Interdependency between mechanical parameters and afferent nerve discharge in hypertrophic intestine of rats

Physiological mechanisms of unexplained (functional) gastrointestinal disorders

Functional gastrointestinal disorders (FGIDs) encompass a range of complex conditions with similar clinical characteristics and no overt pathology. Recent recognition of sub-clinical pathologies in FGIDs, in conjunction with physiological and biochemical abnormalities including increased intestinal permeability, microbial profile alterations, differences in metabolites and extra-intestinal manifestations of disease, call into question the designation of these conditions as 'functional'. This is despite significant heterogeneity in both symptom profile and specifics of reported physiological abnormalities hampering efforts to determine defined mechanisms that drive onset and chronicity of symptoms. Instead, the literature demonstrates these conditions are disorders of homeostatic imbalance, with disruptions in both host and microbial function and metabolism. This imbalance is also associated with extraintestinal abnormalities including psychological comorbidities and fatigue that may be a consequence of gastrointestinal disruption. Given the exploitation of such abnormalities will be crucial for improved therapeutic selection, an enhanced understanding of the relationship between alterations in function of the gastrointestinal tract and the response of the immune system is of interest in identifying mechanisms that drive FGID onset and chronicity. Considerations for future research should include the role of sex hormones in regulating physiological functions and treatment responses in patients, as well as the importance of high-level phenotyping of clinical, immune, microbial and physiological parameters in study cohorts. There is opportunity to examine the functional contribution of the microbiota and associated metabolites as a source of mechanistic insight and targets for therapeutic modulation.

Simulations of Myenteric Neuron Dynamics in Response to Mechanical Stretch

Background

Intestinal sensitivity to mechanical stimuli has been studied intensively in visceral pain studies. The ability to sense different stimuli in the gut and translate these to physiological outcomes relies on the mechanosensory and transductive capacity of intrinsic intestinal nerves. However, the nature of the mechanosensitive channels and principal mechanical stimulus for mechanosensitive receptors are unknown. To be able to characterize intestinal mechanoelectrical transduction, that is, the molecular basis of mechanosensation, comprehensive mathematical models to predict responses of the sensory neurons to controlled mechanical stimuli are needed. This study aims to develop a biophysically based mathematical model of the myenteric neuron with the parameters constrained by learning from existing experimental data. Findings. The conductance-based single-compartment model was selected. The parameters in the model were optimized by using a combination of hand tuning and automated estimation. Using the optimized parameters, the model successfully predicted the electrophysiological features of the myenteric neurons with and without mechanical stimulation.

Conclusions

The model provides a method to predict features and levels of detail of the underlying physiological system in generating myenteric neuron responses. The model could be used as building blocks in future large-scale network simulations of intrinsic primary afferent neurons and their network.

Pressure overload changes mesenteric afferent nerve responses in a stress-dependent way in a fasting rat model

It is well known that overload changes the mechanical properties of biological tissues and fasting changes the responsiveness of intestinal afferents. This study aimed to characterize the effect of overload on mechanosensitivity in mesenteric afferent nerves in normal and fasted Sprague-Dawley rats. Food was restricted for 7 days in the Fasting group. Jejunal whole afferent nerve firing was recorded during three distensions, i.e., ramp distension to 80 cmH₂O luminal pressure (D1), sustained distension to 120 cmH₂O for 2 min (D2), and again to 80 cmH₂O (D3). Multiunit afferent recordings were separated into low-threshold (LT) and wide-dynamic-range (WDR) single-unit activity for D1 and D3. Intestinal deformation (strain), distension load (stress), and firing frequency of mesenteric afferent nerve bundles [spike rate increase ratio (SRIR)] were compared at 20 cmH₂O and 40 cmH₂O and maximum pressure levels among distensions and groups. SRIR and stress changes showed the same pattern in all distensions. The SRIR and stress were larger in the Fasting group compared to the Control group (P < 0.01). SRIR was lower in D3 compared to D1 in controls (P < 0.05) and fasting rats (P < 0.01). Total single units and LT were significantly lower in Fasting group than in Controls at D3. LT was significantly higher in D3 than in D1 in Controls. Furthermore, correlation was found between SRIR with stress (R = 0.653, P < 0.001). In conclusion, overload decreased afferent mechanosensitivity in a stress-dependent way and was most pronounced in fasting rats. Fasting shifts LT to WDR and high pressure shifts WDR to LT in response to mechanical stimulation.

Colonic Dysmotility in Murine Partial Colonic Obstruction Due to Functional Changes in Interstitial Cells

Background/Aims

Interstitial cells play important roles in gastrointestinal (GI) neuro-smooth muscle transmission. The underlying mechanisms of colonic dysmotility have not been well illustrated. We established a partial colon obstruction (PCO) mouse model to investigate the changes of interstitial cells and the correlation with colonic motility.

Methods

Western blot technique was employed to observe the protein expressions of Kit, platelet-derived growth factor receptor-α (Pdgfra), Ca²⁺-activated Cl⁻ (Ano1) channels, and small conductance Ca²⁺- activated K⁺ (SK) channels. Colonic migrating motor complexes (CMMCs) and isometric force measurements were employed in control mice and PCO mice.

Results

PCO mice showed distended abdomen and feces excretion was significantly reduced. Anatomically, the colon above the obstructive silicone ring was obviously dilated. Kit and Ano1 proteins in the colonic smooth muscle layer of the PCO colons were significantly decreased, while the expression of Pdgfra and SK3 proteins were significantly increased. The effects of a nitric oxide synthase inhibitor (L-NAME) and an Ano1 channel inhibitor (NPPB) on CMMC and colonic spontaneous contractions were decreased in the proximal and distal colons of PCO mice. The SK agonist, CyPPA and antagonist, apamin in PCO mice showed more effect to the CMMCs and colonic smooth muscle contractions.

Conclusions

Colonic transit disorder may be due to the downregulation of the Kit and Ano1 channels and the upregulation of SK3 channels in platelet-derived growth factor receptor-α positive (PDGFRα⁺) cells. The imbalance between interstitial cells of Cajal-Ano1 and PDGFRα-SK3 distribution might be a potential reason for the colonic dysmotility.

Stress-strain analysis of duodenal contractility in response to flow and ramp distension in rabbits fed low-fiber diet

BACKGROUND

Previously we demonstrated that low-fiber diet in rabbits affects the passive mechanomorphological properties in the small intestine, resulting in reduced intestinal wall thickness and collagen content, as well as intestinal wall softening. The aim of the present study was to evaluate the contractility in rabbits on long-term low-fiber diet and specifically to compare the contraction threshold, the frequency, and the amplitude of flow-induced and distension-induced contractions in the duodenum between rabbits on normal diet and on long-term low-fiber diet.

METHODS

Ten rabbits were fed a low-fiber diet for 5 months (Intervention group), and five rabbits were fed normal diet (Control group). The duodenal segments were used for determination of mechanical parameters for analyses of contractility. The duodenal experiments were carried out in organ baths containing physiological Krebs solution. Pressure and diameter changes induced by contractions in response to flow and ramp distension were measured. The frequencies and amplitude of contractions were analyzed. Distension-induced contraction thresholds and maximum contraction amplitude of flow-induced contractions were calculated in terms of mechanical stress and strain. Multiple linear regression analyses were applied to study dependencies between contractility parameters and wall thickness, wall area, and muscle layer thickness.

KEY RESULTS

During distension, the pressure, stress, and strain thresholds for induction of phasic contraction were biggest in the Intervention Group (P < 0.05). In addition, the contraction frequencies during flow-induced contraction were highest in the Intervention Group (P < 0.05), whereas the maximum contraction amplitudes in terms of pressure, diameter, stress, and strain were lowest in the Intervention Group (P < 0.05). The contraction thresholds and contraction frequencies were negatively associated with the wall thickness, wall area, and muscle layer thickness, whereas maximum contraction amplitudes were positively associated with the wall thickness, wall area, and muscle layer thickness.

CONCLUSIONS AND INFERENCES

Duodenal contractility in rabbits fed with long-term low-fiber diet exhibited low contraction amplitudes and high contraction thresholds and frequencies. The changes were associated with the low-fiber diet-induced histomorphological remodeling. Studies on detailed structural and functional diet-induced changes in smooth muscle and intestinal nerves are needed for better understanding the remodeling mechanisms.

A Mouse Model of Intestinal Partial Obstruction

Intestinal obstructions, that impede or block peristaltic movement, can be caused by abdominal adhesions and most gastrointestinal (GI) diseases including tumorous growths. However, the cellular remodeling mechanisms involved in, and caused by, intestinal obstructions are poorly understood. Several animal models of intestinal obstructions have been developed, but the mouse model is the most cost/time effective. The mouse model uses the surgical implantation of an intestinal partial obstruction (PO) that has a high mortality rate if it is not performed correctly. In addition, mice receiving PO surgery fail to develop hypertrophy if an appropriate blockade is not used or not properly placed. Here, we describe a detailed protocol for PO surgery which produces reliable and reproducible intestinal obstructions with a very low mortality rate. This protocol utilizes a surgically placed silicone ring that surrounds the ileum which partially blocks digestive movement in the small intestine. The partial blockage makes the intestine become dilated due to the halt of digestive movement. The dilation of the intestine induces smooth muscle hypertrophy on the oral side of the ring that progressively develops over 2 weeks until it causes death. The surgical PO mouse model offers an in vivo model of hypertrophic intestinal tissue useful for studying pathological changes of intestinal cells including smooth muscle cells (SMC), interstitial cells of Cajal (ICC), PDGFRα⁺, and neuronal cells during the development of intestinal obstruction.

Gastrointestinal disorders in joint hypermobility syndrome/Ehlers-Danlos syndrome hypermobility type: A review for the gastroenterologist

BACKGROUND

Joint hypermobility syndrome (JHS)/Ehlers-Danlos syndrome hypermobility type (EDS-HT) is the most common hereditary non-inflammatory disorder of connective tissue, characterized by a wide range of symptoms, mainly joint hyperextensibility and musculoskeletal symptoms. A majority of patients also experiences gastrointestinal (GI) symptoms. Furthermore, JHS/EDS-HT has specifically been shown to be highly prevalent in patients with functional GI disorders, such as functional dyspepsia and irritable bowel syndrome.

PURPOSE

The aim of this review was to examine the nature of GI symptoms and their underlying pathophysiology in JHS/EDS-HT. In addition, we consider the clinical implications of the diagnosis and treatment of JHS/EDS-HT for practicing clinicians in gastroenterology. Observations summarized in this review may furthermore represent the first step toward the identification of a new pathophysiological basis for a substantial subgroup of patients with functional GI disorders.