Summary.Although thrombopoietin itself does not influence platelet aggregation, it enhances platelet activation in response to certain agonists. We evaluated the effects of thrombopoietin on platelet activation using platelet-rich plasma from 16 patients with myeloproliferative disorders (MPD group) and 16 healthy volunteers (control group). Preincubation with thrombopoietin significantly enhanced platelet aggregation stimulated by ADP, collagen, or epinephrine in the MPD group as well as the control group. However, aggregation induced by 3 mM ADP or 16 mM epinephrine showed significantly less augmentation by thrombopoietin in the MPD group than in the control group. Thrombopoietin significantly shortened the lag time between the addition of 3 mM ADP or 16 mM epinephrine and initiation of secondary aggregation and the lag time between addition of 2 mg/ml collagen and initiation of aggregation in both groups. When platelet-rich plasma was used without adjustment of the platelet count, thrombopoietin itself induced aggregation in two patients. Hypoaggregation after addition of 0 . 5 mg/ml collagen was observed in seven out of nine patients with normal thrombopoietin levels and only one of six patients with high levels (P ¼ 0 . 04). Enhancement of 0 . 5 mg/ml collagen-induced aggregation by thrombopoietin was seen in five out of nine patients with normal thrombopoietin levels and none of the six patients with elevated levels (P ¼ 0 . 04). These results indicate that platelet activation by certain agonists is enhanced by thrombopoietin in patients with these diseases as well as in normal controls and that the serum thrombopoietin level may regulate the function of circulating platelets in vivo.

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