STUDIES ON MURINE EMBRYO-DERIVED PLATELET-ACTIVATING-FACTOR (EPAF)

Studies were carried out using the splenectomized mouse bioassay (SMB) to investigate the nature of embryo-derived platelet-activating factor (EPAF) and its relationship to synthetic platelet activating factor (PAF). While both C16-PAF and embryo conditioned media (ECM) induced a significant platelet decline in the SMB at 15 min postinjection, C18-PAF induced a similar effect at 30 min postinjection. The degree of EPAF activity in ECM was not altered with increasing embryo number from 2 to 40/ml of media. In contrast, PAF (Cl6/Cl8 mixture) induced a linear increase in activity with increasing concentration, leading to lethal effects at high concentrations. While EPAF activity was not significantly altered when ECM was diluted 1/1,000, PAF activity was abolished at 1/10 dilution. EPAF in ECM was not inactivated by mouse plasma; however, lipid extracted ECM, like PAF, underwent rapid inactivation in the presence of plasma. Aggregometer studies using horse platelets showed that ECM and lipid-extracted ECM were unable to induce platelet aggregation, while thin-layer chromatography (TLC) purified ECM (Rf 0.23) successfully aggregated horse platelets in vitro. Results suggested that EPAF and PAF are not homologous. EPAF might consist of PAF bound to a regulatory carrier molecule and appears to be associated with EPAF-inhibitor substance(s) in ECM.