The Role of Endogenous Opioid System in the Regulation of Heart Tolerance to Stress-Induced Damage.

In Wistar rats, stress was modeled by 24-h immobilization in a supine posture and stress-induced damage to the heart was assessed by accumulation of 99mTc-pyrophosphate in the myocardium. The intensity of stress reaction was measured by serum levels of cortisol and insulin. Both stressinduced damage to the heart and intensity of stress reaction were examined under control conditions and in rats treated with opioid receptor antagonists naltrexone, methylnaltrexone bromide, MR2266, and ICI174.864. Activation of central μ-opioid receptors with endogenous opioids aggravated stress-induced cardiomyopathy, while stimulation of peripheral μ-opioid receptors produced a cardioprotective effect. The stress-induced damage to the heart was not directly related to up-regulation of cortisol secretion in response to 24-h immobilization. Blockade of the central opioid receptors promoted a decrease in cortisol level in stressed rats.