Glucocorticoid regulation of beta-adrenergic receptors in 3T3-L1 preadipocytes

Treatment of 3T3-L1 preadipocytes (fibroblasts) with 250 nM dexamethasone for 48 hr caused a doubling of total beta-adrenergic receptors and an increase in beta 2-adrenergic receptor subtype proportion from approximately 50% in controls to 85% in treated cells. The responses to epinephrine and norepinephrine in a whole cell cAMP accumulation assay reflected these changes. The effects of dexamethasone on beta-adrenergic receptors were mediated through the glucocorticoid receptor and were time and dose dependent with an EC50 of 2.77 +/- 0.73 nM for an increase in the proportion of beta 2-adrenergic receptors. The rank order of potency of steroids to effect these changes (betamethasone = dexamethasone greater than fludrocortisone greater than hydrocortisone = triamcinolone greater than aldosterone) correlated with their glucocorticoid potency. [3H]Dexamethasone binding to intact cells yielded a KD value of 3.47 +/- 0.38 nM for binding to the glucocorticoid receptor which correlated well with the EC50 for dexamethasone to alter beta-adrenergic receptors. Inhibition of [3H]dexamethasone binding by other steroids confirmed that the ability of steroids to regulate beta-adrenergic receptors correlated with the affinity of each compound for the 3T3-L1 glucocorticoid receptor. Progesterone, which can bind to the glucocorticoid receptor but has only weak agonist activity, competitively inhibited the ability of dexamethasone to alter beta-adrenergic receptors. Protein synthesis, RNA synthesis, and N-linked glycosylation appeared to be necessary for the change in receptor subtype expression and the increase in beta-adrenergic receptor number induced by dexamethasone. The present study suggests that regulation of beta-adrenergic receptor expression in 3T3-L1 preadipocytes by dexamethasone is a glucocorticoid-specific effect which may require gene activation.

Effect of serum, alpha-1 acid glycoprotein, lipoproteins and albumin on human mononuclear leucocyte beta-adrenoceptors

The effects of serum, alpha-1 acid glycoprotein (AAG), serum lipoproteins (SLP) and human serum albumin (HSA) on 3H-(-)-dihydroalprenolol (3H-(-)-DHA) binding and (-)-isoproterenol ((-)-IPR) induced cyclic AMP (cAMP) elevation in human peripheral blood mononuclear leucocytes (MNL) were investigated. The saturable binding of 3H-(-)-DHA was decomposed into two classes of binding sites with maximum binding capacity of approximately 1400 and 30000 sites/cell and with dissociation constants (Kd) of approximately 0.7 and 65 nM. Stimulation of the MNL beta-adrenoceptors by (-)-IPR caused a concentration dependent cAMP accumulation (EC50 approximately 0.2 microM) with maximum level approximately 250% above basal. For all single leucocyte preparations, 30-35 min. exposure to serum, AAG and SLP increased the number of beta-adrenoceptors with 100-200% and the maximal responsiveness to (-)-IPR with 30-90%. The presence of proteins did not change the Kd or the EC50. (-)-Alprenolol inhibited concentration dependently the serum induced increment in (-)-IPR-responsiveness. Serum, AAG and SLP did also increase the number of low affinity binding sites with 25-40% without effect on the Kd. HSA had no consistent effect on beta-adrenergic binding or stimulation. The present study shows that serum, AAG and SLP influence the number and function of MNL beta-adrenoceptors in vitro.

Alkylating beta-blockers: activity of isomeric bromoacetyl alprenolol menthanes

An affinity label for beta-adrenoceptors, N-(bromoacetyl)-N'-[3-(o-allylphenoxy)-2-hydroxypropyl]-1,8-dia min o-p-menthane, has been extensively used in the form of a mixture of four isomers. In the present study, all four isomers were isolated, their structures elucidated, and their interactions with beta-adrenoceptors characterized. The isomer with the aromatic (pharmacophore) group on carbon 1 of p-menthane and with the Z configuration (Z-1) predominates in the mixture and has the highest affinity for beta-adrenoceptors of rat heart (KD = 3 X 10(-8) M) and lungs (KD = 2 X 10(-8) M). This isomer acts as a ligand that binds irreversibly at the drug binding site of the receptor (i.e., after treatment and extensive washing of the membrane preparation, the concentration of the receptors is decreased in a dose-dependent manner), while binding characteristics of the remaining receptors are not changed. The corresponding E diastereomer (E-1) also binds irreversibly to the drug binding site of the receptor. The isomer with the aromatic group on carbon 8 and the Z configuration (Z-8) modifies the receptor noticeably only at higher concentrations and then on a site apparently different from the drug-binding site, i.e., affinity of receptors after the treatment and washing is changed. The corresponding E diastereomer (E-8) modified both the drug-binding and alternative binding site. The results suggest that there is some flexibility in the conformation of the beta-adrenoceptor that enables pairs of ligands, differing by axial or equatorial positions of critical groups, to alkylate the receptor in an analogous manner.

Beta-adrenergic [( 3H]CGP-12177) binding to brain slices and single intact pineal glands

We have characterized and quantified the binding of [3H]CGP-12177 to beta-adrenergic receptor sites in slices (300 microns) of rat cerebral cortex. The receptors are stereospecific, saturable and of high affinity. Binding of [3H]CGP is readily reversible and demonstrates appropriate drug specificity. This assay method allows the demonstration of isoproterenol-induced down-regulation (internalization) of beta-adrenoreceptors. Receptor recycling is observed at 37 degrees C in the absence of beta-agonist but can be blocked by low temperature (0 degree C) or by monensin. beta-Adrenoreceptors can also be labeled and quantified in intact, single pineal glands of rat, mouse and hamster. Rat pineals contain approximately 10 times more binding sites than do hamster or mouse pineals and up to 8 times more sites than found in rat cerebral cortex. Rat pineal [3H]CGP binding can be up- and down-regulated but not to the same degree as seen in brain slices. This assay method is simple, rapid and provides new opportunities for the study of other receptor types in intact tissue.

(-)-S-[3H]CGP-12177 and its use to determine the rate constants of unlabeled beta-adrenergic antagonists

The enantiomers of the hydrophilic beta-adrenergic blocker CGP-12177 have been synthesized and the S-enantiomer radiolabeled with tritium. The dissociation constant (Kd) of the S-enantiomer for binding to the beta-adrenergic receptor is one-half of that of the racemic mixture and at least 2 orders of magnitude lower than that of the R-enantiomer. The kinetic parameters of the latter were determined by analyzing its effect on the association kinetics of (-)-S-[3H]CGP-12177. A computer program was developed that allows the association and dissociation rate constants of unlabeled ligands to be calculated. This method was validated using Monte Carlo simulations. In addition, the rate constants of unlabeled S-CGP-12177 and S-alprenolol calculated using this method were in good agreement with those of S-[3H]CGP-12177 and S-[3H]dihydroalprenolol, respectively, determined independently. The method was also used to measure the rate constants of the enantiomers of pindolol. These antagonists as well as S- and R-CGP-12177 form their receptor complexes with similar association rate constants. In contrast, the dissociation of the R-enantiomers from receptor-ligand complexes were found to be at least 100 times faster than those of the corresponding S-enantiomers.

Non-specific binding of the fluorescent beta-adrenergic receptor probe alprenolol-NBD

The fluorescent beta-adrenergic receptor probe alprenolol-NBD was found to exhibit a high affinity (Kd 3.2 nM) and a low capacity (10 fmol/mg protein) for the beta 2-adrenergic receptor on living Chang liver cells but also a high affinity (Kd 320 nM) for non-beta-adrenergic receptor binding sites with a very high capacity (28,000 fmol/mg protein). Calculations are presented which make clear that less than 3% of the binding of alprenolol-NBD during visualization experiments is beta-adrenergic receptor related. Furthermore, it is shown that besides the downregulation of beta-adrenergic receptors during incubation with isoproterenol, the high-affinity non-beta-receptor binding sites are also deminishing during incubation with isoproterenol. Based on our findings it is concluded that the results of Henis et al. who claimed the visualization of the beta-adrenergic receptor population on Chang liver cells by alprenolol-NBD must be interpreted as an almost completely non-specific fluorescence.

A comparison of the binding characteristics of the beta-adrenoceptor antagonists 3H-dihydroalprenolol and 125I-iodocyanopindolol in rat liver

The binding characteristics of 3H-dihydroalprenolol and 125I-iodocyanopindolol have been compared in a particulate fraction from regenerating rat liver. When total 3H-dihydroalprenolol binding and inhibition of total 3H-dihydroalprenolol binding by (-)isoprenaline, (-)alprenolol and (+/-)cyanopindolol was investigated, it was found that all agents were bound to two classes of saturable binding sites. In the inhibition studies, the presence of two binding components was not obvious until the data were transformed into Hofstee plots and these were decomposed, except in the case of (+/-)cyanopindolol. Only (+/-)cyanopindolol was found to distinguish clearly between the two saturable binding sites identified by 3H-dihydroalprenolol, as indicated by a broad plateau in the inhibition curve. When 125I-iodocyanopindolol was used as radioligand, only one saturable binding site was identified, even in the presence of less selective inhibiting ligands. The lower affinity component of 3H-dihydroalprenolol binding could be inhibited by 10 microM phentolamine. However, binding experiments with 3H-prazosin indicated that the lower affinity component was not identical with the alpha-adrenoceptor. Phentolamine did not influence 125I-iodocyanopindolol binding. Thus, due to its higher specific activity and a high degree of selectivity, 125I-iodocyanopindolol appears to be the ligand of choice.

Rapid vesicle reconstitution of alprenolol-Sepharose-purified beta 1-adrenergic receptors. Interaction of the purified receptor with N

beta-Adrenergic receptors from turkey erythrocyte membranes have been purified 1000-4000-fold using alprenolol-Sepharose affinity chromatography. Addition of deoxycholate solubilized egg phosphatidylcholine to the beta-adrenergic receptor, that is 5-10% pure and in 0.1% digitonin, followed by Sephadex G-50 gel filtration in buffers containing 30 mM MgCl2 results in 65-70% of the receptor being incorporated into phospholipid vesicles. The beta-adrenergic receptor as detected by photoaffinity labeling using [125I]azidobenzylpindolol in membranes and after alprenolol-Sepharose chromatography is a Mr = 40,000 peptide. Addition of deoxycholate extracts of human erythrocyte membranes, which contain the guanine nucleotide stimulatory regulatory protein of adenylate cyclase (Ns) but not beta-adrenergic receptor, were used to reconstitute a guanine nucleotide-mediated change in agonist affinity for the receptor. These results demonstrate that the alprenolol-Sepharose affinity purified beta-adrenergic receptor is functional in both ligand binding and coupling to Ns. The procedure is rapid, efficient and should be generally applicable to beta-adrenergic receptor and Ns from several different membrane systems.

Cyclical antiidiotypic response to anti-hormone antibodies due to neutralization by autologous anti-antiidiotype antibodies that bind hormone

Antiidiotype antibodies were raised against anti-catecholamine ligand antibodies. The antiidiotype response was shown to be cyclical and to correspond to the production of antibodies that could bind to catecholamine beta-adrenergic receptors and stimulate adenylate cyclase. Disappearance of these antibodies from the serum could be correlated with the appearance of a catecholamine ligand-binding activity corresponding to the synthesis of autologous anti-antiidiotype antibodies directed against the induced antiidiotypic molecules. Comparison of the injected versus the induced anti-ligand antibodies reveals striking differences in affinities but similarities in the ability to bind to the antiidiotype antibodies and to the ligand-containing affinity gel. The results support the existence of a functional network of idiotype antiidiotype interactions involving external as well as internal antigens, antibodies, and possibly other types of molecules involved in recognition phenomena, such as hormone receptors.

The interaction of triamterene at the myocardial beta receptor site

Triamterene (TA) inhibits in the microM range, as was previously shown in our laboratory, the positive inotropic action of beta-adrenoceptor agonists like isoproterenol in atrial preparations, when applied first. This interaction may be best explained by a direct influence of TA on the beta-adrenergic receptor-site itself or by an uncoupling of the signal transfer between receptor and adenylate cyclase complex at the inner sarcolemmal membrane site. For clarification we performed biochemical studies at freshly prepared membrane particles from guinea-pig hearts, combining both hypothetical sites of interaction. Binding studies with [3H]-dihydroalprenolol to the myocardial beta-adrenoceptor revealed that TA (1 to 10 mumol/l) did not disturb their functioning excluding a possible interaction at this membrane site. On the other hand we found a non competitive inhibition of the isoprenaline-activated adenylate cyclase while basic activity was not altered in the presence of triamterene (TA, 5 to 200 mumol/l). From the concentration-response curves a Ki-value of TA for half-maximal inhibition of 2.8 mumol/l was calculated. Since the sodium fluoride activated adenylate cyclase was also inhibited by triamterene in the same concentration range we conclude that it interferes with the signal-transfer between the beta-adrenoceptor site and the adenylate cyclase system through the plasmalemma.

Effects of dexamethasone on beta-adrenergic receptors in fetal lung explants

The action of beta-adrenergic agonists on pulmonary surfactant secretion requires lung cell membrane beta-adrenergic receptors. In the fetus, the density of beta-adrenergic receptors in lung increases in the latter stages of gestation. The increase in density can also be induced by maternal glucocorticoid treatment. In this study, we measured beta-adrenergic receptors in developing rat lung by (-) [3H]-dehydroalprenalol (DHA) binding and confirmed the increase in beta-adrenergic receptors late in gestation. To determine if glucocorticoids have a direct effect on fetal lung to regulate beta-adrenergic receptors, we cultured fetal lung explants with dexamethasone. Treated explants had increased DHA binding compared with controls (138.0 +/- 8.8 versus 63.2 +/- 5.0 fmole/mg membrane protein). Scatchard analysis revealed that the increased DHA binding was due to an increase in maximum receptor number. There was also a significant difference in the dissociation constant of the treated and control explants (0.85 +/- 0.07 nM versus 0.43 +/- 0.08 nM, respectively; P less than 0.05), suggesting that the receptors induced by dexamethasone were of lower binding affinity. Cyclohexamide, an inhibitor of protein synthesis, completely eliminated the dexamethasone induced increase in DHA binding. These data indicate that glucocorticoids have a direct effect on fetal lung to increase beta-adrenergic receptor density and that new protein synthesis is required for this effect.

Drug binding to plasma proteins during human pregnancy and in the perinatal period. Studies on cloxacillin and alprenolol

Plasma protein binding of one acidic drug, cloxacillin, and one basic drug, alprenolol, was determined by equilibrium dialysis at +37 degrees C during pregnancy and the 1st postnatal week in 12 women and their newborn infants and in 7 nonpregnant women (controls). A significant increase in fraction free cloxacillin in maternal plasma occurred during pregnancy already from the 2nd trimester compared to the controls (p less than 0.01) and was most pronounced at delivery (median values 0.126 and 0.069, respectively). A similarly increased fraction free cloxacillin was found in cord blood (median value 0.108) which further increased during the 1st postnatal week (range 0.112-0.164). In maternal plasma the binding capacity returned to the values of the controls during the same time period. The binding of cloxacillin was significantly correlated with the concentration of albumin (p less than 0.01). High correlation was also found between binding of the basic drug alprenolol and concentration of orosomucoid (p less than 0.005). This was most obvious in the newborn infants with low concentrations (range 0.1-0.3 g/l) and in the mothers during the puerperium with high concentrations of orosomucoid (range 0.7-2.5 g/l). On the basis of plasma protein binding data in the mother and her child, a maternal to fetal plasma concentration ratio was calculated. For cloxacillin this ratio was close to unity (1.03), while it was significantly above unity for alprenolol (1.72). At equilibrium, therefore, the total plasma concentration of alprenolol in the mother can be expected to exceed the concentration in her infant.

Irreversible blockade of beta adrenoreceptors and their recovery in the rat heart and lung in vivo

The interaction of a bromoacetylated derivative of alprenolol (Alm-CO-CH2Br) with cardiac and lung beta adrenoreceptors was partially characterized. After a short incubation period, the concentration of Alm-CO-CH2Br that inhibited specific [3H]dihydroalprenolol binding by 50% in cardiac and lung membranes was 0.5 and 0.11 microM, respectively. The blockade was time-dependent and Scatchard analysis showed no change in the KD value for specific (-)-[3H]dihydroalprenolol binding but a loss of beta adrenoreceptor content after membrane pretreatment with Alm-CO-CH2Br. The blockade was not reversed by extensive membrane washing, although concurrent treatment with alprenolol fully protected whereas phentolamine had no protective effect. Alm-CO-CH2Br produced a dose-dependent blockade of heart and lung beta adrenoreceptors in vivo and the compound had little or no effect on the growth rate of the rat. Four hours after a single i.p. injection of Alm-CO-CH2Br at 35 mg/kg, the heart and lung beta adrenoreceptor content was decreased by 88 and 90%, respectively. The time required for complete recovery from irreversible beta adrenoreceptor blockade was about 200 hr in the heart and 650 hr in the lung. These results suggest that Alm-CO-CH2Br may be a useful probe for the beta adrenoreceptor both in vitro and for recovery studies in vivo.

Penetration of five beta-adrenergic antagonists into the rabbit eye after ocular instillation

Aqueous humor and serum levels of five beta-adrenergic blockers were measured using gas chromatographic techniques following ocular instillation of 1% solutions to rabbits. Octanol/buffer partition ratios were also determined. No apparent relationship was found between the peak levels of the agents in aqueous humor and their octanol/buffer partition ratios. However, peak aqueous humor levels of timolol (461 ng/100 mg) and practolol (919 ng/100 mg), whose partition ratios were less than unity, occurred somewhat later (1 h) than the peaks of propranolol (859 ng/100 mg at 30 min), oxprenolol (1,771 ng/100 mg at 30 min) or alprenolol (1,004 ng/100 mg at 10 min) whose partition ratios exceeded unity. Peak levels in serum of timolol (8.0 ng/100 microliter), propranolol (4.2 ng/100 microliter), oxprenolol (11.5 ng/100 microliter), and alprenolol (4.5 ng/100 microliter) were achieved within 10 min and reduced to less than 15% within 4 h. Peak serum levels of practolol (9.9 ng/100 microliter) occurred somewhat later (2 h) and remained high (41%) 6 h later.

Propranolol, alprenolol and oxprenolol metabolism in the dog. Identification of N-methylated metabolites

The metabolism of propranolol, alprenolol and oxprenolol was studied in the dog and rat; propranolol in five additional species, including man. Basic, phenolic and neutral metabolites were extracted from urine at pH 9.6 after enzymatic hydrolysis. Separation and identification of parent drug and seven metabolites each for propranolol, alprenolol and oxprenolol in the dog were accomplished by gas chromatography mass spectrometry as the trifluoroacetyl derivatives. A very uniform and predictable fragmentation pattern was observed for all 24 compounds. Seven new metabolites were identified. The metabolism of all three drugs was qualitatively the same, including N-dealkylation followed by N-methylation or deamination of the primary amines. The parent drugs as well as all of their sidechain metabolism products were also partially ring hydroxylated. N-Methylation was only found in the dog and is a minor metabolic pathway. The stereochemical composition of N-methyldesisopropylpropranolol and its immediate precursor N-desisopropylpropranolol showed a marked enrichment of the (+)-isomer.

Alpha-Adrenergic receptors modulate beta-receptor affinity in rat kidney membranes

Adrenergic receptors were first classified into two calsses--alpha and beta--on the basis of the relative pharmacological potencies of agonist compounds, and this classification has been supported by subsequent studies. In some tissues, such as the heart and liver, they exert similar physiological responses, and in other tissues, such as the uterus and vasculature, they have opposing roles. The occurrence of both classes of receptor in the same tissue lewwds to the problem of what determines whether the overall response observed is alpha-type or beta-type, as adrenaline and noradrenaline bind to both receptor classes. Furthermore, direct binding studies have demonstrated that the two receptor classes are distinct and separate entities. We show here that stimulation of alpha-receptors in renal membranes causes a specific decrease in the affinity of the agonist compound isoprenaline for beta-receptors in the same membranes. This demonstrates that interactions occur between renal alpha- and beta-adrenergic receptors. Such interactions may modulate the response of the kidney to sympathetic stimulation.

Hormone-receptor interactions are noncooperative: application to the beta-adrenergic receptor

We have developed kinetic methods with which we have demonstrated that the negatively cooperative hormone-binding model is untenable for the insulin receptor system. These methods have led others to the same conclusion for the thyrotropin and nerve growth factor receptors. We report here the application of these methods to the study of (l)-[propyl-2,3-3H]dihydroalprenolol [(l)-[3H]DHA] binding at 15 degrees C to the beta-adrenergic receptor of the frog erythrocyte--the remaining and most extensively characterized hormone receptor-cyclase system in which negatively cooperative site-site interactions have been reported. Scatchard analysis of the equilibrium binding data for (l)-[3H]DHA in this system is linear. In addition, increasing concentrations of (l)-[3H]DHA during the binding reactions resulted in increasing receptor occupancy but no enhancement of the subsequent dilution-induced dissociation of bound hormone, demonstrating directly that the dissociation rate is independent of occupancy. Furthermore, analysis of the time course of the approach-to-equilibrium for this system at different hormone concentrations was consistent with reversible hormone binding to a homogeneous class of noncooperative receptors, and the analysis yielded a kinetically estimated equilibrium constant consistent with that derived from the linear equilibrium data. The above data indicate that negatively cooperative site-site interactions are not a significant factor for the beta-adrenergic receptor of the frog erythrocyte. These results complete the demonstration that for all known hormone receptor systems, the hormone binds noncooperatively to one or more classes of independent receptor sites. The need for critical reinterpretation of detailed thermodynamic, clinical, and hormone analogue studies based on the negative cooperativity model is discussed.

In vivo receptor binding of iodinated beta-adrenoceptor blockers

Six radiolabeled beta-adrenoceptor blocking agents with a range of affinity constants were evaluated as radioindicators for adrenoceptors in guinea-pig heart and lung. All concentrated in the heart and lung at levels in excess of 0.1% dose/g tissue. On the basis of displacement studies using propranolol, two of the six compounds showed beta-adrenoceptor binding in the lung, and one, H-3 carazolol, showed receptor binding in the heart. These results agree qualitatively with a bi-molecular reversible equilibrium model, and suggest that the beta-adrenoceptor blockers as a group will not be useful in vivo probes of receptor concentration in the heart because of the low affinity constants and high levels of nonreceptor binding associated with the present-day clinical beta blockers. Beta-adrenoceptor blocking agents with affinity constants in excess of 10(9) will be needed to give heart-to-blood ratios of 10.

Macromolecular beta-adrenergic antagonists discriminating between receptor and antibody

The beta-adrenergic antagonist, alprenolol, was attached in an irreversible manner to macromolecular dextran via side arms that differed in length. The ability of these macromolecules to bind to the beta-adrenergic receptor of frog erythrocytes and to catecholamine-binding antibodies raised against partially purified receptors was studied. Compared to the parent drug the potency of binding of macromolecular alprenolol to the receptor decreased about 1/10, 1/600, and 1/8000 when the length of the arm separating alprenolol from the dextran moiety was 13, 8, and 4 atoms, respectively. In contrast, the binding potencies of the parent drug and of all its macromolecular derivatives for the antibody were within the same order of magnitude. Thus, conversion of a drug to a macromolecular form may not only sustain its binding activity but may also lead in a higher selectivity. The macromolecular derivatives described here may be suitable probes for investigation of the location and of the molecular properties of the binding sites for beta-adrenergic drugs.

The molecular structure of adrenergic and dopaminergic substances

At the end of the last century it was established that the different nerve cells along a neuronal path do not come into direct physical contact with one another, but that there are narrow gaps between them, called synapses (Sherrington, 1897; Ramón y Cajal, 1906). Elliot (1905) made the basic experimental observation that the propagation of nerve impulses across a synapse might be mediated by specific chemical agents (see Fig. i). Such substances are now called neurotransmitters, and some 20 different compounds putatively responsible for synaptic transmission in different parts of the nervous system are known at present, e.g. a few recently isolated polypeptides. The most extensively studied transmitters are acetylcholine and the catecholamine group, consisting of dopamine (a), noradrenaline (b), and adrenaline (c).

Neurotransmitter receptors in frontal cortex of schizophrenics

Frontal cerebral cortex brain samples from schizophrenics and controls have been assayed for binding associated with muscarinic cholinergic, serotonin (5HT), gamma-aminobutyric acid (GABA), and beta-adrenergic receptors as well as for the activity of the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD). Binding levels of tritium-LSD, presumably associated with postsynaptic 5HT receptors, were reduced 40% to 50% in samples from schizophrenics in three independent studies, whereas no other consistent alteration was observed in levels of binding associated with other receptors or in the activity of GAD. This change in receptor binding levels does not seem to be attributable to postmortem changes, to influence of drugs received by the patients, or to demographic features of the patient populations.