Mechanism of the inhibition of platelet aggregation produced by prostaglandin F2 alpha

Desmopressin, Misoprostol, nor Carboprost Affect Platelet Aggregability Following Traumatic Brain Injury and Aspirin

INTRODUCTION

Desmopressin (DDAVP) has been utilized clinically in patients taking aspirin (ASA) to improve drug-induced platelet dysfunction. Misoprostol and carboprost, prostaglandin analogs commonly used for postpartum hemorrhage, may also induce platelet aggregation. The aim of this study was to determine the effects of DDAVP, misoprostol, and carboprost administration on platelet aggregability following traumatic brain injury (TBI) in mice treated with ASA.

METHODS

Male C57BL/6 mice were randomized into seven groups (n = 5 each): untouched, ASA only, Saline/TBI, ASA/TBI, ASA/TBI/DDAVP 0.4 μg/kg, ASA/TBI/misoprostol 1 mg/kg, and ASA/TBI/carboprost 100 μg/kg. TBI was induced via a weight drop model 4-h after ASA (50 mg/kg) gavage. Mice were given an intraperitoneal injection of DDAVP, misoprostol, or carboprost 10 minutes after TBI. In vivo testing was completed utilizing tail vein bleed. Mice were sacrificed 30-min posttreatment and blood was collected via cardiac puncture. Whole blood was analyzed via Multiplate impedance aggregometry, rotational thromboelastometry, and TEG6s.

RESULTS

Mice receiving misoprostol after ASA/TBI demonstrated decreased tail vein bleeding times compared to ASA only treated mice. However, mice treated with misoprostol following ASA and TBI demonstrated decreased platelet aggregability compared to untouched mice and TBI only mice within the arachidonic acid agonist pathway. By contrast, DDAVP and carboprost did not significantly change platelet aggregability via adenosine diphosphate or arachidonic acid following ASA and TBI. However, DDAVP did decrease the platelet contribution to clot via rotational thromboelastometry.

CONCLUSIONS

Reversal of medication-induced platelet inhibition has become increasingly controversial after TBI. Based on these results, DDAVP, misoprostol, nor carboprost consistently improve platelet aggregability following TBI in those also treated with ASA.

Effect of Prostanoids on Human Platelet Function: An Overview

Prostanoids are bioactive lipid mediators and take part in many physiological and pathophysiological processes in practically every organ, tissue and cell, including the vascular, renal, gastrointestinal and reproductive systems. In this review, we focus on their influence on platelets, which are key elements in thrombosis and hemostasis. The function of platelets is influenced by mediators in the blood and the vascular wall. Activated platelets aggregate and release bioactive substances, thereby activating further neighbored platelets, which finally can lead to the formation of thrombi. Prostanoids regulate the function of blood platelets by both activating or inhibiting and so are involved in hemostasis. Each prostanoid has a unique activity profile and, thus, a specific profile of action. This article reviews the effects of the following prostanoids: prostaglandin-D₂ (PGD₂), prostaglandin-E₁, -E₂ and E₃ (PGE₁, PGE₂, PGE₃), prostaglandin F_2α (PGF_2α), prostacyclin (PGI₂) and thromboxane-A₂ (TXA₂) on platelet activation and aggregation via their respective receptors.

Eicosanoids in platelets and the effect of their modulation by aspirin in the cardiovascular system (and beyond)

Platelets are important players in thrombosis and haemostasis with their function being modulated by mediators in the blood and the vascular wall. Among these, eicosanoids can both stimulate and inhibit platelet reactivity. Platelet Cyclooxygenase (COX)-1-generated Thromboxane (TX)A₂ is the primary prostanoid that stimulates platelet aggregation; its action is counter-balanced by prostacyclin, a product of vascular COX. Prostaglandin (PG)D₂ , PGE₂ and 12-hydroxyeicosatraenoic acid (HETE), or 15-HETE, are other prostanoid modulators of platelet activity, but some also play a role in carcinogenesis. Aspirin permanently inhibits platelet COX-1, underlying its anti-thrombotic and anti-cancer action. While the use of aspirin as an anti-cancer drug is increasingly encouraged, its continued use in addition to P₂ Y₁₂ receptor antagonists for the treatment of cardiovascular diseases is currently debated. Aspirin not only suppresses TXA₂ but also prevents the synthesis of both known and unknown antiplatelet eicosanoid pathways, potentially lessening the efficacy of dual antiplatelet therapies. LINKED ARTICLES: This article is part of a themed section on Eicosanoids 35 years from the 1982 Nobel: where are we now? To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.8/issuetoc.

Trypanosoma cruzi Produces the Specialized Proresolving Mediators Resolvin D1, Resolvin D5, and Resolvin E2

Trypanosoma cruzi is a protozoan parasite that causes Chagas disease (CD). CD is a persistent, lifelong infection affecting many organs, most notably the heart, where it may result in acute myocarditis and chronic cardiomyopathy. The pathological features include myocardial inflammation and fibrosis. In the Brazil strain-infected CD-1 mouse, which recapitulates many of the features of human infection, we found increased plasma levels of resolvin D1 (RvD1), a specialized proresolving mediator of inflammation, during both the acute and chronic phases of infection (>100 days postinfection) as determined by enzyme-linked immunosorbent assay (ELISA). Additionally, ELISA on lysates of trypomastigotes of both strains Tulahuen and Brazil revealed elevated levels of RvD1 compared with lysates of cultured epimastigotes of T. cruzi, tachyzoites of Toxoplasma gondii, trypomastigotes of Trypanosoma brucei, cultured L₆E₉ myoblasts, and culture medium containing no cells. Lysates of T. cruzi-infected myoblasts also displayed increased levels of RvD1. Lipid mediator metabolomics confirmed that the trypomastigotes of T. cruzi produced RvD1, RvD5, and RvE2, which have been demonstrated to modulate the host response to bacterial infections. Plasma RvD1 levels may be both host and parasite derived. Since T. cruzi synthesizes specialized proresolving mediators of inflammation, as well as proinflammatory eicosanoids, such as thromboxane A₂, one may speculate that by using these lipid mediators to modulate its microenvironment, the parasite is able to survive.

Lung Ischemia Reperfusion Injury

Lung ischemia reperfusion injury (LIRI) is a pathologic process occurring when oxygen supply to the lung has been compromised followed by a period of reperfusion. The disruption of oxygen supply can occur either via limited blood flow or decreased ventilation termed anoxic ischemia and ventilated ischemia, respectively. When reperfusion occurs, blood flow and oxygen are reintroduced to the ischemic lung parenchyma, facilitating a toxic environment through the creation of reactive oxygen species, activation of the immune and coagulation systems, endothelial dysfunction, and apoptotic cell death. This review will focus on the mechanisms of LIRI, the current supportive treatments used, and the many therapies currently under research for prevention and treatment of LIRI.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

Platelet activation

This review article describes the different receptors, second-messengers and mechanisms involved in platelet activation. Several platelet agonists have well-defined receptors at the platelet membrane of which a number are single polypeptides with 7 hydrophobic transmembrane domains. These receptors are connected, via GTP regulatory proteins, with cytoplasmic second-messenger-generating enzymes. Phospholipase C and adenylate cyclase are the two best-known enzymes, generating inositol triphosphate (IP3) and diacyl glycerol from phosphatidylinositol biphosphate and cyclic AMP from ATP respectively. The intraplatelet free calcium level, which is critical for the activation status of the platelet, is increased by IP3 and is lowered in the presence of rising cyclic AMP concentrations. Shape-change occurs with small increases in intraplatelet calcium, while aggregation and secretion of granules take place at higher calcium, levels. The role of myosin and actin filaments and of transmembrane glycoproteins is further discussed.

Functional and ligand binding studies suggest heterogeneity of platelet prostacyclin receptors

1. This study describes attempts to compare prostacyclin (IP-) receptors in human, pig, horse, rabbit and rat platelets and in circular muscle of human, rabbit and dog mesenteric and pig gastroepiploic arteries. Three stable prostacyclin analogues, iloprost, cicaprost and 6a-carba-prostacyclin (6a-carba-PGI2) and a prostaglandin endoperoxide analogue EP 157 (previously shown to mimic prostacyclin on human platelets) were used. 2. Our main conclusion is that prostacyclin receptors on human, pig and horse platelets are similar in nature, but distinct from those on rabbit and rat platelets. Functional studies (inhibition of aggregation) showed that iloprost and cicaprost always had similar potencies whereas 6a-carba PGI2 was much more potent than EP 157 on rabbit and rat platelets (300 and 1000 fold on a molar basis) compared with human, pig and horse platelets (2, 7 and 7 fold respectively). Measurement of initial rates of cyclic AMP production confirmed these orders of potency. 3. Although pig platelets were quite sensitive to inhibition by EP 157 (threshold = 10 nM in some experiments), maximal inhibition of aggregation was not always achieved (20 microM). EP 157 also produced only small elevations of cyclic AMP and inhibited rises in cyclic AMP induced by iloprost. It is possible that EP 157 has a lower efficacy than iloprost at the IP-receptor and on pig platelets it can sometimes act as a partial agonist. 4. Human, pig and horse platelet membranes bound [3H]-iloprost at 30 degrees C and this binding was inhibited by the four prostanoids. On human and pig membranes the order of potency was cicaprost = iloprost greater than 6a-carba PGI2 greater than EP 157. The order of potency may be similar on horse platelet membranes, but the analysis is complicated by the presence of a second component of [3H]-iloprost binding that is inhibited by iloprost and 6a-carba PGI2 but not by cicaprost. This binding may be due to the presence of an EP1-receptor, since iloprost and 6a-carba PGI2 but not cicaprost are known to have potent EP1-receptor agonist actions on smooth muscle preparations. IC50 values for cicaprost inhibition on human, pig and horse membranes were 110, 90 and 165 nM respectively. The need for IP-receptor radioligands of greater specificity is apparent from these studies. 5. Minimal binding of [3H]-iloprost to rabbit and rat platelet membranes was obtained at 30 degrees C. Lowering the incubation temperature to 4 degrees C and ensuring that the temperature did not rise during the filtration process increased binding and allowed inhibition curves to be obtained. The results suggest a lower binding affinity for [3H]-iloprost, associated with a higher dissociation rate for the radioligand-receptor complex. IC50 values for cicaprost were 900nm for rabbit and 640nm for rat platelets. In a similar manner to horse platelet membranes, the presence of a second binding site for [3H]-iloprost was detected on rabbit platelet membranes. 6. Sensitivity to the relaxant action of iloprost on the arterial smooth muscle preparations decreased in the order: human mesenteric, dog mesenteric, rabbit mesenteric, pig gastro-epiploic. Cicaprost was always slightly more potent than iloprost (1.2-2.8 fold). On the pig vessel preparation 6a-carba PGI2 did not produce complete relaxation. The possibility that this is due to an opposing contractile action mediated via EP1 or EP3 receptors is discussed. 7. EP 157 relaxed the human, pig and rabbit arterial preparations at concentrations 100-200 times those of iloprost. This correlates well with its IP-receptor agonist potency on human, pig and horse platelets. The results obtained with EP 157 further demonstrate the potential difficulties in separating platelet inhibitory and vasodilator properties of prostacyclin mimetics in man.

The action of prostanoid receptor agonists and antagonists on smooth muscle and platelets

1. Prostanoid receptors have been characterized in a range of guinea-pig and rat smooth muscle and platelets, according to the scheme of Coleman et al., (1985a), using agonists (prostaglandin D2 (PGD2), PGE1, PGE2, 16,16 dimethyl PGE2, PGF2 alpha, PGI2 and U46619) and putative selective antagonists (AH 6809 and SQ 29,548). 2. The ileum possesses EP1- and IP-receptors; the trachea, EP1-EP2- and TP-receptors; the oesophageal muscularis mucosa, EP1- and TP-receptors; the aorta and the portal vein TP-receptors. The rat colon possesses IP-, FP- and TP-receptors. 3. Guinea-pig platelets possess both IP and DP receptors mediating an inhibition of aggregation and TP receptors mediating proaggregation responses. No evidence was found for the presence of EP1-, EP2- or FP-receptors. 4. Misoprostol and fenprostalene were characterized using the above preparations. Misoprostol acts as a selective EP1-agonist, and has little or no DP, FP, IP and TP activity. In the trachea precontracted with carbachol no evidence of EP2-receptor stimulation was observed. Fenprostalene possesses FP and TP activity but no EP1, EP2, DP or IP activity.

Prostaglandin endoperoxide analogues which are both thromboxane receptor antagonists and prostacyclin mimetics

Two prostaglandin endoperoxide analogues, EP 035 and EP 157, behave as specific thromboxane receptor antagonists on isolated smooth muscle preparations such as rabbit aorta, dog saphenous vein and guinea-pig trachea. However, in human platelet-rich plasma (PRP) they produce an unsurmountable block of aggregation induced by a wide range of agents (ADP, platelet-activating factor, thrombin); this inhibitory profile is typical of that seen with either prostaglandin I2 (PGI2) or PGD2. EP 035 and EP 157 induce large increases in cyclic AMP levels (up to 20 times basal) in human PRP. Simultaneous exposure to PGE1 markedly reduces their effect on cyclic AMP; exposure to PGD2 is much less effective in this respect. The adenylate cyclase inhibitor SQ 22,536 opposes the inhibitory action of EP 035, EP 157, iloprost (a stable PGI2 analogue) and PGD2 on platelet aggregation. However, the xanthone derivative AH 6809 blocks the inhibitory action of PGD2 but does not affect EP 035, EP 157 and PGI2 and its structural analogues. EP 035 and EP 157 displace [3H]-iloprost binding to the PGI2 receptor on human platelet membranes. Displacing ability is ranked as follows: iloprost greater than 6a-carba PGI2 greater than EP 157 greater than EP 035 greater than EP 164 (alpha-dinor derivative of EP 157). This order of potency is the same as that found for activation of adenylate cyclase in homogenates of washed human platelets and for inhibition of aggregation in washed human platelets. The activities of EP 035 and EP 157 were studied in two other systems containing PGI2 receptor-adenylate cyclase complexes, the NCB-20 cell line and human lung tissue. In both cases stimulation of adenylate cyclase was found but maximum rates were below that achieved with iloprost. These effects of EP 035 and EP 157 could be correlated with their abilities to displace [3H]-iloprost binding. 5 These results indicate that EP 035 and EP 157 inhibit the aggregation of human platelets by acting as agonists at the PGI2 receptor linked to adenylate cyclase. They represent a class of compound with both thromboxane receptor blocking activity and prostacyclin mimetic activity.