Two theories have been proposed to account for the mucosal injury associated with intestinal ischemia, hypoxia-countercurrent exchange, and oxygen free radicals. The countercurrent mechanism suggests that mucosal injury should occur predominately during the ischemic period, whereas the oxygen free radical hypothesis predicts that the majority of mucosal injury results from reperfusion of ischemic tissue. Histological specimens obtained during the ischemic period and following reperfusion allowed a systematic evaluation of the time course of development of mucosal lesions in a regional ischemia model. Reperfusion after 3 h of regional hypotension reduced mean mucosal thickness from 1,022.2 +/- 6.3 to 503.6 +/- 10.0 microns. The decrease in mucosal thickness was largely due to a reduction in villus height, inasmuch as the reduction in crypt depth was statistically insignificant. A significantly smaller change in mucosal thickness was observed when the bowel was subjected to 3 h ischemia without reperfusion. The mucosal injury produced by 3 h ischemia and 1 h reperfusion was more severe than that produced by 4 h ischemia without reperfusion. The results of this study suggest that most of the tissue damage produced by the widely employed regional hypotension model occurs at the time of reperfusion.

A growing body of experimental data indicates that reactive oxygen metabolites such as superoxide, hydrogen peroxide, and hydroxyl radicals may mediate the microvascular and parenchymal injury produced by reperfusion of ischemic skeletal muscle. One potential source of these reactive oxygen metabolites is the inflammatory neutrophil. To assess neutrophil accumulation in postischemic skeletal muscle, we measured tissue myeloperoxidase (MPO) activity in skeletal muscle biopsies taken during control, after 4 h of ischemia, and after 1 h of reperfusion. Tissue levels of reduced glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT) were measured in the same samples to identify alterations in tissue free radical defense mechanisms due to ischemia-reperfusion. Reperfusion of ischemic skeletal muscle was associated with a dramatic increase in tissue neutrophil content (as reflected by a 26-fold increase over control in tissue MPO activity after 1 h of reperfusion) and a concurrent 50% decrease in GSH content. Tissue CAT and SOD activities were unaffected by ischemia-reperfusion. These results suggest a possible relationship between ischemia-reperfusion-induced injury, neutrophil infiltration, and the reduction in tissue GSH.

The gastrointestinal epithelium is continuously exposed to reactive oxygen metabolites that are generated within the lumen. In spite of this exposure, the healthy epithelium appears unaffected, suggesting efficient mechanisms for protection against these potentially cytotoxic oxidants. The objective of this study is to characterize the interaction between purified gastric mucin and hydroxyl radicals generated from the interaction between ferric iron and ascorbic acid. We found that both native and pronase-treated mucin effectively scavenged hydroxyl radical and that the scavenging properties were not significantly different. The effective concentration of mucin required for a 50% reduction in malondialdehyde production was approximately 10 mg/ml for both native and pronase-treated mucin. In addition, the iron-ascorbic system produced a dramatic decrease (greater than 50%) in the specific viscosity of mucin that was inhibited by catalase, deferoxamine, and mannitol. Superoxide dismutase had no effect. These data suggest that hydroxyl radicals derived from the iron-catalyzed decomposition of hydrogen peroxide are responsible for the depolymerization of native mucin. We propose that mucin may provide protection to the surface epithelium of the gastrointestinal tract by scavenging oxidants produced within the lumen; however, it does so at the expense of its viscoelastic properties.

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