Optical shape change analysis in stirred and unstirred human platelet suspensions. A comparison of aggregometric and stopped-flow turbidimetric measurements.

To monitor the discoidity of platelets in an aggregometer, a relative, stirring-dependent change in extinction (delta ES) is defined. delta ES is large in highly discoid and small in ADP-activated platelets. The platelet shape change, as monitored by the changes in delta ES, not only precedes ADP-induced aggregation but also outlasts disaggregation. In unstirred samples, ADP induces insignificant changes in extinction in the aggregometer but biphasic changes in extinction in a "zero degree" stopped-flow turbidimeter. This discrepancy apparently arises from differences in the amount of scattered light collected by the optical systems and from wavelength-dependent differences in sensitivity. Similar progress curves were observed in both instruments for the biphasic changes in extinction which accompany the release reaction induced by thrombin or concanavalin A in pre-sphered platelets. The aggregometer is advantageous for monitoring rheooptical effects of the asymmetric platelets while the stopped-flow laser turbidimeter is superior in quantifying the changes in extinction according to the light scattering theory.