Jos Vermylen

Pharmacological Stabilization of Mast Cells Abrogates Late Thrombotic Events Induced by Diesel Exhaust Particles in Hamsters

Background—Particulate air pollution is associated with cardiovascular diseases and myocardial infarction (MI). Methods and Results—We investigated the relationship between airway inflammation and thrombosis 24 hours after intratracheal (IT) instillation of diesel exhaust particles (DEP; 50 &mgr;g/hamster). Mild thrombosis was induced in the femoral vein by endothelial injury, and the consequences of airway inflammation on thrombogenicity were studied via online video microscopy. Lung inflammation and histamine analysis in bronchoalveolar lavage (BAL) and plasma were performed after pretreatment with dexamethasone (DEX) or sodium cromoglycate (SC). DEP induced airway inflammation and histamine release in BAL and in plasma, and increased thrombosis, without elevating plasma von Willebrand factor (vWF) levels. The IT instillation of 400-nm positively charged polystyrene particles (500 &mgr;g/hamster), serving as particles that do not penetrate into the circulation, equally produced airway inflammation, histamine release, and enhanced thrombosis. Histamine in plasma resulted from basophil activation. Intraperitoneal (IP) pretreatment with DEX (5 mg/kg) abolished the DEP-induced histamine increase in BAL and plasma and abrogated airway inflammation and thrombogenicity. The IT pretreatment with DEX (0.5 mg/kg) showed a partial but parallel inhibition of all of these parameters. Pretreatment with SC (40 mg/kg, IP) strongly inhibited airway inflammation, thrombogenicity, and histamine release. Conclusions—Our results are compatible with the triggering of mast cell degranulation and histamine release by DEP. Histamine plays an initial central role in airway inflammation, further release of histamine by circulating basophils, and peripheral thrombotic events. Antiinflammatory pretreatment can abrogate the peripheral thrombogenicity by preventing histamine release from mast cells.

“LUPUS” ANTICOAGULANT AND INHIBITION OF PROSTACYCLIN FORMATION IN PATIENTS WITH REPEATED ABORTION, INTRAUTERINE GROWTH RETARDATION AND INTRAUTERINE DEATH

We discovered a‘lupus’ anticoagulant in 2 out of 24 women with a history of repeated abortions, intrauterine growth retardation and intrauterine death of unknown origin. The‘lupus’ anticoagulant was detected by an abnormal dilute tissue thromboplastin assay (prothrombin time performed with dilute thromboplastin). The production of prostacyclin by fresh or exhausted rings of rat aorta was decreased by the plasma of one of these two patients with a‘lupus’ anticoagulant. In view of the increasing evidence for a physiological role of prostacyclin in pregnancy and fetal life, we suggest that an inhibition of prostacyclin production could compromise fetal outcome.

Thromboxane synthase inhibitors, thromboxane receptor antagonists and dual blockers in thrombotic disorders

Thromboxane A2 (TXA2) plays a pivotal role in platelet activation and is involved in the development of thrombosis. Thromboxane synthase inhibitors suppress TXA2 formation and increase the synthesis of the antiaggregatory prostaglandins PGI2 and PGD2; however, accumulated PGH2 may interact with the platelet and vessel wall TXA2 receptor, thus reducing the antiplatelet effects of this class of drug. TXA2 receptor antagonists block the activity of both TXA2 and PGH2 on platelets and the vessel wall. Very recently, drugs possessing both thromboxane synthase-inhibitory and thromboxane receptor-antagonist properties have been developed. Paolo Gresele and colleagues explain here why these drugs can be more efficacious than traditional antiplatelet agents and review the available experimental studies involving these drugs.

STIMULATION OF PROSTACYCLIN RELEASE FROM VESSEL WALL BY BAY g 6575, AN ANTITHROMBOTIC COMPOUND

Ingestion of 1.2 g Bay g 6575 daily for 1 week by six healthy volunteers had no effect on blood-coagulation, fibrinolysis, or platelet aggregation in vitro, but it seemed to inhibit platelet aggregation in vivo (shown by a smaller reduction in the platelet aggregate ratio after venous occlusion). Plasma drawn from five volunteers after ingestion of a single dose of 1.2 g of the drug stimulated prostacyclin release from slices of rat aorta which had been washed until they stopped releasing anti-aggregating substances, whereas plasma from the same individuals before ingestion of the substance did not. Administration of either Bay g 6575 or dipyridamole alone had no effect on platelet aggregation in vitro, but combined administration resulted in a striking and prolonged inhibition of A.D.P.-induced platelet aggregation. It is proposed that the previously described antithrombotic properties of Bay g 6575 in animals are due to stimulation of prostacyclin release from the vessel wall, and that this effect is also demonstrable in man.

Influence of hydroxyethyl starch on coagulation in patients during the perioperative period

The perioperative use of hydroxyethyl starch (HES) has been implicated as a possible cause of intracranial bleeding. The purpose of this study was to compare the influence on blood coagulation of the isovolemic replacement of 1-L blood loss with either 6% HES (molecular weight [MW] average: 450,000) or 5% human albumin during neurosurgery or lower abdominal surgery. Twenty patients scheduled for brain tumor surgery and 20 patients undergoing transabdominal hysterectomy were studied. The activated partial thromboplastin time, prothrombin time, fibrinogen concentration, factor VIII coagulant, von Willebrand factor antigen, platelet count, and the activated clotting time were compared after induction of anesthesia, after administration of 500 and 1000 mL of colloid solution, and 24 and 48 h postoperatively. All measured coagulation variables remained within physiologic range. Changes in coagulation indices were identical in neurosurgical and hysterectomy patients, except for a larger increase in fibrinogen concentration 24 and 48 h after hysterectomy. The acute phase reaction of factor VIII coagulant and von Willebrand factor, which plays a role in postoperative hypercoagulability, was attenuated by the use of HES. We conclude that isovolemic replacement of 1-L blood loss with either 6% HES (MW average: 450,000) or 5% human albumin does not interfere with normal hemostasis during and after neurosurgery or lower abdominal surgery.

Mouse Carotid Artery Ligation Induces Platelet-Leukocyte–Dependent Luminal Fibrin, Required for Neointima Development

Abstract — The relationship between platelet and leukocyte activation, coagulation, and neointima development was investigated in noninjured murine blood vessels subjected to blood stasis. The left common carotid artery of C57BL/6J mice was ligated proximal to the bifurcation. Tissue-factor expression in luminal leukocytes progressively increased over 2 weeks. On day 3 after ligation, in addition to infiltrated granulocytes, platelet microthrombi and platelet-covered leukocytes as well as tissue-factor–positive fibrin deposits lined the endothelium. Maximal neointima formation in carotid artery cross sections of control mice equaled 28±3.7% (n=11) and 42±5.1% (n=8) of the internal elastic lamina cross-sectional area 1 and 2 weeks after ligation. In FVIII−/− mice, stenosis was significantly lower 1 (11±3.6%, n=8) and 2 (21±4.7%, n=7) weeks after ligation (both P <0.01 versus background-matched controls). In u-PA−/− mice, luminal stenosis was significantly higher 1 (38±7.0%, n=7) and 2 (77±5.6%, n=6) weeks after ligation (P <0.05 and P <0.01, respectively, versus matched controls). In &agr;2-AP−/− mice, stenosis was lower at 1 week (14±2.6%, n=7, P <0.01) but not at 2 weeks. Responses in tissue-type plasminogen activator or plasminogen activator inhibitor-1 gene–deficient mice equaled that in controls. Reducing plasma fibrinogen levels in controls with ancrod or inducing partial thrombocytopenia with busulfan resulted in significantly less neointima, but inflammation was inhibited only in busulfan-treated mice. We conclude that stasis induces platelet activation, leading to microthrombosis and platelet-leukocyte conjugate formation, triggering inflammation and tissue-factor accumulation on the carotid artery endothelium. Delayed coagulation then results in formation of a fibrin matrix, which is used by smooth muscle cells to migrate into the lumen.

A Natural Dominant Negative P2X1 Receptor Due to Deletion of a Single Amino Acid Residue

The P2X1 receptor belongs to a family of oligomeric ATP-gated ion channels with intracellular N and C termini and two transmembrane segments separating a large extracellular domain. Here, we describe a naturally occurring dominant negative P2X1 mutant. This mutant lacks one leucine within a stretch of four leucine residues in its second transmembrane domain (TM2) (amino acids 351–354). Confocal microscopy revealed proper plasma membrane localization of the mutant in stably transfected HEK293 cells. Nevertheless, voltage-clamped HEK293 cells expressing mutated P2X1 channels failed to develop an ATP or ADP-induced current. Furthermore, when co-expressed with the wild type receptor in Xenopus oocytes, the mutated protein exhibited a dose-dependent dominant negative effect on the normal ATP or ADP-induced P2X1 channel activity. These data indicate that deletion of a single apolar amino acid residue at the inner border of the P2X1 TM2 generates a nonfunctional channel. The inactive and dominant negative form of the P2X1 receptor may constitute a new tool for the study of the physiological role of this channel in native cells.

BM 13.177, A SELECTIVE BLOCKER OF PLATELET AND VESSEL WALL THROMBOXANE RECEPTORS, IS ACTIVE IN MAN

BM 13.177, a sulphonamide derivative, prevented platelet aggregation by thromboxane A2 in vitro and selectively inhibited contraction of isolated rabbit femoral arteries induced by two stable endoperoxide analogues. In a double-blind placebo-controlled study, oral BM 13.177 inhibited platelet aggregation induced by arachidonic acid, low dose collagen, and the two stable endoperoxide analogues, and slightly prolonged the bleeding time. Generation of thromboxane or of other prostaglandins was not affected. No side-effects were seen. BM 13.177 appears selectively and safely to block platelet and vessel wall thromboxane receptors and should be useful in elucidating the role of thromboxane A2 in disease.

Role of platelet activation and fibrin formation in thrombogenesis

Further progress in the search for more effective but safe antithrombotic agents is coupled to an improved understanding of the factors involved in arterial and venous thrombogenesis. Although arterial thrombosis is initiated by formation of a layer of platelets on modified endothelium or subendothelial constituents and subsequent recruitment of passing-by platelets, this phenomenon is not sufficient to lead to a full thrombus. Further growth of such a platelet mass depends, to a large extent, on the presence of free thrombin. Thrombin is mainly generated by activation of factor XI on the platelet contact with collagen. In addition, thrombin leads to formation of fibrin, which maintains the stability of the arterial platelet thrombus and is the main component of the venous thrombus. The search for agents that inhibit platelet activation and thrombin formation is, therefore, a logical endeavor.

The Platelet ATP and ADP Receptors

Adenine nucleotides, ADP and ATP, are coreleased from dense granules during platelet activation, as well as from endothelial cells and damaged red blood cells following vascular injury. Through autocrine and paracrine mechanisms, these extracellular signaling molecules interact with the platelet P2 receptors to amplify ongoing platelet activation. Two receptors for ADP, the G(q)-protein-coupled P2Y1 and G(i)-protein-coupled P2Y12 and one receptor for ATP, the P2X1 ion channel, have been identified on platelets. Due to distinct pharmacological properties and differential regulation, the P2Y and P2X receptors essentially operate on different scales of time and distance and trigger selective intracellular signaling cascades. Recent advances in the understanding of the P2Y receptor physiology have reinforced the concept of these receptors as useful targets for antithrombotic therapy. The function of P2X1 in platelet activation only recently started to be unraveled. This review focuses on recent findings on the physiology of these platelet ADP and ATP receptors, their distinct downstream intracellular signaling pathways as well as on the available agonists, antagonists and inhibitors that allow their pharmacological discrimination.