preproenkephalin and Cardiomegaly

The goal of this study was to investigate alterations of the endogenous opioid system in cardiac hypertrophy, to elucidate mechanisms of preproenkephalin (ppENK) gene expression, and to assess effects of endogenous opioids on myocardial contractility and atrioventricular conduction.. Hypertrophy was induced by ligation of a renal artery (2K1C) or chronic isoprenaline infusion (ISO). ppENK and opioid receptor (mu-, delta-, kappa-OR) mRNA expression was quantified by Northern blot and quantitative RT-PCR, respectively. Isolated cardiac myocytes and non-myocytes from neonatal rat heart were used for cell culture experiments.. Overall expression of OR in the heart was markedly lower than in brain tissue, with kappa-OR being the most abundant isoform in the heart. We did not observe differences in kappa-OR expression in ventricular and atrial myocardium. In contrast, delta-OR expression was markedly higher in atria than in ventricles. Mu-OR expression in the heart was below the detection limit of the developed qRT-PCR assay. In left ventricular myocardium, ppENK mRNA levels were significantly increased in 2K1C rats but decreased in ISO rats. Cell culture experiments from neonatal rat hearts revealed that myocytes and non-myocytes express ppENK mRNA. In these cells, receptor-dependent and -independent stimulation of the beta-adrenergic signalling pathway caused an increase in ppENK mRNA. Furthermore, inactivation of inhibitory G-proteins by pertussis toxin increased basal and noradrenaline-stimulated ppENK mRNA expression. The physiological significance of myocardial opioids was investigated in isolated perfused rat hearts. Opioid receptor antagonists (nor-BNI, naltrindol) and the enkephalinase inhibitor CPL had no effect on contractility but significantly altered atrioventricular conduction.. These observations suggest that the cardiac opioid system is activated in cardiac hypertrophy. Pressure overload and stimulation of the beta-adrenergic signalling pathway have been identified as a possible mechanism leading to increased ppENK expression, which may contribute to opioid system activation. Finally, endogenous opioids modulate the dromotropic response to catecholamine stimulation. The latter finding raises the possibility that endogenous opioids may contribute to the pathogenesis of arrhythmias.

Topics: Adrenergic beta-Agonists; Animals; Animals, Newborn; Atrioventricular Node; Blotting, Northern; Cardiomegaly; Enkephalins; Heart Ventricles; Isoproterenol; Male; Muscle Cells; Myocytes, Cardiac; Perfusion; Protein Precursors; Rats; Rats, Sprague-Dawley; Rats, Wistar; Receptors, Opioid; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger

The objective of this study was to examine the expression of preproenkephalin (ppENK) in the heart in cardiac hypertrophy and the effects on cardiac contractility and blood pressure regulation of its peptide products. The ppENK-derived peptides Leu5-enkephalin (LE), Met5-enkephalin (ME), Met5-enkephalin-Arg6-Gly7-Leu8 (MEAGL), and Met5-enkephalin-Arg6-Phe7 (MEAP) were administered intravenously to unanesthetized Sprague-Dawley rats and to an isolated heart preparation from the same species. LE, ME, MEAGL, or MEAP (360 nmol iv) produced an immediate decrease in heart rate, reaching its maximum within 10 s and returning to baseline by 30 s. The blood pressure response for each enkephalin was a small initial decrease followed by a marked and significant increase (P < 0.05 for MEAP). In the isolated heart preparation, neither LE, ME, MEAGL, nor MEAP altered left ventricular contractility. Cardiac hypertrophy was produced in the Dahl salt-dependent model of hypertension with a significantly greater heart weight-to-body weight ratio in the Dahl salt-sensitive (S) compared with the Dahl salt-resistant (R) rat on a high-salt diet (P < 0.05). Tissue RNA was extracted, and Northern blot analysis identified and quantitated mRNA with a 0.93-kilobase cDNA of ppENK A. There was more ppENK mRNA in the left than in the right ventricle and much less in the atria than in the ventricles. The amount of ppENK mRNA was markedly and significantly increased in the left ventricle of the Dahl S compared with the Dahl R rat (P < 0.05). In contrast, there were no differences in ppENK mRNA levels in different brain regions between the R and S rats on a high-salt diet. Interestingly, a larger ppENK mRNA of 1.75 kilobases was abundantly expressed in testicular tissue. These data showing increased ppENK expression raise the possibilities of 1) an autocrine/paracrine role for enkephalins in cardiac hypertrophy and 2) an endocrine role for the hypertrophic heart, with an increased production of enkephalins, especially MEAP, that produces vasoconstriction and further increases in blood pressure.

Topics: Animals; Blood Pressure; Cardiomegaly; Drug Resistance; Enkephalins; In Vitro Techniques; Injections, Intravenous; Male; Myocardium; Peptide Fragments; Protein Precursors; Rats; Rats, Inbred Strains; Rats, Sprague-Dawley; RNA, Messenger; Sodium Chloride