Comparison of the effects of several potassium-channel openers on rat bladder and rat portal vein in vitro

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.

Drugs Currently Undergoing Preclinical or Clinical Trials for the Treatment of Overactive Bladder: A Review

Background

Overactive bladder (OAB) is a common clinical condition for which current drug treatment comprises drugs blocking the cholinergic nerve supply, or augmenting the adrenergic nerve supply, to the detrusor muscle of the urinary bladder. Current treatments have drawbacks, including lack of efficacy and the development of adverse effects in some patients. Hence, new and better drugs for treating OAB will be clinically useful.

Objective

This review is meant to provide information on drugs currently undergoing preclinical or clinical trials for the treatment of OAB published in journal articles or elsewhere.

Methods

The cited articles were retrieved from PubMed and Google Scholar from January 1, 1990, to December 31, 2021. The search terms used were contraction or contractility, detrusor, inhibition, isolated or in vitro, in vivo, overactive bladder, and relaxant effect or relaxation.

Results

There are 4 classes of new drugs under various stages of development for the treatment of OAB. These are drugs acting on the autonomic nerve supply to the detrusor muscle of the urinary bladder that include the anticholinergics tarafenacin and afacifenacin and the β₃ adrenoceptor agonists solabegron and ritobegron; drugs acting on ion channels in the detrusor muscle (eg, potassium channel openers and calcium channel blockers), drugs acting on cellular enzymes like phosphodiesterase-5 inhibitors and Rho kinase inhibitors, and drugs acting on miscellaneous targets (eg, pregabalin and trimetazidine).

Conclusions

Drugs currently used to treat OAB target only the cholinergic and adrenergic cellular signalling pathways. There are many other drugs under trial targeting other cellular pathways that may be useful for treating OAB. Their approval for clinical use might improve the treatment of patients with OAB. (Curr Ther Res Clin Exp. 2022; 83:XXX–XXX)

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.

Effect of the novel BKCa channel opener LDD175 on the modulation of corporal smooth muscle tone

INTRODUCTION

The BKCa channel has been reported to play an important role in erectile function. Recently, novel BKCa channel activator, LDD175, was introduced.

AIM

This study aims to investigate whether LDD175 relaxes corporal smooth muscle (CSM) via BKCa channel activation.

METHODS

After isolation of CSM strip from a male rabbit model, contraction studies using organ bath was performed. Isolating human tissue and cell cultures, electrophysiological studies were done via whole-cell patch-clamp recording.

MAIN OUTCOME MEASURES

Vasodilatory effects of LDD175 were evaluated by cumulative addition ranging from 10(-7) to 10(-4) M in corpus cavernosal strips after precontraction with 10(-5) M phenylephrine via organ bath system. Using cultured human CSM cells, patch-clamp recording was performed. Erectile function was measured by in vivo rat cavernous nerve stimulation.

RESULTS

LDD175 caused an endothelium-independent relaxation of corporal tissues, and this effect was abolished by pretreatment with iberiotoxin. The relaxation effect of 10(-4) M LDD175 was greater than that of 10(-6) M udenafil (54.0 ± 3.1% vs. 34.5 ± 3.9%, P < 0.05); 10(-5) M LDD175 with 10(-6) M udenafil caused a greater relaxation effect on strips than 10(-5) M LDD175 or 10(-6) M udenafil alone (50.7%, 34.1%, vs. 20.7%, respectively, P < 0.001). In patch-clamp recordings, LDD175 increased K(+) currents in a dose-dependent manner, and washout of LDD175 or the addition of iberiotoxin fully reversed the increase. Intravenous LDD175 improved erectile function measured by area under the curve (AUC) of the intracavernosal pressure (ICP)/arterial blood pressure (ABP) ratio (1,612.1 ± 135.6 vs. 1,093.7 ± 123.1, P < 0.05). There was no difference between 10 mg/kg LDD175 and 1 mg/kg udenafil regarding maximal ICP, maximal ICP/ABP ratio, and the AUC of the ICP/ABP ratio (P > 0.05).

CONCLUSIONS

LDD175 leads to an endothelium-independent relaxation of erectile tissue, primarily through the opening of BKCa channels. The results suggest that LDD175 might be a new candidate treatment for erectile dysfunction.

Pharmacotherapy of overactive bladder syndrome

Urinary incontinence is a common and distressing condition that is known to adversely affect quality of life. Overactive bladder (OAB) is the term used to describe the symptom complex of urgency with or without urge incontinence, usually with frequency and nocturia. Drug therapy, in addition to behavioral modification, remains integral in the management of women with OAB, and the development of new drugs, treatment regimens and methods of delivery should improve patient compliance and acceptability. Developments over the last 10 years have led to the launch of several new drugs for the treatment of OAB that may offer greater efficacy while minimizing adverse effects. This article critically reviews the current pharmacological treatment of OAB in addition to providing a rationale for treatment.

New developments in the medical management of overactive bladder

Overactive bladder (OAB) is a clinical syndrome describing the symptom complex of urgency, with or without urgency incontinence and is usually associated with frequency and nocturia. Whilst the majority of women will benefit from initial management with conservative and behavioural intervention a significant number will require medical therapy. Antimuscarinics are currently the most widely prescribed drugs for OAB although very often persistence with medication is limited due to lack of efficacy or intolerable adverse effects. The review, whilst giving a brief overview of OAB syndrome, will focus on new developments in drug therapy for OAB. In addition to evaluating new drugs with different methods of action it will also specifically focus on alternative modalities of treatment and how they may benefit patients with this troublesome and distressing condition.

On benzofuroindole analogues as smooth muscle relaxants

At least two laboratories have independently reported the synthesis of benzofuroindole compounds having potential therapeutic implications in many disease states including those that involve smooth muscle hyperactivity. Through a series of in vitro screenings, they demonstrated the efficacy (and selectivity) of these compounds to potentiate large conductance calcium- (Ca²⁺-) activated K⁺ (BK_Ca) channels, by far, the most characterized of all Ca²⁺-dependent K⁺ channels. Interestingly, promising benzofuroindole derivatives such as compound 7 (10H-benzo[4,5]furo[3,2-b]indole) and compound 22 (4-chloro-7-trifluoromethyl-10H-benzo[4,5]furo[3,2-b]indole-1-carboxylic acid) both exhibited high bladder (versus aorta) selectivity, making them attractive alternative treatments for bladder overactivity. In recent reports, compound 22 (LDD175 or TBIC) also showed inhibition of ileum and uterine contractions, indicating multiple target tissues, which is not surprising as BK_Ca channels are ubiquitously expressed in the animal and human tissues. In this paper, the authors discuss the value of benzofuroindole compounds and the challenges that need to be overcome if they were considered as smooth muscle relaxants.

Levcromakalim and MgGDP activate small conductance ATP-sensitive K+ channels of K+ channel pore 6.1/sulfonylurea receptor 2A in pig detrusor smooth muscle cells: uncoupling of cAMP signal pathways

Pharmacological studies have suggested the existence of ATP-sensitive K(+) (K(ATP)) channel as a therapeutic target in urinary bladders; however, electrical properties have not yet been shown. Patch-clamp techniques were applied to investigate the properties of K(ATP) channels in pig detrusor cells. In whole-cell configuration, levcromakalim, a K(ATP) channel opener, induced a long-lasting outward current in a concentration-dependent manner. The current-voltage curve of the levcromakalim-induced membrane current intersected at approximately -80 mV. This current was abolished by glibenclamide. Intracellular application of 0.1 mM GDP significantly enhanced the levcromakalim-induced membrane current, whereas cAMP did not. Furthermore, neurotransmitters related to cAMP signaling, such as calcitonin gene-related peptide, vasointestinal peptide, adenosine, and somatostatin, had little effect on the membrane current. In cell-attached configuration, levcromakalim activated K(+) channels with a unitary conductance of approximately 12 pS. When the patch configuration was changed to inside-out mode, the K(+) channel activity ran down. Subsequent application of 1 mM GDP reactivated the channels. The openings of the approximately 12 pS K(+) channels in the presence of 1 mM GDP was suppressed by ATP and glibenclamide. In reverse transcription-polymerase chain reaction, K(+) channel pore 6.1 and sulfonylurea receptor (SUR)2A were predominant in pig detrusor cells. The 12 pS K(+) channel activated by levcromakalim in pig detrusor smooth muscle cells is a K(ATP) channel. The predominant expression of SUR2A can account for the lack of effect of neurotransmitters related to cAMP.

Signal transduction underlying the control of urinary bladder smooth muscle tone by muscarinic receptors and beta-adrenoceptors

The normal physiological contraction of the urinary bladder, which is required for voiding, is predominantly mediated by muscarinic receptors, primarily the M₃ subtype, with the M₂ subtype providing a secondary backup role. Bladder relaxation, which is required for urine storage, is mediated by β-adrenoceptors, in most species involving a strong β₃-component. An excessive stimulation of contraction or a reduced relaxation of the detrusor smooth muscle during the storage phase of the micturition cycle may contribute to bladder dysfunction known as the overactive bladder. Therefore, interference with the signal transduction of these receptors may be a viable approach to develop drugs for the treatment of overactive bladder. The prototypical signaling pathway of M₃ receptors is activation of phospholipase C (PLC), and this pathway is also activated in the bladder. Nevertheless, PLC apparently contributes only in a very minor way to bladder contraction. Rather, muscarinic-receptor-mediated bladder contraction involves voltage-operated Ca²⁺ channels and Rho kinase. The prototypical signaling pathway of β-adrenoceptors is an activation of adenylyl cyclase with the subsequent formation of cAMP. Nevertheless, cAMP apparently contributes in a minor way only to β-adrenoceptor-mediated bladder relaxation. BK_Ca channels may play a greater role in β-adrenoceptor-mediated bladder relaxation. We conclude that apart from muscarinic receptor antagonists and β-adrenoceptor agonists, inhibitors of Rho kinase and activators of BK_Ca channels may have potential to treat an overactive bladder.

Management of overactive bladder syndrome

Overactive bladder (OAB) syndrome is the term used to describe the symptom complex of urinary urgency with or without urge incontinence, usually with frequency and nocturia. Drug treatment continues to have an important role in the management of women with OAB. Other treatment options include conservative management with lifestyle interventions, modification of fluid intake, and physiotherapy including bladder retraining. Surgery remains the last resort in the treatment and is usually reserved for intractable detrusor overactivity, as it is associated with significant morbidity. This article reviews the management of the overactive bladder with specific focus on newer developments in the medical treatment of OAB in women.

Overactive bladder treatments in early phase clinical trials

'Overactive bladder' (OAB) is a syndrome that is characterised by symptoms of urgency, with or without urge urinary incontinence, usually with frequency and nocturia [1] . It is a highly prevalent condition affecting 17% of the general population, with a significant negative effect on quality of life, impairing several areas with physical, social, emotional and sexual limitations. The prevalence of OAB increases with age in both men and women [2,3] . The pathophysiology is multifactorial and not yet fully understood. Non-surgical treatment is the mainstay of therapy for OAB. The available options include biofeedback, electrical stimulation, bladder training, pharmacotherapy or a combination of these options. Nevertheless pharmacotherapy is still the treatment of choice for OAB symptoms [4] . The pharmacological treatment of OAB is generally directed towards the central or the peripheral neural control pathways or the detrusor muscle [5] . The antimuscarinic drugs are the most commonly used. In the US, approved antimuscarinics include oxybutynin, tolterodine, trospium chloride, solifenacin and darifenacin. Although this class of drugs has been shown to be more effective than placebo in specific meta-analyses [6] , it has been reported that < or = 80% of the patients discontinue the treatment within 6 months, mainly for the low drug compliance due to the high incidence of side effects [7] . Therefore, there is a strong need to identify drugs with novel mechanisms of action, which could provide equal or even better efficacy and overall greater acceptability than antimuscarinic drugs. At present, several other specific molecular targets identified within detrusor muscle and/or neural systems are under investigation for the development of more specific treatments of OAB. This article provides an up-to date review of drugs that are in investigational preclinical and early stage (Phase I and II) clinical trials for the treatment of OAB.

Actions of ZD0947, a novel ATP-sensitive K+ channel opener, on membrane currents in human detrusor myocytes

BACKGROUND AND PURPOSE

ATP-sensitive K+ channels (K(ATP) channels) play important roles in regulating the resting membrane potential of detrusor smooth muscle. Actions of ZD0947, a novel KATP channel opener, on both carbachol (CCh)-induced detrusor contractions and membrane currents in human urinary bladder myocytes were investigated.

EXPERIMENTAL APPROACH

Tension measurements and patch-clamp techniques were utilized to study the effects of ZD0947 in segments of human urinary bladder. Immunohistochemistry was also performed to detect the expression of the sulphonylurea receptor 1 (SUR1) and the SUR2B antigens in human detrusor muscle.

KEY RESULTS

ZD0947 (> or = 0.1 microM) caused a concentration-dependent relaxation of the CCh-induced contraction of human detrusor, which was reversed by glibenclamide. The rank order of the potency to relax the CCh-induced contraction was pinacidil > ZD0947 > diazoxide. In conventional whole-cell configuration, ZD0947 (> or = 1 microM) caused a concentration-dependent inward K+ current which was suppressed by glibenclamide at -60 mV. When 1 mM ATP was included in the pipette solution, application of pinacidil or ZD0947 caused no inward K+ current at -60 mV. Gliclazide (< or =1 microM), a selective SUR1 blocker, inhibited the ZD0947-induced currents (Ki = 4.0 microM) and the diazoxide-induced currents (high-affinity site, Ki1 = 42.4 nM; low-affinity site, Ki2 = 84.5 microM) at -60 mV. Immunohistochemical studies indicated the presence of SUR1 and SUR2B proteins, which are constituents of KATP channels, in the bundles of human detrusor smooth muscle.

CONCLUSIONS AND IMPLICATIONS

These results suggest that ZD0947 caused a glibenclamide-sensitive detrusor relaxation through activation of glibenclamide-sensitive KATP channels in human urinary bladder.

Investigation of the relaxant effects of pinacidil and cromakalim on the sheep sphincter of Oddi

BACKGROUND/AIMS

ATP-sensitive K+ (KATP) channels play an important role in the regulation of smooth muscle membrane potential. This study was designed to investigate the effects of pinacidil and cromakalim, KATP-sensitive channel activators, on sheep sphincters of Oddi (SO).

METHODS

SO rings were mounted in a tissue bath and tested for changes in isometric tension in response to pinacidil (10(-9)-10(-4)M) and cromakalim (10(-9)-10(-4)M) in the presence or absence of glibenclamide (10(-6)M), a blocker of KATP channels. Furthermore, concentration-dependent contraction responses of carbachol were obtained.

RESULTS

Carbachol (10(-9)-10(-5)M) induced concentration-dependent contraction responses in the SO rings. Pinacidil (10(-9)-10(-4)M) and cromakalim (10(-9)-10(-4)M) induced concentration-dependent relaxation in isolated SO rings precontracted with carbachol (10(-6)M). At their maximum effects, both pinacidil and cromakalim produced nearly full relaxation. In the presence of glibenclamide, concentration-relaxation curves for pinacidil and cromakalim underwent rightward parallel shifts. There were no significant differences between pEC50 and E(max) values of pinacidil and cromakalim in the absence of glibenclamide (10(-6)M) (p > 0.05), but pEC(50) values of pinacidil and cromakalim in the presence of glibenclamide (10(-6)M) were significantly reduced (p < 0.05).

CONCLUSION

These results suggest that the relaxation caused in sheep SO by pinacidil and cromakalim is mediated through the same glibenclamide-sensitive KATP channel. Pinacidil and cromakalim have an equipotent relaxing effect in isolated sheep SO and they can be beneficial as alternative drugs for obtaining selective relaxation during SO manometry after controlled clinical studies.

Pharmacological treatment for overactive bladder in women

Urinary incontinence is a common and distressing condition that is known to adversely affect quality of life. Overactive bladder is the term used to describe the symptom complex of urinary frequency and urgency, with or without urge incontinence. Drug therapy, in addition to behavioral modification, remains integral in the management of women with overactive bladder, and the development of new drugs, treatment regimens and methods of delivery should improve patient compliance and acceptability. This article critically reviews the current pharmacological treatment of overactive bladder in addition to providing a rationale for treatment.

BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine]: a putative potassium channel opener with bladder-relaxant properties

BL-1249 [(5,6,7,8-tetrahydro-naphthalen-1-yl)-[2-(1H-tetrazol-5-yl)-phenyl]-amine] produced a concentration-dependent membrane hyperpolarization of cultured human bladder myocytes, assessed as either a reduction in fluorescence of the voltage-sensitive dye bis-(1,2-dibutylbarbituric acid)trimethine oxonol (EC50 = 1.26 +/- 0.6 microM) or by direct electrophysiological measurement (EC50 = 1.49 +/- 0.08 microM). BL-1249 also produced a membrane hyperpolarization of acutely dissociated rat bladder myocytes. Voltage-clamp studies in human bladder cells revealed that BL-1249 activated an instantaneous, noninactivating current that reversed near E(K). The BL-1249-evoked outward K+ current was insensitive to blockade by glyburide, tetraethylammonium, iberiotoxin, 4-aminopyridine, apamin, or Mg2+. However, the current was inhibited by extracellular Ba2+ (10 mM). In in vitro organ bath experiments, BL-1249 produced a concentration-dependent relaxation of 30 mM KCl-induced contractions in rat bladder strips (EC50 = 1.12 +/- 0.37 microM), yet had no effect on aortic strips up to the highest concentration tested (10 microM). The bladder relaxation produced by BL-1249 was partially blocked by Ba2+ (1 and 10 mM) but not by apamin, iberiotoxin, 4-aminopyridine, glyburide, or tetraethylammonium. In an anesthetized rat model, BL-1249 (1 mg/kg i.v.) decreased the number of isovolumic contractions, without significantly affecting blood pressure. Thus, BL-1249 behaves as a potassium channel activator that exhibits bladder versus vascular selectivity both in vitro and in vivo. A survey of potassium channels exhibiting sensitivity to extracellular Ba2+ at millimolar concentration revealed that the expression of the K2P2.1 (TREK-1) channel was relatively high in human bladder cells versus human aortic cells, suggesting this channel as a possible candidate target for BL-1249.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

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

AIMS

To compare in vivo the efficacy, potency, and bladder-vascular selectivity of ATP-sensitive potassium channel openers (KCOs), YM934 and (-)-cromakalim to a muscarinic antagonist, tolterodine in a novel partial outlet obstructed pig model.

METHODS

Partially obstructed female Landrace pigs were implanted with telemetry transmitters to allow the continuous measurement of intravesical, abdominal and arterial pressures. A subcutaneous port catheter was used to adjust bladder volume. Bladder and arterial pressure were simultaneously monitored under isoflurane anesthesia before and after increasing i.v. doses of test compounds.

RESULTS

Under anesthesia, voiding was completely inhibited, but spontaneous, nonvoiding bladder contractions were observed with mean amplitude of 16 +/- 1 cm H(2)O, duration of 35 +/- 2 seconds, and intercontraction interval of 43 +/- 4 seconds (n = 25). YM934 and (-)-cromakalim both caused dose-dependent decreases in bladder contraction area under the curve (AUC) with effective doses to inhibit AUC by 35% of 3.6 and 14.9 nmol/kg, i.v., respectively. However, concomitant reductions in mean arterial pressure of 12 and 13% were also observed. Tolterodine did not inhibit spontaneous bladder contractions at doses up to 100 nmol/kg, i.v. corresponding to plasma concentrations up to 41 ng/mL.

CONCLUSIONS

The superior efficacy of KCOs to inhibit spontaneous bladder contractions relative to tolterodine support the hypothesis that KCOs may provide an alternate therapeutic mechanism to treat symptoms of overactive bladder if bladder-vascular selectivity can be sufficiently improved. The minimally invasive model described herein appears useful in the preclinical evaluation of potential therapeutics targeted to treat the overactive bladder.

Effects of ZD6169 and ZD0947, 2 potassium adenosine triphosphate channel openers, on bladder function of spinalized rats

PURPOSE

The use of K+ channel openers is emerging as an attractive possibility for treating bladder overactivity. We tested the efficacy of the 2 adenosine triphosphate dependent K channel openers ZD6169 and ZD0947 on detrusor hyperreflexia after spinal cord injury in rats.

MATERIALS AND METHODS

Included in this study were 72 adult Sprague-Dawley rats. Six animals served as normal controls, while 66 underwent spinal cord transection at the 10th thoracic vertebra. Two weeks after spinal cord injury 6 animals underwent filling cystometrography to confirm detrusor hyperreflexia, while another 12 served as control paraplegics. For each drug 24 animals were used and divided into 2 equal groups of 12. Group 1 received the drug in a dose of 3 mg./kg. daily, while group 2 received a dose of 0.3 mg./kg. daily. Each control paraplegic and treatment group was further subdivided into 2 subgroups of 6 rats. In subgroup 1 filling cystometrography was done 3 weeks after spinal cord injury, while in subgroup 2 it was done 4 weeks after spinal cord injury.

RESULTS

Three weeks after spinal cord injury detrusor hyperreflexia developed in all control paraplegic animals with a mean bladder capacity plus or minus standard deviation of 0.7 +/- 0.2 ml. and a mean voiding pressure of 59 +/- 14.2 cm. water. Detrusor hyperreflexia resolved in 66% of the animals that received ZD6169 for 1 week at either dose. For example, mean bladder capacity was 2.5 +/- 1.8 versus 1.8 +/- 1.2 ml. and mean voiding pressure was 42.1 +/- 15.9 versus 43.2 +/- 21.4 cm. water in animals that received 3 versus 0.3 mg./kg. daily, respectively. All animals that received a dose of 3 mg./kg. ZD0947 daily for 1 week showed no detrusor hyperreflexia with a mean bladder capacity of 2.7 +/- 1.8 ml. and mean voiding pressure of 34 +/- 8.5 cm. water, while at 0.3 mg./kg. daily 83% showed no detrusor hyperreflexia with a mean bladder capacity of 2.5 +/- 2.0 ml. and a mean voiding pressure of 41.5 +/- 13.8 cm. water. Each drug produced better urodynamic results when given for 2 weeks.

CONCLUSIONS

ZD6169 and ZD0947 are effective treatment for detrusor hyperreflexia after spinal cord injury and they may provide alternative treatment options for overactive bladder. Each drug has time and dose dependent response effects that reflect their wide range of efficacy. However, ZD0947 shows an efficacy profile that is relatively superior to that of ZD6169.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

A K(ATP) channel opener inhibited myocardial reperfusion action potential shortening and arrhythmias

Low concentrations of certain K(ATP) channel openers have been reported to exert a moderate inhibitory effect on arrhythmias during post-ischaemic early myocardial reperfusion, but the accompanying effects on the time course of changes in action potentials in intact hearts have not yet been studied. We report that in rat isolated hearts, reperfusion following 10 min of regional no-flow ischaemia was associated with both an acute, marked, but transient, shortening of ventricular repolarisation (by 63%) during reperfusion, and a high incidence (90%) of ventricular tachyarrhythmias. The K(ATP) channel opener Ro 31-6930 [2-(6-cyano-2,2-dimethyl-2H-1-benzopyran-4-yl)-pyridine 1-oxide], delivered prior to ischaemia at a relatively low concentration (0.5 microM), significantly reduced the incidence and duration of reperfusion arrhythmias, and prevented the associated acute action potential shortening during reperfusion, each in a glibenclamide (1 microM)-sensitive manner (P<0.05, n=10-15 hearts). This was associated with a moderate and non-arrhythmogenic action potential shortening during ischaemia (a potentially "cardioprotective" effect). However, these data highlight the potential harm these drugs may cause, since a higher concentration of Ro 31-6930 caused marked shortening of action potentials and significant pro-arrhythmia during ischaemia.

Low levels of K(ATP) channel activation decrease excitability and contractility of urinary bladder

Activation of ATP-sensitive potassium (K(ATP)) channels can regulate smooth muscle function through membrane potential hyperpolarization. A critical issue in understanding the role of K(ATP) channels is the relationship between channel activation and the effect on tissue function. Here, we explored this relationship in urinary bladder smooth muscle (UBSM) from the detrusor by activating K(ATP) channels with the synthetic compounds N-(4-benzoylphenyl)-3,3,3-trifluoro-2-hydroxy-2-methylpropionamide (ZD-6169) and levcromakalim. The effects of ZD-6169 and levcromakalim on K(ATP) channel currents in isolated UBSM cells, on action potentials, and on related phasic contractions of isolated UBSM strips were examined. ZD-6169 and levcromakalim at 1.02 and 2.63 microM, respectively, caused half-maximal activation (K1/2) of K(ATP) currents in single UBSM cells (see Heppner TJ, Bonev A, Li JH, Kau ST, and Nelson MT. Pharmacology 53: 170-179, 1996). In contrast, much lower concentrations (K(1/2) = 47 nM for ZD-6169 and K1/2 = 38 nM for levcromakalim) caused inhibition of action potentials and phasic contractions of UBSM. The results suggest that activation of <1% of K(ATP) channels is sufficient to inhibit significantly action potentials and the related phasic contractions.

Non-bacterial cystitis

The concept of non-bacterial cystitis (NBC) combines sterile urine and cystitic symptoms as well as inflammatory changes, in particular in the mucosa and submucosa of the bladder. It includes a multiplicity of vicious circles along the entire continence reflex. An understanding of NBC presupposes knowledge of the origin of the normal urinary urge and its successful control. Against the background of the steadily increasing incidence of interstitial cystitis (often irreversible end-stage NBC), it is suggested here that in the face of a failure of first-line therapeutics (anticholinergics, cyclic antidepressants or oestrogens), one must consider without delay the possible presence of NBC.

Vasorelaxant effects of the potassium channel opener SR 47063 on the isolated human saphenous vein and rat aorta

The vasorelaxant effects of SR 47063 (4-(2-cyanimino-1, 2-dihydropyrid-1-yl)-2,2-dimethyl-6-nitr ochromene), a new K(+)-channel opener structurally related to levcromakalim, were examined in isolated human saphenous vein (HSV) and rat aorta (RA). HSV or RA rings were precontracted with either KCl or noradrenaline and cumulative relaxant concentration-response curves were obtained for SR 47063 (0.1 nM to 1 microM) in the presence or absence of 3 microM glibenclamide. SR 47063 potently relaxed HSV and RA precontracted with 20 mM (but not 60 mM) KCl or 10 microM noradrenaline in a concentration-dependent manner, showing slightly greater activity in the aorta. The potency of the effect of SR 47063 on HSV and RA was 12- and 58-fold greater, respectively, than that reported for the structurally related K(+)-channel opener levcromakalim. The vasorelaxant action of SR 47063 in both blood vessels was strongly inhibited by 3 microM glibenclamide, consistent with a mechanism of action involving ATP-dependent K(+)-channels.

Pharmacological and molecular analysis of ATP-sensitive K(+) channels in the pig and human detrusor

The pharmacological and molecular properties of ATP-sensitive K(+) channels present in pig detrusor smooth muscle were investigated. In isolated pig detrusor strips, ATP-sensitive K(+) channel openers inhibited contractions elicited by low frequency field-stimulation in a concentration-dependent manner. The inhibitory effects of P1075 [N-cyano-N'-(1,1-dimethylpropyl)-N"-3-pyridylguanidine] were attenuated by glyburide with a pA(2) value of 7.38 (slope=1.08). The potency of the inhibitory effects of the K(+) channel openers on the field-stimulated contractions correlated well with those evoked by the muscarinic receptor agonist, carbachol (r=0.93) and furthermore, to relaxation of the pre-contracted (25 mM potassium chloride, KCl) human detrusor (r=0.95). Reverse transcriptase polymerase chain reaction (RT-PCR) analysis showed the presence of mRNA for sulfonylurea receptors SUR1 and SUR2B in both pig and human detrusor. Considering the similarities in the molecular and pharmacological profile of ATP-sensitive K(+) channels between the pig and the human detrusor, it is concluded that the pig detrusor may serve as a suitable in vitro model for the evaluation of novel K(+) channel openers with potential use in urological disorders in humans.

The effect of progesterone on spontaneous and agonist-evoked contractions of the rat aorta and portal vein

The mechanisms underlying the suppression of vasocontractility caused by progesterone were investigated by studying changes in the contractile force of rat isolated aorta and portal vein, induced by altering extracellular concentrations of noradrenaline (NA) potassium ions (K+) and calcium ions (Ca2+). In the aorta, progesterone (10 microM) had a general suppressive effect on NA-, Ca2+- and K+-induced contractions. In contrast, in the portal vein a more selective suppression of contractions was observed. Both tonic and phasic components of contractions induced by cumulative addition of Ca2+ to tissues equilibrated in Ca2+-free saline were suppressed. The phasic but not tonic components of contractions induced by NA addition were suppressed. There was no significant effect on tonic contractions induced by elevated (40-120 mM) K+, but a concentration-dependent suppression of the phasic component of contractions was observed during depolarisation with smaller elevations of K+ concentrations (5-20 mM). These results suggest that on the portal vein the suppressive effect of progesterone is due to a potassium channel opening action, whilst on the aorta a different or additional mechanism of suppression exists.

Novel potassium channel openers. Part 4: transformation of the 1,4-benzoxazine skeleton into 1,4-benzothiazine, 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroquinoxaline, indoline, and 1,5-benzoxazepine

As part of a search for a new potassium channel opener, the 1,4-benzoxazine skeleton derived from the benzopyran skeleton of cromakalim, was transformed into other fused rings such as 1,4-benzothiazine, 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroquinoxaline, indoline, and 1,5-benzoxazepine. The 1,4-benzothiazine derivative displayed approximately 20 times more potent vasorelaxant activity than cromakalim.

Effects of intravesical administration of the K+ channel opener, ZD6169, in conscious rats with and without bladder outflow obstruction

PURPOSE

To investigate the urodynamic effects of the new K(ATP) channel opener, ZD6169, given intravesically, in rats with and without bladder outflow obstruction.

MATERIALS AND METHODS

Female, conscious Sprague-Dawley rats, normal or with bladder hypertrophy and overactivity secondary to bladder outflow obstruction, were given ZD6169 intravesically (10 or 100 ng./ml.), and intra-arterially (1 mg./kg.). Continuous cystometry was performed.

RESULTS

In normal and obstructed rats, intravesical ZD6169 had similar, dose-dependent effects on bladder function. In obstructed rats, ZD6169 (100 ng./ml.) significantly decreased micturition pressure (17%), and increased bladder capacity (32%), micturition volume (18%), residual volume (145%), and inter-contraction interval (71%). There was a marked decrease in both frequency (40%) and amplitude (43%) of the spontaneous bladder activity. When given intra-arterially in obstructed rats ZD6169 increased bladder capacity (19%) and residual volume (47%) and decreased amplitude (51%), but not frequency, of the spontaneous bladder activity.

CONCLUSIONS

In both normal and obstructed rats, intravesical ZD6169, at the investigated doses, significantly affected bladder function. In obstructed rats, the drug markedly reduced bladder overactivity. If the results have clinical validity, intravesical ZD6169 may offer an alternative way of treating bladder overactivity in selected patients.

Biologically selective potassium channel openers having 1,1-diethylpropyl group

To find out selective potassium channel openers (PCOs), we synthesized several 3,5-disubstituted phenylcyanoguanidine derivatives and investigated their structure-activity relationships (SAR). As a result, we discovered selective PCOs having 1,1-diethylpropyl group toward antihypertensive activity.

Effects of ZD6169, a KATP channel opener, on bladder hyperactivity and spinal c-fos expression evoked by bladder irritation in rats

Cystometrographic recording and immunocytochemical techniques were used to examine the effects of ZD6169, an ATP-sensitive K+-channel opener, and capsaicin, an afferent neurotoxin, on urinary bladder hyperactivity and immediate early gene expression in the spinal cord induced by acetic acid (0.25%) irritation of the bladder. Chemical irritation of the bladder of the rat increased the frequency of voiding reflexes by 8 fold and increased c-fos expression in neurons in the dorsal commissure (DCM), sacral parasympathetic nucleus (SPN) as well as the medial and lateral dorsal horn (MDH, LDH) of L6 and S1 segments of the spinal cord. Pretreatment with ZD6169 (30 nM) for 1 h reduced the effect of acetic acid on voiding frequency as reflected by an increase in the intercontraction interval (ICI, 137+/-48% increase, P<0.05). ZD6169 also decreased the number of Fos positive neurons in the L6 spinal cord, in the DCM (62.1+/-7.1% decrease), SPN (48.8+/-7%), MDH (50+/-7.3%) and LDH regions (38. 8+/-10.5%). Similar reductions were noted in the S1 spinal cord: 65. 1+/-10.8% in DCM, 53.8+/-11% in SPN, 56+/-10.4% in MDH and 25.3+/-18. 1% in LDH. Capsaicin pretreatment (125 mg/kg, s.c., 4 days prior to the experiments) also reduced bladder hyperactivity (550% increase in ICI) and decreased the numbers of acetic acid-induced Fos positive neurons 78.8+/-6.3% in DCM, 73+/-7.8% in MDH, 59.2+/-16% in LDH and 45.2+/-17% in SPN of L6 segment of the spinal cord. These results suggest that ZD6169 can influence bladder hyperactivity by suppressing the firing of capsaicin-sensitive C-fiber bladder afferents which are known to modulate the micturition reflex.

Levcromakalim decreases vascular tone, cytoplasmic Ca2+ and Ca2+ sensitivity in canine basilar artery

The involvement of large conductance Ca(2+)-activated K+ channels (BK) and ATP-sensitive K+ (KATP) channels in the regulation of canine basilar arterial tone was estimated in the presence of the agonist and blockers of these channels, by simultaneously measuring the changes in intracellular Ca2+ concentration ([Ca2+]i) with the fura-2 microfluorimetric method. In the resting condition, levcromakalim reduced [Ca2+]i and vascular tone. Levcromakalim suppressed the serotonin-induced increases in [Ca2+]i and force of contraction, the maximum effects of which were much greater than those of nicardipine. The inhibitory effects of levcromakalim were blocked by glibenclamide but not by tetraethylammonium (TEA) or iberiotoxin (IbTX). In the presence of levcromakalim, the curve relating [Ca2+]i with force in the presence of serotonin at different extracellular Ca2+ concentration ([Ca2+]o) was shifted down- and right-ward compared with that in the absence of levcromakalim, suggesting that levcromakalim may reduce the Ca(2+)-sensitivity of the contractile proteins. Thus, levcromakalim may be a good candidate to suppress delayed cerebral vasospasm after subarachnoid hemorrhage.

Nitrergic relaxation of the horse corpus cavernosum. Role of cGMP

The involvement of nitric oxide (NO) and the mechanisms mediating neurogenic relaxation were investigated in the horse corpus cavernosum. NADPH-diaphorase activity was expressed in nerve fibres around arteries and muscular bundles in the horse trabecular tissue. Relaxations in response to electrical field stimulation were tetrodotoxin (10(-6) M)-sensitive, indicating their neurogenic origin. The NO synthase inhibitor, L-NO-arginine (L-NO-Arg, 3 x 10(-5) M), abolished the electrically induced relaxations, which were significantly reversed by L-arginine (3 x 10(-3) M). Exogenous NO (10(-6)-10(-3) M) evoked relaxations which were unaffected by L-NO-Arg. 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 5 x 10(-6) M), an inhibitor of guanylate cyclase activation by NO, reduced the relaxations in response to electrical stimulation and exogenous NO. Iberiotoxin (3 x 10(-8) M) or apamin (5 x 10(-7) M), inhibitors of large and small conductance Ca2+-activated K+ channels, respectively, and glibenclamide (3 x 10(-6) M), a blocker of ATP-sensitive K+ channels, failed to modify the relaxations with NO. It is suggested that NO is present in nerve fibres of the horse corpus cavernosum and relaxes smooth muscle through a guanylate cyclase-dependent mechanism. Neither Ca2+-activated nor ATP-sensitive K+ channels seem to be involved in these relaxations.

Potassium channels participate in gastric mucosal protection in rats with partial portal vein ligation

Glybenclamide, an adenosine triphosphate-dependent potassium (K+(ATP)) channel blocker, lowered portal pressure and attenuated the hyperdynamic splanchnic circulation in rats with partial portal vein ligation (PPVL). The purpose of this report was to confirm these observations and to test the hypothesis that glybenclamide could reduce acidified ethanol-induced gastric mucosal injury in rats with PPVL. Gastric mucosal blood flow (hydrogen gas clearance), systemic blood pressure, and portal pressure were monitored in rats with PPVL or sham operation (SO). Intravenous glybenclamide (20 mg/kg) or vehicle was administered, followed by intragastric acidified ethanol (0.15 N HCl and 15% ethanol). The area of gastric mucosal lesions was assessed by image analysis. In contrast to published findings, there was no significant elevation of portal pressure after glybenclamide administration in rats with PPVL. Glybenclamide did not alter the gastric mucosal hyperemia in these rats. Glybenclamide significantly increased mucosal injury. The data are consistent with the hypothesis that K+(ATP) channels play a role in protecting the gastric mucosa in rats with PPVL.

Effects of the K+ channel opener, ZD6169, on volume and PGE2-stimulated bladder activity in conscious rats

PURPOSE

To investigate the effects of the new K(ATP) channel opener, ZD6169, shown to have an in vivo selectivity for the bladder, on bladder activity in rats.

MATERIALS AND METHODS

ZD6169 was given intra-arterially (i.a., 0.1 and 1 mg./kg.) or orally (3 mg./kg.) to conscious Sprague-Dawley rats undergoing continuous cystometry. Investigations were also performed before and after stimulation of bladder activity by intravesical prostaglandin (PG) E2.

RESULTS

Intra-arterial ZD6169 increased residual volume, but caused no changes in other cystometric parameters. In rats receiving oral ZD6169, cystometric parameters were compared (every hour up to five hours) to those recorded in rats receiving oral vehicle. No differences were found, except in threshold pressure, which was significantly increased. Intravesical PGE2 20 microM increased micturition and basal pressures, and decreased bladder capacity and micturition volume. ZD6169 1 mg./kg., given i.a., reduced or completely prevented the activity induced by intravesical PGE2. Three hours after orally administered ZD6169 (3 mg./kg.), intravesical PGE2 20 microM had no effect. Three hours after oral administration of vehicle, the effects of PGE2 were attenuated, but still statistically significant.

CONCLUSIONS

ZD6169, given i.a. or orally, increased threshold pressure, but had otherwise little effect on volume-induced micturition. However, the drug markedly reduced or prevented PGE2-induced bladder activity when given i.a.; it was also effective when given orally. If ZD6169 has inhibiting effects on bladder contraction in man without any cardiovascular actions, the drug may represent a novel, promising way of treating bladder overactivity.

The overactive bladder: pharmacologic basis of drug treatment

OBJECTIVES

To provide an overview of the basis for drug treatment of the overactive bladder.

METHODS

Published information is evaluated.

RESULTS

The causes of bladder overactivity are not known, but theoretically, increased afferent activity, decreased inhibitory control in the central nervous system (CNS) or peripheral ganglia, and increased sensitivity of the detrusor to efferent stimulation may be involved. Several CNS transmitters can modulate voiding, but few useful drugs with a defined CNS site of action have been developed. Drugs that stimulate gamma-aminobutyric acid receptors are used clinically. Potentially, drugs affecting opioid, 5-hydroxytryptamine, norepinephrine, dopamine, and glutamatergic receptors and mechanisms can be developed, but a selective action on the lower urinary tract may be difficult to obtain. Traditionally, drugs used for treatment of bladder overactivity have had a peripheral site of action, mainly efferent neurotransmission or the detrusor itself. Antimuscarinic drugs, beta-adrenoceptor agonists, alpha-adrenoceptor antagonists, drugs affecting membrane channels, prostaglandin synthetase inhibitors, and several other agents have been used with limited success. New information on the alpha-adrenoceptor and muscarinic receptor subtypes in the human detrusor has emerged and may be the basis for the development of new compounds with effects on bladder overactivity. Decreasing afferent activity seems an attractive therapeutic approach, and drugs affecting afferent nerves by causing release of tachykinins, such as capsaicin and analogs, as well as agents blocking tachykinin receptors, may be of therapeutic interest.

CONCLUSIONS

New drugs, specifically designed for the treatment of bladder overactivity, are desirable.

Effect of cromakalim on the purinergic and cholinergic transmission in the rat detrusor muscle

Contraction of the rat detrusor muscle is mediated by cholinergic and purinergic mechanisms. The present study was carried out to look at the influence of cromakalim, compared with atropine, suramin and nifedipine on the contractile response evoked by single shock and exogenous agonists (carbachol and ATP) in rat urinary bladder. Cromakalim was able to inhibit only partially the response to carbachol and profoundly affected the response to exogenous ATP. Atropine suppressed the response to carbachol and was inactive versus ATP. Suramin was inactive versus carbachol and was able to antagonize the response to ATP. Nifedipine proved to be a non-competitive antagonist versus carbachol (pD2 = 7.66 +/- 0.05) and deeply affected the response to ATP. Cromakalim inhibited only partially the first, purinergic, phase of the electrically evoked response but was able to inhibit in a concentration-dependent manner the second, cholinergic, phase (logIC50 = 6.87 +/- 0.05). Nifedipine blocked both the phases. Atropine blocked partially only the second phase. Suramin inhibited the first phase but, at least partially, also the second one. The combination of atropine and suramin enhanced the inhibition of the second phase. The antagonistic effect of suramin on the second phase could indicate an overlap of the purinergic and cholinergic components. The comparison between pre- and postjunctional effects indicates that cromakalim acts on purinergic transmission predominantly postjunctionally. On the contrary, the action on cholinergic transmission seems to occur mainly at prejunctional level. This conclusion can be relevant in view of the claimed importance of K+ channel openers in the treatment of urinary disorders.

Smooth muscle of the bladder in the normal and the diseased state: pathophysiology, diagnosis and treatment

The smooth muscle of the normal bladder wall must have some specific properties. It must be very compliant and able to reorganise itself during filling and emptying to accommodate the change in volume without generating any intravesical pressure, but whilst maintaining the normal shape of the bladder. It must be capable of synchronous activation to generate intravesical pressure at any length to allow voiding. The cells achieve this through spontaneous electrical activity combined with poor electrical coupling between cells, and a dense excitatory innervation. In the diseased state, alterations of the smooth muscle may lead to failure to store or failure to empty properly. The diseased states discussed are bladder instability and diabetic neuropathy. Bladder instability is characterised urodynamically by uninhibitable rises in pressure during filling, and is seen idiopathically and in association with bladder outflow obstruction and neuropathy. In diabetic neuropathy, many of the smooth muscle changes are a consequence of diuresis, but there is evidence for alterations in the sensory arm of the micturition reflex. In the unstable bladder, additional alterations of the smooth muscle are seen, which are probably caused by the patchy denervation that occurs. The causes of this denervation are not fully established. Nonsurgical treatment of instability is not yet satisfactory; neuromodulation has some promise, but is expensive, and the mechanisms poorly understood. Pharmacological treatment is largely through muscarinic receptor blockade. Drugs to reduce the excitability of the smooth muscle are being sought, since they may represent a better pharmacological option.

Involvement of a glibenclamide-sensitive mechanism in the nitrergic neurotransmission of the pig intravesical ureter

1. The present study was designed to investigate whether potassium (K+) channels are involved in the relaxations to nitric oxide (NO) of pig intravesical ureteral preparations suspended in organ baths for isometric tension recordings. In ureteral strips treated with guanethidine (10(-5) M) and atropine (10(-7) M) to block adrenergic neurotransmission and muscarinic receptors, respectively, NO was either released from nitrergic nerves by electrical field stimulation (EFS, 0.5-10 Hz., 1 ms duration, 20 s trains), or exogenously-applied as an acidified solution of sodium nitrite (NaNO2, 10(-6)-10(-3) M). 2. Incubation with an inhibitor of guanylate cyclase activation by NO, methylene blue (10(-5) M) did not change the basal tension of intravesical ureteral strips but inhibited the relaxation induced by EFS or exogenous NO on ureteral preparations contracted with the thromboxane analogue U46619 (10(-7) M). 3. Incubation with charybdotoxin (3 x 10(-8) M) and apamin (5 x 10(-7) M), which are inhibitors of large and small conductance calcium (Ca2+)-activated K+ channels, respectively, did not modify basal tension or the relaxations induced by EFS and exogenous NO. Treatment with charybdotoxin or apamin plus methylene blue (10(-5) M) significantly reduced the relaxations to EFS and exogenous NO. However, in both cases the reductions were similar to the inhibition evoked by methylene blue alone. The combined addition of charybdotoxin plus apamin did not change the relaxations to EFS or exogenously added NO of the porcine intravesical ureter. 4. Cromakalim (10(-8) 3 x 10(-6) M), an opener of ATP-sensitive K+ channels, evoked a dose-dependent relaxation with a pD2 of 7.3 +/- 0.2 and maximum relaxant effect of a 71.8 +/- 4.2% of the contraction induced by U46619 in the pig intravesical ureter. The blocker of ATP-sensitive K+ channels, glibenclamide (10(-6) M), inhibited markedly the relaxations to cromakalim. 5. Glibenclamide (10(-6) M) had no effect on the basal tone of ureteral preparations but significantly reduced the relaxations induced by both EFS and exogenous NO. Combined treatment with methylene blue (10(-5) M) and glibenclamide (10(-6) M) did not exert an effect greater than that of methylene blue alone on either EFS- or NO-evoked relaxations of the pig ureter. 6. The present results suggest that NO acts as an inhibitory neurotransmitter in the pig intravesical ureter and relaxes smooth muscle through a guanylate cyclase-dependent mechanism which seems to favour the opening of glibenclamide-sensitive K+ channels.

Endothelium-mediated and N omega-nitro-L-arginine methyl ester-sensitive responses to cromakalim and diazoxide in the rat mesenteric bed

The effects of two 'K+ channel openers', (+/-)-6-cyano-3,4-dihydro-2,2-dimethyl-trans-4-(2-oxo-1-pyrrolidyl )-2 H-benzo[b]-pyran-3-ol (cromakalim) and 7-chloro-3-methyl-2 H-1,2,4-benzothiadiazine 1,1-dioxide (diazoxide), were studied on the rat isolated mesenteric bed. Differences in the perfusion pressure were measured as a parameter of vascular resistance. Cromakalim (0.1-700 microM) and diazoxide (1 microM-1 mM) reduced to 60% the contractions elicited by 10 microM noradrenaline and to 30% those evoked by 100 mM KCl. The relaxant effects of cromakalim and diazoxide on the noradrenaline-induced contractions were reduced by the K(+)-ATP channel blocker, 5-chloro-N-[2-[4-[[[(cyclohexylamino) carbonyl]amino]-sulfonyl]phenyl]ethyl]-2-methoxybenzamide (glibenclamide, 0.01-0.3 microM), endothelium removal with 0.1% saponin and pretreatment with the nitric oxide synthesis inhibitor, S(+/-)-N5-[imino(nitroamino)methyl]-L-ornithine methyl ester hydrochloride (L-NAME, 500 microM). Reductions in the relaxant responses after endothelium removal or L-NAME pretreatment were observed with 1-100 microM cromakalim and with 30 microM diazoxide but not with 100 and 300 microM diazoxide. Pretreatment with the inactive stereoisomer D-NAME as well as with the prostanoid synthesis inhibitor, 1-[p-chlorobenzoyl]-5-methoxy-2-methylindole-3-acetic acid (indomethacin, 10 microM), did not affect the reductions in contractile responses to noradrenaline caused by either cromakalim or diazoxide. It is concluded that the relaxant effects of cromakalim and diazoxide in the rat mesenteric bed are endothelium-mediated and L-NAME-sensitive and could at least partially involve the participation of nitric oxide.

Pharmacological characterization of the inwardly-rectifying current in the smooth muscle cells of the rat bladder

1. In freshly-isolated single cells of the rat bladder detrusor, outwardly-rectifying and inwardly-rectifying membrane currents were identified by the whole-cell voltage-clamp technique. 2. The inwardly-rectifying current (IIR) exhibited features of a cation current permeable to both K+ and Na+ but it was unaffected by changes in extracellular Ca2+. It had an activation threshold close to -60 mV and an estimated reversal potential of -29 mV. 3. IIR activated slowly with a voltage-sensitive time-constant of 69 ms at -140 mV and 209 ms at -100 mV but did not exhibit time-dependent inactivation. 4. IIR was unaffected by tetraethylammonium (up to 20 mM) but it was reduced by extracellular Ba2+ (1 mM) and by extracellular Cs+ (1 mM). 5. IIR was reduced by terikalant (100 microM) and markedly inhibited by ciclazindol (100 microM) although at these concentrations, both agents also reduced outward currents. 6. IIR was inhibited by ZD7288 (10-100 microM) in a concentration-dependent manner. At concentrations up to 30 microM, ZD7288 did not reduce the magnitude of outward currents but these were inhibited by 100 microM ZD7288. 7. In strips of bladder detrusor, spontaneous mechanical activity was increased by ZD7288 (0.3-100 microM) and by ciclazindol (0.3-100 microM) but was unaffected by glibenclamide (1-10 microM). 8. It is concluded that IIR closely resembles the hyperpolarization-activated current Ih, previously described in the smooth muscle of rabbit jejunum and in a variety of other cell types. This current may play an important role in modulating detrusor excitability but this could not be confirmed using the inhibitors ZD7288 and ciclazindol.

Comparative effects of K+ channel modulating agents on contractions of rat intestinal smooth muscle

The effects of six K+ channel openers were investigated on contractions of the rat ileum longitudinal muscle-myenteric plexus preparation elicited by electrical field stimulation and by K+. Levcromakalim, pinacidil, RP 49356 (N-methyl-2-(3 pyridyl)-tetrahydrothiopyran-2-carbothioamide-1-oxide) and SDZ PCO 400 ((3S,4R)-3, 4-dihydro-3-hydroxy-2, 2-dimethyl-4-[(3-oxo-1-cyclopenten-1-yl)oxy]-2H-1-benzopyran-6-car bonitrile) completely abolished contractions elicited by electrical stimulation and caused complete relaxation of contractions elicited by K+ with comparable IC50 values. Minoxidil sulphate was much less potent and diazoxide was without effect in either protocol. The relaxant effects of these agents were antagonized by glibenclamide, tetraethylammonium and yohimbine in a manner which was not surmountable. The present study indicates that the relaxant effect of these compounds in intestinal smooth muscle is mediated through glibenclamide-sensitive ATP-dependent K+ channels. These compounds did not preferentially inhibit either direct smooth muscle- or nerve-mediated responses. The present data may point to differences in the channels or their regulatory sites, in intestinal, compared with vascular, smooth muscle.

Leaky urothelium and/or vesical ischemia enable urinary potassium to cause idiopathic urgency/frequency syndrome and urge incontinence

Urine contains up to 10 times more potassium (K+) than blood plasma. Hence, extracellular K+ concentration of the bladder wall can increase secondary to a leaky urothelium (GAG layer deficiency) and/or vesical ischemia (reduced washout) at low filling volumes. Consequent sensory afferentiated excitation/depolarization of the detrusor leads to urgency/frequency and facilitates the onset of 'uninhibited' contractions. This feature, in association with a weak rhabdosphincter, causes urge incontinence. The non-neuromuscular (non-reflexive) origin explains refractoriness to any neurotransmitted inhibition. Even successful interference with contractility (Ca2+) leaves depolarization unaffected. Accordingly, comparative cystometry (saline versus 0.2 M KCl) is recommended in order to comprise better former falsely under-diagnosed 'normals' as well as former undiscovered urge incontinence, and thus indications for bladder neck surgery as well as neuromuscular drug treatment. Future first-line therapy in idiopathic storage disorders should be directed to the GAG layer, vesical blood flow (K+ washout) and the rhabdosphincter.

Potassium channel activation: a potential therapeutic approach?

The physiological role of K+ channel opening by endogenous substances (e.g., neurotransmitters and hormones) is a recognised inhibitory mechanism. Thus, the identification of novel synthetic molecules that 'directly' open K+ channels has led to a new direction in the pharmacology of ion channels. The existence of many different subtypes of K+ channels has been an impetus in the search for new molecules demonstrating channel and, thus, tissue selectivity. This review focuses on the different classes of openers of K+ channels, the intracellular mechanisms involved in the execution of their effects, and potential therapeutic targets.

Effects of pinacidil on arterial and venous resistances and mean circulatory filling pressure in rats

1. The effects of the potassium channel opener, pinacidil, on mean arterial pressure (MAP), mean circulatory filling pressure (MCFP), total peripheral resistance (TPR), cardiac output (CO) and resistance to venous return (Rv) were studied in rats. 2. In pentobarbitone-anaesthetized rats given mecamylamine (ganglionic blocker, 3.7 micrograms kg-1) and noradrenaline (1.5 micrograms kg-1 min-1) to suppress autonomic reflexes, pinacidil (60 and 180 micrograms kg-1 min-1), relative to the vehicle, reduced MAP and TPR in a dose-dependent manner but did not significantly alter CO, MCFP or RV. 3. Pinacidil (10-300 micrograms kg-1 min-1) caused similar increases in MCFP, an inverse index of venous compliance, and similar dose-dependent reductions in mean arterial pressure (MAP) in conscious, intact rats and rats infused with the ganglionic blocker, hexamethonium (150 micrograms kg-1 min-1). In rats with vasomotor tone elevated by the infusion of noradrenaline (1.5 micrograms kg-1 min-1), pinacidil caused markedly greater depressor responses but did not significantly alter MCFP. 4. Our results show that pinacidil is an efficacious vasodilator of arterial resistance blood vessels but has little venodilator activity.