Lipoprotein(a) inhibits collagen-induced aggregation of thrombocytes

Lipoprotein(a) [Lp(a)] is known to interact with human platelets in vitro. In the present study the effect of physiological concentrations of Lp(a) on platelet aggregation was studied. Freshly prepared gel-filtered platelets from healthy donors were incubated for 30 minutes at 37 degrees C with various concentrations of Lp(a); aggregation was triggered with ADP, thrombin, and collagen. Control incubations were performed with Tyrode's solution or LDL. Thrombin- and ADP-triggered aggregations were only slightly influenced by Lp(a), but aggregation of platelets stimulated with collagen (4 mu g/mL) was markedly inhibited. Measurable effects occurred at low concentrations (0.05 mg/mL) of total Lp(a); at 0.5 mg/mL, maximum aggregation of platelets was inhibited by 54+/-20%, and the aggregation rate was attenuated by 47+/-19% compared with platelets incubated with Tyrode's solution. Preincubation of collagen (4 mu g/mL) with Lp(a) yielded similar results. The effect of Lp(a) on platelet aggregation was accompanied by a significant reduction of serotonin release and TXA(2) formation. Higher concentrations of collagen (greater than or equal to 10 mu g/mL) caused the inhibitory effect of Lp(a) on collagen-induced aggregation to disappear. In contrast, incubation of platelets with 5 mg/mL LDL led to a significant increase of aggregation rate, maximum aggregation, serotonin release, and formation of TXA(2) when aggregation was induced with 4 mu g/mL collagen. In an adhesion assay using fresh whole blood, which mimics the in vivo situation of vessel injury, Lp(a) reduced platelet adhesion at shear rates of 300 and 1600/s by 22.6% and 11.6%, respectively, In addition, Lp(a) reduced the size of platelet aggregates significantly (up to 63%); this reduction was more distinct at the higher shear rate. Unlike LDL, Lp(a) is not a proaggregatory lipoprotein; rather, collagen-triggered aggregation in vitro is attenuated by Lp(a).