CELLULAR-DISTRIBUTION OF PLACENTAL-LACTOGEN-II BINDING-SITES IN THE PREGNANT RAT

Recent studies from our laboratories have demonstrated that rat placen tal lactogen II (rPL-II) circulates in fetal rat serum and binds to va rious fetal tissues in late gestation. To identify potential sites of action of rPL-II in the pregnant mother, we characterized the binding of rPL-II to maternal rat liver and examined the cellular distribution of rPL-II binding sites in maternal tissues. In maternal liver, rPL-I I bound with high affinity (Kd 2 nM) to a single class of receptor sit es. Rat prolactin competed for rPL-II binding with one-fifth the poten cy of rPL-II while rat GH was without effect. Affinity cross-linking s tudies demonstrated that the hepatic rPL-II binding site has a molecul ar mass of 41 kDa. In addition to liver, rPL-II bound specifically to maternal ovary, choroid plexus, adrenal, kidney, duodenum and jejunum. In the ovary, rPL-II bound intensly to corpora lutea and to a lesser degree to ovarian follicular cells. In the adrenal gland, rPL-II bindi ng was detected throughout the cortex but was most intense in the zona reticularis. In the kidney, rPL-II bound primarily to cortical tubule s, while in the proximal small intestine, rPL-II binding localized to duodenal and jejunal crypts and villi. The presence of rPL-II in mater nal and fetal serum and the widespread binding of rPL-II to maternal a nd fetal tissues suggest that the hormone plays diverse roles in the a daptation of the mother and fetus to pregnancy.