Effects of pinacidil on detrusor instability in men with bladder outlet obstruction

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.

Suppression of human detrusor smooth muscle excitability and contractility via pharmacological activation of large conductance Ca2+-activated K+ channels

Overactive bladder syndrome is frequently associated with increased detrusor smooth muscle (DSM) contractility. We tested the hypothesis that pharmacological activation of the large-conductance voltage- and Ca(2+)-activated K(+) (BK) channel with NS-1619, a selective BK channel opener, reduces the excitability and contractility of human DSM. We used the amphotericin-perforated whole cell patch-clamp technique on freshly isolated human DSM cells, live-cell Ca(2+) imaging, and isometric DSM tension recordings of human DSM strips obtained from open bladder surgeries. NS-1619 (30 μM) significantly increased the amplitude of the voltage step-induced whole cell BK currents, and this effect was abolished by pretreatment with 200 nM iberiotoxin (IBTX), a selective BK channel inhibitor. In current-clamp mode, NS-1619 (30 μM) significantly hyperpolarized the resting membrane potential, and the hyperpolarization was reversed by IBTX (200 nM). NS-1619 (30 μM) significantly decreased the intracellular Ca(2+) level in isolated human DSM cells. BK channel activation with NS-1619 (30 μM) significantly inhibited the amplitude, muscle force, frequency, duration, and tone of the spontaneous phasic and pharmacologically induced DSM contractions from human DSM isolated strips. IBTX (200 nM) suppressed the inhibitory effects of NS-1619 on spontaneous contractions. The amplitude of electrical field stimulation (0.5-50 Hz)-induced contractions was significantly reduced by NS-1619 (30 μM). Our data suggest that pharmacological activation of BK channels could represent a novel treatment option to control bladder dysfunction in humans.

Inhibitory effects of the ATP-sensitive potassium channel openers cromakalim, pinacidil and minoxidil on the carbachol-response curve in porcine detrusor muscle

Aims

ATP-sensitive potassium channels represent promising drug targets for treating specific bladder diseases. The inhibitory effects of ATP-selective potassium channel openers (PCOs) on the carbachol–response curve in porcine detrusor muscle were examined.

Materials and methods

Each of the three substances used in the study represent one prototype of a different class of PCO: cromakalim belongs to the benzopyran series, pinacidil is a cyanoguanidine derivative, and minoxidil represents a pyrimidine derivative. The porcine detrusor muscle represents one of the best models for human detrusor. Experiments were conducted on muscle strips of porcine detrusor muscle suspended in a tissue bath. Concentration–response curves of carbachol were constructed after pretreatment with cromakalim at 10⁻⁷, 10⁻⁶ and 10⁻⁵ M, and with pinacidil and minoxidil at 10⁻⁶, 10^−5.5 and 10⁻⁵ M, respectively. Each muscle strip was only used to examine one concentration of one substance.

Results

Cromakalim had the greatest inhibitory effect, significantly suppressing the carbachol–response curve at 10⁻⁶ and 10⁻⁵ M. Pinacidil showed a significant inhibitory effect at 10^−5.5 and 10⁻⁵ M, which was smaller than that of cromakalim. Minoxidil did not significantly inhibit the contractions at all examined concentrations.

Conclusions

The examined ATP-sensitive PCOs belonging to the benzopyrans and cyanoguanidines significantly suppressed detrusor contractions. The development of derivatives of these prototypes could open new possibilities for the pharmacological treatment of selected bladder diseases.

Inhibitory effects of different ATP-sensitive potassium channel openers on electrically generated and carbachol-induced contractions of porcine and human detrusor muscle

The inhibitory effects of different potassium channel openers (PCOs) on electrically generated and carbachol-induced contractions of porcine and human detrusor muscle were examined. PCOs could be an interesting substance class for treatment of detrusor overactivity. Experiments were performed on muscle strips suspended in a tissue bath. Human tissue originated from patients who underwent total cystectomy. The concentration-relaxation curves of the first-generation PCOs cromakalim and pinacidil and the untypical PCO minoxidil were performed using carbachol-precontracted detrusor muscle strips of pigs and humans. Additionally, the inhibitory effects of cromakalim, pinacidil and minoxidil on electrically generated contractions of porcine detrusor muscle were examined. Furthermore, the inhibitory effect of the second-generation, bladder-selective PCO ZM 226600 on electrically generated contractions of the human detrusor muscle was determined. Frequency-response curves were performed before and after incubation with one PCO used in two different concentrations. In humans, cromakalim and pinacidil led to a maximum decrease of 73.5 and 68.4% and showed mean pD2 values of 6.65 and 5.5, respectively. In pigs, cromakalim and pinacidil led to a maximum decrease of 90.6 and 93.6% and showed mean pD2 values of 6.39 and 5.01, respectively. Minoxidil did not significantly decrease the precontraction at the highest used concentration in both species. Cromakalim exhibited the biggest inhibitory effect being significant at 10(-5) and 10(-6) M. Pinacidil showed only a significant inhibitory effect at 10(-5) M which was smaller than that of cromakalim. At 3 x 10(-6) M only a very small effect occurred at 1 Hz. Minoxidil did not inhibit the contractions at both examined concentrations except for a very small effect at 1 Hz. In humans, ZM 226600 exhibited at 10(-6) and 10(-5) M a significant inhibitory effect. At 10(-7) M it was only significant at one frequency.

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.

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.

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.

Pharmacology of the lower urinary tract: basis for current and future treatments of urinary incontinence

The lower urinary tract constitutes a functional unit controlled by a complex interplay between the central and peripheral nervous systems and local regulatory factors. In the adult, micturition is controlled by a spinobulbospinal reflex, which is under suprapontine control. Several central nervous system transmitters can modulate voiding, as well as, potentially, drugs affecting voiding; for example, noradrenaline, GABA, or dopamine receptors and mechanisms may be therapeutically useful. Peripherally, lower urinary tract function is dependent on the concerted action of the smooth and striated muscles of the urinary bladder, urethra, and periurethral region. Various neurotransmitters, including acetylcholine, noradrenaline, adenosine triphosphate, nitric oxide, and neuropeptides, have been implicated in this neural regulation. Muscarinic receptors mediate normal bladder contraction as well as at least the main part of contraction in the overactive bladder. Disorders of micturition can roughly be classified as disturbances of storage or disturbances of emptying. Failure to store urine may lead to various forms of incontinence, the main forms of which are urge and stress incontinence. The etiology and pathophysiology of these disorders remain incompletely known, which is reflected in the fact that current drug treatment includes a relatively small number of more or less well-documented alternatives. Antimuscarinics are the main-stay of pharmacological treatment of the overactive bladder syndrome, which is characterized by urgency, frequency, and urge incontinence. Accepted drug treatments of stress incontinence are currently scarce, but new alternatives are emerging. New targets for control of micturition are being defined, but further research is needed to advance the pharmacological treatment of micturition disorders.

K-ATP opener-mediated attenuation of spontaneous bladder contractions in ligature-intact, partial bladder outlet obstructed rats

Symptoms of urinary frequency and urgency secondary to benign prostatic obstruction are common in elderly men. In many patients, these symptoms correspond to the urodynamic finding of involuntary detrusor contractions during filling cystometry (i.e., detrusor instability). Spontaneous non-voiding contractions during filling can be modeled in animals by subchronic, partial urethral obstruction. However, many investigators remove the obstructive ligature a few days prior to cystometrical evaluation (which may not be an ideal representation of the clinical situation where obstruction is still present), and all perform cystometry within 3 days post-bladder catheterization surgery (i.e., while considerable wound healing is present). In the current study, we evaluated the effects, after oral dosing, of three structurally diverse ATP-sensitive potassium channel openers (KCOs) on spontaneous contractions secondary to obstruction in rats with an intact obstructive ligature at the time of testing and 2 weeks post-bladder catheterization. ZD6169, WAY-133537 and a novel dihydropyridine KCO, A-278637, all significantly decreased spontaneous bladder contractions at 30 min post-dosing (p.o.). However, only ZD6169 (10 micromol/kg) and A-278637 (3 micromol/kg) attenuated such bladder contractions at doses that did not concurrently, significantly affect mean arterial blood pressure and heart rate. These data confirm the efficacy of KCOs to inhibit unstable contractions in obstructed rats, and they further demonstrate the positive effect of a novel, bladder-selective KCO, A-278637, in an animal model with potentially less artifact than in previous such models.

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.

Pharmacological agents for the treatment of urinary incontinence due to overactive bladder

Although the exact aetiology of overactive bladder is unknown to date, pharmacological therapy has been targeted to both the central and peripheral nervous systems. Potential CNS targets include GABA, opioid, serotonin (5-HT), dopamine and glutaminergic receptors as well as the alpha-adrenoceptors. Potential PNS targets include muscarinic receptors, calcium and potassium channels and alpha- and beta-adrenergic receptors. Since acetylcholine is the primary excitatory neurotransmitter involved in bladder (detrusor) contraction and emptying, anticholinergic agents are the primary compounds used clinically to decrease involuntary detrusor contractions. Anticholinergic therapy has a stabilising effect on the bladder (detrusor muscle); increases bladder capacity; decreases frequency of involuntary detrusor contractions; and delays the initial urge to void, but does not affect warning time. However, the clinical utility of antimuscarinic therapy is limited by the lack of receptor selectivity, resulting in the classic anticholinergic side effects of dry mouth, blurred vision, constipation and potentially, CNS effects such as somnolence and impaired cognitive function. These unwanted side effects often result in premature discontinuation of therapy and poor compliance. Previous attempts to develop uroselective alpha-adrenergic receptor antagonists have not been successful and although research continues, the hope that this class of agents would be viable alternatives to the anticholinergics remains to be proven in the clinical setting. The recent demise of several potassium channel openers does not augur well for the future of this class of agent. The reasons for the discontinuation of trials with these agents have not been fully elucidated, but one must assume that they were not uroselective and the cardiovascular side effects rendered them less than useful clinically. The serotonin re-uptake inhibitors appear to be promising novel therapeutic agents aimed at controlling bladder over-activity through specific CNS pathways. The sensory side of the micturition reflex is a potential therapeutic target. Agents to desensitise afferent nerve endings involved in C-fibre afferent reflexes include capsaicin and resiniferatoxin. Their clinical applicability is currently being evaluated. Finally, the recent findings related to the role of the P2X3 receptor in the sensory aspects of bladder filling have created new interest in the future development of agents that will improve the management of this prevalent and debilitating condition.

Treatment of overactive bladder: other drug mechanisms

The well-known side effects of antimuscarinic drugs have focused interest on other ways of treating overactive bladder. Targets for pharmacologic intervention may be found in the central nervous system (CNS) or peripherally. Several CNS transmitter systems can modulate voiding, but few drugs with a defined CNS site of action have been demonstrated to be clinically useful. The mechanism of action of imipramine, which may be effective in the treatment of overactive bladder, has not yet been clarified. Like imipramine, duloxetine is an inhibitor of serotonin (5-HT) and norepinephrine reuptake. Duloxetine has shown some promise in the treatment of urinary incontinence, but, as with the selective serotonin reuptake inhibitors, its effectiveness in the treatment of overactive bladder has not been proven. Drugs affecting norepinephrine, dopamine or gamma-aminobutyric acid (GABA) receptors and mechanisms may be useful. There is also evidence for a central site of action of alpha(1)-adrenoceptor (AR) antagonists. Traditionally, drugs for the treatment of overactive bladder have had a peripheral site of action. Drugs acting on ARs or membrane channels, as well as prostaglandin synthase inhibitors and several other agents, have been used with moderate success. However, recent developments may lead to more effective drugs, including potassium channel openers, prostaglandins, selective and nonselective inhibitors of cyclooxygenase and those acting on the beta(3)-ARs in the human detrusor. Drugs that reduce afferent activity represent an attractive therapeutic approach, and transmitters of afferent nerves and their receptors are possible targets for pharmacologic interventions.

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.

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.

A myogenic basis for the overactive bladder

OBJECTIVES

This article summarizes evidence supporting the conclusion that the spontaneous rises in pressure that occur in the overactive bladder, particularly in detrusor instability (DI), regardless of etiology, are myogenic.

METHODS

The evidence quoted has been obtained by several groups of investigators and includes electrophysiologic studies of detrusor myocytes, tension studies of strips of smooth muscle, in vivo experiments on animal models, and light and electron microscopic studies of the structure of the bladder wall.

RESULTS

The results of these studies demonstrate changes in the properties, structure, and innervation of the detrusor. These changes are consistent with the hypothesis that partial denervation of the detrusor may be responsible for altering the properties of the smooth muscle, leading to increased excitability and increased ability of activity to spread between cells, resulting in coordinated myogenic contractions of the whole detrusor.

CONCLUSIONS

It is suggested that alterations in the properties of the detrusor myocytes are a necessary prerequisite for the production of the unstable pressure rises seen in DI of any origin.

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.

Effects of levcromakalim and nucleoside diphosphates on glibenclamide-sensitive K+ channels in pig urethral myocytes

1. Effects of levcromakalim and nucleoside diphosphates (NDPs) on both membrane currents and unitary currents in pig proximal urethra were investigated by use of patch clamp techniques (conventional whole-cell configuration, nystatin perforated patch, cell-attached configuration and inside-out patches). 2. Levcromakalim produced a concentration-dependent outward current at a holding potential of -50 mV. The peak current amplitude showed little variation when measured by either conventional whole-cell or nystatin perforated patch configurations. 3. In conventional whole-cell configuration, the levcromakalim (100 microM)-induced outward current decayed by about 90% in 18 min. In contrast, with the nystatin perforated patch, approximately 86% of the levcromakalim-induced outward current still remained after 18 min. 4. The peak amplitude of the levcromakalim (100 microM)-induced outward membrane current recorded by the conventional whole-cell configuration was greatly reduced by inclusion of 5 mM EDTA in the pipette. The much smaller but significant outward membrane current remaining was abolished by glibenclamide. 5. In conventional whole-cell recordings, inclusion of an NDP in the pipette solution induced a small outward current which slowly reached a maximal amplitude (in 2 to 10 min) and was suppressed by glibenclamide. Addition of 100 microM levcromakalim after the NDP-induced current had peaked activated a further outward current which was larger than that recorded in the absence of NDPs. Approximately 50% of this current still remained at 18 min, even when conventional whole-cell configuration was used. 6. In the cell-attached mode in symmetrical 140 mM K+ conditions, glibenclamide inhibited the 100 microM levcromakalim-activated 43 pS K+ channel in a concentration-dependent manner, showing an inhibitory dissociation constant (Ki) of approximately 520 nM. 7. In inside-out patches in which the glibenclamide-sensitive K+ channel had run down after exposure to levcromakalim, both uridine 5'-diphosphate (UDP) and MgATP were capable of reactivating the channel. Further application of Mg2+ to the UDP-reactivated K+ channels enhanced the channel activity reversibly. 8. In inside-out patches UDP was capable of activating the glibenclamide-sensitive K+ channel without levcromakalim, providing that there was free Mg2+ present (either UDP in 5 mM EGTA or UDP in 5 mM EDTA with Mg2+). Additional application of levcromakalim caused a further reversible activation of channel opening. 9. In the presence of levcromakalim, application of adenosine 5'-triphosphate (ATP) to the inner surface of the membrane patch inhibited UDP-reactivated channel opening in a concentration-dependent manner. 10. Addition of an untreated cytosolic extract of pig proximal urethra reactivated the glibenclamide-sensitive K+ channel in the presence of 100 microM levcromakalim in inside-out patches. 11. These results demonstrate the presence in the pig proximal urethra of a glibenclamide-sensitive K+ channel that is blocked by intracellular ATP and can be activated by levcromakalim. Intracellular UDP can reactivate the channel after rundown. Additionally, intracellular Mg2+ may play an important role in regulating the channel activity.

Potassium channel openers for treatment of bladder hyperactivity

The potassium (K+) channel openers induce hyperpolarization by ATP-sensitive K+ channels. This group of compounds has been demonstrated to effectively relax the human detrusor and reduce bladder hyperactivity in obstructed rats. In patients with overactive bladders, oral administration and intravenous infusion of different K+ channel openers were without significant effect on urodynamic variables. In conscious dogs, a new K+ channel opener was found to increase bladder compliance with reduced micturition frequency. K+ channel openers have an interesting potential for the treatment of bladder hyperactivity, but development of new selective compounds with further clinical experience are demanded.

Activation by levcromakalim and metabolic inhibition of glibenclamide-sensitive K channels in smooth muscle cells of pig proximal urethra

1. The effects of levcromakalim (BRL 38227) on ionic currents recorded from pig proximal urethra were investigated by use of tension measurement and patch clamp techniques (conventional whole-cell configuration, nystatin perforated patch, and cell-attached configuration). 2. Levcromakalim (1 microM) caused a relaxation in the resting tone. This levcromakalim-induced relaxation was inhibited by the pretreatment with 1 microM glibenclamide. 3. The resting membrane potential recorded from single cells in current-clamp mode was-36.1 +/- 4.4 mV (n = 5). 4. Levcromakalim induced a concentration-dependent hyperpolarization with a maximum (at > or = 10 microM) close to the theoretical equilibrium potential of potassium (EK). The membrane hyperpolarization caused by 1 microM levcromakalim (24.7 +/- 5.8 mV, n = 4) was abolished by 1 microM glibenclamide. 5. Levcromakalim (100 microM) caused an outward K current in whole-cell recordings which was unaffected by iberiotoxin (300 nM) but abolished by glibenclamide (10 microM). 6. In cell-attached patches, levcromakalim activated a 43 pS K channel which was inhibited by the application of glibenclamide. 7. The metabolic poison, cyanide (CN), also activated a 43 pS K channel which was suppressed by the application of 10 microM glibenclamide. 8. These results indicate that levcromakalim and metabolic inhibition activate the same 43 pS K channel in pig proximal urethra. The resultant urethral hyperpolarization might reduce the usefulness of K channel openers in the treatment of detrusor instability, but be of value in treating outflow obstruction.

Effects of various phosphodiesterase-inhibitors, forskolin, and sodium nitroprusside on porcine detrusor smooth muscle tonic responses to muscarinergic stimulation and cyclic nucleotide levels in vitro

The cyclic nucleotides cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are second messengers involved in the regulation of contractility in various smooth muscle organs including detrusor smooth muscle. They are synthesized by activation of adenylate and guanylate cyclases, respectively, and inactivated by phosphodiesterases (PDEs). In order to delineate the intracellular regulation of porcine detrusor contractility by cyclic nucleotides and phosphodiesterases, functional organ bath studies and determinations of intracellular cyclic nucleotide contents were performed after incubation of porcine detrusor strips with forskolin (adenylate cyclase activator), sodium nitroprusside (guanylate cyclase activator), and various phosphodiesterase-inhibitors. Significant relaxant responses were achieved only by forskolin, the nonspecific phosphodiesterase-inhibitor papaverine, and the phosphodiesterase 1-inhibitor vinpocetine (62.4 +/- 5.6%, 73 +/- 4.3%, and 53 +/- 7.9%, respectively). Sodium nitroprusside and the selective PDE-inhibitors milrinone, rolipram, zaprinast, and dipyridamole were significantly less efficacious (26.9 +/- 3.9%, 15.5 +/- 3.8%, 15.3 +/- 3.0%, 13 +/- 4.0%, and 13.2 +/- 2.1%, respectively). Forskolin, papaverine, and vinpocetine elevated intracellular cAMP concentrations (7.3-, 1.9-, and 1.7-fold increase at 100 microM, respectively), whereas the other substances failed to enhance cAMP levels. cGMP levels were only increased by sodium nitroprusside (7.8-fold). The adenylate cyclase-cAMP system seems to be the more important signal transduction system involved in the relaxation of carbachol induced smooth muscle tone of the porcine detrusor. The role of the guanylate cyclase-cGMP system is less clear. In addition, the calcium/calmodulin-stimulated PDE I seems to be of major functional importance in regulating cAMP hydrolysis in the porcine detrusor smooth muscle in vitro.

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.

Clinical pharmacology of potassium channel openers

Opening of K+ channels in cell membranes with resulting increase in K+ conductance, shifts the membrane potential in a hyperpolarizing direction towards the K+ equilibrium potential. Hyperpolarization reduces the opening probability of ion channels involved in membrane depolarization and excitation is reduced. K+ channel openers are believed to hyperpolarize smooth muscle cells by a direct action on the cell membrane. The best known members of the group are cromakalim, nicorandil and pinacidil, but several new compounds are being evaluated. In addition, it has recently been shown that also clinically well-known drugs like, e.g. diazoxide and minoxidil exhibit K+ channel opening properties. Nicorandil and new compounds containing nitro groups have a dual mechanism of action, also activating guanylate cyclase, an effect that contributes to their cardiovascular effect profile. K+ channel openers have a wide range of effects. Some of their properties and actions are summarized, and their present applications and/or potential for future application, in e.g. hypertension, angina pectoris, asthma, bladder instability, and several other disorders are discussed. It is concluded that K+ channel openning represents an interesting pharmacological principle with many potential clinical applications. However, most available drugs do not seem to have a sufficient tissue selectivity to be useful therapeutic alternatives. Before the potential of the new members of the group on clinical trials can be properly evaluated, clinical experiences are needed.