Response of human platelets exposed to arachidonate upon stimulation with other agonists: possible role of Ca2+i movement.

Abstract Arachidonate (25 μM) induced only 20% maximal platelet aggregation but 80% maximal cytoplasmic ionized calcium movement (as measured by the photoprotein aequorin). Preincubation with 25 μM arachidonate prevented platelet aggregation and cytoplasmic ionized calcium movement induced by threshold concentrations of the thromboxane A2 analogue U-46619, but did not prevent platelet aggregation induced by threshold concentrations of thrombin, despite a marked reduction of cytoplasmic ionized calcium movement. These findings suggest that thrombin may employ pools of cytoplasmic ionized calcium and/or mechanisms to mobilize it different from those utilized by arachidonate and its metabolites.