Turbidometric evaluations of platelet activation: relative contributions of measured shape change, volume, and early aggregation.

The relative contributions of ADP-induced changes in mean platelet axial ratio (R), mean volume (VT), pseudopod formation, refractive index, and early platelet aggregation to the changes in light transmission (%T) through stirred platelet suspensions (aggregometer) are determined. Measurement of the change in R within the first 10s of ADP addition indicates that the slope of the initial decrease in %T (5s) is in good agreement with that predicted from the theory of Latimer et al. (1977). However, when the changes in %T due to changes in VT are also considered, the observed rate of change of %T is shown to be 20-210% larger than predicted. These observations point to an additional process which produces a decrease in %T. It is shown that platelet aggregation occurring on this early time scale does not contribute to the initial changes in %T. Aggregation does lead to an increase in %T once there are greater than or equal to 30-40% singlet platelets aggregated which occurs at greater than or equal to 4-5s for 1-10 microM ADP. In the absence of aggregation, the extent of %T decrease at greater than 40s is greater than 25% smaller than predicted from the change in R and VT. It is suggested that at early times, changes in platelet refractive index produce significant decreases in %T which are on average offset by comparable increases due to the early volume increase, whereas at later times pseudopod formation has an important influence on %T.