Age-associated increase in rat cardiac opioid production

Several lines of evidence suggest that opioid peptide production in the heart may increase during aging from adulthood through senescence. We tested the hypothesis that cardiac opioid peptides and preproenkephalin (PNK) mRNA would increase with advancing age. Ventricles and atria from male Wistar rats, aged 2, 6, 18, and 22-24 mo of age, were acid extracted and assayed for methionine enkephalin (ME) and leucine enkephalin (LE). Total RNA was extracted from hearts of age-matched rats and probed for PNK mRNA by Northern blot analysis using a cDNA probe. ME and LE peptides were significantly elevated with advancing age in both ventricles [P < 0.001; by analysis of variance (ANOVA)]. Ventricular ME concentration exhibited a biphasic increase with approximately twofold higher peaks at 6 and 22-24 mo of age compared with the 2 mo value of 1.04 +/- 0.05 (SE) pmol/g wet wt. In contrast, ventricular LE concentration was largely unchanged until 22-24 mo of age when it increased approximately threefold over the 2-mo value of 2.19 +/- 0.14 pmol/g wet wt. Left ventricular PNK mRNA increased approximately fivefold between 2 and 18 mo of age (P < 0.01; ANOVA). Thus both enkephalins and the mRNA coding for them were increased in hearts of older vs. younger rats. Because opioid receptor stimulation can negatively modulate several characteristics of cardiac myocyte contraction, these results may have important functional implications for the senescent heart.