Mucosa-submucosa Preparations Research Articles

Roles of endothelial prostaglandin I2 in maintaining synchronous spontaneous Ca2+ transients in rectal capillary pericytes.

In hollow visceral organs, capillary pericytes appear to drive spontaneous Ca2+ transients in the upstream arterioles. Here, mechanisms underlying the intercellular synchrony of pericyte Ca2+ transients were explored. Ca2+ dynamics in NG2 chondroitin sulphate proteoglycan (NG2)-expressing capillary pericytes were examined using rectal mucosa-submucosa preparations of NG2-GCaMP6mice. Spontaneous Ca2+ transients arising from endoplasmic reticulum Ca2+ release were synchronously developed amongst capillary pericytes in a gap junction blocker (3μM carbenoxolone)-sensitive manner and could spread into upstream vascular segments. Spontaneous Ca2+ transients were suppressed by the Ca2+ -activated Cl- channel (CaCC) blocker niflumic acid and their synchrony was diminished by a TMEM16A inhibitor (3μM Ani9) in accordance with TMEM16A immunoreactivity in pericytes. In capillaries where cyclooxygenase (COX)-2 immunoreactivity was expressed in endothelium but not pericytes, non-selective COX inhibitors (1μM indomethacin or 10μM diclofenac) or COX-2 inhibitor (10μM NS 398) disrupted the synchrony of spontaneous Ca2+ transients and raised the basal Ca2+ level. Subsequent prostaglandin I2 (PGI2 ; 100nM) or the KATP channel opener levcromakalim restored the synchrony with a reduction in the Ca2+ level. PGI2 receptor antagonist (1μM RO1138452) also disrupted the synchrony of spontaneous Ca2+ transients and increased the basal Ca2+ level. Subsequent levcromakalim restored the synchrony and reversed the Ca2+ rise. Thus, the synchrony of spontaneous Ca2+ transients in pericytes appears to be developed by the spread of spontaneous transient depolarisations arising from the opening of TMEM16A CaCCs. Endothelial PGI2 may play a role in maintaining the synchrony, presumably by stabilising the resting membrane potential in pericytes. KEY POINTS: Capillary pericytes in the rectal mucosa generate synchronous spontaneous Ca2+ transients that could spread into the upstream vascular segment. Spontaneous Ca2+ release from the endoplasmic reticulum (ER) triggers the opening of Ca2+ -activated Cl- channel TMEM16A and resultant depolarisations that spread amongst pericytes via gap junctions, establishing the synchrony of spontaneous Ca2+ transients in pericytes. Prostaglandin I2 (PGI2 ), which is constitutively produced by the endothelium depending on cyclooxygenase-2, appears to prevent premature ER Ca2+ releases in the pericytes allowing periodic, regenerative Ca2+ releases. Endothelial PGI2 may maintain the synchrony of pericyte activity by stabilising pericyte resting membrane potential by opening of KATP channels.

LPS transport during fat absorption is inhibited in caveolin‐1 knockout murine jejunum

Lipopolysaccharides (LPS) entry from the intestinal lumen to the circulation is associated with systemic inflammation. We have found that LPS is transcellularly transported to portal vein during long‐chain fatty acid (LCFA) exposure via CD36‐ and lipid raft‐mediated pathways in rats, suggesting the involvement of caveolae‐mediated endocytosis. We thus examined LPS transport in caveolin‐1 (Cav1) knockout (KO) murine jejunum.FITC‐LPS was applied to the mucosal bath of Ussing chambered muscle‐stripped jejunal mucosa‐submucosa preparations of Cav1 KO and wild type (WT) mice. Serosal appearance of FITC‐LPS was measured with or without luminal application of oleic acid (OA, 30 mM) with taurocholic acid (TCA, 0.1 mM), or medium‐chain fatty acid sodium caprate (C10, 10 mM).Luminal application of OA/TCA increased FITC‐LPS m‐to‐s transport in WT jejunum, inhibited by the CD36 inhibitor sulfosuccinimidyl oleate (SSO, 0.1 mM) or the lipid raft inhibitor methyl‐β‐cyclodextrin (MβCD, 0.1 mM), not by the clathrin inhibitor chlorpromazine (0.1 mM) or Pitstop2 (30 µM), suggesting that LCFA‐induced LPS transport is mediated by caveolae‐mediated endocytosis, not by clathrin‐mediated endocytosis. In contrast, OA/TCA‐induced FITC‐LPS transport was abolished in Cav1 KO jejunum. Nevertheless, luminal application of C10 increased FITC‐LPS transport both in WT and Cav1 KO jejuna without transepithelial electrical resistance changes, suggesting that C10 enhances transcellular LPS transport via caveolin‐independent endocytosis in the jejunum.These results suggest that LPS transport during LCFA exposure is mediated by Cav1‐mediated endocytosis, whereas MCFA‐induced LPS transport is likely via clathrin‐mediated endocytosis. Modulation of epithelial endocytosis may be a new therapeutic target for LPS‐associated diseases mediated by dietary fatty acids, including metabolic syndrome.

Succinate‐Induced Tuft Cell Hyperplasia Enhances FFA2/5‐HT Mediated Anion Secretion in Mouse Duodenum

Tuft cells are one of luminal chemosensing components in the small intestine, while the role of tuft cell in mucosal secretory responses to luminal chemicals such as short-chain fatty acids (SCFA) has not been elucidated. We thus hypothesized that tuft cell hyperplasia may increase the secretory responses to luminal SCFA due to the increased cholinergic signals since tuft cells express choline acetyltransferase (ChAT). Tuft cell hyperplasia was induced by oral sodium succinate (NaSuc, 100 mM) in the drinking water for 7 days. Sodium propionate (NaP, 10 mM, m), the selective free fatty acid receptor 2 (FFA2) agonist 4-CMTB (10 µM, s), or the TRPA1 agonist allyl isothiocyanate (AITC, 1 mM, m) were applied to the mucosal (m) or serosal (s) baths of an Ussing chamber mounted with muscle-stripped mucosa-submucosa preparations of mouse proximal duodenum or jejunum. The density of tuft cells (doublecortin-like kinase 1; DCLK1), enteroendocrine L cells (proglucagon, glucagon-like peptide-2), and enterochromaffin (EC) cells (5-HT, tryptophan hydroxylase 1; TPH1) were assessed by real-time PCR and immunostaining. NaSuc treatment significantly increased the number of DCLK1-positive tuft cells in the duodenal and jejunal villi. mRNA expression of DCLK1 and FFA2 were increased in the duodenal and jejunal mucosa, whereas TPH1 was increased only in the duodenal mucosa, and ChAT was increased in the jejunal mucosa, but proglucagon was unchanged in both. 5-HT-positive mast cell density rather than EC cell density was increased in the duodenum. Isc in response to luminal NaP was enhanced in the jejunum, which was atropine sensitive. In contrast, Isc responses to serosal 4-CMTB and luminal AITC were enhanced in the duodenum, which were mediated by 5-HT3 and 5-HT4 receptors. These results suggest that NaSuc differentially enhances secretory responses, with 5-HT signals in the duodenum and with cholinergic signals in the jejunum. These data may be implicated in the pathogenesis of functional dyspepsia (FD), since increased duodenal mast cell density and enhanced 5-HT signals are present in FD patients.

Xenin-25 induces anion secretion by activating noncholinergic secretomotor neurons in the rat ileum.

Xenin-25 is a neurotensin-like peptide that is secreted by enteroendocrine cells in the small intestine. Xenin-8 is reported to augment duodenal anion secretion by activating afferent neural pathways. The intrinsic neuronal circuits mediating the xenin-25-induced anion secretion were characterized using the Ussing-chambered, mucosa-submucosa preparation from the rat ileum. Serosal application of xenin-25 increased the short-circuit current in a concentration-dependent manner. The responses were abolished by the combination of Cl--free and -free solutions. The responses were almost completely blocked by TTX (10-6 M) but not by atropine (10-5 M) or hexamethonium (10-4 M). The selective antagonists for neurotensin receptor 1 (NTSR1), neurokinin 1 (NK1), vasoactive intestinal polypeptide (VIP) receptors 1 and 2 (VPAC1 and VPAC2, respectively), and capsaicin, but not 5-hydroxyltryptamine receptors 3 and 4 (5-HT3 and 5-HT4), NTSR2, and A803467, inhibited the responses to xenin-25. The expression of VIP receptors (Vipr) in rat ileum was examined using RT-PCR. The Vipr1 PCR products were detected in the submucosal plexus and mucosa. Immunohistochemical staining showed the colocalization of NTSR1 and NK1 with substance P (SP)- and calbindin-immunoreactive neurons in the submucosal plexus, respectively. In addition, NK1 was colocalized with noncholinergic VIP secretomotor neurons. Based on the results from the present study, xenin-25-induced Cl-/ secretion is involved in NTSR1 activation on intrinsic and extrinsic afferent neurons, followed by the release of SP and subsequent activation of NK1 expressed on noncholinergic VIP secretomotor neurons. Finally, the secreted VIP may activate VPAC1 on epithelial cells to induce Cl-/ secretion in the rat ileum. Activation of noncholinergic VIP secretomotor neurons by intrinsic primary afferent neurons and extrinsic afferent neurons by postprandially released xenin-25 may account for most of the neurogenic secretory response induced by xenin-25. NEW & NOTEWORTHY This study is the first to investigate the intrinsic neuronal circuit responsible for xenin-25-induced anion secretion in the rat small intestine. We have found that nutrient-stimulated xenin-25 release may activate noncholinergic vasoactive intestinal polypeptide (VIP) secretomotor neurons to promote Cl-/ secretion through the activation of VIP receptor 1 on epithelial cells. Moreover, the xenin-25-induced secretory responses are mainly linked with intrinsic primary afferent neurons, which are involved in the activation of neurotensin receptor 1 and neurokinin 1 receptor.

Basolateral FFA2 of enterochromaffin cells contributes to 5‐HT release in rat and mouse duodenum

Enteroendocrine cells (EEC) express nutrient receptors that when activated release hormones such as serotonin (5‐HT). Although nutrient receptors are thought to be localized on the apical membrane of EEC, where they are well placed to sense luminal nutrients, immunohistochemistry mostly localizes the receptors to the cytosol or to either membrane pole. Since villous cells transport luminal nutrients via nutrient transporters or by passive diffusion to the submucosal space, it is possible that absorbed nutrients activate basolaterally‐expressed nutrient receptors rather than detect luminal nutrients. One of the short‐chain fatty acid receptors, FFA2, is expressed on enterochromaffin (EC) cells of rat duodenum; its activation increases HCO3− secretion in vivo, accompanied by 5‐HT release into the portal vein and mediated by 5‐HT4 receptor activation. We thus used muscle‐stripped mucosa‐submucosa preparations of rat and mouse duodenum mounted in Ussing chambers to pharmacologically localize FFA2 to the apical or basolateral membranes of EC cells.The selective and allosteric FFA2 agonist phenylacetamide‐1 (PA1) was applied to the mucosal or serosal bath of the Ussing chamber, where short‐circuit current (Isc) was measured in the presence of serosal indomethacin (10 μM).PA1 (1 – 100 μM) in the mucosal bath had no effect or slightly decreased Isc in both rat and mouse duodenum. In contrast, PA1 in the serosal bath dose‐dependently increased Isc that was greater in mouse than in rat duodenum; in both, the PA1‐induce Isc increase was reduced by the 5‐HT3 receptor antagonist granisetron (Gran, 10 μM) and abolished by the 5‐HT4 receptor antagonist GR113801 (GR; 10 μM). The effect of serosal PA1 was inhibited by the selective FFA2 antagonist GLPG0974 (0.1 mM). Furthermore, the native FFA2 agonists sodium acetate and sodium propionate (pH adjusted to pH 7.4) dose‐dependently increased Isc inhibited by GR, Gran, the selective Nav1.8 antagonist A803467, and the selective neurokinin receptor‐1 (NK‐1) antagonist CP99994, but not by TTX or atropine. Serosal PA1 increased serosal 5‐HT concentrations, which were not affected by TTX+A803467. FFA2 immunoreactivity was limited to EC cells, suggesting that EC cells rather than myenteric neurons are the primary source of 5‐HT.These results suggest that FFA2 is localized at least to the basolateral membrane of EC cells and that its activation increases electrogenic anion secretion. The basolateral location of FFA2 is important for informing the design of experimental and clinical studies aimed at altering 5‐HT release from EC cells, which is related to mucosal defenses and dyspeptic symptoms.Support or Funding InformationVA Merit Review and NIDDK R01 DK54221This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Human-derived gut microbiota modulates colonic secretion in mice by regulating 5-HT3 receptor expression via acetate production.

Serotonin [5-hydroxytryptamine (5-HT)], an important neurotransmitter and a paracrine messenger in the gastrointestinal tract, regulates intestinal secretion by its action primarily on 5-HT3 and 5-HT4 receptors. Recent studies highlight the role of gut microbiota in 5-HT biosynthesis. In this study, we determine whether human-derived gut microbiota affects host secretory response to 5-HT and 5-HT receptor expression. We used proximal colonic mucosa-submucosa preparation from age-matched Swiss Webster germ-free (GF) and humanized (HM; ex-GF colonized with human gut microbiota) mice. 5-HT evoked a significantly greater increase in short-circuit current (ΔIsc) in GF compared with HM mice. Additionally, 5-HT3 receptor mRNA and protein expression was significantly higher in GF compared with HM mice. Ondansetron, a 5-HT3 receptor antagonist, inhibited 5-HT-evoked ΔIsc in GF mice but not in HM mice. Furthermore, a 5-HT3 receptor-selective agonist, 2-methyl-5-hydroxytryptamine hydrochloride, evoked a significantly higher ΔIsc in GF compared with HM mice. Immunohistochemistry in 5-HT3A-green fluorescent protein mice localized 5-HT3 receptor expression to enterochromaffin cells in addition to nerve fibers. The significant difference in 5-HT-evoked ΔIsc between GF and HM mice persisted in the presence of tetrodotoxin (TTX) but was lost after ondansetron application in the presence of TTX. Application of acetate (10 mM) significantly lowered 5-HT3 receptor mRNA in GF mouse colonoids. We conclude that host secretory response to 5-HT may be modulated by gut microbiota regulation of 5-HT3 receptor expression via acetate production. Epithelial 5-HT3 receptor may function as a mediator of gut microbiota-driven change in intestinal secretion.NEW & NOTEWORTHY We found that gut microbiota alters serotonin (5-HT)-evoked intestinal secretion in a 5-HT3 receptor-dependent mechanism and gut microbiota metabolite acetate alters 5-HT3 receptor expression in colonoids.View this article's corresponding video summary at https://www.youtube.com/watch?v=aOMYJMuLTcw&feature=youtu.be.

Xenin Augments Duodenal Anion Secretion via Activation of Afferent Neural Pathways.

Xenin-25, a neurotensin (NT)-related anorexigenic gut hormone generated mostly in the duodenal mucosa, is believed to increase the rate of duodenal ion secretion, because xenin-induced diarrhea is not present after Roux-en-Y gastric bypass surgery. Because the local effects of xenin on duodenal ion secretion have remained uninvestigated, we thus examined the neural pathways underlying xenin-induced duodenal anion secretion. Intravenous infusion of xenin-8, a bioactive C-terminal fragment of xenin-25, dose dependently increased the rate of duodenal HCO3- secretion in perfused duodenal loops of anesthetized rats. Xenin was immunolocalized to a subset of enteroendocrine cells in the rat duodenum. The mRNA of the xenin/NT receptor 1 (NTS1) was predominantly expressed in the enteric plexus, nodose and dorsal root ganglia, and in the lamina propria rather than in the epithelium. The serosal application of xenin-8 or xenin-25 rapidly and transiently increased short-circuit current in Ussing-chambered mucosa-submucosa preparations in a concentration-dependent manner in the duodenum and jejunum, but less so in the ileum and colon. The selective antagonist for NTS1, substance P (SP) receptor (NK1), or 5-hydroxytryptamine (5-HT)3, but not NTS2, inhibited the responses to xenin. Xenin-evoked Cl- secretion was reduced by tetrodotoxin (TTX) or capsaicin-pretreatment, and abolished by the inhibitor of TTX-resistant sodium channel Nav1.8 in combination with TTX, suggesting that peripheral xenin augments duodenal HCO3- and Cl- secretion through NTS1 activation on intrinsic and extrinsic afferent nerves, followed by release of SP and 5-HT. Afferent nerve activation by postprandial, peripherally released xenin may account for its secretory effects in the duodenum.

Hypoxia/Reoxygenation Effects on Ion Transport across Rat Colonic Epithelium.

Ischemia causes severe damage in the gastrointestinal tract. Therefore, it is interesting to study how the barrier and transport functions of intestinal epithelium change under hypoxia and subsequent reoxygenation. For this purpose we simulated hypoxia and reoxygenation on mucosa-submucosa preparations from rat distal colon in Ussing chambers and on isolated crypts. Hypoxia (N2 gassing for 15 min) induced a triphasic change in short-circuit current (Isc): a transient decrease, an increase and finally a long-lasting fall below the initial baseline. During the subsequent reoxygenation phase, Isc slightly rose to values above the initial baseline. Tissue conductance (Gt) showed a biphasic increase during both the hypoxia and the reoxygenation phases. Omission of Cl− or preincubation of the tissue with transport inhibitors revealed that the observed changes in Isc represented changes in Cl− secretion. The radical scavenger trolox C reduced the Isc response during hypoxia, but failed to prevent the rise of Isc during reoxygenation. All changes in Isc were Ca2+-dependent. Fura-2 experiments at loaded isolated colonic crypts revealed a slow increase of the cytosolic Ca2+ concentration during hypoxia and the reoxygenation phase, mainly caused by an influx of extracellular Ca2+. Surprisingly, no changes could be detected in the fluorescence of the superoxide anion-sensitive dye mitosox or the thiol-sensitive dye thiol tracker, suggesting a relative high capacity of the colonic epithelium (with its low O2 partial pressure even under physiological conditions) to deal with enhanced radical production during hypoxia/reoxygenation.

Early life adversity in piglets induces long-term upregulation of the enteric cholinergic nervous system and heightened, sex-specific secretomotor neuron responses.

Early life adversity (ELA) is a risk factor for the later-life onset of gastrointestinal (GI) diseases such as irritable bowel syndrome (IBS); however, the mechanisms are poorly understood. Here, we utilized a porcine model of ELA, early weaning stress (EWS), to investigate the influence of ELA on the development and function of the enteric nervous system (ENS). Female and castrated male (Male-C) piglets were weaned from their sow either at 15 days of age (EWS) or 28 days of age (late weaning control, LWC). At 60 and 170 days of age, ileal mucosa-submucosa preparations were mounted in Ussing chambers and veratridine- and corticotropin releasing factor (CRF)-releasing factor-evoked short circuit current (Isc ) responses were recorded as indices of secretomotor neuron function. Enteric neuron numbers and the expression of select neurotransmitters and their receptors were also measured. Compared with LWC pigs, female, but not Male-C EWS, pigs exhibited heightened veratridine-induced Isc responses at 60 and 170 days of age that were inhibited with tetrodotoxin and atropine. Ileum from EWS pigs had higher numbers of enteric neurons that were choline acetyltransferase positive. Markers of increased cholinergic signaling (increased acetylcholinesterase) and downregulated mucosal muscarinic receptor 3 gene expression were also observed in EWS pigs. This study demonstrated that EWS in pigs induces lasting and sex-specific hypersensitivity of secretomotor neuron function and upregulation of the cholinergic ENS. These findings may represent a mechanistic link between ELA and lifelong susceptibility to GI diseases such as IBS.

Neural FFA3 activation inversely regulates anion secretion evoked by nicotinic ACh receptor activation in rat proximal colon

Luminal short-chain fatty acids (SCFAs) influence gut physiological function via SCFA receptors and transporters. The contribution of an SCFA receptor, free fatty acid receptor (FFA)3, to the enteric nervous system is unknown. FFA3 is expressed in enteric cholinergic neurons. Activation of neural FFA3 suppresses Cl(-) secretion induced by nicotinic ACh receptor activation via a Gi/o pathway. Neural FFA3 may have an anti-secretory function by modulating cholinergic neural reflexes in the enteric nervous system. The proximal colonic mucosa is constantly exposed to high concentrations of microbially-produced short-chain fatty acids (SCFAs). Although luminal SCFAs evoke electrogenic anion secretion and smooth muscle contractility via neural and non-neural cholinergic pathways in the colon, the involvement of the SCFA receptor free fatty acid receptor (FFA)3, one of the free fatty acid receptor family members, has not been clarified. We investigated the contribution of FFA3 to cholinergic-mediated secretory responses in rat proximal colon. FFA3 was immunolocalized to enteroendocrine cells and to the enteric neural plexuses. Most FFA3-immunoreactive nerve fibres and nerve endings were cholinergic, colocalized with protein gene product (PGP)9.5, the vesicular ACh transporter, and the high-affinity choline transporter CHT1. In Ussing chambered mucosa-submucosa preparations (including the submucosal plexus) of rat proximal colon, carbachol (CCh)-induced Cl(-) secretion was decreased by TTX, hexamethonium, and the serosal FFA3 agonists acetate or propionate, although not by an inactive analogue 3-chloropropionate. Serosal application of a selective FFA3 agonist (N-[2-methylphenyl]-[4-furan-3-yl]-2-methyl-5-oxo-1,4,5,6,7,8-hexahydro-quinoline-3-carboxamide; MQC) dose-dependently suppressed the response to CCh but not to forskolin, with an IC50 of 13μm. Pretreatment with MQC inhibited nicotine-evoked but not bethanechol-evoked secretion. The inhibitory effect of MQC was reversed by pretreatment with pertussis toxin, indicating that FFA3 acts via the Gi/o pathway. Luminal propionate induced Cl(-) secretion via the cholinergic pathway, which was reduced by MQC, as well as by TTX, hexamethonium or removal of the submucosal plexus. These results suggest that the SCFA-FFA3 pathway has a novel anti-secretory function in that it inhibits cholinergic neural reflexes in the enteric nervous system.

Communication between mast cells and rat submucosal neurons.

Histamine is a mast cell mediator released e.g. during food allergy. The aim of the project was to identify the effect of histamine on rat submucosal neurons and the mechanisms involved. Cultured submucosal neurons from rat colon express H1, H2 and H3 receptors as shown by immunocytochemical staining confirmed by reverse transcriptase polymerase chain reaction (RT-PCR) with messenger RNA (mRNA) isolated from submucosal homogenates as starting material. Histamine evoked a biphasic rise of the cytosolic Ca(2+) concentration in cultured submucosal neurons, consisting in a release of intracellularly stored Ca(2+) followed by an influx from the extracellular space. Although agonists of all three receptor subtypes evoked an increase in the cytosolic Ca(2+) concentration, experiments with antagonists revealed that mainly H1 (and to a lesser degree H2) receptors mediate the response to histamine. In coculture experiments with RBL-2H3 cells, a mast cell equivalent, compound 48/80, evoked an increase in the cytosolic Ca(2+) concentration of neighbouring neurons. Like the response to native histamine, the neuronal response to the mast cell degranulator was strongly inhibited by the H1 receptor antagonist pyrilamine and reduced by the H2 receptor antagonist cimetidine. In rats sensitized against ovalbumin, exposure to the antigen induced a rise in short-circuit current (I sc) across colonic mucosa-submucosa preparations without a significant increase in paracellular fluorescein fluxes. Pyrilamine strongly inhibited the increase in I sc, a weaker inhibition was observed after blockade of protease receptors or 5-lipoxygenase. Consequently, H1 receptors on submucosal neurons seem to play a pivotal role in the communication between mast cells and the enteric nervous system.

Bacterial overload enhances duodenal anion secretion via PKA upregulation (1109.15)

The duodenal mucosa senses luminal bacteria via pattern recognition receptors. Rats, as coprovores, likely populate their foregut with fecal flora, which we hypothesized would enhance anion secretory responses in the duodenum.We measured short‐circuit current (Isc) in Ussing chambered muscle‐stripped mucosa‐submucosa preparations of rat duodenum. Animals were treated with a suspension of homogenized feces (Fecal, 0.1 g/kg, ig) with or without vehicle (Veh) or omeprazole treatment (OPZ, 10 mg/kg, ig) up to 7 days. Duodenal microflora were assessed by culturing duodenal mucosal scrapes on Brucella blood agar or Lactobacillus‐MRS agar plates.Basolateral application of forskolin (FK, 10 µM) increased Isc, which was further increased by the addition of carbachol (CCh, 10 µM). The Isc increase in response to FK and CCh was markedly enhanced in OPZ+Fecal group, compared to Veh group, whereas no differences were noted among Veh+Fecal, OPZ, or Veh groups. Acetazolamide‐sensitive Isc was increased in OPZ+Fecal group, consistent with enhanced HCO3‐ secretion. Expression of protein kinase A (PKA) catalytic domain a and b, and regulatory domain 1a and 2a was significantly increased in Veh+Fecal and OPZ+Fecal groups, while the expression of the cystic fibrosis transmembrane conductance regulator (CFTR), cyclooxygenase (COX)‐2 or tumor necrosis factor (TNF)‐α was not changed. OPZ treatment increased the detection frequency of lactobacillus in the duodenum, and OPZ+Fecal treatment markedly increased lactobacillus colony counts.These results suggest that increased foregut bacterial exposure enhances the anion secretory response to FK in the duodenal mucosa. Since lactobacillus is a major probiotics, upregulation of PKA by probiotics may be a novel pathway to modulate the duodenal mucosal defenses.Grant Funding Source: Supported by VA Merit Review, NIH R01 DK54221

Selective influence of host microbiota on cAMP‐mediated ion transport in mouse colon

More microbes are resident in the distal colon than any other part of the body, and this microbiota has the capacity to influence enteric nerve development, excitability, and gastrointestinal function. Germ-free (GF) mice are a valuable tool in interrogating the communication between microbiota and host. Despite the intimate relationship which exists between the microbiota and the colonic mucosa-submucosa, there is a paucity of studies examining the influence of the microbiota on secretogogue-evoked responses. To this end, we investigated both epithelial and neural-evoked ion transport, and the response elicited by two commensal organisms, in colonic mucosa-submucosa preparations from GF mice in Ussing chambers. Baseline electrical parameters, short-circuit current and transepithelial resistance, were comparable between tissues from GF and conventional animals. Noteworthy, however, was a hyper-responsiveness of GF colon to forskolin stimulation. In contrast, the absence of the microbiota did not influence the tissue response to bethanechol. Moreover, responses to the sodium-channel activator, veratridine, and the TRPV1 receptor agonist, capsaicin were preserved in GF mice relative to conventional tissues. Similarly, the short-circuit current response to two well-characterized commensal organisms occurred independent of an interaction with the host microbiota. This is the first comprehensive characterization of secretomotor responses in GF colon.

VPAC1 receptors regulate intestinal secretion and muscle contractility by activating cholinergic neurons in guinea pig jejunum

In the gastrointestinal tract, vasoactive intestinal peptide (VIP) is found exclusively within neurons. VIP regulates intestinal motility via neurally mediated and direct actions on smooth muscle and secretion by a direct mucosal action, and via actions on submucosal neurons. VIP acts via VPAC1 and VPAC2 receptors; however, the subtype involved in its neural actions is unclear. The neural roles of VIP and VPAC1 receptors (VPAC1R) were investigated in intestinal motility and secretion in guinea pig jejunum. Expression of VIP receptors across the jejunal layers was examined using RT-PCR. Submucosal and myenteric neurons expressing VIP receptor subtype VPAC1 and/or various neurochemical markers were identified immunohistochemically. Isotonic muscle contraction was measured in longitudinal muscle-myenteric plexus preparations. Electrogenic secretion across mucosa-submucosa preparations was measured in Ussing chambers by monitoring short-circuit current. Calretinin(+) excitatory longitudinal muscle motor neurons expressed VPAC1R. Most cholinergic submucosal neurons, notably NPY(+) secretomotor neurons, expressed VPAC1R. VIP (100 nM) induced longitudinal muscle contraction that was inhibited by TTX (1 μM), PG97-269 (VPAC1 antagonist; 1 μM), and hyoscine (10 μM), but not by hexamethonium (200 μM). VIP (50 nM)-evoked secretion was depressed by hyoscine or PG97-269 and involved a small TTX-sensitive component. PG97-269 and TTX combined did not further depress the VIP response observed in the presence of PG97-269 alone. We conclude that VIP stimulates ACh-mediated longitudinal muscle contraction via VPAC1R on cholinergic motor neurons. VIP induces Cl(-) secretion directly via epithelial VPAC1R and indirectly via VPAC1R on cholinergic secretomotor neurons. No evidence was obtained for involvement of other neural VIP receptors.

Actions of Angeli's salt, a nitroxyl (HNO) donor, on ion transport across mucosa—submucosa preparations from rat distal colon

The aim of this study was to investigate whether nitroxyl (HNO), a redox variant of the radical gasotransmitter nitric oxide (NO) with therapeutically promising properties, affects colonic ion transport. Changes in short-circuit current (Isc) induced by the HNO donor Angeli's salt were recorded in Ussing chambers. Cytosolic Ca2+ concentration was measured with fura-2. The nitroxyl donor induced a concentration-dependent increase in Isc across rat distal colon which was due to a stimulation of chloride secretion. The secretion induced by Angeli's salt (5×10−4mol/l) was not altered by the NO scavenger 2-(4-carboxyphenyl)-4,5-dihydro-4,4,5,5-tetramethyl-1H-imidazolyl-1-oxy-3-oxide (carboxy-PTIO), but was abolished by the HNO scavenger l-cysteine. The response was not dependent on the activity of soluble guanylate cyclase or enteric neurons, but was inhibited by indomethacin. Experiments with apically permeabilized epithelia revealed the activation of basolateral K+ channels and a stimulation of the current carried by the basolateral Na+—K+-pump by Angeli's salt. The secretion induced by Angeli's salt was reduced in the absence of extracellular Ca2+. A prominent increase in the cytosolic Ca2+ concentration was evoked by Angeli's salt predominantly in subepithelial cells within the submucosa, which had the same dependence on extracellular Ca2+ as the Angeli's salt-induced Cl− secretion. Consequently, Angeli's salt induces a soluble guanylate cyclase-independent, Ca2+-dependent Cl− secretion via activation of the Na+—K+-ATPase and of basolateral K+ channels. Cyclooxygenase metabolites produced within the submucosa seem to be involved in this response.

Entacapone promotes cAMP-dependent colonic Cl− secretion in rats

Entacapone is a promising drug used widely for the treatment of Parkinson's disease (PD) as a catechol-O-methyl transferase (COMT) inhibitor. However, entacapone has gastrointestinal side effects. The aim of this study was to investigate the effects of entacapone on the epithelial ion transport in rat distal colon, and explore the underlying mechanism. The study was performed on freshly isolated colonic mucosa-only, submucosa-only and mucosa-submucosa preparations in rat. The short circuit current (I(SC) ) was measured to determine electrogenic ion transport, and a scanning ion-selective electrode technique (SIET) was used to directly measure Cl(-) flux across the epithelium. The content of intracellular cAMP was measured with radioimmunoassay (RIA). Entacapone increased mucosal I(SC) in the rat distal colon. I(SC) was inhibited significantly by apical addition of diphenylamine-2,2'-dicarboxylic acid (DPC), a blocker of the Cl(-) channel, basolateral application of bumetanide, an inhibitor of Na(+) -K(+) -2Cl(-) co-transporter (NKCC), removal of Cl(-) from the bathing solution, and pretreatment with MDL 12330A, an inhibitor of adenylate cyclase. Inhibiting endogenous prostaglandin (PG) synthesis with indomethacin, and eliminating submucosal enteric neural activity with tetrodotoxin (TTX)-inhibited entacapone-evoked I(SC) increases. Similar results were also obtained when Cl(-) flux was measured with SIET. Entacapone significantly increased intracellular cAMP content, which was greatly inhibited by either indomethacin or TTX in the tissues containing submucosal plexus, and by only indomethacin in the mucosa-only preparations. Entacapone stimulates cAMP-dependent Cl(-) secretion in the rat colon, and this process is regulated by endogenous PG and the submucosal enteric nervous system.

Evidence for down-regulation of neurogenic secretion in small intestinal epithelium from weaned piglets suffering from diarrhea

It is well known that active ion transport of the intestinal mucosa is regulated by the enteric nervous system (ENS) at least under physiological conditions. ENS control of the mucosa under pathophysiological situations such as diarrhea is less well studied. To address this question, we used a pig model of secretory diarrhea which is induced within hours in freshly weaned animals after oral treatment with enteropathogenic Escherichia coli Abbotstown (EcA). Two days after challenge jejunal segments were isolated and mucosa–submucosa preparations including neurons of the plexus submucosus were mounted in Ussing chambers. Net electrogenic transport of electrolytes was measured as short-circuit current (I sc) in the absence and presence of pharmacological stimulation. Neurogenic secretion was measured as I sc responses to electrical field stimulation. Basal I sc values indicated higher chloride secretion in EcA animals as in control piglets. EFS induced secretion was only half as high in EcA animals as in controls indicating down-regulation of neurogenic secretory pathways. This assumption was supported by the finding that maximal stimulation of Cl − secretion by forskolin and carbachol was nearly identical in both groups which means similar secretory capacities, but EFS induced I sc responses were significantly lower in EcA animals after forskolin and carbachol pre-stimulation. The results suggest that pro-secretory ENS pathways may be down-regulated in situations when secretory processes are markedly stressed, i.e. in acute secretory diarrhea.

The protein tyrosine kinase pathway is not involved in the regulation of K+ transport across the rat colon.

The protein tyrosine kinase inhibitor, genistein, is known to activate the cystic fibrosis transmembrane regulator (CFTR) Cl- channel and to inhibit K+ currents across the rat colonic epithelium. The aim of the present study is to answer the question whether these effects are involved in the regulation of transepithelial K+ transport. Therefore, the action of genistein on K+ transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake and efflux of Rb+ in mucosa-submucosa preparations. All effects of genistein (5 x 10(-5) mol L(-1)) were tested in the presence of a low concentration of forskolin (2 x 10(-7) mol L(-1)), because prestimulation of the cAMP pathway has been shown to be a prerequisite for a secretory action of genistein. Forskolin caused an increase in the serosa-to-mucosa flux of Rb+ (J(Rb)sm) thereby stimulating net K+ secretion in the proximal and distal colon. None of these effects was further enhanced after administration of genistein. Neither mucosal uptake of Rb+, representing mainly the activity of the H+-K+-ATPase in the distal colon, nor serosal Rb+ uptake, representing, e.g. the activity of the Na+-K+-2Cl- cotransporter, were affected by genistein. Also the efflux of Rb+ across the apical or the basolateral membrane, an indicator for the apical and basolateral K+ conductance, was unchanged in the presence of genistein. These results demonstrate that the K+ channels inhibited by genistein are not involved in transepithelial K+ transport.

Mechanisms of carbachol-induced alterations in K + transport across the rat colon

The effect of carbachol, an agonist of the Ca 2+ pathway, on K + transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake, and efflux of Rb +, a marker for K +, in mucosa–submucosa preparations. Unidirectional ion flux measurements revealed that carbachol stimulated K + secretion in the proximal colon by a marked increase in the serosa-to-mucosa flux ( J sm Rb) and a more moderate rise in the mucosa-to-serosa flux ( J ms Rb). In the distal colon carbachol had no effect on J ms Rb, but J sm Rb was reduced after a transient increase finally resulting in an inhibition of K + secretion. Carbachol caused a stimulation of mucosal Rb + uptake in the distal colon, which was diminished in the presence of inhibitors of the apical H +–K +-ATPase, vanadate and ouabain. In contrast, in the proximal colon the serosal Rb + uptake was enhanced by carbachol, an effect, which could be prevented by bumetanide, an inhibitor of the basolateral Na +–K +–2Cl −-cotransporter. Efflux experiments revealed that carbachol caused a transient increase of apical and basolateral Rb + permeability in both colonic segments. In the distal colon, stimulated K + efflux to the serosal side was reduced by quinine, efflux to the mucosal side was blocked by tetraethylammonium. In the proximal colon, carbachol-activated apical and basolateral K + efflux were inhibited by Ba 2+. In conclusion, these data suggest that in the distal colon carbachol stimulates the H +–K +-ATPase and the basolateral K + efflux through quinine-sensitive K + channels, whereas in the proximal colon carbachol induces K + secretion due to a stimulation of the basolateral Na +–K +–2Cl −-cotransporter and an increased efflux to the luminal side via Ba 2+-sensitive apical K + channels.

Regulation of Ion Transport in the Porcine Intestinal Tract by Enteric Neurotransmitters and Hormones

In the present paper, the mechanisms underlying the neural and hormonal regulation of mucosal ion transport in the pig intestinal tract are reviewed. The active transport of NaCl by isolated sheets of porcine intestinal mucosa is modulated by cholinergic and non-cholinergic neurons of undetermined neurochemical identity that lie in the submucosa. The application of electrical field stimulation to mucosa-submucosa preparations from porcine jejunum, ileum, or colon produces rapid elevations in short-circuit current which are inhibited by tetrodotoxin or omega-conotoxin GVIA, blockers of neuronal Na + and Ca 2+ channels, respectively. In porcine ileum, these elevations in current are mimicked in large part by cholinergic agonists and have been attributed to anion secretion. The majority of classical neurotransmitters and gut peptides that have been examined to date increase active transepithelial anion secretion through interactions with G protein-coupled receptors associated with submucosal neurons or situated on the basolateral membranes of epithelial cells. A small number of neuropeptides interact with neuronal receptors to augment NaCl absorption or decrease anion secretion. Noradrenergic control of intestinal transport differs in the porcine small and large intestines, and displays considerable inter-species variability in its cellular underpinnings. Transport regulation by bombesin-like peptides may be mediated by receptors distributed in both the apical and basolateral membrane domains of epithelial cells in porcine colon. The transport process affected by these peptides may be linked to epithelial growth and differentiation. The pig intestinal tract appears to be a useful biological model for resolving the cellular mechanisms by which gut neurotransmitters and hormones act in regulating transepithelial ion fluxes. Its general relevance to human intestinal function is discussed.