Adenosine and opioid receptors do not trigger the cardioprotective effect of mild hypothermia

Mild hypothermia (32°C-34°C) exerts a potent cardioprotection in animal models of myocardial infarction. Recently, it has been proposed that this beneficial effect is related to survival signaling. We, therefore, hypothesized that the well-known cardioprotective pathways dependent on adenosine and/or opioid receptors could be the trigger of hypothermia-induced salvage. Open-chest rabbits were accordingly exposed to 30 minutes of coronary artery occlusion (CAO) under normothermic (NT) or hypothermic ([HT] 32°C) conditions. In the latter, hypothermia was induced by total liquid ventilation with temperature-controlled perfluorocarbons in order to effect ultrafast cooling and to accurately control cardiac temperature. After 4 hours of reperfusion, infarct and no-reflow zone sizes were assessed and quantified as a percentage of the risk zone. In animals experiencing HT ischemia, the infarct size was dramatically reduced as compared to NT animals (9% ± 3% vs 55% ± 2% of the risk zone, respectively). Importantly, administration of opioid and adenosine receptor antagonists (naloxone [6 mg/kg iv] and 8-(p-sulfophenyl) theophylline [20 mg/kg iv], respectively) did not alter the infarct size or affect the cardioprotective effect of hypothermia. Doses of these 2 antagonists were appropriately chosen since they blunted infarct size reduction induced by selective opioid or adenosine receptor stimulation with morphine (0.3 mg/kg iv) or N (6)-cyclopentyladenosine ([CPA] 100 μg/kg iv), respectively. Therefore, the cardioprotective effect of mild hypothermia is not triggered by either opioid or adenosine receptor activation, suggesting the involvement of other cardioprotective pathways.