Enhancement of platelet functions by low density lipoproteins.

Platelet suspensions, that secreted about 50% of their dense granule contents upon stimulation with alpha-thrombin, showed a dose-dependent increase in secretion after 30 min preincubation with 0.5-3.0 g low density lipoprotein (LDL) protein/1. A 1-5 min preincubation had no effect. The enhancement by LDL only occurred at about 20% secretion or more, indicating that a minimal degree of activation was required for LDL to become effective. Lysine-modified LDL was equally effective as native LDL. The effect of LDL on secretion was accompanied by enhanced thromboxane B2 formation caused by stimulation of the liberation of arachidonate from phosphatidylcholine and/or phosphatidylinositol. However, when thromboxane formation was inhibited or the prostaglandin H2-thromboxane A2-receptor was blocked, LDL remained a potent stimulator of the secretion response. Thus, LDL enhances platelet secretion by a thromboxane A2-dependent and a thromboxane A2-independent mechanism via an effect that is independent of specific binding sites on the platelet.