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.