Joshua M. Bock

Inorganic nitrate supplementation attenuates peripheral chemoreflex sensitivity but does not improve cardiovagal baroreflex sensitivity in older adults

Aging is associated with increased peripheral chemoreceptor activity, reduced nitric oxide (NO) bioavailability, and attenuation of cardiovagal baroreflex sensitivity (BRS), collectively increasing the risk of cardiovascular disease. Evidence suggests that NO may attenuate peripheral chemoreflex sensitivity and increase BRS. Exogenous inorganic nitrate ([Formula: see text]) increases NO bioavailability via the [Formula: see text]-[Formula: see text]-NO pathway. Our hypothesis was that inorganic [Formula: see text] supplementation would attenuate peripheral chemoreflex sensitivity and enhance spontaneous cardiovagal BRS in older adults. We used a randomized, placebo-controlled crossover design in which 13 older (67 ± 3 yr old) adults ingested beetroot powder containing (BRA) or devoid of (BRP) [Formula: see text] and [Formula: see text] daily over 4 wk. Spontaneous cardiovagal BRS was assessed over 15 min of rest and was quantified using the sequence method. Chemoreflex sensitivity was assessed via ~5 min of hypoxia (10% fraction of inspired O2) and reported as the slope of the relationship between O2 saturation (%[Formula: see text]) and minute ventilation (in l/min) or heart rate (in beats/min). Ventilatory responsiveness to hypoxia was reduced after BRA (from -0.14 ± 0.04 to -0.05 ± 0.02 l·min-1·%[Formula: see text]-1, P = 0.01) versus BRP (from -0.10 ± 0.05 to -0.11 ± 0.05 l·min-1·%[Formula: see text]-1, P = 0.80), with no differences in heart rate responsiveness (BRA: from -0.47 ± 0.06 to -0.33 ± 0.04 beats·min-1·%[Formula: see text]-1, BRP: from -0.48 ± 0.07 to -0.42 ± 0.06 beats·min-1·%[Formula: see text]-1) between conditions (interaction effect, P = 0.41). Spontaneous cardiovagal BRS was unchanged after BRA and BRP (interaction effects, P = 0.69, 0.94, and 0.39 for all, up, and down sequences, respectively), despite a reduction in resting systolic and mean arterial blood pressure in the experimental (BRA) group ( P < 0.01 for both). These findings illustrate that inorganic [Formula: see text] supplementation attenuates peripheral chemoreflex sensitivity without concomitant change in spontaneous cardiovagal BRS in older adults. NEW & NOTEWORTHY Exogenous inorganic nitrate supplementation attenuates ventilatory, but not heart rate, responsiveness to abbreviated hypoxic exposure in older adults. Additionally, inorganic nitrate reduces systolic and mean arterial blood pressure without affecting spontaneous cardiovagal baroreflex sensitivity. These findings suggest that inorganic nitrate may attenuate sympathetically oriented pathologies associated with aging.

High-intensity Exercise Enhances Conduit Artery Vascular Function in Older Adults

Purpose Modulation of vascular function follows an exercise intensity–dependent pattern in young adults. This study aimed to investigate the potential intensity–dependent effects of an acute bout of exercise on conduit and resistance artery function in healthy older adults. Methods Eleven healthy older adults (five males/six females, 66 ± 1 yr) completed 30 min of recumbent cycling at 50%–55% (low intensity) and 75%–80% (high intensity) of their age-predicted HRmax on two separate study visits. Doppler ultrasound measures of brachial artery flow-mediated dilation (FMD) and reactive hyperemia were taken at baseline, 10 min postexercise, and 1 h postexercise. In addition, cardiovascular hemodynamics and brachial shear rate were measured every 5 min during exercise. Results Brachial artery FMD was enhanced 10 min after high-intensity exercise (4.8% ± 0.2% to 9.1% ± 0.3%, P < 0.01), but not low-intensity (4.7% ± 0.2% to 6.2% ± 0.3%, P = 0.54) exercise. Peak and total (area under the curve) blood flow during reactive hyperemia (measures of resistance artery function) were enhanced 10 min postexercise for both intensities (peak low intensity, 372 ± 31 to 444 ± 37 mL·min−1; peak high intensity, 391 ± 30 to 455 ± 28 mL·min−1; total low intensity, 142 ± 16 to 205 ± 20 mL; total high intensity, 158 ± 14 to 240 ± 25 mL; main effect of time for both, P < 0.05). However, the magnitude of change in peak and the total blood flow were not different between exercise intensities (interaction effect; P = 0.56 and P = 0.97, respectively). Independent of exercise intensity, FMD returned to baseline 1 h after exercise (high, 5.9% ± 0.3%; low, 5.1% ± 0.1%; both P > 0.05). Conclusion Our data indicate that high-intensity exercise acutely enhances conduit artery function in healthy older adults. In addition, an acute bout of exercise enhances resistance artery function independent of intensity.

Raw and Count Data Comparability of Hip-Worn ActiGraph GT3X+ and Link Accelerometers

To enable inter- and intrastudy comparisons it is important to ascertain comparability among accelerometer models. Purpose The purpose of this study was to compare raw and count data between hip-worn ActiGraph GT3X+ and GT9X Link accelerometers. Methods Adults (n = 26 (n = 15 women); age, 49.1 ± 20.0 yr) wore GT3X+ and Link accelerometers over the right hip for an 80-min protocol involving 12–21 sedentary, household, and ambulatory/exercise activities lasting 2–15 min each. For each accelerometer, mean and variance of the raw (60 Hz) data for each axis and vector magnitude (VM) were extracted in 30-s epochs. A machine learning model (Montoye 2015) was used to predict energy expenditure in METs from the raw data. Raw data were also processed into activity counts in 30-s epochs for each axis and VM, with Freedson 1998 and 2011 count-based regression models used to predict METs. Time spent in sedentary, light, moderate, and vigorous intensities was derived from predicted METs from each model. Correlations were calculated to compare raw and count data between accelerometers, and percent agreement was used to compare epoch-by-epoch activity intensity. Results For raw data, correlations for mean acceleration were 0.96 ± 0.05, 0.89 ± 0.16, 0.71 ± 0.33, and 0.80 ± 0.28, and those for variance were 0.98 ± 0.02, 0.98 ± 0.03, 0.91 ± 0.06, and 1.00 ± 0.00 in the X, Y, and Z axes and VM, respectively. For count data, corresponding correlations were 1.00 ± 0.01, 0.98 ± 0.02, 0.96 ± 0.04, and 1.00 ± 0.00, respectively. Freedson 1998 and 2011 count-based models had significantly higher percent agreement for activity intensity (95.1% ± 5.6% and 95.5% ± 4.0%) compared with the Montoye 2015 raw data model (61.5% ± 27.6%; P < 0.001). Conclusions Count data were more highly comparable than raw data between accelerometers. Data filtering and/or more robust raw data models are needed to improve raw data comparability between ActiGraph GT3X+ and Link accelerometers.

Blunted shear-mediated dilation of the internal but not common carotid artery in response to lower body negative pressure

Shear-mediated dilation in peripheral conduit arteries is blunted with sympathetic nervous system (SNS) activation; however, the effect of SNS activation on shear-mediated dilation in carotid arteries is unknown. We hypothesized that SNS activation reduces shear-mediated dilation in common and internal carotid arteries (CCA and ICA, respectively), and this attenuation is greater in the ICA compared with the CCA. Shear-mediated dilation in the CCA and ICA were measured in nine healthy men (24 ± 1 yr) with and without SNS activation. Shear-mediated dilation was induced by 3 min of hypercapnia (end-tidal partial pressure of carbon dioxide +10 mmHg from individual baseline); SNS activity was increased with lower body negative pressure (LBNP; -20 mmHg). CCA and ICA measurements were made using Doppler ultrasound during hypercapnia with (LBNP) or without (Control) SNS activation. LBNP trials began with 5 min of LBNP with subjects breathing hypercapnic gas during the final 3 min. Shear-mediated dilation was calculated as the percent rise in peak diameter from baseline diameter. Sympathetic activation attenuated shear-mediated dilation in the ICA (Control vs. LBNP, 5.5 ± 0.7 vs. 1.8 ± 0.4%, P < 0.01), but not in the CCA (5.1 ± 1.2 vs. 4.2 ± 1.0%, P = 0.31). Moreover, absolute reduction in shear-mediated dilation via SNS activation was greater in the ICA than the CCA (-3.6 ± 0.7 vs. -0.9 ± 0.8%, P = 0.02). Our data indicate that shear-mediated dilation is attenuated during LBNP to a greater extent in the ICA compared with the CCA. These results potentially provide insight into the role of SNS activation on cerebral perfusion, as the ICA is a key supplier of blood to the brain. NEW & NOTEWORTHY We explored the effect of acute sympathetic nervous system (SNS) activation on shear-mediated dilation in the common and internal carotid arteries (CCA and ICA, respectively) in young healthy men. Our data demonstrate that hypercapnia-induced vasodilation of the ICA is attenuated during lower body negative pressure to a greater extent than the CCA. These data may provide novel information related to the role of SNS activation on cerebral perfusion in humans.

Evidence of a greater functional sympatholysis in habitually aerobic trained postmenopausal women

Habitual aerobic exercise attenuates elevated vasoconstriction during acute exercise (functional sympatholysis) in older men; however, this effect remains unknown in postmenopausal women (PMW). This study tested the hypothesis that PMW who participate in habitual aerobic exercise demonstrate a greater functional sympatholysis compared with their untrained counterparts. Nineteen PMW (untrained n = 9 vs. trained n = 10) performed 5 min of steady-state (SS) forearm exercise at relative [10% and 20% of maximum voluntary contraction (MVC)] and absolute (5 kg) contraction intensities. Lower-body negative pressure (LBNP) was used to increase sympathetic vasoconstriction during rest and forearm exercise. Brachial artery diameter and blood velocities (via Doppler ultrasound) determined forearm blood flow (FBF; ml/min). Forearm muscle oxygen consumption ([Formula: see text]; ml/min) and arteriovenous oxygen difference (a-vO2diff) were estimated during SS-exercise and SS-exercise with LBNP. Forearm vascular conductance (FVC; ml·min-1·100 mmHg-1) was calculated from FBF and mean arterial pressure (MAP; mmHg). Vasoconstrictor responsiveness was determined as the %change in FVC during LBNP. The reduction in FVC (% change FVC) during LBNP was lower in trained compared with untrained PMW at 10% MVC (-7.3 ± 1.2% vs. -13.0 ± 1.1%; P < 0.05), 20% MVC (-4.4 ± 0.8% vs. -8.6 ± 1.4%; P < 0.05), and 5 kg (-5.3 ± 0.8% vs. -8.9 ± 1.4%; P < 0.05) conditions, whereas there were no differences at rest (-32.7 ± 4.4% vs. -33.7 ± 4.0%). Peripheral (FVC, FBF, and [Formula: see text]) and the magnitude change in systemic hemodynamics (heart rate and MAP) did not differ between groups during exercise. Collectively, the findings present the first evidence suggesting that PMW who participate in aerobic exercise demonstrate a greater functional sympatholysis compared with untrained PMW during mild to moderate forearm exercise. NEW & NOTEWORTHY Habitual aerobic exercise attenuates the elevated sympathetic nervous system-induced vasoconstriction during an acute bout of exercise (improved functional sympatholysis) in aging men; however, this effect remains unknown in postmenopausal women (PMW). The novel findings of this study suggest that habitual aerobic exercise results in an enhanced functional sympatholysis in PMW. Conversely, habitual aerobic exercise does not alter blood flow and oxygen utilization during acute forearm exercise compared with PMW who do not habitually exercise.

Impaired modulation of postjunctional α1‐ but not α2‐adrenergic vasoconstriction in contracting forearm muscle of postmenopausal women

Contraction‐mediated blunting of postjunctional α‐adrenergic vasoconstriction (functional sympatholysis) is attenuated in skeletal muscle of ageing males, brought on by altered postjunctional α1‐ and α2‐adrenergic receptor sensitivity. The extent to which postjunctional α‐adrenergic vasoconstriction occurs in the forearms at rest and during exercise in postmenopausal women remains unknown. The novel findings indicate that contraction‐mediated blunting of α1‐ (via intra‐arterial infusion of phenylephrine) but not α2‐adrenergic (via intra‐arterial infusion of dexmedetomidine) vasoconstriction was attenuated in postmenopausal women compared to young women. Additional important findings revealed that postjunctional α‐adrenergic vasoconstrictor responsiveness at rest does not appear to be affected by age in women. Collectively, these results contribute to our understanding of local neurovascular control at rest and during exercise with age in women.

Carotid chemoreceptors: the link between pulmonary and cardiovascular disease?

Chemoreceptors located within the carotid bodies trigger sympathetic outflow in response to an increase in arterial carbon dioxide or reduction in arterial oxygen levels. Principally, this reflex serves to increase respiratory drive as a means to remedy blood gas imbalances, but chronic activation of the carotid chemoreflex leads to sustained elevations in sympathetic tone. Specifically, systemic hypoxaemia (low oxygen saturation) elicits sympathetic alpha-mediated vasoconstriction, thus reducing oxygen delivery to skeletal muscle and other organs, subsequently creating a physiological ‘tug-of-war’ between oxygen supply and demand (Joyner & Casey, 2014). Thus, chronic stimulation of the carotid chemoreceptors via hypoxia is an important experimental and clinical paradigm in the aetiology of cardiovascular diseases, especially in patients with chronic obstructive pulmonary disease (COPD) who demonstrate exaggerated carotid chemoreflex sensitivity. The cascade of biochemical reactions linking hypoxaemia, sympathetic nervous system activity and cardiovascular regulation has, to date, remained poorly understood in COPD. Therefore, a recent study published in this issue of The Journal of Physiology by Phillips and colleagues (2018) attempted to resolve one important piece of this puzzle. Specifically, authors used a single-blind, placebo-controlled cross-over design which investigated carotid chemoreceptor sensitivity in 13 patients with mild–moderate COPD (GOLD Stages 1 and II, not prescribed oxygen therapy). Intravenous dopamine infusion (2 μg kg−1 min−1) and acute exposure to hyperoxia (2 min of F I,O2 = 1.0) were employed as a means to blunt carotid chemoreceptor activity and reduce muscle sympathetic nerve activity. Central (carotid-femoral) and peripheral (carotid-radial) pulse wave velocities (m s−1; index of arterial stiffness) as well as blood velocity in the brachial artery (cm s−1; via Doppler ultrasound) were measured during rest, hyperoxia, and dopamine infusion. Blood velocity, in concert with brachial artery diameter, was used to calculate blood flow (ml min−1), then normalized for blood pressure, yielding vascular conductance (ml min−1 mmHg−1), which served as surrogates of hyperaemia and vasodilatation, respectively. Muscle sympathetic nerve activity (bursts min−1) was also measured via microneurography in a subset of seven COPD patients while breathing room air and hyperoxic gas. As expected, acute dopamine infusion and hyperoxia were successful in blunting the carotid chemoreflex which reduced centraland peripheral-artery pulse wave velocity, indicating reduced arterial stiffness. Patients in this study also demonstrated greater vascular conductance of the brachial artery during dopamine infusion, probably attributable to a reduction in mean arterial pressure (Table 3 in Phillips et al. 2018). In line with this notion, muscle sympathetic nerve activity during hyperoxia was reduced in the subset of seven patients with COPD. Collectively, these findings provide novel insight into the relationship between carotid chemoreceptor overactivity and cardiovascular regulation in patients with COPD by demonstrating that acute blunting of carotid chemoreceptor activity reduces arterial stiffness and improves vasodilatation, presumably through attenuation of sympathetic nerve activity in patients with mild-to-moderate COPD. Targeting the carotid chemoreceptors as a means to improve health-related outcomes is a new and exciting field of biomedical research which has yielded promising results. While Phillips and colleagues (2018) provide novel insight into this field, nitric oxide (NO), historically known for its robust vasodilatory effects, has also been shown to blunt carotid chemoreceptor activity in preclinical and human models. For example, chronic supplementation of inorganic nitrate in the form of beetroot juice increases circulating NO bioavailability via the nitrate-nitrite-nitric oxide pathway, and has been show to attenuate carotid chemoreflex sensitivity (Bock et al. 2018). While this study (Bock et al. 2018) was not designed to investigate vascular function (e.g. arterial stiffness, vasodilatation), it did provide compelling evidence to support the concept that carotid chemoreflex activity can be reduced in a healthy, older population by boosting NO bioavailability using a nutraceutical intervention. The carotid chemoreflex has also been attenuated with NO-boosting therapies in clinical populations. Patients with obstructive sleep apnoea (OSA), much like patients with COPD, are known to have exaggerated carotid chemoreflex sensitivity. These two populations also share elevated levels of inflammation and oxidative stress with an associated reduction in NO (Jelic et al. 2008; Maclay & MacNee, 2013). To this point, Maclay & MacNee (2013) discuss oxidative stress and inflammation being increased with an associated reduction of NO in patients with COPD. Likewise, Jelic and colleagues (2008) demonstrated that patients with OSA had greater nitrotyrosine (oxidative stress) and cyclooxygenase-two (inflammation) but less NO-synthase protein expression on venous endothelial cells. This study also demonstrated a reduction in both oxidative stress and biomarkers of inflammation with concomitant improvement in protein expression of NO-synthase and vascular function (flow-mediated dilatation) following treatment with continuous positive airway pressure (CPAP). Interestingly, and of relevance to Phillips et al. (2018), CPAP therapy attenuates the ventilatory response to hypoxia (carotid chemoreflex sensitivity), increases NO bioavailability which reduces carotid chemoreceptor activity, and ultimately lowers sympathetic nerve activity. Taken together, these studies suggest the link between lower carotid chemoreflex sensitivity and vascular health in patient populations (i.e. COPD and OSA) may be mediated by reducing oxidative stress and inflammation or attenuation in sympathetic nerve activity, although further studies are needed to fully assess this presumption. Despite the present study’s innovation, there are some experimental considerations warranting attention. The cross-sectional design of this study inherently limits

Reduced blood pressure responsiveness to skeletal muscle metaboreflex activation in older adults following inorganic nitrate supplementation

The vasoactive molecule nitric oxide (NO) contributes to regulation of blood pressure (BP) at rest and during exercise. Age-related exaggerated increased BP responses during exercise have been proposed to be due in part to a decreased NO bioavailability and possibly an enhanced skeletal muscle metaboreflex. In the present study we sought to determine if age-related differences in BP responses to skeletal muscle metaboreflex activation exist. Additionally, since NO bioavailability can be improved with exogenous nitrate (NO3-) via the nitrate-nitrite-NO pathway, we tested the hypothesis that inorganic NO3- supplementation would reduce BP responses to muscle metaboreflex activation in healthy older adults. 13 older adults (67 ± 1 years) participated in a randomized, double-blind, placebo controlled crossover study consisting of four weeks of NO3- supplementation [beetroot powder; 250 mg (∼4.03 mmol) of NO3- and 20 mg (∼0.29 mmol) of NO2-] and four weeks of placebo (beetroot powder devoid of NO3-/NO2-). Skeletal muscle metaboreflex testing consisted of isometric handgrip exercise (IHG) at 30% of maximal voluntary contraction immediately followed by post exercise forearm ischemia (PEI), which was achieved by inflation of a rapid pressure cuff (240 mmHg) around the upper arm. BP responses were analyzed as the change (Δ) from baseline to the end of IHG and PEI. An additional 10 young adults (25 ± 1 years) were recruited to serve as a reference cohort and address if BP responses to skeletal muscle metaboreflex activation were greater with aging. BP responses to IHG were similar between the young and older adults. However, older adults demonstrated a greater increase in systolic BP during PEI (P < 0.05). Plasma NO3- and NO2-were increased following NO3- supplementation in older adults (P < 0.01). ΔSystolic BP (19 ± 2 vs. 13±3 mmHg, P < 0.05), ΔDiastolic BP (7 ± 1 vs. 5±1 mmHg, P < 0.05) and ΔMean arterial pressure (11 ± 1 vs. 8±2 mmHg, P < 0.05) were reduced during PEI following four weeks of NO3-supplementation, whereas placebo had no effect on ΔSystolic BP (16 ± 2 vs. 17±2 mmHg), ΔDiastolic BP (5 ± 1 vs. 7±1 mmHg), and ΔMean arterial pressure (8 ± 1 vs. 10±1 mmHg) during PEI (all P > 0.05). These data suggest that inorganic NO3- supplementation attenuates skeletal muscle metaboreflex mediated increases in BP during exercise in older adults.

Inorganic nitrate supplementation enhances functional capacity and lower-limb microvascular reactivity in patients with peripheral artery disease

Peripheral artery disease (PAD) is characterized by functional and vascular impairments as well as elevated levels of inflammation which are associated with reduced nitric oxide (NO) bioavailability. Inorganic nitrate supplementation boosts NO bioavailability potentially improving functional and vasodilatory capacities and may reduce inflammation. Twenty-one patients with PAD were randomly assigned to sodium nitrate (NaNO3) or placebo supplementation groups for eight-weeks. Outcome measures included a 6-min walk test (6 MWT), blood flow and vasodilator function in the forearm and calf, as well as plasma inflammatory and adhesion biomarker concentrations. NaNO3 elevated plasma nitrate (32.3 ± 20.0 to 379.8 ± 204.6 μM) and nitrite (192.2 ± 51.8 to 353.1 ± 134.2 nM), improved 6 MWT performance (387 ± 90 to 425 ± 82 m), peak calf blood flow (BFPeak; 11.6 ± 4.9 to 14.1 ± 5.1 mL/dL tissue/min), and peak calf vascular conductance (VCPeak; 11.1 ± 4.3 to 14.2 ± 4.9 mL/dL tissue/min/mmHg) (p < 0.05 for all). Improvements in calf BFPeak (r = 0.70, p < 0.05) and VCPeak (r = 0.61, p < 0.05) correlated with changes in 6 MWT distance. Placebo supplementation did not change plasma nitrate or nitrite, 6 MWT, calf BFPeak, or calf VCPeak. Forearm vascular function nor inflammatory and adhesion biomarker concentrations changed in either group. Eight-weeks of NaNO3 supplementation improves vasodilatory capacity in the lower-limbs of patients with PAD, which correlated with improvement in functional capacity.

Hypercapnia-induced shear-mediated dilation in the internal carotid artery is blunted in healthy older adults

This study aimed to elucidate the effect of aging on shear-mediated dilation of the common and internal carotid arteries (CCA and ICA, respectively). Hypercapnia-induced shear-mediated dilation in the CCA and ICA were assessed in 10 young (5 women and 5 men, 23 ± 1 yr) and 10 older (6 women/4 men, 68 ± 1 yr) healthy adults. Shear-mediated dilation was induced by two levels of hypercapnia (target end-tidal Pco2, +5 and +10 mmHg from individual baseline values) and was calculated as the percent rise in peak diameter from baseline diameter. There were no differences in shear-mediated dilation between young and older adults in either artery under lower levels of hypercapnia (CCA: 2.8 ± 0.6 vs. 2.0 ± 0.3%, P = 0.35; ICA: 4.6 ± 0.8 vs 3.6 ± 0.4%, P = 0.37). However, shear-mediated dilation in response to higher levels of hypercapnia was attenuated in older compared with young adults in the ICA (4.5 ± 0.5 vs. 7.9 ± 1.2%, P < 0.01) but not in the CCA (3.7 ± 0.6 vs. 4.5 ± 0.8%, P = 0.35). Shear-mediated dilation was significantly correlated to the percent change in shear rate in the ICA (young: r = 0.55, P = 0.01; older: r = 0.49, P = 0.03) but not in the CCA in either young or older adults (young: r = -0.30, P = 0.90; older: r = 0.16, P = 0.50). These data indicate that aging attenuates shear-mediated dilation of the ICA in response to higher levels of hypercapnia, and shear rate is an important stimulus for hypercapnic vasodilation of the ICA in both young and older adults. The present results may provide insights into age-related changes in the regulation of cerebral circulation in healthy adults. NEW & NOTEWORTHY We explored the effect of aging on shear-mediated dilation in the common and internal carotid arteries (CCA and ICA, respectively) in healthy adults. Our findings suggest that 1) aging attenuates shear-mediated dilation of the ICA but not the CCA and 2) shear rate is an important stimulus for hypercapnic vasodilation of the ICA in young and older adults. These findings may provide insights into the age-related changes in cerebrovascular regulation of healthy adults.