation of isolated perfused rat hearts. Hearts were then perfused by working mode, and then made isch emic for 30 min followed by 30 min of reperfusion. Another group of he arts were treated simultaneously with a nitric oxide (NO) blocker, L-n itro-arginine-methyl-ester (L-NAME) (10 mg/kg) and MLA (450 mu g/kg). For arrhythmia studies, 12 hearts were used in each group (total, 48 h earts). Cardiac functions were examined in a separate group of 24 hear ts (n = 6/group). MLA-treated hearts (either dose) were tolerant to is chemic reperfusion injury as evidenced by improved postischemic ventri cular recovery [coronary flow (ml/min) 19.1 +/- 0.8 (300 mu g/kg MLA), 22.6 +/- 1.0(450 mu g/kg MLA) vs. 15.9 +/- 0.7 (control); aortic flow (ml/min) 20.7 +/- 1.8 (300 mu g/kg MLA), 25.8 +/- 1.4 (450 mu g/kg ML A) vs. 11.0 +/- 0.8 (control) left ventricular developed pressure (kPa ) 13.3 +/- 0.6 (300 mu g/kg MLA), 14.6 +/- 0.2 (450 mu g/kg MLA) vs. 1 0.3 +/- 0.7 (control)]. Incidences of ventricular fibrillation and ven tricular tachycardia were decreased compared with the control group on ly in the 450 mu g/kg dose of MLA-treated hearts (92% to 33%). Pretrea tment of the hearts with L-NAME inhibited the preconditioning effect o f MLA. To examine the induction of the iNOS expression, RNAs were extr acted from the control and MLA-treated hearts (after 2, 4,6, 8, 12 and 24 hr of treatment) and Northern blot analyses were performed with a specific cDNA probe for iNOS. A single band of approximately 4.6 kb co rresponding to iNOS mRNA was detected after 4 hr of MLA treatment, whe reas the maximal iNOS expression was found between 6 and 8 hr of MLA t reatment. The results of this study demonstrated that MLA induced the expression of iNOS and protected the myocardium from ischemic reperfus ion injury which is blocked by an inhibitor of NO synthesis, which sug gests a role of NO in MLA-mediated cardioprotection.