TRANSPORT AND METABOLISM OF THYROTROPIN-RELEASING-HORMONE ACROSS THE FETAL MEMBRANE

To determine the transfer and metabolism of TRH by human fetal membran es, the bidirectional transport and uptake of TRH was investigated by adding I-125-labeled TRH (100,000 cpm) or commercial TRH either to the maternal or the fetal compartment of an in vitro model of cultured hu man fetal membranes obtained from term and preterm placenta. Transmemb rane transfer was also studied in the presence of 200 mu M p-hydroxy-m ercuriphenyl-sulphonic acid (p-HMSA), a dipeptidase enzyme inhibitor. Creatinine and heparin were used as an internal markers. Metabolites o f TRH were separated from intact molecules by gel filtration on Sephad ex G-10. The structural integrity of the membrane was confirmed by ele ctron microscopy. The transmembrane transfer of radiolabeled and comme rcial TRH were comparable across both preterm and term placenta. When transport was studied from the maternal to fetal side, the maternal co ncentration of TRH declined rapidly from 100% at time 0 to 19.31 +/- 2 .26% at 8 h with a concomitant increase in the fetal concentration fro m undetectable to a maximum of 2.56 +/- 0.38% with a fetomaternal rati o of 0.16 +/- 0.01. Transfer of TRH from the fetal to maternal compart ment was similar to that of maternal to fetal. Chromatography of mater nal and fetal media showed that TRH was metabolized by the membrane in to small molecular weight fragments. Treatment of the membrane with p- HMSA increased TRH transport from the maternal to fetal compartment to 18.12 +/- 0.91 (P < 0.001) with an fetomaternal ratio of 0.35 +/- 0.0 2 (P < 0.001). Although transmembrane transfer of TRH from the fetal t o maternal side was also increased by p-HMSA, levels achieved were les s than that from maternal to fetal (12.26 +/- 1.50%; P < 0.05). These results suggest that the human fetal membrane acts as an enzymatic bar rier to the bidirectional transfer of TRH from 24 weeks gestation.