Endogenous bacterial toxins are required for the injurious action of platelet-activating factor in rats.

BACKGROUND & AIMS: Platelet-activating factor (PAF), an endogenous mediator for experimental sepsis, has been shown to induce shock and intestinal necrosis in vivo. However, it is unclear whether PAF exerts its injurious effects on the intestinal tissue directly or via synergism with other endogenous products. The aim of this study was to examine the role of endogenous bacterial products, such as endotoxin, in PAF-induced intestinal injury. METHODS: PAF (3 micrograms/kg) was injected intravenously into normally colonized rats, germfree rats, and normal rats pretreated with a combination of antibiotics, and the systemic response and intestinal injury were assessed. RESULTS: PAF did not cause prolonged shock, leukopenia, hemoconcentration, and bowel necrosis in germfree rats. When germfree rats were primed with a low dose (0.5 mg/kg) of endotoxin, the protection was lost. Combined treatment of the normally colonized rats with neomycin, polymyxin B, and metronidazole for 7 days largely protected the animal from PAF-induced shock and intestinal necrosis. CONCLUSIONS: PAF does not directly induce prolonged hypotension, hemoconcentration, persistent leukopenia, and gross intestinal necrosis but causes these changes via a synergism with endogenous bacterial toxins, presumably from the gut flora.

Inflammatory mediators and intestinal injury.

Although the causes of necrotizing enterocolitis (NEC) are not well understood, there is compelling evidence to suggest that the inflammatory mediators play an important role in the pathophysiology of the disease. This article examines the role of platelet-activating factor (PAF) and other mediators on the development of NEC, and attempts to explain the association of the putative NEC risk factors with altered mediator production and subsequent intestinal injury. The authors hypothesize that PAF is a key mediator in the final common pathway leading to NEC.

The role of endogenous nitric oxide and platelet-activating factor in hypoxia-induced intestinal injury in rats.

BACKGROUND/AIMS: Nitric oxide is an endothelium-derived relaxing factor that promotes capillary integrity, inhibits leukocyte adherence and activation, and scavenges oxygen radicals. Because these effects are important in experimental intestinal injury, we studied the role of NO inhibition on hypoxia-induced bowel necrosis in the rat and investigated the interaction between platelet-activating factor (PAF) and NO in this model. METHODS: Sprague-Dawley rats were treated with either hypoxia, NO synthase inhibition (NG-methyl-L-arginine [LNMA] or NG-nitro-L-arginine methyl ester [L-NAME]), hypoxia+LNMA, hypoxia+LNMA+NO donors, or hypoxia+LNMA+PAF receptor inhibition. Evaluations included blood pressure, superior mesenteric artery blood flow, arterial blood gases, histological intestinal injury, intestinal myeloperoxidase activity, and intestinal PAF activity. RESULTS: We found that hypoxia alone for 90 minutes (10% O2, partial O2 pressure = 45 mm Hg) or LNMA alone had no detrimental effects. However, hypoxia+LNMA together caused hypotension, metabolic acidosis, intestinal injury, increased intestinal myeloperoxidase activity, and elevated intestinal PAF concentrations that were prevented by exogenous L-arginine. Furthermore, the hypotension and intestinal injury was prevented by PAF receptor blockade. CONCLUSIONS: We conclude that endogenous NO protects the intestine from hypoxia-induced inflammation and injury, and the balance between local PAF and NO modulates the outcome of hypoxia-stressed intestine.

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.

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.

Endogenous nitric oxide protects against platelet-activating factor-induced bowel injury in the rat.

Platelet-activating factor (PAF) causes bowel necrosis in animal models that is histologically identical to that seen in neonatal necrotizing enterocolitis, but little is known about endogenous mechanisms that might protect against PAF-induced bowel injury. We hypothesized that endogenous nitric oxide might represent such a protective mechanism. Adult male Sprague-Dawley rats were pretreated with 2.5 mg/kg NG-nitro-L-arginine methyl ester (L-NAME), a potent nitric oxide synthase inhibitor, and given injections of 1.5 micrograms/kg PAF 15 min later. Animals treated with normal saline placebo, L-NAME alone, and PAF alone were also studied. Superior mesenteric artery blood flow and blood pressure were continuously recorded. At the end of 2 h or upon death of the animal, hematocrit was measured and intestinal samples were taken for histologic examination and determination of myeloperoxidase activity, a measure of intestinal neutrophil content. Compared with animals given PAF alone, animals pretreated with L-NAME followed by PAF developed significantly worse bowel injury (median injury scores: 2.5 versus 0.5, p = 0.005), hemoconcentration (final hematocrit 65.2 +/- 2.0% versus 53.9 +/- 1.0%, p < 0.001), and intestinal myeloperoxidase activity (12.45 +/- 1.94 U/g versus 6.51 +/- 0.57 U/g, p < 0.01). The last two effects were further accentuated when 10 mg/kg L-NAME was given before PAF. Treatment with sodium nitroprusside, a nitric oxide donor, for 10 min before and after PAF administration reversed the effects of L-NAME. Animals pretreated with phenylephrine rather than L-NAME did not develop worse injury than animals treated with PAF alone despite comparable reductions in superior mesenteric blood flow before PAF treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Increase in plasma platelet-activating factor levels in enterally fed preterm infants.

Because platelet-activating factor (PAF) has been implicated in the pathogenesis of neonatal necrotizing enterocolitis (NEC), we designed a prospective study to examine plasma PAF levels during the first 14 days of feeding in a population of neonates of less than 32 weeks gestation. We found that significantly more patients had detectable plasma PAF levels on days 3 and 14 of feeding when compared to their prefeeding levels (7% on day 0 vs. 26% at day 3, p = 0.04; none on day 0 vs. 18.5% at day 14, p = 0.01). This finding could not be explained by decreased plasma activity of acetylhydrolase, the PAF breakdown enzyme, spontaneous endotoxinemia or a maturational effect. None of the infants who developed detectable PAF levels after feedings were begun went on to develop NEC. We conclude that our findings may reflect increased intestinal PAF production with the provision of feedings to some premature infants. However, this phenomenon by itself does not appear to be a sufficient condition for the subsequent development of NEC.