The basis of platelets: platelets and atherothrombosis: an understanding of the lack of efficacy of aspirin in peripheral arterial disease (PAD) and diabetic patients

Platelet activation subsequent to the adhesion of platelets to the vascular wall results in the release of mediators that promote platelet aggregation, which plays a pivotal role in the development of the polyvascular atherosclerotic disease that can be referred to by the acronym 'ATIS' (AtheroThrombosIS). The currently available antiplatelet drugs used to prevent vascular events in patients with cardiovascular disease, including peripheral arterial disease (PAD), include aspirin and thienopyridines such as clopidogrel. These drugs decrease platelet aggregability, each of them by inhibiting a different pathway of platelet activation and recruitment. Aspirin acts by inhibiting thromboxane A2 (TXA2) formation through the inhibition (acetylation) of cyclo-oxygenase. On the other hand, thienopyridines suppress the platelet aggregation adenosine diphosphate (ADP) pathway by inhibiting the platelet P2Y12 subtype of the ADP receptor. The results of the large ATT (Antithrombotic Trialists' Collaboration) meta-analysis of published clinical studies on aspirin, reported in 2002, confirmed the previous meta-analysis and major trials that treatment with aspirin (mixed with other antiplatelet agents in these large meta-analyses) can prevent vascular events in high-risk patients with cardiovascular disease. However, it must be stressed that specifically in PAD patients no significant effect of aspirin was demonstrated in a more recent meta-analysis. This was also the case for primary and secondary prevention in diabetic patients. In keeping with these observations, neither a five-year follow-up study of Japanese diabetic patients in the JPAD (Japanese Primary Prevention of Atherosclerosis with Aspirin for Diabetes) study, a seven-year follow-up study of UK diabetic patients with PAD in the POPADAD (Prevention of Progression of Arterial Disease and Diabetes) study, nor a very recent Scottish study in the same population of diabetics with PAD revealed a significant beneficial effect for aspirin in preventing ischaemic events. This failure may be a consequence of more rapid recovery of platelet aggregability following each dose of aspirin in these PAD or diabetic populations, with the accelerated platelet turnover resulting in a condition of aspirin resistance. Results of the large scale CAPRIE (Clopidogrel versus Aspirin in Patients at Risk of Ischaemic Events) trial that evaluated clopidogrel in patients with cerebral infarction, myocardial infarction or PAD have found clopidogrel to be significantly more effective than aspirin in preventing ischaemic events in patients with PAD. Furthermore, a subgroup analysis of the study has confirmed the efficacy of clopidogrel in diabetic patients with PAD, showing a significant reduction of events in clopidogrel-treated, compared with aspirin-treated, diabetic patients. These results are also likely to be attributable to the greater frequency of aspirin resistance in aspirin-treated patients in these populations (diabetics and/or PAD). Platelets, through activation and aggregation, have an important role in ATIS. However, although antiplatelet therapy with low-dose aspirin has been reported to prevent vascular events in high-risk patients with cardiovascular disease, recent studies in patients with PAD or diabetes mellitus have failed to support the efficacy of aspirin in preventing vascular events in these patient populations. In contrast, clopidogrel appears to be a useful antiplatelet agent in the prevention of vascular events in patients with PAD or diabetes.     

Influence of combinations of acetylsalicylic acid, acetaminophen, and diclofenac on platelet aggregation

Acetylsalicylic acid (aspirin) is often given together with other nonsteroidal anti-inflammatory drugs and acetaminophen. The latter have been accused in epidemiologic studies to cause an increased cardiovascular risk. We have, therefore, analysed the influence of various such drug combinations on platelet aggregation in vitro. Citrated blood was incubated with either 25 microg/ml acetaminophen, 0.5 microg/ml aspirin, 0.04 microg/ml diclofenac, or buffer; followed by a second of the above-mentioned solutions. After a 20 min incubation, platelet aggregation was assessed with a platelet function analyser (PFA-100), which measures the pore closure time (CT) by aggregating platelets. The length of CT reflects the degree of platelet inhibition. Acetaminophen alone did not affect platelet aggregation. Aspirin and diclofenac both increased CT (184+/-69 s, P<0.01 and 196+/-54 s, P<0.001; control 120+/-13 s). Combinations of either aspirin and diclofenac, aspirin and acetaminophen, or diclofenac and acetaminophen increased CT further (290+/-22 s, 281+/-36 s, 288+/-25 s, respectively, P<0.001). The time sequence of drug application was important: when diclofenac or acetaminophen was added before aspirin, platelet aggregation was less inhibited than when given in opposite order, i.e. aspirin prior to diclofenac or acetaminophen. We conclude that acetaminophen by itself does not affect platelet aggregation, but potentiates the antiaggregatory effect of aspirin or diclofenac. Aspirin given before acetaminophen or diclofenac had a more potent antiaggregatory effect than vice versa. These observations may have clinical implications.     

Efficacy of different doses of aspirin in decreasing blood levels of inflammatory markers in patients with cardiovascular metabolic syndrome

Objectives Inflammation and platelet aggregation and activation are key processes in the initiation of a cardiovascular event. Patients with metabolic syndrome have a high risk of cardiovascular events. This study determined whether small and medium doses of aspirin have anti‐inflammation and antiplatelet aggregation effects in patients with metabolic syndrome. Methods One hundred and twenty‐one consecutive patients with metabolic syndrome were randomized into three groups, receiving 100 mg/day of aspirin, 300 mg/day of aspirin or a placebo, respectively, for 2 weeks. The blood levels of thromboxane B2 (TXB2), a stable product of the platelet aggregation mediator TXA2, 6‐keto‐prostaglandin F1‐α (6‐keto‐PGF1‐α), a stable product of the endogenous cyclooxygenase metabolite prostaglandin I2, and inflammatory mediators including high‐sensitivity C‐reactive protein (hs‐CRP), tumour necrosis factor‐α (TNF‐α) and interleukin‐6 (IL‐6), were determined by ELISA and radioimmunoassay. Key findings The blood levels of hs‐CRP, TNF‐α, IL‐6 and TXB2 were significantly decreased after 2 weeks of treatment with 300 mg/day of aspirin. Patients who received 100 mg/day of aspirin had decreased blood levels of hs‐CRP and TXB2. The blood level of IL‐6 in the 300 mg/day aspirin group was significantly lower than that in the other two groups after 2 weeks of therapy. Aspirin at either dose did not affect the blood level of 6‐keto‐PGF1‐α. Conclusions Aspirin at all doses suppresses the blood levels of inflammatory markers and the platelet aggregation mediator TXA2 in Chinese patients with metabolic syndrome. Since the suppression induced by 300 mg/day of aspirin was greater than that induced by 100 mg/day of aspirin, these data suggest that 300 mg/day of aspirin may be beneficial in decreasing the risk of cardiovascular events in Chinese patients with metabolic syndrome.     

Resistencia a la aspirina: prevalencia,mecanismos de acción y asociación con eventos tromboembólicos. Revisión narrativa

ObjectivesThe purpose of this study is to review the prevalence of aspirin resistance in patients with a high risk of cardiovascular events, and secondly, to investigate its epidemiology and mechanism of action, and the clinical consequences it can provoke.Material and methodsA search was run on PubMed, EMBASE and Reviews Database for English or Spanish articles on aspirin resistance published up to November 2008. Additional studies were obtained by searching the reference lists in the selected articles for articles relevant to our secondary objectives.ResultsAspirin resistance is described as affecting 0 to 57% of the population, and is related to a decreased protective effect against strokes and cardiovascular events. Many modifiable and unmodifiable factors can affect the efficacy of antiplatelet drugs. Possible strategies for overcoming this decreased antiaggregant effect include increasing the aspirin dosage or dual therapy with another antiplatelet agent.ConclusionsLack of response to aspirin decreases its protective effects. However, lack of a standard definition for aspirin resistance, the absence of diagnostic reference methods to identify resistant patients, and the different mechanisms of action involved in platelet aggregation call the clinical importance of this fact into question. Additional well-designed studies are needed to detect patients with real resistance in order to have more effective prevention of cardiovascular morbidity and mortality.     

Thromboxane Receptors Antagonists and/or Synthase Inhibitors

Atherothrombosis is the major cause of mortality and morbidity in Western countries. Several clinical conditions are characterized by increased incidence of cardiovascular events and enhanced thromboxane (TX)-dependent platelet activation. Enhanced TX generation may be explained by mechanisms relatively insensitive to aspirin. More potent drugs possibly overcoming aspirin efficacy may be desirable. Thromboxane synthase inhibitors (TXSI) and thromboxane receptor antagonists (TXRA) have the potential to prove more effective than aspirin due to their different mechanism of action along the pathway of TXA(2). TXSI prevent the conversion of PGH(2) to TXA(2), reducing TXA(2) synthesis mainly in platelets, whereas TXRA block the downstream consequences of TXA(2) receptors (TP) activation.TXA(2) is a potent inducer of platelet activation through its interaction with TP on platelets. TP are activated not only by TXA(2), but also by prostaglandin (PG) D(2), PGE(2), PGF(2α), PGH(2), PG endoperoxides (i.e., 20-HETE), and isoprostanes, all representing aspirin-insensitive mechanisms of TP activation. Moreover, TP are also expressed on several cell types such as macrophages or monocytes, and vascular endothelial cells, and exert antiatherosclerotic, antivasoconstrictive, and antithrombotic effects, depending on the cellular target.Thus, targeting TP receptor, a common downstream pathway for both platelet and extraplatelet TXA(2) as well as for endoperoxides and isoprostanes, may be a useful antiatherosclerotic and a more powerful antithrombotic intervention in clinical settings, such as diabetes mellitus, characterized by persistently enhanced thromboxane (TX)-dependent platelet activation through isoprostane formation and low-grade inflammation, leading to extraplatelet sources of TXA(2). Among TXRA, terutroban is an orally active drug in clinical development for use in secondary prevention of thrombotic events in cardiovascular disease. Despite great expectations on this drug supported by a large body of preclinical and clinical evidence and pathophysiological rationale, the PERFORM trial failed to demonstrate the superiority of terutroban over aspirin in secondary prevention of cerebrovascular and cardiovascular events among ~20,000 patients with stroke. However, the clinical setting and the design of the study in which the drug has been challenged may explain, at least in part, this unexpected finding.Drugs with dual action, such as dual TXS inhibitors/TP antagonist and dual COXIB/TP antagonists are currently in clinical development. The theoretical rationale for their benefit and the ongoing clinical studies are herein discussed.     

阿司匹林抵抗：基础与临床

阿司匹林可通过抑制血栓素A2诱导的血小板聚集,发挥抗血小板疗效,是临床常用的抗血小板药物.然而,临床上部分患者规范服用阿司匹林达不到对血小板的显著抑制,仍有动脉血栓事件发生,称为阿司匹林抵抗.     

Aspirin and Platelet Adenosine Diphosphate Receptor Antagonists in Acute Coronary Syndromes and Percutaneous Coronary Intervention

Platelet activation and aggregation are key components in the cascade of events causing thrombosis following plaque rupture. Antiplatelet therapy is essential in the treatment of patients with acute coronary syndromes (ACS) and for those requiring percutaneous coronary intervention (PCI). Aspirin (acetylsalicylic acid) is a well established antiplatelet therapy and is mandated for secondary prevention of cardiovascular events following ACS. In patients with ACS, the addition of clopidogrel to aspirin is more effective than aspirin alone. For patients undergoing PCI, dual antiplatelet therapy with aspirin and clopidogrel is warranted. Aspirin should be continued indefinitely after PCI. Pretreatment of patients with clopidogrel prior to PCI lowers the incidence of cardiovascular events, yet the optimum timing of drug administration and dose are still being investigated, as is the duration of therapy following PCI. Late-stent thrombosis with drug-eluting stents has pushed the recommendation for duration of clopidogrel therapy up to 1 year and perhaps beyond, in patients without risks for bleeding. The concepts of aspirin and clopidogrel resistance are important clinical questions. No uniform definition exists for aspirin or clopidogrel resistance. Measurements of resistance are often highly variable and do not necessarily correlate with clinical resistance. Noncompliance remains the most prominent mode of resistance. Screening of selected patient populations for resistance or pharmacologic intervention of those patients termed 'resistant' warrants further study.     

Anti-platelet therapy: cyclo-oxygenase inhibition and the use of aspirin with particular regard to dual anti-platelet therapy

Aspirin and P2Y(12) antagonists are commonly used anti-platelet agents. Aspirin produces its effects through inhibition of thromboxane A(2) (TXA(2)) production, while P2Y(12) antagonists attenuate the secondary responses to ADP released by activated platelets. The anti-platelet effects of aspirin and a P2Y(12) antagonist are often considered to be separately additive. However, there is evidence of an overlap in effects, in that a high level of P2Y(12) receptor inhibition can blunt TXA(2) receptor signalling in platelets and reduce platelet production of TXA(2). Against this background, the addition of aspirin, particularly at higher doses, could cause significant reductions in the production of prostanoids in other tissues, e.g. prostaglandin I(2) from the blood vessel wall. This review summarizes the data from clinical studies in which dose-dependent effects of aspirin on prostanoid production have been evaluated by both plasma and urinary measures. It also addresses the biology underlying the cardiovascular effects of aspirin and its influences upon prostanoid production throughout the body. The review then considers whether, in the presence of newer, more refined P2Y(12) receptor antagonists, aspirin may offer less benefit than might have been predicted from earlier clinical trials using more variable P2Y(12) antagonists. The possibility is reflected upon, that when combined with a high level of P2Y(12) blockade the net effect of higher doses of aspirin could be removal of anti-thrombotic and vasodilating prostanoids and so a lessening of the anti-thrombotic effectiveness of the treatment.     

5-Hydroxytryptamine and arachidonic acid metabolites modulate extensive platelet activation induced by collagen in cats in vivo.

1. The pathways contributing to the platelet adhesion/aggregation reaction elicited by collagen microfibrils, administered to cats in vivo, were analysed. 2. The intra-aortic infusion of collagen (100 micrograms kg-1 in 1 min) caused an extensive activation of platelets, as evidenced by the time-dependent drop of free platelet numbers in whole blood, and the increases of 5-hydroxyindoles (5-HI), 5-hydroxytryptamine (5-HT) and thromboxane B2 (TXB2) levels in plasma, prepared from effluent venous blood sampled from the inferior caval vein. 3. 5-HT2 receptor blockade with ketanserin (0.63 mg kg-1 i.v., 10 min) and cyclo-oxygenase inhibition with aspirin (10 mg kg-1 i.v., 10 min) slightly attenuated the peak reduction of free platelets in whole blood in response to collagen without affecting changes in plasma 5-HI. Aspirin, but not ketanserin, reduced the collagen-induced changes in plasma TXB2, prostaglandin E2 (PGE2) and 6K-PGF1 alpha. 4. Dual TXA2 synthetase inhibition/TXA2-prostaglandin endoperoxide receptor antagonism with ridogrel (5 mg kg-1 i.v., 10 min) halved the drop in free platelets, reduced the release of platelet 5-HI, inhibited the increase in plasma TXB2 and elevated that of 6K-PGF1 alpha and PGE2 in response to collagen. 5. Combined treatment with ketanserin and aspirin reduced the collagen-induced drop of free platelets and the release of platelet 5-HI to a similar extent as ridogrel alone; plasma prostanoids were affected as with aspirin alone. 6. Combined administration of ketanserin and ridogrel virtually eliminated the collagen-induced platelet adhesion/aggregation response and release of 5-HI; prostanoids were affected as with ridogrel alone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanisms of aggregation inhibition by aspirin and nitrate-aspirin prodrugs in human platelets

Objectives Aspirin is the mainstay of anti‐platelet therapy in the secondary prevention of cardiovascular disease. However, problems with aspirin safety and resistance demand clinical strategies based on multiple pharmacological approaches. Prodrugs of aspirin may offer beneficial effects in terms of gastro‐intestinal safety and multiple pharmacological approaches. However, the pharmacological profile of aspirin prodrugs in human platelets has not been completed yet. We aimed to compare the effects of aspirin and prodrugs of aspirin ( 1 – 5 ) on human platelet aggregation stimulated by ADP and collagen and associated receptor expression (GPIIb/IIIa and P‐selectin) in platelet‐rich plasma (PRP) and washed platelets (WP). Methods As aspirin is released from prodrugs following esterase hydrolysis we studied the expression and activity of butyrylcholineterase (BuChE) and carboxyesterase (CE) in plasma and platelets. The mechanism of prodrug‐induced platelet aggregation inhibition was explored by studying the effects of plasma and purified human BuChE on aggregation. Finally, the relative contribution of nitric oxide (NO) bioactivity to nitrate‐containing prodrugs of aspirin‐induced inhibition of aggregation was determined using 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxalin‐1‐one (ODQ,) a selective inhibitor of the soluble guanylyl cyclase. Key findings ST0702, 2 , a nicotinic acid‐aspirin codrug was equipotent with aspirin with respect to inhibition of collagen‐induced platelet aggregation. Compound 4 , a NO releasing aspirin was the most potent inhibitor of ADP‐induced platelet aggregation, an effect partially reversed by ODQ. The platelet inhibitory effects of aspirin prodrugs were time‐dependent as the maximal inhibitory effects against collagen‐induced aggregation were achieved by aspirin at 2 min, 1 at 5 min and ST0702 at 15 min. The aspirin prodrugs were significantly less potent in WP than in PRP and the reverse was true of aspirin. In the presence of complete BuChE inhibition in PRP, there was almost complete loss of aspirin prodrug, but not aspirin anti‐aggregatory activity. Interestingly, CE activity was observed in WP and platelet lysate with pNPA substrate. Accordingly, 1 and ST0702 retained 50% and 100% anti‐aggregatory activity at maximal concentrations in WP, which was attenuated in the presence of esterase inhibitor phenylmethylsulphonyl fluoride. Conclusions The inhibitory effect of aspirin prodrugs in PRP is due to prodrug activation by BuChE. In contrast, the platelet‐inhibitory effects of aspirin prodrugs in WP may be mediated through the activity of platelet CE. Compound 4 , a NO‐containing aspirin prodrug, may exert dual inhibitory effects in platelets. Thus, aspirin prodrugs effectively inhibit human platelet aggregation and as such may be an alternative to conventional aspirin.     

Comparison of the effects of drugs on the aggregation of hamster platelets in vivo and in vitro

1. A method is described for measuring the inhibitory effectiveness of drugs on the aggregation by ADP of hamster platelets in vivo.2. The method was used to compare the effects of several drugs, viz. adenosine, imipramine, desmethylimipramine and aspirin, on platelet aggregation in vivo with their in vitro effects measured photometrically.3. The concentrations of adenosine and imipramine present in the cheek pouch after 10 min infusions were measured using radioactively labelled drugs.4. The results show that adenosine (0.4 muM) inhibited platelet aggregation in vivo by 43%, whereas several times this concentration was required to produce the same inhibition in vitro.5. Imipramine and desmethylimipramine (0.4 muM) did not inhibit platelet aggregation in vivo; in vitro, however, desmethylimipramine caused up to 34% inhibition at concentrations as low at 0.25 muM.6. Aspirin (estimated 0.2 mM) inhibited platelet aggregation in vivo by 37% whereas similar inhibition in vitro required about 1 mM aspirin. Sodium salicylate was several times less potent than aspirin in vivo.     

Antiplatelet therapy in atherothrombotic cardiovascular diseases for primary and secondary prevention: a focus on old and new antiplatelet agents

Aspirin has long been the mainstay of primary and secondary prevention against myocardial infarction and ischemic cerebrovascular events. However, the incremental value of aspirin for primary prevention has recently been subject to debate given data from recent large clinical trials, as the net clinical benefit is small. In secondary prevention, aspirin is still strongly recommended. Efforts in obtaining more efficient antiplatelet agents and to reduce cardiovascular morbidity and mortality have led to the development of new adenosine diphosphate (ADP) receptor antagonists, which are superior to clopidogrel. New generation antiplatelet drugs i.e. prasugrel and ticagrelor aim to reduce atherothrombotic events, mortality and stent thrombosis, as well as overcome low- or non-response to clopidogrel. Further agents with antiplatelet properties are being investigated at present. This overview aims to give insights into the rapidly changing field of antiplatelet strategies in cardiovascular diseases.     

Variability in individual responsiveness to aspirin: clinical implications and treatment

Aspirin protects from cardiovascular events because of its antiaggregant effect. The occurrence of new events in patients who take aspirin has been called clinical aspirin resistance. Many authors believe that aspirin resistance must be detected by biochemical tests, although there is no agreement on which is the best. Nor is there agreement on the term aspirin resistance. Tests used in research laboratories are aggregometry (turbidometric and impedance), tests based on activation-dependent changes in platelet surface, and tests based on activation-dependent release from platelets. Point-of-care tests are PFA-100, IMPACT and VerifyNow, which can detect platelet dysfunction that may be due to aspirin effect, but their use for this purpose is not yet recommended. Aspirin response may be modified by different factors: patient's compliance, dose, smoking, hyperlipidemia, hyperglucemia, acute coronary syndrome, percutaneous revascularization, recent stroke, extracorporeal circulation, heart failure, exercise, circadian rhythm, absorption, concomitant medications, polymorphisms. Patients with aspirin resistance may have an increased risk of cardiovascular events, and possible therapeutic options are to increase the dosage, to replace aspirin with another antiaggregant drug or to add another drug. In conclusion, there are many reasons that explain the variability in individual responsiveness to aspirin. The term resistance is probably not exact in describing this phenomenon.     

Clopidogrel and aspirin in cardiovascular medicine: responders or not--current best available evidence

Dual antiplatelet therapy represents an important advance for patients with established coronary artery disease. It is an important strategy for patients with acute coronary syndromes and those undergoing percutaneous transcatheter coronary interventions. Clopidogrel effectively inhibits ADP-induced platelet activation and aggregation by selectively and irreversibly blocking the P2Y(12) receptor on the platelet membrane. Aspirin works by irreversibly acetylating the cyclooxygenase (COX-1) enzyme, thus suppressing the production of thromboxane A(2) (TxA(2)) and inhibiting platelet activation and aggregation. Variable platelet response and potential resistance to therapy has emerged with aspirin and clopidogrel. The definitions of antiplatelet agents variability in responsiveness and nonresponsiveness are discussed. Clopidogrel and aspirin responsiveness as they are measured in the laboratory by various techniques (platelet aggregometry and point-of-care assays such as platelet function analyzer [PFA-100] and rapid platelet function assay [RPFA]) are evaluated. The mechanisms responsible for variations in responsiveness to antiplatelet agents such as clinical, cellular and genetic factors are defined. Aspirin and clopidogrel resistance are emerging clinical entities with potentially severe consequences such as myocardial infarction, stroke or death. The therapeutic interventions to deal with nonresponsiveness are reported, although specific recommendations are not clearly established. In the future, routine measurement of platelet function in patients with cardiovascular disease may become the standard of care. Personalized antithrombotic treatment strategies may be determined by ex-vivo measurements that identify critical pathways influencing thrombotic risk in the individual patient.     

Aspirin resistance: disparities and clinical implications

Abstract Aspirin is one of the most widely prescribed drugs for the prevention of thrombosis in patients with vascular disease. Yet, aspirin is unable to prevent thrombosis in all patients. The term "aspirin resistance" has been used to broadly define the failure of aspirin to prevent a thrombotic event. Whether this is directly related to aspirin itself through biochemical aspirin resistance or treatment failure, or if it is because of aspirin's inability to overcome the thrombogenic aspects of the disease process itself, has not been elucidated. This can have dramatic clinical implications for a variety of vascular disease subsets and is cause for concern, considering the high prevalence of aspirin use for both primary and secondary prevention. Disparities exist in the rates of aspirin resistance among certain patient populations, such as women, patients with diabetes mellitus, and those with heart failure, and across clinical conditions, such as cardiovascular and cerebrovascular disease. Clinical trial data from studies observing resistance have revealed that regardless of study size, dose of aspirin, control for drug interactions and adherence, or assay used to measure platelet function, aspirin resistance is associated with an increased risk for adverse events. Although the evidence is mounting, there has yet to be a consensus on the appropriate clinical response to aspirin resistance.     

Gender differences in the effect of aspirin on retinal ischemia, prostanoid synthesis and nitric oxide production in experimental type 1-like diabetes

BACKGROUND: The protective effect of acetylsalicylic acid (aspirin) against cardiovascular events is known to be weaker in women than in men. The present study was designed to test whether this effect of aspirin differed between sexes in an experimental model of diabetes with retinal ischemia. METHODS: We compared nondiabetic rats and rats after 1, 2 and 3 months of diabetes that were given 2 mg/kg/day p.o. of aspirin from the first day of diabetes. The variables recorded were platelet aggregation, production of thromboxane B(2) (TxB(2)), 6-keto-prostaglandin F(1alpha) and aortic nitric oxide, and the percentage of the retinal surface occupied by horseradish peroxidase (HRP)-permeable vessels. RESULTS: In female rats made diabetic, TxB(2) synthesis was more markedly reduced, and the percentage of HRP-permeable retinal vessels was less markedly reduced, than in their male counterparts. The response to aspirin treatment was weaker in female than in male diabetic rats in terms of inhibition of TxB(2) synthesis, increased nitric oxide production, and prevention of the increase in the percentage of retinal surface covered by HRP-permeable vessels. CONCLUSION: Aspirin was less effective in preventing retinal ischemia in experimental diabetes in female than in male rats.     

Potentiation of ADP-induced aggregation in human platelet-rich plasma by 5-hydroxytryptamine and adrenaline.

1. We have used dose-response curves to quantitate the potentiation of adenosine 5'-diphosphate (ADP)-induced aggregation and thromboxane (TXA2) generation by 5-hydroxytryptamine (5-HT) and adrenaline in human citrated platelet-rich plasma. We have also quantitated the inhibition of these responses by aspirin, ketanserin and yohimbine, singly and in pairs. 2. Ketanserin (5 microM) inhibited TXA2 production and the second wave of platelet aggregation induced by a range of concentrations of ADP alone. This indicates that endogenous 5-HT, released from the platelet dense granules, contributes significantly to responses induced by ADP. 3. When 5-HT (10 microM) was added before ADP, a lower concentration of ADP was required to cause 50% aggregation and TXA2 generation. The ratio of ADP concentrations (CR) to cause 50% aggregation in the presence and absence of 5-HT was 2.1 when only added 5-HT was considered, and 5.0 when endogenous 5-HT was also taken into account. 4. Potentiation of ADP-induced aggregation by 5-HT also occurred in the presence of aspirin, resulting in a CR of 2.3. As expected, ketanserin inhibited potentiation by 5-HT in the presence and absence of aspirin. Although aspirin caused substantial inhibition of aggregation induced by ADP and 5-HT (CR 3.4), further inhibition occurred when ketanserin was also present (CR 6.5). 5. A subthreshold concentration of adrenaline (0.25 microM) caused substantial potentiation of ADP-induced aggregation in the absence (CR 4.0) and presence (CR 2.0) of aspirin. As expected, yohimbine (9 microM) inhibited this potentiation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Triggering by Paf-acether and adrenaline of cyclo-oxygenase-independent platelet aggregation.

Platelet-activating factor (Paf-acether, 1-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine) induced full aggregation and a limited release reaction of human platelets in plasma or in blood. Cyclo-oxygenase inhibition with aspirin only reduced aggregation when induced by threshold amounts of Paf-acether, whereas higher concentrations surmounted inhibition whether tested in citrated or in heparinized platelet-rich plasma or blood. Aspirin-induced inhibition of platelet secretion by Paf-acether was insurmountable and independent of the anti-coagulant used. Paf-acether and adrenaline acted synergistically in inducing aggregation in citrate and heparin. Aspirin in vitro or after oral ingestion at doses that suppressed aggregation induced by arachidonic acid alone, failed to reduce significantly the synergized aggregation induced by Paf-acether alone or combined with adrenaline. Twenty-four hours after the oral ingestion of aspirin, when aggregation by arachidonic acid remained blocked, a slight inhibitory activity on the effect of Paf-acether noted 4 h after aspirin, had ceased. This was probably accounted for by the synthesis of thromboxane A2 by newly formed platelets, since the in vitro addition of aspirin, or of the thromboxane/endoperoxide receptor inhibitor 13-azaprostanoic acid caused the 24 h platelets to behave in a manner similar to platelets collected 4 h after aspirin. The alpha 2-adrenoceptor inhibitor, yohimbine, blocked the direct effect of adrenaline as well as its synergism with Paf-acether. Since the synergistic effect of Paf-acether and adrenaline was maintained when thrombin-degranulated platelets were used, and aspirin remained ineffective against it, it is clear that the augmented platelet responsiveness is not accounted for by the platelet release reaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased platelet expression of glycoprotein IIIa following aspirin treatment in aspirin-resistant but not aspirin-sensitive subjects

Aims

Aspirin is widely used as an anti-platelet agent for cardiovascular prophylaxis. Despite aspirin treatment, many patients experience recurrent thrombotic events, and aspirin resistance may contribute to this. We examined the prevalence of aspirin resistance in a healthy population, and investigated whether the platelet proteome differed in aspirin-resistant subjects.

Methods

Ninety-three healthy subjects received aspirin 300 mg daily for 28 days. Before and at the end of treatment, urine was taken to determine 11-dehydrothromboxane B₂, and blood was taken to measure arachidonic acid (AA)-induced aggregation of platelet-rich plasma and to interrogate the platelet proteome by mass spectrometric analysis with further confirmation of findings using Western blotting.

Results

In two of the 93 subjects, neither AA-induced aggregation nor urinary 11-dehydrothromboxane B₂ was effectively suppressed by aspirin, despite measurable plasma salicylate concentrations, suggesting the presence of true aspirin resistance. Despite no detectable differences in the platelet proteome at baseline, following aspirin a marked increase was seen in platelet glycoprotein IIIa expression in the aspirin-resistant but not aspirin-sensitive subjects. An increase in platelet glycoprotein IIIa expression with aspirin resistance was confirmed in a separate cohort of 17 patients with stable coronary artery disease on long term aspirin treatment, four of whom exhibited aspirin resistance.

Conclusions

In a healthy population, true aspirin resistance is uncommon but exists. Resistance is associated with an increase in platelet glycoprotein IIIa expression in response to aspirin. These data shed new light on the mechanism of aspirin resistance, and provide the potential to identify aspirin-resistant subjects using a novel biomarker.     

糖尿病状态下阿司匹林抵抗的研究进展

阿司匹林作为抗血小板聚集治疗的“金标准”,在预防心脑血管栓塞性疾病方面起着重要的作用。然而在服用阿司匹林后,并非所有的患者均能获得一致的抗血小板聚集效益。临床上,服用治疗剂量的阿司匹林后仍出现心血管事件的现象被称为“阿司匹林抵抗（aspirinresistance,AR）”。发生阿司匹林抵抗的患者中有相当一部分是糖尿病患者。尽管很多糖尿病患者服用阿司匹林进行心血管疾病的一级预防,但获益很低。对于非糖尿病患者,阿司匹林可以降低41％的心血管事件。而在糖尿病患者中心血管事件发生率仅降低了10％。虽然研究显示糖尿病同阿司匹林抵抗高度相关,然而糖尿病患者中高阿司匹林抵抗发生率的原因还不清楚。本文拟就糖尿病患者的阿司匹林抵抗机制及其逆转方法进行探讨。