Journal of Clinical Endocrinology & Metabolism, Vol 66, 283-289, Copyright © 1988 by Endocrine Society

ARTICLES

Functional opioid receptor sites in human placentas

S Belisle, A Petit, N Gallo-Payet, D Bellabarba, JG Lehoux and S Lemaire
Department of Obstetrics and Gynecology, Faculty of Medicine, University of Sherbrooke, Quebec, Canada.

We characterized the presence of opioid peptide receptor sites in plasma membranes and cells from human midterm and term placentas. Incubations with [3H]ethylketo-cyclazocine (EKC) at increasing doses revealed the presence of high affinity, low capacity, opioid peptide receptor-specific binding of the kappa-type. Scatchard analysis of the binding data showed, in the plasma membranes, linear plots at both stages of pregnancy with similar mean equilibrium association constants of 1.31 +/- 0.29 (+/- SE) X 10(9) mol/L-1 (n = 4) at midterm and 0.52 +/- 0.63 X 10(9) mol/L-1 at term (n = 4). In placental cells (n = 4) from term gestations, the binding plots were curvilinear; the first component had a Ka of 5.51 +/- 0.50 X 10(9) mol/L-1, and the second component had a Ka of 1.33 +/- 0.81 X 10(8) mol/L-1 (P less than 0.01). When standardized per mg tissue protein, the number of binding sites in plasma membranes increased from 13.8 +/- 9.8 fmol at midterm to 50.0 +/- 18.6 fmol at term (P less than 0.05). For term placental cells, the concentration of binding sites was 81.2 +/- 36.0 fmol for the high affinity sites and 713 +/- 390 fmol for the lower affinity sites. Specificity for the kappa-type of OPR was found based on the inability of mu- or delta-opioid peptides, as well as LHRH and TRH, to compete for [3H]EKC binding. Term placental cells incubated with various doses of opioid peptides had a 50% increase in placental lactogen production. The increase was significantly higher than controls only with kappa- agonists (P less than 0.05), maximal with 10(-9) mol/L EKC, and completely inhibited by 5 X 10(-6) mol/L naloxone. These results expand on previous data demonstrating the presence of opioid peptide receptor in placental plasma membranes and suggest a role for opioid peptides in regulating secretion of placental lactogen by placental cells.

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