Colloidal gold labelling of fibrinogen receptors in epinephrine- and ADP-activated platelet suspensions.

It has been generally accepted for over twenty years that epinephrine stimulates platelet aggregation without inducing shape change. However, it has been recently reported that discoid platelets are not recruited into ADP- or epinephrine-stimulated aggregates. Previous work in our laboratory has suggested that platelet shape change is necessary for the binding of fibrinogen to its surface receptor, which is a prerequisite for platelet aggregation. These studies seem to indicate that epinephrine-induced platelet aggregation does involve shape change. To investigate this possibility, the extent of shape change and fibrinogen binding in suspensions of epinephrine- and ADP-activated and control platelets was assessed by scanning electron microscopy (SEM). Platelets were incubated with 20 microM epinephrine, 20 microM ADP, or vehicle and labelled with 18 nm gold beads conjugated to fibrinogen or to a monoclonal antibody directed against the glycoprotein IIb/IIIa complex which comprises the fibrinogen receptor. Results indicate that shape change does occur in epinephrine-activated platelets as well as ADP-activated platelets. Although GP IIb/IIIa was shown to be present on both discoid and shape-changed, pseudopodial platelets, a significant degree of fibrinogen binding did not occur earlier than the pseudopodial stage in either activated or control suspensions. Platelet aggregation studies showed that the majority of platelets involved in aggregates had changed shape in both ADP- and epinephrine-treated platelet suspensions. These studies suggest that epinephrine- and ADP-induced platelet aggregation occurs via the exposure of fibrinogen receptors on shape-changed platelets.