Muscarinic receptor subtypes in the submandibular gland and the urinary bladder of the rabbit: in vivo and in vitro functional comparisons of receptor antagonists

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.

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.

β1/2 or M2/3 Receptors Are Required for Different Gastrointestinal Motility Responses Induced by Acupuncture at Heterotopic or Homotopic Acupoints

Acupuncture at homotopic acupoints or heterotopic acupoints is known to either inhibit or facilitate gastrointestinal motility, depending on the acupoint location. However, little effort has been made to investigate the roles of specific receptors (such as adrenergic and muscarinic acetylcholine receptors) in mediating the effects of acupuncture at heterotopic and homotopic acupoints. Different adrenergic receptor subtypes or cholinergic receptor subtypes are predominantly expressed in various sections of the gut, resulting in variations between the effects of acupuncture at heterotopic or homotopic acupoints on gastrointestinal motility. Here, we investigated the role of β₁/β₂ receptors and M₂/M₃ receptors in gastrointestinal motility regulated by acupuncture at ST37, a heterotopic acupoint, and ST25, a homotopic acupoint, by simultaneously recording intraluminal pressures in the distal colon and stomach or jejunum and examining fecal phenol red excretion in β_1/2 receptor-knockout mice and M_2/3 receptor-knockout mice. We found that knockout of the M_2/3 receptor significantly inhibited ST37 acupuncture-induced enhancement of gastric motility, jejunal motility, and colonic motility. Additionally, knocking out of the β_1/2 receptor significantly diminished the ST25 acupuncture-induced inhibition of gastric motility and jejunal motility without significantly altering the enhancement of colonic motility induced by acupuncture at ST25. Acupuncture at ST37 significantly accelerated gastrointestinal transition in β_1/2 receptor-knockout mice and their wild-type littermates. However, this acceleration of gastrointestinal transition was markedly diminished in M_2/3 receptor-knockout mice relative to their wild-type littermates. Acupuncture at ST25 significantly increased gastrointestinal transition in β_1/2 receptor-knockout mice and significantly decreased gastrointestinal transition in M_2/3 receptor-knockout mice without altering gastrointestinal transition in wild-type littermates of either. Our study revealed that M_2/3 receptors are required for the gastrointestinal motility associated with whole gastrointestinal transition enhanced by acupuncture at heterotopic acupoints, whereas β_1/2 receptors are required for the same gastrointestinal motility processes inhibited by acupuncture at homotopic acupoints. Therefore, our findings reveal important biological mechanisms underlying acupuncture treatment of disorders involving gastrointestinal motility dysfunction.

AE9C90CB: a novel, bladder-selective muscarinic receptor antagonist for the treatment of overactive bladder

BACKGROUND AND PURPOSE

AE9C90CB (N- [(1R, 5S, 6R)-3-azabicyclo [3.1.0] hex-6-ylmethyl]-2-hydroxy-N-methyl-2, 2-diphenylacetamide), a novel muscarinic receptor antagonist, was synthesized for the treatment of overactive bladder. Here we describe the in vitro and in vivo profiles of AE9C90CB for action in bladder over salivary gland and compare it with four agents already in clinical use (tolterodine, oxybutynin, solifenacin and darifenacin).

EXPERIMENTAL APPROACH

Radioligand binding assay and isolated tissue-based functional assay were used to evaluate affinity, potency, and receptor subtype selectivity of compounds. Inhibition of carbachol-induced increase in intravesicular pressure and salivary secretion were measured in anaesthetized rabbits to assess the functional selectivity.

KEY RESULTS

In vitro radioligand binding study using human recombinant muscarinic receptors showed that AE9C90CB had greater affinity for M(3) muscarinic receptors with pKi of 9.90 +/- 0.11 and was 20-fold more selective for M(3) than for M(2) muscarinic receptors. AE9C90CB exhibited an unsurmountable antagonism on rat bladder strips (pK(B), 9.13 +/- 0.12). In anaesthetized rabbits after intravenous administration, AE9C90CB dose dependently inhibited carbachol-induced increase in intravesicular pressure and salivary secretion, and exhibited functional selectivity for urinary bladder over salivary gland which was ninefold better than that of oxybutynin.

CONCLUSIONS AND IMPLICATIONS

We have identified AE9C90CB, a compound exhibiting moderate selectivity for M(3) over M(2) receptors but greater selectivity for urinary bladder over salivary gland than oxybutynin, tolterodine, solifenacin and darifenacin. Therefore, AE9C90CB may be a promising compound for the treatment of overactive bladder with reduced potential to cause dry mouth than currently available antimuscarinic drugs.

Expression of neural receptors in mouse meibomian gland

PURPOSE

To investigate gene and protein expression profiles of neural receptors found in the mouse meibomian gland. RNA and protein levels were determined for neuropeptide Y (NPY) receptor, vasoactive intestinal peptide (VIP) receptor, substance P (SP) receptor, and muscarinic cholinergic receptor (mAChR) subtypes M1-M5 in the mouse meibomian gland.

METHODS

Frozen sections of Balb/c mouse eyelids were subjected to laser capture microdissection to isolate pure samples of meibomian gland ductal and acinar cells. Real-time polymerase chain reaction, immunolabeling, and Western blot analysis for SP receptor, VIP receptor, NPY receptor, and mAChR subtypes M1-M5 were performed on meibomian gland ductal and acinar cells.

RESULTS

Expression of NPY1 receptor, VIP receptor 1, SP receptor, and all 5 mAChR subtypes was found in all meibomian gland ductal and acinar cells analyzed by real-time polymerase chain reaction. Immunolabeling and Western blot analysis confirmed the presence of NPY1 receptor, VIP receptor 1, SP receptor, and all 5 mAChR subtypes in the meibomian gland. The levels were variable with the duct showing greater levels of NPY1 receptor, SP receptor, and mAChRs 1, 2, 4, and 5 than with the gland.

CONCLUSIONS

VIP receptor 1, SP receptor, NPY1 receptor, and mAChR subtypes may be involved in the regulation of meibomian gland secretion. Laser capture microdissection in conjunction with gene expression analysis provides an excellent approach for studying meibomian gland cells about which relatively little is known at the molecular level.

Expression of muscarinic receptor subtypes in salivary glands of rats, sheep and man

In rat parotid, submandibular and sublingual glands and in ovine parotid and in human labial glands, the expression of muscarinic receptor subtypes was examined by immunoblotting and immunohistochemistry. Functional correlates were searched for in rat salivary glands. In the rat submandibular and sublingual glandular tissues clear signals of muscarinic M1 and M5 receptors could be detected in the immunoblotting and vague bands for muscarinic M3 and, in particular for, M4 receptors. The rat parotid gland differed. In this gland, the signal was less obvious for the muscarinic M1 receptor, and further, muscarinic M4 receptors appeared more strongly marked than in the submandibular glands. The results from the immunohistochemistry could be interpreted as the muscarinic M4 receptors are located on nerve fibres, since the outer layer of lobuli were densely stained. Intraglandular vessels in the rat submandibular and parotid glands showed expression of M3 receptors. In contrast to the parotid gland, the submandibular vessels also expressed M1 and M2 receptors. Occasionally M5 receptors appeared in the arteries and veins also. The functional studies in the rat confirmed muscarinic M1 receptor mediated secretion in the submandibular gland. Since the M1 receptor blockade did not affect submandibular blood flow, indirect vascular effects could not in total explain the secretory inhibition. Also in the human labial glands, muscarinic M1, M3 and M5 receptors occurred. No or low amounts of muscarinic M2 and M4 receptors could be detected. In patients with Sjögren-like symptoms an up-regulation of M3, M4 and M5 receptors was apparent in the labial glands. In ovine parotid glands all receptors could be detected, but constantly with vague bands for muscarinic M2 receptors. In conclusion, muscarinic M1 receptors seem to be expressed in seromucous/mucous glands. A secretory effect by muscarinic M5 receptors is not to be excluded, since they were expressed in all the glands examined. However, other functions, such as promotion of inflammation, cell growth and proliferation are possible as well.

Comparative in vivo uroselectivity profiles of anticholinergics, tested in a novel anesthetized rabbit model

The aim of this study was to describe a new experimental animal model for simultaneous measurement of carbachol-induced increase in intravesical pressure and salivary secretion in rabbits. Further, we also compared the in vivo potency and urinary bladder versus salivary gland selectivity profiles of Oxybutynin, Tolterodine, Solifenacin and Darifenacin. The intravesical pressure and salivary secretion were evoked by intra-arterial injection of carbachol (1.5 microg/kg). The carbachol-induced increase in intravesical pressure and salivation was simultaneously recorded before and after increasing doses of test drugs administered intravenously. The basal mean changes in intravesical pressure and salivation subsequent to carbachol administration were in the range of 6.7-7.5 mm Hg and 0.5-0.7 g respectively. Repeated administration of vehicle did not elicit any appreciable changes in intravesical pressure and salivary secretion to carbachol administration from the basal values till 3 h. All the test drugs exhibited a dose-dependent inhibition of carbachol-induced increase in intravesical pressure and salivary secretion. Darifenacin demonstrated a greater potency compared to other muscarinic receptor antagonists for inhibiting carbachol-induced increase in intravesical pressure. It also exhibited functional selectivity for the urinary bladder versus salivary gland. In contrast, Oxybutynin was functionally more selective in inhibiting carbachol-induced increase in salivary secretion. The observed urinary bladder versus salivary selectivity values were 0.6+/-0.2, 1.1+/-0.2, 1.7+/-0.5, and 2.3+/-0.5 for Oxybutynin, Tolterodine, Solifenacin and Darifenacin respectively. These results suggest that the functional selectivity of muscarinic receptor antagonists between urinary bladder and salivary glands can be readily detected in this model. Thus rabbits may represent a useful animal model for evaluating putative bladder selective muscarinic receptor antagonists for the treatment of overactive bladder.

Regulation of salivary gland function by autonomic nerves

Oral homeostasis is dependent upon saliva and its content of proteins. Reflex salivary flow occurs at a low 'resting' rate and for short periods of the day more intense taste or chewing stimuli evoke up to ten fold increases in salivation. The secretion of salivary fluid and proteins is controlled by autonomic nerves. All salivary glands are supplied by cholinergic parasympathetic nerves which release acetylcholine that binds to M3 and (to a lesser extent) M1 muscarinic receptors, evoking the secretion of saliva by acinar cells in the endpieces of the salivary gland ductal tree. Most salivary glands also receive a variable innervation from sympathetic nerves which released noradrenaline from which tends to evoke greater release of stored proteins, mostly from acinar cells but also ductal cells. There is some 'cross-talk' between the calcium and cyclic AMP intracellular pathways coupling autonomic stimulation to secretion and salivary protein secretion is augmented during combined stimulation. Other non-adrenergic, non-cholinergic neuropeptides released from autonomic nerves evoke salivary gland secretion and parasympathetically derived vasointestinal peptide, acting through endothelial cell derived nitric oxide, plays a role in the reflex vasodilatation that accompanies secretion. Neuronal type, calcium-activated, soluble nitric oxide within salivary cells appears to play a role in mediating salivary protein secretion in response to autonomimetics. Fluid secretion by salivary glands involves aquaporin 5 and the extent to which the expression of aquaporin 5 on apical acinar cell membranes is upregulated by cholinomimetics remains uncertain. Extended periods of autonomic denervation, liquid diet feeding (reduced reflex stimulation) or duct ligation cause salivary gland atrophy. The latter two are reversible, demonstrating that glands can regenerate provided that the autonomic innervation remains intact. The mechanisms by which nerves integrate with salivary cells during regeneration or during salivary gland development remain to be elucidated.

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.

Submandibular responses to stimulation of the parasympathetic innervation in anesthetized sheep

Submandibular secretory and vascular responses to stimulation of the parasympathetic innervation and the output of vasoactive intestinal peptide (VIP) were investigated in anaesthetized sheep in the presence and absence of atropine (>/=0.5 mg/kg). In the absence of atropine, parasympathetic stimulation caused an increase in the flow of saliva and a decrease in submandibular vascular resistance; the latter response persisted after the administration of atropine and was then significantly reduced at the lowest but not at the higher frequencies tested. The output of VIP from the gland was frequency dependent over the range of 10-20 Hz (continuously) and significantly increased after atropine (P < 0.02). Furthermore, the fall in vascular resistance was linearly related to log VIP output after total muscarinic blockade. Intracarotid infusions of synthetic VIP produced dose-dependent falls in submandibular vascular resistance, together with a corresponding increase in submandibular blood flow. It is concluded that the atropine-resistant vasodilatation that occurs in this gland during parasympathetic stimulation is likely to be due largely, if not entirely, to the release of VIP.

[Molecular biological researches of the lower urinary tract function]

Adrenergic alpha1 and beta receptors are present in the target organs of sympathetic nerve and they participate in the signal transduction mechanism of the lower urinary tract. Adrenergic alpha1 receptors are present in urethral and prostatic smooth muscles, and contract these muscles. Among these receptor subtypes, the alpha1-A receptor has the most important role, and mRNA expression of the corresponding alpha1-a subtype is predominant. In the human urinary bladder detrusor smooth muscle, the expression of adrenergic beta3 receptor subtype mRNA is predominant, and relaxation of detrusor smooth muscle is mediated mainly via beta3 receptor. Afferent nerve with lower threshold can easily transmit bladder sensation and takes an important role in the pathophysiology of urge urinary incontinence. Successful molecular cloning of vanilloid receptors, which are present in these afferent nerves, revealed that vanilloid receptors are ion-channels, sensitive for heat and pH, and termed VR1 and VRL1. Among purinergic receptors, ion channel type P2X3 receptor is found in afferent nerve fibers and plays some roles in the signal transduction of bladder sensation. In the near future, agonist for the adrenergic beta3 receptor and selective antagonists for VR1, VRL1, or P2X3 will possibly become drugs for pollakisuria and urge urinary incontinence.

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.

A benefit-risk assessment of extended-release oxybutynin

Oxybutynin is a muscarinic receptor antagonist, which has been available for a number of years in its original immediate-release (IR) formulation. While oxybutynin IR has proven effective for the treatment of overactive bladder, its extended use can be limited by adverse effects, particularly dry mouth. An extended-release (ER) formulation of oxybutynin based on the OROS system has recently become available, which allows once daily administration. In direct comparison to oxybutynin IR, oxybutynin ER has an increased oral bioavailability for the parent compound oxybutynin which is accompanied by a reduced bioavailability for the active metabolite N-desethyl-oxybutynin. The latter has been implicated in mediating a major part of the adverse effects of oxybutynin treatment. Two double-blind, placebo-controlled, randomised studies in patients with overactive bladder have demonstrated that oxybutynin ER has a similar efficacy as oxybutynin IR but with improved tolerability. This is in line with clinical pharmacological studies demonstrating a smaller impairment of saliva production with oxybutynin ER than with oxybutynin IR. Thus, the ER formulation of oxybutynin maintains the therapeutic benefits and concomitantly improves tolerability.

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.

Studies of muscarinic receptor subtypes in salivary gland function in anaesthetized rats

The in vivo study aimed to examine whether muscarinic receptor subtypes other than muscarinic M3 receptors exert exocrine functional roles in the rat salivary glands. The effects of pirenzepine, methoctramine and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) were examined on secretion from the major salivary glands evoked by acetylcholine (0.001-10 micromol kg(-1) i.v.) in pentobarbitone-anaesthetized rats. Observations were occasionally made on glandular blood flow. 4-DAMP (0.1-100 nmol kg(-1) i.v.) markedly and equipotently inhibited the acetylcholine-evoked fluid responses in all glands. Pirenzepine (0.1 micromol kg(-1) i.v.-10 mmol kg(-1) i.v.) showed significantly lower inhibitory potency than 4-DAMP, most conspicuously in the parotid, while methoctramine (0.1 micromol kg(-1) i.v.-10 mmol kg(-1) i.v.) exerted an even lesser inhibitory effect. Also against acetylcholine-evoked blood flow increases, 4-DAMP showed a conspicuous potency. At 1 and 10 micromol kg(-1) i.v. of pirenzepine, the antagonist reduced the protein concentration in the submandibular saliva, but not in the parotid saliva. While 4-DAMP (1 and 10 nmol kg(-1) i.v.) significantly inhibited acetylcholine-evoked protein secretory responses in the submandibular glands, methoctramine (below 10 micromol kg(-1) i.v.) affected the responses in neither gland. The reduction of the protein concentration in submandibular saliva caused by 4-DAMP and pirenzepine was inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME; 30 mg kg(-1) i.p.), while L-NAME had no or only minute effects on the parotid protein secretion. Thus, in addition to muscarinic M3 receptors, other muscarinic receptors contribute to in vivo functional responses in rat submandibular and sublingual glands. While these other receptors are muscarinic M1 receptors in the sublingual gland, they may be a different subtype, possibly muscarinic M5 receptors, in the submandibular gland. However, muscarinic M1 receptors may induce indirect effects via nitric oxide in the submandibular gland.

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.

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.

Pancreatic exocrine responses to parasympathetic stimulation in anaesthetized pigs

Pancreatic exocrine responses to stimulation of the peripheral ends of the vagus nerves intermittently have been investigated in anaesthetized pigs and compared with the effects of continuous stimulation at corresponding frequencies. At relatively low frequencies < or =20 Hz in bursts or 2 Hz continuously) both the flow of pancreatic juice and the output of protein therein were potentiated by stimulating in bursts. Thus stimulation at 20 Hz in bursts produced a significantly greater flow of pancreatic juice than stimulation at 2 Hz continuously (10.9+/-0.9 compared to 4.8+/-0.7 microl min(-1) (g gland)-1 , respectively; P<0.01). Likewise the output of protein during intermittent stimulation at 20 Hz (144+/-23 microg min(-1) (g gland)-1) far exceeded that produced during continuous stimulation at 2 Hz (49+/-9 microg min(-1) (g gland)-1; P<0.01). Both differences were abolished by atropine (0.5 mg kg(-1) i.v.), which augmented the flow during continuous stimulation (to 8.7 +/- 1.5 microl min(-1) (g gland)-1; P<0.05 at 2 Hz) and substantially reduced the output of protein during intermittent stimulation (to 27+/-7 ng min(-1) (g gland)-1; P<0.01 at 20 Hz in bursts). These results show that a variety of pancreatic exocrine responses can be enhanced by stimulating the parasympathetic innervation in bursts. They are also consistent with the contention that the secretion of protein from the gland, in response to parasympathetic stimulation, is dependent mainly on activation of muscarinic receptors. They confirm that the flow of pancreatic juice is due mainly to the release of VIP and show that, in the absence of atropine, this is restricted by muscarinic inhibition which may be presynaptic as elsewhere.

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.

Presynaptic muscarinic M1 and M2 receptor modulation of auriculotemporal nerve transmission in the rat

Parotid secretory and vascular responses to electrical stimulation of the parasympathetic innervation were measured in anaesthetized rats. Stimulation was performed at 1, 10 and 40 Hz. Atropine (1.5 micromol/kg i.v.) almost abolished the secretion to stimulation of peptide depleted nerves at 40 Hz, thus confirming the existence of a pure cholinergic response. Atropine also reduced secretion by 74% during stimulation of non-depleted nerves at the same frequency. Selective blockade by the muscarinic M1 receptor antagonist pirenzepine and by the muscarinic M2 receptor antagonist methoctramine was found to occur at doses (50 nmol/kg i.v. and of 300 nmol/kg 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 200% at 1 Hz independently of the total number of impulses (10-300), suggesting that M2 receptor activation normally has an inhibitory effect on transmitter release. The magnitude of the increase was inversely related to frequency of stimulation, and changes in the secretory responses occurred at 40 Hz only when non-depleted nerves were stimulated over the longest period employed. The fluid response then increased by 35% and protein concentration by 200%. The vasodilator responses increased at 1 and 10 Hz, but not at 40 Hz. Pirenzepine reduced the secretory and vascular responses at 10 and 40 Hz but only during stimulation over short periods of time. This suggests that M1 receptor activation normally has a facilitatory effect on neurotransmitter release. During stimulation of non-depleted nerves at 10 Hz for 10 impulses, the fluid response was reduced by 29% and the protein concentration by 26%. When the peptide depleted nerves were stimulated at 10 Hz, pirenzepine also reduced the fluid response (by 43%), but not the protein concentration. It is concluded that the release of transmitter from postganglionic nerve fibres in the rat auriculotemporal nerve is modulated by presynaptic muscarinic receptors. Muscarinic M1 receptors normally facilitate cholinergic and peptidergic transmission during short, intense stimulation. On the other hand, muscarinic M2 receptors normally inhibit cholinergic transmission at low frequencies; at higher frequencies, peptidergic transmission is also inhibited, but only after some delay.

Pharmacological characterization of muscarinic receptors in rabbit isolated iris sphincter muscle and urinary bladder smooth muscle

1. The pharmacological characteristics of muscarinic receptors in the rabbit iris sphincter muscle were studied and compared to M3 receptors in rabbit urinary bladder smooth muscle. 2. (+/-)-Cis-dioxolane induced concentration-dependent contractions of the iris sphincter muscle (pEC50 = 6.41+/-0.10, Emax = 181+/-17 mg, n = 38) and urinary bladder smooth muscle (pEC50 = 6.97+/-0.04, Emax = 4.28+/-0.25 g, n = 54). These contractions were competitively antagonized by a range of muscarinic receptor antagonists (pK(B) values are given for the iris sphincter muscle and the bladder smooth muscle, respectively): atropine (9.30+/-0.07 and 9.40+/-0.04), AQ-RA 741 (6.35+/-0.04 and 6.88+/-0.03), darifenacin (9.56+/-0.05 and 9.12+/-0.05), methoctramine (5.75+/-0.07 and 5.81+/-0.06), oxybutynin (8.10+/-0.09 and 8.59+/-0.06), pirenzepine (6.79+/-0.05 and 6.89+/-0.04), secoverine (7.54+/-0.05 and 7.66+/-0.05), p-F-HHSiD (7.55+/-0.09 and 7.50+/-0.05) and zamifenacin (8.69+/-0.10 and 8.36+/-0.06). A significant correlation between the pK(B) values in the bladder and the pK(B) values in the iris was obtained. 3. In both tissues, the pK(B) values correlated most favorably with pKi values for these compounds at human recombinant muscarinic m3 receptors. A reasonable correlation was also noted at human recombinant muscarinic m5 receptors given the poor discriminative ability of ligands between m3 and m5 receptors. 4. Overall, the data from this study suggest that the muscarinic receptors mediating contraction of the rabbit iris sphincter muscle and urinary bladder smooth muscle are similar and equate most closely with the pharmacologically-defined muscarinic M3 receptor.