Agonist binding promotes a guanine nucleotide reversible increase in the apparent size of the bovine anterior pituitary dopamine receptors

Quasi-irreversible binding of agonist to beta-adrenoceptors and formation of non-dissociating receptor-G(s) complex in the absence of guanine nucleotides

Here, we tested the hypothesis that receptor-G protein and agonist may form an irreversible complex in the absence of guanine nucleotides. We used the beta-adrenoceptor-G(s) system of guinea pig lung parenchymal membranes as a model. Two groups of membranes were used in the experiments: (1) washed with nucleotide-free buffer in the presence of isoproterenol (isoproterenol-treated), and (2) washed with buffer alone or with agonist+GDP (both were treated as control). Results were as follows: (1) the iodopindolol binding capacity of isoproterenol-treated membranes was reduced by about 30%. (2) No such reduction was observed in control membranes. (3) Addition of GDP to the isoproterenol-treated membranes completely restored the pindolol binding capacity. We interpreted this result as indicating irreversible agonist-receptor complex is formed when the receptor interacts with nucleotide-free G(salpha). (4) We observed a single peak of beta(2)-adrenoceptor activity in the control group by size-exclusion chromatography of the solubilized membranes. Inclusion of isoproterenol in the washing buffer led to an additional (heavier) peak of beta(2)-adrenoceptor activity. This peak disappeared when GDP was added to the detergent extract before high-pressure liquid chromatography (HPLC) analysis. Western blot analysis of these HPLC fractions showed that the agonist-induced heavier peak contained significantly more G(salpha) protein than did the other fractions. We interpreted this result as indicating that a practically irreversible complex of receptor and G protein is formed in the absence of GDP. We suggest that the tightly bound (nucleotide-free) receptor-G protein complex also contains the agonist, and that this complex can be reversed only by the addition of nucleotides. The implications of these results are also discussed.

Identification of G protein-coupled, inward rectifier potassium channel gene products from the rat anterior pituitary gland

Dopamine (DA) is a physiological regulator of PRL secretion, exerting tonic inhibitory control. DA activates an inward rectifier K(+) (IRK) channel in rat lactotropes, causing membrane hyperpolarization and inhibition of Ca(2+)-dependent action potentials. Both the activation of this effector K(+) channel and the inhibition of PRL release are mediated by D(2)-type receptor activation and pertussis toxin- sensitive G proteins. To study the molecular basis of this physiologically relevant channel, a homology-based PCR approach was employed to identify members of the IRK channel family expressed in the anterior pituitary gland. Nondegenerate primers corresponding to regions specific for IRK channels known to be G protein activated (GIRKs; gene subfamily Kir 3.0) were synthesized and used in the PCR with reverse transcribed female rat anterior pituitary messenger RNA as the template. PCR products of predicted sizes for Kir 3.1, 3.2, and 3.4 were consistently observed by ethidium bromide staining after 16 amplification cycles. The identities of the products were confirmed by subcloning and sequencing. Expression of each of these gene products in anterior pituitary was confirmed by Northern blot analysis. Functional analysis of the GIRK proteins was performed in the heterologous expression system, Xenopus laevis oocytes. Macroscopic K(+) currents were examined in oocytes injected with different combinations of Kir 3.0 complementary RNA (cRNA) and G protein subunit (beta(1)gamma(2)) cRNA. The current-voltage relationships demonstrated strong inward rectification for each individual and pairwise combination of GIRK channel subunits. Oocytes coinjected with any pair of GIRK subunit cRNA exhibited significantly larger inward K(+) currents than oocytes injected with only one GIRK channel subtype. Ligand-dependent activation of only one of the GIRK combinations (GIRK1 and GIRK4) was observed when channel subunits were coexpressed with the D(2) receptor in Xenopus oocytes. Dose-response data fit to a Michaelis-Menten equation gave an apparent K(d) similar to that for DA binding in anterior pituitary tissue. GIRK1 and GIRK4 proteins were coimmunoprecipitated from anterior pituitary lysates, confirming the presence of native GIRK1/GIRK4 oligomers in this tissue. These data indicate that GIRK1 and GIRK4 are excellent candidate subunits for the D(2)-activated, G protein-gated channel in pituitary lactotropes, where they play a critical role in excitation-secretion coupling.

The dopamine D(4) receptor: one decade of research

Dopamine is an important neurotransmitter involved in motor control, endocrine function, reward, cognition and emotion. Dopamine receptors belong to the superfamily of G protein-coupled receptors and play a crucial role in mediating the diverse effects of dopamine in the central nervous system (CNS). The dopaminergic system is implicated in disorders such as Parkinson's disease and addiction, and is the major target for antipsychotic medication in the treatment of schizophrenia. Molecular cloning studies a decade ago revealed the existence of five different dopamine receptor subtypes in mammalian species. While the presence of the abundantly expressed dopamine D(1) and D(2) receptors was predicted from biochemical and pharmacological work, the cloning of the less abundant dopamine D(3), D(4) and D(5) receptors was not anticipated. The identification of these novel dopamine receptor family members posed a challenge with respect to determining their precise physiological roles and identifying their potential as therapeutic targets for dopamine-related disorders. This review is focused on the accomplishments of one decade of research on the dopamine D(4) receptor. New insights into the biochemistry of the dopamine D(4) receptor include the discovery that this G protein-coupled receptor can directly interact with SH3 domains. At the physiological level, converging evidence from transgenic mouse work and human genetic studies suggests that this receptor has a role in exploratory behavior and as a genetic susceptibility factor for attention deficit hyperactivity disorder.

A2a adenosine receptors: guanine nucleotide derivative regulation in porcine striatal membranes and digitonin soluble fraction

We report the characterization of A2a adenosine receptors (A2aARs) in porcine striatal membranes and their solubilization (25%) by the detergent digitonin. After solubilization, the drug specificity and equilibrium [3H]CGS-21680 ([3H]2-(4-(2-carboxyethyl)phenylethylamino)-5'-N-ethyl-carboxamido -adenosine) binding parameters were virtually identical to those obtained in intact membranes, indicating a conservation of the binding site after the removal of receptors from their lipid environment. Gel filtration on a calibrated Superdex 200 HR column revealed a main [3H]CGS-21680 binding peak with an apparent molecular weight of 171,000+/-9000 Da. In membranes, Scatchard analysis of saturation data carried out in a wide range of radioligand concentration (1-100 nM) resulted in a biphasic curve and, in accordance with the two binding sites model, yielded a Kd1 = 7.4+/-0.5 and Kd2 = 53.1+/-3.6 nM, a Bmax1 = 186+/-15 fmol/mg protein and a Bmax2 = 285+/-20 fmol/mg protein, respectively. In the presence of guanosine-5'-O-(3-thiotriphosphate) (GTPgamma[S]) a shift from two affinity states to a single one was evidenced (Kd = 28.5+/-5.9 nM) and a Bmax value of 504+/-10 fmol/mg protein found. In the soluble extract, only one high-affinity state was detected (Kd = 19.3+/-1.1 nM and Bmax = 285+/-20 fmol/mg protein) and, in the presence of GTPgamma[S]), a two site model likewise provided a significantly (P < 0.01) better fit (Kd1 = 13.9+/-1.2 nM and Kd2 = 72.1+/-6.9 nM, Bmax1 = 125+/-10 fmol/mg protein and Bmax2 = 375+/-19 fmol/mg protein, respectively). These results suggest a close relation between the receptor and G protein solubilized as a functional unit and open the way to its purification.

Solubilization and biochemical characterization of the human myometrial corticotrophin-releasing hormone receptor

We have solubilized an active form of the myometrial corticotrophin-releasing hormone (CRH) receptor using 1% w/v digitonin. The solubilized receptor retains its capacity for high-affinity binding as demonstrated by Scatchard analysis, although there was a shift in dissociation constant (Kd) from 83.6 +/- 15-195 +/- 35 pM for the membrane-bound and soluble receptor respectively. There was no difference in the maximum binding site concentrations (Bmax) of 13 +/- 5 and 21.5 +/- 6 fmol/mg protein for the membrane-bound and soluble receptor respectively. Sauvagine unlike CRH had no effect on radiolabeled CRH binding which suggests that the CRH-R2 receptor is not present in the myometrium. The solubilized receptor did not retain guanine-nucleotide sensitivity. The isoelectric focusing (IEF) profile of the human myometrial CRH receptors was significantly different from that of the rat cerebral cortex. Furthermore, solubilization of human myometrial membrane proteins followed by gel filtration and SDS-PAGE revealed a specifically labeled protein with an apparent molecular weight of 42000-47000 kDa. Our results suggest that during solubilization the human myometrial CRH receptor is dissociated from the guanine nucleotide-binding protein (Gs) and that high affinity binding for soluble CRH receptors is not dependent on the coupling of a guanine nucleotide-binding protein.

Cardiac muscarinic receptors. Relationship between the G protein and multiple states of affinity

An expanded version of the mobile receptor model has been assessed in studies on the binding of N-[3H]methylscopolamine and [35S]GTPgammaS to cardiac muscarinic receptors and their attendant G proteins in ventricular membranes from hamster. The model comprises two pools of receptor, one of which lacks G proteins, and a heterogeneous population of G proteins that compete for the receptor within the G protein-containing pool. To guide the formulation of the model itself and to define the various parameters, data were combined from assays performed under various conditions with native membranes and following irreversible blockade of about 80% of the receptors with propylbenzilylcholine mustard. Multiple G proteins are indicated primarily by multiple states of affinity evident in the dose-dependent effect of guanyl nucleotides on the binding of carbachol; G protein-free receptors are indicated by sites of low affinity for carbachol that survive treatment with the mustard. The expanded model generally succeeds where more frugal schemes have been inadequate, but it nevertheless fails to yield a mechanistically consistent description of the data. Guanyl nucleotides and partial alkylation do not affect the inhibitory potency of carbachol in a manner consistent with their supposed effect on the equilibrium between uncoupled and G protein-coupled receptors. As inferred from the model, G proteins are lost upon alkylation of the receptor, and their numbers are regulated by guanyl nucleotides. Parameters estimated via N-[3H]methylscopolamine are wholly inconsistent with the same parameters estimated via [35S]GTPgammaS. The failure of the model suggests that multiple states of affinity may not arise from a ligand-regulated equilibrium between free receptors and G proteins on the one hand and one or more RG complexes on the other.

Regulation of the CHAPS-solubilized muscarinic receptors by an inhibitory GTP binding protein (Gi) in the brain of neonatal and adult rats

CHAPS-solubilized muscarinic receptors (mAChRs) retain selective agonist or antagonist binding and sensitivity to GTP and ADP-ribosylation by pertussis toxin that ribosylates a group of G protein called Gi. This suggests that CHAPS solubilizes the mAChR-Gi complex both from neonatal and adult neurons. The gradient centrifugation of solubilized samples indicated that the neonatal samples contain mostly Gi-coupled mAChR (mAChRn) exhibiting a high affinity for [3H]OxM, but the adult samples contain a comparable quantity of Gi coupled mAChR (mAChRa1) and uncoupled mAChRs (mAChRa2) exhibiting low affinity for the agonist. The binding of [3H]OxM to or the dissociation of [3H]OxM from mAChRn and mAChRa1 was sensitive to GTP. However, the effects of GTP were sensitive to Na+ for mAChRn but not for mAChRa1. ADP-ribosylation by pertussis toxin but not by cholera toxin abolished the effects of Na+ on agonist binding to solubilized mAChRn, suggesting that NaCl affects mAChRn not by interacting with the receptor but by interacting with the mAChR-Gi complex. The subtype composition of mAChRn, mAChRa1 and mAChRa2 was studied by determining the IC50 and the Ki values for the inhibition by subtype-selective antagonists in the binding of [3H]OxM to the receptors. The neonatal and adult samples exhibited differences in the distribution of the mAChR subtypes, the neonatal samples containing mostly the M1-mAChR subspecies but the adult samples containing M1-, M2- and M3-mAChR subspecies. The pirenzepine-induced inhibition in the binding of [3H]OxM both to M1-mAChRn and M1-mAChRa1 was sensitive to GTP. The GTP-induced effects on mAChRn were modulated by Na+, but the effect on M1-mAChRa1 was not. This suggests functional differences between Gi coupled M1-mAChRn and M1-mAChRa1.

Solubilization and characterization of GABAB receptor binding sites from porcine brain synaptic membranes

1. The characteristics of membrane bound GABAB receptors in pig brain are similar to those in rat brain as judged by in vitro binding experiments and sensitivity to GTP. The rank order of affinity of GABAB receptor ligands was CGP 54626 > GABA approximately (-)-baclofen >> CGP 35348 = CGP 36742 > (+)-baclofen in membranes from both species. 2. For solubilization of GABAB receptors from pig brain, washed membranes were preincubated with 5 mM MgSO4 and subsequently incubated with various detergents. 3-[(3-Cholamidopropyl)dimethyl-ammoniol]-1-propane sulphonate (CHAPS) (0.5%) proved to be the most successful, solubilizing 22.7 +/- 4.7% (mean +/- s.e. mean, n = 6) of GABAB receptors. 3. Binding of [3H]-GABA to GABAB receptors solubilized with 0.5% CHAPS exhibited similar characteristics to the binding at membrane bound receptors since, firstly, the Kd and Bmax values (around 30 nM and 450 fmol mg-1 protein, respectively) were comparable; secondly, stereospecific binding for baclofen was obtained in both forms; thirdly, the affinity for the agonists GABA and (-)-baclofen and the antagonists CGP 35348, CGP 36742 and CGP 54626 were the same; fourthly, comparable sensitivity to Ca2+ (2.5 mM) was observed and finally, a similar sensitivity to GTP was apparent. 4. Saturation experiments performed with the GABAB antagonist, [3H]-CGP 54626, indicated a higher Kd value and a lower Bmax value for solubilized (7.7 +/- 2.6 nM and 1033 +/- 41 fmol mg-1 protein, mean +/- s.e. mean, n = 3) than for membrane bound receptors (1.35 +/- 0.08 nM, 1171 +/- 20 fmol mg-1 protein, n = 3).

Cibacron blue-induced enhancement of agonist binding to cholecystokinin (CCK) receptors in solubilized pancreatic membranes

The pancreatic receptor for cholecystokinin (CCK) typifies many G protein-coupled receptors in that its ability to bind agonist can be reduced by GTP or the solubilization of membranes. We found, however, that a dye, cibacron blue, caused up to a 6-fold increase in binding of the CCK receptor agonist, 125I-CCK-8, to rat pancreatic membranes solubilized with digitonin. Binding optimally enhanced in this manner was comparable to binding of 125I-CCK-8 to native membranes with respect to time-course, maximal amount bound, reversibility, and sensitivity to inhibition by various CCK receptor ligands. Increases in affinity of the CCK receptor for CCK-8 accounted fully for the enhancement of binding of 125I-CCK-8. Cibacron blue did not enhance binding of 125I-CCK-8 to native membranes, and also failed to enhance binding of the CCK receptor antagonist, [3H]L-364,718, to solubilized or native membranes. The ability of cibacron blue to enhance binding of agonist but not that of antagonist suggests that this dye may mimic or perhaps stimulate the effects of G protein on CCK receptors. Such a phenomenon may provide new insights into the mechanisms by which receptors distinguish agonists from antagonists.

Solubilization of active GLP-1 (7-36)amide receptors from RINm5F plasma membranes

Glucagon-like peptide-1 (7-36)amide (GLP-1 (7-36)amide) represents a physiologically important incretin in mammals including man. Receptors for GLP-1 (7-36)amide have been described in RINm5F cells. We have solubilized active GLP-1(7-36)amide receptors from RINm5F cell membranes utilizing the detergents octyl-beta-glucoside and CHAPS; Triton X-100 and Lubrol PX were ineffective. Binding of radiolabeled GLP-1(7-36)amide to the solubilized receptor was inhibited concentration-dependently by addition of unlabeled peptide. Scatchard analysis of binding data revealed a single class of binding sites with Kd = 0.26 +/- 0.03 nM and Bmax = 65.4 +/- 21.24 fmol/mg of protein for the membrane-bound receptor and Kd = 22.54 +/- 4.42 microM and Bmax = 3.9 +/- 0.79 pmol/mg of protein for the solubilized receptor. The binding of the radiolabel to the solubilized receptor was dependent both on the concentrations of mono- and divalent cations and the protein/detergent ratio in the incubation buffer. The membrane bound receptor is sensitive to guanine-nucleotides, however neither GTP-gamma-S nor GDP-beta-S affected binding of labeled peptide to solubilized receptor. These data indicate that the solubilized receptor may have lost association with its G-protein. In conclusion, the here presented protocol allows solubilization of the GLP-1(7-36)amide receptor in a functional state, and opens up the possibility for further molecular characterization of the receptor protein.

Human choriogonadotropin-induced coupling of receptor and Gs protein and the effect of hormone deglycosylation

The detergent-soluble extract of rat ovary plasma membranes contained a Gs protein of about 100 kDa as shown by its elution behavior on a Bio Gel A-1.5m column. However, the cell membranes exposed to hCG (37 degrees C, 15 min) contained in addition a higher molecular weight Gs protein complex of 300 kDa comprised of human chorionic gonadotropin (hCG) receptor (hCGR) and Gs. The complex bound with an affinity column of GTP-Sepharose and could be released with Gpp(NH)p and GTP inhibited this binding. The presence of the hCGR in the complex was shown by its binding to 125I-hCG. Furthermore, GTP inhibited the binding of hCG to the complex. These results indicate the presence of hCGR and Gs protein complex in the hCG-treated membranes. hCGR and Gs protein were individually purified and reconstituted into phospholipid vesicles. The protein-phospholipid vesicles showed saturation kinetics of binding of 125I-hCG and 3H-Gpp(NH)p. Incubation of phospholipid vesicles with hCG resulted in a 2-3-fold increase in the binding of 3H-Gpp(NH)p and GTPase activity. Activation of Gs protein was dependent on the length of incubation and the hormone concentration. Deglycosylated hCG was about 10 times less potent than hCG suggesting a role of carbohydrates of hCG in inducing hCG-Gs protein interactions. The data with the in vitro reconstitution system rule out the involvement of a carbohydrate-binding lectin in the function of the hormone.

Properties of adenotin reconstituted into phospholipid vesicles

Adenotin is a low affinity adenosine binding protein that has amino terminal homology with mammalian and avian stress proteins. Human placental adenotin was solubilized and reconstituted into phospholipid vesicles with an overall yield of 30%. The properties of adenotin in vesicles were similar to the native membranes as follows: association has a Kobs of 0.61 +/- 0.03 minute-1; equilibrium is reached in approximately 15 minutes; and the first order dissociation constant is 5.0 +/- 0.3 minute-1. Displacement analysis reveals an agonist potency order and Ki values as follows: N-ethylcarboxamidoadenosine, 0.35 microM; 2-chloroadenosine, 1.5 microM; R-phenylisopropyladenosine, greater than 1000 microM. The addition of 100 microM 5'-guanylylimidodiphosphate did not decrease binding of 5'-N-ethylcarboxamidoadenosine (NECA) at 37 degrees C or 4 degrees C but did decrease the IC50 for PC12 and JAR cell membrane agonist binding from 9.9 to 3.3 microM and increase the binding to 150-211% of the control value at 37 degrees C. The latter studies at 37 degrees C showed high variability. Using binding sites reconstituted into vesicles and gel filtration chromatography and agonist related guanine nucleotide release, the authors investigated whether these changes were related to an interaction between adenotin and a guanine nucleotide regulatory protein. No evidence for such an interaction was found. These data suggest that adenotin retains its binding properties when reconstituted into phospholipid vesicles. The function of this low affinity adenosine binding site remains to be discovered. However, the reconstitution of adenotin into phospholipid vesicles provides a method to study its function.

EQUIL: simulation and data analysis of binding reactions with arbitrary chemical models

We have developed an algorithm for simulation and analysis of arbitrary chemical systems in equilibrium, with emphasis on ligand binding reactions. The program EQUIL can treat reactions involving multiple ligands, multiple binding sites, ternary complex models, allosteric effectors, competitive and noncompetitive binding, conformational changes, cooperativity, and generally any scheme that can be represented as a set of chemical equations. EQUIL is based on a general thermodynamic model of chemical equilibria; it does not involve nonlinear transformation of experimental data, but it does require the user to define the model of interaction between ligands and receptors by writing down the appropriate chemical reactions. EQUIL contains features of particular importance to ligand binding experiments: variable binding capacities, nonspecific binding, and the ability to simultaneously analyze data from different types of experiments. Furthermore, the simulation feature of EQUIL allows the user to investigate the feasibility of experiments that could possibly distinguish between different reaction models. We illustrate the use of this program on personal computers to analyze and simulate simple and complicated interactions between ligands and receptors.

Solubilization of A1 adenosine receptor from pig brain: characterization and evidence of the role of the cell membrane on the coexistence of high- and low-affinity states

The present solubilization strategy recognizes the important role of detergent cocktails in the solubilization and subsequent stability of adenosine A1, receptors from pig brain cortical membranes. The 3-[3-(cholamidopropyl)dimethylammonio]-1-propane-sulfonate-digitonin mixture produced the extraction of up to 52% of the receptor with an enrichment of 1.2-fold with respect to crude membranes. The binding activity of the soluble extract was very stable even in the absence of glycerol. In crude membranes the existence of high- and low-affinity states was detected, but in the soluble extract and in the detergent-treated membranes only the high-affinity state was detected. Association-dissociation curves showed that in crude membranes no interconversion between high- and low-affinity sites is produced by the association of the ligand [3H]R-N6-phenylisopropyladenosine. These results suggest that the high- and low-affinity states are different conformations induced by the structure of the membrane. The modulation of the binding activity by (Gpp(NH)p) 5'-guanylylimidodiphosphate and Mg2+ was studied. In crude membranes Gpp(NH)p shifted the high-affinity state to the low-affinity state, whereas the contrary occurred when Mg2+ was used. The effect of both Mg2+ and Gpp(NH)p was also assayed with the soluble extract and with the detergent-treated membranes. In addition to a decrease of the overall binding capacity, Gpp(NH)p promoted a conversion to all low-affinity states in the detergent-treated membranes or to all very-low-affinity sites in the soluble extract. Mg2+ and Gpp(NH)p counteracted their effects in intact membranes, whereas Mg2+ could not reverse the uncoupling effect of Gpp(NH)p with solubilized or detergent-treated membranes. Thus, it is suggested that Mg2+ acts at sites other than guanine-nucleotide-sensitive sites. If high-affinity states correspond to receptor/G protein complexes and low-affinity states correspond to the uncoupled receptor, we should conclude that Mg2+, as well as the loss of membrane integrity, favours the interaction of A1 receptor molecule with G protein.

Solubilization and reconstitution of dopamine D1 receptor from bovine striatal membranes: effects of agonist and antagonist pretreatment

The bovine striatal dopamine D1 receptor was solubilized with a combination of sodium cholate and NaCl in the presence of phospholipids, following treatment of membranes with a dopaminergic agonist (SKF-82526-J) or antagonist (SCH-23390). The solubilized receptors were subsequently reconstituted into lipid vesicles by gel-filtration. A comparison of ligand-binding properties shows that the solubilized and reconstituted receptors bound [3H]SCH-23390 to a homogeneous site in a saturable, stereospecific and reversible manner with a Kd of 0.95 and 1.1 nM and a Bmax of 918 and 885 fmol/mg protein respectively for agonist- and antagonist-pretreated preparations. These values are very similar to those obtained for membrane-bound receptors. The competition of antagonists for [3H]SCH-23390 binding exhibited a clear D1 dopaminergic order in the reconstituted preparation obtained from either agonist or antagonist-pretreated membranes, except that (+)butaclamol was about four-fold more potent than cis-flupentixol in displacing [3H]SCH-23390 binding in preparation obtained from agonist-pretreated membranes compared to antagonist-pretreated membranes. The agonist/[3H]SCH-23390 competition studies revealed the presence of a high-affinity component of agonist binding in both the reconstituted receptor preparations. The number of high-affinity agonist binding sites, however, is 40-80% higher in reconstituted preparation obtained from antagonist-treated membrane compared to that obtained from the agonist-treated membrane. In both the preparations, 100 microM guanylylimidodiphosphate (Gpp(NH)p) completely abolished the high-affinity component of agonist binding compared to partial abolition in the native membranes, indicating a close association of a G-protein with the solubilized receptors. Whether the receptor was solubilized following agonist or antagonist preincubation of the membranes, the receptor-detergent complex eluted from a steric-exclusion HPLC column with an apparent molecular size of 360,000. Preincubation of the solubilized preparations with Gpp(NH)p had virtually no effect on the elution profile suggesting a lack of guanine nucleotide-dependent dissociation of G-protein receptor complex.

Structural differences between dopamine D2 receptors present in a rat pituitary adenoma and in transplantable rat pituitary tumors 7315a and MtTW15

We have investigated the structure of dopamine (DA) D2 receptors present in an estrone-induced, prolactin (PRL)-secreting, DA-sensitive adenoma and in two PRL-secreting and DA-insensitive transplantable tumors 7315a and MtTW15, in order to identify better the anomalies present in DA-resistant lactotrophs. D2 receptors were found in both a high- and a low-affinity state in adenomatous lactotrophs as shown by displacement studies with the agonist N-propylnorapomorphine (NPA), but only in the low-affinity state in the two DA-resistant tumors. Treatment with the alkylating agent N-ethylmaleimide induced a disappearance of the high-affinity state of the D2 receptor in the adenoma and a reduction in receptor concentration, but did not have any effect on the affinity of receptors present in DA-resistant tumors. Moreover, target size analysis and radiation inactivation studies of D2 receptors, using membranes preincubated with NPA and [3H]spiperone as ligand or using [3H]NPA as ligand on membranes preparations, have shown the presence of distinct structural differences between adenomatous and tumoral D2 receptors and between the two tumoral receptors themselves; these results suggest that the normal functional unit of the D2 receptor is a dimer associated with a guanine nucleotide-binding protein (G protein) subunit and that tumoral D2 receptors may exist in various polymeric forms unassociated with G proteins. The anomalies found to be present in tumoral D2 receptor complexes may be responsible for the insensitivity of these tumors to dopaminergic agonists' inhibitory activity on PRL release and tumor growth.

Reconstitution of the solubilized mu-opioid receptor coupled to a GTP-binding protein

A mu-opioid receptor-GTP binding protein (mu-opioid receptor-G-protein) complex from the 7315c cell was solubilized with CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate) and reconstituted into phospholipid vesicles. Pretreatment of the tissue with either [3H]etorphine or morphine greatly improved recovery of the receptor and maintained it in a GTP-sensitive state. GTP sensitivity was consistent with the hypothesis that a receptor-G-protein complex had been obtained. Other evidence consistent with this hypothesis was that recovery of the solubilized, prelabelled receptor was decreased by approximately 70% by pretreatment of 7315c cells with pertussis toxin. The reconstituted receptor was mu-selective: DAGO (Tyr-D-Ala-Gly-Met-Phe- NH(CH2)2OH), but not ICI 174864 or U50488-H, displaced [3H]etorphine binding with high affinity. The affinity of the reconstituted receptor for [3H]etorphine (1.25 +/- 0.20 nM) was similar to that observed for the membrane-associated receptor (0.53 +/- 0.25 nM). GTP gamma S decreased this affinity 3-fold without changing the number of binding sites. The potencies of GTP gamma S and GTP in diminishing [3H]etorphine binding were similar in the membrane and vesicle preparations, but were 10-fold lower than the potencies observed in diminishing binding to the solubilized receptor. The ability to reconstitute a functional mu-opioid receptor-G-protein complex will facilitate further study of the structure and function of the receptor and the specific identification of the associated GTP-binding protein(s).

Investigation of rat striatal dopamine D-1 receptors solubilized by digitonin with a precipitation method

[3H]SCH 23390 binding sites solubilized from rat striatal membranes by the detergent digitonin were investigated by using a polyethylene glycol precipitation method to separate the bound [3H]SCH 23390 from the free [3H]SCH 23390. The binding of [3H]SCH 23390 to the solubilized preparations was specific and saturable with a KD of 4.99 +/- 0.03 nM and a Bmax of 619 +/- 13 fmol/mg protein. The rank order of potency of dopamine agonists and antagonists for competing with [3H]SCH 23390 binding for the solubilized preparations was appropriate for dopamine D-1 receptors. The competition of SCH 23390 and S(-)-SCH 23388 with [3H]SCH 23390 binding for the solubilized preparations was stereoselective. However, the sensitivity of the dopamine agonist high-affinity binding to guanine nucleotide GTP was almost lost upon digitonin solubilization. Preincubating the membranes with dopamine preserved the guanine nucleotide sensitivity of agonist binding for membranes in solubilized preparations. These results proved that the polyethylene glycol precipitation method can be used for assay of digitonin-solubilized dopamine D-1 receptors in rat striatum.

Mono- and divalent cations modulate the affinities of brain D1 and D2 receptors for dopamine by a mechanism independent of receptor coupling to guanyl nucleotide binding proteins

In order to clarify the question of whether the modulatory effects of cations on dopamine receptor affinities are brought about by shifts in the equilibrium of receptor - G protein - coupling, it was investigated whether mono- and divalent cations were still able to modulate rat striatal D1 and D2 receptor affinities after selective inactivation of the G-proteins linked to the two receptors. The Gs-protein coupled to the D1 receptor was eliminated by mild thermal inactivation, and the Gi- (or Go-) protein associated with the D2 receptor by alkylation with a low concentration of N-ethyl-maleimide. Incubation of striatal membranes at 60 degrees C completely abolished the specific binding of 3H-GTP. Both treatments resulted in an increase of the IC50-values for dopamine as a displacer of 3H-SCH 23390 from D1- and of 3H-spiperone from D2 receptors. Concomitantly, the formerly shallow D1 displacement curves became steeper, with their Hill coefficients increasing. This effect was less evident at D2 receptors. Guanosine triphosphate (GTP), which increased the IC50's of dopamine for both receptors approximately two-fold in control membranes, was without effect in pretreated samples, indicating an effective inactivation of the G-proteins. Na+ ions were still able to lower, and Ca2+ ions to increase the affinities of D1 and D2 receptors for dopamine after such inactivation of the respective G-proteins. It is concluded that the mechanism underlying the regulation of dopamine receptor affinities by mono- and divalent cations is independent of and superimposed upon the coupling of these receptors to guanyl nucleotide binding proteins.

Solubilization of a guanine nucleotide-sensitive form of vasopressin V2 receptors from porcine kidney

Vasopressin (V2) receptors were solubilized from porcine kidney membranes with the detergent egg lysolecithin. Binding of [3H]vasopressin to the solubilized fraction was rapid, specific, and saturable. The agonist dissociation constants observed in membranes and solubilized fractions were 1.7 +/- 0.3 and 2.3 +/- 0.2 nM, respectively. In competition binding experiments, the solubilized fraction exhibited the same pharmacological profile as the membranes. Chemical crosslinking of [125I]vasopressin to the solubilized fraction followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis demonstrated a 62-kDa band which was specifically labeled with [125I]vasopressin. Vasopressin binding sites from the solubilized fractions were resolved by gel filtration and ultracentrifugation on a sucrose gradient. In addition, agonist high affinity binding to V2 receptors and its sensitivity to guanine nucleotides were preserved even after solubilization in the absence of prebound agonist prior to solubilization. Addition of guanine nucleotides such as GTP gamma S decreased the specific binding of [3H]arginine vasopressin to these solubilized fractions in a dose-dependent manner, suggesting the solubilization of a V2 receptor-G protein complex. [32P]ADP ribosylation of the solubilized fraction by cholera and pertussis toxins revealed specifically labeled proteins with molecular weights of 42,000-43,000 and 39,000-41,000, respectively, on sodium dodecyl sulfate polyacrylamide gels. Furthermore [35S]GTP gamma S binding to these solubilized fractions was enhanced by vasopressin, confirming that a significant proportion of the vasopressin receptors must be closely coupled to G proteins even when these receptors are solubilized in the absence of agonist. These results are in contrast with those reported for beta, alpha 2 adrenergic and D2 dopaminergic receptor systems, but in agreement with D1 dopaminergic and A1 adenosine receptors. The molecular mechanism responsible for this difference remains to be determined.

Distribution of the alpha-subunit of the guanine nucleotide-binding protein Gi2 and its comparison to G alpha o

Site specific antisera against a synthetic peptide corresponding to the sequence 3-17 of G alpha 12 have been raised and the specificity examined using purified homogeneous Go, Gi2 and Gi containing a 41 kDa alpha-subunit. The distribution of G alpha i2 was investigated in plasma membranes from different tissues and cells and compared to the distribution of G alpha o and other pertussis toxin sensitive G alpha. Considerable amounts of G alpha i2 were found in endocrine tissue especially in membranes from the adrenal and thyroid, in leucocytes and platelets where it constitutes the major, if not only, pertussis toxin-sensitive G alpha, as well as in some cell lines (C6, NG 108-15, S49 cyc-); erythrocytes contained a 41 kDa G alpha i which was different from G alpha i2. G alpha o was present abundantly in nervous tissue, adrenal medulla and cortex but also found in low amounts in other membranes except for lung, liver and blood cells. Subcellular fractionation of cardiac ventricular muscle demonstrated the presence of G alpha o and low amounts of G alpha i2 in sarcolemma, but only 41kDa G alpha i was present in sarcoplasmic reticulum. The importance of the distinct distribution in terms of signal transduction is discussed.

Direct demonstration of D1 dopamine receptors in the bovine parathyroid gland using the D1 selective antagonist [125I]-SCH 23982

The presence of D1 dopamine receptors in the parathyroid gland has been proposed based on the demonstration of dopaminergic regulation of adenylate cyclase activity and parathyroid hormone release in dispersed bovine parathyroid cells. Using a radioiodinated D1 selective antagonist [125I]-SCH 23982, we have now directly labeled and characterized the D1 dopamine receptors in bovine parathyroid gland membranes. [125I]-SCH 23982 binds in a saturable manner with high affinity and low nonspecific binding to membranes prepared from bovine parathyroid glands. D1 dopamine receptors are present in this preparation at a concentration of approximately 130 fMoles/mg protein and [125I]-SCH 23982 binding increases with increasing protein concentration in a linear fashion. Determination of the Kd using the association (k1) and dissociation (k-1) rate constants revealed good agreement with the Kd determined by saturation analysis (390 pM vs. 682 pM, respectively). Inhibition of 0.3 nM [125I]-SCH 23982 binding by a series of dopaminergic antagonists verified the D1 nature of this binding site, exhibiting appropriate affinities and rank order of potency. The competition curves of all antagonists exhibited Hill coefficients that were not significantly different from 1. Inhibition of [125I]-SCH 23982 binding by dopamine and other dopaminergic agonists revealed the presence of high and low affinity agonist binding sites. Addition of 200 microM GppNHp effected a complete conversion of high affinity dopamine binding sites to a homogeneous population of low affinity dopamine sites. The D1 receptors identified in the parathyroid gland with [125I]-SCH 23982 appear to be pharmacologically identical with those previously characterized in the central nervous system.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization and reconstitution of the D-1 dopamine receptor: potentiation of the agonist high-affinity state of the receptor

The D-1 dopamine receptor was extracted from rat striatal membranes with sodium cholate and NaCl in the presence of a specific agonist and phospholipids. The soluble receptor then was reconstituted into phospholipid vesicles by further addition of phospholipids prior to detergent removal. Of the total membrane receptors, up to 48% were extracted and 36% were reconstituted into phospholipid vesicles. Yields were greatly reduced if the agonist was omitted or replaced with an antagonist. The solubilized and reconstituted D-1 receptors retained the pharmacological properties of the membrane-bound receptors, including the ability to discriminate between active and inactive enantiomers of specific agonists and antagonists. In this regard, the affinity of the reconstituted receptors for the D-1 specific antagonist 125I SCH 23982 was similar to that of the membrane-bound receptors with a Kd of 1.5 nM. Both the soluble and reconstituted forms of the D-1 receptor exhibited two affinity states for the D-1 specific agonist SK&F R-38393. In contrast to the low proportion of the receptors that had a high affinity for the agonists in striatal membranes (less than 6%), there was a dramatic increase following solubilization (22%) and reconstitution (40%). Similar results were obtained by using dopamine; the proportion of high-affinity sites increased from 4% (membrane-bound) to 48% (reconstituted) of the total receptor population. These high-affinity sites were coupled to G proteins, as guanyl nucleotides completely abolished them. Addition of guanyl nucleotides prior to solubilization or to reconstitution, however, had no effect on the subsequent yield of the reconstituted receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Possible mechanism of regulating adenylate cyclase activity in adipocyte membranes from exercise-trained male rats

(-)-Isoproterenol-stimulated adenylate cyclase activities were significantly greater in membranes from exercise-trained male rats than in sedentary male rats. GTP-inhibition of forskolin (10 microM)-stimulated cyclase activities were observed in sedentary membranes, whereas the inhibitory actions of GTP were significantly reduced in membranes from trained rat adipocytes. Treatment of membranes with islet-activating protein, a pertusis toxin, completely abolished the differences in GTP-inhibition of forskolin-stimulated cyclase activities between the two groups. The amounts of the inhibitory regulatory protein (41kDa/40kDa polypeptides) were about 40% less in membranes from trained rats than in sedentary membranes, whereas that of the stimulatory regulatory protein (a 45kDa polypeptide) was equivalent. It is concluded that the enhanced cyclase activities of adipocyte membranes from trained male rats appear to result from, in part, an attenuation of the inhibitory pathway due to a specific decrease in the amount of the inhibitory regulatory proteins.

Evidence for disulfide bonds in membrane-bound and solubilized opioid receptors

The effects of pretreatment with dithiothreitol (DTT) on opioid binding activities of membrane-bound and digitonin-solubilized opioid receptors from bovine adrenal medulla were studied. Pretreatment of membranes with DTT or mercaptoethanol inhibited [3H]diprenorphine binding by reducing the number of binding sites. The inhibitory action of DTT was time and dose dependent. The binding of [3H]D-Ala2-D-Leu5-enkephalin was also inhibited by DTT pretreatment. Pretreatment of digitonin-solubilized binding sites with DTT also reduced the number of [3H]diprenorphine binding sites. The action of DTT was diminished by preincubating the DTT solution with H2O2. [3H]Diprenorphine protected the opioid binding sites from the inhibitory action of DTT. The present results provide evidence that disulfide bonds are implicated in opioid binding activity of the opioid receptor system.

Human placental LHRH receptor: agonist and antagonist labeling produces differences in the size of the non-denatured, solubilized receptor

Membranes of human term placenta were labeled with several photosensitive and non-photosensitive analogues of LHRH. In both groups agonistic and antagonistic peptide structures were tested. The photolabeling amino acid azidophenylalanine was placed either in positions 2, 6 or 7. All compounds were specifically displacing iodinated Buserelin from human placental membranes and from rat pituitary membranes. The iodinated photolabels were displaced by cold Buserelin, some compounds however had a high non-specific binding. Photolabeling experiments on human placental membranes with radioactive photolabels produced all a band at 58,000 daltons in SDS-gel electrophoresis. Solubilization with a non-denaturing detergent and gel filtration produced a major radioactivity peak which was attributed to the LHRH receptor. All labeling experiments with agonist labels produced Kav's indifferent from each other but significantly different from the Kav's of antagonist labeled membranes. This result was confirmed with similar experiments carried out with radioactive Buserelin and a radioactive antagonist under non-photolytic conditions. It is therefore concluded that the placental LHRH receptor contains an LHRH binding component of 58,000 daltons but that the native receptor is composed of several proteins. It is also concluded that agonist occupation of the receptor induces the association of a further component which might be involved in the transmembrane signalling system. The agonist labeled, solubilized native LHRH receptor has a Stokes radius of 52 A and the same, antagonist labeled receptor a radius of 48 A.

Dopamine receptors: molecular structure and function

Two distinct categories of dopamine receptors, termed D1 and D2, have been identified on the basis of pharmacological and biochemical criteria. Some of the progress made in our understanding of the subunit structure, function and signal transduction properties of these important membrane proteins are reviewed.

Characterization of D2 dopamine receptors in dopamine-resistant prolactin-secreting rat pituitary tumors 7315a and MtTW15

We have investigated the structure of D2 receptors present in two prolactin-secreting, dopamine-resistant, transplantable rat pituitary tumors, 7315a and MtTW15. These receptors specifically bind with high affinity the dopamine antagonist [3H]spiroperidol when membrane bound or solubilized by [3-(3-cholamidopropyl)-dimethyl-ammonio]-1-propane sulfonate 10 mM and are pharmacologically characterized as D2 type. Target-size analysis by radiation inactivation indicated a molecular mass of approximately 100,000 and 200,000 daltons for receptors present respectively in 7315a and MtTW15 tumors either membrane bound or solubilized. The minimal size of the D2 binding site was evaluated at 94,000 daltons by photoaffinity labeling with [125I]azido-N-(p-aminophenethyl)-spiperone followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A guanine nucleotide had no effect on the displacing potency of the agonist N-propylnorapomorphine evaluated with membrane-bound or solubilized receptors obtained from either tumor. These results suggest the absence or inactivation of a guanine nucleotide binding protein in the receptorial complex of these tumors. Thus, our data indicate that a structural anomaly is present in the D2 receptorial complex of these prolactin-secreting rat pituitary tumors, which may be responsible for their resistance to the inhibitory effects of dopamine.

Characterization of binding sites for [3H]spiroperidol

Experiments were designed to investigate the biochemical properties of binding sites for [3H]spiroperidol ([3H]SPD) solubilized from canine caudate and to define the effect of detergent on the binding of the radioligand. Extraction of canine caudate with 0.75-1.0% digitonin was found to generate the maximum yield of binding sites for [3H]SPD while minimizing extraction of membrane proteins. Although binding sites were solubilized with 1.0% digitonin, a 10-fold reduction in detergent concentration was necessary to achieve maximal binding of [3H]SPD. The rank order of affinity for agonists and antagonists was consistent with the pharmacologic properties of the D2 subtype of the dopamine receptor. However, the binding of antagonists was found to be complex. Studies with some preparations of pooled canine caudate resulted in competition curves for the D2-selective antagonists domperidone and sulpiride that best fit a single-site model. Other preparations exhibited biphasic inhibition curves with these antagonists. The class of binding sites for [3H]SPD with low affinity for D2-selective antagonists constituted as much as 30-40% of the binding sites. Enrichment of solubilized binding sites for [3H]SPD was achieved by size exclusion HPLC followed by adsorption to DEAE-Sephadex and elution with buffer of increasing ionic strength. Enrichment of binding sites was accompanied by a decrease in the affinity of solubilized sites for [3H]SPD.

Cholecystokinin induces the interaction of its receptor with a guanine nucleotide binding protein

To evaluate the relation between the pancreatic cholecystokinin (CCK) receptor and guanine nucleotide-binding protein(s) we studied the effects of nucleotides on 125I-CCK binding to pancreatic acinar plasma membranes, 125I-CCK binding to solubilized 125I-CCK receptors, and the stability of the solubilized 125I-CCK-receptor complex. In plasma membranes, guanine nucleotides both inhibited CCK binding and increased the dissociation of CCK from its receptor. The potency of the nucleotides studied was GTP gamma S = GMP-PNP greater than GTP much greater than ATP. When membranes were solubilized with digitonin, subsequent binding of CCK was insensitive to guanine nucleotides including GTP, GMP-PNP and GTP gamma S. However, if CCK binding occurred before solubilization of the membranes, guanine nucleotides increased dissociation at concentrations and with a specificity similar to that observed for effects on intact pancreatic membranes. It is concluded that guanine nucleotides act via a protein which is separable from the receptor to induce dissociation of bound CCK. Moreover, CCK binding induces an association in the plasma membrane of the CCK receptor with this guanine nucleotide binding protein.

Interactions of beta-adrenergic receptors with a membrane protein other than the stimulatory guanine nucleotide-binding protein

Beta-adrenergic receptors on membranes prepared from rat lung, wild-type S49 lymphoma cells, and the adenylate cyclase-deficient variant of S49 lymphoma cells (cyc-) bind the agonist [3H]hydroxybenzylisoproterenol ([3H]HBI) with high affinity and this binding of [3H]HBI can be inhibited by GTP. Membranes prepared from these tissues were incubated with the agonist [3H]HBI or the antagonist [125I]iodopindolol ([125I]IPIN), labeled receptors were solubilized with digitonin, and the apparent molecular sizes of the ligand-bound receptor complexes were determined by high-performance size-exclusion chromatography. Results with all three tissues demonstrated that receptors labeled with [125I]IPIN were retained by the size-exclusion columns longer than receptors labeled with [3H]HBI. Thus, the apparent molecular size of soluble beta-adrenergic receptors from rat lung, wild-type S49 cells, and cyc- S49 cells was larger when receptors were occupied with an agonist rather than an antagonist. The results suggest that receptors, including those on cyc- S49 cells, interact with a membrane protein, presumably a guanine nucleotide-binding protein. Since cyc- S49 cells do not contain a functional stimulatory guanine nucleotide-binding protein, but do contain a functional inhibitory guanine nucleotide-binding protein, an interaction between beta-adrenergic receptors and the inhibitory guanine nucleotide-binding protein may be responsible for the apparent increase in the molecular size of the receptor after occupation of the receptor with an agonist.

Dopamine agonist high-affinity state in solubilized D2 receptors in striatum, but not in anterior pituitary

To determine whether brain and pituitary dopamine D2 receptors have similar characteristics on solubilization, canine striatal and bovine anterior pituitary tissues were solubilized with digitonin, and their D2 dopamine receptors were characterized by competition studies and by high pressure liquid chromatography. Solubilized striatal membranes retained the ability to bind agonists with high affinity and retained sensitivity to guanine nucleotides. In contrast, however, solubilized pituitary membranes no longer revealed high-affinity binding of agonists and sensitivity to guanine nucleotide unless they had been solubilized in the presence of agonist. Agonist-pre-labelled pituitary receptors were of larger apparent molecular weight than antagonist-pre-labelled receptors. However, striatal receptors pre-labelled by agonist or antagonist were of similar apparent molecular weight. Thus, unlike the pituitary, striatal receptors probably remain associated with a guanine nucleotide binding protein (N) upon solubilization.

Characterization of high-affinity dopamine D2 receptors and modulation of affinity states by guanine nucleotides in cholate-solubilized bovine striatal preparations

3,4-Dihydroxyphenylethylamine (dopamine) D2 receptors, solubilized from bovine striatal membranes using a cholic acid-NaCl combination, exhibited the typical pharmacological characteristics of both agonist and antagonist binding. The rank order potency of the agonists and antagonists to displace [3H]spiroperidol binding was the same as that observed with membrane-bound receptors. Computer-assisted analysis of the [3H]spiroperidol/agonist competition curves revealed the retention of high- and low-affinity states of the D2 receptor in the solubilized preparations and the proportions of receptor subpopulations in the two affinity states were similar to those reported in membrane. Guanine nucleotide almost completely converted the high-affinity sites to low-affinity sites for the agonists. The binding of the high-affinity agonist [3H]N-n-propylnorapomorphine ([3H]NPA) was clearly demonstrated in the solubilized preparations for the first time. Addition of guanylyl-imidodiphosphate completely abolished the [3H]NPA binding. When the solubilized receptors were subjected to diethylaminoethyl-Sephacel chromatography, the dopaminergic binding sites eluted in two distinct peaks, showing six- to sevenfold purification of the receptors in the major peak. Binding studies performed on both peaks indicated that the receptor subpopulation present in the first peak may have a larger proportion of high-affinity binding sites than the second peak. The solubilized preparation also showed high-affinity binding of [35S]guanosine-5'-(gamma-thio)triphosphate, a result suggesting the presence of guanine nucleotide binding sites, which may interact with the solubilized D2 receptors. These data are consistent with the retention of the D2 receptor-guanine nucleotide regulatory protein complex in the solubilized preparations and should provide a suitable model system to study the receptor-effector interactions.