Evidence for an expression of blood group A antigen on platelet glycoproteins IV and V

Summary. Blood group ABO antigens are known to be carried by several platelet glycoproteins (GP), e. g. GPIb, GPIIa, GPIIb, GPIIIa and PECAM. Beside these proteins, we recently observed that blood group A antigen was also expressed on some other uncharacterized platelet proteins (70–90 kDa) having electrophoretic mobilities closely resembling those of GPIV and GPV. These findings prompted us further to characterize these latter ABO-expressing platelet proteins. By antigen capture ELISA, wherein the monoclonal antibodies (mAbs) CLB-IVC7 and CLB-SW16 were used to hold the corresponding antigens GPIV and GPV, human anti-A specifically bound to these proteins derived from A₁-platelets; neither GPIV nor GPV derived from A₂-, B- or O-platelets bound anti-A. In a Western blot assay using immunoprecipitated GPIV and GPV as antigens, mAb anti-A immunostained GPIV and GPV precipitated from A₁, but not from A₂ and O platelets. These results conclusively demonstrate that blood group A antigen is expressed on platelet GPIV and GPV.     

The Frequency and Nature of Blood Group A3

A special survey of 20,826 random blood donor specimens collected in Ontario, Canada yielded one A₃ blood while a review of blood grouping worksheets of 158,000 individuals in this area also yielded one A₃ blood. These frequencies are much lower than the frequency found in Denmark around 1940 by Gammelgaard, 44 per 10⁵, perhaps due to the present availability of high titer immune anti-A. Elution studies on these two A₃ bloods show that both the agglutination-positive and the agglutination-negative cells absorb anti-A, indicating that A₃ is not always an A₂ + O mixture as has been suggested. Fluorescent antibody studies revealed a marked cell-to-cell variability in fluorescence and, therefore, presumably, cell A antigen content. This finding confirms Gammelgaard's earlier report. This variability, together with a threshold value of A antigen required for agglutination, could explain the characteristic appearance of A₃: agglutinates and free cells.     

Specific Agglutination of Human A3 and Ax Erythrocytes by Snail Anti-A

The anti-A lectin of the snail, Otala lactea , produced specific agglutination of human A₃ and A_x erythrocytes. The snail anti-A was specifically absorbed by these erythrocytes and the eluates showed anti-A activity.     

Glycoprotein IIb/IIIa inhibition attenuates platelet-activating factor-induced platelet activation by reducing protein kinase C activity

Summary.  Glycoprotein (GP)IIb/IIIa inhibition may abolish activated leukocyte-induced platelet activation, in which leukocyte-released platelet-activating factor (PAF) is a major mediator. The present study thus investigated if and how GPIIb/IIIa inhibitors interfere with PAF-induced platelet activation. Platelet and leukocyte activation were monitored by flow cytometry and immunoblotting. GPIIb/IIIa inhibitors (c7E3, non-peptide SR121566, and MAb RFGP56) attenuated PAF-induced, but not adenosine diphosphate (ADP)- or thrombin receptor activating peptide ( TRAP)-induced platelet P-selectin expression in whole blood. GPIIb/IIIa blockade enhanced ADP- or TRAP-induced leukocyte CD11b expression, but not the response to PAF. GPIIb/IIIa blockade attenuated PAF-induced, but enhanced ADP- or TRAP-induced platelet–leukocyte aggregation. Under the present experimental conditions, thromboxane A₂ receptor antagonism did not significantly influence PAF-induced platelet activation, and GPIIb/IIIa inhibition did not interfere with calcium mobilization/influx in platelets. Protein kinase C (PKC) blockade inhibited PAF-induced platelet P-selectin expression, and PAF-induced PKC activity was reduced by GPIIb/IIIa inhibition. PAF (=1 µ m ) did not induce MEK 1/2 or ERK 1/2 phosphorylation, whilst thrombin induced marked responses, which were enhanced by GPIIb/IIIa blockade. Thus, GPIIb/IIIa inhibition attenuates PAF-induced platelet activation via inhibiting PKC activity. GPIIb/IIIa blockade enhances thrombin-induced platelet MEK 1/2 and ERK 1/2 activation, and augments ADP- and TRAP-induced leukocyte activation by enhancing platelet–leukocyte aggregation.     

Change to AXB Phenotype in Genetic A2B Following Carcinoma of the Cervix

In a woman of A₂B genotype, phenotypic change from A₂B to A_xB is documented five yean after a successful operation for carcinoma of the cervix. Serum anti-A activity developed in addition to previously noted anti-A₁. There appears to be an absolute decrease in conversion of H to A substance, occurring independently of hematologic or other malignant disease.     

TRA-418, a novel compound having both thromboxane A2 receptor antagonistic and prostaglandin I2 receptor agonistic activities: its antiplatelet effects in human and animal platelets

Summary.  TRA-418 is a novel compound that has been found in our screening for compounds having both thromboxane A₂ (TP) receptor antagonistic and prostaglandin I₂ (IP) receptor agonistic activities. In the binding assays, TRA-418 showed a 10-fold higher affinity to TP-receptors than IP-receptors. TRA-418 inhibited platelet aggregation induced by the TP-receptor agonist, U-46619 and by arachidonic acid at concentrations lower than those required for inhibition of ADP-induced aggregations. Furthermore, TRA-418 inhibited not only platelet aggregation induced by ADP alone, but also that induced by ADP in the presence of the TP-receptor antagonist, SQ-29548. When the IC₅₀ values of TRA-418 for platelet aggregation were estimated in platelet preparations from monkeys, dogs, cats, and rats using ADP and arachidonic acid as the platelet stimulating agents, it was found that the values estimated in monkey platelets were quite similar to those estimated in human platelets. In ex vivo platelet aggregation in monkeys, TRA-418 exhibited significant inhibitory effects on arachidonic acid-induced aggregation in platelet preparations from monkeys treated at 3 µg kg min⁻¹ or higher doses, where neither a significant decrease in blood pressure nor a significant increase in heart rate was observed. These results are consistent with the fact that TRA-418 has a relatively potent TP-receptor antagonistic activity together with a relatively weak IP-receptor agonistic activity.     

The promoting effect of epinephrine on lipopolysaccharide-induced interleukin-8 production in whole blood may be mediated by thromboxane A2

Summary.  Epinephrine is known to enhance lipopolysaccharide (LPS)-induced interleukin (IL)-8 secretion in a platelet dependent manner. To determine whether thromboxane A₂ (TxA₂; a product from activated platelets) is involved in this process, blood samples drawn either before or 2 h after oral administration of 440 mg acetylsalicylic acid (ASA) were stimulated with LPS (5 ng mL⁻¹) and different concentrations of epinephrine were added (0.1–100.0 µmol L⁻¹). ASA ingestion significantly (global P  < 0.05) reduced the enhancing effect of epinephrine on LPS-induced IL-8 release by 15–28%. To further explore whether TxA₂ may be involved in this process, a TxA₂ agonist (U46619) was added to whole blood together with LPS instead of epinephrine. U46619 mimicked the epinephrine effect: 20 ng mL⁻¹ U46619 enhanced LPS-induced IL-8 release by 39% ( P  < 0.05). Furthermore, preincubation of whole blood with 75 µmol L⁻¹ or 150 µmol L⁻¹ SQ29548, a TxA₂ receptor antagonist, completely blocked epinephrine's promoting effect on LPS-induced IL-8 release. Since thrombin-activated platelets have been reported to be important in the production of IL-8 in monocytes through the activation of monocytes by exposed RANTES in a P-selectin-dependent reaction, we suggest that the epinephrine effect is mediated by enhanced TxA₂ production and subsequent rise in the exposure of RANTES and P-selectin on the platelets of whole blood.     

Identification of the gene encoding pro-phenoloxidase A3 in the fruitfly, Drosophila melanogaster

The fruitfly, Drosophila melanogaster , has three proPO genes ( DoxA1 , CG8193 and CG2952 ). DoxA1 has been shown to encode proPO A₁, one of the two proPO isoforms (A₁ and A₃). However, which of CG8193 or CG2952 encodes proPO A₃ has so far remained elusive. In Northern analysis, CG8193 expression was strong during the larval stage, yet expression of CG2952 was not detected at any stage. Immunoblot analyses with specific antibodies detected CG8193 in the larval hemolymph at the mobility of the endogenous proPO A₃, though no signal for CG2952. These results indicate that the expression of CG2952 is very low and that CG8193 is the gene that encodes proPO A₃. Processing of A₁ and A₃ isoforms in adult homogenate and activity of recombinant proPOs were also investigated.     

Type IIB von Willebrand factor with normal sialic acid content induces platelet aggregation in the absence of ristocetin. Role of platelet activation, fibrinogen, and two distinct membrane receptors.

Three preparations of purified von Willebrand factor (vWF), obtained from unrelated patients affected by type IIB von Willebrand disease, were found to have normal sialic acid content (between 129 and 170 nmol/mg of vWF, as compared with 158 +/- 17 nmol/mg in four normal preparations) and to induce platelet aggregation in the presence of physiologic levels of divalent cations and without addition of ristocetin. A monoclonal antibody that blocks the vWF binding domain of the platelet glycoprotein (GP)Ib caused complete inhibition of IIB vWF-induced aggregation. In contrast, a monoclonal antibody that blocks the receptor for adhesive proteins on the platelet GPIIb/IIIa complex failed to inhibit the initial response of platelets to high concentrations of IIB vWF. Moreover, IIB vWF caused agglutination of formalin-fixed platelets that was blocked only by the anti-GPIb antibody, suggesting that the binding of vWF to GPIb, even in the absence of ristocetin, results in platelet-platelet interaction that is followed by exposure of the GPIIb/IIIa receptors for adhesive proteins. Endogenous ADP, normally active platelet metabolism and fibrinogen binding to GPIIb/IIIa were necessary for maximal and irreversible platelet aggregation. In the absence of fibrinogen, however, aggregation was mediated by vWF binding to GPIIb/IIIa. A 52/48-kD tryptic fragment containing the GPIb binding domain of normal vWF completely blocked the aggregation induced by all three IIB vWF preparations. The present study defines in detail the mechanisms involved in IIB vWF-induced platelet aggregation. Moreover, it establishes that the GPIb binding domain of normal and IIB vWF are closely related and that desialylation is not required for the direct interaction of IIB vWF with GPIb.     

Platelet membrane glycoproteins and microvesicles in blood from postoperative salvage: a study in cardiac bypass patients

BACKGROUND: Transfusion of blood collected by intraoperative and postoperative salvage systems has been linked to the development of thrombocytopenia and disseminated intravascular coagulation. Although functional defects have been reported in platelets from unwashed salvaged blood, platelet membrane glycoprotein (GP) composition, a potentially important determinant of function and survival, has not been studied. STUDY DESIGN AND METHODS: Platelets from 22 patients whose blood was salvaged at the completion of surgery were analyzed and compared to platelets obtained from the venous blood from the same patient. Platelet membranes were stained with fluorescein isothiocyanate-conjugated CD41a monoclonal antibody (anti-GPIIb/IIIa) to identify platelets, a phycoerythrin-conjugated monoclonal antibody, CD62 (anti-P-selectin) to identify activated platelets, and CD42b (anti- GPIb) or anti-GPIb/IX to assess GPIb. Samples were analyzed with a flow cytometer using software. RESULTS: Platelets obtained from salvaged blood demonstrated lower GPIb expression (CD42b and GPIb/IX monoclonal antibody binding), higher P-selectin expression, and greater numbers of platelet-derived microvesicles. CONCLUSION: The clinical significance of transfusing blood containing activated platelets and microvesicles merits investigation.     

Asialo von Willebrand factor interactions with platelets. Interdependence of glycoproteins Ib and IIb/IIIa for binding and aggregation.

Asialo von Willebrand factor (AS-vWf) binds to and aggregates normal human platelets in the absence of ristocetin. Maximal specific binding of AS-vWf is 1-2 micrograms vWf protein/10(8) platelets. Despite the specificity of the binding, only 60% of the bound AS-vWf can be dissociated after equilibrium has been reached. We investigated the site of binding and the mechanism of aggregation of platelets by AS-vWf by (a) pre-incubating platelets with either of two monoclonal antibodies, one against glycoprotein Ib (GPIb) or a second against the glycoprotein IIb/IIIa complex (GPIIb/IIIa), and (b) varying the concentration of fibrinogen in the medium. The results of our studies indicate that AS-vWf binds initially to GPIb. This binding then results in the exposure of receptors for AS-vWf on GPIIb/IIIa. In the presence of plasma fibrinogen, both AS-vWf and fibrinogen bind to GPIIb/IIIa. In the presence of plasma fibrinogen, 50% more AS-vWf binds to the platelet, and this additional AS-vWf binds almost exclusively to GPIIb/IIIa. Despite this enhanced binding of AS-vWf in the absence of fibrinogen, platelet aggregation is much less than that which occurs in the presence of plasma fibrinogen. Comparative studies of AS-vWf binding to normal platelets and the platelets of patients with Glanzmann's thrombasthenia reveal decreased binding to the thrombasthenic platelets and a marked decrease in the extent of platelet aggregation. These studies indicate that AS-vWf binding to, and ensuing aggregation of, platelets is different from that observed with intact vWf protein when platelets are stimulated with either ristocetin or thrombin. The AS-vWf binds to GPIb which, in turn, makes additional AS-vWf receptors available on GPIIb/IIIa. If plasma fibrinogen is present, it competes with the AS-vWf for binding to GPIIb/IIIa and causes aggregation of platelets. In the presence of plasma fibrinogen, more of the AS-vWf binds to GPIIb/IIIa, but this AS-vWf is much less effective than fibrinogen in supporting platelet aggregation.     

Thromboxane A2 and related prostaglandins in airways

Summary— Asthma is now thought to be a chronic inflammatory disease of the airways. The roles of prostanoids, thromboxane A₂ (TXA₂) and the prostaglandins (PGs) in the pathogenesis and pathophysiology of asthma have fostered a wealth of studies but remain controversial. TXA₂ and the bronchoconstrictor PGs, PGD₂ and PGF_2α, are generated in greater amounts in asthmatic than in normal subjects. TXA₂ is a potent constrictor of airway smooth muscle, an inducer of acetylcholine release and of airway microvascular leakage. It may participate in the thickening and the remodeling of the airway wall which may contribute to the airway hyperresponsiveness, a typical feature of asthma. Strategies for inhibition of TXA₂ effects include antagonism of the TXA₂ receptor (TP receptor) and inhibition of the thromboxane synthase. TP receptor antagonists could block the effects of all the bronchoconstrictor prostanoids because TXA₂ as well as the bronchoconstrictor PGs act through activation of lung TP receptor. The recent development of specific and potent TP receptor antagonists and inhibitors of thromboxane synthase has provided tools to assess the role of TXA₂ and bronchoconstrictor PGs in the pathophysiology of asthma.     

Platelet structure and function: immunocytochemical localizations of membrane glycoprotein and alpha-granule protein]

The platelet membrane glycoproteins (GPs) are receptors or binding sites for adhesive proteins. GPIb and GPIIb/IIIa complex are major glycoproteins and have important roles, functionally. GPIb plays an essential role in primary hemostasis as receptor for the von Willebrand factor. The GPIIb/IIIa complex acts as the binding site for adhesive proteins on activated platelets and, as such, is essential for platelet aggregation. On the other hand, four adhesive proteins (fibrinogen, fibronectin, thrombospondin and von Willebrand factor), which are present not only in plasma but also in alpha-granules, mediate or modulate the platelet adhesive response. The interaction between these adhesive proteins and platelet membrane GPs are essential for platelet adhesion and aggregation. The present report will focus on the localization of GPIb and GPIIb/IIIa on the platelet surface and that of adhesive proteins in alpha-granules in both resting and activated human platelets.     

Use of Escherichia coli O86: B7 in the Adsorption of Anti-A and Anti-B from Blood Typing Sera

The Gram-negative bacterium, Escherichia coli O₈₆:B7, has somatic A and B antigens that can be utilized to adsorb anti-A and anti-B from blood typing sera. Adsorption of anti-B is highly effective, one adsorption with heat-killed bacteria for one hour being sufficient to remove anti-B from most type A or O sera. Adsorption of anti-A is less efficient, especially with type O sera.
Adsorption with these organisms caused slight loss in activity of an anti-P₁ serum, but no reduction in antibody titer or avidity was detected in adsorptions using a wide range of other human antisera. The bacterial method of processing is a simple and rapid means whereby sera such as anti-Lu^b, anti-k and anti-Vel may be rendered free of anti-B.     

Renal and Cardiovascular Effect of Prostaglandin A2 in Hypertensive Patients

Abstract. The haemodynamic and renal effects of synthetic prostaglandin A₂ (PGA₂) have been studied in 10 hypertensive subjects during a) a short lasting infusion of hypotensive doses of 3–9μg/kg min. of PGA₂; b) a continuous infusion starting with subdepressive doses and extended with hypotensive doses.—The rapid hypotensive effect observed with high doses was accompanied by an increase in cardiac output and renal blood flow without significant changes in the peripheral vascular bed. Hepatic blood flow was decreased. These observations show a preferential renal redistribution of cardiac output. During the administration of subdepressive doses of PGA., a marked renal effect was observed, consisting in an increase in free water clearance and diuresis with no significant modifications of general haemodynamics. The natriuretic effect was much less pronounced suggesting that PGA₂ is not a specific natriuretic hormone.—The haemodynamic and renal effects of endogenous PGs may play an important role in renal functions and in blood pressure regulation.     

OKT3-induced nephrotoxicity is associated with release of group II secretory phospholipase A2

Abstract. Administration of the murine IgG2a CD3 monoclonal antibody OKT3 exerts a transient nephro-toxic effect. Increased levels of group II secretory phospholipase A₂ (sPLA₂-II) might account for this nephrotoxicity as sPLA₂-II induces the biosynthesis of prostaglandins, vasoactive lipid mediators that influence glomerular haemodynamics and renal function. Furthermore, extracellular phospholipases seem to be involved in proximal tubular cell injury. We studied plasma sPLA₂-II levels in relation to circulating creatinine, tumour necrosis factor α, interleukin 6 and C-reactive protein levels in 15 renal allograft recipients receiving rejection treatment with OKT3. As a control group, we studied 15 renal allograft recipients receiving rejection treatment with methyl-prednisolone. A maximal fourfold increase in sPLA₂-II levels was observed 48 h after the first OKT3 administration, preceded by increased tumour necrosis factor α and interleukin 6 levels and accompanied by increased C-reactive protein levels. Creatinine levels reached a maximal increase 72 h after initiation of treatment. During methylprednisolone treatment no increase in any of the studied parameters was observed. Thus, administration of OKT3 induces increased sPLA₂-II levels, presumably via generation of cytokines. We hypothesize that sPLA₂-II may contribute to the nephrotoxic effect of OKT3 by inducing vasoconstrictive prostaglandins and renal tubular cell injury.     

Disorders of platelet function.

Platelets provide for primary hemostasis by forming a hemostatic plug at sites of vascular damage. They also provide a surface for the assembly of the coagulation protein complexes that generate thrombin, serve as a nidus for fibrin clots, and secrete factors involved in wound repair. Normal platelet function can be divided into four phases: adhesion, aggregation, secretion, and expression of procoagulant activity. Platelet adhesion initiates plug formation as platelets adhere to the connective tissue at the edges of a wound within seconds after vascular damage. When damage occurs in regions of slow blood flow, platelets adhere to subendothelial collagen, fibronectin, and laminin. However, when damage occurs in regions of rapid flow, platelet adhesion requires the presence of subendothelial von Willebrand factor (vWf) and a specific platelet receptor, the glycoprotein Ib/IX (GPIb/IX) complex. Following initial adhesion, platelets aggregate to complete the formation of a hemostatic plug. Platelet aggregation requires active platelet metabolism, platelet stimulation by agonists such as ADP, thrombin, collagen, or epinephrine; the presence of calcium or magnesium ions and specific plasma proteins such as fibrinogen or vWf; and a platelet receptor, the glycoprotein IIb/IIIa (GPIIb/IIIa) complex. Thus, platelet stimulation results in the generation of intracellular second messengers that transmit the stimulus back to the platelet surface, exposing protein binding sites on GPIIb/IIIa. Fibrinogen (or vWf) then binds to GPIIb/IIIa and crosslinks adjacent platelets to produce platelet aggregates. Platelet stimulation also results in platelet secretion and the elaboration of platelet procoagulant activity. During secretion, substances are released to propagate the aggregation response and to promote wound healing; the expression of procoagulant activity localizes thrombin generation to the site of vascular damage. Disorders of platelet function can be divided into those of congenital and those of acquired origin. Although congenital disorders are uncommon, acquired disorders are encountered frequently in clinical practice. Congenital absence of GPIb/IX and GPIIb/IIIa results in the Bernard-Soulier syndrome (BSS) and Glanzmann thrombasthenia (GT), respectively. Each is an autosomal recessive bleeding disorder in which absence of a protein complex renders the affected platelets incapable of undergoing either vWf-mediated adhesion (BSS) or fibrinogen-mediated aggregation (GT).(ABSTRACT TRUNCATED AT 400 WORDS)     

健康成人血小板膜糖蛋白的表达特征

目的　观察健康成人血小板６ 种膜糖蛋白（ Ｇ Ｐ） 在不同性别、不同年龄,静息状态与活化状态时的表达特征。方法　用流式细胞仪测定静息状态下及用凝血酶受体活化肽（ Ｔ Ｒ Ａ Ｐ） 激活的血小板所结合单抗的分子数。结果　男性静息血小板和活化血小板中 Ｇ Ｐ Ⅱｂ／ Ⅲａ 和 Ｇ Ｐ Ⅲａ 显示随年龄减少现象,在静息血小板中 Ｇ Ｐ Ⅱｂ／ Ⅲａ 和 Ｇ ＰⅢａ 值与年龄呈负相关,ｒ 值分别为 － ０ ．４０９ 和－ ０ ．５３４ ,其 Ｐ 值均＜ ０ ．０５ ;而女性中未见到类似结果。 Ｔ Ｒ Ａ Ｐ 激活后 Ｇ Ｐ Ｉｂ 在血小板表面表达下降,而 Ｇ Ｐ Ⅱｂ／Ⅲａ 、Ⅲａ 、 Ｐ选择素均增高,其中 Ｐ选择素在激活后的升高值与激活前的基础值呈负相关。静息血小板的 Ｇ ＰⅠｂ 分子数约为３０ ０００ ,它与 Ｇ Ｐ Ⅴ的比值约３∶１ 。结论　健康人血小板 Ｇ Ｐ表达存在性别和年龄差异。     

Cisplatin-induced renal effects and thromboxane A2 receptor blockade

The aim of this study was to evaluate the renal protective effect of linotroban, a thromboxane A₂ receptor antagonist, in 25 patients with malignant tumours scheduled for cisplatin therapy. Cisplatin was administered 1 h after the start of a 24-h continuous infusion of linotroban or placebo. Glomerular filtration rate and effective renal plasma flow were measured. Infusions of cisplatin decreased glomerular filtration rate by 17 ± 25 mL min⁻¹ ( P  = 0.049 vs. baseline) and effective renal plasma flow by 94 ± 150 mL min⁻¹ ( P  = 0.049 vs. baseline) in the placebo group. In the linotroban group a decrease in glomerular filtration rate by 11 ± 18 mL min⁻¹ ( P  = 0.050 vs. baseline) and in effective renal plasma flow by 26 ± 63 mL min⁻¹ ( P  = 0.2 vs. baseline) was noted. However, no difference was noted between groups in response to treatment. Our findings indicate that linotroban may not be useful for prevention of cisplatin's acute nephrotoxic effects.     

Biosynthesis of prostacyclin and thromboxane A2 during chronic hypoxaemia in children with cyanotic congenital heart disease

The high risk of vaso-occlusive events in children younger than 4 years with cyanotic congenital heart disease and polycythaemia has been attributed to increased thromboxane (Tx) A₂ formation. In older children with cyanotic congenital heart disease, however, the risk of vaso-occlusive events is much lower. We therefore hypothesized that the formation of TxA₂ and prostacyclin is not disturbed in this age group. We measured urinary excretion of stable index metabolites of in vivo TxA₂ and prostacyclin formation by gas chromatography–mass spectrometry in nine children (age 5.9–14.4, median 8.7 years) with cyanotic congenital heart disease, and in nine healthy, age-matched control subjects. The patients excreted less 2,3-dinor-TxB₂ (systemic TxA₂ formation, P =0.03), 2,3-dinor-6-keto-PGF_1α (systemic prostacyclin formation, P =0.03) and TxB₂ (renal TxA₂ formation, P =0.01) than the control subjects. We conclude that in children older than 5 years with cyanotic congenital heart disease, endogenous synthesis of TxA₂ and prostacyclin is not stimulated. This result may explain the lower risk of vaso-occlusive events in this age group as compared with younger children. In addition, our results suggest that chronic hypoxaemia may affect the in vivo formation of TxA₂ and prostacyclin and the metabolic disposition of TxB₂.