Since cold exposure confers cardioprotection, the present study attempted to determine the role of opioid receptors (OR). Stress with cold exposure and restraint for 3 h, shown previously to induce peptic ulcer in a synergistic manner, attenuated infarct size induced by myocardial ischemia and reperfusion in the isolated perfused rat heart from 36.64 ± 1.8 to 22.85 ± 2.6%. This is similar to protecting the rat with morphine at 8 mg/kg, which also attenuated the infarct size from 36.26 ± 1.6 to 20.30 ± 2.1%. The effects of cold-restraint or morphine were abolished by naloxone, a non-selective OR antagonist; nor-binaltorphimine, a selective ĸ-OR antagonist; naltrindole, a selective δ-OR antagonist, or CTOP, a selective µ-OR antagonist. The effects were also attenuated by blockade of protein kinase C or the mitochondrial KATP channel. The finding is first evidence that all three OR subtypes mediate cardioprotection of cold-restraint stress in the rat.

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