1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholines: influence on aggregation and [3H]serotonin release of human thrombocytes

Non-specific PAF binding to embryonal F9 cells and prostacyclin synthesis in human umbilical vein endothelial cells

Our data suggest non-specific Paf binding to intact embryonal F9 cells. Non-specific Paf binding might have some physiological relevance when Paf metabolites interfere with prostacyclin synthesis, for example, in umbilical vein endothelial cells.

Lipoprotein-associated paf (LA-paf) was found in washed human platelets and monocyte/macrophage-like U937 cells

Beyond cholesterol, inflammatory ether phospholipids such as platelet-activating factor (paf) may play a role in atherogenesis. (1) We detected a paf-like compound ('LA-paf') associated with human serum lipoproteins, mainly in LDL but not with the lipoprotein-poor fraction. (2) LA-paf was also found in washed human platelets, from where it was partially released during platelet aggregation in response to paf (50 nM) or thrombin (1 U). In addition, resident monocyte/macrophage-like U937 cells carried huge amounts of LA-paf (41 ng per 10(7) cells) and metabolized added [3H]paf to a labelled compound co-eluting with the retention time of LA-paf in standard HPLC. (3) Functionally, LA-paf had a comparable potency to synthetic paf, because LA-paf aggregated washed aspirin-treated platelets in a concentration-dependent manner. The specific paf receptor antagonist WEB2086 inhibited the platelet aggregation induced by three distinct LA-paf preparations as compared with synthetic paf with similar inhibitory concentrations (IC50: 35.6 +/- 12.8, 24.0 +/- 4.0, 38.0 +/- 15.8 nM for LA-paf, and 43.6 +/- 6.5 nM for synthetic paf), indicating that LA-paf interacted with paf receptors. (4) However, LA-paf had a distinct retention time using high-pressure liquid chromatography (HPLC) as compared with synthetic paf. LA-paf eluted at 9-15 min and synthetic paf at 21-24 min. In addition, total and non-specific [3H]paf binding to intact washed human platelets was affected differently by the two unlabelled agonists: while LA-paf increased total and non-specific (but not specific) binding in a significant manner (P < 0.002 and P < 0.007) as LDL did (P < 0.006 and P < 0.03), synthetic paf decreased total binding (P < 0.03). Similarly, low-density lipoproteins (LDL) increased significantly the total [3H]paf binding. In contrast, paf did not affect specific [125I]LDL binding to human fibroblasts. Our results show the presence of LA-paf in lipoproteins, washed human platelets and monocyte/macrophage-like cells. As LDL and LA-paf purified from the same LDL particles increased significantly the total [3H]paf binding to intact human platelets, it might modulate platelet adherence to vascular endothelial cells.

The role of extracellular Ca2+ and Na+ in paf-acether-induced human platelet activation

The dependence of paf-acether (paf)-induced human platelet activation on extracellular Ca2+ and Na+ was examined by quantitating aggregation, secretion and thromboxane (Tx) formation in the presence of physiological and/or low concentrations of Ca2+ and/or Na+. In the presence of 2 mM Ca2+ and 140 mM Na+, paf induced a dose-dependent reversible aggregation and less than 25% of [14C]serotonin release. These responses were insensitive to aspirin or Tx antagonist SQ 29548 treatment and negligible amounts of Tx were formed. In low Ca2+ buffer, paf induced irreversible aggregation and the 14C-serotonin release could exceed 60%. These increases in platelet response were associated with the formation of Tx and were suppressed by aspirin and SQ 29548 treatments, or by substituting NaCl with N-methylglucamine hydrochloride. Thus, in low Ca2+ medium, Tx synthesis is favored during platelet activation and is dependent on Na+ concentrations. A decrease in extracellular Na+ inhibited the paf-acether-induced Tx synthesis observed in low Ca2+ medium but not that induced by the Tx direct precursor, arachidonic acid (AA). Therefore, the increase observed in low Ca2+ medium, no longer seen when the Na+ level is decreased is not related to an impairment of the cyclooxygenase activity but rather implicates an effect on the activity of phospholipase A2. A decrease in extracellular Na+ (2 mM Ca2+ present), inhibited [14C]serotonin release induced by paf from platelets which had, or had not been, treated with aspirin. In this medium, the AA-induced release reaction was also affected whereas Tx formation was not altered, thus suggesting that other mechanisms involved in platelet response apart from Tx synthesis are dependent on extracellular Na+.

Study of digoxin as inhibitor of the in vivo effects of acetyl glyceryl ether phosphorylcholine (AGEPC) in mice

Acetyl glyceryl ether phosphorylcholine (AGEPC) and the cardiac glycoside digoxin were administered intravenously through the tail vein into ether-anesthetized SWR mice (two months old). The administered doses were 0.18 nmol AGEPC/g b.w. (a lethal one) and 75 or 125 ng digoxin/b.w. Digoxin ameliorates the effects of the lethal dose of AGEPC showing maximum activity when given 5 or 10 min after AGEPC administration to female and male animals respectively. Digoxin shows also a protective action towards the effects of AGEPC and maximum activity appears when it is given 10 min before AGEPC administration. In agreement with the picture of increased survival in digoxin pretreated animals, are our findings on life prolongation of mice which finally die from AGEPC, the amelioration of the expected fall in blood platelet counts after AGEPC administration as well as the improved performance of the animals in a series of physical tests.

BN 52021 displaces [3H]paf-acether from, and inhibits its binding to intact human platelets

Intact platelets incubated at 20 degrees C in the presence of 0.25% (w/v) bovine serum albumin (BSA) bound [3H]paf-acether in a concentration-dependent (0-6.5 nM) manner. Specific [3H]paf-acether binding in the presence of unlabelled paf-acether or BN 52021, a chemically defined ginkgolide, reached a plateau of 14.5 +/- 5 or 17.5 +/- 7.0 fmol at concentrations higher than 0.65 nM [3H]paf-acether. Unlabelled paf-acether or BN 52021 inhibited and displaced [3H]paf-acether binding in a concentration- and time-dependent manner reaching a plateau at 5 nM or 5 microM. Unlabelled paf-acether inhibited [3H]paf-acether binding (75.1 +/- 8%) more strongly if labelled and unlabelled ligands were incubated together than after 15 min preincubation with [3H]paf-acether alone (32.2 +/- 8.6%). The enantiomer of paf-acether (50 nM) failed to displace platelet-bound [3H]paf-acether and the possibility of degradation of [3H]paf-acether by platelets or BN 52021 was excluded under binding conditions. The results demonstrate the presence of specific binding sites for paf-acether in human platelets and the direct competition by BN 52021 for these sites. Serum albumin inhibited total and enhanced specific [3H]paf-acether binding. Radiolabelled albumin itself did not bind to human platelets at 20 degrees C and preincubation of platelets with antibodies to human serum albumin did not inhibit specific paf-acether binding but increased total [3H]paf-acether binding slightly. Serum albumin appears as a necessary phospholipid carrier for specific [3H]paf-acether binding.

Unsaturated platelet-activating factor: influence on aggregation, serotonin release and thromboxane synthesis of human thrombocytes

Unsaturated platelet-activating factor (paf-acether) aggregated thrombocytes of healthy male volunteers like saturated paf-acether. Unsaturated paf-acether released serotonin in the presence of imipramine. Within one minute the release increased depending on the concentrations of unsaturated paf-acether up to 45% of the serotonin. Human thrombocytes synthesized only a small amount of thromboxane B2 (TXB2) after aggregation induced by unsaturated paf-acether. Unsaturated paf-acether prevented the binding of radiolabelled saturated paf-acether to intact washed thrombocytes in the same extent as saturated paf-acether.

Unsaturated 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholines (unsaturated platelet-activating factor): aggregation of human platelets after incubation with indomethacin, creatinephosphate/creatinephosphokinase, xylocain and hirudine, and serotonin release after incubation with indomethacin

Unsaturated platelet-activating factor (PAF) aggregates thrombocytes of healthy female volunteers and releases within 1 min up to 30.95% of the platelet serotonin. Indomethacin does not inhibit the aggregation but reduces the release of serotonin induced by unsaturated PAF in citrated platelet-rich plasma (PRP). Creatinephosphate combined with creatinephosphokinase (CP/CPK) inhibits the second phase, whereas xylocain inhibits the first and second phase of aggregation induced by unsaturated PAF. Hirudine shows no influence on the aggregation induced by unsaturated PAF.