Determination of the Density Distribution of Human Platelets - Methodological Aspects and Comparison with Other Tests for Platelet Activation

The Platelet Aggregation Score and Its Correlation to β- Thromboglobulin and Platelet Factor 4 Levels

We describe a new method of interpreting platelet aggregometry results and its correlation to platelet secretory protein levels. We studied 123 healthy male volunteers, 37-85 years of age. The aggregation response to 2.5 μM ADP was measured in a standardized platelet suspension (SPS) prepared from platelet-rich plasma. Turbidimetic measurements included amplitude of the primary aggregation wave (Amp1), maximum aggregation amplitude (Amax), and area under the aggregation curve (Sagg). A platelet aggregation (PA) score was calculated as ( Amp1 /Amax) ·Sagg ·(whole-blood platelet count/SPS platelet count). β-Thromboglobulin (β-TG)and . platelet factor 4 (PF4) levels were measured by radioimmunoassay. The PA score correlated significantly (p < 0.0001) with β-TG ( r = 0.86) and PF4 ( r = 0.87). It was concluded that the PA score correlates closely with individual subjects' propensities to platelet activation, as reflected by plasma β-TG and PF4 levels. Key Words: Platelets-Platelet aggregometry-Platelet secretory proteins.

Increased platelet cholesterol and decreased percentage volume of platelets as a secondary risk factor for coronary artery disease

Platelet hyperactivity is likely to contribute to the progression of atherogenesis and organized thrombus formation on vascular surfaces. The purpose of this study was to examine the effect of hypercholesterolemia on the cholesterol content of platelets, on platelet responsiveness and other platelet indices using platelets from 5 groups of age-matched subjects (n = 30 each), which includes healthy controls. All groups except controls had a high plasma lipid profile. While subjects in group I had only hyperlipidemia, those in groups II and III had hyperlipidemia in conjunction with diabetes mellitus and hypertension, respectively. The fourth group consisted of patients with confirmed coronary artery disease (CAD). The parameters studied include packed cell volume of platelets (platelet crit), platelet distribution width (PDW), platelet cholesterol and platelet aggregation in response to adenosine diphosphate and collagen. All the patient groups showed increased platelet aggregation (p < 0.05) and low platelet crit compared with controls (p < 0.05). In addition, platelet cholesterol was increased in patients with coronary disease, hyperlipidemia and diabetes mellitus (p < 0.05) but not in patients with hypertension (p > 0.05); PDW was high only in CAD (p < 0.05). A higher PDW indicated a prothrombotic tendency in CAD patients. Our data suggest that hyperlipidemia increases the lipid content in platelets and enhances their reactivity. Hyperactive platelets with increased platelet cholesterol may contribute to accelerated atherogenesis associated with CAD.

Platelet P-selectin and platelet mass, volume and component in sickle cell disease: relationship to genotype

BACKGROUND AND PURPOSE

Excess platelet activation (e.g. increased soluble P selectin [sPsel] and beta thromboglobulin [beta-TG]) is well established in sickle cell disease (SCD) and may contribute to the prothrombotic/hypercoagulable state and vascular occlusion characteristic of the disease. We hypothesised altered whole platelet P-selectin (pPsel), and morphological platelet indices mass, volume and component in SCD and two of its major genotypes.

METHODS

We recruited 35 SCD patients [mean age 31 years, 54% men]. Of these, 16 had homozygous sickle cell (HbSS) disease and 19 had sickle-haemoglobin-C (HbSC) disease. Patients were compared with 29 subjects with normal haemoglobin (HbAA) matched for age and ethnicity. Platelet mass, volume and component were measured by flow cytometry, pPsel in platelet lysate, sP-sel and beta-TG by ELISA.

RESULTS

SCD patients had lower pP-sel and mean platelet volume (MPV) but elevated platelet component (MPC), and, as expected, elevated platelet count, and sP-sel (all p<0.05) compared to HbAA subjects. In both groups, pPsel correlated with MPV, and MPV correlated positively with mean platelet mass (MPM) and negatively with MPC. sPsel correlated with platelet count only in SCD, not in the controls. Platelet count alone was different (higher) in HbSS compared to HbSC, and sPsel correlated with platelet count only in HbSC disease, not in HbSS disease.

CONCLUSION

Patients with SCD have various abnormalities in their platelets regardless of genotype: there are more numerous platelets, which are smaller, contain less P selectin per cell, but have a higher concentration of granules than those of HbAA subjects. These differences may mark and/or promote the prothrombotic state in SCD.

Platelet sialic acid as a potential pathogenic factor in coronary heart disease

It was previously reported that, compared to healthy individuals, patients with coronary heart disease (CHD) exhibit a higher proportion of platelets with lower densities and higher propensity to aggregate. Reasons for this increased tendency to aggregate were unknown but appeared to be independent of the patient's age, gender, or smoking habits. Sialic acid (N-acetyl-neuraminic acid), a negatively charged sugar and constituent of many glycoproteins and gangliosides, is known to confer the bulk of negative charges to mammalian cell surfaces. These negatively charged surfaces can help cells of the bloodstream to maintain a relative distance from each other due to repulsion of the same (negative) charges. In this study, we examined whether differences in platelet sialic acid are a potential pathogenic factor in patients with coronary heart disease. Upon isolating platelets, we found a significantly higher (p < 0.05) proportion of low density platelets in patients compared to healthy controls, which is in accordance with previously published data. We found significantly less (p < 0.05) sialic acid in platelets from patients compared to the control. Most of the platelet total sialic acid was susceptible to cleavage by neuraminidase, demonstrating sialic acid to be preferably localized at the outer platelet surface. We conclude that the lower sialic acid content found in platelets from CHD patients could represent a contributing factor for the observed higher aggregability of platelets from these patients. Due to the lower sialic acid content and resulting lower negative surface charge, less repulsion between the platelets could facilitate aggregation.

The relation among vessel injury, thrombus formation, and platelet survival in rabbits

Continuous or repeated injury of rabbit aortae by indwelling vascular catheters caused the deposition of platelets on the injured vessels and the formation of thrombi rich in platelets and fibrin at sites where flow was most disturbed and injury was most extensive. Incorporation of 51Cr platelets into the thrombi reached a maximum between 3 and 24 hr. The platelet-fibrin-rich thrombi remained reactive to circulating platelets for at least 14 days. Continuing reactivity of thrombi and the turnover of platelets in the thrombi were accompanied by an increase in the proportion of platelets that separated in the least dense fraction on Stractan density gradients. Platelet survival was also shortened (43.5 +/- 5.9 hr in animals with catheters, compared with 62.6 +/- 4.5 hr in animals with a sham operation), indicating that some platelets that had taken part in thrombus formation or had interacted with the injured vessel wall were rapidly cleared from the circulation. Platelets from rabbits that had had indwelling aortic catheters in place for 3 or 6 days survived significantly longer than those from animals with a sham operation upon injection of the platelets into normal animals; thus, continuous turnover of platelets on injured vessels and thrombi, and the clearance of altered platelets, leads to a population of younger platelets that survive longer. The continuing reactivity of thrombi may in part account for repeated occlusive episodes in vascular disease. The contribution of thrombin generation and fibrin formation to the platelet-rich thrombi is substantial and warrants the ongoing evaluation of treatment with a combination of anticoagulant and antiplatelet agents in arterial thrombosis and in thrombus formation on vascular catheters.

Secretory products from thrombin-stimulated human platelets exert an inhibitory effect on NK-cytotoxic activity

We have investigated the interaction between human platelets and the NK-system, with special emphasis on the action of secretory products from platelets in an NK assay with 51Cr-labelled K562 as target cells. Supernatants from thrombin-stimulated platelets added to the NK assay consistently decreased the NK-cytotoxicity by 40% +/- 4.3%, indicating the existence of secreted products from platelets as a source of NK-inhibiting substances. In contrast, no direct cytotoxic effect of these secretory products on the target cells (K562) was seen. Thus, normal human platelets, when stimulated with thrombin, are capable of secreting different, yet undefined factors, which significantly inhibit NK activity in vitro. The results also suggest that the role of products from contaminating in vitro activated platelets should be borne in mind when performing conventional NK assays.

Platelet density analysis: a tool for the detection of acquired storage pool disease in man

This study was designed to evaluate the usefulness of platelet density analysis in the detection of acquired storage pool defects in human patients. Two groups of patients were investigated: 19 subjects affected with a myeloproliferative disorder (group I) where abnormal platelets are released from the megakaryocytes and 11 patients hospitalized in an intensive care unit (Group II) where normal platelets are injured in the circulation. Platelet density distribution after isopycnic centrifugation on a discontinuous stractan density gradient, dense granule markers (serotonin, ATP and ADP) and alpha granule markers (intraplatelet beta-thromboglobulin and platelet factor 4) were simultaneously determined. An increased proportion of the percentage of light platelets was observed in 16 patients of group I and nine of group II; an increased ATP/ADP ratio was observed in 12 patients of group I and 10 of group II. Both the tests were abnormal in 11 patients of group I and nine of group II. In group I, the level of serotonin was low and was related to the percentage of light platelets. The alpha granule specific proteins were normal in the two groups. These results indicate that platelet density analysis may serve as a screening test to detect exhausted platelets in human diseases.

Estimation of platelet size by measurement of intracellular water space using an oil technique

The intracellular water space of human platelets has been measured after equilibration with tritiated water and then separating these cells by centrifugation through phthalate oil of density 1.042. The mean intracellular water space of platelets in citrated plasma was 0.52 +/- 0.09 microliter/10(8) cells for 19 normal subjects. The gravimetric water content of platelets was 784 +/- 4 mg water/g cells. From these values the mean platelet volume was calculated to be 6.2 fl which agrees closely with values based on Coulter size distribution and thrombocytocrit. Gel filtration alters platelets such that a mean 19% of the platelets could not be centrifuged through phthalate oils of density 1.031 or 1.042. The measurement of tritiated water space of platelets centrifuged from their own plasma through oil provides a simple and reliable estimate of the mean platelet size.