Involvement Of Platelet-activating Factor Research Articles

Prevention of Chloroquine-lnduced Electroretinographic Damage by a New Platelet-Activating Factor Antagonist, BN 50730

Chloroquine retinopathy is a severe toxic retinal impairment which may result in loss of vision by alterations of the retinal pigment epithelium and photoreceptors. Currently, there is no specific treatment for this retinopathy. Platelet-activating factor (PAF) is known to modulate retinal function and is one of the major immunomediators of the retina. In order to test the possible involvement of PAF in chloroquine-induced retinopathy and the effectiveness of PAF antagonists in the prevention of this condition, we investigated the effects of BN 50730, a specific PAF antagonist, on the electroretinogram (ERG) of the isolated rat retina exposed to chloroquine. When retinas from normal rats were perfused with chloroquine (10(-6) M), a marked and rapid decrease in b-wave amplitude was observed. In contrast, chloroquine had no effect on the b-wave of the retina isolated from animals pretreated with the PAF antagonist BN 50730 (30 mg/kg/day, i.p., for 5 days). The results obtained indicate that (i) chloroquine is a toxic drug for retinal function, (ii) PAF plays a key role in the mediation of chloroquine retinopathy and (iii) PAF antagonists may constitute valuable agents for the treatment of this retinal impairment.

Effects of a platelet-activating factor antagonist, CV-6209, on gastric mucosal lesions induced by ischemia-reperfusion.

Recent research was shown that oxygen-derived free radicals are involved in the pathogenesis of various diseases, including ischemia-reperfusion injury. We have also reported that oxygen-derived free radicals and lipid peroxidation may play an important role in gastric mucosal injury induced by ischemia-reperfusion. The hypoxanthine-xanthine oxidase system and neutrophils are considered important sources of oxygen-derived free radicals in this process. In recent years, it also has been shown that serum platelet-activating factor (PAF) levels increased during ischemia-reperfusion, and that induction of superoxide generation by neutrophils is one of the important biological effects of PAF. In the present study, we examined the effect of CV-6209, a specific PAF receptor antagonist, on gastric mucosal injury induced by ischemia-reperfusion, to shed some light on the possible involvement of PAF in such lesions. CV-6209 significantly attenuated the gastric mucosal injury induced by ischemia-reperfusion, and inhibited both an increase of thiobarbituric acid reactive substances and a decrease of alpha-tocopherol in gastric mucosa after ischemia-reperfusion. However, CV-6209 had no effect on gastric mucosal blood flow during ischemia-reperfusion. These results suggest that endogenous PAF may play an important role in gastric mucosal injury induced by ischemia-reperfusion, and that CV-6209 exerts its beneficial effect mainly by inhibiting neutrophil superoxide production induced by PAF.

Cardiovascular effects of Acanthaster planci venom in the rat: Possible involvement of PAF in its hypotensive effect

Cardiovascular effects of the crowns-of-thorns starfish ( Acanthaster planci) venom were examined in rats. The crude venom extracted from the spines of A. planci caused systemic hypotension associated with an increase in heart rate and a decrease in renal cortical blood flow when given i.v. The hypotensive effect of the venom was not inhibited by pretreatment with atropine, indomethacin or aprotinin, but was significantly inhibited by SRI 63–441, a platelet activating factor (PAF) antagonist. The venom caused dose-dependent vasorelaxation of the isolated rat aortic ring preparation precontracted by noradrenaline, an effect which was significantly attenuated by pretreatment with SRI 63–441, methylene blue or parabromophenacyl bromide. Denudation of the endothelium also diminished the vasorelaxing effect of the venom. Both the vasorelaxing and the hypotensive effects showed tachyphylaxis. These results suggest the release of PAF or a PAF-like substance from the endothelium by the venom.

Effects of PAF‐antagonists in mouse ear oedema induced by several inflammatory agents

1. Several platelet activating factor (PAF)-antagonists of different chemical structures were tested in the arachidonic acid-, tetradecanoylphorbol acetate-, dithranol-, and benzoic acid-induced mouse ear oedema models. 2. Topical application of UR-10324, UR-11353, CV-6209 and WEB-2086 markedly inhibited ear oedema induced by the four irritants tested, mimicking the profile obtained with dexamethasone. YM-461 was highly effective only in the dithranol-induced ear oedema, while BN-52021 failed to inhibit ear oedema in all models tested. 3. Leukocyte recruitment into the inflamed ears was prevented by PAF-antagonists, as measured by myeloperoxidase activity in the supernatants of ear homogenates. 4. A relationship between PAF-antagonist and anti-inflammatory activities was found in some cases, but other mechanisms cannot be excluded to explain the topical anti-inflammatory effect of these compounds. 5. Our results suggest that topical formulations containing PAF-antagonists could be useful in the treatment of some inflammatory skin diseases and provide evidence on the involvement of PAF in these inflammatory processes.

The Involvement of Platelet-Activating Factor in Thrombocytopenia and Follicular Rupture During Gonadotropin-Induced Superovulation in Immature Rats*

To investigate whether the platelets in the ovaries are activated by the action of platelet-activating factor (PAF) during gonadotropin-induced ovulation, we examined the changes in the platelet count in immature rats after administration of PMSG followed 48 h later by human CG (hCG). The platelet count in the inferior vena cava was significantly decreased 48 h after PMSG administration and was further decreased after hCG administration. When both ovaries of rats were extirpated, the administration of PMSG and hCG did not decrease the platelet count. Subcutaneous administration of a PAF antagonist, Y24180 (0.5-5 mg/kg.6 h), after PMSG injection decreased the number of ova shed in a dose-dependent manner. The decrease in the platelet count induced by the administration of PMSG and hCG was reversed to the level of the untreated control group by Y24180 (2.5 mg/kg.6 h). This inhibitory activity of Y24180 on ovulation and thrombocytopenia was completely reversed by the ip injection of synthetic PAF. Subcutaneous administration of indomethacin (IDM) also reduced the number of ova shed in a dose-dependent manner. However, thrombocytopenia was not reversed by IDM. Moreover, the inhibition of ovulation by IDM was not reversed by synthetic PAF. The present study suggests that: 1) platelets are activated by PAF during gonadotropin-induced ovulation in immature rats; 2) PAF is also involved in the rupture of follicles; 3) the presence of the ovary is indispensable for the generation of PAF in gonadotropin-stimulated immature rats; and 4) the mechanism of PAF action on ovulation may be different from that of prostaglandins.

Platelet-activating factor induces progesterone secretion and changes in morphological appearance in luteinizing granulosa cells in vitro

Recent evidence indicates the involvement of platelet-activating factor (PAF) in ovarian periovulatory processes. The objective of this study was to investigate the influence of PAF on ovarian follicular cellular function. We examined the effects of PAF on progesterone (P) secretion by cultured human luteinizing granulosa cells obtained from women undergoing in vitro fertilization. Follicular cells were cultured in serum-free conditions for 12 to 36 hours with and without PAF (at dosages of 10(-8) to 10(-14) M). Progesterone secretion was significantly increased at 12 and 36 hours after treatment with 10(-8) M PAF (170% +/- 40% and 210% +/- 40%, respectively) and with 10(-10) M PAF (170% +/- 30% and 180% +/- 10%, respectively). Smaller increases were seen with 10(-12) and 10(-14) M PAF. Maximal increases in P secretion in 10(-10) M PAF-treated wells occurred at 3 to 12 hours. At 36 hours, distinct morphological differences between cells in control wells and those containing PAF were noted. These results indicate that PAF may influence follicular cell function during ovulation and corpus luteum formation.

Platelet activating factor involvement in splanchnic artery occlusion shock in rats

Splanchnic artery occlusion shock was induced in anesthetized rats by clamping the splanchnic arteries for 45 min. The survival rate, plasma levels of thromboxane B 2 (TxB 2) and 6-keto-PGF 1α, serum and peritoneal levels of macrophage tumor necrosis factor (TNF α), the phagocytotic and killing activity of peritoneal macrophages and white blood cells count were evaluated. Shocked rats died within 2 h, while all sham-shocked rats survived more than 6 h. Plasma TxB 2 and 6-keto-PGF 1α levels were increased in rats subjected to splanchnic artery occlusion shock compared to the levels in sham-shocked animals. Serum and peritoneal macrophage TNF α levels were undetectable in sham-shocked rats, whereas shocked rats exhibited increased levels of TNF α. Moreover, splanchnic artery occlusion shock reduced peritoneal macrophage phagocytotic and killing activity, and also produced severe leukopenia. A specific receptor antagonist of platelet activating factor (PAF), L-652, 731 (an i.v. bolus of 3.2 mg/kg 2 min after removal of the clamps followed, 5 min thereafter, by a continuous infusion of 0.16 mg/kg per min for 30 min) significantly increased the survival rate, lowered plasma TxB 2 levels and reduced both serum and macrophage TNF α levels in shocked rats. In addition, L-652,731 completely restored macrophage phagocytosis, partially improved macrophage killing and significantly inhibited leukopenia. Finally, the administration of L-652,731 had beneficial effects on the cardiovascular changes induced by splanchnic artery occlusion shock. These finding are consistent with the involvement of PAF in splanchnic artery occlusion shock and indicate that PAF produces shock through direct and indirect (TxB 2-mediated and TNF α-mediated) actions.

Involvement of Platelet-Activating Factor in Endothelin-Induced Vascular Smooth Muscle Cell Contraction

This study was designed to elucidate the participation of platelet-activating factor (PAF) in endothelin-induced vascular constriction in vivo and in vitro. The microvascular hemodynamic changes in rat mesentery induced by the superfusion of endothelin-1 (ET-1) were visualized through an intravital microscopic system. It was revealed in vivo that ET-1 in a range of 100 fM-10 pM caused a sustained arteriolar constriction in a dose-dependent manner. Pretreatment with CV-6209, a selective PAF antagonist, significantly attenuated the constrictive change in arterioles. Changes of intracellular Ca2+ mobilization after treatment with ET-1 were investigated in vitro using a cell line (A-10 cell) derived from rat arterial smooth muscle cells. ET-1 caused a prompt rise in the fura-2-associated fluorescence intensity in the individual A-10 cell and it fell to a lower plateau level that was still higher than the baseline value. CV-6209-pretreated cells did not show the rapid-phase mobilization of Ca2+, but showed the slow late phase of Ca2+ activation. The present study demonstrates that PAF may be involved in endothelin-induced microvascular constriction by mediating the mobilization of Ca2+ in vascular smooth muscle cells.

Prolongation of Parturition in the Pregnant Rat Following Treatment with a Platelet Activating Factor Receptor Antagonist1

The presence of platelet activating factor (PAF) in amniotic fluid of women only in labor is indicative of a role for PAF in parturition. In addition, stimulated amnion membrane produces both PAF and prostaglandin E2, each of which is capable of inducing myometrial contraction. To evaluate the involvement of PAF in the process of parturition, we administered the PAF receptor antagonist, L-659,989, to 17-day timed pregnant rats and followed the events of labor and delivery. Administration by mouth with L-659,989 of three concentrations (1.6, 16, and 48 mg/kg/day) did not alter the gestational period; however, the duration of parturition was increased from 2-fold to 5-fold by such treatment. No toxicity of the analog was apparent; treated dams showed no signs of morbidity, and fetal mortality was not significantly altered by treatment with the antagonist. Based on these experiments, it is suggested that the PAF receptor antagonist interferes with the normal progression of events of parturition and that PAF is an integral mediator in initiating the myometrial contraction necessary for expulsion of the fetus.

Bradykinin‐induced plasma exudation in guinea‐pig airways: involvement of platelet activating factor

1. We studied the effect of bradykinin on plasma exudation in the airways of the anaesthetized guinea-pig in vivo. Tissue content of extravasated Evans blue dye was used as an index of protein exudation in the larynx, trachea, main bronchi and intrapulmonary airways (i.p.a.). 2. Bradykinin increased the content of Evans blue in all tissues studied in a dose-related manner. The response was greatest in the main bronchi and i.p.a., less in the trachea and least in the larynx. A dose of 47 nmol kg-1 was the lowest tested which caused significant (P less than 0.001) plasma exudation with increases in leakage above control values of 256% in the larynx, 405% in the trachea, 394% in the main bronchi and 485% in intrapulmonary airways. 3. Leakage was significantly (P less than 0.05) increased above control values by 1 min after bradykinin (47 nmol kg-1) in the main bronchi and intrapulmonary airways and was maximal in all airways 5 min after bradykinin. Although reduced by 15 min, the tissue content of dye was still significantly (P less than 0.05) increased 2 h after bradykinin. 4. The prolonged tissue dye retention was due to a later phase of slow and maintained exudation preventing full clearance of dye after the initial response. 5. The initial phase of leakage was partially attenuated by the platelet activating factor (PAF) receptor antagonists WEB 2086 or BN 52021, by indomethacin or by inhibiting sensory nerve activation by opioid anaesthesia: it was not affected by mepyramine and cimetidine nor by the sulphidopeptide leukotriene receptor antagonists FPL 55712 or ICI 198,615. Adrenoceptor blockade of the anti-leakage effects of endogenously-released catecholamines significantly (P < 0.05) enhanced leakage. 6. The later phase of plasma leakage was completely inhibited by the PAF antagonists. 7. We conclude that, in guinea-pig airways in vivo, the initial phase of bradykinin-induced plasma exudation is mediated in part by PAF, sensory nerves and prostaglandins, whereas the later, prolonged phase of leakage is mediated exclusively by PAF. If bradykinin is generated in asthma, its potent and prolonged effects on plasma leakage may contribute significantly to airway oedema and may be involved in the development of bronchial hyperresponsiveness.

Involvement of Platelet Activating Factor and Thromboxane A2 in the Renal Response to Unilateral Ureteral Obstruction

Platelet activating factor (PAF) and thromboxane A2 (TxA2) are two vasoactive mediators which can decrease renal blood flow. Both are synthesized by various intrarenal cell types or by macrophages which may infiltrate the kidney during unilateral ureteral obstruction (UUO). In several experimental systems, PAF receptor activation is accompanied by TxA2 release; pharmacological modification of TxA2 synthesis or receptor activation modulates the response to PAF. The involvement of PAF in UUO has not been studied previously, and the role of TxA2 has not been clearly defined by previous investigations. The hemodynamic response to acute UUO is characterized by decreases in renal blood flow (RBF) and glomerular filtration rate and an acute increase in ureteral pressure. In the present experiments, the involvement of either PAF or TxA2 in the acute response to UUO was studied by determining if blockade of either the TxA2 or PAF receptor would affect the renal hemodynamic response to UUO. In addition, the effect of blockade of the TxA2 receptor on the renal response to PAF was determined. Our results indicate that only a small portion of the renal response to PAF is mediated by TxA2, and that neither PAF nor TxA2 can be implicated in the acute hemodynamic response to UUO. TxA2 or PAF involvement in the chronic response to UUO still remains to be determined.

Platelet-activating factor (PAF) induced rhinitis and involvement of PAF in allergic rhinitis in guinea pigs

The effects of inhaled PAF on the guinea pig nasal mucosa were investigated. Intranasal pressure (INP) was recorded as an index of intranasal resistance. To access the capillary premeability of nasal mucosa, exudation of Evans blue into the nasal lavage fluid was determined. Inhalations of histamine and PAF markedly and significantly increased INP and dye exudation into the nasal cavities. The two responses to PAF were about 20-fold and 70-fold stronger than those of histamine, respectively. A PAF antagonist, CV-3988, significantly antagonized both the PAF-induced increases in INP and dye exudation. Indomethacin and OKY-046 had no effect on the PAF-induced responses. FPL-55712 inhibited the PAF-induced increases in INP and dye exudation by 52% and 40%, respectively. Ovalbumin (OA) antigen challenge by inhalation to sensitized guinea pigs resulted in significant increases in both INP and dye exudation. These two responses to 30 mg/ml OA were inhibited by CV-3988 (10 mg/kg, i.v.) by 55% and 40%, respectively. From the above results, it is indicated that: 1) inhalation of PAF evokes rhinitis-like symptoms through activation of PAF receptors, 2) the PAF-induced rhinitis is, in a part, mediated by leukotrienes, and 3) PAF might be involved in allergic rhinitis.

Amplification of platelet response during acute inflammation in rats

Enchanced aggregation of platelets was observed in platelet-rich plasma, but not in washed platelet suspension (WPS), during acute inflammation in rats. Incubation of WPS with inflammatory plasma increased the aggregatory response to ADP, but the plasma itself did not cause aggregation of platelets. It potentiated the aggregatory response of normal platelets, when platelets were stimulated with arachidonic acid, thrombin, calcium ionophore A23187 or phorbol-12-myristate-13-acetate. Pre-treatment of rats with indomethacin (10mg/kg) did not prevent the increased aggregation response of platelets due to inflammation. This response was not due to any of the biogenic amines nor was it due to platelet factor 4. The activity of the inflammatory plasma was reduced when it was incubated with phospholipase A 2, indicating the involvement of platelet-activating factor (PAF). The activity was present both in the lipid and the protein fraction of the inflammatory plasma. The results indicate that a substance(s) released in the circulation during inflammation renders the platelets hyperactive. This substance appears to be a protein which is present in the inflammatory plasma and acts together with PAF to cause increased aggregation of platelets.

O-159 - Involvement of platelet-activating factor and complement activation in zymosan-induced rat pleurisy

O-159 - Involvement of platelet-activating factor and complement activation in zymosan-induced rat pleurisy

P-139 - Involvement of platelet activating factor in endotoxin shock of the rat

P-139 - Involvement of platelet activating factor in endotoxin shock of the rat

Platelet-Activating Factor (PAF)-lnduced Rhinitis and Involvement of PAF in Allergic Rhinitis in Guinea Pigs

The effects of inhaled PAF on the guinea pig nasal mucosa were investigated. Intranasal pressure (INP) was recorded as an index of intranasal resistance. To access the capillary premeability of nasal mucosa, exudation of Evans blue into the nasal lavage fluid was determined. Inhalations of histamine and PAF markedly and significantly increased INP and dye exudation into the nasal cavities. The two responses to PAF were about 20-fold and 70-fold stronger than those of histamine, respectively. A PAF antagonist, CV-3988, significantly antagonized both the PAF-induced increases in INP and dye exudation. Indomethacin and OKY-046 had no effect on the PAF-induced responses. FPL-55712 inhibited the PAF-induced increases in INP and dye exudation by 52% and 40%, respectively. Ovalbumin (OA) antigen challenge by inhalation to sensitized guinea pigs resulted in significant increases in both INP and dye exudation. These two responses to 30 mg/ml OA were inhibited by CV-3988 (10 mg/kg, i.v.) by 55% and 40%, respectively. From the above results, it is indicated that: 1) inhalation of PAF evokes rhinitis-like symptoms through activation of PAF receptors, 2) the PAF-induced rhinitis is, in a part, mediated by leukotrienes, and 3) PAF might be involved in allergic rhinitis.

Possible involvement of platelet activating factor in anaphylaxis of passively sensitised, isolated guinea pig hearts

There is evidence that cardiac tissue may be a target for antigen/antibody reactions. Platelet activating factor (PAF) is released during anaphylaxis and could mediate cardiac damage. To investigate this, guinea pigs were passively sensitised by anti-ovalbumin rabbit serum (6 mg.kg-1 intravenously) and 24 h later their hearts were excised and isolated according to a working heart preparation technique. After a 20 min equilibration period, anaphylactic challenge was induced by a bolus injection of ovalbumin (2 mg in 0.2 ml buffer) via the side arm of the aortic cannula. Heart rate, coronary flow, aortic flow, left ventricular developed pressure (LVDP), its first derivative (LVdp/dtmax) and left ventricular end diastolic pressure (LVEDP) were recorded. After ovalbumin challenge, heart rate and LVEDP were markedly increased, while coronary flow, aortic flow, LVDP, and LVdp/dtmax were profoundly decreased. All these alterations were over within 5 min, and the measured variables returned to approximately the pre-challenge values. BN 52021, a specific PAF receptor antagonist, was dissolved in the perfusion buffer and given in doses of 15, 30 and 60 mumol.litre-1 10 min prior to the induction of anaphylactic challenge until the end of the observation period. BN 52021 inhibited the increase in heart rate and LVEDP and the decrease in coronary and aortic flow, LVDP and LVdp/dtmax in a dose dependent manner. The changes produced by 30 and 60 mumol.litre-1 were statistically significant at the levels of p less than 0.01 and p less than 0.001 when compared to the control values.(ABSTRACT TRUNCATED AT 250 WORDS)

An investigation into the possible involvement of platelet activating factor in experimental allergic encephalomyelitis in rats.

Rats developing experimental allergic encephalomyelitis have been treated with intra-venous injections of Platelet-Activating Factor on days 5, 6 and 7 post adjuvant injection. Other rats have been treated with one of two different PAF antagonists or vehicle. The animals injected with PAF on day 5 post adjuvant developed a more severe form of the disease at an earlier time point than did control animals. Animals treated with the PAF antagonists did not develop the disease to any great extent. Treatment with the vehicle had no effect compared to control. These results implicate PAF in the aetiology of this model of multiple sclerosis and may suggest a role for PAF antagonists in the treatment of this disease.

Occupancy of platelet receptors for platelet-activating factor in patients with septicemia.

The possible involvement of platelet-activating factor (PAF) in the pathogenesis of endotoxemia, was investigated by using a binding assay to patients' platelets, complemented with the extraction and chemical characterization of PAF obtained from patients' platelets. Platelets from 12 human volunteers had 281 +/- 63 freely accessible high affinity binding sites (PAF-receptors) per platelet; whereas this number was of 49 +/- 37 PAF-receptors per platelet, n = 14 samples, P less than 0.01, in a group of 13 patients with positive blood culture. A group of patients with respiratory or cardiovascular disturbances and negative blood culture had 253 +/- 74, accessible receptors per platelet (n = 19 samples from 16 patients, P less than 0.01 as compared to septic patients, which was not significantly different when compared to control individuals). Patients with sepsis possessed significant amounts of PAF associated to their platelets, whereas this mediator could not be isolated from platelets of patients with respiratory or cardiovascular disturbances and negative blood culture, nor from platelets of control individuals. PAF was also assayed in whole blood samples and found at high concentrations in sepsis patients. These data indicate that occupancy of PAF receptors in combination with high amounts of platelet-associated PAF, is a common finding in patients with sepsis.

Endothelin-induced mobilization of Ca 2+ and the possible involvement of platelet activating factor and throm☐ane A 2

Endothelin (ET) caused transient and sustained elevations of cytosolic free Ca 2+ concentrations ({Ca 2+} i) in cultured rat and rabbit vascular smooth muscle cells (VSMC). Specific platelet activating factor (PAF) antagonists (CV-6209 and WEB-2086) and arachidonic acid (AA) cascade blockers (chlorpromazine, indomethacin, CV-4151 and AA-2414) potently inhibited the ET-induced increase in {Ca 2+} i. Additionally, these compounds inhibited the PAF-induced increase in {Ca 2+} i. These results suggest that PAF and throm☐ane A 2 (TXA 2) may be involved in the mechanism of ET-induced mobilization of Ca 2+ in cultured rat and rabbit VSMC.