Oxygen-induced pulmonary vasodilation is mediated by adenosine triphosphate in newborn lambs

In the fetal lamb, oxygen-induced pulmonary vasodilation is attenuated by the combined use of purinergic receptor P1 and P2y antagonists, which block the effect of adenosine and adenosine triphosphate (ATP), respectively, and by N(omega)-nitro-L-arginine [an inhibitor of endothelium-derived nitric oxide (EDNO) synthesis]. In the newborn lamb, oxygen-induced pulmonary vasodilation is not blocked by N(omega)-nitro-L-arginine. We investigated the role of ATP and adenosine in oxygen-induced pulmonary vasodilation in eight newborn lambs with pulmonary hypertension induced by the thromboxane mimic, U46619. The hemodynamic effects of hyperoxia, ATP, adenosine, sodium nitroprusside (SNP), and acetylcholine (ACh) were compared before and after purinergic receptor blockade with Cibacron blue (CB, a P2y-receptor antagonist) and 8-phenyltheophylline (8PT, a P1-receptor antagonist) individually, together, and on a separate day, after infusion of N(omega)-nitro-L-arginine. During pulmonary hypertension, combined pretreatment with 8PT and CB attenuated the decrease in pulmonary arterial pressure caused by hyperoxia (11.3 vs. 35.2%), ATP (10.6 vs. 32.2%), and adenosine (1.9 vs. 33.7%) without change in the effect of ACh or SNP (p < 0.05). N(omega)-Nitro-L-arginine attenuated the pulmonary vasodilation caused by ATP and ACh but not by hyperoxia, adenosine, or SNP. In the newborn lamb, the pulmonary vasodilating effect of both oxygen and ATP are attenuated by combined P1 and P2y purinergic-receptor antagonists. Postnatally, oxygen-induced pulmonary vasodilation appears to be mediated by ATP through purinergic receptors.

Identification, characterization, and functional role of phosphodiesterase type IV in cerebral vessels: effects of selective phosphodiesterase inhibitors

The role of the phosphodiesterase type IV isozyme (PDE IV) in the regulation of cerebrovascular tone was investigated in the canine basilar artery in vitro and in vivo. The PDE isozymes extracted from the canine basilar artery were isolated by diethylaminoethanol (DEAE)-Sepharose affinity chromatography and identified based on sensitivity to isozyme-selective PDE inhibitors. [3H]cAMP hydrolysis was observed in one major and one minor peak of activity. The predominant peak was inhibited by the addition of cGMP (25%), siguazodan (26%), rolipram (39%), and the combination of siguazodan and rolipram (95%). Selective PDE IV inhibitors BRL 61063, rolipram, and denbufylline were equieffective inhibitors of [3H]-ccAMP hydrolysis mediated by PDE IV isolated from the canine basilar artery [concentrations producing 50% inhibition (IC50S) = 0.21 +/- 0.05 microM, 0.67 +/- 0.23 microM, and 0.73 +/- 0.16 microM, respectively]. In precontracted isolated ring segments of the canine basilar artery, selective PDE IV inhibitors produced potent and complete relaxation (IC50S < 150 nM). In contrast, zaprinast (a selective PDE V inhibitor) and siguazodan (a selective PDE III inhibitor) produced only weak relaxation of the basilar artery (IC50S = 4.5 microM and > 10 microM, respectively). Vasorelaxation produced by PDE IV inhibitors was not altered by removing the endothelium, 1-NAME, or adenosine receptor antagonism. In a canine model of acute cerebral vasospasm, all three selective PDE IV inhibitors reversed basilar artery spasm produced by autologous blood without altering mean arterial blood pressure. In contrast, prolonged treatment with BRL 61063 failed to alter the development of basilar spasm in the two hemorrhage canine models of chronic cerebral vasospasm. Denbufylline-induced relaxation in vitro was also significantly impaired in basilar arteries obtained from the model of chronic vasospasm. In conclusion, PDE IV appears to be the predominant isozyme regulating vascular tone mediated by cAMP hydrolysis in cerebral vessels. In addition, vasorelaxation modulated by PDE IV is compromised in chronic cerebral vasospasm associated with subarachnoid hemorrhage.

In vitro pharmacological effects of S 12370 (2-[4-benzhydryloxypiperidinoethyl]isoxindole; an antibronchoconstrictor agent) in normal and sensitized tissue

The effects of S 12370 (2-[4-benzhydryloxypiperidinoethyl]isoxindole), were studied in vitro. In guinea pig isolated tracheal rings, S 12370 induced a similar competitive inhibition of the contractile responses produced by acetylcholine, histamine and serotonin. However, it did not affect the contractions induced by leukotriene D4 (LTD4), substance P and U 46619, a stable analogue of thromboxane A2. S 12370 induced a concentration dependent inhibition of the cholinergic component of the contraction induced by electrical field stimulation, whereas it did not influence the sustained nonadrenergic noncholinergic (NANC) excitatory response observed in guinea pig isolated bronchi. S 12370 did not influence the relaxations induced by prostaglandin E2, isoprenaline and salbutamol, and did not modify the nonadrenergic noncholinergic inhibitory response induced by electrical field stimulation. In isolated left atria, the negative inotropic effect of acetylcholine was competitively inhibited by S 12370. In binding experiments, S 12370 exhibited similar affinity for M1, M2, M3, M4 muscarinic receptors and also recognized 5-HT2 serotonin and H1 histamine receptor subtypes. In ovalbumin-sensitized animals, the contractile response of isolated tracheal rings produced by exposure to the allergen was not influenced by S 12370. Tracheal rings from sensitized animals preexposed in vitro to the allergen developed a hyporesponsiveness to beta-adrenoceptor stimulation. S 12370 prevented the inhibitory effect caused by ovalbumin immune sensitization in the relaxation to isoprenaline. In rat polymorphonuclear neutrophil (PMN) cells, S 12370 up to 10(-5) M did not inhibit the arachidonic acid metabolism. These results suggest that in guinea pig tracheal smooth muscle, S 12370 is a competitive inhibitor of muscarinic, serotonin and histamine receptors and can modulate the beta-adrenergic dysfunction induced by immune sensitization. S 12370 may present some therapeutic interest in inflammatory airway diseases.

Endothelinb receptor agonists produce pulmonary vasodilation in intact newborn lambs with pulmonary hypertension

The hemodynamic effects of endothelin-1 (ET-1) are mediated by at least two distinct receptors: ETa and ETb receptors. Recently, ETb receptor agonists (4 Ala ET-1 and IRL 1620) were developed. To investigate the role of ETb receptor activation on the pulmonary and systemic circulations, we studied the hemodynamic effects of intrapulmonary arterial injections of these receptor agonists in 10 intact newborn lambs. At rest, 4 Ala ET-1 (290-1,725 ng/kg) changed no hemodynamic variables. IRL 1620 (180-1,095 ng/kg) decreased mean pulmonary arterial pressure (PAP, 16.8% +/- 15.0 and 17.8% +/- 8.5, p < 0.05) and left pulmonary artery blood flow (21.6% +/- 22.1 and 33.4% +/- 27.7, p < 0.05) at the two highest doses only. During U46619-induced pulmonary hypertension, both 4 Ala ET-1 (3.2% +/- 8.0 to 15.9% +/- 6.4, p < 0.05) and IRL 1620 (8.7% +/- 6.3 to 21.9% +/- 4.1, p < 0.05) produced selective dose-dependent decreases in PAP. The decrease in mean PAP induced by 4 Ala ET-1 and IRL 1620 was attenuated by N omega-nitro-L-arginine [an inhibitor of endothelium-derived nitric oxide (EDNO) synthesis] (16.6% +/- 3.5 vs. 5.9% +/- 2.3 and 16.2% +/- 3.4 vs. 6.6% +/- 2.8, p < 0.05) and by glybenclamide (a blocker of ATP-dependent potassium channels) (18.2% +/- 7.9 vs. 7.5% +/- 8.3 and 14.7% +/- 3.6 vs. 6.3% +/- 3.2, p < 0.05). ETb receptor activation produces selective pulmonary vasodilation during pulmonary hypertension in intact newborn lambs. The vasodilating properties are mediated in part by release of ENDO and by potassium channel activation.

Thromboxane A2 mimetic U-46619 induces systemic and pulmonary hypertension and delayed tachypnea in the goat

Cardiorespiratory variables were measured continuously in five conscious goats before and after the infusion of U-46619 at a dose of either 2, 4, or 6 micrograms.kg-1.5 min-1. Infusion of U-46619 led to immediate increases in pulmonary arterial blood pressure (ABP) that were sustained for up to 15 min after the end of the infusion. Systemic ABP also increased, but the relative increase from control was less than the pulmonary pressor response. At the highest dose, U-46619 elicited a delayed tachypneic response that was greatest several minutes after the infusion was stopped. U-46619 was also infused simultaneously with sodium nitroprusside to clamp ABP pressure at baseline levels to determine whether stimulation of baroreceptors might contribute to the latency of the tachypneic response. Although sodium nitroprusside infusion prevented the increase in ABP, the increase in breathing frequency was still delayed 3-4 min from the start of the infusion. We conclude that U-46619 elicits pulmonary and systemic arterial hypertension in the conscious goat. At the higher dose U-46619 also elicits a delayed tachypnea that remains delayed even if ABP is normal.

U-46619-induced ischaemic electrocardiographic changes in rats: preventive effects of prostacyclin and nitroglycerin

The anti-anginal effect of nitroglycerin and prostacyclin was examined using, as an index, the ischaemic electrocardiogram (ECG) change (ST elevation) induced by intracoronary arterial injection of 9,11-dideoxy-11 alpha,9 alpha-epoxymethano-PGF2 alpha (U-46619), a stable thromboxane A2 agonist, in anaesthetized rats. The ST elevation induced by U-46619 (5-20 micrograms kg-1, i.c.a.) was dose-dependent and reproducible. U-46619-induced ST elevation was markedly prevented by the pretreatment of intravenous administration of prostacyclin (0.01 micrograms kg-1), and to a lesser extent by nitroglycerin (0.3 mg kg-1). Simultaneously, platelet count decreased significantly in the coronary arterial blood which indicated that platelet aggregation was enhanced by U-46619. The decrease of platelet count in coronary arterial blood at the time of ST elevation was significantly suppressed by prostacyclin (0.1 microgram kg-1, i.v.), but not by nitroglycerin (0.3 mg kg-1, i.v.). These results suggest that the ST elevation induced by intracoronary arterial injection of U-46619 may be derived from spasm of coronary artery and platelet aggregation in the intracoronary artery in rats.

Diethylamine/nitric oxide (NO) adduct, an NO donor, produces potent pulmonary and systemic vasodilation in intact newborn lambs

Nitric oxide (NO), a labile humoral factor produced by vascular endothelial cells, is a potent vasodilator and an important mediator of pulmonary vascular tone. Nucleophile/NO adducts are a new class of compounds that spontaneously and predictively release NO. We investigated the hemodynamic effects of intravenous (i.v.) infusions of a recently developed NO-donor drug, the diethylamine-nitric oxide adduct (DEA/NO), in 17 intact newborn lambs. At rest, DEA/NO (1-2 microgram.kg-1.min-1) produced dose-dependent decreases in mean pulmonary (from 10.6 +/- 8.6 to 21.2 +/- 7.9%, p < 0.05) and systemic arterial pressure (from 13.2 +/- 11.7 to 31.0 +/- 15.4%, p < 0.05). Similarly, during pulmonary hypertension induced by infusion of U46619, DEA/NO (0.5-2.0 micrograms.kg-1.min-1) produced dose-dependent decreases in mean pulmonary (from 7.3 +/- 5.6 to 24.1 +/- 13.3%, p < 0.05) and systemic arterial pressure (from 2.2 +/- 3.8 to 20.3 +/- 12.9%, p < 0.05). Cardiac output (CO), heart rate (HR), systemic arterial blood gases, and pH were unchanged; atrial pressures decreased at higher doses. Equimolar infusions of S-nitroso-N-acetyl-penicillamine, nitroglycerin (NTG), and sodium nitroprusside (SNP) produced similar decreases in pulmonary and systemic arterial pressure. The nucleophile/NO adducts are potent vasodilators; their predictable and quantitative release of NO make them potentially useful research tools. In addition, because these compounds may decrease the incidence of tolerance and the risk from toxic metabolites associated with use of other nitrovasodilators, they may be clinically useful.

Possible involvement of endothelin in thromboxane A2 receptor agonist (U-46619)-induced angina in the rat

The thromboxane A2 receptor agonist, U-46619 ((5Z, 9 alpha, 11 alpha, 13E, 15(S))-9,11-(methanoepoxy)prosta-5,13-dien-1-oic acid) (10 micrograms/kg), induced a typical ischemic change (ST elevation) in the electrocardiogram on intracoronary arterial administration in the rat. The elevation of the ST segment induced by U-46619 was significantly reduced by pretreatment with anti-endothelin-1 rabbit serum. The plasma concentration of endothelin-1 dose dependently increased at the time of ST segment elevation after U-46619. These results indicate that endogenous endothelin-1 partly contributes to coronary spasmodic angina induced by thromboxane A2 in rats.

Design, synthesis, and pharmacology of 3-substituted sodium azulene-1-sulfonates and related compounds: non-prostanoid thromboxane A2 receptor antagonists

A series of novel azulene-1 carboxylic acid derivatives 28-30, azulene-1 sulfonic acid sodium salts 41a-c, and related compounds were synthesized. These compounds were tested for TXA2 receptor antagonistic activity. The inhibitory concentrations (IC50) of these compounds for vascular contraction (TXA2 tau receptor) and platelet aggregation (TXA2 alpha receptor) induced by (15S)-15-hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5(Z),13(E)- dienoic acid (U-46619) were obtained. Azulene-1-sulfonic acid sodium salts 41a-c were over 3 times more potent than azulene-1-carboxylic acids 28-30. The most potent compound, 41b was 4 orders of magnitude more potent than a TXA2 antagonist, BM13,177, in inhibiting vascular contraction (tau receptor) and had an IC50 of 9.0 x 10(-10) M. Compound 41b was also found to be a tau receptor selective antagonist (IC50 of contraction/IC50 of aggregation = 378) and to have no TXA2 synthetase inhibitory activity at concentrations up to 10(-4) M and no partial agonistic activity at concentrations up to 10(-5) M in rabbit aorta (tau receptor) and up to 10(-4) M in rabbit platelet-rich plasma (alpha receptor). In a radioligand binding assay using rabbit gel-filtered platelets, compound 41b had a high-affinity binding for the TXA2 receptor. In an in vivo study, compound 41b inhibited U-46619-induced sudden death in mice at a dose of 0.3 mg/kg and its duration of action was over 8 h when administered orally at 3 mg/kg.

Hyperoxia and alkalosis produce pulmonary vasodilation independent of endothelium-derived nitric oxide in newborn lambs

Supplemental oxygen and alkalosis are the most effective treatments used to lower pulmonary arterial pressure in children with pulmonary hypertensive disorders. However, their mechanisms of action are unknown. Endothelium-derived nitric oxide (EDNO) is an important mediator of pulmonary vascular tone and produces potent pulmonary vasodilation during pulmonary hypertension. In vitro evidence suggests that EDNO may mediate the vasodilating effects of oxygen. To investigate whether EDNO synthesis mediates the pulmonary vasodilation produced by hyperoxia [normocarbic ventilation with 100% oxygen, arterial oxygen tension > 450 torr (60 kPa)] or alkalosis (hyperventilation with 21% oxygen, pH > 7.55) in vivo, eight intact newborn lambs were studied during similar degrees of pulmonary hypertension induced either by the infusion of U46619 (a thromboxane A2 mimic) or N omega-nitro-L-arginine (an inhibitor of EDNO synthesis). The lambs were sedated, paralyzed, and mechanically ventilated. Meclofenamic acid was infused to inhibit prostaglandin synthesis. During pulmonary hypertension induced by U46619, pulmonary arterial pressure and pulmonary vascular resistance were significantly decreased by acetylcholine (an EDNO-dependent vasodilator) (23.1 +/- 3.4% and 43.3 +/- 14.5%, respectively), hyperoxia (26.8 +/- 7.8% and 32.9 +/- 10.6%), and alkalosis (32.1 +/- 10.3% and 36.1 +/- 17.0%) (p < 0.05). During pulmonary hypertension induced by N omega-nitro-L-arginine, the decreases in pulmonary arterial pressure and pulmonary vascular resistance produced by acetylcholine (9.6 +/- 6.4% and 23.9 +/- 14.1%, respectively) were significantly attenuated (p < 0.05), but the decreases produced by hyperoxia or alkalosis were unchanged. Therefore, hyperoxia and alkalosis can produce pulmonary vasodilation independent of EDNO synthesis in the intact newborn lamb.

Alterations by a thromboxane A2 analog (U46619) of calcium dynamics in isolated rat cardiomyocytes

The mechanism by which thromboxane A2 (TXA2) causes its detrimental actions on the myocardium during ischemia and reperfusion injury is unknown. The present study was designed to investigate the influence of U46619, a stable TXA2 analog, on intracellular Ca transients in electrically stimulated single neonatal rat ventricular myocytes by using spectrofluorometric analysis of fura-2-Ca binding. Administration of U46619 increased basal and peak Ca concentrations as well as width of electrically induced Ca transients in a concentration-dependent manner (0.1-1 microM) during a 1-hr exposure. Exposure to 10 microM U46619 caused irregular Ca transients and a marked increase in cytosolic-free Ca concentration. The effects of U46619 were antagonized by the TXA2 receptor antagonist SK&F95585 (2 microM), dibutyryl cyclic AMP (1 mM), verapamil (1 microM) and ryanodine (1 microM). U46619 did not affect the increase in cytosolic Ca induced by KCl (90 mM) depolarization. Caffeine (10 mM)-induced Ca release from the sarcoplasmic reticulum was enhanced markedly in U46619-treated cells. Significant lactate dehydrogenase leakage from the myocytes did not occur at 1 to 10 microM U46619. These results indicate that the increase in Ca transients by U46619 is a receptor-mediated process leading to a Ca accumulation in the sarcoplasmic reticulum which is likely to be responsible for an enhanced cytosolic Ca during excitation-contraction coupling. Thus, the identification of U46619-induced alterations of Ca dynamics appears to provide, at the cellular level, a direct role for TXA2 during myocardial ischemia and reperfusion.

Pharmacological profile of G619, a new platelet aggregation inhibitor

G619, a 4-OH-isophthalic acid derivative, was studied for its capacity to inhibit platelet aggregation. G619 dose-dependently inhibited U46619, collagen, ADP, PAF, thrombin and epinephrine-induced platelet aggregation in vitro. The IC50 values for inhibition of U46619-induced human and rabbit platelet aggregation were 39 and 43 microM, respectively. G619, at 100 microM, inhibited high concentration collagen (10 micrograms/ml)-induced aggregation of rabbit platelets pretreated with indomethacin and increased the level of cAMP in washed rabbit platelets by 30% (p less than 0.01 vs basal). However, G619, did not inhibit fibrinogen binding to GPIIb/IIIa receptor, phosphodiesterase, U46619-induced contractile responses on canine saphenous vein or rabbit aorta, calcium-induced vasoconstriction and thrombin or PAF-induced elevation of [Ca++]i in platelets in vitro. In vivo, the U46619-induced maximal thrombocytopenia in rats was reduced from 40% (vehicle) to 22% and 18% by 10 and 30 mg/kg of G619 i.v., respectively. G619 (30 mg/kg) had no effect on the U46619-induced vasopressor response or sudden death in rats, and had no effect on TxB2 formation. Our results indicate that G619 is a broad-spectrum platelet aggregation inhibitor and may have its effect on a common mechanism for platelet aggregation besides an effect on the thromboxane A2 receptor.

Pharmacological characterization of potent, long-acting thromboxane receptor antagonists, SQ 33,261 and SQ 33,552

SQ 33,261 ([1S-[1 alpha,2 alpha(Z),3 alpha,4 alpha]]-6-[3-[[2- [(phenylamino)carbonyl]hydrazono]methyl]-7-oxabicyclo[2.2.1]hept-2 - yl]-4-hexenoic acid) and SQ 33,552 ([1S-[1 alpha,2 alpha(Z),3 alpha,4 alpha]]-6-[3-[[[[(4- chlorophenyl)amino]carbonyl]hydrazono]methyl]-7- oxabicyclo[2.2.1]hept-2-yl]-4-hexenoic acid) are potent thromboxane (Tx) A2 receptor antagonists. They inhibited platelet aggregation in platelet-rich plasma induced by the TxA2 mimetic, U-46,619 (10 microM), with IC50 values of 200 and 70 nM, respectively. Neither compound inhibited ADP (20 microM)-induced platelet aggregation (IC50 greater than 1000 microM). SQ 33,261 and SQ 33,552 competitively antagonized U-46,619-induced contraction of rat aortic strips with respective pA2 values of 9.0 and 10.1 and KB values of 1.2 and 0.1 nM. They also competitively antagonized U-46,619-induced contraction of guinea pig tracheal strips with pA2 values of 8.9 and 9.9 and KB values of 1.9 and 0.4 nM, respectively. SQ 33,261 and SQ 33,552 (p.o.) were potent inhibitors of U-46,619 (2 mg/kg i.v.)-induced death in mice with ID50 values of 8 and 1 micrograms/kg, respectively. SQ 33,261 and SQ 33,552 (0.2 mg/kg p.o.), also had long duration of action in this assay with 50% survival times of 7 and 15 hr, respectively. SQ 33,261 at 0.01 and 1.0 mg/kg i.v., inhibited arachidonic acid-induced bronchoconstriction and reversed arachidonic acid-induced hypertension to a hypotensive response. SQ 33,552 inhibited TxA2 synthase at high concentrations (IC50 = 307 microM), whereas SQ 33,261 was inactive. Neither compound inhibited cyclooxygenase or caused an elevation of platelet cyclic AMP levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effects of the novel thromboxane A2 receptor antagonist sodium (E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)-ethylidene]-6,11- dihydrodibenz[b,e]oxepine-2-carboxylate monohydrate against 9,11-dideoxy- 9 alpha, 11 alpha-epoxymethano-prostaglandin F2 alpha-induced sudden death in guinea-pigs and rats

Injection of U-46619 (9,11-dideoxy-9 alpha, 11 alpha-epoxymethano- prostaglandinF2 alpha; 130 micrograms/kg i.v.) produced sudden death in anesthetized guinea-pigs and rats within 10-15 min. This sudden death is typified by a precipitous drop in mean arterial blood pressure (MABP) and a dramatic decrease in the circulating platelet counts. In guinea-pigs, KW-3635 (sodium(E)-11-[2-(5,6-dimethyl-1-benzimidazolyl)ethylidene]-6,11- dihydrodibenz[b,e]oxepine-2-carboxylate monohydrate, CAS 127166-41-0) at doses of 0.1 mg/kg or greater dramatically protected animals against sudden death induced by injection of U-46619. Pretreatment with KW-3635 (0.3, 1.0 mg/kg i.v.) inhibited the decrease in circulating platelet counts and the decline in blood pressure associated with the i.v. injection of U-46619. Oral administration of KW-3635 (10, 30 mg/kg) also protected the animals from the U-46619-induced sudden death. The effect of KW-3635 was almost the same as that of daltroban, and was more potent than that of sulotroban. In rats, intravenous administration of KW-3635 at doses of 0.3 mg/kg or greater protected against sudden death. In contrast, acetylsalicylic acid a cyclooxygenase inhibitor, did not protect against sudden death induced by U-46619, indicating that the formation of endogenous thromboxane does not play a major role in the lethal effect of U-46619, and that the blockade of the lethal effects of U-46619 is specific for thromboxane receptor antagonists. Our data show that KW-3635 protects guinea-pigs and rats against U-46619-induced sudden death. Therefore, KW-3635 may be useful for the investigation of diseases where thromboxane is involved.

Synergistic antiplatelet and antithrombotic effects of a prostacyclin analogue (iloprost) combined with a thromboxane antagonist (sulotroban) in guinea pigs and rats

The stable PGI2-analogue iloprost and the TXA2-receptor antagonist sulotroban were investigated for possible cooperative effects on platelet function and experimental thrombus formation in guinea pigs and rats. Iloprost and sulotroban inhibit intravascular platelet aggregation in guinea pigs and rats induced by the stable endoperoxide U 46.619 and collagen, with iloprost being the more potent and (for collagen) more efficacious drug. Combinations of both compounds show synergistic or additive effects on in vivo platelet function. Thrombus formation in rats induced by vascular damage is strongly reduced by combining doses of iloprost and sulotroban (BM 13.177) which given alone are ineffective. These results suggest a cooperative enhancement of antiplatelet and antithrombotic effects for combinations of iloprost and sulotroban. In view of disadvantages of currently used platelet inhibitors this cooperativity may offer a new approach in antiplatelet therapy.

Antagonism of PAF-induced death in mice

The ability of three platelet activating factor (PAF) antagonists, BN52021, L652,731 and 48740RP, and the leukotriene antagonist FPL55712 to block iv PAF-induced death was tested in mice. PAF-induced sudden death has been previously characterized as a model of systemic anaphylaxis and circulatory shock related its hypotensive actions. Of the drugs, BN5201 and L652,731 provided dose-dependent protection against PAF toxicity, whereas the others had no effect. 48740RP was, however active against PAF-induced rabbit platelet aggregation. BN52021 was inactive in three other mouse sudden death models in which arachidonic acid, U46619 or collagen combined with epinephrine is injected iv to provoke a thrombotic/ischemic sudden death. In contrast, the TXA2 antagonist SQ29548 inhibited the acute toxicity of two of these latter challenges (arachidonic acid and thromboxane agonist U46619), but was inactive against PAF lethality. These results suggest that PAF toxicity in mice is a specific model for PAF agonism, and is not mediated by TXA2 or peptido-leukotrienes. Further, PAF-induced mortality should be a simple and useful technique for testing potential PAF antagonists for in vivo activity by various routes of administration.

Antiaggregatory effects of thromboxane receptor antagonists in vivo

The antiaggregatory and antisecretory effects of two newly developed thromboxane receptor antagonists, BM-13,177 and SQ-29,548, were studied in an in vivo model of platelet activation. Arterial platelet count and whole blood ATP concentrations were measured continuously on-line in the arterial blood of anesthetized rabbits. Injections of collagen decreased peripheral platelet count by 25% of initial value. ATP concentrations increased 50 to 100 nM during collagen challenge. SQ-29,548 and BM-13,177 dose-dependently reduced platelet loss to about 50% of that observed in vehicle treated animals. Injection of arachidonic acid (AA) or 9,11-methanoepoxy-PGH2 resulted in sudden death of the animals associated with a 67 to 69% decrease in platelet count and a marked release of ATP. Pretreatment with SQ-29,548 or BM-13,177 increased survival rates from 0 to 100%, and reduced or totally inhibited ATP secretion and decreases in platelet count. In contrast, the thromboxane synthetase inhibitor, dazoxiben, was effective in inhibiting AA induced sudden death, but was without any effect when 9,11-methanoepoxy-PGH2 was used as the challenging agent. We conclude that SQ-29,548 and BM-13,177 are effective in antagonizing the effects of subsequent conversion to thromboxane A2. BM-13,177 and SQ-29,548 are generally considered as specific antagonists of endoperoxide/thromboxane receptors on platelets and smooth muscles in vitro. Preliminary clinical studies with BM-13,177 showed a marked inhibition of ex vivo platelet aggregation in human volunteers and patients (18, 19). These reports encourage further study of thromboxane receptor antagonists as effective anti-thrombotic drugs in the experimental and clinical setting.

Blockade of thromboxane and the prevention of eicosanoid-induced sudden death in mice

We studied the effects of thromboxane-receptor antagonism and thromboxane synthetase inhibition in a thrombotic model of sudden death in mice. Intravenous injection of arachidonic acid (AA; 80 mg/kg) or the prostaglandin-endoperoxide analog U-46,619 (2.3 mg/kg) results in sudden death in approximately 90% of the animals. Pretreatment with the thromboxane receptor antagonist SQ-29,548 (0.3-10 mg/kg) protects dose-dependently against AA and U-46,619-induced sudden death. In contrast, CGS-13,080, a thromboxane synthetase inhibitor, shows a dose-dependent beneficial effect in AA-induced sudden death only. Although PTA2 has partial thromboxane agonistic properties in the rabbit, it protected the mice against AA-induced sudden death, thus demonstrating TxA2 antagonistic properties in this species. These data emphasize the importance of thromboxane A2 as a major mediator of arachidonic acid-induced sudden death and the effectiveness of thromboxane-receptor antagonists in endoperoxide-induced sudden death.

Thromboxane agonism and antagonism in a mouse sudden death model

The effects of the stable thromboxane agonist, U46619, and sodium arachidonate were tested by i.v. injection into male and female mice. U46619 produced dose-dependent mortality in both sexes equally, in contrast to the gender-differentiated effects of arachidonic acid. The thromboxane receptor antagonist, SQ 26,536, protected in a dose-dependent manner against both arachidonate and U46619. The thromboxane antagonist was more effective against arachidonate toxicity in male than in female mice, but was equiactive against U46619 in both sexes. Neither the thromboxane synthetase inhibitor, OKY-1581, nor the cyclooxygenase inhibitor, indomethacin, protected against U46619-induced sudden death. However, cortisone acetate increased survival of mice challenged with U46619. The results support the hypothesis that thromboxane A2 mediates arachidonate-induced sudden death. The effects of arachidonate can be mimicked by the thromboxane agonist and are attenuated by the thromboxane antagonist. The gender difference in arachidonate toxicity is apparently not due to differences in sensitivity to thromboxane A2, as the thromboxane agonist was equally toxic in males and females. The greater protective effect of the thromboxane antagonist against arachidonate toxicity in males suggests that thromboxane A2 is a more important mediator of arachidonate-induced sudden death in males compared to female mice.