Stopped-flow spectrophotometry monitoring the initial processes of platelet activation by platelet-activating factor

The initial processes of platelet activation by platelet-activating factor (PAF) were observed by stopped-flow light scattering and fluorometry. The binding of PAF to rabbit platelets did not change the membrane fluidity, though it caused the removal of calcium from internal stores and induced concomitantly platelet shape changes. These results were quite in contrast to those by thrombin and ADP activation, where the membrane fluidity increased before the calcium release from the internal stores. The increased membrane fluidity in the latter system seemed to be used to transmit an external signal to a GTP-binding protein.