Bleeding disorder due to platelet prostaglandin H synthase-1 (PGHS-1) deficiency

Defective platelet prostaglandin H synthase (PGHS) activity has been recognized as a cause of bleeding disorders, but the defect has not been characterized. We evaluated three female patients aged 37, 48 and 55 who presented with a mild bleeding disorder due to platelet dysfunction. None of the patients had underlying diseases or reported use of aspirin or other nonsteroidal anti-inflammatory drugs. Coagulation screening tests and platelet count were normal in each patient. Platelet aggregation in response to adenosine diphosphate (ADP), collagen and epinephrine were subnormal, characterized by an abnormal second-wave aggregation and propensity for disaggregation. Arachidonate-induced platelet aggregation was defective, whereas PGH₂-induced aggregation was normal. Platelet thromboxane A₂ (TXA₂) production in response to arachidonic acid was reduced in all three patients, i.e. 11.7, 4.6 and 4.4 ng TXB₂/3 10⁸ plt respectively (normal range was 49–81 ng/3 10 ⁸ plt), whereas they were normal in response to exogenous PGH₂, i.e. 71.4, 56.6 and 48.9 ng/3 10⁸ plts, respectively (normal range 49–85 ng/3 10⁸ plt). These results are consistent with a deficiency of platelet PGHS activity. The level of the constitutive platelet PGHS-1 and TXA₂ synthase (TXAS) proteins were determined on platelet microsomal fractions by Western blot analysis using affinity-purified polyclonal antibodies highly specific for human PGHS-1 and TXAS, respectively. In two patients the 70 kD PGHS-1 protein was undetectable, whereas it was normal in the third patient. The 60 kD TXAS band was normal in all three patients. These findings indicate that human platelet PGHS-1 deficiency is due to two types of enzyme defects: type 1 defect is manifested by an undetectable PGHS-1 protein in platelets whereas the type 2 defect is manifested by a normal quantity of PGHS-1 protein which has an impaired catalytic activity.     

Thromboxane A2 and Prostacyclin Release in Bleeding Time Blood during Primary Haemostasis in Healthy Individuals

ABSTRACT It is generally believed that prostacyclin (PGI₂) generation is greatly stimulated when blood vessels are injured, even by minor trauma, such as venepuncture. The Simplate technique for measuring skin bleeding time was adapted to quantify by radioimmunoassay PGI₂ and thromboxane A₂ (TXA₂) in the emerging blood, as the stable degradation products 6-keto-prostaglandin F_1α (6-keto-PGF_1α) and thromboxane B₂ (TXB₂), both of which were measured in venous plasma as well as in serum (clotted at 37°C for 1 h). During bleeding, when platelets aggregate to occlude the injured vessels, the median TXB₂ level in the emerging bleeding time blood was 1.7 ng/ml. The median TXB₂ level in plasma was < 1 ng/ml and in serum 275 ng/ml. The levels of immunoreactive 6-keto-PGF_1α were always below determination limit in bleeding time blood (0.2 ng/ml) and in plasma (0.1 ng/ml), whereas in serum the levels ranged between 0.26 and 0.47 ng/ml. The fact that enhanced PGI₂ production in primary haemostasis in skin incisions could not be demonstrated calls for further investigations of possible PGI₂ production with more sensitive assays or in injured large vessels.     

Effect of a selective thromboxane A2 synthetase inhibitor on the systemic changes induced by circulating pancreatic phospholipase A2

Background In acute pancreatitis, pancreatic phospholipase A₂ (PLA2) in the circulating blood hydrolyzes phospholipids contained in plasma lipoproteins, liberating eicosanoid precursors that are subsequently converted to various eicosanoids. The pathophysiological significance of eicosanoid synthesis via this pathway is unknown. The aim of this study was to clarify the role of thromboxane A₂ (TXA₂) synthesis by circulating pancreatic PLA₂ in the pathogenesis of the systemic complications of acute pancreatitis. Methods Guinea pigs were divided into two groups: a control group and an ozagrel group, which received intravenous administration of ozagrel, a selective TXA₂ synthetase inhibitor. Pancreatic PLA₂ was infused intravenously in both groups for 30 min, and systemic changes during the infusion were examined. Results In the control group, there was an increase in plasma thromboxane B₂ (TXB₂) concentration, a decrease in mean arterial pressure and heart rate, a decrease in arterial base excess (BE), bicarbonate concentration (HCO₃ ⁻), and pH, a decrease in platelet count and plasma fibrinogen concentration, and a shortened prothrombin time during the infusion of pancreatic PLA₂. In the ozagrel-treated group, changes in plasma TXB₂ concentration, BE, HCO₃ ⁻, and platelet count were significantly inhibited. Conclusions TXA₂ synthesis by circulating pancreatic PLA₂ contributes to metabolic acidosis and thrombocytopenia during acute pancreatitis.     

Role of calcium in thromboxane B2-mediated injury to rabbit gastric mucosal cells

A sustained increase in cytosolic Ca²⁺ can damage gastric mucosal cells. The present study has examined the role of Ca²⁺ in thromboxane B₂ (TXB₂)-mediated damage of rabbit isolated gastric mucosal cells. Cells were isolated from rabbit oxyntic mucosa by collagenase-EDTA digestion. Cell metabolic activity and cell damage were estimated by alamar blue dye absorbance and trypan blue uptake, respectively. Cellular Ca²⁺ was monitored by indo-1 dye fluorescence. Addition of TXB₂ (10^–6 and 10^–8 M) to the cell suspension resulted in a decrease in metabolic activity, and this effect was reduced when Ca²⁺ was removed from the incubation Ca²⁺ and incubation of cells with the intracellular Ca²⁺ chelator, BAPTA-AM (20 M), reduced cell injury in response to TXB₂. Incubation of cells with the Ca²⁺ ionophore A23187 (1–25 M) resulted in a dose-dependent increase in trypan blue uptake and a reduction in cell metabolism. Cell unjury in response to A23187 were exacerbated by addition of TXB₂ (10^–8 M) to the cell suspension. TXB₂ treatment reduced cellular content of reduced glutathione (GSH), while exogenous GSH addition (10 mM) reduced TXB₂-mediated cell injury. These data demonstrate that TXB₂ can directly injure gastric mucosal cells. Gastric mucosal cellular damage in response to TXB₂ is mediated in part by a disruption of Ca²⁺ homeostasis as well as a reduction in cellular GSH content.This work was supported by a grant from the Medical Research Council of Canada MT6426.     

Thromboxane A2 Production in Allergen-Induced Immediate and Late Asthmatic Responses: Its Possible Role in Inducing the Late Response

To determine whether thromboxane A₂ (TXA₂) participates in allergen-induced asthmatic responses, we measured plasma TXB₂ levels during allergen-induced immediate and late asthmatic responses (IAR and LAR) in atopic asthmatics. We also examined the effect of a TXA₂ synthetase inhibitor, OKY-046, on the responses.

Plasma TXB₂ levels increased in both IAR and LAR, being 2.2 times higher in LAR than in IAR. Plasma TXB₂ levels in LAR reached the peak 4 hours after allergen challenge when FEV₁ had just begun to decrease in LAR. OKY-046 inhibited both IAR and LAR with decreased plasma TXB₂ levels.

We conclude that TXA₂ is produced in allergen-induced asthmatic responses and participates in the responses, especially in inducing LAR.     

Activation of thromboxane A2 receptors mediates endothelial dysfunction in diabetic mice

Background: Diabetes is one of high-risk factors for cardiovascular disease. Improvement of endothelial dysfunction in diabetes reduces vascular complications. However, the underlying mechanism needs to be uncovered. This study was conducted to elucidate whether and how thromboxane A2 receptor (TPr) activation contributes to endothelial dysfunction in diabetes. Methods and Results: Exposure of human umbilical vein endothelial cells (HUVECs) to either TPr agonists, two structurally related thromboxane A₂ (TxA₂) mimetics, significantly reduced phosphorylations of endothelial nitric oxide synthase (eNOS) at Ser¹¹⁷⁷ and Akt at Ser⁴⁷³. These effects were abolished by pharmacological or genetic inhibitors of TPr. TPr-induced suppression of eNOS and Akt phosphorylation was accompanied by upregulation of PTEN (phosphatase and tension homolog deleted on chromosome 10) and Ser³⁸⁰/Thr^382/383 PTEN phosphorylation. PTEN-specific siRNA restored Akt–eNOS signaling in the face of TPr activation. The small GTPase, Rho, was also activated by TPr stimulation, and pretreatment of HUVECs with Y27632, a Rho-associated kinase (ROCK) inhibitor, rescued TPr-impaired Akt–eNOS signaling. In mice, streptozotocin-induced diabetes was associated with aortic PTEN upregulation, PTEN-Ser³⁸⁰/Thr^382/383 phosphorylation, and dephosphorylation of Akt (at Ser⁴⁷³) and eNOS (at Ser¹¹⁷⁷). Importantly, administration of TPr antagonist blocked these changes. Conclusion: We conclude that TPr activation impairs endothelial function by selectively inactivating the ROCK–PTEN–Akt–eNOS pathway in diabetic mice.     

Ramatroban (BAY u 3405): A Novel Dual Antagonist of TXA2 Receptor and CRTh2, a Newly Identified Prostaglandin D2 Receptor

It is known that thromboxane A₂ (TXA₂) contributes to various diseases such as bronchial asthma, ischemic heart disease, cerebrovascular disorders and allergic rhinitis. A number of TXA₂ synthase inhibitors and TXA₂ receptor (TP receptor) antagonists have been developed to treat these diseases. Ramatroban (BAY u 3405) was developed as a potent TP receptor antagonist with excellent efficacy against allergic rhinitis in many animal models and patients. Recent studies also revealed that ramatroban can block the newly identified PGD₂ receptor, chemoattractant receptor‐homologous molecule expressed on Th2 cells (CRTh2). PGD₂ induces migration and degranulation of eosinophils through CRTh2 and contributes to late‐phase inflammation and cell damage. Accordingly, it was considered that ramatroban suppresses the late‐phase inflammation via TP receptor and CRTh2 blockade. In terms of the efficacy on vascular systems, it was revealed that ramatroban can suppress the expression of monocyte chemoattractant protein‐1 (MCP‐1) and adhesion molecules in endothelial cells and prevent exacerbation of inflammation by blocking these responses. According to our recent studies in hypercholesterolemic rabbits ramatroban prevents macrophage infiltration through MCP‐1 downregulation and neointimal formation after balloon injury and attenuates vascular response to acetylcholine. Therefore, ramatroban may be beneficial in the treatment of atherosclerosis.     

Selective cyclooxygenase-2 inhibitor suppresses renal thromboxane production but not proliferative lesions in the MRL/lpr murine model of lupus nephritis

IntroductionProliferative lupus nephritis (LN) is marked by increased renal thromboxane (TX) A₂ production. Targeting the TXA₂ receptor or TXA₂ synthase effectively improves renal function in humans with LN and improves glomerular pathology in murine LN. This study was designed to address the following hypotheses: (1) TXA₂ production in the MRL/MpJ-Tnfrsf6^lpr/J (MRL/lpr) model of proliferative LN is cyclooxygenase (COX)-2 dependent and (2) COX2 inhibitor therapy improves glomerular filtration rate (GFR), proteinuria, markers of innate immune response and glomerular pathology.MethodsTwenty female MRL/lpr and 20 BALB/cJ mice were divided into 2 equal treatment groups: (1) SC-236, a moderately selective COX2 inhibitor or (2) vehicle. After treatment from the age of 10 to 20 weeks, the effectiveness of inhibition of TXA₂ was determined by measuring urine TXB₂. Response endpoints measured at the age of 20 weeks were renal function (GFR), proteinuria, urine nitrate + nitrite (NO_x) and glomerular histopathology.ResultsSC-236 therapy reduced surrogate markers of renal TXA₂ production during early, active glomerulonephritis. When this pharmacodynamic endpoint was reached, therapy improved GFR. Parallel reductions in markers of the innate immune response (urine NO_x) during therapy were observed. However, the beneficial effect of SC-236 therapy on GFR was only transient, and renal histopathology was not improved in late disease.ConclusionsThese data demonstrate that renal TXA₂ production is COX2 dependent in murine LN and suggest that NO production is directly or indirectly COX2 dependent. However, COX2 inhibitor therapy in this model failed to improve renal pathology, making COX2 inhibition a less attractive approach for treating LN.     

Resolution of Corticosteroid-Induced Diabetes in Allergic Bronchopulmonary Aspergillosis with Omalizumab Therapy: A Novel Approach

There are substantial numbers of reports showing that leukotrienes (LTs) play important roles in adult asthma. No definite evidence has been demonstrated that LTs are involved in asthma attacks in children, although it is highly expected. In this report, we demonstrated that the levels of LTB₄ and LTC₄ but not thromboxane B₂ (TXB₂), a stable metabolite of TXA₂, were significantly elevated in the bronchoalveolar lavage fluid, which was obtained from intubated and mechanically ventilated children with severe asthma attacks. This is direct evidence that LTB₄ and LTC₄ predominantly participate in asthma attacks in pediatric patients.     

Thromboxane A<Subscript>2</Subscript> Inhibition

Bronchial asthma is a disease defined by reversible airway obstruction, bronchial hyperresponsiveness and inflammation. In addition to histamine and acetylcholine, recent studies have emphasized the role of arachidonic acid metabolites (leukotrienes, prostaglandins and thromboxane A₂) in the pathogenesis of asthma. Among these mediators, thromboxane A₂ (TXA₂) has attracted attention as an important mediator in the pathophysiology of asthma because of its potent bronchoconstrictive activity. Thromboxane A₂ is believed to be involved not only in late asthmatic responses but also in bronchial hyperresponsiveness, a typical feature of asthma.Strategies for inhibition of TXA₂ include TXA₂ receptor antagonism and thromboxane synthase inhibition. Results of double-blind, placebo-controlled clinical trials have proven the efficacies of the thromboxane receptor antagonist seratrodast and the thromboxane synthase inhibitor ozagrel in the treatment of patients with asthma. Seratrodast and ozagrel are available in Japan for the treatment of asthma. Ramatroban, another thromboxane receptor antagonist, is currently under phase III clinical evaluation in Europe and Japan for the treatment of asthma.The pharmacological profiles of the thromboxane modulators may be improved by combination with leukotriene D₄ receptor antagonists. A multi-pathway inhibitory agent such as YM 158, which is a novel orally active dual antagonist for leukotriene D₄ and thromboxane A₂ receptors, may have potent therapeutic effects in the treatment of bronchial asthma. Large scale clinical trials are necessary to further define the role of thromboxane modulators in the treatment of patients with asthma.     

Possible modulation by endothelin-1, nitric oxide, prostaglandin I2 and thromboxane A2 of vasoconstriction induced by an α-agonist in mesenteric arterial bed from diabetic rats

Summary We hypothesized that in diabetes arterial reactivity to constrictors is attenuated by certain endothelium-derived substances. We examined the vasoconstriction induced by methoxamine (α₁-agonist) in isolated mesenteric arterial beds from streptozotocin (STZ)-induced diabetic rats and age-matched control rats. The dose-response curve for methoxamine was shifted to the right and the maximum contractile response was impaired in mesenteric arterial beds from diabetic rats. The methoxamine vasoconstriction was reduced in endothelium-denuded preparations from controls rats, but increased in those from diabetic rats. Treatment with the nitric oxide synthase inhibitor N^G-nitro-L-arginine enhanced the vasoconstrictions induced by methoxamine in both control and diabetic rats. Indomethacin had no effect on the methoxamine vasoconstriction in control rats, but it shifted the dose-response curve to the left in diabetic rats. Whether given with or without indomethacin, BQ-123, (an ET_A-receptor antagonist) plus BQ-788 (an ET_B-receptor antagonist) shifted the dose-response curve for methoxamine to the right in control rats (while reducing the maximum response) but to the left in diabetic rats. The methoxamine-stimulated release of 6-keto-prostaglandin F₁α from the mesenteric arterial bed in diabetic rats was approximately four times that seen in the control rats, while the methoxamine-induced release of thromboxane B₂ (TXB₂), a metabolite of thromboxane A₂ (TXA₂), was less in diabetic rats than in the control animals. These results suggest that an increased production of prostaglandin I₂ (PGI₂) and decreased formation of TXA₂ could be responsible for the attenuation of the methoxamine-induced mesenteric vasoconstriction seen in diabetic rats, and these changes in the diabetic state could be partly responsible for the lower blood pressure seen in our diabetic rats. [Diabetologia (1998) 41: 1410–1418] Received: 2 March 1998 and in final revised form: 10 August 1998     

Pharmacology of the Thromboxane Receptor Antagonist and Thromboxane Synthase Inhibitor BM‐531

BM‐531 (N‐tert‐butyl‐N'‐[(2‐cyclohexylamino‐5‐nitrobenzene)sulfonyl]urea), a torasemide derivative, is a novel noncarboxylic thromboxane receptor antagonist and thromboxane synthase inhibitor. Indeed, its affinity for human washed platelet TXA₂ receptors labeled with [3H]SQ‐29548 (IC₅₀= 0.0078 μM) is higher than sulotroban (IC₅₀= 0.93 μM) and SQ‐29548 (IC₅₀= 0.021 μM). Moreover, BM‐531 is characterized by a potent antiaggregatory property. Indeed, on one hand, in human citrated platelet‐rich plasma BM‐531 prevents platelet aggregation induced by arachidonic acid (600 μM) (ED₁₀₀= 0.125 μM), U‐46619, a stable TXA₂ agonist (1 μM) (ED₅₀= 0.482 μM) or collagen (1 μg/mL) (percentage of inhibition: 42.9% at 10 μM) and inhibits the second wave of ADP (2 μM)‐induced aggregation. On the other hand, when BM‐531 is incubated in whole blood from healthy donors, the closure time measured by the recently developed platelet function analyser (PFA‐100®) is significantly prolonged. In addition, at the concentrations of 10 and 1 μM, BM‐531 totally prevents the production of TXB₂ by human platelets activated by arachidonic acid. Finally, at 10 μM, BM‐531 significantly prevents rat fundus contractions induced by U‐46619 but not by prostacyclin. These results suggest that BM‐531, which is devoid of the diuretic property of torasemide, can be regarded as a promising antiplatelet agent.     

Cyclooxygenase inhibition and effects of hypoxia on pulmonary circulation and gas exchange in anesthetized dogs

To investigate whether endogenously produced prostanoids are involved in hypoxic pulmonary vasoconstriction, pulmonary hemodynamic and gas exchange parameters and eicosanoid metabolites were measured in 5 anesthetized, artificially ventilated dogs (mean body weight 27 kg). Hypoxia elicited pulmonary vasoconstriction, but blood plasma levels of thromboxane B₂ (TXB₂) and 6-keto-prostaglandin F_1α (6kPGF_1α) (stable metabolites of TXA₂ and prostaglandin I₂, respectively) remained unchanged. Administration of the cyclooxygenase inhibitor indomethacin blocked the synthesis of prostanoids, so that 6kPGF_1α and TXB₂ levels decreased to values below the detection level (10 pg·ml⁻¹) both during normoxia or hypoxia, but did not affect pulmonary vascular resistance or the alveolar-arterial P_O2 difference (Pai - Pa)_O2. The pulmonary vascular bed remained, however, responsive to TXA₂ as evidenced by infusion of the TXA₂ mimetic, U 46619, which significantly increased the pulmonary vascular resistance and (Pai - Pa)_O2. Our data suggest that prostanoids are not involved in eliciting the effects of hypoxia on pulmonary hemodynamics and gas exchange efficiency.     

Simvastatin effects on portal‐systemic collaterals of portal hypertensive rats

Background and Aim: Portal‐systemic collateral vascular resistance and vasoconstrictor responsiveness are crucial in portal hypertension and variceal bleeding control. Statins enhance vasodilators production, but their influence on collaterals is unknown. This study aimed to survey the effect of simvastatin on collaterals. Methods: Partially portal vein‐ligated rats received oral simvastatin (20 mg/kg/day) or distilled water from ?2 to +7 day of ligation. After hemodynamic measurements on the eighth postoperative day, baseline perfusion pressure (i.e. an index of collateral vascular resistance) and arginine vasopressin (AVP, 0.1 nM–0.1 µM) responsiveness were evaluated with an in situ perfusion model for collateral vascular beds. RT‐PCR of endothelial NO synthase (eNOS), inducible NOS (iNOS), cyclooxygenase‐1 (COX‐1), COX‐2, thromboxane A₂ synthase (TXA₂‐S) and prostacyclin synthase genes was performed in parallel groups for splenorenal shunt (SRS), the most prominent intra‐abdominal collateral vessel. To determine the acute effects of simvastatin, collateral AVP response was assessed with vehicle or simvastatin. SRS RT‐PCR of eNOS, iNOS, COX‐1, COX‐2 and TXA₂‐S, and measurements of perfusate nitrite/nitrate, 6‐keto‐PGF1_α and TXB₂ levels were performed in parallel groups without AVP. Results: Acute simvastatin administration enhanced SRS eNOS expression and elevated perfusate nitrite/nitrate and 6‐keto‐PGF1_α concentrations. Chronic simvastatin treatment reduced baseline collateral vascular resistance and portal pressure and enhanced SRS eNOS, COX‐2 and TXA₂‐S mRNA expression. Neither acute nor chronic simvastatin administration influenced collateral AVP responsiveness. Conclusion: Simvastatin reduces portal‐systemic collateral vascular resistance and portal pressure in portal hypertensive rats. This may be related to the enhanced portal‐systemic collateral vascular NO and prostacyclin activities.     

Platelet activation in patients with the Raynaud phenomenon

Background: The Raynaud phenomenon (RP) is an exaggerated and reversible vasospasm of small arteries triggered by cold or emotional stress. Primary RP (PRP) term is used when the underlying condition is unknown. An altered regulation in vascular tone and/or release of soluble mediators from activated platelets plays a role in PRP through an increased oxidative stress. We assessed platelet activation and oxidative stress in patients with PRP by measuring platelet PAC‐1, an index of glycoprotein (Gp) IIb/IIIa receptor activation, thromboxane A₂ (TXA₂), an index of platelet activation and 8‐epi‐prostaglandin F_2α (8‐epi‐PGF_2α), a marker of endogenous in vivo peroxidation. Methods: Eighteen asymptomatic patients with PRP (age 41.37 ± 16.94 years; 17 women, 1 man) and 18 healthy subjects (age of 35.11 ± 13.16 years; 16 women, 2 men) were studied. PAC‐1 was analysed by flow cytometry while circulating TXB₂, a stable metabolite of TXA₂ and 8‐epi‐PGF_2α levels were assessed by ELISA kit. Results: Our results show a significant platelet activation in PRP patients as indicated by increased PAC‐1 expression (65.29 ± 15.24%; P < 0.001), TXB₂ (1477.83 ± 454.04 pg/mL; P= 0.003) and 8‐epi‐PGF_2α circulating levels (42.50 ± 14.14 ng/mL; P < 0.001). An inverse correlation between the degree of PAC‐1 expression and TXB₂ levels (r=?0.527; P= 0.02) was also found in PRP patients, suggesting that downregulation of GpIIb/IIIa receptor expression may occur during thrombocytopoiesis, as a consequence of the chronic exposure to increased TXB₂ concentration. Conclusions: Our study for the first time shows a marked activation of GpIIb/IIIa receptor in asymptomatic patients with PRP and supports antiplatelet therapy in PRP patients.     

The effects of hepatic steatosis on thromboxane A2 induced portal hypertension

Introduction and aimThromboxane (TX) A₂ was identified as an important vasoconstrictor during Zymosan induced portal perfusion pressure (PP) increase. We aimed at investigating whether hepatic steatosis influences the extent of TXA₂-induced portal hypertension.Materials and methodsSprague–Dawley rats were randomly divided into control and steatosis (induced by the special diet) groups. PP and TXB₂ (stable degradation product of TXA₂) in the perfusate were measured after in situ liver perfusion with Zymosan (150 μg/ml, 40–46 min) or U46619 (TXA₂ analog, 0.1 μM/ml, 40–46 min). The number of Kupffer cell (KC) was measured by immunohistochemistry with CD163.ResultsZymosan induced more TXB₂ production and a higher PP increase in control group than in steatosis group despite more CD163 positive KCs in fatty livers. PP and TXB₂ efflux revealed a strong correlation in control group and a moderate correlation in steatosis group. Contrary to the effect of Zymosan, U46619 induced a much higher PP increase in steatosis group than in control group.ConclusionSevere steatosis increased number of KCs, however, PP increase and TXB₂ efflux caused by Zymosan infusion in fatty livers were lower than those in healthy livers. In contrast, TXA₂ analog caused higher PP increase in fatty livers. Targeting the more sensitive response to TXA₂ in fatty livers might be a potential therapy of severe steatosis.     

The significance of platelet-vessel wall prostaglandin equilibrium during exercise-induced stress

Alterations in platelet-generated thromboxane A₂ (TXA₂) and vessel wall-generated prostacyclin (PGI₂) have been assoclated with myocardial ischemia. To examine TXA₂ - PGI₂ equilibrium at rest and during exercise stress, we studied 13 normal subjects and 15 coronary artery disease patients. Plasma TXB₂ and 6-keto-PGF_1α were measured as stable metabolites of TXA₂ and PGI₂, respectively, by radioimmunoassay. In normal subjects, plasma TXB₂ levels increased 24% during exercise from 135 ± 30 to 168 ± 42 pg/ml (p = NS). Plasma 6-keto-PGF_1α levels increased 224% from 54 ± 17 to 175 ± 57 pg/ml (p < 0.05). In coronary artery disease patients, although resting plasma TXB₂ levels (mean 136 ± 43 pg/ml) were comparable to levels in normal subjects, a greater increase (82%) occurred during exercise (mean 248 ± 70 pg/ml; p < 0.02 compared to resting levels). Resting plasma 6-keto-PGF_1α levels (mean 94 ± 28 pg/ml) were also similar to normal subjects but increased only by 43% during exercise (mean 134 ± 53 pg/ml; p = NS compared to resting levels). These data suggest that: in normal subjects TXA₂ and PGI₂ increase during exercise, PGI₂ increasing more than TXA₂, and although coronary disease patients have resting TXA₂ and PGI₂ levels in the normal range, TXA₂ levels increase more than PGI₂ levels during exercise. These observations may have a bearing on the mechanism of exercise-induced angina pectoris in certain coronary artery disease patients.     

Urinary thromboxane A2 metabolites in patients presenting in the emergency room with acute chest pain

Abstract. Objectives . The diagnosis of acute myocardial infarction (MI) is difficult in emergency rooms where large groups of patients present with chest pain. Confirmation of the diagnosis of MI based on the myocardial band of creatine phosphokinase may take a day. A more rapid diagnostic screening procedure is desirable and for this reason we evaluated urine thromboxane. Design . The study consisted of patients presenting with chest pain. Urine samples were obtained in the emergency room and on the following 5 days for those patients who were admitted to the hospital. The urine samples were used to determine the levels of immunoreactive 11-dehydro-thromboxane B₂ (i-11-dehydro-TXB₂) and 2,3-dinor-thromboxane B₂ (i-2,3-dinor-TXB₂). Myocardial infarction was defined as an increase in the myocardial band fraction of plasma creatine phosphokinase (> 5% of the total) and changes in the electrocardiogram. The patients' diagnoses were retrospectively correlated with thromboxane metabolite levels. Setting . The present study took place in the emergency rooms of two major hospitals: Georgetown University Medical Center. Washington DC, and Fairfax Hospital, Virginia, USA. Subjects . The study comprised 369 patients presenting with acute chest pain and consisted of 247 men and 122 women aged 30–94 years. Main outcome measures . The outcome measure of this study was the predictive value of i-11-dehydro TXB₂ and i-2,3-dinor-TXB₂, for the diagnosis of MI, in patients presenting in the emergency room with chest pain. Results . Patients undergoing an MI had significantly higher levels of both thromboxane metabolites in their urine in the emergency room, when compared to patients undergoing a cardiac event other than an MI or to patients with unstable angina. Thromboxane metabolite levels rapidly returned to normal on the days following admission to the hospital. Aspirin intake appeared to significantly decrease the levels of i-11-dehydro-TXB₂, but not that of i-2,3-dinor-TXB₂. Conclusions . The measurement of thromboxane metabolites in the urine may provide a more rapid, accurate and cost-effective means of diagnosing MIs in patients presenting with chest pain.     

Thromboxane synthase inhibitors and receptor antagonists

Summary Platelet activation plays a major role in myocardial infarction and in reocclusion following successful thrombolysis, as corroborated by several clinical studies using aspirin. However, the overall reduction of new vascular complications in patients with symptomatic arterial disease by aspirin was only around 25%. Therefore, there is great interest in finding new means to inhibit platelet activation more efficiently. One line of research has focused on ways to interfere with the action of thromboxane A₂ in a more selective way than aspirin does. As such, the development of thromboxane synthase inhibitors, followed by thromboxane receptor antagonists, raised hopes for a better treatment. However, both classes of drugs have some drawbacks, which could be overcome by combining them. This aim has led to the development of compounds that intrinsically possess both activities. Ongoing research indicates that such a dual inhibitor may indeed be more powerful than either aspirin or drugs with the single actions.     

Effects of interleukin-2 administration on platelet function in cancer patients

Platelet function in 16 patients with metastatlc renal cell carcinoma and melanoma was studied sequentially over the first 96 hr of treatment with moderate and highdose inter-leukin-2 (IL-2). During the first 96 hr of therapy, an increased ex vivo platelet maximal aggregation (MA) response to ADP, epinephrine, and arachidonlc acld was paralleled by a decrease In the peripheral platelet count. Plasma speclmens from patients receiving the moderate dose schedule showed a significant IL-2 induced secretory response of the platelet α-granule components β-thromboglobulin (BTG) and platelet factor 4 (PF4) and the eicosanoid thromboxane B₂ (TBX₂) as measured by RIA. The increase in TXB₂ was highly correlated with MA when analyzed by bivariate regression analysls, whereas the addition of PF4 to TXB₂ in a multiple regression analysls further Increased their correlation to MA. The observed decrease In peripheral platelet count correlated significantly with MA and PF4 secretion. High-dose IL-2-treated patlents showed a statistically significant increase in the percentage of large platelets exceeding 12 fl in diameter and platelet responsiveness to hypotonic shock. These observations suggest that IL-2 therapy results in a reduced peripheral platelet pool, with an increased proportion of the remaining pool of platelets larger, more viable, and actlvated. © 1994 Wiley-Liss, Inc.