Polyphloretin phosphate temporarily potentiates prostaglandin E 2 on the rat fundus, probably by inhibiting PG15-hydroxydehydrogenase

Polyphloretin phosphate (PPP), an antagonist of some smooth muscle action of prostaglandins, potentiates the effect of prostaglandin E₂ on the rat isolated stomach fundus preparation. No such potentiation could be demonstrated for prostaglandin 15(S) 15 methyl E₂ methyl ester, a synthetic analogue of prostaglandin E₂. PPP also stimulates the longitudinal muscle of human isolated jejunum to contract. Evidence is produced to show that the potentiation of prostaglandin E₂ and the direct stimulant action of PPP are due to inhibition of prostaglandin 15-OH dehydrogenase, an enzyme which inactivates prostaglandin E₂ but not 15(S) 15 methyl E₂ methyl ester.     

Ozone toxicity: Hormone-like oxidation products from arachidonic acid by ozone-catalyzed autoxidation

Arachidonic acid (AA) was rapidly oxidized by a stream of air containing 2.0 ppm of ozone to peroxides having biological activity similar to prostaglandin endoperoxides. Ozone-formed AA peroxides (AAP) (0.5–6.0 nmol) contracted both rabbit aortic strips and rat fundus strips in the presence of indomethacin and Vane's mixture of inhibitors of vasoactive hormones at doses comparable to naturally formed prostaglandin endoperoxides (PGG₂ or PGH₂). Ozone-formed AAP also aggregated human platelets in platelet-rich plasma (PRP) at concentrations similar to PGG₂ or PGH₂ (1.5–3.0 nmol per 0.4 ml), but platelet aggregation was blocked by indomethacin. Active peroxides were not formed in the presence of vitamin E added in molar ratios of from 1:10² to 1:10⁴ to AA prior to oxidation by ozone. Vitamin E had no effect on the activity of AAP once formed, nor were the AAP substrates for human platelet microsomal thromboxane synthetase. Formation of prostaglandin hormone-like peroxides may be part of the toxic action of ozone and other toxicants initiating lipid peroxidation.     

The inhibitory actions of prostaglandins on respiratory smooth muscle

Prostaglandin E₁, in concentrations as low as 1 ng/ml., relaxed isolated tracheal muscle from cat, monkey, rabbit, guinea-pig and ferret. Tracheal muscle from the cat, monkey and rabbit did not exhibit inherent tone and the effect of prostaglandin E₁ on these preparations was seen only after a sustained contraction had been produced by a previous dose of acetylcholine or of another agonist. Prostaglandins E₂, E₃ and F_1α also relaxed isolated cat tracheal muscle which had been stimulated by acetylcholine: their activities relative to that of prostaglandin E₁ were, respectively, 1.0, 0.2 and 0.002. In the anaesthetized cat prostaglandin E₁ increased lung “resistance to inflation” (presumably comparable to bronchial resistance) and the heart rate. In the anaesthetized rabbit and guinea-pig, prostaglandin E₁ antagonized the rise in resistance to inflation of the lungs obtained after vagal stimulation or after the intravenous injection of histamine; it sometimes lowered the resistance to inflation in these species. The possibility that prostaglandin may have a local physiological role in the control of bronchial smooth muscle tone is discussed.     

Synthesis and Platelet Aggregation Inhibiting Activity of Acid Side-chain Modified Hydantoin Prostaglandin Analogues

A series of hydantoin prostaglandin analogues, in which the hexamethylene moiety of the acid side chain was replaced by other spacing groups possessing either ether, sulphide and/or olefin functionality, were prepared and evaluated for platelet aggregation inhibiting activity. The 4-thia analogue 13 proved to be the most potent inhibitor (ca. 22x PGE₁) and the 3-thia- and 3-oxa-analogues, 6 and 10 respectively, are approximately equipotent with BW245C (ca. 14x PGE₁). Z-olefinic analogues (e.g. 11 ) were usually more potent than their E-isomers (e.g. 12 ). Structure-activity relationships are discussed in detail.     

Effects of Gabexate,a Protease Inhibitor,on Smooth Muscle of Guinea-Pig Stomach Fundus

Abstract: A protease inhibitor, gabexate (ethyl-p-6-guanidinohexanoyloxy benzoate), was found to have an antimuscarinic action in muscle strips of the guinea-pig gastric fundus. Gabexate reversibly inhibited carbachol-induced contractions in the presence of prostaglandin synthesis inhibitors (indomethacin or meclofenamate) with a pA₂ of 5.66 for the circular and 5.25 for the longitudinal muscle. The effect was not affected by tetrodotoxin. Gabexate also inhibited contractions produced by prostaglandin E₂ (PGE₂) (21.7 ± 7.3% with 30 μM, n = 12). The inhibition was markedly potentiated by anticholinesterase, diisopropyl fluorophosphate, but converted to contraction by atropine. In the absence of PGE₂, gabexate produced no mechanical response on its own even after atropine application. Treatment with hemicholinium, an acetylcholine synthesis inhibitor, also converted the relaxant effect of gabexate, applied during PGE₂-induced contraction, to contraction. Gabexate also inhibited contracture induced by 30 mM K⁺ weakly (13 ± 2% with 30 μM, n=5). This relaxation was abolished by atropine, without converting to contraction. PGE₂ and excess K⁺ are likely to release acetylcholine from nerve fibres. These results suggest that the inhibitory effect of gabexate is mainly due to the muscarinic receptor blocking action. In addition, gabexate has a potentiating action on the prostaglandin-induced contraction.     

Platelet activating factor and the responses of rat isolated stomach strip to prostaglandin E2

Abstract— The effect of platelet activating factor (PAF) on contractions evoked by acetylcholine, 5-hydroxytryptamine (5-HT) and prostaglandin E₂ (PGE₂) was studied in-vitro on rat stomach strip. Addition of PAF to the organ bath increased PGE- but not 5-HT- or acetylcholine-evoked responses. The effect of PAF was unaffected by atropine, methysergide or indomethacin, but prevented by a specific PAF receptor antagonist BN 52021. The data support a specific interaction between PAF and PGE₂ on rat stomach strip.     

Prostaglandin release from rat stomach following vagal stimulation or administration of acetylcholine

The rat isolated stomach was perfused via the coeliac artery and prostaglandin-like material released from the mucosa into the lumen (luminal fraction) and from blood vessels and surrounding serosal wall (serosal fraction) was bioassayed using rat fundus strips. Prostaglandin F_2α was identified by chromatographic and mass spectrometric techniques. The amount of prostaglandin F_2α released upon their vagal nerve stimulation or acetylcholine infusion increased significantly over basal levels, in association with increased gastric peristaltic activity. The quantity of prostaglandin released into the luminal fraction greatly exceeded the amount liberated into the serosal samples and this increase was dependent upon the rate of stimulation. Infusion of either atropine or hexamethonium markedly reduced both the prostaglandin output and the gastric contraction elicited by stimulation. Pretreating the stomach with indomethacin considerably decreased prostaglandin outflow. The role of endogenous prostaglanding F_2α in relation to parasympathetic neurotransmission is discussed.     

Editorial Review: Palytoxin: Mechanism of Action of a Potent Marine Toxin

Abstract

Palytoxin, the most potent marine toxin known, causes death at doses as low as 0.025 μg/kg. Its chemical structure contains no identifiable subunits, making it one of the most complex natural products known. It exhibits a wide variety of pharmacological actions Including stimulating release of K⁺ from various cell types; depolarization of excitable membranes; contraction of smooth muscle; cardiovascular effects; hemolysis; histamine, prostaglandin and norepinephrine release; platelet aggregation and bone resorption. It also exhibits Inhibition of sperm motility, anti-tumor activity and tumor-promoting activity. Studies on the molecular mechanism of action have led to several proposed mechanisms. The most extensively studied is the proposal that palytoxin interacts with (Na⁺,K⁺)-ATPase at the ouabain binding site, but rather than inhibiting the ion pump as ouabain does, it converts it to an open ion channel, resulting in collapse of ion gradients across the cell membrane. Other possible mechanisms include a similar effect on the acetylcholine receptor, other types of ionophore activity, and release of an endogenous mediator. Many of the pharmacological effects of palytoxin are of types that are mediated by elevated intracellular Ca²⁺. Palytoxin causes elevated intracellular Ca²⁺, but it is not known if it is a direct result of opened ion channels or an indirect effect of altered Na⁺ concentrations.     

Effects of prostaglandin E1 on protein kinase activity and endogenous phosphorylation of intact human platelets.

When intact human platelets were treated with prostaglandin E₁, protein kinase activity of the platelet sonicate was elevated when assayed without cyclic AMP. Total protein kinase activity (assayed with cyclic AMP) remained unchanged. Prostaglandin A₁ mimicked this effect, but other prostaglandins and antiplatelet agents had no effect on protein kinase activity. Phosphoprotein phosphatase activity was not influenced by prostaglandin E₁. Basal phosphorylation of several endogenous substrates for protein kinase was also elevated when platelets were treated with prostaglandin E₁. The results demonstrate that prostaglandin E₁ stimulates protein kinase activity and endogenous phosphorylation in intact platelets and suggest that this effect is mediated by the action of prostaglandin E₁ on cyclic AMP levels. An important role of phosphoprotein in mediating the effect of prostaglandin E₁ on platelet function is suggested.     

TXA2-Antagonistic Properties of Agents Affecting Prostaglandin Synthesis or the Cyclic Nucleotide System in Human Platelets

Abstract: Prostaglandins (PG) E₁ and E₂ as well as 3-isobutyl-1-methylxanthine, nitroprusside, dibutyryl cyclic AMP and N-0164 inhibited platelet aggregation induced by the thromboxane (TX) A₂-mimetic prostaglandin endoperoxide analogue U46619. Non-steroidal anti-inflammatory agents – acetylsalicylic acid, indomethacin, tolfenamic acid, flumizole, nictindole and proquazone – did not demonstrate any antagonistic actions on U46619-induced aggregation at concentrations causing inhibition of prostaglandin/thromboxane synthesis-dependent forms of platelet aggregation. Comparing with the effects of the different test substances on ADP-or arachidonic acid-induced platelet aggregation, it can be suggested that PGE₁ and PGE₂ as well as 3-isobutyl-1-methylxanthine, nitroprusside, and dibutyryl cyclic AMP are functional antagonists and N-0164 is a receptor level antagonist of TXA₂ in platelets.     

Actions of Neurotensin and (Gln4)-Neurotensin on Isolated Tissues

Abstract The actions of neurotensin and (Gln⁴)-neurotensin have been investigated on a number of isolated tissues. They were ineffective in contracting the guinea pig vas deferens, the rabbit aortic strip or the frog rectus abdominis muscle in concentrations up to 0.24 γM. Neurotensin and (Gln⁴)-neurotensin relaxed the rat duodenum at fairly high concentrations (24 nM). The guinea pig ileum contracted in response to increasing doses, although the maximum response were only one half that caused by histamine. Tachyphylaxis was observed at dose intervals of less than 12 minutes, but this tachyphylaxis did not inhibit responses to acetylcholine, histamine, 5-HT or to DMPP, suggesting that the neurotensins may act at specific receptor sites. They contracted the rat fundus strip at concentrations of 0.24 nM and higher. The neurotensins and 5-HT were approximately equipotent on this tissue, although the maximum respones was about 80 % of that to 5-HT. The contractions of the rat fundus strip could not be blocked by atropine, hexamethonium, methysergide, morphine or by 7-OH-THC. These data indicate that neurotensin and (Gln⁴)-neurotensin are equipotent as far as smooth muscle stimulating activity is concerned. Of those organs tested, the rat fundus strip seems to be the most suitable one for studies concerning structure-activity relationships.     

Effects of Prostaglandins on the Smooth Muscle of the Urinary Tract

Abstract A short review is given of the effects of prostaglandins on the smooth muscle of the bladder, urethra and ureter. In several animal species and in man, prostaglandins of the E and F types contract the detrusor. The contractions develop slowly and are not affected by tetrodotoxin, phenoxybenzamine, or atropine, suggesting a direct effect on the smooth muscle cell. However, evidence is also available suggesting a role of the prostaglandins as modulators of transmitter release in the bladder. In animals as well as in man, urethral smooth muscle is contracted by prostaglandin F_2α, but relaxed by prostaglandins E₁ and E₂. The smooth muscle of the ureter seems to react similarly, but published results vary. The detailed mode of action, and the role of prostaglandins in the control of activity in urinary tract smooth muscle remain to be established.     

Studies on prostaglandin antagonists

1. Three prostaglandin antagonists have been examined for their ability to block PGE₂ and PGF_2α on human, guinea-pig and isolated rat gastrointestinal muscle.

2. 7-oxa-13-Prostynoic acid was either a non-selective antagonist, or was ineffective on the tissues studied; it had marked spasmogenic activity on the rat fundus.

3. 1-Acetyl-2-(8-chloro-10,11-dihydrodibenz (b,f)(1,4) oxazepine-10-carbonyl) hydrazine selectively antagonized the excitatory effects of PGE₂ and PGF_2α in guinea-pig and rat tissues, but not in human muscle.

4. Polyphloretin phosphate selectively antagonized the excitatory effects of prostaglandins in both human and guinea-pig muscle preparations, but it caused stimulation of the rat fundus.

5. All the antagonists lowered the tone in many tissues. They also reduced contractions caused by potassium.

6. None of the compounds blocked the inhibitory effect of PGE₂ on intestinal circular muscle.

7. The implication of these results on the nature of prostaglandin receptors, and the value of each compound as a prostaglandin antagonist are discussed.

     

P2Y受体介导大鼠胃体环行肌收缩反应的药理学特点

观察大鼠离体胃体环行肌和胃底环行肌不同的药理学特征，分析核苷及核苷酸类物质诱发胃体环行肌收缩反应的作用特点和受体机制。制备大鼠离体胃体环行肌和胃底环行肌标本，利用受体药理学技术观察药物诱发的收缩反应。在胃体环行肌KCl所致收缩反应与胃底环行肌无显著性差别；但是，CCh收缩胃体环行肌的EC₅₀值 [(0.45 ± 0.15) μmol·L⁻¹] 显著高于胃底环行肌 [(0.20 ± 0.09) μmol·L⁻¹, P < 0.01]。5-HT和His收缩两种标本的EC₅₀值无显著差异 (P > 0.05); 但是, 在胃体环行肌5-HT和His产生收缩反应的E_max值 [(0.81 ± 0.26) 和 (0.88 ± 0.27) g] 显著小于胃底环行肌 [(2.67 ± 0.61) 和 (1.90 ± 0.68) g, P < 0.01]。在预收缩胃体环行肌，ATP (0.1～3 000 μmol·L⁻¹) 诱发浓度依赖性收缩反应，未见舒张反应；在预收缩胃底环行肌标本，同浓度ATP诱发先舒张后收缩的双相反应，并呈浓度依赖性。ATP、UTP、ADP、2-MeSATP和α, β-MeATP浓度依赖性诱发大鼠胃体环行肌收缩反应，2-MeSATP的EC₅₀值为 (7.2 ± 5.2) nmol·L⁻¹比Ach [(3.47 ± 1.20) μmol·L⁻¹] 低500倍；各药物产生收缩反应的效价序列为：2-MeSATP>>ADP>ATP=UTP>α, β-MeATP>>腺苷。酚妥拉明、普萘洛尔、阿托品及河豚毒素不影响ATP和UTP诱发的胃体环行肌收缩反应。研究结果表明, 大鼠胃体环行肌的药理学特征明显不同于胃底环行肌; 核苷酸类物质通过某种特殊的P2Y受体介导胃体环行肌收缩反应，是调节胃体环行肌收缩功能的重要介质。
     

Prostaglandins, oxygen tension and smooth muscle tone

1. By using indomethacin to inhibit their intramural synthesis, we have investigated the contribution of prostaglandins to the maintenance of (a) the intrinsic tone of isolated smooth muscle preparations and (b) contractions produced by drugs or high oxygen concentration.

2. When treated with indomethacin, the rat stomach strip and chick rectum preparation slowly relaxed, whether they were bathed in Krebs solution or blood. Although their sensitivity to added prostaglandin was somewhat enhanced, they became insensitive to changes in oxygen or glucose concentration. However, another smooth muscle preparation, the rat colon, was neither relaxed by indomethacin nor contracted by high oxygen concentration.

3. These results support the hypothesis that intramural generation of prostaglandin maintains the tone of some smooth muscle preparations.

4. Contractions of the guinea-pig isolated colon were induced by histamine. These contractions were normally well maintained but in Krebs solution lacking either oxygen or glucose, only the initial spike contraction remained. In the presence of indomethacin the histamine contraction was also poorly sustained, but maintenance was restored by a low concentration of prostaglandin E₂.

5. Thus, the effects on smooth muscle of oxygen or glucose lack may also be mediated by reduction in the synthesis or effects of an intramural prostaglandin. Extension of this hypothesis to intestinal and vascular smooth muscle in vivo is discussed.

     

Adenylate Cyclase in Rat Synaptosomal Plasma Membranes and Caudate Nucleus Homogenate: Effects of Dopamine,E Prostaglandins,Guanyl-5′-yl-imidodiphosphate and Morphine

Abstract: The adenylate cyclase in two particulate preparations from rat brain, a homogenate from caudate nucleus (CN-homogenate) and a synaptosomal plasma membrane fraction (SPM-fraction) from whole rat brain was investigated. Stimulation of the enzyme by dopamine and prostaglandins E₁ and E₂ was found in the CN-homogenate while only a weak prostaglandin E₁ and E₂ stimulation and no dopamine stimulation could be found in the SPM-fraction. Guanyl-5′-yl-imidophosphate (GppNHp) and NaF could stimulate the adenylate cyclase in both preparations. Morphine up to 10^–5 M altered neither the basal enzyme activity nor any of the stimulated enzyme activities.     

Binding sites for 1,4-dihydropyridine Ca2+-channel modulators in rat intestinal smooth muscle

Summary The contractile response of intestinal smooth muscle to depolarization is characterized by a phasic and a tonic component which are differently sensitive to blockade by 1,4-dihydropyridines. As this difference in sensitivity could be related to different binding sites associated with distinct calcium channels, we analyzed the binding of the calciumantagonist 1,4-dihydropyridine (+)PN 200-110 [isopropyl-4-(2,1,3-benzodiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-methoxycarbonyl-pyridine-3-carboxylate] in longitudinal smooth muscle of the rat ileum.We carried out saturation binding experiments on intact tissue exposed to physiological and depolarizing (100 mmol 1^–1 K⁺) solution, and on different membrane fractions: the total microsomal fraction, the light microsomal fraction (enriched with plasma membranes) and the mitochondrial fraction. Binding of ³H(+)PN 200-110 to the intact longitudinal smooth muscle of rat ileum appeared to be voltage-dependent, K_D decreased in depolarized tissue whereas B_max was unchanged (change in membrane potential was assessed by measuring the distribution of ³H-tetraphenylphosphonium bromide). In membrane fractions two binding sites were detected, a high-affinity site associated with plasma membrane and a low-affinity site presumably associated with mitochondria (abundant in the fractions where the cytochrome c oxidase activity was high, and undetectable in the light microsomes poor in cytochrome c oxidase activity). The K_D value of the high-affinity binding in isolated membrane fractions was similar to the K_D value measured in intact depolarized tissue. The low affinity binding increased at high ionic strength and did not display any stereoselectivity.Therefore, the binding of (+)PN 200-110 did not allow to differentiate more than one type of voltage-operated Ca²⁺-channel sensitive to 1,4-dihydropyridines in the plasma membrane of rat intestinal smooth muscle. Send offprint requests to T. Godfraind at the above address     

Acetal phosphatidic acids: novel platelet aggregating agents

1 Palmitaldehyde, olealdehyde and linolealdehyde acetal phosphatidic acids induced rapid shape change and dose-dependent biphasic aggregation of human platelets in platelet-rich plasma; aggregation was reversible at low doses and irreversible at high doses of the acetal phosphatidic acids. The palmitaldehyde congener elicited monophasic dose-dependent aggregation of sheep platelets in platelet-rich plasma.

2 The threshold concentration for palmitaldehyde acetal phosphatidic acid (PGAP)-induced platelet aggregation was 2.5-5 μM for human platelets and 0.25-0.5 μM for sheep platelets. PGAP was 4-5 times as potent versus human platelets as the olealdehyde and linolealdehyde acetal phosphatidic acids, which were equipotent.

3 PGAP-induced irreversible aggregation of [¹⁴C]-5-hydroxytryptamine ([¹⁴C]-5-HT)-labelled human platelets in platelet-rich plasma was accompanied by release of 44.0±2.4% (s.e.) of the platelet [¹⁴C]-5-HT; reversible aggregation was not associated with release. In contrast, PGAP-induced release of [¹⁴C]-5-HT-labelled sheep platelets was dose-dependent.

4 The adenosine diphosphate (ADP) antagonist, 2-methylthio-AMP, and the cyclo-oxygenase inhibitor, aspirin, abolished PGAP-induced second phase aggregation and release in human platelets but did not affect the first, reversible, phase of aggregation. Both the first and second phases of PGAP-induced aggregation were abolished by chlorpromazine, by the phospholipase A₂ inhibitor, mepacrine, and by nmolar concentrations of prostaglandin E₁ (PGE₁); these agents abolished the second, but not the first phase of ADP-induced aggregation.

5 The related phospholipids, lecithin, lysolecithin and phosphatidic acid, at <100 μM, neither induced aggregation of human platelets in platelet-rich plasma, nor modified PGAP-induced aggregation; 1-palmityl lysophosphatidic acid elicited aggregation of human platelets at a threshold concentration of 100 μM.

6 It is concluded that the acetal phosphatidic acids induce platelet aggregation per se by direct action at the platelet membrane, and that the acetal function is of primary importance in their potent platelet-stimulating activity. Moreover, as the acetal phosphatidic acids are the major components of the smooth muscle-contracting acidic phospholipid tissue extract `Darmstoff' (Vogt, 1949), their potent platelet-aggregating properties may be of physiological or pathological significance.

     

Endothelium-dependent contractions in SHR: a tale of prostanoid TP and IP receptors

In the aorta of spontaneously hypertensive rats (SHR), the endothelial dysfunction is due to the release of endothelium-derived contracting factors (EDCFs) that counteract the vasodilator effect of nitric oxide, with no or minor alteration of its production. The endothelium-dependent contractions elicited by acetylcholine (ACh) involve an increase in endothelial [Ca²⁺]_i, the production of reactive oxygen species, the activation of endothelial cyclooxygenase-1, the diffusion of EDCF and the subsequent stimulation of smooth muscle cell TP receptors. The EDCFs released by ACh have been identified as PGH₂ and paradoxically prostacyclin. Prostacyclin generally acts as an endothelium-derived vasodilator, which, by stimulating IP receptors, produces hyperpolarization and relaxation of the smooth muscle and inhibits platelet aggregation. In the aorta of SHR and Wistar-Kyoto rats, prostacyclin is the principal metabolite of arachidonic acid released by ACh. However, in SHR aorta, prostacyclin does not produce relaxations but activates the TP receptors on vascular smooth muscle cells and produces contraction. The IP receptor is not functional in the aortic smooth muscle cells of SHR as early as 12 weeks of age, but its activity is not reduced in platelets. Therefore, prostacyclin in the rule protects the vascular wall, but in the SHR aorta it can contribute to endothelial dysfunction. Whether or not prostacyclin plays a detrimental role as an EDCF in other animal models or in human remains to be demonstrated. Nevertheless, because EDCFs converge to activate TP receptors, selective antagonists of this receptor, by preventing endothelium-dependent contractions, curtail the endothelial dysfunction in diseases such as hypertension and diabetes.     

ZD9583, an Orally Effective Thromboxane A2 Synthase Inhibitor and Receptor Antagonist with a Sustained Duration of Action in Rat and Dog

The thromboxane A₂ (TXA₂) synthase inhibitory activity and the TXA₂ receptor (TP-receptor) blocking action of ZD9583 ((4Z)-6-[(2S,4S,5R)-2-(1-[2-cyano-4-methylphenoxy]-1-methylethyl)-4-(3-pyridyl)-1,3-dioxan-5-yl]hex-4-enoic acid) has been evaluated in-vitro by use of whole blood and platelets from man, and ex-vivo by use of platelets and whole blood from rats and dogs. ZD9583 caused concentration–dependent inhibition of human platelet microsomal TXA₂ production with an IC50 of 0.017 ± 0.003 μm; this inhibition was associated with an increase in prostaglandin E₂ (PGE₂) and prostaglandin F_2α (PGF_2α) formation. ZD9583 also inhibited collagen-stimulated TXA₂ synthesis in whole blood from man, rat and dog giving IC50 values of 0.027 ± 0.005, 0002 ± 0.006 and 0.013 ± 0.01 μm, respectively. The drug did not modify platelet cyclooxygenase activity as inhibition of thromboxane B₂ (TXB₂) formation was associated with a concomitant increased synthesis of prostaglandin D₂ (PGD₂), PGE₂ and PGF_2α. ZD9583 had little effect on cultured human umbilical vein endothelial cell prostacyclin synthase giving an IC50 of 24.2 ± 4.9 μm. In-vitro ZD9583 caused concentration-dependent inhibition of U46619-induced aggregation responses of platelets from man, rat and dog, yielding apparent log A₂ values of 8.7 ± 0.12, 8.8 ± 0.2 and 9.3 ± 0.2, respectively. The drug was selective; at concentrations up to 100 μm it did not affect 5-hydroxytryptamine or the primary phases of adenosine diphosphate and adrenaline-induced aggregation. ZD9583 (100 μm) did not, furthermore, modify the platelet inhibitory effects of PGD₂, prostaglandin E₁ (PGE₁) and prostacyclin. Oral administration of ZD9583 (3–10 mg kg^?1) to both rats and dogs caused dose-dependant inhibition of collagen-stimulated TXA₂ production ex-vivo which persisted for up to 12 h. The drug also caused profound TXA₂ receptor blockade in both species for in excess of 12-h after an oral dose of 3 mg kg^?1. ZD9583 (3 mg kg^?1, p.o.), when administered to dogs over a five-day period at 12 h intervals, did not cause either tachyphylaxis or an accumulation of effect. We conclude that ZD9583 is a potent, selective, orally active thromboxane synthase inhibitor and TXA₂ receptor antagonist.