Regulation of urinary bladder smooth muscle contractions by ryanodine receptors and BK and SK channels

Activation mechanism and novel binding sites of the BKCa channel activator CTIBD

The large-conductance calcium-activated potassium (BKCa) channel, which is crucial for urinary bladder smooth muscle relaxation, is a potential target for overactive bladder treatment. Our prior work unveiled CTIBD as a promising BKCa channel activator, altering V 1/2 and G max This study investigates CTIBD's activation mechanism, revealing its independence from the Ca2+ and membrane voltage sensing of the BKCa channel. Cryo-electron microscopy disclosed that two CTIBD molecules bind to hydrophobic regions on the extracellular side of the lipid bilayer. Key residues (W22, W203, and F266) are important for CTIBD binding, and their replacement with alanine reduces CTIBD-mediated channel activation. The triple-mutant (W22A/W203A/F266A) channel showed the smallest V 1/2 shift with a minimal impact on activation and deactivation kinetics by CTIBD. At the single-channel level, CTIBD treatment was much less effective at increasing P o in the triple mutant, mainly because of a drastically increased dissociation rate compared with the WT. These findings highlight CTIBD's mechanism, offering crucial insights for developing small-molecule treatments for BKCa-related pathophysiological conditions.

Cells and ionic conductances contributing to spontaneous activity in bladder and urethral smooth muscle

Smooth muscle organs of the lower urinary tract comprise the bladder detrusor and urethral wall, which have a reciprocal contractile relationship during urine storage and micturition. As the bladder fills with urine, detrusor smooth muscle cells (DSMCs) remain relaxed to accommodate increases in intravesical pressure while urethral smooth muscle cells (USMCs) sustain tone to occlude the urethral orifice, preventing leakage. While neither organ displays coordinated regular contractions as occurs in small intestine, lymphatics or renal pelvis, they do exhibit patterns of rhythmicity at cellular and tissue levels. In rabbit and guinea-pig urethra, electrical slow waves are recorded from USMCs. This activity is linked to cells expressing vimentin, c-kit and Ca2+-activated Cl- channels, like interstitial cells of Cajal in the gastrointestinal tract. In mouse, USMCs are rhythmically active (firing propagating Ca2+ waves linked to contraction), and this cellular rhythmicity is asynchronous across tissues and summates to form tone. Experiments in mice have failed to demonstrate a voltage-dependent mechanism for regulating this rhythmicity or contractions in vitro, suggesting that urethral tone results from an intrinsic ability of USMCs to 'pace' their own Ca2+ mobilization pathways required for contraction. DSMCs exhibit spontaneous transient contractions, increases in intracellular Ca2+ and action potentials. Consistent across numerous species, including humans, this activity relies on voltage-dependent Ca2+ influx in DSMCs. While interstitial cells are present in the bladder, they do not 'pace' the organ in an excitatory manner. Instead, specialized cells (PDGFRα+ interstitial cells) may 'negatively pace' DSMCs to prevent bladder overexcitability.

Urothelium-derived prostanoids enhance contractility of urinary bladder smooth muscle and stimulate bladder afferent nerve activity in the mouse

The transitional epithelial cells (urothelium) that line the lumen of the urinary bladder form a barrier between potentially harmful pathogens, toxins, and other bladder contents and the inner layers of the bladder wall. The urothelium, however, is not simply a passive barrier, as it can produce signaling factors, such as ATP, nitric oxide, prostaglandins, and other prostanoids, that can modulate bladder function. We investigated whether substances produced by the urothelium could directly modulate the contractility of the underlying urinary bladder smooth muscle. Force was measured in isolated strips of mouse urinary bladder with the urothelium intact or denuded. Bladder strips developed spontaneous tone and phasic contractions. In urothelium-intact strips, basal tone, as well as the frequency and amplitude of phasic contractions, were 25%, 32%, and 338% higher than in urothelium-denuded strips, respectively. Basal tone and phasic contractility in urothelium-intact bladder strips were abolished by the cyclooxygenase (COX) inhibitor indomethacin (10 µM) or the voltage-dependent Ca2+ channel blocker diltiazem (50 µM), whereas blocking neuronal sodium channels with tetrodotoxin (1 µM) had no effect. These results suggest that prostanoids produced in the urothelium enhance smooth muscle tone and phasic contractions by activating voltage-dependent Ca2+ channels in the underlying bladder smooth muscle. We went on to demonstrate that blocking COX inhibits the generation of transient pressure events in isolated pressurized bladders and greatly attenuates the afferent nerve activity during bladder filling, suggesting that urothelial prostanoids may also play a role in sensory nerve signaling.NEW & NOTEWORTHY This paper provides evidence for the role of urothelial-derived prostanoids in maintaining tone in the urinary bladder during bladder filling, not only underscoring the role of the urothelium as more than a barrier but also contributing to active regulation of the urinary bladder. Furthermore, cyclooxygenase products greatly augment sensory nerve activity generated by bladder afferents during bladder filling and thus may play a role in perception of bladder fullness.

Low-Intensity Extracorporeal Shock Wave Therapy Ameliorates Detrusor Hyperactivity with Impaired Contractility via Transient Potential Vanilloid Channels: A Rat Model for Ovarian Hormone Deficiency

This study explores low-intensity extracorporeal shock wave therapy (LiESWT)'s efficacy in alleviating detrusor hyperactivity with impaired contractility (DHIC) induced by ovarian hormone deficiency (OHD) in ovariectomized rats. The rats were categorized into the following four groups: sham group; OVX group, subjected to bilateral ovariectomy (OVX) for 12 months to induce OHD; OVX + SW4 group, underwent OHD for 12 months followed by 4 weeks of weekly LiESWT; and OVX + SW8 group, underwent OHD for 12 months followed by 8 weeks of weekly LiESWT. Cystometrogram studies and voiding behavior tracing were used to identify the symptoms of DHIC. Muscle strip contractility was evaluated through electrical-field, carbachol, ATP, and KCl stimulations. Western blot and immunofluorescence analyses were performed to assess the expressions of various markers related to bladder dysfunction. The OVX rats exhibited significant bladder deterioration and overactivity, alleviated by LiESWT. LiESWT modified transient receptor potential vanilloid (TRPV) channel expression, regulating calcium concentration and enhancing bladder capacity. It also elevated endoplasmic reticulum (ER) stress proteins, influencing ER-related Ca2+ channels and receptors to modulate detrusor muscle contractility. OHD after 12 months led to neuronal degeneration and reduced TRPV1 and TRPV4 channel activation. LiESWT demonstrated potential in enhancing angiogenic remodeling, neurogenesis, and receptor response, ameliorating DHIC via TRPV channels and cellular signaling in the OHD-induced DHIC rat model.

A Novel Role of Uricosuric Agent Benzbromarone in BK Channel Activation and Reduction of Airway Smooth Muscle Contraction

The uricosuric drug benzbromarone, widely used for treatment of gout, hyperpolarizes the membrane potential of airway smooth muscle cells, but how it works remains unknown. Here we show a novel role of benzbromarone in activation of large conductance calcium-activated K⁺ channels. Benzbromarone results in dose-dependent activation of macroscopic big potassium (BK) currents about 1.7- to 14.5-fold with an EC₅₀ of 111 μM and shifts the voltage-dependent channel activation to a more hyperpolarizing direction about 10 to 54 mV in whole-cell patch clamp recordings. In single-channel recordings, benzbromarone decreases single BKα channel closed dwell time and increases the channel open probability. Coexpressing β1 subunit also enhances BK activation by benzbromarone with an EC₅₀ of 67 μM and a leftward shift of conductance-voltage (G-V) curve about 44 to 138 mV. Site-directed mutagenesis reveals that a motif of three amino acids ³²⁹RKK³³¹ in the cytoplasmic linker between S6 and C-terminal regulator of potassium conductance (RCK) gating ring mediates the pharmacological activation of BK channels by benzbromarone. Further ex vivo assay shows that benzbromarone causes reduction of tracheal strip contraction. Taken together, our findings demonstrate that uricosuric benzbromarone activates BK channels through molecular mechanism of action involving the channel RKK motif of S6-RCK linker. Pharmacological activation of BK channel by benzbromarone causes reduction of tracheal strip contraction, holding a repurposing potential for asthma and pulmonary arterial hypertension or BK channelopathies. SIGNIFICANCE STATEMENT: We describe a novel role of uricosuric agent benzbromarone in big potassium (BK) channel activation and relaxation of airway smooth muscle contraction. In this study, we find that benzbromarone is an activator of the large-conductance Ca²⁺- and voltage-activated K⁺ channel (BK channel), which serves numerous cellular functions, including control of smooth muscle contraction. Pharmacological activation of BK channel by the FDA-approved drug benzbromarone may hold repurposing potential for treatment of asthma and pulmonary arterial hypertension or BK channelopathies.

Ca2+-Sensitive Potassium Channels

The Ca²⁺ ion is used ubiquitously as an intracellular signaling molecule due to its high external and low internal concentration. Many Ca²⁺-sensing ion channel proteins have evolved to receive and propagate Ca²⁺ signals. Among them are the Ca²⁺-activated potassium channels, a large family of potassium channels activated by rises in cytosolic calcium in response to Ca²⁺ influx via Ca²⁺-permeable channels that open during the action potential or Ca²⁺ release from the endoplasmic reticulum. The Ca²⁺ sensitivity of these channels allows internal Ca²⁺ to regulate the electrical activity of the cell membrane. Activating these potassium channels controls many physiological processes, from the firing properties of neurons to the control of transmitter release. This review will discuss what is understood about the Ca²⁺ sensitivity of the two best-studied groups of Ca²⁺-sensitive potassium channels: large-conductance Ca²⁺-activated K⁺ channels, K_Ca1.1, and small/intermediate-conductance Ca²⁺-activated K⁺ channels, K_Ca2.x/K_Ca3.1.

BK Channel in the Physiology and in the Cancer of Pancreatic Duct: Impact and Reliability of BK Openers

BK (KCa 1.1, Slo-1) is a K⁺ channel characterized by an allosteric regulation of the gating mechanism by Ca²⁺ binding and voltage, and a high unitary conductance. The channel is expressed in many different tissues, where it is involved in the regulation or the fine-tuning of many physiological processes. Among other organs, BK is expressed in the pancreatic duct, a part of the gland important for the correct ionic composition of the pancreatic juice. Unfortunately, the pancreatic duct is also the site where one of the deadliest cancer types, the pancreatic duct adenocarcinoma (PDAC), develops. In the past years, it has been reported that continuous exposure of cancer cells to BK openers can have a significant impact on cell viability as well as on the ability to proliferate and migrate. Here, we first summarize the main BK channel properties and its roles in pancreatic duct physiology. Then we focus on the potential role of BK as a pharmacological target in PDAC. Moreover, we discuss how results obtained when employing BK activators on cancer cells can, in some cases, be misleading.

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.

The intracellular Ca2+ release channel TRPML1 regulates lower urinary tract smooth muscle contractility

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel that is localized to late endosomes and lysosomes. Here, we investigated the function of TRPML1 channels in regulating lower urinary tract (LUT) smooth muscle cell (SMC) contractility. We found that TRPML1 forms a stable signaling complex with ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). We further showed that TRPML1 channels are important for initiating an essential Ca²⁺-signaling negative feedback mechanism between RyRs on SR membranes and K⁺ channels on the plasma membrane. Knockout of TRPML1 channels in mice impaired this pathway, resulting in LUT smooth muscle hypercontractility and symptoms of overactive bladder. Our findings demonstrate a critical role for TRPML1 in LUT function.

TRPML1 (transient receptor potential mucolipin 1) is a Ca²⁺-permeable, nonselective cation channel that is predominantly localized to the membranes of late endosomes and lysosomes (LELs). Intracellular release of Ca²⁺ through TRPML1 is thought to be pivotal for maintenance of intravesicular acidic pH as well as the maturation, fusion, and trafficking of LELs. Interestingly, genetic ablation of TRPML1 in mice (Mcoln1^−/−) induces a hyperdistended/hypertrophic bladder phenotype. Here, we investigated this phenomenon further by exploring an unconventional role for TRPML1 channels in the regulation of Ca²⁺-signaling activity and contractility in bladder and urethral smooth muscle cells (SMCs). Four-dimensional (4D) lattice light-sheet live-cell imaging showed that the majority of LELs in freshly isolated bladder SMCs were essentially immobile. Superresolution microscopy revealed distinct nanoscale colocalization of LEL-expressing TRPML1 channels with ryanodine type 2 receptors (RyR2) in bladder SMCs. Spontaneous intracellular release of Ca²⁺ from the sarcoplasmic reticulum (SR) through RyR2 generates localized elevations of Ca²⁺ (“Ca²⁺ sparks”) that activate plasmalemmal large-conductance Ca²⁺-activated K⁺ (BK) channels, a critical negative feedback mechanism that regulates smooth muscle contractility. This mechanism was impaired in Mcoln1^−/− mice, which showed diminished spontaneous Ca²⁺ sparks and BK channel activity in bladder and urethra SMCs. Additionally, ex vivo contractility experiments showed that loss of Ca²⁺ spark–BK channel signaling in Mcoln1^−/− mice rendered both bladder and urethra smooth muscle hypercontractile. Voiding activity analyses revealed bladder overactivity in Mcoln1^−/− mice. We conclude that TRPML1 is critically important for Ca²⁺ spark signaling, and thus regulation of contractility and function, in lower urinary tract SMCs.

Urinary bladder smooth muscle ion channels: expression, function, and regulation in health and disease

Urinary bladder smooth muscle (UBSM), also known as detrusor smooth muscle, forms the bladder wall and ultimately determines the two main attributes of the organ: urine storage and voiding. The two functions are facilitated by UBSM relaxation and contraction, respectively, which depend on UBSM excitability shaped by multiple ion channels. In this review, we summarize the current understanding of key ion channels establishing and regulating UBSM excitability and contractility. They include excitation-enhancing voltage-gated Ca2+ (Cav) and transient receptor potential channels, excitation-reducing K+ channels, and still poorly understood Cl- channels. Dynamic interplay among UBSM ion channels determines the overall level of Cav channel activity. The net Ca2+ influx via Cav channels increases global intracellular Ca2+ concentration, which subsequently triggers UBSM contractility. Here, for each ion channel type, we describe UBSM tissue/cell expression (mRNA and protein) profiles and their role in regulating excitability and contractility of UBSM in various animal species, including the mouse, rat, and guinea pig, and, most importantly, humans. The currently available data reveal certain interspecies differences, which complicate the translational value of published animal research results to humans. This review highlights recent developments, findings on genetic knockout models, pharmacological data, reports on UBSM ion channel dysfunction in animal bladder disease models, and the very limited human studies currently available. Among all gaps in present-day knowledge, the unknowns on expression and functional roles for ion channels determined directly in human UBSM tissues and cells under both normal and disease conditions remain key hurdles in the field.

Reduced Ca2+ spark activity contributes to detrusor overactivity of rats with partial bladder outlet obstruction

We tested whether or not altered Ca²⁺ spark activity accounted for detrusor overactivity (DO) of Wistar rats after partial bladder outlet obstruction (PBOO). We constructed a DO model through PBOO and studied the Ca²⁺ spark activity of detrusor. By way of using confocal microscopy and the patch-clamp technique, Ca²⁺ sparks and spontaneous transient outward currents (STOCs) in detrusor myocytes were measured respectively. Our results indicated that Ca²⁺ spark activity and STOCs were significantly reduced in the DO detrusor myocytes compared to unafflicted control cells, and both of these had levels that were remarkably increased by applications of caffeine (10 μM), a RyR agonist, in DO myocytes. In addition, measures of detrusor contractions were also recorded by using freshly isolated detrusor strips. These results indicated that the spontaneous contraction of DO detrusor was significantly enhanced, and that the effect of caffeine (10 μM) upon detrusor contractions was reversed by applications of iberiotoxin (100 nM) which is a BK channel blocker. Western blotting (WB) analyses indicated that the levels of expression of ryanodine receptor type 2 (RyR2) and FK506 binding protein 12.6 (FKBP12.6) in bladder muscle were respectively decreased and increased in the samples from DO rats. Thus, we considered in the rat DO model wherein PBOO, the reduced Ca²⁺ spark activity in detrusor myocytes partly contributed to overactive detrusor contractions. The impaired Ca²⁺ spark activity may have resulted from decreased RyR2 expression and increased FKBP12.6 expression. Such novel findings in our research might help to provide means for better treatment outcomes for patients afflicted by bladder dysfunction.

Antispasmodic effect of hydroalcoholic and flavonoids extracts of Dracocephalum kotschyi on rabbit bladder

Introduction: Dracocephalum kotschyi extract has antispasmodic activities on smooth muscle including ileum, uterus and trachea. The objective of this research was to investigate antispasmodic activity of hydroalcoholic and flavonoids extracts of D. kotschyi on rabbit bladder contractions. Methods: Rabbits were euthanized by carbon dioxide asphyxiation and the whole bladder was dissected out and immersed in the Tyrode’s solution. Longitudinal bladder strips were mounted vertically in an organ bath at 37°C and gassed continuously with O2 . Bladder strips were contracted with acetylcholine (ACh), KCl, or electrical field stimulation (EFS). Isotonic tension of the tissue was recorded before and after addition of hydroalcoholic or flavonoids rich extracts of D. kotschyi. Nifedipine and propantheline were used as standard drugs. Results: Standard drug propantheline, prevented bladder phasic contraction induced by ACh (1µM) without affecting KCl response. On the other hand, cumulative addition of nifedipine attenuated the tonic contractions induced by KCl (20mM) on bladder smooth muscle. Hydroalcoholic and flavonoids extracts of D. kotschyiat concentration ranges of 10-320 µg/ mL in a concentration dependent way inhibited bladder tonic contraction induced by KCl (n=6). Both extracts also in a concentration-dependent manner relaxed EFS and ACh-induced contractions (range, 20–1280 µg/mL) of bladder smooth muscle in vitro. Complete inhibition was achieved with the highest used concentrations of the extracts. The inhibitory effect of the extract was reversible following washing the tissues with fresh Tyrode’s solution. Conclusion: This study clearly demonstrated that D. kotschyi extracts were able to prevent contractions induced by ACh, KCl or EFS in isolated rabbit bladder. This means that people consuming this medicinal plant may face urinary retention which could be a problem for patients with prostate hypertrophy. On the other hand, this plant might be useful in patients with urinary incontinence. However, its usefulness must be assessed in the controlled clinical trials.

Sarcoplasmic reticulum and calcium signaling in muscle cells: Homeostasis and disease

The sarco/endoplasmic reticulum is an extensive, dynamic and heterogeneous membranous network that fulfills multiple homeostatic functions. Among them, it compartmentalizes, stores and releases calcium within the intracellular space. In the case of muscle cells, calcium released from the sarco/endoplasmic reticulum in the vicinity of the contractile machinery induces cell contraction. Furthermore, sarco/endoplasmic reticulum-derived calcium also regulates gene transcription in the nucleus, energy metabolism in mitochondria and cytosolic signaling pathways. These diverse and overlapping processes require a highly complex fine-tuning that the sarco/endoplasmic reticulum provides by means of its numerous tubules and cisternae, specialized domains and contacts with other organelles. The sarco/endoplasmic reticulum also possesses a rich calcium-handling machinery, functionally coupled to both contraction-inducing stimuli and the contractile apparatus. Such is the importance of the sarco/endoplasmic reticulum for muscle cell physiology, that alterations in its structure, function or its calcium-handling machinery are intimately associated with the development of cardiometabolic diseases. Cardiac hypertrophy, insulin resistance and arterial hypertension are age-related pathologies with a common mechanism at the muscle cell level: the accumulation of damaged proteins at the sarco/endoplasmic reticulum induces a stress response condition termed endoplasmic reticulum stress, which impairs proper organelle function, ultimately leading to pathogenesis.

Attenuated BK channel function promotes overactive bladder in a rat model of obesity

Overactive bladder (OAB) is mostly observed in obese individuals, and is associated with enhanced excitability and contractility of the detrusor smooth muscle (DSM). Large-conductance voltage- and Ca²⁺-activated K⁺ (BK) channels reduce the excitability and contractility of the DSM. We tested whether obesity-induced OAB is associated with altered BK channel expression and activity in the DSM. Seven-week-old female Sprague-Dawley rats (N=80) were fed a normal or high-fat diet (HFD) for 12 weeks. HFD-fed rats exhibited a higher average bodyweight and urodynamically established detrusor overactivity. mRNA levels of the Kcnma1 (BKα subunit) and Kcnmb1 (BKβ1 subunit) in whole tissues and cells from the DSM were reduced in HFD-fed rats. A selective BK channel opener, NS1619, was then applied to DSM cells from the two groups of rats. Patch-clamp techniques revealed that spontaneous transient outward currents, NS1619-induced activation of spontaneous transient outward currents, and whole-cell BK currents, as well as NS1619-induced membrane hyperpolarization, were attenuated in DSM cells from HFD-fed rats. The relaxation effect of NS1619 on contractility was reduced in DSM strips from HFD-fed rats. Thus, impaired expression of Kcnma1 and Kcnmb1 in the DSM contributes to obesity-induced OAB, suggesting that BK channels could be a useful treatment targets in OAB.

Effect of high-fat diet-induced obesity on the small-conductance Ca2+-activated K+ channel function affecting the contractility of rat detrusor smooth muscle

PURPOSE

Obesity usually induces overactive bladder (OAB) associated with detrusor overactivity, which is related to increased contractility of the detrusor smooth muscle (DSM). Small-conductance Ca²⁺-activated K⁺ (SK) channels play a constitutive role in the regulation of DSM contractility. However, the role of SK channels in the DSM changes in obesity-related OAB is still unknown. Here, we tested the hypothesis that obesity-related OAB is associated with reduced expression and activity of SK channels in DSM and that SK channels activation is a potential treatment for OAB.

METHODS

Female Sprague-Dawley rats were fed a normal diet (ND) or a high-fat diet (HFD) and weighed after 12 weeks. Urodynamic studies, quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and isometric tension recording were performed.

RESULTS

Increased average body weights and urodynamically demonstrated OAB were observed in HFD rats. qRT-PCR experiments revealed a decrease in the mRNA expression level of SK channel in DSM tissue of the HFD rats. Isometric tension recordings indicated an attenuated relaxation effect of NS309 on the spontaneous phasic and electrical field stimulation-induced contractions that occurred via SK channel activation in HFD DSM strips.

CONCLUSIONS

Reduced expression and activity of SK channels in the DSM contribute to obesity-related OAB, indicating that SK channels are a potential therapeutic target for OAB.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Non-selective (HCN) Ion Channels Regulate Human and Murine Urinary Bladder Contractility

Purpose: Hyperpolarization-activated cyclic nucleotide gated non-selective (HCN) channels have been demonstrated in the urinary bladder in various species. Since they play a major role in governing rhythmic activity in pacemaker cells like in the sinoatrial node, we explored the role of these channels in human and murine detrusor smooth muscle.

Methods: In an organ bath, human and murine detrusor smooth muscle specimens were challenged with the HCN channel blocker ZD7288. In human tissue derived from macroscopically tumor-free cancer resections, the urothelium was removed. In addition, HCN1-deficient mice were used to identify the contribution of this particular isoform. Expression of HCN channels in the urinary bladder was analyzed using histological and ultrastructural analyses as well as quantitative reverse transcriptase polymerase chain reaction (RT-PCR).

Results: We found that the HCN channel blocker ZD7288 (50 μM) both induced tonic contractions and increased phasic contraction amplitudes in human and murine detrusor specimens. While these responses were not sensitive to tetrodotoxin, they were significantly reduced by the gap junction inhibitor 18β-glycyrrhetic acid suggesting that HCN channels are located within the gap junction-interconnected smooth muscle cell network rather than on efferent nerve fibers. Immunohistochemistry suggested HCN channel expression on smooth muscle tissue, and immunoelectron microscopy confirmed the scattered presence of HCN2 on smooth muscle cell membranes. HCN channels seem to be down-regulated with aging, which is paralleled by an increasing effect of ZD7288 in aging detrusor tissue. Importantly, the anticonvulsant and HCN channel activator lamotrigine relaxed the detrusor which could be reversed by ZD7288.

Conclusion: These findings demonstrate that HCN channels are functionally present and localized on smooth muscle cells of the urinary bladder. Given the age-dependent decline of these channels in humans, activation of HCN channels by compounds such as lamotrigine opens up the opportunity to combat detrusor hyperactivity in the elderly by drugs already approved for epilepsy.

Regulation of Spontaneous Contractions in Intact Rat Bladder Strips and the Effects of Hydrogen Peroxide

Enhanced spontaneous contractions are associated with overactive bladder. Elevated levels of reactive oxygen species might contribute to enhanced spontaneous contractions. We investigated the regulation of spontaneous contractions and the effects of hydrogen peroxide (H₂O₂) in intact rat bladder strips. The spontaneous contractions were measured using a tissue bath system. The vehicle or the specific activators/blockers were applied and followed by the application of 0.003 g% H₂O₂. The basal tension, amplitude, and frequency of spontaneous contractions were quantified. Nisoldipine and bisindolylmaleimide 1 had no effects on spontaneous contractions. SKF96365 and Y27632 decreased basal tension and amplitude. Ryanodine slightly increased frequency. Both iberiotoxin and NS-1619 increased amplitude. Apamin reduced frequency but increased amplitude. NS-309 inhibited both the amplitude and frequency. The basal tension and amplitude increased when H₂O₂ was applied. Pretreatment with NS-309 inhibited H₂O₂-elicited augmented amplitude and frequency, while pretreatment with Y-27632 inhibited the augmented basal tension. The combined application of NS-309 and Y27632 almost eliminated spontaneous contractions and its augmentation induced by H₂O₂. In conclusion, Ca²⁺ influx, Rho kinase activation, and SK channel inactivation play important roles in spontaneous contractions in intact bladder strips, whereas only latter two mechanisms may be involved in H₂O₂-elicited increased spontaneous contractions.

Molecular Determinants of BK Channel Functional Diversity and Functioning

Large-conductance Ca2+- and voltage-activated K+ (BK) channels play many physiological roles ranging from the maintenance of smooth muscle tone to hearing and neurosecretion. BK channels are tetramers in which the pore-forming α subunit is coded by a single gene ( Slowpoke, KCNMA1). In this review, we first highlight the physiological importance of this ubiquitous channel, emphasizing the role that BK channels play in different channelopathies. We next discuss the modular nature of BK channel-forming protein, in which the different modules (the voltage sensor and the Ca2+ binding sites) communicate with the pore gates allosterically. In this regard, we review in detail the allosteric models proposed to explain channel activation and how the models are related to channel structure. Considering their extremely large conductance and unique selectivity to K+, we also offer an account of how these two apparently paradoxical characteristics can be understood consistently in unison, and what we have learned about the conduction system and the activation gates using ions, blockers, and toxins. Attention is paid here to the molecular nature of the voltage sensor and the Ca2+ binding sites that are located in a gating ring of known crystal structure and constituted by four COOH termini. Despite the fact that BK channels are coded by a single gene, diversity is obtained by means of alternative splicing and modulatory β and γ subunits. We finish this review by describing how the association of the α subunit with β or with γ subunits can change the BK channel phenotype and pharmacology.

Differential efficacy of GoSlo-SR compounds on BKα and BKαγ1-4 channels

Large conductance, voltage and Ca²⁺ activated K⁺ channels (BK channels) are abundantly expressed throughout the body and are important regulators of smooth muscle tone and neuronal excitability. Their dysfunction is implicated in various diseases including overactive bladder, hypertension and erectile dysfunction. Therefore, BK channel openers bear significant therapeutic potential to treat the above diseases. GoSlo-SR compounds were designed to be potent and efficacious BK channel openers. Although their structural activity relationships, activation in both BKα and BKαβ channels and the hypothetical mode of action of these compounds has been studied in detail in recent years, their effectiveness to open the BKαγ channels still remains unexplored. In this study, we have examined the efficacy of 3 closely related GoSlo-SR openers, GoSlo-SR-5-6 (SR-5-6), GoSlo-SR-5-44 (SR-5-44) and GoSlo-SR-5-130 (SR-5-130) using inside out patches on BKα channels coexpressed with 4 different LRRC (γ_1-4) subunits in HEK293 cells. Our data suggests that the activation effects due to SR-5-6 were not significantly affected in the presence of γ_1-4 subunits. Interestingly, the effects of more efficacious BK channel opener SR-5-44 were altered by different γ subunits. In cells expressing BKα channels, the shift in V_1/2 (ΔV_1/2) induced by SR-5-44 (3 μM) was -76 ± 3 mV, whereas it was significantly reduced by ∼70 % in BKαγ₁ channels (ΔV_1/2= -23 ± 3, p < 0.001, ANOVA). In BKαγ₂ channels the ΔV_1/2 was -36 ± 1 mV, which was less than that observed in BKαγ₃ and BKαγ₄ channels where the ΔV_1/2 was -47 ± 5 mV, and -82 ± 5 mV, respectively. Additionally, the excitatory effects of a 'β specific' BK channel opener, SR-5-130 were only partially restored in the patches containing BKαγ_1-4 channels. Together this data highlights that subtle modifications in GoSlo-SR structures alter their effectiveness on BK channels with accessory γ subunits and this study might provide a scaffold for the development of more tissue specific BK channel openers.

Original Research: Combined model of bladder detrusor smooth muscle and interstitial cells

Although patients with lower urinary tract symptoms constitute a large and still growing population, understanding of bladder detrusor muscle physiology remains limited. Understanding the interactions between the detrusor smooth muscle cells and other bladder cell types (e.g. interstitial cells, IC) that may significantly contribute to coordinating and modulating detrusor contractions represents a considerable challenge. Computer modeling could help to elucidate some properties that are difficult to address experimentally; therefore, we developed in silico models of detrusor smooth muscle cell and interstitial cells, coupled through gap junctions. The models include all of the major ion conductances and transporters described in smooth muscle cell and interstitial cells in the literature. The model of normal detrusor muscle (smooth muscle cell and interstitial cells coupled through gap junctions) completely reproduced the experimental results obtained with detrusor strips in the presence of several pharmacological interventions (ryanodine, caffeine, nimodipine), whereas the model of smooth muscle cell alone (without interstitial cells) failed to reproduce the experimental results. Next, a model of overactive bladder, a highly prevalent clinical condition in both men and women with increasing incidence at older ages, was produced by modifying several processes as reported previously: a reduction of Ca(2+)-release through ryanodine receptors and a reduction of Ca(2+)-dependent K(+)-conductance with augmented gap junctional coupling. This model was also able to reproduce the pharmacological modulation of overactive bladder. In conclusion, a model of bladder detrusor muscle was developed that reproduced experimental results obtained in both normal and overactive bladder preparations. The results indicate that the non-smooth muscle cells of the detrusor (interstitial cells) contribute significantly to the contractile behavior of bladder detrusor muscle and should not be neglected. The model suggests that reduced Ca(2+)-release through ryanodine receptors and Ca(2+)-dependent K(+)-conductance together with augmented gap junctional coupling might play a major role in overactive bladder pathogenesis.

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.

Regulation of urinary bladder function by protein kinase C in physiology and pathophysiology

BACKGROUND

Protein kinase C (PKC) is expressed in many tissues and organs including the urinary bladder, however, its role in bladder physiology and pathophysiology is still evolving. The aim of this review was to evaluate available evidence on the involvement of PKC in regulation of detrusor contractility, muscle tone of the bladder wall, spontaneous contractile activity and bladder function under physiological and pathophysiological conditions.

METHODS

This is a non-systematic review of the published literature which summarizes the available animal and human data on the role of PKC signaling in the urinary bladder under different physiological and pathophysiological conditions. A wide PubMed search was performed including the combination of the following keywords: "urinary bladder", "PKC", "detrusor contractility", "bladder smooth muscle", "detrusor relaxation", "peak force", "detrusor underactivity", "partial bladder outlet obstruction", "voltage-gated channels", "bladder nerves", "PKC inhibitors", "PKC activators". Retrieved articles were individually screened for the relevance to the topic of this review with 91 citations being selected and included in the data analysis.

DISCUSSION

Urinary bladder function includes the ability to store urine at low intravesical pressure followed by a subsequent release of bladder contents due to a rapid phasic contraction that is maintained long enough to ensure complete emptying. This review summarizes the current concepts regarding the potential contribution of PKC to contractility, physiological voiding, and related signaling mechanisms involved in the control of both the storage and emptying phases of the micturition cycle, and in dysfunctional voiding. Previous studies linked PKC activation exclusively with an increase in generation of the peak force of smooth muscle contraction, and maximum force generation in the lower urinary tract. More recent data suggests that PKC presents a broader range of effects on urinary bladder function including regulation of storage, emptying, excitability of the detrusor, and bladder innervation. In this review, we evaluated the mechanisms of peripheral and local regulation of PKC signaling in the urinary bladder, and their impact on different phases of the micturition cycle under physiological and pathophysiological conditions.

Social stress in mice induces urinary bladder overactivity and increases TRPV1 channel-dependent afferent nerve activity

Social stress has been implicated as a cause of urinary bladder hypertrophy and dysfunction in humans. Using a murine model of social stress, we and others have shown that social stress leads to bladder overactivity. Here, we show that social stress leads to bladder overactivity, increased bladder compliance, and increased afferent nerve activity. In the social stress paradigm, 6-wk-old male C57BL/6 mice were exposed for a total of 2 wk, via barrier cage, to a C57BL/6 retired breeder aggressor mouse. We performed conscious cystometry with and without intravesical infusion of the TRPV1 inhibitor capsazepine, and measured pressure-volume relationships and afferent nerve activity during bladder filling using an ex vivo bladder model. Stress leads to a decrease in intermicturition interval and void volume in vivo, which was restored by capsazepine. Ex vivo studies demonstrated that at low pressures, bladder compliance and afferent activity were elevated in stressed bladders compared with unstressed bladders. Capsazepine did not significantly change afferent activity in unstressed mice, but significantly decreased afferent activity at all pressures in stressed bladders. Immunohistochemistry revealed that TRPV1 colocalizes with CGRP to stain nerve fibers in unstressed bladders. Colocalization significantly increased along the same nerve fibers in the stressed bladders. Our results support the concept that social stress induces TRPV1-dependent afferent nerve activity, ultimately leading to the development of overactive bladder symptoms.

UTP activates small-conductance Ca2+-activated K+ channels in murine detrusor PDGFRα+ cells

Purines induce transient contraction and prolonged relaxation of detrusor muscles. Transient contraction is likely due to activation of inward currents in smooth muscle cells, and prolonged relaxation may be due to activation of small-conductance Ca(2+)-activated K(+) (SK) channels via P2Y1 receptors expressed by detrusor PDGF receptor (PDGFR)α(+) cells. We investigated whether other subtypes of P2Y receptors are involved in the activation of SK channels in PDGFRα(+) cells of detrusor muscles. Quantitative analysis of transcripts revealed that P2ry2, P2ry4, and P2ry14 are expressed in PDGFRα(+) cells of P2ry1-deficient/enhanced green fluorescent protein (P2ry1(-/-)/eGFP) mice at similar levels as in wild-type mice. UTP, a P2Y2/P2Y4 agonist, activated large outward currents in detrusor PDGFRα(+) cells. SK channel blockers and an inhibitor of phospholipase C completely abolished currents activated by UTP. In contrast, UTP activated nonselective cation currents in smooth muscle cells. Under current-clamp (current = 0), UTP induced significant hyperpolarization of PDGFRα(+) cells. MRS2500, a selective P2Y1 antagonist, did not affect UTP-activated outward currents in PDGFRα(+) cells from wild-type mice, and activation of outward currents by UTP was retained in P2ry1(-/-)/eGFP mice. As a negative control, we tested the effect of MRS2693, a selective P2Y6 agonist. This compound did not activate outward currents in PDGFRα(+) cells, and currents activated by UTP were unaffected by MRS2578, a selective P2Y6 antagonist. The nonselective P2Y receptor blocker suramin inhibited UTP-activated outward currents in PDGFRα(+) cells. Our data demonstrate that P2Y2 and/or P2Y4 receptors function, in addition to P2Y1 receptors, in activating SK currents in PDGFRα(+) cells and possibly in mediating purinergic relaxation responses in detrusor muscles.

Neuropilin 1 is essential for gastrointestinal smooth muscle contractility and motility in aged mice

Background and Aims

Neuropilin 1 (NRP1) is a non-tyrosine kinase receptor for vascular endothelial growth factor (VEGF) and class 3 semaphorins, playing a role in angiogenesis and neuronal axon guidance, respectively. NRP1 is expressed in smooth muscle cells (SMC) but the functional role of NRP1 in SMC has not been elucidated. We therefore investigated the biological relevance of NRP1 in SMC in vivo by generating mice with SMC-specific Nrp1 deficiency.

Methods

Conditional gene targeting generated SMC-specific Nrp1 knockout mice (Nrp1^SMKO) in which Cre recombinase is driven by the smooth muscle-specific myosin heavy chain (smMHC) promoter.

Results

SMC-specific Nrp1 deficiency resulted in a significant reduction in intestinal length by 6 months, and, by 18 months, in severe constipation, and enlargement of the intestine consistent with chronic intestinal pseudo-obstruction. These effects were associated with significant thinning of the intestinal smooth muscle, and decreased intestinal contractility. Expression of contractile proteins was reduced in Nrp1^SMKO mice, including the smMHC isoform, SMB, whereas we observed a significant increase in the expression of the small-conductance calcium-activated potassium channel 3 (SK3/KCa2.3), implicated in negative regulation of smooth muscle contraction.

Conclusions

Nrp1 deficiency in visceral SMC results in adult-onset defects in gastrointestinal contractility and motility and causes a shift to a less contractile SMC phenotype. These findings indicate a new role for Nrp1 in the maintenance of the visceral SMC contractile phenotype required for normal GI motility in aged mice.

BK channel activators and their therapeutic perspectives

The large conductance calcium- and voltage-activated K⁺ channel (KCa1.1, BK, MaxiK) is ubiquitously expressed in the body, and holds the ability to integrate changes in intracellular calcium and membrane potential. This makes the BK channel an important negative feedback system linking increases in intracellular calcium to outward hyperpolarizing potassium currents. Consequently, the channel has many important physiological roles including regulation of smooth muscle tone, neurotransmitter release and neuronal excitability. Additionally, cardioprotective roles have been revealed in recent years. After a short introduction to the structure, function and regulation of BK channels, we review the small organic molecules activating BK channels and how these tool compounds have helped delineate the roles of BK channels in health and disease.

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.

Prostaglandin E2 excitatory effects on guinea pig urinary bladder smooth muscle: a novel regulatory mechanism mediated by large-conductance voltage- and Ca2+-activated K+ channels

Prostaglandin E2 (PGE2) is an essential signaling molecule involved in the regulation of detrusor smooth muscle (DSM) function. However, the underlying regulatory mechanism by which PGE2 augments DSM cell excitability and contractility is not well understood. Here, we investigated whether PGE2 inhibits the large conductance voltage- and Ca(2+)-activated K(+) (BK) channels in guinea pig DSM, thereby increasing DSM excitability and contractility. We used a multidisciplinary experimental approach including amphotericin-B perforated patch-clamp electrophysiology and live-cell Ca(2+) imaging in native freshly-isolated DSM cells, isometric tension recordings of intact DSM strips, and pharmacological tools to investigate BK channel regulation by PGE2 in guinea pig DSM. PGE2 increased the spontaneous phasic contractions of isolated DSM strips in a concentration-dependent manner (10 nM-10 µM). BK channel inhibition with paxilline (1 µM) attenuated the PGE2-induced DSM phasic contractions, suggesting that BK channels are involved in the mechanism of PGE2-induced DSM contractions. PGE2 (10 µM) increased the intracellular Ca(2+) levels in freshly-isolated DSM cells. PGE2 (10 µM) also caused an inhibition of the amplitude and frequency of spontaneous transient BK currents in DSM cells. Moreover, PGE2 (10 µM) did not affect the amplitude of whole cell steady-state BK currents in DSM cells. Our findings provide strong experimental evidence that PGE2 leads to an inhibition of the spontaneous transient BK currents, elevation of intracellular Ca(2+) levels in freshly-isolated DSM cells, and augmentation of DSM phasic contractions. Thus, we have revealed a novel mechanism that BK channels mediate PGE2-induced contractions in guinea pig DSM.

Large-conductance voltage- and Ca2+-activated K+ channel regulation by protein kinase C in guinea pig urinary bladder smooth muscle

Large-conductance voltage- and Ca(2+)-activated K(+) (BK) channels are critical regulators of detrusor smooth muscle (DSM) excitability and contractility. PKC modulates the contraction of DSM and BK channel activity in non-DSM cells; however, the cellular mechanism regulating the PKC-BK channel interaction in DSM remains unknown. We provide a novel mechanistic insight into BK channel regulation by PKC in DSM. We used patch-clamp electrophysiology, live-cell Ca(2+) imaging, and functional studies of DSM contractility to elucidate BK channel regulation by PKC at cellular and tissue levels. Voltage-clamp experiments showed that pharmacological activation of PKC with PMA inhibited the spontaneous transient BK currents in native freshly isolated guinea pig DSM cells. Current-clamp recordings revealed that PMA significantly depolarized DSM membrane potential and inhibited the spontaneous transient hyperpolarizations in DSM cells. The PMA inhibitory effects on DSM membrane potential were completely abolished by the selective BK channel inhibitor paxilline. Activation of PKC with PMA did not affect the amplitude of the voltage-step-induced whole cell steady-state BK current or the single BK channel open probability (recorded in cell-attached mode) upon inhibition of all major Ca(2+) sources for BK channel activation with thapsigargin, ryanodine, and nifedipine. PKC activation with PMA elevated intracellular Ca(2+) levels in DSM cells and increased spontaneous phasic and nerve-evoked contractions of DSM isolated strips. Our results support the concept that PKC activation leads to a reduction of BK channel activity in DSM via a Ca(2+)-dependent mechanism, thus increasing DSM contractility.

Ryanodine receptors as leak channels

Ryanodine receptors are Ca(2+) release channels of internal stores. This review focuses on those situations and conditions that transform RyRs from a finely regulated ion channel to an unregulated Ca(2+) leak channel and the pathological consequences of this alteration. In skeletal muscle, mutations in either CaV1.1 channel or RyR1 results in a leaky behavior of the latter. In heart cells, RyR2 functions normally as a Ca(2+) leak channel during diastole within certain limits, the enhancement of this activity leads to arrhythmogenic situations that are tackled with different pharmacological strategies. In smooth muscle, RyRs are involved more in reducing excitability than in stimulating contraction so the leak activity of RyRs in the form of Ca(2+) sparks, locally activates Ca(2+)-dependent potassium channels to reduce excitability. In neurons the enhanced activity of RyRs is associated with the development of different neurodegenerative disorders such as Alzheimer and Huntington diseases. It appears then that the activity of RyRs as leak channels can have both physiological and pathological consequences depending on the cell type and the metabolic condition.

NS309 decreases rat detrusor smooth muscle membrane potential and phasic contractions by activating SK3 channels

BACKGROUND AND PURPOSE

Overactive bladder (OAB) is often associated with abnormally increased detrusor smooth muscle (DSM) contractions. We used NS309, a selective and potent opener of the small or intermediate conductance Ca(2+) -activated K(+) (SK or IK, respectively) channels, to evaluate how SK/IK channel activation modulates DSM function.

EXPERIMENTAL APPROACH

We employed single-cell RT-PCR, immunocytochemistry, whole cell patch-clamp in freshly isolated rat DSM cells and isometric tension recordings of isolated DSM strips to explore how the pharmacological activation of SK/IK channels with NS309 modulates DSM function.

KEY RESULTS

We detected SK3 but not SK1, SK2 or IK channels expression at both mRNA and protein levels by RT-PCR and immunocytochemistry in DSM single cells. NS309 (10 μM) significantly increased the whole cell SK currents and hyperpolarized DSM cell resting membrane potential. The NS309 hyperpolarizing effect was blocked by apamin, a selective SK channel inhibitor. NS309 inhibited the spontaneous phasic contraction amplitude, force, frequency, duration and tone of isolated DSM strips in a concentration-dependent manner. The inhibitory effect of NS309 on spontaneous phasic contractions was blocked by apamin but not by TRAM-34, indicating no functional role of the IK channels in rat DSM. NS309 also significantly inhibited the pharmacologically and electrical field stimulation-induced DSM contractions.

CONCLUSIONS AND IMPLICATIONS

Our data reveal that SK3 channel is the main SK/IK subtype in rat DSM. Pharmacological activation of SK3 channels with NS309 decreases rat DSM cell excitability and contractility, suggesting that SK3 channels might be potential therapeutic targets to control OAB associated with detrusor overactivity.

Apamin modulates electrophysiological characteristics of the pulmonary vein and the Sinoatrial Node

BACKGROUND

Small-conductance calcium-activated potassium (SK) channels play an important role in atrial electrophysiology. Blocking SK channels prolongs action potential (AP) duration and attenuate electrical remodelling. The effects of SK blocking on the pulmonary vein (PV) and the sinoatrial node (SAN) remain unclear.

MATERIALS AND METHODS

Conventional microelectrodes were used to record the AP in SANs and in isolated rabbit PVs with and without denudation before and after treatment with apamin (1, 10 nM). Using the whole-cell patch-clamp technique, the SK current was investigated in isolated single PV and SAN myocytes.

RESULTS

In nondenudated PVs (n = 6), apamin (1 and 10 nM) increased PV spontaneous activity (from 1.8 ± 0.2 Hz to 2.4 ± 0.2 Hz and 2.3 ± 0.2 Hz, P < 0.05) and increased the PV tension by 42 ± 14% and 37 ± 11%. Conversely, in denudated PVs (n = 6), apamin (1 and 10 nM) decreased spontaneous activity (from 2.2 ± 0.3 Hz to 1.9 ± 0.2 Hz and 1.8 ± 0.2 Hz, P < 0.05) and prolonged AP duration without changing PV tensions. Additionally, apamin (1 and 10 nM) decreased spontaneous activity (from 2.8 ± 0.1 Hz to 2.4 ± 0.1 Hz, and 2.4 ± 0.1 Hz, P < 0.05) and prolonged AP duration in SAN (n = 6). SAN myocytes had larger SK currents than did PV cardiomyocytes. Treatment with apamin (10 nM) resulted in a greater decrease (25 ± 8 vs. 20 ± 13%, P < 0.05) in spontaneous activity in isolated single SAN (n = 17) vs. PV cardiomyocytes (n = 16).

CONCLUSION

Small-conductance calcium-activated potassium channels play a key role in SAN and PV spontaneous activity and AP morphology. Apamin may modulate PV electrical activity with and without endothelium dependence.

BRL37344, a β3-adrenergic receptor agonist, decreases nerve-evoked contractions in human detrusor smooth muscle isolated strips: role of BK channels

OBJECTIVE

To investigate the mechanism by which BRL37344, a β3-adrenergic receptor (β3-ARs) agonist, facilitates the inhibition of nerve-evoked contractions in human detrusor smooth muscle (DSM) isolated strips and to identify the role of large-conductance Ca(2+)-activated K(+) (BK) channels in this process.

METHODS

Human DSM specimens were obtained from open bladder surgeries on patients without preoperative history of overactive bladder symptoms. Isometric DSM tension recordings were conducted using force-displacement transducers and thermostatically controlled tissue baths. Nerve-evoked contractions were generated by electrical field stimulation (EFS).

RESULTS

BRL37344, a β3-AR agonist, significantly decreased the amplitude, muscle force, and duration of the DSM contractions induced by 20 Hz EFS, in a concentration-dependent manner. This BRL37344-mediated inhibition of the amplitude and muscle force of the nerve-evoked DSM contraction was significantly reduced by iberiotoxin, a highly selective inhibitor of the BK channel, revealing a role for BK channels in the β3-AR-induced inhibition of human DSM nerve-evoked contractions. We further used atropine, α,β-methylene-ATP, and suramin to separate the cholinergic and purinergic components of human DSM nerve-evoked contractions. We found that the β3-AR agonist, BRL37344, inhibited both components of the EFS-induced (0.5-50 Hz) DSM contractions.

CONCLUSION

This study supports the concept that β3-AR agonists inhibit nerve-evoked contractions in human DSM. We have further revealed that BK channels play a critical role in BRL37344-mediated relaxation of nerve-evoked contractions in human DSM. The study suggests that in addition to β3-ARs, BK channels may also represent promising pharmacologic targets in the treatment of urinary bladder dysfunction.

Overactive bladder mediated by accelerated Ca2+ influx mode of Na+/Ca2+ exchanger in smooth muscle

The Na(+)/Ca(2+) exchanger (NCX) is thought to be a key molecule in the regulation of cytosolic Ca(2+) dynamics. The relative importance of the two Ca(2+) transport modes of NCX activity leading to Ca(2+) efflux (forward) and influx (reverse) in smooth muscle, however, remains unclear. Unexpectedly, spontaneous contractions of urinary bladder smooth muscle (UBSM) were enhanced in transgenic mice overexpressing NCX1.3 (NCX1.3(tg/tg)). The enhanced activity was attenuated by KB-R7943 or SN-6. Whole cell outward NCX current sensitive to KB-R7943 or Ni(2+) was readily detected in UBSM cells from NCX1.3(tg/tg) but not wild-type mice. Spontaneous Ca(2+) transients in myocytes of NCX1.3(tg/tg) were larger and frequently resulted in propagating events and global elevations in cytosolic Ca(2+) concentration. Significantly, NCX1.3(tg/tg) mice exhibited a pattern of more frequent urination of smaller volumes and this phenotype was reversed by oral administration of KB-R7943. On the other hand, KB-R7943 did not improve it in KB-R7943-insensitive (G833C-)NCX1.3(tg/tg) mice. We conclude that NCX1.3 overexpression is associated with abnormal urination owing to enhanced Ca(2+) influx via reverse mode NCX leading to prolonged, propagating spontaneous Ca(2+) release events and a potentiation of spontaneous UBSM contraction. These findings suggest the possibility that NCX is a candidate molecular target for overactive bladder therapy.

Activation of muscarinic M3 receptors inhibits large-conductance voltage- and Ca2+-activated K+ channels in rat urinary bladder smooth muscle cells

Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are key regulators of detrusor smooth muscle (DSM) contraction and relaxation during urine voiding and storage. Here, we explored whether BK channels are regulated by muscarinic receptors (M-Rs) in native freshly isolated rat DSM cells under physiological conditions using the perforated whole cell patch-clamp technique and pharmacological inhibitors. M-R activation with carbachol (1 μM) initially evoked large transient outward BK currents, followed by inhibition of the spontaneous transient outward BK currents (STBKCs) in DSM cells. Carbachol (1 μM) also inhibited the amplitude and frequency of spontaneous transient hyperpolarizations (STHs) and depolarized the DSM cell membrane potential. Selective inhibition of the muscarinic M3 receptors (M3-Rs) with 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; 0.1 μM), but not muscarinic M2 receptors with methoctramine (1 μM), blocked the carbachol inhibitory effects on STBKCs. Furthermore, blocking the inositol 1,4,5-triphosphate (IP3) receptors with xestospongin-C (1 μM) inhibited the carbachol-induced large transient outward BK currents without affecting carbachol inhibitory effects on STBKCs. Upon pharmacological inhibition of all known cellular sources of Ca(2+) for BK channel activation, carbachol (1 μM) did not affect the voltage-step-induced steady-state BK currents, suggesting that the muscarinic effects in DSM cells are mediated by mobilization of intracellular Ca(2+). In conclusion, our findings provide strong evidence that activation of M3-Rs leads to inhibition of the STBKCs, STHs, and depolarization of DSM cells. Collectively, the data suggest the existence of functional interactions between BK channels and M3-Rs at a cellular level in DSM.

Fourier transform analysis of rabbit detrusor autonomous contractions reveals length dependent increases in tone and slow wave development at long lengths

PURPOSE

Bladder wall muscle (detrusor) develops low amplitude rhythmic contractions. Low amplitude rhythmic contraction activity is increased in detrusor from patients with overactive bladder. In this in vitro study we used fast Fourier transforms to assess the length dependence of low amplitude rhythmic contraction components.

MATERIALS AND METHODS

Rabbit detrusor strips were placed in a muscle bath between 2 clips to adjust length and record isometric tension. Tissues stretched from 70% to 130% of a reference muscle length at 10% increments were allowed to develop low amplitude rhythmic contractions at each length for 20 minutes. Low amplitude rhythmic contraction data were analyzed using fast Fourier transforms and represented by a frequency rather than a time spectrum.

RESULTS

Based on fast Fourier transform analysis summarized by signal peaks within specific frequency ranges, rabbit low amplitude rhythmic contraction waveforms were divided into 1 tonic and 2 phasic components, defined as A0 + A1F1 + A2F2, where A0 is a length dependent basal tonic component that increases linearly, A1F1 is a slow wave with a length dependent specific amplitude (A1) and a length independent constant frequency (F1) of approximately 11.2 Hz, and A2F2 is a fast wave with a length dependent amplitude (A2) and frequency (F2) of approximately 0.03 Hz.

CONCLUSIONS

Fast Fourier transform analysis revealed that rabbit low amplitude rhythmic contractions consist of a basal tonic component plus 2 phasic components. The amplitude of all 3 components was length dependent. The frequency of the fast component was not length dependent and the slow component was absent at short muscle lengths, developing only at muscle lengths beyond that producing a maximum active contraction.

Analysis of Ca(2+)-binding sites in the MthK RCK domain by X-ray crystallography

Regulator of K(+) conductance (RCK) domains form a conserved class of ligand-binding domains that control the activity of a variety of prokaryotic and eukaryotic K(+) channels. Structural analysis of these domains by X-ray crystallography has provided insight toward mechanisms underlying ligand binding and channel gating, and thus the experimental strategies aimed at determining structures of liganded and unliganded forms of the domains may be useful in analysis of other ligand-binding domains. Here, we describe a basic strategy for crystallographic analysis of the RCK domain from the MthK channel, for determination of its Ca(2+)-bound structure.

Spontaneous and evoked contractions are regulated by PKC-mediated signaling in detrusor smooth muscle: involvement of BK channels

Protein kinase C (PKC) and large conductance Ca(2+)-activated potassium channels (BK) are downregulated in the detrusor smooth muscle (DSM) in partial bladder outlet obstruction (PBOO). DSM from these bladders display increased spontaneous activity. This study examines the involvement of PKC in the regulation of spontaneous and evoked DSM contractions and whether pharmacologic inhibition of PKC in normal DSM contributes to increased detrusor excitability. Results indicate the PKC inhibitor bisindolylmaleimide 1 (Bim-1) prevented a decline in the amplitude of spontaneous DSM contractions over time in vitro, and these contractions persist in the presence of tetrodotoxin. Bim-1 also reduced the basal DSM tone, and the ability to maintain force in response to electrical field stimulation, but did not affect maximum contraction. The PKC activator phorbol-12,13-dibutyrate (PDBu) significantly reduced the amplitude and increased the frequency of spontaneous contractions at low concentrations (10 nM), while causing an increase in force at higher concentrations (1 μM). Preincubation of DSM strips with iberiotoxin prevented the inhibition of spontaneous contractions by PDBu. The BK channel openers isopimaric acid and NS1619 reduced the Bim-1-induced enhancement of spontaneous contractions in DSM strips. Our data suggest that PKC has a biphasic activation profile in the DSM and that it may play an important role in maintaining the quiescent state of the normal bladder during storage through the effects on BK channel, while helping to maintain force required for bladder emptying. The data also suggest that PKC dysfunction, as seen in PBOO, contributes to detrusor overactivity.

SK channel-selective opening by SKA-31 induces hyperpolarization and decreases contractility in human urinary bladder smooth muscle

Overactive bladder (OAB) is often associated with increased involuntary detrusor smooth muscle (DSM) contractions during the bladder-filling phase. To develop novel therapies for OAB, it is critical to better understand the mechanisms that control DSM excitability and contractility. Recent studies showed that small-conductance Ca(2+)-activated K(+) (SK) channels, SK3 channels, in particular, regulate human DSM function. However, the concept that SK channel-selective pharmacological activation can decrease the excitability and contractility directly in human DSM needs further exploration. Here, we studied the effect of the novel and potent SK channel activator, SKA-31 (or naphtho [1,2-d]thiazol-2-ylamine), on human DSM excitability and contractility at the cellular and tissue level. We used isometric tension recordings on human DSM-isolated strips and the perforated patch-clamp technique on freshly isolated native human DSM cells. SKA-31 significantly decreased spontaneous phasic contractions of DSM-isolated strips. In the presence of the SK channel blocker, apamin, the inhibitory effects of SKA-31 on the DSM spontaneous phasic contractions were significantly reduced. SKA-31 decreased the carbachol- and KCl-induced contractions in human DSM strips. Electrical field stimulation-induced contractions were significantly attenuated in the presence of SKA-31 at all stimulation frequencies (0.5-50 Hz). SKA-31 hyperpolarized the resting membrane potential of human DSM cells. Apamin abolished the hyperpolarizing effect of SKA-31, indicating the involvement of SK channel activation. These results support the concept that pharmacological activation of SK channels with selective openers may represent an attractive new pharmacological approach for decreasing DSM excitability and contractility, thus controlling OAB.

Selective inhibition of phosphodiesterase 1 relaxes urinary bladder smooth muscle: role for ryanodine receptor-mediated BK channel activation

The large conductance voltage- and Ca(2+)-activated K(+) (BK) channel is a major regulator of detrusor smooth muscle (DSM) excitability and contractility. Recently, we showed that nonselective phosphodiesterase (PDE) inhibition reduces guinea pig DSM excitability and contractility by increasing BK channel activity. Here, we investigated how DSM excitability and contractility changes upon selective inhibition of PDE type 1 (PDE1) and the underlying cellular mechanism involving ryanodine receptors (RyRs) and BK channels. PDE1 inhibition with 8-methoxymethyl-3-isobutyl-1-methylxanthine (8MM-IBMX; 10 μM) increased the cAMP levels in guinea pig DSM cells. Patch-clamp experiments on freshly isolated DSM cells showed that 8MM-IBMX increased transient BK currents and the spontaneous transient hyperpolarization (STH) frequency by ∼2.5- and ∼1.8-fold, respectively. 8MM-IBMX hyperpolarized guinea pig and human DSM cell membrane potential and significantly decreased the intracellular Ca(2+) levels in guinea pig DSM cells. Blocking BK channels with 1 μM paxilline or inhibiting RyRs with 30 μM ryanodine abolished the STHs and the 8MM-IBMX inhibitory effects on the DSM cell membrane potential. Isometric DSM tension recordings showed that 8MM-IBMX significantly reduced the spontaneous phasic contraction amplitude, muscle force integral, duration, frequency, and tone of DSM isolated strips. The electrical field stimulation-induced DSM contraction amplitude, muscle force integral, and duration were also attenuated by 10 μM 8MM-IBMX. Blocking BK channels with paxilline abolished the 8MM-IBMX effects on DSM contractions. Our data provide evidence that PDE1 inhibition relaxes DSM by raising cellular cAMP levels and subsequently stimulates RyRs, which leads to BK channel activation, membrane potential hyperpolarization, and decrease in intracellular Ca(2+) levels.

Identification of T-type calcium channels in the interstitial cells of Cajal in rat bladder

OBJECTIVE

To investigate the expression and function of T-type calcium channels in the interstitial cells of Cajal in rat bladders.

METHODS

Bladders were harvested from Sprague-Dawley rats. The expression of T-type calcium channels subtypes (α1G, α1H, and α1I) in interstitial cells of Cajal were identified by double-labeled immunofluorescence analysis and reverse transcription-polymerase chain reaction analysis in whole mount preparations of rat bladders. The function of T-type calcium channels in freshly isolated interstitial cells of Cajal was assessed by detecting the changes of intracellular calcium ([Ca(2+)](i)) with preloading fluo-3 AM, and by evaluating the changes of the phasic contractions of rat bladder strips after treating with mibefradil and glivec.

RESULTS

Three T-type calcium channels subtypes, α1G, α1H, and α1I, colocalized with c-kit in bladder interstitial cells of Cajal by double-labeled immunofluorescence analysis, and this was confirmed using reverse transcription-polymerase chain reaction. The T-type calcium channels selective blocker, mibefradil (1 μM), significantly decreased the intracellular calcium concentration ([Ca(2+)](i)) in isolated interstitial cells of Cajal (P < .01) and inhibited the spontaneous phasic contraction of bladder strips (P < .01). Moreover, the c-kit receptor blocker, glivec, significantly decreased the [Ca(2+)](i) of interstitial cells of Cajal further (P < .01) and the spontaneous phasic contraction of bladder strips.

CONCLUSION

T-type calcium channel subtypes were confirmed to colocalize in interstitial cells of Cajal in rats bladders, which might participate in the spontaneous activity of interstitial cells of Cajal and phasic contractions of bladder strips by modulating [Ca(2+)](i) in interstitial cells of Cajal.

SK but not IK channels regulate human detrusor smooth muscle spontaneous and nerve-evoked contractions

Animal studies suggest that the small (SK) and intermediate (IK) conductance Ca(2+)-activated K(+) channels may contribute to detrusor smooth muscle (DSM) excitability and contractility. However, the ability of SK and IK channels to control DSM spontaneous phasic and nerve-evoked contractions in human DSM remains unclear. We first investigated SK and IK channels molecular expression in native human DSM and further assessed their functional role using isometric DSM tension recordings and SK/IK channel-selective inhibitors. Quantitative PCR experiments revealed that SK3 channel mRNA expression in isolated DSM single cells was ∼12- to 44-fold higher than SK1, SK2, and IK channels. RT-PCR studies at the single-cell level detected mRNA messages for SK3 channels but not SK1, SK2, and IK channels. Western blot and immunohistochemistry analysis further confirmed protein expression for the SK3 channel and lack of detectable protein expression for IK channel in whole DSM tissue. Apamin (1 μM), a selective SK channel inhibitor, significantly increased the spontaneous phasic contraction amplitude, muscle force integral, phasic contraction duration, and muscle tone of human DSM isolated strips. Apamin (1 μM) also increased the amplitude of human DSM electrical field stimulation (EFS)-induced contractions. However, TRAM-34 (1 μM), a selective IK channel inhibitor, had no effect on the spontaneous phasic and EFS-induced DSM contractions suggesting a lack of IK channel functional role in human DSM. In summary, our molecular and functional studies revealed that the SK, particularly the SK3 subtype, but not IK channels are expressed and regulate the spontaneous and nerve-evoked contractions in human DSM.

Mechanisms involved in testosterone-induced relaxation to the pig urinary bladder neck

OBJECTIVES

Testosterone replacement therapy improves bladder capacity in urinary tract dysfunction. There is no information, however, about the role of this steroid hormone on the muscle tension of the bladder outflow region. The current study investigated the mechanisms underlying the testosterone-induced action in the pig bladder neck.

METHODS

Urothelium-denuded bladder neck strips were mounted in myographs for isometric force recordings and for simultaneous measurements of intracellular Ca(2+) concentration ([Ca(2+)](i)) and tension. The relaxations to testosterone, the non-aromatizable metabolite 4,5α-dihydrotestosterone (DHT) and electrical field stimulation (EFS) were carried out on phenylephrine (PhE)-precontracted strips.

RESULTS

Testosterone and DHT evoked similar concentration-dependent relaxations only at very high pharmacological concentrations. The presence of the urothelium and the inhibition of intracellular androgenic receptor (AR), aromatase, 5α-reductase, nitric oxide (NO) synthase, guanylyl cyclase, cyclooxygenase (COX), large-, intermediate- and small-Ca(2+)-activated K(+) channels or ATP-dependent K(+) channels failed to modify the testosterone relaxations. Neuronal voltage-gated Ca(2+) (VOC) channels and voltage-gated K(+) (K(V)) channel blockers potentiated these responses. EFS evoked frequency-dependent relaxations, which were not changed by threshold concentrations of testosterone. In Ca(2+)-free potassium rich physiological saline solution, testosterone inhibited the contractions induced by CaCl(2) and the L-type VOC channel activator (±)-BAY K 8644. Relaxations elicited by testosterone were accompanied by simultaneous decreases in smooth muscle [Ca(2+)](i).

CONCLUSIONS

Testosterone produces relaxation of the pig urinary bladder neck through mechanisms independent of urothelium, AR, aromatase, 5α-reductase, NO synthase, guanylyl cyclase, COX and K(+) channels. Testosterone-induced relaxation is produced via the inhibition of the extracellular Ca(2+) entry through L-type VOC channels.

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.

Do β3-adrenergic receptors play a role in guinea pig detrusor smooth muscle excitability and contractility?

In many species, β3-adrenergic receptors (β3-ARs) have been reported to play a primary role in pharmacologically induced detrusor smooth muscle (DSM) relaxation. However, their role in guinea pig DSM remains controversial. The aim of this study was to investigate whether β3-ARs are expressed in guinea pig DSM and to evaluate how BRL37344 and L-755,507, two selective β3-AR agonists, modulate guinea pig DSM excitability and contractility. We used a combined experimental approach including RT-PCR, patch-clamp electrophysiology, and isometric DSM tension recordings. β3-AR mRNA message was detected in freshly isolated guinea pig DSM single cells. BRL37344 but not L-755,507 caused a slight decrease in DSM spontaneous phasic contraction amplitude and frequency in a concentration-dependent manner. In the presence of atropine (1 μM), only the spontaneous phasic contractions frequency was inhibited by BRL37344 at higher concentrations. Both BRL37344 and L-755,507 significantly decreased DSM carbachol-induced phasic and tonic contractions in a concentration-dependent manner. However, only BRL37344 inhibitory effect was partially antagonized by SR59230A (10 μM), a β3-AR antagonist. In the presence of atropine, BRL37344 and L-755,507 had no inhibitory effect on electrical field stimulation-induced contractions. Patch-clamp experiments showed that BRL37344 (100 μM) did not affect the DSM cell resting membrane potential and K(+) conductance. Although β3-ARs are expressed at the mRNA level, they play a minor to no role in guinea pig DSM spontaneous contractility without affecting cell excitability. However, BRL37344 and L-755,507 have pronounced inhibitory effects on guinea pig DSM carbachol-induced contractions. The study outlines important DSM β3-ARs species differences.

Role of potassium ion channels in detrusor smooth muscle function and dysfunction

Contraction and relaxation of the detrusor smooth muscle (DSM), which makes up the wall of the urinary bladder, facilitates the storage and voiding of urine. Several families of K(+) channels, including voltage-gated K(+) (K(V)) channels, Ca(2+)-activated K(+) (K(Ca)) channels, inward-rectifying ATP-sensitive K(+) (K(ir), K(ATP)) channels, and two-pore-domain K(+) (K(2P)) channels, are expressed and functional in DSM. They control DSM excitability and contractility by maintaining the resting membrane potential and shaping the action potentials that determine the phasic nature of contractility in this tissue. Defects in DSM K(+) channel proteins or in the molecules involved in their regulatory pathways may underlie certain forms of bladder dysfunction, such as overactive bladder. K(+) channels represent an opportunity for novel pharmacological manipulation and therapeutic intervention in human DSM. Modulation of DSM K(+) channels directly or indirectly by targeting their regulatory mechanisms has the potential to control urinary bladder function. This Review summarizes our current state of knowledge of the functional role of K(+) channels in DSM in health and disease, with special emphasis on current advancements in the field.

Pharmacological activation of small conductance calcium-activated potassium channels with naphtho[1,2-d]thiazol-2-ylamine decreases guinea pig detrusor smooth muscle excitability and contractility

Small conductance Ca²⁺-activated K⁺ (SK) and intermediate conductance Ca(2+)-activated K⁺ (IK) channels are thought to be involved in detrusor smooth muscle (DSM) excitability and contractility. Using naphtho[1,2-d]thiazol-2-ylamine (SKA-31), a novel and highly specific SK/IK channel activator, we investigated whether pharmacological activation of SK/IK channels reduced guinea pig DSM excitability and contractility. We detected the expression of all known isoforms of SK (SK1-SK3) and IK channels at mRNA and protein levels in DSM by single-cell reverse transcription-polymerase chain reaction and Western blot. Using the perforated patch-clamp technique on freshly isolated DSM cells, we observed that SKA-31 (10 μM) increased SK currents, which were blocked by apamin (1 μM), a selective SK channel inhibitor. In current-clamp mode, SKA-31 (10 μM) hyperpolarized the cell resting membrane potential, which was blocked by apamin (1 μM) but not by 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34) (1 μM), a selective IK channel inhibitor. SKA-31 (10 nM-10 μM) significantly inhibited the spontaneous phasic contraction amplitude, frequency, duration, and muscle force in DSM isolated strips. The SKA-31 inhibitory effects on DSM contractility were blocked by apamin (1 μM) but not by TRAM-34 (1 μM), which did not per se significantly affect DSM spontaneous contractility. SK channel activation with SKA-31 reduced contractions evoked by electrical field stimulation. SKA-31 effects were reversible upon washout. In conclusion, SK channels, but not IK channels, mediate SKA-31 effects in guinea pig DSM. Pharmacological activation of SK channels reduces DSM excitability and contractility and therefore may provide a novel therapeutic approach for controlling bladder dysfunction.

Large conductance Ca2+ -activated K+ channel activation with NS1619 decreases myogenic and neurogenic contractions of rat detrusor smooth muscle

Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are important in regulating detrusor smooth muscle (DSM) function. Here, we examined systematically how the BK channel pharmacological activation modulates DSM contractility. NS1619, a potent BK channel activator, was utilized as a pharmacological tool to investigate the effect of BK channel activation on rat DSM contractility. Isometric tension recordings of DSM strips isolated from rat urinary bladder were performed systematically under various experimental conditions. NS1619 (30 μM) substantially diminished DSM spontaneous contraction amplitude, muscle force integral, frequency, duration and muscle tone. This effect was blocked by iberiotoxin, a BK channel selective inhibitor. NS1619 inhibited the phasic and tonic contractions in DSM strips pre-contracted with either the cholinergic agonist, carbachol (0.1 μM), or the depolarizing agent, KCl (20mM). In the presence of elevated KCl (60 mM KCl), the inhibitory effect of NS1619 was significantly reduced, indicating that BK channel activation is the underlying mechanism of NS1619 action. BK channel activation with NS1619 dramatically decreased the amplitude of electrical field stimulation (EFS)-induced contractions under a range of stimulation frequencies (0.5-50 Hz). In the presence of specific neurotransmitter inhibitors, BK channel activation with NS1619 significantly decreased both cholinergic and purinergic components of EFS-induced contractions. We conclude that BK channel activation with NS1619 significantly inhibited spontaneous, pharmacologically induced and nerve-evoked DSM contractions. Targeting the BK channel with selective openers may offer a unique opportunity to control DSM contractile activity, including pathophysiological conditions such as overactive bladder and detrusor overactivity, regardless of the underlying cause.