Probucol and other antioxidants prevent the inhibition of endothelium-dependent relaxation by low density lipoproteins

Impact of vitamin C on endothelial function and exercise capacity in patients with a Fontan circulation

OBJECTIVE

  To evaluate the impact of antioxidant therapy on functional health status in Fontan-palliated patients. Design.  Prospective, randomized, double-blind, placebo-controlled trial.

PATIENTS

  Fifty-three generally asymptomatic Fontan patients.

INTERVENTIONS

  Patients were randomized to receive either high-dose ascorbic acid (vitamin C) or placebo for 4 weeks.

OUTCOME MEASURES

  Peripheral vascular function, as measured with endothelium-dependent digital pulse amplitude testing (EndoPAT), and exercise capacity were assessed before and after study drug treatment. Primary outcome measures included the EndoPAT index and peripheral arterial tonometry (PAT) ratio, both validated markers of vascular function. Secondary outcome measures included peak oxygen consumption and work.

RESULTS

  Twenty-three vitamin C- and 21 placebo-assigned subjects completed the protocol (83%). Median age and time from Fontan completion were 15 (interquartile range [IQR] 11.7-18.2) and 11.9 years (IQR 9.0-15.7), respectively. Right ventricular morphology was dominant in 30 (57%). Outcome measures were similar between groups at baseline. Among all subjects, vitamin C therapy was not associated with a statistical improvement in either primary or secondary outcome measures. In subjects with abnormal vascular function at baseline, compared with placebo, vitamin C therapy more frequently resulted in normalization of the EndoPAT index (45% vs. 17%) and PAT ratio (38% vs. 13%).

CONCLUSIONS

  Short-term therapy with vitamin C does not alter endothelial function or exercise capacity in an asymptomatic Fontan population overall. Vitamin C may provide benefit to a subset of Fontan patients with abnormal vascular function.

''Iatrogenic Gilbert syndrome''--a strategy for reducing vascular and cancer risk by increasing plasma unconjugated bilirubin

The catabolism of heme, generating biliverdin, carbon monoxide, and free iron, is mediated by heme oxygenase (HO). One form of this of this enzyme, heme oxygenase-1, is inducible by numerous agents which promote oxidative stress, and is now known to provide important antioxidant protection, as demonstrated in many rodent models of free radical-mediated pathogenesis, and suggested by epidemiology observing favorable health outcomes in individuals carrying high-expression alleles of the HO-1 gene. The antioxidant impact of HO-1 appears to be mediated by bilirubin, generated rapidly from biliverdin by ubiquitously expressed biliverdin reductase. Bilirubin efficiently scavenges a wide range of physiological oxidants by electron donation. In the process, it is often reconverted to biliverdin, but biliverdin reductase quickly regenerates bilirubin, thereby greatly boosting its antioxidant potential. There is also suggestive evidence that bilirubin inhibits the activity or activation of NADPH oxidase. Increased serum bilirubin is associated with reduced risk for atherogenic disease in epidemiological studies, and more limited data show an inverse correlation between serum bilirubin and cancer risk. Gilbert syndrome, a genetic variant characterized by moderate hyperbilirubinemia attributable to reduced hepatic expression of the UDP-glucuronosyltransferase which conjugates bilirubin, has been associated with a greatly reduced risk for ischemic heart disease and hypertension in a recent study. Feasible strategies for boosting serum bilirubin levels may include administration of HO-1 inducers, supplementation with bilirubin or biliverdin, and administration of drugs which decrease the efficiency of hepatic bilirubin conjugation. The well-tolerated uricosuric drug probenecid achieves non-competitive inhibition of hepatic glucuronidation reactions by inhibiting the transport of UDP-glucuronic acid into endoplasmic reticulum; probenecid therapy is included in the differential diagnosis of hyperbilirubinemia, and presumably could be used to induce an ''iatrogenic Gilbert syndrome''. Other drugs, such as rifampin, can raise serum bilirubin through competitive inhibition of hepatocyte bilirubin uptake--although unfortunately rifampin is not as safe as probenecid. Measures which can safely achieve moderate serum elevations of bilirubin may prove to have value in the prevention and/or treatment of a wide range of disorders in which oxidants play a prominent pathogenic role, including many vascular diseases, cancer, and inflammatory syndromes. Phycobilins, algal biliverdin metabolites that are good substrates for biliverdin reductase, may prove to have clinical antioxidant potential comparable to that of bilirubin.

Insulin resistance, a new target for nitric oxide-delivery drugs

In the Western hemisphere, the incidence of insulin resistance and its complications has been growing rapidly and is reaching epidemic proportions. Over the past decade, evidence has accumulated, indicating that nitric oxide (NO) plays a key role in the regulation of metabolic and cardiovascular homeostasis. Defective endothelial nitric oxide synthase (eNOS) driven NO synthesis causes insulin resistance, arterial hypertension and dyslipidemia in mice, and characterizes insulin-resistant humans. On the other hand, stimulation of inducible nitric oxide synthase (iNOS) and NO overproduction in mice, may also cause metabolic insulin resistance, suggesting a Yin-Yang effect of NO in the regulation of glucose homeostasis. Here, we will review the evidence for this novel concept, and thereby provide the conceptual framework for the use of NO-delivery drugs and pharmacological agents that modulate the bioavailability of endogenously produced NO for the treatment of insulin resistance.

Antioxidant vitamins and prevention of cardiovascular disease: epidemiological and clinical trial data

Naturally occurring antioxidants such as vitamin E, beta-carotene, and vitamin C can inhibit the oxidative modification of low density lipoproteins. This action could positively influence the atherosclerotic process and, as a consequence, the progression of coronary heart disease. A wealth of experimental studies provide a sound biological rationale for the mechanisms of action of antioxidants, whereas epidemiologic studies strongly sustain the "antioxidant hypothesis." To date, however, clinical trials with beta-carotene supplements have been disappointing, and their use as a preventive intervention for cancer and coronary heart disease should be discouraged. Only scanty data from clinical trials are available for vitamin C. As to vitamin E, discrepant results have been obtained by the Alpha-Tocopherol, Beta Carotene Cancer Prevention Study with a low-dose vitamin E supplementation (50 mg/d) and the Cambridge Heart Antioxidant Study (400-800 mg/d). The results of the GISSI-Prevenzione (300 mg/d) and HOPE (400 mg/d) trials suggest the absence of relevant clinical effects of vitamin E on the risk of cardiovascular events. Currently ongoing are several large-scale clinical trials that will help in clarifying the role of vitamin E in association with other antioxidants in the prevention of atherosclerotic coronary disease.

Regulation of endothelial nitric oxide synthase: location, location, location

Endothelial nitric oxide synthase (eNOS) is expressed in vascular endothelium, airway epithelium, and certain other cell types where it generates the key signaling molecule nitric oxide (NO). Diminished NO availability contributes to systemic and pulmonary hypertension, atherosclerosis, and airway dysfunction. Complex mechanisms underly the cell specificity of eNOS expression, and co- and post-translational processing leads to trafficking of the enzyme to plasma membrane caveolae. Within caveolae, eNOS is the downstream target member of a signaling complex in which it is functionally linked to both typical G protein-coupled receptors and less typical receptors such as estrogen receptor (ER) alpha and the high-density lipoprotein receptor SR-BI displaying novel actions. This compartmentalization facilitates dynamic protein-protein interactions and calcium- and phosphorylation-dependent signal transduction events that modify eNOS activity. Further understanding of these mechanisms will enable us to take preventive and therapeutic advantage of the powerful actions of NO in multiple cell types.

Impaired endothelium-mediated vasodilation in heart failure: clinical evidence and the potential for therapy

Numerous studies in the last decade have clearly shown an attenuated endothelium-dependent vasodilation in patients with chronic heart failure. This abnormality has been demonstrated in the peripheral, pulmonary, and coronary circulation in patients with both ischemic and nonischemic cardiomyopathy; its magnitude correlates with the severity of symptoms. Endothelial dysfunction in patients with cardiomyopathy and a relatively new onset of symptoms suggests that change in endothelial function occurs early in the course of the disease. In contrast to other circulatory beds, renal circulation has shown significant vasodilatory response to endothelial stimulation. The development of endothelial dysfunction may not be homogeneous, and its magnitude may differ among circulatory systems. Although the clinical implications of the attenuated endothelium-dependent vasodilation in heart failure are not clear, this condition may lead to decreased organ perfusion, impaired exercise tolerance, and progression of disease. Many therapeutic interventions have resulted in improvement of endothelial function in patients with heart failure. Some of these interventions have also proven effective in enhancing exercise capacity, symptoms, and survival in patients with heart failure. This association suggests a therapeutic role for improvement of endothelial function in patients with chronic heart failure.

Endothelial NOS-dependent activation of c-Jun NH(2)- terminal kinase by oxidized low-density lipoprotein

Oxidized low-density lipoprotein (oxLDL) is known to activate a number of signal transduction pathways in endothelial cells. Among these are the c-Jun NH(2)-terminal kinase (JNK), also known as stress-activated protein kinase, and extracellular signal-regulated kinase (ERK). These mitogen-activated protein kinases (MAP kinase) determine cell survival in response to environmental stress. Interestingly, JNK signaling involves redox-sensitive mechanisms and is activated by reactive oxygen and nitrogen species derived from both NADPH oxidases, nitric oxide synthases (NOS), peroxides, and oxidized low-density lipoprotein (oxLDL). The role of endothelial NOS (eNOS) in the activation of JNK in response to oxLDL has not been examined. Herein, we show that on exposure of endothelial cells to oxLDL, both ERK and JNK are activated through independent signal transduction pathways. A key role of eNOS activation through a phosphatidylinositol-3-kinase-dependent mechanism leading to phosphorylation of eNOS is demonstrated for oxLDL-dependent activation of JNK. Moreover, we show that activation of ERK by oxLDL is critical in protection against the cytotoxicity of oxLDL.

The role of natural antioxidants in preserving the biological activity of endothelium-derived nitric oxide

Endothelium-derived nitric oxide (EDNO) is a pivotal molecule in the regulation of vascular tone via the stimulation of vascular smooth muscle cell relaxation and concomitant vasodilation. In addition, EDNO exerts a number of other potent antiatherogenic effects, including inhibition of leukocyte-endothelial interactions, smooth muscle cell proliferation, and platelet aggregation. Endothelial vasodilator dysfunction has been observed in patients with CAD or coronary risk factors such as hypercholesterolemia, hyperhomocysteinemia, essential hypertension, diabetes mellitus, smoking, and aging. Most of these conditions are associated with increased oxidative stress, particularly increased production of superoxide radicals and elevated levels of oxidized LDL, both of which can attenuate the biological activity of EDNO. The levels of superoxide and oxidized LDL can be decreased by administering the small molecule antioxidants vitamins E and C. Vitamin C also spares intracellular thiols, which in turn can stabilize EDNO through the formation of biologically active S-nitrosothiols. Here we review the role that vitamins E and C and thiol compounds play in endothelium-dependent vasodilation. Understanding the mechanisms of the reversal of endothelial dysfunction by natural antioxidants will lead to successful therapeutic interventions of CAD and its clinical sequelae.

High density lipoprotein prevents oxidized low density lipoprotein-induced inhibition of endothelial nitric-oxide synthase localization and activation in caveolae

Oxidized LDL (oxLDL) depletes caveolae of cholesterol, resulting in the displacement of endothelial nitric-oxide synthase (eNOS) from caveolae and impaired eNOS activation. In the present study, we determined if the class B scavenger receptors, CD36 and SR-BI, are involved in regulating nitric-oxide synthase localization and function. We demonstrate that CD36 and SR-BI are expressed in endothelial cells, co-fractionate with caveolae, and co-immunoprecipitate with caveolin-1. Co-incubation of cells with 10 microgram/ml high density lipoprotein (HDL) prevented oxLDL-induced translocation of eNOS from caveolae and restored acetylcholine-induced nitric-oxide synthase stimulation. Acetylcholine caused eNOS activation in cells incubated with 10 microgram/ml oxLDL (10-15 thiobarbituric acid-reactive substances) and blocking antibodies to CD36, whereas cells treated with only oxLDL were unresponsive. Furthermore, CD36-blocking antibodies prevented oxLDL-induced redistribution of eNOS. SR-BI-blocking antibodies were used to demonstrate that the effects of HDL are mediate by SR-BI. HDL binding to SR-BI maintained the concentration of caveola-associated cholesterol by promoting the uptake of cholesterol esters, thereby preventing oxLDL-induced depletion of caveola cholesterol. We conclude that CD36 mediates the effects of oxLDL on caveola composition and eNOS activation. Furthermore, HDL prevents oxLDL from decreasing the capacity for eNOS activation by preserving the cholesterol concentration in caveolae and, thereby maintaining the subcellular location of eNOS.

Effect of L-arginine-nitric oxide system on the metabolism of essential fatty acids in chemical-induced diabetes mellitus

Several studies have shown that the activities of delta-6-desaturase and delta-5-desaturase are depressed in experimental diabetes and in humans with insulin- and non-insulin-dependent diabetes mellitus (type I and type II diabetes mellitus respectively). Furthermore, treatment with insulin is known to correct the defects in desaturases in rats and humans with diabetes, especially in type I. In a recent study, we demonstrated that L-arginine and nitric oxide can prevent alloxan-induced beta cell damage, and the severity of diabetes, and restore the antioxidant status to near normal levels. But, no information is available as to the relationship between L-arginine-nitric oxide system and the metabolism of essential fatty acids in diabetes mellitus. In the present study, it was noted that the plasma levels of saturated fatty acids: stearic and palmitic were increased where as unsaturated fatty acids such as oleic, linoleic, gamma-linolenic and eicosapentaenoic acids (OA, LA, GLA and EPA respectively) were decreased in alloxan-induced diabetic rats. In the liver phospholipid (PL) fraction, GLA, DGLA (dihomo-GLA) and alpha-linolenic acid (ALA) were decreased in the alloxan-treated group; in the muscle PL fraction, LA, GLA and DGLA were low, whereas an increase in the saturated fatty acid content was noted. L-arginine (the precursor of nitric oxide) and sodium nitroprusside (a nitric oxide donor) treatment of alloxan-induced diabetic rats enhanced the levels of LA, GLA and DGLA. Further, nitric oxide synthase inhibitor, L-NMMA, prevented this beneficial action of L-arginine-nitric oxide system on essential fatty acid metabolism. The abnormalities in the essential fatty acid metabolism observed also reverted to normalcy following control of diabetes with insulin. These results indicate that nitric oxide can enhance the activities of delta-6- and delta-5 desaturases, which are depressed in diabetes, and suggests that there is a close interaction between L-arginine-nitric oxide system and the metabolism of essential fatty acids.

Oxidized low density lipoprotein displaces endothelial nitric-oxide synthase (eNOS) from plasmalemmal caveolae and impairs eNOS activation

Hypercholesterolemia-induced vascular disease and atherosclerosis are characterized by a decrease in the bioavailability of endothelium-derived nitric oxide. Endothelial nitric-oxide synthase (eNOS) associates with caveolae and is directly regulated by the caveola protein, caveolin. In the present study, we examined the effects of oxidized low density lipoprotein (oxLDL) on the subcellular location of eNOS, on eNOS activation, and on caveola cholesterol in endothelial cells. We found that treatment with 10 microgram/ml oxLDL for 60 min caused greater than 90% of eNOS and caveolin to leave caveolae. Treatment with oxLDL also inhibited acetylcholine-induced activation of eNOS but not prostacyclin production. oxLDL did not affect total cellular eNOS abundance. Oxidized LDL also did not affect the palmitoylation, myristoylation or phosphorylation of eNOS. Oxidized LDL, but not native LDL, or HDL depleted caveolae of cholesterol by serving as an acceptor for cholesterol. Cyclodextrin also depleted caveolae of cholesterol and caused eNOS and caveolin to translocate from caveolae. Furthermore, removal of oxLDL allowed eNOS and caveolin to return to caveolae. We conclude that oxLDL-induced depletion of caveola cholesterol causes eNOS to leave caveolae and inhibits acetylcholine-induced activation of the enzyme. This process may be an important mechanism in the early pathogenesis of atherosclerosis.

Antioxidant vitamins and prevention of cardiovascular disease: laboratory, epidemiological and clinical trial data

Naturally occurring antioxidants like vitamin E, beta-carotene, and vitamin C can inhibit the oxidative modification of low-density lipoproteins. This action could positively influence the atherosclerotic process and, as a consequence, the progression of coronary heart disease. A wealth of experimental studies provide a sound biological rationale for the mechanisms of action of antioxidants, whereas epidemiological studies strongly sustain the 'antioxidant hypothesis'. To data, however, clinical trials with beta-carotene supplements have been disappointing and their use as a preventive intervention for cancer and coronary heart disease should be discouraged. Only scant data from clinical trials are available for vitamin C. As for vitamin E, discrepant results have been obtained by the Alpha-Tocopherol, Beta-Carotene Cancer Prevention Study with a low-dose vitamin E supplementation (50 mg daily) and the Cambridge Heart Antioxidant Study (400-800 mg daily). Currently ongoing are several large-scale clinical trials that will help in clarifying the role of vitamin E in the prevention of atherosclerotic coronary disease.

Effects of vitamin C and of a cell permeable superoxide dismutase mimetic on acute lipoprotein induced endothelial dysfunction in rabbit aortic rings

Low density lipoprotein (LDL) inhibits endothelium-dependent relaxation. The mechanism is uncertain, but increased production of superoxide anion O2- with inactivation of endothelium-derived NO and formation of toxic free radical species have been implicated. We investigated effects of the cell permeable superoxide dismutase mimetic manganese (III) tetrakis (1-methyl-4-pyridyl) porphyrin (MnTMPyP), the free radical scavenger vitamin C and arginine (which may reduce O2- formation) on acute LDL-induced endothelial dysfunction in rabbit aortic rings, using LDL prepared by ultracentrifugation of plasma from healthy men and aortic rings from New Zealand white rabbits. LDL (150 microg protein ml(-1) for 20 min) markedly inhibited relaxation of aortic rings (in Krebs' solution at 37 degrees C and pre-constricted to 80% maximum tension with noradrenaline) to acetylcholine 82+/-10% (mean percentage difference between sum of relaxations after each concentration of acetylcholine in the presence and absence of LDL, +/-s.e.mean, n=26, P<0.001) but not to the endothelium-independent agonist nitroprusside. MnTMPyP (10 microM) reduced inhibitory effects of LDL from 124+/-27 to 56+/-17% (n=6, P<0.05). Vitamin C (1 mM) reduced inhibitory effects of LDL from 59+/-8 to 22+/-5% (n=6, P<0.05). Inhibitory effects of LDL were similar in the absence or presence of arginine (84+/-12 vs 79+/-16%, n=14, P=0.55). Effects of L-arginine (10 mM) did not differ significantly from those of D-arginine (10 mM). Acute (20 min) exposure of aortic rings to LDL impairs endothelium-dependent relaxation which can be partially restored by MnTMPyP and vitamin C. This is consistent with LDL causing increased O2- generation.

Vitamin C improves endothelial function of conduit arteries in patients with chronic heart failure

BACKGROUND

Chronic heart failure (CHF) is associated with endothelial dysfunction including impaired endothelium-mediated, flow-dependent dilation (FDD). There is evidence for increased radical formation in CHF, raising the possibility that nitric oxide is inactivated by radicals, thereby impairing endothelial function. To test this hypothesis, we determined the effect of the antioxidant vitamin C on FDD in patients with CHF.

METHODS AND RESULTS

High-resolution ultrasound and Doppler was used to measure radial artery diameter and blood flow in 15 patients with CHF and 8 healthy volunteers. Vascular effects of vitamin C (25 mg/min IA) and placebo were determined at rest and during reactive hyperemia (causing endothelium-mediated dilation) before and after intra-arterial infusion of N-monomethyl-L-arginine (L-NMMA) to inhibit endothelial synthesis of nitric oxide. Vitamin C restored FDD in patients with heart failure after acute intra-arterial administration (13.2+/-1.7% versus 8.2+/-1.0%; P<.01) and after 4 weeks of oral therapy (11.9+/-0.9% versus 8.2+/-1.0%; P<.05). In particular, the portion of FDD mediated by nitric oxide (ie, inhibited by L-NMMA) was increased after acute as well as after chronic treatment (CHF baseline: 4.2+/-0.7%; acute: 9.1+/-1.3%; chronic: 7.3+/-1.2%; normal subjects: 8.9+/-0.8%; P<.01).

CONCLUSIONS

Vitamin C improves FDD in patients with CHF as the result of increased availability of nitric oxide. This observation supports the concept that endothelial dysfunction in patients with CHF is, at least in part, due to accelerated degradation of nitric oxide by radicals.

Modulatory role of lipoprotein oxidation on platelet-vessel wall interactions

Oxidation of plasma lipoproteins may play an important role in atherogenesis, but there are also indications that that this process of oxidation may influence other facets of cardiovascular disease, namely blood flow and thrombosis. The evidence that oxidation of low-density lipoproteins may modulate the action of the native lipoproteins with respect to endothelium-dependent relaxation, platelet activation and tissue factor activity is reviewed.

Antioxidants and atherosclerotic heart disease

Epidemiologic studies have provided evidence of an inverse relation between coronary artery disease and antioxidant intake, and vitamin E supplementation in particular. The oxidative-modification hypothesis implies that reduced atherosclerosis is a result of the production of LDL that is resistant to oxidation, but linking the reduced oxidation of LDL to a reduction in atherosclerosis has been problematic. Several important additional mechanisms may underlie the role of antioxidants in preventing the clinical manifestations of coronary artery disease (Fig. 2). Specifically, there is evidence that plaque stability, vasomotor function, and the tendency to thrombosis are subject to modification by specific antioxidants. For example, cellular antioxidants inhibit monocyte adhesion, protect against the cytotoxic effects of oxidized LDL, and inhibit platelet activation. Furthermore, cellular antioxidants protect against the endothelial dysfunction associated with atherosclerosis by preserving endothelium-derived nitric oxide activity. We speculate that these mechanisms have an important role in the benefits of antioxidants.

Physiology and biochemistry of endothelial function in children with chronic renal failure

Premature atherosclerosis is a major cause of morbidity and mortality in chronic renal failure (CRF). Endothelial dysfunction is a key early event in atherogenesis. The aim of this study was to assess the effect of CRF on endothelial function using physiological and biochemical measures. To focus on the effect of CRF itself, 23 children (matched with 23 controls for age and vessel diameter) were selected because they were normotensive, had normal total cholesterol (TC) levels, and were not on vasoactive drugs. Their mean (range) age was 12.0 (7.8 to 17.0) years; GFR 17.5 (8.8 to 34.5) ml/min/1.73 m2. The physiology of endothelial function in the brachial artery was assessed using high resolution ultrasound by measuring its diameter at rest, during reactive hyperemia (endothelium dependent dilation) and after sublingual glyceryl trinitrate (GTN; endothelium independent dilation). Nitric oxide (NO) metabolites and endogenous NO synthetase (eNOS) inhibitors were measured as an assessment of endothelial metabolism. Brachial artery dilation to flow [FMD, mean (SEM)%] was reduced in CRF to 4.9 (0.6) and controls 8.6 (0.6), P < 0.0001. In contrast, the response to GTN was similar in both groups: CRF 25.1 (1.6), controls 23.3 (1.2), P = 0.31. There was no difference in TC, low density lipoprotein (LDL) or high density lipoprotein (HDL) between the patients and the controls. Triglycerides (TG) were higher in the patients but within the normal range. Antibodies against oxidized LDL (ox-LDL) were high in CRF. Endogenous NOS inhibitors were high in CRF, and intermediate NO metabolites were low. There was no correlation between FMD of the brachial artery and lipid subfractions, or with NO metabolites or eNOS inhibitors. Endothelium dependent dilation of the brachial artery is impaired in children with CRF who do not have co-existing risk factors for atherosclerosis. This may represent early evidence of atherogenic vascular disease.

Oxidized low-density lipoproteins inhibit endothelium-dependent relaxations in isolated large and small rabbit coronary arteries

1. Previous studies suggest that oxidatively modified low-density lipoproteins (oxLDL) contribute to the impairment of endothelium-dependent vasodilation in the large arteries of hypercholesterolaemic animals, whereas this may not be the case with regard to the impairment of coronary resistance vessels. For this reason, the effect of lipoproteins on coronary resistance arteries has been examined in this study. 2. The influence of lipoproteins on endothelium-dependent relaxation induced by acetylcholine (ACh) or sodium nitroprusside in PGF2 alpha-preconstricted rings from the large (1st order branch) and small coronary arteries (3rd order branch) and the aorta of New Zealand White rabbits, was investigated. 3. The sensitivity to ACh was greater in the large compared with the small diameter coronary arteries. 4. Endothelium-dependent relaxations were unaffected by native LDL. Oxidized LDL (0.5 and 1 mg protein mL-1) caused a reversible inhibition of relaxations in both preconstricted small and large coronary arteries which was overcome at high ACh concentrations. Similar inhibitions were found in the aorta. 5. Endothelium-independent relaxations elicited by sodium nitroprusside in the large and small coronary arteries were unaffected by the oxidized lipoproteins, indicating that soluble guanylate cyclase activity was unaltered. 6. It is concluded that inhibition of endothelium-dependent relaxation in the small diameter coronary arteries in hypercholesterolaemia may arise from products of oxidative modification of LDL present in the artery itself or released upstream from proximal lesions.

Action of probucol in arteries from normal and hypercholesterolaemic rabbits

1. The effect of probucol on the vascular reactivity of different arteries isolated from rabbits was studied as well as its effects on the development of atherosclerosis in a cholesterol-fed rabbit model. 2. Probucol 10(-6)-5 x 10(-4)M produced a concentration-dependent inhibition of the contractile responses induced by KCI (80 mM), the sequence for the IC50 was: mesenteric artery (5th branch, 4.8 +/- 2.6 x 10(-5) M) > aorta (8.2 +/- 2.3 x 10(-5) M) > femoral artery (> 5 x 10(-4) M). The response to noradrenaline was: mesenteric artery (5th branch, 4.2 +/- 1.3 x 10(-5) M) > aorta (3.2 +/- 3.0 x 10(-4) M) > femoral (> 5 x 10(-4) M). 3. In the aorta, probucol (10(-5)-10(-4) M) shifted the concentration-response curves to Ca2+ downward and to the right. 4. Probucol at 5 x 10(-5) M and 5 x 10(-4) M showed a reduction in the 45Ca2+ uptake in resting, non-stimulated aortic rings as well as the uptake induced by both noradrenaline 10(-6) M and KCI 80 mM. 5. In experiments in vivo, probucol did not affect lipid profiles; however, drug-treatment significantly decreased the cholesterol content of aortic tissue and the extent of intimal surface covered with atherosclerotic lesions. 6. The vascular reactivity was recovered in femoral arteries from rabbits on the atherogenic diet plus probucol. 7. It is concluded that the effect of probucol in vascular smooth muscle can be attributed to an inhibition of Ca2+ entry through both potential- and receptor-operated pathways. Moreover our findings suggest that the effects of probucol on movement of calcium in vascular smooth muscle may play an important role in the mechanism of antiatherogenic properties of this drug.

Hemostatic changes after dietary coenzyme Q10 supplementation in swine

Improved cardiovascular morbidity and mortality have been observed in several clinical studies of dietary supplementation with coenzyme Q10 (CoQ10). We elucidated the effect of CoQ10 on certain hemostatic parameters that may influence the progression of heart disease. Twelve Yorkshire swine were randomized to receive diet supplementation with either CoQ10 or placebo for 20 days. Blood samples were obtained at baseline and at the end of the feeding period. At the end of the protocol, there were no significant differences in hemostatic parameters in the placebo group. A significant increase in total serum CoQ10 level (from 0.39 +/- 0.06 to 0.96 +/- 0.04 microgram/ml, p < 0.001) was noted after the feeding period in the CoQ10-supplemented group. We observed significant inhibition of ADP-induced platelet aggregation (-9.9%) and a decrease in plasma fibronectin (-20.2%), thromboxane B2 (TXB2, -20.6%), prostacyclin (-23.2%), and endothelin-1 (ET-1, -17.9%) level. There were no changes in the plasma concentrations of the natural antithrombotics [antithrombin-III (AT-III), protein S, and protein C] after CoQ10 supplementation. CoQ10 supplementation in a dose of 200 mg daily is associated with mild antiaggregatory changes in the hemostatic profile. Clinical beneficial effects of CoQ10 may be related in part to a diminished incidence of thrombotic complications.

Endothelium-dependent coronary vasomotion relates to the susceptibility of LDL to oxidation in humans

BACKGROUND

Oxidatively modified LDL has been shown to markedly impair endothelium-dependent dilation in experimental studies. The aim of the present study was to determine the relationship between the coronary vasomotor response to the endothelium-dependent agonist acetylcholine and the in vitro susceptibility of LDL to oxidation in patients.

METHODS AND RESULTS

Endothelium-dependent coronary vasomotion in response to acetylcholine (10(-8) to 10(-6) mol/L) was assessed in 23 patients with hypercholesterolemia (mean age, 56 +/- 9 years) after 1 year of therapy with either an American Heart Association Step 1 diet (seven patients), lovastatin and cholestyramine (seven patients), or lovastatin and probucol (nine patients). The susceptibility of LDL to oxidation was determined by measuring the lag phase of conjugated diene formation induced by Cu2+. Patients treated with lovastatin and probucol had prolongation of the lag phase (263 +/- 64 minutes) compared with diet- (91 +/- 22 minutes) or lovastatin and cholestyramine-(118 +/- 57 minutes) treated patients (P<.0001). By univariate analysis, the coronary vasomotor response to acetylcholine was significantly related to the lag phase of conjugated diene formation (P=.002), cholesterol-lowering therapy (P=.002), and serum cholesterol (P=.02). By multivariate analysis, the lag phase remained a significant predictor of the acetylcholine vasomotor response, independent of the effect of cholesterol-lowering treatment.

CONCLUSIONS

In patients treated with lipid-lowering agents, the vasodilator response to acetylcholine is related to the susceptibility of LDL to oxidation. These findings suggest that oxidative stress is an important determinant of the coronary endothelial dysfunction observed in patients with atherosclerosis and hypercholesterolemia.

Antioxidants, lipoprotein oxidation, and arterial function

Lipoproteins have an important influence on arterial tissues. High levels of low density lipoproteins (LDL) may inhibit arterial function in terms of the release of nitric oxide from the endothelium. This inhibition is enhanced by the oxidation of these lipoproteins which may occur during the development of atherosclerosis. Many of these effects are due to lipid oxidation products. The impairment of the release of nitric oxide and vasodilatation may be partially attenuated by antioxidants. LDL may also enhance the activity of blood platelets, especially when they are mildly oxidized. Oral administration of antioxidants may decrease the activation of platelets to a limited extent. Lipid peroxides may play a role in these processes.

Pathophysiology of plaque rupture and the concept of plaque stabilization

Atherosclerosis complicated by plaque rupture or disruption and thrombosis is primarily responsible for the potentially lethal acute coronary syndromes. Plaques with a large extracellular lipid-rich core, thin fibrous cap with reduced collagen content and smooth muscle density, and increased numbers of activated macrophages and mast cells appear to be most vulnerable to rupture. Plaque disruption tends to occur at points at which the plaque surface is weakest and most vulnerable, which coincide with points at which stresses, resulting from biomechanical and hemodynamic forces acting on plaques, are concentrated. Reduced matrix synthesis as well as increased matrix degradation may predispose fibrous caps to rupture spontaneously or in response to extrinsic mechanical or hemodynamic stresses. Modification of endothelial dysfunction and reduction of vulnerability to plaque rupture and thrombosis may lead to plaque stabilization resulting in reduction of the frequency of acute coronary syndromes. This putative concept of plaque stabilization, although attractive, has not yet been rigorously validated in humans. Indirect data from clinical trials involving lipid lowering/modification and lifestyle/risk factor modification, however, provide strong support for this new paradigm. Thus, plaque stabilization may prove to be an important modality for reduction of lethal consequences of coronary atherosclerosis.

Ascorbic acid and atherosclerotic cardiovascular disease

In this chapter, we have briefly reviewed the current scientific knowledge of the role of vitamin C in the prevention of atherosclerosis and its associated clinical manifestations. There is good evidence from animal studies that vitamin C can slow the progression of experimental atherosclerosis. Most of these studies, however, were done either in guinea pigs, using ascorbic acid depletion, or in cholesterol-fed rabbits, using ascorbic acid supplementation. Both animal models have limitations, as guinea pigs are not a well-established (nor well-studied) model of atherosclerosis, and rabbits develop atherosclerosis at high serum beta-VLDL cholesterol levels, and in addition can synthesize ascorbic acid. In contrast, humans develop atherosclerosis spontaneously and readily at moderately elevated serum LDL cholesterol levels and have lost the ability to synthesize ascorbic acid. Thus, the animal studies discussed, although quite promising and suggestive of an anti-atherogenic effect of ascorbic acid, need to be expanded to primates before more definitive conclusions can be drawn. Similar to the animal data, the current evidence from epidemiological studies on the role of vitamin C in the prevention of CVD is inconclusive, with some studies showing a very strong correlation between increased vitamin C intake and incidence of CVD events and other studies showing no correlation at all. Studies on CVD risk factors indicate that vitamin C may moderately decrease total serum cholesterol levels, increase HDL levels, and exert a hypotensive effect. These findings are particularly intriguing and should be pursued vigorously in basic research studies to elucidate biological mechanisms. In addition, it appears that large placebo-controlled, double-blind, randomized trials of vitamin C supplementation (without simultaneous supplementation with vitamin E) in populations with a wide range of vitamin C body levels are needed in order to confirm or refute a role for vitamin C in the prevention of CVD. Unfortunately, no such trials are currently being conducted. The possible mechanisms by which ascorbic acid may affect the development of atherosclerosis and the onset of acute coronary events include effects on arterial wall integrity related to biosynthesis of collagen and GAGs, altered cholesterol metabolism mediated by vitamin C-dependent conversion of cholesterol to bile acids, and effects on triglyceride levels via modulation of lipoprotein lipase activity. A particularly intriguing possible mechanism for the anti-atherogenic effect of vitamin C is prevention of atherogenic, oxidative modification of LDL. Numerous in vitro studies have demonstrated that ascorbic acid strongly inhibits LDL oxidation by a variety of mechanisms. The potential effects of ascorbic acid on platelet function and EDRF metabolism are particularly intriguing, as they might have widespread consequences for the prevention of atherosclerotic lesion development as well as acute clinical events. Thus, both metabolic and antioxidant functions may contribute to the possible reduction of CVD risk by vitamin C.

Endothelial dysfunction in preeclampsia. Part I: Primary prevention. Therapeutic perspectives

Endothelial dysfunction is the final common pathway in the pathogenesis of preeclampsia. Future therapeutic modalities aimed at preventing or treating preeclampsia should either reduce the extent of (or even prevent) endothelial cell dysfunction (primary prevention) or should reduce the consequences of endothelial cell dysfunction (secondary and tertiary prevention). Relevant potential and promising directions for further research will be discussed. Part I of this review concerns the primary prevention of preeclampsia. The etiology of preeclampsia is still unknown. The 4 hypotheses that are the most popular currently are the placental ischemia hypothesis, the immune maladaptation/spiral artery-decidual ¿toxin' hypothesis, the VLDL (very low density lipoproteins)/TxPA (toxicity preventing activity) hypothesis, and the genetic hypothesis. For the near future only the disturbed VLDL/TxPA balance and the imbalance between free radicals and scavengers appear to be amenable for clinical research purposes.

Interactions between endothelial mediators

Prostacyclin, nitric oxide and tissue plasminogen activator constitute a prominent triad of endothelial mediators. Prostacyclin is responsible mainly for maintaining vascular thromboresistance against platelet clumps, inhibits proliferation of vascular smooth muscle and modulates cholesterol turnover, tissue plasminogen activator is a fibrinolytic agent and nitric oxide controls vascular tone and structure. Receptor agonists such as acetylcholine, kinins, endothelins or adenosine diphosphate evoke a coupled release of mediators from endothelial cells. Prostacyclin and nitric oxide synergize in their antiplatelet, fibrinolytic and cardioprotective, but not in their hypotensive actions. Prostacyclin, but not nitric oxide, prevents paradox thrombogenic effects of tissue plasminogen activator. Filogenetically, prostacyclin and tissue plasminogen activator are younger brothers of nitric oxide from which they take over and perfect regulatory properties in circulation. Further studies on interactions of endothelial mediators may lead to a better understanding of mechanisms of thrombosis, atherogenesis, diabetic angiopathies, endotoxic shock and arterial hypertension.

Modulation of particulate nitric oxide synthase activity and peroxynitrite synthesis in cholesterol enriched endothelial cell membranes

Endothelium-derived relaxing factor/nitric oxide (EDRF/NO) is produced by the vascular wall and is a key modulator of vascular tone and blood pressure. Since reduced EDRF/NO release from the endothelium is a major key event in the development of atherosclerosis, we investigated the effect of cholesterol on endothelial cell particulate (membrane-bound) NO synthase activity. Low concentrations (up to 0.2 mM) of liposomal cholesterol progressively activated plasma membrane-bound NO synthase. Increasing cholesterol concentration above that which maximally stimulated enzyme activity produced a progressive inhibition with respect to the control value. In time course experiments using endothelial cell plasma membranes enriched with cholesterol, changes in NO production were followed by analogous changes in soluble guanylate cyclase activity (sGC). N-Monomethyl-L-arginine (L-NMMA) (1 mM) inhibited particulate NO synthase activity at all cholesterol concentrations used with subsequent decreases in cGMP production. Egg lecithin liposomes (free of cholesterol) had no effect on NO synthase activity. A three-fold increase in superoxide (O2-) and a 2.5-fold increase in NO formation followed by an eight-fold increase in peroxynitrite (ONOO-) production by cholesterol-treated microsomes isolated from endothelial cells was observed, one which rose further up to eight-fold in the presence of superoxide dismutase (SOD) (10 U/mL). Cholesterol had no effect on Lubrol-PX solubilized membrane-bound NO synthase or on cytosolic (soluble) NO synthase activities of endothelial cells. Cholesterol modulated lipid fluidity of plasma membranes labelled with 1,6-diphenyl-1,3,5-hexatriene (DPH) as indicated by the steady state fluorescence anisotropy [(ro/r)-1]-1. Arrhenius plots of [(ro/r)-1]-1 indicated that the lipid phase separation of the membranes at 26.2 +/- 1.5 degrees was elevated to 34.4 +/- 1.9 degrees in cholesterol-enriched membranes, consistent with a general decrease in membrane fluidity. Cholesterol-enriched plasma membranes treated with egg lecithin liposomes showed a lipid phase separation at 27.5 +/- 1.6 degrees, indicating the reversible effect of cholesterol on membrane lipid fluidity. Arrhenius plots of NO synthase activity exhibited break point at 26.9 +/- 1.8 degrees which rose to 35.6 +/- 2.1 degrees in 0.5 mM cholesterol-treated plasma membranes and decreased to 21.5 +/- 1.4 degrees in plasma membranes treated with 0.2 mM cholesterol. The allosteric properties of plasma membrane-bound NO synthase inhibited by Mn2+ (as reflected by changes in the Hill coefficient) were changed by cholesterol, consistent with modulations of the fluidity of the lipid microenvironment of the enzyme.(ABSTRACT TRUNCATED AT 400 WORDS)

Proteolysis and fusion of low density lipoprotein particles strengthen their binding to human aortic proteoglycans

Lipid droplets resembling those seen in the extracellular space of the arterial intima were generated in vitro when granule proteases of rat serosal mast cells degraded the apolipoprotein B-100 (apoB-100) component of granule-bound low density lipoprotein (LDL), and the particles fused on the granule surface (Paanenen, K., and Kovanen, P. T. (1994) J. Biol. Chem. 269, 2023-2031). Moreover, the binding of the fused particles to the heparin proteoglycan component of the granules was found to be strengthened. We have now treated LDL particles with alpha-chymotrypsin and examined the strength with which the proteolytically modified LDL binds to human aortic proteoglycans on an affinity column. We found that chymotryptic degradation of the LDL particles triggered particle fusion. The higher the degree of proteolytic degradation, the higher were the degree of fusion and the strength of binding to the aortic proteoglycans. Separation of the proteolyzed particles by size exclusion chromatography into two fractions, unfused and fused particles, and analysis of their binding strengths revealed that not only the fused but also the unfused proteolyzed particles bound more tightly to the proteoglycans than did the native LDL particles. To investigate the mechanism underlying this increase in binding strength, we attached [13C]dimethyl groups to the lysines and used NMR spectroscopy to quantify the active lysine residues of apoB-100, which are thought to be located in basic areas of apoB-100 and involved in binding of LDL to proteoglycans. Analysis of the 13C-labeled particles showed that, despite loss of apoB-100 fragments from the particles, the number of active lysine residues in the unfused proteolyzed particles had not decreased. In the fused proteolyzed particles, the number of active lysine residues was markedly increased. Thus, proteolytic fusion appears to increase the number of basic domains of apoB-100, which would explain the observed increase in the strength of binding of the modified LDL particles to arterial proteoglycans. Since the fused particles resemble the small lipid droplets found in the atherosclerotic arterial intima, this LDL modification offers a plausible mechanism for the focal accumulation of lipid droplets in the extracellular proteoglycan matrix during atherogenesis.

The effect of cholesterol-lowering and antioxidant therapy on endothelium-dependent coronary vasomotion

BACKGROUND

Patients with coronary artery disease and abnormalities of serum lipids often have endothelial vasodilator dysfunction, which may contribute to ischemic cardiac events. Whether cholesterol-lowering or antioxidant therapy can restore endothelium-dependent coronary vasodilation is unknown.

METHODS

We randomly assigned 49 patients (mean serum cholesterol level, 209 +/- 33 mg per deciliter [5.40 +/- 0.85 mmol per liter]) to receive one of three treatments: an American Heart Association Step 1 diet (the diet group, 11 patients); lovastatin and cholestyramine (the low-density lipoprotein [LDL]-lowering group, 21 patients); or lovastatin and probucol (the LDL-lowering-antioxidant group, 17 patients). Endothelium-dependent coronary-artery vasomotion in response to an intracoronary infusion of acetylcholine (10(-8) to 10(-6) M) was assessed at base line and after one year of therapy. Vasoconstrictor responses to these doses of acetylcholine are considered to be abnormal.

RESULTS

Treatment resulted in significant reductions in LDL cholesterol levels of 41 +/- 22 percent in the LDL-lowering-antioxidant group and 38 +/- 20 percent in the LDL-lowering group (P < 0.001 vs. the diet group). The maximal changes in coronary-artery diameter with acetylcholine at base line and at follow-up were -19 and -2 percent, respectively, in the LDL-lowering-antioxidant group, -15 and -6 percent in the LDL-lowering group, and -14 and -19 percent in the diet group (P < 0.01 for the LDL-lowering-antioxidant group vs. the diet group; P = 0.08 for the LDL-lowering group vs. the diet group). (The negative numbers indicate vasoconstriction). Thus, the greatest improvement in the vasoconstrictor response was seen in the LDL-lowering-antioxidant group.

CONCLUSIONS

The improvement in endothelium-dependent vasomotion with cholesterol-lowering and antioxidant therapy may have important implications for the activity of myocardial ischemia and may explain in part the reduced incidence of adverse coronary events that is known to result from cholesterol-lowering therapy.

The effect of probucol and vitamin E treatment on the oxidation of low-density lipoprotein and forearm vascular responses in humans

This study investigates the hypothesis that lipid soluble antioxidants may increase the resistance of low-density lipoprotein (LDL) to oxidation and also enhance vascular endothelial responses in humans. In a double-blind parallel group study, 24 hypercholesterolaemic patients already on treatment with simvastatin (20 mg day-1), were randomized to supplementary treatment with probucol (500 mg bd), vitamin E (400 IU daily) or placebo for 8 weeks. Mean serum cholesterol before antioxidant treatment was 7.00 mmol l-1. Resistance of LDL to oxidation by copper was increased by 830% in the probucol group and by 30% in the vitamin E group. However, thiobarbituric acid reacting substances in whole serum were not altered by either antioxidant. Probucol lowered HDL- and LDL-cholesterol levels and increased the QT interval. Forearm vascular responses, as measured by venous occlusion plethysmography, to acetylcholine, glyceryl trinitrate and NG-monomethyl-L-arginine, were not significantly changed by antioxidant treatment. Probucol has a major, and vitamin E a minor, effect on LDL resistance to oxidation but neither compound appears to alter forearm vascular responses in vivo.