1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho (N,N,N trimethyl) hexanolamine: an analogue of platelet-activating factor with partial agonist activity

Biased signalling from the glucocorticoid receptor: Renewed opportunity for tailoring glucocorticoid activity

Recent landmark studies applying analytical pharmacology approaches to the glucocorticoid receptor (GR) have demonstrated that different ligands can cause differential activation of distinct GR-regulated genes. Drawing on concepts of signalling bias from the field of G protein-coupled receptor (GPCR) biology, we speculate that ligand-dependent differences in GR signalling can be considered analogous to GPCR biased signalling, and thus can be quantitatively analysed in a similar way. This type of approach opens up the possibility of using rational structure-based drug optimisation strategies to improve the therapeutic selectivity of glucocorticoid drugs to maximise their efficacy and minimise adverse effects.

Lysine trimethylation of EF-Tu mimics platelet-activating factor to initiate Pseudomonas aeruginosa pneumonia

Pseudomonas aeruginosa is a ubiquitous microorganism and the most common Gram-negative bacterium associated with nosocomial pneumonia, which is a leading cause of mortality among critically ill patients. Although many virulence factors have been identified in this pathogen, little is known about the bacterial components required to initiate infection in the host. Here, we identified a unique trimethyl lysine posttranslational modification of elongation factor Tu as a previously unrecognized bacterial ligand involved in early host colonization by P. aeruginosa. This modification is carried out by a novel methyltransferase, here named elongation factor Tu-modifying enzyme, resulting in a motif that is a structural mimic of the phosphorylcholine present in platelet-activating factor. This novel motif mediates bacterial attachment to airway respiratory cells through platelet-activating factor receptor and is a major virulence factor, expression of which is a prerequisite to pneumonia in a murine model of respiratory infection.

Phosphorylcholine is an interesting molecule from the microbiological and immunological point of view. It is a crucial epitope for the virulence of many important human pathogens, modulates the host immune response, and is involved in a wide number of processes ranging from allergy to inflammation. Our current work identifies a novel bacterial surface epitope structurally and functionally similar to phosphorylcholine. This novel epitope is crucial for initial colonization of the respiratory tract by Pseudomonas aeruginosa and for development of pneumonia. This opens up new targets for the development of novel drugs to prevent P. aeruginosa pneumonia, which is particularly important given the frequent emergence of multidrug-resistant strains.

Microglial Calcium Responses to Platelet-Activating Factor are Inhibited by Analogue CAS 99103-16-9 and Dihydropyridine PCA 4248 but Not by Ginkgolide A

Calcium signals evoked in N9 microglial cells were monitored using the calcium indicator dye Fluo-4 in a fluorescence imaging plate reader. Platelet activating factor in the range 100 nM to 20 microM elicited graded calcium responses. The analogue CAS 99103-16-9 inhibited the evoked calcium rise with an apparent KB of 1.3 +/- 0.4 microM. The dihydropyridine PCA 4248 inhibited the evoked calcium rise with an apparent KB of 1.2 +/- 0.2 microM. Ginkgolide A at concentrations up to 18 microM had no effect on the evoked calcium rise. While CAS 99103-16-9 and PCA 4248 appear to be simple competitive inhibitors of platelet-activating factor responses, the efficacy of ginkgolide in more complex pharmacological situations may result from an action at a site other than the platelet-activating factor receptor.

Vitamin E attenuates the injurious effects of bioactive phospholipids on human ciliated epithelium in vitro

Bioactive phospholipids (PL), particularly lysophosphatidylcholine (LPC), are being increasingly implicated in the pathogenesis of various acute and chronic inflammatory disorders, particularly those of the airways, while there is emerging evidence that vitamin E may function as a natural antagonist of these lipid mediators of inflammation. The aims of this study were to document the effects of vitamin E on the inhibition of ciliary beating and damage to structural integrity of human ciliated epithelium induced by the PL, platelet-activating factor (PAF), lyso-PAF and LPC in vitro in relation to the anti-oxidative and membrane-stabilizing properties of the vitamin. Ciliary beat frequency was measured by a phototransistor technique, and damage to structural integrity assessed by a visual-scoring index, while superoxide production by polymorphonuclear leukocytes and membrane-stabilizing potential were measured using lucigenin-enhanced chemiluminescence and haemolytic procedures, respectively. All three PL caused inhibition of ciliary beating and structural damage to human ciliated epithelium by membrane-directed cytotoxic mechanisms, which were potentiated by human polymorphonuclear leukocytes due to induction of oxidant-mediated injury. Both direct and phagocyte-inflicted epithelial injury was attenuated by vitamin E. In haemolytic and chemiluminescence assays, vitamin E neutralized both the membrane-destabilizing and pro-oxidative actions of all three PL, while spectrophotometric analysis of mixtures of vitamin E with PAF, lyso-PAF and LPC revealed alterations in peak intensity, as well as peak shifts, indicative of physicochemical interactions between the vitamin and the PL. Vitamin E status may be a determinant of susceptibility to phospholipid-mediated airway inflammation and damage.

Platelet-activating factor increases the expression of metalloproteinase-9 in human bronchial epithelial cells

Platelet-activating factor (PAF) plays an important role in the pathogenesis of bronchial asthma. To investigate the role of PAF in the maintenance and remodeling of the extracellular matrix, we evaluated the expression of matrix metalloproteinase-2 and matrix metalloproteinase-9 from human bronchial epithelial cells after PAF treatment. Gelatin zymography of human bronchial epithelial cell-conditioned media showed major pro-matrix metalloproteinase-9 and minor pro-matrix metalloproteinase-2 expression and these expressions were totally inhibited by the metalloproteinase inhibitor EDTA. The identification of matrix metalloproteinase-9 was confirmed by Western blot analysis. Northern blotting and zymography demonstrated that PAF induced the mRNA of matrix metalloproteinase-9 from human bronchial epithelial cells and an increase in the gelatinolytic activity of pro-matrix metalloproteinase-9 but not in that of pro-matrix metalloproteinase-2. Lyso-PAF did not induce matrix metalloproteinase-9 mRNA or the gelatinolytic activity of pro-matrix metalloproteinase-9. CV6209, an receptor antagonist of PAF, reduced the increases of pro-matrix metalloproteinase-9 mRNA and gelatinolytic activity induced by PAF. Another receptor antagonist of PAF, hexanolamine PAF, did not inhibit the increases in the synthesis or release of pro-matrix metalloproteinase induced by PAF. Based on these results, we propose that matrix metalloproteinase-9 may be actively involved in the PAF-induced physiopathological remodeling in human bronchial epithelial cells.

Endogenous Platelet-Activating Factor Is Critically Involved in Effector Functions of Eosinophils Stimulated with IL-5 or IgG

Eosinophil activation and subsequent release of inflammatory mediators are implicated in the pathophysiology of allergic diseases. Eosinophils are activated by various classes of secretagogues, such as cytokines (e.g., IL-5), lipid mediators (e.g., platelet-activating factor (PAF)), and Ig (e.g., immobilized IgG). However, do these agonists act directly on eosinophils or indirectly through the generation of intermediate active metabolites? We now report that endogenous PAF produced by activated eosinophils plays a critical role in eosinophil functions. Human eosinophils produced superoxide when stimulated with immobilized IgG, soluble IL-5, or PAF. Pretreating eosinophils with pertussis toxin abolished their responses to these stimuli, suggesting involvement of a metabolite(s) that acts on G proteins. Indeed, PAF was detected in supernatants from eosinophils stimulated with IgG or IL-5. Furthermore, structurally distinct PAF antagonists, including CV6209, hexanolamine PAF, and Y-24180 (israpafant), inhibited IgG- or IL-5-induced superoxide production and degranulation. Previous reports indicated that exogenous PAF stimulates eosinophil eicosanoid production through formation of lipid bodies. We found in this study that IgG or IL-5 also induces lipid body formation and subsequent leukotriene C4 production mediated by endogenous PAF. Finally, inhibition of cytosolic phospholipase A2, one of the key enzymes involved in PAF synthesis, attenuated both PAF production and effector functions of eosinophils. These findings suggest that endogenous PAF plays important roles in eosinophil functional responses to various exogenous stimuli, such as cytokines and Igs. Therefore, inhibition of PAF synthesis or action may be beneficial for the treatment of eosinophilic inflammation.

Neutrophils stimulated with a variety of chemoattractants exhibit rapid activation of p21-activated kinases (Paks): separate signals are required for activation and inactivation of paks

Activation of the p21-activated protein kinases (Paks) was compared in neutrophils stimulated with a wide variety of agonists that bind to receptors coupled to heterotrimeric G proteins. Neutrophils stimulated with sulfatide, a ligand for the L-selectin receptor, or the chemoattractant fMet-Leu-Phe (fMLP), platelet-activating factor, leukotriene B4, interleukin-8, or the chemokine RANTES exhibited a rapid and transient activation of the 63- and 69-kDa Paks. These kinases exhibited maximal activation with each of these agonists within 15 s followed by significant inactivation at 3 min. In contrast, neutrophils treated with the chemoattractant and anaphylatoxin C5a exhibited a prolonged activation (>15 min) of these Paks even though the receptor for this ligand may activate the same overall population of complex G proteins as the fMLP receptor. Addition of fMLP to neutrophils already stimulated with C5a resulted in the inactivation of the 63- and 69-kDa Paks. Optimal activation of Paks could be observed at concentrations of these agonists that elicited only shape changes and chemotaxis in neutrophils. While all of the agonists listed above triggered quantitatively similar activation of the 63- and 69-kDa Paks, fMLP was far superior to the other stimuli in triggering activation of the c-Jun N-terminal kinase (JNK) and the p38 mitogen-activated protein kinase (MAPK). These data indicate that separate signals are required for activation and inactivation of Paks and that, in contrast to other cell types, activated Pak does not trigger activation of JNK or p38-MAPK in neutrophils. These results are consistent with the recent hypothesis that G-protein-coupled receptors may initiate signals independent of those transmitted by the alpha and betagamma subunits of complex G proteins.

Phosphatidic acid elicits calcium mobilization and actin polymerization through a tyrosine kinase-dependent process in human neutrophils: a mechanism for induction of chemotaxis

Phospholipids mediate important effects as extracellular messengers in diverse biological systems. We investigated the effects of phosphatidic acid, a biologically active phospholipid potentially involved in the inflammatory process, on calcium mobilization and actin polymerization in human neutrophils and correlated these effects with induction of chemotactic migration. Intermediate-chain length phosphatidic acid (DiC10-PA) induced a biphasic increase in intracellular Ca2+ characterized by a rapid rise commencing immediately upon addition of stimulus followed by a secondary increase which, unlike the initial response, was eliminated by chelation of extracellular Ca2+. Neither of these responses were induced by C10-lysophosphatidic acid or diacylglycerol. The tyrosine kinase inhibitor herbimycin-A (5-10 microg/ml) completely blunted the initial but not the delayed response effected by DiC10-PA. Long-chain phosphatidic acid (DiC18:1) induced only an initial rapid increase in intracellular Ca2+ and this response was similarly markedly attenuated by herbimycin-A. Among several physiologically relevant phospholipids, only phosphatidic acid was able to induce Ca2+ mobilization; phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol -- used individually or in mixed liposomes -- were without effect. Phosphatidic acid conferred calcium-mobilizing activity upon inactive liposome preparations and phosphatidic acid-enriched cellular plasma membranes possessed similar calcium-mobilizing activity. Both DiC10-PA and DiC18:1-PA induced actin polymerization in neutrophils at rates which mirrored the influence of each agent on Ca2+ mobilization. Herbimycin-A blunted the initial increase in actin polymerization effected by phosphatidic acid but had no effect on the delayed, EGTA-sensitive phase. DiC10-PA and DiC18:1-PA also induced neutrophil migration along a concentration gradient. Phospholipids that failed to induce a calcium transient, including phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine, and phosphatidylinositol, likewise failed to induce either actin polymerization or chemotactic migration. Unlike chemotaxis induced by zymosan-activated human serum, phosphatidate-induced chemotaxis was strongly inhibited by pretreatment of cells with herbimycin-A. Consistent with these observations, phosphatidic acid induced the tyrosine phosphorylation of several proteins as early as 10 s after stimulation. Phosphorylation of two distinct proteins with approximate molecular sizes of 72 and 82 kDa was inhibited by levels of herbimycin A used to effectively inhibit calcium mobilization, actin polymerization and chemotaxis. Thus, in neutrophilic leukocytes, extracellular phosphatidic acid induces a unique tyrosine kinase-based signalling pathway that results in calcium mobilization and actin polymerization. These processes may promote directed cellular migration as a consequence of the interaction of phosphatidic acid with neutrophil plasma membranes.

Human monocytes maintained in culture acquire functional responsiveness to platelet-activating factor that is independent of increases in protein tyrosine phosphorylation

1. Acute (day 0) stimulation with platelet-activating factor (PAF) did not elicit superoxide anion (O2-) generation from adherent monocytes. However, by day 2 of culture, PAF induced an increase in O2- generation that was inhibited by pretreatment with the PAF receptor antagonist WEB 2086. 2. The lack of effect of PAF on O2- generation was not due to the absence of receptors, as PAF stimulated an increase in tyrosine phosphorylation and intracellular calcium ([Ca2+]i) on both days 0 and 2 of culture. 3. Pretreatment with the protein tyrosine kinase inhibitor methyl 2,5-dihydroxycinnamate inhibited PAF-induced tyrosine phosphorylation; however, this inhibitor failed to inhibit PAF-induced O2- generation. In contrast, pretreatment with the protein kinase C inhibitor staurosporine had no effect on PAF-induced tyrosine phosphorylation, but did inhibit PAF-induced O2- generation. 4. These results indicate that monocytes maintained in culture acquire a functional response to PAF through a mechanism that appears to be independent of PAF receptor expression, coupling to increases in [Ca2+]i or tyrosine phosphorylation.

Effects of platelet-activating factor (PAF) receptor blockage on mucous glycoprotein secretion in cultured chinchilla middle ear epithelium

Platelet-activating factor (PAF) is a naturally occurring phospholipid that acts as a pleiotropic mediator of inflammation via specific membrane receptors. It has been demonstrated in recent years that PAF is a strong secretagogue of mucous glycoprotein (MGP) in airways and middle ear epithelium. MGP secretion accompanies otitis media with effusion (OME) and prolongs the course of this disease by increasing the viscoelasticity of the fluid. It is important, therefore, to inhibit the pathological secretion of MGP in the treatment of otitis media. In the current study, we investigated the effects of PAF receptor inhibitor WEB 2170 BS on in vitro MGP secretion. At concentrations of 100 mu M, PAF significantly stimulated MGP secretion. WEB 2170 BS significantly inhibited this PAF-simulated MGP secretion at concentrations of 2000 mu M. The action of WEB 2170 BS was concentration-dependent. However, it did not affect MGP secretion stimulated by IL-1beta, suggesting that WEB 2170 BS inhibits PAF-stimulated MGP secretion, specifically. It was noted that WEB 2170 BS did not completely eliminate MGP secretion induced by PAF even though a high concentration was used. The fact that WEB 2170 BS alone does not exhibit an inhibitory or stimulatory effect on the secretion of MGP suggests that WEB 2170 BS competitively binds to PAF receptors and possesses less affinity to PAF receptors than PAF.

Similar coronary vascular effects in the rat perfused heart of platelet-activating factor structural analogues with agonist and antagonist properties

1. Selective blockade of platelet-activating factor (PAF) receptor subtypes by PAF receptor antagonists has been demonstrated. However, selective activation of PAF receptor subtypes by PAF receptor agonists has not been reported. 2. When structural analogues of PAF that have been shown to possess either agonist or antagonist effects were administered by a bolus injection in the rat perfused heart, they all showed agonist effects. Lower amounts produced vasodilation while higher amounts produced vasodilation followed by vasoconstriction. These coronary vascular effects were typical of that observed with PAF. Lyso-PAF did not show the same typical pattern of coronary vascular effect, confirming that the detergent effect of PAF structural analogues did not play a role in the coronary vascular effects. Other PAF antagonists, CV-6209 and WEB 2170, also did not produce the PAF-like response in the rat perfused heart. 3. The coronary vascular effects of hexanolamine-PAF (H-PAF, putative antagonist) and ethanolamine-PAF (E-PAF, agonist) were further studied. Pretreatment with FR-900452 (a PAF receptor antagonist) or MK-886 (a leukotriene synthesis inhibitor) significantly reduced the vasodilator and vasoconstrictor effects of H-PAF and E-PAF. 4. Pretreatment of rat perfused hearts with low concentrations of H-PAF and E-PAF blocked the response to PAF administration in a dose- and time-dependent manner. However, the pretreatment with either H-PAF or E-PAF did not result in a coronary vascular effect expected of a PAF receptor agonist. These results were compatible with H-PAF and E-PAF behaving as PAF receptor antagonists. 5. In summary, our results demonstrate that several PAF structural analogues possess agonist action in the rat perfused heart. Like the coronary vascular effects of PAF, the effects of H-PAF and E-PAF were blocked by a PAF antagonist (FR-900452) and a leukotriene synthesis inhibitor (MK-886). This suggests that both H-PAF and E-PAF mediate their effect through activation of PAF receptors with a subsequent release of leukotrienes that produced vasodilatation and vasoconstriction. Furthermore, pretreatment of perfused hearts with these compounds blocked the response to PAF in these hearts. Thus these compounds can also behave like a PAF receptor antagonist. This latter action may be due to a gradual receptor inactivation or desensitization by the pretreatment of H-PAF and E-PAF through a PAF receptor agonist effect rather than being a PAF receptor antagonist.

Role of eicosanoids but not nitric oxide in the platelet-activating factor-induced increase in vascular permeability in mouse skin

We investigated the role of endogenous eicosanoids and nitric oxide (NO) in the platelet-activating factor (PAF)-induced increase in vascular permeability in mouse skin. Subcutaneous injection of PAF (45-180 pmol/site) induced a dose-related increase in vascular permeability at the injection site. The vascular permeability induced by PAF (180 pmol/site) was significantly inhibited by pretreatment with an intraperitoneal injection of 1-O-hexadecyl-2-acetyl-sn-glycero-3-phospho (N,N,N-trimethyl) hexanolamine (PAF receptor antagonist) (5 and 25 mg/kg) and indomethacin (cyclooxygenase inhibitor) (10 mg/kg), whereas it was not affected by concurrent intravenous administration of NO synthase inhibitors NG-nitro-L-arginine methyl ester (10 mg/kg) or methylene blue (100 micrograms/kg) nor by topical injection of NG-nitro-L-arginine methyl ester. The inhibitory effect of indomethacin was partially reversed by topical administration of prostaglandin E2. These results suggest that PAF increases venular permeability by activating PAF receptors and that plasma extravasation is potentiated by the release of prostanoids which cause arteriolar dilatation. However, NO is not involved in the effect of PAF in mouse skin.

Granulocyte and granulocyte-macrophage colony-stimulating factors exert differential effects on neutrophil platelet-activating factor generation and release

The haemopoietic recombinant human cytokines granulocyte and granulocyte-macrophage colony-stimulating factor (rhG-CSF and rhGM-CSF) are used to facilitate recovery of bone marrow function following cytotoxic chemotherapy. Recent clinical experience indicates that rhG-CSF is better tolerated than rhGM-CSF. Thus, we have compared the priming effects of rhG-CSF and rhGM-CSF on superoxide anion (O2-) generation and platelet-activating factor (PAF) synthesis by neutrophils. During a 60-min incubation of neutrophils with rhGM-CSF (1 nM) or recombinant human tumour necrosis factor-alpha (rhTNF-alpha; 0.3 nM), cell-associated PAF levels increased, and upon stimulation with FMLP (100 nM) there was a striking amplification of PAF formation (8-13-fold) and release (24-36-fold). In contrast, in rhG-CSF (1 nM)-primed cells, there was no increase in cell-associated PAF levels and neither PAF synthesis nor PAF release was amplified following stimulation with FMLP. On the other hand, each of rhG-CSF, rhGM-CSF or rhTNF-alpha increased subsequent FMLP (100 nM)-induced O2- generation (by 89%, 166% and 115%, respectively). These results suggest the existence of distinct intracellular signalling pathways for cytokine priming. Furthermore, some of the more severe adverse reactions to the administration of rhGM-CSF may be a result of the biosynthesis and/or release of the potent inflammatory mediator, PAF.

Partial agonist effect of the platelet-activating factor receptor antagonists, WEB 2086 and WEB 2170, in the rat perfused heart

1. WEB 2086 and WEB 2170 are potent platelet-activating factor (PAF) receptor antagonists and have been used widely as pharmacological tools to investigate the actions of PAF in a variety of biological systems. 2. Low concentrations of WEB 2086 and WEB 2170 blocked the vasoconstrictor action of PAF in the rat perfused heart. In this study, we observed that moderate concentrations of WEB 2086 and WEB 2170 increased the perfusion pressure in rat isolated hearts under constant flow perfusion. The vasoconstrictor actions of WEB 2086 and WEB 2170 were not observed with a structurally different PAF receptor antagonist, FR-900452. 3. To determine whether this vasoconstrictor action of WEB 2086 involved non-specific effects or was via the activation of PAF receptors, hearts were pretreated with 1000 pmol PAF or 50 microM FR-900452. These pretreatments attenuated the vasoconstrictor action of 1 microM WEB 2086, suggesting that the action of WEB 2086 may be mediated via PAF receptors. Pretreatment with the leukotriene receptor antagonist (L-649,923, 5 microM) and the leukotriene synthesis inhibitor (MK-886, 10 microM) that are known to block the vasoconstrictor action of PAF receptor activation also attenuated the vasoconstrictor action of WEB 2086. Pretreatment with PAF or MK-886 attenuated the vasoconstrictor action of 0.5 microM WEB 2170. 4. When PAF receptors were activated by PAF in the perfused heart, significant amounts of leukotriene C4 and leukotriene C4/D4/E4 were detected in the coronary effluent. However, no significant amount of these leukotrienes was detected in the coronary effluent when hearts were perfused with 1 microM WEB 2086 or 0.5 microM WEB 2170. 5. In summary, our results indicate that WEB 2086 and WEB 2170 possess partial agonist effects in the rat perfused heart where they produced vasoconstriction via the activation of PAF receptor. This action could be attenuated by PAF pretreatment or a PAF receptor antagonist. The vasoconstrictor action of WEB 2086 and WEB 2170 involved the production of leukotrienes. But unlike the vasoconstrictor action of PAF, no significant amount of leukotrienes was detected in the effluent suggesting that the vasoconstrictor action of WEB 2086 and WEB 2170 may be explained on the basis of intracellularly or locally produced leukotrienes.

Adenosine inhibits platelet-activating factor, but not tumour necrosis factor-alpha-induced priming of human neutrophils

Regulation of the respiratory burst and its priming by recombinant human tumour necrosis factor-alpha (rhTNF-alpha) and platelet-activating factor (PAF) were investigated in human polymorphonuclear leucocytes (PMN). Adenosine (0.1-10 microM) pretreatment of PMN concentration-dependently inhibited the superoxide anion generation (O2-) in response to formyl-methionyl-leucyl-phenylalanine (FMLP). The priming by PAF (1 microM) for an increased O2- generation by FMLP-stimulated PMN was completely blocked by adenosine pretreatment. In contrast, rhTNF-alpha-induced priming was unaffected by adenosine. In addition, the direct stimulation of PMN O2- by rhTNF-alpha was also unaffected by adenosine as was rhTNF-alpha-induced PAF synthesis. FMLP-induced PAF synthesis was reduced by adenosine to a similar extent as the inhibition of the respiratory burst. Adenosine also inhibited PAF-, but not FMLP-induced increases in intracellular calcium in PMN. These findings indicate that short-term, direct stimulants (FMLP) or priming agents (PAF) are subject to modulation by the endothelial product adenosine, whereas the priming and direct stimulation of the respiratory burst by the longer-acting agent, rhTNF-alpha is unaffected. Moreover, differential inhibition of PMN activation by adenosine reveals important functional differences in the signalling mechanisms initiated by PAF, FMLP and rhTNF-alpha.

Albumin inhibits platelet-activating factor (PAF)-induced responses in platelets and macrophages: implications for the biologically active form of PAF

1. Platelet-activating factor (PAF) binds with high affinity to albumin leading Clay et al. (1990) to suggest that the active form of PAF is the albumin-PAF complex. 2. In the present study the proposal that albumin-bound, rather than monomeric PAF, is the active form of PAF at PAF receptors was critically evaluated by examining the effect of albumin on the potency of PAF in isolated platelets and macrophages. 3. Bovine serum albumin inhibited concentration-dependently PAF-induced responses in platelets and macrophages. The most probable explanation of this finding is that BSA reduced the concentration of free PAF. 4. Thus, we conclude that free PAF, rather than the albumin-PAF complex is the active form. Consequently, local concentrations of albumin will influence profoundly the potency of endogenously released PAF. Moreover, estimates of the affinity of PAF for PAF receptors made in buffers containing BSA, underestimate the true affinity of PAF for its receptors by approximately 3 orders of magnitude.