Human Platelet Age Correlates With Buoyant Density, But Not With Size

Abstract

Following infusion of 51Cr-labelled autologous platelets into normal subjects, high density (HD) and low density (LD) platelet cohorts were isolated by centrifugation in isosmotic arabino- galactan (Stractan) . Specific radioactivity (SA) of LD platelets declined rapidly post-infusion (T1/2 =1.8 days) but SA 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 labelled and transfused. Mean survival of a platelet cohort enriched with HD cells was significantly (p < .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 < .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).These findings imply 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, SA of LD platelets declines rapidly as these platelets move into a more dense compartment and are replaced by newly formed, unlabelled platelets; SA 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