In vivo and in vitro effects of muscarinic receptor antagonists on contractions and release of [3H]acetylcholine in the rabbit urinary bladder

Desipramine, commonly used as a noradrenergic neuroprotectant in 6-OHDA-lesions, leads to local functional changes in the urinary bladder and gastrointestinal tract in healthy rats

The 6-hydroxydopamine (6-OHDA) rat model is one of the most common animal models of Parkinson's disease. When experimentally inducing dopaminergic neurodegeneration in the nigrostriatal pathway using 6-OHDA, the noradrenergic reuptake inhibitor desipramine is often systemically injected in order to protect against damages to the noradrenergic system in the brain. An increasing number of studies are focusing on understanding the pathophysiological changes underlying autonomic non-motor symptoms, in particular urinary bladder and gastrointestinal dysfunctions, of the disease. Several of these studies have investigated the contractile properties and the activation of smooth muscle in the 6-OHDA rat model. Since the injection of desipramine is commonly placed in close proximity to the urinary bladder and gastrointestinal tract, in the current study we wanted to understand if the drug alone has an effect. For this, we have injected a single dose (25 mg/kg) of desipramine either intraperitonially or subcutaneously and investigated smooth muscle contractility in vitro in the urinary bladder, proximal colon and distal ileum four weeks post injection. Our data show that desipramine significantly alters smooth muscle contractility of the urinary bladder and proximal colon in healthy rats. Conclusively, we suggest, based on our data, that desipramine should be omitted when using the 6-OHDA rat model to investigate smooth muscle function in Parkinson's disease research.

In vivo paracrine effects of ATP-induced urothelial acetylcholine in the rat urinary bladder

Mechanical stretch of the urothelium induces the release of ATP that activates bladder afferent nerves. In the rat urinary bladder, ATP is also a contractile co-transmitter in the parasympathetic innervation. In isolated preparations, ATP evokes a urothelial release of acetylcholine that substantially contributes to ATP-evoked contractile responses. Currently we aimed to further examine the interactions of ATP and acetylcholine in the rat urinary bladder in two in vivo models. In the whole bladder preparation, atropine reduced ATP-evoked responses by about 50% in intact but denervated bladders, while atropine had no effect after denudation of the urothelium. In a split bladder preparation, reflex-evoked responses of the contralateral half were studied by applying stimuli (agonists or stretch) to the ipsilateral half. Topical administration of ATP and methacholine as well as of stretch induced contralateral reflex-evoked contractions. While topical administration of atropine ipsilaterally reduced the ATP- and stretch-induced contralateral contractions by 27 and 39%, respectively, the P2X purinoceptor antagonist PPADS reduced them by 74 and 84%. In contrary, the muscarinic M2-(M4)-selective receptor antagonist methoctramine increased the responses by 38% (ATP) and 75% (stretch). Pirenzepine (M1-selective antagonist) had no effect on the reflex. In vitro, in the absence of the reflex, methoctramine did not affect the ATP-induced responses. It is concluded that urothelial ATP potently induces the micturition reflex and stimulates urothelial release of acetylcholine. Acetylcholine subsequently acts on afferents and on the detrusor muscle. While muscarinic M2 and/or M4 receptors in the sensory innervation exert inhibitory modulation, muscarinic M3 receptors cause excitation.

Combination drug therapy against OAB normalizes micturition parameters and increases the release of nitric oxide during chemically induced cystitis

Today, monotherapy is the most common pharmacological treatment option for patients suffering from overactive bladder (OAB). Recent reports have indicated potential benefits of combination therapy, using a muscarinic antagonist and a β3 -adrenoceptor agonist. This may be of particular interest for therapy-resistant patients with OAB and concomitant cystitis. The objective of the current study was to assess how combination therapy affects bladder parameters in health and cystitis and if the efficacy of the drugs can be linked to altered release of nitric oxide (NO). Rats were pretreated with either a combination of the muscarinic antagonist tolterodine and β3 -selective adrenoceptor agonist mirabegron or saline for 10 days. Forty-eight hours prior to assessing micturition parameters in a metabolic cage, the rats were intraperitoneally injected with cyclophosphamide, causing cystitis, or saline. Urine samples were collected and analyzed for NO content. Bladder contractile properties were assessed in an organ bath setup. Induction of cystitis led to bladder overactivity. Combination therapy normalized bladder parameters. Both induction of cystitis and drug treatment increased the release of NO. The innate contractile properties of the bladder were unaffected by combination therapy. This study demonstrates positive effects of combination drug therapy on symptoms of OAB, possibly indicating it to be a good option for treatment of OAB during concomitant cystitis. It remains to be determined if increased release of NO is crucial for successful pharmacological treatment of bladder overactivity during cystitis.

Acetylcholinesterase Inhibitors and Drugs Acting on Muscarinic Receptors- Potential Crosstalk of Cholinergic Mechanisms During Pharmacological Treatment

BACKGROUND

Pharmaceuticals with targets in the cholinergic transmission have been used for decades and are still fundamental treatments in many diseases and conditions today. Both the transmission and the effects of the somatomotoric and the parasympathetic nervous systems may be targeted by such treatments. Irrespective of the knowledge that the effects of neuronal signalling in the nervous systems may include a number of different receptor subtypes of both the nicotinic and the muscarinic receptors, this complexity is generally overlooked when assessing the mechanisms of action of pharmaceuticals.

METHODS

We have search of bibliographic databases for peer-reviewed research literature focused on the cholinergic system. Also, we have taken advantage of our expertise in this field to deduce the conclusions of this study.

RESULTS

Presently, the life cycle of acetylcholine, muscarinic receptors and their effects are reviewed in the major organ systems of the body. Neuronal and non-neuronal sources of acetylcholine are elucidated. Examples of pharmaceuticals, in particular cholinesterase inhibitors, affecting these systems are discussed. The review focuses on salivary glands, the respiratory tract and the lower urinary tract, since the complexity of the interplay of different muscarinic receptor subtypes is of significance for physiological, pharmacological and toxicological effects in these organs.

CONCLUSION

Most pharmaceuticals targeting muscarinic receptors are employed at such large doses that no selectivity can be expected. However, some differences in the adverse effect profile of muscarinic antagonists may still be explained by the variation of expression of muscarinic receptor subtypes in different organs. However, a complex pattern of interactions between muscarinic receptor subtypes occurs and needs to be considered when searching for selective pharmaceuticals. In the development of new entities for the treatment of for instance pesticide intoxication, the muscarinic receptor selectivity needs to be considered. Reactivators generally have a muscarinic M2 receptor acting profile. Such a blockade may engrave the situation since it may enlarge the effect of the muscarinic M3 receptor effect. This may explain why respiratory arrest is the major cause for deaths by esterase blocking.

Neural control of the lower urinary tract

This article summarizes anatomical, neurophysiological, pharmacological, and brain imaging studies in humans and animals that have provided insights into the neural circuitry and neurotransmitter mechanisms controlling the lower urinary tract. The functions of the lower urinary tract to store and periodically eliminate urine are regulated by a complex neural control system in the brain, spinal cord, and peripheral autonomic ganglia that coordinates the activity of smooth and striated muscles of the bladder and urethral outlet. The neural control of micturition is organized as a hierarchical system in which spinal storage mechanisms are in turn regulated by circuitry in the rostral brain stem that initiates reflex voiding. Input from the forebrain triggers voluntary voiding by modulating the brain stem circuitry. Many neural circuits controlling the lower urinary tract exhibit switch-like patterns of activity that turn on and off in an all-or-none manner. The major component of the micturition switching circuit is a spinobulbospinal parasympathetic reflex pathway that has essential connections in the periaqueductal gray and pontine micturition center. A computer model of this circuit that mimics the switching functions of the bladder and urethra at the onset of micturition is described. Micturition occurs involuntarily in infants and young children until the age of 3 to 5 years, after which it is regulated voluntarily. Diseases or injuries of the nervous system in adults can cause the re-emergence of involuntary micturition, leading to urinary incontinence. Neuroplasticity underlying these developmental and pathological changes in voiding function is discussed.

Local Change in Urinary Bladder Contractility Following CNS Dopamine Denervation in the 6-OHDA Rat Model of Parkinson's Disease

BACKGROUND

Urinary problems, including urinary frequency, urgency, and nocturia are some of the non-motor symptoms that correlate most with poor quality of life in Parkinson's disease. However, the mechanism behind these symptoms is poorly understood, in particular regarding peripheral bladder pathophysiology following dopamine degeneration.

OBJECTIVE

In this study, we compared the contractile responsiveness of urinary bladder from the 6-OHDA unilateral rat model of Parkinson's disease with that of normal untreated animals.

METHODS

The contractility of the urinary detrusor muscle was evaluated in bladder strip preparations using electrical field stimulation, and muscarinic and purinoceptor stimulations in an vitro organ bath setup.

RESULTS

Our data show that the overall contractile response following electrical field stimulation was significantly higher (43% at maximum contraction by 20-40 Hz stimulation) in the 6-OHDA-lesioned rats as compared to control animals. This increase was associated with a significant increase in the cholinergic contractile response, where the muscarinic agonist methacholine produced a 44% (at 10 -4 M concentration) higher response in the 6-OHDA-treated rats as compared to controls with a significant left-shift of the dose response. This indicates an altered sensitivity of the muscarinic receptor system following the specific central 6-OHDA-induced dopamine depletion. In addition a 36% larger contraction of strips from the 6-OHDA animals was also observed with purinoceptor activation using the agonist ATP (5×10 -3 M) during atropine treatment.

CONCLUSIONS

Our data shows that it is not only the central dopamine control of the micturition reflex that is altered in Parkinson's disease, but also the local contractile function of the urinary bladder. The current study draws attention to a mechanism of urinary dysfunction in Parkinson's disease that has previously not been described.

Inhibition of nitric oxide synthase prevents muscarinic and purinergic functional changes and development of cyclophosphamide-induced cystitis in the rat

Nitric oxide (NO) has pivotal roles in cyclophosphamide- (CYP-) induced cystitis during which mucosal nitric oxide synthase (NOS) and muscarinic M5 receptor expressions are upregulated. In cystitis, urothelial muscarinic NO-linked effects hamper contractility. Therefore we wondered if a blockade of this axis also affects the induction of cystitis in the rat. Rats were pretreated with saline, the muscarinic receptor antagonist 4-DAMP (1 mg/kg ip), or the NOS inhibitor L-NAME (30 mg/kg ip) for five days. 60 h before the experiments the rats were treated with saline or CYP. Methacholine-, ATP-, and adenosine-evoked responses were smaller in preparations from CYP-treated rats than from saline-treated ones. Pretreatment with 4-DAMP did not change this relation, while pretreatment with L-NAME normalized the responses in the CYP-treated animals. The functional results were strengthened by the morphological observations; 4-DAMP pretreatment did not affect the parameters studied, namely, expression of muscarinic M5 receptors, P1A1 purinoceptors, mast cell distribution, or bladder wall enlargement. However, pretreatment with L-NAME attenuated the differences. Thus, the current study provides new insights into the complex mechanisms behind CYP-induced cystitis. The NO effects coupled to urothelial muscarinic receptors have a minor role in the development of cystitis. Inhibition of NOS may prevent the progression of cystitis.

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.

In vitro functional interactions of acetylcholine esterase inhibitors and muscarinic receptor antagonists in the urinary bladder of the rat

Obidoxime, a weak acetylcholine-esterase (AChE) inhibitor, exerts muscarinic receptor antagonism with a significant muscarinic M2 receptor selective profile. The current examinations aimed to determine the functional significance of muscarinic M2 receptors in the state of AChE inhibition, elucidating muscarinic M2 and M3 receptor interaction. In the in vitro examinations, methacholine evoked concentration-dependent bladder contractile and atrial frequency inhibitory responses. Although atropine abolished both, methoctramine (1 μmol/L) only affected the cholinergic response in the atrial preparations. However, in the presence of methoctramine, physostigmine, an AChE inhibitor, increased the basal tension of the bladder strip preparations (+68%), as well as the contractile responses to low concentrations of methacholine (< 5 μmol/L; +90-290%). In contrast to physostigmine, obidoxime alone raised the basal tension (+58%) and the responses to low concentrations of methacholine (< 5 μmol/L; +80-450%). Physostigmine concentration-dependently increased methacholine-evoked responses, similarly to obidoxime at low concentrations. However, at large concentrations (> 5 μmol/L), obidoxime, because of its unselective muscarinic receptor antagonism, inhibited the methacholine bladder responses. In conclusion, the current results show that muscarinic M2 receptors inhibit muscarinic M3 receptor-evoked contractile responses to low concentrations of acetylcholine in the synaptic cleft. The muscarinic M2 and M3 receptor crosstalk could be a counteracting mechanism in the treatment of AChE inhibition when using reactivators, such as obidoxime.

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.

Muscarinic receptor subtypes involved in urothelium-derived relaxatory effects in the inflamed rat urinary bladder

Functional studies have shown altered cholinergic mechanisms in the inflamed bladder, which partly depend on muscarinic receptor-induced release of nitric oxide (NO). The current study aimed to characterize which muscarinic receptor subtypes that are involved in the regulation of the nitrergic effects in the bladder cholinergic response during cystitis. For this purpose, in vitro examinations of carbachol-evoked contractions of inflamed and normal bladder preparations were performed. The effects of antagonists with different selectivity for the receptor subtypes were assessed on intact and urothelium-denuded bladder preparations. In preparations from cyclophosphamide (CYP; in order to induce cystitis) pre-treated rats, the response to carbachol was about 75% of that of normal preparations. Removal of the urothelium or administration of a nitric oxide synthase inhibitor re-established the responses in the inflamed preparations. Administration of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) inhibited the carbachol-induced contractile responses of preparations from CYP pre-treated rats less potently than controls. Pirenzepine and p-fluoro-hexahydro-sila-diphenidol (pFHHSiD) affected the carbachol-induced contractile responses to similar extents in preparations of CYP pre-treated and control rats. However, the Schild slopes for the three antagonists were all significantly different from unity in the preparations from CYP pre-treated rats. Again, L-NNA or removal of the urothelium eliminated any difference compared to normal preparations. This study confirms that muscarinic receptor stimulation in the inflamed rat urinary bladder induces urothelial release of NO, which counteracts detrusor contraction.

Coupled nitric oxide and autonomic receptor functional responses in the normal and inflamed urinary bladder of the rat

Both divisions of the autonomic nervous system are involved in regulation of urinary bladder function. Several substances, other than noradrenaline and acetylcholine, seem to play important roles in physiology and pathophysiology of lower urinary tract. In the current study, we aimed to examine if there exist interplays between nitric oxide (NO) and autonomic transmitters and if such interactions vary in different parts of the urinary bladder in healthy and cyclophosphamide (CYP)-induced cystitic rats; when administered to the animals (100 mg/kg; i.p.), the cytotoxic CYP metabolite acrolein induces bladder inflammation. In the current study a series of in vitro functional studies were performed on detrusor muscle strip preparations. Stimulation with electrical field stimulation (EFS), methacholine, adenosine 5´-triphosphate (ATP), and adrenaline evoked contractile responses in isolated bladder preparations that were significantly reduced in cyclophosphamide (CYP)-treated rats. While the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (L-NNA; 10(-4) M) did not affect contractile responses in normal, healthy strip preparations, it significantly increased the contractile responses to EFS, methacholine and adrenaline, but not to ATP, in the bladders from the CYP-treated rats. In the CYP-treated rats, the ATP-evoked relaxatory part of its dual response (an initial contraction followed by a relaxation) was 6-fold increased in comparison with that of normal preparations, whereas the isoprenaline relaxation was halved in the CYP-treated. While L-NNA (10(-4) M) had no effect on the isoprenaline-evoked relaxations, it reduced the ATP-evoked relaxations in strip preparations from the bladder body of CYP-treated rats. Stimulation of beta(2)- and beta(3)-adrenoceptors evoked relaxations and both responses were reduced in cystitis, the latter to a larger extent. In the trigone, the reduced ATP-evoked contractile response in the inflamed strips was increased by L-NNA, while L-NNA had no effect on the ATP-evoked relaxations, neither on the relaxations in healthy nor on the larger relaxations in the inflamed trigone. The study shows that both contractile and relaxatory functions are altered in the state of inflammation. The parasympathetic nerve-mediated contractions of the body of the bladder, evoked by the release of ATP and acetylcholine, were substantially reduced in cystitis. The relaxations to beta-adrenoceptor and purinoceptor stimulation were also reduced but only the ATP-evoked relaxation involved NO.

Muscarinic acetylcholine receptors in the urinary tract

Muscarinic receptors comprise five cloned subtypes, encoded by five distinct genes, which correspond to pharmacologically defined receptors (M(1)-M(5)). They belong to the family of G-protein-coupled receptors and couple differentially to the G-proteins. Preferentially, the inhibitory muscarinic M(2) and M(4) receptors couple to G(i/o), whereas the excitatory muscarinic M(1), M(3), and M(5) receptors preferentially couple to G(q/11). In general, muscarinic M(1), M(3), and M(5) receptors increase intracellular calcium by mobilizing phosphoinositides that generate inositol 1,4,5-trisphosphate (InsP3) and 1,2-diacylglycerol (DAG), whereas M(2) and M(4) receptors are negatively coupled to adenylyl cyclase. Muscarinic receptors are distributed to all parts of the lower urinary tract. The clinical use of antimuscarinic drugs in the treatment of detrusor overactivity and the overactive bladder syndrome has focused interest on the muscarinic receptors not only of the detrusor, but also of other components of the bladder wall, and these have been widely studied. However, the muscarinic receptors in the urethra, prostate, and ureter, and the effects they mediate in the normal state and in different urinary tract pathologies, have so far not been well characterized. In this review, the expression of and the functional effects mediated by muscarinic receptors in the bladder, urethra, prostate, and ureters, under normal conditions and in different pathologies, are discussed.

Pharmacological modulation of the micturition pattern in normal and cyclophosphamide pre-treated conscious rats

In the current study, we wanted to assess the influence of muscarinic receptors, nitric oxide and purinoceptors on the micturition pattern of conscious normal and cyclophosphamide (CYP) pre-treated rats. The micturition parameters were assessed using a metabolic cage. Rats were pre-treated with either saline or CYP, to induce cystitis, followed by treatment with either the muscarinic M1/M3/M5 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), the nitric oxide synthase blocker N(ω)-nitro-L-arginine methyl (L-NAME), the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) or a combination of 4-DAMP with PPADS or L-NAME. Voiding volumes per micturition event were significantly lower in CYP pre-treated than in saline pre-treated rats. Neither 4-DAMP nor L-NAME had any effect in the normal rats, whereas PPADS reduced the micturition volume per event. In CYP pre-treated rats, 4-DAMP and L-NAME significantly increased voiding volumes per event and micturition frequency, respectively. 4-DAMP dose-dependently reduced the differences in micturition activity between saline and CYP pre-treated rats. We show that cystitis changes the urodynamics in conscious rats and that this change seems to depend on the production of NO and on altered muscarinic receptor effects. The altered muscarinic receptor responses are likely to per se involve NO-mediated mechanisms.

In vivo and in vitro pharmacological characterization of SVT-40776, a novel M3 muscarinic receptor antagonist, for the treatment of overactive bladder

BACKGROUND AND PURPOSE

Highly selective M(3) muscarinic receptor antagonists may represent a better treatment for overactive bladder syndrome, diminishing side effects. Cardiac side effects of non-selective antimuscarinics have been associated with activity at M(2) receptors as these receptors are mainly responsible for muscarinic receptor-dependent bradycardia. We have investigated a novel antimuscarinic, SVT-40776, highly selective for M(3) over M(2) receptors (Ki = 0.19 nmol.L(-1) for M(3) receptor affinity). This study reports the functional activity of SVT-40776 in the bladder, relative to its activity in atria.

EXPERIMENTAL APPROACH

In vitro and ex vivo (oral dosing) inhibition of mouse detrusor and atrial contractile responses to carbachol were used to study the functional activity of SVT-40776. The in vivo efficacy of SVT-40776 was characterized by suppression of isovolumetric spontaneous bladder contractions in anaesthetized guinea pigs after intravenous administration.

KEY RESULTS

SVT-40776 was the most potent in inhibiting carbachol-induced bladder contractions of the anti-cholinergic agents tested, without affecting atrial contractions over the same range of concentrations. SVT-40776 exhibited the highest urinary versus cardiac selectivity (199-fold). In the guinea pig in vivo model, SVT-40776 inhibited 25% of spontaneous bladder contractions at a very low dose (6.97 microg.kg(-1) i.v), without affecting arterial blood pressure.

CONCLUSIONS AND IMPLICATIONS

SVT-40776 is a potent inhibitor of M(3) receptor-related detrusor contractile activity. The absence of effects on isolated atria preparations represents an interesting characteristic and suggests that SVT-40776 may lack unwanted cardiac effects; a feature especially relevant in a compound intended to treat mainly elderly patients.

Cholinergic nitric oxide release from the urinary bladder mucosa in cyclophosphamide-induced cystitis of the anaesthetized rat

BACKGROUND AND PURPOSE

Previous reports have suggested that nitric oxide (NO) may be released by cholinergic stimuli in the rat bladder in cyclophosphamide-induced cystitis, affecting bladder function. In the current study, we evaluated the effects of cyclophosphamide-induced cystitis on muscarinic whole bladder contractile responses in vivo, and further, if NO might be released from the mucosa by cholinergic stimuli.

EXPERIMENTAL APPROACH

Male rats were pre-treated either with cyclophosphamide (100 mg kg(-1); to induce cystitis) or saline (serving as controls). 60 h later, rats were anaesthetized and bladder pressure monitored.

KEY RESULTS

The muscarinic receptor agonist methacholine (MeCh; 0.5-5 microg kg(-1) i.v.) induced similar contractions (i.e. bladder pressure increases) in inflamed bladders as in controls, which were attenuated dose-dependently by the muscarinic M1/M3/M5 antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; 0.1-1000 microg kg(-1) i.v.). In inflamed bladders, the cholinergic bladder contractions were enhanced after removing the mucosa, while cholinergic contractions were similar in intact and urothelium-denuded inflamed bladders in the presence of the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 30 mg kg(-1) i.v.). L-NAME attenuated the antagonistic effect of 4-DAMP on MeCh-induced contractions in intact inflamed bladders. However L-NAME did not affect the antagonism by 4-DAMP of MeCh-induced contractions of urothelium-denuded bladders, under control conditions or with cyclophosphamide-induced cystitis.

CONCLUSIONS AND IMPLICATIONS

In cyclophosphamide-induced cystitis, the cholinergic function of the bladder is altered. In the inflamed bladder, NO seems to be released via cholinergic stimuli through mucosal muscarinic M3/M5 receptors, presumably on urothelial cells, affecting bladder function.

Changes in muscarinic receptors in the toad urothelial cell line TBM-54 following acrolein treatment

1. In cyclophosphamide-induced cystitis in the rat, cholinergic function of the bladder and muscarinic receptor expression are altered. In the present study, we investigated whether the toad urothelial cell line TBM-54 expresses functional muscarinic receptors and whether changes in muscarinic receptors can be induced in vitro by treating cells with acrolein, a metabolite of cyclophosphamide causing cystitis. 2. The occurrence of muscarinic receptors on cells was assessed by microphysiometry, a method analysing receptor function by measuring changes in the extracellular acidity rate (ECAR) in response to receptor stimulation. 3. Challenging untreated cells with the muscarinic receptor agonist carbachol gave rise to a concentration-dependent increase in changes in ECAR, with a maximal response at 1 mmol/L carbachol of 51 +/- 6%. Pre-incubating cells with different muscarinic receptor antagonists (i.e. pirenzepine (M(1) receptor selective), methoctramine (M(2)/M(4) receptor selective) and 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP; M(3)/M(1)/M(5) receptor selective)), gave rise to a concentration-dependent decrease in the effects of carbachol (0.5 mmol/L) on changes in ECAR. 4. Western blot analysis was used to determine the expression of all muscarinic receptor subtypes (M(1)-M(5)) by the cell line. Following acrolein treatment, cells were markedly less sensitive to carbachol and the expression of muscarinic M(2) receptors was decreased, whereas the expression of muscarinic M(3) receptors was increased. 5. In conclusion, the urothelial cell line TBM-54 expresses functional muscarinic receptors and exposure to acrolein leads to a modulation in the expression of muscarinic receptors. Consequently, acrolein may have direct effects on muscarinic receptor function and expression that contribute to the pathogenesis of cyclophosphamide-induced cystitis.

In vitro characterization of parasympathetic and sympathetic responses in cyclophosphamide-induced cystitis in the rat

In cyclophosphamide-induced cystitis in the rat, detrusor function is impaired and the expression and effects of muscarinic receptors altered. Whether or not the neuronal transmission may be affected by cystitis was presently investigated. Responses of urinary strip preparations from control and cyclophosphamide-pretreated rats to electrical field stimulation and to agonists were assessed in the absence and presence of muscarinic, adrenergic and purinergic receptor antagonists. Generally, atropine reduced contractions, but in contrast to controls, it also reduced responses to low electrical field stimulation intensity (1-5 Hz) in inflamed preparations. In both types, purinoceptor desensitization with alpha,beta-methylene adenosine-5'-triphosphate (alpha,beta-meATP) caused further reductions at low frequencies (<10 Hz). The muscarinic receptor antagonists atropine, 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) ('M(1)/M(3)/M(5)-selective'), methoctramine ('M(2)-selective') and pirenzepine ('M(1)-selective') antagonized the tonic component of the electrical field stimulation-evoked contractile response more potently than the phasic component. 4-DAMP inhibited the tonic contractions in controls more potently than methoctramine and pirenzepine. In inflamed preparations, the muscarinic receptor antagonism on the phasic component of the electrical field stimulation-evoked contraction was decreased and the pirenzepine and 4-DAMP antagonism on the tonic component was much less efficient than in controls. In contrast to controls, methoctramine increased -- instead of decreased -- the tonic responses at high frequencies. While contractions to carbachol and ATP were the same in inflamed and in control strips when related to a reference potassium response, isoprenaline-induced relaxations were smaller in inflamed strips. Thus, in cystitis substantial changes of the efferent functional responses occur. While postjunctional beta-adrenoceptor-mediated relaxations are reduced, effects by prejunctional inhibitory muscarinic receptors may be increased.

Muscarinic receptors in the bladder: from basic research to therapeutics

Muscarinic receptor antagonists (antimuscarinics) serve as the cornerstone in the pharmacological management of overactive bladder (OAB) by relieving the symptoms of urgency, frequency and incontinence. These drugs operate primarily by antagonizing post-junctional excitatory muscarinic receptors (M(2)/M(3)) in the detrusor. The combination of pharmacological and gene knockout studies has greatly advanced our understanding of the functional role of muscarinic receptors in the bladder. M(3) receptors produce direct smooth muscle contraction by a mechanism that relies on entry of extracellular calcium through L-type channels and activation of a rho kinase. M(2) receptors, which predominate in number, appear to facilitate M(3)-mediated contractions. M(2) receptors can also produce bladder contractions indirectly by reversing cAMP-dependent beta-adrenoceptor-mediated relaxation, although the physiological role of beta-adrenoceptors in detrusor relaxation is controversial. Emerging evidence suggests that muscarinic receptors in the urothelium/suburothelium can modulate the release of certain factors, which in turn may affect bladder function at the efferent or afferent axis. Currently, oxybutynin, tolterodine, darifenacin, solifenacin and trospium are the five major antimuscarinics approved for the treatment of OAB. Comparative clinical studies have shown that oxybutynin and solifenacin may be marginally more effective than tolterodine, although the latter seems to be better tolerated. Pharmacokinetic-pharmacodynamic analyses using plasma concentrations of 'total drug' indicate that, at therapeutic doses, the clinical efficacy of darifenacin and solifenacin may be driven primarily by selective M(3) receptor occupation, whereas the pharmacodynamic effects of pan-selective molecules (such as tolterodine, trospium) may potentially involve multiple receptors, including M(2) and M(3). Furthermore, high M(3) receptor occupation is the likely explanation for the greater propensity of darifenacin and oxybutynin to cause dry mouth and/or constipation. Although the recently introduced drugs represent a significant improvement over older drugs, especially with respect to the convenience of dosing schedule, their overall efficacy and tolerability profile is still less than optimal and patient persistence with therapy is low. Recent advances in basic research have not yet offered a clear discovery path for improving the therapeutic index of antimuscarinic molecules. There is still an unmet need for an antimuscarinic medicine with superior clinical effectiveness that can translate into better persistence on therapy.

Postjunctional modulation by muscarinic M2 receptors of responses to electrical field stimulation of rat detrusor muscle preparations

1. The aim of the present study was to examine the modulator influence of muscarinic M(2) receptors on responses of rat urinary bladder detrusor muscle evoked by endogenous stimuli, i.e. by stimulation of the bladder innervation. 2. Responses were evoked by electrical field stimulation (EFS; 2-20 Hz, 0.8 ms, 60 V) of isolated strip preparations mounted in organ baths. The tension of the muscle strips was recorded digitally. EFS was performed by applying stimulation with either a short duration (5 s) or a longer duration (to reach peak response; approximately 20 s). 3. Effects of muscarinic receptor antagonists (muscarinic M(1)/M(3) receptor selective: 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP); muscarinic M(2) receptor selective: methoctramine), a beta-adrenergic antagonist (propranolol) and an adenosine receptor antagonist (8-p-sulfophenyltheophylline) were assessed on contractile activity and on poststimulatory relaxations. 4. Low concentrations of methoctramine (10(-8) m) reduced or tended to reduce the EFS-induced contraction, e.g. at 2 Hz by 12% while methoctramine at 10(-7) m had no significant effect. In addition, in the presence of 4-DAMP (10(-9) m), which tended to inhibit contractions at all frequencies (2-20 Hz; -17 to -25%), methoctramine at 10(-8) and 10(-7) m induced a further reduction of the contractile responses (-5 to -10%; 2-20 Hz). 5. The beta-adrenergic receptor antagonist propranolol (10(-6) m) and the adenosine receptor antagonist 8-p-sulfophenyltheophylline (10(-6) m) both increased contractile responses by 9-21% (2-10 Hz, long duration; P < 0.05-0.001) as a consequence of antagonizing relaxatory stimuli. Neither antagonist affected the contractile responses to EFS with the short duration stimulation. Poststimulatory relaxations were reduced by 30-60% (P < 0.05) by propranolol and by 40-60% (P < 0.001) by 8-p-sulfophenyltheophylline, but for 8-p-sulfophenyltheophylline only after stimulation with the short duration. 6. In the presence of methoctramine (10(-7) m), the 8-p-sulfophenyltheophylline-induced increases of the contractile response to long duration EFS were significantly enhanced at 10 Hz (+12 +/- 4%; P < 0.05), whereas no such enhancement of the propranolol inhibitory effect occurred in the presence of methoctramine. However, poststimulatory beta-adrenoceptor-evoked relaxations after short duration EFS were increased by about 35% in the presence of methoctramine, but not those after long duration. 7. Thus, muscarinic M(2) receptor activation inhibits adenosine receptor- and beta-adrenoceptor-evoked relaxations of the rat detrusor muscle. The inhibition occurs via a transient postjunctional mechanism that mainly affects responses with a short latency.

Distribution and function of muscarinic receptor subtypes in the ovine submandibular gland

The effects of muscarinic receptor antagonists on responses to electrical stimulation of the chorda-lingual nerve were determined in pentobarbitone-anesthetized sheep and correlated to the morphology of tissue specimens. Stimulation at 2 Hz continuously, or in bursts of 1 s at 20 Hz every 10 s, for 10 min induced similar submandibular fluid responses (19 ± 3 vs. 21 ± 3 μl·min−1·g gland−1), whereas vasodilatation was greater during stimulation in bursts (−52 ± 4 vs. −43 ± 5%; P < 0.01). Continuous stimulation at 8 Hz induced substantially greater responses (66 ± 9 μl·min−1·g gland−1 and −77 ± 3%). While atropine (0.5 mg/kg iv) abolished the secretory response at 2 and 20 Hz (1:10 s), a small response persisted at 8 Hz (<5%). The “M1-selective” antagonist pirenzepine (40 μg/kg iv) reduced the fluid response at all frequencies tested ( P < 0.05–0.01), most conspicuously at 2 Hz (reduced by 69%). Methoctramine (“M2/M4-selective”; 100 μg/kg iv; n = 5) had no effect on fluid or the vascular responses but increased the protein output at 2 (+90%, P < 0.05) and 8 Hz (+45%, P < 0.05). The immunoblotting showed distinct bands for muscarinic M1, M3, M4, and M5 receptors, and immunohistochemistry showed muscarinic M1 and M3 receptors to occur in the parenchyma. Thus muscarinic M1 receptors contribute to the secretory response to parasympathetic stimulation but have little effect on the vasodilatation in the ovine submandibular gland. Increased transmitter release caused by blockade of neuronal inhibitory receptors of the M4 subtype would explain the increase in protein output.

Altered muscarinic receptor subtype expression and functional responses in cyclophosphamide induced cystitis in rats

In the in vitro study, it was investigated whether the expression of muscarinic receptors and cholinergic responses were altered in the situation of experimental cystitis. Rats were treated with cyclophosphamide intraperitoneally and the bladders were excised 36-100 h later. Immunohistochemistry and immunoblotting showed all subtypes of the muscarinic receptor (M1-M5) to be present in the specimens from inflamed urinary bladders and controls. In the cyclophosphamide-treated rats, the expression of muscarinic M5 receptors was increased by more than 40 times (p<0.01; n=8) both in the smooth muscle and the urothelium. Both the maximal contractile response to carbachol and to a high potassium concentration was approximately halved in cyclophosphamide-treated tissues, whereas the reduction was substantially greater in response to low carbachol concentrations (<EC(50)). The administration of 4-DAMP inhibited the carbachol-induced contractile responses of inflamed strips less potently than of controls, whereas pirenzepine and methoctramine showed equipotency in the two groups. The nitric oxide synthase inhibitor l-NNA increased the contractile effect of carbachol in inflamed detrusor strips, while it had no effect in controls. Immunoblotting showed endothelial nitric oxide synthase (eNOS) to be up-regulated in cystitis, and immunohistochemistry revealed the change to occur in the urothelium and in the suburothelial layer. The alteration of cholinergic detrusor responses in cyclophosphamide-treated rats depends mainly on a general detriment of contractility but also on indirect effects possibly via nitric oxide synthesis. The most prominent histological alterations occurred in the urothelium in which muscarinic M5 receptors increased in particular. The study further underlines that the urothelium may play significant roles in the pathogenesis of urinary bladder disorders such as interstitial cystitis.

The M2 muscarinic receptor mediates in vitro bladder contractions from patients with neurogenic bladder dysfunction

Bladder muscle specimens from seven patients with neurogenic bladder dysfunction were analyzed to determine whether the muscarinic receptor subtype mediating contraction shifts from M(3) to the M(2) subtype as found in the denervated, hypertrophied rat bladder. Seven bladder specimens were analyzed from six female and one male patients. Six of the patients had traumatic cervical spinal cord injuries (C(4)-C(7)), and the other patient had an L(1) congenital myelomeningocele. This was compared with results from bladder specimens obtained from eight organ transplant donors. The affinities of three subtype-selective muscarinic receptor antagonists for inhibition of carbachol-induced contractions were determined. The affinity of the M(3) selective antagonists darifenacin or p-fluoro-hexahydrosiladifenadol (p-F-HHSiD) was determined in six of the seven spinal injury patient specimens. The affinity was consistent with M(2)-mediated contractions in four of these six specimens, intermediate between M(2) and M(3) in one specimen, and within the M(3) range in one specimen. The other specimen, tested only with the M(2) selective antagonist methoctramine, showed an M(3) affinity. In the organ donors, the affinity of p-F-HHSiD was within the M(2) range for six of seven specimens, whereas the affinity of darifenacin was within the M(3) range for five of six and intermediate between M(2) and M(3) for the other specimen tested. The affinity of methoctramine in both organ donor specimens tested was within the M(3) range. Whereas normal detrusor contractions are mediated by the M(3) receptor subtype, in patients with neurogenic bladder dysfunction as well as certain organ transplant donors, contractions can be mediated by the M(2) muscarinic receptor subtype.

Insulin regulates neuronal M2 muscarinic receptor function in the ileum of diabetic rats

Acetylcholine release from cholinergic nerves in the gastrointestinal tract is limited by neuronal M(2) muscarinic receptors. In diabetic animals, M(2) muscarinic receptor function in the ileum is increased, leading to decreased acetylcholine release and smooth muscle contraction in response to nerve stimulation. The mechanisms responsible for increased M(2) muscarinic receptor function are unknown but may contribute to the gastrointestinal dysmotility that occurs frequently in diabetics. In this study, we investigated whether insulin modulates M(2) muscarinic receptor function in the gastrointestinal tract of diabetic rats. M(2) muscarinic receptor function was tested by measuring the ability of an agonist, pilocarpine, to inhibit and an antagonist, methoctramine, to potentiate electrical field stimulation (EFS)-induced contraction of ileum in vitro. Insulin administration (0.2, 0.6, and 2 U s.c. daily for 7 days) reversed the diabetes-induced increase in M(2) muscarinic receptor function and restored normal contractions to EFS. Insulin had no effect on the function of postjunctional M(3) muscarinic receptors, determined by measuring contractile responses to acetylcholine. These data suggest that insulin tonically inhibits neuronal M(2) muscarinic receptors. Thus, loss of insulin removes this inhibition and increases M(2) muscarinic receptor function leading to decreased acetylcholine release and contraction to EFS. In nondiabetic rats, there was a trend that higher insulin doses (0.6 and 2 U) increased M(2) muscarinic receptor function, suggesting a bell-shaped concentration-response relationship for insulin. In conclusion, lack of insulin or excess insulin increases M(2) muscarinic receptor function in rat ileum. This mechanism may contribute to decreased acetylcholine release in the gastrointestinal tract of diabetics, resulting in dysmotility.

Presynaptic muscarinic receptor mechanisms and submandibular responses to stimulation of the parasympathetic innervation in bursts in rats

Submandibular secretory responses to electrical stimulation of the parasympathetic innervation at variable frequencies were measured in anaesthetized rats. Selective blockade by pirenzepine and by methoctramine occurred at doses (50 and of 300 nmol kg (-1), i.v., respectively) that did not inhibit the responses to exogenous acetylcholine. In the presence of methoctramine, the nerve-evoked fluid responses were increased by 100% at 1 Hz independently of the total number of impulses (10-300), suggesting that M2 receptor activation inhibits transmitter release. The magnitude of the increase was inversely related to frequency of stimulation. The protein concentrations in the fluid responses were not significantly affected by methoctramine. Pirenzepine had an inhibitory effect on the fluid secretory responses, which was dependent of frequency, as well as of number of impulses, suggesting that M1 receptor activation facilitates transmitter release. At 10 Hz given intermittently (for 1 s at 10-s intervals), pirenzepine reduced the fluid response by 25%. The protein release was substantially and significantly reduced by pirenzepine independent of frequency but only during long periods of stimulation (300 impulses). It is concluded that muscarinic M1 receptor activation normally has a facilitatory effect on transmitter release, and that the facilitation occurs during short, intense stimulation. Muscarinic M1 receptors are, however, likely to regulate protein secretion by other mechanisms. Muscarinic M2 receptors, on the other hand, normally inhibit cholinergic transmission at low frequencies. Similar to findings in the alimentary tract of several species, stimulation in bursts at high frequencies is a more efficient stimulation pattern than continuous low frequency stimulation. This pattern of stimulation thus takes advantage of transient facilitation and avoids the inhibition at less intense neuronal activity.

The subtypes of muscarinic receptors for neurogenic bladder contraction in rats

We evaluated in vivo functional selectivity profiles for muscarinic M(2) and M(3) subtypes of four muscarinic antagonists: Compound A (a novel muscarinic receptor antagonist with M(2)-sparing antagonistic activity), darifenacin, (a muscarinic M(3) receptor antagonist); methoctramine (a muscarinic M(2) receptor antagonist) and tolterodine (a nonselective muscarinic receptor antagonist), and compared the inhibition potency on distention-induced bladder contraction in rats. In an in vivo functional study, Compound A (0.03-10 mg/kg, i.v.) showed antimuscarinic activity with high selectivity for M(3) (salivation) over M(2) (bradycardia) (>100-fold). Darifenacin (0.01-0.3 mg/kg, i.v.) showed only slight selectivity for M(3) over M(2) (3.7-fold). Methoctramine (0.003-1 mg/kg, i.v.) showed the reverse selectivity profile (0.077-fold). Tolterodine (0.003-0.3 mg/kg, i.v.) showed less selectivity (1.2-fold). Compound A at M(3) inhibitory doses (0.1 and 0.3 mg/kg, i.v.) showed inhibition in a distention-induced neurogenic bladder contraction model, and its maximal inhibitory effects were about 60% at an even higher dose (3 mg/kg). Methoctramine at M(2) inhibitory doses (0.03 and 0.1 mg/kg, i.v.) did not significantly affect distention-induced bladder contraction. When tolterodine and darifenacin caused inhibition of distention-induced bladder contraction, its maximal inhibitory effects were similar to that of Compound A. Therefore, these findings suggest that Compound A would be an excellent pharmacological tool to give a better understanding of which subtypes of muscarinic receptors act in bladder function so far, and muscarinic M(3), but not M(2), receptors mainly mediate the cholinergic component of distention-induced bladder contraction.

Muscarinic receptors in isolated urinary bladder smooth muscle from different mouse strains

1. The pharmacological characteristics of muscarinic receptors in male and female mouse urinary bladder smooth muscle from different strains (C57Bl/6, 129/SvJ and hybrid backcross N1F2) were studied. 2. (+)-Cis-dioxolane, oxotremorine-M, acetylcholine, carbachol and pilocarpine induced concentration-dependent contractions of the urinary bladder smooth muscle (range for pEC(50)=6.4-6.6, 6.2-6.7, 6.2-6.4, 5.4-6.0 and 0.0-5.1, T(max)=1.9-4.7 g, 1.3-3.4 g, 1.0-3.0 g, 1.4-2.4 and 0.0-0.3 g, respectively, n=4-6 depending on the gender and the strain). In females, these contractions were competitively antagonized by a range of muscarinic receptor antagonists (pK(B) value range, depending on the strain): atropine (8.0-8.9), pirenzepine (6.1-6.4), 4-DAMP (7.6-8.4), methoctramine (5.6-6.1), p-F-HHSiD (7.5-7.7), zamifenacin (7.7-8.4) and darifenacin (8.2-8.7). 3. In recontraction studies, in which the muscarinic M(3) receptor population was decreased, and conditions optimized to study M(2) receptor activation, methoctramine exhibited an affinity estimate consistent with muscarinic M(3) receptors (pK(B)=6.26+/-0.08, pA(2)=6.31+/-0.07; pK(B)=6.09+/-0.22, pA(2)=6.08+/-0.01 for female inbred strain 129/SvJ and hybrid backcross N1F2, respectively) or intermediate between the one expected for this compound at M(2) and M(3) receptors, (pK(B)=6.66+/-0.08, pA(2)=7.00+/-0.27 for female inbred strain C57BL/6). 4. These data study suggest that muscarinic M(3) receptors are the predominant, if not the exclusive, subtype mediating contractile responses to muscarinic agonists in female mouse urinary bladder smooth muscle, with strain differences.

Muscarinic receptors of the urinary bladder: detrusor, urothelial and prejunctional

1. The parasympathetic nervous system is responsible for maintaining normal bladder function, contracting the bladder smooth muscle (detrusor) and relaxing the bladder outlet during micturition. 2. Contraction of the bladder involves direct contraction via M3 receptors and an indirect 're-contraction' via M2-receptors whereby a reduction in adenylate cyclase activity reverses the relaxation induced by beta-adrenoceptor stimulation. 3. Muscarinic receptors are also located on the epithelial lining of the bladder (urothelium) where they induce the release of a diffusible factor responsible for inhibiting contraction of the underlying detrusor smooth muscle. The factor remains unidentified but is not nitric oxide, a cyclooxygenase product or adenosine triphosphate. 4. Finally, muscarinic receptors are also located prejunctionally in the bladder on cholinergic and adrenergic nerve terminals, where M1-receptors facilitate transmitter release and M2 or M4-receptors inhibit transmitter release. 5. In pathological states, changes may occur in these receptor systems resulting in bladder dysfunction. Muscarinic receptor antagonists are the main therapeutic agents available for treatment of the overactive bladder, but whether their therapeutic effect involves actions at all three locations (detrusor, prejunctional, urothelial) has yet to be established.

Actions of the new antimuscarinic compound Lu 25-109 on isolated human and pig detrusor

The aim of this study was to evaluate the effects of a novel antimuscarinic agent, Lu 25-109, a partial M1 receptor agonist, and M2/M3 receptor antagonist in human and pig detrusor to establish its affinity for muscarinic receptors in human detrusor and parotid gland and to compare the results with those obtained with oxybutynin. Effects on the detrusor were determined as regards the ability to inhibit carbachol-induced contractions and contractions induced by electrical field stimulation (EFS). Radioligand binding studies were performed to assess the ability to displace quinuclidinyl benzilate (3H-QNB) from muscarinic receptors in the detrusor and parotid gland. Lu 25-109 produced a concentration-dependent rightward shift of the concentration-response curves for carbachol in both human and pig detrusor, the pK(b) values being 6.2+/-0.1 (n=6) and 5.8+/-0.3 (n=6). Corresponding values for oxybutynin were 7.9+/-0.1 (n=7) and 7.8+/-0.1 (n=6). Contractions induced by EFS in human detrusor were almost completely inhibited by 100 micromol/L Lu 25-109 (84+/-4%; n=4). In contrast, EFS-induced contractions in pig detrusor were less sensitive to Lu 25-109, resulting in a final inhibition of 32+/-6% (n=9) with the highest concentration used (100 micromol/L). This difference in effect between human and pig detrusor was not observed with oxybutynin. Radioligand binding experiments demonstrated a small difference in affinity for Lu 25-109 in the parotid gland compared with the bladder, the pKi values being 6.2+/-0.1 versus 6.5+/-0.1 (n=4). Corresponding values for oxybutynin were 8.5+/-0.1 versus 8.2+/-0.1 (n=4). The results show that Lu 25-109 competitively and effectively antagonizes carbachol-induced contractions and contractions induced by EFS in human detrusor muscle. Even if Lu 25-109 were less potent than oxybutynin, it has an effect profile that makes it of interest to test its ability to counteract bladder overactivity in humans.

On the functional role of muscarinic M2 receptors in cholinergic and purinergic responses in the rat urinary bladder

The functional effects of muscarinic receptor and purinoceptor agonists and antagonists were studied on isolated strip preparations of the rat urinary bladder. The muscarinic "M3/M1-selective" receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) most conspicuously inhibited the carbachol-evoked contractile responses (pA2=9.8), while the muscarinic "M1-selective" receptor antagonist pirenzepine and the muscarinic "M2-selective" receptor antagonist methoctramine were less potent (pA2=7.0 and 6.5, respectively). Administration of 4-DAMP in combination with methoctramine in selective dosages gave no significant additional reduction of carbachol-evoked contractile responses. Adenosine 5'-triphosphate (ATP) elicited transient dose-dependent contractile responses and it caused relaxation of the carbachol-contracted detrusor strips. The relaxatory response was enhanced in the presence of methoctramine and furthermore, was attenuated by the adenosine receptor antagonist 8-p-sulfophenyltheophylline. Administration of 2-chloro-adenosine to pre-contracted strips tended to cause dose-dependent relaxations, which were significantly increased in the presence of methoctramine. The purinergic contractile response, on the other hand, was not affected by methoctramine. Thus, the results are consistent with the cholinergic contractile response in the rat urinary bladder being exerted via activation of muscarinic M3 receptors, while the muscarinic M2 receptors exerted a modulator effect on purine-evoked relaxations in the rat urinary bladder.

Bladder instability: a re-appraisal of classical experimental approaches and development of new therapeutic strategies

1 Despite the growing social interest in human urinary tract disorders, the aetiology of detrusor instability remains poorly understood. Myogenic and neural impairment of detrusor activity caused by CNS or autonomic injuries can results in dysfunctions of normal voiding of the bladder such as urinary incontinence. 2 The contractility of human detrusor smooth muscle is critically dependent on acetylcholine-induced muscarinic receptor activation. Biochemical and functional in vivo and in vitro studies suggest the presence of an heterogeneous population of muscarinic receptor subtypes (M1-M4) localized at muscular and neutral sites. There is increasing evidence on the prejunctional auto- and hetero-regulation of acetylcholine release from parasympathetic nerve endings in modulating detrusor muscle contraction during micturition. 3 Activation of P2X purinoreceptors closely associated with the parasympathetic varicosities seems to be implicated to varying extent in the contractility in normal or instable human detrusor. Interestingly, P2X(1) subtype expression on smooth muscle increases considerably in the symptomatically obstructed bladder. A striking absence of P2X(3) and P2X(5) subtypes was observed in the cholinergic innervation of detrusor from patients with urgent incontinence. Thus, it is likely that alteration of the neural acetylcholine control can play a critical role in pathological states. 4 If the failures in storage and voiding can be recognized urodynamically, considerable difficulties remain in investigating the underlying functional changes especially because the study of the pathophysiology requires techniques that can be justified in animals but not in humans. 5 Recently, to solve this problem an alternative technique using human smooth muscle cells in culture has been developed. Human cell lines may be relevant in investigating the molecular pathways in physiological and pathological conditions. 6 The potential development of novel molecular therapeutic strategies such as gene therapy and tissue engineering is also discussed.

Pharmacological characterization of muscarinic receptors in mouse isolated urinary bladder smooth muscle

The pharmacological characteristics of muscarinic receptors in the male mice urinary bladder smooth muscle were studied. (+)-Cis-dioxolane, oxotremorine-M, acetylcholine, carbachol and pilocarpine induced concentration-dependent contractions of the urinary bladder smooth muscle (pEC(50)=6.6+/-0.1, 6.9+/-0.1, 6.7+/-0.1, 5.8+/-0.1 and 5.8+/-0.1, E(Max)=3.2+/-0.8 g, 2.7+/-0.4 g, 1.0+/-0.1 g, 2.7+/-0.3 and 0.9+/-0.2 g, respectively, n=4). These contractions were competitively antagonized by a range of muscarinic receptor antagonists (pK(B) values): atropine (9.22+/-0.09), pirenzepine (6.85+/-0.08), 4-DAMP (8.42+/-0.14), methoctramine (5.96+/-0.05), p-F-HHSiD (7.48+/-0.09), tolterodine (8.89+/-0.13), AQ-RA 741 (7.04+/-0.12), s-secoverine (8.21+/-0.09), zamifenacin (8.30+/-0.17) and darifenacin (8.70+/-0.09). In this tissue, the pK(B) values correlated most favourably with pK(i) values for these compounds at human recombinant muscarinic M(3) receptors. A significant correlation was also noted at human recombinant muscarinic m5 receptors given the poor discriminative ability of ligands between M(3) and m5 receptors. In recontraction studies, in which the muscarinic M(3) receptor population was decreased, and conditions optimized to study M(2) receptor activation, methoctramine exhibited an affinity estimate consistent with muscarinic M(3) receptors (pK(B)=6.23+/-0.14; pA(2)=6.16+/-0.03). Overall, these data study suggest that muscarinic M(3) receptors are the predominant, if not the exclusive, subtype mediating contractile responses to muscarinic agonists in male mouse urinary bladder smooth muscle.

Characterisation of the prejunctional inhibitory muscarinic receptor on cholinergic nerves in the rat urinary bladder

The nature of the prejunctional inhibitory muscarinic receptor on cholinergic nerve endings in the rat urinary bladder was investigated by measuring stimulated endogenous acetylcholine release via high pressure liquid chromatography (HPLC), in the presence of various selective muscarinic antagonists. The rank order of potencies for the antagonists used was: atropine (-log concentration = 7.8) > 4-DAMP (4-diphenylacetoxy-N-methylpiperidine) (7.6) > tripitramine (7.3) = HHD (hexahydrodifenidol) (7.3) > pFHHSiD (p-fluoro-hexahydrosiladifenidol hydrochloride) (7.0) > himbacine (6.5) > methoctramine (5.9) > or = pirenzepine (5.8) > gallamine (4.3). A comparison of the antagonist potencies obtained, with affinity constants at muscarinic M(1) to M(5) receptors, suggests that the prejunctional inhibitory muscarinic receptor is of the M(4) receptor subtype.

Pharmacological characterization of muscarinic receptors in dog isolated ciliary and urinary bladder smooth muscle

1. The pharmacological characteristics of muscarinic receptors mediating contraction of dog isolated ciliary muscle were determined and compared to those mediating contraction of dog urinary bladder smooth muscle. 2. (+)-Cis-dioxolane induced concentration-dependent contractions of ciliary muscle (pEC50=7.18+/-0.07, Emax=453+/-64 mg, n=19) and urinary bladder isolated smooth muscle (pEC50=6.55+/-0.07, Emax=11+/-1 g, n=19). These responses were antagonized by several muscarinic receptor antagonists (pKb values for the ciliary muscle and the bladder smooth muscle, respectively): atropine (8.25+/-0.14 and 9.21+/-0.09), pirenzepine (6.31+/-0.13 and 6.70+/-0.25), tolterodine (7.97+/-0.14 and 8.68+/-0.12), oxybutynin (7.40+/-0.08 and 7.88+/-0.12), zamifenacin (6.46+/-0.19 and 7.69+/-0.11), S-secoverine (6.66+/-0.14 and 8.13+/-0.07), AQ-RA 741 (6.16+/-0.15 and 7.08+/-0.23), p-F-HHSiD (7.10+/-0.27 and 7.35+/-0.07) and responses were not antagonized by PD 102807 (up to 100 nM). 3. In urinary bladder smooth muscle, the profile of antagonist pKB values correlated significantly with pK(i) values at human recombinant m3 muscarinic receptors, suggesting that M3 muscarinic receptors mediated the response. In the ciliary muscle, a significant (P<0.01) correlation was obtained with human recombinant m3 and m5 receptors. 4. Darifenacin displayed insurmountable antagonism at receptors in the bladder. At receptors in the ciliary muscle, it exhibited two phases of antagonism, comprising an initial low affinity (pKB<6) component and a high affinity phase (pKB>8). 5. The role of pigmentation in the atypical behaviour of darifenacin was examined. In blue coloured eyes, darifenacin produced apparent surmountable, competitive antagonism of the responses to (+)-cis-dioxolane (pKB=8.76+/-0.07). The antagonist profile obtained in this tissue suggested the involvement of a site which has the pharmacological attributes of the M5 receptor. 6. We suggest that the dog urinary bladder contracts in response to M3 muscarinic receptor activation. Contraction of the brown-eyed dog ciliary muscle is more complex and may include involvement of at least two receptors, possibly the M5 and M3 receptor, whereas blue-eyed dog ciliary muscle may involve a single population of M5 muscarinic receptors.

Comparison of the effects of novel antimuscarinic drugs on human detrusor smooth muscle

OBJECTIVE

To evaluate the effects of tolterodine, vamicamide and temiverine, novel antimuscarinic drugs developed for the treatment of frequency and urinary incontinence, on human detrusor smooth muscle. Materials and methods Specimens of human urinary bladder were obtained from 20 patients who underwent total cystectomy for malignant bladder tumour. Using an organ-bath technique, the effects of various drugs on the contractions induced by carbachol, KCl, CaCl2 and electrical field stimulation in the detrusor strips were investigated.

RESULTS

Carbachol (0.001-10000 micromol/L) caused concentration-dependent contractions in human detrusor smooth muscles. Tolterodine (0.01-10 micromol/L), vamicamide (0.01-10 micromol/L), temiverine (0.01-1 micromol/L) and atropine (0.001-1 micromol/L) caused parallel shifts to the right of the concentration-response curves to carbachol. All slopes for the regression line of Schild plots were close to unity, and the rank order of pA2 values was atropine = tolterodine > vamicamide > temiverine. Tolterodine, vamicamide and atropine did not inhibit the maximum contractile responses to carbachol, while temiverine (10 micromol/L) significantly inhibited the maximum contractions. Tolterodine (0.001-1 micromol/L) and vamicamide (0.01-10 micromol/L) did not inhibit the KCl- (80 mmol/L) and CaCl2-induced (5 mmol/L) contractions, while temiverine (0.01-10 micromol/L) significantly inhibited the contractions. Electrical field stimulation (2-60 Hz) caused frequency-dependent contractions in human detrusor smooth muscles, which were significantly inhibited by various drugs. In the presence of 1 micromol/L atropine, tolterodine and vamicamide did not inhibit the contractions induced by electrical field stimulation at all frequencies, while temiverine (10 micromol/L) significantly inhibited the contractions.

CONCLUSION

Tolterodine and vamicamide inhibited contractions of human detrusor smooth muscles only through their antimuscarinic action, while temiverine had both antimuscarinic and calcium-antagonist actions. Furthermore, these novel drugs have different efficacies and potencies for inhibiting human detrusor smooth muscle.

Prejunctional muscarinic receptors modulating acetylcholine release in rabbit detrusor smooth muscles

We investigated the presence and subtypes of functionally prejunctional receptors in cholinergic nerve endings of rabbit detrusor smooth muscle strips using high-performance liquid chromatography coupled with a microdialysis procedure. The effects of pretreatment with various drugs on acetylcholine (ACh) release and contractile responses induced by electrical field stimulation were evaluated. Although atropine (a muscarinic nonselective antagonist) and 4-DAMP (a muscarinic M3 antagonist) did not influence the ACh release, they markedly reduced the contractile responses. Pirenzepine (M1 antagonist) decreased ACh release and contractile responses. Methoctramine (M2 antagonist) increased the ACh release, but did not influence to the contractile responses. These results suggest that the muscarinic receptors in the rabbit detrusor smooth muscle are heterogeneous, prejunctional facilitatory (M1 receptors), and inhibitory (M2 receptors) for ACh release and postjunctional M3 receptors mediating contractile responses.

Muscarinic receptor antagonists in the treatment of overactive bladder

A wealth of clinical evidence supports the view that muscarinic receptor antagonists are effective in the treatment of overactive bladder. However, treatment-limiting adverse effects such as dry mouth, constipation, and blurred vision have restricted the usefulness of previously available agents, such as oxybutynin. A real need therefore existed for effective and well-tolerated agents for the long-term management of the troublesome symptoms of overactive bladder. This review outlines the various approaches that have been used in attempts to overcome the tolerability problems of oxybutynin. It also describes how advances in our understanding of muscarinic receptors and bladder function has led to the potential development of either tissue- or subtype-selective antimuscarinic agents with improved tolerability. Drugs that have been developed in this way include tolterodine and darifenacin, each of which shows some bladder selectivity in animal models. Unlike darifenacin, however, the bladder selectivity of tolterodine has been confirmed by numerous clinical studies. Tolterodine's improved tolerability compared with oxybutynin, along with its equivalent therapeutic efficacy at recommended dosages, permits patients to experience the beneficial effects of long-term treatment. Tolterodine therefore represents a real alternative for the long-term management of overactive bladder. The results of ongoing clinical studies with darifenacin are awaited before it can be concluded that selective antagonism of M(3) receptors leads to improved tolerability over existing agents in the treatment of overactive bladder. Similarly, the potential improvements in tolerability associated with different dosage formulations of oxybutynin, and the clinical utility of S-oxybutynin, are yet to be conclusively demonstrated.

Pharmacological characterization of muscarinic receptors implicated in rabbit detrusor muscle contraction and activation of inositol phospholipid hydrolysis in rabbit detrusor and parotid gland

In the present study, we evaluated the pharmacological characteristics of the functional muscarinic receptors implicated in rabbit detrusor contraction and coupled to inositol phospholipid turnover in rabbit detrusor and parotid gland. The selectivity of several muscarinic antagonists for detrusor vs. salivary gland muscarinic receptors was also examined. The affinities for the muscarinic m1-, m2- and m3-receptor subtypes were determined using membranes from human cloned receptors expressed in CHO-K1 cells using [3H]-N-methyl scopolamine as a radioligand. Anti-muscarinic activity was determined in isolated rabbit detrusor by measuring the displacement of the contractile response to carbachol, and in rabbit detrusor and rabbit parotid by measuring the displacement of inositol phospholipid hydrolysis (total inositol phosphate accumulation) to carbachol. A significant correlation was found between the potencies to antagonize carbachol-induced rabbit detrusor contraction (pK(B)) and the affinities (pKi) for the m3-receptor subtype (r = 0.93, P = 5 x 10(-6)). Lower, but significant, correlations [0.88 (P = 6.3 x 10(-5)), 0.72 (P = 4.6 x 10(-3))] were obtained with m1- or m2-receptor subtypes, respectively. Each muscarinic antagonist tested displayed similar potency to antagonize carbachol-stimulated inositol phospholipid hydrolysis in rabbit detrusor and parotid (r = 0.96, P = 8 x 10(-3)). A significant correlation was found between the potencies to antagonize carbachol-stimulated inositol phospholipid hydrolysis (pK(B)), determined in rabbit detrusor and rabbit parotid, and the affinities (pK(i)) for the m3-receptor subtype [r = 0.96 (P = 0.01), 0.99 (P = 5 x 10(-5)), respectively] and for the m1-receptor subtype [r = 0.98 (P = 3.5 x 10(-3)), 0.94 (P = 0.02), respectively] but not for the m2-receptor subtype [r = 0.33, 0.57, ns, respectively]. In each in vitro assay, methoctramine (preferential M2 selective antagonist) and pirenzepine (preferential M1 selective antagonist) were slightly potent. We suggest that the muscarinic receptor implicated in the response to carbachol in rabbit detrusor and parotid gland corresponds to the M3-subtype. None of the muscarinic antagonists studied in rabbit tissues displayed preferential affinity for the detrusor.

Neuronal soma-dendritic and prejunctional M1-M4 receptors in gastrointestinal and genitourinary smooth muscle

A variety of neurons in gastrointestinal and genitourinary smooth muscle express muscarinic auto- as well as heteroreceptors. These receptors are found on the soma and dendrites of many cholinergic, sympathetic and NANC neurons and on axon terminals. A given neuron may contain both excitatory and inhibitory presynaptic muscarinic receptors. The subtypes involved are species- and tissue-dependent, and neuronal M1 to M4 receptors have been shown to be expressed in smooth muscle tissues. In this study, the ability of several selective muscarinic receptor antagonists to inhibit the effect of arecaidine propargyl ester (APE) on prejunctional muscarinic receptors on sympathetic nerve endings in the rabbit anococcygeus muscle (RAM) was investigated to characterise the receptor subtype involved. Electrical field stimulation (EFS) resulted in a release of noradrenaline (NA) eliciting monophasic contractions due to stimulation of postjunctional alpha1-adrenoceptors. The selective muscarinic agonist APE did not reduce contractions to exogenous NA, but caused a concentration-related and N-methylatropine-sensitive inhibition of neurogenic responses. All muscarinic antagonists investigated failed to affect the EFS-induced contractions, but shifted the concentration-response curve of APE to the right in a parallel and surmountable fashion. Schild analysis yielded regression lines of unit slope, indicating competitive antagonism. The following rank order of antagonist potencies (pA2 values) was found: tripitramine (9.10) > AQ-RA 741 (8.26) > or = himbacine (8.04) > or = (S)-dimethindene (7.69) > pirenzepine (6.46) > or = p-F-HHSiD (6.27). A comparison of the pA2 values determined in the present study with literature binding and functional affinities obtained at native or recombinant M1 to M5 receptors strongly suggests that NA release from sympathetic nerve endings in RAM is inhibited by activation of prejunctional muscarinic M2 receptors.

Function, signal transduction mechanisms and plasticity of presynaptic muscarinic receptors in the urinary bladder

Presynaptic M1 muscarinic receptors on parasympathetic nerve terminals in rat urinary bladder strips are involved in an autofacilitatory mechanism that markedly enhances acetylcholine release during continuous electrical field stimulation. The facilitatory muscarinic mechanism is dependent upon a PKC mediated second messenger pathway and influx of extracellular Ca2+ into the parasympathetic nerve terminals via L and N-type Ca2+ channels. Prejunctional muscarinic facilitation has also been detected in human bladders. The muscarinic facilitatory mechanism is upregulated in hyperactive bladders from chronic spinal cord transected rats; and the facilitation in these preparations is primarily mediated by M3 muscarinic receptors. Presynaptic muscarinic receptors represent a new target for pharmacological treatment of bladder hyperactivity. If presynaptic facilitation is restricted to the bladder and not present in other tissues then drugs acting at this site might be expected to exhibit uroselectivity.

Muscarinic receptor subtypes modulating smooth muscle contractility in the urinary bladder

Normal physiological voiding as well as generation of abnormal bladder contractions in diseased states is critically dependent on acetylcholine-induced stimulation of contractile muscarinic receptors on the smooth muscle (detrusor) of the urinary bladder. Muscarinic receptor antagonists are efficacious in treating the symptoms of bladder hyperactivity, such as urge incontinence, although the usefulness of available drugs is limited by undesirable side-effects. Detrusor smooth muscle is endowed principally with M2 and M3 muscarinic receptors with the former predominating in number. M3 muscarinic receptors, coupled to stimulation of phosphoinositide turnover, mediate the direct contractile effects of acetylcholine in the detrusor. Emerging evidence suggests that M2 muscarinic receptors, via inhibition of adenylyl cyclase, cause smooth muscle contraction indirectly by inhibiting sympathetically (beta-adrenoceptor)-mediated relaxation. In certain diseased states, M2 receptors may also contribute to direct smooth muscle contraction. Other contractile mechanisms involving M2 muscarinic receptors, such as activation of a non-specific cationic channel and inactivation of potassium channels, may also be operative in the bladder and requires further investigation. From a therapeutic standpoint, combined blockade of M2 and M3 muscarinic receptors would seem to be ideal since this approach would evoke complete inhibition of cholinergically-evoked smooth muscle contractions. However, if either the M2 or M3 receptor assumes a greater pathophysiological role in disease states, then selective antagonism of only one of the two receptors may be the more rational approach. The ultimate therapeutic strategy is also influenced by the extent to which pre-junctional M1 facilitatory and M2 inhibitory muscarinic receptors regulate acetylcholine release and also which subtypes mediate the undesirable effects of muscarinic receptor blockade such as dry mouth. Finally, the consequence of muscarinic receptor blockade in the central nervous system on the micturition reflex, an issue which is poorly studied and seldom taken into consideration, should not be ignored.

Pharmacological effects of tolterodine on human isolated urinary bladder

Tolterodine, (R)-N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamine+ ++, is an antimuscarinic drug developed for the treatment of overactive bladder with symptoms of frequency, urgency and urge incontinence. We investigated the effects of tolterodine and its major active metabolite, DD 01 (PNU-200577), (R)-N,N-diisopropyl-3-(2-hydroxy-5-hydroxymethylphenyl)-3-phenylpropa namine, on the contractions induced by carbachol, KCl, CaCl2 and electrical field stimulation in human isolated urinary bladder smooth muscles, using the muscle bath technique. Specimens of human urinary bladder were obtained from 20 patients who underwent total cystectomy due to malignant bladder tumor. The detrusor preparations were taken from the intact part of the dome region of the bladder. Carbachol (10(-9)-10(-2) M) caused concentration-dependent contraction of human detrusor smooth muscles. Tolterodine (10(-9)-10(-6) M), DD 01 (10(-9)-10(-6) M), oxybutynin (10(-8)-10(-6) M), propiverine (10(-8)-10(-6) M), atropine (10(-9)-10(-6) M), pirenzepine (10(-8)-10(-5) M), methoctramine (10(-8)-10(-5) M) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) (10(-9)-10(-6) M) caused typical shifts to the right of the concentration-response curves for carbachol, except for higher concentrations (10(-5) M) of oxybutynin and propiverine, which caused a decrease of about 30% of the maximum contractile responses to carbachol. All the slopes of the regression lines of Schild plots were close to unity, and the rank order of pA2 values was: atropine = DD 01 = tolterodine = 4-DAMP = oxybutynin > propiverine = pirenzepine > methoctramine. Tolterodine (10(-9)-10(-6) M) and DD 01 (10(-9)-10(-6) M) did not inhibit the KCl-induced (80 mM) and CaCl2-induced (5 mM) contractions, while oxybutynin (10(-8)-10(-5) M) and propiverine (10(-8)-10(-5) M) significantly inhibited the contractions. Electrical field stimulation (2-60 Hz) caused frequency-dependent contraction of human detrusor smooth muscles, which were significantly inhibited by various drugs. In the presence of 10(-6) M atropine, tolterodine and DD 01 did not inhibit the residual contractions induced by electrical field stimulation at any of the frequencies, while oxybutynin (10(-5) M) and propiverine (10(-5) M) significantly inhibited the atropine-resistant part of the contractions. The results suggest that the inhibitory effects of tolterodine and DD 01 are mediated only by their antimuscarinic action, which is equal to that of oxybutynin and significantly greater than that of propiverine, and that tolterodine and DD 01 have neither Ca2+ channel antagonist action nor inhibitory effect on the atropine-resistant part of the contractions in human detrusor smooth muscles. These findings support the usefulness of tolterodine as a therapeutic drug for overactive bladder with symptoms of frequency, urgency and urge incontinence.