Spontaneous junction potentials and slow waves in the circular muscle of isolated segments of guinea-pig ileum

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.

A Personal Perspective on the Development of Our Understanding of the Myogenic Control Mechanisms of Gut Motor Function

Myogenic control mechanisms play a role in all motor activities of the gut. Myogenic control systems are defined here as control systems that are intrinsic to the smooth muscle cells and/or interstitial cells of Cajal (ICC) and that can operate without an essential contribution of the intrinsic (ENS) and extrinsic nervous systems. In vivo however, the ENS and the myogenic control systems always work in cooperation. Although myogenic control plays a role in every gut organ, this review focuses on the peristaltic and segmentation activity of the small intestine. It provides some historical perspectives and some discussion on the development of our understanding of the cooperative nature of the myogenic and neurogenic control mechanisms. It highlights how some influential papers inadvertently provided hindrance to full understanding, it discusses how the guinea pig model has hampered acceptance of myogenic control systems and it provides some background into the genesis of our understanding of control mechanisms involving ICC.

Loss of responsiveness of circular smooth muscle cells from the guinea pig ileum is associated with changes in gap junction coupling

Gap junction coupling and neuromuscular transmission to smooth muscle were studied in the first 4 h after preparations were set up in vitro. Intracellular recordings were made from smooth muscle cells of guinea pig ileum. Fast inhibitory junction potentials (IJPs) were small (1.3 ± 1.0 mV) in the first 30 min but increased significantly over the first 120 min to 15.8 ± 0.9 mV (n = 12, P < 0.001). Comparable increases in slow IJPs and excitatory junction potentials were also observed. During the same period, resting membrane potential depolarized from -58.8 ± 1.4 to -47.2 ± 0.4 mV (n = 12, P < 0.001). Input resistance, estimated by intracellular current injection, decreased in parallel (P < 0.05), and dye coupling, measured by intracellular injection of carboxyfluorescein, increased (P < 0.001). Input resistance was higher and dye coupling was less in longitudinal than circular smooth muscle cells. Gap junction blockers [carbenoxolone (100 μM), 18β-glycyrrhetinic acid (10 μM), and 2-aminoethoxydiphenyl borate (50 μM)] hyperpolarized coupled circular smooth muscle cells, reduced the amplitude of fast and slow IJPs and excitatory junction potentials, increased input resistance, and reduced dye coupling. Local application of ATP (10 mM) mimicked IJPs and showed comparable increases in amplitude over the first 120 min; carbenoxolone and 2-aminoethoxydiphenyl borate significantly reduced ATP-evoked hyperpolarizations in coupled cells. In contrast, synaptic transmission between myenteric neurons was not suppressed during the first 30 min. Gap junction coupling between circular smooth muscle cells in isolated preparations was initially disrupted but recovered over the next 120 min to a steady level. This was associated with potent effects on neuromuscular transmission and responses to exogenous ATP.

Sympathetic innervation of the ileocecal junction in horses

The distribution and chemical phenotypes of sympathetic and dorsal root ganglion (DRG) neurons innervating the equine ileocecal junction (ICJ) were studied by combining retrograde tracing and immunohistochemistry. Immunoreactivity (IR) for tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), neuronal nitric oxide synthase (nNOS), calcitonin gene-related peptide (CGRP), substance P (SP), and neuropeptide Y (NPY) was investigated. Sympathetic neurons projecting to the ICJ were distributed within the celiac (CG), cranial mesenteric (CranMG), and caudal mesenteric (CaudMG) ganglia, as well as in the last ganglia of the thoracic sympathetic chain and in the splanchnic ganglia. In the CG and CranMG 91 +/- 8% and 93 +/- 12% of the neurons innervating the ICJ expressed TH- and DBH-IR, respectively. In the CaudMG 90 +/- 15% and 94 +/- 5% of ICJ innervating neurons were TH- and DBH-IR, respectively. Sympathetic (TH-IR) fibers innervated the myenteric and submucosal ganglia, ileal blood vessels, and the muscle layers. They were more concentrated at the ICJ level and were also seen encircling myenteric plexus (MP) and submucosal plexus (SMP) descending neurons that were retrogradely labeled from the ICJ. Among the few retrogradely labeled DRG neurons, nNOS-, CGRP-, and SP-IR nerve cells were observed. Dense networks of CGRP-, nNOS-, and SP-IR varicosities were seen around retrogradely labeled prevertebral ganglia neurons. The CGRP-IR fibers are probably the endings of neurons projecting from the intestine to the prevertebral ganglia. These findings indicate that this crucial region of the intestinal tract is strongly influenced by the sympathetic system and that sensory information of visceral origin influences the sympathetic control of the ICJ.

Mechanisms underlying nutrient-induced segmentation in isolated guinea pig small intestine

Mechanisms underlying nutrient-induced segmentation within the gut are not well understood. We have shown that decanoic acid and some amino acids induce neurally dependent segmentation in guinea pig small intestine in vitro. This study examined the neural mechanisms underlying segmentation in the circular muscle and whether the timing of segmentation contractions also depends on slow waves. Decanoic acid (1 mM) was infused into the lumen of guinea pig duodenum and jejunum. Video imaging was used to monitor intestinal diameter as a function of both longitudinal position and time. Circular muscle electrical activity was recorded by using suction electrodes. Recordings from sites of segmenting contractions showed they are always associated with excitatory junction potentials leading to action potentials. Recordings from sites oral and anal to segmenting contractions revealed inhibitory junction potentials that were time locked to those contractions. Slow waves were never observed underlying segmenting contractions. In paralyzed preparations, intracellular recording revealed that slow-wave frequency was highly consistent at 19.5 (SD 1.4) cycles per minute (c/min) in duodenum and 16.6 (SD 1.1) c/min in jejunum. By contrast, the frequencies of segmenting contractions varied widely (duodenum: 3.6-28.8 c/min, median 10.8 c/min; jejunum: 3.0-27.0 c/min, median 7.8 c/min) and sometimes exceeded slow-wave frequencies for that region. Thus nutrient-induced segmentation contractions in guinea pig small intestine do not depend on slow-wave activity. Rather they result from a neural circuit producing rhythmic localized activity in excitatory motor neurons, while simultaneously activating surrounding inhibitory motor neurons.

Segmentation induced by intraluminal fatty acid in isolated guinea-pig duodenum and jejunum

Small intestinal movements depend on the composition of the chyme with mixing predominating at high nutrient levels and propulsion being prevalent at low nutrient levels. The mechanisms coupling nutrients to motility are unknown. We used computer analysis of video recordings of isolated guinea-pig duodenum, jejunum and ileum to examine movements induced by a fatty acid, decanoic acid. Increasing intraluminal pressure past a threshold using control saline consistently evoked propulsive reflexes: lumen-occluding constrictions appeared at the oral end propagating at 20.4 +/- 2.4 mm s(-1) (mean +/-s.d., jejunum) to the anal end before being repeated until the intraluminal pressure was returned to control. Subthreshold pressure increases sometimes evoked a transient series of constrictions appearing at the oral end and propagating anally at 18.4 +/- 4.7 mm s(-1) (jejunum). At basal pressures, decanoic acid dose-dependently induced motor activity consisting of 40-60 s episodes of constrictions separated by 40-200 s periods of quiescence and lasting up to 2 h. Five contraction patterns were identified within episodes including localized stationary constrictions; constrictions that propagated slowly (5-8 mm s(-1)) for short distances orally or anally; and constrictions that propagated orally or anally for the length of the preparation at 14-20 mm s(-1). Decanoic acid induced motor activity was reversibly abolished by tetrodotoxin (3 microm), hyoscine (1 microm) and hexamethonium (100 microm), but was insensitive to blockade of P2 purinoceptors by PPADS (60 microm). Thus, decanoic acid induces motor activity equivalent to segmentation in guinea-pig small intestine in vitro and this depends on intrinsic neural pathways.

Similarities and differences in the propagation of slow waves and peristaltic waves

The relationship between slow waves and peristaltic reflexes has not been well analyzed. In this study, we have recorded the electrical activity of slow waves together with that generated by spontaneous peristaltic contractions at 240 extracellular sites simultaneously. Recordings were made from five isolated tubular and six sheet segments of feline duodenum superfused in vitro. In all preparations, slow waves propagated as broad wave fronts along the longitudinal axis of the preparation in either the aborad or the orad direction. Electrical potentials recorded during peristalsis (peristaltic waves) also propagated as broad wave fronts in either directions. Peristaltic waves often spontaneously stopped conducting (46%), in contrast to slow waves that never did. Peristaltic waves propagated at a lower velocity than the slow waves (0.98 +/- 0.25 and 1.29 +/- 0.28 cm/s, respectively; P < 0.001; n = 24) and in a direction independent of the preceding slow wave direction (64% in the same direction, 46% in the opposite direction). In conclusion, slow waves and peristaltic waves in the isolated feline duodenum seem to constitute two separate electrical events that may drive two different mechanisms of contraction in the small intestine.

Electrical rhythmicity and spread of action potentials in longitudinal muscle of guinea pig distal colon

Using simultaneous intracellular recordings, we have characterized 1) electrical activity in the longitudinal muscle (LM) of isolated segments of guinea pig distal colon free to contract spontaneously and 2) extent of propagation of spontaneous action potentials around the circumference of the colon. In all animals, rhythmical spontaneous depolarizations (SDs) were recorded that are usually associated with the generation of action potentials. Recordings from pairs of LM cells, separated by 100 microm in the circumferential axis, revealed that each action potential was phase locked at the two electrodes (mean propagation velocity: 3 mm/s). However, at an increased electrode separation distance of 1 mm circumferentially, action potentials and SDs became increasingly uncoordinated at the two recording sites. No SDs or action potentials ever propagated from one circumferential edge to the other (i.e., 13 mm apart). When LM strips were separated from the myenteric plexus and circular muscle, rhythmically firing SDs and action potentials were still recorded. Atropine (1 microM) or tetrodotoxin (1 microM) either reduced the frequency of SDs or temporarily abolished activity, whereas nifedipine (1 microM) always abolished SDs and action potentials. Kit-positive interstitial cells of Cajal were present at the level of the myenteric plexus and circular and longitudinal muscle. In summary, SDs and action potentials in LM propagate over discrete localized zones, usually <1 mm around the circumference of the colon. Furthermore, in contrast to the classic slow wave, rhythmic depolarizations in LM appear to be generated by an intrinsic property of the smooth muscle itself and are critically dependent on opening of L-type Ca(2+) channels.

Propagating contractions of the circular muscle evoked by slow stretch in flat sheets of guinea-pig ileum

Flat sheet preparations of guinea-pig ileum were stretched circumferentially and the propagation of circular muscle contractions along the preparation was investigated. Slow stretch, at 100 microm s-1, of a 50-mm long flat sheet of intestine, evoked circular muscle contraction orally, which propagated, without decrement, for up to 30 mm. This occurred despite circular muscle shortening being prevented, and in the absence of propulsion of contents. Thus, propagation in this flat sheet preparation could not explained on the basis of neuro-mechanical interactions, as previously proposed. Irrespective of the length of preparations, contraction amplitude decreased significantly in the most aboral 10-15 mm of intestine. This was not due to descending inhibitory pathways, but was associated with interruption of ascending excitatory pathways near the aboral end. Slow waves were not detected in circular muscle cells in any preparation (n=8). Smooth muscle action potentials evoked in circular muscle cells, in the presence of tetrodotoxin (TTX, 0.6 micromol L-1), did not propagate for more than 1 mm in the longitudinal axis. Propagation of circular muscle activity, evoked by slow stretch of flat sheet preparations, reveals the presence of a mechanism other than myogenic spread or the neuro-mechanical interactions previously proposed to account for propagation; the nature of this mechanism remains to be determined.

Spatial and temporal coordination of junction potentials in circular muscle of guinea-pig distal colon

1. In isolated, stretched, flat-sheet preparations of guinea-pig distal colon, simultaneous intracellular recordings were made from pairs of circular muscle (CM) cells to map the region of smooth muscle at which spontaneous junction potentials (sJPs) were coordinated in both space and time. 2. Spontaneous inhibitory junction potentials (sIJPs) and excitatory junction potentials (sEJPs) were recorded from all animals with varying frequencies and amplitudes (up to 25 mV). 3. Large amplitude (> or = 9 mV) sIJPs or sEJPs with near-identical amplitudes and time courses were recorded synchronously from two CM cells, even when the two electrodes were separated by up to 11 mm in the circumferential axis and < or = 4 mm in the longitudinal axis. However, smaller (< 9 mV) sIJPs or sEJPs were less coordinated and exhibited greater variability in their times to peak. 4. The standard deviation (S.D.) for the time difference between the peaks of sJPs was related to the amplitude of the events and the distance between the electrodes. The S.D. for large amplitude JPs (approximately 30 ms), which was less than that for small JPs (approximately 150 ms), remained constant across the circumferential axis (at least up to 6 mm), but declined rapidly for distances > or = 2 mm in the longitudinal axis. 5. Current injection (up to 8 nA) into a single CM cell elicited electrotonic potentials in neighbouring CM cells, only when the two electrodes were separated by less than 100 microm circumferentially. Beyond 50 microm electronic potentials were rarely detected. 6. Tetrodotoxin (TTX; 1 microM) abolished all sJPs, whereas hexamethonium (300 microM) either abolished, or substantially reduced all sJPs. 7. Nitro-L-arginine (L-NA; 100 microM) abolished the slow repolarisation phase of sIJPs without any apparent effect on the amplitude of sIJPs. Apamin abolished the fast, initial component of sIJPs, suggesting synchronous release of two inhibitory neurotransmitters during the sIJP. Atropine (1 microM) abolished sEJPs. 8. No sJPs were recorded from the CM layer when it was separated from the myenteric plexus. 9. In conclusion, sIJPs and sEJPs in colonic CM occur synchronously over large regions of the smooth muscle syncitium. The results are discussed in relation to the idea that spontaneous junction potentials in colonic CM are not monoquantal events, but are generated by the simultaneous release of transmitter from many release sites, due to the synchronous activation of many enteric motor neurons.

Simultaneous intracellular recordings from longitudinal and circular muscle during the peristaltic reflex in guinea-pig distal colon

1. Simultaneous intracellular recordings were made from longitudinal muscle (LM) and circular muscle (CM) cells of guinea-pig distal colon during the peristaltic reflex. 2. Spontaneous rhythmical depolarizations with superimposed action potentials (mean amplitude: 19 +/- 2 mV) were regularly recorded from the LM (mean interval: 7 +/- 1 s). In contrast, in the CM layer, spontaneous action potentials occurred with an irregular frequency. Although spontaneous action potentials in LM were rarely correlated in time with those in CM, spontaneous inhibitory junction potentials (sIJPs) were found to occur synchronously in both muscles (5 out of 27 animals; 19 %). 3. Graded inflation of an intra-luminal balloon or mucosal stimulation oral to the recording electrodes elicited gradeable compound IJPs synchronously in both LM (mean amplitude: 6 +/- 1 mV) and CM (mean amplitude: 9 +/- 1 mV) (descending inhibitory reflex). Evoked IJPs were often followed by action potentials in both muscle layers. 4. Mucosal stimuli applied anal to the recording electrodes elicited compound excitatory junction potentials (EJPs) synchronously in both muscles layers that were often associated with the generation of action potentials. In the LM, evoked EJP amplitudes ranged from 3 mV (subthreshold) to 31 mV (including the action potential) and in the CM from 4 mV (subthreshold) to 44 mV (including the action potential). 5. Apamin (500 nM) reduced the evoked IJP in the CM by 55 % (from 11 +/- 2 to 5 +/- 1 mV), but caused no significant reduction in the LM layer (from 8 +/- 1 to 6 +/- 1 mV). Apamin-resistant IJPs in both muscle layers were likely to be due to nitric oxide, since they were abolished by L-NA (100 microM). 6. Atropine (1 microM) abolished the ascending excitatory reflex in both muscles. 7. Injection of neurobiotin into the LM and CM confirmed that simultaneous intracellular recordings were made from different muscle layers. 8. In conclusion, during the peristaltic reflex, the LM and CM layers receive synchronous inhibitory neuromuscular inputs during descending inhibition and synchronous excitatory neuromuscular inputs during ascending excitation. No evidence was found to support reciprocal innervation.

Role of muscle tone in peristalsis in guinea-pig small intestine

We investigated the involvement of muscle tone and circular muscle (CM) contraction in peristalsis in isolated guinea-pig small intestine. A segment of jejunum (approximately 13 cm) was mounted into a three chambered partitioned bath. Peristaltic waves were initiated in the oral chamber either by: (1) infusing fluid into the oral end of the jejunum; the ejected fluid was diverted via a cannula from reaching the intermediate and anal chambers, or by (2) intraluminal balloon distension of the empty oral segment. Tension of the circular muscle was measured in all three chambers. Peristaltic waves elicited by fluid infusion were evoked at an abrupt threshold. In contrast, peristaltic waves elicited by distension could be graded in amplitude according to stimulus intensity. Peristaltic waves evoked in an empty intestine exhibited similar propagation velocities to peristaltic waves associated with fluid propulsion. Nifedipine (200-400 nM) added to the intermediate chamber to block muscle contraction did not prevent peristaltic waves elicited by either stimulus from propagating into the anal chamber, although their amplitude was attenuated. Nifedipine to the site of stimulation (oral chamber) abolished peristaltic waves generated by either stimulus. Tetrodotoxin (1-2 microM), or a low Ca2+-high Mg2+ solution to the intermediate chamber abolished the propagation of peristalsis from the oral to anal chambers. In conclusion, graded peristaltic waves can occur in an empty intestine. Therefore peristalsis is not necessarily an "all-or-none" phenomenon. Peristalsis depends on the spread of nervous activity along the bowel, rather than the reactivation of neural circuits caused by displacement of fluid in the lumen. However, local muscle tone and contraction are important for the initiation and maintenance of peristaltic propagation.

Propagation and neural regulation of calcium waves in longitudinal and circular muscle layers of guinea pig small intestine

BACKGROUND & AIMS

The relative movements of longitudinal muscle (LM) and circular muscle (CM) and the role that nerves play in coordinating their activities has been a subject of controversy. We used fluorescent video imaging techniques to study the origin and propagation of excitability simultaneously in LM and CM of the small intestine.

METHODS

Opened segments of guinea pig ileum were loaded with the Ca(2+) indicator fluo-3. Mucosal reflexes were elicited by lightly depressing the mucosa with a sponge.

RESULTS

Spontaneous Ca(2+) waves occurred frequently in LM (1.2 s(-1)) and less frequently in CM (3.2 min(-1)). They originated from discrete pacing sites and propagated at rates 8-9 times faster parallel (LM, 87 mm/s; CM, 77 mm/s) compared with transverse to the long axis of muscle fibers. The presence of Ca(2+) waves in one muscle layer did not affect the origin, rate of conduction, or range of propagation in the other layer. The extent of propagation was limited by collisions with neighboring waves or recently excited regions. Simultaneous excitation of both muscle layers could be elicited by mucosal stimulation of either ascending or descending reflex pathways. Neural excitation resulted in an increase in the frequency of Ca(2+) waves and induction of new pacing sites without eliciting direct coupling between layers.

CONCLUSIONS

Localized, spontaneous Ca(2+) waves occur independently in both muscle layers, promoting mixing (pendular or segmental) movements, whereas activation of neural reflexes stimulates Ca(2+) waves synchronously in both layers, resulting in strong peristaltic or propulsive movements.

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.

Slow wave and spike action potentials recorded in cell cultures from the muscularis externa of the guinea pig small intestine

Intracellular recordings were obtained to investigate whether slow wave and spike type action potentials are present in cell cultures of the muscularis externa from the guinea pig small intestine. The muscularis externa of the small intestine was dissociated by using specific purified enzymes and gentle mechanical dissociation. Cells were plated on cover slips and maintained in culture for up to 4 weeks. Dissociated cells obtained in this way reorganized themselves in a few days to form small cell clumps showing spontaneous movements. Intracellular recordings of these clumps displayed both spike and slow wave type action potentials. Spikes were observed on top of some slow waves and were abolished by the addition of nifedipine or the removal of extracellular calcium. Slow waves, however, were nifedipine insensitive and temperature sensitive, and were abolished by octanol (a gap junction blocker) and forskolin (an adenyl cyclase activator). Slow waves were never observed in small clumps (<50 µm), suggesting that a critical mass of cells might be required for their generation. These observations demonstrated for the first time the presence of nifedipine-insensitive slow waves in cell cultures of the muscularis externa from the guinea pig small intestine. Cell cultures allow rigorous control of the immediate environment for the cells and this should facilitate future studies on the molecular and cellular mechanisms responsible for the slow waves in the gastrointestinal tract.Key words: smooth muscle, slow waves, spiking activity, gastrointestinal tract, gut, small intestine, electrophysiology, pacemaker activity, guinea pig.

Quantitative analysis of peristalsis in the guinea-pig small intestine using spatio-temporal maps

1. Peristalsis was evoked in guinea-pig small intestine by slow fluid infusion and recorded onto video and digitized. Spatio-temporal maps of diameter and longitudinal movement were constructed and parameters of motion were calculated. 2. During the filling of the isolated segments of intestine, rhythmic local longitudinal movements were observed at several points along the preparation. These phasic longitudinal muscle contractions were associated with small but significant local increases in diameter and probably reflect a passive mechanical coupling by connective tissue in the gut wall. In addition, occasional synchronized longitudinal muscle contractions caused net shortening of the preparation and always preceded the onset of peristaltic emptying. 3. Peristaltic emptying was characterized by a contraction of the circular muscle which usually started at the oral end of the preparation, that propagated aborally, propelling the contents. However, in 19 % of trials, the first circular muscle contraction occurred in the aboral half of the preparation. 4. The propagation of peristalsis consisted of separate sequential circular muscle contractions several centimetres long, particularly in the oral half of the preparation, giving a 'step-like' appearance to the spatio-temporal map. The gut was transiently distended aboral to the propagating circular muscle contraction due to the propulsion of contents. 5. At each point in the preparation, the longitudinal muscle remained contracted during the propulsive part of the circular muscle contraction. Only when the circular muscle contraction became lumen occlusive did lengthening of the longitudinal muscle take place. 6. Spatio-temporal maps are a powerful tool to visualize and analyse the complexity of gastrointestinal motility patterns.

Initiation of peristalsis by circumferential stretch of flat sheets of guinea-pig ileum

1. Segments of isolated guinea-pig intestine, 12 mm long, were distended slowly by intraluminal fluid infusion or by mechanical stretch as either a tube or flat sheet. In all cases, at a constant threshold length, a sudden, large amplitude contraction of the circular muscle occurred orally, corresponding to the initiation of peristalsis. 2. Circumferential stretch of flat sheet preparations evoked graded contractions of the longitudinal muscle (the 'preparatory phase'), which were maintained during circular muscle contraction. This suggests that the lengthening reported during the emptying phase of peristalsis is due to mechanical interactions. 3. The threshold for peristalsis was lower with more rapid stretches and was also lower in long preparations (25 mm) compared with short preparations (5-10 mm), indicating that ascending excitatory pathways play a significant role in triggering peristalsis. 4. Stretching a preparation beyond the threshold for peristalsis evoked contractions of increasing amplitude; thus peristalsis is graded above its threshold. However, during suprathreshold stretch maintained at a constant length, contractions of the circular muscle quickly declined in amplitude and frequency. 5. Circular muscle cells had a resting membrane potential approximately 6 mV more negative than the threshold for action potentials. During slow circumferential stretch, subthreshold graded excitatory motor input to the circular muscle occurred, prior to the initiation of peristalsis. However, peristalsis was initiated by a discrete large excitatory junction potential (12 +/- 2 mV) which evoked bursts of smooth muscle action potentials and which probably arose from synchronized firing of ascending excitatory neuronal pathways.

Anti-c-kit protein immunoreactive cells corresponding to the interstitial cells of Cajal in the guinea-pig small intestine

Interstitial cells of Cajal (ICC) of the guinea-pig small intestine were studied with whole-mount preparations by using the zinc iodide-osmic acid method (ZIO) and immunohistochemistry for vimentin and c-kit receptor tyrosine kinase, and by electron microscopy. The myenteric ICC visualized with ZIO staining are immunopositive to both anti-c-kit antibody (ACK-2) and anti-vimentin antibody (V9), and constitute an independent cellular network from the myenteric plexus. Those cells are characterized by many mitochondria, abundant intermediate filaments, and surface cell membranes not covered with a basal lamina. They are connected with each other by gap junctions at tips of the cytoplasmic processes. It is concluded that the myenteric ICC of the guinea-pig intestine are fibroblast-like cells and that they correspond to the c-kit expressing cells regarded as the intestinal pacemaker.

Effect of longitudinal muscle-myenteric plexus removal and indomethacin on the response to tachykinin NK-2 and NK-3 receptor agonists in the circular muscle of the guinea-pig ileum

1. The effect of removal of the longitudinal muscle-myenteric plexus (LM-MP) and/or indomethacin (10 microM) on the response to the tachykinin NK-2 receptor selective agonist, [beta Ala8]NKA(4-10), or to the NK-3 receptor selective agonist, senktide, was investigated by measuring mechanical activity (isotonic recording) of circular muscle (ring preparation) of the guinea-pig ileum. 2. Indomethacin (10 microM) increased the percentage of ileal rings displaying spontaneous activity, either intact or LM-MP-free. The response to senktide (10 nM and 1 microM) was lower in LM-MP-free than in intact ileal rings, either in the absence or presence of indomethacin. The response to a low concentration (10 nM) of [beta Ala8] NKA (4-10) was enhanced in LM-MP-free rings and by indomethacin. 3. In intact ileal rings, the response to senktide was unaffected by atropine (3 microM) alone or by the tachykinin NK-2 receptor antagonist MEN 10,376 (10 microM) alone while it was reduced by the combined administration of the two antagonists. The response to senktide was greatly reduced by tetrodotoxin (TTX, 1 microM). Senktide-induced contractions (10 nM) were also reduced by the blocker of N-type voltage-sensitive calcium channels, omega-contoxin (CTX, 0.1 microM). 4. In about 30% of preparations tested, an inhibitory response (decrease in spontaneous activity) to 10 nM senktide, was disclosed in CTX-treated intact ileal rings. This inhibitory effect was TTX-sensitive. 5. In LM-MP-free ileal rings, the response to senktide was abolished or reduced by atropine and MEN 10,376, alone or in combination, and was also reduced or abolished by TTX and CTX. 6. The response to [beta Ala8]NKA (4-10) was inhibited by MEN 10,376, in both intact and LM-MP-free ileal rings while it was unaffected by atropine, TTX or CTX. 7. These results indicate that indomethacin pretreatment induces a regular background activity for studying the motor response to tachykinins in the circular muscle of the ileum, probably by blocking the formation of relaxant prostanoids. A further increase in sensitivity to direct smooth muscle stimulation (NK-2 receptor agonist) can be obtained by removal of the LM-MP. The response to NK-3 receptor stimulation is diminished but not abolished by removal of the LM-MP, suggesting that NK-3 receptors are located on neuronal bodies of myenteric neurons, but possibly also at other sites (possibly, nerve terminals).(ABSTRACT TRUNCATED AT 400 WORDS)

Role of nitric oxide in the peristalsis in the isolated guinea-pig ileum

The role of nitric oxide in peristalsis was studied by using NG-nitro-L-arginine, an inhibitor of the biosynthesis of nitric oxide, in the isolated guinea-pig ileum. Constant peristalsis was cyclically induced by distending the ileal wall with intraluminal solution infused at a constant rate. NG-Nitro-L-arginine (10(-6) to 10(-4) M) dose dependently increased the frequency of peristalsis. This effect of NG-nitro-L-arginine was almost hindered by pretreating the ileum with 10(-3) M L-arginine, but not D-arginine. Nitroprusside (5 X 10(-7) M) reversed the frequency increase. In the presence of NG-nitro-L-arginine, peristaltic propulsion occurred at a smaller distension of the ileal wall and the ileum constricted to a smaller diameter at the completion of propulsion. The rate of shortening of longitudinal muscle during distension was raised by NG-nitro-L-arginine, although the peak magnitudes were not changed. Consistent with these effects of NG-nitro-L-arginine on peristalsis, NG-nitro-L-arginine at 10(-5) M increased the contractions of circular muscle in response to electrical field stimulation, but not those of longitudinal muscle. These results suggest that endogenous nitric oxide modulates peristalsis by limiting the contractile activity of the circular muscle of the guinea-pig ileum.

Role of M1 muscarinic receptors in the parasympathetic control of colonic motility in cats and rabbits

1. The effects of pirenzepine (a selective blocking agent of M1 muscarinic receptors) were studied on excitatory junction potentials (EJPs) and inhibitory junction potentials (IJPs) elicited on colonic smooth muscle by stimulation of efferent parasympathetic nerve fibres in anaesthetized cats and rabbits. 2. Pirenzepine (25 micrograms kg-1 to 0.2 mg kg-1, i.v.) decreased the amplitude of EJPs or abolished them. In pirenzepine, parasympathetic stimulation elicited IJPs in most cases. 3. In both species, pirenzepine initiated spontaneous IJPs, indicating an increase in the activity of the non-adrenergic, non-cholinergic (NANC) inhibitory neurones. 4. The results suggest that M1 muscarinic receptors are involved in synaptic transmission within intramural plexuses, at interneuronal synapses in the parasympathetic excitatory pathway to colonic smooth muscle, but are not involved in the pathway to the NANC inhibitory neurones. The facilitation of IJPs by pirenzepine suggests that, under normal physiological conditions, NANC neurones are tonically inhibited by an intramural nervous circuit involving M1 muscarinic receptors.

Effects of a nitric oxide synthase inhibitor on non-cholinergic junction potentials in the circular muscle of the guinea pig ileum

Intracellular microelectrodes were used to record junction potentials from the circular muscle cells of the guinea pig ileum in vitro at 37 degrees C in a modified Krebs solution containing nifedipine (1-2 microM) and hyoscine (1 microM). Transmural nerve stimulation, using volleys of three pulses at 50 Hz, produced a complex response consisting of an inhibitory junction potential (IJP) followed by a prolonged depolarization. Following the addition of the nitric oxide synthase inhibitor NG-nitro-L-arginine (NOLA, 100 microM) the amplitude of the IJP (recorded 10 mm aboral to the stimulating electrodes) was increased by approx. 10% (n = 4). The further addition of apamin (250 nM) abolished the IJP revealing a non-cholinergic excitatory junction potential (EJP). In other experiments (n = 8), preparations were treated with apamin then subjected to substance P desensitization (500 nM, > 20 min). Transmural nerve stimulation now produced a triphasic response (recorded 1 mm aboral to the stimulating electrodes) consisting of: (a) an initial hyperpolarization (approx. 5 mV) lasting about 1 s; followed by (b) a depolarization reaching a peak (approx. 7 mV less negative than the RMP) approx. 2 s after nerve stimulation; and finally (c) a small (approx. 3 mV) hyperpolarization. The addition of NOLA reduced all three phases by 80-90% (n = 8). The subsequent addition of L-arginine (5 mM) partially reversed these effects (n = 3). Conditioning hyperpolarization up to 20 mV increased the amplitude of the NOLA-sensitive IJP and EJP. Further conditioning hyperpolarization reduced the amplitude of the IJP and enhanced the amplitude of the EJP. Large conditioning hyperpolarizations (> 60 mV) reduced the amplitude of both the IJP and EJP. An estimation of the membrane conductance changes occurring during the initial hyperpolarization and depolarization suggest that it was either unchanged or increased. During large conditioning hyperpolarizations in the absence of nerve stimulation, the membrane potential was unstable and began to show spontaneous oscillations (up to 30 mV, every 4-5 s) resembling slow waves. These experiments indicate that NO, or a related compound, appears to mediate the nerve induced apamin-resistant IJP and substance P- and hyoscine-resistant EJP in the circular muscle of the guinea pig ileum.

The cellular network of interstitial cells associated with the deep muscular plexus of the guinea pig small intestine

Systematic examination using electron microscopic montages and serial sections has demonstrated that three types of interstitial cell, namely gap junction-rich cells, glycogen-rich cells and fibroblast-like cells, are densely located along the whole extent of the deep muscular plexus of the guinea pig small intestine. They tend to be distributed in an alternating fashion in the cellular network, connected with muscle cells of the outer, circular layer by means of gap junctions. These three types of interstitial cell show close relations to two types of nerve varicosity: one type is characterized by clear round vesicles with diameters of about 50 nm, and the other by flattened vesicles measuring about 35 nm by 70 nm. Electron-dense patches have been observed at the cytoplasmic side of the axonal membranes. Muscle cells of both inner and outer circular layers also show close relations to these two types of nerve varicosity. These morphological features are discussed with the implication that they may have some regulatory role in intestinal movement.

Interstitial cells associated with the deep muscular plexus of the guinea-pig small intestine, with special reference to the interstitial cells of Cajal

Interstitial cells associated with the deep muscular plexus of the guinea-pig small intestine were studied by electron microscopy, and three-dimensional cell models were reconstructed from serial ultrathin sections with a computer graphic system. Three types of cells were recognized. The first type was similar in shape to smooth muscle cells, but did not contain an organized contractile apparatus. Many large gap junctions comprising about 4% of the cell surface were present; they connected cells of the first type to each other, to the second type of cell and to smooth muscle cells of the outer circular layer. The second type of cell had a well-demarcated cell body with long slender processes and was characterized by a large amount of glycogen comprising about 9% of the cell volume. The third type of cell was similar to fibroblasts, and contained well-developed Golgi apparatus and rough endoplasmic reticulum. Some of these fibroblast-like cells (a possible subtype) formed small gap junctions. All three types of cells showed close relationships with nerve varicosities. This cellular network consisting of gap-junction-rich cells, glycogen-rich cells and smooth muscle cells may be involved in the pacemaking activity of intestinal movement.

Distension-evoked ascending and descending reflexes in the circular muscle of guinea-pig ileum: an intracellular study

Reflex responses evoked by distension of the guinea-pig small intestine were recorded from the circular muscle with intracellular microelectrodes. For this purpose a mechanically stable preparation that allowed the intestinal wall to be distended within 9 mm of the recording site was developed. A segment of intestine was opened along the mesenteric border and pinned mucosa uppermost over a balloon set in the base of an organ bath, so that inflation of the balloon could distend the intestinal wall without simultaneously pushing against the mucosa. Compound excitatory junction potentials (EJPs) and compound inhibitory junction potentials (IJPs) were recorded at sites up to 40 mm oral and anal to the distending stimulus, respectively. The compound EJPs recorded orally had amplitudes of up to 24 mV and declined to baseline during distensions that exceeded 10-15 s. Distensions at intervals of less than 20 s evoked successively smaller oral compound EJPs; after four distensions in 30 the amplitude of the compound EJP had fallen to less than 10%. The amplitude of the oral compound EJP was reduced by hyoscine (1 microM), but the extent of the reduction depended on the degree of distension; responses to mild stimuli were blocked, whereas those to strong stimuli were only slightly reduced. The amplitude of the hyoscine-resistant component of the compound EJP was markedly reduced by antagonists of substance P receptors in the muscle. In the presence of muscarinic and substance P receptor antagonists, a transient compound IJP could be detected on the oral side of the stimulus. The compound IJPs recorded anal to the distension had amplitudes up to 22 mV but the potential returned to baseline during prolonged distension. In the presence of hyoscine (1 microM) some inhibitory activity continued throughout prolonged stimuli. Compound IJP amplitudes were not significantly reduced by repeated distensions separated by more than 6 s. At anal sites a transient depolarization (off-response) was recorded immediately following the termination of a distension in some preparations. The off-response was unaffected by hyoscine and was more readily observed after the further addition of substance P antagonists. The compound IJPs were almost completely blocked by apamin (0.2 microM). The compound EJPs and IJPs recorded orally were blocked by hexamethonium (100 microM), but the amplitudes of compound IJPs recorded anally were significantly reduced by hexamethonium (100-200 microM) only at recording sites greater than 15 mm from the centre of the balloon. The off-response was reduced by hexamethonium at all sites.(ABSTRACT TRUNCATED AT 400 WORDS)