PHYSICAL AND CHEMICAL EFFECTS OF RED-CELLS IN THE SHEAR-INDUCED AGGREGATION OF HUMAN PLATELETS

Both chemical and physical effects of red cells have been implicated i n the spontaneous aggregation of platelets in sheared whole blood (WB) . To determine whether the chemical effect is due to ADP leaking from the red cells, a previously described technique for measuring the conc entration and size of single platelets and aggregates was used to stud y the shear-induced aggregation of platelets in WB flowing through 1.1 9-mm-diameter polyethylene tubing in the presence and absence of the A DP scavenger enzyme system phosphocreatine-creatine phosphokinase (CP- CPK), Significant spontaneous aggregation was observed at mean tube sh ear rates, [G] = 41.9 and 335 s(-1) (42% and 13% decrease in single pl atelets after a mean transit time [t] = 43 s, compared to 89 and 95% d ecrease with 0.2 mu M ADP). The addition of CP-CPK, either at the time of, or 30 min before each run, completely abolished aggregation. In t he presence of 0.2 mu M ADP, CP-CPK caused a reversal of aggregation a t [t] = 17 s after 30% of single cells had aggregated. To determine wh ether red cells exert a physical effect by increasing the time of inte raction of two colliding platelets (thereby increasing the proportion of collisions resulting in the formation of aggregates), an optically transparent suspension of 40% reconstituted red cell ghosts in serum c ontaining 2.5-mu m-diameter latex spheres (3 x 10(5)/mu l) flowing thr ough 100-mu m-diameter tubes was used as a model of platelets in blood , and the results were compared with those obtained in a control suspe nsion of latex spheres in serum alone. Two-body collisions between mic rospheres in the interior of the flowing ghost cell or serum suspensio ns at shear rates from 5 to 90 s(-1) were recorded on cine film. The f ilms were subsequently analyzed, and the measured doublet lifetime, ta u(meas), was compared with that predicted by theory in the absence of interactions with other particles, tau(theor). The mean (tau(meas)/tau (theo)) for doublets in ghost cell suspensions was 1.614 +/- 1.795 (SD ; n = 320), compared to a value of 1.001 +/- 0.312 (n = 90) for double ts in serum. Whereas 11% of doublets in ghost cell suspensions had lif etimes from 2.5 to 5 times greater than predicted, in serum, no double ts had lifetimes greater than 1.91 times that predicted. There was no statistically significant correlation between tau(meas)/tau(theor) and shear rate, but the values of tau(meas)/tau(theor) for low-angle coll isions in ghost cell suspensions were significantly greater than for h igh-angle collisions.