Myogenic electrical control activity in longitudinal muscle of human and dog colon

Interstitial cells of Cajal and human colon motility in health and disease

Our understanding of human colonic motility, and autonomic reflexes that generate motor patterns, has increased markedly through high-resolution manometry. Details of the motor patterns are emerging related to frequency and propagation characteristics that allow linkage to interstitial cells of Cajal (ICC) networks. In studies on colonic motor dysfunction requiring surgery, ICC are almost always abnormal or significantly reduced. However, there are still gaps in our knowledge about the role of ICC in the control of colonic motility and there is little understanding of a mechanistic link between ICC abnormalities and colonic motor dysfunction. This review will outline the various ICC networks in the human colon and their proven and likely associations with the enteric and extrinsic autonomic nervous systems. Based on our extensive knowledge of the role of ICC in the control of gastrointestinal motility of animal models and the human stomach and small intestine, we propose how ICC networks are underlying the motor patterns of the human colon. The role of ICC will be reviewed in the autonomic neural reflexes that evoke essential motor patterns for transit and defecation. Mechanisms underlying ICC injury, maintenance, and repair will be discussed. Hypotheses are formulated as to how ICC dysfunction can lead to motor abnormalities in slow transit constipation, chronic idiopathic pseudo-obstruction, Hirschsprung's disease, fecal incontinence, diverticular disease, and inflammatory conditions. Recent studies on ICC repair after injury hold promise for future therapies.

Comparison of Canine and Human Physiological Factors: Understanding Interspecies Differences that Impact Drug Pharmacokinetics

This review is a summary of factors affecting the drug pharmacokinetics (PK) of dogs versus humans. Identifying these interspecies differences can facilitate canine-human PK extrapolations while providing mechanistic insights into species-specific drug in vivo behavior. Such a cross-cutting perspective can be particularly useful when developing therapeutics targeting diseases shared between the two species such as cancer, diabetes, cognitive dysfunction, and inflammatory bowel disease. Furthermore, recognizing these differences also supports a reverse PK extrapolations from humans to dogs. To appreciate the canine-human differences that can affect drug absorption, distribution, metabolism, and elimination, this review provides a comparison of the physiology, drug transporter/enzyme location, abundance, activity, and specificity between dogs and humans. Supplemental material provides an in-depth discussion of certain topics, offering additional critical points to consider. Based upon an assessment of available state-of-the-art information, data gaps were identified. The hope is that this manuscript will encourage the research needed to support an understanding of similarities and differences in human versus canine drug PK.

Pacemaker function and neural responsiveness of subserosal interstitial cells of Cajal in the mouse colon

KEY POINTS

Rhythmic action potentials and intercellular Ca²⁺ waves are generated in smooth muscle cells of colonic longitudinal muscles (LSMC). Longitudinal muscle excitability is tuned by input from subserosal ICC (ICC-SS), a population of ICC with previously unknown function. ICC-SS express Ano1 channels and generate spontaneous Ca²⁺ transients in a stochastic manner. Release of Ca²⁺ and activation of Ano1 channels causes depolarization of ICC-SS and LSMC, leading to activation of L-type Ca²⁺ channels, action potentials, intercellular Ca²⁺ waves and contractions in LSMC. Nitrergic neural inputs regulate the Ca²⁺ events in ICC-SS. Pacemaker activity in longitudinal muscle is an emergent property as a result of integrated processes in ICC-SS and LSMC.

ABSTRACT

Much is known about myogenic mechanisms in circular muscle (CM) in the gastrointestinal tract, although less is known about longitudinal muscle (LM). Two Ca²⁺ signalling behaviours occur in LM: localized intracellular waves not causing contractions and intercellular waves leading to excitation-contraction coupling. An Ano1 channel antagonist inhibited intercellular Ca²⁺ waves and LM contractions. Ano1 channels are expressed by interstitial cells of Cajal (ICC) but not by smooth muscle cells (SMCs). We investigated Ca²⁺ signalling in a novel population of ICC that lies along the subserosal surface of LM (ICC-SS) in mice expressing GCaMP6f in ICC. ICC-SS fired stochastic localized Ca²⁺ transients. Such events have been linked to activation of Ano1 channels in ICC. Ca²⁺ transients in ICC-SS occurred by release from stores most probably via inositol trisphosphate receptors. This activity relied on influx via store-operated Ca²⁺ entry and Orai channels. No voltage-dependent mechanism that synchronized Ca²⁺ transients in a single cell or between cells was found. Nitrergic agonists inhibited Ca²⁺ transients in ICC-SS, and stimulation of intrinsic nerves activated nitrergic responses in ICC-SS. Cessation of stimulation resulted in significant enhancement of Ca²⁺ transients compared to the pre-stimulus activity. No evidence of innervation by excitatory, cholinergic motor neurons was found. Our data suggest that ICC-SS contribute to regulation of LM motor activity. Spontaneous Ca²⁺ transients activate Ano1 channels in ICC-SS. Resulting depolarization conducts to SMCs, depolarizing membrane potential, activating L-type Ca²⁺ channels and initiating contraction. Rhythmic electrical and mechanical behaviours of LM are an emergent property of SMCs and ICC-SS.

Spontaneous Electrical Activity and Rhythmicity in Gastrointestinal Smooth Muscles

The gastrointestinal (GI) tract has multifold tasks of ingesting, processing, and assimilating nutrients and disposing of wastes at appropriate times. These tasks are facilitated by several stereotypical motor patterns that build upon the intrinsic rhythmicity of the smooth muscles that generate phasic contractions in many regions of the gut. Phasic contractions result from a cyclical depolarization/repolarization cycle, known as electrical slow waves, which result from intrinsic pacemaker activity. Interstitial cells of Cajal (ICC) are electrically coupled to smooth muscle cells (SMCs) and generate and propagate pacemaker activity and slow waves. The mechanism of slow waves is dependent upon specialized conductances expressed by pacemaker ICC. The primary conductances responsible for slow waves in mice are Ano1, Ca2+-activated Cl- channels (CaCCs), and CaV3.2, T-type, voltage-dependent Ca2+ channels. Release of Ca2+ from intracellular stores in ICC appears to be the initiator of pacemaker depolarizations, activation of T-type current provides voltage-dependent Ca2+ entry into ICC, as slow waves propagate through ICC networks, and Ca2+-induced Ca2+ release and activation of Ano1 in ICC amplifies slow wave depolarizations. Slow waves conduct to coupled SMCs, and depolarization elicited by these events enhances the open-probability of L-type voltage-dependent Ca2+ channels, promotes Ca2+ entry, and initiates contraction. Phasic contractions timed by the occurrence of slow waves provide the basis for motility patterns such as gastric peristalsis and segmentation. This chapter discusses the properties of ICC and proposed mechanism of electrical rhythmicity in GI muscles.

Neurogenic and myogenic patterns of electrical activity in isolated intact mouse colon

BACKGROUND

Relatively little is known about the electrical rhythmicity of the whole colon, where long neural pathways are preserved.

METHODS

Smooth muscle electrical activity was recorded extracellularly from the serosa of isolated flat-sheet preparations consisting of the whole mouse colon (n=31).

KEY RESULTS

Two distinct electrical patterns were observed. The first, long intense spike bursts, occurred every 349±256 seconds (0.2±0.2 cpm), firing action potentials for 31±11 seconds at 2.1±0.5 Hz. They were hexamethonium- and tetrodotoxin-sensitive, but persisted in nicardipine as 2 Hz electrical oscillations lacking action potentials. This pattern is called here neurogenic spike bursts. The second pattern, short spike bursts, occurred about every 30 seconds (2.0±0.6 cpm), with action potentials firing at about 1 Hz for 9 seconds (1.0±0.2 Hz, 9±4 seconds). Short spike bursts were hexamethonium- and tetrodotoxin-resistant but nicardipine-sensitive and thus called here myogenic spike bursts. Neurogenic spike bursts transiently delayed myogenic spike bursts, while blocking neurogenic activity enhanced myogenic spike burst durations. External stimuli significantly affected neurogenic but not myogenic spike bursts. Aboral electrical or mechanical stimuli evoked premature neurogenic spike bursts. Circumferential stretch significantly decreased intervals between neurogenic spike bursts. Lesioning the colon down to 10 mm segments significantly increased intervals or abolished neurogenic spike bursts, while myogenic spike bursts persisted.

CONCLUSIONS & INFERENCES

Distinct neurogenic and myogenic electrical patterns were recorded from mouse colonic muscularis externa. Neurogenic spike bursts likely correlate with neurogenic colonic migrating motor complexes (CMMC) and are highly sensitive to mechanical stimuli. Myogenic spike bursts may correspond to slow myogenic contractions, whose duration can be modulated by enteric neural activity.

Relaxin Affects Smooth Muscle Biophysical Properties and Mechanical Activity of the Female Mouse Colon

The hormone relaxin (RLX) has been reported to influence gastrointestinal motility in mice. However, at present, nothing is known about the effects of RLX on the biophysical properties of the gastrointestinal smooth muscle cells (SMCs). Other than extending previous knowledge of RLX on colonic motility, the purpose of this study was to investigate the ability of the hormone to induce changes in resting membrane potential (RMP) and on sarcolemmal ion channels of colonic SMCs of mice that are related to its mechanical activity. To this aim, we used a combined mechanical and electrophysiological approach. In the mechanical experiments, we observed that RLX caused a decay of the basal tone coupled to an increase of the spontaneous contractions, completely abolished by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]-quinoxalin-1-one (ODQ). The electrophysiological results indicate for the first time that RLX directly affects the SMC biophysical properties inducing hyperpolarization of RMP and cycles of slow hyperpolarization/depolarization oscillations. The effects of RLX on RMP were abolished by ODQ as well as by a specific inhibitor of the cGMP-dependent protein kinase, KT5823. RLX reduced Ca(2+) entry through the voltage-dependent L-type channels and modulated either voltage- or ATP-dependent K(+) channels. These effects were abolished by ODQ, suggesting the involvement of the nitric oxide/guanylate cyclase pathway in the effects of RLX on RMP and ion channel modulation. These actions of RLX on membrane properties may contribute to the regulation of the proximal colon motility by the nitric oxide/cGMP/cGMP-dependent protein kinase pathway.

Quantification of in vivo colonic motor patterns in healthy humans before and after a meal revealed by high-resolution fiber-optic manometry

BACKGROUND

Until recently, investigations of the normal patterns of motility of the healthy human colon have been limited by the resolution of in vivo recording techniques.

METHODS

We have used a new, high-resolution fiber-optic manometry system (72 sensors at 1-cm intervals) to record motor activity from colon in 10 healthy human subjects.

KEY RESULTS

In the fasted colon, on the basis of rate and extent of propagation, four types of propagating motor pattern could be identified: (i) cyclic motor patterns (at 2-6/min); (ii) short single motor patterns; (iii) long single motor patterns; and (iv) occasional retrograde, slow motor patterns. For the most part, the cyclic and short single motor patterns propagated in a retrograde direction. Following a 700 kCal meal, a fifth motor pattern appeared; high-amplitude propagating sequences (HAPS) and there was large increase in retrograde cyclic motor patterns (5.6 ± 5.4/2 h vs 34.7 + 19.8/2 h; p < 0.001). The duration and amplitude of individual pressure events were significantly correlated. Discriminant and multivariate analysis of duration, gradient, and amplitude of the pressure events that made up propagating motor patterns distinguished clearly two types of pressure events: those belonging to HAPS and those belonging to all other propagating motor patterns.

CONCLUSIONS & INFERENCES

This work provides the first comprehensive description of colonic motor patterns recorded by high-resolution manometry and demonstrates an abundance of retrograde propagating motor patterns. The propagating motor patterns appear to be generated by two independent sources, potentially indicating their neurogenic or myogenic origin.

A novel animal model of gastrointestinal obstruction for the development of stent

BACKGROUND

The need for newer gastrointestinal (GI) stents has been continuously raised. Newly developed stents are generally tested for physical properties in vitro and directly introduced to clinical practice because there is no reliable animal model of GI obstruction. The aim of this study was to establish an animal model both that can represent obstruction of the GI tract and be used to develop new stents.

MATERIAL AND METHODS

Surgical obstruction of the descending colon by wrapping with a nonabsorbable synthetic mesh and rubber bands was made in 17 healthy mongrel dogs. Four days later, a covered self-expanding metallic stent was placed for the obstructed segment in each dog under fluoroscopic guidance. Patency and migration of the inserted stents were evaluated clinically on a daily basis and fluoroscopically on a weekly basis. After sacrifice of the dogs, the degree and extent of residual colonic obstruction were assessed fluoroscopically. The specimen of the colonic obstructed segment was examined microscopically.

RESULTS

In all 17 mongrel dogs, segmental obstruction in the descending colon was successfully created and confirmed with fluoroscopic examination using a contrast medium. The percentage of luminal narrowing ranged from 99%-100%. Stent placement was technically successful in all 17 dogs. During the follow-up period, stent migration occurred in 12 dogs and indwelling time of a stent ranged from 0-95 d (mean 29.2 ± 38.8 d). On postmortem pathologic examination, it was found that fibrosis had newly formed outside the colonic longitudinal muscle layer in all dogs.

CONCLUSIONS

Our canine colonic obstruction model is the first animal model that can be feasible for developing a new design of stent and provide in vivo data on complications, particularly stent migration.

Role of interstitial cells of Cajal in the generation and modulation of motor activity induced by cholinergic neurotransmission in the stomach

BACKGROUND

Interstitial cells of Cajal (ICC) are intimately linked to the enteric nervous system and a better understanding of the interactions between the two systems is going to advance our understanding of gut motor control. The objective of the present study was to investigate the role of ICC in the generation of gastric motor activity induced by cholinergic neurotransmission.

METHODS

Gastric motor activity was evoked through activation of intrinsic cholinergic neural activity, in in vitro muscle strips by electrical field stimulation, in the in vitro whole stomach by distension and in vivo by fluoroscopy after gavaging the stomach with barium sulfate. The cholinergic activity was assessed as that component of the effect of the stimulus that was sensitive to atropine. These experiments were carried out in wild-type and Ws/Ws rats that have few intramuscular ICC (ICC-IM) in the stomach.

KEY RESULTS

Under all three experimental conditions, cholinergic activity was prominent in both wild-type and W mutant rats providing evidence against the hypothesis that cholinergic neurotransmission to smooth muscle is primarily mediated by ICC-IM. Strong cholinergic activity in Ws/Ws rats was not due to upregulation of muscarinic receptors in ICC but possibly in smooth muscle of the antrum.

CONCLUSIONS & INFERENCES

Pacemaker ICC play a prominent role in the expression of motor activity induced by cholinergic activity and our data suggest that cholinergic neurotransmission to ICC affects the pacemaker frequency.

Two independent networks of interstitial cells of cajal work cooperatively with the enteric nervous system to create colonic motor patterns

Normal motility of the colon is critical for quality of life and efforts to normalize abnormal colon function have had limited success. A better understanding of control systems of colonic motility is therefore essential. We report here a hypothesis with supporting experimental data to explain the origin of rhythmic propulsive colonic motor activity induced by general distention. The theory holds that both networks of interstitial cells of Cajal (ICC), those associated with the submuscular plexus (ICC-SMP) and those associated with the myenteric plexus (ICC-MP), orchestrate propagating contractions as pacemaker cells in concert with the enteric nervous system (ENS). ICC-SMP generate an omnipresent slow wave activity that causes propagating but non-propulsive contractions ("rhythmic propagating ripples") enhancing absorption. The ICC-MP generate stimulus-dependent cyclic depolarizations propagating anally and directing propulsive activity ("rhythmic propulsive motor complexes"). The ENS is not essential for both rhythmic motor patterns since distention and pharmacological means can produce the motor patterns after blocking neural activity, but it supplies the primary stimulus in vivo. Supporting data come from studies on segments of the rat colon, simultaneously measuring motility through spatiotemporal mapping of video recordings, intraluminal pressure, and outflow measurements.

Contractile properties of esophageal striated muscle: comparison with cardiac and skeletal muscles in rats

The external muscle layer of the mammalian esophagus consists of striated muscles. We investigated the contractile properties of esophageal striated muscle by comparison with those of skeletal and cardiac muscles. Electrical field stimulation with single pulses evoked twitch-like contractile responses in esophageal muscle, similar to those in skeletal muscle in duration and similar to those in cardiac muscle in amplitude. The contractions of esophageal muscle were not affected by an inhibitor of gap junctions. Contractile responses induced by high potassium or caffeine in esophageal muscle were analogous to those in skeletal muscle. High-frequency stimulation induced a transient summation of contractions followed by sustained contractions with amplitudes similar to those of twitch-like contractions, although a large summation was observed in skeletal muscle. The results demonstrate that esophageal muscle has properties similar but not identical to those of skeletal muscle and that some specific properties may be beneficial for esophageal peristalsis.

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

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

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.

Regional and transmural density of interstitial cells of Cajal in human colon and rectum

The interstitial cells of Cajal (ICC) are thought to play an important role in the control of gut motility. The regional and transmural pattern of distribution of ICC in the normal human colon and rectum was evaluated with immunohistochemistry using an anti-c-kit antibody. The transmural distribution of ICC was constant throughout the whole colon, the density of ICC was significantly greater at the myenteric plexus than at either the longitudinal or circular muscle layers, and in the rectum the transmural distribution was more even. Regionally, at the myenteric plexus, the transverse colon had a significantly greater density of ICC compared with the right colon (P = 0.038), left colon (P = 0.006), and rectum (P = 0.008). The pattern of distribution of ICC identified in this study is consistent with the proposed roles of ICC as colorectal pacemakers, intermediaries of the neural control of muscle activity, and coordinators of colorectal muscle activity. The highest density of ICC was at the myenteric plexus of the transverse colon, which is the proposed region of pacemaking activity.

Control of motility patterns in the human colonic circular muscle layer by pacemaker activity

1. This study characterized the electrical and mechanical activities of human colonic muscle strips obtained from either the ascending, descending or sigmoid colon of patient volunteers during elective colon resections. 2. Rhythmic contractile activity was observed in colonic circular muscle strips in the absence of external stimuli. This activity persisted in the presence of atropine, phentolamine, propranolol, tetrodotoxin and Nomega-nitro-L-arginine but was abolished by nifedipine. 3. The activity of whole circular muscle (WCM) was compared with that of the myenteric half (MCM), the submucosal half (SCM) and the interior (ICM) of the circular muscle layer. WCM exhibited a prominent 2-4 contractions min-1 contractile pattern which was also present in strips of SCM. In contrast, MCM and ICM exhibited slow (0.3-0.6 contractions min-1), long duration contractions with superimposed higher frequency contractions (17-18 contractions min-1). 4. Resting membrane potential (Vm), recorded at various positions through the thickness of WCM strips did not differ and averaged -50 mV. 5. Slow waves were observed in 83 % of muscles. They averaged 12 mV in amplitude, 9.4 s in duration and had a frequency of 2-4 contractions min-1. Slow waves were greatest in amplitude near the submucosal edge and decreased with distance away from this edge. Each slow wave was associated with a transient contraction. 6. Near the myenteric edge, rapid fluctuations of Vm with a mean frequency of 18 contractions min-1 were recorded in 67 % of muscles. Spiking activity was common and was superimposed upon slow waves and rapid Vm fluctuations. 7. In summary, slow waves were identified in the human colonic circular muscle layer which arise at or near the submucosal edge. These electrical events give rise to a 2-4 contractions min-1 contractile rhythm which is characteristic of the intact muscle layer. Thus, the nature and spatial organization of pacemaker activity in the human colon bears significant resemblance to other animal models, such as the dog and pig.

Effects of metoclopramide on gastrointestinal myoelectric activity in rats

AIM: To investigate the effects of metoclopramide (MCP) action on myoelectric activity in the antrum and small intestine.

METHODS: Ten healthy male Wistar rats, weighing 250-350 g, were anesthetized with ketamine hydrochloride (100 mg/kg, intramuscularly). Four pairs of bipolar stainless steel electrodes 3 mm apart were implanted on the serosal surface of the antrum at one, 10 and 20 cm distal to the pylorus. Five to ten days following the operation, the gastrointestinal myoelectric activity of fasted rats after intramuscular injection of 2.5, six and 12 mg/kg MCP was recorded using a 8-channel EEG machine, and these values were quantitatively compared with the myoelectric activity after saline injection.

RESULTS: In fasted rats, 2.5 mg/kg MCP increased the amplitude of spike activity (402.0 ± 138.4 μV, vs 345 ± 163.4 μV, P < 0.05) and the percentage of the slow wave-containing spike bursts (60.4% ± 22.0% vs 47.4% ± 22.5%, P < 0.01) of small intestine (1 cm distal to the pylorus), but did not affect the myoelectric activity of the antrum. Six and 12 mg/kg MCP increased the amplitude of both the slow wave (332.8 ± 200.1 μV vs 191.2 ± 143.9 μV, P < 0.01; 330.0 ± 197.1 μV vs 191.2 ± 143.9 μV, P < 0.05) and the spike activity of the antrum (180.5 ± 69.7 μV vs 121.8 ± 63.3 μV, P < 0.05; 174.5 ± 71.7 μV vs 123.8 ± 63.3 μV, P < 0.05), while in small intestine (1 cm distal to the pylorus) only the amplitude of spike activity (407.3 ± 179.0 μV vs 345.0 ± 163.4 μV, P < 0.05; 456.0 ± 145.4 μV vs 345.0 ± 163.4 μV, P < 0.05) and the percentage of the slow wave containing spike bursts (61.7% ± 26.5% vs 47.4% ± 22.5%, P < 0.01; 59.1% ± 17.3% vs 47.4% ± 22.5%, P < 0.01) was increased and the latent period significantly prolonged (2.5 ± 0.35 min vs 0.77 ± 0.18 min, P < 0.01).

CONCLUSION: Different mechanisms may be involved in enhancing the myoelectric activity of the antrum and small intestine following MCP administration.

Effects of metoclopramide on gastrointestinal myoelectric activity in rats

AIM

To investigate the effects of metoclopramide (MCP) action on myoelectric activity in the antrum and small intestine.

METHODS

Ten healthy male Wistar rats, weighing 250-350 g, were anesthetized with ketamine hydrochloride (100 mg/kg, intramuscularly). Four pairs of bipolar stainless steel electrodes 3 mm apart were implanted on the serosal surface of the antrum at one, 10 and 20 cm distal to the pylorus. Five to ten days following the operation, the gastrointestinal myoelectric activity of fasted rats after intramuscular injection of 2.5, six and 12 mg/kg MCP was recorded using a 8-channel EEG machine, and these values were quantitatively compared with the myoelectric activity after saline injection.

RESULTS

In fasted rats, 2.5 mg/kg MCP increased the amplitude of spike activity (402.0 ± 138.4 μV, vs 345 ± 163.4 μV, P < 0.05) and the percentage of the slow wave-containing spike bursts (60.4% ± 22.0% vs 47.4% ± 22.5%, P < 0.01) of small intestine (1 cm distal to the pylorus), but did not affect the myoelectric activity of the antrum. Six and 12 mg/kg MCP increased the amplitude of both the slow wave (332.8 ± 200.1 μV vs 191.2 ± 143.9 μV, P < 0.01; 330.0 ± 197.1 μV vs 191.2 ± 143.9 μV, P < 0.05) and the spike activity of the antrum (180.5 ± 69.7 μV vs 121.8 ± 63.3 μV, P < 0.05; 174.5 ± 71.7 μV vs 123.8 ± 63.3 μV, P < 0.05), while in small intestine (1 cm distal to the pylorus) only the amplitude of spike activity (407.3 ± 179.0 μV vs 345.0 ± 163.4 μV, P < 0.05; 456.0 ± 145.4 μV vs 345.0 ± 163.4 μV, P < 0.05) and the percentage of the slow wave containing spike bursts (61.7% ± 26.5% vs 47.4% ± 22.5%, P < 0.01; 59.1% ± 17.3% vs 47.4% ± 22.5%, P < 0.01) was increased and the latent period significantly prolonged (2.5 ± 0.35 min vs 0.77 ± 0.18 min, P < 0.01).

CONCLUSION

Different mechanisms may be involved in enhancing the myoelectric activity of the antrum and small intestine following MCP administration.

Ionic basis of the action potential of guinea pig gallbladder smooth muscle cells

Smooth muscle cells in the intact guinea pig gallbladder had a resting membrane potential of about -45 mV and had spontaneous action potentials that consisted of a rapid depolarization, a transient repolarization, a plateau phase, and a complete repolarization. These action potentials lasted approximately 570 ms and occurred at a frequency of approximately 0.4 Hz. Action potentials were abolished by the dihydropyridine (DHP)-sensitive Ca2+ channel blocker nifedipine (1.0 microM) and were enhanced by the DHP-sensitive Ca2+ channel agonist BAY K 8644 (0.5 microM). The K+ channel blockers tetraethylammonium chloride (5.0 mM) and 4-aminopyridine (4-AP; 2.0 mM) prolonged the action potential, whereas charybdotoxin (100 nM), a blocker of calcium-activated potassium channels, had no effect. Whole cell currents were characterized in enzymatically isolated smooth muscle cells from the same preparation. 4-AP, a blocker of voltage-dependent K+ channels, suppressed 70% of the outward current at 0 mV. Charybdotoxin (100 nM) reduced an additional 15% of the current at 0 mV. Single calcium-activated potassium channels were identified. The potential for half-activation of these channels, at a cytosolic Ca2+ concentration of 100 nM, was 66.8 mV. A fivefold increase in cytosolic Ca2+ resulted in a shift of the activation curve by -53 mV. External tetraethylammonium chloride (200 microM) reduced the mean single channel current by 48% at 0 mV. The whole cell outward current was abolished by replacement of intracellular K+ for Cs+. Ca2+ currents were inhibited by nifedipine and were increased by BAY K 8644. We conclude that DHP-sensitive voltage-dependent Ca2+ channels are responsible for the depolarization of the action potentials and that the repolarization is due to primarily 4-AP-sensitive K+ current.

Immunohistochemical localization of a gap junction protein (connexin43) in the muscularis externa of murine, canine, and human intestine

Electron-microscopic studies have revealed a heterogeneous distribution of gap junctions in the muscularis externa of mammalian intestines. This heterogeneity is observed at four different levels: among species; between small and large intestines; between longitudinal and circular muscle layers; and between subdivisions of the circular muscle layer. We correlated results obtained with two immunomethods, using an antibody to the known gap-junctional protein (connexin43) with ultrastructural findings, and further evaluated the respective sensitivity of these two approaches. For comparative reasons we also included the vascular smooth muscle of coronary arteries into our study. Two versions of the immunotechnique (peroxidase-antiperoxidase and fluorescence methods) were applied to frozen sections of murine, canine, and human small and large intestines, as well as to pig coronary artery. In the small intestine of all three species a very strong reactivity marked the outer main division of the circular muscle layer, while the longitudinal muscle layer as well as the inner thin division of the circular muscle layer were negative. In murine and human colon both muscle layers were negative, while in canine colon the border layer between the circular muscle and the submucosa reacted strongly, and scattered activity was found in the portion of the circular muscle layer (one tenth of its thickness) closest to the submucosa. The remainder of the circular muscle layer and the entire longitudinal muscle layer were negative in the canine colon. In the coronary artery we could not confirm the positive, specific labeling reported by other investigators (l.c.).(ABSTRACT TRUNCATED AT 250 WORDS)

Electrical coupling of circular muscle to longitudinal muscle and interstitial cells of Cajal in canine colon

1. Electrical communication between circular muscle, longitudinal muscle and interstitial cells of Cajal (ICC) was investigated; the hypothesis was tested that the resting membrane potential (RMP) gradient in the circular muscle of canine colon is caused by electrical coupling to neighbouring cells. 2. Isolated longitudinal muscle exhibited spike-like action potentials at a RMP of -45 mV with a frequency and amplitude of 20 cycles/min and 12 mV, respectively. 3. The circular muscle (CM), devoid of longitudinal muscle, myenteric plexus and submuscular ICC-smooth-muscle network, was electrically quiescent at a uniform RMP of -62 mV across the entire circular muscle layer. 4. Preparations consisting of only the submuscular ICC network and a few adjacent layers of circular muscle cells exhibited slow wave-type action potentials at a RMP of about -80 mV. 5. In ICC-CM preparations, consisting of the submuscular ICC network and circular muscle, a RMP gradient of 10 mV was observed near the submucosal border, whereas the RMP was constant at -62 mV in the myenteric half of the circular muscle. 6. In full thickness (FT) preparations, a RMP gradient of 23 mV was observed. The RMP decreased gradually from -71 mV at the submucosal border to -48 mV at the myenteric border of the circular muscle. 7. Coupling of longitudinal muscle to circular muscle caused circular muscle cells at the myenteric surface to depolarize by 14 mV and longitudinal muscle cells to hyperpolarize by 3 mV. 8. In the ICC-CM preparations, the slow wave amplitudes did not decay exponentially away from the ICC network indicating that slow waves propagated actively into the circular muscle; in the FT preparations there was an apparent exponential decay but this was due to the RMP gradient. 9. Spike-like action potentials (SLAPs) superimposed on the plateau phase of slow waves did not decay exponentially away from the myenteric border suggesting that SLAPs were generated within the circular muscle layer. 10. In summary, circular muscle cells possess a uniform intrinsic RMP of -62 mV. The RMP gradient in situ is caused by electrical coupling of circular muscle cells to longitudinal muscle cells and the submuscular network of ICC. In situ, slow wave-type action potentials propagate actively into the circular muscle layer, and, dependent on the level of excitation, circular muscle cells actively generate spikes.

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

Ouabain-induced excitation of colonic smooth muscle due to block of K+ conductance by intracellular Na+ ions

The mechanism by which ouabain causes excitation of canine colonic circular smooth muscle was investigated. Ouabain-induced depolarization and increase in contractility were related to the concentration of extracellular sodium and prevented by complete substitution of sodium ions with N-methyl-D-glucamine or lithium ions. Absence of external sodium ions did not prevent the depolarization and increase in contractility induced by tetraethylammonium. Exposure of the muscle strips to sodium-free solutions produced a transient hyperpolarization and decrease in the input membrane resistance consistent with the hypothesis that intracellular sodium blocks potassium conductance. The relationship between the membrane potential and the extracellular potassium concentration indicated that the resting membrane potential is mainly determined by the membrane potassium conductance. Our data suggest the following mechanism of action for ouabain: (a) ouabain blocks Na+/K+ pump thereby increasing the intracellular sodium concentration; (b) increase in intracellular sodium inhibits membrane potassium conductance, which depolarizes the membrane and prolongs the slow wave plateau, resulting in an increase of the force of contraction. The direct contribution of the sodium pump to the resting membrane potential, if any, can only be minor (< 6 mV).

Acetylcholine-induced relaxation of the rabbit aorta is preload-independent but markedly dependent upon the degree of excitatory agonist-induced tone

1. The aim of the present study was to investigate the relationship between endothelium-dependent relaxation evoked by acetylcholine, tissue preload and the degree of noradrenaline-induced tone in the isolated rabbit aorta. 2. In the aorta preload-response curves were bell-shaped with increasing preload augmenting responses to noradrenaline up to a certain point (optimal value) and then declining. Removal of the endothelium significantly increased responses to low concentrations of noradrenaline (less than 0.1 microM) but did not significantly affect the maximum response of the aorta to this amine or the preload-response curves generated at several concentrations of noradrenaline. 3. The optimal preload value was around 10 g for the aorta and changes in preload did not influence the sensitivity of the tissue to noradrenaline as assessed by pEC50 values to this agonist. 4. Acetylcholine (0.01-10 microM) evoked endothelium-dependent relaxations which in absolute terms increased as the tissue preload was increased. This relationship was much less evident when acetylcholine responses were measured in terms of the percentage inhibition of the respective noradrenaline contraction, when little or no change in the acetylcholine responses was noted. 5. When acetylcholine relaxations were expressed in terms of a percentage of the maximum noradrenaline-induced response evoked at each preload setting, the results confirmed that preload changes had little or no influence upon acetylcholine responses. 6. In contrast, tissue sensitivity to, and the extent of acetylcholine-induced relaxation, were markedly affected by the level of excitatory agonist-induced tone. As noradrenaline-induced tone increased, maximum responses and pIC50 values to acetylcholine were reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Outward currents in longitudinal colonic muscle cells contribute to spiking electrical behavior

Electrical events in longitudinal and circular muscles of the colon are different. Longitudinal muscles generate action potentials superimposed upon small depolarizations termed myenteric potential oscillations and circular muscles generate slow wave events that persist for several seconds. Differences between circular and longitudinal muscles may be related to the potassium channels these cells express. We have studied Ca(2+)-dependent and voltage-dependent K currents of isolated longitudinal cells with the whole cell patch-clamp technique. Test depolarizations positive to -40 mV yielded a transient inward current followed by a large sustained outward current. Blockade of the inward Ca2+ current reduced the amplitude of the outward current. Outward current was also reduced by tetraethylammonium (TEA; 1 mM), suggesting that a component of the outward current is Ca2+ dependent. After blockade of the Ca(2+)-dependent outward current, a voltage- and time-dependent component of outward current remained. The activation and inactivation properties and sensitivity to TEA and 4-aminopyridine (4-AP) were characterized. The voltage-dependent outward current in longitudinal cells had different properties than the voltage-dependent K currents in circular muscle cells (i.e., more negative inactivation, less sensitivity to 4-AP). TEA (1-5 mM) increased the amplitude and frequency of action potentials in intact longitudinal muscles; 4-AP (1 mM) had little effect on electrical activity of longitudinal muscles. The data suggest that differences in electrical behavior of the 2 muscle layers may be related to the expression of different species of K channels.

Effect of voltage and cyclic AMP on frequency of slow-wave-type action potentials in canine colon smooth muscle

1. A non-L-type calcium conductance is involved in the generation of the initial part of the slow-wave-type action potential in colonic smooth muscle. The present study addresses the question whether this conductance is voltage or metabolically activated. 2. Current-induced hyperpolarization increased frequency and amplitude of slow waves measured in Krebs solution. 3. The upstroke potential was 'isolated' from the slow wave by superfusion with 'glucamine-nitrendipine' Krebs solution (NaCl was replaced by glucamine, nitrendipine was added). 4. Hyperpolarization up to -100 mV did not affect the upstroke potential frequency and increased its amplitude. Only hyperpolarization further than -100 mV decreased the frequency less than or equal to 20%, and reduced the amplitude less than or equal to 20%. 5. Depolarization did not affect the upstroke potential frequency. 6. Forskolin, but not 1,9-dideoxyforskolin dramatically decreased the upstroke potential frequency, without affecting other parameters including the resting membrane potential. 7. The effect of forskolin was mimicked by dibutyryl cyclic AMP, 8-bromo-cyclic AMP and 3-isobutyl-1-methylxanthine (IBMX), but not extracellular cyclic AMP. 8. The upstroke potential could not be evoked by depolarizing pulses after inhibition of activity by forskolin. 9. The effect of forskolin could be reversed by the calcium ionophore A23187. 10. In summary, voltage changes up to -40 mV and down to -100 mV do not, but changes in intracellular cyclic AMP do affect the frequency of the upstroke potential. 11. It is likely that intracellular metabolic activity, which may include cyclic AMP but not a voltage change, activates the conductance responsible for the generation of the upstroke potential.

An active feedback system for isotonic studies of smooth muscle

A feedback system used to perform isotonic studies of smooth muscle is presented. This system is capable of applying a constant force to muscle samples regardless of their contractile activities. The force applied to the tissue is controlled using a proportional integral control system that drives a linear motor. The device is integrated into a sucrose gap tissue bath apparatus where measurements of displacement and electrical activity are also possible. The frequency of canine colonic smooth-muscle electrical oscillations is positively related to applied force.

Simultaneous measurement of electrical activity from two colonic smooth muscle layers using a dual sucrose gap apparatus

An apparatus using the sucrose gap technique is presented. With this apparatus simultaneous measurements of contractile and intracellular electrical activity from the two smooth muscle layers of the colon are made. An "L-shaped" muscle preparation consisting of a leg from the circular muscle layer and a leg from the longitudinal muscle layer is used. A theoretical discussion of the device's operation is presented. Finally, experimental results that validate the theory are included.

Role of the sodium pump in pacemaker generation in dog colonic smooth muscle

1. The role of the Na+ pump in the generation of slow wave activity in circular muscle of the dog colon was investigated using a partitioned 'Abe-Tomita' type chamber for voltage control. 2. Blockade of the Na+ pump by omission of extracellular K+, by ouabain, or the combination of 0 mM-Na+ and ouabain, depolarized the membrane up to approximately -40 mV and abolished the slow wave activity. Repolarization back to the control membrane potential by hyperpolarizing current restored the slow wave activity. 3. Slow waves continued to be present in 0 Na+, Li+ HEPES solution. 4. The depolarization induced by the procedures to block Na+ pump activity was associated with an increase in input membrane resistance. 5. Voltage-current relationships show the presence of an inward rectification. 6. Reduction of temperature depolarized the membrane, and decreased the slow wave frequency and amplitude. The slow wave amplitude was restored by repolarization of the membrane. 7. Brief depolarizing pulses evoked premature slow waves. Brief hyperpolarizing pulses terminated the slow waves. 8. We conclude that abolition of slow wave activity by Na+ pump blockade is a direct effect of membrane depolarization and that the Na+ pump is not responsible for the generation of the slow wave. 9. Our results are consistent with the hypothesis that pacemaker activity in smooth muscle is a consequence of membrane conductance changes which are metabolically dependent.

Neurogenic slow depolarizations and rapid oscillations in the membrane potential of circular muscle of mouse colon

1. Intracellular microelectrodes have been used to record the electrical activity of smooth muscle cells of the circular layer from full length strips of mouse colon in vitro. The membrane potential was unstable and showed slow depolarizations (mean amplitude, 10.9 mV; mean frequency, 0.008 Hz; mean duration, 56.4 s). 2. A variable number (mean fifty-six) of rapid oscillations in membrane potential (mean amplitude, 10.2 mV) with a frequency of approximately 2 Hz and a duration of approximately 400 ms were superimposed on each slow depolarization. Occasionally, action potentials arose from the rapid oscillations. The action potentials, but neither the slow depolarizations nor the rapid oscillations, were abolished by 1.0 microM-nifedipine. 3. The majority of the slow depolarizations and the associated rapid oscillations migrated aborally along the colon at a velocity of between 0.5 and 1.5 mm s-1; in the distal colon the slow depolarization was often preceded by a small hyperpolarization. 4. During the rising and plateau phase of the slow depolarization the amplitude of electronic potentials was decreased. Hyperpolarization induced by passing current during the slow depolarization increased the amplitude of the rapid oscillations. 5. Transmural electrical stimulation (single pulses) in the presence of nifedipine evoked (1 mm anal to the stimulating electrodes) an inhibitory junction potential which was sometimes preceded by an excitatory junction potential. The amplitude, of the evoked inhibitory junction potential was decreased during the rising and plateau phase of the slow depolarization. 6. The slow depolarization and the rapid oscillations were abolished by hexamethonium (500 microM), morphine (1-10 microM) and tetrodotoxin (3.1 microM). Atropine (3.5 microM) abolished the rapid oscillations and reduced the amplitude of the slow depolarization. 7. Atropine (3.5 microM) and morphine (10 microM) abolished the evoked excitatory junction potential whilst tetrodotoxin (3.1 microM) abolished both the excitatory and the inhibitory junction potential. 8. It is suggested that the migrating depolarization and accompanying oscillations, which are neurogenic in origin, represent the electrical correlate in the circular muscle layer of the migrating colonic motor complex which has been associated with the propulsion of faecal pellets along the colon.

Pacemaker activity in the proximal lower oesophageal sphincter of the dog

1. The electrical and mechanical activities of different regions of the canine lower oesophageal sphincter were measured using the single sucrose gap technique. 2. Spontaneous electrical activity was found in the region 0-6 mm oral to the squamocolumnar border. 3. The electrical activity consisted of bursts of spikes superimposed on slow waves. The slow-wave frequency ranged from 0.6 to 5 min-1 in different muscle strips. 4. The slow wave-spike complex and associated contraction were insensitive to tetrodotoxin and atropine. 5. In the pacemaker region, electrical stimulation of intrinsic nerves evoked excitatory junction potentials (atropine sensitive), inhibitory junction potentials (non-adrenergic) and post-stimulus excitation. 6. Increase in the frequency of the slow waves was obtained by muscarinic receptor stimulation (carbachol 10(-7) M) and 10 mM-KCl. 7. The distal lower oesophageal sphincter exhibited a high basal tension but did not show spontaneous electrical activity and stimulation of intrinsic nerves revealed only non-cholinergic, non-adrenergic inhibition. 8. The electrical slow-wave activity observed in the proximal sphincter may constitute the control mechanism for the phasic nature of the contractile activity seen during both the postprandial period and phase III of the interdigestive migrating myoelectric complex. 9. The neural cholinergic activity present in the proximal lower oesophageal sphincter suggests the possibility of neural modulation of the myogenic control activity.

Electrical coupling and pacemaker activity in colonic smooth muscle

The effect of heptanol on electrical coupling between submucosal circular muscle cells of the dog colon and consequences for slow-wave activity were investigated. Electrotonic potentials showed exponential decay giving a length constant of 2.6 +/- 0.5 mm and a time constant of 157 +/- 48 ms. Heptanol reversibly abolished electrotonic current spread, and subsequently no slow-wave activity was recorded. The length constant decreased to less than 0.2 mm. The input resistance increased from 3 to 36 M omega, suggesting a change from tissue syncytium to electrically isolated cells. D600 (5 X 10(-6) M) also abolished slow wave activity but had opposite effects on electrotonic current spread. The data are consistent with the hypothesis that heptanol reversibly inhibits intercellular coupling, resulting in loss of spread of extracellularly applied current, uncoupling of cells, and loss of pacemaker activity. Regulation of intercellular communication may be important in the control of intestinal motility.

Interstitial cells of Cajal in the canine colon: a special communication network at the inner border of the circular muscle

The ultrastructure of the region shown to be essential for pacemaking activity of the circular muscle of the canine colon was studied. This region, at the inner border of the circular muscle, consists of a network of several layers of interstitial cells of Cajal type III. These are interconnected to one another and to the adjacent circular muscle cells by numerous gap junctions. Elsewhere in circular muscle, gap junctions are rare and small. In addition, interstitial cells are in close (often less than 20 nm) contact with nerve varicosities containing large granular vesicles or sometimes small granular vesicles. The morphology of interstitial cells resembles that of others of type III. It is suggested that this arrangement of interstitial cells, circular smooth muscles, and nerves allows for a tightly coupled network of membrane oscillators to be subject to neural modulation.

Heterogeneity in spontaneous and tetraethylammonium induced intracellular electrical activity in colonic circular muscle

Marked differences were observed in the intracellular electrical activities (spontaneous and TEA-induced) comparing the submucosal and myenteric plexus surfaces of the circular muscle of the dog colon. Distinct characteristics of the cells at the myenteric plexus surface were: a less (10 mV) polarized membrane, a lower amplitude slow wave, and the occurrence of burst type spiking activity. However, slow waves with a high upstroke amplitude (approximately 2.5 times higher than the plateau) were observed in 40% of the preparations. This high upstroke amplitude was dependent on the occurrence of a regenerative membrane potential change (a spike) during the slow wave propagation into the myenteric plexus surface. Such a spike was mediated by Ca2+-influx and could be evoked or enhanced by electrical pulses or by blocking a TEA-sensitive potassium conductance. In the presence of TEA, spikes occurred in bursts. Both slow waves and spiking activities generated contraction. In conclusion, at least two types of cells exist in the circular muscle layer with marked differences in electrophysiological properties. Slow waves are generated at the submucosal surface, passively propagated to the outermost circular muscle where they induce regenerative membrane potential changes.