PREGNANCY LACTOGENS IN THE RAT CONCEPTUS AND FETUS - CIRCULATING LEVELS, DISTRIBUTION OF BINDING, AND EXPRESSION OF RECEPTOR MESSENGER-RIBONUCLEIC-ACID

To clarify the roles of the rat placental lactogens in embryogenesis a nd fetal development, we measured the concentrations of rat placental lactogen-II (rPL-II) in fetal rat serum and examined the distribution and expression of rPL-I- and rPL-II-binding sites in rat uteroplacenta l and fetal tissues. The concentration of rPL-II in fetal rat serum on day 20 of gestation was 28.3 +/- 0.8 ng/ml (mean +/- SEM; n = 6), app roximately 1/14th its concentration in maternal serum (398.3 +/- 45.3 ng/ml; n = 6). In the midgestational uterus and placenta, rat PL-1 bou nd specifically to mesometrial decidua and to a capsular layer of stro ma overlying the antimesometrial decidua. The binding of radiolabeled rPL-I to these tissues was inhibited by unlabeled rat PRL and human (h ) GH, but not by rat GH, suggesting that the rPL-I-binding sites are l actogenic in nature. In the late gestational fetus, rat PL-II bound sp ecifically to fetal adrenal, kidney, small intestine, liver, and pancr eas; its binding, like that of rPL-I, was inhibited by rPRL, but not b y rGH. rPL-II-binding sites in fetal adrenal were detected as early as day 16, whereas rPL-II-binding sites in fetal kidney and small intest ine were not demonstrable until day 18. Lactogenic binding sites in fe tal liver and pancreas did not appear until days 19-20. The relative a mounts of specific binding of rPL-II to fetal tissues correlated posit ively with tissue levels of expression of the 4.2- and 1.8-kilobase PR L receptor mRNA transcripts. Radiolabeled hGH, which interacts with so matogenic receptors as well as lactogenic receptors, bound specificall y to mesometrial decidua, fetal adrenal, kidney, small intestine, live r, and pancreas. In addition, radiolabeled hGH bound specifically, but with low intensity, to fetal brain. In mesometrial decidua and fetal adrenal, kidney, and small intestine, the binding of hGH was blocked b y rPL-II and rPRL, but not by rGH or ovine GH, suggesting the predomin ance of lactogenic receptors. In contrast, in fetal brain, the binding of hGH was inhibited by rGH, but not by rPL-II, suggesting that the f etal brain contains somatogenic receptors. The presence of rPL-I-bindi ng sites in maternal decidua suggests a paracrine role for the hormone in decidual function at midgestation. The presence of rPL-II in fetal serum and the widespread distribution of rPL-II-binding sites in feta l tissues indicate a role for rPL-II in fetal development.