Adrenaline May Contribute to Prothrombotic Condition via Augmentation of Platelet Procoagulant Response, Enhancement of Fibrin Formation, and Attenuation of Fibrinolysis

Background: Adrenaline is believed to play a role in thrombosis and hemostasis. The complex effect of its clinically relevant concentrations on thrombus formation, coagulation and fibrinolysis in human blood has never been specifically studied. Methods: Confocal microscopy was used to study thrombus formation under flow, exposure of phosphatidylserine (PS) in adhered platelets, to evaluate clots density, and to measure kinetics of fibrin formation and external fibrinolysis under flow. Flow cytometry was utilized to assess PS exposure in non-adhered platelets. Kinetics of clot formation and internal fibrinolysis was evaluated by thromboelastometry. Platelet aggregation was measured by optical aggremometry. Kinetics of clot retraction was assessed by using digital camera. Results: We found that adrenaline (1-10 nM) is able to enhance platelet activation evoked by subthreshold collagen (150 ng/ml), resulting in augmentation of platelet aggregation, thrombus formation under arterial flow conditions, platelet PS exposure, and formation of platelet-fibrin clots. The development of platelet procoagulant response evoked by adrenaline + low collagen was associated with the formation of denser platelet-fibrin clots and the decrease in rate of fibrinolysis despite whether lysis was initiated inside (internal fibrinolysis) or outside the clot (external fibrinolysis). The above phenomena were abolished by the α₂-adrenergic receptor antagonist, rauwolscine. Adrenaline-collagen synergism, expressed as PS exposure, was significantly reduced by cyclooxygenase inhibitor (acetylsalicic acid), GPIIb/IIIa receptor blocker (tirofiban), and P2Y₁₂ receptor antagonist (PSB 0739). Conclusion: Clinically relevant concentrations of adrenaline may significantly augment responses of human platelets in the presence of subthreshold concentrations of collagen, which should be considered during therapies involving adrenaline infusion. Routinely used antiplatelet drugs may reduce the prothrombotic state evoked by adrenaline-collagen synergism.

HAuCl4, Putative General Aquaporins Blocker, Reduces Platelet Spreading, Filopodia Formation, Procoagulant Response, and Thrombus Formation Under Flow

Background: Recent studies indicate that aquaporin (AQP) water channels have a regulatory function in human platelet secretion and in procoagulant response of murine platelets. However, the engagement of AQPs in morphological changes, procoagulant response, and thrombus formation in human blood has never been investigated.

Methods: Confocal microscopy was used to study platelet spreading, filopodia formation, ballooning, and thrombus formation under flow. Flow cytometry was utilized to assess platelet phosphatidylserine (PS) exposure and microparticles shedding. Kinetics of clot formation in vitro was evaluated by thromboelastometry. Mouse model of ferric chloride (III) (FeCl₃)-induced thrombosis was used to investigate thrombus formation in vivo.

Results: We found that chloroauric(III) acid (HAuCl₄), a classical AQP inhibitor (10–100 μM), reduced spreading of human platelets on collagen-coated surfaces and inhibited filopodia formation in a fluid phase. Under flow conditions, HAuCl₄ (100 μM) attenuated thrombi growth on collagen, platelet secretion, and PS exposure. Thrombus formation was restored by the addition of exogenous adenosine diphosphate (ADP). Collagen-evoked platelet procoagulant response (evaluated as PS exposure, shedding of microparticles, platelet-dependent thrombin generation, and membrane ballooning) was distinctly reduced by HAuCl₄ (25–200 μM), as well as the dynamics of clot formation. In mouse model of thrombosis, reduction of surface of PS-positive cells within thrombus was observed in the presence of HAuCl₄ (1–10 mg/kg).

Conclusion: These results suggest that in human platelets AQPs are crucial for agonist-evoked morphological changes, thrombus formation under flow, and in development of procoagulant response. Antithrombotic effect in vivo suggests that nontoxic inhibitors of AQPs may be considered as potential candidates for a novel class of antiplatelet drugs.

Rapid effects of aldosterone on platelets, coagulation, and fibrinolysis lead to experimental thrombosis augmentation

An increase in aldosterone levels positively correlates with an increased risk of acute cardiovascular thrombotic events. The aim of the study was to determine the mechanism of action of prothrombotic aldosterone focusing on the rapid effects of the hormone on platelets, coagulation, and fibrinolysis. A wide panel of advanced ex vivo and in vitro techniques was used for the evaluation of coagulation and fibrinolysis in aldosterone-treated rats. Additionally, two experimental mice models of thrombosis, which allowed for the intravital observation of the first stage of thrombus formation in real time, were used. Acute administration of aldosterone in rats increased the density of fibrin net and platelet aggregates in clots as well as reduced fibrinolysis. These effects were observed within 10 min and were partially suppressed by eplerenone. Moreover, acute administration of aldosterone in mice enhanced platelet accumulation at the site of endothelial injury induced by laser and increased the area of irreversibly activated platelets in FeCl₃-induced thrombus. These results demonstrate that aldosterone acutely affects platelets, coagulation, and fibrinolysis, leading to an enhanced thrombosis. The aldosterone effects were mediated partially via a mineralocorticoid receptor. The mechanism seems to involve non-genomic signaling since the effects were observed within a few minutes of aldosterone administration.

The myeloperoxidase product, hypochlorous acid, reduces thrombus formation under flow and attenuates clot retraction and fibrinolysis in human blood

Hypochlorite (HOCl), a strong oxidant and antimicrobial agent, has been proposed to be associated with hemostatic abnormalities during inflammatory response. However, its complex impact on hemostasis is not completely understood. In this report we studied the effect of clinically relevant (micromolar) HOCl concentrations on thrombus formation under flow, kinetics of platelet-fibrin clot formation, its architecture, retraction, and lysis. We found that HOCl (up to 500 µM) did not affect kinetics of coagulation measured in whole blood. HOCl (500-1000 µM) markedly diminished thrombus formation under flow. Clot retraction rate was reduced by HOCl dose-dependently (50-500 µM). HOCl (125-500 µM) inhibited fibrinolysis in whole blood and in platelet-depleted plasma, dose-dependently. Activity of plasmin was reduced by HOCl at concentrations started from 500 µM. HOCl (up to 500 µM) did not reduce plasminogen binding to fibrin under flow. HOCl (125-500 µM) modulated architecture of fibrin- and platelet-fibrin clots towards structures made of thin and densely packed fibers. Exposure of pure fibrinogen to HOCl (10-1000 µM) resulted in formation of dityrosine and was associated with altered fibrin structure derived from such modified fibrinogen. HOCl-altered fibrin net structure was not related with modulation of platelet procoagulant response, thrombin generation, and factor XIII activity. We conclude that, in human blood, clinically relevant HOCl concentrations may inhibit thrombus formation under flow, clot retraction and fibrinolysis. Fibrinolysis and clot retraction seem to be the most sensitive to HOCl-evoked inhibition. HOCl-modified fibrinogen and altered clot structure associated with it are likely to be primary sources of attenuated fibrinolysis.