Platelet-vessel wall interaction: role of nitric oxide, prostaglandins and endothelins

Characterization of the L-arginine-NO-cGMP pathway in spontaneously hypertensive rat platelets: the effects of pregnancy

Nitric oxide (NO) is a short-lived intercellular messenger that provides an efficient vascular regulatory mechanism to support homeostasis and prevent thrombosis. Endothelial dysfunction and reduced NO bioavailability have a central role in hypertension associated with pregnancy. The purpose of this study was to investigate the impact of pregnancy on the L-arginine-NO-cGMP pathway in platelets and its correlation to platelet function and blood pressure in normotensive rats and spontaneously hypertensive rats (SHRs). Platelets were obtained from blood on the 20th day of pregnancy from female SHRs (SHR-P) and normotensive controls (P) or age-matched nonpregnant rats (SHR-NP and NP). Intraplatelet NO synthase (NOS) activity was reduced in P compared to NP, despite unchanged L-arginine influx. The expression levels of endothelial NOS (eNOS) and inducible NOS (iNOS) were diminished during pregnancy in normotensive rats. Paradoxically, cyclic guanosine monophosphate (cGMP) levels were similar between NP and P, as were phosphodiesterase type 5 (PDE5) expression and platelet aggregation induced by adenosine diphosphate. In SHRs, L-arginine influx was reduced in SHR-P compared to SHR-NP. SHR-P exhibited impaired NOS activity and reduced iNOS expression compared with SHR-NP. Soluble guanylyl cyclase and PDE5 expression in platelets were lower in SHR-P than in SHR-NP, whereas no differences were noted between groups with respect to cGMP levels. However, increased levels of cGMP were observed in SHR-P compared to normotensive groups and platelet aggregability remained unaltered. In conclusion, these observations prompted the hypothesis that normal platelet aggregation in pregnant SHRs may be related to a reduction in PDE5 expression and consequently the maintenance of cGMP levels, independently of reduced platelet NO bioavailability.

Hemostatic and signaling functions of transfused platelets

Metabolic studies have revealed a gradual impairment in platelet integrity during storage, a process termed the platelet storage lesion. Recent evidence shows that stored platelets also lose signaling responses to physiological agonists with impaired integrin activation, secretion, and aggregation of the cells. On the other hand, storage leads to a gain in platelet activation properties, such as release of microparticles and appearance of surface epitopes for their clearance by macrophages. New techniques for measuring flow-induced thrombus formation and platelet-dependent coagulation provide evidence that the hemostatic activity of platelets decreases during storage. Besides pharmacological inhibition, novel storage strategies, like metabolic suppression, should be considered to better preserve platelet functionality while limiting the expression of clearance markers. Understanding the changes that occur in association with the platelet storage lesion and the use of updated storage methods will help to generate platelets for transfusion with optimal hemostatic function and a long circulation time after transfusion.

Prostaglandin A1 inhibits increases in intracellular calcium concentration, TXA(2) production and platelet activation

AIM

In our previous studies we found that cyclopentenane prostaglandin A1 (PGA1) had neuroprotective effects in a rodent ischemic model. In the present study we aimed to investigate the inhibitory effect of PGA1 on platelet function.

METHOD

The rate of aggregation of human platelets was measured by using turbidimetry. The rate of adhesion of platelets to cultured endothelial cells was determined by using [(3)H]-adenine labeled platelets. 5-Hydroxytryptamine release from platelets was measured with O-phthaldialdehyde fluorospectrophotometry. The levels of TXB(2), a stable metabolite of TXA(2), were determined by radioimmuno-assay. Alternations in platelet morphology were observed using an electron microscope, and the intraplatelet free calcium concentrations were measured with Fluo-3/AM FCM assay.

RESULTS

PGA1 significantly inhibited thrombin-, collagen- and ADP-induced aggregation and adhesion of platelets. The morphological changes of platelets induced by thrombin were blocked by PGA1. PGA1 inhibited the release of 5-hydroxytyptamine from dense granules and the synthesis of TXA(2).

CONCLUSION

PGA1 inhibits the activation of platelets probably through blocking increases in intracellular calcium concentration and TXA(2) synthesis.

The role of the platelet in the pathogenesis of atherothrombosis

Platelet adhesion, activation, and aggregation at sites of vascular endothelial disruption caused by atherosclerosis are key events in arterial thrombus formation. Platelet tethering and adhesion to the arterial wall, particularly under high shear forces, are achieved through multiple high-affinity interactions between platelet membrane receptors (integrins) and ligands within the exposed subendothelium, most notably collagen and von Willebrand factor (vWF). Platelet adhesion to collagen occurs both indirectly, via binding of the platelet glycoprotein (GP) Ib-V-IX receptor to circulating vWF, which binds to exposed collagen, and directly, via interaction with the platelet receptors GP VI and GP Ia/IIb. Platelet activation, initiated by exposed collagen and locally generated soluble platelet agonists (primarily thrombin, ADP, and thromboxane A2), provides the stimulus for the release of platelet-derived growth factors, adhesion molecules and coagulation factors, activation of adjacent platelets, and conformational changes in the platelet alpha(IIb)beta3 integrin (GP IIb/IIIa receptor). Platelet aggregation, mediated primarily by interaction between the activated platelet GP IIb/IIIa receptor and its ligands, fibrinogen and vWF, results in the formation of a platelet-rich thrombus. Currently available antiplatelet drugs (aspirin [acetylsalicylic acid], dipyridamole, clopidogrel, ticlopidine, abciximab, eptifibatide, tirofiban) act on specific targets to inhibit platelet activation and aggregation. Elucidation of the multiple mechanisms involved in platelet thrombus formation provides opportunities for selectively inhibiting the pathways most relevant to the pathophysiology of atherothrombosis.

Reactive Oxygen Species

Platelets participate not only in thrombus formation but also in the regulation of vessel tone, the development of atherosclerosis, angiogenesis, and in neointima formation after vessel wall injury. It is not surprising, therefore, that the platelet activation cascade (including receptor-mediated tethering to the endothelium, rolling, firm adhesion, aggregation, and thrombus formation) is tightly regulated. In addition to already well-defined platelet regulatory factors, such as nitric oxide (NO), prostacyclin (PGI2), and adenosine, reactive oxygen species (ROS) participate in the regulation of platelet activation. Although exogenously derived ROS are known to affect the regulation of platelet activation, recent data suggest that the platelets themselves generate ROS. Intracellular ROS signaling in activated platelets could be of significant relevance after transient platelet contact with the vessel wall, during the recruitment of additional platelets, and in thrombus formation. This review discusses the potential cellular and enzymatic sources of ROS in platelets, their molecular mechanisms of action in platelet activation, and summarizes in vitro and in vivo evidence for their physiological and potential therapeutic relevance.

Enhanced in vivo platelet adhesion in vasodilator-stimulated phosphoprotein (VASP)-deficient mice

Platelet adhesion and activation at the vascular wall are the initial steps leading to arterial thrombosis and vascular occlusion. Prostacyclin and nitric oxide inhibit platelet adhesion, acting via cyclic adenosine monophosphate (cAMP)- and cyclic guanosine monophosphate (cGMP)-dependent protein kinases. A major downstream target for both cAMP- and cGMP-dependent protein kinases is the vasodilator-stimulated phosphoprotein (VASP). To test the significance of VASP for the regulation of platelet adhesion in vivo, we studied platelet-vessel wall interactions using VASP-deficient (VASP-/-) mice. Under physiologic conditions, platelet adhesion to endothelial cells was significantly enhanced in VASP null mutants when compared with wild-type mice (P <.05). Platelet recruitment in VASP null mice involved P-selectin and the fibrinogen receptor glycoprotein IIb-IIIa (GPIIb-IIIa). Under pathophysiologic conditions, the loss of VASP increased platelet adhesion to the postischemic intestinal microvasculature, to the atherosclerotic endothelium of ApoE-deficient mice, and to the subendothelial matrix following endothelial denudation (P <.05 vs wild type). Importantly, platelet adhesion in VASP null mutants was unresponsive to nitric oxide. These data show for the first time in vivo that VASP is involved in down-regulation of platelet adhesion to the vascular wall under both physiologic and pathophysiologic conditions.

Hypotension caused by extracorporeal circulation: serotonin from pump-activated platelets triggers nitric oxide release

BACKGROUND

Cardiopulmonary bypass and hemodialysis often cause hypotension. We investigated a possible role of pump-induced platelet activation with consequent serotonin release.

METHODS AND RESULTS

In rats, a heparin-coated extracorporeal shunt was placed between the proximal part of a carotid artery and the distal part of a femoral artery. Autoperfusion did not affect platelets or hemodynamics. Pump perfusion, however, immediately elicited strong platelet aggregation, whereas aortic pressure rapidly fell to 60+/-12% (mean+/-SD) of its prepump value, partially recovered, and then progressively decreased to 70+/-12% at 2 hours. Femoral resistance doubled and then decreased to 59+/-11%. The initial changes in aortic pressure and femoral resistance were proportional to the amount of platelet aggregation, were accompanied by a rise (6-fold) in plasma serotonin levels downstream of the pump, but not in the aorta, and could be mimicked by serotonin-infusion into the leg. All hemodynamic changes were prevented or largely reduced by blockade of 5-hydroxytryptamine (5-HT)2 receptors with pizotifen or ritanserin. The hypotension and femoral resistance decrease could also be prevented or abolished by inhibiting the production of nitric oxide (NO), an intermediate in 5-HT(2B) receptor-induced vasodilation. When the extracorporeal blood was pumped into the aortic arch instead of the femoral artery, the hypotensive effect was similar and also NO dependent, but it was absent with venous return.

CONCLUSIONS

Pump perfusion with arterial return of the blood causes hypotension by endothelial NO-release, which in turn is triggered by serotonin from activated platelets.

Effect of the ingestion of Ginkgo biloba extract on platelet aggregation and urinary prostanoid excretion in healthy and Type 2 diabetic subjects

Enhanced platelet function, particularly in response to collagen, is a common occurrence in diabetes that increases the risk of cardiovascular disease. Ginkgo biloba extract is ingested primarily to improve mental focus but it possesses a blood-thinning potential, which has not been well characterized. This study was designed to compare the effect of ingesting G. biloba extract on platelet aggregation in platelet-rich plasma (PRP) and prostanoid urinary excretion in healthy volunteers and subjects with Type 2 diabetes mellitus (T2DM). Before and after ingesting 120 mg of standardized G. biloba extract for 3 months, platelet aggregation was studied in PRP and urinary metabolites of thromboxane B(2) (TXB(2)) and prostacyclin (PGI(2)) were measured. In healthy volunteers (age, 42+/-11 years; BMI, 28.4+/-4.8 kg/m(2); n=28), the ingestion of G. biloba extract significantly increased fasting insulin and C-peptide (10+/-4 vs. 12+/-6 microU/ml, p<0.007 and 1.3+/-0.8 vs. 2.1+/-1.1 ng/ml, p<0.001, respectively) and significantly reduced collagen but not PAF-mediated platelet aggregation, converting 21 of 28 subjects with [COL+/EPI+] platelets to the [COL-/EPI+] phenotype. This was accompanied by a reduction of 11-dehydro-TXB(2) from 12.4+/-6.1 to 10.3+/-6.1 ng/mg Cr (p<0.04) and PGI(2) metabolites (2,3-dinor-6-keto-PGF(1alpha) and 6-keto-PGF(1alpha)) from 2.2+/-0.8 to 1.8+/-0.8 ng/mg Cr (p<0.05). In the T2DM subjects (age, 54+/-8; BMI, 36.6+/-7.9 kg/m(2); n=19), G. biloba ingestion did not affect pancreatic beta-cell function but significantly reduced platelet aggregation, converting 16 of 19 [COL+/EPI+] platelets to the [COL-/EPI+] phenotype. Unlike the healthy volunteers, this was not accompanied by a reduced urinary prostanoid excretion. G. biloba-induced reduction of both classes of prostanoid metabolites in healthy volunteers, but not in T2DM subjects, may suggest a nonselective inhibition of COX-1-mediated TXA(2) in platelets and COX-2-mediated PGI(2) production by the endothelial cells and perhaps platelet-enriched levels of arachidonic acid or COX-1 activity, or both, in T2DM subjects.

Role of endothelial dysfunction in coronary artery disease

The vascular endothelium plays an essential role in regulating blood flow and other functions of the coronary arteries. Under normal conditions, the endothelium releases a number of factors that regulate arterial vasomotion. One of these factors, endothelial-derived relaxing factor (EDRF), is a vasorelaxant that is identical to, or closely related to, nitric oxide (NO). Endothelial function may be compromised in coronary artery disease (CAD) or in the presence of risk factors for CAD. In this setting, EDRF-NO activity is inhibited, which may lead to vasoconstriction, platelet aggregation, vasospasm, and thrombosis. The relation between endothelial function, arterial vasomotion, and myocardial ischemia is discussed, with particular emphasis on the role of EDRF-NO, and the therapeutic interventions that may be useful in treating the clinical consequences of endothelial dysfunction.

Ultrastructural localisation of nitric oxide synthase, endothelin and binding sites of lectin (from Bandeirea simplicifolia) in the rat carotid artery after balloon catheter injury

An immunocytochemical and cytochemical study has been made on the ultrastructural localisation of type III (endothelial) nitric oxide synthase, endothelin-1 and the binding sites of lectin from Bandeirea simplicifolia to the endothelium surface-associated glycoproteins in the rat left common carotid artery at 1 and 28 d after Fogarty embolectomy balloon catheter-induced injury. Controls were carotid arteries from sham operated rats. In the controls, the immunoreactivity to nitric oxide synthase-III and endothelin-1 was localised in different proportions in vascular endothelial cells (36.9% +/- 4.3 and 7.6% +/- 2.7, respectively); immunoreactivity was confined to the cytoplasm and the membranes of intracellular organelles and structures. In contrast, staining with lectin was localised on the luminal surface of all endothelial cells. 1 d after injury, platelets were adherent to the endothelium-denuded intima. Some of the platelets displayed, immunoreactivity to nitric oxide synthase-III and endothelin-1 and were stained with lectin. 28 d after injury, a neointimal thickening of substantial size was present. Subpopulations of the regrown endothelial cells covering the luminal surface of the neointima showed positive immunoreactivity to nitric oxide synthase-III and endothelin-1 but there was a significant decrease in the proportion of nitric oxide synthase-III-containing endothelial cells (17.2% +/- 1.9; P < 0.001) and a significant increase in the proportion of endothelin-1-containing endothelial cells (36.9% +/- 4.7; P < 0.001) compared with the controls. Staining with lectin was associated with the cell membrane of all endothelial cells and in addition with cells located 'deeper' in the neointima which showed lectin-positive plasmalemma, Golgi complex and multivesicular bodies/lysosomes. In conclusion, regenerated endothelial cells of the neointima showed reduced population (2-fold) of nitric oxide synthase-III-and increased population (5-fold) endothelin-1-positive cells. The subendothelial location of some lectin-stained cells after balloon catheter injury indicates the heterogeneity of the neointima and suggests that some of these cells are involved in early angiogenesis. 24 h and 28 d after injury some platelets showed positive immunoreactivity for nitric oxide synthase-III and endothelin-1.

Effect of diadenosine phosphates on human umbilical vessels: novel platelet-derived vasoconstrictors

The responses of human isolated umbilical artery to the novel platelet-derived mediators, diadenosine phosphates, P1,P5-di(adenosine-5')pentaphosphate (AP5A) and P1,P6-di(adenosine-5')hexaphosphate (AP6A) were studied in vitro. AP5A (1 nM-300 microM; n = 7) and AP6A (1 nM-30 microM; n = 5) produced transient concentration-dependent contractions. Responses to AP5A were unaffected by co-incubation with indomethacin (10 microM; n = 4), NGmonomethyl-L-arginine (L-NMMA; 300 microM; n = 4) and theophylline (1 microM; n = 5). These results demonstrate that diadenosine phosphates have constrictor effects on human blood vessels in vitro and are consistent with a role for these platelet-derived mediators in the regulation of blood vessel tone.