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Placenta as a physiological source for oxytocic factor was implicated in our previous report, where we investigated the serum concentration of oxytocin (OT) and neurophysins during pregnancy, labor and lactation. The existence of immunoreactive(ir)-OT in placenta was suggested first by our study of the serum concentration in the umbilical vein, artery, and maternal circulation. The concentration of ir-OT in the placenta in the second trimester (20.8 ng/g-tissue) was 12-fold of that in the first trimester (1.6 ng/g-tissue). An immunohistochemical study on term placenta with the antiserum to OT revealed that ir-OT was mainly localized in the layer of syncytiotrophoblast. In vitro culture of trophoblast indicated that cycloheximide and prostaglandins (PGs) modulated the synthesis and/or release of ir-OT in the placenta. Further, the placental extract showed sings of mimicry of synthetic OT both in the elution profile through a carboxymethyl cellulose column and in bioactivity assayed by isometric tension of uterine muscle. The bioactivity and immunoreactivity of the extract were compared in the fractionated eluent of the extract on a Sephadex G-25 column. The results indicated that the bioactivity could be parallel with the immunoreactivity assayed by our RIA system, which was consistent with the fact that our antiserum neutralized the bioactivity of OT. We recently obtained a cDNA clone by immunoscreening from a human placental library, and the clone size was approximately 900 nucleotides. The clone was transfected into a Chinese hamster ovary cell line, the cultured media of which contained the recombinant ir-OT proved by assays of both immunoreactivity and bioactivity. A search for the major contractile factor in the initiation of labor is in progress. V

Oxytocin-like Substance in Human Placenta: Search for Uterine Contractile Substance