Activation of Ca2+-activated Cl- current by depolarizing steps in rabbit urethral interstitial cells

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.

Contribution of Cav1.2 Ca2+ channels and store-operated Ca2+ entry to pig urethral smooth muscle contraction

Urethral smooth muscle (USM) generates tone to prevent urine leakage from the bladder during filling. USM tone has been thought to be a voltage-dependent process, relying on Ca²⁺ influx via voltage-dependent Ca²⁺ channels in USM cells, modulated by the activation of Ca²⁺-activated Cl^- channels encoded by Ano1. However, recent findings in the mouse have suggested that USM tone is voltage independent, relying on Ca²⁺ influx through Orai channels via store-operated Ca²⁺ entry (SOCE). We explored if this pathway also occurred in the pig using isometric tension recordings of USM tone. Pig USM strips generated myogenic tone, which was nearly abolished by the Ca_v1.2 channel antagonist nifedipine and the ATP-dependent K⁺ channel agonist pinacidil. Pig USM tone was reduced by the Orai channel blocker GSK-7975A. Electrical field stimulation (EFS) led to phentolamine-sensitive contractions of USM strips. Contractions of pig USM were also induced by phenylephrine. Phenylephrine-evoked and EFS-evoked contractions of pig USM were reduced by ~50-75% by nifedipine and ~30% by GSK-7975A. Inhibition of Ano1 channels had no effect on tone or EFS-evoked contractions of pig USM. In conclusion, unlike the mouse, pig USM exhibited voltage-dependent tone and agonist/EFS-evoked contractions. Whereas SOCE plays a role in generating tone and agonist/neural-evoked contractions in both species, this dominates in the mouse. Tone and agonist/EFS-evoked contractions of pig USM are the result of Ca²⁺ influx primarily through Ca_v1.2 channels, and no evidence was found supporting a role of Ano1 channels in modulating these mechanisms.

Molecular and functional significance of Ca(2+)-activated Cl(-) channels in pulmonary arterial smooth muscle

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca(2+) levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K(+) channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca(2+) channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl(-) and is activated by a rise in intracellular Ca(2+) concentration (Ca(2+)-activated Cl(-) channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca(2+) activating CaCCs, which include stimulation by mobilization from intracellular Ca(2+) stores and Ca(2+) entry through voltage-dependent and voltage-independent Ca(2+) channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.

Calcium signalling in Cajal-like interstitial cells of the lower urinary tract

Interstitial cells of Cajal (ICC) serve several critical physiological roles in visceral smooth muscle organs, including acting as electrical pacemakers to modulate phasic contractile activity and as intermediaries in motor neurotransmission. The major roles of ICC have been described in the gastrointestinal tract, however, ICC-like cells (ICC-LC) can also be found in other visceral organs, including those of the lower urinary tract (LUT), where they provide similar functions, acting as electrical pacemakers and as intermediary cells involved in the modulation of neurotransmission to adjacent smooth muscle cells. The physiological functions of ICC-LC, in particular their role as pacemakers, relies on their ability to generate transient and propagating intracellular Ca(2+) events. The role of ICC-LC as pacemakers and neuromodulators in the LUT is increasingly apparent and the study of their intracellular Ca(2+) dynamics will provide a better understanding of their role in LUT excitability.

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.

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.

Recent advances in studies of spontaneous activity in smooth muscle: ubiquitous pacemaker cells

The general and specific properties of pacemaker cells, including Kit-negative cells, that are distributed in gastrointestinal, urethral and uterine smooth muscle tissues, are discussed herein. In intestinal tissues, interstitial cells of Cajal (ICC) are heterogeneous in both their forms and roles. ICC distributed in the myenteric layer (ICC-MY) act as primary pacemaker cells for intestinal mechanical and electrical activity. ICC distributed in muscle bundles play a role as mediators of signals from autonomic nerves to smooth muscle cells. A group of ICC also appears to act as a stretch sensor. Intracellular Ca2+ dynamics play a crucial role in ICC-MY pacemaking; intracellular Ca2+ ([Ca2+](i)) oscillations periodically activate plasmalemmal Ca2+-activated ion channels, such as Ca2+-activated Cl(-) channels and/or non-selective cation channels, although the relative contributions of these channels are not defined. With respect to gut motility, both the ICC network and enteric nervous system, including excitatory and inhibitory enteric neurons, play an essential role in producing highly coordinated peristalsis.

An outwardly rectifying and deactivating chloride channel expressed by interstitial cells of cajal from the murine small intestine

Recent studies have suggested a role for a chloride current in the modulation of pacemaker potentials generated by interstitial cells of Cajal. Patch-clamp recordings were made from inside-out patches of cultured interstitial cells of Cajal from the murine small intestine. The majority of patches were quiescent immediately after excision, but in some patches currents activated spontaneously after a period of 10 min to 1 h. Currents could also be activated by strongly polarizing the patch. It was found that the currents activated in both cases included a chloride channel. This channel could also be activated by ATP and the catalytic subunit of protein kinase A. The channel had conductance states (+/-SD) of 53 +/- 25.35, 126 +/- 21.44, 180 +/- 12.57 and 211 +/- 8.86 pS. It was outwardly rectifying (as a function of open probability) and deactivated (i.e., gave a tail current) but showed no inactivation. The permeability sequence of the channel was I(-)>>Br(-)>or=Cl(-)>Asp(-). It was unaffected in magnitude or rectification by changing the free Ca2+ concentration of the bath between <10 nM, 100 nM (control) and 2 mM .

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.

Calcium waves in intact guinea pig gallbladder smooth muscle cells

Intracellular Ca(2+) waves and spontaneous transient depolarizations were investigated in gallbladder smooth muscle (GBSM) whole mount preparations with intact mucosal layer [full thickness (FT)] by laser confocal imaging of intracellular Ca(2+) and voltage recordings with microelectrodes, respectively. Spontaneous Ca(2+) waves arose most often near the center, but sometimes from the extremities, of GBSM cells. They propagated regeneratively by Ca(2+)-induced Ca(2+) release involving inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] receptors and were not affected by TTX and atropine (ATS). Spontaneous Ca(2+) waves and spontaneous transient depolarizations were more prevalent in FT than in isolated muscularis layer preparations and occurred with similar pattern in GBSM bundles. Ca(2+) waves were abolished by the Ins(1,4,5)P(3) receptor inhibitors 2-aminoethoxydiphenyl borate and xestospongin C and by caffeine and cyclopiazonic acid. These events were reduced by voltage-dependent calcium channels (VDCCs) inhibitors diltiazem and nifedipine, by PLC inhibitor U-73122, and by thapsigargin and ryanodine. ACh, CCK, and carbachol augmented Ca(2+) waves and induced Ca(2+) flashes. The actions of these agonists were inhibited by U-73122. These results indicate that in GBSM, discharge and propagation of Ca(2+) waves depend on sarco(endo)plasmic reticulum (SR) Ca(2+) release via Ins(1,4,5)P(3) receptors, PLC activity, Ca(2+) influx via VDCCs, and SR Ca(2+) concentration. Neurohormonal enhancement of GBSM excitability involves PLC-dependent augmentation and synchronization of SR Ca(2+) release via Ins(1,4,5)P(3) receptors. Ca(2+) waves likely reflect the activity of a fundamental unit of spontaneous activity and play an important role in the excitability of GBSM.

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.

Regulation of calcium-activated chloride channels in smooth muscle cells: a complex picture is emerging

Calcium-activated chloride channels (ClCa) are ligand-gated anion channels as they have been shown to be activated by a rise in intracellular Ca2+ concentration in various cell types including cardiac, skeletal and vascular smooth muscle cells, endothelial and epithelial cells, as well as neurons. Because ClCa channels are normally closed at resting, free intracellular Ca2+ concentration (approximately 100 nmol/L) in most cell types, they have generally been considered excitatory in nature, providing a triggering mechanism during signal transduction for membrane excitability, osmotic balance, transepithelial chloride movements, or fluid secretion. Unfortunately, the genes responsible for encoding this class of ion channels is still unknown. This review centers primarily on recent findings on the properties of these channels in smooth muscle cells. The first section discusses the functional significance and biophysical and pharmacological properties of ClCa channels in smooth muscle cells, and ends with a description of 2 candidate gene families (i.e., CLCA and Bestrophin) that are postulated to encode for these channels in various cell types. The second section provides a summary of recent findings demonstrating the regulation of native ClCa channels in vascular smooth muscle cells by calmodulin-dependent protein kinase II and calcineurin and how their fine tuning by these enzymes may influence vascular tone.

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.

Activation of chloride currents in murine portal vein smooth muscle cells by membrane depolarization involves intracellular calcium release

The present study describes the first characterization of Ca2+-activated Cl− currents ( IClCa) in single smooth muscle cells from a murine vascular preparation (portal veins). IClCa was recorded using the perforated patch version of the whole cell voltage-clamp technique and was evoked using membrane depolarization. Generation of IClCa relied on Ca2+ entry through dihydropyridine-sensitive Ca2+ channels because IClCa was abolished by 1 μM nicardipine and enhanced by raising external Ca2+ concentration or by application of BAY K 8644. IClCa was characterized by the sensitivity to Cl− channel blockers and the effect of altering the external anion on reversal potential. Activation of IClCa after membrane depolarization was dependent on Ca2+ release from intracellular stores. Thus the amplitude of IClCa was diminished by the SR-ATPase inhibitor cyclopiazonic acid, the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenyl borate (2-APB), and the ryanodine receptor blocker tetracaine. The degree of inhibition produced by the application of 2-APB and tetracaine together was significantly greater than the effect of each agent applied alone. In current-clamp mode, injection of depolarizing current elicited a biphasic action potential, with the later depolarization being sensitive to niflumic acid (NFA; 10 μM). In isometric tension recordings, NFA inhibited spontaneous contractions. These data support a role for this conductance in portal vein excitability.

Purinergic regulation of guinea pig suburothelial myofibroblasts

The Ca(2+)-regulating and electrophysiological properties of guinea-pig suburothelial myofibroblasts have been measured in order to investigate their potential role in the sensation of bladder fullness, due to their strategic position between the urothelium and afferent fibres. Previous work has shown that stretch of the bladder wall releases ATP. Cells that stain positively for vimentin were isolated. About 45% of cells (median membrane capacitance 13.3 pF) exhibited spontaneous depolarizations to about -25 mV with a physiological Cl(-) gradient (frequency 2.6 +/- 1.5 min(-1), duration 14.5 +/- 2.2 s, n= 15). Under voltage-clamp spontaneous inward currents (frequency 1.5 +/- 0.2 min(-1), duration 14.5 +/- 7.0 s, n= 18) were recorded, with a similar reversal potential. The spontaneous currents were preceded by intracellular Ca(2+) transients with a magnitude that was independent of membrane potential. All cells tested responded to ATP by generating an intracellular Ca(2+) transient, followed by inward currents; the currents had a similar reversal potential and slope conductance to their spontaneous counterparts. ATP-generated transients were mimicked by UTP and ADP but not by alpha,beta-methylene-ATP (1-10 microm) or CTP (30 microm), indicating that ATP acts via a P2Y receptor. Transients were partially attenuated by 1 mm suramin but PPADS (80 microm) had no effect. These data indicate that ATP acts via a P2Y receptor, but responses were resistant to the P2Y(1) antagonist MRS2179. ATP-generated transients were abolished by intracellular perfusion with heparin and TMB-8 indicating that IP(3) was the intracellular second messenger. The reversal potentials of the spontaneous and ATP-generated currents were shifted by about +45 mV by a 12-fold reduction of the extracellular [Cl(-)] and the currents were greatly attenuated by 1 mm DIDS. No transients were generated on exposure to the muscarinic agonist carbachol. We propose that these cells may play a regulatory step in the sensation of bladder fullness by responding to ATP. The precise mechanism whereby they couple urothelial ATP release to afferent excitation is the next step to be elucidated.

Localised calcium release events in cells from the muscle of guinea-pig gastric fundus

After enzymatic dispersion of the muscle of the guinea-pig gastric fundus, single elongated cells were observed which differed from archetypal smooth muscle cells due to their knurled, tuberose or otherwise irregular surface morphology. These, but not archetypal smooth muscle cells, consistently displayed spontaneous localized (i.e. non-propagating) intracellular calcium ([Ca(2+)](i)) release events. Such calcium events were novel in their magnitude and kinetic profiles. They included short transient events, plateau events and events which coalesced spatially or temporally (compound events). Quantitative analysis of the events with an automatic detection programme showed that their spatio-temporal characteristics (full width and full duration at half-maximum amplitude) were approximately exponentially distributed. Their amplitude distribution suggested the presence of two release modes. Carbachol application caused an initial cell-wide calcium transient followed by an increase in localized calcium release events. Pharmacological analysis suggested that localized calcium release was largely dependent on external calcium entry acting on both inositol trisphosphate receptors (IP(3)Rs) and ryanodine receptors (RyRs) to release stored calcium. Nominally calcium-free external solution immediately and reversibly abolished all localized calcium release without blocking the initial transient calcium release response to carbachol. This was inhibited by 2-APB (100 microm), ryanodine (10 or 50 microm) or U-73122 (1 microm). 2-APB (100 microm), xestospongin C (XeC, 10 microm) or U-73122 (1 microm) blocked both spontaneous localized calcium release and localized release stimulated by 10 microm carbachol. Ryanodine (50 microm) also inhibited spontaneous release, but enhanced localized release in response to carbachol. This study represents the first characterization of localized calcium release events in cells from the gastric fundus.