Identification of a Gonadotropin-Releasing Hormone-Like Factor in the Rabbit Fetal Placenta 1

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Abstract
The biological and immunological gonadotropin-releasing hormone (GnRH)-like activities in rabbit fetal placentas collected at Day 18 of gestation were investigated. Both crude and partially purified acid extracts of placental tissue were tested. A similarly prepared liver extract served as a control. Immunological GnRH-like activity, determined through a GnRH radioimmunoassay was 1.3-2.0 pg/mg protein for the crude placental extract, 7.1-9.2 pg/mg protein for the partially purified placental extract and was nondetectable for liver extract. Both the crude and partially purified placental extracts increased (P < 0.01) luteinizing hormone (LH) release by dispersed rabbit pituitary cells, whereas the liver extract had no effect. The antagonist [N-acetyl-1,2-(p-chloro-D-phenylalanine)-3-D-tryptophan,6-D-arginine-10-D-alanine]-GnRH was used to determine whether the biological GnRH-like activity in the placental extract was mediated through GnRH receptors. All 3 doses of antagonist (10, 100 and 1000 ng) suppressed the biological GnRH-like activity in the placental extracts. Molecular sieve chromatography of the partially purified placental extract showed that the immunoreactive GnRH-like factor eluted in the same fractions as the GnRH standard. The rabbit fetal placenta has both immunological and biological GnRH-like activity.