Role of μ opioid receptor in morphine preconditioning-induced reduction of myocardial ischemia-reperfusion injury in rats with chronic heart failure: an in vitro experiment

Objective To evaluate the role of μ opioid receptor in morphine preconditioning-induced reduction of myocardial ischemia-reperfusion (I/R) injury in rats with chronic heart failure. Methods Adult male Sprague-Dawley rats, weighing 170-230 g, in which chronic heart failure was induced by injecting doxorubicin via the tail vein, were studied.The rats were sacrificed and their hearts were excised and perfused in a Langendorff apparatus with K-H solution saturated with 95% O2-5% CO2 at 37 ℃.Forty isolated rat hearts with I/R injury were randomly divided into 4 groups (n=10 each): group I/R, morphine preconditioning group (group MP), μ opioid receptor antagonist CTOP plus morphine preconditioning group (group CTOP+ MP) and CTOP group.Myocardial I/R was induced by occlusion of the left coronary artery for 30 min followed by 120 min of reperfusion.In group MP, the hearts were perfused with K-H solution for 15 min, with K-H solution containing 1 μmol/L morphine for 5 min and with K-H solution for 5 min, 3 cycles in total, and then the model of myocardial I/R was established.The hearts were perfused with K-H solution containing 1 μmol/L CTOP starting from 10 min before morphine preconditioning until 5 min of ischemia in group CTOP+ MP.The hearts were perfused with K-H solution containing 1 μmol/L CTOP starting from 40 min before ischemia until 5 min of ischemia in group CTOP.The coronary effluent was collected at 15 min of equilibration (baseline) and 5 and 10 min of reperfusion to detect the activity of lactate dehydrogenase (LDH). Myocardial infarct size (IS) and the area at risk (AAR) were measured by 2, 3, 5-triphenyl-tetrazolium staining, and IS/AAR percentage was calculated.The expression of Bcl-2 and Bax mRNA was determined using uantitative real-time polymerase chain reaction, and the ratio of Bcl-2/Bax was calculated. Results Compared with group I/R, the IS and IS/AAR percentage were significantly decreased, the activity of LDH in coronary effluent was decreased, the expression of Bax mRNA was down-regulated, the expression of Bcl-2 mRNA was up-regulated, and the Bcl-2/Bax ratio was increased in group MP (P 0.05). Compared with group MP, the IS and IS/AAR percentage were significantly increased, the activity of LDH in coronary effluent was increased, the expression of Bax mRNA was up-regulated, the expression of Bcl-2 mRNA was down-regulated, and the Bcl-2/Bax ratio was decreased in group CTOP+ MP (P<0.05). Conclusion The mechanism by which morphine preconditioning reduces myocardial I/R injury may be related to activating μ opioid receptors and thus maintaining the balance between Bcl-2 and Bax gene expression in the rats with chronic heart failure. Key words: Morphine; Receptors, opioid, mu; Heart failure; Myocardial reperfusion injury