Fibrinogen Binding to Human Platelets*

Platelet-activating factor 1-0-alkyl-2-acetyl-englycero-3-phosphocholine (PAF-acether) triggers exposure of fibrinogen binding sites on platelets via binding to specific receptors. Comparison of 13H]PAFacether binding with lZ8I-fibrinogen binding shows that the rate with which PAF-acether binds to a number of receptors and not the degree of receptor occupancy determines how much fibrinogen binds. At low concentrations of PAF-acether (0.1-1.0 nM) binding site exposure is incomplete and parallels the rate of formation of the PAF-acether-receptor complex. Fibrinogen binding then primarily depends on the concentration of PAF-acether. A t a high concentration of PAF-acether (500 nM) binding site exposure is complete within 2-5 min. Fibrinogen binding then depends on the concentration of fibrinogen. Exposure of binding sites in the absence of fibrinogen leads to disappearance of accessible binding sites. At 500 nM PAFacether, this disappearance is exponential in nature and shows the same characteristics after 5-15 min incubation with fibrinogen as after 60 min. Exposure of binding sites is then complete within 5 min and their disappearance is not disturbed by other processes. At 0.5 nM PAF-acether, the same characteristics are found after 60 min incubation with fibrinogen, but shorter incubation times reveal n ongoing binding site exposure that interferes with the disappearance process. These results demonstrate close coupling between the PAF-acether receptors and fibrinogen binding sites and indicate that the rate of formation of the PAFacether-receptor complex is a major factor in the regulation of binding site exposure.