Endothelial dysfunction accompanies a pro-oxidant, pro-diabetic challenge in the insulin resistant, obese Zucker rat in vivo

Review: Endothelial dysfunction in type 2 diabetes

Dysfunction of the endothelial cell monolayer leads to increased vascular tone and permeability and a prothrombotic environment. Type 2 diabetes is a state of insulin resistance, hyperglycaemia and dyslipidaemia characterised by high cardiovascular risk and accelerated atherosclerosis. Many mechanisms by which hyperglycaemia can result in endothelial dysfunction have now been identified. However, the presence of endothelial dysfunction in normoglycaemic first-degree relatives and insulin resistant subjects is less well understood and less readily explained by `confounding' variables. We suggest that insulin's effects on glucose transport in classical target tissues and nitric oxide production in the endothelium are fundamentally linked at a molecular level. It is expected that greater understanding of these underlying mechanisms will lead to novel approaches to prevention of cardiovascular disease in both diabetic and non-diabetic subjects.

Vascular smooth muscle-specific deletion of the leptin receptor attenuates leptin-induced alterations in vascular relaxation

Obesity is a risk factor for cardiovascular disease and is associated with increased plasma levels of the adipose-derived hormone leptin. Vascular smooth muscle cells (VSMC) express leptin receptors (LepR); however, their physiological role is unclear. We hypothesized that leptin, at levels to mimic morbid obesity, impairs vascular relaxation. To test this, we used control and VSM-LepR knockout mice (VSM-LepR KO) created with a tamoxifen-inducible specific Cre recombinase to delete the LepR gene in VSMC. Control (10-12 wk old) and VSM-LepR KO (10-12 wk old) mice were fed a diet containing tamoxifen (50 mg/kg) for 6 wk, after which vascular reactivity was studied in isolated carotid arteries using an organ chamber bath. Vessels were incubated with leptin (100 ng/ml) or vehicle (0.1 mM Tris·HCl) for 30 min. Leptin treatment resulted in significant impairment of vessel relaxation to the endothelial-specific agonist acetylcholine (ACh). When these experiments were repeated in the presence of the superoxide scavenger tempol, relaxation responses to ACh were restored. VSM-LepR deletion resulted in a significant attenuation of leptin-mediated impaired ACh-induced relaxation. These data show that leptin directly impairs vascular relaxation via a VSM-LepR-mediated mechanism, suggesting a potential pathogenic role for leptin to increase cardiovascular risk during obesity.

L-tryptophan ethyl ester dilates small mesenteric arteries by inhibition of voltage-operated calcium channels in smooth muscle

BACKGROUND AND PURPOSE

L-tryptophan (L-W) is a precursor of the vasoconstrictor, 5-HT. However, acute administration of L-W ethyl ester (L-Wee) lowered blood pressure. The mechanism of action is unknown. This study compares the vascular effects of L-W and L-Wee in intact animals, isolated aortic rings, small mesenteric arteries (MA) and explores possible mechanisms by studies in vascular smooth muscle cells (VSMC) of MA.

EXPERIMENTAL APPROACH

Effects of L-W or L-Wee (5-50 mg kg(-1) , i.v.) on mean arterial pressure (MAP) and heart rate (HR) were determined in male Sprague-Dawley rats. The effects of L-W and L-Wee on basal tone and of phenylephrine- or KCl-induced contractions of aortic and MA rings were assessed. Effects of L-Wee and L-W on voltage-operated calcium channels (VOCC) of VSMC of MA were also examined in patch-clamp studies.

KEY RESULTS

Administration of L-Wee, but not L-W, evoked a rapid and transient dose-dependent decrease in MAP and HR. While both agents failed to affect basal tone, L-Wee decreased, concentration-dependently, (I(max) > 98%) tension responses to phenylephrine and KCl in an endothelium-independent manner in aorta (IC(50) 2 mM) and MA (IC(50) 17 µM). L-Wee evoked concentration-dependent inhibition of VOCC currents (IC(50) 12 µM; I(max) 90%) in VSMC of MA.

CONCLUSIONS AND IMPLICATIONS

Esterified L-W (L-Wee), but not L-W, preferentially relaxed resistance vessels rather than conduit vessels. These effects were associated with blockade of VOCC by L-Wee. Our findings suggest that the falls in MAP and HR induced by L-Wee were due to blockade of VOCC by L-Wee.

Differential responses to blood pressure and oxidative stress in streptozotocin-induced diabetic Wistar-Kyoto rats and spontaneously hypertensive rats: effects of antioxidant (honey) treatment

Oxidative stress is implicated in the pathogenesis and/or complications of hypertension and/or diabetes mellitus. A combination of these disorders increases the risk of developing cardiovascular events. This study investigated the effects of streptozotocin (60 mg/kg; ip)-induced diabetes on blood pressure, oxidative stress and effects of honey on these parameters in the kidneys of streptozotocin-induced diabetic Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Diabetic WKY and SHR were randomized into four groups and received distilled water (0.5 mL) and honey (1.0 g/kg) orally once daily for three weeks. Control SHR had reduced malondialdehyde (MDA) and increased systolic blood pressure (SBP), catalase (CAT) activity, and total antioxidant status (TAS). SBP, activities of glutathione peroxidase (GPx) and glutathione reductase (GR) were elevated while TAS was reduced in diabetic WKY. In contrast, SBP, TAS, activities of GPx and GR were reduced in diabetic SHR. Antioxidant (honey) treatment further reduced SBP in diabetic SHR but not in diabetic WKY. It also increased TAS, GSH, reduced glutathione (GSH)/oxidized glutathione (GSSG) ratio, activities of GPx and GR in diabetic SHR. These data suggest that differences in types, severity, and complications of diseases as well as strains may influence responses to blood pressure and oxidative stress.

Protective effects of antioxidant-fortified diet on renal function and metabolic profile in obese Zucker rat

Oxidative stress contributes to the pathophysiology of type 2 diabetes mellitus and its complications, including nephropathy. The current study was designed to test the hypothesis that a diet fortified with antioxidants would be beneficial to delay or prevent the progression of this disease. Male and female Zucker fa/fa rats were fed a control or an antioxidant (AO)-fortified diet starting at 4 weeks of age. Metabolic parameters, renal function, and renal histopathology were analyzed at 6, 13, and 20 weeks of age. Females on the AO diet had significantly lower blood glucose at 6 and 13 weeks, less severe renal pathology at 20 weeks, and higher glomerular filtration rates (GFR) at 20 weeks than age-matched females on the regular diet (P < 0.05). Metabolic parameters including blood glucose, insulin resistance, and serum cholesterol, and mean arterial pressure (MAP), worsened with age in both males and females, as expected. GFR decreased and renal pathology also became more severe with age. Finally, females on the AO diet had higher GFRs and lower MAP at 20 weeks than males on the same diet. This may denote a protective effect of the AO diet in females, but not in males. These findings may have implications for the role of antioxidants as therapy in humans with T2DM.

15-Lipoxygenase metabolites contribute to age-related reduction in acetylcholine-induced hypotension in rabbits

Arachidonic acid (AA) metabolites from the 15-lipoxygenase-1 (15-LO-1) pathway, trihydroxyeicosatrienoic acids (THETAs) and hydroxy-epoxyeicosatrienoic acids (HEETAs), are endothelium-derived hyperpolarizing factors (EDHFs) and relax rabbit arteries. Rabbit vascular 15-LO-1 expression, THETA and HEETA synthesis, and nitric oxide and prostaglandin-independent relaxations to acetylcholine (ACh) and AA decreased with age (neonates to 16-wk-old). We characterized age-dependent ACh-hypotensive responses in vivo in 1-, 4-, 8-, and 16-wk-old rabbits and the contribution of THETAs and HEETAs to these responses. In anesthetized rabbits, blood pressure responses to ACh (4-4,000 ng/kg) were determined in the presence of vehicle or various inhibitors. ACh responses decreased with age (P > 0.001). In the absence or presence of N(omega)-nitro-l-arginine methyl ester (l-NAME) and indomethacin (Indo), maximum responses in 1 (-54.7 +/- 7.4 and -37.9 +/- 3.9%)- and 4 (-48.8 +/- 2.4 and -35.5 +/- 7.8%)-wk-old rabbits were higher than 8 (-30.0 +/- 2.8 and -26.6 +/- 4.4%)- and 16 (-36.7 +/- 3.5 and -27.3 +/- 10%)-wk-old rabbits. A lipoxygenase inhibitor, BW755C, reduced THETA and HEETA synthesis in mesenteric arteries. In the presence of Indo and N(omega)-nitro-l-arginine, ACh relaxations were reduced by BW755C to a greater extent in the mesenteric arteries from the younger rabbits. In 4-wk-old rabbits treated with l-NAME and Indo, the maximum ACh hypotension was reduced by the potassium channel inhibitors apamin and charybdotoxin to -6.9 +/- 0.9%, by apamin alone to -19.5 +/- 1.4%, and by BW755C to -18.8 +/- 3.5%. The present study indicates that the age-related decrease in ACh-induced hypotension is mediated by the decreased synthesis of the 15-LO-1 metabolites THETAs and HEETAs.

Oxidant stress, immune dysregulation, and vascular function in type I diabetes

Although high glucose is an important contributor to diabetic vasculopathies, complications still occur in spite of tight glycemic control, suggesting that some critical event prior to or concurrent with hyperglycemia may contribute to early vascular changes. Utilizing previously published and new experimental evidence, this review will discuss how prior to the hyperglycemic state, an imbalance between oxidants and antioxidants may contribute to early vascular dysfunction and set in motion proinflammatory insults that are further amplified as the diabetes develops. This imbalance results from the resetting of the equilibrium between vessel superoxide/H(2)O(2) production and/or decreased antioxidant defenses. Such an imbalance may cause endothelial dysfunction, characterized by abnormal endothelium-dependent vasoreactivity, as the first sign of blood vessel damage, followed by morphological changes of the vessel wall and inflammation. As such, increased oxidant stress in preglycemic states may be a critically central initiating event that underlies the pathogenesis of life-threatening vascular diseases in autoimmune diabetes. This review focuses on the relationship between oxidative stress, immune dysregulation, and vascular injury in type 1 diabetes, and how the discovery of novel pathways of vascular disease in nonobese diabetic mice may direct future studies in patients with type 1 diabetes.

Flavonoid-rich grapeseed extracts: a new approach in high cardiovascular risk patients?

The management of traditional risk factors such as hypertension and dyslipidaemia has been successful in reducing the development of cardiovascular disease. However, this has not resulted in the amelioration of complications; prompting attention to be focused on novel markers of vascular risk such as endothelial dysfunction (a determinant of vascular tone), vascular inflammation, oxidative stress and insulin resistance. With an ever-growing interest in plant-derived products, agents that could have a beneficial effect on this complex web of pathophysiology have thus been a major area of research and interest. Flavonoids have been a major focus of attention since the days of the French paradox and the presence of high quantity of flavonoids in grapeseed extracts has prompted research looking at its effects on novel markers of vascular risk. This review briefly summarises mechanisms implicated in the development of vascular disease and then focuses upon the potential role of the antioxidant properties of flavonoid-rich grapeseed extracts in the reversal of these processes.

EDHF-mediated rapid restoration of hypotensive response to acetylcholine after chronic, but not acute, nitric oxide synthase inhibition in rats

Several in vitro studies have shown that endothelium-dependent vasodilatation is maintained by endothelium-derived hyperpolarizing factor (EDHF) or prostacyclin in vessels isolated from endothelial nitric oxide synthase knockout mice. Since this has not been addressed by in vivo studies, we sought to define the magnitude and the onset time of this compensation by recording blood pressure responses to endothelium-dependent vasodilators in rats treated acutely or chronically with the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME). Groups of male Sprague-Dawley rats were given plain water (control) or L-NAME (0.7 mg/ml) in drinking water for 1 day, 5 days, 3 wks or 6 wks. Dose-dependent hypotensive responses to acetylcholine, bradykinin and sodium nitroprusside were determined in anesthetized rats before and after acute intravenous infusion of either L-NAME or a combination of apamin plus charybdotoxin that would selectively inhibit EDHF. Acute L-NAME treatment increased the mean arterial pressure and inhibited acetylcholine- and bradykinin-induced fall in blood pressure in control but not in chronic L-NAME treated rats. The endothelium-dependent hypotensive responses to acetylcholine and bradykinin were restored in rats treated with L-NAME after a time period of 24 h along with increased sensitivity to sodium nitroprusside and reduced plasma nitrate+nitrite levels. While apamin+charybdotoxin pretreatment inhibited the responses to acetylcholine and bradykinin in both acute and chronic L-NAME treated groups, it was more pronounced in the latter group. In conclusion, chronic inhibition of nitric oxide synthase results in the development of a compensatory hypotensive response to acetylcholine within 24 h and this is mediated by EDHF.

Twenty-five years since the discovery of endothelium-derived relaxing factor (EDRF): does a dysfunctional endothelium contribute to the development of type 2 diabetes?

Twenty-five years ago, the discovery of endothelium-derived relaxing factor opened a door that revealed a new and exciting role for the endothelium in the regulation of blood flow and led to the discovery that nitric oxide (NO) multi-tasked as a novel cell-signalling molecule. During the next 25 years, our understanding of both the importance of the endothelium as well as NO has greatly expanded. No longer simply a barrier between the blood and vascular smooth muscle, the endothelium is now recognized as a complex tissue with heterogeneous properties. The endothelium is the source of not only NO but also numerous vasoactive molecules and signalling pathways, some of which are still not fully characterized such as the putative endothelium-derived relaxing factor. Dysfunction of the endothelium is a key risk factor for the development of macro- and microvascular disease and, by coincidence, the discovery that NO was generated in the endothelium corresponds approximately in time with the increased incidence of type 2 diabetes. Primarily linked to dietary and lifestyle changes, we are now facing a global pandemic of type 2 diabetes. Characterized by insulin resistance and hyperglycaemia, type 2 diabetes is increasingly being diagnosed in adolescents as well as children. Is there a link between dietary-related hyperglycaemic insults to the endothelium, blood flow changes, and the development of insulin resistance? This review explores the evidence for and against this hypothesis.

Effect of hereditary obesity on renal expressions of NO synthase, caveolin-1, AKt, guanylate cyclase, and calmodulin

BACKGROUND

Obesity has emerged as a major cause of diabetes, cardiovascular disease, and renal insufficiency worldwide. Obese Zucker rats exhibit hyperphagia, obesity, insulin resistance, hyperlipidemia, and glomerulosclerosis and are frequently used as a model to study hereditary form of metabolic syndrome. Nitric oxide plays a major role in preservation of renal function and structure. The present study was designed to test the hypothesis that renal disease in this model may be associated with down-regulation of endothelial (eNOS) and neuromal NO synthases (nNOS) in the kidney. The study further sought to explore expressions of caveolin-1, phospho AKt, and calmodulin, which regulate activities of constituitive NOS isoforms, as well as soluble guanylate cyclase (sGC), which is involved in NO signaling.

METHODS

Twenty-two-week-old male obese and lean Zucker rats were studied. Body weight, serum lipids, urine albumin excretion, and renal tissue abundance of the above proteins were determined.

RESULTS

Serum glucose and arterial pressure were unchanged, whereas urinary NO metabolite (NO(chi)) excretion and renal tissue nitrotyrosine abundance were markedly reduced (denoting depressed NO production) in the obese versus lean Zucker rats. This was accompanied by significant glomerulosclerosis, tubulointerstitial damage, renal immune cell infiltration, marked down-regulations of renal tissue eNOS and nNOS, mild reduction of caveolin-1, and unchanged calmodulin, phospho-AKt, and sGC.

CONCLUSION

Hereditary obesity can result in down-regulations of kidney eNOS and nNOS, marked reduction of NO production, and glomerulosclerosis prior to the onset of frank diabetes and hypertension.

Reduced constrictor reactivity balances impaired vasodilation in the mesenteric circulation of the obese Zucker rat

Obesity causes whole body insulin resistance and impaired vasodilation to nitric oxide (NO). Because NO is a major contributor to the regulation of mesenteric blood flow, the mesenteric circulation of obese animals is faced with reduced capacity to increase flow and increased demand for flow associated with elevated consumption of food. This study hypothesized that insulin resistance impairs NO-mediated dilation but that constrictor reactivity would be reduced to compensate in obese animals. We further hypothesized that elevated superoxide levels caused impaired responses to NO in insulin resistance. Vasodilator reactivity and vasoconstrictor reactivity of mesenteric resistance arteries from lean (LZR) and obese (OZR) Zucker rats were examined in vitro using videomicroscopy. Insulin resistance independent of obesity was induced via fructose feeding in LZR (FF-LZR). Endothelium-dependent NO-mediated dilation was reduced in OZR and FF-LZR compared with LZR. Impairments in NO-mediated dilation were reversed with 1 mM tempol, a SOD mimetic. Constrictor reactivity to norepinephrine was reduced in OZR but not in FF-LZR relative to LZR. Basal mesenteric vascular resistance was similar in LZR and OZR despite impaired NO-dependent dilation in OZR. Mesenteric vascular resistance was increased in FF-LZR relative to LZR. These data indicate that there is reduced constrictor reactivity in OZR that may offset the impaired NO-mediated dilation and preserve mesenteric blood flow in hyperphagic, obese animals.

Sex hormone regulation of systemic endothelial and renal microvascular reactivity in type-2 diabetes: studies in gonadectomized and sham-operated Zucker diabetic rats

BACKGROUND

Male Zucker diabetic rats exhibit a more severe endotheliopathy in comparison with their female diabetic litter mates. The plasma concentrations of both thromboxanes and endothelins are elevated in diabetes, and the receptor cross-talk between TXA(2) and ET-1 receptors may be enhanced in type-2 diabetic Zucker rats.

AIMS

To determine the role of the endogenous sex steroid hormones, testosterone and estradiol on the systemic and renal microvascular reactivity to ET-1, thromboxane-mimetic U46619, ET-TXA(2) receptor interaction, and the nitric oxide vasodilator system in Zucker hypertensive-diabetic rats.

METHODS

Male and female Zucker rats aged 8-10 weeks were each divided into two groups. The male rats were castrated or underwent a sham operation. The female rats were spayed (bilateral ovariectomy and hysterectomy) or had a sham operation. All rats were studied 4-6 weeks after the gonadectomy or sham operations. Blood glucose and insulin as well as plasma concentrations of testosterone and estradiol were determined. Haemodynamic studies were undertaken with determination of the dose-response curve for mean arterial pressure (MAP), renal cortical flow (RCF) and renal medullary blood flow (MBF) in response to ET-1 and U46619, and the effect of interdiction of the ET-TXA(2) interaction with ET-antagonists BQ610 and BQ788. The role of endogenous NO was assessed by its response to graded acetylcholine doses and to a L-NG-nitro-arginine methyl ester (L-NAME) infusion.

RESULTS

Castrated male rats had a significantly lower blood glucose concentration (295 +/- 33 mg dL(-1)) compared with their sham-controls (481 +/- 40 mg dL(-1)), P = 0.008. Mean arterial pressure tended to be lower in the castrated rats. Gonadectomy reduced the plasma testosterone and estradiol concentrations. Castration abolished the hypotensive action of U46619 compared with sham-operated male rats (P < 0.0001, anova). Conversely, the pressor action of U46619 seen in the sham-operated female rats was reversed to a profound hypotensive action in the spayed rats (P < 0.001, anova). The change in MAP after U46619 was inversely correlated to the plasma testosterone concentration (r = -0.73, P = 0.027). The paradoxical hypotensive response elicited by ET-1 in the Zucker diabetic rats of both sexes was abolished by castration only (P < 0.005, anova). Castration caused a significant (P = 0.011) augmentation of the vasodilator response to acetylcholine, while spaying caused a slight attenuation. Castration, but not spaying, resulted in significant increases in MBF after U46619 (P = 0.003, anova), ET-1 (P = 0.005, anova) and acetylcholine (P = 0.053, anova). The ET-(B) antagonist BQ788 augmented the U46619-induced rise in MAP in castrated male rats, and also abolished the U46619-induced increase in MBF (P < 0.01 anova). L-NAME (25 mg kg(-1)) increased MAP and decreased MBF in the gonadectomized and sham-operated rats, except for the castrated male Zucker rats, where it significantly increased MBF (+90 +/- 31 PU) (P = 0.0004, anova) despite the increase in MAP.

CONCLUSIONS

Testosterone and estradiol regulate systemic and microvascular reactivity to TXA(2) receptor stimulation in type-2 diabetic Zucker rats. The impact of testosterone on blood glucose concentration, blood pressure, and the systemic and renal microcirculatory response to ET-1 and NO, as well as the endothelin-thromboxane receptor cross talk, is greater, and opposite to that of estradiol. The effects of testosterone withdrawal may at least in part be mediated by the ET-B receptor subtype and NO generation. Androgen blockade should be investigated further for the reversal or delay of hypertensive-diabetic endotheliopathy.

Protein kinase betaII in Zucker obese rats compromises oxygen and flow-mediated regulation of nitric oxide formation

In severe obesity, microvascular endothelial regulation of nitric oxide (NO) formation is compromised in response to muscarinic stimulation, and major arteries have suppressed flow-mediated dilation. Because normal microvessels are highly dependent on flow-mediated stimulation of NO generation and are responsive to intra- and extravascular oxygen availability, they are likely a major site of impaired endothelial regulation. This study evaluated the blood flow and oxygen-dependent aspects of intestinal microvascular regulation and NO production in Zucker obese rats just before the onset of hyperglycemia. Ruboxistaurin (LY-333531) was used to inhibit PKC-betaII to determine whether flow or oxygen-related NO regulation was improved. Blood flow velocity was increased by forcing arterioles to perfuse approximately 50% larger tissue areas by occlusion of nearby arterioles, and oxygen tension in the bath was lowered to create a modest oxygen depletion. When compared with lean Zucker rats, the periarteriolar NO concentration ([NO]) for obese rats was approximately 30% below normal. At elevated shear rates, the [NO] for arterioles of obese animals was 20-30% below those in the arterioles of lean rats, and the NO response to decreased oxygen was about half normal in obese rats. All of these regulatory problems were essentially corrected in obese rats by PKC blockade with only minor changes in the microvascular behavior in lean rats. Therefore, activation of PKC-betaII in endothelial cells during obesity suppressed NO regulation both at rest and in response to increased flow velocity and decreased oxygen availability.

Oxidative stress as a therapeutic target in diabetes: revisiting the controversy

Oxidative stress has been repetitively shown to be a hallmark of many diseases linked with metabolic or vascular disorders. Therefore diabetes represents an ideal candidate for studying the consequences of oxidative stress and its treatment. Indeed diabetes constitutes a multiple source of free radicals, starting very early in the disease process and worsening over the course of disease. In view of the typical characteristics of diabetes, oxidative stress is expected to have a double impact, on both metabolic and vascular functions. It is therefore particularly disappointing to note the dramatic failure of clinical trials with antioxidants, although it must be pointed out that such studies have not been performed with only diabetic patients. This review describes the many different aspects of oxidative stress in diabetes and proposes possible explanations for the apparent lack of efficacy of antioxidant treatments in patients. Some verifications seem warranted before a definitive conclusion can be drawn about the validity of this therapeutic concept.

Impaired skeletal muscle perfusion in obese Zucker rats

Skeletal muscle arterioles from obese Zucker rats (OZR) exhibit oxidant stress-based alterations in reactivity, enhanced alpha-adrenergic constriction, and reduced distensibility vs. microvessels of lean Zucker rats (LZR). The present study determined the impact of these alterations for perfusion and performance of in situ skeletal muscle during periods of elevated metabolic demand. During bouts of isometric tetanic contractions, fatigue of in situ gastrocnemius muscle of OZR was increased vs. LZR; this was associated with impaired active hyperemia. In OZR, vasoactive responses of skeletal muscle arterioles from the contralateral gracilis muscle were impaired, due in part to elevated oxidant tone; reactivity was improved after treatment with polyethylene glycol-superoxide dismutase (PEGSOD). Arterioles of OZR also exhibited increased alpha-adrenergic sensitivity, which was abolished by treatment with phentolamine (10-5 M). Intravenous infusion of phentolamine (10 mg/kg) or PEG-SOD (2,000 U/kg) in OZR altered neither fatigue rates nor active hyperemia from untreated levels; however, combined infusion improved performance and hyperemia, although not to levels in LZR. Microvessel density in the contralateral gastrocnemius muscle, determined via histological analyses, was reduced by approximately 25% in OZR vs. LZR, while individual arterioles from the contralateral gracilis muscle demonstrated reduced distensibility. These data suggest that altered arteriolar reactivity contributes to reduced muscle performance and active hyperemia in OZR. Further, despite pharmacological improvements in arteriolar reactivity, reduced skeletal muscle microvessel density and arteriolar distensibility also contribute substantially to reduced active hyperemia and potentially to impaired muscle performance.

Impaired Nitric Oxide Function in the Basilar Artery of the Obese Zucker Rat

The effect of insulin-resistance syndrome on vascular function has been examined in isolated basilar arteries using the obese Zucker rat (OZR) and age-matched lean littermate controls (lean Zucker rat; LZR) at 36 weeks of age. The OZR showed significantly reduced oral glucose tolerance and increased body weight, blood pressure, proteinuria, plasma levels of triglycerides, cholesterol, and insulin compared with the LZR. The contractile response to serotonin was significantly increased in the OZR. Furthermore, contractions to serotonin in LZR but not OZR were enhanced in the presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester (NAME). Relaxations to acetylcholine (ACh), histamine, and A23187 were significantly reduced in precontracted arteries from the OZR. In the presence of NAME, histamine responses were significantly reduced whereas ACh and A23187 responses were almost abolished. Relaxations to free-radical nitric oxide (NO) and papaverine were not different in arteries from the OZR, even though responses to sodium nitroprusside were reduced in the OZR. Western blot and immunofluorescent quantitative analyses of eNOS content in cerebral microvessel fractions and basilar artery preparations, respectively, were not significantly different between OZR and LZR. The results suggest impairment in endothelial function resulting in reduced NO function in the basilar artery from the OZR.

Lutein interacts with ascorbic acid more frequently than with alpha-tocopherol to alter biomarkers of oxidative stress in female zucker obese rats

The influence of dietary lutein, with and without moderate amounts of vitamin C (VC) or vitamin E (VE), on biomarkers of oxidative stress was examined in rats. Nine groups of immature Zucker obese (fa/fa) and lean female rats (8/group) consumed ad libitum for 8 wk the AIN-93G diet (Control) to which was added either dl-alpha-tocopherol acetate (VE) at 0.60 mg/kg or ascorbic acid (VC) at 0.75 mg/kg diet. Each of these diets contained lutein oil (FloraGlo) at 0.5 (Lut0.5) or 1.0 (Lut1.0) mg/kg diet. Weight gain, food efficiency and relative liver weight were higher in obese than in lean rats. Although liver malondialdehyde (MDA) concentrations were significantly higher in obese than in lean rats, levels were significantly lower in obese rats fed VE, VE-Lut and VC-Lut0.5 compared with other obese groups. The accumulation of alpha-tocopherol in liver was 6- and 3-times greater in the VE and VE-Lut1.0 groups, respectively, compared with the obese and lean control groups. Lutein reduced the activity of superoxide dismutase (SOD) in obese rats, independent of VC or VE, and raised the activity of glutathione peroxidase to higher levels in lean rats when combined with VC. Plasma insulin levels were dramatically higher in obese compared with lean rats, but significantly lower in obese rats fed VC-Lut0.5, VE-Lut1.0 and Lut1.0 compared with the Control group. These results suggest that lutein independently reduces the activity of SOD and alters more biomarkers of oxidative stress when combined with vitamin C than with vitamin E, and that vitamin E reduces liver lipid peroxidation in obese rats when the accumulation of liver alpha-tocopherol is very high.

Deficiency of adrenomedullin induces insulin resistance by increasing oxidative stress

Hypertension, insulin resistance, and obesity are common age-related metabolic disorders that are often associated with increased oxidative stress and the resultant vascular damage. Underlying mechanisms have been suggested, and age-related overproduction of oxidative stress is one possible candidate. Since we recently found a vasoactive peptide, adrenomedullin, to be an endogenous antioxidant that potently inhibits oxidative stress-induced vascular damage, in the current study we evaluated oxidative stress-induced changes in aged mice. Insulin sensitivities in young and aged adrenomedullin-deficient mice were measured by means of the hyperinsulinemic-euglycemic clamp method; insulin resistance was apparent in aged adrenomedullin-deficient mice with increased urinary excretion of 8-iso-prostaglandin F2alpha, a marker of oxidative stress, but not in young adrenomedullin-deficient mice. Concomitantly, only aged adrenomedullin-deficient mice not only showed increased production of muscular reactive oxygen species, as demonstrated by the electron spin resonance method, but also had significantly decreased insulin-stimulated glucose uptake into the soleus muscle associated with impairment of insulin signals such as insulin receptor substrate-1,2 and phosphatidylinositol-3 kinase activities. In turn, these abnormalities could be nearly reversed by either treatment with 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, a membrane-permeable superoxide dismutase mimetic, or adrenomedullin supplementation. Evidence presented in this report suggests that age-related accumulation of oxidative stress is involved in blood pressure regulation and insulin resistance in aged adrenomedullin-deficient mice, and adrenomedullin is thus an endogenous substance counteracting oxidative stress-induced insulin resistance associated with aging.

Effects of obesity on the pharmacodynamics of nitroglycerin in conscious rats

Literature reports have suggested that hemodynamic response toward organic nitrates may be reduced in obese patients, but this effect has not been studied. We compared the mean arterial pressure (MAP) responses toward single doses of nitroglycerin (NTG, 0.5-50 micro g) in conscious Zucker obese (ZOB), Zucker lean (ZL), and Sprague-Dawley (SD) rats. NTG tolerance development in these animal groups was separately examined. Rats received 1 and 10 micro g/min of NTG or vehicle infusion, and the maximal MAP response to an hourly 30 micro g NTG IVchallenge dose (CD) was measured. Steady-state NTG plasma concentrations were measured during 10 micro g/min NTG infusion. The Emax and ED50 values obtained were 33.9 +/- 3.6 and 3.5 +/- 1.7 micro g for SD rats, 33.2 +/- 4.1 and 3.0 +/- 1.4 micro g for ZL rats, and 34.8 +/- 3.9 and 5.3 +/- 2.8 micro g for ZOB rats, respectively. No difference was found in the dose-response curves among these 3 groups (P >.05, 2-way ANOVA). Neither the dynamics of NTG tolerance development, nor the steady-state NTG plasma concentrations, were found to differ among these 3 animal groups. These results showed that ZOB rats are not more resistant to the hemodynamic effects of organic nitrates compared with their lean controls. Thus, the acute and chronic hemodynamic effects induced by NTG are not sensitively affected by the presence of obesity in a conscious animal model of genetic obesity.

Augmented adrenergic vasoconstriction in hypertensive diabetic obese Zucker rats

This study examined skeletal muscle microvessel reactivity to constrictor stimuli in obese (OZR) versus lean Zucker rats (LZR). Gracilis arteries from both rat groups were isolated, cannulated with glass micropipettes, and viewed via television microscopy. Changes in vessel diameter were measured with a video micrometer. Arterial constriction to norepinephrine was elevated in OZR versus LZR, although vasoconstrictor reactivity to endothelin and angiotensin II was unaltered. Differences in reactivity between vessels of LZR and OZR were not explained by the loss of either endothelial nitric oxide synthase or beta-adrenergic receptor function. Reactivity of in situ cremasteric arterioles of OZR to norepinephrine was elevated versus LZR. Treatment with prazosin increased the diameter of in vivo gracilis arteries of OZR to levels determined in LZR and also normalized blood pressure in OZR. These results suggest that the constrictor reactivity of skeletal muscle microvessels in OZR is heightened in response to alpha-adrenergic stimuli and that development of diabetes in OZR may be associated with impaired skeletal muscle perfusion and hypertension due to microvessel hyperreactivity in response to sympathetic stimulation.

Impaired dilation of skeletal muscle microvessels to reduced oxygen tension in diabetic obese Zucker rats

This study determined alterations to hypoxic dilation of isolated skeletal muscle resistance arteries (gracilis arteries; viewed via television microscopy) from obese Zucker rats (OZR) compared with lean Zucker rats (LZR). Hypoxic dilation was reduced in OZR compared with LZR. Endothelium removal and cyclooxygenase inhibition (indomethacin) severely reduced this response in both groups, although nitric oxide synthase inhibition (N(omega)-nitro-L-arginine methyl ester) reduced dilation in LZR only. Treatment of vessels with a PGH(2)-thromboxane A(2) receptor antagonist had no effect on hypoxic dilation in either group. Arterial dilation to arachidonic acid, iloprost, acetylcholine, and sodium nitroprusside was reduced in OZR versus LZR, although dilation to forskolin and aprikalim was unaltered. Treatment of arteries from OZR with oxidative radical scavengers increased dilation to hypoxia and agonists, with no effect on responses in LZR. The restored hypoxic dilation in OZR was abolished by indomethacin. These results suggest that hypoxic dilation of skeletal muscle microvessels from LZR represents the summated effects of prostanoid and nitric oxide release, whereas the impaired response of vessels in OZR may reflect scavenging of PGI(2) by superoxide anion.

Impaired NO-dependent dilation of skeletal muscle arterioles in hypertensive diabetic obese Zucker rats

This study determined alterations to nitric oxide (NO)-dependent dilation of skeletal muscle arterioles from obese (OZR) versus lean Zucker rats (LZR). In situ cremaster muscle arterioles from both groups were viewed via television microscopy, and vessel dilation was measured with a video micrometer. Arteriolar dilation to acetylcholine and sodium nitroprusside was reduced in OZR versus LZR, although dilation to aprikalim was unaltered. NO-dependent flow-induced arteriolar dilation (via parallel microvessel occlusion) was attenuated in OZR, impairing arteriolar ability to regulate wall shear rate. Vascular superoxide levels, as assessed by dihydroethidine fluorescence, were elevated in OZR versus LZR. Treatment of cremaster muscles of OZR with the superoxide scavengers polyethylene glycol-superoxide dismutase and catalase improved arteriolar dilation to acetylcholine and sodium nitroprusside and restored flow-induced dilation and microvascular ability to regulate wall shear rate. These results suggest that NO-dependent dilation of skeletal muscle microvessels in OZR is impaired due to increased levels of superoxide. Taken together, these data suggest that the development of diabetes and hypertension in OZR may be associated with an impaired skeletal muscle perfusion via an elevated vascular oxidant stress.

Oxidative stress in a rat model of obesity-induced hypertension

The mechanisms underlying the development of hypertension in obesity are not yet fully understood. We recently reported the development of hypertension in a rat model of diet-induced obesity. When Sprague-Dawley rats (n=60) are fed a moderately high fat diet (32 kcal% fat) for 10 to 16 weeks, approximately half of them develop obesity (obesity-prone [OP] group) and mild hypertension (158+/-3.4 mm Hg systolic pressure), whereas the other half (obesity-resistant [OR] group) maintains a body weight equivalent to that of a low fat control group and is normotensive (135.8+/-3.8 mm Hg). We examined the potential role of oxidative stress in the development of hypertension in this model. Lipid peroxides measured as thiobarbituric acid-reactive substances showed a significant increase in the LDL fraction of OP rats (2.8+/-0.32 nmol malondialdehyde/mg protein) compared with OR and control rats (0.9+/-0.3 nmol malondialdehyde/mg protein). Also, aortic and kidney thiobarbituric acid-reactive substances showed a significant (3- and 5- fold) increase in OP rats after 16 weeks of diet. In addition, superoxide generation by aortic rings, measured by lucigenin luminescence, showed a 2-fold increase in the OP group compared with both the OR and control groups. In addition, free isoprostane excretion and nitrotyrosine in the kidney showed an increase in OP rats only. The urine and plasma nitrate/nitrite measured by the LDH method showed a 1.8-fold decrease in OP rats compared with OR rats. However, endothelial NO synthase expression in the kidney cortex and medulla assessed by reverse transcriptase-polymerase chain reaction showed a strong increase in the OP rats versus OR and control rats (endothelial NO synthase/beta-actin ratio 1.3+/-0.04 in OP rats versus 0.44+/-0.02 in OR rats), suggesting a possible shift toward superoxide production by the enzyme. Collectively, the data show a decreased NO bioavailability in OP animals that is due in part to the increased oxidative stress.

Investigation of basal endothelial function in the obese Zucker rat in vitro

(a) We studied basal endothelial function in the insulin-resistant, obese Zucker rat, including the influence of superoxide anion on the regulation of contractile reactivity by nitric oxide (NO), following treatment in vivo with the antioxidant tiron or the pro-oxidant combination hydroquinone+buthionine sulfoximine. (b) The leftward shift in the contractile potency of phenylephrine due to NO synthase inhibition with N(G)-nitro-L-arginine methyl ester (L-NAME) was greater in the isolated aorta of the obese Zucker rat relative to its insulin-sensitive littermate, the lean Zucker rat. (c) Pretreatment with tiron depressed vasoconstriction to phenylephrine and comparably enhanced the L-NAME-mediated leftward shift in contractile reactivity in the obese and lean Zucker rats in hydroquinone+buthionine sulfoximine-sensitive manner. (d) Our data therefore indicate superior endothelial function in the obese relative to the lean Zucker rat, reflected by a greater regulation of vasoconstrictor reactivity by basal NO, while the regulation of NO bioavailability by superoxide anion is similar.