Altered phospholipid composition of plasma membranes from thrombin-stimulated human platelets

The individual phospholipid concentrations, and their respective fatty acid distributions, in whole platelet lysates and plasma membranes derived from unstimulated and thrombin-stimulated intact human platelets were studied. This was of interest, since previous work had led to the suggestion that altered phospholipid concentrations in plasma membranes of intact stimulated cells may be of importance in mediating cellular responses. The concentrations (nmol/mg protein) of phosphatidylinositol in whole platelet lysates and plasma membranes derived from thrombin-activated platelets decreased by 37 and 45%, respectively, a compared to their corresponding controls. As well, concentrations of plasma membrane phosphatidylcholine and phosphatidylethanolamine in thrombin-stimulated platelets decreased by 20 and 9%, respectively, when compared with their control values. The amounts of phosphatidylserine and sphingomyelin in whole platelet lysates and plasma membranes were unchanged by exposure to thrombin. Fatty acid analyses revealed that thrombin stimulation of intact human platelets induced a decrease in the arachidonate content (from 37.7 to 33.1 wt.% of total fatty acid) of plasma membrane phosphatidylinositol. Similar shifts in the wt% of arachidonic acid in plasma membrane phosphatidylcholine were found. These results indicate that thrombin stimulation of intact human platelets produces a significant decrease in the mass of phosphatidylinositol in plasma membranes and raises the suggestion that the preferential depletion of the plasma membrane in arachidonoyl-containing phosphatidylinositol may be of importance in mediating cellular responses to external stimuli.