Kinetics of the Thrombin-Induced Release of Adenosine Triphosphate by Platelets. Comparison with Release of Calcium

The kinetics of the release of ATP from human platelets induced by thrombin has been studied and compared to the previously reported kinetics of Ca²⁺ release. The release of ATP was observed by recording the luminescence resulting from the reaction of released ATP with firefly extract. The reaction was characterized by a delay of several seconds followed by a first-order release. ATP-release curves were analyzed by measuring the length of the delay, the first-order rate constant, and the amount of ATP released. The delay varied inversely with thrombin, while the first-order rate constant was a direct function of thrombin, saturating at about 2 units of thrombin/ml with a value of 0.03 sec^–1. Both the delay and the first-order rate constant were independent of platelet concentration. The amount of ATP released was a function of both platelet and thrombin concentrations, indicating a very slow turnover of thrombin. Although much of the data for ATP release is consistent with a previously proposed model for the reaction of thrombin with platelets, ATP is released by a mechanism that is kinetically distinct from release of Ca²⁺. Direct comparisons of release of ATP and Ca²⁺ indicate that release of Ca²⁺ preceeds release of ATP. The results are discussed in terms of a model in which thrombin causes intracellular release of bound calcium, which in turn mediates other thrombin-induced processes, including the release reaction.