Deglycosylated human chorionic gonadotropin. An antagonist to desensitization and down-regulation of the gonadotropin receptor-adenylate cyclase system

Human chorionic gonadotropin (hCG) was deglycosylated with anhydrous HF and compared with native hCG for binding and biological activity. The deglycosylated hormone (DG-hCG) had the same affinity as hCG for gonadotropin receptors in murine Leydig tumor cells (MLTC-1) but was less than 1% as potent as hCG in stimulating cyclic AMP production in these cells. Exposure of MLTC-1 cells for 30 min to hCG caused a desensitization of hCG-stimulated adenylate cyclase activity, whereas DG-hCG did not induce desensitization even after 4 h. hCG induced down-regulation of hCG receptors; by 4 h, 40% of the receptors had disappeared, whereas there was no receptor loss in cells exposed to DG-hCG for the same time. By 6 h, receptor down-regulation began to occur in the DG-hCG-treated cells and could be mimicked by exposing the cells to dibutyryl cyclic AMP or cholera toxin. Thus, the small increase in cyclic AMP generated by DG-hCG appears to result in some loss of receptors. Cells were incubated with iodinated hCG or DG-hCG for 30 min, washed, and incubated in fresh medium. Both bound ligands were degraded as measured by disappearance of cell-associated radioactivity and appearance of trichloroacetic acid-soluble label in the medium. The half-lives were 3 and 6 h for hCG and DG-hCG, respectively. Our results indicate that DG-hCG in contrast to hCG does not cause either rapid desensitization of hCG-stimulated adenylated cyclase or rapid down-regulation of hCG receptors. Therefore, receptor occupancy alone is insufficient to induce these phenomena.     

Cation-dependent gonadotropin releasing hormone activation of guanylate cyclase

Summary Gonadotropin releasing hormone enhanced guanylate cyclase [E.C.4.6.1.2] two- to threefold in pituitary, testis, liver and kidney. Dose response relationships revealed that at a concentration of 1 nanomolar, gonadotropin releasing hormone caused a maximal augmentation of guanylate cyclase activity and that increasing its concentration to the millimolar range caused no further enhancement of this enzyme. There was an absolute cation requirement for gonadotropin releasing hormone's enhancement of guanylate cyclase activity as there was no increase without any cation present. Gonadotropin releasing hormone could increase guanylate cyclase activity with either calcium or manganese in the incubation medium but more augmentation was observed with manganese. The data in this investigation suggest that guanylate cyclase may play a role in the mechanism of action of gonadotropin releasing hormone.     

Synchronous generation of ovarian hCG binding sites and LH-sensitive adenylate cyclase in immature rats following treatment with pregnant mare serum gonadotropin.

A concomitant increase in the activity of LH-senstive adenylate cyclase and in the number of LH/hCG binding sites was induced in ovaries of immature rats upon administration of pregnant mare serum gonadotropin (PMSG), a hormone preparation known to have predominantly follicle stimulation (FSH-like) activity. When an optimal dose of PMSG (15 i.u./rat) was administered to 25-day-old rats, specific activity of LH-dependent adenylate cyclase and the number of binding sites for LH/hCG per mg protein remained unchanged during the first 24h, but 48h after injection a 2-to 4-fold increase in both parameters was observed. By contrast, there was no change in basal adenylate cyclase activity or in the response of the enzyme to the stimulatory action of guanosine-5'-(beta gamma-imino) triphosphate (Gpp (NH)p), GTP, or NaF. Specific activity of succinate cytochrome c reductase, glucose-6-phosphatase and 5'-nucleotidase were found to be unaffected by the hormonal pretreatment, although total protein determined in these homogenates increased 3-fold in the course of this treatment. It is inferred that during follicular maturation, FSH enhances the responsiveness of ovarian adenylate cyclase to LH by stimulating the insertion of LH/hCG-receptors into the cell membrane.     

Relationship of (8-lysine) vasopressin receptor transition to receptor functional properties in a pig kidney cell line (LLC-PK1)

In intact LLC-PK1 cells, occupancy of vasopressin receptors (Roy, C., and Ausiello, D. A. (1981) J. Biol. Chem. 256, 3415-3522) correlated with cell cAMP production. This relationship was observed as a function of hormone dose, incubation time, and changes in receptor affinity. However, the rate of cAMP production diminished with time in intact cells exposed to high hormone concentrations, even in the presence of a phosphodiesterase inhibitor. A rapid desensitization of adenylate cyclase activity was observed in minutes upon treatment of intact cells with high hormonal concentrations. Desensitization was dose- and time-dependent. Hypertonic sodium chloride, which increased hormonal binding and cell cAMP production, prevented desensitization. The acute decrease in hormone-stimulated adenylate cyclase activity correlated with increased occupancy of low affinity binding sites. EDTA-suspended cells, which have a homogeneous population of binding sites, did not demonstrate desensitization. A proposal is made as to the consequences of this phenomenon at physiological concentrations of vasopressin.     

Thyroid hormone differentially regulates development of beta-adrenergic receptors, adenylate cyclase and ornithine decarboxylase in rat heart and kidney.

In mature animals, thyroid hormone produces parallel up-regulation of beta-adrenergic receptor binding sites and their linkage to adenylate cyclase; during development, these same processes may be critical in establishing the set-point for subsequent adrenergic reactivity. In the current study, we administered triiodothyronine to neonatal rats for the first five days postpartum and evaluated [125I]pindolol binding capabilities and adenylate cyclase activity in membrane preparations from heart and kidney. In the heart, hyperthyroidism elicited an initial increase in receptor density, with subsequent deficits and an eventual return to normal values by young adulthood. In contrast, the ability of isoproterenol, a beta-adrenergic agonist, to stimulate adenylate cyclase was enhanced regardless of whether receptor numbers were increased or decreased; the same effects were also present for basal adenylate cyclase activity and non-receptor-mediated stimulation by forskolin. Enhanced cyclase activity involved both increases in the magnitude of response as well as accelerated onset of the postweaning peak of enzyme activity, results which suggest a direct impact of thyroid status on the ontogenetic expression of adenylate cyclase itself. The kidney, which possesses less efficient beta-receptor coupling to adenylate cyclase in the neonate, was less drastically affected by triiodothyronine for either beta-receptor binding sites or enzyme activity. As we had previously shown that neonatal hyperthyroidism uncouples beta-receptors from growth-related enzymes, such as ornithine decarboxylase, we also evaluated whether the promotion of adenylate cyclase responses was mechanistically linked to effect on ornithine decarboxylase; administration of cyclic AMP analogs to 5 days-old rats led to inhibition of the enzyme in the heart, whereas the same treatment in 9 days-old animals was ineffective. These data suggest that thyroid hormone differentially regulates the development of beta-receptors as well as adenylate cyclase and ornithine decarboxylase, with preferential effects on tissues, such as the heart, that already possess efficient linkage of the receptors to cell transduction mechanisms at birth.     

Selective photoinduced uncoupling of the response of adenylate cyclase to gonadotropins by 5-iodonaphthyl 1-azide

5-Iodonaphthyl 1-azide (INA) has been previously shown to selectively label, on photolysis, only those proteins in contact with the membrane lipids. Low concentrations (less than 10 microM) of INA added to rat ovarian plasma membranes induced, on photoactivation, a selective and complete loss of the response of the adenylate cyclase to stimulation by human chorionic gonadotropin (hCG) or luteinizing hormone (LH). In contrast, this treatment affected neither hCG binding to the receptor nor the stimulation of the enzyme by NaF. That the uncoupling of the receptor from the enzyme by INA occurred within the lipid bilayer can be derived from the finding that the prior presence neither of saturating concentrations of hCG nor of the aqueous nitrene-scavenger glutathione (GSH) prevented this effect. Photolysis at higher concentrations of INA (0.1-1 mM) led to the inhibition of the adenylate cyclase stimulated by fluoride. This effect was totally prevented by glutathione. A similar behavior was obtained with a water-soluble analogue of INA, namely, 5-diazonionapthyl 1-azide (DAN). On photoactivation with 30 microM DAN, the NaF-stimulated adenylate cyclase was inhibited, but this effect was completely prevented by added GSH. At low concentrations where its effects are restricted to the lipid core, INA may represent a useful tool to define receptor coupling with the adenylate cyclase. The capacity of INA at low concentrations to uncouple the hormone receptor from the adenylate cyclase is not restricted to the LH/hCG receptor. Other hormone receptors tested behaved similarly. Therefore, the reported findings appear to represent a general phenomenon.     

Sulfhydryl group(s) in the ligand binding site of the D-1 dopamine receptor: specific protection by agonist and antagonist

An iodinated compound, [125I]-8-iodo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin -7-ol, has been recently reported [Sidhu, A., & Kebabian, J.W. (1985) Eur. J. Pharmacol. 113, 437-440] to be a specific ligand for the D-1 dopamine receptor. Due to its high affinity and specific activity, this ligand was chosen for the biochemical characterization of the D-1 receptor. Alkylation of particulate fractions of rat caudate nucleus by N-ethylmaleimide (NEM) caused an inactivation of the D-1 receptor, as measured by diminished binding of the radioligand to the receptor. The inactivation of the receptor sites by NEM was rapid and irreversible, resulting in a 70% net loss of binding sites. On the basis of Scatchard analysis of binding to NEM-treated tissue, the loss in binding sites was due to a net decrease in the receptor number with a 2-fold decrease in the affinity of the receptor for the radioligand. Receptor occupancy by either a D-1 specific agonist or antagonist protected the ligand binding sites from NEM-mediated inactivation. NEM treatment of the receptor in the absence or presence of protective compound abolished the agonist high-affinity state of the receptor as well as membrane adenylate cyclase activity. The above-treated striatal membranes were fused with HeLa membranes and assayed for dopamine-stimulated adenylate cyclase activity. When the sources of D-1 receptors were from agonist-protected membranes, the receptors retained their ability to functionally couple to the HeLa adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of carbohydrate in human chorionic gonadotropin: Deglycosylation uncouples hormone-receptor complex and adenylate cyclase system

Previous work has shown that deglycosylation of human chorionic gonadotropin (hCG) does not affect its receptor binding characteristics, but its ability to stimulate intracellular cyclic AMP accumulation and steroidogenesis in ovarian cells is abolished. To identify the site at which carbohydrate of hCG is involved in the mechanism of action of the hormone, we have studied adenylate cyclase activity in ovarian membrane preparations in response to deglycosylated and native hCG. The deglycosylated hCG does not stimulate adenylate cyclase of ovarian membrane preparation and also it acts as an inhibitor of hCG action. Data are presented to show that both hCG- and catecholamine receptors are coupled to the same adenylate cyclase complex. Since adenylate cyclase activity in the presence of deglycosylated hCG remains still responsive maximally to catecholamines, it indicates that the adenylate cyclase complex is functional and is unaffected by the interaction of deglycosylated hCG to its receptor. This is further supported by the fact that the deglycosylated hCG does not impair the maximal stimulation of adenylate cyclase by guanine nucleotides. Thus, the site of action of the carbohydrate of hCG is prior to the coupling of the hormone-receptor complex and the adenylate cyclase system.     

Inhibition of adenylate cyclase from luteinized rat ovary by monovalent cations: roles of the stimulatory guanine nucleotide-binding regulatory component and stimulatory hormone receptor

Sodium and other monovalent cations (added as chloride salts) inhibited adenylate cyclase of luteinized rat ovary. Sodium chloride (150 mM) inhibited basal enzyme activity by 20%. Sodium chloride inhibition was enhanced to 34-54% under conditions of enzyme stimulation by guanine nucleotides (GTP and its nonhydrolyzable analog 5'-guanylyl imidodiphosphate), fluoride anion, and agonists (ovine luteinizing hormone (oLH) and the beta-adrenergic catecholamine isoproterenol) acting at stimulatory receptors linked to adenylate cyclase. Sodium chloride inhibition was dependent on salt concentration over a wide range (25-800 mM) as well as the concentrations of GTP and oLH. Inhibition by NaCl was of rapid onset and appeared to be reversible. The order of inhibitory potency of monovalent cations was Li+ greater than Na+ greater than K+. The role of individual components of adenylate cyclase in the inhibitory action of monovalent cations was examined. Exotoxins of Vibrio cholerae and Bordetella pertussis were used to determine respectively the involvement of the stimulatory and inhibitory guanine nucleotide-binding regulatory components (Ns and Ni) in NaCl inhibition. Sodium chloride inhibited cholera toxin-activated adenylate cyclase activity by 29%. Ni did not appear to mediate cation inhibition of adenylate cyclase because pertussis toxin did not attenuate inhibition by NaCl. Enzyme stimulation by agents (forskolin and Mn2+) thought to activate the catalytic component directly was not inhibited by NaCl but was instead significantly enhanced. Sodium chloride (150 mM) increased both the Kd for high-affinity binding of oLH to 125I-human chorionic gonadotropin binding sites and the Kact for oLH stimulation of adenylate cyclase by sevenfold. In contrast, NaCl had no appreciable effect on either isoproterenol binding to (-)-[125I]iodopindolol binding sites or the Kact for isoproterenol stimulation of adenylate cyclase. The results suggest that in luteinized rat ovary monovalent cations uncouple, or dissociate, Ns from the catalytic component and, in a distinct action, reduce gonadotropin receptor affinity for hormone. Dissociation of the inhibitory influence of Ni from direct catalytic activation could account for NaCl enhancement of forskolin- and Mn2+-associated activities. On the basis of these results, the spectrum of divergent stimulatory and inhibitory effects of monovalent cations on adenylate cyclase activities in a variety of tissues may be interpreted in terms of differential enzyme susceptibilities to cation-induced uncoupling of N and catalytic component functions.     

Modulation of adenylate cyclase activity by sulfated glycosaminoglycans. I. Inhibition by heparin of gonadotrophin-stimulated ovarian adenylate cyclase.

Heparin inhibits (I50 = 2 microgram/ml) the activity of luteinizing hormone and human chorionic gonadotropin-stimulated adenylate cyclase in purified rat ovarian plasma membranes. Unstimulated enzyme activity and activity stimulated by NaF, GTP or guanosine 5'-(beta,gamma-imido)triphosphate were inhibited to a lesser extent. Human chorionic gonadotropin binding to this membrane preparation was inhibited by heparin (I50 = 6 microgram/ml). The inhibition with respect to hormone concentration was of a mixed type for hormone binding and adenylate cyclase stimulation. Inhibition by heparin was not eliminated at saturating hormone concentration. The degree of inhibition was unaffected by the order in which enzyme, hormone and heparin were introduced into the assay system. Heparin (3 microgram/ml) did not affect the pH activity relationship of basal and hormone-stimulated adenylate cyclase activity and did not change the dependence of enzyme activity on magnesium ion concentration. The inhibitory action of heparin cannot be solely attributed to interference with either catalysis or hormone binding. The possibility is considered that the highly charged heparin molecule interferes with enzyme receptor coupling, by restricting the mobility of these components or by effecting their conformation.     

小牛睾丸中G蛋白对FSH受体亲和性和腺苷酸环化酶活性的调节

本文用NEM(N-ethylmaleimide)为探针研究了G蛋白(鸟嘌呤核苷酸调节蛋白,Gp)对小牛睾丸中FSH受体的亲和性及腺苷酸环化酶活性的调节作用。证据表明,①在小牛睾丸细胞膜G蛋白上存在两种类型鸟嘌呤核苷酸结合位点(下简称GTP结合位点),高亲和性低容量结合位点及低亲和性高容量结合位点;②高亲和性结合位点(对NEM敏感)调节腺苷酸环化酶活性,而对NEM相对不敏感的低亲和性位点则不直接参与该酶活性的调节;③G蛋白对受体亲和性的调节则不仅要高亲和性位点的参与,而且主要受低亲和性位点的调节。     

Binding of [14,15-3H]14,15-dihydroforskolin to rat liver membranes: comparison with the stimulatory effect of forskolin on adenylate cyclase

The binding of [14,15-3H]14,15-dihydroforskolin ([3H]DHF) to rat liver membranes has been further characterized and was compared with the stimulatory effect of forskolin on adenylate cyclase. The binding equilibrium dissociation constant (KD) for 14,15-dihydroforskolin obtained in inhibition experiments was 0.6 microM, with a maximal binding capacity (Bmax) of 114 pmol/mg protein. A similar KD value (0.5 microM) was derived from kinetics studies that revealed very rapid association and dissociation reactions. For structure-activity relationship studies several forskolin derivatives were synthesized and tested for their ability to inhibit [3H]DHF binding and increase adenylate cyclase activity. Among the tested compounds, forskolin itself was the most potent agonist (K1 = 0.2 microM). Further modification of the molecule in position 7 and (or) 1 decreased or abolished its agonist properties in both adenylate cyclase and binding studies. [3H]DHF binding was not affected by several nucleotides, carbohydrates, lectins, and hormone receptor agonists including isoproterenol, glucagon, and adenosine, but the steroids 17-beta-estradiol, progesterone, and testosterone showed slight inhibitory effects at unphysiologically high concentrations. [3H]DHF binding and forskolin-stimulated adenylate cyclase were sensitive to heat and N-ethylmaleimide treatment. Forskolin protected adenylate cyclase against inactivation by heat but not by N-ethylmaleimide. Preincubation of the membrane with trypsin decreased [3H]DHF binding. The results presented in this study demonstrate that the binding sites identified with [3H]DHF have a high specificity for forskolin and provide evidence that these binding sites are involved in the stimulation of adenylate cyclase by forskolin.     

Association of triiodothyronine binding activity to soluble adenylate cyclase in testicular preparations

Summary Cytosolic adenylate cyclase activity from rat seminiferous tubules was purified by chromatography in DEAE-cellulose, hydroxylapatite and Bio-Gel A-0.5 m as well as by centrifugation in sucrose gradients. In all these purification steps, fractions with adenylate cyclase activity also contained binding activity for L-T₃. Binding studies indicate the existence of two L-T₃ receptor components associated to adenylate cyclase activity. The component exhibiting the highest hormone affinity has the lowest binding capacity.     

Thyrotropin binding to and adenylate cyclase activity of porcine thyroid plasma membranes.

Plasma membranes have been purified from porcine thyroid gland homogenate by discontinuous sucrose gradient centrifugation. The preparations contained specific binding sites for thyrotropin but not for luteinizing hormone or the beta subunits of thyrotropin and luteinizing hormone. Optimum conditions of 125I-labeled thyrotropin binding were pH 6.0-6.5 and 37 degrees C. Thyrotropin binding was reduced by divalent (Ca2+, Mg2+) and monovalent cations (Na+, K+, Li+), 50% inhibition being obtained at 10 mM and 50 mM respectively. Displacement curves of 125I-labeled bovine or porcine thyrotropin by the unlabeled hormone from three species was in the order of increasing concentrations (bovine greater than porcine greater than human) which is the order of decreasing biological activity of these hormone preparations in the assay in vivo in the mouse. The validity of the results was established by controlling that porcine membranes bound the native and the 125I-labeled hormones with equal affinity. A single type of high-affinity (Kd = 0.28 nM) binding sites was detected for bovine and porcine thyrotropins. In contrast, porcine plasma membranes bound human thyrotropin with a lower affinity (Kd = 70 nM). A good correlation was found at equilibrium and in the conditions of the cyclase assay, between receptor occupancy and adenylate cyclase activation for the three hormones.     

Developmental regulation of the pathways of folate-receptor-mediated stimulation of cAMP and cGMP synthesis in Dictyostelium discoideum

Recently, we demonstrated the presence of multiple folate-binding sites on the cell surface of Dictyostelium discoideum. These sites were divided into two major classes, with different ligand specificities (A and B). Each major class consists of several interconvertible subtypes. In the present report, the ability of 13 folate analogs to activate both adenylate and guanylate cyclase in pre- as well as postaggregative cells is examined. The patterns of correlation between binding and activation data indicate that guanylate cyclase activation is mediated by the B-sites in both developmental stages (P less than 0.001). In postaggregative cells, adenylate cyclase also seems to be activated by the B-sites (P less than 0.001). In contrast, adenylate cyclase activation in preaggregative cells was well correlated with the specificity of A-sites (P less than 0.01). Remarkably, the potencies of activation were less affected by molecular modifications than the binding affinities were, as suggested by a slope of 0.4 in a plot of K0.5 values of activation vs. binding. This observation argues against the existence of a transduction mechanism in which the response is proportional to receptor occupancy. For the B-receptor, however, the degree of receptor occupancy appears to determine the response. The existence of folic acid antagonists is demonstrated, some of which are specific for either A-sites coupled to adenylate cyclase or for B-sites coupled to guanylate cyclase.     

Solubilization and chromatographic separation of gonadotropin receptor from adenylate cyclase in ovarian preparations

The distribution of gonadotropin receptor and adenylate cyclase activities has been monitored in Sepharose 6B chromatographic eluates of a Lubrol solubilized fraction of 25 day old rat ovarian tissue. The receptor was resolved from the adenylate cyclase activity. Chromatography of a gonadotropin desensitized rat ovarian preparation showed that the desensitized preparation contained normal amounts of the adenylate cyclase activity when compared with the control ovarian preparation, but there was a marked reduction in the receptor activity in the desensitized ovaries. This study demonstrates that in ovarian tissue adenylate cyclase is a separate entity distinct from the receptor molecule and that the desensitization causes a reduction in the receptor activity with no detectable change in the catalytic and chromatographic properties of the adenylate cyclase.     

Calcitonin-sensitive adenylate cyclase in rat renal tubular membranes.

1. Renal tubular membranes from rat kidneys were prepared, and adenylate cyclase activity was measured under basal conditions, after stimulation by NaF or salmon calcitonin. Apparent Km value of the enzyme for hormone-linked receptor was close to 1 x 10(-8) M. 2. The system was sensitive to temperature and pH. pH was found to act both on affinity for salmon calcitonin-linked receptor and maximum stimulation, suggesting an effect of pH on hormone-receptor binding and on a subsequent step. 3. KCl was without effect areas whereas CoCl and CaCl2 above 100 muM and MnCl2 above 1 muM inhibited F- -and salmon calcitonin-sensitive adenylate cyclase activities. The Ca2+ inhibition of the response reflected a fall in maximum stimulation and not a loss of affinity of salmon calcitonin-linked receptor for the enzyme. 4. The measurement of salmon calcitonin-sensitive adenylate cyclase activity as a function of ATP concentration showed that the hormone increases the maximum velocity of the adenylate cyclase. GTP, ITP and XTP at 200 muM did not modify basal, salmon calcitonin- and parathyroid hormone-sensitive adenylate cyclase activities. 5. Basal, salmon calcitonin- and F- -sensitive adenylate cyclase activities decreased at Mg2+ concentrations below 10 mM. High concentrations of Mg2+ (100 mM) led to an inhibition of the F- -stimulated enzyme. 6. Salmon calcitonin-linked receptor had a greater affinity for adenylate cyclase than human or porcine calcitonin-linked receptors. There was no additive effect of these three calcitonin peptides whereas parathyroid hormone added to salmon calcitonin increased adenylate cyclase activity, thus showing that both hormones bound to different membrane receptors. Human calcitonin fragments had no effect on adenylate cyclase activity. 7. Salmon calcitonin-stimulated adenylate cyclase activity decreased with the preincubation time. This was due to progressive degradation of the hormone and not to the rate of binding to membrane receptors.     

Refractoriness of ovarian adenylate cyclase to continued hormonal stimulation.

Culture of preovulatory rat follicles with luteinizing hormone, follicle-stimulating hormone or prostaglandin E2 for 24 h reduced the subsequent response of adenylate cyclase to the homologous by 80, 50 and 90%, respectively; yet follicles refractory to luteinizing hormone fully responded to follicle-stimulating hormone responded to luteinizing hormone and prostaglandin E2, and those refractory to prostaglandin E2 could be stimulated by either gonadotropin. Desensitization of the adenylate cyclase system by luteinizing hormone was achieved by hormone concentrations of 0.8--2.0 mug/ml in the medium; a lower dose of luteinizing hormone (0.4 mug/ml), though effective in stimulating adenylate cyclase, did not induce refractoriness. Prostaglandin E2 caused partial refractoriness at dose levels of 0.1--0.25 mug/ml; higher dose levels were more effective. These findings suggest that continued exposure to the preovulatory follicle to elevated levels of hormones may cause perturbations in either the interaction between the hormone and its specific receptor or in a subsequent step essential for activation of adenylate cyclase.     

Modulation of adenylate cyclase activity by sulfated glycosaminoglycans I. Inhibition by heparin of gonadotropin- stimulated ovarian adenylate cyclase

Heparin inhibits (I₅₀ = 2 μg/ml) the activity of luteinizing hormone and human chorionic gonadotropin-stimulated adenylate cyclase in purified rat ovarian plasma membranes. Unstimulated enzyme activity and activity stimulated by NaF, GTP or guanosine 5′-(β,γ-imido)triphosphate were inhibited to a lesser extent. Human chorionic gonadotropin binding to this membrane preparation was inhibited by hepatin (I₅₀ = 6 μg/ml). The inhibition with respect to hormone concentration was of a mixed type for hormone binding and adenylate cyclase stimulation. Inhibition by heparin was not eliminated at saturating hormone concentration. The degree of inhibition was unaffected by the order in which enzyme, hormone and heparin were introduced into the assay system. Herapin (3 μg/ml) did not affect the pH activity relationship of basal and hormone-stimulated adenylate cyclase activity and did not change the dependence of enzyme activity on magnesium ion concentration. The inhibitory action of heparin cannot be solely attributed to interference with either catalysis or hormone binding. The possibility is considered that the highly charged herapin molecule interferes with enzyme receptor coupling, by restricting the mobility of these components or by effecting their conformation.     

Early hCG-induced desensitization in Leydig cells.

The early mechanism of hCG induced down regulation of its own receptor as well as steroidogenesis refractoriness of rat Leydig cells to gonadotropin stimulation have been investigated. A single injection of 5, 12, 25, 50 and 100 IU of hCG in rats induced within 8 hours, Leydig cells desensitization. However, apparent receptor loss was significantly lower only in the rats who received 50 and 100 IU of hCG. Cycloheximide inhibits hCG-induced receptors loss but had no effect on hCG-induced desensitization. The most likely explanation for desensitization in the presence of binding sites and a normal adenylate cyclase, is a defective coupling between the receptor sites and the catalytic subunit.