Expression of prolactin-related hormones in the early bovine conceptus, and potential for paracrine effect on the endometrium.

Hormones related to the pituitary hormones PRL and GH are produced by the utero-placental unit of many species. In the cow, these include bovine placental lactogen (bPL) and a distantly related subfamily including the protein encoded by bovine PRL-related complementary DNA I (bPRCI). In the present studies, we defined the onset of expression of these genes in order to begin to study the regulation of their expression and function before implantation. Messenger RNA levels of both bPL and bPRCI were assessed by dot blot analysis of total RNAs prepared from conceptuses on days 15-25. Specific transcripts were not detectable at day 15 but were readily apparent beginning at day 17. To define the portions of the placenta responsible for expressing these genes, RNA was prepared from chorion (separated into cotyledons and intercotyledonary membrane), allantois, and amnion from day 58 gestation and analyzed by Northern hybridization. Transcripts for both these hormones were confined to the chorion, with intercotyledonary RNA containing at least as much as that prepared from cotyledons. To localize the product of these genes, extraembryonic membranes were immunostained with bPL and bPRCI antisera. Mononucleated trophoblastic cells stained for bPL and bPRCI at day 18; similar staining was apparent at days 23 and 24, as well as in some but not all binucleated cells. Microsomes prepared from endometrium exhibited specific binding of bPL throughout pregnancy as well as during the luteal phase of the cycle. At midgestation, these high affinity, low capacity receptors (dissociation constant of 1.27 x 10(-10) M, maximum binding capacity of 44.2 fmol/mg protein) demonstrated a selective affinity for bPL in preference to bGH and bPRL. Using this radioreceptor assay, we demonstrated the presence of bPL at nanomolar concentrations in washings from the uterine lumen, suggesting that bPL may exert paracrine effects on the endometrium.