Role of nitric oxide in lower esophageal sphincter relaxation to swallowing

Achalasia

Achalasia is a rare disorder of the oesophageal smooth muscle characterized by impaired relaxation of the lower oesophageal sphincter (LES) and absent or spastic contractions in the oesophageal body. The key pathophysiological mechanism is loss of inhibitory nerve function that probably results from an autoimmune attack targeting oesophageal myenteric nerves through cell-mediated and, possibly, antibody-mediated mechanisms. Achalasia incidence and prevalence increase with age, but the disorder can affect all ages and both sexes. Cardinal symptoms consist of dysphagia, regurgitation, chest pain and weight loss. Several years can pass between symptom onset and an achalasia diagnosis. Evaluation starts with endoscopy to rule out structural causes, followed by high-resolution manometry and/or barium radiography. Functional lumen imaging probe can provide complementary evidence. Achalasia subtypes have management and prognostic implications. Although symptom questionnaires are not useful for diagnosis, the Eckardt score is a simple symptom scoring scale that helps to quantify symptom response to therapy. Oral pharmacotherapy is not particularly effective. Botulinum toxin injection into the LES can temporize symptoms and function as a bridge to definitive therapy. Pneumatic dilation, per-oral endoscopic myotomy and laparoscopic Heller myotomy can provide durable symptom benefit. End-stage achalasia with a dilated, non-functioning oesophagus may require oesophagectomy or enteral feeding into the stomach. Long-term complications can, rarely, include oesophageal cancer, but surveillance recommendations have not been established.

Neural signalling of gut mechanosensation in ingestive and digestive processes

Eating and drinking generate sequential mechanosensory signals along the digestive tract. These signals are communicated to the brain for the timely initiation and regulation of diverse ingestive and digestive processes - ranging from appetite control and tactile perception to gut motility, digestive fluid secretion and defecation - that are vital for the proper intake, breakdown and absorption of nutrients and water. Gut mechanosensation has been investigated for over a century as a common pillar of energy, fluid and gastrointestinal homeostasis, and recent discoveries of specific mechanoreceptors, contributing ion channels and the well-defined circuits underlying gut mechanosensation signalling and function have further expanded our understanding of ingestive and digestive processes at the molecular and cellular levels. In this Review, we discuss our current understanding of the generation of mechanosensory signals from the digestive periphery, the neural afferent pathways that relay these signals to the brain and the neural circuit mechanisms that control ingestive and digestive processes, focusing on the four major digestive tract parts: the oral and pharyngeal cavities, oesophagus, stomach and intestines. We also discuss the clinical implications of gut mechanosensation in ingestive and digestive disorders.

Enhancing High-Resolution Esophageal Manometry: Use of Ancillary Techniques and Maneuvers

High-resolution manometry evaluates esophageal motor function using 10 supine water swallows. Superimposing impedance over high-resolution manometry pressure topography assesses the relationship between contraction and bolus propulsion and identifies inadequate clearance. Ancillary techniques and maneuvers augment the standard supine high-resolution manometry evaluation by challenging peristaltic function. Increasing bolus volume (rapid drink challenge) and altering bolus consistency (standardized test meal, solid swallows) enhance identification of esophageal outflow obstruction syndromes. Physiologic maneuvers (multiple rapid swallows, abdominal compression) address the ability of the esophageal smooth muscle to augment contraction vigor. Pharmacologic challenge is less commonly used clinically, and elucidates pathophysiology of esophageal motor disorders.

A case study of 10 patients administered HBOC-201 in high doses over a prolonged period: outcomes during severe anemia when transfusion is not an option

BACKGROUND

Hemoglobin-Based Oxygen Carriers (HBOCs) can act as an "oxygen bridge" in acute severe anemia when transfusion is indicated, but not possible. We present data on 10 Expanded Access (EA) patients treated with high cumulative doses of Hemopure (HBOC-201), to assess the ability of HBOC-201 to safely treat life threatening anemia in situations where high volumes of product were administered over an extended period of time.

STUDY DESIGN AND METHODS

Inclusion in this study required that the patient receive at least 10 units of HBOC-201 between 2014 and 2017 under the FDA-sanctioned EA program. Depending on a patient's geographical location, treatment with HBOC-201 was obtained through either a single patient emergency Investigational New Drug (IND) application, or an intermediate size population IND. Of the 41 patients who were treated during this period, 10 patients received 10 or more units of the product. Data were obtained from medical records.

RESULTS

Treatments with HBOC-201 started within 24 hours of signing consent and were administered at an average rate of 1.99 (SD 0.17) units per day over a mean of 8.2 days (SD 2.9), during which patients received on average 16.2 units (SD 5.7 units) of HBOC-201. The median pre-treatment nadir corpuscular hemoglobin (Hb) concentration was 3.3 (SD 0.9) g/dL and post-treatment Hemoglobin was 7.3 (SD 1.7) g/dL. Common side effects included methemoglobinemia, gastrointestinal symptoms, and hypertension. However, no product-related serious adverse events (SAEs) were noted. All patients survived.

CONCLUSIONS

Administration of HBOC-201 over an extended period is a feasible and safe oxygen bridge for severely anemic patients who cannot be transfused with RBC.

Statins and gastroesophageal reflux disease: A meta-analysis

Background/Objectives

Gastroesophageal reflux disease (GERD) is one of the common gastrointestinal disorders worldwide. Recent epidemiologic studies have suggested that use of statins may lower the risk of GERD although the results from different studies were inconsistent. This systematic review and meta-analysis were conducted with the aim to summarize all available data.

Methods

A systematic literature review was performed using MEDLINE and EMBASE database from inception to December 2017. Cohort, case-control, and cross-sectional studies that compared the risk of GERD among statin users versus nonusers were included. Pooled odds ratio (OR) and 95% confidence interval (CI) were calculated using a random-effect, generic inverse variance method.

Results

A total of 4 studies (1 case control, 1 cohort, and 2 cross-sectional studies) with 14,505 participants met the eligibility criteria and were included in the meta-analysis. The risk of GERD among statin users was numerically lower than nonusers with the pooled OR of 0.89 but the result did not achieve statistical significance (95% CI, 0.60-1.33). The statistical heterogeneity in this study was moderate (I² = 54%).

Conclusions

The current meta-analysis found that the risk of GERD was numerically lower among statin users although the pooled result did not reach statistical significance. Therefore, more studies are still needed to further clarify this potential benefit of statins.

Effects of remifentanil on esophageal and esophagogastric junction (EGJ) bolus transit in healthy volunteers using novel pressure-flow analysis

BACKGROUND

Remifentanil is associated with subjective dysphagia and an objective increase in aspiration risk. Studies of opioid effects have shown decreased lower esophageal sphincter relaxation. We assessed bolus transit through the esophagus and esophagogastric junction (EGJ) during remifentanil administration using objective pressure-flow analysis.

METHODS

Data from 11 healthy young participants (23±3 years, 7 M) were assessed for bolus flow through the esophagus and EGJ using high-resolution impedance manometry (Manoscan™, Sierra Scientific Instruments, Inc., LES Angeles, CA, USA) with 36 pressure and 18 impedance segments. Data were analyzed for esophageal pressure topography and pressure-flow analysis using custom Matlab analyses (Mathworks, Natick, USA). Paired t tests were performed with a P-value of < .05 regarded as significant.

KEY RESULTS

Duration of bolus flow through (remifentanil/R 3.0±0.3 vs baseline/B 5.0 ± 0.4 seconds; P < .001) and presence at the EGJ (R 5.1 ± 0.5 vs B 7.1 ± 0.5 seconds; P = .001) both decreased during remifentanil administration. Distal latency (R 5.2 ± 0.4 vs B 7.5 ± 0.2 seconds; P < .001) and distal esophageal distension-contraction latency (R 3.5 ± 0.1 vs B 4.7 ± 0.2 seconds; P < .001) were both reduced. Intrabolus pressures were increased in both the proximal (R 5.3 ± 0.9 vs B 2.6 ± 1.3 mm Hg; P = .01) and distal esophagus (R 8.6 ± 1.7 vs B 3.1 ± 0.8 mm Hg; P = .001). There was no evidence of increased esophageal bolus residue.

CONCLUSIONS AND INFERENCES

Remifentanil-induced effects were different for proximal and distal esophagus, with a reduced time for trans-sphincteric bolus flow at the EGJ, suggestive of central and peripheral μ-opioid agonism. There were no functional consequences in healthy subjects.

Sildenafil improves clinical signs and radiographic features in dogs with congenital idiopathic megaoesophagus: a randomised controlled trial

We evaluated the efficacy of oral sildenafil citrate in dogs with congenital idiopathic megaoesophagus (CIM). Twenty-one puppies were randomly assigned to two groups (treatment and control). The dogs were given sildenafil oral suspension 1 mg/kg every 12 hours for 14 days or placebo in a masked fashion. Clinical signs (frequency of regurgitation and weight gain) and oesophagrams (relative oesophageal diameter, ROD) were evaluated in order to assess the efficacy of drug treatment, by examiners who were unaware of the study protocol. In addition, a set of in vitro experiments on isolated samples of canine lower oesophageal sphincter (LOS) was performed, and the effects of increasing concentrations of sildenafil on basal tone and electrically-stimulated motility were assessed. Sildenafil administration significantly reduced the number of regurgitation episodes (0.88±1.40 v 2.65±1.56, P<0.0001) and significantly increased weight gain in the treated dogs compared to controls (79.76±28.30 per cent v 53.40±19.30 per cent, P=0.034). ROD values, at the end of the treatment period, were significantly decreased in the sildenafil group, compared to pre-treatment values (0.97±0.19 v 0.24±0.14, P<0.0001), in contrast to control subjects (0.98±0.17 v 1.10±0.25, P=0.480). In accordance with the in vivo findings, sildenafil dose-dependently reduced basal tone and increased electrically-induced relaxation of dog LOS samples. These results suggest that sildenafil citrate helps ameliorate clinical and radiographic signs in dogs with CIM by reducing LOS tone, and could represent a novel therapeutic tool for the treatment of this disease.

In vitro effect of nicorandil on the carbachol-induced contraction of the lower esophageal sphincter of the rat

The lower esophageal sphincter (LES) is a specialized region of the esophageal smooth muscle that allows the passage of a swallowed bolus into the stomach. Nitric oxide (NO) plays a major role in LES relaxation. Nicorandil possesses dual properties of a NO donor and an ATP-sensitive potassium channel (KATP channel) agonist, and is expected to reduce LES tone. This study investigated the mechanisms underlying the effects of nicorandil on the LES. Rat LES tissues were placed in an organ bath, and activities were recorded using an isometric force transducer. Carbachol-induced LES contraction was significantly inhibited by KATP channel agonists in a concentration-dependent manner; pinacidil >> nicorandil ≈ diazoxide. Nicorandil-induced relaxation of the LES was prevented by pretreatment with glibenclamide, whereas N(G)-nitro-l-arginine methyl ester (l-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and iberiotoxin were ineffective at preventing nicorandil-induced LES relaxation. Furthermore, nicorandil did not affect high K(+)-induced LES contraction. Reverse-transcription polymerase chain reaction analysis and immunohistochemistry revealed expression of KCNJ8 (Kir6.1), KCNJ11 (Kir6.2), ABCC8 (SUR1) and ABCC9 (SUR2) subunits of the KATP channel in the rat lower esophagus. These findings indicate that nicorandil causes LES relaxation chiefly by activating the KATP channel, and that it may provide an additional pharmacological tool for the treatment of spastic esophageal motility disorders.

Fundamentals of Neurogastroenterology: Physiology/Motility - Sensation

The fundamental gastrointestinal functions include motility, sensation, absorption, secretion, digestion and intestinal barrier function. Digestion of food and absorption of nutrients normally occurs without conscious perception. Symptoms of functional gastrointestinal disorders are often triggered by meal intake suggesting abnormalities in the physiological processes are involved in the generation of symptoms. In this manuscript, normal physiology and pathophysiology of gastrointestinal function, and the processes underlying symptom generation are critically reviewed. The functions of each anatomical region of the digestive tract are summarized. The pathophysiology of perception, motility, mucosal barrier, and secretion in functional gastrointestinal disorders as well as effects of food, meal intake and microbiota on gastrointestinal motility and sensation are discussed. Genetic mechanisms associated with visceral pain and motor functions in health and functional gastrointestinal disorders are reviewed. Understanding the basis for digestive tract functions is essential to understand dysfunctions in the functional gastrointestinal disorders.

Dominant role of interstitial cells of Cajal in nitrergic relaxation of murine lower oesophageal sphincter

Oesophageal achalasia is a disease known to result from reduced relaxation of the lower oesophageal sphincter (LES). Nitric oxide (NO) is one of the main inhibitory transmitters. NO-sensitive guanylyl cyclase (NO-GC) acts as the key target of NO and, by the generation of cGMP, mediates nitrergic relaxation in the LES. To date, the exact mechanism of nitrergic LES relaxation is still insufficiently elucidated. To clarify the role of NO-GC in LES relaxation, we used cell-specific knockout (KO) mouse lines for NO-GC. These include mice lacking NO-GC in smooth muscle cells (SMC-GCKO), in interstitial cells of Cajal (ICC-GCKO) and in both SMC/ICC (SMC/ICC-GCKO). We applied oesophageal manometry to study the functionality of LES in vivo. Isometric force studies were performed to monitor LES responsiveness to exogenous NO and electric field stimulation of intrinsic nerves in vitro. Cell-specific expression/deletion of NO-GC was monitored by immunohistochemistry. Swallowing-induced LES relaxation is strongly reduced by deletion of NO-GC in ICC. Basal LES tone is affected by NO-GC deletion in either SMC or ICC. Lack of NO-GC in both cells leads to a complete interruption of NO-induced relaxation and, therefore, to an achalasia-like phenotype similar to that seen in global GCKO mice. Our data indicate that regulation of basal LES tone is based on a dual mechanism mediated by NO-GC in SMC and ICC whereas swallow-induced LES relaxation is mainly regulated by nitrergic mechanisms in ICC.

Megaesophagus in a line of transgenic rats: a model of achalasia

Megaesophagus is defined as the abnormal enlargement or dilatation of the esophagus, characterized by a lack of normal contraction of the esophageal walls. This is called achalasia when associated with reduced or no relaxation of the lower esophageal sphincter (LES). To date, there are few naturally occurring models for this disease. A colony of transgenic (Pvrl3-Cre) rats presented with megaesophagus at 3 to 4 months of age; further breeding studies revealed a prevalence of 90% of transgene-positive animals having megaesophagus. Affected rats could be maintained on a total liquid diet long term and were shown to display the classic features of dilated esophagus, closed lower esophageal sphincter, and abnormal contractions on contrast radiography and fluoroscopy. Histologically, the findings of muscle degeneration, inflammation, and a reduced number of myenteric ganglia in the esophagus combined with ultrastructural lesions of muscle fiber disarray and mitochondrial changes in the striated muscle of these animals closely mimic that seen in the human condition. Muscle contractile studies looking at the response of the lower esophageal sphincter and fundus to electrical field stimulation, sodium nitroprusside, and L-nitro-L-arginine methyl ester also demonstrate the similarity between megaesophagus in the transgenic rats and patients with achalasia. No primary cause for megaesophagus was found, but the close parallel to the human form of the disease, as well as ease of care and manipulation of these rats, makes this a suitable model to better understand the etiology of achalasia as well as study new management and treatment options for this incurable condition.

Impaired platelet-derived growth factor receptor expression and function in cultured lower esophageal sphincter circular smooth muscle cells from W/W(v) mutant mice

We have previously demonstrated that lower esophageal sphincter (LES) circular smooth muscle (CSM) is functionally impaired in W/W(v) mutant mice that lack interstitial cells of Cajal, and speculated that this could be due to altered smooth muscle differentiation. Platelet-derived growth factor (PDGF) is involved in the maturation and differentiation of smooth muscle. To determine whether PDGF expression and (or) function is altered in W/W(v) mutant mice, PDGF-Rβ expression was measured using RT-PCR, qPCR, and immunocytochemistry, and Ca(2+) imaging and perforated patch clamp recordings performed in isolated LES CSM cells. RT-PCR and immunocytochemistry showed significantly reduced PDGF-Rβ expression in the LES from mutant as opposed to wild-type mice. Quantitative comparison of CSM cell numbers in histological specimens revealed a significantly increased average cell size in the mutant tissue. The specific PDGF-Rβ ligand, PDGF-BB, caused a significant increase in intracellular Ca(2+) in cells from the wild-type mice compared with the mutants. Using a ramp protocol, PDGF-BB caused a 2-fold increase in outward K(+) currents in cells from the wild-type mice, whereas no significant increase was measured in the cells from the mutants. We conclude that the expression and function of PDGF-Rβ in LES CSM from W/W(v) mice is impaired, providing further evidence that LES CSM is abnormal in W/W(v) mutants.

Morphological demonstration of a vagal inhibitory pathway to the lower esophageal sphincter via nitrergic neurons in the rat esophagus

BACKGROUND

The involvement of vagal parasympathetic efferents in esophageal myenteric neurons in vagal inhibitory pathways to the lower esophageal sphincter (LES) is not clear. Thus, this study was performed to demonstrate morphologically the presence of vagal inhibitory pathways to the LES via esophageal neurons.

METHODS

Fast Blue (FB) was injected into the LES of Wistar rats, and 3 days after injection, the animals were subjected to electrical stimulation of the vagus nerve. The esophagus was processed for immunohistochemistry for Fos that was an immediate-early gene as a marker of neuronal activity, nitric oxide synthase (NOS), vasoactive intestinal polypeptide (VIP) and choline acetyltransferase (ChAT). The immunoreactivities were then compared with the FB labeling in esophageal neurons.

KEY RESULTS

Fast Blue-labeled neurons were observed within an esophageal area of 30 mm oral to the LES, with the highest frequency in the esophagus just above the LES. Most of the FB-labeled neurons were positive for NOS and VIP, but a few for ChAT. Following vagal-electrical stimulation, one fourth of the FB-labeled neurons presented nuclei expressing Fos and most of these Fos/FB neurons were NOS-positive.

CONCLUSIONS & INFERENCES

A majority of the FB-labeled esophageal neurons appeared to be descending motor neurons innervating the LES. Moreover, the colocalization of VIP and NOS in most of the LES-projecting neurons suggests that VIP and NO released from these neurons induce LES relaxation, and the innervation of the vagal efferents to the LES-projecting esophageal neurons in the distal esophagus implies a vagal inhibitory pathway responsible for LES relaxation.

Evidence for altered circular smooth muscle cell function in lower esophageal sphincter of W/Wv mutant mice

Nitrergic neurotransmission to gut smooth muscle is impaired in W/W(v) mutant mice, which lack intramuscular interstitial cells of Cajal (ICC-IM). In addition, these mice have been reported to have smaller amplitude unitary potentials (UPs) and a more negative resting membrane potential (RMP) than control mice. These abnormalities have been attributed to absence of ICC-IM, but it remains possible that they are due to alterations at the level of the smooth muscle itself. Amphotericin-B-perforated patch-clamp recordings and Ca(2+) imaging (fura 2) were compared between freshly isolated single circular smooth muscle cells (CSM) from W/W(v) mutant and control mice lower esophageal sphincter (LES). There was no significant difference in seal resistance, capacitance, or input resistance in response to applied electrotonic current pulses between CSM cells from W/W(v) mutants and controls. Compared with control mice, RMP was more negative and UPs significantly smaller in CSM cells from mutant mice LES. Administration of caffeine induced an inward current in cells from both mutant and control mice, but the current density was significantly larger in cells from W/W(v) mutants. Membrane potential hyperpolarization induced by sodium nitroprusside was larger in cells from control mice vs. W/W(v) mutants. In addition, intracellular Ca(2+) transients induced by caffeine were significantly increased in cells from mutants. These findings indicate that LES CSM is abnormal in W/W(v) mutant mice. Thus some physiological functions attributed to ICC-IM based on experiments in smooth muscle of ICC deficient mice may need to be reconsidered.

Variability in the muscle composition of rat esophagus and neural pathway of lower esophageal sphincter relaxation

Several studies from our laboratory show that axial stretch of the lower esophageal sphincter (LES) in an oral direction causes neurally mediated LES relaxation. Under physiological conditions, axial stretch of the LES is caused by longitudinal muscle contraction (LMC) of the esophagus. Because longitudinal muscle is composed of skeletal muscle in mice, vagal-induced LMC and LES relaxation are both blocked by pancuronium. We conducted studies in rats (thought to have skeletal muscle esophagus) to determine if vagus nerve-mediated LES relaxation is also blocked by pancuronium. LMC-mediated axial stretch on the LES was monitored using piezoelectric crystals. LES and esophageal pressures were monitored with a 2.5-Fr solid-state pressure transducer catheter. Following bilateral cervical vagotomy, the vagus nerve was stimulated electrically. LES, along with the esophagus, was harvested after in vivo experiments and immunostained for smooth muscle (smooth muscle α-actin) and skeletal muscle (fast myosin heavy chain). Vagus nerve-stimulated LES relaxation and esophageal LMC were reduced in a dose-dependent fashion and completely abolished by pancuronium (96 μg/kg) in six rats (group 1). On the other hand, in seven rats, LES relaxation and LMC were only blocked completely by a combination of pancuronium (group 2) and hexamethonium. Immunostaining revealed that the longitudinal muscle layer was composed of predominantly skeletal muscle in the group 1 rats. On the other hand, the longitudinal muscle layer of group 2 rats contained a significant amount of smooth muscle (P < 0.05). Our study shows tight coupling between axial stretch on the LES and relaxation of the LES, which suggests a cause and effect relationship between the two. We propose that the vagus nerve fibers that cause LMC induce LES relaxation through the stretch-sensitive activation of inhibitory motor neurons.

Antireflux action of Nissen fundoplication and stretch-sensitive mechanism of lower esophageal sphincter relaxation

BACKGROUND & AIMS

Surgical fundoplication is an effective treatment for gastroesophageal reflux disease. One of the proposed mechanisms for its antireflux action is that it reduces lower esophageal sphincter (LES) relaxation. We investigated whether fundoplication works through a stretch-sensitive mechanism of LES relaxation.

METHODS

Studies were performed in rats. Intravenous and arterial lines were placed and tracheal intubation was performed. A midline laprotomy was performed to place sutures through the esophagus to exert axial stretch on the LES, and the vagus nerve was isolated in the neck for electrical stimulation. The LES pressure was monitored with a 2F solid-state pressure transducer placed through a gastrostomy. Cranial displacement of the LES was recorded using piezoelectric crystals. Data were recorded before and after 360-degree Nissen fundoplication.

RESULTS

Axial stretch and vagus nerve stimulation induced cranial displacement of the LES as well as LES relaxation in a dose-dependent manner. LES relaxation and axial stretch were each significantly reduced after fundoplication (P < .01). Nitric-oxide-induced LES relaxation was not affected by fundoplication. Removal of fundoplication restored axial stretch- and vagus nerve-stimulated LES relaxation as well as LES cranial displacement.

CONCLUSIONS

Fundoplication reduces LES relaxation by interfering with axial stretch on the LES. Based on this mechanism of the antireflux actions of fundoplication, it might be possible to design new surgical strategies to treat reflux disease and reduce complications of fundoplication surgery.

Nitric oxide activation of a potassium channel (BKCa) in feline lower esophageal sphincter

AIM: To assess the effect of nitric oxide (NO) on the large conductance potassium channel (BK_Ca) in isolated circular (CM) and sling (SM) muscle cells and muscle strips from the cat lower esophageal sphincter (LES) to determine its regulation of resting tone and relaxation.

METHODS: Freshly enzymatically-digested and isolated circular smooth muscle cells were prepared from each LES region. To study outward K⁺ currents, the perforated patch clamp technique was employed. To assess LES resting tone and relaxation, muscle strips were mounted in perfused organ baths.

RESULTS: (1) Electrophysiological recordings from isolated cells: (a) CM was more depolarized than SM (-39.7 ± 0.8mV vs -48.1 ± 1.6 mV, P < 0.001), and maximal outward current was similar (27.1 ± 1.5 pA/pF vs 25.7 ± 2.0 pA/pF, P > 0.05); (b) The NO donor sodium nitroprusside (SNP) increased outward currents only in CM (25.9 ± 1.9 to 46.7 ± 4.2 pA/pF, P < 0.001) but not SM (23.2 ± 3.1 to 27.0 ± 3.4 pA/pF, P > 0.05); (c) SNP added in the presence of the BK_Ca antagonist iberiotoxin (IbTX) produced no increase in the outward current in CM (17.0 ± 2.8 vs 13.7 ± 2.2, P > 0.05); and (d) L-NNA caused a small insignificant inhibition of outward K⁺ currents in both muscles; and (2) Muscle strip studies: (a) Blockade of the nerves with tetrodotoxin (TTX), or BK_Ca with IbTX had no significant effect on resting tone of either muscle; and (b) SNP reduced tone in both muscles, and was unaffected by the presence of TTX or IbTX.

CONCLUSION: Exogenous NO activates BK_Ca only in CM of the cat. However, as opposed to other species, exogenous NO-induced relaxation is predominantly by a non-BK_Ca mechanism, and endogenous NO has minimal effect on resting tone.

The influences of g proteins, ca, and k channels on electrical field stimulation in cat esophageal smooth muscle

NO released by myenteric neurons controls the off contraction induced by electrical field stimulation (EFS) in distal esophageal smooth muscle, but in the presence of nitric oxide synthase (NOS) inhibitor, L-NAME, contraction by EFS occurs at the same time. The authors investigated the intracellular signaling pathways related with G protein and ionic channel EFS-induced contraction using cat esophageal muscles. EFS-induced contractions were significantly suppressed by tetrodotoxin (1 microM) and atropine (1 microM). Furthermore, nimodipine inhibited both on and off contractions by EFS in a concentration dependent meaner. The characteristics of 'on' and 'off' contraction and the effects of G-proteins, phospholipase, and K(+) channel on EFS-induced contraction in smooth muscle were also investigated. Pertussis toxin (PTX, a G(i) inactivator) attenuated both EFS-induced contractions. Cholera toxin (CTX, G(s) inactivator) also decreased the amplitudes of EFS-induced off and on contractions. However, phospholipase inhibitors did not affect these contractions. Pinacidil (a K(+) channel opener) decreased these contractions, and tetraethylammonium (TEA, K(+) (Ca) channel blocker) increased them. These results suggest that EFS-induced on and off contractions can be mediated by the activations Gi or Gs proteins, and that L-type Ca(2+) channel may be activated by G-protein alpha subunits. Furthermore, K(+) (Ca)-channel involve in the depolarization of esophageal smooth muscle. Further studies are required to characterize the physiological regulation of Ca(2+) channel and to investigate the effects of other K(+) channels on EFS-induced on and off contractions.

Mechanism of stretch-activated excitatory and inhibitory responses in the lower esophageal sphincter

We recently found that an orally directed stretch of the esophagus activates a neurally mediated relaxation of the lower esophageal sphincter (LES). Goals of our study were to characterize the neural mechanisms responsible for axial and transverse stretch-activated responses in the LES. LES pressure was monitored in anesthetized and artificially ventilated mice. Sutures were placed in the esophagus to exert graded stretch in the longitudinal and transverse directions. Effects of bilateral vagotomy and pharmacological agents on the stretch-activated LES responses were investigated. The relationship between vagally stimulated axial stretch and LES relaxation was also studied. Stretch in the longitudinal and transverse directions caused a dose-dependent LES relaxation and contraction, respectively, that were not affected by bilateral vagotomy and sympathectomy but were blocked by tetrodotoxin. In bilateral vagotomized animals, hexamethonium, atropine, pyridoxalphosphate-6-azophenyl-2',4' disulfonic acid (PPADS), and ondansetron did not block the stretch-activated LES relaxation and contraction. Axial stretch-activated LES relaxation was blocked by nitric oxide inhibitor and transverse stretch-activated LES contraction was blocked by a combination of atropine and substance P antagonist. Electrical stimulation of the vagus nerve induced LES relaxation and axial stretch on the LES, both of which were blocked by rocuronium. Axial and transverse stretch-activated LES relaxation and contraction were present in the W/W(v) mice that lack interstitial cells of Cajal (ICC). Stretch-activated LES relaxation and contraction are mediated through mechanosensitive neurons located in the myenteric plexus, which involves neither synaptic transmission nor ICC.

Inhibitory effect of hypochlorous acid on lower esophageal sphincter tone relaxation by vasoactive intestinal peptide

Under physiological conditions, hypochlorous acid (HOCl) is the major product of myeloperoxidase, a ferric heme enzyme released in inflammatory diseases. In the present study, we investigated the effect of HOCl compared to hydrogen peroxide (H2O2) on the vasoactive intestinal polypeptide (VIP)-induced relaxation of feline lower esophageal sphincter (LES) strips. Isometric tension on LES strips was measured using a force transducer. VIP induced the relaxation of basal LES tone in a concentration-dependent manner. Pretreatment with HOCl (10(-4) M) significantly reduced the VIP-induced relaxation at smaller concentrations than H2O2 (10(-3) M). VIP-induced relaxation is mediated via the Gi/o protein, since pretreatment with Pertussis Toxin (PTX) showed an inhibitory effect on the relaxation. HOCl showed an additional inhibitory effect on the reduced relaxation by PTX, indicating that HOCl might affect another G protein as well as Gi/o. However, HOCl did not affect SNP-, SIN-1-, and 8-br-cGMP-induced relaxation. Nor did HOCl modify the relaxation induced by either forskolin or db-cAMP in LES muscle strips. These results suggest that during short-term treatment, HOCl may damage the upstream events including G protein level, and result in alteration of LES tone in the feline esophagus, similar to the inhibitory effects of H2O2.

Purinergic and nitrergic junction potential in the human colon

The aim of the present work is to investigate a putative junction transmission [nitric oxide (NO) and ATP] in the human colon and to characterize the electrophysiological and mechanical responses that might explain different functions from both neurotransmitters. Muscle bath and microelectrode techniques were performed on human colonic circular muscle strips. The NO donor sodium nitroprusside (10 microM), but not the P2Y receptor agonist adenosine 5'-O-2-thiodiphosphate (10 microM), was able to cause a sustained relaxation. NG-nitro-L-arginine (L-NNA) (1 mM), a NO synthase inhibitor, but not 2'-deoxy-N6-methyl adenosine 3',5'-diphosphate tetraammonium salt (MRS 2179) (10 microM), a P2Y antagonist, increased spontaneous motility. Electrical field stimulation (EFS) at 1 Hz caused fast inhibitory junction potentials (fIJPs) and a relaxation sensitive to MRS 2179 (10 microM). EFS at higher frequencies (5 Hz) showed biphasic IJP with fast hyperpolarization sensitive to MRS 2179 followed by sustained hyperpolarization sensitive to L-NNA; both drugs were needed to fully block the EFS relaxation at 2 and 5 Hz. Two consecutive single pulses induced MRS 2179-sensitive fIJPs that showed a rundown. The rundown mechanism was not dependent on the degree of hyperpolarization and was present after incubation with L-NNA (1 mM), hexamethonium (100 microM), MRS 2179 (1 microM), and NF023 (10 microM). We concluded that single pulses elicit ATP release from enteric motor neurons that cause a fIJP and a transient relaxation that is difficult to maintain over time; also, NO is released at higher frequencies causing a sustained hyperpolarization and relaxation. These differences might be responsible for complementary mechanisms of relaxation being phasic (ATP) and tonic (NO).

Neuro-regulation of lower esophageal sphincter function as treatment for gastroesophageal reflux disease

The junction between the esophagus and the stomach is a specialized region, composed of lower esophageal sphincter (LES) and its adjacent anatomical structures, the gastric sling and crural diaphragm. Together these structures work in a coordinated manner to allow ingested food into the stomach while preventing reflux of gastric contents across the esophago-gastric junction (EGJ) into the esophagus. The same zone also permits retrograde passage of air and gastric contents into esophagus during belching and vomiting. The precise coordination required to execute such a complicated task is achieved by a finely-regulated high-pressure zone. This zone keeps the junction between esophagus and stomach continuously closed, but is still able to relax briefly via input from inhibitory neurons that are responsible for its innervation. Alterations of the structure and function of the EGJ and the LES may predispose to gastroesophageal reflux disease (GERD).

Physiology of normal esophageal motility

The esophagus consists of 2 different parts. In humans, the cervical esophagus is composed of striated muscles and the thoracic esophagus is composed of phasic smooth muscles. The striated muscle esophagus is innervated by the lower motor neurons and peristalsis in this segment is due to sequential activation of the motor neurons in the nucleus ambiguus. Both primary and secondary peristaltic contractions are centrally mediated. The smooth muscle of esophagus is phasic in nature and is innervated by intramural inhibitory (nitric oxide releasing) and excitatory (acetylcholine releasing) neurons that receive inputs from separate sets of preganglionic neurons located in the dorsal motor nucleus of vagus. The primary peristalsis in this segment involves both central and peripheral mechanisms. The primary peristalsis consists of inhibition (called deglutitive inhibition) followed by excitation. The secondary peristalsis is entirely due to peripheral mechanisms and also involves inhibition followed by excitation. The lower esophageal sphincter (LES) is characterized by tonic muscle that is different from the muscle of the esophageal body. The LES, like the esophageal body smooth muscle, is also innervated by the inhibitory and excitatory neurons. The LES maintains tonic closure because of its myogenic property. The LES tone is modulated by the inhibitory and the excitatory nerves. Inhibitory nerves mediate LES relaxation and the excitatory nerves mediate reflex contraction or rebound contraction of the LES. Clinical disorders of esophageal motility can be classified on the basis of disorders of the inhibitory and excitatory innervations and the smooth muscles.

Mechanisms controlling function in the clasp and sling regions of porcine lower oesophageal sphincter

BACKGROUND

Characterization of functional differences between lower oesophageal sphincter (LOS) clasp and sling muscles might aid the development of more specific pharmacological and surgical approaches for the treatment of motility disorders.

METHODS

Circular LOS strips from 25 adult pigs were studied in organ baths to compare the physiology of clasp and sling fibres.

RESULTS

Sling strips developed greater tone than clasp fibres (mean(s.e.m.) 7.59(0.89) versus 4.72(0.67) g; P = 0.017). LOS tone was more dependent on extracellular calcium in clasp strips and on the activity of cholinergic enteric motor neurones (EMNs) in sling strips. The amplitude of maximal relaxation caused by electrical field stimulation (EFS, 3Hz) of EMNs was greater in clasp strips (mean(s.e.m.) 74.5(2.3) versus 58.1(2.2) per cent of tone; P < 0.001). EFS-induced relaxation was reduced in clasp fibres and fully blocked in sling fibres by nitrergic blockade with 10 micromol/l 1H-[1,2,4]oxadiazole-[4,3-alpha]quinoxalin-1-one (ODQ). The amplitude of EFS cholinergic responses was significantly greater in sling fibres. In the clasp region, relaxation caused by stimulation of EMNs with 100 micromol/l nicotine was reduced by ODQ. In sling fibres, nicotine induced relaxation at rest and cholinergic contraction following ODQ.

CONCLUSION

Clasp and sling fibres of the porcine LOS show marked intrinsic functional differences. This should be considered when developing more specific approaches to human LOS motility disorders.

Regulation of basal tone, relaxation and contraction of the lower oesophageal sphincter. Relevance to drug discovery for oesophageal disorders

The lower oesophageal sphincter (LOS) is a specialized region of the oesophageal circular smooth muscle that allows the passage of a swallowed bolus to the stomach and prevents the reflux of gastric contents into the oesophagus. The anatomical arrangement of the LOS includes semicircular clasp fibres adjacent to the lesser gastric curvature and sling fibres following the greater gastric curvature. Such anatomical arrangement together with an asymmetric intrinsic innervation and distinct proportion of neurotransmitters in both regions produces an asymmetric pressure profile. The LOS tone is myogenic in origin and depends on smooth muscle properties that lead to opening of L-type Ca(2+) channels; however it can be modulated by enteric motor neurons, the parasympathetic and sympathetic extrinsic nervous system and several neurohumoral substances. Nitric oxide synthesized by neuronal NOS is the main inhibitory neurotransmitter involved in LOS relaxation. Different putative neurotransmitters have been proposed to play a role together with NO. So far, only ATP or related purines have shown to be co-transmitters with NO. Acetylcholine and tachykinins are involved in the LOS contraction acting through acetylcholine M(3) and tachykinin NK(2) receptors. Nitric oxide can also be involved in the regulation of LOS contraction. The understanding of the mechanisms that originate and modulate LOS tone, relaxation and contraction and the characterization of neurotransmitters and receptors involved in LOS function are important to develop new pharmacological tools to treat primary oesophageal motor disorders and gastro-oesophageal reflux disease.

Achalasia

Achalasia is a rare motor disorder of the oesophagus, characterised by the absence of peristalsis and impaired swallow-induced relaxation. These motor abnormalities result in stasis of ingested food in the oesophagus, leading to clinical symptoms, such as dysphagia, regurgitation of food, retrosternal pain and weight loss. Although it is well demonstrated that loss of myenteric oesophageal neurons is the underlying problem, it still remains unclear why these neurons are preferentially attacked and destroyed by the immune system. This limited insight into pathophysiology explains the fact that treatment is limited to interventions aimed at reducing the pressure of the lower oesophageal sphincter. The most successful therapies are clearly pneumatic dilatation and Heller myotomy with short-term success rates of 70-90%, declining to 50-65% after more than 15 years. The challenge for the coming years will undoubtedly be to get more insight into the underlying disease mechanisms and to develop a treatment to restore function.

Axial stretch: A novel mechanism of the lower esophageal sphincter relaxation

Swallow and esophageal distension-induced relaxations of the lower esophageal sphincter (LES) are associated with an orad movement of the LES because of a concurrent esophageal longitudinal muscle contraction. We hypothesized that the esophageal longitudinal muscle contraction induces a cranially directed mechanical stretch on the LES and therefore studied the effects of a mechanical stretch on the LES pressure. In adult opossums, a silicon tube was placed via mouth into the esophagus and laparotomy was performed. Two needles with silk sutures were passed, 90 degrees apart, through the esophageal walls and silicon tube, 2 cm above the LES. The tube was withdrawn, and one end of each of the four sutures was anchored to the esophageal wall and the other end exited through the mouth to exert graded cranially directed stretch on the LES by using pulley and weights. A cranially directed stretch caused LES relaxation, and with the cessation of stretch there was recovery of the LES pressure. The degree an d duration of LES relaxation increased with the weight and the duration of stretch, respectively. The mean LES relaxation in all animals was 77.7 +/- 4.7%. The required weight to induce maximal LES relaxation differed in animals (714 +/- 348 g). N(G)-nitro-L-arginine, a nitric oxide inhibitor, blocked the axial stretch-induced LES relaxation almost completely (from 78 to 19%). Our data support the presence of an axial stretch-activated inhibitory mechanism in the LES. The role of axial stretch in the LES relaxation induced by swallow and esophageal distension requires further investigation.

Nitric oxide modulates elicitation of reflex swallowing from the pharynx in rats

The pharynx is very important for elicitation of reflex swallowing. The region of the pharynx is innervated by the pharyngeal branch of the glossopharyngeal nerve (GPN-ph). Nitric oxide (NO) plays an important role in various physiological functions. The purpose of this study is to investigate the contribution of NO to reflex swallowing evoked by electrical stimulation of the GPN-ph. Swallowing was evoked in urethane-anesthetized rats by application of repetitive electrical stimulation (10- to 20-μA amplitude, 10- to 20-Hz frequency, 1.0-ms duration) to the central cut end of the GPN-ph or superior laryngeal nerve. Swallowing was identified by electromyographic activity of the mylohyoid muscle. Latency to the first swallow and the interval between swallows were measured. Intravenous administration of NG-nitro-l-arginine (l-NNA, 0.6 mg/kg), a nonselective inhibitor of NO synthase (NOS), extremely prolonged latency to the first swallow and the interval between swallows evoked by the GPN-ph. Intraperitoneal administration of 7-nitroindazole (5.0 mg/kg), a selective inhibitor of neuronal NOS, significantly prolonged latency to the first swallow and the interval between swallows evoked by the GPN-ph. Administration of l-arginine (an NO donor, 500 mg/kg) and sodium nitroprusside (an NO releaser, 0.6 mg/kg) restored the suppression of swallowing induced by the NOS inhibitor. Superior laryngeal nerve-evoked swallowing was suppressed by administration of a higher dose of l-NNA (6.0 mg/kg). Swallowing evoked by water stimulation of the pharynx was also suppressed by l-NNA (0.6 mg/kg). These results suggest that NO plays an important role in signal processing for initiation of reflex swallowing from the pharynx.

Effect of caffeine on lower esophageal sphincter pressure in Thai healthy volunteers

Caffeine affects many aspects of body function including the gastrointestinal system. A single-blinded experimental study was performed to evaluate the effect of caffeine on lower esophageal sphincter (LES) and esophageal peristaltic contractions in healthy Thai adults. The volunteers were six men and six women aged 19-31 years. Subjects drank 100 mL of water. Five wet swallows were performed 30 min after the drink. The basal LES pressure was continuously measured using esophageal manometric technique. They then consumed another 100 mL of water containing caffeine at the dose of 3.5 mg/kg body weight. The swallows and basal LES pressure monitoring were repeated. The results showed no change in basal LES pressure after a water drink while caffeine consumption significantly lowered the pressure at 10, 15, 20 and 25 min. The mean amplitude of contractions and peristaltic velocity were decreased at the distal esophagus at 3 and 8 cm above LES. The mean duration of contraction was decreased at the distal part but increased at the more proximal esophagus. The heart rate, systolic and diastolic blood pressures were increased significantly at 10-20 min after caffeine ingestion. This study indicated that caffeine 3.5 mg/kg affected esophageal function, resulting in a decrease in basal LES pressure and distal esophageal contraction, which is known to promote the reflux of gastric contents up into the esophagus.

Electrically stimulated relaxation is not mediated by GABA in cat lower esophageal sphincter muscle

This study examined the effect of Gamma-Amino butyric acid (GABA) and selective GABA receptor related drugs on the electrically stimulated relaxation in the lower esophageal sphincter muscle (LES) of a cat. Tetrodotoxin (10(-6) M) suppressed the electrically stimulated (0.5-5 Hz) relaxation of the LES. However, guanethidine (10(-6) M) and atropine (10(-6) M) had no effect indicating that the relaxations were neurally mediated via the nonadrenergic and noncholinergic (NANC) pathways. NG-nitro-L-arginine methyl ester (10(-4) M, L-NAME) also inhibited the relaxant response but did not completely abolish the electrically stimulated relaxation with 60 % inhibition, which suggests the involvement of nitric oxide as an inhibitory transmitter. This study examined the role of GABA, an inhibitory neurotransmitter, on neurally mediated LES relaxation. GABA (10(-3)-10(-5) M, non selective receptor agonist), muscimol (10(-3)-10(-5) M, GABA-A agonist), and baclofen (10(-3)-10(-5) M, GABA-B agonist) had no significant effect on the electrically stimulated relaxation. Moreover, bicuculline (10(-5) M, GABA-A antagonist) and phaclofen (10(-5) M, GABA-B antagonist) had no inhibitory effect on the electrically stimulated relaxation. This suggests that GABA and the GABA receptor are not involved in the electrically stimulated NANC relaxation in the cat LES.

Feline lower esophageal sphincter sling and circular muscles have different functional inhibitory neuronal responses

The lower esophageal sphincter (LES) has a circular muscle component exhibiting spontaneous tone that is relaxed by nitric oxide (NO) and a low-tone sling muscle that contracts vigorously to cholinergic stimulation but with little or no evidence of NO responsiveness. This study dissected the responses of the sling muscle to nitrergic innervation in relationship to its cholinergic innervation and circular muscle responses. Motor responses were induced by electrical field stimulation (EFS; 1-30 Hz) of muscle strips from sling and circular regions of the feline LES in the presence of cholinergic receptor inhibition (atropine) or NO synthase inhibition [NG-nitro-L-arginine (L-NNA)+/-atropine]. This study showed the following. First, sling muscle developed less intrinsic resting tone compared with circular muscle. Second, with EFS, sling muscle contracted (most at <or=10 Hz), whereas circular muscle relaxed >50% by 5 Hz. Third, on neural blockade with atropine or L-NNA+/-atropine, 1) sling muscle, although predominantly influenced by excitatory cholinergic stimulation, had a small neural NO-mediated inhibition, with no significant non-NO-mediated inhibition and 2) circular muscle, although little affected by cholinergic influence, underwent relaxation predominantly by neural release of NO and some non-NO inhibitory influence (at higher EFS frequency). Fourth, the sling, precontracted with bethanecol, could relax with NO and some non-NO inhibition. Finally, the tension range of both muscles is similar. In conclusion, sling muscle has limited NO-mediated inhibition to potentially augment or replace sling relaxation effected by switching off its cholinergic excitation. Differences within the LES sling and circular muscles could provide new directions for therapy of LES disorders.

Pharmacologic characterization of intrinsic mechanisms controlling tone and relaxation of porcine lower esophageal sphincter

The neurotransmitters mediating relaxation of lower esophageal sphincter (LES) were studied using circular LES strips from adult pigs in organ baths. LES relaxation by sodium nitroprusside (1 nM-3 microM), vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP; 1 nM-1 microM), ATP (10 microM-30 mM), and tricarbonyldichlororuthenum dimer (1 microM-1 mM) was unaffected by tetrodotoxin (1 microM) or l-N(G)-nitroarginine methyl ester (l-NAME; 100 microM). Calcitonin gene-related peptide (CGRP; 1 nM-1 microM) did not affect LES tone. ATP relaxation was blocked by 1 microM apamin and the P2Y(1) antagonist MRS 2179 (N6-methyl 2'-deoxyadenosine 3',5'-bisphosphate; 10 microM). Apamin inhibited PACAP relaxation. VIP and PACAP relaxation was blocked by 10 U/ml alpha-chymotrypsin. L-NAME (-62.52 +/- 13.13%) and 1H-[1,2,4]oxadiazole-[4,3-alpha]quinoxalin-1-one (ODQ; 10 microM, -67.67 +/- 6.80%) similarly inhibited electrical LES relaxation, and apamin blocked non-nitrergic relaxation. Nicotine relaxation (100 microM) was inhibited by L-NAME (-60.37 +/- 10.8%) and ODQ (-41.90 +/- 7.89%), and apamin also blocked non-nitrergic relaxation. Non-nitrergic and apamin-sensitive LES relaxation by electrical stimulation or nicotine was strongly inhibited by MRS 2179, slightly inhibited by alpha-chymotrypsin and the P2X(1,2,3) receptor antagonist NF 279 (8,8 cent-[carbonylbis(imino-4,1-phenylenecarbonylimino-4,1-phenylenecarbonylimino)]bis-1,3,5-naphthalenetrisulfonic acid hexasodium salt; 10 microM), and unaffected by tin protoporphyrin IX (100 microM). Porcine LES relaxation after stimulation of intrinsic inhibitory motor neurons is mediated by two main neuromuscular pathways: nitric oxide through guanylate cyclase signaling and apamin-insensitive mechanisms and by non-nitrergic apamin-sensitive neurotransmission mainly mediated by ATP, ADP, or a related purine acting on P2Y1 receptors and a minor contribution of purinergic P2X1,2,3 receptors and PACAP. Nitrergic and purinergic co-transmitters show parallel effects of similar magnitude without major interplay. Our study shows no role for CGRP and only a minor one for VIP and carbon monoxide in porcine LES relaxation.

Pathophysiology of gastroesophageal reflux disease

From a pathophysiologic viewpoint, GERD results from the excessive reflux of gastric contents into the distal esophagus. Under normal conditions, this is prevented as a function of the antireflux barrier at the EGJ, the integrity of which is dependent on the delicate interplay of a host of anatomic and physiologic factors, including the integrity of the LES, TLESR, and anatomic degradation of the EGJ inclusive of but not limited to hiatus hernia. Considerable investigative focus is aimed at describing the subtle aberrations of the EGJ that may contribute to the root causes of GERD. The net result is an increased number of reflux events, an increasing diversity of potential mechanisms of reflux, and a diminished ability of the stomach to selectively vent gas, as opposed to gas and gastric juice, during TLESR. Once reflux occurs, the duration of resultant esophageal acid exposure is determined by the effectiveness of esophageal acid clearance, the dominant determinants of which are peristalsis, salivation, and, again, the anatomic integrity of the EGJ. Approximately half of patients who have GERD have abnormal acid clearance and the major contributor to this is hiatus hernia. Abnormalities of acid clearance probably are the major determining factor influencing which patients who have GERD are most prone to developing esophagitis as opposed to symptomatic GERD. In summary, GERD is a multifactorial process involving physiologic and anatomic abnormalities. These abnormalities exhibit a complicated interplay that degrades the ability of the EGJ to contain gastric juice within the stomach and to clear the esophagus of gastric juice effectively once reflux has occurred.

The lower oesophageal sphincter

The lower oesophageal sphincter (LOS) is a specialized segment of the circular muscle layer of the distal oesophagus, accounting for approximately 90% of the basal pressure at the oesophago-gastric junction. Together with the crural diaphragm, it functions as an antireflux barrier protecting the oesophagus from the caustic gastric content. During swallowing or belching, the LOS muscle must relax briefly in order to allow passage of food or intragastric air. These swallow-induced and prolonged transient lower oesophageal sphincter relaxations (TLOSRs) respectively result from activation of the inhibitory motor innervation of the sphincter. Both in man and animals, the main neurotransmitter released by the inhibitory neurones is nitric oxide. The two typical examples of dysfunction of the LOS are achalasia and gastro-oesophageal reflux disease (GORD). Achalasia is characterized by reduction or even absence of the inhibitory innervation to the LOS, leading to impaired LOS relaxation with dysphagia and stasis of food in the oesophagus. On the contrary, GORD results from failure of the antireflux barrier, with increased exposure of the oesophagus to gastric acid. This leads to symptoms such as heartburn and regurgitation, and in more severe cases to oesophagitis, Barrett's oesophagus and even carcinoma. To date, TLOSRs are recognized as the main underlying mechanism, and may represent an important target for treatment. More insight in the pathogenesis of both diseases will undoubtedly lead to new treatments in the near future.

Different responsiveness of excitatory and inhibitory enteric motor neurons in the human esophagus to electrical field stimulation and to nicotine

To compare electrical field stimulation (EFS) with nicotine in the stimulation of excitatory and inhibitory enteric motoneurons (EMN) in the human esophagus, circular lower esophageal sphincter (LES), and circular and longitudinal esophageal body (EB) strips from 20 humans were studied in organ baths. Responses to EFS or nicotine (100 microM) were compared in basal conditions, after N(G)-nitro-l-arginine (l-NNA; 100 microM), and after l-NNA and apamin (1 microM). LES strips developed myogenic tone enhanced by TTX (5 microM) or l-NNA. EFS-LES relaxation was abolished by TTX, unaffected by hexamethonium (100 microM), and enhanced by atropine (3 microM). Nicotine-LES relaxation was higher than EFS relaxation, reduced by TTX or atropine, and blocked by hexamethonium. After l-NNA, EFS elicited a strong cholinergic contraction in circular LES and EB, and nicotine elicited a small relaxation in LES and no contractile effect in EB. After l-NNA and apamin, EFS elicited a strong cholinergic contraction in LES and EB, and nicotine elicited a weak contraction amounting to 6.64 +/- 3.19 and 9.20 +/- 5.51% of that induced by EFS. EFS elicited a contraction in longitudinal strips; after l-NNA and apamin, nicotine did not induce any response. Inhibitory EMN tonically inhibit myogenic LES tone and are efficiently stimulated both by EFS and nicotinic acetylcholine receptors (nAChRs) located in somatodendritic regions and nerve terminals, releasing nitric oxide and an apamin-sensitive neurotransmitter. In contrast, although esophageal excitatory EMN are efficiently stimulated by EFS, their stimulation through nAChRs is difficult and causes weak responses, suggesting the participation of nonnicotinic mechanisms in neurotransmission to excitatory EMN in human esophagus.

Muscarinic modulation of nitrergic neurotransmission in guinea-pig gastric fundus

Muscarinic receptor activation by (4-Hydroxy-2-butynyl)-1-trimethylammonium-m-chlorocarbanilate chloride (McN-A-343) was investigated both on NADPH-d staining and on electrically induced responses in guinea-pig gastric fundus. McN-A-343 (10 micromol L(-1)) significantly increased the optical density of NADPH-d positive neurones, while blockade of nitric oxide synthase with N(omega)-nitro-L-arginine (L-NA) decreased it, suggesting facilitation of nitric oxide (NO) production. Electrical field stimulation (EFS; 2 Hz, 0.2 ms, supramaximal current intensity, 10 s train duration) elicited on-contraction followed by off-relaxation in the circular muscle strips. McN-A-343 (10 micromol L(-1)) transformed the EFS-evoked response from on-contraction into on-relaxation, which was neurogenic, tetrodotoxin-sensitive and hexamethonium-resistant. L-NA partly reduced the EFS-evoked relaxation, revealing two components: a nitrergic and a non-nitrergic one. The effect of McN-A-343 on the amplitude of the EFS-evoked relaxation was not changed by the M(3) receptor antagonist para-fluoro-hexahydro-sila-difenidol hydrochloride, but was significantly enhanced by M(1) receptor blockade with telenzepine. In the presence of telenzepine, the L-NA-dependent nitrergic component of the EFS-induced relaxation predominates. We suggest that cholinergic receptor activation has a dual effect on nitrergic neurotransmission: (i) stimulation of NOS by muscarinic receptor(s) different from M(1) and M(3) subtype, (ii) prejunctional inhibition of NO-mediated relaxation via M(1) receptors. In addition, M(1) receptors may facilitate the non-nitrergic relaxation.

Effects of dietary nitrate on oesophageal motor function and gastro-oesophageal acid exposure in healthy volunteers and reflux patients

BACKGROUND/AIMS

High concentrations of nitric oxide (NO), derived from dietary nitrite in an acid environment, have been demonstrated in the gastric fundus and in the oesophagus. The aim of this study was to investigate whether luminal NO can influence oesophageal smooth muscle performance, lower oesophageal sphincter (LOS) function or gastric and oesophageal acid exposure.

METHODS

Eleven healthy volunteers and 9 patients with chronic gastro-oesophageal reflux disease (GORD) received a diet deprived of nitrate/nitrite but supplemented with placebo or potassium nitrate for 4 days in a randomised order. On day 4 in each trial period, manometry was performed including a sleeve sensor registration of the LOS followed by a simultaneous 24-hour intra-gastric and oesophageal pH registration.

RESULTS

Nitrate supplementation increased the proportion of effective peristalsis when analysed for the entire study population. No other significant effects of dietary nitrate were found on oesophageal motor variables, on the sphincter resting tone or on the number or duration of transient sphincter relaxations. No effect was found on either gastric acidity or gastro-oesophageal reflux variables. Major reflux symptoms were not influenced by nitrate administration.

CONCLUSION

Dietary nitrate did not significantly affect oesophageal motor or LOS function, gastro-oesophageal acid reflux or reflux symptomatology either in healthy volunteers or in GORD patients.

Release of nitric oxide in the central nervous system mediates tonic and phasic contraction of the cat lower oesophageal sphincter

Nitric oxide (NO) in the brainstem is implicated in the control of swallowing and oesophageal peristalsis. This study examines the role of brainstem NO in the maintenance of lower oesophageal sphincter (LOS) tone, relaxation and contraction. In urethane-anaesthetized cats, oesophageal peristalsis and sphincter pressures were continuously monitored. Drugs were administered into the fourth ventricle. Oesophageal peristalsis and sphincter relaxation and contraction were induced by superior laryngeal nerve stimulation or intra-oesophageal balloon distention. Basal sphincter pressure was significantly reduced after the i.c.v. administration of the nitric oxide synthase (NOS) inhibitor, l-Ng-monomethyl arginine. The inhibitor's d-isomer had no significant effect on basal sphincter pressure, while l-arginine partially reversed the effect. The NOS inhibitor had no effect on sphincter relaxation, whereas the contraction of the sphincter following relaxation was significantly inhibited. Central nitric oxide synthase inhibition reduces basal LOS tone and contraction amplitude but has no effect on swallow or balloon distention induced sphincter relaxation. Therefore, central release of NO acts in the pathway to stimulate dorsal motor nucleus of the vagus neurones projecting to excitatory neurones in the sphincter. Inhibition of nitric oxide synthase in the CNS does not prevent relaxation of the LOS, suggesting that other pathways that do not utilize NO are important in the induction of LOS relaxation.

The target of therapies: pathophysiology of gastroesophageal reflux disease

Recognition of the importance of anatomical and mechanical factors in the pathogenesis of GERD has led to attempts that modify these mechanisms by endoluminal therapies. Although these alterations of EGJ anatomy and function appear minor at first glance, one must be cautious not to upset the delicate balance maintained by this complex anatomic zone. Performing a procedure that effects retrograde flow will inevitably also affect antegrade flow to some degree. The challenge will be to find a middle ground whereby GERD symptoms are not exchanged for symptoms such as dysphagia and gas bloat. With respect to endoluminal therapies, the authors are at an early stage of development and much more work is needed to define the relevant mechanical variables that need to be modified and to define the patient populations most appropriate for these interventions.

Central mechanisms of lower esophageal sphincter control

Lower esophageal sphincter (LES) tone is decreased during swallowing, during transient LES relaxations (TLESRs), and before emesis, and this decrease is due primarily to increasing inhibitory vagal output to the LES. Reflex-evoked relaxation of the LES is mediated by long-loop vagovagal reflexes that are coordinated by the dorsal vagal complex in the hindbrain medulla. A sequence of events occurs. Central control of TLESRs has not been studied directly; the information on how drugs may work centrally to reduce TLESRs is extrapolated from knowledge of how the brain evokes LES relaxation. Reduction of the frequency of TLESRs by a GABAB agonist, baclofen, is due to inhibition of vagal afferents, information transfer between the nucleus tractus solitarius and dorsal motor nucleus of the vagus, and vagal efferent outflow. Preliminary data show that cannabinoid receptor activation reduces information transfer between the nucleus tractus solitarius and dorsal motor nucleus of the vagus. The potential therapeutic usefulness of these types of agents that reduce TLESRs by acting centrally is promising.

Peripheral mechanisms affecting the lower esophageal sphincter tone

LES tone is influenced by central and peripheral mechanisms. Peripheral factors influencing the LES tone include inhibitory reflexes originating from the stomach, esophagus, pharynx, and larynx. Although the role of LES relaxation and its associated reflux events induced by gastric distention has been extensively studied, the LES relaxations triggered by stimulation of the aerodigestive tract have only recently been the focus of systematic investigation and have resulted in documentation of the intricate reflex relationship between the upper gastrointestinal tract and the aerodigestive tract.

Lower oesophageal sphincter relaxation evoked by stimulation of the dorsal motor nucleus of the vagus in ferrets

An understanding of the neural control of lower oesophageal sphincter (LOS) relaxation is clinically relevant because transient LOS relaxations (TLOSRs) are a mechanism of acid reflux into the oesophagus. Preganglionic motor neurones innervating the LOS are localized in the dorsal motor nucleus of the vagus (DMV). Based on a single study in cats, it is now widely accepted that these neurones are functionally organized into two separate populations, such that stimulation of the caudal and rostral DMV evokes LOS relaxation and contraction, respectively. Our goal was to map the functional LOS responses to chemical stimulation in the DMV and nucleus tractus solitarius (NTS) of ferrets, an animal model commonly used for conscious studies on TLOSRs, and to test whether DMV-evoked LOS relaxation is mediated through hexamethonium-sensitive vagal-inhibitory pathways to the LOS. We used miniaturized manometry with Dentsleeve to monitor LOS and oesophageal pressures in decerebrate unanaesthetized ferrets. LOS relaxation was evoked readily in response to gastric insufflation, which shows that the vago-vagal reflex was intact in this preparation. Microinjections of l-glutamate (12.5 nmol L-1 in 25 nL) were made into the DMV from approximately - 1.5 to + 2.0 mm relative to the obex. Microinjections into the caudal (- 1.5 to + 0.0 mm behind obex) and intermediate (+ 0.1 to + 1.0 mm rostral to obex) DMV both significantly decreased LOS pressure, and complete LOS relaxation was noted in 28/32 and 11/18 cases, respectively. LOS relaxation responses to DMV microinjection were highly reproducible and abolished by bilateral vagotomy or hexamethonium (15 mg kg-1 intravenously). A nitric oxide synthase inhibitor (l-NAME 100 mg kg-1 intramuscularly) significantly increased the time taken to reach the maximal response. Increases in LOS pressure (24 +/- 4 mmHg; n = 3) were obtained only when stimulation sites were located equal to greater than 1.5 mm rostral to the obex. LOS relaxation (- 78 +/- 10%; n = 6) was evoked by stimulation of the NTS but not immediately outside of the NTS (11 +/- 27%; n = 5). We conclude that there is a very extensive population of 'inhibitory' motor neurones in the DMV that may account for the predominant vagal-inhibitory tone in ferrets. As NTS stimulation evokes LOS relaxation and the predominant response to DMV stimulation is also LOS relaxation, this vago-vagal reflex may involve an excitatory interneurone between the NTS and DMV vagal inhibitory output.

The effect of sildenafil on lower esophageal sphincter and body motility in normal male adults

OBJECTIVE

Sildenafil relaxes smooth muscle by blocking type 5 phosphodiesterase, which destroys nitric oxide-stimulated cyclic guanosine monophosphate. The aim of this study is to investigate the change of lower esophageal sphincter (LES) and body motility with the lapse of time after sildenafil infusion in normal male adults.

METHODS

After basal esophageal manometry in eight healthy male adult volunteers, we infused a 50-mg tablet of sildenafil dissolved in water in the stomach through the manometry catheter and observed the changes of LES and body motility with the lapse of time. We randomized the study population into two groups, and esophageal manometry was repeated in LES and body sequence in four volunteers and in body and LES sequence in the other four volunteers immediately after sildenafil infusion.

RESULTS

LES resting pressure significantly decreased after sildenafil infusion. The body peristaltic amplitude gradually decreased and eventually disappeared, and the latency increased significantly after sildenafil infusion in both the proximal and distal esophagus.

CONCLUSIONS

These data support that nitric oxide mediates LES relaxation and the timing of esophageal peristalsis. In the future, sildenafil can be tried in some esophageal motor disorders, which have defects in nitric oxide neuromuscular communication.

Effect of sildenafil, a phosphodiesterase-5 inhibitor, on oesophageal peristalsis and lower oesophageal sphincter function in cats

The propagation of oesophageal peristaltic contractions and lower oesophageal sphincter (LOS) relaxation depends on neural release of nitric oxide (NO) which acts to increase intracellular cGMP. Sildenafil, a phosphodiesterase-5 inhibitor that increases cGMP, reduces basal LOS pressure in patients with achalasia. We investigated the effect of sildenafil on the propagation of oesophageal contractions and LOS relaxation in the cat. Oesophageal manometry was performed in five cats under light sedation. Peristaltic contractions were monitored at 1, 2, 3, 4 and 8 cm proximal to the LOS, at the LOS using a Dent sleeve, and at 3 cm distal to the upper oesophageal sphincter. Swallow-induced oesophageal contractions and LOS relaxation were recorded during 30 min before and 30 min after intravenous administration of sildenafil. Sildenafil reduced the amplitude of oesophageal contractions only in the smooth muscle oesophagus. The latency from swallow to distal oesophageal contractions was significantly delayed. LOS pressure was significantly reduced but the relaxation nadir was not modified by sildenafil. Sildenafil has profound effects on oesophageal motility: it modifies propagation and amplitude of oesophageal contractions and reduces LOS pressure. Slowing down the propagation of contractions in the transitional zone between the striated and smooth muscle can be a useful tool in patients with segmental aperistalsis or intermittent simultaneous contractions, while the effect on the LOS can benefit patients with achalasia.

Participation of nitric oxide synthase and cyclo-oxygenase in the signal transduction pathway of ileal muscarinic acetylcholine receptors

Parasympathetic activation of ileal motility is essential for intestinal physiology. We have previously demonstrated that carbachol activates muscarinic acetylcholine receptors (mAChR) of rat intestine and stimulates ileal motility via phospholipase C. This activation induces phosphoinositide turnover and intracellular calcium mobilization. We show here that carbachol stimulation of rat ileal motility is potentiated by the nitric oxide synthase (NOS) inhibitor N(G)-monomethyl arginine. Thus, we confirm that carbachol increases, in a dose-dependent manner, the activity of a NOS isoform that depends on calcium-calmodulin binding. Its product, nitric oxide (NO), activates not only guanylyl cyclase, inducing cGMP synthesis, but also cyclo-oxygenase, producing prostaglandin E(2). The prostanoid probably cooperates with NO to induce ileal smooth muscle relaxation.

Peptidergic regulation of gastrointestinal motility in rodents

Peptides involved in the endocrine and enteric nervous systems as well as in the central nervous system exert concerted action on gastrointestinal motility. Mechanical and chemical stimuli which induce peptide release from the epithelial endocrine cells are the earliest step in the initiation of peristaltic activities. Gut peptides exert hormonal effects, but peptide-containing stimulatory (Ach/substance P/tachykinin) and inhibitory (VIP/PACAP/NO) neurons are also involved in the induction of ascending contraction and descending relaxation, respectively. The dorsal vagal complex (DVC), located in the medulla of the brainstem, constitutes the basic neural circuitry of vago-vagal reflex control of gastrointestinal motility. Several gut peptides act on the DVC to modify vagal cholinergic reflexes directly (PYY and PP) or indirectly via afferent fibers in the periphery (CCK and GLP-1). The DVC is also a primary site of action of many neuropeptides (such as TRH and NPY) in mediating gastrointestinal motor activities. The identification over the last few years of a number of neuropeptide systems has greatly changed the field of feeding and body weight regulation. By exploring the brain and gut systems that employ recently identified peptidergic molecules, it will be possible to elaborate on the central and peripheral pathways involved in the regulation of gastrointestinal motility.

Pharmacologic strategies for treating gastroesophageal reflux disease

OBJECTIVE

This paper presents a synopsis of the pathophysiology of gastroesophageal reflux disease (GERD) and the efficacy, safety, and cost of the agents commonly used in its treatment.

BACKGROUND

Symptomatic relief of GERD can be obtained with lifestyle changes (avoidance of factors that may exacerbate symptoms, such as overeating and use of alcohol and tobacco) and use of over-the-counter medications such as antacids or histamine-2 (H2)-receptor antagonists. When these measures are unsuccessful, treatment with prescription-strength medications is required to prevent complications, such as Barrett's esophagus or esophageal adenocarcinoma.

METHODS

Current guidelines for the management of GERD were identified through a MEDLINE search of the English-language literature from January 1995 through December 1999 and a search of the bibliographies of identified articles.

CONCLUSION

Patients who do not respond to initial therapy should be managed with prescription-strength H2-receptor antagonists. Endoscopy should be considered for patients with atypical or refractory symptoms. Patients with a confirmed diagnosis of refractory GERD, severe esophagitis, Barrett's esophagus, or peptic stricture should be treated with and maintained on a proton-pump inhibitor.

Biphasic relaxation of the opossum lower esophageal sphincter: roles of NO., VIP, and CGRP

Vasoactive intestinal polypeptide (VIP) and nitric oxide (NO.) are thought to mediate lower esophageal sphincter (LES) relaxation. Transverse muscle strips from the opossum LES were used to test this hypothesis. Electrical field stimulation (EFS) produced a biphasic LES relaxation: a rapid component during the stimulus was more prominent at lower stimulus frequencies, and a sustained component was more prominent at higher frequencies. N(omega)-nitro-L-arginine and hemoglobin inhibited the rapid component but affected the sustained component less. Exogenous VIP decreased LES tone. A number of purported VIP antagonists blocked neither VIP-induced nor EFS-induced relaxation of the LES. The calcitonin gene-related peptide (CGRP) antagonist CGRP-(8-37) did not alter EFS-induced LES relaxation. EFS-induced relaxation of opossum LES muscle is biphasic, and the initial, rapid component of the relaxation is mediated primarily by NO. The mediator of the sustained component was not identified.