Platelet size as a determinant of platelet function.

The relationship between MPV and platelet function was studied in human platelet subpopulations separated on the basis of size and counterflow centrifugation. The original platelet population and five size-dependent platelet fractions were suspended in buffer or autologous PPP at a platelet count of 2 x 10(8)/ml. Collagen (10 micrograms/ml)-induced aggregation showed a significant negative correlation between MPV and onset of aggregation and positive correlations between the MPV and the rate of extent of aggregation. Thrombin stimulation (1 U/ml) demonstrated similar relationships between MPV and the rate of extent of aggregation. In contrast, ristocetin-induced platelet agglutination occurred at a similar rate and extent in all fractions. To quantitate further the differential response of the platelets, we measured the content and release of ATP and beta-TG. There was a significant correlation between MPV and both ATP and beta-TG content, and a progressive increase in the absolute release of ATP and beta-TG was associated with the increase in MPV through the fractions after stimulation. However, the percent release of total ATP and beta-TG was similar in all fractions. Our data indicate that the intrinsic function of platelets of different sizes is similar but that the absolute ability of platelets to affect their environment, as measured by aggregation and total release of granular content, correlates with their size.