Phytomedicine Vol. 2 (3), pp. 235-242,1996

© 1996 by GustavFischer Verlag, Stuttgart· Jena . New York

Antagonistic effect of yangambin on platelet-activatingfactor (PAF)-induced cardiovascular collapse

E. v. TIBIRICA1, K. MOSQUERA1, M. ABREU1, R. RIBEIR01,F.A. S. CARVALH01, J.M. BARBOSA-FILH02, R. S. B. CORDEIR01

1 Departamentode Fisiologia e Farmacodinamica, Instituto OswaldoCruz, FIOCRUZ, Rio de Janeiro,Brazil.

2 Laboratorio de Tecnologia Farmaceutica, Universidade Federal da Paraiba,Paraiba, Brazil.

Summary

The cardiovascular protective effects of yangambin, a novel and specific naturally-occurringplatelet activating factor (PAP) receptor antagonist, were investigated in the pentobarbital anes­thetized and artificially ventilated rat. Yangambin (3-30 mg kg-l) as well as the reference PAP an­tagonist WEB 2086 (0.1-1.0 mg kg-l) prevented the circulatory collapse elicited by the intrave­nous administration of PAP (0.5Ilg kg-l), in a dose-dependent manner. Yangambin did not inter­fere with the hypotensive effect of several endogenous vasoactive mediators such as acetylcholine,bradykinin, histamine and serotonin. Moreover, when adminstered as a post-treatment the anta­gonist showed the ability to reverse the cardiovascular effects induced by PAP (1.0 ug kg'"). Theprotective effect of yangambin showed to have a duration of action of more than 2 hours. It isconcluded that yangambin is a selective antagonist of the cardiovascular effects of PAF and there­fore constitutes a potential therapeutic agent in different shock states where abnormal PAP re­lease is supposed to play an important role.

Key words: (+)-yangambin; Ocotea duckei VATTIMO (Lauraceae); PAP receptor antagonist;arterial hypotension.

Introduction

In the past few years a wide variety of platelet activatingfactor (PAP) receptor antagonists have been described (fora review Hosford et a!., 1989). These compounds can gen­erally be separated in three classes: i) the PAP-related com­pounds (CV-3988; CV-6209; ONO-6240); ii) synthetic an­tagonists unrelated to the chemical structure of PAF (thetriazolobenzodiazepines like WEB 2086 and WEB 2170)and iii) natural antagonists and their derivatives (kadsure­none; ginkgo Iide B) (Braquet et a!., 1987; Hosford et a!',1989; Handley, 1990).

Interest in PAP receptor antagonists results from the dem­onstration that this mediator has a wide range of biologicalactivities such as activation of platelets and polymorphonu­clear leukocytes, increase in vascular permeability, bron­choconstriction and induction of cardiovascular alterations

(Braquet et a!', 1987). In fact, the systemic administrationof PAP induces marked depressor effects on the cardiovas­cular system in different animal species, which are mainlycharacterized by systemic arterial hypotension (Caillard eta!', 1982; Bessin et a!', 1983; Casals-Stenzel et a!., 1987;Felix et al., 1990). This cardiovascular collapse induced byPAF has already been attributed to dilatation of resistancevessels (Blank et a!., 1979; Handa et a!., 1991; Yakamanaet a!., 1992), negative inotropic effects (Sybertz et a!., 1985;Robertson et a!., 1987, 1988; Delbridge et a!', 1994), pul­monary hypertension (Laurindo et a!', 1989) and decreasedblood volume (MacManus et al., 1980, 1981). Moreover, ithas also been reported that the release of PAF from activat­ed platelets and leukocytes is involved in the induction ofthe cardiovascular collapse observed in systemic anaphy­laxis (Levi et al., 1984; Terashita et a!', 1992) and septicshock (Casals-Stenzel, 1987; Muacevic and Heuer, 1992).

236 E. V. Tibiri~a et al.

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Despite the important progress obtained in the supportivetherapy of the endotoxin shock and consequent multipleorgan failure syndrome, the mortality of this severe patho­logical state remains unacceptably high (Deitch, 1992).

Thus, the pharmacological antagonists of this potent lip­id mediator can be considered of great value as potentialtherapeutic agents.

Here, we describe the pharmacological actions of yan­gambin (Fig. 1) - a novel natural-type PAF receptor antag­onist isolated from a Brazilian plant for its ability to antag­onize the cardiovascular effects of PAF in the anesthetizedrat.

Methods

Animals and hemodynamic measurements

Wistar male rats (from the Oswaldo Cruz Foundationbreeding farm) weighing from 250 to 350 g were anesthe­tized with sodium pentobarbital (75 mg kg') administeredby the intraperitoneal (i. p.) route; anesthesia was comple­mented by another i. p. injection of 15 mg kg! of pentobar­bital before the control period and when necessary (see be­low). The rats were intubated with a polyethylene tube viaa tracheotomy and immobilized with pancuronium bro­mide (1 mg kg>, i. v.)with hourly supplemental doses of 0.2mg kg" and artificially ventilated with room air (tidal vol­ume 10ml kg:", stroke rate 45 min-I). The right jugularvein was catheterized to permit i. v. injections. The instanta­neous arterial pressure was continuously monitoredthrough a catheter placed in the right carotid artery con­nected to a Hewlett Packard quartz transducer (1290 A),which in turn was connected to a pressure processor and re­corder (Hewlett Packard 7754 system with 8805 B amplifi­er). Systolic (SAP) and diastolic (DAP) arterial pressureswere obtained directly from the recordings and mean arte­rial pressure (MAP) was calculated as diastolic pressureplus one-third of the differential pressure; heart rate (HR)was counted from the blood pressure waves by rapid run­ning of the pressure recording. After the completion of thesurgical procedures the animals were allowed to equilibratefor 15 min or until a stable tracing had been obtained (con­trol period). Before the injection of drugs was carried out,the means of 3 values of arterial pressures (SAP, DAP and

MAP) and HR recorded at 5 min intervals were calculatedand considered as the basal hemodynamic values.

Experimental protocol

After the baseline measurements (see above) the test sub­stances (one dose per animal for one drug) or the respectivevehicle were administered i. v. in a volume of 500 III kg'5 min before the i. v. injection of PAF (0.5Ilg kg-I). Eachagent was flushed with 50 III of saline solution. In order totest the specificity of yangambin, different vasoactive sub­stances (one substance per rat) were administered 5 min af­ter yangambin. In another series of experiments yangambinwas intravenously injected when the hypotensive effect ofPAF (Ltlug kg") has reached its maximum (30-60 sec).The maximum hypotensive effet elicited by PAF was de­fined as 100, and the percentage of inhibition or reversal ofthe PAF-induced hypotension by drugs was calculated. Inthe experiments designed to determine the time course ofthe protective effects of yangambin, the antagonist was ad­ministered i. v. via the penile vein under light ether anesthe­sia.

Control experiments were carried out by i. v. administra­tion of saline with 10% (v/v) tween 80 5 min before the in­jection of saline solution with 0.25% (w/v) bovine serumalbumin, as controls of yangambin and PAF injections, re­spectively.

Preparation of yangambin

Spectroscopically pure yangambin was obtained fromOcotea duckei Vattimo (Lauraceae) according to the proce­dures previously described (De Queiroz-Paulo et al., 1991).Leaves of O. duckei were collected in joao Pessoa, State ofParaiba, Brazil, and a voucher specimen is kept at the her­barium of the Federal University of Paraiba, Brazil. Experi­mental data confirm the identity of the isolate with (+)-yan­gambin also as previously described (MacRae and Towers,1985): melting point 123-125°C (from hexane/AcOEt);[a]D = + 50° in CHCI3; ElMS 70 eV,rn/z (%): 446 [M]+ (22),224 (16); 207 (59); 195 (63); 181 (100); 167 (10).

Drugs

The following drugs were used: sodium pentobarbital(Nembutal, Abott Lab., North Chicago, IL; U. S.A.); pan­curonium bromide (Pavulon, Organon Tecknica, Fresnes,France); WEB 2086 3-4-(2-chlorphenyl-)-9-methyl-6 H­thieno- 3,2-f-l ,2,4-triazolo-4,3-a-1,4-diazepin-2-yl-(4-mor­pholiny1)-I-propanone (Boehringer Ingelheim, FRG); ace­tylcholine, histamine, bradykinin, noradrenaline and 5-hy­droxytryptamine (Sigma Chemical CO, St Louis, MO).PAF (1-0-hexadecyl-2-acetyl-sn-glyceryl-3-phosphorylcho­line, powder from Sigma) was dissolved and further dilutedin saline containing 0.25% bovine serum albumin.

Antagonistic effect of yangambin on platelet-activating factor (PAF)-induced cardiovascular collapse 237

All drugs were dissolved in normal saline solution (0.9%NaCI), with the exception of yangambin which was dis­solved in 10% (v/v) tween 80 in saline solution.

Statisticalanalyses

All results are expressed as means ± S. e. m. for eachgroup. The effects of the different treatments on baselinehe­modynamics over time were analyzed by repeated measuresanalysis of variance (ANOVA) while comparisons betweenthe control group (saline-injected) and treated groups (PAFalone or antagonist plus PAF) were made with one wayANOVA. When an overall difference was detected byANO­VA, the Newman-Keuls test was used to localize the statis­tically significant differences. Differences with P values ofless than 0.05 were considered as significant. All the calcu­lations were made by computer-assisted analyses using acommercially available statistical package (Graphpad In­stat, Graphpad Software, University of London, UK).

Results

Inhibitory effects of a pretreatment with the antagonistson PAP-induced systemic hypotension in anesthetized rats

Control experiments and baseline values of the cardio­vascular parameters: The i. v, injection of saline solutionwith 10% (v/v) tween 80 or with 0.25% (w/v) bovine ser­um albumin as vehicles of yangambin and PAF, respective­ly, never induced significant effects on the cardiovascularparameters throughout the recording period. The basal(preinjection) values of mean arterial pressure (MAP) andof heart rate (HR) were not significantly different betweenall the experimental groups, as shown by analyses of vari­ance.

PAP experiments: The i. v. administration of PAF (0.5Ilgkg- l ) to anesthetized rats induced sudden and short-lastingarterial hypotension, which was followed by a recovery ofbasal levels in about 10 min (fig. 2). The hypotensive re­sponse reached its maximum between 30 and 60 sec, whenthe fall in MAP was of 53 ± 5% below baseline levels of125 ± 4 mmHg (n=8, P < 0.05) (Table 1). The arterial hypo­tension was not accompanied by a reduction in heart rate(416 ± 7 bpm before and 422 ± 10 bpm after PAF injection;n=8; n s.).

Yangambin experiments: The prior i. v. administration ofyangambin (3-30 mg kg:", one dose per rat) dose-depen­dently blunted the PAF-induced hypotension (Fig. 2 A). Themaximum fall in MAP, that was of 53 ± 5% (n=8) in the sa­line-injected group was of 42 ± 8% (n=5), 33 ± 6% (n=7)and only of 13 ± 4% (n=5) in the groups of animals pre­treated with doses of yangambin of 3, 10 and 30 mg kg-!,i. v., respectively (Table 1 and Fig. 2). These doses of yan­gambin also reduced the duration of the arterial hypoten­sion elicited by PAF in a dose-related manner (Fig. 2 B).

WEB 2086 experiments: Five min after i. v. injection,WEB2086 at 0.1, 0.3 and 1.0 mg kg' also inhibited PAF­induced hypotension in a dose-related manner: the maxi­mum fall (1 min) in MAP was of 53 ± 5% (n=8) in the non­treated group of animals, while it was of 20 ± 7 (n=5), 28 ±

7 (n=5) and only of 3 ± 2 (n=5) in the groups pre-treated(one dose per rat) with WEB 2086 with doses of 0.1, 0.3and 1.0 mg kg-!, respectively (Fig. 2 C).

Reversing effects of the antagonists on PAP-inducedsystemic hypotension in anesthetized rats

PA P experiments: A higher dose of PAF was used in thisexperimental group (1.0 ug kg") in order to obtain a long­er-lasting hypotensive effect, that is more suitable to the ex-

Table 1. Effects of intravenous PAF (0.5 Ilg kg' ) on the cardi ovascular parameters of pentobarbital-anesth etized and artificially-ventilat­ed Wistar rat s, in the pr esence or absence of a pre-treatment with yanga mbin.

MEAN ARTERIAL PRESSURE (mmHg)Yangambin Doses (mg kg-1) Basal Values After Yangambin (5 min) After PAF (1 min)

o(n=8) 125 ± 4 122 ± 4.5 58 ± 5"3 (n=5) 115 ± 10 110± 9 65 ± 12"10 (n=7) 127 ± 7 125 ± 12 81 ± 7*30 (n=5) 121 ± 3 110± 8 96 ± 8*+

HEART RATE (bpm)Yangambin Doses (mg kg-I ) Basal Values After Yangambin (5 min) After PAF (1 min)

o(n=8) 416 ± 7 N.D. 422 ± 103 (n=5) 393 ± 16 399 ± 17 390 ± 1710 (n=7) 407 ± 12 410 ±11 418 ± 1430 (n=5) 412 ± 5 404 ± 4 404 ± 4

Values are expressed as means ± S. E. M.; n = number of experiments.., P < 0.05; significant difference when compared to baseline values (ANO VA for repeated measures).,+ P < 0.05: significant difference versus non treated group (AN OVA)N. D. = not determined

238 E. V. Tibirica et al.

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periments dealing with the reversion of the effects of PAF

on arterial pressure. Indeed, this dose of the mediator in­duced a more intense and rather stable (5 min) reduction in

MAP, reaching a maximum of 64 ± 2% (n=12; P < 0.05)

from the basal values of 117 ± 3 mmHg (Fig. 3 A). The

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Antagonistic effect of yangambin on platelet-activating factor (PAF)-induced cardiovascular collapse 239

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Fig.4. Specificity of the antagonist activity of yangambin on arte­rial hypotension induced by PAFin pentobarbital anesthetized Wi­star rats. The hypotensive effect of i. v. injected PAF (0.5 ug kg'),histamine (Hist, 100 ug kg"), bradykinin (Brad, 10 ug kg'"), ace­tylcholine (Ach, 0.5 ug kg-]) and of S-hydroxytryptaminc (5-HT,25 ug kg') were compared after i. v. injection of the vehicle (opencolumns) or of yangambin (30 mg kg:", cross-hatched columns).Columns represent the mean :t s. e. mean of 4-7 experiments.'f P < 0.05: significantly different when compared to the non-treat­ed group.

MAP in a dose-related manner (Fig. 3 A). The doses of theantagonist of 6.25,12.5 and 25.0 mg kg-! (one dose per rat)induced a 51 ± 11 %,101 ± 5% and 81 ± 9% reversal of ar­terial hypotension, respectively. It is noteworthy that themaximum hypotensive effect of PAF was virtually the samein all experimental groups (Fig. 3 A and 3 B), reaching thevalues of 42 ± 2.5,62 ± 5, 40.5 ± 3 and 47 ± 1.5 mmHg forcontrol (n=12) and yangambin 6.25 (n=5), 12.5 (n=5) and25.0 (n=5) mg kg'", respectively.

WEB 286 experiments: The PAF receptor antagonistWEB 2086 was used in this same experimental paradigm asa reference. Post-treatment with WEB 2086 (0.1, 0.3 and1.0 mg kg:", i. v., one dose per rat) also reversed the hypo­tensive effect induced by PAE The percentage reversal was83 ± 5, 106 ± 19 and of 92 ± 15% for doses of 0.1,0.3 and1.0 mg kg:", respectively (Fig. 3 B).

Specificityof the antagonistic action of yangambin

The maximum reduction in MAP induced by the i. v. in­jection of various endogenous vasoactive substances beforeand 5 min after (in separate groups of experiments) the in­jection of yangambin (30 mg kg-I, i. v.) were as follows(n=4-7) (Fig. 4): acethylcholine (0.5 ug kg"): 36 ± 3% and35 ± 3%; bradykinin (IOug kg'), 34 ± 2% and 34 ± 5%;histamine (100 ug kg-!): 40 ± 2 % and 41 ± 4.5 and seroto­nin (251lg kg') 39 ± 1% and 34 ± 2%. Thus, yangambinshowed no inhibitory effects on the blood pressure lower­ing effects induced by these various vasoactive substances(Fig. 4).

Fig. 5. The time course of the protective effects of yangambin(30 mg kg! i. v.) against the arterial hypotension induced by i. v.injected PAF (0.5 ug kg-]) in pentobarbital anesthetized Wistarrats. C =controlEach set of points represents the mean :t s. e. mean of 6-9 experi­ments.

We also tested wether the very short-lasting hypotensiveeffect of i. v. injected yangambin (30 mg kg-!) was due to apartial agonist activity of the compound. This dose of yan­gambin elicited a reduction in MAP that reached the maxi­mum of 13 ± 4% below baseline levels 1 to 2 min after in­jection, rising thereafter to attain pre-drug values in 5 min.Pre-treatment with WEB 2086 (1.0mg kg', i. v.), a dose ofthe antagonist that completely inhibits PAF-induced hypo­tension, did not show the ability to inhibit the hypotensiveeffect of yangambin. Thus, yanbambin does not seem topresent any PAF receptor partial agonist effect in the car­diovascular system.

Temporal courseof the protective effects of yangambinon PAF-induced arterial hypotension

The i. v. administration of yangambin 5, 30, 60 and180 min before PAF provided a statistically significant pro­tective effect on the cardiovascular inhibitory effects of themediator. The maximum fall in MAP, that was of 53 ± 5%(n=8) in the saline-injected group was of 13 ± 4% (n=5), 4± 3% (n=7) 10 ± 4.5% (n=9) and of 26 ± 8% (n=9) in thegroups of animals pretreated with yangambin 5, 30, 60 and180 min before PAF (0.5/-lg kg"), respectively (Fig. 5). Inthe group of animals that was treated with yangambin360 min before PAF, the reduction of MAP was of 41 ±

2.5% (n=5), and the inhibitory effect of the antagonist didnot reach statistical significance.

Discussion

The results of the present study demonstrate that yan­gambin is a specific inhibitor of PAF-induced cardiovascu­lar effects.

240 E. V. Tibiri~a et al.

It is well-known that the i. v. administration of ratherlow doses of PAF « Lug kg:") induces severe cardiovas­cular alterations in different animal species, which aremainly characterized by a decrease in arterial blood pres­sure (Caillard et al., 1982; Bessin et al., 1983, Felix et al.,1990) a direct negative chronotropic effect (i. e., cardiode­pression) (Sybertz et al., 1985; Robertson et al., 1987,1988) and by a leakage of plasma from the microvascula­ture resulting from an increased vascular permeability(MacManus et al., 1980, 1981). Because of these pro­found inhibitory effects of PAF on the cardiovascularsystem - which mimic several pathophysiological featuresof the shock state - it has been suggested that PAF is oneof the important mediators involved in systemic anaphy­laxis (Nagaoka et al., 1991) and septic/endotoxic shock(Casals-Stenzel, 1987; Muacevic and Heuer, 1992). In thiscontext, some PAF antagonists have shown the ability toreduce the mortality rate observed after antigen challengein actively sensitized animals (Terashita et al., 1987) andalso to be effective not only as prophylactic but also ascurative agents in endotoxin-induced cardiovascular col­lapse in the rat and guinea pig (Bernat et al., 1992). In re­gard to human anaphylactic reactions, there is a substan­tial body of clinical evidence suggesting that the heartcould be the primary target organ of the endogenous me­diators that are released in these pathological conditions.For instance, some patients exposed to antigen challengedevelop cardiac arrhythmia and circulatory failure beforethe appearance of respiratory distress symptoms (Booth,1970; Austen, 1978; Hirsch, 1982).

In the past few years, some PAF antagonists which are po­tentially useful therapeutic agents in asthma, inflammatoryand allergic diseases have reached the last step of the drugdevelopment process (Snyder, 1990). In this context, plant­derived PAF receptor antagonists i. e., the natural ginkgo­lides (BN 52021 and BN 52063) - that are currently inphase 2 clinical trials, have been shown to be able to reducethe alterations of the pulmonary function elicited by expo­sure to antigen in asthmatic patients (Braquet and Hosford,1991; Guinot et al., 1987). Likewise, a recent multicenter,double-blind and placebo-controlled clinical trial showedthat the administration of BN 52021 during 4 days to pa­tients in septic shock due to gramnegative bacterial infec­tion, induced a significant reduction of the mortality rate (P.Braquet, personal communication). These preliminary find­ings warrant the search for new compounds with PAF an­tagonist activity and low toxicity that are potential thera­peutic agents in the above mentioned pathological states.

Yangambin and its enanti orner epiyangambin were se­lected from a series of lignans isolated from Brazilian plants(De Queiroz-Paulo et al., 1991; Mesquita et al., 1988; Panet al., 1987) for their ability to inhibit PAF-induced rabbitplatelet aggregation (Castro-Faria-Neto et al., 1993). Epi­yangambin also showed the ability to block PAF-inducedthrombocytopenia in rats (Castro-Faria-Netc et al., 1993).

Previous results from our group showed that yangambin,which is a furofuran lignan, competitively displaces[3H]PAF binding (ICso of 1.93 ± 0.53IlM) on rabbit plate­let membrane preparations. Yanbambin also dose-depen­dently and selectively inhibited PAF-induced in vitro rabbitplatelet aggregation (pA2 of 6.45) without decreasing themaximal response. By contrast, yangambin was not ca­pable of inhibiting PAF-induced in vitro rabbit neutrophilchemotaxis, an effect that was prevented by the PAF recep­tor antagonist SR 27417 (Castro-Faria-Neto et al., 1995 a).Yangambin (10 and 20mg kg', i. v.) also dose-depententlyprevented the thrombocytopenia and cardiovascular col­lapse elicited by PAF (31lg kg", i. v.) in anesthetized and ar­tificially ventilated rabbits. Nevertheless, the neutropenicleukopenia elicited by this same dose of PAF in the same ex­perimental conditions was not prevented by the antagonist,whereas reference PAF antagonists (WEB 2086 andSR 27417) significantly inhibited the phenomenon. Theseresults suggest that yangambin is an antagonist that selec­tively blocks PAF receptors in platelets and in the cardio­vascular system (Castro-Faria-Neto et al., 1995 b).

The results of the present study show that yangambin - ina similar manner to the reference PAF antagonistWEB 2086 (Casals-Stenzel et al., 1987) - not only inhibitsbut also reverses the hypotensive effect of PAF in rats, in adose-dependent manner. Indeed, the drastic but reversiblefall in mean arterial pressure elicited by PAF that reached amaximum of more than 50% below preinjection levels, wasvirtually abolished by pretreatment with yangambin. Nev­ertheless, the systemic administration of PAF in our experi­mental conditions (intact animal) did not induce any de­crease in heart rate. Several research teams have already re­ported that PAF induces a negative chronotropic effect.However, these studies employed in vitro preparations asthe isolated perfused heart (Mickelson et al., 1988) and al­so isolated strips of cardiac muscle of animals and man(Camussi et al., 1984; Robertson et al., 1987, 1988). Theabsence of a negative chronotropic effect of PAF in ourstudy could be attributed to the deactivation of the barore­ceptor reflex resulting from the fall in arterial pressure in­duced by PAF, which induces reflex tachycardia that wouldhave masked a direct PAF-induced heart rate reduction.

The blocking effect of yangambin is specific for PAF re­ceptor-mediated hypotension: Indeed, the decrease in arte­rial pressure elicited by other endogenous mediators as his­tamine, serotonin, acetylcholine and bradykinin cannot beabolished by a pretreatment with yangambin. The latter re­sults also rule out the possibility that yangambin could beinterfering with the synthesis and/or release of other va­soactive mediators elicited by the PAF injection. Moreover,it has already been reported that the hypotensive effect ofPAF is neither mediated by histaminergic or cholinergic re­ceptors nor by cyclooxigenase products of arachidonic acidmetabolism (Terashita et al., 1983).

The higher dose of yangambin used in our study (30 mg

Antagonistic effect of yangambin on platelet-activating factor (PAF)-inducedcardiovascular collapse 241

kg") induced significant but short-lasting decreases in arte­rial pressure. Thus, we also tested the hypothesis accordingto which yangambin could be a partial agonist of the PAFreceptor, at least in the cardiovascular system. This is notthe case, as the blood pressure lowering effects of yangam­bin are not antagonized by WEB 2086 in doses that com­pletely block PAF-induced hypotension. This hypotensiveaction also cannot be due to an ucadrenergic blocking ef­fect, as yangambin did not attenuate the hypertensive effectof an i. v. infusion of noradrenaline (data not shown). Any­how, intermediate doses of the antagonist were able to sig­nificantly block the cardiovascular inhibitory effects of PAFeven when arterial pressure had already returned to the pre­injection levels.

Most interestingly, the administration of yangambin assingle i. v. doses at the peak of the hypotensive effect elic­ited by PAF, potently and rapidly reset hemodynamic pa­rameters to pre-injection levels. This is a rather importantfeature of the antagonist as it could exhibit beneficial ef­fects in the early treatment of established circulatory dis­tress states where PAF is supposed to be involved.

In conclusion, yangambin is a selective blocker of PAF-in­duced cardiovascular alterations that will be useful in theconfirmation of the involvement of PAFin the cardiovascu­lar collapse that characterizes the lethal course of anaphy­lactic and septic shocks.

Acknowledgements

We would like to thank the International Foundation for Sci­ence, Sweden (Research Grant nr. F2112-1), the Oswaldo CruzFoundation, Brazil (PAPES grants) and FAPER], Brazil, for finan­cial support. We are also grateful to Mr. Edson Alvarenga (grantfrom the Conselho Nacional de Desenvolvimento Cientifico eTecnol6gico - CNPq - Brazil) for technical assistance.

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Address

E. V. Tibirica, Departamento de Fisiologia e Farrnacodina­mica, Instituto Oswaldo Cruz, Fiocruz, Av. Brasil 4365­Mangu inhos, Caixa Postal 926, 21045-900 - Rio de Janei­ro, Brazil.