Platelet Reactivity In Vitro in Relation to Thromboxane in Healthy Pregnancy

There is substantial evidence of increased platelet reactivity in vivo and in vitro during pregnancy. Previous in vitro studies suggest that platelets from pregnant women show increased sensitivity to agonists, the response to which has a thromboxane dependent component. The aim of this study was to determine whether this is due to increased activity of the thromboxane biosynthetic pathway or to increased platelet sensitivity to the effects of thromboxane. During pregnancy, platelets were more sensitive to the pro-aggregatory effects in vitro of the thromboxane mimetic U46619, in whole blood and in platelet rich plasma, compared to those from non-pregnant controls. The difference in extent of U46619-induced platelet aggregation between groups was abolished in the presence of a high concentration of the specific thromboxane antagonist ICI 192605, but not by prior incubation of blood with aspirin. Platelets from pregnant women were significantly less sensitive to inhibition of arachidonic acid induced activation by the thromboxane synthetase inhibitor dazmegrel, but there was no change in platelet cyclic AMP accumulation under these conditions. Arachidonic acid induced platelet thromboxane B2 production was similar in pregnant and non-pregnant subjects. In conclusion, platelets are more sensitive to the activating effects of thromboxane during pregnancy, but there is no change in the intrinsic reactivity of the thromboxane biosynthetic pathway.

Platelet Aggregation in Whole Blood: The Role of Thromboxane A2 and Adenosine Diphosphate

The platelet aggregation that occurred in whole blood in response to several aggregating agents (collagen, arachidonic acid, adenosine diphosphate, adrenaline and thrombin) was measured using an Ultra-Flo 100 Whole Blood Platelet Counter. The amounts of thromboxane B2 produced were measured by radioimmunoassay. The effects of various inhibitors of thromboxane synthesis and the effects of apyrase, an enzyme that destroys adenosine diphosphate, were determined.

Platelet aggregation was always accompanied by the production of thromboxane B2, and the amounts produced depended on the nature and concentration of the aggregating agent used. The various inhibitors of thromboxane synthesis - aspirin and flurbiprofen (cyclo-oxygenase inhibitors), BW755C (a cyclo-oxygenase and lipoxygenase inhibitor) and dazoxiben (a selective thromboxane synthase inhibitor) - did not markedly inhibit aggregation. Results obtained using apyrase showed that adenosine diphosphate contributed to the aggregation process, and that its role must be acknowledged when devising means of inhibiting platelet aggregation in vivo.

Platelet Behaviour in Normal Pregnancy, Pregnancy Complicated by Essential Hypertension and Pregnancy-Induced Hypertension

Platelet behaviour was studied throughout pregnancy in a group of women who remained normotensive and a group with essential hypertension (EHT). Women who developed pregnancy-induced hypertension (PIH) were also studied together with a group of non-pregnant female controls. We determined the sensitivity of platelets to arachidonic acid (AA) and determined the effects of dazoxiben, a thromboxane synthetase inhibitor, on AA-induced platelet behaviour.

A marked increase in platelet reactivity was evident in all three groups throughout pregnancy; platelets became more sensitive to AA and less sensitive to the inhibitory effects of dazoxiben. The change was apparent as early as 16 weeks gestation. In normotensive pregnancy and in EHT platelet behaviour had returned to normal six weeks after delivery. Platelets from women who developed PIH were more sensitive to AA than those from the other pregnant women and platelet reactivity had not returned to normal six weeks after delivery.

The results indicate that alterations in platelet behaviour may contribute to the vascular complications that are known to be associated with pregnancy and with PIH in particular.

Effects of Prostacyclin and of the Stable Prostacyclin Analogue ZK 36374 on Forearm Blood Flow and Blood Platelet Behaviour in Man

PGI2 and ZK 36374 were each infused into volunteers and the effects on forearm blood flow and on platelet behaviour were determined. Infusions of PGI2 or ZK 36374 did not alter resting forearm blood flow but both agents reduced the extent of the vasoconstriction that occurred in response to cold. ZK 36374 appeared to be a much more potent inhibitor of platelet behaviour than PGI2 when blood was taken while the infusions were in progress, but the effects of both agents were no longer evident one hour after the infusions were terminated. There was an inverse relationship between the extent of cold-induced vasoconstriction and the concentration of sodium arachidonate that was needed to induce platelet aggregation for different individuals. Infusions of PGI2 affected both parameters equally but ZK 36374 had a greater effect on platelet behaviour than on blood flow. It is possible that very low doses of ZK 36374 would result in inhibition of platelet behaviour without producing adverse haemodynamic effects.

Effects of a Thromboxane Synthetase Inhibitor and a cAMP Phosphodiesterase Inhibitor, Singly and in Combination, on Platelet Behaviour

The effects of dazoxiben, a thromboxane synthetase inhibitor, and AH-P 719, a cAMP phosphodiesterase inhibitor, on arachidonic acid (AA)-induced platelet behaviour were determined. The levels of cAMP present in platelet-rich plasma (PRP) after stimulating the platelets with AA in the absence and presence of the agents were also measured.

AH-P 719, as well as dazoxiben, was more effective as an inhibitor of AA-induced platelet behaviour in PRP from some individuals than in PRP from others, and the effectiveness with which it inhibited platelet behaviour paralleled that of dazoxiben. A combination of both agents was more effective than either agent alone.

Both AH-P 719 and dazoxiben increased the level of cAMP in AA-stimulated platelets but again they were more effective in PRP from some individuals than others. A combination of AH-P 719 and dazoxiben always resulted in higher levels of cAMP than either agent alone.

These results imply that cAMP is involved in determining the effects of thromboxane synthetase inhibitors on platelet behaviour, and indicate that the anti-thrombotic potential of a combination of a thromboxane synthetase inhibitor and a cAMP phosphodiesterase inhibitor may be greater than that of the individual agents.

ADP-Induced Platelet Release Reaction in Acute Stroke

The extent of the ADP-induced platelet release reaction has been determined in 107 patients admitted to hospital with acute stroke. In 43 of the patients a precise diagnosis was obtained and in those with proven thromboembolic stroke the mean extent of the release reaction was significantly higher than for those with either primary haemorrhagic stroke or those with subarachnoid haemorrhage. Enhanced platelet reactivity was also found in the patients in whom a precise diagnosis could not be obtained, most of whom probably had cerebral infarction. Comparison of the results obtained for patients with stroke with those obtained for individuals who had not experienced a stroke showed that thromboembolic stroke is associated with platelet hyperactivity while haemorrhagic stroke is associated with platelet hypoactivity. The part that abnormalities of platelet behaviour, whether causal or consequential, might play in occlusive stroke and intra-cranial haemorrhage is discussed and the need for large scale prospective studies is emphasised.

Platelet Behaviour in Non-Insulin-Dependent Diabetes -Influence of Vascular Complications, Treatment and Metabolic Control

Platelet-rich plasma was prepared from 47 patients with noninsulin-dependent diabetes treated with glibenclamide and metformin, and 21 controls. The release of radio-labelled 5-hydroxy-tryptamine in response to aggregating agents (adenosine diphosphate, adrenaline and sodium arachidonate), and the effects on release of a selective thromboxane inhibitor (UK-34787) were investigated. Subsequently, 20 of the diabetic subjects were chosen at random for treatment with insulin; the remainder continued to take tablets. Platelet studies were then repeated, in all patients, after 4 and 6 months.

The results showed an association between platelet behaviour and the presence of vascular complications, and were consistent with previous observations of reduced platelet reactivity in patients taking sulphonylureas. There was no correlation of platelet reactivity with blood glucose, glycosylated haemoglobin or lipid levels.

Blood Platelet Behaviour in Mothers and Neonates

Blood platelet behaviour was compared in mothers at birth and their babies, and in non-pregant, female controls. Platelet responses to arachidonic acid (AA) and to adrenaline were measured in platelet-rich plasma and the inhibitory effects of prostacyclin (PGI2) were determined. Platelets from the mothers differed from those from the neonates and controls in that lower concentrations of AA were needed to induce platelet aggregation and a release reaction. In addition, more PGI2 was needed to inhibit AA-induced platelet aggregation. Platelets from the neonates differed from the mothers and controls in that they were almost completely insensitive to adrenaline. They did not differ from the controls in their sensitivity to AA or PGI2 but the extent of the release reaction induced by AA was significantly reduced.

Platelet Aggregation in Whole Blood Determined Using the Ultra-Flo 100 Platelet Counter

The Ultra-Flo 100 Whole Blood Platelet Counter has proved a useful tool for measuring platelet aggregation in whole blood, the extent of aggregation being deduced from the number of single platelets that remain. The technique has allowed us to show that platelets aggregate spontaneously in citrated blood and in heparinized blood but not in whole blood collected into EDTA. The aggregation occurs during storage but its rate is enhanced by stirring and it occurs more readily when the whole blood has been exposed to plastic rather than glass. It occurs much more readily in whole blood from some individuals than from others and the process may involve adenosine diphosphate (ADP). The rate of aggregation in whole blood is enhanced by several aggregating agents including collagen, ADP and sodium arachidonate which are more usually studied in platelet-rich plasma.

Effects of Ticlopidine Administered to Healthy Volunteers on Platelet Function in Whole Blood

Ticlopidine is thought to be a selective inhibitor of ADP-induced platelet function. Here we have investigated the effects of ticlopidine on platelet function in whole blood induced by ADP and by other platelet agonists. Whole blood was used because it was considered that ADP derived from red cells might act synergistically with other platelet agonists to enhance platelet responses, and that ticlopidine might interfere with this process. Measurements were performed using blood from 16 healthy volunteers before ticlopidine administration, after taking ticlopidine 250 mg daily for 10 days, after taking ticlopidine 250 mg twice daily for a further 10 days, and after 14 days off treatment.

Ticlopidine proved to be a very effective inhibitor of the platelet aggregation induced by ADP; it was most effective in enhancing the reversibility of the aggregation response. The drug modestly but significantly reduced streptokinase, adrenaline, collagen, sodium arachidonate, PAF and U46619 – induced platelet aggregation. The drug significantly reduced the extent of the release reaction (14C-5HT release) induced by ADP, streptokinase, PAF, ristocetin and sodium arachidonate, and also reduced the extent of the synergistic 14C-5HT release induced by combinations of ADP and PAF, ADP and adrenaline and PAF and adrenaline.

The various inhibitory effects of ticlopidine were evident after treatment with 250 mg daily but were more pronounced after 250 mg twice daily. All values had returned to normal after 14 days off treatment. Ticlopidine had no effect on serum thromboxane B2 production nor on several parameters of coagulation and fibrinolysis.

We conclude that ticlopidine is an effective inhibitor of ADP-induced platelet aggregation and also the platelet aggregation and 14C-5HT release induced in whole blood by a number of platelet agonists and combinations of agonists. These latter effects are probably mainly via a selective effect on ADP. The inhibitory effects of the drug are dose-related.

Changes in G-Actin after Platelet Activation in Platelet Rich Plasma

We have used the DNase I inhibition assay to study changes in G-actin after platelet activation in platelet-rich plasma (PRP) induced by ADP. Because of problems associated with depolymerization of F-actin after lysis of ADP-activated platelets in the presence of plasma, G-actin was measured using a lysis buffer that contained formaldehyde to prevent any depolymerization of F-actin.

Different patterns of response were seen depending on the concentration of ADP used, and these were modified by avoiding aggregation by either not stirring the sample or by adding EDTA. The results show rapid conversion of G-actin to F-actin in association with shape change, and there is a further decrease in G-actin associated with irreversible platelet aggregation. Thus evidence is presented that actin polymerization occurs in two phases after ADP stimulation.

A Six-Channel Automated Platelet Aggregometer

A simple automated platelet aggregometer is described which allows six plasma samples to be handled concurrently.

Evidence that Adenosine Diphosphate Can Activate Adenylate Cyclase via Conversion to Adenosine in Platelet-rich Plasma Containing Magnesium

When adenosine diphosphate (ADP) is added to hirudinized platelet-rich plasma (PRP) in which the level of platelet cAMP has been pharmacologically elevated, there is an initial rapid fall in the level of cAMP brought about by inhibition of adenylate cyclase. This may be followed by a subsequent activation of adenylate cyclase that does not occur when citrated PRP is used in place of hirudinized PRP, and is more pronounced in the presence of added Mg2+. Here we provide evidence that a) the Mg2+-dependent activation of adenylate cyclase seen in hirudinized PRP is mediated by adenosine, b) the adenosine produced synergizes with forskolin and with DN9693 to raise the level of cAMP in platelets, but not with iloprost, c) Mg2+ does not influence directly the rate or extent of cAMP production and so is more likely to influence the rate of adenosine production, and d) activation of adenylate cyclase by adenosine can lead to inhibition of platelet aggregation. ARL 66096, a P2T purinoceptor antagonist which inhibits ADP-induced platelet aggregation, prevented inhibition of adenylate cyclase by ADP. Conversely, ARL 66096 did not appear to inhibit conversion of ADP to adenosine and subsequent activation of adenylate cyclase.

A Remote Assay for Measuring Canine Platelet Activation and the Inhibitory Effects of Antiplatelet Agents

BACKGROUND

Antiplatelet medications are increasingly used in dogs. Remote analysis of platelet activity is challenging, limiting assessment of antiplatelet drug efficacy.

HYPOTHESIS/OBJECTIVES

To evaluate a method used in humans for stimulation and remote analysis of canine platelet activity.

ANIMALS

Forty-five dogs of various ages without a coagulopathy or thrombocytopenia. Six were receiving antiplatelet medication.

METHODS

Prospective observational study. Platelets were stimulated with combinations of arachidonic acid (AA) and epinephrine (Epi) or adenosine diphosphate (ADP) and the thromboxane A₂ -mimetic U46619 (U4). PAMFix was added to the blood samples to facilitate delayed analysis of platelet activity. Activity was assessed by flow cytometric measurement of surface P-selectin (CD62P) expression.

RESULTS

Canine platelets could be stimulated with both AA/Epi and ADP/U4. The levels of P-selectin were significantly greater than paired, unstimulated samples (P < 0.001). Inhibition of P-selectin expression occurred after this stimulation by adding antiplatelet drugs in vitro. The efficacy of antiplatelet drugs in samples from treated dogs was also measurable ex vivo using this method. Delayed analysis of platelet activity at time points up to 22 days demonstrated excellent correlation between respective mf values at each time point (r²  = 0.92, P < 0.0001).

CONCLUSIONS AND CLINICAL IMPORTANCE

This study evaluated a new method to remotely assess canine platelet activity. It shows that PAMFix can be used for this purpose. This provides opportunities to interrogate the inhibitory action of antiplatelet drugs in clinical settings.

Evaluation of a whole blood remote platelet function test for the diagnosis of mild bleeding disorders

BACKGROUND

Mild platelet function disorders (PFDs) are complex and difficult to diagnose. The current gold standard test, light transmission aggregometry (LTA), including lumi-aggregometry, is time and labour intensive and blood samples must be processed within a limited time after venepuncture. Furthermore, many subjects with suspected PFDs do not show a platelet abnormality on LTA.

OBJECTIVE

To assess the diagnostic potential of an easy-to-use remote platelet function test (RPFT) as a diagnostic pre-test for suspected PFDs.

METHODS

A remote platelet function test was compared with lumi-aggregometry in participants recruited to the Genotyping and Phenotyping of Platelets Study (GAPP, ISRCTN 77951167). For the RPFT, whole blood was stimulated with platelet agonists, stabilized with PAMFix and returned to the central laboratory for analysis of P-selectin and CD63 by flow cytometry.

RESULTS

For the 61 study participants (42 index cases and 19 relatives) there was a good agreement between lumi-aggregometry and the RPFT, with diagnosis being concordant in 84% of cases (κ = 0.668, P < 0.0001). According to both tests, 29 participants were identified to have a deficiency in platelet function and 22 participants appeared normal. There were four participants where lumi-aggregometry revealed a defect but the RPFT did not, and six participants where the RPFT detected an abnormal platelet response that was not identified by lumi-aggregometry.

CONCLUSION

This study suggests that the RPFT could be an easy-to-use pre-test to select which participants with bleeding disorders would benefit from extensive platelet phenotyping. Further development and evaluation of the test are warranted in a wider population of patients with excessive bleeding and could provide informative screening tests for PFDs.

'VASPFix' for measurement of VASP phosphorylation in platelets and for monitoring effects of P2Y12 antagonists

Vasodilator-stimulated phosphoprotein (VASP) is phosphorylated and dephosphorylated consequent to increases and decreases in cyclic nucleotide levels. Monitoring changes in VASP phosphorylation is an established method for indirect measurement of cyclic nucleotides. Here we describe the use of an innovative cocktail, VASPFix, which allows sensitive and reproducible measurement of phosphorylated VASP (VASP-P) in a simple, single-step procedure using cytometric bead technology. Frozen VASPFix-treated samples are stable for at least six months prior to analysis. We successfully used VASPFix to measure VASP-P in platelets in both platelet-rich plasma and blood in response to compounds that increase (dibutyryl cAMP, adenosine, iloprost, PGE1) and decrease (ADP, PGE1) cAMP, and to determine the effects of certain receptor antagonists on the results obtained. The change in VASP-P brought about by adding ADP to PGE1-stimulated platelets is a combination of the effect of ADP at the P2Y12 receptor and of PGE1 at both IP and EP3 receptors. For iloprost-stimulated platelets EP3 receptors are not involved. A procedure in which iloprost, ADP and VASPFix were used to determine effectiveness of clopidogrel and prasugrel in patients was compared with an established commercial procedure that uses PGE1 and ADP; the latter produced higher platelet reactivity values that were the result of PGE1 interacting with platelet EP3 receptors. We conclude that VASPFix can be used both as a research tool and for clinical investigations and provides better specificity for P2Y12 receptor inhibition. The latter confers a distinct advantage over existing methods used to monitor effects of P2Y12 antagonists on platelet function.

Platelets-the Phoenix arises

Just as the mythical Pheonix arose from the ashes, so too has Platelets. Reluctantly, it had been decided that the December issue of Volume 6, 1996 had to be the final one. Despite considerable success in attracting and publishing high quality papers relating to platelets and platelet-related research, the previous publishers had decided that the journal should come to an end.

Effects on platelet function of an EP3 receptor antagonist used alone and in combination with a P2Y12 antagonist both in-vitro and ex-vivo in human volunteers

EP3 receptor antagonists may provide a new approach to the treatment of atherothrombotic disease by blocking the ability of prostaglandin E2 (PGE2) to promote platelet function acting via EP3 receptors. DG-041 is an EP3 antagonist in the early stage of clinical development. Here, we quantitated effects on platelet function of DG-041 in-vitro and ex-vivo after administration to man when given alone and concomitantly with clopidogrel or clopidogrel and aspirin. With its unique mechanism of action, it was anticipated that DG-041 would potentiate inhibition of platelet function when given in combination with clopidogrel without materially increasing bleeding time. Initially, in-vitro studies were performed to determine inhibitory effects of DG-041 (3 µM) used alone or in combination with the P2Y12 antagonist cangrelor (1 µM), both without and with aspirin (100 µM). Platelet aggregation and P-selectin expression were measured in whole blood (n = 10) following stimulation with the thromboxane A2 (TXA2) mimetic U46619 (0.3 or 1 µM) in combination with either the EP3 agonist sulprostone (0.1 µM), or PGE2 (1 µM). DG-041 alone partially inhibited platelet function in-vitro, as did cangrelor. Addition of both DG-041 and cangrelor in combination provided significantly greater inhibition. An ex-vivo study was then performed using the same experimental approaches. This clinical study was a prospective, randomised, blinded (for DG-041/matching placebo), blocked, crossover study designed to compare the effects of DG-041, clopidogrel, or the combination of DG-041 with either clopidogrel or clopidogrel and aspirin. Healthy volunteers (n = 42) were randomly assigned to receive no background treatment, clopidogrel (300 mg loading dose plus 75 mg daily) or clopidogrel and aspirin (75 mg daily) for 10 days alongside DG-041 (200 mg twice daily) or placebo for 5 days, crossed over to placebo or DG-041 for the next 5 days. Platelet effects and bleeding time were measured at baseline, days 5 and 10. DG-041 partially inhibited platelet function ex-vivo, as did clopidogrel, while administration of both DG-041 and clopidogrel provided significantly greater inhibition. Administration of DG-041 alone did not increase bleeding time, and did not significantly affect the increased bleeding time seen with clopidogrel or clopidogrel with aspirin. Using these experimental approaches, the antiplatelet effects of DG-041 and a P2Y12 antagonist used alone and in combination can be determined both in-vitro and ex-vivo. Results show inhibitory effects of DG-041 on platelet function acting via EP3 receptor blockade, confirmed to be additional to those brought about by P2Y12 blockade. In both in-vitro and ex-vivo studies, aspirin neither promoted nor negated the effects of the other drugs.

Measurement of platelet P-selectin for remote testing of platelet function during treatment with clopidogrel and/or aspirin

There is great interest in assessing the efficacy of treatment with clopidogrel and aspirin in patients with cardiovascular disease using procedures that can be used in a remote setting. Here we have established methods to assess the effects of clopidogrel and aspirin on platelets based on measurements of platelet P-selectin. Platelets were stimulated in whole blood by adding the combination of adenosine diphosphate and the TXA(2) mimetic U46619 (ADP/U4, designed to assess P2Y(12) inhibition) or the combination of arachidonic acid and epinephrine (AA/Epi, designed to assess COX-1 inhibition). The stimulated samples were then fixed using a fixative solution that provides stability for at least 9 days, and sent to a central laboratory for analysis of P-selectin by flow cytometry. Measurements were performed in blood from healthy volunteers and patients with cardiovascular disease. The inhibitory effects of clopidogrel and aspirin were assessed ex vivo and the effects of the direct acting P2Y(12) antagonist cangrelor and aspirin were assessed in vitro. Measurements of platelet aggregation were also performed for comparison. In healthy volunteers clopidogrel ex vivo and cangrelor in vitro markedly inhibited P-selectin expression induced by ADP/U4. Aspirin did not inhibit and did not interfere with the effects of clopidogrel or cangrelor using this test. There was very little overlap of results obtained in the absence and presence of clopidogrel or cangrelor. In contrast, over half of 42 patients with cardiovascular disease did not respond well to clopidogrel treatment, although cangrelor was still effective. Aspirin markedly inhibited P-selectin expression induced by AA/Epi. Clopidogrel had much less effect and did not interfere with the effects of aspirin. There was no overlap of results obtained in the absence and presence of aspirin. Aspirin provided near-complete inhibition in 29 of 30 patients with cardiovascular disease. Aggregometry measurements agreed well with the P-selectin data obtained ex vivo following both clopidogrel and aspirin treatment. It is concluded that measurements of P-selectin performed on fixed blood samples following platelet stimulation in whole blood in a remote setting can be used effectively to monitor the effects of clopidogrel and aspirin.

Activation of leukocytes in whole blood samples by N-formyl-methionyl-leucyl-phenylalanine (FMLP) enhances platelet aggregability but not platelet P-selectin exposure and adhesion to leukocytes

Adhesion of platelets to neutrophils and monocytes is believed to play an important role in intercellular communication. Evidence has been provided that such heterotypic cell-cell contacts via adhesion molecules may be directly involved in intercellular signal transduction as well as facilitate the action of soluble signal transmitters, e.g. cathepsin G, PAF or nitric oxide. With respect to platelet activation, stimulatory and inhibitory effects of leukocytes have been reported, and the results obtained seem to be influenced by the experimental conditions. We investigated the effect of leukocyte stimulation on platelet behaviour in samples of human citrated whole blood. Adding the chemotactic peptide FM LP, which stimulates neutrophils and monocytes but not lymphocytes and platelets, to stirred whole blood samples resulted in a significant enhancement ( P < 0.01) of spontaneous as well as ADP-induced platelet aggregation (25 vs 33% and 66 vs 69% , respectively). In contrast stirring-induced as well as ADP-induced increase of P-selectin exposure (33 and 107% , respectively) was not affected by FMLP. In unstirred whole blood samples, about 10 to 20% of neutrophils and monocytes had bound platelets to their surfaces, and the number of these heterotypic conjugates was enhanced about twofold during spontaneous platelet aggregation. Addition of FMLP significantly reduced the stirring-induced formation of platelet-neutrophil conjugates but not of platelet-monocyte conjugates. These results indicate that neutrophil and/or monocyte activation in whole blood may enhance platelet aggregation, but not secretion (CD62P exposure) and formation of heterotypic platelet-leukocyte conjugates.

GPIIb-IIIa antagonists cause rapid disaggregation of platelets pre-treated with cytochalasin D. Evidence that the stability of platelet aggregates depends on normal cytoskeletal assembly

Platelet activation is accompanied by changes in the composition of the platelet cytoskeleton with rapid incorporation and displacement of certain proteins. Here we have inhibited cytoskeletal assembly by pretreating platelets with cytochalasin D (CyD) and investigated the effect on the stability of the aggregates that form. The experiments were performed in both citrated and hirudinized platelet-rich plasma (PRP) and aggregation was induced by adenosine diphosphate (ADP), collagen, the TXA2-mimetic U46619 and adrenaline. Platelets in the aggregates that formed, underwent rapid disaggregation on addition of EDTA or a GpIIb-IIIa antagonist such as MK-852 and GR144053F, all of which are agents that interfere with the ability of fibrinogen to interact with GpIIb-IIIa. This was the case irrespective of the aggregating agent used and occurred in both citrated and hirudinized PRP. In contrast, the rate of disaggregation brought about by some other agents, iloprost and ARL 66096, appeared to be unaffected by CyD. Information was also obtained on the effects of CyD on the cytoskeletal changes brought about by ADP and the effects on the cytoskeleton of subsequent addition of M K-852. The results show that CyD retards the incorporation of certain proteins (actin, myosin, alpha -actinin, actin binding protein and a 66 K protein) into the cytoskeleton and that subsequent addition of MK-852 results in rapid displacement of some of these with re-incorporation of a 31 K protein. The results suggest that the early changes in the cytoskeleton following platelet activation contribute to the stability of the aggregates that form, and that interference with these early changes results in aggregates that are easily disassembled by agents that interfere with GpIIb-IIIa-fibrinogen complex formation.

Role of GPIIb-IIIa in platelet-monocyte and platelet-neutrophil conjugate formation in whole blood

Platelets in stirred whole blood can be induced to form aggregates and also to form heterotypic platelet-monocyte (P/M) and platelet-neutrophil (P/N) conjugates. Here we have investigated the effects of three GPIIb-IIIa antagonists (GR144053F, MK-852 and Reopro, a CD62P-blocking antibody, GA6, and EDTA on the conjugate formation that occurs on stirring whole blood and in response to adding ADP and PAF. We have confirmed the identities of the conjugates by light microscopy after cell sorting. Platelet aggregation was measured by platelet counting. Monocytes, neutrophils, P/M and P/N were detected and quantitated using immunofluorescence and flow cytometry. Stirring whole blood resulted in both platelet aggregation and formation of P/M but not P/N. Adding ADP or PAF to whole blood caused rapid platelet aggregation and generation of both P/M and P/N. All of the GPIIb-IIIa antagonists studied had similar effects: inhibition of stirring-induced platelet aggregation and P/M formation, and inhibition of ADP-induced platelet aggregation and P/N formation. In contrast, they accelerated ADP induced-P/M conjugate formation and PAF-induced formation of both P/M and P/N. Both EDTA and GA6 completely inhibited P/M and P/N, which is commensurate with CD62P being involved in platelet-leucocyte conjugate formation. The results of these investigations suggest that GPIIb-IIIa has a dual role in determining the interaction between platelets and leukocytes.

Unexpected changes in the catecholamine content of platelets and plasma during exercise

Catecholamines are retained within platelets for several hours after plasma catecholamine concentrations have returned to baseline. To determine whether platelet catecholamine concentrations may provide an index of short-term elevations in plasma adrenaline (A) and noradrenaline (NA), the response of plasma and platelet catecholamines to an interval supramaximal, Max (107% VO(2) Max), and submaximal, Submax (37% VO(2) Max), cycling protocol was examined in seven healthy male volunteers, 22-34 years. Despite large rises in plasma NA and A in the Max study (12- and 8-fold increases above baseline, respectively) and smaller rises in the Submax study, the baseline platelet concentrations of A and NA fell significantly in the first 15 min of exercise in both groups. This fall was greater in the SubMax protocol. Catecholamine concentrations then increased slowly in the second half of exercise, but never returned to baseline. The circulating platelet count almost doubled during the exercise period, increasing from 308 to 569 X 10(3) platelets/ml plasma in both studies, returning close to baseline in recovery. These results indicate that at the beginning of exercise there is large rise in plasma catecholamines and the circulating platelet count, with a fall in the platelet catecholamine concentrations. This suggests that a sequestered platelet population, free of catecholamines, is released at the beginning of exercise. This release most probably occurs from the spleen. If this is the case, the reason for a propagation of platelets in the spleen, free of catecholamines, requires further investigation.

Formation of platelet-leukocyte conjugates in whole blood

The purpose of this investigation was to obtain information on platelet-leukocyte conjugate formation in whole blood and on factors that affect it. We also measured platelet and leukocyte activation by quantitating the expression of CD62P and CD11b. In both cases a flow cytometric approach was used. The results show that platelet-monocyte and platelet-polymorphonuclear leukocyte (PMNL) conjugate formation is enhanced by simply stirring blood, with optimum conjugate formation occurring after 10 min. In the case of monocytes,conjugate formation was enhanced by adenosine diphosphate (ADP). Both monocyte and PMNL conjugate formation was enhanced by phorbol myristate acetate (PMA), but L-formyl methionyl lysyl proline (FMLP) was either without effect (monocytes) or inhibitory (PMNL). EDTA also inhibited conjugate formation (implying involvement of divalent cations), as did dextran sulphate (implying involvement of P-selectin = CD62P). Interestingly GR144053F, which acts at GpIIb-IIIa on platelets to interfere with fibrinogen binding, and also glycyl prolyl arginyl proline (GPRP), a peptide that interferes with the interaction between CD11c on leukocytes and fibrinogen, did not inhibit platelet-monocyte conjugate formation, but did inhibit the platelet-PMNL interaction; this indicates that GpIIb-IIIa on platelets and CD11c on leukocytes and fibrinogen are involved in mediating the interaction between platelets and PMNL but not platelets and monocytes. Surprisingly arginyl-glycyl aspartyl serine (RGDS) inhibited the formation of both types of conjugate but this may be because it also inhibited both platelet and leukocyte activation as measured by CD62P and CD11b exposure and/or interferes with the binding of adhesion molecules other than fibrinogen. The results show that a flow cytometric procedure can be effective in obtaining rapid information on platelet-leukocyte conjugate formation in whole blood and on factors that are involved in its regulation. It is suggested that the technique may be applicable to the study of platelet-leukocyte conjugate formation in whole blood in disease, and also to study the effects of drugs interfering with conjugate formation.

Acyl derivatives of coenzyme A inhibit platelet function via antagonism at P2Y1 and P2Y12 receptors: a new finding that may influence the design of anti-thrombotic agents

We have performed a detailed investigation of the effects on platelet function of coenzyme A (CoA) and several acyl-CoAs. Platelet aggregation was measured by turbidimetry and by platelet counting; platelet shape change was measured using light scattering; P-selectin, Ca2+ mobilization and vasodilator-stimulated phosphoprotein (VASP) phosphorylation were measured by flow cytometry. The compounds investigated inhibited ADP-induced platelet aggregation; those with saturated acyl groups containing 16-18 carbons were most effective. The effects of palmitoyl-CoA (16:0) were studied in depth. It inhibited platelet shape change and Ca2+ mobilization brought about by ADP (but not other agonists) indicating antagonism at P2Y(1) receptors, and also inhibited ADP-induced P-selectin expression. Effects of palmitoyl-CoA on the platelet aggregation and Ca2+ mobilization induced by several different agonists and agonist combinations were compared with those of MRS 2179 (a P2Y(1) antagonist) and AR-C69931 (a P2Y(12) antagonist), and were consistent with palmitoyl-CoA acting mainly at P2Y(1) but also with partial antagonism at P2Y(12) receptors. Antagonism at P2Y(12) receptors was confirmed in studies of VASP-phosphorylation. Palmitoyl-CoA did not act as an antagonist at P2X(1) receptors. The results are discussed in relation to the possibility that acyl-CoAs may contribute as endogenous modulators of platelet function and might serve as lead compounds for the design of novel antithrombotics.

The reversible P2Y antagonist cangrelor influences the ability of the active metabolites of clopidogrel and prasugrel to produce irreversible inhibition of platelet function

BACKGROUND

Agents that act as antagonists at P2Y(12) ADP receptors on platelets are in use (clopidogrel), and in development for use (cangrelor and prasugrel), in patients with cardiovascular disease. Cangrelor is a direct-acting reversible antagonist being developed for short-term infusion; clopidogrel and prasugrel are oral prodrugs that provide irreversible inhibition via transient formation of active metabolites. At the cessation of cangrelor infusion, patients are likely to receive clopidogrel or prasugrel as a means of maintaining antiplatelet therapy.

OBJECTIVES

To apply an experimental in vitro approach to investigate the possibility that cangrelor influences the ability of the active metabolites of clopidogrel and prasugrel to inhibit ADP-mediated platelet function.

METHODS

The effects of cangrelor and the active metabolites of clopidogrel (C-AM) and prasugrel (P-AM) on platelet function were assessed by ADP-induced platelet P-selectin expression in whole blood. The method involved rapid removal of the antagonists by dilution, and measurement of residual platelet inhibition.

RESULTS

Cangrelor, C-AM and P-AM markedly inhibited P-selectin expression. The effect of cangrelor, but not of C-AM and P-AM, was reversible following antagonist removal. Preincubation of blood with cangrelor prior to addition of C-AM or P-AM reduced the ability of metabolites to irreversibly antagonize P2Y(12). Irreversible inhibition was maintained when blood was preincubated with metabolites prior to cangrelor.

CONCLUSIONS

Cangrelor influences the ability of the active metabolites of clopidogrel or prasugrel to inhibit platelet function irreversibly. Careful consideration should be given to the timing of administration of an oral P2Y(12) antagonist following cangrelor infusion.

Raised levels of CD39 in leucocytosis result in marked inhibition of ADP-induced platelet aggregation via rapid ADP hydrolysis

We observed previously that the extent of ADP-induced platelet aggregation in blood from patients with leucocytosis is markedly reduced. We obtained evidence that this is via enhanced ADP metabolism consequent to the high leucocyte count, and speculated that ecto-NTPDase CD39 on leucocytes may be involved. Here we have investigated the association between ADP-induced platelet aggregation, ADP metabolism and expression of ecto-NTPDase CD39 on leucocytes in patients with leucocytosis. Six patients with leucocytosis were compared with six normal controls. Platelet aggregation was measured using platelet counting. ADP metabolism was analysed by HPLC. CD39 on leucocytes from each volunteer and patient was measured by flow cytometry and is presented as the CD39 fluorescence index (CD39FI, the sum of the product of CD39 median fluorescence and cell number for each leucocyte subtype). Compared with the controls, all patients displayed markedly reduced platelet aggregation to ADP in whole blood, markedly enhanced metabolism of ADP to AMP in whole blood, and increased leucocyte CD39FI. The increased CD39FI was due to either a high number of CD39+ve lymphocytes or a high number of CD39+ve neutrophils. In contrast, the measures of aggregation and ADP metabolism performed in platelet-rich plasma from the patients were similar to those obtained for the controls. There was an inverse correlation between ADP-induced aggregation in whole blood and CD39FI, and between the time taken to achieve complete removal of ADP from blood and CD39FI. For two patients with very high CD39FI (60,000 cf 1500 for controls) ADP-induced aggregation was abolished. Reduced aggregation, enhanced ADP metabolism and a raised CD39FI returned to normal in one patient following successful chemotherapy. It is concluded that ADP-induced platelet aggregation in leucocytosis is reduced as a result of enhanced ADP metabolism due to raised levels of leucocyte-associated CD39.

Increased platelet activation in renal transplant patients

BACKGROUND

Patients with functioning renal transplants are at risk of graft thrombosis in the postoperative period, and of fistula thrombosis and other thrombotic events thereafter. Investigation and therapeutic manipulation of haemostasis in these patients offers a means to counter this thrombotic tendency.

METHODS

Platelet aggregation in whole blood, plasma von Willebrand factor and plasma fibrinogen levels were measured in 32 stable renal transplant patients (creatinine <200 micromol/litre, age of graft >4 months) and in 32 age, sex and smoking-habit matched normal controls.

RESULTS

In both patient and control groups, seven patients were smokers and the remaining 25 were non-smokers. There was no significant difference in age between patients and controls [patients, median: 39 (20-64) years; controls: 38 (24-60) years]. Spontaneous platelet aggregation was significantly higher in the patients at all time points studied [30s-6 min; at 4 min: patients median (interquartile range) 19.4 (11.3-27.3)%; controls, 8.0 (5.1-15.0)%, P < 0.0005]. ADP-induced aggregation was also increased at a concentration range of 0.1-3 microM (at 1 microM, 1 min, patients median (interquartile range) was 52.4 (30.5-70.0)%; controls was 16.5 (1.4-31.4)%, P < 0.0001). Transplant patients had significantly higher von Willebrand factor and fibrinogen levels compared with the controls (von Willebrand factor, patients median (range): 158 (13-269)%; controls: 85 (43-223)%, P < 0.00001; fibrinogen, patients: 3.29 (2.12-7.39) g/litre; controls: 2.81 (1.84-4.65) g/litre, P < 0.0002).

CONCLUSION

Patients with stable renal transplants have in vitro evidence of enhanced platelet activation, and increased plasma von Willebrand factor and fibrinogen levels.

Measurement of platelet activation and adhesion to leukocytes during haemodialysis

In this study we used fluorescent-labelled antibodies to measure the extent of platelet adhesion to polymorphonuclear leukocytes (PMNLs), monocytes and lymphocytes. The activation of platelets, PMNLs and monocytes were also measured during the course of haemodialysis treatment using flow cytometric techniques established in our laboratory. Twenty patients were treated with either a cellulose membrane (TFU) or a polycarbonate filter (Pro 500). Blood samples were taken from the output line of the dialyser 2, 15, 30 and 180 min after commencing dialysis and just before starting treatment. Compared with the pre-dialysis sample, there was a marked increase in platelet-PMNL conjugate formation at 2 min, followed by a decrease in conjugates at 15 and 30 min, and a slight increase at 180 min. During extracorporeal circulation PMNLs become activated as measured by a CD11b upregulation at 15, 30 and 180 min, but not at 2 min. Platelet binding to monocytes was increased above 15 min after starting dialysis, and monocyte activation was slightly increased above basal levels during the same period. The activation state of circulating free platelets, as measured by surface P-selectin exposition, initially decreased slightly, but then returned to basal levels over the 3-h period. Changes in cell counts were also detected: there was a massive decrease in circulating PMNLs and monocytes, and a small decrease in circulating platelets, at 15 and 30 min. These reverted to basal values by the end of the 3-h period. There was no change in the number of circulating lymphocytes or erythrocytes. These results show that flow cytometric studies on whole blood samples may provide important information on the behaviour of circulating blood cells, which could supplement conventional clinical measurements, to give a better insight into changes that occur in the circulation during haemodialysis.

Adenine nucleotide metabolism in human blood--important roles for leukocytes and erythrocytes

Adenosine diphosphate (ADP) released into blood induces platelet aggregation and contributes to hemostasis and thrombosis. Released ATP can also induce platelet aggregation and there is evidence that blood leukocytes and also erythrocytes play important roles in this. Rapid metabolism of ADP and ATP by endothelial cells is important in protecting platelets from their effects. Here we have performed a systematic investigation of adenine nucleotide metabolism in human blood and the involvement of blood cells. Conversion of ATP to ADP in blood was due almost exclusively to the presence of leukocytes; plasma, platelets and erythrocytes made little or no contribution. Mononuclear leukocytes (MNLs) and polymorphonuclear leukocytes (PMNLs) were equally effective. Conversion of ADP to AMP was also promoted by leukocytes, with no involvement of platelets or erythrocytes. Some ADP was also converted to ATP in blood, apparently via an enzyme present in plasma, but ATP was then rapidly removed by the leukocytes. Conversion of AMP to adenosine occurred via a plasma enzyme with little or no contribution from any cellular element. As expected, in blood the adenosine produced was removed very rapidly by erythrocytes and then converted to inosine and then hypoxanthine. In the absence of erythrocytes plasma supported only a slow conversion of adenosine to inosine and hypoxanthine, which was not influenced by platelets or leukocytes. This study has demonstrated that leukocytes and erythrocytes play a major role in adenine nucleotide metabolism in blood and that these cells, as well as endothelial cells, may be important determinants of the effects of ATP and ADP on platelets.

Quantitation of platelet aggregation and microaggregate formation in whole blood by flow cytometry

Platelet aggregation and microaggregate formation were measured in samples of stirred whole blood by flow cytometry. Blood samples were stirred in a multi-sample agitator with ADP, fixed and labelled with a platelet-specific CD42a-FITC fluorescent antibody. The blood was then diluted and applied directly to a flow cytometer. Platelets were identified using a gating procedure based on their expression of CD42a and then quantified. Aggregation was monitored as a fall in the number of single platelets. Both reversible and irreversible aggregation responses to ADP were determined and these were found to correlate directly with aggregation responses determined using a well-established single platelet counting technique using the Ultra-Flo 100 Whole Blood Platelet Counter. We found from flow cytometry that ADP-induced aggregation was coupled with a transient formation of platelet microaggregates over the initial 60 s following ADP addition. Assessment of single platelet loss by flow cytometry was found to be a reliable way of monitoring aggregation responses and provided new information on rapid microaggregate formation in ADP-stimulated blood.