title: 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 platelet secretion occurs in parallel with the second wave of platelet aggregation necessitates reassessment of the generally accepted concept that secreted ADP is the cause of second-wave aggregation. The major evidence for an involvement of secreted ADP is inhibition of second-wave aggregation by enzyme-catalyzed removal of ADP, interpreted as removal of secreted ADP. An alternative hypotheses, that the observed inhibition is due to a decrease in potentiation by extracellular ADP present prior to addition of stimulus, has been tested with the enzyme system CP plus CPK (CP/CPK). Low levels of CP/CPK inhibited gamma-thrombin-induced second-wave aggregation only after preincubation. When the second 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 are inconsistent with the idea that CP/CPK blocks second-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. These data demonstrate that inhibition by CP/CPK cannot be taken as evidence for the involvement of secreted ADP.
mesh_terms: D000244:Adenosine Diphosphate / Q000097:blood*; D000255:Adenosine Triphosphate / Q000097:blood; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism; D003402:Creatine Kinase / Q000494:pharmacology*; D006801:Humans; D007700:Kinetics; D010725:Phosphocreatine / Q000494:pharmacology*; D010974:Platelet Aggregation / Q000187:drug effects
chemical_list: D010725:Phosphocreatine; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D003402:Creatine Kinase