Spontaneous activity of lower urinary tract smooth muscles: correlation between ion channels and tissue function

Modification of rat detrusor muscle contraction by ascorbic acid and citric acid involving enhanced neurotransmitter release and Ca2+ influx

AIMS

Consumption of carbonated soft drinks is independently associated with the development of overactive bladder (OR 1.41, 95% Cl 1.02-1.95). We have shown previously that artificial sweeteners, present in carbonated soft drinks, enhanced detrusor muscle contraction. Other constituents of soft drinks are preservatives and antioxidants, we evaluated the effects of two of these, ascorbic acid and citric acid, on the contractile response of isolated rat bladder muscle strips.

METHODS

Detrusor muscle strips were suspended in a perfusion organ bath. We determined the effect of ascorbic acid and citric acid on the contractile responses to electrical field stimulation (EFS) in the absence and presence of atropine, carbachol, alpha, beta methylene ATP, potassium and calcium.

RESULTS

Ascorbic acid and citric acid (10(-7) M to 10(-3) M) enhanced the contractile response to 10 Hz EFS compared to control (P < 0.01). The frequency and amplitude of spontaneous bladder contractions were enhanced in the presence of ascorbic acid and citric acid by 14%, 21%, 21%, and 11% respectively. Ascorbic acid 10(-4) M significantly increased the atropine resistant response to EFS 5 Hz by 37% (P < 0.01) and inhibited contraction in response to carbachol 10(-4) M by 24%, (P < 0.05). Both ascorbic acid 10(-4) M and citric acid 10(-5) M significantly enhanced maximum contractile responses to alpha, beta methylene ATP, KCI and calcium compared to control.

CONCLUSIONS

Ascorbic acid and citric acid augmented bladder muscle contraction possibly by enhanced Ca(2+) influx. Presynaptic neurotransmitter release was enhanced by ascorbic acid. Carbonated beverages containing preservatives may aggravate symptoms of OAB.

Emerging drugs for treatment of overactive bladder and detrusor overactivity

BACKGROUND

Overactive bladder (OAB) signifies the presence of urinary urgency and can have major effects on quality of life and social functioning. Standard antimuscarinic drugs have good initial response rates but substantial adverse effects and long-term compliance problems.

OBJECTIVES

To review the complexities of the mechanisms underlying OAB and the current drugs available for treating its symptoms.

METHODS

The literature was reviewed to define current therapies and drugs in clinical trials. Articles were identified by means of a computerised PubMed and Cochrane Library search (using the following keywords: overactive bladder, detrusor overactivity, urgency and bladder), supported by a search of the PharmaProjects database.

CONCLUSIONS

New drug classes, such as beta-3 adrenergic agonists, may work by reducing contractility or excitability of bladder muscle. Moderation of afferent activity may allow improved OAB symptoms, with lower risk of affecting voiding function. Agents acting on the CNS could influence OAB favourably, but target selection and adverse effects are an issue. The recognition of the functional contribution of the urothelium and the diversity of nerve transmitters has sparked interest in both peripheral and central modulation of OAB pathophysiology.

Stimulation of beta3-adrenoceptors relaxes rat urinary bladder smooth muscle via activation of the large-conductance Ca2+-activated K+ channels

We investigated the role of large-conductance Ca(2+)-activated K(+) (BK) channels in beta3-adrenoceptor (beta3-AR)-induced relaxation in rat urinary bladder smooth muscle (UBSM). BRL 37344, a specific beta3-AR agonist, inhibits spontaneous contractions of isolated UBSM strips. SR59230A, a specific beta3-AR antagonist, and H89, a PKA inhibitor, reduced the inhibitory effect of BRL 37344. Iberiotoxin, a specific BK channel inhibitor, shifts the BRL 37344 concentration response curves for contraction amplitude, net muscle force, and tone to the right. Freshly dispersed UBSM cells and the perforated mode of the patch-clamp technique were used to determine further the role of beta3-AR stimulation by BRL 37344 on BK channel activity. BRL 37344 increased spontaneous, transient, outward BK current (STOC) frequency by 46.0 +/- 20.1%. In whole cell mode at a holding potential of V(h) = 0 mV, the single BK channel amplitude was 5.17 +/- 0.28 pA, whereas in the presence of BRL 37344, it was 5.55 +/- 0.41 pA. The BK channel open probability was also unchanged. In the presence of ryanodine and nifedipine, the current-voltage relationship in response to depolarization steps in the presence and absence of BRL 37344 was identical. In current-clamp mode, BRL 37344 caused membrane potential hyperpolarization from -26.1 +/- 2.1 mV (control) to -29.0 +/- 2.2 mV. The BRL 37344-induced hyperpolarization was eliminated by application of iberiotoxin, tetraethylammonium or ryanodine. The data indicate that stimulation of beta3-AR relaxes rat UBSM by increasing the BK channel STOC frequency, which causes membrane hyperpolarization and thus relaxation.

Beta-adrenergic relaxation of mouse urinary bladder smooth muscle in the absence of large-conductance Ca2+-activated K+ channel

In urinary bladder smooth muscle (UBSM), stimulation of beta-adrenergic receptors (beta-ARs) leads to activation of the large-conductance Ca2+-activated K+ (BK) channel currents (Petkov GV and Nelson MT. Am J Physiol Cell Physiol 288: C1255-C1263, 2005). In this study we tested the hypothesis that the BK channel mediates UBSM relaxation in response to beta-AR stimulation using the highly specific BK channel inhibitor iberiotoxin (IBTX) and a BK channel knockout (BK-KO) mouse model in which the gene for the pore-forming subunit was deleted. UBSM strips isolated from wild-type (WT) and BK-KO mice were stimulated with 20 mM K+ or 1 microM carbachol to induce phasic and tonic contractions. BK-KO and WT UBSM strips pretreated with IBTX had increased overall contractility, and UBSM BK-KO cells were depolarized with approximately 12 mV. Isoproterenol, a nonspecific beta-AR agonist, and forskolin, an adenylate cyclase activator, decreased phasic and tonic contractions of WT UBSM strips in a concentration-dependent manner. In the presence of IBTX, the concentration-response curves to isoproterenol and forskolin were shifted to the right in WT UBSM strips. Isoproterenol- and forskolin-mediated relaxations were enhanced in BK-KO UBSM strips, and a leftward shift in the concentration-response curves was observed. The leftward shift was eliminated upon PKA inhibition with H-89, suggesting upregulation of the beta-AR-cAMP pathway in BK-KO mice. These results indicate that the BK channel is a key modulator in beta-AR-mediated relaxation of UBSM and further suggest that alterations in BK channel expression or function could contribute to some pathophysiological conditions such as overactive bladder and urinary incontinence.

Spontaneous contractions of the pig urinary bladder: the effect of ATP-sensitive potassium channels and the role of the mucosa

OBJECTIVE

To investigate the influence of the mucosa on the inhibitory effects of the ATP-sensitive potassium channel (K(ATP) channel) opener, cromakalim, on the spontaneous contractions of pig bladder strips from the bladder dome and trigone. Little is known about the influence of the mucosa on spontaneous contractions and whether the nature of these contractions differs between the bladder dome and trigone.

MATERIALS AND METHODS

Paired longitudinal strips of female pig bladders were isolated from the dome and trigone. The mucosa was removed from one strip per pair and tissues were set up in organ baths. Spontaneous activity was allowed to develop and recorded, and then cumulative concentration-response curves to cromakalim were obtained. The time needed for spontaneous contractions to develop, the frequency and amplitude of spontaneous contractions, and the effect of cromakalim were analysed. The strips of mucosa removed from the dome to produce denuded strips were also analysed by immunofluorescence using antibodies specific for vimentin and alpha-smooth muscle actin (alpha-SMA).

RESULTS

In the dome removal of the mucosa delayed the development of spontaneous contractions compared with mucosa-intact strips, whilst the trigone strips developed spontaneous contractions soon after set up in the organ baths irrespective of the presence or absence of mucosa. In the dome, cromakalim was more potent in suppressing spontaneous contractions when the mucosa was absent; whilst in the trigone the effects of cromakalim were similar in mucosa-intact and denuded strips. Upon examination of the strips of mucosa by immunofluorescence these strips were shown to contain cells positive for alpha-SMA or vimentin and cells positive for both, suggesting the presence of not only urothelium but also suburothelium and some detrusor smooth muscle bundles.

CONCLUSION

In the dome, the urothelium and suburothelium reduce the inhibitory effect of cromakalim on spontaneous contractions, whilst in the trigone these structures appear to have little influence. The mechanism for generating spontaneous contractions in the intact strips seems to be linked to the urothelium and suburothelium in the dome but not in the trigone.

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.

Effects of augmentation cystoplasty on bladder contractility in rabbits

BACKGROUND/PURPOSE

An experimental study was conducted to investigate the effects of colocystoplasty and prefabricated cystoplasty on the bladder contractility in rabbits.

METHODS

Twenty-eight female New Zealand rabbits were randomly allocated into sham-operated controls, sigmoid enterocystoplasty (SECP), and prefabricated enterocystoplasty (PECP) groups. Augmentation cystoplasty with a 2-cm colon segment was performed in the SECP group. Mucosa was removed from the isolated sigmoid colon and covered with uroepithelial grafts from the bladder mucosa in the PECP group. Two weeks after the replacement of mucosa, enterocystoplasty was performed. Isometric contractions obtained by electrical field stimulation (EFS) and acetylcholine were evaluated in the bladder strips, 3 weeks after the operations.

RESULTS

Bladder strips obtained from the experimental groups displayed similar basal rhythmic activity. Electric field stimulation elicited a frequency-dependent contractile activity, which was lower between 1 to 20 Hz stimulation in the SECP- and PECP-operated animals. Acetylcholine elicited concentration-dependent contractions in all groups. Acetylcholine-induced contractile responses were greater in the PECP group.

CONCLUSIONS

This study demonstrated that an augmented bladder prepared either with a sigmoid colon or prefabricated seromuscular flap displayed contractile activity similar to normal bladder.

Urotheliogenic modulation of intrinsic activity in spinal cord-transected rat bladders: role of mucosal muscarinic receptors

We examined the modulation of intrinsic (i.e., spontaneous) detrusor contractions by the urothelium and the lamina propria through optical mapping approaches. Normal adult and spinal cord-transected (SCT) rat bladders were stained with Ca2+- and voltage-sensitive dyes, and optical activity generated from intrinsic contractions was mapped from the mucosal surface of whole bladder sheets. Both normal adult and SCT rat bladders displayed intrinsic contractions, where normal bladders showed low-amplitude, high-frequency contractions with disorganized patterns of activity. In contrast, in the SCT animals there were high-amplitude, low-frequency contractions that displayed an organized spread of membrane potential and intracellular Ca2+. The difference in contractile activity was mirrored in the Ca2+ and membrane potential maps of bladder sheets. Normal bladders showed multiple initiation sites across the mucosal surface, whereas SCT bladders showed only one or two fixed initiation sites localized to the dome. The magnitude of intrinsic contractions could be enhanced by stretch or low-dose arecaidine (50 nM), a muscarinic-specific agonist. Partial removal of the mucosa decreased the amplitude of the intrinsic contractions and decreased the response to stretch or arecaidine. Optical mapping of mucosa-denuded sheets, where enhanced spontaneous activity was abolished, or application of 1 microM nifedipine to remove smooth muscle signals, but not the mucosal signals, shows that intrinsic activity in pathological bladders is driven by the mucosal layer. In summary, we suggest an urotheliogenic origin for intrinsic activity, where structures within the mucosal layer organize and thereby enhance intrinsic detrusor contractions.

Roles of mechanosensitive ion channels in bladder sensory transduction and overactive bladder

In the storage phase, mechanical stretch stimulates bladder afferents. These signals generate sensations and trigger voiding responses, however the precise mechanisms by which mechanical stimuli excite bladder afferents are yet to be explored. For mechanosensory transduction, the presence of mechanosensors is essential in the peripheral sensory systems including sensory nerve endings, urothelium and others. There is increasing evidence that mechanosensitive ion channels, such as degenerin/epithelial Na(+) channel (ENaC) and transient receptor potential (TRP) channel families, play key roles in the mechanosensory transduction of the urinary bladder. Pharmacological interventions targeting mechanosensitive ion channels may provide a new strategy for the treatment of bladder dysfunction.

Stress urinary incontinence: relative importance of urethral support and urethral closure pressure

PURPOSE

Treatment strategies for stress incontinence are based on the concept that urethral mobility is the predominant causal factor with sphincter function a secondary contributor. To our knowledge the relative importance of these 2 factors has not been assessed in properly controlled studies.

MATERIALS AND METHODS

The Research on Stress Incontinence Etiology project is a case-control study that compared 103 women with stress incontinence and 108 asymptomatic controls in groups matched for age, race, parity and hysterectomy. Urethral closure pressure, urethral and pelvic organ support, levator ani muscle function and intravesical pressure were measured and analyzed using logistic regression and multivariable modeling.

RESULTS

Mean +/- SD maximal urethral closure pressure was 42% lower in cases (40.8 +/- 17.1 vs 70.2 +/- 22.4 cm H(2)O, d = 1.47). Lesser effect sizes were seen for support parameters, including resting urethral axis and urethrovaginal support (d = 0.41 and 0.50, respectively). Other pelvic floor parameters, including genital hiatus size and urethral axis during muscle contraction (d = 0.60 and 0.58, respectively), differed but levator strength and levator defect status did not. Maximum cough pressure, which is an assessment of stress on the continence mechanism, was also different (d = 0.43). After adjusting for body mass index the maximal urethral closure pressure alone correctly classified 50% of cases. Adding the best predictors for urethrovaginal support and cough strength to the model added 11% of predictive ability.

CONCLUSIONS

The finding that maximal urethral closure pressure and not urethral support is the factor most strongly associated with stress incontinence implies that improving urethral function may have therapeutic promise.

C-kit receptor in the human vas deferens: distinction of mast cells, interstitial cells and interepithelial cells

The molecular mechanisms underlying the regulation of vas deferens (VD) motility and semen emission are still poorly understood. Interstitial cells of Cajal (ICC), which harbour the c-kit receptor (CD117), provide the basis of coordinated gut motility. We investigated whether c-kit receptor-positive cells also exist in the normal human VD. Enzyme and fluorescence immunohistochemical techniques were applied on serial sections of human proximal, middle, and distal VD segments (n=49) employing 13 different monoclonal and polyclonal antibodies recognizing the c-kit receptor. The c-kit receptor was detected in either round- or spindle-shaped cells. On account of their antigenic profile, the round- and oval-shaped c-kit receptor-positive cells were identified as mast cells (MC) occurring in all layers of the VD except the epithelium. In contrast, two distinct populations of exclusively c-kit receptor-positive spindle-shaped cells were found within the lamina propria and, rarely, in the inner and outer smooth muscle layers, as well as within the epithelium. Different shaped c-kit receptor-positive MC and IC were present in all layers of the human VD. Our findings demonstrate the presence of different c-kit receptor-positive cells also in the human VD. Their rather ubiquitous distribution within the lamina propria and muscle layers suggests that IC and MC may modulate the neuromuscular transmission and the propagation of electrical signals in multiple systems involved in the draining of fluids. The importance of the c-kit receptor-positive interepithelial cells remains unclear.

Changes in T-type calcium channel and its subtypes in overactive detrusor of the rats with partial bladder outflow obstruction

AIMS

To investigate the activity of the T-type calcium channel (TCC) and the expression of its subtypes in overactive detrusor (OD) myocytes in rats after partial bladder outflow obstruction (PBOO).

METHODS

Thirteen male Wistar rats with OD after PBOO (OD group) and eight sham-operated rats (control group) were studied. The two groups were compared regarding the expression of TCC subtype genes by reverse transcription-polymerase chain reaction (RT-PCR) and the TCC kinetics and cell action potential by whole-cell patch-clamp.

RESULTS

The time course and density of the current were significantly higher in the OD cells than those in the control detrusor. Whole-cell patch-clamp analysis showed that the activation of TCCs in detrusor myocytes in the OD group was faster than the control group, but inactivation was almost the same in both groups, suggesting a significant enhancement of the Ca(2+) "window" current in the OD group. Patch-clamp recording of action potentials in the OD cells indicated an increase in excitability and a decrease in the repolarization interval. RT-PCR assay showed an abnormal expression of alpha1G subtype in the OD cells.

CONCLUSIONS

TCCs could be one of the crucial factors for the abnormal excitation in OD cells. The development of OD after PBOO presumably relates to the increase in TCC current in the bladder cells, the enhancement of the Ca(2+) "window" current for Ca(2+) inflow, the prolongation of the intracellular calcium oscillations, and the acceleration of the cell depolarization.

C-Kit receptor (CD117) in the porcine urinary tract

C-Kit positive interstitial cells of Cajal (ICC) play an important role in the regulation of the smooth muscle motility, acting as internal pacemakers to provide the slow wave activity within various luminal organs. Recently c-Kit-(CD117)-positive interstitial cells (IC) have been shown in the genitourinary tract, but systematic studies on the distribution and density of IC within the urinary tract are still lacking. Therefore the aim of the present study was to analyze systematically the localization and distribution of the c-Kit receptor in the urinary tract of the pig using immunohistochemical and molecular methods. Tissue samples were harvested from the porcine urinary tract including renal calices and pelvis, ureteropelvic junction, proximal, middle and distal ureter, ureteral orifice, fundus, and corpus of the bladder and the internal urethral orifice. Small and large intestine specimen served as controls. Immunohistochemistry (APAAP, IF) was applied on serial frozen sections using four monoclonal and polyclonal antibodies recognizing CD117. Whole mounts of the porcine upper urinary tract were prepared and investigated using conventional and confocal fluorescence microscopy followed by three-dimensional reconstruction. UV-laser microdissection and RT-PCR were applied to confirm the immunohistochemical results. CD117-immunoreactivity labeled bipolar IC and round-shaped mast cells (MC) throughout the adventitia, tunica muscularis and submucosa within the whole porcine urinary tract. While MC were found continuously in all investigated segments, a gradient of bipolar IC was evident. The whole mount preparations gave a detailed cytomorphology of IC within the various layers of the porcine urinary tract. Whole mount preparations revealed closed apposition of bi- and tripolar c-Kit positive IC parallel to the smooth muscle bundles and to veins of the tunica muscularis and adventitia. In the urothelium single CD117-positive interepithelial cells were found. The highest density of CD117-positive cells was found at the ureteropelvic junction, however the differences in between the segments were minimal. Microdissection and RT-PCR confirmed the results uncovered by immunohistochemistry. The ubiquitous distribution of IC and their close relationship to smooth muscle provides strong evidence that IC could contribute to the intrinsic pacemaker activity within the porcine (upper and lower) urinary tract. The role of the interepithelial CD117-positive cells as mechanosensors or as a precursor cell in the regeneration of the urothelium, is conceivable.

Changes of gap junctional cell-cell communication in overactive detrusor in rats

To evaluate the changes in intercellular communication through gap junctions in detrusor overactivity (DO), we studied 23 adult female Wistar rats with DO after partial outflow obstruction (DO group) and 13 sham-operated rats (control group). The two groups were compared by means of urodynamics, light and electron microscopy, expression of Cx40, Cx43, and Cx45 mRNA genes with RT-PCR, Cx43 protein with Western blot analysis, and functional intercellular communication with scrape loading dye transfer (SLDT) and fluorescence recovery after photobleaching (FRAP). The number of gap junctions and the expression of connexin mRNA and Cx43 protein were increased in DO rats, and intercellular communication through gap junctions increased after 6 wk of partial outflow obstruction as assessed with SLDT and FRAP techniques. The findings provide a theoretical rationale for using Cx43 antagonists and gap junction inhibitors in the treatment of patients with overactive detrusor secondary to partial bladder outflow obstruction.

Effects of spermine on the relaxation response of rat detrusor smooth muscles

Endogenous polyamines are known to influence excitation-contraction coupling in smooth muscle. This study was designed to determine the effects of the polyamines spermine, spermidine, and putrescine on the contractile responses of rat detrusor smooth muscles. Under physiological conditions, isometric tension recordings were made of isolated bladder strips from excised rat bladder. The effects of spermine, spermidine, and putrescine (1 mM each) on the bladder contractions induced by various agents, i.e., acetylcholine, bethanechol, high-K, and tetraethylammonium (TEA) were measured. A conventional patch clamp technique was used in whole cell mode with single smooth muscle cells of rat bladder. Calcium channel currents were recorded to determine the effects of spermine on channel activities. Polyamines elicited a concentration-dependent relaxations on the contractile agents induced contractures. Spermine showed the most potent relaxation effect of the polyamines examined, and relaxed the contractions induced by the agents. Calcium channel activities were significantly reduced by adding 1 mM spermine to the bath. We concluded that spermine exerts a potent relaxant effect on rat bladder smooth muscle, and this effect appears to be mediated by calcium channel antagonism.

Smooth muscle and neural mechanisms contributing to the downregulation of neonatal rat spontaneous bladder contractions during postnatal development

Spontaneous bladder contractions (SBCs) in the neonatal rat urinary bladder change from a high-amplitude, low-frequency pattern to a low-amplitude, high-frequency pattern during the first 6 wk of life. Understanding the mechanism of this developmental change may provide insights into the causes of bladder overactivity in adults. In vitro whole bladder preparations from Sprague-Dawley rats were used to study the modulation of SBCs by calcium-activated potassium channels (K(Ca)) and electrical field stimulation from 3 days to 6 wk of life. SBCs in 3-day-old bladders were unmasked by treatment with iberiotoxin (100 nM), an inhibitor of large conductance K(Ca) (BK) channels, or apamin (100 nM), an inhibitor of small conductance K(Ca) (SK) channels. Iberiotoxin significantly increased the magnitude of SBCs at 2-3 wk, whereas apamin was only effective at 6 wk. In 1-2 wk bladders, exposure to room temperature Krebs solution decreased SBCs. This decrease was reversed by activating intramural nerves with electrical field stimulation. The effect of electrical field stimulation was inhibited by atropine (1 microM), suramin (10 microM), or pretreatment with tetrodotoxin (1 microM) but was not reversed by tetrodotoxin applied after electrical field stimulation. BK-alpha mRNA increased threefold, and BK-alpha protein increased fivefold from 3 days to 6 wk. These data suggest that BK channels play an important role in the regulation of SBCs in the neonatal bladder and that both increased BK channel activity, as well as changes in smooth muscle sensitivity to locally released neurotransmitters contribute to the downregulation of SBCs during early postnatal development.

The extracorporeal perfusion of the female pig detrusor as an experimental model for the study of bladder contractility

AIMS

The objective of the study was to establish an experimental model for the extracorporeal perfusion of the pig detrusor. In order to validate this model we examined some biochemical parameters and determined the effect of carbachol on the contractility of perfused female pig bladders.

METHODS

Twenty-six pig bladders were perfused with Krebs-Ringer bicarbonate-glucose buffer for a period up to 5 hr with the aim to preserve a viable organ, which would be responsive to contraction-inducing agents. The intravesical pressure of the bladder as well as the intraarterial pressure of the vesical arteries were recorded before and after administration of carbachol.

RESULTS

The perfusate pH, lactate, partial carbon dioxide tension, and the ATP content in the perfused tissue, all indicators of tissue ischemia or cell necrosis, showed a good preservation of the organ for up to 5 hr. Carbachol was able to induce contractions of the prefilled bladder with a complete draining of the bladder throughout the whole perfusion period.

CONCLUSIONS

We could demonstrate that this perfusion system was able to preserve the pig bladder in a functional condition, appropriate for the study of physiological questions.

Spontaneous activity of mouse detrusor smooth muscle and the effects of the urothelium

AIMS

To characterize the detrusor muscle of the mouse urinary bladder in order to understand more precisely spontaneous contractile behavior of this organ. This study examined the spontaneous electrical activity and Ca(2+) dynamics of the detrusor smooth muscle and investigated the role of the urothelium.

MATERIALS AND METHODS

Detrusor smooth muscle strips were isolated from mouse bladders. The urothelium was either kept intact or removed. Changes in membrane potential were recorded using sharp electrode intracellular recording. To image Ca(2+) dynamics, tissue strips were exposed to 10 microM Oregon Green 488 BAPTA-1 AM for 70 min, and then image series were acquired with a laser-scanning confocal microscope.

RESULTS

(1) Mouse detrusor smooth muscle cells (SMCs) generate nifedipine-sensitive spontaneous action potentials (sAPs) at a low frequency (1.3 +/- 0.9 min(-1), n = 11) in preparations with intact urothelium. This frequency increased when the urothelium was removed (7 +/- 8.3 min(-1), n = 17) (P < 0.05, Student's t test). (2) Frequent ATP-mediated spontaneous depolarizations were recorded in all cells. (3) The frequency of whole cell Ca(2+) flashes of detrusor smooth muscle cells was higher in preparations with the urothelium removed (median 1.2 min(-1), n = 7) than in urothelium denuded preparations (median 0.6 min(-1), n = 7) (P < 0.01, Mann-Whitney U-test).

CONCLUSIONS

Spontaneous activity of the mouse detrusor smooth muscles was characterized enabling future comparative work on gene knock-out strains. Evidence suggesting release of an inhibitory factor by the urothelium was apparent.

Origin of spontaneous activity in neonatal and adult rat bladders and its enhancement by stretch and muscarinic agonists

This study examined the origin of spontaneous activity in neonatal and adult rat bladders and the effect of stretch and muscarinic agonists and antagonists on spontaneous activity. Rats were anesthetized and their bladders were excised, cannulated, and loaded with voltage- and Ca(2+)-sensitive dyes. Intracellular Ca(2+) and membrane potential transients were mapped using photodiode arrays in whole bladders, bladder sheets, or cross-section preparations at 37 degrees C. Intravesical pressure was recorded from whole bladders. In neonatal bladders and sheets, spontaneous Ca(2+) and electrical signals arose at a site near the dome and spread in a coordinated manner throughout the bladder with different dome-to-neck conduction velocities (Ca(2+): 3.7 +/- 0.4 mm/s; membrane potential: 46.2 +/- 3.1 mm/s). In whole bladders, optical signals were associated with spontaneous contractions (10-20 cmH(2)O). By contrast, in adult bladders spontaneous Ca(2+) and electrical activity was uncoordinated, originating at multiple sites and was associated with smaller (2-5 cmH(2)O) contractions. Spontaneous contractions and optical signals were insensitive to tetrodotoxin (2 muM) but were blocked by nifedipine (10 muM). Stretch or low carbachol concentrations (50 nM) applied to neonatal whole bladders enhanced the amplitude (to 20-35 cmH(2)O) of spontaneous activity, which was blocked by atropine. Bladder cross sections revealed that Ca(2+) and membrane potential transients produced by stretch or carbachol began near the urothelial-suburothelial interface and then spread to the detrusor. In conclusion, spontaneous activity in neonatal bladders, unlike activity in adult bladders, is highly organized, originating in the urothelium-suburothelium near the dome. Activity is enhanced by stretch or carbachol and this enhancement is blocked by atropine. It is hypothesized that acetylcholine is released from the urothelium during bladder filling to enhance spontaneous activity.

Relaxation of phasic contractile activity of human detrusor strips by cyclic nucleotide phosphodiesterase type 4 inhibition

OBJECTIVES

Detrusor smooth muscle relaxation is mainly mediated by the cyclic adenosine monophosphate (cAMP) pathway. Elevation of cAMP levels by phosphodiesterase type 4 (PDE4) inhibition relaxes smooth muscles of various origins. We aimed to determine the effect of a PDE4 inhibitor, rolipram, on human detrusor contractions.

METHODS

Human bladder strips (from 20 different donors) with no known overactive bladder (OAB) were studied in organ baths. Detrusor samples with or without urothelium were incubated with carbachol 10(-6)mol/l (in presence or absence of forskolin, 3.10(-7)mol/l) or with KCl 10mmol/l to enhance phasic contractile activity. Concentration response curves for rolipram or vehicle were then performed.

RESULTS

Rolipram (10(-9) to 3.10(-5)mol/l) induced a moderate relaxing effect on carbachol-induced contractions. This effect was enhanced when cAMP levels were increased by forskolin (the maximal effect was 53.0+/-5.1 vs. 83.1+/-5.7%, p<0.01) or in strips with urothelium. In contrast, rolipram (10(-9) to 10(-4)mol/l) drastically inhibited phasic contractile activity: The developed tension, the area under the curve, and the amplitude of phasic activity were reduced to 64.8+/-3.6, 91.2+/-5.3, and 82.3+/-7.3%, respectively, versus 23.6+/-9.5, 34.7+/-18.8, and 18.0+/-16.2% for vehicle, respectively (p<0.05). Frequency of phasic activity was 0.96+/-0.45 contractions per minute versus 2.6+/-0.18 for vehicle (p<0.001). In strips with urothelium, the inhibitory effect of rolipram on phasic contractile activity was similar.

CONCLUSIONS

PDE4 isoenzymes are strongly involved in the regulation of phasic myogenic activity of human bladder strips. Because an increase of this phasic activity may play a role in the pathophysiology of detrusor overactivity, PDE4 inhibitors might represent an attractive strategy for the treatment of OAB.

Enhancement of rat bladder contraction by artificial sweeteners via increased extracellular Ca2+ influx

INTRODUCTION

Consumption of carbonated soft drinks has been shown to be independently associated with the development of overactive bladder symptoms (OR 1.62, 95% CI 1.18, 2.22) [Dallosso, H.M., McGrother, C.W., Matthews, R.J., Donaldson, M.M.K., 2003. The association of diet and other lifestyle factors with overactive bladder and stress incontinence: a longitudinal study in women. BJU Int. 92, 69-77]. We evaluated the effects of three artificial sweeteners, acesulfame K, aspartame and sodium saccharin, on the contractile response of isolated rat detrusor muscle strips.

METHODS

Strips of detrusor muscle were placed in an organ bath and stimulated with electrical field stimulation (EFS) in the absence and presence of atropine, and with alpha,beta methylene ATP, potassium, calcium and carbachol.

RESULTS

Sweeteners 10(-7) M to 10(-2) M enhanced the contractile response to 10 Hz EFS compared to control (p<0.01). The atropine-resistant response to EFS was marginally increased by acesulfame K 10(-6) M, aspartame 10(-7) M and sodium saccharin 10(-7) M. Acesulfame K 10(-6) M increased the maximum contractile response to alpha,beta methylene ATP by 35% (+/-9.6%) (p<0.05) and to KCl by 12% (+/-3.1%) (p<0.01). Sodium saccharin also increased the response to KCl by 37% (+/-15.2%) (p<0.05). These sweeteners shifted the calcium concentration-response curves to the left. Acesulfame K 10(-6) M increased the log EC(50) from -2.79 (+/-0.037) to -3.03 (+/-0.048, p<0.01) and sodium saccharin 10(-7) M from -2.74 (+/-0.03) to 2.86 (+/-0.031, p<0.05). The sweeteners had no significant effect on the contractile response to carbachol but they did increase the amplitude of spontaneous bladder contractions.

DISCUSSION

These results suggest that low concentrations of artificial sweeteners enhanced detrusor muscle contraction via modulation of L-type Ca(+2) channels.

Interstitial cells of Cajal in the urethra

The smooth muscle layer of the urethra generates spontaneous myogenic tone that is thought to make a major contribution to urinary continence. The mechanisms underlying generation of tone remain unclear, however recent studies from our laboratory highlighted a role for a specialised population of pacemaker cells which we originally referred to as interstitial cells (IC) and now term ICC. Urethra ICC possess an electrical pacemaker mechanism characterised by rhythmic activation of Ca(2+)-activated Cl(-) channels leading to spontaneous transient inward currents (STICs) under voltage clamp and spontaneous transient depolarisations (STDs) under current clamp conditions. Both STICS and STDs are now known to be associated with spontaneous Ca(2+) oscillations that result from a complex interplay between release of Ca(2+) from intracellular stores and Ca(2+) influx across the plasma membrane. In this review we will consider some of the precise mechanisms involved in the generation of pacemaker activity and discuss how these are modulated by excitatory and inhibitory neurotransmitters.

Insights into the interstitium of ventricular myocardium: interstitial Cajal-like cells (ICLC)

We have previously described interstitial Cajal-like cells (ICLC) in human atrial myocardium. Several complementary approaches were used to verify the existence of ICLC in the interstitium of rat or human ventricular myocardium: primary cell cultures, vital stainings (e.g.: methylene blue), traditional stainings (including silver impregnation), phase contrast and non-conventional light microscopy (Epon-embedded semithin sections), transmission electron microscopy (TEM) (serial ultrathin sections), stereology, immunohistochemistry (IHC) and immunofluorescence (IF) with molecular probes. Cardiomyocytes occupy about 75% of rat ventricular myocardium volume. ICLC represent approximately 32% of the number of interstitial cells and the ratio cardiomyocytes/ICLC is about 70/1. In the interstitium, ICLC establish close contacts with nerve fibers, myocytes, blood capillaries and with immunoreactive cells (stromal synapses). ICLC show characteristic cytoplasmic processes, frequently two or three, which are very long (tens up to hundreds of microm), very thin (0.1-0.5 microm thick), with uneven caliber, having dilations, resulting in a moniliform aspect. Gap junctions between such processes can be found. Usually, the dilations are occupied by mitochondria (as revealed by Janus green B and MitoTracker Green FM) and elements of endoplasmic reticulum. Characteristically, some prolongations are flat, with a veil-like appearance, forming a labyrinthic system. ICLC display caveolae (about 1 caveola/ 1 microm cell membrane length, or 2-4% of the relative cytoplasmic volume). Mitochondria and endoplasmic reticulum (rough and smooth) occupy 5-10% and 1-2% of cytoplasmic volume, respectively. IHC revealed positive staining for CD34, EGFR and vimentin and, only in a few cases for CD117. IHC was negative for: desmin, CD57, tau, chymase, tryptase and CD13. IF showed that ventricular ICLC expressed connexin 43. We may speculate that possible ICLC roles might be: intercellular signaling (neurons, myocytes, capillaries etc.) and/or chemomechanical sensors. For pathology, it seems attractive to think that ICLC might participate in the process of cardiac repair/remodeling, arrhythmogenesis and, eventually, sudden death.

Interstitial Cajal-like cells (ICLC) in atrial myocardium: ultrastructural and immunohistochemical characterization

We have previously reported (Hinescu & Popescu, 2005) the existence of interstitial Cajal-like cells (ICLC), by transmission electron microscopy, in human atrial myocardium. In the present study, ICLC were identified with non-conventional light microscopy (NCLM) on semi-thin sections stained with toluidine blue and immunohistochemistry (IHC) for CD117/c-kit, CD34, vimentin and other additional antigens for differential diagnosis. Quantitatively, on semi-thin sections, ICLC represent about 1-1.5% of the atrial myocardial volume (vs. approximately 45% working myocytes, approximately 2% endothelial cells, 3-4% for other interstitial cells, and the remaining percentage: extracellular matrix). Roughly, there is one ICLC for 8-10 working atrial myocytes in the intercellular space, beneath the epicardium, with a characteristic (pyriform, spindle or triangular) shape. These ICLC usually have 2-3 definitory processes, emerging from cell body, which usually embrace atrial myocytes (260 nm average distance plasmalemma/sarcolemma) or establish close contact with nerve fibers or capillaries (approximately 420 nm average distance to endothelial cells). Cell prolongations are characteristic: very thin (mean thickness = 0.15+/-0.1 microm), very long for a non-nervous cell (several tens of microm) and moniliform (uneven caliber). Stromal synapses between ICLC and other interstitial cells (macrophages) were found (e.g. in a multicontact type synapse, the average synaptic cleft was approximately 65 nm). Naturally, the usual cell organelles (mitochondria, smooth and rough endoplasmic reticulum, intermediate filaments) are relatively well developed. Caveolae were also visible on cell prolongations. No thick filaments were detected. IHC showed that ICLC were slightly and inconsistently positive for CD117/c-kit, variously co-expressed CD34 and EGF receptor, but appeared strongly positive for vimentin, along their prolongations. Some ICLC seemed positive for a-smooth muscle actin and tau protein, but were negative for nestin, desmin, CD13 and S-100. In conclusion, we provide further evidence of the existence of ICLC in human atrial myocardium, supporting the possible ICLC role in pacemaking, secretion (juxta- and/or paracrine), intercellular signaling (neurons and myocytes). For pathology, ICLC might as well be 'players' in arrhythmogenesis and atrial remodeling.