© 1997 American Heart Association, Inc.
Articles |
Altered Platelet Function Detected by Flow Cytometry
Effects of Coronary Artery Disease and Age
From the Department of Cardiology, Royal Brompton Hospital (C.J.K., C.W., D.C., D.P., K.F.), and the Vascular Cell Biology Laboratory, Royal Free Hospital School of Medicine (M.P., P.S.B., A.P., A.H.G.), London, UK.
Correspondence to Dr Alison Goodall, Division of Chemical Pathology, Clinical Sciences Block, Glenfield Hospital, Leicester LE3 9QP.
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Abstract Platelet activation state and responsiveness to physiological agonists were measured in 65 patients with documented coronary artery disease (54 male and 11 female; mean age, 58 years). Twelve patients (mean age, 52 years), selected at random from the male cohort, were compared with 12 age-matched male control subjects (mean age, 52 years) and with 10 normal, young male subjects (mean age, 25 years). Whole-blood flow cytometry was used to measure platelet activation status ex vivo and platelet responsiveness to physiological agonists in vitro. Peripheral blood samples were analyzed for bound fibrinogen and expression of P-selectin, GPIb, and GPIIb-IIIa at rest and in response to ADP (0.1 to 10 µmol/L) and thrombin (0.02 to 0.32 µ/mL). No significant differences were seen in the basal levels of fibrinogen binding between any of the groups, but P-selectin expression was significantly lower in patients compared with age-matched control subjects (P=.0005). When stimulated with agonists, patients' platelets had significantly decreased fibrinogen binding (P<.03) but no difference in P-selectin expression compared with the age-matched group. Both agonist-induced fibrinogen binding and P-selectin expression were, however, higher in the young subjects compared with either the older control group or the patients (P<.05). GPIb and GPIIb-IIIa expression were lowest in the patients with angina and highest in the young control subjects, with levels in the age-matched control subjects falling between these values. Data from the total patient cohort (n=65) were identical to those in the smaller cohort (n=12). In conclusion, atherosclerosis impairs platelet aggregatory responses (fibrinogen binding) over and above the decreased response seen with age. Platelet degranulation (P-selectin expression) is also impaired in patients with coronary artery disease, but only in comparison with younger subjects, not age-matched controls.
Key Words: platelets • flow cytometry • coronary artery disease
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Introduction |
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Platelets are thought to play a key role in atherogenesis and the development of the thrombotic complications of atheroma,1 2 and antiplatelet therapy is effective in limiting complications in patients with vascular disease3 and angina.4 Despite this, it remains unclear whether platelet behavior is altered in patients with stable atherosclerotic disease. A variety of platelet abnormalities have been described in patients with stable angina. These include an increased tendency to aggregation,5 6 7 8 enhanced prostaglandin generation and abnormal sensitivity to prostacyclin,9 increased TxB2 production,10 decreased platelet survival,11 12 increased levels of ßTG, FPA,13 14 15 and PF4,16 but these have not been consistent findings.17 18 19 20 21 22 23 24 Furthermore, in two large epidemiological studies,25 26 angina was not associated with increased platelet aggregation.
To a large extent these conflicting results reflect the methodological problems of measuring platelet activation in vivo and the inadequacy of isolated in vitro tests of platelet function. Earlier studies relied on indirect measurement of the consequences of platelet activation in vivo, using aggregometry or the measurement of platelet release products. Direct assessment of the activation status of individual platelets can be measured by the sensitive technique of fluorescence-activated flow cytometry, which uses fluorescently labeled antibodies to detect the expression of activation antigens on the platelet surface.27 Activation causes a conformational change in the platelet membrane GPIIb-IIIa complex, exposing the receptor site for fibrinogen and leading to binding of plasma fibrinogen, which can be recognized with specific antibodies.28 29 30 Platelet degranulation is accompanied by translocation of granule membrane to the platelet surface, bringing with them specific membrane glycoproteins such as alpha granule P-selectin,31 which appear as neoantigens. Activation of platelets in vitro can also lead to increased numbers of GPIIb-IIIa complexes and reduced numbers of GPIb-IX complexes on the cell surface.32 Whole-blood flow cytometry of unfixed blood also allows the responsiveness of the platelets to ex vivo stimulation with platelet agonists such as thrombin and ADP to be studied in the presence of other blood components. Thus, in addition to detecting changes in the basal activation state of platelets, the technique can also detect more subtle alterations in platelet behavior.
We used flow cytometry to investigate platelet function in patients with documented coronary disease and stable angina pectoris, comparing these with both healthy, young subjects and age-matched control subjects.
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Subjects and Patients
Platelet function was studied in three groups: patients with stable angina awaiting angioplasty and two groups of healthy male control subjects, one group of age-matched subjects and another group of younger subjects.
Sixty-five patients (54 male and 11 female) with stable angina were
studied. All had limiting angina and were admitted for elective
angioplasty. Patients were between 39 and 79 years of age (mean, 58
years). All subjects received aspirin (150 mg) and a ß-blocker
(atenolol 50 mg or bisoprolol 10 mg OD) as their only antianginal
medication. Thirty of the patients (45%) had single-vessel, 27 (42%)
had double-vessel, and 8 (13%) had triple-vessel disease. Additional
clinical and demographic details (clinical presentation,
previous myocardial infarction and
revascularization, diabetes, smoking, impaired left
ventricular function, and lipid profile) are given in Table 1
. Patients were studied after fasting,
on the morning of angioplasty, before taking their morning
medication.
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Twenty-two healthy male volunteers participated in the study. Twelve male subjects were age-matched to 12 of the patients with angina, who were selected at random from the male cohort. The age range of these subjects and the paired angina patients was 40 to 60 years (mean, 52 years). Ten younger subjects between 21 and 29 years of age (mean, 25 years) were also studied. All control subjects underwent a pretrial examination, including electrocardiography, to verify health status. All had normal resting blood pressure and normal results on resting electrocardiograms with no history of, or risk factors for, coronary artery disease. No subject had taken any platelet-active drug during the 2 weeks preceding the study period, and none of the healthy subjects smoked.
All subjects and patients gave written consent to participate in this study, which was approved by the Ethical Practices Committees of the Royal Brompton and Royal Free Hospitals.
Blood Sampling
All patients and control subjects rested supine for 30 minutes
before collection of blood. All blood samples were obtained by clean
venipuncture, using minimal stasis, via a 21-gauge
butterfly needle into Monovette tubes; conditions designed to prevent
artifactual activation of platelets during phlebotomy. On each
occasion, the first 2.7 mL of blood was drawn into an EDTA tube and
used to obtain blood counts. The next 5 mL was aspirated into a
1/10-vol of 0.106 mol/L trisodium citrate and used, within 10
minutes, for the flow cytometric analysis. A subsequent 5 mL
was drawn into an ice-cold citrate tube and immediately
centrifuged at 4°C for 30 minutes at 3000xg, and
plasma was taken from the center of the tube and frozen in aliquots at
-70°C until analyzed for plasma fibrinogen and soluble
P-selectin. A final 10 mL of blood was collected into a serum/gel tube
for analysis of cholesterol and lipoproteins.
Measurement of Platelet Volume
EDTA samples were stored at room temperature for 2 hours before
routine measurement of red and white blood cell count, hemoglobin, and
MPV in a Coulter Stacks S blood analyzer. Measurement of MPV
has been shown to be stable in EDTA samples under these
conditions.33
Plasma and Serum Assays
Serum Assays
Cholesterol was measured by a standard enzymatic
method (cholesterol oxidase). HDL cholesterol
was measured after first precipitating the apolipoprotein B-containing
fractions with phosphotungstic acid/MgCl2 reagent, and
LDL-cholesterol was calculated using the Friedewald
formula.34 Lp(a) was measured by ELISA.
Plasma Assays
Plasma fibrinogen was determined by the Clauss method on a KC10
coagulometer. sP-selectin was measured by ELISA.
Flow Cytometric Analysis of Platelet
Activation
Reagents
Platelets were identified with a MAb to GPIb
(RFGP37),
and GPIIb-IIIa was identified with a CD41 MAb (RFGP56), both raised in
our laboratory.35 The MAbs were purified from ascites on
protein G-Sepharose and coupled to FITC using standard
techniques.36 Platelet-bound fibrinogen was detected
with an FITC-conjugated polyclonal antibody to human fibrinogen
(Rfgn-FITC), as described by Warkentin et al29 and
Janes et al.30 P-selectin was identified with a
FITC-conjugated immunoglobulin G1 mouse MAb. All antibodies were used
at optimum concentration for maximum fluorescence with minimum
nonspecific binding, determined by titration.
ADP and human -thrombin were purchased from Sigma Chemical Company
Ltd. Samples incubated with thrombin also contained 0.125
mmol/L glycyl-prolyl-L-arginyl-L-proline
peptide to inhibit fibrin polymerization and consequent clot
formation.37 The dilution buffer used was HEPES-buffered
saline (145 mmol/L NaCl, 5 mmol/L KCl, 1
mmol/L MgSO4, and 10 mmol/L HEPES, pH
7.4) that had been passed through a 0.22-µm filter to remove dust
particles.
Assay Procedure
Blood samples were prepared for flow cytometric analysis
using the whole-blood method essentially as described by Janes et
al.30 Five microliters of citrated blood was added to LP3
tubes containing 50 µL of HEPES-buffered saline and 5 µL of
appropriate concentrations of antibodies and agonists. After gentle
mixing, the samples were incubated for 20 minutes, then diluted with
0.5 mL of 0.2% (vol/vol) formyl saline to inhibit further
activation. Incubations were carried out at room temperature (20 to
22°C), and all samples were run in duplicate. Fibrinogen binding was
measured in response to ADP (0.1 to 10 µmol/L) and
thrombin (0.02 to 0.32 µ/mL). P-selectin expression was measured in
response to maximal ADP stimulation (10 µmol/L) and to
thrombin (0.02 to 0.32 µ/mL).
Samples were analyzed, within 2 hours of collection, in a
Coulter EPICS Profile II flow cytometer. The instrument was aligned
daily with Immuno Check and Standard Brite beads to calibrate the light
scatter and fluorescence parameters, respectively.
The platelet population was identified from its light scatter
characteristics, and its identity was confirmed using the anti-GPIb
MAb. An electronic bit map was set around the platelet population
and adjusted for each sample to ensure that >98% of the particles
analyzed were positive for GPIb. The negative cutoff levels for
fluorescence in unstimulated samples were set at
2%.30 Five thousand platelets were analyzed,
and the results represent the means of duplicate samples. The
percentage of platelets positive for the marker and the mean
fluorescence intensity for each sample were used to calculate
the binding index for the marker from the following equation: binding
index=(percent positivexmean fluorescence
intensity)x100-1.
Statistical Analysis
All data are shown as the mean±SD. However, to allow for the
possibility that data were not normally distributed, the relationship
between independent variables was analyzed using
nonparametric (Wilcoxon) tests. Correlation between
variables was plotted using Excel software and found, in all cases,
to be either linear or random. Correlation coefficients were calculated
using Pearson rank sum correlation.
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Results |
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Hematological Parameters
Hemoglobin and blood cell counts were normal in all patients and subjects studied. Platelet counts were normal in all patients and subjects and were not significantly different between any of the groups. Table 2
shows the platelet
counts, MPV, and plasma fibrinogen for the control groups and the
patients with angina, with the latter presented in two groups,
the subgroup of 12 angina patients randomly selected for age-matching
to the healthy control subjects and the total patient group (n=65). MPV
was similar in the patient groups and the age-matched control subjects
but was significantly higher (P<.001) in the younger
subjects compared with all the older groups. Plasma fibrinogen was
within the normal range (2 to 5 g/L) in all subjects (with the
exception of one angina patient who had a level of 5.1 g/L) but
was higher in the patients compared with both the age-matched and the
younger control subjects (P<.05).
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Flow Cytometric Analysis
Baseline Platelet Activation Status
Table 3 shows the level of
fibrinogen binding and expression of P-selectin, GPIb, and
GPIIb-IIIa in the different groups. There was no significant difference
between the main group of patients and the subgroup in expression of
any antigen in the unstimulated samples. There was no significant
difference in fibrinogen binding in unstimulated samples between any of
the groups. However, the patients with angina had significantly lower
expression of P-selectin compared with the age-matched control subjects
(P=.0005). There was no significant difference in P-selectin
expression between angina patients and young control subjects or
between the control groups.
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GPIb and GPIIb-IIIa expression were lowest in the angina patients and
highest in the young control subjects, with levels in the age-matched
control subjects falling between these values. These differences were
significant for both antigens between the angina patients and the young
control subjects (P=.005), but only GPIb expression was
significantly lower between the angina patients and the age-matched
control subjects (P=.001), and only GPIIb-IIIa expression
was significantly different between the two control groups.
The differences in GPIb and GPIIb-IIIa expression between the young cohort of subjects and the other three groups could relate to differences in platelet volume. However, although expression of GPIb per platelet was correlated with MPV in all subjects (r=.433; P=.0001), this was not so for GPIIb-IIIa (P>.3). In addition, differences in platelet volume cannot account for the differences seen between the patients and age-matched control subjects because the platelet volume was no different between these groups.
Platelet Responsiveness to Agonist Stimulation
Fibrinogen binding was studied in response to stimulation
with ADP (0.1 to 10 µmol/L) and thrombin (0.02 to 0.32
µ/mL).
Fig 1 shows dose-response curves for
fibrinogen binding in response to thrombin stimulation. Fig 1, a, shows
data from the subgroup of 12 angina patients and the age-matched
control subjects together with data from the young control group.
Fibrinogen binding was highest in the young control subjects and lowest
in the angina patients, with the curve for the age-matched control
subjects lying in between. The difference between the angina patients
and the control group attained statistical significance
(P<.03), but the differences between the two control groups
did not. Fibrinogen binding in the subgroup of 12 angina patients
selected for age-matching was identical to that seen in the total
patient population (Fig 1, b).
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) with 12 age-matched control subjects (
) and 10
younger male subjects (
). b, Comparison of the 12 paired angina
patients (
).
A very similar pattern was seen for fibrinogen binding in response to
ADP (Fig 2). Again, there was a
statistically significant difference between the angina patients and
the age-matched control group (P=.02) but not between the
two control groups (Fig 2, a). The subgroup of 12 angina patients did
not differ from the main patient group (Fig 2, b).
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Within the patient group, fibrinogen binding in response to ADP and thrombin stimulation showed an inverse relationship with age, but this was significant only with the higher concentrations of thrombin (eg, 0.32 µ/mL thrombin; r=-.442, P=.0004).
P-selectin expression was studied in response to maximal ADP stimulation (10 µmol/L) and to thrombin (0.02 to 0.32 µ/mL) stimulation.
A different pattern was observed from that of fibrinogen binding.
P-selectin expression in response to thrombin stimulation was no
different between the patients and the age-matched control group (Fig 3, a). However, both older groups showed
significantly reduced P-selectin expression compared with the young
control subjects (P=.0006). As before, there was no
significant difference between the subgroup of 12 angina patients and
the main cohort (Fig 3, b).
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A similar trend was observed for P-selectin expression in response to
maximal ADP stimulation (Fig 4), with
higher levels in the young subjects compared with all other groups and
similar levels of expression between the older control subjects and the
patients. However, these differences did not reach statistical
significance (P>.07). Within the patient group, P-selectin
expression did not correlate with age.
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Measurement of sP-Selectin
Levels of sP-selectin were no different in the plasma from the
patients (56.7±3.9; range, 20.8 to 110.6 mg/L) compared with that from
the age-matched control subjects (53.0±8.0; range, 37.8 to 74.4
mg/L).
Effect of Variables
Platelet Count and Plasma Fibrinogen
It has been shown 30 that platelet fibrinogen
binding is not influenced by platelet count or the level of plasma
fibrinogen (both factors that could influence the assay) within the
normal range for these parameters. There was little
variation in platelet count within any of the groups or between the
groups (Table 2
), and there was no consistent correlation
between platelet count or plasma fibrinogen and fibrinogen binding.
For illustration, in the main patient cohort, platelet count showed
an inverse correlation with fibrinogen binding in response to ADP
(10 µmol/L; r=-.300, P=.008) but
not in response to thrombin. Similarly, although in this group there
was a degree of inverse correlation between plasma fibrinogen levels
and fibrinogen binding in response to thrombin (0.32 µ/mL;
r=-.404, P=.003), no correlations were seen with
other stimuli. In addition, similar correlation was observed between
thrombin-induced P-selectin expression and plasma fibrinogen (0.32
µ/mL thrombin; r=-.285, P=.05).
Lipid Variables
Plasma cholesterol was significantly higher in the
patients compared with the age-matched control subjects
(P<.05), but no correlations between plasma
cholesterol (expressed as either total
cholesterol or LDL cholesterol) and
platelet activation were seen within the large patient cohort.
Similarly, there were no statistically significant differences in
platelet activation between patients with total
cholesterol <6.5 mmol/L and those with total
cholesterol >6.5 mmol/L (Table 4)
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Lp(a) levels of >0.3 g/L were seen in 24 (38%) of the
patients, but again, there was no relationship between platelet
activation markers and Lp(a) (Table 4).
Smoking
There was a tendency for platelet activation and platelet
agonist responsiveness to be lower in patients who smoked than in
nonsmokers or ex-smokers (data not shown). However, these differences
were not consistent and were not statistically significant
(P>.05).
Extent of Disease
Patients with two or three diseased vessels had lower levels of
fibrinogen binding than those with single-vessel disease, and these
differences were significant in the resting and ADP-stimulated samples
(P<.05) but not in those stimulated with thrombin (Table 5). No differences were seen, however, in
P-selectin, GPIb or GPIIb-IIIa expression between these groups
(Table 5). Patients with a history of myocardial infarction (n=20)
showed no overall difference in platelet activation or
responsiveness from the overall patient cohort (data not shown).
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We showed that patients with coronary artery disease have abnormalities of platelet activation and function that are measurable by whole-blood flow cytometry. In contrast to some earlier studies using different methodologies, we found that fibrinogen binding (a precursor to aggregation) in response to stimulation with physiological agonists is impaired in patients with coronary artery disease compared with age-matched, healthy control subjects. These differences were over and above the reduced binding seen with age in healthy subjects. In contrast, there was no difference in agonist-induced P-selectin expression (a marker for platelet degranulation) in the patients compared with their age-matched, healthy counterparts, but a marked decrease was seen with age.
Formation of intracoronary thrombus, consequent upon plaque rupture, is thought to be the initiating event in the transition from stable to unstable coronary artery disease. Platelets might be expected to be activated in this situation, and decreased platelet prostacyclin binding,38 increased TxA2-prostaglandin H2 platelet receptor numbers,39 increased TxA2 biosynthesis,23 and increased ßTG and PF4 levels40 have all been reported in patients with unstable angina. The platelet abnormalities described in the acute coronary syndromes may be transient consequences of plaque rupture, but it is also possible that atherosclerosis induces chronic alterations in platelet reactivity, which may precede, and contribute to, the triggering of acute coronary syndromes. No clear consensus on platelet behavior in patients with stable, as opposed to unstable, angina has been reached from existing studies.
Enhanced platelet aggregation to ADP has been reported using a variety of techniques in studies of small numbers of patients,5 6 7 and more recently, a study of 78 patients with coronary artery disease8 showed a marked increase in in vitro aggregation to ADP and collagen when the patients were compared with 113 healthy subjects. Abnormal platelet aggregation was not related in this study to either smoking, increased plasma cholesterol, or hypertension.
In contrast, other studies have found no difference in ADP-induced platelet aggregation in patients with stable angina compared with age- and sex-matched control subjects.17 18 19 Similarly, circulating platelet aggregates, although increased in patients with myocardial infarction, were not increased in patients with stable angina,20 21 and no differences were found in circulating platelet microaggregates from the aorta and pulmonary artery in 41 patients with coronary artery disease undergoing cardiac catheterization compared with 17 subjects with normal coronary arteries.22
Measurement of platelet-release products in patients with stable angina show similar inconsistencies. Small but significant increases in levels of ßTG13 14 and PF416 have been reported in patients with stable angina and were related to the extent of vessel disease,14 but in general the levels were within14 or overlapped13 the normal range.
There is good evidence41 42 that patients with myocardial infarction have larger, younger platelets than normal, probably as a result of increased platelet consumption at the atherosclerotic plaque, and such platelets are thought to have heightened reactivity. In contrast to the findings in myocardial infarction, a recent large study43 in 426 patients with stable angina awaiting coronary artery bypass grafting found no difference in MPV compared with non-age-matched control subjects. This disparity between findings in patients with myocardial infarction and those with stable angina is reflected in large epidemiological studies. In the Caerphilly Collaborative Heart Disease Study,25 investigators studied 1811 men and found platelet aggregation to be associated with electrocardiographic evidence of ischemia and history of myocardial infarction but not angina. Thaulow et al,26 despite finding a link between mortality and platelet count (487 men) and platelet aggregation (150 men) over 13.5 years of follow-up, found no relation between platelet count and the development of angina or positive results on exercise testing. A further study44 in 958 individuals showed a nonsignificant increase in aggregability in patients with a history of ischemic heart disease and abnormal results on electrocardiography.
We found a marked decrease in platelet agonist responsiveness, in particular a decreased degranulation response, in the older subjects. This contrasts with previous reports that suggest increased agonist-induced aggregation in platelet-rich plasma45 46 and increased levels of ßTG and PF447 in the plasma of older individuals.46 However, Reilly and FitzGerald48 found no increase in ßTG, circulating platelet aggregates, or response to arachidonic acid with age despite an increase in Tx excretion. The subjects in the above studies were considerably older than the patients and age-matched control subjects in our study, so the findings may not be directly comparable. It is clear, however, that currently there is little consensus on the effects of advancing age on platelet function. Although we saw no direct statistical correlation between patient age and any of the platelet parameters measured, this may reflect the relatively small age range within this middle-aged population. In addition, the wide prevalence of atherosclerosis in middle-aged men makes it difficult to separate the effects of age from those of disease. The age-matched control subjects were selected because they had a low risk of coronary artery disease, but the majority of such middle-aged men could still be expected to have some clinically insignificant atheroma. It is therefore possible that the differences in vascular disease between the age-matched control subjects and the patients may be relative rather than absolute.
Thus, although studies using a number of techniques have indicated platelet abnormalities in patients with myocardial infarction and acute coronary syndromes, a definitive picture of platelet behavior in stable coronary disease and in older individuals has not emerged. In part this is related to age differences in the patient and control groups and to the different phlebotomy procedures used for patients and control subjects in some studies. Phlebotomy was carefully controlled in the present study; all subjects were sampled under the same conditions, and phlebotomy was performed by the same individual throughout the study.
The lack of consensus reached by investigators also reflects the methodological problems of assessing platelet function in vivo. Most studies use indirect methods to detect platelet activation, many of which are highly susceptible to artifacts induced by phlebotomy and processing. Furthermore, testing of platelet responsiveness in vitro, when platelets are isolated from other blood components, may have limited relevance to the situation in vivo. By using the sensitive technique of fluorescence-activated flow cytometry in whole, unfixed blood, the activation status of individual platelets can be assessed directly. In addition, the technique allows measurement of platelet responsiveness to agonists such as thrombin and ADP to be studied in the presence of other blood components, which may detect more subtle alterations in platelet behavior. These techniques have previously been found to detect activated and hyperresponsive platelets in other clinical conditions.49
Levels of GPIb and GPIIb-IIIa per platelet were significantly lower in patients with angina and in older subjects. Total expression of these antigens has been shown, in certain experimental situations, to alter on activation, with GPIIb-IIIa increasing by translocation from intracellular stores and GPIb decreasing, probably by internalization induced to the platelet cytoskeleton following activation.50 51 The changes seen in the present study do not resemble those seen in stimulated platelets in vitro and probably do not reflect activation in vivo.
Flow cytometry detects the fluorescence intensity of each platelet, which reflects both the surface receptor density and the size of the platelets. Platelet volume was lower in all the older subjects, and this was reflected in lower expression of GPIb and, although not attaining statistical significance, lower expression of GPIIb-IIIa per platelet. Platelet volume differences could not, however, account for the differences in GPIb, GPIIb-IIIa, and fibrinogen binding between the patients and the age-matched control subjects because MPV did not differ between these groups. Measurement of MPV was carefully controlled, and all samples were analyzed 2 hours after collection to account for the swelling seen in EDTA anticoagulant. The decrease in fibrinogen binding did not appear to be related to lower numbers of GPIIb-IIIa complexes per platelet because there was no direct statistical correlation between these parameters under any condition.
Fibrinogen binding was detected with a polyclonal antibody that recognizes both plasma and platelet-bound fibrinogen. Variation in plasma fibrinogen could therefore lead to altered platelet fibrinogen binding by direct competition for the antibody. In a similar way, increased platelet count could lead to reduced antibody binding and hence to lower levels of fluorescence per platelet. We previously showed30 that fibrinogen binding to resting and ADP-stimulated platelets is unaffected by levels of fibrinogen in the plasma, or platelet counts, within the normal range, and this was borne out in the present study. Plasma fibrinogen levels were, with one exception, in the normal range, as were platelet counts, and there was no overall correlation with fibrinogen binding and either of these parameters.
A number of possible mechanisms may underlie the observations of reduced platelet responsiveness with age and disease seen in the present study. First, there is in vitro evidence that platelets exposed to substimulatory levels of agonists such as thrombin are desensitized to subsequent rechallenge with the same agonist,52 a phenomenon that can be detected using the present assay technique.53 Platelets from patients with atherosclerotic arteries could be exhibiting a similar phenomenon in vivo, through constant contact with a low level of thrombin induced by the diseased endothelial surface. Thrombin generation has been reported in patients with stable angina, as evidenced by elevated levels of FPA.14 15
Levels of blood lipids may also affect platelet function. In vitro studies suggest that LDLs that have not undergone any oxidation can inhibit platelet aggregation and fibrinogen binding in a plasma milieu.54 In contrast, low levels of LDL oxidation can enhance aggregation and fibrinogen binding.54 55 Activated and/or hyperresponsive platelets have been reported in hyperlipidemic patients, but this may be associated with alterations in cell membrane lipid composition.56 57 The patients in this study did not show a statistical correlation between platelet responsiveness and lipid parameters. However, platelets from patients with stable coronary artery disease may circulate in a state of hyporesponsiveness, yet switch to abnormal, heightened aggregation in response to alterations in lipid oxidation status. Such a biphasic response, which may underlie the triggering of acute coronary syndromes and myocardial infarction, deserves further study.
An alternative explanation is that platelets that become activated in the circulation are rapidly removed by binding to endothelial cells, leukocytes, or become assimilated into thrombi, leading to a remaining circulating pool of hyporesponsive platelets. In a previous flow cytometric study comparing 16 patients with severe peripheral arterial disease with healthy age-matched control subjects, Galt et al58 found no difference in baseline P-selectin expression and no difference in plasma ßTG between the groups. Interestingly, however, the control subjects formed significantly more platelet aggregates in response to in vitro stimulation with ADP than patients, in keeping with our findings in patients with coronary artery disease. Further studies, analyzing platelet leukocyte aggregates by flow cytometric methods (currently under development) are needed. In the present study levels of sP-selectin were not raised. It is unclear whether sP-selectin in the circulation is derived from platelets or endothelial cells, but our findings do not suggest an increase in sP-selectin from either source.
The patients in our study, but not the control subjects, were taking aspirin and ß-blockers. We previously showed that aspirin, although a powerful antiplatelet agent via inhibition of cyclo-oxygenase, does not affect the flow cytometric detection of platelet activation in response to agonist stimulation. The flow cytometric assay detects aggregation-independent platelet activation events that are therefore independent of the cyclo-oxygenase pathway and are unaffected by aspirin.59 60 These latter studies were carried out in normal subjects, and it is possible that aspirin in patients with hyperresponsive platelets might serve to suppress platelet function. However, this would not explain the lower levels of platelet response seen in the patients compared with the control subjects. Furthermore, we have also shown61 that in healthy volunteers, the administration of atenolol has no effect on fibrinogen binding or P-selectin expression in unstimulated samples and tends to increase the expression of activation markers to agonist stimulation, the opposite effect of that seen in the patients in this study. Thus, the changes in the patients are likely to be disease related rather than pharmacologically induced.
In conclusion, in contrast to previous reports, we have shown that platelets from patients with stable angina are more resistant to agonist-provoked aggregation than those from age-matched control subjects. In addition, there appears to be a decrease in the ability of platelets to degranulate with advancing age.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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C.J.K. is a British Heart Foundation Junior Research Fellow, D.J.P. has been supported as a fellow by the Augustus Newman Foundation, and M.P. is funded by a grant from the Royal Brompton Hospital Clinical Research Committee. This project was funded in part by a grant from Pfizer U.K., Ltd. We thank the Haemophilia Centre and the Department of Chemical Pathology (Royal Free Hospital) for the measurement of plasma fibrinogen and serum lipids, respectively.
Received June 26, 1996; accepted November 28, 1996.
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References |
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