Intravesical adenosine triphosphate stimulates the micturition reflex in awake, freely moving rats

Urinary ATP Levels Are Controlled by Nucleotidases Released from the Urothelium in a Regulated Manner

Adenosine 5′-triphosphate (ATP) is released in the bladder lumen during filling. Urothelial ATP is presumed to regulate bladder excitability. Urinary ATP is suggested as a urinary biomarker of bladder dysfunctions since ATP is increased in the urine of patients with overactive bladder, interstitial cystitis or bladder pain syndrome. Altered urinary ATP might also be associated with voiding dysfunctions linked to disease states associated with metabolic syndrome. Extracellular ATP levels are determined by ATP release and ATP hydrolysis by membrane-bound and soluble nucleotidases (s-NTDs). It is currently unknown whether s-NTDs regulate urinary ATP. Using etheno-ATP substrate and HPLC-FLD detection techniques, we found that s-NTDs are released in the lumen of ex vivo mouse detrusor-free bladders. Capillary immunoelectrophoresis by ProteinSimple Wes determined that intraluminal solutions (ILS) collected at the end of filling contain ENTPD3 > ENPP1 > ENPP3 ≥ ENTPD2 = NT5E = ALPL/TNAP. Activation of adenylyl cyclase with forskolin increased luminal s-NTDs release whereas the AC inhibitor SQ22536 had no effect. In contrast, forskolin reduced and SQ22536 increased s-NTDs release in the lamina propria. Adenosine enhanced s-NTDs release and accelerated ATP hydrolysis in ILS and lamina propria. Therefore, there is a regulated release of s-NTDs in the bladder lumen during filling. Aberrant release or functions of urothelial s-NTDs might cause elevated urinary ATP in conditions with abnormal bladder excitability.

Emerging drugs for the treatment of bladder storage dysfunction

INTRODUCTION

Current drug treatment of lower urinary tract disorders, for example, overactive bladder syndrome and lower urinary tract symptoms associated with benign prostatic hyperplasia, is moderately effective, has a low treatment persistence and some short- and long-term adverse events. Even if combination therapy with approved drugs may offer advantages in some patients, there is still a need for new agents.

AREAS COVERED

New b₃-adrenoceptor agonists, antimuscarinics, the naked Maxi-K channel gene, a novel 5HT/NA reuptake inhibitor and soluble guanylate cyclase activators are discussed. Focus is given to P2X3 receptor antagonists, small molecule blockers of TRP channels, the roles of cannabis on incontinence in patients with multiple sclerosis, and of drugs acting directly on CB1 and CB2 receptor or indirectly via endocannabinoids by inhibition of fatty acid aminohydrolase.

EXPERT OPINION

New potential alternatives to currently used drugs/drug principles are emerging, but further clinical testing is required before they can be evaluated as therapeutic alternatives. It seems that for the near future individualized treatment with approved drugs and their combinations will be the prevailing therapeutic approach.

Transient receptor potential vanilloid type 4 (TRPV4) in urinary bladder structure and function

Bladder pain syndrome (BPS)/interstitial cystitis (IC) is a urologic, chronic pelvic pain syndrome characterized by pelvic pain, pressure, or discomfort with urinary symptoms. Symptom exacerbation (flare) is common with multiple, perceived triggers including stress. Multiple transient receptor potential (TRP) channels (TRPA1, TRPV1, TRPV4) expressed in the bladder have specific tissue distributions in the lower urinary tract (LUT) and are implicated in bladder disorders including overactive bladder (OAB) and BPS/IC. TRPV4 channels are strong candidates for mechanosensors in the urinary bladder and TRPV4 antagonists are promising therapeutic agents for OAB. In this perspective piece, we address the current knowledge of TRPV4 distribution and function in the LUT and its plasticity with injury or disease with an emphasis on BPS/IC. We review our studies that extend the knowledge of TRPV4 in urinary bladder function by focusing on (i) TRPV4 involvement in voiding dysfunction, pelvic pain, and non-voiding bladder contractions in NGF-OE mice; (ii) distention-induced luminal ATP release mechanisms and (iii) involvement of TRPV4 and vesicular release mechanisms. Finally, we review our lamina propria studies in postnatal rat studies that demonstrate: (i) the predominance of the TRPV4+ and PDGFRα+ lamina propria cellular network in early postnatal rats; (ii) the ability of exogenous mediators (i.e., ATP, TRPV4 agonist) to activate and increase the number of lamina propria cells exhibiting active Ca2+ events; and (iii) the ability of ATP and TRPV4 agonist to increase the rate of integrated Ca2+ activity corresponding to coupled lamina propria network events and the formation of propagating wavefronts.

KPR-5714, a selective transient receptor potential melastatin 8 antagonist, improves voiding dysfunction in rats with bladder overactivity but does not affect voiding behavior in normal rats

AIMS

Transient receptor potential melastatin 8 (TRPM8) has a role in the abnormal sensory transduction of the bladder and is involved in the pathophysiology of hyperactivity bladder disorders. The aim of this study is to examine the effects of KPR-5714, a novel and selective TRPM8 antagonist, on voiding dysfunction induced by bladder afferent hyperactivity via mechanosensitive C-fibers in rats.

METHODS

The effects of intragastric administration of KPR-5714 on bladder overactivity induced by intravesical instillation of 10 mM ATP were investigated using cystometry in conscious female rats. We examined the effects of oral administration of KPR-5714 on voiding behavior using a metabolic cage in normal male rats and rats with an intratesticular injection of 3% acetic acid.

RESULTS

In cystometry measurements, the intercontraction interval was decreased by intravesical ATP instillation. KPR-5714 (0.1, 0.3, and 1 mg/kg) dose-dependently prolonged the shortened intercontraction interval provoked by ATP. In voiding behavior measurements, intratesticular injection of acetic acid decreased the mean voided volume and increased voiding frequency. KPR-5714 (0.1 and 0.3 mg/kg) dose-dependently increased the mean voided volume and decreased voiding frequency without affecting the total voided volume in these rats. However, KPR-5714 (1 and 10 mg/kg) did not influence the voiding behavior in normal rats.

CONCLUSION

The present results suggest that KPR-5714 improves voiding dysfunction by inhibiting the enhanced activity of mechanosensitive bladder C-fibers in rats with bladder overactivity and shows no significant change in voiding behavior in normal rats.

Urinary adenosine triphosphate and nitric oxide levels in patients with underactive bladder: a preliminary study

INTRODUCTION

Various in vitro and in vivo animal studies have shown that adenosine triphosphate (ATP) has a stimulatory role and nitric oxide (NO) has an inhibitory role in modulating bladder contractility. However, it is not known what happens to the urinary levels of ATP and NO in humans with underactive bladder (UAB).

METHODS

In this prospective case-control study, we compared ATP and NO levels in twenty six male patients of UAB with a bladder contractility index (BCI) of < 100 and 18 healthy male volunteers without any lower urinary tract symptoms (LUTS).

RESULTS

The mean urinary ATP levels were significantly lower in cases compared to controls (546.1 ± 37.3 pg/µl vs. 610.7 ± 24.9 pg/µl, p value < 0.001) and the mean NO levels were significantly higher in cases compared to controls (1233.4 ± 91.2 pg/µl vs. 1126.3 ± 91.3.4 pg/µl, p value < 0.001). The mean NO/ATP ratio in cases was significantly higher than that of controls (2.26 ± 0.2 vs. 1.84 ± 0.18, p value < 0.000). Using receiver operating curve (ROC) analysis, we noted the area under the curve (AUC) for NO/ATP ratio to be 0.91 in the diagnosis of cases. A cut-off value of 2.06 for NO/ATP ratio had sensitivity, specificity and diagnostic accuracy of 88.5%, 88.9% and 88.6%, respectively, in diagnosing patients with UAB.

CONCLUSION

Patients with UAB have significantly higher levels of urinary NO and decreased levels of urinary ATP. Urinary NO/ATP levels can be considered as a noninvasive alternate test for diagnosing bladder underactivity.

Molecular pathways underlying tissue injuries in the bladder with ketamine cystitis

Ketamine cystitis (KC) is a chronic bladder inflammation leading to urinary urgency, frequency, and pain. The pathogenesis of KC is complicated and involves multiple tissue injuries in the bladder. Recent studies indicated that urothelium disruption, lamina propria fibrosis and inflammation, microvascular injury, neuropathological alterations, and bladder smooth muscle (BSM) abnormalities all contribute to the pathogenesis of KC. Ketamine has been shown to induce these tissue injuries by regulating different signaling pathways. Ketamine can stimulate antiproliferative factor, adenosine triphosphate, and oxidative stress to disrupt urothelium. Lamina propria fibrosis and inflammation are associated with the activation of cyclooxygenase-2, nitric oxide synthase, immunoglobulin E, and transforming growth factor β1. Ketamine contributes to microvascular injury via the N-methyl-D aspartic receptor (NMDAR), and multiple inflammatory and angiogenic factors such as tumor necrosis factor α and vascular endothelial growth factor. For BSM abnormalities, ketamine can depress the protein kinase B, extracellular signal-regulated kinase, Cav1.2, and muscarinic receptor signaling. Elevated purinergic signaling also plays a role in BSM abnormalities. In addition, ketamine affects neuropathological alterations in the bladder by regulating NMDAR- and brain-derived neurotrophic factor-dependent signaling. Inflammatory cells also contribute to neuropathological changes via the secretion of chemical mediators. Clarifying the role and function of these signaling underlying tissue injuries in the bladder with KC can contribute to a better understanding of the pathophysiology of this disease and to the design of effective treatments for KC.

Bladder urothelium converts bacterial lipopolysaccharide information into neural signaling via an ATP-mediated pathway to enhance the micturition reflex for rapid defense

When bacteria enter the bladder lumen, a first-stage active defensive mechanism flushes them out. Although urinary frequency induced by bacterial cystitis is a well-known defensive response against bacteria, the underlying mechanism remains unclear. In this study, using a mouse model of acute bacterial cystitis, we demonstrate that the bladder urothelium senses luminal extracellular bacterial lipopolysaccharide (LPS) through Toll-like receptor 4 and releases the transmitter ATP. Moreover, analysis of purinergic P2X₂ and P2X₃ receptor-deficient mice indicated that ATP signaling plays a pivotal role in the LPS-induced activation of L6–S1 spinal neurons through the bladder afferent pathway, resulting in rapid onset of the enhanced micturition reflex. Thus, we revealed a novel defensive mechanism against bacterial infection via an epithelial-neural interaction that induces urinary frequency prior to bacterial clearance by neutrophils of the innate immune system. Our results indicate an important defense role for the bladder urothelium as a chemical-neural transducer, converting bacterial LPS information into neural signaling via an ATP-mediated pathway, with bladder urothelial cells acting as sensory receptor cells.

The Urothelium: Life in a Liquid Environment

The urothelium, which lines the renal pelvis, ureters, urinary bladder, and proximal urethra, forms a high-resistance but adaptable barrier that surveils its mechanochemical environment and communicates changes to underlying tissues including afferent nerve fibers and the smooth muscle. The goal of this review is to summarize new insights into urothelial biology and function that have occurred in the past decade. After familiarizing the reader with key aspects of urothelial histology, we describe new insights into urothelial development and regeneration. This is followed by an extended discussion of urothelial barrier function, including information about the roles of the glycocalyx, ion and water transport, tight junctions, and the cellular and tissue shape changes and other adaptations that accompany expansion and contraction of the lower urinary tract. We also explore evidence that the urothelium can alter the water and solute composition of urine during normal physiology and in response to overdistension. We complete the review by providing an overview of our current knowledge about the urothelial environment, discussing the sensor and transducer functions of the urothelium, exploring the role of circadian rhythms in urothelial gene expression, and describing novel research tools that are likely to further advance our understanding of urothelial biology.

TRPV4 blockade reduces voiding frequency, ATP release, and pelvic sensitivity in mice with chronic urothelial overexpression of NGF

Transient receptor potential vanilloid family member 4 (TRPV4) transcript and protein expression increased in the urinary bladder and lumbosacral dorsal root ganglia of transgenic mice with chronic urothelial overexpression of nerve growth factor (NGF-OE). We evaluated the functional role of TRPV4 in bladder function with open-outlet cystometry, void spot assays, and natural voiding (Urovoid) assays with the TRPV4 antagonist HC-067047 (1 μM) or vehicle in NGF-OE and littermate wild-type (WT) mice. Blockade of TRPV4 at the level of the urinary bladder significantly (P ≤ 0.01) increased the intercontraction interval (2.2-fold) and void volume (2.6-fold) and decreased nonvoiding contractions (3.0-fold) in NGF-OE mice, with lesser effects (1.3-fold increase in the intercontraction interval and 1.3-fold increase in the void volume) in WT mice. Similar effects of TRPV4 blockade on bladder function in NGF-OE mice were demonstrated with natural voiding assays. Intravesical administration of HC-067047 (1 µM) significantly (P ≤ 0.01) reduced pelvic sensitivity in NGF-OE mice but was without effect in littermate WT mice. Blockade of urinary bladder TRPV4 or intravesical infusion of brefeldin A significantly (P ≤ 0.01) reduced (2-fold) luminal ATP release from the urinary bladder in NGF-OE and littermate WT mice. The results of the present study suggest that TRPV4 contributes to luminal ATP release from the urinary bladder and increased voiding frequency and pelvic sensitivity in NGF-OE mice.

An ex vivo bladder model with detrusor smooth muscle removed to analyse biologically active mediators released from the suburothelium

KEY POINTS

Studies of urothelial cells, bladder sheets or lumens of filled bladders have suggested that mediators released from urothelium into suburothelium (SubU)/lamina propria (LP) activate mechanisms controlling detrusor excitability. None of these approaches, however, has enabled direct assessment of availability of mediators at SubU/LP during filling. We developed an ex vivo mouse bladder preparation with intact urothelium and SubU/LP but no detrusor, which allows direct access to the SubU/LP surface of urothelium during filling. Pressure-volume measurements during filling demonstrated that bladder compliance is governed primarily by the urothelium. Measurements of purine mediators in this preparation demonstrated asymmetrical availability of purines in lumen and SubU/LP, suggesting that interpretations based solely on intraluminal measurements of mediators may be inaccurate. The preparations are suitable for assessments of release, degradation and transport of mediators in SubU/LP during bladder filling, and are superior to experimental approaches previously used for urothelium research.

ABSTRACT

The purpose of this study was to develop a decentralized (ex vivo) detrusor smooth muscle (DSM)-denuded mouse bladder preparation, a novel model that enables studies on availability of urothelium-derived mediators at the luminal and anti-luminal aspects of the urothelium during filling. Urinary bladders were excised from C57BL6/J mice and the DSM was removed by fine-scissor dissection without touching the mucosa. Morphology and cell composition of the preparation wall, pressure-volume relationships during filling, and fluorescent dye permeability of control, protamine sulfate- and lipopolysaccharide-treated denuded bladders were characterized. The preparation wall contained intact urothelium and suburothelium (SubU)/lamina propria (LP) and lacked the DSM and the serosa. The utility of the model for physiological research was validated by measuring release, metabolism and transport of purine mediators at SubU/LP and in bladder lumen during filling. We determined asymmetrical availability of purines (e.g. ATP, ADP, AMP and adenosine) in lumen and at SubU/LP during filling, suggesting differential mechanisms of release, degradation and bilateral transurothelial transport of purines during filling. Some observations were validated in DSM-denuded bladder of the cynomolgus monkey (Macaca fascicularis). The novel model was superior to current models utilized to study properties of the urothelium (e.g. cultured urothelial cells, bladder mucosa sheets mounted in Ussing chambers or isolated bladder strips in organ baths) in that it enabled direct access to the vicinity of SubU/LP during authentic bladder filling. The model is particularly suitable for understanding local mechanisms of urothelium-DSM connectivity and for broad understanding of the role of urothelium in regulating continence and voiding.

Managing urinary incontinence in women - a review of new and emerging pharmacotherapy

INTRODUCTION

The pharmacological treatment of urinary incontinence and overactive bladder (OAB) has been, for a longer time, based on antimuscarinic agents. In recent years, two other pharmacological principles have been introduced for the treatment of OAB and urgency urinary incontinence: the β3-adrenergic agent mirabegron and botulinum neurotoxin. Meanwhile, there is lack of effective drugs for the treatment of stress incontinence.

AREAS COVERED

This literature review presents synthetic compounds aimed to treat female urinary incontinence that are in phase II-III clinical development.

EXPERT OPINION

Antimuscarinic agents will continue to represent the current gold standard for the first-line pharmacological management of OAB and urgency urinary incontinence. The class of β3-agonists will certainly expand with the discovery and clinical development of novel agents. Combination therapy of antimuscarinic agents and β3-agonists could offer an alternative treatment in these patients, including those with symptoms refractory to first-line monotherapy. A huge number of preclinical studies are underway in this field exploring the therapeutic potential of many novel compounds while some have advanced to clinical phases of development.

Neonatal Bladder Irritation Is Associated With Vanilloid Receptor TRPV1 Expression in Adult Rats

PURPOSE

To evaluate whether mild chemical irritation of the bladder in neonatal rats is associated with persistent vanilloid receptor transient receptor potential vanilloid subfamily 1 (TRPV1) activity in adult rats.

METHODS

Female Sprague-Dawley rats were used. Ten-day-old rat pups underwent bladder sensitization via intravesical infusion of 0.2% acetic acid in saline with or without prior bladder desensitization with capsaicin. After 8 weeks, 3 groups of rats (control [group 1], bladder sensitization [group 2], and bladder desensitization [group 3]) underwent cystometry. Inflammation of bladder tissue and the expression of TRPV1 in bladder tissue and dorsal root ganglia (DRG) were also evaluated.

RESULTS

The bladder sensitization group showed more frequent voiding contractions. TRPV1 expression in adult bladder tissue was elevated in group 2. TRPV1 mRNA levels in the bladder and DRG were significantly higher in group 2 than in group 1. Moreover, group 2 had significantly more DRG neurons (identified by uptake of the retrograde label Fast Blue) that exhibited TRPV1 immunoreactivity.

CONCLUSION

We found a significant association between neonatal bladder sensitization and persistent TRPV1 activity in adult rats. This is the first study to focus on the underlying pathogenesis of bladder overactivity from childhood to adulthood. Our findings could lead to the development of new strategies for the treatment and prevention of adult urinary symptoms arising from childhood urinary tract dysfunction.

Role of neurogenic inflammation in local communication in the visceral mucosa

Intense research has focused on the involvement of the nervous system in regard to cellular mechanisms underlying neurogenic inflammation in the pelvic viscera. Evidence supports the neural release of inflammatory factors, trophic factors, and neuropeptides in the initiation of inflammation. However, more recently, non-neuronal cells including epithelia, endothelial, mast cells, and paraneurons are likely important participants in nervous system functions. For example, the urinary bladder urothelial cells are emerging as key elements in the detection and transmission of both physiological and nociceptive stimuli in the lower urinary tract. There is mounting evidence that these cells are involved in sensory mechanisms and can release mediators. Further, localization of afferent nerves next to the urothelium suggests these cells may be targets for transmitters released from bladder nerves and that chemicals released by urothelial cells may alter afferent excitability. Modifications of this type of communication in a number of pathological conditions can result in altered release of epithelial-derived mediators, which can activate local sensory nerves. Taken together, these and other findings highlighted in this review suggest that neurogenic inflammation involves complex anatomical and physiological interactions among a number of cell types in the bladder wall. The specific factors and pathways that mediate inflammatory responses in both acute and chronic conditions are not well understood and need to be further examined. Elucidation of mechanisms impacting on these pathways may provide insights into the pathology of various types of disorders involving the pelvic viscera.

TRPV1 enhances the afferent response to P2X receptor activation in the mouse urinary bladder

Both TRPV1 and P2X receptors present on bladder sensory nerve fibres have been implicated in mechanosensation during bladder filling. The aim of this study was to determine possible interactions between these receptors in modulating afferent nerve activity. In wildtype (TRPV1+/+) and TRPV1 knockout (TRPV1-/-) mice, bladder afferent nerve activity, intravesical pressure, and luminal ATP and acetylcholine levels were determined and also intracellular calcium responses of dissociated pelvic DRG neurones and primary mouse urothelial cells (PMUCs). Bladder afferent nerve responses to the purinergic agonist αβMethylene-ATP were depressed in TRPV1-/- mice (p ≤ 0.001) and also in TRPV1+/+ mice treated with the TRPV1-antagonist capsazepine (10 µM; p ≤ 0.001). These effects were independent of changes in bladder compliance or contractility. Responses of DRG neuron to αβMethylene-ATP (30 µM) were unchanged in the TRPV1-/- mice, but the proportion of responsive neurones was reduced (p ≤ 0.01). Although the TRPV1 agonist capsaicin (1 µM) did not evoke intracellular responses in PMUCs from TRPV1+/+ mice, luminal ATP levels were reduced in the TRPV1-/- mice (p ≤ 0.001) compared to wildtype. TRPV1 modulates P2X mediated afferent responses and provides a mechanistic basis for the decrease in sensory symptoms observed following resiniferatoxin and capsaicin treatment for lower urinary tract symptoms.

Changes in Adenosine Triphosphate and Nitric Oxide in the Urothelium of Patients with Benign Prostatic Hyperplasia and Detrusor Underactivity

PURPOSE

We investigated changes in the levels of adenosine triphosphate and nitric oxide in the urothelium of men with detrusor underactivity and benign prostatic hyperplasia.

MATERIALS AND METHODS

We prospectively enrolled in study 30 men who planned to undergo surgical treatment for benign prostatic hyperplasia. The 15 patients with a bladder contractility index less than 100 were assigned to the detrusor underactivity group while the 15 with a bladder contractility index more than 100 were assigned to the no detrusor underactivity group. Bladder mucosal specimens were collected at surgical prostate resection, and adenosine triphosphate and endothelial nitric oxide synthase were analyzed in these specimens. The levels of adenosine triphosphate and endothelial nitric oxide synthase were compared between the 2 groups. The correlation of urodynamic parameters with adenosine triphosphate and endothelial nitric oxide synthase was assessed in all patients.

RESULTS

Mean ± SEM endothelial nitric oxide synthase did not significantly differ between the detrusor underactivity and no underactivity groups (3.393 ± 0.969 vs 1.941 ± 0.377 IU/ml, p = 0.247). However, the mean level of adenosine triphosphate in the detrusor underactivity group was significantly lower than in the no detrusor underactivity group (1.289 ± 0.320 vs 9.262 ± 3.285 pmol, p = 0.011). In addition, in all patients adenosine triphosphate positively correlated with the bladder contractility index (r = 0.478, p = 0.018) and with detrusor pressure on maximal flow (r = 0.411, p = 0.046).

CONCLUSIONS

Adenosine triphosphate was significantly decreased in the urothelium in men with detrusor underactivity and benign prostatic hyperplasia, reflecting the change in detrusor function.

A novel single compartment in vitro model for electrophysiological research using the perfluorocarbon FC-770

Electrophysiological studies of whole organ systems in vitro often require measurement of nerve activity and/or stimulation of the organ via the associated nerves. Currently two-compartment setups are used for such studies. These setups are complicated and require two fluids in two separate compartments and stretching the nerve across one chamber to the other, which may damage the nerves. We aimed at developing a simple single compartment setup by testing the electrophysiological properties of FC-770 (a perfluorocarbon) for in vitro recording of bladder afferent nerve activity and electrical stimulation of the bladder. Perflurocarbons are especially suitable for such a setup because of their high oxygen carrying capacity and insulating properties. In male Wistar rats, afferent nerve activity was recorded from postganglionic branches of the pelvic nerve in vitro, in situ and in vivo. The bladder was stimulated electrically via the efferent nerves. Organ viability was monitored by recording spontaneous contractions of the bladder. Additionally, histological examinations were done to test the effect of FC-770 on the bladder tissue. Afferent nerve activity was successfully recorded in a total of 11 rats. The bladders were stimulated electrically and high amplitude contractions were evoked. Histological examinations and monitoring of spontaneous contractions showed that FC-770 maintained organ viability and did not cause damage to the tissue. We have shown that FC-770 enables a simple, one compartment in vitro alternative for the generally used two compartment setups for whole organ electrophysiological studies.

Intravital imaging of mouse urothelium reveals activation of extracellular signal-regulated kinase by stretch-induced intravesical release of ATP

To better understand the roles played by signaling molecules in the bladder, we established a protocol of intravital imaging of the bladder of mice expressing a Förster/fluorescence resonance energy transfer (FRET) biosensor for extracellular signal‐regulated kinase (ERK), which plays critical roles not only in cell growth but also stress responses. With an upright two‐photon excitation microscope and a vacuum‐stabilized imaging window, cellular ERK activity was visualized in the whole bladder wall, from adventitia to urothelium. We found that bladder distention caused by elevated intravesical pressure (IVP) activated ERK in the urothelium, but not in the detrusor smooth muscle. When bladder distension was prevented, high IVP failed to activate ERK, suggesting that mechanical stretch, but not the high IVP, caused ERK activation. To delineate its molecular mechanism, the stretch‐induced ERK activation was reproduced in an hTERT‐immortalized human urothelial cell line (TRT‐HU1) in vitro. We found that uniaxial stretch raised the ATP concentration in the culture medium and that inhibition of ATP signaling by apyrase or suramin suppressed the stretch‐induced ERK activation in TRT‐HU1 cells. In agreement with this in vitro observation, pretreatment with apyrase or suramin suppressed the high IVP‐induced urothelial ERK activation in vivo. Thus, we propose that mechanical stretch induces intravesical secretion of ATP and thereby activates ERK in the urothelium. Our method of intravital imaging of the bladder of FRET biosensor‐expressing mice should open a pathway for the future association of physiological stimuli with the activities of intracellular signaling networks.

Increased Urinary Adenosine Triphosphate in Patients With Bladder Outlet Obstruction Due to Benign Prostate Hyperplasia

BACKGROUND

Diagnosis of bladder outflow obstruction (BOO) in patients with lower urinary tract (LUT) symptoms is challenging without using invasive urodynamic tests. Recently, we showed in vitro that urothelial strips from patients with benign prostatic hyperplasia (BPH) release more ATP than controls. Here, we tested whether urinary ATP can be used as a wall tension transducer non-invasive biomarker to detect BOO in patients with BPH.

METHODS

79 male patients with BOO and 22 asymptomatic controls were recruited prospectively. Patients were asked to complete the International Prostate Symptom Score (IPSS) questionnaire and to void at normal desire into a urinary flowmeter; the postvoid residual volume was determined by suprapubic ultrasonography. Urine samples from all individuals were examined for ATP, creatinine, and lactate dehydrogenase.

RESULTS

BOO patients had significantly higher (P < 0.001) urinary ATP normalized by the voided volume (456 ± 36 nmol) than age-matched controls (209 ± 35 nmol). Urinary ATP amounts increased with the voided volume, but the slope of this rise was higher in BOO patients than in controls. A negative correlation was detected between urinary ATP and flow rate parameters, namely maximal flow rate (r = -0.310, P = 0.005), Siroky flow-volume normalization (r = -0.324, P = 0.004), and volume-normalized flow rate index (r = -0.320, P = 0.012). We found no correlation with LUT symptoms IPSS score. Areas under the receiver operator characteristics (ROC) curves were 0.91 (95%CI 0.86-0.96, P < 0.001) for ATP alone and 0.88 (95%CI 0.81-0.94, P < 0,001) when adjusted to urinary creatinine.

CONCLUSIONS

Patients with BOO release higher amounts of ATP into the urine than the control group. The high area under the ROC curve suggests that urinary ATP can be a high-sensitive non-invasive biomarker of BOO, which may have a discriminative value of detrusor competence when comparing BPH patients with low urinary flow rates. Prostate 76:1353-1363, 2016. © 2016 Wiley Periodicals, Inc.

Urothelial purine release during filling of murine and primate bladders

During urinary bladder filling the bladder urothelium releases chemical mediators that in turn transmit information to the nervous and muscular systems to regulate sensory sensation and detrusor muscle activity. Defects in release of urothelial mediators may cause bladder dysfunctions that are characterized with aberrant bladder sensation during bladder filling. Previous studies have demonstrated release of ATP from the bladder urothelium during bladder filling, and ATP remains the most studied purine mediator that is released from the urothelium. However, the micturition cycle is likely regulated by multiple purine mediators, since various purine receptors are found present in many cell types in the bladder wall, including urothelial cells, afferent nerves, interstitial cells in lamina propria, and detrusor smooth muscle cells. Information about the release of other biologically active purines during bladder filling is still lacking. Decentralized bladders from C57BL/6 mice and Cynomolgus monkeys (Macaca fascicularis) were filled with physiological solution at different rates. Intraluminal fluid was analyzed by high-performance liquid chromatography with fluorescence detection for simultaneous evaluation of ATP, ADP, AMP, adenosine, nicotinamide adenine dinucleotide (NAD⁺), ADP-ribose, and cADP-ribose content. We also measured ex vivo bladder filling pressures and performed cystometry in conscious unrestrained mice at different filling rates. ATP, ADP, AMP, NAD⁺, ADPR, cADPR, and adenosine were detected released intravesically at different ratios during bladder filling. Purine release increased with increased volumes and rates of filling. Our results support the concept that multiple urothelium-derived purines likely contribute to the complex regulation of bladder sensation during bladder filling.

Purinergic Mechanisms and Pain

There is a brief introductory summary of purinergic signaling involving ATP storage, release, and ectoenzymatic breakdown, and the current classification of receptor subtypes for purines and pyrimidines. The review then describes purinergic mechanosensory transduction involved in visceral, cutaneous, and musculoskeletal nociception and on the roles played by receptor subtypes in neuropathic and inflammatory pain. Multiple purinoceptor subtypes are involved in pain pathways both as an initiator and modulator. Activation of homomeric P2X3 receptors contributes to acute nociception and activation of heteromeric P2X2/3 receptors appears to modulate longer-lasting nociceptive sensitivity associated with nerve injury or chronic inflammation. In neuropathic pain activation of P2X4, P2X7, and P2Y12 receptors on microglia may serve to maintain nociceptive sensitivity through complex neural-glial cell interactions and antagonists to these receptors reduce neuropathic pain. Potential therapeutic approaches involving purinergic mechanisms will be discussed.

Purinergic signalling in the urinary bladder

It is well established that in most species, exocytotic vesicular release of ATP from parasympathetic neurons contributes to contraction of the bladder. However, ATP is released not only from parasympathetic nerves, but also from the urothelium. During bladder filling, the urothelium is stretched and ATP is released from the umbrella cells thereby activating mechanotransduction pathways. ATP release can also be induced by various mediators present in the urine and and/or released from nerves or other components of the lamina propria. Urothelial release of ATP is mainly attributable to vesicular transport or exocytosis and, to a smaller extent, to pannexin hemichannel conductive efflux. After release, ATP acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex and to mediate the sensation of bladder filling and urgency. ATP also acts on suburothelial interstitial cells/myofibroblasts generating an inward Ca(2+) transient that via gap junctions could provide a mechanism for long-distance spread of signals from the urothelium to the detrusor muscle. ATP release can be affected by urological diseases, e.g., interstitial cystitis and both the mechanisms of release and the receptors activated by ATP may be targets for future drugs for treatment of lower urinary tract disorders.

Pannexin 1 channels mediate the release of ATP into the lumen of the rat urinary bladder

KEY POINTS

ATP is released through pannexin channels into the lumen of the rat urinary bladder in response to distension or stimulation with bacterial endotoxins. Luminal ATP plays a physiological role in the control of micturition because intravesical perfusion of apyrase or the ecto-ATPase inhibitor ARL67156 altered reflex bladder activity in the anaesthetized rat. The release of ATP from the apical and basolateral surfaces of the urothelium appears to be mediated by separate mechanisms because intravesical administration of the pannexin channel antagonist Brilliant Blue FCF increased bladder capacity, whereas i.v. administration did not. Intravesical instillation of small interfering RNA-containing liposomes decreased pannexin 1 expression in the rat urothelium in vivo and increased bladder capacity. These data indicate a role for pannexin-mediated luminal ATP release in both the physiological and pathophysiological control of micturition and suggest that urothelial pannexin may be a viable target for the treatment of overactive bladder disorders.

ABSTRACT

ATP is released from the bladder epithelium, also termed the urothelium, in response to mechanical or chemical stimuli. Although numerous studies have described the contribution of this release to the development of various bladder disorders, little information exists regarding the mechanisms of release. In the present study, we examined the role of pannexin channels in mechanically-induced ATP release from the urothelium. PCR confirmed the presence of pannexin 1 and 2 mRNA in rat urothelial tissue, whereas immunofluorescence experiments localized pannexin 1 to all three layers of the urothelium. During continuous bladder cystometry in anaesthetized rats, inhibition of pannexin 1 channels using carbenoxolone (CBX) or Brilliant Blue FCF (BB-FCF) (1-100 μm, intravesically), or by using intravesical small interfering RNA, increased the interval between voiding contractions. Intravenous administration of BB-FCF (1-100 μg kg(-1) ) did not alter bladder activity. CBX or BB-FCF (100 μm intravesically) also decreased basal ATP concentrations in the perfusate from non-distended bladders and inhibited increases in ATP concentrations in response to bladder distension (15 and 30 cmH2 O pressure). Intravesical perfusion of the ATP diphosphohydrolase apyrase (2 U ml(-1) ), or the ATPase inhibitor ARL67156 (10 μm) increased or decreased reflex bladder activity, respectively. Intravesical instillation of bacterial lipopolysaccharides (LPS) (Escherichia coli 055:B5, 100 μg ml(-1) ) increased ATP concentrations in the bladder perfusate, and also increased voiding frequency; these effects were suppressed by BB-FCF. These data indicate that pannexin channels contribute to distension- or LPS-evoked ATP release into the lumen of the bladder and that luminal release can modulate voiding function.

Activation of P2Y6 receptors increases the voiding frequency in anaesthetized rats by releasing ATP from the bladder urothelium

BACKGROUND AND PURPOSE

Despite the abundant expression of the UDP-sensitive P2Y6 receptor in urothelial cells and sub-urothelial myofibroblasts its role in the control of bladder function is not well understood.

EXPERIMENTAL APPROACH

We compared the effects of UDP and of the selective P2Y6 receptor agonist, PSB0474, on bladder urodynamics in anaesthetized rats; the voided fluid was tested for ATP bioluminescence. The isolated urinary bladder was used for in vitro myographic recordings and [(3) H]-ACh overflow experiments.

KEY RESULTS

Instillation of UDP or PSB0474 into the bladder increased the voiding frequency (VF) without affecting the amplitude (A) and the duration (Δt) of bladder contractions; an effect blocked by the P2Y6 receptor antagonist, MRS2578. Effects mediated by urothelial P2Y6 receptors required extrinsic neuronal circuitry as they were not detected in the isolated bladder. UDP-induced bladder hyperactvity was also prevented by blocking P2X3 and P2Y1 receptors, respectively, with A317491 and MRS2179 applied i.v.. UDP decreased [(3) H]-ACh release from stimulated bladder strips with urothelium, but not in its absence. Inhibitory effects of UDP were converted into facilitation by the P2Y1 receptor antagonist, MRS2179. The P2Y6 receptor agonist increased threefold ATP levels in the voided fluid.

CONCLUSIONS AND IMPLICATIONS

Activation of P2Y6 receptors increased the voiding frequency indirectly by releasing ATP from the urothelium and activation of P2X3 receptors on sub-urothelial nerve afferents. Bladder hyperactivity may be partly reversed following ATP hydrolysis to ADP by E-NTPDases, thereby decreasing ACh release from cholinergic nerves expressing P2Y1 receptors.

Bladder sensory physiology: neuroactive compounds and receptors, sensory transducers, and target-derived growth factors as targets to improve function

Urinary bladder dysfunction presents a major problem in the clinical management of patients suffering from pathological conditions and neurological injuries or disorders. Currently, the etiology underlying altered visceral sensations from the urinary bladder that accompany the chronic pain syndrome, bladder pain syndrome (BPS)/interstitial cystitis (IC), is not known. Bladder irritation and inflammation are histopathological features that may underlie BPS/IC that can change the properties of lower urinary tract sensory pathways (e.g., peripheral and central sensitization, neurochemical plasticity) and contribute to exaggerated responses of peripheral bladder sensory pathways. Among the potential mediators of peripheral nociceptor sensitization and urinary bladder dysfunction are neuroactive compounds (e.g., purinergic and neuropeptide and receptor pathways), sensory transducers (e.g., transient receptor potential channels) and target-derived growth factors (e.g., nerve growth factor). We review studies related to the organization of the afferent limb of the micturition reflex and discuss neuroplasticity in an animal model of urinary bladder inflammation to increase the understanding of functional bladder disorders and to identify potential novel targets for development of therapeutic interventions. Given the heterogeneity of BPS/IC and the lack of consistent treatment benefits, it is unlikely that a single treatment directed at a single target in micturition reflex pathways will have a mass benefit. Thus, the identification of multiple targets is a prudent approach, and use of cocktail treatments directed at multiple targets should be considered.

Purinergic signalling in the urinary tract in health and disease

Purinergic signalling is involved in a number of physiological and pathophysiological activities in the lower urinary tract. In the bladder of laboratory animals there is parasympathetic excitatory cotransmission with the purinergic and cholinergic components being approximately equal, acting via P2X1 and muscarinic receptors, respectively. Purinergic mechanosensory transduction occurs where ATP, released from urothelial cells during distension of bladder and ureter, acts on P2X3 and P2X2/3 receptors on suburothelial sensory nerves to initiate the voiding reflex, via low threshold fibres, and nociception, via high threshold fibres. In human bladder the purinergic component of parasympathetic cotransmission is less than 3 %, but in pathological conditions, such as interstitial cystitis, obstructed and neuropathic bladder, the purinergic component is increased to 40 %. Other pathological conditions of the bladder have been shown to involve purinoceptor-mediated activities, including multiple sclerosis, ischaemia, diabetes, cancer and bacterial infections. In the ureter, P2X7 receptors have been implicated in inflammation and fibrosis. Purinergic therapeutic strategies are being explored that hopefully will be developed and bring benefit and relief to many patients with urinary tract disorders.

ATP released via pannexin-1 hemichannels mediates bladder overactivity triggered by urothelial P2Y6 receptors

In contrast to the well-known signaling role of urothelial ATP to control bladder function, the hypothesis that uracil nucleotides (UTP and/or UDP) also exert autocrine/paracrine actions only recently gained experimental support. Urothelial cells express UDP-sensitive P2Y6 receptors, yet their role in the control of bladder activity has been mostly neglected. This study was designed to investigate the ability of PSB0474, a stable UDP analogue which exhibits selectivity for P2Y6 receptors, to modulate urodynamic responses in the anaesthetized rat in vivo. Instillation of PSB0474 into the bladder increased the voiding frequency (VF) without affecting the amplitude (A) and the duration (Δt) of bladder contractions. PSB0474-induced bladder overactivity was prevented by the selective P2Y6 antagonist, MRS2578. The increase in the VF produced by PSB0474 was also blocked by inhibitors of pannexin-1 hemichannels, (10)Panx or carbenoxolone, when these drugs were applied inside the bladder lumen but not when they were administered intravenously. Reduction of hemichannels pore permeability with H1152 also prevented PSB0474-induced bladder overactivity, but the exocytosis inhibitor, Exo-1, was inactive. PSB0474 increased by 3-fold the urinary ATP content. Implication of hemichannels permeability on PSB0474-induced ATP release was demonstrated by real-time fluorescence video-microscopy measuring the uptake of propidium iodide by intact urothelial cells in the absence and in the presence of MRS2578 or carbenoxolone. Confocal microscopy studies confirmed the co-localization of pannexin-1 and P2Y6 receptors in the rat urothelium. Data indicate that activation of P2Y6 receptors causes bladder overactivity in the anaesthetized rat indirectly by releasing ATP from the urothelium via pannexin-1 hemichannels.

Urinary ATP may be a dynamic biomarker of detrusor overactivity in women with overactive bladder syndrome

BACKGROUND

Nowadays, there is a considerable bulk of evidence showing that ATP has a prominent role in the regulation of human urinary bladder function and in the pathophysiology of detrusor overactivity. ATP mediates nonadrenergic-noncholinergic detrusor contractions in overactive bladders. In vitro studies have demonstrated that uroepithelial cells and cholinergic nerves from overactive human bladder samples (OAB) release more ATP than controls. Here, we compared the urinary ATP concentration in samples collected non-invasively from OAB women with detrusor overactivity and age-matched controls.

METHODS

Patients with neurologic diseases, history of malignancy, urinary tract infections or renal impairment (creatinine clearance <70 ml/min) were excluded. All patients completed a 3-day voiding diary, a 24 h urine collection and blood sampling to evaluate creatinine clearance. Urine samples collected during voluntary voids were immediately freeze-preserved for ATP determination by the luciferin-luciferase bioluminescence assay; for comparison purposes, samples were also tested for urinary nerve growth factor (NGF) by ELISA.

RESULTS

The urinary content of ATP, but not of NGF, normalized to patients' urine creatinine levels (ATP/Cr) or urinary volume (ATP.Vol) were significantly (P<0.05) higher in OAB women with detrusor overactivity (n = 34) than in healthy controls (n = 30). Significant differences between the two groups were still observed by boosting urinary ATP/Cr content after water intake, but these were not detected for NGF/Cr. In OAB patients, urinary ATP/Cr levels correlated inversely with mean voided volumes determined in a 3-day voiding diary.

CONCLUSION

A high area under the receiver operator characteristics (ROC) curve (0.741; 95% CI 0.62-0.86; P<0.001) is consistent with urinary ATP/Cr being a highly sensitive dynamic biomarker for assessing detrusor overactivity in women with OAB syndrome.

Urothelial signaling

The urothelium, which lines the inner surface of the renal pelvis, the ureters, and the urinary bladder, not only forms a high-resistance barrier to ion, solute and water flux, and pathogens, but also functions as an integral part of a sensory web which receives, amplifies, and transmits information about its external milieu. Urothelial cells have the ability to sense changes in their extracellular environment, and respond to chemical, mechanical and thermal stimuli by releasing various factors such as ATP, nitric oxide, and acetylcholine. They express a variety of receptors and ion channels, including P2X3 purinergic receptors, nicotinic and muscarinic receptors, and TRP channels, which all have been implicated in urothelial-neuronal interactions, and involved in signals that via components in the underlying lamina propria, such as interstitial cells, can be amplified and conveyed to nerves, detrusor muscle cells, and ultimately the central nervous system. The specialized anatomy of the urothelium and underlying structures, and the possible communication mechanisms from urothelial cells to various cell types within the bladder wall are described. Changes in the urothelium/lamina propria ("mucosa") produced by different bladder disorders are discussed, as well as the mucosa as a target for therapeutic interventions.

Rodent models for urodynamic investigation

Rodents, most commonly rats, mice, and guinea pigs are widely used to investigate urinary storage and voiding functions, both in normal animals and in models of disease. An often used methodology is cystometry. Micturitions in rodents and humans differ significantly and this must be considered when cystometry is used to interpret voiding in rodent models. Cystometry in humans requires active participation of the investigated patient (subject), and this can for obvious reasons not be achieved in the animals. Cystometric parameters in rodents are often poorly defined and do not correspond to those used in humans. This means that it is important that the terminology used for description of what is measured should be defined, and that the specific terminology used in human cystometry should be avoided. Available disease models in rodents have limited translational value, but despite many limitations, rodent cystometry may give important information on bladder physiology and pharmacology. The present review discusses the principles of urodynamics in rodents, techniques, and terminology, as well as some commonly used disease models, and their translational value.

ATP and P2X purinoceptors in urinary tract disorders

The pharmacological concept of specifically targeting purinoceptors (receptors for ATP and related nucleotides) has emerged over the last two decades in the quest for novel, differentiated therapeutics. Investigations from many laboratories have established a prominent role for ATP in the functional regulation of most tissue and organ systems, including the urinary tract, under normal and pathophysiological conditions. In the particular case of the urinary tract, ATP signaling via P2X1 receptors participates in the efferent control of detrusor smooth muscle excitability, and this function may be heightened in disease and aging. Perhaps of greater interest, ATP also appears to be involved in bladder sensation, operating via activation of P2X3-containing receptors on sensory afferent neurones, both on peripheral terminals within the urinary tract tissues (e.g., ureters, bladder) and on central synapses in the dorsal horn of the spinal cord. Such findings are based on results from classical pharmacological and localization studies in nonhuman and human tissues, gene knockout mice, and studies using recently identified pharmacological antagonists - some of which have progressed as candidate drug molecules. Based on recent advances in this field, it is apparent that the development of selective antagonists for these receptors will occur that could lead to therapies offering better relief of storage, voiding, and sensory symptoms for patients, while minimizing the systemic side effects that curb the clinical effectiveness of current urologic medicines.

Animal models in overactive bladder research

Overactive bladder syndrome (OAB) is a symptom-based diagnosis characterised by the presence of urinary urgency. It is highly prevalent and overlaps with the presence of bladder contractions during urine storage, which characterises the urodynamic diagnosis of detrusor overactivity. Animal models are needed to understand the pathophysiology of OAB, but the subjective nature of the symptom complex means that interpretation of the findings in animals requires caution. Because urinary urgency cannot be ascertained in animals, surrogate markers such as frequency, altered toileting areas, and non-micturition contractions have to be used instead. No model can recapitulate the subjective, objective, and related factors seen in the clinical setting. Models used include partial bladder outlet obstruction, the spontaneous hypertensive rat, the hyperlipidaemic rat, various neurological insults and some gene knock-outs. Strengths and weaknesses of these models are discussed in the context of the inherent difficulties of extrapolating subjective symptoms in animals.

Overactive and underactive bladder dysfunction is reflected by alterations in urothelial ATP and NO release

ATP and NO are released from the urothelium in the bladder. Detrusor overactivity (DO) following spinal cord injury results in higher ATP and lower NO release from the bladder urothelium. Our aim was to study the relationship between ATP and NO release in (1) early diabetic bladders, an overactive bladder model; and (2) "diuretic" bladders, an underactive bladder model. To induce diabetes mellitus female rats received 65mg/kg streptozocin (i.v.). To induce chronic diuresis rats were fed with 5% sucrose. At 28 days, in vivo open cystometry was performed. Bladder wash was collected to analyze the amount of ATP and NO released into the bladder lumen. For in vitro analysis of ATP and NO release, a Ussing chamber was utilized and hypoosmotic Krebs was perfused on the urothelial side of the chamber. ATP was analyzed with luminometry or HPLC-fluorometry while NO was measured with a Sievers NO-analyzer. In vivo ATP release was increased in diabetic bladders and unchanged in diuretic bladders. In vitro release from the urothelium followed the same pattern. NO release was unchanged both in vitro and in vivo in overactive bladders whereas it was enhanced in underactive bladders. We found that the ratio of ATP/NO, representing sensory transmission in the bladder, was high in overactive and low in underactive bladder dysfunction. In summary, ATP release has a positive correlation while NO release has a negative correlation with the bladder contraction frequency. The urinary ATP/NO ratio may be a clinically relevant biomarker to characterize the extent of bladder dysfunction.

Oxybutynin and propiverine suppress adenosine triphosphate-induced bladder overactivity other than through antimuscarinic mechanisms

OBJECTIVES

Adenosine triphosphate (ATP) is released from bladder urothelium in response to stretch and may act as a sensory neurotransmitter. ATP release from the bladder urothelium is augmented in many pathophysiologic conditions, resulting in bladder overactivity. Patients who have bladder overactivity are treated with antimuscarinics with symptom improvement. We investigated the effects of oxybutynin and propiverine on bladder overactivity induced by intravesical instillation of ATP.

MATERIAL AND METHODS

Under urethane anesthesia, cystometry was performed in female Sprague-Dawley rats. After a 2-hour baseline period, protamine sulfate (10 mg/mL) was instilled for 1 hour, and then ATP (60 mM, pH 6.0) was instilled intravesically. Oxybutynin, propiverine, pyridoxal-(5) phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), atropine, 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP), and methoctramine were given intravenously when ATP-induced bladder overactivity was stable.

RESULTS

When protamine sulfate was infused intravesically, the intercontraction interval (ICI) did not decrease significantly, but intravesical instillation of ATP after protamine sulfate treatment decreased the ICI compared with baseline. Oxybutynin, propiverine, and PPADS given intravenously reversed the ATP-induced ICI reduction in a dose-dependent manner. In contrast, ATP-induced ICI reduction was not reversed by intravenous atropine, 4-DAMP, or methoctramine. Maximum voiding pressure did not change with ATP but decreased with antimuscarinics. Pressure threshold (PT) decreased with ATP and stayed reduced after dose of oxybutynin or propiverine.

CONCLUSIONS

Bladder overactivity induced by intravesical instillation of ATP with protamine pretreatment was suppressed by oxybutynin, propiverine, and PPADS, and not by atropine, 4-DAMP, and methoctramine. Oxybutynin and propiverine suppress ATP-induced bladder overactivity other than through antimuscarinic mechanisms.

Bladder afferent signaling: recent findings

PURPOSE

Much current research on lower urinary tract physiology focuses on afferent mechanisms. The main goals are to define and control the signaling pathways by which afferent information is generated and conveyed to the central nervous system. We summarize recent research on bladder afferent mechanisms.

MATERIALS AND METHODS

We systematically reviewed the literature by searching PubMed up to June 2009 with focus on the last 5 years.

RESULTS

At least 2 signaling pathways can be identified, including the urothelial and the myogenic pathway. The urothelial pathway is a functional unit consisting of the urothelium, interstitial cells and afferent nerves in the lamina propria. Signaling occurs via muscle-mucosal mechanoreceptors, mucosal mechanoreceptors and chemoreceptors. The myogenic pathway is activated via in-series mechanoreceptors responding to distention and via spontaneous contractile activity in units of myocytes generating afferent noise.

CONCLUSIONS

To control dysfunctional micturition we must know more about all components involved in normal micturition control, including how afferent information is handled by the central nervous system.

Does adenosine triphosphate released into voided urodynamic fluid contribute to urgency signaling in women with bladder dysfunction?

PURPOSE

Adenosine triphosphate released from urothelium during stretch stimulates afferent nerves and conveys information on bladder fullness. We measured adenosine triphosphate released during cystometric bladder filling in women with idiopathic detrusor overactivity and stress incontinence (controls), and assessed whether the level of released adenosine triphosphate is related to cystometric parameters.

MATERIALS AND METHODS

Routine cystometry was done in 51 controls and 48 women with detrusor overactivity who were 28 to 87 years old. Voided urodynamic fluid was collected and stored at -30 C. Adenosine triphosphate was measured by a bioluminescence assay.

RESULTS

Adenosine triphosphate levels were similar in voided urodynamic fluid of controls and patients with detrusor overactivity (p = 0.79). A significant inverse correlation was seen between adenosine triphosphate and maximal cystometric capacity in controls (p = 0.013), and between voided volume and adenosine triphosphate in controls (p = 0.015) and detrusor overactivity cases (p = 0.019). A significant correlation between first desire to void and adenosine triphosphate was also noted in detrusor overactivity cases (p = 0.033) but not in controls (p = 0.58). No correlation was seen between adenosine triphosphate and detrusor pressure during filling or voiding.

CONCLUSIONS

Adenosine triphosphate measurement in voided urodynamic fluid is a novel approach to understanding signals that may contribute to the urgency sensation (a sudden compelling desire to pass urine). The inverse correlation between adenosine triphosphate in voided urodynamic fluid and first desire to void suggests that adenosine triphosphate has a role in modulating the early filling sensation in patients with detrusor overactivity.

Changes in afferent activity after spinal cord injury

AIMS

To summarize the changes that occur in the properties of bladder afferent neurons following spinal cord injury.

METHODS

Literature review of anatomical, immunohistochemical, and pharmacologic studies of normal and dysfunctional bladder afferent pathways.

RESULTS

Studies in animals indicate that the micturition reflex is mediated by a spinobulbospinal pathway passing through coordination centers (periaqueductal gray and pontine micturition center) located in the rostral brain stem. This reflex pathway, which is activated by small myelinated (Adelta) bladder afferent nerves, is in turn modulated by higher centers in the cerebral cortex involved in the voluntary control of micturition. Spinal cord injury at cervical or thoracic levels disrupts voluntary voiding, as well as the normal reflex pathways that coordinate bladder and sphincter function. Following spinal cord injury, the bladder is initially areflexic but then becomes hyperreflexic due to the emergence of a spinal micturition reflex pathway. The recovery of bladder function after spinal cord injury is dependent in part on the plasticity of bladder afferent pathways and the unmasking of reflexes triggered by unmyelinated, capsaicin-sensitive, C-fiber bladder afferent neurons. Plasticity is associated with morphologic, chemical, and electrical changes in bladder afferent neurons and appears to be mediated in part by neurotrophic factors released in the spinal cord and the peripheral target organs.

CONCLUSIONS

Spinal cord injury at sites remote from the lumbosacral spinal cord can indirectly influence properties of bladder afferent neurons by altering the function and chemical environment in the bladder or the spinal cord.

In vitro release of adenosine triphosphate from the urothelium of human bladders with detrusor overactivity, both neurogenic and idiopathic

BACKGROUND

There is increased evidence to suggest a role for nonadrenergic-noncholinergic neurotransmission in the pathogenesis of bladder dysfunction.

OBJECTIVE

In this set of experiments, we have assessed the contribution of the urothelium to purinergic activity by quantifying the amount of adenosine triphosphate (ATP) released from the urothelium of patients with idiopathic detrusor overactivity (IDO) and with neurogenic detrusor overactivity (NDO) and comparing these releases to those of controls.

DESIGN, SETTING, AND PARTICIPANTS

Bladder tissue with urodynamically and clinically proven NDO (n=8) and IDO (n=8) were included in this study. The carefully dissected urothelium was stimulated by mechanically stretching as well as electrically stimulating and the ATP; thus, release was quantified.

MEASUREMENTS

We used a Lucy Anthos 1 luminometre (Anthos Labtec Instruments GmBH, Wals, Austria) to perform the assay. The results were analysed using Stingray software (Dazdaq Ltd, Brighton, UK).

RESULTS AND LIMITATIONS

Both mechanical stretch and electric field stimulation (EFS) led to increased ATP release in both sets of tissues with overactivity compared to the controls; this rise was even more significant for the IDO urothelium (2416.7±479.8 pmol/g [p<0.005]) than for the NDO urothelium (133.1±22.4 pmol/g [p<0.01]); values for the controls were 77.6±16.2 pmol/g. ATP release following mechanical stretch was more sensitive to tetrodotoxin in bladders with NDO compared to those with IDO as well as to the controls, with ATP levels falling from 233.5±20.7 pmol/g to 107.2±11.6 pmol/g, expressed as percentage of basal levels (p<0.002). The experiments were performed in vitro, and the female patients were a mix of peri- and postmenopausal states.

CONCLUSIONS

These experiments suggested a significant rise in ATP release from the urothelium of bladders with NDO as well as those with IDO in comparison to controls. Most of the ATP released from bladders with NDO is primarily from neuronal sources.

Release of ATP from rat urinary bladder mucosa: role of acid, vanilloids and stretch

BACKGROUND AND PURPOSE

ATP, released from urothelial cells, modulates afferent nerve firing from the urinary bladder. Here, we have characterized ATP release from the rat bladder mucosa in response to acid, capsaicin, electrical field stimulation (EFS) and stretch, using agonists and antagonists at transient receptor potential vanilloid receptor 1 (TRPV1) and acid-sensing ion channels (ASICs).

EXPERIMENTAL APPROACH

Rat mucosal strips (containing urothelium and lamina propria) in Perspex microbaths were superfused with Krebs solution. ATP was measured after exposure of matched strips to acid (pH 6.6-5.0), capsaicin (0.1-10 microM), EFS or stretch (150% of original length).

KEY RESULTS

Median basal ATP release was 3.46 nmol g(-1). The mucosal strips responded to stimuli with potency order (median, IQR): acid (pH 5.6-6.0) 286 (103-555) > 10 microM capsaicin 188 (117-431) > 10 Hz EFS 63.0 (13.3-96.4) > stretch 24.4 (6.73-55.1) nmol ATP g(-1). ATP release in response to acid was pH dependent (P < 0.05). Responses to capsaicin did not desensitize nor were they concentration dependent. TRPV1 antagonist, capsazepine (10 microM) abolished capsaicin-evoked ATP release, and reduced acid-evoked (pH 6.5) release to 30% (P < 0.001). The ASIC channel antagonists gadolinium (0.1 mM) and amiloride (0.3 microM) reduced (P < 0.05) the acid-evoked (pH 6.5) release to 40 and 6.5% respectively. ASIC (ASIC1, ASIC2a, ASIC2b, ASIC3) and two TRPV1 gene products were detected in mucosal and detrusor extracts.

CONCLUSIONS AND IMPLICATIONS

Capsaicin (at TRPV1) and acid (at both TRPV1 and ASIC) induce ATP release from the rat bladder mucosa. This ATP appears to be principally of urothelial origin. This study highlights the importance of ATP and acid as signalling molecules in modulating bladder function.

Purinergic mechanosensory transduction and visceral pain

In this review, evidence is presented to support the hypothesis that mechanosensory transduction occurs in tubes and sacs and can initiate visceral pain. Experimental evidence for this mechanism in urinary bladder, ureter, gut, lung, uterus, tooth-pulp and tongue is reviewed. Potential therapeutic strategies are considered for the treatment of visceral pain in such conditions as renal colic, interstitial cystitis and inflammatory bowel disease by agents that interfere with mechanosensory transduction in the organs considered, including P2X₃ and P2X_2/3 receptor antagonists that are orally bioavailable and stable in vivo and agents that inhibit or enhance ATP release and breakdown.

Purines and sensory nerves

P2X and P2Y nucleotide receptors are described on sensory neurons and their peripheral and central terminals in dorsal root, nodose, trigeminal, petrosal, retinal and enteric ganglia. Peripheral terminals are activated by ATP released from local cells by mechanical deformation, hypoxia or various local agents in the carotid body, lung, gut, bladder, inner ear, eye, nasal organ, taste buds, skin, muscle and joints mediating reflex responses and nociception. Purinergic receptors on fibres in the dorsal spinal cord and brain stem are involved in reflex control of visceral and cardiovascular activity, as well as relaying nociceptive impulses to pain centres. Purinergic mechanisms are enhanced in inflammatory conditions and may be involved in migraine, pain, diseases of the special senses, bladder and gut, and the possibility that they are also implicated in arthritis, respiratory disorders and some central nervous system disorders is discussed. Finally, the development and evolution of purinergic sensory mechanisms are considered.

Ion channel and receptor mechanisms of bladder afferent nerve sensitivity

Sensory nerves of the urinary bladder consist of small diameter A(delta) and C fibers running in the hypogastic and pelvic nerves. Neuroanatomical studies have revealed a complex neuronal network within the bladder wall. Electrophysiological recordings in vitro and in vivo have revealed several distinct classes of afferent fibers that may signal a wide range of bladder stimulations including physiological bladder filling, noxious distension, cold, chemical irritation and inflammation. The exact mechanisms that underline mechanosensory transduction in bladder afferent terminals remain ambiguous; however, a wide range of ion channels (e.g., TTX-resistant Na(+) channels, Kv channels and hyperpolarization-activated cyclic nucleotide-gated cation channels) and receptors (e.g., TRPV1, TRPM8, TRPA1, P2X(2/3), etc) have been identified at bladder afferent terminals and implicated in the generation and modulation of afferent signals. Experimental investigations have revealed that expression and/or function of these ion channels and receptors may be altered in animal models and patients with overactive and painful bladder disorders. Some of these ion channels and receptors may be potential therapeutic targets for bladder diseases.

Cell biology and physiology of the uroepithelium

The uroepithelium sits at the interface between the urinary space and underlying tissues, where it forms a high-resistance barrier to ion, solute, and water flux, as well as pathogens. However, the uroepithelium is not simply a passive barrier; it can modulate the composition of the urine, and it functions as an integral part of a sensory web in which it receives, amplifies, and transmits information about its external milieu to the underlying nervous and muscular systems. This review examines our understanding of uroepithelial regeneration and how specializations of the outermost umbrella cell layer, including tight junctions, surface uroplakins, and dynamic apical membrane exocytosis/endocytosis, contribute to barrier function and how they are co-opted by uropathogenic bacteria to infect the uroepithelium. Furthermore, we discuss the presence and possible functions of aquaporins, urea transporters, and multiple ion channels in the uroepithelium. Finally, we describe potential mechanisms by which the uroepithelium can transmit information about the urinary space to the other tissues in the bladder proper.

Afferent nerve regulation of bladder function in health and disease

The afferent innervation of the urinary bladder consists primarily of small myelinated (Adelta) and unmyelinated (C-fiber) axons that respond to chemical and mechanical stimuli. Immunochemical studies indicate that bladder afferent neurons synthesize several putative neurotransmitters, including neuropeptides, glutamic acid, aspartic acid, and nitric oxide. The afferent neurons also express various types of receptors and ion channels, including transient receptor potential channels, purinergic, muscarinic, endothelin, neurotrophic factor, and estrogen receptors. Patch-clamp recordings in dissociated bladder afferent neurons and recordings of bladder afferent nerve activity have revealed that activation of many of these receptors enhances neuronal excitability. Afferent nerves can respond to chemicals present in urine as well as chemicals released in the bladder wall from nerves, smooth muscle, inflammatory cells, and epithelial cells lining the bladder lumen. Pathological conditions alter the chemical and electrical properties of bladder afferent pathways, leading to urinary urgency, increased voiding frequency, nocturia, urinary incontinence, and pain. Neurotrophic factors have been implicated in the pathophysiological mechanisms underlying the sensitization of bladder afferent nerves. Neurotoxins such as capsaicin, resiniferatoxin, and botulinum neurotoxin that target sensory nerves are useful in treating disorders of the lower urinary tract.

Purinergic signaling in the lumen of a normal nephron and in remodeled PKD encapsulated cysts

The nephron is the functional unit of the kidney. Blood and plasma are continually filtered within the glomeruli that begin each nephron. Adenosine 5' triphosphate (ATP) and its metabolites are freely filtered by each glomerulus and enter the lumen of each nephron beginning at the proximal convoluted tubule (PCT). Flow rate, osmolality, and other mechanical or chemical stimuli for ATP secretion are present in each nephron segment. These ATP-release stimuli are also different in each nephron segment due to water or salt permeability or impermeability along different luminal membranes of the cells that line each nephron segment. Each of the above stimuli can trigger additional ATP release into the lumen of a nephron segment. Each nephron-lining epithelial cell is a potential source of secreted ATP. Together with filtered ATP and its metabolites derived from the glomerulus, secreted ATP and adenosine derived from cells along the nephron are likely the principal two of several nucleotide and nucleoside candidates for renal autocrine and paracrine ligands within the tubular fluid of the nephron. This minireview discusses the first principles of purinergic signaling as they relate to the nephron and the urinary bladder. The review discusses how the lumen of a renal tubule presents an ideal purinergic signaling microenvironment. The review also illustrates how remodeled and encapsulated cysts in autosomal dominant polycystic kidney disease (ADPKD) and remodeled pseudocysts in autosomal recessive PKD (ARPKD) of the renal collecting duct likely create an even more ideal microenvironment for purinergic signaling. Once trapped in these closed microenvironments, purinergic signaling becomes chronic and likely plays a significant epigenetic and detrimental role in the secondary progression of PKD, once the remodeling of the renal tissue has begun. In PKD cystic microenvironments, we argue that normal purinergic signaling within the lumen of the nephron provides detrimental acceleration of ADPKD once remodeling is complete.