Variations in endothelial function and arterial compliance during the menstrual cycle

Age and regional specificity of peak limb vascular conductance in women

The influence of age on limb vasodilator capacity in women is unclear. The objectives of this study were to characterize and compare age-associated changes in forearm and calf peak vascular conductance (VC(peak); a functional index of arterial structure) in women and to identify physiological characteristics predictive of variation in limb-specific VC(peak). Peak conductance (plethysmographic flow/mean arterial pressure), VC(peak) of the forearm (forearm VC(peak)), and calf (calf VC(peak)) after 10 min of arterial occlusion were measured in 58 healthy, normally active women aged 21-79 yr. Aerobic capacity (cycle peak oxygen uptake), arterial health (pulse-wave velocity, ankle-brachial index), total cholesterol, limb-specific tissue composition (dual-energy X-ray absorptiometry), and isometric strength (handgrip, plantar flexion) were also assessed. The relative decline in calf VC(peak) with age (-6.8% per decade, P < 0.001) was greater than the forearm (-4.4% per decade, P = 0.004), in contrast to results previously reported for men (forearm decline > calf decline). Limb VC(peak) per kilogram muscle declined with age in the calf (-6.0% per decade; P = 0.002), but not the forearm (P = 0.12). Age, cholesterol, and regional tissue composition were significant predictors of peak conductance in both limbs; however, age was a stronger predictor of peak conductance in the calf. These results suggest that healthy aging is associated with a linear decline in limb vasodilator capacity in women, but the magnitude of this effect is region specific. Further research will be required to determine whether the decline in lower extremity vasodilator capacity with age explains diminished exercising leg vasodilation in older women.

Intrasubject variability of radial artery flow-mediated dilatation in healthy subjects and implications for use in prospective clinical trials

Flow-mediated dilation (FMD) in the brachial artery is a widely used end point in clinical trials despite large within-subject variability and a small dynamic range. Recent studies suggest that the radial artery may be more advantageous for investigating FMD because of an enhanced vasodilator response. This study therefore assessed the validity and repeatability of radial artery FMD (FMD-R) to evaluate its suitability for the noninvasive evaluation of endothelial function. Thirty-three healthy subjects were recruited over a period of 11 months. Intra- and inter-reader reproducibilities were measured with high-resolution ultrasound at 4 time points: twice 1 morning (1 to 3 hours apart) and twice again within 7 days (range 4 to 9 days between visits). Conduit endothelial-dependent and -independent vasomotion were assessed by responses to reactive hyperemia and nitroglycerin, respectively. FMD-R measurements demonstrated significant intra- and interday variabilities (intraclass correlation coefficients [ICCs] 0.38 and 0.23, p = 0.04 and 0.12, respectively). Bland-Altman plots confirmed the test-retest variation in FMD-R. In contrast, radial artery diameter measurements (intra- and inter-reader) demonstrated a high degree of repeatability (interstudy ICC >0.8, p <0.0001). The number of subjects needed to detect a treatment difference of 2% in FMD-R with a p value of 0.05 and a power of 0.80 would be 118 in a crossover design and 234 in a parallel design for assessing group changes. In conclusion, these findings show that FMD-R is highly variable within subjects, even in a healthy population, after adjusting for multiple technical factors and implies biologic variation in radial artery tone.

Sex steroids modulate human aortic smooth muscle cell matrix protein deposition and matrix metalloproteinase expression

Large artery stiffening increases cardiovascular risk and promotes isolated systolic hypertension which is more prevalent in elderly women than men. Variation in sex steroid levels between males and females and throughout life may modulate arterial stiffness. We hypothesized that sex steroids directly influence expression of important structural proteins which determine arterial biomechanical properties. Human aortic smooth muscle cells were incubated with physiological concentrations of 17beta-estradiol, progesterone, 17beta-estradiol and progesterone, or testosterone for 4 weeks. Collagen, elastin, and fibrillin-1 deposition was examined (histochemistry/immunohistochemistry). Gene and protein expression of 2 important matrix metalloproteinases (MMPs), MMPs 2 and 3, regulating matrix turnover was assessed. All sex steroids reduced collagen deposition relative to control (100%). However, the reduction was greater with female sex steroids than testosterone (control, 100%; 17beta-estradiol plus progesterone, 20+/-2%; testosterone 74+/-12%, P<0.001). Female sex steroids increased elastin deposition compared with control (control, 100%; 17beta-estradiol, 540+/-60%; progesterone, 290+/-40%; 17beta-estradiol plus progesterone, 400+/-80%, all P<0.01). The elastin/collagen ratio was >11-fold higher in the presence of 17beta-estradiol and progesterone compared with testosterone. Fibrillin-1 deposition was doubled in the presence of female sex steroids (17beta-estradiol plus progesterone) compared with testosterone (P<0.01). MMP-2 gene and protein expression was unaffected by any sex steroid. Testosterone increased both gene and protein expression of MMP-3 relative to both control and female sex steroids (P<0.01). This may contribute to degradation of elastic matrix proteins. In conclusion, female sex steroids promote an elastic matrix profile, which likely contributes to variation in large artery stiffness observed between sexes and with changes in hormonal status across the lifespan.

Endothelial function in highly endurance-trained and sedentary, healthy young women

Endothelial function is reduced by age, chronic heart failure, coronary artery disease, hypertension or type 2 diabetes, and it is shown that aerobic exercise may reverse this trend. The effect of a high aerobic training status on endothelial function in young, healthy subjects is however less clear. The present study was designed to determine whether endothelial function is improved in highly endurance-trained young women compared to sedentary, healthy controls. Brachial artery diameter was measured in 16 endurance-trained (age: 23.7 +/- 2.5 years, maximal oxygen uptake (VO2max): 60.6 +/- 4.5 ml/kg per min) and 14 sedentary females (age: 23.7 +/- 2.1 years, VO2max: 40.5 +/- 5.6 ml/kg per min) at rest, during flow-mediated dilation (FMD) and after sublingual glycerol trinitrate administration, using high-resolution ultrasound. FMD did not differ between the endurance-trained and the sedentary females (14.8% vs 16.4%, p = NS), despite a substantial difference in VO2max of 50% (p < 0.001). The endurance-trained group possessed however, a 9% larger resting brachial artery diameter when adjusted for body surface area. The results of the present study suggest that endothelial function is well preserved in young, healthy women, and that a high aerobic training status due to long term aerobic training does not improve the dilating capacity any further.

Variation in non-invasive measurements of vascular function in healthy volunteers during daytime

Although it is often recommended to standardize the time of day when performing non-invasive measurements of vascular function, the exact influence of the time of day on the outcome of IMT (intima-media thickness), PWV (pulse wave velocity), AIX (augmentation index) and FMD (flow-mediated dilatation) measurements has not been reported before. Nineteen healthy volunteers visited our department on two different occasions: the first visit was at 09:00 hours after an overnight fast, and the second visit was at 14:00 hours after a standardized breakfast. Non-invasive measurements of atherosclerosis were performed twice at 09:00 hours and once on the second visit at 14:00 hours. Measurement of IMT, PWV, AIX and FMD was reproducible according to the method of Bland and Altman. The absolute difference between repeated measurements at 09:00 hours showed no significant difference compared with the absolute difference between 09:00 and 14:00 hours for IMT (0.029+/-0.014 compared with 0.021+/-0.014 mm; P = 0.27), PWV (0.63+/-0.50 compared with 0.75+/-0.74 m/s; P = 0.52), AIX (4.0+/-4.0 compared with 5.5+/-5.2%; P = 0.35) and FMD (3.8+/-3.7 compared with 4.2+/-2.9%; P = 0.70). In conclusion, our results show that, in healthy volunteers during the daytime, IMT, PWV, AIX and FMD outcomes are not confounded by variation in the exact time of the examination as long as other (exogenous) conditions, including food intake, smoking and intake of alcohol, are carefully controlled for.

Estrogen Induces Vascular Wall Dilation

Estrogen has been shown to affect vascular cell and arterial function in vitro and in vivo. Here we examined the ability of estradiol (E(2)) to cause rapid arterial dilation of elastic and muscular arteries in vivo and the mechanisms involved. E(2) administration caused a rapid increase in the outer wall diameter of both types of arteries in ovariectomized female mice. This resulted from estrogen receptor (ER)-mediated stimulation of nitric oxide production, demonstrated by preinjecting the mice arteries with a soluble inhibitor of nitric oxide (monomethyl l-arginine) and by showing the absence of E(2) action in eNOS-/- mice. Rapid activation of both ERK/MAP kinase and phosphatidylinositol 3-kinase activity was found in the E(2)-exposed arteries, and inhibiting either kinase prevented the vasodilatory action of E(2). Kinase activation and vasodilator responses to E(2) were absent in either ERalpha or ERbeta knock-out mice, implicating both receptor subtypes as mediating this E(2) action. These results indicate that E(2) modulation of arterial tonus through plasma membrane ER and rapid signaling could underlie many previously observed actions of estrogen reported to occur in women.

A review of methods currently used for assessment of in vivo endothelial function

An intact vascular endothelium is critical to the maintenance of normal arterial tone and coagulation status. Endothelial injury leading to dysfunction is thought to be a precursor to most if not all vascular disease, and has been implicated as a critical event in atherosclerosis. At present there are several methods available for detection of in vivo endothelial function, and the aim of this study was to critically review these methods. Five distinct methods were identified and studied in detail. These methods are diverse and each assesses a different vascular bed. Importantly there is no uniformity among investigators over choice of method and protocol, making it difficult to compare in vivo enothelial dysfunction between groups. These issues need to be addressed in large scale comparative analyses so that investigators can agree a common approach to endothelial function assessment.

Endogenous Estrogens Influence Endothelial Function in Young Men

Males produce endogenous estrogen from testosterone via the enzyme aromatase. Previous studies have suggested a role for endogenous estrogens in cardiovascular function in men. We examined the effects of endogenous estrogen suppression via aromatase inhibition on endothelial function, systemic arterial compliance, and lipoprotein levels in healthy young men. Using a placebo-controlled double-blind randomized design, 20 healthy men, aged 18 to 32 years, were randomized to receive either the aromatase inhibitor anastrozole (1 mg) or matching placebo. Hormone, lipid levels, C-reactive protein (CRP), and homocysteine were measured. Endothelial function, determined by flow-mediated dilation of the brachial artery, and systemic arterial compliance were assessed at baseline and after 6 weeks of treatment. There was a significant decrease in 17beta-estradiol concentrations with aromatase inhibition, from 85.4+/-4.2 to 64.3+/-8.1 pmol/L (mean+/-SD, P=0.042). Compared with baseline, a significant decrease in flow-mediated dilation was observed in subjects taking anastrozole [median, 6.1% (range, 5.2 to 13.4) to 3.5% (2.0 to 5.7), P=0.034] but not in the placebo group. No changes were observed in nitroglycerin-induced endothelium-independent dilation in either group. There was no change in systemic arterial compliance with either aromatase therapy or placebo. There were no significant changes in lipoproteins, testosterone, DHEA, CRP, or homocysteine levels in either the anastrozole or placebo group. We conclude that suppression of endogenous estrogens with an aromatase inhibitor resulted in impairment of flow-mediated dilation without significant changes in lipoproteins, homocysteine, or CRP. Our results suggest that endogenous estrogens play a direct regulatory role in endothelial function in young healthy men.

Gender differences in large artery stiffness pre- and post puberty

Age-related large artery stiffening is more pronounced in women compared with men and is an important cause of isolated systolic hypertension. This study aimed to investigate whether such gender differences are inherent or the result of sex steroid influences. Healthy children prepuberty [26 female (10.3 +/- 0.1 yr), 32 male (10.3 +/- 0.1 yr), mean age +/- SD] and post puberty [30 female (15.9 +/- 0.2 yr), 22 male (15.9 +/- 0.4 yr)] were studied. Large artery stiffness was assessed globally via systemic arterial compliance and regionally via pulse wave velocity. Prepubertal males and females did not differ in body size, cardiac output, or heart rate. Prepubertal females had stiffer large arteries and higher pulse pressure than age-matched males (P < 0.05). Postpubertal males were taller and heavier and had a greater cardiac output and lower heart rate compared with similarly aged females. In relation to pubertal status, females developed more distensible large arteries post puberty whereas males developed stiffer large vessels (P < 0.05). These changes where such that central large artery stiffness was similar between genders in the postpubertal group. Together these data suggest that large artery stiffness varies intrinsically between genders but is also modulated by both male and female sex steroids.