A Novel Assay of Platelet Function reveals Altered Platelet Activity during Healthy Pregnancy.

It is well established that markers of platelet activation are elevated during normal pregnancy. For example, levels of platelet-derived thromboxane are persistently elevated in serum and urine of pregnant women, to levels equivalent to those observed in acute cardiovascular disease. In addition, other markers of platelet activation, such as elevated plasma levels of platelet derived P-Selectin and CD40Ligand, are elevated in pregnancy. However, the clinical significance of platelet activation during pregnancy is not understood. Furthermore, systematic studies of platelet function throughout pregnancy are not widely available.

In this study I aimed therefore to characterize changes in platelet function in women during normal healthy pregnancy. With our collaborators in the Coombe Women’s and Infants University Hospital in Dublin, 20 healthy pregnant women were recruited at their first-trimester hospital visit (T1; 9-14 weeks gestational age). They donated a 12 ml blood sample for analysis of platelet function and plasma thromboxane levels at this first visit and again in the second trimester (T2; 14-27 weeks), the third trimester (T3; 27-37 weeks) and within 8 weeks of delivery of their baby (post-partum; PP). Platelets were prepared from the whole blood samples and analysed as follows: (1) Light Transmission Platelet Aggregation, a gold standard test of platelet function. Platelet aggregation responses are measured in response to a thrombin-derived activator peptide (TRAP), a thromboxane mimic (U46619), a collagen related agonist (CRP), and Arachidonic acid (AA). (2) Platelet ATP/ADP secretion (PAS) assays were used to measure the sensitivity of platelets to secrete their granular contents in response to dose-ranges of agonists (TRAP, U46619, CRP and AA), in order to quantify relative changes in platelet sensitivity during and after pregnancy. Finally, the levels of thromboxane, a prothrombotic prostaglandin, were assessed in the plasma of the women, to gain an insight into levels of platelet activation that might be present in circulating platelets in the women.

Although there have been previous studies that have assessed various aspects of platelet activation and prostaglandin levels during pregnancy, no study has covered all aspects over the full range of gestation from T1 to PP. Thus, this is a novel study that established baseline levels of multiple parameters throughout pregnancy in healthy women. The results of this study will be critical for the design of a follow-on study that will attempt to determine if platelet function differs significantly in mothers at risk of pre-eclampsia or intrauterine growth retardation (IUGR) compared to mothers with normal healthy pregnancies.

The results of this study show that there is substantial evidence for modulation of platelet activation during normal healthy pregnancy. Platelet aggregation in response to standard doses of soluble platelet agonists (TRAP.U46619 and AA) are suppressed in T1 compared to other time-points. In contrast, the responses to CRP, a collagen-related peptide are increased. In PAS assays, differences in response between soluble agonists (TRAP, U46619 and AA) and the collagen-related peptide are also observed. Most strikingly, there is an increase in potency for the collagen related peptide that suggests that this mechanism of platelet activation acquires a new importance in pregnancy. In parallel, responsiveness to other soluble platelet agonists is down regulated. I hypothesize that this subtle regulation of platelet responsiveness during pregnancy reflects a differential regulation of platelet function that is required during a normal healthy pregnancy.