Identification and characterization of the ζ-opioid receptor in developing rat heart

Opioid growth factor (OGF; [Met⁵]enkephalin) inhibits DNA synthesis of epicardial and myocardial cells in the neonatal rat heart in a receptor- mediated fashion. Ligand binding assays using newborn rat heart and [³H][Met⁵]enkephalin were performed to characterize the receptor responsible for the cell replicative effects of OGF in developing heart. Specific and saturable binding was detected, and Scatchard analysis revealed that the data were consistent for a single binding site with a binding affinity of 6.8 ± 0.3 nM and a binding capacity of 21.8 ± 1.9 fmol/mg protein. Subcellular fractionation studies revealed that binding was restricted to the nuclear fraction. Competition experiments showed that cold [Met⁵]enkephalin was the most effective ligand at displacing [³H][Met⁵]enkephalin. Binding was recorded on gestation days 18 and 20, reached its highest level at birth, and steadily decreased from postnatal days 1 to 15; binding at days 21 and 35 and in adults was negligible. The function, pharmacological and biochemical characteristics, distribution, and subcellular location of this OGF receptor in developing mammalian heart are consistent with the ζ-opioid receptor.