Sensorial and intestinointestinal reflex pathways in the human jejunum

Gastric sensitivity and reflexes: basic mechanisms underlying clinical problems

Both reflex and sensory mechanisms control the function of the stomach, and disturbances in these mechanisms may explain the pathophysiology of disorders of gastric function. The objective of this report is to perform a literature-based critical analysis of new, relevant or conflicting information on gastric sensitivity and reflexes, with particular emphasis on the comprehensive integration of basic and clinical research data. The stomach exerts both phasic and tonic muscular (contractile and relaxatory) activity. Gastric tone determines the capacity of the stomach and mediates both gastric accommodation to a meal as well as gastric emptying, by partial relaxation or progressive recontraction, respectively. Perception and reflex afferent pathways from the stomach are activated independently by specific stimuli, suggesting that the terminal nerve endings operate as specialized receptors. Particularly, perception appears to be related to stimulation of tension receptors, while the existence of volume receptors in the stomach is uncertain. Reliable techniques have been developed to measure gastric perception and reflexes both in experimental and clinical conditions, and have facilitated the identification of abnormal responses in patients with gastric disorders. Gastroparesis is characterised by impaired gastric tone and contractility, whereas patients with functional dyspepsia have impaired accommodation, associated with antral distention and increased gastric sensitivity. An integrated view of fragmented knowledge allows the design of pathophysiological models in an attempt to explain disorders of gastric function, and may facilitate the development of mechanistically orientated treatments.

Effect of short-chain fatty acids and acidification on the phasic and tonic motor activity of the human colon

BACKGROUND

The effects of bacterial fermentation on human colonic motor activity could be explained by colonic acidification or short-chain fatty acid (SCFA) production. We compared in healthy volunteers the colonic motor effects of intracolonic infusion of neutral or acidic saline solutions and then of neutral or acidic solutions containing an SCFA mixture.

METHODS

20 healthy volunteers swallowed a probe (with an infusion catheter, 6 perfused catheters and a balloon connected to a barostat) that migrated into the colon. Colonic motor activity was recorded in fasting basal state (1 h), during (3 h) and after (2 h) intracolonic infusion in a random order on two consecutive days of 750 mL of NaCl at pH 7.0 (neutral saline) or 4.5 (acidic saline) in 10 volunteers (first experiment) and of an SCFA mixture (acetic acid 66%, propionic acid 24% and butyric acid 10%; 100 mM) at pH 7.0 or 4.5 in 10 other volunteers (second experiment). We determined for each hour a global motility index (reflecting phasic activity recorded by all catheters), the mean balloon volume (reflecting tonic activity), and the mean number of high-amplitude-propagated contractions (HAPCs).

KEY RESULTS

Intracolonic infusion of neutral or acidic solutions containing saline or an SCFA mixture did not change the global motility index, the barostat balloon volume, or the HAPC number compared with basal values.

CONCLUSIONS & INFERENCES

Under our experimental conditions, these findings suggest that the stimulation of colonic motor activity induced by carbohydrate fermentation is not explained by the acidification of the colonic contents or the resulting production of SCFAs.

Fermentation of starch stimulates propagated contractions in the human colon

BACKGROUND

In healthy humans, up to 30 g of daily ingested starch escape small intestinal digestion, and are fermented in the colon. This physiological starch malabsorption could modify colonic motility through metabolites such as short-chain fatty acids produced by fermentation.

METHODS

Ten healthy volunteers swallowed a probe, consisting of an infusion catheter, six perfused catheters and a balloon connected to a barostat. On two consecutive days colonic motility was recorded in fasting subjects in the basal state (1 h), and then during (3 h), and after (2 h) the intracolonic infusion of 750 mL of isoosmotic and isovolumetric solutions containing sodium chloride with or without 15 g wheat starch. We determined (i) the volume of hydrogen and methane exhaled in breath, (ii) a global motility index and the number of high amplitude propagated contractions (HAPCs), and (iii) the mean balloon volume, reflecting the tonic motor activity.

KEY RESULTS

[median (IQR)] Compared to the basal period, colonic infusion of starch or saline did not modify the colonic motility index and tone. However, the number of HAPCs was significantly higher during and after infusion of starch than of saline [4.5 (2.75-6.5) vs 0.96 (0-2.66)/5 h, starch vs saline respectively; P = 0.011].

CONCLUSIONS & INFERENCES

In healthy humans, colonic fermentation of a physiological malabsorbed amount of starch has no effect on the tonic and phasic colonic motor activities, but produces a significant increase in the number of HAPCs. This may participate in the physiological propulsion of colonic contents.

Intestinal motor activity, endoluminal motion and transit

A programme for evaluation of intestinal motility has been recently developed based on endoluminal image analysis using computer vision methodology and machine learning techniques. Our aim was to determine the effect of intestinal muscle inhibition on wall motion, dynamics of luminal content and transit in the small bowel. Fourteen healthy subjects ingested the endoscopic capsule (Pillcam, Given Imaging) in fasting conditions. Seven of them received glucagon (4.8 microg kg(-1) bolus followed by a 9.6 microg kg(-1) h(-1) infusion during 1 h) and in the other seven, fasting activity was recorded, as controls. This dose of glucagon has previously shown to inhibit both tonic and phasic intestinal motor activity. Endoluminal image and displacement was analyzed by means of a computer vision programme specifically developed for the evaluation of muscular activity (contractile and non-contractile patterns), intestinal contents, endoluminal motion and transit. Thirty-minute periods before, during and after glucagon infusion were analyzed and compared with equivalent periods in controls. No differences were found in the parameters measured during the baseline (pretest) periods when comparing glucagon and control experiments. During glucagon infusion, there was a significant reduction in contractile activity (0.2 +/- 0.1 vs 4.2 +/- 0.9 luminal closures per min, P < 0.05; 0.4 +/- 0.1 vs 3.4 +/- 1.2% of images with radial wrinkles, P < 0.05) and a significant reduction of endoluminal motion (82 +/- 9 vs 21 +/- 10% of static images, P < 0.05). Endoluminal image analysis, by means of computer vision and machine learning techniques, can reliably detect reduced intestinal muscle activity and motion.

New insight into intestinal motor function via noninvasive endoluminal image analysis

BACKGROUND & AIMS

Evaluation of small bowel motility by intestinal manometry is invasive and requires expertise for interpretation. Our aim was to use capsule technology for evaluation of small bowel motor function based on a fully computerized image analysis program.

METHODS

Thirty-six consecutive patients with severe intestinal motor disorders (19 fulfilling manometric criteria of intestinal dysmotility and 17 not) and 50 healthy subjects received the endoscopic capsule (Pillcam; Given Imaging, Yokneam, Israel). Endoluminal image analysis was performed with a computer vision program specifically developed for the detection of contractile patterns (phasic luminal closure and radial wrinkles by wall texture analysis), noncontractile patterns (tunnel and wall appearance by Laplacian filtering), intestinal content (by color decomposition analysis), and endoluminal motion (by chromatic stability). Automatic classification of normal and abnormal intestinal motility was performed by means of a machine-learning technique.

RESULTS

As compared with healthy subjects, patients exhibited less contractile activity (25% less phasic luminal closures, P < .05) and more noncontractile patterns (151% more tunnel pattern, P < .05), static sequences (56% more static images, P < .01), and turbid intestinal content (94% more static turbid images, P < .01). On cross validation, the classifier identified as abnormal all but 1 patient with manometric criteria of dysmotility and as normal all healthy subjects. Out of the 17 patients without manometric criteria of dysmotility, 11 were identified as abnormal and 6 as normal.

CONCLUSIONS

Our study shows that endoluminal image analysis, by means of computer vision and machine-learning techniques, constitutes a reliable, noninvasive, and automated diagnostic test of intestinal motor disorders.

Recto-colonic reflex is impaired in patients with irritable bowel syndrome

Motor and sensory dysfunction of the gut are present in a subset of patients with irritable bowel syndrome (IBS). Recent studies have demonstrated the presence of a recto-colonic inhibitory reflex in healthy humans. It is not known whether this reflex exists in IBS. We studied rectal compliance, perception and the recto-colonic reflex by measuring volume responses of the descending colon to rectal distentions by barostat in 26 IBS patients and 13 healthy controls under both fasting and postprandial conditions. In the fasting state, rectal distention inhibited colonic tone and phasic motility to a similar extent in health and IBS. After a meal, rectal distention inhibited colonic tone and phasic motility to a lesser degree (P < 0.05) in IBS than health. Under postprandial but not fasting conditions, rectal distentions of increasing intensity were associated with higher pain scores in IBS than in health. Rectal distention inhibits tonic and phasic motility of the descending colon in healthy controls and in IBS patients. Postprandially this recto-colonic inhibitory reflex is impaired and attenuated in IBS patients compared with controls. These findings point to an altered reflex function in IBS and have implications for pathophysiology and therapy.

Intestinal tone and gas motion

The intestine propels and evacuates large gas loads without detectable phasic contractions by manometry. We hypothesized that intestinal gas motion is produced by changes in gut tone and capacitance. In 13 healthy subjects, changes in duodenal tone were measured by a barostat during continuous perfusion of lipids (Intralipid, 1 kcal min(-1)) into the duodenum for 60 min. In separate groups, the effects of jejunal gas infusion (N2, CO2 and O2 in venous proportions at 12 mL min(-1) starting after 15 min lipid perfusion) and sham infusion were tested. Gas outflow was collected continuously via an intrarectal cannula. Duodenal lipid perfusion produced a rapid duodenal relaxation (volume increased by 48 +/- 18%; P < 0.01 vs basal). Gas infusion increased gas evacuation (184 +/- 59 mL), and this was associated with a tonic contraction of the duodenum (R = 0.86; P < 0.01) that completely reverted the lipid-induced duodenal relaxation (volume decreased by 42 +/- 13%; P < 0.05). During sham infusion only 52 +/- 28 mL of gas were evacuated (P < 0.05 vs gas infusion), and the duodenum remained relaxed due to the effect of lipids (0 +/- 1% volume reduction; ns). In conclusion, intestinal gas propulsion and clearance is associated with a tonic contraction of the gut wall and reduced gut capacitance.

Effect of gastric distension prior to eating on food intake and feelings of satiety in humans

The factors that regulate food intake and satiation are complex; it has been suggested that signals arising from the small intestine and the stomach play an important role. It is still unknown, to what extent pure mechanical distension of the gastric fundus and antrum can alter food intake. Our aim was therefore to investigate whether transient gastric fundus and antrum distension applied prior to meal ingestion can trigger satiation in healthy humans. Two sequential, randomized, double-blind, four-period cross-over designed studies were performed in 24 healthy male volunteers: (1) 12 subjects underwent four intragastric balloon distension experiments of the fundus (0, 400, 600, 800 ml) before a standard meal intake; (2) 12 subjects underwent intragastric balloon distension experiments of the antrum under the following conditions: 0 ml balloon distension of the antrum plus intraduodenal (ID) saline or ID fat, and 300 ml antrum distension plus ID saline or ID fat. Shortly after the distension period, subjects were free to eat and drink as much as they wished. Neither gastric fundus nor antrum distension showed a reduction in calorie intake. Distending the fundus affected the mean Visual Analogue Scale (VAS) in the premeal period: subjects experienced a reduced degree of hunger and a concomitant feeling of fullness, but the effect was only apparent during distension with a volume of 600 ml or even 800 ml. Cholecystokinin (CCK) and peptide YY (PYY) were not altered by gastric distension. Transient pure mechanical distension of the fundus or the antrum prior to a meal does not trigger satiation.

Intestinal perception: mechanisms and assessment

Physiological stimuli in the gut induce regulatory reflexes to accomplish the digestive process, but are normally not perceived. However, under some circumstances, gut stimuli may activate perception pathways and induce conscious sensations. Experimental evidence gathered during the past decade suggests that patients with functional gut disorders and unexplained abdominal symptoms may have a sensory dysfunction of the gut, so that physiological stimuli would induce symptoms. Assessment of visceral sensitivity is still poorly developed, but in analogy to somatosensory testing, differential stimulation of visceral afferents may be achieved by a combination of stimulation techniques, which may help to characterize sensory dysfunctions. Visceral afferent input is modulated by a series of mechanisms at different levels of the brain-gut axis, and conceivably, a dysfunction of these regulatory mechanisms could cause hyperalgesia. The sensory dysfunction in functional patients seems to be associated with altered reflex activity, and both mechanisms may interact to produce the symptoms. Evidence of a gut sensory-reflex dysfunction as a common pathophysiological mechanism in different functional gastrointestinal disorders would suggest that they are different forms of the same process, and that the clinical manifestations depend on the specific pathways affected.

Effect of rectal distension on abdominal girth

It has been postulated that a viscerosomatic reflex activated by gut distension and inhibiting abdominal wall muscle tone may be one of the mechanisms underlying functional abdominal distension. Any demonstration of such a reflex has to take into account the fact that gut distension may increase abdominal girth as a result of volume displacement. As biomechanical and sensory rectal responses vary at different rates of rectal distension, we hypothesized that different rates of rectal distension might reveal different changes in abdominal girth. Abdominal girth was continuously recorded in 14 healthy subjects using a previously validated extensometer. The rectal distensions were made in a randomized order at rates of 100 mL min(-1) or 10 mL min(-1) up to 150 mL, and sham distensions were used as controls. An increase in abdominal girth was observed at the end of both distensions (P </= 0.008): it was greater after the fast (1.1 +/- 0.5 mm) than after the slow distension (0.8 +/- 0.7 mm), but this difference was not statistically significant (P = 0.2). In conclusion, we were unable to demonstrate the existence of a viscerosomatic reflex activated by gut distension under our experimental conditions.

Analysis of surface geometry of the human stomach using real-time 3-D ultrasonography in vivo

The objective of this study was to develop an analytical method to describe the three-dimensional (3-D) geometry of the gastric antrum, gastric fundus and the whole stomach. The Fourier series method was used to simulate the organ surface geometry obtained from a 3-D ultrasound system. Data generated from eight antrums and three whole stomachs, at pressures of approximately 7 cm H(2)O, were used for lumen curvature calculations. The principal curvatures spatial distributions were non-homogeneous in the gastric antrum, gastric fundus and the stomach due to their complex geometry. The maximum longitudinal principal curvature in the antrum, fundus and total stomach were, respectively, 0.460 +/- 0.066, 0.583 +/- 0.087 and 1.123 +/- 0.328, whereas the maximum circumferential curvature were 1.192 +/- 0.090, 3.649 +/- 1.574 and 8.444 +/- 3.424, respectively. The present study provides an analytical tool for characterizing the complex 3-D geometry of an organ-like the human stomach reconstructed by clinical imaging modalities. Providing an average tension for the stomach does not reflect the large variation in tension throughout the stomach wall.

Rectal function and bowel habit in irritable bowel syndrome

OBJECTIVES

Rectal compliance may influence rectal perception, but their functional implications remain incompletely understood. Our aim was to determine whether rectal function is related to bowel habit in the irritable bowel syndrome.

METHODS

The responses to fixed tension rectal distension applied by means of a tensostat were compared among constipation-predominant (n = 9), diarrhea-predominant (n = 7), alternating habit (n = 11) irritable syndrome subgroups, and healthy controls (n = 15).

RESULTS

Overall, patients had normal rectal compliance and increased perception, but compliance was lower in diarrhea-predominant as compared to constipation-predominant patients (6.7 +/- 0.7 ml/mmHg vs. 9.9 +/- 0.7 ml/mmHg, respectively; p < 0.05) and perception was higher (39 +/- 6 g vs. 64 +/- 9 g tolerance, respectively; p < 0.05).

CONCLUSION

Distinctive tensosensitivity and compliance characterize rectal function in irritable bowel syndrome subgroups with different bowel habit.

Reflex control of intestinal gas dynamics and tolerance in humans

Intestinal transit of gas is normally adapted to the luminal gas load, but in some patients impaired transit may lead to gas retention and symptoms. We hypothesized that intestinal gas transit is regulated by reflex mechanisms released by segmental distension at various gut levels. In 24 healthy subjects, we measured gas evacuation and perception of jejunal gas infusion (12 ml/min) during simultaneous infusion of duodenal lipids mimicking the postprandial caloric load (Intralipid, 1 kcal/min). We evaluated the effects of proximal (duodenal) distension (n = 8), distal (rectal) distension (n = 8), and sham distension, as control (n = 8). Duodenal lipid infusion produced gas retention (366 +/- 106 ml) with low abdominal perception (1.5 +/- 0.8 score). Distension of either the duodenum or rectum during lipid infusion expedited gas transit and prevented retention (-120 +/- 164 and -124 +/- 162 ml retention, respectively; P < 0.05 vs. control). However, the tolerance to the intestinal gas load differed markedly, depending on the site of distension; perception remained low during rectal distension (2.6 +/- 0.7 score; not significant vs. control) but increased during duodenal distension (4.4 +/- 0.7 score; P < 0.05 vs. control). We conclude that focal gut distension, either at proximal or distal sites, accelerates gas transit, but the symptomatic response depends on the site of stimulation.

Review article: bloating in functional bowel disorders

Bloating is a frequently reported symptom in functional bowel disorders. It usually occurs in combination with other symptoms, but may also occur in isolation. The severity of bloating tends to worsen during the course of the day and improves overnight. Although frequently considered to be a subjective phenomenon, recent studies have shown that bloating is associated with a measurable increase in abdominal girth. The pathophysiology of bloating remains elusive, but the evidence supports a sensorimotor dysfunction of the bowel. The possible mechanisms include abnormal gas trapping, fluid retention, food intolerance and altered gut microbial flora. Further studies are needed to define the sensorimotor abnormalities associated with bloating, which might be segmental and transient rather than generalized and persistent. The lack of understanding of this symptom is paralleled by a limited availability of therapeutic options. Conventional medications used in functional bowel disorders are not helpful and may indeed worsen the symptoms. In future, new drugs with activity against serotonin and kappa receptors, or novel approaches such as the use of exclusion diets, probiotics and hypnotherapy, may prove to be useful.

Exaggerated motility of the descending colon with repetitive distention of the sigmoid colon in patients with irritable bowel syndrome

BACKGROUND

Visceral hypersensitivity is one of the mechanisms of irritable bowel syndrome (IBS), but it does not explain the entire symptomatology, i.e., altered bowel habit with abdominal pain relieved by defecation. We tested our hypothesis that an abnormal link between luminal stimulation and mural response may have some role in the pathophysiology of IBS.

METHODS

Patients with IBS (n = 10, median 21 years old, 5 male patients, 5 female patients) and healthy control subjects (n = 10, median 21 years old, 5 men, 5 women) were studied. A manometric catheter with three transducers was inserted to the descending colon and a balloon was placed in the distal sigmoid colon. Another catheter with three transducers was inserted to the duodenum. After baseline for 30min, the sigmoid colon was stimulated by balloon distention for 30min followed by recovery for 30min. Balloon distention was repeated 100 times, and each stimulation consisted of a 5-s inflation and a 10-s deflation, with a volume of 50ml maximum. The sensory threshold of balloon inflation was then examined, and plasma adrenocorticotropic hormone (ACTH) was measured with radioimmunoassay.

RESULTS

Repetitive colonic distention induced a significant increase in motility indices (mmHg s/s%) of the descending colon in the IBS patients (from 118 +/- 25 to 333 +/- 108, P < 0.05) but not of those in controls (from 90 +/- 16 to 89 +/- 19). A significant group difference (P < 0.05), period effect (P < 0.02), and group x period interactions (P < 0.01) were detected with two-way ANOVA. Duodenal motility indices in controls were significantly reduced by colonic distention (from 169 +/- 25 to 104 +/- 14, P < 0.01), but those in the IBS patients were not (from 156 +/- 17 to 124 +/- 20). The sensory threshold of balloon inflation in the IBS patients (74 +/- 10ml) was significantly lower than that in controls (125 +/- 6ml, P < 0.001). There was no significant difference in plasma ACTH levels between the IBS patients and controls.

CONCLUSIONS

Repetitive distention of the distal sigmoid colon below the sensory threshold induced orad exaggerated motility of the colon in IBS patients. The distention inhibited motility of the small intestine in healthy subjects, but this inhibition was blunted in IBS patients. These results suggest that IBS patients may have not only visceral hypersensitivity, but also an abnormal intestinal reflex.

Mechanisms of intestinal gas retention in humans: impaired propulsion versus obstructed evacuation

To explore the clinical role of intestinal gas dynamics, we investigated two potential mechanisms of gas retention, defective propulsion and obstructed evacuation. In healthy subjects, a gas mixture was continuously infused into the jejunum (4 ml/min) 1) during a 2-h control period of spontaneous gas evacuation and 2) during a 2-h test period either with impaired gut propulsion caused by intravenous glucagon (n = 6) or with obstructed (self-restrained) anal evacuation (n = 10) while anal gas evacuation, symptom perception (0-6 scale), and abdominal girth were measured. Impaired gut propulsion and obstructed evacuation produced similar gas retention (558 +/- 68 ml and 407 +/- 85 ml, respectively, vs. 96 +/- 58 ml control; P < 0.05 for both) and abdominal distension (8 +/- 3 mm and 6 +/- 3 mm, respectively, vs. 1 +/- 1 mm control; P < 0.05 for both). However, obstructed evacuation increased symptom perception (2.3 +/- 0.6 score change; P < 0.05), whereas gas retention in the glucagon-induced hypotonic gut was virtually unperceived (-0.4 +/- 0.7 score change; not significant). In conclusion, the perception of intestinal gas accumulation depends on the mechanism of retention.

Esophageal tone in patients with total aperistalsis: gastroesophageal reflux disease versus achalasia

We have evaluated esophageal tone in two different conditions that, in some cases, similarly impair phasic esophageal motility. Studies were performed in 14 healthy volunteers, 10 patients with total esophageal aperistalsis secondary to gastroesophageal reflux disease (GERD), and 25 untreated achalasia patients. We quantified esophageal compliance and relaxation induced by a nitric oxide donor using a barostat. Intraesophageal volume at a minimal distending pressure (2 mmHg) was not significantly different among all three groups (4.1 +/- 0.7, 3.8 +/- 0.7, and 4.2 +/- 1.2 ml for healthy, GERD, and achalasia groups, respectively). Esophageal compliance was significantly increased (P < 0.05 vs. healthy group) in the two groups of patients with aperistalsis (1.9 +/- 0.2, 3.0 +/- 0.2, and 3.1 +/- 0.3 ml/mmHg for healthy, GERD, and achalasia groups, respectively). Esophageal relaxation was decreased in GERD patients (Delta diameter: 0.4 +/- 0.1 cm) and increased in achalasia patients (Delta diameter: 1.3 +/- 0.4 cm) relative to healthy subjects (Delta diameter: 0.9 +/- 0.2 cm) (P < 0.05 for GERD vs. achalasia and healthy groups). Our results indicate that diseases that similarly impair phasic esophageal motility may affect esophageal tone differently.

Gastric wall tension determines perception of gastric distention

BACKGROUND & AIMS

The primary mechanism that originates symptoms in response to gastric distention remains undefined. The aim of this study was to determine which factor, whether intragastric volume, pressure, or wall tension, determines perception of gastric distention.

METHODS

Healthy subjects underwent increasing gastric distentions (2-minute duration at 5-minute intervals) either at fixed pressure levels using a conventional barostat (n = 10) or at fixed tension levels using a newly developed computerized tensostat (n = 12); perception was scored by a 0-6 scale. Distentions were performed during basal conditions (intravenous saline) and during gastric relaxation by glucagon administration (4.8 microgram/kg intravenous bolus plus 9.6 microgram. kg-1. h-1 infusion).

RESULTS

Isobaric distentions with the conventional barostat produced more intense perception during glucagon (95% +/- 40% higher; P < 0.05). However, the factor that determined higher perception could not be ascertained, because at the same pressure levels both intragastric volume and wall tension were greater during glucagon administration (174% +/- 56% and 34% +/- 8% greater, respectively; P < 0.05 vs. saline for both). The tensostat evidenced that perception was selectively related to tension, not to elongation; during glucagon administration, intragastric volumes were significantly larger (80% +/- 28% larger increase; P < 0.05), but perception of isotonic distentions remained the same (27% +/- 22%; nonsignificant change).

CONCLUSIONS

Gastric wall tension, but not intragastric volume, determines perception of gastric distention, at least below nociception.

Functional gut disorders: from motility to sensitivity disorders. A review of current and investigational drugs for their management

Functional gut disorders include several clinical entities defined on the basis of symptom patterns (e.g., functional dyspepsia, irritable bowel syndrome, functional abdominal pain, functional abdominal bloating), for which there is no established pathophysiological mechanism. Because there is no well-defined pathophysiological target, treatment should be aimed at symptom improvement. Prokinetics and antispasmodics have been widely used in the treatment of functional gut disorders on the assumption that disordered motility is the underlying cause of symptoms, and symptom improvement is indeed achievable with these compounds in some, but not all, patients with features of hypo- or hypermotility, respectively. In the first part of this review, we cover the basic pharmacology and discuss the rationale for the clinical use of prokinetics and antispasmodics. On the other hand, in the past few years, the explosive growth in the research focusing on visceral sensitivity and visceral reflexes has suggested that at least some patients with functional gut disorders have altered visceral perception. Thus, the second part of the review covers these developments and focuses on studies addressing the issue of drugs modulating visceral sensitivity.

Tonic and phasic motor activity in the proximal and distal colon of healthy humans

In healthy humans, meals stimulate phasic and tonic motor activity in the unprepared distal colon. The response of the proximal colon remains unknown. In this study, we assessed the effect of a liquid meal on proximal and distal colonic motor activity. In 12 healthy volunteers, colonic tone and phasic motility were simultaneously recorded by using an electronic barostat and perfused catheters in the fasting state and in response to a 1,000-kcal meal. The meal significantly increased the phasic activity in the distal colon (230 +/- 46% of the basal value; P = 0.02) but not in the proximal colon (138 +/- 25% of the basal value; P = 0.2). The intrabag volume of the barostat was significantly more reduced in the distal than in the proximal colon (74 +/- 11 vs. 50 +/- 9% of the basal values, respectively; P = 0.04). We conclude that the postprandial response of the unprepared proximal colon is an immediate tonic contraction that is less pronounced than in the distal colon.

Modulation of gut perception in humans by spatial summation phenomena

1. We have recently shown that perception of intestinal stimuli increases by spatial summation phenomena. Our aim was to determine in humans whether intestinal perception depends on (a) the length of gut stimulated, and (b) the distance between stimuli. 2. In a first series of studies, we compared perception of isobaric intestinal distensions applied over a 3 cm segment and a 36 cm segment by means of two separate barostats (n = 8). In a second series of studies we compared perception of intestinal distensions applied simultaneously by two balloons sited 3, 12 or 48 cm apart (n = 6). 3. Distension of the 36 cm segment induced significantly greater perception than distension of the 3 cm intestinal segment (discomfort perceived at 20 +/- 2 mmHg and 31 +/- 2 mmHg, respectively; P < 0.05). Perception of intestinal balloon distension increased when a second stimulus was simultaneously applied, independently of the distance between the two balloons (the discomfort thresholds were 30 +/- 11, 20 +/- 6 and 28 +/- 7% lower with simultaneous distensions 3, 12 and 48 cm apart, respectively). 4. We conclude that perception of intestinal distension is determined by the extension of the field of stimulation, and the summation effect is similar whether adjacent or distant fields are stimulated.

Perception and gut reflexes induced by stimulation of gastrointestinal thermoreceptors in humans

1. Experimental studies in animals suggest the existence of thermoreceptors in the gastrointestinal tract. Our aim was to investigate the distribution and specificity of upper gut thermoreceptors in humans. 2. In healthy subjects, thermal stimulation of the stomach (n = 8) and the small intestine (n = 6) was produced by means of a thermostat, which recirculates water at adjusted temperatures through an ultrathin intraluminal bag. Progressively warm (42, 47 and 52 degrees C) and cold (32, 22 and 12 degrees C) stimuli of 3 min duration were alternately applied at 13 min intervals. Perception was scored on a scale of 0-6 and gastric tone responses were measured with a barostat. 3. Thermal stimuli induced specific responses: cold stimuli induced abdominal cold sensation and a reflex contraction of the stomach, whereas warm stimuli induced warm sensation and a reflex gastric relaxation. 4. Thermal stimuli induced similar stimulus-related perception in the stomach and small intestine (temperatures between 12 and 49.5 +/- 0.5 degrees C were tolerated). 5. The reflex responses were site specific. Warm and cold stimulation of the stomach induced gastric reflexes (76 +/- 26 ml isobaric expansion at 47 degrees C, and 68 +/- 10 ml contraction at 12 degrees C; P < 0.05 for both). However, only warm, not cold, stimulation of the intestine induced enterogastric reflexes. 6. These results indicate that in humans, warm and cold receptors are distributed along the gastrointestinal tract and project afferent input both into perception and reflex circuits with specific topographic organization.

Biomechanics of the gastrointestinal tract

As the function of the gastrointestinal tract is to a large degree mechanical, it has become increasingly popular to acquire distensibility data in motility research based on various parameters. Hence it is important to know on which geometrical and mechanical assumptions the various parameters are based. Currently, compliance and tone derived from pressure-volume curves are by far the most often used parameters. However, pressure-volume relations obtained in tubular organs must be carefully interpreted as they provide no direct measure of luminal cross-sectional area and other variables useful in plane stress and strain analysis. Thus, erroneous conclusions concerning tissue distensibility may be deduced. Other parameters, such as wall tension, stress and strain, give more useful information about mechanical behaviour. Distensibility data procure significance in fluid mechanics and in the study of tone, peristaltic reflexes, and mechanoreceptor kinematics. Such data are needed for the determination of the interaction between stimulus, electrical responses in neurons and the mechanical behaviour of the gut. Furthermore, from a clinical perspective, investigation of visco-elastic properties is important because GI diseases are associated with growth and remodelling. For example, prestenotic dilatation, increased collagen synthesis, dysmotility and altered distensibility are common features of obstructive diseases. The purpose of this review is to discuss the physiological and clinical importance of acquiring biomechanical data, distensibility parameters and interpretation of these results and their associated errors. We will also discuss some aspects of the relationship between morphology, growth and biomechanics. Finally, we will outline a number of techniques to study the mechanical properties of the GI tract.

Perception and reflex responses to intestinal distention in humans are modified by simultaneous or previous stimulation

BACKGROUND & AIMS

Intestinal reflexes induced by distention in dogs are facilitated by either simultaneous or previous distentions. The aim of this study was to determine whether these phenomena also modulate the responses to intestinal distention, particularly perception, in humans.

METHODS

Perception and intestinal relaxation were measured in 11 healthy subjects in response to increasing jejunal balloon distentions tested (by stimulus-response trials) alone, as control, and with conditioning distentions applied either simultaneously, immediately (10 seconds) before at the same site, or immediately before and 5 cm distant. In 8 additional subjects, the effect of prolonged (90-minute) conditioning distention was tested.

RESULTS

Conditioning had more pronounced effects on perception than on intestinal reflexes. Perception of intestinal distention increased (by 84 +/- 47%; P < 0.05) when a simultaneous distention was applied nearby. By contrast, perception decreased (by 38 +/- 12%; P < 0.05) when a previous distention was applied at the same but not at an adjacent site. Prolonged intestinal distention elicited remarkably stable perception during a 90-minute period. The effects of conditioning were unrelated to intestinal compliance because it remained unchanged.

CONCLUSIONS

In humans, temporospatial interactions of gut stimuli activate modulatory phenomena that determine the perception intensity of the stimuli.

Postural changes in proximal gastric volume and pressure measured using a gastric barostat

A barostat was used to examine the effect of changes in posture on the volume and pressure in a bag positioned in the proximal stomach of 14 normal volunteers. Volumes in the supine position were compared with those in the standing, left lateral and right lateral positions at a constant pressure 2 mmHg above basal intragastric pressure. A separate series of measurements was then used to evaluate the effects of the same postural changes on pressure within the bag whilst its volume was kept constant. Changing from the supine to the left lateral position decreased bag volume by 62% when pressure was controlled; pressure increased by 60% when volume was controlled. In contrast, movement from the supine to the right lateral position resulted in a 68% increase in bag volume and a 31% fall in pressure. Moving from supine to standing had inconsistent effects on bag volume and pressure. There was a negative correlation between the magnitudes of the changes in pressure and volume (r2 = 0.557). The observed effects of posture probably result from changes in the compression of the stomach by abdominal viscera and indicate that subject position must be specified and maintained constant in studies of proximal gastric motor function using a barostat.

Selective dysfunction of mechanosensitive intestinal afferents in irritable bowel syndrome

BACKGROUND/AIMS

Experimental studies have shown gut hypersensitivity in irritable bowel syndrome. The aim of this study was to determine whether heightened perception of gut distention in irritable bowel syndrome is related to either decreased gut compliance, altered mechanosensitive afferents, or nonspecific sensory dysfunction.

METHODS

In 17 patients with irritable bowel syndrome and 15 healthy controls, stimulus-related perception of (1) intestinal balloon distentions, (2) transmucosal electrical nerve stimulation (15 Hz, 100 microseconds), and (3) somatic transcutaneous electrical nerve stimulation (100 Hz, 100 microseconds) was measured. Individual stimuli of 1-minute duration were randomly applied at 5-minute intervals.

RESULTS

Patients tolerated smaller intestinal volumes than controls (33 +/- 3 mL vs. 43 +/- 4 mL, respectively; mean +/- SE; P < 0.05), whereas both intestinal compliance and perception of transmucosal electrical nerve stimulation were normal (patients tolerated 58 +/- 5 mA and healthy subjects tolerated 69 +/- 5 mA). Interestingly, patients perceived both stimuli more diffusely than controls; 48% +/- 9% distentions and 52% +/- 9% electrical stimuli were perceived over more than one abdominal region vs. 21% +/- 9% and 18% +/- 6%, respectively, in controls (P < 0.05 for both). In contrast to gut distentions, patients showed higher tolerance of somatic stimuli than controls (68 +/- 7 mA vs. 42 +/- 6 mA, respectively; P < 0.05).

CONCLUSIONS

Patients with irritable bowel syndrome show selective hypersensitivity of intestinal mechanosensitive pathways associated with a nonspecific, probably central dysfunction of viscerosomatic referral.

Gastric tone determines the sensitivity of the stomach to distention

BACKGROUND/AIMS

Whether meal-related symptoms such as postcibal epigastric fullness and discomfort are caused by hypotonic gastric expansion or gastric hypertension is unknown. This study investigated whether symptoms in healthy individuals in response to gastric distention are produced by gastric expansion or by an increase in intragastric pressure.

METHODS

Increasing gastric distentions (for 5 minutes at 5-minute intervals) at fixed pressure levels (in 2-mm Hg increments) and at fixed volume levels (in 200-mL increments) were performed in 10 healthy subjects per group; perception was measured on a 0-6 scale. Distentions were performed during intravenous infusion of saline (basal) and during gastric relaxation by intravenous administration of glucagon (4.8-micrograms/kg bolus plus 9.6 micrograms.kg-1.h-1 infusion).

RESULTS

The same distending pressure tested produced 30% +/- 9% larger intragastric volumes and 80% +/- 44% higher perception scores when the stomach was relaxed by glucagon (P < 0.05 vs. basal for both). In contrast, the same distending volumes tested produced 25% +/- 7% lower intragastric pressures and 21% +/- 12% lower perception scores when the stomach was relaxed (P < 0.05 vs. basal for both).

CONCLUSIONS

Epigastric symptoms in response to gastric distention are influenced by both the intragastric pressure and the intragastric volume.

Somatic stimulation reduces perception of gut distention in humans

BACKGROUND/AIMS

Perception of gut symptoms may depend on visceral sensory modulation, but the mechanisms are poorly understood. Based on somatosensory data, we hypothesized that somatic stimulation modulates perception of gut stimuli.

METHODS

Perception and gut reflexes were measured in 8 healthy subjects in response to increasing gastric and duodenal distentions (stimulus-response trials) performed alone or with simultaneous application on the hand of either low (just perceivable) or high (nonpainful) transcutaneous electrical nerve stimulation (100 Hz, 100 microseconds). Fixed pressure distentions were performed in stepwise increments by gastric and duodenal barostats to determine the respective thresholds for discomfort. Perception was measured by a questionnaire, and gut reflex responses were measured as isobaric volume changes by the barostats. Individual stimuli of 2 minutes' duration were randomly applied at 10-minute intervals.

RESULTS

Somatic stimuli increased the tolerance to gut distention, and this effect was more pronounced with high stimuli. The gastric threshold for discomfort was 7.3 +/- 0.5 mm Hg with high stimuli vs. 5.5 +/- 0.2 mm Hg without stimuli, and the duodenal discomfort threshold was 15.5 +/- 1.5 mm Hg with high stimuli vs. 13.5 +/- 1.4 mm Hg without stimuli; P < 0.05 for both. Somatic stimuli modified neither basal gut tone nor the relaxatory duodenogastric reflex induced by duodenal distention (gastric expansion by 203 +/- 48 mL with high stimuli and 208 +/- 73 mL without stimuli).

CONCLUSIONS

Somatic stimulation reduces perception of gut distention without interfering with local and reflex gut responses.

Selective gastric hypersensitivity and reflex hyporeactivity in functional dyspepsia

BACKGROUND/AIMS

We have previously shown that patients with functional dyspepsia are hypersensitive to gastric distention. The aim of this study was to establish whether this sensory disturbance was confined to the stomach and whether it was associated with gut reflex dysfunction.

METHODS

In 10 selected patients with dyspepsia and 12 healthy controls, perception and gut reflex responses to gastric distention, duodenal distention, and somatic stimulation were measured. Standardized distentions at fixed pressures were performed by gastric and duodenal barostats. Perception was scored by a detailed symptom questionnaire; gut reflex responses were measured as isobaric volume changes by each barostat. Somatic transcutaneous electrical nerve stimulation was produced on the hand. Individual stimuli (2-minute duration) were randomly applied at 10-minute intervals in stepwise increments in search of the respective threshold for discomfort.

RESULTS

Patients with dyspepsia had gastric hypersensitivity to distention (discomfort threshold at 6.4 +/- 0.4 mm Hg vs. 8.3 +/- 0.6 mm Hg in controls; mean +/- SE; P < 0.05), whereas duodenal and somatic sensitivity was normal. Furthermore, patients with dyspepsia explicitly recognized their clinical symptoms in all gastric but only in 58% +/- 12% of the duodenal distention trials. In addition, patients with dyspepsia showed defective gastric relaxatory responses to duodenal distention (68 +/- 30 mL gastric expansion vs. 239 +/- 12 mL in controls; P < 0.05).

CONCLUSIONS

Patients with dyspepsia are selectively hypersensitive to gastric distention; this sensory dysfunction is associated with impaired reflex reactivity of the stomach.

Response of gastric fundus to rectal distension in healthy persons

We aimed to record fundic motor activity in man using the barostat to ascertain if fundic motility is affected by rectal distension. The distal ends of two barostat tubes were placed in the gastric fundus and rectum in 10 healthy volunteers. The gastric bag was first inflated to a constant pressure level that recorded phasic motor activity as changes in volume of the air-filled bag. Baseline motor activity was recorded before, during, and after a 15-min period of constant rectal distension that was clearly perceived by all subjects but was not painful. In all subjects, continuous phasic volume changes, reflecting fundic motor activity, were recorded at a rate of 1-3/min. During rectal distension, a consistent change in mean contractile force of these phasic volume events was not detected; a decrease of more than 30% occurred in only three subjects. We conclude that fundic phasic volume changes are recordable by the barostat, but these are not substantially inhibited by rectal distension.

Effects of duodenal distention on fasting and postprandial antropyloroduodenal motility in humans

BACKGROUND/AIMS

Mechanoreceptors in the proximal small intestine may play an important role in the regulation of gastric emptying. Balloon distention of the duodenum causes fundic relaxation. The purpose of the present study was to determine the effect of stimulation of duodenal mechanoreceptors on both fasting and postprandial antropyloroduodenal motility in humans.

METHODS

Antropyloroduodenal pressures were recorded in 12 healthy volunteers with a sleeve-sidehole assembly, incorporating two balloons 5 and 20 cm distal to the pylorus. Duplicate proximal and distal duodenal balloon distensions with 10, 20, and 30 mL of air for 2.5 minutes were performed separately and in randomized order both during fasting and after a meal.

RESULTS

During fasting, proximal and distal distention at all volumes increased the number of isolated pyloric pressure waves (P < 0.05) and basal pyloric pressure (P < 0.05), and the response to proximal distention was greater (P < 0.05). Postprandially, proximal and distal distention increased basal pyloric pressure (P < 0.05) with a greater response to proximal distention (P < 0.05), but had no effect on isolated pyloric pressure waves. Both during fasting and postprandially, there were more synchronous and less antegrade antral waves during distention (P < 0.05). The number of duodenal pressure waves increased during proximal (P < 0.05) but not distal distention.

CONCLUSIONS

Stimulation of duodenal mechanoreceptors by balloon distention has significant and region-dependent effects on antropyloroduodenal motility that vary between fasting and postprandial states.