Neutropenia induced by platelet-activating factor (PAF-acether) released from neutrophils: the inhibitory effect of prostacyclin (PGI2)

Platelet-activating factor antagonism: a new concept in the management of regional myocardial ischemia-reperfusion injury

Reperfusion therapies for treatment of myocardial infarction successfully reduce patient mortality; however, regional myocardial ischemia-reperfusion (RMIR) causes its own expression of cardiovascular dysfunction, including myocardial depression, hemodynamic instability, and dysrhythmias, which have increased patient mortality within the first 24 h after starting reperfusion therapy. Current evidence suggests that the release of oxygen-derived reactive substances and subsequent inflammatory mediators during ischemia-reperfusion contribute toward this injury. Platelet-activating factor (PAF), a mediator released during RMIR, has been emphasized by many investigators as playing a central role in causing RMIR injury. Similar cardiovascular dysfunctions that occur during RMIR, including myocardial depression, hemodynamic instability, and dysrhythmias, occur after administration of PAF and are ameliorated with PAF antagonists. Further, PAF antagonists have been shown to be cardioprotective and improve survival when administered before onset of reperfusion. A variety of phospholipid analogues, naturally derived compounds, and synthetic compounds have been developed that form the different classes of PAF antagonists, each with unique antagonizing properties. Several of these compounds have successfully passed safety and efficacy testing in humans; however, to date, no clinical trials have investigated the protective effects of PAF antagonists against RMIR injury. A current theory in the pathogenesis of RMIR injury considers the ischemic and necrotic portion of the myocardium and regional dysfunction due to tissue necrosis to be solely responsible for global cardiac dysfunction leading to hemodynamic instability and death. Evidence now suggests, however, that the global dysfunction is also due to the effect of inflammatory mediators such as PAF, thromboxanes, leukotrienes, and endothelins that are released during RMIR and are distributed throughout the heart on reperfusion. Antagonizing a central inflammatory mediator such as PAF, as adjunct treatment with currently used reperfusion therapies, improves cardiovascular function and survival in animals and should be introduced into clinical trials to investigate if similar protective effects can be provided in humans.

Endogenous corticosteroids mediate the neutrophilia caused by platelet-activating factor in the mouse

Platelet-activating factor (PAF; 100 ng i.v.) transiently modified the number of circulating neutrophils in the mouse, inducing a fast neutropenia (2 min) followed by a late onset neutrophilia (2 h). The potential involvement in PAF-induced neutrophilia of granulocytotic agents such as interleukin-1 and tumor necrosis factor-alpha could be excluded on the basis of the ineffectiveness of interleukin-1 receptor antagonist and of a specific monoclonal antibody anti-murine tumor necrosis factor-alpha. PAF granulocytosis was preceded by a significant rise in plasma corticosterone at 20 min. The involvement of endogenous corticosteroids was confirmed by the experiments with adrenalectomized mice and in animals pretreated with the steroid antagonist RU486 (11 beta-(4-dimethyl amino-phenyl) 17 beta-hydroxy, 17 alpha(prop-1-ynyl) estra 4,9-dien-3-one), where PAF-induced neutrophilia was greatly reduced (approximately 50%). Moreover, sustained increase in plasma corticosterone by administration of adrenocorticotropic hormone was paralleled by an intense neutrophilia. We show evidence that endogenous corticosterone acts through the glucocorticoid-inducible protein lipocortin 1.

Characterization of platelet-activating factor synthesis in glomerular endothelial cell lines

Platelet-activating factor synthesis in two transformed lines of glomerular endothelial cells was characterized and contrasted with platelet-activating factor production in macrovascular-derived endothelial cells as well as with glomerular cells of mesenchymal origin. Platelet-activating factor synthesis was assessed in intact cells and in cell-free preparations. Glomerular endothelial cells constitutively synthesize bio-active alkyl-PAF, and this basal activity can be chronically augmented by various inflammatory and thrombotic agents. In contrast, thrombin-mediated platelet-activating factor formation in bovine pulmonary aortic endothelial cells as well as in glomerular mesangial cells is acute and transient. The potential role of anti-inflammatory prostanoids to function as negative feedback modulators of thrombin- or endothelin-mediated platelet-activating factor synthesis was also investigated, as the synthesis of platelet-activating factor is often associated with the formation of these prostanoids. Indomethacin augmented receptor-mediated platelet-activating factor synthesis while prostanoids of the E and I series reduced agonist-stimulated PAF synthesis. In summary, the unique capacity of glomerular endothelial cells to respond to inflammatory stimuli with sustained platelet-activating factor synthesis is a clear indication of this cell's pivotal role in augmenting the inflammatory response in the limited environment of the glomerulus.

Hypotensive action of IgG preparations containing aggregates is suppressed by PAF-receptor antagonist BN 52021 and by gadolinium chloride (an agent blocking Kupffer cell function)

The hypotensive side effect of four IgG preparations was studied in a rat model. An unmodified, chromatographically isolated preparation (prepn. C) appeared to induce a transient drop in blood pressure via the kininogen kinin system. A preparation made by ethanol fractionation (prepn. E) and an ethanol-fractionated, modified product (prepn. M) produced an action which does not depend on kininogen activation. No blocking of kininase was required for this effect and it was brought about by prepn. M was devoid of any sizeable prekallikrein activator or kallikrein like proteinase content. The effect was associated by the aggregate fraction of prepn. E, but not with IgG monomers of the same preparation. Animals given these latter types of IgG preparations were refractory to a second dose 24 hours later. The involvement of PAF in the kinin-independent hypotensive action is suggested by the finding that PAF-receptor antagonist BN 52021 prevented the effect and returned the blood pressure close to normality when given during hypotension. A blockade of Kupffer cells brought about by administration of GdCl3 was also found to prevent the effect pointing to the involvement of these type of cells in the reactions.

Vasodilatation and inhibition of mediator release represent two distinct mechanisms for prostaglandin modulation of acute mast cell-dependent inflammation

1. Intravital microscopy of the hamster cheek pouch was used to examine the influence of vasodilator prostanoids (prostaglandin E2 (PGE2), PGI2), forskolin, and nitroprusside on the microvascular changes during acute inflammation induced by antigen or histamine. The results extend our previous finding that PGE2 modulates allergic inflammation and histamine release in the cheek pouch model. 2. The microvascular actions of arachidonic acid and different cyclo-oxygenase products (PGE2, PGD2, PGI2, PGF2 alpha, and the thromboxane A2 (TXA2)-analogue U-44069) were first compared with respect to their effects on arteriolar tone. Of the prostaglandins, only PGE2 and PGI2 were potent vasodilators and markedly increased local blood flow. Nitroprusside and forskolin also caused vasodilatation and increased blood flow, but were somewhat less potent than PGE2 and PGI2. 3. Topically applied PGE2 and PGI2 in vasodilator concentrations suppressed the antigen-induced plasma leakage. On the other hand, although the antigen response was predominantly mediated by histamine, both prostaglandins enhanced the plasma leakage evoked by exogenous histamine. 4. In contrast, the vasodilator nitroprusside, in a dose causing an increase in blood flow equal to that of PGE2 and PGI2, potentiated both the histamine-induced plasma leakage, as well as the plasma and leukocyte extravasation after antigen challenge, indicating that the anti-inflammatory actions of the prostaglandins were unrelated to their vasodilator properties per se. 5. Because forskolin, a specific activator of adenylate cyclase, mimicked the actions of PGE2 and PGI2, i.e. inhibition of the antigen-induced plasma extravasation and enhancement of the histamine response, it is possible that the observed antiallergic effects of the prostaglandins were related to accumulation of intracellular adenosine 3': 5'-cyclic monophosphate (cyclic AMP). 6. Taken together, there appears to be a competition between pro- and anti-inflammatory effects of PGE2 and PGI2 in reactions involving release of endogeneous inflammatory mediators in vivo, i.e. enhancement of inflammatory mediator target action on one hand ('two mediator synergism'), and suppression of mediator release on the other. Moreover, the observations indicate that vasodilatation and inhibition of mediator release are two distinct actions of PGE2 and PGI2.

Beneficial effects of prostaglandin E2 in endotoxic shock are unrelated to effects on PAF-acether synthesis

The effects of endotoxic shock on the synthesis of PAF-acether by the stomach, duodenum and lung were examined in the rat. Furthermore, the effect of pretreatment with prostaglandin E2 on endotoxin induced PAF-acether synthesis and changes in vascular permeability were examined. Administration of endotoxin resulted in significant increases in PAF-acether synthesis in all tissues studied. Such increases were apparent within 5-15 minutes of the administration of endotoxin, corresponding to the time when significant hypotension, hemoconcentration and increases in gastrointestinal vascular permeability were first observed. Pretreatment with prostaglandin E2 resulted in a significant reduction of endotoxin-induced hypotension, hemoconcentration and changes in vascular permeability in the gastrointestinal tract. However, prostaglandin pretreatment did not significantly alter endotoxin-induced PAF-acether release from the gastrointestinal tissues studied. These results demonstrate that prostaglandin E2 can significantly attenuate several of the systemic and gastrointestinal manifestations of endotoxic shock. The mechanism responsible for these beneficial actions appears to be unrelated to effects of prostaglandin E2 on PAF-acether synthesis.

Evidence for the role of platelet-activating factor in immune complex vasculitis in the rat

These studies were designed to determine the role of platelet-activating factor (PAF) in the pathogenesis of immune complex (IgG) induced dermal vasculitis in the rat. In vitro, very low (pM and nM) concentrations of PAF "primed" rat neutrophils for enhanced O2-. responses to IgG immune complexes while higher concentrations were directly stimulatory. The PAF receptor antagonist, L-652,731, blocked responses (O2-. production and enzyme release) of rat neutrophils stimulated with PAF but did not block responses triggered by immune complexes, formyl chemotactic peptide or opsonized zymosan particles. When L-652,731 was added to the antibody employed in the reversed passive Arthus reaction, the injury resulting from immune complex-induced vasculitis was significantly attenuated. In order to determine if in vivo protection provided by L-652,731 was related to neutrophils, we developed a new model in which rats are systemically depleted of neutrophils by cyclophosphamide and then locally reconstituted with intact neutrophils in a manner that allows restoration of immune complex-induced vascular injury. With this model, we demonstrated that the effects of neutrophil reconstitution are substantially diminished if the cells are pretreated with L-652,731 and then washed. By priming neutrophils with substimulatory concentrations of PAF, we have also provided in vivo evidence that neutrophil priming can increase the magnitude of vascular injury. These data provide evidence that vascular injury associated with immune complex dermal vasculitis is related to availability of PAF receptors on neutrophils, suggesting a mechanism through which PAF may function as a mediator in the pathogenesis of immune complex vasculitis.

The effect of intravascular neutrophil chemotactic factors on blood neutrophil and platelet kinetics

Intravenous infusion of an analogue (f-met-leu-phe [FMLP]) of a bacterial-derived polymorphonuclear leukocyte (PMNL) chemotactic factor, or of the complement-derived chemotactic stimulus, zymosan-activated plasma (ZAP, containing C5ades Arg) into rabbits induces acute PMNL margination in the pulmonary vasculature. This process also occurs during hemodialysis and the adult respiratory distress syndrome. The pulmonary PMNL sequestration is accompanied by thrombocytopenia. Because of the role platelets and PMNLs play in hemostasis and defense against infection, we studied the fate of these blood elements following sequestration induced by chemotactic factors. By employing 111In-labelled platelets and external radioisotope scanning, platelets were found to sequester in the pulmonary vasculature during FMLP infusion. Simultaneous 51Cr PMNL and 111In-platelet studies showed that following sequestration, PMNLs returned to the circulation and disappeared with a normal half-life (T1/2) whereas the T1/2 of the platelets was markedly shortened (T1/2 of control = 49 +/- 3.0 hr; FMLP or ZAP infused T1/2 = 27 +/- 2.7 hr). Infusion of platelet-activating factor (PAF) induced PMN and platelet sequestration with similar abnormalities in platelet kinetics. Studies with 51Cr- and 14C-serotonin-labelled platelets showed that platelets did not release serotonin during FMLP, ZAP, or low dose PAF-induced sequestration. In contrast to platelet survival, platelet size, platelet aggregation responses, and platelet glycoproteins were not affected by transient sequestration. These results indicate that during PMNL margination induced by relatively "pure" PMNL stimuli such as FMLP, platelets may reversibly marginate and subsequently be cleared at an accelerated rate. The reason for accelerated platelet clearance is not a result of circulating platelet aggregates or detectable proteolytic modification of membrane glycoproteins. Such altered platelet kinetics may contribute to thrombocytopenia during sepsis, the adult respiratory distress syndrome, and other states in which excess PMNL margination occurs.

Inhibitory effect of prostacyclin (PGI2) on neutropenia induced by intravenous injection of platelet-activating-factor (PAF) in the rabbit

Intravenous injection into rabbits of 1-O-octadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (synthetic Platelet-Activating Factor (PAF)) or PAF derived from rabbit basophils caused acute thrombocytopenia and neutropenia which was consequent to the formation of intravascular polymorphonuclear neutrophil (PMN) aggregates and to their sequestration in the microvasculature, primarily of the lung. Infusion of prostacyclin (PGI2; 10 ng/Kg/min to 50 ng/Kg/min) inhibited in a dose-dependent manner PAF-induced thrombocytopenia and neutropenia as well as the sequestration of PMN in the pulmonary capillary network.