Pathophysiological Mechanisms Involved in Overactive Bladder/Detrusor Overactivity

Purpose of Review

To examine the latest published findings on the pathophysiological mechanisms involved in the development of overactive bladder (OAB) and detrusor overactivity (DO), and to identify common pathways linked to the risk factors associated with these conditions.

Recent Findings

Evidence is accumulating, both clinical and experimental, that many of the factors linked to the development of OAB/DO, including ageing, bladder outlet obstruction, psychological stress, and obesity are associated with reduced bladder blood flow. This induces local tissue inflammation with cytokine release and enhanced oxidative stress, ultimately resulting in altered detrusor sensitivity, detrusor hypertrophy and fibrosis, together with afferent hypersensitivity. These mechanisms would explain the symptoms of urgency and frequency observed in OAB patients. Although not a characteristic of OAB, undetected low level bacterial infections of the bladder have been proposed to explain the OAB symptoms in patients resistant to standard treatments. In this condition, inflammatory responses without reductions in perfusion activate the inflammatory pathways.

Summary

Evidence is mounting that poor bladder perfusion and local inflammatory responses are central mechanisms involved in the development of OAB/DO. As our understanding of these pathophysiological mechanisms advances, new avenues for drug development will be identified and ultimately treatment may become more individualized depending on the particular pathway involved and the drugs available.

Mirabegron and solifenacin are effective for the management of the increased urinary frequency induced by psychological stress in female mice

Evidence to support the effectiveness of β3-adrenoceptor agonist mirabegron and anti-muscarinic solifenacin in the management of bladder dysfunction caused by psychological stress is lacking. This study investigates whether mirabegron or solifenacin reduces the bladder overactivity caused by water avoidance stress (WAS) in mice. Female mice were exposed to WAS for 1 h/day for 10 days and received either placebo, solifenacin or mirabegron in drinking water. Controls were age-matched without stress exposure. Voiding behaviour and functional isolated whole bladder responses during distension and in response to pharmacological agents and electrical field stimulation was investigated. Urinary frequency was significantly increased following stress. Mice treated with mirabegron or solifenacin displayed significantly fewer voiding events compared to the stressed mice, and voiding frequency in drug-treated animals was comparable to unstressed controls. The maximal contractile responses of bladders to carbachol were significantly enhanced by stress and reduced by mirabegron but not solifenacin. The frequency of phasic bladder contractions following stimulation with carbachol was significantly enhanced following stress and remained elevated in the mirabegron treated group. However, treatment with solifenacin significantly reduced the frequency of phasic contractions to unstressed control levels. Solifenacin and mirabegron are beneficial in reducing the overall voiding dysfunction caused by WAS in mice.

Partial recovery of voiding function in female mice following repeated psychological stress exposure

Psychological stress causes bladder dysfunction in humans and in rodent models, with increased urinary frequency and altered contractile responses evident following repeated environmental stress exposure. However, whether these changes persist after removal of the stressor is unknown, and the aim of this study was to determine if stress-induced changes in voiding behaviour and bladder function recover following removal of the stressor. Adult female mice were allocated to three groups: Unstressed, Stressed or Stressed + Recovery. Animals in the stressed groups were exposed to water avoidance stress for 1h/day for 10-days, with unstressed animals age-matched and housed under normal conditions. For recovery studies, animals were housed without stress exposure for an additional 10-days. Voiding behaviour was assessed periodically and animals sacrificed on day 10 (Unstressed and Stressed) or day 20 (Unstressed and Stressed + Recovery). Isolated whole bladder studies were used to assess compliance, urothelial mediator release and contractile responses. Exposure to stress increased plasma corticosterone levels almost three-fold (P<0.05) but this returned to baseline during the recovery period. Contractile responses of the bladder to carbachol and KCl were also increased following stress, and again fully recovered after a 10-day stress-free period. In contrast, stress increased urinary frequency four-fold (P<0.001), but this did not return fully to baseline during the recovery period. Bladder compliance was unchanged by stress; however, it was increased in the stressed + recovery group (P<0.05). Thus, following a stress-free period there is partial recovery of voiding behaviour, with an increase in bladder compliance possibly contributing to the compensatory mechanisms.

Chronic psychological stress and lower urinary tract symptoms

It is well established that lower urinary tract symptoms (LUTS), particularly urinary urgency and incontinence, cause stress and anxiety for patients. However, there is mounting evidence that the relationship between these two factors is bidirectional and that chronic psychological stress itself can result in the development of symptoms such as urinary frequency, urgency, incontinence, and pelvic pain. This review considers the evidence that such a relationship exists and reviews the literature from clinical and animal studies to identify some of the mechanisms that might be involved. Inflammatory responses induced by chronic stress appear to offer the strongest link to bladder dysfunction. There is overwhelming evidence, both in patients and animal models, for a release of pro-inflammatory cytokines and chemokines during periods of chronic stress. Furthermore, cytokines have been shown to cause bladder dysfunction and pain via actions in the central nervous system and locally in the bladder. In the brain and spinal cord, pro-inflammatory cytokines influence the regulation of micturition pathways by corticotropin-releasing factor (CRF) and its receptors, while peripherally cytokines affect bladder function, directly causing detrusor hypertrophy and afferent nerve hypersensitivity. There is little information on which treatments may have most benefit for stressed/anxious patients with LUTS, but animal studies suggest traditional drugs for overactive bladder (solifenacin, mirabegron) are more effective on LUTS than anxiolytic drugs (fluoxetine, imipramine). The preliminary preclinical data for CRF receptor antagonists is not consistent. A clearer understanding of the mechanisms involved in stress-induced LUTS should provide a basis for improved treatment of this condition.

Current and emerging pharmacological targets for medical expulsive therapy

The primary goals of medical expulsive therapy are to increase the rate of stone expulsion along the ureter to avoid ureteral obstruction and reduce ureteral colic and thus avoid the need for surgical and more invasive interventions. This review focussed on the findings from in vivo and in vitro animal and human studies that have investigated the pharmacological mechanisms controlling ureteral motility and their translation to current and potentially new clinically used drugs for increasing the rate of stone expulsion along the ureter. The complicated contractility profile of the ureter, which alters with age, tissue segment region, orientation and species contributes to the difficulty of interpreting studies on ureteral pharmacology, which translates to the complexity of discovering ideal drug targets for medical expulsive therapy. Nevertheless, the current drug classes clinically used for patients with stone lodgement include α₁ -adrenoceptor antagonists, calcium channel blockers and NSAIDS, whilst there are promising targets for drug development that require further clinical investigations including the phosphodiesterase type 5 enzyme, β-adrenoceptors and 5-HT receptors.

The anxiolytic sertraline reduces the impact of psychological stress on bladder function in mice

AIMS

To determine if treatment with the selective serotonin reuptake inhibitor (SSRI) sertraline reduces the bladder dysfunction caused by water avoidance stress in mice.

MAIN METHODS

Adult female mice were randomly allocated to (1) Unstressed, (2) Stressed or (3) Stress + Sertraline experimental groups. Stressed mice were subjected to water avoidance for 1 h/day for 10 days and received sertraline or vehicle in drinking water, starting 10-days prior to the first stress exposure. Age matched control/unstressed mice were house under normal conditions without stress exposure. Voiding behaviour was assessed throughout the experimental protocol. After the final stress exposure, a blood sample was taken to measure plasma corticosterone levels and bladders were removed, catheterised and intravesical pressure responses recorded during distension and in response to pharmacological agents.

KEY FINDINGS

Plasma corticosterone levels in sertraline-treated animals were equivalent to unstressed controls and significantly decreased compared to the stressed group. Voiding frequency was significantly increased in the stressed group, and treatment with sertraline significantly decreased voiding frequency, however, this remained elevated compared to unstressed control animals. Bladders from stressed mice displayed enhanced maximal contractile response to the muscarinic agonist carbachol and greater release of ACh in the serosal fluid, which was reduced to control levels by sertraline treatment. Spontaneous phasic contractions were not altered by stress but were significantly reduced in bladders from sertraline treated animals, relative to controls.

SIGNIFICANCE

These results indicate that management of voiding dysfunction caused by psychological stress may be aided by the addition of an SSRI such as sertraline.

Bladder overactivity induced by psychological stress in female mice is associated with enhanced bladder contractility

AIMS

To investigates the effects of water avoidance stress on voiding behaviour and functional bladder responses in mice.

MAIN METHODS

Mice in the Stress group were exposed to water avoidance stress (WAS) for 1 h/day for 10 days, Controls were age-matched and housed normally. Voiding behaviour was measured periodically throughout the stress protocol and bladders were isolated 24-h after final stress exposure to measure bladder compliance, spontaneous phasic activity, contractile responses, and release of urothelial mediators.

KEY FINDINGS

Repeated stress exposure induced a significant increase in plasma corticosterone levels in the WAS group compared to control. An overactive bladder phenotype was observed in WAS mice, causing a significant increase in the number of voiding events observed from as early as day-3, and a 7-fold increase following 10-days' stress. This increase in voiding frequency was associated with a significant decrease in void size, an increase in the number of small voids, but no change in total voided volume. Bladders from stressed mice showed a significant increase in the maximum responses to the muscarinic agonist carbachol (p < 0.01), in addition to enhanced pressure responses to the purinergic agonists ATP (p < 0.05) and αβ-mATP (p < 0.05), and non-receptor mediated contractions to KCl (p < 0.05) compared to controls. Nerve-mediated bladder contractions to electric field stimulation were not significantly affected by stress, nor were spontaneous phasic contractions or release of urothelial ATP and acetylcholine.

SIGNIFICANCE

Repeated exposure to water avoidance stress produced an overactive bladder phenotype, confirmed by increased voiding frequency, and associated with enhanced bladder contractile responses.

Voiding Behavior and Efferent Bladder Function Altered in Mice Following Social Defeat but Not Witness Trauma

Psychological stress is associated with bladder dysfunction, however, the local bladder mechanisms affected are not well understood. This study aimed to determine how psychological stress, caused by social defeat or witness trauma, affects voiding behavior and bladder function. Pairs of male C57Bl/6J mice were placed in a custom-made plexiglass chamber with an aggressor ARC(S) mouse for 1 h/day for 10 days. The social defeat mouse was in physical contact with the aggressor, while the witness was physically separated but could observe interactions between its cage-mate and the aggressor. Age matched control pairs were used for comparison. Voiding analysis was conducted periodically over the 10 days. An ex vivo whole bladder preparation was used to assess functional changes after the period of stress. Plasma corticosterone levels were significantly increased by both social defeat and witness trauma stress when compared to unstressed controls. Voiding analysis revealed a significant decrease in voiding frequency in the social defeat group compared to control animals, indicating an altered voiding phenotype. Witness trauma did not alter voiding behavior. Bladder contractile responses to cholinergic stimulation were not significantly altered in either stress group, nor was relaxation to the beta-adrenoceptor agonist isoprenaline. However, nerve evoked contractile responses were significantly increased at all frequencies in bladders from social defeat but not witness trauma mice. Purinergic contractile responses were also significantly enhanced in this group. Social defeat also resulted in increased urothelial acetylcholine release during bladder distension, with no change in ATP release. In conclusion, functional bladder changes are dependent upon stressor type. Enhanced urothelial acetylcholine may desensitize bladder sensory nerves, which, coupled with more efficient voiding contractions due to enhanced nerve-mediated and purinergic detrusor responses, may account for the altered voiding phenotype observed. This study reports a male model of social defeat stress with reduced urinary frequency, with no voiding changes observed in the witness.

Purinergic receptor mediated calcium signalling in urothelial cells

Non-neuronal ATP released from the urothelium in response to bladder stretch is a key modulator of bladder mechanosensation. Whilst non-neuronal ATP acts on the underlying bladder afferent nerves to facilitate sensation, there is also the potential for ATP to act in an autocrine manner, modulating urothelial cell function. The aim of this study was to systematically characterise the functional response of primary mouse urothelial cells (PMUCs) to ATP. PMUCs isolated from male mice (14–16 weeks) were used for live-cell fluorescent calcium imaging and qRT-PCR to determine the expression profile of P2X and P2Y receptors. The majority of PMUCs (74–92%) responded to ATP (1 μM–1 mM), as indicted by an increase in intracellular calcium (iCa²⁺). PMUCs exhibited dose-dependent responses to ATP (10 nM–1 mM) in both calcium containing (2 mM, EC₅₀ = 3.49 ± 0.77 μM) or calcium free (0 mM, EC₅₀ = 9.5 ± 1.5 μM) buffers. However, maximum iCa²⁺ responses to ATP were significantly attenuated upon repetitive applications in calcium containing but not in calcium free buffer. qRT-PCR revealed expression of P2X_1–6, and P2Y_1–2, P2Y₄, P2Y₆, P2Y_11–14, but not P2X₇ in PMUCs. These findings suggest the major component of ATP induced increases in iCa²⁺ are mediated via the liberation of calcium from intracellular stores, implicating functional P2Y receptors that are ubiquitously expressed on PMUCs.

Diabetes-induced alterations in urothelium function: Enhanced ATP release and nerve-evoked contractions in the streptozotocin rat bladder

Up to 80% of patients with diabetes mellitus develop lower urinary tract complications, most commonly diabetic bladder dysfunction (DBD). The aim of this study was to investigate the impact of diabetes on the function of the inner bladder lining (urothelium). Bladder compliance and intraluminal release of urothelial mediators, adenosine triphosphate (ATP) and acetylcholine (ACh) in response to distension were investigated in whole bladders isolated from 2- and 12-week streptozotocin (STZ)-diabetic rats. Intact and urothelium-denuded bladder strips were used to assess the influence of the urothelium on bladder contractility. Intraluminal ATP release was significantly enhanced at 2 weeks of diabetes, although not at 12 weeks. In contrast, intraluminal ACh release was unaltered by diabetes. Bladder compliance was also significantly enhanced at both 2 and 12 weeks of diabetes, with greatly reduced intravesical pressures in response to distension. Nerve-evoked contractions of bladder strips were significantly greater at 2 weeks of diabetes. When the urothelium was absent, nerve-evoked contractions were reduced, but contractions remained significantly elevated at lower frequencies of stimulation (<5 Hz) in diabetics. Interestingly, although relaxations of bladder strips to isoprenaline were unaltered by diabetes, removal of the urothelium unmasked significantly enhanced relaxations in strips from 2- and 12-week diabetic animals. In conclusion, diabetes alters urothelial function. Enhanced urothelial ATP release may be involved in the hypercontractility observed at early time points of diabetes. These alterations are time-dependent and may contribute to the mechanisms at play during the development of diabetic bladder dysfunction.

Effect of short-term androgen deficiency on bladder contractility and urothelial mediator release

In men, testosterone levels decline by 1% per year after the age of 40. Reduced androgen levels may directly contribute to lower urinary tract symptoms and bladder dysfunction, although the mechanisms are unclear. This study examined the effect of low testosterone and testosterone replacement on key mechanisms involved in local bladder function. Intraluminal release of the mediators ATP and ACh in response to bladder distension was measured in whole bladders from rats 8 weeks following castration, whilst bladder contractility was assessed using isolated strips. Human urothelial cells were cultured under low, physiological and supra-physiological testosterone conditions for 24 h or 5 days, and stretch-induced release of ATP and ACh was measured. Phasic contractile activity of bladder strips, agonist-induced reponses to carbachol and isoprenaline and nerve-evoked contractions were unaffected by castration. The acetylcholinesterase inhibitor neostigmine significantly increased amplitude of phasic activity only in bladder strips following castration, and this was prevented by testosterone replacement. Intraluminal ACh release following bladder distension was significantly reduced following castration, whilst ATP release was unaffected. In contrast, stretch-induced ATP release from urothelial cells was significantly enhanced in low testosterone conditions, whilst ACh release was unaltered. Testosterone-replacement to physiological levels prevented these changes. Whilst androgen deficiency of 8 weeks does not directly affect contractility of bladder smooth muscle, urothelial mediator release is sensitive to changes in testosterone. These changes in mediator release may be an early effect of the decline in testosterone and could affect sensory pathways in the longer term, contributing to the urinary symptoms and bladder dysfunction seen in androgen-deficient men.

Testosterone differentially regulates targets of lipid and glucose metabolism in liver, muscle and adipose tissues of the testicular feminised mouse

Testosterone deficiency is commonly associated with obesity, metabolic syndrome, type 2 diabetes and their clinical consequences-hepatic steatosis and atherosclerosis. The testicular feminised mouse (non-functional androgen receptor and low testosterone) develops fatty liver and aortic lipid streaks on a high-fat diet, whereas androgen-replete XY littermate controls do not. Testosterone treatment ameliorates these effects, although the underlying mechanisms remain unknown. We compared the influence of testosterone on the expression of regulatory targets of glucose, cholesterol and lipid metabolism in muscle, liver, abdominal subcutaneous and visceral adipose tissue. Testicular feminised mice displayed significantly reduced GLUT4 in muscle and glycolytic enzymes in muscle, liver and abdominal subcutaneous but not visceral adipose tissue. Lipoprotein lipase required for fatty acid uptake was only reduced in subcutaneous adipose tissue; enzymes of fatty acid synthesis were increased in liver and subcutaneous tissue. Stearoyl-CoA desaturase-1 that catalyses oleic acid synthesis and is associated with insulin resistance was increased in visceral adipose tissue and cholesterol efflux components (ABCA1, apoE) were decreased in subcutaneous and liver tissue. Master regulator nuclear receptors involved in metabolism-Liver X receptor expression was suppressed in all tissues except visceral adipose tissue, whereas PPARγ was lower in abdominal subcutaneous and visceral adipose tissue and PPARα only in abdominal subcutaneous. Testosterone treatment improved the expression (androgen receptor independent) of some targets but not all. These exploratory data suggest that androgen deficiency may reduce the buffering capability for glucose uptake and utilisation in abdominal subcutaneous and muscle and fatty acids in abdominal subcutaneous. This would lead to an overspill and uptake of excess glucose and triglycerides into visceral adipose tissue, liver and arterial walls.

Three Gaseous Neurotransmitters, Nitric oxide, Carbon Monoxide, and Hydrogen Sulfide, Are Involved in the Neurogenic Relaxation Responses of the Porcine Internal Anal Sphincter

Background/Aims

The internal anal sphincter (IAS) plays an important role in maintaining continence and a number of neurotransmitters are known to regulate IAS tone. The aim of this study was to determine the relative importance of the neurotransmitters involved in the relaxant and contractile responses of the porcine IAS.

Methods

Responses of isolated strips of IAS to electrical field stimulation (EFS) were obtained in the absence and presence of inhibitors of neurotransmitter systems.

Results

Contractile responses of the sphincter to EFS were unaffected by the muscarinic receptor antagonist, atropine (1 μM), but were almost completely abolished by the adrenergic neuron blocker guanethidine (10 μM). Contractile responses were also reduced (by 45% at 5 Hz, P < 0.01) following desensitisation of purinergic receptors with α,β-methylene-ATP (10 μM). In the presence of guanethidine, atropine, and α,β-methylene-ATP, the remaining relaxatory responses to EFS were examined. These responses were not altered by the cyclooxygenase inhibitor, indomethacin (5 μM), the vasoactive intestinal polypeptide receptor antagonist, [_d-p-Cl-Phe6,Leu17]-vasoactive intestinal peptide (PheLeu-VIP; 100 nM), or the purinoceptor antagonists, 8-phenyltheophyline (P1 receptors) or suramin (P2 receptors). However, relaxation responses were reduced by Nω-nitro-L-arginine (L-NNA; 100 μM), an inhibitor of nitric oxide synthesis (40–50% reduction), zinc protoprophyrin IX (10 μM), an inhibitor of carbon monoxide synthesis (20–40% reduction), and also propargylglycine (30 μM) and aminooxyacetic acid (30 μM), inhibitors of hydrogen sulphide synthesis (15–20% reduction).

Conclusions

Stimulation of IAS efferent nerves releases excitatory and inhibitory neurotransmitters: noradrenaline is the predominant contractile transmitter with a smaller component from ATP, whilst 3 gases mediate relaxation responses to EFS, with the combined contributions being nitric oxide > carbon monoxide > hydrogen sulfide.

Testosterone suppresses the expression of regulatory enzymes of fatty acid synthesis and protects against hepatic steatosis in cholesterol-fed androgen deficient mice

AIMS

Non-alcoholic fatty liver disease and its precursor hepatic steatosis is common in obesity and type-2 diabetes and is associated with cardiovascular disease (CVD). Men with type-2 diabetes and/or CVD have a high prevalence of testosterone deficiency. Testosterone replacement improves key cardiovascular risk factors. The effects of testosterone on hepatic steatosis are not fully understood.

MAIN METHODS

Testicular feminised (Tfm) mice, which have a non-functional androgen receptor (AR) and very low serum testosterone levels, were used to investigate testosterone effects on high-cholesterol diet-induced hepatic steatosis.

KEY FINDINGS

Hepatic lipid deposition was increased in Tfm mice and orchidectomised wild-type littermates versus intact wild-type littermate controls with normal androgen physiology. Lipid deposition was reduced in Tfm mice receiving testosterone treatment compared to placebo. Oestrogen receptor blockade significantly, but only partially, reduced the beneficial effects of testosterone treatment on hepatic lipid accumulation. Expression of key regulatory enzymes of fatty acid synthesis, acetyl-CoA carboxylase alpha (ACACA) and fatty acid synthase (FASN) were elevated in placebo-treated Tfm mice versus placebo-treated littermates and Tfm mice receiving testosterone treatment. Tfm mice on normal diet had increased lipid accumulation compared to littermates but significantly less than cholesterol-fed Tfm mice and demonstrated increased gene expression of hormone sensitive lipase, stearyl-CoA desaturase-1 and peroxisome proliferator-activated receptor-gamma but FASN and ACACA were not altered.

SIGNIFICANCE

An action of testosterone on hepatic lipid deposition which is independent of the classic AR is implicated. Testosterone may act in part via an effect on the key regulatory lipogenic enzymes to protect against hepatic steatosis.

Effect of Testosterone on Inflammatory Markers in the Development of Early Atherogenesis in the Testicular-Feminized Mouse Model

BACKGROUND

Low levels of serum testosterone in men are associated with cardiovascular disease. Clinical studies show that testosterone replacement therapy (TRT) can improve symptoms of cardiovascular disease and reduce the inflammatory burden evident in atherosclerosis.

AIM

We used an in vivo animal model to determine whether testosterone influences mediators of vascular inflammation as part of its beneficial effects on atherogenesis.

METHODS

Testicular-feminized (Tfm) mice, which express low endogenous testosterone and a non-functional androgen receptor (AR), were used to assess the effect of androgen status on atheroma formation, serum lipids, and inflammatory mediators. Tfm mice were fed a high-cholesterol diet, received saline or physiological (TRT), and were compared to saline-treated XY littermates.

RESULTS

A total of 28 weeks of high-cholesterol diet caused fatty streak formation in the aortic root of XY littermates and Tfm mice, an effect significantly amplified in Tfm mice. Tfm mice on normal diet showed elevated serum tumor necrosis factor-α (TFN-α) and interleukin-6 compared to XY littermates. High-cholesterol diet induced increased monocyte chemoattractant protein-1 (MCP-1) in Tfm mice, and TFN-α and MCP-1 in XY littermates. TRT reduced fatty streak formation and serum interleukin-6 in Tfm mice but had no significant effects on lipid profiles. Monocyte/macrophage staining indicated local inflammation in aortic root fatty streak areas of all mice, with TRT reducing local inflammation through plaque reduction in Tfm mice. Fractalkine (CX₃CL1) and its receptor (CX₃CR1) were present in fatty streaks of all mice fed a high-cholesterol diet, independent of androgen status.

CONCLUSION

These results are consistent with AR-dependent and AR-independent anti-inflammatory actions of testosterone in atheroprotection, although the local anti-inflammatory mechanisms via which testosterone acts remain unknown.

Phasic activity of urinary bladder smooth muscle in the streptozotocin-induced diabetic rat: effect of potassium channel modulators

Increased phasic activity in the bladder smooth muscle of animal models and patients with detrusor overactivity has been suggested to underlie the pathophysiology of overactive bladder. Potassium (K+) channels are key regulators of bladder smooth muscle tone and thus may play a role in this altered phasic activity. In this study the effects of K+ channel modulators on the phasic activity of bladder strips from the streptozotocin-induced diabetic rat model of bladder dysfunction were investigated. Bladder strips from rats 1 week following streptozotocin administration and age-matched controls were mounted in tissue baths at 37 °C and the effects of K+ channel modulators on resting basal tension or phasic activity induced by a low concentration of carbachol (0.5 μM) were investigated. Activation of BKCa channels by NS1619 had a minor inhibitory effect on carbachol-induced phasic activity of bladder strips from control and diabetic rats, and significantly inhibited amplitude only at 30 μM. Activation of KATP channels by cromakalim inhibited the frequency of carbachol-induced phasic activity of bladder strips, although strips from diabetic rats showed a trend towards being less sensitive to cromakalim. The BKCa channel blocker iberiotoxin was able to induce phasic activity in resting tissues, with diabetic bladder strips demonstrating significantly enhanced phasic activity compared to controls. In contrast, inhibition of SKCa and KATP channels did not induce phasic activity in resting tissues. In conclusion, responses of diabetic rat bladder to BKCa and KATP channel modulators are altered, suggesting altered function and/or expression of channels which may contribute to bladder dysfunction in this model.

The role of c-kit-positive interstitial cells in mediating phasic contractions of bladder strips from streptozotocin-induced diabetic rats

OBJECTIVE

• To investigate the role of c-kit-positive interstitial cells (ICCs) in mediating muscarinic receptor-induced phasic contractions of isolated bladder strips from streptozotocin(STZ)-induced diabetic rats and to confirm the expression and location of ICCs in the rat bladder.

MATERIALS AND METHODS

• Bladders were removed from STZ-induced diabetic rats at 1, 4 and 12 weeks after induction of diabetes and from age-matched controls. • To investigate the functional role of ICCs in mediating phasic contractions, bladder strips were isolated from control and diabetic rats and mounted in tissue baths. • Strips were stimulated with low concentrations of the muscarinic receptor agonist carbachol (CCH; 0.1 µm) to induce phasic contractions and the effect of increasing concentrations (1-50 µm) of imatinib (Glivec® or Gleevec®, formerly STI571), a c-kit tyrosine kinase inhibitor, was then investigated. • For molecular studies, to detect expression of the c-kit tyrosine kinase receptor (c-kit), total cellular RNA was extracted from rat bladders and reverse-transcribed to obtain complementary DNA (cDNA). • Reverse transcription-polymerase chain reaction (RT-PCR) was then performed using primers specific to the c-kit sequence and amplified products separated by agarose gel electrophoresis. • Amplified PCR products were excised from the gel, sequenced and compared with the known c-kit sequence to confirm their identity. • For immunohistochemical detection, whole mount preparations of control rat bladders were fixed in acetone and labelled using antibodies directed to the ICC marker c-kit.

RESULTS

• In functional studies, CCH induced phasic contractions in bladder strips from control and diabetic rats. Bladder strips from 1-week diabetic rats showed CCH-induced phasic contractions, which were greater in amplitude, but had lower frequency, than the controls, whilst no such differences were apparent at later time points of diabetes. • Imatinib decreased the amplitude and the frequency of the CCH-induced phasic contractions in both control and diabetic tissues in a concentration-dependent manner, although in diabetic tissues this effect was only seen at the higher concentrations of imatinib. RT-PCR of bladder cDNA yielded a single amplicon of 480 bp. • The sequence of this amplicon showed a 98% homology with the published c-kit sequence, thus confirming c-kit mRNA expression in both control and 1-week diabetic rat bladder. • Expression of c-kit protein was also detected in a network of cells on the edge of and between smooth muscle bundles of control rat bladders by positive immunoreactivity to c-kit specific antibodies.

CONCLUSION

• These data show the presence of c-kit-positive ICCs in rat urinary bladder and their importance in mediating muscarinic receptor-induced phasic contractions of bladder strips from control and diabetic rats. The role of these ICCs does not seem to be significantly altered by the diabetic state.

Spontaneous contractions of the pig urinary bladder: the effect of ATP-sensitive potassium channels and the role of the mucosa

OBJECTIVE

To investigate the influence of the mucosa on the inhibitory effects of the ATP-sensitive potassium channel (K(ATP) channel) opener, cromakalim, on the spontaneous contractions of pig bladder strips from the bladder dome and trigone. Little is known about the influence of the mucosa on spontaneous contractions and whether the nature of these contractions differs between the bladder dome and trigone.

MATERIALS AND METHODS

Paired longitudinal strips of female pig bladders were isolated from the dome and trigone. The mucosa was removed from one strip per pair and tissues were set up in organ baths. Spontaneous activity was allowed to develop and recorded, and then cumulative concentration-response curves to cromakalim were obtained. The time needed for spontaneous contractions to develop, the frequency and amplitude of spontaneous contractions, and the effect of cromakalim were analysed. The strips of mucosa removed from the dome to produce denuded strips were also analysed by immunofluorescence using antibodies specific for vimentin and alpha-smooth muscle actin (alpha-SMA).

RESULTS

In the dome removal of the mucosa delayed the development of spontaneous contractions compared with mucosa-intact strips, whilst the trigone strips developed spontaneous contractions soon after set up in the organ baths irrespective of the presence or absence of mucosa. In the dome, cromakalim was more potent in suppressing spontaneous contractions when the mucosa was absent; whilst in the trigone the effects of cromakalim were similar in mucosa-intact and denuded strips. Upon examination of the strips of mucosa by immunofluorescence these strips were shown to contain cells positive for alpha-SMA or vimentin and cells positive for both, suggesting the presence of not only urothelium but also suburothelium and some detrusor smooth muscle bundles.

CONCLUSION

In the dome, the urothelium and suburothelium reduce the inhibitory effect of cromakalim on spontaneous contractions, whilst in the trigone these structures appear to have little influence. The mechanism for generating spontaneous contractions in the intact strips seems to be linked to the urothelium and suburothelium in the dome but not in the trigone.

Developments in the pharmacotherapy of the overactive bladder

PURPOSE OF REVIEW

The overactive bladder is a common and distressing condition that has a significant impact on the quality of life of many people worldwide. Anticholinergics remain the first line in pharmacotherapy, however the use of these agents is hindered by adverse effects and limited efficacy. Thus there is a need for more effective treatments. Recently, there has been a move towards targeting novel pathways thought to play a role in overactivity. This review aims to provide an insight into the recent developments in pharmacotherapy of the overactive bladder.

RECENT FINDINGS

With recent advances in our understanding of the basic science of the overactive bladder it is becoming clear that the control of bladder functioning is far more complex than previously believed. Peripherally, a prominent role has emerged for the urothelium and the underlying suburothelium in mechanosensory control, and the role of afferent pathways in pathophysiology is increasingly recognized.

SUMMARY

Recent research has highlighted several potential targets for treatment of the overactive bladder, particularly within the mechanosensory pathways. With the exception of botulinum toxin, however, few new therapies have emerged showing clinical benefits. A clearer understanding of the pathophysiology of the bladder will hopefully lead to more effective and tolerated treatments.

The role of the urothelium in mediating bladder responses to isoprenaline

OBJECTIVES

To investigate whether the responses of the pig bladder to isoprenaline (a nonselective beta-adrenoceptor agonist) are influenced by the presence of an intact urothelium and whether any influence might be attributed to the release of nitric oxide (NO), since stimulation of beta-adrenoceptors induces a direct relaxation of detrusor smooth muscle and beta-adrenoceptors are also present on the urothelium.

MATERIAL AND METHODS

Paired (in the presence or absence of urothelium) longitudinal strips of pig bladder dome were set up in tissue baths and the developed tension recorded. Relaxation responses to isoprenaline were examined after pre-contraction with carbachol. The inhibitory effects of isoprenaline were examined by comparing responses to carbachol in the absence and presence of isoprenaline. To examine a possible role for NO, similar experiments were performed in the presence of the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA).

RESULTS

In the presence of the urothelium, both the potency (pEC(50)) and the maximum contractile responses to carbachol were depressed. In relaxation experiments, isoprenaline relaxed carbachol pre-contracted tissues by approximately 75%, and the potency and maximum relaxation were similar in the absence and presence of the urothelium. In the inhibition experiments, the presence of isoprenaline caused rightward parallel shifts of the concentration-response curves to carbachol, but isoprenaline did not influence the maximum contractions. In the presence of the urothelium there was a greater shift with 0.1 microm isoprenaline than in denuded tissues. Incubation with L-NNA did not affect the influence of the urothelium on responses to isoprenaline in any experimental group.

CONCLUSIONS

The relaxation responses of the bladder to isoprenaline do not appear to involve the urothelium or NO release in vitro. However, contractile responses to carbachol were inhibited in the presence of an intact urothelium, and this might reflect the release of an inhibitory factor other than NO.

Muscarinic receptor function, density and G-protein coupling in the overactive diabetic rat bladder

1 Bladder smooth muscle sensitivity to muscarinic agonists is increased in the overactive bladder. Treatment of rats with streptozotocin induces a diabetic state in which the bladder muscle is overactive and also supersensitive to muscarinic agonists. This study has examined bladder contraction, muscarinic receptor density and receptor/G-protein coupling in the streptozotocin-induced overactive bladder of the rat. 2 Diabetes was induced by a single intraperitoneal dose of streptozotocin. Seven days later contraction of isolated detrusor muscle strips was assessed in tissue bath experiments, while receptor density was assayed in saturation experiments with [3H]-QNB (quinuclidinyl benzilate, L-[benzilic-4,4'-3H]) and receptor/G-protein coupling was determined in agonist displacement experiments with this radioligand. 3 Isolated detrusor strips from diabetic animals displayed an enhanced degree of spontaneous activity (0.060 +/- 0.016 g mg(-1), compared with 0.015 +/- 0.004 g mg(-1), P < 0.05). Carbachol produced contractile responses in tissues from both control and diabetic rats, but the diabetic tissues were more sensitive to this agonist, the pEC50 being 6.52 +/- 0.17 compared with 5.93 +/- 0.06 in controls (P < 0.001). Maximum responses to carbachol were similar in both groups of animals. The increase in carbachol potency was accompanied by a 40% increase in receptor density from 158 +/- 5 to 221 +/- 22 fmol mg(-1) protein (P < 0.05), but this was not enough to fully account for the change in tissue sensitivity. 4 In the absence of GTP-gamma-S, carbachol displaced [3H]-QNB from two binding sites, the high-affinity site (pKi = 7.06 +/- 0.26) which represents the receptors coupled to G-proteins made up 43.1 +/- 5.9% of the total binding sites in control tissues and this value was similar (41.0 +/- 4.0%) in the diabetic tissues (pKi = 6.64 +/- 0.31). In the presence of GTP-gamma-S, carbachol displaced [3H]-QNB from a single binding site which had a low-affinity, similar to the low-affinity site observed in the absence of GTP-gamma-S. 5 These data demonstrate that detrusor supersensitivity is observed after only 1 week of untreated diabetes in the rat. The overactivity is associated with an enhanced sensitivity to carbachol, which is partly explained by an increase in receptor density, but there appears to be no change in receptor/G-protein coupling.

Depressed contractile responses to neurokinin A in idiopathic but not neurogenic overactive human detrusor muscle

OBJECTIVE

The role of tachykinins such as neurokinin A in regulating bladder function is unclear, but NK2 receptors seem to mediate contraction in the human bladder and it has been suggested that these peptides may have a role in the pathophysiology of bladder dysfunction. The present study investigates neurokinin receptor-mediated contractility of detrusor muscle in the idiopathic overactive and neurogenic overactive bladder and investigates the neurokinin receptor subtypes involved.

METHODS

Human bladder was obtained from patients undergoing cystectomy (normal) or clam cystoplasty (idiopathic overactive) and from patients with spinal injuries (neurogenic overactive). Strips of isolated detrusor muscle were mounted in physiological Krebs-bicarbonate solution and cumulative concentration-response curves to 1 nM to 300 microM neurokinin A (NKA) were obtained in the absence and presence of neurokinin receptor antagonists, either the NK2 receptor-selective antagonist SR 48968 or the NK3 receptor-selective antagonist SB 223412.

RESULTS

NKA evoked concentration-dependent contraction of normal, idiopathic, and neurogenic overactive detrusor strips. In idiopathic overactive detrusor muscle, NKA-induced contraction was significantly reduced relative to normal detrusor (0.031 +/- 0.005 mg/g, n = 11 versus 0.193 +/- 0.039 mg/g, n = 7). Sensitivity to the peptide was also significantly (p < 0.01) reduced in idiopathic overactive detrusor, with mean pEC50 values (concentration producing 50% maximal response) of 6.62+/-0.16 (n = 11) compared to 7.47+/-0.19 (n = 7) in normal detrusor. In contrast, NKA-induced responses of neurogenic overactive detrusor were similar to those in normal detrusor, with a mean maximum contraction of 0.199 +/- 0.036 mg/g (n = 10) and mean pEC50 value of 7.85+/-0.16 (n = 10). NKA curves in all groups were shifted to the right by the NK2 receptor-selective antagonist SR 48968 with high affinity, pK(B) values being similar in normal, idiopathic, and neurogenic overactive detrusor (8.85 + 0.08, n = 14; 8.97 +/- 0.13, n = 12; 8.73 +/- 0.12, n = 8, respectively). In contrast the NK3 receptor-selective antagonist SB 223412 had a minimal effect on NKA responses and affinity values were low (pK(B) 5.81 +/- 0.11, n = 12 in normal; 5.75 +/- 0.08, n = 12 in idiopathic overactive, and 5.77 +/- 0.13, n = 11 in neurogenic overactive).

CONCLUSION

These data indicate that NKA-induced responses are impaired in detrusor muscle from idiopathic overactive human bladder, but not in detrusor muscle from neurogenic overactive bladder. The NK2 receptor subtype appears to mediate NKA responses in the normal, idiopathic overactive, and neurogenic overactive detrusor. This is important evidence suggesting a difference between the bladder pathophysiology observed in idiopathic versus neurogenic overactive detrusor.

Serotonin-induced potentiation of cholinergic responses to electrical field stimulation in normal and neurogenic overactive human detrusor muscle

OBJECTIVE

To compare the serotonin (5-HT)4-receptor-mediated effects of 5-HT on the potentiation of cholinergic responses to electrical-field stimulation (EFS) in isolated strips of detrusor muscle from patients with normal or neurogenic overactive bladders.

MATERIAL AND METHODS

Strips of detrusor muscle were field-stimulated (10 Hz, 0.01 ms duration, 60 V for 5 s) at 100-s intervals until consistent responses were obtained. In the presence of methiothepin, ketanserin and ondansetron (all 1 mumol/L) to block 5-HT1, 5-HT2 and 5-HT3 receptors, respectively, the cumulative administration of 5-HT or the selective 5-HT4 agonist cisapride, produced concentration-dependent enhancement of responses to EFS in both types of tissue.

RESULTS

The maximum potentiation induced by 5-HT in neurogenic overactive detrusor muscle was reduced (P < 0.05) by about half compared to normal detrusor muscle, but EC50 values obtained in normal and overactive tissue were not significantly different. Cisapride was less potent than 5-HT and acted as a partial agonist relative to 5-HT. The selective 5-HT4 receptor antagonist RS-100235 was a potent antagonist of the 5-HT-induced potentiation of responses to EFS. At 3 nmol/L RS-100235 antagonized the effects of 5-HT in both groups of tissues without affecting the maximum responses. The affinity estimates (apparent pKB values of 9.2-9.5) for this antagonist were similar in normal and overactive detrusor muscle.

CONCLUSIONS

These results indicate that 5-HT4 receptor-mediated potentiation of field-stimulated responses is lower in the neurogenic overactive detrusor muscle than in normal tissue. 5-HT4 receptor antagonist affinity is unchanged in the neurogenic overactive bladder.

Investigation of neurokinin-2 and -3 receptors in the human and pig bladder

OBJECTIVE

To investigate the role of neurokinin (NK)-2 and -3 receptors in mediating the contraction of detrusor muscle strips from human and pig, to determine whether the pig is a good model for the study of tachykinin receptors in the human bladder, as the biological actions of tachykinins, e.g. substance P and NKA are mediated via three distinct receptor subtypes, NK-1, -2 and -3.

MATERIALS AND METHODS

Strips of detrusor muscle were obtained from the bladder dome and neck of female pigs and from patients undergoing cystectomy. Cumulative concentration-response curves to NKA were obtained in the absence and presence of either the NK-2 receptor-selective antagonist SR48968 or the NK-3 receptor-selective antagonist SB223412.

RESULTS

NKA produced concentration-dependent contractions in the human and pig detrusor muscle; the curves were shifted to the right by SR48968, with high affinity (pKB 8.9, 8.3 and 8.0 in the human, pig dome and pig neck, respectively), whereas SB223412 had a minimal effect (pKB 5.8, 5.8 and 6.3, respectively).

CONCLUSION

These data confirm that the NK-2 receptor subtype mediates NKA-induced contraction of the human and pig detrusor muscle. The NK-3 receptor appears to have no role in detrusor contraction of either species. The results also provide evidence that the NK-2 receptor in human and pig are the same, and the latter may be an appropriate species to study tachykinin-induced contractions in human bladder.

The role of alpha1D-adrenoceptors in prostatic contraction examined using protection studies

1 The aim of the study was to investigate the role of the alpha1D-adrenoceptor in alpha1-adrenoceptor-induced contraction of human prostate by means of protection experiments. 2 Responses of human prostate strips to noradrenaline were recorded, along with responses of rat aorta and vas deferens, tissues possessing predominantly alpha1D- and alpha1A-adrenoceptors respectively, for comparison. alpha1-adrenoceptors were then inactivated by incubation with the irreversible antagonist phenoxybenzamine. In some tissues alpha1A- or alpha1D-adrenoceptors were 'protected' from inactivation by incubation in the presence of the selective alpha1A- or 1D-adrenoceptor antagonists 5-methylurapidil and BMY 7378 before recording further responses to noradrenaline. 3 Phenoxybenzamine reduced the maximum noradrenaline-induced response and the potency of noradrenaline in all tissues. In rat vas deferens and human prostate, 5-methylurapidil protected alpha1A-adrenoceptors in a concentration-dependent manner. In rat aorta, 10 nM BMY 7378 almost fully protected alpha1D-adrenoceptors. However, concentrations of BMY 7378 up to 30-fold higher failed to protect receptors in the human prostate. 4 These results suggest that in human prostate functional alpha1D-adrenoceptors do not contribute to noradrenaline-induced contractile responses.

Potential therapeutic targets for treatment of the overactive bladder

Muscarinic receptor antagonists remain the main therapy for the treatment of the overactive bladder yet severe adverse effects make them unsuitable for a large number of patients. The development of new drugs with novel mechanisms of action for the treatment of this condition is therefore essential. This article considers some of the targets currently under investigation for the development of such compounds. Beta-adrenoceptor agonists and KATP channel openers inhibit detrusor muscle activity and remain targets for drug development. There is also evidence that alpha-adrenoceptor antagonists may be effective in the overactive bladder, but the mechanism involved in this action is unclear. Finally the role of tachykinins in regulating bladder function through both the sensory and the motor innervation make them a potential target for drug development, but as with the the others, a selective action on the bladder must remain the goal of drug development.

The minor population of M3-receptors mediate contraction of human detrusor muscle in vitro

1 The objective was to determine the role of muscarinic receptor subtypes in mediating contraction of the human detrusor smooth muscle in vitro. 2 Contractile responses of human detrusor muscle strips to carbachol were obtained in the absence and presence of a range of muscarinic antagonists (pirenzepine, methoctramine, 4-diphenylacetoxy-N-methyl piperidine methiodide (4-DAMP), tropicamide, oxybutynin and tolterodine). Affinity estimates (pKB values) were calculated for the antagonists and correlated with values at the cloned muscarinic receptor subtypes quoted in the literature. 3 Pirenzepine, methoctramine and tropicamide drugs that have high affinities at M1, M2 and M4-receptors, respectively, all had low affinities on the human detrusor (pKB values of 6.8, 6.9 and 6.5, respectively), whilst the M3-selective antagonist 4-DAMP had a high affinity (9.5). Schild plots for all four antagonists had slopes of unity indicating an action at a single receptor. Oxybutynin and tolterodine also acted as competitive antagonists with affinity estimates of 7.6 and 8.1, respectively. 4 When the antagonist affinities obtained on the bladder were plotted against the values published for these antagonists at the cloned muscarinic receptor subtypes, the best correlations were obtained for the m3- and m5-muscarinic receptor subtypes. 5 These data suggest that direct contractile responses of the human detrusor muscle to muscarinic receptor stimulation in vitro are mediated solely via the M3-muscarinic receptor subtype with no contribution from the major M2-receptor population.

The effect of streptozotocin-induced diabetes on cardiac beta-adrenoceptor subtypes in the rat

1. The present study investigates the effect of short-term experimental diabetes of 14-days duration on the beta-adrenoceptor subtypes of the rat heart. 2 .beta-adrenoceptor-mediated functional responses to submaximal doses of isoprenaline were enhanced in Langendorff-perfused hearts from diabetic rats, manifested as greater changes in tension, heart rate and rates of tension development (+dT/dt) and decline (-dT/dt). 3. Radioligand binding data demonstrated that total cardiac beta-adrenoceptor density and affinity for [3H]-dihydroalprenolol was unchanged by diabetes, although a decrease in beta1-adrenoceptor density and increase in beta2-adrenoceptor density was observed. 4. In conclusion, hearts from 14-day streptozotocin-induced diabetic rats demonstrate a number of alterations within the beta-adrenoceptor system. However, the enhanced beta-adrenoceptor-mediated responses to isoprenaline were not caused by an overall increase in density of beta-adrenoceptors, but were accompanied by changes in the ratio of the beta-adrenoceptor subtypes.

5-Hydroxytryptamine-induced potentiation of cholinergic responses to electrical field stimulation in pig detrusor muscle

OBJECTIVE

To determine whether detrusor muscle from the pig can be used as a model to study presynaptic 5-hydroxytryptamine (HT)4-receptor-mediated effects on the electrical field stimulated (EFS) cholinergic responses previously identified in the human bladder. Materials and methods Strips of detrusor muscle were mounted in physiological Krebs' solution under 1 g tension and field stimulated (25 Hz, 0.01 ms duration, 60 V for 5 s) at 100-s intervals and allowed to equilibrate. Concentration-response curves to 5-HT (1 nmol/L to 300 micromol/L) were constructed in the presence and absence of the 5-HT4-selective antagonists RS-100235 and GR-113808 (both at 0.3, 1 and 3 nmol/L). All experiments were conducted in the presence of methiothepin and ondansetron (both 1 micromol/L) to block 5-HT1, -HT2 and -HT3 receptors.

RESULTS

5-HT potentiated the cholinergic responses to EFS in pig bladder strips in a concentration-dependent manner, with a maximum mean (SEM) potentiation of 48.3 (7.7)% of the resting tension (n = 25). Both RS-100235 and GR-113808 antagonized the effect of 5-HT with high affinity, yielding apparent pKB values which were consistent with the responses being mediated via the 5-HT4 receptor subtype.

CONCLUSION

These data indicate that 5-HT can potentiate EFS responses in isolated pig bladder strips and that the 5-HT4 receptor subtype mediates this response. Therefore, the pig may be used as an effective model to study presynaptic 5-HT4 receptors previously reported in the human bladder.

The effects of streptozotocin-induced diabetes and aldose reductase inhibition with sorbinil, on left and right atrial function in the rat

Diabetes mellitus is frequently associated with the complications of cardiovascular disease. Activation of the aldose reductase (or polyol) pathway has long been implicated as an underlying factor for the development of many diabetic complications and indeed, treatment with aldose reductase inhibitors has been shown to prevent or reverse many of these diabetic complications. This study examines the effects of 14-day streptozotocin-induced diabetes on alpha1- and beta-adrenoceptor-mediated responses in rat isolated left and right atria. The effects of treatment with the aldose reductase inhibitor (ARI) sorbinil were also studied. A positive inotropic response was observed to both isoprenaline and phenylephrine in left atria. Diabetes of 14 days duration resulted in a supersensitivity of these tissues to the beta-adrenoceptor agonist in comparison with controls, while responses to the alpha1-adrenoceptor agonist were unaltered. Spontaneously beating right atria from diabetic rats was found to have a depressed resting rate compared with control tissues, although positive chronotropic beta-adrenoceptor-mediated responses were not affected by diabetes. Phenylephrine produced alpha1-adrenoceptor-mediated chronotropic responses in right atrial tissues, and these were found to be enhanced in rats with diabetes. Treatment of diabetic rats with the ARI sorbinil was successful in preventing only one of the observed diabetes-induced changes in atrial function, namely the supersensitivity of left atria to isoprenaline. Sorbinil treatment did, however, alter responses of control left and right atria in a manner similar to diabetes. In conclusion, streptozotocin-induced diabetes of 14 days duration was found to cause a number of alterations in the functioning of both left and right atria. ARI treatment with sorbinil failed to prevent all but one of these changes, and in addition altered responses of atria from control rats, having a similar effect to that produced by diabetes. These data suggest that sorbinil may have effects in addition to, and independent of, aldose reductase inhibition in the cardiovascular system.

M3 muscarinic receptors but not M2 mediate contraction of the porcine detrusor muscle in vitro

1. The objective of the study was to determine the role of muscarinic receptor subtypes in mediating contraction of the porcine detrusor smooth muscle in vitro. 2. Strips of pig detrusor muscle were set up in physiological salt solution and the tensions developed by the tissues were recorded. Responses to carbachol were obtained in the absence and presence of a range of muscarinic antagonists (4-DAMP, methoctramine, darifenacin, oxybutynin, tolterodine and pirenzepine). Antagonist affinity values (pKB values) were calculated and compared with those quoted in the literature for these antagonists at each of the muscarinic receptor subtypes. 3. The M3-selective antagonists, 4-DAMP and darifenacin had high affinities (pKB values of 9.4 and 8.6, respectively). Oxybutynin, tolterodine and pirenzepine had affinities of 8.2, 8.1 and 6.8, respectively, whilst the M2-selective agent methoctramine had a relatively low affinity (pKB = 6.1). The rank order of affinities was, therefore, 4-DAMP > darifenacin > oxybutynin > tolterodine > pirenzepine > methoctramine for the pig detrusor. Correlation of the antagonist affinities obtained on the bladder with those published for these antagonists at the five muscarinic receptor subtypes identified the M3(m3)-receptor as the muscarinic subtype mediating detrusor contractile responses in vitro. 4. These data suggest that a small population of M3-muscarinic receptors must mediate direct contractile responses of the pig detrusor muscle to muscarinic receptor stimulation in vitro.

The effect of sorbinil, an aldose reductase inhibitor, on aortic function in control and streptozotocin-induced diabetic rats

1. The present study investigates the effect of treatment of 14-day streptozotocin-diabetic rats with the aldose reductase inhibitor, sorbinil, on changes ex vivo in aortic vasoconstriction and vasodilation. 2. Maximum contractile responses and aortic sensitivity to phenylephrine were significantly enhanced in aortae from 14-day diabetic rats, in accordance with our previous data. 3. Endothelium-dependent relaxations to carbachol were, in contrast, depressed, although endothelium-independent relaxations to forskolin and sodium nitroprusside were unaltered. 4. Sorbinil treatment of diabetic animals failed to prevent any of these diabetes-induced alterations in aortic function, and indeed exacerbated some of these alterations. In addition, sorbinil treatment caused altered aortic responses in control animals, which sometimes mirrored those found in diabetic animals. 5. It can be concluded that sorbinil may have actions in addition to, and independent of, polyol pathway inhibition. Thus, sorbinil may not be an effective tool for the investigation of aldose reductase inhibition within the vascular system of the rat.

In vitro studies on the reactivation by oximes of phosphylated acetylcholinesterase--II. On the formation of O,O-diethyl phosphorylated AChE and O-ethyl methylphosphonylated AChE and their reactivation by PS2

The in vitro reactivation profiles of O,O-diethyl phosphorylated AChE and O-ethyl methyl phosphonylated AChE by P2S (2-hydroxy iminomethyl-1-methyl-pyridinium methane sulphonate) have been determined. Whilst reinhibition of the reactivated AChE by phosphorylated oxime (POX) is not important in determining the reactivation profile of O,O-diethyl phosphorylated AChE, reinhibition of the reactivated AChE by phosphonylated oxime can, however, be important in determining the reactivation profile of O-ethyl methylphosphonylated AChE and the extent of this reinhibition is determined by the initial concentration of phosphonylated AChE. Kinetic analysis of the reactivation profiles demonstrated that the generally accepted scheme for this reactivation process is incorrect and that phosphylated AChE cannot be considered as a single species although an adequate description of the present data is afforded by a model using a 1:1 mixture of two species each with its own rate of reactivation. In the case of O,O-diethyl phosphorylated AChE the main kinetic difference between these two species is found not in the formation or stability of the phosphorylated AChE-P2S complex but in its subsequent reaction. From results with O-ethyl methylphosphonylated AChE prepared from two pairs of enantiomers as well as from the racemic fluoridate it was concluded that phosphonylation of AChE may not always occur via a mechanism involving inversion of configuration at phosphorus but can also occur with retention of configuration. Reactivation by P2S of O-ethyl methylphosphonylated AChE prepared from (S) organophosphates proceeds with inversion of configuration at phosphorus. Inversion also occurs in the reinhibition of AChE by the POX produced in the initial reactivation.

In vitro studies on the reactivation by oximes of phosphylated acetylcholinesterase--I. On the reactions of P2S with various organophosphates and the properties of the resultant phosphylated oximes

The rates of formation and decomposition of a series of phosphylated oximes derived from P2S (2-hydroxy-iminomethyl-1-methylpyridinium methane-sulphonate) have been measured. The rates of inhibition of AChE by these phosphylated oximes and the parent (and related) organophosphates have also been measured. Possession of these rate data now permits a detailed analysis of the reactivation of phosphylated AChE by P2S to be made (see following paper).

The reactivation by oximes of phosphonylated acetylcholinesterase: the possible erroneous interpretation of reactivating potency

A comparative study of the reactivation by two oximes of acetylcholinesterase inhibited by several organophosphates has been made, with particular reference to the dependence of the degree of reactivation produced by an oxime (reactivating potency) upon the concentration of inhibited enzyme. In the case of one inhibitor it is demonstrated that the relative reactivating potency of the two oximes can be reversed by a change in experimental conditions. It is concluded that the measurement of the reactivation produced by two or more oximes, particularly when carried out under standardized conditions, is of little value in determining their relative reactivating potencies, and of negligible value in predicting their likely therapeutic effectiveness against organophosphate poisoning.

Interactions between acetylcholinesterase and tetra-N-alkylammonium ions

A study of the mechanism of interaction of acetylcholinesterase with some simple tetra-N-alkyammonium ions has been made. Kinetic schemes have been proposed which are consistent with the experimental results observed in the enzyme-tetra-N-alkylammonium system in the presence of substrate and in the presence of an organophosphorus inhibitor.