Muscle blood flow responses to dynamic exercise in young obese humans

Endothelial and exercise vasodilation are reduced in postmenopausal females with obesity versus lean and overweight

BACKGROUND

Obesity (OB) is highly prevalent in females after menopause, especially visceral adipose tissue (VAT) accumulation which contributes to endothelial dysfunction. The endothelium assists in regulating blood flow (BF) during exercise and is attenuated in females with OB. The purpose of this study was to examine upper and lower limb flow-mediated dilation (FMD) and BF regulation during graded low-intensity submaximal exercises in postmenopausal females with BMI in the lean (LN), overweight (OW) and OB categories.

METHODS

Participants were grouped by body mass index (BMI) into LN (BMI 18.5-24.9 kg/m2; n = 11), OW (BMI 25.0-29.9 kg/m2; n = 15), and OB (BMI 30.0-39.9 kg/m2; n = 13). FMD of the brachial (BA-FMD) and superficial femoral arteries (FA-FMD) were assessed. Subsequently, BF and vascular conductance (VC) in the upper (BA-BF and BA-VC) and lower limbs (FA-BF and FA-VC) were measured during separate 3-stage incremental rhythmic handgrip and plantarflexion exercises.

RESULTS

Significantly lower FA-FMD (P < 0.05) were seen in OB than LN and OW groups with no differences in BA-FMD. Increases in FA-BF and FA-VC were attenuated during the last stage of plantarflexion exercise at 30% of 1RM in OB (both P < 0.001) compared to LN and OW, while upper-body exercise vasodilation was unchanged. FA-BF and FA-VC during plantarflexion exercise were correlated to FA-FMD (FA-BF: r = 0.423, P = 0.007, FA-VC: r = 0.367, P = 0.021) and BMI (FA-BF: r = -0.386, P = 0.015, FA-VC: r = -0.456, P = 0.004).

CONCLUSION

Postmenopausal females with OB have reduced lower-limb endothelial and exercise vasodilator function during submaximal dynamic plantarflexion exercise compared to LN and OW. Our findings indicate that obesity may predict diminished leg endothelial function, BF and VC during exercise in postmenopausal females.

Obesity does not alter vascular function and handgrip exercise hemodynamics in middle-aged patients with hypertension

Lifestyle modification including exercise training is often the first line of defense in the treatment of obesity and hypertension (HTN), however, little is known regarding how these potentially compounding disease states impact vasodilatory and hemodynamic responses at baseline and exercise. Therefore, this study sought to compare the impact of obesity on vascular function and hemodynamics at baseline and during handgrip (HG) exercise among individuals with HTN. Non-obese (13M/7F, 56 ± 16 yr, 25 ± 4 kg/m2) and obese (17M/4F, 50 ± 7 yr, 35 ± 4 kg/m2) middle-aged individuals with HTN forwent antihypertensive medication use for ≥2 wk before assessment of vascular function by brachial artery flow-mediated dilation (FMD) and exercise hemodynamics during progressive HG exercise at 15-30-45% maximal voluntary contraction (MVC). FMD was not different between Non-Obese (4.1 ± 1.7%) and Obese (5.2 ± 1.9%, P = 0.11). Systolic blood pressure (SBP) was elevated by ∼15% during the supine baseline and during HG exercise in the obese group. The blood flow response to HG exercise at 30% and 45% MVC was ∼20% greater (P < 0.05) in the obese group but not different after normalizing for the higher, albeit, nonsignificant differences in workloads (MVC: obese: 24 ± 5 kg, non-obese: 21 ± 5 kg, P = 0.11). Vascular conductance and the brachial artery shear-induced vasodilatory response during HG were not different between groups (P > 0.05). Taken together, despite elevated SBP during HG exercise, obesity does not lead to additional impairments in vascular function and peripheral exercising hemodynamics in patients with HTN. Obesity may not be a contraindication when prescribing exercise for the treatment of HTN among middle-aged adults, however, the elevated SBP should be appropriately monitored.NEW & NOTEWORTHY This study examined vascular function and handgrip exercise hemodynamics in obese and nonobese individuals with hypertension. Obesity, when combined with hypertension, was neither associated with additional vascular function impairments at baseline nor peripheral hemodynamics and vasodilation during exercise compared with nonobese hypertension. Interestingly, systolic blood pressure and pulse pressure were greater in the obese group during supine baseline and exercise. These findings should not be ignored and may be particularly important for rehabilitation strategies.

Non-invasive MR imaging techniques for measuring femoral arterial flow in a pediatric and adolescent cohort

Magnetic Resonance Imaging (MRI) is well‐suited for imaging peripheral blood flow due to its non‐invasive nature and excellent spatial resolution. Although MRI is routinely used in adults to assess physiological changes in chronic diseases, there are currently no MRI‐based data quantifying arterial flow in pediatric or adolescent populations during exercise. Therefore the current research sought to document femoral arterial blood flow at rest and following exercise in a pediatric‐adolescent population using phase contrast MRI, and to present test‐retest reliability data for this method. Ten healthy children and adolescents (4 male; mean age 14.8 ± 2.4 years) completed bloodwork and resting and exercise MRI. Baseline images consisted of PC‐MRI of the femoral artery at rest and following a 5 × 30 s of in‐magnet exercise. To evaluate test‐retest reliability, five participants returned for repeat testing. All participants successfully completed exercise testing in the MRI. Baseline flow demonstrated excellent reliability (ICC = 0.93, p = 0.006), and peak exercise and delta rest‐peak flow demonstrated good reliability (peak exercise ICC = 0.89, p = 0.002, delta rest‐peak ICC = 0.87, p = 0.003) between‐visits. All three flow measurements demonstrated excellent reliability when assessed with coefficients of variance (CV’s) (rest: CV = 6.2%; peak exercise: CV = 7.3%; delta rest‐peak: CV = 7.1%). The mean bias was small for femoral arterial flow. There was no significant mean bias between femoral artery flow visits 1 and 2 at peak exercise. There were no correlations between age or height and any of the flow measurements. There were no significant differences between male and female participants for any of the flow measurements. The current study determined that peripheral arterial blood flow in children and adolescents can be evaluated using non‐invasive phase contrast MRI. The MRI‐based techniques that were used in the current study for measuring arterial flow in pediatric and adolescent patients demonstrated acceptable test‐retest reliability both at rest and immediately post‐exercise.

The Impact of Obesity on the Regulation of Muscle Blood Flow during Exercise in Patients with Heart Failure with a Preserved Ejection Fraction

Obesity is now considered a primary comorbidity in heart failure with preserved ejection fraction (HFpEF) pathophysiology, mediated largely by systemic inflammation. While there is accumulating evidence for a disease-related dysregulation of blood flow during exercise in this patient group, the role of obesity in the hemodynamic response to exercise remain largely unknown. Small muscle mass handgrip (HG) exercise was utilized to evaluate exercising muscle blood flow in non-obese (BMI < 30 kg/m²,n=14) and obese (BMI > 30 kg/m²,n=40) patients with HFpEF. Heart rate (HR), stroke index (SI), cardiac index (CI), mean arterial pressure (MAP), forearm blood flow (FBF) and vascular conductance (FVC) were assessed during progressive intermittent HG exercise (15-30-45% maximal voluntary contraction, MVC). Blood biomarkers of inflammation (C-reactive protein (CRP) and Interleukin-6 (IL-6)) were also determined. Exercising FBF was reduced in obese patients with HFpEF at all work rates (15%: 304±42 vs. 229±15ml/min; 30%: 402±46 vs. 300±18ml/min; 45%: 484±55 vs. 380±23ml/min, non-obese vs. obese, p=0.025), and was negatively correlated with BMI (R=-.47, p<0.01). In contrast, no differences in central hemodynamics (HR, SI, CI, MAP) were found between groups. Proinflammatory biomarkers were markedly elevated in obese patients (CRP: 2133±418 vs. 4630±590ng/ml, p=0.02; IL-6: 2.9±0.3 vs. 5.2±0.7pg/ml, p = 0.04, non-obese vs. obese), and both biomarkers were positively correlated with BMI (CRP: R=0.40, p=0.03; IL-6: R=0.57, p<0.01). Together, these findings demonstrate the presence of obesity and an accompanying milieu of systemic inflammation as important factors in the dysregulation of exercising muscle blood flow in patients with HFpEF.

Skeletal Muscle Microvascular Dysfunction in Obesity-Related Insulin Resistance: Pathophysiological Mechanisms and Therapeutic Perspectives

Obesity is a worrisomely escalating public health problem globally and one of the leading causes of morbidity and mortality from noncommunicable disease. The epidemiological link between obesity and a broad spectrum of cardiometabolic disorders has been well documented; however, the underlying pathophysiological mechanisms are only partially understood, and effective treatment options remain scarce. Given its critical role in glucose metabolism, skeletal muscle has increasingly become a focus of attention in understanding the mechanisms of impaired insulin function in obesity and the associated metabolic sequelae. We examined the current evidence on the relationship between microvascular dysfunction and insulin resistance in obesity. A growing body of evidence suggest an intimate and reciprocal relationship between skeletal muscle microvascular and glucometabolic physiology. The obesity phenotype is characterized by structural and functional changes in the skeletal muscle microcirculation which contribute to insulin dysfunction and disturbed glucose homeostasis. Several interconnected etiologic molecular mechanisms have been suggested, including endothelial dysfunction by several factors, extracellular matrix remodelling, and induction of oxidative stress and the immunoinflammatory phenotype. We further correlated currently available pharmacological agents that have deductive therapeutic relevance to the explored pathophysiological mechanisms, highlighting a potential clinical perspective in obesity treatment.

Heart failure with preserved ejection fraction diminishes peripheral hemodynamics and accelerates exercise-induced neuromuscular fatigue

This study investigated the impact of HFpEF on neuromuscular fatigue and peripheral hemodynamics during small muscle mass exercise not limited by cardiac output. Eight HFpEF patients (NYHA II-III, ejection-fraction: 61 ± 2%) and eight healthy controls performed dynamic knee extension exercise (80% peak workload) to task failure and maximal intermittent quadriceps contractions (8 × 15 s). Controls repeated knee extension at the same absolute intensity as HFpEF. Leg blood flow (Q_L) was quantified using Doppler ultrasound. Pre/postexercise changes in quadriceps twitch torque (ΔQ_tw; peripheral fatigue), voluntary activation (ΔVA; central fatigue), and corticospinal excitability were quantified. At the same relative intensity, HFpEF (24 ± 5 W) and controls (42 ± 6 W) had a similar time-to-task failure (∼10 min), ΔQ_tw (∼50%), and ΔVA (∼6%). This resulted in a greater exercise-induced change in neuromuscular function per unit work in HFpEF, which was significantly correlated with a slower Q_L response time. Knee extension exercise at the same absolute intensity resulted in an ∼40% lower Q_L and greater ΔQ_tw and ΔVA in HFpEF than in controls. Corticospinal excitability remained unaltered during exercise in both groups. Finally, despite a similar ΔVA, ΔQ_tw was larger in HFpEF versus controls during isometric exercise. In conclusion, HFpEF patients are characterized by a similar development of central and peripheral fatigue as healthy controls when tested at the same relative intensity during exercise not limited by cardiac output. However, HFpEF patients have a greater susceptibility to neuromuscular fatigue during exercise at a given absolute intensity, and this impairs functional capacity. The patients' compromised Q_L response to exercise likely accounts, at least partly, for the patients' attenuated fatigue resistance.NEW & NOTEWORTHY The susceptibility to neuromuscular fatigue during exercise is substantially exaggerated in individuals with heart failure with a preserved ejection fraction. The faster rate of fatigue development is associated with the compromised peripheral hemodynamic response characterizing these patients during exercise. Given the role of neuromuscular fatigue as a factor limiting exercise, this impairment likely accounts for a significant portion of the exercise intolerance typical for this population.

Assessment of resistance vessel function in human skeletal muscle: guidelines for experimental design, Doppler ultrasound, and pharmacology

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions.

Metabolic and muscular factors limiting aerobic exercise in obese subjects

PURPOSE

The aim of the present study was to understand the role of central (cardiovascular O₂ delivery) and peripheral factors (muscle level) in limiting the maximal aerobic performance in obese (OB) subjects.

METHODS

Fifteen OB (mean age ± SD 25 ± 7 years; BMI 43 ± 7 kg/m²) and 13 lean sedentary subjects (CTRL, age 27 ± 7 years; BMI 22 ± 3 kg/m²) participated in this study. Oxygen uptake (VO₂), hearth rate (HR) and cardiac output (CO) were measured during cycle ergometer (CE) and knee extension (KE) incremental tests. Maximal voluntary contractions (MVCs) of knee extensor muscles were performed before and immediately after the two tests.

RESULTS

VO₂peak, HR peak and CO peak were significantly higher in CE than KE (+ 126%, + 33% and + 46%, respectively, p < 0.001), both in OB and CTRL subjects, without differences between the two subgroups. Maximal work rate was lower in OB than CTRL (191 ± 38 vs 226 ± 39 W, p < 0.05) in CE, while it was similar between the two subgroups in KE. Although CE and KE determined a reduction of MVC in both subgroups, MVC resulted less decreased after CE than KE exercises (- 14 vs - 32%, p < 0.001) in OB, while MVC decrements were similar after the two exercises in CTRL (- 26% vs - 30%, p > 0.05, for CE and KE, respectively).

CONCLUSIONS

The lower muscle fatigue observed in OB after CE compared to KE test suggests that central factors could be the most important limiting factor during cycling in OB.

Preserved ability to blunt sympathetically-mediated vasoconstriction in exercising skeletal muscle of young obese humans

Sympathetic vasoconstriction is attenuated in exercising muscles to assist in matching of blood flow with metabolic demand. This "functional sympatholysis" may be impaired in young obese individuals due to greater sympathetic activation and/or reduced local vasodilatory capacity of both small and large arteries, but this remains poorly understood. We tested the hypothesis that functional sympatholysis is impaired in obese individuals compared with normal-weight counterparts. In 36 obese and normal-weight young healthy adults (n = 18/group), we measured forearm blood flow and calculated forearm vascular conductance (FVC) responses to reflex increases in sympathetic nerve activity induced by lower body negative pressure (LBNP) at rest and during rhythmic handgrip exercise at 15% and 30% of the maximal voluntary contraction (MVC). FVC was normalized to lean forearm mass. In normal-weight individuals, LBNP evoked a decrease in FVC (-16.1 ± 5.7%) in the resting forearm, and the reduction in FVC (15%MVC: -8.1 ± 3.3%; 30%MVC: -1.0 ± 4.0%) was blunted during exercise in an intensity-dependent manner (P < 0.05). Similarly, in obese individuals, LBNP evoked a comparable decrease in FVC (-10.9 ± 5.7%) in the resting forearm, with the reduction in FVC (15%MVC: -9.7 ± 3.3%; 30%MVC: -0.3 ± 4.0%) also blunted during exercise in an intensity-dependent manner (P < 0.05). The magnitude of sympatholysis was similar between groups (P > 0.05) and was intensity-dependent (P < 0.05). Our findings suggest that functional sympatholysis is not impaired in young obese individuals without overt cardiovascular diseases.

The Contributions of Arterial Cross-Sectional Area and Time-Averaged Flow Velocity to Arterial Blood Flow

Background:

Ultrasound has been used for noninvasive assessments of endothelial function in both clinical and athletic settings and to identify changes in muscle blood flow in response to exercise, nutritional supplementation, and occlusion. The purposes of the present study were to examine the reliability and relative contributions of arterial cross-sectional area and time-averaged flow velocity to predict muscle blood flow as a result of fatiguing exercise in men and women.

Methods:

Eighteen healthy men and 18 healthy women performed 50 consecutive eccentric repetitions of the elbow flexors at 60% of their pretest eccentric peak torque at a velocity of 180° s⁻¹. Test-retest reliability and stepwise linear regression analyses were performed to determine the ability of arterial cross-sectional area and time-averaged flow velocity to predict brachial artery muscle blood flow for the men, women, and combined sample.

Results:

There was no systematic test versus retest mean differences (P > 0.05) for any of the ultrasound determined variables. The two-variable regression models significantly improved the ability to predict muscle blood flow and were associated with smaller standard error of the estimates (3.7%–10.1% vs. 16.8%–37.0% of the mean baseline muscle blood flow values) compared to the one-variable models.

Conclusions:

The findings of the present study supported the use of ultrasound for reliable assessments of arterial diameter, arterial cross-sectional area, time-averaged flow velocity, and muscle blood flow from the brachial artery in men and women. Furthermore, time-averaged flow velocity was a more powerful predictor of muscle blood flow than arterial cross-sectional area.

Exercise-induced hyperemia is associated with knee extensor fatigability in adults with type 2 diabetes

The aim of this study was to compare fatigability, contractile function, and blood flow to the knee extensor muscles after dynamic exercise in patients with type 2 diabetes mellitus (T2DM) and controls. The hypotheses were that patients with T2DM would demonstrate greater fatigability than controls, and greater fatigability would be associated with a lower exercise-induced increase in blood flow and greater impairments in contractile function. Patients with T2DM ( n = 15; 8 men; 62.4 ± 9.0 yr; 30.4 ± 7.7 kg/m²; 7,144 ± 3,294 steps/day) and 15 healthy control subjects (8 men, 58.4 ± 6.9 yr; 28.4 ± 4.6 kg/m²; 7,893 ± 2,323 steps/day) were matched for age, sex, body mass index, and physical activity. Fatigability was quantified as the reduction in knee extensor power during a 6-min dynamic exercise. Before and after exercise, vascular ultrasonography and electrical stimulation were used to assess skeletal muscle blood flow and contractile properties, respectively. Patients with T2DM had greater fatigability (30.0 ± 20.1% vs. 14.6 ± 19.0%, P < 0.001) and lower exercise-induced hyperemia (177 ± 90% vs. 194 ± 79%, P = 0.04) than controls but similar reductions in the electrically evoked twitch amplitude (37.6 ± 24.8% vs. 31.6 ± 30.1%, P = 0.98). Greater fatigability of the knee extensor muscles was associated with postexercise reductions in twitch amplitude ( r = 0.64, P = 0.001) and lesser exercise-induced hyperemia ( r = -0.56, P = 0.009). Patients with T2DM had greater lower-limb fatigability during dynamic exercise, which was associated with reduced contractile function and lower skeletal muscle blood flow. Thus, treatments focused on enhancing perfusion and reversing impairments in contractile function in patients with T2DM may offset lower-limb fatigability and aid in increasing exercise capacity. NEW & NOTEWORTHY Although prior studies compare patients with type 2 diabetes mellitus (T2DM) with lean controls, our study includes controls matched for age, body mass, and physical activity to more closely assess the effects of T2DM. Patients with T2DM demonstrated no impairment in macrovascular endothelial function, evidenced by similar flow-mediated dilation to controls. However, patients with T2DM had greater fatigability and reduced exercise-induced increase in blood flow (hyperemia) after a lower-limb dynamic fatiguing exercise compared with controls.

Adults with initial metabolic syndrome have altered muscle deoxygenation during incremental exercise

OBJECTIVE

Reduced aerobic power is independently associated with metabolic syndrome (MetS) incidence and prevalence in adults. This study investigated whether muscle deoxygenation (proxy of microvascular O₂ extraction) during incremental exercise is altered in MetS and associated with reduced oxygen consumption ( V˙O_2peak ).

METHODS

Twelve men with initial MetS (no overt diseases and medication-naive; mean ± SD, age 38 ± 7 years) and 12 healthy controls (HCs) (34 ± 7 years) completed an incremental cycling test to exhaustion, in which pulmonary ventilation and gas exchange (metabolic analyzer), as well as vastus lateralis deoxygenation (near infrared spectroscopy), were measured.

RESULTS

Subjects with MetS, in contrast to HCs, showed lower V˙O_2peak normalized to total lean mass, similar V˙O₂ response to exercise, and earlier break point (BP) in muscle deoxygenation. Consequently, deoxygenation slope from BP to peak exercise was greater. Furthermore, absolute V˙O_2peak was positively associated with BP in correlations adjusted for total lean mass.

CONCLUSIONS

MetS, without overt diseases, altered kinetics of muscle deoxygenation during incremental exercise, particularly at high-intensity exercise. Therefore, the balance between utilization and delivery of O₂ within skeletal muscle is impaired early in MetS natural history, which may contribute to the reduction in aerobic power.

Impaired skeletal muscle vasodilation during exercise in heart failure with preserved ejection fraction

BACKGROUND

Exercise intolerance is a hallmark symptom of heart failure patients with preserved ejection fraction (HFpEF), which may be related to an impaired ability to appropriately increase blood flow to the exercising muscle.

METHODS

We evaluated leg blood flow (LBF, ultrasound Doppler), heart rate (HR), stroke volume (SV), cardiac output (CO), and mean arterial blood pressure (MAP, photoplethysmography) during dynamic, single leg knee-extensor (KE) exercise in HFpEF patients (n=21; 68 ± 2 yrs) and healthy controls (n=20; 71 ± 2 yrs).

RESULTS

HFpEF patients exhibited a marked attrition during KE exercise, with only 60% able to complete the exercise protocol. In participants who completed all exercise intensities (0-5-10-15 W; HFpEF, n=13; Controls, n=16), LBF was not different at 0 W and 5 W, but was 15-25% lower in HFpEF compared to controls at 10 W and 15 W (P<0.001). Likewise, leg vascular conductance (LVC), an index of vasodilation, was not different at 0 W and 5 W, but was 15-20% lower in HFpEF compared to controls at 10 W and 15 W (P<0.05). In contrast to these peripheral deficits, exercise-induced changes in central variables (HR, SV, CO), as well as MAP, were similar between groups.

CONCLUSIONS

These data reveal a marked reduction in LBF and LVC in HFpEF patients during exercise that cannot be attributed to a disease-related alteration in central hemodynamics, suggesting that impaired vasodilation in the exercising skeletal muscle vasculature may play a key role in the exercise intolerance associated with this patient population.

Rapid onset vasodilation with single muscle contractions in the leg: influence of age

The influence of aging on contraction-induced rapid vasodilation has been well characterized in the forearm. We sought to examine the impact of aging on contraction-induced rapid vasodilation in the leg following single muscle contractions and determine whether potential age-related impairments were similar between limbs (leg vs. arm). Fourteen young (23 ± 1 years) and 16 older (66 ± 1 years) adults performed single leg knee extensions at 20%, 40%, and 60% of work rate maximum. Femoral artery diameter and blood velocity were measured using Doppler ultrasound. Limb vascular conductance (VC) was calculated using blood flow (mL·min⁻¹) and mean arterial pressure (mmHg). Peak and total vasodilator responses in the leg (change [Δ] in VC from baseline) were blunted in older adults by 44–50% across exercise intensities (P < 0.05 for all). When normalized for muscle mass, age-related differences were still evident (P < 0.05). Comparing the rapid vasodilator responses between the arm and the leg of the same individuals at similar relative intensities (20% and 40%) reveals that aging influences peak and total vasodilation equally between the limbs (no significant age × limb interaction at either intensity, P = 0.28–0.80). Our data demonstrate that (1) older adults exhibit an attenuated rapid hyperemic and vasodilator response in the leg; and (2) the age-related reductions in rapid vasodilation are similar between the arm and the leg. The mechanisms contributing to the age-related differences in contraction-induced rapid vasodilation are perhaps similar to those seen with the forearm model, but have not been confirmed.

Lack of independent effect of type 2 diabetes beyond characteristic comorbidities and medications on small muscle mass exercising muscle blood flow and exercise tolerance

Persons with type 2 diabetes (T2D) are believed to have reduced exercise tolerance; this may be partly due to impaired exercising muscle blood flow (MBF). Whether there is an impact of T2D on exercising MBF within the typical constellation of comorbidities (hypertension, dyslipidemia, obesity) and their associated medications has not been investigated. We tested the hypothesis that small muscle mass exercise tolerance is reduced in persons with T2D versus Controls (matched for age, body mass index, fitness, comorbidities, non-T2D medications) and that this is related to blunted MBF. Eight persons with T2D and eight controls completed a forearm critical force (fCF_impulse) test as a measure of exercise tolerance (10-min intermittent maximal effort forearm contractions; the average contraction impulse in the last 30 sec quantified fCF_impulse). Forearm blood flow (FBF; ultrasound) and mean arterial pressure (MAP; finger photoplethysmography) were measured; forearm vascular conductance (FVK) was calculated. Data are means ± SD, T2D versus Control. fCF_impulse was not different between groups (136.9 ± 47.3  N·sec vs. 163.1 ± 49.7 N·sec, P = 0.371) nor was the ΔFBF from rest to during exercise at fCF_impulse (502.9 ± 144.6 vs. 709.1 ± 289.2 mL/min, P = 0.092), or its determinants ΔFVK and ΔMAP (both P > 0.05), although there was considerable interindividual variability. ΔFBF was strongly related to fCF_impulse (r = 0.727, P = 0.002), providing support for the relationship between oxygen delivery and exercise tolerance. We conclude that small muscle mass exercising MBF and exercise tolerance are not impaired in representative persons with T2D versus appropriately matched controls. This suggests that peripheral vascular control impairment does not contribute to reduced exercise tolerance in this population.

Exercise-mediated vasodilation in human obesity and metabolic syndrome: effect of acute ascorbic acid infusion

We tested the hypothesis that infusion of ascorbic acid (AA), a potent antioxidant, would alter vasodilator responses to exercise in human obesity and metabolic syndrome (MetSyn). Forearm blood flow (FBF, Doppler ultrasound) was measured in lean, obese, and MetSyn adults (n = 39, 32 ± 2 yr). A brachial artery catheter was inserted for blood pressure monitoring and local infusion of AA. FBF was measured during dynamic handgrip exercise (15% maximal effort) with and without AA infusion. To account for group differences in blood pressure and forearm size, and to assess vasodilation, forearm vascular conductance (FVC = FBF/mean arterial blood pressure/lean forearm mass) was calculated. We examined the time to achieve steady-state FVC (mean response time, MRT) and the rise in FVC from rest to steady-state exercise (Δ, exercise - rest) before and during acute AA infusion. The MRT (P = 0.26) and steady-state vasodilator responses to exercise (ΔFVC, P = 0.31) were not different between groups. Intra-arterial infusion of AA resulted in a significant increase in plasma total antioxidant capacity (174 ± 37%). AA infusion did not alter MRT or steady-state FVC in any group (P = 0.90 and P = 0.85, respectively). Interestingly, higher levels of C-reactive protein predicted longer MRT (r = 0.52, P < 0.01) and a greater reduction in MRT with AA infusion (r = -0.43, P = 0.02). We concluded that AA infusion during moderate-intensity, rhythmic forearm exercise does not alter the time course or magnitude of exercise-mediated vasodilation in groups of young lean, obese, or MetSyn adults. However, systemic inflammation may limit the MRT to exercise, which can be improved with AA.

The role of muscle mass in exercise-induced hyperemia

Exercise-induced hyperemia is often normalized for muscle mass, and this value is sometimes evaluated at relative exercise intensities to take muscle recruitment into account. Therefore, this study sought to better understand the impact of muscle mass on leg blood flow (LBF) during exercise. LBF was assessed by Doppler ultrasound in 27 young healthy male subjects performing knee-extensor (KE) exercise at three absolute (5, 15, and 25 W) and three relative [20, 40, and 60% of maximum KE (KEmax)] workloads. Thigh muscle mass (5.2-8.1 kg) and LBF were significantly correlated at rest (r = 0.54; P = 0.004). Exercise-induced hyperemia was linearly related to absolute workload, but revealed substantial between-subject variability, documented by the coefficient of variation (5 W: 17%; 15 W: 16%; 25 W: 16%). Quadriceps muscle mass (1.5-2.7 kg) and LBF were not correlated at 5, 15, or 25 W (r = 0.09-0.01; P = 0.7-0.9). Normalizing blood flow for quadriceps muscle mass did not improve the coefficient of variation at each absolute workload (5 W: 21%; 15 W: 21%; 25 W: 22%), while the additional evaluation at relative exercise intensities resulted in even greater variance (20% KEmax: 29%; 40% KEmax: 29%; 60% KEmax: 27%). Similar findings were documented when subjects were parsed into high and low aerobic capacity. Thus, in contrast to rest, blood flow during exercise is unrelated to muscle mass, and simply normalizing for muscle mass or comparing normalized blood flow at a given relative exercise intensity has no effect on the inherent blood flow variability. Therefore, during exercise, muscle mass does not appear to be a determinant of the hyperemic response.

Increased leg blood flow and improved femoral artery shear patterns in metabolic syndrome after a diet and exercise programme

BACKGROUND

Altered vascular shear profiles may contribute to the development of atherosclerosis. Physical activity promotes anti-atherogenic shear patterns, resulting in reduced cardiovascular disease risk. Adults with metabolic syndrome (MetSyn) are at increased risk of developing atherosclerosis and cardiovascular disease. Thus, we hypothesized that conduit artery antegrade shear rate (ASR) would increase and retrograde shear rate (RSR) and oscillatory shear indices (OSI) would decrease in MetSyn patients (n = 16, 51 ± 2 years) after participation in a diet and exercise programme (DEP).

METHODS

Blood flow (Doppler ultrasound, brachial and femoral arteries) was measured, and shear rates were calculated in MetSyn patients before and after 12 weeks of DEP participation. In addition, plasma samples were collected to measure atherogenic markers.

RESULTS

Diet and exercise programme participation increased resting leg blood flow and femoral artery ASR (P ≤ 0·05), and tended to decrease OSI (P = 0·09); RSR did not change (P>0·05). No changes in resting arm blood flow or ASR were observed (P>0·05), and both RSR and OSI increased after participation (P≤0·05). DEP participation reduced plasma vascular cell adhesion molecule(VCAM)-1 (P = 0·03), with a trend for reduced intercellular cell adhesion molecule(ICAM)-1 (P = 0·09) (i.e. atherogenic markers).

CONCLUSION

Modest changes in diet and physical activity result in limb-specific improvements in vascular shear profiles and reduced systemic markers of atherosclerotic risk in MetSyn patients. These data provide novel physiologic insight into adaptations that may limit the progression of atherosclerosis in patients with MetSyn.

Microvascular function in younger adults with obesity and metabolic syndrome: role of oxidative stress

Older adults with cardiovascular disease exhibit microvascular dysfunction and increased levels of reactive oxygen species (ROS). We hypothesized that microvascular impairments begin early in the disease process and can be improved by scavenging ROS. Forearm blood flow (Doppler ultrasound) was measured in 45 young (32 ± 2 yr old) adults (n = 15/group) classified as lean, obese, and metabolic syndrome (MetSyn). Vasodilation in response to endothelial (ACh) and vascular smooth muscle [nitroprusside (NTP) and epoprostenol (Epo)] agonists was tested before and after intra-arterial infusion of ascorbic acid to scavenge ROS. Vasodilation was assessed as a rise in relative vascular conductance (ml·min(-1)·dl(-1)·100 mmHg(-1)). ACh and NTP responses were preserved (P = 0.825 and P = 0.924, respectively), whereas Epo responses were lower in obese and MetSyn adults (P < 0.05) than in lean controls. Scavenging of ROS via infusion of ascorbic acid resulted in an increase in ACh-mediated (P < 0.001) and NTP-mediated (P < 0.001) relative vascular conductance across all groups, suggesting that oxidative stress influences vascular responsiveness in adults with and without overt cardiovascular disease risk. Ascorbic acid had no effect on Epo-mediated vasodilation (P = 0.267). These results suggest that obese and MetSyn adults exhibit preserved endothelium-dependent vasodilation with reduced dependence on prostacyclin and are consistent with an upregulation of compensatory vascular control mechanisms.

Rapid onset vasodilatation is blunted in obese humans

AIM

Conduit artery function in obese humans is frequently assessed at rest, but very little is known about resistance artery function in response to muscle contraction. We tested the hypothesis that obese adults will exhibit reduced contraction-induced rapid onset vasodilatation. Single and brief forearm contractions were used to isolate the local effects of muscle contraction on the forearm vasodilatory response, independent of systemic haemodynamic and sympathetic neural influence.

METHODS

We measured forearm blood flow (Doppler ultrasound), blood pressure (finger photoplethysmography) and heart rate (electrocardiogram) on a beat-by-beat basis in 14 obese (body mass index = 36.2 ± 1.7 kg m(-2)) and 14 lean (body mass index = 21.6 ± 0.7 kg m(-2)) young (18-40 years) adults. Percent changes from baseline in forearm vascular conductance (FVC(%) ) were calculated in response to single, brief forearm contractions performed in random order at 15, 20, 25, 30, 40 and 50% of maximal voluntary contraction (MVC).

RESULTS

In both groups, each single contraction evoked a significant (P < 0.05), immediate (within one cardiac cycle) and graded FVC(%) increase from one up to six cardiac cycles post-contraction. Immediate (20-50% MVC), peak (15-50% MVC) and total (area under the curve, 20-50% MVC) vasodilatory responses were reduced with obesity. The degree of impaired vasodilatation increased with increasing workloads.

CONCLUSIONS

These novel findings demonstrate a blunted contraction-induced rapid onset vasodilatation with obesity that is exercise intensity dependent. Impaired rapid onset vasodilatation may negatively impact haemodynamic responses to everyday intermittent activities performed by obese humans.

Blood Pressure Control at Rest and during Exercise in Obese Children and Adults

The hemodynamic responses to exercise have been studied to a great extent over the past decades, and an exaggerated blood pressure response during an acute exercise bout has been considered as an indicator of cardiovascular risk. Obesity is a major factor influencing the blood pressure response to exercise since evidence indicates that the arterial pressure response to exercise is exacerbated in obese compared with lean adults. Signs of augmented responses (such as an exaggerated blood pressure response) to physical exertion appear early in life (from the prepubertal years) in obese individuals. Understanding the mechanisms that drive the altered hemodynamic responses during exercise in obese individuals and prevent the progression to hypertension is vitally important. This paper focuses on the evidence linking obesity with alterations of the autonomic nervous system and discusses the potential mechanisms and consequences of the altered sympathetic nervous system behavior in obese individuals at rest and during exercise. Furthermore, this paper presents the alterations in the reflex regulatory mechanisms ("exercise pressor reflex" and baroreflex) in obese children and adults and addresses the effects of training on obesity-related disturbances.

Heterogeneous vascular responses to hypoxic forearm exercise in young and older adults

We aimed to assess age-related differences in compensatory hypoxic vasodilation during moderate-to-high dynamic exercise at absolute workloads. We hypothesized healthy older adults (n = 12, 61 ± 1 years) would exhibit impaired hypoxic vasodilation at a moderate absolute workload, and this effect would be exaggerated at a higher workload when compared to young adults (n = 17, 27 ± 2 years). Forearm blood flow (FBF) was measured with Doppler ultrasound. Dynamic forearm exercise (20 contractions/min) was completed at two absolute workloads (8 and 12 kg) under normoxic (0.21 FiO2, ~98% SpO2) and isocapnic hypoxic (~0.10 FiO2, 80% SpO2) conditions performed in random order. FBF was normalized as forearm vascular conductance (FBF / mean arterial blood pressure = FVC) to control for differences in blood pressure and to assess vasodilation. FVC increased with exercise and hypoxia (main effects, p < 0.05); vascular responses were not different between young and older adults (interaction effect exercise × group p = 0.37 and hypoxia × group p = 0.96). Results were confirmed when analyzed as either an absolute or relative change in FVC (ΔFVC and %ΔFVC, respectively). Although group responses to hypoxia were not different, individual results were highly variable (i.e., some adults constricted and others dilated to hypoxia). These data suggest (1) compensatory hypoxic vasodilation in older adults is not impaired during forearm exercise at both moderate and higher absolute exercise intensities, and (2) vascular responses to hypoxia are heterogeneous in both young and older adults. Results suggest unique individual differences exist in factors regulating vascular responses to hypoxia.

Effect of obesity and metabolic syndrome on hypoxic vasodilation

This study was designed to test whether obese adults and adults with metabolic syndrome (MetSyn) exhibit altered hyperemic responses to hypoxia at rest and during forearm exercise when compared with lean controls. We hypothesized blood flow responses due to hypoxia would be lower in young obese subjects (n = 11, 24 ± 2 years, BMI 36 ± 2 kg m(-2)) and subjects with MetSyn (n = 8, 29 ± 3 years BMI 39 ± 2 kg m(-2)) when compared with lean adults (n = 13, 29 ± 2 years, BMI 24 ± 1 kg m(-2)). We measured forearm blood flow (FBF, Doppler Ultrasound) and arterial oxygen saturation (pulse oximetry) during rest and steady-state dynamic forearm exercise (20 contractions/min at 8 and 12 kg) under two conditions: normoxia (0.21 F(i)O(2), ~98% S(a)O(2)) and hypoxia (~0.10 F(i)O(2), 80% S(a)O(2)). Forearm vascular conductance (FVC) was calculated as FBF/mean arterial blood pressure. At rest, the percent change in FVC with hypoxia was greater in adults with MetSyn when compared with lean controls (p = 0.02); obese and lean adult responses were not statistically different. Exercise increased FVC from resting levels in all groups (p < 0.05). Hypoxia caused an additional increase in FVC (p < 0.05) that was not different between groups; responses to hypoxia were heterogeneous within and between groups. Reporting FVC responses as absolute or percent changes led to similar conclusions. These results suggest adults with MetSyn exhibit enhanced hypoxic vasodilation at rest. However, hypoxic responses during exercise in obese adults and adults with MetSyn were not statistically different when compared with lean adults. Individual hypoxic vasodilatory responses were variable, suggesting diversity in vascular control.

Alpha-adrenergic control of blood flow during exercise: effect of sex and menstrual phase

Sex differences exist in autonomic control of the cardiovascular system. This study was designed to directly test sex or female menstrual phase-related differences in α-adrenergic control of blood flow during exercise. We hypothesized that women would exhibit reduced α-adrenergic vasoconstriction compared with men during exercise; in addition, women would constrict less during the early luteal than the early follicular phase of the female menses. Young men (n = 10) were studied once and women (n = 9) studied twice, once during the early follicular phase and once during the early luteal phase of female menses. We measured forearm blood flow (FBF; Doppler ultrasound of the brachial artery) during rest and steady-state dynamic exercise (15 and 30% of maximal voluntary contraction, 20 contractions/min). A brachial artery catheter was inserted for the local administration of α-adrenergic agonists [phenylephrine (PE; α(1)) or clonidine (CL; α(2))]. Blood flow responses to exercise [forearm vascular conductance (FVC)] were similar between all groups. At rest, infusion of PE or CL decreased FVC in all groups (40-60% reduction). Vasoconstriction to PE was abolished in all groups at 15 and 30% exercise intensity. Vasoconstriction to CL was reduced at 15% and abolished at 30% intensity in all groups; women had less CL-induced constriction during the early luteal than early follicular phase (P < 0.017, 15% intensity). These results indicate that vasodilator responses to forearm exercise are comparable between men and women and are achieved through similar paths of α-adrenergic vascular control at moderate intensities; this control may differ at low intensities specific to the female menstrual phase.