Prolactin binding in the developing rat fetal liver

Developmental expression of the growth hormone receptor gene in rabbit tissues

Total RNA from several adult (6-18-month-old) rabbit tissues was characterized using an oligonucleotide probe derived from the extracellular domain of the nucleotide sequence of the rabbit growth hormone receptor (GH-R) cDNA. Multiple GH-R mRNA species of approximately 4.6, approximately 3.3, 2.1 and approximately 1.4 kb were detected. The major 4.6 kb transcript was detectable in all tissues examined but with quite marked abundance differences. The highest level of expression was observed in liver followed closely by muscle. A qualitative assessment of insulin-like growth factor I (IGF-I) mRNA abundance was made in these same tissues. The data showed that the tissue abundance of GH-R mRNA was not necessarily parallel to that of IGF-I mRNA. The ontogeny of GH-R mRNA was studied in rabbit liver, muscle, heart and kidney. Low levels of GH-R mRNA were detectable in all fetal tissues studied except kidney which showed relatively high levels, suggesting that GH may play an important role in early kidney development. The overall developmental pattern of GH-R mRNA was similar in heart, muscle and liver, being low in fetal and early neonatal (day 3) periods and reaching maximal levels between 2 and 6 months. However, in kidney the pattern contrasted markedly. Relatively high levels of GH-R mRNA were observed in fetal and early neonatal (day 3) kidney with little change throughout development. The developmental pattern of IGF-I gene expression was not necessarily co-ordinately regulated with the ontogenic pattern of GH-R gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Placental lactogen receptors in maternal sheep liver: effects of fasting and refeeding

In a recent study we demonstrated that fasting of the pregnant ewe reduces the number of placental lactogen (PL) receptors in fetal sheep liver. In the present study we examined the effects of a 72-h fast on the number and affinity of PL receptors in maternal sheep liver. Fasting caused a 57% reduction in the number of hepatic ovine PL receptors; this effect was reversed by refeeding. There were no changes in the affinity of the PL receptor, the subunit structure of the receptor, or the extent of occupancy of the receptor by endogenous circulating maternal hormones. The number of hepatic PL receptors was correlated positively with the maternal plasma concentrations of glucose and insulin, suggesting that these factors may regulate PL binding to maternal tissues during pregnancy. In addition, PL receptor number was correlated positively with maternal plasma insulin-like growth factor I (IGF-I) concentrations, suggesting that a reduction in hepatic ovine PL binding may contribute to the reduction in maternal IGF-I during fasting. Fasting produced a 72% reduction in the number of ovine growth hormone receptors in maternal liver and an 83% increase in hepatic insulin binding. These findings indicate that fasting of the pregnant ewe reduces the number of PL receptors in maternal and fetal liver. The reduction in PL binding may contribute to maternal and fetal hyposomatomedinemia and may play a role in the pathogenesis of the intrauterine growth retardation that accompanies maternal nutritional deprivation.

Purification of a distinct placental lactogen receptor, a new member of the growth hormone/prolactin receptor family

Recent findings from this laboratory suggest that the biological actions of placental lactogen (PL) in mammalian fetal tissues are mediated through binding of the hormone to a distinct and unique PL receptor. We have now purified this receptor from fetal and maternal sheep liver, characterized its binding to PL, growth hormone (GH), and prolactin (PRL), and determined its molecular weight by SDS-PAGE and by affinity cross-linking techniques. Soluble extracts containing specific, high-affinity (Kd 0.5 nM) PL binding activity were prepared by incubating ovine fetal and maternal liver microsomes with 1% Triton X-100. The detergent solubilized PL receptor was purified two- to threefold by ion-exchange chromatography and an additional twofold by gel exclusion chromatography on Sepharose 6B. The PL receptor was then purified 75,000- to 125,000-fold by affinity chromatography using a column of ovine PL (oPL) coupled to Affi-Gel 10. The molecular weight of the oPL receptor as determined by SDS-PAGE and by cross-linking techniques was 44,000 +/- 2,000 (range 40,000-48,000). The purified receptor bound 125I-oPL specifically and with high affinity (Kd 0.5 nM) but did not bind either radiolabeled ovine GH or ovine PRL. In addition, in competition studies using 125I-oPL as the radioligand, the purified PL receptor bound unlabeled oPL with a potency 30-50 times greater than that of ovine GH and 500-1,000 times greater than that of ovine PRL. These findings demonstrate the presence of a specific PL receptor in fetal and maternal sheep liver. The PL receptor, together with the GH and PRL receptors, constitute a family of distinct but related hormone receptors that differ in their relative affinities for PL, GH, and PRL. Changes in the expression of the three receptors may mediate changes in the hormonal control of growth during the transition from fetal to postnatal life.

The ontogeny of specific prolactin binding sites in the rat choroid plexus

The development of prolactin receptors in the choroid plexus of the rat was examined using the in vivo autoradiographic approach employing the principle of competitive binding. Experimental animals were injected with [125I]prolactin alone (total binding) while control animals received [125I]prolactin and a 500-fold excess of unlabelled prolactin (non-specific binding). Newborns as well as animals 10, 14 and 18 days postnatal were studied. Three minutes following hormone injection animals received an intracardiac perfusion with fixative and tissues were prepared for quantitative light microscopic autoradiography. The choroid plexus first demonstrated specific binding of prolactin, i.e. a statistically significant difference in the autoradiographic reactions between experimental and control animals, at 14 days postnatal. The lactogen specificity of these binding sites was further defined by the ability of [125I]prolactin to be displaced by unlabelled human growth hormone, which is lactogenic in rats, and not by unlabelled insulin, which is structurally dissimilar to prolactin. Morphometric analyses were performed on electron micrographs of choroid plexus from 10- and 14-day postnatal rats. The volume densities of constituents known to be enriched in polypeptide hormone receptors were measured and compared. Small cytoplasmic vesicles and tubules were statistically significantly more abundant in 10-day-old rats than in 14-day-old animals. It is conjectured that these vesicles and tubules contain an intracellular pool of prolactin receptors whose decrease at 14 days parallels the expression of specific binding sites at the cell surface.