Effects of ATP-sensitive K+ channel openers and tolterodine on involuntary bladder contractions in a pig model of partial bladder outlet obstruction

Non-radioactive Rb+ Efflux Assay for Screening KATP Channel Modulators

ATP-sensitive potassium (KATP) channels function as metabolic sensors that link cell membrane excitability to the cellular energy status by controlling potassium ion (K+) flow across the cell membrane according to intracellular ATP and ADP concentrations. As such, KATP channels influence a broad spectrum of physiological processes, including insulin secretion and cardiovascular functions. KATP channels are hetero-octamers, consisting of four inward rectifier potassium channel subunits, Kir6.1 or Kir6.2, and four sulfonylurea receptors (SURs), SUR1, SUR2A, or SUR2B. Different Kir6 and SUR isoforms assemble into KATP channel subtypes with distinct tissue distributions and physiological functions. Mutations in the genes encoding KATP channel subunits underlie various human diseases. Targeted treatment for these diseases requires subtype-specific KATP channel modulators. Rubidium ions (Rb+) also pass through KATP channels, and Rb+ efflux assays can be used to assess KATP channel function and activity. Flame atomic absorption spectroscopy (Flame-AAS) combined with microsampling can measure Rb+ in small volume, which provides an efficient tool to screen for compounds that alter KATP channel activity in Rb+ efflux assays. In this chapter, we describe a detailed protocol for Rb+ efflux assays designed to identify new KATP channel modulators with potential therapeutic utilities.

Mechanism of Social Stress-Related Erectile Dysfunction in Mice: Impaired Parasympathetic Neurotransmission and Ketamine

This study aimed to investigate the mechanism underlying social stress (SS)-induced erectile dysfunction (ED) and evaluate the effects of a single subanesthetic dose of ketamine on SS-related ED. Male FVB mice were exposed to retired male C57BL/6 mice for 60 min daily over a 4-week period. In the third week, these FVB mice received intraperitoneal injections of either saline (SSS group) or ketamine (SSK group). Erectile function was assessed by measuring the intracavernosal pressure (ICP) during electrical stimulation of the major pelvic ganglia. Corpus cavernosum (CC) strips were utilized for wire myography to assess their reactivity. Both SSS and SSK mice exhibited significantly lower ICP in response to electrical stimulation than control mice. SS mice showed increased contractility of the CC induced by phenylephrine. Acetylcholine-induced relaxation was significantly reduced in SSS and SSK mice. Sodium nitroprusside-induced relaxation was higher in SSS mice compared to control and SSK mice. Nicotine-induced neurogenic and nitric oxide-dependent relaxation was significantly impaired in both SSS and SSK mice. An immunohistochemical analysis revealed co-localization of tyrosine hydroxylase and neuronal nitric oxide synthase-immunoreactive fibers in the CC. These findings highlight the complex nature of SS-related ED and suggest the limited efficacy of ketamine as a therapeutic intervention.

Benefits and limitations of animal models in partial bladder outlet obstruction for translational research

The functions of the lower urinary tract have been investigated for more than a century. Lower urinary tract symptoms, such as incomplete bladder emptying, weak urine stream, daytime urinary frequency, urgency, urge incontinence and nocturia after partial bladder outlet obstruction, is a frequent cause of benign prostatic hyperplasia in aging men. However, the pathophysiological mechanisms have not been fully elucidated. The use of animal models is absolutely imperative for understanding the pathophysiological processes involved in bladder dysfunction. Surgical induction has been used to study lower urinary tract functions of numerous animal species, such as pig, dog, rabbit, guinea pig, rat and mouse, of both sexes. Several morphological and functional modifications under partial bladder outlet obstruction have not only been observed in the bladder, but also in the central nervous system. Understanding the changes of the lower urinary tract functions induced by partial bladder outlet obstruction would also contribute to appropriate drug development for treating these pathophysiological conditions. In the present review, we discuss techniques for creating partial bladder outlet obstruction, the characteristics of several species, as well as issues of each model, and their translational value.

Research Findings on Overactive Bladder

Several physiopathologic conditions lead to the manifestation of overactive bladder (OAB). These conditions include ageing, diabetes mellitus, bladder outlet obstruction, spinal cord injury, stroke and brain injury, Parkinson's disease, multiple sclerosis, interstitial cystitis, stress and depression. This review has discussed research findings in human and animal studies conducted on the above conditions. Several structural and functional changes under these conditions have not only been observed in the lower urinary tract, but also in the brain and spinal cord. Significant changes were observed in the following areas: neurotransmitters, prostaglandins, nerve growth factor, Rho-kinase, interstitial cells of Cajal, and ion and transient receptor potential channels. Interestingly, alterations in these areas showed great variation in each of the conditions of the OAB, suggesting that the pathophysiology of the OAB might be different in each condition of the disease. It is anticipated that this review will be helpful for further research on new and specific drug development against OAB.

Phasic activity of urinary bladder smooth muscle in the streptozotocin-induced diabetic rat: effect of potassium channel modulators

Increased phasic activity in the bladder smooth muscle of animal models and patients with detrusor overactivity has been suggested to underlie the pathophysiology of overactive bladder. Potassium (K+) channels are key regulators of bladder smooth muscle tone and thus may play a role in this altered phasic activity. In this study the effects of K+ channel modulators on the phasic activity of bladder strips from the streptozotocin-induced diabetic rat model of bladder dysfunction were investigated. Bladder strips from rats 1 week following streptozotocin administration and age-matched controls were mounted in tissue baths at 37 °C and the effects of K+ channel modulators on resting basal tension or phasic activity induced by a low concentration of carbachol (0.5 μM) were investigated. Activation of BKCa channels by NS1619 had a minor inhibitory effect on carbachol-induced phasic activity of bladder strips from control and diabetic rats, and significantly inhibited amplitude only at 30 μM. Activation of KATP channels by cromakalim inhibited the frequency of carbachol-induced phasic activity of bladder strips, although strips from diabetic rats showed a trend towards being less sensitive to cromakalim. The BKCa channel blocker iberiotoxin was able to induce phasic activity in resting tissues, with diabetic bladder strips demonstrating significantly enhanced phasic activity compared to controls. In contrast, inhibition of SKCa and KATP channels did not induce phasic activity in resting tissues. In conclusion, responses of diabetic rat bladder to BKCa and KATP channel modulators are altered, suggesting altered function and/or expression of channels which may contribute to bladder dysfunction in this model.

Altered neurogenic and mechanical responses to acetylcholine, ATP and substance P in detrusor from rat with outlet obstruction

The well-known side effects of anticholinergic compounds used to treat urinary incontinence caused by detrusor overactivity have addressed the interest on other pharmacological intervention. The purpose of the present work was to investigate the possible changes in purinergic and cholinergic components of parasympathetic neurotransmission in obstructed rat bladders with detrusor overactivity, and to examine the effect of the association of suramin, atropine and indomethacin on nerve-mediated responses to electrical field stimulation (EFS). Mechanical responses to exogenous acetylcholine, ATP and substance P were also evaluated. Altered sensitivities to acetylcholine and to the sensory neurotransmitter substance P, but unchanged sensitivity to the stable ATP analogue alpha,beta-methyleneATP were observed in bladders from obstructed rats. Suramin and atropine inhibited purinergic and cholinergic components of the neurogenic responses evoked by EFS in detrusor strips from control and obstructed rats. Interestingly, suramin enhanced the antagonistic effect of atropine on neurogenic responses of detrusor strips at all frequencies of stimulation tested. Our results suggest that the association between an antimuscarinic drug and an antagonist of P2X purinoceptors such as suramin might be helpful to reduce the therapeutic dosage of the antimuscarinic drug, along with its side effects. This approach may be of interest in the therapy of patients with bladder incontinence caused by detrusor overactivity, which do not even respond to a maximal dosage of antimuscarinic drug.

Pharmacological characterization of urinary bladder smooth muscle contractility following partial bladder outlet obstruction in pigs

Partial bladder outlet obstruction of the pig is considered as a valuable preclinical model for evaluating the profile of compounds for the treatment of bladder overactivity. In this study, we characterized the pharmacological properties of isolated bladder smooth muscle from pigs following partial outlet obstruction and its sensitivity to potassium channel openers. Bladder strips from obstructed animals showed significantly lower maximal efficacy (E(max)) and sensitivity to stimulation by ATP and carbachol, but not to those evoked by serotonin, compared to age-matched controls. Tissue strips from obstructed animals also showed a 2.5-fold increase in the potency and significantly reduced maximum response following K+ depolarization. With respect to spontaneous activity, bladder strips from control strips demonstrated little spontaneous phasic activity at all preloads examined. In contrast, bladder strips from obstructed animals showed large preload-dependent increases in spontaneous phasic activity at preload values of 16-32 g. The potencies of K(ATP) channel openers to relax carbachol-evoked contractions showed a good 1:1 correlation (r(2)=0.90) between obstructed and control bladder strips. These studies demonstrate that obstructed pig bladders show enhanced spontaneous phasic activity especially at elevated preloads, which may underlie unstable myogenic bladder contractions reported in cystometrographic measurements in vivo. The impaired responses to electrical field stimulation could be attributed to reduced efficacies and/or lower sensitivities of muscarinic and purinergic signaling pathways. K(ATP) channel sensitivities remain essentially unimpaired in the obstructed bladder and could be effectively modulated by openers with potential for the treatment of overactive bladder secondary to outlet obstruction.

Animal models in urological disease and sexual dysfunction

There are several conditions associated with dysfunction of the lower urinary tract or which result in a reduction in the ability to engage in satisfactory sexual function and result in significant bother to sufferers, partners and/or carers. This review describes some of the animal models that may be used to discover safe and effective medicines with which to treat them. While alpha adrenoceptor antagonists and 5-alpha-reductase inhibitors deliver improvement in symptom relief in benign prostatic hyperplasia sufferers, the availability of efficacious and well-tolerated medicines to treat incontinence is less well served. Stress urinary incontinence (SUI) has no approved medical therapy in the United States and overactive bladder (OAB) therapy is limited to treatment with muscarinic antagonists (anti-muscarinics). SUI and OAB are characterised by high prevalence, a growing ageing population and a strong desire from sufferers and physicians for more effective treatment options. High patient numbers with low presentation rates characterizes sexual dysfunction in men and women. The introduction of Viagra in 1998 for treating male erectile dysfunction and the success of the phosphodiesterase type 5 inhibitor class (PDE5 inhibitor) have indicated the willingness of sufferers to seek treatment when an effective alternative to injections and devices is available. The main value of preclinical models in discovering new medicines is to predict clinical outcomes. This translation can be established relatively easily in areas of medicine where there are a large number of drugs with different underlying pharmacological mechanisms in clinical usage. However, apart from, for example, the use of PDE5 inhibitors to treat male erectile dysfunction and the use of anti-muscarinics to treat OAB, this clinical information is limited. Therefore, current confidence in existing preclinical models is based on our understanding of the biochemical, physiological, pathophysiological and psychological mechanisms underlying the conditions in humans and how they are reflected in preclinical models. Confidence in both the models used and the pharmacological data generated is reinforced if different models of related aspects of the same disorder generate confirmatory data. However, these models will only be fully validated in retrospect once the pharmacological agents they have helped identify are tested in humans.

K(ATP) channel therapeutics at the bedside

The family of potassium channel openers regroups drugs that share the property of activating adenosine triphosphate-sensitive potassium (K(ATP)) channels, metabolic sensors responsible for adjusting membrane potential-dependent functions to match cellular energetic demands. K(ATP) channels, widely represented in metabolically-active tissue, are heteromultimers composed of an inwardly rectifying potassium channel pore and a regulatory sulfonylurea receptor subunit, the site of action of potassium channel opening drugs that promote channel activity by antagonizing ATP-induced pore inhibition. The activity of K(ATP) channels is critical in the cardiovascular adaptive response to stress, maintenance of neuronal electrical stability, and hormonal homeostasis. Thereby, K(ATP) channel openers have a unique therapeutic spectrum, ranging from applications in myopreservation and vasodilatation in patients with heart or vascular disease to potential clinical use as bronchodilators, bladder relaxants, islet cell protector, antiepileptics and promoters of hair growth. While the current experience in practice with potassium channel openers remains limited, multitude of ongoing investigations aims at defining the benefit of this emerging family of therapeutics in diverse disease conditions associated with metabolic distress.

Effect of the ATP-sensitive potassium channel opener ZM226600 on cystometric parameters in rats with ligature-intact, partial urethral obstruction

The activity of a recent K(ATP) channel opener, the N-(4-Phenylsulfonylphenyl)-3,3,3-trifluoro-2-hydroxy-2-methylpropionamide (ZM226600) was investigated on a female rat model of overactive bladder with outlet obstruction. Both ZM226600 and pinacidil instilled into the bladder (10(-7) M, 30 min) or following systemic administration (10, 100 nmol/kg e.v.) almost completely abolished bladder overactivity and improved residual volume and frequency of micturition. However, pinacidil affected arterial pressure. Oxybutynin instilled into the bladder (10(-7), 10(-6), 10(-5) M, 30 min) decreased detrusor overactivity by about 16%, 25% and 46% respectively, but also blocked micturition reflexes at highest doses tested. Oxybutynin reduced detrusor overactivity by about 50% and 80%, after systemic administration (10, 100 nmol/kg e.v.), but also blocked micturition reflexes at the highest dose tested. In conclusion, ZM226600 is more active than oxybutynin in reducing bladder overactivity, and it is devoid of vascular side effects observed with pinacidil. Its short duration of action (about 1 h) is probably the main problem to solve, in order to consider this compound a valid alternative to antimuscarinics in the therapy of bladder overactivity.

(-)-(9S)-9-(3-Bromo-4-fluorophenyl)-2,3,5,6,7,9-hexahydrothieno[3,2-b]quinolin-8(4H)-one 1,1-dioxide (A-278637), a novel ATP-sensitive potassium channel opener: hemodynamic comparison to ZD-6169, WAY-133537, and nifedipine in the anesthetized canine

The therapeutic utility of KATP channel opening agents (KCOs) in the treatment of overactive bladder may be limited by hypotension as a result of insufficient selectivity in vivo for bladder versus vasculature smooth muscle. Recently, we demonstrated that the putative uroselective KCOs, A-278637, ZD-6169, and WAY-133537 suppress unstable bladder contraction in an in vivo pre-clinical pig model of detrusor instability secondary to partial outlet obstruction. In the present study in the anesthetized dog we targeted plasma concentrations 3-, 10-, and 30-fold above a common index of in vivo efficacy (EC35) for suppression of unstable bladder contraction in pigs, to provide a comprehensive cardiovascular profile of these compounds. When compared at similar multiples of efficacy, dose-dependent reductions in SVR were greater in ZD-6169 and WAY-133537-treated animals versus A-278637. A-278637, unlike ZD-6169 or WAY-133537, produced no effect on MAP at concentrations 10-fold above the EC35. At concentrations 30-fold above the EC35, MAP in A-278637-treated animals was reduced -11% from baseline versus -24% and -42% for ZD-6169 and WAY-133537. Accordingly, at plasma concentrations approximately 30-fold above the EC35 reflex-mediated increases in HR were modest for A-278637-treated animals (15% above baseline) versus ZD-6169 (22%) or WAY-133537 (35%). Increases in both dP/dt and cardiac output occurred at lower therapeutic multiples and were greater in magnitude for animals treated with WAY-133537 (66% and 64% above baseline, respectively, 60 minutes into compound infusion) and ZD-6169 (10% and 13%) versus A-278637 (-2% and 6%). Thus, A-278637 exerted lesser effects on cardiovascular function at equivalent multiples of the EC35 than either ZD-6169 or WAY-133537. These data suggest that A-278637 possesses a greater functional selectivity for urinary bladder versus vascular smooth muscle in vivo and that A-278637 may exhibit a more favorable therapeutic index than either ZD-6169 or WAY-133537.

In vitro and in vivo characterization of a novel naphthylamide ATP-sensitive K+ channel opener, A-151892

1. Openers of ATP-sensitive K(+) channels are of interest in several therapeutic indications including overactive bladder and other lower urinary tract disorders. This study reports on the in vitro and in vivo characterization of a structurally novel naphthylamide N-[2-(2,2,2-trifluoro-1-hydroxy-1-trifluoromethyl-ethyl)-naphthalen-1-yl]-acetamide (A-151892), as an opener of the ATP-sensitive potassium channels. 2. A-151892 was found to be a potent and efficacious potassium channel opener (KCO) as assessed by glibenclamide-sensitive whole-cell current and fluorescence-based membrane potential responses (-log EC(50)=7.63) in guinea-pig bladder smooth muscle cells. 3. Evidence for direct interaction with KCO binding sites was derived from displacement of binding of the 1,4-dihydropyridine opener [(125)I]A-312110. A-151892 displaced [(125)I]A-312110 binding to bladder membranes with a -log Ki value of 7.45, but lacked affinity against over 70 neurotransmitter receptor and ion channel binding sites. 4. In pig bladder strips, A-151892 suppressed phasic, carbachol-evoked and electrical field stimulus-evoked contractility in a glibenclamide-reversible manner with -log IC(50) values of 8.07, 7.33 and 7.02 respectively, comparable to that of the potencies of the prototypical cyanoguanidine KCO, P1075. The potencies to suppress contractions in thoracic aorta (-log IC(50)=7.81) and portal vein (-log IC(50)=7.98) were not substantially different from those observed for suppression of phasic contractility of the bladder smooth muscle. 5. In vivo, A-151892 was found to potently suppress unstable bladder contractions in obstructed models of unstable contractions in both pigs and rats with pED(35%) values of 8.05 and 7.43, respectively. 6. These results demonstrate that naphthylamide analogs exemplified by A-151892 are novel K(ATP) channel openers and may serve as chemotypes to exploit additional analogs with potential for the treatment of overactive bladder and lower urinary tract symptoms.