PRODUCTION AND METABOLISM OF PLATELET-ACTIVATING-FACTOR IN THE NORMAL AND ISCHEMIC FETAL-RAT BRAIN

Production and metabolism of platelet-activating factor (PAF) in the fetal rat brain under normal and under ischemic stress conditions were examined. Endogenous PAF levels, determined by a bioassay using PAF-stimulated platelet release of [H-3]serotonin, averaged 2.32 +/- 2.14 pg/mg in control brains and was reduced to 1.10 +/- 1.06 pg/mg after 20 min of maternal-fetal blood flow occlusion. [H-3]PAF administered intracranially into the fetuses in utero was removed in a biphasic, time-dependent manner: a rapid component with an estimated elimination rate constant of 0.067 min(-1) and t(1/2) of 10 min and a slower component with an elimination rate of 0.017 min(-1) and t(1/2) of 41 min. In fetal brains subjected to ischemia a delayed elimination of [H-3] PAF was noticed in the slow component (t(1/2) = 59 min), indicating a possible difference between the clearance of exogenous and endogenous PAF. The disappearance of [H-3]PAF was accompanied by an increase in the radioactivity associated with lyso-PAF that reached a plateau after 2.5 min, possibly indicating the degradation of the fast component. A steady increase in the alkyl-acyl-glycerophosphorylcholine radioactivity commenced after 5 min and continued up to 30 min. The endogenous production of PAF and the rapid degradation due to maternal-fetal blood flow occlusion indicate an additional target for therapeutic intervention in the pathology of intrauterine ischemia. Addition of the calcium ionophore A23187 stimulated in vitro formation of PAF and lyso-PAF from [H-3]choline-labeled fetal brain phospholipids, suggesting that intracellular calcium may play a major stimulatory role in PAF production. Degradation of polyphosphoinositides by a phospholipase C may constitute a major target for PAF generated either by decapitation or after blood flow occlusion, as evident from the protective effect of the in vivo administered BN52021 PAF antagonist.