Tetrahydrobiopterin improves endothelial function in human saphenous veins

Minimizing Hyperglycemia-Induced Vascular Endothelial Dysfunction by Inhibiting Endothelial Sodium-Glucose Cotransporter 2 and Attenuating Oxidative Stress: Implications for Treating Individuals With Type 2 Diabetes

This overview deals with mechanisms whereby hyperglycemia-induced oxidative stress compromises vascular endothelial function and provides a background for a recently published study illustrating the beneficial impact of endothelial sodium-glucose cotransporter 2 (SGLT2) inhibitors in attenuating hyperglycemia-induced vascular dysfunction in vitro. The data provide new insight that can possibly lead to improved drug therapy for people with type 2 diabetes. The working hypotheses that underpinned the experiments performed are provided, along with the findings of the study. For the causes of hyperglycemia-induced vascular endothelial dysfunction, the findings point to the key roles of: 1) functional endothelial SGLT2; 2) oxidative stress-induced signalling pathways including mammalian sarcoma virus kinase, the EGF receptor-kinase and protein kinase C; and 3) mitochondrial dysfunction triggered by hyperglycemia was mitigated by an SGLT2 inhibitor in the hyperglycemic mouse aorta vascular organ cultures. The overview sums up the approaches implicated by the study that can potentially counteract the detrimental impact of hyperglycemia on vascular function in people with diabetes, including the clinical use of SGLT2 inhibitors for those with type 2 diabetes already being treated, for example, with metformin, along with dietary supplementation with broccoli-derived sulforaphane and tetrahydrobiopterin. The caveats associated with the study for extending the findings from mice to humans are summarized, pointing to the need to validate the work using vascular tissues from humans. Suggestions for future clinical studies are made, including the assessment of the impact of the therapeutic strategies proposed on measurements of blood flow in subjects with diabetes.

Neuronal nitric oxide synthase-derived hydrogen peroxide effect in grafts used in human coronary bypass surgery

Recently, H₂O₂ has been identified as the endothelium-dependent hyperpolarizing factor (EDHF), which mediates flow-induced dilation in human coronary arteries. Neuronal nitric oxide synthase (nNOS) is expressed in the cardiovascular system and, besides NO, generates H₂O₂ The role of nNOS-derived H₂O₂ in human vessels is so far unknown. The present study was aimed at investigating the relevance of nNOS/H₂O₂ signaling in the human internal mammary artery (IMA) and saphenous vein (SV), the major conduits used in coronary artery bypass grafting. In the IMA, but not in the SV, ACh (acetylcholine)-induced vasodilatation was decreased by selective nNOS inhibition with TRIM or Inhibitor 1, and by catalase, which specifically decomposes H₂O₂ Superoxide dismutase (SOD), which generates H₂O₂ from superoxide, decreased the vasodilator effect of ACh on SV. In the IMA, SOD diminished phenylephrine-induced contraction in endothelium-containing, but not in endothelium-denuded vessels. Importantly, while exogenous H₂O₂ produced vasodilatation in IMA, it constricted SV. ACh increased H₂O₂ production in both sets of vessels. In the IMA, the increase in H₂O₂ was inhibited by catalase and nNOS blockade. In SV, H₂O₂ production was abolished by catalase and reduced by nNOS inhibition. Immunofluorescence experiments showed the presence of nNOS in the vascular endothelium and smooth muscle cells of both the IMA and SV. Together, our results clearly show that H₂O₂ induced endothelium-dependent vascular relaxation in the IMA, whereas, in the SV, H₂O₂ was a vasoconstrictor. Thus, H₂O₂ produced in the coronary circulation may contribute to the susceptibility to accelerated atherosclerosis and progressive failure of the SV used as autogenous graft in coronary bypass surgery.

Metformin improves endothelial function in aortic tissue and microvascular endothelial cells subjected to diabetic hyperglycaemic conditions

The cellular mechanisms whereby metformin, the first line drug for type 2 diabetes (T2DM), mediates its antidiabetic effects remain elusive, particularly as to whether metformin has a direct protective action on the vasculature. This study was designed to determine if a brief 3-h exposure to metformin protects endothelial function against the effects of hyperglycaemia. We investigated the protective effects of metformin on endothelial-dependent vasodilatation (EDV) in thoracic aortae from T2DM db/db mice and on high glucose (HG, 40 mM) induced changes in endothelial nitric oxide synthase (eNOS) signaling in mouse microvascular endothelial cells (MMECs) in culture. Exposure of aortae from db+/? non-diabetic control mice to high glucose (HG, 40 mM) containing Krebs for 3-h significantly (P<0.05) reduced acetylcholine (ACh)-induced EDV compared to ACh-induced EDV in aortae maintained in normal glucose (NG, 11 mM) Krebs. The reduction of EDV was partially reversed following a 3-h exposure to 50 μM metformin; metformin also improved ACh-induced EDV in aortae from diabetic db/db mice. Immunoblot analysis of MMECs cultured in HG versus NG revealed a significant reduction of the ratio of phosphorylated (p-eNOS)/eNOS and p-Akt/Akt, but not the expression of total eNOS or Akt. The 3-h exposure of MMECs to metformin significantly (P<0.05) reversed the HG-induced reduction in phosphorylation of both eNOS and Akt; however, no changes were detected for phosphorylation of AMPK or the expression of SIRT1. Our data indicate that a 3-h exposure to metformin can reverse/reduce the impact of HG on endothelial function, via mechanisms linked to increased phosphorylation of eNOS and Akt.

Nitroxyl: a vasodilator of human vessels that is not susceptible to tolerance

Pre-clinical studies have identified nitroxyl (HNO), the reduced congener of nitric oxide (NO•), as a potent vasodilator which is resistant to tolerance development. The present study explores the efficacy of HNO in human blood vessels and describes, for the first time, a vasodilator for humans that is not susceptible to tolerance. Human radial arteries and saphenous veins were obtained from patients undergoing coronary artery graft surgery and mounted in organ baths. Repeated vasodilator responses to the HNO donor Angeli's salt (AS) and NO• donor glyceryl trinitrate (GTN) were determined. AS- and GTN-induced concentration-dependent vasorelaxation of both human radial arteries (AS pEC50: 6.5 ± 0.2; -log M) and saphenous veins (pEC50: 6.7 ± 0.1) with similar potency. In human radial arteries, GTN-induced relaxation was reduced by the NO• scavenger hydroxocobalamin (HXC; P<0.05) but was unaffected by the HNO scavenger L-cysteine. Alternately, AS was unaffected by HXC but was reduced by L-cysteine (5-fold shift, P<0.05). The sGC (soluble guanylate cyclase) inhibitor ODQ abolished responses to both AS and GTN in arteries and veins (P<0.05). Inhibition of voltage-dependent potassium channels (Kv channels) with 4-AP also significantly reduced responses to AS (pEC50: 5.5) and GTN, suggesting that the relaxation to both redox congeners is cGMP- and Kv channel-dependent. Critically, a concentration-dependent development of tolerance to GTN (1 and 10 μM; P<0.05), but not to AS, was observed in both saphenous veins and radial arteries. Like GTN, the HNO donor AS causes vasorelaxation of human blood vessels via activation of a cGMP-dependent pathway. Unlike GTN, however, it does not develop tolerance in human blood vessels.

The implication of periodontitis in vascular endothelial dysfunction

BACKGROUND

Periodontitis is the most common oral infection seen in humans worldwide. It is characterized by gradual destruction of tooth supporting tissues, eventually leading to loss of tooth. The periodontal biofilm associated with periodontitis comprises of gram-positive and gram-negative bacteria, instrumental for the initiation and progression of periodontitis. Evidence-based literature has identified the nature of periodontal infection as a possible causative condition in the inducement of 'low-grade systemic inflammation and infection'. The periodontal pathogens exert systemic effects via the haematogenous route.

AIM

The present review provides an insight into the pathophysiology of the endothelial dysfunction with reference to periodontal infection and highlights the association between periodontitis and endothelial dysfunction. Various studies addressing the implication of periodontitis on endothelial dysfunction will be described, with a focus of periodontal treatment on improvement of endothelial function.

MATERIALS AND METHODS

Studies examining the effects of periodontitis on vascular endothelial function were segregated. Studies conducted on both animal and human models were identified using MEDLINE database search with key search terms such as 'Periodontitis', 'vascular endothelium', 'endothelial dysfunction', 'periodontal bacteria' and 'periodontal therapy'. Systematic reviews and meta-analysis were also screened. Only studies published in English language were considered. The review has been prepared by screening MEDLINE database from 1989 to 2012.

RESULTS AND CONCLUSIONS

Chronic periodontitis results in altered vascular response, increased expression of pro-inflammatory cytokines and adhesion molecules inducing vascular endothelial dysfunction. Periodontal therapy may ameliorate the perturbed vascular endothelial function.

Tetrahydrobiopterin in cardiovascular health and disease

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

Endothelial dysfunction in diabetes mellitus: possible involvement of endoplasmic reticulum stress?

The vascular complications of diabetes mellitus impose a huge burden on the management of this disease. The higher incidence of cardiovascular complications and the unfavorable prognosis among diabetic individuals who develop such complications have been correlated to the hyperglycemia-induced oxidative stress and associated endothelial dysfunction. Although antioxidants may be considered as effective therapeutic agents to relieve oxidative stress and protect the endothelium, recent clinical trials involving these agents have shown limited therapeutic efficacy in this regard. In the recent past experimental evidence suggest that endoplasmic reticulum (ER) stress in the endothelial cells might be an important contributor to diabetes-related vascular complications. The current paper contemplates the possibility of the involvement of ER stress in endothelial dysfunction and diabetes-associated vascular complications.

Tetrahydrobiopterin: biochemistry and pathophysiology

BH4 (6R-L-erythro-5,6,7,8-tetrahydrobiopterin) is an essential cofactor of a set of enzymes that are of central metabolic importance, including four aromatic amino acid hydroxylases, alkylglycerol mono-oxygenase and three NOS (NO synthase) isoenzymes. Consequently, BH4 is present in probably every cell or tissue of higher organisms and plays a key role in a number of biological processes and pathological states associated with monoamine neurotransmitter formation, cardiovascular and endothelial dysfunction, the immune response and pain sensitivity. BH4 is formed de novo from GTP via a sequence of three enzymatic steps carried out by GTP cyclohydrolase I, 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase. An alternative or salvage pathway involves dihydrofolate reductase and may play an essential role in peripheral tissues. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase, except for NOSs, in which the BH4 cofactor undergoes a one-electron redox cycle without the need for additional regeneration enzymes. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I. BH4 biosynthesis is controlled in mammals by hormones and cytokines. BH4 deficiency due to autosomal recessive mutations in all enzymes, except for sepiapterin reductase, has been described as a cause of hyperphenylalaninaemia. A major contributor to vascular dysfunction associated with hypertension, ischaemic reperfusion injury, diabetes and others, appears to be an effect of oxidized BH4, which leads to an increased formation of oxygen-derived radicals instead of NO by decoupled NOS. Furthermore, several neurological diseases have been suggested to be a consequence of restricted cofactor availability, and oral cofactor replacement therapy to stabilize mutant phenylalanine hydroxylase in the BH4-responsive type of hyperphenylalaninaemia has an advantageous effect on pathological phenylalanine levels in patients.

A review of endothelial dysfunction in diabetes: a focus on the contribution of a dysfunctional eNOS

Although the etiology of vascular dysfunction in diabetes has been extensively investigated in both humans as well as animal models of human diabetes, the relative importance of the cellular pathways involved is still not fully understood. In this review, we focus on reviewing the literature that provides insights into how an acute exposure to hyperglycemia results in a dysregulation of endothelial nitric oxide synthase function, the subsequent downstream effects of endothelial nitric oxide synthase dysregulation, and the development of endothelial dysfunction.

Endothelial dysfunction in diabetes: multiple targets for treatment

Robert Furchgott's discovery of the obligatory role that the endothelium plays in the regulation of vascular tone has proved to be a major advance in terms of our understanding of the cellular basis of diabetic vascular disease. Endothelial dysfunction, as defined by a reduction in the vasodilatation response to an endothelium-dependent vasodilator (such as acetylcholine) or to flow-mediated vasodilatation, is an early indicator for the development of the micro- and macroangipathy that is associated with diabetes. In diabetes, hyperglycaemia plays a key role in the initiation and development of endothelial dysfunction; however, the cellular mechanisms involved as well as the importance of dyslipidaemia and co-morbidities such as hypertension and obesity remain incompletely understood. In this review, we discuss the mechanisms whereby hyperglycaemia, oxidative stress and dyslipidaemia can alter endothelial function and highlight their effects on endothelial nitric oxide synthase (eNOS), the endothelium-dependent hyperpolarising factor (EDHF) pathway(s), as well as on the role of endothelium-derived contracting factors (EDCFs) and adipocyte-derived vasoactive factors such as adipose-derived relaxing factor (ADRF).

Role of marginal vitamin C deficiency in atherogenesis: in vivo models and clinical studies

Vitamin C is a pivotal redox modulater in many biological reactions of which several remain poorly understood. Naturally, vitamin C has been the subject of many investigations over the past decades in relation to its possible beneficial effects on cardiovascular disease primarily based on its powerful yet general antioxidant properties. However, growing epidemiological, clinical and experimental evidence now suggests a more specific role of ascorbate in vasomotion and in the prevention of atherosclerosis. For example, in contrast to most other biological antioxidants, administration of vitamin C can apparently induce vasodilation. Millions of people worldwide can be diagnosed with vitamin C deficiency according to accepted definitions. In this perspective, the present review examines the evidence for a specific link between vitamin C deficiency and increased risk of atherosclerosis as well as the possible mechanisms by which vitamin C may exert its protective function.

Roles of Oxidants, Nitric Oxide, and Asymmetric Dimethylarginine in Endothelial Function

Vascular endothelium plays a crucial role in ensuring normal function and morphology of blood vessels, and many risk factors of atherosclerosis act via their effects on endothelial cells. However, endothelial dysfunction is induced by very different pathomechanisms. In principle, it is caused by an impaired bioavailability of nitric oxide (NO) due to an inhibited synthesis (eg, by asymmetric dimethylarginine [ADMA]) or increased consumption of formed NO (by reactive oxygen species [ROS]). ROS can be synthesized in the organism (eg, by different enzymes) or can be administered from the environment (eg, by cigarette smoking), whereas ADMA is the subject of endogenous metabolism only. Many studies have elucidated the system of pathomechanisms and targeted some as potential goals for therapeutic interventions. This review demonstrates roles of ROS, NO, ADMA, endothelin, and estrogen in endothelial function and dysfunction focusing on homocysteinemia and diabetes mellitus and provide examples for the medical treatment of endothelial dysfunction.

Influence of tangential stress on mechanical responses to vasoactive agents in human saphenous vein with and without perivascular adipose tissue

AIM

Improvement in short-term patency of vein grafts harvested with the surrounding tissue and no distention has been noted. The influence of transient tangential stress on mechanical function to vasoactive agents in isolated human saphenous veins stripped or with attached perivascular adipose tissue was assessed.

METHODS

Concentration-response curves to noradrenaline, 5-hydroxytryptamine, methylcholine, sodium nitroprusside and nicardipine were constructed for veins exposed to no, low (approximately 120 mmHg) or high (approximately 240 mmHg) tangential stress.

RESULTS

Tangential stress did not affect contractile effects of noradrenaline or relaxant effects of methylcholine and sodium nitroprusside. Regression analysis of the concentration-response curve to 5-hydroxytryptamine revealed a significant (P=0.042) increase in sensitivity in saphenous veins without perivascular adipose tissue exposed to no tangential stress, compared with veins with attached adipose tissue. Exposure to high stress significantly (P=0.024) increased the potency of 5-hydroxytryptamine in blood vessels without perivascular adipose tissue, as opposed to veins with adipose tissue. Relaxant responses to nicardipine in veins with perivascular adipose tissue were significantly (P=0.001) affected by exposure to low tangential stress compared with no or high tangential stress. A parallel comparison revealed that intact veins compared with those without perivascular adipose tissue exposed to low stress were significantly (P=0.020) more resistant to the relaxant effects of nicardipine.

CONCLUSION

The findings of the present report support the view that tangential stress has an impact on the actions of vasoactive agents, but this influence is variable and factor(s) released from perivascular adipose tissue may have a bearing on the observed effect.

Effects of resveratrol on vascular tone and endothelial function of human saphenous vein and internal mammary artery

BACKGROUND

The polyphenolic compound resveratrol presented in red wine has potent cardiovascular effect in animal. Here, we investigated the ability of resveratrol to relax human coronary bypass grafts, saphenous vein and internal mammary artery and also its effect on their endothelial reactivity.

METHODS

Vascular rings were obtained from 38 male patients undergoing coronary artery bypass operation. The relaxant effects of resveratrol (10-70 microM) and acetylcholine (10(-8)-10(-4) M) were examined on precontracted saphenous vein and internal mammary artery rings.

RESULTS

Resveratrol, at concentration of 70 microM caused relaxations of 34.2+/-5.7% in saphenous vein and 35.2+/-5.4% in internal mammary artery. Endothelium removal and l-NOARG (nitric oxide synthase inhibitor, 10(-4) M) pretreatment almost completely inhibited the relaxation to resveratrol in internal mammary artery but partially in saphenous vein rings. Indomethacin (cyclooxygenase inhibitor, 10(-5) M) slightly, but not significantly enhanced the relaxation to resveratrol in both vessels. The endothelium-dependent relaxations to acetylcholine were significantly improved in the presence of resveratrol of 20 microM in both grafts (E(max): 33.8+/-3.7% versus 46.8+/-4% in saphenous vein n=9; p<0.05; 54. 4+/-5.3% versus 69.3+/-5.4% in internal mammary artery, n=8, p<0.05). The relaxations to acetylcholine were fully eliminated by combination of resveratrol with l-NOARG (10(-4) M) in both vessels.

CONCLUSIONS

Resveratrol produced mainly endothelium-dependent and nitric oxide-mediated vasodilation in human internal mammary artery but partially in saphenous vein rings and improved their endothelial reactivity. This may have a therapeutic potential in cardiovascular diseases.

Tetrahydrobiopterin deficiency exaggerates intimal hyperplasia after vascular injury

Decreased levels of tetrahydrobiopterin (BH4), an absolute cofactor for nitric oxide synthase (NOS), lead to uncoupling of NOS into a superoxide v. nitric oxide producing enzyme, and it is this uncoupling that links it to the development of vascular disease. However, the effects of in vivo deficiency of BH4 on neointimal formation after vascular injury have not been previously investigated. Hph-1 mice, which display 90% deficiency in guanine triphosphate cyclohydrolase I, the rate limiting enzyme in BH4 synthesis, were used. Hph-1 and wild-type mice, treated with either vehicle or BH4 (n = 15 per group), were subjected to wire-induced femoral artery injury, and NOS expression and activity, inflammation, cell proliferation, superoxide production, and neointimal formation were assessed. The major form of NOS expressed over vessel wall after vascular injury was endothelial NOS. Hph-1 mice exhibited lower NOS activity (2.8 +/- 0.3 vs. 4.5 +/- 0.4 pmol/min/mg protein, P < 0.01), and higher aortic superoxide content (5.2 +/- 2.0 x 10(5) cpm vs. 1.6 +/- 0.7 x 10(5) cpm, P < 0.01) compared with wild-type controls, indicating uncoupling of NOS. Treatment of hph-1 mice with BH4 significantly increased NOS activity (from 2.8 +/- 0.3 to 4.1 +/- 0.4 pmol.min(-1).mg protein(-1), P < 0.05), and attenuated superoxide production (from 5.2 +/- 2.0 x 10(5) cpm to 0.8 +/- 0.7 x 10(5) cpm, P < 0.05). Hph-1 mice also had higher inflammatory reactions and more cell proliferation after vascular injury. Furthermore, hph-1 mice responded by a marked increase in neointimal formation at 4 wk after vascular injury, compared with wild-type controls (intima:media ratio: 4.5 +/- 0.5 vs. wild-type 0.7 +/- 0.1, P < 0.001). Treatment of hph-1 mice with BH4 prevented vascular injury-induced increase in neointimal formation (intima:media ratio: 1.4 +/- 0.1 vs. hph-1, P < 0.001). Treatment had no effect on wild-type controls. In summary, we describe, for the first time, that in vivo BH4 deficiency facilitates neointimal formation after vascular injury. Modulation of BH4 bioavailability is an important therapeutic target for restenosis.

Therapeutic potential of nitric oxide donors in the prevention and treatment of atherosclerosis

Well-known risk factors for atherosclerosis include hypercholesterolaemia, hypertension, diabetes, and smoking. These conditions are associated with endothelial dysfunction, which itself is associated with reduced endothelial generation of nitric oxide (NO). This is an overview of the implications of NO generation in atherosclerosis and of the potential therapeutic benefit of drugs which donate NO, such as organic nitrates, nicorandil, and sydnonimines, or those which increase the availability of endogenous NO, such as statins, angiotensin-converting enzyme inhibitors, L-arginine, and tetrahydrobiopterin.

Tetrahydrobiopterin and nitric oxide: mechanistic and pharmacological aspects

In previous minireviews in this journal, we discussed work on induction of tetrahydrobiopterin biosynthesis by cytokines and its significance for nitric oxide (NO) production of intact cells as well as functions of H4-biopterin identified at this time for NO synthases (Proc Soc Exp Biol Med 203: 1-12, 1993; Proc Soc Exp Biol Med 219: 171-182, 1998). Meanwhile, the recognition of the importance of tetrahydrobiopterin for NO formation has led to new insights into complex biological processes and revealed possible novel pharmacological strategies to intervene in certain pathological conditions. Recent work could also establish that tetrahydrobiopterin, in addition to its allosteric effects, is redox-active in the NO synthase reaction. In this review, we summarize the current view of how tetrahydrobiopterin functions in the generation of NO and focus on pharmacological aspects of tetrahydrobiopterin availability with emphasis on endothelial function.

Evaluation of a novel sutureless anastomotic connector: From endothelial function to mid-term clinical and angiographic follow-up

OBJECTIVES

We evaluated the effect of the St Jude Medical sutureless anastomotic connector on endothelium-dependent and -independent saphenous vein graft relaxation, as well as on clinical outcomes and graft patency in patients.

METHODS

Human saphenous vein grafts were assigned to control or connector groups (loaded for 1 or 5 minutes; n = 18). Isometric dose-response curves to endothelium-dependent and -independent (sodium nitroprusside) vasodilators were constructed in saphenous vein grafts precontracted with phenylephrine. Thrombin-mediated vasorelaxation, an early determinant of saphenous vein graft failure, was also evaluated. Percent maximum relaxation was compared between groups. Patients in whom the St Jude Medical connector was employed underwent clinical follow-up, stress tests, and angiography 6 to 12 months postoperatively.

RESULTS

A23187-induced endothelium-mediated relaxation, sodium nitroprusside-induced endothelium-independent relaxation, and thrombin-mediated vasorelaxation did not differ between control and connector saphenous vein grafts at either time point studied. Twenty-seven patients received St Jude Medical connectors. There was no hospital mortality; patients were followed for 679 +/- 241 days. There was 1 late death; the connector saphenous vein graft was patent at postmortem. All connector saphenous vein grafts were patent at follow-up angiography. Four grafts had stenoses (30%-60%), without symptoms or requirement for intervention. All hand-sewn saphenous vein grafts were also patent.

CONCLUSIONS

The St Jude Medical connector does not impair endothelium-dependent vasorelaxation. In patients, patency of the connector saphenous vein grafts 6 to 12 months postoperatively was 100% but 22% of grafts had non-flow-limiting stenoses at or near the connector. Further long-term studies are required to confirm the safety of the St Jude Medical connector with regards to endothelial function and restenosis.

Tetrahydrobiopterin restores endothelial dysfunction induced by an oral glucose challenge in healthy subjects

An oral glucose challenge causes transient impairment of endothelial function, probably because of increased oxidative stress. During oxidative stress, endothelial nitric oxide (NO) synthase (eNOS) becomes uncoupled because of decreased bioavailability of tetrahydrobiopterin (BH4), an essential cofactor of eNOS. Therefore, we examined whether an acute supplement of BH4 could restore endothelial dysfunction induced by an oral glucose challenge. Healthy subjects were examined in 53 experiments. Forearm blood flow was measured by venous occlusion plethysmography. Dose-response studies were obtained during intra-arterial infusion of serotonin to elicit endothelium-dependent, NO-specific vasodilation and during sodium nitroprusside (SNP) infusion to elicit endothelium-independent vasodilation. Subjects were examined before (fasting) and 1 and 2 h after an oral glucose challenge (75 g) with serotonin (n = 10) and SNP (n = 8). On different days (6R)-5,6,7,8-tetrahydro-l-biopterin dihydrochloride (6R-BH4; n = 10), the active cofactor of eNOS or its stereoisomer (6S)-5,6,7,8-tetrahydro-l-biopterin sulfate (6S-BH4; n = 10), which is inactive as a cofactor, was added 10 min (500 microg/min) before and during the 1-h postchallenge serotonin dose-response study. In vitro studies showed that 6R-BH4 and 6S-BH4 were equipotent antioxidants. Serotonin response was reduced by 24 +/- 7% (at the highest dose) at 1 h postchallenge compared with fasting (P = 0.001) and was restored 2 h postchallenge. The reduction was reversed by the administration of 6R-BH4 but not by 6S-BH4. SNP responses were slightly increased 1 and 2 h postchallenge (increased by 15 +/- 13% at third dose 2 h postchallenge, P = 0.0001). An oral glucose challenge causes transient, NO-specific, endothelial dysfunction, which may be reversed by BH4. Transient postprandial endothelial dysfunction may be partly explained by reduced bioavailability of BH4 and NO.

Tetrahydrobiopterin attenuates homocysteine induced endothelial dysfunction

Homocysteine is an independent risk factor for atherosclerotic vascular disease. It impairs endothelial function via increasing superoxide production and quenching nitric oxide (NO) release. Tetrahydrobiopterin (BH4) is a critical cofactor that couples nitric oxide synthase and facilitates the production of nitric oxide (vs. superoxide anions). In the first study, the effects of hyperhomocysteinemia (0.1 mM, 3 h) on endothelium-dependent vasorelaxation to ACh and A23187 were examined in isolated segments of rat aortae in the presence or absence of BH4 (0.1 mM). In the second study, the effects of hyperhomocysteinemia (24 h) on nitric oxide production and superoxide release (using lucigenin chemiluminescence) were studied in human umbilical vein endothelial cells in the absence or presence of BH4 (10 microM). Homocysteine incubation impaired receptor-dependent and -independent endothelial function to ACh and A23187. This effect was attenuated by BH4. Furthermore, homocysteine exposure increased superoxide production and impaired agonist-stimulated nitric oxide release. These effects were attenuated by BH4 (p < 0.05). Hyperhomocysteinemia impairs endothelial function, in part due to a diminished bioavailability of BH4 with resultant uncoupling of nitric oxide synthase. BH4 may represent an important target for strategies aimed at improving endothelial dysfunction secondary to hyperhomocysteinemia.

Nitric oxide, atherosclerosis and the clinical relevance of endothelial dysfunction

The endothelium plays a key role in vascular homeostasis through the release of a variety of autocrine and paracrine substances, the best characterized being nitric oxide. A healthy endothelium acts to prevent atherosclerosis development and its complications through a complex and favorable effect on vasomotion, platelet and leukocyte adhesion and plaque stabilization. The assessment of endothelial function in humans has generally involved the description of vasomotor responses, but more widely includes physiological, biochemical and genetic markers that characterize the interaction of the endothelium with platelets, leukocytes and the coagulation system. Stable markers of inflammation such as high sensitivity C-reactive protein are indirect and potentially useful measures of endothelial function for example. Attenuation of the effect of nitric oxide accounts for the majority of what is described as endothelial dysfunction. This occurs in response to atherosclerosis or its risk factors. Much remains to be learned about the molecular and genetic pathophysiological mechanisms of endothelial cell abnormalities. However, pharmacological intervention with a growing list of medications can favorably modify endothelial function, paralleling beneficial effects on cardiovascular morbidity and mortality. In addition, several small studies have provided tantalizing evidence that measures of endothelial health might provide prognostic information about an individual patient's risk of subsequent events. As such, the sum of this evidence makes the clinical assessment of endothelial function an attractive surrogate marker of atherosclerosis disease activity. The review will focus on the role of nitric oxide in atherosclerosis and the clinical relevance of these findings.

Novel cardioprotective effects of tetrahydrobiopterin after anoxia and reoxygenation: Identifying cellular targets for pharmacologic manipulation

OBJECTIVES

Contemporary cardioprotective strategies to prevent perioperative ischemia-reperfusion injury have focused on the l-arginine nitric oxide pathway. Tetrahydrobiopterin is an absolute cofactor required for the enzyme nitric oxide synthase and is thus a critical determinant of nitric oxide production. We hypothesized that ischemia-reperfusion results in diminished levels of tetrahydrobiopterin, which might represent a key cellular defect underlying endothelial and myocyte dysfunction after ischemia-reperfusion. To this aim, we examined the effects of tetrahydrobiopterin supplementation in (1) an in vivo experimental model of global ischemia-reperfusion and (2) an in vitro human ventricular heart cell model of simulated ischemia-reperfusion. Measures of endothelial function, oxidant production, cell survival, and cardiac function were used to assess outcome.

METHODS

In study 1 Wistar rats were divided into one of 2 groups (n = 10 per group). One group received tetrahydrobiopterin (25 mg x kg(-1) x d(-1) for 7 days), and the other group served as the control group. Hearts were subjected to 30 minutes of ischemia followed by 30 minutes of reperfusion, and left ventricular developed pressure, left ventricular systolic pressure, and left ventricular end-diastolic pressure were determined by using the modified Langendorff technique. In study 2 we quantitated myocardial malondialdehyde, a marker of lipid peroxidation, in ventricular tissues from both groups of animals using butanol phase extraction and spectrophotometric analysis. In study 3 coronary vascular responses were determined in vascular segments of the left coronary artery in both groups of animals after ischemia-reperfusion. Endothelium-dependent and endothelium-independent vasodilatation to acetylcholine and sodium nitroprusside, respectively, were compared between groups. In study 4, using a human ventricular heart cell model of simulated ischemia-reperfusion, we studied the effects of tetrahydrobiopterin (20 micromol/L) on cellular injury (as assessed by means of trypan blue uptake).

RESULTS

After ischemia-reperfusion, myocardial dysfunction was evidenced by a decrease in left ventricular developed pressure and an increase in left ventricular end-diastolic pressure (P =.01 compared with baseline). Hearts from tetrahydrobiopterin-treated rats exhibited protection against ischemia-reperfusion injury (left ventricular developed pressure: 74 +/- 4 vs control 42 +/- 8 mm Hg, P =.01; left ventricular end-diastolic pressure: 12 +/- 3 vs 34 +/- 7 mm Hg, P =.01). Furthermore, tetrahydrobiopterin treatment attenuated the rise in malondialdehyde levels after ischemia-reperfusion (P =.01). After reperfusion, coronary endothelial function to acetylcholine was attenuated (P =.003 vs sham-treated mice), whereas responses to sodium nitroprusside remained unchanged. Tetrahydrobiopterin-treated rats exhibited an improvement in acetylcholine-mediated vasorelaxation (P =.01 vs ischemia-reperfusion group). Cellular injury, as assessed by means of trypan blue uptake, was higher in human ventricular heart cells subjected to simulated ischemia-reperfusion; this effect was prevented with tetrahydrobiopterin treatment (P =.001).

CONCLUSIONS

Supplemental tetrahydrobiopterin provides a novel cardioprotective effect on left ventricular function, endothelial-vascular reactivity, oxidative damage, and cardiomyocyte injury after ischemia-reperfusion injury and might represent an important cellular target for future operative myocardial protection strategies.

Cellular basis of endothelial dysfunction in small mesenteric arteries from spontaneously diabetic (db/db -/-) mice: role of decreased tetrahydrobiopterin bioavailability

1. Endothelium-dependent and -independent regulation of vascular tone in small mesenteric arteries (SMA) from control (db/db +/?) and diabetic (db/db -/-) mice was compared. 2. Phenylephrine-induced maximum contraction, but not sensitivity, of SMA in db/db -/- compared to db/db +/? was enhanced. 3. Acetylcholine (ACh), but not sodium nitroprusside (SNP), -induced relaxation was reduced in SMA from db/db -/- compared to db/db +/?. 4. ACh-induced relaxation of SMA was inhibited by a combination of N(omega)-nitro-L-arginine and indomethacin in db/db +/?, but not in db/db -/-. 5. Acute incubation of SMA with tetrahydrobiopterin (BH(4), 10 microM) and sepiapterin (100 microM) enhanced ACh-induced relaxation in SMA from db/db -/-, but not from db/db +/? 2,4-diamino-6-hydroxypyrimidine, an inhibitor of GTP cyclohydrolase I, (10 mM), impaired the sensitivity of SMA from db/db +/? to ACh, which was restored by co-incubation with BH(4) (10 microM). 6. BH(4) and superoxide dismutase (SOD, 150 u ml(-1)), either alone or in combination, had no effect on either ACh or SNP-induced relaxation in SMA from eNOS -/- mice. 7. Incubation of SMA with SOD (150 iu ml(-1)), catalase (200 iu ml(-1)) and L-arginine (1 mM) had no effect on ACh-induced relaxation of SMA. However, the combination of polyethylene glycol-SOD (200 iu ml(-1)) and catalase (80 u ml(-1)) improved the sensitivity of ACh-induced relaxation in db/db -/-, but not in db/db +/?. 8. These data suggest that increased production of superoxide anions and decreased availability of BH(4) result in an 'uncoupling' of nitric oxide synthase and endothelial dysfunction in SMA from db/db -/- mice.

Endothelin blockade potentiates endothelial protective effects of ACE inhibitors in saphenous veins

BACKGROUND

Angiotensin II and endothelin-1 are potent endothelium-derived contracting factors. The effects of acute endothelin antagonism on endothelial function in saphenous vein from patients treated with and without angiotensin-converting enzyme inhibitors were compared.

METHODS

Vascular segments of saphenous vein were obtained perioperatively from 14 patients on angiotensin-converting enzyme inhibitors and 29 controls. In vitro endothelium-dependent and -independent responses to acetylcholine and sodium nitroprusside were assessed by constructing isometric dose-response curves in precontracted rings in the presence and absence of bosentan (endothelinA/B receptor antagonist) and BQ-123 (endothelinA antagonist) using isolated organ baths. Percent maximum relaxation and sensitivity were compared between interventions.

RESULTS

Endothelium-dependent relaxation to acetylcholine was augmented in the angiotensin-converting enzyme inhibitor-treated group (p < 0.005). Both specific and mixed endothelin receptor blockade improved acetylcholine-mediated relaxation in the angiotensin-converting enzyme inhibitor-treated and untreated groups (p < 0.02). The effects of these antagonists were endothelium specific as endothelium-independent responses to sodium nitroprusside remain unaltered.

CONCLUSIONS

These data demonstrate that (1) chronic angiotensin-converting enzyme inhibition improves endothelial function in saphenous veins, and (2) this effect can be further augmented by acute endothelin blockade. These data suggest that antagonism of both angiotensin II and endothelin may be important in attenuating saphenous vein arteriosclerosis.

Vascular endothelial dysfunction: does tetrahydrobiopterin play a role?

Tetrahydrobiopterin is one of the most potent naturally occurring reducing agents and an essential cofactor required for enzymatic activity of nitric oxide synthase (NOS). The exact role of tetrahydrobiopterin in the control of NOS catalytic activity is not completely understood. Existing evidence suggests that it can act as allosteric and redox cofactors. Suboptimal concentration of tetrahydrobiopterin reduces formation of nitric oxide and favors "uncoupling" of NOS leading to NOS-mediated reduction of oxygen and formation of superoxide anions and hydrogen peroxide. Recent findings suggest that accelerated catabolism of tetrahydrobiopterin in arteries exposed to oxidative stress may contribute to pathogenesis of endothelial dysfunction present in arteries exposed to hypertension, hypercholesterolemia, diabetes, smoking, and ischemia-reperfusion. Beneficial effects of acute and chronic tetrahydrobiopterin supplementation on endothelial function have been reported in experimental animals and humans. Furthermore, it appears that beneficial effects of some antioxidants (e.g., vitamin C) on vascular function could be mediated via increased intracellular concentration of tetrahydrobiopterin. In this review, the potential role of tetrahydrobiopterin in the pathogenesis of vascular endothelial dysfunction and mechanisms underlying beneficial vascular effects of tetrahydrobiopterin will be discussed.

Endothelium-derived relaxing factors: A focus on endothelium-derived hyperpolarizing factor(s)

Endothelium-derived hyperpolarizing factor (EDHF) is defined as the non-nitric oxide (NO) and non-prostacyclin (PGI2) substance that mediates endothelium-dependent hyperpolarization (EDH) of vascular smooth muscle cells (VSMC). Although both NO and PGI2 have been demonstrated to hyperpolarize VSMC by cGMP- and cAMP-dependent mechanisms, respectively, and in the case of NO by cGMP-independent mechanisms, a considerable body of evidence suggests that an additional cellular mechanism must exist that mediates EDH. Despite intensive investigation, there is no agreement as to the nature of the cellular processes that mediates the non-NO/PGI2 mediated hyperpolarization. Epoxyeicosatrienoic acids (EET), an endogenous anandamide, a small increase in the extracellular concentration of K+, and electronic coupling via myoendothelial cell gap junctions have all been hypothesized as contributors to EDH. An attractive hypothesis is that EDH is mediated via both chemical and electrical transmissions, however, the contribution from chemical mediators versus electrical transmission varies in a tissue- and species-dependent manner, suggesting vessel-specific specialization. If this hypothesis proves to be correct then the potential exists for the development of vessel and organ-selective vasodilators. Because endothelium-dependent vasodilatation is dysfunctional in disease states (i.e., atherosclerosis), selective vasodilators may prove to be important therapeutic agents.Key words: endothelium, nitric oxide, potassium channels, hyperpolarization, gap junctions.