Influence of vortex speed on fresh versus stored platelet aggregation in the absence and presence of extracellular ATP

Platelets are subjected to vastly differing shear forces under laminar and nonlaminar flow patterns throughout the tortuous cardiovascular system. Dif ferent activation pathways appear to be associated with platelet adhesion a nd aggregation under high shear rates vs. low shear rates. We found that pl atelets continue to aggregate at very low stirring rates (100 RPM) and low shear forces although significantly less than at high stirring rates (1000 RPM). These conditions may model vortices encountered in vivo, such as down stream of partially occluded blood vessels, The extent of agonist-induced p latelet aggregation, at varying stir rates, remained essentially unchanged between 1200 and 600 RPM. This was true for both freshly prepared and store d platelets even though the extent of aggregation was significantly reduced with stored platelets. Agonists used were thrombin, thrombin receptor acti vating peptide (TRAP), SFLLRNP, the thromboxane A2 mimetic, U46619, plus ep inephrine and ADP+epinephrine. At lower stir rates (100-400 RPM), little or no difference in aggregation levels was observed between fresh and stored platelets, depending upon agonist used. This may indicate that old and youn g platelets, in vivo, would be equally active at vessel walls exposed to bl ood flowing through a slow vortex at low shear rates. ATP, released from ac tivated platelets, may act as a potent regulator of platelet aggregation wi thin a vortex where the resident time of platelets and bioactive molecules is greater than in laminar flow regions. High levels of extracellular ATP ( 100 mu M) inhibited agonist-induced aggregation of fresh platelets to a gre ater extent than stored platelets, except with ADP+epinephrine where the co nverse was observed, Inhibition, in general, appeared to be inversely relat ed to stir rates. Low levels of extracellular ATP (10 nM, 1 mu M) generally stimulated agonist-induced aggregations independent of stir rates and to a greater extent with stored platelets than fresh platelets. Unraveling how hemostasis functions within microenvironments may facilitate ways to furthe r regulate this process. (C) 2000 Elsevier Science Ltd. All rights reserved .