A haemorrhagic platelet disorder associated with altered stimulus-response coupling and abnormal membrane phospholipid composition

Volatile anaesthetics induce changes in the expression of P-selectin and glycoprotein Ib on the surface of platelets in vitro

Halothane has been shown to inhibit platelet aggregation and may, therefore, prevent intraoperative platelet activation. The aim of this study was to compare the effects of volatile anaesthetics on platelet activation, including the transformation into an adhesive platelet phenotype. After in vitro exposure to volatile anaesthetics, the expression of the adhesion molecule P-selectin and the internalization of the receptor for the von Willebrand factor (GPlb) were analysed by flow cytometry. In contrast to desflurane or N2O, sevoflurane (> or = 0.5 MAC, P < 0.05), halothane (> or = 1.0 MAC, P < 0.01) and isoflurane (> or = 2.0 MAC, P < 0.01) induced a significantly higher expression of P-selectin on the surface of platelets, indicating the degranulation of alpha-granules. In the presence of desflurane (> or = 0.5 MAC, P < 0.05), halothane (> or = 1.0 MAC, P < 0.01), isoflurane and sevoflurane (both > or = 2.0 MAC, P < 0.01), a redistribution of GPlb occurred, indicating platelet activation. N2O had no effect. In conclusion, several of the volatile anaesthetics tested in vitro induced changes in the expression of P-selectin and GPlb, being characteristic of platelet activation. None of the anaesthetics investigated interfered with the platelet response to ADP stimulation.

Congenital disorders of platelet function: disorders of signal transduction and secretion

Congenital defects in platelet function are associated with bleeding manifestations of variable intensity and arise by diverse mechanisms. Defects in platelet-vessel wall interaction (disorders of adhesion) may arise because of qualitative or quantitative abnormalities in plasma von Willebrand factor (von Willebrand disease) or in platelet glycoprotein Ib, the binding site on platelets for von Willebrand factor (Bernard-Soulier syndrome). Disorders characterized by abnormal platelet-platelet interaction (disorders of aggregation) arise because of absence of plasma fibrinogen (congenital afibrinogenemia) or because of qualitative or quantitative abnormalities in platelet glycoprotein IIb-IIIa complex (Glanzmann's thrombasthenia). Patients with abnormalities in platelet secretion and signal transduction are a heterogeneous group characterized by impaired aggregation responses and secretion of granule contents. A small proportion of these patients have deficiency of granule stores (storage pool deficiency [SPD]) or impaired production of thromboxane A2; in most, the mechanisms underlying the platelet dysfunction are unknown. Evidence is accumulating that at least some patients have abnormalities in early signal transduction events. Abnormalities in phospholipase C activation, G-protein activation, and other events (eg, protein phosphorylation) have been documented. Platelets play a major role in blood coagulation, and in Scott syndrome, there is an abnormality in platelet contribution to the mechanisms leading to thrombin generation. In most patients with inherited platelet dysfunction, the underlying mechanisms remain to be delineated. Future studies of these patients should yield valuable new information on normal platelet mechanisms.

Platelet activation with combination of ionophore A23187 and a direct protein kinase C activator induces normal secretion in patients with impaired receptor mediated secretion and abnormal signal transduction

Defects in signal transduction mechanisms may underlie the impaired aggregation and secretion in patients with congenital platelet function defects (CPD). Both protein kinase C (PKC) induced pleckstrin phosphorylation and cytoplasmic Ca2+ mobilization play a major role in secretion. We postulated that combined platelet activation with a cell permeable direct PKC activator 1,2-dioctanoyl-sn-glycerol (DiC8) and ionophore A23187, which possibly bypass the steps involved in the intracellular synthesis of two major mediators (inositol trisphosphate, diacylglycerol), may induce normal dense granule secretion in patients with impaired receptor mediated secretion. We studied eight CPD patients with abnormal aggregation and secretion in response to several different surface receptor-mediated agonists despite the presence of normal dense granule contents. Receptor mediated Ca2+ mobilization and/or pleckstrin phosphorylation were abnormal in seven patients. Platelet activation with a combination of ADP (8 microM) with DiC8 (200 microM) or A23187 (10 microM) improved secretion in four patients. However, platelet activation with a combination of 200 microM DiC8 with 10 microM A23187, or 100 microM DiC8 with 5 microM A23187 induced normal secretion in platelet-rich plasma in all patients. These studies suggest that in such patients with CPD the ultimate process of exocytosis or secretion per se is intact and impaired secretion results from abnormalities in early signal transduction events, possibly upstream of diacylglycerol formation and calcium mobilization. Detailed studies are needed to delineate the specific abnormalities in these heterogenous patients with signal transduction defects.

Platelet signal transduction defect with Galpha subunit dysfunction and diminished Galphaq in a patient with abnormal platelet responses

G proteins play a major role in signal transduction upon platelet activation. We have previously reported a patient with impaired agonist-induced aggregation, secretion, arachidonate release, and Ca2+ mobilization. Present studies demonstrated that platelet phospholipase A2 (cytosolic and membrane) activity in the patient was normal. Receptor-mediated activation of glycoprotein (GP) IIb-IIIa complex measured by flow cytometry using antibody PAC-1 was diminished despite normal amounts of GPIIb-IIIa on platelets. Ca2+ release induced by guanosine 5'-[gamma-thio]triphosphate (GTP[gammaS]) was diminished in the patient's platelets, suggesting a defect distal to agonist receptors. GTPase activity (a function of alpha-subunit) in platelet membranes was normal in resting state but was diminished compared with normal subjects on stimulation with thrombin, platelet-activating factor, or the thromboxane A2 analog U46619. Binding of 35S-labeled GTP[gammaS] to platelet membranes was decreased under both basal and thrombin-stimulated states. Iloprost (a stable prostaglandin I2 analog) -induced rise in cAMP (mediated by Galphas) and its inhibition (mediated by Galphai) by thrombin in the patient's platelet membranes were normal. Immunoblot analysis of Galpha subunits in the patient's platelet membranes showed a decrease in Galphaq (<50%) but not Galphai, Galphaz, Galpha12, and Galpha13. These studies provide evidence for a hitherto undescribed defect in human platelet G-protein alpha-subunit function leading to impaired platelet responses, and they provide further evidence for a major role of Galphaq in thrombin-induced responses.