Kathryn A. Heel

The pathobiology of peritonitis.

The peritoneum is more than a mechanical covering that allows for the easy gliding of opposed peritoneal surfaces. The peritoneal mesothelial cells facilitate the action of powerful innate immune mechanisms. In addition, the peritoneal-associated lymphoid tissues contain unique cells that may play a crucial role in the localization of intraperitoneal infection. A clearer understanding of the molecular and cellular events underlying peritoneal functions in both the unstimulated and stimulated state will aid future treatment of peritonitis.

REVIEW: Peyer's patches

The function of Peyers patches as antigenic sampling sites involves the complex interplay of a variety of mechanisms that aim to recognize luminal antigens, induce an immunological response and decrease the incidence of antigen translocation across the mucosal epithelium. This is achieved by M cells, which facilitate the uptake of luminal antigens, a vascular architecture that promotes the retention of absorbed antigens within the patch interstitium (allowing for maximal antigenic activation of lymphocytes) and the presence of lymphoid follicles that contain antigen‐presenting cells and lymphocytes. Lymphocytes encountering antigen in the Peyers patches proliferate, differentiate into fully mature antigen‐specific effector cells and migrate to the mesenteric lymph nodes where they undergo final maturation. The mature lymphocytes then enter the systemic circulation and migrate throughout the other mucosa‐associated lymphoid tissues of the body and ‘home’ into the gut via high endothelial venules and gut‐associated lymphoid tissue‐specific adhesion molecules, providing antigen‐specific lymphocytes at sites likely to re‐encounter the antigen.

The role of glutaminase in the small intestine.

Glutaminase is the enzyme which hydrolyses glutamine, the main respiratory fuel of the intestine, to yield glutamate and ammonia. Glutaminase has a central role in intestinal metabolism: the products of the reaction catalyzed by glutaminase can be transaminated, catabolized to yield energy or used for the biosynthesis of pyrimidine nucleotides. Experimental treatments which deprive the intestine of glutamine induce intestinal atrophy. In this review, attention is paid to the role of glutaminase in intestinal metabolism. Background information on the structure, kinetics and distribution of glutaminase precede a discussion of the metabolism of glutamine within the intestine. In closing, we review the factors known to regulate glutaminase activity and emphasise that the regulation of glutaminase within the intestine is poorly understood.

The effect of minimum luminal nutrition on bacterial translocation and atrophy of the jejunum during parenteral nutrition

In situations of catabolic stress, the gut becomes atrophic and may have diminished barrier function as evidenced by an increase in bacterial translocation. The aim of this study was to examine the effect of minimum luminal nutrition during parenteral nutrition on the extent of jejunal atrophy and rate of bacterial translocation. Central venous lines were inserted into 30 rats before they underwent randomization to receive nutritional support with: (a) conventional parenteral nutrition; (b) conventional parenteral nutrition with 3 g/day of rat food (i.e., minimum luminal nutrition); or (c) rat food ad libitum. The rats were assessed after 10 days for nutritional status, extent of jejunal atrophy, caecal flora, as well as the extent of bacterial translocation to the mesenteric lymph nodes, liver and spleen. Rats in the rat food ad libitum group lost the smallest amount of weight and had the least amount of jejunal atrophy, yet had a similar rate of bacterial translocation as the parenterally nourished groups. When compared with the conventional parenteral nutrition group, the minimum luminal nutrition group had better preservation of the weight of the small bowel and its isolated mucosa (P < 0.01), but had a similar rate of bacterial translocation. Minimum luminal nutrition reduced the extent of atrophy of the gut but did not affect the incidence of bacterial translocation. It is inferred that there is no direct relationship between the extent of mucosal atrophy and incidence of bacterial translocation.

The effect of minimum luminal nutrition on mucosal cellularity and immunity of the gut

Many catabolic patients can only consume small volumes of enteral nutrients. The aim of this study was to evaluate markers of cellularity and immunity in the small intestine of rats randomized to receive 6 days of parenteral nutrition, 25% enteral and 75% parenteral nutrition (i.e. minimum luminal nutrition) or enteral nutrition. The same glutamine‐enriched solution was used for both parenteral and enteral nutrition. Enteral nutrition was associated with the least amount of jejunal atrophy (P < 0.01), with the results from the minimum luminal nutrition group approximating those of the parenteral nutrition group. Parenteral nutrition was associated with the greatest number of CD2+ cells ( P < 0.05) and the lowest CD4/CD8 cell ratio ( P < 0.01) in the jejunal mucosa. In essence, we failed to demonstrate that there are any appreciable benefits associated with the enteral consumption of 25% of a nutrient load.

Warm Ischaemia Time in a Model for Small Bowel Transplantation

Intestinal transplantation is associated with high rates of mortality and morbidity. This paper details our initial experience with 82 heterotopic small bowel transplants based upon the original rat model described by Monchik and Russell (Surgery 70:693–702, 1971). A key issue associated with mortality was a warm ischaemia time of more than 40 min (P < 0.01). Sixty‐eight percent of the recipients (44/65) survived for more than 24 hr when the warm ischaemia time of the donor bowel was reduced to less than 40 min. Investigators establishing an animal model of heterotopic small bowel transplantation should pay particular attention to the warm ischaemia time of the donor bowel.

The influence of ischemia/reperfusion injury on the jejunum.

Ischemia/reperfusion injury (IRI) after free tissue transfer of the small intestine results in transmural tissue damage. This study examined the effects of IRI on the jejunum. Wistar rats served either as controls (N = 10) or underwent clamping of the infrarenal aorta for 1 hour followed by 1 hour of reperfusion (N = 10). Both ischemia and reperfusion reduced the protein and deoxyribonucleic acid content of the jejunal mucosa (p < 0.05). Myeloperoxidase activity in the jejunal mucosa remained relatively low. The expression of leukocyte function-associated antigen 1 and intercellular adhesion molecule 1 (ICAM-1) on the surface of mucosal cells was not altered significantly by the ischemic insult, but was reduced after the period of reperfusion (p < 0.05). This coincided with an increase in messenger ribonucleic acid (mRNA) for ICAM-1 within isolated mucosal cells (p < 0.05). The specific activity of glutaminase in isolated jejunal mucosal cells was diminished after ischemia and reperfusion (p < 0.05), and this was not associated with an appreciable change in glutaminase mRNA expression. These results have identified some molecular mechanisms underlying IRI of the small intestine that are possible candidates for therapeutic intervention.

Enteral branched-chain amino acids increase the specific activity of jejunal glutaminase and reduce jejunal atrophy

Branched‐chain amino acid (BCAA)‐enriched nutrient solutions reduce gut atrophy associated with parenteral nutrition. We hypothesized that this effect was mediated by phosphate‐dependent glutaminase. Thirty male Wistar rats (300–350 g) underwent a standardized surgical procedure and were then randomized into three groups to receive 6 days of ad libitum enteral nutrition. The animals were fed a solution of conventional nutrients, a solution of conventional nutrients enriched with 2.0% BCAA or a solution of conventional parenteral nutrients enriched with 2.5% glutamine. When compared with rats fed conventional nutrients, rats fed BCAA and glutamine had less jejunal atrophy (P< 0.05) and a greater specific activity of phosphate‐dependent glutaminase in the jejunum (131%; P< 0.05). It is concluded that enteral BCAA reduce atrophy of the jejunum via the generation of glutamine.

The effect of branched-chain amino acid-enriched parenteral nutrition on gut permeability.

In situations of catabolic stress, the gut becomes atrophic and has a diminished barrier function as evidenced by an increased permeability to a variety of molecules. It is known that the parenteral administration of branched-chain amino acids (BCAA) reduce gut atrophy. The aim of this study was to examine the effect of BCAA-enriched solutions of parenteral nutrients on gut permeability. A secondary aim was to observe the association between gut permeability and variables that have been used to assess jejunal atrophy. Central venous lines were inserted into 30 rats before randomization to receive nutritional support with: (1) a conventional parenteral solution (CPN), (2) A 2.0% BCAA-enriched solution (BCAA), or (3) rat food ad lib (Rat Food). The rats were assessed after 7 d for nutritional status, gut morphology, and gut permeability ratio (ratio of the permeability to 14C raffinose and 3H mannitol). We found that rats in the Rat Food Group lost the least amount of weight, had the least amount of jejunal atrophy, and had better preservation of barrier function as determined by gut permeability. When compared with the CPN Group, the BCAA Group had better preservation of jejunal morphology and protein content (p < 0.05), but a similar gut permeability. A cross-correlation matrix demonstrated a significant negative correlation between permeability to mannitol and mucosal weight, mucosal protein content and mucosal DNA content. Branched-chain amino acid-enriched parenteral nutrition reduced gut atrophy but not the gut permeability associated with parenteral nutrition. In the parenterally nourished rat model, atrophy of the jejunum is associated with increased permeability to small molecules.

The Role of Enterocytes in Gut Dysfunction

Stem cells in the intestinal epithelium give rise to enterocytes, goblet cells, enteroendocrine cells, and Paneth cells. Each of these cell lines plays a role in cytoprotection of the intestinal mucosa. In particular, it has been demonstrated that mature enterocytes can act as antigen presenting cells. Parenteral and enteral nutrition are used to nourish critically ill patients. However, these regimens are unfortunately associated with gut atrophy. Glutamine, the preferred intestinal nutrient, reverses this gut atrophy and plays a key role in maintaining the barrier function of the gut. Specific nutrients (putrescine, spermidine, spermine) have been used to modulate intestinal adaption. In addition, ornithine has been shown to act as a regulator of intestinal adaption. In this review, we discuss the relationship between the biology of enterocytes and failure of the gut barrier.