Protective effects of a platelet activating factor (PAF) antagonist and its combined treatment with prostaglandin (PG) E1 in traumatic shock

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Effect of recombinant soluble P-selectin glycoprotein ligand-1 on leukocyte-endothelium interaction in vivo. Role in rat traumatic shock

-Traumatic shock induces profound pathophysiological alterations and initiates inflammatory reactions in many tissues, thus resulting in acute multiple organ damage (eg, intestine, pancreas, and liver). In the rat, Noble-Collip drum trauma increases P-selectin expression on the vascular endothelium as a result of loss of endothelium-derived NO. Here we postulated that blockade of the earliest steps in leukocyte adhesion (ie, leukocyte rolling) via administration of a recombinant soluble form of P-selectin glycoprotein ligand-1 (PSGL-1; the recombinant soluble form is rsPSGL.Ig) would attenuate selectin-mediated events observed in the rat during traumatic shock. Using intravital microscopy of the rat mesenteric microvasculature, we found that intravenous infusion of rsPSGL.Ig significantly inhibited leukocyte-endothelium interaction (ie, leukocyte rolling, adherence, and transmigration) induced by traumatic shock as well as by activation of the microvascular endothelium with 50 micromol/L NG-nitro-L-arginine methyl ester. Immunohistochemical detection of P-selectin on the mesenteric venular endothelial surface demonstrated that rsPSGL.Ig functionally neutralizes effects of P-selectin on the endothelial cell surface rather than attenuating P-selectin expression. Systemic administration of rsPSGL.Ig to traumatized rats prolonged survival time and survival rate, significantly attenuating ileal myeloperoxidase activity and significantly preserving mesenteric endothelial function. Furthermore, PSGL-1 mRNA levels were significantly increased in the blood of traumatized rats and were reduced after systemic administration of rsPSGL.Ig. Thus, soluble recombinant forms of PSGL-1 are able to ameliorate acute shock states by suppressing selectin-mediated leukocyte-endothelium interaction at both the functional and molecular levels.

Experimental model of cardiovascular post-resuscitation syndrome--no effect of platelet activating factor antagonism

The cardiovascular instability seen in the reperfusion phase after resuscitation from cardiac arrest may contribute to secondary brain injury. The aim of the present study was to characterise post-resuscitation cardiovascular instability in an experimental model of cardiac arrest and to test if cardiovascular stability could be improved by pre-treatment with the platelet activating factor (PAF) antagonist BN52021. Ten anaesthetised pigs received pre-treatment with BN52021 before induction of ventricular fibrillation (arrest/BN52021 group), while ten animals received only the vehicle (arrest/vehicle group). After a non-intervention interval of 9 min, resuscitation was attempted. Resuscitated animals were observed for 5 h and compared to a sham arrest group of seven animals. The haemodynamic situation after resuscitation was characterised by a low cardiac output that was inadequate in relation to the oxygen demand, as reflected by a low mixed venous oxygen saturation. The arterial blood pressure was to some extent reduced and the filling pressures in both the right and left heart were increased, but urinary output was not reduced. The severe haemodynamic compromise was not adequately reflected by standard monitoring variables such as arterial blood pressure and urinary output. Pre-treatment with BN52021 was unable to improve any aspect of short-term survival or haemodynamic stability.

Cellular mechanisms of heparinase III protection in rat traumatic shock

Pentobarbital-anesthetized rats subjected to traumatic shock developed a shock state characterized by marked hypotension to 65-70 mmHg, a survival time of 88 +/- 13 min, significant increases in ileal myeloperoxidase activity (P < 0.01), and severe endothelial dysfunction as evidenced by a significant (P < 0.01) decrease in vasorelaxation to endothelium-dependent dilators. Treatment with heparinase III (45 microg . kg-1 . min-1) 10 min posttrauma prolonged survival time to 223 +/- 19 min (P < 0.001), significantly attenuated ileal myeloperoxidase activity (P < 0.01), and significantly preserved endothelial function (P < 0.05). Intravital microscopy of the rat mesentery showed that infusion of heparinase III (45-67 microg . kg-1 . min-1) significantly (P < 0.01) attenuated both leukocyte rolling and adherence in the rat mesenteric microvasculature in response to NG-nitro-L-arginine methyl ester stimulation. Immunohistochemical localization of surface-expressed P-selectin on mesenteric venules showed that heparinase III infusion at 45-67 microg . kg-1 . min-1 significantly (P < 0.05) attenuated the increase in surface P-selectin expression. The beneficial effects of heparinase III are mediated at least in part by attenuating leukocyte-endothelial cell interactions via a P-selectin-dependent mechanism.

PAF-induced death in NMRI mice--a suitable shock model for testing new PAF receptor antagonists. Correlation with eicosanoid related substances

The effects of PAF antagonists, of substances which influence the arachidonic acid metabolism, and of dexamethasone and ketotifen were evaluated in an acute PAF-induced mortality model in female NMRI mice. We established a dependence of sensitivity to PAF on strain (AB mice showed no dose dependence) and on sex of the animals as well as on the PAF charges used in our experiments. PAF produced resistance in surviving animals against the PAF-induced death on repeated application. The PAF antagonists, WEB 2170 and WEB 2086, provided the best dose-dependent protection against PAF toxicity, followed by dexamethasone, by the COX/LOX synthetase inhibitor X 86 (a BW 755 C-analogue) and by the PAF receptor antagonist BN 52021. Particularly remarkable was the excellent prevention by aspirin. Aspirin may not only inhibit the cyclooxygenase pathway but also endogenous PAF synthesis. Other drugs, i.e. indomethacin, the thromboxane receptor antagonist, BM 13177, the thromboxane synthetase inhibitor, HOE 944, as well as the lipoxygenase inhibitors (NDGA, esculetin, SHAM and phenidone) exerted a dose-dependent protection only at high doses.

Effects of stimulation and inhibition of acid secretion on gastric damage induced by platelet-activating factor

Intravenous administration of platelet-activating factor (PAF) induces extensive damage in rat gastric mucosa. The aim of the present study was to examine whether the presence or absence of acid in the gastric lumen could modify the PAF-induced gastric damage. The effects of inhibition of basal and pentagastrin-stimulated acid secretion by ranitidine on the deep histological gastric damage induced by 30 min of infusion of PAF (100 ng/kg/min) were assessed by using a histological score. Inhibition of basal gastric acid secretion did not prevent the histological gastric damage induced by PAF. Stimulation of gastric acid secretion by pentagastrin significantly increased PAF-induced gastric damage, and this effect was reversed by a dose of ranitidine that returns acid secretion to baseline levels. This acid-related damage was confined to the deep mucosa, since scanning electron microscope analysis ruled out an additional surface damage in PAF-infused rats when gastric acid was stimulated. The data indicate that a certain amount of acid may worsen the deep gastric mucosal damage induced by PAF.

Platelet-activating factor in cardiovascular stress situations

Since the elucidation of its chemical structure two decades ago, platelet-activating factor (PAF) has emerged as an important mediator of various cardiovascular stress situations. Most notably, PAF was implicated as a key factor in the septic shock syndrome, based on the similarities between endotoxin and PAF biological effects, the elevation of circulating and tissue levels of PAF during endotoxemia, and the protective effect of PAF antagonists in the septic state. In addition, accumulating data suggest the involvement of PAF in the pathophysiological processes associated with ischemia, hemorrhage and trauma, where PAF exerts its effects directly on cells and blood elements or indirectly through interactions with other mediators such as cytokines and prostaglandins. Nevertheless, the relative contribution of PAF to the pathophysiological processes in endotoxemia is still unknown and should await further investigations. The primary aims of this chapter are: to delineate the effects of PAF on the cardiovascular system, to summarize the data which suggest the involvement of PAF in stress situations of the cardiovascular system, and to identify areas where future experimental efforts should be focused.

Pooling of blood in postischemic shock is modulated by platelet-activating factor (PAF)

In experiments on dogs, i.v. administration of platelet-activating factor (PAF) (500 ng/kg) was shown to induce hypotension which, apart from decreased myocardial contractility, was characterized by blood pooling in veins (82.6 +/- 6.8 mL/kg). This was accompanied by restriction of venous return to the heart and reduction of cardiac output (CO). During postischemic shock the cardio- and hemodynamic disturbances were similar to those induced by i.v. administration of PAF. In the postischemic shock model, preliminary blockage of PAF receptors with the PAF receptor antagonist BN 52021 (6 mg/kg, i.v.) significantly decreased the amount of blood pooled in shock from 38.7 +/- 5 to 18.3 +/- 2 mL/kg (p less than 0.01). Simultaneously, the reduction of CO and blood pressure, induced by reperfusion of the continuously ischemized tissues of a rear limb, was less significant in pretreated vs. the nontreated group. The data suggest that PAF may be involved in postischemic blood pooling and that PAF antagonists could be used to correct postischemic cardio- and hemodynamic disturbances.

Influence of some prostaglandins and prostaglandin analogues on PAF-induced shock in mice

Prostaglandins and Prostaglandin-analogues were investigated for their ability to protect mice from platelet-activating factor (PAF) induced shock. 75% mortality in female NMRI mice was induced by i.v. injection of 75 micrograms/kg PAF. Nileprost and PGE1, the most potent substances, produced a dose dependent protection against PAF. Iloprost and PGI2 were less effective. PGE2, nalador, flunoprost and U 46619 were neither protective nor deleterious. The strong difference in the effectiveness between the two prostaglandins of the E-series and the poor effect of PGI2 and the PGI2 analogue is remarkable. Flunoprost and U 46619 that increased the TXB2 synthesis or release in two experimental models did not enhance the PAF mortality; TXA2 seems to be only a secondary mediator of the acute PAF-induced death.

Protective effects of a specific platelet activating factor (PAF) antagonist, WEB 2086, in traumatic shock

We have investigated the role of platelet activating factor (PAF) in the pathogenesis of a murine model of traumatic shock using WEB 2086, a specific antagonist of PAF. WEB 2086 (0.5 mg/kg) significantly reversed the decrease in mean arterial blood pressure (MABP) induced by PAF (0.3 micrograms/kg) in anesthetized rats. Anesthetized rats were subjected to Noble-Collip drum trauma. Traumatized rats treated with WEB 2086 (0.5 mg/kg bolus followed by infusion at 0.5 mg/kg/hr) maintained a higher MABP than those receiving only the vehicle (0.9% NaCl). Improvement in MABP paralleled a significant increase in overall survival time (p less than 0.01) in rats receiving WEB 2086 (0.5 mg/kg). WEB 2086 also significantly attenuated the plasma accumulation of the lysosomal hydrolase, cathepsin D and of free amino-nitrogen compounds, compared to shocked rats receiving only the vehicle. Furthermore, the production of the cardiotoxic peptide, myocardial depressant factor (MDF) was also blunted by WEB 2086. These results suggest that PAF may be an important mediator in the pathogenesis of traumatic shock in rats. Furthermore, PAF receptor antagonists may be useful as therapeutic agents when given early in the course of ischemic and shock states.