The effects of PAF antagonists on arrhythmias and platelets during acute myocardial ischaemia and reperfusion

Anti-atherosclerotic effects and molecular targets of ginkgolide B from Ginkgo biloba

Bioactive compounds derived from herbal medicinal plants modulate various therapeutic targets and signaling pathways associated with cardiovascular diseases (CVDs), the world's primary cause of death. Ginkgo biloba , a well-known traditional Chinese medicine with notable cardiovascular actions, has been used as a cardio- and cerebrovascular therapeutic drug and nutraceutical in Asian countries for centuries. Preclinical studies have shown that ginkgolide B, a bioactive component in Ginkgo biloba , can ameliorate atherosclerosis in cultured vascular cells and disease models. Of clinical relevance, several clinical trials are ongoing or being completed to examine the efficacy and safety of ginkgolide B-related drug preparations in the prevention of cerebrovascular diseases, such as ischemia stroke. Here, we present a comprehensive review of the pharmacological activities, pharmacokinetic characteristics, and mechanisms of action of ginkgolide B in atherosclerosis prevention and therapy. We highlight new molecular targets of ginkgolide B, including nicotinamide adenine dinucleotide phosphate oxidases (NADPH oxidase), lectin-like oxidized LDL receptor-1 (LOX-1), sirtuin 1 (SIRT1), platelet-activating factor (PAF), proprotein convertase subtilisin/kexin type 9 (PCSK9) and others. Finally, we provide an overview and discussion of the therapeutic potential of ginkgolide B and highlight the future perspective of developing ginkgolide B as an effective therapeutic agent for treating atherosclerosis.

Trapped platelets activated in ischemia initiate ventricular fibrillation

BACKGROUND

We tested the hypothesis that ischemia-induced ventricular fibrillation (VF) is facilitated by platelets, trapped regionally in the ischemic zone and activated to release arrhythmogenic secretome.

METHODS AND RESULTS

In a randomized study in blood-free, buffer-perfused isolated rat hearts, ischemic zone territory (34±1% of left ventricle) was selected so that ischemia evoked VF in only 42% of controls. VF incidence was increased to 91% (P<0.05) by coronary ligation-induced trapping of freshly prepared autologous platelets (infused before and during coronary ligation, with trapping confirmed by 111In-labeled platelet autoradiographic imaging). Trapping of platelet secretome prepared ex vivo, or platelet-sized fluorospheres, did not increase ischemia-induced VF incidence. Secretome alone did, however, evoke VF in 2 sham coronary-ligated hearts. Perfusion did not activate infused platelets in sham coronary-ligated hearts, whereas ligation activated trapped platelets (assessed by thromboxane release). In a separate study, trapping whole-heparinized blood mimicked the ability of trapped platelets to increase VF incidence. This effect was not prevented by >5 days oral pretreatment in vivo with clopidogrel (10 mg/kg per day) or indomethacin (2.4 mg/kg per day).

CONCLUSIONS

Platelets facilitate VF during acute ischemia independently of their ability to participate in occlusive thrombosis. Moreover, the effect is unresponsive to antiplatelet drugs commonly used. Labile secretome constituents appear to be responsible. This opens a novel avenue for antiarrhythmic drug research.

Platelets and cardiac arrhythmia

Sudden cardiac death (SCD) remains one of the most prevalent modes of death in industrialized countries, and myocardial ischemia due to thrombotic coronary occlusion is its primary cause. The role of platelets in the occurrence of SCD extends beyond coronary flow impairment by clot formation. Here we review the substances released by platelets during clot formation and their arrhythmic properties. Platelet products are released from three types of platelet granules: dense core granules, alpha-granules, and platelet lysosomes. The physiologic properties of dense granule products are of special interest as a potential source of arrhythmic substances. They are released readily upon activation and contain high concentrations of serotonin, histamine, purines, pyrimidines, and ions such as calcium and magnesium. Potential arrhythmic mechanisms of these substances, e.g., serotonin and high energy phosphates, include induction of coronary constriction, calcium overloading, and induction of delayed after-depolarizations. Alpha-granules produce thromboxanes and other arachidonic-acid products with many potential arrhythmic effects mediated by interference with cardiac sodium, calcium, and potassium channels. Alpha-granules also contain hundreds of proteins that could potentially serve as ligands to receptors on cardiomyocytes. Lysosomal products probably do not have an important arrhythmic effect. Platelet products and ischemia can induce coronary permeability, thereby enhancing interaction with surrounding cardiomyocytes. Antiplatelet therapy is known to improve survival after myocardial infarction. Although an important part of this effect results from prevention of coronary clot formation, there is evidence to suggest that antiplatelet therapy also induces anti-arrhythmic effects during ischemia by preventing the release of platelet activation products.

Regulation of platelet-activating factor (PAF) activity in human diseases by phospholipase A2 inhibitors, PAF acetylhydrolases, PAF receptor antagonists and free radical scavengers

The aim of this review is to present recent findings indicating the likely involvement of platelet-activating factor (PAF) in human diseases, and possible ways of alleviating its harmful effects. PAF is a potent proinflammatory mediator and promotes adhesive interactions between leukocytes and endothelial cells, leading to transendothelial migration of leukocytes, by a process of juxtacrine intercellular signalling. This process leads to activation of leukocytes and the release of reactive oxygen radicals, lipid mediators, cytokines and enzymes. These reaction products subsequently contribute to the pathological features of various inflammatory diseases. The reactive oxygen radicals cause low density lipoprotein (LDL) oxidation which mediates the development of atherosclerosis. Oxidized LDL may damage cellular and subcellular membranes, leading to tissue injury and cell death. Among the therapeutic approaches considered are agents that inhibit/degrade proinflammatory mediators and thereby have anti-inflammatory and/or anti-atherogenic potential. These include inhibitors of phospholipase A2 activity, PAF-acetylhydrolases, PAF antagonists and free radical scavengers/antioxidants, the latter protecting against oxidized LDL-induced cytotoxicity.

Potential interactions between iloprost and SIN-1 on platelet aggregation and myocardial infarct size in vivo

Nitric oxide and prostacyclin are endothelial-derived vasodilators which inhibit platelet aggregation in a synergistic manner. Experiments were designed to examine whether 3-morpholino-sydnonimine (SIN-1) and iloprost have synergistic cardioprotective actions and whether their effects on infarct size are related to inhibition of platelet aggregation. Anaesthetized rabbits (n = 9-10 per group) were subject to 40 min myocardial ischaemia followed by 3 h reperfusion. Infarct size (percentage of area at risk) was not altered significantly by 3 microg kg(-1) min(-1) SIN-1 (29.7 +/- 1.9%), but was reduced by 0.03 microg kg(-1) min(-1) iloprost (24.6 +/- 1.6%) and to a greater extent by the combination of SIN-1 and iloprost (18.8 +/- 1.7%), compared to controls (33.6 +/- 4.7%). In control rabbits, there were reductions in the ex vivo aggregation of platelets in response to ADP or collagen after ischaemia and reperfusion. SIN-1 and iloprost caused some alterations in platelet responses, but combined administration of both drugs did not produce greater effects. Although the reduction in myocardial infarct size was greatest with both drugs, this did not appear to be a synergistic interaction and was not dependent on the effects of the drugs on haemodynamics or platelet aggregation.

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.

Novel Insights Into the Pathogenesis and Therapy of Circulatory Shock

Several lines of evidence suggest central roles for nitric oxide, oxygen free radicals, cytokines, and adhesion molecules in circulatory shock. Lipid peroxidation fre quently accompanies ischemia-reperfusion injury; re cruitment of leukocytes exposed to ischemia-reperfu sion injury is largely dependent on leukocytes and endothelial cell adhesion interaction, enhanced nitric oxide stimulation by proinflammatory cytokines contrib utes to increased gut mucosal permeability in sepsis, and tumor necrosis factor (TNF) administration mimics the hemodynamic changes characteristic of circulatory shock. Recent advances in molecular and cellular biol ogy have paved the way for innovative approaches to the treatment of circulatory shock. Novel pharmaco logic approaches are being directed at three distinct levels of the inflammatory cascade: the inciting insult (ie, endotoxin), the mediators (ie, TNF, interleukin-1, oxygen free radicals), and the effector cells (ie, neutro phils). The intent of this review is to discuss the biology of mediators in circulatory shock as well as the experi mental and clinical evidence implicating their role in development and pathogenesis of the injury. Current status of novel adjunctive therapies for circulatory shock is reviewed.

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.

Platelet activation during myocardial ischaemia: a contributory arrhythmogenic mechanism

Experimental and clinical observations of the involvement of platelets in the pathophysiology of myocardial ischaemia indicate the importance of interactions between these formed elements and the heart. The aim of this review is to outline evidence linking platelet activation, myocardial ischaemia and infarction, and to present evidence for a link between platelet activation, arrhythmogenesis and sudden death. A brief review of platelet physiology and pharmacology is provided, with a review of the cardiac electrophysiological effects of ischaemia and the electrophysiological effects of platelet-derived substances. The concept that platelet activation during myocardial ischaemia is a contributory arrhythmogenic mechanism is discussed.

Effects of platelet-activating factor antagonists WEB 2086 and BN 50730 on digoxin-induced arrhythmias

Effects of platelet-activating receptor antagonists WEB 2086 (1.0-30.0 mg.kg-1 intravenously) and BN 50730 (10.0 mg.kg-1 intravenously) alone or in combination with CGS 8515 (a specific 5-lipoxygenase inhibitor, 0.3 mg.kg-1 intravenously) and Dazmegrel (a thromboxane synthase inhibitor, 1.0 mg.kg-1.hr-1 intravenous infusion) on digoxin-induced arrhythmias were investigated in anaesthetised guinea-pigs. ECG, mean arterial blood pressure, heart rate and arrhythmias were recorded, starting 30 min. before digoxin administration and continuing for 60 min. afterwards. WEB 2086 (10.0 mg.kg-1 intravenously) reduced the mortality rate and arrhythmia score significantly compared to the control values. However, in combination with CGS 8515, it did not affect the mortality rate. BN 50730 (10.0 mg.kg-1) reduced the incidence of ventricular fibrillation and also arrhythmia score. BN 50730 in combination with Dazmegrel was reduced the arrhythmia score, incidence of ventricular fibrillation and mortality rate significantly, compared to control values. Digoxin-induced acute rise in mean arterial blood pressure was not affected by any of drug treatment except WEB 2086 (10.0 mg.kg-1) in combination with CGS 8515. Heart rate values did not differ between groups. However, pressure-rate index was reduced by WEB 2086 alone or in combination with CGS 8615. Results showed that although two different platelet-activating factor antagonists have different effects on the incidence of ventricular fibrillation and mortality, they improved the digoxin-induced arrhythmias when they were used either separately or in combination with CGS 8515 or Dazmegrel by implicating that platelet-activating factor has a role on digoxin-induced arrhythmias.

Dual inhibition of platelet-activating factor and arachidonic acid metabolism by ajmaline and effect on carrageenan-induced rat paw oedema

The effects of ajmaline on human platelet aggregation, arachidonate metabolism and platelet activating factor (PAF)-induced lethality in rabbits were examined. Platelet aggregation induced by several stimuli (ADP, collagen, and PAF) was inhibited by increasing concentrations of ajmaline. The potency of ajmaline was higher when PAF was employed as stimulating agent in comparison with other agonists (IC50 70, 270 and 380 microM for PAF, ADP and collagen, respectively) whereas ajmaline had no effect against arachidonic acid-induced aggregation. In contrast however, ajmaline inhibited arachidonate metabolism by platelet homogenates. The formation of both thromboxane A2 and 12-hydroxy-eicosatetraenoic acid was inhibited by ajmaline with comparable potencies. Pretreatment of rabbits with ajmaline (50 mg kg-1) completely abolished the lethal effects of PAF (11 micrograms kg-1) given intravenously (P < 0.001). In addition, ajmaline at doses ranging from 50 to 100 mg kg-1 inhibited carrageenan-induced rat paw oedema (P < 0.001). In this test ajmaline was three times more potent than aspirin. In the light of these results we conclude that ajmaline, a known anti-arrhythmic agent is a PAF antagonist and a dual inhibitor of platelet cyclo-oxygenase and lipoxygenase enzymes with anti-inflammatory properties.

Pathophysiology and mediators of ischemia-reperfusion injury with special reference to cardiac surgery. A review

Although necessary for the ultimate tissue survival, reperfusion may paradoxically exacerbate the ischemic injury. Ischemia and reperfusion injury is intimately woven together. The relative role of reperfusion injury is not clarified and probably varies with the ischemic insult: Reperfusion is always preceded by ischemia, and some of the reperfusion-related events may represent a process continuing from the ischemic period; thus the proper designation should be ischemia-reperfusion injury. The reperfusion-related events are: arrhythmias, myocardial stunning with both systolic and diastolic dysfunction, and low reflow and microvascular stunning. Of pathogenetic importance are the mode and speed of reperfusion as well as the initiation of an intracoronary inflammatory reaction during reperfusion, including endothelium-leukocyte interaction, platelets, generation of oxygen free radical, generation and release of arachidonic acid metabolites, platelet activating factor, endothelium derived relaxing factor, endothelins, kinins, and histamine, complement activation, disturbances in calcium homeostasis, and disturbances in lipid and fatty acid metabolism.

Variations of blood PAF-acether levels during coronary artery surgery

Extracorporeal circulation (ECC) is associated with thrombocytopenia and transient leukopenia. After ECC and coronary artery bypass graft (CABG) surgery, some patients can develop pulmonary and cardiac dysfunction, which might be related to the release of various mediators such as thromboxane A2, C5a, and C3a anaphylatoxins. The involvement of PAF-acether (PAF), a potent vasoactive thrombocytopenic and leukoneutropenic agent, has not been determined. Therefore, 10 patients were studied during and after CABG. The release of PAF, lipo PAF (PAF bound to blood lipoproteins), and lyso PAF (PAF precursor and metabolite) was measured in blood from the left atrium, radial artery, and pulmonary artery before and after CABG. PAF, lipo PAF, and lyso PAF were also determined during ECC at the entry and exit points of the oxygenator. Hemodynamic parameters, platelet, and leukocyte counts in the pulmonary artery were measured simultaneously. PAF did not increase significantly during ECC; it showed a transitory six-fold increase immediately after CABG in the radial artery (0.18 +/- 0.13 v 1.09 +/- 0.36 ng/mL, P < 0.05), but not in the pulmonary artery (0.10 +/- 0.03 v 0.56 +/- 0.21 ng/mL, P > 0.05). Blood PAF amounts in the radial artery were significantly higher than in the left atrium following ECC (1.09 +/- 0.36 v 0.06 +/- 0.04, P < 0.05), probably indicating PAF production in the heart. No variation of blood lipo PAF and lyso PAF was observed. No correlation was seen between PAF amounts and blood cell count.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Interactions of prostanoids with the platelet activating factors

The platelet-activating factor (PAF) induced a marked increase of the thromboxane (TX) B2-formation in the incubation medium of isolated myocardium and tissue from other organs. The content of the 6-oxo-prostaglandin (PG)F1 alpha, the inactive metabolite of PGI2, remained uninfluenced or showed a small decrease. PAF, given in a concentration of 2.10(-9) mol/l or a single dose of 100 ng, significantly reduced the contraction force and the coronary flow of isolated guinea-pig hearts. This effect was connected with a high efflux of TXA2. The PAF-antagonist, WEB 2086, nearly abolished the cardiac effects of PAF, and iloprost or a pretreatment with indomethacin markedly reduced the PAF-influence on the heart. The TXA2-antagonist BM 13177 was ineffective. The results indicate a close interaction between the myocardial PAF-effect and the TXA2-formation of the heart tissue, but gave no suggestion for a mediation of the PAF-effect by TXA2. The PAF-antagonistic action of WEB 2086, iloprost and indomethacin could be of some interest in the therapy of cardiovasculatory diseases.

The effects of a PAF antagonist on ischemic myocardial damage and arrhythmia in the dog

Myocardial ischemia is associated with accumulation of lyso-phospholipids, including lyso-platelet activating factor, the degradation product and precursor of platelet activating factor. These compounds produce cellular and microvascular damage and, in the myocardium, depression of contractility and arrhythmia. The potent platelet activating factor antagonist, WEB 2086, or placebo, was infused (IV) 10 min before constriction of the proximal left anterior descending coronary artery in open-chest dogs. Two protocols were followed: the dose of WEB 2086 was 0.5 mg/kg in those subjected to 20 min ischemia with 10 min reperfusion (n = 40) and 5 mg/kg preceding 60 min ischemia alone (n = 24). There was no significant difference in the number of ventricular premature complexes between WEB 2086 and placebo treated dogs during either period of ischemia. On reperfusion in those surviving 20 min of ischemia, 5 of the 18 WEB 2086 and 9 of the 18 placebo treated dogs developed ventricular fibrillation (NS). After 60 min of myocardial ischemia, there was no statistical difference in histological changes (nuclear swelling, aggregation of chromatin, myofibrillar separation) between groups. Hence, no substantial effect of relatively large doses of WEB 2086 on ischemia-induced histological change or arrhythmia was found in this preparation.

Effect of BN 52256 and other mediator antagonists on ouabain-induced ventricular fibrillation in sensitized guinea-pigs and on ischemia-induced fibrillation in rats

The threshold dose of ouabain necessary to induce ventricular fibrillation is decreased in sensitized guinea-pigs signalizing an enhanced arrhythmogenicity. Esculetine or WEB 2170 antagonists of histamine and PAF, respectively and especially a combination of both can increase the threshold dose of ouabain-induced fibrillation demonstrating an antiarrhythmic effect. BN 52256 a substance with multi-antagonistic properties shows antiarrhythmic effect in this method, too. WEB 2170 and BN 52256 decrease the incidence of ventricular fibrillation after coronary ligation in rats.

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.