Evidence that platelet buoyant density,but not size,correlates with platelet age in man

Following infusion of ⁵¹Cr-labeled autologous platelets into normal subjects, high-density (HD) and low-density (LD) platelet cohorts were isolated by prolonged centrifugation in isosmotic arabino-galactan (Stractan). Specific radioactivity of LD platelets declined rapidly post-infusion (T_1/2 = 1.5 days), but specific radioactivity of HD platelets remained constant or increased over a 3–4-day period and gradually declined for 6–7 days thereafter. These differences were exaggerated when platelet cohorts enriched in LD or HD cells by slow centrifugation in high-density albumin were labeled and transfused. Mean survival of a platelet cohort enriched with HD cells was significantly (P < 0.02) shorter (7.73 days) than that of a cohort enriched with LD cells (9.33 days). In normal subjects treated with aspirin, capacity for thromboxane synthesis was regained more rapidly (P < 0.05) in LD than in HD platelets. HD and LD platelets differed only slightly in mean volume (HD platelets = 7.57 μ³, LD platelets = 6.87 μ³, 0.05 < P < 0.01). We believe the most logical interpretation of these findings is that under normal conditions in man, newly formed platelets are less dense on the average than total platelets and become more dense as they age in the circulation. Thus, specific radioactivity of LD platelets declines rapidly as these platelets move into a more dense compartment and are replaced by newly formed, un-labelled cells; specific radioactivity of HD platelets remains constant or increases as labelled platelets enter this compartment in numbers equal to or greater than the number leaving it at the end of their life span. The similarity in mean volumes of LD and HD platelets suggests that platelet size is unrelated to platelet age under normal conditions.