Interaction between longitudinal and circular muscle in intestine of cat

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.

Interstitial cells: regulators of smooth muscle function

Smooth muscles are complex tissues containing a variety of cells in addition to muscle cells. Interstitial cells of mesenchymal origin interact with and form electrical connectivity with smooth muscle cells in many organs, and these cells provide important regulatory functions. For example, in the gastrointestinal tract, interstitial cells of Cajal (ICC) and PDGFRα(+) cells have been described, in detail, and represent distinct classes of cells with unique ultrastructure, molecular phenotypes, and functions. Smooth muscle cells are electrically coupled to ICC and PDGFRα(+) cells, forming an integrated unit called the SIP syncytium. SIP cells express a variety of receptors and ion channels, and conductance changes in any type of SIP cell affect the excitability and responses of the syncytium. SIP cells are known to provide pacemaker activity, propagation pathways for slow waves, transduction of inputs from motor neurons, and mechanosensitivity. Loss of interstitial cells has been associated with motor disorders of the gut. Interstitial cells are also found in a variety of other smooth muscles; however, in most cases, the physiological and pathophysiological roles for these cells have not been clearly defined. This review describes structural, functional, and molecular features of interstitial cells and discusses their contributions in determining the behaviors of smooth muscle tissues.

Are interstitial cells of Cajal plurifunction cells in the gut?

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

Anisotropic propagation in the small intestine

Abstract Measuring propagation anisotropy may help in determining the tissue layers involved in the propagation of electrical impulses in the intestine. We used 240 extracellular electrograms recorded from the isolated feline duodenum. The conduction velocities of slow waves and of individual spikes were measured from their site of origin into all directions. Both slow waves and spikes propagate anisotropically in the small intestine but in different directions and to a different degree. Slow waves propagated anisotropically faster in the circumferential (1.7 +/- 0.8 cm s(-1)) than in the axial direction (1.3 +/- 0.5 cm s(-1); P < 0.001). Spikes, on the other hand, propagated faster in the longitudinal direction (7.8 +/- 4.5 cm s(-1)) than in the circumferential direction (3.3 +/- 4.3 cm s(-1); P < 0.001). Furthermore, the average conduction velocity of spikes (6.3 +/- 4.5 cm s(-1)) was significantly higher than that of slow waves (1.5 +/- 1.1 cm s(-1); P < 0.001). The anisotropic propagation of spikes supports the argument that these propagate in the longitudinal muscle layer. The anisotropic propagation of slow waves may be the result of the interaction between the myenteric layer of interstitial cells of Cajal and their electrotonic connection to both the longitudinal and the circular muscle layer.

The interstitial cells of Cajal and a gastroenteric pacemaker system

In spite of a claim by Kobayashi (1990) that they do not correspond to the cells originally depicted by CAJAL, a particular category of fibroblast-like cells have been identified in the gut by electron microscopy (Faussone-Pellegrini, 1977; Thuneberg, 1980) and by immunohistochemistry for Kit protein (Maeda et al., 1992) under the term of the "interstitial cells of Cajal (ICC)". Generating electrical slow waves, the ICC are intercalated between the intramural neurons and the effector smooth muscular cells, to form a gastroenteric pacemaker system. ICC at the level of the myenteric plexus (IC-MY) are multipolar cells forming a reticular network. The network of IC-MY which is believed to be the origin of electrical slow waves is morphologically independent from but associated with the myenteric plexus. On the other hand, intramuscular ICC (IC-IM) usually have spindle-shaped contours arranged in parallel with the bulk smooth muscle cells. Associated with nerve bundles and blood vessels, the IC-IM possess receptors for neurotransmitters and such circulating hormones as cholecystokinin, suggesting their roles in neuromuscular and hormone-muscular transmissions. In addition, gap junctions connect the IC-MY and IC-IM, thereby realizing the electrically synchronized integrity of ICC as a pacemaker system in the gut. The smooth muscle cells are also coupled with ICC via gap junctions, and the functional unit thus formed enables rhythmically synchronized contractions and relaxations. It has recently been found that a lack of Kit-expressing cells may induce hyper-contractility of the tunica muscularis in vitro, whereas a decrease in Kit expression within the muscle wall causes dysmotility-like symptoms in vivo. The pacemaker system in the gut thus seems to play a critical role in the maintenance of both moderate and normal motility of the digestive tract. A loss of Kit positive cells has been detected in several diseases with an impaired motor activity, including diabetic gastroenteropathy. Pathogenesis of these diseases is thought to be accounted for by impaired slow waves and neuromuscular transmissions; a pacemaker disorder may possibly induce a dysmotility-like symptom called 'gastroenteric arrhythmia'. A knowledge of the structure and function of the ICC and the pacemaker system provides a basis for clarifying the normal mechanism and the pathophysiology of motility in the digestive tract.

The spatial behaviour of spike patches in the feline gastroduodenal junction in vitro

In the isolated feline gastroduodenal region, the spatial propagation of slow waves and of individual spikes was reconstructed. Recordings were performed simultaneously from 240 extracellular electrodes positioned on the serosal surface across the junction. Results from nine experiments (22 slow waves) showed that the slow wave never propagated across the gastroduodenal region and that this block was due to the presence of a zone of quiescence caudal to the pylorus. In contrast, spikes (n=155) were able to propagate into the quiescent zone, either from the antrum (15.4%) or from the duodenum (34.0%) and occasionally, were able to propagate from one organ to the other (10.9%). However, in all cases, spike conduction was self-limited and activated a local area termed a 'patch'. The length of the patches located in the gastroduodenal region was significantly longer than in the rest of the duodenum (20.2 mm +/- 9. 7 vs. 9.5 mm +/- 3.2; P < 0.001) indicating a possible enhancement of spike propagation in this region. In conclusion, in spite of the total conduction block for slow waves, individual spikes are able to propagate across the gastroduodenal region, albeit in self-limited areas or 'patches'. These spike patches could form the building blocks for gastroduodenal coordination.

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.

Two populations of smooth muscle cells in the guinea-pig gastric antrum

K+ outward currents (I[K]) expressed by guinea-pig antral smooth muscle cells were studied using the whole-cell voltage-clamp technique. In about 88% of cells depolarization steps applied from Vh = -70 mV activated a fast transient component (I[K(to)]) with voltage-dependent characteristics, and a noninactivating component with slow activation kinetics (I[K(sl)]). Both components were carried by K+ ions. Apamin (10 nM to 1 microM) selectively depressed I(K[to]) in a concentration-dependent manner. I(K(sl)) was blocked by 1 mM tetraethylammonium or 0.1 microM charybdotoxin. 10 mM tetraethylammonium abolished both components of I(K). Nicardipine (1 microM) did not affect the voltage- and time-dependent characteristics of the net I(K), but reduced the current density of I(K[sl]) from 22.36+/-1.38 microA/cm2 to 13.06+/-0.92 microA/cm2 at +40 mV. In about 12% of the cells depolarization-evoked I(K) could be separated as two pharmacologically distinguishable components: a glipizide-sensitive current (forming about 70% of the net I[K]) and a charybdotoxin-sensitive current (30% of the net I[K]). Nicardipine (1 microM) affected neither the amplitude nor the time-course of I(K) of this cell population. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM) or ryanodine (1 microM) led to a 50-200% increase of I(K[sl]) in the majority of cells and to an about 30% increase of the net I(K) in 12% of cells. The data obtained suggest the existence of at least two populations of cells in guinea-pig antral smooth muscle. Twelve percent of cells seem to be responsible for the generation of slow wave potentials, while 88% of cells most probably respond passively to the electrotonically spread depolarization.

Phasic contractions of the muscular components of human esophagus and gastroesophageal junction in vitro

This study was performed to assess the repetitive phasic mechanical and (or) electrical activity of the muscle from different regions of the human gastroesophageal junction (GEJ). Muscle strips from the circular and longitudinal layers of the gastric fundus and esophagus and of the clasp and sling components of the GEJ were obtained from surgical specimens and prepared for in vitro recording of contractile or electrical activity. Phasic contractions occurred in all regions except the longitudinal muscle of the gastric fundus and that overlying the sling. Robust phasic activity (2.6 +/- 0.6 min-1) was most frequent (92% of specimens) in longitudinal muscle overlying the clasp, arising spontaneously in 67%. Stretch or carbachol stimulation increased the frequency of these contractions. Transmural electrical stimulation produced a transient cessation of phasic activity. Electrical recording showed slow waves with superimposed spiking coinciding with phasic contractions. These activities were unaltered by 1 microM atropine or 1 microM tetrodotoxin, but inhibited by 2 microM verapamil. In conclusion, several muscles of the human esophagus and GEJ manifest repetitive contractions in vitro, particularly the longitudinal muscle overlying the clasp muscle fibers. These oscillations are due to electrical slow waves, can potentially be modulated by intrinsic nerves, and may play a role in the intermittent phasic contractions of lower esophageal sphincter pressure in vivo.

Intercellular communication in smooth muscle

The functioning of a group of cells as a tissue depends on intercellular communication; an example is the spread of action potentials through intestinal tissue resulting in synchronized contraction. Recent evidence for cell heterogeneity within smooth muscle tissues has renewed research into cell coupling. Electrical coupling is essential for propagation of action potentials in gastrointestinal smooth muscle. Metabolic coupling may be involved in generation of pacemaker activity. This review deals with the role of cell coupling in tissue function and some of the issues discussed are the relationship between electrical synchronization and gap junctions, metabolic coupling, and the role of interstitial cells of Cajal in coupling.

Intercellular dye-coupling in intestinal smooth muscle. Are gap junctions required for intercellular coupling?

The dye Lucifer Yellow was injected into single smooth muscle cells in the guinea pig small intestine in order to study intercellular coupling. Dye-coupling was observed in both the circular and longitudinal muscle layers and was markedly reduced when the intercellular pH was lowered. These results suggest the presence of gap junctions among intestinal muscle cells, but are inconsistent with previous ultrastructural studies that failed to demonstrate such junctions in the longitudinal muscle.

Three-dimensional observation of the fibroblast-like cells associated with the rat myenteric plexus, with special reference to the interstitial cells of Cajal

An extensive cellular network becomes visible over the myenteric plexus of the rat after removal of the overlying tissues under the scanning electron microscope. The cells are mainly stellate and have many slender processes via which they interconnect. They form a three-dimensional network and are closely associated with the ganglia and nerve bundles, and also extend over the smooth muscle cells. They are considered to correspond to the interstitial cells of Cajal because of their peculiar arrangement and their topography. Transmission electron-microscopic evidence demonstrates that the majority of those cells have features of fibroblasts. Gap junctions and intermediate junctions are observed between these fibroblast-like cells, and also between them and smooth muscle cells. Examination of serial thin sections reveals that single fibroblast-like interstitial cells connect to both circular and longitudinal muscle cells via gap junctions. It is suggested that the network of interstitial cells conducts electrical signals.

Free-calcium and force transients during depolarization and pharmacomechanical coupling in guinea-pig smooth muscle

1. Fura2 was loaded by permeation and hydrolysis of the acetoxymethyl ester into smooth muscle cells of intact thin sheets of the longitudinal layer of the small intestine of the guinea-pig, to record Ca2+ transients during contraction. 2. Cytoplasmic Ca2+ ([Ca2+]i) was monitored by computing the ratio of the fluorescence signal excited at 340 and 380 nm wavelengths. The dye loading and the exposure to UV light required for the experiments had no significant effect on the contractile parameters observed. 3. Spontaneous, rhythmic increases in [Ca2+]i were often observed, preceding the onset of force. Removal of extracellular Ca2+ caused a very transient increase in [Ca2+]i accompanied by a phasic force transient; this was followed by a decline in [Ca2+]i and tension below control levels. Elevated Ca2+ from 1.2 to 15 mM also caused a fall in [Ca2+]i and a relaxation of basal tension. 4. Elevation of [K+]o increased [Ca2+]i. Graded concentrations of K+ caused graded changes in both fluorescence ratio and tension. 5. Carbachol evoked a transient increase in [Ca2+]i and contraction. Thereafter, in spite of the continued presence of the drug, both signals declined, presumably as the result of cholinergic desensitization. The initial phasic force response to carbachol was usually followed by an 'after-contraction', that was only occasionally accompanied by a similar (small) secondary rise in the fluorescence signal. 6. In depolarized smooth muscle, both in the presence and in the absence of extracellular Ca2+, carbachol induced a transient increase in [Ca2+]i, indicating that Ca2+ release from intracellular stores is a major mechanism of pharmacomechanical coupling. 7. In some preparations an applied stretch caused, after a few seconds, a rise in [Ca2+]i and force development.

Origin and propagation of electrical slow waves in circular muscle of canine proximal colon

Experiments to determine the site of slow-wave origin and the mechanism of propagation were performed on muscles of the canine proximal colon. Cells along the submucosal border of the circular layer had resting membrane potentials (RMP) averaging -78 mV, and slow waves, 40 mV in amplitude. The RMP of cells through the thickness of the circular layer decreased exponentially with distance from the submucosal border, such that RMPs of circular cells at the myenteric border were only -43 mV. Slow waves decreased in amplitude through the thickness such that slow waves could not be detected adjacent to the myenteric border. When a thin strip of muscle along the submucosal border was removed, slow waves were not recorded from the bulk of the circular layer and could not be evoked by acetylcholine. Slow waves were still present in the excised strip. Experiments to determine the rate of slow-wave propagation were also performed. Two cells were impaled, one at the submucosal surface, and another at some distance through the circular layer. Slow waves occurred nearly simultaneously at both sites. What latency was observed could be explained on the basis of electrotonic conduction. The results support the hypothesis that in the canine proximal colon slow waves are generated at the extreme submucosal surface of the circular layer. The bulk of the circular layer does not possess either pacemaker or regenerative mechanisms, and slow waves propagate passively toward the myenteric border. The cable properties of the circular muscle syncytium furnish a barrier to invasion of the longitudinal layer by the slow wave event.

Motoneurones of the submucous plexus regulate electrical activity of the circular muscle of canine proximal colon

The hypothesis that the circular muscle of the canine proximal colon receives motor input from neurones in the submucous plexus was tested. Circular muscle cells were impaled with micro-electrodes and submucous plexus neurones were stimulated by electrical field stimulation and microejection of acetylcholine (ACh). In the presence of atropine to block the direct muscarinic effects, microejection of ACh onto the submucosa where intact submucous ganglia were suspended evoked: (i) an inhibitory junction potential (i.j.p.) that reduced the amplitude, duration and rate of rise of the subsequent slow wave; (ii) a slow wave of increased duration following the initial inhibitory response. These responses were enhanced by increasing the volume of ACh administered. Responses to ACh were blocked by hexamethonium, 10(-4) M; d-tubocurarine, 10(-4) M; or tetrodotoxin (TTX), 10(-6) M, suggesting they were neural in origin. Both inhibitory and excitatory responses were the result of non-cholinergic and non-adrenergic nerves. The transmitters mediating these effects are unknown. Removal of the longitudinal muscle, myenteric plexus, and the serosal portion of the circular muscle had no apparent effect on the responses to application of ACh to submucosal ganglia. In these preparations the responses to field stimulation were identical to those produced by ACh. The submucous plexus also provides cholinergic input to the circular muscle. When ACh was discretely applied to the submucosa cholinergic responses were elicited at the muscle cell which were significantly reduced by hexamethonium or TTX. These findings suggest that the cholinergic responses were the result of ACh release by neurones at the effector and not by overflow of the exogenous ACh. Cholinergic responses were also elicited in preparations in which the myenteric plexus had been removed. Slow waves in circular muscle of the proximal colon yield excitation-contraction coupling in the absence of Ca2+ action potentials. Therefore the influence of submucous neurones on electrical slow waves has direct consequences on motor activity. Reduction in the amplitude and duration of slow wave by i.j.p.s. results in reduction in the amplitude and duration of phasic contractions. Excitatory inputs enhance contractions. The data support a new concept: motoneurones emanating from submucous ganglia innervate the circular muscle and provide inhibitory and excitatory inputs to regulate slow wave activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Enteric neural regulation of slow waves in circular muscle of the canine proximal colon

The spontaneous electrical activities of circular muscle cells of the canine proximal colon were studied with intracellular micro-electrodes. All circular muscle cells exhibited slow waves at frequencies ranging between 2.8 and 7.0 cycles/min. The slow waves consisted of an upstroke phase followed by a plateau phase of variable duration (2-40 s). Many cells displayed a slow diastolic depolarization, or 'pre-potential' between slow waves. Slow waves spontaneously varied in duration and frequency in most preparations, creating distinctive slow wave patterns. Atropine, 2 X 10(-6) M, decreased the durations of slow waves in many preparations and often changed the pattern to a series of relatively uniform slow waves. A further reduction in mean slow wave duration was produced by additional treatment with tetrodotoxin, 10(-6) M. These results suggested that slow wave duration and pattern were affected by spontaneous discharge from both cholinergic and non-cholinergic excitatory nerves. Transmural nerve stimulation caused a short latency increase in slow wave duration (up to 38 s) that was abolished by atropine. In the presence of atropine, transmural stimulation evoked inhibitory junction potentials that reduced the amplitude and duration of the subsequent slow wave. The slow wave of reduced amplitude was followed by a slow wave of increased duration. The increase in duration of the slow wave did not appear to be related to the size of the preceding hyperpolarization, suggesting it was mediated by the release from non-cholinergic excitatory nerves. All responses to transmural stimulation were blocked by tetrodotoxin. Microejection of acetylcholine on to the muscle adjacent to the micro-electrode also produced an atropine-sensitive increase in slow wave duration. Tissues that had been stored in the cold overnight to reduce intrinsic neural activity exhibited regular slow waves of short duration. It is proposed that the basic myogenic pattern of spontaneous slow wave activity consists of regularly occurring slow waves of short duration (2-5 s). Intrinsic cholinergic and non-cholinergic excitatory nerves appear to modulate slow wave activity in vitro, producing distinctive slow wave patterns of variable instantaneous frequency and duration.

Electrophysiology of smooth muscle of the small intestine of some mammals

Intracellular recordings were made from cells located in the longitudinal, inner and outer circular muscle layers of the dog, cat, rabbit, opossum and human small intestine. In whole-thickness preparations in all five species, longitudinal muscle cells generated slow waves and spikes. However, in isolated longitudinal muscle preparations, all cells tested were electrically silent. In whole-thickness and in isolated preparations, cells in the inner circular muscle layer generated spontaneous spikes superimposed on slow potentials. However, the occurrence of spikes and slow potentials was more regular in whole-thickness preparations. In whole-thickness preparations, cells in the outer circular muscle layer generated slow waves which were coupled with phasic contractions. However, in isolated outer circular muscle preparations, all cells tested were electrically silent and spontaneous phasic contractions were absent. In whole-thickness preparations, non-neural cells located on the serosal side of the outer circular muscle layer generated slow waves. The data suggest that spontaneous slow waves of the small intestine of the dog, cat, rabbit, opossum and human are generated in non-neural cells located between the longitudinal and outer circular muscle layer and by non-neural cells located between the outer and inner circular muscle layers.

Gradient in excitation-contraction coupling in canine gastric antral circular muscle

Slow waves decay in amplitude as they propagate through the thickness of circular muscle of the canine antrum. Slow waves are the excitable events that initiate contractions in the antrum. Excitation-contraction coupling occurs if slow wave depolarizations surpass a 'mechanical threshold'. The amplitude of slow waves recorded from circular muscle cells near the submucosa was insufficient to reach the mechanical threshold previously determined for muscle near the myenteric plexus, suggesting that either submucosal cells are normally mechanically quiescent, or that contractions of submucosal cells are initiated at more polarized levels. Experiments were performed to determine the voltage-tension relationships in adjacent 'myenteric' and 'submucosal' circular muscles. Membrane potentials of the muscles were depolarized by elevated concentrations of potassium. Submucosal muscles were stimulated to contract at lower potassium concentrations than were myenteric muscles. Contractions of submucosal muscles at each potassium concentration studied were more forceful than contractions of myenteric muscles. Plots of membrane potential vs. potassium concentration on a logarithmic scale showed that the membrane potential of myenteric cells was more dependent upon the potassium gradient than the membrane potential of submucosal cells. The potassium permeability of both groups of cells increased when depolarized, and the slopes of these plots approached Nernstian levels when depolarized below -55 mV. Force developed in submucosal strips at more polarized levels than in myenteric muscles. The 'mechanical threshold' of submucosal muscles was 5-10 mV above resting potential, whereas myenteric muscles had to be depolarized by 25-30 mV before contraction was initiated. The mechanisms responsible for the difference in mechanical thresholds are not known, but differences in the voltage dependence of calcium channels, in calcium release mechanisms, or in the sensitivity of the contractile proteins to calcium could be involved.

Slow wave heterogeneity within the circular muscle of the canine gastric antrum

A cross-sectional preparation was designed in which multiple micro-electrodes can be precisely positioned to impale smooth muscle cells anywhere from the serosa to the submucosa. Intracellular electrical recordings were obtained from gastric antral circular muscle cells from the myenteric plexus to the submucosa. The resting membrane potential changed linearly as a function of distance from the myenteric plexus to the submucosa. Slow wave upstroke dV/dt, upstroke potential amplitude, and plateau potential amplitude changed linearly as a function of distance from the myenteric plexus to the submucosa. When slow waves were recorded simultaneously from a circular cell near the myenteric plexus and from a cell near the submucosa, the event always occurred first in the cell near the myenteric plexus. Electrical differences did not appear to be caused by electrotonic decay of slow waves as they propagated through the circular muscle. Electrical differences could not be explained on the basis of differences in intrinsic neural activity or prostaglandin synthesis. Membrane polarization could not explain the differences in slow waves between myenteric and submucosal circular muscle cells. The conclusion of this paper is that fundamental differences exist between the excitability mechanisms and/or passive membrane properties of cells near the myenteric plexus and the submucosa. These differences might be manifest in different motor performance of these two muscle cell populations.

Effects of alterations in calcium levels on cat small intestinal slow waves

Intact segments of cat intestinal muscle and strips of isolated longitudinal muscle were treated with agents that reduce intracellular calcium concentration: incubation in 0-calcium saline, treatment with calcium conductance blockers, elevated extracellular magnesium concentration, or alkalinization with NH4Cl. These treatments reduced amplitude and frequency of slow waves in intact segments but only reduced frequency in isolated longitudinal muscle. The reduction in frequency was characterized by prolongation of the hyperpolarized phase of the slow waves. Treatments that would moderately increase intracellular calcium concentration, i.e., increasing external calcium to four times normal levels or lowering pH by CO2, increased slow-wave frequency. Increased frequency was associated with reduced amplitude and shortening of the hyperpolarized phase of the slow waves. Greater than four times normal calcium levels and intense spiking reduced slow-wave frequency. Chlorotetracycline fluorescence, an indicator of intracellular calcium concentration, showed fluctuations synchronous with slow waves. It is concluded that the reactions that pace the generation of slow waves are dependent on the level of intracellular calcium.

Depolarization-induced contractile activity of smooth muscle in calcium-free solution

In calcium-free solution, strips of cat intestinal muscle developed slow, rhythmic electrical potential changes that triggered contractions. Some strips failed to develop spontaneous electrical activity in calcium-free solution but responded with contractions to depolarization by direct electrical stimulation or by treatment with barium chloride, potassium chloride, or acetylcholine. Similar results were obtained with segments of cat stomach, colon, esophagus, bladder, uterus, and vena cava, as well as with rabbit vena cava. In calcium-free saline, rat small intestinal muscle showed fast electrical activity with accompanying development of a tetanuslike contraction. After 60 min in calcium-free solution, cat small intestinal muscle retained 17.7% of its original concentration of calcium. It is concluded that in some smooth muscles, depolarization-triggered release of intracellular calcium does not require an associated influx of calcium.

Dominance of longitudinal muscle in propagation of intestinal slow waves

The purpose of these experiments was to test the hypothesis that circular muscle plays an active role in the propagation of intestinal slow waves, specifically be providing excitatory current through a process of regenerative amplification. With volume-recording techniques and microelectrode recordings we obtained the following results that are not consistent with such a mechanism: 1) slow waves propagated without delay or decrease in amplitude along segments of cat jejunum devoid of a ring of circular muscle up to 3 mm wide, i.e., across a longitudinal muscle bridge more than 4 space constants long (9 of 11 preparations) but did not propagate across a circumferential cut through the longitudinal muscle layer (14 of 14 preparations); 2) the membrane current associated with the slow wave had a pronounced inward component when recorded from either the serosal or the mucosal side of the longitudinal muscle bridge but was entirely outward when recorded from either the mucosal or the serosal side of exposed circular muscle, including those preparations in which various thicknesses of circular muscle were removed from the mucosal side of the recording area; 3) slow-wave amplitudes recorded intracellularly from intact (n = 9) and isolated (n = 8) longitudinal muscle preparations were not significantly different (27.0 +/- 4.3 vs. 25.4 +/- 5.3 (SD) mV); 4) after 30 min in 4.4 X 10(-6) M verapamil, slow-wave amplitude did not significantly decrease, although contractile activity had long since terminated. These results are more consistent with the hypothesis that longitudinal muscle provides most, if not all, of the current required for slow-wave propagation in the small intestine.

Comparative study of the smooth muscle layers of the rabbit duodenum

1. Intracellular electrodes were used to compare the electrical activity of smooth muscle cells from the longitudinal and circular layers of the rabbit duodenum and their responses to stimulation of the intramural nerves. 2. The longitudinal muscle cells had an average membrane potential of 52 mV when measured between slow waves. Spontaneous action potentials were superimposed on every slow wave. 3. The circular muscle cells had a higher membrane potential of 64 mV although the amplitude of the slow waves was similar to that of the longitudinal muscle cells. Spontaneous action potentials were rarely observed in the circular muscle cells. 4. Lowering the temperature from 36 to 30 degrees C caused a reduction in the membrane potential of the longitudinal muscle cells but not in the circular muscle cells. However, the amplitude of the slow waves of the two layers was reduced to a similar extent. 5. Electrical stimulation produced advances of the slow wave cycles if the stimuli were applied between slow waves. The responses of the cells from the two layers were identical. 6. Under normal conditions, electrotonic coupling was observed only in cells of the muscle layer whose long axis was aligned along the direction of the applied current. 7. In the longitudinal muscle, cholinergic responses blocked by atropine were observed. Inhibitory potentials were the predominant response in the circular muscle. 8. Excitatory responses were recorded in 9% of the circular muscle cells. "Off' excitation following termination of a train of repetitive stimulation pulses was also observed. 9. The differences in membrane potentials, spontaneous spiking activities, neural responses, and the failure to demonstrate good electrotonic coupling between the muscle layers suggest that there was poor electrotonic interaction between the muscle layers. The amplitude of the slow waves of the two layers was nevertheless similar. Thus the validity of the hypothesis that slow waves were transmitted passively from the longitudinal layer into the circular layer through electrotonic coupling must be questioned.

The interaction of amine local anaesthetics with muscarinic receptors

1 Amine local anaesthetics inhibited the binding of (-)-[3H]-quinuclidinyl benzilate ((-)-[3H]-QNB) to muscarinic receptors in crude synaptosomal preparations from guinea-pig brain. The order of potency was SKF 525A greater than tetracaine greater than procaine congruent to quinidine greater than procainamide greater than bupivacaine greater than lignocaine greater than prilocaine. 2 The concentration of tetracaine or prilocaine causing 50% inhibition of the receptor-specific binding of [3H]-QNB varied linearly with the concentration of [3H]-QNB present for the range of concentrations of prilocaine used and at lower concentrations of tetracaine, thus providing evidence for a competitive interaction. The affinity constant for tetracaine was 2.6 +/- 0.2 x 10(5) M-1 and that for prilocaine 2.6 +/- 0.8 x 10(3) M-1. At higher concentrations of tetracaine the interaction appears to diverge from simple competitive kinetics. 3 The log dose-response curve for the contractile response of longitudinal muscle strips from guinea-pig intestine to carbachol was shifted in a parallel fashion by low concentrations of tetracaine, but flattened by higher doses. A similar effect was observed for both lignocaine and prilocaine. The affinity constants for tetracaine and prilocaine calculated from the parallel shifts, 1 x 10(5) M-1 and 4 x 10(3) M-1, respectively, were in reasonable accord with the binding data. 4 The curve for the inhibition of [3H]-QNB binding by carbachol was not significantly altered, either in position or shape, in the presence of 1 mM prilocaine. Thus there is no evidence that prilocaine, which increases the affinity of nicotinic acetylcholine receptors for agonists, has any similar effect on agonist binding to muscarinic receptors.

The electrical activity recorded from smooth muscle of the circular layer of the human stomach

The membrane properties of circular muscles of 55 human stomachs were investigated by microelectrode and double sucrose gap methods. The membrane potential of the circular muscle of the corpus region was -57 mV and no regional difference was evident as compared with tissues from the antrum and cardia. The stomach muscle presented cable like properties, and the length constant measured in the corpus region was 1.34 mm. The circular muscle of all regions of the stomach exhibited slow waves. The amplitude and duration of slow waves varied markedly (the mean values were 18 mV and 6 s, respectively). The Q10 value for the slow wave was 2.4. The slow wave could be divided into two different components (first and second component) by application of electrical current or by using solutions with various ionic environments. Na ions had more effect on the spike component and Ca ions on the second component. The generation of the first component of the slow wave was blocked by either Na-free, K-free, Ca-free, or Cl-deficient solution but this component reappeared by application of outward current pulse, except in Cl-deficient solution. These results suggest that the generation of slow wave depends on more than one type of ion and that metabolic factors do indeed play a role. Membrane properties of the human stomach were compared with those of the guinea-pig stomach.