Electrophysiological properties of gastric pacemaker potentials

Ca2+ dynamics in interstitial cells: foundational mechanisms for the motor patterns in the gastrointestinal tract

The gastrointestinal (GI) tract displays multiple motor patterns that move nutrients and wastes through the body. Smooth muscle cells (SMCs) provide the forces necessary for GI motility, but interstitial cells, electrically coupled to SMCs, tune SMC excitability, transduce inputs from enteric motor neurons, and generate pacemaker activity that underlies major motor patterns, such as peristalsis and segmentation. The interstitial cells regulating SMCs are interstitial cells of Cajal (ICC) and PDGF receptor (PDGFR)α+ cells. Together these cells form the SIP syncytium. ICC and PDGFRα+ cells express signature Ca2+-dependent conductances: ICC express Ca2+-activated Cl- channels, encoded by Ano1, that generate inward current, and PDGFRα+ cells express Ca2+-activated K+ channels, encoded by Kcnn3, that generate outward current. The open probabilities of interstitial cell conductances are controlled by Ca2+ release from the endoplasmic reticulum. The resulting Ca2+ transients occur spontaneously in a stochastic manner. Ca2+ transients in ICC induce spontaneous transient inward currents and spontaneous transient depolarizations (STDs). Neurotransmission increases or decreases Ca2+ transients, and the resulting depolarizing or hyperpolarizing responses conduct to other cells in the SIP syncytium. In pacemaker ICC, STDs activate voltage-dependent Ca2+ influx, which initiates a cluster of Ca2+ transients and sustains activation of ANO1 channels and depolarization during slow waves. Regulation of GI motility has traditionally been described as neurogenic and myogenic. Recent advances in understanding Ca2+ handling mechanisms in interstitial cells and how these mechanisms influence motor patterns of the GI tract suggest that the term "myogenic" should be replaced by the term "SIPgenic," as this review discusses.

Interstitial cells of Cajal - pacemakers of the gastrointestinal tract

Gastrointestinal (GI) organs display spontaneous, non-neurogenic electrical, and mechanical rhythmicity that underlies fundamental motility patterns, such as peristalsis and segmentation. Electrical rhythmicity (aka slow waves) results from pacemaker activity generated by interstitial cells of Cajal (ICC). ICC express a unique set of ionic conductances and Ca2+ handling mechanisms that generate and actively propagate slow waves. GI smooth muscle cells lack these conductances. Slow waves propagate actively within ICC networks and conduct electrotonically to smooth muscle cells via gap junctions. Slow waves depolarize smooth muscle cells and activate voltage-dependent Ca2+ channels (predominantly CaV1.2), causing Ca2+ influx and excitation-contraction coupling. The main conductances responsible for pacemaker activity in ICC are ANO1, a Ca2+ -activated Cl- conductance, and CaV3.2. The pacemaker cycle, as currently understood, begins with spontaneous, localized Ca2+ release events in ICC that activate spontaneous transient inward currents due to activation of ANO1 channels. Depolarization activates CaV 3.2 channels, causing the upstroke depolarization phase of slow waves. The upstroke is transient and followed by a long-duration plateau phase that can last for several seconds. The plateau phase results from Ca2+ -induced Ca2+ release and a temporal cluster of localized Ca2+ transients in ICC that sustains activation of ANO1 channels and clamps membrane potential near the equilibrium potential for Cl- ions. The plateau phase ends, and repolarization occurs, when Ca2+ stores are depleted, Ca2+ release ceases and ANO1 channels deactivate. This review summarizes key mechanisms responsible for electrical rhythmicity in gastrointestinal organs.

Ca2+ transients in ICC-MY define the basis for the dominance of the corpus in gastric pacemaking

Myenteric interstitial cells of Cajal (ICC-MY) generate and actively propagate electrical slow waves in the stomach. Slow wave generation and propagation are altered in gastric motor disorders, such as gastroparesis, and the mechanism for the gradient in slow wave frequency that facilitates proximal to distal propagation of slow waves and normal gastric peristalsis is poorly understood.  Slow waves depend upon Ca2+-activated Cl- channels (encoded by Ano1). We characterized Ca2+ signaling in ICC-MY in situ using mice engineered to have cell-specific expression of GCaMP6f in ICC. Ca2+ signaling differed in ICC-MY in corpus and antrum. Localized Ca2+ transients were generated from multiple firing sites and were organized into Ca2+ transient clusters (CTCs). Ca2+ transient refractory periods occurred upon cessation of CTCs, but a relatively higher frequency of Ca2+ transients persisted during the inter-CTC interval in corpus than in antrum ICC-MY. The onset of Ca2+ transients after the refractory period was associated with initiation of the next CTC. Thus, CTCs were initiated at higher frequencies in corpus than in antrum ICC-MY. Initiation and propagation of CTCs (and electrical slow waves) depends upon T-type Ca2+ channels, and durations of CTCs relied upon L-type Ca2+ channels. The durations of CTCs mirrored the durations of slow waves. CTCs and Ca2+ transients between CTCs resulted from release of Ca2+ from intracellular stores and were maintained, in part, by store-operated Ca2+ entry. Our data suggest that Ca2+ release and activation of Ano1 channels both initiate and contribute to the plateau phase of slow waves.

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.

Electrogastrography in adults and children: the strength, pitfalls, and clinical significance of the cutaneous recording of the gastric electrical activity

Cutaneous electrogastrography (EGG) is a non-invasive technique to record gastric myoelectrical activity from the abdominal surface. Although the recent rapid increase in the development of electrocardiography, EGG still suffers from several limitations. Currently, computer analysis of EGG provides few reliable parameters, such as frequency and the percentage of normal and altered slow wave activity (bradygastria and tachygastria). New EGG hardware and software, along with an appropriate arrangement of abdominal electrodes, could detect the coupling of the gastric slow wave from the EGG. At present, EGG does not diagnose a specific disease, but it puts in evidence stomach motor dysfunctions in different pathological conditions as gastroparesis and functional dyspepsia. Despite the current pitfalls of EGG, a multitasking diagnostic protocol could involve the EGG and the (13)C-breath testing for the evaluation of the gastric emptying time-along with validated gastrointestinal questionnaires and biochemical evaluations of the main gastrointestinal peptides-to identify dyspeptic subgroups. The present review tries to report the state of the art about the pathophysiological background of the gastric electrical activity, the recording and processing methodology of the EGG with particular attention to multichannel recording, and the possible clinical application of the EGG in adult and children.

Targeting ion channels for the treatment of gastrointestinal motility disorders

Gastrointestinal (GI) functional and motility disorders are highly prevalent and responsible for long-term morbidity and sometimes mortality in the affected patients. It is estimated that one in three persons has a GI functional or motility disorder. However, diagnosis and treatment of these widespread conditions remains challenging. This partly stems from the multisystem pathophysiology, including processing abnormalities in the central and peripheral (enteric) nervous systems and motor dysfunction in the GI wall. Interstitial cells of Cajal (ICCs) are central to the generation and propagation of the cyclical electrical activity and smooth muscle cells (SMCs) are responsible for electromechanical coupling. In these and other excitable cells voltage-sensitive ion channels (VSICs) are the main molecular units that generate and regulate electrical activity. Thus, VSICs are potential targets for intervention in GI motility disorders. Research in this area has flourished with advances in the experimental methods in molecular and structural biology and electrophysiology. However, our understanding of the molecular mechanisms responsible for the complex and variable electrical behavior of ICCs and SMCs remains incomplete. In this review, we focus on the slow waves and action potentials in ICCs and SMCs. We describe the constituent VSICs, which include voltage-gated sodium (Na(V)), calcium (Ca(V)), potassium (K(V), K(Ca)), chloride (Cl(-)) and nonselective ion channels (transient receptor potentials [TRPs]). VSICs have significant structural homology and common functional mechanisms. We outline the approaches and limitations and provide examples of targeting VSICs at the pores, voltage sensors and alternatively spliced sites. Rational drug design can come from an integrated view of the structure and mechanisms of gating and activation by voltage or mechanical stress.

On the origin of rhythmic calcium transients in the ICC-MP of the mouse small intestine

Interstitial cells of Cajal associated with the myenteric plexus (ICC-MP) are pacemaker cells of the small intestine, producing the characteristic omnipresent electrical slow waves, which orchestrate peristaltic motor activity and are associated with rhythmic intracellular calcium oscillations. Our objective was to elucidate the origins of the calcium transients. We hypothesized that calcium oscillations in the ICC-MP are primarily regulated by the sarcoplasmic reticulum (SR) calcium release system. With the use of calcium imaging, study of the effect of T-type calcium channel blocker mibefradil revealed that T-type channels did not play a major role in generating the calcium transients. 2-Aminoethoxydiphenyl borate, an inositol 1,4,5 trisphosphate receptor (IP(3)R) inhibitor, and U73122, a phospholipase C inhibitor, both drastically decreased the frequency of calcium oscillations, suggesting a major role of IP(3) and IP(3)-induced calcium release from the SR. Immunohistochemistry proved the expression of IP(3)R type I (IP(3)R-I), but not type II (IP(3)R-II) and type III (IP(3)R-III) in ICC-MP, indicating the involvement of the IP(3)R-I subtype in calcium release from the SR. Cyclopiazonic acid, a SR/endoplasmic reticulum calcium ATPase pump inhibitor, strongly reduced or abolished calcium oscillations. The Na-Ca exchanger (NCX) in reverse mode is likely involved in refilling the SR because the NCX inhibitor KB-R7943 markedly reduced the frequency of calcium oscillations. Immunohistochemistry revealed 100% colocalization of NCX and c-Kit in ICC-MP. Testing a mitochondrial NCX inhibitor, we were unable to show an essential role for mitochondria in regulating calcium oscillations in the ICC-MP. In summary, ongoing IP(3) synthesis and IP(3)-induced calcium release from the SR, via the IP(3)R-I, are the major drivers of the calcium transients associated with ICC pacemaker activity. This suggests that a biochemical clock intrinsic to ICC determines the pacemaker frequency, which is likely directly linked to kinetics of the IP(3)-activated SR calcium channel and IP(3) metabolism.

Analysis of pacemaker activity in the human stomach

Extracellular electrical recording and studies using animal models have helped establish important concepts of human gastric physiology. Accepted standards include electrical quiescence in the fundus, 3 cycles per minute (cpm) pacemaker activity in corpus and antrum, and a proximal-to-distal slow wave frequency gradient. We investigated slow wave pacemaker activity, contractions and distribution of interstitial cells of Cajal (ICC) in human gastric muscles. Muscles were obtained from patients undergoing gastric resection for cancer, and the anatomical locations of each specimen were mapped by the operating surgeon to 16 standardized regions of the stomach. Electrical slow waves were recorded with intracellular microelectrodes and contractions were recorded by isometric force techniques. Slow waves were routinely recorded from gastric fundus muscles. These events had similar waveforms as slow waves in more distal regions and were coupled to phasic contractions. Gastric slow wave frequency was significantly greater than 3 cpm in all regions of the stomach. Antral slow wave frequency often exceeded the highest frequency of pacemaker activity in the corpus. Chronotropic mechanisms such as muscarinic and prostaglandin receptor binding, stretch, extracelluar Ca(2+) and temperature were unable to explain the observed slow wave frequency that exceeded accepted normal levels. Muscles from all regions through the thickness of the muscularis demonstrated intrinsic pacemaker activity, and this corresponded with the widespread distribution in ICC we mapped throughout the tunica muscularis. Our findings suggest that extracellular electrical recording has underestimated human slow wave frequency and mechanisms of human gastric function may differ from standard laboratory animal models.

Factors which determine the duration of follower potentials in longitudinal smooth muscle isolated from the guinea-pig stomach antrum

In isolated longitudinal muscle tissues of the guinea-pig stomach antrum, recording electrical responses from smooth muscle cells revealed a periodical generation of follower potentials with variable durations. The I-D relationship, made by plotting the duration as a function of the interval before generating follower potential, was linear. Experiments were carried out to investigate the effects of chemicals which had been known to modulate the release of Ca(2+) from the internal stores (2-aminoethoxy-diphenyl-borate, cyclopiazonic acid, caffeine), inhibit mitochondrial metabolic activity (m-chlorophenyl hydrazone, 2-deoxy-D-glucose, potassium cyanide, rotenone), inhibit ATP-sensitive K-channels distributed in mitochondria (glibenclamide, 5-hydroxydecanoic acid) and inhibit the activity of proteinkinase C (chelerythrine), on the I-D relationship of follower potentials. The effects of depolarization on follower potentials were assessed by stimulating tissues with high potassium solution. Experiments were carried out mainly in the presence of nifedipine which minimized the movements of muscles with no modulation of follower potentials. Cycropiazonic acid and caffeine reduced the slope of I-D relationship, with associated reduction of the duration and frequency of follower potentials. 2-Aminoethoxydiphenyl borate reduced the duration and amplitude and increased the frequency of follower potentials, with depolarization of the membrane, and the effects were simulated by high potassium solution. m-Chlorophenyl hydrazone, potassium cyanide, 2-deoxy-D-glucose, rotenone, 5-hydroxydecanoic acid and glibenclamide reduced the slope of I-D relationship, with associated reduction of the frequency of follower potentials. Chelerythrine did not modulate the slope of I-D relationship, with reduced frequency of follower potentials. It seemed likely that the amount of Ca(2+) released from the internal stores and also mitochondrial function had causal relationship to the duration of pacemaker potentials, suggesting that internal Ca-stores and mitochondria are taking the central role for determining the duration of the pacemaker activity. Proteinkinase C did not seem to participate to the function of mitochondria and internal Ca(2+) stores.

Influence of Shenqing Recipe on morphology and quantity of colonic interstitial cells of Cajal in trinitrobenzene sulfonic acid induced rat colitis

OBJECTIVE

To observe the influence of Shenqing Recipe (SQR), a kind of Traditional Chinese Medicine, on the morphology and quantity of colonic interstitial cells of Cajal (ICC) in trinitrobenzene sulfonic acid (TNBS)-induced rat colitis, and to investigate the possible mechanism of SQR in regulating intestinal dynamics.

METHODS

Sixty rats were randomly divided into normal control, model 1, model 2, mesalazine, and high-dose, and low-dose SQR groups with 10 rats in each group. TNBS (10 mg) dissolved in 50% ethanol was instilled into the lumen of the rat colon of the latter five groups to induce colitis. On the 4th day after administration of TNBS, each treatment group was administered one of the following formulations by enteroclysis gavage once a day for 7 days: 600 mg•kg⁻¹•d⁻¹ mesalazine, 2.4 g•kg⁻¹•d⁻¹ SQR, and 1.2 g•kg⁻¹•d⁻¹ SQR. Model 2 rats received normal saline solution. After 7 days colonic samples were collected. While the colonic samples of model 1 group were collected on the 3rd day after TNBS administered. Ultrastructure of ICC in the damaged colonic tissues was observed with transmission electron microscope. Expression of c-kit protein in colonic tissue was determined by immunohistochemical staining and Western blot.

RESULTS

The ultrastructure of colonic ICC in the rat model of TNBS-induced colitis showed a severe injury, and administration of SQR or mesalazine reduced the severity of injury. Similarly, the expression of c-kit protein of TNBS-induced colitis rat model was significantly decreased compared with the normal control group (P < 0.05). Treatment with SQR or mesalazine significantly increased the expression of c-kit protein compared with the administration of control formulations (P < 0.05), especially the high-dose SQR group.

CONCLUSION

SQR could alleviate and repair the injured ICC, and improve its quantity, which might be involved in regulating intestinal motility.

Chronic water immersion-restraint stress-induced ultrastructural injury to interstitial cells of Cajal in the rat gastric antrum

AIM: To investigate the alterations in the ultrastructure of interstitial cells of Cajal (ICC) in the gastric antrum of rats undergoing chronic water immersion-restraint stress.

METHODS: Forty-eight male Sprague-Dawley rats were randomly and equally divided into six groups: three experimental groups and three matched control groups. The three experimental groups underwent water immersion-restraint stress for one hour daily for 3, 7 and 28 days, respectively, while the three control groups were allowed free access to food and water. On days 4, 8 and 29, the rats in both the experimental and control groups were sacrificed. Two pieces of antrum tissues were taken from each of three rats in each group and fixed in 3% glutaraldehyde for electron microscopic examination. The severity of injury was then scored.

RESULTS: Compared to the control groups, the ICC in the gastric antrum of rats in the experimental groups showed widened perinuclear space, discontinuous basement membrane, cytoplasmic dissolution and vacuolation, decreased number of gap junctions, mitochondrial swelling and vacuolation, dilated endoplasmic reticulum, decreased amount of rough endoplasmic reticulum, and nuclear abnormality. With the prolongation of stress duration, the ultrastructural injury to ICC was aggravated, particularly prominent in cytoplasmic dissolution and vacuolation and the decrease in the amount of rough endoplasmic reticulum.

CONCLUSION: Chronic water immersion-restraint stress can induce ultrastructural injury to ICC in the rat gastric antrum.

Hypoxia differentially modulates the activity of pacemaker and smooth muscle cells in the guinea pig stomach antrum

Effects of hypoxic solution (O(2) tension, 161 +/- 11 mmHg) on electrical responses of the membrane (slow waves), intracellular Ca(2+)-responses measured by Fura-2 fluorescence (Ca-transients) and isometric mechanical responses (phasic contraction) were observed in circular smooth muscles isolated from the guinea-pig stomach antrum. In normoxic solution (O(2) tension, 362 +/- 28 mmHg), muscle cells generated slow waves spontaneously, and switching to hypoxic solution caused an increase in frequency and decrease in duration of slow waves, with no significant change in the resting membrane potential. Hypoxia also reduced the amplitude and duration and increased the frequency of Ca-transients. The increase in frequency of slow waves by hypoxia was prevented by cyclopiazonic acid (CPA) but not by carbonyl cyanide m-chlorophenyl-hydrazone (CCCP), potassium cyanide (KCN) or low-Ca solution. The reduction by hypoxia of the duration of slow waves was prevented by CCCP or KCN but not by CPA or low-Ca solution. Hypoxia resulted in an increase in frequency and decrease in amplitude of phasic contractions, and the changes were prevented by CPA but not by CCCP. These results suggested that in antrum smooth muscle tissues, the increase in frequency of spontaneous activity by hypoxia is related to the enhanced function of the CPA-sensitive internal Ca-stores in pacemaker cells, while the inhibition in amplitude of phasic contractions by hypoxia may be mainly related to the decrease in Ca(2+) release from the CPA-sensitive internal stores in smooth muscle cells. It is concluded that in hypoxic solution, the function of internal Ca(2+) stores is enhanced in ICC-MY and is inhibited in smooth muscle cells in the guinea-pig stomach antrum.

Serum from rats medicated with Dachengqitang significantly enhances the expression of inositol 1, 4, 5-triphosphate receptor mRNA in interstitial cells of Cajal in rat jejunum

AIM: To investigate the effect of serum from rats medicated with Dachengqitang on the expression of inositol 1, 4, 5-triphosphate receptor (IP₃R) mRNA in interstitial cells of Cajal (ICC) in rat jejunum.

METHODS: After rats were intragastrically administered Dachengqitang, they were killed to prepare medicated rat serum. The ICCs in rat jejunum were then isolated and treated with Dulbecco's Modified Eagle's Medium/Ham's Nutrient Mixture F-12 (DMEM-F12; normal control group), 100% medicated rat serum in DMEM-F12 (high-dose treatment group), and 20% medicated rat serum in DMEM-F12 (low-dose treatment group), respectively. The mRNA expression of IP3R subtypes in ICCs were determined by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS: All subtypes of IP₃R were expressed in ICCs in rat jejunum. The mRNA expression levels of IP3R subtypes were significantly higher in the two serum treatment groups than in the normal control group (all P < 0.05). Significant differences were also noted in the mRNA expression levels of IP3R subtypes between the high-dose and low-dose treatment groups (IP3R1/β-actin: 1.012 ± 0.129 vs 0.625 ± 0.075; IP₃R2/β-actin: 0.813 ± 0.098 vs 0.476 ± 0.031; IP₃R3/β-actin: 0.924 ± 0.113 vs 0.583 ± 0.046, all P < 0.05).

CONCLUSION: Serum from rats medicated with Dachengqitang can significantly increase the expression of IP3R mRNA in ICCs in rat jejunum in a dose-dependent manner.

Gastric electrical stimulation with long pulses in humans and animals: can data obtained in animals be replicated in humans?

AIMS

  The aim of this study was to investigate and compare effective parameters for gastric electrical stimulation (GES) to modulate gastric muscle functions in different species.

METHODS

  Four species: Pigs, dogs, rats, and mice implanted with two pairs of electrodes on the serosal surface of the stomach were studied, respectively. Experiment 1 was designed to entrain/pace gastric slow waves and included a series of 5-min periods with long-pulse GES of different pulse widths and frequencies. Experiment 2 was designed to induce gastric dysrhythmia with long-pulse GES of different frequencies. Gastric slow waves were recorded during the entire experiment.

RESULTS

  1) The minimum pulse width for GES to completely entrain the slow waves was similar (100-400 msec) in all four species. 2) With fixed amplitude (4 mA) and pulse width (400 msec), the highest frequency at which slow waves could be paced was similar (about 10-60% higher than the intrinsic slow wave frequency) in all species. 3) With fixed pulse width of 400 msec and amplitude of 6 mA, GES with nine to 18 cycles per min (cpm) was able to induce dysrhythmia in dogs. In addition, there was no significant difference among these frequencies of 9-18 cpm. 4) GES with 400 msec, 6 mA, and 9 cpm was able to induce dysrhythmia in all species. These effective GES parameters in results 1-4 were similar to those used in humans in the literature.

CONCLUSIONS

  There is no significant difference in stimulation parameters when GES is applied to alter gastric slow waves in different animal models. Furthermore, the effective parameters for GES to alter slow waves are similar between the humans and various animal models. These findings suggest that stimulation parameters obtained from animal studies are applicable in humans.

Cellular mechanism of the voltage-dependent change in slow potentials generated in circular smooth muscle of the guinea-pig gastric corpus

The cellular mechanism of the voltage-dependent properties of slow potentials were investigated in single bundles of circular smooth muscle isolated from the gastric corpus of guinea-pig using conventional microelectrode recordings. Hyperpolarization of the membrane by current injection decreased the frequency and increased the amplitude of slow potentials linearly. At potentials negative of -80 mV, slow potential generation was abolished and a periodic generation of clustered unitary potentials was evident. Application of cyclopiazonic acid (CPA, 20 microM) or thapsigargin (1 microM; inhibitors of Ca(2+)-ATPase), carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 0.1 microM; mitochondrial protonophore) or 2-aminoethoxydiphenyl borate (2-APB, 20 microM; inhibitor of IP(3) receptor-mediated Ca(2+) release) depolarized the membrane and reduced or inhibited the amplitude and frequency of slow potentials: repolarization of the membrane to the resting level by current injection resulted in a recovery of the amplitude of slow potentials in the presence of CPA or CCCP, but not 2-APB. The slow potentials abolished by thapsigargin did not recover upon membrane repolarization. The altered frequency of slow potentials by 2-APB, CPA or CCCP was not reversed by membrane repolarization to control potentials. Depolarization of the membrane by about 10 mV with high-potassium solution also reduced the amplitude and increased the frequency of slow potentials in a manner restored by repolarization to control potentials upon current injection, suggesting that membrane depolarization did not affect the voltage dependency of pacemaker activity. The results indicate that in corpus circular muscles the voltage dependency of the frequency and amplitude of slow potentials requires a functional Ca(2+) store and mitochondria.

Quantitative cellular description of gastric slow wave activity

Interstitial cells of Cajal (ICC) are responsible for the spontaneous and omnipresent electrical activity in the stomach. A quantitative description of the intracellular processes whose coordinated activity is believed to generate electrical slow waves has been developed and is presented here. In line with recent experimental evidence, the model describes how the interplay between the mitochondria and the endoplasmic reticulum in cycling intracellular Ca(2+) provides the primary regulatory signal for the initiation of the slow wave. The major ion channels that have been identified as influencing slow wave activity have been modeled according to data obtained from isolated ICC. The model has been validated by comparing the simulated profile of the slow waves with experimental recordings and shows good correspondence in terms of frequency, amplitude, and shape in both control and pharmacologically altered conditions.

Alteration of the responses of gastric smooth muscle to endothelin in streptozotocin-induced diabetic rats

Diabetic gastropathy is suggested to be the result of not only an autonomic neuropathy but also to disorder of the spontaneous rhythmic motility of the gastric smooth muscle. Attempts were made to investigate the alteration of the effects of endothelin-1 (ET-1), which is known to enhance the spontaneous activity of gastrointestinal smooth muscle, on gastric activity in streptozotocin (STZ)-induced diabetic rats. STZ-induced diabetic rats were prepared by the injection of Sprague-Dawley (SD) rats with STZ (i.p.). Isometric mechanical responses were recorded in isolated circular smooth muscle strips of the stomach antrum, to measure changes in the rhythmicity of the smooth muscle. ET-1 (10 nM) significantly elevated the resting tension and the frequency of spontaneous contraction, but did not alter the amplitude of the spontaneous oscillatory contractions in normal rats. In diabetic rats, ET-1 elevated the resting tension, and spontaneous contractions were increased in frequency, however they were decreased in amplitude. In normal rats, sarafotoxin S6c (S6c, 10 nM), a selective ET(B) receptor agonist, elevated the resting tension slightly and increased both the frequency and amplitude of the spontaneous contractions. However, S6c significantly elevated the resting tension alone in STZ-induced diabetic rats. Selective stimulation of endothelin type A (ET(A)) receptors with ET-1, in the presence of a selective antagonist of ET(B) receptors, produced similar responses in the gastric muscle of both normal and diabetic rats. These results indicate that ET-1 elevates the resting tension and increases the frequency of the spontaneous oscillatory contractions in both normal and STZ-induced diabetic rats, to a similar extent. However, the specific actions on ET(B) receptors were quite different between the two: the elevating actions on the resting tension were much greater in STZ-diabetic rats than in normal rats. The results suggested the facilitation of ET(B) receptor signaling in the antrum during the pathogenesis of diabetic gastropathy.

Neuronal conduction of excitation without action potentials based on ceramide production

Background

Action potentials are the classic mechanism by which neurons convey a state of excitation throughout their length, leading, after synaptic transmission, to the activation of other neurons and consequently to network functioning. Using an in vitro integrated model, we found previously that peripheral networks in the autonomic nervous system can organise an unconventional regulatory reflex of the digestive tract motility without action potentials.

Methodology/Principal Findings

In this report, we used combined neuropharmacological and biochemical approaches to elucidate some steps of the mechanism that conveys excitation along the nerves fibres without action potentials. This mechanism requires the production of ceramide in membrane lipid rafts, which triggers in the cytoplasm an increase in intracellular calcium concentration, followed by activation of a neuronal nitric oxide synthase leading to local production of nitric oxide, and then to guanosine cyclic monophosphate. This sequence of second messengers is activated in cascade from rafts to rafts to ensure conduction of the excitation along the nerve fibres.

Conclusions/Significance

Our results indicate that second messengers are involved in neuronal conduction of excitation without action potentials. This mechanism represents the first evidence—to our knowledge—that excitation is carried along nerves independently of electrical signals. This unexpected ceramide-based conduction of excitation without action potentials along the autonomic nerve fibres opens up new prospects in our understanding of neuronal functioning.

Factors modifying the frequency of spontaneous activity in gastric muscle

The cellular mechanisms that determine the frequency of spontaneous activity were investigated in gastric smooth muscles isolated from the guinea-pig. Intact antral muscle generated slow waves periodically; the interval between slow waves was decreased exponentially by depolarization of the membrane to reach a steady interval value of about 7 s. Isolated circular muscle bundles produced slow potentials spontaneously or were evoked by depolarizing current stimuli. Evoked slow potentials appeared in an all-or-none fashion, with a refractory period of approximately 2-3 s. Low concentrations of chemicals that modify intracellular signalling revealed that the refractory period was causally related to the activity of protein kinase C (PKC). Activation of PKC increased and inhibition of PKC activity decreased the frequency of slow potentials. Chemicals that inhibit mitochondrial functions reduced the frequency of slow waves. Inhibition of internal Ca(2+)-store activity decreased the amplitude, but not the frequency of slow potentials, suggesting that the amplitude is causally related to Ca(2+) release from the internal store. The results suggest that changes in [Ca(2+)](i) caused by the activity of mitochondria may play a key role in determining the frequency of spontaneous activity in gastric pacemaker cells.

Metabolic component of the temperature-sensitivity of slow waves recorded from gastric muscle of the guinea-pig

The effects of changes in temperature on slow waves were investigated in smooth muscle tissues isolated from the guinea-pig gastric antrum. Within the range 24 degrees C to 42 degrees C, elevation of temperature increased the frequency and maximum rate of rise of the upstroke phase (dV/dt) of slow waves and decreased their duration, with no alteration to amplitude or resting membrane potential. These observations also applied to follower potentials and pacemaker potentials recorded from longitudinal muscle and myenteric interstitial cells, respectively. Slow waves were comprised of 1st and 2nd components, and the latency for generating the 2nd component was decreased exponentially by elevating temperature, reaching a stable value of about 1 s above 32 degrees C. The temperature coefficient was >2 for the frequency, dV/dt and latency of the 2nd component, about 1.7 for the duration and about 1 for amplitude. Potassium cyanide (KCN), an inhibitor of mitochondrial metabolic activity, reduced the frequency and duration of slow waves, with no alteration to other parameters (amplitude, dV/dt, latency). In the presence of 30 microM KCN, the temperature-dependency of the frequency of slow waves was diminished or abolished, while other parameters of slow waves remained unaltered. These results indicate that in slow waves the frequency may be related to metabolic activities, while the temperature-dependent changes in the dV/dt, latency for the 2nd component and duration of slow waves are produced largely by mechanisms other than metabolic activity.

The effects of flufenamic acid on spontaneous activity of smooth muscle tissue isolated from the guinea-pig stomach antrum

The effects of flufenamic acid were investigated on slow waves, follower potentials and pacemaker potentials recorded respectively from circular smooth muscle cells, longitudinal smooth muscle cells and interstitial cells of Cajal distributed in the myenteric layers (ICC-MY) of the guinea-pig stomach antrum. Flufenamic acid (>10(-5) M) inhibited the amplitude and rate of rise of the upstroke phase of the slow waves, with no marked alteration in their frequency of occurrence. The inhibitory actions of flufenamic acid appeared to be mainly on slow potentials recorded from circular smooth muscle cells, but not on follower or pacemaker potentials. After abolishing spontaneous slow potentials with flufenamic acid, depolarizing current stimuli could evoke slow potentials with an amplitude that was much smaller than in the absence of flufenamic acid, with no significant alteration to the input resistance of the membrane. The time elapsed for the generation of the 2nd component of the slow waves or the slow potentials evoked during depolarizing current pulse stimulation was increased by flufenamic acid. The rate of rise of unitary potentials, but not the frequency of occurrence, was inhibited by flufenamic acid. These results indicate that the inhibitory actions of flufenamic acid appear to be mainly on the circular muscle layer including the interstitial cells of Cajal distributed within the muscle bundles (ICC-IM). Nifedipine-sensitive spike potentials were not inhibited by flufenamic acid. It is concluded that the selective inhibition of the 2nd component of slow waves by flufenamic acid may be mainly due to the inhibition of ion channels, possibly Ca2+-sensitive Cl--channels, activated during generation of slow potentials in the ICC-IM distributed in the circular muscle layer.

Dual concentration-dependent effects of phorbol 12, 13-dibutyrate on spontaneous and acetylcholine-induced electrical responses recorded from isolated circular smooth muscle of the guinea-pig stomach antrum

Intracellular recordings of electrical activity were made from circular smooth muscle cells in small segments of tissue isolated from the guinea-pig stomach antrum. Every cell that was impaled exhibited a rhythmic generation of slow potentials. Experiments were carried out to test the effects of three different concentrations (1, 10 and 100 nM) of phorbol 12, 13-dibutyrate (PDBu) on these slow potentials and on the responses produced by acetylcholine (ACh), in the presence of nifedipine and N(omega)-nitro-L-arginine (nitroarginine), known inhibitors of L-type Ca-channels and nitric oxide synthase, respectively. The resting membrane potential was -62 +/- 7 mV, while the frequency and amplitude of the slow potentials were 1.6 +/- 0.1 cycle per min (cpm) and 33 +/- 1 mV, respectively. Application of 1 nM PDBu increased the frequency of slow potentials, with no significant change in the membrane potential and amplitude of slow potentials. At a concentration of 100 nM, PDBu depolarized the membrane by about 6 mV, and either decreased the amplitude and frequency of the slow potentials or abolished them. The amplitude and frequency of the slow potentials were not significantly changed in the presence of 10 nM PDBu. In the presence of chelerythrine (1-2 microM), a known inhibitor of protein kinase C (PKC), the increase in frequency of slow potentials by 1 nM PDBu and depolarization produced by 100 nM PDBu were not elicited. The increase in frequency of slow potentials by 100 nM ACh was inhibited by PDBu, in a concentration-dependent manner, and ACh-responses were abolished in the presence of 100 nM PDBu. These results indicate that PDBu has dual actions on the spontaneous activity of antral circular muscle, with low concentrations increasing and high concentrations inhibiting the frequency of the slow potentials. The former may be produced by activation of protein kinase C (PKC). As the ACh-induced excitation of slow potentials is inhibited by PDBu, a possible causal relationship between the inhibition and over-activation of PKC is considered.

Effects of endogenous and exogenous nitric oxide on electrical responses of circular smooth muscle isolated from the guinea-pig stomach antrum

The effects of endogenous and exogenous nitric oxide (NO) on electrical activity were investigated in circular smooth muscle preparations isolated from the guinea-pig stomach antrum. The actions of endogenous NO were evaluated from the effects of inhibition of NO synthesis by N(omega)-nitro-L-arginine (nitroarginine), while those of exogenous NO were assessed from the effects of SIN-1, an NO donor. Antral circular smooth muscle generated slow potentials periodically at a frequency of about 1 cycle per min (cpm), and unitary potentials were also generated in a random fashion in the interval between slow potentials. Application of nitroarginine (10(-5) M) increased the frequency of slow potentials, with no significant alteration of the resting membrane potential and amplitude of slow potentials. Frequency analysis of unitary potentials revealed that nitroarginine also increased the spectral density at 0.01-1 Hz frequency. The refractory period for the generation of slow potentials evoked by depolarizing pulses was about 10 s, but was decreased to 6 s by nitroarginine. In the presence of nitroarginine, SIN-1 (10(-9)-10(-7) M) reduced the amplitude and frequency of slow potentials: low concentrations (<10(-8) M) reduced only the frequency of slow potentials, while higher concentrations (10(-8)-10(-7) M) reduced both the amplitude and frequency of slow potentials, in a concentration-dependent manner, before abolishing the slow potentials. The power spectrum of the unitary potentials indicated that SIN-1 (>10(-8) M) reduced the spectral density at 0.01-1 Hz frequency. The refractory period for the generation of slow potentials was increased again to about 10 s by SIN-1. Thus, the excitatory effects of nitroarginine could be antagonized by SIN-1, suggesting that the inhibitory effects of endogenous NO are comparable to those of exogenous NO produced by SIN-1. The results also suggested that the effects of NO on smooth muscle are insignificant and NO selectively inhibits the activity of intramuscular interstitial cells of Cajal (ICC-IM).

Inhibitory actions of cilostazol on electrical responses of smooth muscle isolated from the guinea-pig stomach antrum

We have investigated the effects of cilostazol, a type III phosphodiesterase inhibitor, on the electrical responses of smooth muscle tissue isolated from the guinea-pig stomach antrum. Cilostazol (10(-5) M) inhibited slow waves recorded from circular muscle cells, but did not significantly alter the pacemaker potentials and follower potentials recorded from myenteric interstitial cells and longitudinal muscle cells respectively. Slow potentials generated in isolated circular muscle bundles without attached myenteric interstitial cells were inhibited by cilostazol (>10(-7) M), while all membrane activities were abolished by 10(-5) M cilostazol. In circular muscle bundles, the input resistance of smooth muscle cells and the refractory period for the generation of slow potentials were not altered during the inhibition of spontaneous activity with cilostazol. While cilostazol at 10(-7) and 10(-6) M did not elevate the tissue content of cyclic AMP, at 10(-5) M cyclic AMP was elevated by about 30%. A similar elevation was also produced by 10(-7) M forskolin. The content of cyclic AMP was not significantly increased in preparations stimulated with 10(-3) M caffeine. The potency for inhibiting slow waves was in the order caffeine (10(-3) M) > forskolin (10(-7) M) > cilostazol (10(-5) M). The frequency of slow waves was decreased by caffeine or forskolin but not by cilostazol, while the duration was reduced by caffeine but not by cilostazol or forskolin. Follower potentials were modulated by caffeine and forskolin, but not by cilostazol: the duration was reduced by caffeine, the frequency was reduced by caffeine or forskolin, and the amplitude was not significantly altered by any of them. The results indicate that cilostazol has high selectivity in inhibiting the activity of circular muscle much more than that of longitudinal muscle or pacemaker cells, with no causal relation to the tissue content of cyclic AMP as appears to be the case for the inhibitory actions of caffeine and forskolin.

Role of protein kinase C in the excitatory action of cholinergic nerve stimulation on spontaneous activity of circular smooth muscle isolated from the guinea-pig stomach antrum

Following inhibition of NO production with nitroarginine, circular muscle isolated from the guinea-pig gastric antrum generated periodic slow potentials and unitary potentials. Transmural nerve stimulation (TNS) during the interval between slow potentials evoked an apamin-sensitive inhibitory junction potential (IJP) followed by an atropine-sensitive depolarization; the latter was either a transient depolarization with enhanced generation of unitary potentials or a slow potential. After inhibition of unitary potentials and slow potentials with 1 mM caffeine, TNS evoked an IJP and subsequent cholinergic depolarization, the latter developing slowly and lasting for about 10 s. TNS was unable to elicit a slow potential until a certain period of time had elapsed following the cessation of a slow potential. The period during which TNS could not evoke slow potentials (termed the high-threshold period) was about 10 s, and this period was increased by chelerythrine and decreased by phorbol esters. It is concluded that cholinergic nerve-mediated excitation of gastric muscle involves the activation of protein kinase C (PKC), and that the high-threshold period, during which the generation of slow potentials by TNS is inhibited, may be a consequence of reduced activity of PKC.

Spontaneous activity and its cholinergic modulation in circular smooth muscle isolated from guinea-pig stomach antrum

Circular smooth muscle isolated from the guinea-pig gastric antrum generated periodic slow potentials in the presence of nifedipine and nitroarginine to prevent the activity of voltage-gated L-type Ca-channels and endogenous production of NO respectively. Chelerythrine, an inhibitor of protein kinase C (PKC), in the concentration range 10(-7)-3 x 10(-7) M reduced the frequency but not the amplitude of spontaneous slow potentials without altering the resting membrane potential. 2-Aminoethoxydiphenyl borate (2-APB, 3 x 10(-6) M), an inhibitor at inositol-1,4,5-trisphosphate (IP(3)) receptors, depolarized the membrane, increased the frequency and reduced the amplitude of the slow potentials; the latter actions were independent of depolarization. Two different phorbol esters, phorbol 12,13-dibutyrate and phorbol-12-myristate-13-acetate, increased the frequency of slow potentials, without altering the amplitude or changing the resting membrane potential; the effects of phorbol esters were antagonized by chelerythrine. Stimulation of muscarinic receptors with acetylcholine (ACh), in concentrations below those causing membrane depolarization (3 x 10(-8)-10(-7) M), increased the amplitude and frequency of slow potentials. Chelerythrine inhibited the ACh-induced increase in the frequency of slow potentials but did not prevent the increase in their amplitude. 2-APB inhibited the ACh-induced increase in the amplitude of slow potentials but did not prevent the increase in their frequency. These results suggest that the frequency of spontaneous slow potentials is regulated by PKC and their amplitude by IP(3) production. ACh increases both the amplitude and frequency of slow potentials; the former is related to the activation of PKC, while the latter is related to activation of IP(3)-receptors.

Propagation of pacemaker activity in the guinea-pig antrum

Cyclical periods of depolarization (slow waves) underlie peristaltic contractions involved in mixing and emptying of contents in the gastric antrum. Slow waves originate from a myenteric network of interstitial cells of Cajal (ICC-MY). In this study we have visualized the sequence and propagation of Ca(2+) transients associated with pacemaker potentials in the ICC network and longitudinal (LM) and circular muscle (CM) layers of the isolated guinea-pig gastric antrum. Gastric antrum was dissected to reveal the ICC-MY network, loaded with Fluo-4 AM and activity was monitored at 37 degrees C. Ca(2+) waves propagated throughout the ICC-MY network at an average velocity of 3.24 +/- 0.12 mm s(-1) at a frequency of 4.87 +/- 0.16 cycles min(-1) (n= 4). The propagation of the Ca(2+) wave often appeared 'step-like', with separate regions of the network being activated after variable delays. The direction of propagation was highly variable (Delta angle of propagation 44.3 +/- 10.9 deg per cycle) and was not confined to the axes of the longitudinal or circular muscle. Ca(2+) waves appeared to spread out radially from the site of initiation. The initiating Ca(2+) wave in ICC-MY was correlated to secondary Ca(2+) waves in intramuscular interstitial cells of Cajal, ICC-IM, and smooth muscle cells, and the local distortion (contraction) in a field of view. TTX (1 microm) had little effect on slow wave or pacemaker potential activity, but 2-APB (50 microm) blocked all Ca(2+) waves, indicating a pivotal role for intracellular Ca(2+) stores. Nicardipine (2 microm) eliminated the Ca(2+) transient generated by smooth muscle, but did not affect the fast upstroke associated with ICC-MY. These results indicate that slow waves follow a sequence of activation, beginning with the ICC-MY and ICC-IM network, followed later by a sustained Ca(2+) transient in the muscle layers that is responsible for contraction.

Effects of endothelin-1 on the membrane potential and slow waves in circular smooth muscle of rat gastric antrum

Electrophysiological effects of endothelin-1 (ET-1) on circular smooth muscle of rat gastric antrum were investigated by using intracellular membrane potential recording techniques. ET-1 (10 nM) caused an initial hyperpolarization of the membrane which was followed by a sustained depolarization. ET-1 also increased the frequency but not the amplitude of slow waves. In the presence of the endothelin type A (ETA) receptor antagonist, BQ123 (1 microM), ET-1 (10 nM) depolarized the membrane and increased the frequency of slow waves, but without the initial hyperpolarization. The selective endothelin type B (ETB) receptor agonist, sarafotoxin S6c (10 nM), also depolarized the membrane and increased the frequency of slow waves. In the presence of the ETB receptor antagonist, BQ788 (1 microM), ET-1 (10 nM) hyperpolarized the membrane. However, in the presence of BQ788, ET-1 caused neither the depolarization nor the increase in the frequency of the slow waves. The ET-1-induced hyperpolarization was completely abolished by apamin (0.1 microM). In the presence of apamin, ET-1 depolarized the membrane and increased the frequency of slow waves. The ET-1-induced depolarization was significantly attenuated by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS, 0.3 mM). The increase of the frequency by ET-1 was observed both in the presence and absence of DIDS. These results suggest that, ET-1 hyperpolarizes the membrane by the activation of Ca2+-activated K+ channels via ETA receptors, and depolarizes the membrane by the activation of Ca2+-activated Cl- channels via ETB receptors. ET-1 also appears to increase the frequency of slow waves via ETB receptors, however this mechanism would seem to be independent of membrane depolarization.