Platelet and Fibrin Deposition on Subendothelium: Opposite Dependence on Blood Shear Rate

The rate of platelet deposition on subendothelium (adhesion + adhesion-induced aggregation) from anticoagulated blood is shear rate (~ v/d) dependent and thus increases with increasing blood flow velocity (v) and decreasing vessel diameter (d). To investigate concomitant fibrin deposition, subendothelium of rabbit aorta was exposed to native blood under controlled flow conditions (Schweiz. Med. Wschr. 106, 1367 (1976)). Native blood was circulated by a roller pump for 3 min from a carotid artery through an annular perfusion chamber (maintained at 37°c) into a jugular vein of a rabbit at flow rates of 5, 20 and 40 ml/min, producing wall shear rates at the exposed subendothelial surface of 500, 2000 and 4000 s^-1, respectively. Subendothelium and adhering blood elements were immediately fixed by perfusion of glutaraldehyde avoiding any blood-air interphase. Platelet interaction with subendothelium and fibrin deposition were determined morphometrically. At a shear rate of 500, 2000 and 4000 s^-1, 23±3, 43±4 and 66±3% (mean ± SE) of the surface were covered with platelets and 67±12, 26±12 and 10±4% with fibrin, respectively. The corresponding values for adhesion-induced aggregation were 12±3, 38±6 and 56±5% in native and about 4, 17±5 and 29±4% in citrated (15 mM) blood indicating that even a low citrate concentration inhibits adhesion-induced aggregation. Similar results were obtained with human blood drawn from a cubital vein.

This is direct evidence that fibrin deposition on subendothelium predominates at low (veins), platelet adhesion and aggregation at high (arteries + small vessel) shear rates.