Oxidative stress may explain how hypertension is maintained by normal levels of angiotensin II

Physiological and Biochemical Vascular Reactivity Parameters of Angiotensin II and the Action of Biased Agonist TRV023

Vascular reactivity experiments using isolated aortic rings have been widely used as a model for physiological and pharmacological studies since the early sixties. Here, we suggest several parameters that the researcher should pay attention to when investigating angiotensin II in their experimental models. Angiotensin II is one of the active peptides of the renin-angiotensin system and exerts its effect through the AT1 and AT2 receptors. Some studies seek to understand the effects of angiotensin II receptors at the vascular level by using vascular reactivity experiments. However, because of the large number of variations, there are only a handful of reactivity studies that seek to use this method. Thus, the objective of this study was to standardize experimental methods with angiotensin II, through vascular reactivity protocols. For this, variables such as basal tension, concentration interval, single concentration, curve concentration response, and multiple experiments using the same aortic ring were developed using the technique of vascular reactivity in an organ bath. This is the first study that has standardized the vascular reactivity protocol. In addition, we demonstrated the effects of TRV023-biased ligand of the AT1R at vascular sites.

A translational model of chronic kidney disease in swine

Animal models of chronic kidney disease (CKD) are critical for understanding its pathophysiology and for therapeutic development. The cardiovascular and renal anatomy and physiology of the pig are virtually identical to humans. This study aimed to develop a novel translational model of CKD that mimics the pathological features of CKD in humans. CKD was induced in seven domestic pigs by bilateral renal artery stenosis and diet-induced dyslipidemia. Animals were observed for a total of 14 wk. Renal hemodynamics and function were quantified in vivo using multi-detector CT after 6, 10, and 14 wk of CKD. Urine and blood were collected at each time-point, and blood pressure was continuously measured (telemetry). After completion of in vivo studies, pigs were euthanized, kidneys were removed, and microvascular (MV) architecture (μCT), markers of renal injury, inflammation, and fibrosis were evaluated ex vivo. Additional pigs were used as controls ( n = 7). Renal blood flow and glomerular filtration were reduced by 50% in CKD, accompanied by hypertension and elevated plasma creatinine, albumin-to-creatinine ratio and increased urinary KIM-1 and NGAL, suggesting renal injury. Furthermore, 14 wk of CKD resulted in cortical and medullary MV remodeling and loss, inflammation, glomerulosclerosis, tubular atrophy, and tubule-interstitial fibrosis compared with controls. The current study characterizes a novel model of CKD that mimics several of the pathological features observed in human CKD, irrespective of the etiology. Current approaches only slow rather than halt CKD progression, and this novel model may offer a suitable platform for the development of new treatments in a translational fashion.

Oxidative stress increases the risk of pancreatic β cell damage in chronic renal hypertensive rats

Hypertension often occurs in conjunction with insulin resistance. The purpose of this study was to evaluate whether sustained renal hypertension increases the risk of diabetes mellitus in rats, and to define the underlying mechanisms. Two-kidney, one-clip hypertensive (2K1C) rats received captopril (50 mg/kg/day), α-lipoic acid (100 mg/kg/day), or vehicle treatment for 3 months after surgery. Blood pressure was measured by tail cuff plethysmography. Oral glucose tolerance test (OGTT), immunohistochemistry, and western blotting were performed. In addition, insulin secretion from islet cells was measured. OGTT yielded abnormal results, and the number of islet cells and the size of pancreatic β/α cells were decreased in 2K1C rats. Basal insulin levels were also reduced in the plasma. Insulin secretion from pancreatic islet cells in response to high glucose was also attenuated in 2K1C rats compared with sham rats. The levels of oxidative stress markers, including 8-hydroxydeoxyguanosine and NADPH oxidase-4, were increased in pancreatic tissue and pancreatic islets in 2K1C rats. The abnormalities observed in 2K1C rats were improved by captopril or α-lipoic acid treatment. These findings indicate that sustained renal hypertension may lead to pancreatic dysfunction, increasing oxidative stress in pancreatic islets.

Antihypertensive Effects of Hydroalcoholic Extract of Crataegus Azarolus Subspecies Aronia Fruit in Rats with Renovascular Hypertension: An Experimental Mechanistic Study

BACKGROUND

Hawthorn species decreases blood pressure and relaxes precontracted vessels. This study aimed at examining the antihypertensive effect and related mechanisms of hydroalcoholic extract of Crataegus azarolus subspecies aronia fruit in rats with renovascular hypertension.

METHODS

Six groups of male Sprague-Dawley rats, each containing 6 to 8 rats, were studied. The groups comprised of one sham group and 5 renal artery-clipped groups. The sham group received vehicle (distilled water 0.5 ml/day) and the renal artery-clipped groups received vehicle or the extract at 5, 10, 20 or 30 mg/kg/day. Oral vehicle or extract was administered daily for 4 weeks following sham-operation or induction of hypertension. Systolic blood pressure and heart rate were measured weekly. Isolated aorta study was performed by last week and serum superoxide dismutase and glutathione reductase were measured. The findings were analyzed using one-way analysis of variance and Duncan's multiple range tests at P≤0.05 using SigmaStat software.

RESULTS

The data obtained after 4 weeks of treatment showed that the renal artery-clipped group receiving vehicle had significantly higher systolic blood pressure (P=0.002) and phenylephrine maximal response (P=0.01); and lower acetylcholine maximal response (P=0.01), serum superoxide dismutase (P=0.006) and serum glutathione reductase (P=0.006) than those of the sham group. The renal artery-clipped group receiving extract had significantly lower systolic blood pressure (P=0.03) and phenylephrine maximal response (P=0.01); and significantly higher acetylcholine maximal response (P=0.01), serum superoxide dismutase (P=0.015), and serum glutathione reductase (P=0.015) than those of the renal artery-clipped group receiving vehicle.

CONCLUSION

Our findings show that the hydroalcoholic extract of Crataegus azarolus subspecies aronia fruit has antihypertensive effects, which may be partly due to antioxidant and nitric oxide releasing effects.

Antenatal nicotine induces heightened oxidative stress and vascular dysfunction in rat offspring

BACKGROUND AND PURPOSE

Antenatal nicotine exposure causes aberrant vascular reactivity and increased blood pressure in adult male rat offspring in a sex-dependent manner. The present study tested the hypothesis that maternal nicotine administration increases the production of reactive oxygen species resulting in the vascular hypertensive reactivity in male offspring.

EXPERIMENTAL APPROACH

Nicotine was administered to pregnant rats via subcutaneous osmotic minipumps throughout the gestation. The vascular oxidative damage and dysfunction were determined in 5-month-old male offspring. Contraction studies were performed on isolated aortas and their expression of NADPH oxidase (Nox2)/gp91 and nox4 determined by Western blot analysis. In addition, oxidative damage in the vessel wall was determined by measuring malondialdehyde concentrations, vascular superoxide production and SOD activity.

KEY RESULTS

Antenatal nicotine significantly increased angiotensin II-induced arterial contractions in the offspring. The exaggerated vascular contractions were inhibited by both apocynin (a Nox inhibitor) and tempol (a SOD mimetic) in a concentration-dependent manner. In addition, ACh-induced relaxations were impaired in aortas isolated from the nicotine-treated offspring, which were restored by both apocynin and tempol in a concentration-dependent manner. The nicotine treatment significantly decreased the superoxide dismutase activity and increased malondialdehyde, superoxide and nitrotyrosine protein levels in the vascular wall. Consistently, antenatal nicotine exposure significantly enhanced the protein expression of NADPH oxidase Nox2/gp91, but not Nox4 in the aorta.

CONCLUSIONS AND IMPLICATIONS

The present findings suggest that antenatal nicotine exposure results in the programming of heightened oxidative stress and vascular hypertensive reactivity via a Nox2-dependent mechanism, leading to an increased risk of hypertension in adult offspring.

Isoprostanes 8-iPF2alpha-III: risk markers of premature rupture of fetal membranes?

AIMS

Isoprostanes may serve as sensitive and specific markers of in vivo oxidative stress intensity. We wanted to determine, whether or not isoprostane concentration may be considered as a risk marker of premature rupture of fetal membranes (PROM).

METHODS

On the basis of the presence of PROM and gestational maturity, a total of 128 patients were divided into: (1) preterm PROM (pPROM) group; (2) PROM at term group; (3) control preterm (C1) group and (4) control at term (C2) group. The concentrations of 8-iPF(2alpha)-III were determined using the enzyme-linked immunosorbent assay method.

RESULTS

The mean free isoprostane concentrations, examined in amniotic fluid and maternal plasma in the PROM at term patients were significantly higher than in C2 individuals (p < 0.01). The mean concentrations of free 8-iPF(2alpha)-III measured in blood plasma from women in the C1 group were significantly lower than in patients from the pPROM, PROM at term and C2 groups (p < 0.001, p < 0.00001 and p < 0.00001, respectively).

CONCLUSION

The measurement of free isoprostane concentration in maternal plasma and amniotic fluid may be considered as a laboratory marker of a PROM-risk pregnancy.

Exogenous L-arginine ameliorates angiotensin II-induced hypertension and renal damage in rats

Experiments were performed to determine whether exogenous L-arginine could ameliorate angiotensin II-induced hypertension and renal damage. Rats were instrumented with chronic indwelling femoral venous and arterial catheters for infusions of drugs and measurement of conscious arterial pressure. Arterial blood pressure significantly increased from 124+/-1 to 199+/-4 mm Hg, after 9 days of continuous infusion of angiotensin II (20 ng/kg per minute; IV; n=6 to 9). In contrast, the increase in arterial pressure after 9 days of angiotensin II infusion was significantly blunted by 45% (P=0.0003) in rats coadministered L-arginine (300 microg/kg per minute; IV; n=7 to 9). The glomerular injury index was significantly greater in rats administered angiotensin II in comparison with rats administered saline vehicle (P<0.001). Coinfusion of L-arginine significantly increased plasma nitrate/nitrite concentrations (P<0.001) and completely prevented angiotensin II-induced glomerular damage (P<0.001). Angiotensin II infusion alone and combined angiotensin II plus L-arginine infusion significantly increased urinary albumin excretion. Albuminuria in rats administered angiotensin II plus L-arginine is likely to be because of increased intraglomerular pressure. Our experiments demonstrate that L-arginine can blunt angiotensin II-induced hypertension and associated renal damage. This latter observation is most exciting because it indicates that increasing NO bioavailability, in addition to lowering arterial pressure, can greatly reduce hypertension-induced renal damage.

Vascular biology of angiotensin and the impact of physical activity

The renin-angiotensin system (RAS) is important for regulating blood pressure and extracellular fluid. The concept of the RAS has recently evolved from a classical systemic endocrine system to an appreciation of local RASs functioning in a paracrine manner, including in the vascular wall. Angiotensin II (AII), the main effector of the RAS, is a potent vasoconstrictor formed by the action of angiotensin-converting enzyme (ACE). ACE is multifunctional and also destroys the endogenous vasodilator bradykinin. A recently discovered novel ACE2 enzyme is responsible for forming a vasodilatory compound, angiotensin 1-7, from AII. Thus, the actions of ACE and ACE2 are antagonistic. Tissue actions of AII are mediated by specific receptors, AT1 and AT2, with AT1 mediating the classical actions. AT1-stimulated vasoconstricton occurs via phospholipase-D-mediated second messenger generation directly, and indirectly via the coupling of AT1 to the prooxidant enzyme NADPH oxidase. Since the vascular NADPH oxidase is a major source of vascular reactive oxygen species generation and is responsible for the breakdown of the vasodilator nitric oxide (NO), there is another potential link between RAS and regulation of vasodilatory pathways. AT2 signaling is antagonistic to AT1 signaling, and results in bradykinin and NO formation. Chronic AII signaling induces vascular dysfunction, whereas pharmacological management of the RAS can not only control blood pressure, but also correct endothelial dysfunction in hypertensives. Exercise training can also improve endothelial function in hypertensives, raising the question of whether there is a potential role for RAS in mediating the vascular effects of exercise training. Recent studies have demonstrated reductions in the expression of NADPH oxidase components in the vascular wall in response to exercise training, thus tempering one of the main cellular effectors of AII, and this is associated with reduced vascular ROS production and enhanced NO bioavailability. Importantly, it has now been demonstrated in human arteries that exercise training also tempers vascular AT1 receptor expression and AII-induced vasoconstriction, while enhancing endothelium-dependent dilation. The signals responsible for these chronic adaptations are not clearly understood, and may include changes in RAS components prompted by acute exercise. ACE genotype may have an effect on physical activity levels and on the cardiovascular responses to exercise training, and the II genotype (compared with ID and DD) is associated with the largest endothelium-dependent dilations in athletes compared with those in sedentary individuals. Thus, the tissue location of the RAS, the complement of ACE/ACE2, the receptor expression of AT1/AT2, and the ACE genotype are all variables that could impact the vascular responses to exercise training, but the responses of most of these variables to regular exercise training and the mechanisms responsible have not been systematically studied.

Mechanisms of nitrate tolerance: potential roles of folate

More than 100 years since their introduction in cardiovascular therapy, nitrates continue to be widely used in ischaemic heart disease despite incomplete knowledge of their intimate mechanism of action. Particularly, the development of a progressive attenuation of their efficacy over prolonged use (tolerance) continues to be the subject of current investigation. Newer findings point to the role of increased intracellular oxidative stress as a mechanism for tolerance and to folic acid derivatives as pharmacologic means to attenuate its development. This paper reviews nitrate mechanism of action, the history of nitrate tolerance and newer findings related to the use of folate to prevent this phenomenon.

Renal mechanisms involved in stress-induced antinatriuresis and antidiuresis in rats

The present study was conducted to investigate if changes in sodium and water excretion in stressed animals were due to modifications in the glomerular filtration rate (GFR) and to determine the participation of angiotensin II (Ang II) and alpha and beta-adrenoceptors on sodium and water renal excretion in rats subjected to immobilization stress (IMO). Male Wistar rats (250-300 g) were randomly separated into five different groups and vehicle (0.9% NaCl) via intraperitoneal (i.p.) or propanolol (3 mg/kg i.p.) or captopril (6 mg/kg i.p.) or yohimbine (3 mg/kg i.p.) or prazosin (1 mg/kg i.p.) were injected respectively. During experimental measurements, the animals were kept in metabolic cages for 6 h and sodium, potassium and water renal excretion and saline (1.5% NaCl) and water intake were determined at day 1 (drug effect) and day 7 (drug + IMO effects). GFR was measured by creatinine clearance in control and IMO rats. A stress-induced antinatriuresis and antidiuresis was reversed by alpha 1 and alpha 2-adrenoceptor antagonists, while captopril inhibited only the antidiuresis and propranolol had no effect on either parameter. No differences were observed in creatinine clearance in the studied groups. Since yohimbine blocks alpha 2-adrenoceptors and prazosin blocks alpha 1-adrenoceptors and alpha 2B-adrenoceptors, the stress-induced renal sodium reabsorption mainly could be attributed to alpha 2B-adrenoceptors. The present results indicate that beta-adrenoceptors do not participate in this response and, Ang II only reverses the antidiuresis and shows a slight participation in antinatriuresis. The increment in sodium and water reabsorption caused by IMO occurred without changes in the glomerular filtration rate.

Involvement of TP and EP3 receptors in vasoconstrictor responses to isoprostanes in pulmonary vasculature

Although isoprostanes generally act on smooth muscle via TXA(2)-selective prostanoid receptors (TPs), some suggest other prostanoid receptors or possibly even a novel isoprostane-selective receptor might be involved. We studied contractions to several isoprostanes in porcine pulmonary vasculature using organ bath techniques. 8-iso-prostaglandin E(2) (PGE(2)) was the most potent and efficacious of the isoprostanes, with a log EC(50) of -7.0 +/- 0.2 in the pulmonary artery and -6.8 +/- 0.2 in the pulmonary vein. The responses to all the isoprostanes were essentially completely blocked by the TP receptor antagonist ICI 192605 [4(Z)-6-[(2,4,5-cis)2-(2-chlorophenyl)-4-(2-hydroxyphenyl)1,3-dioxan-5-yl]hexenoic acid], and the equilibrium dissociation constants for ICI 192605 competing with U46619 or 8-iso-PGE(2) were both approximately 2 nM, indicating that isoprostane-evoked responses involve primarily TP receptors. Only 8-iso-PGE(2) was able to evoke substantial contractions in the presence of ICI 192605 and only in the pulmonary vein. The EC(50) of these ICI 192605-insensitive responses was -6.1 +/- 0.2. Using a variety of prostanoid agonists, we found the pulmonary vein lacked excitatory PGF(2alpha)-selective prostanoid receptor (FP) or PGD(2)-selective prostanoid receptor (DP) but expressed excitatory EP(3) receptors. The ICI 192605-insensitive responses to 8-iso-PGE(2) were unaffected by the EP(1) antagonist SC-19220 [8-chloro-debenz[b,f][1,4]oxazepine-10(11H)-carboxy-(2-acetyl) hydrazine; 10(-5) M] but were antagonized by the less selective DP/EP(1)/EP(2) antagonist AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid; 10(-5) M) or by cyclopiazonic acid (10(-5) M; depletes the internal Ca(2+) store). Our data indicate that, whereas 8-iso-PGE(2) constricts pulmonary vasculature primarily through TP receptors, a substantial portion of this response is also directed through EP(3) receptors or possibly a novel isoprostane receptor.

Differential effects of pentaerythritol tetranitrate and nitroglycerin on the development of tolerance and evidence of lipid peroxidation: a human in vivo study

OBJECTIVES

We investigated the development of nitrate tolerance after continuous exposure to nitroglycerin (GTN) as compared with pentaerythritol tetranitrate (PETN) in humans.

BACKGROUND

Sustained therapy with GTN causes tolerance and has been associated with increased production of free oxygen radicals by the endothelium. Pentaerythritol tetranitrate is an organic nitrate that has been used in the therapy of angina. There have been no investigations concerning the development of tolerance to PETN in humans. Animal investigations suggested that continuous therapy with PETN does not cause increased free radical production or hemodynamic tolerance.

METHODS

We randomized 30 healthy volunteers to continuous GTN (0.6 mg/h/24 h), long-acting PETN (60 mg orally three times a day) or no treatment (control group) for seven days. We studied systemic blood pressure responses and venous volume responses to GTN with strain-gauge plethysmography. The levels of cytotoxic aldehydes and isoprostanes were measured as markers of free radical-mediated lipid peroxidation.

RESULTS

Tolerance, as demonstrated by blood pressure and forearm plethysmography, developed in the GTN group and was absent in the PETN group (p < 0.05). Therapy with GTN was associated with a significant increase in plasma markers of lipid peroxidation. This response was not observed in those treated with PETN (isoprostanes: control: 38 +/- 5; GTN: 59 +/- 6; PETN: 38 +/- 3 microg/ml; p < 0.005).

CONCLUSIONS

Treatment with PETN does not cause tolerance and is not associated with evidence of increased free radical production.