Captopril restores endothelium-dependent relaxation of rat aortic rings after exposure to homocysteine

This study was designed to investigate the effects of captopril, an angiotensin-converting enzyme inhibitor, on inhibition of endothelium-dependent relaxation induced by homocysteine in isolated rat aorta. Isometric tension recordings were used to assess inhibitory effects of homocysteine and protective effects of captopril on endothelium-dependent relaxation of aortic rings. Exposure of aortic rings to homocysteine (0.3 approximately 3 mmol/L) for 30 min induced a significant concentration-dependent inhibition of endothelium-dependent relaxation response to acetylcholine (ACh), but did not affect endothelium-independent relaxation response to sodium nitroprusside. Pre-incubation of aortic rings with captopril (3 approximately 30 micromol/L) for 15 min and co-incubation of aortic rings with homocysteine (1 mmol/L) for another 30 min attenuated the inhibition of homocysteine in a dose-dependent manner. Moreover, superoxide dismutase (SOD, 200 U/mL), a scavenger of superoxide anions, reduced homocysteine-induced inhibition. L-Arginine (3 mmol/L), a precursor of nitric oxide (NO), also attenuated the impairment of vasorelaxation induced by homocysteine. However, in the combined presence of SOD and L-arginine, the inhibitory effect of homocysteine was reversed, which was very similar to the effect of 30 micromol/L captopril. These results suggest that captopril can prevent the inhibition of endothelium-dependent relaxation induced by homocysteine in isolated rat aorta, which may be related to scavenging oxygen free radicals and enhancing NO production.     

Captopril restores endothelium-dependent relaxation induced by advanced oxidation protein products in rat aorta

To explore whether advanced oxidation protein products (AOPP) can cause endothelial dysfunction in vitro, and whether captopril exerts beneficial effect on impaired endothelium-dependent relaxation induced by exogenous advanced oxidation protein products and to investigate the potential mechanisms. Both the Acetylcholine (ACh)-induced endothelium-dependent relaxation (EDR), sodium nitroprusside-induced endothelium-independent relaxation of aortic rings were measured by recording isometric tension after the rings were exposed to AOPP-BSA in the absence or presence of captopril to assess the injury effect of AOPP-BSA and the protective effect of captopril on the aortic endothelium, respectively. Co-incubation of aortic rings with AOPP-BSA (3 mmol/L) for 90 minutes resulted in a significant inhibition of EDR to ACh, but had no effects on endothelium-independent relaxation to SNP. After incubation of the rings in the co-presence of captopril (3 to 30 micromol/L) or enalaprilat (30 micromol/L) with AOPP-BSA (3 mmol/L) for 90 minutes, captopril significantly and enalaprilat only partly attenuated the inhibition of EDR induced by AOPP-BSA. This protective effect of captopril (30 micromol/L) was abolished by N-nitro-L-arginine methyl ester (10 micromol/L), an inhibitor of nitric oxide synthase. Furthermore, the superoxide anion scavenger superoxide dismutase (SOD, 200 U/mL), and the nitric oxide precursor L-arginine (3 mmol/L) also ameliorated the impaired EDR caused by AOPP-BSA. But D-arginine had no effect on the impaired EDR caused by AOPP-BSA. AOPP-BSA can trigger endothelial dysfunction and captopril can protect the endothelium against functional damage induced by AOPP-BSA in rat aorta, increase nitric oxide bioavailability. The mechanisms of endothelial dysfunction induced by AOPP-BSA may include the decrease of NO and the generation of oxygen-free radicals.     

Characterization of three inhibitors of endothelial nitric oxide synthase in vitro and in vivo.

1. Three analogues of L-arginine were characterized as inhibitors of endothelial nitric oxide (NO) synthase by measuring their effect on the endothelial NO synthase from porcine aortae, on the vascular tone of rings of rat aorta and on the blood pressure of the anaesthetized rat. 2. NG-monomethyl-L-arginine (L-NMMA), N-iminoethyl-L-ornithine (L-NIO) and NG-nitro-L-arginine methyl ester (L-NAME; all at 0.1-100 microM) caused concentration-dependent inhibition of the Ca2(+)-dependent endothelial NO synthase from porcine aortae. 3. L-NMMA, L-NIO and L-NAME caused an endothelium-dependent contraction and an inhibition of the endothelium-dependent relaxation induced by acetylcholine (ACh) in aortic rings. 4. L-NMMA, L-NIO and L-NAME (0.03-300 mg kg-1, i.v.) induced a dose-dependent increase in mean systemic arterial blood pressure accompanied by bradycardia. 5. L-NMMA, L-NIO and L-NAME (100 mg kg-1, i.v.) inhibited significantly the hypotensive responses to ACh and bradykinin. 6. The increase in blood pressure and bradycardia produced by these compounds were reversed by L-arginine (30-100 mg kg-1, i.v.) in a dose-dependent manner. 7. All of these effects were enantiomer specific. 8. These results indicate that L-NMMA, L-NIO and L-NAME are inhibitors of NO synthase in the vascular endothelium and confirm the important role of NO synthesis in the maintenance of vascular tone and blood pressure.     

Endothelial nitric oxide production stimulated by the bioflavonoid chrysin in rat isolated aorta

In the present study, the effects of the bioflavonoid chrysin (5,7-dihydroxyflavone) were analysed on nitric oxide (NO) production from vascular endothelium. In aortic rings, incubation with chrysin or acetylcholine (both at 10 microM) increased L-NAME-sensitive endothelial NO release as measured using the fluorescent probe 4,5-diaminofluorescein diacetate (DAF-2 DA). Moreover, chrysin increased cGMP accumulation only in aortic rings with endothelium. However, at this concentration, chrysin had no effect either on basal or on NADPH-stimulated vascular superoxide production. Moreover, at this low concentration, chrysin, similar to acetylcholine, induced aortic relaxation, which was abolished by both endothelial deprivation and NO synthase inhibition. Endothelium-dependent relaxation induced by chrysin was unaltered by removal of extracellular calcium and incubation with the intracellular calcium chelator BAPTA, while the phosphatidylinositol (PI)-3 kinase inhibitor wortmannin suppressed the endothelial dependence. In conclusion, chrysin stimulated NO release from endothelial cells leading to vascular cGMP accumulation and subsequent endothelium dependent aortic relaxation. Chrysin-stimulated NO release is calcium independent and possibly mediated via PI3-kinase.     

Mechanisms of methyclothiazide-induced inhibition of contractile responses in rat aorta

Methyclothiazide (MCTZ), a thiazide diuretic, inhibits the contractile response induced by norepinephrine in aortic rings from 12-week-old spontaneously hypertensive rats (SHR). Although not modified by indomethacin, this inhibition was attenuated by either mechanical removal of the endothelium or N omega-nitro-L-arginine (NOLA) treatment. These results suggest that the MCTZ effects on the norepinephrine-evoked vascular response are mediated by an endothelium-dependent mechanism involving endothelium-dependent relaxing factor (EDRF)/nitric oxide (NO) release. MCTZ was also found to alter the contractile response induced by the addition of Ca(2+) to a depolarizing solution, and this inhibitory effect was partially abolished by NOLA application. Our data led us to propose that MCTZ relaxes aortic rings, resulting in an endothelium-dependent relaxation phenomenon that could even be reinforced under high-K(+) depolarizing conditions.     

Estradiol relaxes rat aorta via endothelium-dependent and -independent mechanisms

The effects of estrogen on arterial function are heterogeneous with respect to vessel and/or species. We have investigated 17beta-estradiol-induced relaxation in isolated rat aorta with regard to the role of the vascular endothelium and ionic mechanisms. Estrogen induced a concentration-dependent relaxation of 46.5 +/- 7.9% and 70.1 +/- 12.2% (10(-8) and 10(-7)M), which was reduced by endothelial denudation. Furthermore, L-nitroarginine methyl ester completely abrogated this effect; however, estradiol did not relax KCl-contracted rings. Tetraethyl ammonium (1 mmol/l) completely blocked estradiol-induced relaxation. Estradiol increased [cGMP] in isolated aortic rings via NO, but did not significantly affect NOS activity in endothelial cells. Thus, estrogen can relax rat aorta in vitro via both endothelium-dependent and -independent mechanisms involving the NO/cGMP and potassium channel signaling system.     

A novel capsaicin derivative VOA induced relaxation in rat mesenteric and aortic arteries: involvement of CGRP, NO, cGMP, and endothelium-dependent activities

The vasorelaxant effects of N-[4-O-[2-methoxy, phenoxyethylaminobutyl]-3-methoxy benzyl]-nonamide (VOA), a novel capsaicin derivative, and associated releasing activities of nitric oxide (NO) and calcitonin gene-related peptide (CGRP) were investigated in this study. Systemic administration of VOA decreased blood pressure and heart rate in a dose-dependent manner in both normotensive as well as spontaneously hypertensive rats. Nw-nitro-L-arginine methyl ester (L-NAME), glibenclamide, and capsazepine inhibited VOA-induced hypotension. In phenylephrine-precontracted rat aortic rings and mesenteric arteries with intact endothelium, VOA caused a concentration-dependent relaxation. This relaxation was reduced after endothelium was removed or pretreated with L-NAME, methylene blue, 1 H-[1,2,4]oxidazolol [4,3-a] quinoxalin-1-one, tetraethylammonium, glibenclamide, CGRP (8-37), or capsazepine, respectively. In endothelially denuded vessel rings, tetraethylammonium, glibenclamide, CGRP (8-37), and capsazepine also reduced VOA-induced relaxation. In high potassium (80 mmol/L)-precontracted rat aortic rings with intact endothelium, VOA failed to induce relaxation. VOA induced a concentration-dependent increase of CGRP-like enzyme immunoreactivity, which was also significantly inhibited by capsazepine. In human umbilical vein endothelial cells, VOA increased NO release and guanosine-3', 5'-cyclic monophosphate level, which were significantly inhibited by L-NAME. The Western blot analysis on human umbilical vein endothelial cells indicated that VOA increased the expression of endothelium nitric oxide synthase. In conclusion, VOA might exert its relaxation effects in rat vascular smooth muscle through the CGRP/KATP channel and the NO/ cGMP pathway.     

Relaxant effect of oxime derivatives in isolated rat aorta: role of nitric oxide (NO) formation in smooth muscle

Various oxime derivatives were evaluated as nitric oxide (NO) donors in arteries. Relaxation of rat aortic rings was used for bioassay of NO production, and electron paramagnetic resonance spectroscopy for demonstration of NO elevation. In rings with or without endothelium or adventitia, hydroxyguanidine and hydroxyurea were almost inactive, whereas formamidoxime, acetaldoxime, acetone oxime, acetohydroxamic acid and formaldoxime elicited relaxation. Active compounds increased NO levels in endothelium-denuded rings. Formaldoxime was the most potent agent for both relaxation and NO elevation in aortic rings, and it also increased NO in human aortic smooth muscle cells. In endothelium-denuded rings, relaxation was inhibited by a NO scavenger (2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and by inhibitors of soluble guanylyl-cyclase (1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one) or cyclic GMP-dependent protein kinases (Rp-8-bromo cyclic GMP monophosphorothioate). Neither N(omega)-nitro-l-arginine methylester (a NO synthases inhibitor) nor proadifen (a cytochrome P450 inhibitor) decreased the effect of oxime derivatives. However, 7-ethoxyresorufin (7-ER, an inhibitor of P4501A(1) which can also inhibit various NADPH-dependent reductases) abolished the relaxant effect of these compounds, without affecting the one of glyceryl trinitrate (GTN) or 2-(N,N-diethylamino)-diazenolate-2-oxide. 7-ER also abolished formaldoxime-induced NO increase in aortic rings. In rings tolerant to GTN, formaldoxime-induced relaxation and NO elevation were not different from those obtained in control rings. In conclusion, some oxime derivatives release NO by 7-ER-sensitive pathways in aortic smooth muscle, thus eliciting vasorelaxation. Pathways of NO formation are likely distinct from NO synthases and from those responsible for GTN biotransformation. Oxime derivatives could be useful for NO delivery in arteries in which endothelial NO synthase activity is impaired.     

RENAL VASOCONSTRICTION IS MODULATED BY NITRIC OXIDE

1. The effect of the nitric oxide synthesis inhibitor, NG-nitro-L-arginine (NOLA), has been examined on vascular reactivity in the rat isolated perfused kidney. 2. NOLA (10 mumol/L) had no effect on basal perfusion pressure, but significantly enhanced the vasoconstrictor responses to sympathetic nerve stimulation (1-16 Hz, 10 s) and noradrenaline (10-300 pmol). The enhancements were greater with the lower frequencies of stimulation and lower doses of noradrenaline. 3. The enhancing effect of 10 mumol/L NOLA on vasoconstrictor responses to nerve stimulation was partially prevented by 100 mumol/L L-arginine while 100 mumol/L D-arginine had no effect. 4. The results suggest that nitric oxide attenuates vasoconstrictor responses in the rat kidney, and provide evidence that nitric oxide has a physiological role in the modulation of vascular reactivity.     

Rapid effect of progesterone on the contraction of rat aorta in-vitro

Progesterone induced rapid relaxation of KCl-induced contraction of rat aortic rings. The relaxant effect of progesterone on aortic rings was concentration-dependent (over the range of 10(-10) to 10(-5) M) and partially dependent on the endothelium. Application of a nitric oxide (NO) synthase antagonist N(G)-monomethyl-L-arginine (L-NMMA, 10(-5) M) after progesterone treatment partially inhibited the relaxant effects of progesterone. This suggested that part of the effect was through the production of nitric oxide. Washing out the steroid hormone in the bath solutions could quickly reverse the inhibitory effects of progesterone on phasic tension generation in aortic rings. Five minutes after washout, the tension generation in aortic rings was completely restored. Cultured endothelial cells from rat aorta increased release of NO into culture media in response to a 60-min exposure to progesterone. Aldosterone and dexamethasone were also tested, and failed to relax KCl-induced contraction of aortic rings. These data suggest that the vascular effects of progesterone are not mediated by a genomic action of this steroid, and that the vascular effects are mediated partially through endothelial NO production.     

Modulation of the vasodepressor actions of acetylcholine, bradykinin, substance P and endothelin in the rat by a specific inhibitor of nitric oxide formation.

1. The effects of the specific inhibitor of nitric oxide (NO) formation, NG-monomethyl-L-arginine (L-NMMA), on resting systemic arterial blood pressure (BP) and on the actions of both endothelium-dependent and endothelium-independent vasodilators were investigated in the anaesthetized, normotensive rat. 2. Intravenous administration of L-NMMA (12.5-50 mg kg-1; 47-188 mumol kg-1) but not its enantiomer, D-NMMA, induced a dose-related increase in BP, which was reversed by the intravenous administration of L-arginine (150-600 mumol kg-1), but not D-arginine. 3. The vasodepressor responses to intravenous administration of the endothelium-dependent vasodilators, acetylcholine, bradykinin and substance P were significantly inhibited by L-NMMA (94 and 188 mumol kg-1 i.v.), but not by D-NMMA. 4. The inhibition by L-NMMA of these vasodepressor responses was reversed by administration of L-arginine, but not D-arginine. 5. Endothelin (ET-1) induced dose-related vasodepressor responses following bolus intravenous administration, which were significantly inhibited by L-NMMA but not by D-NMMA. This inhibition was reversed by administration of L-arginine. 6. The vasodepressor effects of the endothelium-independent vasodilators, glyceryl trinitrate or prostacyclin, were not significantly inhibited by L-NMMA. 7. These findings with L-NMMA suggest that resting blood pressure in the rat is modulated by endogenous NO biosynthesis and that endothelium-dependent vasodilators act through the formation of endogenous NO to exert their actions in vivo.     

Protective effects of cariporide on endothelial dysfunction induced by high glucose

Aim: To explore the effects of cariporide, a selective sodium-hydrogen antiporter inhibitor, on endothelial dysfunction induced by high glucose. Methods: Acetylcholine (ACh)-induced endothelium-dependent relaxation (EDR), sodium nitroprusside (SNP)-induced endothelium-independent relaxation and biochemical parameters including malondialdehyde (MDA), superoxide dismutase (SOD), and nitric oxide (NO) were measured in rat isolated aorta. Results: A 6-h incubation of aortic rings with high glucose (44 mmol/L) resulted in a significant inhibition of EDR, but had no effects on endothelium-independent relaxation. After the 6-h incubation of aortic rings in the co-presence of cariporide (0.01, 0.1, and 1μmol/L) with high glucose, cariporide prevented the inhibition of EDR caused by high glucose in concentration-dependent manners. Similarly, high glucose decreased SOD activity and contents of NO, and increased MDA concentration in aortic tissue. Cariporide (1 μmol/L) significantly resisted the decrease of NO content and SOD activity, and elevation of MDA concentration caused by high glucose in aortic tissues. Mannitol (44 mmol/L) or cariporide (1μmol/L) alone had no effect on EDR, endothelium-independent relaxation and biochemical parameters. Conclusion: Cariporide significantly prevented endothelial dysfunction induced by high glucose. The mechanisms of endothelial dysfunction induced by high glucose may involve the activation of sodium-hydrogen antiporter and the generation of oxygen-free radicals, but it is not related to the change of osmolarity.     

Inhibition of the release of endothelium-derived relaxing factor in vitro and in vivo by dipeptides containing NG-nitro-L-arginine.

1. We have shown that dipeptides containing NG-nitro-L-arginine (NO2Arg) inhibit the biosynthesis of endothelium-derived relaxing factor (EDRF) in vitro and in vivo. 2. In anaesthetized rats, intravenous administration at 1-30 mg kg-1 of the methyl ester of NO2Arg, NO2-Arg-L-phenylalanine (NO2Arg-Phe), L-alanyl-NO2Arg (Ala-NO2Arg) or NO2Arg-L-arginine (NO2Arg-Arg) produced dose-related increases in mean arterial blood pressure (MABP) which were unaffected by D-arginine (D-Arg; 20 mg kg-1 min-1 for 15 min), but prevented by co-infusions of L-arginine (L-Arg; 20 mg kg-1 min-1 for 15 min) or by their parent dipeptides. 3. NO2Arg methyl ester, NO2Arg-Phe methyl ester or Ala-NO2Arg methyl ester (10 mg kg-1, i.v.) also inhibited the reduction in MABP caused by the endothelium-dependent vasodilator, acetylcholine (30 micrograms kg-1 min-1 for 3 min), but not those induced by glycerly trinitrate (20 micrograms kg-1 min-1 for 3 min) or iloprost (6 micrograms kg-1 min-1 for 3 min) which act directly on the vascular smooth muscle. 4. Moreover, NO2Arg methyl ester, NO2Arg-Phe methyl ester or NO2Arg-Arg methyl ester (100 microM) inhibited the acetylcholine-induced relaxation of rabbit aortic strips, and NO2Arg-Phe methyl ester (30 microM) blocked the stimulated (bradykinin, 30 pmol) release of EDRF from bovine aortic endothelial cells grown on microcarrier beads.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of eNOS phosphorylation at Ser-116 in regulation of eNOS activity in endothelial cells

Endothelial nitric oxide synthase (eNOS) catalyzes the conversion of L-arginine to L-citrulline and nitric oxide (NO), an important modulator of vascular function. eNOS is regulated post-translationally through phosphorylation/dephosphorylation at a number of specific phosphorylation sites including Ser-116 in the bovine eNOS sequence. Whether phosphorylation of eNOS at Ser-116 in endothelial cells is stimulatory or inhibitory has not previously been definitively determined. In this study we show that mimicking phosphorylation of eNOS at Ser-116 by Asp mutation reduces basal NO release from endothelial cells. Preventing phosphorylation at this site by Ala mutation increases the amount of NO release from endothelial cells in response to agonist stimulation. In addition, mimicking phosphorylation of Ser-116 increases eNOS association with caveolin-1 and reduces the vascular reactivity of intact aortic rings. eNOS phosphorylation at Ser-116, therefore, appears to contribute to negative modulation of eNOS activity and hence to regulation of vascular tone.     

l—S.R—蝙蝠葛苏林碱通过减少一氧化氮的产生保护培养的海马神经元对 …

AIM: To explore mechanisms of l-S.R-daurisoline (DS)-mediated protection of cultured hippocampal neurons from sodium glutamate (Glu) cytotoxicity. METHODS: Cultured neurons obtained from rat hippocampus were used to examine the protective effect of DS against Glu neurotoxicity. Cell viability was estimated using trypan blue dye exclusion method and [3-(4, 5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] (MTT) assay. Release of nitric oxide (NO) from the hippocampus was assayed using rat thoracic aorta in vitro. RESULTS: DS 0.01-10 mumol.L-1 concentration-dependently inhibited Glu cytotoxicity and increased cell viability with 50% prevention of cell death 2.8 mumol.L-1 (95% confidence limit 1.2-5.9 mumol.L-1). This protection was mostly attenuated by L-arginine (Arg) 1 mmol.L-1. DS 0.01-10 mumol.L-1 did not prevent sodium nitropusside (SNP) 500 mumol.L-1-induced cytotoxicity. DS 10 mumol.L-1 blocked Glu-elicited relaxation of the endothelium-denued rat aortic rings contracted by norepinephrine (NE) 10 mumol.L-1 in the presence of hippocampal tissue, but did not affect that induced by SNP. This indicated that DS inhibited Glu-triggered NO generation but did not prevent the effects of NO. CONCLUSION: DS prevented neurons from Glu neurotoxicity by inhibiting Glu-triggered NO generation.     

Na+/Ca2+ exchanger inhibitor ameliorates impaired endothelium-dependent Na+ relaxation induced by high glucose in rat aorta

The present study was designed to investigate the effects of KB-R7943, an inhibitor of the Na+/Ca2+ exchanger, on impaired endothelium-dependent relaxation (EDR) induced by high glucose in rat isolated aorta. Both acetylcholine (ACh)-induced EDR and sodium nitroprusside (SNP)-induced endothelium-independent relaxation (EIR) were measured after aortic rings had been exposed to high glucose in the absence and presence of KB-R7943. Coincubation of aortic rings with high glucose (25 mmol/L) for 24 h resulted in a significant inhibition of EDR, but had no effect on EIR. After incubation of aortic rings in the presence of both KB-R7943 (0.1-10 micromol/L) and high glucose for 24 h, significantly attenuation of impaired EDR was observed. This protective effect of KB-R7943 (10 micromol/L) was abolished by superoxide dismutase (SOD; 200 U/mL) and l-arginine (3 mmol/L), whereas d-arginine (3 mmol/L) had no effect. Similarly, high glucose decreased SOD activity and the release of nitric oxide (NO) and increased superoxide anion (O2(-)) production in aortic tissue. KB-R7943 significantly decreased O2(-) production and increased SOD activity and NO release. These results suggest that KB-R7943 can restore impaired EDR induced by high glucose in rat isolated aorta, which may be related to the scavenging of oxygen free radicals and enhanced NO production.     

Impairment of endothelium-dependent relaxation of rat aortas by homocysteine thiolactone and attenuation by captopril

To explore the effects of angiotensin-converting enzyme (ACE) inhibitors on endothelial dysfunction induced by homocysteine thiolactone (HTL). Both endothelium-dependent relaxation and nondependent relaxation of thoracic aortic rings in rats induced by acetylcholine (Ach) or sodium nitroprusside (SNP) and biochemical parameters including malondialdehyde (MDA) and nitric oxide (NO) were measured in rat isolated aorta. Exposure of aortic rings to HTL (3 to 30 mM) for 90 minutes made a significant inhibition of endothelium-dependent relaxation induced by Ach, decreased contents of NO, and increased MDA concentration in aortic tissue. After incubation of aortic rings with captopril (0.003 to 0.03 mM) attenuated the inhibition of endothelium-dependent relaxation (EDR) and significantly resisted the decrease of NO content and elevation of MDA concentration caused by HTL (30 mmol/L) in aortic tissues, a similarly protective effect was observed when the aortic rings were incubated with both N-acetylcysteine (0.05 mM). Treatment with enalaprilat (0.003 to 0.01 mM) made no significant difference with the HTL (30 mM) group regarding EDR, but enalaprilat (0.03 mM) and losartan (0.03 mM) could partly restore the EDR in response to HTL (30 mM). Captopril was more effective than enalaprilat and losartan in attenuation of the inhibition of on acetylcholine-stimulated aortic relaxation by HTL in the same concentration. Moreover, superoxide dismutase (SOD, 200 U/mL), which is a scavenger of superoxide anions, apocynin (0.03 mM), which is an inhibitor of NADPH oxidase, and l-Arginine (3 mmol/L), a precursor of nitric oxide (NO), could reduce HTL (30 mM)-induced inhibition of EDR. After pretreatment with not only the NO synthase inhibitor Nomega-nitro-l-arginine methyl ester (L-NAME, 0.01 mM) but also the free sulfhydryl group blocking agent p-hydroxymercurybenzoate (PHMB, 0.05 mM) could abolish the protection of captopril and N-acetylcysteine, respectively. These results suggest that mechanisms of endothelial dysfunction induced by HTL may include the decrease of NO and the generation of oxygen free radicals and that captopril can restore the inhibition of EDR induced by HTL in isolated rat aorta, which may be related to scavenging oxygen free radicals and may be sulfhydryl-dependent.     

The impairment of endothelium-dependent arterial relaxation by 7-ketocholesterol is associated with an early activation of protein kinase C

Among components of oxidized low density lipoproteins, cholesterol derivatives oxidized in position 7 inhibit endothelium-dependent arterial relaxation by decreasing the release of the main endothelium-derived relaxing factor, nitric oxide (NO). The aim of the present study was to bring new insights into the molecular mechanism by which 7-ketocholesterol can block the endothelium-dependent arterial relaxation. Superoxide dismutase did not prevent the inhibitory effect of 7-ketocholesterol on endothelium-dependent relaxation, and consistent observations were made whether superoxide dismutase was conjugated or not to polyethylene glycol. In addition, neither glutathione supplementation, nor oxypurinol, i.e. a xanthine oxidase inhibitor could reverse the effect of 7-ketocholesterol, indicating that NO was not inactivated by superoxide anion. A direct alteration of the activity of the calcium-dependent NO synthase could also be ruled out, since identical relaxing effects of the calcium ionophore A23187 were observed whether arterial rings were treated or not with 7-ketocholesterol. 4 Whereas the above observations come in support of an early, inhibitory action of 7-ketocholesterol, the specific blockade of one given subtype of membrane receptors could be discarded, and similar inhibitions were observed when either muscarinic or purinergic receptors were stimulated. Finally, the blockade of protein kinase C activity by chelerythrine arose as the sole relevant tool in preventing the effect of 7-ketocholesterol on the endothelium-dependent relaxation of rabbit aortic rings. In addition, complementary studies on cultured bovine aortic endothelial cells came in direct support of the ability of 7-ketocholesterol to activate PKC. In conclusion, 7-ketocholesterol that is present in human hypercholesterolaemic plasma, in atherosclerotic arteries, and in many processed foods can block the release of NO by vascular endothelial cells through its ability to activate PKC.     

Effect of chronic lithium administration on endothelium-dependent relaxation in rat aorta

1. The effects of chronic lithium administration on the relaxant responses of rat thoracic aortic rings in the presence of indomethacin (a cyclo-oxygenase inhibitor) and/or NG-nitro-L-arginine (L-NOARG; a nitric oxide synthase inhibitor) to acetylcholine (ACh) or sodium nitroprusside were investigated in the present study. 2. Acetylcholine produced a concentration-dependent relaxation in vessels precontracted by phenylephrine (PE), while in lithium-treated rats the maximal relaxation was significantly increased. 3. Indomethacin (20 mumol/L) significantly potentiated the ACh-induced relaxation in lithium-treated and control rats. 4. NG-Nitro-L-arginine (1 mumol/L) decreased the ACh-induced relaxation in both control and lithium-treated rats. In contrast, indomethacin (20 mumol/L) reversed the inhibitory effect of L-NOARG. 5. Sodium nitroprusside produced similar concentration-dependent relaxations of vessels from both control and lithium-treated rats, which was not affected by indomethacin. In endothelium-denuded rings, indomethacin (20 mumol/L) caused a rightward shift in the concentration-contraction curve to PE. 6. These data support evidence for a possible increase in endothelium-dependent relaxation induced by ACh during long-term administration of lithium in rat aortic rings.