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

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

Bicarbonate ion transport by the electrogenic Na+ /HCO3- cotransporter, NBCe1, is required for normal electrical slow-wave activity in mouse small intestine

BACKGROUND

Normal gastrointestinal motility depends on electrical slow-wave activity generated by interstitial cells of Cajal (ICC) in the tunica muscularis of the gastrointestinal tract. A requirement for HCO₃^- in extracellular solutions used to record slow waves indicates a role for HCO₃^- transport in ICC pacemaking. The Slc4a4 gene transcript encoding the electrogenic Na⁺ /HCO₃^- cotransporter, NBCe1, is enriched in mouse small intestinal myenteric region ICC (ICC-MY) that generate slow waves. This study aimed to determine how extracellular HCO₃^- concentrations affect electrical activity in mouse small intestine and to determine the contribution of NBCe1 activity to these effects.

METHODS

Immunohistochemistry and sharp electrode electrical recordings were used.

KEY RESULTS

The NBCe1 immunoreactivity was localized to ICC-MY of the tunica muscularis. In sharp electrode electrical recordings, removal of HCO₃^- from extracellular solutions caused significant, reversible, depolarization of the smooth muscle and a reduction in slow-wave amplitude and frequency. In 100 mM HCO₃^- , the muscle hyperpolarized and slow wave amplitude and frequency increased. The effects of replacing extracellular Na⁺ with Li⁺ , an ion that does not support NBCe1 activity, were similar to, but larger than, the effects of removing HCO₃^- . There were no additional changes to electrical activity when HCO₃^- was removed from Li⁺ containing solutions. The Na⁺ /HCO₃^- cotransport inhibitor, S-0859 (30µM) significantly reduced the effect of removing HCO₃^- on electrical activity.

CONCLUSIONS & INFERENCES

These studies demonstrate a major role for Na⁺ /HCO₃^- cotransport by NBCe1 in electrical activity of mouse small intestine and indicated that regulation of intracellular acid:base homeostasis contributes to generation of normal pacemaker activity in the gastrointestinal tract.

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.

Sodium-calcium exchanger 1 is the key molecule for urinary potassium excretion against acute hyperkalemia

The sodium (Na⁺)-chloride cotransporter (NCC) expressed in the distal convoluted tubule (DCT) is a key molecule regulating urinary Na⁺ and potassium (K⁺) excretion. We previously reported that high-K⁺ load rapidly dephosphorylated NCC and promoted urinary K⁺ excretion in mouse kidneys. This effect was inhibited by calcineurin (CaN) and calmodulin inhibitors. However, the detailed mechanism through which high-K⁺ signal results in CaN activation remains unknown. We used Flp-In NCC HEK293 cells and mice to evaluate NCC phosphorylation. We analyzed intracellular Ca²⁺ concentration ([Ca²⁺]_in) using live cell Ca²⁺ imaging in HEK293 cells. We confirmed that high-K⁺-induced NCC dephosphorylation was not observed without CaN using Flp-In NCC HEK29 cells. Extracellular Ca²⁺ reduction with a Ca²⁺ chelator inhibited high-K⁺-induced increase in [Ca²⁺]_in and NCC dephosphorylation. We focused on Na⁺/Ca²⁺ exchanger (NCX) 1, a bidirectional regulator of cytosolic Ca²⁺ expressed in DCT. We identified that NCX1 suppression with a specific inhibitor (SEA0400) or siRNA knockdown inhibited K⁺-induced increase in [Ca²⁺]_in and NCC dephosphorylation. In a mouse study, SEA0400 treatment inhibited K⁺-induced NCC dephosphorylation. SEA0400 reduced urinary K⁺ excretion and induced hyperkalemia. Here, we identified NCX1 as a key molecule in urinary K⁺ excretion promoted by CaN activation and NCC dephosphorylation in response to K⁺ load.

Na+/Ca2 + Exchange and Pacemaker Activity of Interstitial Cells of Cajal

Interstitial cells of Cajal (ICC) are pacemaker cells that generate electrical slow waves in gastrointestinal (GI) smooth muscles. Slow waves organize basic motor patterns, such as peristalsis and segmentation in the GI tract. Slow waves depend upon activation of Ca²⁺-activated Cl^– channels (CaCC) encoded by Ano1. Slow waves consist of an upstroke depolarization and a sustained plateau potential that is the main factor leading to excitation-contraction coupling. The plateau phase can last for seconds in some regions of the GI tract. How elevated Ca²⁺ is maintained throughout the duration of slow waves, which is necessary for sustained activation of CaCC, is unknown. Modeling has suggested a role for Na⁺/Ca²⁺ exchanger (NCX) in regulating CaCC currents in ICC, so we tested this idea on murine intestinal ICC. ICC of small and large intestine express NCX isoforms. NCX3 is closely associated with ANO1 in ICC, as shown by immunoprecipitation and proximity ligation assays (PLA). KB-R7943, an inhibitor of NCX, increased CaCC current in ICC, suggesting that NCX, acting in Ca²⁺ exit mode, helps to regulate basal [Ca²⁺]_i in these cells. Shifting NCX into Ca²⁺ entry mode by replacing extracellular Na⁺ with Li⁺ increased spontaneous transient inward currents (STICs), due to activation of CaCC. Stepping ICC from −80 to −40 mV activated slow wave currents that were reduced in amplitude and duration by NCX inhibitors, KB-R7943 and SN-6, and enhanced by increasing the NCX driving force. SN-6 reduced the duration of clustered Ca²⁺ transients that underlie the activation of CaCC and the plateau phase of slow waves. Our results suggest that NCX participates in slow waves as modeling has predicted. Dynamic changes in membrane potential and ionic gradients during slow waves appear to flip the directionality of NCX, facilitating removal of Ca²⁺ during the inter-slow wave interval and providing Ca²⁺ for sustained activation of ANO1 during the slow wave plateau phase.

Role of Pericytes in the Initiation and Propagation of Spontaneous Activity in the Microvasculature

The microvasculature is composed of arterioles, capillaries and venules. Spontaneous arteriolar constrictions reduce effective vascular resistance to enhance tissue perfusion, while spontaneous venular constrictions facilitate the drainage of tissue metabolites by pumping blood. In the venules of visceral organs, mural cells, i.e. smooth muscle cells (SMCs) or pericytes, periodically generate spontaneous phasic constrictions, Ca²⁺ transients and transient depolarisations. These events arise from spontaneous Ca²⁺ release from the sarco-endoplasmic reticulum (SR/ER) and the subsequent opening of Ca²⁺-activated chloride channels (CaCCs). CaCC-dependent depolarisation further activates L-type voltage-dependent Ca²⁺ channels (LVDCCs) that play a critical role in maintaining the synchrony amongst mural cells. Mural cells in arterioles or capillaries are also capable of developing spontaneous activity. Non-contractile capillary pericytes generate spontaneous Ca²⁺ transients primarily relying on SR/ER Ca²⁺ release. Synchrony amongst capillary pericytes depends on gap junction-mediated spread of depolarisations resulting from the opening of either CaCCs or T-type VDCCs (TVDCCs) in a microvascular bed-dependent manner. The propagation of capillary Ca²⁺ transients into arterioles requires the opening of either L- or TVDCCs again depending on the microvascular bed. Since the blockade of gap junctions or CaCCs prevents spontaneous Ca²⁺ transients in arterioles and venules but not capillaries, capillary pericytes appear to play a primary role in generating spontaneous activity of the microvasculature unit. Pericytes in capillaries where the interchange of substances between tissues and the circulation takes place may provide the fundamental drive for upstream arterioles and downstream venules so that the microvasculature network functions as an integrated unit.

Spontaneous activity in the microvasculature of visceral organs: role of pericytes and voltage-dependent Ca(2+) channels

The microvasculature plays a primary role in the interchange of substances between tissues and the circulation. In visceral organs that undergo considerable distension upon filling, the microvasculature appears to display intrinsic contractile properties to maintain their flow. Submucosal venules in the bladder or gastrointestinal tract generate rhythmic spontaneous phasic constrictions and associated Ca(2+) transients. These events are initiated within either venular pericytes or smooth muscle cells (SMCs) arising from spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) and the opening of Ca(2+) -activated chloride channels (CaCCs) that trigger Ca(2+) influx through L-type voltage-dependent Ca(2+) channels (VDCCs). L-type VDCCs also play a critical role in maintaining synchrony within the contractile mural cells. In the stomach myenteric layer, spontaneous Ca(2+) transients originating in capillary pericytes appear to spread to their neighbouring arteriolar SMCs. Capillary Ca(2+) transients primarily rely on SR Ca(2+) release, but also require Ca(2+) influx through T-type VDCCs for their synchrony. The opening of T-type VDCCs also contribute to the propagation of Ca(2+) transients into SMCs. In visceral microvasculature, pericytes act as either spontaneously active contractile machinery of the venules or as pacemaker cells generating synchronous Ca(2+) transients that drive spontaneous contractions in upstream arterioles. Thus pericytes play different roles in different vascular beds in a manner that may well depend on the selective expression of T-type and L-type Ca(2+) channels.

TRP-Na(+)/Ca(2+) Exchanger Coupling

Na(+)/Ca(2+) exchangers (NCXs) have traditionally been viewed principally as a means of Ca(2+) removal from non-excitable cells. However there has recently been increasing interest in the operation of NCXs in reverse mode acting as a means of eliciting Ca(2+) entry into these cells. Reverse mode exchange requires a significant change in the normal resting transmembrane ion gradients and membrane potential, which has been suggested to occur principally via the coupling of NCXs to localised Na(+) entry through non-selective cation channels such as canonical transient receptor potential (TRPC) channels. Here we review evidence for functional or physical coupling of NCXs to non-selective cation channels, and how this affects NCX activity in non-excitable cells. In particular we focus on the potential role of nanojunctions, where the close apposition of plasma and intracellular membranes may help create the conditions needed for the generation of localised rises in Na(+) concentration that would be required to trigger reverse mode exchange.

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+).

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.

T-type Ca2+ channels and the urinary and male genital tracts

T-type Ca(2+) channels are widely expressed throughout the urinary and male genital tracts, generally alongside L-type Ca(2+) channels. The use of pharmacological blockers of these channels has suggested functional roles in all regions, with the possible exception of the ureter. Their functional expression is apparent not just in smooth muscle cells but also in interstitial cells that lie in close proximity to muscle, nerve and epithelial components of these tissues. Thus, T-type Ca(2+) channels can contribute directly to modulation of muscle function and indirectly to changes of epithelial and nerve function. T-type Ca(2+) channel activity modulates phasic contractile activity, especially in conjunction with Ca(2+)-activated K(+) channels, and also to agonist-dependent responses in different tissues. Upregulation of channel density occurs in pathological conditions associated with enhanced contractile responses, e.g. overactive bladder, but it is unclear if this is causal or a response to the pathological state. Moreover, T-type Ca(2+) channels may have a role in the development of prostate tumours regulating the secretion of mitogens from neuroendocrine cells. Although a number of selective channel blockers exist, their relative selectivity over L-type Ca(2+) channels is often low and makes evaluation of T-type Ca(2+) channel function in the whole organism difficult.

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.

Reverse mode of the sodium/calcium exchanger subtype 3 in interstitial cells of Cajal from rat bladder

OBJECTIVE

To investigate how the sodium/calcium exchanger subtype 3 (NCX3) and its reverse mode contribute to the function of interstitial cells of Cajal (ICCs) from the rat bladder.

METHODS

The study used 20 female Wistar rats. We observed the expression of the NCX3 expression in the bladder using reverse transcriptase-polymerase chain reaction and Western blotting. The NCX3 in ICCs was also confirmed by double-labeled fluorescence. NCX3 functions in reverse mode of ICCs were observed using confocal microscopy with preload fluo-3AM, and its currents were evaluated using the whole-cell patch clamp technique, with or without the NCX3 inhibitor KB-R7943 (5 and 30μM), with an afterward identification of ICCs using single-cell polymerase chain reaction.

RESULTS

NCX3 was confirmed in rat bladder ICCs. The time required for the intracellular calcium concentration [Ca(2+)]i of NCX3 was enhanced by KB-R7943 (5μM, P ≤.01). Moreover, KB-R7943 (5 and 30μM) significantly decreased the currents generated by the reverse mode of NCX3 from the ICCs (P <.05).

CONCLUSION

NCX3 is expressed in rat bladder ICCs. The reverse mode of NCX3 can generate [Ca(2+)]i of the bladder ICCs.

Functional and morphological properties of pericytes in suburothelial venules of the mouse bladder

BACKGROUND AND PURPOSE

In suburothelial venules of rat bladder, pericytes (perivascular cells) develop spontaneous Ca(2+) transients, which may drive the smooth muscle wall to generate spontaneous venular constrictions. We aimed to further explore the morphological and functional characteristics of pericytes in the mouse bladder.

EXPERIMENTAL APPROACH

The morphological features of pericytes were investigated by electron microscopy and fluorescence immunohistochemistry. Changes in diameters of suburothelial venules were measured using video microscopy, while intracellular Ca(2+) dynamics were visualized using Fluo-4 fluorescence Ca(2+) imaging.

KEY RESULTS

A network of α-smooth muscle actin immunoreactive pericytes surrounded venules in the mouse bladder suburothelium. Scanning electron microscopy revealed that this network of stellate-shaped pericytes covered the venules, while transmission electron microscopy demonstrated that the venular wall consisted of endothelium and adjacent pericytes, lacking an intermediate smooth muscle layer. Pericytes exhibited spontaneous Ca(2+) transients, which were accompanied by phasic venular constrictions. Nicardipine (1 μM) disrupted the synchrony of spontaneous Ca(2+) transients in pericytes and reduced their associated constrictions. Residual asynchronous Ca(2+) transients were suppressed by cyclopiazonic acid (10 μM), 2-aminoethoxydiphenyl borate (10 μM), U-73122 (1 μM), oligomycin (1 μM) and SKF96365 (10 μM), but unaffected by ryanodine (100 μM) or YM-244769 (1 μM), suggesting that pericyte Ca(2+) transients rely on Ca(2+) release from the endoplasmic reticulum via the InsP(3) receptor and also require Ca(2+) influx through store-operated Ca(2+) channels.

CONCLUSIONS AND IMPLICATIONS

The pericytes in mouse bladder can generate spontaneous Ca(2+) transients and contractions, and thus have a fundamental role in promoting spontaneous constrictions of suburothelial venules.

On the origin of rhythmic calcium transients in the ICC-MP of the mouse small intestine

Interstitial cells of Cajal associated with the myenteric plexus (ICC-MP) are pacemaker cells of the small intestine, producing the characteristic omnipresent electrical slow waves, which orchestrate peristaltic motor activity and are associated with rhythmic intracellular calcium oscillations. Our objective was to elucidate the origins of the calcium transients. We hypothesized that calcium oscillations in the ICC-MP are primarily regulated by the sarcoplasmic reticulum (SR) calcium release system. With the use of calcium imaging, study of the effect of T-type calcium channel blocker mibefradil revealed that T-type channels did not play a major role in generating the calcium transients. 2-Aminoethoxydiphenyl borate, an inositol 1,4,5 trisphosphate receptor (IP(3)R) inhibitor, and U73122, a phospholipase C inhibitor, both drastically decreased the frequency of calcium oscillations, suggesting a major role of IP(3) and IP(3)-induced calcium release from the SR. Immunohistochemistry proved the expression of IP(3)R type I (IP(3)R-I), but not type II (IP(3)R-II) and type III (IP(3)R-III) in ICC-MP, indicating the involvement of the IP(3)R-I subtype in calcium release from the SR. Cyclopiazonic acid, a SR/endoplasmic reticulum calcium ATPase pump inhibitor, strongly reduced or abolished calcium oscillations. The Na-Ca exchanger (NCX) in reverse mode is likely involved in refilling the SR because the NCX inhibitor KB-R7943 markedly reduced the frequency of calcium oscillations. Immunohistochemistry revealed 100% colocalization of NCX and c-Kit in ICC-MP. Testing a mitochondrial NCX inhibitor, we were unable to show an essential role for mitochondria in regulating calcium oscillations in the ICC-MP. In summary, ongoing IP(3) synthesis and IP(3)-induced calcium release from the SR, via the IP(3)R-I, are the major drivers of the calcium transients associated with ICC pacemaker activity. This suggests that a biochemical clock intrinsic to ICC determines the pacemaker frequency, which is likely directly linked to kinetics of the IP(3)-activated SR calcium channel and IP(3) metabolism.

Sodium-calcium exchange in intracellular calcium handling of human airway smooth muscle

Enhanced airway contractility following inflammation by cytokines such as tumor necrosis factor alpha (TNFα) or interleukin-13 (IL-13) involves increased intracellular Ca²⁺ ([Ca²⁺]_i) levels in airway smooth muscle (ASM). In ASM, plasma membrane Ca²⁺ fluxes form a key component of [Ca²⁺]_i regulation. There is now growing evidence that the bidirectional plasma membrane Na⁺/Ca²⁺ exchanger (NCX) contributes to ASM [Ca²⁺]_i regulation. In the present study, we examined NCX expression and function in human ASM cells under normal conditions, and following exposure to TNFα or IL-13. Western blot analysis showed significant expression of the NCX1 isoform, with increased NCX1 levels by both cytokines, effects blunted by inhibitors of nuclear factor NF-κB or mitogen-activated protein kinase. Cytokine-mediated increase in NCX1 involved enhanced transcription followed by protein synthesis. NCX2 and NCX3 remained undetectable even in cytokine-stimulated ASM. In fura-2 loaded human ASM cells, NCX-mediated inward Ca²⁺ exchange as well as outward exchange (measured as rates of change in [Ca²⁺]_i) was elicited by altering extracellular Na⁺ and Ca²⁺ levels. Contribution of NCX was verified by measuring [Na⁺]_i using the fluorescent Na⁺ indicator SBFI. NCX-mediated inward exchange was verified by demonstrating prevention of rising [Ca²⁺]_i or falling [Na⁺]_i in the presence of the NCX inhibitor KBR7943. Inward exchange-mode NCX was increased by both TNFα and IL-13 to a greater extent than outward exchange. NCX siRNA transfection substantially blunted outward exchange and inward exchange modes. Finally, inhibition of NCX expression or function blunted peak [Ca²⁺]_i and rate of fall of [Ca²⁺]_i following histamine stimulation. These data suggest that NCX-mediated Ca²⁺ fluxes normally exist in human ASM (potentially contributing to rapid Ca²⁺ fluxes), and contribute to enhanced [Ca²⁺]_i regulation in airway inflammation.

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.

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.

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.

Presence of cyclic nucleotide-gated channels in the rat urethra and their involvement in nerve-mediated nitrergic relaxation

We have addressed the distribution of cGMP-gated channels (CNG) in the rat urethra for the first time, as well as their putative role in mediating of the relaxation elicited by electrical field stimulation of nitrergic nerves. Functional studies have shown that specifically blocking CNG with L-cis-diltiazem leads to the rapid inhibition of urethral relaxation induced either by nitric oxide (NO) released by the nerves or by soluble guanylate cyclase activated with YC-1. By contrast, nerve-mediated noradrenergic contractions were only slowly and mildly reduced by L-cis-diltiazem. This effect was mimicked by lower concentrations of the D-diltiazem isomer, probably due to the nonspecific inhibition of voltage-dependent calcium channels. However, D-diltiazem did not affect relaxation responses. The expression of heteromeric retinal-like CNGA1 channels was demonstrated by conventional PCR on mRNA from the rat urethra. These channels were located in a subpopulation of intramuscular interstitial cells of Cajal (ICC) as well as in smooth muscle cells, although they were less abundant in the latter. CNG channels could not be visualized in any nervous structure within the urethral wall, in agreement with the emerging view that a subset of ICC serves as a target for NO. These channels could provide a suitable ionic mechanism to associate the changes in cytosolic calcium with the activation of the nitric NO-cGMP pathway and relaxation although the precise mechanisms involved remain to be elucidated.

Smooth muscle cell calcium activation mechanisms

Smooth muscle cell (SMC) contraction is controlled by the Ca2+ and Rho kinase signalling pathways. While the SMC Rho kinase system seems to be reasonably constant, there is enormous variation with regard to the mechanisms responsible for generating Ca2+ signals. One way of dealing with this diversity is to consider how this system has been adapted to control different SMC functions. Phasic SMCs (vas deferens, uterus and bladder) rely on membrane depolarization to drive Ca2+ influx across the plasma membrane. This depolarization can be induced by neurotransmitters or through the operation of a membrane oscillator. Many tonic SMCs (vascular, airway and corpus cavernosum) are driven by a cytosolic Ca2+ oscillator that generates periodic pulses of Ca2+. A similar oscillator is present in pacemaker cells such as the interstitial cells of Cajal (ICCs) and atypical SMCs that control other tonic SMCs (gastrointestinal, urethra, ureter). The changes in membrane potential induced by these cytosolic oscillators does not drive contraction directly but it functions to couple together individual oscillators to provide the synchronization that is a characteristic feature of many tonic SMCs.

Role of mitochondria in modulation of spontaneous Ca2+ waves in freshly dispersed interstitial cells of Cajal from the rabbit urethra

Interstitial cells of Cajal (ICC) isolated from the rabbit urethra exhibit pacemaker activity that results from spontaneous Ca(2+) waves. The purpose of this study was to investigate if this activity was influenced by Ca(2+) uptake into mitochondria. Spontaneous Ca(2+) waves were recorded using a Nipkow spinning disk confocal microscope and spontaneous transient inward currents (STICs) were recorded using the whole-cell patch clamp technique. Disruption of the mitochondrial membrane potential with the electron transport chain inhibitors rotenone (10 microm) and antimycin A (5 microm) abolished Ca(2+) waves and increased basal Ca(2+) levels. Similar results were achieved when mitochondria membrane potential was collapsed using the protonophores FCCP (0.2 microm) and CCCP (1 microm). Spontaneous Ca(2+) waves were not inhibited by the ATP synthase inhibitor oligomycin (1 microm), suggesting that these effects were not attributable to an effect on ATP levels. STICs recorded under voltage clamp at -60 mV were also inhibited by CCCP and antimycin A. Dialysis of cells with the mitochondrial uniporter inhibitor RU360 (10 microm) also inhibited STICS. Stimulation of Ca(2+) uptake into mitochondria using the plant flavonoid kaempferol (10 microm) induced a series of propagating Ca(2+) waves. The kaempferol-induced activity was inhibited by application of caffeine (10 mm) or removal of extracellular Ca(2+), but was not significantly affected by the IP(3) receptor blocker 2-APB (100 microm). These data suggest that spontaneous Ca(2+) waves in urethral ICC are regulated by buffering of cytoplasmic Ca(2+) by mitochondria.

[Molecular pharmacologic approaches to functional analysis of new biological target molecules for drug discovery]

This review focuses on two pharmacologic approaches to the functional evaluation of new target molecules for drug discovery. One is the development of a novel specific antagonist of the Na(+)-Ca(++) exchanger (NCX) SEA0400. The other is a comprehensive analysis of the functions of pituitary adenylate cyclase-activating polypeptide (PACAP), a neuropeptide ligand for G protein-coupled receptors. NCX is the one of the last target molecules regulating the cellular Ca(++) concentration. There was no efficient way to address the pathophysiologic roles of NCX until a specific antagonist, 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), was developed. Our recent studies using SEA0400 clearly showed the possible roles of NCX in several pathologic states of cardiovascular and nervous tissues. In our second approach including gene-targeting methods, we found new, unexpected roles of PACAP in higher brain functions, such as psychomotor, cognition, photoentrainment, and nociception. Based on these experimental findings, a genetic association study in schizophrenia patients revealed that the single-nucleotide polymorphisms of the PACAP gene are significantly associated with the hypofunction of the hippocampus. Regarding the peripheral roles of PACAP, we found that PACAP is involved not only in the regulation of insulin secretion in pancreatic islets, but also in the regulation of islet turnover. In subsequent phenotypic analysis of PACAP transgenic mice, we identified novel candidate genes that probably have promising functional roles.

Sodium and asthma: something borrowed, something new?

Some early studies have called attention to the potential contribution of sodium (both dietary and serum levels) in airway-related disease, although the picture was not entirely clear. Two recent developments may now allow a more careful consideration of this: first, the greatly improved understanding of the role of salt in hypertension (particularly the identification of subgroups of salt-sensitive individuals within the general population), and second, the recent discovery of the role of the Na(+)/Ca(2+) exchanger in smooth muscle function. Here, we first review those two developments and then apply them to airway smooth muscle and asthma.

Differential gene expression in functional classes of interstitial cells of Cajal in murine small intestine

Interstitial cells of Cajal (ICC) have important functions in regulation of motor activity in the gastrointestinal tract. In murine small intestine, ICC are gathered in the regions of the myenteric plexus (ICC-MY) and the deep muscular plexus (ICC-DMP). These two classes of ICC have different physiological functions. ICC-MY are pacemaker cells and generate the slow-wave electrical rhythmicity of gastrointestinal organs. ICC-DMP form synaptic connections with the varicose nerve terminals of enteric motor neurons and are involved in reception and transduction of motor neurotransmission. Gene expression underlying specific functions of ICC classes is incompletely understood. In the present study, we used recently developed highly selective techniques to isolate the two functional ICC classes from enzymatically dispersed intestinal muscles by fluorescence-activated cell sorting. The transcriptomes of ICC-MY and ICC-DMP were investigated using oligonucleotide microarray analysis. Differential expression of functional groups of genes defined by standard gene ontology terms was also studied. There were substantial numbers of genes expressed more abundantly in ICC than in the tunica muscularis, and we also detected marked phenotypic differences between ICC-MY and ICC-DMP. Notably, genes related to cell junction, process guidance, and vesicle trafficking were upregulated in ICC. Consistent with their specific functions, metabolic and Ca(2+) transport genes were relatively upregulated in ICC-MY, whereas genes for signaling proteins involved in transduction of neurotransmitter functions were relatively upregulated in ICC-DMP. Our results may lead to the identification of novel biomarkers for ICC and provide directions for further studies designed to understand ICC function in health and disease.

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.

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).

The reverse mode of the Na(+)/Ca(2+) exchanger provides a source of Ca(2+) for store refilling following agonist-induced Ca(2+) mobilization

Agonist-induced contraction of airway smooth muscle (ASM) can be triggered by an elevation in the intracellular Ca(2+) concentration, primarily through the release of Ca(2+) from the sarcoplasmic reticulum (SR). The refilling of the SR is integral for subsequent contractions. It has been suggested that Ca(2+) entry via store-operated cation (SOC) and receptor-operated cation channels may facilitate refilling of the SR. Indeed, depletion of the SR activates substantial inward SOC currents in ASM that are composed of both Ca(2+) and Na(+). Accumulation of Na(+) within the cell may regulate Ca(2+) handling in ASM by forcing the Na(+)/Ca(2+) exchanger (NCX) into the reverse mode, leading to the influx of Ca(2+) from the extracellular domain. Since depletion of the SR activates substantial inward Na(+) current, it is conceivable that the reverse mode of the NCX may contribute to the intracellular Ca(2+) pool from which the SR is refilled. Indeed, successive contractions of bovine ASM, evoked by various agonists (ACh, histamine, 5-HT, caffeine) were significantly reduced upon removal of extracellular Na(+); whereas contractions evoked by KCl were unchanged by Na(+) depletion. Ouabain, a selective inhibitor of the Na(+)/K(+) pump, had no effect on the reductions observed under normal and zero-Na(+) conditions. KB-R7943, a selective inhibitor of the reverse mode of the NCX, significantly reduced successive contractions induced by all agonists without altering KCl responses. Furthermore, KB-R7943 abolished successive caffeine-induced Ca(2+) transients in single ASM cells. Together, these data suggest a role for the reverse mode of the NCX in refilling the SR in ASM following Ca(2+) mobilization.

Ca2+ signalling in urethral interstitial cells of Cajal

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

Interstitial cells of Cajal in the urethra

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