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) u nder shear stress in the cone and plate(let) analyser (CPA) results in plat elet adhesion, followed by release reaction and aggregation of circulating platelets on the adherent platelets. The properties of circulating non-adhe red platelets in the CPA was studied by exposure of WB to ECM at a high she ar rate (1300/s) for 2 min (1st run), followed by transfer of the suspensio n to a new ECM-coated well for a second run (2nd run) under similar conditi ons. The results of the 2nd run demonstrated transient adhesion refractorin ess associated with platelet microaggregate formation in the suspension. Th e adhesion refractoriness was dependent on platelet activation during the 1 st run and was prevented by addition of apyrase (ADP scavenger) or ADP rece ptor inhibitor, suggesting a role for ADP in mediating this response. Furth ermore, exposure of WB samples to suboptimal concentrations of ADP (0.4-1 m u mol/l) or a thrombin receptor activating peptide (TRAP) (5 mu mol/l) for 2 min resulted in a similar transient platelet adhesion refractoriness to E CM under flow conditions. The transient platelet refractoriness and microag gregate formation induced by ADP was associated with a transient reduction in glycoprotein (GP)Ib, increased P-selectin expression and increased fibri nogen binding by circulating platelets. These data suggest a role for plate let agonists at suboptimal concentrations in modulating platelet function a nd limiting the expansion of the thrombus.