Comparison of the beta-adrenoceptor affinity and selectivity of cetamolol, atenolol, betaxolol, and ICI-118,551

The objective of the present study was to compare the quantitative differences in the beta 1- vs. beta 2-adrenoceptor affinity and selectivity of cetamolol and its enantiomers to the reference compounds atenolol, betaxolol, and ICI-118,551, using isolated tissues obtained from the dog, guinea pig, and rat. Cetamolol antagonized the beta-adrenoceptor-mediated responses induced by isoproterenol, epinephrine, norepinephrine, and salbutamol, in tissues from both the dog and guinea pig, in a concentration-dependent manner. For a given tissue, the beta-adrenoceptor antagonist activity of cetamolol (measured as a pA2 or pKB value) was independent of the agonist used. In the dog tissues, cetamolol was more potent at inhibiting responses in the coronary artery (beta 1-adrenoceptors) than in the saphenous vein (beta 2-adrenoceptors). In the guinea pig tissues, the potency of cetamolol was approximately the same in the trachea (mixed beta 1- and beta 2-adrenoceptors) and atria (predominately beta 1-adrenoceptors), but lower in the soleus muscle (beta 2-adrenoceptors). Studies with the S-(-) and R-(+) enantiomers of cetamolol demonstrated that the S-(-) enantiomer was approximately 100-fold more potent at beta 1-adrenoceptors than the R-(+) enantiomer. In rat brain, cetamolol displaced [3H]-dihydroalprenolol bound to homogenates of cortex (beta 1-adrenoceptor binding sites) and cerebellum (beta 2-adrenoceptor binding sites). The potency of cetamolol at beta 1-adrenoceptors was found to be similar to that of betaxolol but greater than that of atenolol. However, the magnitude of the beta 1-adrenoceptor selectivity displayed by atenolol and betaxolol was greater than that displayed by cetamolol. In contrast, ICI-118,551 was found to possess potent and selective affinity for beta 2-adrenoceptors.     

Beta 1- and beta 2-adrenoceptor antagonist activities of ICI-215001, a putative beta 3-adrenoceptor agonist.

1. The present study was undertaken to characterize the beta 3-adrenoceptor agonist activity of ICI-215001 and to determine whether it exhibits additional activities on beta 1- and beta 2-adrenoceptors in isolated spontaneously beating atrium, trachea and ileum of guinea-pig. 2. In guinea-pig atrium, isoprenaline, a non-selective beta-adrenoceptor agonist, caused concentration-dependent, positive chronotropic effects that were inhibited by atenolol, a selective beta 1-antagonist. ICI-215001 also competitively antagonized the increase in heart rate caused by isoprenaline. 3. ICI-215001 exhibited low intrinsic activity at increasing the beating rate of atrium and no activity on resting or induced tone of tracheal strips. 4. In strips of guinea-pig trachea, contracted submaximally with carbachol, isoprenaline, caused concentration-dependent relaxations. Both ICI-118551, a selective beta 2-adrenoceptor antagonist, and ICI-215001 competitively inhibited the relaxations caused by isoprenaline. 5. In isolated strips of guinea-pig ileum longitudinal smooth muscle contracted with histamine, isoprenaline and ICI-215001 caused relaxations which were inhibited by alprenolol, a beta-adrenoceptor antagonist with modest affinity for beta 3-adrenoceptors, but were resistant to ICI-118551 and atenolol. 6. These results indicate that ICI-215001 exhibits beta 3-adrenoceptor agonist activity as demonstrated by relaxations mediated via atypical beta-adrenoceptors in the longitudinal smooth muscle of guinea-pig ileum. Further, the studies demonstrate that ICI-215001 can act as an antagonist at beta 1- and beta 2-adrenoceptors in situations where its intrinsic agonist activity is low.     

Characteristics of 125I-iodocyanopindolol binding to beta-adrenergic and serotonin-1B receptors of rat brain: selectivity of beta-adrenergic agents

The present study was designed to examine the specificity of beta-adrenergic antagonists for beta 1-, beta 2-adrenergic and 5HT1B-serotonergic receptors by the competitive interaction with 125I-iodocyanopindolol (125I-ICYP) as a radioligand. The beta 1-adrenoceptors were preferred by acebutolol, atenolol, betaxolol, practolol, and l-, dl- and d-metoprolol, while butoxamine and lCl-118,551 preferred beta 2-adrenoceptors. The selectivities of these beta 1- and beta 2-antagonists are well-known, but alprenolol which is known as a non-selective antagonist was 7.2-fold more selective for the beta 2-adrenoceptors in the present study. All beta-antagonists used were more selective towards beta-adrenoceptors as compared with 5HT1B-receptors. Good correlations were observed between the potencies of beta-adrenoceptor antagonists for inhibition of 125I-ICYP binding to beta 1- and beta 2-adrenoceptor sites and their potencies for inhibiting the binding of the same radioligand to 5HT1B-serotonergic receptor sites. These results suggest that beta-adrenoceptor antagonists can bind to beta-adrenoceptors and 5HT1B-receptors.     

Autoradiographic localization of beta-adrenoceptors in pig lung using [125I]-iodocyanopindolol.

The binding of the beta-adrenoceptor radioligand [125I]-iodocyanopindolol (I-CYP) has been studied in pig lung parenchyma and the distribution of binding sites visualised by light microscopic autoradiography. I-CYP binding was saturable (maximum binding capacity Bmax = 51 +/- 3 fmol mg-1 protein), involving sites with high affinity (dissociation constant KD = 73 +/- 10 pM). Specific I-CYP binding was displaceable both by beta-adrenoceptor agonists ((-)-isoprenaline greater than (-)-adrenaline greater than (+/-)-fenoterol greater than (-)-noradrenaline greater than (+)-isoprenaline greater than (+/-)-RO363) and antagonists ((+/-)-propranolol greater than ICI-118551 greater than atenolol), indicating a predominance of beta 2-adrenoceptors. Further analysis showed that displacement data for the beta 1-selective antagonist atenolol and the beta 2-selective antagonist ICI-118551 were fitted best to a 2 binding site model and that both beta 1- and beta 2-adrenoceptors were present in pig lung in the ratio 28:72 respectively. Autoradiographic grains were localized over tissue and were most dense over alveolar walls greater than vascular endothelium greater than vascular smooth muscle greater than bronchial smooth muscle = bronchial epithelium. Atenolol (10(-5) M) caused a 31% reduction in specific grain density over alveolar wall tissue, while a 10 fold lower concentration of ICI-118551 (10(-6) M) caused a 50% decrease. These results are consistent with binding data in pig lung parenchyma demonstrating a mixed population of beta-adrenoceptors with a predominance of the beta 2 subtype.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta 2-adrenoceptor-mediated increase in the slow inward calcium current in atrial cells

The effects of procaterol (beta 2-adrenoceptor agonist) on the slow inward calcium current (Isi) were examined in single cells of guinea-pig heart. Procaterol increased Isi in atrial cells, in the presence of atenolol (beta 1-adrenoceptor antagonist). The effect was abolished by ICI-118551 (beta 2-adrenoceptor antagonist). However, procaterol did not modify the membrane currents in ventricular cells. These results suggest that beta 2-adrenoceptor agonists bind to beta 2-adrenoceptors in atrial cells and augment Isi, but beta 2-adrenoceptors are not present in ventricular cells.     

Effects of selective beta 2-adrenoceptor blockade on serum potassium and exercise performance in normal men.

1. The differential effects of beta-adrenoceptor subtypes on potassium fluxes and exercise capacity were compared in eight healthy young men using single oral doses of the selective beta 2-adrenoceptor antagonist ICI-118551, the selective beta 1-adrenoceptor antagonist atenolol or the non-selective beta-adrenoceptor antagonist propranolol. The study was randomized, double-blind and placebo controlled. 2. Potassium in the venous effluent from the exercising muscles increased progressively with increasing exercise intensity. This response was augmented by propranolol, whereas neither atenolol nor ICI-118551 modified the response. After exercise potassium concentration fell exponentially with no difference between the treatment regimens. 3. Cumulative work was significantly reduced by ICI-118551 (6.4%, P = 0.04) and by propranolol (12.4%, P less than 0.01), whereas the reduction with atenolol (5.6%) did not reach statistical significance. 4. Atenolol and propranolol reduced peak heart rate by 23% and 29%, and peak systolic blood pressure by 9% and 11% respectively during maximal exercise. ICI-118551 caused a non-significant reduction in heart rate during submaximal exercise, with a significant reduction at maximum exercise (6% reduction), whereas systolic blood pressure was not different from placebo. Diastolic blood pressures were similar across all treatment regimens. 5. Similar glucose concentrations were obtained at baseline and at exhaustion during all treatment regimens. Lactate concentrations were comparable for any given exercise intensity irrespective of treatment regimens. Propranolol reduced lactate concentrations from the exercising muscles at maximum exercise in proportion to the reduction of maximal exercise capacity. 6. The subjective perception of fatigue was not affected by either beta 1- or beta 2-adrenoceptor blockade.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional distribution of beta-adrenoceptors in the human heart: coexistence of functional beta 1- and beta 2-adrenoceptors in both atria and ventricles in severe congestive cardiomyopathy

We evaluated the amount of beta 1- and beta 2-adrenoceptors in human right and left atrium as well as in right and left ventricular wall obtained from heart transplant recipients who suffered from end-stage congestive cardiomyopathy. The total number of myocardial beta-adrenoceptors was assessed with the nonsubtype selective beta-adrenoceptor radioligand (-)[125I]iodocyanopindolol (ICYP); concomitantly, the number of beta 1-adrenoceptors was determined with the selective beta 1-adrenoceptor radioligand (-)[3H]bisoprolol. The number of beta 2-adrenoceptors was calculated by subtracting (-)[3H]bisoprolol binding sites from ICYP binding sites. With this technique, a beta 1/beta 2-ratio of approximately 65/35% for both atria and of approximately 75/25% for both ventricles was found. Identical results were obtained when the beta 1/beta 2-ratio was calculated indirectly by nonlinear regression analysis of competition curves of the selective beta 1-adrenoceptor antagonist bisoprolol and the selective beta 2-adrenoceptor antagonist ICI 118,551 with ICYP binding. In addition, on atria and on ventricles, adenylate cyclase was activated by norepinephrine (presumably by beta 1- and beta 2-adrenoceptor stimulation) and by procaterol (by beta 2-adrenoceptor stimulation). It is concluded that in the human heart functional beta 1- and beta 2-adrenoceptors coexist on both atria and both ventricles. In end-stage congestive cardiomyopathy, there appears to be a selective down-regulation of cardiac beta 1-adrenoceptors, whereas beta 2-adrenoceptors are obviously not affected. This may explain the beneficial effects of beta 2-adrenoceptor agonists in severe heart failure.     

The distribution of beta-adrenoceptors in dog kidney: an autoradiographic analysis

The distribution of beta-adrenoceptors in slide-mounted dog kidney sections was determined using the radioligand (-)-[125I]cyanopindolol ((-)-[125I]CYP) and autoradiography. Using conditions designed to prevent (-)-[125I]CYP binding to non-beta-adrenoceptor sites, biochemical studies revealed that (-)-[125I]CYP binding equilibrated within 150 min (K1 = 3.2 X 10(8) M-1 min-1), was saturable (KD = 30.72 +/- 2.96 pM; Bmax = 0.57 +/- 0.03 fmol/section, n = 4) and stereoselective with respect to the stereoisomers of propranolol and pindolol. Delineation of beta-adrenoceptor subtypes with the selective beta 1-adrenoceptor antagonist betaxolol and beta 2-adrenoceptor antagonist ICI 118,551 demonstrated that the proportions of beta 1-: beta 2-adrenoceptors was between 1:6 and 1:11. Autoradiographic studies showed that beta 1-adrenoceptors were localized on the juxtaglomerular apparatus and glomeruli, while beta 2-adrenoceptors were localized on medullary rays. The distribution of beta-adrenoceptors with respect to renal function in the dog kidney is discussed.     

Beta-adrenoceptors and human skeletal muscle characterisation of receptor subtype and effect of age.

1. Rectus abdominis muscle biopsies were obtained from 28 patients undergoing abdominal surgery. In membranes prepared from these biopsies beta-adrenoceptor binding was examined. The apparent affinity (KD) and the density (Bmax) of the receptors for the radioligand (-)-[125I]cyanopindolol were 28.5 +/- 2.7 (pM) and 25.9 +/- 2.1 (fmol mg-1 protein) (mean +/- s.e. mean) respectively. In forceps biopsies from vastus lateralis muscle from four healthy volunteers the values for KD and Bmax were 22.5 +/- 4.4 (pM) and 16.4 +/- 2.2 (fmol mg-1 protein). The binding characteristics for the radioligand were similar in the biopsies from the two muscle sites. 2. Inhibition of the radioligand binding by the selective beta 2-adrenoceptor antagonist ICI 118551 (KI = 117 +/- 45 nM) and selective beta 1-adrenoceptor antagonist metoprolol (KI = 15229 +/- 5046 nM) suggests the dominance of beta 2-adrenoceptor subtype in human skeletal muscle. 3. There were no significant differences in the skeletal muscle beta-adrenoceptor densities or affinities between the young and older patients.     

Effect of SR59230A on atypical beta-adrenoceptor mediating relaxation in the Guinea Pig gastric fundus

The purpose of the present study was to clarify whether atypical beta-adrenoceptors which presented in the guinea pig gastric fundus are beta(3)-adrenoceptors or putative beta(4)-adrenoceptors. In the presence of both the selective beta(1)-adrenoceptor antagonist atenolol (10(-4) mol/l) and the selective beta(2)-adrenoceptor antagonist butoxamine (10(-4) mol/l), the selective beta(3)-adrenoceptor antagonist SR59230A caused a concentration-dependent rightward shift of the concentration-response curve to catecholamines (isoprenaline, noradrenaline and adrenaline) and beta(3)-adrenoceptor agonists (BRL37344 and CGP12177A) in the guinea pig gastric fundus. Schild plot analyses of SR59230A against these agonists gave pA(2) values of 7.35 +/- 0.03 (isoprenaline), 7.26 +/- 0.04 (noradrenaline), 7.26 +/- 0.05 (adrenaline), 7.79 +/- 0.03 (BRL37344) and 6.74 +/- 0.03 (CGP12177A), respectively, and all Schild slopes were not significantly different from unity. These results suggest that atypical beta-adrenoceptors mediating relaxant responses of these agonists in the guinea pig gastric fundus are beta(3)-adrenoceptors rather than putative beta(4)-adrenoceptors.     

Identification and characterisation of beta-adrenoceptors in guinea-pig skeletal muscle

(-)-[125I]Iodocyanopindolol (ICYP) was used to characterise beta-adrenoceptors in skeletal muscle of the guinea-pig. The binding if ICYP to soleus and gastrocnemius muscles was saturable and reversible with KD values of 10.7 +/- 1.1 and 11.6 +/- 1.4 pM and Bmax values of 84.0 +/- 5.7 and 59.9 +/- 8.6 fmol/mg protein for gastrocnemius and soleus muscles respectively. Hofstee plots for the selective beta 1-adrenoceptor antagonists atenolol and metoprolol and for the selective beta 2-adrenoceptor antagonist ICI 118551 were linear in both skeletal muscles suggesting the presence of homogenous populations of beta-adrenoceptors. Furthermore, from the Ki values for atenolol (8381 +/- 2063 nM), metoprolol (585 +/- 80 nM) and ICI 118551 (0.39 +/- 0.05 nM) in gastrocnemius and ICI 118551 (0.47 +/- 0.09 nM) in soleus muscle, it is concluded that the beta-adrenoceptors in skeletal muscle of the guinea-pig are predominantly if not exclusively of the beta 2-subtype.     

Specific and non-specific effects of beta-adrenoceptor agonists on guinea pig alveolar macrophage function

The existence of beta-adrenoceptors on guinea pig alveolar macrophage membranes was determined by means of radioligand binding studies. Saturable binding with [125I]cyanopindolol demonstrated 38 +/- 6 fmol binding sites per 10(6) alveolar macrophages with a Kd of 0.85 +/- 0.15 nM. With timolol, atenolol and ICI 118.551 for competition of [125I]cyanopindolol binding it became clear that guinea pig alveolar macrophages possessed adrenergic binding sites of the beta 2-subtype. The cyclic AMP levels of alveolar macrophages could be increased by selective beta 2-adrenoceptor agonists but not by selective beta 1-adrenoceptor agonists. The influence of non-selective beta- and selective beta 1- and beta 2-adrenoceptor agonists on the phagocytic and metabolic responsiveness of alveolar macrophages was also studied. Addition of beta-adrenoceptor agonists had no effect on the uptake of bacteria by alveolar macrophages. Incubation of alveolar macrophages with increasing amounts of non-selective and selective beta 1-agonists resulted in a dose-dependent decrease in the detection of hydrogen peroxide released by alveolar macrophages. This effect was due to the scavenging properties of these drugs. The selective beta 2-receptor agonists, salbutamol and terbutaline, had no effect on the oxidative metabolism of alveolar macrophages. We conclude that guinea pig alveolar macrophages possess beta 2-adrenoceptors on their cell surface and that these receptors are not involved in the phagocytic activity of alveolar macrophages.     

The role of beta(3)-adrenoceptors in mediating relaxation of porcine detrusor muscle.

1. beta-adrenoceptors mediate relaxation of bladder detrusor smooth muscle. This study investigates the contribution of beta(3)-adrenoceptors to relaxation of the pig urinary bladder. 2. Cell membranes were prepared from detrusor muscle of the pig bladder dome and competition experiments with [(3)H]-dihydroalprenolol (DHA), a non-selective beta-adrenoceptor antagonist was used as a specific radioligand to determine the presence of beta-adrenoceptor subtypes. In functional experiments, isolated detrusor muscle strips were used to determine the potency of agonists and the affinity of antagonists. 3. In competition binding experiments, CGP20712A (beta(1)-adrenoceptor selective) displaced [(3)H]-DHA from a single binding site with a low affinity. In contrast, displacement data for ICI 118551 (beta(2)-adrenoceptor antagonist) and SR59230A (beta(3)-adrenoceptor antagonist) best fitted a two-site model suggesting a predominant (70%) population of beta(3)-adrenoceptors. 4. In functional studies, isoprenaline and salbutamol (beta(2)-adrenoceptor agonist) relaxed KCl precontracted muscle strips with high potency (pEC(50) 7.7 and 7.2, respectively), whilst CGP12177 and BRL37344 (beta(3)-adrenoceptor agonists) had low potency and were partial agonists. CGP20712A and atenolol (beta(1)-adrenoceptor antagonists) antagonised responses with a low affinity. ICI118551 antagonized responses to isoprenaline and salbutamol with a high affinity (pK(B)=7.8 and 8.7, respectively), but the Schild slopes were low suggesting that responses were mediated by more than one beta-adrenoceptor. The Schild plot for SR59230A was biphasic, apparent pK(B) values for 3 - 10 nM SR59230A being 8.6 and those for 30 nM - 1 microM being 7.7. 5. These data suggest that beta(3)-adrenoceptors are the predominant beta-adrenoceptor subtype present in the pig bladder and that beta-adrenoceptor mediated responses of this tissue are mediated via both the beta(2)- and beta(3)-adrenoceptor subtypes.     

The beta 1- and beta 2-adrenoceptor affinity of atenolol and metoprolol. A receptor-binding study performed with different radioligands in tissues from the rat, the guinea pig and man

The radioligand binding technique was used to perform a systematic investigation of the beta 1- and beta 2-adrenoceptor affinity of atenolol and metoprolol in tissues from the rat, the guinea pig and man. Radioligands, [125I](+/-)hydroxybenzylpindolol, [125I](-)pindolol, [3H](-)dihydroalprenolol and [3H](-)CGP12177, with different degrees of lipophilicity were used in the binding experiments. In membrane preparations of rat ventricular myocardium and uterus, the number of specific binding sites was similar when comparing experiments performed with the different radioligands. The percentage of the beta 1- and beta 2-adrenoceptor subpopulations in the tissues studied was not dependent on the radioligand or displacing compound used. Furthermore, the affinity of metoprolol and atenolol for beta 1- and beta 2-adrenoceptors was independent of the radioligand used or the tissue studied. The beta 1-adrenoceptor affinity of metoprolol was about 6-7 times higher than that of atenolol, while the beta 1-adrenoceptor selectivity was similar (about 30-fold) for the two beta-blockers. It is concluded that the physical-chemical properties of the radioactive ligands and beta-blockers studied do not affect the results obtained from beta-adrenoceptor-binding experiments in cellular membrane fractions. The beta 1- and beta 2-adrenoceptor affinities did not change in any experiments performed in tissues from the rat, the guinea pig and man for either atenolol or metoprolol.     

Ligand binding properties of putative beta 3-adrenoceptors compared in brown adipose tissue and in skeletal muscle membranes.

1. The beta-adrenoceptor population was characterized in membrane preparations from rat brown adipose tissue (BAT) and from soleus muscle by use of the radioligand [125I]-iodocyanopindolol ([125I]-ICYP). In addition, atypical binding sites for [125I]-ICYP found in both tissues were examined, and the relationship between these sites and the putative rat beta 3-adrenoceptor is discussed. 2. It was established that BAT membranes host a mixed population of beta 1- and beta 2-adrenoceptors. Of these two sites, 55% showed a high affinity for the beta 1-selective ligand CGP 20712A (pK 8.5), and 45% showed a high affinity for the beta 2-selective antagonist ICI 118551 (pK 8.6). Soleus muscle membranes were found to host a population of beta 2-adrenoceptors, characterized by a high affinity for ICI 118551 (pK 9.1), but beta 1-adrenoceptors could not be detected in this preparation. 5-Hydroxytryptamine receptors were not detected in either preparation. 3. In addition to beta 1- and beta 2-adrenoceptors, atypical binding sites were identified in both tissues using high concentrations of radioligand (0.5-0.6 nM) and in the presence of 1 microM (-)-propranolol. The atypical sites were abundant, representing 80 and 81% of the total [125I]-ICYP binding sites in BAT and soleus muscle respectively. When the pK values for 11 ligands were compared, the correlation coefficient for atypical sites in BAT and soleus muscle was 0.94.(ABSTRACT TRUNCATED AT 250 WORDS)     

Distribution of beta 1- and beta 2-adrenoceptors in mouse trachea and lung: a quantitative autoradiographic study.

1. Binding and quantitative autoradiography were used to detect [125I]-iodocyanopindolol (I-CYP) associated with beta 1- and beta 2-adrenoceptors in mouse tracheal epithelium and airway smooth muscle as well as in lung parenchymal tissue. 2. Specific I-CYP binding to slide-mounted tissue sections of both trachea and parenchyma was of high affinity (KD = 49.0 pM, n = 3, trachea; KD = 118.9 pM, n = 3, parenchyma) and saturable, involving single populations of non-interacting binding sites (Hill coefficient nH = 1.00 +/- 0.02, trachea; nH = 0.99 +/- 0.03, parenchyma). 3. Direct measurement of tissue radioactivity also showed that specific I-CYP binding was competitively inhibited in the presence of the beta-adrenoceptor antagonists (-)-propranolol (non-selective), CGP 20712A (beta 1-selective) and ICI 118,551 (beta 2-selective). Analysis of the competition binding curves for the two selective antagonists revealed mixed populations of beta 1- and beta 2-adrenoceptors in the approximate proportions 33% and 67% respectively in mouse trachea and 28% and 72% respectively in mouse lung parenchyma. 4. Densities of autoradiographic grains derived from specific I-CYP binding to alveolar wall tissue and to tracheal epithelium and airway smooth muscle were quantified by a computer-assisted image analysis system, which allowed the construction of competition binding curves in the presence of the selective beta-adrenoceptor antagonists CGP 20712A and ICI 118,551. Analysis of these data demonstrated that in alveolar wall, beta 1- and beta 2-adrenoceptors co-existed in the proportions 18% and 82%, respectively. 5. Quantitative autoradiographic analyses also showed that beta 1- and beta 2-adrenoceptors were differentially distributed in tracheal epithelium and airway smooth muscle. The beta 2-adrenoceptor subtype accounted for 71% of all beta-adrenoceptors in epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Characterization of rat liver beta-adrenoceptors during perinatal development as determined by [125I]-iodopindolol radioligand binding assays.

1. The subtype specificity of beta-adrenoceptors in foetal (20 days post coitum) rat liver membrane preparations has been determined by use of [125I]-iodopindolol binding assays and the characteristics of radioligand binding have been resolved. 2. The kinetics of radioligand association and dissociation (in the presence of 5 x 10(-4) M isoprenaline) showed an association rate constant of 1.5 x 10(7) M-1 S-1 and dissociation rate constant of 9.1 x 10(-4) S-1, corresponding to a dissociation constant for [125I]-iodopindolol of 60.7 pM. A similar dissociation constant (75 pM) was determined by saturation binding assays. 3. The rank order of potency for displacement of [125I]-iodopindolol binding was consistent with binding to a predominantly beta 2-adrenoceptor population (i.e. ICI 118551 greater than isoprenaline greater than adrenaline greater than noradrenaline greater than atenolol). Computer analysis of displacement curves in the presence of a beta 1-subtype selective agent (atenolol) or a beta 2-subtype selective agent (ICI 118551) revealed the presence of beta 2- and beta 1-adrenoceptor subtypes in a ratio of about 80:20%. 4. Saturation binding assays by use of [125I]-iodopindolol were carried out at different perinatal ages to determine total beta-adrenoceptor concentrations and beta 2-subtype (in the presence of 5 x 10(-7) M atenolol) adrenoceptor concentrations. Competition binding assays with atenolol confirmed that at all ages apparent beta 2-adrenoceptor binding accounted for 84-95% of the total beta-adrenoceptor binding. The total beta- and beta 2-adrenoceptor binding capacity increased by 2.3 fold from 20 days post coitum to birth, and then decreased postnatally at 1 and 2 days post partum. The dissociation constant for [125I]-iodopindolol binding did not show any change with age. 5. The change in beta 2-adrenoceptor concentration with age is discussed in relation to the changing beta-adrenoceptor-mediated responsiveness of glucose production by rat liver during perinatal development.     

The importance of choice of agonist in studies designed to predict β 2:β 1 adrenoceptor selectivity of antagonists from pA2 values on guinea-pig trachea and atria

1. pA2 values have been obtained for propranolol, butoxamine, H35/25 and atenolol on guinea-pig isolated trachea and atria (rate) using noredrenaline (beta 1-selective), isoprenaline (non-selective) and fenoterol (beta 2-selective) as agonists. 2. pA2 values varied with the agonist used on trachea but not on atria and, therefore, trachea : atria selectivity values varied with the agonist used. 3. It is suggested that the best estimate of the selectivity of an antagonist between beta 2- and beta 1-adrenoceptors is obtained by comparing its pA2 value obtained on trachea using a beta 2-selective agonist with that obtained on atria using a beta 1-selective agonist. The reasons for this are discussed. 4. The quantitative values for beta 2 : beta 1 selectivity obtained using the above pA2 values were butoxamine 17.0 H35/25 13.5, propranolol 2.75 and atenolol 0.036, i.e. butoxamine and H35/25 were beta 2-selective, propranolol was non-selective and atenolol was beta 1-selective. 5. The results support the hypotheses that guinea-pig trachea contains a mixture of beta 1- and beta 2-adrenoceptors and that guinea-pig atria contain only beta 1-adrenoceptors.     

Prenalterol is an agonist at beta 2- as well as at beta 1-adrenoceptors

Prenalterol exerted agonist activity in cat, but not guinea-pig, isolated atria, which contain predominantly beta 1-adrenoceptors. Prenalterol relaxed K+ -contracted rat uterus, but not histamine-contracted cat lung strips; both contain predominantly beta 2-adrenoceptors. The effect of prenalterol in rat uterus was antagonised by the selective beta 2-adrenoceptor antagonist ICI 118551 but not by the selective beta 1-adrenoceptor antagonist atenolol. Thus the ability of prenalterol to exert beta-adrenoceptor activity is tissue-dependent, rather than beta-adrenoceptor subtype-dependent.     

FUNCTION, CHARACTERIZATION AND AUTORADIOGRAPHIC LOCALIZATION AND QUANTITATION OF β-ADRENOCEPTORS IN CARDIAC TISSUES

1. This paper demonstrates the use of organ bath, radioligand binding and autoradiography to detect beta 1- and beta 2-adrenoceptors in human and guinea-pig cardiac tissues. 2. In organ bath experiments, non-selective and beta 1- and beta 2-adrenoceptor selective agonists produced concentration-dependent inotropic responses in human right atrial appendage. Both subtypes mediate inotropic responses. In guinea-pig right atria chronotropic responses were mediated predominantly through beta 1-adrenoceptors. 3. Receptor binding studies using (-)[125I]-cyanopindolol (CYP) and beta 1- and beta 2-adrenoceptor selective antagonists showed that beta 2-adrenoceptors comprised 25% of the total population of beta-adrenoceptors in guinea-pig right atria. In human right atria the proportion is higher (40%). 4. Quantitative autoradiography was used to determine the location and densities of beta 1- and beta 2-adrenoceptors in guinea-pig heart. Both beta 1- and beta 2-adrenoceptors were distributed on myocardium. The atrioventricular conducting system had a higher density of beta 2-adrenoceptors compared with myocardium.