Platelet-endothelial Cell Interactions Research Articles

Induction and Suppression of Endothelial Cell Apoptosis by Sphingolipids: A Possible In Vitro Model for Cell-Cell Interactions Between Platelets and Endothelial Cells

Because sphingosine (Sph) is actively incorporated into platelets and rapidly converted to sphingosine 1-phosphate (Sph-1-P), which is then released extracellularly, it is important to study the effects of Sph and Sph-1-P on endothelial cells from the viewpoint of platelet-endothelial cell interaction. In this study, we found that Sph, as well as ceramide, induces apoptosis in human umbilical vein endothelial cells (HUVECs). In contrast, Sph-1-P acts as a HUVEC survival factor; this bioactive lipid was shown to protect HUVECs from apoptosis induced by the withdrawal of growth factors and to stimulate HUVEC DNA synthesis. In metabolic studies, [3H]Sph, incorporated into HUVECs, was converted to [3H]Cer and further to [3H]sphingomyelin in a time-dependent manner, whereas [3H]Sph-1-P formation from [3H]Sph was weak and transient. These findings in HUVECs are very different from those of platelets, which possess a highly active Sph kinase but lack Sph-1-P lyase. As a result, platelets abundantly store Sph-1-P, whereas HUVECs contain much less Sph-1-P. Finally, HUVECs, in contrast to platelets, failed to release Sph-1-P extracellularly, indicating that HUVECs themselves are not able to supply the survival factor Sph-1-P, but receive it from activated platelets. Our results suggest that platelets may maintain the integrity of endothelial cells by incorporating Sph and releasing Sph-1-P.

Increased adhesion and aggregation of platelets lacking cyclic guanosine 3',5'-monophosphate kinase I.

Atherosclerotic vascular lesions are considered to be a major cause of ischemic diseases, including myocardial infarction and stroke. Platelet adhesion and aggregation during ischemia-reperfusion are thought to be the initial steps leading to remodeling and reocclusion of the postischemic vasculature. Nitric oxide (NO) inhibits platelet aggregation and smooth muscle proliferation. A major downstream target of NO is cyclic guanosine 3', 5'-monophosphate kinase I (cGKI). To test the intravascular significance of the NO/cGKI signaling pathway in vivo, we have studied platelet-endothelial cell and platelet-platelet interactions during ischemia/reperfusion using cGKI-deficient (cGKI-/-) mice. Platelet cGKI but not endothelial or smooth muscle cGKI is essential to prevent intravascular adhesion and aggregation of platelets after ischemia. The defect in platelet cGKI is not compensated by the cAMP/cAMP kinase pathway supporting the essential role of cGKI in prevention of ischemia-induced platelet adhesion and aggregation.

Cardioprotective effects of abciximab (ReoPro) in an isolated perfused rat heart model of ischemia and reperfusion.

Ischemia followed by reperfusion in the presence of polymorphonuclear leukocytes (PMNs) results in cardiac contractile dysfunction as well as myocardial injury, due in large part to endothelial dysfunction, upregulation of cell adhesion molecules and subsequent neutrophil induced cardiac injury. We studied the effects of abciximab (ReoPro), an anti-IIb/IIIa antibody, which has been shown to attenuate platelet interactions, in a neutrophil-platelet mediated isolated perfused rat heart model of ischemia (I) (20 min) and reperfusion (R) (45 min). Administration of abciximab (6.5 micrograms/kg) 10 min prior to the perfusion of the PMN + platelet perfused I/R heart improved post-reperfusion coronary flow and preserved post-reperfusion left ventricular developed pressure (LVDP) and +dP/dt max as indices of cardiac contractile function. Abciximab-treated hearts reperfused in the presence of PMNs and platelets preserved all indices of cardiac contractile function. I/R heart perfused with PMNs and platelets produced a profound injury to the hearts which was attenuated with the treatment of abciximab. In addition, abciximab significantly reduced PMN accumulation in the ischemic myocardium from 38 +/- 1 PMNs/mm2 in untreated hearts to 7 +/- 1 in rats given abciximab. Similar results were obtained with PMN perfused I/R rat hearts without platelets. These results provide evidence that abciximab is a potent and effective cardioprotective agent that inhibits leukocyte-endothelial cell interactions as well as platelet-endothelial cell interaction and preserves cardiac contractile function and coronary perfusion following myocardial ischemia and reperfusion. Therefore, IIb/IIIa may be important in attenuating both platelet and neutrophil-mediated myocardial dysfunction.

Platelet-Endothelial Cell Interactions During Ischemia/Reperfusion: The Role of P-Selectin

Growing evidence supports a pathophysiological role for platelets during the manifestation of postischemic reperfusion injury; in the current study, we investigated the nature and the molecular determinants of platelet-endothelial cell interactions induced by ischemia/reperfusion (I/R). Platelet-endothelium and leukocyte-endothelium interactions after 1 hour of ischemia were monitored in vivo within mouse small intestine. By intravital fluorescence microscopy, we observed that platelets, like leukocytes, roll along or firmly adhere to postischemic microvascular endothelial cells. In contrast, few leukocyte-endothelial cell interactions were detected in sham-operated controls. Monoclonal antibodies against P-selectin significantly attenuated platelet rolling and adherence in response to I/R. To identify whether platelet or endothelial P-selectin plays the major role in mediating postischemic platelet-endothelial cell interactions, P-selectin-deficient or wild-type platelets were transfused into wild-type or P-selectin-deficient mice, respectively. Whereas platelets lacking P-selectin rolled along or adhered to postischemic wild-type endothelium, interactions between wild-type platelets with mutant endothelium were nearly absent, indicating that I/R-induced platelet-endothelium interactions are dependent on the expression of P-selectin by endothelial cells. Concomitantly, P-selectin expression in the intestinal microvasculature was enhanced in response to I/R, whereas no upregulation of P-selectin was observed on circulating platelets. In summary, we provide first in vivo evidence that platelets accumulate in the postischemic microvasculature early after reperfusion via P-selectin-ligand interactions. Platelet recruitment and subsequent activation might play an important role in the pathogenesis of I/R injury.

Platelet-Endothelial Cell Interactions During Ischemia/Reperfusion: The Role of P-Selectin

AbstractGrowing evidence supports a pathophysiological role for platelets during the manifestation of postischemic reperfusion injury; in the current study, we investigated the nature and the molecular determinants of platelet-endothelial cell interactions induced by ischemia/reperfusion (I/R). Platelet-endothelium and leukocyte-endothelium interactions after 1 hour of ischemia were monitored in vivo within mouse small intestine. By intravital fluorescence microscopy, we observed that platelets, like leukocytes, roll along or firmly adhere to postischemic microvascular endothelial cells. In contrast, few leukocyte-endothelial cell interactions were detected in sham-operated controls. Monoclonal antibodies against P-selectin significantly attenuated platelet rolling and adherence in response to I/R. To identify whether platelet or endothelial P-selectin plays the major role in mediating postischemic platelet-endothelial cell interactions, P-selectin-deficient or wild-type platelets were transfused into wild-type or P-selectin-deficient mice, respectively. Whereas platelets lacking P-selectin rolled along or adhered to postischemic wild-type endothelium, interactions between wild-type platelets with mutant endothelium were nearly absent, indicating that I/R-induced platelet-endothelium interactions are dependent on the expression of P-selectin by endothelial cells. Concomitantly, P-selectin expression in the intestinal microvasculature was enhanced in response to I/R, whereas no upregulation of P-selectin was observed on circulating platelets. In summary, we provide first in vivo evidence that platelets accumulate in the postischemic microvasculature early after reperfusion via P-selectin-ligand interactions. Platelet recruitment and subsequent activation might play an important role in the pathogenesis of I/R injury.

Chronic dietary supplementation with L-arginine inhibits platelet aggregation and thromboxane A2 synthesis in hypercholesterolaemic rabbits in vivo.

L-arginine exerts anti-atherosclerotic effects in hypercholesterolaemic rabbits via modulating endogenous NO production. We investigated whether L-arginine inhibits thromboxane formation in vivo and platelet aggregation ex vivo in this animal model. The urinary excretion rates of 2,3-dinor-6-keto-PGF1 alpha (major urinary metabolite of PGI2) and 2,3-dinor-TXB2 (major urinary metabolite of thromboxane A2) were used as indicators of platelet-endothelial cell interactions in vivo. Rabbits were fed 1% cholesterol (Cholesterol group, N = 8), 1% cholesterol plus 2.25% L-arginine (Cholesterol + L-arginine, N = 8), or normal rabbit chow (Control, N = 4) for 12 weeks. Urine samples were collected in weekly intervals. At the end of the study period platelet aggregation ex vivo and endothelium-dependent and -independent vascular function of isolated aortic rings in vitro was assessed. Urinary 2,3-dinor-TXB2 excretion significantly increased in the cholesterol group (p < 0.05), and endogenous NO formation (measured as urinary nitrate excretion) decreased (p < 0.05). Both parameters were significantly correlated with each other (R = 0.48, p < 0.01). L-arginine partly restored urinary nitrate excretion and significantly reduced TXA2 production to values even below those in the control group (p < 0.001). Urinary 2,3-dinor-6-keto-PGF1 alpha excretion increased in early hypercholesterolaemia and returned to control values in the second half of the study period. The early increase in urinary 2,3-dinor-6-keto-PGF1 alpha excretion was attenuated by L-arginine. Platelet aggregation was significantly enhanced in cholesterol-fed rabbits and attenuated by dietary L-arginine. L-arginine also improved the impaired endothelium-dependent relaxations to ADP, and normalized the vasoconstrictor effects of 5-HT in isolated aortic rings. Cholesterol-feeding enhances platelet aggregation and TXA2 formation, and stimulates platelet-endothelial cell interaction in rabbits. These effects are probably due to impaired NO elaboration, as indicated by decreased urinary nitrate excretion. Chronic dietary supplementation with L-arginine elevates systemic NO elaboration and significantly increases the PGI2/TXA2 ratio. It thus beneficially influences the homeostasis between vasodilator and vasoconstrictor prostanoids in vivo.

Quantitative analysis of small intestinal microcirculation in the mouse.

Impairment of intestinal nutritive perfusion and accumulation of inflammatory cells in the intestinal microvasculature are well-known sequelae of mesenteric ischemia/reperfusion, sepsis, and shock. However, the molecular mechanisms underlying these alterations are still not fully understood. The mouse is particularly suitable for the study of these mechanisms since in this species the involvement of, for example, adhesion receptors or pro-/anti-adhesive mediators can be selectively investigated by the use of monoclonal antibodies or gene-targeted strains. The aim of our present study was, therefore, to establish a model to investigate the microcirculation in the mouse small intestine. Under anesthesia by inhalation of isoflurane-N2O, Balb/c mice (n = 16) were laparotomized, and a segment of the jejunum was exteriorized for intrvital fluorescence microscopy. Using FITC-dextran (MW 150,000) as a plasma marker, functional capillary density (FCD) of both the intestinal mucosa and muscle layer was analyzed. Nutritive perfusion was homogeneous in both compartments with values for FCD of 512 +/- 15 cm-1 in mucosa and 226 +/- 21 cm-1 in the muscle layer. No significant changes were observed throughout the observation period of 2 h (FCD values at the end of the observation period: 524 +/- 31 cm-1 and 207 +/- 7 cm-1 in mucosa and muscle, respectively). Besides capillary perfusion, leukocyte-endothelial cell interaction was analyzed in postcapillary venules of the intestinal submucosa using rhodamine-6G as an in vivo leukocyte stain. Under physiological conditions only a few white blood cells were found rolling along or firmly adherent to the microvascular endothelium (number of rolling leukocytes 1 +/- 0.2 cells/mm per second; number of adherent leukocytes: 18 +/- 7 cells/mm2). In a separate group rhodamine-6G-labeled syngeneic platelets were infused to analyze platelet-endothelial cell interactions quantitatively in vivo. Platelets rolled along or attached to the endothelium in a manner similar to leukocytes. However, in contrast to leukocytes the interactions were not restricted to venules, but were also observed in small arterioles. The newly established model allows for the visualization and quantitative assessment of both nutritive perfusion and platelet/leukocytendothelial cell interactions within the distinct layers of the mouse small intestine. Using this model in combination with gene-targeted mice or monoclonal antibodies it is possible to investigate the molecular mechanisms of intestinal inflammation reactions.

Calcium signalling by G protein-coupled sphingolipid receptors in bovine aortic endothelial cells.

Besides its role as a putative second messenger releasing Ca2+ from intracellular stores, sphingosine-1-phosphate (SPP) has recently been identified as an extracellularly acting ligand activating a high affinity G protein-coupled membrane receptor in various cell types. Since SPP can be released from activated platelets, we examined in the present study whether endothelial cells express receptors for SPP and related sphingolipids. In bovine aortic endothelial cells loaded with fura-2, addition of SPP caused a rapid and transient increase in intracellular Ca2+ concentration ([Ca2+]i), amounting to maximally about 230 nM. Removal of extracellular Ca2+ revealed that SPP-induced [Ca2+]i elevations were due to both release of Ca2+ from intracellular stores and influx of extracellular Ca2+. Pretreatment of the cells with pertussis toxin inhibited the SPP-induced increase in [Ca2+]i by 83%, in line with the previously reported involvement of G proteins of the Gi/o family in SPP signalling in other cell types. In contrast to other [Ca2+]i-elevating agonists, e.g., ATP and bradykinin, SPP did not activate phospholipase C in bovine aortic endothelial cells, suggesting the involvement of a novel, unidentified signalling pathway in SPP-induced release of intracellular Ca2+. Furthermore, SPP also did not cause activation of either phospholipase D or A2. Out of various related sphingolipids studied, only sphingosylphosphorylcholine (SPPC) induced a similar maximal increase in [Ca2+]i as SPP, and its effect was also fully pertussis toxin-sensitive. However, the potencies of the two sphingolipids to increase [Ca2+]i differed by more than two orders of magnitude, with the EC50 values being 0.8 nM and 260 nM for SPP and SPPC, respectively. These results identify SPP and SPPC as novel and potent endothelial agonists, inducing calcium signalling by activation of a Gi/o protein-coupled receptor(s). Given the recently reported release of SPP from thrombin-activated platelets, SPP may represent a novel mediator of platelet-endothelial cell interactions.

Roles of prostacyclin, EDRF and active oxygens in leukocyte‐dependent platelet adhesion to endothelial cells induced by platelet‐activating factor in vitro

1. The mechanism of polymorphonuclear leukocyte (PMN)-dependent platelet adhesion to cultured endothelial cells induced by platelet-activating factor (PAF) was investigated to determine whether PMNs release or generate any factor(s) capable of inducing platelet adhesion, and the roles of prostacyclin and endothelium-derived relaxing factor (EDRF). 2. Cell-free supernatants, sonicates or rapid filtrates of PAF-stimulated PMN suspensions did not induce platelet adhesion to endothelial cells, but the PMN sonicates induced platelet adhesion when endothelial cells were pretreated with both aspirin and NG-nitro-L-arginine (L-NOARG). Its microphotograph showed that mainly platelet aggregates adhered to the endothelial cell surface. 3. Platelet adhesion induced by the PMN sonicates to aspirin- and L-NOARG-pretreated endothelial cells was dose-dependently prevented by OP-41483 (1-100 nM), and slightly by L-arginine (1 mM). The inhibition of platelet adhesion by OP-41483 and L-arginine was potentiated by their combination. 4. WEB 2170 (3 microM), a PAF antagonist, inhibited platelet adhesion induced by the PMN sonicates. However, PAF alone did not induce significant platelet adhesion to aspirin- and L-NOARG-treated endothelial cells. 5. Platelet adhesion induced by the PMN sonicates was not suppressed by AA-861 and indomethacin. However, both superoxide dismutase and catalase significantly inhibited platelet adhesion, and, in combination, their inhibitory effect was synergistically potentiated. Mannitol had no effect. It was also significantly inhibited by alpha 1-antitrypsin, whereas chymostatin and elastatinal had no effect. 6. PAF-induced platelet adhesion to endothelial cells in the presence of intact PMNs was not suppressed by indomethacin and AA-861, or by protease inhibitors. SOD alone, and in combination with catalase, caused a slight but significant inhibition, while catalase and mannitol by themselves had no effect.7. PMN-induced platelet adhesion was slightly inhibited by OP-41483 (100 nM). L-Arginine (1 mM)alone had no effect, but slightly potentiated the effect of OP-41483. This platelet adhesion was not accompanied by suppression of prostacyclin synthesis.8. The results with the PMN sonicates show that prostacyclin, EDRF and active oxygens are important modulators of intercellular interactions between platelets and endothelial cells. These results further suggest that the mechanism of intact PMN-dependent platelet adhesion is primarily through platelet endothelial cell interactions in which leukocyte-derived active oxygens play a role, but does not involve platelet-platelet interactions inhibitable by prostacyclin.

Interactions between arachidonic and eicosapentaenoic acids during their dioxygenase-dependent peroxidation

Eicosapentaenoic acid (EPA), a major polyunsaturated fatty acid of fish has been widely proposed as a potential nutrient for decreasing platelet-endothelial cell interactions and the subsequent atherogenesis and thrombogenesis. This is mainly based upon the decrease of arachidonic acid (AA) oxygenation into bioactive molecules like thromboxane A 2. In addition, EPA may be oxygenated into its own active derivatives via cell dioxygenases. We report evidence for the requirement of specific peroxides, adequately provided by AA, to allow EPA to be oxygenated into its bioactive products like prostaglandin I 3, a prostacyclin mimetic. On the other hand, we present some data that argue for a decreased basal AA dioxygenation (specific peroxidation) by small concentrations of EPA. The interactions between AA and EPA are then dual, EPA being able to counteract AA oxygenation whereas EPA requires AA to be efficiently oxygenated.

Modulation of endothelin-1 (ET-1) release and degradation by neutrophil- and platelet-endothelial cell interactions

Modulation of endothelin-1 (ET-1) release and degradation by neutrophil- and platelet-endothelial cell interactions

N G-monomethyl-l-arginine increases platelet deposition on damaged endothelium in vivo. A scanning electron microscopic study

There is increasing evidence that nitric oxide (NO) synthetized in vascular endothelium and in platelets by NO synthase influences vascular tone, down regulates platelet function and platelet-vessel wall interaction both in vitro and in vivo . We investigated the effect of a NO synthase inhibitor, N G-mono-methyl-L-arginine (L-NMMA, 100 mg/kg iv) on platelet-endothelial cell interaction in rabbit arteries ex vivo using scanning electron microscope (SEM). The effect of L-NMMA was examined on intact endothelium and on that damaged by arterial constriction. The infusion of L-NMMA increased systemic blood pressure and decreased carotid blood flow, however, it did not change the appearance of an intact endothelium and did not result in platelet activation on intact endothelial cells. In contrast, SEM of endothelial areas damaged by constriction showed extensive platelet adhesion and aggregation on subendothelium. These morphological changes were not detected in control animals with intact or damaged by arterial constriction endothelium. These results show that under physiological conditions, the inhibition of NO synthase alone does not result in platelet activation in vivo . However, when combined with endothelial injury it may lead to platelet activation and thrombosis.

Modulation of prostanoid formation by various polyunsaturated fatty acids during platelet-endothelial cell interactions

Previous studies have reported that polyunsaturated fatty acids (PUFAs) of nutritional interest may influence arachidonic acid (20:4n-6) metabolism in both platelets and endothelium, when tested separately. In the present study, platelets (PL) and cultured endothelial cells (EC) were first pre-enriched with eight different PUFAs for a two hour incubation in the presence of free fatty acid albumin pre-coated with each acid. EC, PL or both cell populations in combination, were then stimulated by thrombin (0.1 U/ml) for five minutes. Prostanoids were extracted, purified by thin-layer chromatography, and TxB 2, 6-keto-PGF 1α and PGE 2 were quantitated by radioimmunoassays. Prostanoids or dihomoprostanoids formed from cyclooxygenase substrates other than 20:4n-6 were measured by gas chromatography-negative chemical ionisation mass-spectrometry (GC-MS). When co-incubated with EC, PL produced less TxB 2 (−15 and −85% in the absence and presence of thrombin, respectively). In contrast, 6-keto-PGF 1α increased by 189 (basal conditions) and 358% (thrombin stimulation) when PL were added to EC, in agreement with PGH 2 transfers from PL to EC. PGE 2, produced by both cell populations, reached amounts which roughly represent the sum of those measured in PL and EC alone, except when cells were pre-enriched with linoleic (18:2n-6) and the n-3 family fatty acids (18:3-, 20:5- and 22:6n-3). 6-keto-PGF 1α was markedly inhibited by adrenic acid (22:4n-6), while this acid was converted into dihomo-6-keto-PGF 1α the stable metabolite of dihomoprostacyclin. 22:4n-6 also inhibited TxB 2 formation and was converted into dihomo-TxA 2. Finally, we found that 20:5n-3 was oxygenated by both cell populations into TxB 3 and Δ17-6keto-PGF 1 α but at a much lower level than TxB 2 and 6-keto-PGF 1α from endogenous 20:4n-6.

Interaction of platelets with endothelial cells: Activation of a novel neutral protease

The activation of a new neutral protease of MW 85,000 was demonstrated on interaction between porcine aortic endothelial cells and human and porcine platelets in culture. The activity of this enzyme, PECAP (platelet endothelial cell activated protease), was detected by electrophoresis on polyacrylamide gels impregnated with the substrate casein. Our results showed that the platelet-endothelial cell interaction did not involve induction of synthesis of de novo enzyme, but rather an activation of a latent enzyme. PECAP cleaved casein and fibrinogen, but had no activity against gelatin or elastin. It was not inhibited by inhibitors of metalloproteases (EDTA, 1.10 phenanthroline), serine proteases (phenylmethylsulfonyl fluoride, elastatinal), or cysteine proteases (iodoacetate, N-ethylmaleimide) and it seems to be unrelated to the previously known proteases of mesenchymal and hematopoietic cells.

Modulation of prostacyclin/thromboxane formation by molsidomine during plateletendothelial cell interactions

Platelet and endothelial cell metabolism of both exogenous and endogenous arachidonic acid, via the cyclooxygenase pathway, was evaluated according to thromboxane and prostacyclin formations. This was investigated in platelets, endothelial cells alone or during their interactions, in the presence or absence of SIN-1, the active anti-anginal metabolite of molsidomine. This revealed that, in contrast to the generation of thromboxane, which was decreased in the presence of endothelial cells, especially from endogenous arachidonate, that of prostacyclin increased under basal conditions as well as from endogenous arachidonate to a lesser extent. SIN-1 reduced thromboxane formation solely from endogenous arachidonate, and this was more pronounced when both cell populations interacted. In contrast, SIN-1 failed to decrease prostacyclin formation, which would emphasize its anti-aggregating potential. We conclude that the liberation of arachidonic acid leading to prostanoid synthesis may be differently regulated in platelets and endothelial cells, and that molsidomine might be a potential anti-aggregating drug in altering specifically thromboxane formation.

Platelet-endothelial cell interactions in vitro following freeze-thaw injury or detergent treatment.

Platelet interactions with cultured bovine endothelial cells were studied following freeze-thaw damage or detergent treatment. Platelets from whole blood, platelet-rich plasma, or gel-filtered plasma did not interact directly with freeze-thaw-damaged endothelial cells. Freezing and thawing did result in the exposure of an extracellular matrix located beneath the cells, which proved very thrombogenic. Platelets from all sources attached to both microfilament and amorphous components of the extracellular matrix, although only platelets from whole blood demonstrated aggregation and extensive pseudopodia formation. Treatment of cells with Triton-X detergent resulted in exposure of an intracellular cytoskeleton. Most platelets attached to the cytoskeleton were located near the cell border and had one or more pseudopodia either in contact with extracellular or intracellular material. Adhesion of platelets to the extracellular matrix may represent platelet-collagen or platelet-fibronectin interactions since both are produced by an incorporated into the extracellular matrix. Platelet interaction with endothelial cytoskeletons may represent contact of pseudopodia with the now exposed matrix located beneath the cells. The possibility that platelets also adhered to intracellular components could not be eliminated. These findings are in agreement with data from a freeze-thaw injury model of perfused aorta. In addition, they tend to indicate that physical insult is not sufficient to induce platelet interaction with the endothelial surface, but that chemical modification enhances platelet deposition.

Relationship between cyclic AMP and thromboxane formation in platelet-endothelial cell interactions

Human platelet aggregation was triggered in the presence of various numbers of cultured endothelial cells. Thromboxane B 2 formation and platelet cyclic AMP were also measured. Using a low concentration of thrombin (0.025U/ml) as aggregating agent, the inhibition of platelet aggregation correlated with that of thromboxane formation and was directly related to both the number of endothelial cells and platelet cyclic AMP. In contrast, using arachidonic acid (10 −5M)instead of thrombin, platelet aggregation could be abolished although thromboxane formation was not affected. These results suggest that platelet aggregation induced by low concentrations of thrombin might be dependent on prostaglandin endoperoxides/thromboxane A 2 production which could be inhibited by cyclic AMP. The normal synthesis of thromboxane B 2 from exogenous arachidonate indicates that cyclic AMP is only active upon the liberation of endogenous arachidonate from platelet phospholipids.

Effect of an alpha-blocking agent, nicergoline, on the interaction between blood platelets, elastin and endothelial cells

The action on an α-blocking agent, Nicergoline, and of a β-blocking agent, Acebutolol, on platelet-elastin and platelet-endothelial cell interaction was investigated. Nicergoline (10 −4M) , but not Acebutolol, added to PRP, decreased the adhesion of platelets to Sepharose-elastin columns by about 60 per cent. Nicergoline at 10 −7M completely inhibited the adhesion of platelets to endothelial cells ‘activated’ by epinephrine. The same inhibition was obtained when the endothelial cells or the platelets were preincubated with the α-blocking agent before mixing. Acebutolol was inefficient. These results confirm the efficiency of this α-blocking agent in preventing platelet adhesion to elastic fibers and to endothelial cells. They confirm the inefficiency of the β-blocking agent in both models used. They also suggest that platelet α-receptors may be involved in platelet-elastin interaction as well as in plateletendothelial cell interaction.