Kinetics and mobilization from the spleen of indium-111-labeled platelets during platelet apheresis

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Abstract
The rate and extent of platelet mobilization from the spleen were measured and their relationship to the removal of platelets from the peripheral blood during discontinuous flow platelet apheresis was determined in 4 normal volunteers. Autologous plateltes were labeled with 111In-oxine and in vivo whole body and organ 111In radioactivity quantitated with a scintillation camera and a computer-assisted imaging system. Dynamic changes in splenic radioactivity were monitored during 12 cycles of platelet apheresis. The number of platelets harvested and changes in whole body and blood 111In activity were determined during the procedure. The platelet life-span was estimated and the sites of sequestion of labeled platelets was measured. Platelet apheresis removed a mean of 64% of platelets in the circulation, i.e., 48% of all platelets in the body. During the procedures, 28.0 .+-. 9.4% 111In-labeled platelets in the body were removed, splenic radioactivity decreased by 36.5 .+-. 13.2% and whole body activity decreased by 34.5 .+-. 9.7%. 111In activity in the spleen and whole body decreased in parallel, indicating a dynamic equilibrium between these pools. The life-span of the labeled platelets was 226 .+-. 25 h, similar to that of normal subjects. The major sites of sequestration of senescent platelets were the spleen (37.9 .+-. 20%) and liver (30.3 .+-. 5.6%); this is similar to that found in normal subjects. As platelets are removed from the peripheral blood, the blood pool is rapidly and effectively replenished from the splenic platelet pool. These 2 pools are in dynamic equilibrium and permit removal of large numbers of platelets without resultant thrombocytopenia. Platelet apheresis does not adversely effect platelet life-span, and the sequestration pattern in the reticuloendothelial system is normal.

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