ENDOTHELIUM - THE NEXT FRONTIER IN BIOCOMPATIBILITY

Vascular endothelium plays a central role in two specific functional s ystems. It controls vascular tone, hemostasis, and substance transport . The endothelium is the ''docking station'' for trapping, deactivatio n, and regeneration of activated blood compounds and provides the prin cipal clearance mechanism for biologically active mediators released b y different cell types. The second function is a regenerational one. D uring the period between insults (or between dialysis sessions), the e ndothelium has to restore the ''first line of defense,'' that is, to r egenerate the injured athrombogenic surface of the vessel wall and its antioxidative potential, defoliate damaged endothelial cells, and int erpolated new ones. These two important endothelial activities are req uired over and above its basic functions. Future research in artificia l organs must take into account that continuous or intermittent blood- membrane contact creates an altered endothelial response. These altere d responses may result in adaptional reactions that may differ substan tially in the acutely ill patient on continuous venovenous hemofiltrat ion (CVVH) or in a stable patient on maintenance hemodialysis. By a re duction in such factors as immediate or delayed cell-cell interactions (direct or indirect), it may be possible to influence the long-term o utcome of chronic hemodialysis patients. Other research should strive to enhance those factors of endothelial function that are essential in the defensive and restorative properties of endothelial tissue. This is especially important in such continuous therapies as CVVH, long-ter m membrane oxygenation, and artificial heart and blood vessels. Curren tly, there are more unanswered questions than possible answers concern ing endothelial functions in long-term hemodialysis patients, but it i s clear that excluding endothelial cell behavior from investigation of extracorporeal therapy in the future would be a substantial omission.