Platelet factor 4 (PF4) and heparin-released platelet factor 4 (HR-PF4) in patients with cardiovascular disorders

The Effect of Intensive Dietary Intervention on the Level of RANTES and CXCL4 Chemokines in Patients with Non-Obstructive Coronary Artery Disease: A Randomised Study

Simple Summary

The dietary approaches to stop hypertension (DASH) diet contains meals with plenty of vegetables, fruits and low-fat dairy products, as well as whole grains, fish, poultry and nuts. One of the potential mechanisms of the beneficial effects of the DASH diet on the cardiovascular system may be modification of the inflammatory process. In the present study, we assessed the impact of the DASH diet on selected inflammatory markers in patients with atherosclerosis. The study lasted 12 months and involved 79 patients (40 followed the DASH diet, while 39 were in the control group). The results indicate that the DASH diet lowers the level of the inflammatory markers, which may contribute to the inhibition of atherosclerosis.

Abstract

Background: Inflammation is the key pathophysiological mechanism of the initiation and progression of atherosclerosis. The study objective was to assess the effects of a dietary intervention based on the model of the dietary approaches to stop hypertension (DASH) diet on the levels of chemokines RANTES and CXCL4 in patients with non-obstructive coronary artery disease. Methods: As part of Dietary Intervention to Stop Coronary Atherosclerosis in Computed Tomography (DISCO-CT) study, patients were randomised to an intervention group (n = 40), where the DASH diet was introduced along with optimal pharmacotherapy, and to a control group (n = 39), with optimal pharmacotherapy alone. In the DASH group, systematic dietary counselling was provided for the follow-up period. RANTES and CXCL4 levels were determined using ELISA. Results: In the DASH group, the RANTES level insignificantly reduced from 42.70 ± 21.1 ng/mL to 38.09 ± 18.5 ng/mL (p = 0.134), and the CXCL4 concentration significantly reduced from 12.38 ± 4.1 ng/mL to 8.36 ± 2.3 ng/mL (p = 0.0001). At the same time, an increase in the level of both chemokines was observed in the control group: RANTES from 34.69 ± 22.7 to 40.94 ± 20.0 ng/mL (p = 0.06) and CXCL4 from 10.98 ± 3.6 to 13.0 5± 4.8 ng/mL (p = 0.009). The difference between the changes in both groups was significant for both RANTES (p = 0.03) and CXCL4 (p = 0.00001). The RANTES/CXCL4 ratio reduced in the control group (from 3.52 ± 2.8 to 3.35 ± 2.8; p = 0.006), while in the DASH group, an increase was observed (from 3.54 ± 1.7 to 4.77 ± 2.4; p = 0.001). Conclusions: A 12-month-long intensive dietary intervention based on DASH diet guidelines as an addition to optimal pharmacotherapy causes changes in the levels of chemokines CXCL4 and RANTES and their mutual relationship in comparison to conventional treatment.

The antigenic complex in HIT binds to B cells via complement and complement receptor 2 (CD21)

Heparin-induced thrombocytopenia is a prothrombotic disorder caused by antibodies to platelet factor 4 (PF4)/heparin complexes. The mechanism that incites such prevalent anti-PF4/heparin antibody production in more than 50% of patients exposed to heparin in some clinical settings is poorly understood. To investigate early events associated with antigen exposure, we first examined the interaction of PF4/heparin complexes with cells circulating in whole blood. In healthy donors, PF4/heparin complexes bind preferentially to B cells (>90% of B cells bind to PF4/heparin in vitro) relative to neutrophils, monocytes, or T cells. Binding of PF4 to B cells is heparin dependent, and PF4/heparin complexes are found on circulating B cells from some, but not all, patients receiving heparin. Given the high proportion of B cells that bind PF4/heparin, we investigated complement as a mechanism for noncognate antigen recognition. Complement is activated by PF4/heparin complexes, co-localizes with antigen on B cells from healthy donors, and is present on antigen-positive B cells in patients receiving heparin. Binding of PF4/heparin complexes to B cells is mediated through the interaction between complement and complement receptor 2 (CR2 [CD21]). To the best of our knowledge, these are the first studies to demonstrate complement activation by PF4/heparin complexes, opsonization of PF4/heparin to B cells via CD21, and the presence of complement activation fragments on circulating B cells in some patients receiving heparin. Given the critical contribution of complement to humoral immunity, our observations provide new mechanistic insights into the immunogenicity of PF4/heparin complexes.

CXCL4 Plasma Levels Are Not Associated with the Extent of Coronary Artery Disease or with Coronary Plaque Morphology

BACKGROUND

CXCL4 is a platelet chemokine released at micromolar concentrations upon platelet activation. CXCL4 has been shown to promote atherogenesis by various mechanisms. However, data on CXCL4 plasma levels in patients with coronary artery disease are largely inconclusive. Computed coronary artery angiography (CCTA) represents an excellent tool to quantify and characterize coronary atherosclerotic plaques. We hypothesized that increased CXCL4 plasma levels may be associated with features of plaque instability resulting in adverse cardiovascular events. Specifically, we sought to determine whether CXCL4 levels are correlated with specific features of coronary artery disease including (1) plaque volume, (2) calcium score, (3) degree of stenosis, or (4) vascular remodeling.

METHODS AND RESULTS

CXCL4 plasma levels were measured by ELISA in 217 patients undergoing CCTA for suspected CAD (mean age 64.2 ± 9.4 years, 107 (49.3%) male). Mean CXCL4 plasma levels were 12.5 ± 4.6 ng/mL. There was no significant correlation between CXCL4 levels and any clinical or demographic parameters including cardiovascular risk factors. CXCL4 plasma levels did not differ between patient with or without coronary artery disease (CAD: 12.5 ± 4.5 ng/ml, no CAD: 12.5 ± 4.8 ng/ml). Neither univariate nor multivariate analysis showed an association between CXCL4 levels and plaque volume, total calcium score, degree of stenosis, or vascular remodeling. Subgroup analysis of patients with CAD as confirmed by CCTA did not show any association of CXCL4 levels with the extent of CAD.

CONCLUSIONS

While CXCL4 may be present and active within the arterial wall, local increase of CXCL4 may not translate into systemically elevated CXCL4 levels. Further studies will have to test whether CXCL4 may still represent a suitable therapeutic target in human atherosclerosis.

Emphasis on the Role of PF4 in the Incidence, Pathophysiology and Treatment of Heparin Induced Thrombocytopenia

Heparin Induced Thrombocytopenia (HIT) is caused by antibodies that recognize platelet factor 4 (PF4) associated with polyanionic glycosaminoglycan drugs or displayed on vascular cell membranes. These antibodies are elicited by multimolecular complexes that can occur when heparin is administered in clinical settings associated with abundant PF4. Heparin binding alters native PF4 and elicits immune recognition and response. While the presence of heparin is integral to immunogenesis, the HIT antibody binding site is within PF4. Thus HIT antibodies develop and function to cause thrombocytopenia and/or thrombosis only in the presence of PF4. Future emphasis on understanding the biology, turnover and regulation of PF4 may lead to insights into the prevention and treatment of HIT.

Profiling of heparin-induced thrombocytopenia antibody levels in patients with and without diabetes

Heparin/platelet factor 4 (H:PF4) antibodies are the causative agent in heparin-induced thrombocytopenia (HIT). The antibodies are frequently formed after exposure to heparin, most commonly without any signs of clinical HIT. Heparin-induced thrombocytopenia antibodies have been detected by enzyme-linked immunosorbent assay (ELISA) in individuals who have not been exposed to heparin. It is possible that the antibodies could be elicited by PF4 associated with endogenous, heparin-like glycosaminoglycans (GAGs). This risk would be higher in individuals with endothelial dysfunction and chronic platelet activation. In the setting of an outpatient endocrinology clinic, both diabetic and nondiabetic patients were studied and compared with healthy volunteers. Heparin/platelet factor 4 antibody titers were measured by ELISA and analyzed to determine the frequency of clinically seropositive responses, and median and interquartile ranges of baseline antibody titers. The study found no increase in frequency of ELISA-positive patients among diabetic patients. Moreover, the diabetic population had lower overall level of H:PF4 antibody titer, especially the subgroups treated with thiazolidinedione drugs or angiotensin receptor blockers. Further studies are needed to determine whether subthreshold titers of HIT antibody may be reflective of the physiological state of platelet/endothelial balance.

Heparin Displaces Interferon-γ–Inducible Chemokines (IP-10, I-TAC, and Mig) Sequestered in the Vasculature and Inhibits the Transendothelial Migration and Arterial Recruitment of T Cells

Background— Heparin, used clinically as an anticoagulant, also has antiinflammatory properties and has been described to inhibit interferon (IFN)-γ responses in endothelial cells. We investigated the effects of heparin on the IFN-γ–inducible chemokines IP-10/CXCL10, I-TAC/CXCL11, and Mig/CXCL9, which play important roles in the vascular recruitment of IFN-γ–producing Th1 cells through interactions with their cognate receptor, CXCR3.

Methods and Results— Patients undergoing coronary artery bypass grafting were studied because coronary atherosclerosis is recognized as a Th1-type inflammatory disease and the subjects required systemic heparinization. Plasma levels of IP-10, I-TAC, and Mig increased immediately after heparin administration and diminished promptly after heparin antagonism with protamine. These effects were independent of detectable circulating IFN-γ or the IFN-γ inducer interleukin-12. We confirmed previous reports that heparin inhibits the IFN-γ–dependent production of CXCR3 chemokine ligands using atherosclerotic coronary arteries in organ culture. In addition to prolonged treatment decreasing chemokine secretion, heparin rapidly displaced membrane-associated IP-10 from cultured endothelial cells that did not express CXCR3 and reduced the IP-10–dependent transendothelial migration of T helper cells under conditions of venular shear stress. Finally, heparin administration to immunodeficient mouse hosts decreased both the recruitment and accumulation of memory T cells within allogeneic human coronary arteries.

Conclusions— Besides inhibiting IFN-γ responses, heparin has further immunomodulatory effects by competing for binding with IP-10, I-TAC, and Mig on endothelial cells. Disruption of CXCR3 + Th1 cell trafficking to arteriosclerotic arteries may contribute to the therapeutic efficacy of heparin in inflammatory arterial diseases, and nonanticoagulant heparin derivatives may represent a novel antiinflammatory strategy.

PF4/heparin complexes are T cell-dependent antigens

Heparin-induced thrombocytopenia (HIT) is a life-threatening, thrombotic disorder associated with development of anti-platelet factor 4 (anti-PF4)/heparin autoantibodies. Little is known about the antigenic and cellular requirements that initiate the immune response to these complexes. To begin to delineate mechanisms of autoantibody formation in HIT, we studied the immunizing effects of murine PF4 (mPF4)/heparin in mice with and without thymic function. Euthymic mice were injected with mPF4/heparin complexes, mPF4, heparin, or buffer. Mice injected with mPF4/heparin, but not mPF4 or heparin alone, developed heparin-dependent autoantibodies that shared serologic and functional characteristics of human HIT antibodies, including preferential binding to mPF4/heparin complexes and causing heparin- and FcRgammaIIA-dependent platelet activation. In contrast, athymic mice did not develop HIT-like antibodies. Taken together, these studies establish that PF4/heparin complexes are highly immunogenic and elicit self-reacting anti-PF4/heparin antibodies in a T cell-dependent manner.

Candidate-based proteomics in the search for biomarkers of cardiovascular disease

The key concept of proteomics (looking at many proteins at once) opens new avenues in the search for clinically useful biomarkers of disease, treatment response and ageing. As the number of proteins that can be detected in plasma or serum (the primary clinical diagnostic samples) increases towards 1000, a paradoxical decline has occurred in the number of new protein markers approved for diagnostic use in clinical laboratories. This review explores the limitations of current proteomics protein discovery platforms, and proposes an alternative approach, applicable to a range of biological/physiological problems, in which quantitative mass spectrometric methods developed for analytical chemistry are employed to measure limited sets of candidate markers in large sets of clinical samples. A set of 177 candidate biomarker proteins with reported associations to cardiovascular disease and stroke are presented as a starting point for such a 'directed proteomics' approach.

The Release of Tissue Factor Pathway Inhibitor and Platelet Factor 4 After Heparin Injection in Patients with Thrombocytosis

Platelet factor 4 (PF4) and tissue factor pathway inhibitor (TFPI) are two proteins with high affinity for heparin. They are each stored in platelets, as well as on endothelial cell surfaces, from where both are displaced or released following an injection of heparin with a rapid and marked increase in serum levels. Prior work has demonstrated that the platelet count is one of the factors affecting the levels of heparin-releasable PF4. We therefore characterized the response to a dose of intravenous heparin previously demonstrated to completely displace PF4 from the non-platelet pool in subjects with normal or increased platelet counts. Seventeen patients with essential thrombocytosis (ET), 10 patients with polycythemia vera and high platelet counts (PV-H), 7 patients with polycythemia vera and normal platelet counts (PV-N) and 10 controls received an initial bolus of 40 I.U./kg of unfractionated heparin, followed 2 hours later by a 2nd bolus of a fixed dose of 1000 I.U. TFPI activity did not show any variation among the different groups, either before (TFPI) or after (HR-TFPI) the first bolus of heparin: ET, TFPI 92.6 ± 21.5%, HR-TFPI 298.3 ± 165.8; PV-H, TFPI 91.5 ± 32.0, HR-TFPI 210 ± 1.0; PV-N, TFPI 69.4 ± 24.0, HR-TFPI 203.0 ± 79.0; C, TFPI 109.5 ± 33.5, HR-TFPI 234.0 ± 60.4. TFPI activity returned to basal values prior to the 2nd injection of heparin, which again elicited a rise in TFPI, albeit smaller due to the lower level of heparin injected. In contrast to the lack of any difference between groups with respect to TFPI, the level of heparin-releasable PF4 (HR-PF4) was significantly higher in ET and PV-H patients compared to PV-N patients or controls. However when normalized for platelet count, both PV-H and PV-N had HR-PF4 levels after the 1st heparin injection that were significantly higher than observed in ET patients (PV-H 1.163 + 0.108, PV-N 1.411 + 0.019, ET 0.737 + 0.086 ng/10/3 platelets) supporting an increased platelet activation in PV. Thus, although platelets contain approximately 5-10% of the total amount of TFPI in plasma, they do not affect the major intravascular pool of TFPI mobilizable by heparin. However, since the concentration at the site of vessel wall injury is enhanced several-fold, TFPI could play a role in competing with PF4 to limit thrombus formation in patients with high platelet count.

Low molecular weight heparin and the heparin mobilisable pool of platelet factor 4 are reduced in young survivors of myocardial infarction

The platelet factor 4 (PF4) mobilisation properties of low molecular weight heparin (Fraxiparine, Sanofi Winthrop, France) in young survivors of myocardial infarction (YSMI) and healthy volunteers have been investigated. The study group consisted of 42 YSMI less than 44 years old, all of them with angiographically proven occlusive coronary artery disease, studied 6 to 24 months after the acute event. The control group was composed of 30 healthy men of similar age. Subjects from the study and control groups were allocated to the following subgroups: those receiving 60 or 120 IU/kg b.w. of standard heparin (Polfa Kutno, Poland) and those receiving 60, 120 or 180 IC anti-Xa U/kg b.w. of low molecular weight heparin (Fraxiparine, Sanofi Winthrop, France) as a single intravenous injection. Additionally, in five YSMI patients the influence of prolonged aspirin administration (0.3g daily for more than 30 days) on the Fraxiparine mobilsable pool of PF4 and beta-thromboglobulin (beta-TG) concentration in the plasma was determined after injection of 180 IC anti-Xa U/kg b.w. of the drug. The PF 4 and beta-TG concentration in the plasma was evaluated using enzyme immunoassay methods before heparin or Fraxiparine intravenous injection and 2, 5, 10, 20, 30 and 60 min after. In both, the control and YSMI groups baseline PF4 levels were found to be normal. Moreover, similar marked dose-dependent increases of PF4 concentration in the plasma measured after 60 and 120 IU/kg b.w. of heparin as well as after 60 and 120 IC anti-Xa U/kg b.w. of Fraxiparine was found. The administration of 120 IU/kg b.w. of heparin resulted in a reduced rise in plasma PF 4 in YSMI as compared to healthy controls. The same phenomenon was observed when 180 IC anti-Xa U/kg b. w. of Fraxiparine was injected intravenously. In YSMI treatment with aspirin had no influence on the Fraxiparine mobilisable pool of PF 4 or the beta-TG concentration in the plasma. These results suggest that mobilisable pool of platelet factor 4 in young survivors of myocardial infarction derives from the "nonplatelet pool" and that reduction of heparin- or Fraxiparine-releasable pool of PF4 may reflect an impaired endothelium function, probably due to atherosclerosis.

Reversal of heparin anticoagulation by recombinant platelet factor 4 in humans

BACKGROUND

Protamine is used to reverse the anticoagulant effects of heparin, but it can have important side effects. Platelet factor 4 (PF4) is a protein found in platelet alpha granules that binds to and thereby neutralizes heparin. We evaluated the safety and effectiveness of intravenous recombinant PF4 to neutralize heparin anticoagulation after cardiac catheterization in a phase 1, open-label trial.

METHODS AND RESULTS

The study group consisted of 18 patients having diagnostic cardiac catheterization. Heparin (5000 U) was given after vascular access was obtained. In the first 12 patients, additional heparin was given at the conclusion of the procedure so that all patients had activated coagulation times > 300 seconds before rPF4 was given. Three patients each received 0.5, 1.0, 2.5, or 5.0 mg/kg rPF4 over a period of 3 minutes at the conclusion of the catheterization procedure. In 6 additional patients, extra heparin was not given at the conclusion of the procedure, and 1.0 mg/kg rPF4 was given. Hemodynamic measurements, cardiac output, and serial blood tests were performed 5, 10, 20, and 30 minutes after rPF4 and then into the next 24 hours. There were no serious side effects in any patient, despite transient rPF4 levels as high as 14,870 ng/mL in the patients receiving 5.0 mg/kg. One patient receiving 2.5 mg/kg had a slight transient rise in liver enzymes possibly related to the rPF4. There were no important hemodynamic effects of rPF4 administration at any dose used. Doses of 2.5 and 5.0 mg/kg were uniformly effective in reversing the anticoagulant effect of heparin. At lower doses, rPF4 neutralized the effects of heparin in most but not all patients. Pharmacokinetic analysis suggested a monophasic and one-compartment clearance of the PF4-heparin complex. No neutralizing factors to rPF4 were detected in the samples collected 7 days after dosing.

CONCLUSIONS

rPF4, in doses ranging from 0.5 to 5.0 mg/kg over 3 minutes, had no serious side effects. Given in sufficient amounts, rPF4 can completely and rapidly reverse the anticoagulant effects of heparin.

Production of novel monoclonal antibodies against rabbit platelet factor four

Ten hybridomas producing monoclonal antibodies (Mabs) against rabbit platelet factor 4 (PF4) were obtained from the fusion of splenocytes from mice immunized with purified rabbit PF4 and NSO mouse myeloma cells. When the reactivities of these monoclonal antibodies were determined by enzyme-linked immunosorbent assay and immunoblotting with human and rabbit PF4, they showed a high degree of specificity. Only one Mab recognized an epitope common to the human and rabbit molecules, the other nine reacted only with the rabbit protein. All the antibodies recognized, in crude platelet lysates, a band that comigrates with the purified PF4 protein. None of these antibodies cross-reacted with major rabbit or human platelet-poor plasma proteins. The significance of the Mabs in immunological and physiological studies is discussed.

Heparin-releasable platelet factor 4 in patients with coronary artery disease

Recently, platelet factor 4 (PF4) release by heparin (heparin-releasable PF4) has been examined as a useful marker of the interaction between the substances liberated from circulating platelets and the vascular endothelium. We compared the plasma levels of PF4 and beta-thromboglobulin (beta-TG) after intravenous heparin injection in patients with coronary artery disease (CAD) and normal control subjects. We also studied the effects of low-dose aspirin (81 mg/day) on the plasma level of heparin-releasable PF4 in the CAD patients. Blood samples were obtained before and 5 min after the intravenous injection of heparin (1,000 IU) from 23 patients with CAD and 15 normal control subjects. Although the plasma beta-TG level remained unchanged after heparin injection, the plasma PF4 level markedly increased in both groups. There was a significant difference in plasma PF4 levels at 5 min after heparin injection between the CAD group (100.1 +/- 38.1) and the control group (61.0 +/- 24.0) (p less than 0.01). The PF4/beta-TG ratio after heparin injection was also higher in the CAD group than in the control group (p less than 0.01). There was a correlation between the PF4/beta-TG ratio after heparin and the Gensini CAD score, which defines the severity of coronary atherosclerosis (r = 0.489, n = 23, p less than 0.01). Low-dose aspirin was administered to 11 CAD patients for 246.0 +/- 28.8 days. Blood samples for the assay of PF4 and beta-TG were obtained as stated above, and platelet aggregation, thromboxane B2 (TxB2), and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) levels were also measured before and during aspirin administration.(ABSTRACT TRUNCATED AT 250 WORDS)

High heparin released platelet factor 4 in uncomplicated type 1 diabetes mellitus

The role of platelet activation in diabetic microangiopathy is still controversial. We evaluated the degree of platelet activation in relation to vessel wall damage in three selected, well matched groups of subjects (10 healthy controls; 20 insulin dependent diabetic patients, 10 without microangiopathy and 10 with microangiopathy). We measured beta TG and PF4 plasma levels before and 5, 15 and 90 min after a heparin bolus i.v. (5000 U). beta TG basal levels were increased only in diabetic patients with microangiopathy. Diabetic patients without microangiopathy showed significantly higher levels of heparin released-PF4 (HR-PF4) in comparison with healthy controls. High levels of HR-PF4 seem to be an early marker of in vivo increased platelet activation in uncomplicated diabetes mellitus.

Platelet function in acute respiratory failure

To assess the role of platelets in thrombohemorrhagic complications of acute respiratory failure (ARF), we studied platelet function in 13 ARF patients admitted for intensive care, in six acutely ill intensive care patients without evidence of acute lung injury (non-ARF), and in 10 normal subjects. Platelet counts in ARF and non-ARF patients were similar to the normal range. The bleeding time of the ARF patients (8.5 +/- 0.9 min) was significantly longer (p less than 0.01) than the normal (4.8 +/- 0.2 min) but similar to non-ARF patients (5.4 +/- 0.8 min). The bleeding time prolongations in ARF patients were unrelated to platelet concentration. Platelet aggregation induced by ADP and thrombin was normal in both ARF and non-ARF patient groups. The epinephrine response was impaired in one non-ARF patient and in three ARF patients; collagen-induced aggregation was absent in two ARF patients, with a prolonged bleeding time. Levels of VIII:C and vWF in both groups of patients were similar to the normal level, but VIIIR:Ag levels in ARF patients (407 +/- 45% of normal) were higher (p less than 0.01) than in both non-ARF patients (210% +/- 10%) and normal subjects (106% +/- 4). The electrophoretic mobility of VIIIR:Ag was abnormal in ARF patients. The prolonged bleeding time in ARF patients appears to result from the qualitative and quantitative VIIIR:Ag defect. beta-Thromboglobulin levels were greater (p less than 0.01) in ARF patients (87.6 +/- 6.9 ng/ml; p less than 0.001) than in non-ARF patients (46.2 +/- 3.1 ng/ml) or in normal subjects (25.3 +/- 2.5 ng/ml p less than 0.0001). However, platelet factor 4 plasma levels in ARF patients (18 +/- 1.6 ng/ml) did not differ from those in non-ARF patients (15.0 +/- 3.0 ng/ml), but both were significantly different from normal (6.1 +/- 0.8 ng/ml). Plasma thromboxane B2 (T X B2) levels were not different from normal values in either ARF or non-ARF patients, but 6-keto-PGF1 alpha levels were significantly reduced (p less than 0.01) in ARF patients (215 +/- 43 pg/ml) compared to normal values (381 +/- 34 pg/ml). Non-ARF patients had 6-keto-PGF1 alpha levels (285 +/- 111 pg/ml) midway between the normal values and those of ARF patients. Our results suggest that in vivo platelet activation occurs in ARF. ARF patients have quantitative and qualitative platelet defects that may contribute to thrombotic and hemorrhagic complications.

Plasma levels of platelet secretory proteins

Platelets contain three types of secretory organelles: the dense granules, the alpha granules, and the lysosomes. Most of the proteins secreted from platelets are stored in the alpha granules, whereas the dense granules contain substances such as adenine nucleotides, serotonin, Ca++, and inorganic pyrophosphate types as well as a heparatinase. Three of the secreted alpha granule proteins have been measured by radioimmunoassay and it has been suggested that levels of these proteins in patient plasmas provide an index of in vivo platelet activation and secretion. These three are beta-thromboglobulin, platelet factor 4, and thrombospondin. In this chapter the chemistry of these proteins will be considered briefly, as will their clearance from the circulation, and then the clinical studies will be reviewed critically. Since radioimmunoassays were developed for these proteins (the first was reported in 1975), there has been a profusion of reports on levels of one or another of these proteins in a wide range of disease states, and these reports have indicated secreted platelet protein levels ranging from normal to grossly elevated in a given disease state. Possible reasons for such variability will be discussed.

Serial studies of platelet factor 4 and beta thromboglobulin during exercise in patients with coronary artery disease

In vivo activation of platelets can be accurately measured by radioimmunoassays of platelet factor 4 (PF4) and beta thromboglobulin (beta TG). Studies that attempt to correlate increases in PF4 and beta TG levels with exercise-induced myocardial ischemia have yielded conflicting results. To further examine the natural history of release of PF4 and beta TG we used a method of serial samplings of these proteins during and after exercise in nine normal subjects and 24 patients with coronary artery disease (CAD). Mean values for PF4 and beta TG at rest, during each stage, and immediately after treadmill exercise were the same for normal subjects and for patients with positive and negative responses to exercise-tolerance tests (ETTs). However, nonparametric analysis and regression equations disclosed differences in trends of PF4 level during exercise; PF4 levels increased in normal subjects during exercise, while patients with positive ETTs had no change in PF4 levels and patients with negative ETTs actually showed a decrease in PF4. This investigation confirmed that exercise-induced myocardial ischemia is not associated with platelet aggregation as manifested by the release of the platelet-specific proteins PF4 and beta TG. Statistical analysis suggested that prior reports of elevated levels of PF4 during exercise could have been caused by technical and methodologic difficulties that were associated with the collection and handling of the samples.