Inhibitor of fibrinolysis from mesothelium

Peritoneal response to abdominal surgery: the role of equine abdominal adhesions and current prophylactic strategies

Intra-abdominal adhesions constitute a significant clinical and surgical problem that can lead to complications such as pain and bowel occlusion or subocclusion. These adhesions are frustrating and potentially fatal, representing a major postoperative complication in abdominal surgery. It is estimated that 32% of horses undergoing laparotomy will present clinical symptoms due to adhesions, but the true prevalence is not known because a large proportion of animals with postoperative recurrent colics are medically treated or submitted to euthanasia without necropsy. Adhesions are highly cellular, vascularized, dynamic structures that are influenced by complex signaling mechanisms. Understanding their pathogenesis could assist in applying better therapeutic strategies and in developing more effective antiadhesion products. Currently, there are no definitive strategies that prevent adhesion formation, and it is difficult to interpret the results of existing studies due to nonstandardization of an induction model and evaluation of their severity. The best clinical results have been obtained from using minimally traumatic surgical techniques, anti-inflammatory agents, antimicrobials, anticoagulants, and mechanical separation of serosal surfaces by viscous intraperitoneal solutions or physical barriers. This paper aims to review adhesion formation pathogenesis, guide the understanding of major products and drugs used to inhibit adhesion formation, and address their effectiveness in the equine species.

Plasminogen activators and inhibitors in peritoneal tissue

Serosal trauma elicits an inflammatory response which leads to the deposition of fibrin at injured sites, the residuals of which appear to be essential in excessive tissue repair and formation of intraabdominal adhesions. Local plasminogen activity may modulate this early phase of tissue repair. The present study was undertaken to investigate the distribution and cellular expression of plasminogen activators and their inhibitors in human peritoneal normal and inflamed tissue. Tissue-type plasminogen activator (t-PA) was expressed in subserosal capillary walls, and in normal mesothelium, but not in inflammation. Immunoreactivity for the plasminogen activator inhibitor type 1 (PAI-1) was present in normal mesothelium, and substantially increased in inflammation, where, in addition, immunoreactivity was found throughout the submesothelial tissue. This PAI-1 was partly co-localized with macrophages, as was the urokinase plasminogen activator (u-PA), suggesting an involvement of these cells in peritoneal tissue fibrinolysis. Inflammation or abrasion of the mesothelium during surgery is likely to cause a depletion of the local t-PA source and expose the potentially PAI-1-containing submesothelial tissue, thus promoting persistence of fibrin and formation of adhesions.

Plasminogen activator inhibitor-2 expression in inflamed appendix

Plasminogen activator inhibitors are thought to be responsible for the abolition of fibrinolytic activity in inflamed peritoneum. This reduction in the fibrin clearing capacity of the peritoneum promotes the formation of intra-abdominal adhesions. High concentrations of plasminogen activator inhibitor-2 (PAI-2) have been previously found in inflamed peritoneal tissue using immunoassays, but it is undetectable in normal peritoneum. The aim of this study was to localize plasminogen activator inhibitor-2 production in tissue by in situ mRNA hybridisation. Sections of normal and inflamed human appendix were hybridised with a digoxigenin labelled cDNA probe. In normal appendix staining was confined to macrophages in the mucosa. Macrophage staining was also seen in inflamed tissue but with a wider distribution throughout the appendix wall. PAI-2 was also localized to mesothelial cells of inflamed but not normal appendix. Cell identities were confirmed using immunohistochemistry directed against cell specific markers. Staining was absent from control slides incubated with plasmid DNA or PAI-2 probe following ribonuclease digestion. The identification of the cells expressing the PAI-2 gene in peritoneum increases our understanding of the pathophysiological process leading to fibrin deposition within the abdomen during peritonitis.

Plasminogen activator inhibitor 2 reduces peritoneal fibrinolytic activity in inflammation

Fibrinolysis in peritoneal tissue may play a role in the development of intra-abdominal adhesions. The plasminogen-activating capacity of human peritoneum results largely from the presence of tissue plasminogen activator (tPA). Inflammation reduces peritoneal plasminogen-activating activity and leads to the appearance of plasminogen activator inhibitor (PAI) type 1. The role of PAI-2 in the inhibition of peritoneal fibrinolysis during inflammation was investigated in this study. The plasminogen-activating activity of peritoneal biopsy homogenates (seven inflamed, seven normal), measured using a fibrin plate technique, was reduced in inflamed compared with normal tissue (median < 0.07 versus 13.9 units/cm2, P < 0.01); tPA antigen levels were not significantly different (median 1.02 versus 1.34 ng/ml). PAI-1 and PAI-2 antigens were not detected in normal human peritoneum but were present in inflamed peritoneum (median concentration 8.8 ng/ml for PAI-1, 26.7 ng/ml for PAI-2). These inhibitors may be important factors in adhesion formation by contributing to the abolition of peritoneal plasminogen-activating activity.

Localization of plasminogen activator inhibitor-1 production in inflamed appendix by in situ mRNA hybridization

The peritoneum has been shown to possess fibrinolytic activity which is thought to play a role in the prevention of intra-abdominal adhesion formation. Recently inflamed peritoneal tissue has been shown to have reduced fibrinolytic activity secondary to increased levels of plasminogen activator inhibitor-1 (PAI-1). The aim of this study was to localize the production of PAI-1 in appendix tissue using in situ mRNA hybridization. Sections of normal and inflamed appendix were hybridized with a digoxigenin-labelled cDNA probe. PAI-1 production was localized to both mesothelium and serosal blood vessel endothelium in all inflamed appendix samples. Cell identities were confirmed using immunohistochemistry directed against mesothelial and endothelial cell markers. Staining was not seen on 1 probe following ribonuclease digestion). The identification of the cells expressing the PAI-1 gene in peritoneum increases our understanding of the pathophysiological changes in fibrinolytic activity which occur in inflammation and may lead to adhesion formation.

The mesothelium: its fibrinolytic properties

We have investigated the fibrinolytic properties of the mesothelium by developing a test system in which standard tissue samples can be assessed in a comparative and semi-quantitative manner. The problem whether plasminogen activators were generated by the mesothelium itself or were transported from a subserosal site was also studied. A standard system was developed by using a uniform gelatin disc to obtain a monolayer imprint of mesothelium and subsequently incorporating several of these samples on a fibrin plate. Pure cultures of mesothelial cells were able to induce fibrinolysis.