DIACYLGLYCEROL ELEVATIONS IN CONTROL PLATELETS ARE UNACCOMPANIED BY PLECKSTRIN PHOSPHORYLATION - IMPLICATIONS FOR THE ROLE OF DIACYLGLYCEROL IN PLATELET ACTIVATION

Several laboratories have reported that diacylglycerol levels in human platelets (approximate to 100 pmol/10(9) platelets) increased several fold in response to 0.5-1 U/ml thrombin. We report here fluctuations i n diacylglycerol mass in control platelets, the magnitude of which wer e 60-90% of that measured in platelets treated with 0.2-0.5 U/ml of th rombin. These control platelets were not activated by such criteria as absence of aggregation, secretion, phosphatidic acid production and p hosphorylation of the protein kinase C substrate, pleckstrin. Thrombin treatment evoked all of the above responses. Analysis of the diacylgl ycerol molecular species by reverse-phase HPLC of the dimethylated, ph osphorylated derivatives showed that all of the molecular species that were present in control platelets were also present in thrombin-treat ed platelets. Most of the species appeared to fluctuate at random in c ontrol platelets with the exception of 1-stearoyl-2-arachidonoyl-sn-gl ycerol which was more or less stable and increased severalfold over co ntrol values only upon thrombin treatment. Furthermore, only this spec ies accumulated as [P-32]phosphorylated PtdOH in thrombin-treated plat elets prelabelled with [P-32]P-i. Our findings show that, in platelets elevation of diacylglycerol molecular species other than the 1-stearo yl-2-arachidonoyl species occurs, but these changes are not necessaril y linked to activation of protein kinase C as measured by pleckstrin p hosphorylation which was observed only upon elevation of 1-stearoyl-2- arachidonoyl-sn-glycerol.