Properties of spontaneous depolarizations in circular smooth muscle cells of rabbit urethra

Interstitial cell of Cajal-like cells (ICC-LC) exhibit dynamic spontaneous activity but are not functionally innervated in mouse urethra

Urethral smooth muscle cells (USMC) contract to occlude the internal urethral sphincter during bladder filling. Interstitial cells also exist in urethral smooth muscles and are hypothesized to influence USMC behaviours and neural responses. These cells are similar to Kit+ interstitial cells of Cajal (ICC), which are gastrointestinal pacemakers and neuroeffectors. Isolated urethral ICC-like cells (ICC-LC) exhibit spontaneous intracellular Ca2+ signalling behaviours that suggest these cells may serve as pacemakers or neuromodulators similar to ICC in the gut, although observation and direct stimulation of ICC-LC within intact urethral tissues is lacking. We used mice with cell-specific expression of the Ca2+ indicator, GCaMP6f, driven off the endogenous promoter for Kit (Kit-GCaMP6f mice) to identify ICC-LC in situ within urethra muscles and to characterize spontaneous and nerve-evoked Ca2+ signalling. ICC-LC generated Ca2+ waves spontaneously that propagated on average 40.1 ± 0.7 μm, with varying amplitudes, durations, and spatial spread. These events originated from multiple firing sites in cells and the activity between sites was not coordinated. ICC-LC in urethra formed clusters but not interconnected networks. No evidence for entrainment of Ca2+ signalling between ICC-LC was obtained. Ca2+ events in ICC-LC were unaffected by nifedipine but were abolished by cyclopiazonic acid and decreased by an antagonist of Orai Ca2+ channels (GSK-7975A). Phenylephrine increased Ca2+ event frequency but a nitric oxide donor (DEA-NONOate) had no effect. Electrical field stimulation (EFS, 10 Hz) of intrinsic nerves, which evoked contractions of urethral rings and increased Ca2+ event firing in USMC, failed to evoke responses in ICC-LC. Our data suggest that urethral ICC-LC are spontaneously active but are not regulated by autonomic neurons.

Commonality and heterogeneity of pacemaker mechanisms in the male reproductive organs

During emission, the first phase of ejaculation, smooth muscle in organs of the male reproductive tract (MRT) vigorously contract upon sympathetic nerve excitation to expel semen consisting of sperm and seminal plasma. During inter-ejaculation phases, the epididymis, seminal vesicles and prostate undergo spontaneous phasic contractions (SPCs), this transporting and maintaining the quality of sperm and seminal plasma. Recent studies have revealed platelet-derived growth factor receptor α-expressing (PDGFRα+) subepithelial interstitial cells in seminal vesicles subserve the role of pacemaker cells that electrically drive SPCs in this organ. PDGFRα+ smooth muscle cells in the epididymis also appear to function as pacemaker cells implicating PDGFRα as a potential signature molecule in MRT pacemaking. The dominant mechanism driving pacemaking in these organs is the cytosolic Ca2+ oscillator. This operates through entrainment of the release-refill cycle of Ca2+ stores, the released Ca2+ ions opening Ca2+-activated chloride channels, including in some cases ANO1 (TMEM16A), with the resultant pacemaker potential activating L-type voltage-dependent Ca2+ channels in the smooth muscle causing contraction (viz. SPCs). A second pacemaker mechanism, namely the membrane oscillator also has a role in specific cases. Further investigations into the commonality and heterogeneity of MRT pacemakers will open an avenue for understanding the pathogenesis of male infertility associated with deterioration of seminal plasma.

Pattern Formation in a Spatially Extended Model of Pacemaker Dynamics in Smooth Muscle Cells

Spatiotemporal patterns are common in biological systems. For electrically coupled cells, previous studies of pattern formation have mainly used applied current as the primary bifurcation parameter. The purpose of this paper is to show that applied current is not needed to generate spatiotemporal patterns for smooth muscle cells. The patterns can be generated solely by external mechanical stimulation (transmural pressure). To do this we study a reaction-diffusion system involving the Morris-Lecar equations and observe a wide range of spatiotemporal patterns for different values of the model parameters. Some aspects of these patterns are explained via a bifurcation analysis of the system without coupling - in particular Type I and Type II excitability both occur. We show the patterns are not due to a Turing instability and that the spatially extended model exhibits spatiotemporal chaos. We also use travelling wave coordinates to analyse travelling waves.

Mechanisms underlying spontaneous phasic contractions and sympathetic control of smooth muscle in the rat caudal epididymis

Here we investigate mechanisms underlying spontaneous phasic contractions (SPCs) and sympathetic control of contractility in the rat epididymis, a long tubular duct involved in transportation and maturation of sperm. Longitudinal contractions of short segments (~ 1.5 mm) of rat proximal and distal caudal epididymal duct were measured + / - nerve stimulation. The extent of sympathetic innervation of these duct regions was determined by immunohistochemistry. Proximal caudal duct segments (150-300 μm dia.) exhibited SPCs, while distal segments (350-500 μm) were quiescent in ~ 80% of preparations. SPC amplitude and frequency were reduced by the L-type voltage-dependent Ca²⁺ channel (LVDCC) blocker nifedipine (1 μM), with the T-type voltage-dependent Ca²⁺ channel (TVDCC) blocker ML218 (1 μM) specifically decreasing SPC frequency. SPCs were inhibited upon blockade of the SR/ER Ca²⁺-ATPase (CPA 10 μM). SPCs were also inhibited by caffeine (1 μM), 2-APB (100 μM), niflumic acid (100 μM), or by lowering extracellular [Cl^-] from 134.4 to 12.4 mM but not by ryanodine (25 μM) or tetracaine (100 μM). Electrical field stimulation (EFS) at 2 Hz for 60 s caused a sustained α₁-adrenoceptor-sensitive contraction in distal segments and enhanced and/or induced α₂-adrenoceptor-sensitive oscillatory phasic contractions in proximal and distal segments, the latter mimicked by application of the α₂-adrenoceptor agonist clonidine. We hypothesise that SPCs in the proximal cauda are triggered by pacemaker mechanisms involving rhythmic IP₃ receptor-operated SR/ER store Ca²⁺ release and resultant activation of CaCC with TVDCCs and possibly LVDCCs subserving in this process. Sympathetic nerve-released noradrenaline induces α₂-adrenoceptor-mediated phasic contractions in the proximal and distal cauda. These findings provide new pharmacological targets for male infertility and contraception.

Anoctamin 1 antagonism potentiates conventional tocolytic-mediated relaxation of pregnant human uterine smooth muscle

BACKGROUND

Currently available tocolytic agents are not effective treatment for preterm labor beyond 48 h. A major reason is the development of maternal side effects which preclude the maintenance of an effective steady-state drug concentration. One strategy that can mitigate these side effects is utilizing synergistic drug combinations to reduce the drug concentrations necessary to elicit a clinical effect. We have previously shown that three anoctamin 1 (ANO1) antagonists mediate potent relaxation of precontracted human uterine smooth muscle (USM). In this study, we aimed to determine whether a combination of sub-relaxatory doses of tocolytic drugs in current clinical use [the L-type voltage-gated calcium channel (VGCC) blocker, nifedipine (NIF); and the β2-adrenergic (β2AR) agonist, terbutaline (TRB)] will potentiate USM relaxation with two ANO1 antagonists [benzbromarone (BB) and MONNA (MN)].

OBJECTIVE

This study sought to examine the synergistic potency and mechanistic basis of two ANO1 antagonists with currently available tocolytic drugs. Functional endpoints assessed included relaxation of pre-contracting pregnant human USM tissue, inhibition of intracellular calcium release, and reduction of spontaneous transient inward current (STIC) recordings in human uterine smooth muscle cells.

METHODS

Human myometrial strips and primary human USM cells were used in organ bath and calcium flux experiments with different combinations of sub-threshold doses of ANO1 antagonists and terbutaline or nifedipine to determine if ANO1 antagonists potentiate tocolytic drugs.

RESULTS

The combination of sub-threshold doses of two ANO1 antagonists and current tocolytic drugs demonstrate a significant degree of synergy to relax human pregnant USM compared to the effects achieved when these drugs are administered individually.

CONCLUSION

A combination of sub-threshold doses of VGCC blocker and β2AR agonist with ANO1 antagonists potentiates relaxation of oxytocin-induced contractility and calcium flux in human USM ex vivo. Our findings may serve as a foundation for novel tocolytic drug combinations.

Role of Ano1 Ca2+-activated Cl- channels in generating urethral tone

Urinary continence is maintained in the lower urinary tract by the contracture of urethral sphincters, including smooth muscle of the internal urethral sphincter. These contractions occlude the urethral lumen, preventing urine leakage from the bladder to the exterior. Over the past 20 years, research on the ionic conductances that contribute to urethral smooth muscle contractility has greatly accelerated. A debate has emerged over the role of interstitial cell of Cajal (ICC)-like cells in the urethra and their expression of Ca²⁺-activated Cl^- channels encoded by anoctamin-1 [Ano1; transmembrane member 16 A (Tmem16a) gene]. It has been proposed that Ano1 channels expressed in urethral ICC serve as a source of depolarization for smooth muscle cells, increasing their excitability and contributing to tone. Although a clear role for Ano1 channels expressed in ICC is evident in other smooth muscle organs, such as the gastrointestinal tract, the role of these channels in the urethra is unclear, owing to differences in the species (rabbit, rat, guinea pig, sheep, and mouse) examined and experimental approaches by different groups. The importance of clarifying this situation is evident as effective targeting of Ano1 channels may lead to new treatments for urinary incontinence. In this review, we summarize the key findings from different species on the role of ICC and Ano1 channels in urethral contractility. Finally, we outline proposals for clarifying this controversial and important topic by addressing how cell-specific optogenetic and inducible cell-specific genetic deletion strategies coupled with advances in Ano1 channel pharmacology may clarify this area in future studies.NEW & NOTEWORTHY Studies from the rabbit have shown that anoctamin-1 (Ano1) channels expressed in urethral interstitial cells of Cajal (ICC) serve as a source of depolarization for smooth muscle cells, increasing excitability and tone. However, the role of urethral Ano1 channels is unclear, owing to differences in the species examined and experimental approaches. We summarize findings from different species on the role of urethral ICC and Ano1 channels in urethral contractility and outline proposals for clarifying this topic using cell-specific optogenetic approaches.

Functional comparison of anoctamin 1 antagonists on human uterine smooth muscle contractility and excitability

Background: Pre-term birth is a major health care challenge throughout the world, and preterm labor represents a potentially reversible component of this problem. Current tocolytics do not improve preterm labor beyond 48 h. We have previously shown that anoctamin 1 (ANO1) channel blockade results in relaxation of pre-contracted human uterine smooth muscle (USM). Three drug classes with reported medicinal effects in humans also have members with ANO1 antagonism. In this study, we compared the ability of representatives from these 3 classes to reduce human USM contractility and excitability. Objective: This study sought to examine the comparative potency of 3 ANO1 antagonists on pregnant human USM relaxation, contraction frequency reduction, inhibition of intracellular calcium release and membrane hyperpolarization. Methods: Experiments were performed using: 1) Ex vivo organ bath (human pregnant tissue), 2) Oxytocin-induced calcium flux (in vitro human USM cells) and 3) Membrane potential assay (in vitro human USM cells). Results: Benzbromarone (BB) demonstrated the greatest potency among the compounds tested with respect to force, frequency inhibition, reducing calcium elevation and depolarizing membrane potential. Conclusion: While all 3 ANO1 antagonists attenuate pregnant human uterine tissue contractility and excitability, BB is the most potent tocolytic drug. Our findings may serve as a foundation for future structure-function analyses for novel tocolytic drug development.

Mucosa-Dependent, Stretch-Sensitive Spontaneous Activity in Seminal Vesicle

Seminal vesicles (SVs), a pair of male accessory glands, contract upon sympathetic nerve excitation during ejaculation while developing spontaneous phasic constrictions in the inter-ejaculatory storage phase. Recently, the fundamental role of the mucosa in generating spontaneous activity in SV of the guinea pig has been revealed. Stretching the mucosa-intact but not mucosa-denuded SV smooth muscle evokes spontaneous phasic contractions arising from action potential firing triggered by electrical slow waves and associated Ca²⁺ flashes. These spontaneous events primarily depend on sarco-endoplasmic reticulum (SR/ER) Ca²⁺ handling linked with the opening of Ca²⁺-activated chloride channels (CaCCs) resulting in the generation of slow waves. Slow waves in mucosa-intact SV smooth muscle are abolished upon blockade of gap junctions, suggesting that seminal smooth muscle cells are driven by cells distributed in the mucosa. In the SV mucosal preparations dissected free from the smooth muscle layer, a population of cells located just beneath the epithelium develop spontaneous Ca²⁺ transients relying on SR/ER Ca²⁺ handling. In the lamina propria of the SV mucosa, vimentin-immunoreactive interstitial cells including platelet-derived growth factor receptor α (PDGFRα)-immunoreactive cells are distributed, while known pacemaker cells in other smooth muscle tissues, e.g. c-Kit-positive interstitial cells or α-smooth muscle actin-positive atypical smooth muscle cells, are absent. The spontaneously-active subepithelial cells appear to drive spontaneous activity in SV smooth muscle either by sending depolarizing signals or by releasing humoral substances. Interstitial cells in the lamina propria may act as intermediaries of signal transmission from the subepithelial cells to the smooth muscle cells.

Noradrenaline inhibits neurogenic propulsive motor patterns but not neurogenic segmenting haustral progression in the rabbit colon

BACKGROUND

Excessive sympathetic inhibition may be a cause of colon motor dysfunction. Our aim was to better understand the mechanisms of sympathetic inhibition on colonic motor patterns using the rabbit colon, hypothesizing that noradrenaline selectively inhibits propulsive motor patterns.

METHODS

Changes in motor patterns of the rabbit colon were studied ex vivo using noradrenaline and adrenoceptor antagonists and analyzed using spatiotemporal diameter maps.

KEY RESULTS

Noradrenaline abolished propulsive contractions: it abolished the long-distance contractions (LDCs) from a baseline frequency of 0.8 ± 0.3 and the clusters of fast propagating contractions (FPCs) at a frequency of 14.4 ± 2.8 cpm. Both motor patterns recovered after addition of the α₂ -adrenoceptor antagonist yohimbine to a frequency of 0.5 ± 0.2  and 9.9 ± 3.3 cpm, respectively. The β-adrenoceptor antagonist propranolol did not prevent the loss of propulsive motor patterns with noradrenaline. Noradrenaline did not inhibit haustral boundary contractions and increased the frequency of the myogenic ripples from 8.3 ± 1.4 to 10.5 ± 1.3 cpm which was not affected by yohimbine, propranolol nor the α₁ -adrenoceptor blocker prazosin.

CONCLUSIONS AND INFERENCES

Noradrenergic inhibition of propulsive motor patterns is mediated by the α₂ -adrenoceptor to inhibit the neurogenic LDCs and the neurogenic clustering of FPCs. The neurogenic haustral boundary contractions are not affected, suggesting that α_2- receptors are on selective neural circuits. The excitatory effect of noradrenaline on ripples may be due to the activation of adrenoceptors on interstitial cells of Cajal, but action on α_1- receptors was excluded. No role for the β-adrenoceptor was found.

Venous Vasomotion

Veins exhibit spontaneous contractile activity, a phenomenon generally termed vasomotion. This is mediated by spontaneous rhythmical contractions of mural cells (i.e. smooth muscle cells (SMCs) or pericytes) in the wall of the vessel. Vasomotion occurs through interconnected oscillators within and between mural cells, entraining their cycles. Pharmacological studies indicate that a key oscillator underlying vasomotion is the rhythmical calcium ion (Ca²⁺) release-refill cycle of Ca²⁺ stores. This occurs through opening of inositol 1,4,5-trisphosphate receptor (IP₃R)- and/or ryanodine receptor (RyR)-operated Ca²⁺ release channels in the sarcoplasmic/endoplasmic (SR/ER) reticulum and refilling by the SR/ER reticulum Ca²⁺ATPase (SERCA). Released Ca²⁺ from stores near the plasma membrane diffuse through the cytosol to open Ca²⁺-activated chloride (Cl^-) channels, this generating inward current through an efflux of Cl^-. The resultant depolarisation leads to the opening of voltage-dependent Ca²⁺ channels and possibly increased production of IP₃, which through Ca²⁺-induced Ca²⁺ release (CICR) of IP₃Rs and/or RyRs and IP₃R-mediated Ca²⁺ release provide a means by which store oscillators entrain their activity. Intercellular entrainment normally involves current flow through gap junctions that interconnect mural cells and in many cases this is aided by additional connectivity through the endothelium. Once entrainment has occurred the substantial Ca²⁺ entry that results from the near-synchronous depolarisations leads to rhythmical contractions of the mural cells, this often leading to vessel constriction. The basis for venous/venular vasomotion has yet to be fully delineated but could improve both venous drainage and capillary/venular absorption of blood plasma-associated fluids.

Ca2+ signalling in mouse urethral smooth muscle in situ: role of Ca2+ stores and Ca2+ influx mechanisms

KEY POINTS

Contraction of urethral smooth muscle cells (USMCs) contributes to urinary continence. Ca2+ signalling in USMCs was investigated in intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs were spontaneously active in situ, firing intracellular Ca2+ waves that were asynchronous at different sites within cells and between adjacent cells. Spontaneous Ca2+ waves in USMCs were myogenic but enhanced by adrenergic or purinergic agonists and decreased by nitric oxide. Ca2+ waves arose from inositol trisphosphate type 1 receptors and ryanodine receptors, and Ca2+ influx by store-operated calcium entry was required to maintain Ca2+ release events. Ca2+ release and development of Ca2+ waves appear to be the primary source of Ca2+ for excitation-contraction coupling in the mouse urethra, and no evidence was found that voltage-dependent Ca2+ entry via L-type or T-type channels was required for responses to α adrenergic responses.

ABSTRACT

Urethral smooth muscle cells (USMCs) generate myogenic tone and contribute to urinary continence. Currently, little is known about Ca2+ signalling in USMCs in situ, and therefore little is known about the source(s) of Ca2+ required for excitation-contraction coupling. We characterized Ca2+ signalling in USMCs within intact urethral muscles using a genetically encoded Ca2+ sensor, GCaMP3, expressed selectively in USMCs. USMCs fired spontaneous intracellular Ca2+ waves that did not propagate cell-to-cell across muscle bundles. Ca2+ waves increased dramatically in response to the α1 adrenoceptor agonist phenylephrine (10 μm) and to ATP (10 μm). Ca2+ waves were inhibited by the nitric oxide donor DEA NONOate (10 μm). Ca2+ influx and release from sarcoplasmic reticulum stores contributed to Ca2+ waves, as Ca2+ free bathing solution and blocking the sarcoplasmic Ca2+ -ATPase abolished activity. Intracellular Ca2+ release involved cooperation between ryanadine receptors and inositol trisphosphate receptors, as tetracaine and ryanodine (100 μm) and xestospongin C (1 μm) reduced Ca2+ waves. Ca2+ waves were insensitive to L-type Ca2+ channel modulators nifedipine (1 μm), nicardipine (1 μm), isradipine (1 μm) and FPL 64176 (1 μm), and were unaffected by the T-type Ca2+ channel antagonists NNC-550396 (1 μm) and TTA-A2 (1 μm). Ca2+ waves were reduced by the store operated Ca2+ entry blocker SKF 96365 (10 μm) and by an Orai antagonist, GSK-7975A (1 μm). The latter also reduced urethral contractions induced by phenylephrine, suggesting that Orai can function effectively as a receptor-operated channel. In conclusion, Ca2+ waves in mouse USMCs are a source of Ca2+ for excitation-contraction coupling in urethral muscles.

Tocolysis: Present and future treatment options

In the United States, the generally accepted indication for tocolytic therapy centers on suppression of preterm labor. This may be in the form of preventative therapy with progesterone in women with prior spontaneous preterm birth or as an acute intervention to suppress established uterine contractions associated with cervical change occurring at less than 37 weeks gestation. This article seeks to apply this perspective to tocolytic therapy. Here, we provide a review of current tocolytic options and what the last decade of discovery has revealed about the regulation of myometrial excitability and quiescence. Moving forward, we must incorporate the emerging molecular data that is amassing in order to develop novel and effective tocolytic therapeutic options to prevent preterm labor and spontaneous preterm birth (sPTB).

Effects of new-generation TMEM16A inhibitors on calcium-activated chloride currents in rabbit urethral interstitial cells of Cajal

Interstitial cells of Cajal (ICC) isolated from the rabbit urethra exhibit Ca²⁺-activated Cl^- currents (I _ClCa) that are important for the development of urethral tone. Here, we examined if TMEM16A (ANO1) contributed to this activity by examining the effect of "new-generation" TMEM16A inhibitors, CACC_inh-A01 and T16A_inh-A01, on I _ClCa recorded from freshly isolated rabbit urethral ICC (RUICC) and on contractions of intact strips of rabbit urethra smooth muscle. Real-time quantitative PCR experiments demonstrated that TMEM16A was highly expressed in rabbit urethra smooth muscle, in comparison to TMEM16B and TMEM16F. Single-cell RT-PCR experiments revealed that only TMEM16A was expressed in freshly isolated RUICC. Depolarization-evoked I _ClCa in isolated RUICC, recorded using voltage clamp, were inhibited by CACC_inh-A01 and T16A_inh-A01 with IC₅₀ values of 1.2 and 3.4 μM, respectively. Similarly, spontaneous transient inward currents (STICs) recorded from RUICC voltage clamped at -60 mV and spontaneous transient depolarizations (STDs), recorded in current clamp, were also inhibited by CACC_inh-A01 and T16A_inh-A01. In contrast, spontaneous Ca²⁺ waves in isolated RUICC were only partially reduced by CACC_inh-A01 and T16A_inh-A01. Finally, neurogenic contractions of strips of rabbit urethra smooth muscle (RUSM), evoked by electric field stimulation (EFS), were also significantly reduced by CACC_inh-A01 and T16A_inh-A01. These data are consistent with the idea that TMEM16A is involved with CACCs in RUICC and in contraction of rabbit urethral smooth muscle.

Role of mucosa in generating spontaneous activity in the guinea pig seminal vesicle

KEY POINTS

The mucosa may have neuron-like functions as urinary bladder mucosa releases bioactive substances that modulate sensory nerve activity as well as detrusor muscle contractility. However, such mucosal function in other visceral organs remains to be established. The role of mucosa in generating spontaneous contractions in seminal vesicles (SVs), a paired organ in the male reproductive tract, was investigated. The intact mucosa is essential for the generation of spontaneous phasic contractions of SV smooth muscle arising from electrical slow waves and corresponding increases in intracellular Ca²⁺ . These spontaneous events primarily depend on Ca²⁺ handling by sarco-endoplasmic reticulum Ca²⁺ stores. A population of mucosal cells developed spontaneous rises in intracellular Ca²⁺ relying on sarco-endoplasmic reticulum Ca²⁺ handling. The spontaneously active cells in the SV mucosa appear to drive spontaneous activity in smooth muscle either by sending depolarizing signals and/or by releasing humoral substances.

ABSTRACT

The role of the mucosa in generating the spontaneous activity of guinea-pig seminal vesicle (SV) was explored. Changes in contractility, membrane potential and intracellular Ca²⁺ dynamics of SV smooth muscle cells (SMCs) were recorded using isometric tension recording, intracellular microelectrode recording and epi-fluorescence Ca²⁺ imaging, respectively. Mucosa-intact but not mucosa-denuded SV preparations generated TTX- (1 μm) resistant spontaneous phasic contractions that were abolished by nifedipine (3 μm). Consistently, SMCs developed mucosa-dependent slow waves (SWs) that triggered action potentials and corresponding Ca²⁺ flashes. Nifedipine (10 μm) abolished the action potentials and spontaneous contractions, while suppressing the SWs and Ca²⁺ flashes. Both the residual SWs and spontaneous Ca²⁺ transients were abolished by cyclopiazonic acid (CPA, 10 μm), a sarco-endoplasmic reticulum Ca²⁺ -ATPase (SERCA) inhibitor. DIDS (300 μm) and niflumic acid (100 μm), blockers for Ca²⁺ -activated Cl^- channels (CACCs), or low Cl^- solution also slowed or prevented the generation of SWs. In SV mucosal preparations detached from the muscle layer, a population of mucosal cells generated spontaneous Ca²⁺ transients that were blocked by CPA but not nifedipine. These results suggested that spontaneous contractions and corresponding Ca²⁺ flashes in SV SMCs arise from action potential generation due to the opening of L-type voltage-dependent Ca²⁺ channels. Spontaneous Ca²⁺ transients appear to primarily result from Ca²⁺ release from sarco-endoplasmic reticulum Ca²⁺ stores to activate CACCs to develop SWs. The mucosal cells firing spontaneous Ca²⁺ transients may play a critical role in driving spontaneous activity of SV smooth muscle either by sending depolarizing signals or by releasing humoral substances.

Inherent rhythm of smooth muscle cells in rat mesenteric arterioles: An eigensystem formulation

On the basis of experimental data and mathematical equations in the literature, we remodel the ionic dynamics of smooth muscle cells (SMCs) as an eigensystem formulation, which is valid for investigating finite variations of variables from the equilibrium such as in common experimental operations. This algorithm provides an alternate viewpoint from frequency-domain analysis and enables one to probe functionalities of SMCs' rhythm by means of a resonance-related mechanism. Numerical results show three types of calcium oscillations of SMCs in mesenteric arterioles: spontaneous calcium oscillation, agonist-dependent calcium oscillation, and agonist-dependent calcium spike. For simple single and double SMCs, we demonstrate properties of synchronization among complex signals related to calcium oscillations, and show different correlation relations between calcium and voltage signals for various synchronization and resonance conditions. For practical cell clusters, our analyses indicate that the rhythm of SMCs could (1) benefit enhancements of signal communications among remote cells, (2) respond to a significant calcium peaking against transient stimulations for triggering globally oscillating modes, and (3) characterize the globally oscillating modes via frog-leap (non-molecular-diffusion) calcium waves across inhomogeneous SMCs.

Spatial Noise in Coupling Strength and Natural Frequency within a Pacemaker Network; Consequences for Development of Intestinal Motor Patterns According to a Weakly Coupled Phase Oscillator Model

Pacemaker activities generated by networks of interstitial cells of Cajal (ICC), in conjunction with the enteric nervous system, orchestrate most motor patterns in the gastrointestinal tract. It was our objective to understand the role of network features of ICC associated with the myenteric plexus (ICC-MP) in the shaping of motor patterns of the small intestine. To that end, a model of weakly coupled oscillators (oscillators influence each other's phase but not amplitude) was created with most parameters derived from experimental data. The ICC network is a uniform two dimensional network coupled by gap junctions. All ICC generate pacemaker (slow wave) activity with a frequency gradient in mice from 50/min at the proximal end of the intestine to 40/min at the distal end. Key features of motor patterns, directly related to the underlying pacemaker activity, are frequency steps and dislocations. These were accurately mimicked by reduction of coupling strength at a point in the chain of oscillators. When coupling strength was expressed as a product of gap junction density and conductance, and gap junction density was varied randomly along the chain (i.e., spatial noise) with a long-tailed distribution, plateau steps occurred at pointsof low density. As gap junction conductance was decreased, the number of plateaus increased, mimicking the effect of the gap junction inhibitor carbenoxolone. When spatial noise was added to the natural interval gradient, as gap junction conductance decreased, the number of plateaus increased as before but in addition the phase waves frequently changed direction of apparent propagation, again mimicking the effect of carbenoxolone. In summary, key features of the motor patterns that are governed by pacemaker activity may be a direct consequence of biological noise, specifically spatial noise in gap junction coupling and pacemaker frequency.

Ion channels of the mammalian urethra

The mammalian urethra is a muscular tube responsible for ensuring that urine remains in the urinary bladder until urination. In order to prevent involuntary urine leakage, the urethral musculature must be capable of constricting the urethral lumen to an extent that exceeds bladder intravesicular pressure during the urine-filling phase. The main challenge in anti-incontinence treatments involves selectively-controlling the excitability of the smooth muscles in the lower urinary tract. Almost all strategies to battle urinary incontinence involve targeting the bladder and as a result, this tissue has been the focus for the majority of research and development efforts. There is now increasing recognition of the value of targeting the urethral musculature in the treatment and management of urinary incontinence. Newly-identified and characterized ion channels and pathways in the smooth muscle of the urethra provides a range of potential therapeutic targets for the treatment of urinary incontinence. This review provides a summary of the current state of knowledge of the ion channels discovered in urethral smooth muscle cells that regulate their excitability.

The role of Ca(2+) influx in spontaneous Ca(2+) wave propagation in interstitial cells of Cajal from the rabbit urethra

KEY POINTS

Tonic contractions of rabbit urethra are associated with spontaneous electrical slow waves that are thought to originate in pacemaker cells termed interstitial cells of Cajal (ICC). ICC pacemaker activity results from their ability to generate propagating Ca(2+) waves, although the exact mechanisms of propagation are not understood. In this study, we have identified spontaneous localised Ca(2+) events for the first time in urethral ICC; these were due to Ca(2+) release from the endoplasmic reticulum (ER) via ryanodine receptors (RyRs) and, while they often remained localised, they sometimes initiated propagating Ca(2+) waves. We show that propagation of Ca(2+) waves in urethral ICC is critically dependent upon Ca(2+) influx via reverse mode NCX. Our data provide a clearer understanding of the intracellular mechanisms involved in the generation of ICC pacemaker activity. Interstitial cells of Cajal (ICC) are putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous propagating Ca(2+) waves that are modulated by [Ca(2+)]o and whose propagation is inhibited by inositol tri-phosphate receptor (IP3 R) blockers. The purpose of this study was to further examine the role of Ca(2+) influx and Ca(2+) release in the propagation of Ca(2+) waves. Intracellular Ca(2+) was measured in Fluo-4-loaded ICC using a Nipkow spinning disc confocal microscope at fast acquisition rates (50 fps). We identified previously undetected localised Ca(2+) events originating from ryanodine receptors (RyRs). Inhibiting Ca(2+) influx by removing [Ca(2+)]o or blocking reverse mode sodium-calcium exchange (NCX) with KB-R 7943 or SEA-0400 abolished Ca(2+) waves, while localised Ca(2+) events persisted. Stimulating RyRs with 1 mm caffeine restored propagation. Propagation was also inhibited when Ca(2+) release sites were uncoupled by buffering intracellular Ca(2+) with EGTA-AM. This was reversed when Ca(2+) influx via NCX was increased by reducing [Na(+)]o to 13 mm. Low [Na(+)]o also increased the frequency of Ca(2+) waves and this effect was blocked by tetracaine and ryanodine but not 2-aminoethoxydiphenyl borate (2-APB). RT-PCR revealed that isolated ICC expressed both RyR2 and RyR3 subtypes. We conclude: (i) RyRs are required for the initiation of Ca(2+) waves, but wave propagation normally depends on activation of IP3 Rs; (ii) under resting conditions, propagation by IP3 Rs requires sensitisation by influx of Ca(2+) via reverse mode NCX; (iii) propagation can be maintained by RyRs if they have been sensitised to Ca(2+).

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

The role of cAMP dependent protein kinase in modulating spontaneous intracellular Ca²⁺ waves in interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) serve as electrical pacemakers in the rabbit urethra. Pacemaking activity in ICC results from spontaneous intracellular Ca(2+) waves that rely on Ca(2+) release from endoplasmic reticulum (ER) stores. The purpose of this study was to investigate if the action of protein kinase A (PKA) affected the generation of Ca(2+) waves in ICC. Intracellular [Ca(2+)] was measured in fluo-4 loaded ICC, freshly isolated from the rabbit urethra using a Nipkow spinning disc confocal microscope. Application of the PKA inhibitor H-89 (10 μM) significantly inhibited the generation of spontaneous Ca(2+) waves in ICC and this was associated with a significant decrease in the ER Ca(2+) load, measured with 10mM caffeine responses. Ca(2+) waves could be rescued in the presence of H-89 by stimulating ryanodine receptors (RyRs) with 1mM caffeine but not by activation of inositol 1,4,5 tri-phosphate receptors (IP3Rs) with 10 μM phenylephrine. Increasing intracellular PKA with the cAMP agonists forskolin and 8-bromo-cAMP failed to yield an increase in Ca(2+) wave activity. We conclude that PKA may be maximally active under basal conditions in ICC and that inhibition of PKA with H-89 leads to a decreased ER Ca(2+) load sufficient to inactivate IP3Rs but not RyRs.

The effect of high [K(+)]o on spontaneous Ca(2+) waves in freshly isolated interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) act as putative pacemaker cells in the rabbit urethra. Pacemaker activity in ICC results from spontaneous global Ca²⁺ waves that can be increased in frequency by raising external [K⁺]. The purpose of this study was to elucidate the mechanism of this response. Intracellular [Ca²⁺] was measured in fluo‐4‐loaded smooth muscle cells (SMCs) and ICC using a Nipkow spinning disk confocal microscope. Increasing [K⁺]_o to 60 mmol/L caused an increase in [Ca²⁺]_i accompanied by contraction in SMCs. Raising [K⁺]_o did not cause contraction in ICC, but the frequency of firing of spontaneous calcium waves increased. Reducing [Ca²⁺]_o to 0 mmol/L abolished the response in both cell types. Nifedipine of 1 μmol/L blocked the response of SMC to high [K⁺]_o, but did not affect the increase in firing in ICC. This latter effect was blocked by 30 μmol/L NiCl₂ but not by the T‐type Ca²⁺ channel blocker mibefradil (300 nmol/L). However, inhibition of Ca²⁺ influx via reverse‐mode sodium/calcium exchange (NCX) using either 1 μmol/L SEA0400 or 5 μmol/L KB‐R7943 did block the effect of high [K⁺]_o on ICC. These data suggest that high K⁺ solution increases the frequency of calcium waves in ICC by increasing Ca²⁺ influx through reverse‐mode NCX.

Pacemaker activity in ICC results from spontaneous global Ca²⁺ waves that can be increased in frequency by raising external [K⁺]. The experiments described support the hypothesis that high K⁺ solution increases the frequency of calcium waves in ICC by increasing Ca²⁺ influx through reverse‐mode NCX.

Mechanisms underlying activation of transient BK current in rabbit urethral smooth muscle cells and its modulation by IP3-generating agonists

We used the perforated patch-clamp technique at 37°C to investigate the mechanisms underlying the activation of a transient large-conductance K(+) (tBK) current in rabbit urethral smooth muscle cells. The tBK current required an elevation of intracellular Ca(2+), resulting from ryanodine receptor (RyR) activation via Ca(2+)-induced Ca(2+) release, triggered by Ca(2+) influx through L-type Ca(2+) (CaV) channels. Carbachol inhibited tBK current by reducing Ca(2+) influx and Ca(2+) release and altered the shape of spike complexes recorded under current-clamp conditions. The tBK currents were blocked by iberiotoxin and penitrem A (300 and 100 nM, respectively) and were also inhibited when external Ca(2+) was removed or the CaV channel inhibitors nifedipine (10 μM) and Cd(2+) (100 μM) were applied. The tBK current was inhibited by caffeine (10 mM), ryanodine (30 μM), and tetracaine (100 μM), suggesting that RyR-mediated Ca(2+) release contributed to the activation of the tBK current. When IP3 receptors (IP3Rs) were blocked with 2-aminoethoxydiphenyl borate (2-APB, 100 μM), the amplitude of the tBK current was not reduced. However, when Ca(2+) release via IP3Rs was evoked with phenylephrine (1 μM) or carbachol (1 μM), the tBK current was inhibited. The effect of carbachol was abolished when IP3Rs were blocked with 2-APB or by inhibition of muscarinic receptors with the M3 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (1 μM). Under current-clamp conditions, bursts of action potentials could be evoked with depolarizing current injection. Carbachol reduced the number and amplitude of spikes in each burst, and these effects were reduced in the presence of 2-APB. In the presence of ryanodine, the number and amplitude of spikes were also reduced, and carbachol was without further effect. These data suggest that IP3-generating agonists can modulate the electrical activity of rabbit urethral smooth muscle cells and may contribute to the effects of neurotransmitters on urethral tone.

Electrical slow waves in the mouse oviduct are dependent upon a calcium activated chloride conductance encoded by Tmem16a

Myosalpinx contractions are critical for oocyte transport along the oviduct. A specialized population of pacemaker cells-oviduct interstitial cells of Cajal-generate slow waves, the electrical events underlying myosalpinx contractions. The ionic basis of oviduct pacemaker activity is unknown. We examined the role of a new class of Ca(2+)-activated Cl(-) channels (CaCCs)-anoctamin 1, encoded by Tmem16a-in oviduct slow wave generation. RT-PCR revealed the transcriptional expression of Tmem16a-encoded CaCCs in the myosalpinx. Intracellular microelectrode recordings were performed in the presence of two pharmacologically distinct Cl(-) channel antagonists, anthracene-9-carboxylic acid and niflumic acid. Both of these inhibitors caused membrane hyperpolarization, reduced the duration of slow waves, and ultimately inhibited pacemaker activity. Niflumic acid also inhibited propagating calcium waves within the myosalpinx. Slow waves were present at birth in wild-type and heterozygous oviducts but failed to develop by birth in mice homozygous for a null allele of Tmem16a (Tmem16a(tm1Bdh/tm1Bdh)). These data suggest that Tmem16a-encoded CaCCs contribute to membrane potential and are responsible for the upstroke and plateau phases of oviduct slow waves.

Calcium oscillations and pacemaking

Calcium plays important role in biological systems where it is involved in diverse mechanisms such as signaling, muscle contraction and neuromodulation. Action potentials are generated by dynamic interaction of ionic channels located on the plasma-membrane and these drive the rhythmic activity of biological systems such as the smooth muscle and the heart. However, ionic channels are not the only pacemakers; an intimate interaction between intracellular Ca(2+) stores and ionic channels underlie rhythmic activity. In this review we will focus on the role of Ca(2+) stores in regulation of rhythmical behavior.

Spontaneous Ca2+ signaling of interstitial cells in the guinea pig prostate

PURPOSE

We investigated whether prostate interstitial cells generate spontaneous Ca(2+) oscillation, a proposed mechanism underlying pacemaker potentials to drive spontaneous activity in stromal smooth muscle cells.

MATERIALS AND METHODS

Intracellular free Ca(2+) in portions of guinea pig prostate and freshly isolated, single prostate interstitial cells were visualized using fluo-4 Ca(2+) fluorescence. Spontaneous electrical activity was recorded in situ with intracellular microelectrodes.

RESULTS

In whole tissue preparations spontaneous Ca(2+) flashes firing synchronously across all smooth muscle cells within the field of view resulted in muscle wall contractions. Nonpropagating Ca(2+) waves were also recorded in individual smooth muscle cells. Nifedipine (Sigma®) (1 μM) largely decreased or abolished these Ca(2+) flashes and suppressed slow wave discharge upon blockade of their superimposed action potentials. Isolated prostate interstitial cells were readily distinguished from smooth muscle cells by their spiky processes and lack of contraction during intracellular Ca(2+) increases. Prostate interstitial cells generated spontaneous Ca(2+) transients in the form of whole cell flashes, intracellular Ca(2+) waves or localized Ca(2+) sparks. All 3 Ca(2+) signals were abolished by nicardipine (1 μM), cyclopiazonic acid (10 μM), caffeine (Sigma) (10 mM) or extracellular Ca(2+) removal.

CONCLUSIONS

Prostate interstitial cells generate spontaneous Ca(2+) transients that occur at a frequency comparable to Ca(2+) flashes in situ or slow waves relying on functional internal Ca(2+) stores. However, unlike other interstitial cells in the urinary tract, Ca(2+) influx through L-type Ca(2+) channels is fundamental to Ca(2+) transient firings in prostate interstitial cells. Thus, it is not possible to conclude that prostate interstitial cells are responsible for pacemaker potential generation.

Role of mitochondria in contraction and pacemaking in the mouse uterus

BACKGROUND AND PURPOSE

Uterine spontaneous contraction and pacemaking are poorly understood. This study investigates the role of the mitochondrial Ca(2+) store in uterine activity.

EXPERIMENTAL APPROACH

We investigated the effects of mitochondrial and sarco-endoplasmic reticulum (SER) inhibitors on contraction, membrane potential (Vm) and cytosolic Ca(2+) concentration ([Ca(2+) ](c) ) in longitudinal smooth muscle of the mouse uterus.

KEY RESULTS

The mitochondrial agents rotenone, carbonylcyanide-3-chlorophenylhydrazone (CCCP), 7-chloro-5-(2-chlorophenyl)-1,5-dihydro-4,1-benzothiazepin-2(3H)-one (CGP37157) and kaempferol decreased the force of contractions. The ATP synthase inhibitor oligomycin had no significant effect. The effects of these agents were compared with those of SER inhibitors cyclopiazonic acid (CPA), 2-amino ethoxyphenylborate (2-APB) and caffeine. All agents, except CPA and oligomycin, decreased contractile force. CPA and CCCP transiently increased contraction frequency, which returned to control levels, whereas rotenone, CGP37157, kaempferol and 2-APB decreased frequency and caffeine had no significant effect. Application of the mitochondrial agents when CPA functionally inhibited stores did not change contraction frequency but, with the exception of kaempferol, decreased force. CCCP caused depolarization and maintained increase in [Ca(2+) ](c) or depolarization/transient hyperpolarization and transient increase in [Ca(2+) ](c) for oestrus and di-oestrus tissues respectively. Rotenone caused hyperpolarization and maintained increase in [Ca(2+) ](c) . CGP37157 and kaempferol caused hyperpolarization but no measurable change in [Ca(2+) ](c) . Application of a range of K(+) channel blockers indicated a role of Ca(2+) -activated K(+) (K(Ca) ) channels in the CCCP- and CGP37157-induced actions.

CONCLUSIONS AND IMPLICATIONS

Mitochondria have a modulatory role on uterine contractions, with mitochondrial inhibition reducing contraction amplitude and pacemaker frequency by changes in Vm, [Ca(2+) ](c) and/or Ca(2+) influx.

Role of perinuclear mitochondria in the spatiotemporal dynamics of spontaneous Ca2+ waves in interstitial cells of Cajal-like cells of the rabbit urethra

BACKGROUND AND PURPOSE

Although spontaneous Ca(2+) waves in interstitial cells of Cajal (ICC)-like cells (ICC-LCs) primarily arise from endoplasmic reticulum (ER) Ca(2+) release, the interactions among mitochondrial Ca(2+) buffering, cellular energetics and ER Ca(2+) release in determining the spatiotemporal dynamics of intracellular Ca(2+) remain to be elucidated.

EXPERIMENTAL APPROACH

Spontaneous Ca(2+) transients in freshly isolated ICC-LCs of the rabbit urethra were visualized using fluo-4 Ca(2+) imaging, while the intracellular distribution of mitochondria was viewed with MitoTracker Red.

KEY RESULTS

Spontaneous Ca(2+) waves invariably originated from the perinuclear region where clusters of mitochondria surround the nucleus. Perinuclear Ca(2+) dynamics were characterized by a gradual rise in basal Ca(2+) that preceded each regenerative Ca(2+) transient. Caffeine evoked oscillatory Ca(2+) waves originating from anywhere within ICC-LCs. Ryanodine or cyclopiazonic acid prevented Ca(2+) wave generation with a rise in basal Ca(2+), and subsequent caffeine evoked a single rudimentary Ca(2+) transient. Inhibition of glycolysis with 2-deoxy-glucose or carbonyl cyanide 3-chlorophenylhydrazone, a mitochondrial protonophore, increased basal Ca(2+) and abolished Ca(2+) waves. However, caffeine still induced oscillatory Ca(2+) transients. Mitochondrial Ca(2+) uptake inhibition with RU360 attenuated Ca(2+) wave amplitudes, while mitochondrial Ca(2+) efflux inhibition with CGP37157 suppressed the initial Ca(2+) rise to reduce Ca(2+) wave frequency.

CONCLUSIONS AND IMPLICATIONS

Perinuclear mitochondria in ICC-LCs play a dominant role in the spatial regulation of Ca(2+) wave generation and may regulate ER Ca(2+) release frequency by buffering Ca(2+) within microdomains between both organelles. Glycolysis inhibition reduced mitochondrial Ca(2+) buffering without critically disrupting ER function. Perinuclear mitochondria may function as sensors of intracellular metabolites.

Ion channel modulators and urinary tract function

The membrane potential fulfils an important role in initiating smooth muscle contraction, through its depolarization and the subsequent influx of Ca(2+) through voltage-gated Ca(2+) channels. Changes in membrane potential can also coordinate contraction across great distances, utilizing the speed of electrical current flow through gap junctions. Hence, regulating membrane potential can greatly influence smooth muscle function. In this chapter, we will consider the influence of ion channels, as dynamic gatekeepers of membrane permeability, on urogenital function. Through their ability to act as key regulators of both the resting membrane potential and its dynamic changes, they provide important pharmacological targets for influencing urogenital function.Urogenital smooth muscle and urothelia contain a diverse range of molecularly and functionally distinct K(+) channels, which are key to regulating the resting membrane and for re-establishing the normal membrane potential following both active and passive changes. The voltage-gated Ca(2+) channels are key to initiating contraction and causing rapid depolarization, supplemented in some smooth muscles by rapid Na(+) conductances. The Cl(-) channels, often assumed to be passive, can actively change the membrane potential, and hence, cellular function, because Cl(-) is not usually at its equilibrium potential. The useful ways in which these ion channels can be targeted therapeutically in the ureter, bladder and urethra are discussed, focussing particularly on treatments for ureteric obstruction and detrusor overactivity. Current treatments for many urinary tract disorders, particularly the overactive bladder, are complicated by side effects. While ion channels have traditionally been considered as poor therapeutic targets by the pharmaceutical industry, our increasing knowledge of the molecular diversity of K(+) and Cl(-) channels gives new hope for more narrowly focused drug targeting, while the exciting discoveries of active currents in interstitial cells give us a new set of cellular targets for drugs.

Interstitial cells of Cajal in the urinary tract

The study of novel interstitial cells in the tissues of the urinary tract has defined advances in the field in the last decade. These intriguing cells belong to the same family as the better known interstitial cells of Cajal (ICC) of the gastrointestinal tract, and their discovery has been interpreted to suggest that pacemaker cells may be present in the urinary tract, driving the spontaneous or myogenic activity of the neighboring smooth muscle. This scenario may be true for the urethra where ICC have been described as "loose pacemakers" providing multiple, random inputs to modulate urethral smooth muscle activity. However, there is a paucity of direct evidence available to support this hypothesis in the bladder (where the smooth muscle cells are spontaneously active) or the renal pelvis (where atypical smooth muscle cells are the pacemakers), and it now seems more likely that urinary tract ICC act as modulators of smooth muscle activity.Interestingly, the literature suggests that the role of urinary tract ICC may be more apparent in pathophysiological conditions such as the overactive bladder. Several reports have indicated that the numbers of ICC present in overactive bladder tissues are greater than those from normal tissues; moreover, the contractility of tissues from overactive bladders in vitro appears to be more sensitive to the Kit antagonist, glivec, than those from normal bladder. Future research on urinary tract ICC in the short to medium term is likely to be dynamic and exciting and will lead to increasing our understanding of the roles of these cells in both normal and dysfunctional bladder.

Functions of ICC-like cells in the urinary tract and male genital organs

Interstitial cells of Cajal (ICC)-like cells (ICC-LCs) have been identified in many regions of the urinary tract and male genital organs by immunohistochemical studies and electron microscopy. ICC-LCs are characterized by their spontaneous electrical and Ca²⁺ signalling and the cellular mechanisms of their generation have been extensively investigated. Spontaneous activity in ICC-LCs rises from the release of internally stored Ca²⁺ and the opening of Ca²⁺-activated Cl⁻ channels to generate spontaneous transient depolarizations (STDs) in a manner not fundamentally dependent on Ca²⁺ influx through L-type voltage-dependent Ca²⁺ channels. Since urogenital ICC-LCs have been identified by their immunoreactivity to Kit (CD117) antibodies, the often-used specific marker for ICC in the gastrointestinal tract, their functions have been thought likely to be similar. Thus ICC-LCs in the urogenital tract might be expected to act as either electrical pacemaker cells to drive the smooth muscle wall or as intermediaries in neuromuscular transmission. However, present knowledge of the functions of ICC-LCs suggests that their functions are not so predetermined, that their functions may be very region specific, particularly under pathological conditions. In this review, we summarize recent advances in our understanding of the location and function of ICC-LCs in various organs of the urogenital system. We also discuss several unsolved issues regarding the identification, properties and functions of ICC-LCs in various urogenital regions in health and disease.

Novel excitatory effects of adenosine triphosphate on contractile and pacemaker activity in rabbit urethral smooth muscle

PURPOSE

Adenosine triphosphate is thought to be an important neurotransmitter in urethral smooth muscle but its physiological role is still unclear. We characterized the effects of adenosine triphosphate on contractile and pacemaker activity in rabbit urethral smooth muscle.

MATERIALS AND METHODS

Tension recordings were made from strips of rabbit proximal urethral smooth muscle. Membrane currents from freshly isolated smooth muscle cells and interstitial cells of Cajal were recorded using the patch clamp technique. Intracellular Ca(2+) was measured using confocal microscopy.

RESULTS

Exogenous application of adenosine triphosphate (10 microM) evoked robust contractions that were inhibited by the type 2 purinergic receptor blocker suramin (100 microM) and the selective type 2 purinergic Y1 receptor antagonist MRS2500 (Tocris Bioscience, Ellisville, Missouri) (100 nM). Application of the type 2 purinergic Y receptor agonist 2-MeSADP (1 microM) mimicked the effects of adenosine triphosphate. When smooth muscle cells were studied under voltage clamp at -60 mV, adenosine triphosphate evoked a large single inward current (greater than 1.2 nA) but 2-MeSADP produced a small current (about 16 pA). In contrast, when interstitial cells of Cajal were held at -60 mV, they showed spontaneous transient inward currents that were increased in frequency by adenosine triphosphate and 2-MeSADP. These excitatory effects were inhibited by suramin and MRS2500. Interstitial cells of Cajal showed spontaneous Ca(2+) waves that were increased in frequency by adenosine triphosphate and 2-MeSADP. These effects were also inhibited by suramin and MRS2500.

CONCLUSIONS

Contractile effects of adenosine triphosphate in urethral smooth muscle are mediated by the activation of type 2 purinergic Y receptors on interstitial cells of Cajal.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

Synchronization of Ca2+ oscillations: a coupled oscillator-based mechanism in smooth muscle

Entrained oscillations in Ca(2+) underlie many biological pacemaking phenomena. In this article, we review a long-range signaling mechanism in smooth muscle that results in global outcomes of local interactions. Our results are derived from studies of the following: (a) slow-wave depolarizations that underlie rhythmic contractions of gastric smooth muscle; and (b) membrane depolarizations that drive rhythmic contractions of lymphatic smooth muscle. The main feature of this signaling mechanism is a coupled oscillator-based synchronization of Ca(2+) oscillations across cells that drives membrane potential changes and causes coordinated contractions. The key elements of this mechanism are as follows: (a) the Ca(2+) release-refill cycle of endoplasmic reticulum Ca(2+) stores; (b) Ca(2+)-dependent modulation of membrane currents; (c) voltage-dependent modulation of Ca(2+) store release; and (d) cell-cell coupling through gap junctions or other mechanisms. In this mechanism, Ca(2+) stores alter the frequency of adjacent stores through voltage-dependent modulation of store release. This electrochemical coupling is many orders of magnitude stronger than the coupling through diffusion of Ca(2+) or inositol 1,4,5-trisphosphate, and thus provides an effective means of long-range signaling.

Spontaneous electrical waveforms in aging guinea pig prostates

PURPOSE

We characterized spontaneous electrical activity in the aging guinea pig prostate.

MATERIALS AND METHODS

Membrane potential recordings were made using conventional single microelectrode recording techniques.

RESULTS

Three types of spontaneous waveforms were recorded, including spikes, slow waves and spontaneous transient depolarizations. Spikes were classified as hyperactive or active. Active cells showed a mean +/- SEM frequency of 5.06 +/- 0.63 minutes(-1), significantly different from that in hyperactive cells (362.05 +/- 151.82 minutes(-1), p <0.05). After hyperpolarization amplitude was also significantly different in the active and hyperactive groups (17.80 +/- 1.98 vs 9.96 +/- 1.05 mV). Spike activity was abolished by 1 microM nifedipine in 7 preparations (p <0.05). Slow wave activity occurred at a frequency of 5.2 +/- 0.5 minutes(-1). The spike component of slow wave activity was abolished by 1 microM nifedipine, although the depolarizing transient remained unaltered from control values (8.1 +/- 3.1 mV, paired Student's t test p >0.05). Spontaneous transient depolarizations were recorded in the presence of slow waves in 10 preparations and of spikes in 13, and in quiescent cells in 9. Spontaneous transient depolarization frequency was highest in otherwise quiescent cells (24.55 +/- 6.48 minutes(-1)) compared to that in the presence of slow waves or spikes. Adding 1 microM nifedipine in 5 preparations did not significantly affect any measured parameters (p >0.05). Pacemaker potentials were not recorded in the aging prostate.

CONCLUSIONS

With increased age there is an increase in spike activity, which could conceivably explain the increased prostatic tone that accompanies aging. Spike activity and the spike component of the slow wave were abolished by nifedipine, suggesting a role for L-type channels. Finally, spontaneous transient depolarizations were unaffected by nifedipine, suggesting that mechanisms other than Ca(2+) entry via L-type channels are responsible for their generation and maintenance.

Spontaneous transient depolarizations in lymphatic vessels of the guinea pig mesentery: pharmacology and implication for spontaneous contractility

Guinea pig mesenteric lymphatic vessels exhibit rhythmic constrictions induced by action potential (AP)-like spikes and initiated by entrainment of spontaneous transient depolarizations (STDs). To characterize STDs and the signaling mechanisms responsible for their occurrence, we used intracellular microelectrodes, Ca2+ imaging, and pharmacological agents. In our investigation of the role of intracellular Ca2+ released from Ca2+ stores, we observed that intracellular Ca2+ transients accompanied some STDs, although there were many exceptions where Ca2+ transients occurred without accompanying STDs. STD frequency and amplitude were markedly affected by activators/inhibitors of inositol 1,4,5-trisphosphate receptors (IP3Rs) but not by treatments known to alter Ca2+ release via ryanodine receptors. A role for Ca2+-activated Cl(-) (Cl(Ca)) channels was indicated, as STDs were dependent on the Cl(-) but not Na+ concentration of the superfusing solution and were inhibited by the Cl(Ca) channel blockers niflumic acid (NFA), anthracene 9-carboxylic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid but not by the volume-regulated Cl(-) blocker DIDS. Increases in STD frequency and amplitude induced by agonist stimulation were also inhibited by NFA. Nifedipine, the hyperpolarization-activated inward current blocker ZD-7288, and the nonselective cation/store-operated channel blockers SKF-96365, Gd3+, and Ni2+ had no or marginal effects on STD activity. However, nifedipine, 2-aminoethoxydiphenyl borate, NFA, SKF-96365, Gd3+, and Ni2+ altered the occurrence of spontaneous APs. Our findings support a role for Ca2+ release through IP3Rs and a resultant opening of Cl(Ca) channels in STD generation and confirm the importance of these events in the initiation of lymphatic spontaneous APs and subsequent contractions. The abolition of spontaneous APs by blockers of other excitatory ion channels suggests a contribution of these conductances to lymphatic pacemaking.

Effects of imatinib mesylate on spontaneous electrical and mechanical activity in smooth muscle of the guinea-pig stomach

BACKGROUND AND PURPOSE

Effects of imatinib mesylate, a Kit receptor tyrosine kinase inhibitor, on spontaneous activity of interstitial cells of Cajal (ICC) and smooth muscles in the stomach were investigated.

EXPERIMENTAL APPROACH

Effects of imatinib on spontaneous electrical and mechanical activity were investigated by measuring changes in the membrane potential and tension recorded from smooth muscles of the guinea-pig stomach. Its effects on spontaneous changes in intracellular concentration of Ca(2+) ([Ca(2+)](i)) (Ca(2+) transients) were also examined in fura-2-loaded preparations.

KEY RESULTS

Imatinib (1-10 microM) suppressed spontaneous contractions and Ca(2+) transients. Simultaneous recordings of electrical and mechanical activity demonstrated that imatinib (1 microM) reduced the amplitude of spontaneous contractions without suppressing corresponding slow waves. In the presence of nifedipine (1 microM), imatinib (10 microM) reduced the duration of slow waves and follower potentials in the antrum and accelerated their generation, but had little affect on their amplitude. In contrast, imatinib reduced the amplitude of antral slow potentials and slow waves in the corpus.

CONCLUSIONS AND IMPLICATIONS

Imatinib may suppress spontaneous contractions of gastric smooth muscles by inhibiting pathways that increase [Ca(2+)](i) in smooth muscles rather than by specifically inhibiting the activity of ICC. A high concentration of imatinib (10 microM) reduced the duration of slow waves or follower potentials in the antrum, which reflect activity of ICC distributed in the myenteric layers (ICC-MY), and suppressed antral slow potentials or corporal slow waves, which reflect activity of ICC within the muscle bundles (ICC-IM), presumably by inhibiting intracellular Ca(2+) handling.

A survey of commonalities relevant to function and dysfunction in pelvic and sexual organs

Micturition, defecation and sexual function are all programmed through spinal reflexes that are under descending control from higher centres. Interaction between these reflexes can clearly be perceived, and evidence is accumulating the dysfunction in one reflex is often associated with dysfunction in another. In this article, we describe some of the basic properties and neural control of the smooth muscles mediating the reflexes, reviewing the common features that underlie these reflex functions, and what changes may be responsible for dysfunction. We propose that autonomic control within the pelvis predisposes pelvic and sexual organs to crosstalk, with the consequence that diseases and conditions of the pelvis are subject to convergence on a functional level. It should be expected that disturbance of the function of one system will inevitably impact adjacent systems.

SPONTANEOUS OSCILLATORY CONTRACTIONS IN AORTAS OF RATS WITH ARTERIAL PRESSURE LABILITY CAUSED BY SINOAORTIC DENERVATION

1. The spontaneous variation of blood pressure is defined as arterial pressure lability. Sinoaortic denervation (SAD) is characterized by arterial pressure lability without sustained hypertension. 2. The phenomenon of spontaneous oscillatory contractions (SOCs) occurs more frequently in the vascular beds of hypertensive animals. In large arteries, such as the aorta, SOCs occur only occasionally or they can be initiated by application of chemical stimuli. 3. In the present study, we investigated whether the arterial pressure lability evoked by SAD could be related to the emergence of SOCs in the aorta of rats submitted to SAD compared with sham-operated rats (SO). Three days after surgery (SAD or SO), aortic rings were placed in an organ chamber and the incidence (percentage of rats presenting SOCs), frequency (number of SOCs in 10 min) and amplitude (mN) of SOCs were measured. The participation of external Ca(2+) and K(+) channels in the maintenance of SOCs was also verified. 4. The incidence and frequency of SOCs were higher in endothelium-denuded aortas from SAD rats (82% and 38 +/- 4 SOCs/10 min, respectively) than in aortas from SO rats (40% and 14 +/- 2 SOCs/10 min, respectively). In aortas from SAD rats, verapamil (0.2 micromol/L), pinacidil (0.3 micromol/L) and tetraethylammonium (TEA; 5 mmol/L) totally inhibited SOCs, whereas increasing the CaCl(2) concentration to 2.0 and 2.5 mmol/L increased the frequency of SOCs. Interestingly, increasing the concentration of CaCl(2) to 3.5 mmol/L inhibited these contractions in aortas from SAD rats. 5. These results show that although SAD rats did not become hypertensive, their aortas were capable of initiating SOCs without the application of any chemical stimuli. The SOCs seem to be dependent on Ca(2+) influx sensitive to verapamil and also involve K(+) channels sensitive to pinacidil and TEA.

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.

K+ channel modulation of slow wave activity in the guinea-pig prostate

BACKGROUND AND PURPOSE

The aim of this study was to investigate the role of different K(+) channel populations and the inhibitory effect of various exogenously applied K(+) channel openers in the regulation of slow wave activity in the guinea-pig prostate.

EXPERIMENTAL APPROACH

Recordings of membrane potential were made using intracellular microelectrodes.

KEY RESULTS

Tetraethylammonium (TEA 300 micro M and 1 mM), iberiotoxin (150 nM) and 4-aminopyridine (4-AP 1 mM) increased the frequency of slow wave discharge. Apamin (1-200 nM) and glibenclamide (1 micro M) had no effect on slow wave activity. Lemakalim (1 micro M) and PCO-400 (1 micro M) abolished the slow waves, as did sodium nitroprusside (SNP 10 micro M) and calcitonin gene-related peptide (CGRP 100 nM). The inhibitory effect of these agents was independent of a significant change in membrane potential. In the presence of 4-AP (1 mM), TEA (1 mM) or glibenclamide (1 micro M) the inhibitory actions of SNP (10 micro M) were attenuated. The inhibitory actions of CGRP (100 nM) were also reversed by glibenclamide (1 micro M). In contrast, isoprenaline (1 micro M) did not alter the frequency of slow wave discharge.

CONCLUSIONS AND IMPLICATIONS

These results demonstrate that BK(Ca) and 4-AP-sensitive K(+) channels regulate the frequency of prostatic slow wave discharge. SNP and CGRP abolish slow waves in a hyperpolarisation-independent manner, partially via opening of K(ATP) channels. BK(Ca) and 4-AP-sensitive K(+) channels also play an important role in the SNP-induced inhibition of slow wave activity. The lack of membrane hyperpolarisation associated with the SNP- and CGRP-induced inhibition implies that the channels involved in this action are not predominantly located on the smooth muscle cells.