oPL Binding Research Articles

Ovine placental lactogen specifically binds to endometrial glands of the ovine uterus.

A hormonal servomechanism has been proposed to regulate differentiation and function of the endometrial glandular epithelium (GE) in the ovine uterus during pregnancy. This mechanism involves sequential actions of estrogen, progesterone, ovine interferon tau (IFNtau), placental lactogen (oPL), and placental growth hormone (oGH). The biological actions of oPL in vitro are mediated by homodimerization of the prolactin receptor (oPRLR) and heterodimerization of the oPRLR and oGH receptor. The objectives of the study were to determine the effects of intrauterine oPL, oGH, and their combination on endometrial histoarchitecture and gene expression and to localize and characterize binding sites for oPL in the ovine uterus in vivo using an in situ ligand binding assay. Intrauterine infusion of oPL and/or oGH following IFNtau into ovariectomized ewes treated with progesterone daily differentially affected endometrial gland number and expression of uterine milk proteins and osteopontin. However, neither hormone affected PRLR, insulin-like growth factor (IGF)-I, or IGF-II mRNA levels in the endometrium. A chimeric protein of placental secretory alkaline phosphatase (SEAP) and oPL was used to identify and characterize binding sites for oPL in frozen sections of interplacentomal endometrium from pregnant ewes. Specific binding of SEAP-oPL was detected in the endometrial GE on Days 30, 60, 90, and 120 of pregnancy. In Day 90 endometrium, SEAP-oPL binding to the endometrial GE was displaced completely by oPL and prolactin (oPRL) but only partially by oGH. Binding experiments using the extracellular domain of the oPRLR also showed that iodinated oPL binding sites could be competed for by oPRL and oPL but not by oGH. Collectively, results indicate that oPL binds to receptors in the endometrial glands and that oPRL is more effective than oGH in competing for these binding sites. Thus, effects of oPL on the endometrial glands may be mediated by receptors for oPRL and oGH.

Identification of a tyrosine residue in ovine placental lactogen as essential for its binding to receptors

Nitration of ovine placental lactogen (oPL) with a 10-fold molar excess of tetranitromethane over protein content resulted in the modification of 0.8 tyrosine residue. No conformational changes were observed by either fourth-derivative spectral analysis or circular dichroism. Nitration significantly decreased the binding capacity of the hormone to lactogenic and somatogenic rat liver receptors. This binding capacity was not restored by reduction of the nitro groups, thus indicating that the decrease was not due to the difference in p K between tyrosine and nitrotyrosine. The nitroytrosine-containing peptide was isolated from a tryptic digest by HPLC and its modification extent was of 67%, which is consistent with the decrease in binding capacities (65% and 70%). Its amino acid sequence was determined and the modified tyrosine residue was identified as Tyr-46. These results provide the first evidence of the involvement of a tyrosine residue in the binding of oPL to both lactogenic and somatogenic receptors. This tyrosine appears to be a shared binding epitope between oPL and the prolactins.

Ontogeny of a specific high-affinity binding site for ovine placental lactogen in fetal and postnatal liver

Ovine placental lactogen (oPL) exerts actions in sheep and rodent fetal tissues that growth hormone (GH) does not. However, in postnatal tissues, both oPL and GH possess these activities. Although a high-affinity binding site for oPL in ovine fetal liver has been reported, some investigators believe this to be the GH receptor. It was our objective to discriminate between oPL and GH binding to fetal liver microsomes using competitive saturation analyses. Microsomal membranes from fetal liver (Days 60, 90, 105, 120, and 135 of gestation) and postnatal liver (1 wk of age) were incubated with increasing amounts of [ 125I]oPL in the absence or presence of a 100-fold molar excess of unlabeled oPL. Saturable binding of [ 125I]oPL was observed with fetal liver and postnatal liver microsomes. The K d of the oPL-binding site in fetal liver was 122.1 ± 8.2 pM (mean ± standard error), and receptor concentrations remained relatively constant (9.8 ± 1.1 fmol/mg of membrane protein) across gestation. The highest concentration of oPL binding was detected in 1-wk postnatal liver microsomes (53.0 fmol/mg of membrane protein). Saturation analyses using [ 125I]GH and [ 125I] prolactin (PRL) were also conducted with fetal liver membrane preparations. Although specific binding for these two radiolabeled ligands was observed in control tissues, no specific binding was observed in fetal liver. These data are in agreement with earlier reports that a high-affinity binding site for oPL exists in fetal tissues. The fact that saturable binding could not be demonstrated for either GH or PRL with fetal liver microsomes contradicts recent suggestions that oPL is binding the GH receptor.

Characterization of ovine growth hormone (oGH) and ovine placental lactogen (oPL) binding to fetal and adult hepatic tissue in sheep: evidence that oGH and oPL interact with a common receptor

The GH receptor (GHR) plays a key role in postnatal growth regulation. Although plasma concentrations of GH are high during fetal life, its role during fetal development is not well understood. Recent data suggest that GHR are present in fetal hepatic tissue as early as 51 days gestation. However, the levels of GHR expression are markedly lower in fetal hepatic tissue compared to postnatal values, and there are conflicting data suggesting that ovine placental lactogen (oPL) and oGH share a common receptor. Given the uncertainty about whether oPL acts via the oGHR or a distinct receptor, we performed ligand binding and affinity cross-linking studies on hepatic microsomal membranes from adult castrated male, pregnant female, and fetal sheep. Ligand binding assays at a constant concentration of membranes showed that [125I]oPL yielded consistently higher (P < 0.001) specific binding (59.5 +/- 6.4%, 30.5 +/- 5.7%, and 7.6 +/- 2.4% for castrated male, pregnant female, and fetal sheep, respectively) compared to [125I]oGH (17.8 +/- 4.7%, 5.0 +/- 1.6%, and 1.2 +/- 0.4% for castrated male, pregnant female, and fetal sheep, respectively). Cross-reactivity studies showed that unlabeled oPL was consistently more potent than unlabeled oGH in displacing either of the labeled ligands. The dissociation constant (Kd) for oPL binding ranged from 0.16-0.40 nM and was not changed by solubilization with Triton X-100. Equilibrium binding analysis for oGH showed lower affinity for hepatic microsomal membranes (Kd, 1.7-3.2 nM) in each of the three groups of animals. Affinity cross-linking of microsomal membranes from castrated male and pregnant female sheep liver showed four major cross-linked complexes with both [125I]oPL and [125I]oGH, with mol wt of 150, 97, 75, and 60 kilodaltons. All four bands were identified with both ligands. Unlabeled oPL showed markedly higher potency than unlabeled oGH in reducing the signal of the [125I]oPL cross-linked complexes, whereas unlabeled oGH and oPL showed comparable potencies in reducing the signal of the [125I]oGH complexes. Immunoprecipitation of detergent-solubilized hepatic microsomal membranes from pregnant and fetal sheep using a panel of monoclonal antibodies raised against the extracellular region of the rabbit GHR showed potent immunological recognition of the [125I]oPL-receptor complexes. We suggest that oGH and oPL bind to a common or a related receptor protein(s). It is possible that differences in receptor dimerization or association with other membrane proteins are the basis of the differences in affinity and biological actions of the two hormones.

Characterization of ovine growth hormone (oGH) and ovine placental lactogen (oPL) binding to fetal and adult hepatic tissue in sheep: evidence that oGH and oPL interact with a common receptor.

The GH receptor (GHR) plays a key role in postnatal growth regulation. Although plasma concentrations of GH are high during fetal life, its role during fetal development is not well understood. Recent data suggest that GHR are present in fetal hepatic tissue as early as 51 days gestation. However, the levels of GHR expression are markedly lower in fetal hepatic tissue compared to postnatal values, and there are conflicting data suggesting that ovine placental lactogen (oPL) and oGH share a common receptor. Given the uncertainty about whether oPL acts via the oGHR or a distinct receptor, we performed ligand binding and affinity cross-linking studies on hepatic microsomal membranes from adult castrated male, pregnant female, and fetal sheep. Ligand binding assays at a constant concentration of membranes showed that [125I]oPL yielded consistently higher (P < 0.001) specific binding (59.5 +/- 6.4%, 30.5 +/- 5.7%, and 7.6 +/- 2.4% for castrated male, pregnant female, and fetal sheep, respectively) compared to [125I]oGH (17.8 +/- 4.7%, 5.0 +/- 1.6%, and 1.2 +/- 0.4% for castrated male, pregnant female, and fetal sheep, respectively). Cross-reactivity studies showed that unlabeled oPL was consistently more potent than unlabeled oGH in displacing either of the labeled ligands. The dissociation constant (Kd) for oPL binding ranged from 0.16-0.40 nM and was not changed by solubilization with Triton X-100. Equilibrium binding analysis for oGH showed lower affinity for hepatic microsomal membranes (Kd, 1.7-3.2 nM) in each of the three groups of animals. Affinity cross-linking of microsomal membranes from castrated male and pregnant female sheep liver showed four major cross-linked complexes with both [125I]oPL and [125I]oGH, with mol wt of 150, 97, 75, and 60 kilodaltons. All four bands were identified with both ligands. Unlabeled oPL showed markedly higher potency than unlabeled oGH in reducing the signal of the [125I]oPL cross-linked complexes, whereas unlabeled oGH and oPL showed comparable potencies in reducing the signal of the [125I]oGH complexes. Immunoprecipitation of detergent-solubilized hepatic microsomal membranes from pregnant and fetal sheep using a panel of monoclonal antibodies raised against the extracellular region of the rabbit GHR showed potent immunological recognition of the [125I]oPL-receptor complexes. We suggest that oGH and oPL bind to a common or a related receptor protein(s). It is possible that differences in receptor dimerization or association with other membrane proteins are the basis of the differences in affinity and biological actions of the two hormones.

The induction of hepatic somatotrophic receptors after birth in sheep is dependent on parturition-associated mechanisms

While the GH receptor (GHR) plays a key role during adult life, its role during fetal development is not well understood. Recent data suggest that GHRs are present in ovine fetal hepatic tissue at mid-gestation. However, the levels of GHR expression are markedly lower in fetal hepatic tissue in comparison with postnatal values. The present study investigates whether the neonatal induction of the hepatic GHR and plasma levels of IGF-I follow a pattern of strictly age-related development or whether birth-associated processes stimulate this increase. Stereotaxic electrolytic lesioning of the fetal paraventricular nuclei was employed to prolong gestation markedly. We compared the hepatic binding of ovine placental lactogen (oPL) and oGH and plasma levels of IGF-I in these post-mature fetuses with those in pre-term fetuses, pregnant mothers and lambs which were of the same conceptional age as the post-mature fetuses. While specific binding of both 125I-labelled oGH and 125I-labelled oPL to hepatic microsomal membranes was fully reversible in all groups, the specific binding of 125I-labelled oGH was significantly (P < 0.001) lower than specific binding of 125I-labelled oPL in all groups of animals. There was no difference in specific 125I-labelled oGH or 125I-labelled oPL binding in livers or plasma levels of IGF-I in post-mature fetuses in comparison with pre-term fetuses. In contrast, a major increase (P < 0.001) in 125I-labelled oGH and 125I-labelled oPL binding and plasma IGF-I levels was observed in lambs.(ABSTRACT TRUNCATED AT 250 WORDS)

Placental lactogen-binding sites in isolated fetal fibroblasts: characterization, processing, and regulation.

Placental lactogen (PL) stimulates amino acid transport, DNA synthesis, and insulin-like growth factor production in isolated fetal fibroblasts and myoblasts. To clarify the mechanisms by which PL exerts its effects in fetal tissues, we have examined the binding and processing of PL and its receptor in cultured ovine fetal skin fibroblasts. Fetal sheep fibroblasts bound ovine PL (oPL) with high affinity (Kd, 0.2 nM) and ovine (o) GH (Kd, 1.6 nM) and oPRL (Kd, > 200 nM) with lower affinities, as recently reported. Maximal specific binding of oPL was noted after a 24-h incubation at 4 C. When fibroblasts were incubated with [125I]oPL at 4 C and then warmed to 37 C, the radiolabeled hormone was internalized within 2-4 min. Most of the internalized hormone, however, was recycled rapidly to the cell surface by 6-10 min and released intact by retroendocytosis into the incubation medium. Of the small amount of remaining internalized radioligand, approximately 50% appeared to be degraded, as assessed by solubility in trichloracetic acid. To directly examine the cellular processing of receptor proteins, we used the membrane-impermeant cross-linking reagent bis-(sulfosuccinimidyl)suberate (BS3) to detect oPL-binding sites confined to the cell surface. These studies detected a specific oPL-receptor complex with a mol wt of 130 kilodaltons, suggesting that the mol wt of the membrane receptor is 108 kilodaltons. At 37 C, oPL-receptor complexes were internalized rapidly, reducing cell surface binding activity to 25% of the initial values within 4 min. oPL receptor activity reappeared on the cell surface at 6 min, suggesting receptor recycling, but declined precipitously during the ensuing 30 min, reflecting receptor processing and turnover. After proteolysis of surface receptors with trypsin, binding activity was recovered fully during a 12- to 24-h incubation in serum-containing medium. Recovery of surface receptors was blocked by cyclohexamide (2 microM), suggesting that repopulation of receptors depends upon new protein synthesis. Specific binding of PL was 1.5- to 5-fold higher in cells preincubated in medium containing dexamethasone (0.1 microM), suggesting that glucocorticoids may regulate expression of the fetal PL receptor. These studies provide insight into the cellular mechanisms by which the fetus may regulate its sensitivity and response to PL.

Tissue distribution and ontogeny of growth hormone receptor messenger ribonucleic acid and ligand binding to hepatic tissue in the midgestation sheep fetus.

While circulating GH concentrations are high in fetal life, skeletal growth is only slightly reduced by GH deficiency in utero. This has been explained by the relatively low binding of GH to fetal hepatic tissue, suggesting a lack of GH receptors (GHR). The GHR also recognizes ovine placental lactogen (oPL), which may have a specific role either as a fetal growth-promoting hormone or in regulating fetal metabolism. We investigated GHR expression and membrane binding of ovine (o) GH and oPL in various ovine fetal tissues and in maternal liver at different gestational stages. Singleton-bearing ewes at 51, 95, and 120 days gestation were killed. Liver, muscle, kidney, and brain samples were taken from the fetuses as well as placentas and livers from the ewes (n = 3/gestational age). GHR mRNA measured by Northern blot analysis was expressed at high levels in maternal liver at all gestational stages. A major band was observed at 4.4 kilobases (kb), and three minor bands were observed at 2.5, 1.7, and 8.1 kb. In fetal and placental tissue, only the 4.4-kb band was detected. This was present as early as day 51 of gestation in liver, kidney, lung, heart, and placenta and increased slightly with advancing gestation. On day 51, the expression of GHR mRNA in muscle was negligible, but by day 95, muscle expressed higher concentrations than fetal liver. Placental samples showed only a slight signal, with no change over the gestational range studied. In situ hybridization revealed the placental mRNA to be primarily associated with the decidua. Hepatic tissue showed specific binding to [125I]oGH and [125I]oPL from 51 days gestation. [125I]oPL showed a higher [51 days, 17.9 +/- 1.9% (mean +/- SEM); 95 days, 11.5 +/- 1.6%; 120 days, 16.3 +/- 0.9%] specific binding to the liver membranes than [125I]oGH (51 days, 2.1 +/- 0.7%; 95 days, 2.6 +/- 0.3%; 120 days, 3.5 +/- 0.4%). We conclude that oGHR are present as early as day 51 of gestation in various tissues, including liver. The message appears later in skeletal muscle than in liver. As the GH receptor binds oPL with higher potency than oGH, the parallel ontogenic changes in [125]oGH and [125]oPL binding in the liver do not support the presence of a PL receptor under independent developmental regulation.

Binding of placental lactogen and growth hormone to fetal sheep fibroblasts.

Growth hormone (GH) regulates growth and development in the postnatal period but lacks somatotropic activity in the fetus. In contrast, the placental hormone placental lactogen (PL) stimulates amino acid transport, DNA synthesis, and somatomedin production in isolated fetal tissues, suggesting that PL may function as a "fetal GH." To elucidate the mechanisms by which PL exerts GH-like effects in fetal tissues, we examined the binding of PL, GH, and prolactin to cultured skin fibroblasts obtained from midgestational fetal lambs. Ovine fetal fibroblasts bound radiolabeled ovine PL (oPL) specifically and with high affinity (EC50 0.20 nM). In competitive displacement assays using 125I-oPL as the radioligand, the potency of unlabeled oPL was eight to 12 times greater than that of ovine GH and congruent to 1000 times greater than that of ovine prolactin. Covalent cross-linking of 125I-oPL (22 kD) to ovine fetal fibroblasts revealed a specific hormone-receptor complex with an apparent M(r) of 130,000, suggesting that the high affinity oPL binding site has a molecular mass of approximately 108 kD. The specific bindings of radiolabeled ovine GH (0.6% per 250 micrograms protein) and ovine prolactin (0.04% per 250 micrograms protein) were only 1/15 and 1/230 that of radiolabeled oPL (9.1% per 250 micrograms protein), and no specific cross-linking of 125I-ovine GH or 125I-ovine prolactin to ovine fetal fibroblasts was detected. These findings demonstrate preferential binding of PL by isolated fetal sheep fibroblasts in culture, providing a cellular mechanism whereby PL may exert growth-promoting effects in the fetus.

The placental lactogen receptor in maternal and fetal sheep liver: regulation by glucose and role in the pathogenesis of fasting during pregnancy.

To clarify the roles of glucose and insulin in the regulation of the PL receptor in fetal and maternal sheep liver, we administered iv glucose to pregnant ewes during a 72-h fast. The binding of ovine PL (oPL) to hepatic membranes from glucose-infused ewes and their fetuses was compared with the binding of oPL to tissues of fasted, saline-infused sheep and sheep fed normally ad libitum. Fasting of pregnant ewes caused a 58-70% reduction in the number of PL receptors in fetal and maternal liver. Intravenous administration of glucose during fasting increased the number of PL receptors in fetal liver by 137.4%. In contrast, glucose administration during fasting had no effect on the number of PL receptors in maternal liver. The number of PL binding sites in fetal liver correlated positively with fetal weight (r = 0.59) and length (r = 0.54) and with fetal plasma glucose (r = 0.69) and insulin (r = 0.55) concentrations. In contrast, PL binding was inversely related to fetal plasma oPL concentrations (r = -0.70). These findings suggested that glucose, insulin, and/or oPL may regulate PL binding in the ovine fetus. To determine whether glucose or insulin exert direct effects on the PL receptor in ovine fetal tissues, we examined the binding of radiolabeled oPL to ovine fetal hepatocytes and fibroblasts in culture. The specific binding of oPL to fetal hepatocytes was low and variable (1.0 +/- 0.5%) and it was not possible to assess reliably the effects of glucose or insulin supplementation. The specific binding of oPL to fetal fibroblasts (5.4 +/- 0.6%/mg) was unaffected by variations in media glucose concentrations (5.5-16.5 mM) or by pretreatment with insulin (10-1000 ng/ml). The results of these studies demonstrate that glucose and other nutritional factors regulate the expression of the PL receptor in fetal and maternal sheep liver. Alterations in PL binding play roles in the metabolic adaptation of the mother and fetus to nutritional deprivation and stress.

Nutritional Regulation of the Placental Lactogen Receptor in Fetal Liver: Implications for Fetal Metabolism and Growth*

We have recently identified and purified from fetal liver a distinct receptor that mediates the effects of placental lactogen (PL) on amino acid transport, glycogen synthesis, and somatomedin production in fetal tissues. At present, the factors that regulate the number and affinity of PL receptors in the fetus are unknown. Since maternal nutrition plays a critical role in fetal metabolism and growth, we have examined the role of nutrition in the regulation of the PL receptor in fetal lambs. Pregnant ewes at 123-126 days gestation were fed ad libitum (FED), fasted for 3 days (FASTED), or fasted for 3 days and then refed for an additional 3 days (REFED). The ewes were then killed, and the binding of [125I]ovine (o) PL to hepatic microsomes from the fetal lambs was examined. Maternal fasting caused a 60-75% reduction in the specific binding of oPL to fetal liver; the effect of fasting was reversed in part by refeeding [specific binding per mg protein: FED, 11.8 +/- 2.2% (n = 8); FASTED, 2.8 +/- 0.4% (n = 7); REFED, 7.2 +/- 2.6% (n = 3)]. The decrease in oPL binding resulted from an 80% reduction in the number of fetal oPL-binding sites (Scatchard analysis); there were no changes in the affinity of the oPL receptor (Kd, 0.6 nM), the subunit structure of the receptor, or the degree of occupancy of the receptor in vivo by endogenous fetal hormones. The specific bindings of GH (0.6%), PRL (0.3%), and insulin (35%) to fetal liver were not affected by maternal fasting, indicating that caloric restriction exerted a specific effect on oPL binding in the fetus. The number of fetal oPL-binding sites was positively correlated with the fetal liver glycogen content (r = 0.69; P less than 0.01) and the fetal plasma concentrations of glucose (r = 0.68; P less than 0.01) and insulin-like growth factor-I (r = 0.74; P less than 0.001), suggesting a role for the PL receptor in the regulation of fetal carbohydrate metabolism and growth. The number of fetal PL receptors was inversely correlated with the fetal plasma oPL concentration (r = 0.47; P less than 0.05). These studies demonstrate that fasting of the pregnant ewe reduces the number of PL receptors in ovine fetal liver. The reduction in fetal hepatic PL receptors may contribute to the mobilization and depletion of fetal liver glycogen stores and may play a role in the pathogenesis of the fetal growth retardation that accompanies maternal caloric deprivation.

Inhibition of Binding of Ovine Placental Lactogen to Growth Hormone and Prolactin Receptors by Monoclonal Antibodies*

From a single cell fusion, five stable hybridomas secreting antiovine placental lactogen (oPL) antibodies were obtained. Three of these secrete immunoglobulin (Ig)G subclass, and the other two secrete IgM class antibodies. Ascites fluids were raised in mice for each clone and were used as the antibody component for the development of solid phase RIA. Three solid-phase RIAs were successfully established using individual IgG subclass monoclonal antibodies, but the IgM class antibodies were ineffective. In all three individual solid-phase RIAs, the binding of [125I]iodo-oPL to the immobilized antibody was inhibited by unlabeled oPL, but not by ovine pituitary PRL (oPRL), ovine GH (oGH), or ovine pituitary extract. Two of the IgG subclass antibodies were able to inhibit the binding of [125I] iodo-oPL to PRL receptors(s) and to GH receptor(s) in rabbit mammary gland and liver, respectively. One of these two IgG subclass antibodies was more effective at inhibiting the binding of oPL to PRL receptor(s) in rabbit mammary gland, whereas the other one is more effective in inhibiting the binding of oPL to GH receptor(s) in rabbit liver. These antibodies, however, could only weakly inhibit the binding of [125I]iodo-oPRL to rabbit mammary gland and were ineffective in inhibiting the binding of [125I]iodo-oGH to rabbit liver. The addition of monoclonal antibodies in both radioreceptor assay (RRA) for PRL (RRA-PRL) and for GH (RRA-GH) did not affect the parallelism of the displacement curve of oPL standard. Our results suggest that oPL might contain two distinct binding sequence(s): one responsible for the binding of oPL to PRL receptor(s) and the other responsible for the binding of oPL to GH receptor(s). These two binding sequences might overlap or be located adjacent to one another. The interaction of monoclonal antibodies with these binding sequences of oPL may block the binding of oPL with PRL and GH receptor(s). Alternatively, our studies suggest that the monoclonal antibodies do not bind to hormone receptor(s)-binding sequence(s) in oPL, but the interaction between oPL and monoclonal antibody might alter the conformational structure of the oPL which will consequently lead to a lower binding of oPL to PRL and GH receptor(s).

Binding inhibition by monoclonal antibodies of ovine placental lactogen to growth hormone receptors in human liver

In the radioreceptor assay for growth hormone (RRA-GH) using [ 125I]iodo-hGH, hGH and human liver membrane particulate fractions as tracer, hormone standard and receptors, respectively, ovine placental lactogen (oPL) is capable of inhibiting the binding of [ 125I]iodo-hGH in a parallel manner with hGH and in equipotency. Similarly, in the RRA-GH by employing [ 125I]iodo-oPL, oPL and human liver membrane particulate fractions as tracer, hormone standard and receptors, respectively, hGH is also equipotent as oPL in inhibiting the binding of [ 125I]iodo-oPL in a parallel fashion. The addition of monoclonal antibodies against oPL in the assay was effective in inhibiting the binding of [ 125I]iodo-oPL to human liver, but could not, however, inhibit the binding of [ 125I]iodo-hGH to human liver. Furthermore, the addition of the monoclonal antibodies in the RRA-GH did not affect the parallelism of the oPL standard but lowered the total binding of oPL. Our studies indicate that the structure of the binding sequence in oPL which binds to the GH receptor of human liver is not identical to the equivalent sequence of hGH and that the monoclonal antibodies compete with GH receptors in human liver for the binding of oPL.

Distribution of Binding Sites for Ovine Placental Lactogen in the Sheep*

Specific binding sites for ovine placental lactogen (oPL) have been detected in the 100,000 × g pellet obtained from tissues derived from non-pregnant and pregnant ewes as well as from fetal tissues. Specific binding of ovine [125I]iodo-PL (percentage per milligram of protein) was found in non-pregnant ewe liver (28%), adipose tissue (20%), ovary (10%), corpus luteum (6%), uterus (5%), and fetal liver (5%). The specific binding of oPL was less than 3% in the following maternal tissues: mammary gland, adrenal, kidney, spleen, pancreas, lung, heart, skeletal muscle brain (cortex), and uterus near term (130–135 days’ gestation). Less than 3% specific binding was found also in several fetal tissues including adipose tissue, adrenal, kidney, spleen, pancreas, lung, heart, skeletal muscle, brain (cortex), and placenta near term (both fetal cotyledons and maternal caruncles). The binding of oPL to maternal liver, adipose tissue, ovary, corpus luteum, and fetal liver was inhibited by ovine GH but not by ovine PRL or ovine LH. In the radioreceptor assay for oPL using liver (100,000 × g pellets) from the ewe, ovine [125I]iodo-PL was inhibited by several GH preparations and by human placental lactogen. PRL preparations from several species as well as other pituitary hormones did not compete for the binding sites. Conversely, oPL only slightly inhibited the binding of ovine [125I]iodo-PRL in the radioreceptor assay for oPRL using pregnant ewe uterus. These observations suggest that oPL may act more like a GH than a PRL, and hence may mediate important metabolic effects during the greater part of pregnancy in the ewe. (Endocrinology102: 632, 1978)