Platelet antithrombin deficiency: a new clinical entity.

A kindred with a history of multiple thromboses was studied for coagulant abnormalities. A deficiency of serum antithrombin III was found in approximately half of the 13 family members by either coagulant or immunologic assay. No clear relationship between antithrombin III deficiency and a history of thrombosis was present. Platelet antithrombin assays were also studied in the same subjects. Ten of the 13 members were deficient. None of the remaining three had a history of thrombosis. On the basis of these findings, the hypothesis is proposed that in some cases familial hypercoagulability may be due to a platelet antithrombin deficiency and that the serum antithrombin III deficiency in some cases is a secondary rather than a primary effect.     

Hereditary antithrombin III deficiency: Effect of antithrombin III deficiency on platelet function

Antithrombin III (AT III) is the main physiologic inhibitor of thrombin, and activated factors X and IX as well. Normal levels of AT III appear to be necessary to maintain blood fluidity and to prevent thrombosis. Four families with AT III deficiency and recurrent venous thromboembolism have been reported on. We present an additional family with AT III deficiency and a high incidence of thromboembolism. AT III levels were determined by both a functional and an immunologic assay. Results of platelet function tests, not previously reported in persons with AT III deficiency, were found to be normal. Following gel filtration, the platelets were very sensitive to thrombin. Thrombin-induced platelet aggregation appears to be dependent on a balance between the amount of thrombin and AT III present.     

Platelet aggregation defect in megakaryoblastic leukemia

Platelet aggregation induced in vitro with ADP, adrenalin and ristocetin was tested in 7 patients with megakaryoblastic leukemia (MKL). All patients had normal or high platelet counts and presented with hemorrhagic diathesis including purpura ecchymosis and epistaxis. Platelet morphology was grossly abnormal and electron microscopy revealed few, or absence of, alpha-granules. Platelet aggregation was reduced in all the cases with at least one aggregating agent. Our studies confirm that MKL is often accompanied by a thrombocytopathy which should be taken into account in the management of these patients.     

Platelet aggregation and antithrombin III levels in diabetic children.

We studied platelet function and antithrombrin III levels in 30 insulin-dependent diabetic children with no clinically evident vascular complications. 9 were in-patients and 21 were out-patients. The disease had been discovered within the previous 10 years. 25 control subjects of comparable age and body weight were studied simultaneously. Template bleeding time, threshold concentrations of ADP or adrenaline required to induce irreversible platelet aggregation and plasma antiherparin activity (platelet factor 4) did not differ significantly in control and patient groups. In contrast, the immunological levels of plasma antithrombine III were significantly higher in the diabetic group. These results suggest that diabetic children, with no clinical signs of microanigopathy, show no laboratory changes suggesting increased platelet function. The unxpected increase in the antithrombin III level could reflect a very early defense mechanism against activation of the blood clotting system.     

Platelet dose for prophylactic platelet transfusions

Although guidelines exist to deal with some aspects of platelet transfusion practice, many important clinical issues have not been addressed in large randomized controlled trials (RCTs). Slichter et al. conducted a RCT of prophylactic platelet transfusions to determine the effects of the dose of platelets on clinical signs of bleeding, the use of platelet and red cell transfusions, changes in the recipient's post-transfusion platelet count, days to next transfusion and adverse events (Effects of Prophylactic Platelet Dose on Transfusion Outcomes [PLADO] trial). The primary end point of the study (i.e., the percentage of patients in each group with at least one episode of bleeding of grade 2 or higher according to the WHO criteria) was not significantly different (71, 69 and 70% of patients in the low-, medium- and high-dose group, respectively). According to these data, one can conclude that the dose of platelets transfused has no significant effect on the incidence of bleeding in patients with hypoproliferative thrombocytopenia and platelet counts no greater than 10 × 10?/l.     

Platelet calmodulin correlates with platelet turnover

We measured the calmodulin content in platelets in 13 normal persons and in 62 patients with hematological diseases. The level of platelet calmodulin was higher in patients with idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus, myeloproliferative disorders, acute leukemia in a recovery phase, aplastic anemia, thrombosis and hypersplenism as compared to the controls. Among the patients with ITP, calmodulin was lower in responders than in nonresponders and those at the initial diagnosis. We also measured the volume, life-span and aggregation of the platelets and demonstrated a significant relationship between the calmodulin level and the platelet volume, and a negative relationship between the calmodulin level and platelet life-span, there was no correlation between the calmodulin level and platelet aggregation. We thus conclude that platelet calmodulin is inversely correlated with platelet turnover.     

Absence of platelet activating factor (PAF) mediated platelet aggregation: A new platelet defect

The failure of normal human platelets to aggregate in response to platelet activating factor (PAF) has not been previously observed. We report here the first case of a patient whose platelets did not aggregate to PAF on multiple occasions.     

Platelet receptor proteolysis: a mechanism for downregulating platelet reactivity

The platelet plasma membrane is literally at the cutting-edge of recent research into proteolytic regulation of the function and surface expression of platelet receptors, revealing new mechanisms for how the thrombotic propensity of platelets is controlled in health and disease. Extracellular proteolysis of receptors irreversibly inactivates receptor-mediated adhesion and signaling, as well as releasing soluble fragments into the plasma where they act as potential markers or modulators. Platelet-surface sheddases, particularly of the metalloproteinase-disintegrin (ADAM) family, can be regulated by many of the same mechanisms that control receptor function, such as calmodulin association or activation of signaling pathways. This provides layers of regulation (proteinase and receptor), and a higher order of control of cellular function. Activation of pathways leading to extracellular shedding is concomitant with activation of intracellular proteinases such as calpain, which may also irreversibly deactivate receptors. In this review, platelet receptor shedding will be discussed in terms of (1) the identity of proteinases involved in receptor proteolysis, (2) key platelet receptors regulated by proteolytic pathways, and (3) how shedding might be regulated in normal physiology or future therapeutics. In particular, a focus on proteolytic regulation of the platelet collagen receptor, glycoprotein (GP)VI, illustrates many of the key biochemical, cellular, and clinical implications of current research in this area.     

Platelet coagulation factor Va: the major secretory platelet phosphoprotein

Platelet-derived coagulation factor Va is the primary secreted substrate for a thrombin-stimulation-dependent platelet kinase. Human platelet factor Va, consisting of a molecular weight (M(r)) 105,000 heavy chain and an M(r) 74,000 light chain, incorporates phosphate in at least two sites on the light chain. Phosphorylated factor Va represents 50% of the secreted protein-associated phosphate. This modification occurs exclusively at serine residues and is inhibited by H-7 and staurosporine, which suggests a protein kinase C (PKC)-mediated event. Purified plasma factor V and Va are phosphorylated in the light chain region by rat brain PKC. The activity of platelet factor Va in prothrombinase on platelets is not altered when phosphorylation is inhibited by staurosporine. Plasma-derived factor Va in the presence of thrombin stimulated platelets is phosphorylated on both the heavy chain and the light chain. Plasma factor V and factor Va heavy chain phosphorylation occurs without light chain phosphorylation in the presence of added 32P gamma-ATP and non-stimulated or collagen- stimulated platelets or casein kinase II. This differential phosphorylation of factor Va heavy and light chain shows two independent platelet kinase activities that act on factor Va. The heavy chain factor V/Va kinase activity is similar to casein kinase II, which we have demonstrated previously to act on factor Va and accelerate activated protein C inactivation of the cofactor. Our data show platelet-dependent phosphorylation of platelet and plasma factor V and Va resulting in significant covalent modifications of the cofactor. These modifications may play a role in directing the extracellular distribution of factor V and factor Va.     

Megaloblastic anemia and platelet function--a qualitative platelet defect in pernicious anemia

In order to know the entities of platelet defect in pernicious anemia, we investigated platelet functions in ten cases with pernicious anemia. Three of the cases had total gastrectomy. Five cases showed thrombocytopenia and four cases revealed prolongation of Ivy bleeding time. Decreased adhesiveness was observed in three cases. Various abnormalities in platelet aggregation were observed. However, almost all of the cases showed remarkable improvement of decreased platelet functions after the therapy of Vitamin B12 injection. The results of adenine nucleotides in platelets and the release of them following collagen or epinephrine aggregation were analysed in comparison with normal platelets. The ADP was definitely decreased and the ATP/ADP ratio was increased. In addition, the release of ATP and ADP at collagen or epinephrine induced aggregation was markedly decreased, and after the therapy of Vitamin B12, the decrease of adenine nucleotide release remarkably increased. In summary, the acquired defects of platelet function in pernicious anemia are regarded as a secondary storage pool disease, and its defects improve after Vitamin B12 therapy.     

Platelet associated IgG, platelet survival, and platelet sequestration in thrombocytopenic states

S ummary . Platelet-associated IgG (PAIgG), platelet mean life span (MLS), and platelet sequestration sites were studied in 69 patients with immune (ITP) and presumed nonimmune thrombocytopenias (NTP). A shortened MLS was associated with elevated PAIgG (N=46), and with normal PAIgG (N=15), Four patients had a normal MLS, but elevated PAIgG, four patients were normal for both parameters. The highest PAIgG values occurred in ITP patients with a very short MLS. Nine NTP patients had also elevated PAIgG, but a normal or slightly shortened MLS. There was a significant double log correlation between PAIgG and MLS for ITP, but not for NTP patients. Judged from the coefficient of determination, only 10% of PAIgG were directly related to a shortened MLS.
70% of patients (N= 63) had exclusively splenic and 30% hepatosplenic sequestration. PAIgG was elevated in 29/44 patients with splenic (66%) and in 16/19 patients with hepatosplenic sequestration (84%). In ITP, PAIgG-positive cases were observed in 69% of splenic v 82% of hepatosplenic sequestration, while in NTP the corresponding figures were 6/11 and 2/2. No significant correlation between PAIgG and either sequestration type was demonstrable.
We conclude that in immunologically mediated thrombocytopenia only a small portion of PAIgG accounts for a decreased MLS, and that the concentration of PAIgG per se does not determine the platelet sequestration type.