Rosiglitazone, a peroxisome proliferator-activated receptor γ stimulant, abrogates diabetes-evoked hypertension by rectifying abnormalities in vascular reactivity

Arginase overexpression and NADPH oxidase stimulation underlie impaired vasodilation induced by advanced glycation end products

BACKGROUND

Advanced glycation endproducts (AGEs) play a major role in the development of many vascular complications that are mediated by endothelial dysfunction. The present work aimed to investigate the mechanism by which AGEs impair vasodilation.

METHODS

The effect of AGEs on vasodilation induced by acetylcholine or D NONOate was examined by incubating isolated rat aortae with different AGEs concentrations. ACh-induced nitric oxide generation was assessed using the fluorescent probe diaminofluorecein (DAF-FM). The effect of AGEs on expression of mRNA for arginase 2, NADPH oxidase and endothelial nitric oxide synthase (eNOS) were determined by real-time PCR.

RESULTS

One-hour in vitro incubation of rat aortae with AGEs impaired endothelial-dependent vasodilation produced by ACh, while increasing D NONOate-induced vasodilation. Preincubation of aortae with l-ornithine, an arginase 2-inhibitor, prevented the impairment effect induced by AGEs on endothelial-dependent vasodilation. Superoxide scavenging by tempol or NADPH oxidase inhibition by apocynin also blocked the effect of AGEs. AGEs decreased ACh-induced NO production and this was inhibited by both l-ornithine and apocynin. Furthermore, AGEs exposure increased arginase mRNA expression but decreased mRNA expression for eNOS in isolated rat aortae.

CONCLUSION

The present results indicate that AGEs impairs endothelial-dependent vasodilation, and this effect is mediated via arginase overexpression and NADPH oxidase stimulation.

Phenolics from Garcinia mangostana alleviate exaggerated vasoconstriction in metabolic syndrome through direct vasodilatation and nitric oxide generation

BACKGROUND

Exaggerated vasoconstriction plays a very important role in the hypertension, a major component of metabolic syndrome (MetS). In the current work, the potential protective effect of methanol extract of fruit hulls of Garcinia mangostana L. on the exaggerated vasoconstriction in MetS has been investigated. In addition, the bioactive fraction and compounds as well as the possible mechanism of action have been illustrated.

METHODS

The effect of methanol extract of G. mangostana (GMT) fruit hulls on the vascular reactivity of aorta isolated from animals with MetS was investigated through bioassay-guided fractionation procedures. GMT was partitioned with chloroform (I) and the remaining mother liquor was fractionated on a Diaion HP-20 with H2O, 50 and 100 % methanol to give fractions II, III, and IV, respectively. The effect of total extract (GMT), bioactive fraction and the bioactive compounds on the vasoconstriction were examined in aortae isolated from animals with MetS by incubation for 30 min before exposing aortae to cumulative concentrations of phenylephrine (PE). The direct relaxant effect was also examined by adding cumulative concentrations of the bioactive fraction and its bioactive compounds to PE precontracted vessels. In addition, aortic nitric oxide (NO) and reactive oxygen species (ROS) production was investigated.

RESULTS

Bioassay-guided fractionation of GMT revealed isolation of garcimangosone D (1), aromadendrin-8-C-β-D-glucopyranoside (2), 2,4,3'-trihydroxy benzophenone-6-O-β-D-glucopyranoside (3), maclurin-6-O-β-D-glucopyranoside (rhodanthenone) (4), epicatechin (5), and 2,3',4,5',6-pentahydroxy benzophenone (6). Only compounds 2, 4, and 5 significantly alleviated the exaggerated vasoconstriction of MetS aortae and in the same time showed significant vasodilation of PE pre-contracted aortae. To further illustrate the mechanism of action, the observed vasodilation was completely blocked by the nitric oxide (NO) synthase inhibitor, Nω-nitro-L-arginine methyl ester hydrochloride and inhibited by guanylate cyclase inhibitor, methylene blue. However, vasodilation was not affected by the potassium channel blocker, tetraethylammonium or the cyclooxygenase inhibitor, indomethacin. In addition, compounds 2, 4, and 5 stimulated NO generation from isolated aortae to levels comparable with acetylcholine. Furthermore, 4 and 5 inhibited reactive oxygen species generation in MetS aortae.

CONCLUSION

The phenolic compounds 2, 4, and 5 ameliorated the exaggerated vasoconstriction in MetS aortae through vasodilatation-NO generation mechanism.

Phenolics from Garcinia mangostana Inhibit Advanced Glycation Endproducts Formation: Effect on Amadori Products, Cross-Linked Structures and Protein Thiols

Accumulation of Advanced Glycation Endproducts (AGEs) in body tissues plays a major role in the development of diabetic complications. Here, the inhibitory effect of bioactive metabolites isolated from fruit hulls of Garcinia mangostana on AGE formation was investigated through bio-guided approach using aminoguanidine (AG) as a positive control. Including G. mangostana total methanol extract (GMT) in the reaction mixture of bovine serum albumin (BSA) and glucose or ribose inhibited the fluorescent and non-fluorescent AGEs formation in a dose dependent manner. The bioassay guided fractionation of GMT revealed isolation of four bioactive constituents from the bioactive fraction; which were identified as: garcimangosone D (1), aromadendrin-8-C-glucopyranoside (2), epicatechin (3), and 2,3',4,5',6-pentahydroxybenzophenone (4). All the tested compounds significantly inhibited fluorescent and non-fluorescent AGEs formation in a dose dependent manner whereas compound 3 (epicatechin) was found to be the most potent. In search for the level of action, addition of GMT, and compounds 2-4 inhibited fructosamine (Amadori product) and protein aggregation formation in both glucose and ribose. To explore the mechanism of action, it was found that addition of GMT and only compound (3) to reaction mixture increased protein thiol in both glucose and ribose while compounds 1, 2 and 4 only increased thiol in case of ribose. In conclusion, phenolic compounds 1-4 inhibited AGEs formation at the levels of Amadori product and protein aggregation formation through saving protein thiol.

Baicalein protects against hypertension associated with diabetes: effect on vascular reactivity and stiffness

The present work investigated the possible protective effect of baicalein, a natural lipoxygenase enzyme inhibitor, on both insulin deficiency (ID) and insulin resistance (IR)-induced macro-vascular impairment. ID and IR were induced by STZ or fructose for 8 or 12 weeks respectively while baicalein was administered in the last six weeks. Blood pressure (BP) was recorded and isolated aorta reactivity to phenylephrine (PE) and acetylcholine (ACh) were studied. Blood levels of glucose, insulin, advanced glycation end products (AGEs) and tumour necrosis factor-α (TNF-α) were determined. Aortic nuclear transcription factor-κB (NF-κB) activation was assessed. Both models resulted in elevated BP, increased vasoconstriction and impaired relaxation KCl, elevated TNF-α and AGEs, NF-κB activation, marked infiltration of leukocytes in the adventitia, pyknosis of endothelial cells and marked collagen deposition. Baicalein ameliorated elevations in BP in models, prevented exaggerated vasoconstriction IR model and improved relaxation in ID model. Baicalein reduced AGEs and TNF-α level, decreased NF-κB activation and inhibited histopathological changes in both models. Baicalein offsets the hypertensive and the vascular impairment associated with both diabetic models via ameliorating functional and structural derangements of blood vessels.

Arginase inhibition alleviates hypertension in the metabolic syndrome

BACKGROUND AND PURPOSE

We have previously shown that arginase inhibition alleviates hypertension associated with in a diabetic animal model. Here, we investigated the protective effect of arginase inhibition on hypertension in metabolic syndrome.

EXPERIMENTAL APPROACH

Metabolic syndrome was induced in rats by administration of fructose (10% in drinking water) for 12 weeks to induce vascular dysfunction. Three arginase inhibitors (citrulline, norvaline and ornithine) were administered daily in the last 6 weeks of study before and tail BP was recorded in conscious animals. Concentration response curves for phenylephrine (PE), KCl and ACh in addition to ACh-induced NO generation were obtained in thoracic aorta rings. Serum glucose, insulin, uric acid and lipid profile were determined as well as reactive oxygen species (ROS) and arginase activity.

KEY RESULTS

Arginase activity was elevated in metabolic syndrome while significantly inhibited by citrulline, norvaline or ornithine treatment. Metabolic syndrome was associated with elevations in systolic and diastolic BP, while arginase inhibition significantly reduced elevations in diastolic and systolic BP. Metabolic syndrome increased vasoconstriction responses of aorta to PE and KCl and decreased vasorelaxation to ACh, while arginase inhibition completely prevented impaired responses to ACh. In addition, arginase inhibition prevented impaired NO generation and exaggerated ROS formation in metabolic syndrome. Furthermore, arginase inhibition significantly reduced hyperinsulinaemia and hypertriglyceridaemia without affecting hyperuricaemia or hypercholesterolaemia associated with metabolic syndrome.

CONCLUSIONS AND IMPLICATIONS

Arginase inhibition alleviates hypertension in metabolic syndrome directly through endothelial-dependent relaxation/NO signalling protection and indirectly through inhibition of insulin resistance and hypertriglyceridaemia.

Characterization of vascular complications in experimental model of fructose-induced metabolic syndrome

Vascular dysfunction is an important complication associated with metabolic syndrome (MS). Here we fully characterized vascular complications in a rat model of fructose-induced MS. MS was induced by adding fructose (10%) to drinking water to male Wistar rats of 6 weeks age. Blood pressure (BP) and isolated aorta responses phenylephrine (PE), KCl, acetylcholine (ACh), and sodium nitroprusside (SNP) were recorded after 6, 9, and 12 weeks of fructose administration. In addition, serum levels of glucose, insulin, uric acid, tumor necrosis factor α (TNFα), lipids, advanced glycation end products (AGEs), and arginase activity were determined. Furthermore, aortic reactive oxygen species (ROS) generation, hemeoxygenase-1 expression, and collagen deposition were examined. Fructose administration resulted in a significant hyperinslinemia after 6 weeks which continued for 12 weeks. It was also associated with a significant increase in BP after 6 weeks which was stable for 12 weeks. Aorta isolated from MS animals showed exaggerated contractility to PE and KCl and impaired relaxation to ACh compared with control after 6 weeks which were clearer at 12 weeks of fructose administration. In addition, MS animals showed significant increases in serum levels of lipids, uric acid, AGEs, TNFα, and arginase enzyme activity after 12 weeks of fructose administration. Furthermore, aortae isolated from MS animals were characterized by increased ROS generation and collagen deposition. In conclusion, adding fructose (10%) to drinking water produces a model of MS with vascular complications after 12 weeks that are characterized by insulin resistance, hypertension, disturbed vascular reactivity and structure, hyperuricemia, dyslipidemia, and low-grade inflammation.

Aldose reductase inhibitors zopolrestat and ferulic acid alleviate hypertension associated with diabetes: effect on vascular reactivity

This study investigated the effect of aldose reductase (AR) inhibitors on hypertension in diabetes. Diabetes was induced with streptozotocin, while AR inhibitors zopolrestat and ferulic acid were administered at 2 weeks after streptozotocin treatment and for 6 weeks afterwards. Then, blood pressure (BP) and serum level of glucose were determined. Concentration–response curves for phenylephrine (PE), KCl, and acetylcholine (ACh) were obtained in isolated aorta. In addition, ACh-induced NO and reactive oxygen species (ROS) generation in aorta and histopathology were examined. Compared with the control animals, diabetes increased diastolic and systolic BP. AR inhibitors reduced diastolic BP elevation without affecting the developed hyperglycaemia. Diabetes increased the contractile response of aorta to KCl, and decreased the relaxation response to Ach, while administering AR inhibitors prevented an impaired response to ACh. Incubation of aorta isolated from diabetic animals with AR inhibitors did not affect the impaired relaxation response to ACh. In addition, AR inhibitors negated the impaired Ach-stimulated NO generation seen in aorta isolated from diabetic animals. Furthermore, diabetes was accompanied with marked infiltration of leukocytes in aortic adventitia, endothelial cell pyknosis, and increased ROS formation. AR inhibitors reduced leukocyte infiltration and inhibited endothelial pyknosis and ROS formation. In conclusion, AR inhibitors negate diabetes-evoked hypertension via ameliorating impaired endothelial relaxation and NO production.

Heme oxygenase-1 induction protects against hypertension associated with diabetes: effect on exaggerated vascular contractility

Disturbances in vascular reactivity are important components of diabetes-evoked hypertension. Heme oxygenase-1 (HO-1) is a homeostatic enzyme upregulated in stress. This study aims to investigate the protective effect of HO-1 against diabetes-evoked hypertension. Rats were left 8 weeks after diabetes induction with streptozotocin to induce vascular dysfunction in the diabetic groups. HO-1 inducers, hemin and curcumin, were daily administrated in the last 6 weeks in the treated groups after 2 weeks of induction. Then, at the end of the study (8 weeks), HO-1 protein level was assessed by immunofluorescence; blood pressure (BP) was recorded; isolated aorta reactivity to phenylephrine (PE) and KCl was studied; reactive oxygen species (ROS) generation was determined; and serum level of glucose, advanced glycation end products (AGEs), and tumor necrosis factor alpha (TNF-α) were determined. While not affected by diabetes, HO-1 protein expression was strongly induced by hemin or curcumin administration. Compared with control animals, diabetes increased systolic and pulse BP. Induction of HO-1 by hemin or curcumin significantly reduced elevated systolic BP and abolished elevated pulse BP without affecting the developed hyperglycemia or AGEs level. The possibility that alterations in vascular reactivity contributed to diabetes-HO-1 BP interaction was investigated. Diabetes increased contractile response of the aorta to PE and KCl, while HO-1 induction by curcumin or hemin prevented aorta-exaggerated response to PE and KCl. Furthermore, the competitive HO inhibitor, tin protoporphyrin, abolished the protective effect of hemin. Diabetes was accompanied with elevated level of TNF-α and ROS generation, while HO-1 induction abrogated increased TNF-α and ROS generation. Collectively, induction of HO-1 protects against hypertension associated with diabetes via ameliorating exaggerated vascular contractility by reducing TNF-α and aortic ROS levels.