Blockade of Voltage-Sensitive K+ Channels Increases Contractility More in Transverse Than in Longitudinal Rat Detrusor Strips

4-aminopyridine improves lower urinary tract symptoms in a patient with benign prostatic hyperplasia and downbeat nystagmus syndrome

Aminopyridines are potassium channel blockers that increase the excitability of nerve cells and axons; therefore, they are widely used to treat different neurological disorders. Here we present a patient with idiopathic downbeat nystagmus and lower urinary tract symptoms (LUTS) due to benign prostatic hyperplasia who was treated with the sustained-release form of 4-aminopyridine (4-AP). During treatment with 4-AP, the LUTS improved. This improvement was monitored by using uroflowmetry and the International Prostate Symptom Score. A significant improvement of symptoms was observed in relation to the voided volume. This included an improved emptying of the bladder without an increase in residual urine. In animal studies, both nonselective K(+) channel blockade and selective voltage-sensitive potassium blockade by 4-AP resulted in increased contraction on rat detrusor strips. To our knowledge, this is the first clinical observation of the mode of action of 4-AP in urological symptoms in humans.

Low concentrations of niflumic acid enhance basal spontaneous and carbachol-induced contractions of the detrusor

PURPOSE

The urinary bladder expresses Ca(2+)-activated Cl(-) channels (CACC), but its physiological role in governing contractility remains to be defined. The CACC modulator niflumic acid (NFA) is widely used despite the variable results arisen from different drug concentrations used. This study was designed to examine the effects of NFA at low concentrations on detrusor strip contractility.

METHODS

Rat detrusor strips with mucosa-intact (+MU) and mucosa-denuded (-MU) were prepared in transverse (Tr) and longitudinal (Lg) with respect to the bladder orientation. Isometric force measurements were made at baseline (for spontaneous phasic contractile activity) and during drug stimulation (by carbachol, CCh) with and without NFA.

RESULTS

NFA (1 and 10 μmol/L) pretreatment enhanced CCh-induced contractions more in +MU than -MU strips with no selectivity on contractile direction. For spontaneous phasic contractions, NFA-treated strips in the Tr direction showed increased phasic amplitude, while phasic frequency was unchanged.

CONCLUSIONS

The findings suggest low concentrations of NFA having a potentiating effect on detrusor contractions that was sensitive to the MU and contractile direction.

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.

The role of the urothelium and ATP in mediating detrusor smooth muscle contractility

OBJECTIVES

To examine the contractility of urothelium-intact (+UE) and urothelium-denuded (-UE) rat detrusor strips under adenosine triphosphate (ATP) treatment. Purinergic signaling exists in the bladder but both the inhibitory effect of ATP on detrusor contractions and the function of urothelial ATP are not established.

METHODS

Detrusor strips were obtained from bladders of young adult rats. Isometric tension from both transverse and longitudinal contractions was measured using a myograph. The muscarinic agonist carbachol (CCh) was used to induce contractions, which were under the influences of different concentrations of ATP.

RESULTS

In both +UE and -UE strips, 1 mM ATP suppressed CCh-induced contractions. In longitudinal contractions, ATP added to the inhibitory effect of urothelium on CCh responses. Removal of the urothelium, but with exogenous ATP added, recovered the CCh responses to the same level as in +UE strips with no added ATP. Transverse contractions were less susceptible to ATP in the presence of urothelium.

CONCLUSIONS

We showed that the urothelium and ATP suppressed CCh-induced contractions to a similar extent. The findings suggest an inhibitory role of urothelial ATP in mediating detrusor smooth muscle contractility, which may be impaired in diseased bladders.

β-Adrenoceptor-mediated differences in transverse and longitudinal strips from the rat detrusor

Contractions and relaxations of the urinary bladder occur in all directions to facilitate urine release and storage. Transverse and longitudinal contractility of detrusor smooth muscle have been studied before using various pharmacologic stimuli but not β agonists. Given the importance of β-adrenoceptors in mediating bladder relaxation, the effects of isoprenaline (IPNA) in transverse and longitudinal contractility were examined. Pretreatment with a low concentration of IPNA (0.1 or 1 μM) suppressed carbachol (CCh)-induced contractions, more in the transverse than longitudinal direction. Increasing the IPNA concentration to 10 or 100 μM resulted in greater inhibition of longitudinal contractions. Also in the longitudinal direction, IPNA-induced relaxation was greater than in the transverse direction. When precontracted with a submaximal concentration of CCh (1 μM), IPNA increased the phasic activity in the longitudinal direction only. In summary, β-adrenoceptor-mediated differences between transverse and longitudinal contractility were revealed. In testing the relaxant properties of selective β-agonists, the findings here should be considered such that other than the conventional longitudinal contractions, measurements are also made in other directions.

Functional effects of the KCNQ modulators retigabine and XE991 in the rat urinary bladder

The anticonvulsant retigabine has previously been reported to inhibit bladder overactivity in rats in vivo but the mechanism and site of action are not known. In the present study we investigated the effect of retigabine in isolated rat bladder tissue. Bladders from Sprague-Dawley rats were cut transversally into rings and mounted on an isometric myograph. The average tension, the amplitude and frequency of bladder muscle twitches were measured. The bladder tissue was stimulated with carbachol, KCl (5, 10 and 60mM), and by electric field stimulation. Dose-response curves were obtained with increasing concentrations of the KCNQ((2-5)) selective positive modulator, retigabine or with the KCNQ((1-5)) negative modulator XE991. Retigabine experiments were repeated in the presence of 10 microM XE991. Retigabine reduced both the contractility and the overall tonus of bladder tissue independent of the mode of stimulation with EC(50) values ranging from 3.3 microM (20mM KCl) to 8.3 microM (0.2 microM carbachol). In support of a KCNQ-specific effect, retigabine had only weak effects after 60mM KCl pre treatment and all retigabine effects could be reversed by XE991. XE991 increased both the amplitude and mean tension of the bladder but was more potent at increasing the number rather than the size of the stimulated twitches. In conclusion, this study demonstrates an efficacious KCNQ dependent effect of retigabine and XE991 on rat bladder contractility.