Rhythmic spontaneous contractions in the rat proximal colon

Roles of interstitial cells of Cajal in regulating gastrointestinal motility: in vitro versus in vivo studies

The aim of this article is to provide a better understanding of the roles of interstitial cells of Cajal (ICC) in regulating gastrointestinal motility by reviewing in vitro and in vivo physiological motility studies. Based on the in vitro studies, ICC are proposed to have the following functions: to generate slow waves, to mediate neurotransmission between the enteric nerves and the gastrointestinal muscles and to act as mechanoreceptors. However, there is limited evidence available for these hypotheses from the in vivo motility studies. In this review, we first introduce the major subtypes of ICC and their established functions. Three Kit mutant mouse and rodent models are presented and the loss of ICC subtypes in these mutants is reviewed. The physiological motility findings from various in vitroand in vivo experiments are discussed to give a critical review on the roles of ICC in generating slow waves, regulating gastrointestinal motility, mediating neural transmission and serving as mechanoreceptors. It is concluded that the role of ICC as pacemakers may be well established, but other cells may also be involved in the generation of slow waves; the theory that ICC are mediators of neurotransmission is challenged by the majority of the in vivo motility studies; the hypothesis that ICC are mechanoreceptors has not found supportive evidence from the in vivo studies yet. More studies are needed to explain discrepancies in motility findings between the in vitro and in vivo experiments.

Roles of interstitial cells of Cajal in regulating gastrointestinal motility: in vitro versus in vivo studies

The aim of this article is to provide a better understanding of the roles of interstitial cells of Cajal (ICC) in regulating gastrointestinal motility by reviewing in vitro and in vivo physiological motility studies. Based on the in vitro studies, ICC are proposed to have the following functions: to generate slow waves, to mediate neurotransmission between the enteric nerves and the gastrointestinal muscles and to act as mechanoreceptors. However, there is limited evidence available for these hypotheses from the in vivo motility studies. In this review, we first introduce the major subtypes of ICC and their established functions. Three Kit mutant mouse and rodent models are presented and the loss of ICC subtypes in these mutants is reviewed. The physiological motility findings from various in vitro and in vivo experiments are discussed to give a critical review on the roles of ICC in generating slow waves, regulating gastrointestinal motility, mediating neural transmission and serving as mechanoreceptors. It is concluded that the role of ICC as pacemakers may be well established, but other cells may also be involved in the generation of slow waves; the theory that ICC are mediators of neurotransmission is challenged by the majority of the in vivo motility studies; the hypothesis that ICC are mechanoreceptors has not found supportive evidence from the in vivo studies yet. More studies are needed to explain discrepancies in motility findings between the in vitro and in vivo experiments.

Are interstitial cells of Cajal plurifunction cells in the gut?

The proposed functions of the interstitial cells of Cajal (ICC) are to 1) pace the slow waves and regulate their propagation, 2) mediate enteric neuronal signals to smooth muscle cells, and 3) act as mechanosensors. In addition, impairments of ICC have been implicated in diverse motility disorders. This review critically examines the available evidence for these roles and offers alternate explanations. This review suggests the following: 1) The ICC may not pace the slow waves or help in their propagation. Instead, they may help in maintaining the gradient of resting membrane potential (RMP) through the thickness of the circular muscle layer, which stabilizes the slow waves and enhances their propagation. The impairment of ICC destabilizes the slow waves, resulting in attenuation of their amplitude and impaired propagation. 2) The one-way communication between the enteric neuronal varicosities and the smooth muscle cells occurs by volume transmission, rather than by wired transmission via the ICC. 3) There are fundamental limitations for the ICC to act as mechanosensors. 4) The ICC impair in numerous motility disorders. However, a cause-and-effect relationship between ICC impairment and motility dysfunction is not established. The ICC impair readily and transform to other cell types in response to alterations in their microenvironment, which have limited effects on motility function. Concurrent investigations of the alterations in slow-wave characteristics, excitation-contraction and excitation-inhibition couplings in smooth muscle cells, neurotransmitter synthesis and release in enteric neurons, and the impairment of the ICC are required to understand the etiologies of clinical motility disorders.

Interstitial cells of Cajal and neuromuscular transmission in the rat lower oesophageal sphincter

The distribution of interstitial cells of Cajal (ICC) and neurotransmission were investigated in lower oesophageal sphincter (LES) circular muscle strips from Sprague-Dawley (SD) rats, Ws/Ws mutant rats and their wild-type (+/+) siblings. Intramuscular c-Kit-positive cells, confirmed to be ICC-IM by electron microscopy, were observed throughout both muscle layers from SD and +/+ rats. In contrast, c-Kit-positive, ultrastructurally typical ICC-IM were absent in Ws/Ws. LES strips from Ws/Ws rats showed increased spontaneous contractile activity. Strips from SD and +/+ rats, responded to electrical neuronal stimulation with a relaxation that was in part L-NNA and in part apamin sensitive, followed by a contraction which was decreased by atropine. In Ws/Ws rats, similar to +/+ rats, neurally mediated relaxation was L-NNA and apamin sensitive and the contraction was decreased by atropine. We conclude that in the rat LES, relaxation is mediated by NO and an apamin-sensitive mediator, and contraction primarily by acetylcholine. Despite the absence of c-Kit-positive ICC, nerve-muscle interaction can be accomplished likely by diffusion of neurotransmitters to the smooth muscle cells. The lack of c-Kit-positive ICC is related to an increase in the basal tone and spontaneous contractile activity. The presence of fibroblast-like ICC in Ws/Ws rats might represent immature ICC whose possible functions need further investigation.

Pacemaker activity and inhibitory neurotransmission in the colon of Ws/Ws mutant rats

The aim of this study was to characterize the pacemaker activity and inhibitory neurotransmission in the colon of Ws/Ws mutant rats, which harbor a mutation in the c-kit gene that affects development of interstitial cells of Cajal (ICC). In Ws/Ws rats, the density of KIT-positive cells was markedly reduced. Wild-type, but not Ws/Ws, rats showed low- and high-frequency cyclic depolarization that were associated with highly regular myogenic motor patterns at the same frequencies. In Ws/Ws rats, irregular patterns of action potentials triggered irregular muscle contractions occurring within a bandwidth of 10-20 cycles/min. Spontaneous activity of nitrergic nerves caused sustained inhibition of muscle activity in both wild-type (+/+) and Ws/Ws rats. Electrical field stimulation of enteric nerves, after blockade of cholinergic and adrenergic activity, elicited inhibition of mechanical activity and biphasic inhibitory junction potentials both in wild-type and Ws/Ws rats. Apamin-sensitive, likely purinergic, inhibitory innervation was not affected by loss of ICC. Variable presence of nitrergic innervation likely reflects the presence of direct nitrergic innervation to smooth muscle cells as well as indirect innervation via ICC. In summary, loss of ICC markedly affects pacemaker and motor activities of the rat colon. Inhibitory innervation is largely maintained but nitrergic innervation is reduced possibly related to the loss of ICC-mediated relaxation.

Spontaneous electrical rhythmicity and the role of the sarcoplasmic reticulum in the excitability of guinea pig gallbladder smooth muscle cells

Spontaneous action potentials and Ca(2+) transients were investigated in intact gallbladder preparations to determine how electrical events propagate and the cellular mechanisms that modulate these events. Rhythmic phasic contractions were preceded by Ca(2+) flashes that were either focal (limited to one or a few bundles), multifocal (occurring asynchronously in several bundles), or global (simultaneous flashes throughout the field). Ca(2+) flashes and action potentials were abolished by inhibiting sarcoplasmic reticulum (SR) Ca(2+) release via inositol (1,4,5)-trisphosphate [Ins(1,4,5)P(3)] channels with 2-aminoethoxydiphenyl borate and xestospongin C or by inhibiting voltage-dependent Ca(2+) channels (VDCCs) with nifedipine or diltiazem or nisoldipine. Inhibiting ryanodine channels with ryanodine caused multiple spikes superimposed upon plateaus of action potentials and extended quiescent periods. Depletion of SR Ca(2+) stores with thapsigargin or cyclopiazonic acid increased the frequency and duration of Ca(2+) flashes and action potentials. Acetylcholine, carbachol, or cholecystokinin increased synchronized and increased the frequency of Ca(2+) flashes and action potentials. The phospholipase C (PLC) inhibitor U-73122 did not affect Ca(2+) flash or action potential activity but inhibited the excitatory effects of acetylcholine on these events. These results indicate that Ca(2+) flashes correspond to action potentials and that rhythmic excitation in the gallbladder is multifocal among gallbladder smooth muscle bundles and can be synchronized by excitatory agonists. These events do not depend on PLC activation, but agonist stimulation involves activation of PLC. Generation of these events depends on Ca(2+) entry via VDCCs and on Ca(2+) mobilization from the SR via Ins(1,4,5)P(3) channels.

In vivo recording of colonic motility in conscious rats with deficiency of interstitial cells of Cajal, with special reference to the effects of nitric oxide on colonic motility

BACKGROUND

We recorded in vivo colonic motility in rats with a deficiency of interstitial cells of Cajal (ICC) (Ws/Ws rats) and in wild-type rats (+/+ rats), with special reference to the effects of nitric oxide (NO) on colonic motility in both types of rats, in order to ascertain the role of ICC in colonic motility, and the relationship between NO and ICC in regard to colonic motility.

METHODS

Miniature strain-gauge force transducers were sutured on the surface of the ascending and sigmoid colon of Ws/Ws rats and +/+ rats as controls. After 1 week and a fasting period of 24 h, colonic motility in +/+ and Ws/Ws rats was recorded. We also studied the effect of NO on colonic motility in both types of rats, by means of the administration of N-nitro-L-arginine methyl ester (L-NAME) or L-arginine.

RESULTS

In +/+ rats, there were contractions with high amplitude and long duration in both the ascending and sigmoid colon. The number, amplitude, and duration of contractions in the ascending colon were 9.9/20 min, 6.1 g, and 22.7 s, respectively. These findings in the sigmoid colon were 5.2/20 min, 5.2 g, and 23.0 s, respectively. The number of contractions in the ascending and sigmoid colon in Ws/Ws rats (2.3 and 1.0/20 min) was significantly lower than that in +/+ rats (P < 0.05). The number of contractions in the ascending and sigmoid colon in +/+ rats (9.7 and 5.1/20 min before treatment) was significantly increased by L-NAME administration (28.7 and 13.9/40-60 min after treatment; P < 0.05), but that in Ws/Ws rats was not influenced. The number of contractions in the ascending and sigmoid colon in +/+ rats (10.2 and 5.2/20 min before treatment) was significantly decreased by L-arginine administration (3.6 and 2.1/40-60 min after treatment; P < 0.05), but that in Ws/Ws rats was not influenced.

CONCLUSIONS

ICC must be related to the occurrence of a normal number of colonic contractions. NO may be involved in the inhibitory regulation of colonic motility, and the effect of NO on the occurrence of contractions appears to be mediated by ICC.

Absence of peristalsis in the ileum of W/W(V) mutant mice that are selectively deficient in myenteric interstitial cells of Cajal

It is well known that the enteric nervous system plays a key role in the generation of gastrointestinal peristaltic movements. Recently, the networks of interstitial cells of Cajal (ICC) have been found to be essential in the generation of spontaneous gastrointestinal movements. However, the role of ICC in the mechanisms involved in the generation of peristaltic movements is still controversial. The aim of the present study was to reveal how pacemaker myenteric ICC (ICC-MY) and the enteric nervous system contribute to the mechanisms involved in the generation of intestinal peristalsis. We compared spontaneous peristaltic movements of the ileum in wild type (WT) mice with those in W/W(V) mutant mice which are selectively deficient in ICC-MY. Simultaneous recordings were made from both the circular and longitudinal muscle of a 4-cm long segment of ileum under hydrostatic pressure of 0--0.5 cm H(2)O. Mechanical activity and continuous video-images of the ileum were compared between WT and W/W(V) mutant mice under control conditions, in the presence of N-nitro-L-arginine methyl ester (L-NAME) and after tetrodotoxin (TTX). In the WT mouse ileum, peristaltic waves to propagate from the oral to the anal end were frequently observed. The frequency of these peristaltic waves and their associated synchronous longitudinal and circular muscle contractions was increased by L-NAME. The peristaltic waves were abolished by TTX. In the W/W(V) mutant mouse ileum, no peristaltic waves to propagate from the oral to the anal end were observed in control and even after L-NAME, although the local spontaneously generated longitudinal and circular muscle contractions were enhanced by L-NAME. These local contractions were not abolished by TTX. The results presented here suggested that ICC-MY are essential for the generation of spontaneous intestinal peristaltic movements. It is conceivable that ICC-MY may determine the polarity of the excitation of the intestine such that longitudinal and circular muscle contractions propagate from the oral to the anal end of the intestinal segments, although the question of why ICC-MY are necessary for the neural pathways remains unresolved.

Different modulation of spontaneous activities by nitrergic inhibitory nerves between ileum and jejunum in W/Wv mutant mice

We compared the spontaneous electrical and mechanical activities between the jejunum and ileum in the W/Wv mutant mouse, where ICC in the myenteric region (ICC-MY) are deficient. Electrical slow waves (SWs) superimposed with spike potentials, and synchronous circular and longitudinal muscle mechanical activities at a regular rhythm under approximately 1 cm H2O were recorded in the jejunum and ileum of wild-type mice. However, in the jejunum and ileum of W/Wv mice, irregular electrical and mechanical activities without discernable SWs were recorded. N-nitro-L-arginine methyl ester (L-NAME) significantly decreased the mean interval of longitudinal muscle contractions from 4.43+/-3.39 to 2.50+/-1.23 s in the ileum of W/W(V) mice. L-NAME also significantly decreased mean coefficient of variance (decreased irregularity) in the intervals from 2.59+/-0.84 to 0.48+/-0.46 in the ileum. Tetrodotoxin also significantly decreased mean interval and coefficient of variance in the ileum. Neither L-NAME nor tetrodotoxin affected contractile activity in the jejunum. These results suggest that enteric nitrergic nerves in the ileum, but not the jejunum, mediate a steady-state inhibition of myogenic activity in W/Wv mice.

Gut pacemaker cells: the interstitial cells of Cajal (ICC)

This review will focus on the pacemaker mechanisms underlying gastrointestinal autonomic rhythmicity in an attempt to elucidate the differences and similarities between the pacemaker mechanisms in the heart and gut. Interstitial cells of Cajal (ICC) form networks that are widely distributed within the submucosal (ICC-SM), intra-muscular (ICC-IM, ICC-DMP) and inter-muscular layers (ICC-MY) of the gastrointestinal tract from the esophagus to the internal anal sphincter. The ICC generate spontaneously active pacemaker currents that may be recorded as plateau and slow potentials. These pacemaker currents drive the spontaneous electrical and mechanical activities of smooth muscle cells. The enteric nervous system, composed of both the myenteric (inter-muscular) plexus and the submucosal plexus, is also distributed in the gastrointestinal tract from the esophagus to the internal anal sphincter. The role of the ICC and the enteric nervous system in the integrative control of gastrointestinal function and especially of spontaneous rhythmic activity, is still unknown. Nevertheless, at least from the results presented in this review of studies of the jejunum, ileum and proximal colon of the mouse, it is convincing that the ICC drive spontaneous rhythmic motility, although a role for the enteric nervous system in the regulation of spontaneous rhythmic motility cannot be overlooked. Furthermore, intracellular Ca2+ handling has a critical role in the generation of pacemaker activity in the gut and heart, although respective players such as the Ca2+-ATPase of the sarcoplasmic reticulum (endoplasmic reticulum), IP3 receptors, ryanodine receptors and plasma membrane ion channels may have divergent roles in the Ca2+-release refilling cycles. In conclusion, intracellular Ca2+ handling plays a key role in the gut pacemaker responsible for spontaneous rhythmicity, as well as in the cardiac pacemaker responsible for spontaneous beating. Pharmacotherapeutic targeting of intracellular Ca2+ handling mechanisms may be a promising approach to the treatment and cure of gut motility dysfunction.

Interstitial Cells in the Musculature of the Gastrointestinal Tract: Cajal and Beyond

Expression of the receptor tyrosine kinase KIT on cells referred to as interstitial cells of Cajal (ICC) has been instrumental during the past decade in the tremendous interest in cells in the interstitium of the smooth muscle layers of the digestive tract. ICC generate the pacemaker component (electrical slow waves of depolarization) of the smooth musculature and are involved in neurotransmission. By integration of ICC functions, substantial progress has been made in our understanding of the neuromuscular control of gastrointestinal motility, opening novel therapeutic perspectives. In this article, the ultrastructure and light microscopic morphology, as well as the functions and the development of ICC and of neighboring fibroblast-like cells (FLC), are critically reviewed. Directions for future research are considered and a unifying concept of mesenchymal cells, either KIT positive (the "ICC") or KIT negative "non-Cajal" (including the FLC and possibly also other cell types) cell types in the interstitium of the smooth musculature of the gastrointestinal tract, is proposed. Furthermore, evidence is accumulating to suggest that, as postulated by Santiago Ramon y Cajal, the concept of interstitial cells is not likely to be restricted to the gastrointestinal musculature.