Direct Preconditioning of Cardiac Myocytes via Opioid Receptors and K ATP Channels

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Abstract
—Previous studies demonstrated that opioid receptor activation mimics the cardioprotective effect of ischemic preconditioning via KATP channels in the intact heart. However, it is unknown whether this beneficial effect is exerted at the level of the cardiac myocyte or coronary vasculature or is mediated via the sarcolemmal or the mitochondrial KATP channel. Thus, the purpose of the present study was to investigate whether opioid receptor stimulation could mimic the cardioprotective effect of preconditioning in a cardiac myocyte model of simulated ischemia. Cardiac ventricular myocytes cultured from chick embryos 14 days in ovo were used as an in vitro model for ischemic preconditioning. A 5-minute exposure of the myocytes to the opioid receptor agonist morphine protected the myocytes during a subsequent 90-minute period of simulated ischemia, which was manifested as a pronounced reduction in the percentage of cardiac cells killed and the amount of creatine kinase released during ischemia. The preconditioning-like effect of morphine was concentration-dependent, reached a maximal effect at 1 μmol/L, and was reversed by naloxone (0.1 to 10 μmol/L). When KATP channel antagonists, such as glibenclamide, or the mitochondrial selective inhibitor 5-hydroxydecanoic acid were present during preexposure to morphine, they abolished the protective effect of morphine. Thus, cardiac myocytes express functional opioid receptors, and their activation mimics the cardioprotective effect of ischemic preconditioning. These results provide direct evidence that the preconditioning-like effect of morphine in the intact heart can be exerted at the level of cardiac myocytes and is most likely the result of mitochondrial KATP channel activation.