SummaryThe interaction of β2-glycoprotein-I (β2-G-I), a plasma constituent of unknown function, with blood platelets was studied. The following results were obtained: 1) β2-G-I binds to washed human platelets isolated by centrifugation (WP) at one kind of specific, saturable binding sites. The dissociation constant was found to be approx. 1 × 10−6M.2) In the presence of physiological concentrations of Ca++ (2.5 mM), this specific binding is markedly reduced. Unspecific binding of β2-G-I to platelets, however, is not influenced by Ca++.3) Platelets prepared by gel filtration (GFP), differing in their in vitro aggregability from WP, exhibit no specific binding of β2-G-I. Binding to GFP is also not induced by activation with thrombin, collagen or ADP.4) β2-G-I causes significant alteration of the ADP-induced aggregation of GFP. Aggregation induced by thrombin, collagen, arachidonic acid or PAF-acether, however is not altered by β2G-I.It is suggested, that pelleting during centrifugation causes irreversible rearrangements in the membrane of platelets.
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 micrograms/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 micrograms/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 TXA2 formation. Higher concentrations of collagen ( > or = 10 micrograms/ mL) caused the inhibitory effect on 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 TXA2 when aggregation was induced with 4 micrograms/mL collagen. In an adhesion assay using fresh whole blood, which mimics the in vitro 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 distant 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).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.