Expression of the endothelial thrombomodulin (TM) on the ischemic rat flap model: preliminary study

Thrombomodulin (TM), a cell surface-expressed glycoprotein predominantly synthesized by vascular endothelial cells, is a critical cofactor for thrombin-medicated activation of protein C. TM thus has an impact on coagulation, inflammation, and fibrinolysis. In this study, we investigated expression of endothelial TM in the dorsal skin of the rat as an ischemic flap model. Twenty male Wistar rats weighing between 250 and 350 g were used in the study. Nine by 3-cm, full-thickness, caudally based random pattern dorsal skin flaps were elevated. The rats were randomized into 2 subgroups according to the evaluation time. Tissue blood flow of the skin flaps was measured 4 times (before the operation and on days 1, 3, and 7) at 1, 3, and 5 cm distal to the baseline of the skin flap. Skin flap samples including subcutaneous tissue were taken from killed rats at day 3 (n = 10) and 7 (n = 10) for histologic assessment. These samples were also taken from the midline at 1, 3, and 5 cm distal to the baseline of the skin flaps. The survival rate of the skin flaps was measured on day 7. According to the blood flow rate change, we found that there were significant differences between the 1-, 3-, and 5-cm samples throughout the experiment. The most profound change was that it was at 5 cm in which there was an initial pattern of reduced perfusion followed by cessation of perfusion. On the third day, positive immunoperoxidase staining specific for TM was detected in all the specimens of the skin biopsies taken from 1 and 3 cm. The 5-cm samples demonstrated very little evidence of necrosis and had negative immunoperoxidase staining for TM. The 1-cm samples were found to have preserved morphologic features present on days 3 and 7. The 3-cm samples showed no evidence of necrosis, though some of the capillary vessels were filled with aggregated blood cells. The vascular wall had negative peroxidase staining for endothelial TM. At 5 cm, there was clear evidence of necrosis, some found within the capillary vessels, which were filled with aggregated blood cells. These samples also had negative peroxidase staining for TM. On day 7, the survival rate of skin flaps was 45.35%. In this study, we demonstrate that tissue ischemia is an important factor, particularly in down-regulating TM transcription. TM depletion from the vascular cells and microthrombus formation occurred in the ischemic areas, playing an important role in secondary aggravation of tissue ischemia.