Platelet size: measurement, physiology and vascular disease

Platelet volume is a marker of platelet function and activation. It is readily measured as mean platelet volume (MPV) by clinical haematology analysers using sodium citrate as the anticoagulant. Measurement in EDTA can be unreliable since MPV increases significantly in a time-dependent manner. MPV correlates with platelet function and activation, whether measured as aggregation, thromboxane synthesis, beta-thromboglobulin release, procoagulant function, or adhesion molecule expression. MPV is increased in certain vascular risk factor states, including hypercholesterolaemia and diabetes mellitus, but not essential hypertension. It is increased in acute myocardial infarction, acute ischaemic stroke, pre-eclampsia and renal artery stenosis. Importantly, an elevated MPV predicts a poor outcome following myocardial infarction, restenosis following coronary angioplasty, and the development of pre-eclampsia. Research into the epidemiology of MPV is now required to determine whether thrombomegaly is a risk factor for developing vascular disease. Similarly, the physiological mechanisms which regulate MPV within the megakaryocyte need to be elucidated. Whether MPV ever becomes a routinely requested test remains to be seen but changes in methodology will be required first.

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Cigarette smoking sensitizes and desensitizes impedance-measured ADP-induced platelet aggregation in whole blood

The effect of smoking on platelet aggregation appears to produce conflicting results, with some studies indicating an enhancement and others a decrease of aggregation. This epidemiological study of 120 male smokers, a subset of the Caerphilly Heart Disease Study, examined the relationship of two dimensions of smoking (time proximity of last cigarette before venepuncture and serum nicotine concentration) with threshold dose of adenosine diphosphate (ADP) necessary to induce platelet aggregation in whole blood. Means (range) of ADP threshold dose and nicotine concentration were 1.66 (0.5-2.5, censored) microM and 12.2 (0-35.2) ng/ml. Men smoking within 30 min of venepuncture demonstrated lower ADP threshold doses (-0.48 microM lower [95% C.I.: -0.95, -0.02])--reflecting increased sensitivity. Men with higher nicotine concentration had higher ADP threshold doses (Regression Coefficient: +0.032 microM per ng/ml [95% C.I.: 0.003, 0.062])--reflecting decreased sensitivity. Men smoking 30 min or more before venepuncture who also had high nicotine concentration (25-30 ng/ml) demonstrated the highest ADP threshold doses compared to never smokers and to men smoking the previous day (approximately 2.20 vs 1.86 and 1.81 microM). Relations involving nicotine concentration do not necessarily reflect a pharmacological effect although the potential for a short term nicotine mediated tolerance effect cannot be dismissed. These observations support an hypothesis suggesting a temporal sequence of platelet sensitization and desensitization during smoking.

von Willebrand factor, a possible indicator of endothelial cell damage, decreases during long-term compliance with a lipid-lowering diet

OBJECTIVES

To test whether serum von Willebrand factor (vWf) would be lower in men with atherosclerosis who had been consuming a lipid-lowering diet for 3 years than in a control group of men with atherosclerosis who had been following their normal diet.

DESIGN

A randomized, population-based case-control study.

SETTING

A tertiary health care referral centre at a University Hospital.

SUBJECTS

Men age less than 66 years with angiographically proven coronary atherosclerosis and a cholesterol level > 6 mmol L-1. Sixty started the study and 50 completed it.

INTERVENTIONS

Subjects were randomized to a lipid-lowering diet or to taking their normal diet for approximately 3 years.

MAIN OUTCOME MEASURES

The components of the subjects' diets were assessed and blood was obtained for total, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) cholesterol, triglycerides and for vWf.

RESULTS

Men on the lipid-lowering diet consumed less total, saturated and monounsaturated fats (all P < 0.001), cholesterol and retinol (both P < 0.002) but increased polyunsaturated fats (P < 0.001), fibre, vitamin E (both P < 0.005) and carbohydrate (P < 0.05). Those on the lipid-lowering diet also had lower serum levels of total and LDL cholesterol (P < 0.002 and P < 0.05, respectively), triglycerides (P < 0.02) and vWf (P < 0.05) than the men on their normal diet. There was no difference in HDL cholesterol. Levels of vWf correlated with both total cholesterol (P < 0.005) and inversely with dietary polyunsaturated fats (P < 0.02).

CONCLUSION

von Willebrand factor, a possible indicator of endothelial cell damage, decreases during long-term compliance with a lipid-lowering diet.

Increased monocyte tissue factor expression in coronary disease

OBJECTIVE

To investigate whether monocyte expression of tissue factor is increased in patients with acute coronary syndromes and chronic stable angina.

DESIGN

Cross sectional study of monocyte tissue factor expression in patients with ischaemic heart disease and control subjects.

BACKGROUND

Unstable angina and myocardial infarction are associated with enhanced mononuclear cell procoagulant activity. Procoagulant activity of blood monocytes is principally mediated by tissue factor expression. Tissue factor initiates the coagulation cascade and monocyte tissue factor expression may therefore be increased in these syndromes.

METHODS

Monocyte tissue factor expression was measured cytometrically in whole blood flow using a polyclonal rabbit antihuman tissue factor antibody.

PATIENTS

30 patients with acute myocardial infarction, 17 with unstable angina, 13 with chronic stable angina, and 11 normal control subjects.

RESULTS

Increased proportions of monocytes expressing tissue factor (> 2.5%) were found in none of 11 (0%) normal subjects, five 13 (38%) patients with stable angina, 11 of 17 (64%) patients with unstable angina, and 16 of 30 (53%) patients with myocardial infarction (2P = 0.006). Blood from all subjects showed similar monocyte tissue factor expression similar monocyte tissue factor expression (46.1 (15.1)%) after lipopolysaccharide stimulation.

CONCLUSION

Hypercoagulability associated with acute myocardial infarction, unstable angina, and chronic stable angina may be induced by tissue factor expressed on circulating monocytes.

Megakaryocyte DNA content is increased in patients with coronary artery atherosclerosis

OBJECTIVE

Increased megakaryocyte size and megakaryocyte ploidy have been observed in man following myocardial infarction and sudden cardiac death. The aim of this study was to compare megakaryocyte ploidy in patients with and without coronary artery atherosclerosis.

METHODS

Sternal megakaryocytes were obtained immediately prior to cardiac surgery in 26 patients with significant coronary artery atherosclerosis requiring surgical bypass grafting and in 14 age matched control patients requiring cardiac valve replacement or repair with minimal or no coronary atherosclerosis on arteriography. Megakaryocytes were identified using a monoclonal antibody to the CD41 (GPIIb/IIIa) surface antigen and ploidy was studied using flow cytometry.

RESULTS

Megakaryocyte modal ploidy was significantly higher in the coronary atherosclerosis patients (modal ploidy 8N, n = 0 subjects; 16N, n = 16; 32N, n = 10) as compared with the control group (modal ploidy 8N, n = 5 subjects; 16N, n = 7; 32N, n = 2), p = 0.008. Ploidy correlated with the degree of coronary atherosclerosis defined as the number of atherosclerotic vessels observed on angiography (r = 0.51, p < 0.001, 95% confidence limits 0.25 to 0.70) and the serum total cholesterol concentration (r = 0.50, p = 0.006, 95% confidence limits 0.18 to 0.72). Bleeding time was not different [315(semiquartile range 45) s v 315(75) s] between the two groups in spite of the fact that 20 of 26 coronary atherosclerosis patients were taking aspirin versus one of 14 valve patients.

CONCLUSIONS

Megakaryocyte ploidy is increased in human coronary atherosclerosis and correlates with serum lipids. Hypercholesterolaemia may induce increases in megakaryocyte ploidy, and hence the production of hyperfunctional platelets, which will contribute to atherogenesis.

Increased platelet volume and platelet mass in patients with atherosclerotic renal artery stenosis

1. Platelet volume was measured in citrated blood in two groups of patients at risk of having atherosclerotic renal artery stenosis, namely (i) 30 patients with severe hypertension, and (ii) 44 patients with peripheral vascular disease. 2. Platelet volume was increased in patients with hypertension who had atherosclerotic renal artery stenosis diagnosed by angiography: no renal artery stenosis, median 7.2 (interquartile range 0.5) x 10(-15)/l; renal artery stenosis 7.8 (0.8) x 10(-15)/l; platelet mass also increased with increasing severity of renal artery stenosis. Platelet volume correlated with severity of renal artery stenosis (rs = 0.391, 2p = 0.033, n = 30). Similarly, platelet volume correlated with severity of renal artery stenosis in patients with peripheral vascular disease (rs = 0.319, 2p = 0.035, n = 44). Serum immunoreactive platelet-derived growth factor (predominantly released from platelets) and plasma immunoreactive interleukin-6 (a cytokine which has been postulated to regulate platelet volume) concentrations were not different between hypertensive patients with and without renal artery stenosis. 3. Since large platelets are hyperactive, increased platelet volume may contribute to the development of atherosclerotic renal artery stenosis. However, the present data do not lend support to the hypothesis that platelet-derived growth factor is central to renal artery atherogenesis. Interleukin-6 does not appear to regulate platelet volume.

Angiotensin-1 converting enzyme (ACE) polymorphism in patients presenting with myocardial infarction or unstable angina

A deletion/insertion polymorphism in the ACE gene has been reported previously as a potent factor for myocardial infarction. We have tested the frequency of the deletion (D) allele of the ACE gene in 308 consecutive patients admitted to coronary care with chest pain. The gene frequencies were compared with those of 348 controls recruited from the London area. Of 108 Caucasian patients with myocardial infarction, the DD genotype was found more frequently than the combined DI and II genotypes (Chi-square, chi 2 = 5.07, 2P = 0.024). The overall D gene frequency was higher in myocardial infarction patients (125 of 216, 58%) than in controls (347 of 696, 49.9%) (chi 2 = 3.79, 2P = 0.052). In contrast, the DD genotype and D allele frequencies in patients with unstable angina were similar to those found in our normal population. A nonsignificant difference in allele frequency between myocardial infarction and unstable angina patients was observed but the small numbers of subjects studied precludes a more formal comparison. Since unstable angina and myocardial infarction represent a spectrum of coronary thrombosis, it is possible that the DD genotype favours the development of myocardial infarction, perhaps through the presence of higher serum ACE concentrations.

Platelet volume is not increased in essential hypertension

Platelet size, a determinant of platelet function, is a newly emerging risk factor for atherothrombosis. Platelet volume has previously been shown, using less than ideal methodology, to be increased in essential hypertension. Mean platelet volume was measured in 38 patients with essential hypertension and 38 paired normotensive control subjects matched for age and sex. Platelet volume, median (interquartile ranges) 7.7 (7.2-8.5) fl vs. 7.8 (7.5-8.4) fl (2P = 0.40), platelet count 242 (220-288) x 10(9)/l vs. 243 (215-292) x 10(9)/l (2P = 0.68) and platelet mass 1.91 (1.67-2.18) ml/l vs. 1.84 (1.70-2.23) ml/l (2P = 0.90) were similar in the hypertensive patients (supine BP: SBP 168 (153-178) mmHg, DBP 103 (98-110) mmHg) and control subjects (SBP 126 (112-138) mmHg, DBP 78 (74-82) mmHg). Serum lipids and plasma glucose and creatinine concentrations were not different between the groups. Mean BP did not correlate with platelet volume (rs = -0.100, 2P = 0.39) or platelet count (rs = 0.111, 2P = 0.34). These data suggest that platelet volume is not altered in essential hypertension.

The routine measurement of platelet size using sodium citrate alone as the anticoagulant

Mean platelet volume (MPV), a measure of platelet size, is becoming recognised as an important marker of platelet function. However, platelets swell in edetic acid (EDTA), the standard haematology anticoagulant, in a time-dependent manner making such measurements potentially unreliable. The effect of incubation time on MPV, and platelet distribution width (PDW), as measured in EDTA, low (1:9 volume/volume with blood) or high (1:4 v/v with blood) concentration sodium citrate was studied. MPV measured in high concentration sodium citrate did not change with time in contrast to MPV measured in either low concentration sodium citrate or EDTA which both increased in an inverse exponential fashion. MPV and PDW, measured in high concentration sodium citrate, had similar within-assay and between-assay coefficients of variation as other platelet, red cell and white cell haematology variables measured in EDTA: MPV 1.4%, 2.1%; PDW 1.4%, 1.5%; MCV 0.4%, 0.7%; PC 3.1%, 6.1%; WCC 1.5%, 7.3%; Hb 2.1%, 2.4% respectively. MPV measured in EDTA and corrected for incubation time approximated to, but was higher than, the MPV measured in high concentration sodium citrate. PDW correlated inversely with platelet count (r = -0.415, 2p < 0.001). MPV may be measured in sodium citrate (at 1:4 v/v with blood) alone with a better accuracy and reproducibility than similar measurements made in EDTA. Furthermore, such measurements are not influenced by incubation time, unlike for EDTA.

The effect of nitric oxide-donating vasodilators on monocyte chemotaxis and intracellular cGMP concentrations in vitro

The effects of sodium nitroprusside (SNP) and 3-morpholino sydnonimine (SIN-1), isosorbide dinitrate (ISDN) and glyceryl trinitrate (GTN), and molsidomine (the inactive precursur of SIN-1) on monocyte chemotaxis and cyclic GMP (cGMP) concentration were studied. SNP and SIN-1 inhibited monocyte N-formyl-methionyl-leucyl-phenylalanine-stimulated migration and increased cGMP concentrations in a dose-dependent (> 10(-5) mol.l-1) and time-dependent manner. Furthermore, 8-bromo cGMP inhibited monocyte chemotaxis in a dose-dependent fashion. In contrast, ISDN, GTN and molsidomine did not alter monocyte migration or cGMP concentration. These results support earlier observations that nitric oxide inhibits monocyte function in vitro via a cGMP-mediated mechanism. The differential effects of the spontaneous and thiol-dependent NO-donating nitrovasodilators on monocyte function suggests that monocytes, like platelets, are not able to directly metabolise ISDN and GTN. If similar observations can be made in vivo, it is possible that certain nitrovasodilators might be used therapeutically to inhibit monocyte function, for example during atherogenesis.

The relationship between increased platelet count and megakaryocyte size in bronchial carcinoma

This study investigates megakaryocyte size in bronchial carcinoma and its relationship to platelet count, mean platelet volume and platelet function, the latter determined in vivo by measurement of the cutaneous bleeding time. Measurements were performed in 19 patients with bronchial carcinoma and 11 patients with a hiatus hernia (controls). Platelet count (335 +/- 123 v. 241 +/- 72 x 10(9)/l) and megakaryocyte total area (772 +/- 135 v. 666 +/- 105 microns 2) (mean +/- standard deviation) were both significantly increased in patients with bronchial carcinoma when compared to controls whilst the bleeding time was shorter in the patient group (253 +/- 116 v. 321 +/- 80 s). Patients with bronchial carcinoma were older (60.2 +/- 8.4 v. 48.8 +/- 13.2 years) and smoked more heavily (760 +/- 511 v. 92 +/- 175 cigarette years) than controls. Megakaryocyte size, platelet count and bleeding time all correlated with smoking habit (r = 0.395, P = 0.031; r = 0.622, P less than 0.001; r = 0.515, P = 0.004 respectively). There were no statistically significant differences between the groups with respect to gender or mean platelet volume. When considering all patients, significant correlations existed between platelet count and megakaryocyte area (r = 0.400, P = 0.029), and bleeding time and megakaryocyte area (r = -0.365, P = 0.047). The megakaryocyte nuclear and cytoplasmic areas were positively correlated (r = 0.855, P less than 0.001). This study suggests that the elevated platelet count in patients with bronchial carcinoma is mediated by an increase in the size of bone marrow megakaryocytes although the results are compounded by the association between smoking and megakaryocyte/platelet parameters.

Influence of platelet size on outcome after myocardial infarction

Although platelet characteristics have an important influence on ischaemic heart disease, the nature of the association of platelet size and platelet count with death and reinfarction after an index heart attack is unknown. Mean platelet volume (MPV), a determinant of platelet reactivity, was measured in 1716 men six months after myocardial infarction (MI). Deaths and recurrent ischaemic heart disease events were then assessed at two years. MPV was greater in 126 men who had a further ischaemic event (fatal or non-fatal) than in the 1590 men who had no further MI (p less than 0.001). In addition, the MPV was larger in men who died than in those who did not (p less than 0.001). There was no difference in platelet count between these groups. When analysed by quartiles, consistent trends of increasing age-adjusted relative odds of death and recurrent ischaemic events were noted for MPV. MPV did not correlate with known ischaemic heart disease risk factors such as blood pressure, blood lipids, fibrinogen, white cell count, or plasma viscosity. We believe that MPV is a further independent risk factor for recurrent MI.

Serum platelet-derived growth factor and endothelin concentrations in human hypercholesterolaemia

Platelet-derived growth factor (PDGF) and endothelin are both smooth muscle cell mitogens, and may have an important role in mediation of the proliferative phase of atherogenesis. Since hypercholesterolaemia is an important risk factor for atherosclerosis, we have measured serum endothelin and PDGF concentrations in matched pairs of hypercholesterolaemic patients and control subjects. Both endothelin and PDGF concentrations were higher in the hypercholesterolaemic patients (23.8 +/- 4.3 pM vs. 21.7 +/- 2.4 pM, P = 0.019, median difference 3.0, 95% confidence limits 0.6-7.3, and 115.4 +/- 60.3 pM vs. 64.7 +/- 22.7 pM P = 0.033, median difference 27.2, 95% confidence limits 1.3-82.5, respectively), while serum endothelin and PDGF concentrations were positively correlated with each other (r = 0.764, P less than 0.0001, 95% confidence limits 0.505-0.897). PDGF concentrations were correlated with the peripheral blood platelet count (r = 0.560, P = 0.007, 95% confidence limits 0.180-0.794). These findings suggest that PDGF, and possibly endothelin, may be involved in hypercholesterolaemia-related atherogenesis.

CD11/CD18 cell surface adhesion glycoproteins: discordance of monocyte function and expression in response to stimulation

The adherence of blood leukocytes to vascular endothelium precedes their diapedesis into the extravascular space. These processes require the expression of adherence glycoproteins on the cell surface of the leukocyte. The relative importance of these adherence molecules is so far poorly understood. However, there is evidence to suggest that a disparity exists between the surface receptor expression of these glycoproteins and leukocyte adherence to vascular endothelial cells in culture. We have investigated the importance of each of the adhesion glycoproteins CD11a, CD11b, and CD11c in mediating the adherence of human monocytes to endothelial cells in culture. We have also investigated the chronological relationship between changes in monocyte adherence to endothelial cells and the surface expression of CD11a, CD11b, and CD11c following stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP). The increase in adherence occurred within 1 minute, but declined if monocytes were preincubated with fMLP for up to 30 minutes. The surface expression of adherence molecules demonstrated a significant increase in CD11a and CD11b in the presence of fMLP after 10 min and was maintained while monocyte adherence to endothelium declined. These changes in surface receptor expression were quantitated using an immunolabeling technique. It is suggested that fMLP stimulation of monocyte adherence is unlikely to be solely dependent on increased surface receptor expression of adhesion molecules.

Human monocyte characteristics are altered in hypercholesterolaemia

Peripheral blood monocytes are involved during atherogenesis in adhering to endothelium, migrating into the subendothelial space and taking-up lipoproteins to become macrophage/foam cells. We have assessed whether peripheral blood monocyte characteristics are altered in human hyperlipidaemia in age/sex/smoking status matched pairs of patients and controls. Monocytes from the hypercholesterolaemic patients, as opposed to the controls, were more sensitive to stimulation by the agonist, N-formyl-methionyl-leucyl-phenylalanine, with respect to chemokinesis (stimulation index 1.48 +/- 0.17 vs. 1.10 +/- 0.14), chemotaxis (4.05 +/- 0.55 vs. 2.72 +/- 0.24) and adhesion to porcine aortic endothelial monolayers (1.26 +/- 0.05 vs. 1.17 +/- 0.06). The patients' monocyte total surface expression of the adhesion glycoprotein CD11b/CD18 (37.5 +/- 7.1 vs. 36.0 +/- 7.1), but not CD11c/CD18 (31.6 +/- 7.2 vs. 31.4 +/- 6.8), was increased; however, the monocytes in hyperlipidaemia were larger (9.15 +/- 0.11 microns vs. 8.98 +/- 0.11 microns) such that the surface density of CD11b/CD18 was not altered (0.144 +/- 0.029 vs. 0.142 +/- 0.029). The data suggest that circulating monocytes are functionally different in hypercholesterolaemia. This may explain the increased involvement by monocytes in hypercholesterolaemia-related atherogenesis.

Nitric oxide and prostacyclin. Divergence of inhibitory effects on monocyte chemotaxis and adhesion to endothelium in vitro

Monocyte-endothelial interactions are of fundamental importance in determining the movement of monocytes from the blood stream into the vessel wall. This study reports that two endothelium-derived factors, nitric oxide and prostacyclin, alter in vitro monocyte behavior. Nitric oxide (greater than 10(-5) M) inhibited monocyte adhesion to porcine aortic endothelial cell monolayers, whereas prostacyclin (10(-9) to 10(-5) M) had no effect. Both nitric oxide and prostacyclin inhibited monocyte chemotaxis stimulated by N-formyl-methionyl-leucyl-phenylalanine and induced dose-dependent increases in intracellular cyclic guanosine monophosphate and cyclic adenosine monophosphate concentrations, respectively. The cell surface expression of the CD11b/CD18 adhesion receptor, a glycoprotein complex known to mediate monocyte intracellular adhesion, was not altered by either nitric oxide or by prostacyclin. Thus, endothelium-derived nitric oxide and prostacyclin may have a physiological role in modulating monocyte-vascular wall interactions. Alterations in this system may contribute to the increased monocyte emigration from the blood stream into the vessel wall observed in atherogenesis.

Endothelin and PDGF do not stimulate peripheral blood monocyte chemotaxis, adhesion to endothelium, and superoxide production

Endothelin is a potent vasoconstrictory endothelial-derived peptide which can induce smooth muscle proliferation and therefore may be proatherogenic. Platelet-derived growth factor is also a potent mitogenic vasoconstrictory protein which is proatherogenic. We report that neither endothelin nor PDGF stimulate superoxide production, or monocyte adhesion to porcine aortic endothelial cell monolayers; additionally endothelin is not a chemoattractant factor for monocytes. If endothelin and PDGF are important in atherogenesis it is unlikely that a monocyte response to these mediators is involved.