Stimulation of large-conductance calcium-activated potassium channels inhibits neurogenic contraction of human bladder from patients with urinary symptoms and reverses acetic acid-induced bladder hyperactivity in rats

Emerging therapies for overactive bladder: preclinical, phase I and phase II studies

INTRODUCTION

Overactive bladder syndrome is a common chronic condition with a significant impact on quality of life and economic burden. Persistence with pharmacologic therapy has been limited by efficacy and side effects. A greater understanding of the pathophysiology of overactive bladder has led to the initial evaluation of several drugs affecting ion channels, the autonomic nervous system, and enzymes which may provide useful alternatives for the management of overactive bladder.

AREAS COVERED

A comprehensive review was performed using PubMed and Cochrane databases as well as reviewing clinical trials in the United States. The current standard of care for overactive bladder will be discussed, but this paper focuses on investigational drugs currently in preclinical studies and phase I and II clinical trials.

EXPERT OPINION

Current therapies for overactive bladder have limitations in efficacy and side effects. A greater understanding of the pathophysiology of overactive bladder has identified the role(s) of other pathways in the overactive bladder syndrome. Targeting alternative pathways including ion channels and enzymes may provide alternative therapies of overactive bladder and a more tailored approach to the management of overactive bladder.

Inhibition of prejunctional parasympathetic pathways by β3-adrenoceptor agonists in the isolated pig detrusor: comparison with human detrusor studies

Adrenergic receptors of the β₃-subtype (β₃-ADRs) seem to represent a new target for a more effective pharmacological treatment of overactive bladder (OAB), a wide spread urinary disorder. A promising opportunity for OAB therapy might rely on the development of selective β₃-ADR agonists, but an appropriate preclinical screening, as well as investigation of their pharmacological mechanism(s), is limited by poor availability of human bladder samples and of translational animal models. In this study, we used the porcine urinary bladder as experimental tool to ascertain the functions of β₃-ADRs in the control the parasympathetic motor drive. Tritiated acetylcholine ([³H]-ACh), mainly originated from neural stores, was released by electrical field stimulation (EFS) in epithelium-deprived detrusor strips from pigs bred without estrogens. EFS produced simultaneously [³H]-ACh release and smooth muscle contraction allowing to asses neural (pre-junctional) and myogenic (postjunctional) effects in the same experiment. Isoprenaline and mirabegron produced on the EFS-evoked effects a concentration-dependent inhibition antagonized by L-748,337, a high selective β₃-ADR antagonist. The analysis of the resultant pharmacodynamic parameters supports the notion that in pig detrusors, as well as in previously described human detrusors, the activation of inhibitory β₃-ADRs can modulate neural parasympathetic pathways. In such inhibitory control, the involvement of membrane K⁺ channels, mainly of the SK type, seems to play a pivotal role similarly to what previously described in humans. Therefore, the isolated porcine detrusor can provide a suitable experimental tool to study the mechanisms underlying the clinical efficacy of selective β₃-ADR compounds for human use.

Large‐Conductance Calcium‐Activated Potassium Channel Opener, NS1619, Protects Against Mesenteric Artery Remodeling Induced by Agonistic Autoantibodies Against the Angiotensin II Type 1 Receptor

Background

Agonistic autoantibodies against the angiotensin II type 1 receptor (AT1‐AAs) extensively exist in patients with hypertensive diseases and have been demonstrated to play crucial roles in the pathophysiological process of vascular remodeling. However, the treatment options are limited. The large‐conductance calcium‐activated potassium (BK) channel is a critical regulator and potential therapeutic target of vascular tone and architecture. We have previously observed that AT1‐AAs have an inhibitory effect on BK channels. However, whether BK channel dysfunction is involved in AT1‐AAs‐induced vascular remodeling and the therapeutic effect of BK channel opener is unclear.

Methods and Results

In our study, mesenteric arteries from AT1‐AAs‐positive rats exhibited increased wall thickness, narrowing of the arteriolar lumen, and increased collagen accumulation. Patch clamp test results showed that the voltage sensitivity of BK channel declined in mesenteric arteriolar smooth muscle cells from AT1‐AAs‐positive rats. Experiments with freshly isolated mesenteric arteriolar smooth muscle cells showed that AT1‐AAs reduced the opening probability, open levels, open dwell time, and calcium sensitivity of BK channel. Experiments with HEK293T cells transfected with GFP‐ZERO‐BK α‐subunit plasmids suggested a BK channel α‐subunit‐dependent mechanism. BK channel α‐subunit deficient, namely KCNMA1^−/− rats showed a phenotype of mesenteric artery remodeling. The administration of NS1619, a specific BK channel opener targeting the α‐subunit, reversed the phenotypic transition and migration induced by AT1‐AAs in cultured mesenteric arteriolar smooth muscle cells. Finally, perfusion of NS1619 significantly relieved the pathological effects induced by AT1‐AAs in vivo.

Conclusions

In summary, we provide compelling evidence that BK channel α‐subunit dysfunction mediates AT1‐AAs‐induced mesenteric artery remodeling. Preservation of BK channel activity may serve as a potential strategy for the treatment of AT1‐AAs‐induced maladaptive resistance artery remodeling.

The Large-Conductance, Calcium-Activated Potassium Channel: A Big Key Regulator of Cell Physiology

Large-conductance Ca²⁺-activated K⁺ channels facilitate the efflux of K⁺ ions from a variety of cells and tissues following channel activation. It is now recognized that BK channels undergo a wide range of pre- and post-translational modifications that can dramatically alter their properties and function. This has downstream consequences in affecting cell and tissue excitability, and therefore, function. While finding the “silver bullet” in terms of clinical therapy has remained elusive, ongoing research is providing an impressive range of viable candidate proteins and mechanisms that associate with and modulate BK channel activity, respectively. Here, we provide the hallmarks of BK channel structure and function generally, and discuss important milestones in the efforts to further elucidate the diverse properties of BK channels in its many forms.

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.

L-cysteine/hydrogen sulfide pathway induces cGMP-dependent relaxation of corpus cavernosum and penile arteries from patients with erectile dysfunction and improves arterial vasodilation induced by PDE5 inhibition

The aim was to evaluate and characterize H₂S-induced relaxation of human corpus cavernosum (HCC) and penile resistance arteries (HPRA) from patients with erectile dysfunction (ED). HCC and HPRA were obtained from men with ED at the time of penile prosthesis insertion. H₂S-mediated relaxations were evaluated by exposing these tissues to the stable analogue, NaHS, and to the precursor of H₂S, L-cysteine (CYS). The effects of NaHS and CYS were also evaluated on cGMP accumulation in HCC and on acetylcholine- and sildenafil-mediated relaxations in HCC and HPRA. NaHS consistently relaxed HPRA and HCC and more potently than human prostate and bladder. NaHS-induced relaxations in HCC and HPRA were unaffected by the ATP-sensitive K⁺-channel blocker, glibenclamide or the NO synthase inhibitor, L-NAME, slightly reduced by the Ca²⁺-activated K⁺-channel blocker, tetraethylammonium, and markedly inhibited by the soluble guanylyl cyclase inhibitor, ODQ. NaHS caused a cGMP increase in HCC that was inhibited by ODQ. CYS produced relaxations of HCC and HPRA that were sensitive to ODQ and to inhibition of the H₂S synthesizing enzymes, cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS). CYS also increased cGMP in HCC. In contrast to NaHS, CYS-induced relaxations were prevented by endothelium removal in HPRA. Only in HPRA, treatment with CYS (30 μM) potentiated acetylcholine- and sildenafil-induced relaxations. This effect was prevented by CSE/CBS inhibition and by removing the endothelium. Exogenous and endogenous H₂S relaxes HCC and HPRA from ED patients through cGMP accumulation and potentiates vasodilatory capacity of PDE5 inhibition, supporting the therapeutic potential of modulating H₂S pathway.

Intestinal, urinary and uterine antispasmodic effects of isoespintanol, metabolite from Oxandra xylopioides leaves

BACKGROUND

Isoespintanol is a monoterpene isolated from the leaves of Oxandra xylopioides Diels. (Annonaceae) with antioxidant and antiinflammatory effects. It was of interest to know whether it has antispasmodic effects such as other known drugs, phloroglucinol and trimethoxybenzene, used in therapeutics for treating biliary, urinary and uterine spasms.

PURPOSE

To assess whether isoespintanol possesses antispasmodic effects on intestine, uterus and bladder.

STUDY DESIGN

A preclinical study was performed in which isoespintanol, phloroglucinol and trimethoxybenzene were evaluated with concentration-contractile response curves (CRC) of carbachol in isolated rat intestine and bladder, and with CRC of serotonin (5-HT) in rat uterus. Moreover, it was assessed whether isoespintanol interferes with Ca²⁺ influx by making CRC of Ca²⁺ in high-K⁺ medium in intestine and bladder.

RESULTS

Isoespintanol non-competitively inhibited the CRC of carbachol with affinity constant (pK) of 4.78 ± 0.09 in intestine and 4.60 ± 0.09 in bladder. Phloroglucinol and trimethoxybenzene were also non-competitive antagonists, but isoespintanol was 8 times more potent than trimethoxybenzene and similarly potent than phloroglucinol in intestine. In bladder, isoespintanol resulted 8 times more potent than trimethoxybenzene. The maximal inhibition of contraction followed the order of isoespintanol > trimethoxybenzene > phloroglucinol in intestine, and isoespintanol > trimethoxybenzene in bladder. Moreover, isoespintanol also completely and non-competitively inhibited the CRC of Ca²⁺, with a pK of 5.1 ± 0.1 in intestine, and 4.32 ± 0.07 in bladder. In uterus isoespintanol reduced, completely and non-competitively, the contraction produced by 5-HT with pK of 5.05 ± 0.07.

CONCLUSION

Results demonstrate that isoespintanol is a very good intestinal, urinary and uterine antispasmodic, with higher potency than the other drugs used in therapeutics. The mechanism of action of isoespintanol is the interference with Ca²⁺ influx, at a difference of trimethoxybenzene and phloroglucinol.

Mitochondrial protein Fus1/Tusc2 in premature aging and age-related pathologies: critical roles of calcium and energy homeostasis

Decreased energy production and increased oxidative stress are considered to be major contributors to aging and aging-associated pathologies. The role of mitochondrial calcium homeostasis has also been highlighted as an important factor affecting different pathological conditions. Here, we present evidence that loss of a small mitochondrial protein Fus1 that maintains mitochondrial homeostasis results in premature aging, aging-associated pathologies, and decreased survival. We showed that Fus1KO mice develop multiple early aging signs including lordokyphosis, lack of vigor, inability to accumulate fat, reduced ability to tolerate stress, and premature death. Other prominent pathological changes included low sperm counts, compromised ability of adult stem cells to repopulate tissues, and chronic inflammation. At the molecular level, we demonstrated that mitochondria of Fus1 KO cells have low reserve respiratory capacity (the ability to produce extra energy during sudden energy demanding situations), and show significantly altered dynamics of cellular calcium response.

Our recent studies on early hearing and memory loss in Fus1 KO mice combined with the new data presented here suggest that calcium and energy homeostasis controlled by Fus1 may be at the core of its aging-regulating activities. Thus, Fus1 protein and Fus1-dependent pathways and processes may represent new tools and targets for anti-aging strategies.

Molecular structure and function of big calcium-activated potassium channels in skeletal muscle: pharmacological perspectives

The large-conductance Ca2+-activated K+ (BK) channel is broadly expressed in various mammalian cells and tissues such as neurons, skeletal muscles (sarco-BK), and smooth muscles. These channels are activated by changes in membrane electrical potential and by increases in the concentration of intracellular calcium ion (Ca2+). The BK channel is subjected to many mechanisms that add diversity to the BK channel α-subunit gene. These channels are indeed subject to alternative splicing, auxiliary subunits modulation, posttranslational modifications, and protein-protein interactions. BK channels can be modulated by diverse molecules that may induce either an increase or decrease in channel activity. The linkage of these channels to many intracellular metabolites and pathways, as well as their modulation by extracellular natural agents, have been found to be relevant in many physiological processes. BK channel diversity is obtained by means of alternative splicing and modulatory β- and γ-subunits. The association of the α-subunit with β- or with γ-subunits can change the BK channel phenotype, functional diversity, and pharmacological properties in different tissues. In the case of the skeletal muscle BK channel (sarco-BK channel), we established that the main mechanism regulating BK channel diversity is the alternative splicing of the KCNMA1/slo1 gene encoding for the α-subunit generating different splicing isoform in the muscle phenotypes. This finding helps to design molecules selectively targeting the skeletal muscle subtypes. The use of drugs selectively targeting the skeletal muscle BK channels is a promising strategy in the treatment of familial disorders affecting muscular skeletal apparatus including hyperkalemia and hypokalemia periodic paralysis.

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.

Emerging drugs for overactive bladder

INTRODUCTION

Overactive bladder (OAB) is a common problem which can have disastrous effects on the quality of life of the sufferer. There are established treatments for the problem but they have significant adverse effects. Better drugs and new treatment modalities are necessary to deal with OAB.

AREA COVERED

Antimuscarinics, mirabegron and intravesical injection of botulinum toxin A are established treatments for OAB. Sacral neuromodulation is more invasive but has been successful in treating OAB. Phase II and III trials are in progress for newer β3-agonists and various combinations of antimuscarinics, β3-agonists and antidiuretics. Targeted secretion inhibitors (TSI) can increase efficacy and reduce adverse effects. Liposome integrated botulinum toxin A has an advantage of effective administration by intravesical instillation. Both medicines are in Phase II trials. Many other drugs which have promising results are discussed.

EXPERT OPINION

Newer antimuscarinics have better tolerability. Long-term data for mirabegron has shown that it is more effective in severe OAB. Combination drugs may prove to be more effective with less adverse effects. Emerging treatments with TSI, lipotoxin and gene therapy appear promising.

Involvement of β3-adrenoceptors in the inhibitory control of cholinergic activity in human bladder: Direct evidence by [(3)H]-acetylcholine release experiments in the isolated detrusor

Bladder overactivity (OAB) is a multifactorial bladder disorder that requires therapeutics superior to the current pharmacological treatment with muscarinic antagonists. β3-adrenoceptor (β3-ADR) agonists represent a novel promising approach that differently addresses the parasympathetic pathway, but the clinical efficacy of these drugs has not been fully elucidated to date. Therefore, we aimed to study the pharmacological mechanisms activated by β3-ADR agonists at muscular and neural sites in the isolated human bladder. Detrusor smooth muscle strips obtained from male patients undergoing total cystectomy were labelled with tritiated choline and stimulated with electrical field stimulation (EFS). EFS produced smooth muscle contraction and simultaneous acetylcholine ([(3)H]-ACh) release, which mostly reflects the neural origin of acetylcholine. Isoprenaline (INA), BRL37344 and mirabegron inhibited the EFS-evoked contraction and [(3)H]-ACh release in a concentration-dependent manner, yielding concentration-response curves (CRCs) that were shifted to the right by the selective β3-ADR antagonists L-748,337 and SR59230A. Based on the agonist potency estimates (pEC50) and apparent affinities (pKb) of antagonists evaluated from the CRCs of agonists, our data confirm the occurrence of β3-ADRs at muscle sites. Moreover, our data are consistent with the presence of inhibitory β3-ADRs that are functionally expressed at the neural site. Taken together, these findings elucidate the mechanisms activated by β3-ADR agonists because neural β3-ADRs participate in the inhibition of detrusor motor drive by reducing the amount of acetylcholine involved in the cholinergic pathway.

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.