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Preservation of global cardiac function in the rabbit following protracted ischemia/reperfusion using monophosphoryl
L Zhao1, C C Kirsch, S R Hagen
1Department of Pharmaceutical Sciences, Ribi ImmunoChem Research Inc., Hamilton, Montana, USA.
Abstract:
Monophosphoryl lipid A (MLA), a derivative of the minimal substructure of lipopolysaccharide (lipid A) possesses immunomodulatory activity of the parent lipid A yet enjoys reduced toxicity. It has previously been reported that pretreatment with MLA reduces myocardial infarct size and stunning in dogs following ischemia and reperfusion. The aim of this study was to evaluate the ability of monophosphoryl lipid A (MLA) to preserve global cardiac function and peripheral hemodynamics in a rabbit model of prolonged regional ischemia (90 min), and reperfusion (6 h). An evaluation of potential mechanisms by which MLA may preserve cardiac function was also undertaken. Single dose pretreatment with MLA (35 micrograms/kg i.v.) 24 h prior to ischemia resulted in significant improvement in left ventricular developed pressure, dP/dt, rate-pressure product and mean arterial pressure during reperfusion (P < 0.05 v control). Although in this model of prolonged ischemia MLA pretreatment did not reduce infarct size (54.5 +/- 11.4% in control v 63.3 +/- 8.3% in MLA, P = N.S.), evaluation of myocardial adenylate and adenosine catabolite pools at the end of ischemia indicated a preservation of ATP and ADP and a decreased production of downstream adenosine catabolites including inosine, xanthine and uric acid. Adenosine kinase, but not 5'-nucleotidase (5'-NTase) or adenosine deaminase activity determined following reperfusion was 76% and 60% higher (P < 0.05) in non-risk and post-ischemic myocardium of MLA pretreated rabbits compared with controls. Although there was a trend toward lower tissue myeloperoxidase activity in post-ischemic myocardium from treated rabbits, the results were not significantly different from control animals. These results suggest that a 24-h pretreatment with MLA, without further treatment during ischemia or reperfusion was associated with: (1) preservation of global myocardial function during reperfusion; (2) preservation of myocardial high energy adenylates and reduced formation of adenosine catabolites during ischemia; (3) elevated myocardial adenosine kinase activity. Increased recycling of adenosine to phosphorylated nucleotides may result from MLA's affect on adenosine kinase, which could explain the drugs effect on adenylate and adenosine metabolite pools.
Insights
Monophosphoryl lipid A (MLA) pretreatment improved cardiac function and hemodynamics in rabbits during prolonged ischemia and reperfusion. This cardioprotective effect may stem from preserved high-energy adenylates and enhanced adenosine kinase activity.
Area of Science:
- Cardiology
- Immunology
- Biochemistry
Background:
- Monophosphoryl lipid A (MLA), a less toxic derivative of lipopolysaccharide (lipid A), exhibits immunomodulatory properties.
- Previous studies indicated MLA pretreatment reduces myocardial infarct size and stunning in canine models of ischemia-reperfusion injury.
Purpose of the Study:
- To assess MLA's efficacy in preserving global cardiac function and peripheral hemodynamics in a rabbit model of prolonged regional ischemia (90 min) and reperfusion (6 h).
- To investigate potential mechanisms underlying MLA's cardioprotective effects.
Main Methods:
- Rabbits received a single intravenous dose of MLA (35 µg/kg) 24 hours prior to inducing ischemia.
- Global cardiac function (left ventricular developed pressure, dP/dt, rate-pressure product) and mean arterial pressure were monitored during reperfusion.
- Myocardial adenylate and adenosine catabolite pools were analyzed post-ischemia; enzyme activities (adenosine kinase, 5'-NTase, adenosine deaminase) and myeloperoxidase were assessed post-reperfusion.
Main Results:
- MLA pretreatment significantly improved left ventricular developed pressure, dP/dt, rate-pressure product, and mean arterial pressure during reperfusion (P < 0.05).
- MLA did not significantly reduce infarct size but preserved myocardial ATP and ADP levels, decreasing downstream adenosine catabolites (inosine, xanthine, uric acid) post-ischemia.
- Adenosine kinase activity was significantly elevated (76-60%, P < 0.05) in both non-risk and post-ischemic myocardium of MLA-treated rabbits; 5'-NTase and adenosine deaminase activities were unchanged.
Conclusions:
- A single 24-hour pretreatment with MLA preserves global myocardial function and peripheral hemodynamics during prolonged ischemia-reperfusion in rabbits.
- MLA's cardioprotective effects are associated with preserved myocardial high-energy adenylates, reduced adenosine catabolite formation, and elevated myocardial adenosine kinase activity.
- Increased adenosine recycling to phosphorylated nucleotides via enhanced adenosine kinase may explain MLA's beneficial effects on adenylate and adenosine metabolite pools.