Intracellular viscosity changes during activation of blood platelets: studies by fluorescence polarization

The intracellular viscosity changes that occur in washed human platelets as a result of activation by thrombin or ADP were studied by the use of a fluorescent probe. Results obtained showed a sharp and quick decrease of the intracellular viscosity when platelets were activated by thrombin. This decreased preceded both the release and the aggregation. When platelets were activated by ADP, the decrease in the polarization of fluorescence (and the viscosity) was more moderate. The fluorescent probe is bound to small proteins and peptides in the cytoplasm and not in the granules. Therefore, these changes in the fluorescence polarization reflect changes in the cytoplasmic viscosity which might be due to reorganization of the contractile proteins' system.