Interstitial cells of Cajal mediate inhibitory neurotransmission in the stomach

Effect of lipopolysaccharide on small intestinal L-leucine transport in rabbit

In the present study, we have investigated whether the lipopolysaccharide (LPS) endotoxin from Escherichia coli is able to alter the jejunal transport of L-leucine when the tissue is exposed to endotoxin. The results have shown that the LPS at 3 x 10(-5) microg/ml decreases the uptake of L-leucine into the enterocyte, as well as the mucosal to serosal flux of L-leucine. The secretagogue effect of LPS on the gut did not affect the inhibitory effect of LPS on the intestinal absorption of the amino acid. The endotoxin did not modify amino acid diffusion across the intestinal epithelium. However, from the mediated transport, only the Na+-dependent transport system was affected by LPS with a diminution of the transporter affinity (the apparent Km was increased). In addition, we found a reduction of the Na+, K+-ATPase activity, which could explain the L-leucine Na+-dependent transport inhibition.

Interstitial cells of Cajal--their role in pacing and signal transmission in the digestive system

Interstitial cells of Cajal (ICC) are located in most parts of the digestive system. Although they were discovered over 100 years ago, their function began to be unravelled only recently. Morphological observations have led to a number of hypotheses on the possible physiological roles of ICC: (1) these cells may be the source of slow electrical waves recorded in gastrointestinal (GI) muscles; (2) they participate in the conduction of electrical currents, and (3) mediate neural signals between enteric nerves and muscles. These hypotheses were supported by experiments in which the ICC-containing layer was removed surgically, or when ICC were ablated chemically, and as a consequence the slow waves were absent. Electrophysiological experiments on isolated cells confirmed that ICC can generate rhythmic electrical activity and can also respond to messenger molecules known to be released from enteric nerves. In mice mutants deficient in ICC, or in mice treated with antibody against the protein c-Kit, slow wave activity was impaired. These results support the role of ICC as pacemaker cells. Physiological studies have shown that ICC in certain GI regions are important for signal transmission between nerves and smooth muscle. There is evidence that pathological changes in ICC may be associated with GI motility disorders. The full interpretation of the role of ICC in disease conditions will require much further study on the physiology and pharmacology of these cells.

Altered electrical activity in colonic smooth muscle cells from dystrophic (mdx) mice

Because the colon from dystrophic (mdx) mice shows an altered motor pattern, probably due to neural disorders, our aim was to examine the electrophysiological properties of muscle cells and the functionality of nitrergic transmission in circular muscle from normal and mdx colon. Normal colonic cells (resting membrane potential [RMP] about -50 mV) showed spontaneous hyperpolarizations (inhibitory junction potentials; IJPs) and cyclic slow depolarizations were sometimes recorded. Mdx colon had a depolarized RMP (about -36 mV) and spontaneous IJPs, but the cyclic activity was never observed. In the normal colon, Nomega-nitro-L-arginine methyl ester (L-NAME) induced depolarization and abolished the cyclic activity. In the mdx colon, L-NAME caused a slight depolarization. Both preparations displayed the same value of RMP in the presence of L-NAME. In normals, neural stimulation induced nonadrenergic, noncholinergic IJPs composed of fast hyperpolarizations followed by a nitrergic slow hyperpolarization, selectively abolished by L-NAME. In the mdx colon the evoked IJPs were composed only of the initial fast hyperpolarization, the nitrergic component being absent. The hyperpolarization to sodium nitroprusside was not significantly different in both preparations. We conclude that the colon from animals lacking in dystrophin displays different electrophysiological features because of an impairment of nitric oxide function.

Differential gene expression in the small intestines of wildtype and W/W(V) mice

Much of the evidence demonstrating the role of interstitial cells of Cajal (ICC) in pacemaking and neurotransmission in the gastrointestinal tract comes from studies of W/W(V) mice. These animals have few pacemaker ICC in the small bowel due to reduced functional Kit protein. We examined gene expression in the small intestines of wildtype and W/W(V) mice. RNA expression in the jejunums of wildtype and W/W(V) mutants was studied using a differential gene expression

METHOD

Seven known genes were differentially expressed in wildtype and W/W(V) mice. COX7B (cytochrome c oxidase, subunit VIIb) and SORCIN (encoding multidrug-resistance complex, class 4) were suppressed in both fed and fasted W/W(V) mice. Expression of another five genes was increased in W/W(V) mice: ADA (adenosine deaminase), MDH1 (malate dehydrogenase), RPL-8 (ribosomal protein L8), SPTB2 (spectrin, nonerythroid, beta subunit), and p6-5 (encoding phosphorylcholine [PC] T-cell suppressor factor [TsF]). Differential expression was the same in fasted and fed animals, suggesting that the differences were independent of the dietetic state. We conclude that several genes are differentially expressed in the small intestines of W/W(V) mice where the major lesion is loss of pacemaker ICC. Differential gene display may help develop a molecular profile of motility disorders in which ICC are lost.

Immunohistochemical demonstration of the NK(1) tachykinin receptor on muscle and epithelia in guinea pig intestine

BACKGROUND AND AIMS

Previous immunohistochemical studies failed to reveal neurokinin (NK)(1) tachykinin receptors on intestinal muscle, despite convincing pharmacologic data indicating their presence. This study aimed to apply optimal immunohistochemical methods to reveal the receptors.

METHODS

NK(1)-receptor immunoreactivity was examined by confocal microscopy in tissue incubated with or without 10(-7) mol/L substance P (SP), 10(-7) mol/L SP plus 10(-6) mol/L NK(1) receptor antagonist (CP99994), or with fluorescent cyanine 3.18 (Cy3) SP.

RESULTS

Without incubation, NK(1)-receptor immunoreactivity was strong on muscle of the rectum and distal colon and weak in proximal colon and small intestine. NK(1) receptor was located on the surface of muscle cells in all gut regions. Exposure to SP increased the intensity of immunoreactivity, and the receptor moved into the cytoplasm. Mobilization of the receptor by SP was blocked by the NK(1)-receptor antagonist CP99994. Cy3-SP was internalized by muscle cells and colocalized with the receptor. NK(1)-receptor immunoreactivity occurred on crypt epithelial cells in the small intestine and the base of glands in the proximal colon.

CONCLUSIONS

The NK(1) receptor occurs on the external muscle throughout the small and large intestines. SP binds and triggers NK(1)-receptor aggregation and internalization in the muscle.

Mechanical responses evoked by nerve stimulation in gastric muscles of mouse lacking inositol trisphosphate receptor

Alteration of mechanical responses elicited by transmural nerve stimulation (TNS) was investigated in pylorus muscle of stomach isolated from mutant mice lacking expression of IP, type-1 receptor. In wild and mutant mice. TNS inhibited spontaneous contractions and generated an off-response at the cessation. The effects of inhibitors of neurotransmission revealed that in wild mice, acetylcholine and nitric oxide were involved as excitatory and inhibitory mediators, respectively. In mutant mice, a lack of nitroxidergic component with associated attenuation of cholinergic transmission was found. The off-response was inhibited by apamin in both mice. In mutant mice, spantide-sensitive excitatory response appeared in the presence of apamin. Acetylcholine and substance P enhanced while noradrenaline and sodium nitroprusside inhibited spontaneous contractions, in both wild and mutant mice; the actions were weaker in mutant mice than in wild mice for any agonists. The results indicate that pylorus smooth muscles receive cholinergic excitatory and nitroxidergic and non-adrenergic non-cholinergic inhibitory projections, and a lack of IP, type-1 receptor results in an impairment of cholinergic and nitroxidergic components, with no alteration of non-adrenergic non-cholinergic inhibitory projections. In addition, the mutation induces a substance P projection which is not detected in wild mice.

Selective knockout of intramuscular interstitial cells reveals their role in the generation of slow waves in mouse stomach

1. Intracellular recording techniques were used to compare the patterns of electrical activity generated in the antral region of the stomachs of wild-type and W/W(V) mutant mice. Immunohistochemical techniques were used to determine the distribution of c-kit-positive interstitial cells of Cajal (ICC) within the same region of the stomach. 2. In wild-type mice interstitial cells were found at the level of the myenteric plexus (ICC(MY)) and distributed within the smooth muscle bundles (ICC(IM)). In these preparations slow waves, which consisted of initial and secondary components, were detected. 3. In W/WV mutant mice ICC(MY) could be identified at the level of the myenteric plexus but ICC(IM) were not detected within smooth muscle bundles. Intracellular recordings revealed that smooth muscle cells generated waves of depolarization; these lacked a secondary component. 4. These results indicate that the secondary regenerative component of a slow wave is generated by ICC(IM). Thus the depolarization arising from the pacemaker cells, ICC(MY), is augmented by ICC(IM), so causing a substantial membrane depolarization in the circular muscle layer. Rather than contributing directly to rhythmical electrical activity, smooth muscle cells appear to depolarize at the command of the two subpopulations of ICC.

Interstitial cells of Cajal: primary targets of enteric motor innervation

For many years morphologists have noted the close relationship between interstitial cells of Cajal (ICC) and nerve fibers within the tunica muscularis of gastrointestinal (GI) organs. These observations led to speculations about a role for ICC in mediating neural inputs to the GI tract. Immunohistochemical and functional studies demonstrated the presence of receptors for the neurotransmitters utilized by enteric motor neurons, and changes in second messengers in ICC after field stimulation of intrinsic enteric neurons showed that ICC were functionally innervated in GI muscles. Recent double labeling experiments have shown that both excitatory and inhibitory enteric motor neurons are closely associated with ICC in the deep muscular plexus (IC-DMP) of the small intestine and intramuscular ICC (IC-IM) of the proximal and distal GI tract. Enteric motor neurons form synaptic-like structures with IC-IM and IC-DMP. Far fewer close contacts are found between enteric motor neurons and smooth muscle cells. Experiments on W/W(V) mutants that lack IC-IM in the stomach, lower esophageal sphincter, and pylorus have shown that these ICC are critical components of the neuromuscular junction. Cholinergic excitatory and nitrergic inhibitory neurotransmission are severely decreased in tissues lacking IC-IM, yet there is no loss of cholinergic or nitrergic neurons in W/W(V) mutants. These data suggest that either the post-junctional mechanisms responsible for receiving and transducing neurotransmitter signals are specifically expressed by ICC, or that the large extracellular spaces typically between nerve terminals and smooth muscle cells may not allow effective concentrations of neurotransmitters to reach receptors expressed by smooth muscle cells. These findings indicate an important role for certain classes of ICC in enteric neurotransmission and predict that loss of ICC in human motor disturbances may significantly compromise neural regulation of GI motility.

Interstitial cells of Cajal: primary targets of enteric motor innervation

For many years morphologists have noted the close relationship between interstitial cells of Cajal (ICC) and nerve fibers within the tunica muscularis of gastrointestinal (GI) organs. These observations led to speculations about a role for ICC in mediating neural inputs to the GI tract. Immunohistochemical and functional studies demonstrated the presence of receptors for the neurotransmitters utilized by enteric motor neurons, and changes in second messengers in ICC after field stimulation of intrinsic enteric neurons showed that ICC were functionally innervated in GI muscles. Recent double labeling experiments have shown that both excitatory and inhibitory enteric motor neurons are closely associated with ICC in the deep muscular plexus (IC-DMP) of the small intestine and intramuscular ICC (IC-IM) of the proximal and distal GI tract. Enteric motor neurons form synaptic-like structures with IC-IM and IC-DMP. Far fewer close contacts are found between enteric motor neurons and smooth muscle cells. Experiments on W/W(V) mutants that lack IC-IM in the stomach, lower esophageal sphincter, and pylorus have shown that these ICC are critical components of the neuromuscular junction. Cholinergic excitatory and nitrergic inhibitory neurotransmission are severely decreased in tissues lacking IC-IM, yet there is no loss of cholinergic or nitrergic neurons in W/W(V) mutants. These data suggest that either the post-junctional mechanisms responsible for receiving and transducing neurotransmitter signals are specifically expressed by ICC, or that the large extracellular spaces typically between nerve terminals and smooth muscle cells may not allow effective concentrations of neurotransmitters to reach receptors expressed by smooth muscle cells. These findings indicate an important role for certain classes of ICC in enteric neurotransmission and predict that loss of ICC in human motor disturbances may significantly compromise neural regulation of GI motility.

Vagal afferent innervation of smooth muscle in the stomach and duodenum of the mouse: morphology and topography

Intraganglionic laminar endings (IGLEs) and intramuscular arrays (IMAs), the two putative mechanoreceptors that the vagus nerve supplies to the gastrointestinal smooth muscle, have been characterized almost exclusively in the rat. To provide normative inventories of these afferents for the mouse, the authors examined the endings in the stomach and small intestine of three strains used as backgrounds for gene manipulations (i.e., C57, 129/SvJ, and WBB6). Animals received nodose ganglion injections of wheat germ agglutinin-horseradish peroxidase or dextran-tetramethylrhodamine conjugated to biotin. The horseradish peroxidase tissue was processed with tetramethylbenzidine and was used to map the distributions and densities of the two endings; the dextran material was counterstained with c-Kit immunohistochemistry to assess interactions between intramuscular arrays and interstitial cells of Cajal. IGLEs and IMAs constituted the vagal innervation of mouse gastric and duodenal smooth muscle. IGLE morphology and distributions, with peak densities in the corpus-antrum, were similar in the three strains of mice and comparable to those observed in rats. IMAs varied in complexity from region to region but tended to be simpler (fewer telodendria) in mice than in rats. IMAs were most concentrated in the forestomach and sphincters in mice, as in rats, but the topographic distributions of the endings varied both between strains of mice (subtly) and between species (more dramatically). IMAs appeared to make appositions with both interstitial cells and smooth muscle fibers. This survey should make it practical to assay the effects of genetic (e.g., knockout) and experimental (e.g., regeneration) manipulations affecting visceral afferents and their target tissues.

A Novel Pacemaker Mechanism Drives Gastrointestinal Rhythmicity

Electric pacemaker activity drives peristaltic and segmental contractions in the gastrointestinal tract. Interstitial cells of Cajal (ICC) are responsible for spontaneous pacemaker activity. ICC remain rhythmic in culture and generate voltage-independent inward currents via a nonselective cation conductance. Ca(2+) release from endoplasmic reticulum and uptake by mitochondria initiates pacemaker currents. This novel mechanism provides the basis for electric rhythmicity in gastrointestinal muscles.

Tension and stretch receptors in gastrointestinal smooth muscle: re-evaluating vagal mechanoreceptor electrophysiology

Electrophysiological and morphological analyses of vagal mechanoreceptors in the gut wall suggest conflicting conclusions. Electrophysiology has distinguished a single general class of ending in smooth muscle, one characterized as an 'in series' tension receptor. Morphology, in contrast, has characterized two distinct specializations of vagal afferent endings in the muscle wall of the gastrointestinal (GI) tract. These two structures differ in terms of their target tissues, terminal architectures and regional distributions; they also develop on different ontogenetic timetables and depend on different trophic support in the muscle wall. On the basis of these features, we have proposed that one of the putative mechanoreceptors, the intraganglionic laminar ending (IGLE), has characteristics of a tension receptor and the other, the intramuscular array (IMA), has features of a stretch or length receptor. In a functional analogy with striated muscle proprioceptors, IGLEs should have similarities to Golgi tendon organs, whereas IMAs should have equivalencies with muscle spindle afferents. The present survey re-examines the recording analyses in light of the structural observations. This review indicates that previous electrophysiological studies are too inconclusive to refute the inference that the vagus supplies two distinct types of mechanoreceptors to the muscle wall of the GI tract. Multiple methodological constraints and sources of variance have limited the resolution of electrophysiological experiments. Specifically, these experiments have conventionally used distension stimuli that confound tension and stretch. In addition, sampling strategies have biased recording experiments towards a focus on one type of ending, the IGLE. Furthermore, putative functional properties (e.g., broad tuning) of vagal mechanoreceptors suggest that distinguishing two recording patterns will require exacting protocols. Combining a recognition of the methodological difficulties that have limited electrophysiological analyses with an understanding of the structural features of the endings, however, suggests several critical electrophysiological experiments with the resolution to distinguish two classes of response profiles. Until such experiments can be conducted, sensory physiology's axiom that 'function varies with form', taken together with a re-assessment of the existing data, suggests that the vagus nerve supplies stretch receptors as well as tension receptors to the wall of the GI tract.

Muscular innervation of the proximal duodenum of the guinea pig

We investigated the muscular structure and innervation of the gastroduodenal junction in the guinea pig. In the gastroduodenal junction, the innermost layer of the circular muscle contained numerous nerve fibers and terminals. Since this nerve network continued onto the deep muscular plexus (DMP) of the duodenum, we surmised that the numerous nerve fibers in the gastroduodenal junction were specialized DMP in the most proximal part of the duodenum. The innermost layer containing many nerve fibers was about 1,000 microm in length and 100 microm in thickness in the proximal duodenum. This layer contained numerous connective tissue fibers composed of collagen and elastic fibers. Five to 30 smooth muscle cells lay in contact with each other and were surrounded by fine connective tissue. The nerve fibers in the proximal duodenum contained nerve terminals immunoreactive for choline acetyltransferase, dynorphin, enkephalin, galanin, gastrin-releasing peptide, nitric oxide synthase, substance P, and vasoactive intestinal polypeptide. Adrenergic fibers which contained tyrosine hydroxylase immunoreactivity were rare in the proximal duodenum. In the innermost layer of the proximal duodenum, there were numerous c-Kit immunopositive cells that were in contact with nerve terminals. This study allowed us to clarify the specific architecture of the most proximal portion of the duodenum. The functional significance of the proximal duodenum in relation to the electrical connection and neural cooperation of the musculature between the antrum and the duodenum is also discussed.

Vagal inhibition in the antral region of guinea pig stomach

The effects of vagal stimulation in the presence of a muscarinic antagonist were examined on three distinct rhythmically active cells located in guinea pig antrum. Vagal stimulation inhibited contractions of the circular muscle layer but did not change their rate of occurrence. With the use of intracellular recording techniques, these stimuli were found to initiate inhibitory junction potentials in the circular layer but produced smaller potential changes in driving and follower cells. Inhibition of the circular muscle layer involved two separate components. The dominant component was independent of changes in membrane potential and was abolished by nitro-L-arginine. After abolishing Ca(2+) entry into smooth muscle cells with a Ca(2+) antagonist, vagal stimulation continued to inhibit the residual contractions associated with each slow wave. When the cyclic changes in intracellular Ca(2+) concentration associated with each slow wave were measured, they were found to be unchanged by vagal stimulation. The observations suggest that vagal inhibition of stomach movements does not alter pacemaker activity in the stomach; rather, it results from a change in the sensitivity of smooth muscle contractile proteins to Ca(2+).

Molecular markers expressed in cultured and freshly isolated interstitial cells of Cajal

Located within the tunica muscularis of the gastrointestinal (GI) tract are networks of cells known as interstitial cells of Cajal (ICC). ICC are critical for important basic functions of GI motility such as generation and propagation of slow-wave pacemaker activity and reception of regulatory inputs from the enteric nervous system. We have developed a novel procedure to identify and isolate individual ICC from freshly dispersed cell preparations of the murine small intestine and gastric fundus and to determine differential transcriptional expression We have compared the expression profiles of pacemaker ICC isolated from the murine small intestine (IC-MY) and ICC involved in neurotransmission from the gastric fundus (IC-IM). We have also compared expression profiles between ICC and smooth muscle cells (SMC) and between freshly isolated ICC and cultured ICC. Cultured ICC express smooth muscle myosin, whereas freshly dispersed ICC do not. All cell types express muscarinic receptor types M(2) and M(3), neurokinin receptors NK(1) and NK(3), and inhibitory receptor VIP-1, whereas only cultured ICC and SMC express VIP-2. Both cultured and freshly dispersed IC-IM and IC-MY express the soluble form of stem cell factor, whereas SMC from the gastric fundus express only the membrane-bound form.

Arginosuccinate synthetase, arginosuccinate lyase and NOS in canine gastrointestinal tract: immunocytochemical studies

Nitric oxide synthase (NOS) requires the substrate L-arginine for NO production to support multiple gastrointestinal functions. We asked, 'Where do enzymes to regenerate L-arginine from L-citrulline exist?'. We examined loci of immunoreactivities in the canine gastrointestinal tract for arginosuccinate synthetase and arginosuccinate lyase, enzymes that resynthesize L-arginine from L-citrulline, in relation to the distribution of nNOS immunoreactivity or NADPH-diaphorase histochemistry. Arginosuccinate synthetase and lyase were present in many neurones and nerve fibres in the myenteric plexus of the lower oesophageal sphincter (LOS), antrum, pylorus, ileum and colon; in the submucosal plexus of ileum and colon; in longitudinal muscle of ileum and colon; and in nerve bundles in circular muscle everywhere. LOS muscle was also immunoreactive for both enzymes. Circular and longitudinal muscle cells of the ileum and colon and cells resembling interstitial cells of Cajal in the deep muscular plexus of the ileum and the submuscular plexus of the colon also appeared immunoreactive. In neurones, arginosuccinate synthetase and nNOS were usually co-localized. NADPH diaphorase activity was present in LOS and likely in pylorus, but not in muscularis externa of ileum or colon. We conclude that resynthesis of L-arginine probably occurs in enteric nerves, interstitial cells of Cajal (ICC) and LOS muscle; also apparently in some cells without NOS to utilize it.

Cellular mechanisms of myogenic activity in gastric smooth muscle

In many regions of the intestine, a thin layer of interstitial cells of Cajal (ICC) lie in the myenteric region, between the circular and longitudinal muscle layers. ICC are connected by gap junctions to surrounding ICC and also with circular and longitudinal smooth muscle cells, forming a large electrical syncytium. Damage of the ICC causes a disorder in the patterns of rhythmic activity. Isolated ICC produce a rhythmic oscillation of the membrane potential. All these observations have led to the suggestion that ICC may be the pacemaker cell responsible for intestinal activity. Gastric smooth muscles generate slow oscillatory membrane potential changes (slow waves) and spike potentials. The activity is considered to be linked to the metabolism in the cell. Three types of cells located in the gastric wall (circular and longitudinal smooth muscle cells and ICC) produce synchronized electrical responses with different shapes. The electrical responses appear to originate in ICC and then spread to the smooth muscle layers, indicating that ICC may also be the pacemaker cells responsible for gastric activity. However, isolated circular smooth muscle tissues spontaneously generate regenerative potentials, suggesting that there are at least two sites for the initiation of spontaneous activity in the stomach. Regenerative potentials persist in the presence of Ca-antagonists and are inhibited by agents which disrupt intracellular Ca(2+) homeostasis. Depolarization of the membrane elicits regenerative potentials after a long delay and the potentials have long refractory periods. This suggests that an unidentified 2nd messenger may be formed during the delay between membrane depolarization and the initiation of a regenerative potential. In gastric muscles of mutant mice which do not express inositol trisphosphate (InsP(3)) receptors, spike potentials but not slow waves are generated, suggesting the possible involvement of InsP(3) in the initiation of spontaneous activity.

Ultrastructural observations of fibroblast-like cells forming gap junctions in the W/W(nu) mouse small intestine

The ultrastructure of the wild-type (+/+) mice small intestine was compared with c-kit mutant (W/W(nu)) mice which only have few interstitial cells of Cajal (ICC) associated with Auerbach's plexus, in order to elucidate whether the specialized membrane contacts are general features of so-called fibroblast-like cells that are widely distributed in the tunica muscularis of the alimentary tract. Fibroblast-like cells in the Auerbach region were found in approximately equal number in W/W(nu) mice as in +/+ mice, while ICC associated with Auerbach's plexus (ICC-AP) could not be demonstrated in W/W(nu) mice in the present investigation. Fibroblast-like cells were characterized by cytoplasm of moderate to high electron density, well developed rough endoplasmic reticulum and nuclei with thick peripheral accumulations of heterochromatin. There were no basal lamina and caveolae along the cell membrane. It was observed that single fibroblast-like cells formed probable small gap junctions with muscle cells of both circular and longitudinal layers. Fibroblast-like cells with the same features were also observed in the region of the deep muscular plexus in both +/+ and W/W(nu) mice. The present observation, together with our previous studies on rats and guinea-pigs, suggest the common presence of gap junctions or gap junction-like structures on fibroblast-like cells in the gastrointestinal musculature and their involvement in the regulatory system of gastrointestinal motility by passing electrical or molecular signals to influence the state of muscle tonus.

Idiopathic gastric perforation in neonates and abnormal distribution of intestinal pacemaker cells

BACKGROUND/PURPOSE

The etiology of idiopathic gastric perforation (IGP) in neonates is unclear. Interstitial cells of Cajal (ICC) express tyrosine kinase receptor C-kit, and act as gastrointestinal pacemaker cells. Stem cell factor (SCF) is a C-kit ligand and plays an important role in immune system homeostasis in the gastrointestinal tract. The authors hypothesized that abnormal distribution of ICC or SCF in the gastric wall (ie, abnormal motility or impaired immunity) could predispose the stomach to IGP.

METHODS

Stomachs obtained at postmortem from neonates who died of IGP (n = 7) and other causes (control group; n = 10) were used. Biopsy sections were taken at random from various sites in the stomach, including macroscopically intact areas, and labeled immunohistochemically using antibodies to C-kit(a marker for ICC) and SCF.

RESULTS

In all control specimens, ICC were present between the muscle layers and around the myenteric plexuses of the stomach wall. In contrast, ICC were absent in all biopsy sections from 3 of the 7 IGP stomachs. In the remaining 4 IGP stomachs, there were fewer ICC in the muscle layers compared with controls, and ICC were absent around the myenteric plexuses. The distribution of SCF immunoreactivity in IGP and control specimens was similar.

CONCLUSION

The findings suggest that a lack of ICC (ie, gastric hypomotility) may be implicated in the etiology of IGP in neonates.

Differences in circular muscle contraction and peristaltic motor inhibition caused by tachykinin NK1 receptor agonists in the guinea-pig small intestine

The tachykinin NK1 receptor agonist substance P methyl ester (SPOME) impedes intestinal peristalsis by releasing nitric oxide (NO) from inhibitory motor neurones. Since NK1 receptor agonists differ in their receptor interaction, we set out to compare a range of NK1 receptor agonists including SPOME, septide and GR-73 632 in their effects on propulsive peristalsis and circular muscle activity in the guinea-pig isolated small intestine. SPOME (100-300 nM) inhibited peristalsis by a rise of the pressure threshold at which peristaltic waves were triggered, whereas septide and GR-73 632 (30-300 nM) interrupted peristalsis by causing circular muscle spasms. Separate experiments showed that all three NK1 receptor agonists caused contraction of the circular muscle, which was enhanced by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (300 mM) and the P2X purinoceptor antagonist suramin (300 mM). In contrast, tetrodotoxin (300 nM) augmented the contractile effect of septide and GR-73 632 but not that of SPOME. It is concluded that the motor response to NK1 receptor agonists involves release of NO and adenosine triphosphate from inhibitory motor neurones. However, the NK1 receptor agonists differ in the mechanism by which they cause inhibitory transmitter release, which corresponds to differences in their antiperistaltic action.

Interstitial cells of Cajal mediate cholinergic neurotransmission from enteric motor neurons

Interstitial cells of Cajal (ICC) are interposed between enteric neurons and smooth muscle cells in gastrointestinal muscles. The role of intramuscular ICC (IC-IM) in mediating enteric excitatory neural inputs was studied using gastric fundus muscles of wild-type animals and W/W(v) mutant mice, which lack IC-IM. Excitatory motor neurons, labeled with antibodies to vesicular acetylcholine transporter or substance-P, were closely associated with IC-IM. Immunocytochemistry showed close contacts between enteric neurons and IC-IM. IC-IM also formed gap junctions with smooth muscle cells. Electrical field stimulation yielded fast excitatory junction potentials in the smooth muscle that were blocked by atropine. Neural responses were greatly reduced in muscles of W/W(v) animals. Loss of cholinergic responses in W/W(v) muscles seemed to be caused by the loss of close synaptic contacts between motor neurons and IC-IM, because these muscles were not less responsive to exogenous acetylcholine than were wild-type muscles. W/W(v) muscles also responded to excitatory nerve stimulation when the breakdown of acetylcholine was blocked by neostigmine. The density of cholinergic nerve bundles within the muscles was not significantly different in wild-type and W/W(v) muscles, and similar amounts of (14)[C]choline were released from preloaded wild-type and W/W(v) muscles in response to nerve stimulation. The impact of losing IC-IM on gastric compliance was also evaluated in intact stomachs. Pressure increased as a function of fluid volume and infusion rate in wild-type animals, but W/W(v) animals showed little basal tone and minimal increases in pressure with fluid infusions. These data suggest that IC-IM play a major role in receiving cholinergic excitatory inputs from the enteric nervous system in the murine fundus.

Interstitial cells of Cajal mediate cholinergic neurotransmission from enteric motor neurons

Interstitial cells of Cajal (ICC) are interposed between enteric neurons and smooth muscle cells in gastrointestinal muscles. The role of intramuscular ICC (IC-IM) in mediating enteric excitatory neural inputs was studied using gastric fundus muscles of wild-type animals and W/W(v) mutant mice, which lack IC-IM. Excitatory motor neurons, labeled with antibodies to vesicular acetylcholine transporter or substance-P, were closely associated with IC-IM. Immunocytochemistry showed close contacts between enteric neurons and IC-IM. IC-IM also formed gap junctions with smooth muscle cells. Electrical field stimulation yielded fast excitatory junction potentials in the smooth muscle that were blocked by atropine. Neural responses were greatly reduced in muscles of W/W(v) animals. Loss of cholinergic responses in W/W(v) muscles seemed to be caused by the loss of close synaptic contacts between motor neurons and IC-IM, because these muscles were not less responsive to exogenous acetylcholine than were wild-type muscles. W/W(v) muscles also responded to excitatory nerve stimulation when the breakdown of acetylcholine was blocked by neostigmine. The density of cholinergic nerve bundles within the muscles was not significantly different in wild-type and W/W(v) muscles, and similar amounts of (14)[C]choline were released from preloaded wild-type and W/W(v) muscles in response to nerve stimulation. The impact of losing IC-IM on gastric compliance was also evaluated in intact stomachs. Pressure increased as a function of fluid volume and infusion rate in wild-type animals, but W/W(v) animals showed little basal tone and minimal increases in pressure with fluid infusions. These data suggest that IC-IM play a major role in receiving cholinergic excitatory inputs from the enteric nervous system in the murine fundus.

The search for the origin of rhythmicity in intestinal contraction; from tissue to single cells

More than a century ago, rhythmic propulsive contractile activity was observed in the intestine after blockade of nerve conduction, thus demonstrating a form of peristalsis that appeared to be under myogenic control. During this century, light and electron microscopic investigations provided the hypothesis that interstitial cells of Cajal (ICC) could be the cells of origin for this rhythmicity. In recent years, physiological studies demonstrated a link between the presence of electrical slow wave activity and the presence of ICC. The recognition that the ICC cell membrane harbours the Kit protein sparked rapid advancement in ICC research, and has been essential in the identification of ICC in tissue and in culture through Kit immunohistochemistry and kit mRNA reverse transcriptase polymerase chain reaction (RT-PCR). With these techniques, electrophysiology was carried out on positively identified single ICC in culture. These methods revealed that single ICC generate spontaneous rhythmic inward currents and slow waves in membrane potential, thus providing strong evidence that ICC generate the electrical pacemaker activity for the gut musculature.

Interstitial cells of Cajal in human gut and gastrointestinal disease

This paper reviews the distribution of interstitial cells of Cajal (ICC) in the human gastrointestinal (GI) tract, based on ultrastructural and immunohistochemical evidence. The distribution and morphology of ICC at each level of the normal GI tracts is addressed from the perspective of their functional significance. Alterations of ICC reported in achalasia of cardia, infantile hypertrophic pyloric stenosis, chronic intestinal pseudoobstruction, Hirschsprung's disease, inflammatory bowel diseases, slow transit constipation, and some other disorders of GI motility as well as in gastrointestinal stromal tumors are reviewed, with emphasis on the place of ICC in the pathophysiology of disease.

Impaired relaxation of stomach smooth muscle in mice lacking cyclic GMP-dependent protein kinase I

1. Guanosine 3', 5'-cyclic monophosphate (cyclic GMP)-dependent kinase I (cGKI) is a major receptor for cyclic GMP in a variety of cells. Mice lacking cGKI exhibit multiple phenotypes, including severe defects in smooth muscle function. We have investigated the NO/cGMP- and vasoactive intestinal polypeptide (VIP)/adenosine 3', 5'-cyclic monophosphate (cyclic AMP)-signalling pathways in the gastric fundus of wild type and cGKI-deficient mice. 2. Using immunohistochemistry, similar staining patterns for NO-synthase, cyclic GMP- and VIP-immunoreactivities were found in wild type and cGKI-deficient mice. 3. In isolated, endothelin-1 (3 nM - 3 microM)-contracted, muscle strips from wild type mice, electrical field stimulation (1 - 16 Hz) caused a biphasic relaxation, one initial rapid, followed by a more slowly developing phase. In preparations from cGKI-deficient mice only the slowly developing relaxation was observed. 4. The responses to the NO donor, SIN-1 (10 nM - 100 microM), and to 8-Br-cyclic GMP (10 nM - 100 microM) were markedly impaired in strips from cGKI-deficient mice, whereas the responses to VIP (0.1 nM - 1 microM) and forskolin (0.1 nM - 1 microM) were similar to those in wild type mice. 5. These results suggest that cGKI plays a central role in the NO/cGMP signalling cascade producing relaxation of mouse gastric fundus smooth muscle. Relaxant agents acting via the cyclic AMP-pathway can exert their effects independently of cGKI.

Gap junctions in intestinal smooth muscle and interstitial cells of Cajal

This manuscript reviews gap junctions' roles in control of intestinal motility. Gap junctions (GJs) of small intestine (SmIn) are found between circular muscle (CM) cells, between interstitial cells of Cajal (ICC) of deep muscular plexus (DMP) and between them and adjacent outer circular muscle (OCM). GJs between longitudinal muscle (LM) cells or between cells of inner circular muscle (ICM) have not been reported. Occasional GJs have been reported between ICC of the myenteric plexus (MyP) and rarely between these ICC and adjacent LM or CM cells, or between ICC within CM and smooth muscle cells. In the colon (Co) of several species a special network of ICC lines the inner border of CM, the submuscular plexus (SP). GJs are found between ICCs and between them and CM cells. The ICC of MyP of Co are associated with LM and CM; occasional GJs exist between ICC and each muscle layer. Small GJs are missed by electron microscopy or light microscopic Immunocytochemistry. Therefore, GJ coupling may exist without demonstrated GJs. The consequences for the pacemaking functions of ICC networks of varied densities of GJ between ICC and between ICC of MyP or DMP or of SP and CM are considered. Connexins (Cxs) that compose intestinal GJs may affect coupling, but are incompletely known. Understanding of the role of GJs in coordinating intestinal motility requires knowing: (1) what passes through gap junctions to couple ICC to smooth muscle cells; (2) what Cx with what conductances and what modulatory controls connect ICC and smooth muscle cells; (3) whether smooth muscles can generate slow waves independent of ICC networks; and (4) what happens to motility, slow waves, and IJPs when GJs are selectively uncoupled.

Receptors in interstitial cells of Cajal: identification and possible physiological roles

Interstitial cells of Cajal (ICCs) are specialized cells of the gastrointestinal tract forming distinct populations depending on their location in the gut wall. Morphological observations and functional data have led to the hypothesis of two functions for the ICCs: (1) as pacemakers of the rhythmic activity; (2) as intermediaries in neural inputs to the muscle. The identification of specific receptors on the ICCs has represented an important step in the knowledge of these cells. Immunohistochemical labeling of these receptors provided information on both ICC morphology and contacts (particularly those with nerve endings) and on the functions of these cells. All ICC possess the Kit receptor, which represents the best tool to identify these cells under the light microscope. It has been demonstrated that this receptor is essential for ICC differentiation, and, by using mutant mice lacking the Kit-related gene, it has been possible to discriminate among all the ICC those with a primary role as pacemakers. The ileal ICC, in particular those at the deep muscular plexus, express the tachykinin receptor NK1 and a subtype of somatostatin receptors and contain nitric oxide synthase. All these data support a primary role of these ICC in neural transmission.

Interstitial cells of Cajal in human gut and gastrointestinal disease

This paper reviews the distribution of interstitial cells of Cajal (ICC) in the human gastrointestinal (GI) tract, based on ultrastructural and immunohistochemical evidence. The distribution and morphology of ICC at each level of the normal GI tracts is addressed from the perspective of their functional significance. Alterations of ICC reported in achalasia of cardia, infantile hypertrophic pyloric stenosis, chronic intestinal pseudoobstruction, Hirschsprung's disease, inflammatory bowel diseases, slow transit constipation, and some other disorders of GI motility as well as in gastrointestinal stromal tumors are reviewed, with emphasis on the place of ICC in the pathophysiology of disease.

Visualization of interstitial cells of Cajal in living, intact tissues

Interstitial cells of Cajal (ICC) appear to be a major element in pacing and signal transmission in the gastrointestinal tract. A prominent problem in the study of ICC has been the difficulty in observing them in intact tissues. We used several methods to visualize living ICC in freshly-dissected tissues: (1) Placing small crystals of the lipophilic dye DiI in the submucosal-circular muscle border in the mouse colon resulted in the labeling of living ICC-like cells. Two main morphological cell types, bipolar and multipolar, were noted. The DiI stain could be converted into a stable, electron-opaque product. Electron-microscopic observations showed that the labeled cells had the typical appearance of ICC reported in previous studies. (2) Living ICC in the region of the myenteric plexus (ICC-MP) in the small intestines of mice and guinea-pigs were observed with Nomarski optics. This enabled the visualization of ICC in living tissues, and the impalement of the cells with Lucifer yellow-filled microelectrodes. The dye-labeled cells had the morphological features of ICC-MP, and about 30% of them were found to be dye coupled to 1-21 other ICC. The identity of the cells as ICC was verified by electron-microscopy following photoconversion, and by c-kit immunohistochemistry. (3) Living ICC were labeled with a c-kit antibody that does not require tissue fixation. This resulted in the fluorescent staining of the entire ICC network. Single cells were labeled by dye injection, which provided a detailed picture of ICC morphology. This method was found to be suitable for a wide range of tissues. We expect that these three methods for identifying ICC in intact, living tissues will be useful for physiological and pharmacological investigations of ICC in a variety of gastrointestinal tissues.

One hundred years of interstitial cells of Cajal

This review is a portrayal of the evolution of ideas involving the interstitial cells of Cajal in changing disguises as dull fibroblasts, not very exciting Schwann cells, or perhaps quite important, though primitive neurons. However, today unmasked (we believe), they reveal themselves as myoid cells, a role that, judging by current interest, is far more exciting than former ones. Close to 500 publications from 1860-1999 have contributed to the discussion in one way or the other. This literature contains a wealth of correct observations but obviously also wrong interpretations, which are seen as a result of too blind a belief in specificities of visualization methods, combined with a desire to interpret even the hidden detail. It has been my objective to attempt to trace the origins of viable ideas, and I have therefore focused on relatively few authors. The most recent development from 1980 until today is so well covered by easily accessible reviews that I have resorted to a mere, but hopefully complete, list of them. Modern ICC'ists have so far been caught in the external muscle of the gut and kept their hands off its internal affairs. However, while working my way through the literature it struck me that a number of recent studies may provide the elements of a plausible model for the villous contraction mechanism. In the present context, an important point is that the very first published interstitial "neurons" from Cajal's hand-of the intestinal villus, 1889-may achieve new significance as a possible correlate to the regulatory ICC of the intestinal muscularis. Partly to make this point, I have taken the liberty of giving a short account of recent results from our lab.

Guide to the identification of interstitial cells of Cajal

The interstitial cell of Cajal, abbreviated ICC, is a specific cell type with a characteristic distribution in the smooth muscle wall throughout the alimentary tract in humans and laboratory mammals. The number of publications relating to ICC is rapidly increasing and demonstrate a rich variation in the structure and organization of these cells. This variation is species-, region-, and location-dependent. We have chosen to define a "reference ICC," basically the ICC in the murine small intestine, as a platform for discussion of variability. The growing field of ICC markers for light and electron microscopy is reviewed. Although there is a rapidly increasing number of approaches applicable to bright field and fluorescence microscopy, the location of markers by electron microscopy still suffers from inadequate preservation of ultrastructural detail. Finally, we summarize evidence related to ICC ultrastructure under conditions differing from those of the normal, adult individual (during differentiation, in pathological conditions, transplants, mutants, and in cell culture).

Development and plasticity of interstitial cells of Cajal

Interstitial cells of Cajal (ICC) are the pacemakers in gastrointestinal (GI) muscles, and these cells also mediate or transduce inputs from the enteric nervous system. Different classes of ICC are involved in pacemaking and neurotransmission. ICC express specific ionic conductances that make them unique in their ability to generate and propagate slow waves in GI muscles or transduce neural inputs. Much of what we know about the function of ICC comes from developmental studies that were made possible by the discoveries that ICC express c-kit and proper development of ICC depends upon signalling via the Kit receptor pathway. Manipulating Kit signalling with reagents to block the receptor or downstream signalling pathways or by using mutant mice in which Kit or its ligand, stem cell factor, are defective has allowed novel studies into the specific functions of the different classes of ICC in several regions of the GI tract. Kit is also a surface antigen that can be used to conveniently label ICC in GI muscles. Immunohistochemical studies using Kit antibodies have expanded our knowledge about the ICC phenotype, the structure of ICC networks, the interactions of ICC with other cells in the gut wall, and the loss of ICC in some clinical disorders. Preparations made devoid of ICC have also allowed analysis of the consequences of losing specific classes of ICC on GI motility. This review describes recent advances in our knowledge about the development and plasticity of ICC and how developmental studies have contributed to our understanding of the functions of ICC. We have reviewed the clinical literature and discussed how loss or defects in ICC affect GI motor function.

Control systems of gastrointestinal motility are immature at birth in dogs

Networks of interstitial cells of Cajal (ICC) in the myenteric plexus (Myp) or circular muscle (CM) function as pacemakers for gastrointestinal slow waves. ICC in contact with muscle and closely associated with nerves in the CM may mediate inhibitory neurotransmission. We wondered if ICC in Myp and CM and their connections are immature at birth and mature first in the proximal gut in association with nerves. Tissues from lower esophageal sphincter (LES), pylorus (PYL), small intestine (SI) and colon (CO) of 18 term fetal dogs taken from six females were fixed and prepared for ultrastructural examination and studied. Ganglia were present where expected in the Myp and submucous plexus (SMP). ICC cells were present in the Myp of PYL, SI and CO and appeared to have normal relationships to the outer border of CM as in adults. ICC in CM were found associated with nerves in the LES and in PYL, but not in SI or CO. However, axons in CM were everywhere usually free of glial covering, indicating ongoing migration or development. No organized deep muscular plexus (DMP) in SI or submuscular plexus (SP) in colon was present. Visible gap junctions were absent everywhere except for very rare ones between circular muscle cells. We conclude that at birth the neural and ICC networks of CM are more immature in intestine and colon than in oesophagus and stomach. Development of nerve and ICC of CM in oesophagus and stomach apparently precedes that in the remaining gut. However networks in these regions have not achieved adult organization and ICC and smooth muscle cells are anatomically poorly coupled. These findings suggest the reasons that gut motility at birth will not be adult in pattern are because ICC, nerve and muscle control systems are not fully differentiated. Further developmental delays in ICC and nerve maturation could have serious consequences for feeding of infant animals.

Development of interstitial cells of Cajal and pacemaking in mice lacking enteric nerves

BACKGROUND & AIMS

Development of interstitial cells of Cajal (ICC) requires signaling via Kit receptors. Kit is activated by stem cell factor (SCF), but the source of SCF in the bowel wall is unclear and controversy exists about whether enteric neurons express the SCF required for ICC development.

METHODS

Glial cell line-derived neurotrophic factor (GDNF) knockout mice, which lack enteric neurons throughout most of the gut, were used to determine whether neurons are necessary for ICC development. ICC distributions were determined with Kit immunofluorescence, and function of ICC was determined by intracellular electrical recording.

RESULTS

ICC were normally distributed throughout the gastrointestinal tracts of GDNF-/- mice. Intracellular recordings from aganglionic gastrointestinal muscles showed normal slow wave activity at birth in the stomach and small intestine. Slow waves developed normally in aganglionic segments of small bowel placed into organ culture at birth. Quantitative polymerase chain reaction showed similar expression of SCF in the muscles of animals with and without enteric neurons. Expression of SCF was demonstrated in isolated intestinal smooth muscle cells.

CONCLUSIONS

These data suggest that enteric neurons are not required for the development of functional ICC. The circular smooth muscle layer, which develops before ICC, may be the source of SCF required for ICC development.

Heme oxygenase immunoreactive neurons in the rat intestine and their relationship to nitrergic neurons

BACKGROUND

Carbon monoxide (CO), like nitric oxide (NO), is a putative gaseous neurotransmitter. CO is produced by the enzyme heme oxygenase (HO) acting on a family of heme-containing compounds. Two isomers of HO have been characterized (HO-1, HO-2). In the CNS and in peripheral ganglia HO-2 occurs in a majority of neurons. NO and CO function as transmitters of enteric neurons but the relative distribution of enteric neurons utilizing these gaseous transmitters is unknown in rodent. We have studied the distribution of HO-2 immunoreactivity and NO synthase (NOS) activity within the rat ileum.

METHODS

Tissue sections and primary neuronal cell cultures were incubated with a HO-2 specific antibody, and then assessed or reprocessed for NOS activity using NADPH-dependent diaphorase staining.

RESULTS

HO-2 immunoreactivity was expressed in subpopulations of myenteric and submucosal neurons. Approximately 45% of the ganglion cells in tissue section were HO-2 positive. This was similar in proportion to those found to stain for NOS activity, and 10% of HO-2 positive neurons also contained NOS. HO-2 immunoreactivity was also found in epithelial cells within the villi, and in interstitial cells around the myenteric plexus and within the smooth muscle. In culture, the distribution and colocalisation of HO-2 and NOS positive neurons was similar to that in tissue sections. We identified labelled neurons as either Dogiel Type I or II; only Type II cells colocalized NOS and HO-2.

CONCLUSION

Neurons, endocrine-like cells and interstitial cells with the capacity for CO production are distributed throughout the ileum and some neurons have the capacity to synthesize both NO and CO as gaseous messengers.

Interstitial cells of cajal generate electrical slow waves in the murine stomach

1. The gastric corpus and antrum contain interstitial cells of Cajal (ICC) within the tunica muscularis. We tested the hypothesis that ICC are involved in the generation and regeneration of electrical slow waves. 2. Normal, postnatal development of slow wave activity was characterized in tissues freshly removed from animals between birth and day 50 (D50). Slow wave amplitude and frequency increased during this period. Networks of myenteric ICC (IC-MY) were present in gastric muscles at birth and did not change significantly in appearance during the period of study as imaged by confocal immunofluorescence microscopy. 3. IC-MY networks were maintained and electrical rhythmicity developed in organ culture in a manner similar to normal postnatal development. Electrical activity was maintained for at least 48 days in culture. 4. Addition of a neutralizing antibody (ACK2) for the receptor tyrosine kinase, Kit, to the culture media caused progressive loss of Kit-immunoreactive cells. Loss of Kit-immunoreactive cells was associated with loss of slow wave activity. Most muscles became electrically quiescent after 3-4 weeks of exposure to ACK2. 5. In some muscles small clusters of Kit-immunoreactive IC-MY remained after culturing with ACK2. These muscles displayed slow wave activity but only in the immediate regions in which Kit-positive IC-MY remained. These data suggest that regions without Kit-immunoreactive cells cannot generate or regenerate slow waves. 6. After loss of Kit-immunoreactive cells, the muscles could not be paced by direct electrical stimulation. Stimulation with acetylcholine also failed to elicit slow waves. The data suggest that the generation of slow waves is an exclusive property of IC-MY; smooth muscle cells may not express the ionic apparatus necessary for generation of these events. 7. We conclude that IC-MY are an essential element in the spontaneous rhythmic electrical and contractile activity of gastric muscles. This class of ICC appears to generate slow wave activity and may provide a means for regeneration of slow waves.

An immunohistochemical study of interstitial cells of Cajal (ICC) in the equine gastrointestinal tract

The interstitial cells of Cajal (ICC) are c-kit immunoreactive cells of the gastrointestinal tract which are suggested to have a role in the control of intestinal motility. Cells with c-kit immunoreactivity have not been previously described in the gastrointestinal tract of the horse. Immunoreactivity for c-kit was revealed using immunohistochemical labelling with an anti-c-kit polyclonal antibody. Sections of normal gastrointestinal tissue were examined from 13 anatomically defined sites from stomach to small colon taken from horses free from gastrointestinal disease. Three types of c-kit immunoreactive cells were identified: spindle-shaped cells in the region of the myenteric plexus, stellate or bipolar cells in the circular muscle layer, and round cells in the submucosa. The round cells were shown to be mast cells with the use of toluidine blue staining, whereas the other c-kit immunoreactive cells did not exhibit metachromasia and were classified as ICC. This study will serve as a basis for future pathological studies in the horse.

Activation of outward K+ currents: effect of VIP in oesophagus

1. Electrical field stimulations (EFS) of the opossum and canine lower oesophageal sphincters (OLOS and CLOS respectively) and opossum oesophageal body circular muscle (OOBCM) induce non-adrenergic, non-cholinergic (NANC) relaxations of any active tension and NO-mediated hyperpolarization. VIP relaxes the OLOS and CLOS and any tone in OOBCM without major electrophysiological effects. These relaxations are not blocked by NOS inhibitors. Using isolated smooth muscle cells, we tested whether VIP acted through myogenic NO production. 2. Outward currents were similar in OOBCM and OLOS and NO increased them regardless of pipette Ca2+(i), from 50-8000 nM. L-NAME or L-NOARG did not block outward currents in OLOS at 200 nM pipette Ca2+. 3. Outward currents in CLOS cells decreased at 200 nM pipette Ca2+ or less but NO donors still increased them. VIP had no effect on outward currents in cells from OOBCM, OLOS or CLOS under conditions of pipette Ca2+ at which NO donors increased outward K+ currents. 4. We conclude, VIP does not mimic electrophysiological effects of NO donors on isolated cells of OOBCM, OLOS or CLOS. VIP relaxes the OLOS and CLOS and inhibits contraction of OOBCM by a mechanism unrelated to release of myogenic NO or an increase in outward current. 5. Also, the different dependence of outward currents of OOBCM and OLOS on pipette Ca2+ from those of CLOS suggests that different K+ channels are involved and that myogenic NO production contributes to K+ channel activity in CLOS but not in OLOS or OOBCM.

Parallel pathways mediate inhibitory effects of vasoactive intestinal polypeptide and nitric oxide in canine fundus

1. The gastric adaptation reflex is activated by the release of non-adrenergic, non-cholinergic (NANC) inhibitory transmitters, including nitric oxide (NO) and vasoactive intestinal polypeptide (VIP). The role of NO in this reflex is not disputed, but some investigators suggest that NO synthesis is stimulated by VIP in post-junctional cells or in nerve terminals. We investigated whether the effects of these transmitters are mediated by independent pathways in the canine gastric fundus. 2. VIP and NO produced concentration-dependent relaxation of the canine fundus. Nomega-nitro-L-arginine (L-NNA) reduced relaxation induced by electrical field stimulation (EFS; 0.5-8 Hz), but had no effect on responses to exogenous VIP and sodium nitroprusside (SNP, 10 microM). 3. Oxyhaemoglobin reduced relaxations produced by EFS and SNP. Oxyhaemoglobin also reduced relaxation responses to low concentrations of VIP (<10 nM), but these effects were non-specific and mimicked by methaemoglobin which had no effect on nitrergic responses. 4. A blocker of guanylyl cyclase, 1H-[1,2,4]oxidiazolo [4,3,-a]quinoxalin-1-one, (ODQ) inhibited responses to EFS, SNP and DETA/NONOate (an NO.donor), but had no effect on responses to VIP. cis-N-(2-phenylcyclopentil)-azacyclotridec-1en-2-amine monohydrochloride (MDL 12,330A), a blocker of adenylyl cyclase, reduced responses to EFS, VIP and forskolin, but did not affect responses to SNP. 5. Levels of cyclic GMP were enhanced by the NO donor S-nitroso-n-acetylpenicillamine (SNAP) but were unaffected by VIP (1 microM). The increase in cyclic GMP in response to SNAP was blocked by ODQ. 6. The results suggest that at least two transmitters, possibly NO and VIP, mediate relaxation responses in the canine fundus. NO and VIP mediate responses via cyclic GMP- and cyclic AMP-dependent mechanisms, respectively. No evidence was found for a serial cascade in which VIP is coupled to NO-dependent responses.

The enteric nervous system and regulation of intestinal motility

The enteric nervous system exerts local control over mixing and propulsive movements in the small intestine. When digestion is in progress, intrinsic primary afferent neurons (IPANs) are activated by the contents of the intestine. The IPANs that have been physiologically characterized are in the intrinsic myenteric ganglia. They are numerous, about 650/mm length of small intestine in the guinea pig, and communicate with each other through slow excitatory transmission to form self-reinforcing assemblies. High proportions of these neurons respond to chemicals in the lumen or to tension in the muscle; physiological stimuli activate assemblies of hundreds or thousands of IPANs. The IPANs make direct connections with muscle motor neurons and with ascending and descending interneurons. The circular muscle contracts as an annulus, about 2-3 mm in minimum oral-to-anal extent in the guinea pig small intestine. The smooth muscle cells form an electrical syncytium that is innervated by about 300 excitatory and 400 inhibitory motor neurons per mm length. The intrinsic nerve circuits that control mixing and propulsion in the small intestine are now known, but it remains to be determined how they are programmed to generate the motility patterns that are observed.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

Regulation of gastrointestinal (GI) motility is intimately coordinated with the modulation of ionic conductance expressed in GI smooth muscle and nonmuscle cells. Interstitial cells of Cajal (ICC) act as pacemaker cells and possess unique ionic conductances that trigger slow wave activity in these cells. The slow wave mechanism is an exclusive feature of ICC: Smooth muscle cells may lack the basic ionic mechanisms necessary to generate or regenerate slow waves. The molecular identification of the components for these conductances provides the foundation for a complete understanding of the ionic basis for GI motility. In addition, this information will provide a basis for the identification or development of therapeutics that might act on these channels. It is much easier to study these conductances and develop blocking drugs in expression systems than in native GI muscle cells. This review focuses on the relationship between ionic currents in native GI smooth muscle cells and ICC and their molecular counterparts.

Immunohistochemical distribution of c-Kit-positive cells and nitric oxide synthase-positive nerves in the guinea-pig small intestine

Anatomical relationships between c-Kit-positive cells and nitric oxide synthase-positive nerves in the small intestine were examined by double-labeling immunohistochemistry. Cryosections and whole mount preparations of the guinea-pig small intestine were double-immunolabeled using anti-c-Kit and neuronal nitric oxide synthase antibodies, and were observed using confocal laser scanning microscopy. The c-Kit-like immunoreactivity constituted dense reticular networks in the deep muscular plexus and myenteric plexus of the intestinal wall. The nitric oxide synthase-like immunoreactivity occurred in the circular muscle layer, most densely at the deep muscular plexus, as well as within the ganglion strands or connecting strands of the myenteric plexus. Close association between c-Kit-like immunoreactivity and nitric oxide synthase-like immunoreactivity was evident in the deep muscular plexus. Specimens immunolabeled with the anti-nitric oxide synthase antibody were further examined under transmission electron microscopy. Axon profiles with nitric oxide synthase-like immunoreactivity lay closely adjacent to the interstitial cells in the deep muscular plexus as well as to smooth muscle cells of the circular muscle layer, whereas there was a considerable distance (> 500 nm) between interstitial cells and axon profiles with nitric oxide synthase-like immunoreactivity in the myenteric plexus. These results suggest that the interstitial cells in the deep muscular plexus serve as mediators of the nitrergic neurotransmission to the musculature in the small intestine, playing a role in the regulation of intestinal movement.

Voltage sensitivity of slow wave frequency in isolated circular muscle strips from guinea pig gastric antrum

In circular muscle preparations isolated from the guinea pig gastric antrum, regular spontaneous electrical activity (slow waves) was recorded. Under normal conditions (6 mM K+), the frequency and shape of the slow waves were similar to those observed in ordinary stomach smooth muscle preparations. When the resting membrane potential was hyperpolarized and depolarized by changing the extracellular K+ concentration (2-18 mM), the frequency of slow waves decreased and increased, respectively. Application of cromakalim hyperpolarized the cell membrane and reduced the frequency of slow waves in a dose-dependent manner. Cromakalim (3 microM) hyperpolarized the membrane, and slow waves ceased in most preparations. In the presence of cromakalim, subsequent increases in the extracellular K+ concentration restored the frequency of slow waves accompanied by depolarization. Also, glibenclamide completely antagonized this effect of cromakalim. In smooth muscle strips containing both circular and longitudinal muscle layers, such changes in the slow wave frequency were not observed. It was concluded that the maneuver of isolating circular smooth muscle altered the voltage dependence of the slow wave frequency.

Slow waves in circular muscle of porcine ileum: structural and electrophysiological studies

The structural and functional bases of pacemaking (slow waves) in porcine ileal circular muscle were studied. The myenteric plexus contained two, structurally distinct types of interstitial cells of Cajal (ICC) interconnected by gap junctions and connected by close contacts to muscle layers. At the deep muscular plexus, ICC were present, not regularly close to nerve axons or in gap junction contact with one another or outer circular muscle, which had many gap junctions. Slow waves (5.2 +/- 2 mV amplitude and 4.6 +/- 0.7 s duration) occurred at 9.9 +/- 1.1 counts/min. Tissue length and time constants were 2.00 +/- 0.3 mm and 111 +/- 37 ms, respectively. Large electrical field-induced hyperpolarizations or depolarizations reduced amplitudes but not frequencies or durations of slow waves; hyperpolarizations progressively reduced inhibitory junction potentials as if the K+ channel opening mediated them. In conclusion, the myenteric plexus ICC of pig ileum, which appears to pace the muscle layers, appears insensitive to voltages applied to the syncytium of circular muscle cells. Limited coupling between ICC and circular muscle or voltage-insensitive pacemaking activity may explain these findings.

c-Kit immunoreactive interstitial cells in the human gastrointestinal tract

c-Kit immunopositive cells are considered to be pacemakers and/or mediators of neurotransmission in the gastrointestinal tract. They also correspond to the interstitial cells of Cajal (ICs) in mice. The normal distribution of c-Kit positive cells and their relation to ICs in the human gastrointestinal tract remain unclear. In this study we examine the distribution of c-Kit positive cells and their ultrastructure in normal human tissue. We then classified them and examined their relationship to ICs. Thirty nine samples of gut from the esophagus to the sigmoid colon from humans (ranging in age from a 16 week old fetus to a 57 year old and without motility disorders), were processed for immunohistochemistry, electronmicroscopy and immuno-electronmicroscopy. c-Kit immunopositive cells were located in the external muscle from the lower esophagus to the sigmoid colon, wherever the external muscle was composed of smooth muscle cells, and they were classified morphologically into two groups. Cells in the first group were mainly spindle-shaped bipolar cells with few branches; these cells ran parallel to nearby smooth muscle. Ultrastructurally, they possessed many intermediate filaments and caveolae. The spindle-shaped cells were present in the esophagus, stomach and small intestine. The second group of cells were located only in the colon, and were multipolar or bipolar cells with numerous branches. Cells in the second group were also rich in caveolae and/or smooth endoplasmic reticulum, but intermediate filaments were not prominent. Although both groups of c-Kit immunopositive cells corresponded to ICs, some ICs in the human gut do not appear to express c-Kit immunoreactivity.

Searching for intrinsic properties and functions of interstitial cells of Cajal

Evidence is mounting that interstitial cells of Cajal may function as pacemaker cells and have a role in NO-mediated neurotransmission. Several colonic motor disorders may be associated with abnormal ICC function.