Abstract: AIM: To study the effects of naloxone on β-endorphin (β-EP) in plasma and myocardium homogenate during myocardial ischemia-reperfusion (I R) injury in rabbits.METHODS: 45 New Zealand rabbits were randomly assigned into 5 groups (5 rabbits in control group, and each 10 rabbits in ischemia group, naloxone protection group, naloxone treatment group and ischemiareperfusion group).Myocardial ischemia models andmyo- cardial ischemia-reperfusion injury models in rabbits were made by means of ligating the left anterior descending branch of coronary artery and then investigated the change of β-EP in plasma and myocardium homogenate during I R injury after the protection and treatment with naloxone (an antagonist of opiate receptor).The blood and myocardium homogenate were taken at different times in each group.The levels of β-EP were detected with radioimmunology method.RESULTS: The levels of β-EP in plasma began to decrease after 2 h of ischemia, which were strikingly different to those before ischemia(P<0.05), and the levels of β-EP in myocardium homogenate also decreased.Comparing with those before ischemia, the levels of β-EP increased significantly after I R injury (P<0.05 or P<0.01), whereas its levels began to decrease after 4 h of I R and had no significant difference in naloxone protection group(P >0.05), but the treatment group showed no significant change(P >0.05).The levels of β-EP in myocardium homogenate had no difference to those before ischemia in all of three groups.CONCLUSION: Naloxone may effectively reduce the levels of β-EP during myocardial ischemia and I R injury, which decrease the injury to blood vessel and myocardium.

Key words: naloxone, myocardial ischemia, reperfusion injury, β-endorphin