Factors controlling the intrasplenic transit of platelets

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Abstract
The kinetics of 111In-labeled platelets were investigated in human subjects by a gamma camera and computer system. Time activity curves were recorded over the spleen, chest and liver, following i.v. bolus injection of autologous radiolabeled platelets. Splenic blood flow, expressed as the percentage of total blood volume entering the spleen in unit time, and intrasplenic platelet transit time were calculated from the time activity curves. Spleen size was recorded from the camera images. Splenic blood flow increased with increasing spleen size (rt = 0.56, P < 0.001). Platelet transit time showed an inverse relationship with a semiquantitative estimate of splenic perfusion (i.e., splenic blood flow/unit vol spleen; r = -0.64, P < 0.001) but was not related to spleen size. At high rates of estimated splenic perfusion transit time appeared to reach a minimum value of .apprx. 7 min, whereas at low rates of estimated perfusion, transit time rose sharply to reach levels > 20 min. The effect of polycythemia, taken to indicate high intrasplenic hematocrit on platelet transit time, was also investigated. Patients with secondary polycythemia had elevated transit times (19 min SEM [standard error of mean] 1.6, n = 10) compared with primary polycythemia (9.7 min SEM 0.8, n = 5) and normals (9.6 min SEM 0.5, n = 5). Splenic perfusion was important in determining the duration of platelet transit.