Spatial and temporal mapping of pacemaker activity in interstitial cells of Cajal in mouse ileum in situ

Optogenetic techniques for understanding the gut peristalsis during chicken embryonic development

Gut peristaltic movements transport ingested materials along the gut axis, which is critical for food digestion and nutrient absorption. While a large amount of studies have been devoted to analyzing the physiological functions of peristalsis in adults, little is known about how the peristaltic system is established during embryogenesis. In recent years, the chicken developing gut has emerged as an excellent model, in which specific sites along the gut axis can be genetically labeled enabling live imaging and optogenetic analyses. This review provides an overview of recent progress in optogenetic studies of gut peristalsis. Analyses with an improved channelrhodopsin-2 variant demonstrated that the peristalsis can artificially be generated in the developing gut. These studies unveiled novel functional coordination between different regions along the gut axis. In addition, imaging with GCaMP6s, a genetically encoded calcium indicator, enabled a fine mapping of developmental changes in the peristaltic patterns as Ca2+ signals. These advanced techniques will broaden our knowledge of how embryonic peristalsis is established at the cellular and molecular level, leading to the understanding of physiological and pathological processes in adult peristalsis.

Effects of natural products on functional constipation: analysis of active ingredient and mechanism

Constipation is a prevalent clinical ailment of the gastrointestinal system, yet its pathogenesis remains ambiguous. Despite the availability of numerous treatment modalities, they are insufficient in resolving the issue for patients. This work conducted a comprehensive review of the existing literature pertaining to the utilization of natural products for the treatment of constipation, with a focus on the efficacy of natural products in treating constipation, and to provide a comprehensive summary of their underlying mechanisms of action. Upon conducting a thorough review of the extant literature, we found that natural products can effectively treat constipation as modern synthetic drugs and compounded drugs with acetylcholinesterase (AChE) effects, rich in fiber and mucus, and the effects of increasing the tension of the ileum and gastrointestinal tract muscle, mediating signaling pathways, cytokine, excitability of the smooth muscle of the gastrointestinal tract, and regulating the homeostasis of intestinal flora. However, there is a wide variety of natural products, and there are still relatively few studies; the composition of natural products is complex, and the mechanism of action of natural products cannot be clarified. In the future, we need to further improve the detailed mechanism of natural products for the treatment of constipation.

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.

Insights on gastrointestinal motility through the use of optogenetic sensors and actuators

The muscularis of the gastrointestinal (GI) tract consists of smooth muscle cells (SMCs) and various populations of interstitial cells of Cajal (ICC), platelet-derived growth factor receptor α⁺ (PDGFRα⁺ ) cells, as well as excitatory and inhibitory enteric motor nerves. SMCs, ICC and PDGFRα⁺ cells form an electrically coupled syncytium, which together with inputs from the enteric nervous system (ENS) regulate GI motility. Early studies evaluating Ca²⁺ signalling behaviours in the GI tract relied upon indiscriminate loading of tissues with Ca²⁺ dyes. These methods lacked the means to study activity in specific cells of interest without encountering contamination from other cells within the preparation. Development of mice expressing optogenetic sensors (GCaMP, RCaMP) has allowed visualization of Ca²⁺ signalling behaviours in a cell specific manner. Additionally, availability of mice expressing optogenetic modulators (channelrhodopsins or halorhodospins) has allowed manipulation of specific signalling pathways using light. GCaMP expressing animals have been used to characterize Ca²⁺ signalling behaviours of distinct classes of ICC and SMCs throughout the GI musculature. These findings illustrate how Ca²⁺ signalling in ICC is fundamental in GI muscles, contributing to tone in sphincters, pacemaker activity in rhythmic muscles and relaying enteric signals to SMCs. Animals that express channelrhodopsin in specific neuronal populations have been used to map neural circuitry and to examine post junctional neural effects on GI motility. Thus, optogenetic approaches provide a novel means to examine the contribution of specific cell types to the regulation of motility patterns within complex multi-cellular systems. Abstract Figure Legends Optogenetic activators and sensors can be used to investigate the complex multi-cellular nature of the gastrointestinal (GI tract). Optogenetic activators that are activated by light such as channelrhodopsins (ChR2), OptoXR and halorhodopsinss (HR) proteins can be genetically encoded into specific cell types. This can be used to directly activate or silence specific GI cells such as various classes of enteric neurons, smooth muscle cells (SMC) or interstitial cells, such as interstitial cells of Cajal (ICC). Optogenetic sensors that are activated by different wavelengths of light such as green calmodulin fusion protein (GCaMP) and red CaMP (RCaMP) make high resolution of sub-cellular Ca²⁺ signalling possible within intact tissues of specific cell types. These tools can provide unparalleled insight into mechanisms underlying GI motility and innervation. This article is protected by copyright. All rights reserved.

Ca2+ Signaling Is the Basis for Pacemaker Activity and Neurotransduction in Interstitial Cells of the GI Tract

Years ago gastrointestinal motility was thought to be due to interactions between enteric nerves and smooth muscle cells (SMCs) in the tunica muscularis. Thus, regulatory mechanisms controlling motility were either myogenic or neurogenic. Now we know that populations of interstitial cells, c-Kit⁺ (interstitial cells of Cajal or ICC), and PDGFRα⁺ cells (formerly "fibroblast-like" cells) are electrically coupled to SMCs, forming the SIP syncytium. Pacemaker and neurotransduction functions are provided by interstitial cells through Ca²⁺ release from the endoplasmic reticulum (ER) and activation of Ca²⁺-activated ion channels in the plasma membrane (PM). ICC express Ca²⁺-activated Cl^- channels encoded by Ano1. When activated, Ano1 channels produce inward current and, therefore, depolarizing or excitatory effects in the SIP syncytium. PDGFRα⁺ cells express Ca²⁺-activated K⁺ channels encoded by Kcnn3. These channels generate outward current when activated and hyperpolarizing or membrane-stabilizing effects in the SIP syncytium. Inputs from enteric and sympathetic neurons regulate Ca²⁺ transients in ICC and PDGFRα⁺ cells, and currents activated in these cells conduct to SMCs and regulate contractile behaviors. ICC also serve as pacemakers, generating slow waves that are the electrophysiological basis for gastric peristalsis and intestinal segmentation. Pacemaker types of ICC express voltage-dependent Ca²⁺ conductances that organize Ca²⁺ transients, and therefore Ano1 channel openings, into clusters that define the amplitude and duration of slow waves. Ca²⁺ handling mechanisms are at the heart of interstitial cell function, yet little is known about what happens to Ca²⁺ dynamics in these cells in GI motility disorders.

Analysis of Intestinal Movements with Spatiotemporal Maps: Beyond Anatomy and Physiology

Over 150 years ago, methods for quantitative analysis of gastrointestinal motor patterns first appeared. Graphic representations of physiological variables were recorded with the kymograph after the mid-1800s. Changes in force or length of intestinal muscles could be quantified, however most recordings were limited to a single point along the digestive tract.In parallel, photography and cinematography with X-Rays visualised changes in intestinal shape, but were hard to quantify. More recently, the ability to record physiological events at many sites along the gut in combination with computer processing allowed construction of spatiotemporal maps. These included diameter maps (DMaps), constructed from video recordings of intestinal movements and pressure maps (PMaps), constructed using data from high-resolution manometry catheters. Combining different kinds of spatiotemporal maps revealed additional details about gut wall status, including compliance, which relates forces to changes in length. Plotting compliance values along the intestine enabled combined DPMaps to be constructed, which can distinguish active contractions and relaxations from passive changes. From combinations of spatiotemporal maps, it is possible to deduce the role of enteric circuits and pacemaker cells in the generation of complex motor patterns. Development and application of spatiotemporal methods to normal and abnormal motor patterns in animals and humans is ongoing, with further technical improvements arising from their combination with impedance manometry, magnetic resonance imaging, electrophysiology, and ultrasonography.

Engaging biological oscillators through second messenger pathways permits emergence of a robust gastric slow-wave during peristalsis

Peristalsis, the coordinated contraction—relaxation of the muscles of the stomach is important for normal gastric motility and is impaired in motility disorders. Coordinated electrical depolarizations that originate and propagate within a network of interconnected layers of interstitial cells of Cajal (ICC) and smooth muscle (SM) cells of the stomach wall as a slow-wave, underly peristalsis. Normally, the gastric slow-wave oscillates with a single period and uniform rostrocaudal lag, exhibiting network entrainment. Understanding of the integrative role of neurotransmission and intercellular coupling in the propagation of an entrained gastric slow-wave, important for understanding motility disorders, however, remains incomplete. Using a computational framework constituted of a novel gastric motility network (GMN) model we address the hypothesis that engaging biological oscillators (i.e., ICCs) by constitutive gap junction coupling mechanisms and enteric neural innervation activated signals can confer a robust entrained gastric slow-wave. We demonstrate that while a decreasing enteric neural innervation gradient that modulates the intracellular IP₃ concentration in the ICCs can guide the aboral slow-wave propagation essential for peristalsis, engaging ICCs by recruiting the exchange of second messengers (inositol trisphosphate (IP₃) and Ca²⁺) ensures a robust entrained longitudinal slow-wave, even in the presence of biological variability in electrical coupling strengths. Our GMN with the distinct intercellular coupling in conjunction with the intracellular feedback pathways and a rostrocaudal enteric neural innervation gradient allows gastric slow waves to oscillate with a moderate range of frequencies and to propagate with a broad range of velocities, thus preventing decoupling observed in motility disorders. Overall, the findings provide a mechanistic explanation for the emergence of decoupled slow waves associated with motility impairments of the stomach, offer directions for future experiments and theoretical work, and can potentially aid in the design of new interventional pharmacological and neuromodulation device treatments for addressing gastric motility disorders.

The coordinated contraction and relaxation of the muscles of the stomach, known as peristalsis is important for normal gastric motility and primarily governed by electrical depolarizations that originate and propagate within a network of interconnected layers of interstitial cells of Cajal (ICCs) and smooth muscle cells of the stomach wall as a slow-wave. Under normal conditions, a gastric slow-wave oscillates with a single period and uniform rostrocaudal lag, exhibiting network entrainment. However, the understanding of intrinsic and extrinsic mechanisms that ensure propagation of a robust entrained slow-wave remains incomplete. Here, using a computational framework, we show that in conjunction with an enteric neural innervation gradient along the rostrocaudal ICC chain, and intercellular electrical coupling, the intercellular exchange of inositol trisphosphate between ICCs prevents decoupling by extending the longitudinal entrainment range along the stomach wall, even when variability in intercellular coupling exists. The findings from our study indicate ways that ensure the rostrocaudal spread of a robust gastric slow-wave and provide a mechanistic explanation for the emergence of decoupled slow waves associated with motility impairments of the stomach.

Ca2+ transients in ICC-MY define the basis for the dominance of the corpus in gastric pacemaking

Myenteric interstitial cells of Cajal (ICC-MY) generate and actively propagate electrical slow waves in the stomach. Slow wave generation and propagation are altered in gastric motor disorders, such as gastroparesis, and the mechanism for the gradient in slow wave frequency that facilitates proximal to distal propagation of slow waves and normal gastric peristalsis is poorly understood.  Slow waves depend upon Ca2+-activated Cl- channels (encoded by Ano1). We characterized Ca2+ signaling in ICC-MY in situ using mice engineered to have cell-specific expression of GCaMP6f in ICC. Ca2+ signaling differed in ICC-MY in corpus and antrum. Localized Ca2+ transients were generated from multiple firing sites and were organized into Ca2+ transient clusters (CTCs). Ca2+ transient refractory periods occurred upon cessation of CTCs, but a relatively higher frequency of Ca2+ transients persisted during the inter-CTC interval in corpus than in antrum ICC-MY. The onset of Ca2+ transients after the refractory period was associated with initiation of the next CTC. Thus, CTCs were initiated at higher frequencies in corpus than in antrum ICC-MY. Initiation and propagation of CTCs (and electrical slow waves) depends upon T-type Ca2+ channels, and durations of CTCs relied upon L-type Ca2+ channels. The durations of CTCs mirrored the durations of slow waves. CTCs and Ca2+ transients between CTCs resulted from release of Ca2+ from intracellular stores and were maintained, in part, by store-operated Ca2+ entry. Our data suggest that Ca2+ release and activation of Ano1 channels both initiate and contribute to the plateau phase of slow waves.

Nitric Oxide Is Essential for Generating the Minute Rhythm Contraction Pattern in the Small Intestine, Likely via ICC-DMP

Nitrergic nerves have been proposed to play a critical role in the orchestration of peristaltic activities throughout the gastrointestinal tract. In the present study, we investigated the role of nitric oxide, using spatiotemporal mapping, in peristaltic activity of the whole ex vivo mouse intestine. We identified a propulsive motor pattern in the form of propagating myogenic contractions, that are clustered by the enteric nervous system into a minute rhythm that is dependent on nitric oxide. The cluster formation was abolished by TTX, lidocaine and nitric oxide synthesis inhibition, whereas the myogenic contractions, occurring at the ICC-MP initiated slow wave frequency, remained undisturbed. Cluster formation, inhibited by block of nitric oxide synthesis, was fully restored in a highly regular rhythmic fashion by a constant level of nitric oxide generated by sodium nitroprusside; but the action of sodium nitroprusside was inhibited by lidocaine indicating that it was relying on neural activity, but not rhythmic nitrergic nerve activity. Hence, distention-induced activity of cholinergic nerves and/or a co-factor within nitrergic nerves such as ATP is also a requirement for the minute rhythm. Cluster formation was dependent on distention but was not evoked by a distention reflex. Block of gap junction conductance by carbenoxolone, dose dependently inhibited, and eventually abolished clusters and contraction waves, likely associated, not with inhibition of nitrergic innervation, but by abolishing ICC network synchronization. An intriguing feature of the clusters was the presence of bands of rhythmic inhibitions at 4-8 cycles/min; these inhibitory patches occurred in the presence of tetrodotoxin or lidocaine and hence were not dependent on nitrergic nerves. We propose that the minute rhythm is generated by nitric oxide-induced rhythmic depolarization of the musculature via ICC-DMP.

Ca2+ signaling driving pacemaker activity in submucosal interstitial cells of Cajal in the murine colon

Interstitial cells of Cajal (ICC) generate pacemaker activity responsible for phasic contractions in colonic segmentation and peristalsis. ICC along the submucosal border (ICC-SM) contribute to mixing and more complex patterns of colonic motility. We show the complex patterns of Ca²⁺ signaling in ICC-SM and the relationship between ICC-SM Ca²⁺ transients and activation of smooth muscle cells (SMCs) using optogenetic tools. ICC-SM displayed rhythmic firing of Ca²⁺transients ~ 15 cpm and paced adjacent SMCs. The majority of spontaneous activity occurred in regular Ca²⁺ transients clusters (CTCs) that propagated through the network. CTCs were organized and dependent upon Ca²⁺ entry through voltage-dependent Ca²⁺ conductances, L- and T-type Ca²⁺ channels. Removal of Ca²⁺ from the external solution abolished CTCs. Ca²⁺ release mechanisms reduced the duration and amplitude of Ca²⁺ transients but did not block CTCs. These data reveal how colonic pacemaker ICC-SM exhibit complex Ca^2+-firing patterns and drive smooth muscle activity and overall colonic contractions.

A high throughput machine-learning driven analysis of Ca2+ spatio-temporal maps

High-resolution Ca2+ imaging to study cellular Ca2+ behaviors has led to the creation of large datasets with a profound need for standardized and accurate analysis. To analyze these datasets, spatio-temporal maps (STMaps) that allow for 2D visualization of Ca2+ signals as a function of time and space are often used. Methods of STMap analysis rely on a highly arduous process of user defined segmentation and event-based data retrieval. These methods are often time consuming, lack accuracy, and are extremely variable between users. We designed a novel automated machine-learning based plugin for the analysis of Ca2+ STMaps (STMapAuto). The plugin includes optimized tools for Ca2+ signal preprocessing, automated segmentation, and automated extraction of key Ca2+ event information such as duration, spatial spread, frequency, propagation angle, and intensity in a variety of cell types including the Interstitial cells of Cajal (ICC). The plugin is fully implemented in Fiji and able to accurately detect and expeditiously quantify Ca2+ transient parameters from ICC. The plugin's speed of analysis of large-datasets was 197-fold faster than the commonly used single pixel-line method of analysis. The automated machine-learning based plugin described dramatically reduces opportunities for user error and provides a consistent method to allow high-throughput analysis of STMap datasets.

Shifting into high gear: how interstitial cells of Cajal change the motility pattern of the developing intestine

The first contractile waves in the developing embryonic gut are purely myogenic; they only involve smooth muscle. Here, we provide evidence for a transition from smooth muscle to interstitial cell of Cajal (ICC)-driven contractile waves in the developing chicken gut. In situ hybridization staining for anoctamin-1 (ANO1), a known ICC marker, shows that ICCs are already present throughout the gut, as from embryonic day (E)7. We devised a protocol to reveal ICC oscillatory and propagative calcium activity in embryonic gut whole mount and found that the first steady calcium oscillations in ICCs occur on (E14). We show that the activation of ICCs leads to an increase in contractile wave frequency, regularity, directionality, and velocity between E12 and E14. We finally demonstrate that application of the c-KIT antagonist imatinib mesylate in organ culture specifically depletes the ICC network and inhibits the transition to a regular rhythmic wave pattern. We compare our findings to existing results in the mouse and predict that a similar transition should take place in the human fetus between 12 and 14 wk of development. Together, our results point to an abrupt physiological transition from smooth muscle mesenchyme self-initiating waves to ICC-driven motility in the fetus and clarify the contribution of ICCs to the contractile wave pattern.NEW & NOTEWORTHY We reveal a sharp transition from smooth muscle to interstitial cell of Cajal (ICC)-driven motility in the chicken embryo, leading to higher-frequency, more rhythmic contractile waves. We predict the transition to happen between 12 and 14 embryonic wk in humans. We image for the first time the onset of ICC activity in an embryonic gut by calcium imaging. We show the first KIT and anoctamin-1 (ANO1) in situ hybridization micrographs in the embryonic chicken gut.

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.

Pacemaker function and neural responsiveness of subserosal interstitial cells of Cajal in the mouse colon

KEY POINTS

Rhythmic action potentials and intercellular Ca²⁺ waves are generated in smooth muscle cells of colonic longitudinal muscles (LSMC). Longitudinal muscle excitability is tuned by input from subserosal ICC (ICC-SS), a population of ICC with previously unknown function. ICC-SS express Ano1 channels and generate spontaneous Ca²⁺ transients in a stochastic manner. Release of Ca²⁺ and activation of Ano1 channels causes depolarization of ICC-SS and LSMC, leading to activation of L-type Ca²⁺ channels, action potentials, intercellular Ca²⁺ waves and contractions in LSMC. Nitrergic neural inputs regulate the Ca²⁺ events in ICC-SS. Pacemaker activity in longitudinal muscle is an emergent property as a result of integrated processes in ICC-SS and LSMC.

ABSTRACT

Much is known about myogenic mechanisms in circular muscle (CM) in the gastrointestinal tract, although less is known about longitudinal muscle (LM). Two Ca²⁺ signalling behaviours occur in LM: localized intracellular waves not causing contractions and intercellular waves leading to excitation-contraction coupling. An Ano1 channel antagonist inhibited intercellular Ca²⁺ waves and LM contractions. Ano1 channels are expressed by interstitial cells of Cajal (ICC) but not by smooth muscle cells (SMCs). We investigated Ca²⁺ signalling in a novel population of ICC that lies along the subserosal surface of LM (ICC-SS) in mice expressing GCaMP6f in ICC. ICC-SS fired stochastic localized Ca²⁺ transients. Such events have been linked to activation of Ano1 channels in ICC. Ca²⁺ transients in ICC-SS occurred by release from stores most probably via inositol trisphosphate receptors. This activity relied on influx via store-operated Ca²⁺ entry and Orai channels. No voltage-dependent mechanism that synchronized Ca²⁺ transients in a single cell or between cells was found. Nitrergic agonists inhibited Ca²⁺ transients in ICC-SS, and stimulation of intrinsic nerves activated nitrergic responses in ICC-SS. Cessation of stimulation resulted in significant enhancement of Ca²⁺ transients compared to the pre-stimulus activity. No evidence of innervation by excitatory, cholinergic motor neurons was found. Our data suggest that ICC-SS contribute to regulation of LM motor activity. Spontaneous Ca²⁺ transients activate Ano1 channels in ICC-SS. Resulting depolarization conducts to SMCs, depolarizing membrane potential, activating L-type Ca²⁺ channels and initiating contraction. Rhythmic electrical and mechanical behaviours of LM are an emergent property of SMCs and ICC-SS.

Age-associated changes of the intrinsic nervous system in relation with interstitial cells in the pre-weaning goat rumen

In this study, we investigated the neural changes and their relationships with interstitial cells (ICs) in the rumen of pre-weaning goats by transmission electron microscopy, western blot and immunofluorescence (antibody: general neuronal marker-Protein Gene Product (PGP9.5)/ IC marker-vimentin). The immunofluorescence results showed that PGP9.5-positive reaction was widely distributed in neuronal soma (NS) and nerve fibre (NF). The NSs were observed in the ganglia of the myenteric plexus (MP) but not in the submucosal plexus. The mean optical density (MOD) of the whole of PGP9.5-positive nerves and the protein expression level of PGP.5 in the rumen wall both decreased significantly with age. However an obvious increase MOD of PGP.5-positive NFs within the rumen epithelium were observed. In the MP, the nerves and ICs were interwoven to form two complex networks that gradually tightened with age. Furthermore, NSs and nerve trunks were surrounded by a ring-boundary layer consisting of several ICs that became physically closer with aging. Moreover, ICs were located nearby NFs within the ML, forming connections between ICs, smooth muscle cells and axons. This study describes the pattern of neural distribution and its association with ICs in the developing rumen which shed light on the postpartum development of ruminants.

Ca2+ signalling behaviours of intramuscular interstitial cells of Cajal in the murine colon

KEY POINTS

Colonic intramuscular interstitial cells of Cajal (ICC-IM) exhibit spontaneous Ca²⁺ transients manifesting as stochastic events from multiple firing sites with propagating Ca²⁺ waves occasionally observed. Firing of Ca²⁺ transients in ICC-IM is not coordinated with adjacent ICC-IM in a field of view or even with events from other firing sites within a single cell. Ca²⁺ transients, through activation of Ano1 channels and generation of inward current, cause net depolarization of colonic muscles. Ca²⁺ transients in ICC-IM rely on Ca²⁺ release from the endoplasmic reticulum via IP₃ receptors, spatial amplification from RyRs and ongoing refilling of ER via the sarcoplasmic/endoplasmic-reticulum-Ca²⁺ -ATPase. ICC-IM are sustained by voltage-independent Ca²⁺ influx via store-operated Ca²⁺ entry. Some of the properties of Ca²⁺ in ICC-IM in the colon are similar to the behaviour of ICC located in the deep muscular plexus region of the small intestine, suggesting there are functional similarities between these classes of ICC.

ABSTRACT

A component of the SIP syncytium that regulates smooth muscle excitability in the colon is the intramuscular class of interstitial cells of Cajal (ICC-IM). All classes of ICC (including ICC-IM) express Ca²⁺ -activated Cl^- channels, encoded by Ano1, and rely upon this conductance for physiological functions. Thus, Ca²⁺ handling in ICC is fundamental to colonic motility. We examined Ca²⁺ handling mechanisms in ICC-IM of murine proximal colon expressing GCaMP6f in ICC. Several Ca²⁺ firing sites were detected in each cell. While individual sites displayed rhythmic Ca²⁺ events, the overall pattern of Ca²⁺ transients was stochastic. No correlation was found between discrete Ca²⁺ firing sites in the same cell or in adjacent cells. Ca²⁺ transients in some cells initiated Ca²⁺ waves that spread along the cell at ∼100 µm s^-1 . Ca²⁺ transients were caused by release from intracellular stores, but depended strongly on store-operated Ca²⁺ entry mechanisms. ICC Ca²⁺ transient firing regulated the resting membrane potential of colonic tissues as a specific Ano1 antagonist hyperpolarized colonic muscles by ∼10 mV. Ca²⁺ transient firing was independent of membrane potential and not affected by blockade of L- or T-type Ca²⁺ channels. Mechanisms regulating Ca²⁺ transients in the proximal colon displayed both similarities to and differences from the intramuscular type of ICC in the small intestine. Similarities and differences in Ca²⁺ release patterns might determine how ICC respond to neurotransmission in these two regions of the gastrointestinal tract.

Pharmacological modulation of the spatiotemporal disposition of micromotions in the intact resting urinary bladder of the rabbit; their pattern is under both myogenic and autonomic control

OBJECTIVES

To explore and characterize the disposition and dynamics of micromotions in the wall of the intact resting teradotoxinized urinary bladder of the rabbit before and after the administration of adrenergic and cholinergic pharmaceutical agents.

METHODS

Spatiotemporal maps and related intravesical pressure were used to analyse propagating patches of contractions (PPCs) and their component individual myogenic contractions [propagating individual contractions (PICs)] in the wall of the tetradotoxinized urinary bladder.

RESULTS

The bladder wall exhibited two contractile states that were of similar frequencies to those of the two types of electrophysiological discharge described in previous studies; the first, in which cyclic PPCs predominated, the second in which small irregular PICs predominated. The addition of carbachol increased the size, frequency, speed and distance of propagation of PPCs, whereas the addition of isoprenaline temporarily halted the incorporation of PICs into PPCs, and reduced patch size and total area undergoing contraction. The RhoA kinase (ROCK) inhibitor Y-27632 reduced both largest patch index and mean patch size. Both carbenoxolone and ROCK inhibition decreased the duration of PPCs. Carbenoxolone also prolonged duration and accelerated PPC propagation velocity. The authors postulate that these differences arise from differing effects of these agents on myocytes and interstitial cells within the stress environment of the bladder, influencing the development, coordination and propagation of PPCs.

CONCLUSIONS

The timings and structure of spontaneous micromotions in the wall of the isolated bladder change when it is treated with sympathetic/parasympathetic agonists and with myogenically active agents. Correspondingly, disorders of bladder wall contraction may result from disorders of either neurogenic or myogenic signalling and may be amenable to treatment with combinations of agents that influence both.

Spontaneous Electrical Activity and Rhythmicity in Gastrointestinal Smooth Muscles

The gastrointestinal (GI) tract has multifold tasks of ingesting, processing, and assimilating nutrients and disposing of wastes at appropriate times. These tasks are facilitated by several stereotypical motor patterns that build upon the intrinsic rhythmicity of the smooth muscles that generate phasic contractions in many regions of the gut. Phasic contractions result from a cyclical depolarization/repolarization cycle, known as electrical slow waves, which result from intrinsic pacemaker activity. Interstitial cells of Cajal (ICC) are electrically coupled to smooth muscle cells (SMCs) and generate and propagate pacemaker activity and slow waves. The mechanism of slow waves is dependent upon specialized conductances expressed by pacemaker ICC. The primary conductances responsible for slow waves in mice are Ano1, Ca2+-activated Cl- channels (CaCCs), and CaV3.2, T-type, voltage-dependent Ca2+ channels. Release of Ca2+ from intracellular stores in ICC appears to be the initiator of pacemaker depolarizations, activation of T-type current provides voltage-dependent Ca2+ entry into ICC, as slow waves propagate through ICC networks, and Ca2+-induced Ca2+ release and activation of Ano1 in ICC amplifies slow wave depolarizations. Slow waves conduct to coupled SMCs, and depolarization elicited by these events enhances the open-probability of L-type voltage-dependent Ca2+ channels, promotes Ca2+ entry, and initiates contraction. Phasic contractions timed by the occurrence of slow waves provide the basis for motility patterns such as gastric peristalsis and segmentation. This chapter discusses the properties of ICC and proposed mechanism of electrical rhythmicity in GI muscles.

Quantifying Patterns of Smooth Muscle Motility in the Gut and Other Organs With New Techniques of Video Spatiotemporal Mapping

The uses and limitations of the various techniques of video spatiotemporal mapping based on change in diameter (D-type ST maps), change in longitudinal strain rate (L-type ST maps), change in area strain rate (A-type ST maps), and change in luminous intensity of reflected light (I-maps) are described, along with their use in quantifying motility of the wall of hollow structures of smooth muscle such as the gut. Hence ST-methods for determining the size, speed of propagation and frequency of contraction in the wall of gut compartments of differing geometric configurations are discussed. We also discuss the shortcomings and problems that are inherent in the various methods and the use of techniques to avoid or minimize them. This discussion includes, the inability of D-type ST maps to indicate the site of a contraction that does not reduce the diameter of a gut segment, the manipulation of axis [the line of interest (LOI)] of L-maps to determine the true axis of propagation of a contraction, problems with anterior curvature of gut segments and the use of adjunct image analysis techniques that enhance particular features of the maps.

Hwangryunhaedok-tang induces the depolarization of pacemaker potentials through 5-HT3 and 5-HT4 receptors in cultured murine small intestine interstitial cells of Cajal

AIM

To investigate the effects of a water extract of Hwangryunhaedok-tang (HHTE) on the pacemaker potentials of mouse interstitial cells of Cajal (ICCs).

METHODS

We dissociated ICCs from small intestines and cultured. ICCs were immunologically identified using an anti-c-kit antibody. We used the whole-cell patch-clamp configuration to record the pacemaker potentials generated by cultured ICCs under the current clamp mode (I = 0). All experiments were performed at 30 °C-32 °C

RESULTS

HHTE dose-dependently depolarized ICC pacemaker potentials. Pretreatment with a 5-HT₃ receptor antagonist (Y25130) or a 5-HT₄ receptor antagonist (RS39604) blocked HHTE-induced pacemaker potential depolarizations, whereas pretreatment with a 5-HT₇ receptor antagonist (SB269970) did not. Intracellular GDPβS inhibited HHTE-induced pacemaker potential depolarization and pretreatment with a Ca²⁺-free solution or thapsigargin abolished the pacemaker potentials. In the presence of a Ca²⁺-free solution or thapsigargin, HHTE did not depolarize ICC pacemaker potentials. In addition, HHTE-induced pacemaker potential depolarization was unaffected by a PKC inhibitor (calphostin C) or a Rho kinase inhibitor (Y27632). Of the four ingredients of HHT, Coptidis Rhizoma and Gardeniae Fructus more effectively inhibited pacemaker potential depolarization.

CONCLUSION

These results suggest that HHTE dose-dependently depolarizes ICC pacemaker potentials through 5-HT₃ and 5-HT₄ receptors via external and internal Ca²⁺ regulation and via G protein-, PKC- and Rho kinase-independent pathways.

Clustering of Ca2+ transients in interstitial cells of Cajal defines slow wave duration

Electrical slow waves in the small intestine are generated by pacemaker cells called interstitial cells of Cajal. Drumm et al. record clusters of Ca²⁺ transients in these cells that are entrained by voltage-dependent Ca²⁺ entry and which define the duration of the electrical slow waves.

Interstitial cells of Cajal (ICC) in the myenteric plexus region (ICC-MY) of the small intestine are pacemakers that generate rhythmic depolarizations known as slow waves. Slow waves depend on activation of Ca²⁺-activated Cl⁻ channels (ANO1) in ICC, propagate actively within networks of ICC-MY, and conduct to smooth muscle cells where they generate action potentials and phasic contractions. Thus, mechanisms of Ca²⁺ regulation in ICC are fundamental to the motor patterns of the bowel. Here, we characterize the nature of Ca²⁺ transients in ICC-MY within intact muscles, using mice expressing a genetically encoded Ca²⁺ sensor, GCaMP3, in ICC. Ca²⁺ transients in ICC-MY display a complex firing pattern caused by localized Ca²⁺ release events arising from multiple sites in cell somata and processes. Ca²⁺ transients are clustered within the time course of slow waves but fire asynchronously during these clusters. The durations of Ca²⁺ transient clusters (CTCs) correspond to slow wave durations (plateau phase). Simultaneous imaging and intracellular electrical recordings revealed that the upstroke depolarization of slow waves precedes clusters of Ca²⁺ transients. Summation of CTCs results in relatively uniform Ca²⁺ responses from one slow wave to another. These Ca²⁺ transients are caused by Ca²⁺ release from intracellular stores and depend on ryanodine receptors as well as amplification from IP₃ receptors. Reduced extracellular Ca²⁺ concentrations and T-type Ca²⁺ channel blockers decreased the number of firing sites and firing probability of Ca²⁺ transients. In summary, the fundamental electrical events of small intestinal muscles generated by ICC-MY depend on asynchronous firing of Ca²⁺ transients from multiple intracellular release sites. These events are organized into clusters by Ca²⁺ influx through T-type Ca²⁺ channels to sustain activation of ANO1 channels and generate the plateau phase of slow waves.

Regulation of Gastrointestinal Smooth Muscle Function by Interstitial Cells

Interstitial cells of mesenchymal origin form gap junctions with smooth muscle cells in visceral smooth muscles and provide important regulatory functions. In gastrointestinal (GI) muscles, there are two distinct classes of interstitial cells, c-Kit(+) interstitial cells of Cajal and PDGFRα(+) cells, that regulate motility patterns. Loss of these cells may contribute to symptoms in GI motility disorders.

Conditional genetic deletion of Ano1 in interstitial cells of Cajal impairs Ca2+ transients and slow waves in adult mouse small intestine

Myenteric plexus interstitial cells of Cajal (ICC-MY) in the small intestine are Kit+ electrical pacemakers that express the Ano1/TMEM16A Ca2+-activated Cl- channel, whose functions in the gastrointestinal tract remain incompletely understood. In this study, an inducible Cre-LoxP-based approach was used to advance the understanding of Ano1 in ICC-MY of adult mouse small intestine. KitCreERT2/+;Ano1Fl/Fl mice were treated with tamoxifen or vehicle, and small intestines (mucosa free) were examined. Quantitative RT-PCR demonstrated ~50% reduction in Ano1 mRNA in intestines of conditional knockouts (cKOs) compared with vehicle-treated controls. Whole mount immunohistochemistry showed a mosaic/patchy pattern loss of Ano1 protein in ICC networks. Ca2+ transients in ICC-MY network of cKOs displayed reduced duration compared with highly synchronized controls and showed synchronized and desynchronized profiles. When matched, the rank order for Ano1 expression in Ca2+ signal imaged fields of view was as follows: vehicle controls>>>cKO(synchronized)>cKO(desynchronized). Maintenance of Ca2+ transients' synchronicity despite high loss of Ano1 indicates a large functional reserve of Ano1 in the ICC-MY network. Slow waves in cKOs displayed reduced duration and increased inter-slow-wave interval and occurred in regular- and irregular-amplitude oscillating patterns. The latter activity suggested ongoing interaction by independent interacting oscillators. Lack of slow waves and depolarization, previously reported for neonatal constitutive knockouts, were also seen. In summary, Ano1 in adults regulates gastrointestinal function by determining Ca2+ transients and electrical activity depending on the level of Ano1 expression. Partial Ano1 loss results in Ca2+ transients and slow waves displaying reduced duration, while complete and widespread absence of Ano1 in ICC-MY causes lack of slow wave and desynchronized Ca2+ transients.NEW & NOTEWORTHY The Ca2+-activated Cl- channel, Ano1, in interstitial cells of Cajal (ICC) is necessary for normal gastrointestinal motility. We knocked out Ano1 to varying degrees in ICC of adult mice. Partial knockout of Ano1 shortened the widths of electrical slow waves and Ca2+ transients in myenteric ICC but Ca2+ transient synchronicity was preserved. Near-complete knockout was necessary for transient desynchronization and loss of slow waves, indicating a large functional reserve of Ano1 in ICC.

Calcium Signaling in Interstitial Cells: Focus on Telocytes

In this review, we describe the current knowledge on calcium signaling pathways in interstitial cells with a special focus on interstitial cells of Cajal (ICCs), interstitial Cajal-like cells (ICLCs), and telocytes. In detail, we present the generation of Ca²⁺ oscillations, the inositol triphosphate (IP₃)/Ca²⁺ signaling pathway and modulation exerted by cytokines and vasoactive agents on calcium signaling in interstitial cells. We discuss the physiology and alterations of calcium signaling in interstitial cells, and in particular in telocytes. We describe the physiological contribution of calcium signaling in interstitial cells to the pacemaking activity (e.g., intestinal, urinary, uterine or vascular pacemaking activity) and to the reproductive function. We also present the pathological contribution of calcium signaling in interstitial cells to the aortic valve calcification or intestinal inflammation. Moreover, we summarize the current knowledge of the role played by calcium signaling in telocytes in the uterine, cardiac and urinary physiology, and also in various pathologies, including immune response, uterine and cardiac pathologies.

Interstitial cell modulation of pyeloureteric peristalsis in the mouse renal pelvis examined using FIBSEM tomography and calcium indicators

Typical and atypical smooth muscle cells (TSMCs and ASMCs, respectively) and interstitial cells (ICs) within the pacemaker region of the mouse renal pelvis were examined using focused ion beam scanning electron (FIB SEM) tomography, immunohistochemistry and Ca²⁺ imaging. Individual cells within 500-900 electron micrograph stacks were volume rendered and associations with their neighbours established. 'Ribbon-shaped', Ano1 Cl^- channel immuno-reactive ICs were present in the adventitia and the sub-urothelial space adjacent to the TSMC layer. ICs in the proximal renal pelvis were immuno-reactive to antibodies for Ca_V3.1 and hyperpolarization-activated cation nucleotide-gated isoform 3 (HCN3) channel sub-units, while basal-epithelial cells (BECs) were intensely immuno-reactive to Kv7.5 channel antibodies. Adventitial to the TSMC layer, ASMCs formed close appositions with TSMCs and ICs. The T-type Ca²⁺channel blocker, Ni²⁺ (10-200 μM), reduced the frequency while the L-type Ca²⁺ channel blocker (1 μM nifedipine) reduced the amplitude of propagating Ca²⁺ waves and contractions in the TSMC layer. Upon complete suppression of Ca²⁺ entry through TSMC Ca²⁺ channels, ASMCs displayed high-frequency (6 min^-1) Ca²⁺ transients, and ICs distributed into two populations of cells firing at 1 and 3 min^-1, respectively. IC Ca²⁺ transients periodically (every 3-5 min^-1) summed into bursts which doubled the frequency of ASMC Ca²⁺ transient firing. Synchronized IC bursting and the acceleration of ASMC firing were inhibited upon blockade of HCN channels with ZD7288 or cell-to-cell coupling with carbenoxolone. While ASMCs appear to be the primary pacemaker driving pyeloureteric peristalsis, it was concluded that sub-urothelial HCN3(+), Ca_V3.1(+) ICs can accelerate ASMC Ca²⁺ signalling.

Problems with extracellular recording of electrical activity in gastrointestinal muscle

Motility patterns of the gastrointestinal tract are important for efficient processing of nutrients and waste. Peristalsis and segmentation are based on rhythmic electrical slow waves that generate the phasic contractions fundamental to gastrointestinal motility. Slow waves are generated and propagated actively by interstitial cells of Cajal (ICC), and these events conduct to smooth muscle cells to elicit excitation-contraction coupling. Extracellular electrical recording has been utilized to characterize slow-wave generation and propagation and abnormalities that might be responsible for gastrointestinal motility disorders. Electrode array recording and digital processing are being used to generate data for models of electrical propagation in normal and pathophysiological conditions. Here, we discuss techniques of extracellular recording as applied to gastrointestinal organs and how mechanical artefacts might contaminate these recordings and confound their interpretation. Without rigorous controls for movement, current interpretations of extracellular recordings might ascribe inaccurate behaviours and electrical anomalies to ICC networks and gastrointestinal muscles, bringing into question the findings and validity of models of gastrointestinal electrophysiology developed from these recordings.

Nitric oxide-induced oxidative stress impairs pacemaker function of murine interstitial cells of Cajal during inflammation

The pacemaker function of interstitial cells of Cajal (ICC) is impaired during intestinal inflammation. The aim of this study is to clarify the pathophysiological mechanisms of ICC dysfunction during inflammatory condition by using intestinal cell clusters. Cell clusters were prepared from smooth muscle layer of murine jejunum and treated with interferon-gamma and lipopolysaccharide (IFN-γ+LPS) for 24h to induce inflammation. Pacemaker function of ICC was monitored by measuring cytosolic Ca(2+) oscillation in the presence of nifedipine. Treatment with IFN-γ+LPS impaired the pacemaker activity of ICC with increasing mRNA level of interleukin-1 beta, tumor necrosis factor-alpha and interleukin-6 in cell clusters; however, treatment with these cytokines individually had little effect on pacemaker activity of ICC. Treatment with IFN-γ+LPS also induced the expression of inducible nitric oxide synthase (iNOS) in smooth muscle cells and resident macrophages, but not in ICC. Pretreatment with NOS inhibitor, L-NAME or iNOS inhibitor, 1400W ameliorated IFN-γ+LPS-induced pacemaker dysfunction of ICC. Pretreatment with guanylate cyclase inhibitor, ODQ did not, but antioxidant, apocynin, to suppress NO-induced oxidative stress, significantly suppressed the impairment of ICC function induced by IFN-γ+LPS. Treatment with IFN-γ+LPS also decreased c-Kit-positive ICC, which was prevented by pretreatment with L-NAME. However, apoptotic ICC were not detected in IFN-γ+LPS-treated clusters, suggesting IFN-γ+LPS stimulation just changed the phenotype of ICC but not induced cell death. Moreover, ultrastructure of ICC was not disturbed by IFN-γ+LPS. In conclusion, ICC dysfunction during inflammation is induced by NO-induced oxidative stress rather than NO/cGMP signaling. NO-induced oxidative stress might be the main factor to induce phenotypic changes of ICC.

Stimulus-induced pacemaker activity in interstitial cells of Cajal associated with the deep muscular plexus of the small intestine

BACKGROUND

The ICC-DMP have been proposed to generate stimulus-dependent pacemaker activity, rhythmic transient depolarizations, that take part in orchestrating segmentation and clustered propulsive motor patterns in the small intestine. However, little is known about the fundamental properties of ICC-DMP.

METHODS

This study was undertaken to increase our understanding of intrinsic properties of the ICC-DMP through calcium imaging and intracellular electrical recordings.

KEY RESULTS

Without stimulation, most ICC-DMP were quiescent. In some preparations ICC-DMP generated rhythmic low-frequency calcium oscillations (<10 cpm) with or without high frequency activity superimposed (>35 cpm). Immunohistochemistry proved the existence of NK1R on the ICC-DMP and close contacts between ICC-DMP and substance P-positive nerves. Substance P (25 nM) induced low-frequency calcium oscillations that were synchronized across the ICC-DMP network. Substance P also induced low frequency rhythmic transient depolarizations (<10cpm) in circular muscle cells close to the ICC-DMP. An intracellular recording from a positively identified ICC-DMP showed rhythmic transient depolarizations with superimposed high frequency activity. To investigate if quiescent ICC-DMP were chronically inhibited by nitrergic activity, nNOS was inhibited, but without effect.

CONCLUSIONS & INFERENCES

Substance P changes non-synchronized high frequency flickering or quiescence in ICC-DMP into strong rhythmic calcium transients that are synchronized within the network; they are associated with rhythmic transient depolarizations within the same frequency range. We hypothesize that Substance P, released from nerves, can evoke rhythmicity in ICC-DMP, thereby providing it with potential pacemaker activity.

Effects of the roots of Liriope Platyphylla Wang Et tang on gastrointestinal motility function

ETHNOPHARMACOLOGICAL RELEVANCE

Liriope platyphylla Wang et Tang continues to be used in Korea as a traditional medicine for the treatment of gastrointestinal (GI) disorders related to constipation and abnormal GI motility.

AIM OF THE STUDY

Because GI disorders, especially GI motility dysfunctions, are major lifelong problems, the authors investigated the effects of a water extract of the roots of L. platyphylla Wang et Tang (LPE) on the pacemaker potentials (PPTs) of interstitial cells of Cajal (ICCs) and on GI motility in male ICR mice.

MATERIALS AND METHODS

Enzymatic digestions were used to dissociate ICCs from small intestines, and the whole-cell patch-clamp configuration was used to record PPTs generated by cultured ICCs in vitro. In vivo effects of LPE on GI motility were investigated by measuring intestinal transit rates (ITRs) of Evans blue in normal mice and in acetic acid (AA) and streptozotocin (STZ)-induced diabetic mouse models of GI motility dysfunction.

RESULTS

LPE dose-dependently depolarized PPTs in ICCs. Pretreatment with methoctramine (a muscarinic M2 receptor antagonist) did not block LPE-induced PPT depolarization. However, pretreatment with 4-DAMP (a muscarinic M3 receptor antagonist) blocked LPE-induced PPT depolarization. In addition, treatment with LY294002 (a phosphoinositide 3-kinase (PI3K) inhibitor) also blocked LPE-induced PPT depolarization. Intracellular GDPβS inhibited LPE-induced PPT depolarization, and LPE-induced PPT depolarization was found to occur in a phospholipase C (PLC)- and a protein kinase C (PKC)-dependent manner. Pretreatment with Ca(2+)free solution or thapsigargin (a Ca(2+)-ATPase inhibitor in endoplasmic reticulum) abolished PPTs, and under these conditions, LPE did not depolarize ICC PPTs. In normal mice, ITRs were significantly and dose-dependently increased by LPE (0.01-1g/kg administered intragastrically (i.g.)). In addition, LPE (i.g.) significantly recovered GI motility dysfunctions in both animal models.

CONCLUSION

LPE dose-dependently depolarizes ICC PPTs through M3 receptors via external and internal Ca(2+)regulation and via G protein-, PI3K-, PLC- and PKC- dependent pathways in vitro. Also, in vivo, LPE increased ITRs in treatment naïve mice and our two mouse models of GI dysfunction. Therefore, this study shows that LPE offers a basis for the development of a prokinetic agent that prevents or alleviates GI motility dysfunctions.

Spontaneous Ca(2+) transients in interstitial cells of Cajal located within the deep muscular plexus of the murine small intestine

KEY POINTS

Interstitial cells of Cajal at the level of the deep muscular plexus (ICC-DMP) in the small intestine generate spontaneous Ca(2+) transients that consist of localized Ca(2+) events and limited propagating Ca(2+) waves. Ca(2+) transients in ICC-DMP display variable characteristics: from discrete, highly localized Ca(2+) transients to regionalized Ca(2+) waves with variable rates of occurrence, amplitude, duration and spatial spread. Ca(2+) transients fired stochastically, with no cellular or multicellular rhythmic activity being observed. No correlation was found between the firing sites in adjacent cells. Ca(2+) transients in ICC-DMP are suppressed by the ongoing release of inhibitory neurotransmitter(s). Functional intracellular Ca(2+) stores are essential for spontaneous Ca(2+) transients, and the sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump is necessary for maintenance of spontaneity. Ca(2+) release mechanisms involve both ryanodine receptors (RyRs) and inositol triphosphate receptors (InsP3 Rs). Release from these channels is interdependent. ICC express transcripts of multiple RyRs and InsP3 Rs, with Itpr1 and Ryr2 subtypes displaying the highest expression.

ABSTRACT

Interstitial cells of Cajal in the deep muscular plexus of the small intestine (ICC-DMP) are closely associated with varicosities of enteric motor neurons and generate responses contributing to neural regulation of intestinal motility. Responses of ICC-DMP are mediated by activation of Ca(2+) -activated Cl(-) channels; thus, Ca(2+) signalling is central to the behaviours of these cells. Confocal imaging was used to characterize the nature and mechanisms of Ca(2+) transients in ICC-DMP within intact jejunal muscles expressing a genetically encoded Ca(2+) indicator (GCaMP3) selectively in ICC. ICC-DMP displayed spontaneous Ca(2+) transients that ranged from discrete, localized events to waves that propagated over variable distances. The occurrence of Ca(2+) transients was highly variable, and it was determined that firing was stochastic in nature. Ca(2+) transients were tabulated in multiple cells within fields of view, and no correlation was found between the events in adjacent cells. TTX (1 μm) significantly increased the occurrence of Ca(2+) transients, suggesting that ICC-DMP contributes to the tonic inhibition conveyed by ongoing activity of inhibitory motor neurons. Ca(2+) transients were minimally affected after 12 min in Ca(2+) free solution, indicating these events do not depend immediately upon Ca(2+) influx. However, inhibitors of sarco/endoplasmic reticulum Ca(2+) -ATPase (SERCA) pump and blockers of inositol triphosphate receptor (InsP3 R) and ryanodine receptor (RyR) channels blocked ICC Ca(2+) transients. These data suggest an interdependence between RyR and InsP3 R in the generation of Ca(2+) transients. Itpr1 and Ryr2 were the dominant transcripts expressed by ICC. These findings provide the first high-resolution recording of the subcellular Ca(2+) dynamics that control the behaviour of ICC-DMP in situ.

Electrophysiological and Mechanical Characteristics in Human Ileal Motility: Recordings of Slow Waves Conductions and Contractions, In vitro

Little human tissue data are available for slow waves and migrating motor complexes, which are the main components of small bowel motility. We investigated the electrophysiological and mechanical characteristics of human ileal motility, in vitro. Ileum was obtained from patients undergoing bowel resection. Electrophysiological microelectrode recordings for membrane potential changes and mechanical tension recordings for contraction from smooth muscle strips and ileal segments were performed. Drugs affecting the enteric nervous system were applied to measure the changes in activity. Slow waves were detected with a frequency of 9~10/min. There were no cross-sectional differences in resting membrane potential (RMP), amplitude or frequency between outer and inner circular muscle (CM), suggesting that electrical activities could be effectively transmitted from outer to inner CM. The presence of the interstitial cell of Cajal (ICC) at the linia septa was verified by immunohistochemistry. Contractions of strips and segments occurred at a frequency of 3~4/min and 1~2/min, respectively. The frequency, amplitude and area under the curve were similar between CM and LM. In segments, contractions of CM were associated with LM, but propagation varied with antegrade and retrograde directions. Atropine, N^W-oxide-L-arginine, and sodium nitroprusside exhibited different effects on RMP and contractions. There were no cross-sectional differences with regard to the characteristics of slow waves in CM. The frequency of contractions in smooth muscle strips and ileal segments was lower than slow waves. The directions of propagation were diverse, indicating both mixing and transport functions of the ileum.

Colonic migrating motor complex: Generation and propagation mechanism

The colonic migrating motor complex (CMMC) is a critical neurally mediated, cyclical contractile and electrical event. CMMC is the primary motor pattern underlying fecal pellet propulsion along the murine colon. Abnormal CMMC has important implications in a number of gastrointestinal disorders, especially slow transit constipation. This review focuses on the mechanisms involved in producing and propagating the CMMC, which is likely dependent on mucosal and neuronal serotonin and pacemaker interstitial cells of Cajal networks and how peristaltic reflexes or occult reflexes affect them, and emphasizes the important role of intrinsic primary afferent neurons, ascending excitatory and descending inhibitory neural pathways. In addition to these, we also introduce some new tools to detect specific neuronal activity so as to offer some exciting insights into the role of 5-hydroxytryptamine in colonic motility.

Pacemaker role of pericytes in generating synchronized spontaneous Ca2+ transients in the myenteric microvasculature of the guinea-pig gastric antrum

Properties of spontaneous Ca(2+) transients in the myenteric microvasculature of the guinea-pig stomach were investigated. Specifically, we explored the spatio-temporal origin of Ca(2+) transients and the role of voltage-dependent Ca(2+) channels (VDCCs) in their intercellular synchrony using fluorescence Ca(2+) imaging and immunohistochemistry. The microvasculature generated spontaneous Ca(2+) transients that were independent of both Ca(2+) transients in interstitial cells of Cajal (ICC) and neural activity. Spontaneous Ca(2+) transients were highly synchronous along the length of microvasculature, and appeared to be initiated in pericytes and spread to arteriolar smooth muscle cells (SMCs). In most cases, the generation or synchrony of Ca(2+) transients was not affected by blockers of L-type VDCCs. In nifedipine-treated preparations, synchronous spontaneous Ca(2+) transients were readily blocked by Ni(2+), mibefradil or ML216, blockers for T-type VDCCs. These blockers also suppressed the known T-type VDCC dependent component of ICC Ca(2+) transients or slow waves. Spontaneous Ca(2+) transients were also suppressed by caffeine, tetracaine or cyclopiazonic acid (CPA). After the blockade of both L- and T-type VDCCs, asynchronous Ca(2+) transients were generated in pericytes on precapillary arterioles and/or capillaries but not in arteriolar SMCs, and were abolished by CPA or nominally Ca(2+) free solution. Together these data indicate that pericytes in the myenteric microvasculature may act as the origin of synchronous spontaneous Ca(2+) transients. Pericyte Ca(2+) transients arise from Ca(2+) release from the sarco-endoplasmic reticulum and the opening of T-type Ca(2+) VDCCs is required for their synchrony and propagation to arteriolar SMCs.

Ion channel expression in the developing enteric nervous system

The enteric nervous system arises from neural crest-derived cells (ENCCs) that migrate caudally along the embryonic gut. The expression of ion channels by ENCCs in embryonic mice was investigated using a PCR-based array, RT-PCR and immunohistochemistry. Many ion channels, including chloride, calcium, potassium and sodium channels were already expressed by ENCCs at E11.5. There was an increase in the expression of numerous ion channel genes between E11.5 and E14.5, which coincides with ENCC migration and the first extension of neurites by enteric neurons. Previous studies have shown that a variety of ion channels regulates neurite extension and migration of many cell types. Pharmacological inhibition of a range of chloride or calcium channels had no effect on ENCC migration in cultured explants or neuritogenesis in vitro. The non-selective potassium channel inhibitors, TEA and 4-AP, retarded ENCC migration and neuritogenesis, but only at concentrations that also resulted in cell death. In summary, a large range of ion channels is expressed while ENCCs are colonizing the gut, but we found no evidence that ENCC migration or neuritogenesis requires chloride, calcium or potassium channel activity. Many of the ion channels are likely to be involved in the development of electrical excitability of enteric neurons.

Effects of gap junction inhibition on contraction waves in the murine small intestine in relation to coupled oscillator theory

Waves of contraction in the small intestine correlate with slow waves generated by the myenteric network of interstitial cells of Cajal. Coupled oscillator theory has been used to explain steplike gradients in the frequency (frequency plateaux) of contraction waves along the length of the small intestine. Inhibition of gap junction coupling between oscillators should lead to predictable effects on these plateaux and the wave dislocation (wave drop) phenomena associated with their boundaries. It is these predictions that we wished to test. We used a novel multicamera diameter-mapping system to measure contraction along 25- to 30-cm lengths of murine small intestine. There were typically two to three plateaux per length of intestine. Dislocations could be limited to the wavefronts immediately about the terminated wave, giving the appearance of a three-pronged fork, i.e., a fork dislocation; additionally, localized decreases in velocity developed across a number of wavefronts, ending with the terminated wave, which could appear as a fork, i.e., slip dislocations. The gap junction inhibitor carbenoxolone increased the number of plateaux and dislocations and decreased contraction wave velocity. In some cases, the usual frequency gradient was reversed, with a plateau at a higher frequency than its proximal neighbor; thus fork dislocations were inverted, and the direction of propagation was reversed. Heptanol had no effect on the frequency or velocity of contractions but did reduce their amplitude. To understand intestinal motor patterns, the pacemaker network of the interstitial cells of Cajal is best evaluated as a system of coupled oscillators.

Intracellular Ca(2+) release from endoplasmic reticulum regulates slow wave currents and pacemaker activity of interstitial cells of Cajal

Interstitial cells of Cajal (ICC) provide pacemaker activity in gastrointestinal muscles that underlies segmental and peristaltic contractions. ICC generate electrical slow waves that are due to large-amplitude inward currents resulting from anoctamin 1 (ANO1) channels, which are Ca(2+)-activated Cl(-) channels. We investigated the hypothesis that the Ca(2+) responsible for the stochastic activation of ANO1 channels during spontaneous transient inward currents (STICs) and synchronized activation of ANO1 channels during slow wave currents comes from intracellular Ca(2+) stores. ICC, obtained from the small intestine of Kit(+/copGFP) mice, were studied under voltage and current clamp to determine the effects of blocking Ca(2+) uptake into stores and release of Ca(2+) via inositol 1,4,5-trisphosphate (IP3)-dependent and ryanodine-sensitive channels. Cyclocpiazonic acid, thapsigargin, 2-APB, and xestospongin C inhibited STICs and slow wave currents. Ryanodine and tetracaine also inhibited STICs and slow wave currents. Store-active compounds had no direct effects on ANO1 channels expressed in human embryonic kidney-293 cells. Under current clamp, store-active drugs caused significant depolarization of ICC and reduced spontaneous transient depolarizations (STDs). After block of ryanodine receptors with ryanodine and tetracaine, repolarization did not restore STDs. ANO1 expressed in ICC has limited access to cytoplasmic Ca(2+) concentration, suggesting that pacemaker activity depends on Ca(2+) dynamics in restricted microdomains. Our data from studies of isolated ICC differ somewhat from studies on intact muscles and suggest that release of Ca(2+) from both IP3 and ryanodine receptors is important in generating pacemaker activity in ICC.

The effects of eupatilin (stillen®) on motility of human lower gastrointestinal tracts

Gastrointestinal motility consists of phasic slow-wave contractions and the migrating motor complex (MMC). Eupatilin (Stillen®) has been widely used to treat gastritis and peptic ulcers, and various cytokines and neuropeptides are thought to be involved, which can affect gastrointestinal motility. We performed a study to identify the effects of eupatilin on lower gastrointestinal motility with electromechanical recordings of smooth muscles in the human ileum and colon. Ileum and colon samples were obtained from patients undergoing bowel resection. The tissues were immediately stored in oxygenated Krebs-Ringer's bicarbonate solution, and conventional microelectrode recordings from muscle cells and tension recordings from muscle strips and ileal or colonic segments were performed. Eupatilin was perfused into the tissue chamber, and changes in membrane potentials and contractions were measured. Hyperpolarization of resting membrane potential (RMP) was observed after administration of eupatilin. The amplitude, AUC, and frequency of tension recordings from circular and longitudinal smooth muscle strips and bowel segments of the ileum and colon were significantly decreased after admission of eupatilin. Eupatilin elicited dose-dependent decreases during segmental tension recordings. In conclusion, eupatilin (Stillen®) showed inhibitory effects on the human ileum and colon. We propose that this drug may be useful for treating diseases that increase bowel motility, but further studies are necessary.

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 significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Ano1, a Ca2+-activated Cl- channel, coordinates contractility in mouse intestine by Ca2+ transient coordination between interstitial cells of Cajal

Interstitial cells of Cajal (ICC) are pacemaker cells that generate electrical activity to drive contractility in the gastrointestinal tract via ion channels. Ano1 (Tmem16a), a Ca(2+)-activated Cl(-) channel, is an ion channel expressed in ICC. Genetic deletion of Ano1 in mice resulted in loss of slow waves in smooth muscle of small intestine. In this study, we show that Ano1 is required to maintain coordinated Ca(2+) transients between myenteric ICC (ICC-MY) of small intestine. First, we found spontaneous Ca(2+) transients in ICC-MY in both Ano1 WT and knockout (KO) mice. However, Ca(2+) transients within the ICC-MY network in Ano1 KO mice were uncoordinated, while ICC-MY Ca(2+) transients in Ano1 WT mice were rhythmic and coordinated. To confirm the role of Ano1 in the loss of Ca(2+) transient coordination, we used pharmacological inhibitors of Ano1 activity and shRNA-mediated knock down of Ano1 expression in organotypic cultures of Ano1 WT small intestine. Coordinated Ca(2+) transients became uncoordinated using both these approaches, supporting the conclusion that Ano1 is required to maintain coordination/rhythmicity of Ca(2+) transients. We next determined the effect on smooth muscle contractility using spatiotemporal maps of contractile activity in Ano1 KO and WT tissues. Significantly decreased contractility that appeared to be non-rhythmic and uncoordinated was observed in Ano1 KO jejunum. In conclusion, Ano1 has a previously unidentified role in the regulation of coordinated gastrointestinal smooth muscle function through coordination of Ca(2+) transients in ICC-MY.

The significance of interstitial cells in neurogastroenterology

Smooth muscle layers of the gastrointestinal tract consist of a heterogeneous population of cells that include enteric neurons, several classes of interstitial cells of mesenchymal origin, a variety of immune cells and smooth muscle cells (SMCs). Over the last number of years the complexity of the interactions between these cell types has begun to emerge. For example, interstitial cells, consisting of both interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive (PDGFRα(+)) cells generate pacemaker activity throughout the gastrointestinal (GI) tract and also transduce enteric motor nerve signals and mechanosensitivity to adjacent SMCs. ICC and PDGFRα(+) cells are electrically coupled to SMCs possibly via gap junctions forming a multicellular functional syncytium termed the SIP syncytium. Cells that make up the SIP syncytium are highly specialized containing unique receptors, ion channels and intracellular signaling pathways that regulate the excitability of GI muscles. The unique role of these cells in coordinating GI motility is evident by the altered motility patterns in animal models where interstitial cell networks are disrupted. Although considerable advances have been made in recent years on our understanding of the roles of these cells within the SIP syncytium, the full physiological functions of these cells and the consequences of their disruption in GI muscles have not been clearly defined. This review gives a synopsis of the history of interstitial cell discovery and highlights recent advances in structural, molecular expression and functional roles of these cells in the GI tract.

Mitochondrial calcium handling within the interstitial cells of Cajal

The interstitial cells of Cajal (ICC) drive rhythmic pacemaking contractions in the gastrointestinal system. The ICC generate pacemaking signals by membrane depolarizations associated with the release of intracellular calcium (Ca(2+)) in the endoplasmic reticulum (ER) through inositol-trisphosphate (IP3) receptors (IP3R) and uptake by mitochondria (MT). This Ca(2+) dynamic is hypothesized to generate pacemaking signals by calibrating ER Ca(2+) store depletions and membrane depolarization with ER store-operated Ca(2+) entry mechanisms. Using a biophysically based spatio-temporal model of integrated Ca(2+) transport in the ICC, we determined the feasibility of ER depletion timescale correspondence with experimentally observed pacemaking frequencies while considering the impact of IP3R Ca(2+) release and MT uptake on bulk cytosolic Ca(2+) levels because persistent elevations of free intracellular Ca(2+) are toxic to the cell. MT densities and distributions are varied in the model geometry to observe MT influence on free cytosolic Ca(2+) and the resulting frequencies of ER Ca(2+) store depletions, as well as the sarco-endoplasmic reticulum Ca(2+) ATP-ase (SERCA) and IP3 agonist concentrations. Our simulations show that high MT densities observed in the ICC are more relevant to ER establishing Ca(2+) depletion frequencies than protection of the cytosol from elevated free Ca(2+), whereas the SERCA pump is more relevant to containing cytosolic Ca(2+) elevations. Our results further suggest that the level of IP3 agonist stimulating ER Ca(2+) release, subsequent MT uptake, and eventual activation of ER store-operated Ca(2+) entry may determine frequencies of rhythmic pacemaking exhibited by the ICC across species and tissue types.

Developmental changes in postnatal murine intestinal interstitial cell of Cajal network structure and function

The mammalian gastrointestinal (GI) tract undergoes rapid development during early postnatal life in order to transition from a milk to solid diet. Interstitial cells of Cajal (ICC) are the pacemaker cells that coordinate smooth muscle contractility within the GI tract, and hence we hypothesized that ICC networks undergo significant developmental changes during this early postnatal period. Numerical metrics for quantifying ICC network structural properties were applied on confocal ICC network imaging data obtained from the murine small intestine at various postnatal ages spanning birth to weaning. These imaging data were also coupled to a biophysically-based computational model to simulate pacemaker activity in the networks, to quantify how changes in structure may alter function. The results showed a pruning-like mechanism which occurs during postnatal development, and the temporal course of this phenomenon was defined. There was an initial ICC process overgrowth to optimize network efficiency and increase functional output volume. This was followed by a selective retaining and strengthening of processes, while others were discarded to further elevate functional output volume. Subsequently, new ICC processes were formed and the network was adjusted to its adult morphology. These postnatal ICC network developmental events may be critical in facilitating mature digestive function.

Functional alterations in gut contractility after connexin36 ablation and evidence for gap junctions forming electrical synapses between nitrergic enteric neurons

Neurons in the enteric nervous system utilize numerous neurotransmitters to orchestrate rhythmic gut smooth muscle contractions. We examined whether electrical synapses formed by gap junctions containing connexin36 also contribute to communication between enteric neurons in mouse colon. Spontaneous contractility properties and responses to electrical field stimulation and cholinergic agonist were altered in gut from connexin36 knockout vs. wild-type mice. Immunofluorescence revealed punctate labelling of connexin36 that was localized at appositions between somata of enteric neurons immunopositive for the enzyme nitric oxide synthase. There is indication for a possible functional role of gap junctions between inhibitory nitrergic enteric neurons.

Expression and function of a T-type Ca2+ conductance in interstitial cells of Cajal of the murine small intestine

Interstitial cells of Cajal (ICC) generate slow waves in gastrointestinal (GI) muscles. Previous studies have suggested that slow wave generation and propagation depends on a voltage-dependent Ca(2+) entry mechanism with the signature of a T-type Ca(2+) conductance. We studied voltage-dependent inward currents in isolated ICC. ICC displayed two phases of inward current upon depolarization: a low voltage-activated inward current and a high voltage-activated current. The latter was of smaller current density and blocked by nicardipine. Ni(2+) (30 μM) or mibefradil (1 μM) blocked the low voltage-activated current. Replacement of extracellular Ca(2+) with Ba(2+) did not affect the current, suggesting that either charge carrier was equally permeable. Half-activation and half-inactivation occurred at -36 and -59 mV, respectively. Temperature sensitivity of the Ca(2+) current was also characterized. Increasing temperature (20-30°C) augmented peak current from -7 to -19 pA and decreased the activation time from 20.6 to 7.5 ms [temperature coefficient (Q10) = 3.0]. Molecular studies showed expression of Cacna1g (Cav3.1) and Cacna1h (Cav3.2) in ICC. The temperature dependence of slow waves in intact jejunal muscles of wild-type and Cacna1h(-/-) mice was tested. Reducing temperature decreased the upstroke velocity significantly. Upstroke velocity was also reduced in muscles of Cacna1h(-/-) mice, and Ni(2+) or reduced temperature had little effect on these muscles. Our data show that a T-type conductance is expressed and functional in ICC. With previous studies our data suggest that T-type current is required for entrainment of pacemaker activity within ICC and for active propagation of slow waves in ICC networks.

Enteric sensory neurons communicate with interstitial cells of Cajal to affect pacemaker activity in the small intestine

Enteric sensory neurons (the AH neurons) play a role in control of gastrointestinal motor activity; AH neuron activation has been proposed to change propulsion into segmentation. We sought to find a mechanism underlying this phenomenon. We formulated the hypothesis that AH neurons increase local ICC-MP (interstitial cells of Cajal associated with the myenteric plexus) pacemaker frequency to disrupt peristalsis and promote absorption. To that end, we sought structural and physiological evidence for communication between ICC-MP and AH neurons. We designed experiments that allowed us to simultaneously activate AH neurons and observe changes in ICC calcium transients that underlie its pacemaker activity. Neurobiotin injection in AH neurons together with ICC immunohistochemistry proved the presence of multiple contacts between AH neuron varicosities and the cell bodies and processes of ICC-MP. Generating action potential activity in AH neurons led to increase in the frequency and amplitude of calcium transients underlying pacemaker activity in ICC. When no rhythmicity was seen, rhythmic calcium transients were evoked in ICC. As a control, we stimulated nitrergic S neurons, which led to reduction in ICC calcium transients. Hence, we report here the first demonstration of communication between AH neurons and ICC. The following hypothesis can now be formulated: AH neuron activation can disrupt peristalsis directed by ICC-MP slow wave activity, through initiation of a local pacemaker by increasing ICC pacemaker frequency through increasing the frequency of ICC calcium transients. Evoking new pacemakers distal to the proximal lead pacemaker will initiate both retrograde and antegrade propulsion causing back and forth movements that may disrupt peristalsis.

Circumferential and functional re-entry of in vivo slow-wave activity in the porcine small intestine

BACKGROUND

Slow-waves modulate the pattern of small intestine contractions. However, the large-scale spatial organization of intestinal slow-wave pacesetting remains uncertain because most previous studies have had limited resolution. This study applied high-resolution (HR) mapping to evaluate intestinal pacesetting mechanisms and propagation patterns in vivo.

METHODS

HR serosal mapping was performed in anesthetized pigs using flexible arrays (256 electrodes; 32 × 8; 4 mm spacing), applied along the jejunum. Slow-wave propagation patterns, frequencies, and velocities were calculated. Slow-wave initiation sources were identified and analyzed by animation and isochronal activation mapping.

KEY RESULTS

Analysis comprised 32 recordings from nine pigs (mean duration 5.1 ± 3.9 min). Slow-wave propagation was analyzed, and a total of 26 sources of slow-wave initiation were observed and classified as focal pacemakers (31%), sites of functional re-entry (23%) and circumferential re-entry (35%), or indeterminate sources (11%). The mean frequencies of circumferential and functional re-entry were similar (17.0 ± 0.3 vs 17.2 ± 0.4 cycle min(-1) ; P = 0.5), and greater than that of focal pacemakers (12.7 ± 0.8 cycle min(-1) ; P < 0.001). Velocity was anisotropic (12.9 ± 0.7 mm s(-1) circumferential vs 9.0 ± 0.7 mm s(-1) longitudinal; P < 0.05), contributing to the onset and maintenance of re-entry.

CONCLUSIONS & INFERENCES

This study has shown multiple patterns of slow-wave initiation in the jejunum of anesthetized pigs. These results constitute the first description and analysis of circumferential re-entry in the gastrointestinal tract and functional re-entry in the in vivo small intestine. Re-entry can control the direction, pattern, and frequency of slow-wave propagation, and its occurrence and functional significance merit further investigation.

Rapid high-amplitude circumferential slow wave propagation during normal gastric pacemaking and dysrhythmias

BACKGROUND

Gastric slow waves propagate aborally as rings of excitation. Circumferential propagation does not normally occur, except at the pacemaker region. We hypothesized that (i) the unexplained high-velocity, high-amplitude activity associated with the pacemaker region is a consequence of circumferential propagation; (ii) rapid, high-amplitude circumferential propagation emerges during gastric dysrhythmias; (iii) the driving network conductance might switch between interstitial cells of Cajal myenteric plexus (ICC-MP) and circular interstitial cells of Cajal intramuscular (ICC-IM) during circumferential propagation; and (iv) extracellular amplitudes and velocities are correlated.

METHODS

An experimental-theoretical study was performed. High-resolution gastric mapping was performed in pigs during normal activation, pacing, and dysrhythmia. Activation profiles, velocities, and amplitudes were quantified. ICC pathways were theoretically evaluated in a bidomain model. Extracellular potentials were modeled as a function of membrane potentials.

KEY RESULTS

High-velocity, high-amplitude activation was only recorded in the pacemaker region when circumferential conduction occurred. Circumferential propagation accompanied dysrhythmia in 8/8 experiments was faster than longitudinal propagation (8.9 vs 6.9 mm s(-1) ; P = 0.004) and of higher amplitude (739 vs 528 μV; P = 0.007). Simulations predicted that ICC-MP could be the driving network during longitudinal propagation, whereas during ectopic pacemaking, ICC-IM could outpace and activate ICC-MP in the circumferential axis. Experimental and modeling data demonstrated a linear relationship between velocities and amplitudes (P < 0.001).

CONCLUSIONS & INFERENCES

The high-velocity and high-amplitude profile of the normal pacemaker region is due to localized circumferential propagation. Rapid circumferential propagation also emerges during a range of gastric dysrhythmias, elevating extracellular amplitudes and organizing transverse wavefronts. One possible explanation for these findings is bidirectional coupling between ICC-MP and circular ICC-IM networks.