Bladder Volume Alters Cholinergic Responses of the Isolated Whole Mouse Bladder

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

AIMS

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

MAIN METHODS

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

KEY FINDINGS

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

SIGNIFICANCE

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

β3-Adrenoceptor agonists inhibit purinergic receptor-mediated contractions of the murine detrusor

β₃-Adrenoceptor (β₃-AR) agonists are used to treat overactive bladder syndrome; however, their mechanism of action has not been determined. The aims of this study were to compare the effects of β₃-AR agonists on cholinergic versus purinergic receptor-mediated contractions of the detrusor and to examine the mechanisms underlying inhibition of the purinergic responses by β₃-AR agonists. Isometric tension recordings were made from strips of murine detrusor and whole cell current recordings were made from freshly isolated detrusor myocytes using the patch-clamp technique. Transcriptional expression of exchange protein directly activated by cAMP (EPAC) subtypes in detrusor strips was assessed using RT-PCR and real-time quantitative PCR. The β₃-AR agonists BRL37344 and CL316243 (100 nM) inhibited cholinergic nerve-mediated contractions of the detrusor by 19 and 23%, respectively, but did not reduce contractions induced by the cholinergic agonist carbachol (300 nM). In contrast, BRL37344 and CL316243 inhibited purinergic nerve-mediated responses by 55 and 56%, respectively, and decreased the amplitude of contractions induced by the P2X receptor agonist α,β-methylene ATP by 40 and 45%, respectively. The adenylate cyclase activator forskolin inhibited purinergic responses, and these effects were mimicked by a combination of the PKA activator N⁶-monobutyryl-cAMP and the EPAC activator 8-pCPT-2'-O-methyl-cAMP-AM (007-AM). Application of ATP (1 μM) evoked reproducible P2X currents in isolated detrusor myocytes voltage-clamped at -60 mV. These responses were reduced in amplitude in the presence of BRL37344 and also by 007-AM. This study demonstrates that β₃-AR agonists reduce postjunctional purinergic responses in the detrusor via a pathway involving activation of the cAMP effector EPAC.

Modulation of Bladder Wall Micromotions Alters Intravesical Pressure Activity in the Isolated Bladder

Micromotions are phasic contractions of the bladder wall. During urine storage, such phasic activity has little effect on intravesical pressure, however, changed motile activity may underlie urodynamic observations such as detrusor overactivity. The potential for bladder motility to affect pressure reflects a summation of the overall movements, comprising the initiation, propagation, and dissipation components of micromotions. In this study, the influence of initiation of micromotions was investigated using calcium activated chloride channel blocker niflumic acid, and the effect of propagation using blockers of gap junctions. The overall bladder tone was modulated using isoprenaline. Isolated tissue strips and whole bladder preparations from juvenile rats were used. 18β-glycyrrhetinic acid was used to block gap junctions, reducing the amplitude and frequency of micromotions in in vitro and ex vivo preparations. Niflumic acid reduced the frequency of micromotions but had no effect on the amplitude of pressure fluctuations. Isoprenaline resulted in a reduction in pressure fluctuations and a decrease in pressure baseline. Using visual video data analysis, bladder movement was visible, irrespective of lack of pressure changes, which persisted during bladder relaxation. However, micromotions propagated over shorter distances and the overall bladder tone was reduced. All these results suggest that phasic activity of the bladder can be characterised by a combination of initiation and propagation of movement, and overall bladder tone. At any given moment, intravesical pressure recordings are an integration of these parameters. This synthesis gives insight into the limitations of clinical urodynamics, where intravesical pressure is the key indicator of detrusor activity.

β3-adrenoceptor agonists inhibit carbachol-evoked Ca2+ oscillations in murine detrusor myocytes

OBJECTIVE

To test if carbachol (CCh)-evoked Ca²⁺ oscillations in freshly isolated murine detrusor myocytes are affected by β3-adrenoceptor (β-AR) modulators.

MATERIALS AND METHODS

Isometric tension recordings were made from strips of murine detrusor, and intracellular Ca²⁺ measurements were made from isolated detrusor myocytes using confocal microscopy. Transcriptional expression of β-AR sub-types in detrusor strips and isolated detrusor myocytes was assessed using reverse transcriptase-polymerase chain reaction (RT-PCR) and real-time quantitative PCR (qPCR). Immunocytochemistry experiments, using a β3-AR selective antibody, were performed to confirm that β3-ARs were present on detrusor myocytes.

RESULTS

The RT-PCR and qPCR experiments showed that β1-, β2- and β3-AR were expressed in murine detrusor, but that β3-ARs were the most abundant sub-type. The selective β3-AR agonist BRL37344 reduced the amplitude of CCh-induced contractions of detrusor smooth muscle. These responses were unaffected by addition of the BK channel blocker iberiotoxin. BRL37344 also reduced the amplitude of CCh-induced Ca²⁺ oscillations in freshly isolated murine detrusor myocytes. This effect was mimicked by CL316,243, another β3-AR agonist, and inhibited by the β3-AR antagonist L748,337, but not by propranolol, an antagonist of β1- and β2-ARs. BRL37344 did not affect caffeine-evoked Ca²⁺ transients or L-type Ca²⁺ current in isolated detrusor myocytes.

CONCLUSION

Inhibition of cholinergic-mediated contractions of the detrusor by β3-AR agonists was associated with a reduction in Ca²⁺ oscillations in detrusor myocytes.

Neural control of the lower urinary tract: peripheral and spinal mechanisms

This review deals with individual components regulating the neural control of the urinary bladder. This article will focus on factors and processes involved in the two modes of operation of the bladder: storage and elimination. Topics included in this review include: (1) The urothelium and its roles in sensor and transducer functions including interactions with other cell types within the bladder wall ("sensory web"), (2) The location and properties of bladder afferents including factors involved in regulating afferent sensitization, (3) The neural control of the pelvic floor muscle and pharmacology of urethral and anal sphincters (focusing on monoamine pathways), (4) Efferent pathways to the urinary bladder, and (5) Abnormalities in bladder function including mechanisms underlying comorbid disorders associated with bladder pain syndrome and incontinence.

Urinary bladder function and somatic sensitivity in vasoactive intestinal polypeptide (VIP)-/- mice

Vasoactive intestinal polypeptide (VIP) is an immunomodulatory neuropeptide widely distributed in neural pathways that regulate micturition. VIP is also an endogenous anti-inflammatory agent that has been suggested for the development of therapies for inflammatory disorders. In the present study, we examined urinary bladder function and hindpaw and pelvic sensitivity in VIP(-/-) and littermate wildtype (WT) controls. We demonstrated increased bladder mass and fewer but larger urine spots on filter paper in VIP(-/-) mice. Using cystometry in conscious, unrestrained mice, VIP(-/-) mice exhibited increased void volumes and shorter intercontraction intervals with continuous intravesical infusion of saline. No differences in transepithelial resistance or water permeability were demonstrated between VIP(-/-) and WT mice; however, an increase in urea permeability was demonstrated in VIP(-/-) mice. With the induction of bladder inflammation by acute administration of cyclophosphamide, an exaggerated or prolonged bladder hyperreflexia and hindpaw and pelvic sensitivity were demonstrated in VIP(-/-) mice. The changes in bladder hyperreflexia and somatic sensitivity in VIP(-/-) mice may reflect increased expression of neurotrophins and/or proinflammatory cytokines in the urinary bladder. Thus, these changes may further regulate the neural control of micturition.

The integrative physiology of the bladder

Normal bladder function is complex, resulting from the co-operative interaction of numerous regulatory cell types, of which the interstitial cells and the peripheral neurones are particularly interesting. Collectively, these comprise the myovesical plexus, which appears to confer structural and functional characteristics on the bladder loosely akin to those of the gut. These include functional modularity, which gives rise to the potential for localised and propagating peristalsis-like movements in the bladder wall according to the prevailing physiological conditions. Localised modular activity during filling may contribute to normal generation of sensation and exaggerated modular activity may give rise to urinary urgency. Enhanced co-ordination of modular activity occurs in various models of detrusor overactivity; it leads to surges of contraction over a large part of the bladder wall, generating phasic changes in intravesical pressure. During voiding, the myovesical plexus sustains detrusor contraction at the behest of the brainstem, monitoring state of bladder fullness as it does so, as a guide to the required duration for which it has to keep up the effort. Accordingly, the bladder wall itself may house structures which render the bladder the effector level in a hierarchy of lower urinary tract regulation. Dysfunction in these vital regulatory structures is an underestimated factor in the pathophysiology of clinical bladder problems.

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.

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.

The use of the isolated mouse whole bladder for investigating bladder overactivity

The isolated mouse whole bladder was used to study in vitro bladder overactivity evoked by intramural nerve sensitization with bradykinin, mimicking neurogenic bladder overactivity secondary to bladder inflammation. Intravesical pressure responses to intramural electrical stimulation of intramural nerves were measured under isovolumetric condition. Validation showed that carbachol produced a dose-response curve closely mirroring that observed in the isolated muscle strips and demonstrated the dual nature of electrically evoked neurotransmission, consisting of a cholinergic component largely mediated by M(3) receptors and a purinergic component mediated by P2X receptors. ATP generated a biphasic dose-response curve, suggesting that the P2X receptors may be heterogeneous in distribution. Characterization of bradykinin receptors showed bradykinin to be extremely potent in exciting the bladder, producing a dose-response curve with an EC(50) of 90 nM, and bradykinin also enhanced electrically evoked bladder contractions. These effects were inhibited by the B(2) receptor antagonist HOE 140 (d-Arg(0)-Arg(1)-Pro(2)-Hyp(3)-Gly(4)-Thi(5)-Ser(6)-d-Tic(7)-Oic(8)-Arg(9)) but not the B(1) receptor antagonist desArg(10) HOE 140 (H-d-Arf-Arg-Pro-Hyp-Gly-Thi-Ser-d-Tic-Oic-OH) and were also modulated by alpha,beta,methyleneATP. The isolated mouse whole bladder has proved a viable, robust model in which to demonstrate the pharmacological characteristic of the bladder and adds to the repertoire of in vitro tools for investigating potential therapeutic agents.

Location of interstitial cells and neurotransmitters in the mouse bladder

INTRODUCTION

To investigate whether interstitial cells (ICs) are present in the adult mouse bladder, and what transmitters characterize adjacent nerve fibres, as ICs in human and guinea-pig bladder lie close to nerve fibres but transmitters present in these nerves have not yet been reported.

MATERIALS AND METHODS

Sections of the bladder wall from 12 adult male mice (six each, aged 3-4 or 18-24 months) were incubated in carboxygenated Krebs' solution containing isobutyl-methyl-xanthene (1 mm), followed by the nitric oxide (NO) donor diethylamino-NONOate; control tissues remained in Krebs' solution. Samples were fixed in 4% paraformaldehyde and processed for immunofluorescence histochemistry for cGMP, neuronal NO synthase (nNOS), vesicular acetylcholine transferase (VAChT), calcitonin gene-related polypeptide (CGRP) and protein gene product (PGP) 9.5. ICs were identified as non-neuronal cells of appropriate morphology manifesting an increase in cGMP after exposure to the NO donor.

RESULTS

ICs were apparent in the outer muscle, but not the inner muscle or suburothelial region. nNOS- and CGRP-immunoreactive fibres were close to and alongside IC processes. In contrast, nerve fibres containing VAChT were only occasionally found close to ICs and rarely running alongside them. ICs showed no immunoreactivity to c-kit. There was no overt difference in IC cell distribution between young and aged adult specimens. Older mice showed patchy denervation of the detrusor, but ICs were not specifically affected.

CONCLUSIONS

ICs are confined to the outer part of the bladder wall in the mouse and may receive peptidergic and nitrergic innervation, which might serve to modulate their putative functional role. Alterations in the overall IC population do not appear to underlie ageing-related changes in lower urinary tract function.