The Mucus Layer is Critical in Protecting Against Ischemia-reperfusion-mediated Gut Injury and in the Restitution of Gut Barrier Function

It is well documented that the gut injury plays a critical role in the development of systemic inflammation and distant organ injury in conditions associated with splanchnic ischemia. Consequently, understanding the mechanisms leading to gut injury is important. In this context, recent work suggests a protective role for the intestinal mucus layer and an injury-inducing role for luminal pancreatic proteases. Thus, we explored the role of the mucus layer in gut barrier function by observing how the removal of the mucus layer affects ischemia-reperfusion-mediated gut injury in rats as well as the potential role of luminal pancreatic proteases in the pathogenesis of gut injury. Ischemia was induced by the ligation of blood vessels to segments of the ileum for 45 min, followed by up to 3 h of reperfusion. The ileal segments were divided into five groups. These included a nonischemic control, ischemic segments exposed to saline, the mucolytic N-acetylcysteine (NAC), pancreatic proteases, or NAC + pancreatic proteases. Changes in gut barrier function were assessed by the permeation of fluorescein isothiocyanate dextran (molecular weight, 4,000 d) in ileal everted sacs. Gut injury was measured morphologically and by the luminal content of protein, DNA, and hemoglobin. The mucus layer was assessed functionally by measuring its hydrophobicity and morphologically. Gut barrier function was promptly and effectively reestablished during reperfusion, which was accompanied by the restoration of the mucus layer. In contrast, treatment of the gut with the mucolytic NAC for 10 min during ischemia resulted in a failure of mucus restitution and further increases in gut permeability and injury. The presence of digestive proteases by themselves did not exacerbate gut injury, but in combination with NAC, they caused an even greater increase in gut injury and permeability. These results suggest that the mucus layer not only serves as a barrier between the luminal contents and gut surface epithelia, but also plays a critical role in the maintenance and restitution of gut barrier function.