Impaired endothelium-dependent vascular relaxation in patients with hypercholesterolemia extends beyond the muscarinic receptor.

Patients with hypercholesterolemia have impaired endothelium-dependent vasodilation. However, previous human studies have invariably used muscarinic agents to assess endothelial function. The purpose of this investigation was to determine whether impaired endothelium-dependent vasodilation of hypercholesterolemic patients is related to a specific and isolated defect of the muscarinic receptor, or to a broader abnormality of the endothelial cells. The forearm vascular responses to the endothelium-dependent agents acetylcholine (7.5, 15, and 30 micrograms/min) and substance P (1, 2, and 4 pmol/min), and to the direct smooth muscle dilator sodium nitroprusside (0.8, 1.6, and 3.2 micrograms/min) were studied in 16 hypercholesterolemic patients (8 men and 8 women; age [mean +/- SD] 50 +/- 7 years; serum cholesterol > 250 mg/dl) and 16 normal volunteers (8 men and 8 women; age 47 +/- 8 years; serum cholesterol < 200 mg/dl). Drugs were infused into the brachial artery and the response of the forearm vasculature was measured by strain-gauge plethysmography. The vasodilator response to acetylcholine was reduced in hypercholesterolemic patients compared with normal controls; at the highest dose (30 micrograms/min) the increase in forearm blood flow was 13.5 +/- 7 ml/min/100 ml in controls and 7.54 +/- 6 in patients (p < 0.05). The response to substance P was also blunted in hypercholesterolemic patients; at the highest dose (4 pmol/min), the increase in forearm blood flow was 12.1 +/- 5 ml/min/100 ml in controls and 7.6 +/- 4 in patients (p < 0.03). A significant correlation was found between the highest blood flow responses with acetylcholine and with substance P (r = 0.58; p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of antioxidant vitamins on low density lipoprotein oxidation and impaired endothelium-dependent vasodilation in patients with hypercholesterolemia.

OBJECTIVES: The aims of this study were to determine whether antioxidant vitamins could reduce the susceptibility of low density lipoprotein (LDL) to oxidation and improve endothelium-dependent vasodilator responsiveness in patients with hypercholesterolemia. BACKGROUND: Animals and humans with hypercholesterolemia have exhibited impaired endothelium-dependent vasodilation. In vitro studies suggest that oxidatively modified LDL can impair nitric oxide production. METHODS: Forearm blood flow was measured with strain gauge plethysmography and brachial artery drug infusions in 19 patients, aged 52 +/- 9 years, with hypercholesterolemia (mean +/- SD total cholesterol 283 +/- 22 mg/dl, LDL 197 +/- 31 mg/dl) and in 14 subjects, aged 48 +/- 8 years, with normal cholesterol levels (total cholesterol 169 +/- 20 mg/dl, LDL 102 +/- 25 mg/dl). Acetylcholine (7.5, 15 and 30 micrograms/min) was utilized as an endothelium-dependent vasodilator, and sodium nitroprusside (0.8, 1.6 and 3.2 micrograms/min) was used to test endothelium-independent vasodilation. Oxidative susceptibility of LDL was measured by a spectrophotometric assay of conjugated diene production after the addition of copper chloride. Hypercholesterolemic patients then received daily antioxidant vitamin supplements (beta-carotene [30 mg], ascorbic acid [vitamin C] [1,000 mg], vitamin E [800 IU]) for 1 month, with repeat measurement of both forearm blood flow responsiveness to the same agonists and LDL oxidizability. RESULTS: The maximal flow in response to acetylcholine was impaired in patients compared with that in normal subjects (9.8 +/- 7.8 vs. 15.9 +/- 8.1 ml/min per 100 ml, p = 0.03), with similar maximal flow responses to sodium nitroprusside (9.5 +/- 4.2 vs. 9.0 +/- 2.8 ml/min per 100 ml, p = 0.72). After 1 month of vitamin therapy, the onset of LDL oxidation was prolonged over baseline measurements by 71 +/- 67%, and the maximal rate of oxidation was decreased by 26 +/- 25% (both p < 0.001). However, the maximal forearm blood flow response to acetylcholine remained unchanged from baseline values (maximal flow after acetylcholine 9.0 +/- 6.2 vs. 9.8 +/- 7.8 ml/min per 100 ml, p = 0.57). This study had 80% power (alpha = 0.05) to exclude a 45% increase over baseline value in acetylcholine-stimulated flow during vitamin therapy. CONCLUSIONS: Although 1 month of administration of antioxidant vitamin supplements in hypercholesterolemic patients reduced the susceptibility of LDL to oxidation, impairment in endothelial function remained unaltered. The use of nonvitamin antioxidants or concomitant reduction in LDL levels, as well as more sensitive techniques for measuring vascular responsiveness, may be required to show a beneficial effect on endothelial vasodilator function.

Contribution of endothelium-derived nitric oxide to exercise-induced vasodilation.

BACKGROUND: Endothelium-derived nitric oxide is an important modulator of resting vascular tone in animals and humans. However, the contribution of nitric oxide to exercise-induced vasodilation is unknown. METHODS AND RESULTS: The effect of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthesis, on exercise-induced vasodilation was studied in 18 healthy subjects (mean +/- SD, 40 +/- 10 years; 10 women). Acetylcholine was used to test the efficacy of L-NMMA in inhibiting stimulation of nitric oxide synthesis and sodium nitroprusside to test the specificity of L-NMMA in inhibiting endothelium-dependent vasodilation. Intermittent handgrip exercise and infusions of acetylcholine and sodium nitroprusside were performed during intra-arterial infusion of 5% dextrose (control) and L-NMMA (4 to 16 mumol/min). Forearm blood flow was determined by strain-gauge plethysmography. Forearm oxygen extraction was measured from arterial and venous oxygen saturations. In a separate study, 10 subjects performed exercise during infusions of 5% dextrose, L-arginine (the substrate for nitric oxide production), and D-arginine (the stereoisomer that is not a substrate for nitric oxide production). L-NMMA reduced exercise blood flow by 7 +/- 13% (P = .04), increased exercise resistance by 18 +/- 20% (P = .02), and increased exercise oxygen extraction by 16 +/- 17% (P < .001). The degree of inhibition of acetylcholine-induced vasodilation with L-NMMA correlated positively with the degree of reduction in exercise blood flow (r = .55, P = .02). The highest dose of L-NMMA (16 mumol/min) produced the greatest effect; exercise blood flow was reduced by 11 +/- 14% (P = .03), and vascular resistance increased by 26 +/- 23% (P = .005). L-NMMA did not affect the forearm vasodilation produced by sodium nitroprusside. Exercise blood flow, resistance, and oxygen extraction were not significantly modified by infusions of either L- or D-arginine. CONCLUSIONS: Inhibition of nitric oxide synthesis reduces exercise-induced vasodilation in the human forearm, indicating that nitric oxide plays a role in exercise-induced vasodilation. Increased availability of nitric oxide substrate does not enhance exercise-induced vasodilation in healthy subjects. These findings have important implications for disease states in which endothelium-derived nitric oxide production is impaired.

Impaired endothelium-dependent vasodilation in patients with essential hypertension: evidence that the abnormality is not at the muscarinic receptor level.

OBJECTIVES: The purpose of this study was to determine whether the impaired endothelium-dependent vasodilation of hypertensive patients is related to a specific defect of the muscarinic receptor or to a broader abnormality of the vascular endothelium. BACKGROUND: Patients with essential hypertension have abnormal endothelium-dependent vasodilator response to acetylcholine. However, whether this results from an isolated dysfunction of the endothelial cell muscarinic receptor is unknown. METHODS: The responses of the forearm vasculature to acetylcholine and substance P (endothelium-dependent agents acting on different receptors) and to sodium nitroprusside (a direct dilator of vascular smooth muscle) were studied in eight hypertensive patients (six men, two women; mean age [+/- SD] 50 +/- 12 years) and eight normal control subjects (four men, four women; mean age 49 +/- 9 years). To determine the nitric oxide contribution to substance P-induced vasodilation, the vascular responses to substance P were also measured after inhibition of nitric oxide synthesis with NG-monomethyl-L-arginine. Drugs were infused into the brachial artery, and forearm blood flow was measured by strain gauge plethysmography. RESULTS: The response to acetylcholine was significantly blunted in hypertensive patients (highest blood flow [mean +/- SD] 8.4 +/- 4 vs. 13.8 +/- 4 ml/min per 100 ml in control subjects, p < 0.03). Similarly, the vasodilator effect of substance P was significantly reduced in hypertensive patients (highest blood flow [mean +/- SD] 8.8 +/- 4 vs. 13.9 +/- 4 ml/min per 100 ml in control subjects, p < 0.03). A significant correlation was found between the maximal blood flow with acetylcholine and that with substance P (r = 0.68, p < 0.004). The vasodilator response to sodium nitroprusside was similar in patients and control subjects. The nitric oxide contribution to substance P-induced vasodilation was reduced in hypertensive patients, such that the responses to substance P measured during infusion of NG-monomethyl-L-arginine were not significantly different between the two groups. CONCLUSIONS: These findings indicate that the endothelial abnormality of patients with essential hypertension is not restricted to the muscarinic cell receptor.

Investigation of decreased availability of nitric oxide precursor as the mechanism responsible for impaired endothelium-dependent vasodilation in hypercholesterolemic patients.

OBJECTIVES: The purpose of this study was to determine whether the impaired endothelium-dependent vasodilation of hypercholesterolemic patients is due to decreased availability of L-arginine, the substrate for nitric oxide. BACKGROUND: Patients with hypercholesterolemia have impaired endothelium-dependent vasodilation that is related to a defect in the endothelium-derived nitric oxide system. However, the precise location of this abnormality has not been determined. METHODS: The study included 12 hypercholesterolemic patients (6 men, 6 women; 52 +/- 9 years old; serum cholesterol > 240 mg/dl) and 15 normal volunteers (8 men, 7 women; 50 +/- 6 years old; serum cholesterol < 210 mg/dl). The forearm vascular responses to intraarterial infusion of acetylcholine, an endothelium-dependent vasodilator (7.5, 15, 30 micrograms/min), and sodium nitroprusside, a direct smooth muscle dilator (0.8, 1.6, 3.2 micrograms/min) were studied before and during infusion of L- or D-arginine (a stereoisomer of arginine that is not a nitric oxide precursor). RESULTS: The response to acetylcholine was lower in hypercholesterolemic patients than in control subjects. However, no significant difference was observed with sodium nitroprusside infusion. L-Arginine augmented the response to acetylcholine in normal subjects (maximal blood flow increased from 14.4 +/- 7 to 18.9 +/- 10 ml/min per 100 ml, p < 0.002). In contrast, in the hypercholesterolemic patients, only a mild but not significant improvement in the response to acetylcholine was observed with the infusion of L-arginine (maximal blood flow increased from 6.8 +/- 4 to 8.4 +/- 5 ml/min per 100 ml; p = 0.16); however, a similar mild but not significant change was also observed with D-arginine (maximal blood flow increased from 6.8 +/- 4 to 8.3 +/- 4 ml/min per 100 ml, p = 0.07). L-Arginine did not modify the response to sodium nitroprusside in either group. CONCLUSIONS: The augmentation of endothelium-dependent vasodilation by L-arginine, the nitric oxide precursor, is defective in hypercholesterolemic patients. This supports the concept of an abnormal endothelium-derived nitric oxide system in hypercholesterolemia and indicates that decreased availability of nitric oxide substrate is not responsible for the impaired endothelial function in this condition.

The role of nitric oxide in endothelium-dependent vasodilation of hypercholesterolemic patients.

BACKGROUND: Patients with hypercholesterolemia have a reduced response to endothelium-dependent vasodilators. However, the regulatory function of the endothelium on vascular tone is mediated through the release of several vasoactive substances; therefore, a reduced response to endothelium-dependent agents does not identify which of the factors released by the endothelium is involved in this abnormality. METHODS AND RESULTS: To investigate the role of nitric oxide in the endothelium-dependent vasodilation in hypercholesterolemia, we studied the effect of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of endothelial nitric oxide synthesis, on basal vascular tone and on the responses to acetylcholine, an endothelium-dependent vasodilator, and to sodium nitroprusside, a direct smooth muscle dilator. The study included 33 hypercholesterolemic patients (17 men; 51 +/- 8 years; plasma cholesterol, > or = 240 mg/dL) and 23 normal controls (12 men; 48 +/- 7 years; plasma cholesterol, < 210 mg/dL). Drugs were infused into the brachial artery, and the response of the forearm vasculature was measured by strain-gauge plethysmography. Basal blood flow and vascular resistance were similar in hypercholesterolemic patients and normal controls (3.1 +/- 1 versus 2.6 +/- 0.8 mL/min per 100 mL and 32.1 +/- 13 versus 36.1 +/- 12 mm Hg/mL-1.min-1.100 mL-1, respectively). The reduction in basal blood flow and increase in vascular resistance produced by L-NMMA were not significantly different between the two groups. L-NMMA markedly blunted the response to acetylcholine in normals (maximum flow decreased from 16.4 +/- 8 to 7.0 +/- 3; P < .005); however, the arginine analogue did not significantly modify the response to acetylcholine in the hypercholesterolemic patients (maximum flow, 11.1 +/- 8 versus 10.0 +/- 8). L-NMMA did not modify the vasodilator response to sodium nitroprusside in either controls or patients. CONCLUSIONS: These findings indicate that hypercholesterolemic patients have a defect in the bioactivity of nitric oxide that may explain their impaired endothelium-dependent vascular relaxation.

Role of endothelium-derived nitric oxide in the abnormal endothelium-dependent vascular relaxation of patients with essential hypertension.

BACKGROUND: Patients with essential hypertension have abnormal endothelium-dependent vasodilation. Because the endothelium exerts its action on the vascular smooth muscle through the release of several substances, it is important to identify which of these factors is involved in the abnormal response of hypertensive arteries. METHODS AND RESULTS: To investigate the role of endothelium-derived nitric oxide in this abnormality, we studied the vascular effect of the arginine analogue NG-monomethyl-L-arginine, an inhibitor of the endothelial synthesis of nitric oxide, under baseline conditions and during infusion of acetylcholine, an endothelium-dependent vasodilator, and sodium nitroprusside, a direct smooth muscle dilator. The study included 11 hypertensive patients (seven men; age, 46.5 +/- 9 years) and 10 normal control subjects (seven men; age, 45.7 +/- 7 years). Drugs were infused into the brachial artery, and the response of the forearm vasculature was measured by strain-gauge plethysmography. Basal blood flow was similar in normal control subjects and hypertensive patients (2.97 +/- 0.7 versus 2.86 +/- 1.1 mL.min-1.100 mL-1, respectively). NG-monomethyl-L-arginine produced a significantly greater decrease in blood flow in control subjects than in patients (1.08 +/- 0.6 versus 0.32 +/- 0.4 mL.min-1.100 mL-1; p < 0.004). The vasodilator response to acetylcholine was reduced in patients compared with control subjects (maximum flow, 8.2 +/- 4 versus 16.4 +/- 8 mL.min-1.100 mL-1; p < 0.001). NG-monomethyl-L-arginine blunted the vasodilator response to acetylcholine in control subjects (maximum flow decreased from 16.4 +/- 8 to 7.01 +/- 3 mL.min-1.100 mL-1; p < 0.004); however, the arginine analogue did not significantly alter the response to acetylcholine in hypertensive patients (maximum flow, 8.2 +/- 4 versus 8.01 +/- 5 mL.min-1.100 mL-1). NG-monomethyl-L-arginine did not modify the vasodilator response to sodium nitroprusside in either control subjects or patients. CONCLUSIONS: These findings indicate that patients with essential hypertension have a defect in the endothelium-derived nitric oxide system that may at least partly account for both the increased vascular resistance under basal conditions and the impaired response to endothelium-dependent vasodilators.

Effect of increased availability of endothelium-derived nitric oxide precursor on endothelium-dependent vascular relaxation in normal subjects and in patients with essential hypertension.

BACKGROUND: Patients with essential hypertension have a deficit in the endothelium-derived nitric oxide system that results in impaired endothelium-dependent vascular relaxation. The objective of this study was to determine whether this abnormality is caused by a deficiency of substrate for nitric oxide synthesis. METHODS AND RESULTS: The vascular responses to acetylcholine (an endothelium-dependent vasodilator infused at 7.5, 15, and 30 micrograms/min) and sodium nitroprusside (a direct smooth muscle dilator infused at 0.8, 1.6, and 3.2 micrograms/min) were studied during combined administration of dextrose 5% or L-arginine (substrate for nitric oxide synthesis infused at 40 mumol/min) in 12 normal control subjects (seven men and five women; age, 49.3 +/- 7 years) and 14 hypertensive patients (nine men and five women; age, 48.4 +/- 7 years). In addition, the effect of D-arginine (stereoisomer of arginine that is not a precursor of nitric oxide) on the vascular responses to acetylcholine was studied in eight normal control subjects and seven hypertensive patients. Drugs were infused into the brachial artery, and the response of the forearm vasculature was measured by strain gauge plethysmography. The vasodilator response to acetylcholine was significantly blunted in hypertensive patients compared with normal control subjects (maximum flow, 8.9 +/- 5 versus 15.7 +/- 6 mL.min-1.100 mL-1, respectively; p < 0.007); however, no difference was observed in the response to sodium nitroprusside (11.4 +/- 6 and 11.7 +/- mL.min-1.100 mL-1, respectively). L-Arginine did not significantly change basal blood flow or vascular resistance in either group. In normal control subjects, the infusion of L-arginine significantly augmented the vasodilator response to acetylcholine (maximum flow, 15.7 +/- 6 versus 21.4 +/- 8 mL.min-1.100 mL-1 before and after L-arginine, respectively; p < 0.001). In contrast, in hypertensive patients, the infusion of L-arginine did not alter the response to acetylcholine (maximum flow, 8.9 +/- 5 and 8.4 +/- 4 mL.min-1.100 mL-1 before and after L-arginine, respectively). The administration of L-arginine did not modify the response to sodium nitroprusside in either group. Similarly, the infusion of D-arginine did not alter the response to acetylcholine in either group. CONCLUSIONS: In normal humans, availability of substrate for production of nitric oxide is a rate-limiting step for endothelium-dependent vascular relaxation. In contrast, increased availability of nitric oxide precursor does not modify endothelium-mediated vasodilation in hypertensive patients. These findings provide further evidence of a defect in the endothelium-derived nitric oxide system in hypertension and indicate that this abnormality is not related to decreased availability of substrate for nitric oxide production.