A role for changes in platelet production in the cause of acute coronary syndromes

Platelets are heterogeneous with respect to their size, density, and reactivity. Large platelets are more active hemostatically, and platelet volume has been found to be increased both in patients with unstable angina and with myocardial infarction. Furthermore, platelet volume is a predictor of a further ischemic event and death when measured after myocardial infarction. Platelets which are anucleate cells with no DNA are derived from their precursor, the megakaryocyte. Therefore, it is suggested that changes in platelet size are determined at thrombopoiesis in the megakaryocyte and that those changes might precede acute cardiac events. Understanding of the signaling system that controls platelet production may also further elucidate the cascade of events leading to acute vascular occlusion in some patients.     

Inherited thrombocytopenia: novel insights into megakaryocyte maturation,proplatelet formation and platelet lifespan

The study of patients with inherited bleeding problems is a powerful approach in determining the function and regulation of important proteins in human platelets and their precursor, the megakaryocyte. The normal range of platelet counts in the bloodstream ranges from 150 000 to 400 000 platelets per microliter and is normally maintained within a narrow range for each individual. This requires a constant balance between thrombopoiesis, which is primarily controlled by the cytokine thrombopoietin (TPO), and platelet senescence and consumption. Thrombocytopenia can be defined as a platelet count of less than 150 000 per microliter and can be acquired or inherited. Heritable forms of thrombocytopenia are caused by mutations in genes involved in megakaryocyte differentiation, platelet production and platelet removal. In this review, we will discuss the main causative genes known for inherited thrombocytopenia and highlight their diverse functions and whether these give clues on the processes of platelet production, platelet function and platelet lifespan. Additionally, we will highlight the recent advances in novel genes identified for inherited thrombocytopenia and their suggested function.     

Platelet transfusions in haematology patients: are we using them appropriately?

Background and Objectives A large proportion of all platelet components are given to haematology patients. As there are risks associated with their transfusion, costs associated with production, and shortages may occur, it is important that their use is appropriate. Study Design and Methods The study was split into two parts, a survey to assess local practice guidelines and an assessment of platelet usage. A total of 123 hospitals completed the survey and 168 hospitals submitted data of 40 haematology patients over a 3‐month period. Results The organizational survey found that 36% of hospitals routinely give prophylactic platelet transfusions to patients with long‐term bone‐marrow failure. Also, a significant minority of hospitals administer platelet transfusions if the platelet count is below a certain threshold prior to performing a bone‐marrow aspirate (11%) or a bone‐marrow aspirate and trephine (23%); both of these are contrary to UK platelet transfusion guidelines. Data were collected on a total of 3402 patients, of which 3296 cases were eligible for analysis. They received approximately 46% of all platelet components issued to participating hospitals in England during the study period. The majority (69%) of platelet transfusions were prophylactic; of these only 33% were given when the platelet count was ≤10 × 10⁹/l. Using an algorithm, based on current UK guidelines, 60% of prophylactic transfusions were appropriate, 6% could not be assessed and 34% were inappropriate. A total of 10% of all prophylactic transfusions were double the standard adult dose. Conclusions There is considerable potential for decreased use of platelet transfusions with a consequent improvement in their appropriate use and cost reduction.     

Diagnosis of von Willebrand's disease. A comparative study of diagnostic tests on nine families with von Willebrand's disease and its differential diagnosis from hemophilia and thrombocytopathy.

Nine probands with von Willebrand's disease, and their family members, totalling 43 people, were examined. Twenty-seven had a history of bleeding; 29 had an increased factor VIII activity:factor VIII related antigen ratio; 24 had a decreased factor VIII related antigen; 23 had a prolonged bleeding time; 19 had a reduced platelet adhesiveness; 16 had a decreased factor VIII activity; and 14 had an abnormal ristocetin-induced platelet aggregation. Eight members with both normal beleeding time and normal factor VIII activity were found to have other abnormal tests: elevated ratio of factor VIII activity to factor VIII related antigen in seven; decreased factor VIII related antigen in four; and reduced platelet adhesiveness in one. Therefore, ratio of factor VIII activity to factor VIII related antigen and factor VIII related antigen are more sensitive and may be used for the detection of heterozygous carriers of von Willebrand's disease. Although patients with thrombocytopathy may have a prolonged bleeding time, decreased platelet adhesiveness and reduced platelet aggregation by ristocetin, their factor VIII activity, factor VIII related antigen and ratio of factor VIII activity to factor VIII related antigen are normal and their abnormal ristocetin test cannot be corrected by the addition of factor VIII concentrate. Hemophilic subjects and hemophilic carriers, who are deficient in factor VIII activity, usually have a normal bleeding time, normal platelet adhesiveness, and normal ristocetin test. In contrast to patients with von Willebrand's disease, their factor VIII related antigen is normal or slightly increased and their ratio of factor VIII activity to factor VIII related antigen is significantly reduced. We conclude that ratio of factor VIII activity to factor VIII related antigen and factor VIII related antigen are not only more sensitive but also more specific for the diagnosis of von Willebrand's disease.     

Detection and identification of occult HBV in blood donors in Taiwan using a commercial,multiplex, multi‐dye nucleic acid amplification technology screening test

Platelet components became routinely available to many institutions in the late 1960s and since then utilization has steadily increased. Platelets are produced by three principal methods and their manufacturing process is regulated by multiple agencies. As the field of platelet transfusion has evolved, a broad array of strategies to improve platelet safety has developed. This review will explore the evolution of modern platelet component therapy, highlight the various risks associated with platelet transfusion and describe risk reduction strategies that have been implemented to improve platelet transfusion safety. In closing, the reader will be briefly introduced to select investigational platelet and platelet‐mimetic products that have the potential to enhance platelet transfusion safety in the near future.     

Platelets and diabetic vascular disease

Tests of platelet behaviour in vitro, particularly aggregation and retention and in vivo tests such as measurement of platelet survival and plasma levels of beta-thromboglobulin are frequently abnormal in diabetic patients, particularly in those with vascular disease. The concept has therefore arisen that platelet hyper-reactivity is one factor responsible for diabetic microangiopathy. Whereas there is experimental and histological evidence for the mediation of platelets in the pathogenesis of atherosclerosis, direct evidence of platelet involvement in microangiopathy is scanty. Similar alterations in platelet behaviour have been observed in a variety of other conditions with vessel wall damage in common and evidence is presented which suggests that these platelet abnormalities may be secondary to vessel wall injury. In diabetic subjects, some changes in platelet behaviour are reversed by improved glycaemic control. Evidence that platelets are involved in the pathogenesis of diabetic microangiopathy therefore remains circumstantial, though current trials of anti-platelet agents may enable a more precise evaluation of their role.     

The isolation of human platelet factor V [published erratum appears in Blood 1987 Jul;70(1):339]

Human platelet factor V has been isolated using either a monoclonal or polyclonal antibody directed against human plasma factor V. The largest peptide observed upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of purified human platelet factor V comigrates with purified human plasma factor V. However, a significant portion of the isolated protein is represented by peptides of lower apparent molecular weight (Mr). These lower Mr species that copurify with platelet factor V have been shown to be platelet factor V components by their immunological cross-reactivity with monoclonal and polyclonal antibodies to purified human plasma factor V. Platelets isolated from whole blood drawn directly into inhibitors to prevent proteolysis and platelet activation demonstrate the pattern of fragmented platelet factor V. The components of purified platelet factor V demonstrate apparent Mr ranging between 115 K and 330 K and are detectably different from the intermediates and end products observed during the thrombin cleavage of single-chain plasma factor V. Upon treatment with thrombin the platelet factor V components are cleaved and the end products are indistinguishable from those obtained upon thrombin activation of plasma factor V to plasma factor Va. Examination of the components by immunoblotting demonstrates that some of the cleavages which have occurred in the platelet factor V molecule are within the 150-K activation peptide. Bioassay indicates that platelet factor V exists as a procofactor and cleavage by thrombin yields the active cofactor, platelet factor Va. These data suggest that human platelet factor V is stored in the platelet as a partially fragmented procofactor that can be activated by thrombin to yield human platelet factor Va, the active cofactor in the human prothrombinase complex.     

The Role of Platelets in Essential Hypertension

Recent research is helping us understand the complex interactions that occur between platelets and their environment. The several intracellular events that occur during platelet activation are being identified as are their effects on other platelets, the endothelium and coagulation factors. Heightened platelet activation is seen early in essential hypertension and probably plays an important role in the initiation and progression of atherosclerosis and the disorders associated with it. This review identifies some of the changes in platelet structure and function in essential hypertension and their role in the pathogenesis of hypertensive vascular disease.     

Angeborene Thrombozytenfunktionsstörungen

Inherited thrombocytopathies are much less frequent in comparison to acquired platelet function disorders. However, congenital disorders can lead to severe bleeding tendency and are often not diagnosed. They are induced by different platelet defects based on disorders of platelet adhesion, receptors, secretion and signal transduction. In some cases they are associated with thrombocytopenia, giant platelets and various comorbidities. This article gives an overview regarding diverse defects, their diagnosis and treatment options.     

Impairment of platelet aggregation in hemolytic uremic syndrome: evidence for platelet "exhaustion"

Thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure are the hallmarks of hemolytic-uremic syndrome (HUS). This report presents the results on platelet studies from 10 consecutive HUS patients in childhood. During their acute illness, they all displayed a characteristic pattern of impaired platelet function: no aggregating responses to epinephrine, some to ADP, and moderate to collagen. In addition, platelet contents of beta-thromboglobulin (beta TG) were markedly reduced. As these patients improved clinically, their platelet- aggregating responses also normalized despite their uremic state. Incubation of platelets with uremic plasma or guanidino-succinic acid, a uremic toxin, had minor effects on platelet-aggregating activity. Since low levels of platelet beta TG suggest that these platelets were in an exhausted state, in vitro experiments were performed to exhaust normal platelets by incubation at 37 degrees C. A proportional impairment of platelet-aggregating responses and decreasing levels of platelet beta TG were noted. Furthermore, the pattern of impairment was similar to that found in the platelet-aggregating activities of HUS patients. Thus, "exhaustion," in addition to azotemia and thrombocytopenia, are factors that contribute to the functional impairment of platelets in these patients. Further studies to reveal mechanisms that lead to platelet exhaustion in HUS are of fundamental importance in the understanding of this illness.     

Platelet autoantibodies are common in hepatitis C infection, irrespective of the presence of thrombocytopenia

OBJECTIVE: To investigate the generation of platelet antibodies in hepatitis C virus (HCV)-infected individuals and their relation to the development to thrombocytopenia with the aim of using their detection as a diagnostic aid of immune thrombocytopenia in these patients. MATERIALS AND METHODS: We tested by the monoclonal antibody-specific immobilization of platelet antigen assay (MAIPA) for the presence of platelet antibodies against specific glycoprotein (GP) targets (GPIIb/IIIa, GPIb/IX, GPIa/IIa, GPIIIb, GPV, and FcRgammaIIa) in 48 HCV-infected individuals of various stages of disease and compared the results with those from 35 patients with alcoholic liver cirrhosis. RESULTS: Thirty-two HCV-infected individuals (66%) had detectable platelet antibodies. The most common target was GPIIb/IIIa, but all other GP were also targets. Results were not different from patients with alcoholic liver cirrhosis. There was no correlation between antibodies and platelet counts, or the stage of disease, or the viral genotype, or a discernible influence of treatment with alpha-interferon. CONCLUSION: While platelet autoantibodies are common in individuals with HCV infection, their detection does not assist in the diagnosis of immune thrombocytopenia.     

Pathophysiology and thrombokinetics in autoimmune thrombocytopenia

Studies that have measured platelet survival by autologous platelet labeling with (111)In and (51)Cr have differed in their results. Although all studies have revealed a significant decrease in platelet life span, the rates of platelets entering the circulation, a calculated and inferred determination, have been found to be moderately decreased to as much as five times normal. Several mechanisms have been proposed to explain an apparent decrease in platelet production, including a true decrease due to damage to the megakaryocytes by autoantibody, versus a decrease only in 'effective' production due to intramedullary destruction by the reticuloendothelial system. Recently, the identification of the cytokine, thrombopoietin, has allowed the evaluation of another aspect of the pathophysiology of thrombocytopenic states. Megakaryocyte growth factor levels are increased 10 to 20 times in patients who are thrombocytopenic due to chemotherapy or aplastic anemia, but may be decreased, normal, or only modestly raised in patients with immune platelet destruction. Autologous platelet survival measurements, prior to and while on therapy with a stable platelet count, reveal that removal of part of the reticuloendothelial system with splenectomy leads to increased platelet survival and platelet number. Similar studies reveal that corticosteroid treatment for immune thrombocytopenic purpura (ITP) effectively increases the rate of platelet production but does not change platelet survival. It may be that other therapies are effective by a combination of these mechanisms. Stimulation of thrombopoietin production or administering exogenous cytokine may have a role to play in future management of patients with ITP.     

Formation and fate of platelet microparticles

There is increasing evidence that platelet microparticles participate in thrombus formation. Yet the origin of platelet microparticles and their fate in the circulation remain poorly defined. It is unknown, for example, whether circulating platelet microparticles found in healthy individuals are derived from activated platelets or generated during megakaryopoiesis. The life span of platelet microparticles and the mechanism of their clearance have also not been determined. This article addresses these fundamental aspects of the physiology of platelet microparticles.     

Procoagulant potential of platelet alpha granules

In this brief review of the literature it is pointed out that during platelet activation and degranulation platelet alpha granules leave the platelet interior through blebs in platelet plasma membrane and through the tips of the pseudopods, and then accumulate in the external milieu. There they undergo disintegration and secondary adhesion to the platelet plasma membranes. During their disintegration they expose their tightly packed GPIIb-IIIa complexes, annexin V stainable aminophospholipids, factor V, and the membrane markers CD62 and CD63. There is also demasking of lysosomal acid phosphatase activity and microvesicle formation. Lysosomal nature of platelet alpha granules is mentioned. It is suggested that platelet alpha granules are the sole source of platelet procoagulant activity and platelet microparticles (MP) or microvesicles (MV). The implications of this concept for antiplatelet therapy are discussed. A relationship of this process to tissue factor exposure and apoptosis is suggested.     

Tests of platelet function

PURPOSE OF REVIEW: Platelets play a vital role in the normal hemostasis, and derangements of their function can lead to hemorrhage or thrombosis. While we have made progress in elucidating the molecular mechanisms leading to platelet adhesion, aggregation, shape change, and secretion, clinically useful tests of platelet function have lagged behind. The following is a review of some of the currently available tests of platelet function, their advantages and drawbacks, as well as the clinical scenarios in which they are likely to be useful. RECENT FINDINGS: Attention is now being paid to standardization and optimization of older tests such as platelet aggregometry, in addition to better defining the role of newer tests such as the platelet function analyzer and thromboelastography in diagnosing and managing disorders of primary hemostasis and platelet function. SUMMARY: Platelet function is complex and may be disrupted at any of a number of steps, including adhesion, aggregation, shape change and secretion. We are better defining the role of the currently available tests, while identifying gaps in our ability to diagnose disorders of platelet function.     

The effects of nonsteroidal anti-inflammatory drugs on platelet function and severity of upper gastrointestinal haemorrhage

Nonsteroidal anti-inflammatory drugs (NSAIDs) cause gastrointestinal (GI) damage primarily due to the inhibition of prostaglandin synthesis in gastric mucosa, which is an important factor in mucosa protection. Platelets are a cardinal feature of vascular repair. A variety of angiogenic stimulators are stored in platelets and are released during clotting at the wound. When there is a defect in any of these functions and/or platelet number, haemostasis is usually impaired and there may be an associated increased risk and severity of bleeding. While the mechanism of mucosal injury and bleeding are well documented with the use of NSAIDs, very little is known about the platelet function abnormalities and their effects on severity of upper GI bleedings. We performed a prospective analysis of 49 patients who had a history of NSAIDs use to investigate the association between the platelet function impairment associated with NSAIDs and severity of upper GI haemorrhages. Thirty-six of 49 patients (73.5%) had deteriorated platelet function. Mean severity score of patients with deteriorated platelet functions was 3.39, and that of patients with normal platelet functions was 2.46. Mean severity score was statistically significantly higher in patients with deteriorated platelet functions. In conclusion, impaired platelet functions associated with NSAIDs may cause more severe upper GI bleeding. Clinicians should be alert for GI complications especially in older patients and in those with a history of ulcer bleeding.     

Platelet granule disorders

The present review has cataloged the inherited and acquired disorders of platelet granules. Unfortunately, a mere listing of different conditions in which dense bodies, alpha granules, or both are decreased, absent, or fused does little to define their importance in human platelet physiology or as a causative factor in hemorrhagic disease. The inherited disorders serve as the best index of granule involvement in platelet hemostatic function. Our experience with storage pool deficiency in patients with Hermansky-Pudlak syndrome has suggested that in many individuals virtual absence of dense bodies and their contents does not present a serious threat to hemostasis. Placing HPS patients on aspirin did not cause spontaneous hemorrhage, suggesting that secretion of dense body contents and synthesis of endoperoxides and thromboxane A2 are not absolutely essential for platelet function. However, the literature strongly suggests that many patients with HPS and SPD face a serious risk from bleeding, and hemorrhage may cause death. We can only conclude that some patients with HPS have platelet defects or other hemostatic problems that render SPD a far more serious threat than in other patients who appear to have the same disease. Dense bodies of and by themselves do not appear absolutely required for platelet function. Isolated deficiency of alpha granules presents the same enigma. Only a few patients with this rare inherited disease have been reported. They are generally considered to have mild to severe hemorrhagic problems. However, the past medical history of our two patients with GPS has recently been reviewed and platelet function studies repeated. Despite the mild thrombocytopenia, they are free of any significant bleeding episodes and their platelet function appears virtually normal. Our findings do not support the concept that alpha granules are essential for platelet function. The only condition that seems to support a critical role for storage organelles in hemostasis is the combined alpha-granule, dense body deficiency in one patient reported by Weiss. This patient does have bleeding problems. However, it is difficult to draw conclusions based on a single patient, and the discovery of other patients will help to clarify the hemostatic problem of patients with dual storage organelle deficiencies. In the meantime, we have prepared platelets from normal individuals free of storage granules by sedimentation through gradients containing cytochalasin B. The function of the normal agranular platelets is compromised, but they do respond to some aggregating agents.(ABSTRACT TRUNCATED AT 400 WORDS)     

Diagnostic evaluation of platelet function disorders

Platelet function disorders are inherited and acquired conditions that represent a common cause of bleeding. Their clinical findings are generally similar to von Willebrand disease. It is often challenging to diagnose common platelet function disorders due to heterogeneity in their features, uncertainties about their pathogenesis and genetic cause, variability in the procedures used to assess platelet function in diagnostic laboratories and the lack of diagnostic criterion. Some inherited platelet function disorders have been established to increase risks for bleeding and bleeding scores. However, bleeding history assessment tools are not validated for use in diagnosing platelet function disorders. Standardized tests that assess aggregation function, dense granule deficiency and dense granule secretion are useful for diagnosing common platelet function disorders, in addition to some rare conditions. Guidelines have emerged to improve and standardize the laboratory tests for diagnosing platelet function disorders, including how to interpret aggregometry findings. Nonetheless, there is need to further evaluate the features, pathogenesis and genetic cause of many platelet function disorders, including the inherited conditions that impair granule secretion.     

Analysis of anti-platelet antibody and platelet volume in idiopathic thrombocytopenic purpura

We investigated platelets and plasma from patients with idiopathic thrombocytopenic purpura (ITP) to elucidate the antigenic determinants at which their autoantibodies are directed, and studied the relationship between anti-platelet antibody and platelet volume. We used flow cytometry to detect platelet-associated IgG (PAIgG), C3 (PAC3), IgM (PAIgM) and platelet volume, and also to determine the binding rate of monoclonal anti-platelet antibodies in patients with ITP. The following results were obtained. 1. Both anti-GPIIb/IIIa autoantibodies (21 of 71 patients) and anti-GPIb autoantibodies (3 of 71 patients) were found in ITP. 2. The decrease in platelet count in patients without anti-GPIIb/IIIa autoantibodies was significant. 3. The increase in platelet volume was found more frequently in patients with a platelet count less than 50,000 and in untreated patients. 4. There was a positive correlation between the platelet volume and PAIgM in patients with a platelet count less than 30,000 and high levels of PAIgM.     

Effects of nabumetone on platelet function in healthy volunteers

Non-steroidal anti-inflammatory drugs (NSAID) are frequently prescribed but gastrointestinal haemorrhages and inhibition of platelet function are two side effects that limit their use. Nabumetone belongs to a new prostaglandin-sparing class of NSAID with a low potential for causing gastrointestinal mucosal irritancy and inhibition of platelet function. We have used flow cytometry and in vitro bleeding time (IVBT) to measure the effects of nabumetone on platelet function in healthy volunteers. Nabumetone was found to cause a significant decrease in platelet-bound fibrinogen after adenosine diphosphate (ADP) activation using flow cytometry and a significant increase in IVBT when using CaCl(2) as activating substance. The platelet inhibitory effect was less pronounced than the changes seen with low dose aspirin. Flow cytometry and IVBT are two sensitive methods well suited for clinical use and could both be used to monitor drug-induced inhibition of platelet function.