Endocannabinoids have been implicated in protective effects in the heart and brain, but the mechanism of possible
infarct-size-reducing effects remains controversial. Using a model of delayed preconditioning (PC), rats received the
nitric oxide (NO) donor
nitroglycerin (0.15 mg/h/kg) for 24 hours via transdermal application. Two days later, rat isolated perfused hearts were subjected to global, no-flow
ischemia (20 min), and reperfusion (120 min).
Cannabinoid receptor antagonists were given before no-flow throughout the protocol.
Endocannabinoids were detected by liquid chromatography and mass spectrometry. NO-induced PC reduced the left ventricular
infarct size from 40.9 +/- 3.9% to 27.5 +/- 3.8% (P < 0.05). Treatment with the specific
CB1 cannabinoid receptor antagonist
AM-251 (0.3 microM) prevented the protective effect of PC on
infarct size (40.2 +/- 4.7%, P > 0.05 vs. controls). On the contrary, the specific
CB2 receptor antagonist
AM-630 (0.3 microM) did not alter
infarct size (31.6 +/- 6.3%, P > 0.05 vs. PC alone). Recovery of left ventricular developed pressure and coronary flow was incomplete in control and NO-pretreated hearts and not consistently altered by
cannabinoid receptor antagonists. PC increased the heart tissue content of the
endocannabinoid 2-arachidonylglycerol (2-AG) from 4.6 +/- 1.0 nmol/g in controls to 12.0 +/- 2.1 nmol/g (P < 0.05). Tissue levels of the
endocannabinoid arachidonylethanolamide (
anandamide) remained unchanged (19.8 +/- 3.9 pmol/g vs. 19.5 +/- 4.8 pmol/g). 2-AG (1 microM) or its metabolically stable derivative noladinether (0.1 microM), given 30 minutes before
ischemia/reperfusion in unpreconditioned hearts, mimicked the cardioprotective effects of PC and reduced
infarct size. We conclude that delayed PC through transdermal
nitroglycerin application increases the production of the
endocannabinoid 2-AG which elicits protective effects against
myocardial infarction via CB1
cannabinoid receptors which represents one new mechanism of NO-mediated PC.