The calcium requirement in GnRH-stimulated LH release is not mediated through a specific action on receptor binding

GPCR-radioligand binding assays

Radioligand binding assays provide sensitive and quantitative information about guanine nucleotide protein G protein-coupled receptor (GPCR) expression and affinity for a wide variety of ligands, making them essential for drug structure-activity studies and basic GPCR research. Three basic radioligand binding protocols, saturation, indirect (competition, displacement, or modulation), and kinetic binding assays, are used to assess GPCR expression (Bmax), equilibrium dissociation constants for radioligands (Kd) and nonradioactive ligands (Ki), association and dissociation rates, and to distinguish competitive and allosteric mechanisms of GPCR-ligand interactions. Nonspecific radioligand binding may be mitigated by appropriate choices of reaction conditions. Radioligand depletion (bound radioactivity >10% of total radioligand), which compromises accuracy of Kd and Ki measurements, can be limited by adjusting receptor concentration and appropriate radioligand choice. Accurate Kd and Ki values in saturation and indirect binding assays depend on binding equilibrium. Equilibration time for high-affinity ligands, with slow dissociation rates, may require much extended incubation times or increased incubation temperature.

Four naturally occurring mutations in the human GnRH receptor affect ligand binding and receptor function

In the present study, we performed functional analyses of four mutations in the human GnRH receptor (GnRHR) gene, identified in patients with idiopathic hypogonadotropic hypogonadism. These mutations result in amino acid substitutions in the extracellular N-terminal domain (Thr32Ile), second extracellular loop (Cys200Tyr), third intracellular loop (Leu266Arg) and sixth transmembrane helix (Cys279Tyr). Immunocytochemical analysis of cells transfected with HA-tagged GnRHR constructs revealed that all four mutant receptors were present on the cell surface. However, all four mutant receptors failed to exhibit measurable specific GnRH binding and, except for Thr32Ile, any significant inositol phosphate accumulation after GnRH stimulation. In addition, Leu266Arg and Cys279Tyr receptors were unable to stimulate gonadotropin subunit or GnRHR gene promoter activity in response to GnRH. Interestingly, the Cys200Tyr mutant was able to stimulate gonadotropin subunit and GnRHR promoter activity, albeit with a higher EC(50) and a markedly reduced maximal response compared to wild type receptor. The Thr32Ile mutant was also able to stimulate gonadotropin subunit and GnRHR promoters, but with a further significant increase in EC(50). Similarly, this mutant partially retained the ability to activate extracellular signal-regulated kinase 1 and stimulate CRE-luciferase activity with an identical shift in EC(50). Taken together, the studies suggest that the Thr32Ile mutation reduces hGnRHR function primarily by reducing ligand binding affinity, and the Cys200Tyr mutation reduces cell surface receptor expression. All four amino acid substitutions interfered with ligand binding, and affected signal transduction and stimulation of gonadotropin and GnRHR gene expression in response to GnRH.

Regulation of the gonadotropin-releasing hormone receptor (GnRHR) by RGS proteins: role of the GnRHR carboxyl-terminus

The cytoplasmic carboxyl-terminus of G-protein coupled receptors (GPCRs), absent in the mammalian gonadotropin-releasing hormone receptor (GnRHR), plays an important role in receptor expression, desensitization, internalization and efficiency of coupling to G proteins. Regulators of G protein signaling (RGS) likewise are involved in regulating GPCR-G protein mediated responses and can regulate transcription of other genes. In this study, we evaluate differential expression, ligand binding and effector coupling of the rat GnRHR (rGnRHR) and a chimera of rGnRHR with the pre-mammalian carboxyl domain (rGnRHR-C-tail). Membrane expression of the chimeric receptor and G(q)alpha and G(s)alpha-mediated signaling was increased 2- and 1.5-fold, respectively by RGS10, while RGS3 did not interfere with rGnRHR and rGnRHR-C-tail cell surface expression in spite of negatively regulating GnRH-stimulated G(q)alpha-mediated signaling by both receptors. The rGnRHR and rGnRHR-C-tail showed similar internalization rates in the presence of either RGS protein, indicating that the modification of rGnRHR expression and regulation in the presence of a carboxyl-terminus by RGS10 was not caused by alteration of the internalization rate. The observations in this study implicate the carboxyl domain of the receptor as a site of interaction for RGS10, but not RGS3. This is the first evidence of an altered cell surface expression and regulation of the GnRHR bearing a carboxyl-terminus by RGS proteins.

Conserved mammalian gonadotropin-releasing hormone receptor carboxyl terminal amino acids regulate ligand binding, effector coupling and internalization

The mammalian gonadotropin-releasing hormone receptor (GnRHR), with 327 amino acids, is among the smallest G protein coupled receptors identified. Absent from this receptor is the cytoplasmic tail, characteristic of other members of this superfamily, which frequently mediates desensitization and down-regulation. The fifteen carboxyl terminal residues in the mammalian GnRHR are absolutely conserved, suggesting important roles for these residues. In the current study, mutations of the mammalian GnRHR were made to study the carboxyl terminus. The receptor mutant GnRHR(Ser(326)Ala) was reduced in ligand affinity (117% reduction compared to wild type (wt)), while receptor numbers and internalization remained unchanged. GnRHR(Ser(326)Tyr) was decreased in effector coupling, while ligand affinity remained unchanged compared to wt. These studies also show that, while mutation of Ser(326) caused a change in ligand binding and effector coupling, truncation at this residue (GnRHR[des(326-327)]) had no measurable effect on GnRHR ligand binding, effector coupling or internalization, functions which appear to require different structural determinants than expression and routing. Removal of all three carboxyl terminal residues (Phe(325), Ser(326) and Leu(327)) or mutation of the receptor (GnRHR[Phe(325)Ala]) caused a complete loss of measurable ligand binding and effector coupling, clearly suggesting an unexplained role for Phe(325).

Activin A augments GnRH-mediated transcriptional activation of the mouse GnRH receptor gene

The response of pituitary gonadotropes to GnRH correlates directly with the concentration of GnRH receptors (GnRHRs) on the cell surface, which is mediated in part at the level of GnRHR gene expression. We have previously localized GnRH responsiveness in the mouse GnRHR (mGnRHR) gene promoter to two elements: activating protein-1 and sequence underlying responsiveness to GnRH-1. This study was designed to investigate potential synergy between GnRH and activin A in transcriptional activation of the mGnRHR gene. In functional transfection studies using alphaT3-1 cells, GnRH agonist stimulation of the mGnRHR gene promoter (-765/+62) resulted in a 10.9-fold increase in activity, which was further increased 2-fold (to 21.3-fold) following activin pretreatment. Activin pretreatment alone had no effect. Deletion of region -387/-308 or mutation of a putative SMAD-binding element at -331/-324 (5'-GTCTAG[T]C-3') abrogated the augmented response to GnRH in the presence of activin but not the response to GnRH alone. Overexpression of SMAD2 and SMAD3 along with SMAD4 increased transcriptional activity of the mGnRHR gene, which was further increased by GnRH agonist stimulation. These data demonstrate that activin augments GnRH-mediated transcriptional activation of the mGnRHR gene and suggest that this effect may be mediated through SMAD transcription factors.

Two novel mutations in the gonadotropin-releasing hormone receptor gene in Brazilian patients with hypogonadotropic hypogonadism and normal olfaction

Several point mutations in the GnRH receptor gene have been described in an autosomal recessive form of congenital isolated hypogonadotropic hypogonadism (HH). We investigated 17 Brazilian patients (10 males and 7 females) from 14 different families, with HH and normal olfaction. The diagnosis of HH was based on absent or incomplete sexual development after 17 yr of age associated with low or normal levels of LH in both sexes and low levels of testosterone in males and of estradiol in females. All patients presented with a normal sense of smell in an olfactory specific test. The coding region of the GnRH receptor gene was amplified by PCR and directly sequenced. A novel missense mutation, Arg(139)His, located in the conserved DRS motif at the junction of the third transmembrane and the second intracellular loop of the GnRH receptor was identified in the homozygous state in one female with complete HH. The Arg(139)His mutation completely eliminated detectable GnRH-binding activity and prevented GnRH-induced stimulation of inositol phosphate accumulation in vitro. In another family, a new compound heterozygous mutation (Asn(10)Lys and Gln(106)Arg) was identified in four siblings (two males and two females) with partial HH. The Gln(106)Arg mutation, located in the first extracellular loop, has been previously described, and in vitro analysis indicated that the mutant receptor was able to bind GnRH, but with a reduced affinity. The Asn(10)Lys mutation in the extracellular amino-terminal domain of the receptor also reduced the affinity for GnRH in vitro. In this family we also identified a previously described silent polymorphism at amino acid residue 151 in the second intracellular loop that segregated with the two inactivating mutations of the GnRH receptor. This polymorphism was also found in two unrelated patients with sporadic HH without GnRH receptor loss of function mutations. No mutations were identified in the remaining cases. A good correlation between genotype and phenotype was found in our patients. The woman, who is homozygous for the completely inactivating Arg(139)His mutation, has complete HH with undetectable serum basal LH and FSH levels that failed to respond to GnRH stimulation. In addition, the affected patients who are compound heterozygotes for the Asn(10)Lys/Gln(106)Arg mutations, have partial HH with low serum basal LH levels that were responsive to GnRH stimulation. No clinical or hormonal differences were found between HH patients with and without mutations in the GnRH receptor gene, indicating that these data do not contribute to the identification of HH patients with GnRH receptor mutations. In conclusion, we report the first naturally occurring mutation within the conserved DRS motif of the GnRH receptor in a female with complete HH and a novel compound heterozygous mutation (Asn(10)Lys and Gln(106)Arg) in a family with partial HH, increasing the repertoire of the inactivating mutations of the GnRH receptor.

Combined modification of intracellular and extracellular loci on human gonadotropin-releasing hormone receptor provides a mechanism for enhanced expression

The mammalian gonadotropin-releasing hormone (GnRH) receptor (GnRH-R) has been a therapeutic target for human and animal medicine. This receptor is a unique G-protein-coupled receptor that lacks the intracellular C-terminal domain commonly associated with this family. Development of highthrough put screens for agents active in humans has been hampered by low expression levels of the hGnRH-R in cellular models. Two sites have attracted the interest of laboratories studying regulation of expression. The chimeric addition of the C-terminal tail from catfish GnRH-R (cfGnRH-R) to the rat GnRH-R significantly augmented receptor expression in GH3 cells. In addition, rodent GnRH-R contains 327 amino acids, but cow, sheep, and human GnRH-R (hGnRH-R) contain 328 residues, the "additional" residue being a Lys 191. Deletion of Lys 191 (del 191) from the hGnRH-R resulted in increased receptor expression levels and decreased internalization rates in both COS-7 and HEK 293 cells. In this study, the combined effect of the addition of the C-tail from cfGnRH-R and deletion of the Lys 191 from the hGnRH-R was compared to expression of the wild-type (WT) or either alteration alone in a transient expression system using primate cells. The altered receptor (hGnRH-R[del 191]-C-tail) showed significantly increased receptor expression at the cell surface compared with the WT or either modification alone. The inositol phosphate response to stimulation was also significantly elevated in response to GnRH agonist. After treatment with a GnRH agonist, the altered receptors showed a slower internalization rate. The homologous steady-state regulation of the WT and the altered receptors was similar, although the response of the altered receptors was significantly decreased. These results suggest that the conformational change in the receptor as a result of the deletion of Lys 191 and the addition of the C-terminus tail substantially increased the steady-state receptor expression and decreased internalization and homologous regulation. Because the effects on expression are greater than additive, it appears that these alterations exert their effects by differing means. These techniques for expression of the hGnRH-R in transfected mammalian cells provide the basis for a therapeutic screen for GnRH analogs, agonists, and antagonists of the hGnRH.

Mutations at the consensus phosphorylation sites in the third intracellular loop of the rat gonadotropin-releasing hormone receptor: effects on receptor ligand binding and signal transduction

In this study, site-directed mutagenesis of potential phosphorylation sites (Thr238, Ser253, and Thr264) for protein kinase C and C-terminal portion (Ala260-Leu265) of the third intracellular loop of the rat GnRH receptor (rGnRHR) was performed to assess the significance of these regions in the function of the GnRHR. Mutation at one or all of the three potential phosphorylation sites had differential effects on receptor ligand binding. Mutation of Ser253 or Thr264 to Ala did not significantly affect the receptor-binding affinity but decreased the number of measurable binding sites. Mutation of Thr238 to Ala or triple mutation of Thr238, Ser253, and Thr264 impaired or abolished receptor-binding affinity. Mutations of the potential phosphorylation sites affected receptor-mediated inositol phospholipid (IP) production and correlated with alterations in receptor binding after mutation, but they did not significantly affect receptor-mediated cAMP production or cAMP-mediated prolactin release. In addition, mutation of Ser253 or Thr264 to Ala did not affect the GnRH-provoked desensitization in terms of GnRH agonist-stimulated IP production. Deletion of the C-terminal portion (Ala260-Leu265) of the third intracellular loop of the rGnRHR, including a potential phosphorylation site (Thr264), abolished the receptor-binding affinity and receptor-mediated signal transduction. Replacement of the deleted C-terminal portion with a C-terminal portion (Ala-Ala-Arg-Thr-Leu-Ser) of the third intracellular loop of the Gq/11-coupled rat M1 muscarinic acetylcholine receptor did not restore receptor function. These results suggest that the potential phosphorylation sites or the region around the phosphorylation site of the third intracellular loop of the GnRHR is important for the structural integrity and expression of the receptor but that phosphorylation at these sites is not required for desensitization.

Visualization of unoccupied and occupied gonadotropin-releasing hormone receptors in living cells

Three chimeras of the rat GnRH receptor (rGnRHR) and an enhanced green fluorescent protein (GFP) were assessed to examine their suitability as probes of the receptor in transfected GH3 cells. Direct fusion of GFP to the N or C terminus of the rGnRHR abolished the receptor ligand binding affinity and the chimeric receptors were intracellularly localized. In contrast, rGnRHR-Ctail-GFP, a fusion of the N-terminus of the GFP to the C-terminus of the rGnRHR with the intracellular C-terminal tail of the catfish GnRHR as an intermediate spacer, was functional in terms of plasma membrane localization, ligand binding ability, receptor-mediated signal transduction and pattern of homologous down-regulation. The functional chimera of GnRHR and GFP provided a useful model for observation of GnRHR distribution and agonist-stimulated trafficking in living cells.

Characterization of gonadotropin-releasing hormone (GnRH) binding sites in the pituitary and testis of the frog, Rana esculenta

Frog, Rana esculenta, pituitary and testis gonadotropin-releasing hormone (GnRH) receptors were characterized by using 125I-chicken IIGnRH (cIIGnRH) as radiolabeled ligand. At 4 C equilibrium binding of 125I-cIIGnRH to pituitary homogenates was achieved after 90 min of incubation; binding of 125I-cIIGnRH to testis membrane fractions reached its maximum at 60 min of incubation. Binding of the radioligand was a function of tissue concentration, with a positive correlation over the range 0.5-2 tissue equivalents per tube. One pituitary and one testis per tube were used as standard experimental condition. Incubation of the pituitary homogenate with increasing concentrations of 125I-cIIGnRH indicated saturable binding at radioligand concentrations of 1 nM and above while for the testis membrane preparation saturation was achieved using 5 nM 125I-cIIGnRH. The binding of 125I-cIIGnRH was found to be reversible after addition of the cold analog and the displacement curves could be resolved into one linear component for both tissues. Scatchard analysis suggested the presence of one class of binding sites for both pituitary and testis (Pituitary: Kd = 1.25 +/- 0.14 nM and Bmax = 8.55 +/- 2.72 fmol/mg protein; testis: Kd = 2.23 +/- 0.89 nM and Bmax = 26.48 +/- 7.39 fmol/mg protein). Buserelin displaced the labeled 125I-cIIGnRH with a lower IC50 as compared with cIIGnRH cold standard, while Arg-vasopressin (AVP) was completely ineffective, confirming the specificity of binding.

Exogenous action of 5-lipoxygenase by its metabolites on luteinizing hormone release in rat pituitary cells

The stimulatory effect of exogenously administered potato 5-lipoxygenase (0.1-0.3 U/2 ml) on luteinizing hormone (LH) release was demonstrated in rat anterior pituitary cells in a superfusion system. Nordihydroguaiaretic acid (NDGA), an inhibitor of 5-lipoxygenase, abolished the effect of the enzyme on LH secretion. The secretory effect on LH after 5-lipoxygenase administration was biphasic and dependent on Ca2+ indicating that 5-lipoxygenase affects LH release through its oxygenation reaction. Another series of experiments demonstrated that activation of 5-lipoxygenase, expressed as production of leukotriene (LT) B4 and C4 (728 +/- 127 pg/10(6) cells and 178 +/- 23 pg/10(6) cells, respectively) occurs in rat pituitary cells after addition of Ca2+ ionophore A23187. However, LTB4 and LTC4 were not formed by pituitary cells that had previously been desensitized by gonadotropin-releasing hormone (GnRH), the physiological ligand of LH release. These results are consistent with a role of 5-lipoxygenase metabolites in the mechanism of GnRH-induced LH secretion.

Characterization of gonadotropin-releasing hormone binding sites in the pituitary of the three-spined stickleback, Gasterosteus aculeatus

Binding sites for gonadotropin-releasing hormone (GnRH) in stickleback pituitary homogenates were characterized using an iodinated, superactive analog of salmon GnRH (sGnRH), D-Arg6-Pro9-sGnRH-NEt (sGnRHa). Binding of 125I-sGnRHa reached equilibrium after 60 min incubation at 4 degrees and was a function of tissue concentration. The specificity of 125I-sGnRHa binding was demonstrated by displacement with sGnRHa, sGnRH, and Buserelin [D-Ser(t-Bu)6-Pro9-GnRH-NEt]. Both Scatchard analyses of saturation data and displacement curves revealed a single class of high-affinity binding sites (Ka = 0.71 +/- 0.03 X 10(9) M-1, Bmax = 1087 +/- 165 fmol/mg protein).

Effects of lanthanum in cellular systems. A review

Lanthanum belongs to the group of elements known as "lanthanons," which also includes cerium, europium, promethium, and thulium. It is the most electropositive element of the rare earth group, is uniformly trivalent, and is similar in its chemical properties to the alkaline earth elements. The effects of this element and its compounds on cellular systems are of considerable interest because of their increasing use in industry and as a substitute or antagonist for calcium in a variety of cellular reactions. Lanthanum is also being employed extensively in studying anatomical barriers, membrane structure, and subcellular transport systems, particularly the calcium pathway.

Gonadotropin-releasing hormone (GnRH) antibodies formation in hypogonadotropic azoospermic men treated with pulsatile GnRH--diagnosis and possible alternative treatment

Five hypogonadotropic azoospermic men received pulsatile, intravenous gonadotropin-releasing hormone (GnRH) treatment over prolonged period. In three patients, the spouses were successful in achieving five pregnancies, three of which generated five healthy newborns, one ended in a first-trimester abortion, and one is ongoing. In one patient, anti-GnRH antibodies were detected, secondary to initial response. This was associated with deterioration of gonadotropin levels and diminution in testosterone to pretreatment levels. The cross-reactivity of the antibodies with five GnRH agonistic analogs was examined. Possible treatment with pulsatile GnRH analogs in such patients is discussed. In light of the relatively long period of treatment needed to achieve spermatogenesis and fertility in patients with hypogonadotropic azoospermia, monitoring the appearance of GnRH antibodies seems appropriate to assess whether therapy should be concluded. The superiority of intravenous pulsatile GnRH treatment to subcutaneous GnRH treatment or to human menopausal gonadotropin (hMG)/human chorionic gonadotropin (hCG) treatment is discussed.

Characterization of the receptor for gonadotropin-releasing hormone in the pituitary of the African catfish,Clarias gariepinus

Receptors for gonadotropin-releasing hormone (GnRH) were characterized using a radioligand prepared from a superactive analog of salmon GnRH (sGnRH), D-Arg(6)-Pro(9)-sGnRH-NEt (sGnRHa). Binding of(125)I-sGnRHa to catfish pituitary membrane fractions reached equilibrium after 2 h incubation at 4°C. Displacement experiments with several GnRH analogs as well as other peptides, demonstrated the specificity of(125)I-sGnRHa binding. Specific binding was enhanced in the presence of the cation chelator ethylene bis (oxyethylenenitrilo) tetra-acetic acid (EGTA), indicating an inhibitory effect of cations on GnRH-receptor binding. The binding of(125)I-sGnRHa to pituitary membranes was found to be saturable at radioligand concentrations of 5 nM and above. A Scatchard analysis of the saturation data suggested the presence of a single class of high-affinity binding sites (Ka=0.901±0.06×10(9)M(-1), Bmax=1678±150 fmol/mg protein). A comparative study on(125)I-sGnRHa binding to pituitary membrane fractions of male and female catfish, indicated that there were no differences in binding affinity and binding capacity between both sexes. The results demonstrate the presence of specific, saturable GnRH receptors in the African catfish pituitary.

Characterization of luteinizing hormone-releasing hormone receptor binding in rat pituitary cell monolayer cultures; influence of intercellular communication

Receptors for the luteinizing hormone-releasing hormone (LHRH) were characterized in rat pituitary cells cultured for 3 days as monolayers on coverslips using 125I-[D-Ala6-Pro9-LHRH-NEt] as the labeled ligand. The monolayers were left intact during the binding assay. Specific binding displayed the various characteristics of binding to the physiological LHRH receptor. Various kinetic data corresponded to those reported previously. However, in these cultured cells, in which binding was tested in a physiological medium, the dose response of competition for binding by LHRH agonists ranged over a smaller concentration range (less than 2 orders of magnitude) than that by LHRH antagonists. In a cation-free buffer competition curves of agonists and antagonists were parallel but the apparent dissociation constant was lower than in the physiological medium. In cultures of pituitary cell populations separated by unit gravity sedimentation, the specific binding increased with the proportional number of gonadotrophs in the various populations. However, when the gonadotroph-richest population (approximately equal to 70% gonadotrophs) was cultured after recombination with gonadotroph-poor populations, binding capacity significantly increased. Microscopic examinations suggested that this phenomenon was the consequence of disrupting cellular contacts among gonadotrophs. It is concluded that certain characteristics of LHRH receptors tested on cells in a tissue-like organization and in a physiological environment are different from those reported previously in disrupted cells or monodispersed cell suspensions and that intercellular communication is an important factor controlling LHRH receptors.

Copper and thiol regulation of gonadotropin releasing hormone binding and luteinizing hormone release

The effects of copper, sulfhydryl and disulfide reagents on gonadotropin, releasing hormone (GnRH) binding to pituitary membrane preparations and on luteinizing hormone (LH) release from pituitaries of immature female rats were studied. Copper ions reduced the specific binding of 125I-labeled [D-Ser(t-Bu)6, des-Gly10-ethylamide]GnRH (Buserelin) to pituitary membranes in a dose responsive manner. Fifty percent inhibition of the specific binding was obtained at 3 x 10(-5)M Cu+2. The decreased binding stems from a 8-fold decrease in the apparent affinity of Buserelin, in the presence of copper ions. Cupric sulfate was examined for its ability to affect basal and GnRH stimulated LH release. Copper stimulated basal LH release 18-fold and 8-fold at 10(-4)M and 10(-5)M, respectively. The LH release in response to Cu+2 was calcium dependent. The effect of Cu+2 on GnRH stimulated LH release was a combined effect of Cu+2 on GnRH binding. GnRH on LH release and Cu+2 on LH release. N-ethylmaleimide, hydrogen peroxide and dithiothreitol did not alter significantly the specific binding of 125I-labeled Buserelin. N-ethylmaleimide and hydrogen peroxide did not affect GnRH stimulated LH release, whereas dithiothreitol (1 mM) significantly inhibited GnRH stimulated LH release. The present findings may explain the induction of ovulation by copper and the reduction in serum LH after injection of reducing agents.

Gonadotropin releasing-hormone receptors: photoaffinity labeling with an antagonist

A photoaffinity antagonist of gonadotropin releasing hormone (GnRH), D pGlu-D-Phe-D-Trp-Ser-D-Lys6(N epsilon-azidobenzoyl)-Leu-Arg-Pro-Gly-NH2 (photoaffinity antagonist) was prepared by reacting [D-pGlu1, D-Phe2, D-Trp3, D-Lys6]GnRH with the N-hydroxysuccinimide ester of 4-azidobenzoic acid. The analog appeared homogeneous when analyzed by thin-layer chromatography and its photoreactivity was demonstrated by spectral changes when exposed to light. The photoaffinity antagonist retained high affinity binding to the GnRH receptor of pituitary membrane preparations and exhibited antagonistic activity when assayed in vitro in whole pituitaries. Pituitary membrane preparations were incubated with the radioactive photoaffinity GnRH antagonist and irradiated with light. Sodium dodecyl sulfate gel electrophoresis after solubilization and reduction showed the specific labeling of a single specific protein with an apparent molecular weight of 60,000 daltons. These results indicate that GnRH agonists and antagonists bind to the same receptor.

Does calcitonin modulate anterior pituitary hormone secretion?

In a group of 5 healthy subjects salmon CT (sCT) infusion was unable to induce significant variations on basal secretory levels of LH, FSH, PRL and TSH. In a second group of 5 normal subjects, GnRH and TRH tests were performed both during sCT and saline infusion; a clear inhibition of TSH-stimulated levels and of PRL area was documented, while gonadotropin secretion was not significantly affected by sCT infusion. These results suggest that CT effect might be attributed to a change in intracellular calcium of pituitary cells; however the different behavior between TRH-and GnRH-stimulated hormones might be due to a different hormonal release mechanism. Furthermore the widespread recognition of CT-like immunoreactivity in adenohypophysis and in portions of the central nervous system suggests that CT may be a neurotransmitter or paracrine regulator.

Some characteristics of GnRH receptors in rat-pituitary membranes: differences between an agonist and and antagonist

The interaction of gonadotropin-releasing hormone (GnRH) agonists and antagonists with pituitary membranes was studied by using 125I-labeled agonist, [D-Ser(t-Bu)6, des-Gly10-ethylamide]-GnRH, and antagonist [D-pGlu1, D-Phe2, Trp3,6]-GnRH. Their binding was affected differently by cations, and by pretreatment of membranes with proteolytic enzymes and sulfhydryl-blocking reagents. Monovalent cations at millimolar concentrations (10-100 mM) and divalent cations at lower concentrations (0.5-5 mM) reduced more significantly the binding of the agonist than that of the antagonist. Pretreatment of the membranes with trypsin and chymotrypsin abolished the specific binding of both agonist and antagonist, in a dose-response manner, with the former being less affected. Pretreatment of the membranes with sulfhydryl-blocking reagents did not alter the binding of the antagonist but enhanced the binding of the agonist. This enhancement in the specific binding was found to be due to an increase in the apparent affinity of the agonist. These results may suggest that GnRH agonists and antagonists bind differently to the same receptor.

Role of calcium in the modulation of ornithine decarboxylase activity in isolated pig granulosa cells in vitro

We examined the role of Ca(2+) in the control of basal and hormone-stimulated ornithine decarboxylase activity in isolated pig granulosa cells maintained under chemically defined conditions in vitro. Omission of Ca(2+) from the incubation medium (measured Ca(2+) concentration 5mum) decreased basal enzymic activity, and significantly (P<0.01) impaired the response to maximally stimulating doses of either lutropin or follitropin. No significant alteration occurred in the concentration of either gonadotropin required to elicit half-maximal effects. The addition of EGTA (1.27-2.0mm) to chelate residual extracellular Ca(2+) further decreased hormone-induced rises in ornithine decarboxylase activity. Despite the presence of 1.27mm concentrations of extracellular Ca(2+), the administration of presumptive Ca(2+) antagonists, believed to impair trans-membrane Ca(2+) influx [verapamil (10-100mum), nifedipine (1-100mum) or CoCl(2) (1mm)] suppressed hormone-stimulated ornithine decarboxylase activity. The inhibitory effects of verapamil or of Ca(2+) omission from the medium were not overcome by the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (0.25mm), or by cholera toxin, or by an exogenously supplied cyclic AMP analogue, 8-bromo cyclic AMP. Conversely, micromolar concentrations of a putative bivalent-cation ionophore, A23187, increased significantly the stimulation of ornithine decarboxylase activity by saturating concentrations of lutropin or 8-bromo cyclic AMP. Thus the present observations implicate Ca(2+) ions in the modulation of hormone action and cellular function in normal ovarian cells.

Perifusion of rat pituitaries: requirements for optimal GnRH-stimulated LH release

Perifusion of rat hemipituitaries was used as an in vitro model for luteinizing hormone (LH) release in response to gonadotropin-releasing hormone (GnRH). LH release during continuous stimulation with GnRH (10(-7) M) was inhibited by EGTA (1.5 or 5 mM given 1 h prior to GnRH) or by D-600 (methoxyverapamil, 1 mM, given concomitantly with GnRH). These findings suggested that GnRH-stimulated LH release was a Ca2+-dependent process. Inhibition of LH release caused by Ca2+ chelation with EGTA was reversed when hemipituitaries were returned to medium without EGTA. Elevation of medium K+ (to 50 mM, without GnRH) stimulated Ca2+-dependent LH release during the 1st h. GnRH-stimulated LH release was 60% inhibited by cycloheximide (25 micrograms/ml) after the 1st h. LH release was not stimulated by dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP), although this cyclic nucleotide was shown to have biological activity and to enter pituitary cells as judged by its ability to stimulate prolactin release from the same tissue. The data suggest that optimal LH release in response to GnRH requires extracellular Ca2+ and depends on protein synthesis.

Receptor-mediated internalization of fluorescent gonadotropin-releasing hormone by pituitary gonadotropes

A bioactive, fluorescent derivative of gonadotropin-releasing hormone, < Glu-His-Trp-Ser-Tyr-D-Lys(N epsilon-tetramethylrhodamine)-Leu-Arg-Pro-Gly-NH2, was prepared. This peptide retained high-affinity binding (apparent dissociation constant, 3 nM) to the receptor for gonadotropin-releasing hormone and was utilized for microscopic visualization and localization of gonadotropin-releasing hormone receptors in cultured rat pituitary cells. The fluorescently labeled receptors were initially distributed uniformly on the cell surface and formed patches, which subsequently internalized (at 37 degrees C) into endocytic vesicles. These processes were dependent on specific binding sites for the rhodamine-labeled peptide to gonadotrope cells. Cluster formation and internalization were markedly reduced in the absence of Ca2+, which is required for gonadotropin secretion. It is possible that cluster formation, microaggregation, and internalization of gonadotropin-releasing hormone receptors may be important in eliciting biological effects or for the observed loss of tissue responsiveness after desensitization due to exposure to the homologous hormone.