William MacKendrick

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.

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.

Endogenous Nitric Oxide Protects against Platelet-Activating Factor-Induced Bowel Injury in the Rat

ABSTRACT: 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 μg/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. Treatment with superoxide dismutase and catalase did not ameliorate the effects of L-NAME on PAF-induced injury, hemoconcentration, and neutrophil infiltration into the bowel. We conclude that endogenous nitric oxide defends against PAF-induced bowel injury by antagonizing certain immediate effects of PAF, particularly those leading to capillary leakage and neutrophil infiltration into the bowel. Our studies also suggest that the mechanism of action does not involve vasodilatation or scavenging of oxygen radicals.

Serial Diffusion Tensor Imaging Detects White Matter Changes That Correlate with Motor Outcome in Premature Infants

The objective of the study was to assess predictive value of serial diffusion tensor MRI (DTI) for the white matter injury and neurodevelopmental outcome in a cohort of premature infants. Twenty-four infants less than 32 weeks’ gestation were stratified to a control group (n = 11), mild brain injury with grades 1–2 of intraventricular hemorrhage (n = 6) and severe brain injury with grades 3–4 intraventricular hemorrhage (n = 4). Serial DTI studies were performed at around 30 and 36 weeks’ gestation. Fractional anisotropy (FA) and apparent diffusion coefficient were calculated. Twelve infants were followed up for developmental outcome. Developmental testing was performed with the Bayley Scales of Infant Development to obtain psychomotor index (Performance Developmental Index). Apparent diffusion coefficient was higher in the severe injury group at the second MRI in the central and occipital white matter, and corona radiata; FA was lower in optic radiation compared to controls. Performance Developmental Indexscore correlated with FA on the scan taken at the 30th week and inversely with the change of FA between scans in internal capsule and occipital white matter. A low value of FA at 30 weeks and a higher change of FA predicted less favorable motor outcome at 2 years and suggests that early subtle white matter injury can be detected in premature infants even without obvious signs of injury.

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.

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.

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.

Use of clonidine in the prevention and management of neonatal abstinence syndrome

Introduction. Neonatal abstinence syndrome (NAS) is a complicated medical condition with treatment regimens that traditionally have included methadone and other opioids, barbiturates, and benzodiazepines. We describe a case series in which clonidine was used for the prevention and management of patients with NAS. Patients and Methods. Medical records of infants treated with clonidine for NAS from January 2003 to March 2006 were reviewed for gestational age, birth weight, NAS score, dose of clonidine, duration of treatment, and additional medications required. Results. Fourteen patients were identified. The mean gestational age was 30.1 weeks (range 24.4–40.7 weeks); three patients were full-term. Eleven had been on intravenous fentanyl for sedation; three were born to opioid-dependent mothers. All patients were treated with clonidine, administered in doses of 0.5–1.0 mcg/kg orally every 6 h. No patient received opioids. Mean duration of treatment was 6.8 days (range 4–15). Mean abstinence scores were 6.4 pretreatment (range 0–20) and 1.9 posttreatment (range 0–5). No patients suffered an adverse event (hypotension, bradycardia, excessive sedation, and oxygen desaturation) from clonidine administration, and no seizures were identified. Conclusions. Our data suggest that clonidine may be a reasonable alternative to more traditional agents used to prevent or treat NAS. We agree with the statement of the American Academy of Pediatrics Committee on Drugs that states that larger trials and pharmacologic data are needed before the routine use of clonidine can be recommended.

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.

TRANSFORMING GROWTH FACTOR-β PREVENTS THE ACUTE PLATELET ACTIVATING FACTOR-INDUCED INCREASE IN INTESTINAL MUCOSAL PERMEABILITY IN JUVENILE RATS.• 721

TRANSFORMING GROWTH FACTOR-β PREVENTS THE ACUTE PLATELET ACTIVATING FACTOR-INDUCED INCREASE IN INTESTINAL MUCOSAL PERMEABILITY IN JUVENILE RATS. • 721