Kinetics of platelet density subpopulations in splenectomized mongrel dogs

Autologous ⁵¹Cr-platelet kinetic studies were performed in splenectomized mongrel dogs. Mean survival time of PRP-platelets was 5.4 ± 1.5 (SD) days (n = 6). The curves, though slightly curvilinear, showed mostly a linear type of decay, denoting that platelet removal from the circulation is mainly determined by aging of the cells. High-density (HD) and low-density (LD) platelet cohorts were isolated in Stractan gradients from samples drawn daily after infusion of labeled platelets. Specific radioactivity in HD cohorts declined rapidly postinfusion (T1/2 = 1.3 days), but specific radioactivity in LD platelets increased for 2 days and steadily declined for 4 days thereafter (n = 6). Labeled HD platelets, comprising 11.7% of the total population, lived significantly longer in circulation than LD platelets (19.1% of the total population) (n = 3). The patterns of decay of the radioactivity, however, do not have all the characteristics of pure age-cohort survival curves; 3.7 days after the infusion of labeled HD platelets, the specific radioactivity in LD cohorts was six times higher than on day 1, but attained only 20% of the initial specific radioactivity in HD platelets. After the infusion of labeled LD platelets no radioactivity was recovered in circulating HD cohorts. These findings indicate that mongrel dog platelets decrease in density with aging, but also that platelet density heterogeneity is in part determined during the thrombopoietic process. These data are consistent with those of other authors in rabbits and rhesus monkeys, but contrast with the observations that platelets in humans, baboons, and Macaca fasicularis monkeys increase in density with age, suggesting that the displacement of platelets toward compartments of either higher or lower density depends on the species under study.