Aggregation mechanism of blood platelets studied by the time-resolved light scattering method

A laser light-scattering aggregometer was applied to study epinephrine-induced aggregation of human blood platelets. We improved the analytical software for the aggregometer; thus, it has become feasible to use a time-resolved light scattering method (TRLSM) based on Mie's theory, allowing one to monitor changes in both size and particle concentration as a function of time. By combination of TRLSM with microscopic analysis, it was found that there is a critical condition at which almost all the single platelets aggregate with one another to result in small size aggregates (AG(S)), at 2 min after addition of epinephrine (1.5 +/- 0.5 muM). The resulting AG(S) was spherical in shape, 15 mum in diameter (as a number-averaged value), and it was composed of about 1900 single platelets closely packed. After the formation of AG(S) particles went essentially to completion, they aggregated with one another to result in large particles (AG(L)), with an accompanying decrease in the AG(S) concentration. The AG(L) formed is not spherical but consists of irregularly bound AG(S) particles. The formation of AG(L) was inhibited upon the addition of aspirin, while there was no critical condition at which the AG(L) with a characteristic particle size is formed. These physical aspects were discussed in connection with biochemical mechanism for epinephrine-induced platelet aggregation in the presence and the absence of aspirin as the inhibitor.