Aim Postconditioning is brief cycles of reperfusion and ischaemia during the early phase of reperfusion following a prolonged ischaemic insult. Opioids are well-known endogenous triggers of preconditioning. Because postconditioning shares the protective pathways with preconditioning, G protein–coupled receptor activation may serve as an essential mechanism that triggers protection of postconditioning. Receptor binding studies showed that κ opioid receptor (κ-OR) is a predominant opioid receptor in heart. Therefore, we determined whether endogenous agonist of κ-OR, dynorphin, triggers postconditioning, especially reduces apoptosis of I/R myocardium and to identify its underlying mechanism.
Methods Besides the vehicle, the other SD rats underwent a 30 min left anterior descending occlusion followed by 120 min of reperfusion with or without a postconditioning stimulus (three cycles of 10 s reperfusion and 10 s reocclusion) initiated at the onset of reperfusion. The selective κ opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 2 mg/kg, intravenously), administered 5 min before the reperfusion. The blood plasma was analysed spectrophotometrically for determination of CK and LDH levels. Myocardial apoptosis was quantitatively analysed by detection of TUNEL with an apoptosis detection kit. Six fields from the peri-infarct zone were analysed and the number of TUNEL positive cardiomyocytes was counted on 400 high power fields. Immunoreactive Dynorphin were measured by an antigen competitive ELISA.
Results CK (U/L) and LDH (U/L) were significantly higher in I/R group than those in the control (3401±251 vs 689±76, 2329±216 vs 753±97, p<0.01). Postconditioning significantly reduced the release of CK and LDH from I/R myocardium (2026±268 vs 3401±251, 1543±169 vs 2329±216, p<0.01). These reduction were abolished by nor-BNI (p<0.01). Regional myocardial I/R resulted in a significant increase in cardiomyocyte apoptosis (18.7±2.5 vs 1±0.25, p<0.01). Postconditioning exerted a significant anti-apoptotic effect (10.4±1.3 vs 18.7±2.5, p<0.01). This protective effect was attenuated by pretreatment with Nor-BNI (p>0.05). Immunoreactive dynorphin content (pg/ml) in serum significantly increased after postconditioning (78.5±12 vs 37.3±6.5, p<0.01). Increased dynorphin did not reduced by κ opioid receptor antagonist Nor-BNI (p>0.05).
Conclusions We find that cardiac protection and anti-apoptotic effect of postconditioning is mediated by activating κ opioid receptor. And cardiac protective and anti-apoptosis effect of postconditioning is mediated by enhanced dynorphin express in rats. Recently, clinical use of postconditioning as a treatment for cardiovascular disease has been an increasing attention, and opioid receptor triggers postconditoning, so the study of the relationship between κ opioid receptor and ischaemia reperfusion injury (IRI) may provide a new insight for the curing of IRI.