Two Methods for Intraplatelet Serotonin Evaluation: Their Use in Thrombocytosis

The content of platelet dense bodies is considered to be decreased in thrombocytosis associated myeloproliferative disorders and normal in reactive thrombocytosis. Intraplatelet serotonin i.e., 5-hydroxytryptamine (5HT), content is usually considered a particularly good marker. We report our experience of platelet 5HT levels evaluated by radioimmunoassay (RIA) and fluorimetric assays in patients with high platelet counts due to essential thrombocythemia (48 patients) or reactive thrombocytosis (34 patients). The diagnosis of essential thrombocythemia was made according to Polycythemia Vera Study Group criteria. A significant statistical difference was observed in serotonin platelet content between essential thrombocythemia and secondary thrombocytosis patients both with fluorimetric and RIA assays. On the contrary, no difference was evident between reactive thrombocytosis patients and normal control subjects. However, a significant difference was observed between 5HT in serum and platelet pellets when evaluated by RIA. Statistical analysis with the X2test showed that the sensitivity of the fluorimetric assay is slightly higher than that of the RIA assay; however, specificity is about the same with the two methods. It is possible that the two assays measure different molecules. At any rate, the fluorimetric assay seems to be better in evaluating the platelet content of 5HT at least in patients with high platelet counts. Key Words: Thrombocytosis—Serotonin.

Dysfunction of the PI3 kinase/Rap1/integrin α(IIb)β(3) pathway underlies ex vivo platelet hypoactivity in essential thrombocythemia

Patients with myeloproliferative disorders (MPDs), such as essential thrombocythemia (ET) have increased risk of thrombosis and bleeding, which are major sources of morbidity and mortality. Most MPD patients have a gain of function mutation in Janus kinase 2 (JAK2V617F), but little is known how JAK2V617F affects platelet function. Here, we demonstrate that platelets from ET patients have impaired SFLLRN-mediated fibrinogen binding and have lost the potentiating effect of thrombopoietin (which couples to JAK2) on this pathway. In contrast, SFLLRN-mediated P-selectin expression, ATP secretion, phosphorylation of the PKC substrate pleckstrin, and Ca(2+) mobilization were unaffected in JAK2V617F positive platelets. In addition, thrombopoietin-mediated JAK2 phosphorylation was unchanged, suggesting that signaling pathways activated downstream of JAK2 are impaired. Indeed, we found that platelets from JAK2V617F positive ET patients have significantly reduced phosphorylation of the PI3 kinase substrate Akt, and have reduced activation of Rap1 in response to thrombopoietin, IGF-1,ADP, SFLLRN, and thrombin. This effect was independent of Giα P2Y12 purinergic receptor function as ADP-mediated inhibition of VASP phosphorylation was unchanged. These results demonstrate that the PI3 kinase/Rap1 pathway is intrinsically impaired in platelets from JAK2V617F-positive ET patients, resulting in diminished thrombin and thrombopoietin-mediated integrin α(IIb)β(3) activation.

Lactate production by thrombin-activated platelets of patients with primary thrombocythemia

INTRODUCTION

Platelet activation needs a high energy demand which is supplied by the degradation of glucose into lactate. Platelet response to agonists in patients with primary thrombocythemia is defective. We studied the production of lactate by the platelets of patients with this disease and defective platelet aggregation.

MATERIAL AND METHODS

Ten patients suffering from primary thrombocythemia and ten controls were included in this study. The lactate generation was measured in resting and thrombin activated platelets in absence or presence of glucose.

RESULTS

Resting platelets incubated for 30 min in phosphate-buffered saline (PBS) generated the same amount of lactate in patients (44.6+/-21.6 micromol/10(11) cells) and controls (41.0+/-17.3 micromol/10(11) cells). Addition of glucose led to similar increases in lactate formation by platelets in patients (82.2+/-26.4 micromol/10(11) cells) and controls (88.1+/-34.5 micromol/10(11) cells). The addition of thrombin in absence of glucose did not modify the lactate formation respective to PBS. Finally, the incubation of platelets with both glucose and thrombin caused further increases in the generation of lactate in both groups, patients (236.9+/-83.9 micromol/10(11) cells) and controls (228.6+/-63.5 micromol/10(11) cells) without differences between them. The production of lactate in both groups was also similar when platelets were incubated for 10 min or 20 min with both thrombin and glucose. However at 5 min, platelets of patients generated more lactate (97.8+/-23.7 micromol/10(11) cells) than controls (66.5+/-38.7 micromol/10(11) cells, p<0.05).

CONCLUSIONS

These results suggest that thrombin is able to induce an initial hyperactivity of those pathways involved in the platelet energy production of patients with primary thrombocythemia.

Bleeding time and platelet function in essential thrombocythemia and other myeloproliferative syndromes

Bleeding time (BT) and platelet function tests have been widely used in patients with essential thrombocythemia (ET), with the aim to support diagnosis and to identify laboratory predictors of haemorrhagic and thrombotic complications. BT is significantly prolonged in 7-19% of ET patients and several functional abnormalities have been observed in platelet structure, biochemistry and survival. However, the attempt to relate these in vivo and in vitro platelet dysfunctions with diagnosis or clinical sequelae has been generally disappointing. Therefore, BT and platelet function tests are currently not recommended in the initial evaluation or during the follow-up of patients with ET, unless in the setting of a clinical or biological study. A noteworthy exception is represented by a subset of patients characterized by very high platelet count (> 1500 x 10(9)/L) and bleeding symptoms, who can have an acquired von Willebrand disease. In these cases, prolonged BT and abnormal multimeric pattern of von Willebrand factor are useful for diagnosing and monitoring this acquired hemorrhagic disease. BT and platelet function tests should be included in the baseline evaluation of ET patients enrolled in prospective clinical trials aiming assess their predictive role on clinical end-points.

Disorders of platelet function

Qualitative platelet disorders are described and reviewed above. The acquired platelet function defects are very common, and sometimes result in hemorrhage, especially in association with trauma or surgery. However, the specific biochemical defect is absent, and no characterized platelet abnormalities have been recognized. On the other hand, the hereditary qualitative platelet defects are rare, but the platelet abnormalities are characteristic. The study of these patients had led to an increased understanding of the normal primary hemostatic mechanism. Recently, the molecular basis analysis of the platelet defects has been developed. This will help us understand the molecular events involved in platelet adhesion and aggregation.

Adhesion efficiency, platelet density and size

We have previously shown that adhesion of human platelets to immobilized collagen is extremely rapid, with initial rates approaching 3% of single particles adhering per 10 ms. Here, we have investigated adhesion efficiency to collagen as a function of platelet density. Platelet subpopulations: low-density (1.040 < d < 1.065 g/ml), intermediate-density (1.065 < d < 1.070 g/ml) and high-density (1.070 < d < 1.080 g/ml) were separated by Percoll density gradient centrifugation. They constituted 24%, 47% and 29% of the total platelet population and had mean volumes of 6.01, 7.37 and 8.21 fl, respectively. Using a continuous-flow, micro-affinity column, we found that the most dense (large) platelets exhibited initial rate of adhesion 4 times greater than the least dense (small) platelets. They were also less sensitive to inhibition by prostacyclin (PGI2). In contrast, there was no significant difference in aggregation induced by high doses of ADP and collagen, indicating that the most dense platelets were not preferentially involved in aggregation induced by high doses of agonists. These results suggest that normal circulating platelets can be distinctly heterogeneous in their ability to adhere to collagen under arterial-flow conditions. The greater efficiency of high-density platelets may be related to increased content of the glycoprotein Ia/IIa (GPIa/IIa) complex.

Platelets of patients affected with essential thrombocythemia are abnormal in plasma membrane and adenine nucleotide content

The adenine nucleotides present in distinct cellular compartments of platelets of 27 patients affected with essential thrombocythemia have been measured. In order to quantify granule-bound nucleotides and adenylic cytoplasmic pool, platelets have been stimulated with thrombin or treated with increasing digitonin concentrations, respectively. Among patients, we have identified two groups: 12 patients (Group 1) had normal platelet level of ATP and ADP both in dense granules as well as in cytoplasmic pool. The other patients (Group 2) had granule ATP and ADP significantly lower and ATP/ADP ratio significantly higher than controls. In these patients an increase in hypoxanthine level, derived from metabolic ATP degradation occurring during stimulation, was observed. In addition, in the latter patients an increased resistance of plasma membrane to digitonin was shown, suggesting that membrane fluidity should be reduced owing to a modified cholesterol/phospholipid ratio.

In vitro platelet adhesion to vascular subendothelium: studies in patients with polycythaemia vera

In this work we studied platelet adhesion to subendothelial surfaces in 10 patients with polycythaemia vera and 10 healthy volunteers at 40% Hct (corresponding to the mean value of our control group) and 55% Hct (a value roughly corresponding to the mean Hct in polycythaemic patients). Platelet concentration was kept constant at 2.0-2.5 X 10(11)/l. The results indicate that there was a statistically significant increase in adhesion both in controls and in patients with Hct varying from 40% to 55%. The contribution of the higher Hct in promoting platelet adhesion was comparable in the two groups. When red blood cells (RBC) from the patients were tested with platelets from healthy volunteers in cross-over experiments, they promoted adhesion in the same way as control RBC. Similarly, when patients' platelets were mixed with control RBC, adhesion was the same as control platelets. These data indicate that platelet and RBC contribution to this parameter are not significantly modified in this group of polycythaemic patients, provided that platelet and RBC values are adjusted to control range.

Essential thrombocythemias. Clinical evolutionary and biological data

This retrospective study reviewed 94 patients, aged 6 to 90 years, with thrombocythemia. The women/men ratio was 1.76. At onset or, less commonly, during the course of the disease, 45% of the patients presented hemorrhagic manifestations, 29% thrombotic complications, and only 14% the association of hemorrhage and thrombosis. The average platelet count was 1200 X 10(9)/1, with no difference according to sex or age. Platelet hypoaggregation was seen in practically all the cases (94%), although myelofibrosis was less frequent (54%). The frequency of hemorrhages increased when the platelet count was above 1000 X 10(9)/1 (P less than 0.01), but the occurrence of thrombosis was correlated neither with platelet count nor with thrombopathy. Survival time was lengthy: 80% of survival (standard error 6%) was observed at 100 months. Transformation to acute leukemia was observed in five patients. Because the disease course is most often prolonged, therapeutic measures must be conservative: anti-aggregating drugs in small doses, and chemotherapy beginning with nonalkylating agents.

Platelet 5-hydroxytryptamine transport and storage in myeloproliferative disorders

Platelets of patients with myeloproliferative disorders (MD) such as polycythaemia vera (PV), chronic myelogenous leukaemia (CML), idiopathic myelofibrosis (IM) and essential thrombocythaemia (ET) have been found to have low 5HT levels measured both by a fluorimetric and a liquid chromatographic assay. Km and Vmax for platelet active uptake of 3H-5HT were not significantly different in controls and patients. Inhibition of 5HT reuptake by imipramine or induction of moderate release by fenfluramine were not sufficient to distinguish the group of MD platelets from controls, although some patients had less of a tendency to retain intraplatelet amine. The low platelet 5HT content found in our patients seems not to be the consequence of disturbed active transport of 5HT across platelet membrane. Although defective storage of this amine within the cell is probable, the results of the present study do not rule out the possibility that platelets from MD patients undergo in vivo activation by endogenous stimuli not inhibited by aspirin. 10 d treatment with aspirin did not result in any significant rise in intraplatelet 5HT concentration.

[Temperature-dependent spontaneous platelet aggregation in polycythemia vera and primary thrombocythemia: a diagnostic criterion]

In twenty patients suffering from myeloproliferative syndromes (nine with polycythaemia vera, 11 with primary thrombocythaemia) platelet aggregation was tested following incubation of blood samples at 4 degrees C, room temperature, 30 degrees C and 37 degrees C. A spontaneous platelet aggregation following incubation at room temperature took place in 65% of patients with primary thrombocythaemia. At the two higher temperatures, positive results were seen in 72% and 92% of cases, respectively. Seven of nine patients (78%) showed a pathological spontaneous platelet aggregation at room temperature. Pathological spontaneous platelet aggregation at 30 degrees C occurred in eight of nine (89%) cases. The spontaneous platelet aggregation test is a simple method to diagnose primary thrombocythaemia and asymptomatic polycythaemia vera, the sensitivity of which increases considerably following incubation of blood samples at two and three different temperatures.

Platelet density analysis: a tool for the detection of acquired storage pool disease in man

This study was designed to evaluate the usefulness of platelet density analysis in the detection of acquired storage pool defects in human patients. Two groups of patients were investigated: 19 subjects affected with a myeloproliferative disorder (group I) where abnormal platelets are released from the megakaryocytes and 11 patients hospitalized in an intensive care unit (Group II) where normal platelets are injured in the circulation. Platelet density distribution after isopycnic centrifugation on a discontinuous stractan density gradient, dense granule markers (serotonin, ATP and ADP) and alpha granule markers (intraplatelet beta-thromboglobulin and platelet factor 4) were simultaneously determined. An increased proportion of the percentage of light platelets was observed in 16 patients of group I and nine of group II; an increased ATP/ADP ratio was observed in 12 patients of group I and 10 of group II. Both the tests were abnormal in 11 patients of group I and nine of group II. In group I, the level of serotonin was low and was related to the percentage of light platelets. The alpha granule specific proteins were normal in the two groups. These results indicate that platelet density analysis may serve as a screening test to detect exhausted platelets in human diseases.

Serotonin uptake and storage in human platelet density subpopulations

Serotonin uptake and storage were studied in human platelet density subpopulations which were isolated after isopycnic centrifugation on a discontinuous iso-osmolar stractan gradient. Kinetic parameters of the serotonin uptake were calculated (Km, Vi max) and the granular storage capacity was determined by comparing the total amount taken up in the presence or absence of reserpine, a specific inhibitor of the uptake at the granular level. Mean platelet volume and the number of mepacrine-labelled dense bodies were also determined. The results show that the active metabolic process of serotonin uptake is identical whatever the platelet subpopulation but the storage capacity is greater in the densest fraction which contains more dense bodies than the lightest one. Thus active serotonin uptake appears as another well-defined platelet metabolic process which is independent of platelet density; this argues against the conception that light platelets could be old platelets with reduced functional capacities.