Release of platelet-derived growth factor from human platelets by arachidonic acid

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Abstract
Platelet .alpha.-granules contain a factor that stimulates the proliferation of arterial smooth muscle cells and may play a role in atherogenesis. The role of arachidonic acid in mediating the release of the platelet-derived growth factor (PDGF) from human platelets was studied. PDGF was assayed by stimulation of [3H]thymidine incorporation into DNA of mouse [fibroblast] 3T3 cells. Platelet aggregation and the release of platelet factor 4, .beta.-thromboglobulin, and serotonin were also studied. A biphasic response pattern was observed when gel-filtered platelets were incubated with arachidonate over the concentration range 0.01-0.4 mM. At low arachidonate levels (approximately 0.025-0.1 mM), specific concentration-dependent aggregation and release of PDGF and of the other components were observed. This effect was not seen with any of 5 other fatty acids tested and was suppressed by indomethacin (24 .mu.M). At higher arachidonate concentrations (approximately 0.15-0.35 mM), a concentration-dependent turn-off of aggregation and release occurred. At these concentrations the platelets remained functional, and no release of lactate dehydrogenase was observed. A similar biphasic pattern of arachidonate-induced aggregation and release was observed with platelet-rich plasma, over a similar range of arachidonate to albumin mole ratios. PDGF and other .alpha.-granule constituents can be released from platelets specifically by arachidonate via an indomethacin-sensitive pathway, most probably involving the platelet cyclooxygenase and conversion of arachidonate to prostaglandin metabolites. The mechanisms responsible for the turn-off of the specific arachidonate-mediated responses at higher arachidonate concentrations remain to be defined.

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