Effect of lipid feeding on endothelium dependent relaxation in rabbit aortic preparations

Canagliflozin ameliorates aortic and hepatic dysfunction in dietary-induced hypercholesterolemia in the rabbit

AIMS

Canagliflozin is an antidiabetic agent which lowers blood glucose levels by inhibiting the glucose reabsorption machinery in the proximal tubules. There have not been conducted any study on its direct impact on hypercholesterolemia and associated vascular disorders independently of blood glucose lowering activity.

MATERIALS AND METHODS

Rabbits were arranged in 3 groups: Group 1 (Control): regular rabbit chow; Group 2 (HCD): 1% cholesterol-enriched chow was given to rabbits for 4 weeks; Group 3 (HCD-CANA): 1% cholesterol-enriched chow was fed to rabbits concurrently with canagliflozin (10 mg/kg/day, orally) for 4 weeks. At the end of experiment, blood and tissue samples were obtained for biochemical, histological, immunohistochemical, and vascular reactivity assessment.

KEY FINDINGS

When statistically compared to Control (P < 0.05), HCD showed a significant increase in the serum triglycerides, low-density lipoprotein, total cholesterol, C-reactive protein, alkaline phosphatase, alanine aminotransferase and aspartate aminotransferase. Furthermore, a significant decrease was seen in both liver and aortic levels of glutathione peroxidase and superoxide dismutase concurrently with a significant elevation in malondialdehyde levels. Aortic levels of nitrate/nitrite ratio were significantly elevated. Acetylcholine-induced relaxation was impaired as the E_max decreased significantly in aortae. Moreover, a significant increase was seen in the level of aortic intima/media ratio. Canagliflozin treatment significantly improved vascular function, lipid profile and inflammation and reduced liver injury.

SIGNIFICANCE

Our data suggest that SGLT-2 inhibition via canagliflozin not only possesses an antihyperglycemic activity, but also improves hypercholesterolemia, vascular and liver function in dietary-induced hypercholesterolemia in the rabbit.

Sulforaphane attenuates the development of atherosclerosis and improves endothelial dysfunction in hypercholesterolemic rabbits

The aim of the present work was to explore possible protective effects of sulforaphane (SFN) against atherosclerosis development and endothelial dysfunction in hypercholesterolemic rabbits. Rabbits were assigned to three groups of five: group I fed normal chow diet for four weeks, group II fed 1% high cholesterol diet (HCD) and group III fed HCD + SFN (0.25 mg/kg/day). Blood samples were collected for measurement of serum triglycerides (TGs), total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C), lactate dehydrogenase (LDH) and C-reactive protein (CRP). Aortic malondialdehyde (MDA), reduced glutathione (GSH), superoxide dismutase (SOD) and total nitrite/nitrate (NOx) were measured. Vascular reactivity and intima/media (I/M) ratio were analyzed. Nuclear factor-kappa B (NF-κB) activation in aortic endothelial cells was identified immunohistochemically. HCD induced significant increases in serum TGs, TC, LDL-C, LDH, and CRP, and aortic MDA and SOD. Moreover, HCD caused significant reductions in serum HDL-C, aortic GSH and NOx. SFN administration significantly decreased HCD-induced elevations in serum TC, LDL-C, CRP, and LDH. while significantly increased HDL-C and GSH levels and normalized aortic SOD and NOx. Additionally, SFN significantly improved rabbit aortic endothelium-dependent relaxation to acetylcholine. Moreover, SFN significantly reduced the elevation in I/M ratio. This effect was confirmed by aortic histopathologic examination. The expression of NF-κB in aortic tissue showed a marked reduction upon treatment with SFN. In conclusion, this study reveals that SFN has the ability to ameliorate HCD-induced atherosclerotic lesions progression and vascular dysfunction, possibly via its lipid-lowering and antioxidant effects and suppression of NF-κB-mediated inflammation.

Attenuation of oxidative stress, inflammation, and endothelial dysfunction in hypercholesterolemic rabbits by allicin

Allicin, the active substance of garlic, exerts a broad spectrum of pharmacological activities and is considered to have potential therapeutic applications. The present study was designed to investigate the possible beneficial effects of allicin against oxidative stress, inflammation, and endothelial dysfunction in hypercholesterolemic rabbits. Male New Zealand white rabbits were used in this study. Rabbits randomly received 1 of the following treatments: normal chow diet for 4 weeks, 1% high cholesterol diet (HCD), HCD plus allicin (10 mg/kg/day), or HCD plus atorvastatin (10 mg/kg/day). Blood samples were collected at the end of experimental diets for measurement of serum total cholesterol (TC), triglycerides (TGs), high-density lipoprotein cholesterol (HDL-C), C-reactive protein (CRP), malondialdehyde (MDA), reduced glutathione (GSH), and superoxide dismutase (SOD). In addition, the aorta was removed for measurement of vascular reactivity, histopathological changes, intima/media (I/M) ratio, and immunohistochemical staining of both tumor necrosis-alpha (TNF-α) and nuclear factor (NF)-κB. HCD induced significant increases in serum TC, TGs, low-density lipoprotein cholesterol (LDL-C), CRP, and MDA. Moreover, HCD caused significant decrease in serum GSH and SOD. In addition, aortic relaxation response to acetylcholine (ACh) was impaired. Immunohistochemical staining of aortic specimens from HCD-fed rabbits revealed high expression levels of both TNF-α and the oxidant-induced transcription factor, NF-κB. Allicin supplementation significantly decreased serum MDA and CRP, increased serum HDL-C, GSH, and SOD levels while nonsignificantly affecting HCD-induced elevations in serum TC and LDL-C. Additionally, allicin significantly protected against HCD-induced attenuation of rabbit aortic endothelium-dependent relaxation to ACh and elevation in I/M ratio. This effect was confirmed by histopathological examination of the aorta. Moreover, allicin has substantially beneficial effects on aortic expression of TNF-α and NF-κB compared with HCD-fed rabbits. In conclusion, these findings demonstrate that allicin may be useful in reducing oxidative stress, inflammation, vascular dysfunction, and the aortic pathology in hypercholesterolemic rabbits.

Nitric oxide function in atherosclerosis

Atherosclerosis is a chronic inflammatory process in the intima of conduit arteries, which disturbs the endothelium-dependent regulation of the vascular tone by the labile liposoluble radical nitric oxide (NO) formed by the constitutive endothelial nitric oxide synthase (eNOS). This defect predisposes to coronary vasospasm and cardiac ischaemia, with anginal pain as the typical clinical manifestation. It is now appreciated that endothelial dysfunction is an early event in atherogenesis and that it may also involve the microcirculation, in which atherosclerotic lesions do not develop. On the other hand, the inflammatory environment in atherosclerotic plaques may result in the expression of the inducible NO synthase (iNOS) isozyme. Whether the dysfunction in endothelial NO production is causal to, or the result of, atherosclerotic lesion formation is still highly debated. Most evidence supports the hypothesis that constitutive endothelial NO release protects against atherogenesis e.g. by preventing smooth muscle cell proliferation and leukocyte adhesion. Nitric oxide generated by the inducible isozyme may be beneficial by replacing the failing endothelial production but excessive release may damage the vascular wall cells, especially in combination with reactive oxygen intermediates.

Anti-atherogenic effects of seabuckthorn (Hippophaea rhamnoides) seed oil

Seabuckthorn (SBT) seed oil is a rich source of unsaturated fatty acids, phytosterols, carotenoids and flavonoids, which are known to have significant anti-atherogenic and cardioprotective activity. The anti-atherogenic activity of supercritical CO(2) extracted SBT seed oil was evaluated in white albino rabbits fed on high cholesterol diet for 60 days. The study was performed on 20 male healthy rabbits divided into four groups of 5 animals each. Group I - control, group II - SBT seed oil, group III - cholesterol (1%) for 60 days, group IV - cholesterol+SBT seed oil. After 30 days of high cholesterol diet, group IV rabbits received 1 ml of SBT seed oil daily for 30 days. Blood total cholesterol (TC), LDL-cholesterol (LDL-C), HDL-cholesterol (HDL-C) and triglyceride (TG) levels were measured before and after the administration of SBT seed oil. The vasorelaxant activity of the seed oil was studied in vitro using aortic ring model technique and changes in isometric force were recorded using a polygraphic recording system. Accumulation of cholesterol in the aorta was studied using Sudan-IV staining technique. SBT seed oil feeding to normal rabbits for 18 days caused a significant decline in plasma cholesterol, LDL-C, atherogenic index (AI) and LDL/HDL ratio. The HDL-C levels, HDL-C/TC ratio (HTR) and vasorelaxant activity of the aorta were significantly increased. In cholesterol-fed animals the TC, TG, LDL-C and AI were significantly increased and showed a decline following seed oil administration. The increase in HDL-C was more marked in seed oil treated hypercholesterolemic animals. The acetylcholine-induced vasorelaxant activity was significantly decreased in cholesterol-fed animals and could be restored to that of normal values by seed oil administration. These observations suggest that supercritical CO(2) extracted SBT seed oil has significant anti-atherogenic and cardioprotective activity.

Endothelial dysfunction and improvement of the angiotensin II-reactivity in hypercholesterolemic rabbits: role of cyclooxygenase metabolites

The aim of this paper was to study the effect of high cholesterol diet on endothelial function and vascular reactivity to angiotensin II and to test the role of vasoconstrictor cyclooxygenase metabolites in this experimental condition. Rabbits were fed with either normal chow or a diet containing 1% cholesterol for 6-7-week. Isometric contractions were measured in rubbed or unrubbed aortic rings. Arteries were contracted with noradrenaline and then exposed to one cumulative dose-response curve to acetylcholine in absence (control) or in presence of indomethacin, (N-[2-cyvlohexyloxy)-4-nitrophenyl]-methanesulfonamide) (NS 398) or 4-hydroxy-2,2,6,6-tetraethylpiperidine-N-oxyl (tempol). After washing the arteries, one cumulative dose-response curve to angiotensin II was constructed in absence or presence of indomethacin, NS 398, [1S-[1 alpha,2 beta (5Z),3 beta,4 alpha]-7-[3-[[2-[(phenylamino) carbonyl]hydrazino]methyl]-7-oxabicyclo[2.2.1] hept-2-yl]-5-heptenoic acid (SQ29548) or 17-octadecynoic acid (17-ODYA). In other group, resting potential was recorded in basal and angiotensin II-stimulated conditions. Indomethacin, NS 398 or 17-ODYA were added to the bath before angiotensin II-stimulation. Rabbits fed on a diet enriched with cholesterol showed higher plasma levels of total cholesterol and LDL. Hypercholesterolemic diet impaired acetylcholine relaxation. Indomethacin normalized endothelium-dependent relaxation whereas NS 398 and tempol had no effect on this phenomenon. Angiotensin II-reactivity was increased in endothelium intact hypercholesterolemic aortic rings and indomethacin, SQ29548 or 17-ODYA blocked this effect. The resting potential of unrubbed hypercholesterolemic arteries was significantly less negative to control after angiotensin II-stimulation. 17-ODYA but not indomethacin prevented angiotensin II-depolarization. High cholesterol diet caused endothelial dysfunction and increased the angiotensin II-reactivity. Both effects were cyclooxygenase1-dependent. Deficit in the NO-production might improve 20-hydroxyeicosatrienoic acid availability, which induces depolarization and angiotensin II-sensitization. In addition, 20-hydroxyeicosatrienoic acid would be metabolized by cyclooxygenase1 to 20-endoperoxides which act through thromboxane A(2)/prostaglandin H(2) receptors contributing to angiotensin II-reactivity increase.

Comparison of endothelial function in the carotid artery between normal and short-term hypercholesterolemic rabbits

The present study was undertaken to investigate and compare the vascular function in carotid arteries isolated from normal short-term hypercholesterolemic rabbits. Rabbits were fed normal or 0.5% cholesterol chow for 5 weeks. The tension of isolated carotid artery rings was measured isometrically. Serum lipid levels were measured and morphometric analysis was performed. And content of nitrate/nitrite in the carotid artery was also determined. In the carotid artery precontracted by phenylephrine, the cholesterol chow diet administered for 5 weeks decreased acetylcholine-induced relaxation at only middle concentrations, though it significantly increased the content of nitrate/nitrite, the sum of stable nitric oxide metabolites, in the carotid artery. Cholesterol chow for 5 weeks had no influence on sodium nitroprusside-induced relaxation in the carotid artery. The N(G)-nitro-L-arginine- and indomethacin-resistant endothelium-dependent relaxation induced by acetylcholine was significantly decreased in rabbits receiving the cholesterol chow as compared to rabbits receiving the control diet. The resistant part of acetylcholine-induced relaxation was significantly inhibited when the carotid artery was treated with glibenclamide, a selective inhibitor of ATP-sensitive K(+) channels, 4-aminopyridine, an inhibitor of voltage-dependent K(+) channels, or charybdotoxin, an inhibitor of large and intermediate conductance Ca(2+)-activated K(+) channels, and it was significantly inhibited by tetraethylammonium, a non-selective inhibitor of Ca(2+)-activated K(+) channels and N,N-di-ethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF 525a), a nonselective cytochrome P-450 monooxygenase (CYP) inhibitor, or ketoconazole, a selective CYP3A inhibitor in only normal rabbits. These results suggest that short-term hypercholesterolemia decreased EDHF-induced relaxation mediated through K(+) channels in rabbit carotid artery and that it may be due partially to the inhibition of CYP3A system in the carotid artery at an early stage of hypercholesterolemia.

Chronic L-arginine supplementation improves endothelial cell vasoactive functions in hypercholesterolemic and atherosclerotic monkeys

Chronic exposure to L-arginine results in regression of atherosclerotic lesions and reversal of endothelial dysfunction. We investigated whether chronic L-arginine supplementation induces regression of atherosclerotic lesions and reversal of endothelial dysfunction in atherogenic rhesus monkeys and the mechanism which leads to these effects. About 12 male rhesus monkeys were fed 1% cholesterol and 18 g butter for 6 months to create an experimental model of hypercholesterolaemia and atherosclerosis (Group I) and 12 monkeys were fed standard stock diet for 6 months (Group II). After, 6 months these two groups were further divided into 2 sub-groups which in addition to their respective diets were fed 2.5% L-arginine in drinking water for additional 6 months (Group III and Group IV). Systemic nitric oxide (NO) formation was assessed as plasma nitrite and cGMP formation every 3 months. Oxygen free radical (OFR) generation and malondialdehyde production as an index of lipid peroxidation were determined. Changes in isometric tension were compared in isolated ring segments of thoracic aorta from normal and hypercholesterolemic animals. Cholesterol feeding progressively reduced plasma nitrite and cGMP generation (p < 0.05). Dietary L-arginine partly restored the levels of plasma nitrite and cGMP (p < 0.05) but did not change plasma cholesterol levels. L-arginine significantly reduced aortic intimal thickening, blocked the production of carotid and coronary intimal plaques and completely preserved endothelium-dependent vasodilator function. Further, L-arginine significantly inhibited generation of the reactive oxygen species (ROS) generation and lipid peroxidation. Chronic oral supplementation with L-arginine blocks the progression of plaques via restoration of nitric oxide synthase substrate availability and reduction of vascular oxidative stress.

Alteration in endothelial function and modulation by treatment with pioglitazone in rabbit renal artery from short-term hypercholesterolemia

The present study was undertaken to investigate endothelial function and epoxyeicosatrienoic acids (EETs), which is a cytochrome P-450 monooxygenase (CYP) metabolite and one of the candidates as an endothelium-derived hyperpolarizing factor (EDHF) in the renal artery isolated from short-term hypercholesterolemic rabbits, and also to characterize the effects of pioglitazone on it. Rabbits were fed normal, 0.5% cholesterol chow, or 0.5% cholesterol chow plus 300 ppm pioglitazone for 5 weeks. The tension of isolated renal artery rings was measured isometrically. Serum lipid levels were measured and morphometric analysis was performed. EET contents in the renal artery were also determined. The cholesterol chow diet for 5 weeks increased serum lipid levels, and pioglitazone had no influence on it. In the phenylephrine precontracted renal artery, the cholesterol chow did not affect acetylcholine-induced relaxation. The N(G)-nitro-l-arginine- and indomethacin-resistant endothelium-dependent relaxation induced by acetylcholine was significantly enhanced in rabbits receiving the cholesterol chow as compared to rabbits receiving the control diet, and pioglitazone normalized it. The resistant part of acetylcholine-induced relaxation was significantly inhibited when the renal artery was treated with charybdotoxin, an inhibitor of large- and intermediate-conductance Ca(2+)-activated K(+) channels, or N,N-di-ethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF 525a), a nonselective CYP inhibitor, and it was significantly inhibited by sulfaphenazole, a selective CYP2C9 inhibitor in rabbits receiving only the cholesterol chow. In KCl-precontracted renal artery, the cholesterol chow inhibited acetylcholine-induced relaxation and pioglitazone normalized it. The cholesterol chow increased the production of EETs and reduced nitrate/nitrite contents in the renal artery, and pioglitazone strongly suppressed them. These results suggest that the EETs may be one of the EDHFs in the rabbit renal artery and beneficial effects of pioglitazone on alterations in endothelial function induced by cholesterol feeding are due, in part, to the protective action on the nitric oxide system and/or the suppression of increased production of EETs.

Characterization of endothelium-dependent relaxation and modulation by treatment with pioglitazone in the hypercholesterolemic rabbit renal artery

The present study was undertaken to investigate vascular function in hypercholesterolemic rabbits and also to characterize the effects of pioglitazone on it. Rabbits were fed normal, 0.5% cholesterol chow, or 0.5% cholesterol chow plus 300 ppm pioglitazone for 5 or 10 weeks. The tension of isolated renal artery rings was measured isometrically, and morphometric analysis was performed. The cholesterol chow diet administered for 5 weeks did not affect acetylcholine-induced relaxation in the renal artery but that for 10 weeks decreased it. The N(G)-nitro-L-arginine (L-NOARG)- and indomethacin-resistant endothelium-dependent relaxation induced by acetylcholine in the renal artery was enhanced in rabbits receiving the cholesterol chow for 5 or 10 weeks, as compared to rabbits receiving the control diet, and the percentage of plaque area formation was increased in the renal artery by the cholesterol chow for 10 weeks. Pioglitazone normalized them without lowering serum lipid levels. The resistant parts of acetylcholine-induced relaxation was significantly inhibited when the renal artery was treated with charybdotoxin, an inhibitor of large and intermediate conductance Ca(2+)-activated K(+) channels, or N,N-diethylaminoethyl-2,2-diphenylvalerate hydrochloride (SKF 525a), a cytochrome P-450 monooxygenase inhibitor. Results indicate that hypercholesterolemia enhances endothelium-derived hyperpolarizing factor (EDHF)-mediated relaxation in the rabbit renal artery and pioglitazon normalizes it without lowering serum lipid levels, and suggest that the maintenance of endothelial function by pioglitazon is related to the mechanisms for its anti-atheromatous activity.

[Effect of HMG-CoA reductase inhibition on endothelial dysfunction-inducing protein in hypercholesterolemic rabbits]

INTRODUCTION AND OBJECTIVES

In our laboratory, we recently obtained evidence that cultured bovine endothelial cells contain cytosolic proteins that form complexes with the 3'-unstranslated region of endothelial nitric oxide synthase (eNOS) mRNA and are associated with its destabilization. The aim of this study was to determine the presence of such proteins and the level of eNOS expression in hypercholesterolemic rabbits as an in vivo model of endothelial dysfunction.

METHODS AND RESULTS

Endothelium-dependent relaxation in response to acetylcholine was reduced in aortic segments from hypercholesterolemic rabbits compared with controls. Treatment of hypercholesterolemic rabbits with simvastatin (25 mg/kg body weight/day) restored endothelium-dependent relaxation. Aortic eNOS expression was reduced in hypercholesterolemic rabbits and was accompanied by enhanced binding activity of a 60-KDa cytosolic protein and reduced stability of eNOS mRNA. Simvastatin treatment upregulated eNOS expression and reduced the interaction of cytosolic protein with the 3'-untranslated region of eNOS mRNA.

CONCLUSIONS

These results demonstrate the presence of a 60-KDa protein that binds to eNOS mRNA and reduces eNOS expression in the vascular wall.

Polyethylene glycol-superoxide dismutase, a conjugate in search of exploitation

Without a doubt PEG-SOD has been the enzyme most studied in PEGylation. One can say that it represents the preferred model to assess chemistries for PEG activation, analytical procedures suitable for conjugate characterization, the influence of PEG size in conjugate removal from circulation and elimination of immunogenicity and antigenicity, and the effect of route of administration. The effect of PEG conjugation was studied in vitro and in vivo models in comparison with the free enzyme and the following conclusions may be drawn: (1) At the blood vessel level, PEG-SOD has been shown to provide a greater resistance to oxidant stress, to improve endothelium relaxation and inhibit lipid oxidation. (2) In the heart, PEG-SOD proved to be at least as effective as native SOD in treatment of reperfusion-induced arrhythmias and myocardial ischemia. (3) In the lung, PEG-SOD appeared to be able to reduce oxygen toxicity and E. coli-induced lung injury, but not in the treatment of lung physiopathology associated with endotoxin-induced acute respiratory failure and in the reduction of asbestos-induced cell damage. (4) On cerebral ischemia/reperfusion injuries the effect of PEG-SOD was uncertain, also due to the difficulty of cerebral cell penetration. (5) In kidney and liver ischemia both enzyme forms were found to ameliorate reperfusion damage. In view of so much positive research on PEG-SOD, it is surprising that no approved application in human therapy has been established and approved.

Two angiotensin AT1 receptor antagonists, irbesartan and losartan, effects in cholesterol-fed rabbits

This study was performed to examine the long-term effects of irbesartan and losartan, two angiotensin (AT(1)) receptor antagonists, on lipoproteins and vascular responsiveness in vessels isolated from hypercholesterolemic rabbits. Four groups of rabbits (n=40) were used: Group 0 (control group), Group 1 [hypercholesterolemic group, 0.5% (wt./wt.) cholesterol-enriched diet], Group 2 (hypercholesterolemic+irbesartan 10 mg/kg/day) and Group 3 (hypercholesterolemic+losartan 10 mg/kg/day). After 17 weeks of treatment, total cholesterol and low-density lipoproteins levels in irbesartan- and losartan-treated groups were significantly lower than those of Group 1 (alpha=0.05). Furthermore, levels of high-density lipoproteins were higher in the treated groups than in the hypercholesterolemic (alpha=0.05) when we consider the same level of total cholesterol in the hypercholesterolemic and the treated groups. Despite the effect of the drugs on the abovementioned parameters, treatment with irbesartan or losartan did not improve endothelium-dependent and independent relaxation in aortic and mesenteric rings. Treatment with irbesartan and losartan decreased noradrenaline-induced contraction in aortic rings with respect to that in the hypercholesterolemic group (alpha=0.05). In addition, irbesartan treatment improved the increase in serotonin-induced contraction in proximal coronary arteries with respect to that in the hypercholesterolemic group (alpha=0.05). These results indicate that irbesartan and losartan restore noradrenaline-induced contraction in hypercholesterolemic rabbit-isolated arteries and improve lipoprotein profile in cholesterol-fed rabbits.

Lipids and endothelium-dependent vasodilation--a review

Studies using both in vitro and in vivo techniques have repeatedly shown that endothelium-dependent vasodilation (EDV) is impaired in different forms of experimental as well as human hypercholesterolemia. Clearly this impaired EDV can be reversed by lowering cholesterol levels by diet or medical therapy. Competitive blocking of L-arginine, changes in nitric oxide synthase activity, increased release of endothelin-1, and inactivation of nitric oxide due to superoxide ions all contribute to the impairment in EDV during dyslipidemia. The oxidation of low density lipoprotein, with its compound lysophosphatidylcholine, plays a critical role in these events. However, data on the role of triglycerides and fat-rich meals regarding EDV are not so consistent as data for cholesterol, although a view that the compositions of individual fatty acids and antioxidants are of major importance is emerging. Thus, this review shows that while impaired EDV is a general feature of hypercholesterolemia, the mechanisms involved and the therapeutic opportunities available still have to be investigated. Furthermore, discrepancies regarding the role of triglycerides and fat content in food may be explained by divergent effects of different fatty acids on the endothelium.

Study of garlic extracts and fractions on cholesterol plasma levels and vascular reactivity in cholesterol-fed rats

Garlic is known for its pharmacologic and nutritional properties. In previous studies, garlic elicited a reduction in plasma levels of lipids by inhibiting hepatic cholesterol synthesis. The aim of this study was to investigate in an in vivo model the effects of garlic extract and some fractions on cholesterol levels and vascular reactivity in cholesterol-fed rats. Rats were fed a cholesterol-enriched diet for 16 wk and were divided into 10 groups as follows: control and hypercholesterolemic diet groups, 4 groups fed frozen garlic fractions and 4 groups fed raw garlic fractions with different doses. Blood samples were obtained to analyze HDL and LDL cholesterol levels. After treatment, rats were killed. The heart, liver and kidneys were weighed; the aorta was isolated, mounted in organ chambers and vascular reactivity was tested. Plasma concentration of cholesterol was 58 mg/dL (100%) at the beginning of the study and increased to 102 mg/dL (153%; hypercholesterolemic group) at the end of the treatment. Plasma total cholesterol decreased in all groups treated with garlic; moreover, this effect was higher in rats fed raw garlic fractions and extracts. LDL decreased significantly with respect to the hypercholesterolemic group in all groups treated with garlic fractions and extracts (P: < 0.01); however, an increase in HDL was found in those treated with frozen fractions and extracts. The liver:body weight ratio decreased in all treated groups. The relaxing effect of acetylcholine (ACh) was enhanced in arteries contracted with noradrenaline (NE). These data suggest that garlic fractions could prevent diet-induced hypercholesterolemia and vascular alterations in the endothelium-dependent relaxation associated with atherosclerosis.

Atherogenic lipids and endothelial dysfunction: mechanisms in the genesis of ischemic syndromes

Atherogenic lipids, particularly oxidized low-density lipoprotein, are responsible for a wide range of cellular dysfunctions within the vessel wall. The effects on endothelial cells disrupt normal control of vasomotion, with a reduction of effective nitric oxide activity, the development of a procoagulant surface, chronic low-grade inflammation, and abnormal cell growth. These changes are central not only in the development of atherosclerosis but also in the evolution of both stable and unstable ischemic syndromes. There is growing evidence that these abnormal changes in cell function respond rapidly to changes in the atherogenic lipids. Certain cell functions can improve within hours or days of cholesterol lowering.

Retrobulbar and peripheral capillary blood flow in hypercholesterolemic subjects

PURPOSE

To assess retrobulbar blood flow velocities and finger nailfold capillary blood flow velocities (two readily accessible vascular beds) in subjects with primary hypercholesterolemia before and during lipid-lowering therapy.

METHODS

Retrobulbar blood flow velocities and finger nailfold capillary blood flow velocities were assessed in 15 hypercholesterolemic subjects and in 15 age-matched and sex-matched healthy control subjects. In addition, the change in blood flow velocities after a 3-month period of lipid-lowering therapy was evaluated.

RESULTS

No alterations in retrobulbar blood flow velocities after lipid-lowering therapy could be observed in hypercholesterolemic patients. Nailfold capillary blood flow velocity was slower in hypercholesterolemic subjects than control subjects immediately after local cooling (P = .0052), but this vascular dysregulation did not improve after lipid-lowering therapy (P = .58).

CONCLUSIONS

Blood flow alterations, potentially caused by perturbed vascular endothelial function, may occur in nailfold capillaries of hypercholesterolemic subjects. Such alterations were not seen in retrobulbar vessels, and blood flow measures did not change during lipid-lowering therapy.

N-Substituted analogues of S-nitroso-N-acetyl-D,L-penicillamine: chemical stability and prolonged nitric oxide mediated vasodilatation in isolated rat femoral arteries

Previous studies show that linking acetylated glucosamine to S-nitroso-N-acetyl-D,L-penicillamine (SNAP) stabilizes the molecule and causes it to elicit unusually prolonged vasodilator effects in endothelium-denuded, isolated rat femoral arteries. Here we studied the propanoyl (SNPP; 3 carbon side-chain), valeryl (SNVP; 5C) and heptanoyl (SNHP; 7C) N-substituted analogues of SNAP (2C), to further investigate other molecular characteristics that might influence chemical stability and duration of vascular action of S-nitrosothiols. Spectrophotometric analysis revealed that SNVP was the most stable analogue in solution. Decomposition of all four compounds was accelerated by Cu(II) and cysteine, and neocuproine, a specific Cu(I) chelator, slowed decomposition of SNHP. Generation of NO from the compounds was confirmed by electrochemical detection at 37 degrees C. Bolus injections of SNAP (10 microl; 10(-8)-10(-3) M) into the perfusate of precontracted, isolated rat femoral arteries taken from adult male Wistar rats (400-500 g), caused concentration-dependent, transient vasodilatations irrespective of endothelial integrity. Equivalent vasodilatations induced by SNVP and SNHP were transient in endothelium-intact vessels but failed to recover to pre-injection pressures at moderate and high concentrations (10(-6)-10(-3) M) in those denuded of endothelium. This sustained effect (> 1 h) was most prevalent with SNHP and was largely reversed by the NO scavenger, haemoglobin. We suggest that increased lipophilicity of SNAP analogues with longer sidechains facilitates their retention by endothelium-denuded vessels; subsequent slow decomposition within the tissue generates sufficient NO to cause prolonged vasodilatation. This is a potentially useful characteristic for targeting NO delivery to areas of endothelial damage.

Lovastatin maintains nitric oxide--but not EDHF-mediated endothelium-dependent relaxation in the hypercholesterolemic rabbit carotid artery

The endothelium contributes to the regulation of vascular tone by producing nitric oxide (NO) and the endothelium-derived hyperpolarising factor (EDHF). In hypercholesterolemia, endothelium-dependent relaxation is impaired but can be restored by treatment with lovastatin (LOVAS). We investigated the effects of LOVAS on NO and EDHF-mediated relaxation. Rabbits were fed 1% cholesterol diet for 4 weeks and 0.5%) cholesterol for the following 12 weeks (CHOL-group). The LOVAS group additionally received 10 mg of lovastatin over the last 12-week period. Experiments were performed in carotid artery rings. Relaxant responses to acetylcholine (ACh) were recorded in the presence of indomethacin. Nitro-L-arginine (NOARG, 100 microM) and potassium chloride (KCl, 35 mM) were used to differentiate between NO- and EDHF-mediated relaxations. Cholesterol impaired ACh-induced relaxations and this effect was prevented by LOVAS (control 100+/-1%, CHOL 81+/-6%, LOVAS 98+/-1%). In the presence of NOARG, relaxations to ACh were not different between the LOVAS and CHOL groups (control 78+/-4%, CHOL 64+/-6%, LOVAS 64+/-5%). When KCl was used, ACh-induced relaxations were similar in the LOVAS and control group (control 75+/-5%, CHOL 49+/-6%, LOVAS 76+/-2%). In arteries treated with NOARG and KCl together, no relaxations were observed. Relaxations of arteries from the control group were not affected by 18 h preincubation with lovastatin (10 microM). Lovastatin selectively maintains nitric oxide-mediated endothelium-dependent relaxation in hypercholesterolemic rabbit carotid arteries.

Effects of long-term smoking on myocardial blood flow, coronary vasomotion, and vasodilator capacity

BACKGROUND

The effect of long-term smoking on coronary vasomotion and vasodilator capacity in healthy smokers is unknown.

METHODS AND RESULTS

Myocardial blood flow (MBF) was quantified with [13N]ammonia and positron emission tomography (PET) at rest, during cold pressor testing (endothelium-dependent vasomotion), and during dipyridamole-induced hyperemia in 16 long-term smokers and 17 nonsmokers. MBF at rest did not differ between the 2 groups. Cold induced similar increases in rate-pressure product (RPP) in smokers and nonsmokers. However, MBF increased only in nonsmokers and was, during cold, higher than in smokers (0.91+/-0.18 versus 0.78+/-0.14 mL x g(-1) x min(-1), P<0.05). MBF normalized to the RPP (derived from the ratio of MBF ([milliliters per gram per minute] to RPP [beats per minute times millimeters of mercury] times 10000) declined in smokers but remained unchanged in nonsmokers (0.86+/-0.10 versus 0.72+/-0.11, P=0.0006, and 0.99+/-0.25 versus 0.96+/-0.27, P=NS). The hyperemic response to dipyridamole and the myocardial flow reserve did not differ between the 2 groups. In a multiple regression model adjusted for age, sex, serum lipid levels, years of smoking, and pack-years, years of smoking was the strongest predictor of the normalized blood flow response to cold (P<0.001), followed by the HDL/LDL ratio.

CONCLUSIONS

The normal hyperemic response to dipyridamole in long-term smokers indicates a preserved endothelium-independent coronary vascular smooth muscle relaxation, whereas the abnormal response to cold suggests a defect in coronary vasomotion likely located at the level of the coronary endothelium. Its severity depends on the total exposure time to smoking.

Prolonged effect of a novel S-nitrosated glyco-amino acid in endothelium-denuded rat femoral arteries: potential as a slow release nitric oxide donor drug

1. The vasodilator properties of a novel S-nitrosated glyco-amino acid (RIG200) were investigated in isolated rat femoral arteries and compared with those of the parent S-nitrosothiol compound, S-nitroso-N-acetylpenicillamine (SNAP). 2. Spectrophotometric analysis revealed that 2.5 mM solutions of RIG200 decomposed more slowly (half-life (t1/2) = 216.2 +/- 26.7 min) than SNAP (t1/2 = 37.2 +/- 13.8 min) in Krebs buffer at 24 degrees C. Furthermore, the rate of decomposition of SNAP, but not of RIG200, was significantly reduced by the Cu(I) chelator, neocuproine. We concluded that the relative stability of RIG200 is due, at least in part, to its resistance to trace Cu(I)-catalyzed decomposition. Nitric oxide (NO) generation from SNAP and RIG200 was confirmed by use of an NO electrode. 3. Experiments to investigate the vasodilator effects of RIG200 were carried out on isolated femoral arteries taken from adult male Wistar rats (400-550 g). Lengths of artery (7-8 mm long) were cannulated, dissected free and perfused at constant flow rate (0.6 ml min-1) with Krebs buffer. Vessels were precontracted with phenylephrine (10.2 +/- 0.3 microM) and developed pressures of 91.8 +/- 4 mmHg, detected upstream by a differential pressure transducer. 4. Concentration-dependent vasodilator responses to bolus injections of SNAP or RIG200 (10 microliters; 10(-8)-10(-3) M) made into the perfusate of endothelium-intact vessels were transient, recovering the preinjection pressure in < 20 min. 5. Responses to equivalent bolus injections of SNAP in endothelium-denuded vessels were also transient but those in response to concentrations of RIG200 > 10(-5) M were sustained. Responses to 10(-3) M RIG200 were sustained for periods > 4 h. Sustained vasodilatation was reversed by the NO scavenger, ferrohaemoglobin (10 microM) but was unaffected by the NO synthase inhibitor, N omega-nitro-L-arginine methyl ester (200 microM), indicating involvement of NO from a source other than NO synthase. 6. We suggest that a possible explanation for the prolonged effect of RIG200 is retention of the compound by the vascular wall, facilitated by endothelial denudation. Slow decomposition of RIG200 in situ would release sufficient NO to maintain a 'vasodilator tone' which persists for more than 4 h. Selective retention by damaged vessels could have important therapeutic implications with regard to targeted delivery of NO, restoring protection to areas deprived of endogenous NO, whilst avoiding unwanted hypotension.

N(G)-nitro-L-arginine- and indomethacin-resistant endothelium-dependent relaxation in the rabbit renal artery: effect of hypercholesterolemia

Studies were designed to compare the N(G)-nitro-L-arginine- and indomethacin-resistant, endothelium-dependent relaxation to acetylcholine in isolated renal artery rings from normal and cholesterol-fed rabbits. It was assumed that the resistant part in response to acetylcholine is mediated by the endothelial-derived hyperpolarizing factor (EDHF). Rabbits were fed normal (n = 15) or cholesterol enriched chow (n = 13, 1% cholesterol for 4 weeks, 0.5% for 12 weeks). In organ chamber experiments, renal artery rings were precontracted with 0.1-1 microM phenylephrine or 35 mM KCl, and relaxed with acetylcholine (0.001-10 microM) in the presence of 10 microM indomethacin. Studies were performed in the presence or absence of: 100 microM N(G)-nitro-L-arginine (L-NOARG) to inhibit the nitric oxide pathway, 100 nM charybdotoxin (CTX) or 1 mM tetrabutylammonium (TBA) to inhibit Ca2+-activated K+ channels, and 100 microM SKF 525a to inhibit cytochrome P450 monoxygenase pathway. In normal arteries, L-NOARG partially inhibited acetylcholine-induced relaxation. The resistant part was almost abolished when the arteries were depolarized with KCl, or when L-NOARG was combined with either CTX, TBA or SKF 525a. In arteries from hypercholesterolemic animals, the relaxation to acetylcholine was only slightly impaired as compared to normal animals. However, in comparison to arteries from normal animals, the L-NOARG-resistant part of acetylcholine-induced endothelium-dependent relaxation was enhanced. It is speculated that differences in the balance between nitric oxide (NO)- and EDHF-mediated control of vascular tone may maintain acetylcholine-induced vasodilatation of the renal artery in hypercholesterolemia.

Effect of diet induced hypercholesterolemia and selenium supplementation on nitric oxide synthase activity

The aim of the present study was to examine the activity of nitric oxide synthase (NOS, EC 1.14.23) in plasma of high fat diet (HFD, 2% cholesterol and 100 g table butter/kg diet) and HFD + selenium (Se, 1 ppm as sodium selenite) fed rabbits for three months. Significant increase in the serum cholesterol and triglyceride levels in HFD fed group was observed. The activity of NOS also increased very significantly. However in Se supplemented animals, there was a significant reduction in serum cholesterol as well as in plasma NOS activity relative to HFD fed animals. It is concluded that the protective effect of Se on HFD induced NOS activity acts probably through its antioxidant/inhibitory action.

Simvastatin, an HMG-coenzyme A reductase inhibitor, improves endothelial function within 1 month

BACKGROUND

Cholesterol-lowering therapy can improve cardiovascular morbidity and mortality in patients with atherosclerosis. Although the mechanisms responsible are unclear, these benefits precede macroscopic changes in the vasculature. Emerging evidence that improvement in endothelial function may occur requires substantiation; in particular, it is unclear how early any such improvement would be detectable after initiation of therapy.

METHODS AND RESULTS

This randomized, double-blind, placebo-controlled crossover study evaluated the effect of simvastatin (20 mg daily for 4 weeks) on endothelium-dependent and endothelium-independent vasodilation and on the response to the inhibitor of nitric oxide synthesis, NG-monomethyl-L-arginine (L-NMMA), in the forearm vasculature of subjects with moderate elevation of total serum cholesterol (6.0 to 10.0 mmol/L) by use of strain-gauge plethysmography. Studies were repeated after 3 more months of open therapy. When the results are expressed as percentage changes in flow in the infused arm relative to the noninfused arm, the vasodilator response to acetylcholine was significantly increased after 4 weeks of treatment with simvastatin (P < .0005), and this improvement was further enhanced after 3 months (P < .005). Concurrently, simvastatin augmented the vasoconstrictor response to L-NMMA, an effect that was maintained at 3 months (P < .0005). The response to the endothelium-independent vasodilator sodium nitroprusside was unaltered.

CONCLUSIONS

These observations indicate that within 1 month of treatment with simvastatin, both the stimulated and basal nitric oxide dilator functions of the endothelium are augmented, and the benefits of this HMG-coenzyme A reductase inhibitor persist with continued therapy.

Circulating plasma xanthine oxidase contributes to vascular dysfunction in hypercholesterolemic rabbits

Reactive oxygen species play a central role in vascular inflammation and atherogenesis, with enhanced superoxide (O2.-) production contributing significantly to impairment of nitric oxide (.NO)-dependent relaxation of vessels from cholesterol-fed rabbits. We investigated potential sources of O2.- production, which contribute to this loss of endothelium-dependent vascular responses. The vasorelaxation elicited by acetylcholine (ACh) in phenylephrine-contracted, aortic ring segments was impaired by cholesterol feeding. Pretreatment of aortic vessels with either heparin, which competes with xanthine oxidase (XO) for binding to sulfated glycosaminoglycans, or the XO inhibitor allopurinol resulted in a partial restoration (36-40% at 1 muM ACh) of ACh-dependent relaxation. Furthermore, O2.(-)-dependent lucigenin chemiluminescence, measured in intact ring segments from hypercholesterolemic rabbits, was decreased by addition of heparin, allopurinol or a chimeric, heparin-binding superoxide dismutase. XO activity was elevated more than two-fold in plasma of hypercholesterolemic rabbits. Incubation of vascular rings from rabbits on a normal diet with purified XO (10 milliunits/ml) also impaired .NO-dependent relaxation but only in the presence of purine substrate. As with vessels from hypercholesterolemic rabbits, this effect was prevented by heparin and allopurinol treatment. We hypothesize that increases in plasma cholesterol induce the release of XO into the circulation, where it binds to endothelial cell glycosaminoglycans. Only in hypercholesterolemic vessels is sufficient substrate available to sustain the production of O2.- and impair NO-dependent vasorelaxation. Chronically, the continued production of peroxynitrite, (ONOO-) which the simultaneous generation of NO and O2.- implies, may irreversibly impair vessel function.

Vasodilator responses of rat isolated tail artery enhanced by oxygen- dependent, photochemical release of nitric oxide from iron-sulphur-nitrosyls

1. The vasodilator properties and photochemical decomposition of two synthetic iron-sulphur-nitrosyl clusters (cluster A: [Fe4S4(NO)4], tetranitrosyl-tetra-mu 3-sulphido-tetrahedro-tetrairon; and B:[Fe4S3 (NO)7]-1, heptanitrosyl-tri-mu 3-thioxotetraferrate(-1)) have been investigated. Experiments were carried out on isolated, internally-perfused segments of rat tail artery. 2. Bolus injections (10 microliters) of A or B ( > 0.25 mM) delivered into the internal perfusate generated sustained (or S-type) vasodilator responses, characterized by a persistent plateau of reduced tone due to NO released from clusters which enter and become trapped within endothelial cells. Clusters were therefore irradiated with visible laser light (lambda = 457.9 or 514.5 nm) either (a) in solution, while passing through a glass tube en route to the artery; or (b) when retained within the endothelium, by illuminating the artery directly during the plateau of an S-type response. Irradiation produced an additional vasodilator response, the magnitude of which depended upon wavelength and laser beam energy. 3. The nitric oxide synthase inhibitor, NG-monomethyl-L-arginine (100 microM), had no effect on light-induced vasodilator responses. However, they were (a) blocked entirely by adding oxyhaemoglobin (5 microM) to the internal perfusate; and (b) greatly enhanced by the enzyme superoxide dismutase (150 u ml-1). 4. Photolysis of cluster B was measured by absorption spectroscopy and by detecting NO released with an electrochemical sensor. The photochemical reaction was found to be oxygen-dependent. The half-time for inactivation of cluster-derived NO was measured by interposing different lengths of tubing (i.e. time delays) between the photolysis tube and NO sensor. The steady-state probe current decayed exponentially with increasing delay time, with a t 1/2 of 21 s. The amplitudes of vasodilator responses of the tail artery also decreased exponentially by increasing the time delay (t 1/2 = 58 s). Superoxide dismutase (150 u ml-1) prevented this from happening, showing that "inactivation' of cluster-derived NO was caused by reaction with superoxide anions formed during photolysis. 5. We conclude that potentiation of vasodilator responses to iron-sulphur-nitrosyl clusters by visible light is due to an oxygen-dependent photochemical reaction which accelerates the release of ligated nitrosyl groups as free NO. Based on our measurements, we estimate that ca 100 pM NO is sufficient to produce a just-detectable additional vasodilatation and that the ED50 dose is ca 3.7 nM.

Endothelial dysfunction in the early stage of atherosclerosis precedes appearance of intimal lesions assessable with intravascular ultrasound

The objective of this study was to clarify whether morphologic evaluation of the in vivo artery with intravascular ultrasound provides as sensitive a marker as endothelial dysfunction or microscopic histologic assessment. Endothelial dysfunction assessed with the changes in the vessel diameter during acetylcholine infusion has been used as a more sensitive marker of atherosclerosis than the angiographic estimates of morphologic structure of the vessel. Recent advent of intravascular ultrasound has provided such high-resolution images of the vessels that morphologic changes in the vessel structure are sensitively and accurately detected. Twenty-two rabbits were divided into three groups: six rabbits fed a cholesterol-rich diet for 2 weeks as the hypercholesterolemia group, eight rabbits fed with the diet for 8 weeks as the atherosclerosis group, and eight rabbits fed a normal diet as the normal group. After evaluating the atherosclerotic lesions by intravascular ultrasound, the cross-sectional area was measured in the baseline and during the infusion of acetylcholine (0.05, 0.5, and 5 micrograms/kg/min) and nitroglycerin (5 micrograms/kg/min). No atherosclerotic lesions were detectable with intravascular ultrasound in any rabbit despite the presence of microscopic intimal lesions in the vessels in the rabbits of the atherosclerosis group. The cross-sectional area increased during acetylcholine infusion in the rabbits of the normal and the hypercholesterolemia groups. In contrast, in the rabbits of the atherosclerosis group, the cross-sectional area did not significantly increase during acetylcholine infusion at the rate of 0.5 microgram/kg/min and even tended to decrease at the rate of 5 micrograms/kg/min (-3.8% +/- 3.7%, P < 0.05 vs the normal group). Dilating responses to nitroglycerin infusion were similar among all three groups. In conclusion, impairment of the endothelium-dependent vasodilating response assessed with intravascular ultrasound in the in vivo vessel precedes the appearance of echographic atherosclerotic findings. Thus intravascular ultrasound, if used in combination with drug intervention to assess endothelial function, would provide even more accurate assessment of the vessels than conventional intravascular ultrasound alone.

Evidence that expression of inducible nitric oxide synthase in response to endotoxin is augmented in atherosclerotic rabbits

Atherosclerotic lesions contain monocytes/macrophages and vascular smooth muscle cells and thus may have an increased capacity for generation of nitric oxide by inducible nitric oxide synthase (NOS). We used three approaches (contractile responses, generation of L-citrulline from L-arginine, and staining with NADPH-diaphorase) to test the hypothesis that after administration of lipopolysaccharide (LPS) in vivo, generation of nitric oxide by inducible NOS is augmented in atherosclerotic arteries. New Zealand White (normal, n = 18) and Watanabe heritable hyperlipidemic (atherosclerotic, n = 21) rabbits were anesthetized and injected intravenously with vehicle or LPS. Contractile responsiveness of aortic segments was examined in vitro 4 hours after injection of LPS in vivo. There was a substantial (approximately fivefold) decrease in contractile sensitivity of aortas from LPS-treated atherosclerotic rabbits and a small (approximately twofold) decrease in normal rabbits. Incubation of aortic segments with aminoguanidine, which inhibits inducible NOS, restored contractile responsiveness after LPS treatment. In vitro assay of conversion of [14C]L-arginine to [14C]L-citrulline by aortic segments demonstrated marked (approximately fivefold) increase in calcium-independent conversion of [14C]L-arginine by LPS-treated atherosclerotic, but not normal, aortas. NADPH-diaphorase staining demonstrated positive cells only in the endothelium of normal rabbits and in the lesions and media of the atherosclerotic aortas in both vehicle- and LPS-treated rabbits. The general distribution of these NADPH-diaphorase-positive cells resembled that of smooth muscle cells and not macrophages. Thus, impairment of contractile responses, generation of L-citrulline, and staining with NADPH-diaphorase suggest that atherosclerotic arteries have increased capacity for generation of nitric oxide by inducible NOS.

Low concentration of oxidized low-density lipoprotein and lysophosphatidylcholine upregulate constitutive nitric oxide synthase mRNA expression in bovine aortic endothelial cells

Endothelium-dependent relaxation is markedly reduced in atherosclerotic arteries. Recently, the endothelium-dependent relaxing factor has been identified as nitric oxide (NO). We used RNase protection assay and immunoblotting to elucidate the effect of atherogenic lipoprotein on the expression of constitutive NO synthase (cNOS) mRNA and protein levels in bovine aortic endothelial cells. Twenty-four-hour exposure to a low concentration of oxidized low-density lipoprotein (10 micrograms protein/mL) upregulated cNOS mRNA levels (2.4 +/- 0.4-fold, P < .01). However, native low-density lipoprotein and high-density lipoprotein did not have any effect on cNOS mRNA levels. Furthermore, 5 micrograms/mL of lysophosphatidylcholine (LPC) also upregulated cNOS mRNA levels (2.6 +/- 0.5-fold, P < .01) at 8 hours. This action of LPC was abolished with cycloheximide but not with staurosporine. We concluded that atherogenic lipoproteins upregulate cNOS mRNA and protein levels in bovine aortic endothelial cells. This observation supports the hypothesis that an impairment of endothelium-dependent vasodilatation in atherosclerotic vessels may not be due to a decrease in cNOS expression. Moreover, the LPC action on cNOS mRNA levels requires new protein synthesis.

Long-term inhibition of NO synthesis promotes atherosclerosis in the hypercholesterolemic rabbit thoracic aorta. PGH2 does not contribute to impaired endothelium-dependent relaxation

We examined whether prostaglandin (PG) H2, as an endothelium-dependent contracting factor, or the disturbed production of endothelium-derived relaxing factor, impairs endothelium-dependent relaxation and whether long-term inhibition of nitric oxide (NO) synthesis aggravates atherosclerosis in hypercholesterolemic rabbits. Male New Zealand White rabbits were fed one of the following diets: (1) standard chow; (2) 2% cholesterol-supplemented chow; (3) standard chow with 80 micrograms/mL N omega-nitro-L-arginine methylester (L-NAME), an NO synthetase inhibitor, in their drinking water; or (4) 2% cholesterol-supplemented chow with 80 or 160 micrograms/mL L-NAME in their drinking water. The rabbits were fed these diets for 8 or 12 weeks. Then aortic rings were obtained, and changes in isometric tension were recorded. Intimal atherosclerotic areas of the thoracic aortas were subsequently measured by planimetry. The cholesterol-supplemented diet significantly impaired endothelium-dependent aortic relaxation to acetylcholine. Pretreatment with the thromboxane A2/PGH2 receptor antagonist ONO-3708 did not reverse this impaired response. Vessels from both normocholesterolemic and hypercholesterolemic rabbits given L-NAME showed more impaired endothelium-dependent relaxation than those from their dietary counterparts not given L-NAME. Morphometric analysis revealed marked enlargement of intimal atherosclerotic areas in aortas from L-NAME-treated hypercholesterolemic rabbits compared with those from untreated hypercholesterolemic rabbits. These findings suggest that PGH2 does not contribute to impaired endothelium-dependent relaxation and that long-term administration of L-NAME promotes atherosclerosis by inhibition of NO synthesis in the hypercholesterolemic rabbit thoracic aorta.

Superoxide and peroxynitrite in atherosclerosis

The role of reactive oxygen species in the vascular pathology associated with atherosclerosis was examined by testing the hypothesis that impaired vascular reactivity results from the reaction of nitric oxide (.NO) with superoxide (O2-), yielding the oxidant peroxynitrite (ONOO-). Contractility studies were performed on femoral arteries from rabbits fed a cholesterol-supplemented diet. Cholesterol feeding shifted the EC50 for acetylcholine (ACh)-induced relaxation and impaired the maximal response to ACh. We used pH-sensitive liposomes to deliver CuZn superoxide dismutase (SOD; superoxide:superoxide oxidoreductase, EC 1.15.1.1) to critical sites of .NO reaction with O2-. Intravenously injected liposomes (3000 units of SOD per ml) augmented ACh-induced relaxation in the cholesterol-fed group to a greater extent than in controls. Quantitative immunocytochemistry demonstrated enhanced distribution of SOD in both endothelial and vascular smooth muscle cells as well as in the extracellular matrix. SOD activity in vessel homogenates of liposome-treated rabbits was also increased. Incubation of beta very low density lipoprotein with ONOO- resulted in the rapid formation of conjugated dienes and thiobarbituric acid-reactive substances. Our results suggest that the reaction of O2- with .NO is involved in the development of atherosclerotic disease by yielding a potent mediator of lipoprotein oxidation, as well as by limiting .NO stimulation of vascular smooth muscle guanylate cyclase activity.

Beneficial effect of cholesterol-lowering therapy on coronary endothelium-dependent relaxation in hypercholesterolaemic patients

Since hypercholesterolaemia is associated with impaired endothelium-dependent vasodilation, a study was conducted to find out whether cholesterol reduction will improve endothelial function in patients with hypercholesterolaemia and normal coronary arteries. 25 men (mean age 51 [SD 8] years) with total serum cholesterol > 6.2 mmol/L) and angiographically normal coronary arteries had their coronary vasomotor responses to intracoronary acetylcholine and nitroglycerin assessed by computer-assisted quantitative angiography at baseline and after 6 months of cholesterol-reducing diet and cholestyramine. Between baseline and follow-up mean total serum cholesterol level fell by 28.7 (SD 5.6)% (p < 0.001); mean low-density lipoprotein (LDL) cholesterol level by 35.6 (8.7)% (p < 0.001); and mean total cholesterol to high-density lipoprotein (HDL) cholesterol ratio by 29.4 (10.6)% (p < 0.001). Acetylcholine significantly reduced the mean segment diameter at baseline, by 21.7 (14.0)% (p < 0.01), but it increased the diameter at follow-up, by 6.16 (13.3)% (p < 0.01), the difference between the two occasions being significant (p < 0.001). Nitroglycerin significantly increased the mean segment diameter, both at baseline, by 18.7 (11.5)% (p < 0.01), and at follow-up, by 19.3 (12.1)% (p < 0.01), the difference between the two responses being not significant. At baseline total cholesterol and LDL cholesterol did not correlate with acetylcholine response, but they did at follow-up (total cholesterol, r = 0.67, p < 0.01; LDL cholesterol, r = 0.64, p < 0.01). Impairment of endothelium-dependent (acetylcholine-induced) dilation of the epicardial coronary arteries in hypercholesterolaemic patients with angiographically normal coronary arteries is thus reversible by reducing serum cholesterol. In addition, the degree of impairment of acetylcholine-induced vasomotor response is related to the cholesterol concentrations after therapy.

Ramipril prevents impaired endothelium-dependent relaxation in arteries from rabbits fed an atherogenic diet

Endothelium-dependent relaxation in arteries is attenuated in clinical and experimental atherosclerosis. This study investigates the endothelial preservation properties of the angiotensin converting enzyme inhibitor, ramipril, by assessing its ability to restore endothelium-dependent responsiveness in blood vessels from rabbits fed an atherogenic diet (0.25% cholesterol; 3% coconut oil; 12 weeks). Seven rabbits fed the atherogenic diet received ramipril (3 mg/kg mixed into their food daily) and 6 rabbits were maintained on the atherogenic diet alone. Control rabbits (n = 6) were fed a standard diet and did not receive ramipril. At the end of the dietary intervention, the rabbits were killed and blood was collected for measurement of the lipid profile. The thoracic aorta was isolated and half was frozen for pathologic review while the other half was cut into rings and placed in a muscle bath for measurement of isometric force development. Dose response curves to phenylephrine (10(-9) to 10(-5) M) and angiotensin II (10(-10) to 3 x 10(-7) M) were completed. There was a minimal decrease in responsiveness to phenylephrine in vessels from rabbits eating the atherogenic diet compared with controls and no significant differences in the response to angiotensin II for any of the vessels. Following contraction by phenylephrine, acetylcholine (10(-9) to 10(-5) M) and nitroglycerin (10(-10) to 10(-5) M) dose response curves were completed. Relaxation to acetylcholine in aortic rings from control rabbits was observed, although in arteries from atherogenic rabbits relaxation was attenuated. This effect was prevented in the atherogenic rabbits fed ramipril. Responsiveness to the endothelium-independent vasodilator, nitroglycerin, was similar in arteries from the three rabbit groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia causes an abnormal contractile response in the atherosclerotic rabbit aorta. Implication of reduced nitric oxide and cGMP production

Both atherosclerotic lesions and hypoxia alter the contractile properties of the arterial wall and, in particular, may interfere with the relaxation mechanisms dependent or not on the endothelium. The present study was designed to test the effect of severe hypoxia on the contractile behavior of the atherosclerotic rabbit aorta. Segments of aortas obtained from control, cholesterol-fed, or Watanabe hereditary hyperlipidemic rabbits were mounted in organ chambers for isometric tension recording. A change of the bath PO2 from "normoxic" conditions (95% O2-5% CO2) to "hypoxic" conditions (95% N2-5% CO2) caused relaxation in the precontracted control aortas (by approximately 85%) but a transient contraction (approximately 20% of the maximal contraction obtained with 30 mM KCl) followed by a relaxation in the precontracted atherosclerotic aortas. Both types of responses were observed in aortas contracted with aggregating platelets, 5-hydroxytryptamine (5-HT), norepinephrine, endothelin, and prostaglandin F2 alpha. The hypoxic contractions in atherosclerosis were not dependent on the presence of an intact endothelium. They could not be antagonized by blockers of alpha-adrenoceptors, 5-HT2 receptors, histamine receptors, thromboxane receptors, and muscarinic cholinoreceptors. Inhibitors of cyclooxygenase, lipoxygenase, Na+, K(+)-ATPase, and free radical scavengers or an activator of endothelium-derived relaxing factor did not significantly affect the hypoxic contraction; the absence of effect of some inhibitors of protein synthesis seems to rule out the involvement of endothelin, angiotensin II, and bradykinin. The hypoxic contraction was not influenced by omission of Ca2+ from the medium or by inhibition of Ca2+ influx but was prevented by blockade of intracellular Ca2+. The inhibitor of nitric oxide synthase (nitro-L-arginine, 100 microM) and the guanylyl cyclase inhibitor (methylene blue, 10 microM) both enhanced the initial contractile responses to 5-HT to a similar extent as hypoxia and completely prevented the hypoxic contraction in the atherosclerotic tissues. The cyclic nucleotide analogues 8-bromo-cGMP and dibutyryl cAMP also inhibited the hypoxic contraction in the atherosclerotic aorta. The cGMP levels were markedly decreased and the cAMP levels were moderately decreased in the aortas of the cholesterol-fed rabbits as compared with the control aortas. Hypoxia further decreased cGMP but not the cAMP levels in atherosclerotic aortas with and without endothelium. Our data thus demonstrate the occurrence of an unusual vasoconstrictor response in atherosclerotic arteries; this constrictor response depends on the availability of intracellular Ca2+ and seems to be due to the further inhibition of an already impaired cGMP production.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth regulatory properties of endothelins

Endothelins are produced by endothelial and epithelial cells, macrophages, fibroblasts, and many other types of cells. Their receptors are present in numerous cells, including smooth muscle cells, myocytes, and fibroblasts. Evidence now suggests that the three isoforms of endothelins (ET-1 and the other two related isopeptides, ET-2 and ET-3) regulate growth in several of these cells. Endothelin-1 influences DNA synthesis, the expression of protooncogenes, cell proliferation, and hypertrophy. The participation of ET in mitogenesis involves activation of multiple transduction pathways, such as the production of second messengers, the release of intracellular pools of calcium, and influx of extracellular calcium. Moreover, ET-1 acts in synergism with various factors, such as EGF, PDGF, bFGF, TGFs, insulin, etc., to potentiate cellular transformation or replication. Several of these factors may in turn stimulate the synthesis and/or the release of endothelins. The production and release of endothelins are also increased in acute and chronic pathological processes, e.g., atherosclerosis, postangioplastic restenosis, hypertension, and carcinogenesis. It is postulated that endothelins act in a paracrine/autocrine manner in growth regulation and play an important role mediating vascular remodeling in some cardiovascular diseases. The present review analyses the implication of endothelins (ET-1, -2, and -3) in physiopathology related to their growth regulatory properties.

Induction of a calcium-independent NO synthase by hypercholesterolaemia in the rabbit

Endothelium-dependent and -independent relaxation of aortic ring preparations was assessed and nitric oxide (NO) synthase activity measured in the lung, and cerebellum of cholesterol-fed and normal rabbits. Endothelium-dependent relaxation of acetylcholine and ATP was depressed while that to the calcium ionophore, A23187, was unaltered in the cholesterol-fed group. Relaxation to sodium nitroprusside was however greater in aortae from the cholesterol-fed animals. Neither Ca(2+)-dependent nor Ca(2+)-independent NO synthase activity could be detected in aortae or hearts taken from either group of animals. Activity of both enzymes was unaltered in cerebellae from both groups of animals. Activity of the Ca(2+)-independent enzyme was however significantly greater (ca. 2 fold) in lungs from the cholesterol-fed rabbits though the activity of the Ca(2+)-dependent NO synthase was not significantly altered. This finding may account for the increased production of nitrogen oxides previously observed in this model of hypercholesterolaemia.

Iron-sulphur cluster nitrosyls, a novel class of nitric oxide generator: mechanism of vasodilator action on rat isolated tail artery

1. Two iron-sulphur cluster nitrosyls have been investigated as potential nitric oxide (NO.) donor drugs (A: tetranitrosyltetra-mu 3-sulphidotetrahedro-tetrairon; and B: heptanitrosyltri-mu 3-thioxotetraferrate(1-)). Both compounds are shown to dilate precontracted, internally-perfused rat tail arteries. 2. Bolus injections (10 microliters) of compound A or B generate two kinds of vasodilator response. Doses below a critical threshold concentration (DT) evoke transient (or T-type) responses, which resemble those seen with conventional nitrovasodilators. Doses > DT produce sustained (or S-type) responses, comprising an initial, rapid drop of pressure, followed by incomplete recovery, resulting in a plateau of reduced tone which can persist for several hours. 3. T- and S-type responses are attenuated by ferrohaemoglobin (Hb) and by methylene blue (MB), but not by inhibitors of endothelial NO. synthase. Addition of either Hb or MB to the internal perfusate can restore agonist-induced tone when administered during the plateau phase of an S-type response. Moreover, subsequent removal of Hb causes the artery to re-dilate fully. 4. We conclude that T- and S-type responses are both mediated by NO.. It is postulated that S-type responses represent the sum of two vasodilator components: a reversible component, superimposed upon a non-recoverable component. The former is attributed to free NO., preformed in solution at the time of injection; and the latter to NO. generated by gradual decomposition of a 'store' of iron-sulphur-nitrosyl complexes within the tissue. This hypothesis is supported by histochemical studies which show that both clusters accumulate in endothelial cells.

Role of thromboxane in impaired renal vasodilatation response to acetylcholine in hypercholesterolemic rats

Short-term cholesterol feeding has been shown to cause impaired vasodilatation in response to acetylcholine. The present study of renal hemodynamics was carried out to examine the role of thromboxane/PGH2 in mediating this abnormal response. In normal rats (ND), infusion of acetylcholine into the suprarenal aorta caused marked increases in renal blood flow, GFR, single nephron glomerular filtration rate, single nephron afferent plasma flow, and ultrafiltration coefficient, accompanied by a fall in preglomerular resistance. In cholesterol fed rats (CSD), the response to acetylcholine was markedly blunted. Infusion of L-arginine, the precursor to nitric oxide (NO), caused comparable renal vasodilatation in ND and CSD rats, implying that the ability to synthesize NO from its precursor was not severely impaired in the CSD animals. The observations do not exclude, however, the possibility of impaired synthesis of NO from endogenous precursor. In additional experiments, we infused a TxA2/PGH2 receptor antagonist in CSD rats and then administered acetylcholine. Renal vasodilatation occurred to a degree indistinguishable from that in ND rats given acetylcholine alone. When ND rats were infused with the same combination of the TxA2/PGH2 receptor antagonist and acetylcholine, renal vasodilatation was also significantly greater than with acetylcholine alone. This suggests that acetylcholine initiates release of vasoconstrictor prostanoids as well as NO from vascular endothelium. This was observed in ND as well as in CSD animals. Because LDL increases the supply of arachidonic acid for prostaglandin synthesis, we postulate that greater amounts of PGH2/TxA2 are synthesized via calcium activation of phospholipase A2 when acetylcholine is administered to CSD animals. This may account in large measure for the blunted vasodilatation to acetylcholine.

Impaired endothelium-dependent cholinergic coronary vasodilation in patients with angina and normal coronary arteriograms

The coronary vasomotor responses to selective infusion of graded concentrations (10(-6) to 10(-4) M) of acetylcholine into the left anterior descending artery were assessed by quantitative coronary arteriography in 24 patients with normal coronary arteriograms (12 patients with atypical symptoms and 12 patients with typical anginal pain) and 36 patients with coronary artery disease with different degrees of atherosclerosis of the left anterior descending artery. In the patients with normal coronary arteries and atypical chest pain, acetylcholine induced predominantly a vasodilator response, which was maximal during a 10(-5) M acetylcholine infusion. In contrast, in patients with coronary artery disease, acetylcholine caused dose-dependent vasoconstriction, which was observed even if the left anterior descending artery itself was smooth. Marked vasoconstriction was also induced in the patients with typical anginal pain and angiographically normal coronary arteries. In nine of these patients, this constrictor response was associated with anginal pain and electrocardiographic evidence of myocardial ischemia. Intracoronary administration of isosorbide dinitrate (1 mg) relieved the anginal pain and dilated all vessels. These data suggest that 1) patients with normal coronary arteriograms and angina pectoris manifest impairment of endothelium-dependent vasodilation similar to that observed in patients with overt coronary atherosclerosis; and 2) abnormal coronary vasoconstrictor responses resulting from this impairment may contribute to the pathogenesis of myocardial ischemia and angina in these patients.

Chronic treatment with polyethylene-glycolated superoxide dismutase partially restores endothelium-dependent vascular relaxations in cholesterol-fed rabbits

The endothelium-derived relaxing factor is rapidly inactivated by superoxide radicals, and atherosclerotic vessels generate excess radical species. We tested the hypothesis that an imbalance between intrinsic superoxide dismutase (SOD) activity and the generation of superoxide radicals in atherosclerotic arteries may result in augmented inactivation of endothelium-derived relaxing factor. Vascular SOD was increased in normal and cholesterol-fed (1% cholesterol for 4 months) rabbits approximately twofold by treatment with polyethylene-glycolated SOD (PEG-SOD; 41,000 units/kg/day i.m.) for 1 week. Aortic rings from these animals and nontreated control and atherosclerotic rabbits subsequently were studied in organ chambers. Endothelium-dependent relaxations to acetylcholine and the calcium ionophore A23187 were improved by PEG-SOD in atherosclerotic but not in normal rabbits. PEG-SOD pretreatment did not alter endothelium-independent relaxations to nitroprusside. Thus, treatment with PEG-SOD can partially restore impaired endothelium-dependent relaxation of atherosclerotic arteries. We conclude that generation of oxygen-derived radicals likely contributes to endothelial dysfunction of atherosclerotic arteries.

Serum platelet-derived growth factor and endothelin concentrations in human hypercholesterolaemia

Platelet-derived growth factor (PDGF) and endothelin are both smooth muscle cell mitogens, and may have an important role in mediation of the proliferative phase of atherogenesis. Since hypercholesterolaemia is an important risk factor for atherosclerosis, we have measured serum endothelin and PDGF concentrations in matched pairs of hypercholesterolaemic patients and control subjects. Both endothelin and PDGF concentrations were higher in the hypercholesterolaemic patients (23.8 +/- 4.3 pM vs. 21.7 +/- 2.4 pM, P = 0.019, median difference 3.0, 95% confidence limits 0.6-7.3, and 115.4 +/- 60.3 pM vs. 64.7 +/- 22.7 pM P = 0.033, median difference 27.2, 95% confidence limits 1.3-82.5, respectively), while serum endothelin and PDGF concentrations were positively correlated with each other (r = 0.764, P less than 0.0001, 95% confidence limits 0.505-0.897). PDGF concentrations were correlated with the peripheral blood platelet count (r = 0.560, P = 0.007, 95% confidence limits 0.180-0.794). These findings suggest that PDGF, and possibly endothelin, may be involved in hypercholesterolaemia-related atherogenesis.

Aortic response to relaxing agents in Watanabe heritable hyperlipidemic (WHHL) rabbits of different age

Serum and aortic tissue cholesterol levels in parallel with aortic relaxation to endothelium-dependent and independent drugs were determined in Watanabe heritable hyperlipidemic (WHHL) rabbits in comparison with New Zealand (N.Z.) normocholesterolemic rabbits, aged 4-14 months. Serum cholesterol was elevated (626 +/- 99 mg/100 ml) in 4-6-month-old WHHL rabbits and significantly lower in 12-14-month-old animals (344 +/- 51 mg/100 ml). Cholesterol infiltration in thoracic aorta was high in young WHHL compared with N.Z. rabbits (0.88 +/- 0.3 mg/100 mg fresh tissue vs. 0.08 +/- 0.003 mg/100 mg, respectively) and it did not vary with age. In N.Z. rabbits, serum and aortic cholesterol levels were low from 4 to 14 months of age. The aortic relaxation to acetylcholine (0.03-3 microM) on EC50 noradrenaline precontracted rings was similar in 4-6-month-old WHHL and N.Z. rabbits of the same age. In WHHL rabbits, the relaxation to acetylcholine was significantly reduced in 7-11- (-35% at maximum) and in 12-14-month-old rabbits (-40% at maximum). In N.Z. rabbits the response to acetylcholine was not modified in the 3 age groups. The relaxation to ATP (30 microM to 3 mM) was reduced by age both in N.Z. and in WHHL rabbits, but in 12-14-month-old WHHL rabbits the maximal relaxing response was significantly more elevated than in age-matched N.Z. rabbits (50.1 +/- 2.5% vs. 35.1 +/- 3.2%, respectively). The aortic relaxation to NaNO2 (10 microM to 3 mM) was reduced by age both in N.Z. and in WHHL rabbits.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-derived relaxing factor: basic review and clinical implications

EDRF is a potent, endogenous vasodilator that is produced and released from endothelial cells and subsequently causes the relaxation of VSM through the activation of soluble guanylate cyclase and an increase in VSM cyclic GMP. Structurally, EDRF is likely to be NO or a related nitrogen oxide-containing compound. It is synthesized in endothelial and other cell types from L-arginine by a calcium-calmodulin and NADPH-dependent enzyme. Its action is very similar to the nitrovasodilators that act directly on VSM. EDRF is present in all vascular beds, large and small vessels, and in a wide range of species. Its role in human vascular physiology and pathophysiology is just beginning to be understood. EDRF is a potent endogenous vasodilator and inhibitor of platelet aggregation and adhesion. Its activity is impaired in hypertension and atherosclerosis, and its absence due to endothelial damage may play a role in cerebral and coronary vasospasm. It is a mediator of flow-dependent vasodilation, and its inhibition by hypoxia may contribute to the hypoxic pulmonary vasoconstrictor response. Endothelial cell damage and impairment of EDRF production may also contribute to acute and chronic pulmonary hypertension. A further understanding of the chemical nature and synthetic pathways of EDRF should lead to the production of analogs and antagonists, which may play an important role in future treatments for atherosclerosis, myocardial infarction, angina, hypertension, and other vascular diseases. The recent realization that EDRF serves as the second messenger for guanylate cyclase activation and cyclic GMP production in a variety of cell types outside of the cardiovascular system, including renal and respiratory epithelium, cerebellar neurons, macrophages, and adrenocytes, suggests even broader implications. The importance of EDRF to the anesthesiologist may go beyond an understanding of its role in cardiovascular physiological and pathophysiological states. Initial studies have shown that the endothelium may play a role in mediating the vascular actions of anesthetics, and that anesthetics can inhibit the production, release, or action of EDRF. How are these interactions mediated? Are there significant differences between anesthetics with regard to their effects on EDRF? Is there a clinically significant effect of anesthetics on basal activity of EDRF, or only in response to exogenous stimulation? Conversely, it is important to determine if alterations in endothelial cell function by various disease states such as hypertension, atherosclerosis, adult respiratory distress syndrome, cerebral vasospasm, and others cause changes in the vascular actions of anesthetics. The potential interactions of anesthetics with EDRF production and action in cell types other than the endothelium have not yet been explored.(ABSTRACT TRUNCATED AT 400 WORDS)