Concurrent measurement of platelet ionized calcium concentration and aggregation: studies with the lumiaggregometer

Platelet aggregation and aequorin-indicated cytoplasmic ionized calcium concentration ([Cai2+]) were measured simultaneously using a modification of a commercially available instrument, the Chronolog Whole Blood Lumiaggregometer. The [Cai2+] rise in response to arachidonic acid, the Ca2+-ionophore A23187, collagen, and epinephrine occurred either before or simultaneously with platelet shape change or aggregation. PGD2 caused concentration-dependent inhibition of both the [Cai2+] rise and aggregation. However, suppression of the [Cai2+] rise alone did not fully account for the inhibition of aggregation due to the elevated cAMP produced by PGD2. Agonist-induced elevation of [Cai2+] did not depend on platelet aggregation, but was enhanced by close platelet-to-platelet contact. Concurrent measurement of platelet aggregation and [Cai2+] may further clarify pathways to platelet activation.