Disorders of platelet function

4-Аminopyridine sequesters intracellular Ca2+ which triggers exocytosis in excitable and non-excitable cells

4-aminopyridine is commonly used to stimulate neurotransmitter release resulting from sustained plasma membrane depolarization and Ca²⁺-influx from the extracellular space. This paper elucidated unconventional mechanism of 4-aminopyridine-stimulated glutamate release from neurons and non-neuronal cells which proceeds in the absence of external Ca²⁺. In brain nerve terminals, primary neurons and platelets 4-aminopyridine induced the exocytotic release of glutamate that was independent of external Ca²⁺ and was triggered by the sequestration of Ca²⁺ from intracellular stores. The initial level of 4-aminopyridine-stimulated glutamate release from neurons in the absence or presence of external Ca²⁺ was subequal and the difference was predominantly associated with subsequent tonic release of glutamate in Ca²⁺-supplemented medium. The increase in [Ca²⁺]_i and the secretion of glutamate stimulated by 4-aminopyridine in Ca²⁺-free conditions have resulted from Ca²⁺ efflux from endoplasmic reticulum and were abolished by intracellular free Ca²⁺ chelator BAPTA. This suggests that Ca²⁺ sequestration plays a profound role in the 4-aminopyridine-mediated stimulation of excitable and non-excitable cells. 4-Aminopyridine combines the properties of depolarizing agent with the ability to sequester intracellular Ca²⁺. The study unmasks additional mechanism of action of 4-aminopyridine, an active substance of drugs for treatment of multiple sclerosis and conditions related to reduced Ca²⁺ efflux from intracellular stores.

Prevalence of disease and relationships between laboratory phenotype and bleeding severity in platelet primary secretion defects

BACKGROUND

The prevalence of platelet primary secretion defects (PSD) among patients with bleeding diathesis is unknown. Moreover, there is paucity of data on the determinants of bleeding severity in PSD patients.

OBJECTIVE

To determine the prevalence of PSD in patients with clinical bleeding and to study the relationships between the type of platelet defect and bleeding severity.

METHODS

Data on patients referred for bleeding to the Angelo Bianchi Bonomi Hemophilia and Thrombosis Center, Milan (Italy) in the years between 2008 and 2012 were retrieved to study the prevalence of PSD. Demographic, clinical and laboratory information on 32 patients with a diagnosis of PSD was used to compare patients with or without associated medical conditions and to investigate whether or not the type and extension of platelet defects were associated with the bleeding severity score (crude and age-normalized) or with the age at first bleeding requiring medical attention.

RESULTS

The estimated prevalence of PSD among 207 patients with bleeding diathesis and bleeding severity score above 4 was 18.8% (95% confidence interval [CI]: 14.1-24.7%). Patients without associated medical conditions had earlier age of first bleeding (18 vs 45 years; difference: -27 years; 95% CI: -46 to -9 years) and different platelet functional defect patterns (Fisher's exact test of the distribution of patterns, P = 0.007) than patients with accompanying medical conditions. The type and extension of platelet defect was not associated with the severity of bleeding.

CONCLUSIONS

PSD is found in approximately one fifth of patients with clinical bleeding. In patients with PSD, the type and extension of laboratory defect was not associated with bleeding severity.

Role of platelets in neurodegenerative diseases: a universal pathophysiology

Platelets play an important role in a variety of disorders, namely, cardiovascular, psychosomatic, psychiatric, thrombosis, HIV/AIDS in addition to various neurodegenerative diseases (NDDs). Recent evidence indicates that platelet react to diverse stressors, thereby offering an interesting vantage point for understanding their potential role in contemporary medical research. This review addresses the possible role of platelets as a systemic probe in various NDDs, such as amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, Alzheimer's disease, multiple sclerosis, etc. The current review based on published literature, describes a probable link between platelets and pathophysiology of various NDDs. It also discusses how platelets epitomize ultrastructural, morphological, biochemical and molecular changes, highlighting their emerging role as systemic tools in different NDDs.

Our expanding view of platelet functions and its clinical implications

Platelets are the primary cell mediator of thrombosis. A deficiency of platelets can result in severe bleeding defects. "Overactive" platelets contribute to life-threatening outcomes in diseases such as heart attack, stroke, and cancer. The use of platelet inhibitors for thrombosis prevention must therefore seek a delicate balance between inhibiting platelet activation and an associated increased bleeding risk. There are currently few platelet inhibitors clinically available, making the search for novel anti-platelet drug targets a major research priority. Several newly identified pathways of platelet activation may hold hope in this area. In addition, important roles for platelets beyond hemostasis have been discovered. Platelets have recently been described as mediators of diverse inflammatory diseases such as arthritis, hepatitis, malaria, and atherosclerosis. This has heightened the need to broaden our understanding of platelet functions and platelet-derived inflammatory mediators. It has also heightened interest in a continued search for new platelet inhibitors and presents new opportunities for platelet inhibitors to be used in a wide array of disease treatment strategies.

Platelet-dense granule deficiency causes postoperative hemorrhage in patients receiving enoxaparin: a novel observation with dramatic clinical implications

BACKGROUND

Inherent hemorrhage risk has impeded the universal adoption of low-molecular-weight heparin (LMWH) for venous thromboembolic prophylaxis in surgical patients. Coagulation pathway parameters and platelet numbers routinely are evaluated preoperatively; scant attention has been directed toward evaluation of platelet function. We hypothesized that administration of LMWH may unmask latent platelet dysfunction and result in postoperative hemorrhage.

METHODS

Postoperative hemorrhage occurred in 15 (3.5%) of 423 consecutive patients undergoing laparoscopic gastric bypass. All patients received LMWH (enoxaparin, 40 mg) preoperatively. Hematologic evaluation included measurement of von Willebrand's factor level and activity, factor VIII level, and electron microscopic enumeration of platelet-dense granules.

RESULTS

All patients had normal preoperative platelet counts and coagulation profiles. Ten patients underwent hematologic evaluation: coagulation pathway parameters were normal in all; however, all patients had a markedly decreased number of platelet-dense granules.

CONCLUSIONS

Platelet-dense granule deficiency may cause postoperative hemorrhage in patients receiving LMWH.

Review article: blood platelet number and function in chronic liver disease and cirrhosis

BACKGROUND

The liver plays a central role in coagulation and fibrinolysis but is also closely intertwined with the function and number of blood platelets.

AIM

To describe and integrate all literature concerning blood platelets and liver disease by performing a thorough literature research.

METHODS

A thorough literature research on 'blood platelets' and 'liver disease' was performed.

RESULTS

Thrombocytopenia is a marked feature of chronic liver disease and cirrhosis. Traditionally, this thrombocytopenia was attributed to passive platelet sequestration in the spleen. More recent insights suggest an increased platelet breakdown and to a lesser extent decreased platelet production plays a more important role. Besides the reduction in number, other studies suggest functional platelet defects. This platelet dysfunction is probably both intrinsic to the platelets and secondary to soluble plasma factors. It reflects not only a decrease in aggregability, but also an activation of the intrinsic inhibitory pathways. The net effect, finally, is a decreased platelet function in the various types of chronic liver diseases and cirrhosis. Finally, recent data suggest that platelets are not only affected by but can also contribute to the liver disease process, as for instance, in viral hepatitis and cholestatic liver disease.

CONCLUSION

Platelet research in liver disease is a growing area of investigation and could provide new pathophysiological insights.

Species variability in platelet aggregation response to different agonists

Conflicting data on platelet function in animal species are reported in the literature. In this study, the response of buffalo, horse, pig and sheep platelets to different agonists was assessed. Blood samples were collected from the jugular vein of six healthy subjects of each species and platelet-rich plasma was obtained by centrifugation. Platelet aggregation responses to increasing doses of adenosine 5'-diphosphate (ADP), arachidonic acid, collagen, platelet activating factor (PAF) and ristocetin were measured by a turbidimetric method. Horse platelets were the most responsive to ADP, collagen and PAF, whereas sheep platelets were the most responsive to ristocetin. The response to arachidonic acid varied least between species. PAF was the most effective agonist, inducing a maximum aggregation response at a concentration of 1 micro M for platelets of each species. Conversely, concentrations of ristocetin higher than 1mg/ml induced a maximum aggregation response only with sheep and horse platelets. The different responses of platelets from the four animal species to various agonists may reflect either (1). structural differences (including composition of the platelet membrane and presence of specific agonist receptors), or (2). activation of distinct signalling pathways by the agonist.

Pathogenetic analysis of three cases with a bleeding disorder characterized by defective platelet aggregation induced by Ca2+ ionophores

We report three cases of platelet dysfunction characterized by defective Ca2+ ionophore-induced platelet aggregation without impaired production of thromboxane A2 (TXA2). The patients had mild to moderate bleeding tendencies, and their platelet aggregation and secretion induced by ADP, collagen, arachidonic acid, stable TXA2 (STA2) and Ca2+ ionophore A23187 was defective or much reduced. However, ristocetin- or thrombin-induced platelet aggregation was normal. The analysis of second messenger formation showed that inositol 1,4,5-triphosphate formation or Ca2+ mobilization induced by thrombin, STA2 or A23187 was normal. Furthermore, the phosphorylation of 47 kDa protein (pleckstrin) and 20 kDa protein (myosin light chain, MLC) in response to those agonists was normal. These findings suggest that the defective site in the patients' platelets lies in the process distal to or independent of protein kinase C activation, Ca2+ mobilization and MLC phosphorylation.

Platelets and psychiatry: lessons learned from old and new studies

Platelets play an important role not only in hemostasis but also in the pathophysiology of coronary artery disease. The complex interactions among the vascular endothelium, platelets, and blood components are one of the most exciting research areas today. This review addresses some fundamentals of platelet physiology and examines why platelets are interesting probes for neurophysiology. Results of current studies suggest that platelets are affected by diverse stressors, including psychological ones, and that platelets offer an interesting vantage point for understanding the neurophysiology of various psychiatric disorders. We also describe how platelets have been used for various types of research, including studies of stress associated with cardiovascular disease and studies of platelets in psychopharmacological research. Finally, we examine some of the psychiatric literature related to platelets; these studies range from case studies from the 1920s to contemporary experimental studies.