Phosphodiesterase-linked inhibition of nonmicturition activity in the isolated bladder

Role of PTHrP in attenuating transient pressure rises and associated afferent nerve activity of the rat bladder

Parathyroid hormone-related protein (PTHrP) released from detrusor smooth muscle (DSM) as the bladder fills acts as an endogenous DSM relaxant to facilitate bladder storage function. Here, the effects of exogenous PTHrP on transient pressure rises (TPRs) in the bladder and associated afferent nerve activity during bladder filling were investigated. In anaesthetized rats, changes in the intravesical pressure were measured while the bladder was gradually filled with saline. Afferent nerve activity was simultaneously recorded from their centrally disconnected left pelvic nerves. In DSM strips, spontaneous and nerve-evoked contractions were isometrically recorded. The distribution of PTHrP receptors (PTHrPRs) in the bladder wall was also examined by fluorescence immunostaining. The bladders in which the contralateral pelvic nerve was also centrally disconnected developed nifedipine, an L-type voltage-dependent Ca²⁺ channel blocker-sensitive TPRs (< 3 mmHg). Intravenous administration of PTHrP suppressed these TPRs and associated bursts of afferent nerve activity. In the bladders with centrally connected contralateral pelvic nerves, atropine, a muscarinic receptor antagonist-sensitive large TPRs (> 3 mmHg) developed in the late filling phase. PTHrP diminished the large TPRs and corresponding surges of afferent nerve activity. In DSM strips, bath-applied PTHrP (10 nM) suppressed spontaneous phasic contractions, while less affecting nerve-evoked contractions. PTHrPRs were expressed in DSM cells but not in intramural nerve fibers. Thus, PTHrP appears to suppress bladder TPRs and associated afferent nerve activity even under the influence of low degree of parasympathetic neural input during storage phases. Endogenous PTHrP may indirectly attenuate afferent nerve activity by suppressing TPRs to facilitate urinary accommodation.

Relevance of dog as an animal model for urologic diseases

We utilize animal models in urologic research to improve understanding of urinary physiology, determine the etiology of many urologic diseases, and discover and test novel therapeutic interventions. Dogs have a similar urinary tract anatomy and physiology to human and they develop many urologic diseases spontaneously. This chapter offers detailed comparisons of urinary tract anatomy, physiology, and the most common urologic diseases between humans and dogs. Dogs offer a unique opportunity for urologic research because they can be studied in research colonies and in client owned cohorts. Dogs also are among a limited number of non-human species that require continence and socially appropriate urinary behaviors (ex. going to the bathroom outside, training to not have submissive urination, etc.). These features make dogs unique in the animal kingdom and make them an ideal animal model for urologic research.

Role of detrusor PDGFRα+ cells in mouse model of cyclophosphamide-induced detrusor overactivity

Cyclophosphamide (CYP)-induced cystitis is a rodent model that shares many features common to the cystitis occurring in patients, including detrusor overactivity (DO). Platelet-derived growth factor receptor alpha positive (PDGFRα⁺) cells have been proposed to regulate muscle excitability in murine bladders during filling. PDGFRα⁺ cells express small conductance Ca²⁺-activated K⁺ channels (predominantly SK3) that provide stabilization of membrane potential during filling. We hypothesized that down-regulation of the regulatory functions of PDGFRα⁺ cells and/or loss of PDGFRα⁺ cells generates the DO in CYP-treated mice. After CYP treatment, transcripts of Pdgfrα and Kcnn3 and PDGFRα and SK3 protein were reduced in detrusor muscle extracts. The distribution of PDGFRα⁺ cells was also reduced. Inflammatory markers were increased in CYP-treated detrusor muscles. An SK channel agonist, CyPPA, increased outward current and hyperpolarization in PDGFRα⁺ cells. This response was significantly depressed in PDGFRα⁺ cells from CYP-treated bladders. Contractile experiments and ex vivo cystometry showed increased spontaneous contractions and transient contractions, respectively in CYP-treated bladders with a reduction of apamin sensitivity, that could be attributable to the reduction in the SK conductance expressed by PDGFRα⁺ cells. In summary, PDGFRα⁺ cells were reduced and the SK3 conductance was downregulated in CYP-treated bladders. These changes are consistent with the development of DO after CYP treatment.

Molecular and functional characterization of detrusor PDGFRα positive cells in spinal cord injury-induced detrusor overactivity

Volume accommodation occurs via a novel mechanism involving interstitial cells in detrusor muscles. The interstitial cells in the bladder are PDGFRα+, and they restrain the excitability of smooth muscle at low levels and prevents the development of transient contractions (TCs). A common clinical manifestation of spinal cord injury (SCI)-induced bladder dysfunction is detrusor overactivity (DO). Although a myogenic origin of DO after SCI has been suggested, a mechanism for development of SCI-induced DO has not been determined. In this study we hypothesized that SCI-induced DO is related to loss of function in the regulatory mechanism provided by PDGFRα+ cells. Our results showed that transcriptional expression of Pdgfra and Kcnn3 was decreased after SCI. Proteins encoded by these genes also decreased after SCI, and a reduction in PDGFRα+ cell density was also documented. Loss of PDGFRα+ cells was due to apoptosis. TCs in ex vivo bladders during filling increased dramatically after SCI, and this was related to the loss of regulation provided by SK channels, as we observed decreased sensitivity to apamin. These findings show that damage to the mechanism restraining muscle contraction during bladder filling that is provided by PDGFRα+ cells is causative in the development of DO after SCI.

Changes in the expression and function of the PDE5 pathway in the obstructed urinary bladder

Our study aims to explore changes in bladder contractility and the phosphodiesterase type 5 (PDE5) signalling pathway in response to partial bladder outlet obstruction (PBOO). A surgically induced male rat PBOO model and human obstructed bladder tissues were used. Histological changes were examined by H&E and Masson's trichrome staining. Bladder strip contractility was measured via organ bath. The expressions of nitric oxide synthase (NOS) isoforms, PDE5, muscarinic cholinergic receptor (CHRM) isoforms and PDE4 isoforms in bladder were detected by RT‐PCR and Western blotting. The immunolocalization of the PDE5 protein and its functional activity were also determined. PBOO bladder tissue exhibited significant SM hypertrophy and elevated responsiveness to KCl depolarization and the muscarinic receptor agonist carbachol. NOS isoforms, PDE5, CHRM2, CHRM3 and PDE4A were up‐regulated in obstructed bladder tissue, whereas no change in PDE4B and PDE4D isoform expression was observed. With regard to PDE5, it was expressed in the SM bundles of bladder. Interestingly, obstructed bladder exhibited less relaxation responsiveness to sodium nitroprusside (SNP), but an exaggerated PDE5 inhibition effect. The up‐regulation of PDE5 could contribute to the lack of effect on Q_max for benign prostatic hyperplasia/lower urinary tract symptom (BPH/LUTS) patients treated with PDE5 inhibitors. Moreover, PDE5 (with presence of NO) and PDE4 may serve as new therapeutic targets for bladder diseases such as BPH‐induced LUTS and overactive bladder (OAB).

Effects of chlorogenic acid on carbachol-induced contraction of mouse urinary bladder

Chlorogenic acid (CGA) is a polyphenol found in coffee and medicinal herbs such as Lonicera japonica. In this study, the effect of CGA-induced relaxation on carbachol (CCh)-induced contraction of mouse urinary bladder was investigated. CGA (30-300 μg/ml) inhibited CCh- or U46619-induced contraction in a concentration-dependent manner. SQ22536 (adenylyl cyclase inhibitor) recovered CGA-induced relaxation of CCh-induced contraction; however, ODQ (guanylyl cyclase inhibitor) did not have the same effect. In addition, 3-isobutyl-1-methylxanthine (IBMX) enhanced CGA-induced relaxation; however, forskolin or sodium nitroprusside did not have the same effect. Moreover, Ro 20-1724, a selective phosphodiesterase (PDE) 4 inhibitor, enhanced CGA-induced relaxation, but vardenafil, a selective PDE5 inhibitor, did not have the same effect. In the presence of CCh, CGA increased cyclic adenosine monophosphate (cAMP) level, whereas SQ22536 inhibited the increase of cAMP levels. Moreover, higher cAMP levels were obtained with CGA plus IBMX treatment than the total cAMP levels obtained with separate CGA and IBMX treatments. In conclusion, these results suggest that CGA inhibited CCh-induced contraction of mouse urinary bladder by partly increasing cAMP levels via adenylyl cyclase activation.

Inhibition of vascular smooth muscle inward-rectifier K+ channels restores myogenic tone in mouse urinary bladder arterioles

Prolonged decreases in urinary bladder blood flow are linked to overactive and underactive bladder pathologies. However, the mechanisms regulating bladder vascular reactivity are largely unknown. To investigate these mechanisms, we examined myogenic and vasoactive properties of mouse bladder feed arterioles (BFAs). Unlike similar-sized arterioles from other vascular beds, BFAs failed to constrict in response to increases in intraluminal pressure (5-80 mmHg). Consistent with this lack of myogenic tone, arteriolar smooth muscle cell membrane potential was hyperpolarized (-72.8 ± 1.4 mV) at 20 mmHg and unaffected by increasing pressure to 80 mmHg (-74.3 ± 2.2 mV). In contrast, BFAs constricted to the thromboxane analog U-46619 (100 nM), the adrenergic agonist phenylephrine (10 µM), and KCl (60 mM). Inhibition of nitric oxide synthase or intermediate- and small-conductance Ca²⁺-activated K⁺ channels did not alter arteriolar diameter, indicating that the dilated state of BFAs is not attributable to overactive endothelium-dependent dilatory influences. Myocytes isolated from BFAs exhibited BaCl₂ (100 µM)-sensitive K⁺ currents consistent with strong inward-rectifier K⁺ (K_IR) channels. Notably, block of these K_IR channels "restored" pressure-induced constriction and membrane depolarization. This suggests that these channels, in part, account for hyperpolarization and associated absence of tone in BFAs. Furthermore, smooth muscle-specific knockout of K_IR2.1 caused significant myogenic tone to develop at physiological pressures. This suggests that 1) the regulation of vascular tone in the bladder is independent of pressure, insofar as pressure-induced depolarizing conductances cannot overcome K_IR2.1-mediated hyperpolarization; and 2) maintenance of bladder blood flow during bladder filling is likely controlled by neurohumoral influences.

Transient contractions of urinary bladder smooth muscle are drivers of afferent nerve activity during filling

Activation of afferent nerves during urinary bladder (UB) filling conveys the sensation of UB fullness to the central nervous system (CNS). Although this sensory outflow is presumed to reflect graded increases in pressure associated with filling, UBs also exhibit nonvoiding, transient contractions (TCs) that cause small, rapid increases in intravesical pressure. Here, using an ex vivo mouse bladder preparation, we explored the relative contributions of filling pressure and TC-induced pressure transients to sensory nerve stimulation. Continuous UB filling caused an increase in afferent nerve activity composed of a graded increase in baseline activity and activity associated with increases in intravesical pressure produced by TCs. For each ∼4-mmHg pressure increase, filling pressure increased baseline afferent activity by ∼60 action potentials per second. In contrast, a similar pressure elevation induced by a TC evoked an ∼10-fold greater increase in afferent activity. Filling pressure did not affect TC frequency but did increase the TC rate of rise, reflecting a change in the length-tension relationship of detrusor smooth muscle. The frequency of afferent bursts depended on the TC rate of rise and peaked before maximum pressure. Inhibition of small- and large-conductance Ca(2+)-activated K(+) (SK and BK) channels increased TC amplitude and afferent nerve activity. After inhibiting detrusor muscle contractility, simulating the waveform of a TC by gently compressing the bladder evoked similar increases in afferent activity. Notably, afferent activity elicited by simulated TCs was augmented by SK channel inhibition. Our results show that afferent nerve activity evoked by TCs represents the majority of afferent outflow conveyed to the CNS during UB filling and suggest that the maximum TC rate of rise corresponds to an optimal length-tension relationship for efficient UB contraction. Furthermore, our findings implicate SK channels in controlling the gain of sensory outflow independent of UB contractility.

Rationale for the use of prostaglandins and phosphodiesterase inhibitors in the treatment of functional bladder disorders

In this paper a general discussion of the available data on the role of prostaglandin (PG) and phosphodiesterase is discussed. Functional studies would be a next step to understand the functional meaning of the data described in this paper. The data presented are a basis for further research on selective modulation of the EP1 and EP2 receptor which could be a therapeutic target in functional bladder disorders such as OAB. PDE inhibitors are closer to clinical use, as these drugs have been studied and registered for other indications such as erectile dysfunction in men. Therefore, in vivo studies in human subjects can be conducted on short term. However, from a scientific point of view, it is very important to unravel the exact site of action and role of PDE inhibition with in vitro and in vivo studies as is the case with PG. In this way, a combination of drugs targeting different mechanisms involved in bladder physiology such as PG, cGMP, cAMP, and muscarinic receptors, could reduce side effects and improve efficacy.

Inhibition of phosphodiesterases relaxes detrusor smooth muscle via activation of the large-conductance voltage- and Ca²⁺-activated K⁺ channel

Detrusor smooth muscle (DSM) exhibits increased spontaneous phasic contractions under pathophysiological conditions such as detrusor overactivity (DO). Our previous studies showed that activation of cAMP signaling pathways reduces DSM contractility by increasing the large-conductance voltage- and Ca(2+)-activated K(+) (BK) channel activity. Here, we tested the hypothesis whether inhibition of phosphodiesterases (PDEs) can reduce guinea pig DSM excitability and contractility by increasing BK channel activity. Utilizing isometric tension recordings of DSM isolated strips and the perforated patch-clamp technique on freshly isolated DSM cells, we examined the mechanism of DSM relaxation induced by PDE inhibition. Inhibition of PDEs by 3-isobutyl-1-methylxanthine (IBMX), a nonselective PDE inhibitor, significantly reduced DSM spontaneous and carbachol-induced contraction amplitude, frequency, duration, muscle force integral, and tone in a concentration-dependent manner. IBMX significantly reduced electrical field stimulation-induced contractions of DSM strips. Blocking BK channels with paxilline diminished the inhibitory effects of IBMX on DSM contractility, indicating a role for BK channels in DSM relaxation mediated by PDE inhibition. IBMX increased the transient BK currents (TBKCs) frequency by ∼3-fold without affecting the TBKCs amplitude. IBMX increased the frequency of the spontaneous transient hyperpolarizations by ∼2-fold and hyperpolarized the DSM cell resting membrane potential by ∼6 mV. Blocking the BK channels with paxilline abolished the IBMX hyperpolarizing effects. Under conditions of blocked Ca(2+) sources for BK channel activation, IBMX did not affect the depolarization-induced steady-state whole cell BK currents. Our data reveal that PDE inhibition with IBMX relaxes guinea pig DSM via TBKCs activation and subsequent DSM cell membrane hyperpolarization.

Expression of cAMP-dependent protein kinase isoforms in the human prostate: functional significance and relation to PDE4

OBJECTIVES

To investigate the expression of isoforms of the cyclic AMP (cAMP)-dependent protein kinase (cAK) in the transition zone of the human prostate and the functional significance of the enzyme in the control of prostate smooth muscle.

METHODS

Using Western blot analysis and immunohistochemistry, the expression and distribution in the prostate of cAKIalpha, cAKIbeta, cAKIIalpha, and cAKIIbeta in relation to alpha-actin and the phosphodiesterase PDE4 (types A and B) were investigated. The effects of the cAK inhibitor Rp-8-CPT-cAMPS on the reversion of the adrenergic tension of isolated prostate tissue induced by forskolin, rolipram, sodium nitroprusside (SNP), and tadalafil were examined by means of the organ bath technique.

RESULTS

Immunosignals specific for cAKIalpha, cAKIIalpha, and cAKIIbeta were observed in the smooth musculature and glandular structures of the prostate. Double stainings revealed the colocalization of alpha-actin and PDE4 with the cAK isoforms. The expression of the cAK isoforms was confirmed by Western blot analysis. The relaxation of the tension induced by norepinephrine brought about by forskolin, rolipram, SNP, and tadalafil was significantly attenuated by Rp-8-CPT-cAMPS.

CONCLUSIONS

The colocalization of smooth muscle alpha-actin and PDE4 with cAK, as well as the results from the organ bath experiments, provide further evidence for a pivotal role of the cAMP-dependent signaling in the regulation of prostate smooth muscle contractility. Compounds interacting with the cAMP/cAK pathway might represent a new therapeutic avenue to treat symptoms of benign prostatic hyperplasia and lower urinary tract symptomatology.

Detrusor myocyte activity and afferent signaling

AIMS

To discuss (1) mechanisms involved in the generation and control of myocyte contractions and consequent afferent nerve activity and (2) these mechanisms as targets for drugs aimed for treatment of overactive bladder (OAB) symptoms and detrusor overactivity (DO).

METHODS

Literature review of myocyte activation, bladder afferent nerves, mediators in the bladder, and translational aspects of the findings.

RESULTS

During bladder filling, there is normally no parasympathetic outflow from the spinal cord. Despite this, the bladder develops tone during filling and also exhibits non-synchronized local contractions and relaxations that are caused by a basal myogenic mechanical activity that may be reinforced by release of, for example, acetylcholine from non-neuronal and/or neuronal sources or local mediators, such as prostaglandins and endothelins. It is suggested that these spontaneous contractions are able to generate activity in afferent nerves ("afferent noise") that may contribute to DO and OAB.

CONCLUSIONS

Spontaneous bladder myocyte contractions and factors that are able to modulate them, as well as the consequent afferent nerve activity, may be targets for drugs meant for treatment of OAB/DO.

PDE5 inhibitors for LUTS

To review the current literature regarding the relationship between lower urinary tract symptoms (LUTS) and erectile dysfunction (ED), and the role of phosphodiesterase-5 (PDE5) inhibitors for the treatment of LUTS. Review of recently published (1990-2009) data regarding epidemiologic and pathophysiologic mechanisms are involved in LUTS-ED, focusing on PDE5 inhibitors particularly evidenced from level 1 clinical trials. Search terms included phosphodiesterase inhibitors, nitric oxide, autonomic hyperactivity, Rho-kinase, atherosclerosis, LUTS, benign prostatic hypertrophy, and ED. Results of several epidemiologic studies show a possible causal relationship between LUTS and ED. Four possible mechanisms have been proposed to explain this association. Multiple large clinical trials have shown a benefit in LUTS after PDE5-inhibitors treatment. PDE5 inhibitors show promise as a future treatment for LUTS, either in conjunction with existing therapies or as a primary treatment.

Prospective pharmacologic therapies for the overactive bladder

Lower urinary tract symptoms (LUTS), overactive bladder syndrome (OAB) and detrusor overactivity (DO) are all conditions that can have major effects on quality of life and social functioning. Antimuscarinic drugs are first-line treatment-they often have good initial response rates, but adverse effects and decreasing efficacy cause long-term compliance problems, and alternatives are needed. The recognition of the functional contribution of the urothelium, the spontaneous myocyte activity during bladder filling, and the diversity of nerve transmitters has sparked interest in both peripheral and central modulation of LUTS/OAB/DO pathophysiology. There may be several new possibilities to treat LUTS/OAB/DO. β(3)-AR agonists (YM178), PDE 5 inhibitors (sildenafil, tadalafil, vardenafil), vitamin D analogs (elocalcitol), combinations (α(1)-AR antagonist + antimuscarinic), and drugs with a central mode of action (tramadol, aprepitant) all have Randomized controlled trial (RCT) documented efficacy. Which of these therapeutic principles will be developed to clinically useful treatments remains to be established.

Effect of phosphodiesterase type 4 inhibitor rolipram on cyclophosphamide-induced cystitis in rats

Cyclophosphamide induces a severe haemorrhagic cystitis characterized by bladder overactivity. The study was conducted to examine effects of a phosphodiesterase 4 (PDE4) inhibitor rolipram on bladder overactivity in rats with cyclophosphamide treatment. 42 female Wistar rats were used. 30 rats received a single i.p. injection of cyclophosphamide, and after 72 h, bladder function was evaluated by (1) in vitro preparations of whole bladders and (2) cystometry with continuous saline infusion under urethane anesthesia. Cyclophosphamide-treatment dramatically potentiated the basal spontaneous contractions of isolated whole bladders compared to control rats. Atropine, guanethidine or suramin was ineffective on the spontaneous contractions whereas nifedipine completely abolished. Rolipram (5-80 microM) induced a significant concentration-dependent decrease on the amplitude, frequency (contractions/min) and area under the curve of spontaneous contractions. Carbachol elicited phasic contractions superimposed on a tonic contraction. Rolipram caused a relaxation on the tonic contraction whereas it could not affect the phasic contractions induced by carbachol. In anesthetized rats, during continuous infusion cystometry, intercontraction interval was significantly shorter in cyclophosphamide-injected rats than in control rats. Rolipram at 5-40 microM has no significant effect on the intercontraction interval and contraction pressure while it significantly decreased pressure threshold. At 80 microM, it significantly decreased the intercontraction interval and contraction pressure. In conclusion, PDE4 inhibitor rolipram caused a significant decrease on the amplitude, frequency and area under the curve of basal spontaneous contractions in cyclophosphamide-treated rats, at doses that have no effect on the carbachol-induced phasic contractions and cystometric parameters. PDE4 inhibitors may be considered as an attractive strategy for the treatment of cyclophosphamide-induced bladder overactivity.

Phosphodiesterases (PDEs) and PDE inhibitors for treatment of LUTS

Lower urinary tract (LUT) smooth muscle can be relaxed by drugs that increase intracellular concentrations of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Both of these substances are degraded by phosphodiesterases (PDEs), which play a central role in the regulation of smooth muscle tone. The distribution and functional significance of PDE enzymes vary in different tissues of the LUT. Targeting specific PDE isoenzymes should thus allow organ selectivity. PDE 4 and 5 appear to predominate in the prostate, PDE 1 and 4 are thought to influence detrusor smooth muscle function, and PDE 5 may be functionally important in the urethra and vasculature. Studies on the use of PDE inhibitors to treat various LUT symptoms (LUTS), have yielded favorable results. Thus, positive effects of the PDE 5 inhibitors sildenafil and tadalafil on symptoms and quality of life in men with LUTS, erectile dysfunction, and BPH have also been demonstrated. These effects may be due to effects on cGMP signaling and/or modification of afferent input from bladder, urethral, and prostate tissue. This review gives an update on the distribution of PDEs in structures relevant for LUT function, and discusses how inhibition of these enzymes can contribute to beneficial effects on LUTS. Information for the review was obtained from searches of the PubMed database, and from the authors' files.

The effects of a type 4 phosphodiesterase inhibitor and the muscarinic cholinergic antagonist tolterodine tartrate on detrusor overactivity in female rats with bladder outlet obstruction

OBJECTIVE

To investigate the effects of the selective phosphodiesterase (PDE) type 4 inhibitor IC485 and the widely used antimuscarinic drug tolterodine tartrate on bladder activity in rats with bladder outlet obstruction (BOO), as inhibition of PDE4 leads to elevation of intracellular cAMP levels and relaxation of smooth muscle.

MATERIALS AND METHODS

BOO was induced in female Sprague-Dawley rats by tying a silk ligature around the urethra. Six weeks after inducing BOO, conscious rats were assessed by cystometry with the urethral ligature intact. The effects of IC485 (5, 10 and 50 mg/kg intravenous, i.v.) were examined and compared with those of tolterodine (0.01, 0.1 and 1 mg/kg i.v.).

RESULTS

IC485 (5-50 mg/kg i.v.) decreased the number and amplitude of non-voiding contractions during the storage phase by 63-88% and 49-83%, respectively; IC485 also increased bladder capacity by 28-37%. There was no change in blood pressure after applying IC485. Tolterodine tartrate (0.1 and 1.0 mg/kg) significantly decreased the number and amplitude of non-voiding contractions by 38-74% and 29-44%, respectively, and increased bladder capacity by 19-51%. Whereas voiding efficiency and maximum voiding pressure were not altered by IC485 at any dose, tolterodine significantly reduced both, by 35-67% and 19-34%, respectively.

CONCLUSION

Both IC485 and tolterodine tartrate reduced detrusor overactivity in rats with BOO. In addition, doses of IC485 that suppressed non-voiding contractions had no effect on voiding function. Therefore, selective PDE4 inhibitors deserve further study as potential agents for treating detrusor overactivity in patients with BOO.

Therapeutic receptor targets for lower urinary tract dysfunction

The functions of the lower urinary tract, to store and periodically release urine, are dependent on the activity of smooth and striated muscles in the bladder, urethra, and external urethral sphincter. During urine storage, the outlet is closed, and the bladder smooth muscle is quiescent. When bladder volume reaches the micturition threshold, activation of a micturition center in the dorsolateral pons (the pontine micturition center) induces a bladder contraction and a reciprocal relaxation of the urethra, leading to bladder emptying. During voiding, sacral parasympathetic (pelvic) nerves provide an excitatory input (cholinergic and purinergic) to the bladder and inhibitory input (nitrergic) to the urethra. These peripheral systems are integrated by excitatory and inhibitory regulation at the levels of the spinal cord and the brain. Injury or diseases of the nervous system, as well as drugs and disorders of the peripheral organs, can produce lower urinary tract dysfunction. In the overactive bladder (OAB) condition, therapeutic targets for facilitation of urine storage can be found at the levels of the urothelium, detrusor muscles, autonomic and afferent pathways, spinal cord, and brain. There is increasing evidence showing that the urothelium has specialized sensory and signaling properties including: (1) expression of nicotinic, muscarinic, tachykinin, adrenergic, bradykinin, and transient receptor potential (TRP) receptors, (2) close physical association with afferent nerves, and (3) ability to release chemical molecules such as adenosine triphosphate (ATP), acetylcholine, and nitric oxide. Increased expression and/or sensitivity of these urothelial-sensory molecules that lead to afferent sensitization have been documented as possible pathogenesis of OAB. Targeting afferent pathways and/or bladder smooth muscles by modulating activity of ligand receptors (e.g., neurokinin, ATP, or beta3-adrenergic receptors) and ion channels (e.g., TRPV1 or K) could be effective to suppress OAB. In the stress urinary incontinence condition, pharmacotherapies targeting the neurally mediated urethral continence reflex during stress conditions such as sneezing or coughing could be effective for increasing the outlet resistance. Therapeutic targets include adrenergic and serotonergic receptors in the spinal cord as well as adrenergic receptors at the urethral sphincter, which can enhance urethral reflex activity during stress conditions and increase baseline urethral pressure, respectively.

Cholinergic activation of phasic activity in the isolated bladder: possible evidence for M3- and M2-dependent components of a motor/sensory system

OBJECTIVES

To analyse pressure changes induced by muscarinic agonists on the isolated bladder in order to examine whether there are different responses representing different components of a motor/sensory system within the bladder wall.

MATERIALS AND METHODS

Whole isolated bladders from 19 female guinea-pigs (280-400 g) were used. A cannula was inserted into the urethra to monitor intravesical pressure and the bladder was suspended in a heated chamber containing carboxygenated physiological solution at 33-36 degrees C. Initially, the responses to the cholinergic agonists, arecaidine but-2-ynyl ester tosylate and carbachol were assessed. Then, in an attempt to identify the muscarinic receptor subtypes involved, the effects of selective muscarinic antagonists on the arecaidine-induced bladder responses were assessed. The antagonists used were the relatively M(3)-selective 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) and darifenicin, and relatively M(2)-selective AFDX-116. All drugs were added to the solution bathing the ablumenal surface of the bladder.

RESULTS

The whole bladders exposed to cholinergic agonists respond with complex changes in intravesical pressure. Immediately after application of the agonist there was a burst of high frequency transient contractions. During continued application of agonist the frequency of the transients decreased and their amplitude increased. Thus, there appear to be two components to the response: an initial fast phase and a later slow component. The maximum frequency of the initial burst increased with increasing concentrations of agonist. By contrast, the frequency of the transients in the steady state showed little dependence on agonist concentration. There were quantitative differences between the responses to arecaidine and carbachol. Arecaidine was less effective in generating the initial burst of high-frequency activity and the transients were significantly larger. At low dose, arecaidine was more effective in producing the large transients in the steady state. Pre-exposure of the bladder to 4-DAMP (0.1-10 nM) or darifenicin (0.1-10 nM) significantly reduced the frequency of the initial burst of activity; 0.3 nM 4-DAMP reduced the frequency by half. In this concentration range, 4-DAMP reduced the amplitude of the initial transients but did not affect the frequency of the transients in the steady state. There were similar results with darifenicin. However, darifenicin was less effective in reducing the amplitude of the initial transients. By contrast, ADFX-116 had little effect on the frequency of the initial transients but did reduce amplitude; 300 nM AFDX-116 was needed to reduce the frequency of the initial burst by half.

CONCLUSIONS

This analysis suggests that there are different but interrelated mechanisms in the isolated bladder contributing to complex contractile activity. Three components can be identified: a mechanism operating during voiding to produce a global contraction of the whole bladder and two mechanisms, pacemaker and conductive, involved in generating and propagating local contractions in the bladder wall. The pacemaker component is more sensitive to darifenicin and 4-DAMP than to AFDX-116 suggesting that the underlying processes rely predominantly on M(3) receptors and less so on M(2) (M(3) > M(2)). The phasic activity in the later stages is less affected by M(3) antagonists and might therefore involve predominantly M(2) receptors (M(2) > M(3)). The potential importance of these results in terms of the general physiology and pharmacology of the bladder is discussed.

Suppression of detrusor overactivity in rats with bladder outlet obstruction by a type 4 phosphodiesterase inhibitor

OBJECTIVE

To investigate the effects of a selective type 4 cyclic nucleotide phosphodiesterase (PDE4) inhibitor, IC486051, on bladder activity in normal rats and those with and bladder outlet obstruction (BOO), as inhibition of PDE4 leads to elevation of intracellular cAMP levels and relaxation of smooth muscle.

MATERIALS AND METHODS

BOO was induced in female Sprague-Dawley rats by tying a silk ligature around the urethra. At 4 or 6 weeks after inducing BOO, conscious rats were assessed by cystometry with the urethral ligature intact. In unobstructed rats, blood pressure was also measured.

RESULTS

In unobstructed rats, IC486051 (0.1 mg/kg intravenously) produced no significant changes in cystometric variables, while at a dose of 0.5 mg/kg maximum voiding pressure was reduced by 34%. At both doses, there was a small, transient increase in blood pressure. In both 4- and 6-week BOO rats IC486051 dose-dependently decreased the number and amplitude of non-voiding bladder contractions by up to 80%, relative to pre-treatment values. At doses of 0.1 and 0.5 mg/kg IC486051 had no significant effect on voiding variables. In the 4-week BOO rats, a dose of 1.0 mg/kg decreased bladder capacity, voided volume and residual volume by 21%, 32% and 18%, respectively. In 6-week BOO rats, a dose of 1.0 mg/kg decreased maximal voiding pressure by 17% and pressure threshold for voiding by 28%. In both groups of rats with BOO, voiding efficiency was unchanged.

CONCLUSIONS

A selective PDE4 inhibitor can effectively suppress detrusor overactivity in rats with BOO, at doses that have no effect on voiding bladder contractions. Thus, selective PDE4 inhibitors should be considered for the treatment of overactive bladder in patients with BOO.

Relaxation of phasic contractile activity of human detrusor strips by cyclic nucleotide phosphodiesterase type 4 inhibition

OBJECTIVES

Detrusor smooth muscle relaxation is mainly mediated by the cyclic adenosine monophosphate (cAMP) pathway. Elevation of cAMP levels by phosphodiesterase type 4 (PDE4) inhibition relaxes smooth muscles of various origins. We aimed to determine the effect of a PDE4 inhibitor, rolipram, on human detrusor contractions.

METHODS

Human bladder strips (from 20 different donors) with no known overactive bladder (OAB) were studied in organ baths. Detrusor samples with or without urothelium were incubated with carbachol 10(-6)mol/l (in presence or absence of forskolin, 3.10(-7)mol/l) or with KCl 10mmol/l to enhance phasic contractile activity. Concentration response curves for rolipram or vehicle were then performed.

RESULTS

Rolipram (10(-9) to 3.10(-5)mol/l) induced a moderate relaxing effect on carbachol-induced contractions. This effect was enhanced when cAMP levels were increased by forskolin (the maximal effect was 53.0+/-5.1 vs. 83.1+/-5.7%, p<0.01) or in strips with urothelium. In contrast, rolipram (10(-9) to 10(-4)mol/l) drastically inhibited phasic contractile activity: The developed tension, the area under the curve, and the amplitude of phasic activity were reduced to 64.8+/-3.6, 91.2+/-5.3, and 82.3+/-7.3%, respectively, versus 23.6+/-9.5, 34.7+/-18.8, and 18.0+/-16.2% for vehicle, respectively (p<0.05). Frequency of phasic activity was 0.96+/-0.45 contractions per minute versus 2.6+/-0.18 for vehicle (p<0.001). In strips with urothelium, the inhibitory effect of rolipram on phasic contractile activity was similar.

CONCLUSIONS

PDE4 isoenzymes are strongly involved in the regulation of phasic myogenic activity of human bladder strips. Because an increase of this phasic activity may play a role in the pathophysiology of detrusor overactivity, PDE4 inhibitors might represent an attractive strategy for the treatment of OAB.

Interstitial cells and phasic activity in the isolated mouse bladder

OBJECTIVE

To describe the distribution of interstitial cells (ICs, defined as cells which show an increase in cGMP in response to nitric oxide, NO) in the isolated mouse bladder, and changes in phasic contractile activity after exposure to a NO donor.

MATERIALS AND METHODS

The whole bladder was removed from 17 female mice, killed by cervical dislocation. For immunohistochemistry (six mice) the bladder was incubated in carboxygenated Krebs' solution at 36 degrees C, containing 1 mm of the phosphodiesterase inhibitor isobutyl-methyl-xanthine. Individual pieces of tissue were exposed to 100 microm of the NO donor diethylamine NONOate for 10 min; control tissues remained in Krebs' solution. Tissues were then fixed in 4% paraformaldehyde and processed for cGMP immunohistochemistry. Bladder pressure was measured in bladders from 11 mice; the bladders were cannulated via the urethra and suspended in a heated chamber containing carboxygenated Tyrode solution at 33-35 degrees C and intravesical pressure recorded. All drugs were added to the solution bathing the abluminal surface.

RESULTS

NO induced an increase in cGMP in cells in the outer layers of the bladder wall, forming two distinct types based on their location; cells lying on the surface of the muscle bundles (surface muscle ICs) and cells within the muscle bundles (intramuscular ICs). Cholinergic nerve fibres were identified by the expression of vesicular acetylcholine transporter and neuronal NO synthase (nNOS). Choline acetyltransferase- and nNOS-positive nerves also had high cGMP levels in response to 100 microm diethylamine NONOate. In vitro exposure of an isolated whole unstimulated bladder to 100 microm diethylamine NONOate had no effect on resting bladder pressure. When whole bladders were exposed to muscarinic stimulation (30-100 nm arecaidine) there was an initial large transient rise in pressure followed by complex phasic changes in pressure. Adding 100 microm diethylamine NONOate abolished this phasic activity. Interestingly, the phasic activity was inhibited midway between the peak and trough of a phasic cycle. Such a pattern of inhibition might reflect the complexity of the phasic activity involving both excitatory and inhibitory components.

CONCLUSIONS

These data show the presence of NO/cGMP-sensitive ICs in the outer muscle layers of the mouse bladder. Activating these cells alters the pattern of muscarinic-induced phasic activity. We suggest that the role of the ICs in the outer muscle layers is to generate and modulate phasic activity. If so, then this is the first report of a functional role for ICs in the bladder.

Phasic non-micturition contractions in the bladder of the anaesthetized and awake rat

OBJECTIVE

To characterize the contractile activity that occurs in the bladder during the filling phase of the micturition cycle (non-micturition contractions, NMCs), which generate transient rises in intravesical pressure not associated with urine flow.

MATERIALS AND METHODS

The experiments were conducted using anaesthetized (chloral hydrate) and un-anaesthetized rats. In un-anaesthetized rats bladder contractile activity was measured using an intravesical cannula implanted under full surgical anaesthesia 3 days previously. In the anaesthetized rats the bladder was exteriorized and a cannula inserted through the dome. In these experiments electrical activity within the detrusor was also measured with a suction electrode on the bladder surface. For each rat, the experimental protocol involved filling the bladder at a constant rate (10 mL/h) to evoke micturition cycles, or infusion of a fixed volume and recording made under effective isovolumetric conditions.

RESULTS

In both anaesthetized and un-anaesthetized rats there were transient rises in bladder pressure (0.5-3 cmH2O). In the anaesthetized rats the amplitude of the transients increased throughout the filling phase, with little change in frequency. The phasic NMCs generating these pressure transients were accompanied by electrical changes in the detrusor. In the middle phase of bladder filling the slow pressure changes were accompanied by slow waves of electrical activity which changed in the pressure cycles immediately before micturition to high-frequency low-amplitude signals. In the un-anaesthetized rats there was a period immediately after voiding where there was no activity. As filling proceeded, low-amplitude low-frequency NMCs appeared that gradually increased in frequency and amplitude during the filling phase. However, the frequency of the transients decreased immediately before micturition despite an increase in amplitude. Similar responses were seen during isovolumetric recording.

CONCLUSION

The present results show the presence of NMCs in the rat bladder, identify volume-dependent changes in the pattern of this activity during the micturition cycle, and show that NMCs are accompanied by electrical changes in the detrusor. The physiological significance of NMCs is not known but it might be linked to the generation of afferent discharge from mechanoreceptors in the wall, so contributing to sensations related to bladder volume.

Interstitial cells and cholinergic signalling in the outer muscle layers of the guinea-pig bladder

OBJECTIVES

To explore the relationship between cholinergic mechanisms and interstitial cells (ICs) in the outer muscle layer of the bladder.

MATERIALS AND METHODS

In bladder tissue from male guinea-pigs, ICs were identified by their response to nitric oxide (NO) with a rise in cGMP. Sections of the lateral bladder wall were incubated in Krebs' solution containing 1 mm of the nonspecific phosphodiesterase inhibitor isobutyl-methyl-xanthene. Tissues were then exposed to 100 microm of the NO donor NONOate for 10 min, control tissues remained in Krebs' solution. Tissues were then processed for immunohistochemistry for cGMP, choline acetyltransferase (ChAT), neurofilament protein, and the nonspecific neuronal marker protein gene product (PGP) 9.5.

RESULTS

cGMP-positive ICs were found mainly in the outer muscle layers of the bladder wall. Three types were identified based on location; on the outer surface of the bladder wall, on the surface of the muscle bundles, and within the muscle bundles. Some of the intramuscular ICs stained for ChAT, but they did not stain with PGP 9.5. Nerve fibres were seen in close contact with the ChAT-positive intramuscular ICs, and these nerves expressed ChAT and neurofilament protein.

CONCLUSIONS

A subpopulation of intramuscular ICs can synthesise acetylcholine, and might release acetylcholine onto the underlying muscle. These cells are in close contact with nerves, suggesting that they might be activated by neural inputs. Thus there may be a system in the detrusor involving cholinergic nerves acting on ICs which can activate the smooth muscle via a complex cholinergic input.

Comparison study of autonomous activity in bladders from normal and paraplegic rats

AIM

To identify differences in the pattern of pressure generated by isolated bladders from normal and paraplegic rats.

MATERIALS AND METHODS

Nine female Wister rats were made paraplegic by spinal cord transsection at the vertebral level T8-T9 and sacrificed between D21 and D28. A further group (n = 9) was used as a control group. Each bladder was excised and placed in an organ bath where intravesical pressures were measured. Pressure changes were divided in two well-defined groups: macro-transients and spikes. The effects of intravesical volume load and muscarinic (M) agonists were studied.

RESULTS

We demonstrated a higher frequency, a longer duration, and a higher variance of duration in macro-transients in the neurogenic group. Intravesical volume load influenced the amplitude and frequency of macro-transients in both groups similarly. The effects of the muscarinic (M(2))-selective agonist arecaïdine were different in neurogenic bladder; the effects of the non-selective muscarinic (M)-agonist carbachol were similar in both groups.

CONCLUSION

We showed that the pattern of autonomous activity was significantly different between normal and neurogenic rat bladders. We also found evidence for alterations in the muscarinic response of isolated neurogenic rat bladders. This model offers an exciting new research tool to evaluate the detrusor activity in neurogenic and normal conditions.

Regulation of cyclic nucleotides in the urinary tract

Cyclic nucleotide levels are controlled through their synthesis from nucleotide triphosphates by cyclases and their degradation to 5'-monophosphates by phosphodiesterases (PDEs). Components controlling cyclic AMP-induced relaxation in the urinary tract include receptors, inhibitory and stimulatory G-proteins, isoforms of adenylyl cyclase and PDEs. The responsiveness of PDEs to a variety of physiological challenges is related to the presence of multiple families of isoenzymes with specific localization within tissues and within cells. At least 11 families of PDEs encode more than 50 PDE proteins produced in mammalian cells. In the urinary tract, characterization of PDE isoforms has lagged behind other systems and much of the literature was published prior to identification of PDE7, 8, 9, 10, 11. Specific PDE inhibitors regulate smooth muscle function in the bladder, urethra, prostate and ureter. The pharmacological potential of these inhibitors may include treatment of urge incontinence and the low compliance bladder, and treatment of prostate cancer. G-proteins also regulate cyclic AMP production. Changes in specific G- protein isoforms with aging, most prominently Gialpha2, cause decreased relaxation response in the aging bladder. As we have seen here with aging and certainly in other disease processes, levels of the components of adenylyl cyclase/phosphodiesterase/protein kinase can change and thus affect the relaxation response. By exploitation of differences in PDE expression in disease, such as the overexpression of PDEs in cancer, treatment options may present themselves.

Recent advances in basic science for overactive bladder

PURPOSE OF REVIEW

Detrusor overactivity is a relatively common yet embarrassing symptom complex with significant impact on quality of life. The mainstay of current pharmacological treatment involves the use of muscarinic receptor antagonists, but their therapeutic effectiveness is limited by a combination of limited efficacy and troublesome side effects and has recently been challenged by Herbison et al. Recognition of the limitations of existing therapy has started the search for pharmacotherapeutic agents acting on alternative pathways underlying detrusor overactivity with the intention of improving storage symptoms of urgency, frequency and urge incontinence.

RECENT FINDINGS

Recent research has suggested that several transmitters may modulate bladder storage. However, no agents currently available, acting via mechanisms other than muscarinic receptors have entered clinical practice so far. It is clear that far from being a passive container for urine, the urothelium is a crucial area within the bladder wall and its functions are complex and only now beginning to be appreciated. The release of several neurotransmitters from urothelium in response to distension and its action on receptors on sensory neurons is being increasingly recognized. The role for this afferent stimulation on the micturition reflex is gradually gaining importance in the pathophysiology of detrusor overactivity.

SUMMARY

In this article, the recent developments in basic science related to the pathogenesis and pharmacological basis for future drug targets for effective management of overactive bladder are discussed.