Tetrahydrobiopterin improves vascular endothelial function in ovariectomized rats

Acute ascorbic acid infusion increases left ventricular diastolic function in postmenopausal women

OBJECTIVES

We tested the hypothesis that oxidative stress contributes to reductions in left ventricular diastolic (LV) function in estrogen-deficient postmenopausal women, related in part to reduced nitric oxide (NO) bioavailability.

STUDY DESIGN

LV diastolic function - recorded using transthoracic echocardiography and determined as the peak early (E) to late (A) mitral inflow velocity ratio and the E to peak early (e') mitral annular velocity ratio - and brachial artery flow mediated dilation (FMD), a biomarker of NO bioavailability, were measured during acute systemic infusions of saline (control) and ascorbic acid (experimental model to decrease oxidative stress) in healthy premenopausal women (N=14, 18-40 years) and postmenopausal women (N=23, 45-75 years).

RESULTS

The E/A ratio was lower (1.16[1.06-1.33] vs 1.65[1.5-2.3]; median[interquartile range]) and the E/e' ratio was elevated (8.8[7.6-9.9] vs. 6.6[5.5-7.3]) in postmenopausal compared with premenopausal women, indicating reduced LV diastolic function. E/A and E/e' were correlated with FMD (r=0.54 and r=-0.59, respectively, both P<0.01). Ascorbic acid infusion improved both FMD (5.4±2.0% to 7.8±2.6%) and E/e' (to 8.1[7.2-9.7], P=0.01) in postmenopausal women but not in premenopausal women. Ascorbic acid did not change E/A in either group.

CONCLUSION

The current study provides evidence that oxidative stress contributes to reduced LV diastolic function in estrogen-deficient postmenopausal women, possibly by reducing the availability of NO.

Hyperaggregability and impaired nitric oxide production in platelets from postmenopausal women

OBJECTIVE

Cardiovascular mortality increases after menopause in women. Nitric oxide is essential for proper platelet function inhibiting its aggregation and maintaining vascular haemostasis. Here, we investigated whether platelet function and intraplatelet l-arginine-nitric oxide pathway are impaired in postmenopausal women.

STUDY DESIGN

Cross-sectional.

MAIN OUTCOMES MEASURES

Blood was collected from 16 premenopausal and 12 postmenopausal women without any additional risk factor for cardiovascular disease. Platelet reactivity was measured by light transmission aggregometry. l-Arginine-nitric oxide pathway was assessed measuring transmembrane l-[(3)H]-arginine transport, nitric oxide synthase activity by the citrulline assay, and arginase activity by the conversion of l-[(14)C]arginine to l-[(14)C]-urea. The activity of antioxidant enzymes was measured by spectrophotometric assays. Protein expression was determined by Western blotting.

RESULTS

Platelet aggregation was increased in postmenopausal compared to premenopausal women. Postmenopausal women demonstrated reduced plasma levels of l-arginine, a lower nitric oxide synthase activity, similar endothelial and inducible nitric oxide synthase expression, and a compensatory increase in l-arginine transmembrane transport. Arginase expression and activity did not differ between groups. In regard to oxidative stress, no differences between groups were observed NAPDH oxidase subunits expression and protein carbonylation. However, the activity of the antioxidant enzyme superoxide dismutase and catalase protein levels in platelets were higher in postmenopausal women.

CONCLUSION

Postmenopausal women present increased platelet reactivity, which may be due to a reduction in intraplatelet nitric oxide synthesis. Platelet hyperaggregability is known to be associated with arterial and venous thromboembolic event; therefore, it may contribute to the heightened risk of cardiovascular adverse events in this population.

Vascular Aging across the Menopause Transition in Healthy Women

Vascular aging, featuring endothelial dysfunction and large artery stiffening, is a major risk factor for developing cardiovascular disease (CVD). In women, vascular aging appears to be accelerated during the menopause transition, particularly around the late perimenopausal period, presumably related to declines in ovarian function and estrogen levels. The mechanisms underlying endothelial dysfunction and large artery stiffening with the menopause transition are not completely understood. Oxidative stress and the proinflammatory cytokine tumor necrosis factor-α contribute to endothelial dysfunction and large artery stiffening in estrogen-deficient postmenopausal women. Habitual endurance exercise attenuates the age-related increase in large artery stiffness in estrogen-deficient postmenopausal women and can reverse arterial stiffening to premenopausal levels in estrogen-replete postmenopausal women. In contrast, estrogen status appears to play a key permissive role in the adaptive response of the endothelium to habitual endurance exercise in that endothelial improvements are absent in estrogen-deficient women but present in estrogen-replete women. We review here the current state of knowledge on the biological defects underlying vascular aging across the menopause transition, with particular focus on potential mechanisms, the role of habitual exercise in preserving vascular health, and key areas for future research.

Tetrahydrobiopterin improves endothelial function and decreases arterial stiffness in estrogen-deficient postmenopausal women

The mechanisms mediating arterial stiffening with aging and menopause are not completely understood. We determined whether administration of tetrahydrobiopterin (BH(4)), a critical cofactor for endothelial nitric oxide synthase to produce nitric oxide, would increase vascular endothelial-dependent vasodilatory tone and decrease arterial stiffness in estrogen-deficient postmenopausal women. Additionally, we examined whether the beneficial effects of estrogen on vascular function were possibly related to BH(4). Arterial stiffness (carotid artery compliance) and endothelial-dependent vasodilation [brachial artery flow-mediated dilation (FMD)] were measured in postmenopausal (n = 24; 57 ± 1 yr, mean ± SE) and eumenorrheic premenopausal (n = 9; 33 ± 2 yr) women before and 3 h after the oral administration of BH(4). Subsequently, in postmenopausal women, vascular testing (before and after BH(4)) was repeated following randomization to either 2 days of transdermal estradiol or placebo. Baseline carotid artery compliance and brachial artery FMD were lower in postmenopausal than in premenopausal women (P < 0.0001). BH(4) administration increased carotid artery compliance (0.61 ± 0.05 to 0.73 ± 0.04 mm(2)·mmHg(-1)·10(-1) vs. baseline, P < 0.0001) and brachial artery FMD (P < 0.001) in postmenopausal women but had no effect in premenopausal women (P = 0.62). Carotid artery compliance (0.59 ± 0.05 to 0.78 ± 0.06 mm(2)·mmHg(-1)·10(-1), P < 0.001) and FMD increased in postmenopausal women in response to estradiol (P = 0.02) but were not further improved with the coadministration of BH(4), possibly because estrogen increased BH(4) bioavailability. Carotid artery compliance and FMD increased with BH(4) in the placebo group (P = 0.02). Although speculative, these results suggest that reduced vascular BH(4) may be an important contributor to arterial stiffening in estrogen-deficient postmenopausal women, related in part to reduced endothelial-dependent vasodilatory tone.

Tetrahydrobiopterin restores diastolic function and attenuates superoxide production in ovariectomized mRen2.Lewis rats

After oophorectomy, mRen2.Lewis rats exhibit diastolic dysfunction associated with elevated superoxide, increased cardiac neuronal nitric oxide synthase (nNOS) expression, and diminished myocardial tetrahydrobiopterin (BH₄) content, effects that are attenuated with selective nNOS inhibition. BH₄ is an essential cofactor of nNOS catalytic activity leading to nitric oxide production. Therefore, we assessed the effect of 4 wk BH₄ supplementation on diastolic function and left ventricular (LV) remodeling in oophorectomized mRen2.Lewis rats compared with sham-operated controls. Female mRen2.Lewis rats underwent either bilateral ovariectomy (OVX) (n = 19) or sham operation (n = 13) at 4 wk of age. Beginning at 11 wk of age, OVX rats were randomized to receive either BH₄ (10 mg/kg · d) or saline, whereas the sham rats received saline via sc mini-pumps. Loss of ovarian hormones reduced cardiac BH₄ when compared with control hearts; this was associated with impaired myocardial relaxation, augmented filling pressures, increased collagen deposition, and thickened LV walls. Additionally, superoxide production increased and nitric oxide decreased in hearts from OVX compared with sham rats. Chronic BH₄ supplementation after OVX improved diastolic function and attenuated LV remodeling while restoring myocardial nitric oxide release and preventing reactive oxygen species generation. These data indicate that BH₄ supplementation protects against the adverse effects of ovarian hormonal loss on diastolic function and cardiac structure in mRen2.Lewis rats by restoring myocardial NO release and mitigating myocardial O₂⁻ generation. Whether BH₄ supplementation is a therapeutic option for the management of diastolic dysfunction in postmenopausal women will require direct testing in humans.

Resveratrol inhibits rat aortic vascular smooth muscle cell proliferation via estrogen receptor dependent nitric oxide production

Vascular smooth muscle cell (VSMC) proliferation is pivotal in the progression of hypertension, atherosclerosis, and restenosis. Resveratrol is a grape polyphenol that is implicated as an important contributor to red wine's vascular protective effects. Its antimitogenic action on VSMC is attributed to an array of pleiotropic effects, including modulation of the estrogen receptor (ER). To elucidate the mechanisms underlying resveratrol-mediated ER modulation and its inhibition of VSMC proliferation, we treated VSMC with resveratrol with or without the ER antagonist ICI 182,780 and measured cell proliferation and nitric oxide (NO) production. Resveratrol dose-dependently decreased VSMC DNA synthesis, with a half maximal inhibitory concentration (IC50) of 3.73+/-0.57 microM, and dramatically slowed cell growth, but did not induce VSMC apoptosis. Resveratrol-mediated decrease in proliferation was reversed by cotreatment with ICI 182,780, and resveratrol effectively competed with 17beta-estradiol for binding to the ER, exhibiting an IC50 of 8.92+/-0.14 microM. Resveratrol induced a sustained increase in ER-dependent NO production. Further, resveratrol-mediated decrease in VSMC proliferation was blunted by cotreatment with the general nitric oxide synthase (NOS) inhibitor N5-(1-Iminomethyl)-L-ornithine, dihydrochloride or with the inducible NOS (iNOS)-selective inhibitor S,S'-1,4-phenylene-bis (1,2-ethanediyl)bis-isothiourea, dihydrobromide, but not with the neuronal NOS-selective inhibitor 7-nitroindazole. Though resveratrol did not alter iNOS protein levels, it dose-dependently increased levels of iNOS activity, of the iNOS cofactor tetrahydrobiopterin (BH4), and of guanosine triphosphate cyclohydrolase I protein, the rate-limiting enzyme in BH4 biosynthesis. In addition, all of these effects were abolished by cotreatment with ICI 182,780. Thus, the antimitogenic effects of resveratrol on VSMC may be mediated by an ER-induced increase in iNOS activity.

Discoordinate regulation of renal nitric oxide synthase isoforms in ovariectomized mRen2. Lewis rats

Estrogen depletion markedly exacerbates hypertension in female congenic mRen2. Lewis rats, a model of tissue renin overexpression. Because estrogen influences nitric oxide synthase (NOS) and NO may exert differential effects on blood pressure, the present study investigated the functional expression of NOS isoforms in the kidney of ovariectomized (OVX) mRen2. Lewis rats. OVX-mRen2. Lewis exhibited an increase in systolic blood pressure (SBP) of 171 +/- 5 vs. 141 +/- 7 mmHg (P < 0.01) for intact littermates. Renal cortical mRNA and protein levels for endothelial NOS (eNOS) were reduced 50-60% (P < 0.05) and negatively correlated with blood pressure. In contrast, cortical neuronal NOS (nNOS) mRNA and protein levels increased 100 to 300% (P < 0.05). In the OVX kidney, nNOS immunostaining was more evident in the macula densa, cortical tubules, and the medullary collecting ducts compared with the intact group. To determine whether the increase in renal nNOS expression constitutes a compensatory response to the reduction in renal eNOS, we treated both intact and OVX mRen2. Lewis rats with the selective nNOS inhibitor L-VNIO from 11 to 15 wk of age. The nNOS inhibitor reduced blood pressure in the OVX group (185 +/- 3 vs. 151 +/- 8 mmHg, P < 0.05), but pressure was not altered in the intact group (146 +/- 4 vs. 151 +/- 4 mmHg). In summary, exacerbation of blood pressure in the OVX mRen2. Lewis rats was associated with the discoordinate regulation of renal NOS isoforms. Estrogen sensitivity in this congenic strain may involve the influence of NO through the regulation of both eNOS and nNOS.

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.

Decreases in renal functional reserve and proximal tubular fluid output in conscious oophorectomized rats: normalization with sex hormone substitution

Age-dependent glomerulosclerosis with reduced GFR develops earlier among men than among women. Therefore, whether female sex hormones could prevent the age-dependent decrease in GFR was investigated. The kidney function in oophorectomized rats treated with placebo (OOX group), estrogen (OOX+E(2) group), or estrogen plus progesterone (OOX+E(2)+P group) for 5 mo and in sham-operated rats (sham group) was examined. The rats were 13 mo of age at the time of the investigation. They were conscious and chronically instrumented. The results demonstrated that estrogen, alone or in combination with progesterone, was without effect on the baseline GFR and effective renal plasma flow but prevented severe decreases in the renal functional reserve and fractional proximal tubular fluid output (lithium clearance technique, fractional lithium excretion), which were observed in the OOX group. The renal functional reserve (estimated by stimulation with glycine) was -223 +/- 151, 483 +/- 129, 675 +/- 76, and 208 +/- 140 micro l/min in the OOX, OOX+E(2), OOX+E(2)+P, and sham groups, respectively. Fractional lithium excretion was 12.4 +/- 3.1, 26.8 +/- 2.0, 31.8 +/- 2.5, and 23.6 +/- 3.3% in the OOX, OOX+E(2), OOX+E(2)+P, and sham groups, respectively. In conclusion, oophorectomy at the age of 8 mo did not produce a decrease in baseline GFR in female rats within a period of 5 mo. However, oophorectomy led to severe decreases in the renal functional reserve and fractional proximal tubular fluid output. Both effects were prevented with administration of estrogen. Sham-operated rats demonstrated values for renal functional reserve and fractional lithium excretion that were between those observed for the OOX group and the groups treated with sex hormones.

The two faces of endothelial nitric oxide synthase in the pathophysiology of atherosclerosis

In the endothelium, nitric oxide (NO) is constitutively generated from the conversion of L-arginine to L-citrullin by the enzymatic action of endothelial NO synthase (eNOS). An impairment of endothelium-dependent relaxation (EDR) is present in atherosclerotic vessels even before vascular structural changes occur, and represents the reduced eNOS-derived NO activity. Because of its multiple biological actions, NO from eNOS is believed to act as an anti-atherogenic molecule. On the other hand, there is increased production of superoxide in atherosclerotic vessels, which promotes atherogenesis. Recently it is revealed that eNOS becomes dysfunctional and produces superoxide rather than NO under various pathological conditions in which tissue levels of BH4 are reduced. The pathological role of dysfunctional eNOS has attracted attentions in vascular disorders including atherosclerosis, in which abnormal pteridine metabolisms in vascular tissue including decreased BH4 levels and increased BH2 levels have been demonstrated. The presence of dysfunctional eNOS may not only impair EDR but also accelerate lesion formation in atherosclerotic vessels. This review focuses on two faces of eNOS as both an NO- as well as superoxide-producing enzyme depending on tissue pteridine metabolisms in the pathophysiology of atherosclerosis.

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.