Role of different opioid receptors in morphine postconditioning-induced reduction of ischemia-reperfusion injury in isolated rat hearts

Abstract

Objective To evaluate the role of different opioid receptors in morphine postconditioning-induced reduction of ischemia-reperfusion (I/R) injury in isolated rat hearts. Methods Healthy adult male Sprague-Dawley rats, aged 7-8 weeks, weighing 250-300 g, were anesthetized with chloral hydrate.The hearts were removed and retrogradely perfused with oxygenated K-H solution in a Langendorff apparatus.Forty-eight Langendorff-perfused rat hearts were divided into 4 groups (n=12 each) by a random number table method: I/R group, morphine postconditioning group (M group), δ receptor antagonist naltrexone plus morphine postconditioning group (N+ M group) and κ receptor antagonist nor-binaltorphimine plus morphine postconditioning group (B+ M group). Hearts were subjected to 4 cycles of perfusion with K-H solution containing 1 μmol/L morphine for 15 s and then with K-H solution containing no morphine for 15 s in group M. In N+ M group and B+ M group, hearts were perfused with 5 μmol/L naltrindole and 5 μmol/L nor-binaltorphimine, respectively, starting from 10 min of equilibration until 5 min of reperfusion, and morphine postconditioning was similar to those previously described in group M. Heart rate and the maximum rate of increase or decrease in left ventricular pressure (±dp/dtmax) were recorded at 20 min of equilibration and 30 and 60 min of reperfusion.Coronary effluent was collected at 20 min of equilibration and 60 min of reperfusion for measurement of the activity of creatine kinase by colorimetric assay.Eight hearts were obtained at 60 min of reperfusion for determination of myocardial infarct size.Four hearts were obtained at 60 min of reperfusion for detection of the expression of microtubule-associated protein 1 light chain 3 Ⅰ (LC3Ⅰ) and LC3Ⅱ by Western blot.LC3Ⅱ/LC3Ⅰ ratio was calculated. Results Compared with the baseline value at 20 min of equilibration, heart rate and ±dp/dtmax were significantly decreased at 30 and 60 min of reperfusion, and the activity of creatine kinase in coronary effluent was increased at 60 min of reperfusion in four groups (P<0.05). Compared with group I/R, the activity of creatine kinase in coronary effluent, percentage of myocardial infarct size and LC3Ⅱ/LC3Ⅰ ratio were significantly decreased in group M (P<0.05). Compared with group M, the activity of creatine kinase in coronary effluent, percentage of myocardial infarct size and LC3Ⅱ/LC3Ⅰ ratio were significantly increased in N+ M and B+ M groups (P<0.05). Conclusion δ and κ opioid receptors are involved in morphine postconditioning-induced reduction of I/R injury in isolated rat hearts, and the mechanism may be related to inhibiting the level of autophagy. Key words: Receptors, opioid; Morphine; Ischemic postconditioning; Myocardial reperfusion injury