MCF-7; a human breast cancer cell line with estrogen, androgen, progesterone, and glucocorticoid receptors.

We have identified receptors for glucocorticoids, progestins, and androgens in a human breast tumor cell line (MCF-7) known to have estrogen receptor. Sucrose density gradients show that MCF-7 cytosol contains approximately 100 fm/mg protein estradiol (E2-3H) receptor, more than 300 fm/mg protein progesterone receptor (measured with R5020-3H), about 40 fm/mg protein 5alpha-dihydrotestosterone (5alpha-DHT-3H) receptor, and 800 fm/mg glucocorticoid receptor (measured with dexamethasone-3H). Dissociation constants obtained by Scatchard analyses were approximately 0.6 x 10(-10)M (E2), 1 x 10(-9)M (R5020), 2.8 x 10(-10)M (5alpha-DHT) and 8 x 10(-9)M (dexamethasone). No cross competition was found for estrogen receptor, but progestins competed for androgen and glucocorticoid binding. The androgen, but not the glucocorticoid, partially competed for R5020 binding to progesterone receptor. This first demonstration of 4 classes of steroid receptors in human breast cancer means that MCF-7 may be an excellent in vitro model for studying the mechanism of tumor response to endocrine therapy as well as the complex relationships between binding and biological actions of these hormones.     

Alpha- and beta-adrenergic receptors of the rat salivary gland. Elevation after chemical sympathectomy.

Binding of [3H]dihydroergokryptine and [3H]dihydroalprenolol to membrane preparations from rat submaxillary gland was measured to characterize the alpha- and beta-adrenergic receptors, respectively. Kinetic analysis of the data revealed a high affinity binding site for each radioligand. Inhibition of binding at each site was stereospecific for the active isomer of the catecholamine used. The greater ability of a beta1 than beta2 specific beta-adrenergic antagonist to displace [3H]dihydroalprenolol binding indicated that this binding site was of the beta1 type. Chemical sympathectomy with reserpine or 6-hydroxydopamine resulted in a significant increase in both [3H]dihydroalprenolol and [3H]dihydroergokryptine binding in the rat submaxillary gland. 3scatchard analysis of the data indicated that these increases in binding were due to a change in total number of binding sites for [3H]dihydroergokryptine and [3H]dihydroalprenolol with little change in apparent affinities. This suggests that changes in alpha- and beta-adrenergic receptor density may be important in the development of supersensitivity in salivary glands after reserpine and 6-hydroxydopamine treatment.     

Biochemical characterization of the BETA-adrenergic receptor of the frog erythrocyte

Summary The beta-adrenergic receptor which is coupled to adenylate cyclase in the frog erythrocycte plasma membrane provides a convenient model system for probing the molecular characteristics of an adenylate cyclase coupled hormone receptor. Direct radioligand binding studies with beta-adrenergic agonists and antagonists such as [³H]hydroxybenzylisoproterenol and [³H]dihydroalprenolol have shed new light on the biochemical properties of the receptor as well as on its mode of interaction with other components of the adenylate cyclase system. Agonist binding to the receptor induces a high affinity state of the receptor which can be selectively reverted to a low agonist affinity state by guanyl nucleotides. This agonist-induced high affinity state of the receptor appears to correspond to a receptor moiety which has larger apparent molecular weight and which is probably a complex of the beta-adrenergic receptor and nucleotide regulatory binding protein. Antagonists do not appear capable of inducing or stabilizing the formation of this high affinity receptor-nucleotide site complex.The beta-adrenergic receptors have been solubilized using the plant glycoside digitonin as the detergent and have been highly purified by biospecific affinity chromatography on an alprenolol-agarose affinity support. These highly purified receptor preparations retain all of the binding characteristics observed in the unpurified soluble receptor preparations.Remarkably, antibodies raised in rabbits against affinity chromatography purified preparations of the receptor, themselves bind beta-adrenergic ligands with typical beta-adrenergic specificity. Such antibodies which possess binding sites similar to those of physiological receptors provide useful model systems for further probing the molecular characteristics of beta-adrenergic binding sites.     

Regulation of alpha- and beta-adrenergic receptor subclasses by gonadal steroids in human myometrium

Both alpha- and beta-adrenergic receptors have been identified in the human myometrium by radioligand binding. Both adrenergic receptor subclasses have been shown to mediate the contractile response of the uterus upon catecholamine stimulation: alpha-adrenergic receptors cause uterine contraction while beta-adrenergic receptors induce relaxation. We have identified alpha 1- and alpha 2-adrenergic receptors in myometrial membranes using the newly developed radiolabelled specific antagonists [3H]-prazosin and [3H]-rauwolscine. This enabled us to characterize both receptor subclasses individually. Beta adrenergic receptors were identified using the radiolabelled antagonist (-)-[3H]-dihydroalprenolol. Binding of radioligands to the myometrial membrane receptors was rapid, readily reversible, of high affinity and stereoselective. The total number of alpha 1-, alpha 2- and beta-receptors was determined by Scatchard analysis of radioligand saturation binding and the beta/beta 2-receptor ratio was determined by computer analysis of the beta 2-selective antagonist ICI 118 551) (-)-[3H]-dihydroalprenolol competition binding curves. This enabled us to study the regulation of both alpha- and beta-receptor subclasses under various physiological and pharmacological conditions in the human, i.e., during different phases of the menstrual cycle, in postmenopausal women and during depo-progestin (Medroxyprogesterone acetate) therapy. Only the alpha 2- and beta 1-adrenergic receptor concentrations were found to be subjected to gonadal steroid regulation. The number of alpha 2- and beta 1-adrenergic receptors increased concomitantly with circulating plasma oestradiol levels. This effect was counteracted by progesterone. The number of alpha 1- and beta 2-adrenergic receptors was unaffected by the gonadal steroid environment. These results are an example of the heteroregulation of membrane receptors by oestrogens and progesterone and cast new light on the regulatory mechanisms involved in uterine contractility in the human.     

A potential role for catecholamines in the development and progression of carcinogen-induced mammary tumors: hormonal control of beta-adrenergic receptors and correlation with tumor growth.

In order to gain further knowledge on the beta-adrenergic receptor system in DMBA-induced rat mammary tumors, we have studied the correlation between changes in tumoral beta-adrenergic receptor concentration and distribution, progesterone receptor status and tumor growth after ovariectomy and treatment with various ovarian and adrenal steroids, or induction of hyperprolactinemia. Autoradiographic localization of beta-adrenergic receptors in ovariectomized (OVX) animals shows very weak labeling with [125I]cyanopindolol. In these tumors, the connective tissue is predominant, while the epithelial cell content is very low. Similarly, when direct measurements of [125I]cyanopindolol are performed with membrane preparations, beta-adrenergic receptor concentration is sharply reduced 2-3 weeks following ovariectomy or treatment with LHRH against [D-Trp6, des-Gly-NH2(10)]LHRH ethylamide. This effect on the beta-adrenergic receptor population in the tumor is accompanied by the well known effect of castration on tumor growth and progesterone receptor levels, namely a marked regression of tumor growth and a significant decrease in progesterone receptor concentration. Treatment of OVX rats with 17 beta-estradiol (E2) alone or in combination with progesterone (P) caused a highly significant increase in beta-adrenergic and progesterone receptor levels, as well as tumor growth. A similar sharp increase in the value of the three parameters studied was observed following daily treatment of OVX rats with dehydroepiandrosterone (DHEA) or androst-5-ene-3 beta,17 beta-diol (5-ene-diol). The autoradiographic localization of beta-adrenergic receptors in OVX rats treated with 5-ene-diol showed that the epithelial cells were numerous with a high degree of labeling. On the other hand, treatment of OVX animals with the androgen dihydrotestosterone (DHT) did not produce significant changes in beta-adrenergic receptor levels or tumor growth. Finally, endogenously-induced hyperprolactinemia by implanting three anterior pituitary glands under the kidney capsule of OVX animals resulted in a significant increase in beta-adrenergic and progesterone receptor levels as well as tumor growth. The positive correlation observed between changes in beta-adrenergic receptor concentration, progesterone receptor levels and tumor growth indicates a high sensitivity of the beta-adrenergic receptor population of DMBA-induced rat mammary tumors to the hormonal milieu, and suggests that the beta-adrenergic receptor system may represent a valuable parameter of hormone responsiveness.     

Effects of androgen on alpha- and beta-adrenergic receptors in membranes from the rat seminal vesicle.

The effects of castration and androgen-replacement on adrenergic receptors in membranes from the rat seminal vesicle were studied. Membranes from seminal vesicles showed saturable and high-affinity binding sites for the beta-adrenergic receptor antagonist, [3H]dihydroalprenolol ([3H]DHA), and the alpha 1-adrenergic receptor antagonist, [3H]prazosin. Castration markedly reduced beta-adrenergic receptors with decreasing the effect of GTP modulating the receptor-ligand affinity, suggesting defects in both the receptor per se and the guanine-nucleotides-regulating mechanism after castration. In contrast, castration increased alpha 1-adrenergic receptors and androgen-replacement reversed this change. The effects of GTP decreasing the alpha 1-receptor binding affinity to the radioligand were observed to a similar extent in the castrated and control membranes. These results demonstrate an inverse regulation by androgen on beta- and alpha 1-adrenergic receptors in membranes of the rat seminal vesicle.     

Effect of hydrocortisone on beta-adrenergic receptors in lung membranes.

It has been observed that glucocorticoids potentiate beta-adrenergic stimulation of cardiovascular and airway tissues. In order to investigate the mechanism of this potentiating action, we examined the effect of glucocorticoids on the number and affinity of beta-adrenergic receptors in animal lung tissues, by a direct binding technique using [125]I-Iodohydroxybenzylpindolol ([125]I-HYP), a potent beta-adrenergic receptor antagonist. Specific binding of [125]I-HYP to rat lung membranes was saturable with 386 fmol of [125]I-HYP/mg protein at saturation. The apparent equilibrium dissociation constant of [125]I-HYP for beta-receptors was 221 nM. Chronic administration of hydrocortisone increased the density of beta-adrenergic receptors by 70% from 386 fmol to 657 fmol/mg with some decrease in the affinity of [125]I-HYP for beta-adrenergic receptors. By contrast, adrenalectomy produced a 29% fall in the number of beta-adrenergic receptors without altering the affinity of [125]I-HYP for beta-receptors, and this change was reversed by exogenous adminstration of hydrocortisone. The present study suggests that glucocorticoids may participate in regulating the density of beta-adrenergic receptors, and may potentiate beta-adrenergic receptors stimulation, at least in part by increasing beta-receptor density in tissue membranes.     

Pituitary and brain D2 receptor density measured in vitro and in vivo in EEDQ treated male rats.

The effect of the alkylating compound N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (20 mg/kg, 24 h) on dopamine D2 receptor density in rat pituitary and brain was measured using in vitro and in vivo radioligand binding techniques. In the in vitro radioligand binding experiments EEDQ was found to reduce the density (Bmax) of [3H]-spiperone binding sites in the striatum by 86% while in the pituitary the corresponding decrease was only 37%. The affinity (KD) of the remaining striatal and pituitary D2 receptors was not different in EEDQ treated animals as compared to controls. When D2 receptor density was measured in vivo the effect of EEDQ was less pronounced. Thus, in rats given EEDQ the specific binding of either of the two D2 ligands [3H]-raclopride or [3H]-spiperone (administered in a single dose) in striatum and in the limbic forebrain was reduced by 45-62%; moreover, no significant decrease in pituitary D2 receptor density was observed. The data are discussed in relation to the finding (presented in a separate paper) that the same dose of EEDQ that failed to influence pituitary D2 receptor density as measured in vivo effectively antagonizes the prolactin decreasing effect of the partial D2 agonist (-)-3-(3-hydroxyphenyl)-N-n-propyl-piperidine [(-)-3-PPP].     

Characteristics of beta-adrenergic receptors in isolated cells and in crude membranes of brown adipose tissue

In relation to decreased metabolic sensitivity to catecholamines observed, in vitro, in brown fat of cold-acclimated rats, beta-adrenergic receptors were studied in isolated cells and in a crude membrane preparation from rat interscapular brown adipose tissue. [3H] dihydroalprenolol binding had the same characteristics in both types of preparation; competition studies of [3H] dihydroalprenolol binding led to the characterization of beta 1 subtype adrenergic receptors with a lower affinity of beta-adrenergic agonists for [3H] dihydroalprenolol binding sites in membranes than that found in isolated cells. Cold acclimation produced, in isolated cells only, a decrease of 41% in the [3H] dihydroalprenolol binding sites and a beta-adrenergic agonist affinity increase. It is concluded that beta-adrenergic receptor decrease could be a factor, at the hormone receptor interaction level, in the regulation of the transmission of biological action responsible for the cold-induced decrease in catecholamine responsiveness in brown adipose tissue. For a study of the desensitization process in brown fat, isolated cells seem to offer certain advantages over a crude membrane preparation.     

Identification of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian palate

A direct radioligand binding technique utilizing a beta-adrenergic antagonist [3H]Dihydroalprenolol [( 3H]DHA) was employed in the identification and characterization of fetal palatal beta-adrenergic receptors. [3H]DHA binding was saturable (Bmax 16 fmol/mg protein) with high affinity and an apparent equilibrium dissociation constant (KD) of 1.5 nM. Binding of [3H]DHA was displaced by the competitive beta-adrenergic antagonist propranolol in a concentration-dependent manner. Dissociation kinetic studies demonstrated almost complete reversibility of radioligand binding within 60 min. The functionality of these beta-adrenergic receptors was demonstrated by showing that fetal palatal mesenchymal cells responded to catecholamine agonists with dose-dependent accumulations of intracellular cAMP. This effect could be entirely blocked by the beta-antagonist, propranolol. The relative potency order of catecholamines in eliciting an elevation of cellular cAMP was characteristic of a beta 2-adrenergic receptor-mediated response: (-) isoproterenol greater than (-) epinephrine greater than (-) norepinephrine. In addition, this response was found to be stereospecific with (-) isoproterenol being significantly more potent than (+) isoproterenol. Both the [3H]DHA binding characteristics and the catecholamine sensitivity of fetal palatal tissue support the presence of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian secondary palate.     

Involvement of norepinephrine activity in the regulation of alpha 1 adrenergic receptors in the medial preoptic nucleus of estradiol-treated rats

To establish whether the diurnal decrease in the density of alpha 1 receptors observed in the medial preoptic nucleus (MPN) of estrogen (E2)-treated rats is related to the concomitant diurnal increase in norepinephrine (NE) turnover rates, we quantitated the density of [3H]-Prazosin binding to alpha 1 receptors after blockade of NE turnover with alpha-methyl-paratyrosine (alpha MPT). A series of preliminary studies was performed to rule out an interference of this drug with [3H]-Prazosin binding to alpha 1 adrenergic receptors in vitro and in vivo. Incubation of brain slices with alpha MPT produced a dose-dependent inhibition of [3H]-Prazosin binding to alpha 1 adrenergic receptors with an IC50 of approximately 6 mM. Scatchard analysis demonstrated that alpha MPT exhibited a simple competitive interaction with [3H]-Prazosin binding sites as shown by an increase in the apparent dissociation constant (Kd) of the ligand and no change in the number of alpha 1 receptors (Bmax). In contrast, preincubation of brain slices with alpha MPT and prior in vivo administration of alpha MPT did not affect [3H]-Prazosin binding to alpha 1 adrenergic receptors. Once we established that alpha MPT could be used to suppress NE turnover without interfering with the measurement of alpha 1 receptor densities, we repeatedly injected this drug to ovariectomized (OVX) and E2-implanted rats. The density of alpha 1 adrenergic receptors in MPN was quantitated autoradiographically. Blockade of NE turnover with alpha MPT only partially prevented the reduction in alpha 1 receptor density observed in the E2-treated rats, suggesting that the decrease in the level of [3H]-Prazosin binding sites cannot be completely ascribed to increased NE turnover rates.     

Muscarinic Acetylcholine Receptors in Neuroblastoma Cells: Lack of Effect of Veratrum Alkaloids on Receptor Number

The effect of compounds that activate sodium channels on the number of muscarinic acetylcholine receptors in neuroblastoma NIE 115 cells has been investigated. The cells were used in electrically unexcitable ("control" cells) and excitable ("differentiated" cells) states. Although receptor assays using a single concentration of the radioligand [3H]scopolamine methyl chloride indicated a loss of receptors after a 6-h incubation of cells with veratrine, no true loss of receptors was seen with any of the compounds tested (veratridine, veratrine, aconitine) when full saturation analyses were performed in either control or differentiated cells. The apparent receptor loss seen with veratrine was due to a muscarinic receptor-active component of veratrine (not veratridine) occluded by the cells and released into the binding assays upon cell breakage. Veratridine and aconitine have a very low affinity for muscarinic acetylcholine receptors, and the binding of carbamoylcholine to the receptors is unaffected by tetrodotoxin, so that there is no evidence in this system for interaction between muscarinic receptors and sodium channels.     

Guanyl nucleotide and divalent cation regulation of cortical S2 serotonin receptors

Computer-assisted quantitative analysis of radioligand binding to rat cortical S2 serotonin receptors indicates the existence of two affinity states of the same receptor population. Monophasic antagonist competition curves for [3H]ketanserin-labelled sites suggest a uniform population of receptors with one affinity state for antagonists. Biphasic competition curves of agonists suggest that agonists discriminate high- and low-agonist-affinity forms of the S2 receptors. The affinities of agonists for the high- and low-affinity states, and the apparent percentages of high agonist-affinity forms varies with different agonists. The guanine nucleotides GTP and guanyl-5'-imido-diphosphate [Gpp(NH)p], as well as divalent cations, modulate the proportion of the sites with high affinity for agonists as evidenced by their ability to shift the agonist competition curves for [3H]ketanserin-labelled S2 receptors. GTP and Gpp(NH)p effects appear to be agonist-specific, as they do not affect antagonist competition for [3H]ketanserin-labelled S2 receptors, or [3H]ketanserin binding to S2 receptors. ATP and ADP have little or no effect on the binding properties of S2 serotonin receptors, whereas GDP is less potent than GTP. The presence of these specific nucleotide effects are the first evidence suggesting involvement of a guanine nucleotide-binding protein in the mechanism of agonist interaction with the S2 serotonin receptor. In general, the binding properties of [3H]ketanserin-labelled S2 serotonin receptors strongly resemble those of adenylate-cyclase coupled receptors such as the beta-adrenergic, the alpha 2-receptor, and the D-2 dopamine receptor. This may indicate the S2 serotonin receptor is coupled to adenylate cyclase activity, through a GTP binding protein.     

Characterization of Neurotransmitter Receptors in the Rat Hippocampal Formation

The hippocampal formation has been extensively research in terms of its putative neurotransmitters, anatomical connections, and behavioral relevance. An aspect of importance is the assessment of apparent neurotransmitter receptors by using receptor binding assays. In the present study, such assays were done in vitro to investigate alpha 1-adrenergic, alpha 2-adrenergic, beta-adrenergic, muscarinic cholinergic, benzodiazepine, and opiate receptors in the rat hippocampal formation. The corresponding radioligands for these receptors were [3H]prazosin, [3H]p-aminoclonidine, [3H]dihydroalprenolol, [3H]quinuclidinyl benzilate, [3H]flunitrazepam, and [3H]naloxone. An analysis of the binding parameters for the ligands indicated saturable binding of a high affinity and the following rank order of maximal binding capacities: [3H]flunitrazepam greater than [3H]quinuclidinyl benzilate greater than [3H]naloxone greater than [3H]p-aminoclonidine greater than [3H]prazosin greater than [3H]dihydroalprenolol. Competition experiments with pharmacologic agonists and antagonists confirmed the specificity of each ligand. The results are integrated with information on other types of receptors and with neurotransmitter concentrations, and discussed in terms of hippocampal function.     

Differential DNA binding by calf uterine estrogen and progesterone receptors results from differences in oligomeric states

The studies presented here provided evidence that the calf uterine estrogen and progesterone receptors exhibit different DNA-binding properties in vitro as a result of having different dimerization constants. The affinity of the estrogen and progesterone receptors for DNA was measured by using isocratic elution from DNA-Sepharose. The hormone-free estrogen receptor had a 10-fold higher affinity for DNA than did the hormone-free progesterone receptor when measured at receptor concentrations of 6-12 nM and 180 mM KCl. No effect on DNA binding by binding progesterone to its receptor was detected. This contrasts with the increased affinity for DNA and increased number of ions released upon DNA binding exhibited by the hormone-bound estrogen receptor. Between 2 and 3 ions were released when the progesterone receptor and the diluted estrogen receptor bound DNA. These observations suggested the progesterone receptor was in the monomeric state, whereas the estrogen receptor was in the dimeric state at receptor concentrations of 6-12 nM. When the dimerization constant of the progesterone receptor was measured, the value of approximately 7 nM obtained was 20-fold higher than the value of 0.3 nM reported for the estrogen receptor. This makes it likely the two receptors exist in different forms at the same concentration in vitro. It is also suggested the predominant form of the estrogen and progesterone receptors in vivo could differ.     

The effect of propylbenzilylcholine mustard on contraction and radioligand binding parameters of muscarinic receptors in guinea pig ileum

The receptor occupancy-biological effect relationship for muscarinic receptors in guinea pig ileal smooth muscle has been studied by comparison of radioligand binding and contractile response. Muscarinic receptors in homogenates of ileal smooth muscle were labeled with [3H]-1-Quinuclidinyl benzilate. Treatment with propylbenzilylcholine mustard (PrBCM), to inactivate irreversibly muscarinic receptors, caused a large dose dependent rightward shift of the dose-response curve to three agonistic furtrethonium derivatives with a concomitant decrease in maximal response. Using those data, the fraction of receptors remaining unoccupied (q-values) and "true affinity constants" (-log KA-values) were calculated. Exposure to 20 or 60 nM PrBCM for 15 minutes resulted in a 39% and a 61% reduction in specific [3H]-1-Quinuclidinyl benzilate binding sites respectively to be compared with a 62% and a 85% decrease expected from calculated q-values. KA-values for the methyl and ethyl derivative agreed well with the dissociation constants for the high affinity agonist sites determined from displacement of [3H-]-1-Quinuclidinyl benzilate. The KA-value for the propylfurtrethonium corresponds to the low affinity agonist dissociation constant. The fraction of receptors in the high affinity agonist state differs considerably for the three furtrethonium derivatives investigated. Neither the fraction of receptors in the high affinity agonist state, nor the ratio of dissociation constants for these states is affected by the alkylation of 85% of the functional muscarinic receptors. The inactivation of components of the effector system by PrBCM seems unlikely.     

Assessment of mechanistic proposals for the binding of agonists to cardiac muscarinic receptors

N-[3H]Methylscopolamine has been used to characterize muscarinic receptors in crude homogenates prepared from hearts of Syrian golden hamsters. The Hill coefficient is one for specific binding of the radioligand itself and for its inhibition by muscarinic antagonists; markedly lower values are obtained for its inhibition by muscarinic agonists. The binding patterns of agonists have been analyzed in terms of a mixture of sites differing in affinity for the drug and reveal the following. All agonists discern at least two classes of receptor in atrial and ventricular homogenates. The number of classes and the relative size of each differ for different agonists in the same region and for the same agonist in different regions. Atrial and ventricular affinities are in good agreement for some agonists but differ for others. Guanylyl imidodiphosphate (GMP-PNP) is without effect on the specific binding of the radioligand but alters the binding of carbachol via an apparent redistribution of receptors from one class to another; the apparent affinity at either class remains unchanged. Carbachol reveals two classes of sites in ventricular preparations, and the nucleotide mediates an interconversion from higher to lower affinity; three classes are revealed in atrial preparations, and the nucleotide eliminates the sites of highest affinity with a concomitant increase in the number of sites of lowest affinity. Taken together, the data are incompatible with the notion of different, noninterconverting sites; rather, there appear to be several possible states of affinity such that the equilibrium distribution of receptors among the various states is determined by the tissue, by the agonist, and by neurohumoral modulators such as guanylyl nucleotides. The effects of agonists and GMP-PNP cannot be rationalized in terms of a ternary complex model in which the low Hill coefficients arise from a spontaneous equilibrium between receptor (R) and G protein (G) and in which agonists bind preferentially to the RG complex.     

Cytoplasmic progesterone receptors in the hypothalamus-preoptic area of the mouse: effect of estrogen priming

A synthetic progestin, R5020, was used to identify cytoplasmic progestin receptors in the hypothalamuspreoptic area (HPOA) of ovariectomized mice. These high-affinity receptors exhibited an apparent dissociation constant of approx. 1 nM. The receptors were specific for progestins. [3H]R5020 binding was inhibited by more than 50% with a 50-fold excess of either radioinert R5020 or progesterone. 5 alpha-Dihydroprogesterone inhibited binding to a lesser extent. 3 alpha-Hydroxy-5 alpha-pregnane-20-one and cortisol did not compete for [3H]R5020 binding. Administration of estradiol benzoate (10 micrograms), 48 h prior to death, resulted in a 54% increase in the HPOA progestin receptor concentration when compared to oil-injected controls. These data demonstrate that there are specific and saturable cytoplasmic progestin receptors in the mouse HPOA and that the concentration of these receptors is increased after estrogen treatment.     

Guanine nucleotides modulate the affinity of antagonists at beta-adrenergic receptors

Investigation of the properties of the binding of the radiolabelled antagonists (125I)-iodohydroxybenzylpindolol, (125I)-iodopindolol, and (125I)-iodocyanopindolol to beta-adrenergic receptors of L6 myoblast membranes revealed that guanine nucleotides caused a 2 to 4.5 fold increase in the apparent affinity of these antagonists. No significant effects of GTP were observed on the density of binding sites determined with each radioligand. GTP, GDP, and GMPPNP were of similar high affinity in producing this effect, while GMP was much less potent, and ATP was without effect. Under similar assay conditions GTP reduced the apparent binding affinity of the agonist isoproterenol for the beta-adrenergic receptors of L6 cells. The results indicate that, contrary to previous observations, guanine nucleotides affect not only the interactions of agonists with beta-adrenergic receptors, but also the interaction of antagonists with these adenylate cyclase-linked receptors.