Unique properties of muscularis mucosae smooth muscle in guinea pig urinary bladder

The role of intracellular calcium and Rho kinase pathways in G protein-coupled receptor-mediated contractions of urinary bladder urothelium and lamina propria

The influence of extracellular and intracellular calcium on smooth muscle contractile activity varies between organs. In response to G protein-coupled receptor (GPCR) stimulation, the urinary bladder detrusor muscle has shown a 70% dependence on extracellular calcium, whereas the urothelium and lamina propria (U&LP) has a 20%-50% dependence. However, as this only accounts for partial contractile activity, the contribution of intracellular calcium and calcium sensitization pathways remains unclear. This study assessed the role of intracellular signaling pathways on GPCR-mediated urinary bladder U&LP contraction. Porcine U&LP responses to activation of the G_q/11-coupled muscarinic, histamine, 5-hydroxytryptamine (serotonin), neurokinin, prostaglandin, and angiotensin II receptors were assessed with three selective inhibitors of store-released intracellular calcium, 2-aminoethyl diphenylborinate (2-APB), cyclopiazonic acid (CPA), and ruthenium red, and three Rho kinase inhibitors, fasudil, Y-27632, and GSK269962. There was no discernible impact on receptor agonist-induced contractions of the U&LP after blocking intracellular calcium pathways, suggesting that this tissue is more sensitive to alterations in the availability of extracellular calcium. However, an alternative mechanism of action for GPCR-mediated contraction was identified to be the activation of Rho kinase, such as when Y-27632 significantly reduced the GPCR-mediated contractile activity of the U&LP by approximately 50% (P < 0.05, n = 8). This suggests that contractile responses of the bladder U&LP do not involve a significant release of calcium from intracellular stores, but that G_q/11-coupled receptor activation causes calcium sensitization via Rho kinase. This study highlights a key role for Rho kinase in the urinary bladder, which may provide a novel target in the future pharmaceutical management of bladder contractile disorders.

The Dependence of Urinary Bladder Responses on Extracellular Calcium Varies Between Muscarinic, Histamine, 5-HT (Serotonin), Neurokinin, Prostaglandin, and Angiotensin Receptor Activation

With many common bladder diseases arising due to abnormal contractions, a greater understanding of the receptor systems involved may aid the development of future treatments. The aim of this study was to identify any difference in the involvement of extracellular calcium (Ca²⁺) across prominent contractile-mediating receptors within cells lining the bladder. Strips of porcine urothelium and lamina propria were isolated from the urinary bladder dome and mounted in isolated tissue baths containing Krebs-bicarbonate solution, perfused with carbogen gas at 37°C. Tissue contractions, as well as changes to the frequency and amplitude of spontaneous activity were recorded after the addition of muscarinic, histamine, 5-hydroxytryptamine, neurokinin-A, prostaglandin E2, and angiotensin II receptor agonists in the absence and presence of 1 µM nifedipine or nominally zero Ca²⁺ solution. The absence of extracellular Ca²⁺ influx after immersion into nominally zero Ca²⁺ solution, or the addition of nifedipine, significantly inhibited the contractile responses (p < 0.05 for all) after stimulation with carbachol (1 µM), histamine (100 µM), 5-hydroxytryptamine (100 µM), neurokinin-A (300 nM), prostaglandin E2 (10 µM), and angiotensin II (100 nM). On average, Ca²⁺ influx from extracellular sources was responsible for between 20–50% of receptor-mediated contractions. This suggests that although the specific requirement of Ca²⁺ on contractile responses varies depending on the receptor, extracellular Ca²⁺ plays a key role in mediating G protein-coupled receptor contractions of the urothelium and lamina propria.

Functional nitrergic innervation of smooth muscle structures in the mucosa of pig lower urinary tract

Neurally released nitric oxide (NO) functions as an inhibitory neurotransmitter of urethral but not detrusor smooth muscles while relaxing bladder vasculature and muscularis mucosae (MM). Here, the distribution of nitrergic nerves was examined in the mucosa of pig lower urinary tract using immunohistochemistry, and their vasodilatory functions were studied by measuring arteriolar diameter changes. Properties of smooth muscle cells in the lamina propria (SMC-LP) of urethra and trigone were also investigated using florescence Ca²⁺ imaging. In the bladder mucosa, neuronal nitric oxide synthase (nNOS)-immunoreactive nitrergic fibres projected to suburothelial arterioles and venules. Perivascular nitrergic nerves were intermingled with but distinct from tyrosine hydroxylase (TH)-immunoreactive sympathetic or calcitonin gene-related peptide (CGRP)-immunoreactive afferent nerves. MM receive a nitrergic but not sympathetic or afferent innervation. In the mucosa of urethra and trigone, nitrergic nerves were in close apposition with sympathetic or afferent nerves around suburothelial vasculature but did not project to SMC-LP. In suburothelial arterioles of bladder and urethra, N ω-nitro-L-arginine (L-NA, 100 μM), an NOS inhibitor, enhanced electrical field stimulation (EFS)-induced sympathetic vasoconstrictions, while tadalafil (10 nM), a phosphodiesterase type 5 (PDE5) inhibitor, suppressed the vasoconstrictions. SMC-LP developed asynchronous spontaneous Ca²⁺ transients without responding to EFS. The spontaneous Ca²⁺ transients were enhanced by acetylcholine (1 μM) and diminished by noradrenaline (1 μM) but not SIN-1 (10 μM), an NO donor. In the lower urinary tract mucosa, perivascular nitrergic nerves appear to counteract the sympathetic vasoconstriction to maintain the mucosal circulation. Bladder MM but not SMC-LP receive an inhibitory nitrergic innervation.

Comparative effects of angiotensin II on the contractility of muscularis mucosae and detrusor in the pig urinary bladder

To explore contractile actions of angiotensin II (ATII) on the muscularis mucosae (MM) of the bladder, ATII-induced contractions were compared between MM and the detrusor smooth muscle (DSM) of the pig bladder by isometric tension recordings. Effects of ATII on spontaneous Ca²⁺ transients in MM were visualized using Cal-520 fluorescence. ATII receptor type 1 (ATR1) expression in MM and DSM was also examined by immunohistochemistry. ATII (1 nM-1 μM) caused phasic contractions of MM in a concentration-dependent manner, while ATII (10 nM-10 μM) had no or marginal effects on DSM contractility. ATII (100 nM)-induced MM contractions had an amplitude of approximately 70% of carbachol (1 μM)-induced or 90% of U46619 (100 nM)-induced contractions. Candesartan (10 nM), an ATR1 blocker, prevented the contractile effects of ATII (1 nM) in MM, while ATR1 immunofluorescence was greater in MM than DSM. ATII (10-100 pM) increased the frequency but not the amplitude of spontaneous Ca²⁺ transients in MM. Both urothelium-intact and -denuded MM strips developed comparable spontaneous phasic contractions, but ATII, carbachol and U46619-induced contractions were significantly larger in urothelium-denuded than urothelium-intact MM strips. In conclusion, the MM appears to have a much greater sensitivity to ATII compared with DSM that could well sense circulating ATII, suggesting that MM may be the predominant target of contractile actions induced by ATII in the bladder while the urothelium appears to inhibit MM contractility.

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.

A Novel in situ Approach to Studying Detrusor Smooth Muscle Cells in Mice

The aim of our study was to develop a novel approach to investigating mouse detrusor smooth muscle cell (SMC) physiological activity, utilizing an acute tissue dissection technique and confocal calcium imaging. The bladder of a sacrificed adult female NMRI mouse was dissected. We used light and transmission electron microscopy to assess morphology of SMCs within the tissue. Calcium imaging in individual SMCs was performed using confocal microscopy during stimulation with increasing concentrations of carbamylcholine (CCh). SMCs were identified according to their morphology and calcium activity. We determined several parameters describing the SMC responses: delays to response, recruitment, relative activity, and contraction of the tissue. CCh stimulation revealed three different SMC phenotypes: spontaneously active SMCs with and without CCh-enhanced activity and SMCs with CCh-induced activity only. SMCs were recruited into an active state in response to CCh-stimulation within a narrow range (1-25 µM); causing activation of virtually all SMCs. Maximum calcium activity of SMCs was at about 25 µM, which coincided with a visible tissue contraction. Finally, we observed shorter time lags before response onsets with higher CCh concentrations. In conclusion, our novel in situ approach proved to be a robust and reproducible method to study detrusor SMC morphology and physiology.

Role of Ryanodine Type 2 Receptors in Elementary Ca2+ Signaling in Arteries and Vascular Adaptive Responses

Background Hypertension is the major risk factor for cardiovascular disease, the most common cause of death worldwide. Resistance arteries are capable of adapting their diameter independently in response to pressure and flow-associated shear stress. Ryanodine receptors (RyRs) are major Ca2+-release channels in the sarcoplasmic reticulum membrane of myocytes that contribute to the regulation of contractility. Vascular smooth muscle cells exhibit 3 different RyR isoforms (RyR1, RyR2, and RyR3), but the impact of individual RyR isoforms on adaptive vascular responses is largely unknown. Herein, we generated tamoxifen-inducible smooth muscle cell-specific RyR2-deficient mice and tested the hypothesis that vascular smooth muscle cell RyR2s play a specific role in elementary Ca2+ signaling and adaptive vascular responses to vascular pressure and/or flow. Methods and Results Targeted deletion of the Ryr2 gene resulted in a complete loss of sarcoplasmic reticulum-mediated Ca2+-release events and associated Ca2+-activated, large-conductance K+ channel currents in peripheral arteries, leading to increased myogenic tone and systemic blood pressure. In the absence of RyR2, the pulmonary artery pressure response to sustained hypoxia was enhanced, but flow-dependent effects, including blood flow recovery in ischemic hind limbs, were unaffected. Conclusions Our results establish that RyR2-mediated Ca2+-release events in VSCM s specifically regulate myogenic tone (systemic circulation) and arterial adaptation in response to changes in pressure (hypoxic lung model), but not flow. They further suggest that vascular smooth muscle cell-expressed RyR2 deserves scrutiny as a therapeutic target for the treatment of vascular responses in hypertension and chronic vascular diseases.

β3 -Adrenoceptors in the normal and diseased urinary bladder-What are the open questions?

β₃ -Adrenoceptor agonists are used in the treatment of overactive bladder syndrome. Although the relaxant response to adrenergic stimulation in human detrusor smooth muscle cells is mediated mainly via β₃ -adrenoceptors, the plasma concentrations of the therapeutic dose of mirabegron, the only clinically approved β₃ -adrenoceptor agonist, are considerably lower than the EC₅₀ for causing direct relaxation of human detrusor, suggesting a mechanism of action other than direct relaxation of detrusor smooth muscle. However, the site and mechanism of action of β₃ -adrenoceptor agonists in the bladder have not been firmly established. Postulated mechanisms include prejunctional suppression of ACh release from the parasympathetic nerves during the storage phase and inhibition of micro-contractions through β₃ -adrenoceptors on detrusor smooth muscle cells or suburothelial interstitial cells. Implications of possible desensitization of β₃ -adrenoceptors in the bladder upon prolonged agonist exposure and possible causes of rarely observed cardiovascular effects of mirabegron are also discussed. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Role of K+ channels in regulating spontaneous activity in the muscularis mucosae of guinea pig bladder

To explore the roles of various K⁺ channels in regulating the spontaneous activity of bladder muscularis mucosae (MM) that is considered to play an important role in maintaining mucosal function. Effects of K⁺ channel modulators on electrical and contractile activity in the guinea-pig bladder MM were examined using intracellular microelectrode and isometric tension recording. The MM predominately generated bursting spontaneous action potentials (SAPs) and phasic contractions (SPCs) that were blocked by nifedipine (1µM). NS309 (10µM), a small-conductance Ca²⁺-activated K⁺ (SK) channel opener, dramatically prolonged after-hyperpolarisation (AHP) and converted bursting SAPs into individually action potentials in an apamin (100nM)-sensitive manner. Apamin alone increased the number of SAPs during bursts. NS1619 (10µM), a large-conductance Ca²⁺-activated K⁺ (BK) channel opener, abolished SAPs in a manner reversed by iberiotoxin (IbTX, 100nM), a BK channel blocker. IbTX alone enlarged SAPs and abolished their AHPs. Flupirtine (10µM), a voltage-dependent K⁺ channel (K_v7) opener, diminished SAPs in a manner reversed by XE991 (10µM), a K_v7 channel blocker. XE991 alone exerted modest excitatory effects on SAPs. These K⁺ channel modulators had corresponding effects on SPCs. Bursting SAP firing appears to result from a lower level activation of SK channels in MM than that DSM. BK channels play a predominant role in regulating SAP configuration, while K_v7 channels have only a marginal role. The prevention of bursting SAPs and associated reduction in SPCs upon the pharmacological activation of a reserved population of SK channels may well have a considerable therapeutic potential.

Host Responses to Urinary Tract Infections and Emerging Therapeutics: Sensation and Pain within the Urinary Tract

Urinary tract infection (UTI) pathogenesis is understood increasingly at the level of the uropathogens and the cellular and molecular mediators of host inflammatory responses. However, little is known about the mediators of symptoms during UTI and what distinguishes symptomatic events from asymptomatic bacteriuria. Here, we review bladder physiology and sensory pathways in the context of an emerging literature from murine models dissecting the host and pathogen factors mediating pain responses during UTI. The bladder urothelium is considered a mediator of sensory responses and appears to play a role in UTI pain responses. Virulence factors of uropathogens induce urothelial damage that could trigger pain due to compromised bladder-barrier function. Instead, bacterial glycolipids are the major determinants of UTI pain independent of urothelial damage, and the O-antigen of lipopolysaccharide modulates pain responses. The extent of pain modulation by O-antigen can have profound effects, from abolishing pain responses to inducing chronic pain that results in central nervous system features reminiscent of neuropathic pain. Although these effects are largely dependent upon Toll-like receptors, pain is independent of inflammation. Surprisingly, some bacteria even possess analgesic properties, suggesting that bacteria exhibit a wide range of pain phenotypes in the bladder. In summary, UTI pain is a complex form of visceral pain that has significant potential to inform our understanding of bacterial pathogenesis and raises the specter of chronic pain resulting from transient infection, as well as novel approaches to treating pain.

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

AIMS

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

On the origin of spontaneous activity in the bladder

OBJECTIVES

To characterise separately the pharmacological profiles of spontaneous contractions from the mucosa and detrusor layers of the bladder wall and to describe the relationship in mucosa between adenosine triphosphate (ATP) release and spontaneous contractions.

MATERIALS AND METHODS

Spontaneous contractions were measured (36 °C) from isolated mucosa or detrusor preparations, and intact (mucosa + detrusor) preparations from guinea-pig bladders. Potential modulators were added to the superfusate. The percentage of smooth muscle was measured in haematoxylin and eosin stained sections. ATP release was measured in superfusate samples from a fixed point above the preparation using a luciferin-luciferase assay.

RESULTS

The magnitude of spontaneous contractions was in the order intact >mucosa >detrusor. The percentage of smooth muscle was least in mucosa and greatest in detrusor preparations. The pharmacological profiles of spontaneous contractions were different in mucosa and detrusor in response to P2X or P2Y receptor agonists, adenosine and capsaicin. The intact preparations showed responses intermediate to those from mucosa and detrusor preparations. Low extracellular pH generated large changes in detrusor, but not mucosa preparations. The mucosa preparations released ATP in a cyclical manner, followed by variations in spontaneous contractions. ATP release was greater in mucosa compared with detrusor, augmented by carbachol and reversed by the M2 -selective antagonist, methoctramine.

CONCLUSIONS

The different pharmacological profiles of bladder mucosa and detrusor, implies different pathways for contractile activation. Also, the intermediate responses from intact preparations implies functional interaction. The temporal relationship between cyclical variation of ATP release and amplitude of spontaneous contractions is consistent with ATP release controlling spontaneous activity. Carbachol-mediated ATP release was independent of active contractile force.

Enhanced urothelial ATP release and contraction following intravesical treatment with the cytotoxic drug, doxorubicin

Intravesical administration of the cytotoxic drug doxorubicin is a common treatment for superficial carcinoma of the bladder, but it is associated with significant urological adverse effects. The aim of this study was to identify doxorubicin-induced changes in the local mechanisms involved in regulating bladder function. As a model of intravesical doxorubicin administration in patients, doxorubicin (1 mg/mL) was applied to the luminal surface of porcine bladders for 60 min. Following treatment, the release of urothelial/lamina propria mediators (acetylcholine (Ach), ATP and prostaglandin E2 (PGE2) and contractile responses of isolated tissue strips was investigated. Doxorubicin pretreatment did not affect contractile responses of detrusor muscle to carbachol, but did enhance neurogenic detrusor responses to electrical field stimulation (219 % at 5 Hz). Contractions of isolated strips of urothelium/lamina propria to carbachol were also enhanced (30 %) in tissues from doxorubicin pretreated bladders. Isolated strips of urothelium/lamina propria from control bladders demonstrated a basal release of all three mediators (Ach > ATP > PGE2), with increased release of ATP when tissues were stretched. In tissues from doxorubicin-pretreated bladders, the basal release of ATP was significantly enhanced (sevenfold), while the release of acetylcholine and PGE2 was not affected. The application of luminal doxorubicin, under conditions that mimic intravesical administration to patients, affects urothelial/lamina propria function (increased contractile activity and ATP release) and enhances efferent neurotransmission without affecting detrusor smooth muscle. These actions would enhance bladder contractile activity and sensory nerve activity and may explain the adverse urological effects observed in patients following intravesical doxorubicin treatment.

Physiological and pathophysiological implications of micromotion activity in urinary bladder function

'Micromotions' is a term signifying the presence of localized microcontractions and microelongations, alongside non-motile areas. The motile areas tend to shift over the bladder surface with time, and the intravesical pressure reflects moment-by-moment summation of the interplay between net contractile force generated by micromotions and general bladder tone. Functionally, the bladder structure may comprise modules with variable linkage, which supports presence of localized micromotions (no functional linkage between modules), propagating contractions (where emergence of linkage allows sequential activation) and the shifting of micromotions over time. Detrusor muscle, interstitial cells and intramural innervation have properties potentially relevant for initiating, coordinating and modulating micromotions. Conceptually, such activity could facilitate the generation of afferent activity (filling state reporting) in the absence of intravesical pressure change and the ability to transition to voiding at any bladder volume. This autonomous activity is an intrinsic property, seen in various experimental contexts including the clinical setting of human (female) overactive bladder. 'Disinhibited autonomy' may explain the obvious micromotions in isolated bladders and perhaps contribute clinically in neurological disease causing detrusor overactivity. Furthermore, any process that could increase the initiation or propagation of microcontractions might be anticipated to have a functional effect, increasing the likelihood of urinary urgency and detrusor overactivity respectively. Thus, models of bladder outlet obstruction, neurological trauma and ageing provide a useful framework for detecting cellular changes in smooth muscle, interstitial cells and innervation, and the consequent effects on micromotions.

Central role of the BK channel in urinary bladder smooth muscle physiology and pathophysiology

The physiological functions of the urinary bladder are to store and periodically expel urine. These tasks are facilitated by the contraction and relaxation of the urinary bladder smooth muscle (UBSM), also known as detrusor smooth muscle, which comprises the bladder wall. The large-conductance voltage- and Ca(2+)-activated K(+) (BK, BKCa, MaxiK, Slo1, or KCa1.1) channel is highly expressed in UBSM and is arguably the most important physiologically relevant K(+) channel that regulates UBSM function. Its significance arises from the fact that the BK channel is the only K(+) channel that is activated by increases in both voltage and intracellular Ca(2+). The BK channels control UBSM excitability and contractility by maintaining the resting membrane potential and shaping the repolarization phase of the spontaneous action potentials that determine UBSM spontaneous rhythmic contractility. In UBSM, these channels have complex regulatory mechanisms involving integrated intracellular Ca(2+) signals, protein kinases, phosphodiesterases, and close functional interactions with muscarinic and β-adrenergic receptors. BK channel dysfunction is implicated in some forms of bladder pathologies, such as detrusor overactivity, and related overactive bladder. This review article summarizes the current state of knowledge of the functional role of UBSM BK channels under normal and pathophysiological conditions and provides new insight toward the BK channels as targets for pharmacological or genetic control of UBSM function. Modulation of UBSM BK channels can occur by directly or indirectly targeting their regulatory mechanisms, which has the potential to provide novel therapeutic approaches for bladder dysfunction, such as overactive bladder and detrusor underactivity.

Diabetes attenuates urothelial modulation of detrusor contractility and spontaneous activity

OBJECTIVES

To investigate the effect of diabetes on urothelial modulation of bladder contractility.

METHODS

Bladder strips (urothelium intact or denuded) were prepared from 8-week-old streptozotocin-induced diabetic (n = 19) and non-diabetic control rats (n = 10). The effect of modulators of MaxiK (iberiotoxin and tetraethylammonium) and Kv7 (XE991 and retigabine) potassium channel activity were investigated for their effects on both carbachol-induced force generation and spontaneous contractile activity.

RESULTS

In bladder strips from non-diabetic animals, the presence of the urothelium resulted in marked sensitivity to carbachol-induced force generation by modulators of MaxiK and Kv7 channel activity, whereas in the diabetic animal urothelial sensitivity to these agents was significantly diminished. Urothelial-intact bladder strips from non-diabetic animals were more sensitive to modulators of Kv7 activity in reducing the amplitude of spontaneous phasic contractions than urothelial-denuded bladder strips, whereas in diabetic animals the presence or absence of the urothelium did not alter the sensitivity to modulators of Kv7 activity. Spontaneous activity in the presence of tetraethylammonium was not affected by the urothelium in bladder strips from either diabetic or non-diabetic animals.

CONCLUSIONS

The presence of the urothelium in bladders from non-diabetic animals modulates the activity of potassium blockers to affect bladder contractility, whereas in the diabetic bladder this effect is attenuated. These findings could help to explain the lack of success of pharmaceutical treatments targeting potassium channels to treat bladder pathology in patients with diseases imparing urothelial function.

Ethanol-mediated relaxation of guinea pig urinary bladder smooth muscle: involvement of BK and L-type Ca2+ channels

Mechanisms underlying ethanol (EtOH)-induced detrusor smooth muscle (DSM) relaxation and increased urinary bladder capacity remain unknown. We investigated whether the large conductance Ca(2+)-activated K(+) (BK) channels or L-type voltage-dependent Ca(2+) channels (VDCCs), major regulators of DSM excitability and contractility, are targets for EtOH by patch-clamp electrophysiology (conventional and perforated whole cell and excised patch single channel) and isometric tension recordings using guinea pig DSM cells and isolated tissue strips, respectively. EtOH at 0.3% vol/vol (~50 mM) enhanced whole cell BK currents at +30 mV and above, determined by the selective BK channel blocker paxilline. In excised patches recorded at +40 mV and ~300 nM intracellular Ca(2+) concentration ([Ca(2+)]), EtOH (0.1-0.3%) affected single BK channels (mean conductance ~210 pS and blocked by paxilline) by increasing the open channel probability, number of open channel events, and open dwell-time constants. The amplitude of single BK channel currents and unitary conductance were not altered by EtOH. Conversely, at ~10 μM but not ~2 μM intracellular [Ca(2+)], EtOH (0.3%) decreased the single BK channel activity. EtOH (0.3%) affected transient BK currents (TBKCs) by either increasing frequency or decreasing amplitude, depending on the basal level of TBKC frequency. In isolated DSM strips, EtOH (0.1-1%) reduced the amplitude and muscle force of spontaneous phasic contractions. The EtOH-induced DSM relaxation, except at 1%, was attenuated by paxilline. EtOH (1%) inhibited L-type VDCC currents in DSM cells. In summary, we reveal the involvement of BK channels and L-type VDCCs in mediating EtOH-induced urinary bladder relaxation accommodating alcohol-induced diuresis.

Molecular expression and pharmacological evidence for a functional role of kv7 channel subtypes in Guinea pig urinary bladder smooth muscle

Voltage-gated Kv7 (KCNQ) channels are emerging as essential regulators of smooth muscle excitability and contractility. However, their physiological role in detrusor smooth muscle (DSM) remains to be elucidated. Here, we explored the molecular expression and function of Kv7 channel subtypes in guinea pig DSM by RT-PCR, qRT-PCR, immunohistochemistry, electrophysiology, and isometric tension recordings. In whole DSM tissue, mRNAs for all Kv7 channel subtypes were detected in a rank order: Kv7.1~Kv7.2Kv7.3~Kv7.5Kv7.4. In contrast, freshly-isolated DSM cells showed mRNA expression of: Kv7.1~Kv7.2Kv7.5Kv7.3~Kv7.4. Immunohistochemical confocal microscopy analyses of DSM, conducted by using co-labeling of Kv7 channel subtype-specific antibodies and α-smooth muscle actin, detected protein expression for all Kv7 channel subtypes, except for the Kv7.4, in DSM cells. L-364373 (R-L3), a Kv7.1 channel activator, and retigabine, a Kv7.2-7.5 channel activator, inhibited spontaneous phasic contractions and the 10-Hz electrical field stimulation (EFS)-induced contractions of DSM isolated strips. Linopiridine and XE991, two pan-Kv7 (effective at Kv7.1-Kv7.5 subtypes) channel inhibitors, had opposite effects increasing DSM spontaneous phasic and 10 Hz EFS-induced contractions. EFS-induced DSM contractions generated by a wide range of stimulation frequencies were decreased by L-364373 (10 µM) or retigabine (10 µM), and increased by XE991 (10 µM). Retigabine (10 µM) induced hyperpolarization and inhibited spontaneous action potentials in freshly-isolated DSM cells. In summary, Kv7 channel subtypes are expressed at mRNA and protein levels in guinea pig DSM cells. Their pharmacological modulation can control DSM contractility and excitability; therefore, Kv7 channel subtypes provide potential novel therapeutic targets for urinary bladder dysfunction.

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.

Phasic contractions in urinary bladder from juvenile versus adult pigs

Aims

Alterations in properties of the bladder with maturation are relevant physiologically and pathophysiologically. The aim of this study was to investigate alterations in bladder properties with maturation in juvenile vs. adult pig, focussing on differences between layers of the bladder wall (mucosa vs. detrusor) and the presence and functional contribution of interstitial cells (ICs).

Methods

Basal and cholinergic-induced phasic contractions (PCs) in mucosal and denuded-detrusor strips from juvenile and adult pigs were assessed. Expression of c-kit, a marker of ICs, was investigated in the mucosa and the detrusor layers of the pig bladder. The functional role of ICs in mediating PCs was examined using imatinib.

Results

Mucosal strips from juvenile and adult pig bladders demonstrated basal PCs whilst denuded-detrusor strips did not. PCs of mucosal strips from juvenile pigs were significantly greater than those from adult bladders. Immunoreactivity for c-kit was detected in mucosa and detrusor layers of pig bladder. Histological studies demonstrated a distinct layer of smooth muscle between the urothelium and bladder detrusor, termed the muscularis mucosa. Imatinib was only effective in inhibiting PCs in mucosal strips from juvenile pigs. Imatinib inhibited the carbachol-induced PCs of both juvenile and adult denuded-detrusor strips, although strips from juvenile bladders demonstrated a trend towards being more sensitive to this inhibition.

Conclusions

We confirm the presence of c-kit positive ICs in pig urinary bladder. The enhanced PCs of mucosal strips from juvenile animals could be due to altered properties of ICs or the muscularis mucosa in the bladders of these animals.

Non-adrenergic, non-cholinergic, non-purinergic contractions of the urothelium/lamina propria of the pig bladder

Acetylcholine, and to a lesser extent ATP, mediates neurogenic contractions of bladder smooth muscle. Recently, the urothelium and lamina propria have also been shown to have contractile properties, but the neurotransmitters involved in mediating responses to nerve stimulation have not been investigated. Isolated strips of porcine urothelium with lamina propria were electrically field stimulated and contractions recorded. Drugs interfering with neurotransmission were then employed to identify which neurotransmitters mediated responses. Strips of urothelium/lamina propria developed spontaneous contractions with a frequency of 3.5±0.1 cycles min⁻¹ and amplitude of 0.84±0.06 g. Electrical field stimulation at 5, 10, and 20 Hz resulted in frequency-related contractions (1.13±0.36 g, 1.59±0.46 g and 2.20±0.53 g, respectively, n=13), and these were reduced in the presence of tetrodotoxin (1 μm) by 77±20% at 5 Hz, 79±7% at 10 Hz and 74±12% at 20 Hz (all P<0.01), indicating they were predominantly neurogenic in nature. Neither the muscarinic antagonist atropine (10 μm), the adrenergic neurone blocker guanethidine (10 μm) nor desensitization of the purinergic receptors with α,β-methylene ATP (10 μm) affected the contractile amplitude. Similarly, responses were not affected by the nitric oxide synthase inhibitor L-NNA (100 μm) or drugs that interfere with peptide neurotransmission (capsaicin, NK2 antagonist GR159897, protease inhibitors). In conclusion, electrical depolarization of the nerves present in the porcine urothelium/lamina propria results in frequency-dependent contractions, which are predominantly neurogenic in nature. These contractions are resistant to drugs that inhibit the adrenergic, cholinergic and purinergic systems. The neurotransmitter involved in the responses of this tissue is therefore unknown but does not appear to be a peptide.

Functional innervation of Guinea-pig bladder interstitial cells of cajal subtypes: neurogenic stimulation evokes in situ calcium transients

Several populations of interstitial cells of Cajal (ICC) exist in the bladder, associated with intramural nerves. Although ICC respond to exogenous agonists, there is currently no evidence of their functional innervation. The objective was to determine whether bladder ICC are functionally innervated. Guinea-pig bladder tissues, loaded with fluo-4AM were imaged with fluorescent microscopy and challenged with neurogenic electrical field stimulation (EFS). All subtypes of ICC and smooth muscle cells (SMC) displayed spontaneous Ca²⁺-oscillations. EFS (0.5 Hz, 2 Hz, 10 Hz) evoked tetrodotoxin (1 µM)-sensitive Ca²⁺-transients in lamina propria ICC (ICC-LP), detrusor ICC and perivascular ICC (PICC) associated with mucosal microvessels. EFS responses in ICC-LP were significantly reduced by atropine or suramin. SMC and vascular SMC (VSM) also responded to EFS. Spontaneous Ca²⁺-oscillations in individual ICC-LP within networks occurred asynchronously whereas EFS evoked coordinated Ca²⁺-transients in all ICC-LP within a field of view. Non-correlated Ca²⁺-oscillations in detrusor ICC and adjacent SMC pre-EFS, contrasted with simultaneous neurogenic Ca²⁺ transients evoked by EFS. Spontaneous Ca²⁺-oscillations in PICC were little affected by EFS, whereas large Ca²⁺-transients were evoked in pre-EFS quiescent PICC. EFS also increased the frequency of VSM Ca²⁺-oscillations. In conclusion, ICC-LP, detrusor ICC and PICC are functionally innervated. Interestingly, Ca²⁺-activity within ICC-LP networks and between detrusor ICC and their adjacent SMC were synchronous under neural control. VSM and PICC Ca²⁺-activity was regulated by bladder nerves. These novel findings demonstrate functional neural control of bladder ICC. Similar studies should now be carried out on neurogenic bladder to elucidate the contribution of impaired nerve-ICC communication to bladder pathophysiology.

Regional distribution and molecular interaction of caveolins in bladder smooth muscle

What's known on the subject? and What does the study add? Caveolae are specialised regions of bladder smooth muscle (BSM) cell membranes where specific signalling pathways are regulated. Caveolin proteins are involved in caveolar biogenesis and function as signal transduction regulators. Expression of caveolin-1, -2, and -3 has been previously identified in the bladder; however, the distribution and relative expression of these proteins have not been defined. The present data show significant differences in the spatial distribution of caveolin proteins throughout the bladder wall. Region dependent variations in the co-localisation of caveolin subtypes in detrusor SM were also detected. These findings support the premise that the unique spatial pattern of caveolin proteins associated with BSM cells may enable regionally distinct functional responses to common stimuli.

OBJECTIVE

• To determine the regional expression profile of caveolin isoforms (integral membrane proteins abundant in caveolae), the spatial relationships among caveolin proteins within specific smooth muscle (SM) regions and the extent of their molecular interactions in bladder SM (BSM).

MATERIALS AND METHODS

• Regional differences in the expression of caveolin family members were determined by quantitative reverse transcriptase-polymerase chain reaction and Western blot of RNA and protein extracted from the base, body and dome of rat bladders. • To evaluate the distribution of caveolin-1 (Cav-1), Cav-2 and Cav-3 within the bladder, longitudinal tissue sections from the base to dome were processed for confocal microscopy and quantified for intensity of immunoreactivity (IR) and extent of co-localisation. • Interactions among Cav-1, Cav-2 and Cav-3 were determined by co-immunoprecipitation.

RESULTS

• Differential expression of Cav-1 and Cav-3 was detected among bladder regions, with lowest expression in the bladder base relative to the dome. • Cav-1 was highly expressed in all regions, although an increase in IR from submucosa to serosa was detected in each region. • The distribution of Cav-2 IR generally paralleled Cav-1, but progressively decreased from submucosa to serosa in each region. • Cav-3 expression predominated in the medial region of BSM increasing progressively from base to dome, but was poorly expressed in the outer SM layer particularly in the dome. • Cav-1 co-precipitated extensively with both Cav-2 and Cav-3. Co-precipitation between Cav-3 and Cav-2 was also detected.

CONCLUSIONS

• The isoform-specific spatial distribution and distinct molecular interactions among caveolins in BSM may contribute to the contractile heterogeneity of BSM cells and facilitate differential modulation of responses to local stimuli. • As BSM caveolae regulate key signalling processes involved in contraction, altered expression of caveolin proteins may generate a regional imbalance in contraction/relaxation responses, thus leading to bladder dysfunction.

Sympathetic nerve stimulation induces local endothelial Ca2+ signals to oppose vasoconstriction of mouse mesenteric arteries

It is generally accepted that the endothelium regulates vascular tone independent of the activity of the sympathetic nervous system. Here, we tested the hypothesis that the activation of sympathetic nerves engages the endothelium to oppose vasoconstriction. Local inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) signals ("pulsars") in or near endothelial projections to vascular smooth muscle (VSM) were measured in an en face mouse mesenteric artery preparation. Electrical field stimulation of sympathetic nerves induced an increase in endothelial cell (EC) Ca(2+) pulsars, recruiting new pulsar sites without affecting activity at existing sites. This increase in Ca(2+) pulsars was blocked by bath application of the α-adrenergic receptor antagonist prazosin or by TTX but was unaffected by directly picospritzing the α-adrenergic receptor agonist phenylephrine onto the vascular endothelium, indicating that nerve-derived norepinephrine acted through α-adrenergic receptors on smooth muscle cells. Moreover, EC Ca(2+) signaling was not blocked by inhibitors of purinergic receptors, ryanodine receptors, or voltage-dependent Ca(2+) channels, suggesting a role for IP(3), rather than Ca(2+), in VSM-to-endothelium communication. Block of intermediate-conductance Ca(2+)-sensitive K(+) channels, which have been shown to colocalize with IP(3) receptors in endothelial projections to VSM, enhanced nerve-evoked constriction. Collectively, our results support the concept of a transcellular negative feedback module whereby sympathetic nerve stimulation elevates EC Ca(2+) signals to oppose vasoconstriction.