Platelet activation detected by turbidometric shape-change analysis: differential influence of cytochalasin B and prostaglandin E1.

Activation of platelets by specific stimuli is associated with changes in the light transmission of the platelet suspension. EDTA avoids aggregation of activated platelets but does not interfere with platelet activation. Three optical effects which require increasing stimulus concentrations are characterized: 1) Disc-sphere transformation of platelets (=shape change I) is reflected by a decrease in the light transmission (LT) and disappearance of the change in LT when the stirring mechanism is switched on or off. 2) Higher inducer concentrations are necessary to elicit a further decrease in LT (=shape change II) which is detected independently on stirring. 3) A final increase in LT follows the shape change with stronger stimuli and indicates secretion. 2) and 3) are also found during stimulation of spheroid platelets obtained under certain washing conditions or by pre-treatment with cold. Cytochalasin B (CB) 60 μM impairs the disc-sphere transformation. Low concentrations of CB (2 μM) strongly enhance the final increase in LT and the extrusion of serotonin from platelets stimulated by activators of the release but not by ADP, epinephrine, or diamide. Prostaglandin E₁ (PGE₁) inhibits the three optical effects and the release. The requirement for the stepwise increase in PGE₁ concentrations to inhibit disc-sphere transformation, shape change II, and the extrusion phase of the release reaction confirms the conclusion that the optical parameters indicate varying degrees of platelet activation.