Diseases of the cardiovascular system are the leading cause of morbidity and mortality in men and women in developed countries, and CVD is becoming more prevalent in developing countries. Atherosclerotic cardiovascular disease (CVD) prevalence in men is greater than in women until menopause, when the prevalence of CVD increases in women until it exceeds that of men. Endothelial function is a barometer of vascular health and a predictor of atherosclerosis that may provide insights into sex differences in CVD, as well as how and why CVD risk drastically changes with menopause. Studies of sex differences in endothelial function are conflicting, with some studies showing earlier decrements in endothelial function in men compared to women, while others showing similar age-related declines between the sexes. Because the increase in CVD risk coincides with menopause, it is generally thought that female hormones, in particular estrogens, are cardioprotective. Further, it is often proposed that androgens are detrimental. In truth, the relationships are more complex. This review will first address female and male sex hormones and their receptors, and how these interact with the cardiovascular system, particularly the endothelium, in healthy young women and men. Second, we will address sex differences in sex steroid receptor-independent mechanisms controlling endothelial function, focusing on the vascular endothelin and the renin-angiotensin systems, in healthy young women and men. Finally, we will discuss sex differences in age-associated endothelial dysfunction, focusing on the role of attenuated circulating sex-hormones in these effects.

Local skin heating is used to assess microvascular function in clinical populations because NO is required for full expression of the response; however, controversy exists as to the precise NO synthase (NOS) isoform producing NO. Human aging is associated with attenuated cutaneous vasodilation but little is known about the middle aged, an age cohort used for comparison with clinical populations. We hypothesized that endothelial NOS (eNOS) is the primary isoform mediating NO production during local heating, and eNOS-dependent vasodilation would be reduced in middle-aged skin. Vasodilation was induced by local heating (42°C) and during acetylcholine dose-response (ACh-DR: 0.01, 0.1, 1.0, 5.0, 10.0, 50.0, 100.0 mmol/l) protocols. Four microdialysis fibers were placed in the skin of 24 men and women; age cohorts were 12 middle-aged (53 ± 1 yr) and 12 young (23 ± 1 yr). Sites served as control, nonselective NOS inhibited [N(G)-nitro-l-arginine methyl ester (l-NAME)], inducible NOS (iNOS) inhibited (1400W), and neuronal NOS (nNOS) inhibited (N(ω)-propyl-l-arginine). After full expression of the local heating response, l-NAME was perfused at all sites. Cutaneous vascular conductance was measured and normalized to maximum (%CVC(max): Nitropress). l-NAME reduced %CVCmax at baseline, all phases of the local heating response, and at all ACh concentrations compared with all other sites. iNOS inhibition reduced the initial peak (53 ± 2 vs. 60 ± 2%CVC(max); P < 0.001); however, there were no other differences between control, nNOS-, and iNOS-inhibited sites during the phases of local heating or ACh-DR. When age cohorts were compared, NO-dependent vasodilation during local heating (52 ± 6 vs. 68 ± 4%CVC(max); P = 0.013) and ACh perfusion (50 mmol/l: 83 ± 3 vs. 93 ± 2%CVC(max); 100 mmol/l: 83 ± 4 vs. 92 ± 3%CVC(max); both P = 0.03) were reduced in middle-aged skin. There were no differences in NOS isoform expression obtained from skin biopsy samples between groups (all P > 0.05). These data suggest that eNOS mediates the production of NO during local heating and that cutaneous vasodilation is attenuated in middle-aged skin.

Essential hypertension is a pro-inflammatory, pro-constrictor disease coinciding with endothelial dysfunction and inward vessel remodeling. Using the skin circulation our aim was to determine if iNOS upregulation attenuates NO-dependent cutaneous vasodilation in hypertensive humans. We hypothesized that with hypertension (1) localized iNOS inhibition would restore vasodilation in response to NO-dependent stimuli and (2) iNOS expression would be increased and phosphorylated vasodilator-stimulated phosphoprotein (pVASP) would be decreased. In vivo protocols: four intradermal microdialysis (MD) fibers were placed in 9 hypertensive and 10 normotensive (SBP: 146 ± 4 vs.113 ± 2 mmHg, p<0.001) men and women. MD fibers served as control, iNOS-inhibited (1400W), nNOS-inhibited (NPLA), and non-selective NOS-inhibited (L-NAME). Cutaneous vascular conductance was calculated (%CVCmax; sodium nitroprusside) during standardized local heating (42°C) and acetylcholine (ach) dose-response protocols (0.01, 0.1, 1, 5, 10, 50, 100 mmol/L). The NO-dependent local heating response was attenuated at control (95 ± 2 vs. 76 ± 2 %CVCmax, p<0.05) and nNOS-inhibited sites (94 ± 4 vs. 77 ± 3 %CVCmax, p<0.01) in hypertensives. iNOS inhibition augmented the NO-dependent local heating response (93 ± 2 vs. 89 ± 10 %CVCmax). Ach-induced vasodilation was attenuated in control sites at doses ≥ 0.1mM Ach in hypertensives, and was restored with iNOS inhibition (0.1 mM p<0.05; 1, 5, 10 mM p<0.001; 50, 100 mM p<0.01). In vitro iNOS expression was increased (p=0.006) and pVASP/VASP was decreased in skin from hypertensive humans (p=0.04). These data suggest that iNOS is upregulated in essential hypertensive humans and contributes to reduced NO-dependent cutaneous vasodilation.

Key pointsr The reduction in skin blood flow during whole-body cooling is impaired in healthy older adults. However, the relative contributions of altered skin sympathetic nerve activity (SSNA), transduction of this efferent neural outflow to the cutaneous vasculature, and peripheral vascular responsiveness to adrenergic stimuli to the impaired reflex vasoconstrictor response to whole-body cooling in human ageing remain unclear.r We report that the SSNA response to whole-body cooling is blunted in healthy older adults, and this attenuated sympathetic response is related to a marked impairment in reflex cutaneous vasoconstriction. Further, the reflex SSNA response to a non-thermoregulatory stimulus was preserved in older adults during cooling.r We additionally show that cutaneous vascular responsiveness to adrenergic stimuli is not reduced in older adults.r These results further our understanding of the physiological mechanisms underlying impaired thermal-cardiovascular integration in healthy ageing.Abstract Reflex cutaneous vasoconstriction is impaired in older adults; however, the relative roles of altered skin sympathetic nerve activity (SSNA) and end-organ peripheral vascular responsiveness are unclear. We hypothesized that in older adults whole-body cooling would elicit a blunted SSNA response and cutaneous adrenergic responsiveness would be reduced. Twelve young adults (Y; 24 ± 1 years) and 12 older adults (O; 57 ± 2 years) participated in two protocols. In Protocol 1, SSNA (peroneal microneurography) and red cell flux in the affected dermatome (laser Doppler flowmetry; dorsum of foot) were measured during whole-body cooling (mean skin temperature (T sk ) 30.5°C; water-perfused suit). Mental stress was performed at mean T sk 34.0°C (thermoneutral) and at 30.5°C. In Protocol 2, an intradermal microdialysis fibre was placed in the skin of the lateral calf for graded infusions of noradrenaline (norepinephrine) (NA; 10 −12 to 10 −2 M). Cutaneous vascular conductance (CVC = flux/mean arterial pressure) was expressed as a change from baseline ( CVC base ). Vasoconstriction was attenuated in O. SSNA increased significantly during cooling in Y (+184 ± 37%; P < 0.05) but not O (+51 ± 12%; P > 0.05). Mental stress at T sk 30.5°C further increased SSNA in both groups. There was no age-related difference in adrenergic responsiveness to exogenous NA (logEC 50 : −6.41 ± 0.24 in Y, −6.37 ± 0.25 in O; P > 0.05). While the SSNA response to whole-body cooling is impaired with ageing, SSNA can be further increased by a non-thermoregulatory stimulus. Cutaneous adrenergic sensitivity is not reduced in O. These findings suggest that alterations in afferent signalling or central processing likely contribute to blunted SSNA responses to cooling and subsequent impairments in reflex cutaneous vasoconstriction in ageing. Abbreviations BP, blood pressure; CVC, cutaneous vascular conductance; MAP, mean arterial pressure; NA, noradrenaline (norepinephrine); SSNA, skin sympathetic nerve activity; T sk , skin temperature.

Women who have had preeclampsia have increased CVD risk, however the mechanism(s) responsible for this association remain unclear. Microvascular damage sustained during a preeclamptic pregnancy may persist post-partum. The putative mechanisms mediating this dysfunction include a reduction in nitric oxide (NO)-dependent dilation, and an increased sensitivity to angiotensin II (ang II). In this study, we evaluated endothelium-dependent dilation, ang II sensitivity, and the therapeutic effect of ang II receptor blockade (losartan) on endothelium-dependent dilation in vivo in the microvasculature of women with a history of preeclampsia (PrEC, n=12) and control women who had a healthy pregnancy (HC, n=12). We hypothesized that PrEC would have 1) reduced endothelium-dependent dilation, 2) reduced NO-mediated dilation, and 3) increased sensitivity to ang II. We further hypothesized that localized losartan would increase endothelium-dependent vasodilation in PrEC. We assessed microvascular endothelium-dependent vasodilator function by measurement of cutaneous vascular conductance responses to graded infusion of acetylcholine (ACh; 10−7–102mmol/L) and a standardized local heating protocol in control sites and sites treated with 15mmol/L L-NAME (NO-synthase inhibitor) or 43μmol/L losartan. Further, we assessed microvascular vasoconstrictor sensitivity to ang II (10−20–10−4mol/L). PrEC had significantly reduced endothelium-dependent dilation (−0.3±0.5 vs −1.0±0.4 logEC50; P<0.001) and NO-dependent dilation (16±3 vs 39±6%; P=0.006). PrEC also had augmented vasoconstrictor sensitivity to ang II (−10.2±1.3 vs −8.3±0.5; P=0.006). AT1R-inhibition augmented endothelium-dependent vasodilation and NO-dependent dilation in PrEC but had no effect in HC. These data suggest that women who have had preeclampsia have persistent microvascular dysfunction post-partum, mediated in part by increased sensitivity to ang II.

Folic acid is a member of the B-vitamin family and is essential for amino acid metabolism. Adequate intake of folic acid is vital for metabolism, cellular homeostasis, and DNA synthesis. Since the initial discovery of folic acid in the 1940s, folate deficiency has been implicated in numerous disease states, primarily those associated with neural tube defects in utero and neurological degeneration later in life. However, in the past decade, epidemiological studies have identified an inverse relation between both folic acid intake and blood folate concentration and cardiovascular health. This association inspired a number of clinical studies that suggested that folic acid supplementation could reverse endothelial dysfunction in patients with cardiovascular disease (CVD). Recently, in vitro and in vivo studies have begun to elucidate the mechanism(s) through which folic acid improves vascular endothelial function. These studies, which are the focus of this review, suggest that folic acid and its active metabolite 5-methyl tetrahydrofolate improve nitric oxide (NO) bioavailability by increasing endothelial NO synthase coupling and NO production as well as by directly scavenging superoxide radicals. By improving NO bioavailability, folic acid may protect or improve endothelial function, thereby preventing or reversing the progression of CVD in those with overt disease or elevated CVD risk.

Reflex cutaneous vasodilation in response to passive heating is attenuated in human aging. This diminished response is mediated, in part, by age-associated reductions in endothelial function; however, the contribution of altered skin sympathetic nervous system activity (SSNA) is unknown. We hypothesized that 1) healthy older adults would demonstrate blunted SSNA responses to increased core temperature compared with young adults and 2) the decreased SSNA response would be associated with attenuated cutaneous vasodilation. Reflex vasodilation was elicited in 13 young [23 ± 1 (SE) yr] and 13 older (67 ± 2 yr) adults using a water-perfused suit to elevate esophageal temperature by 1.0°C. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry) in the innervated dermatome (the dorsum of foot) were continuously measured. SSNA was normalized to, and expressed as, a percentage of baseline. Cutaneous vascular conductance (CVC) was calculated as flux/mean arterial pressure and expressed as a percentage of maximal CVC (local heating, 43°C). Reflex vasodilation was attenuated in older adults (P < 0.001). During heating, SSNA increased in both groups (P < 0.05); however, the response was significantly blunted in older adults (P = 0.01). The increase in SSNA during heating was linearly related to cutaneous vasodilation in both young (R = 0.87 ± 0.02, P < 0.01) and older (R = 0.76 ± 0.05, P < 0.01) adults; however, slope of the linear regression between ΔSSNA and ΔCVC was reduced in older compared with young (older: 0.05 ± 0.01 vs. young: 0.08 ± 0.01; P < 0.05). These data demonstrate that age-related impairments in reflex cutaneous vasodilation are mediated, in part, by blunted efferent SSNA during hyperthermia.

Functional constitutive nitric oxide synthase (NOS) is required for full expression of reflex cutaneous vasodilation that is attenuated in aged skin. Both the essential cofactor tetrahydrobiopterin (BH(4)) and adequate substrate concentrations are necessary for the functional synthesis of nitric oxide (NO) through NOS, both of which are reduced in aged vasculature through increased oxidant stress and upregulated arginase, respectively. We hypothesized that acute local BH(4) administration or arginase inhibition would similarly augment reflex vasodilation in aged skin during passive whole body heat stress. Four intradermal microdialysis fibers were placed in the forearm skin of 11 young (22 ± 1 yr) and 11 older (73 ± 2 yr) men and women for local infusion of 1) lactated Ringer, 2) 10 mM BH(4), 3) 5 mM (S)-(2-boronoethyl)-l-cysteine + 5 mM N(ω)-hydroxy-nor-l-arginine to inhibit arginase, and 4) 20 mM N(G)-nitro-l-arginine methyl ester (l-NAME) to inhibit NOS. Red cell flux was measured at each site by laser-Doppler flowmetry (LDF) as reflex vasodilation was induced. After a 1.0°C rise in oral temperature (T(or)), mean body temperature was clamped and 20 mM l-NAME was perfused at each site. Cutaneous vascular conductance was calculated (CVC = LDF/mean arterial pressure) and expressed as a percentage of maximum (%CVC(max); 28 mM sodium nitroprusside and local heat, 43°C). Vasodilation was attenuated at the control site of the older subjects compared with young beginning at a 0.3°C rise in T(or). BH(4) and arginase inhibition both increased vasodilation in older (BH(4): 55 ± 5%; arginase-inhibited: 47 ± 5% vs. control: 37 ± 3%, both P < 0.01) but not young subjects compared with control (BH(4): 51 ± 4%CVC(max); arginase-inhibited: 55 ± 4%CVC(max) vs. control: 56 ± 6%CVC(max), both P > 0.05) at a 1°C rise in T(or). With a 1°C rise in T(or), local BH(4) increased NO-dependent vasodilation in the older (BH(4): 31.8 ± 2.4%CVC(max) vs. control: 11.7 ± 2.0%CVC(max), P < 0.001) but not the young (BH(4): 23 ± 4%CVC(max) vs. control: 21 ± 4%CVC(max), P = 0.718) subject group. Together these data suggest that reduced BH(4) contributes to attenuated vasodilation in aged human skin and that BH(4) NOS coupling mechanisms may be a potential therapeutic target for increasing skin blood flow during hyperthermia in older humans.