Jennifer R. Vranish

Endothelial dysfunction following prolonged sitting is mediated by a reduction in shear stress

We and others have recently reported that prolonged sitting impairs endothelial function in the leg vasculature; however, the mechanism(s) remain unknown. Herein, we tested the hypothesis that a sustained reduction in flow-induced shear stress is the underlying mechanism by which sitting induces leg endothelial dysfunction. Specifically, we examined whether preventing the reduction in shear stress during sitting would abolish the detrimental effects of sitting on popliteal artery endothelial function. In 10 young healthy men, bilateral measurements of popliteal artery flow-mediated dilation were performed before and after a 3-h sitting period during which one foot was submerged in 42°C water (i.e., heated) to increase blood flow and thus shear stress, whereas the contralateral leg remained dry and served as internal control (i.e., nonheated). During sitting, popliteal artery mean shear rate was reduced in the nonheated leg (pre-sit, 42.9 ± 4.5 s(-1); and 3-h sit, 23.6 ± 3.3 s(-1); P < 0.05) but not in the heated leg (pre-sit, 38.9 ± 3.4 s(-1); and 3-h sit, 63.9 ± 16.9 s(-1); P > 0.05). Popliteal artery flow-mediated dilation was impaired after 3 h of sitting in the nonheated leg (pre-sit, 7.1 ± 1.4% vs. post-sit, 2.8 ± 0.9%; P < 0.05) but not in the heated leg (pre-sit: 7.3 ± 1.5% vs. post-sit, 10.9 ± 1.8%; P > 0.05). Collectively, these data suggest that preventing the reduction of flow-induced shear stress during prolonged sitting with local heating abolishes the impairment in popliteal artery endothelial function. Thus these findings are consistent with the hypothesis that sitting-induced leg endothelial dysfunction is mediated by a reduction in shear stress.

Influence of sex on microvascular and macrovascular responses to prolonged sitting

Increased daily sitting time is associated with greater cardiovascular risk, and, on average, women are more sedentary than men. Recent reports have demonstrated that prolonged sitting reduces lower leg microvascular (reactive hyperemia) and macrovascular [flow-mediated dilation (FMD)] vasodilator function. However, these studies have predominately included men, and the effects of sitting in young women are largely unexplored. This becomes important given known sex differences in vascular function. Thus, herein, we assessed popliteal artery reactive hyperemia and FMD before and after a 3-h sitting period in healthy young women (n = 12) and men (n = 8). In addition, resting popliteal artery hemodynamics (duplex Doppler ultrasound) and calf circumference were measured before, during, and after sitting. Resting popliteal artery shear rate was reduced to a similar extent in both groups during the sitting period (women: -48.5 ± 8.4 s-1 and men: -52.9 ± 12.3 s-1, P = 0.45). This was accompanied by comparable increases in calf circumference in men and women (P = 0.37). After the sitting period, popliteal artery FMD was significantly reduced in men (PreSit: 5.5 ± 0.9% and PostSit: 1.6 ± 0.4%, P < 0.001) but not women (PreSit: 4.4 ± 0.6% and PostSit: 3.6 ± 0.6%, P = 0.29). In contrast, both groups demonstrated similar reductions in hyperemic blood flow area under the curve (women: -28,860 ± 5,742 arbitrary units and men: -28,691 ± 9,685 arbitrary units, P = 0.99), indicating impaired microvascular reactivity after sitting. These findings indicate that despite comparable reductions in shear rate during 3 h of uninterrupted sitting, macrovascular function appears protected in some young women but the response was variable, whereas men exhibited more consistent reductions in FMD. In contrast, the leg microvasculature is susceptible to similar sitting-induced impairments in men and women.NEW & NOTEWORTHY We demonstrate that leg macrovascular function was consistently reduced in young men but not young women after prolonged sitting. In contrast, both men and women exhibited similar reductions in leg microvascular reactivity after sitting. These data demonstrate, for the first time, sex differences in vascular responses to prolonged sitting.

Exaggerated Vasoconstriction to Spontaneous Bursts of Muscle Sympathetic Nerve Activity in Healthy Young Black MenNovelty and Significance

Blacks have the highest prevalence of hypertension, putting them at greater risk of cardiovascular disease and death. Previous studies have reported that, relative to whites, healthy black men have augmented pressor responses to sympathoexcitatory stressors. Although important, these studies do not inform about the resting state and the influence of spontaneous changes in resting muscle sympathetic nerve activity (MSNA). Likewise, little is known about the transduction of MSNA into a vascular response at rest on a beat-to-beat basis. Accordingly, we tested the hypothesis that relative to whites, blacks would exhibit greater vasoconstriction and pressor responses following spontaneous bursts of MSNA. Mean arterial pressure, common femoral artery blood flow, and MSNA were continuously recorded during 20 minutes of supine rest in 35 young healthy men (17 blacks and 18 whites). Signal averaging was used to characterize changes in leg vascular conductance, total vascular conductance, and mean arterial pressure following spontaneous MSNA bursts. Blacks demonstrated significantly greater decreases in leg vascular conductance (blacks: −15.0±1.0%; whites: −11.5±1.2%; P=0.042) and total vascular conductance (blacks: −8.6±0.9%; whites: −5.1±0.4%; P=0.001) following MSNA bursts, which resulted in greater mean arterial pressure increases (blacks: +5.2±0.6 mm Hg; whites: +3.9±0.3 mm Hg; P=0.04). These exaggerated responses in blacks compared with whites were present whether MSNA bursts occurred in isolation (singles) or in combination (multiples) and were graded with increases in burst height. Collectively, these findings suggest that healthy young black men exhibit augmented sympathetic vascular transduction at rest and provide novel insight into potential mechanism(s) by which this population may develop hypertension later in life.African Americans (AA) have the highest prevalence of hypertension, putting them at greater risk of cardiovascular disease and death. Previous studies have reported that, relative to Caucasian Americans (CA), healthy AA men have augmented pressor responses to sympatho-excitatory stressors. While important, these studies do not inform about the resting state and the influence of spontaneous changes in resting muscle sympathetic nerve activity (MSNA). Likewise, little is known regarding the transduction of MSNA into a vascular response at rest on a beat-to-beat basis. Accordingly, we tested the hypothesis that relative to CA, AA would exhibit greater vasoconstriction and pressor responses following spontaneous bursts of MSNA. Mean arterial pressure (MAP), common femoral artery blood flow, and MSNA were continuously recorded during 20 minutes of supine rest in 35 young healthy men (17 AA and 18 CA). Signal-averaging was used to characterize changes in leg (LVC) and total vascular conductance (TVC) and MAP following spontaneous MSNA bursts. AA demonstrated significantly greater decreases in LVC (AA: -15.0±1.0, CA: -11.5±1.2%; P=0.042) and TVC (AA: -8.6±0.9, CA: -5.1±0.4%; P=0.001) following MSNA bursts, which resulted in greater MAP increases (AA: +5.2±0.6, CA: +3.9±0.3 mmHg; P=0.04). These exaggerated responses in AA compared to CA were present whether MSNA bursts occurred in isolation (singles) or in combination (multiples) and were graded with increases in burst height. Collectively, these findings suggest that healthy young AA men exhibit augmented sympathetic vascular transduction at rest, and provide novel insight into potential mechanism(s) by which this population may develop hypertension later in life.

Influence of physical inactivity on arterial compliance during a glucose challenge

What is the central question of this study? To understand better the effects of acute hyperglycaemia on arterial stiffness in healthy young individuals, we assessed arterial stiffness in physically active men before and after reduced ambulatory physical activity to decrease insulin sensitivity. What is the main finding and its importance? During an oral glucose tolerance test, we identified an increase in leg arterial stiffness (i.e. reduced femoral artery compliance) only when subjects were inactive for 5 days (<5000 steps day−1) and not when they were engaging in regular physical activity (>10,000 steps day−1). These results demonstrate the deleterious consequence of acute reductions in daily physical activity on the response of the peripheral vasculature to acute hyperglycaemia.

Acute reduction in posterior cerebral blood flow following isometric handgrip exercise is augmented by lower body negative pressure

The mechanism(s) for the increased occurrence of a grayout or blackout, syncope, immediately after heavy resistance exercise are unclear. It is well‐known that orthostatic stress increases the occurrence of postexercise syncope. In addition, previous findings have suggested that hypo‐perfusion, especially in the posterior cerebral circulation rather than anterior cerebral circulation, may be associated with the occurrence of syncope. Herein, we hypothesized that the postexercise decrease in posterior, but not anterior, cerebral blood flow (CBF) would be greater during orthostatic stress. Nine healthy subjects performed 3‐min isometric handgrip (HG) at 30% maximum voluntary contraction without (CONTROL) and during lower body negative pressure (LBNP; −40 Torr) while vertebral artery (VA) blood flow, as an index of posterior CBF, and middle cerebral artery blood velocity (MCAv), as an index of anterior CBF, were measured. Immediately after HG (0 to 15 sec of recovery phase), mean arterial pressure decreased but there was no difference in this reduction between CONTROL and LBNP conditions (−15.4 ± 4.0% and −17.0 ± 6.2%, P = 0.42). Similarly, MCAv decreased following exercise and was unaffected by the application of LBNP (P = 0.22). In contrast, decreases in VA blood flow immediately following HG during LBNP were significantly greater compared to CONTROL condition (−24.2 ± 9.5% and ‐13.4 ± 6.6%, P = 0.005). These findings suggest that the decrease in posterior CBF immediately following exercise was augmented by LBNP, whereas anterior CBF appeared unaffected. Thus, the posterior cerebral circulation may be more sensitive to orthostatic stress during the postexercise period.

Regulation of Regional Cerebral Blood Flow During Graded Reflex-Mediated Sympathetic Activation via Lower Body Negative Pressure

The role of the sympathetic nervous system in cerebral blood flow (CBF) regulation remains unclear. Previous studies have primarily measured middle cerebral artery blood velocity to assess CBF. Recently, there has been a transition towards measuring internal carotid artery (ICA) and vertebral artery (VA) blood flow using duplex Doppler ultrasound. Given that the VA supplies autonomic control centers in the brainstem, we hypothesized that graded sympathetic activation via lower body negative pressure (LBNP) would reduce ICA but not VA blood flow. ICA and VA blood flow were measured during two protocols: Protocol-1, low-to-moderate LBNP (-10, -20, -30, -40 Torr) and Protocol-2, moderate-to-high LBNP (-30, -50, -70 Torr). ICA and VA blood flow, diameter, and blood velocity were unaffected up to -40 LBNP. However, -50 and -70 LBNP evoked reductions in ICA and VA blood flow (e.g., -70 LBNP: %∆VA-baseline= -27.6±3.0) that were mediated by decreases in both diameter and velocity (e.g., -70 LBNP: %∆VA-baseline diameter= -7.5±1.9 and %∆VA-baseline velocity= -13.6±1.7), which were comparable between vessels. Since hyperventilation during -70 LBNP reduced PETCO2, this decrease in PETCO2 was matched via voluntary hyperventilation. Reductions in ICA and VA blood flow during hyperventilation alone were significantly smaller than during -70 LBNP and were primarily mediated by decreases in velocity (%∆VA-baseline velocity= -8.6±2.4; %∆VA-baseline diameter= -0.05±0.56). These data demonstrate that both ICA and VA were unaffected by low-to-moderate sympathetic activation, whereas robust reflex-mediated sympatho-excitation caused similar magnitudes of vasoconstriction in both arteries. Thus, contrary to our hypothesis, the ICA was not preferentially vasoconstricted by sympathetic activation.

Brief periods of inactivity reduce leg microvascular, but not macrovascular, function in healthy young men

What is the central question of this study? We aimed to examine leg vascular responses to brief periods of inactivity. What is the main finding and its importance? We demonstrate that a mere 10 min of sitting is sufficient to impair leg microvascular function (reactive hyperaemia). However, conduit artery vasodilatation (flow‐mediated dilatation) was unaffected, indicating maintained macrovascular function. Interestingly, immobile supine rest also resulted in a reduction in microvascular function alone that was prevented when calf muscle contractions were performed. Collectively, these data highlight the susceptibility of the microcirculation to short periods of inactivity and the beneficial role of skeletal muscle contraction for vascular health.

Sex differences in the mechanisms mediating blunted cutaneous microvascular function in young black men and women

The black population exhibits attenuated vasodilatory function across their lifespan, yet little is known regarding the mechanisms of this impairment. Recent evidence suggests a potential role for oxidative stress. Therefore, we tested the hypothesis that NADPH oxidase (NOX) and/or xanthine oxidase (XO) contribute to blunted nitric oxide (NO)-mediated cutaneous microvascular function in young black adults. In 30 white and black subjects (8 men and 7 women in each group), local heating was performed while NOX and XO were inhibited by apocynin and allopurinol, respectively, via intradermal microdialysis. The plateau in cutaneous vascular conductance (red blood cell flux/mean arterial pressure) during 39°C local heating at each site was compared with a control site perfused with lactated Ringer solution. Subsequent inhibition of NO synthase via Nω-nitro-l-arginine methyl ester allowed for quantification of the NO contribution to vasodilation during heating. Black individuals, relative to white individuals, had a blunted cutaneous vascular conductance plateau at the control site (45 ± 9 vs. 68 ± 13%max, P < 0.001) that was increased by both apocynin (61 ± 15%max, P < 0.001) and allopurinol (58 ± 17%max, P = 0.005). Black men and black women had similar responses to heating at the control site ( P = 0.99), yet apocynin and allopurinol increased this response only in black men (both P < 0.001 vs. control). The NO contribution was also increased via apocynin and allopurinol exclusively in black men. These findings suggest that cutaneous microvascular function is reduced because of NOX and XO activity in black men but not black women, identifying a novel sex difference in the mechanisms that contribute to blunted vascular responses in the black population. NEW & NOTEWORTHY We demonstrate that cutaneous microvascular responses to local heating are consistently reduced in otherwise healthy young black men and women relative to their white counterparts. Inhibition of NADPH oxidase and xanthine oxidase via apocynin and allopurinol, respectively, augments microvascular function in black men but not black women. These data reveal clear sex differences in the mechanisms underlying the racial disparity in cutaneous microvascular function.

Quantification of Sympathetic Transduction in Type 2 Diabetes Patients: 2825 Board #345 June 2 9

Type 2 Diabetes patients (T2D) have been shown to have greater alpha-adrenergic sensitivity. How this impacts the transduction of muscle sympathetic nerve activity (MSNA) to arterial blood pressure under resting conditions using spontaneous fluctuations in MSNA, as well as during stressors known to elicit sympatho-excitation (e.g., cold pressor test (CPT)) is unclear. PURPOSE: We tested the hypothesis that T2D patients would exhibit greater sympathetic transduction compared to ageand BMI-matched, healthy controls. METHODS: MSNA (microneurography), heart rate (ECG), and beat-to-beat arterial blood pressure (finger photoplethysmography) were continuously recorded during a 10 minute baseline period, and in response to a 2-minute CPT in six T2D patients and six ageand BMI-matched, healthy controls (CON).To quantify sympathetic transduction at rest, normalized burst heights were divided into four quartiles (smallest to largest), related to the corresponding peak change in mean arterial pressure (MAP) within those quartiles and a slope was determined. To quantify sympathetic transduction in response to a stressor, the change in MAP was related to the change in MSNA from rest to the last minute of CPT. RESULTS: There were no differences in resting sympathetic transduction between groups (CON slope: 0.0103±0.0023 mmHg/AU, T2D slope: 0.0095±0.0016 mmHg/AU; p=0.78). Indeed, signal averaging of MSNA bursts indicated a similar peak increase in blood pressure in CON (+4.2±0.6 mmHg) and T2D (+4.0±0.9 mmHg) (p=0.66). Although the peak increase in blood pressure to CPT tended to be higher in T2D (T2D: +31.6±3.4 mmHg, CON: +21.4±3.7 mmHg; p=0.096), the Δ MAP/ Δ MSNA relationship during CPT was not different between groups (CON: 0.4158±0.21, T2D: 0.1862±0.05; p=0.36). CONCLUSIONS: Despite clear sympathetically-mediated increases in blood pressure in T2D patients and healthy CON subjects both at rest and during the CPT, neither of the methodologies used to estimate sympathetic transduction, with respect to changes in arterial blood pressure, detected group differences.

Methodological assessment of sympathetic vascular transduction

Direct measurement of muscle sympathetic nerve activity (MSNA) originated in Sweden in the 1960s and has been used in thousands of studies (Vallbo et al. 2004). While these measures have been performed extensively to characterize resting MSNA and sympathetic responsiveness, the transduction of MSNA into changes in vascular tone is much less commonly studied. This is surprising, given that changes in MSNA must influence the peripheral vasculature in order to affect blood flow and blood pressure (BP). Indeed, the amount of MSNA is only one aspect of sympathetic control and a focus on transduction of the nerve signal into a vascular and/or BP response can provide further insight into sympathetic regulation. Transduction of an MSNA burst into a vascular response requires successful release of neurotransmitters into the synaptic cleft, receptor binding on smooth muscle, and sufficient vascular smooth muscle contraction. Several groups have used cardiovascular stressors to cause large reflex-mediated increases in sympathetic outflow and assess transduction of MSNA into a vascular or a BP response (Minson et al. 2000). While this reflects transduction of MSNA during sympathoexcitation, it does not represent sympathetic vascular transduction under resting conditions. In a recent issue of The Journal of Physiology, Briant et al. (2016) attempt to better understand resting sympathetic transduction in humans with a central goal of developing a measure of transduction for use in disease populations with elevated sympathetic nerve activity. In their study, Briant et al. (2016) utilized a technique to determine the transduction of MSNA into changes in diastolic blood pressure (DBP). Subjects were divided into four groups: young women (YW), post-menopausal women (PMW), young men (YM), and older men (OM). Subjects rested in the supine position, while DBP (finger-cuff photoplethysmography or arterial catheter) and MSNA were continuously recorded for 5 min. Linear regression of DBP to MSNA (the inverse of the relationship commonly used to investigate baroreflex sensitivity, i.e. MSNA to DBP) at a lag of 6–8 cardiac cycles was used to describe MSNA transduction. This is to say, a given DBP was traced back 6–8 cardiac cycles to investigate the influence of MSNA on DBP. MSNA burst area and burst height were normalized to the largest burst in the recording, binned into (1% × s) bins and associated with the corresponding DBP. An important question addressed in the paper was related to sex and ageing differences in the regulation of BP. The index of transduction used by Briant et al. (2016) supported differences between groups. YM had the highest transduction, with PMW and OM significantly lower than YM, but not significantly different from each other. YW had significantly lower transduction than all other groups, which may result from other vasodilatory mechanisms present in YW at rest (e.g. upregulation of nitric oxide caused by the chronic presence of oestrogen). Interestingly, this paper showed that YW had a significant increase in transduction following β-blockade while PMW and YM had no systematic changes. This evidence supports a greater role of β2-mediated sympathetic vasodilatation at rest in young women (Hart et al. 2011). In the current study, women were shown to have an increase in transduction of sympathetic nerve activity as they age, whereas men were shown to have an age-related decline in sympathetic transduction (Briant et al. 2016). Interestingly, these findings are in contrast to previous reports (Vianna et al. 2012), which found that both sexes had age-related declines in transduction of sympathetic nerve activity. This may be explained, in part, by differing methodologies, with Vianna et al. (2012) using beat-to-beat data and signal averaging of each individual burst and its ensuing blood pressure increase over a period of 10–15 cardiac cycles. Overall, the new method described by Briant et al. (2016) may offer a useful tool for estimating sympathetic transduction. There are, however, some considerations worthy of discussion. The small transduction slopes reported by Briant et al. (2016) suggest that MSNA does not have a great influence on resting DBP, whereas previous studies have suggested MSNA to be a primary effector of vascular tone at rest. This may be partly due to the methodological approach wherein all DBP values were analysed, regardless of whether or not there were any MSNA bursts at the 6–8 cardiac cycle lag for that DBP. This presents a challenge associated with the analysis and interpretation of the data because each group has very different resting MSNA, but similar heart rates, and therefore including DBP values associated with no MSNA may have altered (i.e. minimized) the slope of the relationship between MSNA and its transduction into changes in DBP. Effectively, some of the difference in transduction slopes amongst groups may be due to varying levels of resting nerve activity. Running the analysis exclusively with DBPs associated with the presence of a sympathetic burst may provide additional, important information. Interestingly, the transduction estimates using MSNA and total peripheral resistance (TPR) were not as strong as the DBP association. Although this may have been due to the small sample size included in this sub-analysis, this warrants future consideration. The use of α-receptor blockade would be important to further demonstrate the use of this measure, by providing an example of ‘minimized sympathetic transduction’. Finally, although this technique has the potential for being used to estimate transduction in various disease populations with high nerve activity, at present the studies have only been performed in healthy populations. While ageing is typically associated with higher nerve activity (OM 35 ± 3 bursts min−1; PMW 38 ± 2 bursts min−1), this activity is much less than that of disease populations (e.g. heart failure 45 ± 4 bursts min−1; Middlekauff et al., 2004). Utilization of this technique in populations with higher nerve activity will be important to test. Despite the given considerations, Briant et al. (2016) have developed a new methodology for quantifying transduction of MSNA at rest. This is important because the study of MSNA transduction