Role of endogenous opioid peptides in the infarct size-limiting effect of adaptation to chronic continuous hypoxia

Aims The objective of this study was to examine the involvement of endogenous opioid peptides and opioid receptor (OR) subtypes in the cardioprotective effect of adaptation to chronic hypoxia in rats. Main methods Rats were exposed to continuous normobaric hypoxia (CNH; 12% oxygen) for 3 weeks. Myocardial ischemia was induced by 20-min coronary artery occlusion followed by 3-h reperfusion in anesthetized open-chest animals. Various OR antagonists were administered to rats prior to ischemia. The size of myocardial infarction and the incidence of ischemic ventricular arrhythmias were assessed. Myocardial and plasma concentrations of opioid peptides (met-enkephalin, β-endorphin, and endomorphins) were determined. Key findings Adaptation to CNH significantly increased myocardial and plasma concentrations of opioids, potentiated their further elevation by ischemia/reperfusion, and reduced myocardial infarct size, but it did not affect the incidence of ischemic arrhythmias. The infarct size-limiting effect of CNH was abolished by OR antagonists naltrexone (non-selective), naloxone methiodide (non-selective peripherally acting), TIPP[ψ] (δ-OR), naltriben (δ₂- OR), or CTAP (μ-OR), while BNTX (δ₁-OR) and nor-binaltorphimine (κ-OR) had no effect. Significance The results suggest that the infarct size-limiting effect afforded by adaptation to CNH is mediated by activation of peripheral δ₂- and μ-ORs by elevated levels of endogenous opioid peptides.