β-Adrenergic receptor subtypes

Synthesis and adrenergic activity of a new series of N-aryl dicyclopropyl ketone oxime ethers: SAR and stereochemical aspects

A novel series of 31 N-aryl dicyclopropyl ketone oxime ethers were synthesized and tested for their activity at alpha- and beta-adrenergic receptors. All of the compounds showed greater affinity for beta-than for alpha1-receptor sites. Some compounds had pure antagonist effects whereas some were partial agonists. Several compounds had an antagonist effect matching that of propranolol in in vitro (binding data and pA2 values on rat heart ventricle homogenates and guinea pig spontaneously beating right and electrically driven left atrial isolated preparations, respectively) and in in vivo tests (measurement of antagonism toward isoprenaline-induced tachycardia in anesthetized rats). Furthermore, all of the compounds showed a beta1-adrenergic selectivity (beta2-affinity > 1500 nM).

Prediction of the therapeutic dose for beta-stimulants based on preclinical data: application of oral dosage forms and aerosols to asthmatic patients

The therapeutic doses of beta-stimulants in the treatment of bronchial asthma show wide variation among the drugs. In the present study, we tried to construct a system for prediction of the optimum clinical dose of beta-stimulants as oral dosage forms and aerosols using the data from preclinical pharmacological studies and pharmacokinetic data in humans. Values of EC50 (drug concentration that gives the half-maximum bronchodilator effect) in the in vitro studies based on isolated trachea of guinea pigs were collected from the literature. The ratios of the EC50 value of each drug to that of isoproterenol were used as the indicator of in vitro pharmacological activity. There were significant correlations between the EC50 ratios and the oral dose or the maximum plasma concentration after single oral administration, but the correlation coefficients were relatively small (r < .9). On the other hand, a linear log-log relationship was observed between the EC50 ratio and the maximum plasma unbound concentration (Cu(max)) (slope = 0.907, r = .955, p < 0.001). In the case of aerosols, a good correlation was observed between the EC50 ratio and the dose (slope = 0.770, r = .900, p < 0.01). The predicted concentration of beta-stimulants in the lung after aerosol administration tended to be higher than the Cu(max) after oral administration, suggesting the contribution of nonspecific binding in the lung tissue. These findings indicate the possibility of predicting the appropriate dose of beta-stimulants based on the preclinical pharmacological data and the pharmacokinetic data in humans.

Antagonism of 8-OH-DPAT-induced behaviour in rats

Selective activation of the 5-HT1A receptor induces lower lip retraction (LLR) in rats. 8-Hydroxy-dipropylamino tetralin (8-OH-DPAT)-induced LLR could not be antagonised by the 5-HT antagonists methysergide, metergoline or mesulergine. In fact, some 5-HT antagonists induced LLR. However, 8-OH-DPAT-induced LLR could be antagonised by pindolol, spiperone, spiroxatrine and NAN-190, but not by the beta 1-adrenoceptor antagonist metoprolol, the beta 2-adrenoceptor antagonist butoxamine or the dopamine antagonist haloperidol. This antagonism was competitive as the dose-response curve of 8-OH-DPAT was shifted to the right. Pindolol, spiperone, spiroxatrine and NAN-190 all have a high affinity for the 5-HT1A receptor. This indicates that blockade of 8-OH-DPAT-induced LLR is only possible by selective blockade of 5-HT1A receptors. A possible mechanism of action is discussed. The increased defecation induced by 8-OH-DPAT could be antagonised by pindolol and NAN-190. The effect of spiroxatrine and haloperidol on the 8-OH-DPAT-induced increase in defecation was bimodal: an increase after a low and a decrease after a high dose of 8-OH-DPAT. Metoprolol and butoxamine had no effect on the 8-OH-DPAT-induced increase in defecation, thereby excluding an influence of beta-adrenoceptors.

In vitro selectivity of agonists and antagonists for beta 1- and beta 2-adrenoceptor subtypes in rat brain

A radioreceptor binding assay was developed to determine the selectivity of beta-adrenoceptor agents in rat brain. This was achieved by using the highly selective unlabelled antagonists CGP 20712A and ICI 118-551 to block beta 1- or beta 2-sub-populations respectively in rat cerebral cortex membranes. This permitted the selective labelling of beta-adrenoceptors with the antagonist (-)-[125I]pindolol. Using this method, compounds could be routinely screened and selectivity profiles for binding in the CNS determined with a high degree of sensitivity and resolution.

Drug-induced penile erections in rats: indications of serotonin1B receptor mediation

The induction of penile erections by a variety of compounds with a direct or indirect effect on serotonin (5HT) receptors was investigated in rats. L-5-Hydroxy-tryptophan (L-5HTP) induced penile erections when co-administered with nialamide and the peripheral decarboxylase inhibitor benserazide, indicating that the site of action for inducing penile erections is within the central nervous system. Penile erections were also induced by the 5HT uptake inhibitors zimelidine, fluoxetine, citalopram, Org 6997, by the 5HT-releasing agent fenfluramine and by the putative 5-HT1B receptor agonist 1-(3'-chlorophenyl)-piperazine (mCPP). The 5HT1A-agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) did not induce penile erections. The less selective 5HT receptor agonists 5-methoxy-N,N-dimethyl-tryptamine (5MeODMT), 5-methoxytryptamine (5MeOT), dl-lysergic acid diethylamide (LSD) and Ru 24969 were also ineffective. Induction of penile erections by quipazine appeared only when this compound was co-administered with the 5HT2 receptor antagonist pirenperone. Receptor antagonists were tested against penile erections induced by Org 6997. The beta-adrenoceptor antagonists that also have 5HT1 antagonistic properties, (S)-pindolol and dl-propranolol, antagonized Org 6997-induced penile erections but butoxamine and metoprolol did not. Spiperone and pirenperone in doses selective for 5HT1A and 5HT2 receptors respectively were also inactive. Haloperidol, 0.46 mg/kg, partially attenuated penile erections induction. The data are discussed in the light of the hypothesis that penile erections induction by serotonin-mimetic compounds is mediated by 5HT1B receptors in the striatum.

Influence of urapidil on alpha- and beta-adrenoreceptors in pithed rats

The interaction of urapidil with pre- and postsynaptic alpha-adrenoreceptors and with postsynaptic beta-adrenoreceptors was studied in pithed normotensive rats and compared to the effects of clonidine, prazosin, and atenolol. I.v. injection of urapidil did not substantially change blood pressure, while clonidine raised blood pressure. Urapidil dose-dependently antagonized the pressor effects of the alpha 1-agonist L-phenylephrine (pDR2 6.8) and of the alpha 2-agonist azepexole (pDR2 5.2). Compared to urapidil, prazosin was a more potent antagonist of phenylephrine at postsynaptic vascular alpha 1-adrenoreceptors (pDR2 8.7) and of azepexole at alpha 2-adrenoreceptors (pDR2 5.6). Urapidil inhibited the tachycardia produced by discontinuous or continuous electrical stimulation of the thoracic spinal outflows (ID50 4.8 and 27.2 mumol/kg, respectively). In contrast to the inhibitory action of clonidine (ID50 0.039 and 0.023 mumol/kg, respectively), the inhibition by urapidil was not reversed by the selective alpha 2-antagonist rauwolscine (10 mumol/kg). Prazosin did not change stimulation-evoked tachycardia but atenolol caused pronounced inhibition (ID50 0.158 mumol/kg, discontinuous stimulation). Urapidil dose-dependently antagonized the tachycardic effect of isoprenaline at beta 1-adrenoreceptors (pDR2 5.1) but also exhibited intrinsic activity by increasing basal heart rate (maximum effect of urapidil was 30% of that of isoprenaline). Urapidil did not change the vasodilatory beta 2-adrenoreceptor-mediated effect of isoprenaline. The results suggest that urapidil is an antagonist at postsynaptic vascular alpha 1- and alpha 2-adrenoreceptors, with a greater potency against alpha 1-adrenoreceptors. An agonistic interaction of urapidil with presynaptic alpha 2-adrenoreceptors could not be demonstrated in pithed rats. Instead, the inhibition by urapidil of stimulation-evoked tachycardia could be accounted for by its beta 1-adrenoreceptor antagonistic effect.

Effects of gonadal steroids and adrenergic agonists on avian growth and feed efficiency

The effects of gonadal steroids and adrenergic agonists in growing poultry are reviewed. Estrogens, used commercially to caponize, improve muscle palatability and weight gain but are hyperlipidemic and adversely affect feed efficiency. Their ability to caponize results from the fact that negative feedback regulation of gonadotropin secretion is mediated by estrogen synthesized within the male's central nervous system. Androgens are strongly anabolic in avian species but are deleterious to growing bone. Thus, androgens depress growth and efficiency when given before epiphyseal closure but stimulate gain and improve efficiency when given afterwards. The beta-adrenergic agonist clenbuterol improves the growth and efficiency of broiler chickens. The effects of beta-agonists on muscle, adipogenic tissue, and the endocrine system are presented as possible mechanisms for these actions.

Repeated treatment with antidepressant drugs prevents salbutamol-induced hypoactivity in rats

We have found earlier that a number of antidepressant drugs, administered repeatedly, prevent the salbutamol-induced hypoactivity in rats. Our further experiments show that a similar effect is produced by repeated, but not acute, treatment with other antidepressants: clomipramine, mianserin and nialamide. Two non-antidepressant psychotropic drugs, haloperidol and diazepam, were inactive after repeated administration. These results seem to support our earlier hypothesis that prevention of the salbutamol-induced hypoactivity may be regarded as functional evidence at the behavioral level for the subsitivity of beta-adrenoceptors.

Effects of (-)-(R)-1-(p-hydroxyphenyl)-2-[3,4-dimethoxyphenethyl)amino]ethanol (TA-064), a new cardiotonic agent, on circulating parameters of carbohydrate and lipid metabolism in the rat

Effects of the new cardiotonic and selective beta 1-adrenergic agonist TA-064, (-)-(R)-1-(p-hydroxyphenyl)-2-[(3,4-dimethoxyphenethyl)amino] ethanol, on circulating concentrations of glucose, lactate, free fatty acids (FFA), glycerol, cyclic AMP and the pancreatic hormones insulin (IRI) and glucagon (IRG) were examined in rats. TA-064, administered orally or intraperitoneally at the dose of 10 mg/kg (ca. 50 times the therapeutic dose) or higher, caused a slight transient rise followed by a persistent lowering of blood glucose concentrations, but it did not affect blood lactate levels at all. The same treatment with TA-064 elevated the concentrations of blood FFA, glycerol and plasma IRI and IRG. These changes induced by TA-064 were inhibited by pretreatment with propranolol (a non-selective beta-adrenergic antagonist) and practolol (a selective beta 1-adrenergic antagonist). The non-selective beta-adrenergic agonist isoproterenol and the selective beta 2-adrenergic agonist terbutaline elevated both blood glucose and lactate when administered intraperitoneally. They also brought about sustained rises in blood glycerol and plasma IRI, but only transiently increased the plasma IRG level. The cardiotonic agent prenalterol, claimed to be a selective beta 1-agonist, elevated blood glucose, lactate, and glycerol only slightly, and plasma IRI significantly, but it had no effect on plasma IRG. The cardiotonic agents dobutamine and amrinone also elevated blood glucose. Thus, TA-064 is unique among the beta-adrenergic and other cardiotonic agents in that it produces sustained hypoglycemia while it has no lactacidemic effect. Since this hypoglycemic action of TA-064 was always preceded by a rise in plasma IRI and abolished in streptozotocin-diabetic rats, we conclude that increased secretion of pancreatic insulin and the lack of hyperglycemic action are responsible for the hypoglycemia by high doses of TA-064.

Studies on the mechanisms by which clenbuterol, a beta-adrenoceptor agonist, enhances 5-HT-mediated behaviour and increases metabolism of 5-HT in the brain of the rat

The head twitch response in mice produced by injection of 5-hydroxytryptophan (100 mg/kg i.p.) and carbidopa (25 mg/kg i.p.) was enhanced by administration of clenbuterol (0.5 mg/kg i.p.), a beta-adrenoceptor agonist. Clenbuterol also enhanced the hyperactivity syndrome in rats produced by quipazine (25 mg/kg i.p.), a 5-hydroxytryptamine (5-HT) agonist. This enhancement was not prevented by depletion of 5-HT in brain with p-chlorophenylalanine or after pretreatment with prazosin. The behavioural responses of the rats to administration of the alpha 2-adrenoceptor agonist, clonidine, was unaltered by acute or longer-term administration of clenbuterol. Following chronic administration of clenbuterol (5 mg/kg daily for 14 days), a procedure resulting in down-regulation of central beta-adrenoceptors, a larger dose of clenbuterol was necessary to enhance the quipazine-induced hyperactivity, suggesting that the mechanism of enhancement involved central post-synaptic beta-adrenoceptors. Further evidence for this conclusion was that a lesion of central noradrenaline pathways produced by 6-hydroxydopamine did not abolish the clenbuterol-induced enhancement of the quipazine-mediated behaviour. The binding characteristics of 5-HT2-receptors were unchanged by acute or chronic administration of clenbuterol. Clenbuterol (5 mg/kg) increased the percentage of plasma free (non-albumin bound) tryptophan, plasma free fatty acid concentration and the concentration of tryptophan and 5-hydroxyindoleacetic acid (5-HIAA) in the brain. The increase in 5-HT turnover in brain was prevented by pretreatment with the beta 1-adrenoceptor antagonist atenolol, which enters the brain poorly. It is therefore suggested that the clenbuterol-induced increase in 5-HT metabolism results from the increase in the concentration of plasma free fatty acid which increases plasma free tryptophan and thus increases the concentration of tryptophan in brain and 5-HT synthesis in brain. The clenbuterol-induced enhancement of 5-HT-mediated behaviour is therefore not associated with its effect on 5-HT metabolism. The data are discussed in relation to that obtained after administration of antidepressant drugs.

The effect of chronic antidepressant administration on beta-adrenoceptor function of the rat pineal

1 The beta-adrenoceptor agonist, isoprenaline (1.5-3.0 mg kg(-1) intravenously), produced a dose-related increase in rat pineal melatonin content. This increase was prevented by pretreatment with the selective beta(1)-adrenoceptor antagonist, atenolol (2 mg kg(-1)), but not by the beta(2)-adrenoceptor antagonist, butoxamine (2 mg kg(-1)). The beta(2)-adrenoceptor agonist, terbutaline (5.0 mg kg(-1)), produced a moderate increase in pineal melatonin content.2 Repeated daily administration of desmethylimipramine (10 mg kg(-1) for 10 days) and maprotiline (10 mg kg(-1) for 10 days), antidepressants predominantly inhibiting noradrenaline (NA) uptake, reduced the isoprenaline-induced increase in pineal melatonin content. Amitriptyline (20 mg kg(-1) for 14 days), a drug which inhibits both NA and 5-hydroxytryptamine (5-HT) uptake, had a similar effect. The beta-adrenoceptor agonist, clenbuterol (5 mg kg(-1) for 14 days), also attenuated the increase in pineal melatonin produced by isoprenaline.3 In contrast, chronic administration of the selective 5-HT uptake inhibitor, fluoxetine (10 mg kg(-1) for 10 days), or the antidepressants, iprindole and mianserin (both 20 mg kg(-1) for 14 days), which do not inhibit monoamine uptake, failed to reduce the increase in pineal melatonin following isoprenaline. Repeated electroconvulsive shock was similarly without effect.4 Ten hours after the final dose of desmethylimipramine (10 mg kg(-1)) once daily for 10 days there was no change in the usual dark phase increase in pineal melatonin.5 The data suggest that repeated administration of certain antidepressant drugs results in reduced pineal beta-adrenoceptor sensitivity. However the lack of change in the dark phase increase in pineal melatonin following repeated desmethylimipramine, implies that the reduced ss-adrenoceptor sensitivity may be part of an adaptive process which maintains normal pineal function. Therefore the decrease in beta-adrenoceptor number in the brain reported after chronic antidepressant administration may not be associated with a change in overall synaptic function.

beta-Adrenoceptor agonists enhance 5-hydroxytryptamine-mediated behavioural responses

The beta-adrenoceptor agonists, salbutamol, terbutaline and clenbuterol, were investigated for their effect on 5-hydroxytryptamine-mediated (5-HT) hyperactivity. 2 The lipophilic beta-adrenoceptor agonist, clenbuterol (5 mg/kg) enhanced the behaviours induced by quipazine (25 mg/kg), including headweaving, forepaw treading and hind-limb abduction and thus increased automated activity recording. Clenbuterol (5 mg/kg) also enhanced the hyperactivity syndrome produced by the 5-HT agonist, 5-methoxy N,N-dimethyltryptamine (2 mg/kg) and the combination of tranylcypromine (10 mg/kg) and L-tryptophan (50 mg/kg). Salbutamol and terbutaline potentiated quipazine-induced hyperactivity only when given at the higher dose of 20 mg/kg. 3 The effect of clenbuterol in enhancing quipazine hyperactivity was blocked by the centrally acting beta 1-adrenoceptor antagonist, metoprolol (5 mg/kg), but not by the beta 2-adrenoceptor antagonist, butoxamine (5 mg/kg) or the peripherally acting beta 1-adrenoceptor antagonist, atenolol (5 mg/kg). 4 Clenbuterol (5 mg/kg) did not enhance the circling responses produced by methamphetamine (0.5 mg/kg) in unilateral nigrostriatal-lesioned rats. 5 The results suggest that beta-adrenoceptor agonists in common with some established antidepressant treatments produce enhancement of 5-HT-mediated behavioural responses.

Mapping of mammalian beta-adrenoreceptors by use of macromolecular alprenolol derivatives. A comparison with amphibian erythrocyte receptors

THe interaction of macromolecular alprenolol derivatives with beta-adrenoreceptors of rat heart, lung, and erythrocytes and frog erythrocytes has been studied. Macromolecular derivatives were prepared by covalently coupling alprenolol to dextrans containing a homologous series of spacer arms of various lengths. The affinity of these macromolecules for frog erythrocyte membranes increased with increasing length of spacer arm. In contrast, the affinity of these macromolecules for all mammalian membrane preparations was weak and insensitive to the length of the spacer arm. The inhibition of [3H]dihydroalprenolol binding to rat heart, lung, and erythrocyte membrane preparations by these macromolecular derivatives was more than 1000-fold less potent than inhibition by alprenolol. The results suggest different structural characteristics between mammalian and amphibian beta-adrenoreceptors; however, apparently only small differences between mammalian receptors could be distinguished with these probes.