Passive and active membrane properties of canine gastric antral circular muscles

Direct optogenetic stimulation of smooth muscle cells to control gastric contractility

Rationale: Antral peristalsis is responsible for gastric emptying. Its failure is called gastroparesis and often caused by dysfunction of enteric neurons and interstitial cells of Cajal (ICC). Current treatment options, including gastric electrical stimulation, are non-satisfying and may improve symptoms but commonly fail to restore gastric emptying. Herein, we explore direct optogenetic stimulation of smooth muscle cells (SMC) via the light-gated non-selective cation channel Channelrhodopsin2 (ChR2) to control gastric motor function.

Methods: We used a transgenic mouse model expressing ChR2 in fusion with eYFP under the control of the chicken-β-actin promoter. We performed patch clamp experiments to quantify light-induced currents in isolated SMC, Ca²⁺ imaging and isometric force measurements of antral smooth muscle strips as well as pressure recordings of intact stomachs to evaluate contractile responses. Light-induced propulsion of gastric contents from the isolated stomach preparation was quantified in video recordings. We furthermore tested optogenetic stimulation in a gastroparesis model induced by neuronal- and ICC-specific damage through methylene blue photo-toxicity.

Results: In the stomachs, eYFP signals were restricted to SMC in which blue light (460 nm) induced inward currents typical for ChR2. These depolarizing currents led to contractions in antral smooth muscle strips that were stronger than those triggered by supramaximal electrical field stimulation and comparable to those evoked by global depolarization with high K⁺ concentration. In the intact stomach, panoramic illumination efficiently increased intragastric pressure achieving 239±46% (n=6) of the pressure induced by electrical field stimulation and triggered gastric transport. Within the gastroparesis model, electric field stimulation completely failed but light still efficiently generated pressure waves.

Conclusions: We demonstrate direct optogenetic stimulation of SMC to control gastric contractility. This completely new approach could allow for the restoration of motility in gastroparesis in the future.

Extracellular Cl- regulates electrical slow waves and setting of smooth muscle membrane potential by interstitial cells of Cajal in mouse jejunum

NEW FINDINGS

What is the central question of this study? The aim was to investigate the roles of extracellular chloride in electrical slow waves and resting membrane potential of mouse jejunal smooth muscle by replacing chloride with the impermeant anions gluconate and isethionate. What is the main finding and its importance? The main finding was that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in interstitial cells of Cajal (ICCs) is a major contributor to the shape of electrical slow waves. Furthermore, the data confirm that ICCs set the smooth muscle membrane potential and that altering Cl^- homeostasis in ICCs can alter the smooth muscle membrane potential. Intracellular Cl^- homeostasis is regulated by anion-permeable channels and transporters and contributes to excitability of many cell types, including smooth muscle and interstitial cells of Cajal (ICCs). Our aims were to investigate the effects on electrical activity in mouse jejunal muscle strips of replacing extracellular Cl^- (Cl^-_o ) with the impermeant anions gluconate and isethionate. On reducing Cl^-_o , effects were observed on electrical slow waves, with small effects on smooth muscle membrane voltage (E_m ). Restoration of Cl^- hyperpolarized smooth muscle E_m proportional to the change in Cl^-_o concentration. Replacement of 90% of Cl^-_o with gluconate reversibly abolished slow waves in five of nine preparations. Slow waves were maintained in isethionate. Gluconate and isethionate substitution had similar concentration-dependent effects on peak amplitude, frequency, width at half peak amplitude, rise time and decay time of residual slow waves. Gluconate reduced free ionized Ca²⁺ in Krebs solutions to 0.13 mm. In Krebs solutions containing normal Cl^- and 0.13 mm free Ca²⁺ , slow wave frequency was lower, width at half peak amplitude was smaller, and decay time was faster. The transient hyperpolarization following restoration of Cl^-_o was not observed in W/W^v mice, which lack pacemaker ICCs in the small intestine. We conclude that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in ICCs plays a major role in generation or propagation of slow waves. Furthermore, these data support a role for ICCs in setting smooth muscle E_m and that altering Cl^- homeostasis in ICCs can alter smooth muscle E_m .

Carbon monoxide, hydrogen sulfide, and nitric oxide as signaling molecules in the gastrointestinal tract

Carbon monoxide (CO) and hydrogen sulfide (H2S) used to be thought of simply as lethal and (for H2S) smelly gaseous molecules; now they are known to have important signaling functions in the gastrointestinal tract. CO and H2S, which are produced in the gastrointestinal tract by different enzymes, regulate smooth muscle membrane potential and tone, transmit signals from enteric nerves, and can regulate the immune system. The pathways that produce nitric oxide, H2S, and CO interact; each can inhibit and potentiate the level and activity of the other. However, there are significant differences between these molecules, such as in half-lives; CO is more stable and therefore able to have effects distal to the site of production, whereas nitric oxide and H2S are short lived and act only close to sites of production. We review their signaling functions in the luminal gastrointestinal tract and discuss how their pathways interact. We also describe other physiological functions of CO and H2S and how they might be used as therapeutic agents.

Ultrafast Ca2+ wave in cultured vascular smooth muscle cells aligned on a micropatterned surface

Communication between vascular smooth muscle cells (SMCs) allows control of their contraction and so regulation of blood flow. The contractile state of SMCs is regulated by cytosolic Ca2+ concentration ([Ca2+]i) which propagates as Ca2+ waves over a significant distance along the vessel. We have characterized an intercellular ultrafast Ca2+ wave observed in cultured A7r5 cell line and in primary cultured SMCs (pSMCs) from rat mesenteric arteries. This wave, induced by local mechanical or local KCl stimulation, had a velocity around 15 mm/s. Combining of precise alignment of cells with fast Ca2+ imaging and intracellular membrane potential recording, allowed us to analyze rapid [Ca2+]i dynamics and membrane potential events along the network of cells. The rate of [Ca2+]i increase along the network decreased with distance from the stimulation site. Gap junctions or voltage-operated Ca2+ channels (VOCCs) inhibition suppressed the ultrafast Ca2+ wave. Mechanical stimulation induced a membrane depolarization that propagated and that decayed exponentially with distance. Our results demonstrate that an electrotonic spread of membrane depolarization drives a rapid Ca2+ entry from the external medium through VOCCs, modeled as an ultrafast Ca2+ wave. This wave may trigger and drive slower Ca2+ waves observed ex vivo and in vivo.

Review article: carbon monoxide in gastrointestinal physiology and its potential in therapeutics

BACKGROUND

While carbon monoxide (CO) is a known toxin, it is now recognised that CO is also an important signalling molecule involved in physiology and pathophysiology.

AIMS

To summarise our current understanding of the role of endogenous CO in the regulation of gastrointestinal physiology and pathophysiology, and to potential therapeutic applications of modulating CO.

METHODS

This review is based on a comprehensive search of the Ovid Medline comprehensive database and supplemented by our ongoing studies evaluating the role of CO in gastrointestinal physiology and pathophysiology.

RESULTS

Carbon monoxide derived from haem oxygenase (HO)-2 is predominantly involved in neuromodulation and in setting the smooth muscle membrane potential, while CO derived from HO-1 has anti-inflammatory and antioxidative properties, which protect gastrointestinal smooth muscle from damage caused by injury or inflammation. Exogenous CO is being explored as a therapeutic agent in a variety of gastrointestinal disorders, including diabetic gastroparesis, post-operative ileus, organ transplantation, inflammatory bowel disease and sepsis. However, identifying the appropriate mechanism for safely delivering CO in humans is a major challenge.

CONCLUSIONS

Carbon monoxide is an important regulator of gastrointestinal function and protects the gastrointestinal tract against noxious injury. CO is a promising therapeutic target in conditions associated with gastrointestinal injury and inflammation. Elucidating the mechanisms by which CO works and developing safe CO delivery mechanisms are necessary to refine therapeutic strategies.

Mentation on the immunological modulation of gastrointestinal motility

The immunological modulation of gastrointestinal motility is currently one of the most dynamic and fascinating areas of enteric research, as investigators are beginning to focus their studies on the pathophysiology of various gastrointestinal dysmotilities. The new fruits of this investigative initiative has resulted in the appearance of a fascinating series of articles which demonstrate that intestinal inflammatory events alter a distinct population of enteric neurons and that these alterations last long past the apparent resolution of the inciting event. Studies over the past few years have unequivocally demonstrated that the muscularis externa itself is an active and complex immunological compartment with unique features. The rodent muscularis externa is constitutively populated by a dense network of muscularis macrophages throughout the entire gastrointestinal tract. Although few other leukocytes are present in the rodent, the human muscularis is densely populated by both macrophages and mast cells. Postoperative ileus and endotoxin-induced ileus have turned out to be extremely useful rodent models to elucidate the importance of muscularis leukocytes in causing intestinal dysfunction. Using models of ileus, studies have demonstrated that a complex molecular inflammatory scenario is triggered within the muscularis externa, which consists of MAP kinase phosphorylation, transcriptior factor activation and the subsequent induction of various cytokines, chemokines and, importantly, smooth muscle inhibitory substances, such as nitric oxide and prostaglandins from iNOS and COX-2. This local molecular inflammatory milieu leads to leukocyte extravasation. Data suggests that the muscularis macrophage network is the conductor of the molecular and cellular inflammatory responses which causes ileus.

Membrane potential gradient is carbon monoxide-dependent in mouse and human small intestine

The aims of this study were to quantify the change in resting membrane potential (RMP) across the thickness of the circular muscle layer in the mouse and human small intestine and to determine whether the gradient in RMP is dependent on the endogenous production of carbon monoxide (CO). Conventional sharp glass microelectrodes were used to record the RMPs of circular smooth muscle cells at different depths in the human small intestine and in wild-type, HO2-KO, and W/W(V) mutant mouse small intestine. In the wild-type mouse and human intestine, the RMP of circular smooth muscle cells near the myenteric plexus was -65.3 +/- 2 mV and -58.4 +/- 2 mV, respectively, and -60.1 +/- 2 mV and -49.1 +/- 1 mV, respectively, in circular smooth muscle cells at the submucosal border. Oxyhemoglobin (20 microM), a trapping agent for CO, and chromium mesoporphyrin IX, an inhibitor of heme oxygenase, abolished the transwall gradient. The RMP gradients in mouse and human small intestine were not altered by N(G)-nitro-l-arginine (200 microM). No transwall RMP gradient was found in HO2-KO mice and W/W(V) mutant mice. TTX (1 microM) and 1H-[1,2,4-]oxadiazolo[4,3-a]quinoxalin-1-one (10 microM) had no effect on the RMP gradient. These data suggest that the gradient in RMP across the thickness of the circular muscle layer of mouse and human small intestine is CO dependent.

Systematic review: applications and future of gastric electrical stimulation

BACKGROUND

Over the past 20 years, gastric electrical stimulation has received increasing attention among researchers and clinicians.

AIM

To give a systematic review on the effects, mechanisms and applications of gastric electrical stimulation.

METHODS

Medline was used to identify the articles to be included in this review. Key words used for the search included gastric electrical stimulation, gastric pacing, electrical stimulation, stomach, gastrointestinal motility, central nervous system, gastroparesis, nausea and vomiting; obesity and weight loss. Combinational uses of these keywords were made to identify relevant articles. Most of the articles included in this review ranged from 1985 to 2006.

RESULTS

Based on the general search, the review was structured as follows: (i) peripheral and central effects and mechanisms of gastric electrical stimulation; (ii) clinical applications of gastric electrical stimulation for gastroparesis and obesity and (iii) future development of gastric electrical stimulation.

CONCLUSIONS

Great progress has been made during the past decades. Gastric electrical stimulation has been shown to be effective in normalizing gastric dysrhythmia, accelerating gastric emptying and improving nausea and vomiting. Implantable device has been made available for treating gastroparesis as well as obesity. However, development of a new device and controlled clinical studies are required to further prove clinical efficacy of gastric electrical stimulation.

Voltage-dependent calcium entry underlies propagation of slow waves in canine gastric antrum

Electrical slow waves in gastrointestinal (GI) muscles are generated by interstitial cells of Cajal (ICC), and these events actively propagate through networks of ICC within the walls of GI organs. The mechanism by which spontaneously active pacemaker sites throughout ICC networks are entrained to produce orderly propagation of slow waves is unresolved. A three-chambered partition bath was used to test the effects of agents that affect metabolism, membrane potential and voltage-dependent Ca(2+) entry on slow wave propagation in canine antral smooth muscle strips. Slow waves evoked by electrical field stimulation actively propagated from end to end of antral muscle strips with a constant latency between two points of recording. When the central chamber of the bath was perfused with low-temperature solutions, mitochondrial inhibitors, reduced extracellular Ca(2+) or blockers of voltage-dependent Ca(2+) channels, active propagation failed. Depolarization or hyperpolarization of the tissue within the central chamber also blocked propagation. Blockade of propagation by reduced extracellular Ca(2+) and inhibitors of dihydropyridine-resistant Ca(2+) channels suggests that voltage-dependent Ca(2+) entry may be the 'entrainment factor' that facilitates active propagation of slow waves in the gastric antrum.

The role of carbon monoxide in the gastrointestinal tract

Carbon monoxide (CO) is a biologically active product of haem metabolism that contributes to the normal physiology of the gastrointestinal tract. In this article, we review recent data showing that CO is an integral regulator of gastrointestinal motility and an important factor in the response to gastrointestinal injury. CO is generated by haem oxygenase-2 (HO-2), which is constitutively expressed in many inhibitory neurones of the vertebrate enteric nervous system. The membrane potential gradients along and across the muscle layers of the gastrointestinal tract require the generation of CO by haem oxygenase-2. The presence of CO is also necessary for normal inhibitory neurotransmission in circular smooth muscle and appears to permit nitric oxide-mediated inhibitory neurotransmission. Genetic deletion of the haem oxygenase-2 gene in mice slows gut transit. The other major CO synthetic enzyme, haem oxygenase-1 (HO-1) is induced under conditions of stress or injury. Recent studies have demonstrated that up-regulation of haem oxygenase-1 protects the gut from several types of gastrointestinal injury, suggesting that CO or induction of HO-1 may find therapeutic use in gastrointestinal diseases and injuries. Furthermore, it is anticipated that the understanding of CO-mediated signalling in the gastrointestinal tract will inform studies in other tissues that express haem oxygenases.

Carbon monoxide is an endogenous hyperpolarizing factor in the gastrointestinal tract

In all mammalian species examined to date, there is a 10 mV or more gradient in resting membrane potential across the wall of the gastric antrum, small intestine and colon, and an even larger gradient along the long axis of the stomach. These voltage gradients, which may be considered biological rheostats, are central to the ability of circular smooth muscle to vary the strength of contraction from weak to propulsive and occluding. In this short review, we consider recent data that support the hypothesis that carbon monoxide generated in interstitial cells of Cajal is a hyperpolarizing factor for circular smooth muscle and the root of the essential voltage gradients.

Distribution of pacemaker function through the tunica muscularis of the canine gastric antrum

1. Interstitial cells of Cajal (ICC) have been shown to generate pacemaker activity in gastrointestinal (GI) muscles. Experiments were performed to characterize the ICC within the canine gastric antrum and to determine the site(s) of pacemaker activity and whether active propagation pathways exist within the thick-walled tunica muscularis of large mammals. 2. Immunohistochemistry and electron microscopy revealed four populations of ICC within the antral muscularis on the basis of anatomical location. Typical ICC were found in the myenteric region of the small intestine (IC-MY). Intramuscular ICC (IC-IM) were intermingled between muscle fibres of circular and longitudinal muscle layers. ICC were also found within septa (IC-SEP) between muscle bundles and along the submucosal surface of the circular muscle layer (IC-SM). ICC were identified in each location by ultrastructural features. 3. Intracellular electrical recordings demonstrated nifedipine-insensitive slow waves throughout the circular muscle layer. Separation of interior and submucosal circular muscle strips from the dominant (myenteric) pacemaker region dramatically slowed frequency but did not block spontaneous slow waves, suggesting that pacemaker cells populate all regions of the circular muscle. 4. Slow waves could be evoked in interior and submucosal circular muscles at rates above normal antral frequency by electrical pacing or by acetylcholine (0.3 microM). Active slow wave propagation occurred in all regions of the circular muscle, and propagation velocities were similar in each region. 5. In summary, antral muscles of the canine stomach have pacemaker capability throughout the circular muscle. Normally, a dominant pacemaker near the myenteric plexus drives slow waves that actively propagate throughout the circular layer. Pacemaker activity and the active propagation pathway may occur in networks of ICC that are distributed in the region of the myenteric plexus and throughout the circular muscle layer.

Cellular and molecular basis for electrical rhythmicity in gastrointestinal muscles

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

Fos-like immunoreactivity in the brain of homozygous diabetes insipidus Brattleboro and normal Long-Evans rats

Water deprivation induces the production of the transcription factor Fos in neurons of the neurohypophysial system. These neurons, which are located primarily in the hypothalamic paraventricular (PVN) and supraoptic nuclei (SON), produce the antidiuretic hormone vasopressin. The present immunocytochemical study has analyzed the distribution of Fos in brain regions involved in osmoregulation and compared the extent of Fos immunoreactivity (Fos-IR) in vasopressin-deficient Brattleboro and normal Long-Evans rats under stimulated and non-stimulated conditions. Rats were osmotically challenged by means of a single intraperitoneal injection of 1.5 M/L NaCl. Since Fos may be induced by the stress of handling of animals, non-injected and isotonic saline-injected rats were used as controls. Faint nuclear Fos immunostaining was found in the organum vasculosum of the lamina terminalis (OVLT), the median preoptic nucleus (MnPO), subfornical organ (SFO), and SON of non-injected and isotonic saline-injected Brattleboro but not Long-Evans rats. Hypertonic saline injection specifically induced Fos-IR in neurons located in the SFO, OVLT, MnPO, PVN, SON, hypothalamic accessory nuclei (including the nucleus circularis), and arcuate hypothalamic nucleus (Arc) in both Long Evans and Brattleboro rats. No differences in distribution of the induced immunostaining were found between the strains. Stress of handling and (isotonic saline) injection induced Fos-IR in the lateral septal nuclei, central amygdaloid nuclei, medial amygdaloid nucleus, medial preoptic area, the bed nucleus of the stria terminalis, cingulate- and piriform cortex, the lateral hypothalamic area, ventromedial hypothalamic nucleus, and the habenular nucleus. The data are consistent with a role for Fos in the regulation of vasopressin gene expression during acute hyperosmotic stimulation. In addition, this study demonstrated that during chronic osmotic stimulation, as experienced by homozygous Brattleboro rats, Fos-IR is limited but apparently present constantly and that it increased in these animals following acute osmotic challenge. Our observations suggest that c-fos regulatory controls in homozygous Brattleboro rats are different from those in Long-Evans rats.