Membrane alterations in connection with the release reaction in human platelets as studied by the lactoperoxidase-iodination technique and by agglutination with bovine factor VIII-related protein

Transient adhesion refractoriness of circulating platelets under shear stress: the role of partial activation and microaggregate formation by suboptimal ADP concentration

Exposure of whole blood (WB) to subendothelial extracellular matrix (ECM) under shear stress in the cone and plate(let) analyser (CPA) results in platelet adhesion, followed by release reaction and aggregation of circulating platelets on the adherent platelets. The properties of circulating non-adhered platelets in the CPA was studied by exposure of WB to ECM at a high shear rate (1300/s) for 2 min (1st run), followed by transfer of the suspension to a new ECM-coated well for a second run (2nd run) under similar conditions. The results of the 2nd run demonstrated transient adhesion refractoriness associated with platelet microaggregate formation in the suspension. The adhesion refractoriness was dependent on platelet activation during the 1st run and was prevented by addition of apyrase (ADP scavenger) or ADP receptor inhibitor, suggesting a role for ADP in mediating this response. Furthermore, exposure of WB samples to suboptimal concentrations of ADP (0.4-1 micromol/l) or a thrombin receptor activating peptide (TRAP) (5 micromol/l) for 2 min resulted in a similar transient platelet adhesion refractoriness to ECM under flow conditions. The transient platelet refractoriness and microaggregate formation induced by ADP was associated with a transient reduction in glycoprotein (GP)Ib, increased P-selectin expression and increased fibrinogen binding by circulating platelets. These data suggest a role for platelet agonists at suboptimal concentrations in modulating platelet function and limiting the expansion of the thrombus.

Influence of thrombin in suspension, surface activation, and high shear on platelet surface GPIb/IX distribution

Studies in our laboratory have shown that glycoprotein Ib/IX (GPIb/IX), the receptor for von Willebrand factor (vWf), is not decreased in number or cleared from exposed surfaces to the internal membranes of platelets activated in suspension by thrombin alone, by interaction with formvar surfaces alone, or by a combination of the two modes of stimulation. The present study has examined the influence of three different types of stimulation including activation by thrombin in suspension followed by surface activation on formvar, then exposure to high shear stress in a flat chamber. Samples were fixed for study in the electron microscope after each single stimulus, after the combination of two modes of activation, and after the combination of suspension, surface, and shear activation and were stained by an immunogold procedure using monoclonal antibodies to localize GPIb/IX on singly, doubly, or multiply activated cells. The results demonstrate that GPIb/IX receptors remain on activated platelets from edge to edge and that there is no difference in the number or distribution of receptor complexes on thrombin-activated platelets, surface-activated cells, or platelets exposed to the combination of suspension, surface, and shear activation. The findings add additional support to the concept that GPIb/IX is not a mobile receptor and is not cleared from exposed surfaces to internal membranes under physiologic conditions.

Influence of ionized calcium on thrombin-induced down regulation of GPIb/IX receptors on human platelets

The influence of ionized calcium on the down-regulation of GPIb/IX receptors on human platelets was evaluated by flow cytometry using monoclonal antibodies. Addition of EDTA alone to a washed platelet suspension did not cause decreased monoclonal antibody binding to the cells. However, introduction of thrombin to the washed platelets containing EDTA resulted in a marked decrease in binding of monoclonal antibodies to the GPIb/IX receptors. If calcium at 1-3 mmol/L was added to buffered platelets instead of EDTA before thrombin, down-regulation was prevented or significantly reduced. Restoring calcium to EDTA platelets after the thrombin-stimulated down-regulation had been in progress for 1-3 minutes caused reversal of decreased antibody binding by GPIb/IX to near resting levels. Results demonstrate that extracellular calcium is a major factor regulating thrombin-induced down-regulation.

Aggregated-disaggregated, refractory platelets retain sensitivity to ristocetin

The present investigation has used an aggregation-disaggregation-reaggregation model to determine if exposure to potent aggregating agents causing a refractory state in deaggregated platelets results in down-regulation and clearance of GPIb/IX, the receptor for von willebrand factor. Thrombin, the thrombin receptor activating peptide (TRAP) and the thromboxane A2 mimic, U46619, caused irreversible aggregation and secretion when stirred on an aggregometer with plateletrich plasma (PRP). Addition of prostaglandin E1 (PGE1) after the plateau of maximum aggregation was reached caused rapid dissociation of platelet aggregates. Dissociated platelets were refractory to a second exposure to the primary stimulus or to other aggregating agents. Exposure of the refractory cells to epinephrine before the primary agent restored sensitivity resulting in a second wave of irreversible aggregation. Deaggregated, refractory platelets, however, retained their sensitivity to ristocetin. Amounts of the antibiotic causing agglutination of resting PRP also caused agglutination of disaggregated, refractory platelets. Addition of epinephrine to samples of refractory platelets less sensitive to low concentrations of the antibiotic restored their capacity to undergo irreversible ristocetin-induced agglutination. Analysis of the frequency of gold particles associated with monoclonal antibodies directed against GPIb/IX on control platelets and disaggregated, refractory platelets showed no significant difference in the number of receptors. The findings indicate that significant numbers of GPIb/IX receptors remain on platelet surfaces following exposure to potent aggregating agents.

Membrane changes associated with platelet activation. Exposure of actin on the platelet surface after thrombin-induced secretion

The effect of aggregation and secretion on membrane proteins was studied in washed human platelets. Reversible aggregation without secretion was stimulated by ADP and secretion without aggregation was stimulated by thrombin in the presence of EDTA. No loss of platelet surface glycoproteins occurred during reversible ADP-induced platelet aggregation, as measured by quantitative polyacrylamide gel electrophoresis analysis of platelets that were labeled with (125)I-diazotized diiodosulfanilic acid (DD(125)ISA) before ADP stimulation. Also, no new proteins became exposed on the platelet surface after ADP aggregation, as determined by DD(125)ISA labeling after stimulation. Thrombin-induced platelet secretion also caused no loss of platelet surface glycoproteins. However, after platelet secretion two new proteins were labeled by DD(125)ISA: (a) actin and (b) the 149,000-mol wt glycoprotein (termed GP-G), which is contained in platelet granules and secreted in response to thrombin. The identity of DD(125)ISA-labeled actin was confirmed by four criteria: (a) comigration with actin in three different sodium dodecyl sulfate-polyacrylamide gel electrophoresis systems, (b) elution from a particulate fraction in low ionic strength buffer, (c) co-migration with actin in isoelectric focusing, and (d) binding to DNase I. The identity of actin and its appearance on the platelet surface after thrombin-induced secretion was also demonstrated by the greater binding of an anti-actin antibody to thrombin-treated platelets, measured with (125)I-staphylococcal protein A.Therefore, major platelet membrane changes occur after secretion but not after reversible aggregation. The platelet surface changes occurring with secretion may be important in the formation of irreversible platelet aggregates and in the final retraction of the blood clot.