Partial Agonism and Functional Selectivity: A Study on β-Adrenoceptor Mediated Effects in Tracheal,Cardiac and Skeletal Muscle

Abstract: Colterol, procaterol, sulfonterol, terbutaline and three monophenolic derivatives of terbutaline were examined with respect to their ability to react in vitro on β-adrenoceptors in tissues isolated from guinea-pig. The effects measured were a) relaxation of the tracheal smooth muscle (mostly β₂), b) depression of subtetanic contractions of the soleus muscle (β₂), and c) increase in the force of the papillary muscle of the left ventricle (β₁). 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 β₂-selectivity, in general characterized by a higher intrinsic activity at β₂- than at β₁-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 β₁-adrenoceptors is an important factor for functional selectivity of β₂-adrenoceptor agonists. On the other hand there seems to be no useful differences between the maximum effect elicited by a partial β₂-adrenoceptor agonist on the skeletal muscle as compared with airway smooth muscle.     

The interaction between salmeterol and beta 2-adrenoceptor agonists with higher efficacy on guinea-pig trachea and human bronchus in vitro.

1. In guinea-pig tracheal preparations precontracted with 1 mumol l-1 carbachol, formoterol, procaterol, fenoterol, salmefamol, salbutamol and terbutaline (in that order of potency) caused a concentration-dependent and almost complete, relaxation. However, under these conditions, the maximum relaxation by salmeterol was approximately 30% of the maximum attainable relaxation. 2. We have therefore explored the ability of salmeterol to inhibit the relaxant response to beta 2-adrenoceptor agonists of different chemical structure and relatively higher efficacy in smooth muscle preparations from guinea-pig trachea and human bronchus. 3. With 1 mumol l-1 salmeterol in the organ bath, the concentration-effect curves for the other agonists were shifted to the right in a variable way by 1.8-2.8 log units, fenoterol and salbutamol being the extremes. 4. When 20 mumol l-1 sulfonterol, another low efficacy beta 2-adrenoceptor agonist, was substituted for salmeterol, the difference in the magnitude of the rightward shift between fenoterol and salbutamol was eliminated. 5. In the human bronchus, formoterol and terbutaline had a higher apparent efficacy than salmeterol. With 1 mumol l-1 salmeterol in the organ bath, the concentration-effect curve for formoterol was shifted 2.7 log units to the right. 6. Salmeterol inhibits, competitively, relaxant responses to beta 2-adrenoceptor agonists with higher efficacy. The degree of inhibition seems to be dependent on the agonist used. This contrasts with results obtained with sulfonterol and suggests that salmeterol interacts with the beta 2-adrenoceptor in a complex way.     

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.     

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.     

Is there a third heart beta-adrenoceptor?

A series of partial agonists with high affinity for myocardial beta 1- and beta 2-adrenoceptors cause stimulant effects in heart that are resistant to blockade of beta 1- and beta 2-adrenoceptors. The concentrations of partial agonist that cause stimulant effects greatly exceed those that cause blockade. Alberto Kaumann suggests that such non-conventional partial agonists, often analogues of pindolol, may act through a third heart beta-adrenoceptor, which resembles the beta 3-adrenoceptor of white adipocytes and smooth muscle of airways and ileum.     

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.     

The in vitro pharmacology of xamoterol (ICI 118,587).

The effect of xamoterol and (-)-isoprenaline have been compared for their activity at beta-adrenoceptor sites in a number of in vitro cardiac and smooth muscle preparations. Xamoterol produced weak positive chronotropic effects in guinea-pig, rat and cat atria (intrinsic activity less than 0.55, (-)-isoprenaline = 1). Positive inotropic effects were obtained in driven left atria of the cat but were absent in guinea-pig left atrial and right ventricular strip preparations. Agonistic effects were due to beta 1-adrenoceptor stimulation. Xamoterol was without beta-adrenoceptor-mediated inhibitory effects in guinea-pig ileal, tracheal and uterine preparations and in the rat vas deferens and oestrogen-primed uterus. Weak beta 2-adrenoceptor-mediated relaxation was obtained in progesterone-primed rat uteri. Xamoterol produced non-specific inhibitory effects in guinea-pig ileal and tracheal preparations. Xamoterol acted as a competitive antagonist at beta 1-(pA2 range = 7.4 to 7.8) and beta 2-adrenoceptors (pA2 range 5.2 to 6.2) and displaced [125I]-iodocyanopindolol from guinea-pig left atrial (pKD = 7.25) and uterine (pKD 5.24) membrane preparations. It is concluded that xamoterol displays a selective affinity for beta 1-adrenoceptors. Although its partial agonistic actions are more evident at beta 1-adrenoceptor sites, like prenalterol, xamoterol displays a degree of tissue rather than receptor-dependent selectivity.     

Demonstration of the beta 2-adrenoceptor intrinsic efficacy of AY-28,925 using the PGF2 alpha-contracted guinea-pig trachea

The abilities of AY-28,925, labetalol and medroxalol to relax the PGF2 alpha-contracted isolated guinea-pig trachea have been investigated to compare their activities at beta 2-adrenoceptors. Maximum relaxation induced by AY-28,925 was significantly greater than that induced by either labetalol or medroxalol. This relaxation occurred in a concentration-dependent manner over a range of concentrations consistent with the previously determined affinity of AY-28,925 for beta-adrenoceptors. ICI-118,551 inhibited AY-28,925-induced relaxation in a concentration-dependent manner with a pA2 value similar to that determined for ICI-118,551 inhibition of the selective beta 2-adrenoceptor agonist salbutamol, but not the selective beta 1-adrenoceptor agonist norepinephrine. The Schild plot slope for ICI-118,551 inhibition of AY-28,925 or salbutamol did not differ significantly from unity, while that for inhibition of isoproterenol (a non-selective beta-adrenoceptor agonist) did. It is concluded that AY-28,925 is a more efficacious relaxant of tracheal smooth muscle than either labetalol or medroxalol, and that this relaxant activity is the result of its greater intrinsic efficacy at the beta 2-adrenoceptor.     

PHOSPHODIESTERASE IV INHIBITORS SYNERGISTICALLY POTENTIATE RELAXATION INDUCED BY FORSKOLIN IN GUINEA-PIG TRACHEA

1. β-Adrenoceptor receptor agonists are the principal bronchodilator agents used in the treatment of bronchial asthma. However, the regular use of β-adrenoceptor agonists in asthmatic patients is likely to increase asthma severity because of a defective β-adrenoceptor function. Bronchodilators that bypass this defective function are therefore needed. 2. Our objectives in this study were: (i) to assess the effects of an agent that directly activates adenylate cyclase (forskolin) on guinea-pig tracheal smooth muscle; (ii) to study the interactions between selective cyclic nucleotide phosphodiesterase (PDE) inhibitors and forskolin by measuring isometric tension; and (iii) to compare these results with the interaction between PDE inhibitors and terbutaline, a β-adrenoceptor agonist. 3. The relaxant effects of forskolin alone, which is now under development as a new bronchodilator for bronchial asthma therapy, were slightly weaker than those of terbutaline on guinea-pig tracheal smooth muscle. 4. Both denbufylline and Ro 20–1724, cyclic nudeotide PDEIV inhibitors, synergistically increased the relaxant effects of forskolin and terbutaline, while other PDE isozyme inhibitors (amrinone, vinpocetine and zaprinast) had only a minor influence. 5. In conclusion, a good synergistic interaction between forskolin and PDE IV inhibitors, especially denbufylline, may provide a means for bypassing β-adrenoceptors. Thus, the combination of forskolin and PDE inhibitors would become useful in the treatment of bronchial asthma.     

Beta 1-adrenoceptors mediate smooth muscle relaxation in mouse isolated trachea.

1. The relaxant effects to the beta-adrenoceptor agonists isoprenaline, adrenaline, noradrenaline, RO363, procaterol and fenoterol were investigated in carbachol-contracted mouse isolated tracheal preparations. 2. The order of potencies for those beta-adrenoceptor agonists that induced full relaxation of carbachol-contracted mouse tracheal preparations was isoprenaline greater than RO363 greater than noradrenaline = adrenaline greater than fenoterol. The EC50 value of isoprenaline for relaxation was 46 nM. The beta 1-adrenoceptor-selective agonist, RO363 was ten times more potent than the beta 2-adrenoceptor-selective agonist, fenoterol. The highly beta 2-adrenoceptor-selective agonist procaterol was a partial relaxant and induced only 28 +/- 4% relaxation. 3. Relaxations induced by noradrenaline and isoprenaline were not significantly affected by the neuronal uptake inhibitor, cocaine (10 microM) or by the extraneuronal uptake inhibitor, deoxycorticosterone acetate (25 microM) respectively. The alpha-adrenoceptor agonist methoxamine induced no observable elevation of mouse tracheal smooth muscle tone. 4. Schild plots for the beta-adrenoceptor antagonists, atenolol and betaxolol (beta 1-adrenoceptor-selective) and ICI 118,551 (beta 2-adrenoceptor-selective) were linear, with slope values approaching unity. Mean pA2 values derived for atenolol, betaxolol and ICI 118,551 for antagonism of beta-adrenoceptor-mediated relaxation were 7.1, 8.4 and 7.2, respectively. These data were independent of the use of isoprenaline or noradrenaline as the agonist. 5. These findings indicate that beta-adrenoceptor-mediated relaxations of mouse isolated trachea occur predominantly through activation of beta 1-adrenoceptors.     

Effects of beta-adrenoceptor agonists in human bronchial smooth muscle.

1. We have investigated the potency and duration of action of isoprenaline and a range of beta-adrenoceptor agonists as relaxants of inherent tone in human superfused, isolated bronchial smooth muscle, a tissue reported to contain a homogeneous population of beta 2-adrenoceptors. 2. All of the beta-adrenoceptor agonists caused concentration-related inhibition of inherent tone, with isoprenaline having an EC50 of 27 nM. The rank order of agonist potency was: formoterol > or = -salmeterol > or = clenbuterol > fenoterol = isoprenaline > terbutaline > or = salbutamol > quinprenaline. 3. Relaxant responses to salmeterol were fully reversed by the selective beta 2-adrenoceptor blocking drug, ICI 118551, demonstrating the involvement of beta 2-adrenoceptors. 4. Rt50, i.e. the time taken for 50% recovery from the effects of an EC50 concentration of agonist, differed considerably between the different beta 2-adrenoceptor agonists. Most agonists were short-acting, having Rt50 values less than 13 min. Quinprenaline was of moderate duration, with an Rt50 value of > or = 20 min. In contrast, salmeterol was extremely long-acting, with no sign of recovery within 4 h. 5. Estimates of relative potency and duration of action were similar to those previously determined for these agonists in the guinea-pig isolated trachea. These results suggest, therefore, that guinea-pig trachea is a suitable alternative to human bronchus for the evaluation of the actions of beta-adrenoceptor agonists on airways smooth muscle.     

Beta 3-adrenoceptor-mediated relaxation of guinea-pig gastric funds smooth muscle: cAMP-independent characteristics and a primary role of 4-aminopyridine-sensitive voltage-dependent K+ (Kv) channels

beta-Adrenoceptor subtypes which mediate relaxation of guinea-pig gastrointestinal smooth muscles in response to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline) and beta 3-adrenoceptor agonists (BRL37344 and (+/-)-CGP12177A) are predominantly beta 3-adrenoceptors. Although cAMP-PKA system is thought to play a substantial role in the smooth muscle relaxation mediated via beta 1- and beta 2-adrenoceptors, there is little information on the role of cAMP in beta 3-adrenoceptor-meidated relaxation. The present study was carried out to elucidate the role of cAMP in beta 3-adrenoceptor-meidated relaxation of guinea-pig gastric fundus smooth muscle. Furthermore, possible contribution of two types of K+ (voltage-dependent and Ca(2+)-activated K+, BKCa; voltage-dependent, Kv) channels was also examined pharmaco-mechanically. In gastric fundus smooth muscle, catecholamines and beta 3-adrenoceptor agonists elicited potent relaxations in the presence of beta 1- and beta 2-adrenoceptor antagonists. All of these relaxations were not diminished by an adenylate cyclase inhibitor, SQ-22,536 (100 microM), which indicates their characteristic of cAMP-independency. SQ-22,536-resistant, beta 3-adrenoceptor-mediated relaxations were strongly attenuated by a Kv channel blocker, 4-aminopyridine (3 mM), but not by iberiotoxin (100 nM), a selective blocker of BKCa channel. The present results indicate that 4-aminopyridine-sensitive Kv channels play a primary role in cAMP-independent relaxation of guinea-pig gastric fundus smooth muscle in response to the stimulations of beta 3-adrenoceptors.     

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.     

Effects of fenoterol on beta-adrenoceptor and muscarinic M2 receptor function in bovine tracheal smooth muscle.

Prolonged (18 h) incubation of isolated bovine tracheal smooth muscle with the beta2-adrenoceptor agonist fenoterol (10 microM) induced desensitization of isoprenaline-induced adenylyl cyclase activity in bovine tracheal smooth muscle membranes, characterized by a 25% decrease in maximal effect (Emax) (P < 0.05), while the sensitivity to the agonist (pEC50) was unchanged. The Emax value of isoprenaline-induced smooth muscle relaxation of submaximal methacholine-induced contractile tones was similarly reduced by about 25% (P < 0.001), while the pEC50 value was diminished by 1.0 log unit (P < 0.001). As determined by 30 microM gallamine-induced muscarinic M2 receptor antagonism and pertussis toxin-induced inactivation of G(i alpha), muscarinic M2 receptor-mediated functional antagonism did not play a role in isoprenaline-induced relaxation of bovine tracheal smooth muscle contracted by methacholine, both in control and in 18-h fenoterol-treated tissue. In line with these observations, we found no enhanced muscarinic M2 receptor-mediated inhibition of 1 microM forskolin-stimulated adenylyl cyclase activity after 18-h fenoterol treatment. These data indicate that 18-h fenoterol treatment of bovine tracheal smooth muscle induces beta2-adrenoceptor desensitization and reduced functional antagonism of methacholine-induced contraction by beta-adrenoceptor agonists, without a change of muscarinic M2 receptor function.     

Evidence for the presence of beta-3-adrenoceptors mediating relaxation in the human oviduct

Beta1- and beta2-adrenoceptors mediate relaxation in the oviductal smooth muscle. This study examines the existence and function of beta3-adrenoceptors in the human oviduct. Ring segments of the oviduct were set up for isometric tension recording. The effect of isoprenaline and BRL 37344 on smooth muscle tone was examined. The expression of beta3-adrenoceptors in the oviduct was also examined. Isoprenaline and BRL 37344 concentration-dependently relaxed circular muscles of the oviduct. BRL 37344 was less potent than isoprenaline and was a partial agonist. Propranolol shifted isoprenaline but not BRL 37344 concentration-response curve to the right without reducing the maximum response. Cyanopindolol (1 micromol/l), a beta3-adrenoceptor antagonist, shifted the isoprenaline concentration response curve to the right. The -log K(B) value of 7.8 indicates activation of beta3-adrenoceptors by isoprenaline. mRNA for beta3-adrenoceptors was expressed in the oviduct. These results suggest that beta3-adrenoceptors, mediating relaxation, are expressed in the human oviduct.     

α-Methyl-5-HT, a 5-HT2 receptor agonist, stimulates β2-adrenoceptors in guinea pig airway smooth muscle

Alpha-methyl-5-HT is widely used as a high-affinity 5-HT(2) receptors agonist, though some studies have postulated that this drug also activates other serotonergic receptors. In the present work, we found that a wide range of concentrations of alpha-methyl-5-HT induced biphasic responses (contraction followed by relaxation) in guinea pig tracheal rings. The relaxing phase caused by 32microM alpha-methyl-5-HT was blocked by 0.1microM propranolol. Furthermore, during an ongoing histamine-induced contraction, alpha-methyl-5-HT (0.1-100microM) produced a concentration-dependent relaxation starting at 10microM. This relaxation was fully abolished by 0.1microM propranolol or 1microM ICI 118,551 (a selective beta(2)-adrenoceptor antagonist). Additionally, in electrophysiological recordings, 32microM alpha-methyl-5-HT also enhanced the membrane K(+) currents of single tracheal myocytes, an effect reverted by propranolol and ICI 118,551, and mimicked by 0.1microM salbutamol. Thus, we concluded that alpha-methyl-5-HT activates beta(2)-adrenoceptors in guinea pig tracheal smooth muscle at concentrations >or=10microM. This effect must be taken into account when this drug is used in airway smooth muscle and in other tissues expressing beta(2)-adrenoceptors.     

Atypical responses of rat ileum to pindolol, cyanopindolol and iodocyanopindolol.

1. Pindolol, cyanopindolol (CYP) and iodocyanopindolol (IodoCYP) have been reported to act either as antagonists, agonists or partial agonists at the beta 3-adrenoceptor in different preparations. A comprehensive investigation has not yet been described with these compounds tested in one tissue from one species. This study was conducted to delineate the pharmacological effects of pindolol, CYP and IodoCYP and to provide data on their affinities at the predominant beta-adrenoceptor in rat ileum. 2. The beta-adrenoceptors present in rat ileum were characterized in the presence of CGP 20712A and ICI 118 551, atropine and corticosterone, with (-)-isoprenaline used as an agonist. The role of the beta 1 and beta 2-adrenoceptors was determined by the omission of either CGP 20712A, ICI 118 551, or both, from the buffers. Conversely, the effectiveness of the beta 1- and beta 2-adrenoceptor blockade was examined by use of the beta 1-adrenoceptor-selective agonist, RO 363 and the beta 2-adrenoceptor-selective agonist, salbutamol. 3. There was no evidence for the presence of functional beta 1-adrenoceptors, and no strong evidence that beta 2-adrenoceptor stimulation contributed to the relaxant effects of (-)-isoprenaline. (-)-Phenylephrine did not produce relaxation of the tissue and 5-hydroxytryptamine produced contraction. 4. The beta 3-adrenoceptor-selective agonist, BRL 37344 and (-)-isoprenaline were potent full agonists (pD2 8.35 +/- 0.04 and 7.76 +/- 0.14 respectively), whereas ICI D7114 was less potent (pseudo pD2 6.92 +/- 0.15). These results indicate that the predominant functional beta-adrenoceptors in rat ileum are beta 3-adrenoceptors. 5. Partial agonist effects were produced by CYP (pD2 5.28 +/- 0.26) and IodoCYP (pD2 7.0 +/- 0.26), but not pindolol. All three compounds antagonized the effects of (-)-isoprenaline with pKb values of 6.68 +/- 0.10, 7.59 +/- 0.07 and 7.59 +/- 0.11 for pindolol, CYP and IodoCYP respectively. Likewise, CYP and IodoCYP antagonized the effects of BRL 37344 with pKb values of 7.20 +/- 0.22 and 7.21 +/- 0.14 respectively. This study provides the first functional data on the effects of IodoCYP, the ligand with the highest known affinity for the beta 3-adrenoceptor, at the characterized rat ileum beta 3-adrenoceptor. 6. In conclusion, whereas pKb values suggest that CYP and IodoCYP have a similar affinity for the beta 3-adrenoceptor in rat ileum, the higher potency of IodoCYP suggests that it promotes a greater coupling efficiency, or that its partial agonist effects are produced through a site other than the beta 3-adrenoceptor. The similar pKb values for CYP and IodoCYP at the beta 3-adrenoceptor contrast with their order of known affinities at the beta 1- and beta 2-adrenoceptors, where IodoCYP is far more potent than CYP. This provides evidence of further differences in the characteristics of the beta 3-adrenoceptors compared to the beta 1- and beta 2-adrenoceptors. Finally, the utility of IodoCYP as a beta 3-adrenoceptor antagonist would appear to be limited because of the greater magnitude of partial agonist effects that it produces.     

Beta1- and beta3-adrenoceptors mediate relaxation in ovine trachealis smooth muscle

Isoprenaline (non-selective) and noradrenaline (beta1-selective) concentration-dependently relaxed ovine tracheal strips precontracted with carbachol. The pD2 values were 7.07 +/- 0.08 and 6.13 +/- 0.10 for isoprenaline and noradrenaline, respectively. In the same preparation, salbutamol either produced weak relaxation or in some cases, contractile responses indicating the presence of very little or no beta2-adrenoceptors in this preparation. Isoprenaline-and noradrenaline-induced relaxations were antagonized by propranolol and atenolol with pA2 values in the range reported in the literature for an action on beta1-adrenoceptors. ICI 118551 also antagonized isoprenaline- and noradrenaline-induced relaxation but at concentrations much higher than are required to block beta2-adrenoceptors, confirming that beta2-adrenoceptors do not contribute significantly to these responses. The selective beta3-adrenoceptor agonist, BRL 37344A produced concentration-dependent relaxation of tracheal strips. BRL 37344A was a full agonist producing 100% relaxation of carbachol-induced tone. BRL 37344A-induced relaxation was weakly antagonized by propranolol confirming an action, mainly, on beta3-adrenoceptors. Cyanopindolol antagonized isoprenaline-induced relaxation (in the presence of propranolol, 10(-7) M) with a pA2 value of 8.06 +/- 0.24. It was therefore concluded that beta1- and beta3-adrenoceptors mediated agonist-induced relaxation in sheep tracheal strips.     

SELECTIVE DEVELOPMENT OF TOLERANCE TO β-ADRENOCEPTOR AGONISTS IN SKELETAL MUSCLE AS COMPARED WITH AIRWAY SMOOTH MUSCLE FROM THE GUINEA-PIG

1. Guinea-pig were fed with a diet containing terbutaline or placebo for 4--5 days. The trachea, soleus muscle and the extensor digitorum longus (EDL) from these animals were prepared for recording of isometric contractions in vitro. 2. After treatment with terbutaline in vivo, the response of the pilocarpine-contracted trachea to terbutaline and isoprenaline was slightly suppressed with no change in maximum relaxation. 3. After treatment with terbutaline in vivo the maximum depression of the incomplete tetanic contractions of the soleus muscle brought about by terbutaline or isoprenaline was diminished by about 70%. The response of the EDL was also attenuated after previous treatment with terbutaline in vivo. 4. These data indicate a selective development of tolerance to the effects of beta-adrenoceptor agonists in skeletal muscle as compared with tracheal smooth muscle. 5. The present results provide an experimental analogue to the clinical observation that patients being treated with beta-adrenoceptor agonists become tolerant to the tremorogenic rather than to the bronchodilating effect.     

Characterization of the beta-adrenergic inhibition of motility in cat colon strips

The inhibition of spontaneous contractile activity in isolated cat colon strips by isoprenaline, prenalterol and terbutaline was studied. Isoprenaline produced a concentration-dependent complete inhibition with an EC50 of 6.18 nM. Prenalterol and terbutaline produced a concentration-dependent partial inhibition of 68 and 75% respectively, with EC50 of 0.52 and 0.43 microM respectively. The inhibitory effects of the three agonists were blocked by metoprolol and IPS 339 in such a way as to suggest that the prenalterol effect was mediated by beta 1-adrenoceptors and the terbutaline effect was mediated by beta 2-adrenoceptors, whereas the effect of isoprenaline appeared to be mediated by both types of receptors. Thus, the results indicate that both beta 1- and beta 2-adrenoceptors are involved in the adrenergic inhibition of spontaneous contractile activity in the cat colon strip. Qualitative effects of prenalterol and terbutaline on the spontaneous contractile activity in the cat colon strip indicated that the beta 1-adrenoceptor-mediated effects may be related to inhibition of the activity in cholinergic neurons, whereas the beta 2-adrenoceptors may be acting on the smooth muscle cells per se.