Platelets in nonresponders to epinephrine stimulation showed reduced response to ADP

Serotonin secretion by blood platelets: accuracy of high-performance liquid chromatography-electrochemical technique compared with the isotopic test and use in a clinical laboratory

Background

Mild secretion defects are the most frequent and challenging blood platelet disorders to diagnose. Most δ-granule secretion tests lack validation, are not quantitative, or have unreliable response to weak platelet agonists.

Objectives

To compare platelet serotonin secretion by HPLC-electrochemical detection technique (HPLC-ECD) with the reference isotopic test (3H-5-HT), evaluating its performance in clinical laboratories.

Methods

The assay validation followed STARD-2015 recommendations. HPLC-ECD measured the nonsecreted serotonin remaining in platelet pellets after aggregation, comparing it with the reference 3H-5-HT assay. We studied subjects with inherited and aspirin-induced blood platelet disorders and assessed the HPLC-ECD operation for routine clinical diagnosis.

Results

Calibration curves were linear (R2 = 0.997), with SD for residuals of 3.91% and analytical sensitivity of 5ng/mL. Intra- and interassay imprecision bias ranged between -8.5% and 2.1% and -9% and 3.1%, respectively. Serotonin recovery and stability were >95%, and the variability range of measurements was -5.5% to 4.6%. Statistical differences detected between tests were biologically irrelevant, with bias of 1.48% (SD, 8.43) and CI agreement of -18% to 15%. Both assays distinctly detected platelet secretion induced by 10 μM epinephrine and 4 μmM adenosine diphosphate. However, HPLC-ECD is quantitative and more sensitive to low serotonin content in blood platelets. Reference cutoffs for each agonist were determined in 87 subjects. Initially, the HPLC-ECD requires relatively expensive equipment and trained operators but has remarkably cheap running costs and a turn-around time of 24-36 hours. We have used this diagnostic tool routinely for >8 years.

Conclusion

HPLC-ECD assay for platelet serotonin secretion is highly accurate, has advantages over the reference 3H-5-HT test, and is suitable as a clinical laboratory technique.

Individual variations in platelet reactivity towards ADP, epinephrine, collagen and nitric oxide, and the association to arterial function in young, healthy adults

INTRODUCTION

Platelet aggregation and secretion can be induced by a large number of endogenous activators, such as collagen, adenosine diphosphate (ADP) and epinephrine. Conversely, the blood vessel endothelium constitutively release platelet inhibitors including nitric oxide (NO) and prostacyclin. NO and prostacyclin are also well-known vasodilators and contribute to alterations in local blood flow and systemic blood pressure.

MATERIALS AND METHODS

In this study we investigated individual variations in platelet reactivity and arterial functions including blood pressure and flow-mediated vasodilation (FMD) in 43 young, healthy individuals participating in the Lifestyle, Biomarkers and Atherosclerosis (LBA) study. Platelet aggregation and dense granule secretion were measured simultaneously by light transmission and luminescence. FMD was measured with ultrasound.

RESULTS

The platelet function assay showed inter-individual differences in platelet reactivity. Specifically, a sub-group of individuals had platelets with an increased response to low concentrations of ADP and epinephrine, but not collagen. When the NO-donor S-nitroso-N-acetyl-DL-penicillamine (SNAP) was combined with high doses of these platelet activators, the results indicated for sub-groups of NO-sensitive and NO-insensitive platelets. The individuals with NO-sensitive platelets in response to SNAP in combination with collagen had a higher capacity of FMD of the arteria brachialis.

CONCLUSIONS

Platelet reactivity towards ADP, epinephrine and NO differs between young, healthy individuals. Some individuals have a more effective response towards NO, both in the aspect of platelet inhibition ex vivo, as well as vasodilation in vivo.

PKC inhibitors RO 31-8220 and Gö 6983 enhance epinephrine-induced platelet aggregation in catecholamine hypo-responsive platelets by enhancing Akt phosphorylation

Impaired responsiveness of platelets to epinephrine (epi) and other catecholamines (CA) has been reported in approximately 20% of the healthy Korean and Japanese populations. In the present study, platelet aggregation induced by epi was potentiated by RO 31-8220 (RO) or Gö 6983 (Gö). Phosphorylated Akt (p-Akt) was very low in epi-stimulated PRP from CA-hypo-responders (CA-HY), whereas it was detected in those from CA-good responders (CA-GR). RO and Gö increased p-Akt, one of the major downstream effectors of phosphoinositol-3 kinase (PI3K), in epi-stimulated PRP from both groups. Wortmannin, a PI3K inhibitor, attenuated the RO or Gö-induced potentiation of p-Akt in epi-stimulated PRP, suggesting positive effects for RO and Gö on PI3K. TXA(2) formation was increased by the addition of either RO or Gö in epi-stimulated platelets. The present data also suggest that impaired Akt phosphorylation may be responsible for epinephrine hypo-responsiveness of platelets.

The Similarity Between Aspirin Treatment and the Lack of Response to Epinephrine and the Frequency of Aspirin Resistance in Healthy Males

Objectives: The lack of response of platelets against epinephrine has been discovered with a frequency of 14% to 40% in previous studies. There are studies that have demonstrated the effect of aspirin on platelets may resemble the lack of response to epinephrine. In this study, the extent of the effects of aspirin treatment on aggregation and secretion in healthy males with a lack of response to epinephrine and the frequency of aspirin resistance were investigated. Methods: Blood samples were collected at the beginning and at the end of a 10-day aspirin usage in 52 healthy males. Epinephrine, adenosine diphosphate (ADP), collagen, arachidonic acid (AA) and thrombin aggregations, and adenosine triphosphate (ATP) secretion were studied. Participants were assigned to nonresponder (<20%), semiresponder (20%-60%), and responder (>60%) groups, depending on their maximum aggregation responses to epinephrine. Participants who displayed an aggregation to AA at the end of the aspirin treatment were accepted to be aspirin resistant. Results: Of the 52 participants, 4 were found to be nonresponders and 3 of 52 of the participants were found to be semiresponders. Although the lack of response to epinephrine and aspirin treatment displayed similarities in aggregations using epinephrine, ADP, collagen, and thrombin, they differed in aggregations using AA and for ATP secretion. The ratio of aspirin resistance was determined to be 4:52. Conclusions: The observation of AA aggregation in the participants with a lack of response to epinephrine demonstrates that epinephrine nonresponse cannot substitute aspirin treatment. The fact that aspirin resistance is observed in healthy males supports the view that aspirin resistance exists even before the first usage.

Elevated plasma concentration of NO and cGMP may be responsible for the decreased platelet aggregation and platelet leukocyte conjugation in platelets hypo-responsive to catecholamines

Impaired responsiveness to epinephrine and other catecholamines (CA) were previously reported in platelets of 20 approximately 30% healthy Japanese and Koreans. In the present study, the possible mechanisms of different responsiveness to CA in platelets of CA hypo-responders (CA-HY) and CA good-responders (CA-GR) were investigated. Increased platelet-leukocyte conjugate (PLC) formations were observed with whole blood of CA-GR than with that of CA-HY in both non-stimulated [mean fluorescence intensity (MFI) values: 1.33 +/- 0.26 vs. 1.16 +/- 0.19] and ADP (MFI: 5.54 +/- 3.46 vs. 2.15 +/- 1.13) or TRAP (MFI: 5.11 +/- 2.32 vs. 3.38 +/- 1.47) activated states. The platelets of CA-GR, when stimulated with ADP (10 microM), released approximately twice the amount of ATP than those of CA-HY (0.88 +/- 0.65 and 0.45 +/- 0.36 nmole, respectively). Nitric oxide (NO) and cyclic guanosine monophosphate (cGMP) levels were significantly higher in non-stimulated PRP of CA-HY than in that of CA-GR (70.3 +/- 24.1 microM and 14.1 +/- 4.9 nM vs. 41.1 +/- 15.8 microM and 6.7 +/- 2.4 nM, respectively). The platelet-monocyte conjugation induced with either ADP or TRAP was significantly reduced in CA-GR with the addition of linsidomine, a NO donor, (MFI: 2.78 +/- 0.43 vs. 3.73 +/- 0.90, or 4.28 +/- 0.95 vs. 5.76 +/- 1.33, respectively). Moreover, the degree of platelet aggregation and the ATP secretion induced by epinephrine in CA-GR were significantly retarded with the addition of either linsidomine or 8-Bromo-cGMP (a cGMP analog) with more substantial effects on ATP release than aggregation. The results suggested that elevated NO and/or cGMP plasma levels may be responsible for the lower platelet aggregation and PLC formation observed in CA-HY than that in CA-GR.

Evaluation of platelet function and lack of response to epinephrine in pregnant women

Previous studies in healthy subjects have demonstrated a lack of response of platelets to epinephrine at a rate of 16-40% on an aggregometer. An association between the increased procoagulant factors during pregnancy and venous thromboembolism is known, and it has also been shown that prolactin levels increase platelet aggregation. We evaluated whether platelet functions in pregnant women and also assessed the lack of response to epinephrine during this period. We compared 27 healthy and volunteering pregnant women with 26 similar control subjects. Platelet functions were assessed with an aggregometer and a Platelet Function Analyzer (PFA-100). Less than 40% response to epinephrine on the aggregometer was defined as an impaired epinephrine response. The aggregation response of epinephrine was normal in 25 of the 27 pregnant women, while two of them showed a late-rising response. Eight of the 26 subject control group (30.8%) showed an impaired response to epinephrine. When we compared the 25 pregnant and 18 control subjects with normal aggregation responses, the maximum aggregation responses to ADP and epinephrine, and the Col/Epi and Col/ADP cartridge closure time values were significantly lower in pregnant women. There were no difference between second and third trimesters as regards platelet function parameters. The fact that no impaired response to epinephrine was detected in pregnant women while a 30% rate was observed in non-pregnant women indicates that the platelet malfunction caused by a disorder in the Gi protein and intracellular mechanisms is bypassed during pregnancy thanks to some physiological changes.

Adhesion of human platelets to albumin is synergistically increased by lysophosphatidic acid and adrenaline in a donor-dependent fashion

Lysophosphatidic acid (LPA) and adrenaline are weak platelet activators considered important for thrombus formation, and were previously shown to synergistically increase platelet aggregation. Here we investigate synergistic activation by LPA and adrenaline when measuring platelet adhesion. Platelet-rich plasma from healthy blood donors together with adrenaline and/or LPA were added to protein-coated microplates. Platelets were allowed to adhere and the amount of adhesion detected enzymatically. The LPA and adrenaline combination induced a synergistic increase of platelet adhesion to a normally non-adhesive albumin surface. The degree of synergy varied markedly between individuals; these variations could not be explained by age, gender, blood type or different amounts of platelets, oxidized low-density lipoprotein, insulin or glucose in plasma. There was a trend indicating increased synergistic effect for platelets sensitive to adrenaline stimulation. The synergistic effect was blocked by the alpha2-adrenoceptor antagonist yohimbine and inhibited by the ADP scavenger system creatine phosphate/creatine phosphokinase and antibodies against alphaIIbbeta3. Furthermore, platelets adhering to albumin after adrenaline and LPA treatment expressed P-selectin. In conclusion, LPA and adrenaline act synergistically to increase alphaIIbbeta3-mediated platelet adhesion to albumin, dependent on alpha2-adrenoceptor signalling and platelet secretion. We also confirm that synergistic platelet activation achieved with LPA and adrenaline is highly donor dependent.

The difference between clopidogrel responsiveness and posttreatment platelet reactivity

BACKGROUND

Aggregation is the most common measure of platelet reactivity. The relative inhibition of platelet aggregation between pretreatment and posttreatment is the most common estimate of clopidogrel responsiveness. However, patients responsive to clopidogrel may remain with highly reactive platelets and thus have increased thrombotic risk.

METHODS

Platelet reactivity was determined by ADP-induced aggregation (%) in 62 patients undergoing elective coronary stenting at pretreatment and 5 days postprocedure. All patients were on aspirin (325 mg) and received 300 mg of clopidogrel immediately poststenting and 75 mg qd. Pretreatment reactivity was divided into tertiles. Based on clopidogrel drug responsiveness, nonresponders were defined as <10% relative inhibition of pretreatment aggregation, semiresponders as 10-30%, and responders as >30%. We determined the relation between clopidogrel responsiveness and platelet reactivity.

RESULTS

Pretreatment reactivity tertiles by 5 microM ADP were: low (47+/-9%), moderate (64+/-4%), and high (78+/-6%). Eight patients were nonresponders, 18 were semiresponders, and 36 were responders. Clopidogrel responsiveness directly correlated with pretreatment reactivity, 86% of responders had moderate or high pretreatment reactivity, whereas 75% of nonresponders had low pretreatment reactivity. Despite being more responsive, 16% of patients with high pretreatment reactivity and 17% with moderate pretreatment reactivity remained with moderate posttreatment reactivity.

CONCLUSION

Measuring clopidogrel responsiveness may overestimate the risk of stent thrombosis in nonresponders with low pretreatment reactivity and underestimate risk in those responders who remain with high posttreatment platelet reactivity. Posttreatment platelet reactivity is a better measure of thrombotic risk than responsiveness to clopidogrel.

The vasculopathy of Raynaud's phenomenon and scleroderma

The scleroderma (SSc) disease process involves dramatic dysfunction in acute and chronic vascular regulatory mechanisms; it presents initially with heightened vasoconstrictor or vasospastic activity and progresses to structural derangement or vasculopathy of the microcirculation. This article discusses the regulatory mechanisms that contribute to this dysfunction and the vascular changes in the context of the other aspects of the SSc disease process in a novel attempt to integrate the individual pathologies of the disease process.