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The shape change and aggregation of washed platelets induced by 10 AM arachidonic acid (AA) can be reversed by 20 ng/ml prostacyclin (PGI2), but these platelets can be reactivated by treatment with 30 AM epinephrine and subsequent addition of 10 AM AA mixture . These events may be modulated by CAMP since 2 mM dibutyryl CAMP also reversed activation without reactivation by epinephrine and AA. We examined protein phosphorylation and formation of cytoskeletal cores resistant to 1 % Triton X-100 extraction of these platelets and correlated these processes with aggregation, fibrinogen binding, and changes in ultrastructure. Unactivated platelet cores contained <15% of the total actin and no detectable myosin or actin-binding protein . AA-induced cytoskeletal cores, which contained 60-80% of the total actin, myosin, and actin-binding protein as the major components, were disassembled back to unactivated levels by PGI2 and then fully reassembled by epinephrine and AA. Phosphorylation of myosin light chain and a 40,000-dalton protein triggered by AA (two-to fivefold) was reversed to basal levels by PG1 2 but was completely restored to peak levels upon addition of the epinephrine and AA mixture . The reversibility of actin-binding protein phosphorylation could not be established clearly because both PGI2 and dibutyryl CAMP caused its phosphorylation independent of activation . With this possible exception, cytoskeletal assembly with associated protein phosphorylation, aggregation, fibrinogen binding, and changes in ultrastructure triggered by activation are readily and concertedly recyclable .

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The present study has evaluated the hypothesis stating that glycoprotein (GP) Ib/IX, the receptor for von Willebrand factor (vWF), is downregulated and cleared from exposed surfaces to channels of the open canalicular system (OCS) on platelets activated by thrombin in suspension. Cryosections of resting and thrombin-activated platelets fixed at intervals of 1 to 30 minutes after stimulation by thrombin and stained with antiglycocalicin antibody and protein A gold showed no decrease in the density of GPIb/IX receptors on the platelet surface or increase on linings of the OCS at any interval after stimulation by thrombin. Thin sections of platelets exposed to thrombin in suspension followed by settling onto a plastic chamber for intervals of 1 to 30 minutes revealed retention of GPIb/IX receptors on exposed surfaces detected by vWF, anti-vWF, and protein A gold throughout the 30-minute period of study. Results of this investigation indicate that GPIb/IX receptors remain on the surface of platelets activated by thrombin in suspension, are not cleared to the OCS, and retain the ability to bind vWF for at least 30 minutes.

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The receptor for von Willebrand factor (vWF) on human platelets, glycoprotein (GP) Ib/IX, has been shown in our studies to be an immobile complex when stimulated in suspension or on surfaces. Recent investigations have revealed that GP Ib/IX remains immobile on platelets activated in suspension followed by exposure to formvar surfaces that cause the cells to spread. However, since channels of the open canalicular system (OCS) are evaginated back on to the exposed surface during spreading, it was suggested that our study missed the clearance of GP Ib/IX from the exposed surface to internal membranes. The present study has added cytochalasin B after exposure of platelets to thrombin or TRAP in suspension in order to prevent spreading and movement of GP Ib/IX during subsequent exposure to surface activation on formvar grids. Results indicate that GP Ib/IX receptors remain randomly dispersed from edge to edge on platelets activated by thrombin or TRAP in suspension 10 minutes before treatment with CB followed by surface activation. Statistical analysis of the frequency of immunogold particles binding to monoclonal antibodies attached to GP Ib/IX revealed no significant reduction in frequency, translocation from cell edges or concentration of GP Ib/IX receptors in or around channels of the OCS. Results support the concept that GP Ib/IX is not cleared from exposed surfaces to the OCS of platelets activated by thrombin or TRAP and surface activation.

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