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From the *Department of Anesthesiology, University of Hong Kong, Hong Kong; Department of Anesthesiology, Anhui Medical University, China; and Department of Physiology, University of Hong Kong, Hong Kong.
Address correspondence and reprint requests to Dr. Gordon T.C. Wong, Room 424, K Block, Queen Mary Hospital, Pokfulam Rd., Hong Kong. Address e-mail to gordon{at}hkucc.hku.hk.
Abstract
BACKGROUND: Small doses of intrathecal morphine provide cardioprotection similar to that conferred by IV morphine. However, the extent of intrathecal morphine preconditioning (IT-MPC) relative to that resulting from ischemic preconditioning (IPC) is unknown. Further, it is uncertain whether IT-MPC is mediated by opioid receptor dependent pathways. In this study, we compared the extent of cardioprotection conferred by IT-MPC with IPC and investigated the role of opioid receptors in this effect.
METHODS: Eighty anesthetized, open-chest, male Sprague-Dawley rats were assigned to 1 of 13 groups (n = 6–7) after successful intrathecal catheter placement. Rats in the IPC group were subjected to three 5-min cycles of myocardial ischemia (induced by occlusion of the left main coronary artery) interspersed with 5 min of reperfusion. After IPC, myocardial ischemia and reperfusion injury was induced by 30 min of left main coronary artery occlusion followed by 2 h of reperfusion. In the IT-MPC groups, the rats were given 3 consecutive 5 min intrathecal morphine infusions (0.03, 0.3, 3.0, or 30.0 µg/kg, respectively) interspersed with 5 min infusion-free periods, before myocardial ischemia reperfusion injury. In 2 other groups either 300µg/kg of IV morphine or 10 µL of intrathecal normal saline were given. The selective delta, kappa, and mu opioid receptor antagonists naltrindole, nor-binaltorphimine, and D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), respectively, were given to groups of animals receiving IT-MPC to evaluate the relative role of the specific opioid receptor subtypes in IT-MPC preconditioning. Myocardial infarct size (IS), as a percentage of the area at risk (AAR), was determined by 2,3,5-triphenyltetrazolium staining.
RESULTS: Intrathecal morphine 0.3 to 30 µg/kg reduced myocardial IS compared with intrathecal normal saline control animals. The IS/AAR were 33% ± 10% (0.3 µg/kg), 29% ± 10% (3 µg/kg) and 29% ± 16% (30 µg/kg), versus 53% ± 8% for the control group (P < 0.01). The reduction in IS/AAR with IT-MPC was similar to that produced by IV morphine (33% ± 6%, P = 0.84) and IPC (22% ± 4%, P = 0.41). Myocardial preconditioning due to IT-MPC was attenuated by co-administration of any one of the opioid receptor antagonists (IT-MPC + naltrindole 50% ± 9%, IT-MPC + nor binaltorphimine 43% ± 6%, IT-MPC + CTOP 53% ± 9%, P = 0.14).
CONCLUSIONS: IT-MPC produced comparable cardioprotection to myocardial IPC and IV morphine. Myocardial preconditioning from intrathecal morphine seems to involve delta, kappa, and mu opioid receptors.
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