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Thrombopoietin increases platelet adhesion under flow and decreases rolling

Summary. Thrombopoietin (TPO) is known to sensitize platelets to other agonists at 20 ng/ml, and above 100 ng/ml it is an independent activator of aggregation and secretion. In studies with a perfusion chamber, TPO, between 0·01 ng/ml and 1 ng/ml, increased platelet adhesion to surface‐coated fibrinogen, fibronectin and von Willebrand Factor (VWF) but not to a collagen‐coated surface. Increased adhesion was observed at shear rates of 300/s and 800/s in perfusions with whole blood as well as in suspensions of platelets and red blood cells reconstituted in plasma. The by the cyclooxygenase inhibitor, indomethacin, and the thromboxane A2‐receptor blocker, SQ30741, abolished the stimulation by TPO. The effect of TPO was mimicked by a very low concentration (10 nmol/l) of the thromboxane TxA2 analogue, U46619. Real‐time studies of platelet adhesion to a VWF‐coated surface at a shear of 1000/s showed that about 20% of the platelets were in a rolling phase before they became firmly attached. TPO (1 ng/ml) pretreatment reduced this number to < 5%, an effect again abolished by indomethacin. Thus, TPO potentiates the direct and firm attachment of platelets to surface‐coated ligands for αIIbβ3, possibly by increasing the ligand affinity of the integrin.