METABOLISM OF THROMBOPOIETIN (TPO) IN-VIVO - DETERMINATION OF THE BINDING-DYNAMICS FOR TPO IN MICE

Previous in vivo studies have established that plasma thrombopoietin ( TPO) levels are regulated by binding to c-MpI on platelets and that, i n vitro, platelets bind and degrade TPO, To determine if the in vivo m etabolism of TPO was specific and saturable, we injected normal CD-1 m ice IV with trace amounts of I-125-rmTPO with or without a saturating concentration of rmTPO, The amount of radioactivity present in the spl een, blood cell fraction, platelet fraction, tibia/fibula, and femur w as significantly greater in the mice receiving I-125-rmTPO alone, Conv ersely, the amount of radioactivity present in the plasma was signific antly greater in the mice receiving both I-125-rmTPO and rmTPO, thus s uggesting the uptake of rmTPO by the spleen, platelets, and bone marro w in vivo was saturable, Platelet and spleen homogenates from animals receiving I-125-rmTPO alone showed a degradation pattern of I-125-rmTP O similar to that observed in vitro using mouse platelet rich plasma. To determine the in vivo binding dynamics for rmTPO, mice were injecte d with I-125-rmTPO alone or with increasing concentrations of rmTPO; s pleen and blood cell-associated radioactivity was determined at 2 hour s postinjection. A 4-parameter curve fit of the data indicated that th e ''in vivo binding affinity'' for rmTPO was approximately 6.4 mu g/kg . These data indicate that after a dose of approximately 6.4 mu g/kg, 50% of all c-MpI receptors will be saturated with rmTPO. Electron micr oscopy indicated that radioactivity was present bound to and within me gakaryocytes and platelets in both sternum and spleen and platelets in circulation. Together these data demonstrate that in vivo, I-125-rmTP O is mainly metabolized by platelets and to a small extent by cells of the megakaryocyte lineage, via a specific and saturable mechanism. (C ) 1997 by The American Society of Hematology.