Effect of pre-contraction on β-adrenoceptor-mediated relaxation of rat urinary bladder

Does coupling to ADP ribosylation factor 6 explain differences between muscarinic and other receptors in interaction with β-adrenoceptor-mediated smooth muscle relaxation?

Numerous studies in airways, ileum, and urinary bladder have demonstrated that relaxation by β-adrenoceptor agonists has lower potency and/or efficacy when contraction was elicited by muscarinic receptor agonists as compared to other G-protein-coupled receptors, KCl, or basal tone, but the molecular mechanisms behind this relative resistance remain unclear. A paper by Huang et al. in this issue demonstrates that NAV2729, an inhibitor of ADP ribosylation factor 6, inhibits contraction of isolated blood vessels elicited by muscarinic receptor agonists, but not by α₁-adrenoceptor agonists or KCl. Against this background, we discuss the role of ADP ribosylation factor 6 in cellular responses to G-protein-coupled receptor stimulation. While ADP ribosylation factor 6 apparently is the only promising molecular explanation for the relative resistance of smooth muscle contraction elicited by muscarinic agonists, the existing data are insufficient for a robust conclusion.

Clinical Utility of β3-Adrenoreceptor Agonists for the Treatment of Overactive Bladder: A Review of the Evidence and Current Recommendations

This nonsystematic review provides a summary of current evidence on the use of β₃-adrenoreceptor agonists (β₃-ARAs) for the treatment for lower urinary tract symptoms. Soon after their discovery in 1989, β₃-ARs were identified as a predominant adrenoreceptor subtype in the human urinary bladder. Although it is widely believed that β₃-ARAs cause detrusor relaxation, the effect on bladder afferent signaling likely plays an important role in their mechanism of action as well. In 2011 and 2012, mirabegron was approved for clinical use in overactive bladder (OAB) patients. Pooled analysis of data from prospective randomized studies on >60,000 OAB patients showed that when compared to placebo, mirabegron was superior with respect to reducing the frequency, number, and severity of urgency episodes, number of incontinence episodes and increasing dry rate, but not in reduction of nocturia episodes. The only side effect showing significantly higher incidence than placebo was nasopharyngitis. Mirabegron is approved for OAB treatment in all age-groups and in pediatric patients with neurogenic bladder. Vibegron is another β₃-ARA approved for OAB treatment in the US and Japan. Several large, multicenter, double-blind, randomized trials have documented statistically significant superiority of vibegron over placebo on all efficacy end points. Other β₃-ARAs are being developed; however, to date none has been introduced to clinical use. All β₃-ARAs provide efficacy similar to anticholinergics. They have a favorable safety profile and are well tolerated. Due to their different mechanisms of action, combination of β₃-ARAs with anticholinergic compounds allows for increased efficacy.

Validation of Fenoterol to Study β2-Adrenoceptor Function in the Rat Urinary Bladder

Fenoterol is a β2-adrenoceptor (AR)-selective agonist that is commonly used to investigate relaxation responses mediated by β2-AR in smooth muscle preparations. Some data have questioned this because fenoterol had low potency in the rat urinary bladder when a muscarinic agonist was used as a pre-contraction agent and because some investigators proposed that fenoterol may act in part via β3-AR. We designed the present study to investigate whether fenoterol is a proper pharmacological tool to study β2-AR-mediated relaxation responses in the rat urinary bladder. Firstly, we have compared the effect of pre-contraction agents on fenoterol potency and found that fenoterol potency was about 1.5 log units greater against KCl than carbachol (pEC50 7.19 ± 0.66 and 5.62 ± 1.09 of KCl and of carbachol, respectively). To test the selectivity of fenoterol, we have determined the effects of the β2-AR antagonist ICI 118,551 and the β3-AR antagonist L 748,337 on relaxation responses to fenoterol. While 300 nM L 748,337 had little effect on the potency of fenoterol (pEC50 6.56 ± 0.25 and 6.33 ± 0.61 in the absence and presence of L 748,337, respectively), the relaxation curve for fenoterol was right-shifted in the presence 300 nM ICI 118,551 (pEC50 5.03 ± 0.18). Thus, we conclude that fenoterol is a proper pharmacological tool to assess β2-AR-mediated responses in the rat urinary bladder and most likely in other smooth-muscle preparations containing multiple subtypes of the β-AR.

β3 -Adrenoceptors in the normal and diseased urinary bladder-What are the open questions?

β₃ -Adrenoceptor agonists are used in the treatment of overactive bladder syndrome. Although the relaxant response to adrenergic stimulation in human detrusor smooth muscle cells is mediated mainly via β₃ -adrenoceptors, the plasma concentrations of the therapeutic dose of mirabegron, the only clinically approved β₃ -adrenoceptor agonist, are considerably lower than the EC₅₀ for causing direct relaxation of human detrusor, suggesting a mechanism of action other than direct relaxation of detrusor smooth muscle. However, the site and mechanism of action of β₃ -adrenoceptor agonists in the bladder have not been firmly established. Postulated mechanisms include prejunctional suppression of ACh release from the parasympathetic nerves during the storage phase and inhibition of micro-contractions through β₃ -adrenoceptors on detrusor smooth muscle cells or suburothelial interstitial cells. Implications of possible desensitization of β₃ -adrenoceptors in the bladder upon prolonged agonist exposure and possible causes of rarely observed cardiovascular effects of mirabegron are also discussed. 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.

Pathophysiology, Clinical Importance, and Management of Neurogenic Lower Urinary Tract Dysfunction Caused by Suprasacral Spinal Cord Injury

Management of persistent lower urinary tract dysfunction resulting from severe thoracolumbar spinal cord injury can be challenging. Severe suprasacral spinal cord injury releases the spinal cord segmental micturition reflex from supraspinal modulation and increases nerve growth factor concentration in the bladder wall, lumbosacral spinal cord, and dorsal root ganglion, which subsequently activates hypermechanosensitive C-fiber bladder wall afferents. Hyperexcitability of bladder afferents and detrusor overactivity can cause urine leaking during the storage phase. During urine voiding, the loss of supraspinal control that normally coordinates detrusor contraction with sphincter relaxation can lead to spinal cord segmental reflex-mediated simultaneous detrusor and sphincter contractions or detrusor-sphincter dyssynergia, resulting in inefficient urine voiding and high residual volume. These disease-associated changes can impact on the quality of life and life expectancy of spinal-injured animals. Here, we discuss the pathophysiology and management considerations of lower urinary tract dysfunction as the result of severe, acute, suprasacral spinal cord injury. In addition, drawing from experimental, preclinical, and clinical medicine, we introduce some treatment options for neurogenic lower urinary tract dysfunction that are designed to: (1) prevent urine leakage arising because of detrusor overactivity during bladder filling, (2) preserve upper urinary tract integrity and function by reducing intravesical pressure and subsequent vesicoureteral reflux, and (3) prevent urinary tract and systemic complications by treating and preventing urinary tract infections.

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.

Actions of cAMP on calcium sensitization in human detrusor smooth muscle contraction

OBJECTIVES

To clarify the effect of cAMP on the Ca(2+) -sensitized smooth muscle contraction in human detrusor, as well as the role of novel exchange protein directly activated by cAMP (Epac) in cAMP-mediated relaxation.

MATERIALS AND METHODS

All experimental protocols to record isometric tension force were performed using α-toxin-permeabilized human detrusor smooth muscle strips. The mechanisms of cAMP-mediated suppression of Ca(2+) sensitization activated by 10 μm carbachol (CCh) and 100 μm GTP were studied using a selective rho kinase (ROK) inhibitor, Y-27632, and a selective protein kinase C (PKC) inhibitor, GF-109203X. The relaxation mechanisms were further probed using a selective protein kinase A (PKA) activator, 6-Bnz-cAMP and a selective Epac activator, 8-pCPT-2'-O-Me-cAMP.

RESULTS

We observed that CCh-induced Ca(2+) sensitization was inhibited by cAMP in a concentration-dependent manner. GF-109203X (10 μm) but not Y-27632 (10 μm) significantly enhanced the relaxation effect induced by cAMP (100 μm). 6-Bnz-cAMP (100 μm) predominantly decreased the tension force in comparison with 8-pCPT-2'-O-Me-cAMP (100 μm).

CONCLUSIONS

We showed that cAMP predominantly inhibited the ROK pathway but not the PKC pathway. The PKA-dependent pathway is dominant, while Epac plays a minor role in human detrusor smooth muscle Ca(2+) sensitization.

β3-Adrenoceptor-mediated relaxation of rat and human urinary bladder: roles of BKCa channels and Rho kinase

Previous studies suggest that the large-conductance Ca(2+)-activated K(+) (BKCa) channel and Rho-kinase play major roles in the control of urinary bladder tone. Here, we investigated their involvement in β-adrenoceptor (AR)-mediated relaxation of rat and human bladder. Concentration-response curves of isoprenaline and mirabegron-induced bladder relaxation were generated against passive tension and KCl- and carbachol-induced tone, in the absence or presence of the BKCa channel inhibitor iberiotoxin (100 nM) or the Rho-kinase inhibitor Y27,632 (1 μM). Myosin light chain (MLC) phosphorylation was studied by Western blot. In rat, iberiotoxin only slightly altered isoprenaline- and mirabegron-induced relaxation against KCl-induced tone but attenuated relaxation by both agonists against carbachol-induced tone. Y27,632 enhanced isoprenaline- or mirabegron-induced relaxation only against carbachol-induced tone. In humans, iberiotoxin slightly enhanced relaxation by both agonists against carbachol-induced pre-contraction. Y27,632 did not change isoprenaline-induced relaxation but enhanced that by mirabegron. Under passive tension, MLC phosphorylation was markedly reduced by both β-AR agonists, an effect insensitive to Y27,632. In the presence of carbachol, both β-AR agonists increased MLC phosphorylation, an effect reduced by Y27,632 only in the presence of 1 μM carbachol. These results indicate that the extent of BKCa channel and Rho-kinase involvement in relaxation induced by β-AR agonists depends on pre contractile stimulus and species.

Long-term anodal block stimulation at sacral anterior roots promoted recovery of neurogenic bladder function in a rabbit model of complete spinal cord injury

A complete spinal cord injury model was established in experimental rabbits using the spinal cord clip compression method. Urodynamic examination was performed 2 weeks later to determine neurogenic bladder status. The rabbits were treated with anodal block stimulation at sacral anterior roots for 4 weeks. Electrical stimulation of sacral anterior roots improved urodynamic parameters of neurogenic bladder in rabbit models of complete spinal cord injury, effectively promoted urinary function, and relieved urinary retention. Immunohistochemistry results showed that a balance was achieved among expression of muscarinic receptor subunits M2, M3, ATP-gated ion channel P2X3 receptors, and β2-adrenergic receptor, and nerve growth factor expression decreased. These results suggested that long-term sacral anterior root stimulation of anodal block could be used to treat neurogenic bladder in a rabbit model of complete spinal cord injury.

β-Adrenoceptor-Mediated Relaxation of Carbachol-Pre-Contracted Mouse Detrusor

Aims: To study the β-adrenoceptor subtypes involved in the relaxation responses to (-)-isoprenaline in carbachol-pre-contracted (CCh) mouse detrusor muscle with intact and denuded mucosa. Methods: Isolated muscle strips from the urinary bladder of male C57BL6 mice or β2-adrenoceptor knockout mice were pre-contracted with CCh, 1 µM and relaxed with increasing concentrations of the β-adrenoceptor (β-AR) agonist (-)-isoprenaline and forskolin. For estimating the β-AR subtypes involved, subtype-selective receptor blockers were used, that is, CGP 20712A (β1-ARs), ICI 118,551 (β2-ARs), and L748,337 (β3-ARs). Results: Unlike in KCl-pre-contracted muscle, the mucosa did not affect the sensitivity of the relaxation response to (-)-isoprenaline in CCh-pre-contracted murine detrusor strips. Increasing concentrations of (-)-isoprenaline produced a biphasic concentration-relaxation response without any difference both during the presence and absence of mucosa. The relaxation fraction produced by low (-)-isoprenaline concentrations was mediated by β2-AR as evidenced by a shift of the concentration-response curve to higher concentrations with ICI 118,551, but not with CGP 20712A and L748,337, and by the absence of this fraction in β2-AR-KO mice. The relaxation response with low sensitivity to (-)-isoprenaline was not affected by any of the β-AR subtype-selective blockers and was the only response detected in detrusor strips from β2-AR-KO mice. Conclusions: In CCh-pre-contracted mouse detrusor, β2-ARs are responsible for the relaxation component with high sensitivity to (-)-isoprenaline as indicated by the conversion of a biphasic into a monophasic CRC with ICI 118,551 or by its absence in β2-AR KO mice. The mucosa does not impair relaxation under these conditions.

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.

Does hypercholesterolemia affect the relaxation of the detrusor smooth muscle in rats? In vitro and in vivo studies

To evaluate the effects of hypercholesterolemia on the relaxation function of the urinary bladder, we examined the physiological mechanisms involved in the isoproterenol-induced relaxation in isolated detrusor strips in vitro and voiding behavior in vivo in rats. Adult male Sprague-Dawley rats were fed standard (control, N = 16) or 4 % cholesterol diet (hypercholesterolemia, N = 17) for 4 weeks. Concentration-response curves for isoproterenol-induced relaxations in carbachol-precontracted detrusor muscle strips were recorded. The contributions of β2- and β3-adrenoceptors and ATP-dependent and Ca(2+)-dependent potassium channels to the relaxation response were investigated by using selective adrenergic agonists salbutamol and BRL 37344 and specific potassium channel inhibitors glibenclamide and charybdotoxin, respectively. Cystometrography was performed to assess bladder function. Hypercholesterolemic rats had higher serum cholesterol and low- and high-density lipoprotein levels than the controls with no sign of atherosclerosis. Isoproterenol-induced relaxation was significantly enhanced in the hypercholesterolemia group. Preincubation with the M2 receptor antagonist attenuated the relaxation response in both groups. The relaxation responses to isoproterenol and salbutamol were similar in both groups, while BRL 37344 appeared to produce a greater relaxant effect in the hypercholesterolemic rats. Also, the inhibitory effects of potassium channel inhibitors on relaxation responses were comparable among the groups. The cystometric findings revealed that threshold and basal pressure values were higher in the hypercholesterolemia group compared with controls. We showed that hypercholesterolemia leads to greater relaxation responses to isoproterenol, appears to impair the braking function of M2 cholinergic receptors on adrenoceptor-induced relaxations in the isolated detrusor muscle, and affects the voiding function in rats.

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

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

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

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Opposition involves transmitter release, signal transduction and receptor expression.

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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.

Discovery history and clinical development of mirabegron for the treatment of overactive bladder and urinary incontinence

INTRODUCTION

Overactive bladder (OAB) and urinary incontinence, although not life-threatening, are very bothersome chronic health conditions. The limitations of current pharmacological treatment urge the need for novel drugs with alternative mechanisms of action. Huge efforts in this area of research led to the synthesis of several selective and potent β3-adrenoceptor agonists that gained relevance through research during the late 80s and 90s. Mirabegron was the first compound of this new class of drugs that showed preclinical efficacy in several models of storage bladder dysfunction, together with a favorable human pharmacological profile. Having passed the proof-of-concept stage, an extensive clinical development and pharmacology program was performed during the last 10 years, involving >10,000 individuals, before mirabegron was granted marketing approval.

AREAS COVERED

In this case history, the authors review the milestones in mirabegron's discovery based on a systematic literature review.

EXPERT OPINION

Thanks to its tolerability and safety/efficacy balance, mirabegron has potential to fill a need for new treatment options for OAB, and paves the way for further development of a completely new class of drugs aimed to treat this condition. However, the exact role of mirabegron in clinical practice has yet to be defined. Further studies are needed in order to clarify, together with post-launch information, critical safety issues and cost-effectiveness in head-to-head comparison with current standard treatments.

Do β-adrenoceptor agonists induce homologous or heterologous desensitization in rat urinary bladder?

β3-Adrenoceptor agonists have recently been introduced for the symptomatic treatment of the overactive bladder syndrome. As such treatment is not curative, long-term treatment is anticipated to be required. As the susceptibility of β3-adrenoceptors to undergo agonist-induced desensitization is cell type- and tissue-dependent, we have explored whether pre-treatment with a β-adrenoceptor agonist will attenuate subsequent relaxation responses to freshly added agonist using rat urinary bladder as a model. We have used the prototypical β-adrenoceptor agonist isoprenaline, the β2-selective fenoterol and the β3-selective CL 316,243 and mirabegron as well as the receptor-independent bladder relaxant forskolin. We show that a 6-h pre-treatment with agonist can significantly reduce subsequent relaxation against KCl-induced smooth muscle tone, but agonist-induced desensitization was also observed with longer pre-treatments or against passive tension. The agonist-induced desensitization was prominent for the β2 component of rat bladder relaxation but much weaker or even absent for the β3 component. Moreover, β-adrenoceptor agonist pre-treatment reduced contractile responses to the muscarinic agonist carbachol and the receptor-independent stimulus KCl. Taken together these data do not support the hypothesis that the long-term clinical efficacy of β3-adrenoceptor agonists in the treatment of the overactive bladder syndrome will be limited by receptor desensitization. Rather they raise the possibility that such treatment may not only cause smooth muscle relaxation but also may attenuate hyper-contractility of the bladder.

BRL37344, a β3-adrenergic receptor agonist, decreases nerve-evoked contractions in human detrusor smooth muscle isolated strips: role of BK channels

OBJECTIVE

To investigate the mechanism by which BRL37344, a β3-adrenergic receptor (β3-ARs) agonist, facilitates the inhibition of nerve-evoked contractions in human detrusor smooth muscle (DSM) isolated strips and to identify the role of large-conductance Ca(2+)-activated K(+) (BK) channels in this process.

METHODS

Human DSM specimens were obtained from open bladder surgeries on patients without preoperative history of overactive bladder symptoms. Isometric DSM tension recordings were conducted using force-displacement transducers and thermostatically controlled tissue baths. Nerve-evoked contractions were generated by electrical field stimulation (EFS).

RESULTS

BRL37344, a β3-AR agonist, significantly decreased the amplitude, muscle force, and duration of the DSM contractions induced by 20 Hz EFS, in a concentration-dependent manner. This BRL37344-mediated inhibition of the amplitude and muscle force of the nerve-evoked DSM contraction was significantly reduced by iberiotoxin, a highly selective inhibitor of the BK channel, revealing a role for BK channels in the β3-AR-induced inhibition of human DSM nerve-evoked contractions. We further used atropine, α,β-methylene-ATP, and suramin to separate the cholinergic and purinergic components of human DSM nerve-evoked contractions. We found that the β3-AR agonist, BRL37344, inhibited both components of the EFS-induced (0.5-50 Hz) DSM contractions.

CONCLUSION

This study supports the concept that β3-AR agonists inhibit nerve-evoked contractions in human DSM. We have further revealed that BK channels play a critical role in BRL37344-mediated relaxation of nerve-evoked contractions in human DSM. The study suggests that in addition to β3-ARs, BK channels may also represent promising pharmacologic targets in the treatment of urinary bladder dysfunction.

Autonomic nervous control of the urinary bladder

The autonomic nervous system plays an important role in the regulation of the urinary bladder function. Under physiological circumstances, noradrenaline, acting mainly on β(3) -adrenoceptors in the detrusor and on α(1) (A) -adrenoceptors in the bladder outflow tract, promotes urine storage, whereas neuronally released acetylcholine acting mainly on M(3) receptors promotes bladder emptying. Under pathophysiological conditions, however, this system may change in several ways. Firstly, there may be plasticity at the levels of innervation and receptor expression and function. Secondly, non-neuronal acetylcholine synthesis and release from the urothelium may occur during the storage phase, leading to a concomitant exposure of detrusor smooth muscle, urothelium and afferent nerves to acetylcholine and noradrenaline. This can cause interactions between the adrenergic and cholinergic system, which have been studied mostly at the post-junctional smooth muscle level until now. The implications of such plasticity are being discussed.

Pharmacological profile of β3-adrenoceptor agonists in clinical development for the treatment of overactive bladder syndrome

β(3)-Adrenoceptor agonists are an emerging drug class for the treatment of the overactive bladder syndrome, and clinical proof-of-concept data have been obtained for three representatives of this class, mirabegron, ritobegron, and solabegron. We review here the pharmacological profile of these three drugs and discuss the potential clinical relevance of differences between them. In the absence of direct comparative studies, it appears that all three are strong agonists selective for β(3)- vs. β(1)- and β(2)-adrenoceptors in studies with cloned receptor subtypes. The potency of these agonists may be species-dependent, with all three having high potency in the human detrusor. All three agonists were effective in one or more animal models of bladder dysfunction, which typically involved reductions of micturition frequency. Agonist doses effective for bladder function lowered blood pressure in some cases, but the relevance of this for clinical use is difficult to determine due to species differences in the importance of cardiovascular β(3)-adrenoceptors. While limited effects on other organ systems are expected for β(3)-adrenoceptor agonists, this requires further investigation.

In vitro and in vivo pharmacological profile of the selective β3-adrenoceptor agonist mirabegron in rats

To investigate the pharmacological properties of mirabegron in in vitro and in vivo, the effects on cAMP accumulation in Chinese hamster ovary (CHO) cells expressing rat β-adrenoceptors, the relaxant activity in isolated rat bladder smooth muscle, and the voiding effects in cerebral infarcted rats were evaluated. Mirabegron increased cAMP accumulation with EC(50) value and intrinsic activity of 19 nmol/L and 1.0, respectively, in CHO cells expressing rat β(3)-adrenoceptors. The EC(50) values and the intrinsic activities of mirabegron were 610 nmol/L and 0.6 for rat β(1)-adrenoceptors and were sumless and 0.1 for β(2)-adrenoceptors, respectively. Mirabegron showed concentration-dependent relaxant and full agonistic effects in rat bladder strips under passive tension with EC(50) value of 290 nmol/L. The concentration-response curve of mirabegron was affected neither by the β(1)-adrenoceptor selective antagonist CGP-20712A nor by the β(2)-adrenoceptor selective antagonist ICI-118,551. In in vivo studies with cerebral infarcted rats, a significant decrease in the volume voided per micturition compared with sham-operated rats was observed. Mirabegron dose-dependently increased the volume voided per micturition. In conclusion, we have extended the selectivity profile of mirabegron to rats and demonstrated that it is effective via stimulation of β(3)-adrenoceptors in a rat cerebral infarction model of detrusor overactivity.

Effect of mirabegron, a novel β3-adrenoceptor agonist, on bladder function during storage phase in rats

Mirabegron, a selective β(3)-adrenoceptor agonist, facilitates urine storage function by exerting a relaxing effect on bladder smooth muscle. Here, we investigated the effect of mirabegron on bladder function during the storage phase. We assessed the effect of mirabegron on the resting intravesical pressure in anesthetized rats and also tested antimuscarinics (oxybutynin and tolterodine) under the same experimental conditions. Mirabegron dose-dependently decreased the resting intravesical pressure, while oxybutynin and tolterodine showed no statistically significant effects on resting intravesical pressure. We also investigated the effect of mirabegron on bladder function using cystometry technique in conscious rats with bladder outlet obstruction. While mirabegron dose-dependently decreased the frequency of nonvoiding contractions, considered an index of abnormal response in bladder storage, no significant effects were noted on the amplitude of nonvoiding contractions, micturition pressure, threshold pressure, voided volume, residual volume, or bladder capacity. Neither oxybutynin nor tolterodine affected the frequency of nonvoiding contractions; however, oxybutynin increased residual volume and tended to decrease voided volume in a dose-dependent manner, and tolterodine dose-dependently decreased voided volume. Taken together, these results shed light on the suggestion of mirabegron as a therapeutic agent, compared with antimuscarinics, with its most prominent effect being the facilitation of bladder storage.

Insight into new potential targets for the treatment of overactive bladder and detrusor overactivity

Although overactive bladder (OAB) and detrusor overactivity (DO) are not synonyms, they share therapeutic options and partially underlying physiopathological mechanisms. The aim of this overview is to give insight into new potential targets for the treatment of OAB and DO. A narrative review was done in order to reach this goal. Ageing, pelvic floor disorders, hypersensitivity disorders, morphologic bladder changes, neurological diseases, local inflammations, infections, tumors and bladder outlet obstruction may alter the normal voluntary control of micturition, leading to OAB and DO. The main aim of pharmacotherapy is to restore normal control of micturition, inhibiting the emerging pathological involuntary reflex mechanism. Therapeutic targets can be found at the levels of the urothelium, detrusor muscles, autonomic and afferent pathways, spinal cord and brain. Increased expression and/or sensitivity of urothelial-sensory molecules that lead to afferent sensitization have been documented as a possible pathogenesis of OAB. Targeting afferent pathways and/or bladder smooth muscles by modulating activity of ligand receptors and ion channels could be effective to suppress OAB.

Functional investigation of β-adrenoceptors in human isolated detrusor focusing on the novel selective β3-adrenoceptor agonist KUC-7322

This study aimed to characterize the β-adrenoceptor (β-AR) subtype mediating relaxation of isolated human bladder strips and to explore relaxation by the novel β3-AR-selective agonist KUC-7322 for its relaxant effect on the human isolated detrusor and for its effect on the carbachol (CCh)-induced contractile response. In two parallel studies, relaxation of isolated human bladder strips was tested for the β-AR agonists isoproterenol, clenbuterol, BRL 37344, and KUC-7322. For the isoproterenol and KUC-7322 responses, antagonism by CGP 20712A, ICI 118551, and SR59230A was determined. The potency and efficacy of the reference agonists for detrusor relaxation was in line with their known β3-AR activity. KUC-7322 relative to isoproterenol was a full agonist with a pEC(50) of 5.95 ± 0.09 and 5.92 ± 0.11 in the two studies. SR59230A exhibited antagonism of the expected potency against isoproterenol (apparent pK (B) 7.2) but not against KUC-7322. Neither isoproterenol nor KUC-7322 nor forskolin significantly attenuated CCh-induced contraction. These results suggest that KUC-7322 displays full agonistic activity in relaxing the human detrusor without inhibiting the contraction induced by cholinergic stimulation. These characteristics, if proven in vivo, may be beneficial for the treatment of overactive bladder, as increased bladder capacity with a negligible effect on voiding contractions may be anticipated.

Urothelial beta-3 adrenergic receptors in the rat bladder

AIMS

To investigate the distribution of beta-3 adrenergic receptors (β(3)ARs) in the rat bladder and to examine the contribution of urothelial β(3)ARs to agonist-induced suppression of bladder reflexes and relaxation of smooth muscle.

METHODS

Bladder tissue was collected from 8- to 10-month old female SD rats. In some samples, the urothelium was surgically separated from the smooth muscle. The expression and localization of βAR mRNA and β(3)AR protein were determined using RT-PCR and immunohistochemistry. Contractile responses to the specific β(3)AR agonists TAK-677 and BRL37344 were measured in bladder strips with or without the urothelium. The contribution of urothelial β(3)ARs to the micturition reflex was assessed in continuous cystometry in urethane anesthetized rats using intravesical delivery of β(3)AR agonists.

RESULTS

RT-PCR detected mRNA of all βARs in urothelium and smooth muscle. Immunostaining detected β(3)ARs throughout the urothelium, in the smooth muscle, myofibroblast-like cells, and in the peripheral nerves. Ovariectomy did not change the distribution of β(3)ARs in any bladder structure. Intravesical administration of TAK-677 and BRL37344 (1-5 × 10(-4) M) decreased voiding frequency and amplitude of bladder contractions. In bladder strips in vitro both β(3)AR agonists (10(-12) to 10(-4) M) relaxed the smooth muscle in a concentration-dependent manner to the same extent in strips with and without the urothelium.

CONCLUSIONS

In addition to their presence in bladder smooth muscle, β(3)ARs are present in the urothelium where their activation may alter reflex voiding via release of factor(s) that act on non-myocyte structures including the afferent and/or efferent nerves to influence bladder contractility.

Muscarinic receptor subtypes and signalling involved in the attenuation of isoprenaline-induced rat urinary bladder relaxation

β-Adrenoceptors are important mediators of smooth muscle relaxation in the urinary bladder, but the concomitant presence of a muscarinic agonist, e.g., carbachol, can attenuate relaxation responses by reducing potency and/or efficacy of β-adrenoceptor agonists such as isoprenaline. Therefore, the present study was designed to explore the subtypes and signalling pathways of muscarinic receptors involved in the attenuation of isoprenaline-induced isolated rat detrusor preparations using novel subtype-selective receptor ligands. In radioligand binding studies, we characterized BZI to be a M₃-sparing muscarinic agonist, providing selective M₂ stimulation in rat bladder, and THRX-182087 as a highly M₂-selective antagonist. The use of BZI and of THRX-182087 in the presence of carbachol enabled experimental conditions with a selective stimulation of only M₂ or M₃ receptors, respectively. Confirming previous findings, carbachol attenuated isoprenaline-induced detrusor relaxation. M₂-selective stimulation partly mimicked this attenuation, indicating that both M₂ and M₃ receptors are involved. During M₃-selective stimulation, the attenuation of isoprenaline responses was reduced by the phospholipase C inhibitor U 73,122 but not by the protein kinase C inhibitor chelerythrine. We conclude that both M₂ and M₃ receptors contribute to attenuation of β-adrenoceptor-mediated relaxation of rat urinary bladder; the signal transduction pathway involved in the M₃ component of this attenuation differs from that mediating direct contractile effects of M₃ receptors.

β-adrenoceptor agonist effects in experimental models of bladder dysfunction

β-adrenoceptor stimulation can enhance the storage function of the urinary bladder by acting on detrusor smooth muscle tone, mediator release from the urothelium and/or afferent nerve activity. In humans this may occur predominantly if not exclusively via the β₃-subtype. The effects of β-adrenoceptor agonists including several β₃-selective agonists have been studied in vitro and in vivo, in healthy animals of both genders and various age groups and in a wide range of animal (mostly rat) models of genetic or acquired bladder dysfunction. Such models included bladder irritation by intravesical instillation of acetic acid or prostaglandin E₂, bladder outlet obstruction, stroke, diabetes, spontaneously hypertensive rats, and NO synthase inhibition. Across all of these models β-adrenoceptor agonists had effects consistent with improved bladder storage function. β₃-adrenoceptor effects are resistant to agonist-induced desensitization in many cell types, but whether this also applies to the human bladder is unknown. The efficacy of β-adrenoceptor agonists appears to be largely unaffected by common polymorphisms of the β₃-adrenoceptor gene. Taken together these findings suggest that β₃-adrenoceptor agonists may become useful drugs for the treatment of bladder storage dysfunction, a view supported by recent phase III clinical studies for one such agent, mirabegron.

Human Urinary Bladder Strip Relaxation by the β-Adrenoceptor Agonist Isoprenaline: Methodological Considerations and Effects of Gender and Age

The present study was primarily designed to explore various methodological aspects related to organ bath experiments evaluating human detrusor relaxation by the β-adrenoceptor agonist isoprenaline. Data are based upon a series of 30 consecutive patients, and this cohort was also used to explore possible effects of gender and age. KCl-induced contraction was related to strip length but not weight or cross-sectional area, indicating that the former is most suitable for data normalization. Storage of detrusor strips in cold buffer for up to 2 days did not affect contractile responses to KCl or efficacy of isoprenaline to cause relaxation but significantly affected the isoprenaline potency. No such alterations were observed with up to 1 day of cold storage. The type (KCl vs. passive tension) or strength of contractile stimulus had only minor effects on isoprenaline responses although these differences reached statistical significance in some cases. Similarly, gender and age had only minor if any effects on KCl-induced contraction or isoprenaline-induced relaxation, but the current data are too limited for robust conclusions. In summary we have evaluated experimental conditions for the testing of human detrusor strip contraction and relaxation which should be useful for future larger studies.

β-Adrenergic Receptor Subtypes in the Urinary Tract

Within the urinary tract, β-adrenergic receptors (AR) are found largely on smooth muscle cells but are also present, at least in the bladder, in the urothelium and on afferent nerves. Our understanding of β-AR subtype expression and function is hampered by a lack of well-validated tools, particularly with regard to β(3)-AR. Moreover, the β-AR subtypes involved in a specific function may differ between species. In the ureter, β-AR can modulate pacemaker activity and smooth muscle tone involving multiple subtypes. In the human bladder, β-AR promote urine storage. Bladder smooth muscle relaxation primarily involves β(3)-AR, and the agonists selective for this subtype are in clinical development to treat bladder dysfunction. While prostate and urethra also express β-AR, the overall physiological role in these tissues remains unclear.

Lack of evidence that nebivolol is a β₃-adrenoceptor agonist

Nebivolol is a selective β₁-adrenoceptor antagonist which, in addition, displays endothelium-dependent vasodilating properties in humans and other species. β₃-adrenoceptors have been proposed to be a molecular target of nebivolol-induced vasodilatation. Therefore, we have investigated possible β₃-adrenoceptor agonism by nebivolol for relaxation of the human and rat urinary bladder (prototypical β₃-adrenoceptor-mediated responses) as well as for cAMP accumulation in Chinese hamster ovary cells stably transfected with the human β-adrenoceptor subtypes. Nebivolol concentration-dependently relaxed both human and rat isolated urinary bladder strips but with low potency, similar to that reported for vasodilatation. However, nebivolol-induced bladder relaxation in either species was not inhibited by the β₃-adrenoceptor antagonist SR 59,230A (10μM), although this compound inhibited the isoprenaline-induced relaxation with the expected potency. In radioligand binding studies nebivolol had lower affinity for human β₃-adrenoceptors than the other two β-adrenoceptor subtypes, but this low affinity was in line with its potency to relax the bladder or isolated blood vessels. In functional studies nebivolol even in high concentrations did not stimulate cAMP formation via any of the three cloned human β-adrenoceptors or in rat bladder smooth muscle cells. Taken together these data demonstrate that nebivolol can relax not only vascular but also urinary bladder smooth muscle. However, they do not support the hypothesis that nebivolol is an agonist at cloned human β₃-adrenoceptors or in rat or human urinary bladder.

β-Adrenoceptor-mediated differences in transverse and longitudinal strips from the rat detrusor

Contractions and relaxations of the urinary bladder occur in all directions to facilitate urine release and storage. Transverse and longitudinal contractility of detrusor smooth muscle have been studied before using various pharmacologic stimuli but not β agonists. Given the importance of β-adrenoceptors in mediating bladder relaxation, the effects of isoprenaline (IPNA) in transverse and longitudinal contractility were examined. Pretreatment with a low concentration of IPNA (0.1 or 1 μM) suppressed carbachol (CCh)-induced contractions, more in the transverse than longitudinal direction. Increasing the IPNA concentration to 10 or 100 μM resulted in greater inhibition of longitudinal contractions. Also in the longitudinal direction, IPNA-induced relaxation was greater than in the transverse direction. When precontracted with a submaximal concentration of CCh (1 μM), IPNA increased the phasic activity in the longitudinal direction only. In summary, β-adrenoceptor-mediated differences between transverse and longitudinal contractility were revealed. In testing the relaxant properties of selective β-agonists, the findings here should be considered such that other than the conventional longitudinal contractions, measurements are also made in other directions.