title: Postconditioning prevents reperfusion injury by activating delta-opioid receptors.
abstract: BACKGROUND
  While postconditioning has been proposed to protect the heart by targeting the mitochondrial permeability transition pore (mPTP), the detailed mechanism underlying this action is unknown. The authors hypothesized that postconditioning stimulates opioid receptors, which in turn protect the heart from reperfusion injury by targeting the mPTP.
  
  
  METHODS
  Rat hearts (both in vivo and in vitro) were subjected to 30 min of ischemia and 2 h of reperfusion. Postconditioning was elicited by six cycles of 10-s reperfusion and 10-s ischemia. To measure nitric oxide concentration, cardiomyocytes loaded with 4-amino-5-methylamino-2',7'-difluorofluorescein were imaged using confocal microscopy. Mitochondrial membrane potential was determined by loading cardiomyocytes with tetramethylrhodamine ethyl ester.
  
  
  RESULTS
  In open chest rats, postconditioning reduced infarct size, an effect that was reversed by both naloxone and naltrindole. The antiinfarct effect of postconditioning was also blocked by the mPTP opener atractyloside. In isolated hearts, postconditioning reduced infarct size. Morphine mimicked postconditioning to reduce infarct size, which was abolished by both naltrindole and atractyloside. N-nitro-l-arginine methyl ester and guanylyl cyclase inhibitor 1H-[1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one blocked the action of morphine. Further experiments showed that morphine produces nitric oxide in cardiomyocytes by activating delta-opioid receptors. Moreover, morphine could prevent hydrogen peroxide-induced collapse of mitochondrial membrane potential in cardiomyocytes, which was reversed by naltrindole, N-nitro-l-arginine methyl ester, and the protein kinase G inhibitor KT5823.
  
  
  CONCLUSIONS
  Postconditioning protects the heart by targeting the mPTP through activation of delta-opioid receptors. The nitric oxide-cyclic guanosine monophosphate-protein kinase G pathway may account for the effect of postconditioning on the mPTP opening.
authors.affiliation: Department of Anesthesiology, The University of North Carolina at Chapel Hill, North Carolina 27599, USA.
authors.forename: Youngho
authors.initials: Y
authors.lastname: Jang
authors.forename: Jinkun
authors.initials: J
authors.lastname: Xi
authors.forename: Huihua
authors.initials: H
authors.lastname: Wang
authors.forename: Robert A
authors.initials: RA
authors.lastname: Mueller
authors.forename: Edward A
authors.initials: EA
authors.lastname: Norfleet
authors.forename: Zhelong
authors.initials: Z
authors.lastname: Xu
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004195:Disease Models, Animal; D006321:Heart / Q000502:physiology* / Q000503:physiopathology; D006861:Hydrogen Peroxide / Q000494:pharmacology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D018613:Microscopy, Confocal; D008929:Mitochondria, Heart / Q000187:drug effects / Q000502:physiology; D009203:Myocardial Infarction / Q000503:physiopathology*; D015428:Myocardial Reperfusion Injury / Q000517:prevention & control*; D032383:Myocytes, Cardiac / Q000502:physiology / Q000648:ultrastructure; D009569:Nitric Oxide / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D017465:Receptors, Opioid, delta / Q000502:physiology*
chemical_list: D017465:Receptors, Opioid, delta; D009569:Nitric Oxide; D006861:Hydrogen Peroxide