Decay of the activating signal after platelet stimulation with 1-0-alkyl-2-acetyl-SN-glycero-3-phosphorylcholine: changes in calcium permeability.

The decay of the platelet-activating signal after platelet stimulation with the platelet-activating factor 1-0-alkyl-2-acetyl-SN-glycero-3-phosphorylcholine (AGEPC) was examined. Washed human platelets in calcium-poor buffer were stimulated with AGEPC followed 0.25 to 10 min later by reconstitution with 1.8 mM calcium. AGEPC did not induce platelet aggregation in calcium-poor buffer but aggregation occurred upon addition of calcium. The capacity of calcium-reconstitution to elicit platelet aggregation decayed rapidly after platelet exposure to AGEPC (T 1/2 = 1 min). A parallel decay of AGEPC-induced increased calcium permeability (R = 0.93) was demonstrated using the fluorescent probe Quin 2. Studies with calcium channel blockers suggest that AGEPC opens a calcium channel which is distinct from those present in unstimulated platelets.