脑梗死患者阿司匹林抵抗与血小板环氧合酶2相关性的研究

目的探讨脑梗死患者阿司匹林抵抗与血小板环氧合酶2(cyclooxygenase-2,COX-2)的关系。方法 95例脑梗死患者服用阿司匹林(100mg/d)14d后,用二磷酸腺苷(adenosine phosphate,ADP)和花生四烯酸(arachidonic acid,AA)作诱导剂,测定血小板最大聚集率,并通过测定其尿11-脱氢-血栓素B2(11-dehydrothromboxane B2,11-dh-TXB2)的水平观察体内阿司匹林对血小板功能的抑制程度(11-dh-TXB2≤1500pg/mg判定为阳性),同时用蛋白质印迹技术检测了血小板COX-2的表达水平。结果脑梗死患者阿司匹林抵抗和半抵抗的发生率为27.4%,阿司匹林抵抗组尿11-dh-TXB2水平比敏感组显著升高(P<0.001),而阳性率显著降低(P<0.001),血小板COX-2表达明显增多。结论血小板COX-2可能参与了脑梗死患者阿司匹林抵抗的发生机制。     

Evaluation of the platelet count drop method for assessment of platelet function in comparison with “gold standard” light transmission aggregometry

Introduction

Hyporesponsiveness to antiplatelet agents has been linked to an increased risk of major adverse cardiovascular events. However, light transmission aggregometry (LTA), the gold standard methodology for assessing platelet function, requires expertise and is labour-intensive, which render its use in clinical settings impractical. We assessed whether platelet count drop (PCD), a technique widely available in any haematology laboratory, could replace LTA in testing for inhibition of platelet aggregation induced by antiplatelet agents.

Materials and methods

One hundred and sixty-one coronary artery disease patients taking aspirin alone and 91 patients taking a combination of aspirin and clopidogrel were enrolled. Platelet aggregation was measured by LTA and PCD stimulated with 1.6 mM of arachidonic acid (AA) for aspirin and 5 and 20 μM of adenosine diphosphate (ADP) for clopidogrel.

Results

Correlation between AA-induced LTA and PCD was inexistent (r = - 0.043, p = 0.587), while correlation between ADP-induced LTA and PCD was low (r = 0.374, p < 0.0001 for ADP 5 μM and r = 0.402, p < 0001 for ADP 20 μM). PCD, whether stimulated with AA or ADP, overestimated platelet aggregation as assessed by LTA, by 13-18%. The wide 95% limits of agreement suggest that the assays can disagree significantly in individual patients.

Conclusions

Although the PCD method is widely available in non-specialized laboratories, our results demonstrate that there is poor correlation with the current gold standard, i.e. LTA. Thus, PCD should not be used in replacement of LTA to assess antiplatelet responsiveness.     

Ouabain affects platelet reactivity as measured in vitro

Ouabain, a digitalis glycoside and an inhibitor of the Mg2+-dependent Na+-K+ ATPase, was used to probe the role of intracellular Na+ levels in the regulation of platelet reactivity. Platelets preincubated with 10 to 150 microM ouabain exhibited a potentiated aggregation response to collagen (14.4 to 180 micrograms/mL), ADP (4 to 12 microM) and thrombin (0.03 to 0.10 unit/mL). Ouabain markedly decreased the time interval between addition of collagen and the onset of shape change. At submaximal concentrations of collagen, thrombin and ADP, preincubation with ouabain increased the rate and amplitude of the aggregation response. Irreversible aggregation was achieved in ouabain-treated platelets by using concentrations of ADP which induced only reversible aggregation in the absence of ouabain. In addition, chelation of extracellular calcium with EGTA or EDTA (2 mM) failed to block reactivity to collagen, ADP or thrombin in ouabain-treated platelets. These results suggest that ouabain induces a "preactivation state" in platelets, perhaps via modulation of intracellular Na+ levels.     

Simultaneous measurement of adenosine triphosphate release and aggregation potentiates human platelet aggregation responses for some subjects, including persons with Quebec platelet disorder

Platelet aggregometry and dense granule adenosine triphosphate (ATP) release assays are helpful to diagnose platelet disorders. Some laboratories simultaneously measure aggregation and ATP release using Chronolume? a commercial reagent containing D-luciferin, firefly luciferase and magnesium. Chronolume? can potentiate sub-maximal aggregation responses, normalising canine platelet disorder findings. We investigated if Chronolume? potentiates human platelet aggregation responses after observing discrepancies suspicious of potentiation. Among patients simultaneously tested by light transmission aggregometry (LTA) on two instruments, 18/43 (42%), including 14/24 (58%) with platelet disorders, showed full secondary aggregation with one or more agonists only in tests with Chronolume?. As subjects with Quebec platelet disorder (QPD) did not show the expected absent secondary aggregation responses to epinephrine in tests with Chronolume?, the reason for the discrepancy was investigated using samples from 10 QPD subjects. Like sub-threshold ADP (0.75 μM), Chronolume? significantly increased QPD LTA responses to epinephrine (p<0.0001) and it increased both initial and secondary aggregation responses, leading to dense granule release. This potentiation was not restricted to QPD and it was mimicked adding 1-2 mM magnesium, but not D-luciferin or firefly luciferase, to LTA assays. Chronolume? potentiated the ADP aggregation responses of QPD subjects with a reduced response. Furthermore, it increased whole blood aggregation responses of healthy control samples to multiple agonists, tested at concentrations used for the diagnosis of platelet disorders (p values <0.05). Laboratories should be aware that measuring ATP release with Chronolume? can potentiate LTA and whole blood aggregation responses, which alters findings for some human platelet disorders, including QPD.     

Biological efficacy of low against medium dose aspirin regimen after coronary surgery: analysis of platelet function

The failure of aspirin to inhibit platelet function has been documented in patients undergoing coronary artery bypass graft (CABG) surgery, but the causes of "aspirin-resistance" remain uncertain. The aim of this study was to investigate the efficacy of aspirin in patients undergoing CABG surgery receiving either 100 mg or 325 mg of oral aspirin for 5-days. Platelet function was tested the day before surgery and on day +1 and day +5, and evaluated by changes in collagen-induced thromboxane-A2 (TxA2) release and platelet aggregation following stimulation with collagen, ADP and epinephrine. In all patients, baseline platelet aggregation was significantly inhibited by pre-incubation with in vitro aspirin (150 micromol/l), with a mean reduction in TxA2-release of >or=95.5% (82.3,99.1). After 5-days of oral aspirin, platelet aggregation was significantly inhibited, and was not further inhibited by in vitro aspirin. Oral aspirin was also associated with a >or=99.5% (97.8, 99.7) reduction in TxA2-release, and with the reversal of the second-phase of ADP-induced aggregation which is TxA2-dependent. In addition a single-dose of 325 mg aspirin on the first post-operative morning may have a greater inhibitory effect on collagen-induced aggregation than 100 mg aspirin. Western blot analysis provided no evidence for the presence of COX-2 in platelets, while the up-regulation of p38-MAPK following platelet-stimulation and surgery was seen. The inhibition of COX-2 (NS398) or p38-MAPK (SB203580) activity did not affect platelet aggregation and TxA2-release on day +5. In summary, there was no evidence for inherent or acquired aspirin-resistance in this surgical population, or for the involvement of either COX-2 or p38-MAPK.     

Gingerols and related analogues inhibit arachidonic acid-induced human platelet serotonin release and aggregation.

Gingerols, the active components of ginger (the rhizome of Zingiber officinale, Roscoe), represent a potential new class of platelet activation inhibitors. In this study, we examined the ability of a series of synthetic gingerols and related phenylalkanol analogues (G1-G7) to inhibit human platelet activation, compared to aspirin, by measuring their effects on arachidonic acid (AA)-induced platelet serotonin release and aggregation in vitro. The IC(50) for inhibition of AA-induced (at EC(50)=0.75 mM) serotonin release by aspirin was 23.4+/-3.6 microM. Gingerols and related analogues (G1-G7) inhibited the AA-induced platelet release reaction in a similar dose range as aspirin, with IC(50) values between 45.3 and 82.6 microM. G1-G7 were also effective inhibitors of AA-induced human platelet aggregation. Maximum inhibitory (IC(max)) values of 10.5+/-3.9 and 10.4+/-3.2 microM for G3 and G4, respectively, were approximately 2-fold greater than aspirin (IC(max)=6.0+/-1.0 microM). The remaining gingerols and related analogues maximally inhibited AA-induced platelet aggregation at approximately 20-25 microM. The mechanism underlying inhibition of the AA-induced platelet release reaction and aggregation by G1-G7 may be via an effect on cyclooxygenase (COX) activity in platelets because representative gingerols and related analogues (G3-G6) potently inhibited COX activity in rat basophilic leukemia (RBL-2H3) cells. These results provide a basis for the design of more potent synthetic gingerol analogues, with similar potencies to aspirin, as platelet activation inhibitors with potential value in cardiovascular disease.     

Evidence that the rat is not an appropriate model to study the role of prostaglandins in normal or abnormal platelet aggregation

Abnormal platelet aggregation seen in experimentally induced diabetic, hypercholesterolemic and spontaneously hypertensive rats (SHR) has been linked with increased prostaglandin synthesis. The present study was conducted to examine the role of prostaglandins in rat platelet activation using normal Wistar Kyoto (WKY) and SHR rats. Up to 30 microM ADP did not induce secondary phase of platelet aggregation in rat PRP and up to 30 microM epinephrine did not produce any response in rat PRP. In other experiments ADP (1.0 microM) and epinephrine (2.0 microM) induced typical biphasic aggregation responses in human PRP. Up to 20 microM U46619, a stable analog of prostaglandin H2, did not induce platelet aggregation in rat PRP or washed rat platelets. In contrast 2.0 microM U46619 caused maximal aggregation in human PRP and washed human platelets. Arachidonic acid (1.5-2.0 mM) induced aggregation in washed rat platelets. However, this was associated with excessive (67% and 94%) loss of cytoplasmic LDH. The low concentrations of thrombin (0.04 and 0.05 U/ml), induced two to three-fold increase in aggregation response in SHR platelets as compared to WKY platelets. Higher concentrations of thrombin (0.1 and 0.3 U/ml) induced similar aggregation responses in SHR and WKY platelets. Thrombin (0.04-0.3 U/ml) induced serotonin secretion in a concentration dependent manner. The extent of secretion was the same in SHR and WKY platelets at all concentrations. Thrombin-induced synthesis of thromboxane A2 (TXA2) in WKY and SHR platelets was quantified using a radioimmunoassay for TXB2. Thrombin (0.04-0.3 U/ml) produced TXB2 in WKY and SHR platelets in a concentration dependent manner. The SHR platelets produced significantly larger amounts of TXB2 as compared to WKY platelets. In other experiments aspirin (500 microM) inhibited thrombin (0.05 U/ml) induced TXB2 synthesis by 75% in both WKY and SHR platelets but failed to inhibit aggregation or secretion in either WKY or SHR platelets. Based on these data it is suggested that: (a) rat platelets inspite of their ability to synthesize TXA2 do not require TXA2 for aggregation; and (b) the rat may not be an appropriate model to study the role of prostaglandins in normal or abnormal platelet aggregation.     

Evaluation of laboratory methods routinely used to detect the effect of aspirin against new reference methods

Background

Aspirin, a commonly used antiplatelet agent, blocks platelet thromboxane A₂ (TXA₂) formation from arachidonic acid (AA) by acetylating platelet cyclooxygenase-1 (COX-1). Laboratory methods currently used to detect this antiplatelet effect of aspirin provide variable results. We have reported three methods that assess platelet COX-1 acetylation (inactivation) by aspirin and its direct consequences. The first and second assays use monoclonal anti-human-COX-1 antibodies that only detect acetylated (inactivated) COX-1 and active (non-acetylated) COX-1, respectively. The third method measures platelet production of TXB₂ (the stable metabolite of TXA₂) in vitro in response to AA. We compared the results of these three reference methods with other routinely used methods for assessing the functional consequences aspirin treatment.

Methods

108 healthy volunteers were treated with low-dose aspirin for 7 days. On day 7 following aspirin treatment COX-1 in the platelets was fully acetylated whereas only non-acetylated COX-1 was present in the day 0 platelets. Further, TXB₂ production by day 7 platelets was completely blocked. The following tests were performed on the samples obtained from study participants before and after seven days of aspirin treatment: PFA-100 closure time with collagen/epinephrine cartridge, VerifyNow® (VN) Aspirin Assay, platelet aggregation and ATP secretion using AA, ADP, epinephrine and collagen as agonists.

Results

Comparing the pre-treatment and day 7 values, methods that use AA as platelet agonist (AA-induced platelet aggregation/secretion and VN Aspirin Assay) showed high discriminative power. In contrast, results of the other tests showed considerable overlap between day 7 and day 0 values.

Conclusions

Only assays that clearly distinguish between acetylated and non-acetylated platelet COX-1 are useful for establishing the antiplatelet effect of aspirin. The other tests are not suitable for this purpose.     

The effect of agonists and antagonists of platelet aggregation on von Willebrand factor-mediated platelet agglutination

Ristocetin-induced platelet agglutination (RIPA) in EDTA-treated citrated platelet-rich plasma was reduced to 49 +/- 11% by 1.25 microM ADP, 41 +/- 14% by 1 microM A23187, and 26 +/- 7% by 0.1 microgram/ml platelet activating factor (PAF). The effect of 5-110 microM epinephrine was not dose-dependent, but varied between donors, with RIPA from 56-100% of the control. The inhibitory effects of these agonists were not altered by prior treatment of platelets with aspirin. Prior addition of 200 microM ATP (an ADP receptor antagonist acting at both high and low affinity ADP receptors) prevented the inhibitory action of ADP but not that of A23187 or PAF, suggesting that the inhibitory actions of the latter are not mediated by released ADP. As 700 microM 8-bromoadenosine 5-diphosphate (an ADP receptor antagonist acting mainly at the high affinity receptor) did not prevent ADP-induced inhibition of RIPA, interaction of ADP with the low affinity receptor is presumably responsible for its inhibitory action. As A23187, but not phorbol myristate acetate (0.1 microM) inhibited RIPA, an increase in intracellular calcium ions rather than direct stimulation of protein kinase C appears to mediate agonist-induced inhibition. Cytochalasin B (10.5-21 microM), dibucaine (0.5-1 mM), and prostaglandin E1 (25 nM), added before or after the agonist, prevented or reversed ADP-, A23187-, and PAF-induced inhibition of RIPA, suggesting that the state of the platelet cytoskeleton affects inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of tris on responses of human and rabbit platelets to aggregating agents

Despite reports that Tris [tris (hydroxymethyl)aminomethane] affects platelets, it is often used to buffer suspending media. Human or rabbit platelets were washed and resuspended in Tyrode solution containing apyrase and 0.35% albumin. Addition of 15 mM Tris partially inhibited primary aggregation induced by 10 microM ADP and inhibited aggregation and release of 14C-serotonin from prelabelled platelets stimulated with low concentrations of thrombin (0.05-0.2 U/mL), or collagen. Platelets resuspended in 15 mM Tris, 0.15 M NaCl, 0.35% albumin, pH 7.5, did not aggregate in response to 10 microM ADP whereas platelets in Tyrode-albumin aggregated extensively. Ca2+ (5 mM) did not overcome the inhibition of thrombin-induced aggregation. Tris (15 or 1.5 mM) potentiated aggregation and release induced by sodium arachidonate (20-50 microM) or the ionophore A23187 (0.6-1 microM). Pretreatment of platelets with aspirin did not prevent potentiation by A23187, indicating that it is not mediated through activation of the arachidonate pathway. The inhibitory and potentiating effects of Tris are similar to those of amino sugars, lysine, arginine and primary amines such as methylamine and cadaverine, and may represent general effects of amines on platelets. Potentiation of the effects of some aggregating agents and inhibition of others re-emphasizes the concept that there are several different mechanisms through which aggregation can occur. Tris-based buffers are unsuitable for platelet suspending media and their use as solvents for aggregating agents or inhibitors should be limited.     

Binding of aggregated immunoglobulins to the human platelet Fc receptors: a mechanism of platelet to platelet bridging

Aggregated immunoglobulins react with human platelets by occupying the Fc receptors present on their surface, inducing aggregation and the release reaction. We studied the effect of heat aggregated gammaglobulins (HAGG) on ADP-induced aggregation of platelets. We used the minimum concentration of ADP required to induce a reversible aggregation of platelets without any substantial amount of serotonin (14C-5HT) release. EDTA (5 mM) added at the peak of platelet aggregation resulted in rapid deaggregation of these platelets. However, incubation of platelets with HAGG at a dose that did not by itself induce any aggregation or release reaction, followed by ADP addition resulted in an irreversible platelet aggregation of greater magnitude accompanied by a substantial release of 14C-5HT. The addition of EDTA at the peak of platelet aggregation failed to deaggregate these platelets. To determine whether the augmented aggregation response and the inhibition of deaggregation was due to HAGG or a consequence of platelet release products, we used thrombin-degranulated platelets. The augmented aggregation response and the inhibition of deaggregation due to HAGG and ADP could be demonstrated using these platelets. To confirm that the binding of HAGG to the platelet Fc receptors was responsible for these observations, we incubated platelets with an excess of Fc fragments of IgG prior to the addition of HAGG and ADP. This abolished the aggregation response observed previously. From this study we conclude that interplatelet bridging by HAGG renders the platelets hyperaggregable and appears to be a mechanism involved in maintaining platelet aggregates.     

Synergistic actions of paf-acether and sodium arachidonate in human platelet aggregation. 2. Unexpected results after aspirin intake

The effect of sodium arachidonate and paf-acether on the activation of human platelet rich plasma from volunteers 2.30 to 36 hours after 500 mg of aspirin intake was studied. Concentrations of paf-acether which induce a reversible aggregation in platelet rich plasma (PRP) (0.29-0.029 microM) and concentrations of sodium arachidonate (AA) which don't produce aggregation (0.75-1mM) on the PRP from these volunteers, induced full aggregation when added together. But no cooperation activity was achieved in the 2.30 hours sample. Contrarily to the in vitro studies performed in human normal PRP, ASA (200 micrograms/ml) or indomethacin(12 microM) added to the PRP were unable to suppress the cooperative aggregation effect; neither did apyrase (12U/ml), esculetin (10 microM) or nordihydroguaiaretic acid (0.1 microM) have any action on the activated platelets but the synergistic action is completely suppressed by BW 755C (0.1 mM). TXB2 formation is very low in all these activated samples and insufficient to cause platelet aggregation. These results suggest 2 behaviors of platelets: synergistic activity of paf-acether and exogenous AA in vitro on normal human PRP is mediated mainly through active metabolites of AA formed via cyclooxygenase, as was previously published. When cyclooxygenase is inhibited in vivo by administration of 500 mg ASA, the cooperative effect of agonists is still present but the active aggregating product(s) is probably, formed through a pathway different of that of the cyclooxygenase or lypoxygenase.     

Collagen-induced platelet aggregation:--evidence against the essential role of platelet adenosine diphosphate

The hypothesis that platelet ADP is responsible for collagen-induced aggregation has been re-examined. It was found that the concentration of ADP obtaining in human PRP at the onset of aggregation was not sufficient to account for that aggregation. Furthermore, the time-course of collagen-induced release in human PRP was the same as that in sheep PRP where ADP does not cause release. These findings are not consistent with claims that ADP alone perpetuates a collagen-initiated release-aggregation-release sequence. The effects of high doses of collagen, which released 4-5 microM ADP, were not inhibited by 500 microM adenosine, a concentration that greatly reduced the effect of 300 microM ADP. Collagen caused aggregation in ADP-refractory PRP and in platelet suspensions unresponsive to 1 mM ADP. Thus human platelets can aggregate in response to collagen under circumstances in which they cannot respond to ADP. Apyrase inhibited aggregation and ATP release in platelet suspensions but not in human PRP. Evidence is presented that the means currently used to examine the role of ADP in aggregation require investigation.     

Low molecular weight heparin as an anticoagulant for in vitro platelet function studies

We evaluated the suitability of low molecular weight (LMW) heparin as an anticoagulant for in vitro platelet function tests in 11 normal volunteers. Results with citrated platelets were considered as the standard. Spontaneous platelet aggregation and the aggregation responses to ADP, epinephrine, collagen, ristocetin and thrombin were measured turbidimetrically in an aggregometer. Dose-response and dose-rate curves were constructed for ADP- and epinephrine-induced aggregation. The maximum aggregation response (EDmax) and rate (EDRmax), and the estimated dose of agonist to induce 50% of the maximum response (ED50) and rate (EDR50) were calculated from these curves. The inhibition of ADP-induced aggregation with PGI2 was expressed as per cent inhibition. The release of ATP and TxA2, from platelets aggregated with collagen was measured. No spontaneous aggregation occurred with either anticoagulant. The ED50 and the EDR50 for heparinized platelets were significantly lower for ADP induced aggregation (0.8 +/- 0.3 microM vs 2.1 +/- 1.0 microM [p = 0.001] and 0.4 +/- 0.1 microM vs 0.8 +/- 0.3 microM [p = 0.003]). The EDRmax with ADP was significantly higher (p = 0.004) for heparinized platelets (64.6 +/- 17.0 units/ml vs 50.4 +/- 7.6 units/ml). The heparinized platelets aggregated slightly, but significantly, less in response to ristocetin than the citrated platelets. The response of washed heparinized and citrated platelets to thrombin was not significantly different. The per cent inhibition of ADP aggregation with PGI2, was significantly lower with heparinized platelets. The release of TxA2 and ATP was similar for both anticoagulants. These results indicate that LMW heparin is a satisfactory anticoagulant for platelet aggregation tests.     

LY53857, a 5HT2 receptor antagonist, delays occlusion and inhibits platelet aggregation in a rabbit model of carotid artery occlusion.

The present study was designed to evaluate the effectiveness of the ergoline 5HT2 receptor antagonist, LY53857 in a rabbit model of vascular arterial occlusion. LY53857 (1 and 10 microM) inhibited serotonin amplified platelet aggregation responses to threshold concentrations of ADP in rabbit platelets in vitro. LY53857 (1 microM) not only inhibited the serotonin component of rabbit platelet aggregation, but also inhibited in vitro aggregation induced by ADP (48.7 +/- 16.7% inhibition), collagen (76.1 +/- 15.9% inhibition) and U46619 (65.2 +/- 12.3% inhibition). The effectiveness of this ergoline 5HT2 receptor antagonist in blocking aggregation to ADP, collagen and U46619 may be related to its ability to inhibit a serotonin component of platelet aggregation since rabbit platelets possess high concentrations of serotonin that may be released during aggregation produced by other agents. Based on the effectiveness of LY53857 to inhibit rabbit platelet aggregation, we explored the ability of LY53857 to extend the time to carotid artery occlusion in rabbits following electrical stimulation of the artery. Reproducible carotid artery occlusion was induced in rabbits by moderate stenosis coupled to arterial cross clamping, followed by electrical stimulation. With this procedure, occlusion occurred at 47.0 +/- 7 min (n = 30) after initiation of the electrical stimulation. Animals pretreated with LY53857 (50 to 500 micrograms/kg i.v.) showed a delay in the time to carotid artery occlusion (at 100 micrograms/kg i.v. occlusion time extended to 164 +/- 16 min). Furthermore, ex vivo platelet aggregation from animals treated with LY53857 (300 micrograms/kg i.v.) resulted in 40.5% inhibition of platelet aggregation in response to the combination of ADP (1 microM) and serotonin (1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)     

ADP induced depolarization of human platelet membrane potential

The transmembrane potential of human blood platelets suspended in plasma was investigated by studying the distribution of a radiolabeled permeant ion [14C] thiocyanate. The membrane potential of resting platelets was found to be -54.50 mV +/- 9.23 S.D. with a range of -39 to -76 mV (n = 27). The possibility that platelet activation alters membrane potential or that changes in membrane potential serve as an activation trigger was investigated. Stimulation by ADP (10 microM) resulted in a significant (p less than 0.05) depolarization of the membrane potential. Preincubation with 6 mM EGTA failed to inhibit ADP-induced depolarization even though EGTA effectively prevented primary and secondary aggregation but not shape change. Preincubation with PGE1 inhibited shape change, aggregation, and the ADP-induced depolarization. No significant change in membrane potential was observed following stimulation by epinephrine (50 microM). These results suggest that the initial interaction of ADP and its receptor may involve an inward positive current which can be determined by thiocyanate distribution.     

Abciximab treatment in vitro after aspirin treatment in vivo has additive effects on platelet aggregation, ATP release, and P-selectin expression

To prevent arterial thrombosis, abciximab is administered together with aspirin. However, whether or not there are benefits to combine abciximab with aspirin is not yet well defined. Healthy volunteers were studied for the effect of aspirin+abciximab using sodium arachidonate and adenosine diphosphate (ADP) alone or in combination to induce platelet activation/aggregation. Abciximab produced complete inhibition of platelet aggregation induced with ADP but only 40% inhibition of aggregation induced by 0.75-mmol/l sodium arachidonate. Abciximab added in vitro to platelet-rich plasma (PRP) from platelets from aspirin-treated donors produced an almost complete inhibition of platelet aggregation. Aspirin, and abciximab alone, did not inhibit adenosine triphosphate (ATP) release as thoroughly as aspirin+abciximab did. Abciximab (3–5 μg/ml) produced inhibition of P-selectin expression induced with 5 (from 46.2±6.0% to 27.4±7.0%, P=.002) and 20-μmol/l ADP (from 53.1±8.1% to 35.1±11.0%, P=.019), but no effect was observed when 0.75-mmol/l sodium arachidonate was used (P=.721). Aspirin diminished P-selectin expression in sodium arachidonate-stimulated platelets (from 77.7±11.8% to 40.2±3.6%, P<.0001) in non-aspirinated and platelets from aspirin-treated donors, respectively. Abciximab (3, 4, and 5 μg/ml) added to platelets from aspirin-treated donors decreased P-selectin expression in platelets stimulated with sodium arachidonate from 40.2±8.6% to 25.6±11.5% (P=.027), to 20.5±3.5% (P<.0001), and to 22.5±1.8% (P<.0001). We concluded that the antiplatelet effect of abciximab is greatly increased by aspirin.     

Kinetic parameters of platelet aggregation as an expression of platelet responsiveness

The kinetics of platelet aggregation induced by collagen and by ADP were studied. The maximum aggregation (deltaLTmax) and the ADP and collagen concentrations required to produce half-maximum aggregation (Kd) were determined using platelets obtained from normal individuals, individuals who ingested aspirin and individuals whose platelet-rich plasma (PRP) demonstrated spontaneous aggregation. The Kd and deltaLTmax for ADP-induced platelet aggregation were variable and markedly affected by the citrate concentration. Conversely, kinetic parameters of collagen-induced aggregation were more reproducible and less affected by citrate. The Kd for collagen in platelet aggregation was increased following aspirin ingestion and decreased in samples of PRP that demonstrated spontaneous aggregation. These results suggest that kinetic parameters of platelet aggregation may be useful to express the responsiveness of platelets.     

Aspirin treatment does not attenuate platelet or leukocyte activation as monitored by whole blood flow cytometry

Despite undoubtful clinical evidence of anti-platelet effects of aspirin, previous studies demonstrate little inhibition by aspirin treatment of single platelet activation as measured by flow cytometry. This discrepancy was further evaluated using flow cytometric measurements of circulating platelet-leukocyte aggregates (PLAs), which may be a more sensitive marker of platelet activation in vivo than measurements of activation markers on single platelets. Blood samples were obtained from 15 healthy subjects before and after aspirin treatment (75 and 500 mg daily for 1 week). Platelet (P-selectin expression) and leukocyte (CD11b expression) activation and platelet-leukocyte aggregation were monitored by whole blood flow cytometry. Approximately 1% platelets in unstimulated samples were P-selectin-positive, i.e., circulating activated platelets, and about 3% of leukocytes were circulating as PLAs; neither of these parameters were reduced by aspirin treatment. Circulating platelet micro-aggregates were not influenced by aspirin either. In vitro stimulation with ADP, thrombin, or PAF increased platelet P-selectin expression and thus PLA formation, but these responses were not affected by aspirin. Leukocyte CD11b expression, a marker of leukocyte secretion, was not significantly influenced by aspirin either in unstimulated samples or upon in vitro stimulation. Thus, the present data support the concept that thromboxane generation is of little importance for the activation of single platelets; the platelet inhibiting effect of aspirin is seen mainly in the presence of close cell-cell contact, which enhances the importance of thromboxane. Multiple mechanisms may contribute to the remarkable clinical anti-thrombotic effect of aspirin.