Regulation of beta 1- and beta 2-adrenoceptors following chronic treatment with beta-adrenoceptor antagonists

The effect of chronic pretreatment of guinea pigs with various beta-adrenoceptor antagonists on the binding characteristics of the radioligand 125I-cyanopindolol (ICYP) and responsiveness of adenylate cyclase to isoprenaline in the gastrocnemius muscle (beta 2-adrenoceptors) and the left ventricle (beta 1-adrenoceptors) were compared. Pretreatment of guinea pigs with propranolol or ICI 118,551 for one week significantly increased the density of the beta 2-adrenoceptors in the gastrocnemius muscle. Atenolol pretreatment for one week did not affect the density of the receptors. Pretreatment of the animals with pindolol for one week reduced the density of beta 2-adrenoceptors in skeletal muscle. In the left ventricle pretreatment of the guinea pigs with any of the antagonists did not alter either the density or the KD of beta 1-adrenoceptors. The responsiveness of adenylate cyclase to isoprenaline (10(-4) M) in the left ventricle or skeletal muscle was not affected when the guinea pigs were pretreated with propranolol. Pretreatment of the guinea pigs with reserpine (0.5 mg.kg-1) intraperitoneally for one week, to deplete catecholamines did not affect beta-adrenoceptor density or KD in the left ventricle or skeletal muscle. We conclude that the regulation of beta-adrenoceptors by the antagonist may not be caused by the prevention of the access of endogenous agonists to beta-adrenoceptors and it is dependent on the selectivity of the antagonist and on the susceptibility of the receptors to regulation.     

Effect of pretreatment with bronchodilator drugs on in vitro responsiveness of guinea pig lung adenylate cyclase.

Pretreatment of guinea pigs with 5 microgram/kg isoprenaline or 10 microgram/kg salbutamol s.c. thrice daily for 7 days reduced the responsiveness of lung slice and tracheal ring adenylate cyclase to isoprenaline, but not to prostaglandin E1. Pretreatment of guinea pigs with isoprenaline also reduced the sensitivity of tracheal smooth muscle strip adenylate cyclase to isoprenaline. Cross-tolerance developed to noradrenaline in lung slices obtained from guinea pigs pretreated with isoprenaline. Propranolol blocked the response of lung slice adenylate cyclase of control and isoprenaline-pretreated animals to approximately the same degree. The presence of phentolamine in the incubation medium did not affect the reduced sensitivity to isoprenaline. Possible mechanisms of development of tolerance to sympathomimetic bronchodilator drugs are discussed.     

The use of the guinea-pig lung parenchyma preparation in studies of the beta-adrenoceptor adenylate cyclase system.

The binding, adenylate cyclase activation, and functional effect of four beta-adrenoceptor agonists were studied in the guinea-pig lung parenchyma preparation and the results were compared with those obtained earlier in guinea-pig left-heart ventricle (beta 1-adrenoceptors) and soleus muscle (beta 2-adrenoceptors) preparations. The pKi-values of the unselective compounds, isoprenaline and orciprenaline, were in good agreement with those obtained in the heart and soleus muscle. The beta 2-adrenoceptor selective compounds KWD 2026 and terbutaline were bound to two sites, one corresponding to the beta 1-adrenoceptors and the other to the beta 2-adrenoceptors. The pKi-value of isoprenaline was in good agreement with its pKact-value indicating that maximum adenylate cyclase activity is obtained when the occupancy of the receptors is maximal. Further, the relative intrinsic efficacy calculated from the functional effect and receptor occupancy agreed well with the relative maximum adenylate cyclase activation by the agonists which was also found earlier for the guinea-pig heart ventricle and soleus muscle preparations. Relative effects were obtained from both functional experiments and from affinity and adenylate cyclase activating studies. There was good agreement between relative effects obtained in these two ways. It is concluded that the guinea-pig lung parenchyma preparation may be useful for the study of the beta-adrenoceptor adenylate cyclase system.     

An analysis of the beta 2-adrenoceptor selectivity in three series of beta-adrenoceptor agonists

The aim of the study was to analyse the beta 2-adrenoceptor selectivity earlier found in two series of catecholamines and one series of resorcinolamines (Johansson et al. 1986). The affinity of the compounds was assessed in binding studies in preparations from the guinea-pig left heart ventricle (beta 1-adrenoceptors) and the soleus muscle (beta 2-adrenoceptors) using 3H-CGP-12177 as radioligand. Further, the activation of the adenylate cyclase by the compounds was studied in the same preparations. Selectivity quotients were obtained from both functional effects and from affinity and adenylate cyclase activating studies. There was a good correlation between the selectivity quotients obtained in these two ways. Tertiary butyl substitution on the amino nitrogen gave the highest beta 2-adrenoceptor selectivity in both the catechol and resorcinol series. In comparison with their isopropyl substituted analogues the beta 2-adrenoceptor selectivity of these compounds (KWD 2026 and terbutaline) was mainly due to a change in affinity for the beta 1- and beta 2-adrenoceptors and, to a lesser degree, a change in intrinsic efficacy.     

Relaxant beta-receptors in the trachea, but not prejunctional alpha 2-receptors in the tracheal cholinergic neurones, are subjected to homologous desensitization

Guinea-pigs were pretreated with either isoprenaline, terbutaline or the alpha 2 agonist B-HT 920 in order to asses the hypothesis that beta-and alpha 2 receptors in trachea are subjected to homologous desensitization. In these experiments the beta receptor activity was investigated on tracheal ring preparations contracted with carbacholine. Both in the isoprenaline and the terbutaline pretreated group the relaxant responses to beta agonists were diminished. Pretreatment with B-HT 920 did not affect the sensitivity of the trachea to beta stimulation. In order to asses the responsiveness of alpha 2 receptors the trachea was contracted by electrical field stimulation in the presence of propranolol. During these conditions contractions were mediated by activation of cholinergic neurones and inhibitory effects of alpha stimulation were due to inhibition of the cholinergic neurotransmission by stimulation of prejunctional alpha 2 receptors. In these tests neither isoprenaline, terbutaline nor B-HT 920 pretreatment affected the responsiveness of alpha stimulation to inhibit the electrically induced contractions.     

Regulation of beta-adrenoceptors by catecholamines in the rabbit skeletal muscle

(-)-[125I]Cyanopindolol (ICYP) was used to characterise beta-adrenoceptors in the gastrocnemius muscle of the rabbit. The binding of ICYP was saturable. The KI value for the selective beta 2-adrenoceptor antagonist, ICI 118551, was 0.16 +/- 0.01 nM (mean +/- S.E.M) and for the selective beta 1-adrenoceptor antagonist, metoprolol, was 300.1 +/- 33.3 nM. Hofstee plots for both antagonists were linear indicating the presence of homogeneous beta 2-adrenoceptors in the skeletal muscle of the rabbit. In gastrocnemius muscles obtained from rabbits chronically pretreated in vivo with vehicle (0.1% ascorbic acid), adrenaline (40-50 nmol.kg-1.h-1) or noradrenaline (80-100 nmol.kg-1.h-1) via osmotic mini-pumps implanted for 10 days, the Bmax values were 44 +/- 3, 25 +/- 4 (P less than 0.05) and 41 +/- 7 (fmol.mg-1 protein) and KD values were 7.3 +/- 0.9, 6.3 +/- 0.8 and 9.3 +/- 2.5 (pM), respectively. Thus pretreatment with the circulating hormone adrenaline but not noradrenaline down-regulated the number of skeletal muscle beta 2-adrenoceptors but did not influence the affinity.     

Partial agonism and functional selectivity: a study on beta-adrenoceptor mediated effects in tracheal, cardiac and skeletal muscle

Colterol, procaterol, sulfonterol, terbutaline and three monophenolic derivatives of terbutaline were examined with respect to their ability to react in vitro on beta-adrenoceptors in tissues isolated from guinea-pig. The effects measured were a) relaxation of the tracheal smooth muscle (mostly beta 2), b) depression of subtetanic contractions of the soleus muscle (beta 2), and c) increase in the force of the papillary muscle of the left ventricle (beta 1). Antagonistic effects were measured against isoprenaline as an agonist. The compounds studied showed a wide variation in selectivity, potency and intrinsic activity. All agonists showed a pronounced beta 2-selectivity, in general characterized by a higher intrinsic activity at beta 2- than at beta 1-adrenoceptors, while differences in affinity, as judged from the pA2-values were small. Partial agonists, such as sulfonterol, which did not cause a complete relaxation of a moderately contracted tracheal muscle, produced identical concentration-response curves from the trachea and soleus muscle. It is concluded that partial agonism at beta 1-adrenoceptors is an important factor for functional selectivity of beta 2-adrenoceptor agonists. On the other hand there seems to be no useful differences between the maximum effect elicited by a partial beta 2-adrenoceptor agonist on the skeletal muscle as compared with airway smooth muscle.     

Desensitisation of mast cell beta2-adrenoceptor-mediated responses by salmeterol and formoterol

1. The long-acting beta(2)-adrenoceptor agonist formoterol (10(-10)-10(-6) m) inhibited the IgE-dependent release of histamine from human lung mast cells in a concentration-dependent manner. Formoterol was more potent and a full agonist relative to the nonselective beta-adrenoceptor agonist isoprenaline. By contrast, the long-acting beta(2)-adrenoceptor agonist salmeterol (10(-10)-10(-6) m) was about two-thirds less efficacious than either formoterol or isoprenaline as an inhibitor of histamine release. 2. Isoprenaline, formoterol and salmeterol (all at 10(-5) m) increased total cell cAMP levels in mast cells over basal by 361+/-90 (P<0.05), 321+/-89 (P<0.05) and 64+/-24% (P>0.05), respectively. 3. Long-term (24 h) incubation of mast cells with formoterol (10(-6) m) or salmeterol (10(-6) m) essentially abolished the subsequent ability of isoprenaline to inhibit histamine release. Both formoterol and salmeterol were more effective at inducing the functional desensitisation than isoprenaline (10(-6) m) or the short-acting beta(2)-adrenoceptor agonist salbutamol (10(-6) m). 4. The desensitisation induced by long-term treatments with salmeterol and formoterol was specific for beta(2)-adrenoceptor-mediated inhibition of histamine release as the inhibitory effects of alternative cAMP-elevating compounds, prostaglandin E(2), a receptor-mediated activator of adenylate cyclase, and forskolin, a direct activator of adenylate cyclase, were unaffected by desensitising treatments. 5. Radioligand binding studies were performed to determine beta(2)-adrenoceptor density in cell membranes after pretreatment (24 h) of cells with agonists. Isoprenaline, formoterol and salmeterol (all at 10(-6) m) reduced beta(2)-adrenoceptor density by 13+/-5 (P>0.05), 49+/-13 (P<0.05) and 35+/-17% (P>0.05), respectively. 6. These data indicate that long-term exposure of mast cells to both salmeterol and formoterol can cause substantial levels of desensitisation to beta(2)-adrenoceptor-mediated responses in mast cells.     

Islet-activating protein impairs α 2-adrenoceptor-mediated inhibitory regulation of human platelet adenylate cyclase

Human platelet adenylate cyclase is stimulated by prostaglandin E1 (PGE1) and inhibited by adrenaline acting via alpha 2-adrenoceptors. Both hormonal factors increase GTP hydrolysis in platelet membranes by stimulation of a high affinity GTPase. The influence of the Bordetella pertussis toxin, islet-activating protein (IAP), was studied on adrenaline and PGE1-induced GTPase stimulation and on adrenaline-induced adenylate cyclase inhibition. Pretreatment of platelet membranes with IAP greatly reduced the adrenaline-induced inhibition of the platelet adenylate cyclase. At similar concentrations of the toxin, stimulation of GTP hydrolysis caused by adrenaline was impaired. In contrast, pretreatment of platelet membranes with IAP had no effect on stimulation of GTP hydrolysis by the adenylate cyclase-stimulating agent, PGE1, both when studied alone or in combination with adrenaline. On the other hand, cholera toxin reduced the PGE1-induced GTPase stimulation but did not affect the adrenaline-stimulated GTP hydrolysis. The data indicate that there are two separate GTP-hydrolyzing systems in human platelet membranes and that cholera toxin and pertussis toxin selectively affect stimulation of GTP hydrolysis by hormones that stimulate and inhibit adenylate cyclase, respectively.     

On minimum cyclic AMP formation rates associated with positive inotropic effects mediated through β1-adrenoceptors in kitten myocardium

The agonist (-)-denopamine was used as a tool to study relationships between pharmacological effects and adenylate cyclase stimulation mediated through beta 1-adrenoceptors. 1. (-)-Denopamine was a full agonist in kitten papillary muscles (force), kitten left atria (force) and kitten and guinea-pig atria (sinoatrial frequency). (-)-Denopamine was a strong partial agonist in guinea-pig tracheae (relaxation). None of these effects was influenced by blockade of beta 2-adrenoceptors. beta 1-Adrenoceptors mediated all effects of (-)-denopamine in atria and effects of low (-)-denopamine concentrations in papillary muscles and tracheae, as assessed with beta 1-selective antagonists. 2. High (-)-denopamine concentrations caused positive inotropic effects in papillary muscles and tracheorelaxant effects that were resistant to blockade by beta 1-, beta 2- and alpha-adrenoceptor antagonists (non-adrenergic effects). 3. (-)-Denopamine stimulated the adenylate cyclase of membranes derived from kitten ventricle and calf tracheal cells with an intrinsic activity of 0.3 and 0.2, respectively, compared to catecholamines. The contribution of beta 1- and beta 2-adrenoceptors to cyclase stimulation was assessed by selective blockade. Cyclase stimulation through beta 2-adrenoceptors by (-)-denopamine was 12% in ventricle and 82% in trachea but is not associated with positive inotropic effects and tracheal relaxation. 4. (-)-Denopamine exhibited only a 2- to 5-fold selectivity for beta 1-adrenoceptors compared to beta 2-adrenoceptors, as estimated consistently from binding assays and blockade of cyclase stimulation in myocardial and tracheal cell membranes. 5. Desensitization of kitten ventricular tissues, caused by a 3 h exposure to 30 mumol/l (-)-isoprenaline followed by 5 h washout, reduced the inotropic sensitivity of papillary muscles without decreasing the maximum inotropic effects of (-)-denopamine. In desensitized tissues, the nonadrenergic effect contributed by 30% to the maximum inotropic effect of (-)-denopamine. 6. In membranes, derived from desensitized tissues, the maximum adenylate cyclase stimulation induced by (-)-isoprenaline, (-)-denopamine and xamoterol was reduced to 1/2 of the corresponding stimulations observed in membranes from sham desensitized tissues. The density of beta-adrenoceptors, assessed with 3H-(-)-CGP 12,177, was not changed by the desensitization procedure suggesting that part of the receptors was uncoupled from the adenylate cyclase. The partial inotropic agonist xamoterol, which has an intrinsic activity of 0.5 in non-desensitized tissues, failed to cause positive inotropic effects in desensitized papillary muscles suggesting that not all cyclic AMP possesses inotropic relevance.(ABSTRACT TRUNCATED AT 400 WORDS)     

Cardiostimulatory effects of prenalterol,a beta-1 adrenoceptor partial agonist,in vivo and in vitro

Summary The cardiostimulatory effects of prenalterol, a beta-1-adrenoceptor partial agonist, were studied in vivo and in vitro and compared to those evoked by isoprenaline, a full agonist, and to those of other partial agonists.In the anaesthetized rat, prenalterol and terbutaline were found not to elevate the myocardial cyclic AMP content; this was in sharp contrast to isoprenaline. Both partial agonists did, however, produce significant effects on heart rate.In the anaesthetized cat, prenalterol exhibited chronotropic and inotropic intrinsic activities of 88 and 76% respectively in relation to isoprenaline. No statistically significant increase in myocardial cyclic AMP content could however be detected.Prenalterol did not stimulate adenylate cyclase significantly in the cat myocardial homogenate. This was also true of the beta-2-adrenoceptor selective partial agonist procaterol. In this preparation, isoprenaline, noradrenaline and adrenaline acted as full agonists. Furthermore, prenalterol produced a concentration-dependent inhibition of isoprenaline-activated adenylate cyclase.Our data indicate that maximal cardiac stimulation occurs at a low level of adenylate cyclase activation and low myocardial cyclic AMP concentration when provoked by a full beta-adrenoceptor agonist. The maximal physiological effects of a partial agonist such as prenalterol may consequently be achieved at a marginal activation of the adenylate cyclase.The present data may thus support the hypothesis of a large beta-adrenoceptor reserve for full agonists in the heart.     

Effects of in vivo treatment with isoprenaline or prenalterol on beta-adrenoceptor mechanisms in the heart and soleus muscle of the cat

Summary The full agonist isoprenaline (5.3–6.6 nmol/kg min) and the partial -adrenoceptor agonist prenalterol (10.6–13.3 nmol/kg · min) were administered to cats continuously via osmotic minipumps (i.p.). After seven days the functional and adenylate cyclase responsiveness to the agonists, as well as the -adrenoceptor-binding characteristics, were studied in cardiac and soleus muscle preparations in vitro.After isoprenaline pretreatment, the papillary muscles and soleus muscle strips were 15–18 times less sensitive to isoprenaline compared with muscles from control cats. The stimulatory potency (pD₂) of prenalterol in the papillary muscle was not changed significantly. The affinity of the agonists to the -adrenoceptors was unaffected in both tissues by the pretreatment, but the densities of -adrenoceptors were significantly reduced, by 36% (myocardium) and 47% (soleus) respectively. In the cat papillary muscle the intrinsic sympathomimetic activity (ISA) of prenalterol on contractile parameters was reduced from 84 (T _max), 69 (dT/dt _max) and 71% (dT/dt _min) in control animals, to 33, 22 and 28%, respectively in the animals pretreated with isoprenaline.Prenalterol pretreatment did not induce any marked changes, either in the stimulatory potency or affinity of the agonists in the two tissuer or in the maximal response (ISA) of prenalterol in the papillary muscle.The marked reduction in the stimulatory potency of isoprenaline and the reduced ISA of prenalterol in the myocardium after isoprenaline pretreatment can not be explained by the reduction in -adreoceptor density alone. Since the affinity to the -adrenoceptors is unaffected, a reduced efficiency in the signal transmission must be the main cause. This alteration in signal transmission seems to be an event located distal to adenylate cyclase, since the relative decrease in the enzyme activity is even less than the loss of -adrenoceptors in both the myocardium and soleus muscle.The present results demonstrate that the effects on -adrenoceptors and functional responsiveness are different after prolonged treatment with a full -adrenoceptor agonist and a partial agonist, such as prenalterol.Some of these data were presented at the 66th Annual Meeting of the Federation of American Societies for Experimental Biology, April 15–23, 1982 New Orleans, USA     

5-HT(7) receptor and beta(2)-adrenoceptor share in the inhibition of porcine uterine contractility in a muscle layer-dependent manner.

To compare the inhibition of uterine contractility mediated by beta-adrenoceptors and 5-HT(7) receptors, the effects of catecholamines and 5-HT on spontaneous contractions were examined in longitudinal and circular muscles isolated from three different regions (cornu, corpus and cervix) of the non-pregnant proestrus porcine uterus. In addition, the distribution of beta-adrenoceptors between muscle layers was characterized by means of adenylate cyclase activity assay, cyclic AMP assay and [(3)H]dihydroalprenolol binding studies. In the cornu, isoprenaline, adrenaline and noradrenaline inhibited the spontaneous contraction of longitudinal and circular muscles but longitudinal muscle was more sensitive to catecholamines than was circular muscle. The inhibitory response to isoprenaline was antagonized by propranolol (300 nM) or (+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol (ICI 118,551; 100 nM). The rank order of potency was isoprenaline > or =adrenaline > noradrenaline. The beta(2)-adrenoceptor-selective agonist, clenbuterol, was more potent than xamoterol (beta(1)-selective) and (+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxyacetic acid (BRL 37344; beta(3)-selective) to inhibit the spontaneous contraction of longitudinal muscles. Isoprenaline increased adenylate cyclase activity in both muscle layers, but the activity in the longitudinal muscle was greater than that in the circular muscle. Cyclic AMP production by isoprenaline was also more conspicuous in the longitudinal muscle than in the circular muscle. Although both muscle layers contained a single class of [3H]dihydroalprenolol binding sites with similar K(d) values (longitudinal muscle, 3.1+/-0.94 nM, n=4; circular muscle, 2.4+/-0.73 nM, n=4), B(max) in the longitudinal muscle (175.7+/-32.8 fmol/mg protein, n=4) was significantly higher than that in the circular muscle (53.1+/-5.1 fmol/mg protein, n=4). As previously reported [Br. J. Pharmacol. 130 (2000) 79], 5-HT also inhibited the spontaneous contraction of both muscle layers from the cornu and the 5-HT(7) receptor antagonist, 2a-[4-(4-phenyl-1,2,3,6-tetrahydropyridyl)butyl]-2a,3,4,5-tetrahydro-benzo[cd]indol-2(1H)-one (DR4004; 100 nM, n=4) blocked the 5-HT-induced inhibition of spontaneous contractions in the circular muscles, and reversed the less marked inhibition in the longitudinal muscles. In other regions of the uterus (corpus and cervix), 5-HT inhibited the spontaneous contraction of the circular muscles but contracted the longitudinal muscle strips. On the other hand, isoprenaline caused muscle layer-dependent inhibition (longitudinal muscle > circular muscle) in both regions, and the responsiveness tended to increase toward the cervix. In conclusion, beta(2)-adrenoceptors are present heterogeneously in the porcine uterus (longitudinal muscle > circular muscle) and share the inhibition of uterine contractility with 5-HT(7) receptors in a layer-dependent manner (longitudinal muscle: beta(2)-adrenoceptors, circular muscle: 5-HT(7) receptors).     

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.     

Human cardiac beta-adrenoceptors: both beta 1- and beta 2-adrenoceptors are functionally coupled to the adenylate cyclase in right atrium

We investigated properties of catecholamine-sensitive adenylate cyclase in membranes from human right atria. Basal adenylate cyclase was activated by Mg2+-ions and guanyl nucleotides [Gpp(NH)p, GTP] in a concentration-dependent manner; guanyl nucleotide activation was strongly dependent on the presence of Mg2+-ions. Catecholamines stimulated adenylate cyclase activity in the following order of potency: (-)-isoprenaline greater than (-)-adrenaline = (-)-noradrenaline greater than phenylephrine, indicating that, in human right atrium, beta 1-adrenoceptors predominate. The beta 1-agonist dobutamine and the beta 2-agonists fenoterol and procaterol activated adenylate cyclase with an intrinsic activity of 0.5-0.7 (isoprenaline = 1.0). Adenylate cyclase activation by dobutamine or procaterol was not additive with the activation induced by isoprenaline. On the contrary, combination of dobutamine (100 microM) and procaterol (10 microM) resulted in activation of adenylate cyclase which was not different from that evoked by saturating concentration of isoprenaline (10 microM), indicating that dobutamine (beta 1) and procaterol (beta 2) produce adenylate cyclase activation through stimulation of different beta-adrenoceptor subtypes. The beta 2-selective antagonist ICI 118,551 was much more potent in inhibiting procaterol-than isoprenaline-stimulated adenylate cyclase activity, whereas the beta 1-selective antagonist betaxolol inhibited isoprenaline-stimulated activity more potently. We conclude that in human right atrium, both beta 1- and beta 2-adrenoceptors are functionally coupled to the adenylate cyclase system.     

Desensitization by terbutaline of beta-adrenoceptors in the guinea-pig soleus muscle: biochemical alterations associated with functional changes.

1 The effects of adrenaline and terbutaline on cyclic adenosine 3',5'-monophosphate (cyclic AMP) content, 22Na-efflux, 42K-influx and subtetanic contractions have been assessed in soleus muscles isolated from guniea-pigs which had been maintained on food with or without terbutaline for 5 days. 2 Terbutaline and adrenaline increased cyclic AMP content and suppressed subtetanic contractions, and regression analysis indicates a statistically significant correlation between these two effects (P less than 0.01). 3 In muscles obtained from terbutaline-treated animals, the effects of terbutaline and adrenaline on cyclic AMP content, active Na-K-transport and subtetanic contractions were all considerably suppressed, but insulin stimulated 22Na-efflux and affected subtetanic contractions to the same extent as in the muscles obtained from the control group. 4 The results suggest that terbutaline treatment leads to a reduction in the number of beta 2-adrenoceptors in skeletal muscle or an impairment of their function. 5 The results provide further support for the idea that the effect of adrenaline or insulin on skeletal muscle contractions is the outcome of stimulation of active Na-K-transport.     

Steric aspects of agonism and antagonism at beta-adrenoceptors: experiments with the enantiomers of terbutaline and pindolol

The enantiomers of terbutaline, a beta 2-selective adrenoceptor agonist, and pindolol, an unselective antagonist with partial agonist activity, were examined with respect to their ability to react in vitro on adrenoceptors in the trachea (mostly beta 2), the soleus muscle (beta 2) and in the papillary muscle of the left ventricle (beta 1) from the guinea-pig (+)-terbutaline was more than 3,000 times less potent than (-)-terbutaline in relaxing the trachea and in depressing subtetanic contractions of the soleus muscle. (+)-terbutaline did not inhibit the effects of (-)-terbutaline in these tissues. The effect of (-)-terbutaline on the papillary muscle was about 200 times weaker than on the soleus. (+)-terbutaline had a negligible inotropic effect on the papillary muscle and it did not inhibit the effect of isoprenaline. The enantiomers of pindolol did not show any consistent agonistic activity under the present experimental conditions. (-)-pindolol inhibited competitively the effect of isoprenaline to the same extent in all three tissues. (+)-pindolol was about 200 times less potent in this respect. Our data do not reveal any qualitative differences in the pharmacological properties between the optical isomers of terbutaline and pindolol, respectively.     

Xamoterol activates beta 1- but not beta 2-adrenoceptors in mammalian myocardium: comparison of its affinity for beta 1- and beta 2-adrenoceptors coupled to the adenylate cyclase in feline and human ventricle with positive inotropic effects

The mode of action of xamoterol, a beta 1-selective partial agonist, was investigated in feline myocardium. Xamoterol bound with an 18-fold greater affinity to ventricular beta 1-adrenoceptors (labeled with [3H](-)-bisoprolol) than to beta 2-adrenoceptors (labeled with [3H]ICI 118,551). Xamoterol had a 10-20-fold higher affinity for ventricular beta 1-adrenoceptors coupled to the adenylate cyclase than for cyclase-coupled beta 2-adrenoceptors. The intrinsic activity of xamoterol with respect to (-)-norepinephrine was 0.5 in right ventricular papillary muscles (force), 0.6 in left atria (force); 0.6 in right atria (sinoatrial rate) and 0.1-0.2 in ventricular membranes (cyclase). The stimulant effects of xamoterol were antagonized by beta 1-specific CGP 20,712 A but not by beta 2-selective ICI 118,551. Xamoterol activated only beta 1-adrenoceptors, while beta 2-adrenoceptors occupied by xamoterol remained silent. The positive inotropic effects of a nearly maximally effective xamoterol concentration were associated with a considerably greater beta 1-mediated cyclase stimulation than the same inotropic effect of (-)-norepinephrine. In human ventricular membranes xamoterol stimulated marginally the adenylate cyclase and antagonized the effects of (-)-norepinephrine with a 30-fold greater affinity for beta 1- than for beta 2-adrenoceptors.     

Antigen challenge modifies the cyclic AMP response of inflammatory mediators and beta-adrenergic drugs in alveolar macrophages

Adenylate cyclase activity was determined in alveolar macrophages (AMs) obtained from bronchoalveolar lavage (BAL) fluids of naive and antigen-challenged guinea pigs. After the anaphylactic reaction in ovalbumin-sensitized guinea pigs, the basal levels of cyclic AMP in AMs were significantly increased compared to the levels in naive AMS (1.87 +/- 0.22 versus 5.26 +/- 0.45 pmol cyclic AMP/5.10(6) cells). Prostaglandin E2 (PGE2), prostacyclin (DC-PGI2), histamine, isoprenaline and salbutamol stimulated adenylate cyclase activity more effectively in AMs obtained from sensitized guinea pigs after the booster injection compared to AMs obtained from non-treated animals. Moreover, DC-PGI2 and histamine, which were hardly able to induce a rise in cyclic AMP levels in naive AMs, become effective activators in AMs obtained after antigen challenge (100 and 60% increase in the response, respectively). Using selective receptor ligands, we have shown that beta 2-adrenoceptors and H2-subtype histamine receptors are functionally coupled to macrophage adenylate cyclase activity. The present data indicate that sensitization does not affect the configuration of the receptor on the outer membrane (no change in affinity constants), but affects other parts of the transmembrane signal system leading to the intracellular production of cyclic AMP (e.g. regulatory binding proteins or increases in the number of receptors).