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Preconditioning of rat heart with monophosphoryl lipid A: a role for nitric oxide
A Tosaki1, N Maulik, G T Elliott
1Cardiovascular Division, Department of Surgery, University of Connecticut School of Medicine, Farmington, Connecticut, USA.
Abstract:
Preconditioning with monophosphoryl lipid A (MLA) protects rabbit hearts from prolonged ischemic reperfusion injury by a mechanism involving inducible nitric oxide synthase (iNOS) activation. This study was undertaken to determine whether MLA also could precondition rat hearts in a similar manner. Rats were injected with two different doses of MLA (300 microg/kg or 450 microg/kg i.v.) or vehicle (control), and after 24 hr the animals were sacrificed for preparation of isolated perfused rat hearts. Hearts were then perfused by working mode, and then made ischemic for 30 min followed by 30 min of reperfusion. Another group of hearts were treated simultaneously with a nitric oxide (NO) blocker, L-nitro-arginine-methyl-ester (L-NAME) (10 mg/kg) and MLA (450 microg/kg). For arrhythmia studies, 12 hearts were used in each group (total, 48 hearts). Cardiac functions were examined in a separate group of 24 hearts (n = 6/group). MLA-treated hearts (either dose) were tolerant to ischemic reperfusion injury as evidenced by improved postischemic ventricular recovery [coronary flow (ml/min) 19.1 +/- 0.8 (300 microg/kg MLA), 22.6 +/- 1.0 (450 microg/kg MLA) vs. 15.9 +/- 0.7 (control); aortic flow (ml/min) 20.7 +/- 1.8 (300 microg/kg MLA), 25.8 +/- 1.4 (450 microg/kg MLA) vs. 11. 0 +/- 0.8 (control); left ventricular developed pressure (kPa) 13.3 +/- 0.6 (300 microg/kg MLA), 14.6 +/- 0.2 (450 microg/kg MLA) vs. 10. 3 +/- 0.7 (control)]. Incidences of ventricular fibrillation and ventricular tachycardia were decreased compared with the control group only in the 450 microg/kg dose of MLA-treated hearts (92% to 33%). Pretreatment of the hearts with L-NAME inhibited the preconditioning effect of MLA. To examine the induction of the iNOS expression, RNAs were extracted 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 corresponding to iNOS mRNA was detected after 4 hr of MLA treatment, whereas the maximal iNOS expression was found between 6 and 8 hr of MLA treatment. The results of this study demonstrated that MLA induced the expression of iNOS and protected the myocardium from ischemic reperfusion injury which is blocked by an inhibitor of NO synthesis, which suggests a role of NO in MLA-mediated cardioprotection.
Insights
Monophosphoryl lipid A (MLA) preconditioning protects rat hearts from ischemic reperfusion injury by inducing nitric oxide synthase (iNOS). This protective effect, mediated by nitric oxide (NO), was blocked by L-NAME, confirming NO
Area of Science:
- Cardiology
- Immunology
- Biochemistry
Background:
- Ischemic reperfusion injury remains a significant clinical challenge.
- Monophosphoryl lipid A (MLA) has shown cardioprotective effects in rabbits via inducible nitric oxide synthase (iNOS).
Purpose of the Study:
- To investigate whether MLA can precondition rat hearts against ischemic reperfusion injury.
- To elucidate the role of iNOS and nitric oxide (NO) in MLA-mediated cardioprotection in rats.
Main Methods:
- Rats were treated with MLA (300 or 450 microg/kg) or vehicle, followed by isolated working heart perfusion, 30 min ischemia, and 30 min reperfusion.
- Cardiac function and arrhythmias were assessed.
- Hearts were analyzed for iNOS mRNA expression using Northern blot.
- The effect of L-nitro-arginine-methyl-ester (L-NAME), a NO blocker, was evaluated.
Main Results:
- MLA treatment significantly improved post-ischemic cardiac function (coronary flow, aortic flow, developed pressure) in a dose-dependent manner.
- The 450 microg/kg MLA dose reduced the incidence of ventricular arrhythmias.
- MLA induced iNOS mRNA expression starting at 4 hours, peaking between 6-8 hours.
- L-NAME pretreatment abolished the protective effects of MLA.
Conclusions:
- Monophosphoryl lipid A (MLA) effectively preconditions rat hearts against ischemic reperfusion injury.
- The cardioprotection afforded by MLA is mediated by the induction of inducible nitric oxide synthase (iNOS) and the subsequent production of nitric oxide (NO).
- These findings suggest a potential therapeutic role for MLA in preventing myocardial damage during ischemic events.