Supplemention with tetrahydrobiopterin suppresses the development of hypertension in spontaneously hypertensive rats

Effects of supplementation of BH4 after prolonged ischemia in skeletal muscle

PURPOSE

To determine whether the supplementation of tetrahydrobiopterin (BH(4), an essential cofactor of nitric oxide synthase; NOS) could attenuate endothelial dysfunction and improve NOS activity and cell viability in skeletal muscle after ischemia/reperfusion (I/R).

METHODS

A vascular pedicle isolated rat cremaster muscle model was used. Cremaster muscles were subjected to 4 h of ischemia followed by 2 h of reperfusion. Rats were given either normal saline or BH(4) by intravenous injection at 1 min prior to reperfusion. After reperfusion, average arteriole diameter, capillary perfusion, endothelial-dependent/-independent vasodilatation, NOS activity, and muscle cell viability were evaluated.

RESULTS

Supplementation of BH(4) prior to reperfusion significantly attenuated reperfusion-induced vasoconstriction, poor capillary perfusion, and endothelial dysfunction and enhanced cNOS activity and slightly improved cell viability in the skeletal muscle after I/R.

CONCLUSION

Supplementation of BH(4) during reperfusion provided a significant protection against I/R injury in rat skeletal muscle.

Mechanisms of disease: oxidative stress and inflammation in the pathogenesis of hypertension

Animal studies have shown that oxidative stress and renal tubulointerstitial inflammation are associated with, and have major roles in, the pathogenesis of hypertension. This view is supported by the observations that alleviation of oxidative stress and renal tubulointerstitial inflammation reduce arterial pressure in animal models. Conversely, hypertension has been shown to cause oxidative stress and inflammation in renal and cardiovascular tissues in experimental animals. Taken together, these observations indicate that oxidative stress, inflammation and arterial hypertension participate in a self-perpetuating cycle which, if not interrupted, can lead to progressive cardiovascular disease and renal complications. These events usually occur in an insidious and asymptomatic manner over an extended period following the onset of hypertension. Severe target organ injury can, however, occasionally occur precipitously in the course of malignant or accelerated hypertension. Given the high degree of heterogeneity of hypertensive disorders, the factor(s) initiating the vicious cycle described vary considerably in different forms of hypertension. For instance, oxidative stress in the kidney and vascular tissue is the primary mediator in the pathogenesis of angiotensin-induced, and perhaps lead-induced, hypertension. By contrast, increased arterial pressure is probably the initiating trigger in salt-sensitive hypertension. Although the initiating factor might vary between hypertensive disorders, according to the proposed model, the three components of the cycle eventually coalesce in all forms of hypertension.

Age-related changes in endothelial nitric oxide synthase phosphorylation and nitric oxide dependent vasodilation: evidence for a novel mechanism involving sphingomyelinase and ceramide-activated phosphatase 2A

Aging is the single most important risk factor for cardiovascular diseases (CVD), which are the leading cause of morbidity and mortality in the elderly. The underlying etiologies that elevate CVD risk are unknown, but increased vessel rigidity appears to be a major hallmark of cardiovascular aging. We hypothesized that post-translational signaling pathways become disrupted with age and adversely affect endothelial nitric oxide synthase (eNOS) activity and endothelial-derived nitric oxide (NO) production. Using arterial vessels and isolated endothelia from old (33-month) vs. young (3-month) F344XBrN rats, we show a loss of vasomotor function with age that is attributable to a decline in eNOS activity and NO bioavailability. An altered eNOS phosphorylation pattern consistent with its inactivation was observed: phosphorylation at the inhibitory threonine 494 site increased while phosphorylation at the activating serine 1176 site declined by 50%. Loss of phosphorylation on serine 1176 was related to higher ceramide-activated protein phosphatase 2 A activity, which was driven by a 125% increase in ceramide in aged endothelia. Elevated ceramide levels were attributable to chronic activation of neutral sphingomyelinases without a concomitant increase in ceramidase activity. This imbalance may stem from an observed 33% decline in endothelial glutathione (GSH) levels, a loss known to differentially induce neutral sphingomyelinases. Pretreating aged vessel rings with the neutral sphingomyelinase inhibitor, GW4869, significantly reversed the age-dependent loss of vasomotor function. Taken together, these results suggest a novel mechanism that at least partly explains the persistent loss of eNOS activity and endothelial-derived NO availability in aging conduit arteries.

Tetrahydrobiopterin protects against guanabenz-mediated inhibition of neuronal nitric-oxide synthase in vitro and in vivo

It is established that guanabenz inhibits neuronal nitric-oxide (NO) synthase (nNOS) and causes the enhanced proteasomal degradation of nNOS in vivo. Although the time- and NADPH-dependent inhibition of nNOS has been reported in studies where guanabenz was incubated with crude cytosolic preparations of nNOS, the exact mechanism for inhibition is not known. Moreover, even less is known about how the inhibition of nNOS triggers its proteasomal degradation. In the current study, we show, with the use of purified nNOS, that guanabenz treatment leads to the oxidation of tetrahydrobiopterin and formation of a pterin-depleted nNOS, which is not able to form NO. With the use of 14C-labeled guanabenz, we were unable to detect any guanabenz metabolites or guanabenz-nNOS adducts, indicating that reactive intermediates of guanabenz probably do not play a role in the inhibition. Superoxide dismutase, however, prevents the guanabenz-mediated oxidation of tetrahydrobiopterin and inhibition of nNOS, suggesting the role of superoxide as an intermediate. Studies in rats show that administration of tetrahydrobiopterin prevents the inhibition and loss of penile nNOS due to guanabenz, indicating that the loss of tetrahydrobiopterin plays a major role in the effects of guanabenz in vivo. Our findings are consistent with the destabilization and enhanced degradation of nNOS found after tetrahydrobiopterin depletion. These studies suggest that drug-mediated destabilization and subsequent enhanced degradation of protein targets will likely be an important toxicological consideration.

Cardiovascular responses produced by central injection of hydrogen peroxide in conscious rats

Reactive oxygen species (ROS) have been shown to modulate neuronal synaptic transmission and may play a role on the autonomic control of the cardiovascular system. In this study we investigated the effects produced by hydrogen peroxide (H(2)O(2)) injected alone or combined with the anti-oxidant agent N-acetil-l-cysteine (NAC) or catalase into the fourth brain ventricle (4th V) on mean arterial pressure and heart rate of conscious rats. Moreover the involvement of the autonomic nervous system on the cardiovascular responses to H(2)O(2) into the 4th V was also investigated. Male Holtzman rats (280-320 g) with a stainless steel cannula implanted into the 4th V and polyethylene cannulas inserted into the femoral artery and vein were used. Injections of H(2)O(2) (0.5, 1.0 and 1.5 micromol/0.2 microL, n=6) into the 4th V produced transient (for 10 min) dose-dependent pressor responses. The 1.0 and 1.5 micromol doses of H(2)O(2) also produced a long lasting bradycardia (at least 24 h with the high dose of H(2)O(2)). Prior injection of N-acetyl-l-cysteine (250 nmol/1 microL/rat) into the 4th V blockade the pressor response and attenuated the bradycardic response to H(2)O(2) (1 micromol/0.5 microL/rat, n=7) into the 4th V. Intravenous (i.v.) atropine methyl bromide (1.0 mg/kg, n=11) abolished the bradycardia but did not affect the pressor response to H(2)O(2). Prazosin hydrochloride (1.0 mg/kg, n=6) i.v. abolished the pressor response but did not affect the bradycardia. The increase in the catalase activity (500 UEA/1 microL/rat injected into the 4th V) also abolished both, pressor and bradycardic responses to H(2)O(2). The results suggest that increased ROS availability into 4th V simultaneously activate sympathetic and parasympathetic outflow inducing pressor and bradycardic responses.

A yeast 2-hybrid analysis of human GTP cyclohydrolase I protein interactions

The yeast 2-hybrid system was used to identify protein domains involved in the oligomerization of human guanosine 5'-triphosphate (GTP) Cyclohydrolase I (GCH1) and the interaction of GCH1 with its regulatory partner, GCH1 feedback regulatory protein (GFRP). When interpreted within the structural framework derived from crystallography, our results indicate that the GCH1 N-terminal alpha-helices are not the only domains involved in the formation of dimers from monomers and also suggest an important role for the C-terminal alpha-helix in the assembly of dimers to form decamers. Moreover, a previously unknown role of the extended N-terminal alpha-helix in the interaction of GCH1 and GFRP was revealed. To discover novel GCH1 protein binding partners, we used the yeast 2-hybrid system to screen a human brain library with GCH1 N-terminal amino acids 1-96 as prey. This protruding extension of GCH1 contains two canonical Type-I Src homology-3 (SH3) ligand domains located within amino acids 1-42. Our screen yielded seven unique clones that were subsequently shown to require amino acids 1-42 for binding to GCH1. The interaction of one of these clones, Activator of Heat Shock 90 kDa Protein (Aha1), with GCH1 was validated by glutathione-s-transferase (GST) pull-down assay. Although the physiological relevance of the Aha1-GCH1 interaction requires further study, Aha1 may recruit GCH1 into the endothelial nitric oxide synthase/heat shock protein (eNOS/Hsp90) complex to support changes in endothelial nitric oxide production through the local synthesis of BH4.

Endothelial dysfunction: a multifaceted disorder (The Wiggers Award Lecture)

Endothelial cells synthesize and release various factors that regulate angiogenesis, inflammatory responses, hemostasis, as well as vascular tone and permeability. Endothelial dysfunction has been associated with a number of pathophysiological processes. Oxidative stress appears to be a common denominator underlying endothelial dysfunction in cardiovascular diseases. However, depending on the pathology, the vascular bed studied, the stimulant, and additional factors such as age, sex, salt intake, cholesterolemia, glycemia, and hyperhomocysteinemia, the mechanisms underlying the endothelial dysfunction can be markedly different. A reduced bioavailability of nitric oxide (NO), an alteration in the production of prostanoids, including prostacyclin, thromboxane A2, and/or isoprostanes, an impairment of endothelium-dependent hyperpolarization, as well as an increased release of endothelin-1, can individually or in association contribute to endothelial dysfunction. Therapeutic interventions do not necessarily restore a proper endothelial function and, when they do, may improve only part of these variables.

Estrogen therapy replenishes vascular tetrahydrobiopterin and reduces oxidative stress in ovariectomized rats

OBJECTIVE

We investigated whether the effect of estrogen therapy on vascular endothelial function is mediated through increasing the bioavailability of tetrahydrobiopterin (BH4) and associated antioxidant capacity in ovariectomized (Ovx) rats.

DESIGN

Aortas of sham-operated, Ovx, and Ovx plus estrogen therapy (Ovx + ET) female Sprague-Dawley rats were used to measure vascular reactivity. Plasma levels of nitric oxide (NO) metabolites, total antioxidant capacity, aortic superoxide anion (O2.), and BH4 contents were determined.

RESULTS

Vascular reactivity, assessed on isolated aortic segments, indicated that phenylephrine-induced contraction in the Ovx group was significantly greater than that in the sham and Ovx + ET groups. The vasodilator responses to acetylcholine (10 to 10 M) and L-arginine (L-Arg; 10 M) in the sham and Ovx + ET groups were significantly greater than those in the Ovx group. Pretreatment with BH4 (10 M) enhanced the vasodilator responses to L-Arg in the Ovx group compared with the untreated Ovx group. An inhibitor of BH4 synthesis, 2,4-diamino-6-hydroxypyrimidine (2 mM), significantly attenuated the vasodilator response to L-Arg in the sham and Ovx + ET groups. In addition, Ovx significantly increased O2. production in aortic tissues and decreased plasma NO metabolites levels, whereas ET significantly prevented these effects. Pretreatment with BH4 also significantly decreased aortic O2. production in the Ovx group; both plasma total antioxidant capacity and aortic BH4 contents in the Ovx group decreased significantly compared with those in the sham group, which were also improved by ET. There were no significant differences in the protein expression of endothelial NO synthase in aortas in these groups.

CONCLUSIONS

ET increases the availability of vascular BH4 to attenuate O2. production and restores total antioxidant capacity, leading to improved NO-mediated vasodilation in Ovx rats.

Supplement of tetrahydrobiopterin by a gene transfer of GTP cyclohydrolase I cDNA improves vascular dysfunction in insulin-resistant rats

Deficiency of tetrahydrobiopterin (BH4) in the vascular tissue contributes to endothelial dysfunction in the insulin-resistant state. We intended to develop a new gene transfer method by overexpression of its biosynthetic enzyme, GTP cyclohydrolase I (GTP-CH1). The GTP-CH1 cDNA was inserted into a pCAGGS vector, and then plasmid DNA was mixed with atelocollagen, and the aliquot was injected into thigh muscles of insulin-resistant Zucker fatty rats. After 4 weeks, pteridine derivative levels, superoxide anion (O2-), activity of endothelial nitric oxide synthase (eNOS), and endothelium-dependent relaxation were evaluated in the aortas obtained from Zucker lean or fatty rats. The BH4 contents and GTP-CH1 activity in Zucker fatty rats were 50%-55% less than those of Zucker lean rats. However, those impairments were significantly improved by a plasmid DNA injection, and aortic BH4 content reached more than 80% of the level of Zucker lean rats. Increased A23187-stimulated O2- production as well as decreased eNOS activity and endothelial function in insulin-resistant Zucker fatty rats were improved by a plasmid DNA injection to a level similar to that in Zucker lean rats. These findings suggest that intramuscular GTP-CH1 gene transfer using atelocollagen serves as a useful method of long-term systemic delivery of BH4 and the treatment of endothelial dysfunction.

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.

Angiotensin-II-induced oxidative stress elicits hypoadiponectinaemia in rats

AIMS/HYPOTHESIS

Hypertension, endothelial dysfunction and insulin resistance are associated conditions that share oxidative stress and vascular inflammation as common features. Adiponectin is an abundant plasma adipokine that plays a physiological role in modulating lipid metabolism and exerts a potent anti-inflammatory activity. We hypothesised that adiponectin levels decrease in response to oxidative stress and that this may promote the development of hypertension, endothelial dysfunction and insulin resistance.

METHODS

Rats were infused with angiotensin II (AngII) or its vehicle, either alone or in combination with tempo1 (4-hydroxy-2,2,6,6-tetramethyl piperidinoxyl), a membrane-permeable metal-independent superoxide dismutase mimetic, or tetrahydrobiopterin (BH4), one of the most potent naturally occurring reducing agents and an essential cofactor for nitric oxide synthase activity. Heart rate, systolic blood pressure, body weight and serum levels of adiponectin were measured on day 7 of treatment, and then the animals were killed. Vessel tone and superoxide production were measured ex vivo in thoracic vascular rings. The expression of adiponectin mRNA in adipose tissue was assessed by Northern blotting, and in 3T3-L1 adipocytes exposed to H2O2 by real-time PCR. The expression of NAD(P)H oxidase subunit mRNAs in the rats was assessed by RT-PCR and real-time PCR.

RESULTS

Hypertension and endothelial dysfunction were induced in rats by infusion of AngII and reversed by administration of tempol. Plasma concentrations of adiponectin and adipose tissue levels of adiponectin mRNA were decreased in AngII-infused rats, and this effect was prevented by cotreatment with tempol or BH4. The production of superoxide anions (O2-) was significantly increased in the aortae of AngII-treated rats, and this increase was prevented by the administration of tempol or BH4. Levels of mRNAs that encode NAD(P)H oxidase components, including p22phox, gp91phox, p47phox and Rac1, were similarly increased in adipose tissue, aortae and hearts of AngII-infused rats. Cotreatment of rats with tempol or BH4 reversed AngII-induced increases in NAD(P)H oxidase subunit mRNAs. Fully differentiated 3T3-L1 adipocytes, also exhibited diminished adiponectin mRNA levels when exposed to low concentrations of H2O2.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that AngII-induced oxidative stress and endothelial dysfunction are accompanied by a decrease in adiponectin gene expression. Since antioxidants were observed to prevent the actions of AngII, and H2O2 on its own suppressed adiponectin expression, we conclude that adiponectin gene expression is negatively modulated by oxidative stress. Plasma adiponectin levels may provide a useful indicator of oxidative stress in vivo, and suppressed levels may contribute to the proinflammatory and metabolic derangements associated with type 2 diabetes, coronary artery disease and the metabolic syndrome.

Expression of NOX-I, gp91phox, p47phox and P67phox in the aorta segments above and below coarctation

Aorta coarctation results in hypertension (HTN) in the arterial tree proximal to stenosis and, as such, provides an ideal model to discern the effects of different levels of blood pressure on the vascular tissue in the same animal. Compelling evidence has emerged supporting the role of oxidative stress as a cause of HTN. However, whether or not HTN (independent of the circulating humoral factors) can cause oxidative stress is less certain. NAD(P)H oxidase isoforms are the main source of reactive oxygen species (ROS) in the vascular tissues. We therefore compared the expressions of NOX-I, gp91phox and the regulatory subunits of the enzyme in the aorta segments residing above and below coarctation in rats with abdominal aorta banding. Rats were studied 4 weeks after aorta banding above the renal arteries or sham operation. Subunits of NAD(P)H oxidase and its NOX-I isoform as well as endothelial NO synthase (eNOS) and nitrotyrosine (footprint of NO oxidation by superoxide) were measured in the aorta segments above and below coarctation. The gp91phox, p47phox, and p67phox subunits of NAD(P)H oxidase, NOX-I isoform, eNOS and nitrotyrosine were markedly increased in the aorta segment above coarctation (hypertensive zone), but were virtually unchanged in the segment below coarctation. Since, excepting blood pressure, all other conditions were constant, the upregulation of NAD(P)H oxidase isoforms and the increased NO oxidation in the aorta segment above, but not below, coarctation prove that HTN, per se, independent of circulating mediators can cause oxidative/nitrosative stress in the arterial wall. These observations suggest that HTN control may represent a specific form of antioxidant therapy for hypertensive disorders.

Hypertension promotes integrin expression and reactive oxygen species generation by circulating leukocytes

BACKGROUND

Compelling evidence has emerged pointing to the role of oxidative stress in the pathogenesis of hypertension (HTN) in experimental animals. Excess production of reactive oxygen species (ROS) in the renal and vascular tissues has been shown to raise arterial pressure via inactivation of nitric oxide and generation of isoprostanes. Accumulation of inflammatory cells in the renal interstitium has been shown to increase ROS generation in the kidneys of hypertensive animals. In addition, considerable evidence has emerged pointing to spontaneous activation of circulating leukocytes in animals and humans with hereditary HTN. This study was designed to explore whether induction of HTN in genetically normal animals can lead to spontaneous activation and ROS production in circulating leukocytes.

METHODS

Integrin expression, superoxide, and hydrogen peroxide production were assessed by flow cytometry in the circulating and splenic leukocytes of Sprague-Dawley rats rendered hypertensive by abdominal aorta banding above the renal arteries, and in sham-operated control rats.

RESULTS

The hypertensive animals studied 4 weeks after abdominal aorta banding exhibited a significant increase in superoxide and H(2)O(2) production in the circulating granulocyte, and a marked increase in H(2)O(2) production in the blood and splenic helper and cytotoxic T-lymphocytes. This was coupled with a significant up-regulation of CD18 and CD11a in splenic helper T-cells and cytotoxic T cells, and of CD18 in the circulating helper T cells.

CONCLUSION

Induction of HTN in genetically normotensive rats causes a spontaneous increase in ROS generation in the circulating and splenic leukocytes. This phenomenon can contribute to systemic oxidative stress, inflammation, cardiovascular and renal complications in hypertensive animals.

Programming blood pressure in adult SHR by shifting perinatal balance of NO and reactive oxygen species toward NO: the inverted Barker phenomenon

The “programming hypothesis” proposes that an adverse perinatal milieu leads to adaptation that translates into cardiovascular disease in adulthood. The balance between nitric oxide (NO) and reactive oxygen species (ROS) is disturbed in cardiovascular diseases, including hypertension. Conceivably, this balance is also disturbed in pregnancy, altering the fetal environment; however, effects of perinatal manipulation of NO and ROS on adult blood pressure (BP) are unknown. In spontaneously hypertensive rats (SHR), NO availability is decreased and ROS are increased compared with normotensive Wistar-Kyoto rats, and, despite the genetic predisposition, the perinatal environment can modulate adult BP. Our hypothesis is that a disturbed NO-ROS balance in the SHR dam persistently affects BP in her offspring. Dietary supplements, which support NO formation and scavenge ROS, administered during pregnancy and lactation resulted in persistently lower BP for up to 48 wk in SHR offspring. The NO donor molsidomine and the superoxide dismutase mimic tempol-induced comparable effects. Specific inhibition of inducible nitric oxide synthase (NOS) reduces BP in adult SHR, suggesting that inducible NOS is predominantly a source of ROS in SHR. Indeed, inducible NOS inhibition in SHR dams persistently reduced BP in adult offspring. Persistent reductions in BP were accompanied by prevention of proteinuria in aged SHR. We propose that in SHR the known increase in ANG II type 1 receptor density during development leads to superoxide production, which enhances inducible NOS activity. The relative shortage of substrate and cofactors leads to uncoupling of inducible NOS, resulting in superoxide production, activating transcription factors that subsequently again increase inducible NOS expression. This vicious circle probably is perpetuated into adult life.

Treatment with tetrahydrobiopterin reduces blood pressure in male SHR by reducing testosterone synthesis

Treatment with tetrahydrobiopterin (BH(4)) reduces blood pressure in spontaneously hypertensive rats (SHR). In the present study, we tested the hypothesis that chronic BH(4) reduces blood pressure in male SHR by reducing testosterone biosynthesis mediated by increasing nitric oxide (NO). Male SHR, aged 17-18 wk, intact or castrated, were treated for 1 wk with BH(4) (20 mg.kg(-1).day(-1) ip). After 1 wk, mean arterial pressure (MAP), serum testosterone, and nitrate/nitrite excretion (NO(x)) were measured. MAP was significantly higher in intact males than castrated males (179 +/- 2 vs. 155 +/- 4 mmHg, P < 0.001). In intact males, BH(4) caused a 17% reduction in MAP (148 +/- 2 mmHg), had no effect on NO(x), and reduced serum testosterone by 85% (24.09 +/- 2.37 vs. 3.72 +/- 0.73 ng/dl; P < 0.001). In castrated males, BH(4) had no effect on MAP (152 +/- 5 mmHg) but increased NO(x) by 38%. When castrated males were supplemented with testosterone, MAP increased to the same level as in intact males (180 +/- 7 mmHg), and BH(4) had no effect on MAP (182 +/- 7 mmHg) or NO(x). NO has been shown to decrease testosterone biosynthesis. Chronic sodium nitrite (70 mg.kg(-1).day(-1) x 1 wk) decreased MAP in intact males (150 +/- 4 mmHg) but had no effect on serum testosterone (21.46 +/- 3.08 ng/dl). The data suggest that BH(4) reduces testosterone synthesis and thereby reduces MAP in male SHR, an androgen-dependent model of hypertension. The mechanism(s) by which BH(4) reduces serum testosterone levels are not clear, but the data do not support a role for NO as a mediator.

The role of shear stress in the pathogenesis of atherosclerosis

Although the pathobiology of atherosclerosis is a complex multifactorial process, blood flow-induced shear stress has emerged as an essential feature of atherogenesis. This fluid drag force acting on the vessel wall is mechanotransduced into a biochemical signal that results in changes in vascular behavior. Maintenance of a physiologic, laminar shear stress is known to be crucial for normal vascular functioning, which includes the regulation of vascular caliber as well as inhibition of proliferation, thrombosis and inflammation of the vessel wall. Thus, shear stress is atheroprotective. It is also recognized that disturbed or oscillatory flows near arterial bifurcations, branch ostia and curvatures are associated with atheroma formation. Additionally, vascular endothelium has been shown to have different behavioral responses to altered flow patterns both at the molecular and cellular levels and these reactions are proposed to promote atherosclerosis in synergy with other well-defined systemic risk factors. Nonlaminar flow promotes changes to endothelial gene expression, cytoskeletal arrangement, wound repair, leukocyte adhesion as well as to the vasoreactive, oxidative and inflammatory states of the artery wall. Disturbed shear stress also influences the site selectivity of atherosclerotic plaque formation as well as its associated vessel wall remodeling, which can affect plaque vulnerability, stent restenosis and smooth muscle cell intimal hyperplasia in venous bypass grafts. Thus, shear stress is critically important in regulating the atheroprotective, normal physiology as well as the pathobiology and dysfunction of the vessel wall through complex molecular mechanisms that promote atherogenesis.

Oxidant stress in kidneys of spontaneously hypertensive rats involves both oxidase overexpression and loss of extracellular superoxide dismutase

Oxidant stress is an important contributor to renal dysfunction and hypertension. We have previously demonstrated that regulation of renal oxygen consumption by nitric oxide (NO) is impaired in the kidney of spontaneously hypertensive rats (SHR) due to increased superoxide production. We further explored the mechanisms of enhanced oxidant stress in the kidney of SHR. Suppression of cortical oxygen consumption by bradykinin (BK) or enalaprilat (Enal), which act through stimulation of endogenous NO, was impaired in SHR (BK: -14.1 +/- 1.2%; Enal: -15.5 +/- 1.2%) and was restored by addition of apocynin, an inhibitor of assembly of the NAD(P)H oxidase complex (BK: -21.0 +/- 0.6%; Enal: -25.3 +/- 1.4%), suggesting this as the source of enhanced superoxide production. Addition of an angiotensin type 1 receptor blocker, losartan, also restored responsiveness to control levels (BK: -22.0 +/- 1.1%; Enal: -23.6 +/- 1.3%), suggesting that ANG II is responsible for enhanced oxidase activity. A similar defect in responsiveness to BK and Enal could be induced in Wistar-Kyoto kidneys by ANG II and was reversed by a superoxide scavenger (tempol), apocynin or losartan. Immunoblotting of cortical samples demonstrated enhanced expression of endothelial NO synthase (eNOS 1.9x) and NAD(P)H oxidase components (gp91(phox) 1.6x and Rac-1 4.5x). Expression of SOD-1 and -2 were unchanged, but SOD-3 was significantly decreased in SHR (0.5x). Thus NO bioavailability is impaired in SHR owing to an ANG II-mediated increase in superoxide production in association with enhanced expression of NAD(P)H oxidase components, despite increased expression of eNOS. Loss of SOD-3, an important superoxide scavenger, may also contribute to enhanced oxidant stress.

Roles of oxidative stress and antioxidant therapy in chronic kidney disease and hypertension

PURPOSE OF REVIEW

Oxidative stress is frequently associated with, and is partly involved in, the pathogenesis of chronic renal failure, hypertension and their complications. In the past few years, considerable progress has been made in deciphering the impact and the molecular mechanism of oxidative stress in these disorders. This article is intended to provide an overview of oxidative stress in hypertension and chronic renal failure.

RECENT FINDINGS

Recent studies have provided irrefutable evidence that oxidative stress can cause hypertension and hypertension can cause oxidative stress. The upregulation of nicotinamide adenine dinucleotide phosphate (reduced form) oxidase and the tubulointerstitial accumulation of activated T cells, macrophages and superoxide-producing cells are partly responsible for oxidative stress in several models of hypertension. Antioxidant therapy alleviates hypertension, averts nuclear factor kappa B activation, and mitigates tubulointerstitial inflammation in hypertensive animals. Oxidative stress contributes to hypertension, endothelial dysfunction and brain disorders in chronic renal failure animals, and is partly caused by the upregulation of nicotinamide adenine dinucleotide phosphate (reduced form) oxidase and the downregulation of superoxide dismutase.

SUMMARY

Oxidative stress, hypertension and inflammation are closely interrelated and involve a spiralling vicious cycle that can lead to progressive deterioration of hypertension and target organ damage.

Effect of chronic tetrahydrobiopterin supplementation on blood pressure and proteinuria in 5/6 nephrectomized rats

BACKGROUND

Tetrahydrobiopterin (BH4) is a key cofactor of nitric oxide (NO) synthase. Reduced BH4 levels may mediate endothelial NO synthase uncoupling, resulting in reduced NO synthesis and enhanced oxidative stress. In rats after 5/6 nephrectomy (Nx), administration of BH4 prevents the onset of hypertension, typically observed 10 days after Nx. This effect is associated with an increased synthesis of NO. The aim of the present study was to evaluate the effect of chronic BH4 therapy on blood pressure and renal morphology.

METHODS

During an 8 week period, five groups of rats were studied: untreated 5/6 Nx rats, BH4-treated Nx rats (BH4, 10 mg/kg body weight/day administered intraperitoneally), l-arginine treated Nx rats (LA, 130 mg/kg/day), diltiazem-treated Nx rats (DILT, 30 mg/kg/day) and sham-operated rats. Treatments were commenced 24 h after surgery. Systolic blood pressure values (SBP), 24 h proteinuria (UP) and creatinine clearance rate (CCR) were assessed before and at weeks 4 and 8 of the study period. Histological changes in the kidney were evaluated at the end of the study (week 8).

RESULTS

Compared with baseline, in Nx rats both SBP and UP increased significantly (112+/-1 to 136+/- 1.4 mmHg, P<0.01 and 23+/-2 to 127 +/- 26 mg/day, P<0.01, respectively). Treatment with BH4 normalized SBP values as did treatment with LA and DILT (109+/-3, 115+/-2 and 114+/-2 mmHg, respectively). UP was markedly reduced by BH4, the reduction being similar to that obtained by LA and significantly more marked than that of DILT rats (20+/-2, 28+/-3 and 62+/- 14 mg/day, respectively). CCR was equally decreased in all Nx groups. Histological evaluation showed the development of mesangial expansion in Nx rats, an effect that was significantly blunted by all treatments.

CONCLUSIONS

In rats after 5/6 nephrectomy, BH4 supplementation initiated 24 h after surgery and maintained for 8 weeks preserved SBP, reduced UP and prevented the development of glomerular mesangial expansion.

Reactive oxygen species in vascular biology: implications in hypertension

Reactive oxygen species (ROS), including superoxide (*O2-), hydrogen peroxide (H2O2), and hydroxyl anion (OH-), and reactive nitrogen species, such as nitric oxide (NO) and peroxynitrite (ONOO-), are biologically important O2 derivatives that are increasingly recognized to be important in vascular biology through their oxidation/reduction (redox) potential. All vascular cell types (endothelial cells, vascular smooth muscle cells, and adventitial fibroblasts) produce ROS, primarily via cell membrane-associated NAD(P)H oxidase. Reactive oxygen species regulate vascular function by modulating cell growth, apoptosis/anoikis, migration, inflammation, secretion, and extracellular matrix protein production. An imbalance in redox state where pro-oxidants overwhelm anti-oxidant capacity results in oxidative stress. Oxidative stress and associated oxidative damage are mediators of vascular injury and inflammation in many cardiovascular diseases, including hypertension, hyperlipidemia, and diabetes. Increased generation of ROS has been demonstrated in experimental and human hypertension. Anti-oxidants and agents that interrupt NAD(P)H oxidase-driven *O2- production regress vascular remodeling, improve endothelial function, reduce inflammation, and decrease blood pressure in hypertensive models. This experimental evidence has evoked considerable interest because of the possibilities that therapies targeted against reactive oxygen intermediates, by decreasing generation of ROS and/or by increasing availability of antioxidants, may be useful in minimizing vascular injury and hypertensive end organ damage. The present chapter focuses on the importance of ROS in vascular biology and discusses the role of oxidative stress in vascular damage in hypertension.

Phenylalanine Improves Dilation and Blood Pressure in GTP Cyclohydrolase Inhibition-Induced Hypertensive Rats

GTP cyclohydrolase (GTPCH), the rate-limiting enzyme in the production of the nitric oxide synthase cofactor tetrahydrobiopterin (BH4), is partly regulated by the GTPCH feedback regulatory protein (GFRP). GFRP can inhibit GTPCH by end-product negative feedback, and L-phenylalanine (L-Phe) reverses this inhibition and increases BH4 biosynthesis in vitro. We hypothesized that L-Phe would increase endothelium-dependent relaxation and decrease blood pressure in rats made hypertensive by GTPCH inhibition. Di-amino-hydroxypyrimidine (DAHP, 10 mmol/L), a known inhibitor of GTPCH, was given with or without L-Phe or D-Phe (2 mmol/L) in the drinking water of rats for 3 days and blood pressure was measured via tail-cuff. Endothelium-intact aortic segments were hung in organ chambers for measurement of isometric force generation. Systolic blood pressure was increased significantly in DAHP-treated rats compared with controls. The addition of L-Phe attenuated the hypertensive effect, whereas D-Phe had no effect. Acetylcholine- and A23187-induced relaxation was decreased in aortas from DAHP-treated rats compared with controls, but was restored in aortas from DAHP+L-Phe-treated rats. Following NOS inhibition, sensitivity to sodium nitroprusside was increased in aortas from DAHP-treated rats, but restored in DAHP+L-Phe-treated rats. These results suggest that L-Phe can reverse GTPCH inhibition in vivo leading to increased vasodilation and decreased blood pressure.

Role of tetrahydrobiopterin in adrenocorticotropic hormone-induced hypertension in the rat

Adrenocorticotropic hormone (ACTH)-induced hypertension in the rat is characterized by nitric oxide deficiency. Tetrahydrobiopterin (BH4) is an essential cofactor for the enzyme nitric oxide synthase and glucocorticoids have been reported to reduce cytokine-induced BH4 production. Accordingly we hypothesized that ACTH-induced hypertension would be reversed by BH4 supplementation. Male Sprague-Dawley rats (n = 33) were treated with BH4 in vehicle (10 mg/kg/day i.p.) or vehicle alone (5 mg/kg/day i.p. of ascorbic acid in 4 mM HCl) for 10 days. ACTH (0.2 mg/kg s.c.) or saline daily injection was started 2 days after BH4 or vehicle treatment and continued for 8 days. Systolic blood pressure (SBP) was measured on alternate days using the tail cuff method. Treatment with HCl, ascorbic acid or BH4 alone had no effect on SBP. In saline treated rats, neither BH4 nor its vehicle modified SBP. In ACTH treated rats, SBP was increased in both BH4 (from 128 +/- 6 to 142 +/- 4 mmHg, T0 to T10, P < 0.0005, one way ANOVA) and vehicle groups (from 127 +/- 3 to 158 +/- 7 mmHg, T0 to T10, P < 0.001, one way ANOVA). There was no significant difference in SBP between BH4 + ACTH treated and vehicle + ACTH treated rats. Thus, daily injection of BH4 (10 mg/kg i.p.) failed to prevent the development of ACTH-induced hypertension in rat.

Effect of high-salt diet on NO release and superoxide production in rat aorta

Sprague-Dawley rats were fed either a high-salt (HS) diet (4.0% NaCl) or a low-salt (LS) diet (0.4% NaCl) for 3 days. Nitric oxide (NO) and superoxide production were assessed in the thoracic aorta by evaluating the fluorescence signal intensity from 4,5-diaminofluorescein (DAF-2DA) and dihydroethidine, respectively. Methacholine caused increased NO release in the aortas from rats on a LS but not HS diet. The SOD mimetic tempol restored methacholine-induced NO release in aortas from rats on a HS diet. Methacholine also caused superoxide production in the aortas of rats on a HS diet but not in the aortas of rats on a LS diet. Tempol and NG-monomethyl-l-arginine eliminated methacholine-induced superoxide production in the aortas of rats on a HS diet. Aortic rings from rats on the HS diet showed impaired methacholine-induced relaxation, which was improved by tempol. Tempol alone caused a NO-dependent relaxation of norepinephrine-precontracted aortas that was significantly greater in the aortas of rats on the HS diet than in vessels from rats on the LS diet. These data suggest that a HS diet impairs endothelium-dependent relaxation via reduced NO levels and increased superoxide production.

Sepiapterin decreases vasorelaxation in nitric oxide synthase inhibition-induced hypertension

Exogenous BH4 (tetrahydrobiopterin) has been shown to improve endothelial function in cardiovascular disease; however, in the presence of elevated superoxide levels and decreased nitric oxide synthase (NOS) activity, BH4 may become autoxidized, resulting in reduced vasodilation. The authors tested the hypothesis that increasing BH4 will further reduce endothelium-dependent relaxation in aortas from rats made hypertensive by NOS inhibition. N omega-nitro-L-arginine (L-NNA, approximately 49 mg/kg/d) was administered in the rats' drinking water for 4 days. Systolic blood pressures, measured by tail-cuff technique, were significantly increased in L-NNA-treated rats. Endothelium-intact aortic segments were isolated and hung in organ chambers for the measurement of isometric force generation. Aortas from L-NNA-treated rats had decreased relaxation to acetylcholine compared with controls, and this was further decreased after incubation with sepiapterin. Superoxide dismutase (SOD) restored relaxation in aortas from L-NNA-treated rats to that of control. In addition, SOD or ascorbic acid reversed the sepiapterin-induced decrease in relaxation in aortas from L-NNA treated rats. Aortas from L-NNA-treated rats in the absence and presence of sepiapterin, and sepiapterin-treated control aortas, had increased dihydroethidium staining for superoxide compared with untreated controls. These results support the hypothesis that sepiapterin further reduces vasodilation in the presence of NOS inhibition and may be caused by BH4 autoxidation.

GTP cyclohydrolase 1 inhibition attenuates vasodilation and increases blood pressure in rats

GTP cyclohydrolase 1 is the rate-limiting enzyme in production of tetrahydrobiopterin, a necessary cofactor for endothelial nitric oxide synthase. We tested the hypothesis that inhibition of tetrahydrobiopterin synthesis impairs endothelium-dependent relaxation and increase blood pressure in rats. 2,4-Diamino-6-hydroxypyrimidine (DAHP), a GTP cyclohydrolase 1 inhibitor, was given in drinking water (approximately 120 mg.kg(-1).day(-1)) to male Sprague-Dawley rats for 3 days. Systolic blood pressures were measured (tail-cuff procedure) for 3 days before and each day during DAHP treatment. Blood pressure was significantly increased after DAHP treatment (122 +/- 2 vs. 154 +/- 3 mmHg before and after DAHP, respectively; P < 0.05). Endothelium-intact aortic segments from pentobarbital sodium-anesthetized rats were isolated and hung in organ chambers for measurement of isometric force generation. Aortas from DAHP-treated rats exhibited a decreased maximal relaxation to ACh compared with controls [% relaxation from phenylephrine (10-7 M)-induced contraction: DAHP 57 +/- 6% vs. control 79 +/- 4%; P < 0.05]. Relaxation responses to A-23187 were also decreased in aortas from DAHP-treated rats compared with controls. Incubation with sepiapterin (10-4 M, 1 h), which produces tetrahydrobiopterin via a salvage pathway, restored relaxation to ACh in aortas from DAHP-treated rats. Superoxide dismutase significantly increased ACh-induced relaxation in aortas from DAHP-treated rats, whereas catalase had no effect. Endothelium-independent relaxation to sodium nitroprusside in aortas from DAHP-treated rats was not different from control rats; however, nitric oxide synthase inhibition increased sensitivity to sodium nitroprusside in aortas from DAHP-treated rats. These results support the hypothesis that GTP cyclohydrolase 1 inhibition decreases relaxation and increases blood pressure in rats.

Chronic oral supplementation with sepiapterin prevents endothelial dysfunction and oxidative stress in small mesenteric arteries from diabetic (db/db) mice

We previously reported that acute incubation with tetrahydrobiopterin (BH4) or sepiapterin, a cofactor for endothelial nitric oxide synthase and a stable precursor of BH4, respectively, enhanced the acetylcholine (Ach)-induced relaxation of isolated small mesenteric arteries (SMA) from diabetic (db/db) mice. In this study, we investigated the effect of chronic oral supplementation of sepiapterin (10 mg x kg-1 x day-1) to db/db mice on endothelium function, biopterin levels and lipid peroxidation in SMA. Oral dietary supplementation with sepiapterin had no effect on glucose, triglyceride, cholesterol levels and body weight. SMA from db/db mice showed enhanced vascular reactivity to phenylephrine, which was corrected with sepiapterin supplementation. Furthermore, Ach, but not sodium nitroprusside-induced relaxation, was improved with sepiapterin supplementation in db/db mice. BH4 levels and guanosine triphosphate cyclohydrolase I activity in SMA were similar in db/+ and db/db mice. Sepiapterin treatment had no effects on BH4 or guanosine triphosphate cyclohydrolase I activity. However, the level of dihydrobiopterin+biopterin was higher in SMA from db/db mice, which was corrected following sepiapterin treatment. Thiobarbituric acid reactive substance, malondialdehyde, a marker of lipid peroxidation, was higher in SMA from db/db mice, and was normalized by sepiapterin treatment. These results indicate that sepiapterin improves endothelial dysfunction in SMA from db/db mice by reducing oxidative stress. Furthermore, these results suggest that decreased biosynthesis of BH4 may not be the basis for endothelial dysfunction in SMA from db/db mice.

Oxidative stress promotes blood cell-endothelial cell interactions in the microcirculation

Oxidative stress occurs when the production of reactive oxygen species (ROS) exceeds the capacity of the cell to detoxify these potentially injurious oxidants using endogenous antioxidant defense systems. Conditions associated with oxidative stress include ischemia/reperfusion, hypercholesterolemia, diabetes, and hypertension. The adhesion of circulating blood cells (leukocytes, platelets) to vascular endothelium is a key element of the pro-inflammatory and prothrombogenic phenotype assumed by the vasculature in these and other disease states that are associated with an oxidative stress. There is a growing body of evidence that links the blood cell endothelial cell interactions in these conditions to the enhanced production of ROS. Potential enzymatic sources of ROS within the microcirculation include xanthine oxidase, NAD(P)H oxidase, and nitric oxide synthase. ROS can promote a pro-inflammatory/prothrombogenic phenotype within the microvasculature by a variety of mechanisms, including the inactivation of nitric oxide, the activation of redox-sensitive transcription factors (e.g., nuclear factor-kappaB) that govern the expression of endothelial cell adhesion molecules (e.g., P-selectin), and the activation of enzymes (e.g., phospholipase A(2)) that produce leukocyte-stimulating inflammatory mediators (e.g., platelet-activating factor). The extensively documented ability of different oxidant-ablating interventions to attenuate blood cell endothelial cell interactions underscores the importance of ROS in mediating the dysfunctional microvascular responses to oxidative stress.

Folic acid says NO to vascular diseases

OBJECTIVES

The possible link between folic acid or folate and tetrahydrobiopterin (H(4)B), vitamin C, polyunsaturated fatty acids (PUFAs), and nitric oxide (NO), which may explain the beneficial actions of these nutrients in various vascular conditions, was investigated.

METHODS

The literature pertaining to the actions of folic acid/folate, H(4)B, vitamin C, PUFAs, and NO was reviewed.

RESULTS

Impaired endothelial NO (eNO) activity is an early marker for cardiovascular disease. Most risk factors for atherosclerosis are associated with impaired endothelium-dependent vasodilatation due to reduced NO production. Folate not only reduces plasma homocysteine levels but also enhances eNO synthesis and shows anti-inflammatory actions. It stimulates endogenous H(4)B regeneration, a cofactor necessary for eNO synthesis, inhibits intracellular superoxide generation, and thus enhances the half-life of NO. H(4)B in turn enhances NO generation and augments arginine transport into the cells. Folic acid increases the concentration of omega-3 PUFAs, which also enhance eNO synthesis. Vitamin C augments eNO synthesis by increasing intracellular H(4)B and stabilization of H(4)B. Insulin stimulates H(4)B synthesis and PUFA metabolism, suppresses the production of proinflammatory cytokine tumor necrosis factor-alpha and superoxide anion, and enhances NO generation. The ability of folate to augment eNO generation is independent of its capacity to lower plasma homocysteine levels.

CONCLUSIONS

The common mechanism by which folic acid, H(4)B, vitamin C, omega-3 fatty acids, and L-arginine bring about their beneficial actions in various vascular diseases is by enhancing eNO production. Hence, it remains to be determined whether a judicious combination of folic acid, vitamins B12, B6, and C, H(4)B, L-arginine, and omega-3 fatty acids in appropriate amounts may form a novel approach in the prevention and management of various conditions such as hyperlipidemias, coronary heart disease, atherosclerosis, peripheral vascular disease, and some neurodegenerative conditions.

Enhanced depressor response to endothelial nitric oxide synthase gene transfer into the nucleus tractus solitarii of spontaneously hypertensive rats

Previously, we demonstrated that endothelial nitric oxide synthase (eNOS) gene transfer into the nucleus tractus solitarii (NTS) decreased blood pressure, heart rate and sympathetic nerve activity in conscious normotensive Wistar-Kyoto rats (WKY). In order to determine whether overexpression of eNOS in the NTS causes different effects on blood pressure and heart rate between spontaneously hypertensive rats (SHR) and WKY, we transfected adenovirus vectors encoding either eNOS (AdeNOS) or beta-galactosidase (Ad beta gal) into the NTS of SHR and WKY in vivo. The local expression of eNOS in the NTS was confirmed by Western blot analysis for eNOS protein, and the magnitude of expression did not differ between SHR and WKY. Blood pressure and heart rate were monitored by the use of a radio-telemetry system in a conscious state before and 7 days after the gene transfer. Systolic blood pressure (SBP) and heart rate decreased on day 7 in both AdeNOS-transfected SHR and WKY. However, the magnitude of decreases in SBP of AdeNOS-transfected SHR was greater than that of AdeNOS-transfected WKY (-24.1 +/- 2.9 vs. -15.9 +/- 2.1 mmHg, p < 0.05). Transfection of Ad beta gal into the NTS did not alter SBP in either group. A depressor response evoked by microinjection of L-glutamate into the NTS did not differ between the two strains. These results suggest that overexpression of eNOS in the NTS causes a greater depressor response in SHR than in WKY in a conscious state. An abnormality of the L-arginine-NO pathway in the NTS may be related to the hypertensive mechanism(s) of SHR.

Role of oxidative stress in atherosclerosis

The common risk factors for atherosclerosis increase production of reactive oxygen species (ROS) by endothelial, vascular smooth muscle, and adventitial cells. These ROS initiate processes involved in atherogenesis through several important enzyme systems, including xanthine oxidase, nicotinamide adenine dinucleotide phosphate (NADPH) oxidases, and nitric oxide synthase. Physical forces also regulate vascular production of ROS. Oscillatory shear, which is present at sites where atherosclerosis develops, seems a particularly potent stimulus of superoxide production. The signaling cascade for activation of the NAD(P)H oxidase by angiotensin II has recently been elucidated and seems to involve a feed-forward mechanism that permits ongoing production of ROS for prolonged periods. Oxidative stress in humans with coronary artery disease is also exacerbated by a reduction of vascular extracellular superoxide dismutase, normally an important protective enzyme against the superoxide anion.

Regulation of endothelial-type NO synthase expression in pathophysiology and in response to drugs

In many types of cardiovascular pathophysiology such as hypercholesterolemia and atherosclerosis, diabetes, cigarette smoking, or hypertension (with its sequelae stroke and heart failure) the expression of endothelial NO synthase (eNOS) is altered. Both up- and downregulation of eNOS have been observed, depending on the underlying disease. When eNOS is upregulated, the upregulation is often futile and goes along with a reduction in bioactive NO. This is due to an increased production of superoxide generated by NAD(P)H oxidase and by an uncoupled eNOS. A number of drugs with favorable effects on cardiovascular disease upregulate eNOS expression. The resulting increase in vascular NO production may contribute to their beneficial effects. These compounds include statins, angiotensin-converting enzyme inhibitors, AT1 receptor antagonists, calcium channel blockers, and some antioxidants. Other drugs such as glucocorticoids, whose administration is associated with cardiovascular side effects, downregulate eNOS expression. Stills others such as the immunosuppressants cyclosporine A and FK506/tacrolimus or erythropoietin have inconsistent effects on eNOS. Thus regulation of eNOS expression and activity contributes to the overall action of several classes of drugs, and the development of compounds that specifically upregulate this protective enzyme appears as a desirable target for drug development.

Overexpression of endothelial nitric oxide synthase accelerates atherosclerotic lesion formation in apoE-deficient mice

Nitric oxide (NO) derived from endothelial NO synthase (eNOS) is regarded as a protective factor against atherosclerosis. Therefore, augmentation of eNOS expression or NO production by pharmacological intervention is postulated to inhibit atherosclerosis. We crossed eNOS-overexpressing (eNOS-Tg) mice with atherogenic apoE-deficient (apoE-KO) mice to determine whether eNOS overexpression in the endothelium could inhibit the development of atherosclerosis. After 8 weeks on a high-cholesterol diet, the atherosclerotic lesion areas in the aortic sinus were unexpectedly increased by more than twofold in apoE-KO/eNOS-Tg mice compared with apoE-KO mice. Also, aortic tree lesion areas were approximately 50% larger in apoE-KO/eNOS-Tg mice after 12 weeks on a high-cholesterol diet. Expression of eNOS and NO production in aortas from apoE-KO/eNOS-Tg mice were significantly higher than those in apoE-KO mice. However, eNOS dysfunction, demonstrated by lower NO production relative to eNOS expression and enhanced superoxide production in the endothelium, was observed in apoE-KO/eNOS-Tg mice. Supplementation with tetrahydrobiopterin, an NOS cofactor, reduced the atherosclerotic lesion size in apoE-KO/eNOS-Tg mice to the level comparable to apoE-KO mice, possibly through the improvement of eNOS dysfunction. These data demonstrate that chronic overexpression of eNOS does not inhibit, but accelerates, atherosclerosis under hypercholesterolemia and that eNOS dysfunction appears to play important roles in the progression of atherosclerosis in apoE-KO/eNOS-Tg mice.

Impaired regulation of renal oxygen consumption in spontaneously hypertensive rats

Abnormalities of nitric oxide (NO) and oxygen radical synthesis and of oxygen consumption have been described in the spontaneously hypertensive rat (SHR) and may contribute to the pathogenesis of hypertension. NO plays a role in the regulation of renal oxygen consumption in normal kidney, so the response of renal cortical oxygen consumption to stimulators of NO production before and after the addition of the superoxide scavenging agent tempol (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) was studied. Baseline cortical oxygen consumption was similar in SHR and Wistar-Kyoto (WKY) rats (SHR: 600 +/- 55 nmol O(2)/min per g, WKY: 611 +/- 51 nmol O(2)/min per g, P > 0.05). Addition of bradykinin, enalaprilat, and amlodipine decreased oxygen consumption significantly less in SHR than WKY (SHR: bradykinin -13.9 +/- 1.9%, enalaprilat -15.3 +/- 1.6%, amlodipine -11.9 +/- 0.7%; WKY: bradykinin -22.8 +/- 1.0%, enalaprilat -24.1 +/- 2.0%, amlodipine -20.7 +/- 2.3%; P < 0.05), consistent with less NO effect in SHR. Addition of tempol reversed the defects in responsiveness to enalaprilat and amlodipine, suggesting that inactivation of NO by superoxide contributes to decreased NO availability. The response to an NO donor was similar in both groups and was unaffected by the addition of tempol. These results demonstrate that NO availability in the kidney is decreased in SHR, resulting in increased oxygen consumption. This effect is due to enhanced production of superoxide in SHR. By lowering intrarenal oxygen levels, reduced NO may contribute to susceptibility to injury and renal fibrosis. Increasing NO production, decreasing oxidant stress, or both might prevent these changes by improving renal oxygenation.

Tetrahydrobiopterin improves vascular endothelial function in ovariectomized rats

The goal of the present study is to investigate the role of tetrahydrobiopterin (BH4) in the vascular response in ovariectomized rats. Rats were randomly assigned to two groups: (1) sham group: sham-operated female rats, and (2) Ovx group: rats were ovariectomized. Our results have shown that the plasma 17 beta-estradiol levels in the Ovx group at the end of the experiment were significantly lower than in the sham group. Vasoreactivity assessed with intact aortic rings indicated that the phenylephrine-induced vasocontractile response to aortic rings from the Ovx group was greater than that of the sham group. In contrast, the vasodilator responses to acetylcholine and L-arginine (L-Arg) in the sham group were significantly greater than in the Ovx group. Differences in vasoreactivity in denuded aorta between the two groups were not noted. Moreover, exogenous BH4 significantly restored L-Arg-induced vasodilator responses in the Ovx group. However, this improvement effect was not found in the sham group. In addition, there were significant increases in superoxide anion production in aortic tissue and significant decreases in plasma nitric oxide levels in the Ovx group. Furthermore, BH4 contents in the aorta in the Ovx group were significantly decreased compared with the sham group. In conclusion, the present study demonstrates that the impairment of vascular reactivity was found in the ovariectomized rats. The possible mechanism of this defect may have resulted from the deficiency of available BH4. Thus, this study may provide a novel therapeutic strategy for the treatment of postmenopausal cardiovascular disorders.