THROMBIN INDUCES A BIPHASIC 1,2-DIACYLGLYCEROL PRODUCTION IN HUMAN PLATELETS

The 1,2-diacylglycerol (DAG) mass content was measured in thrombin-stimulated human platelets. Thrombin stimulates a biphasic accumulation of DAG, with an early phase reaching a peak at 10 s and a later phase reaching a peak at 2-3 min. The time course of first-phase DAG production corresponded well to that of Ins(1,4,5)P3 formation, which was rapid and transient. The second phase of DAG accumulation occurred after the level of Ins(1,4,5)P3 returned to nearly basal. Thrombin stimulated the decrease in PtdIns and phosphatidylcholine contents. The source of second-phase DAG was examined in platelets prelabelled with three radioactive fatty acids, i.e. arachidonic, palmitic and myristic. Thrombin stimulated the increase in radioactivity of DAG with decline of PtdIns in platelets labelled with [H-3]arachidonic acid or [H-3]palmitic acid, in which PtdIns wasconsiderably labelled. In contrast, significant accumulation of [H-3]DAG was not observed in [H-3]myristic acid-labelled platelets, in which PtdIns was poorly labelled. In platelets prelabelled with [H-3]inositol, an increase in InsP in response to thrombin was seenfor more than 5 min. In contrast, upon stimulation, significant increases in [H-3]phosphocholine and [H-3]choline were not observed in [methyl-H-3]choline-labelled platelets. Thrombin induced a small production of phosphatidylethanol, when ethanol was present during stimulation. However, the formation of DAG and phosphatidic acid was not significantly affected by ethanol. These results suggest thrombin stimulates a biphasic accumulation of DAG, initially from PtdInsP2 and later from PtdIns in human platelets.