FACTORS THAT CONTRIBUTE TO SPONTANEOUS PLATELET-AGGREGATION AND STREPTOKINASE-INDUCED AGGREGATION IN WHOLE-BLOOD

When whole blood is stirred there is a ''spontaneous'' platelet aggreg ation (SPA) which is presumed to be caused by proaggregatory factors r eleased from platelets and other blood cells. Adding streptokinase (SK ) to stirred whole blood frequently increases the rate and extent of t he platelet aggregation that occurs; this is likely to be via immune c omplex formation between SK and natural anti-SK antibodies leading to increased release of pro-aggregatory factors. In this investigation we have examined the effects of several inhibitors and antagonists in an attempt to identify the proaggregatory factors that contribute to bot h SPA and SK-induced aggregation (SKA) and to evaluate different means of inhibiting both processes. The effects of the inhibitors/antagonis ts were determined in vitro after adding them to citrated whole blood obtained from healthy volunteers. Platelet aggregation was measured us ing a platelet counting technique. Inhibition of both SPA and SKA by a pyrase and by FPL 66096 (a P-2T receptor antagonist) demonstrated the involvement of ADP in both processes. Inhibition by chlorpromazine ind icated that the most likely source of the ADP is red cells. The effect s of sulotroban (a TXA(2) antagonist) indicated involvement of TXA(2) in SKA but not in SPA. The lack of effect of specific antagonists at S -2, alpha(2) and PAF receptors suggested lack of involvement of seroto nin, catecholamines and platelet-activating factor in either SPA or SK A. Both SPA and SKA were potently inhibited by low concentrations of i loprost (a PGI(2) analogue), but a high concentration of SIN-1 (a NO d onor) was much less effective. SPA and SKA were prevented by EDTA and by RGDS indicating the importance of divalent cations and of the RGD s equence in adhesive proteins in mediating the platelet aggregation tha t occurred. We also determined the effects on SPA and SKA of adding Mg Cl2 to whole blood. In this case we used blood containing hirudin as a nticoagulant. MgCl2 (1 mM) appeared to delay the onset of SPA and mark edly inhibited SKA. The results of this systematic study provide clear information on the nature of the pro-aggregatory factors that contrib ute to SPA and SKA in stirred whole blood and on effective means of in hibiting these processes.