Effects Of PAF-acether Research Articles

Vivo effects of PAF-acether and arterial thrombosis.

Platelet-activating factor (PAF) was first described in 1972 (1), after a soluble intermediate was demonstrated originating from rabbit leukocytes and thought to be responsible for the liberation of histamine from rabbit platelets (2). Structural analysis indicated that PAF was a 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (3); as such it was termed PAF-acether.

Inhibition of in vivo and in vitro pulmonary effects of paf-acether by BN 52021

Inhibition of in vivo and in vitro pulmonary effects of paf-acether by BN 52021

Inhibition of the vascular effect of PAF-acether

Inhibition of the vascular effect of PAF-acether

In vitro models for studying drug modulation of paf-acether effects

In vitro models for studying drug modulation of paf-acether effects

Desensitization to PAF-induced bronchoconstriction and to activation of alveolar macrophages by repeated inhalations of PAF in the guinea pig

Guinea-pig alveolar macrophages are activated in the presence of PAF-acether (PAF), as shown by O 2 •− production, suggesting that these cells, abundant in the lungs, are involved in PAF-induced bronchoconstriction. Alveolar macrophages collected after in vivo desensitization to the bronchoconstrictor effect of PAF became refractory to it in vitro , whereas the O 2 •− production in response to f-met-leu-phe persisted, although it was diminished suggesting a partial cross-desensitization. A similar desensitization to PAF was also observed in alveolar macrophages in vitro , demonstrating a stimulus-specific process. This study suggests that alveolar macrophages may be involved in bronchoconstriction induced by aerosol of PAF.

Electrophysiological effects of platelet-activating factor (PAF-acether) in guinea-pig papillary muscles

The effects of PAF-acether (10 −11 to 10 −7 M) were studied on the electrical and mechanical activity of guinea-pig papillary muscles. At 10 −11 M PAF-acether did not modify the amplitude and V max of the upstroke or the resting membrane potential. At higher concentrations PAF-acether produced a dose-dependent increase in the amplitude and V max of the upstroke, shortened the action potential duration and hyperpolarized the resting membrane potential. These effects were accompanied by a biphasic effect on ventricular contractile force. The shortening of the APD was inhibited in muscles pretreated with tetraethylammonium or verapamil. In papillary muscles depolarized by 27 mM K Tyrode solution PAF-acether induced slow action potentials which were blocked by verapamil. PAF-acether produced a dose-dependent increase in amplitude and V max of the upstroke on the slow action potentials elicited by isoproterenol, prolonged the action potential duration and hyperpolarized the resting membrane potential. These results suggest that in guinea-pig papillary muscles PAF-acether increased Ca influx via the slow inward current.

The effects of PAF-acether on the cardiovascular system and their inhibition by a new highly specific receptor antagonist BN 52021: Vasospasm in isolated guinea-pig heart and contraction of isolated rat portal vein

The effects of PAF-acether on the cardiovascular system and their inhibition by a new highly specific receptor antagonist BN 52021: Vasospasm in isolated guinea-pig heart and contraction of isolated rat portal vein

The effects of PAF-acether on the cardiovascular system and their inhibition by a new highly specific receptor antagonist BN 52021 2. Hypotensive shock in the normal anaesthetized rat. Comparison with the effects of salbutamol, a beta-2 agonist

The effects of PAF-acether on the cardiovascular system and their inhibition by a new highly specific receptor antagonist BN 52021 2. Hypotensive shock in the normal anaesthetized rat. Comparison with the effects of salbutamol, a beta-2 agonist

Inhibition by the paf acether antagonist 48740 RP of the bronchopulmonary effects of paf-acether

Inhibition by the paf acether antagonist 48740 RP of the bronchopulmonary effects of paf-acether

Triggering by Paf-acether and adrenaline of cyclo-oxygenase-independent platelet aggregation.

Platelet-activating factor (Paf-acether, 1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) induced full aggregation and a limited release reaction of human platelets in plasma or in blood. Cyclo-oxygenase inhibition with aspirin only reduced aggregation when induced by threshold amounts of Paf-acether, whereas higher concentrations surmounted inhibition whether tested in citrated or in heparinized platelet-rich plasma or blood. Aspirin-induced inhibition of platelet secretion by Paf-acether was insurmountable and independent of the anti-coagulant used. Paf-acether and adrenaline acted synergistically in inducing aggregation in citrate and heparin. Aspirin in vitro or after oral ingestion at doses that suppressed aggregation induced by arachidonic acid alone, failed to reduce significantly the synergized aggregation induced by Paf-acether alone or combined with adrenaline. Twenty-four hours after the oral ingestion of aspirin, when aggregation by arachidonic acid remained blocked, a slight inhibitory activity on the effect of Paf-acether noted 4 h after aspirin, had ceased. This was probably accounted for by the synthesis of thromboxane A2 by newly formed platelets, since the in vitro addition of aspirin, or of the thromboxane/endoperoxide receptor inhibitor 13-azaprostanoic acid caused the 24 h platelets to behave in a manner similar to platelets collected 4 h after aspirin. The alpha 2-adrenoceptor inhibitor, yohimbine, blocked the direct effect of adrenaline as well as its synergism with Paf-acether. Since the synergistic effect of Paf-acether and adrenaline was maintained when thrombin-degranulated platelets were used, and aspirin remained ineffective against it, it is clear that the augmented platelet responsiveness is not accounted for by the platelet release reaction. 6 Paf-acether and adrenaline act synergistically and stimulate platelets by cyclo-oxygenaseindependent mechanisms, which may be relevant in human physiopathological conditions.

Interactions between thyreostimulin and PAF-acether on thyroid cell metabolism

1-O-Alkyl, 2-acetyl sn-glycerylphosphorylcholine (platelet activating factor (PAF)-acether) originally described as a platelet activating factor, is effective on various parameters in different cells. It seemed interesting to us to test it on porcine thyroid cells cultured for 1 to 5 days in the absence (control cells) or in the presence of 0.1 mU/ml thyreostimulin (TSH cells). At concentrations ranging from 0.5 to 2.5 μM, PAF-acether inhibited significantly the accumulation of cyclic AMP resulting from a 5 min incubation of the cells with TSH (40 mU/ml) or forskolin (0.1 mM). PAF-acether alone did not affect basal cyclic AMP accumulation. The maximal inhibition was obtained on a 3 day culture and amounted to 40–50%. The inhibition was transient and vanished after a 30 min incubation. The effects of PAF-acether (0.5 μM) on phospholipid metabolism depended closely upon the physiological state of the cells and upon the age of the culture. When PAF-acether was incubated for 2 h with [ 32P]phosphate, it mimicked the effects of TSH, i.e. it increased phosphatidylinositol (PI) labelling on 1 day control cells (expected effect) and decreased it with 1 day TSH cells (reverse effects). The PAF-acether effect was rapid in onset. After cell prelabelling for 2 h in the presence of TSH, PAF-acether added for 15 min completely counteracted the hormone effects on PI and phosphatidylcholine (PC) but increased the phosphatidic acid (PA) labelling. The effect of PAF-acether on PI labelling was partially antagonized by forskolin. Protein iodination, which is very dependent on the culture conditions, reached a very high rate in 3 or 4 day TSH-treated cells. This rate was not markedly enhanced by acute TSH stimulation and was decreased by 37% in 45 min by 2.5 μM PAF. This negative effect was completely abolished in the presence of TSH. On the contrary PAF, like TSH but more efficiently, increased the protein iodination of control cells by 60% in a 45 min period. The positive effect of PAF was not additive to that of TSH. The partial sharing of TSH properties by forskolin on the one hand and PAF on the other suggests that the cyclic AMP-dependent effects of the hormone are controlled by yet uncharacterized agents linked to phospholipid turnover and vice-versa.

The platelet-independent release of thromboxane A2 by Paf-acether from guinea-pig lungs involves mechanisms distinct from those for leukotriene.

Intra-arterial injections of platelet-activating factor (Paf-acether, 10-300 ng) to the perfused guinea-pig lung induced a dose-related bronchoconstriction, followed by contraction of the rat aorta superfused with the lung effluent, indicating the release of thromboxane A2 (TXA2) activity. These effects were matched with injections of bradykinin (Bk) at 100-1000 ng, leukotriene C4(LTC4) at 10-300 ng or arachidonic acid (AA) at 30-300 micrograms. Repeated doses of Paf-acether led to a specific desensitization of the release of TXA2, under conditions where Bk, LTC4 and arachidonic acid retained their ability to release TXA2. Bronchoconstriction and the release of TXA2 induced by Paf-acether were suppressed when the lungs were perfused with acetylsalicylic acid, but not with salicylic acid. The phospholipase A2 inhibitor, p-bromophenacyl bromide suppressed the release of TXA2 by Bk, but did not interfere with its formation from AA, nor with its release with Paf-acether and LTC4. The lipoxygenase inhibitor, nordihydroguaiaretic acid, inhibited to a similar extent the release of TXA2 by Bk, LTC4 and Paf-acether but also reduced directly the formation of TXA2 from arachidonic acid, invalidating its use as a specific antilipoxygenase agent. The leukotriene C4/D4 antagonist, FPL 55712, suppressed the TXA2 releasing effects of LTC4, and was completely inactive against Paf-acether, Bk or arachidonic acid. The aerosol of Paf-acether was tested in the anaesthetized guinea-pig and resulted in bronchoconstriction, unaccompanied by thrombocytopenia. Unlike bronchoconstriction induced by intravenous Paf-acether, which is refractory to cyclo-oxygenase inhibitors, the effects of the aerosol were suppressed by aspirin. Platelet depletion, which blocks the intravenous effects of Paf-acether, failed to interfere with those of the aerosol. Paf-acether induced a marked contraction of the superfused guinea-pig isolated parenchyma lung strip, which was followed by total and irreversible desensitization to itself. The contractile effect was not inhibited by aspirin or indomethacin, atropine, mepyramine, methysergide, phenoxybenzamine or propranolol, indicating that cyclo-oxygenase products, cholinergic stimuli, histamine, 5-hydroxytryptamine and catecholamine mechanisms are not involved. Our results indicate that Paf-acether interacts with pulmonary sites distinct from those for Bk, LTC4 or AA, since no cross-desensitization between Paf-acether and the other agonists was noted, p-bromophenacyl bromide inhibited Bk only and FPL 55712 inhibited only LTC4. The phospholipase A2 involved with the release of the arachidonate needed for the formation of TXA2 by Paf-acether or LTC4-stimulated lungs may differ from the enzyme accounting for its formation by Bk. The cellular sites with which Paf-acether interacts may also be distinct and less readily accessible to p-bromophenacyl bromide.

Paf-acether generates chemiluminescence in human neutrophils in the absence of cytochalasin b

In the absence of cytochalasin B, synthetic Paf-acether (0.1–10 μM) induced oxygen radical production in polymorphonuclear neutrophils as measured by the luminol-dependent chemiluminescence (LDCL) test. This effect was observed after a lag period of 10 s and was maximal between 5 and 15 min. In the presence of cytochalasin B, the kinetics were shortened, but the lag period was not modified and the same concentrations of the agonist had to be used to induce LDCL. None of the structural analogs tested (2-lyso Paf-acether, Paf-acether enantiomer, 1 ester analog of Paf-acether, lyso-phosphatidycholine) were active, irrespective of the presence of cytochalasin B. Paf-acether (10 μM) shortened the kinetics of opsonized zymosan (10 μg/ml)-induced LDCL and enhanced it by 550% and 250% at 5 min and 10 min respectively, without affecting the peak value. Similar results were obtained using non-opsonized zymosan (100 μg/ml). Lower concentrations of Paf-acether (0.1 μM) were also able to increase oxygen radical production induced by low doses of zymosan and opsonized zymosan. The triggering and enhancing effects of Paf-acether on oxygen radical production by resting and stimulated polymorphonuclear neutrophils support the role of Paf-acether in inflammation.

Effect of platelet activating factor on the human nasal mucosa.

In order to determine the effect of PAF-acether on the vascular bed in the human nose, 11 healthy volunteers were studied. Rhinomanometry was performed to determine the effect on the capacitance vessels, and the 133Xe wash-out method enabled determination of the mucosal blood flow, regulated mainly by the resistance vessels. These measurements were made before and after topical administration of PAF-acether in three different doses; 0.26 micrograms, 2.6 micrograms and 26 micrograms per nostril. The results indicate that PAF-acether induces a dose-dependent vasoconstriction of the capacitance vessels and a reduction in mucosal blood flow. That PAF-acether should be a principal mediator in the allergic reaction in the human nose is not supported by these findings.

Cooperative effects of 1-O-alkyl-2-O-acetyl- [formula omitted]-glycero-3-phosphocholine (PAF-acether) and exogenous arachidonic acid in stimulation of human blood platelets

Cooperative effects of PAF-acether and arachidonic acid in blood platelet activation were studied in human platelet-rich plasma. Using a combination of low concentrations of PAP-acether and subthreshold amounts of exogenous arachidonic acid an enhancement of aggregation and an increased formation of malondialdehyde are obtained. Both effects are completely suppressed either by acetylsalicylic acid or by use of PAF-acether desensitized platelets, indicating that cyclooxygenase products and intact receptors for PAP-acether are of primary importance in the observed synergism. Stimulating activity of PAF-acether on the formation of cyclooxygenase products is also seen with use of higher concentrations of arachidonic acid during mild stimulation of human platelets. This effect of PAP-acether is dependent on the concentration used and is obviously due to changes in the platelet membrane as derived from experiments with lysed platelets. Enhanced capacity of platelets to synthesize endoperoxides and thromboxane in consequence of an in vivo release of PAF-acether into the circulation might be of pathobiochemical relevance in some circumstances.

Acute microvascular effects of PAF-acether, as studied by intravital microscopy

Application of PAF-acether (platelet-activating factor) (final concentration 1–20 nM) to the microcirculation of the hamster cheek pouch resulted in dose-dependent vasoconstriction and an increase in macromolecular permeability, as assessed by leakage of intravascular fluorescein-labelled dextran ( M w 150 000 ). An increase in adhering leukocytes in venules was only seen with 20 nM PAF-acether. Animals made neurtropenic by treatment with antineutrophil serum raised in rabbits showed a reduced leakage response to 20 nM PAF-acether, whereas the leakage induced with 5 nM PAF-acether was unaffected. This indicates that the increase in vascular permeability induced by PAF-acether occurs both as a presumably direct effect independent of polymorphonuclear leukocytes (PMNL), and via an indirect PMNL-dependent mechanism. Intravenous administration of PAF-acether (10 μg/kg body weight) to hamsters resulted in rapid and transient thrombocytopenia and leukopenia whereas 1 μg/kg only affected the leukocytes. These potent microvascular effects of PAF-acether support its proposed role as a mediator of allergic and inflammatory reactions.

The actions of Paf-acether (platelet-activating factor) on guinea-pig isolated heart preparations.

Paf-acether (platelet-activating factor) is a phospholipid capable of stimulating platelets to release their granular contents and cause platelet aggregation. When Paf-acether was administered to isolated heart preparations from normal guinea-pigs there was a significant concentration-dependent reduction in coronary flow and contractile force. The high concentration of Paf-acether was equally effective in reducing these cardiac parameters in the presence of atropine. The non-acetylated Paf-acether analogue, 2-lyso Paf-acether, the enantiomer, and a closely related phospholipid 1, alpha-lysophosphatidylcholine palmitoyl, did not affect coronary flow and contractile force, indicating the specificity of Paf-acether. These data demonstrate a potent effect of Paf-acether on cardiac function. Whether or not these effects are direct or mediated through generation of endogenous mediators remains to be established.

Dependency of the Paf-acether induced bronchospasm on the lipoxygenase pathway in the guinea-pig

The Paf-acether (platelet-activating factor) induced bronchospasm (Paf-BCS) was studied in the anesthetized guinea-pig. The SRS antagonist, FPL-55712, as well as inhibitors of both lipoxygenase and cyclooxygenase, phenidone, nordihydroguaiaretic acid (NDGA), and benoxaprofen, caused a dose-related antagonism of Paf-BCS. By contrast, selective inhibitors of cyclooxygenase, indomethacin and aspirin, exerted moderate antagonism at intermediate doses, but had no effect at high doses. Furthermore, diethylmaleate (DEM), which impairs leukotriene synthesis by interfering with glutathione (GSH), suppressed Paf-BCS. Taken together, these results demonstrate that the lipoxygenase pathway plays a major part in the bronchospasmogenic effect of Paf-acether in the guinea-pig.

Hypotensive activity of PAF-acether in rats.

PAF-acether caused a dose-related decrease in blood pressure of conscious SHR rats (5–20 μg/kg i.p.) and anaesthetized normotensive rats (0.06–0.50 μg/kg i.v.). In anaesthetized normotensive rats, PAF-acether-induced hypotension was not associated with tachycardia and not modified by various pretreatments such as atropine, mepyramine, cimetidine, propranolol, sulpiride, ketoprofen (an inhibitor of prostaglandin cyclooxygenase) and teprotide. PAF-acether (0.5 μg/kg i.v.) reduced the pressor effect of norepinephrine but not that of angiotensine II. In pithed rats, PAF-acether (0.12–0.50 μg/kg i,.v.) shifted the dose-response curve for norepinephrine-induced hypertension to the right in a parallel manner and did not reverse the inhibitory action of clonidine on cardiac nerve stimulation. These results suggest that the hypotensive effect of PAF-acether in anaesthetized rats may be mediated by α-adrenergic blockade and PAF-acether looks like a powerful postsynaptic adrenoceptor blocking agent. However,in vitro, up to 100 μg/l, PAF-acether did not modify the norepinephrine-induced contractions on non-vascular (vas deferens) and vascular (thoracic aorta) preparations of rats. Moreover, PAF-acether did not inhibitin vitro [3H]WB-4101 and [3H]p-aminoclonidine binding in rat brain preparations. This absence ofin vitro activity suggest that thein vivo adrenolytic activity of PAF-acether is in fact an indirect one either via a directly active metabolite or possibly through the release of an unidentified endogenous adrenolytic factor.

Pharmacological properties of PAF-acether in the guinea-pig: platelet-dependent and independent reactions.

The in vivo effects of PAF-acether in the guniea-pig are reviewed, particularly with respect to bronchoconstriction and to platelets, the former being suppressed when the latter are depleted. Bronchoconstriction by PAF-acether is not inhibited by aspirin, and when the latter was associated to mepyramine and to methysergide, bronchoconstriction was blocked, whereas thrombocytopenia persisted. Our results indicated that platelets collected from aspirin--mepyramine--methysergide-treated animals are less reactive for PAF-acether than control platelets, with respect to secretion of ATP, but aggregated fully. Secretion and aggregation are thus dissociable, the former being involved with bronchoconstriction. Results obtained with sulphinpyrazone and with reserpine are also discussed, and lead to the concept that platelet may release a presently unknown mediator when stimulated with PAF-acether.