Role of prostaglandins in controlling plasma fibronectin levels

Fibronectin is a normal glycoprotein component of plasma, interstitial fluid, and extracellular matrix which has binding sites for collagen, gelatin, actin, glycosaminoglycans, fibrin, Staphylococcus aureus, and some cells. Since it is a dimer, it can crosslink these substances to each other or to extracellular components of basement membrane, thereby affecting many physiological processes. The level of circulating fibronectin is markedly reduced following even moderate blunt or operative trauma, thermal injury, starvation, advanced cancers, hemorrhage, etc. Replacement therapy has been tried with some success in patients who become septic following multiple injuries. The reduction in plasma fibronectin has been attributed to several causes including consumption by binding to cell debris at the site of injury, binding to circulating cell debris and its subsequent removal by elements of the phagocytic system, and degradation by proteolytic cleavage. However, the amount of fibronectin removed from circulation raises some question about this. In this paper, we used indomethacin, ibuprofen, imidazole, and essential fatty acid deprivation to inhibit the synthesis of prostaglandins in young adult rats. Thirty minutes after ip administration of one of the inhibitors, the rats were subjected to a midline laparotomy and mild intestinal manipulation. Blood samples were taken at intervals following closure of the incision and analyzed for fibronectin. In all cases, the normal decline in plasma fibronectin seen in untreated rats was abrogated by inhibiting prostaglandin synthesis. Since imidazole specifically inhibits thromboxane A synthesis, this strongly suggests that thromboxanes directly or indirectly control the trauma-induced reduction in circulating fibronectin. This was confirmed by ip injection of thromboxane into the rats which resulted in a decline in plasma fibronectin levels.