The magnitude of cardiac hypertrophy is altered with advancing age and is accompanied by an increase in cardiac opioid peptide production. Previous work in the laboratory has shown that opioid peptides can directly modulate cardiac myocyte contractile function and inhibit catecholamine induced inotropy by affecting signal transduction through the beta- adrenergic pathway. Since cardiac hypertrophy is believed to be a regulated in part by the beta-adrenergic signal transduction pathway, we hypothesized that cardiac opioids regulate the hypertrophic process. The objectives of this study were to determine whether the cardiac response to hypertrophic stress is associated with changes in opioid mRNA and peptide levels in rat heart, and whether opioid gene expression in the heart is mediated by adrenergic stimulation. Cardiac opioid production was upregulated 2-3-fold with advancing age, and proenkephalin (PNK) gene expression was elevated in the heart during aging (7-fold), and during chronic pressure overload hypertrophy leading to heart failure (2-fold). Paradoxically, PNK gene expression is transiently downregulated after 3 days of aortic constriction, and this transient decrease was abolished by alpha-adrenergic blockade. Results of studies on cultured neonatal cardiac myocytes indicate that: 1) PNK mRNA abundance is increased in myocyte cultures supplemented with norepinephrine (NE), peaking at 4 hours after addition, 2) alpha- and beta-adrenergic receptor stimulation act synergistically to increase the expression of PNK, and 3) myocyte cultures of high purity permit the induction of hypertrophy marker genes but not PNK by NE treatment, while the opposite is true for cultures containing cardiac fibroblasts. The mutually exclusive expression of PNK and markers of hypertrophy suggests that opioid peptides may inhibit cardiac myocyte hypertrophy. In addition, the level of PNK induction achieved in mixed cultures by NE treatment is sufficiently higher than that in cultures of cardiac fibroblasts, suggesting that a myocyte- fibroblast interaction exists which enhances the PNK expression in one or both of these cell types.
