Endothelial CB1-receptors limit infarct size through NO formation in rat isolated hearts

The aim of the present study was to document the presence of cannabinoid receptors in the rat heart, and to assess the cardioprotective properties of CB1- and CB2-receptor agonists. Rat isolated hearts were exposed to low-flow ischemia and reperfusion, with selective cannabinoid agonists administered prior to and during the ischemic period. In some hearts, RT-PCR, Western blots, and immunohistological techniques were used to identify and localize both cannabinoid-receptor subtypes. The effect of cannabinoids on infarct size was evaluated in additional hearts using TTC staining. Protein and mRNA for both CB1- and CB2-receptors were found in rat heart extracts. CB1-receptors were localized almost exclusively on arterial and capillary endothelial cells in intact hearts, whereas CB2-receptors appeared on cardiomyocytes and endothelial cells of larger arteries. Both the CB1-agonist, ACEA (50 nM), and the CB2-agonist, JWH015 (50 nM), reduced infarct size. However, only the cardioprotective effect of the CB1-agonist was blocked by the NO-synthase inhibitor, NG-nitro-l-arginine (30 μM). In conclusion, CB1-receptors are present mainly on endothelial cells in the rat heart, and exert their effect through production of NO. In contrast, CB2-receptors present on cardiomyocytes exert a cardioprotective effect independent of this endothelial factor.