Antagonists of PAF and histamine inhibit ouabain-induced cardiac arrhythmias in sensitized guinea-pigs

Platelet-activating factor (PAF) seems to be involved in different pathophysiological conditions, including cardiac arrhythmia. The arrhythmogenic potency of PAF has been shown experimentally by different methods. PAF antagonists inhibit the PAF-induced enhanced arrhythmogenicity. The present paper demonstrates that the threshold dose of ouabain-induced arrhythmia is decreased in sensitized guinea-pigs. Antagonists of PAF (BN 52021, WEB 2086, WEB 2170) and histamine antagonist clemastine can increase the threshold dose of ouabain-induced arrhythmia. A combination of WEB 2170 and clemastine, each of the drug is a low dose which is without effect when applying one of them only, shows a highly significant antiarrhythmic effect in this method. The threshold dose of ouabain necessary to induce ventricular flutter was increased from 89 micrograms/kg to 129 micrograms/kg and the threshold dose of ventricular fibrillation was enhanced from 101 micrograms/kg to 137 micrograms/kg. In dependence on the pathophysiological conditions, different mediators seem to be involved in the occurrence of cardiac arrhythmia. Therapeutically influencing these mediators could be a real chance to optimise the treatment of such kind of cardiac arrhythmias.     

Protective effect of WEB 2086, a novel antagonist of platelet activating factor, in endotoxin shock

WEB 2086, a novel specific platelet activating factor (PAF) antagonist derived from triazolodiazepines, inhibited in a dose-related manner the hypotensive and lethal effect of PAF as well as of E. coli endotoxin in the rat. The hypotension induced by endotoxin (15 mg/kg i.v.) or PAF (30 ng/(kg X min) i.v.) in anaesthetized rats was prevented by oral (1-10 mg/kg) and inhibited or reversed by i.v. (0.1-5.0 mg/kg or 0.1-1.0 mg/kg) doses of WEB 2086. Similar oral and i.v. doses of WEB 2086 protected conscious rats from PAF (15 micrograms/kg i.v.)- and endotoxin (7.5 mg/kg i.v.)-induced death. The results obtained with WEB 2086 confirm that PAF has an important role in the pathophysiology of endotoxin shock. This compound may have a therapeutic effect in human septic shock.     

Protective effects of dazmegrel on the PAF potential of ouabain-induced cardiac arrhythmias.

The ouabain threshold to induce cardiac arrhythmias in urethane-anaesthetized guinea-pigs was not modified by the administration of either dazmegrel, 4 mg/kg i.v., or lysine-acetylsalicylate, 13.5 mg/kg i.v., 5 min before the infusion of ouabain,10g/kg per min i.v. The previous administration of platelet-activating factor (PAF), 0.01 to 1 nmol/animal i.v., 2 min prior to ouabain, caused a significant, dose-dependent decrease of the ouabain threshold for the cardiac rhythm disturbances. Lysine-acetylsalicylate lacked any effect on this PAF potentiation. Pretreatment with dazmegrel 5 min before PAF, 0.05 nmol/animal i.v., abolished the PAF potentiation of the digitalis-induced arrhythmias. These results suggest that thromboxane synthesis is involved in this PAF effect and indicate an ability of PAF to induce thromboxane generation even in the case of cyclooxygenase inhibition.     

Effects of WEB 2086, a novel antagonist of platelet activating factor, in active and passive anaphylaxis

WEB 2086, a novel specific platelet activating factor (PAF) antagonist derived from triazolodiazepines, inhibited in a dose-related manner the PAF-dependent component of anaphylaxis as well as PAF-induced effects in mice and guinea pigs. In mice a lethal anaphylactic shock and a PAF-induced (100 micrograms/kg i.v.) death was inhibited by i.v. WEB 2086. The ED50 values were 13.6 and 0.37 mg/kg i.v., respectively. In actively sensitized guinea pigs, the anaphylactic lung reaction (bronchoconstriction), but not the corresponding hypotension, was prevented by oral (0.05-0.5 mg/kg) doses of WEB 2086. In contrast, in passively sensitized animals a dose-dependent inhibition of the pulmonary (bronchoconstriction) and blood pressure (hypotension) reaction due to anaphylaxis was achieved by i.v. WEB 2086. Similarly, oral (0.05-0.5 mg/kg) and i.v. (0.005-0.05 mg/kg) WEB 2086 inhibited PAF-induced reduction in respiratory flow (bronchoconstriction) and hypotension in guinea pigs. The ED50 values were 0.070 and 0.066 mg/kg p.o., and 0.017 and 0.015 mg/kg i.v., respectively. In conclusion, PAF seems to play a more major role in passive than in active anaphylaxis in guinea pigs. These results provide further evidence for an important role of PAF in anaphylaxis and support the hypothesis that PAF is involved in asthma and other allergic diseases.     

The effect of the specific PAF antagonist BN 52021 and the calcium blocker diltiazem on PAF induced arrhythmogenicity

Platelet-activating factor (PAF) in a concentration of 10(-11) mole per animal decreased the threshold doses for onsets of arrhythmia, ventricular flutters and fibrillation in ouabain induced arrhythmia in guinea-pigs in a statistically significant manner. The specific PAF antagonist BN 52021 (20 mg/kg, orally) completely inhibited the PAF effect for all types of arrhythmia, while the Ca2+ antagonistic drug diltiazem (0.1 mg/kg, i.v.) failed to counteract the PAF action. BN 52021 (20 mg/kg, per os) alone did not exert any effect on ouabain induced arrhythmia, but as expected, diltiazem (0.1 mg/kg, i.v.) showed antiarrhythmic effects. These results confirm the specific PAF antagonistic activity of BN 52021, its usefulness for PAF related cardiac rhythm disturbances and indicate that the method used could be a further useful tool to screen PAF antagonistic substances.     

Characterization of cardiovascular events mediated by platelet activating factor during systemic anaphylaxis.

Previous studies have shown that an anaphylactic reaction in the isolated perfused heart is characterized by drastic coronary constriction, arrhythmias, and severe impairment of contractility. In vivo anaphylaxis is associated with myocardial ischemia and rapid cardiovascular failure. Recently, not only histamine but also platelet activating factor (PAF) has been implicated in cardiac manifestation of anaphylaxis. The present study was designed to separate the effects of PAF from those of histamine on cardiovascular function during systemic anaphylaxis. In guinea pigs, sensitization was produced by subcutaneous (s.c.) application of ovalbumin. Fourteen days after sensitization, the effects of an intravenous (i.v.) infusion of ovalbumin were tested in anesthetized artificially ventilated guinea pigs. The renewed application of the antigen induced severe cardiac dysfunction. Within 3 min, cardiac output (CO) had already decreased by 90% and left ventricular end-diastolic pressure (LVEDP) increased significantly, indicating left ventricular pump failure. Concurrently, ECG recordings uniformly showed signs of acute myocardial ischemia. In addition, arrhythmias occurred in terms of atrioventricular block. After 4 min, blood pressure (BP) rapidly decreased. All animals died within 10 min. Pretreatment with the H1-receptor antagonist mepyramine (1 mg/kg i.v.) in combination with the H2-receptor antagonist cimetidine (10 mg/kg i.v.) delayed onset of myocardial ischemia, arrhythmias and cardiac pump failure. After 10 min, however, LV contractility and BP steadily decreased, leading to severe hypotension within 30 min. If the selective PAF antagonist WEB 2086 (1 mg/kg i.v.) was administered in addition to cimetidine and mepyramine, myocardial ischemia and LV contractile failure were markedly inhibited further. In contrast, pretreatment with WEB 2086 alone had no beneficial effects on the anaphylactic cardiovascular changes. Therefore, we conclude that histamine is the predominant mediator during the early phase of systemic anaphylaxis whereas PAF-mediated effects are involved in cardiac dysfunction during the protracted late phase of anaphylaxis.     

The role of nitric oxide and platelet-activating factor in the inhibition by endotoxin of pentagastrin-stimulated gastric acid secretion.

Administration of E. coli endotoxin (1 mg/kg i.v.) abolished the acid secretory response induced by a bolus injection of pentagastrin (100 micrograms/kg i.v.) in the continuously perfused stomach of the anaesthetized rat. Endotoxin administration did not modify mean systemic arterial blood pressure. Pretreatment with NG-nitro-L-arginine methyl ester (L-NAME; 5-20 mg/kg i.v.), but not dexamethasone (5 mg/kg s.c. twice) or indomethacin (5 mg/kg i.m.), substantially restored the secretory responses to pentagastrin. The actions of L-NAME were reversed by the prior administration of L-arginine (100 mg/kg i.v.), but not by its enantiomer D-arginine (100 mg/kg i.v.). L-NAME (10 mg/kg i.v.) increased blood pressure but this does not seem to be the mechanism by which endotoxin-induced acid inhibition was prevented, since similar systemic pressor responses induced by noradrenaline (15 micrograms/kg per min i.v.) had no such effect. The platelet-activating factor (PAF) receptor antagonist, WEB 2086 (2 mg/kg), induced a partial reversal of the inhibition by endotoxin of acid responses to pentagastrin. In endotoxin-treated rats, the combined administration of L-NAME (10 mg/kg) and WEB 2086 (2 mg/kg) completely restored the degree of H+ output induced by pentagastrin to levels similar to those of control, vehicle-treated animals. These findings suggest that endotoxin-induced acute inhibition of acid responses to pentagastrin involves NO synthesis and the release of PAF.     

Interference of azelastine with anaphylaxis induced by ovalbumin challenge in actively sensitized rats.

The inhibition of the haematological alterations and prevention of death due to systemic anaphylaxis after antigen challenge were investigated in rats after various drug treatments. The i.v. injection of ovalbumin (250 micrograms/kg) into actively sensitized rats induced marked thrombocytopenia and haemoconcentration within 5 min and significant leukocytosis within 30 min, lasting for 2 h after the challenge. Pretreatment with meclizine or terfenadine (15-30 mg/kg i.p.) inhibited antigen-induced haemoconcentration, whereas WEB 2086 (2-10 mg/kg i.p.) and PCA 4248 (5-10 mg/kg p.o.), two platelet-activating factor (PAF) antagonists, interfered with thrombocytopenia only. Azelastine (1-20 mg/kg p.o.) dose dependently inhibited antigen-induced haemoconcentration and thrombocytopenia but failed to block leukocytosis. Azelastine also inhibited the thrombocytopenia observed after the i.v. administration of PAF (4 micrograms/kg). Administration of ovalbumin at a dose of 1.5 mg/kg resulted in a lethal anaphylactic reaction in about 85% of the rats. Pretreatment with WEB 2086 (10 mg/kg i.p.), meclizine (30 mg/kg i.p.) or both increased the survival rate from 15 to 57, 68 and 87%, respectively. Azelastine alone (20 mg/kg p.o.) completely blocked the lethal reaction. It was concluded that the ability of azelastine to antagonize histamine and PAF is important for its effectiveness against anaphylactic shock.     

Platelet-activating factor antagonists in experimental shock.

Platelet-activating factor (PAF) at 10 micrograms/kg i.v. induced profound hyperkalemia, changes of hematological parameters, patterns of ECG, and acid-base balance in rats. In separate experiments infusions of PAF at 30 ng/kg/min or injection of endotoxin from E. coli (15 mg/kg i.v.) induced a marked drop in blood pressure. All these changes were antagonized by pretreatment or post-treatment (in case of hypotension) by the selective hetrazepinoic PAF antagonists apafant (WEB 2086, CAS 105219-56-5; 0.1-5 mg/kg i.v.), bepafant (WEB 2170, CAS 114776-28-2; 0.05-1.0 mg/kg i.v.), or STY 2108 (0.01-0.1 mg/kg), respectively. The results support the view that PAF can mimic features of endotoxin shock and that the hetrazepines (like apafant, bepafant) are useful tools to clarify the role of PAF in such conditions.     

Inhibition of platelet-activating factor fails to limit ischemia and reperfusion-induced myocardial damage.

To determine the role of platelet-activating factor (1-O-hexa-decyl-2-acetyl-sn-glyceryl-phosphoryl-choline, PAF) in myocardial ischemic and reperfusion-induced injury, the effects of a PAF receptor antagonist (WEB 2086) were studied in an anesthetized canine model of ischemia (90 min) and reperfusion (6 h). Thirty minutes after onset of ischemia, WEB 2086 was administered as a bolus (20 mg/kg intravenously, i.v.) followed by a continuous 6-h infusion (10 mg/kg/h i.v.). Controls received vehicle alone (0.9% saline). Platelet aggregation was studied at baseline and at 1, 2, 4, and 6 h of drug administration and at the end of the reperfusion period. WEB 2086 treatment did not significantly affect platelet aggregation stimulated by ADP or arachidonic acid (AA). After 1 h of drug infusion, the ex vivo aggregatory response to exogenous (200 nM) PAF was ablated in WEB 2086-treated animals. WEB 2086 administration did not affect heart rate (HR) or mean arterial blood pressure (MAP) during the occlusion or reperfusion phases. During reperfusion of the ischemic tissue, left circumflex coronary artery (LCX) blood flow of WEB 2086-treated animals increased (p < 0.05) above control value. The area of the left ventricle at risk of infarct was not different between control and WEB 2086-treated groups. Infarct size was not significantly reduced in WEB 2086-treated animals. The results of our investigation using a 90-min ischemic period followed by 6-h reperfusion show that pharmacologic antagonism of PAF by WEB 2086 does not protect the heart against ischemia and reperfusion-induced injury.     

Effect of BN 52256 and other mediator antagonists on ouabain-induced cardiac arrhythmia in a model of anaphylaxis in guinea-pigs.

Different mediators released by anaphylaxis seem to be involved in different pathophysiological conditions, including cardiac arrhythmia. Histamine, 5-HT and platelet-activating factor (PAF) could participate in the enhanced arrhythmogenicity during anaphylaxis in guinea-pigs. The threshold dose of ouabain-induced arrhythmia is decreased in actively sensitized guinea-pigs by i.p. administration of ovalbumin. The purpose of the present paper was to investigate the effect of different mediator antagonists. Antagonists of PAF (WEB 2170), histamine (clemastine) and 5-HT (cyproheptadine) in doses of 5.0 mg/kg, 5.0 mg/kg and 0.5 mg/kg, respectively, can increase the threshold dose of ouabain-induced arrhythmias signalling an antiarrhythmic effect. A combination of WEB 2170 and clemastine, each of them in inactive doses (2.0 mg/kg and 1.0 mg/kg, respectively) showed a statistically significant antiarrhythmic effect. A combination of the same dose of WEB 2170 and cyproheptadine (0.1 mg/kg) under the same conditions induced an antiarrhythmic effect, too. BN 52256 is a new antiallergic drug synthesized on the basis of a novel concept of combining inhibitory activity against various inflammatory mediators in one molecule. BN 52256 in doses of 20-80 micrograms/kg exhibited a statistically significant antiarrhythmic effect. BN 52256 needed a 12.5-125 fold lower dose to induce the same antiarrhythmic effect compared to the antagonists of PAF, histamine or 5-HT investigated in this study. Depending on the pathophysiological conditions, different mediators seem to be involved in the occurrence of cardiac arrhythmia. A complex inhibition of these mediators could induce a more specific influence on such kinds of cardiac arrhythmias.     

The booster injection of antigen during active sensitization of guinea-pig modifies the anti-anaphylactic activity of the PAF antagonist WEB 2086.

1. Serum IgG levels of sensitized guinea-pigs bled at various times after the booster injection were evaluated and its capacity to sensitize passively lung strips from normal guinea-pigs assessed. Following the booster injection, both serum IgG and the ability to sensitize passively lung strips increased during the first week and decreased slowly thereafter. 2. The PAF antagonist WEB 2086 (3 mg kg-1, i.v.) blocked the anaphylactic bronchoconstriction induced by intravenous administration of ovalbumin (1 mg kg-1) when guinea-pigs were challenged 2 and 4 days after the booster injection, but became ineffective when tested in guinea-pigs challenged 7, 28 and 56 days after the booster injection. 3. The ability of WEB 2086 to reduce anaphylactic bronchoconstriction of guinea-pigs challenged 2 and 4 days after the booster injection was unrelated to either the selective involvement of one type of immunoglobulin, low IgG titres in sera or a reduced sensitizing capacity. 4. The booster injection, which accounts for the loss of efficacy of WEB 2086 from the fourth day thereafter, probably operates as a PAF-independent inflammatory challenge. 5. The protocol for immunisation and the day of experiment after the booster injection determines the sensitivity of the anaphylactic bronchoconstriction to inhibition of PAF antagonists.     

Interactions between the autonomic nervous system and the cardiovascular effects of ouabain in guinea-pigs

Anaesthetized guinea-pigs were intoxicated with an intravenous infusion of ouabain. This infusion induced a marked pressor response which was reduced in bilaterally adrenalectomized or pithed animals. Ouabain produced initial bradyarrhythmias in 60% of guinea-pigs. Bilateral vagotomy or pretreatment with atropine abolished the bradyarrhythmias and sensitized the animals to the arrhythmic effects of ouabain. Pithing or beta-adrenoceptor blockade reduced the potency of ouabain for producing arrhythmias, but bilateral adrenalectomy did not give protection. Preferential alpha 2-adrenoceptor stimulation with clonidine (10-300 micrograms . kg-1 i.v.) also reduced the arrhythmogenic effects of ouabain, whereas no protection was found with St 91, a clonidine related compound which does not cross the blood-brain barrier. The effect of clonidine was antagonized by piperoxan. Preferential alpha 2-adrenoceptor blockade with piperoxan (6 mg . kg-1 i.v.) did not change the pressor response to ouabain, but sensitized the animals to the arrhythmogenic effects of ouabain. In contrast, the preferential alpha 1-antagonistic agent AR-C 239 (0.3 mg . kg-1 i.v.) abolished the pressor response to ouabain and in addition increased the dose of ouabain required to produced ventricular premature beats and ventricular fibrillation. These experiments indicate: (i) that ouabain produces in guinea-pigs a pressor response which seems to be due to catecholamine release from the adrenal medulla probably by an action on the central nervous system; (ii) that the ventricular arrhythmias induced by ouabain are due in part to stimulation of the central nervous system leading to an increase in beta-adrenoceptor activity; (iii) that central alpha-adrenoceptors appear to be involved in the arrhythmogenic effects of ouabain, as clonidine reduced these effects. On the other hand piperoxan, a preferential alpha 2-adrenoceptor antagonist did not change the pressor response to ouabain and increased the arrhythmogenic effects whereas AR-C 239 had opposite effects.     

Protective effects of yangambin on cardiovascular hyporeactivity to catecholamines in rats with endotoxin-induced shock

The protective effects of a new, selective, plant-derived platelet-activating factor (PAF) antagonist, yangambin, on the cardiovascular alterations and mortality due to endotoxic shock were investigated in anaesthetized rats. We also studied the involvement of PAF in the induction of the vascular and cardiac hyporesponsiveness to adrenergic stimulation observed during endotoxaemia. The animals were sensitized to the lethal effects of Escherichia coli lipopolysaccharide (LPS) with D(+)-galactosamine (50 mg/kg, i.v.) 15 min before LPS injection. LPS (3 mg/kg, i.v.) induced a progressive and marked decrease in mean arterial blood pressure from 85+/-4 to 30+/-3 mmHg and a reduction of cardiac output (CO) from 180+/-7 to 37+/-3 ml/min (120 min) accompanied by a maintenance of systemic vascular resistance, suggesting that cardiovascular collapse resulted mainly from myocardial depression. The maximum pressor responses to noradrenaline (0.3-3.0 microg/kg, i.v.) fell from 72+/-9 (control) to 5+/-1 mmHg (LPS) while the CO responses decreased from 81+/-5 to 8+/-3 ml/min. Pre-treatment with yangambin (30 mg/kg, i.v.) or with WEB 2086 (5 mg/kg, i.v.), a reference PAF receptor antagonist, completely prevented the LPS-induced cardiovascular collapse and abolished the sharp reductions of the arterial blood pressure and CO responses to noradrenaline observed during endotoxaemia. Post-treatment with yangambin 90 min after LPS administration did not reverse the arterial hypotension, cardiac failure or cardiovascular hyporesponsiveness to catecholamines. Finally, the acute (150 min) survival rates of endotoxic shock increased from 0% (LPS group) to 100% in the groups pretreated with either yangambin or WEB 2086. The long-term (7-day) survival also increased from 0% (LPS group) to 85% (yangambin pre-treatment group). In conclusion, these data suggest a role for PAF in the pathogenesis of endotoxin-induced vascular and cardiac hyporesponsiveness to catecholamines and confirm its involvement in the complex cascade of multiple mediators released during endotoxic/septic shock. Yangambin proved to be an effective pharmacological agent against cardiovascular collapse and mortality in endotoxin shock.     

Suppression of the Arthus reaction by Y-24180, a potent and specific antagonist of platelet-activating factor.

A novel and potent antagonist of platelet-activating factor (PAF), Y-24180 (4-(2-chlorophenyl)-2-[2-(4-isobutylphenyl)ethyl]-6,9-dimethyl-6H- thieno[3,2-f][1,2,4]triazolo[4,3-a][1,4] diazepine) was investigated for the effects on the skin reactions induced by chemical mediators and the Arthus reactions. In the rat dorsal skin, Y-24180 (0.1-10 mg/kg, p.o.) inhibited increase in vascular permeability by the intradermal PAF injection in a dose dependent manner and the inhibitory activity was 60 times more potent than that of WEB 2086. While even at doses as large as 10 mg/kg, p.o., it had no effect on vascular permeability in the rat skin induced by histamine, serotonin, bradykinin and leukotriene D4. On a reversed passive Arthus reaction in rat dorsal skin, Y-24180 (0.1-1 mg/kg, p.o.) markedly inhibited vascular permeability in a dose dependent manner and the inhibitory activity was 15 times more potent than that of WEB 2086. Y-24180 also inhibited the Arthus dermal reaction in rabbits (0.03-0.3 mg/kg, p.o.) and guinea pigs (0.1-1 mg/kg, p.o.). In addition, Y-24180 (0.1-10 mg/kg, p.o.) significantly reduced the exudate volume and the number of infiltrated inflammatory cells in the reversed passive Arthus pleural reaction in rats. Furthermore, in rat passive Arthus pancreatitis, Y-24180 (0.3-10 mg/kg, p.o.) significantly inhibited the dye extravasation from the pancreas. These results provide strong evidence that endogenous PAF plays an important role as a mediator in the type III allergic inflammation.     

Differential actions of platelet-activating factor (PAF) receptor antagonists on the vasodilator and vasoconstrictor effects of PAF in the rat perfused heart.

Selectivity for blocking the coronary vasodilator and vasoconstrictor effects of platelet-activating factor (PAF) in the rat perfused heart was observed with different PAF antagonists. CV-6209 showed selectivity for blocking the vasodilator effect of PAF and a higher concentration (10 fold) was required to block the vasoconstrictor effect. The remaining PAF antagonists (FR-900452, WEB 2086 and BN-50739) showed selectivity for blocking the vasoconstrictor effect of PAF (10, 200 and 1000 fold respectively). A combination of low concentrations of CV-6209 (10 nM) with FR-900452 (5 microM) or WEB 2086 (0.5 microM) was effective in blocking both the vasodilator and vasoconstrictor effects of PAF. CV-6209 and WEB 2086 did not affect the vasodilator action of leukotriene B4 (LTB4) and the vasoconstrictor action of LTC4 and LTD4. Our results support the hypothesis that the functionally opposite effects of PAF in the rat perfused heart may be mediated by different PAF receptor subtypes.     

Antiarrhythmic and hemodynamic effects of prifuroline

The antiarrhythmic activity of 4-(2-benzofuranyl)-2-(dimethylamino)-1-pyrroline (prifuroline) has been evaluated in rats, guinea-pigs and dogs. Prifuroline dose-dependently antagonizes the arrhythmogenic action of aconitine in rats, when administered either intravenously (5, 10 or 20 mg/kg) or intraduodenally (10, 20 or 50 mg/kg); it exhibits effectiveness by the digestive route at doses only twice as greater as the active i.v. doses: its intravenous anti-aconitine activity is comparable to that of disopyramide, and superior to that of quinidine; lidocaine is inactive in this test. Prifuroline also diminishes ventricular susceptibility to electrical stimulation in open-chest rats; its effect is comparable to that of disopyramide and amiodarone at the same dose levels; quinidine and lidocaine are less effective. Only prifuroline and propranolol were able to antagonize ouabain toxicity in guinea-pigs, quinidine showing only borderline activity, and disopyramide, lidocaine and verapamil being ineffective. In a model of arrhythmias induced by anoxic stress in rats, all the tested compounds were found active, with prifuroline and disopyramide providing complete protection at high dose levels. The arrhythmias induced in dogs by coronary artery ligation were markedly antagonized by prifuroline after doses of 5 and 10 mg/kg i.v. or 30 mg/kg intraduodenally; the duration of its antiarrhythmic activity in this model of arrhythmias in conscious dogs was much longer after intraduodenal than after i.v. administration. Prifuroline was also able to restore sinus rhythm in guinea-pigs after intracardiac conduction blockade with acetylcholine, although being devoid of anticholinergic activity. It also diminishes the maximal frequency of guinea-pig atria electrically stimulated in viro (EC25 = 5 X 10(-6) g/ml).(ABSTRACT TRUNCATED AT 250 WORDS)     

Role for intracellular platelet-activating factor in the circulatory failure in a model of gram-positive shock.

1. This study investigates the effects of two structurally different antagonists of platelet-activating factor (PAF), BN52021 and WEB2086, on the circulatory and renal failure elicited by lipoteichoic acid (LTA) from Staphylococcus aureus (an organism without endotoxin) in anaesthetized rats. 2. Administration of LTA (10 mg kg-1, i.v.) caused hypotension and vascular hyporeactivity to noradrenaline (1 microgram kg-1, i.v.) WEB2086 (5 mg kg-1, i.v., 20 min before and 150 min after LTA) inhibited the delayed fall in mean arterial blood pressure (at 300 min: 99 +/- 6 mmHg vs. 75 +/- 6 mmHg, P < 0.01) and prevented the decrease in pressor response to noradrenaline (at 300 min: 36 +/- 5 mmHg min vs. 17 +/- 5 mmHg min, P < 0.01). Surprisingly, BN52021 (20 mg kg-1, i.v., 20 min before and 150 min after LTA) neither prevented the hypotension (74 +/- 6 mmHg) nor the vascular hyporeactivity (21 +/- 5 mmHg min). However, BN52021 inhibited the hypotension to injections of PAF as well as the circulatory failure elicited by lipopolysaccharides (10 mg kg-1, i.v.). 3. LTA caused an increase in plasma concentration of creatinine from 39 +/- 5 microM (sham-operated) to 70 +/- 8 microM and urea from 4.7 +/- 0.1 to 13.1 +/- 1.6 mM. The renal failure elicited by LTA was significantly inhibited by WEB2086 (creatinine: 45 +/- 4 microM and urea: 5.7 +/- 0.7 mM), but not by BN52021. 4. The induction of nitric oxide synthase activity in lungs by LTA was attenuated by WEB2086 from 98 +/- 17 to 40 +/- 15 pmol L-citrulline 30 min-1 mg-1 protein (P < 0.01), but not by BN52021 (148 +/- 21 pmol L-citrulline 30 min-1 mg-1 protein). Similarly, WEB2086, but not BN52021, inhibited the increase in plasma nitrite concentration associated with the delayed circulatory failure caused by LTA. The release of tumour necrosis factor-alpha (TNF-alpha) after injection of LTA was not attenuated by WEB2086. 5. The induction of nitrite release by cultured macrophages activated with LTA (10 micrograms ml-1 for 24 h) was inhibited by 74 +/- 4% by WEB2086 (3 x 10(-4) M), but not by BN52021, indicating that only WEB2086 acts on intracellular PAF receptors. 6. Thus, the intracellular release of PAF contributes to the circulatory and renal failure and induction of nitric oxide synthase elicited by LTA in anaesthetized rats. The difference between the two structurally different PAF antagonists in our septic shock models using either LTA or lipopolysaccharide (LPS), shows the importance of models for Gram-positive sepsis in the elucidation of the pathophysiology of septic shock and for the evaluation of potential drugs.     

The antiarrhythmic effect of (-)-U-50,488 in rats with acute ischemia and reperfusion of heart is mediated by kappa1-opioid receptor activation

Pretreatment with a selective kappa1 opioid receptor (OR) agonist (-)-U-50,488 (1 mg/kg, i.v.) prevented the development of arrhythmias induced by occlusion (10 min) and reperfusion (10 min) in ketamine anesthetized rats, while the treatment with a less active enantiomer (+)-U-50,488 in the same dose produced no such effects. Preliminary intravenous administration of a selective kappa1 OR antagonist norbinaltorphimine (9 mg/kg) fully abolished the antiarrhythmic effect of (-)-U-50,488, while the kappa2 OR antagonist quadazocine (3 mg/kg) did not eliminate this effect. The injections of norbinaltorphimine or quadazocine alone did not influence the incidence of model arrhythmias caused by the occlusion and reperfusion. It was concluded that kappa1 OR stimulation favors an increase in cardiac tolerance to the arrhythmogenic action of occlusion and reperfusion.     

Platelet activating factor as a proinflammatory mediator in acetic-induced colitis in the rat.

Platelet activating factor (PAF) induces neutrophilia and produces a variety of gastrointestinal lesions. The role of PAF as a proinflammatory mediator was examined by measuring the production of PAF and the efficacy of the PAF antagonists WEB 2086 and Ro 24-0238 in acetic acid (HOAc)-induced colitis. PAF levels within the colonic mucosa, as measured by radioimmunoassay, increased from 259 +/- 119 ng/mg in control tissue, to 616 +/- 266 ng/mg in HOAc inflamed tissue. The accumulation of neutrophils within the mucosa was decreased by 53 +/- 10% by pretreatment with 3 mg/kg WEB 2086, and by 43 +/- 11% by 3 mg/kg Ro 24-0238. PAF-induced neutrophilia had no effect on the severity of HOAc-induced colitis, therefore, PAF my be involved in sustaining the chronic inflammation of colitis.