Specialised pacemaking cells in the rabbit urethra

Properties of spontaneous Ca2+ transients recorded from interstitial cells of Cajal-like cells of the rabbit urethra in situ

Interstitial cells of Cajal-like cells (ICC-LCs) in the urethra may act as electrical pacemakers of spontaneous contractions. However, their properties in situ and their interaction with neighbouring urethral smooth muscle cells (USMCs) remain to be elucidated. To further explore the physiological role of ICC-LCs, spontaneous changes in [Ca(2+)](i) (Ca(2+) transients) were visualized in fluo-4 loaded preparations of rabbit urethral smooth muscle. ICC-LCs were sparsely distributed, rather than forming an extensive network. Ca(2+) transients in ICC-LCs had a lower frequency and a longer half-width than those of USMCs. ICC-LCs often exhibited Ca(2+) transients synchronously with each other, but did not often show a close temporal relationship with Ca(2+) transients in USMCs. Nicardipine (1 microm) suppressed Ca(2+) transients in USMCs but not in ICC-LCs. Ca(2+) transients in ICC-LCs were abolished by cyclopiazonic acid (10 microm), ryanodine (50 microm) and caffeine (10 mm) or by removing extracellular Ca(2+), and inhibited by 2-aminoethoxydiphenyl borate (50 microm) and 3-morpholino-sydnonimine (SIN-1; 10 microm), but facilitated by increasing extracellular Ca(2+) or phenylephrine (1-10 microm). These results indicated that Ca(2+) transients in urethral ICC-LCs in situ rely on both Ca(2+) release from intracellular Ca(2+) stores and Ca(2+) influx through non-L-type Ca(2+) channel pathways. ICC-LCs may not act as a coordinated pacemaker electrical network as do ICC in the gastrointestinal (GI) tract. Rather they may randomly increase excitability of USMCs to maintain the tone of urethral smooth muscles.

Urethral stromal tumor with pacemaker cell phenotype

Penile malignancies are rare in developed countries. The authors present a case of a penile urethral mesenchymal tumor occurring in a 51-year-old Caucasian male and displaying light microscopic, immunohistochemical, and ultrastructural features suggestive of a pacemaker cell type, combined with a lack of diagnostic features of any other established tumor category. The immunohistochemical profile was intensely positive for vimentin, PKC theta, and NSE and weakly positive to nonreactive for CD34 and smooth muscle actin, and entirely negative for CD117 (c-kit), S-100, and other markers. C-kit and PDGFRA gene analysis showed no mutations. Electron microscopy revealed tumor cells with plentiful cytoplasm and cytoplasmic processes/filopodia, both filled with intermediate filaments and occasional solitary focal densities. There were also prominent smooth endoplasmic reticulum cisternae, caveolae, neurosecretory granules, particularly concentrated in cytoplasmic processes, and synaptic-type structures. Poorly formed basal lamina, gap junctions, and intercellular collagen aggregates, consistent with skeinoid-type fibers, were also noted. Interstitial cells with potential pacemaker function have been recently described in the lower urinary tract, including the urethra, and this tumor may be related to this cellular phenotype.

Control of uterine Ca2+ by membrane voltage: toward understanding the excitation-contraction coupling in human myometrium

Myometrial contractility is a complex and dynamic physiological process that changes substantially during pregnancy and culminates in childbirth. Uterine contractions are initiated by transient rises in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)), which in turn are triggered and controlled by myometrial action potentials. The sequence of events between the action potential generation and the contraction initiation is referred to as excitation-contraction coupling. Hormones and other physiologically active substances affect myometrial contractility by modulating different steps in the excitation-contraction coupling process. It is therefore imperative that we understand that process to understand the regulation of myometrial contractility. The complex action potentials generated by human myometrium result from the activity of many ion channels, transporters, and pumps. Two types of myometrial action potential waveform have been described in the literature: a plateau type and a spike type. Parameters of the myometrial [Ca(2+)](i) transients and contractions differ depending on the type of action potential that triggers them. Some aspects of the excitation-contraction coupling are unique to human myometrium and cannot be found in animal models; some others are common between many species. This article reviews the current state and discusses future directions of physiological research on human myometrial excitation-contraction coupling.

Interstitial Cajal-like cells in human uterus and fallopian tube

Recently, parallels have been drawn between enteric interstitial cells of Cajal (ICC) and similar cells outside the gut-interstitial Cajal-like cells (ICLC). This article reviews our laboratory findings on ICLC in the female reproductive tract. Since the morphology and function of ICLC are still a subject of debate, our purpose was to investigate whether ICLC are present in the fallopian tube and/or uterus, and if they share ultrastructural and immunohistochemical (IHC) features and/or functional roles. We studied ICLC presence in the human fallopian tube and myometrium primarily by light microscopy, and then by transmission electron microscopy (TEM), in tissue samples and at a single cell level. Taking advantage of our ICLC studies of several organs (pancreas, mammary gland, myocardium), we assembled a set of criteria, derived from ultrastructural features of ICLC, called "platinum standard." Besides the putative pacemaker function, ICLC might have other physiological roles, depending on tissue type (e.g., intercellular signaling, immune surveillance, steroid sensors). Consequently, there is a great urge for a conceptual framework that could allow a better understanding, from a functional point of view, and more so, as the ICLC processes are the longest cellular prolongations (except neurons).

Kit-positive interstitial cells in the rabbit urethra: structural relationships with nerves and smooth muscle

OBJECTIVE

To identify interstitial cells (ICs) in the wall of the rabbit urethra using antibodies to the Kit receptor, and to examine their location, morphology and relationship with nerves and smooth muscle cells (SMCs), as studies of enzymatically isolated cells from the rabbit urethra have established that there are specialized cells that show spontaneous electrical activity and have morphological properties of ICs.

MATERIALS AND METHODS

Urethral tissues from rabbits were fixed, labelled with antibodies and examined with confocal microscopy. Some specimens were embedded in paraffin wax and processed for histology. Histological sections from the most proximal third and mid-third region of rabbit urethra were stained with Masson's Trichrome to show their cellular arrangement.

RESULTS

Sections from both regions had outer longitudinal and inner circular layers of SM, and a lamina propria containing connective tissue and blood vessels; the lumen was lined with urothelial cells. The mid-third region had a more developed circular SM layer than the most-proximal samples, and had extensive inner longitudinal SM bundles in the lamina propria. Labelling with anti-Kit revealed immunopositive cells within the wall of the rabbit urethra, in the circular and longitudinal layers of the muscularis. Double-labelling with an antibody to SM myosin showed Kit-positive cells on the boundary of the SM bundles, orientated parallel to the axis of the bundles. Others were in spaces between the bundles and often made contact with each other. Kit-positive cells were either elongated, with several lateral branches, or stellate with branches coming from a central soma. Similar cells could be labelled with vimentin antibodies. Their relationship with intramural nerves was examined by double immunostaining with an anti-neurofilament antibody. There were frequent points of contact between Kit-positive cells and nerves, with similar findings in specimens double-immunostained with anti-neuronal nitric oxide synthase (nNOS).

CONCLUSION

Kit-positive ICs were found within the SM layers of the rabbit urethra, in association with nerves, on the edge of SM bundles and in the interbundle spaces. The contact with nNOS-containing neurones might imply participation in the nitrergic inhibitory neurotransmission of the urethra.

Effects of temperature on pacemaker potentials in the mouse small intestine

The effects of temperature on the generation of pacemaker potentials recorded from myenteric interstitial cells of Cajal (ICC-MY) distributed in the mouse small intestine were investigated using intracellular recording techniques. In response to increasing temperatures in the range of 26-40 degrees C, the frequency and maximum rate of rise (dV/dt (max)) of pacemaker potentials were increased while their duration was decreased. The resting membrane potential and amplitude of the pacemaker potentials were not affected by change in temperature. Elevation of temperature decreased the amplitude, duration, and rise time of unitary potentials generated spontaneously during intervals between the pacemaker potentials. Metabolic inhibition (KCN and iodoacetic acid) decreased the frequency of pacemaker potentials with no alteration to the amplitude and dV/dt (max). Cyclopiazonic acid (3 muM), an inhibitor of the internal Ca(2+) pump, abolished pacemaker potentials in low-temperature conditions (<29 degrees C) but not at high-temperature conditions (>38 degrees C). These results suggest that the primary and plateau components of pacemaker potentials have different temperature sensitivities: the primary component is highly temperature-sensitive and is activated at higher temperatures, while the plateau component is formed by activation of temperature-insensitive mechanisms. The results also suggest that the mitochondria-induced intracellular Ca(2+) handling system seems to be involved in the initiation of the generation of pacemaker potentials but not in their configuration.

Heterogeneous CPA sensitivity of spontaneous excitation in smooth muscle of the rabbit urethra

1. To investigate the role of intracellular Ca stores in generating spontaneous excitation of the urethra, the effects of cyclopiazonic acid (CPA) on spontaneous contractions, transient increases in intracellular calcium concentration ([Ca2+]i; Ca transients) and depolarizations were examined in smooth muscles of the rabbit urethra. 2. In about 90% of circular smooth muscle (CSM) preparations, CPA (10 microM) increased the amplitude of spontaneous contractions by about 180% and reduced their frequency to some 25% of control values (CPA-resistant), while it readily abolished the contractions in the remaining preparations. 3. In about 70% of CSM preparations, CPA prevented the generation of spontaneous depolarizations termed slow waves, but increased their amplitude and duration in the remainder. CPA also prevented the generation of spontaneous Ca transients in about 40% of CSM preparations, while increasing their amplitude and duration in the remaining preparations. In CPA-resistant preparations that had been exposed to nicardipine (1 microM), subsequent CPA invariably abolished residual spontaneous depolarizations or Ca transients. CPA abolished caffeine-induced Ca transients in Ca-free solutions, suggesting that it effectively depleted intracellular Ca stores. 4. Longitudinal smooth muscles generated spontaneous action potentials, which had a shape distinct from that of slow waves in CSM. Spontaneous action potentials were abolished by nicardipine but not CPA. 5. Transmural nerve stimulation increased the frequency of Ca transients to give a sustained rise in [Ca2+]i, but inhibited their generation after blocking alpha-adrenoceptors with phentolamine (1 microM). These nerve-evoked responses were preserved in preparations that had been exposed to CPA. Similarly, both in control and CPA-treated CSM preparations, spontaneous Ca transients were accelerated by noradrenaline (NAd, 1 microM) and were suppressed by 3-morpholino-sydnonimine (SIN-1, 10 microM), a nitric oxide (NO) donor. 6. In conclusion, CSM of the urethra generates spontaneous activity, which depends on Ca release from intracellular Ca stores. However, after blocking this primary pacemaking mechanism, L-type Ca channel-dependent action potentials may drive CSM. Irrespective of the origin of pacemaking, neurally-released NAd and NO are capable of modulating spontaneous excitation.

Interstitial cells of Cajal in the murine gallbladder

OBJECTIVE

The presence of interstitial cells of Cajal (ICC) has been described throughout the digestive tract. In this study, we investigated whether ICC also exist in the gallbladder wall of CD1 mice.

MATERIAL AND METHODS

Immunofluorescent confocal microscopy was used to identify the morphology and distribution of ICC in either whole-mount flat preparations or enzyme-dispersed cells from the gallbladder of CD1 mice. Methylene blue staining and transmission electron microscopy were performed to detect the presence of ICC, and reverse transcription-polymerase chain reaction (RT-PCR) and Western blot were used to confirm the expressions of ICC-specific marker c-kit in the gallbladder tissues.

RESULTS

The presence of c-kit-positive cells was demonstrated in both the gallbladder wall and the enzyme-dispersed cells. The ICC were distributed throughout the wall of the gallbladder and organized into a network. Under transmission electron microscopy, ICC were characterized by their well-developed perinuclear endoplasmic reticulum, abundant mitochondria, free ribosomes and intermediate filaments, and distinctive caveolae and lack of myosin filaments. The expressions of ICC-specific marker c-kit were also confirmed in the gallbladder tissue.

CONCLUSION

The murine gallbladder wall contains ICC.

Interstitial cells and phasic activity in the isolated mouse bladder

OBJECTIVE

To describe the distribution of interstitial cells (ICs, defined as cells which show an increase in cGMP in response to nitric oxide, NO) in the isolated mouse bladder, and changes in phasic contractile activity after exposure to a NO donor.

MATERIALS AND METHODS

The whole bladder was removed from 17 female mice, killed by cervical dislocation. For immunohistochemistry (six mice) the bladder was incubated in carboxygenated Krebs' solution at 36 degrees C, containing 1 mm of the phosphodiesterase inhibitor isobutyl-methyl-xanthine. Individual pieces of tissue were exposed to 100 microm of the NO donor diethylamine NONOate for 10 min; control tissues remained in Krebs' solution. Tissues were then fixed in 4% paraformaldehyde and processed for cGMP immunohistochemistry. Bladder pressure was measured in bladders from 11 mice; the bladders were cannulated via the urethra and suspended in a heated chamber containing carboxygenated Tyrode solution at 33-35 degrees C and intravesical pressure recorded. All drugs were added to the solution bathing the abluminal surface.

RESULTS

NO induced an increase in cGMP in cells in the outer layers of the bladder wall, forming two distinct types based on their location; cells lying on the surface of the muscle bundles (surface muscle ICs) and cells within the muscle bundles (intramuscular ICs). Cholinergic nerve fibres were identified by the expression of vesicular acetylcholine transporter and neuronal NO synthase (nNOS). Choline acetyltransferase- and nNOS-positive nerves also had high cGMP levels in response to 100 microm diethylamine NONOate. In vitro exposure of an isolated whole unstimulated bladder to 100 microm diethylamine NONOate had no effect on resting bladder pressure. When whole bladders were exposed to muscarinic stimulation (30-100 nm arecaidine) there was an initial large transient rise in pressure followed by complex phasic changes in pressure. Adding 100 microm diethylamine NONOate abolished this phasic activity. Interestingly, the phasic activity was inhibited midway between the peak and trough of a phasic cycle. Such a pattern of inhibition might reflect the complexity of the phasic activity involving both excitatory and inhibitory components.

CONCLUSIONS

These data show the presence of NO/cGMP-sensitive ICs in the outer muscle layers of the mouse bladder. Activating these cells alters the pattern of muscarinic-induced phasic activity. We suggest that the role of the ICs in the outer muscle layers is to generate and modulate phasic activity. If so, then this is the first report of a functional role for ICs in the bladder.

Interaction between interstitial cells and smooth muscles in the lower urinary tract and penis

Smooth muscles in the lower urinary tract and corporal tissue exhibit spontaneous contractile activity which depends on L-type Ca(2+) channels. The mechanism underlying this activity is spontaneous electrical activity which shows varied form and property between these tissues. Recent studies revealed that interstitial cells (ICs) are widely distributed in the genitourinary system, and suggested their involvement in spontaneous muscle activity. ICs in the system are not a simple analogy of interstitial cells of Cajal (ICC) in the gut, which act as electrical pacemaker, but represent variability amongst tissues which may account for individual characteristics of each organ. In the bladder and corporal tissue, where smooth muscle cells are capable of generating spontaneous electrical activity, ICs may modulate smooth muscle activity. ICs in corporal tissue release prostaglandins via cyclooxygenase-2 (COX-2) activity and reinforce not only spontaneous but also nerve-mediated alpha-adrenergic contractions. In the bladder, their fundamental role in the integration of signals between populations of cells has been proposed, and thus changes in ICs may contribute to an overactive bladder, a pathological condition which results from increased excitability in detrusor smooth muscles. In the urethra, ICs may act as electrical pacemakers as do ICC. However, overall contractility of urethral smooth muscles does not necessarily rely on pacemaking of ICs, and thus some population of smooth muscles may also have their own excitability.

Ca2+ signalling in urethral interstitial cells of Cajal

Interstitial cells of Cajal (ICC) in the urethra have been proposed as specialized pacemakers that are involved in the generation of urethral tone and therefore the maintenance of urinary continence. Recent studies on freshly dispersed ICC from the urethra of rabbits have demonstrated that pacemaker activity in urethra ICC is characterized by spontaneous transient depolarizations (STDs) under current clamp and spontaneous transient inward currents (STICs) under voltage clamp. When these events were simultaneously recorded with changes in intracellular Ca(2+) (using a Nipkow spinning disk confocal microscope) they were found to be associated with global Ca(2+) oscillations. In this short review we will consider some of these recent findings regarding the contribution of intracellular Ca(2+) stores and Ca(2+) influx to the generation of pacemaker activity in urethral ICC with particular emphasis on the contribution of reverse Na(+)/Ca(2+) exchange (NCX).

Organization and function of ICC in the urinary tract

ICC are found in both the upper and lower urinary tract. They are not found in the ureter itself but are confined to the lamina propria of the renal pelvis and pelvi-calyceal junction. They do not appear to have a primary pacemaker role (this is ascribed to atypical smooth muscle cells in the same location) but rather conduct and amplify the pacemaker signals generated by the atypical smooth muscle cells. In the bladder, ICC are widely distributed in the sub-urothelial region, in the lamina propria and at the margins of the detrusor smooth muscle bundles. Again they appear not to have a pacemaking role and such evidence as there is would suggest that they have a role in the modulation of signal transduction. The strongest evidence that ICC in the urinary tract act as pacemakers comes from studies of those in the urethra. Isolated ICC show regular spontaneous depolarizations in current clamp which resemble very closely the slow waves recorded from intact tissue. In voltage clamp they show abundant calcium-activated chloride current and spontaneous transient inward currents which can be blocked by chloride channel blockers. However, their role in the modulation of urethral tone has yet to be fully elucidated.

Interstitial cells of Cajal in erectile dysfunction

The corpora cavernosa (CC) evokes electric activity. Slow waves (SWs) appear to originate from interstitial cells of Cajal (ICCs), which seem to control the activity of the smooth muscle cells (SMC). The ICCs were demonstrated to exist in the CC. We investigated the hypothesis that the ICC distribution differs with each of the various ED types. The study comprised 62 men with ED: 16 neurogenic (NGED), 15 arteriogenic (AGED), 11 venogenic (VGED) and 22 psychogenic (PGED). 15 volunteers with normal erections acted as controls. The patients underwent a complete diagnostic evaluation. A biopsy of 3 x 3 mm from the CC was subjected to C-kit immunohistochemistry examination. Specificity control of the antisera consisted of incubation of the tissue with normal rabbit serum substituted for the primary antiserum. C-kit positive stellate-appearing cells resembling those of ICC were detected in the controls. The branches were either laterally located (multipolar) or lying at each pole (bipolar). They were distinguishable from the SMC, which were C-kit negative. ICC were detected in all specimens from patients with NGED and VGED, absent in 13/15 with AGED and scanty in PGED. ICC distribution was different in the various types of ED. It is suggested that this distribution interferes with SW discharge and the control of SMC activity with a resulting ED.

Interstitial cells of Cajal: a new perspective on smooth muscle function

Interstitial cells of Cajal (ICC) were described more than 100 years ago by Ramon y Cajal. For many years these cells were identified only by non-specific histological stains and later, more reliably, by electron microscopy. Ultrastructural features and the anatomical locations of ICC suggested important physiological roles for these cells. A breakthrough occurred in our ability to study ICC when it was recognized that antibodies for Kit could be used to identify ICC, even in living tissues. Signalling via Kit, a receptor tyrosine kinase, is also necessary for ICC development and maintenance of phenotype. Thus, blocking Kit, by a variety of techniques, caused loss of ICC in experimental animals and demonstrated the critical physiological functions of these cells in gastrointestinal motility. Loss of ICC in human gastrointestinal diseases may contribute to the motor pathologies observed. Unrestrained Kit signalling leads to the transformation of ICC and the development of gastrointestinal stromal tumours. Now ICC-like cells have been identified in a variety of smooth muscle tissues, and the race is on to discover whether these cells have equivalent or even novel functions in organs outside the gastrointestinal tract. This perspectives article gives a short overview of the history of ICC research and directions for future investigation.

Multiple mechanisms involved in oxytocin-induced modulation of myometrial contractility

Oxytocin is a small peptide hormone with multiple sites of action in human body. It regulates a large number of reproduction-related processes in all species. Particularly important is its ability to stimulate uterine contractility. This is achieved by multiple mechanisms involving sarcoplasmic reticulum Ca2+ release and sensitization of the contractile apparatus to Ca2+. In this paper, we review the data published by us and other groups on oxytocin-induced modulation of uterine contractility. We conclude that sensitization of contractile apparatus to Ca2+ is the most relevant physiological effect of oxytocin on human myometrium.

Localization of M 2 and M 3 Muscarinic Receptors in Human Bladder Disorders and Their Clinical Correlations

PURPOSE

We studied the cellular localization of muscarinic receptor subtypes 2 and 3 in the human bladder and related any changes in overactive and painful bladder syndromes to measures of clinical dysfunction.

MATERIALS AND METHODS

Bladder specimens obtained from patients with painful bladder syndrome (11), idiopathic detrusor overactivity (12) and from controls with asymptomatic microscopic hematuria (16) were immunostained using specific antibodies to muscarinic receptor subtypes 2 and 3, and to vimentin, which is a marker for myofibroblasts. Immunostaining results were quantified with computerized image analysis and correlated with clinical dysfunction using frequency and urgency scores.

RESULTS

Muscarinic receptor subtype 2 and 3 immunoreactivity was observed in the urothelium, nerve fibers and detrusor layers. In addition, strong myofibroblast-like cell staining, similar to vimentin, was present in the suburothelial region and detrusor muscle. A significant increase in suburothelial myofibroblast-like muscarinic receptor subtype 2 immunoreactivity was seen in patients with painful bladder syndrome (p = 0.0062) and idiopathic detrusor overactivity (p = 0.0002), and in muscarinic receptor subtype 3 immunoreactivity in those with idiopathic detrusor overactivity (p = 0.0122) with a trend in painful bladder syndrome. Muscarinic receptor subtype 2 and 3 immunoreactivity significantly correlated with the urgency score (p = 0.0002 and 0.0206, respectively) and muscarinic receptor subtype 2 immunoreactivity correlated with the frequency score (p = 0.0029). No significant difference was seen in urothelial and detrusor muscarinic receptor subtypes 2 and 3 or vimentin immunostaining.

CONCLUSIONS

To our knowledge this is the first study to show the cellular localization of muscarinic receptor subtypes 2 and 3 in the human bladder. The increase in muscarinic receptor subtypes 2 and 3 immunostaining in myofibroblast-like cells in clinical bladder syndromes and its correlation with clinical scores suggests a potential role in pathophysiological mechanisms and the therapeutic effect of anti-muscarinic agents.

Calcium currents in interstitial cells from the guinea-pig bladder

OBJECTIVE

To determine whether there are inward currents in interstitial cells (IC) isolated from the guinea-pig detrusor and if so, to characterise them using the patch-clamp technique and pharmacological agents.

MATERIALS AND METHODS

Using the whole-cell patch-clamp technique, inward currents were studied in IC enzymatically isolated from the detrusor of the guinea-pig bladder. Currents were evoked by stepping positively from a holding potential of - 80 mV.

RESULTS

Outward K+ currents were blocked by Cs+ internal solution to reveal inward currents, which activated at voltages more positive than - 50 mV, peaked at 0 mV, reversed near + 50 mV and were half-maximally activated at - 27 mV. The inward currents showed voltage-dependent inactivation and were half-maximally inactivated at - 36 mV. Fitting the activation and inactivation data with a Boltzmann function revealed a window current between - 40 mV and + 20 mV. The decay of the current evoked at 0 mV could be fitted with a single exponential with a mean time-constant of 88 ms. Replacing external Ca2+ with Ba2+ significantly increased this to 344 ms. The current amplitude was augmented by Ba2+, and by Bay K 8644. Inward currents were significantly reduced by 1 microm nifedipine, across the voltage range, but the blockade was more effective on the current evoked at 0 mV than that evoked by a step to - 20 mV, perhaps indicating voltage-dependence of the action of nifedipine or another component of inward current. Increasing the concentration of the drug to 10 microm caused no further significant reduction either at 0 mV or at -20 mV. However, in the presence of 1 microm nifedipine the latter current was significantly reduced by 100 microm Ni2+. Both currents were significantly reduced in Ca2+-free solution.

CONCLUSIONS

IC from the guinea-pig detrusor possess inward currents with typical characteristics of L-type Ca2+ current. They also have a component of inward Ca2+ current, which was resistant to nifedipine, but sensitive to Ni2+. Further work is needed to characterise the latter conductance.

Role of intracellular stores in the regulation of rhythmical [Ca2+]i changes in interstitial cells of Cajal from rabbit portal vein

Interstitial cells of Cajal (ICCs) freshly isolated from rabbit portal vein and loaded with the Ca(2+)-sensitive indicator fluo-3 revealed rhythmical [Ca(2+)](i) changes occurring at 0.02-0.1 Hz. Each increase in [Ca(2+)](i) originated from a discrete central region of the ICC and propagated as a [Ca(2+)](i) wave towards the cell periphery, but usually became attenuated before reaching the ends of the cell. In about 40% of ICCs each rhythmical change in [Ca(2+)](i) consisted of an initial [Ca(2+)](i) increase (phase 1) followed by a faster rise in [Ca(2+)](i) (phase 2) and then a decrease in [Ca(2+)](i) (phase 3); the frequency correlated with the rate of rise of [Ca(2+)](i) during phase 1, but not with the peak amplitude. Rhythmical [Ca(2+)](i) changes persisted in nicardipine, but were abolished in Ca(2+)-free solution as well as by SK&F96365, cyclopiazonic acid, thapsigargin, 2-APB, xestospongin C or ryanodine. Intracellular Ca(2+) stores visualised with the low-affinity Ca(2+) indicator fluo-3FF were found to be enriched with ryanodine receptors (RyRs) detected with BODIPY TR-X ryanodine. Rhythmical [Ca(2+)](i) changes originated from a perinuclear S/ER element showing the highest RyR density. Immunostaining with anti-TRPC3,6,7 antibodies revealed the expression of these channel proteins in the ICC plasmalemma. This suggests that these rhythmical [Ca(2+)](i) changes, a key element of ICC pacemaking activity, result from S/ER Ca(2+) release which is mediated via RyRs and IP(3) receptors and is modulated by the activity of S/ER-Ca(2+)-ATPase and TRP channels but not by L-type Ca(2+) channels.

Contribution of reverse Na+-Ca2+ exchange to spontaneous activity in interstitial cells of Cajal in the rabbit urethra

Interstitial cells of Cajal (ICC) isolated from the rabbit urethra exhibit regular Ca2+ oscillations that are associated with spontaneous transient inward currents (STICs) recorded under voltage clamp. Their frequency is known to be very sensitive to external Ca2+ concentration but the mechanism of this has yet to be elucidated. In the present study experiments were performed to assess the role of Na+-Ca2+ exchange (NCX) in this process. Membrane currents were recorded using the patch clamp technique and measurements of intracellular Ca2+ were made using fast confocal microscopy. When reverse mode NCX was enhanced by decreasing the external Na+ concentration [Na+]o from 130 to 13 mM, the frequency of global Ca2+ oscillations and STICs increased. Conversely, inhibition of reverse mode NCX by KB-R7943 and SEA0400 decreased the frequency of Ca2+ oscillations and STICs. Application of caffeine (10 mM) and noradrenaline (10 microM) induced transient Ca2+-activated chloride currents (I(ClCa)) at -60 mV due to release of Ca2+ from ryanodine- and inositol trisphosphate (IP3)-sensitive Ca2+ stores, respectively, but these responses were not blocked by KB-R7943 or SEA0400 suggesting that neither drug blocked Ca2+-activated chloride channels or Ca2+ release from stores. Intact strips of rabbit urethra smooth muscle develop spontaneous myogenic tone. This tone was relaxed by application of SEA0400 in a concentration-dependent fashion. Finally, single cell RT-PCR experiments revealed that isolated ICC from the rabbit urethra only express the type 3 isoform of the Na+-Ca2+ exchanger (NCX3). These results suggest that frequency of spontaneous activity in urethral ICC can be modulated by Ca2+ entry via reverse NCX.

Activation of the cGMP/PKG pathway inhibits electrical activity in rabbit urethral interstitial cells of Cajal by reducing the spatial spread of Ca2+ waves

In the present study we used a combination of patch clamping and fast confocal Ca2+ imaging to examine the effects of activators of the nitric oxide (NO)/cGMP pathway on pacemaker activity in freshly dispersed ICC from the rabbit urethra, using the amphotericin B perforated patch configuration of the patch-clamp technique. The nitric oxide donor, DEA-NO, the soluble guanylyl cyclase activator YC-1 and the membrane-permeant analogue of cGMP, 8-Br-cGMP inhibited spontaneous transient depolarizations (STDs) and spontaneous transient inward currents (STICs) recorded under current-clamp and voltage-clamp conditions, respectively. Caffeine-evoked Cl- currents were unaltered in the presence of SP-8-Br-PET-cGMPs, suggesting that activation of the cGMP/PKG pathway does not block Cl- channels directly or interfere with Ca2+ release via ryanodine receptors (RyR). However, noradrenaline-evoked Cl- currents were attenuated by SP-8-Br-PET-cGMPs, suggesting that activation of cGMP-dependent protein kinase (PKG) may modulate release of Ca2+ via IP3 receptors (IP3R). When urethral interstitial cells (ICC) were loaded with Fluo4-AM (2 microm), and viewed with a confocal microscope, they fired regular propagating Ca2+ waves, which originated in one or more regions of the cell. Application of DEA-NO or other activators of the cGMP/PKG pathway did not significantly affect the oscillation frequency of these cells, but did significantly reduce their spatial spread. These effects were mimicked by the IP3R blocker, 2-APB (100 microm). These data suggest that NO donors and activators of the cGMP pathway inhibit electrical activity of urethral ICC by reducing the spatial spread of Ca2+ waves, rather than decreasing wave frequency.

Phosphodiesterase 5 in the female pig and human urethra: morphological and functional aspects

OBJECTIVE

To characterize the distribution of phosphodiesterase 5 (PDE-5), cGMP and cGMP-dependent protein kinase I (PKG1), and to evaluate the effect of pharmacological inhibition of PDE-5 in isolated preparations of pig and human urethra, as the nitric oxide (NO)/cGMP pathway generates the main inhibitory signals to reduce resistance in the bladder outlet and urethra during emptying of the bladder.

MATERIALS AND METHODS

After obtaining ethics committee approval, urethral specimens were obtained from three female patients during cystectomy, and from young female pigs. The specimens were prepared for immunohistochemical investigations and for functional studies in organ baths. Effects of sildenafil, vardenafil and tadalafil (1 nm to 30 microm) were studied in l-noradrenaline (1 microm)-activated or spontaneously contracted preparations, and on relaxations induced by electrical-field stimulation (EFS). Levels of cGMP were determined by radioimmunoassay.

RESULTS

After stimulation with the NO donor, DETA NONO-ate (1 mm), there was greater cGMP-immunoreactivity (IR) in urethral and vascular smooth muscles. There was a wide distribution of cGMP- and vimentin-positive interstitial cells between pig urethral smooth muscle bundles. There was also cGMP-IR within NO-synthase-IR endothelium. There was PDE-5 IR within the urethral and vascular smooth muscle cells, but also in vascular endothelial cells that expressed cGMP-IR. In pig and human sections, there was strong PKG1-IR in alpha-actin-IR urethral smooth muscle cells that also contained IR for cGMP. Sildenafil, vardenafil and tadalafil caused mean (sem) concentration-dependent relaxations of the pig urethra which, at 30 microm, were 80 (3)% (11 samples), 81 (5)% (12 samples) and 64 (4)% (10 samples) of the spontaneous tone. The relaxation of L-noradrenaline-contracted female human urethra was 100% in response to 10 microm sildenafil, and 85 (15)% and 47 (13)% for 30 microm of vardenafil and tadalafil, respectively (three samples). Vardenafil or sildenafil (30 microm) doubled cGMP levels in pig specimens. There were no effects on cGMP levels with tadalafil. EFS (1-32 Hz) caused l-NG-nitroarginine-sensitive relaxations of pig urethral muscle that were increased in amplitude and duration by PDE-5 inhibition. At 0.1 microm, sildenafil, vardenafil or tadalafil significantly prolonged the mean (sem) duration of the relaxation at 4 Hz by 55 (19)%, 45 (14)% and 51 (12)%, respectively.

CONCLUSIONS

PDE-5-, cGMP- and PKG1-IR is widely distributed in human and pig urethral tissues. Nerve-induced relaxations of urethral preparations were enhanced at low concentrations of sildenafil, vardenafil and tadalafil, whereas there were direct smooth muscle-relaxant actions of the PDE-5 inhibitors at high concentrations. Inhibition of PDE-5 might be an interesting option to facilitate cGMP-mediated relaxation of the outflow region.

Mechanisms of Disease: specialized interstitial cells of the urinary tract--an assessment of current knowledge

Scientists interested in the smooth muscles of the urinary tract, and their control, have recently been studying cells in the interstitium of tissues that express the c-kit antigen (Kit(+) cells). These cells have morphologic features that are reminiscent of the well-described pacemaker cells in the gut, the interstitial cells of Cajal (ICC). The spontaneous contractile behavior of muscles in the urinary tract varies widely, and it is clear that urinary tract Kit(+) interstitial cells cannot be playing an identical role to that played by the ICC in the gut. Nevertheless, there is increasing evidence that they do play a role in modulating the contractile behavior of adjacent smooth muscle, and might also be involved in mediating neural control. This review outlines the properties of ICC in the gut, and gives an account of the discovery of cells in the interstitium of the main components of the urinary tract. The physiologic properties of such cells and the functional implications of their presence are discussed, with particular reference to the bladder. In this organ, Kit(+) cells are found under the lamina propria, where they might interact with the urothelium and with sensory nerves, and also between and within the smooth-muscle bundles. Confocal microscopy and calcium imaging are being used to assess the physiology of ICC and their interactions with smooth muscles. Differences in the numbers of ICC are seen in smooth muscle specimens obtained from patients with various pathologies; in particular, bladder overactivity is associated with increased numbers of these cells.

The functional effects of a c-kit tyrosine inhibitor on guinea-pig and human detrusor

OBJECTIVE

To describe the effect of a specific c-kit receptor inhibitor (imatinib mesylate) on human detrusor strips in vitro and guinea-pig cystometry in vivo, and to show histological data suggesting differences in the distribution of interstitial cells of Cajal (ICC)-like cells in 'normal' and overactive human detrusor, as these cells have been identified as possible mediators of spontaneous activity and excitability in bladder smooth muscle.

MATERIALS AND METHODS

Specimens of human detrusor were stained immunohistochemically with a c-kit antibody. Human detrusor strips were mounted in a superfused organ-bath apparatus, and smooth muscle contraction was evoked with carbachol and electrical field stimulation in the presence and absence of imatinib mesylate. Also, guinea-pig urodynamic studies were conducted before and after i.v. administration of imatinib mesylate, and changes in bladder variables and spontaneous activity were recorded.

RESULTS

Imatinib mesylate (10(-6)M) inhibited evoked smooth muscle contraction and spontaneous activity in overactive human detrusor, with less effect on normal human tissue. Imatinib mesylate (10(-5)M) improved bladder capacity, compliance, voided volumes, urinary frequency, and reduced contraction thresholds and spontaneous activity during guinea-pig cystometry. c-kit labelling showed significantly more ICC-like cells in overactive human detrusor than in normal specimens.

CONCLUSION

c-kit receptor blockers have inhibitory effects on guinea-pig and overactive human detrusor, possibly via c-kit receptors on bladder ICC-like cells. This and the possibility that there are more ICC-like cells in overactive bladder suggest that the c-kit receptor may provide a novel target for treating detrusor overactivity.

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.

Contractile properties of the proximal urethra and bladder in female pig: morphology and function

AIMS

To compare the contractile properties of proximal urethral and bladder muscle of the female pig.

MATERIALS AND METHODS

In two proximal segments (I and II) of the urethra, small muscle bundles were excised to measure the force-length (maximum force) and the force-velocity (unloaded shortening velocity) relation using the stop-test. The rate of force development was calculated using phase plots. Contractile properties of urethral and bladder segments were statistically compared using the Mann-Whitney U-test. Immunohistochemical staining of whole circumference urethral cross sections was used to identify the location of smooth and striated muscle fibres.

RESULTS

On isometric force development, the urethral muscle bundles revealed a fast ( approximately 0.5 sec) and a slow ( approximately 2.1 sec) time constant, whereas in bladder only a slow ( approximately 2.3 sec) component was measured. On average, isometric force was highest in bladder. The length range over which force was produced was smallest in urethral segment II, followed by urethral segment I and finally bladder. The unloaded shortening velocity was 0.15, 0.25 and 0.35 1/sec, respectively. Histological preparations showed that smooth as well as striated muscle was present in proximal urethra. In urethral muscle bundles, spontaneous contractions were measured with a frequency of 0.4 Hz.

CONCLUSIONS

Differences in contractility found between urethra and bladder may be ascribed to the presence of striated muscle in the proximal urethra. The regulation of tone and spontaneous contractions may be part of the continence mechanism in the female pig urinary tract.

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

Smooth muscles from the urethra and bladder display characteristic patterns of spontaneous contractile activity in the filling phase of the micturition cycle. Tonic contractions are seen in the urethral smooth muscles, and phasic contractions occur in the detrusor. Overactivity in the detrusor is a common clinical problem. The ion channels in the smooth muscle membranes play an important role in determining the functional properties, and are obvious targets for treatment of the overactive bladder. Recent evidence suggests that interstitial cells may also play a role in determining the pattern of spontaneous activity, although their precise role is less well established in the urinary tract than in the gut. The ion channels involved in these cells are also of interest. This review discusses what is known of ion channels in these tissues, and their implications for function.

Inherent pacemaker function of duodenal GIST

Gastrointestinal stromal tumours (GIST) are thought to derive from interstitial cells of Cajal (ICCs), which are putative pacemaker cells for gut motility. Isolated cells were obtained by enzymatic treatment of human duodenum GIST tissue having a frequent gain-of-function gene mutation. After cell culturing, c-Kit immunoreactivity was preserved and the cells developed long processes. Whole cell patch clamp recordings revealed voltage-dependent outward currents, without transient inward currents. Intracellular Ca(2+) measurements showed oscillation-like spontaneous activity in some GIST cells. RT-PCR revealed expression of ion channels (Kv1.1, Kv1.6 and KCNH2; IP3R1, and IP3R2; TRPC1, 3, 6 and 7; Cx43), which have been suggested to play important roles in pacemaker activity. However, SCN5A, a TTX-resistant Na(+) channel known to be expressed in human ICCs, was below detectable levels. These data suggest that GIST cells appear to preserve some, but not all ionic mechanisms underlying pacemaker activity in ICC.

Origin of spontaneous rhythmicity in smooth muscle

Rhythmic electrical activity is a feature of most smooth muscles but the mechanical consequences can vary from regular rapid phasic contractions to sustained contracture. For many years it was thought that spontaneous electrical activity originated in smooth muscle cells but recently it has become apparent that there are specialized pacemaker cells in many organs that are morphologically and functionally distinct from smooth muscle and that the former cells are the source of spontaneous electrical activity. Such a pacemaker function is well documented for the ICC of the gastrointestinal tract but evidence is accumulating that ICC-like cells play a similar role in other types of smooth muscle. We have recently shown that there are specialized pacemaking cells in the rabbit urethra which are spontaneously active when freshly isolated, readily distinguishable from smooth muscle cells under bright field illumination and relatively easy to study using patch-clamp and confocal imaging techniques. Recent results suggest that calcium oscillations in isolated rabbit urethral interstitial cells are initiated by calcium release from ryanodine sensitive intracellular stores, that oscillation frequency is very sensitive to the external calcium concentration and that conversion of the primary oscillation to a propagated calcium wave depends upon IP3-induced calcium release.

Pacemaker activity in urethral interstitial cells is not dependent on capacitative calcium entry

The aim of the present study was to investigate the properties and role of capacitative Ca(2+) entry (CCE) in interstitial cells (IC) isolated from the rabbit urethra. Ca(2+) entry in IC was larger in cells with depleted intracellular Ca(2+) stores compared with controls, consistent with influx via a CCE pathway. The nonselective Ca(2+) entry blockers Gd(3+) (10 microM), La(3+) (10 microM), and Ni(2+) (100 microM) reduced CCE by 67% (n = 14), 65% (n = 11), and 55% (n = 9), respectively. These agents did not inhibit Ca(2+) entry when stores were not depleted. Conversely, CCE in IC was resistant to SKF-96365 (10 microM), wortmannin (10 microM), and nifedipine (1 microM). Spontaneous transient inward currents were recorded from IC voltage-clamped at -60 mV. These events were not significantly affected by Gd(3+) (10 microM) or La(3+) (10 microM) and were only slightly decreased in amplitude by 100 microM Ni(2+). The results from this study demonstrate that freshly dispersed IC from the rabbit urethra possess a CCE pathway. However, influx via this pathway does not appear to contribute to spontaneous activity in these cells.

C-kit immunopositive interstitial cells (Cajal-type) in human myometrium

Previous reports describing Cajal-like interstitial cells in human uterus are contradictory in terms of c-kit immunoreactivity: either negative (but vimentin-positive) in pregnant myometrium, or positive, presumably in the endometrium. The aim of this study was to verify the existence of human myometrial Cajal-like interstitial cells (m-CLIC). Six different, complementary approaches were used: 1) methylene-blue supravital staining of tissue samples (cryosections), 2) methylene blue and Janus green B vital staining (m-CLIC and mitochondrial markers, respectively), and 3) extracellular single-unit electrophysiological recordings in cell cultures, 4) non-conventional light microscopy on glutaraldehyde/osmium fixed, Epon-embedded semi-thin sections (less than 1 microm) stained with toluidine blue (TSM), 5) transmission electron microscopy (TEM), and 6) immunofluorescence (IF). We found m-CLIC in myometrial cryosections and in cell cultures. In vitro, m-CLIC represented approximately 7% of the total cell number. m-CLIC had 2-3 characteristic processes which were very long (approximately 60 microm), very thin (< or =0.5 microm) and moniliform. The dilated portions of processes usually accommodated mitochondria. In vitro, m-CLIC exhibited spontaneous electrical activity (62.4+/-7.22 mV membrane potentials, short duration: 1.197+/-0.04 ms). Moreover, m-CLIC fulfilled the usual TEM criteria, the so-called 'gold' or 'platinum' standards (e.g. the presence of discontinuous basal lamina, caveolae, endoplasmic reticulum, and close contacts between each other, with myocytes, nerve fibers and/or capillaries etc.). IF showed that m-CLIC express CD117/c-kit, sometimes associated with CD34, with vimentin along their processes. In conclusion, we describe myometrial Cajal-like interstitial cells that have affinity for methylene blue and Janus green B vital dyes, fulfill (all) TEM criteria, express CD117/c-kit and have spontaneous electric activity.

Interstitial cells in the vasculature

Interstitial cells of Cajal are believed to play an important role in gastrointestinal tissues by generating and propagating electrical slow waves to gastrointestinal muscles and/or mediating signals from the enteric nervous system. Recently cells with similar morphological characteristics have been found in the wall of blood vessels such as rabbit portal vein and guinea pig mesenteric artery. These non-contractile cells are characterised by the presence of numerous processes and were easily detected in the wall of the rabbit portal vein by staining with methylene blue or by antibodies to the marker of Interstitial Cells of Cajal c-kit. These vascular cells have been termed "interstitial cells" by analogy with interstitial cells found in the gastrointestinal tract. Freshly dispersed interstitial cells from rabbit portal vein and guinea pig mesenteric artery displayed various Ca2+-release events from endo/sarcoplasmic reticulum including fast localised Ca2+ transients (Ca2+ sparks) and longer and slower Ca2+ events. Single interstitial cells from the rabbit portal vein, which is a spontaneously active vessel, also demonstrated rhythmical Ca2+ oscillations associated with membrane depolarisations, which suggests that in this vessel interstitial cells may act as pacemakers for smooth muscle cells. The function of interstitial cells from the mesenteric arteries is yet unknown. This article reviews some of the recent findings regarding interstitial cells from blood vessels obtained by our laboratory using electron microscopy, immunohistochemistry, tight-seal patch-clamp recording, and fluorescence confocal imaging techniques.

About the presence of interstitial cells of Cajal outside the musculature of the gastrointestinal tract

Santiago Ramon y Cajal observed a special cell type that appeared to function as endstructures of the intrinsic nervous system in several organs. These cells were structurally and functionally further characterized in the gut musculature and named interstitial cells of Cajal (ICC). In recent years, interstitial cells have been identified in the vasculature, urinary tract, glands and other organs. Their morphologies and functions are just beginning to be clarified. It is likely that amongst them, subtypes will be discovered that warrant the classification of interstitial cells of Cajal. This "point of view" continues the discussion on the criteria that should be used to identify ICC outside the musculature of the gut.

Sulphonylurea receptors differently modulate ICC pacemaker Ca2+ activity and smooth muscle contractility

Appropriate gastrointestinal motility is essential to properly control the body energy level. Intracellular Ca2+ ([Ca2+]i) oscillations in interstitial cells of Cajal (ICCs; identified with c-Kit immunoreactivity) are considered to be the primary mechanism for the pacemaker activity in gastrointestinal motility. In the present study, RT-PCR examinations revealed predominant expression of the type 1 isoform of sulphonylurea receptors (SUR1) in ICCs of the mouse ileum, but expression of SUR2 was predominant in smooth muscle. In cell clusters prepared from the same tissue, smooth muscle contractility and pacemaker [Ca2+]i activity in ICCs were found to be differentially modulated by K(ATP) channel openers and sulphonylurea compounds, in accordance with the expression of SUR isoforms. 1 microM cromakalim nearly fully suppressed the mechanical activity in smooth muscle, whereas ICC pacemaker [Ca2+]i oscillations persisted. Greater concentrations (approximately 10 microM) of cromakalim attenuated pacemaker [Ca2+]i oscillations. This effect was not reversed by changing the reversal potential of K+, but was prevented by glibenclamide. Diazoxide at 30 muM terminated ICC pacemaker [Ca2+]i oscillations, but again treatment with high extracellular K+ did not restore them. These results suggest that SUR can modulate pacemaker [Ca2+]i oscillations via voltage-independent mechanism(s), and also that intestinal pacemaking and glucose control are closely associated with SUR.

Identification of interstitial cells of Cajal in human urinary bladder: concept of vesical pacemaker

OBJECTIVES

To determine whether, given the presence of interstitial cells of Cajal (ICCs) in organs that discharge slow waves, such as the gastrointestinal tract, such cells could also be present in the human urinary bladder (UB) and might exist in great numbers in the bladder dome, constituting the "primary pacemaker" from which slow waves spread to the body of the UB. This hypothesis was based on previous studies that had demonstrated the vesical electric waves starting in the dome and spreading caudad.

METHODS

Specimens (0.5 x 0.5 cm) from different vesical locations were obtained from the cystectomized UB of 18 patients (12 men, 6 women, mean age 42.6 +/- 3.8 years) with bladder cancer. Fixed sections were prepared and subjected to c-kit immunohistochemistry. Controls for the antiserum specificity consisted of incubation of the tissue with normal rabbit serum substituted for the primary antiserum.

RESULTS

Fusiform, c-kit positive, and ICC-like cells were detected in the vesical muscle layers; they had dendritic processes. They occurred separately or were connected through the dendritic processes to form a cellular network. An accumulation of ICCs surrounded by a connective tissue layer was detected in all the dome specimens. Mast cells occurred in the vesical mucosa and submucosa; they were c-kit positive but had a rounded body with no dendritic processes.

CONCLUSIONS

We identified cells in the UB with morphologic and immunologic phenotypes similar to the ICCs of the gut. An accumulation of these cells, detected in the vesical dome, seems to constitute the "primary" vesical pacemaker that initiates the slow waves that spread to the other vesical walls. We believe that a deficiency or absence of these cells may be involved in vesical motility disorders. Additional studies are needed to delineate the role of these cells in UB physiology and pathologic conditions.

Age-dependence of the spontaneous activity of the rat urinary bladder

Abnormal mechanical function of the bladder is manifested in a number of ways including higher frequency of involuntary detrusor contractions associated with reduced compliance of the bladder that is responsible for an increase in intraluminal pressure during filling. There are basically two ways to approach experimentally these problems: (1) by studying the neural control of the lower urinary tract function, and (2) by measuring the properties of smooth muscle cells in the bladder wall. Studies on smooth muscle function often do not take the origin of smooth muscle cells into account i.e., whether they were harvested from normal or overactive bladders. Although, this simplistic view may be beneficial to understanding the generation of the spontaneous activity of the bladder, however, it does not sufficiently explain the cell-to-cell propagation of the spontaneous smooth muscle activity. The spontaneous activity of smooth muscle is an important factor that works against the bladder compliance in the filling phase, and may inversely affect the neurally evoked response during micturition. The intensity of spontaneous activity is the age-dependent; it is high in neonatal bladders it is small or almost non-existent in adults and reemerges in older bladders. This review focuses on these age-dependent alterations of spontaneous bladder contractions and describes the possible mechanisms which may have important role in regulating the spontaneous contractions using the rat as an animal model.

Interstitial cell of Cajal-like cells in the upper urinary tract

Autorhythmicity in the upper urinary tract (UUT) has long been considered to arise in specialized atypical smooth muscle cells (SMC) predominately situated in the most proximal regions of the pyeloureteric system. These atypical SMC pacemakers have been thought to trigger adjacent electrically-quiescent typical SMC to fire action potentials which allow an influx of Ca2+ and the generation of muscle contraction. More recently, the presence of cells with many of the morphological, electrical and immunohistochemical characteristics of interstitial cells of Cajal (ICC), the pacemaker cells of the gastrointestinal tract, have been located in many regions of both the upper and lower urinary tract. This article reviews the evidence from the literature and from our laboratory supporting a role of both atypical SMC and ICC-like cells in the initiation and propagation of pyeloureteric peristalsis in the UUT. We propose a new model in which there are 2 populations of pacemaker cells, high frequency atypical SMC and lower frequency ICC-like cells, both of which can drive electrically-quiescent typical SMC. The relative presence of these 2 populations of pacemaker cells and the relatively-long refractoriness of typical SMC determines the decreasing frequency of contraction with distance from the renal fornix. In the absence of the proximal pacemaker drive from atypical SMC after pyeloureteral/ureteral obstruction or surgery, ICC-like cell pacemaking provides a compensatory mechanism allowing the ureter to maintain rudimentary peristaltic waves and movement of urine from the pyelon towards the bladder.

Recent advances in basic science for overactive bladder

PURPOSE OF REVIEW

Detrusor overactivity is a relatively common yet embarrassing symptom complex with significant impact on quality of life. The mainstay of current pharmacological treatment involves the use of muscarinic receptor antagonists, but their therapeutic effectiveness is limited by a combination of limited efficacy and troublesome side effects and has recently been challenged by Herbison et al. Recognition of the limitations of existing therapy has started the search for pharmacotherapeutic agents acting on alternative pathways underlying detrusor overactivity with the intention of improving storage symptoms of urgency, frequency and urge incontinence.

RECENT FINDINGS

Recent research has suggested that several transmitters may modulate bladder storage. However, no agents currently available, acting via mechanisms other than muscarinic receptors have entered clinical practice so far. It is clear that far from being a passive container for urine, the urothelium is a crucial area within the bladder wall and its functions are complex and only now beginning to be appreciated. The release of several neurotransmitters from urothelium in response to distension and its action on receptors on sensory neurons is being increasingly recognized. The role for this afferent stimulation on the micturition reflex is gradually gaining importance in the pathophysiology of detrusor overactivity.

SUMMARY

In this article, the recent developments in basic science related to the pathogenesis and pharmacological basis for future drug targets for effective management of overactive bladder are discussed.

Subset of cells immunopositive for neurokinin-1 receptor identified as arterial interstitial cells of Cajal in human large arteries

In the adventitia of large arteries, dendritic cells are located between nerve fibers, some of which contain substance P. The aim of the present study was to examine whether neurokinin 1 receptor (NK-1R) was expressed by dendritic cells in the arterial wall. Parallel sections of aortic and carotid artery segments were immunostained with anti-NK-1R and cell-type-specific antibodies. Dendritic cells in the arterial wall expressed NK-1R, albeit at a low level. Other cells, which intensely expressed NK-1R, were located along the border between the media and adventitia. They did not co-express any dendritic cell markers, including fascin, CD1a, S100, or Lag-antigen, and were negative for CD68, CD3, and mast cell tryptase. These NK-1R(+) cells were laser-capture microdissected and studied by means of electron-microscopic analysis. The microdissected cells were in direct contact with nerve endings, and their ultrastructure was typical of the interstitial cells of Cajal present in the gastrointestinal tract. Further systematic electron-microscopic analysis revealed that the cells displaying the features typical of interstitial cells of Cajal were a basic element of the human arterial wall architectonics. Arterial interstitial cells of Cajal were negative for c-kit but they expressed vasoactive intestinal peptide receptor 1 (VIPR1). Destructive alterations of contacts between arterial interstitial cells of Cajal and nerve endings were observed in arterial segments with atherosclerotic lesions. The functional significance of the arterial interstitial cells of Cajal and their possible involvement in atherosclerosis and other vascular diseases need clarification.

Interstitial cells of Cajal in pancreas

We show here (presumably for the first time) a special type of cell in the human and rat exocrine pancreas. These cells have phenotypic characteristics of the enteric interstitial cells of Cajal (ICC). To identify pancreatic interstitial cells of Cajal (pICC) we used routine light microscopy, non-conventional light microscopy (less than 1 mum semi-thin sections of Epon-embedded specimens cut by ultramicrotomy and stained with Toluidine blue), transmission electron microscopy (TEM), and immunocytochemistry. The results showed that pICC can be recognized easily by light microscopy, particularly on semi-thin sections, as well as by TEM. Two-dimensional reconstructions from serial photos suggest a network-like spatial distribution of pICC. pICC represent 3.3+/-0.5% of all pancreatic cells, and seem to establish close spatial relationships with: capillaries (43%), acini (40%), stellate cells (14%), nerve fibres (3%). Most of pICC (88%) have 2 or 3 long processes (tens of mum) emerging from the cell body. TEM data show that pICC meet the criteria for positive diagnosis as ICC (e.g. numerous mitochondria, 8.7+/-0.8% of cytoplasm). Immunocytochemistry revealed that pICC are CD117/c-kit and CD34 positive. We found pICC positive (40-50%) for smooth muscle alpha-actin or S-100, and, occasionally, for CD68, NK1 neurokinin receptor and vimentin. The reactions for desmin and chromogranin A were negative in pICC. At present, only hypotheses and speculations can be formulated on the possible role of the pICC (e.g., juxtacrine and/or paracrine roles). In conclusion, the quite-established dogma: "ICC only in cavitary organs" is overpassed.

Involvement of ryanodine receptors in pacemaker Ca2+ oscillation in murine gastric ICC

Using a cell cluster preparation from the stomach smooth muscle tissue of mice, we measured intracellular Ca(2+) oscillations in interstitial cells of Cajal (ICCs) in the presence of nifedipine. Pacemaker [Ca(2+)](i) activity in ICCs was significantly suppressed by caffeine application and restored after washout. Application of either ryanodine or FK-506 terminated the pacemaker [Ca(2+)](i) activity irreversibly. Immunostaining of smooth muscle tissue showed that c-Kit-immunopositive cells (that form network-like structure cells in the myenteric plexus, equivalent to ICCs) clearly express ryanodine receptors (RyR). RT-PCR revealed that ICCs (identified with c-Kit-immunoreactivity) predominantly express type 3 RyR (RyR3). Furthermore, the FK-binding proteins 12 and 12.6, both of which would interact with RyR3, were detected. In conclusion, we provide first evidence for the essential contribution of RyR to generating pacemaker activity in gastric motility. Similar mechanisms might account for spontaneous rhythmicity seen in smooth muscle tissues distributed in the autonomic nervous system.

Morphology and localization of interstitial cells in the guinea pig bladder: structural relationships with smooth muscle and neurons

PURPOSE

In the current study we examined the location of interstitial cell of Cajal (ICC)-like cells in the guinea pig bladder wall and studied their structural interactions with nerves and smooth muscle cells.

MATERIALS AND METHODS

Whole mount samples and cryosections of bladder tissue were labeled with primary and fluorescent secondary antibodies, and imaged using confocal and multiphoton microscopy.

RESULTS

Kit positive ICC-like cells were located below the urothelium, in the lamina propria region and throughout the detrusor. In the suburothelium they had a stellate morphology and appeared to network. They made connections with nerves, as shown by double labeling experiments with anti-kit and anti-protein gene product 9.5. A network of vimentin positive cells was also found, of which many but not all were kit positive. In the detrusor kit positive cells were most often seen at the edge of smooth muscle bundles. They were elongated with lateral branches, running in parallel with the bundles and closely associated with intramural nerves. Another population of kit positive cells was seen in the detrusor between muscle bundles. These cells had a more stellate-like morphology and made connections with each other. Kit positive cells were seen tracking nerve bundles and close to intramural ganglia. Vimentin positive cells were present in the detrusor, of which some were also kit positive.

CONCLUSIONS

There are several populations of ICC-like cells throughout the guinea pig bladder wall. They differ in morphology and orientation but all make connections with intramural nerves and in the detrusor they are closely associated with smooth muscle cells.

Inwardly rectifying chloride channel activity in intestinal pacemaker cells

Cl(-) channels are proposed to play a role in gut pacemaker activity, but little is known about the characteristics of Cl(-) channels in interstitial cells of Cajal (ICC), the intestinal pacemaker cells. The objective of the present study was to identify whole cell Cl(-) currents in ICC associated with previously observed single-channel activity and to characterize its inward rectification. Whole cell patch-clamp studies showed that ICC express an inwardly rectifying Cl(-) current that was not sensitive to changes in cation composition of the extracellular solutions. Currents were not affected by replacing all cations with N-methyl-d-glucamine (NMDG(+)). Whole cell currents followed the Cl(-) equilibrium potential and were inhibited by DIDS and 9-anthracene carboxylic acid. Ramp protocols of single-channel activity showed that inward rectification was due to reduction in single-channel open probability, not a reduction in single-channel conductance. Single-channel data led to the hypothesis that strong cooperation exists between 30-pS channels that show less cooperation at potentials positive to the reversal potential. Hence, an inwardly rectifying Cl(-) channel plays a prominent role in determining pacemaker activity in the gut.

Novel type of interstitial cell (Cajal-like) in human fallopian tube

We describe here--presumably for the first time--a Cajal-like type of tubal interstitial cells (t-ICC), resembling the archetypal enteric ICC. t-ICC were demonstrated in situ and in vitro on fresh preparations (tissue cryosections and primary cell cultures) using methylene-blue, crystal-violet, Janus-Green B or MitoTracker-Green FM Probe vital stainings. Also, t-ICC were identified in fixed specimens by light microscopy (methylene-blue, Giemsa, trichrome stainings, Gomori silver-impregnation) or transmission electron microscopy (TEM). The positive diagnosis of t-ICC was strengthened by immunohistochemistry (IHC; CD117/c-kit+ and other 14 antigens) and immunofluorescence (IF; CD117/c-kit+ and other 7 antigens). The spatial density of t-ICC (ampullar-segment cryosections) was 100-150 cells/mm2. Non-conventional light microscopy (NCLM) of Epon semithin-sections revealed a network-like distribution of t-ICC in lamina propria and smooth muscle meshwork. t-ICC appeared located beneath of epithelium, in a 10-15 microm thick 'belt', where 18+/-2% of cells were t-ICC. In the whole lamina propria, t-ICC were about 9%, and in muscularis approximately 7%. In toto, t-ICC represent ~8% of subepithelial cells, as counted by NCLM. In vitro, t-ICC were 9.9+/-0.9% of total cell population. TEM showed that the diagnostic 'gold standard' (Huizinga et al., 1997) is fulfilled by 'our' t-ICC. However, we suggest a 'platinum standard', adding a new defining criterion- characteristic cytoplasmic processes (number: 1-5; length: tens of microm; thickness: < or =0.5 microm; aspect: moniliform; branching: dichotomous; organization: network, labyrinthic-system). Quantitatively, the ultrastructural architecture of t-ICC is: nucleus, 23.6+/-3.2% of cell volume, with heterochromatin 49.1+/-3.8%; mitochondria, 4.8+/-1.7%; rough and smooth endoplasmic-reticulum (1.1+/-0.6%, 1.0+/-0.2%, respectively); caveolae, 3.4+/-0.5%. We found more caveolae on the surface of cell processes versus cell body, as confirmed by IF for caveolins. Occasionally, the so-called 'Ca2+-release units' (subplasmalemmal close associations of caveolae+endoplasmic reticulum+mitochondria) were detected in the dilations of cell processes. Electrophysiological single unit recordings of t-ICC in primary cultures indicated sustained spontaneous electrical activity (amplitude of membrane potentials: 57.26+/-6.56 mV). Besides the CD117/c-kit marker, t-ICC expressed variously CD34, caveolins 1&2, alpha-SMA, S-100, vimentin, nestin, desmin, NK-1. t-ICC were negative for: CD68, CD1a, CD62P, NSE, GFAP, chromogranin-A, PGP9.5, but IHC showed the possible existence of (neuro)endocrine cells in tubal interstitium. We call them 'JF cells'. In conclusion, the identification of t-ICC might open the door for understanding some tubal functions, e.g. pace-making/peristaltism, secretion (auto-, juxta- and/or paracrine), regulation of neurotransmission (nitrergic/purinergic) and intercellular signaling, via the very long processes. Furthermore, t-ICC might even be uncommitted bipotential progenitor cells.

Calcium oscillations in interstitial cells of the rabbit urethra

Measurements were made (using fast confocal microscopy) of intracellular Ca2+ levels in fluo-4 loaded interstitial cells isolated from the rabbit urethra. These cells exhibited regular Ca2+ oscillations which were associated with spontaneous transient inward currents recorded under voltage clamp. Interference with D-myo-inositol 1,4,5-trisphosphate (IP3) induced Ca2+ release using 100 microm 2-aminoethoxydiphenyl borate, and the phospholipase C (PLC) inhibitors 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate and U73122 decreased the amplitude of spontaneous oscillations but did not abolish them. However, oscillations were abolished when ryanodine receptors were blocked with tetracaine or ryanodine. Oscillations ceased in the absence of external Ca2+, and frequency was directly proportional to the external Ca2+ concentration. Frequency of Ca2+ oscillation was reduced by SKF-96365, but not by nifedipine. Lanthanum and cadmium completely blocked oscillations. These results suggest that Ca2+ oscillations in isolated rabbit urethral interstitial cells are initiated by Ca2+ release from ryanodine-sensitive intracellular stores, that oscillation frequency is very sensitive to the external Ca2+ concentration and that conversion of the primary oscillation to a propagated Ca2+ wave depends upon IP3-induced Ca2+ release.

cGMP-generating cells in the bladder wall: identification of distinct networks of interstitial cells

OBJECTIVE

To identify cells which might contribute to the complex physiological responses of the guinea-pig bladder, and specifically to describe the distribution and types of cell in the bladder wall of the guinea pig which respond to nitric oxide (NO) with an increase in intracellular cGMP, i.e. putative interstitial cells (ICs).

MATERIALS AND METHODS

The whole bladder was removed from 11 male guinea pigs killed by cervical dislocation. Sections of the bladder wall, from the dome lateral wall and base, were isolated and incubated separately in Krebs' solution at 36 degrees C, gassed with 95% O(2) and 5% CO(2), and containing 1 mmol/L of the nonspecific phosphodiesterase inhibitor isobutyl-methyl-xanthene. Individual pieces of tissue were then exposed to 100 micromol/L of the NO donor NONOate for 10 min; control tissues remained in Krebs' solution. Tissues were then fixed in 4% paraformaldehyde and processed for immunohistochemistry. cGMP and neuronal NO synthase (nNOS) were subsequently visualized using appropriate primary and secondary antibodies.

RESULTS

Cells responding to NO with an increase in cGMP were detected in the dome, lateral wall and base, with positive cells in the thin outer surface of the wall (muscle coat), associated with muscle bundles in an outer layer of muscle, and in a region immediately beneath the urothelium. These cells (not urothelium, smooth muscle or vascular) are described as interstitial cells. Superficial urothelial umbrella cells were apparent and were strongly cGMP-positive. A high density of interstitial cells was associated with muscle bundles on the outer aspects of the wall, while few cells were detected on inner bundles. Thus there appeared to be two distinct types of muscle, inner and outer, with no obvious orientation of the fibres in each layer. Both muscle groups contained fibres expressing nNOS. In the outer muscle layer most of these fibres co-localized with cGMP, suggesting that different populations of nerves innervate each layer. There were more nNOS-positive fibres in the base of the bladder than in the dome. Three populations of cGMP-positive interstitial cells were associated with the outer muscle layer; cells in the outer surface (muscle coat interstitial cells, MC-ICs), cells on the surface of the bundles (superficial, SM-ICs) and cells within the muscle bundles (intramuscular, IM-ICs). The IM-ICs form a network in close apposition to the smooth muscle cells while the SM-ICs may connect adjacent muscle bundles and connect to the MC-ICs. Thus, there is a network linking potentially the muscle cells in the outer muscle bundles. cGMP-positive cells were also detected in the suburothelial layer (suburothelial, SU-ICs) which had a different structure to the cells associated with muscle, had a oval cell bodies with bifurcating processes and appeared to form a complex network; they were prevalent in the base and virtually absent in the dome.

CONCLUSIONS

There are structures within the bladder wall that can be identified and categorized by the ability of the constituent cells to increase intracellular cGMP in response to NO; these cells have been defined as ICs. Two distinct networks were identified, one associated with the outer muscle layers and another lying immediately beneath the urothelium, predominantly in the base of the bladder. The functions of these cells and networks are unknown; their possible roles in complex motor activity, urothelial signalling and bladder pathophysiology are discussed.

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.

Identification of interstitial cells of Cajal in corporal tissues of the guinea-pig penis

This study shows for the first time the presence of interstitial cells of Cajal (ICC) and their possible role in the initiation of spontaneous excitation in the corporal tissue of the guinea-pig penis. ICC, which were identified by their c-kit immunoreactivity, were abundantly distributed in the corporal smooth muscle meshwork. Spontaneous increases in the intracellular calcium concentration ([Ca(2+)](i); calcium transients) were visualized in preparations loaded with the fluorescent dye fura-2. Ca transients originated from the boundary of muscle bundles and then spread throughout the meshwork (Ca waves). Ca waves were strongly suppressed by either CPA (10 microm), ryanodine (50 microm) or 2-APB (10 microm), and their synchronicity was disrupted by 18beta-GA (30 microm). These results suggest that ICC in the corporal tissue may have a role as pacemakers to drive the bulk of smooth muscles, and that intracellular Ca(2+) stores and gap junctions are critical for the generation of spontaneous excitation.