Persistent impairment of endothelium-dependent relaxation to acetylcholine and progression of atherosclerosis following 6 weeks of cholesterol feeding in the rabbit

The synthesis and (or) release of endothelium-dependent relaxant factor released by acetylcholine is impaired in New Zealand white rabbits fed an atherogenic diet. Experiments were designed to investigate whether the synthesis and (or) release of the endothelium-dependent relaxant factor from rabbit aortas are restored after reversal from an atherogenic diet to a non-atherogenic diet. Atherosclerosis was induced by feeding a diet containing lipids and 2% cholesterol for 6 weeks. Rabbits were sacrificed after 6 weeks on the atherogenic diet and 36 weeks after return to a standard laboratory diet. Synthesis and (or) release of the factor from the thoracic aorta was assayed using a bioassay system. The relaxant responses produced in the assay tissue were impaired both in the acute stage and after 36 weeks on non-atherogenic food. This impaired relaxation is probably due to a persistent functional abnormality in the aortic endothelium resulting in the failure to synthesize and (or) release endothelium-dependent relaxation factor 36 weeks after induction of atherosclerosis.     

Chronic exposure to nicotine alters endothelium-dependent arteriolar dilatation: effect of superoxide dismutase.

The first goal of this study was to determine whether chronic injection of nicotine alters endothelium-dependent arteriolar dilatation. We measured the diameter of cheek pouch resistance arterioles (approximately 50 microm in diameter) in response to endothelium-dependent (acetylcholine and ADP) and -independent (nitroglycerin) agonists in control hamsters and hamsters treated with nicotine (2 microg. kg-1. day-1 for 2-3 wk). In control hamsters, acetylcholine (0.1 and 1.0 microM) dilated arterioles by 13 +/- 2 and 31 +/- 3%, respectively, and ADP (1.0 and 10 microM) dilated arterioles by 18 +/- 1 and 30 +/- 1%, respectively. In contrast, acetylcholine (0.1 and 1.0 microM) dilated arterioles by only 5 +/- 2 and 12 +/- 3%, respectively, and ADP (1.0 and 10 microM) dilated arterioles by only 7 +/- 2 and 13 +/- 3%, respectively, in animals treated with nicotine (P < 0.05 vs. response in control hamsters). Nitroglycerin produced similar dose-related dilatation of cheek pouch arterioles in control and nicotine-treated hamsters. Our second goal was to examine a possible mechanism for impaired endothelium-dependent arteriolar dilatation during chronic treatment with nicotine. We found that superfusion of the cheek pouch microcirculation with superoxide dismutase (150 U/ml) restored impaired endothelium-dependent, but did not alter endothelium-independent, arteriolar dilatation in hamsters treated with nicotine. Superfusion with superoxide dismutase did not alter endothelium-dependent or -independent arteriolar dilatation in control hamsters. We suggest that chronic exposure to nicotine produces selective impairment of endothelium-dependent arteriolar dilatation via a mechanism related to the synthesis/release of oxygen-derived free radicals.     

Chronic ouabain treatment increases the contribution of nitric oxide to endothelium-dependent relaxation

The aim of this study was to analyze the contribution of nitric oxide, prostacyclin and endothelium-dependent hyperpolarizing factor to endothelium-dependent vasodilation induced by acetylcholine in rat aorta from control and ouabain-induced hypertensive rats. Preincubation with the nitric oxide synthase inhibitor N-omega-nitro-l-arginine methyl esther (L-NAME) inhibited the vasodilator response to acetylcholine in segments from both groups but to a greater extent in segments from ouabain-treated rats. Basal and acetylcholine-induced nitric oxide release were higher in segments from ouabain-treated rats. Preincubation with the prostacyclin synthesis inhibitor tranylcypromine or with the cyclooxygenase inhibitor indomethacin inhibited the vasodilator response to acetylcholine in aortic segments from both groups. The Ca²⁺-dependent potassium channel blocker charybdotoxin inhibited the vasodilator response to acetylcholine only in segments from control rats. These results indicate that hypertension induced by chronic ouabain treatment is accompanied by increased endothelial nitric oxide participation and impaired endothelium-dependent hyperpolarizing factor contribution in acetylcholine-induced relaxation. These effects might explain the lack of effect of ouabain treatment on acetylcholine responses in rat aorta.     

Gene transfer of superoxide dismutase isoforms reverses endothelial dysfunction in diabetic rabbit aorta

Increased production of oxygen free radicals is an important mechanism of endothelial dysfunction in diabetes mellitus. Our goal was to test whether adenovirus (Ad)-mediated gene transfer of copper/zinc (CuZn) or manganese superoxide dismutase (Mn SOD) improves relaxation of diabetic vessels. The aortas from 9 alloxan-induced diabetic mellitus (DM) and 16 control rabbits were used. Control and DM rings were transduced ex vivo with Ad vectors encoding Mn SOD (AdMn SOD), CuZn SOD (AdCuZn SOD), beta-galactosidase (Ad(beta)gal), or diluents. In the absence of gene transfer, SOD activity was significantly increased in DM aortas. Transgene expression in DM AdCuZn SOD and DM AdMn SOD-transduced vessels was confirmed by Western blot analysis and by increased SOD activity (DM AdCuZn SOD, 76.2 +/- 9.3; DM AdMn SOD, 65.2 +/- 4.8; P < 0.05 vs. DM Ad(beta)gal; 50.9 +/- 4.4 U/mg protein). Superoxide production was increased in DM Ad(beta)gal-transduced aorta and relaxations to acetylcholine were impaired in these vessels. Gene transfer of CuZn SOD and Mn SOD corrected both of these defects. Thus Ad-mediated gene transfer CuZn and Mn SOD to the diabetic aorta improves endothelium-dependent relaxation.     

Hydroxyl radicals mediate injury to endothelium-dependent relaxation in diabetic rat

The purpose of this study was to determine the radical species which mediates the toxic effects of exogenous oxygenderived free radicals on endothelial function of chronic diabetic rat aorta. Endothelium-dependent relaxation to acetylcholine was impaired in diabetic vessels. Exposure to the exogenous free radical generating system of xanthine plus xanthine oxidase selectively impaired endothelium-dependent relaxation to acetylcholine in control and diabetic aorta with relaxations essentially abolished in diabetic aorta. The loss of relaxation to acetylcholine in diabetic aorta was prevented or attenuated by pretreatment with catalase, dimethylthiourea or desferrioxamine, but not by mannitol or superoxide dismutase. These results suggest that hydroxyl radicals play an important role in the endothelial injury produced by oxygen-derived free radicals in chronic diabetic rat aorta. Furthermore, the site of the injury is likely due to intracellular generation of hydroxyl radicals.     

Chronic hypertriglyceridemia in young watanabe heritable hyperlipidemic rabbits impairs endothelial and medial smooth muscle function

Several studies have suggested that hypertriglyceridemia is a common risk factor for coronary heart disease. Although increasing serum levels of triglyceride correlate with hypercoagulability, little is known about the contribution of hypertriglyceridemia to vascular function. We successfully segregated two lines of rabbits with genetically-determined severely high (TGH; 2764 +/- 413 mg/dl) and moderately high (TGL; 191 +/- 12 mg/dl) levels of triglyceride, but with comparable levels of total cholesterol, from Watanabe heritable hyperlipidemic rabbits. To determine whether hypertriglyceridemia was involved in alterations of vascular function, we conducted isometric tension studies and analyzed protein expression on thoracic aortic rings isolated from young (3-4 month) TGH, TGL and Japanese White rabbit (JW). No difference in percentage of plaque area in the thoracic aorta was found between TGH and TGL. Relaxing responses, evoked by sodium nitroprusside were similar in JW, TGL and TGH, but endothelium-dependent relaxation to acetylcholine was impaired in TGH compared with JW or TGL (maximal relaxation in JW; 83.5 +/- 2.7%, TGL; 79.9 +/- 5.3%, TGH; 59.1 +/- 5.7%, p<0.05). Relaxation to A23187 was also attenuated in TGH compared with JW, but not significantly different between TGL and JW. Endothelium-independent relaxation elicited by isoproterenol in TGH was significantly decreased compared with JW or TGL (maximal relaxation in JW; 95.2 +/- 2.6% TGL; 91.0 +/- 4.9%, TGH; 75.1 +/- 5.2%, p<0.05). Protein expression of angiotensin II type-1 receptor was increased in TGH and that of nitric oxide synthases-3 was attenuated in TGH compared with TGL. This is the first study showing that endothelium-dependent and -independent vascular relaxation under the condition of combined hyperlipidemia was severely impaired as compared to that under only hypercholesterolemia. These results suggest that hypertriglyceridemia aggravates functional impairment induced by hypercholesterolemia in endothelial and smooth muscle cells.     

Thiopental inhibits nitric oxide production in rat aorta

We studied whether thiopental affects endothelial nitric oxide dependent vasodilator responses and nitrite production (an indicator of nitric oxide production) elicited by acetylcholine, histamine, and A23187 in rat aorta (artery in which nitric oxide is the main endothelial relaxant factor). In addition, we evaluated the barbiturate effect on nitric oxide synthase (NOS) activity in both rat aorta and kidney homogenates. Thiopental (10-100 microg/mL) reversibly inhibited the endothelium-dependent relaxation elicited by acetylcholine, histamine, and A23187. On the contrary, this anesthetic did not modify the endothelium-independent but cGMP-dependent relaxation elicited by sodium nitroprusside (1 nM - 1 microM) and nitroglycerin (1 nM - 1 microM), thus excluding an effect of thiopental on guanylate cyclase of vascular smooth muscle. Thiopental (100 microg/mL) inhibited both basal (87.8+/-14.3%) and acetylcholine- or A23187-stimulated (78.6+/-3.9 and 39.7+/-5.6%, respectively) production of nitrites in aortic rings. In addition the barbiturate inhibited (100 microg/mL) the NOS (45+/-4 and 42.8+/-9%) in aortic and kidney homogenates, respectively (measured as 14C-labeled citrulline production). In conclusion, thiopental inhibition of endothelium-dependent relaxation and nitrite production in aortic rings strongly suggests an inhibitory effect on NOS. Thiopental inhibition of the NOS provides further support to this contention.     

Gene transfer of extracellular superoxide dismutase improves relaxation of aorta after treatment with endotoxin

Lipopolysaccharide (LPS) impairs vascular function, in part by generation of reactive oxygen species. One goal of this study was to determine whether gene transfer of extracellular SOD (ECSOD) improves vascular responsiveness in LPS-treated rats. A second goal was to determine whether effects of ECSOD are dependent on the heparin-binding domain of the enzyme, which facilitates binding of ECSOD to the outside of cells. Adenoviruses containing ECSOD (AdECSOD), ECSOD with deletion of its heparin-binding domain (AdECSOD-HBD), or a control virus (AdLacZ) were injected intravenously into rats. Three days later, vehicle or LPS (10 mg/kg ip) was injected. After 24 h, vascular reactivity was examined in aortic rings in vitro. Maximum relaxation to acetylcholine was 95 +/- 1% (means +/- SE) after AdlacZ plus vehicle and 77 +/- 3% after AdlacZ plus LPS (P < 0.05). Responses to calcium ionophore A-23187 and submaximal concentrations of nitroprusside also were impaired by LPS. Gene transfer of ECSOD, but not AdECSOD-HBD, improved (P < 0.05) relaxation to acetylcholine and A-23187 after LPS. Maximum relaxation to acetylcholine was 88 +/- 3% after LPS plus AdECSOD. Superoxide was increased in aorta after LPS, and the levels were reduced after AdECSOD but not AdECSOD-HBD. LPS-induced adhesion of leukocytes to aortic endothelium was reduced by AdECSOD but not by AdECSOD-HBD. We conclude that after gene transfer in vivo, binding of ECSOD to arteries effectively decreases the numbers of adherent leukocytes and levels of superoxide and improves impaired endothelium-dependent relaxation produced by LPS.     

Antibody against cholesteryl ester transfer protein (CETP) elicited by a recombinant chimeric enzyme vaccine attenuated atherosclerosis in a rabbit model

The recombinant chimeric enzyme of AnsB-TTP-CETPC, which comprised asparagianse, tetanus toxin helper T-cell epitope and CETP B-cell epitope, was used to vaccinate New Zealand white rabbits in alum adjuvant. After anti-CETP antibodies were successfully produced, rabbits were fed with a high cholesterol diet for fifteen weeks until atherosclerotic lesions formed in arteries. The results showed that after CETP was inhibited by anti-CETP antibodies the fraction of plasma cholesterol in HDL increased and the fraction of plasma cholesterol in LDL decreased in rAnsB-TTP-CETPC immunized rabbits. The average size of aorta atherosclerotic plaques in rabbits treated with rAnsB-TTP-CETPC was 42.3% less than in rabbits treated with OVA (neutral control), or 47.6% less than in rabbits treated with rHSP 65 (negative control). The average thickness of hyperplastic coronary artery in rAnsB-TTP-CETPC immunized rabbits was 159+/-12 microm, which was significantly lower than in rHSP 65 immunized rabbits (187+/-15 microm) or OVA immunized rabbits (248+/-18 microm). The data reported here demonstrated that rAnsB-TTP-CETPC could significantly attenuate the development of atherosclerosis in rabbits fed with high cholesterol diet, and might be developed to an anti-atherosclerosis vaccine in the future.     

L-Arginine does not improve endothelium-dependent relaxation inin vitro Watanabe rabbit thoracic aorta

Summary The findings that even a singlein vitro addition of L-arginine is able to normalize endothelium function in cerebral vessel from diet-induced hypercholesterolemic rabbits prompted us to investigate if similar results could be obtained on Watanabe rabbits thoracic aorta, in which we previously demonstrated low content of the amino acid.L-Arginine (1 mM) preincubated for 45 minutes before the addition of drugs for studing endothelium-dependent vascular relaxation, did not modify the effect of acetylcholine on aortic isolated preparations. The lack of any effect by L-arginine indicates that the amino acid deficiency is not main cause of the impairment of endothelium function. The muscarinic receptor functionality affected by atherosclerotic process and/or the increased synthesis of EDCFs could account for the reduced endothelium-dependent relaxation.     

Increased production of prostacyclin (PGI2) by vascular intimal smooth muscle cells from atherosclerotic rabbit aorta

In vitro PGI2 synthesis by aortic strips obtained from thoracic aorta of rabbits fed a high cholesterol diet was examined and compared with that of control rabbits fed a normal diet. In this report, the amounts of PGI2 produced were shown as 6-keto-PGF1 alpha per microgram of aortic tissue DNA instead of per mg wet weight. We also investigated PGI2 synthesis by cultured smooth muscle cells (SMC) obtained from atherosclerotic intima. Basal PGI2 production by aortic strips from atherosclerotic rabbit aorta was significantly augmented compared with that of controls. Arachidonic acid (AA)-induced PGI2 production by atherosclerotic aorta was also significantly higher than that of controls. PGI2 producing capacities of intimal and medial layers, separated from atherosclerotic aorta, were examined and the intimal layer was found to elicit a significantly greater PGI2 production than the medial layer. Furthermore, cultured intimal SMC obtained from atherosclerotic rabbit aorta produced a greater amount of PGI2 than medial SMC from normal rabbit aorta at various cultured conditions. These results suggest that the possibility of enhanced PGI2 production by atherosclerotic aorta may well be considered as a defence mechanism of the vessel wall against damaging stimuli.     

Cholinergic reactivity of cerebral arteries in the developing fetal and newborn lamb.

Cerebral arteries of newborn pigs and baboons constrict to acetylcholine, suggesting that endothelium-dependent dilator mechanisms may be lacking in immature cerebral arteries. The present study tested this possibility in the immature sheep by examining the response of cerebral arterioles in fetal and newborn sheep to endothelium-dependent dilator, acetylcholine. Pial arteriolar diameter was measured in 9 anaesthetized foetuses in utero (4 preterm, 90-111 days gestation and 5 term, 128-143 days gestation) and in 5 anaesthetized, newborn lambs (14 days) using a closed cranial window with intravital microscopy. Application of acetylcholine to the pial surface induced dose-dependent increase in pial arteriolar diameter in all age groups; EC50 for acetylcholine was 0.10 +/- 0.03, 0.28 +/- 0.08 and 0.26 +/- 0.17 microM for preterm fetal, term fetal, and newborn lambs, respectively. The data demonstrate a sensitive dilator response to acetylcholine in immature fetuses as well as newborn lambs suggesting that cholinergic-mediated release of endothelium-dependent relaxing factor is functional early in gestation. The contractile response to acetylcholine observed in newborn pigs and premature baboons may reflect a species difference rather than maturational lack of endothelium-dependent dilator mechanisms.     

Alteration of p66shc is associated with endothelial dysfunction in the abdominal aortic coarctation of rats

To examine the role of p66shc in endothelial dysfunction, we investigated the endothelium-dependent relaxation, protein expression and superoxide production in abdominal aortic coarctation rats. Endothelium-dependent relaxation to acetylcholine was impaired only in the aortic segments above the aortic coarctation (35.0+/-7.1% vs. 86.6+/-6.0% for sham control at 1 microM Ach). The aortic segments exposed to increased blood pressure showed a decreased phosphorylation of endothelial nitric oxide synthase, an increased phosphorylation of p66shc, and an increased superoxide production. Angiotensin II elicited a significantly increased phosphorylation of p66shc in the endothelial cells. Taken together, these findings suggest that the increased phosphorylation of p66shc is one of the important mediators in the impaired endothelium-dependent relaxation of aortic coarctation rats.     

ACE inhibitors and statins acutely improve endothelial dysfunction of human coronary arterioles

Long-term treatment with angiotensin-converting enzyme (ACE) inhibitors as well as angiotensin II type 1 (AT(1)) receptor antagonists and statins reduces cardiovascular mortality in patients with coronary artery disease as well as chronic heart failure. Little is known about the acute effects of these compounds on vascular reactivity of coronary resistance vessels. Coronary arterioles were obtained from patients undergoing coronary bypass operation (atherosclerosis group) or valve replacement (control group). Responses to endothelium-dependent agonists (histamine, serotonin, and acetylcholine) as well as to the endothelium-independent agonist sodium nitroprusside (SNP) were investigated under baseline conditions and after incubation (15 min) with lisinopril (ACE inhibitor), candesartan (AT(1) receptor antagonist), or fluvastatin. In atherosclerotic vessels, vasorelaxation was significantly reduced to all endothelium-dependent agonists but not, however, to SNP (77 +/- 8, -24 +/- 16, -46 +/- 24, and 98 +/- 8% relaxation for histamine, serotonin, acetylcholine, and SNP, respectively). Lisinopril and fluvastatin but not candesartan significantly improved the responses to the endothelium-dependent agonists (lisinopril: 94 +/- 4, 17 +/- 22, and -20 +/- 13%; fluvastatin: 96 +/- 8, 23 +/- 21, and -25 +/- 18% relaxation for histamine, serotonin, and acetylcholine, respectively). The effect of lisinopril was prevented by pretreatment with a bradykinin antagonist (HOE-130) and dichloroisocoumarine, an inhibitor of kinine-forming enzymes. Pretreatment with a nitric oxide (NO) synthase inhibitor abolished the improvement of endothelial function by lisinopril and fluvastatin. Vascular reactivity in the control group was not influenced by any of the pharmacological interventions. The data demonstrate that in atherosclerosis, endothelium-dependent relaxation of coronary resistance arteries is severely compromised. The impairment can acutely be reversed by ACE inhibitors and statins via increasing the availability of NO.     

Augmentation of NO-mediated vasodilation in metabolic acidosis

Reduction of perivascular pH in acidemia produces hyporesponsiveness of vascular bed to vasoconstrictors. In the present study, we examined the effects of modest acidification on dilatory responses of isolated rat thoracic aorta. Acetylcholine produced endothelium-dependent relaxation in phenylephrine-precontracted aorta, which was markedly enhanced by acidification of Krebs-Henseleit solution from pH 7.4 to 7.0. A similar augmentation was observed in the relaxing responses to NO donors (SNP, SIN-1, SNAP), 8-Br-cGMP and NS-1619 (a putative K(Ca) channel opener and/or Ca channel inhibitor) in endothelium-denuded, phenylephrine-contracted aorta. However, papaverine-induced relaxation was not affected by the change in pH. At pH 7.4, the relaxing responses to acetylcholine and SNP were partially inhibited by charybdotoxin (K(Ca) channel inhibitor) but not glibenclamide (K(ATP) channel inhibitor), while at pH 7.0 the relaxation induced by either drug was not affected by K(+) channel inhibitors. Relaxation induced by 8-Br-cGMP or NS-1619 was not inhibited by charybdotoxin or glibenclamide. Acidification to pH 7.0 increased the cGMP production in response to acetylcholine in endothelium-intact aorta and to SNP in endothelium-denuded aorta. These results show that modest acidification augments NO-mediated relaxation in rat aorta, probably due to an enhancement of cGMP-dependent but K(+) channel-unrelated relaxation mechanisms.     

Increased production of prostacyclin (PGI2) by vascular intimal smooth muscle cells from atherosclerotic rabbit aorta

PGI₂ synthesis by aortic strips obtained from thoracic aorta of rabbits fed a high cholesterol diet was examined and compared with that of control rabbits fed a normal diet. In this report, the amounts of PGI₂ produced were shown as 6-keto-PGF_1α per μg of aortic tissue DNA instead of per mg wet weight. We also investigated PGI₂ synthesis by cultured smooth muscle cells (SMC) obtained from atherosclerotic intima.Basal PGI₂ production by aortic strips from atherosclerotic rabbit aorta was significantly augmented compared with that of controls. Arachidonic acid (AA)-induced PGI₂ production by atherosclerotic aorta was also significantly higher than that of controls. PGI₂ producing capacities of intimal and medial layers, separated from atherosclerotic aorta, were examined and the intimal layer was found to elicit a significantly greater PGI₂ production than the medial layer.Furthermore, cultured intimal SMC obtained from atherosclerotic rabbit aorta produced a greater amount of PGI₂ than medial SMC from normal rabbit aorta at various cultured conditions. These results suggest that the possibility of enhanced PGI₂ production by atherosclerotic aorta may well be considered as a defence mechanism of the vessel wall against damaging stimuli.     

Effects of chelator treatment on aorta and corpus cavernosum from diabetic rats.

Transition-metal catalyzed reactions contribute to oxidative stress, which has been implicated in the pathogenesis of diabetic complications. The aim was to evaluate the effects of treatment with the transition metal chelator trientine on endothelium-dependent relaxation of aorta and corpus cavernosum from streptozotocin-induced diabetes of 8 weeks duration in rats. Effects on cavernosum autonomic innervation were also examined. Diabetes caused a 30.1 +/- 3.8% reduction in maximum aorta endothelium-dependent relaxation to acetylcholine (ACh), which was markedly attenuated (72.7 +/- 10.6%) by trientine treatment. Reversal treatment (4 weeks untreated diabetes, 4 weeks trientine) did not effect endothelium-dependent relaxation compared with aortas from rats with 4 weeks of diabetes, however, there was a 22.5 +/- 6.2% improvement compared with 8 weeks of diabetes. Eight weeks of diabetes caused a 41.5 +/- 6.6% reduction in corpus cavernosum endothelium-dependent maximum relaxation to ACh that was 70.1 +/- 16.9% prevented by trientine. Cavernosum nonadrenergic, noncholinergic (NANC) nerve stimulation caused frequency-dependent relaxation to a maximum of 40.9 +/- 2.4%, which was reduced by diabetes to 24.2 +/- 2.1%. Trientine partially prevented this deficit, maximum relaxation being 31.9 +/- 2.3%. Thus, metal chelator treatment has beneficial effects on aorta and cavernosum endothelium-dependent relaxation and on cavernosum NANC innervation.     

Evidence for endothelin-1 release from resistance vessels of rats in response to hypoxia

To elucidate further the contribution of endothelin into endothelium-dependent vasoconstriction evoked by hypoxia, we observed endothelin release during hypoxia. Endothelin was detectable in perfusate from mesenteric artery. Immunoreactive endothelin was confirmed as endothelin-1 by a reverse phase-HPLC. Endothelin release increased 4.1 +/- 1.3 to 12.4 +/- 2.0 pg/30 min without changing perfusion pressure. Thirty minutes of hypoxia stimulated endothelin release by 71 +/- 11% (P less than 0.05) and was associated with an elevation of perfusion pressure. These results suggest that endothelin contributes to endothelium-dependent vasoconstriction by hypoxia in mesenteric artery and may play an important role in the local peripheral vascular tone.     

Cholesterol feeding impairs endothelium-dependent relaxation of rabbit aorta

Experiments were designed to assess the effect of cholesterol feeding on the endothelium-mediated relaxation of the rabbit aorta to acetylcholine. Age-matched male New Zealand white rabbits were fed either a 2% cholesterol diet or standard rabbit chow. The animals were anaesthetized with sodium pentobarbitone and sacrificed after 4 and 8 weeks on these diets. Rings were prepared from the proximal thoracic aorta and examined in tissue baths. These rings were contracted first with norepinephrine (-6 log mol/L) and acetylcholine was added to demonstrate the endothelium-mediated relaxation. The endothelium-dependent relaxation was significantly less in aortas from rabbits fed the 2% cholesterol diet than in aortas from animals fed the conventional diet. This impairment of relaxation was apparent after both 4 and 8 weeks of cholesterol feeding. In both groups of animals no relaxation was seen in rings from which the endothelium was removed. These results show that cholesterol feeding leads to an impairment of endothelium-mediated relaxation of the rabbit aorta to acetylcholine.     

Coronary endothelial dysfunction is not rapidly reversible with ascorbic acid

In humans with cardiovascular risk factors, increased vascular production of superoxide anion may contribute to endothelial dysfunction by its reacting with nitric oxide and reducing its biological activity. High concentrations of ascorbic acid scavenge superoxide anion and restore normal endothelium-dependent vasodilation in humans with cardiovascular risk factors. To investigate the contribution of increased superoxide anion to endothelial dysfunction in atherosclerotic coronary arteries, we examined the effect of sequential infusions of ascorbic acid (final concentration 0.1, 1.0, and 10 mmol/L) or placebo on coronary endothelial function in 26 subjects referred for cardiac catheterization to evaluate coronary artery disease. Coronary vasomotor function was evaluated using intracoronary agonist infusion, quantitative angiography, and intracoronary Doppler measurements. At baseline, endothelium-dependent vasodilation of epicardial arteries and coronary microvessels was impaired to an equivalent extent in the ascorbic acid and placebo groups. Sequential ascorbic acid infusions had no effect on the acetylcholine-induced change in coronary artery diameter (-11+/-8, -12+/-10, and -9+/-9%) compared with the effect of placebo (-14+/-13, -16+/-10, and -13+/-9%) infusions (p=0.98). Similarly, the changes in coronary blood flow during acetylcholine infusions were equivalent during ascorbic acid (51+/-44, 67+/-66, and 62+/-52%) and placebo (61+/-104, 55+/-93, and 50+/-69%) infusions (p=0.63). Ascorbic acid also had no effect on the dilator response to intracoronary nitroglycerin (p=0.19). These data argue against an important role for superoxide-mediated "inactivation" of nitric oxide or another rapidly reversible form of oxidative stress as a mechanism of coronary endothelial dysfunction in patients with coronary atherosclerosis.