Apoprotein (A) antagonises THE GPIIB/IIIA receptor on collagen and adp-stimulated human platelets.

The nature of the lipoprotein (a) (Lp(a))/agonist-stimulated platelet interaction is unclear. The objective was to determine whether Lp(a) inhibits platelet aggregation by displacing fibrinogen from the platelet GPIIb/IIIa receptor. Platelets were washed in Tyrode's buffer and stimulated using 10 micromolar ADP or 2 micrograms/ml collagen. Lp(a) was isolated from plasma using lectin affinity chromatography followed by ultracentrifugation. Lp(a) inhibited aggregation of collagen- and ADP-stimulated platelets with IC-50's of about 5 mg/dl. Lp(a) inhibited 125I-labeled fibrinogen binding to collagen-stimulated platelets with an IC-50 of less than 5 mg/dl. MAb 3B1, specific for apo(a), restored platelet aggregation to control levels, inhibited 125I-labelled Lp(a) binding, and increased 125I-labelled fibrinogen binding by displacing Lp(a) from the fibrinogen binding site. In conclusion, binding of Lp(a) results in displacement of fibrinogen from its receptor, leading to decreased platelet aggregation. This antagonism suggests a novel role for Lp(a) in modulating fibrinogen binding to the GPIIb/IIIa receptor on collagen- and ADP-stimulated platelets.