Aggregation-induced alterations in human platelet membranes: A spin label study

Washed human platelets were labeled with stearate or methyl stearate spin probe. The order parameter of stearate spin probe was decreased markedly when the platelets were aggregated by thrombin, ADP or epinephrine. Central peak width of methyl stearate spin probe was also reduced in the aggregation induced by thrombin and ADP. However, isolated plasma membrane did not show any decrease of the order parameter by the addition of thrombin. Aspirin inhibited both the platelet aggregation and the decrease of the order parameter. The results suggest that ESR spectral changes might be caused by reorganization of membrane constituents rather than their quantitative or qualitative alteration.