| Brief periods of ischemia lead to a reduced severity of cardiac injury following a subsequent or more sustained ischemia, a mechanism termed ischaemic preconditioning. Ischemic preconditioning has 2 phases - an early phase or first window of protection that lasts for 1 to 3 h after the ischemic preconditioning and a delayed phase or second window of protection that appears 12 to 24 h after the ischemic preconditioning and may last for 24 to 72 h. Studies have shown that the delayed cardioprotection of ischemic preconditioning can be reproduced pharmacologically with clinically relevant agents, suggesting that this phenomenon might be exploited for therapeutic purposes.The present study at first time defined the delayed cardioprotection of cardiac K-opioid receptor stimulation with a selective K-opioid receptor agonist U50,488H in vivo by observing the myocardial infarct in the heart upon sublethal ischemia /reperfusion. Secondly, we detewrmined the mechanisms of delayed cardioprotection of cardiac K-opioid receptor stimulation by observing the intracellur Ca2+ ([Ca2+]i) homeostasis, mitochondrialfunction and expression of heat shock protein 70. We also attempted to define the roles of ATP sensitive potassium channels (KATP) in delayed cardioprotection of cardiac K-opioid receptor stimulation. The main contents of this study include:1. Different does of U50,488H were given intravenously to rats, which provides pharmacological preconditioning. After 12-72 h, we determined the myocardial infarct in the heart upon sublethal ischemia/reperfusion.2. We determined the alterations in resting [Ca2+]j and electrically induced [Ca2+]j transient in isolated ventricular myocytes upon metabolic inhibition 24 h after intravenous administration of U50,488H.3. We defined the role of KATP channels in infarct sparing effect of cardiac K-opioid receptor stimulation by blocking the sarcolemmal or mitochondrial KATP channel with their selective blockers, HUM-1098 and 5-HD, at the same time of preconditiong or just before ischemia.4. We defined the role of KATP channels in [Ca2+]j homeostasis stabling effect of cardiac K-opioid receptor stimulation by blocking the sarcolemmal or mitochondrial KATP channel with their selective blockers, HRM-1098 and 5-HD, at the same time of preconditiong or just before ischemia.5. We observed the mitochondrial ATP synthesis in isolated perfused rat heart subjected to sublethal ischernia/reperfusion to determine the protective effect of K-opioid receptor stimulation on the mitochondrial function.6.We defined the role of KATP channels in the protective effect of K-opioid receptor stimulation on the mitochondrial function.7. We observed the expression of heat shock protein 70 in isolated perfused rat heart subjected to sublethal ischemia/ reperfusion to determine the role of heat shock protein 70 in the protective effect of K-opioid receptor stimulation.Main results:1. Myocardial ischemia for 30 min and reperfusion for 2 h induced myocardial infarct as expected. A significant reduction ininfarct size appeared when U50,488H was administered at the range of 3 -15 mg/kg 24 h before ischemia/reperfusion. Significant infarct size reductions occurred at 18, 24 and 48 h after administration of 10 mg/kg U50,488H with the greatest reduction at 24 h. There was no significant reduction in infarct size 12 or 72 h after administration of U50,488H.. The effect of U50,488H was completely abolished when the selective K-opioid receptor antagonist nor-BNI at 10 mg/kg was administered 10 min prior to administration of U50,488H, which took place at 24 h before ischemia/reperfusion.2. The elevation in resting [Ca2+]j and an reductiong in the electrically induced [Ca2+]j transient upon ischemic insults were significantly attenuated by administration with 10 mg/kg U50,488H, which was abolished in the presence of nor-BNI.3. When co-administered with U50,488H, both HMR-1098 and 5-HD significantly attenuated the U50,488H-induced reduction in infarct size.When HMR-1098 o...
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