Role Of Prostaglandins In Platelet Fibrinogen Receptor Exposure

Fibrinogen binding to membrane receptors exposed by agonists such as ADP and thrombin is a prerequisite for platelet aggregation. However, a role for platelet prostaglandins in this process remains to be clarified. To assess whether platelet prostaglandins can expose fibrinogen receptors, we examined the effects of aspirin (ASA) and indomethacin on fibrinogen binding to ADP and epinephrine- stimulated platelets. Fibrinogen binding was measured by incubating gel-filtered human platelets (GFP) with ¹²⁵I- labeled fibrinogen, CaCl₂ and ADP or epinephrine at 37°C for 3 min without stirring. Free and platelet-bound I-fibrinogen were separated by rapid sedimentation of the platelets through silicone oil. The platelet release reaction, measured as ¹⁴C-serotonin secretion, did not occur under these conditions. Fibrinogen binding in response to l-2μM ADP or 10μM epinephrine was inhibited 40-60% by preincubation of platelet-rich plasma with 1 mM ASA for 20 min at 37° C. Similar results were produced by adding 25μM indomethacin to GFP. The inhibitory effect of ASA on ADP-stimulated fibrinogen binding could be overcome by increasing the ADP concentration while the inhibition of epinephrine-stimulated fibrinogen binding could not. However, stimulation of aspirin-treated platelets with both 10μM epinephrine and the prostaglandin endoperoxide PGH₂(1μM) restored the extent of fibrinogen binding to that of control platelets stimulated by 10μM epinephrine alone. Scatchard analysis demonstrated that ASA decreased the number but not the affinity of the exposed fibrinogen binding sites. Identical results were obtained if ASA was ingested before blood donation. These studies demonstrate that prostaglandins synthesized in response to platelet stimulation can expose fibrinogen receptors on the platelet surface. Furthermore, these studies support the concept that platelet aggregation can occur through mechanisms independent of platelet ADP secretion.