Potentiation by adrenaline of human platelet activation and the inhibition by the alpha-adrenergic antagonist nicergoline of platelet adhesion, secretion and aggregation

Adrenaline (1 to 10 microM) can induce the aggregation of human platelets suspended in citrated plasma but does not induce the aggregation of washed human platelets at doses as high as 1 mM, although these platelets respond normally to ADP, PAF-acether, collagen, arachidonic acid, thrombin, the endoperoxide analog U-46619 and the Ca2+ ionophore A23187. Adrenaline (0.5 microM) potentiates the aggregation and secretion induced by all the previous agonists in citrated platelet-rich plasma (cPRP) or in washed platelets. The activation by adrenaline of human platelets is mediated by alpha 2-adrenergic receptors, as demonstrated by inhibition with a series of adrenergic antagonists. The alpha-adrenergic antagonist nicergoline inhibits the activation of human platelets by adrenaline in the following situations: nicergoline inhibits the aggregation and secretion caused by adrenaline in cPRP (IC50 0.22 microM and 0.28 microM respectively); nicergoline inhibits the aggregation and secretion induced by the combination of adrenaline and each aggregating agent listed above in cPRP (IC50 ranging from 0.1 to 2.5 microM) or in washed platelets (IC50 ranging from 0.1 to 0.8 microM); nicergoline inhibits the binding of 3H-yohimbine to washed human platelets (IC50 0.26 microM); the intravenous administration of nicergoline (0.5 mg/kg per day) to patients inhibits significantly the ex vivo response of their platelets to adrenaline in cPRP. High concentrations of nicergoline also inhibit the aggregation and secretion induced by the aggregating agents listed above in cPRP (IC50 range 108 to 670 microM) and in washed platelets (IC50 range 27 to 140 microM) and the adhesion of platelets to collagen-coated surfaces. This latter effect is not mediated through blockade of alpha-adrenoceptors. A possible role of adrenaline in platelet activation in vivo could justify the use of nicergoline (Sermion), an alpha-adrenergic antagonist in combination therapy to prevent arterial thrombosis.     

FAK-mediated inhibition of vascular smooth muscle cell migration by the tetraspanin CD9

Migration of vascular smooth muscle cells (SMC) towards the intima is a key event in vascular proliferative diseases. We investigated a potential role for the tetraspanin CD9 in this process in a wound migration assay. Aortic SMC from CD9 knock-out mice had higher migration rates and the presumably stimulatory anti-CD9 antibody ALMA-1 inhibited migration of human SMC. The signaling pathways responsible for this inhibitory effect were investigated. In migrating CD9-/- SMC, stress fiber formation was decreased and focal adhesions were smaller and more diffusely distributed, consistent with an inhibition of integrin clustering. In migrating mouse SMC expressing CD9, focal adhesion kinase (FAK) tyrosine phosphorylation was doubled. No differences in intracellular calcium signaling were observed between CD9+/+ and CD9-/- SMC during migration. We suggest that CD9 in hibits SMC migration by a stimulation of both stress fiber formation and integrin clustering, leading to a stimulation of FAK phosphorylation.     

Culture of vascular cells in tridimensional (3-D) collagen: a methodological review

Experimental models based on the culture of cells within tridimensional (3-D) gels add a 3-D organization to the classical 2-D culture of vascular cells. They allow the study of cell structure in an environment which is more representative of the in vivo situation and the investigation of cellular functions which cannot be studied using the basic 2-D models. This review shows examples of the use of cultures of vascular cells (endothelial cells, smooth muscle cells as well as fibroblasts) in 3-D collagen matrices for the study of cellular functions as diverse as angiogenesis, extracellular matrix reorganization, migration through 3-D collagen gels or phenotype modulation. It also describes recent advances in the in vitro reconstruction of biological blood vessels by bioengineering. A method for the preparation of 3-D collagen gels is described.     

A simple in vitro model of mechanical injury of confluent cultured endothelial cells to study quantitatively the repair process

A model of in vitro mechanical injury of confluent human endothelial cells (EC) in culture was developed. Human EC were obtained from umbilical veins and grown to confluence. Application on the EC monolayer of a calibrated disk of cellulose polyacetate paper resulted in removal of the EC, leaving a continuous subendothelial extracellular matrix (ECM) on the culture dish. The regeneration time depended on the original size of the lesion. Regeneration was similar with EC grown on different substrates such as human fibronectin, human subendothelial ECM, bovine collagen type I or surfaces coated with Transglutine, a surgical glue containing adhesive proteins. A human brain extract containing growth factor activity accelerated significantly the repair of the lesion, especially at low serum concentration. This simple in vitro model of mechanical injury allows the quantitative study of the effects of matrices, growth factors and pharmacological agents on the repair process.     

Increased aggregation and secretion responses of human platelets when loaded with the calcium fluorescent probes quin2 and fura-2

Incorporation into human platelets of the calcium fluorescent indicators quin2 or fura-2 at low concentrations used to measure intracellular free calcium leads to the potentiation of the effects of agonists on platelets. This was shown by increased aggregatory and secretory responses of quin2 or fura-2 loaded platelets after stimulation with ADP, PAF and with low concentrations of thrombin, collagen, the endoperoxide analog U-46619 and the calcium ionophore A 23187. Quin2 and fura-2 mediated platelet sensitisation could be due to altered arachidonic acid metabolism since it was inhibited by prior treatment with the cyclooxygenase inhibitor acetylsalicylate. In contrast, platelets loaded with higher concentrations of calcium chelators exhibited diminished aggregation responses to all aggregating agents. This latter effect was accompanied by increased fluidity of the platelet plasma membrane bilayer and by the exposure of a new pool of membranes to the outer surface of platelets, as monitored with trimethyl-ammonium-diphenylhexatriene (TMA-DPH) in platelets loaded with the non-fluorescent calcium probe analog MAPT. In contrast, low concentrations of quin2 did not potentiate shape change of platelets activated with ADP. Thus, shape change and aggregation can be influenced separately by intracellular Ca2+ chelators. We conclude that platelet responses are altered by the incorporation of intracellular calcium chelators at concentrations used to monitor intracellular calcium changes.     

Inhibition of aggregation and secretion of human platelets by quercetin and other flavonoids: structure-activity relationships

Quercetin and 12 other natural flavonoid aglycones inhibit washed human platelet aggregation and secretion of serotonin induced by ADP, collagen or thrombin. The inhibitory effect of flavonoids is of the same order of magnitude as IBMX and dipyridamole. The structural features required for a flavonoid to inhibit human platelet function are similar to those previously reported by us to inhibit cyclic nucleotide phosphodiesterase. The inhibitory effect of flavonoids on human platelet function was diminished by saturation of the C-2, C-3 double bond, lack of the C-4 carbonyl, glycosylation at C-3 and a high number of hydroxyl substituents.     

Role of cyclic AMP in promoting the thromboresistance of human endothelial cells by enhancing thrombomodulin and decreasing tissue factor activities.

1. The effects of forskolin, prostaglandin E1 (PGE1), dibutyryl cyclic AMP (db cyclic AMP), dibutyryl cyclic GMP (db cyclic GMP) and 3-isobutyl-l-methyl-xanthine (IBMX) were investigated on the expression of tissue factor and thrombomodulin activities on the surface of human saphenous vein endothelial cells (HSVEC) in culture. 2. Forskolin (10(-6) to 10(-4) M), PGE1 (10(-7) to 10(-5) M) and db cyclic AMP (10(-4) to 10(-3) M) caused a concentration-dependent decrease of cytokine-induced tissue factor activity. 3. Similar concentrations of forskolin, PGE1 and db cyclic AMP enhanced significantly constitutive thrombomodulin activity and reversed the decrease of this activity caused by interleukin-1 (IL-1). 4. IBMX (10(-4) M) decreased tissue factor activity and enhanced the effect of forskolin on tissue factor and thrombomodulin activities. 5. Forskolin (10(-4) M) decreased the IL-1-induced tissue factor mRNA and increased the thrombomodulin mRNA level. IL-1 did not change the thrombomodulin mRNA level after 2 h of incubation with HSVEC in culture. 6. Dibutyryl cyclic GMP (10(-4) M to 10(-3) M) did not influence tissue factor or thrombomodulin activity. 7. Our data suggest that elevation of intracellular cyclic AMP levels may participate in the regulation of tissue factor and thrombomodulin expression, thus contributing to promote or restore antithrombotic properties of the endothelium.