Abstract
Platelets adhere to artificial surfaces in the initial stage of thrombus formation, but the subsequent steps in signal transduction that lead to platelet activation by artificial surfaces are not understood. When 0.325-micron diameter beads composed of a hydrophobic polymer, polymethylmethacrylate (PMMA), were added to gel-filtered aequorin-loaded platelets suspended in media containing Ca2+, the platelets aggregated; addition of fibrinogen was not required. Platelet aggregation was preceded by an increase in cytoplasmic Ca2+ and was accompanied by phosphorylation of the 47-Kd substrate of protein kinase C (PKC), 5-hydroxytryptamine (5-HT) release, and accumulation of phosphatidic acid. All these effects were partially inhibited by apyrase and aspirin. Monoclonal antibodies (MoAbs) 7E3 and M148 and the synthetic peptides RGDS and fibrinogen gamma chain fragment 400–411, all of which bind to the platelet fibrinogen receptor glycoprotein IIb- IIIa (GPIIb-IIIa) and inhibit fibrinogen binding, prevented PMMA- induced aggregation but did not inhibit the Ca2+ increase. Chymotrypsin- treated platelets aggregated after addition of fibrinogen, but not PMMA. We conclude that platelets interact initially with PMMA at membrane sites other than those required for fibrinogen binding, leading to activation of membrane phospholipases and PKC, an increase in cytoplasmic Ca2+, release of 5-HT, ADP, and fibrinogen from storage granules, and to platelet aggregation.
