Pathogenesis of early necrotizing enterocolitis in the hypoxic neonatal dog

What animal model should I use to study necrotizing enterocolitis?

Unfortunately, we are all too familiar with the statement: "Necrotizing enterocolitis remains the leading cause of gastrointestinal surgical emergency in preterm neonates". It's been five decades since the first animal models of necrotizing enterocolitis (NEC) were described. There remains much investigative work to be done on identifying various aspects of NEC, ranging from the underlying mechanisms to treatment modalities. Experimental NEC is mainly focused on a rat, mouse, and piglet models. Our aim is to not only highlight the pros and cons of these three main models, but to also present some of the less-used animal models that have contributed to the body of knowledge about NEC. Choosing an appropriate model is essential to conducting effective research and answering the questions asked. As such, this paper reviews some of the variations that come with each model.

Feeding associated neonatal necrotizing enterocolitis (Primary NEC) is an inflammatory bowel disease

Neonatal necrotizing enterocolitis which develops after feeding preterm infants is characterized by severe intestinal inflammation and profound systemic metabolic acidosis. The fermentation of undigested dietary carbohydrate by colonic flora yields gases (CO₂ and H₂) and short chain organic acids. These organic acids can disrupt the intestinal mucosa and initiate inflammation driven predominantly by resident mast cells and by granulocytes which are recruited from blood. A systemic acidosis ensues derived from intestinal acids, not classic lactic acidosis produced from anaerobic metabolism. The systemic acidosis further compromises inflamed bowel leading to bowel necrosis.

Nitric oxide levels in the intestines of mice submitted to ischemia and reperfusion: L-arginine effects

OBJECTIVE

Usually an experimental necrotizing enterocolitis experimental model, we Investigated nitric oxide levels in intestinal tissues of newborn mice with or without l-arginine therapy during sessions of ischemia and reoxygenation.

METHODS

Twenty-six newborn mice from the Wistar EPM-1 lineage, weighing from 4.5 to 6.2 g, were randomly assigned to three groups: G-I/R, hypoxia and reoxygenation; G-Arg, l-arginine treatment I/R; and G-CTL, controls. G-I/R and G-Arg mice underwent twice a day during their first 3 days of life exposure to gas chambers with 100% CO(2) for 5 minutes at 22 degrees C before reoxygenation with 100% O(2) for another 5 minutes. After 12 hours, all animals were sedated, laparotomized, and had samples of ileum and colon taken and- either formalin fixed histopathologic examinations or frozen to -80 degrees C for estimation of tissue nitric oxide levels. Intestinal injuries were classified according to the criteria of Chiu et al.

RESULTS

The G-I/R and G-Arg groups showed injuries characteristic of necrotizing enterocolitis (NEC) with an improved structural preservation rate in G-Arg. The concentration of nitric oxide in the Ileum was much higher with G-Arg (16.5 +/- 4.9; P = 0.0019) G-I/R (7.3 +/- 2.0). This effect was not observed in the colon: G-I/R = 10.7 +/- 4.6 versus G-Arg = 15.5 +/- 8.7 (P = .2480).

CONCLUSION

Supply of L-arginine increased tissue levels of nitricoxide and reduced morphologic intestinal injury among mice undergoing I/R.

Neonatal and pediatric care of the puppy and kitten

Although the interval from birth to weaning is brief for dogs and cats, it is a very intensive period of adjustment to the extrauterine environment and preparation for the relatively greater independence of post-weaning. This rapid progression of events is preceded by critical developmental transitions, including organogenesis and parturition, and it is characterized by postparturient transitions that include the 3-4d neonatal period, the 21-28 maturation, and weaning itself. The purpose of this discussion is to characterize the primary events of these five transitional zones and to describe practical methods for evaluating progression through each transition, for establishing differential diagnoses, and for supportive actions and responses.

Intestinal trefoil factor in treatment of neonatal necrotizing enterocolitis in the rat model

AIM

Neonatal necrotizing enterocolitis (NEC) is the most common gastrointestinal disease of premature infants. The role of cytokines and growth factors in the pathophysiology of NEC is not yet clearly defined. Among these factors, the intestinal trefoil factor (ITF) is known as cytoprotective to the gut. We studied the cytoprotective effect of trefoil factor in the 1-day-old Wistar rat pup model following hypoxic-ischemic cold stress.

MATERIALS AND METHODS

In the present study, thirty 1-day-old Wistar rat pups were randomly divided into three groups: Group 1, normal controls: Group 2, NEC; Group 3, NEC+ITF. Experimental NEC was induced by exposure to hypoxia for 60 s followed by cold stress at 4 degrees C for 10 min. The animals were euthanized at development of NEC, and at 96 h the intestinal tissue was processed and examined for histological changes of NEC.

RESULTS

The pathological lesions indicated severe separation of the submucosa and lamina propria and tissue necrosis in Group 2, and slight submucosal and lamina propria separation in Group 3. There were no histopathological changes in the controls. The mean of histological grade of group 2 was 2.8 (range 2-4), and 1.2 (range 0-2) in group 3. A difference was found when the two groups were compared (P<0.05).

CONCLUSION

ITF may provide a new way for the therapy of NEC in rats.

Protective effects of recombinant human granulocyte colony stimulating factor in a rat model of necrotizing enterocolitis

The role of cytokines and growth factors in the pathophysiology of neonatal necrotizing enterocolitis (NEC) is not defined clearly yet. The aim of this study was to determine the effects of recombinant human granulocyte colony stimulating factor (G-CSF) on intestinal cells in hypoxia-induced experimental NEC in rats. The study was experimented on Sprague Dawley rat pups. Group 1 (untreated, n = 7) rats were subjected to hypoxia-reoxygenation (H/O) and then were returned to standard conditions. Group 2 (G-CSF treated, n = 7) rats were subjected to H/O, and then were treated with G-CSF (100 microg/kg enterally) for 5 days. Group 3 was served as nonhypoxic controls. All animals were killed on day five, and histological examination was performed on intestinal samples. There were no histopathological changes in the control group. The histological findings in untreated rats were similar to those seen in neonatal NEC, with destruction of villi and crypts with extension to the muscularis layer. Intestinal damage was mild in group 2 and these histological changes were better than group 1, and worse than group 3. The mean of histologic grade of group 1 was 2.4 (range 2-3), and in the group 2, it was 1.2 (range 0-2). A difference was found when two groups were compared with each other (P < 0.05). In an experimental model of NEC, G-CSF could have a protective effect on intestinal damage.

Glycine reduces tissue lipid peroxidation in hypoxia-reoxygenation-induced necrotizing enterocolitis in rats

PURPOSE

To assess the protective effect of glycine in an experimental model of Neonatal Necrotizing Enterocolitis (NEC).

METHODS

Fifty (50) neonatal Wistar rats, from a litter of six female rats and weighing 4 to 6 grams, were used. Five animals were cannibalized and the 45 remaining were distributed into three groups: the G1 normal control group (n=12); the G2 Group (n=16), of animals that underwent hypoxia-reoxygenation (HR); the G3 Group of animals (n=17) that underwent HR following a 5% intraperitoneal glycine infusion. The animals underwent hypoxia in a CO2 chamber receiving an air flow of 100% CO2 for 5 minutes and reoxygenation receiving an O2 flow at 100% for 5 minutes. One centimeter long small bowel and colon segments were prepared for histological analysis. The rest of the bowel was removed in a block and frozen at minus 80 degrees C for homogenization and determination of tissue malondialdehyde (MDA). Tissue lesions were classified as Grade 0 to Grade 5, according to the level of damaged mucosa.

RESULTS

The animals in Group G1 had levels of small bowel and colon lesion significantly smaller as compared to the animals in Groups G2 and G3. The G2 group had mean MDA values significantly higher than the animals in the G1 (p = .015) and G3 (p=0.021) groups. MDA values did not differ significantly (p = 0.992) for the animals in groups G1 and G3.

CONCLUSION

Glycine reduces tissue MDA levels (a measurement of lipid peroxidation) following HR in neonatal rats.

[Evaluation of an experimental model of necrotizing enterocolitis in rats]

OBJECTIVE

To evaluate an experimental model of necrotizing enterocolitis in rats proposed by OKUR e col. in 1995.

METHODS

On their first day of life, 28 EPM-Wistar rats weighing between 4 and 6 grams were submitted to hypoxia (H) by placing them in a CO2 gas chamber for rodents' sacrifice, where they received a 100% CO2 air flow for 5 minutes. After the hypoxia the animals were reanimated (R) with a 100% O2 air flow, also for 5 minutes. The animals were allocated in two groups: G1: control (n=12): rats not submitted to H-R; G2: (n=16): rats submitted to H-R. Segments of the small intestine and colon were prepared for histological analysis. The remaining intestine was used to measure tissular malondialdehyde.

RESULTS

Mean malondialdehyde dosages were 1.05 (0.44-2.03) and 2.60 (0.59- 6.4) nmol MDA/mg protein for G1 and G2, respectively. G2's mean value was significantly higher than in the control group (p<0.002). Significant statistical difference between the studied groups was found in relation the level of injury, with G1 presenting significantly lower levels than G2.

CONCLUSIONS

The model showed that neonatal hypoxia may cause intestinal wall injury in rats. Despite the discreet histological injuries found, the method is suitable for evaluation of tissular free radicals.

Resuscitation with 100% oxygen causes intestinal glutathione oxidation and reoxygenation injury in asphyxiated newborn piglets

OBJECTIVE

To compare mesenteric blood flow, oxidative stress, and mucosal injury in piglet small intestine during hypoxemia and reoxygenation with 21%, 50%, or 100% oxygen.

SUMMARY BACKGROUND DATA

Necrotizing enterocolitis is a disease whose pathogenesis likely involves hypoxia-reoxygenation and the generation of oxygen-free radicals, which are known to cause intestinal injury. Resuscitation of asphyxiated newborns with 100% oxygen has been shown to increase oxidative stress, as measured by the glutathione redox ratio, and thus may predispose to free radical-mediated tissue injury.

METHODS

Newborn piglets subjected to severe hypoxemia for 2 hours were resuscitated with 21%, 50%, or 100% oxygen while superior mesenteric artery (SMA) flow and hemodynamic parameters were continuously measured. Small intestinal tissue samples were analyzed for histologic injury and levels of oxidized and reduced glutathione.

RESULTS

SMA blood flow decreased to 34% and mesenteric oxygen delivery decreased to 9% in hypoxemic piglets compared with sham-operated controls. With reoxygenation, SMA blood flow increased to 177%, 157%, and 145% of baseline values in piglets resuscitated with 21%, 50%, and 100% oxygen, respectively. Mesenteric oxygen delivery increased to more than 150% of baseline values in piglets resuscitated with 50% or 100% oxygen, and this correlated significantly with the degree of oxidative stress, as measured by the oxidized-to-reduced glutathione ratio. Two of eight piglets resuscitated with 100% oxygen developed gross and microscopic evidence of pneumatosis intestinalis and severe mucosal injury, while all other piglets were grossly normal.

CONCLUSIONS

Resuscitation of hypoxemic newborn piglets with 100% oxygen is associated with an increase in oxygen delivery and oxidative stress, and may be associated with the development of small intestinal hypoxia-reoxygenation injury. Resuscitation of asphyxiated newborns with lower oxygen concentrations may help to decrease the risk of necrotizing enterocolitis.

Neonatal necrotizing enterocolitis: clinical considerations and pathogenetic concepts

Necrotizing enterocolitis (NEC), a disease affecting predominantly premature infants, is a leading cause of morbidity and mortality in neonatal intensive care units. Although several predisposing factors have been identified, such as prematurity, enteral feeding, and infection, its pathogenesis remains elusive. In the past 20 years, we have established several animal models of NEC in rats and found several endogenous mediators, especially platelet-activating factor (PAF), which may play a pivotal role in NEC. Injection of PAF induces intestinal necrosis, and PAF antagonists prevent the bowel injury induced by bacterial endotoxin, hypoxia, or challenge with tumor necrosis factor-a (TNF) plus endotoxin in adult rats. The same is true for lesions induced by hypoxia and enteral feeding in neonatal animals. Human patients with NEC show high levels of PAF and decreased plasma PAF-acetylhydrolase, the enzyme degrading PAF. The initial event in our experimental models of NEC is probably polymorphonuclear leukocyte (PMN) activation and adhesion to venules in the intestine, which initiates a local inflammatory reaction involving proinflammatory mediators including TNF, complement, prostaglandins, and leukotriene C4. Subsequent norepinephrine release and mesenteric vasoconstriction result in splanchnic ischemia and reperfusion. Bacterial products (e.g., endotoxin) enter the intestinal tissue during local mucosal barrier breakdown, and endotoxin synergizes with PAF to amplify the inflammation. Reactive oxygen species produced by the activated leukocytes and by intestinal epithelial xanthine oxidase may be the final pathway for tissue injury. Protective mechanisms include nitric oxide produced by the constitutive (mainly neuronal) nitric oxide synthase, and indigenous probiotics such as Bifidobacteria infantis. The former maintains intestinal perfusion and the integrity of the mucosal barrier, and the latter keep virulent bacteria in check. The development of tissue injury depends on the balance between injurious and protective mechanisms.

A model of hypoxia-induced necrotizing enterocolitis: the role of distension

BACKGROUND/PURPOSE

This study was performed to investigate additional effects of intestinal distension in the damage to the gut caused by hypoxia-reperfusion.

METHODS

Five groups each consisting of ten 1-day-old Wistar albino rat pups were studied; Group 1, hypoxia-reoxygenation; Group 2, hypoxia-reoxygenation and distension; Group 3, distension and hypoxia-reoxygenation; Group 4, distension; and Group 5, control. Hypoxia was induced by placing the rat pups in a 100% carbon dioxide chamber for 5 minutes. After the hypoxia, the pups were exposed to 100% oxygen for reoxygenation for 5 minutes. The intestinal distension was carried out with a fine 21-gauge SILASTIC cannula via rectal route. The rats were killed on the third day, and all the intestine were harvested from duodenum to sigmoid colon. Malondialdehyde (MDA) levels were determined as an index of lipid peroxidation related to free radical reaction in the intestine. The histopathologic investigation was carried out by light microscopy in a blinded fashion.

RESULTS

The MDA levels of Group 3 animals were significantly higher than those in Group 1, 4, and the control group (P < .05). The MDA level of Group 2 did not differ significantly from that of the Group 3 (P > .05). All MDA levels of the study groups also were significantly higher than the control group (P < .05).

CONCLUSION

The results demonstrated that intestinal distension increased the damaging effects of hypoxia-reoxygenation in the gut.

Acute hypoxia does not increase bacterial translocation in newborn rabbits

PURPOSE

We have previously demonstrated that spontaneous bacterial translocation (BT) occurs in newborn rabbits and correlates strongly with small bowel colonization (BC). Birth stress, specifically hypoxia, is believed to increase this pathologic process and thus lead to sepsis. This study investigated the relationship between BT and acute hypoxia in newborn rabbits.

METHODS

Four hundred seventeen rabbit pups (aged 0, 2 to 4, 6, and 28 days) were divided into four groups according to the type of hypoxic stress: 9% O(2) for 1 hour, 9% O(2) + 12% CO(2) for 1 or 4 hours, and 21% O(2) (control animals). The animals were killed 1.5 or 20 hours after the stress. Sterile specimens of mesenteric lymph nodes (MIN), spleen, liver, small bowel, and large bowel were incubated aerobically at 37 degrees C for 24 hours in thioglycolate broth, and subsequently plated on both MacConkey and Colistin Naladixic Acid media. After 24 hours, the growth on both plates was recorded. X(2) analysis was used, and P values of less than .05 were considered significant.

RESULTS

BC of the small bowel and BT to the MLN were low in the first 4 days of life in the hypoxic groups (range, 0% to 21% BC, 0% to 6% BT) and the control group (range, 4% to 30% BC, 3% to 12% BT). After an increase in BC at 6 days of age, the rate of BT increased to 25% to 29% in control animals. The rate of BT in the hypoxic groups (25%) did not differ significantly from that of the controls (P > .05). Additionally, killing at 20 hours (v 1.5 hours) was not associated with an increase in the incidence of BT. None of the stress groups had a significant increase in BT compared with the controls. Importantly, although 4 hours of 9% O(2) + 12% CO(2) resulted in a 30% mortality rate, the incidence of BT was no higher than that of the control animals (13% v 29%; P > .05).

CONCLUSION

Severe hypoxic stress in newborn rabbits does not increase the incidence of BT. Because the incidence of BT correlates with that of BC, and because BC is the same in the control and hypoxic animals, the sepsis observed in hypoxic newborns probably is not related to an increased incidence of BT.

Hypoxia-induced necrotizing enterocolitis in the immature rat: the role of lipid peroxidation and management by vitamin E

The authors developed an experimental model of necrotizing enterocolitis (NEC) by hypoxia-reoxygenation, and determined the content of malondialdehyde levels as an index of lipid peroxidation, related with a free-radical reaction in the gastrointestinal tract of newborn rats. They also investigated the role of vitamin E, an antioxidant, in this free-radical injury. The study was performed on 1-day-old rats. The 30 rat pups were divided into three groups. Hypoxia was induced by placing the pups in a 100% carbon dioxide chamber for 5 minutes. The pups were reoxygenated with 100% oxygen for 5 minutes. Group 1 (n = 10) was subjected to hypoxia-reoxygenation and killed 3 days after hypoxia. Group 2 (n = 10) was subjected to hypoxia-reoxygenation and treated with vitamin E (30 IU/kg/d intraperitoneally) for the next 3 days, and killed. Group 3 (n = 10) rats served as controls. The histopathology of the intestinal lesions in group 1 animals was characteristic of ischemic injury and ranged from superficial epithelial damage with villous shortening to transmural necrosis. In the vitamin E-treated animals these lesions were milder. The malondialdehyde levels of group 1 were significantly higher than those of the other two groups (P < .001). This study shows that oxidant-mediated lipid peroxidation injury plays a central role in mediating hypoxia-induced intestinal necrosis and suggests that vitamin E may play a therapeutic role in NEC.

Role of asphyxia and feeding in a neonatal rat model of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a common gastrointestinal disorder affecting premature infants. To investigate critically the importance of the purported risk factors of NEC (formula feeding, asphyxia, bacteria, and prematurity), we developed a neonatal rat model that closely mimics the human disease. Full-term and premature newborn rats were stressed with formula feeding, asphyxia, and/or exogenous bacterial colonization and subsequently evaluated grossly and histologically for the development of intestinal injury. We found that most animals treated with asphyxia, formula feeding, and bacteria developed NEC (77%) and died (86%) by 96 h. All maternally fed animals treated with asphyxia and bacterial colonization survived and had normal intestinal histology. Furthermore, asphyxia was a critical instigating factor, because formula and bacterial exposure without asphyxia resulted in normal intestine and minimal mortality (12%). Enteral bacterial colonization was not a significant determinant of NEC in this model. We conclude that the neonatal rat model is an excellent test system for the study of NEC. As in the human disease, asphyxia and formula feeding play an important role in the pathophysiology of experimental NEC.

An animal model of necrotizing enterocolitis induced by infant formula and ischemia in developing piglets

BACKGROUND/AIMS

The lipid component of piglet formula (0.5% fat) causes increased mucosal permeability in 1-day-old piglets after ischemia/reperfusion. The present study examined if luminal exposure to infant formulas (3.5% fat) and ischemia/reperfusion result in an animal model of necrotizing enterocolitis and if injury is dependent on the formula fat composition.

METHODS

Plasma-to-lumen clearance of 51Cr-ethylenediamine-tetraacetic acid was measured, and morphology was evaluated during luminal perfusion with preterm, term, and delipidated preterm cow milk-based infant formulas before and after ischemia/reperfusion in 1-day-old and 1-month-old piglet jejunoileum. In a separate set of experiments, a 1-2-cm segment of ileum was exteriorized and opened to expose the mucosal surface, and the villi were superfused with the above formulas (no ischemia).

RESULTS

Before ischemia, clearances were markedly higher for intestinal loops perfused with preterm formula than for loops perfused with term and delipidated formulas in 1-day-old animals. After ischemia, clearances in loops perfused with preterm formula were significantly greater and grossly hemorrhagic and histologically necrotic compared with loops perfused with delipidated formula (minimal injury). Superfusion with preterm formula caused diffuse hyperemia and hemorrhage into intestinal villi.

CONCLUSIONS

Luminal perfusion of 1-day-old piglet jejunoileum with predigested and bile acid-solubilized preterm infant formula, in combination with ischemia/reperfusion, produces an animal model of necrotizing enterocolitis, but only if the lipid fraction of the formula is present.

Platelet-activating factor, tumor necrosis factor, hypoxia and necrotizing enterocolitis

The pathogenesis of necrotizing enterocolitis (NEC) is poorly understood. We have established several animal models of NEC by using a combination of various stimuli and stress, including endotoxin, PAF, TNF, and hypoxia. We discuss the mechanism of their actions and the possible roles of these factors in the pathogenesis of human NEC.

Mucosal permeability in the immature rat intestine: effects of ischemia-reperfusion, cold stress, hypoxia, and drugs

Increased mucosal permeability may represent an important factor in the etiology of necrotizing enterocolitis (NEC). In the present study we used an immature rat model to assess the permeability effects of a number of stresses commonly seen in infants with NEC. In 10-day-old rats, mucosal permeability to 51Cr EDTA was measured after subjecting the animals to 10-minute ischemia-reperfusion injury, 30 minutes of hypoxia (14% oxygen), cold stress (4 degree C for 4 minutes), and intraperitoneal indomethacin (0.2 or 2.0 mg/kg) or theophylline (40 or 200 mg/kg). When compared with appropriate controls, mucosal permeability was found to be significantly increased by ischemia-reperfusion injury, hypoxia, and high-dose indomethacin, but not by cold, theophylline, or low-dose indomethacin. Renal clearance studies confirmed that elevated blood levels of 51Cr EDTA were due to increased permeability rather than decreased renal excretion of the probe. These studies confirm that mucosal permeability in the immature rat is increased by a variety of insults, and may represent a "common pathway" in the etiology of NEC.

Necrotizing enterocolitis. New thoughts about pathogenesis and potential treatments

Necrotizing enterocolitis (NEC) remains a major cause of morbidity and mortality in premature infants. An incomplete understanding of its pathogenesis has hampered efforts to devise an effective preventative strategy. New insights into the pathogenesis of NEC, particularly at the cellular and biochemical level, however, offer a rational basis for the development of new approaches to this disease.

Necrotizing enterocolitis: a review of pathogenetic mechanisms and implications for prevention

Necrotizing enterocolitis (NEC) is a devastating gastrointestinal disease of premature neonates that accounts for 3000 to 4000 deaths each year in the United States. The pathogenesis is not well understood, however theories suggest that prematurity, enteral feeding, bacterial colonization, and intestinal ischemia contribute to the intestinal injury. Furthermore, recent studies have shown that platelet activating factor and perhaps other inflammatory mediators mediate bowel necrosis in animals and possibly in humans. Although no specific intervention for NEC treatment exists, preventive therapy using either enteral IgA supplementation, breast milk feeding, antibiotic prophylaxis, or exogenous steroid administration have reduced the incidence of this overwhelming disease in small randomized trials. These modalities and perhaps PAF antagonists or other inflammatory mediator inhibitors may reduce the incidence or severity of NEC in the next several years.

Histological investigations into the relationship between low birth weight and spontaneous bowel damage in the neonatal piglet

Experimental models for the study of neonatal necrotizing enterocolitis (NEC) using the small for gestational age (SGA), viable, term-delivered piglet have recently been described. A spontaneous model, especially of the early phases of NEC, has not been reported. Pig litters usually include one or two lethally or sublethally SGA "runted" piglets culled from the litters by breeders. We have examined four groups of piglets: group A, SGA, lethally runted animals who died spontaneously before 12 h postpartum; group B, SGA, sublethally runted animals, showing some signs of vitality and left alive for 24 h before culling; group C, appropriate for gestation age (AGA) animals subjected to hypoxia and hyperviscosity known to induce NEC-like lesions; and group D, AGA control animals. Acute, multiorgan pathology common to pre- and dysmature neonates was seen in groups A and B. Animals in group B also showed evidence of early postnatal complications such as urinary tract infection and diffuse pulmonary damage. All animals of groups A and B showed unequivocal early changes in the distal ileum, with mucosal and submucosal necrosis, suggestive of ischemic injury. One animal in group B had developed mural necrosis involving the whole of the submucosa. No similar changes were seen in control group D animals. The very-SGA, lethally and sublethally runted, spontaneously term-delivered piglet offers sufficient spontaneous early NEC-like changes for studies of the early pathogenesis of this condition.

Collateral blood flow in the distal ileum of neonatal piglets: a clue to the pathogenesis of necrotizing enterocolitis

Normal blood flow was measured in two regions of the ileum (distal and proximal) of normal birth weight (NBW) and low birth weight (LBW) neonatal piglets. Compensatory collateral blood flow in response to occlusion of vessels in the mesenteric vascular arcades was also measured in distal and proximal ileum of NBW and LBW neonatal piglets. Under normal control conditions the blood flow in the distal ileum of NBW piglets is reduced (40% less than proximal) and in LBW animals this reduction is greater (55% less than proximal). Both proximal and distal ileum of NBW animals but the proximal ileum only of LBW animals could mount compensatory collateral flow, whereas the distal ileum of LBW animals was unable to do so. This decompensation in LBW distal ileum may explain the predilection of NEC lesions for the distal ileum and proximal colon.

Hypoxia, PAF, and necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is an important neonatal disease with a high mortality rate. The pathophysiology is unclear but epidemiologic studies suggest that hypoxia and infection are important risk factors. In this review we discuss the effect of hypoxia and platelet-activating factor (PAF) on intestinal blood flow and intestinal necrosis, and implicate PAF as an important mediator in hypoxia-induced intestinal injury. Finally we provide evidence that PAF may be important in neonatal NEC.

Hypoxia causes ischemic bowel necrosis in rats: the role of platelet-activating factor (PAF-acether)

We have previously shown that injection of platelet-activating factor causes necrotizing enterocolitis in the rat and that platelet-activating factor is an endogenous mediator in lipopolysaccharide-induced bowel necrosis. Because hypoxia is a known predisposing factor for neonatal necrotizing enterocolitis, we investigated the effect of hypoxia on platelet-activating factor formation and intestinal necrosis. Young male Sprague-Dawley rats were made severely hypoxic by placing them in a 100% N2 chamber for 2 minutes; moderate hypoxia was accomplished using 10% O2 for 15 or 30 minutes. To evaluate the role of platelet-activating factor on intestinal perfusion and injury, two platelet-activating factor antagonists, SRI 63-441 and WEB 2086, were injected 10 minutes before the hypoxic exposure. We found that plasma platelet-activating factor levels were significantly elevated after 2 minutes of severe hypoxia (13.8 +/- 2.9 ng/mL vs. control 2.1 +/- 0.8 ng/mL) and after 30 minutes of moderate hypoxia (41.1 +/- 11.7 ng/mL). This increase in platelet-activating factor level was not caused by decreased degradation, because neither plasma nor intestinal platelet-activating factor acetylhydrolase was decreased in the hypoxic rats. (Intestinal acetylhydrolase activity was actually increased). Intestinal perfusion was markedly decreased at 30 minutes in hypoxic animals. In contrast, all platelet-activating factor antagonist-treated animals had normal intestinal perfusion. Histological examination of affected bowel from hypoxic animals showed early intestinal necrosis which was completely prevented by pretreatment with SRI 63-441 and WEB 2086. Because 30 minutes of hypoxia also resulted in metabolic acidosis, we further investigated if acidosis alone could induce platelet-activating factor release and bowel injury. We found that acidosis alone resulted in moderate increase of plasma platelet-activating factor but did not produce bowel injury. We conclude that platelet-activating factor plays a central role in mediating hypoxia-induced intestinal necrosis. Acidosis may enhance the effect of hypoxia on platelet-activating factor production.