Ion transport and enteric nervous system (ENS) in rat rectal colon: mechanical stretch causes electrogenic Cl-secretion via plexus Meissner and amiloride-sensitive electrogenic Na-absorption is not affected by intramural neurons

Impaired Intestinal Permeability of Tricellular Tight Junctions in Patients with Irritable Bowel Syndrome with Mixed Bowel Habits (IBS-M)

BACKGROUND

The underlying pathophysiology of irritable bowel syndrome (IBS) is still unclear. Our aim was to investigate the pathophysiological mechanisms of diarrhea, constipation, and antigen uptake in mixed-type IBS (IBS-M).

METHODS

Colonoscopic biopsies were obtained from IBS-M patients. Epithelial transport and barrier function of colonic mucosae were characterized in Ussing chambers using impedance spectroscopy. Mucosal permeability to macromolecules was measured. Western blotting for tight junction (TJ) proteins was performed and their subcellular localization was visualized by confocal microscopy. RNA-sequencing was performed for gene expression and signaling pathway analysis.

RESULTS

In IBS-M, epithelial resistance and ENaC-dependent sodium absorption were unchanged, while short-circuit current reflecting chloride secretion was reduced. Concomitantly, epithelial permeability for fluorescein and FITC-dextran-4000 increased. TJ protein expression of occludin decreased, whereas claudins were unaltered. Confocal microscopy revealed the de-localization of tricellulin from tricellular TJs. Involved pathways were detected as proinflammatory cytokine pathways, LPS, PGE2, NGF, and vitamin D.

CONCLUSIONS

Decreased anion secretion explains constipation in IBS-M, while ion permeability and sodium absorption were unaltered. Reduced occludin expression resulted in the delocalization of tricellulin from the tricellular TJ, leading to increased macromolecular permeability that contributes to antigen influx into the mucosa and perpetuates a low-grade inflammatory process.

Distension evoked mucosal secretion in human and porcine colon in vitro

It was suggested that intestinal mucosal secretion is enhanced during muscle relaxation and contraction. Mechanisms of mechanically induced secretion have been studied in rodent species. We used voltage clamp Ussing technique to investigate, in human and porcine colonic tissue, secretion evoked by serosal (Pser) or mucosal (Pmuc) pressure application (2-60 mmHg) to induce distension into the mucosal or serosal compartment, respectively. In both species, Pser or Pmuc caused secretion due to Cl- and, in human colon, also HCO3- fluxes. In the human colon, responses were larger in proximal than distal regions. In porcine colon, Pmuc evoked larger responses than Pser whereas the opposite was the case in human colon. In both species, piroxicam revealed a strong prostaglandin (PG) dependent component. Pser and Pmuc induced secretion was tetrodotoxin (TTX) sensitive in porcine colon. In human colon, a TTX sensitive component was only revealed after piroxicam. However, synaptic blockade by ω-conotoxin GVIA reduced the response to mechanical stimuli. Secretion was induced by tensile rather than compressive forces as preventing distension by a filter inhibited the secretion. In conclusion, in both species, distension induced secretion was predominantly mediated by PGs and a rather small nerve dependent response involving mechanosensitive somata and synapses.

Reduction of epithelial secretion in male rat distal colonic mucosa by bile acid receptor TGR5 agonist, INT-777: role of submucosal neurons

BACKGROUND

Recent evidence from rat neuron-free mucosa study suggests that the membrane bile acid receptor TGR5 decreases colonic secretion under basal and stimulated conditions. As submucosal neurons are key players in secretory processes and highly express TGR5, we investigated their role in TGR5 agonist-induced inhibition of secretion and the pathways recruited.

METHODS

TGR5 expression and localization were assessed in rat proximal (pC) and distal (dC) colon by qPCR and immunohistochemistry with double labeling for cholinergic neurons in whole-mount preparations. The influence of a selective (INT-777) or weak (ursodeoxycholic acid, UDCA) TGR5 agonist on colonic secretion was assessed in Ussing chambers, in dC preparation removing seromuscular ± submucosal tissues, in the presence of different inhibitors of secretion pathways.

KEY RESULTS

TGR5 mRNA is expressed in full thickness dC and pC and immunoreactivity is located in colonocytes and pChAT-positive neurons. Addition of INT-777, and less potently UDCA, decreased colonic secretion in seromuscular stripped dC by -58.17± 2.6%. INT-777 effect on basal secretion was reduced in neuron-free and TTX-treated mucosal-submucosal preparations. Atropine, hexamethonium, indomethacin, and L-NAME all reduced significantly INT-777's inhibitory effect while the 5-HT₄ antagonist, RS-39604, and lidocaine abolished it. INT-777 inhibited stimulated colonic secretion induced by nicotine, but not cisapride, carbachol or PGE2.

CONCLUSIONS & INFERENCES

TGR5 activation inhibits basal and stimulated distal colonic secretion in rats by acting directly on epithelial cells and also inhibiting submucosal neurons. This could represent a counter-regulatory mechanism, at the submucosal level, of the known prosecretory effect of bile acids in the colon.

Prostaglandin I(2) sensory input into the enteric nervous system during distension-induced colonic chloride secretion in rat colon

AIM

Intestinal pressure differences or experimental distension induce ion secretion via the enteric nervous system, the sensorial origin of which is only poorly understood. This study aimed to investigate sensorial inputs and the role of afferent and interneurones in mechanically activated submucosal secretory reflex circuits.

METHODS

Distension-induced rheogenic chloride secretion was measured as increase in short-circuit current 10 min after distension (DeltaI(SC)(10); distension parameters +/- 100 microL, 2 Hz, 20 s) in partially stripped rat distal colon in the Ussing-chamber in vitro. PGE(2) and PGI(2) were measured by radioimmunoassay.

RESULTS

DeltaI(SC)(10) was 2.0 +/- 0.2 micromol h(-1) cm(-2) and could be attenuated by lobeline, mecamylamine and dimethylphenylpiperazine, indicating an influence of nicotinergic interneurones. Additionally, a contribution of afferent neurones was indicated from the short-term potentiation of DeltaI(SC)(10) by capsaicin (1 microm). As evidence for its initial event, indomethacin (1 microm) inhibited distension-induced secretion and the release of PGI(2) was directly detected after distension. Furthermore, serotoninergic mediation was confirmed by granisetron (100 microm) which was functionally localized distally to PGI(2) in this reflex circuit, as granisetron inhibited an iloprost-induced I(SC), while indomethacin did not affect serotonin-activated ion secretion.

CONCLUSIONS

Distension-induced active electrogenic chloride secretion in rat colon is mediated by a neuronal reflex circuit which includes afferent neurones and nicotinergic interneurones. It is initiated by distension-induced PGI(2) release from subepithelial cells triggering this reflex via serotoninergic 5-HT(3) receptor transmission. Functionally, this mechanism may help to protect against intestinal stasis but could also contribute to luminal fluid loss, e.g. during intestinal obstruction.

Amiloride-sensitive epithelial Na+channel currents in surface cells of rat rectal colon

Surface cells of the mammalian distal colon are shown to molecularly express the amiloride-sensitive epithelial Na+channel composed of three homologous subunits (α-, β-, and γ-ENaC). However, because basic electrophysiological properties of amiloride-sensitive Na+channels expressed in these cells are largely unknown at the cellular level, functional evidence for the involvement of the subunits in the native channels is incomplete. Using electrophysiological techniques, we have now characterized functional properties of native ENaC in surface cells of rectal colon (RC) of rats fed a normal Na+diet. Ussing chamber experiments showed that apical amiloride inhibited a basal short-circuit current in mucosal preparation of RC with an apparent half-inhibition constant ( Ki) value of 0.20 μM. RT-PCR analysis confirmed the presence of transcripts of α-, β-, and γ-rENaC in rectal mucosa. Whole cell patch-clamp experiments in surface cells of intact crypts acutely isolated from rectal mucosa identified an inward cationic current, which was inhibited by amiloride with a Kivalue of 0.12 μM at a membrane potential of –64 mV, the inhibition being weakly voltage dependent. Conductance ratios of the currents were Li+(1.8) > Na+(1) >> K+(≈0), respectively. Amiloride-sensitive current amplitude was almost the same at 15 or 150 mM extracellular Na+, suggesting a high Na+affinity for current activation. These results are consistent with the hypothesis that a heterooligomer composed of α-, β-, and γ-ENaC may be the molecular basis of the native channels, which are responsible for amiloride-sensitive electrogenic Na+absorption in rat rectal colon.

Effects of nicotinic receptor blockade on the colonic mucosal response to luminal bile acids in anaesthetized rats

AIM

To test the hypothesis that nicotinic receptor mechanisms mediate the effects of bile acids on the colonic mucosa.

METHODS

The epithelial transport response to 4 mm deoxycholic acid (DCA) was studied in vitro and in vivo, in rat colon. In vitro, epithelial resistance was measured by square pulse analysis, and net membrane current was calculated from the transmucosal potential difference (PD) and resistance. In vivo, we measured PD and net fluid transport.

RESULTS

In vitro, DCA significantly increased membrane current and induced a progressive decrease in epithelial resistance, which in the distal colon eventually resulted in a significant PD reduction. This response was not significantly affected by hexamethonium. In vivo, DCA reduced PD with a significantly larger response in distal colon, but had no consistent effect on net fluid absorption. Nicotinic receptor blockade per se increased net fluid absorption and slightly reduced PD in proximal colon, and inhibited spontaneous net fluid secretion and markedly reduced PD in distal colon. Nicotinic receptor blockade significantly attenuated the bile-acid induced PD response.

CONCLUSION

The data do not support the theory that a bile acid-activated secretory reflex exists in rat colon. The reduced PD response after hexamethonium suggests that a mechanism involving nicotinic receptors may potentiate the permeability response to luminal bile acids.

Peptide YY-induced alteration of colonic electrolyte transport in the rat

The effect of peptide YY (PYY) on active electrolyte transport in rat colon was studied under short-circuited conditions. PYY (10(-6) M) decreased the basal short-circuit current (Isc) in both the distal and proximal segments of the colon. The decrease in Isc induced by PYY in the distal colon was about 3 times larger than that in the proximal colon. The response to PYY was inhibited by diphenylamine 2-carboxylate, a specific blocker of the Cl-channel, but not by amiloride, a Na-channel blocker. Unidirectional flux measurements in the distal colon revealed that PYY increased the net Na and Cl absorption and decreased the serosal-to-mucosal Cl flux. PYY inhibited the neurally mediated secretory response to electrical field stimulation in a concentration-dependent manner. PYY was also shown to reduce the direct action of the cholinergic agonist bethanechol on the epithelium. These results suggest that PYY inhibits electrogenic Cl secretion and stimulates electroneutral NaCl absorption via both presynaptic and postsynaptic sites in the distal colon.

Epithelial barrier and ion transport in coeliac sprue: electrical measurements on intestinal aspiration biopsy specimens

Epithelial barrier function and ion transport was studied in coeliac sprue using a miniaturised Ussing device for measurements on diagnostic aspiration biopsy specimens from the jejunum of untreated or gluten free nourished sprue patients, or from healthy controls. Pure epithelial resistance (Re) indicating epithelial barrier function was determined by transmural alternating current impedance analysis. It was reduced by 56% in acute sprue mean (SEM) (9 (1) omega.cm2) compared with controls (20(2) omega.cm2). In gluten free nourished sprue patients Re was only partly recovered (15 (1) omega.cm2). Subepithelial resistance (Rsub) was also changed from 28 (1) omega.cm2- in control to 17 (1) omega.cm2 in acute sprue because of the change in mucosal architecture, but was unchanged in gluten free nourished sprue patients (29 (4) omega.cm2). In acute sprue, unidirectional Na+ and Cl- fluxes were increased in both directions as a consequence of the decreased resistance. However, short circuit current (ISC) as well as Na+ and Cl- net fluxes were not significantly different from control. Subsequently, the electrogenic Cl- secretory system was investigated. After maximal stimulation with theophylline and prostaglandin E1, a Cl(-)-dependent increase in ISC was obtained in the sprue mucosa and control jejunum. It showed saturation characteristics and was blockable by serosal bumetanide. When compared with control, neither Km nor Vmax of this electrogenic Cl- secretion was significantly changed in coeliac sprue. In conclusion, a miniaturised Ussing device was used for transport measurements on intestinal biopsy specimens. In acute coeliac disease, the epithelial barrier of the jejunum was seriously disturbed. The active electrogenic Cl- secretory transport system was present in the sprue mucosa, but was not activated in the Ussing chamber in vitro when compared with control jejunum.

The role of extrinsic nervous system in jejunal absorption during elevation of intraluminal pressure in anesthetized dogs

The aim of this study was to investigate the role of the extrinsic nervous system in jejunal absorption under elevated intraluminal pressure of the jejunum. Increase in intraluminal pressure from 0 to 70-100 mmHg decreased net absorption of fluid, Na+ and Cl-, from 7.6 +/- 0.5 to 5.6 +/- 0.3 ml/15 min, 1.1 +/- 0.1 to 0.7 +/- 0.1 mEq/15 min, and 1.2 +/- 0.1 to 0.8 +/- 0.1 mEq/15 min, respectively. To examine the role of the extrinsic nervous system in the depressed net jejunal absorption induced by the increase in intraluminal pressure, jejunal afferent and efferent nerve activities were measured in response to the increase in intraluminal pressure. Both afferent and efferent nerve activities increased to 224 +/- 14 and 236 +/- 18% in response to the increase in intraluminal pressure. In the extrinsic denervated jejunal loop, the responses of net absorption to the elevation of intraluminal pressure were quite different from those in innervated jejunum. That is, in the denervated jejunal loop the elevation of intraluminal pressure converted jejunal absorption to jejunal secretion. These results indicate that elevation of intraluminal pressure elicits the jejuno-jejunal reflex, and this mechanism counteracts jejunal secretion.

Mechanisms of the secretory response to luminal propionate in rat descending colon in vitro

Short chain fatty acids stimulate Cl secretion in rat descending colon in vitro via an enteric reflex involving mucosa and cholinergic nerves. We used the short circuit current as the measure of Cl secretion caused by Na propionate (NaP) (0.5 mM) in luminal bath fluid and studied the mechanism of the response. The NaP response was decreased 81% by atropine and 76% by lidocaine. It was unaffected by tetrodotoxin, omega-conotoxin or by tachyphylaxis to capsaicin, CGRP, substance P, histamine or PGE2. It was not reduced by inhibitors of 5-HT2 or 5HT3 receptors or by partial tachyphylaxis to 5-HT. However, superficial mucosal injury with hypertonic Na sulfate (2 M) or xylose (4.5 M) reduced the NaP response by 90% and 86%, respectively, and mucosal concanavalin A (1 mg/ml) reduced it by 73%. Neither piroxicam (10 microM) nor nordihydroguaretic acid (10 microM) affected the NaP response. We hypothesize that NaP stimulates the superficial epithelium to release an unidentified agonist that depolarizes predominantly cholinergic nerve terminals and causes colonic secretion.

Electrogenic Na+ absorption of rat distal colon is confined to surface epithelium: a voltage-scanning study

There is no quantitative assignment of large intestinal electrogenic Na+ absorption to surface epithelium and crypts so far. We determined the spatial distribution of electrogenic Na+ absorption to crypts and surface epithelium of rat late distal colon using a modified voltage-scanning technique. Voltage deflections resulting from external 30-Hz current were sensed by an extracellular microelectrode stepping at 0.7 Hz above crypt openings or surface epithelium. Local conductances were calculated applying a planar model of electrical field distribution to surface epithelium and a electrostatic disk source model to the crypts. These models were confirmed by methodological experiments where the electrode position was varied in vertical and horizontal direction. Electrogenic Na+ absorption was detected by blocking apical Na+ channels by mucosal 0.1 mM amiloride. Under control conditions surface epithelium contributed 44% (2.0 +/- 0.2 mS/cm2) and crypts 56% (2.6 +/- 0.2 mS/cm2) to the total conductance of 4.6 +/- 0.4 mS/cm2. Electrogenic Na+ absorption was induced by 6 h in vitro incubation in a medium containing 3 nM aldosterone. This caused a short-circuit current (ISC) of 12.1 +/- 0.8 mumol.h-1.cm-2, which was paralleled by a 2.5-fold increase in surface epithelial conductance to 5.1 +/- 0.4 mS/cm2, whereas crypt conductance was not significantly altered (3.0 +/- 0.2 mS/cm2). Amiloride reversed ISC to -0.8 +/- 0.1 mumol.-1.cm-2 and decreased surface epithelium conductance to 2.3 +/- 0.3 mS/cm2 but again had no significant effect on crypt conductance (2.5 +/- 0.3 mS/cm2). Sham incubation (no hormones added) for 6 h neither induced electrogenic transport nor altered local epithelial conductances.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of neuronally mediated secretion in rat colonic mucosa by prostaglandin D2

The effect of prostaglandin D2 (PGD2) on ion transport across the mucosa of the descending colon was studied in rats. PGD2 dose-dependently decreased baseline short-circuit current of mucosa-submucosal preparations mounted either in the Ussing chamber or mounted as an everted sac. However, with the everted sac technique, the tissue was about 1000 times more sensitive to PGD2. Concomitant with the decrease in short-circuit current, PGD2 increased the mucosal-to-serosal fluxes of sodium and chloride and decreased the serosal-to-mucosal flux of chloride. PGD2 inhibited the secretory action of the PGI2 analogue iloprost, PGD2 alpha, and neurotensin. The action of these secretagogues was dependent on the presence of the submucosal plexus. In contrast, PGD2 had no effect on the increase in short-circuit current caused by PGD2, substance P, or serotonin, the actions of which were not dependent on the presence of the submucosal plexus. The results indicate that the action site of the antisecretory mechanism of PGD2 is localized in the secretomotor neurons.

Ion transport in the experimental short bowel syndrome of the rat

The adaptational changes of epithelial ion transport in the short bowel syndrome were studied. Ileal remnants of rats were investigated 8 weeks after 70% proximal small intestinal resection. Pure epithelial resistance measured by impedance analysis decreased from 27 +/- 1 to 21 +/- 1 omega.cm2, and polyethylene glycol 4000 fluxes increased from 2.5 +/- 0.3 to 3.6 +/- 0.3 nmol.h-1.cm-2, indicating increased permeability of the short bowel. Unidirectional flux measurements in control ileum showed absorptive net fluxes of Na+ and Cl- that were assigned to electroneutral NaCl absorption and a short-circuit current that was accounted for by the residual flux (HCO3- secretion). Neither NaCl absorption nor HCO3- secretion were altered in the short bowel. Also, electrogenic Cl- secretion, defined after maximal stimulation by theophylline and prostaglandin E1 was not changed in the short bowel. In contrast, electrogenic Na+/glucose cotransport increased in Vmax from 2.0 +/- 0.3 in controls to 5.0 +/- 1.0 mumol.h-1.cm-2 in the short bowel. Tight junction structure was studied by freeze-fracture electron microscopy. The number of horizontal strands was unchanged, whereas tight junction depth was slightly increased in the short bowel. Microvillus area of short bowels was increased by 20% in villus regions. Under the light microscope, villus height was increased by 30%. In conclusion, the short bowel mucosa undergoes adaptive responses to reduced overall absorptive area by increasing glucose-dependent electrogenic Na+ absorption to 250%, which is partly caused by increased villus and microvillus surface area. Electrogenic Cl- and HCO3- secretion and electroneutral NaCl absorption remained unchanged. The decreased epithelial resistance is caused by mucosal surface amplification.

Failure of cholinergic stimulation to induce a secretory response from the rectal mucosa in cystic fibrosis

The secretory response to cholinergic stimulation was investigated in rectal biopsy specimens from children with cystic fibrosis and a control group using a modified Ussing chamber technique. Acetylcholine (10(-3) mol/l) increased the short circuit current in 12 control specimens by mean (SEM) 83.0 (16.4) microA/cm2, but samples from five children with cystic fibrosis failed to exhibit such a response (-1.4 (3.2) microA/cm2). Amiloride (10(-4) mol/l), which will inhibit electrogenic sodium absorption in viable tissues, caused similar reductions in the short circuit current of both control and cystic fibrosis tissues (control = -37.7 (7.7) microA/cm2; cystic fibrosis = -44.0 (9.3) microA/cm2). Thus, the failure of chloride secretion observed in the small intestine also exists in the rectal mucosa. This observation could be used both to aid diagnosis and to study the basic defect.

Distension-induced secretion in the rat colon: mediation by prostaglandins and submucosal neurons

Distension of the rat colon descendens in vitro by a hydrostatic gradient induced an increase in short-circuit current (Isc). In a mucosa-submucosa preparation containing the plexus submucosus, the increase in Isc was biphasic with a half-time of about 200 s for the spontaneous returning to the baseline. The time course was monophasic in a mucosa preparation without the plexus submucosus. The increase in Isc in the mucosa-submucosa preparation was inhibited by an inhibitor of phospholipase A2, quinacrine, and by indomethacin, tetrodotoxin or atropine; each of these compounds also abolished the second phase of the response. In contrast, only indomethacin was effective in reducing the increase in Isc in the mucosa preparation. In both preparations the response to distension was inhibited by scilliroside, by replacement of Cl- with gluconate, and by administration of frusemide or the chloride channel blocker, anthracene-9-carboxylic acid. The results indicate that distension induces chloride secretion by causing the release of prostaglandins, which act indirectly, i.e. mediated by the submucosal plexus, and directly at the epithelium.