Platelet interaction with high and low density lipoproteins

Gel-filtered platelets (GFP) from normal human subjects bound both low density lipoproteins (LDL) and high density lipoproteins (HDL). This binding was saturable and 125I-labelled lipoprotein uptake was inhibited by plasma. Platelets are also able to degrade lipoproteins but only to a limited extent. LDL appeared to compete with 125I-labelled HDL for platelet uptake, whereas the ability of HDL to displace 125I-LDL was limited. Cyclohexanedione-treated LDL (CHD-LDL), unlike CHD-HDL, did not compete with [125I]LDL for platelet accumulation, suggesting that arginine residues are necessary for LDL but not HDL binding. Addition of HDL or LDL to GFP did not alter platelet aggregation. However, in the presence of thrombin (0.5 U/ml), 1 mg/ml LDL incubated for 1 h at 23 degrees C enhanced platelet aggregation (215% increase) whereas HDL under similar conditions decreased aggregation by 53%. LDL also shortened the time of maximal aggregation whereas HDL had the opposite effect.