Early changes in human vein grafts: the role of reactive oxygen species produced by NADPH oxidase
10 / 2001 - 10 / 2003
Survival rates of human vein grafts following coronary artery bypass grafting are generally less favourable than those of selected arterial grafts. However, vein grafts are still necessary in most patients, related to surgical limitations of arterial grafts. Various studies have shown that degenerative processes affect graft patency after implantation: thrombosis (<1 month after surgery), intimal hyperplasia (between 1 month and 1 year after surgery) and in the end atherosclerosis-like changes. The process of thrombosis is promoted by endothelial loss, probably partly related to overdistension (=increased wall tension), and downregulation of thrombomodulin in the vein graft wall. Furthermore, intimal hyperplasia, characterized by proliferation of smooth muscle cells, is augmented by distension-induced endothelial loss, next to ischemic injury (due to the loss of vasa vasorum) and increased wall tension (due to distension). Therefore, the process of overdistension of the venous graft apparently plays a crucial role in the early phase of graft failure. Experiments in animals have shown that inhibition of overdistension via an external stent surrounding the vein graft results in preservation of vein graft wall structures. However, in the human situation the acute changes and structural damage in implanted vein grafts is difficult to study. Therefore we recently developed a novel experimental set-up, in which segments of human vein grafts were placed during surgery in a loop of the extracorporeal circulation. In this way the direct response of vein grafts to arterial blood flow (autologous, oxygenated) and pressure at physiological conditions can be studied. In this system we already observed complete de-endothelialization subsequent to 60 minutes of perfusion, which could be prevented by external stenting (PolyTetraFluyoroEthylene stent). Overdistension is hypothesized to result in the activation of different intracellular processes, leading to cell damage of the vessel wall. A possible crucial mediator in these processes is the production of reactive oxygen species (ROS). Interestingly recent publications suggest non-phagocytic NADPH oxidase as a source of ROS in arteries. In collaboration with Prof. Roos (Dept. of Immunohaematology, CLB), we found in tissue homogenates of venous grafts (with or without endothelium) all 4 components of the phagocytic NADPH oxidase (p22phox, p47phox, p67phox and gp91phox), suggesting a role of this NADPH oxidase or a very similar system in ROS production by veins. In this project human vessel explants (veins) will be studied in a whole blood perfusion system, up to 24 hours of perfusion. We will focus on the role of overdistension of the vein graft on functional properties of the vessel wall (coagulation, adhesion molecules, apoptosis, inflammation). Molecular mechanisms that explain functional changes due to overdistension will then be characterized. For these changes, an important role of NADPH oxidase is hypothesized. Therefore, NADPH oxidase will be characterized furthermore and the effect of ROS will be studied, partly by using more or less specific inhibitors. Next to this, the efficacy of external stenting to prevent and/or reduce functional changes will also be studied. Therefore, our unique experimental set-up gives us the possibility to study the important early changes in venous grafts, which is of paramount clinical importance.