A ROLE FOR MUSCARINIC RECEPTORS OR RHO-KINASE IN HYPERTENSION ASSOCIATED RAT BLADDER DYSFUNCTION?

Normalization of organ bath contraction data for tissue specimen size: does one approach fit all?

Organ bath experiments are a key technology to assess contractility of smooth muscle. Despite efforts to standardize tissue specimen sizes, they vary to a certain degree. As it appears obvious that a larger piece of tissue should develop greater force, most investigators normalize contraction data for specimen size. However, they lack agreement which parameter should be used as denominator for normalization. A pre-planned analysis of data from a recent study was used to compare denominators used for normalization, i.e., weight, length, and cross-sectional area. To increase robustness, we compared force with denominator in correlation analysis and also coefficient of variation with different denominators. This was done concomitantly with urinary bladder strips and aortic rings and with multiple contractile stimuli. Our urinary bladder data show that normalization for strip weight yielded the tightest but still only moderate correlation (e.g., r² = 0.3582 for peak carbachol responses based on 188 strips). In aorta, correlations were even weaker (e.g., r² = 0.0511 for plateau phenylephrine responses normalized for weight based on 200 rings). Normalization for strip size is less effective in reducing data variability than previously assumed; the normalization denominator of choice must be identified separately for each preparation.

Urinary Bladder Weight and Function in a Rat Model of Mild Hyperglycemia and Its Treatment With Dapagliflozin

Hypertrophy and dysfunction of the urinary bladder are consistently observed in animal models of type 1 and less consistently in those of type 2 diabetes. We have tested the effects of mild hyperglycemia (n = 10 per group) in a randomized, blinded study and, in a blinded pilot study, of type 2 diabetes (n = 6 per group) and its treatment with dapagliflozin (1 mg/kg per day) on weight, contraction, and relaxation of the rat bladder. Based on a combination of high-fat diet and a low dose of streptozotocin, animals in the main study reached a mean peak blood glucose level of about 300 mg/dl, which declined to 205 mg/dl at study end. This was associated with a small, if any, increase in bladder weight. In a pooled analysis of all animals of the main and the pilot study, we detected a correlation of moderate strength between blood glucose and bladder weight (r² = 0.2013; P = 0.0003 for Pearson correlation coefficient). Neither the main nor the pilot study found evidence for an altered contractility (responses to carbachol or KCl) or relaxation (responses to isoprenaline, fenoterol, CL 316,243, or forskolin). Treatment with dapagliflozin in the absence of hyperglycemia increased diuresis in the main study by 43% relative to control and increased bladder weight by 15% in the pooled groups of both studies (post hoc analysis). We conclude that mild hyperglycemia has no major effects on bladder hypertrophy or function.

Mucosal modulation of contractility in bladder strips from normal and overactive rat models and the effect of botulinum toxin A on overactive bladder strips

AIMS

To investigate the local, regulatory role of the mucosa on bladder strip contractility from normal and overactive bladders and to examine the effect of botulinum toxin A (BoNT-A).

METHODS

Bladder strips from spontaneously hyperactive rat (SHR) or normal rats (Sprague Dawley, SD) were dissected for myography as intact or mucosa-free preparations. Spontaneous, neurogenic and agonist-evoked contractions were investigated. SHR strips were incubated in BoNT-A (3 h) to assess effects on contractility.

RESULTS

Spontaneous contraction amplitude, force-integral or frequency were not significantly different in SHR mucosa-free strips compared with intacts. In contrast, spontaneous contraction amplitude and force-integral were smaller in SD mucosa-free strips than in intacts; frequency was not affected by the mucosa. Frequency of spontaneous contractions in SHR strips was significantly greater than in SD strips. Neurogenic contractions in mucosa-free SHR and SD strips at higher frequencies were smaller than in intact strips. The mucosa did not affect carbachol-evoked contractions in intact versus mucosa-free strips from SHR or SD bladders. BoNT-A reduced spontaneous contractions in SHR intact strips; this trend was also observed in mucosa-free strips but was not significant. Neurogenic and carbachol-evoked contractions were reduced by BoNT-A in mucosa-free but not intact strips. Depolarisation-induced contractions were smaller in BoNT-A-treated mucosa-free strips.

CONCLUSIONS

The mucosal layer positively modulates spontaneous contractions in strips from normal SD but not overactive SHR bladder strips. The novel finding of BoNT-A reduction of contractions in SHR mucosa-free strips indicates actions on the detrusor, independent of its classical action on neuronal SNARE complexes.

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

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

Detrusor underactivity: a plea for new approaches to a common bladder dysfunction

AIMS

Detrusor underactivity (DU) is defined by the International Continence Society as a contraction of reduced strength and/or duration resulting in prolonged or incomplete emptying of the bladder but has yet received only little attention. The purpose of this report is to summarize the ICI-RS meeting in Bristol in 2010 exploring current knowledge on DU and outline directions for future research.

METHODS

A think tank discussion was held and the summary of discussions was presented to all ICI-RS participants. This report is based on the final discussions.

RESULTS

The understanding of the pathophysiology, epidemiology, assessment, and treatment of DU remains rudimentary. DU is defined by pressure-flow analysis but no consensus exists regarding which of the available formulae should be used for quantification of detrusor work. DU is likely to be multifactorial. Aging causes a decay in detrusor activity but other concomitant causes, either myogenic or neurogenic, may aggravate the problem resulting in decrease of detrusor contractility. No effective pharmacotherapy for the condition exists. Only a few surgical therapeutic strategies have been explored, such as neuromodulation and skeletal muscle myoplasties. Consequently, the management of affected individuals remains unsatisfactory.

CONCLUSIONS

Future directions recommended by the ICI-RS panel include assessment of pathogenesis by developing novel animal models in addition to new non-invasive tests allowing longitudinal trials. Furthermore, optimizing the existing evaluation algorithms to support standard testing for DU and further epidemiological studies to quantify the size of the problem are required for the development of future treatment modalities.

Characterization of the urinary bladder dysfunction in renovascular hypertensive rats

AIMS

Association between arterial hypertension and urinary bladder dysfunction has been reported in humans and spontaneously hypertensive rats. However, no study exists evaluating the bladder dysfunction in conditions of renovascular hypertension. The purpose of this study was to characterize the bladder dysfunction in two kidney-one clip (2K-1C) hypertensive rats.

METHODS

A silver clip was placed around the renal artery of male Wistar rats. After 8 weeks, cystometric study, concentration-response curves to contractile and relaxant agents, frequency-dependent contractions, histomorphometry, muscarinic M(2) /M(3) mRNA expression and cyclic AMP measurements were performed.

RESULTS

2K-1C rats showed enhanced bladder volume, wall thickness and smooth muscle density. 2K-1C rats also exhibited increases in bladder capacity and non-void contractions, and decreases in the inter-contraction intervals. In isolated detrusor smooth muscle (DSM), contractions to carbachol and electrical-field stimulation (EFS) were significantly greater in 2K-1C rats. The Rho-kinase inhibitor Y27632 (10 µM) significantly reduced the carbachol-induced contractions in SHAM and 2K-1C rats, but DSM remained overactive in 2K-1C rats in presence of Y27632. Concentration-dependent contractions to the P2X receptor agonist α,β-methylene ATP, KCl and extracellular Ca(2+) did not change between SHAM and 2K-1C groups. In 2K-1C rats, isoproterenol, metaproterenol and BRL 37-344 (non-selective, β(2) - and β(3) -selective adrenoceptor agonists, respectively) produced significantly lower relaxations and decreased cAMP levels, whereas relaxant responses to sodium nitroprusside and BAY 41-2272 remained unchanged. Muscarinic M(3) mRNA expression receptors were higher in 2K-1C group.

CONCLUSIONS

Renovascular hypertensive rats exhibit bladder dysfunction that involves tissue remodeling and enhanced muscarinic M(3) -mediated contractions associated with reduced β-adrenoceptor-mediated signal transduction.

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

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

Rho-kinase, a common final path of various contractile bladder and ureter stimuli

Normal urinary bladder function is based on the proper contraction and relaxation of smooth muscle (SM), which constitutes the majority of the bladder wall. The contraction and relaxation of all SM involves a phosphorylation-dephosphorylation pathway involving the enzymes smooth muscle myosin light chain kinase (SMMLCK) and smooth muscle myosin light chain phosphatase (SMMLCP), respectively. Although originally thought to function just as a passive opposition to SMMLCK-driven SM contraction, it is now clear that SMMLCP activity is under an extremely complex molecular regulation via which SMMLCP inhibition can induce "calcium sensitization." This review provides a thorough summary of the literature regarding the molecular regulation of the SMMLCP with a focus on one of its major inhibitory pathways that is RhoA/Rho-kinase (ROK) including its activation pathways, effector molecules, and its roles in various pathological conditions associated with bladder dysfunction. Newly emerging roles of ROK outside of SM contractility are also discussed. It is concluded that the RhoA/ROK pathway is critical for the maintenance of basal SM tone of the urinary bladder and serves as a common final pathway of various contractile stimuli in rabbits, rats, mice, and pigs as well as humans. In addition, this pathway is upregulated in response to a number of pathological conditions associated with bladder SM dysfunction. Similarly, RhoA/Rho-kinase signaling is essential for normal ureteral function and development and is upregulated in response to ureteral outlet obstruction. In addition to its critical role in bladder SM function, a role of ROK in the urothelium is also beginning to emerge as well as roles for ROK in bladder infection and invasion and metastasis of bladder cancer.

Effects of fasudil, a Rho-kinase inhibitor, on contraction of pig bladder tissues with or without urothelium

OBJECTIVES

To investigate the effects of fasudil, a Rho-associated serine-threonine protein kinase inhibitor, on contraction of the pig urinary bladder tissues with or without urothelium.

METHODS

Cumulative concentration-response curves (CRCs) to carbachol were obtained with and without 3-10 microM fasudil. Drug effects were evaluated in detrusor with and without urothelium. Inhibitory responses to fasudil were also examined in tissues precontracted with KCl and carbachol, and in response to electrical field stimulation, in pig bladder with and without urothelium.

RESULTS

In detrusor without urothelium, maximum contraction (E(max)) decreased after administration of fasudil at 3 or 10 micromol/L (both P < 0.01), or 30 micromol/L (72.5 + or - 7.43%, 58.4 + or - 8.04% and 68.4 + or - 9.6%, respectively, of the first curve). In detrusor with urothelium, E(max) decreased significantly (all P < 0.05) after the addition of 3, 10 or 30 micromol/L of fasudil (84.9 + or - 6.7%, 67.9 + or - 5.2% and 35.2 + or - 4.1%, respectively). In tissues precontracted with 80 mmol/L KCl or 100 micromol/L carbachol, tension after administration of fasudil (1 nmol/L to 100 micromol/L) decreased (by approximately 40%), only after administration of fasudil at high concentration (>1 micromol/L), in detrusor both with and without urothelium. In tissues with and without urothelium, responses to electrical field stimulation at 1-50 Hz decreased significantly in a concentration-dependent manner after addition of fasudil (3 to 30 micromol/L).

CONCLUSIONS

Fasudil seems to provoke relaxation of the bladder detrusor via both urothelium-dependent and independent pathways.

An extract (THC-002) of Ba-Wei-Die-Huang-Wan inhibits expression of tachykinins, and P2X3 and TRPV1 receptors, and inhibits ATP-induced detrusor overactivity in spontaneously hypertensive rats

AIMS

To investigate possible mechanisms of action of THC-002 (HARNCARE), a galenical produced from the traditional Chinese herbal mixture Ba-Wei-Die-Huang-Wan, which has been reported to improve lower urinary tract symptoms (LUTS) in patients.

METHODS

Forty-five female SHRs were randomly separated into three groups. Two groups were given 20 ml physiological saline solution (PSS) per kg-body weight orally daily for 1 week. An hour after the administration of PSS, one of the groups received 20 mg THC-002 per kg body weight, and the other a similar volume of THC-002-free saline. The third group received no treatments. The bladders were analyzed by real time RT-PCR (n = 6) and immunohistochemistry (n = 3) for the expression of tachykinins and P2X3 and TRPV1 receptors. Cystometric investigation (n = 6) was conducted after intravesical instillation of saline followed by 5 mg/ml ATP solution.

RESULTS

Treatment with PSS caused and upregulation of tachykinins and P2X3 and TRPV1 receptors, which was prevented in the group treated with THC-002. In the normal (non-treated) and non-THC-002-treated SHRs, instillation of the ATP solution decreased voiding interval, micturition volume, and bladder capacity compared to the instillation of saline. However, in the THC-002-treated SHRs, ATP instillation had no effect.

CONCLUSIONS

In SHRs, THC-002 reduced the bladder expression of tachykinins and P2X3 and TRPV1 receptors, and inhibited ATP-induced detrusor overactivity. These effects may explain part of its beneficial effects on LUTS.

Glucose-dependent enhancement of diabetic bladder contraction is associated with a rho kinase-regulated protein kinase C pathway

Urinary bladder dysfunction, which is one of the most common diabetic complications, is associated with alteration of bladder smooth muscle contraction. However, details regarding the responses under high-glucose (HG) conditions in diabetes are poorly understood. The objective of this study was to identify a relationship between extracellular glucose level and bladder smooth muscle contraction in diabetes. Bladder smooth muscle tissues were isolated from spontaneously type II diabetic (ob/ob mouse; 16-20 weeks of age, male) and age-matched control (C57BL mouse) mice. Carbachol (CCh) induced time- and dose-dependent contractions in ob/ob and C57BL mice; however, maximal responses differed significantly (14.34 +/- 0.32 and 12.69 +/- 0.22 mN/mm(2) after 30 microM CCh treatment, respectively; n = 5-8). Pretreatment of bladders under HG conditions (22.2 mM glucose; concentration is twice that of normal glucose for 30 min) led to enhancement of CCh-induced contraction solely in diabetic mice (15.9 +/- 0.26 mN/mm(2); n = 5). Basal extracellular glucose-dependent enhancement of bladder contraction in diabetes was documented initially in this study. The correlation between intracellular calcium concentration and contraction was enhanced only in the ob/ob mouse. This enhancement of contraction and total protein kinase C (PKC) activity were inhibited by pretreatment with not only a PKC inhibitor (rottlerin) but also with a rho kinase inhibitor, fasudil [1-(5-isoquinolinesulfonyl)homopiperazine HCl]. These reagents also suppressed the differences between ob/ob and C57BL mouse bladder contractions under HG conditions. The data indicated that glucose-dependent enhancement of contraction in diabetic bladder is involved in the activation of the rho kinase and calcium-independent PKC pathways. This dysfunction may contribute to bladder complications such as detrusor overactivity and reduced bladder capacity in diabetes.

Signal transduction underlying the control of urinary bladder smooth muscle tone by muscarinic receptors and beta-adrenoceptors

The normal physiological contraction of the urinary bladder, which is required for voiding, is predominantly mediated by muscarinic receptors, primarily the M₃ subtype, with the M₂ subtype providing a secondary backup role. Bladder relaxation, which is required for urine storage, is mediated by β-adrenoceptors, in most species involving a strong β₃-component. An excessive stimulation of contraction or a reduced relaxation of the detrusor smooth muscle during the storage phase of the micturition cycle may contribute to bladder dysfunction known as the overactive bladder. Therefore, interference with the signal transduction of these receptors may be a viable approach to develop drugs for the treatment of overactive bladder. The prototypical signaling pathway of M₃ receptors is activation of phospholipase C (PLC), and this pathway is also activated in the bladder. Nevertheless, PLC apparently contributes only in a very minor way to bladder contraction. Rather, muscarinic-receptor-mediated bladder contraction involves voltage-operated Ca²⁺ channels and Rho kinase. The prototypical signaling pathway of β-adrenoceptors is an activation of adenylyl cyclase with the subsequent formation of cAMP. Nevertheless, cAMP apparently contributes in a minor way only to β-adrenoceptor-mediated bladder relaxation. BK_Ca channels may play a greater role in β-adrenoceptor-mediated bladder relaxation. We conclude that apart from muscarinic receptor antagonists and β-adrenoceptor agonists, inhibitors of Rho kinase and activators of BK_Ca channels may have potential to treat an overactive bladder.

Effects of ovariectomy and oestrogen replacement on the function and expression of Rho-kinase in rat bladder smooth muscle

OBJECTIVE

To investigate the effects of ovariectomy and oestrogen replacement on the function and expression of Rho-kinase in rat bladder smooth muscle, as the actual effects of oestrogen deprivation on bladder smooth muscle are unclear.

MATERIALS AND METHODS

Female Sprague-Dawley rats were placed into one of three groups: sham-operated, bilateral ovariectomy-only, and bilateral ovariectomy plus oestrogen replacement groups. In the last group, oestrogen was replaced by weekly injection of beta-estradiol 17-cypionate (250 microg/kg subcutaneously for 6 weeks) beginning at 1 week after ovariectomy, whereas the other groups received vehicle-only injections for 6 weeks. After treatment, the bladder was removed for muscle strip studies to evaluate the effects of Y-27632, a specific inhibitor of Rho-kinase, on baseline tension and carbachol-induced tonic contractions. Also, the protein expression of RhoA and Rho-kinase isoenzymes was assessed by Western blot analysis.

RESULTS

Of the three groups, incubation with 10 microm Y-27632 resulted in the largest decrease in baseline tension of strips from the bilateral ovariectomy-only group, but this was not statistically significant (P > 0.05). For carbachol-induced tonic contractions, strips from the bilateral ovariectomy-only group were attenuated the most among the three groups after adding Y-27632 (P < 0.05). However, there were no significant differences in the levels of RhoA and the two Rho-kinase isoenzymes in bladder tissues from the three groups.

CONCLUSION

Our data show that oestrogen might inhibit the function of Rho-kinase in bladder smooth muscle, while having no significant effect on its expression. This finding might help to explain the greater incidence of urinary tract symptoms suggestive of overactive bladder after the menopause in women.

Rho kinase: a target for treating urinary bladder dysfunction?

Urinary incontinence and other urinary storage symptoms are frequent in the general population but available treatments have limited efficacy and tolerability. Rho kinase (ROCK) has a central role in the regulation of smooth muscle contraction, including that of the urinary bladder. Recent experimental evidence indicates that this role could be deregulated and exacerbated in local and systemic pathological conditions that affect the bladder. In vitro studies with prototypical ROCK inhibitors such as Y27632 and in vivo data from animal models indicate that such drugs have potential as future treatments for bladder dysfunction.

Effects of gender, age and hypertension on beta-adrenergic receptor function in rat urinary bladder

beta-Adrenoceptors mediate urinary bladder relaxation, and gender, age and hypertension have been linked to bladder dysfunction. Therefore, we have studied whether any of these factors affects the ability of beta-adrenoceptor agonists to relax rat bladder detrusor muscle in vitro. For this purpose we have compared male and female Wistar rats, young and old male Wistar rats, and male normotensive and spontaneously hypertensive rats (SHR). Comparisons were done using KCl-precontracted bladder strips (length about 15-20 mm) and the endogenous agonist noradrenaline, the synthetic non-subtype-selective agonist isoprenaline, and the prototypical beta(3)-adrenoceptor agonists BRL 37,344 and CGP 12,177. While all agonists yielded numerically weaker relaxation in female as compared to male rats (for example for noradrenaline E(max) 40+/-4% vs 53+/-6% relaxation, pEC(50) 5.41+/-0.13 vs 5.60+/-0.14), this difference reached statistical significance only for the weak partial agonist CGP 12,177. Responses to all agonists were attenuated in old as compared to young rats, largely due to a reduced maximum effect, although the difference did not reach statistical significance for isoprenaline. The maximum relaxation responses to noradrenaline and isoprenaline were significantly lower in SHR than in normotensive rats, but both strains exhibited similar responses to the partial agonist BRL 37,344. We conclude that factors associated with bladder dysfunction, such as gender, age and hypertension, can be associated with impaired beta-adrenoceptor-mediated bladder relaxation. However, these alterations are not always consistent across various agonists, and the extent of the differences can be small. Therefore, we propose that beta-adrenoceptor dysfunction may contribute to the pathophysiology of such conditions, but is unlikely to be the only or even the major factor in this regard. We speculate that beta-adrenoceptor agonists may be effective in the treatment of bladder dysfunction under all of these conditions.

Animal models in urological disease and sexual dysfunction

There are several conditions associated with dysfunction of the lower urinary tract or which result in a reduction in the ability to engage in satisfactory sexual function and result in significant bother to sufferers, partners and/or carers. This review describes some of the animal models that may be used to discover safe and effective medicines with which to treat them. While alpha adrenoceptor antagonists and 5-alpha-reductase inhibitors deliver improvement in symptom relief in benign prostatic hyperplasia sufferers, the availability of efficacious and well-tolerated medicines to treat incontinence is less well served. Stress urinary incontinence (SUI) has no approved medical therapy in the United States and overactive bladder (OAB) therapy is limited to treatment with muscarinic antagonists (anti-muscarinics). SUI and OAB are characterised by high prevalence, a growing ageing population and a strong desire from sufferers and physicians for more effective treatment options. High patient numbers with low presentation rates characterizes sexual dysfunction in men and women. The introduction of Viagra in 1998 for treating male erectile dysfunction and the success of the phosphodiesterase type 5 inhibitor class (PDE5 inhibitor) have indicated the willingness of sufferers to seek treatment when an effective alternative to injections and devices is available. The main value of preclinical models in discovering new medicines is to predict clinical outcomes. This translation can be established relatively easily in areas of medicine where there are a large number of drugs with different underlying pharmacological mechanisms in clinical usage. However, apart from, for example, the use of PDE5 inhibitors to treat male erectile dysfunction and the use of anti-muscarinics to treat OAB, this clinical information is limited. Therefore, current confidence in existing preclinical models is based on our understanding of the biochemical, physiological, pathophysiological and psychological mechanisms underlying the conditions in humans and how they are reflected in preclinical models. Confidence in both the models used and the pharmacological data generated is reinforced if different models of related aspects of the same disorder generate confirmatory data. However, these models will only be fully validated in retrospect once the pharmacological agents they have helped identify are tested in humans.

Effects of ageing on muscarinic receptor subtypes and function in rat urinary bladder

We compared the density and function of M2 and M3 muscarinic acetylcholine receptor subtypes in the urinary bladder of young adult (3 months) and old (23 months) male Wistar rats. Old rats had a reduced density of muscarinic receptors (96+/-10 vs. 156+/-21 fmol/mg protein), but competition experiments with the M3-selective darifenacin did not indicate alterations in the relative roles of M2 and M3 receptors, with the former being more abundant. The amount of immunodetectable alpha-subunits of various G-proteins potentially linked to muscarinic receptor function was unchanged. The potency of carbachol to contract bladder strips was also unaltered; its maximum effects as well as those of a single KCl concentration were unchanged if raw data or those corrected for strip length were analysed, but somewhat reduced when those corrected for strip weight were analysed. Antagonistic effects of atropine, the M2-selective Ro 320-6206 and the M3-selective darifenacin were unchanged. Agonistic effects of the M3-sparing agonist 4-(2-oxo-2,3-dihydro-benzoimidazol-1-yl)-[1,4']bipiperidinyl-1'-carboxylic acid ethyl ester were similarly poor in young and old rats. Additional experiments were concomitantly performed in submandibular glands from the same animals. While total muscarinic receptor density in submandibular glands was not significantly affected by age (56+/-5 vs. 61+/-4 fmol/mg protein), the relative contribution of M3 receptors significantly declined from 68+/-3% to 57+/-2% based upon darifenacin competition curves. We conclude that aged Wistar rats express fewer muscarinic receptors in their urinary bladder, but there is no change in the relative abundance of M2 and M3 receptors; this is accompanied by only minor if any alterations in receptor responsiveness. In contrast, submandibular gland expresses similar receptor numbers in young and old rats, but slightly fewer M3 receptors in old animals.