Endothelium-dependent relaxation by acetylcholine is impaired in hypertriglyceridemic humans with normal levels of plasma LDL cholesterol.

OBJECTIVES: Patients with high triglyceride (of which very low density lipoproteins [VLDL] are the main carriers), but with normal low density lipoprotein (LDL) cholesterol levels, were examined for in vivo endothelium function status. BACKGROUND: Very low density lipoproteins inhibit endothelium-dependent, but not -independent, vasorelaxation in vitro. METHODS: Three groups were studied: 1) healthy volunteers (n = 10; triglyceride 1.24+/-0.14 mmol/liter, LDL cholesterol 2.99+/-0.24 mmol/liter); 2) hypertriglyceridemic (n = 11; triglyceride 6.97+/-1.19 mmol/liter, LDL cholesterol 2.17+/-0.2 mmol/liter, p < 0.05); and 3) hypercholesterolemic (n = 10; triglyceride 2.25+/-0.29 mmol/liter, LDL cholesterol 5.61+/-0.54 mmol/liter; p < 0.05) patients. Vasoactive responses to acetylcholine, sodium nitroprusside, noradrenaline, N(G)-monomethyl-L-arginine and postischemic hyperemia were determined using forearm venous occlusion plethysmography. RESULTS: Responses to acetylcholine (37 microg/min) were significantly dampened both in hypercholesterolemic (% increase in forearm blood flow: 268.2+/-62) and hypertriglyceridemic patients (232.6+/-45.2) when compared with controls (547.8+/-108.9; ANOVA p < 0.05). Responses to sodium nitroprusside (at 1.6 microg/min: controls vs. hypercholesterolemics vs. hypertriglyceridemic: 168.7+/- 25.1 vs. 140.6+/-38.9 vs. 178.5+/-54.5% increase), noradrenaline, N(G)-monomethyl-L-arginine and postischemic hyperemic responses were not different among the groups examined. CONCLUSIONS: Acetylcholine responses are impaired in patients with pathophysiologic levels of plasma triglycerides but normal plasma levels of LDL cholesterol. The impairment observed was comparable to that obtained in hypercholesterolemic patients. We conclude that impaired responses to acetylcholine normally associated with hypercholesterolemia also occur in hypertriglyceridemia. These findings identify a potential mechanism by which high plasma triglyceride levels may be atherogenic independent of LDL cholesterol levels.

Non-specific inhibition by human lipoproteins of endothelium dependent relaxation in rat aorta may be attributed to lipoprotein phospholipids.

OBJECTIVE: In vitro incubation of low-density lipoprotein (LDL) is reported to attenuate endothelium dependent relaxation mediated by acetylcholine (ACh) while not affecting endothelium-independent relaxation. This study was designed to examine the effects of other lipid-carrying lipoproteins as well as to study their effects on responses mediated by endothelium dependent agonists other than ACh. METHODS: The effects of human LDL, very-low-density lipoprotein (VLDL) and high density lipoprotein (HDL) on endothelium-dependent relaxation by ACh, histamine and the calcium ionophore, A23187, and endothelium-independent relaxation by sodium nitroprusside (SNP) were investigated is rat isolated aortic rings. The effects of combined LDL and HDL incubation on responses mediated by ACh were also examined. Control experiments included experiments examining the effects of bovine serum albumin on responses mediated by ACh. Thiobarbituric-acid-reactive substances (TBARS) measured before and after organ bath incubation indicated little oxidation of the lipoproteins used. RESULTS: Maximal responses to ACh were inhibited by LDL, VLDL and HDL (0.02 and 0.2 mg protein/ml), to histamine by LDL (0.2 mg protein/ml), VLDL (0.02 and 0.2 mg protein/ml) and HDL (0.02 mg protein/ml) and to A23187 by LDL (0.2 mg protein/ml). VLDL (0.2 mg protein/ml) and HDL (0.02 and 0.2 mg protein/ml). A small but significant correlation (r = 0.54, P = 0.01) was observed between the level of inhibition of the endothelium-dependent responses and lipoprotein phospholipid concentration in the organ bath but not between the level of inhibition and cholesterol (free and esterified) or triglyceride concentrations. Responses to SNP were unaffected by LDL, VLDL and HDL. Combined incubation of tissues with LDL (0.2 mg protein/ml) and HDL (0.2 mg protein/ml) significantly increased maximal responses to ACh (pre-lipoproteins 81.8 +/- 5.7 vs plus-LDL/HDL 100 +/- 0.0; P < 0.05). Bovine serum albumin had no effect on the maximal responses to ACh. CONCLUSIONS: We conclude that inhibition by human lipoproteins of endothelium-dependent agonists occurs with LDL, HDL and VLDL and suggest that this may be due to the phospholipid content of each lipoprotein. However, combined incubation of HDL with LDL negates this effect and an increased maximal response to ACh is reported.

Effects of ET(A) - and ET(B)-receptor antagonists on regional kidney blood flow, and responses to intravenous endothelin-1, in anaesthetized rabbits.

OBJECTIVE: To determine the roles of endothelin (ET)-receptor subtypes in the effects of exogenous and endogenous ETs on regional kidney blood flow in anaesthetized rabbits. DESIGN AND METHODS: The effects on regional kidney blood flow of the ET(A) antagonist BQ610, and the ET(B) antagonist BQ788, were tested. We also examined the effects of intravenous and renal arterial bolus doses of ET-1, and how these responses are modified by pretreatment with BQ610 and BQ788. RESULTS: BQ610 reduced mean arterial pressure (MAP, 3%), and increased total renal blood flow (RBF, 10%), cortical perfusion (CBF, 11%) and medullary perfusion (MBF, 16%). BQ788 increased MAP (6%) and reduced RBF (16%) and CBF (13%) but not MBF. The effects of BQ788 were abolished by pretreatment with BQ610. Intravenous ET-1 (300 ng/kg) reduced RBF and CBF, but increased MBF. BQ788 potentiated ET-1 mediated reductions in CBF, and abolished increases in MBF. BQ610 blunted reductions in RBF and CBF produced by ET-1, but did not significantly affect MBF responses. The renal vascular effects of intravenous ET-1 were mimicked by lower doses (1-30 ng/kg) administered into the renal artery. CONCLUSIONS: Endogenous ETs act at ET(A)-receptors to reduce MBF and CBF, but ET(B)-receptors have little direct role in physiological control of renal haemodynamics. Bolus doses of ET-1 act at ET(B)-receptors in the kidney to increase MBF. The effects of bolus ET-1 on the cortical vasculature appear to result from the competing influences of ET(A)-mediated vasoconstriction and ET(B)-mediated vasodilatation.

Responses to endothelium-dependent agonists in subcutaneous arteries excised from hypercholesterolaemic men.

1. Vasomotor function of the vascular endothelium was examined in human subcutaneous arteries excised from 8 hypercholesterolaemic and 7 normolipidaemic subjects. 2. Left gluteal skin biopsies were performed under local anaesthesia. Subcutaneous arteries were isolated and two vessels from each subject mounted in separate myographs. A 20 ml fasting blood sample was taken at the time of the biopsy. 3. Hypercholesterolaemic subjects had either never been treated with lipid lowering therapy or therapy had been stopped at least two weeks before the study (n = 2). At the time of the study total plasma cholesterol levels (control: 4.6+/-0.3 vs hypercholesterolaemic: 8.3+/-0.6 mmol l(-1): P < 0.01) were significantly elevated in hypercholesterolaemic subjects when compared with controls. 4. Full concentration-response curves to the vasoconstrictor noradrenaline and the vasodilators acetylcholine and substance P were constructed. A single point concentration-response to sodium nitroprusside (10 microM) was also obtained. Dilator responses were obtained in vessels pre-constricted with a submaximal concentration of noradrenaline. Vessels were then incubated for 30 min with either L- or D-arginine (10 microM) and the concentration-response curves to the three dilator agonists repeated in the presence of the amino acid. 5. Maximum relaxation responses to acetylcholine (control vs hypercholesterolaemic: 83.3+/-6.1% vs 47.4+/-13.5%; P < 0.05), but not to substance P or sodium nitroprusside, were dampened in the hypercholesterolaemic group when compared with controls. 6. Neither incubation with L-arginine nor D-arginine had any effect on maximum relaxation responses to acetylcholine in either the control group (pre L-arginine vs plus L-arginine: 83.3+/-6.1 vs 82.3+/-5.5%, pre D-arginine vs plus D-arginine: 98.9+/-1.2 vs 98.2+/-1.1%) or the hypercholesterolaemic group (pre L-arginine vs plus L-arginine: 47.4+/-13.5 vs 55.3+/-14.3%, pre D-arginine vs plus D-arginine: 43.3+/-13.6 vs 65.4+/-12.3%). 7. When results from the two study groups were pooled, the strongest predictor of maximum relaxation obtained to acetylcholine was apolipoprotein A1 (r = 0.67; P = 0.001). 8. In conclusion, relaxation responses mediated by the endothelium-dependent agonist acetylcholine, but not by substance P, are impaired in hypercholesterolaemic patients. L-Arginine did not improve the impaired relaxation responses to acetylcholine. We suggest that impaired endothelium-dependent relaxation is specific to acetylcholine and not to an abnormal L-arginine-nitric oxide pathway in subcutaneous arteries excised from this study group.

The vascular activity of some isoflavone metabolites: implications for a cardioprotective role.

Legume-derived isoflavones such as genistein, diadzein and equol have been associated with a reduction in risk of cardiovascular disease. In the current study, we explore the vascular activity of several isoflavone metabolites namely dihydrodaidzein, cis and trans-tetrahydrodaidzein and dehydroequol for potential cardioprotective properties. Rat isolated aortic rings were used. 17beta-oestradiol, equol, and all four of the metabolites studied significantly antagonized contractile responses to noradrenaline. The direct vasodilatory action of these compounds were examined and in contrast to 17beta-oestradiol, the vasodilatory effect of which was demonstrated to be endothelium independent, the dilatory action of all four compounds could be inhibited by endothelium denudation. Further, the dilatory action of both dihydrodaidzein and cis-tetrahydrodaidzein were inhibited by the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine (NOLA), by the soluble guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and by 40 mM KCl. Dilatory responses to dehydroequol and trans-tetrahydrodaidzein, on the other hand, were inhibited by 40 mM KCL but not by NOLA nor ODQ. Finally, we examined the protective potential of these compounds in inhibiting endothelium damage by oxidized low density lipoprotein (ox-LDL). Trans-tetrahydrodaidzein was at least 10 fold more potent than 17beta-oestradiol in protecting against ox-LDL induced damage. We conclude that the isoflavone metabolites, dihydrodaidzein, cis- and trans-tetrahydrodaidzein and dehydroequol, may potentially represent a novel series of cardioprotective therapeutics.

Exercise training increases basal nitric oxide production from the forearm in hypercholesterolemic patients.

The objective of this study was to investigate the effects of cycle training on basal nitric oxide (NO) production and endothelium-dependent dilator capacity in hypercholesterolemic patients in whom acetylcholine responsiveness is impaired. Nine sedentary hypercholesterolemic volunteers (total plasma cholesterol >6.0 mmol/L; 2 female) aged 44+/-3 years (mean+/-SEM) participated in the study. Subjects remained sedentary for 4 weeks and performed 4 weeks of home-based cycle training (3 x 30 minutes/week at 65% maximum oxygen consumption [VO(2)max]) in a randomized order. Arteriovenous nitrate/nitrite (NO(x)) gradient was assessed and plethysmography was used to measure the forearm blood flow responses to arterial infusions of acetylcholine, sodium nitroprusside, and N(G)mono methyl L-arginine. Training increased VO(2)max from 30.4+/-1.9 to 34.3+/-1.4 mL x kg(-1) x min(-1) (P=0.01). Intrabrachial diastolic blood pressure was reduced from 70+/-3 to 68+/-3 mm Hg (P=0.02) with training, whereas systolic pressure did not change. Plasma triglycerides and total, LDL, and HDL cholesterol were not different between interventions. In the sedentary state, there was a positive forearm arteriovenous difference in plasma NO(x) indicating net extraction (6.8+/-4.0 nmol x 100 mL(-1) x min(-1)), whereas in the trained state this difference was negative, indicating net production (-5.8+/-5.8 nmol x 100 mL(-1) x min(-1); P=0.03). N(G)mono methyl L-arginine, at a dose of 4 micromol/min, caused a greater vasoconstriction after training (79.6+/-3.4% versus 69.9+/-6.8%; P=0.05). Acetylcholine and sodium nitroprusside induced dose-dependent elevations in forearm blood flow that were unaffected by training. These data suggest that basal release of endothelium-derived NO is increased with 4 weeks of home based training in hypercholesterolemic patients, independently of lipid profile modification. This may contribute to the cardiovascular protective effects of exercise training, including reduced blood pressure.

Modulation of vascular function by diet and exercise.

Clinical research is conducted in free living individuals who are always subject to the influences on vascular function and the major cardiovascular regulators of their lifestyle. The purpose of this paper is to review some lifestyle influences on cardiovascular function, particularly the sympathetic nervous system and endothelially mediated vasodilatation. There are highly differentiated sympathetic responses to feeding, and to acute exercise. Over a longer period obesity has a typical pattern of sympathetic activity. Reduced dietary salt intake elicits profound localised increases in sympathetic activity to the kidney. Marine oil supplementation attenuates the sympathetic responses to psychological stress and improves endothelially mediated vasodilatation in hypercholesterolaemics. Exercise training reduced total noradrenaline spillover, the major beds affected being the renal and skeletal muscle. These examples illustrate the dynamic nature of vascular dilatation and that, like the sympathetic nervous system, it is modulated by short, medium and long term influences. In both cases there is regulation both at a local and systemic level. Habitual, and recent, lifestyle can exert important cardiovascular effects which must be taken into account in clinical and epidemiological research.