The role of a low beta 1-adrenoceptor selectivity of [3H]CGP-12177 for resolving subtype-selectivity of competitive ligands

Molecular dissection of dual pseudosymmetric solute translocation pathways in human P-glycoprotein

The human multispecific drug efflux transporter P-glycoprotein (P-gp) causes drug resistance and modulates the pharmacological profile of systemically administered medicines. It has arisen from a homodimeric ancestor by gene duplication. Crystal structures of mouse MDR1A indicate that P-gp shares the overall architecture with two homodimeric bacterial exporters, Sav1866 and MsbA, which have complete rotational symmetry. For ATP-binding cassette transporters, nucleotide binding occurs in two symmetric positions in the motor domains. Based on the homology with entirely symmetric half-transporters, the present study addressed the key question: can biochemical evidence for the existence of dual drug translocation pathways in the transmembrane domains of P-gp be found? P-gp was photolabeled with propafenone analogs, purified, and digested proteolytically, and peptide fragments were identified by high-resolution mass spectrometry. Labeling was assigned to two regions in the protein by projecting data into homology models. Subsequently, symmetric residue pairs in the putative translocation pathways were identified and replaced by site-directed mutagenesis. Transport assays corroborated the existence of two pseudosymmetric translocation pathways. Although rhodamine123 has a preference to take one path, verapamil, propafenones, and vinblastine preferentially use the other. Two major findings ensued from this study: the existence of two solute translocation pathways in P-gp as a reflection of evolutionary origin from a homodimeric ancestor and selective but not exclusive use of one of these pathways by different P-gp solutes. The pseudosymmetric behavior reconciles earlier kinetic and thermodynamic data, suggesting an alternative concept of drug transport by P-gp that will aid in understanding the off-target quantitative structure activity relationships of P-gp interacting drugs.

Preclinical evaluation of an 18F-labelled beta1-adrenoceptor selective radioligand based on ICI 89,406

PURPOSE

Radioligand binding studies indicate a down-regulation of myocardial beta(1)-adrenoceptors (beta(1)-AR) in cardiac disease which may or may not be associated with a decrease in beta(2)-ARs. We have chosen ICI 89,406, a beta(1)-selective AR antagonist, as the lead structure to develop new beta(1)-AR radioligands for PET and have synthesised a fluoro-ethoxy derivative (F-ICI).

METHODS

(S)-N-[2-[3-(2-Cyano-phenoxy)-2-hydroxy-propylamino]-ethyl]-N'-[4-(2-[(18)F]fluoro-ethoxy)-phenyl]-urea ((S)-[(18)F]F-ICI) was synthesised. Myocardial uptake of radioactivity after intravenous injection of (S)-[(18)F]F-ICI into adult CD(1) mice or Wistar rats was assessed with positron emission tomography (PET) and postmortem dissection. Metabolism was assessed by high-performance liquid chromatography analysis of plasma and urine.

RESULTS

The heart was visualised with PET after injection of (S)-[(18)F]F-ICI but neither unlabelled F-ICI nor propranolol (non-selective beta-AR antagonist) injected 15 min after (S)-[(18)F]F-ICI affected myocardial radioactivity. Ex vivo dissection demonstrated that predosing with propranolol or CGP 20712 (beta(1)-selective AR-antagonist) did not affect myocardial radioactivity. Radiometabolites rapidly appeared in plasma and both (S)-[(18)F]F-ICI and radiometabolites accumulated in urine.

CONCLUSIONS

Myocardial uptake of (S)-[(18)F]F-ICI after intravenous injection was mainly at sites unrelated to beta(1)-ARs. (S)-[(18)F]F-ICI is not a suitable beta(1)-selective-AR radioligand for PET.

Regional distribution and cellular localization of beta2-adrenoceptors in the adult zebrafish brain (Danio rerio)

The beta(2)-adrenergic receptors (ARs) are G-protein-coupled receptors that mediate the physiological responses to adrenaline and noradrenaline. The present study aimed to determine the regional distribution of beta(2)-ARs in the adult zebrafish (Danio rerio) brain by means of in vitro autoradiographic and immunohistochemical methods. The immunohistochemical localization of beta(2)-ARs, in agreement with the quantitative beta-adrenoceptor autoradiography, showed a wide distribution of beta(2)-ARs in the adult zebrafish brain. The cerebellum and the dorsal zone of periventricular hypothalamus exhibited the highest density of [(3)H]CGP-12177 binding sites and beta(2)-AR immunoreactivity. Neuronal cells strongly stained for beta(2)-ARs were found in the periventricular ventral telencephalic area, magnocellular and parvocellular superficial pretectal nuclei (PSm, PSp), occulomotor nucleus (NIII), locus coeruleus (LC), medial octavolateral nucleus (MON), magnocellular octaval nucleus (MaON) reticular formation (SRF, IMRF, IRF), and ganglionic cell layer of cerebellum. Interestingly, in most cases (NIII, LC, MON, MaON, SRF, IMRF, ganglionic cerebellar layer) beta(2)-ARs were colocalized with alpha(2A)-ARs in the same neuron, suggesting their interaction for mediating the physiological functions of nor/adrenaline. Moderate to low labeling of beta(2)-ARs was found in neurons in dorsal telencephalic area, optic tectum (TeO), torus semicircularis (TS), and periventricular gray zone of optic tectum (PGZ). In addition to neuronal, glial expression of beta(2)-ARs was found in astrocytic fibers located in the central gray and dorsal rhombencephalic midline, in close relation to the ventricle. The autoradiographic and immunohistochemical distribution pattern of beta(2)-ARs in the adult zebrafish brain further support the conserved profile of adrenergic/noradrenergic system through vertebrate brain evolution.

Antipsychotic drugs influence transport of the β-adrenergic antagonist [3H]-dihydroalprenolol into neuronal and blood cells

The amine hypothesis suggests that the cause of schizophrenic or depressive psychosis is dysfunction of noradrenergic or serotonergic neurotransmission. We investigated pharmacological properties of [3H]-dihydroalprenolol (DHA) transport into C6, IMR32, native lymphocytes, B-lymphoblastoids and MOLT-3 cells. DHA transport was inhibited by a heterogeneous group of structurally related compounds exhibiting an amine group and various aromatic ring structures. It was verified on cells of neuronal/glial and blood cell origin but in detail on B-lymphoblastoids. The latter once showed strongest inhibition of DHA transport using tricyclic antidepressants (amitriptyline: IC50 = 2.86 microM, imipramine: IC50 = 3.33 microM) and haloperidol (IC50 = 3.98 microM) as a neuroleptic. Antipsychotics like clozapine (IC50 = 11 microM), olanzapine (IC50 = 15 microM), spiperone (IC50 = 66 microM) and EMD 49980 (ICso >> 100 microM) were less effective. In contrast to cells of blood origin, a stimulation of DHA transport by antipsychotics was not detectable using neuronal cells. As antipsychotics showed a distinct inhibition and, concerning cells of blood origin, a stimulation of transport after pre-incubation, further investigations seem to be of interest in respect to its involvement in the cellular uptake of drugs and therefore its impact on the quality of therapy of psychiatric patients.

Labelling of Rat Brain ß-Adrenoceptors: (3H)CGP-12177 or (125I)Iodocyanopindolol?

Binding of (125I)iodocyanopindolol (ICYP) and (3H)CGP-12177 to rat brain homogenates was characterized and compared. ICYP was shown to bind to both beta-adrenergic and serotonin1B (5HT1B) receptors whereas (3H)CGP-12177 only labelled the first ones. The addition of 10 microM serotonin (5HT) prevented ICYP binding to 5HT receptors and under these experimental conditions both ligands labelled a similar total number of beta-adrenoceptors in the different rat brain regions. ICYP displayed a higher affinity for cerebellar (mainly beta 2-subtype) than for cerebral cortex beta-adrenoceptors (mainly beta 1-subtype) suggesting a subtype selectivity. A multiple displacement binding approach using CGP-20712A, a beta 1-subtype ligand, as competitor revealed a 2.6 fold selectivity of ICYP for the beta 2-adrenoceptor subtype. On the other hand, (3H)CGP-12177 binds only to beta-adrenoceptors and is not subtype selective in the rat brain homogenate. Considering both its high specificity and its lack of subtype selectivity (3H)CGP-12177 seems to be a more suitable ligand than ICYP to non-selectively label beta-adrenoceptors in rat brain.

Are [O-methyl-11C]derivatives of ICI 89,406 beta1-adrenoceptor selective radioligands suitable for PET?

PURPOSE

Radioligand binding studies show that beta(1)-adrenoceptor (beta(1)-AR) density may be reduced in heart disease without down regulation of beta(2)-ARs. Radioligands are available for measuring total beta-AR density non-invasively with clinical positron emission tomography (PET) but none are selective for beta(1)- or beta(2)-ARs. The aim was to evaluate ICI 89,406, a beta(1)-AR-selective antagonist amenable to labelling with positron emitters, for PET.

METHODS

The S-enantiomer of an [O-methyl-(11)C] derivative of ICI 89,406 ((S)-[(11)C]ICI-OMe) was synthesised. Tissue radioactivity after i.v. injection of (S)-[(11)C]ICI-OMe (< 2 nmol x kg(-1)) into adult Wistar rats was assessed by small animal PET and post mortem dissection. Metabolism was assessed by HPLC of extracts prepared from plasma and tissues and by measuring [(11)C]CO(2) in exhaled air.

RESULTS

The heart was visualised by PET after injection of (S)-[(11)C]ICI-OMe but neither unlabelled (S)-ICI-OMe nor propranolol (non-selective beta-AR antagonist) injected 15 min after (S)-[(11)C]ICI-OMe affected myocardial radioactivity. Ex vivo dissection showed that injecting unlabelled (S)-ICI-OMe, propranolol or CGP 20712A (beta(1)-selective AR antagonist) at high dose (> 2 mumol x kg(-1)) before (S)-[(11)C]ICI-OMe had a small effect on myocardial radioactivity. HPLC demonstrated that radioactivity in myocardium was due to unmetabolised (S)-[(11)C]ICI-OMe although (11)C-labelled metabolites rapidly appeared in plasma and liver and [(11)C]CO(2) was detected in exhaled air.

CONCLUSION

Myocardial uptake of (S)-[(11)C]ICI-OMe after i.v. injection was low, possibly due to rapid metabolism in other tissues. Injection of unlabelled ligand or beta-AR antagonists had little effect indicating that binding was mainly to non-specific myocardial sites, thus precluding the use of (S)-[(11)C]ICI-OMe to assess beta(1)-ARs with PET.

Pharmacological characterization of CGP 12177 at the human beta(2)-adrenoceptor

1 It has recently been reported that CGP 12177 can act as an agonist at a novel secondary site within the human beta(1)-adrenoceptor. The aim of this study was to undertake a detailed pharmacological study of the effects of CGP 12177 on the human beta(2)-adrenoceptor. 2 CGP 12177 acted as a potent partial agonist of (3)H-cyclic AMP accumulation (log EC(50)-8.90+/-0.06) and CRE-mediated reporter gene transcription (log EC(50)-9.66+/-0.04) in CHO-K1 cells expressing the human beta(2)-adrenoceptor. These CGP-induced responses were antagonized by the beta(2)-selective antagonist ICI 118551 (apparent log K(D) values of -8.84+/-0.15 and -9.51+/-0.02 for the cyclic AMP and reporter gene responses respectively). 3 CGP 12177 was also able to antagonize both cyclic AMP and reporter gene responses to more efficacious beta(2)-agonists with similar log K(D) values (e.g. -9.57+/-0.15 and -10.04+/-0.096 respectively with salbutamol as agonist). 4 (3)H-CGP 12177 binding to beta(2)-adrenoceptors in intact CHO-beta(2) cells yielded a log K(D) value of -9.84+/-0.06, but indicated that the ligand dissociates very slowly from the receptor (t(1/2) for dissociation=65 min). However, studies with a Green Fluorescent Protein (GFP)-tagged beta(2)-adrenoceptor indicated that CGP 12177 does not stimulate beta(2)-adrenoceptor internalization. 5 This study demonstrates that CGP 12177 is a high affinity partial agonist of both cAMP accumulation and CRE-mediated gene transcription at the human beta(2)-adrenoceptor. It provides no evidence that CGP 12177 can discriminate a secondary site on the beta(2)-adrenoceptor analogous to that observed for the human beta(1)-adrenoceptor. However, despite its very weak actions on cAMP accumulation, the potent agonist effects of CGP 12177 on CRE-mediated gene transcription at the human beta(2)-adrenoceptor, coupled with its long duration of action, offers a potential lead for drug development for the treatment of chronic inflammatory airway diseases.

Comparison of the affinity of beta-blockers for two states of the beta 1-adrenoceptor in ferret ventricular myocardium

We compared the potency of 11 clinically available beta-blockers as antagonists of the positive inotropic effects of (-)-isoprenaline and CGP12177 on ferret ventricular myocardium. (-)-CGP12177, (-)-pindolol and (-)-alprenolol were non-conventional partial agonists with intrinsic activity of 0.7, 0.2 and 0.1 respectively. All beta-blockers antagonized in a concentration-dependent and surmountable manner the positive inotropic effects of both (-)-isoprenaline and CGP12177. The potency of each beta-blocker was consistently higher against (-)-isoprenaline than against CGP12177. Two groups of beta-blockers were identified. In one group the difference between the pK(B) values of blockade against (-)-isoprenaline and CGP12177 was 1.1 - 1.6 log units ((-)-alprenolol, (-)-pindolol, (-)-bupranolol, nadolol and carvedilol). In the other group the pK(B) difference was of 2.1 - 3.0 log units ((-)-atenolol, metoprolol, bisoprolol, sotalol, (-)-propranolol and (-)-timolol). The beta-blockers competed with (-)-[(125)I]-cyanopindolol for binding to ventricular beta(1)-adrenoceptors. The binding affinities correlated with the corresponding blocking potencies against (-)-isoprenaline. On average the pK(i) values were 0.5 log units smaller than the pK(B) values against (-)-isoprenaline but 1.6 log units greater than the pK(B) values against CGP12177. In ferret ventricle the effects of (-)-isoprenaline appear to be antagonized by beta-blockers through the state of the beta(1)-adrenoceptor for which (-)-[(125)I]-cyanopindolol and beta-blockers have high affinity. The cardiostimulant effects of CGP12177 appear to be mediated through a low-affinity state of the beta(1)-adrenoceptor for which beta-blockers have low affinity.

Synthesis and in vivo evaluation of [11C]ICI 118551 as a putative subtype selective beta2-adrenergic radioligand

Erytro-(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[ iso-propylamino]-2-butanol (ICI 118551) a potent clinically used beta2 adrenergic antagonist, was labelled with carbon-11 (t1/2 = 20.4 min) as a potential radioligand for the non-invasive assessment of beta2 adrenergic receptors in the lung with positron emission tomography (PET). The radiolabelled compound was prepared by reductive N-alkylation of its des-isopropyl precursor with [2-11C]acetone. (+/-)-[11C]ICI 118551 was obtained in greater than 98% radiochemical purity in 30 min with a radiochemical yield of 15 + 5% (non-decay corrected) and a specific radioactivity 2.5 +/- 0.5 Ci/micromol. The biological evaluation of racemic erythro (+/-)-[11C]ICI 118551 in rats and Macaca Nemestrina shows a high radioactivity uptake in lung and heart. However, in both animal models no detectable displacement of lung radioactivity concentration was observed after pre-treatment with propranolol or ICI 118551, which indicates that in this organ, radioligand uptake is mostly due to non-specific binding. The biological data suggest that erythro (+/-)-[11C]ICI 118551 is not adequate to be further developed as a tracer for beta2 adrenergic receptor imaging in vivo.

Contribution of beta-adrenoceptor subtypes to relaxation of colon and oesophagus and pacemaker activity of ureter in wildtype and beta(3)-adrenoceptor knockout mice

The smooth muscle relaxant responses to the mixed beta(3)-, putative beta(4)-adrenoceptor agonist, (-)-CGP 12177 in rat colon are partially resistant to blockade by the beta(3)-adrenoceptor antagonist SR59230A suggesting involvement of beta(3)- and putative beta(4)-adrenoceptors. We now investigated the function of the putative beta(4)-adrenoceptor and other beta-adrenoceptor subtypes in the colon, oesophagus and ureter of wildtype (WT) and beta(3)-adrenoceptor knockout (beta(3)KO) mice. (-)-Noradrenaline and (-)-adrenaline relaxed KCl (30 mM)-precontracted colon mostly through beta(1)-and beta(3)-adrenoceptors to a similar extent and to a minor extent through beta(2)-adrenoceptors. In colon from beta(3)KO mice, (-)-noradrenaline was as potent as in WT mice but the effects were mediated entirely through beta(1)-adrenoceptors. (-)-CGP 12177 relaxed colon from beta(3)KO mice with 2 fold greater potency than in WT mice. The maintenance of potency for (-)-noradrenaline and increase for (-)-CGP 12177 indicate compensatory increases in beta(1)- and putative beta(4)-adrenoceptor function in beta(3)KO mice. In oesophagi precontracted with 1 microM carbachol, (-)-noradrenaline caused relaxation mainly through beta(1)-and beta(3)-adrenoceptors. (-)-CGP 12177 (2 microM) relaxed oesophagi from WT by 61.4+/-5.1% and beta(3)KO by 67.3+/-10.1% of the (-)-isoprenaline-evoked relaxation, consistent with mediation through putative beta(4)-adrenoceptors. In ureter, (-)-CGP 12177 (2 microM) reduced pacemaker activity by 31.1+/-2.3% in WT and 31.3+/-7. 5% in beta(3)KO, consistent with mediation through putative beta(4)-adrenoceptors. Relaxation of mouse colon and oesophagus by catecholamines are mediated through beta(1)- and beta(3)-adrenoceptors in WT. The putative beta(4)-adrenoceptor, which presumably is an atypical state of the beta(1)-adrenoceptor, mediates the effects of (-)-CGP 12177 in colon, oesophagus and ureter.

Regional brain distribution of noradrenaline uptake sites, and of alpha1-alpha2- and beta-adrenergic receptors in PCD mutant mice: a quantitative autoradiographic study

The mouse "Purkinje cell degeneration" (pcd) is characterized by a primary loss of Purkinje cells, as well as by retrograde and secondary partial degeneration of cerebellar granule cells and inferior olivary neurons; this neurological mutant can be considered as an animal model of human degenerative ataxia. To determine the consequences of this cerebellar pathology on the noradrenergic system, noradrenaline transporters as well as alpha1-, alpha2- and beta-adrenergic receptors were evaluated by quantitative ligand binding autoradiography in adult control and pcd mice using, respectively, [3H]nisoxetine, [3H]prazosin, [3H]idazoxan and [3H]CGP12177. In cerebellar cortex and deep nuclei of pcd mutants, [3H]nisoxetine labelling of noradrenaline transporters was higher than in control mice. However, when binding densities were corrected by surface area, they remained unchanged in the cerebellar cortex but associated with 25% and 40% lower levels of labelling of alpha1 and beta receptors, as well as a very important increase (275%) of alpha2 receptors. In deep cerebellar nuclei, surface corrections did not reveal any changes either in transporter or in receptor densities. Higher densities of [3H]nisoxetine labelling were found in several regions related with the cerebellum, namely inferior olive, inferior colliculus, vestibular, reticular, pontine, raphe and red nuclei, as well as in primary motor and sensory cerebral cortex; they may reflect an increased noradrenergic innervation related to motor adjustments for the cerebellar dysfunction. Increased [3H]nisoxetine labelling was also measured in vegetative brainstem regions and in dorsal hypothalamus, implying altered autonomic functions and possible compensation in pcd mutants. Other changes found in extracerebellar regions affected by the mutation, such as thalamus and the olfactory system implicated both noradrenaline transporters and adrenergic receptors. In contrast to the important alterations of the noradrenergic system in cerebellar cortex, the lack of receptor changes in deep cerebellar nuclei suggests that local adaptations may be sufficient to minimize the consequence of the cerebellar atrophy on motor control. An intense labelling by [3H]idazoxan of the inner third of the molecular layer was a novel, albeit unexplained finding, and could represent a postsynaptic subset of alpha2-adrenergic receptors.

Putative beta 4-adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca2+: comparison with atrial receptors and relationship to (-)-[3H]-CGP 12177 binding

1. We identified putative beta4-adrenoceptors by radioligand binding, measured increases in ventricular contractile force by (-)-CGP 12177 and (+/-)-cyanopindolol and demonstrated increased Ca2+ transients by (-)-CGP 12177 in rat cardiomyocytes. 2. (-)-[3H]-CGP 12177 labelled 13 - 22 fmol mg-1 protein ventricular beta1, beta2-adrenoceptors (pKD approximately 9.0) and 50 - 90 fmol mg-1 protein putative beta4-adrenoceptors (pKD approximately 7.3). The affinity values (pKi) for (beta1,beta2-) and putative beta4-adrenoceptors, estimated from binding inhibition, were (-)-propranolol 8.4, 5.7; (-)-bupranolol 9.7, 5.8; (+/-)-cyanopindolol 10.0,7.4. 3. In left ventricular papillary muscle, in the presence of 30 microM 3-isobutyl-1-methylxanthine, (-)-CGP 12177 and (+/-)-cyanopindolol caused positive inotropic effects, (pEC50, (-)-CGP 12177, 7.6; (+/-)-cyanopindolol, 7.0) which were antagonized by (-)-bupranolol (pKB 6.7 - 7.0) and (-)-CGP 20712A (pKB 6.3 - 6.6). The cardiostimulant effects of (-)-CGP 12177 in papillary muscle, left and right atrium were antagonized by (+/-)-cyanopindolol (pKP 7.0 - 7.4). 4. (-)-CGP 12177 (1 microM) in the presence of 200 nM (-)-propranolol increased Ca2+ transient amplitude by 56% in atrial myocytes, but only caused a marginal increase in ventricular myocytes. In the presence of 1 microM 3-isobutyl-1-methylxanthine and 200 nM (-)-propranolol, 1 microM (-)-CGP 12177 caused a 73% increase in Ca2+ transient amplitude in ventricular myocytes. (-)-CGP 12177 elicited arrhythmic transients in some atrial and ventricular myocytes. 5. Probably by preventing cyclic AMP hydrolysis, 3-isobutyl-1-methylxanthine facilitates the inotropic function of ventricular putative beta4-adrenoceptors, suggesting coupling to Gs protein-adenylyl cyclase. The receptor-mediated increases in contractile force are related to increases of Ca2+ in atrial and ventricular myocytes. The agreement of binding affinities of agonists with cardiostimulant potencies is consistent with mediation through putative beta4-adrenoceptors labelled with (-)-[3H]-CGP 12177.

Preliminary evaluation of 2-[4-[3-tert-butylamino)-2-hydroxypropoxy]phenyl]-3-methyl-6-me thoxy-4(3H)-quinazolinone ([+/-]HX-CH 44) as a selective beta1-adrenoceptor ligand for PET

(+/-)-3-[11C]Methyl-2-[4-[3-(tert-butylamino)-2-hydroxypropoxy]phenyl]-6 -methoxy-4(3H) quinazolinone ([+/-]-[11C]HX-CH 44) was labeled with carbon-11 using [11C]iodomethane with the corresponding N-demethylated precursor. Then, 30-90 mCi (1.10-3.33 GBq) of pure [11C]HX-CH 44 were obtained 30 min after end of bombardment with specific radioactivities of 500-1,400 mCi/micromol (18.5-51.8 GBq/micromol). Myocardial uptake in dogs was 0.340+/-0.043 pmol/mL tissue per nanomole injected, 10-15 min postinjection. Heart-to-lung ratio was 3 from the 5th to the 30th minute. Only 35% of the myocardial radioactivity could be displaced. Tissue uptake could not be blocked with appropriate compounds. Therefore, (+/-)-[11C]HX-CH 44 does not appear to be a suitable ligand for the study of myocardial beta1-adrenoceptors in positron emission tomography.

Characterization of beta-adrenergic receptors in fish and amphibian lymphoid organs

Cells from goldfish and amphibian lymphoid organs, mainly leukocytes, express high affinity beta-adrenergic receptors specific for beta-adrenergic ligands (agonists: adrenaline, noradrenaline, terbutaline, and fenoterol; antagonists: CGP-12177, dihydroalprenolol, propranolol, atenolol, and butoxamine). The rank order of ligand potency does not allow their being classified into any known mammalian subtype. Among features that distinguish them from mammalian beta1 and beta2-adrenoceptors is much lower affinity for (-)-CGP-12177, obtained in both saturation and kinetic experiments (about 25 nM for goldfish head kidney cells). The density of receptors on goldfish and anuran cells is organ-dependent and comparable to that estimated on mammalian leukocytes. The extraordinarily high receptor density on salamander splenic cells (about 183,000) correlates with the large size of urodele cells. The competition experiments on goldfish cells with propranolol and CGP-12177 suggest the existence of yet another binding site, which may be either another beta-AR subtype, or a serotonergic receptor.

A novel beta-adrenoceptor ligand for positron emission tomography: evaluation in experimental animals

Myocardial and pulmonary beta-adrenoceptors can be imaged and quantified with the antagonist (S)-4-[3[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-b enzimidazol-2-[11C]-one (S-[11C]CGP-12177). The synthesis of this ligand (based on the reaction of a precursor with [11C]phosgene) is laborious and in many centers the final product has a low and variable specific activity. This prevents widespread use of S-[11C]CGP-12177 for studies in patients. We prepared S-[11C]CGP-12388, the isopropyl analogue of CGP-12177, by a reliable one-pot procedure and evaluated the radiopharmaceutical for beta-adrenoceptor imaging. Blocking experiments with subtype-selective beta-adrenergic drugs showed that myocardial and pulmonary uptake of S-[11C]CGP-12388 in anesthetized rats reflects ligand binding to beta1- and beta2-adrenoceptors. In this animal model, clearance, metabolism and tissue/plasma ratios of S-[11C]CGP-12388 were similar to those of S-[11C]CGP-12177. A [18F]fluoroisopropyl analogue of CGP-12177 showed less favorable characteristics. S-[11C]CGP-12388 was therefore selected for evaluation in humans and it may become the tracer of choice for clinical studies since it is easily prepared.

Validity of (-)-[3H]-CGP 12177A as a radioligand for the 'putative beta4-adrenoceptor' in rat atrium

1. We have recently suggested the existence in the heart of a 'putative beta4-adrenoceptor' based on the cardiostimulant effects of non-conventional partial agonists, compounds that cause cardiostimulant effects at greater concentrations than those required to block beta1- and beta2-adrenoceptors. We sought to obtain further evidence by establishing and validating a radioligand binding assay for this receptor with (-)-[3H]-CGP 12177A ([-]-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one) in rat atrium. We investigated (-)-[3H]-CGP 12177A for this purpose for two reasons, because it is a non-conventional partial agonist and also because it is a hydrophilic radioligand. 2. Increasing concentrations of (-)-[3H]-CGP 12177A, in the absence or presence of 20 microM (-)-CGP 12177A to define non-specific binding, resulted in a biphasic saturation isotherm. Low concentrations bound to beta1- and beta2-adrenoceptors (pKD 9.4+/-0.1, Bmax 26.9+/-3.1 fmol mg(-1) protein) and higher concentrations bound to the 'putative beta4-adrenoceptor' (pKD 7.5+/-0.1, Bmax 47.7+/-4.9 fmol mg(-1) protein). In other experiments designed to exclude beta1- and beta2-adrenoceptors, (-)-[3H]-CGP 12177A (1-200 nM) binding in the presence of 500 nM (-)-propranolol was also saturable (pKD 7.6+/-0.1, Bmax 50.8+/-7.4 fmol mg(-1) protein). 3. The non-conventional partial agonists (-)-CGP 12177A (pKi 7.3+/-0.2), (+/-)-cyanopindolol (pKi 7.6+/-0.2), [-]-pindolol (pK1 6.6+/-0.1) and (+/-)-carazolol (pKi 7.2+/-0.2) and the antagonist (-)-bupranolol (pKi 6.6+/-0.2), all competed for (-)-[3H]-CGP 12177A binding in the presence of 500 nM (-)-propranolol at the 'putative beta4-adrenoceptor', with affinities closely similar to potencies and affinities determined in organ bath studies. 4. The catecholamines competed with (-)-[3H]-CGP 12177A at the 'putative beta 4-adrenoceptor' in a stereoselective manner, (-)-noradrenaline (pKiH 6.3+/-0.3, pKiL 3.5+/-0.1), (-)-adrenaline (pKiH 6.5+/-0.2, pKiL 2.9+/-0.1), (-)-isoprenaline (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1), (+)-isoprenaline (pKi< 1.7), (-)-RO363 ((-)-(1-(3,4-dimethoxyphenethylamino)-3-(3,4-dihydroxyphenoxy++ +)-2-propranol)oxalate, pKi 5.5+/-0.1). 5. The inclusion of guanosine 5-triphosphate (GTP 0.1 mM) had no effect on binding of (-)-CGP 12177A or (-)-isoprenaline to the 'putative beta4-adrenoceptor'. In competition binding studies, (-)-CGP 12177A competed with (-)-[3H]-CGP 12177A for one receptor state in the absence (pKi 7.3+/-0.2) or presence of GTP (pKi 7.3+/-0.2). (-)-Isoprenaline competed with (-)-[3H]-CGP 12177A for two states in the absence (pKiH 6.6+/-0.3, pKiL 3.5+/-0.1; % H 25+/-7) or presence of GTP (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1; % H 37+/-6). In contrast, at beta1-adrenoceptors, GTP stabilized the low affinity state of the receptor for (-)-isoprenaline. 6. The specificity of binding to the 'putative beta 4-adrenoceptor' was tested with compounds active at other receptors. High concentrations of the beta 3-adrenoceptor agonists, BRL 37344 ((RR+SS)[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]phenoxy]acetic acid, 6 microM), SR 58611A (ethyl{(7S)-7-[(2R)-2-(3-chlorophenyl)-2-hydroxyethylamino]-5,6,7,8-tetrahydronaphtyl2-yloxy} acetate hydrochloride, 6 microM), ZD 2079 ((+/-)-1-phenyl-2-(2-4-carboxymethylphenoxy)-ethylamino)-ethan-1-ol, 60 microM), CL 316243 (disodium (R,R)-5-[2-[2-(3-chlorophenyl)-2-hydroxyethyl-amino]propyl]- 1,3-benzodioxole-2,2-dicarboxylate, 60 microM) and antagonist SR 59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-2S-2-propanol oxalate, 6 microM) caused less than 22% inhibition of (-)[3H]-CGP 12177A binding in the presence of 500 nM (-)propranolol. Histamine (1mM), atropine (1 microM), phentolamine (10 microM), 5-HT (100 microM) and the 5-HT4 receptor antagonist SB 207710 ((1-butyl-4-piperidinyl)-methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate, 10nM) caused less than 26% inhibition of binding. 7.Non-conventional partial agonists, the antagonist (-)bupranolol and catecholamines all competed for (-)[3H]-CGP 12177A binding in the absence of (-)propranolol at beta1-adrenoceptors, with affinities (pKi) ranging from 1.6-3.6 log orders greater than at the 'putative beta 4-adrenoceptor'. 8.We have established and validated a radioligand binding assay in rat atrium for the 'putative beta 4-adrenoceptor' which is distinct from beta1-, beta2- and beta 3-adrenoceptors. The stereoselective interaction with the catecholamines provides further support for the classification of the receptor as 'putative beta 4-adrenoceptor'.

Proposal for the interaction of non-conventional partial agonists and catecholamines with the 'putative beta 4-adrenoceptor' in mammalian heart

1. Evidence for a 'putative beta 4-adrenoceptor' originated over 20 years ago when cardiostimulant effects were observed to non-conventional partial agonists. These agonists were originally described as beta 1- and beta 2-adrenoceptor antagonists; however, they cause cardiostimulant effects at much higher concentrations than those required to block beta 1- and beta 2-adrenoceptors. Cardiostimulant effects of non-conventional partial agonists have been observed in mouse, rat, guinea-pig, cat, ferret and human heart tissues. 2. The receptor is expressed in several heart regions, including the sinoatrial node, atrium and ventricle. 3. The receptor is resistant to blockade by most antagonists that possess high affinity for beta 1- and beta 2-adrenoceptors, but is blocked with moderate affinity by (-)-bupranolol and CGP 20712A. 4. The receptor is pharmacologically distinct from the beta 3-adrenoceptor. Micromolar concentrations of beta 3-adrenoceptor agonists have no agonist or blocking activity. The receptor is also resistant to blockade by a beta 3-adrenoceptor-selective antagonist. 5. The receptor mediates increases in cAMP levels and cAMP-dependent protein kinase (PK) A activity in cardiac tissues. Phosphodiesterase inhibition potentiates the positive chronotropic and inotropic effects of non-conventional partial agonists. 6. The receptor mediates hastening of atrial and ventricular relaxation, which is consistent with involvement of a cAMP-dependent pathway. 7. The non-conventional partial agonist (-)-[3H]-CGP 12177A labels the cardiac putative beta 4-adrenoceptor. Non-conventional partial agonists compete for binding with affinities that are closely similar to their agonist potencies. Catecholamines compete for binding in a stereoselective manner with a rank order of affinity of (-)-RO363 > (-)-isoprenaline > (-)-noradrenaline > or = (-)-adrenaline >> (+)-isoprenaline, suggesting that catecholamines can interact with the receptor. 8. The putative beta 4-adrenoceptor appears to be coupled to the Gs-adenylyl cyclase system, which could serve as a guide to its future cloning. Activation of the receptor may plausibly improve diastolic function but could also mediate arrhythmias.

Anomalous behavior of CGP 12177A on beta 1-adrenergic receptors

CGP 12177A originally was developed as a hydrophilic antagonist to detect cell surface beta 1- and beta 2-adrenergic receptors, and subsequently was found to be a partial agonist for the atypical or beta 3-adrenergic receptor. Using hamster cells stably expressing either the human beta 1-, human beta 2- or rat beta 1-adrenergic receptor, we found that CGP 12177A behaved as an agonist of beta 1-adrenergic receptors. Whereas at low concentrations, CGP 12177a behaved as an antagonist and inhibited isoproterenol stimulation of adenylyl cyclase activity, at higher concentrations, it stimulated a response even in the absence of isoproterenol. The agonistic properties of CGP 12177A were positively correlated with the level of beta 1-adrenergic receptor expression. Thus, at low receptor of densities, CGP 12177A behaved as a weak, partial agonist whereas as high receptor densities, the drug was a full agonist. At similar high densities of the beta 2-adrenergic receptor, CGP 12177A acted only as a partial agonist. Competition binding studies to membranes from cells expressing beta 1-adrenergic receptors indicated that approximately 90% of the receptors were in a high affinity, guanine nucleotide-insensitive state for CGP 12177A whereas approximately 10% of the receptors were in a lower affinity, guanine nucleotide-sensitive state for CGP 12177A. We propose that the latter receptors are precoupled to stimulatory G proteins and recognize CGP 12177A as an agonist whereas the high affinity, uncoupled receptors recognize CGP 12177A as an antagonist.

Beta-adrenoceptor subtypes in young and old rat ventricular myocytes: a combined patch-clamp and binding study

1. We used electrophysiological and binding techniques to assess the presence of beta 1- and beta 2-adrenoceptors (beta 1AR and beta 2AR) in rat cardiac myocytes and to determine their ratio during aging. Experiments were performed in left ventricular myocytes enzymatically dissociated from the heart of 3-(young) or 22-month-old (old) Wistar Kyoto rats. 2. In patch-clamp experiments, myocytes from old rats showed a prolonged action potential duration (at -20 mV: 41.7 +/- 3.6 vs 26.2 +/- 3.1 ms; at -60 mV: 154.4 +/- 17.7 vs 87.1 +/- 6.9 ms, P < 0.05) and an augmented membrane capacitance (an index of cell size) (271.7 +/- 20.2 vs 164.3 +/- 14.6 pF, P < 0.05) compared to young rats. beta 2AR stimulation, achieved by superfusing myocytes with the selective beta 2AR agonist, zinterol (10 microM) or with (-)-isoprenaline (1 microM) in the presence of the selective beta 1AR antagonist, CGP 20712A (0.1 microM), significantly increased L-type calcium current (ICa,L) in rat ventricular myocytes. The percentage increase was similar in both young and old rats, either with zinterol (26.9 +/- 3.6% and 24.2 +/- 2.8%, respectively) or isoprenaline plus CGP 20712A (30.4 +/- 3.7% and 22.4 +/- 4.1%, respectively). Isoprenaline alone (beta 1AR and beta 2AR stimulation) caused a much smaller increase in ICa,L in old rats (58.4 +/- 12.1%) than in younger ones (95.3 +/- 8.1%) (P = 0.067). 3 The number of ,BAR mg-' protein, measured with saturation binding assays of the non selective ,betaAR antagonist [3H]-CGP 12177 was 1989.4+/-189.5 for 3- and of 1580.7 +/-161.5 for 22-month-old rats.Competition for [3H]-CGP 12177 binding by CGP 20712A gave biphasic curves which demonstrated two classes of binding sites. Densities (as percentages of total PAR density), and affinities for the two binding sites were: 80.4 +/- 2.2% (Ki = 6.6 +/- 1.3 nM) betaAR and 19.6 +/- 2.2% (Ki = 6.9 +/- 2.2 microM) beta2AR in young rats and 66.1 +/- 1.2% (Ki=8.3+/- 1.1 nM) beta1 AR and 33.9+/- 1.2% (Ki=5.2+/-0.6 PM) P2AR in old rats. Thebeta1AR/beta2AR ratio was significantly (P<0.01) reduced in old rats with respect to the younger ones.4 By combining electrophysiological and binding measurements, we calculated beta1AR and beta2ARdensities as number of receptors per microM2 of cell surface. In old rats, beta1 density was significantly decreased compared to young rats (8.4+/-2.0 vs 15.4+/-3.7 receptors microM-2, P<0.05), while beta2AR density remained unchanged at both 3 and 22 months (3.8 +/- 0.7 and 4.2+/-1.1 receptors microM-2, respectively).5 Our results demonstrate that both beta1AR and beta2AR are functionally present in rat ventricular myocytes of young and old rats. The decreased responsiveness to betaAR stimulation during aging appears to be associated with a selective reduction in the density of beta1AR.

Quantification of an 11C-labelled beta-adrenoceptor ligand, S-(-)CGP 12177, in plasma of humans and rats

beta-Adrenoceptors in human lungs and heart can be imaged with the radioligand 4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3- dihydro-2H-benzimidazol-2-11C-one (CGP 12177, [11C]I). For quantification of receptor density with compartment models by adjustment of rate constants, an 'input function' is required which consists of the integral of the concentration of unmodified ligand in arterial plasma over time. A discrepancy in the literature regarding metabolic stability of [11C]I prompted us to study metabolism in rats by reversed-phase HPLC (RP-HPLC) of trichloroacetic acid extracts of arterial plasma after i.v. injection of [11C]I (> 11.1 TBq/mmol, 11 MBq/kg). Some plasma samples were also directly applied to an internal-surface reversed-phase (ISRP) column. In parallel experiments, tritiated [11C]I was employed and methanol extracts of arterial plasma were analyzed by straight-phase TLC. The three methods were in excellent agreement. Unmodified [11C]I decreased from > 98.5% (3H) or > 99.9% (11C) initially to 57 +/- 7% at 80 min post injection due to formation of two polar metabolites. Using the RP-HPLC method, no metabolism was detectable in humans up to 30 min after injection of [11C]I (1851 MBq). Deproteinization of plasma with acetonitrile resulted in the formation of a radioactive species (artifact) which eluted immediately after the void volume in RP-HPLC and which could be mistakenly interpreted as a metabolite. Plasma protein binding was low (ca. 30%) in both humans and rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Independent and coordinate regulation of beta 1- and beta 2-adrenergic receptors in rat C6 glioma cells

Rat C6 glioma cells have both beta 1- and beta 2-adrenergic receptors in approximately 7:3 ratio. When the cells were exposed to the beta-adrenergic agonist isoproterenol, there was a rapid sequestration of up to 50% of the surface receptor population over a 30-min period as measured by the loss of binding of the hydrophilic ligand [3H] CGP-12177 to intact cells. Using the beta 1-selective antagonist CGP 20712A to quantify the proportion of the two subtypes, it was found that although both beta 1 and beta 2 receptors were sequestered, the latter were sequestered initially twice as fast as the former. More prolonged agonist exposure led to a down-regulation of approximately 90% of the total receptor population by 6 h as measured by the loss of binding of the more hydrophobic ligand [125I]iodocyanopindolol to cell lysates. The two subtypes, however, underwent down-regulation with similar kinetics. Treatment of the cells with agents that raise cyclic AMP levels such as cholera toxin and forskolin resulted in a slower, but still coordinated down-regulation of both subtypes. Thus, there appears to be both independent and coordinate regulation of endogenous beta 1-and beta 2-adrenergic receptors in the same cell line.

Demonstration of the suitability of CGP 12177 for in vivo studies of beta-adrenoceptors

1. Positron emission tomography (PET) with appropriate radioligands offers the possibility of studying receptors non-invasively in man. The suitability of CGP 12177, a hydrophilic non-selective beta-adrenoceptor antagonist which can be labelled with the positron emitter 11C, as a ligand for in vivo studies of beta-adrenoceptors was assessed in rats. 2. [3H]-CGP 12177 was injected into the tail veins of restrained conscious rats. Serial blood samples were taken from tail arteries to determine clearance from plasma. Rats were killed and tissues removed to determine tissue uptake. Radioactivity was assessed by liquid scintillation counting. 3. The uptake of (-)-[3H]-CGP 12177 in various tissues was compared to that of (+/-)-[3H]-CGP 12177. Maximum tissue:plasma ratios obtained for the (-)-enantiomer in lung, heart and liver were 170, 42 and 13 compared with 60, 15 and 12 for the racemate. Prior injection of excess unlabelled (+/-)-CGP 12177 blocked the uptake of both (-)- and (+/-)-[3H]-CGP 12177 in lung and heart but not liver, tissue:plasma ratios for both tracers being reduced to 7, 3 and 7 respectively. 4. Clearance of (-)-[3H]-CGP 12177 from plasma was rapid during the first 5 min but showed only small changes during 5 to 90 min. Uptake in lung and heart reached a maximum at 1 to 5 min and showed a slow decrease during 5-90 min. Prior injection of unlabelled (+/-)-CGP 12177 reduced uptake in lung and heart to 10% and 20% respectively. Injection of unlabelled ( +/-)-CGP 12177 at 15 mind is placed ~75% of the radioactivity by 90 min.5. ( +/- )-Propranolol had a similar effect to that of unlabelled ( +/-)-CGP 12177. Prior injection reduced uptake of radioactivity in lung and heart to 15% and 20% respectively. Injection of ( +/- )-propranolol at 15 min displaced ~ 60% of the radioactivity by 90 min indicating that the tracer binds to beta-adrenoceptor sites in vivo.6. In vivo saturation curves, obtained by injection of (-)-[3H]-CGP 12177 with increasing amounts of unlabelled (- )-CGP 12177, gave values of Bmax for lung of ~45 pmol per g wet weight of tissue and for heart of ~6 pmol per g wet weight of tissue. KD could only be expressed as nmol injected per kg bodyweight, that for lung (2.5 nmol kg-1) being greater than that for heart (1.3 nmol kg-1).7. Competition studies carried out by co-injecting (-)-[3H]-CGP 12177 with unlabelled (+/- )-CGP12177 or (-)-propranolol gave similar values for Bmax (lung 44 pmol g-1, heart 6 pmol g-1,). Values of KD for (+/-)-CGP 12177 (lung 4.7 pmol kg-1, heart 2.6 pmol kg-1) were approximately twice those for(-)-CGP 12177. Values of KD for (-)-propranolol (lung 38 nmol kg-1, heart 104 nmol kg-1) were greater.8. The results show that (-)-[3H]-CGP 12177 is a suitable ligand for assessing beta-adrenoceptors in vivo.

Uptake of radioligands by rat heart and lung in vivo: CGP 12177 does and CGP 26505 does not reflect binding to beta-adrenoceptors

The biodistribution of (-)-4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H-benzimidazol-2-one (CGP12177, a non-selective beta-adrenoceptor antagonist) and 1-[2-(3-carbamoyl-4-hydroxy)-(5-3H-phenoxy)]-2-propanol methanesulfonate, (CGP26505, a beta 1-adrenoceptor antagonist) was studied in rats pretreated with various alpha- and beta-adrenoceptor blocking drugs (5 min before 3H injection, in dosages at which the drugs demonstrated the expected selectivity). Cardiac and pulmonary radioactivity were measured after 10 min, when specific binding was maximal. Uptake of [3H]CGP12177 was linked to binding to beta-adrenoceptors since it was not affected by prazosin or yohimbine, and was equally well inhibited by propranolol, unlabelled CGP12177 and isoprenaline. Moreover, atenolol and CGP20712A inhibited [3H]CGP12177 uptake in heart (predominantly beta 1-adrenoceptors) more potently than ICI 118,551, while in lungs (predominantly beta 2-adrenoceptors) ICI 118,551 was more potent than atenolol or CGP20712A. In contrast, [3H]CGP26505 uptake in the target organs was equally effectively inhibited by propranolol and ICI 118,551, and significantly lowered by alpha-adrenoceptor antagonists. We conclude that [11C]CGP12177, but not [11C]CGP2605 will be suitable for positron emission tomography imaging of beta-adrenoceptors in animals.

Suitability of CGP-12177 and CGP-26505 for quantitative imaging of beta-adrenoceptors

[3H]CGP-12177, a non-selective beta-adrenoceptor antagonist, and [3H]CGP-26505, a beta 1-selective beta-adrenoceptor antagonist, were intravenously administered to rats. 94-97% of the injected radioactivity disappeared from plasma with t1/2 0.2 and 0.5 min. Total/non-specific binding ratios of 5.4 and 6.9 (CGP-12177) or 2.0 and 2.8 (CGP-26505) were maintained in heart and lung from 10 to 40 min post-injection. Labelled plasma metabolites appeared after greater than 20 min (CGP-12177) or within 2 min (CGP-26505). No metabolites were found in the heart. CGP-12177 binds to blood cells, but CGP-26505 does not. CGP-12177 can be used for PET imaging of total (beta 1 and beta 2) adrenoceptors in the heart and lung of experimental animals, but CGP-26505 is less suitable for in vivo analysis of the beta 1-subpopulation.

Coexistence of three beta-adrenoceptor subtypes in white fat cells of various mammalian species

The nature of the beta-adrenoceptors (beta-ARs) of the white fat cells of five mammalian species (rat, hamster, rabbit, dog and humans) was reassessed. The coexistence of at least three beta-ARs on the fat cell (except human) was demonstrated. Comparative binding and lipolysis studies were performed, using recently synthesized compounds selective for the atypical beta-AR of the rat brown fat cell and of the rat colon. beta 1- and beta 2-ARs have previously been identified in all the mammalian white fat cells using [125I]cyanopindolol ([125]CYP) or [3H]dihydroalprenolol. In addition to these receptors, we now demonstrated the existence of a third beta-AR directly involved in adrenergic-mediated lipolysis, and identified it in the white fat cells of the most commonly studied animal species, except humans. This receptor is not detected by the classically used beta-antagonist radioligands, explaining the discrepancies in reports on the nature of the beta-ARs of the adipose tissue. Pharmacological delineation of the third type of beta-AR-induced lipolysis showed this receptor to be rather similar to the previously proposed atypical beta-AR of brown and white rat fat cells. Its pharmacological properties were clarified, using new selective full agonists and partial agonists also acting as non-selective beta 1/beta 2-antagonists. The limits of [125]CYP as a radioligand were reported and the usefulness of BRL 37344, (+/-)-CGP 12177 and phenylethanolaminotralines derivatives (having an atypical beta-activity on intestinal motility) as major tools usable for atypical beta-AR activation was demonstrated. Moreover, confirming our previous results about the nature of the beta-ARs (beta 1- and beta 2-ARs) located in the fat cells of women (Mauriège et al., J. Lipid Res., 1987, 17, 156), no atypical beta-AR-mediated lipolysis was identified in abdominal adipose tissue from healthy women. The possible differences and similarities between this receptor and the recently cloned beta 3-AR are discussed.

Desensitization pattern of cardiac beta-adrenoceptor subtypes by prolonged in vivo infusion of pindolol and celiprolol in rats

The regulatory effects of pindolol and celiprolol on cardiac beta-adrenoceptor density were studied in vivo in order to assess the subtype selectivity of their partial agonistic activity (PAA). The substances were continuously administered to rats for 1 week by means of implanted osmotic minipumps. The density of beta-adrenoceptor subtypes were estimated from ICYP saturation binding experiments performed on cardiac ventricular plasma membranes in the presence of a highly selective antagonist (CGP 20172 A or ICI 118,551). Both antagonists were employed at concentrations as high as to block one subtype only without affecting the complementary subtype. For control purposes, rats were also treated with isoprenaline (0.4 mg/kg/h) and propranolol (0.15 mg/kg/h), or vehicle. Pindolol (0.036 mg/kg/h) and celiprolol (0.36 mg/kg/h) reduced the density of ventricular beta 2-adrenoceptors by 46% and 23%, respectively, which--in the case of pindolol--was significant when compared to the non-treated controls. Both compounds, however, produced a small, but distinct increase in the number of beta 1-adrenoceptors by approximately 26%. This finding is in contrast to the propranolol--induced up-regulation of both beta 1- and beta 2-adrenoceptors by approximately 80%. Since supramaximal doses of each drug were administered, a significant smaller increase of beta 1-adrenoceptors by pindolol and celiprolol--as compared to the increase produced by propranolol--can be interpreted as evidence for a PAA of pindolol and celiprolol on beta 1-adrenoceptors as well. Isoprenaline as a full agonist caused a marked loss of of both beta-adrenoceptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)