Palmitic acid-labeled lipids selectively incorporated into platelet cytoskeleton during aggregation

Previous experiments showed that during the early stages (20–30 seconds) of aggregation induced by adenosine diphosphate (ADP, 2 μM) or thrombin (0.1 U/mL) of rabbit or human platelets prelabeled with ³H]palmitic acid, labeled lipid became associated with the cytoskeleton isolated after lysis with 1% Triton X-100, 5 mM EGTA ethylene glycol-bis-(β-aminoethyl ether)]-N,N,N',N'-tetra-acetic acid. The association appeared to be related to the number of sites of contact and was independent of the release of granule contents. We have now investigated the nature of the labeled lipids by thin-layer and column chromatography and found differences between the distribution of the label in intact platelets (both stimulated and unstimulated) and the isolated cytoskeletons. In both species and with either ADP or thrombin as aggregating agent, 70–85% of the label in both intact platelets and in the cytoskeletons was in phospholipids. The distribution of label among the phospholipids in the cytoskeletons was similar to that in intact platelets except that the percentage of label in phosphatidylcholine was significantly higher in the cytoskeletons of human platelets than in the intact platelets, and the percentage of label in phosphatidylserine/phosphatidylinositol was significantly lower in the cytoskeletons of rabbit platelets and thrombin-aggregated human platelets than in intact platelets. The cytoskeletons contained a lower percentage of label in triacylglycerol, diacylglycerol, and cholesterol ester than the intact platelets. Contrary to a report in the literature, we found no evidence for the incorporation of diacylglycerol and palmitic acid into the cytoskeleton. Although intact rabbit platelets had more label in ceramide (6.7±2.9%) than intact human platelets (1.5±0.9%), platelets of both species exhibited a three- to four-fold enrichment of labeled ceramide in the cytoskeletons. Thus phospholipids and ceramide that are readily labeled with palmitic acid are selectively incorporated into the cytoskeleton during the initial stages of platelet aggregation.