Platelet Size Affects both Micro- and Macro-Aggregation: Contributions of Platelet Number, Volume Fraction and Cell Surface

Summary

The relationship between platelet size (v), platelet number (N₀), platelet volume fraction (φ) and platelet aggregation was evaluated in human platelet subpopulations separated on the basis of volume using counterflow centrifugation. The original platelet population and three size-dependent platelet fractions were concentrated and resuspended in autologous citrated platelet-poor plasma at varying N₀. Micro-aggregation (PA) and macro-aggregation (TA) were determined respectively by electronic particle counting and light transmission (turbidometry). At similar N₀, large platelets were about two-fold more sensitive and more rapidly recruited into both micro (PA) and macro (TA) aggregates in response to ADP than the small platelets (v were respectively 7.3 ± 0.2 and 4.2 ± 0.2 fl, each 16 ± 4% of the total population). At volume fractions favouring the small platelets, however, the above differences persisted for PA, but not for TA. Similar size-dependent differences were observed for two other receptor-mediated platelet activators, namely a stable thromboxane A₂ analogue (U46619) and platelet activating factor (PAF), but not for ristocetin-induced agglutination. Increasing platelet size appears to optimize platelet aggregation due to (i) the simple geometric advantage of large-sized platelets associated with any given N₀, seen for both PA und TA, and (ii) intrinsic differences in size-dependent subpopulations which favour more efficient platelet membrane surface changes associated with receptor-mediated micro-aggregation (PA), for which a likely model is proposed.

Keywords

Platelet count - Platelet size - Platelet volume fraction - Platelet micro- and macro-aggregation - Shape change

• References

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