Prostaglandin-induced recovery of barrier function in porcine ileum is triggered by chloride secretion

Environmental stressors affect intestinal permeability and repair responses in a pig intestinal ischemia model

The pig is a powerful model for intestinal barrier studies, and it is important to carefully plan animal care and handling for optimal study design as psychological and physiological stressors significantly impact intestinal mucosal barrier function. Here, we report the effects of a period of environmental acclimation versus acute transport stress on mucosal barrier repair after intestinal ischemic injury. Jejunal ischemia was induced in young pigs which had been allowed to acclimate to a biomedical research housing environment or had been transported immediately prior to experimental injury (non-acclimated). Mucosa was then incubated ex vivo on Ussing chambers. In uninjured mucosa, there was no difference in transepithelial electrical resistance (TEER) or epithelial integrity between groups. However, acclimated pigs had increased macromolecular flux as compared to non-acclimated pigs during the first hour of ex vivo incubation. Ischemia induced greater epithelial loss in non-acclimated pigs as compared to acclimated pigs, yet this group achieved greater wound healing during recovery. Non-acclimated pigs had more robust TEER recovery ex vivo following injury versus acclimated pigs. The expression pattern of the tight junction protein claudin-4 was disrupted in acclimated pigs following recovery but showed enhanced localization to the apical membrane in non-acclimated pigs following recovery. Acute transport stress increases mucosal susceptibility to epithelial loss but also primes the tissue for a more robust barrier repair response. Alternatively, environmental acclimation increases leak pathway and diminishes barrier repair responses after ischemic injury.

Epithelial restitution defect in neonatal jejunum is rescued by juvenile mucosal homogenate in a pig model of intestinal ischemic injury and repair

Intestinal ischemic injury results sloughing of the mucosal epithelium leading to host sepsis and death unless the mucosal barrier is rapidly restored. Volvulus and neonatal necrotizing enterocolitis (NEC) in infants have been associated with intestinal ischemia, sepsis and high mortality rates. We have characterized intestinal ischemia/repair using a highly translatable porcine model in which juvenile (6-8-week-old) pigs completely and efficiently restore barrier function by way of rapid epithelial restitution and tight junction re-assembly. In contrast, separate studies showed that younger neonatal (2-week-old) pigs exhibited less robust recovery of barrier function, which may model an important cause of high mortality rates in human infants with ischemic intestinal disease. Therefore, we aimed to further refine our repair model and characterize defects in neonatal barrier repair. Here we examine the defect in neonatal mucosal repair that we hypothesize is associated with hypomaturity of the epithelial and subepithelial compartments. Following jejunal ischemia in neonatal and juvenile pigs, injured mucosa was stripped from seromuscular layers and recovered ex vivo while monitoring transepithelial electrical resistance (TEER) and ³H-mannitol flux as measures of barrier function. While ischemia-injured juvenile mucosa restored TEER above control levels, reduced flux over the recovery period and showed 93±4.7% wound closure, neonates exhibited no change in TEER, increased flux, and a 11±23.3% increase in epithelial wound size. Scanning electron microscopy revealed enterocytes at the wound margins of neonates failed to assume the restituting phenotype seen in restituting enterocytes of juveniles. To attempt rescue of injured neonatal mucosa, neonatal experiments were repeated with the addition of exogenous prostaglandins during ex vivo recovery, ex vivo recovery with full thickness intestine, in vivo recovery and direct application of injured mucosal homogenate from neonates or juveniles. Neither exogenous prostaglandins, intact seromuscular intestinal layers, nor in vivo recovery enhanced TEER or restitution in ischemia-injured neonatal mucosa. However, ex vivo exogenous application of injured juvenile mucosal homogenate produced a significant increase in TEER and enhanced histological restitution to 80±4.4% epithelial coverage in injured neonatal mucosa. Thus, neonatal mucosal repair can be rescued through direct contact with the cellular and non-cellular milieu of ischemia-injured mucosa from juvenile pigs. These findings support the hypothesis that a defect in mucosal repair in neonates is due to immature repair mechanisms within the mucosal compartment. Future studies to identify and rescue specific defects in neonatal intestinal repair mechanisms will drive development of novel clinical interventions to reduce mortality in infants affected by intestinal ischemic injury.

Hypoxia-induced intestinal barrier changes in balloon-assisted enteroscopy

KEY POINTS

Balloon-assisted enteroscopy (BAE) is an emerging standard procedure by utilizing distensible balloons to facilitate deep endoscopy in the small and large intestine. Sporadic cases of bacteraemia were found after BAE. Balloon distension by BAE caused gut tissue hypoxia. The impact of balloon distension-induced hypoxia on intestinal barriers remains unclear. Murine models of BAE by colonic balloon distension showed that short- and long-term hypoxia evoked opposite effects on epithelial tight junctions (TJs). Short-term hypoxia fortified TJ integrity, whereas long-term hypoxia caused damage to barrier function. Our data showed for the first time the molecular mechanisms and signalling pathways of epithelial barrier fortification and TJ reorganization by short-term hypoxia for the maintenance of gut homeostasis. The findings suggest avoiding prolonged balloon distension during BAE to reduce the risk of hypoxia-induced gut barrier dysfunction.

ABSTRACT

Balloon-assisted enteroscopy (BAE) is an emerging standard procedure that uses distensible balloons to facilitate deep endoscopy. Intestines are known to harbour an abundant microflora. Whether balloon distension causes perturbation of blood flow and gut barrier dysfunction, and elicits risk of bacterial translocation remains unknown. Our aims were to (1) conduct a prospective study to gather microbiological and molecular evidence of bacterial translocation by BAE in patients, (2) establish a murine model of colonic balloon distension to investigate tissue hypoxia and intestinal barrier, and (3) assess the effect of short- and long-term hypoxia on epithelial permeability using cell lines. Thirteen patients were enrolled for BAE procedures, and blood samples were obtained before and after BAE for paired comparison. Four of the 13 patients (30.8%) had positive bacterial DNA in blood after BAE. Post-BAE endotoxaemia was higher than the pre-BAE level. Nevertheless, no clinical symptom of sepsis or fever was reported. To mimic clinical BAE, mice were subjected to colonic balloon distension. Local tissue hypoxia was observed during balloon inflation, and reoxygenation after deflation. A trend of increased gut permeability was seen after long-term distension, whereas a significant reduction of permeability was observed by short-term distension in the proximal colon. Human colonic epithelial Caco-2 cells exposed to hypoxia for 5-20 min exhibited increased tight junctional assembly, while those exposed to longer hypoxia displayed barrier disruption. In conclusion, sporadic cases of bacteraemia were found after BAE, without septic symptoms. Short-term hypoxia by balloon distension yielded a protective effect whereas long-term hypoxia caused damage to the gut barrier.

ClC-2 regulation of intestinal barrier function: Translation of basic science to therapeutic target

The ClC-2 chloride channel is a member of the voltage-gated chloride channel family. ClC-2 is involved in various physiological processes, including fluid transport and secretion, regulation of cell volume and pH, maintaining the membrane potential of the cell, cell-to-cell communication, and tissue homeostasis. Recently, our laboratory has accumulated evidence indicating a critical role of ClC-2 in the regulation of intestinal barrier function by altering inter-epithelial tight junction composition. This review will detail the role of ClC-2 in intestinal barrier function during intestinal disorders, including experimental ischemia/reperfusion injury and dextran sodium sulfate (DSS)-induced inflammatory bowel disease. Details of pharmacological manipulation of ClC-2 via prostone agonists will also be provided in an effort to show the potential therapeutic relevance of ClC-2 regulation, particularly during intestinal barrier disruption.

Fatty acids, inflammation and intestinal health in pigs

The intestine is not only critical for nutrient digestion and absorption, but also is the largest immune organ in the body. However, in pig production, inflammation induced by numerous factors, such as pathogen infection and stresses (e.g., weaning), results in intestinal mucosal injury and dysfunction, and consequently results in poor growth of pigs. Dietary fatty acids not only play critical roles in energy homeostasis and cellular membrane composition, but also exert potent effects on intestinal development, immune function, and inflammatory response. Recent studies support potential therapeutic roles for specific fatty acids (short chain and medium chain fatty acids and long chain polyunsaturated fatty acids) in intestinal inflammation of pigs. Results of these new lines of work indicate trophic and cytoprotective effects of fatty acids on intestinal integrity in pigs. In this article, we review the effect of inflammation on intestinal structure and function, and the role of specific fatty acids on intestinal health of pigs, especially under inflammatory conditions.

Porcine models of digestive disease: the future of large animal translational research

There is increasing interest in nonrodent translational models for the study of human disease. The pig, in particular, serves as a useful animal model for the study of pathophysiological conditions relevant to the human intestine. This review assesses currently used porcine models of gastrointestinal physiology and disease and provides a rationale for the use of these models for future translational studies. The pig has proven its utility for the study of fundamental disease conditions such as ischemia-reperfusion injury, stress-induced intestinal dysfunction, and short bowel syndrome. Pigs have also shown great promise for the study of intestinal barrier function, surgical tissue manipulation and intervention, as well as biomaterial implantation and tissue transplantation. Advantages of pig models highlighted by these studies include the physiological similarity to human intestine and mechanisms of human disease. Emerging future directions for porcine models of human disease include the fields of transgenics and stem cell biology, with exciting implications for regenerative medicine.

Influence of dietary supplementation with flaxseed and lactobacilli on the mucosal morphology and proliferative cell rate in the jejunal mucosa of piglets after weaning

The aim of the study was to investigate the influence of flaxseed and lactobacilli supplementation to the diet of piglets during the time period between 10 days before and 21 days after weaning. The morphometry of the jejunal mucosa and proliferative ratio of both epithelial and lamina propria cells were compared with those found in a group of piglets fed with the usual diet added with sunflower oil during the same time period. The addition of flaxseed oil to the diet significantly increased the crypt depth in comparison with both groups supplemented with sunflower (P < 0.05 and 0.001 respectively) on the weaning day. Moreover, the flaxseed addition caused a significant decrease in villus height (P < 0.01) and crypt depth (P < 0.01) 21 days postweaning in comparison with the sunflower group. The proliferative ratio of the epithelial cells in the sunflower group on the weaning day was significantly higher than in both flaxseed groups (P < 0.01). Paradoxically, significantly higher proliferative activity in the mucosal connective tissue in the group with flaxseed supplementation in comparison with the sunflower group was observed on the day of weaning, as well as 3 days later (P < 0.05 both). A combination of flaxseed with lactobacilli showed significantly lower proliferative activity in the connective tissue cells from weaning up to 7 days after weaning (P < 0.05 all) in comparison with the flaxseed group.

Animal models of ischemia-reperfusion-induced intestinal injury: progress and promise for translational research

Research in the field of ischemia-reperfusion injury continues to be plagued by the inability to translate research findings to clinically useful therapies. This may in part relate to the complexity of disease processes that result in intestinal ischemia but may also result from inappropriate research model selection. Research animal models have been integral to the study of ischemia-reperfusion-induced intestinal injury. However, the clinical conditions that compromise intestinal blood flow in clinical patients ranges widely from primary intestinal disease to processes secondary to distant organ failure and generalized systemic disease. Thus models that closely resemble human pathology in clinical conditions as disparate as volvulus, shock, and necrotizing enterocolitis are likely to give the greatest opportunity to understand mechanisms of ischemia that may ultimately translate to patient care. Furthermore, conditions that result in varying levels of ischemia may be further complicated by the reperfusion of blood to tissues that, in some cases, further exacerbates injury. This review assesses animal models of ischemia-reperfusion injury as well as the knowledge that has been derived from each to aid selection of appropriate research models. In addition, a discussion of the future of intestinal ischemia-reperfusion research is provided to place some context on the areas likely to provide the greatest benefit from continued research of ischemia-reperfusion injury.

Ex vivo effect of gold nanoparticles on porcine synovial membrane

Gold nanoparticles (AuNPs) have great potential as carriers for local drug delivery and as a primary therapeutic for treatment of inflammation. Here we report on the AuNP-synovium interaction in an ex vivo model of intra-articular application for treatment of joint inflammation. Sheets of porcine femoropatellar synovium were obtained post mortem and each side of the tissue samples was maintained in a separate fluid environment. Permeability to AuNPs of different sizes (5-52 nm) and biomarker levels of inflammation were determined to characterize the ex vivo particle interaction with the synovium. Lipopolysaccharide or recombinant human interleukin-1β were added to fluid environments to assess the ex vivo effect of pro-inflammatory factors on permeability and biomarker levels. The synovium showed size selective permeability with only 5 nm AuNPs effectively permeating the entire tissues' width. This process was further governed by particle stability in the fluid environment. AuNPs reduced matrix metalloproteinase and lactate dehydrogenase activity and hyaluronic acid concentrations but had no effect on prostaglandin E2 levels. Exposure to pro-inflammatory factors did not significantly affect AuNP permeation or biomarker levels in this model. Results with ex vivo tissue modeling of porcine synovium support an anti-inflammatory effect of AuNPs warranting further investigation.

Diets high in heat-treated soybean meal reduce the histamine-induced epithelial response in the colon of weaned piglets and increase epithelial catabolism of histamine

We examined the influence of dietary fermentable protein (fCP) and fermentable carbohydrates (fCHO) on the colonic epithelial response to histamine in pigs. Thirty-two weaned piglets were fed 4 diets in a 2 × 2 factorial design with low fCP/low fCHO, low fCP/high fCHO, high fCP/low fCHO and high fCP/high fCHO. After 21-23 days, the pigs were killed and tissue from the proximal colon was stimulated with carbachol, histamine, PGE2 or sodium hydrogen sulphide in Ussing chambers. Changes in short-circuit current and tissue conductance were measured. Diamine oxidase, histamine N-methyltransferase, stem cell growth factor receptor, Fc-epsilon receptor I and cystic fibrosis transmembrane conductance regulator gene expression was determined. Activities of diamine oxidase and histamine N-methyltransferase and numbers of colonic mast cells were measured. The change in the short-circuit current in response to histamine was lower (P = 0.002) and tended to be lower for PGE2 (P = 0.053) in high fCP groups compared to low fCP groups, irrespective of fCHO. Additionally, the change in tissue conductance after the application of histamine was lower (P = 0.005) in the high fCP groups. The expression of histamine N-methyltransferase mRNA (P = 0.033) and the activities of diamine oxidase (P = 0.001) and histamine N-methyltransferase (P = 0.006) were higher with high fCP in comparison with low fCP. The expression of mast cell markers, stem cell growth factor receptor (P = 0.005) and Fc-epsilon receptor I (P = 0.049) was higher with high fCP diets compared to diets low in fCP, whereas the mast cell count did not differ between groups. The expression of the cystic fibrosis transmembrane conductance regulator was reduced (P = 0.001) with high fCP diets compared to low fCP diets. The lower epithelial response to histamine and PGE2 and elevated epithelial histamine inactivation suggests an adaptation to high fCP diets.

Influence of fermentable carbohydrates or protein on large intestinal and urinary metabolomic profiles in piglets

It was recently shown that variations in the ratio of dietary fermentable carbohydrates (fCHO) and fermentable protein (fCP) differentially affect large intestinal microbial ecology and the mucosal response. Here we investigated the use of mass spectrometry to profile changes in metabolite composition in colon and urine associated with variation in dietary fCHO and fCP composition and mucosal physiology. Thirty-two weaned piglets were fed 4 diets in a 2 × 2 factorial design with low fCP and low fCHO, low fCP and high fCHO, high fCP and low fCHO, and high fCP and high fCHO. After 21 to 23 d, all pigs were euthanized and colon digesta and urine metabolite profiles were obtained by mass spectrometry. Analysis of mass spectra by partial least squares approach indicated a clustering of both colonic and urinary profiles for each pig by feeding group. Metabolite identification and annotation using the Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathways revealed increased abundance of metabolites associated with arachidonic acid metabolism in colon of pigs fed a high concentration of fCP irrespective of dietary fCHO. Urinary metabolites did not show as clear patterns. Mass spectrometry can effectively differentiate metabolite profiles in colon contents and urine associated with changes in dietary composition. Whether metabolite profiling is an effective tool to identify specific metabolites (biomarkers) or metabolite profiles associated with gut function and integrity needs further elucidation.

Cell lineage identification and stem cell culture in a porcine model for the study of intestinal epithelial regeneration

Significant advances in intestinal stem cell biology have been made in murine models; however, anatomical and physiological differences between mice and humans limit mice as a translational model for stem cell based research. The pig has been an effective translational model, and represents a candidate species to study intestinal epithelial stem cell (IESC) driven regeneration. The lack of validated reagents and epithelial culture methods is an obstacle to investigating IESC driven regeneration in a pig model. In this study, antibodies against Epithelial Adhesion Molecule 1 (EpCAM) and Villin marked cells of epithelial origin. Antibodies against Proliferative Cell Nuclear Antigen (PCNA), Minichromosome Maintenance Complex 2 (MCM2), Bromodeoxyuridine (BrdU) and phosphorylated Histone H3 (pH3) distinguished proliferating cells at various stages of the cell cycle. SOX9, localized to the stem/progenitor cells zone, while HOPX was restricted to the +4/‘reserve’ stem cell zone. Immunostaining also identified major differentiated lineages. Goblet cells were identified by Mucin 2 (MUC2); enteroendocrine cells by Chromogranin A (CGA), Gastrin and Somatostatin; and absorptive enterocytes by carbonic anhydrase II (CAII) and sucrase isomaltase (SIM). Transmission electron microscopy demonstrated morphologic and sub-cellular characteristics of stem cell and differentiated intestinal epithelial cell types. Quantitative PCR gene expression analysis enabled identification of stem/progenitor cells, post mitotic cell lineages, and important growth and differentiation pathways. Additionally, a method for long-term culture of porcine crypts was developed. Biomarker characterization and development of IESC culture in the porcine model represents a foundation for translational studies of IESC-driven regeneration of the intestinal epithelium in physiology and disease.

Nutritional factors influencing intestinal health of the neonate

Dietary nutrients are essential for gastrointestinal (GI) growth and function, and nutritional support of GI growth and development is a significant component of infant care. For healthy full-term neonates, nutritional provisions of the mother's milk and/or formula will support normal maturation of structure and function of the GI tract in most infants. The composition of breast milk affects GI barrier function and development of a competent mucosal immune system. The functional nutrients and other bioactive components of milk support a microenvironment for gut protection and maturation. However, premature infants struggle with feeding tolerance impairing normal GI function, leading to intestinal dysfunction and even death. The high prevalence worldwide of enteric diseases and dysfunction in neonates has led to much interest in understanding the role of nutrients and food components in the establishment and maintenance of a functioning GI tract. Neonates who do not receive enteral feeding as either mother's milk or formula are supported by total parental nutrition (TPN). The lack of enteral nutrition can compound intestinal dysfunction, leading to high morbidity and mortality in intestinally compromised infants. Reciprocally, enteral stimulation of an immature GI tract can also compound intestinal dysfunction. Therefore, further understanding of nutrient interactions with the mucosa is necessary to define nutritional requirements of the developing GI tract to minimize intestinal complications and infant morbidity. Piglet models of intestinal development and function are similar to humans, and this review summarizes recent findings regarding nutrient requirements for growth and maintenance of intestinal health. In particular, this article reviews the role of specific amino acids (arginine, glutamine, glutamate, and threonine), fatty acids (long chain polyunsaturated, medium chain, and short chain), various prebiotic carbohydrates (short-chain fructo-oligosaccharide, fructo--oligosaccharide, lacto-N-neotetraose, human milk oligosaccharide, polydextrose, and galacto-oligosaccharide), and probiotics that have been examined in the suckling piglet model of intestinal health.

Dietary long-chain PUFA enhance acute repair of ischemia-injured intestine of suckling pigs

Infant formula companies have been fortifying formulas with long-chain PUFA for 10 y. Long-chain PUFA are precursors of prostanoids, which stimulate recovery of intestinal barrier function. Supplementation of milk with PUFA increases the content of arachidonic acid (ARA) in enterocyte membranes; however, the effect of this enrichment on intestinal repair is not known. The objective of these experiments was to investigate the effect of supplemental ARA on intestinal barrier repair in ischemia-injured porcine ileum. One-day-old pigs (n = 24) were fed a milk-based formula for 10 d. Diets contained no PUFA (0% ARA), 0.5% ARA, 5% ARA, or 5% EPA of total fatty acids. Following dietary enrichment, ilea were subjected to in vivo ischemic injury by clamping the local mesenteric blood supply for 45 min. Following the ischemic period, control (nonischemic) and ischemic loops were mounted on Ussing chambers. Transepithelial electrical resistance (TER) was measured over a 240-min recovery period. Ischemia-injured ileum from piglets fed 5% ARA (61.0 ± 14%) exhibited enhanced recovery compared with 0% ARA (16 ± 14) and 0.5% ARA (22.1 ± 14)-fed pigs. Additionally, ischemia-injured ileum from 5% EPA (51.3 ± 14)-fed pigs had enhanced recovery compared with 0% ARA-fed pigs (P < 0.05). The enhanced TER recovery response observed with ischemia-injured 5% ARA supplementation was supported by a significant reduction in mucosal-to-serosal flux of (3)H-mannitol and (14)C-inulin compared with all other ischemia-injured dietary groups (P < 0.05). A histological evaluation of ischemic ilea from piglets fed the 5% ARA showed reduced histological lesions after ischemia compared with the other dietary groups (P < 0.05). These data demonstrate that feeding elevated levels of long-chain PUFA enhances acute recovery of ischemia-injured porcine ileum.

Mucosal injury and inflammatory cells in response to brief ischaemia and reperfusion in the equine large colon

REASON FOR PERFORMING STUDY

Intestinal ischaemia and reperfusion (I/R) can activate inflammatory cells in the equine colon, although effects on different types of inflammatory cells have received little attention.

OBJECTIVES

To assess early mucosal injury, the reaction of mucosal neutrophils, eosinophils, mast cells and macrophages, and cyclooxygenase (COX)-1 and -2 expression in response to I/R in the equine large colon.

METHODS

Large colon ischaemia was induced for 1 h (1hI) followed by 4 h of reperfusion in 6 horses, and mucosal biopsies were sampled before and after ischaemia, and after 1, 2 and 4 h of reperfusion. Semithin sections (500 nm) of epon-embedded biopsies were stained with toluidine blue for histomorphometric evaluation. The number and distribution of mucosal macrophages (CD163), neutrophils (calprotectin), eosinophils (LUNA) and mast cells (toluidine blue) were determined, and mucosal COX-1 and -2 expression was identified.

RESULTS

Ischaemia caused epithelial cell and nuclear swelling (mean ± s.e. nuclear width; control: 2.7 ± 0.2 µm vs. 1hI: 4.2 ± 0.2 µm; P<0.01), subepithelial oedema (control: 0.2 ± 0.1 µm vs. 1hI: 3.2 ± 0.2 µm; P<0.01) and increased epithelial apoptosis (control: 14.3 ± 4.1 apoptotic cells/mm mucosa vs. 1hI: 60.4 ± 14.0 apoptotic cells/mm mucosa; P<0.01). COX-2 expression (P<0.01) was evident after ischaemia. Reperfusion caused paracellular fluid accumulation (control: 0.9 ± 0.1 µm vs. 1hI: 0.6 ± 0.6 µm vs. 1hI + 4hR: 1.6 ± 0.2 µm; P<0.05). Epithelial repair started at 1 h of reperfusion (P<0.001), followed by migration of neutrophils into the mucosa after 2 h (control: 72.3 ± 18.4 cells/mm(2) mucosa vs. 1hI + 2hR: 1149.9 ± 220.6 cells/mm(2) mucosa; P<0.01). Mucosal eosinophils, mast cells and macrophages did not increase in numbers but were activated.

CONCLUSIONS

Epithelial injury and COX-2 expression caused by short-term hypoxia were followed by intense inflammation associated with epithelial repair during reperfusion.

POTENTIAL RELEVANCE

Equine colonic mucosa subjected to a brief period of ischaemia can repair during reperfusion, despite increased mucosal inflammation.

Astrovirus infection induces sodium malabsorption and redistributes sodium hydrogen exchanger expression

Astroviruses are known to be a leading cause of diarrhea in infants and the immunocompromised; however, our understanding of this endemic pathogen is limited. Histological analyses of astrovirus pathogenesis demonstrate clinical disease is not associated with changes to intestinal architecture, inflammation, or cell death. Recent studies in vitro have suggested that astroviruses induce actin rearrangement leading to loss of barrier function. The current study used the type-2 turkey astrovirus (TAstV-2) and turkey poult model of astrovirus disease to examine how astrovirus infection affects the ultrastructure and electrophysiology of the intestinal epithelium. These data demonstrate that infection results in changes to the epithelial ultrastructure, rearrangement of F-actin, decreased absorption of sodium, as well as redistribution of the sodium/hydrogen exchanger 3 (NHE3) from the membrane to the cytoplasm. Collectively, these data suggest astrovirus infection induces sodium malabsorption, possibly through redistribution of specific sodium transporters, which results in the development of an osmotic diarrhea.

Effects of cyclooxygenase inhibitors flunixin and deracoxib on permeability of ischaemic-injured equine jejunum

REASONS FOR PERFORMING STUDY

Recent studies have shown that flunixin prevented recovery of equine jejunum post ischaemia. However, the use of a purported cyclooxygenase (COX)-2 preferential inhibitor, etodolac, also prevented recovery. These findings may have implications for the use of nonsteroidal anti-inflammatory drugs in colic patients.

OBJECTIVE

To compare the effects of deracoxib, a highly selective canine COX-2 inhibitor, with flunixin on in vitro recovery of ischaemic-injured equine jejunum.

METHODS

Six horses underwent 2 h jejunal ischaemia, after which mucosa was mounted in Ussing chambers and recovered for 240 mins. Transepithelial electrical resistance (TER) and mucosal-to-serosal fluxes of 3H-mannitol were monitored as indices of barrier function in the presence of flunixin or deracoxib.

RESULTS

The TER of ischaemic-injured tissue recovered significantly over 240 mins in the presence of no treatment, but not in the presence of flunixin or deracoxib. In addition, flunixin-treated ischaemic jejunum was significantly more permeable to mannitol when compared with untreated tissue by the end of the recovery period, whereas deracoxib treatment did not increase permeability. Addition of the PGE1 analogue misoprostol to flunixin-treated tissue restored recovery of TER.

CONCLUSIONS AND POTENTIAL RELEVANCE

Treatment of horses with ischaemic jejunal disease with flunixin may result in a prolonged permeability defect in recovering mucosa. Addition of misoprostol or replacement of flunixin with deracoxib may ameliorate effects of COX inhibitors on recovering mucosa.

Comparison of the chloride channel activator lubiprostone and the oral laxative Polyethylene Glycol 3350 on mucosal barrier repair in ischemic-injured porcine intestine

AIM: To investigate the effects of lubiprostone and Polyethylene Glycol 3350 (PEG) on mucosal barrier repair in ischemic-injured porcine intestine.

METHODS: Ileum from 6 piglets (approximately 15 kg body weight) was subjected to ischemic conditions by occluding the local mesenteric circulation for 45 min in vivo. Ileal tissues from each pig were then harvested and mounted in Ussing chambers and bathed in oxygenated Ringer’s solution in vitro. Intestinal barrier function was assessed by measuring transepithelial electrical resistance (TER) and mucosal-to-serosal fluxes of ³H-mannitol and ¹⁴C-inulin. Statistical analyses of data collected over a 120-min time course included 2-way ANOVA for the effects of time and treatment on indices of barrier function.

RESULTS: Application of 1 μmol/L lubiprostone to the mucosal surface of ischemic-injured ileum in vitro induced significant elevations in TER compared to non-treated tissue. Lubiprostone also reduced mucosal-to-serosal fluxes of ³H-mannitol and ¹⁴C-inulin. Alternatively, application of a polyethylene laxative (PEG, 20 mmol/L) to the mucosal surface of ischemic tissues significantly increased flux of ³H-mannitol and ¹⁴C-inulin.

CONCLUSION: This experiment demonstrates that lubiprostone stimulates recovery of barrier function in ischemic intestinal tissues whereas the PEG laxative had deleterious effects on mucosal repair. These results suggest that, unlike osmotic laxatives, lubiprostone stimulates repair of the injured intestinal barrier.

Mice lacking the Na+/H+ exchanger 2 have impaired recovery of intestinal barrier function

Ischemic injury induces breakdown of the intestinal barrier. Recent studies in porcine postischemic tissues indicate that inhibition of NHE2 results in enhanced recovery of barrier function in vitro via a process involving interepithelial tight junctions. To further study this process, recovery of barrier function was assessed in wild-type (NHE2(+/+)) and NHE2(-/-) mice in vivo and wild-type mice in vitro. Mice were subjected to complete mesenteric ischemia in vivo, after which barrier function was measured by blood-to-lumen mannitol clearance over a 3-h recovery period or measurement of transepithelial electrical resistance (TER) in Ussing chambers immediately following ischemia. Tissues were assessed for expression of select junctional proteins. Compared with NHE2(+/+) mice, NHE2(-/-) mice had greater intestinal permeability during the postischemic recovery process. In contrast to prior porcine studies, pharmacological inhibition of NHE2 in postischemic tissues from wild-type mice also resulted in significant reductions in TER. Mucosa from NHE2(-/-) mice displayed a shift of occludin and claudin-1 expression to the Triton-X-soluble membrane fractions and showed disruption of occludin and claudin-1 localization patterns following injury. This was qualitatively and quantitatively recovered in NHE2(+/+) mice compared with NHE2(-/-) mice by the end of the 3-h recovery period. Serine phosphorylation of occludin and claudin-1 was downregulated in NHE2(-/-) postischemia compared with wild-type mice. These data indicate an important role for NHE2 in recovery of barrier function in mice via a mechanism involving tight junctions.

Restoration of barrier function in injured intestinal mucosa

Mucosal repair is a complex event that immediately follows acute injury induced by ischemia and noxious luminal contents such as bile. In the small intestine, villous contraction is the initial phase of repair and is initiated by myofibroblasts that reside immediately beneath the epithelial basement membrane. Subsequent events include crawling of healthy epithelium adjacent to the wound, referred to as restitution. This is a highly regulated event involving signaling via basement membrane integrins by molecules such as focal adhesion kinase and growth factors. Interestingly, however, ex vivo studies of mammalian small intestine have revealed the importance of closure of the interepithelial tight junctions and the paracellular space. The critical role of tight junction closure is underscored by the prominent contribution of the paracellular space to measures of barrier function such as transepithelial electrical resistance. Additional roles are played by subepithelial cell populations, including neutrophils, related to their role in innate immunity. The net result of reparative mechanisms is remarkably rapid closure of mucosal wounds in mammalian tissues to prevent the onset of sepsis.

Recovery of mucosal barrier function in ischemic porcine ileum and colon is stimulated by a novel agonist of the ClC-2 chloride channel, lubiprostone

Previous studies utilizing an ex vivo porcine model of intestinal ischemic injury demonstrated that prostaglandin (PG)E(2) stimulates repair of mucosal barrier function via a mechanism involving Cl(-) secretion and reductions in paracellular permeability. Further experiments revealed that the signaling mechanism for PGE(2)-induced mucosal recovery was mediated via type-2 Cl(-) channels (ClC-2). Therefore, the objective of the present study was to directly investigate the role of ClC-2 in mucosal repair by evaluating mucosal recovery in ischemia-injured intestinal mucosa treated with the selective ClC-2 agonist lubiprostone. Ischemia-injured porcine ileal mucosa was mounted in Ussing chambers, and short-circuit current (I(sc)) and transepithelial electrical resistance (TER) were measured in response to lubiprostone. Application of 0.01-1 microM lubiprostone to ischemia-injured mucosa induced concentration-dependent increases in TER, with 1 microM lubiprostone stimulating a twofold increase in TER (DeltaTER = 26 Omega.cm(2); P < 0.01). However, lubiprostone (1 microM) stimulated higher elevations in TER despite lower I(sc) responses compared with the nonselective secretory agonist PGE(2) (1 microM). Furthermore, lubiprostone significantly (P < 0.05) reduced mucosal-to-serosal fluxes of (3)H-labeled mannitol to levels comparable to those of normal control tissues and restored occludin localization to tight junctions. Activation of ClC-2 with the selective agonist lubiprostone stimulated elevations in TER and reductions in mannitol flux in ischemia-injured intestine associated with structural changes in tight junctions. Prostones such as lubiprostone may provide a selective and novel pharmacological mechanism of accelerating recovery of acutely injured intestine compared with the nonselective action of prostaglandins such as PGE(2).

Prostaglandin-mediated inhibition of Na+/H+ exchanger isoform 2 stimulates recovery of barrier function in ischemia-injured intestine

Prostaglandins stimulate repair of the ischemia-injured intestinal barrier in the porcine ileum through a mechanism involving cAMP-dependent Cl- secretion and inhibition of electroneutral Na+/H+ exchanger (NHE) activity. In the present study, we focused on the role of individual NHE isoforms in the recovery of barrier function. Ischemia-injured porcine ileal mucosa was mounted on Ussing chambers. Short-circuit current (I(sc)), transepithelial electrical resistance (TER), and isotopic fluxes of 22Na were measured in response to PGE2 and selective inhibitors of epithelial NHE isoforms. Immunoassays were used to assess the expression of NHE isoforms. Forty-five minutes of intestinal ischemia resulted in a 45% reduction in TER (P < 0.01). Near-complete restitution occurred within 60 min. Inhibition of NHE2 with HOE-694 (25 microM) added to the mucosal surface of the injured ileum stimulated significant elevations in TER, independent of changes in I(sc) and histological evidence of restitution. Pharmacological inhibition of NHE3 or NHE1 with mucosal S-3226 (20 microM) or serosal cariporide (25 microM), respectively, had no effect. Ischemia-injured tissues treated with mucosal S-3226 or HOE-694 exhibited equivalent reductions in mucosal-to-serosal fluxes of 22Na+ (by approximately 35%) compared with nontreated ischemia-injured control tissues (P < 0.05). Intestinal ischemia resulted in increased expression of the cytoplasmic NHE regulatory factor EBP50 in NHE2 but not in NHE3 immunoprecipitates. Selective inhibition of NHE2, and not NHE3, induces recovery of barrier function in the ischemia-injured intestine.

Electroneutral sodium absorption and electrogenic anion secretion across murine small intestine are regulated in parallel

Electrolyte transport processes of small intestinal epithelia maintain a balance between hydration of the luminal contents and systemic fluid homeostasis. Under basal conditions, electroneutral Na(+) absorption mediated by Na(+)/H(+) exchanger 3 (NHE3) predominates; under stimulated conditions, increased anion secretion mediated by CFTR occurs concurrently with inhibition of Na(+) absorption. Homeostatic adjustments to diseases that chronically affect the activity of one transporter (e.g., cystic fibrosis) may include adaptations in the opposing transport process to prevent enterosystemic fluid imbalance. To test this hypothesis, we measured electrogenic anion secretion (indexed by the short-circuit current) across NHE3-null [NHE3(-)] murine small intestine and electroneutral Na(+) absorption (by radioisotopic flux analysis) across small intestine of mice with gene-targeted disruptions of the anion secretory pathway, i.e., CFTR-null [CFTR(-)] or Na(+)-K(+)-2Cl(-) cotransporter-null [NKCC1(-)]. Protein expression of NHE3 and CFTR in the intestinal epithelia was measured by immunoblotting. In NHE3(-), compared with wild-type small intestine, maximal and bumetanide-sensitive anion secretion following cAMP stimulation was significantly reduced, and there was a corresponding decrease in CFTR protein expression. In CFTR(-) and NKCC1(-) intestine, Na(+) absorption was significantly reduced compared with wild-type. NHE3 protein expression was decreased in the CFTR(-) intestine but was unchanged in the NKCC1(-) intestine, indicating that factors independent of expression also downregulate NHE3 activity. Together, these data support the concept that absorptive and secretory processes determining NaCl and water movement across the intestinal epithelium are regulated in parallel to maintain balance between the systemic fluid volume and hydration of the luminal contents.

Nitric oxide synthase stimulates prostaglandin synthesis and barrier function in C. parvum-infected porcine ileum

Cell culture models implicate increased nitric oxide (NO) synthesis as a cause of mucosal hyperpermeability in intestinal epithelial infection. NO may also mediate a multitude of subepithelial events, including activation of cyclooxygenases. We examined whether NO promotes barrier function via prostaglandin synthesis using Cryptosporidium parvum-infected ileal epithelium in residence with an intact submucosa. Expression of NO synthase (NOS) isoforms was examined by real-time RT-PCR of ileal mucosa from control and C. parvum-infected piglets. The isoforms mediating and mechanism of NO action on barrier function were assessed by measuring transepithelial resistance (TER) and eicosanoid synthesis by ileal mucosa mounted in Ussing chambers in the presence of selective and nonselective NOS inhibitors and after rescue with exogenous prostaglandins. C. parvum infection results in induction of mucosal inducible NOS (iNOS), increased synthesis of NO and PGE2, and increased mucosal permeability. Nonselective inhibition of NOS (NG-nitro-L-arginine methyl ester) inhibited prostaglandin synthesis, resulting in further increases in paracellular permeability. Baseline permeability was restored in the absence of NO by exogenous PGE2. Selective inhibition of iNOS [L-N6-(1-iminoethyl)-L-lysine] accounted for approximately 50% of NOS-dependent PGE2 synthesis and TER. Using an entire intestinal mucosa, we have demonstrated for the first time that NO serves as a proximal mediator of PGE2 synthesis and barrier function in C. parvum infection. Expression of iNOS by infected mucosa was without detriment to overall barrier function and may serve to promote clearance of infected enterocytes.

Physiological concentrations of bile salts inhibit recovery of ischemic-injured porcine ileum

We have previously shown rapid in vitro recovery of barrier function in porcine ischemic-injured ileal mucosa, attributable principally to reductions in paracellular permeability. However, these experiments did not take into account the effects of luminal contents, such as bile salts. Therefore, the objective of this study was to evaluate the role of physiological concentrations of deoxycholic acid in recovery of mucosal barrier function. Porcine ileum was subjected to 45 min of ischemia, after which mucosa was mounted in Ussing chambers and exposed to varying concentrations of deoxycholic acid. The ischemic episode resulted in significant reductions in transepithelial electrical resistance (TER), which recovered to control levels of TER within 120 min, associated with significant reductions in mucosal-to-serosal (3)H-labeled mannitol flux. However, treatment of ischemic-injured tissues with 10(-5) M deoxycholic acid significantly inhibited recovery of TER with significant increases in mucosal-to-serosal (3)H-labeled mannitol flux, whereas 10(-6) M deoxycholic acid had no effect. Histological evaluation at 120 min revealed complete restitution regardless of treatment, indicating that the breakdown in barrier function was due to changes in paracellular permeability. Similar effects were noted with the application of 10(-5) M taurodeoxycholic acid, and the effects of deoxycholic acid were reversed with application of the Ca(2+)-mobilizing agent thapsigargin. Deoxycholic acid at physiological concentrations significantly impairs recovery of epithelial barrier function by an effect on paracellular pathways, and these effects appear to be Ca(2+) dependent.

Neutrophils augment recovery of porcine ischemia-injured ileal mucosa by an IL-1beta- and COX-2-dependent mechanism

Polymorphonuclear neutrophils (PMNs) play a critical role in intestinal mucosal injury and repair. To study effects of PMNs on acutely injured mucosa, we applied PMNs isolated from circulation or peritoneal fluid from animals with chemically induced peritonitis to ischemia-injured porcine ileal mucosa. In preliminary experiments, PMNs enhanced recovery of transepithelial electrical resistance (TER), and this action was inhibited by pretreatment with the nonselective cyclooxygenase (COX) inhibitor indomethacin. Because COX-2 is upregulated by inflammatory mediators such as IL-1beta, which is released by PMNs, we postulated that PMNs enhance recovery of ischemia-injured mucosa by a pathway involving IL-1beta and COX-2. Application of 5 x 10(6) PMNs to the serosal surface of ischemia-injured mucosa significantly enhanced recovery of TER (P < 0.05), an effect that was inhibited by the selective COX-2 inhibitor NS-398 (5 microM) and by an IL-1beta receptor antagonist (0.1 mg/ml). Addition of 10 ng/ml IL-1beta to the serosal surface of injured tissues caused a significant increase in TER (P < 0.05) that was inhibited by pretreatment with NS-398. Western blot analysis of mucosal homogenates revealed dramatic upregulation of COX-2 in response to IL-1beta or peritoneal PMNs, and the latter was inhibited by an IL-1beta receptor antagonist. Real-time PCR revealed that increased mRNA COX-2 expression preceded increased COX-2 protein expression in response to IL-1beta. We concluded that PMNs augment recovery of TER in ischemia-injured ileal mucosa via IL-1beta-dependent upregulation of COX-2.

Effects of flunixin meglumine or etodolac treatment on mucosal recovery of equine jejunum after ischemia

OBJECTIVE

To examine the effects of flunixin meglumine and etodolac treatment on recovery of ischemic-injured equine jejunal mucosa after 18 hours of reperfusion.

ANIMALS

24 horses.

PROCEDURE

Jejunum was exposed to 2 hours of ischemia during anesthesia. Horses received saline (0.9% NaCl) solution (12 mL, i.v., q 12 h), flunixin meglumine (1.1 mg/kg, i.v., q 12 h), or etodolac (23 mg/kg, i.v., q 12 h). Tissue specimens were obtained from ischemic-injured and nonischemic jejunum immediately after ischemia and 18 hours after recovery from ischemia. Transepithelial electric resistance (TER) and transepithelial flux of tritium-labeled mannitol measured mucosal permeability. Denuded villous surface area and mean epithelial neutrophil count per mm2 were calculated. Western blot analysis for cyclooxygenase (COX)-1 and -2 was performed. Pharmacokinetics of flunixin and etodolac and eicosanoid concentrations were determined.

RESULTS

Ischemic-injured tissue from horses treated with flunixin and etodolac had significantly lower TER and increased permeability to mannitol, compared with that from horses treated with saline solution. Epithelial denudation after ischemia and 18 hours after recovery was not significantly different among treatments. Both COX-1 and -2 were expressed in ischemic-injured and nonischemic tissues. Ischemia caused significant upregulation of both COX isoforms. Eicosanoid concentrations were significantly lower in tissues from flunixin and etodolac-treated horses, compared with that from horses treated with saline solution.

CONCLUSIONS AND CLINICAL RELEVANCE

Flunixin and etodolac treatment retarded recovery of intestinal barrier function in jejunal mucosa after 18 hours of reperfusion, whereas tissues from horses treated with saline solution recovered baseline values of TER and permeability to mannitol.

Mitogen-activated protein kinases regulate COX-2 and mucosal recovery in ischemic-injured porcine ileum

Mitogen-activated protein kinase (MAPK) pathways transduce signals from a diverse array of extracellular stimuli. The three primary MAPK-signaling pathways are the extracellular regulated kinases (ERK1/2), p38 MAPK, and c-Jun NH(2)-terminal kinase (JNK). Previous research in our laboratory has shown that COX-2-elaborated prostanoids participate in recovery of mucosal barrier function in ischemic-injured porcine ileum. Because COX-2 expression is regulated in part by MAPKs, we postulated that MAPK pathways would play an integral role in recovery of injured mucosa. Porcine mucosa was subjected to 45 min of ischemia, after which tissues were mounted in Ussing chambers, and transepithelial electrical resistance (TER) was monitored as an index of recovery of barrier function. Treatment of tissues with the p38 MAPK inhibitor SB-203580 (0.1 mM) or the ERK1/2 inhibitor PD-98059 (0.1 mM) abolished recovery. Western blot analysis revealed that SB-203580 inhibited upregulation of COX-2 that was observed in untreated ischemic-injured mucosa, whereas PD-98059 had no effect on COX-2 expression. Inhibition of TER recovery by SB-203580 or PD-98059 was overcome by administration of exogenous prostaglandin E(2) (1 microM). The JNK inhibitor SP-600125 (0.1 mM) significantly increased TER and resulted in COX-2 upregulation. COX-2 expression appears to be positively and negatively regulated by the p38 MAPK and the JNK pathways, respectively. Alternatively, ERK1/2 appear to be involved in COX-2-independent reparative events that remain to be defined.

Lateral intercellular space volume as a determinant of CFTR-mediated anion secretion across small intestinal mucosa

Studies of full-thickness, small intestinal preparations have shown that maximal anion secretion [indexed by short-circuit current (I(sc))] during intracellular cAMP (cAMP(i)) stimulation is transient and followed by a decline toward baseline. Declining I(sc) is preceded by decreases in transepithelial conductance (G(t)), which in the small intestine reflects the lateral intercellular space (LIS) volume of the paracellular pathway. We hypothesized that decreases in LIS volume limit the magnitude and duration of cAMP(i)-stimulated anion secretion. Experimental manipulations to increase the patency of the LIS (assessed by G(t) and electron microscopy) were investigated for an effect on the magnitude of cAMP(i)-stimulated anion secretion (assessed by the I(sc) and isotopic fluxes) across murine small intestine. In control studies, changes of G(t) after cAMP(i) stimulation were associated with a morphological "collapse" of the LIS, which did not occur in intestine of CFTR-null mice. Removal of the outer intestinal musculature, exposure to a serosal hypertonic solution, or increased serosal hydrostatic pressure minimized reductions in G(t) and increased the cAMP(i)-stimulated I(sc) response. Increased I(sc) primarily resulted from increased Cl(-) secretion that was largely bumetanide sensitive. However, bumetanide-insensitive I(sc) was also increased, and similar increases occurred in the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1)-null intestine, indicating that activities of non-NKCC1 anion uptake proteins are also affected by LIS volume. Thus LIS patency is an important determinant of the magnitude and duration of CFTR-mediated anion secretion in murine small intestine. Decreases in LIS volume may limit the pool of available anions to basolateral transporters involved in transepithelial secretion.

PG-mediated closure of paracellular pathway and not restitution is the primary determinant of barrier recovery in acutely injured porcine ileum

Small bowel epithelium is at the frontline of intestinal barrier function. Restitution is considered to be the major determinant of epithelial repair, because function recovers in parallel with restitution after acute injury. As such, studies of intact mucosa have largely been replaced by migration assays of cultured epithelia. These latter studies fail to account for the simultaneous roles played by villous contraction and paracellular permeability in recovery of barrier function. NSAIDs result in increased intestinal permeability and disease exacerbation in patients with inflammatory bowel disease (IBD). Thus we examined the reparative attributes of endogenous PGs after injury of ileal mucosa by deoxycholate (6 mM) in Ussing chambers. Recovery of transepithelial electrical resistance (TER) from 20-40 Omega.cm2 was abolished by indomethacin (Indo), whereas restitution of 40-100% of the villous surface was unaffected despite concurrent arrest of villous contraction. In the presence of PG, resident crypt and migrating epithelial cells were tightly apposed. In tissues treated with Indo, crypt epithelial cells had dilated intercellular spaces that were accentuated in the migrating epithelium. TER was fully rescued from the effects of Indo by osmotic-driven collapse of the paracellular space, and PG-mediated recovery was significantly impaired by blockade of Cl- secretion. These studies are the first to clearly distinguish the relative contribution of paracellular resistance vs. restitution to acute recovery of epithelial barrier function. Restitution was ineffective in the absence of PG-mediated paracellular space closure. Failure of PG-mediated repair mechanisms may underlie barrier failure resulting from NSAID use in patients with underlying enteropathy.

PI3K signaling is required for prostaglandin-induced mucosal recovery in ischemia-injured porcine ileum

We have previously shown that PGE(2) and PGI(2) induce recovery of transepithelial resistance (TER) in ischemia-injured porcine ileal mucosa, associated with initial increases in Cl(-) secretion. We believe that the latter generates an osmotic gradient that stimulates resealing of tight junctions. Because of evidence implicating phosphatidylinositol 3-kinase (PI3K) in regulating tight junction assembly, we postulated that this signaling pathway is involved in PG-induced mucosal recovery. Porcine ileum was subjected to 45 min of ischemia, after which TER was monitored for a 180-min recovery period. Endogenous PG production was inhibited with indomethacin (5 microM). PGE(2) (1 microM) and PGI(2) (1 microM) stimulated recovery of TER, which was inhibited by serosal application of the osmotic agent urea (300 mosmol/kgH(2)O). The PI3K inhibitor wortmannin (10 nM) blocked recovery of TER in response to PGs or mucosal urea. Immunofluorescence imaging of recovering epithelium revealed that PGs restored occludin and zonula occludens-1 distribution to interepithelial junctions, and this pattern was disrupted by pretreatment with wortmannin. These experiments suggest that PGs stimulate recovery of paracellular resistance via a mechanism involving transepithelial osmotic gradients and PI3K-dependent restoration of tight junction protein distribution.

Intestinal NaCl transport in NHE2 and NHE3 knockout mice

Sodium/proton exchangers [Na(+)/H(+) (NHEs)] play an important role in salt and water absorption from the intestinal tract. To investigate the contribution of the apical membrane NHEs, NHE2 and NHE3, to electroneutral NaCl absorption, we measured radioisotopic Na(+) and Cl(-) flux across isolated jejuna from wild-type [NHE(+)], NHE2 knockout [NHE2(-)], and NHE3 knockout [NHE3(-)] mice. Under basal conditions, NHE(+) and NHE2(-) jejuna had similar rates of net Na(+) (approximately 6 microeq/cm(2) x h) and Cl(-) (approximately 3 microeq/cm(2) x h) absorption. In contrast, NHE3(-) jejuna had reduced net Na(+) absorption (approximately 2 microeq/cm(2) x h) but absorbed Cl(-) at rates similar to NHE(+) and NHE2(-) jejuna. Treatment with 100 microM 5-(N-ethyl-N-isopropyl) amiloride (EIPA) completely inhibited net Na(+) and Cl(-) absorption in all genotypes. Studies of the Na(+) absorptive flux (J) indicated that J in NHE(+) jejunum was not sensitive to 1 microM EIPA, whereas J in NHE3(-) jejunum was equally sensitive to 1 and 100 microM EIPA. Treatment with forskolin/IBMX to increase intracellular cAMP (cAMP(i)) abolished net NaCl absorption and stimulated electrogenic Cl(-) secretion in all three genotypes. Quantitative RT-PCR of epithelia from NHE2(-) and NHE3(-) jejuna did not reveal differences in mRNA expression of NHE3 and NHE2, respectively, when compared with jejunal epithelia from NHE(+) siblings. We conclude that 1) NHE3 is the dominant NHE involved in small intestinal Na(+) absorption; 2) an amiloride-sensitive Na(+) transporter partially compensates for Na(+) absorption in NHE3(-) jejunum; 3) cAMP(i) stimulation abolishes net Na(+) absorption in NHE(+), NHE2(-), and NHE3(-) jejunum; and 4) electroneutral Cl(-) absorption is not directly dependent on either NHE2 or NHE3.

PGE2 triggers recovery of transmucosal resistance via EP receptor cross talk in porcine ischemia-injured ileum

16,16-Dimethyl-PGE2 (PGE2) may interact with one of four prostaglandin type E (EP) receptors, which signal via cAMP (via EP2 or EP4 receptors) or intracellular Ca(2+) (via EP1 receptors). Furthermore, EP3 receptors have several splice variants, which may signal via cAMP or intracellular Ca(2+). We sought to determine the PGE2 receptor interactions that mediate recovery of transmucosal resistance (R) in ischemia-injured porcine ileum. Porcine ileum was subjected to 45 min of ischemia, after which the mucosa was mounted in Ussing chambers. Tissues were pretreated with indomethacin (5 microM). Treatment with the EP1, EP2, EP3, and EP4 agonist PGE2 (1 microM) elevated R twofold and significantly increased tissue cAMP content, whereas the EP2 and EP4 agonist deoxy-PGE1 (1 microM) or the EP1 and EP3 agonist sulprostone (1 microM) had no effect. However, a combination of deoxy-PGE1 and sulprostone stimulated synergistic elevations in R and tissue cAMP content. Furthermore, treatment of tissues with deoxy-PGE1 and the Ca(2+) ionophore A-23187 stimulated synergistic increases in R and cAMP, indicating that PGE2 triggers recovery of R via EP receptor cross talk mechanisms involving cAMP and intracellular Ca(2+).

Genistein augments prostaglandin-induced recovery of barrier function in ischemia-injured porcine ileum

We have previously shown that PGE(2) enhances recovery of transmucosal resistance (R) in ischemia-injured porcine ileum via a mechanism involving chloride secretion. Because the tyrosine kinase inhibitor genistein amplifies cAMP-induced Cl(-) secretion, we postulated that genistein would augment PGE(2)-induced recovery of R. Porcine ileum subjected to 45 min of ischemia was mounted in Ussing chambers, and R and mucosal-to-serosal fluxes of [(3)H]N-formyl-methionyl-leucyl phenylalanine (FMLP) and [(3)H]mannitol were monitored as indicators of recovery of barrier function. Treatment with genistein (10(-4) M) and PGE(2) (10(-6) M) resulted in synergistic elevations in R and additive reductions in mucosal-to-serosal fluxes of [(3)H]FMLP and [(3)H]mannitol, whereas treatment with genistein alone had no effect. Treatment of injured tissues with genistein and either 8-bromo-cAMP (10(-4) M) or cGMP (10(-4) M) resulted in synergistic increases in R. However, treatment of tissues with genistein and the protein kinase C (PKC) agonist phorbol myristate acetate (10(-5)-10(-6) M) had no effect on R. Genistein augments recovery of R in the presence of cAMP or cGMP but not in the presence of PKC agonists.