K(ATP) channels mediate the beta(2)-adrenoceptor agonist-induced relaxation of rat detrusor muscle

A biophysically constrained computational model of the action potential of mouse urinary bladder smooth muscle

Urinary incontinence is associated with enhanced spontaneous phasic contractions of the detrusor smooth muscle (DSM). Although a complete understanding of the etiology of these spontaneous contractions is not yet established, it is suggested that the spontaneously evoked action potentials (sAPs) in DSM cells initiate and modulate the contractions. In order to further our understanding of the ionic mechanisms underlying sAP generation, we present here a biophysically detailed computational model of a single DSM cell. First, we constructed mathematical models for nine ion channels found in DSM cells based on published experimental data: two voltage gated Ca2+ ion channels, an hyperpolarization-activated ion channel, two voltage-gated K+ ion channels, three Ca2+-activated K+ ion channels and a non-specific background leak ion channel. The ion channels' kinetics were characterized in terms of maximal conductances and differential equations based on voltage or calcium-dependent activation and inactivation. All ion channel models were validated by comparing the simulated currents and current-voltage relations with those reported in experimental work. Incorporating these channels, our DSM model is capable of reproducing experimentally recorded spike-type sAPs of varying configurations, ranging from sAPs displaying after-hyperpolarizations to sAPs displaying after-depolarizations. The contributions of the principal ion channels to spike generation and configuration were also investigated as a means of mimicking the effects of selected pharmacological agents on DSM cell excitability. Additionally, the features of propagation of an AP along a length of electrically continuous smooth muscle tissue were investigated. To date, a biophysically detailed computational model does not exist for DSM cells. Our model, constrained heavily by physiological data, provides a powerful tool to investigate the ionic mechanisms underlying the genesis of DSM electrical activity, which can further shed light on certain aspects of urinary bladder function and dysfunction.

Effects of K(+) channel openers on spontaneous action potentials in detrusor smooth muscle of the guinea-pig urinary bladder

The modulation of spontaneous excitability in detrusor smooth muscle (DSM) upon the pharmacological activation of different populations of K(+) channels was investigated. Effects of distinct K(+) channel openers on spontaneous action potentials in DSM of the guinea-pig bladder were examined using intracellular microelectrode techniques. NS1619 (10μM), a large conductance Ca(2+)-activated K(+) (BK) channel opener, transiently increased action potential frequency and then prevented their generation without hyperpolarizing the membrane in a manner sensitive to iberiotoxin (IbTX, 100nM). A higher concentration of NS1619 (30μM) hyperpolarized the membrane and abolished action potential firing. NS309 (10μM) and SKA31 (100μM), small conductance Ca(2+)-activated K(+) (SK) channel openers, dramatically increased the duration of the after-hyperpolarization and then abolished action potential firing in an apamin (100nM)-sensitive manner. Flupirtine (10μM), a Kv7 channel opener, inhibited action potential firing without hyperpolarizing the membrane in a manner sensitive to XE991 (10μM), a Kv7 channel blocker. BRL37344 (10μM), a β3-adrenceptor agonist, or rolipram (10nM), a phosphodiesterase 4 inhibitor, also inhibited action potential firing. A higher concentration of rolipram (100nM) hyperpolarized the DSM and abolished the action potentials. IbTX (100nM) prevented the rolipram-induced blockade of action potentials but not the hyperpolarization. BK and Kv7 channels appear to predominantly contribute to the stabilization of DSM excitability. Spare SK channels could be pharmacologically activated to suppress DSM excitability. BK channels appear to be involved in the cyclic AMP-induced inhibition of action potentials but not the membrane hyperpolarization.

ATP-sensitive potassium channels modulate in vitro tocolytic effects of β₂-adrenergic receptor agonists on uterine muscle rings in rats in early but not in late pregnancy

Aim

To investigate whether ATP-sensitive potassium (K_ATP) channels modulate the tocolytic effect of β₂-AR agonists (ritodrine and salmeterol) in early-pregnant (day 6) and late-pregnant (day 22) rat uterus in vitro, in order to examine the relation between the K_ATP channel sulphonylurea-binding regulatory subunit (SUR) expression and pharmacological reactivity of β₂-AR agonists.

Methods

The tocolytic effects of ritodrine and salmeterol (10^-10-10^-5 M) on spontaneous rhythmic contractions were investigated cumulatively, alone, or in the presence of the K_ATP channel blocker glibenclamide (10^-6 M) and the K_ATP channel opener pinacidil (10^-9-10^-7 M) after 5-min preincubation.

Results

β₂-AR agonist induced myometrial relaxation was inhibited by glibenclamide and enhanced by pinacidil on day 6, when SUR1 expression levels were high. Neither glibenclamide nor pinacidil mediated tocolytic effect was measured on day 22.

Conclusion

Low expression of the K_ATP channels at the end of gestation may facilitate enhanced excitability and contractility in the rat myometrium. The combination of a betamimetic and a K_ATP channel opener will therefore not be of therapeutic relevance in the treatment of preterm delivery.

Mirabegron, a novel, non-antimuscarinic drug for the overactive bladder: An up-to-dated review

Mirabegron opened a new era in the treatment of overactive bladder (OAB). For the first time physicians dealing with OAB have an effective alternative to the pharmacological mainstay of the therapy for this disorder, the antimuscarinic drugs. This first-in-class, potent β3-adrenoceptors agonist has recently received approval by regulatory authorities in Japan, United States and Europe, based on the favourable efficacy-tolerability profile demonstrated in multiple randomized, multinational, controlled trials, both short and long-term. There is substantial consistency through the studies in reporting the cardiovascular safety of treatment with mirabegron. The main advantage of mirabegron is the placebo-like incidence of classic adverse effects caused by antimuscarinics, dry mouth and constipation, that is expected to improve long-term adherence of patients to treatment. Mirabegron can be used in patients with contraindications to antimuscarinics and in those who discontinued previous antimuscarinic therapy. Herein, we reviewed the published literature on mirabegron, focusing on the rationale of β3-agonism for OAB treatment and on the preclinical and clinical evidence of efficacy and safety available on this new pharmacological principle.

Bladder contractility is mediated by different K+ channels in the urothelium and detrusor smooth muscle

The roles played by K(+) channels in the urothelium (UE) and detrusor smooth muscle (DSM) in regulating agonist-induced bladder contraction is not known at present. Thus, the effects in carbachol (CCh)-induced contraction in UE-intact (+UE) and UE-denuded (-UE) rat detrusor strips pretreated with K(+)-channel blockers were investigated here. The K(+)-channel blockers used were 4-aminopyridine (4-AP), glibenclamide (Glib), iberiotoxin (IbTx), charybdotoxin (ChTx), and apamin. In the absence of K(+)-channel blockers, control CCh-induced contractions were more potent in -UE than +UE strips. Treatment with IbTx and apamin resulted in more potent CCh-induced contractions in +UE strips. In -UE strips, CCh potency was increased by ChTx and Glib, but decreased by 4-AP. Different K(+) channels in the UE and DSM were thus involved in regulating bladder contractions. Contractile mediatory function of these channels, specific to the UE or DSM, may be potential drug targets in the management of bladder disorders.

Detrusor myocyte activity and afferent signaling

AIMS

To discuss (1) mechanisms involved in the generation and control of myocyte contractions and consequent afferent nerve activity and (2) these mechanisms as targets for drugs aimed for treatment of overactive bladder (OAB) symptoms and detrusor overactivity (DO).

METHODS

Literature review of myocyte activation, bladder afferent nerves, mediators in the bladder, and translational aspects of the findings.

RESULTS

During bladder filling, there is normally no parasympathetic outflow from the spinal cord. Despite this, the bladder develops tone during filling and also exhibits non-synchronized local contractions and relaxations that are caused by a basal myogenic mechanical activity that may be reinforced by release of, for example, acetylcholine from non-neuronal and/or neuronal sources or local mediators, such as prostaglandins and endothelins. It is suggested that these spontaneous contractions are able to generate activity in afferent nerves ("afferent noise") that may contribute to DO and OAB.

CONCLUSIONS

Spontaneous bladder myocyte contractions and factors that are able to modulate them, as well as the consequent afferent nerve activity, may be targets for drugs meant for treatment of OAB/DO.

Prospective pharmacologic therapies for the overactive bladder

Lower urinary tract symptoms (LUTS), overactive bladder syndrome (OAB) and detrusor overactivity (DO) are all conditions that can have major effects on quality of life and social functioning. Antimuscarinic drugs are first-line treatment-they often have good initial response rates, but adverse effects and decreasing efficacy cause long-term compliance problems, and alternatives are needed. The recognition of the functional contribution of the urothelium, the spontaneous myocyte activity during bladder filling, and the diversity of nerve transmitters has sparked interest in both peripheral and central modulation of LUTS/OAB/DO pathophysiology. There may be several new possibilities to treat LUTS/OAB/DO. β(3)-AR agonists (YM178), PDE 5 inhibitors (sildenafil, tadalafil, vardenafil), vitamin D analogs (elocalcitol), combinations (α(1)-AR antagonist + antimuscarinic), and drugs with a central mode of action (tramadol, aprepitant) all have Randomized controlled trial (RCT) documented efficacy. Which of these therapeutic principles will be developed to clinically useful treatments remains to be established.

Potentiation of potassium currents by beta-adrenoceptor agonists in human urinary bladder smooth muscle cells: a possible electrical mechanism of relaxation

We examined the effects of beta-adrenoceptor agonists on the membrane currents of smooth muscle cells from the human urinary bladder using a whole-cell patch clamp to investigate the involvement of Ca(2+)-activated K(+) (K(Ca)) channels in relaxation by beta-adrenergic agonists. With 0.05 mmol/l EGTA in the patch pipette, depolarizing pulses evoked outward rectifying currents. Isoproterenol (1 micromol/l) significantly increased the membrane currents by 75% at +80 mV with 0.05 mmol/l EGTA pipette solution. BRL 37344 (1 micromol/l) significantly increased the membrane currents by 44% at +80 mV. Iberiotoxin (100 nmol/l) significantly decreased the membrane currents by 60% at +80 mV. In the presence of iberiotoxin, the potentiation of the outward currents by isoproterenol was greatly suppressed and, in the presence of iberiotoxin and apamin (1 micromol/l), the potentiation by isoproterenol was totally abolished. On the other hand, with 5 mmol/l EGTA pipette solution, depolarizing pulses evoked smaller outward currents. Isoproterenol (1 micromol/l) did not change the membrane currents with 5 mmol/l EGTA pipette solution. The real-time PCR analysis revealed the expression of beta(2)-adrenoceptors in the cells. These results suggest that Ca(2+)-activated and iberiotoxin- and apamin-sensitive currents via both large-conductance and small-conductance K(Ca) channels could be increased by stimulation of beta(2)-adrenoceptors.

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.

Effects of potassium channel modulators on human detrusor smooth muscle myogenic phasic contractile activity: potential therapeutic targets for overactive bladder

OBJECTIVES

Increased urinary bladder detrusor smooth muscle phasic contractility has been suggested to be associated with idiopathic bladder overactivity (OAB). We examined the role of voltage-dependent L-type calcium channels, adenosine triphosphate-sensitive potassium (K(ATP)) channels, and calcium-activated potassium (BK(Ca) and SK(Ca)) channels in the regulation of human detrusor phasic contractile activity.

METHODS

Isolated human bladder strip phasic contractions were measured and quantified as the mean area under the force-time curve, amplitude, and frequency of phasic contractions in 22 bladder samples.

RESULTS

Human detrusor strips displayed myogenic phasic contractions in the presence of atropine (10(-6) M), phentolamine (10(-6) M), propranolol (10(-6) M), suramin (10(-5) M), and tetrodotoxin (10(-6) M). The L-type calcium channel inhibitor nifedipine (300 nM) abolished the contractile activity. Blockade of K(ATP) channels by glibenclamide (1 and 10 microM) did not alter myogenic contractions. In contrast, the K(ATP) channel opener pinacidil (10 microM) markedly inhibited phasic contractility. Iberiotoxin (100 nM) and apamin (100 nM), potent and selective inhibitors of BK(Ca) and SK(Ca) channels, respectively, significantly increased the area under the force-time curve and the amplitude of contractions.

CONCLUSIONS

Phasic contractions of human detrusor are dependent on calcium entry through L-type calcium channels. BK(Ca) and SK(Ca) channels play a key role in the modulation of human detrusor smooth muscle phasic contractility. Furthermore, these observations support the concept that increasing conductance through K(ATP), BK(Ca), and SK(Ca) channels may represent attractive pharmacologic targets for decreasing phasic contractions of detrusor smooth muscle in OAB.

Effects of 138–355, a β3-adrenoceptor selective agonist, on relaxation of the human detrusor muscle in vitro

AIMS

Beta-adrenoceptors are the predominant beta-adrenoceptor subtype present in the bladder and urethra. This study investigates the effects of 138-355, an active-metabolite of TT-138 and beta(3)-adrenoceptor agonist, on relaxation of the human detrusor in vitro.

METHODS

Tumor-free tissue samples of human bladder muscle from 39 patients undergoing total cystectomy due to bladder cancer were obtained, and the mucosa and serosa were removed. Tissues were mounted in 5 or 10 ml organ baths containing Krebs solution, which was gassed with 95% O(2) and 5% CO(2). Resting tension of 1 g was obtained. When the contraction had stabilized, increasing concentrations of beta adrenoceptor agonists (non-selective, isoprenaline; beta(2)-selective, clenbuterol; beta(3)-selective, 138-355 and BRL37344) and propiverine (a non-selective anti-muscarinic antagonist) were added cumulatively and concentration-relaxation curves (CRCs) were obtained. CRCs to 138-355 were obtained in the absence and presence of SR59230A, a beta(3)-selective antagonist, and antagonist affinity values (pA(2)) were calculated from the Schild plot.

RESULTS

Isoproterenol, clenbuterol, 138-355 and BRL37344 concentration-dependently relaxed isolated human urinary bladder strips with pEC(50) value being 6.76+/-0.17, 5.23+/-0.22, 5.80+/-0.26 and 5.90+/-0.28, respectively. Following antagonist assay, it was observed that concentration-relaxation curves to 138-355 was competitively antagonized by beta(3)- adrenoceptor antagonist, SR59230A, with a pA(2) value of 7.01+/-0.45 and with a Schild slope of 0.72+/-0.07.

CONCLUSIONS

Involvement of the beta(3)-adrenoceptor appears to be greater than that of the beta(3)-adrenoceptor for relaxations of the human bladder. The relaxation response of 138-355 appears to be mediated via the beta(3)-adrenoceptor stimulation.

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.

Identification of beta-adrenoceptor subtypes in lower urinary tract of the female pig

PURPOSE

We investigated the presence and functional role of beta-adrenoceptor subtypes in the bladder base and proximal urethra of the female pig.

MATERIALS AND METHODS

Saturation experiments were done with 7 concentrations (0.25 to 16 nM.) of [(3)H]-dihydroalprenolol (NEN Life Science Products, Boston, Massachusetts). Competition experiments with [(3)H]-dihydroalprenolol were performed with unlabeled antagonists (beta1 selective CGP20712A, beta2 selective ICI118551 and beta3 selective SR59230A). In functional studies concentration-relaxation curves to the beta3-agonist BRL37344 were obtained and antagonist affinities for SR59230A were determined.

RESULTS

CGP20712A displaced [(3)H]-dihydroalprenolol with low affinity, suggesting that beta1-adrenoceptors were not present. Displacement with ICI118551 in the bladder base and urethra best fitted a 2-site model with 20% and 28% high affinity sites (beta2), respectively. Displacement experiments with SR59230A in the bladder base demonstrated that 59% of binding sites had high affinity (beta3). In the urethra displacement with SR59230A best fitted a 1-site model but with a pK(i) of 7.2 that was intermediate between that expected for beta2 and beta3-adrenoceptors. In functional studies BRL37344 induced relaxation with pEC50 values of 5.5 and 8, and a maximum relaxation response relative to 30 microM. isoprenaline of 79% and 90% in the bladder base and urethra, respectively. The affinity value of SR59230A for the response to BRL37344 was 7.87 and 7.71 in the bladder base and urethra, respectively, which were intermediate between those of beta2 and beta3-adrenoceptors.

CONCLUSIONS

Apparently beta3-adrenoceptors are the predominant beta-adrenoceptor subtype present in the lower urinary tract of the pig.