Inhibition by a stable analogue of adenosine triphosphate of platelet aggregation by adenosine diphosphate

1 In citrated platelet-rich plasma, freshly prepared from rabbit blood, the velocity of platelet aggregation was within limits proportional to the log of the concentration of added adenosine diphosphate (ADP). 2 Addition of either adenosine triphosphate (ATP) or its beta,y-methylene analogue inhibited aggregation similarly except that the analogue was about half as potent as ATP. beta,y-Methylene ATP also reversed the optical effects associated with the shape change of platelets very similarly to ATP itself. 3 As beta,y-methylene ATP is not a substrate for nucleoside diphosphokinase, these observations do not support the proposition that inhibition of aggregation by added ATP is due to its utilization by the nucleoside diphosphokinase of platelets.