RELEASED ADENOSINE-DIPHOSPHATE STABILIZES THROMBIN-INDUCED HUMAN PLATELET AGGREGATES

Normal human platelets aggregated by thrombin undergo the release reaction and are not readily deaggregated by the combination of inhibitors hirudin. chymotrypsin, and prostaglandin E1 (PGE1). In contrast, thrombin-induced aggregates of platelets from patients with delta-storage pool deficiency (δ-SPD), which lack releasable nucleotides, are readily deaggregated by the same combination of inhibitors. The ease with which δ-SPD platelets are deaggregated is caused by the lack of stabilizing effects of released ADP, since: (1) exogenous adenosine diphosphate (ADP) (10 μmol/L), but not serotonin (2 μmol/L), abolishes the ability of these inhibitors to deaggregate δ-SPD platelets; (2) thrombin-induced aggregates of platelets from a patient (V.R.) (whose platelets have a severe, selective impairment of sensitivity to ADP, but normal amounts of releasable nucleotides) can be readily deaggregated, and addition of ADP does not stabilize the platelet aggregates; (3) apyrase or creatine phosphate (CP)/creatine phosphokinase (CPK), added before thrombin, make control platelets more easily deaggregated by hirudin, chymotrypsin, and PGE1. and do not change the deaggregation response of δ-SPD platelets and of V.R.'s platelets. Thrombin-induced aggregation and release of β-thromboglobulin in control, δ-SPD, and in V.R.'s platelets was similar and not inhibited by apyrase or CP/CPK. The stabilizing effect of ADP on platelet aggregates is specific, since epinephrine in the presence of apyrase to remove traces of released ADP does not stabilize the aggregates of control, δ-SPD. or of V.R.'s platelets. Because epinephrine increases fibrinogen binding to thrombin-stimulated platelets to a greater extent than ADP, but does not stabilize the aggregates, it is unlikely that the additional fibrinogen binding sites induced by ADP have a major role in inhibiting deaggregation by the combination of inhibitors.