PLATELET AGONISTS ENHANCE THE IMPORT OF PHOSPHATIDYLETHANOLAMINE INTOHUMAN PLATELETS

It is unknown whether the endocytosis-independent transfer of phosphol ipids from lipoproteins to platelets is regulated by platelet agonists such as thrombin. The movements of the choline phospholipids phosphat idylcholine and sphingomyelin (labeled with either C-14 or the fluores cent pyrenedecanoic acid) between low density lipoproteins and platele ts were unaffected by thrombin (0.5 unit/ml). In contrast, thrombin ac celerated the import of diacyl phosphatidylethanolamine (PE) and alken ylacyl phosphatidylethanolamine into platelets by about 4-fold. Simila rly, thrombin receptor-activating peptide (15 mu M), collagen (10 mu g /ml), and ADP (10 mu M) enhanced PE uptake. High density lipoprotein p articles and egg phosphatidylcholine vesicles were also donors for sti mulation of platelet PE import. Part of the [C-14]arachidonic acid-lab eled PE transferred from low density lipoprotein to platelets activate d by thrombin and collagen was metabolized to C-14-eicosanoids, Inhibi tors of protein kinase C partially prevented thrombin-induced [C-14]PE uptake, while direct activators of protein kinase C increased incorpo ration of [C-14]PE into platelets. Proteinaceous factor(s) recovered i n the extracellular medium from ADP- and thrombin-activated platelet s uspensions were found to accelerate the transfer of pyrenedecanoic aci d-labeled PE between donor and acceptor lipid vesicles. The stimulatio n of import of ethanolamine phospholipids led to a 2-fold enhancement of the prothrombinase activity of thrombin-activated platelets. Our st udy demonstrates that physiological platelet stimuli increase specific ally the transfer of ethanolamine phospholipids from lipoproteins to p latelets through a secretion-dependent mechanism. This might contribut e to the increase of procoagulant activity of stimulated platelets.