Blood Clots Are Rapidly Assembled Hemodynamic Sensors Flow Arrest Triggers Intraluminal Thrombus Contraction

Objective-Blood clots form under flow during intravascular thrombosis or vessel leakage Prevailing hemodynamics influence thrombus structure and may regulate contraction processes. A microfluidic device capable of flowing human blood over a side channel plugged with collagen (+/- tissue factor) was used to measure thrombus permeability (kappa) and contraction a controlled transthrombus pressure drops. Methods and Results-The collagen (kappa(collagen)= 1.98x10-(11) cm(2)), supported formation of a 20-mu m thick platelet layer, which unexpectedly underwent massive platelet retraction on flow arrest. This contraction resulted in a 5.34-fold increase in permeability because of collagen restructuring. Without stopping flow, platelet deposits (no fibrin) had a permeability of kappa(platelet)=5.45x10-(14) cm(2) and platelet-fibrin thrombi had kappa(thrombus)=2.71x10(-14) cm(2) for Delta P=20.7 to 23.4 mmHg, the first ever measurements for clots formed under arterial flow (1130 s(-1) wall shear rate). Platelet sensing of flow cessation triggered a 4.6- to 6.5-fold (n=3, P<0.05) increase in contraction rate, which was also observed in a rigid, impermeable parallel-plate microfluidic device. This triggered contraction was blocked by the myosin IIA inhibitor blebbistatin and by inhibitors of thromboxane A2 (TXA(2)) and ADP signaling. In addition, flow arrest triggered platelet intracellular calcium mobilization, which was blocked by TXA(2)/ADP inhibitors. As clots become occlusive or blood pools following vessel leakage, the flow diminishes, consequently allowing full platelet retraction. Conclusion-Flow dilution of ADP and thromboxane regulates platelet contractility with prevailing hemodynamics, a newly defined flow-sensing mechanism to regulate clot function. (Arterioscler Thromb Vasc Biol. 2012;32:2938-2945.)