Spontaneous transient hyperpolarizations in the rabbit small intestine

5-Hydroxytryptamine Enhances the Pacemaker Activity of Interstitial Cells of Cajal in Mouse Colon

We examined the localization of the 5-hydroxytryptamine (5-HT) receptor and its effects on mouse colonic interstitial cells of Cajal (ICCs) using electrophysiological techniques. Treatment with 5-HT increased the pacemaker activity in colonic ICCs with depolarization of membrane potentials in a dose-dependent manner. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blockers blocked pacemaker activity and 5-HT-induced effects. Moreover, an adenylate cyclase inhibitor inhibited 5-HT-induced effects, and cell-permeable 8-bromo-cAMP increased the pacemaker activity. Various agonists of the 5-HT receptor subtype were working in colonic ICCs, including the 5-HT4 receptor. In small intestinal ICCs, 5-HT depolarized the membrane potentials transiently. Adenylate cyclase inhibitors or HCN blockers did not show any influence on 5-HT-induced effects. Anoctamin-1 (ANO1) or T-type Ca2+ channel blockers inhibited the pacemaker activity of colonic ICCs and blocked 5-HT-induced effects. A tyrosine protein kinase inhibitor inhibited pacemaker activity in colonic ICCs under controlled conditions but did not show any influence on 5-HT-induced effects. Among mitogen-activated protein kinase (MAPK) inhibitors, a p38 MAPK inhibitor inhibited 5-HT-induced effects on colonic ICCs. Thus, 5-HT's effect on pacemaker activity in small intestinal and colonic ICCs has excitatory but variable patterns. ANO1, T-type Ca2+, and HCN channels are involved in 5-HT-induced effects, and MAPKs are involved in 5-HT effects in colonic ICCs.

Interstitial cells of Cajal - pacemakers of the gastrointestinal tract

Gastrointestinal (GI) organs display spontaneous, non-neurogenic electrical, and mechanical rhythmicity that underlies fundamental motility patterns, such as peristalsis and segmentation. Electrical rhythmicity (aka slow waves) results from pacemaker activity generated by interstitial cells of Cajal (ICC). ICC express a unique set of ionic conductances and Ca2+ handling mechanisms that generate and actively propagate slow waves. GI smooth muscle cells lack these conductances. Slow waves propagate actively within ICC networks and conduct electrotonically to smooth muscle cells via gap junctions. Slow waves depolarize smooth muscle cells and activate voltage-dependent Ca2+ channels (predominantly CaV1.2), causing Ca2+ influx and excitation-contraction coupling. The main conductances responsible for pacemaker activity in ICC are ANO1, a Ca2+ -activated Cl- conductance, and CaV3.2. The pacemaker cycle, as currently understood, begins with spontaneous, localized Ca2+ release events in ICC that activate spontaneous transient inward currents due to activation of ANO1 channels. Depolarization activates CaV 3.2 channels, causing the upstroke depolarization phase of slow waves. The upstroke is transient and followed by a long-duration plateau phase that can last for several seconds. The plateau phase results from Ca2+ -induced Ca2+ release and a temporal cluster of localized Ca2+ transients in ICC that sustains activation of ANO1 channels and clamps membrane potential near the equilibrium potential for Cl- ions. The plateau phase ends, and repolarization occurs, when Ca2+ stores are depleted, Ca2+ release ceases and ANO1 channels deactivate. This review summarizes key mechanisms responsible for electrical rhythmicity in gastrointestinal organs.

A myogenic motor pattern in mice lacking myenteric interstitial cells of Cajal explained by a second coupled oscillator network

The interstitial cells of Cajal associated with the myenteric plexus (ICC-MP) are a network of coupled oscillators in the small intestine that generate rhythmic electrical phase waves leading to corresponding waves of contraction, yet rhythmic action potentials and intercellular calcium waves have been recorded from c-kit-mutant mice that lack the ICC-MP, suggesting that there may be a second pacemaker network. The gap junction blocker carbenoxolone induced a "pinstripe" motor pattern consisting of rhythmic "stripes" of contraction that appeared simultaneously across the intestine with a period of ~4 s. The infinite velocity of these stripes suggested they were generated by a coupled oscillator network, which we call X. In c-kit mutants rhythmic contraction waves with the period of X traveled the length of the intestine, before the induction of the pinstripe pattern by carbenoxolone. Thus X is not the ICC-MP and appears to operate under physiological conditions, a fact that could explain the viability of these mice. Individual stripes consisted of a complex pattern of bands of contraction and distension, and between stripes there could be slide waves and v waves of contraction. We hypothesized that these phenomena result from an interaction between X and the circular muscle that acts as a damped oscillator. A mathematical model of two chains of coupled Fitzhugh-Nagumo systems, representing X and circular muscle, supported this hypothesis. The presence of a second coupled oscillator network in the small intestine underlines the complexity of motor pattern generation in the gut.NEW & NOTEWORTHY Physiological experiments and a mathematical model indicate a coupled oscillator network in the small intestine in addition to the c-kit-expressing myenteric interstitial cells of Cajal. This network interacts with the circular muscle, which itself acts as a system of damped oscillators, to generate physiological contraction waves in c-kit (W) mutant mice.

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.

Inhibitory Neural Regulation of the Ca 2+ Transients in Intramuscular Interstitial Cells of Cajal in the Small Intestine

Gastrointestinal motility is coordinated by enteric neurons. Both inhibitory and excitatory motor neurons innervate the syncytium consisting of smooth muscle cells (SMCs) interstitial cells of Cajal (ICC) and PDGFRα+ cells (SIP syncytium). Confocal imaging of mouse small intestines from animals expressing GCaMP3 in ICC were used to investigate inhibitory neural regulation of ICC in the deep muscular plexus (ICC-DMP). We hypothesized that Ca2+ signaling in ICC-DMP can be modulated by inhibitory enteric neural input. ICC-DMP lie in close proximity to the varicosities of motor neurons and generate ongoing Ca2+ transients that underlie activation of Ca2+-dependent Cl- channels and regulate the excitability of SMCs in the SIP syncytium. Electrical field stimulation (EFS) caused inhibition of Ca2+ for the first 2-3 s of stimulation, and then Ca2+ transients escaped from inhibition. The NO donor (DEA-NONOate) inhibited Ca2+ transients and Nω-Nitro-L-arginine (L-NNA) or a guanylate cyclase inhibitor (ODQ) blocked inhibition induced by EFS. Purinergic neurotransmission did not affect Ca2+ transients in ICC-DMP. Purinergic neurotransmission elicits hyperpolarization of the SIP syncytium by activation of K+ channels in PDGFRα+ cells. Generalized hyperpolarization of SIP cells by pinacidil (KATP agonist) or MRS2365 (P2Y1 agonist) also had no effect on Ca2+ transients in ICC-DMP. Peptidergic transmitter receptors (VIP and PACAP) are expressed in ICC and can modulate ICC-DMP Ca2+ transients. In summary Ca2+ transients in ICC-DMP are blocked by enteric inhibitory neurotransmission. ICC-DMP lack a voltage-dependent mechanism for regulating Ca2+ release, and this protects Ca2+ handling in ICC-DMP from membrane potential changes in other SIP cells.

Extracellular Cl- regulates electrical slow waves and setting of smooth muscle membrane potential by interstitial cells of Cajal in mouse jejunum

NEW FINDINGS

What is the central question of this study? The aim was to investigate the roles of extracellular chloride in electrical slow waves and resting membrane potential of mouse jejunal smooth muscle by replacing chloride with the impermeant anions gluconate and isethionate. What is the main finding and its importance? The main finding was that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in interstitial cells of Cajal (ICCs) is a major contributor to the shape of electrical slow waves. Furthermore, the data confirm that ICCs set the smooth muscle membrane potential and that altering Cl^- homeostasis in ICCs can alter the smooth muscle membrane potential. Intracellular Cl^- homeostasis is regulated by anion-permeable channels and transporters and contributes to excitability of many cell types, including smooth muscle and interstitial cells of Cajal (ICCs). Our aims were to investigate the effects on electrical activity in mouse jejunal muscle strips of replacing extracellular Cl^- (Cl^-_o ) with the impermeant anions gluconate and isethionate. On reducing Cl^-_o , effects were observed on electrical slow waves, with small effects on smooth muscle membrane voltage (E_m ). Restoration of Cl^- hyperpolarized smooth muscle E_m proportional to the change in Cl^-_o concentration. Replacement of 90% of Cl^-_o with gluconate reversibly abolished slow waves in five of nine preparations. Slow waves were maintained in isethionate. Gluconate and isethionate substitution had similar concentration-dependent effects on peak amplitude, frequency, width at half peak amplitude, rise time and decay time of residual slow waves. Gluconate reduced free ionized Ca²⁺ in Krebs solutions to 0.13 mm. In Krebs solutions containing normal Cl^- and 0.13 mm free Ca²⁺ , slow wave frequency was lower, width at half peak amplitude was smaller, and decay time was faster. The transient hyperpolarization following restoration of Cl^-_o was not observed in W/W^v mice, which lack pacemaker ICCs in the small intestine. We conclude that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in ICCs plays a major role in generation or propagation of slow waves. Furthermore, these data support a role for ICCs in setting smooth muscle E_m and that altering Cl^- homeostasis in ICCs can alter smooth muscle E_m .

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.

ANO1 in intramuscular interstitial cells of Cajal plays a key role in the generation of slow waves and tone in the internal anal sphincter

KEY POINTS

The internal anal sphincter develops tone important for maintaining high anal pressure and continence. Controversy exists regarding the mechanisms underlying tone development. We examined the hypothesis that tone depends upon electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC-IM) by activation of Ca²⁺ -activated Cl^- channels (ANO1, encoded by Ano1) and voltage-dependent L-type Ca²⁺ channels (Cav_L , encoded by Cacna1c). Measurement of membrane potential and contraction indicated that ANO1 and Cav_L have a central role in SW generation, phasic contractions and tone, independent of stretch. ANO1 expression was examined in wildtype and Ano1^/+egfp mice with immunohistochemical techniques. Ano1 and Cacna1c expression levels were examined by quantitative PCR in fluorescence-activated cell sorting. ICC-IM were the predominant cell type expressing ANO1 and the most likely candidate for SW generation. SWs in ICC-IM are proposed to conduct to smooth muscle where Ca²⁺ entry via Cav_L results in phasic activity that sums to produce tone.

ABSTRACT

The mechanism underlying tone generation in the internal anal sphincter (IAS) is controversial. We examined the hypothesis that tone depends upon generation of electrical slow waves (SWs) initiated in intramuscular interstitial cells of Cajal (ICC-IM) by activation of Ca²⁺ -activated Cl^- channels (encoded by Ano1) and voltage-dependent L-type Ca²⁺ channels (encoded by Cacna1c). Phasic contractions and tone in the IAS were nearly abolished by ANO1 and Cav_L antagonists. ANO1 antagonists also abolished SWs as well as transient depolarizations that persisted after addition of Cav_L antagonists. Tone development in the IAS did not require stretch of muscles, and the sensitivity of contraction to ANO1 antagonists was the same in stretched versus un-stretched muscles. ANO1 expression was examined in wildtype and Ano1^/+egfp mice with immunohistochemical techniques. Dual labelling revealed that ANO1 expression could be resolved in ICC but not smooth muscle cells (SMCs) in the IAS and rectum. Ano1, Cacna1c and Kit gene expression were the same in extracts of IAS and rectum muscles. In IAS cells isolated with fluorescence-activated cell sorting, Ano1 expression was 26.5-fold greater in ICC than in SMCs while Cacna1c expression was only 2-fold greater in SMCs than in ICC. These data support a central role for ANO1 and Cav_L in the generation of SWs and tone in the IAS. ICC-IM are the probable cellular candidate for ANO1 currents and SW generation. We propose that ANO1 and Cav_L collaborate to generate SWs in ICC-IM followed by conduction to adjacent SMCs where phasic calcium entry through Cav_L sums to produce tone.

Trypsin induces biphasic muscle contraction and relaxation via transient receptor potential vanilloid 1 and neurokinin receptors 1/2 in porcine esophageal body

Duodenal reflux of fluids containing trypsin relates to refractory gastroesophageal reflux disease (GERD). Esophageal peristalsis and clearance are important factors in GERD pathogenesis. However, the function of trypsin in esophageal body contractility is not fully understood. In this study, effects of trypsin on circular smooth muscle (CSM) and longitudinal smooth muscle (LSM) of the porcine esophageal body were examined. Trypsin elicited a concentration dependent biphasic response, a major contraction and a subsequent relaxation only in CSM. In CSM, contraction occurred at trypsin concentrations of 100nM and relaxation at 1μM. A proteinase-activated receptor (PAR)2 activating peptide, SLIGKV-NH₂ (1mM), induced a monophasic contraction. Those responses were unaffected by tetrodotoxin though abolished by the gap junction uncouplers carbenoxolone and octanol. They were also partially inhibited by a transient receptor potential vanilloid type 1 (TRPV1) antagonist and abolished by combination of neurokinin receptor 1 (NK₁) and NK₂ antagonists, but not by an NK₃ antagonist, suggesting a PAR2-TRPV1-substance P pathway in sensory neurons. Substance P (100nM), an agonist for various NK receptors (NK₁, NK₂ and NK₃) with differing affinities, induced significant contraction in CSM, but not in LSM. The contraction was also blocked by the combination of NK₁ and NK₂ antagonists, but not by the NK₃ antagonist. Moreover, substance P-induced contractions were unaffected by the TRPV1 antagonist, but inhibited by a gap junction uncoupler. In conclusion, trypsin induced a biphasic response only in CSM and this was mediated by PAR2, TRPV1 and NK_1/2. Gap junctions were indispensable in this tachykinin-induced response.

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.

Characterization of slow waves generated by myenteric interstitial cells of Cajal of the rabbit small intestine

Slow waves (slow wavesICC) were recorded from myenteric interstitial cells of Cajal (ICC-MY) in situ in the rabbit small intestine, and their properties were compared with those of mouse small intestine. Rabbit slow wavesICC consisted of an upstroke depolarization followed by a distinct plateau component. Ni(2+) and nominally Ca(2+)-free solutions reduced the rate-of-rise and amplitude of the upstroke depolarization. Replacement of Ca(2+) with Sr(2+) enhanced the upstroke component but decreased the plateau component of rabbit slow wavesICC. In contrast, replacing Ca(2+) with Sr(2+) decreased both components of mouse slow wavesICC. The plateau component of rabbit slow wavesICC was inhibited in low-extracellular-Cl(-)-concentration (low-[Cl(-)]o) solutions and by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), an inhibitor of Cl(-) channels, cyclopiazonic acid (CPA), an inhibitor of internal Ca(2+) pumps, or bumetanide, an inhibitor of Na(+)-K(+)-2Cl(-) cotransporter (NKCC1). Bumetanide also inhibited the plateau component of mouse slow wavesICC. NKCC1-like immunoreactivity was observed mainly in ICC-MY in the rabbit small intestine. Membrane depolarization with a high-K(+) solution reduced the upstroke component of rabbit slow wavesICC. In cells depolarized with elevated external K(+), DIDS, CPA, and bumetanide blocked slow wavesICC. These results suggest that the upstroke component of rabbit slow wavesICC is partially mediated by voltage-dependent Ca(2+) influx, whereas the plateau component is dependent on Ca(2+)-activated Cl(-) efflux. NKCC1 is likely to be responsible for Cl(-) accumulation in ICC-MY. The results also suggest that the mechanism of the upstroke component differs in rabbit and mouse slow wavesICC in the small intestine.

Temporal sequence of activation of cells involved in purinergic neurotransmission in the colon

KEY POINTS

Platelet derived growth factor receptor α (PDGFRα(+) ) cells in colonic muscles are innervated by enteric inhibitory motor neurons. PDGFRα(+) cells generate Ca(2+) transients in response to exogenous purines and these responses were blocked by MRS-2500. Stimulation of enteric neurons, with cholinergic and nitrergic components blocked, evoked Ca(2+) transients in PDGFRα(+) and smooth muscle cells (SMCs). Responses to nerve stimulation were abolished by MRS-2500 and not observed in muscles with genetic deactivation of P2Y1 receptors. Ca(2+) transients evoked by nerve stimulation in PDGFRα(+) cells showed the same temporal characteristics as electrophysiological responses. PDGFRα(+) cells express gap junction genes, and drugs that inhibit gap junctions blocked neural responses in SMCs, but not in nerve processes or PDGFRα(+) cells. PDGFRα(+) cells are directly innervated by inhibitory motor neurons and purinergic responses are conducted to SMCs via gap junctions.

ABSTRACT

Interstitial cells, known as platelet derived growth factor receptor α (PDGFRα(+) ) cells, are closely associated with varicosities of enteric motor neurons and suggested to mediate purinergic hyperpolarization responses in smooth muscles of the gastrointestinal tract (GI), but this concept has not been demonstrated directly in intact muscles. We used confocal microscopy to monitor Ca(2+) transients in neurons and post-junctional cells of the murine colon evoked by exogenous purines or electrical field stimulation (EFS) of enteric neurons. EFS (1-20 Hz) caused Ca(2+) transients in enteric motor nerve processes and then in PDGFRα(+) cells shortly after the onset of stimulation (latency from EFS was 280 ms at 10 Hz). Responses in smooth muscle cells (SMCs) were typically a small decrease in Ca(2+) fluorescence just after the initiation of Ca(2+) transients in PDGFRα(+) cells. Upon cessation of EFS, several fast Ca(2+) transients were noted in SMCs (rebound excitation). Strong correlation was noted in the temporal characteristics of Ca(2+) transients evoked in PDGFRα(+) cells by EFS and inhibitory junction potentials (IJPs) recorded with intracellular microelectrodes. Ca(2+) transients and IJPs elicited by EFS were blocked by MRS-2500, a P2Y1 antagonist, and absent in P2ry1((-/-)) mice. PDGFRα(+) cells expressed gap junction genes, and gap junction uncouplers, 18β-glycyrrhetinic acid (18β-GA) and octanol blocked Ca(2+) transients in SMCs but not in neurons or PDGFRα(+) cells. IJPs recorded from SMCs were also blocked. These findings demonstrate direct innervation of PDGFRα(+) cells by motor neurons. PDGFRα(+) cells are primary targets for purinergic neurotransmitter(s) in enteric inhibitory neurotransmission. Hyperpolarization responses are conducted to SMCs via gap junctions.

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.