血小板活化过程中膜糖蛋白Ⅱb/Ⅲa构象变化的研究

目的　对血小板活化过程中膜糖蛋白（ Ｇ Ｐ） Ⅱｂ／ Ⅲａ 构象变化进行初步探讨。方法　分别用供体荧光（ Ｆ Ｉ Ｔ Ｃ） 与受体荧光（ Ｔ Ｒ） 标记识别 Ｇ ＰⅡｂ／ Ⅲａ 上不同抗原决定簇的单抗。用流式细胞仪检测活化血小板在５３０ ｎｍ 处的荧光强度值,并计算荧光供受体间的荧光共振能量转移值（ Ｆ Ｒ Ｅ Ｔ Ｖ） 。结果　不论何种单抗作为荧光供体,静息态血小板均可被测得一低而稳定的 Ｆ Ｒ Ｅ Ｔ Ｖ（ 平均５ ．５ ％ ） 。血小板被激活时 Ｆ Ｒ Ｅ Ｔ Ｖ 会有显著升高,表明 Ｇ ＰⅡｂ／ Ⅲａ 内的亚单位间发生了位置或（ 和） 方向上的变化,该变化也可因胞外钙离子的清除而发生,但不依赖于纤维蛋白原与其受体的结合。结论　血小板活化时 Ｆ Ｒ Ｅ Ｔ Ｖ 的升高可定性反映出荧光标记单抗所结合的 Ｇ ＰⅡｂ／ Ⅲａ 内部亚单位间所发生的重新排列,这种构象的改变可最终导致纤维蛋白原受体的表达。     

血小板膜糖蛋白酶联免疫测定法的建立及应用

目的用酶联免疫法对血小板膜糖蛋白（ＧＰ）进行定量和定性测定。方法应用ＳＺ系列抗人血小板单克隆抗体建立了以竞争性酶联免疫法定量测定ＧＰⅠｂ、ＧＰⅡｂ、ＧＰⅢａ及α－颗粒膜蛋白１４０（ＧＭＰ－１４０）在不同血小板表面的表达量；又建立了固相间接酶联免疫法定性测定血小板特异性同种抗原。结果每个正常血小板表面含有ＧＰⅠｂ２１８８４±２４９１分子，ＧＰⅡｂ３９１４１±３６１８分子，ＧＰⅢａ４５２７７±５０７６分子。凝血酶活化血小板表达９００２±１０９５个ＧＭＰ－１４０分子／血小板。血小板无力症纯合子或杂合子可被明确鉴别。对１０种血小板特异性同种抗原的表达进行的人群调查结果表明，人类血小板抗原（ＨＰＡ）－１和ＨＰＡ－４同种抗原的表达频率不同于国外报道的人群频率。结论两种酶联免疫测定法有助于血小板ＧＰ以及血小板免疫学的研究，具有较大的临床推广应用价值。     

糖尿病患者血小板膜糖蛋白的改变及其临床意义

目的观察Ⅱ型糖尿病患者血小板膜糖蛋白的变化及其临床意义。方法应用流式细胞术（ＦＣＭ）观察７０例Ⅱ型糖尿病患者及２８名正常人血小板膜上ＧＰⅠｂ、Ⅱｂ、Ⅲａ及Ｐ选择素的表达。以免疫放射法（ＩＲＭＡ）测定血小板膜Ｐ选择素分子数,并以酶联免疫法（ＥＬＩＳＡ）对血浆中的Ｐ选择素进行检测。结果ＦＣＭ测得的血小板膜上Ｐ选择素表达阳性率,糖尿病伴血管病患者为（１０．２８±４．８６）％,明显高于正常人的（３．７５±１．８３）％（Ｐ＜０．０１）及无血管病变者的（４．２５±２．２９）％（Ｐ＜００１）。用ＩＲＭＡ法测得的结果与ＦＣＭ类似。血浆中Ｐ选择素浓度伴血管病变组（６．１８±３．２０）μｇ／Ｌ亦高于正常对照组的（３．４３±１．５６）μｇ／Ｌ（Ｐ＜０．０５）及无血管病变组的（３．７１±２．１２）μｇ／Ｌ（Ｐ＜００５）。相反,血小板上ＧＰⅠｂ的表达在糖尿病伴血管病变组为（８６．３５±１１．００）％明显低于正常对照组的（９２６３±７．５５）％（Ｐ＜０．０５）。而血小板ＧＰⅡｂ、Ⅲａ的表达无改变。结论有血管病变的Ⅱ型糖尿患者血小板膜上和血浆中Ｐ选择素升高,而ＧＰⅠｂ降低。     

流式细胞术分析急性脑缺血患者血小板膜糖蛋白改变

应用流式细胞术（ＦＣＭ）检测急性脑缺血患者血小板膜糖蛋白Ⅱｂ／Ⅲａ复合物（ＧＰⅡｂ／Ⅲａ），ＧＰ１ｂ，α－颗粒膜蛋白（ＧＭＰ－１４０），假血管性血友病因子（ｖＷＦ）等的改变，结果显示病例组ＧＰ阳性细胞百分率及平均荧光强度均较对照线明显升高（Ｐ〈０．００１～０．０５），表明急性离缺血患者血小板处于激活状态，并探讨了ＧＰ在脑缺血发病中的作用ＦＣＭ为研究脑缺血患者血小板活化程度和抗血小板药物疗效提供了一     

EDRF／NO抑制血小板表面膜糖蛋白表达

血管损伤后，血小板活化引起的粘附、聚集增强及其在损伤处血管壁的沉积，在动脉粥样硬化病理形成中有重要意义。大量的证据表明，ＥＤＲＦ／ＮＯ可抑制血小板功能，但其机制不详。可能与其影响血小板表面万分有关。最新的研究证实，ＥＤＲＦ／ＮＯ抑制血酶和ＴＸＡ２类似物诱导的血小板表面糖蛋白Ｐ－选择素、ＣＤ６３和ＧＰⅡｂ－Ⅲａ复合物表达的增强。本文就ＥＤＲＦ／ＮＯ对血小板表面膜糖蛋白表达的抑进行综述，以期揭示ＥＤＲ     

儿童特发性血小板减少性紫癜骨髓巨核细胞成熟状态

用血小板凝血酶敏感素（ＴＳＰ）、血小板膜糖蛋白（ＧＰ）Ⅱｂ／Ⅲａ复合物和ＩｇＧ单克隆抗体，用碱性磷酸酶－抗碱性磷酸酶染色法对７例正常儿童和２７例特发性血小板减少性紫癜（ＩＴＰ）患儿骨髓巨核细胞进行研究。结果发现：正常儿童巨核细胞ＴＳＰ和ＧＰⅡｂ／Ⅲａ含量与细胞成熟程度成正比；２７例患儿的巨核细胞ＧＰⅡｂ／Ⅲａ含量均降低，２３例ＴＳＰ含量减少，１２例巨核细胞上ＩｇＧ含量增高，且ＩｇＧ增高与ＴＳＰ降低有一定关系。提示，在儿童ＩＴＰ的发病过程中，不仅存在血小板破坏增加，而且巨核细胞的成熟受阻，这种受阻可能是血小板抗体作用于巨核细胞的结果。     

PTA1单克隆抗体诱导人血小板活化聚集和胞浆Ca^2＋水平的升高

目的：探讨血小板和Ｔ细胞活化抗原１（ＰＴＡ１）诱导人血小板活化聚集和对血小板胞浆Ｃａ２＋水平影响的机制。方法：血小板聚集与ＡＴＰ释放试验及血小板胞浆Ｃａ２＋水平测定。结果：ＰＴＡ１单克隆抗体（ＭｃＡｂ）体外可诱导人血小板活化聚集，ＥＧＴＡ与ＰＧＩ２可以完全抑制ＰＴＡ１ＭｃＡｂ诱导的血小板活化聚集，ＰＴＡ１ＭｃＡｂＦ（ａｂ′）２对ＣＤ９或ＣＤ４１诱导的血小板活化聚集无明显影响。ＰＴＡ１ＭｃＡｂ促进血小板胞浆Ｃａ２＋水平升高。结论：ＰＴＡ１ＭｃＡｂ的刺激作用与血小板膜表面Ｆｃ受体和ＣＤ４１／ＣＤ６１（ⅡｂⅢａ）复合物有关，促进血小板胞浆Ｃａ２＋水平升高为胞外Ｃａ２＋内流与胞浆内Ｃａ２＋储存释放共同作用所致。     

整合素αⅡbβ3和血小板信号传递

整合素αⅡｂβ３（ＧＰⅡｂⅢａ）是血小板上含量最多的膜糖蛋白，是一种钙依赖性异源二聚体复合物。αⅡｂβ３作为一种粘附分子受体可结合含有ＲＧＤ序列的配体，β３分子上第１０９～１７１和２１１～２２２位氨基酸是识别ＲＧＤ的部位。αⅡｂβ３还可通过αⅡｂ链第２９４～３１４位氨基酸识别和结合纤维蛋白原γ链Ｃ端１２肽（ＨＨＬＧＧＡＫＱＡＧＤＶ）。αⅡｂβ３同时介导血小板跨膜双向信号传递，内外信号传递可调节受体自身结合配体的亲和力状态；外内信号传递可引起继发的血小板反应，包括颗粒内容物的分泌，第二相血小…     

血小板膜糖蛋白Ⅱb／Ⅲa基因Leu33Pro多态性与缺血性脑卒中的 …

目的：探讨中国人缺血性脑卒中与血小板膜糖蛋白（ＧＰ）Ⅱｂ／Ⅲａ基因Ｌｅｕ３３Ｐｒｏ多态性之间的关系。方法：利用ＰＣＲ技术和分子杂交技术对北京地区２９４例缺血性脑卒中患者进行ＧＰⅡｂ／Ⅲａ基因Ｌｅｕ３３Ｐｒｏ多态性的检测和分析。并与２７９例北京地区的非卒中对照进行比较。结果：缺血性脑卒中患者ＧＰⅡｂ／Ⅲａ基因Ｌｅｕ３３Ｐｒｏ多态性的基因型频率和等位基因频率与对照组相比无明显差异。结论：ＧＰⅡｂ／Ⅲａ     

用流式细胞术分析人血小板膜糖蛋白Ⅱb/Ⅲa

用流式细胞术分析人血小板膜糖蛋白Ⅱｂ／Ⅲａ王美健林其燧潘家绮赖悦云血小板膜糖蛋白（ＧＰ）Ⅱｂ／Ⅲａ属整合素家族成员，是纤维蛋白原受体，介导血小板聚集。在先天性血小板无力症时，血小板ＧＰⅡｂ／Ⅲａ较少、缺失或异常。单克隆抗体作为分子探针对于研究血小板膜...     

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.     

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.     

Hemoglobin-based oxygen carriers do not alter platelet functions: study of three chemically modified hemoglobin solutions

OBJECTIVE: Chemically modified hemoglobins are being developed as potential oxygen-carrying blood substitutes (HBOCs). Clinical and preclinical data demonstrate the vasoactive properties of HBOCs by trapping of nitric oxide, which is also known to have platelet inhibitory activities properties. This study evaluated the effects of three structurally different HBOCs (Hb-Dex-BTC, alphaalpha-Hb, and o-raffinose-poly-Hb) on platelet functions in vitro to compare to those elicited by plasma substitutes, such as hydroxyethylstarch. DESIGN: Platelet activation state was assessed using platelet-rich plasma diluted to 20% (v/v) with the different solutions, by main measuring glycoproteins (GPIb, GPIIb/IIIa, and P-selectin) using flow cytometry. Aggregation was assessed by impedance aggregometry on whole blood hemodiluted to 20% (v/v) with the solutions. SETTING: Biological hematology department of the university hospital of Nancy-Brabois. PATIENTS AND PARTICIPANTS: Ten healthy volunteers consent and informed of the study who denied taking any drugs at the time of the experiment. MEASUREMENTS AND RESULTS: None of these solutions induced activation nor modified reactivity of platelets as measured by the surface expression of glycoproteins GPIb, GPIIb/IIIa, and P-selectin. Moreover, none of these solutions induced platelet aggregation when added alone, nor modified the aggregation patterns of platelets induced by collagen (0.5 microg/ml) and thrombin receptor agonist peptide (12.5 microM). CONCLUSIONS: The three tested structurally different HBOCs, as with hydroxyethylstarch, did not alter platelet functions in vitro.     

Blood group A antigen expression in platelets is prominently associated with glycoprotein Ib and IIb. Evidence for an A1/A2 difference

Blood group ABH antigens are associated with platelets as intrinsic determinants and extrinsically adsorbed antigens, and exist both on glycosphingolipids and on glycoproteins (GPs). We now provide direct evidence that the blood group ABH antigens are prominently associated with platelet GPIb and GPIIb. By immunoprecipitation, a murine monoclonal anti-A antibody precipitated surface-biotin-labelled blood group A₁ platelet membrane proteins with electrophoretic characteristics identical to those of GPIb/IX and GPIIb/IIIa. By immunoblotting of SDS-PAGE separated blood group A₁ platelet proteins the monoclonal anti-A antibody bound to proteins with electrophoretic characteristics identical to those of GPIb and GPIIb. When immunoaffinity purified GPIb/IX and GPIIb/IIIa, derived from blood group O, A₁ and A₂ platelets, were employed for immunoblotting, GPIb and GPIIb only from A₁ platelets bound the monoclonal anti-A antibody. By ELISA, wherein monoclonal antibodies specific for GPIb (AP1) and the GPIIb/IIIa complex (AP2) were used to capture and hold antigens from platelet lysate, human anti-A antibodies reacted with these proteins derived from blood group A₁ platelets; proteins from blood group A₂, O and B platelets showed no reactivity. These results indicate that blood group A antigen is associated with GPIb and GPIIb derived from blood group A₁ but not A₂ platelets.     

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.     

Effect of amphotericin B and fluconazole on platelet membrane glycoproteins

BACKGROUND : Fever, chills, and reduced platelet recovery may result when platelets are transfused simultaneously with amphotericin B. Amphotericin B reportedly increases the pitting of membranes in stored platelets. STUDY DESIGN AND METHODS : The effects of amphotericin B and another antifungal agent, fluconazole, on platelet membrane glycoproteins (GP) were examined by the incubation of split aliquots of fresh and stored platelet concentrates (PCs) with these drugs for 3 days in storage bags. To determine the effect of storage, PCs were stored for 5 days, and aliquots removed on Days 1 through 5 were placed in platelet storage bags with 4 micrograms per mL of amphotericin B for 2 to 6 hours. Membrane glycoprotein expression was assessed by flow cytometry with fluorescein isothiocyanate-labeled monoclonal antibodies (MoAbs) directed against the following antigens: GPIb (CD42b), CD63 (an activation protein), P-selectin (CD62), and GPIIb/IIIa (CD41a). RESULTS : Amphotericin B produced a concentration-dependent decrease in the surface binding of CD42b MoAb with no consistent changes in the binding of CD41a, CD63, or CD62 MoAbs after a 3-day exposure. Stored but not fresh PCs showed decreased binding of MoAb CD42b after a 6-hour exposure to amphotericin B (4 micrograms/mL). Fluconazole produced no changes. When the binding of MoAb CD42b to permeabilized platelets was used to measure total platelet content, amphotericin B (4 micrograms/mL) decreased MoAb CD42b binding to a similar degree in fresh and stored platelets. Inhibition of aggregation to ADP and collagen and ADP and epinephrine was seen in stored but not fresh PCs. CONCLUSION : Therapeutic levels of amphotericin B resulted in partial loss of total platelet GPIb in fresh and stored PCs, but decreased surface expression of platelet membrane GPIb only in stored platelets. This difference between fresh and stored platelets may be related to the limited reservoir of GPIb available for redistribution to the membrane in the previously stored PCs and may account for the decreased recovery of transfused platelets observed in some patients receiving amphotericin B.     

Differences in platelet function in patients with acute myeloid leukemia and myelodysplasia compared to equally thrombocytopenic patients with immune thrombocytopenia

Summary. Background: Severe thrombocytopenia is a major risk factor for hemorrhage, but platelet function and bleeding risk at low platelet counts are poorly understood, because of the limitations of platelet function testing at very low platelet counts. Objectives: To examine and compare platelet function in severely thrombocytopenic patients with acute myeloid leukemia (AML) or myelodysplasia (MDS) with that in patients with immune thrombocytopenia (ITP). Methods: Whole blood flow cytometric measurement of platelet activation and platelet reactivity to agonists was correlated with the immature platelet fraction (IPF) and bleeding symptoms. Results: Patients with AML/MDS had smaller platelets, lower IPF and substantially lower platelet surface expression of activated glycoprotein (GP)IIb–IIIa and GPIb, both with and without addition of ex vivo ADP or thrombin receptor‐activating peptide, than patients with ITP. In both ITP and AML/MDS patients, increased platelet surface GPIb on circulating platelets and expression of activated GPIIb–IIIa and GPIb on ex vivo activated platelets correlated with a higher IPF. Whereas platelet reactivity was higher for AML/MDS patients with bleeding than for those with no bleeding, platelet reactivity was lower for ITP patients with bleeding than for those with no bleeding. Conclusions: AML/MDS patients have lower in vivo platelet activation and ex vivo platelet reactivity than patients with ITP. The proportion of newly produced platelets correlates with the expression of platelet surface markers of activation. These differences might contribute to differences in bleeding tendency between AML/MDS and ITP patients. This study is the first to define differences in platelet function between AML/MDS patients and ITP patients with equivalent degrees of thrombocytopenia.     

A new murine monoclonal antibody reports an activation-dependent change in the conformation and/or microenvironment of the platelet glycoprotein IIb/IIIa complex.

Considerable evidence indicates that the glycoprotein (GP) IIb/IIIa complex on human platelets functions as a receptor for fibrinogen, but little is known about the mechanism of receptor "exposure." To investigate this mechanism, our previously described murine monoclonal antibody (10E5) and a new monoclonal antibody (7E3), both of which block the binding of fibrinogen to platelets and bind to GPIIb and/or GPIIIa, were radiolabeled and their rates of binding to native and ADP-activated platelets were studied. At low concentrations, 125I-10E5 bound nearly equally rapidly to both native and activated platelets, whereas 125I-7E3 bound slowly to native platelets and much more rapidly to activated platelets. This increased rate of 7E3 binding is unlikely to be due to an increase in the number of GPIIb/IIIa sites on the surface of activated platelets because: (a) the rate of 10E5 binding was unchanged; (b) the total number of surface GPIIb/IIIa sites increased by only 2-10% with activation as judged by equilibrium binding of near-saturating concentrations of 10E5 and 7E3, and (c) there was less than 1% release of platelet factor 4 with activation, indicating minimal fusion of alpha-granule membranes (a potential source of GPIIb/IIIa) with the plasma membrane. Other activators (epinephrine, thrombin, and ionophore A 23187) also increased the rate of 7E3 binding, as did digestion of platelets with chymotrypsin. Aspirin did not affect the rate of binding of 7E3, whereas apyrase, prostaglandin E1, and dibucaine all inhibited the enhancement of the 7E3-binding rate produced by ADP. These data provide evidence for an activation-dependent change in the conformation and/or microenvironment of the GPIIb/IIIa complex, and offer a method of studying the receptor exposure mechanism that does not rely on the binding of fibrinogen itself.