Human platelet stimulation by acetyl glyceryl ether phosphorylcholine

Is there an interplay between the SARS-CoV-2 spike protein and Platelet-Activating factor?

Previous publications have reported a potent effect of COVID-19 on platelet function and that the Spike protein enhances washed human platelet aggregation induced by various agonists. This study aims to evaluate whether mRNA vaccination for COVID-19 affects human platelet-rich plasma (hPRP) aggregation response, whether a recombinant Spike protein modulates PAF-induced aggregation in hPRP and in washed rabbit platelets (WRP), and to investigate the effect of recombinant Spike protein on the PAF production in the U-937 cell line. Our results showed that PRP from vaccinated individuals exhibited ex vivo lower EC₅₀ values in response to PAF, ADP, and collagen. Platelet incubation with the Spike protein alone did not induce aggregation either in hPRP or in WRP, but resulted in augmentation of in vitro PAF-induced aggregation in hPRP from non-vaccinated individuals and in WRP. When PRP from vaccinated individuals was incubated with the Spike protein and PAF was subsequently added, elimination of the secondary wave of the biphasic aggregation curve was recorded compared with the aggregation induced by PAF alone. Collagen-induced in vitro aggregation was dose-dependently reduced when platelets were pre-incubated with the Spike protein in all tested aggregation experiments. Stimulation of U-937 by the Spike protein induced an increase in intracellular PAF production accompanied by elevation of the activities of all three PAF biosynthetic enzymes. In conclusion, since the Spike protein appears to modulate PAF production and activity, the use of compounds that act as PAF inhibitors, could be considered at least in mild cases of patients infected with SARS-CoV-2.

Dissecting the Mechanism of Intracellular Mycobacterium smegmatis Growth Inhibition by Platelet Activating Factor C-16

Mycobacterium tuberculosis (M.tb) infection results in approximately 1.3 million human deaths each year. M.tb resides primarily inside macrophages, and maintains persistent infection. In response to infection and inflammation, platelet activating factor C-16 (PAF C-16), a phospholipid compound, is released by various cells including neutophils and monocytes. We have recently shown that PAF C-16 can directly inhibit the growth of two representative non-pathogenic mycobacteria, Mycobacterium bovis BCG and Mycobacterium smegmatis (M. smegmatis), by damaging the bacterial cell membrane. Here, we have examined the effect of PAF C-16 on M. smegmatis residing within macrophages, and identified mechanisms involved in their growth inhibitory function. Our results demonstrated that exogenous PAF C-16 inhibited the growth of M. smegmatis inside phagocytic cells of monocytic cell line, THP-1; this effect was partially blocked by PAF receptor antagonists, suggesting the involvement of PAF receptor-mediated signaling pathways. Arachidonic acid, a downstream metabolite of PAF C-16 signaling pathway, directly inhibited the growth of M. smegmatis in vitro. Moreover, the inhibition of phospholipase C and phospholipase A₂ activities, involved in PAF C-16 signaling pathway, increased survival of intracellular M. smegmatis. Interestingly, we also observed that inhibition of inducible nitric oxide synthase (iNOS) enzyme and antibody-mediated neutralization of TNF-α partially mitigated the intracellular growth inhibitory effect of PAF C-16. Use of a number of PAF C-16 structural analogs, including Lyso-PAF, 2-O-methyl PAF, PAF C-18 and Hexanolamino PAF, revealed that the presence of acetyl group (CH₃CO) at sn-2 position of the glycerol backbone of PAF is important for the intracellular growth inhibition activity against M. smegmatis. Taken together, these results suggest that exogenous PAF C-16 treatment inhibits intracellular M. smegmatis growth, at least partially, in a nitric oxide and TNF-α dependent manner.

Aspirin discontinuation syndromes: clinical implications of basic research studies

Abrupt discontinuation of many drugs used in medicine causes withdrawal syndromes, some of which can be fatal. Discontinuation of a number of cardiovascular drugs can increase the risk of cardiovascular events. Whereas aspirin administration is known to decrease the risk of vascular ischemic problems, aspirin withdrawal may temporarily increase the risk of thrombotic events. Indeed, aspirin withdrawal has been associated with an increased risk of thrombosis both in clinical and fundamental research studies. Such complications occur within the first month after interrupting aspirin therapy and their mechanism remains unexplained. We have previously demonstrated that aspirin, when injected as a single high dose (100 mg/kg), induces a prothrombotic state in the rat, similar to that described above, 8 and 10 days after administration. This effect in the rat may be reproduced 1 hour after a single injection of ultra-low-dose aspirin. Caution is therefore required regarding the possibility of drug discontinuation effects within the framework of drug safety evaluation.

Platelet activating factor in heart failure: potential role in disease progression and novel target for therapy

Heart failure (HF) is a complex syndrome with cardiac, renal, neurohormonal and sympathetic nervous system's manifestations, the pathogenesis of which among others is connected to inflammation. PAF has local and systemic effects pertaining to HF progression since it causes a negative inotropic effect, it induces arrhythmias, it induces apoptosis and it is involved in inflammation and atherosclerosis. In the present review the role of PAF in HF will be thoroughly presented along with the relevant data on PAF enzymes and the potential role of PAF metabolic circuit as a novel pharmacological target.

Platelet-activating factor (PAF)-antagonists of natural origin

Presently herbal medicines are being used by about 80% of the world population for primary health care as they stood the test of time for their safety, efficacy, cultural acceptability and lesser side effects. The discovery of platelet activating factor antagonists (PAF antagonists) during these decades are going on with different framework, but the researchers led their efficiency in studying in vitro test models. Since it is assumed that PAF play a central role in etiology of many diseases in humans such as asthma, neuronal damage, migraine, cardiac diseases, inflammatory, headache etc. Present days instinctively occurring PAF antagonist exists as a specific grade of therapeutic agents for the humans against these and different diseases either laid hold of immunological or non-immunological types. Ginkgolide, cedrol and many other natural PAF antagonists such as andrographolide, α-bulnesene, cinchonine, piperine, kadsurenone, different Piper species' natural products and marine origin plants extracts or even crude drugs having PAF antagonist properties are being used currently against different inflammatory pathologies. This review is an attempt to summarize the data on PAF and action of natural PAF antagonists on it, which were evaluated by in vivo and in vitro assays.

Fish Oil Supplementation Inhibits Platelet Aggregation and ATP Release Induced by Platelet-activating Factor and Other Agonists

The anti-inflammatory effects of fish oil may partly be due to the inhibition of platelet activation induced by platelet-activating factor (PAF) and other agonists. To investigate this hypothesis, the diets of 12 healthy volunteers were supplemented with 12 fish oil capsules or 12 olive oil capsules daily for 4 weeks in a double blind crossover study. Aggregation induced by PAF (18 and 12.5 nM) and collagen (20 μg/ml)tended to be reduced after fish oil but the effect was statistically significant only in subjects receiving fish oil in the 6rst 4 weeks of the study (P 0.05, n=6). The effect of fish oil supplementation on platelet ATP release was more marked with significant inhibition of ATP release induced by PAF (1200 and 36 nM, P 0.01, n = 12), collagen (20 μg/ml, P 0.005, n = 12) and ADP (15,10 and 5 μM, P 0.05, n = 12). Olive oil supplementation appeared to inhibit ATP release induced by collagen (45 and 30 μg/ml, P> 0.025, n = 12), while aggregation and ATP release induced by arachidonic acid and adrenaline were unaffected by the supplements. Plasma fibrinogen was significantly reduced after olive oil (P 0.01, n = 12) while prothrombin time was reduced after fish oil (P 0.001, n = 12) and olive oil (P 0.0025). Reduced platelet aggregation and more importantly, inhibition of platelet release induced by PAF and other agonists may contribute to the anti-inflammatory effects of fish oil supplementation in a number of disease states but olive oil may also independently affect platelet function and influence the effect offish oil.

Effect of fast-food Mediterranean-type diet on type 2 diabetics and healthy human subjects' platelet aggregation

OBJECTIVE

The aim of our work was to carry out a randomized clinical trial with a fast-food Mediterranean type diet rich in platelet activating factor (PAF) antagonist to investigate the effect on type 2 diabetics and healthy human subject's platelet aggregation.

RESEARCH DESIGN AND METHODS

We extracted lipids from fast-food Mediterranean type foodstuffs, and tested them in vitro for their ability to inhibit or antagonize PAF towards washed rabbit platelets. We chose the foodstuffs that exerted the most potent in vitro anti-PAF activity and fed 22 healthy (group A) and 23 type 2 diabetics (group B) subjects on a diet containing the chosen foodstuffs. The 22 type 2 diabetics (group C) subjects were kept on their regular diet that was being followed before entering the study. Before and after a 4-week diet, all enrolled subjects underwent the following examinations; measurement of total cholesterol, low density lipoprotein (LDL-cholesterol), high density lipoprotein (HDL-cholesterol), triglycerides, glucose, HbA(1c), body mass index (BMI), and platelet aggregation in response to PAF, adenosine 5' diphosphate (ADP) and arachidonic acid (AA).

RESULTS

The chosen diet significantly increased the EC(50) values of PAF and ADP to groups A and B (p<0.05). No statistical difference was observed on the EC(50) value of group C. No statistical differences were detected among Cholesterol, LDL-cholesterol, triglycerides, glucose, HBA(1c), BMI, and EC(50) for AA values, for any of the three groups.

CONCLUSIONS

Consumption of a fast-food Mediterranean type diet rich in PAF antagonist improved platelet response of type 2 diabetics and healthy human subjects against thrombotic, inflammatory and proatherogenic factors.

Antiplatelet drug discontinuation is a risk factor for ischemic stroke

Antiplatelet drugs (APD) are widely used in the prevention of ischemic cardio- and cerebrovascular diseases. The authors studied the frequency of stroke occurring after APD discontinuation, the cause of discontinuation, and the delay between APD disruption and stroke. Only 4.49% of strokes were related to a recent APD discontinuation, but all cases occurred between 6 and 10 days after drug discontinuation (p < 0.0001). This temporal pattern has biologic plausibility because the inhibited platelets circulate in the blood for about 10 days.

Ultrastructural examination of rabbit aortic wall following high-fat diet feeding and selenium supplementation: a transmission electron microscopy study

Experiments were carried out to examine the changes occurring in the wall of rabbit aortae following high-fat diet (HFD) feeding as well as HFD + selenium supplementation. Male New Zealand White rabbits were divided into three groups-control, HFD-fed and HFD + Se supplementation-and were treated for three months. The study depicted that levels of serum total cholesterol and triglycerides were markedly increased in the HFD-fed group as compared with control animals. However, in the HFD + Se-fed group, these levels were markedly suppressed vis-à-vis animals fed on HFD only. Development of atherogenic and atheromatic plaques has been shown at the light microscopy level in HFD-fed rabbits, whereas these developments were not visible in the HFD + Se-fed rabbits. Transmission electron microscopy findings indicated altered ultrastructure in the endothelial cells of the intimal layer as well as smooth-muscle cells of the medial layer in HFD-fed animals. However, these findings indicated normal ultrastructure in most of the cells, with little ultrastructural alterations from animals supplemented with Se along with HFD feeding. The study on the whole depicted the ability of Se to inhibit the onset of progression of aortic disease and hence has relevance to its therapeutic potential.

The platelet-activating factor signaling system and its regulators in syndromes of inflammation and thrombosis

OBJECTIVES

To review the platelet-activating factor (PAF) signaling system, its regulation, and its dysregulation in acute inflammation and thrombosis and in syndromes that involve these cascades, including sepsis.

DATA SOURCES

A summary of published literature from MEDLINE search files and published reviews. DATA EXTRACTION, SYNTHESIS, AND SUMMARY: PAF, a phospholipid signaling molecule, transmits outside-in signals to intracellular transduction systems and effector mechanisms in a variety of cell types, including key cells of the innate immune and hemostatic systems: neutrophils, monocytes, and platelets. Thus, the PAF signaling system is a point of convergence at which injurious stimuli can trigger and amplify both acute inflammatory and thrombotic cascades. The biological activities of PAF are regulated by several precise mechanisms that, together, constrain and control its action in physiologic inflammation. Unregulated synthesis of PAF or defects in the mechanisms that limit its biological activities have the potential to cause pathologic inflammation and thrombosis. In addition, nonenzymatic generation of oxidized phospholipids that are recognized by the PAF receptor can trigger inflammatory and thrombotic events. There is evidence that the PAF signaling system is dysregulated in sepsis, shock, and traumatic injury and that interruption or termination of its effector responses leads to beneficial outcomes. Plasma PAF acetylhydrolase, an enzyme that hydrolyzes PAF and structurally related oxidized phospholipids, yielding products that are no longer recognized by the PAF receptor, may be a particularly important signal terminator.

CONCLUSION

The PAF signaling system can trigger inflammatory and thrombotic cascades, amplify these cascades when acting with other mediators, and mediate molecular and cellular interactions (cross talk) between inflammation and thrombosis. Evidence from in vitro experiments, studies of experimental animals, and clinical observations in humans indicates that the PAF signaling system is important in sepsis and other syndromes of inflammatory injury and that therapeutic strategies to interrupt or terminate signaling via the PAF signaling system may be useful in these conditions.

Platelet-activating-factor-hydrolyzing phospholipase C in the salivary glands and saliva of the mosquitoCulex quinquefasciatus

A phospholipase C activity specific for platelet-activating factor (PAF), named PAF phosphorylcholine hydrolase, was found in the salivary glands and saliva of the human-feeding mosquito Culex quinquefasciatus. The enzymatic activity was demonstrated by inhibition of PAF-induced platelet aggregation, and by identification of substrate consumption and production of diacyl glyceride by electrospray-ionisation mass spectrometry. The activity has a neutral optimal pH and an apparent molecular mass of 40–50 kDa. Two anthropophilic mosquito species, Aedes aegypti and Anopheles gambiae, do not have this salivary activity. The results are interpreted within the evolutionary context of the genera Culex, Aedes and Anopheles.

Time related neutralization of two doses acetyl salicylic acid

Aspirin has a well established role in the prevention of arterial thrombosis. Discussion on the efficacy and safety of aspirin in the treatment and prophylaxis of thrombosis has become an important issue. In fact, hemorrhage complications are often associated with its use. On the other hand, previous studies showed unexpected thrombotic potencies associated with the presence of this drug at ultra low doses (ULD) in the circulation. In this study, we aimed to evaluate the effect of aspirin at ULD, injected 1, 2, or 3 hours after the administration of aspirin at 100 mg/kg, on hemostasis and bleeding in rats. We used an experimental model of thrombosis induced by laser beams to evaluate these effects. Platelet aggregation was determined by Cardinal and Flower method. Results from this investigation demonstrate that the neutralizing effect of aspirin at ULD did not operate significantly 1 hour after the injection of aspirin at 100 mg/kg. This effect was observed 2 and 3 hours after. The use of aspirin at ULD to neutralize the side effects of aspirin at high doses will reduce the hemorrhagic risk during extra corporeal circulation. The therapeutic benefit and safety of aspirin therapy in the treatment of cardiovascular diseases can be obtained.

Thromboembolic complications several days after a single-dose administration of aspirin

The antithrombotic properties of acetyl salicylic acid (ASA) used at current doses are largely demonstrated. However, our previous study showed unexpected thrombotic potencies associated with the use of this drug. In this study we investigate the effect of aspirin on an experimental thrombosis induced by laser beams, according to its in vivo plasma concentration. Experiments were done on nine groups of seven Wistar male rats. The groups are defined by the delay between aspirin administration time and the laser-induced thrombosis time. Results from this study showed an enhancement of thromboembolic complications when thrombosis was induced 8 or 10 days after aspirin administration; the number of emboli and the duration of embolization are increased, compared to the control group. The prothrombotic properties of ASA demonstrated in this study, might limit its therapeutic benefit and might explain thromboembolic complications observed in some ASA-treated patients. These results also suggest a biological monitoring several days after aspirin administration to patients.

Regulation of platelet-activating factor (PAF) biosynthesis via coenzyme A-independent transacylase in the macrophage cell line IC-21 stimulated with lipopolysaccharide

The regulation of PAF synthesis by the macrophage cell line IC-21 challenged with bacterial endotoxin was investigated. The LPS-induced increase in cellular PAF levels was rapid, sustained and attained maximal levels within 30 min following LPS stimulation. PAF accumulation was accompanied by the activation of the CoA-independent transacylase and acetyl-CoA: lyso-PAF acetyltransferase, whereas the release of free [3H]arachidonic acid in prelabeled cells reflecting the activation of phospholipase A2, occurred primarily within the initial 1-5 min of treatment with LPS. Cell lysates from LPS-stimulated macrophages exhibited a markedly increased enzymatic activity that was capable of both acylation of 1-[3H]alkyl-2-lyso-GPC (lyso-PAF) and deacylation of 1-[3H]alkyl-2-acyl-GPC generating [3H]lyso-PAF via CoA-independent transacylation of exogenous lysoplasmenylethanolamine compared with extracts from resting macrophages. Pretreatment of the cells with LPS for 5 and 30 min enhanced significantly the transfer of [14C]arachidonic acid from 1-[3H]alkyl-2-[14C]arachidonoyl-GPC into plasmenylethanolamine in prelabeled cell homogenates following the addition of exogenous lysoplasmenylethanolamine. Taken together, these data suggest that the CoA-independent transacylase, but not phospholipase A2, is a key enzyme responsible for the prolonged generation of lyso-PAF and that the increased capability of CoA-independent transacylation followed by CoA-dependent acetylation of lyso-PAF can sustain the biosynthesis of PAF in LPS-stimulated IC-21 macrophages.

Pharmacology of ABT-491, a highly potent platelet-activating factor receptor antagonist

ABT-491 (4-ethynyl-N, N-dimethyl-3-[3-fluoro-4-[(2-methyl-1H-imidazo-[4,5-c]pyridin-1-yl)methy l]benzoyl]-1H- indole-1-carboxamide hydrochloride) is a novel PAF (platelet-activating factor) receptor antagonist with a K(i) for inhibiting PAF binding to human platelets of 0.6 nM. Binding kinetics of ABT-491 to the PAF receptor is consistent with a relatively slow off-rate of the antagonist when compared to PAF. Inhibition of PAF binding is selective and is correlated with functional antagonism of PAF-mediated cellular responses (Ca2+ mobilization, priming, and degranulation). Administration of ABT-491 in vivo leads to potent inhibition of PAF-induced inflammatory responses (increased vascular permeability, hypotension, and edema) and PAF-induced lethality. Oral potency (ED50) was between 0.03 and 0.4 mg/kg in rat, mouse, and guinea-pig. When administered intravenously in these species, ABT-491 exhibited ED50 values between 0.005 and 0.016 mg/kg. An oral dose of 0.5 mg/kg in rat provided > 50% protection for 8 h against cutaneous PAF challenge. ABT-491 administered orally was also effective in inhibiting lipopolysaccharide-induced hypotension (ED50 = 0.04 mg/kg), gastrointestinal damage (0.05 mg/kg, 79% inhibition), and lethality (1 mg/kg, 85% vs. 57% survival). The potency of this novel antagonist suggests that ABT-491 will be useful in the treatment of PAF-mediated diseases.

Effects of platelet-activating factor on left ventricular performance in dogs

To elucidate the mechanism by which platelet-activating factor (PAF) decreases cardiac output (CO), its effects on left ventricular (LV) performance were studied using a LV pressure-volume model in 9 anesthetized dogs. LV volume was measured with a conductance catheter and LV pressure with a manometer-tipped catheter. The slopes of the end-systolic pressure-volume (Ees), stroke work-end-diastolic volume (Mw), and arterial end-systolic pressure-stroke volume relations (Ea), and of end-diastolic pressure-volume relations (EDPVRs), were obtained to evaluate changes in LV performance. The time constant of isovolumetric relaxation (T) was computed by a logarithmic and a derivative method. After intravenous administration of PAF (1 microgram/kg), LV end-diastolic volume and pressure, end-systolic pressure, maximum dP/dt and CO decreased. Ees and Mw decreased significantly, minimum dP/dt decreased, and T increased. The arterial end-systolic pressure-stroke volume relation shifted leftward with an increase in the slope (Ea), whereas EDPVRs were shifted to the left and superimposed on their potential passive diastolic properties. These findings indicate the PAF decreases CO by reducing preload and contractility and by increasing arterial load. However, despite a prolongation of relaxation, PAF may not affect passive diastolic chamber stiffness.

Azelastine hydrochloride inhibits platelet activating factor-like activity in human eosinophils

We investigated the inhibitory effect of azelastine hydrochloride (azelastine), an anti-asthmatic drug, on platelet-activating factor (PAF)-like activity in eosinophils obtained from asthmatic and non-asthmatic patients. Eosinophils were preincubated with or without azelastine and stimulated with f-Met-Leu-Phe (fMLP, 10 mumol) for 15 min. PAF-like activity was detected by aggregation of washed guinea-pig platelets. PAF-like activity released from asthmatic eosinophils without preincubation of azelastine was 2.36 [1.02] (mean [SD], ng/10(7) cells) in supernatants and 13.87 [4.77] in cell pellets. After preincubation with 10(-8), 10(-6), and 10(-4) M azelastine, PAF-like activity reduced to 1.85 [0.46] (mean [SD], ng/10(7) cells), 1.11 [0.14], and 0.88 [0.09] (n = 15) in the supernatants, and 11.83 [2.93], 8.32 [1.41], and 6.27 [1.25] (n = 15) in the cell pellets, respectively. PAF-like activity in non-asthmatic eosinophils without preincubation of azelastine was 2.01 [0.86] (mean [SD], ng/10(7) cells) in supernatants and 7.44 [0.99] in cell pellets. After preincubation with 10(-8), 10(-6), and 10(-4) M azelastine, PAF-like activity reduced to 1.73 [0.64] (mean [SD], ng/10(7) cells), 1.12 [0.23], and 0.84 [0.17] (n = 20) in the supernatants, and 6.26 [2.08], 4.65 [0.88], and 3.02 [0.43] (n = 20) in the cell pellets, respectively. Our results showed that preincubation with azelastine caused a dose-dependent inhibition of intra and extracellular PAF-like activity from asthmatic and non-asthmatic eosinophils in the same manner.

Inhibition of PAF-induced platelet aggregation by WEB 2086 'in-vitro', an antagonist to the receptor for platelet-activating factor, in bovine

The sensitivity of bovine platelet aggregation in response to PAF stimulation and the ability of WEB 2086 (a thieno-triazolodiazepine) to inhibit response to PAF-induced platelet aggregation were investigated in the blood from five healthy male Belgian Blue calves. The recorded response to PAF showed a plateau which was dependent on the PAF concentration. Platelet aggregation induced by PAF consists of two mechanisms: reversible and irreversible aggregations which are accompanied by the release of platelet granule contents. Reversible aggregation occurred above (2 . 10(-9) mol/l) PAF, and irreversible aggregation occurred above (2 . 10(-7) mol/l) PAF. Addition of WEB 2086 to bovine platelets in vitro induced a rightward shift in the dose-response curve to PAF. WEB 2086 inhibited PAF-induced aggregation in a competitive reversible manner (pA2 = 7.61). The results of our study show that PAF induces platelet aggregation in platelet-rich plasma (PRP) and that addition of WEB 2086 to bovine platelets in vitro inhibits PAF-induced Platelet Aggregation.

Enhancement by cyclo-oxygenase inhibitors of platelet-activating factor production in thapsigargin-stimulated macrophages

1. Thapsigargin stimulated the accumulation of cell-associated platelet-activating factor (PAF) and extracellular prostaglandin E2 (PGE2) in rat peritoneal macrophages. PAF in the conditioned medium was less than the detectable amount. To obtain further insight into the mechanism of PAF accumulation, the role of PGE2 in PAF accumulation was investigated. 2. When macrophages were incubated in medium containing thapsigargin (30 ng ml(-1), 46.1 nM) and cyclo-oxygenase inhibitors such as indomethacin, naproxen or ibuprofen, the PAF content of the cells at 10 min was increased in a concentration-dependent manner in accordance with inhibition of PGE2 production. The stimulation by thapsigargin, cyclo-oxygenase inhibitors did not increase PAF accumulation. 3. In thapsigargin-stimulated macrophages, when PGE2(10(-7) M) was added to the medium, the cyclo-oxygenase inhibitor-induced stimulation of PAF accumulation at 10 min was markedly inhibited. 4. The accumulation of PAF induced by thapsigargin alone at 10 min was inhibited by exogenous PGE2 (10(-8) and 10(-7) M), or arachidonic acid (10(-6) and 10(-5) M) in accordance with the increase in PGE2 production. 5. The accumulation of PAF induced by thapsigargin alone or by thapsigargin and indomethacin (10(-6) M) was inhibited by dibutyryl cyclic AMP. 6. These results indicate that the concurrently produced PGE2 in thapsigargin-stimulated macrophages down-regulates PAF accumulation by increasing intracellular cyclic AMP levels, and that cyclo-oxygenase inhibitors increase PAF accumulation by inhibiting PGE2 production.

Biochemical evidence of platelet-activating factor (PAF) in human middle ear effusions

Platelet-activating factor (PAF) is one of the most potent biological lipid mediators. This is especially true in relation to inflammation. In order to characterize the biochemical features of otitis media with effusion, the authors characterized and determined the concentration of the PAF present in human middle ear effusions obtained from 23 patients with otitis media with effusion. Each sample of middle ear effusion was divided into two groups: serous (n = 12) and mucoid effusions (n = 11). The platelet-activating factor activity was found mainly in mucoid middle ear effusions, and the amounts of PAF were higher in mucoid type (3.55 +/- 1.19 ng/g, mean +/- standard deviation [SD]) than in the serous type (0.44 +/- 0.19 ng/g). Phospholipids obtained from the middle ear effusions contained a large amount of lyso-platelet-activating factor, the biologically inactive precursor or breakdown product of platelet-activating factor. Based on these findings, it is suggested that platelet-activating factor may play an important role as a mediator of the inflammatory responses in the pathogenesis of otitis media with effusion.

Eicosanoid generation and effects on the aggregation of thrombocytes from the rainbow trout, Oncorhynchus mykiss

Fish blood lacks anucleate platelets but contains a nucleated cell type termed the thrombocyte that is thought to be functionally analogous. Thrombocytes were purified from the peripheral blood of the rainbow trout, Oncorhynchus mykiss, by a two step gradient centrifugation method. Following this procedure, the recovered thrombocytes were 78-86% pure as defined by immunoreactivity to a panel of monoclonal antibodies and were of variable morphology from round to spindle-shaped. Incubation of thrombocyte suspensions with either calcium ionophore, A23187, platelet-activating factor or a thromboxane (TX) mimetic, U-46619, generated a range of eicosanoids derived from arachidonic acid including 12-hydroxyeicosatetraenoic acid (12-HETE), TXB2, prostaglandin (PG) E2, leukotriene (LT) B4 and lipoxin (LX) A4. The equivalent products derived from eicosapentaenoic acid were also formed. Co-incubation of thrombocytes with either erythrocytes or granulocytes/monocytes in the presence of calcium ionophore did not result in the formation of any further new lipoxygenase products. Incubation of isolated thrombocytes in plasma-free conditions with U-46619 (0.03-10 microM) resulted in a rapid, dose-dependent aggregatory response. This effect was markedly augmented in the presence of mammalian fibrinogen (400 micrograms ml-1). Thrombin (0.1-1.3 units ml-1), like U-46619, was also a potent proaggregatory compound for trout thrombocytes. LXA4 and LTB4 had limited aggregatory potential and then only at high concentrations (10 microM), while 12-HETE and PAD had no significant effect at all concentrations tested. These results demonstrate that some of the eicosanoids released during the activation of trout thrombocytes are involved in the aggregatory behaviour of this cell type.

Effects of the PAF antagonists BN50726 and BN50739 on arrhythmogenesis and extent of necrosis during myocardial ischaemia/reperfusion in rabbits

1. The effects of two novel platelet activating factor (PAF) antagonists BN50726 and BN50739 on arrhythmias, haemodynamics and extent of necrosis during myocardial ischaemia and reperfusion were investigated in anaesthetized rabbits subjected to coronary artery ligation. 2. BN50739 reduced heart rate prior to coronary artery occlusion (P < 0.005) but had no other significant haemodynamic effects at this time. BN50739 and BN50726 did not significantly alter heart rate or blood pressure during 30 min of ischaemia or 30 min of reperfusion, compared to control hearts. 3. BN50739 and BN50726 had no effect on the incidence of arrhythmias during ischaemia. BN50726 significantly reduced the incidence of reperfusion ventricular fibrillation compared to controls (0% v 40%, P < 0.05), and improved survival (80% v 39%, P < 0.05). Similar trends were observed with BN50739. 4. BN50726 reduced the extent of necrosis compared to control hearts (18 +/- 2% v 30 +/- 3%, P < 0.01). A similar trend was observed with BN50739. 5. These results demonstrate that PAF antagonism with BN50726 attenuates reperfusion-induced arrhythmias and preserves myocardium in the early phase of ischaemia, independently of haemodynamic effects.

Xanthines inhibit human platelet aggregation induced by platelet-activating factor

1. Platelet-activating factor (PAF) may be involved in the pathogenesis of asthma, and therefore the effects of the anti-asthma drugs theophylline and enprofylline on human platelet aggregation and adenosine triphosphate (ATP) release induced by PAF and adenosine diphosphate (ADP) were studied. 2. Enprofylline (50% inhibitory concentration [IC50] = 94.8 +/- 13.2 mumol/L) was more potent than theophylline (IC50 = 934.1 +/- 40.1 mumol/L) as an inhibitor of PAF-induced aggregation, and the xanthines were twice as potent as inhibitors of PAF-induced aggregation when compared with ADP-induced aggregation. ATP release was 1.4 times more sensitive to inhibition by the xanthines than aggregation. 3. Although high concentrations of xanthines inhibited platelet aggregation and ATP release induced by PAF, therapeutic concentrations are unlikely to inhibit PAF-induced effects.

Inhibition of cyclic flow variations and reocclusion after thrombolysis in dogs by a novel antagonist of platelet-activating factor

To determine whether platelet-activating factor is a specific mediator of cyclic flow variations in damaged stenotic arteries and whether it contributes to reocclusion after thrombolysis, femoral arteries in anesthetized dogs were subjected to mural injury and high grade stenosis to induce cyclic flow variations (28 +/- 4/h) or methods selected to elicit platelet-rich and fibrin-rich thrombosis. Oral administration of a novel triazolobenzodiazepine (U46,195 [10 mg/kg]) that selectively inhibits platelet-activating factor abolished cyclic flow variations within 120 min and for greater than or equal to 2 h thereafter compared with persistent flow variations in dogs given saline solution. Platelet aggregation induced ex vivo with platelet-activating factor was inhibited in parallel with in vivo inhibition of cyclic flow variations after administration of U46,195. However, buccal mucosa bleeding time was not affected. After thrombosis, administration of U46,195 before thrombolysis was induced with human recombinant tissue-type plasminogen activator (1.7 mg/kg intravenously over 60 min) prevented reocclusion within 120 min in six of eight and six of seven arteries by platelet-rich and fibrin-rich thrombosis, respectively. In contrast, in dogs given saline solution, reocclusion occurred in eight of eight (p = 0.007 compared with U46,195) and five of eight arteries by platelet-rich and fibrin-rich thrombosis, respectively. Thus, both cyclic flow variations and reocclusion after thrombolysis appear to be mediated in part by platelet-activating factor. The results suggest that inhibition of platelet-activating factor with specific antagonists may be useful in reducing platelet-mediated occlusion of coronary arteries without eliciting bleeding.

Studies on the role of platelet-activating factor in blood pressure regulation

Circulating levels of 1-O-hexadecyl-2-acetyl-sn-glycero-3-phosphocholine (C16PAF) in human subjects were measured by gas chromatography/mass spectrometry using negative ion chemical ionization. The mean (+/- S.D.) circulating C16PAF levels in patients with essential hypertension (18.1 +/- 5.3 pg/mL, n = 16) were not significantly different from those in normotensive subjects (17.2 +/- 7.2 pg/mL, n = 14). During a salt balance study, high salt intake (20 g/day) significantly increased the circulating level of C16PAF, and changes in circulating C16PAF significantly and positively correlated with changes in mean arterial blood pressure (r = 0.47, p less than 0.05). Changes in C16PAF also correlated with changes in creatinine clearance (r = 0.55, p less than 0.05), but did not correlate with changes in plasma sodium concentration, plasma chloride concentration and plasma volume. An intravenous injection of 50 micrograms of human atrial natriuretic peptide (hANP) decreased circulating C16PAF levels from 20.0 +/- 2.7 to 13.9 +/- 2.4 pg/mL of blood (n = 10, p less than 0.01) in healthy subjects. The data appear to indicate that C16PAF levels are changed by salt intake-induced mild increase in blood pressure, and that hANP may be an endogenous factor which lowers circulating C16PAF.

Platelet response to the aggregatory effect of platelet activating factor (PAF) ex vivo in patients with acute myocardial infarction

Platelets from patients with acute myocardial infarction exhibit an increased sensitivity to the aggregatory effect of PAF, in vitro, the first 48 h after the onset of the symptoms. This sensitivity, expressed as PAF EC50 values, seems to be transient after the 2 day period. Also, a remarkable decreased sensitivity to the inhibitory effect of PGI2 against the aggregation induced by PAF appears to the platelets of those patients the first hours after the onset of the symptoms, and persists for at least 14 days. Treatment of patients by drugs with a known inhibitory effect on platelet aggregation in vivo and in vitro (aspirin, nifedipine, indomethacin), does not influence the increase in platelet sensitivity to PAF, but inhibits the secondary aggregation induced by the released aggregating factors from the PAF activated platelets. The increase in platelet sensitivity to PAF is not unique to the AMI since it is also observed in patients with acute bacterial pneumonia. However, we cannot support the theory that it is a general phenomenon of acute tissue injury since it is general phenomenon of acute tissue injury since it is not observed in patients with acute muscular injury.

Interaction of platelet-activating factor with cultured guinea pig tracheal epithelial cells

The present study has examined the interaction of platelet-activating factor (PAF) with cultured guinea pig tracheal epithelial cells (GTE). PAF stimulated GTE to release endogenous arachidonic acid and metabolize it to prostaglandins E2 and F2 alpha (PGE2 and PGF2 alpha). Prostanoid production by GTE in response to PAF was dose-dependent (0.1-100 nM) and was maximal within 5 min. PGE2 and PGF2 alpha levels increased by 3.3 +/- 0.8 and 3.2 +/- 0.6 ng/10(6) cells respectively over basal levels in response to 100 nM-PAF. The ability of GTE to synthesize and/or catabolize PAF was also examined. GTE readily incorporated [3H]acetate into a product which migrated on t.l.c. with PAF. However, further characterization of this product suggested that label had not been incorporated into PAF, but rather that it was incorporated into another lipid product with chromatographic characteristics similar to those of PAF. In contrast, GTE readily metabolized PAF to inactive products. When [3H]PAF was incubated with GTE, 50% of the total [3H]PAF added was catabolized in approx. 15 min. The major route of catabolism of PAF by GTE was the deacetylation-reacylation pathway, which yielded 1-O-[3H]alkyl-2-acyl-sn-glycerophosphocholine. Determination of the nature of the long-chain acyl group incorporated into the sn-2 position of the newly synthesized products revealed that oleic and linoleic acids were the major fatty acids present. Taken together, these results suggest that respiratory epithelial cells respond to stimulation by PAF with enhanced production of PGE2 and PGF2 alpha, and also have the capacity to modulate inflammatory reactions in the airways by their ability to degrade this potent inflammatory mediator.

Interferon-gamma enhances PAF-acether production by stimulated human polymorphonuclear leucocytes

Human polymorphonuclear leucocytes (PMN) stimulated with either immune complexes (IC), phorbol myristate acetate (PMA) or N-formyl-methionyl-leucyl-phenylalanine (FMLP) generate platelet-activating factor (PAF-acether). The present study demonstrates that treatment of PMN with recombinant human interferon-gamma (IFN-gamma) significantly enhanced the production of PAF-acether by stimulated cells, in a concentration-dependent mode. On the contrary, alpha and beta IFN were completely unable to increase PAF-acether synthesis by stimulated PMN. The significance of these results is discussed.

Electrophysiological and arrhythmogenic effects of platelet activating factor during normal perfusion, myocardial ischaemia and reperfusion in the guinea-pig

1. Platelet activating factor (PAF) is often used to study the effects of platelet activation. While direct myocardial electrophysiological effects of PAF have been described in superfused myocardial tissue, little is known about its actions on the whole heart. 2. The cellular electrophysiological and arrhythmogenic effects of PAF (10(-11)M, 10(-10)M and 10(-9)M) were studied during normal perfusion, global myocardial ischaemia and reperfusion in Langendorff-perfused guinea-pig hearts at 32 degrees C. 3. PAF (10(-9)M) increased the incidence of ventricular fibrillation during ischaemia and reduced action potential duration (APD) during normal perfusion and early myocardial ischaemia (10(-9)M and 10(-10)M). PAF also reduced refractory period (RP) during normal perfusion (10(-9)M) and early ischaemia (10(-9)M and 10(-10)M). PAF prevented recovery of APD (10(-9)M) and RP (10(-9)M and 10(-10)M) during reperfusion. PAF at a concentration of 10(-11)M had no electrophysiological effects. 4. PAF (10(-9)M) increased the QRS width of the electrocardiogram during late ischaemia while 10(-10)M PAF raised pacing threshold during late ischaemia. 5. Perfusion pressure was increased, and developed tension decreased by 10(-9)M PAF. 6. These results demonstrate that PAF has direct myocardial electrophysiological effects in the whole heart which occur during normal perfusion and are capable of augmenting the effects of myocardial ischaemia, but are independent of the presence of platelets.

Mucosal dysfunction and damage induced by platelet activating factor (PAF)

The effect of PAF on human nasal mucosa was investigated in vitro. Normal paranasal sinus mucosa was obtained from the ethmoid sinuses by surgical procedure and incubated in the form of tissue culture. Ciliary movement was viewed under an inverted microscope and recorded on video tapes, and its activity was measured photoelectrically. Morphological alterations were examined by light and electron microscopy. PAF inhibited ciliary activity of human nasal mucosa, in a time and a dose dependent manner, at concentrations from 10(-6)M to 10(-10)M, while no significant change was observed at 10(-11) M. Lyso-PAF exhibited minimal effect on the mucosa at a concentration of 10(-6) M. Morphological alterations of the epithelial layer of the mucosa such as edema, cell exfoliation and desquamation were found to increase across time. Ultrastructural alterations were observed prior to inhibition of ciliary activity. These data indicate the cytotoxic effect of PAF on human paranasal sinus mucosa.

Regulation of platelet-activating factor receptor and PAF receptor-mediated arachidonic acid release by protein kinase C activation in rat Kupffer cells

Phorbol 12-myristate 13-acetate (PMA), a potent protein kinase C activator, caused down-regulation of receptors for platelet-activating factor (AGEPC) on the plasma membrane of rat Kupffer cells (40-50% reduction) but had a relatively minor effect on the binding affinity of the receptors for AGEPC (Kd = 0.30 nM vs 0.56 nM) when incubated with the cells for a short period of time (30-60 min). As a consequence, the AGEPC receptor-mediated arachidonic acid release was attenuated. The PMA-induced down-regulation of AGEPC receptors was concentration-dependent, specific, and transient (the maximal effect was observed at about 1 h and the level of specific [3H]AGEPC binding gradually returned to the control level within 8.5 h and even higher than the control level at 24 h after addition of PMA). Upon removing PMA from the culture medium, more than half of the lost receptors were replaced within 1 h at 37 degrees C and the recovery process appeared to be independent of protein synthesis. The ability of PMA to down-regulate the AGEPC receptors was lost in cells "down-regulated" for protein kinase C, suggesting that the receptor-regulatory effect of PMA is protein kinase C-dependent. Protein kinase C appeared to be involved in the AGEPC-induced arachidonic acid release since 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine dihydrochloride, a protein kinase C inhibitor, attenuated the stimulatory effect of AGEPC in this system. In addition, AGEPC-induced [3H]arachidonic acid release was inhibited significantly in cells down-regulated for protein kinase C. The present study thus demonstrates that protein kinase C has dual actions in the regulation of AGEPC-mediated events, i.e., a positive forward action, regulating AGEPC-stimulated arachidonic acid release, and a negative action, which inactivates or down-regulates AGEPC receptors.

Inhibition of thrombin- and collagen-induced phosphoinositides breakdown in rabbit platelets by a PAF antagonist--denudatin B, an isomer of kadsurenone

Denudatin B, an isomer of kadsurenone, was isolated from Magnolia fargesii. It inhibited the aggregation and ATP release of washed rabbit platelets caused by platelet-activating factor (PAF) in a concentration-dependent manner. The IC50 on PAF (2 ng/ml)-induced aggregation was about 10 micrograms/ml. High concentration of denudatin B (greater than 50 micrograms/ml) also inhibited the aggregation and ATP release of platelets caused by ADP, collagen, arachidonic acid and thrombin. However, shape change of platelets still existed. Prolongation of the incubation time with platelets could not cause further inhibition, and the aggregability of platelets could be restored after denudatin B was washed out from platelets. Thrombin-induced thromboxane B2 formation was almost completely suppressed. In the absence of extracellular calcium (EGTA 1 mM), ATP release caused by thrombin was inhibited. Thrombin-induced rise of the intracellular calcium concentration was suppressed by denudatin B, but not by BN52021 or kadsurenone. The generation of inositol phosphate in washed platelets caused by collagen, PAF and thrombin was also suppressed. The data indicate that PAF antagonist denudatin B has nonspecific antiplatelet action at high concentration by inhibiting phosphoinositides breakdown induced by collagen and thrombin.

Pregnancy suppression by a platelet activating factor antagonist, ONO-6240, in mice

Mouse embryos produce an embryo-derived platelet activating factor (EDPAF) during their early developmental stages. The present study was aimed at investigating the physiological significance of EDPAF by using a specific PAF antagonist, ONO-6240. EDPAF-induced platelet decrease was antagonized by ONO-6240 in a dose-dependent manner, although this alone did not affect circulating platelet concentrations. ONO-6240, when administered to maternal mice, increased the proportion of mice conceiving small litter sizes, whereas it did not affect in vivo or in vitro development of embryos. EDPAF was suggested to play an important role in establishing pregnancy in mice by promoting implantation.

Detection of lyso-platelet-activating factor by high-performance liquid chromatography after derivatisation with fluorescent fatty acids

Lyso-platelet-activating factor (lyso-PAF) was derivatised with 9-anthracenepropionic acid in the presence of dicyclohexylcarbodiimide, p-toluenesulfonic acid and 4-dimethylaminopyridine. The reaction yield exceeded 90% when the fatty acid was present in double molar amounts versus lyso-PAF. The procedure was equally effective in the derivatisation of other lysophospholipids. The derivatized phospholipids are detected by ultraviolet absorption (lambda = 253 nm) or fluorescence detection (using excitation at 254 nm and emission at 450 nm). The technique was applied successfully to the detection of lyso-PAF in complement activated rabbit plasma.

Platelet activating factor and sheep platelets: a sensitive new bioassay

The in vitro response of sheep platelets to platelet activating factor (PAF) was investigated. Sheep platelet-rich plasma aggregated in response to PAF with an EC50 of 10 nM. Platelets isolated via arabinogalactan density gradient centrifugation displayed an EC50 of 50 pM with a threshold response at 0.1 pM. PAF-induced release of 14C-serotonin from isolated sheep platelets was comparable with an EC50 of 50 pM and threshold release at 10 fM. PAF-induced aggregation was specific in that it could be blocked by the competitive receptor antagonists Alprazolam (Upjohn, IC50 = 40 microM), L-652,731 (MSD, IC50 = 6 microM), and WEB 2086 (Boehringer Ingelheim, IC50 = 0.8 microM). At micromolar concentrations, WEB 2086 did not inhibit ADP- or thrombin-induced aggregation nor thrombin-induced serotonin release. However, at higher concentrations of WEB 2086 some inhibition of thrombin-induced platelet aggregation and release was observed. Subsequent experiments demonstrated that high concentrations of WEB 2086 can inhibit thrombin-induced clotting (Ki = 866 microM) and cleavage of the chromogenic substrate Spectrozyme-TH (Ki = 842 microM). In summary, the response of isolated sheep platelets to PAF was specifically inhibitable and was 10 to 100 times more sensitive than washed rabbit platelets, the most popular bioassay currently in use.

Mechanisms of platelet activating factor-induced aggregation and secretion in human platelets

The role of TXA2 in PAF-induced aggregation and secretion of human platelets is unclear. We have studied the relationship between aggregation, synthesis of TXA2 and release of 5-HT during the time course of aggregation induced by PAF and collagen. For PAF-induced aggregation there was strong aggregation and secretion with minimal production of TXA2 in contrast to collagen in which a surge in TXA2 synthesis preceded both aggregation and secretion. To determine the role of calcium flux in PAF-induced aggregation we have similarly studied the temporal relationships between aggregation, secretion and TXA2 synthesis for calcium ionophore A23187 induced aggregation but found these to be distinctly different from those determined for PAF. A method for measuring absolute amounts of 5HT released from platelets in small volumes of plasma is described. We conclude that TXA2 is not important in the mechanism of PAF induced aggregation and that an increase in the level of intraplatelet calcium per se is not sufficient to explain the mediation of PAF-induced aggregation.

Selenium deficiency inhibits prostacyclin release and enhances production of platelet activating factor by human endothelial cells

Selenium is an essential component of glutathione peroxidase, an enzyme which protects cells against peroxidation and controls concentrations of intracellular peroxides. Since selenium deficiency is clinically associated with an increased degree of atherosclerosis, the effects of selenium deficiency on prostacyclin (PGI2) and platelet activating factor (PAF) production by cultured human umbilical vein endothelial cells (HUVEC) were investigated. In selenium-deficient HUVEC, histamine-induced PGI2 synthesis was significantly decreased when compared to selenium-supplemented HUVEC; in contrast, histamine-induced PAF production was increased by selenium deficiency. Histamine-induced inositol trisphosphate and [Ca2+]i responses and the conversion of PGG2 and PGH2 to PGI2 were not altered by selenium deficiency. However, selenium deficiency decreased the conversion of exogenous arachidonate to PGI2 and markedly suppressed glutathione peroxidase activity. These results suggest that selenium deficiency, by decreasing glutathione peroxidase activity, makes HUVEC susceptible to peroxide-induced inhibition of the cyclooxygenase activity of PGH2 synthase, resulting in decreased PGI2 production. These changes may alter platelet function in vivo and thus play a role in the increased incidence of atherosclerosis reported in selenium-deficient individuals.

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+.

Sheep platelets as a model for human platelets: evidence for specific PAF (platelet activating factor) receptors

In a number of animals, the platelet response to Platelet Activating Factor (PAF) has been shown to differ considerably from that in humans. However, aggregation, release and particularly shape change were quite similar for human and sheep platelets. In this study, aggregation and shape change analysis were used to measure the response of sheep platelets to various synthetic analogs of PAF. Response is greatly reduced with no alkyl group in position 2 of PAF and decreases progressively as the number of carbons of the alkyl gets larger than three. A reduction of activity is also seen as the ether linkage at position 1 of PAF is replaced by an ester linkage. These changes are indicative of a specific membrane receptor for PAF in sheep platelets and confirm the usefulness of sheep platelets as a model for PAF-platelet interaction in humans.

Platelet-activating factor causes goblet cell depletion in the conjunctiva

Platelet-activating factor (PAF) (1-O-hexadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine) produced dose-dependent depletion of the goblet cell population associated with the conjunctival epithelium. Reductions in goblet cell numbers did not correspond to leukocyte infiltration and were consistent with a direct effect of PAF. In contrast, LTD4 and LTE4 did not affect the goblet cell population although they caused massive eosinophil infiltration into the conjunctival epithelium. Histamine also produced conjunctival goblet cell depletion, but this appeared secondary to eosinophil degranulation and resultant epithelial desquamation. In addition to goblet cell expulsion, PAF produced an increase in conjunctival microvascular permeability over an identical dose-range. PAF-induced leukocyte emigration was small or absent and comprised a neutrophil infiltrate which exhibited no clear dose-dependent relationship. Lyso-PAF produced effects only at the highest dose employed where pathological changes and a distinct increase in conjunctival microvascular permeability were evident. Lyso-PAF- and PAF-induced increases in conjunctival microvascular permeability were virtually abolished by the PAF antagonist CV-6209. The pronounced inhibitory activity of CV-6209 suggests that high doses of lyso-PAF may either weakly stimulate conjunctival PAF receptors or that there may be sufficient conversion of lyso-PAF to biologically active levels of PAF.

Metabolism of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and lyso-PAF (1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) by cultured rat Kupffer cells

The metabolism of platelet-activating factor (PAF; identified as AGEPC: 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) and lyso-PAF (lyso-GEPC: 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine) was investigated in cultured rat Kupffer cells. The rat Kupffer cells accumulated [3H]AGEPC and deacetylated this compound to the corresponding [3H]lyso-GEPC, which was the major metabolic product of [3H]AGEPC. [3H]Lyso-GEPC was distributed primarily in the supernatant fraction of incubated cells throughout the experimental interval. Only a very small portion of the [3H]lyso-GEPC was further converted to 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine (alkylacyl-GPC), indicating that this acylation process was not particularly active in these cells. When [3H]lyso-GEPC was incubated with Kupffer cells, the conversion of lyso-GEPC to AGEPC via the acetyltransferase reaction increased up to 30 min and declined thereafter. Bovine serum albumin (BSA) had a substantial influence on both the cellular uptake and the metabolism of [3H]AGEPC. An increase in the BSA concentration in the incubation media reduced the cellular uptake of [3H]AGEPC and the subsequent formation of lyso-GEPC. The results of this study suggest that the hepatic Kupffer cells play an important role in the metabolism of PAF. Moreover, these results infer that the regulation of the PAF level in certain hepatic pathophysiological situations may be a consequence of the production and subsequent metabolism of this potent lipid autacoid in the Kupffer cells of the liver.