The role of endogenously formed diacylglycerol in the propagation and termination of platelet activation. A biochemical and functional analysis using the novel diacylglycerol kinase inhibitor, R 59 949

The putative roles for the second messenger, diacylglycerol, were investigated in intact platelets using a novel diacylglycerol kinase inhibitor, R 59 949, or (3-[2-[4-[bis(4-fluorophenyl)methylene]-1-piperidinyl]ethyl]-2,3- dihydro-2-thioxo-4(1H)-quinazolinone). The compound inhibited the diacylglycerol kinase in a concentration-dependent manner (10(-8) to 10(-5) M) in isolated platelet membranes and in intact platelets. When platelets were stimulated with vasopressin in the presence of the compound, protein kinase C activity was markedly increased; the formation of inositol phosphates, the increase in intracellular Ca2+ and shape-change reaction were antagonized while the vasopressin-induced polyphosphoinositide synthesis was amplified, and this in a distinct inositolphospholipid pool. In the presence of R 59 949, vasopressin- as well as collagen-induced release reaction and aggregation was strongly increased, independently of the formation of arachidonate metabolites. It is concluded that diacylglycerol formed after receptor activation, likely by activating the protein kinase C, plays an important role in the propagation of platelet functional responses in casu aggregation and secretion and controls the termination of the primary receptor coupled responses.