Distinct placental lactogen and prolactin (lactogen) receptors in bovine endometrium

Roles of conceptus secretory proteins in establishment and maintenance of pregnancy in ruminants

Reproduction in ruminant species is a highly complex biological process requiring a dialogue between the developing conceptus (embryo-fetus and associated placental membranes) and maternal uterus which must be established during the peri-implantation period for pregnancy recognition signaling and regulation of gene expression by uterine epithelial and stromal cells. The uterus provide a microenvironment in which molecules secreted by uterine epithelia and transported into the uterine lumen represent histotroph, also known as the secretome, that are required for growth and development of the conceptus and receptivity of the uterus to implantation by the elongating conceptus. Pregnancy recognition signaling as related to sustaining the functional lifespan of the corpora lutea, is required to sustain the functional life-span of corpora lutea for production of progesterone which is essential for uterine functions supportive of implantation and placentation required for successful outcomes of pregnancy. It is within the peri-implantation period that most embryonic deaths occur in ruminants due to deficiencies attributed to uterine functions or failure of the conceptus to develop appropriately, signal pregnancy recognition and/or undergo implantation and placentation. The endocrine status of the pregnant ruminant and her nutritional status are critical for successful establishment and maintenance of pregnancy. The challenge is to understand the complexity of key mechanisms that are characteristic of successful reproduction in humans and animals and to use that knowledge to enhance fertility and reproductive health of ruminant species in livestock enterprises.

Native and recombinant bovine placental lactogens

The bovine placenta produces a wide variety of proteins that are structurally and functionally similar to the pituitary proteins from the GH/PRL gene family. Bovine placental lactogen (bPL) is a 200-amino acid long glycoprotein hormone that exhibits both lactogenic and somatogenic properties. The apparent molecular masses of purified native (n) bPL molecules (31-33 kDa) exceed 23 041 Da, which is the theoretical molecular mass of the protein core. At least six isoelectric variants (pI: 4.85-6.3) of bPL were described in cotyledonary extracts and three different bPL isoforms (pI: 4.85-5.25) were found in fetal sera. The bPL molecules that are detected in higher concentrations in peripheral circulation exhibit a more acidic pI than those present in placental homogenates. This may reflect an important glycosylation process occurring just prior to the bPL secretion. The bPL mRNA is transcribed in trophectoderm binucleate cells starting from Day 30 of pregnancy until the end of gestation. In mothers, bPL is involved in the regulation of ovarian function, mammogenesis, lactogenesis, and pregnancy stage-dependent adaptation of nutrient supplies to the fetus. Due to the higher fetal, compared to maternal concentrations of circulating hormone, it has been suggested that bPL primarily targets fetal tissues.

Gene expression and maintenance of pregnancy in bovine: roles of trophoblastic binucleate cell-specific molecules

Cell to cell interaction plays a pivotal role in the regulation of placentogenesis and exchange of stage-specific developmental signals between the fetal and maternal units. Specifically, these interactions are paramount for programmed fetal growth, maternal adaptation to pregnancy and coordination of parturition. However, little is known about the precise regulation of placentation and maintenance of gestation in cattle. Therefore, the aim of the present study was to decipher the complex networks of cell communication to gain an insight into the multifaceted developmental process and understand the profound consequences of flawed communication. In the ruminant, the binucleate cell plays a central role in forming the structures and secretions at the fetomaternal interface that are crucial in establishing and maintaining pregnancy. Herein, we summarise differences in the abundance of specific RNA transcripts in the bovine cotyledon and caruncle using global gene expression profiling and further investigate the relationship of mRNA abundance for selected pregnancy-specific genes of interest (identified from microarray studies) that are localised exclusively to the binucleate cell, such as placental lactogen, prolactin-related proteins and pregnancy-associated glycoproteins. The results suggest that a well-orchestrated transcriptional command from binucleate cells is pivotal to the establishment and progression of pregnancy in cattle.

Expression and purification of human placenta lactogen in Escherichia coli

There are many growth factors secreted by placenta including growth hormone, placenta lactogen (PL), prolactin, follicle stimulating hormone, luteinizing hormone, thyroid stimulating hormone, and chorionic gonadotropin. For a systematic study of how these growth factors work together to result in the various biological functions and future clinical applications, it is needed to produce enough quantities of each protein. In this paper, we report the cloning of human PL (hPL) and expression by Escherichia coli (E. coli). Four kinds of expression vectors containing the hPL gene were transformed into several kinds of suitable host strains and grown at 37 and/or 30 degrees C. Determination of the yield of recombinant hPL by SDS-PAGE reveals that among the various conditions, pQE30-PL in E. coli strain M15[pREP4] expressed the largest amount of recombinant hPL at 37 degrees C. However, the expressed recombinant hPL was accumulated in inclusion body forms. The inclusion bodies were solubilized in 8M urea and purified by a His6 tagged affinity column under denaturing condition and the final yield of hPL was determined to be 48 mg/L. Intra-chain disulfide bonds could be formed either by oxidation in the refolding buffer or by air oxidation in the presence of urea. The biological activity was examined by the fact that hPL could stimulate erythroid maturation by the formation of hemoglobin in K-562 cells in the presence of erythropoietin. Initial optimization studies resulted in the production of 282.4 mg/L of hPL.

Temporospatial expression of placental lactogen and prolactin-related protein-1 genes in the bovine placenta and uterus during pregnancy

The anatomical location of binucleate cells (BNC) influences protein expression but not steroid synthesis in ruminants. In order to determine if BNC in disparate locations differentially express bovine placental lactogen (bPL) and prolactin-related protein-1 (bPRP-1), we quantitated bPL and bPRP-1 transcripts in placentomal (cotyledonary, caruncular) and interplacentomal (intercotyledonary, intercaruncular) tissues throughout pregnancy in the bovine using real-time reverse transcription PCR (RT-PCR) and in situ hybridization. Levels of both bPL and bPRP-1 transcripts at peri-implantation were significantly higher (P < 0.01) in the fetal membrane than in caruncular and intercaruncular tissues. Thereafter, mRNA for these related proteins demonstrated different spatial as well as temporal patterns of expression. Levels of bPRP-1 transcripts peaked at day 60 of pregnancy. Between day 60 and 100, bPRP-1 transcripts fell by approximately sevenfold (P < 0.01) in cotyledonary and intercotyledonary tissues, and fourfold in caruncular (P < 0.01) tissue. Levels of bPRP-1 transcripts remained low in the cotyledonary, intercotyledonary, and caruncular tissues until peripartum. In contrast, bPL expression in placentomes increased with progression of gestation (P < 0.01), but decreased in interplacentomal tissue around peripartum. To conclude, disparate patterns of bPRP-1 and bPL genes are transcribed in the placentomal and interplacentomal tissues during gestation in the bovine, suggesting that these prolactin-like hormones serve distinct functions and are regulated differently in the uteroplacental unit in this species.

Evaluation of the somatogenic activity of bovine placental lactogen with cell lines transfected with the bovine somatotropin receptor

Studies have shown that bovine placental lactogen (bPL) has partial somatogenic activity in vivo even though binding results clearly indicate bPL does not cause homodimerization of the bovine somatotropin receptor (bST-R). To help understand the receptor binding versus biological activity of bovine somatotropin (bST) and bPL we have developed a homologous model system. Full length bST-R was stably transfected into a murine lymphoid cell line, Ba/F3 and a hamster kidney cell line, BHK. From both transfected cell lines, clones were isolated (Ba/F3-C1 and BHK-24) which demonstrated specific binding of bST and, or bPL. Bovine ST stimulated proliferation of the Ba/F3-C1 clonal line over a dose range of 10 to 3000 pM with an EC50 of 100 pM. A bST variant (des 1-4 bST) and porcine ST (pST) which both have approximately 10% of the binding affinity for bST-R as native bST were 1 and 10% as potent as bST in this bioassay, respectively. This suggests that affinity and biological activity are correlated for this system. Proliferation was initiated through the bST-R because addition of a monoclonal antibody which recognizes the extracellular domain of bST-R and inhibits binding of bST to its receptor, inhibited bST-stimulated mitosis. However, even though the affinity of bPL for the bST-R is similar to that of bST, bPL antagonized the proliferative action of bST with an IC50 of 1 nM. Components of the somatogenic signal transduction pathway were also evaluated in both cell lines. Addition of bST to the cell cultures increased phosphorylation of JAK2 in Ba/F3-C1 and BHK-24 cells in a dose-responsive manner but bPL failed to increase phosphorylation of JAK2 in either cell line. In summary, these data support the hypothesis that ST-R homodimerization is necessary for bioactivity in this model system but fail to explain apparent somatogenic activity of bPL in vivo.

Ruminant placental lactogens act as antagonists to homologous growth hormone receptors and as agonists to human or rabbit growth hormone receptors

Growth hormone receptor (GHR)-mediated activity of ruminant placental lactogens (PLs) and ovine (o) GH was compared, using cells transfected with full size human (h), rabbit (rb), and oGHRs. All three PLs acted as agonists in heterologous bioassays, whereas in homologous bioassays in cells transfected with oGHRs they antagonized the oGH activity. Despite these differences, oGH and PLs bound with similar affinity to the oGHR extracellular domain (oGHR-ECD), indicating that the binding occurs through hormone site I. Gel filtration of complexes between oPL and oGHR-ECD showed a 1:1 stoichiometry, confirming this conclusion. The oPL T185D and bPL T188D, which exhibited weak biological activity mediated through GHRs, behaved as site I antagonists, whereas oPL G130R and bPL G133R formed a 1:1 complex with GHR-ECDs and bound to h/rb/oGHR-ECDs with affinity similar to that of wild-type oPL. They had no agonistic activity in all models transfected with h/rb and oGHRs, but were antagonistic to all of them. In conclusion, ruminant PLs antagonize the activity of oGH in homologous systems, because they cannot homodimerize oGHRs, whereas in heterologous systems they act as agonists. The structural analysis hints that minor differences in the sequence of the GHR-ECDs may account for this difference. Since the initial step in the activity transduced through cytokine/hemapoietic receptors family is receptor homodimerization or heterodimerization, we suggest that the question of homologous versus heterologous interactions should be reexamined.

Site-directed mutagenesis of recombinant bovine placental lactogen at lysine-73 leads to selective attenuation of its somatogenic activity

Bovine placental lactogen (bPL) is capable of binding and transducing biological activity via somatogenic and lactogenic receptors. To modify this capability, three analogs, bPL(K73D), bPL(K73F) and bPL(K73A), mutated at position 73, and corresponding to R64 in human GH (hGH), were produced in Escherichia coli. Circular dichroic spectrum analyses indicated proper refolding in all cases. Biological activity of these analogs was tested in vitro. In a lactogenic-receptor-mediated Nb2 rat lymphoma cell bioassay, bPL and its analogs acted similarly. In another lactogenic bioassay that measures beta-casein synthesis by HC-11 mouse mammary-gland cells, the analogs were 30-40% as potent as bPL. In contrast, somatogenic receptor-mediated bioactivity in FDC-P1 cells transfected with either rabbit (rb) or hGH receptor (R) was almost completely abolished in these analogs. In receptor binding assays, the effect was more conspicuous and the mutations affected not only somatogenic but also lactogenic binding. Binding to rat (r) and rabbit PRL receptor extracellular domains (ECDs) or membrane-embedded receptors was only slightly changed, except for bPL (K73D), which displayed very low affinity. In somatogenic binding assays to intact IM-9 human lymphocytes, hGHR-ECD or bovine liver membranes, bPL (K73D) did not bind at all, and bPL(K73F) or bPL(K73A) binding was drastically reduced. Binding experiments performed in real time using a BIAcore apparatus revealed that the decreased binding could be mainly attributed to increased k(off) rather than decreased k(on) values. The complex with hGHR-ECD revealed a 2:1 stoichiometry with bPL, bPL(K73F) and bPL(K73A), although the complex with these analogs was less stable than with bPL, whereas bPL(K73D) scarcely assembled a 1:1 complex. In contrast, bPL and the three analogs formed stable 1:2 complexes with rPRL-ECD. These results suggest that position 73 in bPL is more important for somatogenic than lactogenic properties and concurs with results from other groups, which have shown that R64, the analogous amino acid in hGH holds the same differential importance with respect to somatogenic binding.

Differential expression of the growth hormone receptor and its transcript in bovine uterus and placenta

By reverse transcription-polymerase chain reaction (RT-PCR) the transcript of the growth hormone receptor (GHR) was proved in bovine placentae of different gestational stages. Using non-radioactive in situ hybridization (ISH) the mRNA encoding GHR was localized in the uterine epithelium, glands, vessels and amniochorion of the placenta from the sixth week until the ninth month of gestation. Production of mRNA was low during the first trimester, significantly increased in the second trimester and peaked at the end of the sixth month of pregnancy. No transcript of GHR was found in the endometrium of non-pregnant uteri. The colocalization of the protein using a monoclonal antibody showed the same distribution pattern as the mRNA. Immunoblotting revealed a protein with a molecular weight of 120 kDa in the endometrium of pregnant cows which was not found in non-pregnant uteri. Our results imply that growth hormone is involved in placental metabolism and embryonic development from the early beginning of pregnancy until birth.

Purification and properties of placental prolactin-related protein-I

We used sucrose density gradient centrifugation, size exclusion chromatography, and high-pressure reversed-phase chromatography in the purification of bovine prolactin-related protein-I (bPRP-I) to homogeneity from a secretory granule-enriched fraction of fetal cotyledon. Amino terminal sequence was unambiguous, consistent with the nucleic acid sequence of the cDNA 50 codons distal to the initial AUG in the open reading frame, and began with the residues: RKSFTDRFMNAASLSHDFY. This is distinct from the signal peptide cleavage site predicted by the algorithm of von Heijne (1986) as well as that expected by comparison with other members of the growth hormone/prolactin family of hormones. The level of bPRP-I in uterine fluid was sufficient to detect by Western blot of unfractionated material and estimated as at least 0.65 microM. In contrast, bPRP-I was undetectable in the serum by this method. Interaction of [125I]-bPRP-I with high molecular weight serum components interfered with its measurement by radio-immunoassay, and could be replicated with purified alpha 2-macroglobulin with an apparent KD of about 0.41 microM. Thus, the bPRP-I gene product is processed secreted and distributed in a manner consistent with a paracrine action at the materno-fetal interface.

Recombinant analogues of prolactin, growth hormone, and placental lactogen: correlations between physical structure, binding characteristics, and activity

The availability of recombinant growth hormones, prolactins, placental lactogens and a few soluble extracellular domains of their receptors have extended our ability to study the interaction of somatogenic and lactogenic hormones with their receptors. Modifications of their respective cDNAs have enabled the preparation of sufficient amounts of the corresponding proteins. The present review summarizes two aspects of these interactions: (a) the relationship between binding, the apparent ability to dimerize the receptors and biological activities in vitro and in vivo; and (b) the effect of mutations on selective changes in the ability of human growth hormone and bovine placental lactogen to interact with somatogenic and lactogenic receptors. In view of this summary, strategies for preparing a second generation of biologically relevant recombinant hormones are discussed.

Expression of alternate growth hormone receptor messenger RNA in ovary and uterus of cattle

Growth hormone (GH) receptor mRNA is found within the corpus luteum and endometrium of cattle. However, binding sites for placental lactogen (PL) but not GH are found within these tissues. The objectives were to isolate cDNA for the GH receptor within the reproductive tissues of cattle and to examine these cDNA as potential variants of the GH receptor that bind PL. Ten cDNA clones were isolated from a bovine endometrial cDNA library with a 32P-labeled cDNA of the GH receptor extracellular domain. On the basis of restriction enzyme digestion, 2 of the 10 cDNA clones contained exon 1. The DNA sequence of these clones was determined by dideoxy nucleotide sequencing. The exon 1 DNA sequence of each clone (exon 1B) was different from the previously reported exon 1 for the bovine GH receptor cDNA isolated from liver (exon 1A). Analyses of these cDNA sequences showed that exon 1B contained significant homology with placental forms of the GH receptor found in mouse and human. Unlike the human cDNA, the bovine cDNA isolated from endometrium contained an intact third exon. Amplification of GH receptor mRNA by reverse transcriptase polymerase chain reaction, with exon 1A- and 1B-specific forward primers, showed that exon 1B was expressed in liver, corpus luteum, ovary, endometrium, and myometrium. Exon 1A was found almost exclusively in liver, and little was found in reproductive tissues. The predicted initiation of protein coding for the GH receptor was within the second exon and was not changed by the splicing of the alternate first exon. This suggests that the alternate mRNA results in the expression of intact GH receptor protein that is similar to that found within liver. Alternative promoters (1B) may control the expression of the receptor outside the liver. Furthermore, mechanisms other than the differential splicing of GH receptor protein may dictate the specificity of PL binding within the endometrium and corpus luteum.

Selective modification of recombinant bovine placental lactogen by site-directed mutagenesis at its C terminus

Five recombinant analogues of bovine placental lactogen (bPL) ((bPL(S184H), bPL(S187A), bPL(S187F), bPL(T188F), bPL(T188F,I190F)) were prepared, expressed in Escherichia coli, and purified to homogeneity. Circular dichroism analysis revealed no or minor structural changes, except in bPL(T188F,I190F). Binding and biological activities of bPL(T188F,I190F) were almost completely abolished, whereas bPL analogues mutated at position 187 retained their full activity. Point mutation T188F resulted in selective modification; binding to somatogenic receptors, their extracellular domains (ECDs), and to bPLR in the endometrium as well as somatogenic receptor-mediated biological activities were reduced or abolished, whereas binding to lactogenic receptors, their ECDs, and subsequent biological activity was fully or almost fully retained. This selective modification most likely results from a steric hindrance induced by a bulky Phe-188 chain of bPL which interacts with the Arg-43 of the human or Leu-43 of the non-human GHRs. Point mutation S184H abolished the interaction with hGHR, most likely due to the unfavorable charge-charge interaction, possibly accompanied by steric hindrance between Arg-43 of the receptor and the newly introduced His-184 and possible interference with the putative interaction between the alkyl portion of Thr-188 and Lys-185 of bPL with Trp-104 of hGHR. In contrast, bPL(S184H) retained its capacity to interact with nonhuman GHRs. Decrease in the biological activity of bPL(S184H) was also observed in two lactogenic receptor-mediated bioassays most likely due to the elimination of the intermolecular hydrogen bond of Ser-184 with a side chain of Tyr-127, which appears in all lactogenic receptors.

Two forms of the prolactin receptor messenger ribonucleic acid are Present in ovine fetal liver and adult ovary

Previous binding studies indicated that there is little to no specific prolactin binding in ovine fetal liver and adult ovary. Therefore, we sought to determine if ovine prolactin receptor (PRLR) mRNA is present in those tissues. Primers were designed from the bovine PRLR cDNA sequence for use in reverse transcriptase-polymerase chain reaction (RT-PCR). RT-PCR analysis of ovine fetal liver total cellular RNA (tcRNA) isolated from days 60, 90, 105, 120 and 135 of gestation, and luteal tcRNA isolated from days 3, 7, 10, 13 and 16 of the estrous cycle revealed that PRLR mRNA was present in these tissues. However, two RT-PCR products were generated from both tissues. The two RT-PCR products did not differ between the two tissue sources in sequence, and were designated oPRLR-1 and oPRLR-2. Ovine PRLR-1 is 513 bp in length and is 96.4% identical to the bovine cDNA. Ovine PRLR-2 is identical to oPRLR-1 until nucleotide (nt) 420 at which point a 39 bp insertion occurs. This insertion occurs between Homology Boxes 1 and 2 within the cytoplasmic domain of the receptor, resulting in an 11 amino acid divergent sequence, followed by two stop codons. Ribonuclease-protection assay revealed that oPRLR-1 mRNA is the most abundant in these tissues. Our data indicate that two forms of oPRLR mRNA are Present in fetal liver and adult ovary, and that one form (oPRLR-2) is predicted to encode a truncated PRLR.

Effects of somatotropin on the conceptus, uterus, and ovary during maternal recognition of pregnancy in cattle

Effects of recombinant bovine somatotropin (rbST) on ovarian and uterine function and the production of components of the insulin-like growth factor (IGF) system were examined during the period of maternal recognition of pregnancy in cattle. Lactating dairy cows were treated with 25 mg/d rbST (n = 8) or saline (n = 8) for 16 d after estrus. Ovaries, uteri, and conceptuses were collected on Day 17 after estrus. The length (millimeters) of the conceptus was recorded. The concentration of IGF-I and the content of IGF-binding proteins (BP) in uterine flushings were determined. Corpora lutea (CL) were weighed, and the number of follicles (> or = 2 mm in diameter) were counted. Follicular fluid from the largest and second-largest follicles was assayed for the concentration of IGF-I, IGFBP, progesterone, and estradiol. The length of the conceptus and the total amount of IGF-I in uterine fluid were similar for rbST and control. Recombinant bST increased 1) the weight of the CL, 2) the number of largest follicles (10 to 15 mm in diameter), 3) the concentration of IGF-I in the follicular fluid, 4) the follicular fluid content of IGFBP of the largest estrogenic follicle, and 5) the quantity of IGFBP in uterine flushings. The concentration of progesterone in the follicular fluid tended to be increased in rbST-treated cows, whereas the concentration of estradiol was similar to that of control cows. The concentration of progesterone in plasma was similar for rbST compared with control. In conclusion, the administration of rbST in lactating dairy cows for 16 d after estrus did not alter the growth of the conceptus collected on Day 17. The greatest responses to rbST were found within the ovary, where rbST increased the weight of the CL and altered the amount of IGF-I and IGFBP in the follicular fluid.

Bovine placental prolactin-related hormones

The bovine placenta, like that of rodents and primates, synthesizes members of the PRL/GH gene family, which may assist the pituitary hormones or perform unique functions during pregnancy. Bovine placental lactogen (bPL) potentially may act through.three receptors: as an agonist or partial antagonist at the PRL and GH receptors, and via an apparently specific receptor in the endometrium. A large distinct subfamily of diverse primary structure, including bovine PRL-related protein I (bPRP-I), evidently does not act via these receptors. Advances in our understanding of hormone-receptor interactions for this gene family have provided new tools to study the role of these hormones in the successful pregnancy.

Molecular cloning of the bovine prolactin receptor and distribution of prolactin and growth hormone receptor transcripts in fetal and utero-placental tissues

We have isolated a bovine prolactin (bPRL) receptor cDNA from an endometrial cDNA library, which predicts a 557 amino acid transmembrane protein similar to the long forms of other characterized prolactin receptors. The predicted cytoplasmic domain is slightly truncated primarily by a stop codon located 36 codons 5' from the stop utilized in the human hepatic transcript. When expressed in COS cells, this cDNA was shown to encode a protein which bound bovine placental lactogen (bPL) and bPRL with nearly equal affinity (KD for bPL, 2.03 x 10(-10) M; bPRL, 3.07 x 10(-10) M). Northern analysis demonstrated multiple transcripts, with maternal liver, corpus luteum, intestine, endometrium and fetal liver containing a major transcript of about 3.8 kb, and maternal corpus luteum and endometrium, a second sized transcript of apparently equal abundance of 4.4 kb. This difference did not appear to be within the coding region. Primer extension analysis of maternal hepatic and endometrial transcripts revealed considerable heterogeneity. Examination of the distribution of prolactin and growth hormone receptor transcripts at mid-pregnancy by semi-quantitative reverse transcriptase polymerase chain reaction showed that both are widespread in bovine fetal and placental tissues. This isolation of bovine prolactin receptor cDNA, and description of receptor distribution will facilitate study of the action of the placental and pituitary members of this gene family during pregnancy.

Biological activity of bovine placental lactogen in 3T3-F442A preadipocytes is mediated through a somatogenic receptor

Bovine placental lactogen (bPL) exhibited antimitogenic differentiation-promoting biological activity in 3T3-F442A preadipocytes. Competitive binding studies and affinity labelling revealed bPL activity to be mediated through a somatogenic type of receptor that recognizes human growth hormone (hGH) and bovine GH, but not ovine prolactin or human PL. The bioactivity of bPL was sixfold lower than that of hGH despite that bPL is binding to the somatogenic receptors with fivefold higher affinity. This discrepancy may result from the relatively low ability of bPL to induce post-receptoral effects such as receptor dimerization.

Stimulation of milk yield and feed intake by bovine placental lactogen in the dairy cow

A 6 x 6 Latin square design was used to test the effects of recombinant bovine placental lactogen on milk yield, milk composition, feed intake, and blood hormone and metabolite levels in nonpregnant lactating cows. The six treatments (5, 10, 20, and 40 mg/d of placental lactogen, water as negative control, and 20 mg/d of bST as positive control) were administered by subcutaneous injection twice daily for 9 d. Blood samples were taken during the last 5 d of the treatment period. The three highest doses of placental lactogen increased milk yield, and there was a linear dose effect, although placental lactogen was less potent than bST. Milk concentrations of lactose, protein, and fat were not altered by any of the treatments. Dry matter intake was increased by two of the doses of placental lactogen, but not by bST. Blood urea N concentration was decreased in a dose-dependent manner by placental lactogen and was also decreased by bST. Similarly, serum insulin-like growth factor-I was increased in a dose-dependent manner by placental lactogen and was also increased by bST. Plasma concentrations of NEFA and glucose were increased by bST, but placental lactogen had little or no effect on either of these parameters. Thus, placental lactogen appears to act, in part, as a weak somatotropin agonist; however, it also appears to have specific activities, e.g., stimulating feed intake.