Ontogeny of Endogenous Secretion of Immunoreactive-Thyrotropin Releasing Hormone by the Human Placenta

We studied endogenous production of immunologically active TRH (ir-TRH) by human placenta throughout gestation. Fragments (20 g) of placentae obtained between 7 and 41 weeks' gestation were incubated in TC-199 media with or without TRH degrading enzyme inhibitors (1 mM of dithiothreitol, 200 microm O-phenanthroline, 0.8 mM EDTA) at 37 C for 24 h. TRH was quantitated by RIA. Release of ir-TRH was also studied in an in vitro model of dually-perfused isolated lobule of human term placenta. Cellular localization of TRH was performed by staining early-, mid- and late-gestation placentae with anti-TRH rabbit polyclonal antibody, using the indirect avidin-biotin complex immunoperoxidase method. TRH was produced by placental fragments from 7 to 41 weeks' gestation. Placental TRH secretion was maximal between 7-12 weeks gestation both in presence (655 +/- 79 pg/10 mg protein) and absence (423 +/- 75 pg/10 mg protein) of enzyme inhibitors. Secretion of TRH declined with increasing gestation both with (y = 779 - 15x; r = 0.90; P < 0.001; n = 15) and without (y = 525 - 12x; r = 0.87; P < 0.001; n = 15) enzyme inhibitors. In the perfusion experiments, endogenous TRH was released predominantly into the fetal circulation, and its concentration was markedly higher in the presence of enzyme inhibitors (146 +/- 27 vs. 34 +/- 7 pg/mL; P < 0.001). Immunostaining of chorionic villi localized TRH to the cytoplasm of the syncytial layer, and the intensity declined with advancing gestational age. These data suggest that immunoactive TRH is produced by human placenta begin at 7 weeks gestation, and production declines with gestational age.