Key role of ADP receptor in thrombus stability in the absence and presence of coagulation. (A) Human PPACK-anticoagulated blood was flowed over VWF/collagen at 1000 s^–1 for 4 minutes. Thrombi were formed on coverslips (i). These were perfused again for 10 minutes with HEPES buffer (pH 7.45) containing 2 mM CaCl₂ and 2 mM MgCl₂. Vehicle (ii) or ADP (20 μM) (iii) was added during secondary perfusion. Representative phase-contrast images (180 × 180 μm) are given after 10 minutes. Strings of platelets connected by tethers (dotted line) detaching during 27 seconds of perfusion (iv-v). (B) Citrated blood was recalcified with CaCl₂/MgCl₂ before flow over VWF/collagen at 1000 s^–1. Fibrin-containing thrombi on coverslips were perfused again for 10 minutes with the same HEPES buffer supplemented with AR-C69931MX (30 μM) (Video S3). Representative phase-contrast images are given of 1 field before and after secondary perfusion (i-ii). Note dissolution of all but one of the fibrin-containing thrombi. Arrows indicate fibrin-containing thrombi. Shown are representative images (n = 3-5 experiments).