Arterial flow conditions downregulate thrombomodulin on saphenous vein endothelium

Background-The antithrombogenic properties of venous endothelium may be att enuated when vein is implanted in the arterial circulation. Such changes ma y facilitate thrombosis, which is the final common pathway for saphenous ve in arterial bypass graft occlusion. Methods and Results-Using human saphenous vein in a validated ex vivo flow circuit, we investigated (1) the possibility that arterial flow conditions (mean pressure, 100 mm Hg, 90 cpm, approximate to 200 mL/min) alter the con centration of proteins involved in regulating thrombosis at the vessel wall and (2) the influence of ion channel blockade on such effects, Concentrati ons of thrombomodulin and tissue factor were quantified by Western blotting (ratio of von Willebrand factor staining) and immunohistochemistry (as a p ercentage of CD31-staining area). Thrombomodulin concentrations after 90 mi nutes of venous and arterial flow conditions were quantified by immunostain ing (68.9+/-4.8% and 41.0+/-3.0% CD31, respectively; P<0.01) and by Western blotting (1.35+/-0.20 and 0.15+/-0.03 ratio of von Willebrand factor, resp ectively; P<0.01). The ability of endothelial cells to generate activated p rotein C also decreased from 62+/-14 to 19+/-10 ng . min(-1) . 1000 cells(- 1) (P=0.01). The significant reduction in thrombomodulin was attenuated if calcium was removed from the perfusate but not by external vein stenting. I nclusion in the vein perfusate of drugs that reduce calcium entry (includin g Gd3+, to block stretch-activated ion channels, and nifedipine) abolished the reduction in thrombomodulin concentration observed after arterial flow conditions. In freshly excised vein, negligible concentrations of tissue fa ctor were detected on the endothelium and concentrations did not increase a fter 90 minutes of arterial flow conditions, although the inclusion of nife dipine caused the immunostaining to increase from 3.0+/-0.4% to 8.5+/-0.7% CD31 (P<0.02). Conclusions-In saphenous vein endothelium exposed to arterial flow conditio ns, there is rapid downregulation of thrombomodulin, sufficient to limit pr otein C activation, by a calcium-dependent mechanism.