Release of human chorionic gonadotropin (hCG) and its alpha-subunit (hCG-alpha) from perifused human placenta

The release of human chorionic gonadotropin (hCG) and its alpha-subunit (hCG-alpha) from the normal human placenta and the effect of some stimulatory agents on their release were studied in vitro using a perfusion system. Each perfusate was assayed for hCG and hCG-alpha in its own homologous radioimmunoassay systems. Both hCG and hCG-alpha were released from the placenta at any stage of gestation in our perfusion system. Much more hCG than hCG-alpha was released from the placenta in early gestation. By comparison, however, hCG-alpha increased gradually with the gestational age. The amount of hCG-alpha released was almost equal to that of hCG in the placenta in the 17th gestational week. After the 22nd gestational week, hCG-alpha was released in larger quantities than hCG, and about 10 times more hCG-alpha than hCG was released from the term placenta. These results were also confirmed by gel filtration of perfusates on a Sephadex G-100 column. hCG-alpha, compared with hCG, was present in excess in gel filtrated perfusates in the last two trimesters. By adding 1 mM dibutyryl cyclic AMP to the perifusion medium, the release of both hCG and hCG-alpha was stimulated significantly. Synthetic luteinizing hormone releasing hormone (LH-RH) at concentrations of 10 ng/ml and 100 ng/ml had no effect, but at a high concentration (1 microgram/ml), LH-RH stimulated the release of them. Moreover, mouse epidermal growth factor (EGF) stimulated not only the release of hCG and hCG-alpha but also their production, because both hCG and hCG-alpha levels rose progressively with the time course in the presence of EGF. The present studies demonstrate that the perifusion system of chorionic tissues is a useful method for investigating the release of hCG and its subunits in vitro.     

Specific, high-affinity binding sites for human luteinizing hormone (hLH) and human chorionic gonadotrophin (hCG) in Candida species

The presence of specific binding sites for [125I]-labelled hLH and hCG is described in Candida species. Binding was present in three strains of Candida albicans, and in Candida tropicalis, and was greatest in microsomes, though binding was also present in cytosol fractions. hLH and hCG mutually competed for these binding sites. Other hormones did not bind and did not compete for hLH binding sites. Scatchard plots showed two classes of binding sites, one with high affinity, low capacity and the other with lower affinity, high capacity binding in both microsomes and cytosol. This is the first report of specific binding sites for mammalian peptide hormones in a yeast.     

SDS-PAGE electrophoretic property of human chorionic gonadotropin (hCG) and its beta-subunit

The microheterogeneity property of hCG with regards to its sialic acid contents resulted in variable mobility of the glycoprotein in SDS-PAGE. The intact hCG molecule is composed of two dissimilar subunits, namely alpha- and beta-subunits. The identification of hCG bands in SDS-PAGE was accomplished by the immunoblotting experiment, whereby the antibody directed toward the specific region of beta-subunit of hCG was used. The data shows that the different mobility of intact hCG was attributed to the different degree of desialylation of the glycoprotein. Nevertheless, unlike the intact hCG, the mobility of its beta-subunit was not affected by its variety sialic acid content. This characteristic of beta-hCG is beneficial when semi-quantification of total hCG is required. Quantification of hCG using the HPLC-reversed phase C18 analytical column is not possible as the glycoprotein was eluted in multiple fractions at different retention times. The identification of denatured hCG (HPLC eluted fractions) was carried out by immunoblotting experiment whilst immunoassay technique failed to detect its presence in any fraction.     

Reduction of testicular human chorionic gonadotropin receptors by human chorionic gonadotropin in vivo and in vitro

Changes in rat and human testicular human chorionic gonadotropin (hCG) binding sites induced by hCG were estimated in vivo and in vitro. After a single administration of hCG, the specific 125I-hCG bindings were significantly reduced for 7 and 5 days in rat and human testes, respectively. Thereafter, 125I-hCG bindings had recovered to pretreatment values by the 14th day after the administration. Occupied hCG bindings accounted for about half of the reduced bindings on the day after administration of hCG. After this time, however, the occupancy did not contribute so much to the reduction of the bindings. In experiments in vitro using the organ culture technique, an exposure to hCG for 24 h induced a dose-related significant loss of the specific 125I-hCG bindings for 7 and 5 days in rat and human testes, respectively. Thereafter, the loss was gradually recovered. These patterns of changes in 125I-hCG bindings in vitro were similar to those in vivo. These findings suggest that the reduction in hCG binding sites by hCG is due to not only occupancy but also downregulation of the binding sites and that the testicular organ culture method used in the present study is useful to study hormonal regulation of testicular function, especially in human testes.     

The biological and clinical significance of nicks in human chorionic gonadotropin and its free beta-subunit.

Human chorionic gonadotropin (hCG) is a glycoprotein hormone composed of two dissimilar subunits, alpha and beta. Nicks or missing peptide linkages have been found in the beta 44-52 region of the beta-subunit of hCG, whether from pregnancy or trophoblast disease. This article reviews recent reports about the location of nicks in hCG, their origin and occurrence, their effects on the steroidogenic and receptor-binding activities of hCG, and on the immunological activities of hCG and its free beta-subunit. Taken together, the reports show: (1) nicks occur primarily between beta 47 and beta 48, and to a lesser extent between beta 44 and beta 45; (2) the extent of nicking in hCG samples varies widely, from undetectable to 100 percent of molecules; (3) nicks greatly reduce the steroidogenic activity of hCG in vitro (nicked molecules have less than 20 percent of the activity of the intact hormone); (4) nicks may occur at the trophoblast-myometrial interface or in the circulation by the action of human leucocyte elastase or similar leucocytic protease; (5) hCG testing kits using dimer-specific antibodies may not detect nicked molecules and may give different results from those using other antibodies; (6) hCG international reference preparations and the CR series of hCG standards are variably nicked (10 percent to 20 percent), complicating the problem of discordant hCG results in nick-sensitive assays; (7) results from commonly used immunoassays for measurement of the hCG free beta-subunit vary by as much as tenfold because some of the antibodies employed do not detect nick free beta-subunit.     

Tumor- and pregnancy-derived isoforms of human chorionic gonadotropin: biological and diagnostic relevance

OBJECTIVES: Human chorionic gonadotropin (hCG) and hCG variants are of high clinical importance for the diagnosis of pregnancy, monitoring of abnormal and ectopic pregnancies, testing for Down's syndrome or monitoring therapy of hCG-secreting malignancies. In serum and urine, hCG appears in microheterogeneous isoforms with respect to protein backbone structure and the extent of glycosylation. The present study reports on the identification, immunological characterization, biological activity of glycosylation isoforms of pregnancy (preg) and tumor-derived (tu) hCG, and the impact of glycosylation on diagnostic immunoassays. METHODS: Twenty-two urinary preg- and tu-hCG isoforms were separated by preparative isoelectrofocusing (hCG-pI variants) and characterized by Western blot. Number, topography and accessibility pattern of epitopes on their surface was evaluated by two-site radioimmunoassays using 14 different monoclonal antibodies (mabs). Binding of hCG isoforms to four different LH/CG receptors was investigated in radioreceptor assays, and their biological activity determined by measuring cAMP elevation. RESULTS: All 22 hCG glycosylation variants appeared immunologically intact: each isoform, even when highly acidic, expressed all 14 surface epitopes which were arranged in a topographical manner indistinguishable from crude hCG. hCG isoforms were able to bind to four different receptor variants, with slightly varying affinities, but orientations indistinguishable from each other as shown by identical epitope accessibility patterns. Each of the hCG-pI variants was able to activate the LH/CG-Rs, but with varying reactivities. CONCLUSIONS: We conclude that in contrast to deglycosylated hCG, all hCG glycosylation isoforms investigated act as receptor agonists. Moreover, there is no overspecificity of mabs to certain hCG isoforms due to carbohydrate variability that exclude others from diagnostic measurement.     

Regulation of progesterone secretion in human syncytiotrophoblast in culture by human chorionic gonadotropin.

In the present investigation we sought to define the specific sites in the pathway of placental progesterone biosynthesis that underlie the action of human chorionic gonadotropin (hCG). When the cells were challenged with dibutryl cAMP (dbcAMP), forskolin or isobutylmethylxanthine, they produced significantly higher amounts of progesterone which in the presence of the hCG antibody was reduced to the level of the control set of cells. Trophoblast cells cultured in serum free medium with 25-hydroxycholesterol (25-OHC) produced increased amounts of progesterone. In the presence of hCG antibody at a concentration which neutralized the secreted hCG, the steroid production was completely blocked, even when the 25-OHC was added to the medium. Also, direct quantitation of the cytochrome P450 SCC enzyme in the absence of hCG indicated a significant decrease. The exogenous addition of low density lipoproteins (LDL) increased the progesterone secretion by the trophoblast cells in culture. Neutralization of hCG by the antibodies, however, drastically reduced the LDL induced progesterone secretion, which was restored by the addition of dbcAMP to the medium. Based on these findings, we suggest a stimulatory effect of hCG on normal trophoblast cells at the level of LDL utilization and cytochrome P450 SCC enzyme. Since dbcAMP could mimic these actions of hCG, the data suggest a possible autocrine/paracrine role of hCG on the trophoblast cells. An additive effect of hCG and cAMP on progesterone secretion observed in our studies, indicate that apart from hCG, adenylate cyclase activity may also be regulated by other factors.     

A method for determination of the specific activity of receptor-bound human chorionic gonadotropin (hCG).

Precise knowledge of the specific activity (S.A., muc/mug) of the receptor bound radiolabelled hormone is required for study of the stoichiometry of peptide hormone-receptor interactios. A radioligand receptor assay using 131-I-hCG as tracer and transplantable mouse luteoma homogenates (Biol. Reprod. 8:550, 1973) as a source of receptor was used as a model to determine the specific activity of receptor bound 125-I-hCG. Progressive saturation of the gonadotropin receptor by 125-I-hCG suggests the presence of a high affinity-low capacity binding event (saturating between 14 and 37 ng/100 mg homogenate) that does not distinguish between non-radioactive hCG and 125-I-hCG, and a low affinity-high capacity binding event (saturating between 240 and 270 ng/100 mg homogenate) that shows a preference for non-radioactive hCG over 125-I-hCG. Parallelism between bound 125-I-hCG and non-radioactive hCG in terms of competition with tracer 131-I-hCG could only be demonstrated for the high affinity event.     

Topographic antigenic determinants recognized by monoclonal antibodies on human choriogonadotropin beta-subunit

We describe a first attempt to study the antibody-combining sites recognized by monoclonal antibodies raised against the beta-subunit of human choriogonadotropin (hCG). Two groups of antibodies were first defined by their ability to recognize only the free beta-subunit or the free and combined subunit. Antibodies FBT-11 and FBT-11-L bind only to hCG beta-subunit but not to hCG, whereas antibodies FBT-10 and D1E8 bind to both the beta-subunit and the hormone. In both cases, the antigenic determinants were localized to the core of the protein (residues 1-112), indicating the weak immunogenicity of the specific carboxyl-terminal extension of hCG-beta. Nine synthetic peptides spanning different regions of hCG-beta and lutropin-beta were assessed for their capacity to inhibit antibody binding. A synthetic peptide inclusive of the NH2-terminal region (residues 1-7) of the hCG beta-subunit was found to inhibit binding to the radiolabeled subunit of a monoclonal antibody specific for free hCG-beta (FBT-11). Further delineation of the antigenic site recognized by this antibody provided evidence for the involvement of fragment 82-92. Moreover, monoclonal antibody FBT-11 inhibited the recombination of hCG-beta to hCG-alpha, indicating that its antigenic determinant might be located nearby or in the hCG-beta portion interacting with the alpha-subunit. Binding of monoclonal antibody FBT-10, corresponding to the second antigenic determinant, was weakly inhibited by fragment 82-105 and did not impair the recombination of the hCG beta-subunit to the hCG alpha-subunit. Its combining site appeared to be located in a region of the intact native choriogonadotropin present at the surface of the hormone-receptor complex.     

Unmasking by neuraminidase of specific chorionic gonadotrophin binding activity of human placental syncytiotrophoblast

Purified human placental syncytiotrophoblast consistently failed to bind specifically to 125I-labelled hCG. Treatment of the syncytiotrophoblast with neuraminidase resulted in the ability to bind 125I-labelled hCG that was displaceable by excess of unlabelled hCG. Neuraminidase treatment removed 73.8% of the total neuraminic acid of syncytiotrophoblast. The specific binding of 125I-labelled hCG increased linearly with increasing amount of neuraminidase-treated syncytiotrophoblast, was saturable and had a Ka = 1.6 x 10(7) M-1. Excess of GH, prolactin, placental lactogen or insulin did not inhibit the binding, whereas LH did so completely and FSH partly.     

Assay system for simultaneous measurement of three steroidogenic enzyme activities in rat and human testis--effect of human chorionic gonadotropin

An assay system that measures the enzymatic activities (17 alpha-hydroxylase, 17,20-desmolase, and 17 beta-hydroxysteroid dehydrogenase) in the delta 4 pathway of testosterone biosynthesis using rat and human testicular homogenate was examined. This system involves the simultaneous separation of the steroid intermediates by a three-step TLC procedure. The observed Rf values were 0.78 for progesterone (P), 0.59 for 17 alpha-hydroxyprogesterone (17 alpha-HP), 0.70 for androstenedione (A), 0.5 for testosterone, 0.64 for dihydrotestosterone, and 0.45 for 3 alpha, 17 beta-androstanediol. The identification of these steroid intermediates was further accomplished by acetylation and rechromatography of the representative samples along with the authentic standards and by recrystallization to constant specific activity until three consecutive crystallizations were within +/- 5% of the mean value. Incubation time up to 30 min and increasing protein concentrations showed a linear relationship with respect to these three enzymatic activities. The optimum temperature for these enzymatic activities varied from 32 to 34 degrees C, with a sharp decline between 37 and 40 degrees C. The Michaelis constants (Km) for the rat testis homogenate samples were 0.17 microM for P, 0.22 microM for 17 alpha-HP, and 2.5 microM for A, while for the human testis the Km values were 1.2, 2.2, and 2.3 microM, respectively, for these substrates. The concentrations of the endogenous steroid substrates present in these homogenate samples did not alter the Km or Vmax values. The effect of human chorionic gonadotropin (hCG) in vitro on these steroidogenic enzyme activities was also studied. In the rat testis, 10 IU of hCG produced a significant rise in all the three enzyme activities whereas in the human testis 10 and 30 IU of hCG showed no significant change in any of these enzymatic activities. However, 100 IU of hCG resulted in a significant increase in 17 alpha-hydroxylase and 17,20-desmolase activities in the human testis. These studies suggest that this assay system for the measurement of these enzymatic activities using a testicular homogenate sample provides consistent and reproducible results. Based on the sensitivities of the measurements and our experience with testicular biopsy technique, we conclude that a routine testicular biopsy in the human should provide sufficient tissue to run these enzymatic assays.     

Immunoaffinity trapping of urinary human chorionic gonadotropin and its high-performance liquid chromatographic-mass spectrometric confirmation

A method has been developed for confirmation of the glycopeptide human chorionic gonadotropin (hCG) in urine. A solid-phase immunoaffinity trapping technique utilizing a monoclonal antibody recognizing both intact hCG and free hCG β-subunits was developed for the extraction of hCG from urine. Recovery of hCG from a urine matrix was essentially quantitative. The hCG was quantitatively eluted with 6 M guanidine hydrochloride, reductively alkylated with vinylpyridine, and subjected to tryptic digestion. The tryptic digest was analysed by HPLC-MS. Ions from three tryptic fragments were monitored with selected ion monitoring to provide specific detection of hCG. The signal observed for a concentration of 25 mIU/ml of hCG could be clearly distinguished from background with a signal-to-noise ratio of 12:1.     

Analysis of amylin cleavage products provides new insights into the amyloidogenic region of human amylin

Human chorionic gonadotropin (hCG), is a placental hormone which exerts its major effect by stimulating progesterone production, crucially sustaining the early weeks of pregnancy. Detection of hCG with specific monoclonal antibodies (mAbs) has become the chosen means for pregnancy diagnosis. We have used antibody Fv fragments derived from two high-affinity mAbs, one against the alpha and the other against the beta-hCG subunit to enable the crystallisation of intact or desialylated hCG. Crystals of a ternary complex composed of Fv anti-alpha/hCG/Fv anti-beta were found to diffract to 3.5 A resolution, and the structure was solved by molecular replacement. In the crystal, the two Fvs keep hCG as in a molecular cage, providing good protein-protein contacts and leaving enough space for the saccharides to be accommodated in the cell solvent. The two Fvs were found not to interact directly through their complementary-determining regions with the hCG saccharides, but only with the protein. The hCG structure in the ternary complex was very close to that of the HF partially deglycosylated hormone, thus indicating that neither the saccharides nor the Fvs had any substantial influence on hormone structure.     

Production of monoclonal antibodies to human chorionic gonadotropin

Production of monoclonal antibodies against human chorionic gonadotropin (hCG) has been studied using hCG as an immunogen. Spleen cells of BALB/c mice immunized with hCG were fused with NS-1 mouse myeloma cells. This study reports the successful isolation of a hybrid clone secreting a monoclonal antibody specific for hCG. By using PEG 4,000 as a fusion agent, the fusion rates were between 42.0 and 50.2%. In total 842 hybridomas were produced. Among them, 403 hybridomas had hCG antibody production. After cloning twice by limiting dilution and alternately screening by enzyme immunoassay and by radioimmunoassay, there were 39 cell lines having specific antibody production. Among them, the No. 57-42-2 had the highest reactivity. By Ouchterlony test, the monoclonal antibody was shown to be IgG1. The affinity constant of the antibody to hCG was 0.6 x 10(9) 1/mole. In radioimmunoassay, the cross reactivity of the antibody to human luteinizing hormone (LH) and human follicle-stimulating hormone (FSH) was 1.5% and 0.7%, respectively. There was no cross reaction with human thyrotropin-stimulating hormone (TSH).     

Expression of biologically active human chorionic gonadotropin and its subunits by recombinant vaccinia virus

Vaccinia virus (VV) expression vector was used to clone the genes for coding alpha and beta subunits of human chorionic gonadotropin (hCG). Recombinant viruses VSL3 and VSS1 containing these genes were selected as blue coloured plaques on the basis of co-expression of Escherichia coli beta-galactosidase in the infected cells. CV-1 cells when infected with VSL3 or VSS1 secreted 2.4 and 1.8 micrograms of alpha and beta hCG subunits, respectively, per 3 x 10(6) cells after 24 h of infection. The subunit proteins expressed individually had immunoreactivity with monoclonal and polyclonal antibodies specific to hCG. The subunit hormonal peptides associated with each other during co-infection to form the complete hCG dimer, which was biologically active as evident from the induction of steroidogenesis in a mouse Leydig cell system.     

Tumor-associated antigen human chorionic gonadotropin beta contains numerous antigenic determinants recognized by in vitro-induced CD8+ and CD4+ T lymphocytes

The beta subunit of human chorionic gonadotropin (hCG beta) is markedly overexpressed by neoplastic cells of differing histological origin including those present in colon, breast, prostate and bladder tumors. We have previously shown that some patients with hCG beta-producing urothelial tumors have circulating T cells that proliferate in response to hCG beta. To make a comprehensive study of hCG beta as a potential target for cancer immunotherapy, we investigated whether hCG beta peptides could induce CD4+ or CD8+ T-cell responses in vitro. By stimulating peripheral blood mononuclear cells (PBMCs) from three donors with mixtures of overlapping 16-mer synthetic peptides analogous to portions of either the hCG beta 20-71 or the hCG beta 102-129 region, we established six CD4+ T-cell lines that proliferated specifically in response to five distinct determinants located within these two hCG beta regions. Three antigenic determinants (hCG beta 52-67, 106-121 and 114-125) were presented by HLA-DR molecules, while the two other antigenic determinants (hCG beta 48-63 and 56-67) were presented by HLA-DQ molecules. Interestingly, one T-cell line specific for peptide hCG beta 106-121 recognized hCG beta peptides comprising, at position 117, either an alanine or an aspartic acid residue, with the latter residue being present within the protein expressed by some tumor cells. In addition, three other hCG beta-derived peptides that exhibited HLA-A*0201 binding ability were able to stimulate CD8+ cytotoxic T cells from two HLA-A*0201 donors. These three immunogenic peptides corresponded to regions hCG beta 40-48, hCG beta 44-52 and hCG beta 75-84. Our results indicate that the tumor-associated antigen hCG beta possesses numerous antigenic determinants liable to stimulate CD4+ and CD8+ T lymphocytes, and might thus be an effective target antigen for the immunotherapy of hCG beta-producing tumors.     

Chemical synthesis of peptide fragments of the hormone-specific beta-subunit of human follicle-stimulating hormone

In order to determine the specific antigenic determinants of human follicle-stimulating hormone (hFSH), hFSH-beta peptides with amino acid residues 33-49 (V2), 95-118 (V3), 76-118 (V3 + 1/2 C2), 1-33 (V1 + C1), 22-33 (1/2C1), and 95-107 (V3 + 1/4C2) according to the nomenclature of Stewart and Stewart [Stewart, M., & Stewart, F. (1977) J. Mol. Biol. 116, 175] as well as additional peptides with the residues 93-107, 91-107, 89-107, 87-107, and 85-107 were chemically synthesized. The peptides were examined in radioimmunoassay systems of FSH, luteinizing hormone (LH), or human chorionic gonadotropin (hCG). V3 + 1/2C2 and V1 + C1 showed immunological activity, whereas the other peptides did not. Antibodies were raised in rabbits against these peptides and examined for specific binding with hFSH, LH, thyroid-stimulating hormone (TSH), and hCG. V3 + 1/2C2 as well as V1 + C1 produced antisera, which specifically bound hFSH, hLH, and hTSH, indicating that the amino acid sequences contained in hFSH-beta peptides V3 + 1/2C2 and V1 + C1 share common antigenic sites with hLH and hTSH. Antisera were produced in rabbits against hFSH-beta, against reduced and S-aminoethylated hFSH-beta (AE-FSH-beta), and against AE-FSH-beta coupled to hemocyanin. Reduced and S-aminoethylated beta-subunit of FSH-beta coupled with hemocyanin produced antisera in rabbits that specifically bound only hFSH and not hLH, hTSH, or hCG.     

Adsorptive stripping voltammetry of human chorionic gonadotropin and two of its specific antibodies

The electrochemical behaviour of human chorionic gonadotropin (hCG), as well as rat anti-βhCG and rabbit anti-βhCG antibodies, has been studied using cyclic voltammetry and adsorptive stripping voltammetry. Conditions have been optimised for the determination of these species by differential pulse adsorptive stripping voltammetry with respect to accumulation potential, accumulation time, scan rate, pulse amplitude and drop size. This technique has also been used to investigate the interaction of hCG with each of its specific antibodies in solution.     

Fertility of dairy cattle treated with human chorionic gonadotropin (hCG) to stimulate progesterone secretion

Two experiments were conducted to determine if progesterone secretion and fertility would be affected by administration of human chorionic gonadotropin (hCG) before or after the first insemination. In Experiment 1, 48 Holstein heifers received 1000 IU of hCG or 1 ml of saline on Days 2, 3, and 4 of an estrous cycle. They were inseminated at the subsequent estrus. In Experiment 2, 110 Jersey and 105 Holstein cows received a single injection of 5000 IU of hCG or 5 ml of saline on Day 3 after estrus. These cows were first inseminated either at the estrus immediately preceding treatment or at the subsequent estrus. In both experiments, blood samples for determination of progesterone were collected thrice weekly for 3 to 4 wk following treatment. In Experiment 1, progesterone concentrations during mid-cycle were higher in hCG-treated heifers than in saline-treated controls. Treatment with hCG resulted in an 11% increase in the first service conception rate (P < 0.48). In Experiment 2, hCG-treated cows displayed higher progesterone secretion during mid-cycle than saline-treated herdmates. The conception rate of cows inseminated prior to hCG-treatment was not affected by treatment, but cows inseminated after treatment had a marginally lower fertility rate. The conception rate of cows receiving a repeat insemination following hCG treatment was higher than for the controls. We conclude that treatment with hCG did not improve the conception rate at the first insemination, but it may be beneficial for cows that require a repeat service.     

Effect of cyclic 3':5'-AMP derivatives prostaglandins and related agents on human chorionic gonadotropin secretion in human malignant trophoblast in culture.

The secretion of human chorionic gonadotropin (hCG) is stimulated by addition of N6, O2'-dibutyryl cyclic 3':5'-AMP (dbcAMP) or theophylline to normal term placenta and human malignant trophoblast cells in vitro. To understand better the specificity of this process, malignant trophoblast cultures were incubated with 3':5'-cyclic AMP (cAMP) derivatives, prostaglandins and other agents for 1 to 3 days, and the secretion of radioimmunoassayable hCG was measured. Whereas dbcAMP was the most potent agent in stimulating secretion of hCG, the N6--and O2'-monobutyryl derivatives of cAMP and phosphodiesterase inhibitors (theophylline, papaverine, 3-isobutyl-1-methylxanthine) also increased the secretion of the hormone. A slight increase in hCG secretion was observed following addition of adenine. By contrast, butyrate, cAMP, cyclic 3':5'-GMP (cGMP), dbcGMP, 5'-AMP, adenosine, L-epinephrine and prostaglandins E1, E2, F1a and F2a were ineffective. Particulate fractions from sonicates of malignant trophoblast cultures contained adenylate cyclase activity which was stimulated more than 10-fold by NaF, but not by either catecholamines or prostaglandins. The relatively specific stimulation of hCG secretion suggested that a regulatory process involving cAMP may have physiological significance in the trophoblast.