Differences in thromboxane production between neonatal and adult platelets in response to arachidonic acid and epinephrine

50 Years of Pediatric Hemostasis: Knowledge, Diagnosis, and Treatment

Studies from the past 50 years have contributed to the expanding knowledge regarding developmental hemostasis. This is a dynamic process that begins in the fetal phase and is characterized by physiological variations in platelet counts and function, and concentrations of most coagulation factors and the native coagulation inhibitors in early life, as compared with adulthood. The developmental hemostasis studies since the 1980 to 1990s established the laboratory reference values for coagulation factors. It was only a decade or two later, that thromboelastography (TEG) or (rotational thromboelastometry [ROTEM]) as well as thrombin generation studies, provided special pediatric reference values along with the ability to evaluate clot formation and lysis. In addition, global whole blood-based clotting assays provided point of care guidance for proper transfusion support to children hospitalized in intensive care units or undergoing surgery. Although uncommon, thrombosis in children and neonates is gaining increasing recognition, typically as a secondary complication in sick children. Bleeding in children, and particularly intracerebral hemorrhage in newborns, still represent a therapeutic challenge. Notably, our review will outline the advancements in understanding developmental hemostasis and its manifestations, with respect to the pathophysiology of thrombosis and bleeding complications in young children. The changes of transfusion policy and approach to thrombophilia testing during the last decade will be mentioned. Subsequently, a brief summary of the data on anticoagulant treatments in pediatric patients will be presented. Finally, we will point out the 10 most cited articles in the field of pediatric and neonatal hemostasis.

Ontogeny of platelet function

Although the hemostatic potential of adult platelets has been investigated extensively, regulation of platelet function during fetal life is less clear. Recent studies have provided increasing evidence for a developmental control of platelet function during fetal ontogeny. Fetal platelets feature distinct differences in reactive properties compared with adults. These differences very likely reflect a modified hemostatic and homeostatic environment in which platelet hyporeactivity contributes to prevent pathological clot formation on the one hand but still ensures sufficient hemostasis on the other hand. In this review, recent findings on the ontogeny of platelet function and reactivity are summarized, and implications for clinical practice are critically discussed. This includes current platelet-transfusion practice and its potential risk in premature infants and neonates.

Neonatal platelets: mediators of primary hemostasis in the developing hemostatic system

The human hemostatic system is developmentally regulated, resulting in qualitative and quantitative differences in the mediators of primary and secondary hemostasis as well as fibrinolysis in neonates and infants. Although gestational age-related differences in coagulation factor levels occur, the existence of a unique neonatal platelet phenotype remains controversial. Complicated by difficulties in obtaining adequate neonatal blood volumes with which to perform functional assays, ambiguity surrounds the characterization of neonatal platelets. Thus, much of the current knowledge of neonatal platelet function has been based on studies from cord blood samples. Studies suggest that cord blood-derived platelets, as a surrogate for neonatal platelets, are hypofunctional when compared with adult platelets. This relative platelet dysfunction, combined with a propensity toward thrombocytopenia in the neonatal intensive care unit population, creates a clinical conundrum regarding the appropriate administration of platelet transfusions. This review provides an appraisal of the distinct functional phenotype of neonatal platelets. Neonatal platelet transfusion practices and the impact of the relatively hypofunctional neonatal platelet on those practices will be considered.

Developmental haemostasis: secondary haemostasis

The haemostatic system is a complex interaction between the vasculature, cellular components and plasma proteins that interact to maintain haemostasis in the healthy body. The haemostatic system can be further defined as primary, secondary and tertiary haemostasis to better define the interdependent mechanisms that combine to maintain haemostasis. The term 'developmental haemostasis' was first introduced by Maureen Andrews in the 1980s to describe the age-related physiological changes of the coagulation system as it develops progressively over time from fetal, neonatal, paediatric to adult and geriatric systems. This paper will focus on developmental changes in secondary haemostasis, that is, the plasma protein changes that occur with age, particularly during the fetal and neonatal period, when the changes are most marked compared to the adult system.

Differences in adhesion receptor expression between immature and older platelets and red blood cells of neonates and adults

PURPOSE

To examine the hypothesis that reticulated platelets and reticulocytes show elevated adhesion receptor expression compared with mature cells in both adult and neonatal cells.

METHODS

Flow cytometry was used to examine laminin, fibronectin (VLA-6), and thrombospondin (glycoprotein IV [GPIV]) expression in reticulated red cells, reticulated platelets, and older peripherally circulating mature red cells and mature platelets in seven newborn cord blood samples and blood samples from eight adult volunteers.

RESULTS

The difference in the neonatal reticulated platelet percentage of 9.2+/-14.8% was not statistically significant from the adult reticulated platelet percentage of 5.0+/-1.5% in this small population. There was a statistically significant difference between the reticulated cord blood red cell mean of 7.7+/-1.8% and the adult mean of 3.1+/-0.43%. Mean expression of VLA-6 was 96% in adult reticulated platelets, 79% in adult mature platelets, 81% in cord reticulated platelets and 65% in cord mature platelets. Mean expression of GPIV was similar, with corresponding values of 90%, 71%, 78%, and 57%. Reticulated red cells in adults averaged 44% VLA-4 and 46% GPIV; cord reticulocytes were 9% and 15%, respectively.

CONCLUSIONS

Reticulated cells newly released from the bone marrow express more adhesive receptors than mature cells in both groups. Cord blood samples showed hypoexpression of both receptor types in red blood cells and platelets.

Performing and interpreting the bleeding time in the neonatal intensive care unit

In summary, the bleeding time is a helpful clinical tool to detect and investigate certain hemostatic defects in neonates and to evaluate the adequacy of treatments. A prolonged bleeding time alone is sometimes not sufficient to diagnose specific conditions requiring further investigations. Platelet hyporeactivity in the first days of life, gestational age, platelet dysfunction secondary to various neonatal or maternal pathologic conditions, neonatal or maternal drug administration, and hematocrit must be considered for the correct interpretation of the bleeding time test in neonates. In addition, reliable test results can be ensured only by the scrupulous execution of the procedure.

Imparied platelet--dense granule release in neonates

PURPOSE

The aim of this study is to examine differences in platelet dense granule (PDG) uptake and release between preterm infants, term infants, and adults.

METHODS

PDG uptake and release was examined by flow cytometry using mepacrine fluorescent staining in phycoerythrin-anti-GPIIb/IIIa bound platelets taken from cord blood of eight term infants and seven preterm infants and venous blood from eight adults.

RESULTS

Analysis of the baseline fluorescence in the untreated versus thrombin-treated samples revealed significant differences in the way infant PDGs responded to thrombin stimulation when compared with adults. Initial uptake of mepacrine in both term and preterm platelets was similar to that in adult platelets. Statistically significant differences between adults and both term and preterm infants, at two concentrations of mepacrine, were observed after stimulation with thrombin.

CONCLUSION

Persistent mepacrine staining of infant PDGs, when compared with those of adults, after thrombin stimulation implies defective infant PDG release. This may partially explain why infants have impaired response to agonists requiring ATP release from PDGs.

Impaired signal transduction in neonatal platelets

Previous in vitro studies of cord blood platelets from full-term and preterm neonates have demonstrated decreased responses to most physiologic agonists. This hyporesponsiveness is, in part, related to both deficient synthesis of, and response to, an important mediator of platelet function, thromboxane A2(TxA2). The poor response of neonatal platelets to TxA2 is not due to differences in TxA2 receptor binding characteristics, when compared with platelets from adult controls. Therefore, the postreceptor signal transduction pathway was investigated. The TxA2 receptor is linked via the trimeric GTP-binding protein, Gq, to phospholipase C-beta (PLC beta), and stimulation of platelets with the stable TxA2 mimetic, U46619, leads to activation of PLC beta and subsequent intracellular signaling events. U46619-induced 32P-phosphatidic acid formation, an index of PLC beta activation, was decreased in platelets of neonates (166 +/- 10%) when compared with adult controls (206 +/- 22%) (p < 0.05). Mobilization of intracellular calcium was impaired in platelets of newborns (175 +/- 49 nM) in comparison to adult controls (506 +/- 130 nM) (p < 0.01), after stimulation with U46619. U46619-stimulated GTPase activity was blunted in platelet membrane fractions from full-term neonates and almost absent in platelet membranes from preterm infants. Immunoblotting studies of the platelet membrane fractions, quantified by densitometric analysis, showed that levels of the G alpha q subunit were not significantly different between adult and neonate, and were not the cause of the marked differences in GTPase activity. These data suggest that signal transduction through the TxA2 receptor is affected by decreased activity of Gq in platelets of neonates, and that this defect in signal transduction through PLC beta contributes to the observed poor response of newborns' platelets to TxA2 and consequently to TxA2-dependent agonists such as collagen.

Number and activation of circulating polymorphonuclear leukocytes and platelets are associated with neonatal respiratory distress syndrome severity

OBJECTIVE

To determine whether number and activation of circulating polymorphonuclear leukocytes (PMNs) and platelets are associated with disease severity in neonatal respiratory distress syndrome (RDS).

DESIGN

Prospective study.

SETTING

Tertiary neonatal intensive care unit.

PATIENTS

Preterm infants with severe (n = 18) or mild to moderate (n = 18) RDS who were consecutively admitted.

INTERVENTIONS

PMN and platelet counts and plasma concentrations of elastase-alpha1-proteinase inhibitor (E-alpha1-PI) and thromboxane B2 (TxB2) were recorded each day during the first 5 days of life. E-alpha1-PI-to-PMN and TxB2-to-platelet ratios were calculated to correct for the influence of the PMN and platelet count on elastase and thromboxane release.

RESULTS

From day 2, the severe RDS group had lower median PMN counts (1.5 vs 4.5 x 10/L), lower mean platelet counts (136 vs 230 x 10/L), and more elastase and thromboxane release, indicated by higher median E-alpha1-PI-to-PMN (39.2 vs 13.0 ng/10 PMNs on day 2) and TxB2-to-platelet (2.61 vs 0.52 pg/10 platelets on day 3) ratios than the mild-to-moderate group. Lower PMN and platelet counts and higher elastase and thromboxane release were correlated with birth asphyxia (lower 5-minute Apgar scores and umbilical arterial PH values), higher respiratory requirements (fraction of inspired oxygen and peak inspiratory pressure), and decreased values for continuous measures of RDS severity (ventilatory efficiency index and PaO2-to-alveolar oxygen tension ratio).

CONCLUSION

Decreased PMN and platelet counts and increased elastase and thromboxane release are correlated with increased RDS severity. Birth asphyxia (hypoxia and acidosis) and tissue injury caused by high-pressure ventilation and hyperoxia may promote this activation process.

Deficient thromboxane synthesis and response in platelets from premature infants

In vitro function of cord blood platelets from 35 premature infants (gestational age 32 +/- 3.2 wk) was compared with that of 12 full-term infants and 14 adult control subjects. In comparison with adult platelets, preterm platelets showed impaired aggregation, in response to thrombin, collagen, ADP, and U46619 [a stable analog of thromboxane A2 (TxA2)], and impaired [14C]serotonin secretion in response to collagen and U46619. The production of TxB2 (the stable TxA2 metabolite) in response to collagen was reduced in preterm platelets (30.2 +/- 5.5 ng/mL) compared with full-term (52.7 +/- 12.6 ng/mL) or adult control platelets (132.3 +/- 38.7 ng/mL). The deficient TxB2 production and U46619 response prompted further investigation of TxA2 receptor number and binding characteristics. Immunoblotting using an anti-TxA2 receptor antibody (anti-P2) identified a single, identical 55-kD band in solubilized membranes of control, full-term, and preterm platelets. Flow cytometry using anti-P2 produced histograms that did not differ between adults and neonates. Ligand binding studies using [3H]U46619 were carried out on 10 samples from each group. Scatchard analysis yielded a single class of binding sites with no significant difference among the Kd values (85 +/- 16 versus 99 +/- 12 versus 100 +/- 12 nM) or number of binding sites per platelet (1876 +/- 460 versus 2450 +/- 478 versus 2777 +/- 536) for preterm and full-term infants and adults. Therefore platelets of preterm infants show impaired TxA2 production and response. The poor response is not related to altered binding characteristics of the TxA2 receptor but may lie in the postreceptor signal transduction pathway.

Platelet disorders in newborn infants: diagnosis and management

Platelets are small, disc-shaped, anucleated cells formed by fragmentation of megakaryocytes in the bone marrow. They circulate in blood with a lifespan of 7 to 10 days and, together with fibrin, form hemostatic plugs at sites of vessel injury. Abnormalities of platelets, either quantitative or qualitative, may cause clinically significant bleeding with resultant morbidity and, on occasion, mortality. This review will focus on platelet disorders in neonates, defined as infants of up to 4 months of age. Special emphasis will be given to the physiology of platelet function in healthy and sick newborn infants. The review will be divided into sections as follows: role of platelets in hemostasis, platelet function in newborn infants, quantitative platelet disorders, qualitative platelet disorders, and platelet transfusion therapy.

Impaired mobilization of intracellular calcium in neonatal platelets

Abnormalities in platelet functions including aggregation and the release reaction have long been recognized to be present in neonatal platelets. Because calcium is an important mediator of many platelet functions, we have investigated the mobilization of calcium in neonatal platelets. All umbilical cord blood samples were obtained from healthy, full term gestations. Changes in cytoplasmic calcium levels were monitored using Fura-2 as a fluorescent probe. Fura-2-loaded washed platelets were stimulated with the agonists collagen (2 micrograms/mL) or thrombin (1.0 U/mL). When compared with adult controls, intracellular calcium release in the platelets of the neonate was significantly impaired in response to these agonists. Mean levels for calcium release in adults versus neonates in response to collagen were 168 +/- 120 nM (+/-SD, n = 10), and 61 +/- 69 nM (n = 7, p < 0.05). A decrease in response to thrombin was also observed [1296 +/- 503 nM (n = 8) in adults versus 603 +/- 482 nM (n = 7) in neonates, p < 0.025]. Results similar to those observed with unpaired neonatal and adult platelets were also obtained when neonatal platelets (n = 5) were compared with their paired maternal controls. In further studies, we have documented that the calcium content of the dense tubular system was normal in the neonatal platelet, indicating that the observed impairment in calcium mobilization in the neonate was not due to a decrease in calcium stores. The previously documented abnormalities in neonatal platelet function appear to be due to the impaired mobilization of this important intracellular mediator.

Neonatal and maternal platelet cytosolic calcium in normotensive and hypertensive pregnancies

A prospective study investigated platelet cytosolic calcium in non-pregnant volunteers (n = 30) and samples from the umbilical veins of babies from both normotensive (n = 18) and hypertensive (n = 15) primigravidae, and their mothers. There was no significant difference between the neonatal umbilical venous platelet cytosolic calcium concentration (p[Ca2+]i) in babies born to normotensive primigravidae or to those whose pregnancies were complicated by gestational hypertension (88 x 9 (SE) 2 x 5) in normotensive primagravidae, 80 x 6 (2 x 8) in pregnancy induced hypertension without proteinuria, and 89 x 3 (3 x 2) nmol/l in pre-eclampsia. There was also no significant difference in the p[Ca2+]i from the umbilical veins of the pregnancies studied and those of non-pregnant female volunteers in the follicular phase of their menstrual cycle. This was despite a gradual and significant rise in p[Ca2+]i with increasing severity of disease in the mothers of the babies studied (119 x 9 (4 x 1) in normotensive primagravidae, 130 x 8 (7 x 3) in pregnancy induced hypertension without proteinuria, and 148 x 2 (4 x 5 ) nmol/l in pre-eclampsia). The mean maternal p[Ca2+]i in the three samples returned to concentrations comparable with those in non-pregnant subjects by 12 weeks after birth. These data demonstrate no significant difference between the mean p[Ca2+]i in non-pregnant women and those obtained from the umbilical venous blood of normotensive or hypertensive primigravidae. They suggest that the functional hypoactivity of neonatal platelets is probably not secondary to a decrease in basal p[Ca2+]i. They also suggest that the progressively raised p[Ca2+]i in normal and hypertensive pregnancies might be due to a pregnancy specific factor that does not cross the placenta,