A method of testing platelet aggregation in native whole blood

The association of depression with platelet activation: evidence for a treatment effect

BACKGROUND

Depression is associated with an increased risk of cardiovascular disease (CVD). Although the mechanism is uncertain, prothrombotic and inflammatory factors may play a role.

OBJECTIVES

As platelets play a key role in CVD, we determined first, whether depressed individuals had more activated platelets than non-depressed individuals and second, whether treatment of depression reduced platelet activation levels.

PATIENTS/METHODS

We recruited 108 depressed outpatients and 45 control subjects all without a history of CVD. After psychological assessment, the depressed patients were offered treatment with medication and/or psychotherapy. Flow cytometric markers of platelet activation and level of depression were assessed at baseline and at 4 weeks and 6 months after treatment.

RESULTS

Depression was associated with increased platelet activation with a higher number of circulating CD62p (0.76x10(9) L(-1) vs. 0.46, P=0.019) and CD63 (P=0.05) positive platelets compared with controls. Patients with depression also had more circulating platelet-leukocyte aggregates than controls (P<0.001). There was a positive correlation between the severity of depression and the level of platelet activation. Platelets from depressed patients were also hyperreactive to adenosine 5 -diphosphate (ADP) stimulation with increased CD62p and CD63 exposure (P=0.003 and 0.019, respectively). Six months of treatment resulted in a reduced number of circulating CD62p and CD63 positive platelets (29.84% and 53.38% decrease) and a 20.9% reduction in CD63 exposure after ADP activation.

CONCLUSIONS

Depression is associated with increased in vivo platelet activation and resolution of depression using psychotherapy and/or medication reduces platelet activation. These findings provide insights into the link between depression and cardiovascular risk.

Monitoring modulators of platelet aggregation in a microtiter plate assay

Platelets play a central role in maintaining biological hemostasis. Inappropriate platelet activation is responsible for thrombotic diseases such as myocardial infarction and stroke. Therefore, novel agents that can inhibit platelet activation are necessary. However, assays that monitor platelet aggregation are generally time-consuming and require high volumes of blood and specialized equipment. Therefore, a medium- to high-throughput assay that can monitor platelet aggregation would be considered useful. Such an assay should be sensitive, comparable to the "gold standard" assay of platelet aggregometry, and able to monitor multiple samples simultaneously but with low assay volumes. We have developed such a microtiter assay. It can assay an average of 60 independent treatments per 60 ml blood donation and demonstrates greater sensitivity than the current gold standard assay, namely platelet aggregation in stirring conditions in a platelet aggregometer. The microtiter plate (MTP) assay can detect known inhibitors of platelet function such as indomethacin, aspirin, and ReoPro. It is highly reproducible when using standard doses of agonists such as thrombin receptor-activating peptide (20 microM) and collagen (0.19 mg/ml). Finally, the MTP assay is rapid and sensitive and can detect unknown platelet-modulating agents from a library of compounds.

A whole blood flow cytometric determination of platelet activation by unfractionated and low molecular weight heparin in vitro

The influence of unfractionated (Heparin-Natrium) and low-molecular heparin (Fragmin(R)) on platelet activation in whole blood was investigated by FACS analysis in vitro using antibodies against glycoprotein (gp) IIb/IIIa (CD 41), GMP 140 (CD 62P), gp 53 (CD 63) and fibrinogen. Samples were also labeled with anti-gp Ib (CD 42b). Neither unfractionated heparin (UFH) nor low molecular weight heparin (LMWH) led to significant (i.e., p<0.05) changes in fluorescence intensities of platelets labeled with anti-gp IIb/IIIa or anti-gp 53. Significant platelet activation due to unfractionated heparin could be observed by labeling with anti-GMP 140 (UFH: p=0.009; LMWH: p=0.16). The proportion of platelets with surface-bound fibrinogen was significantly increased (UFH: p=0.00006; LMWH: p=0.008). After incubation with heparins, activation ability of platelets by adenosine diphosphate (ADP) was significantly increased. The potentiating action of unfractionated heparin was larger. Therefore, flow cytometric results of platelet activation in patients receiving heparin should be interpreted carefully.

Increased lipid binding to thrombi in coronary artery disease. Findings in patients without premedication in native (not anticoagulated) test systems

Thrombi and clots were produced from native (i.e., not anticoagulated) platelet-rich and platelet-poor plasma from patients with coronary artery disease and control subjects who had not taken any medication known to influence plasma lipids, coagulation, or platelet aggregation. The clotting times were recorded, and the lipid content of clots, thrombi, platelets, plasma, and high density lipoprotein was analyzed. Thrombi produced from native platelet-rich plasma were 46% heavier in coronary artery disease patients and contained about 20% more phospholipids and free cholesterol and about twice the amount of triglycerides and esterified cholesterol in both absolute and relative amounts with respect to the corresponding lipids of plasma plus platelets. The elevated content of lipids not only increases the size of the thrombi but also changes their quality because of an increased content in plasmatic lipids, as platelets contain only trace amounts of triglycerides and cholesterol esters. In agreement herewith, fibrinogen and maximal amplitude on the thrombelastogram were increased in coronary artery disease patients, whereas the thrombus-forming time and clotting times of platelet-poor and platelet-rich plasma were shortened, indicating accelerated coagulation and activation of platelets. Analysis of these results suggests a disturbed interrelation in coronary artery disease between lipids and hemostasis, in which platelets, high density lipoprotein, and lipoproteins rich in triglycerides and cholesterol esters may play a role.

Rapid and selective measurement of platelet-activating factor using a quantitative bioassay of platelet aggregation

A bioassay for the measurement of platelet-activating factor (PAF) based on the quantification of platelet aggregation was developed. The method used a platelet analyzer in conjunction with a multiwelled micromixing device and whole blood collected from male New Zealand White rabbits. This bioassay can be nonselective and used to quantitate platelet aggregation induced by any activator. The EC50 for platelet-activating factor, arachidonic acid, and adenosine diphosphate were 0.0232, 55, and 10 microM, and at these concentrations the half maximal aggregation response occurred at 7.5, 10.0, and 12.5 min, respectively. The bioassay was capable of the sensitive quantification of platelet aggregation in small volumes (50 microL) of citrated rabbit whole blood, using a short (i.e., 15-min) incubation period. This enabled multiple bioassays to be performed without the need for a large volume of whole blood to be collected from the rabbit. Selective measurement of platelet-activating factor was achieved by adding inhibitors of arachidonic acid- and adenosine diphosphate-dependent pathways of platelet activation, that is, acetylsalicyclic acid and phosphoenolpyruvate/pyruvate kinase, respectively, to the citrated rabbit whole blood immediately before bioassay. These inhibited arachidonic acid- and adenosine diphosphate-induced platelet aggregation, but had no effect on platelet aggregation induced by platelet-activating factor. Platelet-activating factor was selectively inhibited by its receptor antagonist BN 52021. This method for measuring platelet-activating factor was reproducible; at the EC50, the inter- and intrabioassay coefficients of variation were within acceptable limits at 13.17% and 9.75%, respectively.

Influence of oral contraceptives on coagulation tests in native blood and plasma

Routine coagulation laboratory tests, clotting times in native (not anticoagulated) whole blood, platelet-rich and platelet-poor plasma, and recalcification times in citrated whole blood, platelet-rich and platelet-poor plasma were performed in 14 healthy premenopausal women. Blood was taken before and after one or two cycles of low-dose oral contraceptives. After oral contraceptives a reduction in clotting time in native platelet-rich plasma and activated partial thromboplastin time were observed. Recalcification times in whole blood and platelet-rich plasma were shorter than clotting times in their native counterparts. The observed changes are compatible with a procoagulant effect seen soon after the start of oral contraceptive use. The absence of these changes in the recalcification times in citrate systems suggests a masking effect of citrate. The reduction in clotting times in native platelet-rich but not in platelet-poor plasma indicates that the hypercoagulability in oral contraceptives users is mainly related to platelets.

Whole blood platelet function in acute ischemic stroke. Importance of dense body secretion and effects of antithrombotic agents

We studied platelet function in whole blood, a situation that better reflects the in vivo state, from 85 patients with acute ischemic stroke and from 19 healthy controls. Patients receiving no antithrombotic drugs demonstrated increased platelet dense body secretion without an associated increase in platelet aggregation, thus raising the possibility that dense body secretion may be of separate importance in cerebral infarction. Our results also suggest that dense body secretion may occur independently of aggregation. Heparin and heparin plus warfarin were ineffective in reducing the high level of dense body secretion seen in acute cerebral infarction, whereas treatment with aspirin plus dipyridamole inhibited both dense body secretion and platelet aggregation. It seems worthwhile to investigate the usefulness of antiplatelet drugs in the treatment of acute ischemic stroke wherein clinical outcome is correlated with the extent of suppression of platelet dense body secretion.

Observations on human platelet aggregation in native whole blood: synergism and sensitivity to aggregating agents in vitro

Threshold sensitivity levels, as well as the synergistic effects of agonists for blood platelets have been evaluated by impedance aggregometry using whole blood without anticoagulation. Platelet aggregation was studied using adenosine diphosphate (ADP), epinephrine, and thrombin. Threshold responses to ADP, epinephrine, and thrombin were observed at lower concentrations in diluted native whole blood (NWB) than reported values for citrated whole blood, and appear to be similar to those of citrated platelet rich plasma (PRP). Potentiation of aggregation occurred when both ADP and epinephrine, or epinephrine and thrombin but not ADP and thrombin were present in combination. Synergism between agonists thus is similar but not identical to that of PRP, and NWB appears to provide a sensitive system with a more physiologic milieu than other in vitro aggregation systems.

Sex-related differences in platelet aggregation in native whole blood

To clarify discrepant reports about sex-related differences platelet aggregation (PA) was tested by a new impedance method in native whole blood (NWB) and compared to the results of citrated whole blood (CWB) and the turbidometric method (TM). In NWB collagen or ADP-induced PA was stronger in women than men similar to the TM indicating that this is not or not only caused by citrate. With collagen this difference persisted after ingestion of 125 mg acetylsalicylic acid (ASA) which might in part explain the greater therapeutic benefit of ASA for men. The results exclude haematocrit or platelet count as a cause of the differences of PA between women and men. Sodium arachidonate (NaAA)-induced PA was inhibited by 125 mg ASA in all tested methods; since inhibition of ADP-induced PA was not observed in NWB or CWB and the impedance methods failed to show a second wave of ADP-induced PA the significance of the second wave appears doubtful. The impedance method of NWB was found to be more sensitive to collagen, ADP or NaAA than the impedance method of CWB and to collagen or NaAA more sensitive than the TM. The impedance methods in NWB or CWB are not suited to test epinephrine-induced PA.

Whole blood platelet aggregation is not affected by cigarette smoking but is sex-related

1. In normal subjects, 18-49 years old, the effects of the smoking habit (greater than 10 cigarettes/day) and the act of smoking two cigarettes over 10 min were studied on whole blood platelet aggregation (in vitro impedance method). 2. Acute smoking (n = 10) did not affect platelet aggregation to ADP, collagen or to platelet activating factor (PAF) nor thromboxane B2 production during aggregation. There was no difference between smokers (n = 13) and non-smokers (n = 10). However, aggregation to all aggregants was greater in females (n = 11) than males (n = 12) (ADP and collagen, P less than 0.001; PAF, P less than 0.01; ANOVA). 3. Although others have obtained diverse results studying platelet-rich plasma, the absence of an effect of cigarette smoking on whole blood platelet aggregation is consistent with many of those observations. Greater in vitro aggregability in females than males is consistent with the few studies of platelet-rich plasma. It seems unlikely that the role of cigarette smoking as a risk factor for ischaemic heart disease is related to a direct effect on platelet aggregability.

Effects of bacterial endotoxin and platelet activating factor (PAF) on human platelet aggregation in native whole blood

The effect of bacterial endotoxins E. coli 0111:B4 or S. minnesota and platelet activating factor (1-0-alkyl-2-acetyl-SN-glycero-3-phosphorylcholine) on platelet aggregation in native whole blood (NWB) were evaluated by impedance aggregometry. In the absence of anticoagulants the patterns of impedance changes associated with aggregation were distinct from those of clotting. Both E. coli 0111:B4 and S. minnesota endotoxins shortened the time to clot formation, but impedance changes suggestive of accelerated platelet aggregation were minimal or absent. In contrast, PAF caused an increased impedance, with oscillations characteristic of aggregation, which in some instances was superseded by the smooth impedance change associated with clotting. E. coli 0111:B4 endotoxin blocked aggregation and delayed the onset of clotting after PAF, whereas S. minnesota endotoxin accelerated platelet aggregation by PAF in ten of thirteen experiments. Incubation of E. coli 0111:B4 endotoxin and PAF markedly enhanced aggregation by PAF, whereas the effect of S. minnesota was variable. Although E. coli 0111:B4 and S. minnesota endotoxins accelerated clotting but not platelet aggregation of human NWB in vitro, their interaction with PAF is complex, depending on the type of endotoxin and individual reactivity. The findings suggest that endotoxin could interact with PAF to significantly augment possible hemorrhagic and/or thrombotic complications of septic shock in humans.