Platelet Activating Factor (PAF)-Induced Aggregation And Release Responses In Vitro In Three Species

Abstract

PAF-induced aggregation in human, rabbit and guinea pig platelets was studied, along with concomitant dense granule (ATP) release. Platelet rich plasma (PRP) was prepared from citrated whole blood (9:1) and adjusted to 5 × 10° plt/ ml. Aggregation and release assays were performed in a Chronolog Lumi-Aggregometer. PAF was dissolved in Tris- Tyrode-Albumin (0.25%) buffer, pH 7.4. ATP standards were run prior to each assay.ATP release was most sensitive in the guinea pig (range 10-10 (max) to 10-11(min) M PAF), less in the rabbit (10-8 to 10-9M PAF) and least sensitive to PAF in human platelets. In human platelets PAF elicited a biphasic release of ATP, the first immediately following addition of PAF and the second at_4-5 min. Release was induced in human platelets by 2 ß l0-7 to 6 × 10-7M PAF.Throughout the range of PAF concentrations which caused a dose-related release of ATP in guinea pig platelets, platelet aggregatory response remained constant (85% or more). Similar findings were observed in rabbit platelets.Human PRP exhibited biphasic release and irreversible aggregation at 10-6M PAF. At 10-7M the initial phase of ATP release disappeared, while a biphasic aggregatory response remained, accompanied by a secondary release pattern. Aspirin (ASA 200 μM final concentration) inhibited both the secondary phases of release and aggregation.PAF-induced aggregation in rabbit and guinea pig platelets appears to be independent of the release reaction. In human platelets, PAF gives a biphasic release pattern, the second phase being inhibited by ASA. The significance of the novel first phase of release is being studied.