The new radioligand [(3)H]-L 748,337 differentially labels human and rat β3-adrenoceptors

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

The β3 adrenoceptor in proliferative retinopathies: "Cinderella" steps out of its family shadow

In the retina, hypoxic condition leads to overgrowing leaky vessels resulting in altered metabolic supply that may cause impaired visual function. Hypoxia-inducible factor-1 (HIF-1) is a central regulator of the retinal response to hypoxia by activating the transcription of numerous target genes, including vascular endothelium growth factor, which acts as a major player in retinal angiogenesis. In the present review, oxygen urge by the retina and its oxygen sensing systems including HIF-1 are discussed in respect to the role of the beta-adrenergic receptors (β-ARs) and their pharmacologic manipulation in the vascular response to hypoxia. In the β-AR family, β1- and β2-AR have long been attracting attention because their pharmacology is intensely used for human health, while β3-AR, the third and last cloned receptor is no longer increasingly emerging as an attractive target for drug discovery. Here, β3-AR, a main character in several organs including the heart, the adipose tissue and the urinary bladder, but so far a supporting actor in the retina, has been thoroughly examined in respect to its function in retinal response to hypoxia. In particular, its oxygen dependence has been taken as a key indicator of β3-AR involvement in HIF-1-mediated responses to oxygen. Hence, the possibility of β3-AR transcription by HIF-1 has been discussed from early circumstantial evidence to the recent demonstration that β3-AR acts as a novel HIF-1 target gene by playing like a putative intermediary between oxygen levels and retinal vessel proliferation. Thus, targeting β3-AR may implement the therapeutic armamentarium against neovascular pathologies of the eye.

Antagonism of α1-adrenoceptors by β3-adrenergic agonists: Structure-function relations of different agonists in prostate smooth muscle contraction

Effects of β₃-adrenergic agonists on prostate smooth muscle contraction are poorly characterized, although mirabegron is used for treatment of lower urinary tract symptoms. Off-target effects of several β₃-adrenergic agonists include antagonism of α₁-adrenoceptors. Proposed, but unconfirmed explanations include phenylethanolamine backbones, found in some β₃-adrenergic agonists and imparting interaction with catecholamine binding pockets of adrenoceptors. Here, we examined effects of β₃-adrenergic agonists on contractions of human prostate tissues, including ZD7114 (without phenylethanolamine moiety), ZD2079 (phenylethanolamine backbone), BRL37344 and CL316243 (chloride-substituted phenylethanolamine deriatives). Prostate tissues were obtained from radical prostatectomy. Contractions by α₁-adrenergic agonists and electric field stimulation (EFS) were studied in an organ bath. ZD7114 (10 µM) right-shifted concentration responses curves for α₁-adrenergic agonists, resulting in increased EC₅₀ values for phenylephrine, methoxamine and noradrenaline up to one magnitude, without affecting E_max values. ZD7114 (10 µM) inhibited EFS-induced contractions, resulting in reduced E_max values. All effects of ZD7114 were resistant to the β₃-adrenergic antagonist L-748337, including increases in EC₅₀ values for α₁-adrenergic agonists, up to more than two magnitudes. Using 10 µM, neither ZD2079, BRL37344 or CL316243 affected α₁-adrenergic or EFS-induced contractions. At escalated concentrations, BRL37344 (200 µM) right-shifted concentration response curves for phenylephrine, increased EC₅₀ values for phenylephrine, and inhibited EFS-induced contractions, while CL316243 (300 µM) did not affect phenylephrine- or EFS-induced contractions. In conclusion, phenylethanolamine backbones are not decisive to impart α₁-adrenoceptor antagonism to β₃-agonists. Effects of β₃-adrenergic agonists on prostate smooth muscle contraction are limited to off-target effects, including α₁-adrenoceptor antagonism by ZD7114 and BRL37344.

β3-Adrenergic receptors regulate human brown/beige adipocyte lipolysis and thermogenesis

β3-Adrenergic receptors (β3-ARs) are the predominant regulators of rodent brown adipose tissue (BAT) thermogenesis. However, in humans, the physiological relevance of BAT and β3-AR remains controversial. Herein, using primary human adipocytes from supraclavicular neck fat and immortalized brown/beige adipocytes from deep neck fat from 2 subjects, we demonstrate that the β3-AR plays a critical role in regulating lipolysis, glycolysis, and thermogenesis. Silencing of the β3-AR compromised genes essential for thermogenesis, fatty acid metabolism, and mitochondrial mass. Functionally, reduction of β3-AR lowered agonist-mediated increases in intracellular cAMP, lipolysis, and lipolysis-activated, uncoupling protein 1-mediated thermogenic capacity. Furthermore, mirabegron, a selective human β3-AR agonist, stimulated BAT lipolysis and thermogenesis, and both processes were lost after silencing β3-AR expression. This study highlights that β3-ARs in human brown/beige adipocytes are required to maintain multiple components of the lipolytic and thermogenic cellular machinery and that β3-AR agonists could be used to achieve metabolic benefit in humans.

Effect of β1 /β2 -adrenoceptor blockade on β3 -adrenoceptor activity in the rat cremaster muscle artery

BACKGROUND AND PURPOSE

The physiological role of vascular β₃ -adrenoceptors is not fully understood. Recent evidence suggests cardiac β₃ -adrenoceptors are functionally effective after down-regulation of β₁ /β₂ -adrenoceptors. The functional interaction between the β₃ -adrenoceptor and other β-adrenoceptor subtypes in rat striated muscle arteries was investigated.

EXPERIMENTAL APPROACH

Studies were performed in cremaster muscle arteries isolated from male Sprague-Dawley rats. β-adrenoceptor expression was assessed through RT-PCR and immunofluorescence. Functional effects of β₃ -adrenoceptor agonists and antagonists and other β-adrenoceptor ligands were measured using pressure myography.

KEY RESULTS

All three β-adrenoceptor subtypes were present in the endothelium of the cremaster muscle artery. The β₃ -adrenoceptor agonists mirabegron and CL 316,243 had no effect on the diameter of pressurized (70 mmHg) cremaster muscle arterioles with myogenic tone, while the β₃ -adrenoceptor agonist SR 58611A and the nonselective β-adrenoceptor agonist isoprenaline caused concentration-dependent dilation. In the presence of β_1/2 -adrenoceptor antagonists nadolol (10 μM), atenolol (1 μM) and ICI 118,551 (0.1 μM) both mirabegron and CL 316,243 were effective in causing vasodilation and the potency of SR 58611A was enhanced, while responses to isoprenaline were inhibited. The β₃ -adrenoceptor antagonist L 748,337 (1 μM) inhibited vasodilation caused by β₃ -adrenoceptor agonists (in the presence of β_1/2 -adrenoceptor blockade), but L 748,337 had no effect on isoprenaline-induced vasodilation.

CONCLUSION AND IMPLICATIONS

All three β-adrenoceptor subtypes were present in the endothelium of the rat cremaster muscle artery, but β₃ -adrenoceptor mediated vasodilation was only evident after blockade of β_1/2 -adrenoceptors. This suggests constitutive β_1/2 -adrenoceptor activity inhibits β₃ -adrenoceptor function in the endothelium of skeletal muscle resistance arteries.

Mirabegron elicits rat corpus cavernosum relaxation and increases in vivo erectile response

Mirabegron is the first β3-adrenoceptor agonist approved on the market and may offer beneficial pharmacological action in patients with overactive bladder and erectile dysfunction. Here, we further investigate the mechanisms by which mirabegron induces rat corpus cavernosum (CC) relaxation. Adult male Wistar rats were used. The CC were isolated for in vitro functional assays and β-adrenoceptors subtypes mRNA expression evaluation. Animals were treated orally with mirabegron (30 mg/kg, 3 h), tadalafil (10 mg/kg, 3 h) or both for intracavernous pressure (ICP). Intracellular levels of cAMP and cGMP were also determined. The β₁-, β₂- and β₃-adrenoceptors subtypes were expressed in rat CC. Mirabegron produced concentration-dependent CC relaxations that were unaffected by the β₁-, β₂- or β₃-adrenoceptor antagonists atenolol (1 μM), ICI-118,551 (1 μM) and L748,337 (10 μM), respectively. Mirabegron-induced relaxations were not affected by the phosphodiesterase type 4 inhibitor, rolipram, or the adenylyl cyclase selective inhibitor, SQ 22,536. Potassium channel- or calcium influx-blockade are not involved in mirabegron-induced relaxations. In contrast, mirabegron produced rightward shifts in the contractile response induced by the α1-adrenoceptor agonist, phenylephrine. Finally, cavernous nerve stimulation caused frequency-dependent ICP increases, which were significantly increased in rats treated with mirabegron in a similar degree of tadalafil-treated rat, without promoting a significant cAMP or cGMP accumulation. Together, our results demonstrate that mirabegron induced CC relaxation through α1-adrenoceptor blockade. Care should be taken to translate the effect of mirabegron into the clinic, especially when using rat as an animal model of erectile dysfunction.

Desensitization of cAMP Accumulation via Human β3-Adrenoceptors Expressed in Human Embryonic Kidney Cells by Full, Partial, and Biased Agonists

β₃-Adrenoceptors couple not only to cAMP formation but, at least in some cell types, also to alternative signaling pathways such as phosphorylation of extracellular signal-regulated kinase (ERK). β₃-Adrenoceptor agonists are used in long-term symptomatic treatment of the overactive bladder syndrome; it is only poorly understood which signaling pathway mediates the clinical response and whether it undergoes agonist-induced desensitization. Therefore, we used human embryonic kidney cells stably transfected with human β₃-adrenoceptors to compare coupling of ligands with various degrees of efficacy, including biased agonists, to cAMP formation and ERK phosphorylation, particularly regarding desensitization. Ligands stimulated cAMP formation with a numerical rank order of isoprenaline ≥ L 755,507 ≥ CL 316,243 > solabegron > SR 59,230 > L 748,337. Except for the weakest agonist, L 748,337, pretreatment with any ligand reduced cAMP responses to freshly added isoprenaline or forskolin to a similar extent. On the other hand, we were unable to detect ERK phosphorylation despite testing a wide variation of conditions. We conclude that a minor degree of efficacy for cAMP formation may be sufficient to induced full desensitization of that response. Transfected human embryonic kidney cells are not suitable to study desensitization of ERK phosphorylation by β₃-adrenoceptor stimulation.

Cardiac β3 -adrenoceptors-A role in human pathophysiology?

As β₃ -adrenoceptors were first demonstrated to be expressed in adipose tissue they have received much attention for their metabolic effects in obesity and diabetes. After the existence of this subtype had been suggested to be present in the heart, studies focused on its role in cardiac function. While the presence and functional role of β₃ -adrenoceptors in the heart has not uniformly been detected, there is a broad consensus that they become up-regulated in pathological conditions associated with increased sympathetic activity such as heart failure and diabetes. When detected, the β₃ -adrenceptor has been demonstrated to mediate negative inotropic effects in an inhibitory G protein-dependent manner through the NO-cGMP-PKG signalling pathway. Whether these negative inotropic effects provide protection from the adverse effects induced by overstimulation of β₁ /β₂ -adrenoceptors or in themselves are potentially harmful is controversial, but ongoing clinical studies in patients with congestive heart failure are testing the hypothesis that β₃ -adrenceptor agonism has a beneficial effect. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Everything You Always Wanted to Know about β3-AR * (* But Were Afraid to Ask)

The beta-3 adrenergic receptor (β₃-AR) is by far the least studied isotype of the beta-adrenergic sub-family. Despite its study being long hampered by the lack of suitable animal and cellular models and inter-species differences, a substantial body of literature on the subject has built up in the last three decades and the physiology of β₃-AR is unraveling quickly. As will become evident in this work, β₃-AR is emerging as an appealing target for novel pharmacological approaches in several clinical areas involving metabolic, cardiovascular, urinary, and ocular disease. In this review, we will discuss the most recent advances regarding β₃-AR signaling and function and summarize how these findings translate, or may do so, into current clinical practice highlighting β₃-AR’s great potential as a novel therapeutic target in a wide range of human conditions.

Agonist-induced desensitisation of β3 -adrenoceptors: Where, when, and how?

β₃ -Adrenoceptor agonists have proven useful in the treatment of overactive bladder syndrome, but it is not known whether their efficacy during chronic administration may be limited by receptor-induced desensitisation. Whereas the β₂ -adrenoceptor has phosphorylation sites that are important for desensitisation, the β₃ -adrenoceptor lacks these; therefore, it had been assumed that β₃ -adrenoceptors are largely resistant to agonist-induced desensitisation. While all direct comparative studies demonstrate that β₃ -adrenoceptors are less susceptible to desensitisation than β₂ -adrenoceptors, desensitisation of β₃ -adrenoceptors has been observed in many models and treatment settings. Chimeric β₂ - and β₃ -adrenoceptors have demonstrated that the C-terminal tail of the receptor plays an important role in the relative resistance to desensitisation but is not the only relevant factor. While the evidence from some models, such as transfected CHO cells, is inconsistent, it appears that desensitisation is observed more often after long-term (hours to days) than short-term (minutes to hours) agonist exposure. When it occurs, desensitisation of β₃ -adrenoceptors can involve multiple levels including down-regulation of its mRNA and the receptor protein and alterations in post-receptor signalling events. The relative contributions of these mechanistic factors apparently depend on the cell type under investigation. Which if any of these factors is applicable to the human urinary bladder remains to be determined. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Mirabegron, a β3-adrenoceptor agonist reduced platelet aggregation through cyclic adenosine monophosphate accumulation

Mirabegron is a β₃-adrenoceptor agonist and released on the marked for the treatment of overactive bladder. Because mirabegron is the only β₃-adrenoceptor agonist available and substances that increase the levels of cyclic adenosine monophosphate (cAMP) inhibit platelet activity, we tested the hypothesis that mirabegron could have antiplatelet activity. Collagen- and thrombin induced platelet aggregation, thromboxane B2 (TXB₂) and cyclic nucleotides quantification and calcium (Ca²⁺) mobilization were determined in the absence and presence of mirabegron in human washed platelets. Our results revealed that mirabegron (10-300 µM) produced significant inhibitions on platelet aggregation induced by collagen- or thrombin, accompanied by greater intracellular levels of cAMP. The β₃-adrenoceptor antagonist L 748,337 (1 µM) and the adenylate cyclase inhibitor, SQ 22,536 (100 µM) reversed the inhibition induced by mirabegron in thrombin-stimulated platelets. The selective antagonists for β₁-and β₂-adrenoceptors, atenolol and ICI 117,551 (3 µM), respectively did not interfere on the inhibition induced by mirabegron. In Fluo-4 loaded platelets, mirabegron reduced the total and intracellular Ca²⁺ levels. Pre-incubation with mirabegron almost abolished the levels of TXB2. Mirabegron did not augment the intracellular levels of cyclic guanosine monophosphate. In conclusion, mirabegron inhibited human platelet aggregation through cAMP accumulation, thus suggesting that substances that activate β₃-adrenoceptor could be beneficial as adjuvant antiplatelet therapy.

β-Adrenoceptor-mediated Relaxation of Urinary Bladder Muscle in β2-Adrenoceptor Knockout Mice

Background and Objective: In order to characterize the β-adrenoceptor (AR) subtypes involved in agonist-stimulated relaxation of murine urinary bladder we studied the effects of (-)-isoprenaline and CL 316,243 on tonic contraction and spontaneous contractions in detrusor strips of wild-type (WT) and β₂-AR knockout (β₂-AR KO) mice.

Materials and Methods: Urinary bladders were isolated from male WT and β₂-AR KO mice. β-AR subtype expression was determined with quantitative real-time PCR. Intact muscle strips pre-contracted with KCl (40 mM) were exposed to cumulatively increasing concentrations of (-)-isoprenaline or β₃-AR agonist CL 316,243 in the presence and absence of the subtype-selective β-AR blockers CGP 20712A (β₁-ARs), ICI 118,551 (β₂-ARs), and L748,337 (β₃-ARs).

Results: Quantitative real-time PCR confirmed lack of β₂-AR expression in bladder tissue from β₂-AR KO mice. In isolated detrusor strips, pre-contraction with KCl increased basal tone and enhanced spontaneous activity significantly more in β₂-AR KO than in WT. (-)-Isoprenaline relaxed tonic tension and attenuated spontaneous activity with similar potency, but the concentrations required were two orders of magnitude higher in β₂-AR KO than WT. The concentration-response curves (CRCs) for relaxation were not affected by CGP 20712A (300 nM), but were shifted to the right by ICI 118,551 (50 nM) and L748,337 (10 μM). The -logEC₅₀ values for (-)-isoprenaline in WT and β₂-AR KO tissue were 7.98 and 6.00, respectively, suggesting a large receptor reserve of β₂-AR. (-)-CL 316,243 relaxed detrusor and attenuated spontaneous contractions from WT and β₂-AR KO mice with a potency corresponding to the drug’s affinity for β₃-AR. L743,337 shifted the CRCs to the right.

Conclusion: Our findings in β₂-AR KO mice suggest that there is a large receptor reserve for β₂-AR in WT mice so that this β-AR subtype will mediate relaxation of tone and attenuation of spontaneous activity under physiological conditions. Nevertheless, upon removal of this reserve, β₃-AR can also mediate murine detrusor relaxation.

Safety and tolerability of β3-adrenoceptor agonists in the treatment of overactive bladder syndrome - insight from transcriptosome and experimental studies

INTRODUCTION

We have reviewed the safety and tolerability of β3-adrenoceptor agonists, specifically mirabegron and solabegron, a newly emerging drug class for the treatment of the overactive bladder syndrome. We discuss them mechanistically in the context of expression and other preclinical data.

AREAS COVERED

Based on a systematic PubMed search, incidence of overall adverse events, hypertension, dry mouth, and constipation are comparable between mirabegron or solabegron and placebo. Hypertension is the most frequently observed adverse event, but has a similar incidence with mirabegron and placebo. Nevertheless, severe uncontrolled hypertension has become a contraindication for use of mirabegron based on observation of severe hypertension in association with mirabegron exposure. The overall incidence of adverse events is also similar between mirabegron and the muscarinic receptor antagonist tolterodine, but the incidence of dry mouth is much lower with mirabegron.

EXPERT OPINION

The high β3-adrenoceptor mRNA expression in the human ovaries is not associated with reproductive side effects. Generally, β3-adrenoceptors exhibit a rather restricted expression in human tissues, which may explain the overall good tolerability of agonists acting on this receptor. We propose that expression profiles and functional preclinical studies can be important tools in the prediction of adverse event profiles in first-in-class drugs.

β3-Adrenoceptor agonists for overactive bladder syndrome: Role of translational pharmacology in a repositioning clinical drug development project

β3-Adrenoceptor agonists were originally considered as a promising drug class for the treatment of obesity and/or type 2 diabetes. When these development efforts failed, they were repositioned for the treatment of the overactive bladder syndrome. Based on the example of the β3-adrenoceptor agonist mirabegron, but also taking into consideration evidence obtained with ritobegron and solabegron, we discuss challenges facing a translational pharmacology program accompanying clinical drug development for a first-in-class molecule. Challenges included generic ones such as ligand selectivity, species differences and drug target gene polymorphisms. Challenges that are more specific included changing concepts of the underlying pathophysiology of the target condition while clinical development was under way; moreover, a paucity of public domain tools for the study of the drug target and aspects of receptor agonists as drugs had to be addressed. Nonetheless, a successful first-in-class launch was accomplished. Looking back at this translational pharmacology program, we conclude that a specifically tailored and highly flexible approach is required. However, several of the lessons learned may also be applicable to translational pharmacology programs in other indications.

Mirabegron relaxes urethral smooth muscle by a dual mechanism involving β3 -adrenoceptor activation and α1 -adrenoceptor blockade

LINKED ARTICLE

This article is commented on by Michel, M. C., pp. 429-430 of this issue. To view this commentary visit http://dx.doi.org/10.1111/bph.13379.

BACKGROUND AND PURPOSE

Mirabegron is the first β3 -adrenoceptor agonist approved for treatment of overactive bladder syndrome. This study aimed to investigate the effects of β3 -adrenoceptor agonist mirabegron in mouse urethra. The possibility that mirabegron also exerts α1 -adrenoceptor antagonism was also tested in rat smooth muscle preparations presenting α1A - (vas deferens and prostate), α1D - (aorta) and α1B -adrenoceptors (spleen).

EXPERIMENTAL APPROACH

Functional assays were carried out in mouse and rat isolated tissues. Competition assays for the specific binding of [(3) H]prazosin to membrane preparations of HEK-293 cells expressing each of the human α1 -adrenoceptors, as well as β-adrenoceptor mRNA expression and cyclic AMP measurements in mouse urethra, were performed.

KEY RESULTS

Mirabegron produced concentration-dependent urethral relaxations that were shifted to the right by the selective β3 -adrenoceptor antagonist L-748,337 but unaffected by β1 - and β2 -adrenoceptor antagonists (atenolol and ICI-118,551 respectively). Mirabegron-induced relaxations were enhanced by the PDE4 inhibitor rolipram, and the agonist stimulated cAMP synthesis. Mirabegron also produced rightward shifts in urethral contractions induced by the α1 -adrenoceptor agonist phenylephrine. Schild regression analysis revealed that mirabegron behaves as a competitive antagonist of α1 -adrenoceptors in urethra, vas deferens and prostate (α1A -adrenoceptor, pA2  ≅ 5.6) and aorta (α1D -adrenoceptor, pA2  ≅ 5.4) but not in spleen (α1B -adrenoceptor). The affinities estimated for mirabegron in functional assays were consistent with those estimated in radioligand binding with human recombinant α1A - and α1D -adrenoceptors (pKi  ≅ 6.0).

CONCLUSION AND IMPLICATIONS

The effects of mirabegron in urethral smooth muscle are the result of β3 -adrenoceptor agonism together with α1A and α1D -adrenoceptor antagonism.

Role of host β1- and β2-adrenergic receptors in a murine model of B16 melanoma: functional involvement of β3-adrenergic receptors

Complex interactions between tumor cells and their surrounding compartment are strongly influenced by the host in which the tumor grows. In melanoma, for instance, stress-associated norephinephrine (NE), acting at β-adrenergic receptors (β-ARs), stimulates melanoma cell proliferation and tumor angiogenesis. Among β-ARs, β3-ARs play a role acting not only at tumor cells but also at non-neoplastic stromal cells within the melanoma. In the present study, we used a murine model of B16 melanoma to evaluate the role of the host β1- and β2-ARs in melanoma growth and we determined whether the role of β3-ARs can be influenced by the absence of stromal β1- and β2-ARs. As compared to wild-type mice, β1/2-AR knockout mice displayed (i) increased intratumoral levels of both NE and β3-ARs, as evidentiated at both messenger and protein levels; (ii) increased tumor vascularization; (iii) decreased tumor cell proliferation but increased tumor cell apoptosis; and (iv) increased responsiveness to intratumoral injection of the β3-AR blocker L-748,337 in terms of decrease in tumor growth, tumor vascular response, tumor cell proliferation, and increase in tumor cell death. These findings together validate the role of β-AR signaling in melanoma microenvironment suggesting that non-neoplastic stromal cells may be targeted by β-AR-related drugs. The additional fact that β3-ARs play an important role in melanoma growth suggests selective β3-AR antagonists as important proapoptotic agents.

Selectivity of pharmacological tools: implications for use in cell physiology. A review in the theme: Cell signaling: proteins, pathways and mechanisms

Pharmacological inhibitors are frequently used to identify the receptors, receptor subtypes, and associated signaling pathways involved in physiological cell responses. Based on the effects of such inhibitors conclusions are drawn about the involvement of their assumed target or lack thereof. While such inhibitors can be useful tools for a better physiological understanding, their uncritical use can lead to incorrect conclusions. This article reviews the concept of inhibitor selectivity and its implication for cell physiology. Specifically, we discuss the implications of using inhibitor vs. activator approaches, issues of direct vs. indirect pathway modulation, implications of inverse agonism and biased signaling, and those of orthosteric vs. allosteric, competitive vs. noncompetitive, and reversible vs. irreversible inhibition. Additional problems can result from inconsistent estimates of inhibitor potency and differences in potency between cell-free systems and intact cells. These concepts are illustrated by several examples of inhibitors displaying affinity for related but distinct targets or even unrelated targets. Of note, many of the issues being addressed are also applicable to genetic inhibition strategies. The main practical conclusion following from these concepts is that investigators should be critical in the choice of inhibitor, its concentrations, and its mode of application. When this advice is adhered to, small-molecule pharmacological inhibitors can be important experimental tools in the hand of physiologists.

The odd sibling: features of β3-adrenoceptor pharmacology

β3-Adrenoceptor agonists have recently been introduced for the treatment of overactive urinary bladder syndrome. Their target, the β3-adrenoceptor, was discovered much later than β1- and β2-adrenoceptors and exhibits unique properties which make extrapolation of findings from the other two subtypes difficult and the β3-adrenoceptor a less-understood subtype. This article discusses three aspects of β3-adrenoceptor pharmacology. First, the ligand-recognition profile of β3-adrenoceptors differs considerably from that of the other two subtypes, i.e., many antagonists considered as nonselective actually are β3-sparing, including propranolol or nadolol. Many agonists and antagonists classically considered as being β3-selective actually are not, including BRL 37,344 ((±)-(R*,R*)-[4-[2-[[2-(3-chlorophenyl)-2-hydroxyethyl]amino]propyl]phenoxy] acetic acid sodium hydrate) or SR 59,230 (3-(2-ethylphenoxy)-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-(2S)-2-propanol oxalate). Moreover, the binding pocket apparently differs between the human and rodent β3-adrenoceptor, yielding considerable species differences in potency. Second, the expression pattern of β3-adrenoceptors is more restricted than that of other subtypes, particularly in humans; this makes extrapolation of rodent findings to the human situation difficult, but it may result in a smaller potential for side effects. The role of β3-adrenoceptor gene polymorphisms has insufficiently been explored and may differ even between primate species. Third, β3-adrenoceptors lack the phosphorylation sites involved in agonist-induced desensitization of the other two subtypes. Thus, they exhibit downregulation and/or desensitization in some, but not other, cell types and tissues. When desensitization occurs, it most often is at the level of mRNA or signaling molecule expression. All three of these factors have implications for future studies to better understand the β3-adrenoceptor as a novel pharmacological target.

Mucosa of murine detrusor impairs β2 -adrenoceptor-mediated relaxation

AIMS

To investigate the role of the mucosa in (-)-isoprenaline-induced relaxation of mouse detrusor muscle and to characterize the β-adrenoceptor subtypes involved.

METHODS

Isolated intact and mucosa-denuded muscle strips from the urinary bladder of male C57BL6 mice were pre-contracted with KCl (40 mM) and were relaxed with increasing concentrations of the β-adrenoceptor (β-AR) agonist (-)-isoprenaline and forskolin in the presence and absence of the subtype-selective β-AR blockers CGP20712A (β1 -ARs), ICI118,551 (β2 -ARs), and L748,337 (β3 -ARs).

RESULTS

Force development in response to KCl was larger in mucosa-denuded than in intact preparations and was almost completely relaxed with increasing concentrations of (-)-isoprenaline. Mucosa-denuded muscles were about 10-fold more sensitive to (-)-isoprenaline than intact muscles. CGP20712A did not affect the concentration-response curves (CRCs) to (-)-isoprenaline, ICI118,551 shifted the CRC further to the right in denuded than in intact strips so that the difference between them was abolished. Combined exposure to β1 -AR and β2 -AR blocker yielded the same result. L748,337 did not significantly affect the CRC to (-)-isoprenaline but caused additional blockade to ICI118,551 in the presence of intact mucosa.

CONCLUSIONS

The mucosa of mouse detrusor strips impairs KCl-induced force development and reduces the sensitivity to β-AR-induced relaxation. The relaxing response to (-)-isoprenaline as well as the mucosa effect thereupon are mainly mediated by β2 -ARs. A minor involvement of β3 -ARs becomes apparent particularly at high (-)-isoprenaline concentrations.

The pharmacological rationale for combining muscarinic receptor antagonists and β-adrenoceptor agonists in the treatment of airway and bladder disease

•

Muscarinic receptors increase smooth muscle tone in airways and urinary bladder.

•

β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.

•

Opposition involves transmitter release, signal transduction and receptor expression.

•

This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.

Muscarinic receptor antagonists and β-adrenoceptor agonists are used in the treatment of obstructive airway disease and overactive bladder syndrome. Here we review the pharmacological rationale for their combination. Muscarinic receptors and β-adrenoceptors are physiological antagonists for smooth muscle tone in airways and bladder. Muscarinic agonism may attenuate β-adrenoceptor-mediated relaxation more than other contractile stimuli. Chronic treatment with one drug class may regulate expression of the target receptor but also that of the opposing receptor. Prejunctional β₂-adrenoceptors can enhance neuronal acetylcholine release. Moreover, at least in the airways, muscarinic receptors and β-adrenoceptors are expressed in different locations, indicating that only a combined modulation of both systems may cause dilatation along the entire bronchial tree. While all of these factors contribute to a rationale for a combination of muscarinic receptor antagonists and β-adrenoceptor agonists, the full value of such combination as compared to monotherapy can only be determined in clinical studies.