ADENOSINE 5'-DIPHOSPHATE AS A FACTOR IN PLATELET-AGGREGATION INDUCED BY HUMAN PLASMA REMNANT LIPOPROTEINS

The action of lipoprotein lipase on chylomicrons (CM) and very low den sity lipoproteins (VLDL) produces remnant lipoproteins (RLP) which are rich in triglycerides, cholesterol and apolipoprotein E (apo E). Apo E serves as a ligand for uptake of RLP by macrophages, platelets, endo thelial cells and other cells expressing the LDL-receptor or the remna nt receptor, thus having a major role in the clearance of plasma chole sterol and triglycerides, but at the same time, uptake of apo E-bearin g RLP can profoundly alter the physiology of these cells and promote a therosclerosis. Like RLP, blood platelets also have roles in atheroscl erosis and thrombosis, hence it is likely that RLP influence platelet activity as well. RLP derived from normal human plasma VLDL and CM wer e prepared using two monoclonal antibodies, anti-apo B-100 (JI-H) and anti-ape A-I (H-12) coupled to Sepharose 4B gel to form an immunoaffin ity column. Lipoproteins containing apo B-100 including VLDL and LDL a dsorb to (JI-H)-gel, while CM and HDL with ape A-I adsorb to (H-12)-ge l. The particles in the unbound fraction (RLP) are rich in apo B-48, a po E and apo B-100 containing particles with multiple molecules of apo E. The RLP fraction with a total triglyceride of 14+/-3.2 mg/ml; chol esterol, 0.39+/-0.1 mg/ml and protein, 0.78+/-0.24 mg/ml (n = 19) was added to aliquots of blood of man, rabbits, guinea pigs, mice, and rat s at protein equivalents of 0.98 to 78 mu g/ml blood and agitated gent ly at 37 degrees C for 40 sec. Platelet aggregation was measured as a fall in single platelet count. RLP induced aggregation of platelets in man (p < 0.005) rabbit (p < 0.0005), guineapig (p < 0.002) and mouse (p < 0.0001), but no RLP induced platelet aggregation was observed in the rat blood. Scanning electron microscopy revealed that in the prese nce of RLP, platelets had adhered to and formed aggregates on red cell s. The platelet response to RLP was inhibited by apyrase known to scav enge ADP, by 5 mu M 2-chloroadenosine, a platelet ADP receptor antagon ist and by 3.4 mu M cilostazol, a phosphodiesterase type III inhibitor known to raise cyclic AMP level in platelets. It is thought that RLP cause leakage of ADP from red cells which then mediates platelet aggre gation.