Why single daily dose of aspirin may not prevent platelet aggregation

The effect of different doses of aspirin on the synergistic activity of sodium arachidonate plus platelet activating factor (paf) ADP or collagen in platelet aggregation was studied in human volunteers. Aggregation studies in platelet rich plasma (PRP) showed that aspirinated platelets, unresponsive to arachidonate, when stirred with threshold concentrations of paf, ADP or collagen, reacted differently according to the dose of aspirin and the time elapsed since ingestion. After a single or daily 50 mg dose for 7-10 days independent of elapsed time until blood withdrawal, a complete synergistic activity was obtained. In PRP samples obtained 24 hours after the last aspirin intake, a complete synergistic aggregation was achieved after a single dose or after 7-10 days of 500 mg aspirin ingestion; synergistic effect did not appear when blood was drawn 2.5 hours after intake. The thromboxane B2 concentrations were very low in all samples after PRP stimulation with sodium arachidonate or paf or both. As rationale is that platelet activation in vivo occurs in response to several stimuli, the therapeutic implications of our results is that aspirin may not prevent the agonist potentiation effect when low dose or daily high dose (500mg) are administrated. This may explain the erratic results of most aspirin trials in which this drug was used to suppress platelet function.     

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.     

Exercise-induced changes in platelet aggregation; a comparison of whole blood and platelet rich plasma techniques

Studies have been performed to assess the effect of exercise on spontaneous platelet aggregation in shaken whole blood, and on agonist-induced platelet aggregation in whole blood and platelet rich plasma (PRP). Spontaneous platelet aggregation in shaken whole blood was increased following exercise compared to pre-exercise values. The increase in spontaneous aggregation after exercise correlated inversely with the increase in white cell count in whole blood. Platelet sensitivity in whole blood to adrenaline, collagen and adenosine diphosphate (ADP) was increased following exercise. Changes in platelet sensitivity to adrenaline following exercise correlated with increases in plasma noradrenaline levels but not with changes in blood cell counts. In PRP, platelet sensitivity to ADP and to collagen was increased following exercise when the pre and post-exercise PRP platelet counts were not corrected to allow for the increase in platelet count which occurred with exercise. When the PRP platelet counts were corrected, no changes in platelet sensitivity to any agonist after exercise were observed.     

The effects of argon laser on in vitro aggregation of platelets in platelet rich plasma and whole blood

The effects of an Argon laser on platelet aggregation were studied, since platelets may be exposed to laser energy when used intravascularly. Various preparations of platelets in platelet rich plasma (PRP) and whole blood, with or without aspirin, were tested with the aggregating agents ADP, collagen, thrombin, and epinephrine. Simultaneous release of ATP was also measured in PRP. At relatively low levels of irradiation, platelet aggregation was potentiated. Enhancement was evidenced by an increase in percent aggregation, earlier onset of the reaction, and reduction in the amount of aggregating agent required. In PRP, the mechanism of laser potentiation appeared to be the release of endogenous ATP from platelets. At relatively high levels of irradiation, platelets were destroyed and aggregation abolished. In whole blood, the mechanism was somewhat more complicated since release of ATP occurred from RBCs as well as platelets. Spontaneous aggregation following laser treatment occurred in isolated instances in PRP and in every trial in whole blood preparations. Aspirin ingestion inhibited the laser's effects in PRP but not in whole blood. These results may have important clinical implications for laser angioplasty, and the potentiated aggregation response may prove useful in laboratory studies of platelet function.     

Multiple electrode aggregometry: a new device to measure platelet aggregation in whole blood

Several methods are used to analyse platelet function in whole blood. A new device to measure whole blood platelet aggregation has been developed, called multiple electrode platelet aggregometry (MEA). Our aim was to evaluate MEA in comparison with the single platelet counting (SPC) method for the measurement of platelet aggregation and platelet inhibition by aspirin or apyrase in diluted whole blood. Platelet aggregation induced by different concentrations of ADP, collagen and TRAP-6 and platelet inhibition by apyrase or aspirin were determined in citrateor hirudin-anticoagulated blood by MEA and SPC. MEA indicated that spontaneous platelet aggregation was lower, and stimulated platelet aggregation was higher in hirudin- than citrate-anticoagulated blood. In hirudin-anticoagulated, but not citrate-anticoagulated blood, spontaneous platelet aggregation measured by MEA was inhibited by apyrase. For MEA compared with SPC the dose response-curves of agonist-induced platelet aggregation in citrate- and hirudin-blood showed similar EC50 values for TRAP, and higher EC50 values for ADP (non-significant) and collagen (p < 0.05). MEA and the SPC method gave similar results concerning platelet-inhibition by apyrase and aspirin. MEA was more sensitive than SPC to the inhibitory effect of aspirin in collagen-induced aggregation. In conclusion, MEA is an easy, reproducible and sensitive method for measuring spontaneous and stimulated platelet aggregation, and evaluating antiplatelet drugs in diluted whole blood. The use of hirudin as an anticoagulant is preferable to the use of citrate. MEA is a promising technique for experimental and clinical applications.     

Synergistic effects of platelet-activating factor and other platelet agonists in human platelet aggregation and release: The role of adp and products of the cyclooxygenase pathway

The synergistic effects of platelet-activating factor (PAF) with ADP, collagen, thrombin, A23187, adrenaline, sodium arachidonate and ristocetin in human platelet aggregation and ß-thromboglobulin (ß-TG) release were investigated in citrated platelet-rich plasma (PRP). Synergism in both aggregation and release was present with all agonists except ristocetin.Upon oral intake of aspirin (ASA) the PAF-induced irreversible aggregation as well as the synergistic irreversible aggregation became reversible. Both prior to and after ASA ingestion ADP removal by creatine phosphate/creatine phosphokinase (CP/CPK) resulted in a reduced, reversible platelet aggregation induced by PAF alone or in combination with the other agonists. The ADP-removal and ASA-ingestion also strongly inhibited the ß-TG release. The synergistic aggregation and release were also inhibited by ASA and indomethacin in vitro as well as by the competitive ADP-inhibitor ATP.It is concluded that not only the activation of human platelets by low doses of PAF itself, but also the synergism of PAF and other platelet agonists is highly dependent upon ADP and products of the cyclooxygenase pathway.     

Anesthetics and anticoagulants used in the preparation of rat platelet-rich-plasma alter rat platelet aggregation

Aggregation of platelets in heparin- and citrate-anticoagulated platelet-rich-plasma (PRP) from rats anesthetized with methoxyflurane (M), diethyl ether (E), acepromazine/ketamine (A/K), or sodium pentobarbital (P) is described, as are platelet counts. Platelet counts were highest in heparin- or citrate-PRP from E and A/K anesthetized rats. Collagen and arachidonic acid (AA) induced aggregation in heparin-PRP only, and ADP induced greater aggregation in heparin-PRP than in citrate-PRP. Differences between citrate-PRP and heparin-PRP are probably due to citrate inhibition of platelet aggregation, since addition of citrate to heparin-PRP decreased aggregation, while addition of heparin to citrate-PRP did not alter aggregation. Aggregation of hirudin-PRP was slightly less than heparin-PRP. Anesthetics affected rat platelet aggregation: the rank order of the maximal extent of ADP-induced aggregation in citrate-PRP was M greater than E = A/K greater than P, and that for AA and collagen in heparin-PRP was E = A/K greater than M = P. The correlation between the effect of the anesthetics and activation of the sympathoadrenal system is discussed. It appeared that of the commonly used anticoagulants and anesthetics, heparin and methoxyflurane had the least influence on rat platelet aggregation.     

Newborn platelet dysfunction: a storage pool and release defect.

Per cent aggregation, release and content of adenine nucleotides, and specific radioactivity were evaluated in citrated platelet-rich plasma (PRP) prepared from paired samples of maternal and cord blood. Platelets of newborn infants aggregated normally in response to highdose ADP (20 muM), strong collagen suspensions, and thrombin; however, when compared with PRP from the mothers or from normal adults, per cent aggregation in response to lower concentrations of ADP (2 muM), weak collagen, and part particularly epinephrine was markedly reduced. Nucleotide release after stimulation of the newborns' PRP with the latter two inducers was also impaired. ATP and ADP content of the newborns' platelets was also significantly less than that of their mothers or of normal adults, but specific activity was normal. The data suggest that the impairment of ADP release in the platelets of newborn infants is due to decreased sensitivity to external stimuli. Since metabolic ATP is necessary for the platelet release reaction, it is postulated that the platelet dysfunction results from a lack of metabolic ATP.     

Increased platelet sensitivity among individuals with aspirin resistance - platelet aggregation to submaximal concentration of arachidonic acid predicts response to antiplatelet therapy

Aspirin 'resistance' (AR) is a phenomenon of uncertain etiology describing decreased platelet inhibition by aspirin. We studied whether (i) platelets in AR demonstrate increased basal sensitivity to a lower degree of stimulation and (ii) platelet aggregation with submaximal stimulation could predict responses to aspirin. Serum thromboxane B(2) (TxB(2)) levels and platelet aggregation with light transmission aggregometry (LTA) were measured at baseline and 24 hours after 325 mg aspirin administration in 58 healthy subjects. AR was defined as the upper sixth of LTA (> or = 12%) to 1.5 mM AA. Baseline platelet aggregation with submaximal concentrations of agonists [ADP 2 microM, arachidonic acid (AA) 0.75 mM, collagen 0.375 and 0.5 microg/ml] was greater in AR subjects compared with non-AR subjects, but not with higher concentrations (ADP 5 microM and 20 microM, AA 1.5 mM and collagen 1 microg/ml). Post-aspirin platelet aggregation was elevated in AR subjects with both submaximal and maximal stimulation. Baseline and post-aspirin serum TxB(2) were higher in AR subjects and decreased further with ex-vivo COX-1 inhibition, suggesting incompletely suppressed COX-1 activity. Pre-aspirin platelet aggregation to 0.75 AA demonstrated a dichotomous response with 29/58 subjects having aggregation < or = 15% and 29/58 subjects having aggregation > or = 75%. In the high aggregation group 28% had AR compared to 6% in the non-AR group (p = 0.04). In conclusion, platelets in AR subjects demonstrate increased basal sensitivity to submaximal stimulation, which could predict responses to antiplatelet therapy.     

Effects of bortezomib on platelet aggregation and ATP release in human platelets, in vitro

INTRODUCTION: Proteasome inhibitor bortezomib (PS-341) has been the first proteasome inhibitor that has entered clinical trials with its antiproliferative and proapoptotic effects in patients with multiple myeloma. Recent studies indicate that proteasome inhibitors can be useful in prevention of experimental arterial thrombosis in renovascular hypertensive rat models. The aim of the present study is to investigate the effect of bortezomib on in vitro platelet aggregation and adenosine triphosphate (ATP) release of human platelets. MATERIALS AND METHODS: For this purpose, platelet aggregation was induced in the platelet-rich plasma (PRP) using 3 microg ml(-1) collagen, 5 microM adenosine diphosphate (ADP), 10 microM epinephrine and 1 U ml(-1) thrombin and ATP release was induced by collagen. RESULTS AND CONCLUSIONS: Bortezomib showed an inhibitory effect on platelet aggregation induced by ADP in human PRP in a dose- and time-dependent manner, whereas it had no effect on collagen-, epinephrin and thrombin-induced aggregation. ATP-release reaction induced by collagen was inhibited dose- and time-dependently by bortezomib, even though collagen-induced platelet aggregation was apparently not affected in human PRP. These findings indicate that bortezomib may be an antiaggregating agent and its' effects may be related to adenine nucleotide receptor dependent regulatory proteins which are important for physiological and pathophysiological cellular processes. However, our in vitro studies suggest that this hypothesis is inadequate to explain the observations completely. This phenomenon and its clinical implication justify further clinical investigations.     

Studies on the effect of platelet inhibitors on platelet adhesion to collagen and collagen-induced human platelet activation

The effect of pyridoxal 5'-phosphate (PALP) and trifluoperazine (TFPZ), the calmodulin antagonist, on in vitro platelet adhesion to collagen and collagen-induced platelet activation was studied using platelet-rich-plasma (PRP) or washed platelets (WPL). Platelet aggregation and [14C]-5HT release induced by "threshold" or low concentrations of collagen (0.6 micrograms/ml) in PRP were completely abolished by PALP (24 mM), TFPZ (250 microM) as well as indomethacin (10 microM). At higher concentrations of collagen (10-15 micrograms/ml) in PRP and WPL, the use of stirred and unstirred platelets treated with collagen enabled a distinction to be made between aggregation and adhesion-mediated release reaction. Platelet aggregation and the aggregation-mediated release reaction induced by these concentrations of collagen in stirred platelets were completely abolished by PALP, TFPZ and indomethacin although neither adhesion to collagen nor the adhesion-mediated release reaction of unstirred platelets was significantly affected by these inhibitors. Interestingly, both adhesion and the adhesion-mediated release reaction were abolished by concentrations of PALP 10-40 fold higher than those required to abolish aggregation. Collagen-induced platelet aggregation, but not platelet adhesion, was inhibited in resuspended platelets pretreated with PALP and NaBH4 indicating a separation in the membrane sites involved in aggregation and adhesion. The results further emphasize the distinction between adhesion and aggregation-mediated events with regards to collagen with the latter being more susceptible to inhibition by antiplatelet agents such as PALP and TFPZ.     

Ticlopidine selectively inhibits human platelet responses to adenosine diphosphate.

Platelet aggregation and fibrinogen binding were studied in 15 individuals before and 7 days after the oral administration of ticlopidine (250 mg b.i.d.). Ticlopidine significantly inhibited platelet aggregation induced by adenosine diphosphate (ADP), the endoperoxide analogue U46619, collagen or low concentrations of thrombin, but did not inhibit platelet aggregation induced by epinephrine or high concentrations of thrombin. Ticlopidine inhibited 125I-fibrinogen binding induced by ADP, U46619 or thrombin (1 U/ml). The ADP scavengers apyrase or CP/CPK, added in vitro to platelet suspensions obtained before ticlopidine, caused the same pattern of aggregation and 125I-fibrinogen binding inhibition as did ticlopidine. Ticlopidine did not inhibit further platelet aggregation and 125I-fibrinogen binding induced in the presence of ADP scavengers. After ticlopidine administration, thrombin or U46619, but not ADP, increased the binding rate of the anti-GPII b/III a monoclonal antibody 7E3 to platelets. Ticlopidine inhibited clot retraction induced by reptilase plus ADP, but not that induced by thrombin or by reptilase plus epinephrine, and prevented the inhibitory effect of ADP, but not that of epinephrine, on the PGE1-induced increase in platelet cyclic AMP. The number of high- and low-affinity binding sites for 3H-ADP on formalin-fixed platelets and their Kd were not modified by ticlopidine. These findings indicate that ticlopidine selectively inhibits platelet responses to ADP.     

Characterization of rat platelet serotonin receptors with tryptamine agonists and the antagonists: ketanserin and SCH 23390.

Current literature suggests that the platelet 5-HT2 receptor, thought to be the only active platelet serotonin (5-HT) receptor, may be both heterogeneous and not the sole 5-HT receptor subtype on platelet membranes. The present studies used more selective tryptamine agonists, and the antagonists ketanserin and SCH 23390 to characterize rat platelet 5-HT receptors in vitro. The present studies also addressed anticoagulant effects (citrate versus heparin) on platelet 5-HT aggregation in the rat. 5-HT was less potent at enhancing ADP-induced aggregation in heparinized rat platelet rich plasma (PRP) as compared to citrated PRP. However, potency and maximum aggregation to ADP were greater in heparinized platelets. In citrated rat PRP, the selective tryptamine agonists, 5-carboxamidotryptamine (5-CT) and 2-CH3-5-HT, produced little change in the baseline ADP-induced aggregation and induced platelet shape change only in higher concentrations (greater than 1 microM). In contrast, alpha-CH3-5-HT-induced shape change and enhancement of ADP aggregation were superimposable with that of 5-HT itself suggesting 5-HT2 receptor activation. The antagonists, ketanserin and SCH 23390, inhibited 5-HT enhancement of ADP-induced aggregation with affinity constants consistent with the presence of 5-HT2 receptors as well. Studies with heparinized rat PRP did not unmask activity to 5-CT or 2-CH3-5-HT. Thus, although reports of multiple platelet 5-HT receptors exist, the only detectable, functional 5-HT receptor to enhance aggregation in rat platelets was probably of the 5-HT2 type.     

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.     

Singlet oxygen (1O(2)) disrupts platelet aggregates.

INTRODUCTION: Important mediators of activated polymorphonuclear leukocytes (PMN) are the oxidants HOCl and chloramine, which generate the nonradical photon-emitting oxidant singlet oxygen (1O(2)). Since 1O(2) inhibits platelet aggregation, we became interested in a possible oxidant mediated reversibility of platelet aggregation. METHODS: Chloramine T (CT) is a stable 1O(2) generator that mimics the natural chloramine N-chloro-taurine. Platelet-rich plasma (PRP) was incubated with CT 0-8 min after addition of the aggregation agonist (10 microM adenosine-5'-diphosphate, ADP, or 5 microg/ml collagen) and the aggregation was monitored. Platelet function was also analyzed by the platelet function analyzer, PFA-100. Fifty microliters of 200 micromol/l ADP was added to 400 microl PRP. After 1 min at 37 degrees C, 50 microl of 0 or 30 mmol/l CT was added, and after an incubation for 3 min at 37 degrees C, 50 microl of 25% glutaraldehyde was added. The samples were analyzed in a transmission microscope at x3000 and x7000 magnification. RESULTS: Chloramines inhibit platelet function in PRP: about 1 mM CT suppresses 50% of the aggregatory capacity of thrombocytes in normal PRP (effective dose 50%, ED(50)=1 mM chloramine), which is identical to the ED(50) for CT in whole blood. The ADP- or collagen-induced platelet aggregation can be reversed by addition of CT: up to 2 min after the addition of ADP as the aggregation inducer, the aggregation is reversible to more than 70% by addition of a 1O(2) release-inducer (3 mM CT). In contrast, addition of CT 8 min after the addition of ADP results only in about 50% reversal of platelet aggregation. The electron microscopic images of platelets before ADP, after incubation for 4 min at 20 micromol/l ADP, after incubation for 1 min at 20 micromol/l ADP, and a further incubation for 3 min at 3 mmol/l CT demonstrate an ADP-dependent formation of platelet aggregates, which are disrupted by 1O(2) into the single platelets; a phenomenon comparable to the decomposition of a puzzle or the continental drift of the major earth plates. The morphology of oxidized and unoxidized platelets is similar. CONCLUSION: This study demonstrates that 1O(2) inhibits and reverses platelet aggregation. The physiologic signal action and the direct anticoagulant action of 1O(2) might be a new principle for pharmacologic intervention in atherothrombosis.     

Platelet aggregation following exposure of phospholipase-treated platelet rich plasma to hydrogen peroxide

Hydrogen peroxide at micromolar concentrations (250–500 μM) can induce platelet aggregation of phospholipase-treated PRP. This effect, which occurs independently of the release of ADP, is blocked by aspirin, furosemide, catalase and 2-mercaptoethanol. PRP preincubated with H₂O₂ for 2–5 min. does not respond to Phl-H₂O₂ or collagen. This inhibitory effect is abolished with longer preincubations. In the presence of ADP or epinephrine, H₂O₂ enhances aggregation, if added to PRP with the inducers, and decreases the platelet response to the inducers, if preincubated with PRP for 2 min. The data suggest that micromolar concentrations of H₂O₂, which could be generated at sites of platelet plug formation by granulocytes, could influence the processes of hemostasis and thrombosis.     

Influence of time, temperature and platelet concentration on dog platelet aggregation.

Previously encountered variability in platelet aggregation responses in laboratory and clinical investigations prompted this study of common physical factors that affect aggregability of platelets (specifically, canine platelets). Valid comparison of platelet responses under the various conditions of this study was made possible by calibrating the aggregometer-recorder system using the method described below. Using this technique a direct relationship was found between platelet concentration and the amplitude of the aggregation response. For the first 90 min after venepuncture, the magnitude of platelet aggregation was stable, but deterioration in platelet aggregability occurred after two hr in vitro. Choice of short-term (one hr) storage temperature affected the amplitude of aggregation induced by 30 uM epinephrine (EPI), but not by 21 uM adenosine diphosphate (ADP). EPI-induced aggregation was significantly greater after short term storage at room temperature (20°C) than after storage at freezing (0°C) or body (37°C and 40°C) temperatures. Comparing aggregating agents, the average amplitude of aggregation in response to ADP was significantly greater than to EPI even though, in administering equal weights of agents per volume of platelet rich plasma (PRP), the resulting molar concentration of EPI was stronger than that of ADP (30 uM EPI vs 21 uM ADP).     

The role of platelet prostanoids and dense granule compounds in initial attachment, spreading and aggregation of platelets on collagen substrates

The role of platelet prostanoids, ADP and 5HT in initial attachment, spreading and aggregation of platelets on collagen substrates (CI, CIII, CIV, CV, CC) was studied. A positive linear correlation was found between thrombi-like aggregate formation on collagen substrates and production of platelet prostanoids. No correlation was established between platelet aggregation and 14C-5HT release. Thrombi-like aggregate formation was completely inhibited by indomethacin and TXA2/PGH2 antagonists (13-APA and BM 13.177). Both 13-APA and BM 13.177 had no effect on platelet spreading, while indomethacin inhibited this process by 25%. The ADP-scavenger system (CP/CPK) inhibited platelet aggregation and spreading by 25-30%. Initial attachment was not influenced by aspirin, indomethacin and CP/CPK. The data obtained indicate that platelet aggregation on collagen substrates is mediated by PGH2 and TXA2 production. These compounds slightly affect the platelet spreading. Both platelet spreading and aggregation on collagen substrates are only partially mediated by ADP and 5HT release. Initial attachment of platelets does not depend on the release reaction and PGH2/TXA2 synthesis.     

Effect of alpha-tocopherol administration on platelet function in man

The effect of vitamin E administration on platelet function was evaluated in a group of normal, healthy volunteers. Platelet aggregation induced by collagen, ADP and epinephrine and platelet adhesion to collagen were measured at weekly intervals in 20 men and 27 women divided into 3 experimental groups of 12 individuals each and one control group of 5 men and 6 women. One experimental group was on a 6-week regimen of vitamin E in increasing dosages (400 I. U.--1,200 I. U.), the second group received aspirin, 300 mg every other day, and the third group was on a combination of vitamin E and aspirin. In the control group, platelet function was measured at weekly intervals. In women, vitamin E by itself produced a small but significant reduction of collagen-induced platelet aggregation. A similar trend was seen in men. However, the reduction never reached statistical significance. Adhesiveness to collagen was not affected by aspirin ingestion but showed a highly significant reduction in vitamin E and vitamin E + aspirin treated individuals. These results suggest that vitamin E administration could have a beneficial effect in patients suffering from arterial thromboembolic diseases.     

Glycoprotein Ia/IIa-mediated activation-dependent platelet adhesion to collagen.

In the presence of anti-glycoprotein (GP) IIb/IIIa antibody at the concentration which completely inhibit platelet aggregation, ADP (0.5-3 microM) increased platelet adhesion to collagen in a concentration-dependent manner under static conditions when platelet-rich plasma (PRP) was used for the assay instead of washed platelets. This was also supported by the results of scanning electron microscopic analyses. The ADP-induced platelet adhesion to collagen was inhibited by PGI2 or beraprost, a stable analogue of PGI2, in a concentration-dependent manner (1-10 ng/ml). These findings suggested the presence of activation-dependent platelet adhesion to collagen. ADP-induced platelet adhesion to collagen was almost completely inhibited by anti-GPIa/IIa and anti-GPIIa antibodies. In the present study, we provide the first direct evidence that the activation-dependent platelet adhesion to collagen is induced by ADP and that GPIa/IIa also plays an important role in the mechanisms of this adhesion.