Vasopressin acts on platelets to generate procoagulant activity

Study on the Mechanism of the Adrenaline-Evoked Procoagulant Response in Human Platelets

Adrenaline has recently been found to trigger phosphatidylserine (PS) exposure on blood platelets, resulting in amplification of the coagulation process, but the mechanism is only fragmentarily established. Using a panel of platelet receptors' antagonists and modulators of signaling pathways, we evaluated the importance of these in adrenaline-evoked PS exposure by flow cytometry. Calcium and sodium ion influx into platelet cytosol, after adrenaline treatment, was examined by fluorimetric measurements. We found a strong reduction in PS exposure after blocking of sodium and calcium ion influx via Na+/H+ exchanger (NHE) and Na+/Ca2+ exchanger (NCX), respectively. ADP receptor antagonists produced a moderate inhibitory effect. Substantial limitation of PS exposure was observed in the presence of GPIIb/IIIa antagonist, phosphoinositide-3 kinase (PI3-K) inhibitors, or prostaglandin E1, a cyclic adenosine monophosphate (cAMP)-elevating agent. We demonstrated that adrenaline may develop a procoagulant response in human platelets with the substantial role of ion exchangers (NHE and NCX), secreted ADP, GPIIb/IIIa-dependent outside-in signaling, and PI3-K. Inhibition of the above mechanisms and increasing cytosolic cAMP seem to be the most efficient procedures to control adrenaline-evoked PS exposure in human platelets.

Thrombocytopathies: Not Just Aggregation Defects-The Clinical Relevance of Procoagulant Platelets

Platelets are active key players in haemostasis. Qualitative platelet dysfunctions result in thrombocytopathies variously characterized by defects of their adhesive and procoagulant activation endpoints. In this review, we summarize the traditional platelet defects in adhesion, secretion, and aggregation. In addition, we review the current knowledge about procoagulant platelets, focusing on their role in bleeding or thrombotic pathologies and their pharmaceutical modulation. Procoagulant activity is an important feature of platelet activation, which should be specifically evaluated during the investigation of a suspected thrombocytopathy.

GC-MS and LC-MS/MS pilot studies on the guanidine (NG)-dimethylation in native, asymmetrically and symmetrically NG-dimethylated arginine-vasopressin peptides and proteins in human red blood cells

Protein-arginine methyltransferases catalyze the methylation of the guanidine (N^G) group of proteinic L-arginine (Arg) to produce monomethyl and dimethylarginine proteins. Their proteolysis releases the free amino acids monomethylarginine (MMA), symmetric dimethylarginine (SDMA) and asymmetric dimethylarginine (ADMA), respectively. MMA, SDMA and ADMA are inhibitors of the nitric oxide synthase (NOS) activity. High circulating and low urinary concentrations of ADMA and SDMA are considered risk factors in the cardiovascular and renal systems, mainly due to their inhibitory action on NOS activity. Identity, biological activity and concentration of N^G-methylated proteins are largely unknown. The present study addressed these issues by using GC-MS and LC-MS/MS approaches. GC-MS was used to quantify free ADMA released by classical HCl-catalyzed hydrolysis of three synthetic Arg-vasopressin (V) peptides and of unknown endogenous N^G-dimethylated proteins. The cyclic (c) disulfide forms of Arg-vasopressin analogs, i.e., Arg-vasopressin (cV-Arg-Gly-NH₂), asymmetrically N^G-dimethylated vasopressin (cV-ADMA-Gly-NH₂) and symmetrically N^G-dimethylated vasopressin (cV-SDMA-Gly-NH₂) were used as model peptides in quantitative GC-MS analyses of ADMA, SDMA and other expected amino acids from the hydrolyzed Arg-vasopressin analogs. cV-ADMA-Gly-NH₂ and cV-SDMA-Gly-NH₂ were discriminated from cV-Arg-Gly-NH₂ by LC-MS and LC-MS/MS, yet they were indistinguishable from each other. The same applies to the respective open (o) reduced and di-S-acetamide forms of oV-ADMA-Gly-NH₂, oV-SDMA-Gly-NH₂ and oV-Arg-Gly-NH₂. Our LC-MS and LC-MS/MS studies suggest that the Arg-vasopressin analogs form [(M-H)]⁺ and [(M-H)+H]⁺ in the positive ESI mode and undergo in part conversion of their terminal Gly-NH₂ (NH₂, 16 Da) group to Gly-OH (OH, 17 Da). The product ion mass spectra of the di-S-acetamide forms are complex and contain several intense mass fragments differing by 1 Da. cV-ADMA-Gly-NH₂ and cV-SDMA-Gly-NH₂ induced platelet aggregation in platelet-rich human plasma with moderately different initial velocity and maximal aggregation rates compared to cV-Arg-Gly-NH₂. Previous studies showed that human red blood cells are rich in large (>50 kDa) ADMA-containing proteins of unknown identity. Our LC-MS/MS proteomic study identified several membrane and cytosolic erythrocytic N^G-dimethylated proteins, including spectrin-α (280 kDa), spectrin-β (247 kDa) and protein 4.1 (80 kDa). Being responsible for the stability of the erythrocyte membrane, the newly identified main targets for N^G-dimethylation in human erythrocytes should be given a closer look in erythrocytic diseases like hereditary spherocytosis.

Noncanonical effects of vasopressin in angiogenesis

The molecular action of vasopressin depends on the localization of hormonal receptors. The basic physiological effects of vasopressin are manifested in the blood vasculature, renal inner medulla and brain. To date, new information concerning the tissue-specific spreading of vasopressin receptors has been accumulated, and it needs to be summarized. Platelets and endotheliocytes expressing V1a and V2 receptor types, respectively, are related to less investigated targets of the hormone. Vasopressin induces the initial reversible stage of platelet activation, required for interaction with intercellular matrix proteins. Platelet adhesion on endothelium activates cellular secretion of growth factors and enzymes for intercellular matrix glucosamine metabolism. Platelet hyaluronidase HYAL2 hydrolyses high-molecular hyaluronic acid to shorter fragments. Unlike intact hyaluronic acid with a molecular weight of several megadaltons, generally showing distinctive antiangiogenic properties, intermediate fractions of hyaluronan hydrolysis in a range from 2.5 to 200 kilodaltons have a stimulating effect on angiogenesis. Intercellular contacts between platelets and endotheliocytes are stabilized due to adhesive transmembrane glycoprotein PECAM-1 interaction. Resulting PECAM-1 heterodimers acquire conformation with high affinity to integrins αvβ3. Integrin activation forms contact links between endothelium and fibrillar proteins. Activated endotheliocytes secrete von Willebrand factor and P-selectin. These proteins are accumulated in Weibel–Palade bodies. Vasopressin stimulates cAMP-dependent ACAP-regulated exocytosis of Weibel–Palade bodies. von Willebrand factor possesses adhesive properties and additionally accelerates interaction of cells with the intercellular matrix. Adhesion on fibrillar collagen and membrane glycoproteins in cooperation with effects of PECAM-1–αvβ3 integrin complexes fixes cell aggregates in the surrounding interstitium and promotes proliferating endotheliocyte migration in according to the direction of local growth factor gradients during angiogenesis. Neurohormonal regulation of platelet and endotheliocyte secretory activity functionally link proliferation and migration of endotheliocytes during angiogenesis and integrate it according to the adaptive capacity of the entire organism.

Involvement of hyperglycemia in the development of platelet procoagulant response: the role of aldose reductase and platelet swelling

: Rise in mean platelet volume (MPV) has been demonstrated to be associated with increased platelet reactivity. In diabetes patients, augmented MPV was proposed to contribute to increased risk of thrombotic complications. Therefore, the aim of this study was to investigate whether under hyperglycemic conditions, aldose reductase (AR)-mediated sorbitol formation and associated rise in cell volume, which subsequently results in platelet hyperactivation. Platelets were obtained from 30 healthy volunteers and 13 patients with diabetes. We evaluated changes in platelet size, their reactivity (measured as aggregation and secretion), and sorbitol content evoked by glucose. Measurement of procoagulant activity and thromboelastography were performed to assess how hyperglycemia affects coagulation. We have found that incubation of platelets with glucose (>10 mmol/l) leads to increased MPV, potentiation of collagen-evoked platelet aggregation, secretion, and procoagulant response (measured as platelet-dependent thrombin generation and phosphatidylserine expression). Glucose-treated platelets had higher sorbitol content and demonstrated enhanced tubulin polymerization. All the above-mentioned phenomena were reduced following the blocking of AR or by vincristine (microtubule destabilizing agent). Thromboelastography measurements demonstrated that hyperglycemia is associated with reduction of clotting time (R) and increase in the alpha angle (reflects platelet activation). Addition of sorbinil (AR inhibitor) or vincristine normalized R variable and alpha angle. The hyperglycemic conditions may accelerate platelet-related thrombin generation through the activation of polyol pathway, enhanced tubulin polymerization and associated with it rise in platelet volume.

Early and sustained vasopressin infusion augments the hemodynamic efficacy of restrictive fluid resuscitation and improves survival in a liver laceration model of hemorrhagic shock

BACKGROUND

Current management of hemorrhagic shock favors restrictive fluid resuscitation before control of the bleeding source. We investigated the additional effects of early and sustained vasopressin infusion in a swine model of hemorrhagic shock produced by liver laceration.

METHODS

Forty male domestic pigs (32-40 kg) had a liver laceration inflicted with an X-shaped blade clamp, 32 received a second laceration at minute 7.5, and 24 received two additional lacerations at minute 15. Using a two-by-two factorial design, animals were randomized 1:1 to receive vasopressin infusion (0.04 U/kg per minute) or vehicle intraosseously from minute 7 until minute 240 and 1:1 to receive isotonic sodium chloride solution (12 mL/kg) intravenously at minute 30 or no fluids.

RESULTS

Kaplan-Meier curves showed greater survival after vasopressin with isotonic sodium chloride solution (8/10) compared to vasopressin without isotonic sodium chloride solution (4/10), vehicle with isotonic sodium chloride solution (3/10), or vehicle without isotonic sodium chloride solution (3/10), but the differences were not statistically significant (p = 0.095 by log-rank test). However, logistic regression showed vasopressin to elicit a statistically significant benefit on survival (p = 0.042). Vasopressin augmented mean aortic pressure between 10 and 20 mm Hg without intensifying the rate of bleeding from liver laceration, which was virtually identical to that of vehicle-treated animals (33.9 ± 5.1 and 33.8 ± 4.8 mL/kg). Vasopressin increased systemic vascular resistance and reduced transcapillary fluid extravasation, augmenting the volume of isotonic sodium chloride solution retained (6.5 ± 2.7 vs 2.4 ± 2.0 mL/kg by minute 60). The cardiac output and blood flow to the myocardium, liver, spleen, kidney, small bowel, and skeletal muscle at minute 120 and minute 180 were comparable or higher in the vasopressin group.

CONCLUSIONS

Early and sustained vasopressin infusion provided critical hemodynamic stability during hemorrhagic shock induced by liver laceration and increased the hemodynamic efficacy of restrictive fluid resuscitation without intensifying bleeding or compromising organ blood flow resulting in improved 240-minute survival.

Peroxynitrite may affect fibrinolysis via the reduction of platelet-related fibrinolysis resistance and alteration of clot structure

We tested the hypothesis that in vitro peroxynitrite (ONOO(-), a product of activated inflammatory cells) may affect fibrinolysis in human blood through the reduction of platelet-related fibrinolysis resistance. It was found that ONOO(-) (25-300 µM) accelerated lysis of platelet-fibrin clots (in PRP) dose-dependently, whereas fibrinolysis of platelet-free clots was slightly inhibited by ≥ 1000 µM stressor. Concentrations of ONOO(-) affecting the lysis of platelet-rich clots, inhibited clot retraction (CR) in a dose-dependent manner. Thromboelastometry (ROTEM) measurements performed in PRP showed that treatment with ONOO(-) (threshold conc. 100 µM) prolongs clotting time, and reduces alpha angle, and clot formation velocity parameters indicating for reduced thrombin formation rate. In PRP, ONOO(-) (threshold conc. 100 µM) reduced the collagen-evoked exposure of phosphatidylserine (PS) on platelets' plasma membrane, the shedding of platelet-derived microparticles (PMP), and inhibited platelet-dependent thrombin generation (measured in artificial system), dose-dependently. As judged by confocal microscopy, similar ONOO(-) concentrations altered the architecture of clots formed in collagen-treated PRP. Clots formed in the presence of ONOO(-) were less dense and were composed of thicker fibers, which make them more susceptible to lysis. In platelet-depleted plasma, ONOO(-) (up to milimolar concentration) did not alter clot structure. Blockage of PS exposed on platelets resulted in an alteration of clot architecture toward more prone to lysis. ONOO(-), at lysis-affecting concentrations, inhibited the collagen-evoked secretion of fibrinolytic inhibitors from platelets. We conclude that physiologically relevant ONOO(-) concentrations may accelerate the lysis of platelet-fibrin clots predominantly via downregulation of platelet-related mechanisms including: platelet secretion, clot retraction, platelet procoagulant response, and the alteration in clot architecture associated with it.

Platelet activation after presyncope by lower body negative pressure in humans

Central hypovolemia elevates hemostatic activity which is essential for preventing exsanguination after trauma, but platelet activation to central hypovolemia has not been described. We hypothesized that central hypovolemia induced by lower body negative pressure (LBNP) activates platelets. Eight healthy subjects were exposed to progressive central hypovolemia by LBNP until presyncope. At baseline and 5 min after presyncope, hemostatic activity of venous blood was evaluated by flow cytometry, thrombelastography, and plasma markers of coagulation and fibrinolysis. Cell counts were also determined. Flow cytometry revealed that LBNP increased mean fluorescence intensity of PAC-1 by 1959±455 units (P<0.001) and percent of fluorescence-positive platelets by 27±18%-points (P = 0.013). Thrombelastography demonstrated that coagulation was accelerated (R-time decreased by 0.8±0.4 min (P = 0.001)) and that clot lysis increased (LY₆₀ by 6.0±5.8%-points (P = 0.034)). Plasma coagulation factor VIII and von Willebrand factor ristocetin cofactor activity increased (P = 0.011 and P = 0.024, respectively), demonstrating increased coagulation activity, while von Willebrand factor antigen was unchanged. Plasma protein C activity and tissue-type plasminogen activator increased (P = 0.007 and P = 0.017, respectively), and D-dimer increased by 0.03±0.02 mg l⁻¹ (P = 0.031), demonstrating increased fibrinolytic activity. Plasma prothrombin time and activated partial thromboplastin time were unchanged. Platelet count increased by 15±13% (P = 0.014) and red blood cells by 9±4% (P = 0.002). In humans, LBNP-induced presyncope activates platelets, as evidenced by increased exposure of active glycoprotein IIb/IIIa, accelerates coagulation. LBNP activates fibrinolysis, similar to hemorrhage, but does not alter coagulation screening tests, such as prothrombin time and activated partial thromboplastin time. LBNP results in increased platelet counts, but also in hemoconcentration.