Nitric oxide synthase inhibition and platelet function

Honey and cardiovascular risk factors, in normal individuals and in patients with diabetes mellitus or dyslipidemia

Diabetes mellitus, hypercholesteremia, hypertension (HTN), and obesity are well-known risk factors for cardiovascular diseases (CVD). Various medications are currently in use for management of these comorbidities. Undesirable side effects are unavoidable and the ultimate and ideal goal is hardly achieved. Honey and other bee products are widely used in traditional medicine for management of many diseases. Others and the authors have found potent biological activities of these products. Honey is now reintroduced in modern medicine as part of wound and burn management. Honey has antioxidant, anti-inflammatory, and antimicrobial activities. More studies are exploring other aspects of honey activity such as its effect on blood sugar, body weight, lipid profile, C-reactive protein, nitric oxide, proinflammatory prostaglandins, and homocysteine. Growing evidence and scientific data support the use of honey in patients with diabetes, HTN, dyslipidemia, obesity, and CVD. This review discusses clinical and preclinical studies on potential influence of honey on diabetes mellitus and cardiovascular risk factors, and emphasizes the importance of conducting more clinical and controlled studies.

Endothelin attenuates endothelium-dependent platelet inhibition in man

AIM

The vascular endothelium produces several substances, including nitric oxide (NO) and endothelin-1 (ET-1), which participate in the regulation of vascular tone in humans. Both these substances may exert other actions of importance for cardiovascular disease, e.g. effects on vascular smooth muscle cell proliferation and inflammation, and NO inhibits platelet function. Experiments were designed to investigate the effect of ET-1 on endothelium-dependent vasodilatation and attenuation of platelet activation.

METHODS

In 25 healthy male subjects (25 +/- 1 years), forearm blood flow was measured by venous occlusion plethysmography, and platelet activity was assessed by whole blood flow cytometry (platelet fibrinogen binding and P-selectin expression) in unstimulated and adenosine diphosphate (ADP)-stimulated samples during administration of ET-1, the endothelium-dependent vasodilator acetylcholine and the NO synthase inhibitor l-NMMA.

RESULTS

Acetylcholine increased forearm blood flow and significantly inhibited platelet activation in both unstimulated and ADP-stimulated samples. In samples stimulated with 0.3 microm ADP, fibrinogen binding decreased from 41 +/- 4% to 31 +/- 3% (P < 0.01, n = 11) after acetylcholine administration. The vasodilator response to acetylcholine was significantly impaired during infusions of ET-1 and l-NMMA. ET-1 did not affect platelet activity per se, whereas l-NMMA increased platelet P-selectin expression. Both ET-1 and l-NMMA attenuated the acetylcholine-induced inhibition of platelet activity.

CONCLUSIONS

Our study indicates that, further to inhibiting endothelium-dependent vasodilatation, ET-1 may also attenuate endothelium-dependent inhibition of platelet activation induced by acetylcholine. An enhanced ET-1 activity, as suggested in endothelial dysfunction, may affect endothelium-dependent platelet modulation and thereby have pathophysiological implications.

Platelet activation in allergic asthma patients during allergen challenge with Dermatophagoides pteronyssinus

BACKGROUND

Animal models of allergic asthma indicate that intravascular platelet activation is necessary for the development of allergen-induced chronic airway inflammation.

OBJECTIVE

To evaluate whether the development of a late asthmatic response (LAR) in allergic asthma patients challenged with a relevant allergen is consequent to platelet activation.

METHODS

Thirty-three house dust mite sensitive asthmatic patients were challenged intrabronchially with Dermatophagoides pteronyssinus (Dp) extract. Twelve non-atopic healthy subjects (HC) were used as controls. Platelet count and plasma levels of beta-thromboglobulin (beta-TG), platelet factor-4 (PF-4) and soluble P-selectin (sP-selectin) were assessed before the challenge (T(0)) and 30 min (T(EAR)), 6 h (T(LAR)) and 24 h (T(24)) after the challenge.

RESULTS

Eleven patients responded to allergen challenge with an isolated early asthmatic response (single responders, SR). In 22 patients dual asthmatic response was demonstrated (dual responders, DR). At T(0) neither the platelet count nor the mean plasma level of beta-TG in DR or SR were different from HC, the mean plasma level of PF-4 in SR was significantly greater than in HC (P=0.01) or DR (P=0.001), the mean plasma level of sP-selectin was significantly greater in DR than in HC (0.0002) but not statistically different from SR (P=0.055). A significant decrease in the platelet count and increase in the plasma level of all the studied markers was seen at T(EAR), which was followed by a gradual return to the baseline values in the SR. Elevated plasma levels of platelet activation markers and decreased platelet count were seen in the DR even at T(24). Strong correlation was found between the increase in plasma concentration of beta-TG at T(EAR) and the maximum fall in forced expiratory volume in 1 s at T(LAR) (r=-0.57; P=0.0006).

CONCLUSION

In allergic asthma patients development of prolonged airway inflammation after allergen challenge is associated with intravascular platelet activation.

Nitric oxide and its relationship to thrombotic disorders

Nitric oxide (NO) is released by the endothelium preventing platelet adhesion to the vessel wall. When released by platelets, NO inhibits further recruitment of platelets to a growing thrombus. Modulation of endogenous NO release may be a mechanism by which the thrombotic response can be regulated as suggested by several clinical diseases associated with impaired bioactive NO. Diseases including atrial fibrillation and coronary atherothrombotic disease have been associated with impaired NO release or decrease in NO bioavailability.

Isosorbide-5-mononitrate treatment prevents cyclosporin A-induced platelet hyperactivation and the underlying nitric oxide-cyclic guanosine-3',5'-monophosphate disturbances

INTRODUCTION

The clinical use of cyclosporin A (CsA) is commonly associated with the development of hypertension and increased risk of thromboembolic events. Decreased endothelium-dependent relaxation and increased platelet activation seems to be involved on those side effects, but the underlying mechanisms are not yet elucidated. The present study aimed to evaluate the CsA effect on the platelet NO-cyclic guanosine-3',5'-monophosphate (cGMP) pathway and the putative benefits of concomitant isosorbide-5-mononitrate (IS-5-MN) administration on CsA-induced hypertension and on platelet hyperactivation.

MATERIALS AND METHODS

Blood pressures, platelet NO synthase activity and cGMP content, intracellular free calcium concentration ([Ca2+]i) and whole blood platelet aggregation were assessed in three rat groups orally treated, during 7 weeks, with the following diets: orange juice (control group), 5 mg/kg/day of CsA (CsA group) and 150 mg/kg/day, b.i.d., of IS-5-MN for 2 weeks and IS-5-MN plus 5 mg/kg/day of CsA for 7 weeks (IS-5-MN+CsA group).

RESULTS

IS-5-MN treatment has prevented hypertension development obtained in the solely CsA-treated rats. CsA treatment has inhibited NOS activity, which was reverted by the concomitant IS-5-MN and CsA administration. On the contrary, platelets from CsA-treated rats had cGMP content increased when compared with the control rats. The variation obtained when ISMN was present was less predominant. Therefore, the organic nitrate treatment has prevented platelet hyperactivation, namely, by decreasing thrombin-evoked [Ca2+]i and collagen-evoked platelet aggregation, when compared with the solely CsA-treated group. The preventive effect of IS-5-MN was reinforced by electron microscopy studies of platelet activation.

CONCLUSIONS

By increasing [Ca2+]i and aggregation, CsA induces platelet hyperactivation and simultaneously increases cGMP content, which might represent a compensatory inhibitory mechanism. The concomitant IS-5-MN treatment prevents the above-mentioned platelet hyperreactivity and tends to normalize the NO-cGMP pathway as well as the development of hypertension.

Vasoactive substances: nitric oxide and endothelial dysfunction in atherosclerosis

The endothelium synthesizes and releases nitric oxide (NO) to maintain homeostatic function. Under basal conditions, endothelium-derived NO maintains a nonthrombogenic surface, prohibits leukocyte attachment, and promotes vascular relaxation. In the setting of clinical syndromes associated with the development of atherothrombosis, there is decreased bioavailable NO owing to diminished synthesis and release in addition to increased generation of reactive oxygen species. These biochemical changes perturb significantly vascular homeostatic mechanisms and promote platelet aggregation, inflammatory cell diapedesis, and vasoconstriction. Endothelial dysfunction may be evaluated using invasive and/or noninvasive techniques, including coronary artery reactivity to acetylcholine and brachial artery ultrasonography, respectively. NO replacement therapies may be initiated to restore deficits associated with dysfunctional endothelium. Strategies to replenish bioavailable NO include the administration of organic nitrosovasodilators or NO donor compounds, therapies to improve NO synthase function, and gene therapy.

Effect of nitric oxide modulation on systemic haemodynamics and platelet activation determined by P-selectin expression

Inhibiting platelet and endothelial nitric oxide production favours platelet adhesion and aggregation, and arterial vasoconstriction. This study investigated the effect of NG-nitro-l-arginine methyl ester (L-NAME), a stereospecific inhibitor of nitric oxide synthesis, on P-selectin expression on platelets, platelet-derived microparticles and platelet-leucocyte aggregates, and on soluble P-selectin levels. Twelve healthy male volunteers were infused intravenously with L-NAME and then with a 10% solution of either l- or d-arginine. Blood pressure responses were recorded and whole blood and serum collected at baseline and after each infusion. P-selectin expression was analysed in all samples by flow cytometry. Serum levels of soluble P-selectin were batch analysed using an enzyme-linked immunosorbent assay at the end of the study. P-selectin expression on platelets, platelet-derived microparticles and platelet-leucocyte aggregates did not vary significantly from baseline levels following the infusion of L-NAME or l- or d-arginine. However, endothelial nitric oxide synthase inhibition caused a marked elevation of arterial blood pressure (P < 0.01) that was restored to pretreatment values by l- but not d-arginine. Serum levels of the soluble form decreased significantly (P = 0.001) following the infusion of l- and d-arginine compared with samples taken at baseline and following L-NAME infusion. In conclusion, inhibition of constitutive nitric oxide synthase in the endothelium and platelets produced significant increases in blood pressure but did not alter platelet membrane expression of P-selectin.

Role of the recombinant protein of the platelet receptor for type I collagen in the release of nitric oxide during platelet aggregation

Nitric oxide plays an important role in platelet function and platelets possess the endothelial isoform of nitric oxide synthase. Several reports have indicated that nitric oxide is released upon exposure of platelets to collagen. We have reported that a non-integrin platelet protein of 65 kDa is a receptor for type I collagen. By direct measurement of NO release from washed human platelets suspended in Tyrode buffer with a ISO-NO Mark II, World Precision Instruments, Sarasota, FL, USA, p30 sensor, type I collagen, but not ADP and epinephrine, induces the release of NO in a time-dependent manner. The production of NO is inhibited either by preincubation of type I collagen with the platelet type I collagen receptor recombinant protein or by preincubation of platelets with the antibody to the receptor protein, the anti-65 antibody. However, preincubation of platelets with anti-P-selectin and anti-glycoprotein IIb/IIIa did not affect the release of NO by platelets. These results suggest that the 65 kDa platelet receptor for type I collagen is specifically linked to the generation of NO, and that the 65 kDa platelet receptor for type I collagen plays an important new role in platelet function.

Nitric oxide protects the ultrastructure of pancreatic acinar cells in the course of caerulein-induced acute pancreatitis

Nitric oxide (NO) as a unique biological mediator that has been implicated in many physiological and pathophysiological processes may have a significant influence on the course of acute pancreatitis and the recovery process. The aim of the study was to evaluate the effect of a NO synthase inhibitor or a substrate for NO endogenous production on the ultrastructural features of the acinar cells in the course of caerulein-induced acute pancreatitis. Acute pancreatitis was induced in the rats by a supramaximal dose of caerulein. During acute pancreatitis induction, the rats were treated with L-arginine (the substrate for NO synthesis), NG-nitro-L-arginine (L-NNA, NO synthase inhibitor), L-arginine + L-NNA or saline. Light and electron microscopy examinations were performed in all groups after pancreatitis induction and additionally after 7 and 14 days of recovery. The study demonstrated that the NO synthase inhibitor given during pancreatitis induction in rats enhances the damage to the acinar cells, detected ultrastructurally, and increases the cellular inflammatory infiltration. In the later period, the considerable damage to the mitochondria and the changes in secretory compartment were observed, including dilated cisternae of Golgi apparatus, focal degranulation of rough endoplasmic reticulum, and reduced number of zymogen granules and condensing vacuoles. L-arginine reversed to some extent the deleterious effect of L-NNA, although when administered alone it had no apparent effect on the ultrastructure of pancreatic acinar cells compared with untreated animals. The obtained results indicate that the NO synthase inhibitor enhances the ultrastructural degenerative alterations in the pancreatic acinar cells in the course of caerulein-induced acute pancreatitis and confirm the protective role of endogenous nitric oxide in this disease.

Deficient platelet-derived nitric oxide and enhanced hemostasis in mice lacking the NOSIII gene

Endothelial nitric oxide synthase (eNOS) has been identified in human platelets. Although platelet-derived nitric oxide (NO) has been shown to inhibit platelet recruitment in vitro, its role in the regulation of the hemostatic response in vivo has not been characterized. To define the role of platelet-derived NO in vivo, we studied mice that lacked a functional eNOS gene (NOSIII). Surface P-selectin expression in platelets from eNOS-deficient mice was not significantly altered; however, bleeding times were markedly decreased in eNOS-deficient versus wild-type mice (77.2+/-3 versus 133.4+/-3 seconds, P<0.00005). To determine the contribution of endothelium- versus platelet-derived NO to the bleeding time, isolated platelets from either eNOS-deficient or wild-type mice were transfused into a thrombocytopenic eNOS-deficient mouse and the bleeding time was measured. The bleeding times in mice transfused with eNOS-deficient platelets were significantly decreased compared with mice transfused with wild-type platelets (Deltableeding time, -24.6+/-9.1 and -3.4+/-5.3 seconds, respectively; P<0.04). Platelet recruitment was studied by measuring serotonin release from a second recruitable population of platelets that were added to stimulated platelets at the peak of NO production. There was 40.3+/-3.7% and 52. 0+/-2.1% serotonin release for platelets added to wild-type or eNOS-deficient platelets, respectively (P<0.05). In summary, mice that lacked eNOS had markedly decreased bleeding times even after endothelial NO production was controlled. These data suggest that the lack of platelet-derived NO alters in vivo hemostatic response by increasing platelet recruitment. Thus, these data support a role for platelet-derived NO production in the regulation of hemostasis.

Impaired platelet production of nitric oxide predicts presence of acute coronary syndromes

BACKGROUND

Thrombus formation within a coronary vessel is the acute precipitating event in most acute coronary syndromes. Recently, constitutive nitric oxide synthase (cNOS) has been identified in human platelets, and platelet-derived nitric oxide has been shown to inhibit platelet recruitment after aggregation. However, its role in regulating platelet responses under normal or pathologic conditions has not yet been elucidated.

METHODS AND RESULTS

We examined nitric oxide (NO) production by platelets isolated from 87 patients undergoing coronary angiography, 37 with stable angina and 50 with unstable angina or a myocardial infarction within 2 weeks. After stimulation with 5 micromol/L ADP, platelet aggregation and NO production were simultaneously measured with an NO-selective microelectrode adapted for use in a standard platelet aggregometer. Mean (+/-SEM) platelet-derived NO production was 1.78+/-0.36 pmol/10(8) and 0.26+/-0.05 pmol/10(8) platelets in coronary patients with stable angina and acute coronary syndromes, respectively (P=0. 0001). By logistic regression analysis, heparin treatment (odds ratio 6.6, CI 1.9 to 22.8, P=0.003), lower platelet-NO production (odds ratio 4.0, CI 1.3 to 11.5, P=0.01), and extent of atherosclerosis (odds ratio 1.5, CI 1.1 to 2.0, P=0.02) were independent predictors of an acute coronary syndrome. In the subset of patients with angiographic evidence of atherosclerosis (n=83), logistic regression demonstrated that platelet NO production (odds ratio 3.9, CI 1.3 to 11.1, P=0.01) and heparin treatment (odds ratio 6.4, CI 1.9 to 22.0, P=0.004) were independent predictors of an acute coronary syndrome, whereas extent of atherosclerosis was not.

CONCLUSIONS

In summary, aggregating platelets from patients with acute coronary syndromes produce less NO. Since platelet aggregation and thrombus formation are implicated in unstable angina and myocardial infarction, impaired platelet-derived NO production may contribute to the development of acute coronary syndromes.

Nitric oxide and endothelin in the development of cardiac allograft vasculopathy. Potential targets for therapeutic interventions

Extensive research has been carried out in recent years to discover the potential risk factors contributing to cardiac allograft atherogenesis. Injury to endothelial cells has been regarded as an important early mechanism in the development of transplant atherosclerosis; it leads to the manifestation of epicardial and microvascular endothelial dysfunction and development of intimal hyperplasia. Moreover, continuous minor endothelial cell damage contributes to endothelial dysfunction which reflects one of the first measurable steps in the cascade of atherogenesis without macroscopic evidence of vascular lesions. The discovery of two important vasoactive substances nitric oxide (NO) and endothelin (ET) has brought new insights but also new unsolved questions regarding the mechanisms leading to atherosclerosis. To date it is known that both substances play a major role in both prevention and development of atherosclerosis. NO appears to be protective in low concentrations by inhibiting leukocyte and platelet activation/adherence and smooth muscle cell proliferation. Impaired endothelial NO production, as one cause of endothelial dysfunction may occur in early stages of atherosclerosis before macroscopic lesions are evident. In addition, increased endothelin release also results in endothelial dysfunction by inducing vasoconstriction; it promotes vascular lesion formation due to endothelial- and vascular smooth muscle cell proliferation. Direct and indirect manipulation of both the NO and ET signal transduction systems may provide novel preventive and therapeutic approaches for limiting transplant atherogenesis and to treat native atherosclerosis. This review summarizes important experimental and clinical evidence which points to nitric oxide and endothelin as potential therapeutic targets in the process of cardiac allograft vasculopathy.

Nitric oxide and platelet function: implications for neonatology

Nitric oxide (NO) is a mediator that modulates vessel wall tone and hemostatic-thrombotic balance. Platelet function is regulated by NO generated from platelets, endothelial cells and leukocytes. Nitric oxide has been shown to inhibit platelet adhesion, aggregation, and stimulate disaggregation of preformed platelet aggregates. Many of the effects of NO are mediated by its stimulation of guanylate cyclase and the formation of cyclic GMP and its subsequent transduction mechanism. In vivo, NO is likely to interact with prostacyclin, metabolites of ecto-nucleotidase, and lipoxygenase to modulate platelet function in a synergistic manner. An imbalance of NO production (deficiency or overproduction) has been implicated in the pathogenesis of various vascular disorders including thrombosis, atherosclerosis, septicemia, and ischemia-reperfusion injury. It is likely that some of detrimental effects of NO are mediated through its reaction with superoxide anion to form the potent oxidant, peroxynitrite. Nitric oxide gas and NO donors are used for the pharmacological treatment of various vascular disorders. Because inhaled NO has been documented to improve systemic oxygenation and reduce the need for extracorporeal membrane oxygenation, it has been widely used in neonates with severe hypoxemia. An inhibition of platelet function, resulting in a prolonged bleeding time, has been shown in adults receiving inhaled NO. Because bleeding complications may occur in high-risk infants, it is important to evaluate the effect of inhaled NO on platelet function and its correlation with clinical consequences such as intracranial hemorrhage. For these reasons, hemostasis should be carefully monitored during the administration of inhaled NO to critically ill neonates.

Nitric oxide released from activated platelets inhibits platelet recruitment

Vessel injury and thrombus formation are the cause of most ischemic coronary syndromes and, in this setting, activated platelets stimulate platelet recruitment to the growing thrombus. Recently, a constitutive nitric oxide synthase (NOS) has been identified in human platelets. To further define the capacity of platelets to produce nitric oxide (NO), as well as to study the role of this NO in platelet recruitment, we adapted a NO-selective microelectrode for use in a standard platelet aggregometer, thereby permitting simultaneous measurement of platelet aggregation and NO production. Treatment of platelets with the NO synthase inhibitor -NG-nitroarginine methyl ester (L-NAME), reduced NO production by 92+/-8% in response to 5 microM ADP compared to control but increased aggregation by only 15+/-2%. In contrast, L-NAME had a more pronounced effect on platelet recruitment as evidenced by a 35+/-5% increase in the extent of aggregation, a 33+/-3% decrease in cyclic GMP content, and a 31+/-5% increase in serotonin release from a second recruitable population of platelets added to stimulated platelets at the peak of NO production. To study platelet recruitment accurately, we developed an assay that monitors two platelet populations simultaneously. Nonbiotinylated platelets were incubated with L-NAME or vehicle and activated with ADP. At peak NO production, biotinylated platelets were added. As measured by three-color flow cytometry, there was a 56+/-11% increase in the number of P selectin- positive platelets in the nonbiotinylated population treated with L-NAME as compared to control. When biotinylated platelets were added to the L-NAME-treated nonbiotinylated population, the number of P selectin positive biotinylated plate-lets increased by 180+/-32% as compared to biotinylated platelets added to the control. In summary, stimulated platelets produce NO that modestly inhibits platelet activation but markedly inhibits additional platelet recruitment. These data suggest that platelet-derived NO may regulate platelet recruitment to a growing thrombus.

Nitric oxide--biological mediator, modulator and factor of injury: its role in the pathogenesis of atherosclerosis

Nitric oxide (NO) is generated from L-arginine by the family of isoenzymes called NO synthases (NOS). Gene cloning has identified neuronal, endothelial and cytokine-inducible isoforms of NOS. The effects of NO depend on its microenvironment and result from interactions with oxygen, heme proteins and thiols. NO regulates vascular homeostasis by controlling vascular resistance, blood pressure, cell-cell contact and proliferation. Atherogenesis leads to decreased bioactivity of NO and this, in turn, can precipitate enhanced cell adhesion, proliferation, vasoconstriction and accelerate the generation of atherosclerotic lesions. It is possible that some of the detrimental effects of atherosclerosis on the NO pathway result from the generation of secondary oxidants such as peroxynitrite, a product of the reaction of NO with superoxide. The pharmacologic strategies including the stimulation of generation of endogenous NO, NO-replacement therapy and decreasing oxidative stress may be useful for ameliorating the clinical course of atherosclerosis.