The effects of citrate and extracellular calcium ions on the platelet release reaction induced by adenosine diphosphate and collagen.

The ADP-induced release of 3H-serotonin from human platelets in heparinized platelets rich plasma is markedly stimulated by the addition of sodium citrate. The aggregation and release that is induced by collagen is less affected by citrate. Data is presented that supports the view that the effects of citrate on both ADP- and collagen-induced release are largely via alteration of the concentration of ionized calcium in plasma. Collagen can induce release of 3H-serotonin via extracellular calcium-independent and -dependent mechanisms. The possibility that the calcium-dependent mechanism is aggregation-dependent and that the calcium is required for platelet aggregation rather than directly involved in the release reaction is discussed.