Muscle metaboreflex activation during dynamic exercise vasoconstricts ischemic active skeletal muscle

Hemodynamic responses to the cold pressor test in individuals with metabolic syndrome: a case-control study in a multiracial sample of adults

Previous research shows that exercise pressor and metaboreflex responses are significantly exaggerated in individuals with metabolic syndrome, but it is unclear if these exaggerated responses extend to the cold pressor test (CPT). This study tested the hypothesis that, contrary to previously reported exaggerated responses during exercise, CPT responses would not be significantly exaggerated in individuals with MetS compared to matched controls. Eleven individuals with MetS and eleven control participants matched by age, race, sex, and ethnicity completed a cardiometabolic prescreening and a CPT. Each CPT required participants to immerse their hand in ice water for two minutes while beat-by-beat blood pressure, heart rate (HR), and leg blood flow (LBF) were continuously measured. Leg vascular conductance (LVC) was calculated as LBF divided by mean arterial pressure (MAP). The precent changes in MAP, systolic blood pressure (SBP), diastolic blood pressure (DBP), HR, LBF, and LVC were compared across time (BL vs. Minutes 1 and 2 of CPT) and between groups (MetS vs. Control) using repeated measures analyses of variance. As expected, MAP (f = 32.11, p < 0.001), SBP (f = 23.18, p < 0.001), DBP (f = 40.39, p < 0.001), and HR (f = 31.81, p < 0.001) increased during the CPT, and LBF (f = 4.75, p = 0.014) and LVC (f = 13.88, p < 0.001) decreased. However, no significant main effects of group or group by time interactions were observed (f ≤ 0.391, p ≥ 0.539). These findings indicate that the hemodynamic responses to the CPT are not significantly exaggerated in MetS, and therefore, previous reports of exaggerated exercise pressor and metaboreflex responses in MetS cannot be attributed to generalized sympathetic overexcitability.

Cardiac output-mediated regulation of cerebral blood flow during exercise: Clinical perspectives on the indirect impact of muscle metaboreflex

The muscle metaboreflex stimulates the elevation of arterial blood pressure, aiming to rectify the oxygen deficit by enhancing oxygen delivery to support muscle activity. Moreover, activating the muscle metaboreflex significantly increases cardiac output (CO) by increasing factors such as heart rate, ventricular contractility, preload, stroke volume and mobilization of central blood volume. Previous studies indicate that ageing and cardiovascular diseases modify the muscle metaboreflex during exercise, limiting the ability to increase CO during physical activity. Alongside reduced exercise capacity, the attenuated rise in CO due to abnormal muscle metaboreflex in these patients impedes the increase in cerebral blood flow during exercise. Considering that CO plays a pivotal role in regulating cerebral blood flow adequately during exercise, this occurrence might contribute to an elevated risk of cerebral diseases, and it could also, at least, reduce the effective role of exercise in preventing cerebral disease and dementia among elderly individuals and patients with cardiovascular conditions. Therefore, it is important to consider this phenomenon when optimizing the effectiveness of exercise rehabilitation in patients with cardiovascular disease to prevent cerebral diseases and dementia.

Mechanisms mediating muscle metaboreflex control of cardiac output during exercise: Impaired regulation in heart failure

The ability to increase cardiac output during dynamic exercise is paramount for the ability to maintain workload performance. Reflex control of the cardiovascular system during exercise is complex and multifaceted involving multiple feedforward and feedback systems. One major reflex thought to mediate the autonomic adjustments to exercise is termed the muscle metaboreflex and is activated via afferent neurons within active skeletal muscle which respond to the accumulation of interstitial metabolites during exercise when blood flow and O2 delivery are insufficient to meet metabolic demands. This is one of the most powerful cardiovascular reflexes capable of eliciting profound increases in sympathetic nerve activity, arterial blood pressure, central blood volume mobilization, heart rate and cardiac output. This review summarizes the mechanisms meditating muscle metaboreflex-induced increases in cardiac output. Although much has been learned from studies using anaesthetized and/or decerebrate animals, we focus on studies in conscious animals and humans performing volitional exercise. We discuss the separate and interrelated roles of heart rate, ventricular contractility, ventricular preload and ventricular-vascular coupling as well as the interaction with other cardiovascular reflexes which modify muscle metaboreflex control of cardiac output. We discuss how these mechanisms may be altered in subjects with heart failure with reduced ejection fraction and offer suggestions for future studies.

Altered Autonomic Function in Metabolic Syndrome: Interactive Effects of Multiple Components

Metabolic syndrome (MetS) describes a set of disorders that collectively influence cardiovascular health, and includes hypertension, obesity, insulin resistance, diabetes, and dyslipidemia. All these components (hypertension, obesity, dyslipidemia, and prediabetes/diabetes) have been shown to modify autonomic function. The major autonomic dysfunction that has been documented with each of these components is in the control of sympathetic outflow to the heart and periphery at rest and during exercise through modulation of the arterial baroreflex and the muscle metaboreflex. Many studies have described MetS components in singularity or in combination with the other major components of metabolic syndrome. However, many studies lack the capability to study all the factors of metabolic syndrome in one model or have not focused on studying the effects of how each component as it arises influences overall autonomic function. The goal of this review is to describe the current understanding of major aspects of metabolic syndrome that most likely contribute to the consequent/associated autonomic alterations during exercise and discuss their effects, as well as bring light to alternative mechanisms of study.

Muscle metaboreflex stimulates the cardiac sympathetic afferent reflex causing positive feedback amplification of sympathetic activity: effect of heart failure

Exercise intolerance is a hallmark symptom of heart failure and to a large extent stems from reductions in cardiac output that occur due to the inherent ventricular dysfunction coupled with enhanced muscle metaboreflex-induced functional coronary vasoconstriction, which limits increases in coronary blood flow. This creates a further mismatch between O2 delivery and O2 demand, which may activate the cardiac sympathetic afferent reflex (CSAR), causing amplification of the already increased sympathetic activity in a positive-feedback fashion. We used our chronically instrumented conscious canine model to evaluate if chronic ablation of afferents responsible for the CSAR would attenuate the gain of muscle metaboreflex before and after induction of heart failure. After afferent ablation, the gain of the muscle metaboreflex control of mean arterial pressure was significantly reduced before (-239.5 ± 16 to -95.2 ± 8 mmHg/L/min) and after the induction of heart failure (-185.6 ± 14 to -95.7 ± 12 mmHg/L/min). Similar results were observed for the strength (gain) of muscle metaboreflex control of heart rate, cardiac output, and ventricular contractility. Thus, we conclude that the CSAR contributes significantly to the strength of the muscle metaboreflex in normal animals with heart failure serving as an effective positive-feedback amplifier thereby further increasing sympathetic activity.NEW & NOTEWORTHY The powerful pressor responses from the CSAR arise via O2 delivery versus O2 demand imbalance. Muscle metaboreflex activation (MMA) simultaneously elicits coronary vasoconstriction (which is augmented in heart failure) and profound increases in cardiac work thereby upsetting oxygen balance. Whether MMA activates the CSAR thereby amplifying MMA responses is unknown. We observed that removal of the CSAR afferents attenuated the strength of the muscle metaboreflex in normal and subjects with heart failure.

Augmented sympathoexcitation slows postexercise heart rate recovery

Slow heart rate recovery following exercise may be influenced by persistent sympathoexcitation. This study examined 1) the effect of muscle metaboreflex activation (MMA) on heart rate recovery following dynamic exercise; and 2) whether the effect of MMA on heart rate recovery is reversible by reducing sympathoexcitation [baroreflex activation via phenylephrine (PE)] in canines. Twenty-two young adults completed control and MMA protocols during cycle ergometry at 110% ventilatory threshold with 5 min recovery. Heart rate recovery kinetics [tau (τ), amplitude, end-exercise, and end-recovery heart rate] and root mean square of successive differences (RMSSD) were measured. Five chronically instrumented canines completed control, MMA (50%-60% imposed reduction in hindlimb blood flow), and MMA with end-exercise PE infusion (MMA + PE) protocols during moderate exercise (6.4 km·h-1) and 3 min recovery. Heart rate recovery kinetics and MAP were measured. MAP increased during MMA versus control in canines (P < 0.001). Heart rate recovery τ was slower during MMA versus control in humans (17% slower; P = 0.011) and canines (150% slower; P = 0.002). Heart rate recovery τ was faster during MMA + PE versus MMA (40% faster; P = 0.034) and was similar to control in canines (P = 0.426). Amplitude, end-exercise, and end-recovery heart rate were similar between conditions in humans (all P ≥ 0.122) and in canines (all P ≥ 0.084). MMA decreased RMSSD in early recovery (P = 0.004). MMA-induced sympathoexcitation slows heart rate recovery and this effect is markedly attenuated with PE. Therefore, elevated sympathoexcitation via MMA impairs heart rate recovery and inhibition of this stimulus normalizes, in part, heart rate recovery.NEW & NOTEWORTHY Augmented sympathoexcitation, via muscle metaboreflex activation, functionally slows heart rate recovery in both young healthy adults and chronically instrumented canines. Furthermore, elevated sympathoexcitation corresponded with lower parasympathetic activity, as assessed by heart rate variability, during the first 3 min of recovery. Finally, sympathoinhibition, via phenylephrine infusion, normalizes heart rate recovery during muscle metaboreflex activation.

Blood flow restriction training activates the muscle metaboreflex during low-intensity sustained exercise

Blood flow restriction training (BFRT) employs partial vascular occlusion of exercising muscle and has been shown to increase muscle performance while using reduced workload and training time. Numerous studies have demonstrated that BFRT increases muscle hypertrophy, mitochondrial function, and beneficial vascular adaptations. However, changes in cardiovascular hemodynamics during the exercise protocol remain unknown, as most studies measured blood pressure before the onset and after the cessation of exercise. With reduced perfusion to the exercising muscle during BFRT, the resultant accumulation of metabolites within the ischemic muscle could potentially trigger a large reflex increase in blood pressure, termed the muscle metaboreflex. At low workloads, this pressor response occurs primarily via increases in cardiac output. However, when increases in cardiac output are limited (e.g., heart failure or during severe exercise), the reflex shifts to peripheral vasoconstriction as the primary mechanism to increase blood pressure, potentially increasing the risk of a cardiovascular event. Using our chronically instrumented conscious canine model, we utilized a 60% reduction in femoral blood pressure applied to the hindlimbs during steady-state treadmill exercise (3.2 km/h) to reproduce the ischemic environment observed during BFRT. We observed significant increases in heart rate (+19 ± 3 beats/min), stroke volume (+2.52 ± 1.2 mL), cardiac output (+1.21 ± 0.2 L/min), mean arterial pressure (+18.2 ± 2.4 mmHg), stroke work (+1.93 ± 0.2 L/mmHg), and nonischemic vascular conductance (+3.62 ± 1.7 mL/mmHg), indicating activation of the muscle metaboreflex.NEW & NOTEWORTHY Blood flow restriction training (BFRT) increases muscle mass, strength, and endurance. There has been minimal consideration of the reflex cardiovascular responses that could be elicited during BFRT sessions. We showed that during low-intensity exercise BFRT may trigger large reflex increases in blood pressure and sympathetic activity due to muscle metaboreflex activation. Thus, we urge caution when employing BFRT, especially in patients in whom exaggerated cardiovascular responses may occur that could cause sudden, adverse cardiovascular events.

Effectiveness of Respiratory Muscles Training by Voluntary Isocapnic Hyperpnea Versus Inspiratory Threshold Loading on Intercostales and Vastus Lateralis Muscles Deoxygenation Induced by Exercise in Physically Active Adults

Respiratory muscle training (RMT) improves physical performance, although it is still debated whether this effect depends on the type of training. The purpose of this study was to compare the effects of two different types of RMT, i.e., voluntary isocapnic hyperpnea (VIH) and inspiratory threshold loading (ITL), on the deoxygenation of intercostal (ΔSmO₂-m. intercostales) and vastus lateralis (ΔSmO₂-m. vastus lateralis) muscles during exercise. Twenty-four participants performed eight weeks of RMT by: (i) VIH (3 days·week^-1 for 12 min at 60% maximal voluntary ventilation) or (ii) ITL (5 sets·week^-1 of 30 breaths·minute^-1 at 60% maximal inspiratory pressure). Cardiopulmonary exercise testing (CPET) included ΔSmO₂ (the change from baseline to end of test) of intercostal and vastus lateralis muscles. After RMT, both groups showed decreased ΔSmO₂-m. intercostales (VIH = 12.8 ± 14.6%, p = 0.04 (effect size, ES = 0.59), and ITL = 8.4 ± 9.8%, p = 0.04 (ES = 0.48)), without a coincident change of ∆SmO_2-m. vastus lateralis. ITL training induced higher V˙O_2-peak absolute values than VIH (mean Δ post-pre, ITL = 229 ± 254 mL·min^-1 [95% CI 67-391] vs. VIH, 39 ± 153 mL·min^-1 [95% CI -58-136.0], p = 0.01). In conclusion, both RMT improved the balance between supply and oxygen consumption levels of m. intercostales during CPET, with ITL also inducing an increase of aerobic capacity.

Arterial Baroreflex Inhibits Muscle Metaboreflex Induced Increases in Effective Arterial Elastance: Implications for Ventricular-Vascular Coupling

The ventricular-vascular relationship assesses the efficacy of energy transferred from the left ventricle to the systemic circulation and is quantified as the ratio of effective arterial elastance to maximal left ventricular elastance. This relationship is maintained during exercise via reflex increases in cardiovascular performance raising both arterial and ventricular elastance in parallel. These changes are, in part, due to reflexes engendered by activation of metabosensitive skeletal muscle afferents-termed the muscle metaboreflex. However, in heart failure, ventricular-vascular uncoupling is apparent and muscle metaboreflex activation worsens this relationship through enhanced systemic vasoconstriction markedly increasing effective arterial elastance which is unaccompanied by substantial increases in ventricular function. This enhanced arterial vasoconstriction is, in part, due to significant reductions in cardiac performance induced by heart failure causing over-stimulation of the metaboreflex due to under perfusion of active skeletal muscle, but also as a result of reduced baroreflex buffering of the muscle metaboreflex-induced peripheral sympatho-activation. To what extent the arterial baroreflex modifies the metaboreflex-induced changes in effective arterial elastance is unknown. We investigated in chronically instrumented conscious canines if removal of baroreflex input via sino-aortic baroreceptor denervation (SAD) would significantly enhance effective arterial elastance in normal animals and whether this would be amplified after induction of heart failure. We observed that effective arterial elastance (E_a), was significantly increased during muscle metaboreflex activation after SAD (0.4 ± 0.1 mmHg/mL to 1.4 ± 0.3 mmHg/mL). In heart failure, metaboreflex activation caused exaggerated increases in E_a and in this setting, SAD significantly increased the rise in E_a elicited by muscle metaboreflex activation (1.3 ± 0.3 mmHg/mL to 2.3 ± 0.3 mmHg/mL). Thus, we conclude that the arterial baroreflex does buffer muscle metaboreflex induced increases in E_a and this buffering likely has effects on the ventricular-vascular coupling.

Chronic ablation of TRPV1-sensitive skeletal muscle afferents attenuates the muscle metaboreflex

Exercise intolerance is a hallmark symptom of cardiovascular disease and likely occurs via enhanced activation of muscle metaboreflex-induced vasoconstriction of the heart and active skeletal muscle which, thereby limits cardiac output and peripheral blood flow. Muscle metaboreflex vasoconstrictor responses occur via activation of metabolite-sensitive afferent fibers located in ischemic active skeletal muscle, some of which express transient receptor potential vanilloid 1 (TRPV1) cation channels. Local cardiac and intrathecal administration of an ultrapotent noncompetitive, dominant negative agonist resiniferatoxin (RTX) can ablate these TRPV1-sensitive afferents. This technique has been used to attenuate cardiac sympathetic afferents and nociceptive pain. We investigated whether intrathecal administration (L4-L6) of RTX (2 µg/kg) could chronically attenuate subsequent muscle metaboreflex responses elicited by reductions in hindlimb blood flow during mild exercise (3.2 km/h) in chronically instrumented conscious canines. RTX significantly attenuated metaboreflex-induced increases in mean arterial pressure (27 ± 5.0 mmHg vs. 6 ± 8.2 mmHg), cardiac output (1.40 ± 0.2 L/min vs. 0.28 ± 0.1 L/min), and stroke work (2.27 ± 0.2 L·mmHg vs. 1.01 ± 0.2 L·mmHg). Effects were maintained until 78 ± 14 days post-RTX at which point the efficacy of RTX injection was tested by intra-arterial administration of capsaicin (20 µg/kg). A significant reduction in the mean arterial pressure response (+45.7 ± 6.5 mmHg pre-RTX vs. +19.7 ± 3.1 mmHg post-RTX) was observed. We conclude that intrathecal administration of RTX can chronically attenuate the muscle metaboreflex and could potentially alleviate enhanced sympatho-activation observed in cardiovascular disease states.

Ventricular contraction and relaxation rates during muscle metaboreflex activation in heart failure: are they coupled?

NEW FINDINGS

What is the central question of this study? Does the muscle metaboreflex affect the ratio of left ventricular contraction/relaxation rates and does heart failure impact this relationship. What is the main finding and its importance? The effect of muscle metaboreflex activation on the ventricular relaxation rate was significantly attenuated in heart failure. Heart failure attenuates the exercise and muscle metaboreflex-induced changes in the contraction/relaxation ratio. In heart failure, the reduced ability to raise cardiac output during muscle metaboreflex activation may not solely be due to attenuation of ventricular contraction but also alterations in ventricular relaxation and diastolic function.

ABSTRACT

The relationship between contraction and relaxation rates of the left ventricle varies with exercise. In in vitro models, this ratio was shown to be relatively unaltered by changes in sarcomere length, frequency of stimulation, and β-adrenergic stimulation. We investigated whether the ratio of contraction to relaxation rate is maintained in the whole heart during exercise and muscle metaboreflex activation and whether heart failure alters these relationships. We observed that in healthy subjects the ratio of contraction to relaxation increases from rest to exercise as a result of a higher increase in contraction relative to relaxation. During muscle metaboreflex activation the ratio of contraction to relaxation is significantly reduced towards 1.0 due to a large increase in relaxation rate matching contraction rate. In heart failure, contraction and relaxation rates are significantly reduced, and increases during exercise are attenuated. A significant increase in the ratio was observed from rest to exercise although baseline ratio values were significantly reduced close to 1.0 when compared to healthy subjects. There was no significant change observed between exercise and muscle metaboreflex activation nor was the ratio during muscle metaboreflex activation significantly different between heart failure and control. We conclude that heart failure reduces the muscle metaboreflex gain and contraction and relaxation rates. Furthermore, we observed that the ratio of the contraction and relaxation rates during muscle metaboreflex activation is not significantly different between control and heart failure, but significant changes in the ratio in healthy subjects due to increased relaxation rate were abolished in heart failure.

Recent advances in exercise pressor reflex function in health and disease

Autonomic alterations at the onset of exercise are critical to redistribute cardiac output towards the contracting muscles while preventing a fall in arterial pressure due to excessive vasodilation within the contracting muscles. Neural mechanisms responsible for these adjustments include central command, the exercise pressor reflex, and arterial and cardiopulmonary baroreflexes. The exercise pressor reflex evokes reflex increases in sympathetic activity to the heart and systemic vessels and decreases in parasympathetic activity to the heart, which increases blood pressure (BP), heart rate, and total peripheral resistance through vasoconstriction of systemic vessels. In this review, we discuss recent advancements in our understanding of exercise pressor reflex function in health and disease. Specifically, we discuss emerging evidence suggesting that sympathetic vasoconstrictor drive to the contracting and non-contracting skeletal muscle is differentially controlled by central command and the metaboreflex in healthy conditions. Further, we discuss evidence from animal and human studies showing that cardiovascular diseases, including hypertension, diabetes, and heart failure, lead to an altered exercise pressor reflex function. We also provide an update on the mechanisms thought to underlie this altered exercise pressor reflex function in each of these diseases. Although these mechanisms are complex, multifactorial, and dependent on the etiology of the disease, there is a clear consensus that several mechanisms are involved. Ultimately, approaches targeting these mechanisms are clinically significant as they provide alternative therapeutic strategies to prevent adverse cardiovascular events while also reducing symptoms of exercise intolerance.

Difference in the integrated effects of sympathetic vasoconstriction and local vasodilation in human skeletal muscle and skin microvasculature

We investigated the integration of sympathetic vasoconstriction and local vasodilation in the skeletal muscle and skin microvasculature of humans. In 39 healthy volunteers, we simultaneously measured the blood flow index in the flexor carpi radialis muscle using diffuse correlation spectroscopy and the skin using laser‐Doppler flowmetry. We examined the effects of acute sympathoexcitation induced by forehead cooling on relatively weak and robust vasodilatory responses during postocclusive reactive hyperemia (PORH) induced by 70‐sec and 10‐min arterial occlusion in the upper arm. To increase sympathetic tone during PORH, forehead cooling was begun 60 sec before the occlusion release and ended 60 sec after the release. In the 70‐sec occlusion trials, acute sympathoexcitation reduced the peak and duration of vasodilation in both skeletal muscle and skin. The inhibition of vasodilation by sympathoexcitation was blunted in both tissues by the robust vasodilatory stimulation produced by the 10‐min occlusion, and the degree of blunting was greater in skeletal muscle than in skin, especially the initial and peak responses. Sympathoexcitation reduced the peak vasodilation only in skin, while it accelerated the initial vasodilation only in skeletal muscle. However, the decline in vasodilation after the peak was significantly hastened in skeletal muscle, shortening the duration of the vasodilation. We conclude that, in humans, the integration of sympathetic vasoconstriction and local vasodilation has different effects in skeletal muscle and skin and is likely an important contributor to the selective control of perfusion in the microcirculations of different tissues.

Muscle metaboreflex-induced increases in effective arterial elastance: effect of heart failure

Dynamic exercise elicits robust increases in sympathetic activity in part due to muscle metaboreflex activation (MMA), a pressor response triggered by activation of skeletal muscle afferents. MMA during dynamic exercise increases arterial pressure by increasing cardiac output via increases in heart rate, ventricular contractility, and central blood volume mobilization. In heart failure, ventricular function is compromised, and MMA elicits peripheral vasoconstriction. Ventricular-vascular coupling reflects the efficiency of energy transfer from the left ventricle to the systemic circulation and is calculated as the ratio of effective arterial elastance (E_a) to left ventricular maximal elastance (E_max). The effect of MMA on E_a in normal subjects is unknown. Furthermore, whether muscle metaboreflex control of E_a is altered in heart failure has not been investigated. We utilized two previously published methods of evaluating E_a [end-systolic pressure/stroke volume (E_aPV)] and [heart rate × vascular resistance (E_aZ)] during rest, mild treadmill exercise, and MMA (induced via partial reductions in hindlimb blood flow imposed during exercise) in chronically instrumented conscious canines before and after induction of heart failure via rapid ventricular pacing. In healthy animals, MMA elicits significant increases in effective arterial elastance and stroke work that likely maintains ventricular-vascular coupling. In heart failure, E_a is high, and MMA-induced increases are exaggerated, which further exacerbates the already uncoupled ventricular-vascular relationship, which likely contributes to the impaired ability to raise stroke work and cardiac output during exercise in heart failure.

Muscle pump-induced inhibition of sympathetic vasomotor outflow during low-intensity leg cycling is attenuated by muscle metaboreflex activation

Muscle sympathetic nerve activity (MSNA) decreases during leg cycling at low intensity because of muscle pump-induced increases in venous return and loading of the cardiopulmonary baroreceptors. However, MSNA increases during leg cycling when exercise is above moderate intensity or for a long duration, suggesting that the sympathoinhibitory effect of the cardiopulmonary baroreflex can be overridden by a powerful sympathoexcitatory drive, such as the skeletal muscle metaboreflex. Therefore, we tested the hypothesis that high-intensity muscle metaboreflex activation attenuates muscle pump-induced inhibition of MSNA during leg cycling. MSNA (left radial nerve) was recorded during graded isolation of the muscle metaboreflex in the forearm with postexercise ischemia (PEI) after low (PEI-L)- and high (PEI-H)-intensity isometric handgrip exercise (20% and 40% maximum voluntary contraction, respectively). Leg cycling (15–20 W) was performed alone and during each PEI trial (PEI-L+Cycling, PEI-H+Cycling). Cycling alone induced a significant decrease in MSNA burst frequency (BF) and total activity (TA). MSNA BF and TA also decreased when cycling was performed during PEI-L. However, the magnitude of decrease in MSNA during PEI-L+Cycling [∆BF: –19 ± 2% ( P < 0.001), ∆TA: –25 ± 4% ( P < 0.001); mean ± SE] was less than that during cycling alone [∆BF: –39 ± 5% ( P = 0.003), ∆TA: –45 ± 5% ( P = 0.002)]. More importantly, MSNA did not decrease during cycling with PEI-H [∆BF: –1 ± 2% ( P = 0.845), ∆TA: +2 ± 3% ( P = 0.959)]. These results suggest that muscle pump-induced inhibition of sympathetic vasomotor outflow during low-intensity leg cycling is attenuated by muscle metaboreflex activation in an intensity-dependent manner.

NEW & NOTEWORTHY There are no available data concerning the interaction between the sympathoinhibitory effect of muscle pump-induced cardiopulmonary baroreflex loading during leg cycling and the sympathoexcitatory influence of the muscle metaboreflex. In this study, muscle metaboreflex activation attenuated the inhibition of muscle sympathetic nerve activity (MSNA) during leg cycling. This may explain, in part, the response of MSNA to graded-intensity dynamic exercise in which low-intensity leg cycling inhibits MSNA whereas high-intensity exercise elicits graded sympathoexcitation.

Clinical safety of blood flow-restricted training? A comprehensive review of altered muscle metaboreflex in cardiovascular disease during ischemic exercise

Blood flow restriction training (BFRT) is an increasingly widespread method of exercise that involves imposed restriction of blood flow to the exercising muscle. Blood flow restriction is achieved by inflating a pneumatic pressure cuff (or a tourniquet) positioned proximal to the exercising muscle before, and during, the bout of exercise (i.e., ischemic exercise). Low-intensity BFRT with resistance training promotes comparable increases in muscle mass and strength observed during high-intensity exercise without blood flow restriction. BFRT has expanded into the clinical research setting as a potential therapeutic approach to treat functionally impaired individuals, such as the elderly, and patients with orthopedic and cardiovascular disease/conditions. However, questions regarding the safety of BFRT must be fully examined and addressed before the implementation of this exercise methodology in the clinical setting. In this respect, there is a general concern that BFRT may generate abnormal reflex-mediated cardiovascular responses. Indeed, the muscle metaboreflex is an ischemia-induced, sympathoexcitatory pressor reflex originating in skeletal muscle, and the present review synthesizes evidence that BFRT may elicit abnormal cardiovascular responses resulting from increased metaboreflex activation. Importantly, abnormal cardiovascular responses are more clearly evidenced in populations with increased cardiovascular risk (e.g., elderly and individuals with cardiovascular disease). The evidence provided in the present review draws into question the cardiovascular safety of BFRT, which clearly needs to be further investigated in future studies. This information will be paramount for the consideration of BFRT exercise implementation in clinical populations.

The magnitude of the exercise pressor reflex is influenced by the active skeletal muscle mass in the decerebrate rat

The exercise pressor reflex is composed of two components, namely the muscle mechanoreflex and the muscle metaboreflex. The afferents evoking the two components are either thinly myelinated (group III) or unmyelinated (group IV); in combination they are termed "thin fiber afferents." The exercise pressor reflex is often studied in unanesthetized, decerebrate rats. However, the relationship between the magnitude of this reflex and the number of thin fiber afferents stimulated by muscle contraction is unknown. This lack of knowledge prompted us to test the hypothesis that the magnitude of the exercise pressor reflex was directly proportional to the amount of muscle mass activated. Muscle mechanoreceptors were stimulated by stretching the calcaneal tendon. Likewise, muscle metaboreceptors were stimulated by injecting lactic acid into the arterial supply of the hindlimb muscles. In addition, both muscle mechanoreceptors and metaboreceptors were stimulated by statically contracting the hindlimb muscles. We found that simultaneous bilateral (both hindlimbs) stimulation of thin fiber afferents with stretch, lactic acid, and static contraction evoked significantly greater pressor responses than did unilateral (one hindlimb) stimulation of these afferents. In addition, the magnitude of the pressor responses to bilateral simultaneous stimulation of thin fiber afferents evoked by stretch, lactic acid, and contraction was not significantly different from the magnitude of the sum of the pressor responses evoked by unilateral stimulation of these afferents by stretch, lactic acid, and contraction. We conclude that the magnitude of the exercise pressor reflex and its two components is dependent on the number of afferents stimulated.

Central cardiovascular hemodynamic response to unilateral handgrip exercise with blood flow restriction

AIM

Exercise training with blood flow restriction (BFR) increases muscle size and strength. However, there is limited investigation into the effects of BFR on cardiovascular health, particularly central hemodynamic load.

PURPOSE

To determine the effects of BFR exercise on central hemodynamic load (heart rate-HR, central pressures, arterial wave reflection, and aortic stiffness).

METHODS

Fifteen males (age = 25 ± 2 years; BMI = 27 ± 2 kg/m², handgrip max voluntary contraction-MVC = 50 ± 2 kg) underwent 5-min bouts (counter-balanced, 10 min rest between) of rhythmic unilateral handgrip (1 s squeeze, 2 s relax) performed with a moderate-load (60% MVC) with and without BFR (i.e., 71 ± 5% arterial inflow flow reduction, assessed via Doppler ultrasound), and also with a low-load (40% MVC) with BFR. Outcomes included HR, central mean arterial pressure (cMAP), arterial wave reflection (augmentation index, AIx; wave reflection magnitude, RM%), aortic arterial stiffness (pulse wave velocity, aPWV), and peripheral (vastus lateralis) microcirculatory response (tissue saturation index, TSI%).

RESULTS

HR increased above baseline and time control for all handgrip bouts, but was similar between the moderate load with and without BFR conditions (moderate-load with BFR =  + 9 ± 2; moderate-load without BFR =  + 8 ± 2 bpm, p < 0.001). A similar finding was noted for central pressure (e.g., moderate load with BFR, cMAP =  + 14 ± 1 mmHg, p < 0.001). No change occurred for RM% or AIx (p > 0.05) for any testing stage. TSI% increased during the moderate-load conditions (p = 0.01), and aPWV increased above baseline following moderate-load handgrip with BFR only (p = 0.012).

CONCLUSIONS

Combined with BFR, moderate load handgrip training with BFR does not significantly augment central hemodynamic load during handgrip exercise in young healthy men.

Neural Control of Cardiovascular Function During Exercise in Hypertension

During both static and dynamic exercise hypertensive subjects can experience robust increases in arterial pressure to such an extent that heavy exercise is often not recommended in these patients due to the dangerously high levels of blood pressure sometimes observed. Currently, the mechanisms mediating this cardiovascular dysfunction during exercise in hypertension are not fully understood. The major reflexes thought to mediate the cardiovascular responses to exercise in normotensive healthy subjects are central command, arterial baroreflex and responses to stimulation of skeletal muscle mechano-sensitive and metabo-sensitive afferents. This review will summarize our current understanding of the roles of these reflexes and their interactions in mediating the altered cardiovascular responses to exercise observed in hypertension. We conclude that much work is needed to fully understand the mechanisms mediating excessive pressor response to exercise often seen in hypertensive patients.

Altered arterial baroreflex-muscle metaboreflex interaction in heart failure

Two powerful reflexes controlling cardiovascular function during exercise are the muscle metaboreflex and arterial baroreflex. In heart failure (HF), the strength and mechanisms of these reflexes are altered. Muscle metaboreflex activation (MMA) in normal subjects increases mean arterial pressure (MAP) primarily via increases in cardiac output (CO), whereas in HF the mechanism shifts to peripheral vasoconstriction. Baroreceptor unloading increases MAP via peripheral vasoconstriction, and this pressor response is blunted in HF. Baroreceptor unloading during MMA in normal animals elicits an enormous pressor response via combined increases in CO and peripheral vasoconstriction. The mode of interaction between these reflexes is intimately dependent on the parameter (e.g., MAP and CO) being investigated. The interaction between the two reflexes when activated simultaneously during dynamic exercise in HF is unknown. We activated the muscle metaboreflex in chronically instrumented dogs during mild exercise (via graded reductions in hindlimb blood flow) followed by baroreceptor unloading [via bilateral carotid occlusion (BCO)] before and after induction of HF. We hypothesized that BCO during MMA in HF would cause a smaller increase in MAP and a larger vasoconstriction of ischemic hindlimb vasculature, which would attenuate the restoration of blood flow to ischemic muscle observed in normal dogs. We observed that BCO during MMA in HF increases MAP by substantial vasoconstriction of all vascular beds, including ischemic active muscle, and that all cardiovascular responses, except ventricular function, exhibit occlusive interaction. We conclude that vasoconstriction of ischemic active skeletal muscle in response to baroreceptor unloading during MMA attenuates restoration of hindlimb blood flow. NEW & NOTEWORTHY We found that baroreceptor unloading during the muscle metaboreflex in heart failure results in occlusive interaction (except for ventricular function) with significant vasoconstriction of all vascular beds. In addition, restoration of blood flow to ischemic active muscle, via preferentially larger vasoconstriction of nonischemic beds, is significantly attenuated in heart failure.

Voluntary apnea during dynamic exercise activates the muscle metaboreflex in humans

Voluntary apnea during dynamic exercise evokes marked bradycardia, peripheral vasoconstriction, and pressor responses. However, the mechanism(s) underlying the cardiovascular responses seen during apnea in exercising humans is unknown. We therefore tested the hypothesis that the muscle metaboreflex contributes to the apnea-induced pressor response during dynamic exercise. Thirteen healthy subjects participated in apnea and control trials. In both trials, subjects performed a two-legged dynamic knee extension exercise at a workload that elicited heart rates at ~100 beats/min. In the apnea trial, after reaching a steady state, subjects began voluntary apnea. Immediately after cessation of the apnea, arterial occlusion was initiated at both thighs and the subjects stopped exercising. The occlusion was sustained for 3 min in the postexercise period. In the control trial, the occlusion was started without subjects performing the apnea. The apnea induced marked bradycardia, pressor responses, and decreases in arterial O₂ saturation, cardiac output, and total vascular conductance. In addition, arterial blood pressure was significantly higher and total vascular conductance was significantly lower in the apnea trials than the control trials throughout the occlusion period. In separate sessions, we measured apnea-induced changes in exercising leg blood flow in the same subjects. Leg blood flow was significantly reduced by apnea and reached the resting level at the peak of the apnea response. We conclude that the muscle metaboreflex is activated by the decrease in O₂ delivery to the working muscle during apnea in exercising humans and contributes to the large pressor response. NEW & NOTEWORTHY We demonstrated that apnea during dynamic exercise activates the muscle metaboreflex in humans. This result indicates that a reduction in O₂ delivery to working muscle triggers the muscle metaboreflex during apnea. Activation of the muscle metaboreflex is one of the mechanisms underlying the marked apnea-induced pressor response.

Exaggerated cardiovascular responses to treadmill running in rats with peripheral arterial insufficiency

Patients with atherosclerotic peripheral artery disease have an augmented pressor response to treadmill walking, but the underlying mechanisms remain poorly understood and difficult to isolate because of the confounding presence of numerous cardiovascular risk factors. In the present study, we tested the hypothesis that a chronic deficit in muscle blood flow capacity would be sufficient to trigger an exaggerated pressor response to dynamic exercise. Sprague-Dawley rats (5 male and 5 female) were instrumented with radiotelemetry devices to measure the cardiovascular responses to treadmill running before and after bilateral femoral artery ligation, which has been previously shown to reduce the blood flow capacity of distal hindlimb muscles by >60%. Treadmill running evoked reproducible increases in mean arterial pressure (MAP) and heart rate (HR), which were significantly augmented 3 days after femoral artery ligation in both male rats [ΔMAP: +10 ± 1 (SE) vs. +18 ± 3 mmHg and ΔHR: +94 ± 12 vs. +148 ± 15 beats/min, P < 0.05] and female rats (ΔMAP: +16 ± 3 vs. +30 ± 5 mmHg and ΔHR: +128 ± 20 vs. +178 ± 19 beats/min, P < 0.05). Similar exaggerated MAP and HR responses were observed at repeated intervals between 3 and 65 days postligation. These findings indicate that a chronic deficit in muscle blood flow capacity is an important, persistent cause of the abnormal pressor and cardioaccelerator responses to dynamic exercise in both male and female rats with peripheral arterial insufficiency. NEW & NOTEWORTHY Using radiotelemetry to assess cardiovascular effects of exercise, we showed that femoral artery obstruction in male and female rats is an important, persistent cause of exaggerated pressor and cardioaccelerator responses to treadmill running. This translational model reproduces the abnormal cardiovascular response to exercise seen in patients with peripheral artery disease. Listen to this article's corresponding podcast at http://ajpheart.podbean.com/e/treadmill-bp-in-simulated-peripheral-artery-disease/ .

Muscle metaboreflex-induced vasoconstriction in the ischemic active muscle is exaggerated in heart failure

When oxygen delivery to active muscle is insufficient to meet the metabolic demand during exercise, metabolites accumulate and stimulate skeletal muscle afferents, inducing a reflex increase in blood pressure, termed the muscle metaboreflex. In healthy individuals, muscle metaboreflex activation (MMA) during submaximal exercise increases arterial pressure primarily via an increase in cardiac output (CO), as little peripheral vasoconstriction occurs. This increase in CO partially restores blood flow to ischemic muscle. However, we recently demonstrated that MMA induces sympathetic vasoconstriction in ischemic active muscle, limiting the ability of the metaboreflex to restore blood flow. In heart failure (HF), increases in CO are limited, and metaboreflex-induced pressor responses occur predominantly via peripheral vasoconstriction. In the present study, we tested the hypothesis that vasoconstriction of ischemic active muscle is exaggerated in HF. Changes in hindlimb vascular resistance [femoral arterial pressure ÷ hindlimb blood flow (HLBF)] were observed during MMA (via graded reductions in HLBF) during mild exercise with and without α₁-adrenergic blockade (prazosin, 50 µg/kg) before and after induction of HF. In normal animals, initial HLBF reductions caused metabolic vasodilation, while reductions below the metaboreflex threshold elicited reflex vasoconstriction, in ischemic active skeletal muscle, which was abolished after α₁-adrenergic blockade. Metaboreflex-induced vasoconstriction of ischemic active muscle was exaggerated after induction of HF. This heightened vasoconstriction impairs the ability of the metaboreflex to restore blood flow to ischemic muscle in HF and may contribute to the exercise intolerance observed in these patients. We conclude that sympathetically mediated vasoconstriction of ischemic active muscle during MMA is exaggerated in HF. NEW & NOTEWORTHY We found that muscle metaboreflex-induced vasoconstriction of the ischemic active skeletal muscle from which the reflex originates is exaggerated in heart failure. This results in heightened metaboreflex activation, which further amplifies the reflex-induced vasoconstriction of the ischemic active skeletal muscle and contributes to exercise intolerance in patients.

Exaggerated coronary vasoconstriction limits muscle metaboreflex-induced increases in ventricular performance in hypertension

Increases in myocardial oxygen consumption during exercise mainly occur via increases in coronary blood flow (CBF) as cardiac oxygen extraction is high even at rest. However, sympathetic coronary constrictor tone can limit increases in CBF. Increased sympathetic nerve activity (SNA) during exercise likely occurs via the action of and interaction among activation of skeletal muscle afferents, central command, and resetting of the arterial baroreflex. As SNA is heightened even at rest in subjects with hypertension (HTN), we tested whether HTN causes exaggerated coronary vasoconstriction in canines during mild treadmill exercise with muscle metaboreflex activation (MMA; elicited by reducing hindlimb blood flow by ~60%) thereby limiting increases in CBF and ventricular performance. Experiments were repeated after α₁-adrenergic blockade (prazosin; 75 µg/kg) and in the same animals following induction of HTN (modified Goldblatt 2K1C model). HTN increased mean arterial pressure from 97.1 ± 2.6 to 132.1 ± 5.6 mmHg at rest and MMA-induced increases in CBF, left ventricular dP/dt_max, and cardiac output were markedly reduced to only 32 ± 13, 26 ± 11, and 28 ± 12% of the changes observed in control. In HTN, α₁-adrenergic blockade restored the coronary vasodilation and increased in ventricular function to the levels observed when normotensive. We conclude that exaggerated MMA-induced increases in SNA functionally vasoconstrict the coronary vasculature impairing increases in CBF, which limits oxygen delivery and ventricular performance in HTN.

NEW & NOTEWORTHY

We found that metaboreflex-induced increases in coronary blood flow and ventricular contractility are attenuated in hypertension. α₁-Adrenergic blockade restored these parameters toward normal levels. These findings indicate that the primary mechanism mediating impaired metaboreflex-induced increases in ventricular function in hypertension is accentuated coronary vasoconstriction.

Interaction between the muscle metaboreflex and the arterial baroreflex in control of arterial pressure and skeletal muscle blood flow

The muscle metaboreflex and arterial baroreflex regulate arterial pressure through distinct mechanisms. During submaximal exercise muscle metaboreflex activation (MMA) elicits a pressor response virtually solely by increasing cardiac output (CO) while baroreceptor unloading increases mean arterial pressure (MAP) primarily through peripheral vasoconstriction. The interaction between the two reflexes when activated simultaneously has not been well established. We activated the muscle metaboreflex in chronically instrumented canines during dynamic exercise (via graded reductions in hindlimb blood flow; HLBF) followed by simultaneous baroreceptor unloading (via bilateral carotid occlusion; BCO). We hypothesized that simultaneous activation of both reflexes would result in an exacerbated pressor response owing to both an increase in CO and vasoconstriction. We observed that coactivation of muscle metaboreflex and arterial baroreflex resulted in additive interaction although the mechanisms for the pressor response were different. MMA increased MAP via increases in CO, heart rate (HR), and ventricular contractility whereas baroreflex unloading during MMA caused further increases in MAP via a large decrease in nonischemic vascular conductance (NIVC; conductance of all vascular beds except the hindlimb vasculature), indicating substantial peripheral vasoconstriction. Moreover, there was significant vasoconstriction within the ischemic muscle itself during coactivation of the two reflexes but the remaining vasculature vasoconstricted to a greater extent, thereby redirecting blood flow to the ischemic muscle. We conclude that baroreceptor unloading during MMA induces preferential peripheral vasoconstriction to improve blood flow to the ischemic active skeletal muscle.