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Necrotizing enterocolitis is an important cause of morbidity and mortality in low birth weight infants(1). Many conditions such as asphyxia, hypoxia, hypotension, hyperviscosity, infection, and feedings are thought to predispose low birth weight infants to necrotizing enterocolitis. Tissue hypoxia and ischemia appear to be common elements in many of the risk factors for this disorder(2). Recognition that ischemia-reperfusion impairs endothelial vascular function in the mesenteric artery of newborn pigs supports the contention that ischemia is an important component of intestinal injury in young subjects(2).

Increased production of oxygen free radicals during reperfusion of ischemic gut has been suggested to play a role in the pathogenesis of necrotizing enterocolitis(3). In adult cats, oxygen free radicals have been shown to contribute to intestinal injury after ischemia-reperfusion(4). An oxygen free radical scavenger, superoxide dismutase, has been shown to prevent increases in intestinal capillary permeability and attenuate epithelial necrosis(4). Furthermore, accumulation of arachidonic acid and increased prostanoid production have been demonstrated during reperfusion of ischemic myocardium(5), and increased prostanoid production has been observed during reperfusion of ischemic gut in adult subjects(6). In addition, there appears to be an interaction between oxygen free radicals and prostanoid metabolism(5). Prostanoids may also play a role in the changes occurring during reperfusion of ischemic gut in newborn subjects. Although the newborn pig appears to have a limited capacity to generate oxidants via xanthine oxidase in nonischemic gut or in the resident granulocytes within the mucosa/submucosa(7), it remains possible that oxygen free radicals generated by the prostanoid system(5) and/or white blood cells recruited into ischemic gut might contribute to intestinal injury in newborn pigs.

The present study was designed: 1) to examine the effects of superior mesenteric artery occlusion for 1 h and reperfusion for 2 h in vivo on the development of histopathologic intestinal injury, 2) to determine whether products of arachidonic acid metabolism are produced during reperfusion of ischemic gut, and 3) to determine whether oxygen free radical scavengers attenuate the gut injury in newborn pigs. To study this, we measured intestinal perfusion, oxygenation, prostanoid flux,e.g. intestinal perfusion times arterial minus portal prostanoid concentrations, and tissue pathology in response to mesenteric artery occlusion and reperfusion in newborn pigs pretreated with placebo or SOD plus CAT before ischemia. Placebo-pretreated sham-operated nonischemic pigs were enrolled to serve as a control group. Because hemodynamic changes are also important in the response to necrotizing enterocolitis, hemodynamic variables including vascular pressures, heart rate, cardiac output, and its splanchnic distribution were also measured.

METHODS

Animals. Nineteen 2-4-d-old farm-bred pigs weighing 1.0-1.6 kg were the subjects of this study. The study was approved by the Institutional Animal Care and Use Committee of Brown University and Women and Infants' Hospital of Rhode Island.

Surgical procedures. One day before the study, catheters and a superior mesenteric artery vascular occluder were surgically placed under isoflurane (1-2%), nitrous oxide (10-30%), oxygen (68-69%), and local lidocaine (1%) anesthesia. Polyvinyl catheters were placed into the left ventricle via the carotid artery for radionuclide-labeled microsphere administration, the distal aorta via a femoral artery for radionuclide-labeled microsphere reference sample withdrawal and study sampling, abdominal aorta via the contralateral femoral artery for arterial blood pressure and heart rate monitoring, the inferior vena cava via a femoral vein for transfusion and study of drug administration, and the portal via the umbilical vein for venous sampling. An incision was then made on the right flank of the piglet in the direction of the last rib. The superior mesenteric artery was extraperitoneally isolated, and a vascular occluder placed around the artery 1 cm distal to its origin. Caution was observed to avoid damage to the perivascular nerves, the occluder was carefully secured to avoid traction on the artery, and the flank was closed in layers. After surgery, the pigs received antibiotics, ampicillin (100 mg·kg-1) and gentamicin (5 mg·kg-1), and catheters were filled with heparin (20 U·mL-1) and sealed. The occluder and catheters were placed in a cloth pouch on the flank of the pig. The pig was then dressed in a stockinette to prevent access to the catheters. They were able to ambulate within 15-30 min and were gavage-fed every 3 h with 5% dextrose and 0.45 saline until 8 h before the study. This regimen was selected rather than pig milk replacer to lessen the potential stress of a postprandial hyperemia(8) shortly after surgical manipulation of the superior mesenteric artery. All pigs were studied 18-24 h after surgery.

Experimental protocol. On the day of study, the pigs were assigned to one of three groups as follows: placebo-pretreated, sham-operated control group not exposed to ischemia, designated as placebo-pretreated nonischemic group (n = 5); placebo-pretreated ischemic group exposed to ischemia and reperfusion, designated as placebo-pretreated ischemic group(n = 7); and polyethylene glycol-SOD (5000 U·kg-1) plus polyethylene glycol-CAT (100 000 U·kg-1)-pretreated group, also exposed to ischemia and reperfusion, designated as SOD plus CAT-pretreated ischemic group (n = 7). Before the study, the pigs were placed in a sling, and the deep rectal temperature maintained between 38°C and 39 °C(9). Five sets of determinations were obtained: at baseline, after 1 h of ischemia, and after 20, 60, and 120 min of reperfusion or at 80, 120, and 180 min of study, respectively. The pigs were pretreated with placebo (diluent) or the enzymes 15 min before the onset of ischemia or sham ischemia. Ischemia was induced by occluding the superior mesenteric artery for 1 h. Vascular occlusion was discontinued, and reperfusion was continued for 2 h. In the placebo-pretreated nonischemic group, ischemia was replaced with a sham treatment, which consisted of approaching the piglet and touching the external portion of the occluder.

Enzyme doses and timing of pretreatment were based upon those shown to prevent postasphyxic cerebral hypoperfusion in newborn lambs(10). Polyethylene glycol-SOD and polyethylene glycol-CAT were used, because the polyethylene glycol moiety blocks renal clearance, allowing recirculation of active enzymes for long periods, facilitates active enzyme transport into cells without interfering with their action, and reduces immunogenicity(11).

Each measurement consisted of radionuclide-labeled microsphere determinations of total and mucosal-submucosal and muscularis-serosal intestinal blood flow, cardiac output, and the distribution of cardiac output to the splanchnic organs. Before each blood flow measurement, arterial and portal venous pH, blood gases, oxygen content, prostacyclin, measured as 6-keto-PGF1α, and thromboxane A2, measured as thromboxane B2, and arterial hematocrit were obtained. After each determination, blood was returned to the piglet from an age-and hematocrit-matched donor piglet. At the termination of the study, the piglet was killed by an overdose of pentobarbital (100-200 mg·kg-1). Necropsy was performed for confirmation of catheter placement and procurement of tissues. Four centimeters of the terminal ileum proximal to the ileocecal valve were obtained for tissue pathology. These samples were fixed in 10% buffered formalin for histology. The gastrointestinal tract was removed and gently washed of its contents. The stomach, intestines, colon, liver, pancreas, lungs, and kidneys were weighed and fixed in formalin until tissue processing.

Physiologic and analytical methodologies. According to previously established techniques(8, 9, 12), blood flow was measured with microspheres 15 ± 5 μm in diameter labeled with one of the following six randomly assigned radionuclides:46 Sc, 51Cr, 57Co, 95Nb, 103Ru, or113 Sn (DuPont NEN, Boston, MA). Approximately 1 × 106 microspheres suspended in a 10% dextran solution with 0.01% Tween 80 were continuously agitated and injected into the left ventricle over 30 s. The catheter was flushed with 2 mL of 0.9 normal saline. A reference blood sample was withdrawn at 1.03 mL·min-1 (model 940, Harvard Apparatus, Millis, MA) from the abdominal aorta beginning 10 s before the microsphere injection and lasting for 120 s.

After 7 d of fixation, the intestines were reweighed and divided into mucosa-submucosa and muscularis-serosa by blunt dissection(8). Blood flow was measured separately to the mucosa-submucosa and muscularis-serosa. Total blood flow to the intestines was the sum of these layers. Fixed and carbonized tissue samples were packed to a 1-cm height in glass counting vials(9, 12). Blood and tissue specimens were counted in a well-type γ-scintillation spectrophotometer [Tracor Analytic sample changer (model 1185, Elkgrove Village, IL)] interfaced to a multichannel analyzer (model 4203, Canberra Industries, Meriden, CT). Blood flow data were generated with a MacIntosh computer (Apple Computer, Inc., Cupertino, CA) that corrected for background, decay, and spillover using the least squares method of analysis(12). Blood flow was determined using the followinequation(12): All tissue and reference blood samples had sufficient microspheres, except for a few small regions of the intestines during ischemia to assure blood flow accuracy to within ±10%(12). Paired samples of lungs and kidneys (data not shown) demonstrated the absence of shunting through a ductus arteriosus and errors due to streaming.

Heart rate and mean arterial blood pressure were continuously monitored using a Hewlett-Packard transducer (model 1280, Lexington, MA) and recorded on a Hewlett-Packard Polygraph (model 7754 A series, Waltham, MA). Blood gases and pH were determined on a Corning 168 blood gas analyzer (Corning Scientific, Medford, MA) corrected for body temperature, arterial and venous oxygen content in duplicate on a Lex-O2-Con apparatus (Lexington Instruments Corp., Waltham, MA), and arterial hematocrit in duplicate by a microhematocrit technique.

Prostanoid methodology. Arterial and portal venous whole blood samples obtained for 6-keto-PGF1α and thromboxane B2 were slowly withdrawn into iced nonheparinized polypropylene tubes containing 28 mg·mL-1 EDTA and 40 mg·mL-1 indomethacin and immediately centrifuged, and the duplicate was plasma stored at -70 °C until the time of assay. Prostanoids were extracted using hydrochloric acid to reduce the pH to 3-4. Ethyl acetate was added, the organic and aqueous phases were separated, and the samples were allowed to precipitate to dryness. The residue was dissolved in a 0.01 M phosphate buffer (pH 7) containing 0.1% bovine γ-globulin and 0.1% sodium azide. After 2 h of incubation at 25°C, the bound and free analyte were separated using dextran-coated charcoal. The samples were assayed with RIA (Advanced Magnetics, Cambridge, MA) as adapted by Chemtob et al.(13). Radioactivity was determined in a liquid scintillation counter (T.M. Analytic, Elk Grove, IL). Antibodies to the sampled prostanoids exhibited <1% cross-reactivity with the other prostanoids measured. Efficiency of extraction for prostanoids was >85%. Interassay variability was 6%.

Histologic technique. Terminal ileum samples obtained for histology were approximately 4 × 0.6 × 0.4 cm in size. Sections from each sample were embedded in paraffin, and 5-μm sections were cut and stained with hematoxylin and eosin. Morphologic and morphometric evaluations were performed on coded sections by two pathologists (C.E.O. and H.P.), who were not aware of the piglet's treatment. A grading system was adapted to evaluate the lesions that had features characteristic of ischemic bowel injury(14, 15). The extent of necrosis, hemorrhage, edema, and cellular infiltrate was evaluated as follows. Zero indicated the absence of changes; 1, mild; 2, moderate; and 3, severe changes. A total score reflecting the overall severity of changes evaluated separately in the mucosa and submucosa was calculated by the sum of the grade of necrosis times 3, hemorrhage times 2, edema and cellular infiltrate each times 1. Thus, the total range of possible scores varied from 0 to 42. Because there were no changes in the muscularis propria or serosa, these regions were not scored.

Computations and statistical analysis. Intestinal oxygen uptake(˙Vo2) was calculated using the following formula derived from the Fick principle as previously described(8):˙Vo2 = ˙Q × (Cao2 - Cpvo2), where ˙Q is blood flow, Cao2 is arterial oxygen content, and Cpvo2 is portal venous oxygen content.

Prostanoid venous effluents are thought to reflect concentrations in specific organ vasculatures(16). Arterial and portal venous prostanoid concentrations were measured, and intestinal flux of prostanoids was calculated. Although the equation outlined above for uptake is most accurately applied to substrates with unidirectional flux such as oxygen, this equation was used to quantitate intestinal prostanoid influx (+˙V 6-keto-PGF1α) and efflux (-˙V 6-keto-PGF1α). The same procedure has previously been used to examine cerebrovasculature prostanoid flux(16). Cardiac output was determined as the summation of blood flow to the individually measured organs and carcass and expressed as mL·min-1·kg-1. Blood flow was expressed as mL·min-1·100 g-1 fresh tissue weight,˙Vo2 was expressed as μmol·min-1·100 g-1, and prostanoid efflux and influx as pg·mL-1·100 g-1 fresh tissue weight.

ANOVA for repeated measures was used for the examination of significant differences within and among the study groups. If a significant difference was found by ANOVA, Neuman-Keuls post hoc testing was used to detect statistically significant differences among the study periods. To further describe and enhance statistically significant groups by time interactions, additional analyses were performed. Separate ANOVA values for repeated measures on each of the three groups and pairwise group comparisons were also performed. When a significant difference between groups was detected, an unpaired t test with the Bonferroni correction was used to identify differences between groups at each study period. ANOVA for repeated measures was also used to compare the responses of blood flow within the mucosa-submucosa with that of the muscularis. When a significant difference was found, the paired t test with the Bonferroni correction was used to identify differences between the layers at each study period. ANOVA for repeated measures was also used to define influx or efflux of prostanoids compared with a value of zero (no net flux). The histopathologic intestinal scores were compared among the groups by the nonparametric Mann Whitney U test. All data were expressed as mean ± SEM; p< 0.05 was considered statistically significant unless otherwise specified.

RESULTS

Arterial and portal pH and base excess decreased during the ischemia and reperfusion study periods in the placebo- and SOD plus CAT-pretreated groups(Table 1). Although small decreases in arterial base excess were also observed in the placebo-pretreated nonischemic group, the changes were greater (ANOVA, interactions: p < 0.05) in both pretreated ischemic groups. Small but significant decreases in arterial Paco2 were observed during the same study periods in the placebo-pretreated ischemic group. Hematocrit values (mean baseline = 27± 1) were equivalent among the groups and did not change during the studies (data not shown).

Table 1 Arterial and portal pH, Po2, Pco2, and base excess values for study groups by study period
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Mean arterial blood pressure increased during ischemia in both pretreated groups (Table 2). Heart rate increased during and after ischemia in the placebo- and not in the SOD plus CAT-pretreated group. Cardiac output decreased significantly during ischemia and reperfusion in the placebo-pretreated ischemic group, during reperfusion in the SOD plus CAT-pretreated group, and at 60 min (120 min of study) of reperfusion in the placebo-pretreated nonischemic group. Portal venous pressure (mean baseline = 6 ± 2 mm Hg) was similar among the groups and did not change significantly during the studies (data not shown).

Table 2 Hemodynamic variables for study groups by study period
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Baseline values of 6-keto-PGF1α flux were similar among the groups (Fig. 1). In the placebo and SOD plus CAT-pretreated ischemic groups, 6-keto-PGF1α efflux was evident(p < 0.05 versus no or zero efflux) 20 min (80 min of study) after ischemia. Thereafter, efflux of this prostanoid decreased. Changes were not observed in the placebo-pretreated nonischemic group. Significant differences in thromboxane B2 (mean baseline = 0.4 ± 0.3 pg·min-1·100 g-1·104) flux were not observed among the groups or over the study periods (data not shown).

Figure 1
figure 1

6-Keto-PGF1α flux plotted against study time in minutes before ischemia at baseline, during ischemia and reperfusion for the three study groups: placebo-pretreated nonischemic (n = 5); placebo-pretreated ischemic (n = 7); and SOD plus CAT-pretreated ischemic (n = 6); n = number of measurements unless indicated in parentheses. *p < 0.05 vs baseline,+p < 0.05 vs no or zero efflux, §p < 0.01 vs placebo-pretreated nonischemic group for the same study period, ‡ANOVA, interactions: p < 0.05 vs placebo-pretreated nonischemic group over time.

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Superior mesenteric artery occlusion for 1 h was associated with an expected reduction in intestinal blood flow, which increased during reperfusion, but remained significantly below baseline values (Fig. 2). The patterns of change in the intestinal mucosa-submucosa blood flow were also similar to that of the total intestine during ischemia and reperfusion (Fig. 3). After ischemia, blood flow to the muscularis was similar to baseline values. The patterns of change in blood flow to the mucosa-submucosa and muscularis differed significantly (ANOVA, interactions: p < 0.05) between the pretreated ischemic groups and the placebo-pretreated nonischemic group. The patterns of change in blood flow were similar in both pretreated ischemic groups. Blood flow to the mucosa-submucosa was higher (p < 0.05) than to the muscularis for all study periods within the placebo-pretreated nonischemic group. The patterns of change in blood flow to the mucosa-submucosa differed significantly (ANOVA, interactions: p < 0.05) from that of the muscularis within both of the pretreated ischemic groups, and not the nonischemic pretreated group. Changes in blood flow (Figs. 2 and 3) were not observed during the study periods in the placebo-pretreated nonischemic group.

Figure 2
figure 2

Total intestinal blood flow plotted against study time in minutes for the three study groups: placebo-pretreated nonischemic(n = 5); placebo-pretreated ischemic (n = 7); and SOD plus CAT-pretreated ischemic (n = 7); n = number of measurements unless indicated in parentheses. *p < 0.05vs baseline, §p < 0.01 vs placebo-pretreated nonischemic group for the same study period, ‡ANOVA, interactions: p < 0.05 vs placebo-pretreated nonischemic group over time.

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Figure 3
figure 3

Small bowel mucosa and muscularis blood flow values plotted against study time in minutes for the three study groups: placebo-pretreated nonischemic (n = 5), placebo-pretreated ischemic(n = 7), and SOD plus CAT-pretreated ischemic (n = 7);n = number of measurements unless indicated in parentheses. Symbol are as in Figure 2. ‡ANOVA: interactions p < 0.05 for both mucosa and muscularis of the placebo and SOD plus CAT-pretreated ischemic groups vs placebo-pretreated nonischemic group over time.

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After the ischemia-related reduction in the percent of cardiac output distributed to the gastrointestinal tract, there was a postocclusive increase that was similar to baseline in both ischemic groups (Table 3). The percent of cardiac output distributed to the pancreas decreased significantly during ischemia and reperfusion and to the liver did not change. Changes were not observed in the placebo-pretreated nonischemic group.

Table 3 Percent of cardiac output distributed to the splanic circulation for study groups by study period
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Oxygen uptake was reduced during ischemia and similar to baseline values during reperfusion in the placebo and SOD plus CAT-pretreated ischemic groups (Fig. 4). Changes in intestinal oxygenation were not observed in the placebo-pretreated nonischemic group.

Figure 4
figure 4

Oxygen uptake plotted against study time in minutes for the three study groups: placebo-pretreated nonischemic (n = 5), placebo-pretreated ischemic (n = 6), and SOD plus CAT-pretreated ischemic (n = 6); n = number of measurements unless indicated in parentheses. *p < 0.05 vs baseline,‡ANOVA, interactions: p < 0.05 for the placebo and SOD plus CAT-pretreated ischemic groups vs placebo-pretreated nonischemic group over time.

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Based upon our scoring system with a range of possible total scores from 0 to 42, the placebo-pretreated nonischemic pigs had scores of 3 with a range of 0-7 (Table 4). The total scores of 18 with a range of 14-23 in the placebo-pretreated and of 17 with a range of 10-23 in the SOD plus CAT-pretreated ischemic groups, each differed significantly from that of the placebo-treated nonischemic group. Necrosis of the mucosa, destruction of the surface epithelium, lamina propria congestion, hemorrhage, edema, and infiltration with cells were evident. Submucosal congestion, edema, and infiltration with cells were also observed. There were no pathologic changes in muscularis propria and serosa. Findings were similar between the placebo and the SOD plus CAT-pretreated, ischemic groups. In Figure 5, the histopathologic changes in the ileal mucosa of the placebo and SOD plus CAT-pretreated ischemic pigs are compared with nonischemic pig mucosa.

Table 4 Intestinal histopathology by study group
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Figure 5
figure 5

(A) Ileal mucosa from nonischemic pig.(B) Mucosa and submucosa from a placebo-pretreated ischemic pig.(C) Wall of ileum from a SOD plus CAT-pretreated ischemic pig. Panel A shows deep crypts (C) lined by mucin-secreting (goblet) cells. The cells of the lamina propria (L) are stromal fibrocytes, occasional lymphocytes, and fewer eosinophils. In panel B only a few degenerated epithelial cells (white arrow) persist deep in the crypts. Capillaries in the lamina propria are markedly congested (black arrow). The increased cellularity in the lamina propria is due to lymphocytes and an occasional neutrophil. Similar changes are noted in panel C. The absence of changes in the muscularis propria (between arrowheads) is apparent. In other areas in both placebo and SOD plus CAT-pretreated ischemic pigs, hemorrhage in the lamina propria was prominent. Bar in panel A equals 100 μm. All photographs taken using×10 objective. Hematoxylin and eosin stain.

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DISCUSSION

The purpose of our study was to examine 1) the effects of superior mesenteric artery occlusion and reperfusion in vivo on the development of intestinal histopathologic injury; 2) to determine whether products of arachidonic acid metabolism, e.g. prostanoids were increased during reperfusion of ischemic gut; and 3) to determine whether oxygen free radical scavengers attenuated the gut injury in newborn piglets. Ischemia-reperfusion resulted in superior mesenteric artery occlusive shock reflected by portal venous metabolic acidosis (Table 1), most likely because of anaerobic intestinal metabolism during ischemia. The increase in mean arterial blood pressure during superior mesenteric artery occlusion was probably a result of increases in blood volume from the availability of blood normally perfusing the intestines(Table 2). The tachycardia in the placebo-pretreated ischemic group may represent a compensatory response to offset decreases in the cardiac output and possibly systemic effects of substances released from ischemic intestines. Likewise, the mechanisms of the reduced cardiac output in both pretreated ischemic groups are probably multifactorial, including hormonal, sympathetic stimulation, volume contraction, because of fluid losses to the intestinal lumen and effects of substances released from the injured gut. The basis for the decreased cardiac output at 120 min of the study in the pretreated nonischemic group remains unexplained. However, this reduction in cardiac output was not accompanied by changes in intestinal perfusion, oxygenation, prostanoid flux, or histopathology, suggesting that the placebo-pretreated nonischemic group was an appropriate control group.

The effects of superior mesenteric artery occlusion and reperfusion were examined because intestinal perfusion is compromised in many conditions associated with necrotizing enterocolitis, such as asphyxia, hypoxia, hypotension, sepsis, and cold stress(9, 15, 17–20). In addition, ischemia potentially participates in etiology of this disorder(2). In most previous studies, changes have not been examined after termination of the insult, and histopathologic intestinal injury has not been simultaneously examined in vivo in newborn subjects. Previous work in adult dogs demonstrated that reperfusion of severely ischemic small bowel resulted in a progressive decline in blood flow(21). Consistent with these findings, in our study intestinal blood flow remained below baseline during reperfusion. This pattern of perfusion was associated with severe histopathologic changes in our newborn pigs (Table 4, Fig. 5).

Previous work in newborn pigs has evaluated the effects of ischemia-reperfusion on intestinal circulation by studying in vitro intestinal segments(22). In contrast to our findings, reperfusion of in vitro ischemic intestinal segments from pigs at a similar age resulted in blood flow values that were equivalent to those of controls(22). Because in vitro denervated intestinal segments evaluate intrinsic rather than intrinsic and extrinsic vascular regulation, it is likely that extrinsic factors may have played a more important role in our study.

The mucosal and muscularis layers of the intestines exhibited different patterns of blood flow responses (Fig. 3). The mucosa remained underperfused after ischemia, whereas muscularis perfusion was similar to baseline during reperfusion, suggesting the presence of a limited reactive hyperemia in this layer. This finding may be related to enhanced intestinal motility, which has been reported after arterial occlusion in adult subjects(23). In this regard, we have previously shown that blood flow to the jejunal muscularis and not the mucosa increased after hypoxia(20). In adult dogs, the superficial mucosa is more sensitive to ischemia than is the more resistant muscularis(24, 25). In our study, compromised mucosal-submucosal perfusion most likely contributed to selective mucosal degeneration, including subepithelial edema and interruption of epithelial integrity. In contrast, the relatively sustained muscularis perfusion was associated with resistance to histopathologic injury (Fig. 5).

In contrast to the established role for prostanoids in vascular regulation of normal and ischemic gut in adult subjects(6, 21), little information is available regarding prostanoid metabolism in the immature intestine. Knowledge of prostanoid metabolism in newborn subjects is of interest because of the widespread use of indomethacin to prevent intraventricular hemorrhage and to treat patent ductus arteriosus(26, 27). Prostacyclin (6-keto-PGF1α) and PGE2 are vasodilator prostanoids that represent the majority of the vasoactive prostanoids in the normal adult gut(28). During reperfusion of ischemic gut, altered arachidonic acid metabolism is thought to be responsible for many vascular changes including neutrophil infiltration and migration, platelet aggregation and vasomotor changes(11, 21). In premature sheep with patent ductus arteriosus, cyclooxygenase inhibition by indomethacin decreases intestinal blood flow and blunts the regulation of oxygen metabolism in the ileum(29). In newborn pigs, we established that there was an early reperfusion-related efflux of the vasodilator prostacyclin(6-keto-PGF1α, Fig. 1), suggesting that this vasodilator prostanoid represents a component of the response to mesenteric ischemia. Efflux of this prostanoid after markedly decreased intestinal perfusion may represent its accumulation during ischemia. Given the evidence that cyclooxygenase inhibition adversely affects intestinal perfusion and oxygen metabolism in premature lambs with patent ductus arteriosus(29), and our findings that 6-keto-PGF1α efflux is evident after ischemia suggests that prostanoids may participate in the response to altered states of intestinal perfusion in immature subjects.

Controversy exists regarding potential interactions between oxygen free radicals and prostanoids(5, 30–32). Although oxygen-derived free radicals are generated during conversion of PGG2 to PGH2(5), oxygen free radical scavengers stimulate the breakdown of arachidonic acid and enhance conversion of PGG2 to PGH2(5). Thus, it appears that free radicals generated during PG metabolism exert a feedback loop that reduces the activity of the cyclooxygenase system. Likewise, reductions in oxygen free radicals stimulate cyclooxygenase activity by preventing this feedback loop(5, 30, 31). In contrast, superoxide dismutase has been reported to attenuate 6-keto-PGF1α release in a rabbit model of necrotizing enterocolitis induced by a luminal insult which causes local generation of free radicals(32). In our study, 6-keto-PGF1α efflux was similar in the placebo- and SOD plus CAT-pretreated pigs, suggesting that the oxygen free radical scavengers did not alter PG metabolism after ischemia (Fig. 1).

Ischemic intestinal injury occurs when blood flow is reduced to such an extent that tissue oxygenation is severely restricted. In our study, the reduction in perfusion during ischemia was sufficient to severely restrict oxygen metabolism. Oxygen uptake remains independent of perfusion until intestinal blood flow is reduced to such an extent that it becomes perfusion dependent(33). The ability to maintain oxygen uptake during ischemia is a critical factor in providing protection from mucosal damage(33). Therefore, severe restrictions in perfusion during ischemia, resulting in decreased oxygen uptake, probably contributed to the histopathologic mucosal damage in our newborn pigs.

The mucosal damage in our pigs could have been accounted for by the cytotoxic effects of free radicals produced during reperfusion of ischemic gut(34–36). Although the newborn pig has a limited capacity to generate oxidants via xanthine oxidase in nonischemic gut and in the resident granulocytes within the mucosa/submucosa(7), oxygen free radicals are also generated during prostanoid metabolism and by white cells recruited to ischemic gut(5, 37–39). Moreover, although negligible quantities of xanthine oxidase were detected in adult pig heart, oxygen free radical scavengers improve functional recovery of reperfused ischemic myocardium(37). Taken together the findings outlined above suggest that there was reason to hypothesize that oxygen free radical scavengers might have been protective in reperfused tissues that do not contain xanthine oxidase, such as gut in newborn pigs. Nonetheless, contrary to our hypothesis, superoxide dismutase and catalase did not attenuate the ischemic damage in our study. It is possible that protection was not conferred because of the dose and/or timing of our pretreatment regimen or that oxygen free radicals do not play a major role in intestinal ischemic-reperfusion injury in newborn pigs.

In summary, after superior mesenteric artery occlusion for 1 h and reperfusion for 2 h, severe histopathologic intestinal injury was detected in vivo, prostanoid efflux was detected during early reperfusion, and SOD plus CAT given just before occlusion did not attenuate the extent of injury in newborn pigs.