Identification and quantification of beta-adrenoceptors sites in red blood cells from rats

Effect of fenoterol stereochemistry on the β2 adrenergic receptor system: ligand-directed chiral recognition

The β(2) adrenergic receptor (β(2)-AR) is a model system for studying the ligand recognition process in G protein-coupled receptors. Fenoterol (FEN) is a β(2)-AR selective agonist that has two centers of chirality and exists as four stereoisomers. Radioligand binding studies determined that stereochemistry greatly influences the binding affinity. Subsequent Van't Hoff analysis shows very different thermodynamics of binding depending on the stereoconfiguration of the molecule. The binding of (S,x')-isomers is almost entirely enthalpy controlled whereas binding of (R,x')-isomers is purely entropy driven. Stereochemistry of FEN molecule also affects the coupling of the receptor to different G proteins. In a rat cardiomyocyte contractility model, (R,R')-FEN was shown to selectively activate G(s) protein signaling while the (S,R')-isomer activated both G(i) and G(s) protein. The overall data demonstrate that the chirality at the two chiral centers of the FEN molecule influences the magnitude of binding affinity, thermodynamics of local interactions within the binding site, and the global mechanism of β(2)-AR activation. Differences in thermodynamic parameters and nonuniform G-protein coupling suggest a mechanism of chiral recognition in which observed enantioselectivities arise from the interaction of the (R,x')-FEN stereoisomers with a different receptor conformation than the one with which the (S,x')-isomer interacts.

Comparative Molecular Field Analysis of the Binding of the Stereoisomers of Fenoterol and Fenoterol Derivatives to the β2Adrenergic Receptor

Stereoisomers of fenoterol and six fenoterol derivatives have been synthesized and their binding affinities for the beta2 adrenergic receptor (Kibeta2-AR), the subtype selectivity relative to the beta1-AR (Kibeta1-AR/Kibeta2-AR) and their functional activities were determined. Of the 26 compounds synthesized in the study, submicromolar binding affinities were observed for (R,R)-fenoterol, the (R,R)-isomer of the p-methoxy, and (R,R)- and (R,S)-isomers of 1-naphthyl derivatives and all of these compounds were active at submicromolar concentrations in cardiomyocyte contractility tests. The Kibeta1-AR/Kibeta2-AR ratios were >40 for (R,R)-fenoterol and the (R,R)-p-methoxy and (R,S)-1-naphthyl derivatives and 14 for the (R,R)-1-napthyl derivative. The binding data was analyzed using comparative molecular field analysis (CoMFA), and the resulting model indicated that the fenoterol derivatives interacted with two separate binding sites and one steric restricted site on the pseudo-receptor and that the chirality of the second stereogenic center affected Kibeta2 and subtype selectivity.

Enantioselective separation and online affinity chromatographic characterization of R,R- and S,S-fenoterol

BACKGROUND

rac-Fenoterol is a beta2-adrenoceptor agonist (beta2-AR) used in the treatment of asthma. It has two chiral centers and is marketed as a racemic mixture of R,R'- and S,S'-fenoterol (R-F and S-F). Here we report the separation of the R-F and S-F enantiomers and the evaluation of their binding to and activation of the beta2-AR.

METHODS

R-F and S-F were separated from the enantiomeric mixture by chiral chromatography and absolute configuration determined by circular dichroism. Beta2-AR binding was evaluated using frontal affinity chromatography with a stationary phase containing immobilized membranes from HEK-293 cells that express human beta2-AR and standard membrane binding studies using the same membranes. The effect of R-F and S-F on cardiomyocyte contractility was also investigated using freshly isolated adult rat cardiomyocytes.

RESULTS

Chiral chromatography of rac-fenoterol yielded separated peaks with an enantioselectivity factor of 1.21. The less retained peak was assigned the absolute configuration of S-F and the more retained peak R-F. Frontal chromatography using membrane-bound beta2-AR as the stationary phase and rac-3H-fenoterol as a marker ligand showed that addition of increasing concentrations of R-F to the mobile phase produced concentration-dependent decreases in rac-3H-fenoterol retention, while similar addition of S-F produced no change in rac-3H-fenoterol retention. The calculated dissociation constant of R-F was 472 nM and the number of available binding sites 176 pmol/column, which was consistent with the results from the membrane binding study 460 +/- 55 nM (R-F) and 109,000 +/- 10,400 nM (S-F). In the cardiomyocytes, R-F increased maximum contractile response from (265 +/- 11.6)% to (306 +/- 11.8)% of resting cell length (P < 0.05) and reduced EC50 from -7.0 +/- 0.270 to -7.1 +/- 0.2 log[M] (P < 0.05), while S-F had no significant effect.

DISCUSSION

Previous studies have shown that rac-fenoterol acts as an apparent beta2-AR/G(s) selective agonist and fully restores diminished beta2-AR contractile response in cardiomyocytes from failing hearts of spontaneously hypertensive rats (SHR). Here we report the separation of the enantiomers of rac-fenoterol and that R-F is the active component of rac-fenoterol. Further evaluation of R-F will determine if it has enhanced selectivity and specificity for beta2-AR/G(s) activation and if it can be used in the treatment of congestive heart failure.

A functional assay for G-protein-coupled receptors using stably transformed insect tissue culture cell lines

Insect cells are an underexplored resource for functional G-protein-coupled receptor (GPCR) assays, despite a strong record in biochemical (binding) assays. Here we describe the use of vectors capable of creating stably transformed insect cell lines to generate a cell-based functional GPCR assay. This assay employs the luminescent photoprotein aequorin and the promiscuous G-protein subunit Galpha16 and is broadly applicable to human GPCRs. We demonstrate that the assay can quantitate ligand concentration-activity relationships for seven different human GPCRs, can differentiate between partial and full agonists, and can determine rank order potencies for both agonists and antagonists that match those seen with other assay systems. Human Galpha16 improves signal strength but is not required for activity with some receptors. The coexpression of human and bovine betagamma subunits and/or phospholipase Cbeta makes no difference to agonist efficacy or potency. Two different receptors expressed in the same cell line respond to their specific agonists, and two different cell lines (Sf9 and High 5) are able to functionally detect the same expressed GPCR. Sf9 cells have the capability to produce fully functional human receptors, allied to a low background of endogenous receptors, and so are a valuable system for investigating orphan GPCRs and receptor dimerization.

Extent of beta 1- and beta 2-receptor occupancy in plasma assesses the antagonist activity of metoprolol, pindolol, and propranolol in the elderly

We estimated antagonist activity of metoprolol, pindolol, and propranolol in elderly cardiovascular patients by determining the extent to which the drugs occupied rabbit lung beta 1- and rat reticulocyte beta 2-adrenoceptors in plasma samples during drug treatment. The randomized, double-blind, crossover study was carried out by administering twice daily 100 mg metoprolol, 5 mg pindolol, and 80 mg propranolol for 7 days to 20 hypertensive subjects with a mean age of about 70 years. A 2-week interval was kept between administration of the different regimens. Receptor occupancy was measured at 1 hour before and 2 hours after administration of the last dose of each regimen by adding rabbit lung beta 1- and rat reticulocyte beta 2-receptors to plasma samples and by labeling the receptors with a radiolabeled beta-antagonist, (-)-[3H]CGP-12177. The results and conclusions were the following: (a) The extent to which metoprolol, pindolol, and propranolol occupied rabbit lung beta 1- and rat reticulocyte beta 2-adrenoceptors in plasma samples estimated accurately the intensity of beta-receptor antagonism in the patients who did not tolerate physiological and pharmacological tests measuring the degree of beta 1- and beta 2-adrenoceptor blockade. (b) The mean beta 1- and beta 2-receptor occupancy of pindolol and propranolol varied between 76% and 99% during the treatments. The mean beta 1-receptor occupancy of the metoprolol regimen varied between 54% and 92%, and its beta 2-receptor occupancy varied between 6% and 38%. Thus the antagonist activity of the metoprolol regimen differed significantly from that of the other regimens (ANOVA for repeated measures, p < 0.05 and 0.001, for the beta 1- and beta 2-occupancy, respectively). (c) The extent of beta 1- and beta 2-receptor occupancy in plasma samples was in conformity with the literature on the intensity, selectivity, and duration of beta-blockade after similar drug doses. (d) The data on the receptor occupancy of beta-blocking drugs in plasma samples appear to be valuable in analyzing their effects, and it may be a method for optimizing drug therapy for aged cardiovascular patients.

Preparation and in vivo binding of [11C]carazolol, a radiotracer for the beta-adrenergic receptor

Carazolol is a high affinity beta-adrenergic receptor antagonist which is relatively non-specific for the receptor subtypes. The labeling of the two enantiomers of this compound with carbon-11, including the synthesis of the required labeling precursors, is reported. The yield and specific activity are sufficient for use in positron tomography. Biodistribution and specific receptor binding studies show the labeled material to be of interest for further investigation as a radiopharmaceutical for positron tomography.

A sensitive radioligand binding assay for timolol in plasma

A sensitive and reproducible radioreceptor assay (RRA) is described for the determination of low picogram levels of timolol in plasma. The plasma or serum samples (1 mL) are prepared by selectively extracting timolol with lipophilic solvents or, alternatively, only plasma (serum) proteins are precipitated prior to binding assay. The recovery of timolol is at least 90% during the sample preparation. In the radioligand binding assay, timolol and the nonselective beta-antagonist [3H](-)-CGP-12177 compete for the binding sites present in the rat reticulocyte membranes. The detection limit for timolol (30 pg/mL) exceeds 50 times the sensitivity of the GC-MS techniques for timolol. The RRA results and GC results obtained from the same clinical samples correlate excellently (r = 0.99). There was no evidence for interference caused by timolol metabolites in the RRA. Timolol concentrations in plasma following its oral and even ocular administration were possible to monitor using the RRA. The method can be modified to also measure several other beta-antagonist drugs in picogram per milliliter quantities.

Evaluation of 3H-CGP 12177 and 3H-DHA binding to beta 2-adrenoceptors of rat reticulocytes by means of affinity spectra

3H-CGP 12177 and 3H-DHA binding to rat reticulocyte beta 2-adrenoceptors by using an excess of 1 x 10(-5) mol l-1 propranolol was evaluated by means of affinity spectra. To describe the 3H-CGP 12177 interaction with rat reticulocytes a binding isotherm valid for one single class of specific binding sites was found to be satisfactory. On the other hand, a more complex binding isotherm had to be used for the description of the 3H-DHA interaction with rat reticulocytes. This observation further supports the preferred application of 3H-CGP 12177 over 3H-DHA for radio-receptor assay of beta-adrenoceptor blockers.

Stereoselective formation of fenoterol-para-glucuronide and fenoterol-meta-glucuronide in rat hepatocytes and enterocytes

The glucuronidation of fenoterol (Berotec, Partusisten) in isolated rat hepatocytes and enterocytes was investigated. Two different glucuronides, fenoterol para-glucuronide and fenoterol meta-glucuronide, were formed in proportions, that were constant over the concentration range investigated (0-1 mM). The fraction of para-glucuronide formed was 0.40 +/- 0.01 for hepatocytes and 0.54 +/- 0.01 for enterocytes. Fenoterol consists of a racemic mixture of SS'(+)fenoterol and RR'(-)fenoterol. The maximum glucuronidation rate of the (-)enantiomer (Vmax = 3.6 +/- 0.3 nmol/min/mg in hepatic microsomes and 3.4 +/- 0.1 nmol/min/mg in intestinal microsomes) is significantly lower than the same values of the (+)isomer (Vmax = 6.7 +/- 0.8 nmol/min/mg in hepatic microsomes and 5.8 +/- 0.4 nmol/min/mg in intestinal microsomes). Kmapp-values for the (-)enantiomer were lower than for the (+)enantiomer. Similar, but less pronounced, differences in Vmax were observed in isolated cells: Vmax = 148 +/- 13 and 372 +/- 50 pmol/min/mg [(-)fenoterol in hepatocytes and enterocytes], Vmax = 173 +/- 12 and 444 +/- 57 pmol/min/mg [(+)fenoterol in hepatocytes and enterocytes]. Calculation of intrinsic metabolic clearance (Clint = Vmax/Kmapp) from the cellular data suggests that the (+)enantiomer may be more efficiently eliminated by liver metabolism in vivo than the (-)enantiomer. This can result in stereoselective first-pass metabolism of the fenoterol enantiomers.

Effect of GTP on the affinity of denopamine, a new cardiotonic agent, for beta-adrenergic receptors of turkey erythrocytes and rat reticulocyte membranes

Affinities of denopamine, a new cardiotonic agent, and several beta-adrenergic drugs for turkey erythrocyte membranes (TEM) and rat reticulocyte membranes (RRM) which contain homogeneous beta 1- and beta 2-receptors, respectively, were studied by receptor binding. The order of potencies of denopamine and several beta-adrenergic agonists in displacing 3H-dihydroalprenolol binding (Ki, nM) in TEM was isoproterenol (Iso, 27) greater than norepinephrine (Nor, 360) greater than epinephrine (Epi, 860) greater than dobutamine (DB, 1380) greater than denopamine (1540) greater than dopamine (DA, 49500). The order in RRM was Iso (7.3) greater than Epi (58) greater than DB (750) greater than Nor (1090) greater than denopamine (2300) greater than DA (26800). In the presence of GTP, competition curves for full agonists like Iso, Epi and Nor shifted to the low affinity side (Ki values increased by 300-500% in TEM and 200-460% in RRM), and the slopes were steepened in both membrane preparations. The Ki value for denopamine increased only in TEM (70%) and that in RRM was not influenced by GTP. This suggests that denopamine has an agonist property at the beta 1-receptor but not at the beta 2-receptor and that the intrinsic activity at the beta 1-receptor of the drug is lower than full agonists. Affinities of DB and DA for TEM were influenced by GTP as well as those for RRM, although the extent of the rightward shift was less than full agonists.

Baboon erythrocyte ghosts contain beta-adrenergic receptors

We have used the beta-adrenergic antagonist [3H]dihydroalprenolol [( 3H]DHA) to identify binding sites on the erythrocyte membrane of the primate Papio ursinus. Analysis of the saturation isotherm revealed binding to be saturable with a maximal number of binding sites of 499 fmol/mg protein. [3H]DHA binds specifically to the erythrocyte ghosts with an apparent dissociation constant (Kd) of 0.57 +/- 0.06 nM. A similar value for Kd (0.46 +/- 0.07 nM) was evaluated from the rate constants of association (0.013 +/- 0.003 X nM-1 X min-1) and dissociation (0.006 +/- 0.001 X min-1). beta-adrenergic agonists compete for the binding sites with an order of potency (dl-isoproterenol greater than l-epinephrine greater than l-norepinephrine) typical of a beta 2-adrenergic receptor. Binding was shown to be stereospecific with l-stereoisomers being more potent than their corresponding d-stereoisomers in causing half-maximal inhibition. Isoproterenol stimulated the production of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) in a concentration-dependent manner, maximal levels (1.130 +/- 0.358 pmol cAMP/10(8) cells) being four times the basal levels. The results demonstrate the existence of a large number of beta-adrenergic receptors on baboon erythrocyte ghosts.

Identification of beta-adrenergic receptors in teleost red blood cells

Beta-adrenoceptors have been identified in eel erythrocyte membranes using [3H]dihydroalprenolol binding. These receptors exhibit a mean dissociation constant (KD) of 1.36 nM and a mean maximum number of binding sites (Bmax) of 315 fmol/mg protein. These receptors do not appear to belong to the beta 1-subtype.

Simple and reliable radioreceptor assay for beta-adrenoceptor antagonists and active metabolites in native human plasma

A radioreceptor assay (RRA) for the assay of beta-adrenoceptor antagonists in native human plasma is described. The hydrophilic antagonist 3H-CGP 12177 was used as the radioligand. In contrast to the hydrophobic radioligand 3H-dihydroalprenolol, which was investigated in parallel, the beta-adrenoceptor binding of 3H-CGP 12177 by rat reticulocyte membranes was found not to be affected by inclusion of increasing proportions (0-66% of incubation volume) of human plasma in the assay. Thus, solvent extraction of drug and/or active metabolites was not necessary to avoid binding of the radioligand tracer to plasma added in the RRA. The assay of unprocessed samples was possible. Drug concentrations in plasma after oral administration of propranolol (240 mg) or carteolol (30 mg) to 6 healthy volunteers were measured by the RRA and in parallel by a chemical method. The results from both methods agreed when the plasma concentration kinetics of propranolol were investigated (elimination half-life:3.9 h). In contrast, plasma concentrations of carteolol were consistently higher according to the RRA after oral administration of the drug. Identical concentrations, however, were found by the RRA and chemical method using plasma samples spiked with carteolol. Plasma concentrations of carteolol detected by the chemical method decline monoexponentially (elimination half-life: 5.4 h). A similar half-life of elimination for parent drug was found by the RRA (5.9 h), but an additional term describing the appearance of an active metabolite was necessary to account for the biphasic drug elimination (elimination half-life of metabolite: 17.3 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of functionally active beta-adrenoceptors in rat mast cells. Correlation between (--)[3H]-dihydroalprenolol binding and inhibition of histamine release

The correlation between the binding of a beta-adrenoceptor antagonist, (--)[3H]-dihydroalprenolol (DHAP), and the adrenergic inhibition of histamine release by acetylcholine and by compound 48/80 was studied with isolated purified rat mast cells and in rat mast cell crude membrane fractions. Acetylcholine-evoked histamine release was inhibited by catecholamines, in the order isoprenaline greater than adrenaline greater than noradrenaline. Pretreatment of cells with (--)alprenolol antagonized the inhibitory effect of isoprenaline on acetylcholine-induced histamine release. 40/80-evoked histamine release was blocked by isoprenaline at significantly higher concentrations than those required to inhibit cholinergic histamine release. The inhibitory effect of isoprenaline was equally antagonized by preincubating mast cells with (--)alprenolol. Specific binding sites for DHAP have been demonstrated in rat mast cell membranes. The specific binding of DHAP was inhibited by adrenoceptor agonists and antagonists according to the stereospecificity of these compounds. A close correlation between the binding-inhibitory potency of various adrenergic compounds and the data obtained in the pharmacological experiments was found, thus indicating the presence of beta-adrenoceptors in rat mast cells.

Receptor binding sites for beta-adrenergic ligands on human erythrocytes

Affinity, specificity and kinetics for [3H]-DHA binding to human red cell ghost were determined by ultra-filtration. At 2 degree an apparent dissociation constant of 0.96 nM was found with maximum specific binding of 29 fmoles per mg protein. The low dissociation constant was confirmed by kinetic studies with a value of 0.86 nM. Propranolol and isoproterenol inhibited [3H]-DHA binding stereo specifically. Agonist potency (IPR greater than EPI greater than NE) indicated that human erythrocytes had an adrenergic receptor of beta-2 subtype. Isoproterenol in the presence of theophylline resulted in a concentration-dependent increase of intracellular cAMP levels in intact cells. Basal and maximal levels were 2.3 and 7.5 pmoles/108 cells respectively after 2.5 min stimulation. EC50 for isoproterenol was 0.27 microM. Propranolol shifted the isoproterenol concentration response curve to the right. The present results show that human erythrocytes possess recognition sites for beta-adrenergic ligands with binding characteristics similar to that of adrenergic receptors of beta-2 subtype. At least a small number of these binding sites are functionally coupled to adenylate cyclase.