Young steady-state rabbit platelets do not have an enhanced capacity to expose procoagulant phosphatidylserine

Platelets are recognized to be physiologically and functionally heterogeneous. An example of the diversity in reactivity is the formation of a distinct subpopulation of procoagulant phosphatidylserine (PS)-exposing platelets upon activation. Platelet age has been proposed as a determinant of platelet function, and it has been reported that young platelets are more reactive in exposing PS; using the same methodology of thiazole orange (TO) staining to distinguish young and old platelets, the percentages of procoagulant platelets produced by thrombin plus collagen activation of platelets from healthy controls were examined by flow cytometry. The procoagulant subpopulation formed by TO-positive platelets (with high TO fluorescence), purported to be young reticulated platelets, was observed to be significantly larger than that formed by TO-negative platelets (with low TO fluorescence), purported to be older platelets. However, it was noted that TO fluorescence in the total platelet population was unimodal and increased with platelet size, assessed by forward scatter. This observation raised the concern that TO-positive platelets are not necessarily the youngest platelets in the condition of steady-state platelet production. Thus, to unequivocally determine whether platelet age is a factor in procoagulant platelet formation, a different approach to identify young, steady-state platelets was employed. Rabbits were injected with biotin to label >95% of circulating platelets in vivo; 24 hours post-biotinylation, the non-biotinylated platelets in the circulation, detected flow cytometrically, are the youngest, newly-formed platelets. It was demonstrated that these youngest platelets were not larger in size than older, biotinylated platelets, and that they did not have an enhanced capacity to expose PS.