Measurement of platelet ionized calcium.

We measured ionized cytoplasmic calcium in human blood platelets by two techniques: one based on the fluorescence of intracellular Quin 2 and the other based on the luminescence of the calcium sensitive photoprotein, aequorin. Platelet activation (shape change, aggregation, secretion) induced by thrombin, ADP, epinephrine, collagen, A23187, or phorbol ester was invariably preceded or accompanied by a rise in platelet free calcium indicated by aequorin, but in some cases Quin 2 failed to detect the elevation of calcium concentration. Aequorin appears to be sensitive to localized changes in cytoplasmic calcium levels, while Quin 2 reflects average or diffuse calcium values. Failure of Quin 2 to indicate a rise in free calcium levels in association with a platelet activity does not establish the process as "calcium independent".