Platelets, Coagulation, and the Liver

Introduction

In his first edition, Chesley included a chapter entitled Disseminated Intravascular Coagulation and began by stating the widely held hypothesis at that time that this was a fundamental feature of preeclampsia–eclampsia. In his usual thorough fashion, he reviewed data that had accrued up to that time, and he concluded that there was evidence for slightly increased coagulation and fibrinolysis during normal pregnancy. He went on to say, however, that many women with severe preeclampsia and eclampsia show no detectable evidence of increased coagulation and fibrinolysis. He concluded that disseminated intravascular coagulation did not appear to be a fundamental feature of the disease.

Many of Chesley's predecessors and contemporaries who espoused activation of intravascular coagulation with preeclampsia syndrome drew their conclusions from autopsy findings that undoubtedly led to some of these erroneous findings. Although observed as early as 1924, it had been proven that platelet concentrations were decreased in some women with the preeclampsia syndrome—especially severe cases that included those with eclampsia. In the first large study, Pritchard et al. reported a mean platelet count of 206,000/μL in 91 consecutive women with eclampsia. In 25% of this cohort the platelet count was<150,000/μL, in 15% it was<100,000 μL, and in 3% it was<50,000/μL. From these and other studies, Chesley concluded that thrombocytopenia was a feature of the preeclampsia syndrome, but that it was not caused by consumptive coagulopathy.

Somewhat parallel to the coagulation story, it had been long known that severe preeclampsia and eclampsia were associated with gross and microscopical changes in the liver. The search for a serum analyte for hepatocellular necrosis yielded documentation of elevated serum glutamic oxaloacetic transaminase (SGOT) levels. In another first edition chapter entitled The Liver , Chesley summarized 11 studies of SGOT measurements, and he cited abnormal values in 84% of women with eclampsia, half of those with severe preeclampsia, and a fourth who had mild preeclampsia. And while hepatocellular damage is a known cause of coagulopathy, Chesley concluded that damage to the liver was generally not severe enough to cause significant liver dysfunction. But the link between thrombocytopenia and liver involvement characterized by elevated serum transaminase levels did evolve as a marker for the severity of preeclampsia. To call attention to this, Weinstein coined the term HELLP syndrome — H emolysis, E levated L iver enzymes, and L ow P latelets.

Thus coagulation, thrombocytopenia, and hepatic changes of the preeclampsia syndrome became accepted as interrelated. As with any review concerning preeclampsia, a major difficulty is the use of variable or imprecise criteria for its diagnosis, as discussed in Chapter 1. This caveat must be considered even when comparing studies cited in this chapter.

Platelets

Platelets are the smallest of the formed blood components, with a diameter of 1.5–3 μm and a volume approximating 7 fL. Megakaryocytes present in bone marrow are the precursors of platelets released into the circulation. Exhausted circulating platelets are sequestered by the liver and spleen, and in pregnancy, the placenta also sequesters platelets, where they undergo phagocytosis. Healthy adults produce 10 ¹¹ platelets per day, which are replenished at the rate of 10% per day and have a life span of 8–9 days.

Platelets are extremely complex morphologically and biochemically, with a huge range of functions. These anucleate discoid elements consist of a plasma membrane, cytoskeletal elements, and several organelles, some of which communicate with the surface via an open canalicular system. Numerous membrane receptors serve to discharge platelet functions, the primary one being their adaptation to adhere to damaged blood vessels, interact with one another, and stimulate thrombin generation. Through platelet adhesion, activation, aggregation, and their interactions with soluble factors in the coagulation cascade, platelets are vitally important to the formation of stable blood clots.

Critical to platelet function are platelet surface receptors that bind adhesive glycoproteins. These receptors include the GPIb/IX/V complex that binds von Willebrand factor (vWF), the integrin GPIIb/IIIa (αIIbβ3) receptor, which binds fibrinogen and vWF, and GPIa/IIa, which binds collagen. Other receptors bind additional matrix glycoproteins, while the P-selectin receptor mediates interactions with leukocytes to incite a proinflammatory response.

Ligand binding by platelet cell surface receptors may induce platelet activation, through “outside-in” signaling. An idea of the complexity of this process comes from consideration of the multiple agonists shown in Table 16.1 . Various stimuli, including collagen, thrombin, serotonin, epinephrine, thromboxane A ₂ , platelet-activating factor, and ADP, all stimulate platelet activation. Blood vessel injury or atherosclerotic plaque rupture causes endothelial disruption, exposing collagen to which platelets adhere. Platelet activation requires vWF and results in a morphological change from discoid to spherical, with numerous fine filopodia or pseudopodia and release of intracellular calcium. Most platelets that accumulate at a site of injury do not adhere directly to subendothelial surfaces, but aggregate with each other, mediated primarily through the effects of platelet GPIIbIIIa, which adopts an active conformation capable of binding fibrinogen as a consequence of platelet activation. Inherent to the platelet activation process is secretion of the contents of dense granules and alpha granules that contain a variety of bioactive substances, some of which are listed in Table 16.1 . ADP released and thromboxane A2 produced by activated platelets serve to promote further platelet activation and cause local vasoconstriction as a stable blood clot is formed.

Platelets in Normal Pregnancies and with Preeclampsia

A number of changes to platelets and their various functions occur in normal pregnancy. As discussed in Chapters 7 and 9 , pregnancy itself is a transient state of heightened inflammation and endothelial stress, where changes in platelet function serve to promote maternal coagulation around the time of delivery. Some of the pregnancy-associated changes in platelet numbers, morphology, and function are summarized in Table 16.2 .

During normal pregnancy, there is increased platelet turnover and more immature circulating platelets with higher mean platelet volumes (MPV). These immature platelets are prone to aggregate with less stimulation and contribute to a cycle of continued activation, aggregation, and exhaustion. Several large, population-based studies have demonstrated that in uncomplicated pregnancies, a reduction in platelet count with advancing gestation reaches a nadir of 10% reduction by term. In one study, the mean platelet count in 6770 pregnant women near term was 213,000/μL, compared with 248,000/μL in nonpregnant female controls. The 2.5th percentile in the pregnant group, used to define the lower limit of normal platelet concentrations, was 116,000/μL. The genesis of this net decrease is related to increased plasma volume and predisposition of immature platelets to activate. Additionally, splenic volume expands in pregnancy by as much as 35%, facilitating sequestration of circulating platelets. Reference ranges for platelet count and MPV for nonpregnant adults and in pregnancy, by trimester, are shown below in Table 16.3 .

In pregnancies affected by preeclampsia, the observed effects on platelet count and maturity of circulating platelets are often enhanced. ^, A recent meta-analysis of platelet function in preeclampsia demonstrated an overall increase in MPV, without a significant effect on platelet aggregation. The variation of frequency and intensity of thrombocytopenia between may be more indicative of analytical methods; nevertheless, up to half of women with preeclampsia develop thrombocytopenia and the extent is generally proportional to disease severity. Ironically, preservation of platelet count in preeclampsia may be ominous, indicating diminished placental function with decreased platelet sequestration. The pathogenesis of preeclampsia-associated thrombocytopenia is likely multifactorial. The elevated levels of thromboxane A ₂ metabolites in the urine of preeclamptic patients, as well as the increased plasma levels of the platelet α-granule proteins β-thromboglobulin and platelet factor 4 (PF4), support the argument that platelet activation contributes to accelerated platelet clearance.

Although only the most severe cases of preeclampsia are associated with a coagulation profile suggestive of disseminated intravascular coagulation (DIC), the plasma of most patients with preeclampsia contains increased levels of thrombin–antithrombin complexes, and approximately 40% of cases have increased levels of fibrin D-dimers. This suggests subclinical activation of the coagulation system at a minimum. Increased thrombin generation may be one mechanism that promotes platelet activation, but platelets also may be stimulated through contact with dysfunctional endothelium and/or exposed subendothelium of injured placental vasculature. Platelet adhesion may be promoted by reduced levels of ADAMTS-13 as well as elevated levels and larger, more active multimers of vWF and other adhesive proteins such as cellular fibronectin.

Platelet volume increases normally across pregnancy. As shown in Table 16.3 , preeclampsia is associated with a further increase in mean platelet volume, ^{, ,} reflecting increased platelet turnover and resulting in an increased proportion of young platelets in the circulation. Other factors include complex changes in the pattern of platelet production and release by megakaryocytes. Optimal methods to investigate platelet life span require radiolabeling, which is prohibited in pregnancy. Using the method of platelet malondialdehyde production, disparate findings have been reported. ^, Specifically, in their longitudinal study, Pekonen et al. did not find a significant change in platelet life span in preeclampsia. Conversely, Rakoczi et al. reported a significant decrease. The demonstration of a shorter platelet production time is consistent with a shorter platelet half-life. Importantly, the degree of thrombocytopenia is related to severity, and significant thrombocytopenia is usually associated with severe preeclampsia and eclampsia. These changes are direct, rather than immune-mediated, and found only in maternal platelets. Even in the presence of marked maternal thrombocytopenia, neither cord blood nor fetal platelet counts were affected.

Redman et al. demonstrated that the platelet count fell at an early stage in the evolution of preeclampsia. Despite this, the absolute platelet count is of limited predictive or prognostic value. Of clinical significance are the findings of Leduc et al., who showed that in the absence of thrombocytopenia, women with severe preeclampsia do not have significant clotting abnormalities. And Barron et al. showed that a combination of a normal platelet count plus a normal serum lactate dehydrogenase level had a negative-predictive value of 100% for clinically significant clotting abnormalities in women with preeclampsia. Thus, studies to assess coagulation, namely prothrombin and activated partial thromboplastin times and plasma fibrinogen concentration, should be reserved for women with platelet counts <100,000/μL.

HELLP Syndrome

HELLP syndrome defines a preeclampsia variant defined by Hemolysis, Elevated Liver transaminases, and Low platelets. Thrombocytopenia occurring in HELLP syndrome is generally more severe than with uncomplicated preeclampsia. It has been reported that the rate of fall of the platelet count is a predictor of the eventual severity of HELLP, with women whose counts decrease by > 50,000/μL per day having a higher probability of developing moderate to severe thrombocytopenia (an absolute platelet count <100,000/μL). There also appears to be a correlation between the extent of thrombocytopenia and the degree of liver dysfunction in women with HELLP syndrome. The platelet nadir is usually reached approximately 24 h postpartum, with normalization occurring over the next 6–11 days. As with preeclampsia, the thrombocytopenia of the HELLP syndrome reflects a multifactorial pathogenesis with platelet activation by contact with damaged endothelium, platelet consumption secondary to thrombin generation, and microangiopathic hemolysis.

Platelet Activation and Platelet Function

There is clear evidence that platelet activation is increased during pregnancy, and this is further increased in women with preeclampsia ( Table 16.2 ). Circulating levels of factors stored within platelet granules demonstrate that platelet activation has occurred. Plasma levels of β-thromboglobulin, a platelet α-granule protein, are increased in normal pregnancy, and higher plasma levels of β-thromboglobulin have been demonstrated in preeclamptic women compared with normal pregnant controls. ^, Janes and Goodall reported that these increased β-thromboglobulin concentrations were associated with degranulation, as evidenced by elevated levels of the lysosomal-granule membrane antigen, CD63. Socol et al. found that this measure of platelet α-granule release correlated with increasing levels of proteinuria and serum creatinine, suggesting a link between platelet activation and renal microvascular changes.

There is increased expression of other platelet membrane antigens that signify activation. Platelets taken from preeclamptic women express increased levels of CD40L and its circulating soluble component, sCD40L. There also are increased levels of the antigens CD41-and CD62P+ and their respective circulating microparticles along with platelet–monocyte aggregates. ^, This topic is discussed further in Chapter 8 by Vatish et al.

Elevation of β-thromboglobulin levels precedes the clinical development of preeclampsia by at least 4 weeks. In contrast with normal pregnancy, levels of β-thromboglobulin did not correlate with increased fibrinopeptide A—a marker of thrombin generation—reported in preeclampsia. These findings suggest that mechanisms other than thrombin-mediated platelet stimulation are responsible for platelet activation. An immune mechanism may be a contributory factor. Burrows et al. reported increased serum levels of platelet-associated immunoglobulin G, which correlated with disease severity. In a prospective study, Samuels et al. measured platelet-bound and circulating platelet-bindable immunoglobulin. They reported a higher frequency of abnormal platelet antiglobulin found in preeclamptic women compared with normotensive pregnant women. Alterations in platelet-bound immunoglobulins might be from the deposition of autoreactive antibodies or immune complexes caused by placental tissue antigens. Alternatively, platelet activation at sites of microvascular injury could lead to externalization of IgG and other proteins in platelet α-granules.

Serum levels of platelet factor 4 (PF4), another α-granule protein, are not significantly elevated in preeclampsia. Because PF4 is cleared by binding to the endothelium rather than by renal excretion, a contribution of impaired renal function to increased β-thromboglobulin levels cannot be excluded. Serotonin is also released when platelets aggregate, and Middelkoop et al. found decreased serotonin concentrations in platelet-poor plasma from preeclamptic women compared with levels in plasma from normal pregnant women. These results are consistent with platelet aggregation and consumption.

Janes and Goodall used flow cytometry to detect activated platelets in whole blood, identified by bound fibrinogen or by CD63 antigen expression. They reported that activated platelets were detected prior to the development of preeclampsia. These findings were confirmed by Konijnenberg et al., who also demonstrated enhanced expression of anti-P-selectin, another marker of α-granule secretion.

The most reliable method of assessing in vivo thromboxane production is by measurement of stable urinary metabolites of thromboxane A. Urinary excretion of 2,3-dinor-thromboxane B2 and 11-dehydro-thromboxane B2 is increased in normal pregnancy. These urinary metabolites are further increased in women with preeclampsia compared with normotensive pregnant women and may predate clinical signs. ^, These observations provide further evidence of increased platelet activation in preeclampsia. Thromboxane generation promotes activation of surrounding platelets.

Garzetti et al. studied platelet membranes and found increased fluidity and cholesterol concentration with preeclampsia. These changes are consistent with increased unsaturated fatty acid content. These latter compounds are both a substrate for lipid oxidation and participate in thromboxane formation. Increased thromboxane production may thus reflect altered platelet membranes in preeclampsia.

There may also be an activated-platelet interaction with the ADAMTS-13 metalloproteinase system that cleaves vWF. Platelets interact with factor VIII to promote normal ADAMTS-13 action; however, disruption of this impairs vWF proteolysis both in vivo and in vitro. The effects of this on the normally deficient ADAMTS-13 state for pregnancy and the genesis of preeclampsia are speculative at this time.

In summary, there is ample evidence for platelet activation in preeclampsia. There is reduced platelet concentration, increased size, reduced life span, increased α-granule release, enhanced expression of cell adhesion molecules, and increased thromboxane production. This increased activation, which occurs early in the disease, may either result from an extrinsic factor such as endothelial damage with platelet activation, or it might be intrinsic and antedate pregnancy, evidenced by platelet-binding-site alterations (also see Chapter 10 by Goulopoulou et al.).

While the evidence indicates increased platelet activation in vivo in normal pregnancy, and even more so in preeclamptic women, the data are in contrast to findings from numerous in vitro studies examining platelet activation and function. These studies demonstrate increased platelet activation in normal pregnancy compared with nonpregnant women; however, reduced activation is consistently reported in vitro with preeclampsia; the apparent discrepancy can be resolved by suggesting that platelet activation occurred in vivo, with granule release prior to in vitro studies. Burke et al. studied platelet aggregation across normal pregnancy in response to exposure to collagen and arachidonic acid in vitro. Platelet aggregation decreased somewhat in early pregnancy, but thereafter increased.

Horn et al. studied radiolabeled serotonin release in vitro in platelet-rich plasma from preeclamptic patients in response to arachidonic acid. This response was diminished compared with nonhypertensive control subjects. ^,

Platelets, like endothelial cells, contain a constitutive form of nitric oxide synthase. The activity of this enzyme has been found to be significantly lower in platelets from women with preeclampsia compared with those from normal pregnant women. At the same time, however, platelet nitric oxide and peroxynitrite levels are increased in preeclamptic women compared with normal controls. Although platelet nitric oxide synthesis may contribute to the vasodilatation of normal pregnancy, it is unclear whether lower activity in preeclampsia contributes to its pathogenesis.

Other in vitro studies have attempted to elucidate mechanisms responsible for increased in vivo platelet activation in preeclampsia. There is evidence that the inhibitory mechanisms that switch off platelet activation responses may be less effective in preeclampsia. Prostacyclin acts via cyclic adenosine monophosphate (cAMP) to inhibit platelet aggregation. Vascular endothelial cell production of prostacyclin is diminished in preeclampsia compared with normal pregnancy. The resulting tendency to vasoconstriction and platelet aggregation is accentuated by a reduced platelet sensitivity to prostacyclin. Horn et al. found that there was no alteration in sensitivity to prostacyclin—or other modifiers of cAMP such as thromboxane synthase inhibitors—when platelets from women with gestational hypertension were compared with platelets from normal pregnant women. In reports focusing on preeclampsia, however, this reduced susceptibility to inhibition by prostacyclin was more marked—up to 50%—in preeclampsia. ^, The finding of increased numbers of platelet thromboxane A receptors in women with preeclampsia is also consistent with increased in vivo platelet activation. Arachidonic acid, ADP, and epinephrine all have a thromboxane-dependent component in their mechanism of action. Finally, increased thromboxane receptor density would be expected to lead to increases in vivo reactivity.

Aspirin, an Antiplatelet Agent to Prevent Preeclampsia

The apparent imbalance in opposing prostaglandins in preeclampsia led to the hypothesis that low-dose aspirin might have a role as a preventative agent. Low-dose aspirin favors inhibition of thromboxane while exerting a very minor effect on prostacyclin production, resulting in a significant reduction of the plasma, serum, and placental thromboxane/PGI ₂ ratios. Aspirin is the oldest and most widely applied antiplatelet agent and continues to be the subject of over a 1000 clinical trials annually. Aspirin's antiplatelet effects were first demonstrated in 1971, leading to the award of the Nobel Prize. From this point onward, research to apply these effects for risk reduction, beginning in cardiovascular medicine, gained momentum.

Aspirin, acetylsalicylic acid, is a weak acid completely absorbed by passive diffusion in the upper gastrointestinal tract. Aspirin is quickly hydrolyzed by carboxylesterases (CEs) in the gastric mucosa, and later by CEs in plasma, erythrocytes, and the liver, which deacetylate and inactivate aspirin to salicylic acid. ^, Following oral dosing with immediate release preparations, peak plasma levels are achieved in 40 min with a half-life of only 15–20 min. Salicylic acid is further metabolized by the hepatic microsomal enzyme system and excreted renally; the dominant urinary metabolite being salicyluric acid. Elimination of low-dose aspirin is constant when related to plasma concentration and follows first-order pharmacokinetics. There is a paucity of pregnancy-specific pharmacokinetic information for aspirin, despite its widespread use. However, well-documented physiological changes in pregnancy including increased intestinal transit time, circulatory volume expansion, changes to plasma transport proteins, and glomerular filtration rate underpin the observations of decreased uptake rate and volume of distribution.

Aspirin's main pharmacodynamic target is the cyclooxygenase 1 enzyme (COX-1), a membrane-bound glycoprotein found on the endoplasmic reticulum of many cells, including platelets. COX-1 catalyses oxidation of arachidonic acid and, in so doing, regulates production of cyclic prostanoids, including thromboxane A2 (TXA ₂₎ , a vasoconstrictor and promoter of platelet aggregation, and prostacyclin, a vasodilator. Aspirin's inhibition of COX-1 takes place rapidly, prior to hepatic metabolism, within the portal system. Inhibition of platelet activity is evident within one h of oral dosing, before salicylic acid is detectable in peripheral blood. Platelets are anucleate and do not have the capacity to regenerate COX, meaning COX-1 activity is irreversibly inhibited for the life span of the platelet. Additionally, aspirin also acts on bone marrow megakaryocyte platelet precursors, so that with long-term therapy, new platelets enter the circulation already suppressed.

In 2007, a Cochrane systematic review and an individual patient data meta-analysis assessed the effectiveness of aspirin as a preventative treatment for preeclampsia for pregnant women at high risk. ^, The Cochrane review compared any antiplatelet agent, including aspirin, with placebo or no antiplatelet agent and included 59 RCTs with 37,560 women. Forty-three of the studies included focused on aspirin. The authors reported a significant reduction in the risk of preeclampsia, delivery before 37 weeks' gestation, and reduction in fetal and neonatal deaths by 17%, 8%, and 14%, respectively associated with low-dose aspirin use. The individual patient data meta-analysis reported a 10% reduction in preeclampsia and a 10% reduction in preterm birth associated with low-dose aspirin use, but was not powered to identify a subgroup deriving most benefit.

In 2017, the combined multimarker screening and randomized patient treatment with aspirin for evidence-based preeclampsia prevention (ASPRE) study, evaluated the efficacy of 150 mg aspirin. ASPRE was a double-blind placebo controlled RCT; participants were women at high risk of preeclampsia. The primary outcome, preterm preeclampsia, occurred in 1.6% of the aspirin group versus 4.3% of the placebo group (OR in the aspirin group 0.38; 95% CI 0.20–0.74; p = .004), and the findings were strongly supportive of enhanced risk reduction with an increase aspirin dose within the “low-dose” range.

When using aspirin as a preventative treatment, the overall aims are to identify the lowest effective dose that inhibits platelet COX-1, spares endothelial COX, and has a low risk of adverse effects. In pregnancy these aims have the added effect of redressing the imbalance in the TXA ₂ : prostacyclin ratio noted in women that go on to develop preeclampsia. Martin et al. demonstrated that in pregnancy, daily aspirin doses as low as 20 mg can improve the TXA ₂ : prostacyclin ratio. Strong inhibition of platelet TXA ₂ with no significant effect on prostacyclin is evident with doses of 50 and 60 mg. Maximal inhibition of platelet aggregation can be achieved with doses ranging from 40 to 160 mg daily.

Unfortunately, aspirin's inhibition of prostaglandin pathways, responsible for its antiplatelet effects, is also associated with adverse effects on the gastrointestinal and renal systems. Aspirin intolerance can affect up to 20% of individuals and hypersensitivity affects 0.6%–2.4%.

However, these effects are dose-related and in the low dose ranges used to prevent preeclampsia gastrointestinal and renal effects are minimal.

In pregnancy, both acetylsalicylic acid and salicylic acid cross the placenta. The ability of aspirin to prevent formation of placental microthrombi relies on this access to placental vasculature. However, there is a significant body of evidence and clinical experience indicating no teratogenic effects of low-dose aspirin and a reliable safety profile for mother and fetus. Reassuring individual patient data meta-analysis and a further systematic review including an analysis of harms found no increased risk of bleeding in aspirin-treated women. ^, Another systematic review reported no increase in the risk of adverse effects on pregnancy outcome with low-dose aspirin and a meta-analysis of 22 studies demonstrated no increase in congenital abnormalities in women who had used low-dose aspirin.

Aspirin Nonresponsiveness

A proportion of individuals demonstrate suboptimal response to aspirin, referred to as “aspirin nonresponsiveness” and “aspirin resistance.” States of accelerated platelet turnover, including pregnancy, have been postulated as potential causes of aspirin nonresponsiveness. The reported prevalence ranges from 5% to 65% influenced by the tests, cutoffs used, and population.

Biochemically, suboptimal response to aspirin equates to inadequate suppression of COX-dependent platelet activation during aspirin treatment, whereby an individual's platelets retain the ability to generate TXA ₂ . This can arise due to incomplete COX inhibition or by TXA ₂ produced by alternative pathways. Currently, there is no universally accepted definition of aspirin nonresponsiveness.

A systematic review and meta-analysis of 20 studies of aspirin nonresponsiveness in cardiology patients demonstrated a high burden of morbidity associated with aspirin nonresponsiveness, reporting higher incidences of second cardiac events and cardiac death with odds ratios of 3.85 (95% CI 3.08–4.80 ( p < .001) and 5.99 (95% CI 2.28–15.72 ( p < .003), respectively. Several observational studies explored the concept of aspirin nonresponsiveness in high-risk pregnancies, describing associations between aspirin nonresponsiveness, preeclampsia, and fetal growth restriction. However, in a prospective cohort of high-risk women who underwent longitudinal testing with multiple COX-specific platelet function tests and adherence testing for aspirin metabolites, no persistent aspirin nonresponsiveness was identified, and there were no significant associations between platelet response to aspirin and preeclampsia or fetal growth restriction. Thus, when evaluating platelet response to aspirin, it is essential to discriminate true aspirin nonresponsiveness from suboptimal adherence to aspirin.

Expression of Platelet Receptors in Pregnancy and Preeclampsia

As indicated above, the platelet surface hosts a plethora of glycoprotein receptors that mediate signals from the extracellular milieu to the platelet signaling machinery, often using specific G-protein-coupled receptors as intermediaries. Some receptors, such as GPIIbIIIa, the platelet fibrinogen receptor, undergo transition to an active conformation only upon platelet activation—“inside-out signaling”—enabling the binding of fibrinogen that bridges platelets during platelet activation. In addition to receptors that bind adhesive ligands, platelets display numerous receptors for platelet agonists, such as protease-activated receptors, particularly PAR-1, which bind thrombin, P2Y(12) receptors, which bind ADP, and receptors for PGI2, TXA2, and other mediators. The integrated function of these receptors regulates the platelet activation response, and antiplatelet therapies target many of these receptors. However, there are very few data available on the expression of these receptors in normal pregnancy or hypertensive pregnancy disorders. A study demonstrated that pregnancy-specific glycoproteins, secreted by syncytiotrophoblast, bind GPIIbIIIa and inhibit fibrinogen binding; however, the role of this interaction in regulation of platelet aggregation during pregnancy is uncertain. Polymorphisms in glycoprotein platelet receptors may regulate platelet responsiveness in vitro and potentially impact thrombosis risk, but their roles in pregnancy outcomes have not been thoroughly studied; a polymorphism in platelet GP6, which mediates collagen signaling, is associated with platelet hyperaggregability and an increased risk of fetal loss in a retrospective analysis, though an increased risk of pregnancy-associated hypertension was not reported. Likewise, a polymorphism of platelet GPIaIIa (integrin α2β1) was also retrospectively associated with early-onset fetal loss, but not hypertensive disease. ^,

Changes in the expression of P2Y(12), PGI2, or TXA2 receptors involved in platelet activation and signaling have not been reported in pregnancy or pregnancy-induced hypertensive disease and are a potentially fruitful area of study. ^, One report described decreased affinity of PGI2 receptors in preeclampsia, but this finding has not been reproduced.