REASSESSMENT OF THE EVIDENCE FOR THE ROLE OF SECRETED ADP IN BIPHASIC PLATELET-AGGREGATION - MECHANISM OF INHIBITION BY CREATINE-PHOSPHATE PLUS CREATINE-PHOSPHOKINASE

Abstract

The observation that [human] platelet secretion occurs in parallel with the 2nd wave of platelet aggregation necessitates reassessment of the generally accepted concept that secreted ADP is the cause of 2nd-wave aggregation. The major evidence for involvement of secreted ADP is inhibition of 2nd-wave aggregation by enzyme-catalyzed removal of ADP, interpreted as removal of secreted ADP. An alternative hypothesis that the observed inhibition is due to a decrease in potentiation by extracellular ADP present prior to addition of stimulus, was tested with the enzyme system CP [creatine phosphate] plus CPK [creatine phosphokinase] (CP/CPK). Low levels of CP/CPK inhibited .gamma.-thrombin-induced 2nd-wave aggregation only after preincubation. When the 2nd wave of .gamma.-thrombin- or epinephrine-induced aggregation was inhibited by very high levels of CP/CPK, the inhibition was overcome by an increase in the level of stimulus. These results were inconsistent with the idea that CP/CPK blocks 2nd-wave aggregation by removing secreted ADP, but they are consistent with a decreased potentiation of the platelets due to a lower level of extracellular ADP prior to stimulation. Freshly prepared platelet-rich plasma contained a mean (n = 16) of 12 nM extracellular ADP. Inhibition by CP/CPK cannot be taken as evidence for involvement of secreted ADP.