Muscle metaboreflex activation during dynamic exercise evokes epinephrine release resulting in β2-mediated vasodilation

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.

Autonomic and Vascular Responses during Reactive Hyperemia in Healthy Individuals and Patients with Sickle Cell Anemia

Background and Objectives: To compare autonomic and vascular responses during reactive hyperemia (RH) between healthy individuals and patients with sickle cell anemia (SCA). Materials and Methods: Eighteen healthy subjects and 24 SCA patients were subjected to arterial occlusion for 3 min at the lower right limb level. The pulse rate variability (PRV) and pulse wave amplitude were measured through photoplethysmography using the Angiodin^® PD 3000 device, which was placed on the first finger of the lower right limb 2 min before (Basal) and 2 min after the occlusion. Pulse peak intervals were analyzed using time-frequency (wavelet transform) methods for high-frequency (HF: 0.15-0.4) and low-frequency (LF: 0.04-0.15) bands, and the LF/HF ratio was calculated. Results: The pulse wave amplitude was higher in healthy subjects compared to SCA patients, at both baseline and post-occlusion (p < 0.05). Time-frequency analysis showed that the LF/HF peak in response to the post-occlusion RH test was reached earlier in healthy subjects compared to SCA patients. Conclusions: Vasodilatory function, as measured by PPG, was lower in SCA patients compared to healthy subjects. Moreover, a cardiovascular autonomic imbalance was present in SCA patients with high sympathetic and low parasympathetic activity in the basal state and a poor response of the sympathetic nervous system to RH. Early cardiovascular sympathetic activation (10 s) and vasodilatory function in response to RH were impaired in SCA patients.

Gene and protein expression of dorsal root ganglion sensory receptors in normotensive and hypertensive male rats

The exercise pressor reflex (EPR), a neurocirculatory control mechanism, is exaggerated in hypertensive humans and rats. Disease-related abnormalities within the afferent arm of the reflex loop, including mechano- and metabosensitive receptors located at the terminal end of group III/IV muscle afferents, may contribute to the dysfunctional EPR in hypertension. Using control (WKY) and spontaneous hypertensive (SHR) rats, we examined dorsal root ganglion (DRG) gene and protein expression of molecular receptors recognized as significant determinants of the EPR. Twelve lumbar DRGs (6-left, 6-right) were harvested from each of 10 WKY (arterial blood pressure (MAP): 96±9mmHg) and 10 SHR (MAP: 144±9mmHg). DRGs from the left side were used for protein expression (Western blotting; normalized to GAPDH), whereas right-side DRGs (i.e., parallel structure) were used to determine mRNA levels (RNA-sequencing, normalized to TPM). Analyses focused on metabosensitive (ASIC3, Bradykinin-receptor B2, EP4, P2X3, TRPv1) and mechanosensitive (Piezo1/2) receptors. While Piezo1 was similar in both groups (P=0.75), protein expression for all other receptors was significantly higher in SHR compared to WKY. With the exception of a greater Bradykinin-receptor B2 in SHR (P<0.05), mRNA expression of all other receptors was not different between groups (P>0.18). The higher protein content of these sensory receptors in SHR indirectly supports the previously proposed hypothesis that the exaggerated EPR in hypertension is, in part, due to disease-related abnormalities within the afferent arm of the reflex loop. The upregulated receptor content, combined with normal mRNA levels, insinuates that posttranscriptional regulation of sensory receptor protein expression might be impaired in hypertension.

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.

Physical Exercise Exerts Neuroprotective Effect on Memory Impairment by Mitigate the Decline of Striatum Catecholamine and Spine Density in a Vascular Dementia Rat Model

OBJECTIVE

The present study aims to investigate the underlying neurochemical mechanism of physical exercise on striatum synapsis and memory function in vascular dementia model.

METHODS

32 Sprague-Dawley (SD) rats were randomly divided into 4 groups: control group (C group, n = 6), vascular dementia group (Vascular dementia group, n = 7), physical exercise and vascular dementia group (Exe-VD group, n = 6), physical exercise and black group (Exe group, n = 6). 4 weeks of voluntary wheel running were used as pre-exercise training. Vascular dementia model was established by bilateral common carotid arteries occlusion (BCCAo) for 1 week. Passive avoidance test (PAT) were used to test memory function. The level of striatum catecholamine in the microdialysate were detected by enzyme linked immunosorbent assy (ELISA). Golgi staining was used to analyze striatum neuronal spine density.

RESULTS

Behavioral data indicated that 4 weeks of physical exercise ameliorated memory impairment in vascular dementia model. Striatum catecholamine level significantly decreased in VD group when compared with C group (P < .001). But this phenomenon can be rescue by physical exercise (P < .001). In addition, compared with C group, neuronal spine density significantly decreased in VD group (P < .01), but 4 weeks of physical exercise can rescue this phenomenon (P < .05).

CONCLUSION

4 weeks of physical exercise improves memory function by mitigate the decline of striatum catecholamine and spine density in VD model.

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.

Gender Differences in Hemodynamic Regulation and Cardiovascular Adaptations to Dynamic Exercise

Exercise is a major challenge for cardiovascular apparatus since it recruits chronotropic, inotropic, pre-load, and afterload reserves. Regular physical training induces several physiological adaptations leading to an increase in both cardiac volume and mass. It appears that several gender-related physiological and morphological differences exist in the cardiovascular adjustments and adaptations to dynamic exercise in humans. In this respect, gender may be important in determining these adjustments and adaptations to dynamic exercise due to genetic, endocrine, and body composition differences between sexes. Females seem to have a reduced vasoconstriction and a lower vascular resistance in comparison to males, especially after exercise. Significant differences exist also in the cardiovascular adaptations to physical training, with trained women showing smaller cardiac volume and wall thickness compared with male athletes. In this review, we summarize these differences.

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.

Systemic and regional hemodynamic response to activation of the exercise pressor reflex in patients with peripheral artery disease

Patients with peripheral artery disease (PAD) have an accentuated exercise pressor reflex (EPR) during exercise of the affected limb. The underlying hemodynamic changes responsible for this, and its effect on blood flow to the exercising extremity, are unclear. We tested the hypothesis that the exaggerated EPR in PAD is mediated by an increase in total peripheral resistance (TPR), which augments redistribution of blood flow to the exercising limb. Twelve patients with PAD and 12 age- and sex-matched subjects without PAD performed dynamic plantar flexion (PF) using the most symptomatic leg at progressive workloads of 2-12 kg (increased by 1 kg/min until onset of fatigue). We measured heart rate, beat-by-beat blood pressure, femoral blood flow velocity (FBV), and muscle oxygen saturation (SmO2) continuously during the exercise. Femoral blood flow (FBF) was calculated from FBV and baseline femoral artery diameter. Stroke volume (SV), cardiac output (CO), and TPR were derived from the blood pressure tracings. Mean arterial blood pressure and TPR were significantly augmented in PAD compared with control during PF. FBF increased during exercise to an equal extent in both groups. However, SmO2 of the exercising limb remained significantly lower in PAD compared with control. We conclude that the exaggerated pressor response in PAD is mediated by an abnormal TPR response, which augments redistribution of blood flow to the exercising extremity, leading to an equal rise in FBF compared with controls. However, this increase in FBF is not sufficient to normalize the SmO₂ response during exercise in patients with PAD.NEW & NOTEWORTHY In this study, peripheral artery disease (PAD) patients and healthy control subjects performed graded, dynamic plantar flexion exercise. Data from this study suggest that previously reported exaggerated exercise pressor reflex in patients with PAD is driven by greater vasoconstriction in nonexercising vascular territories which also results in a redistribution of blood flow to the exercising extremity. However, this rise in femoral blood flow does not fully correct the oxygen deficit due to changes in other mechanisms that require further investigation.

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.

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.

The Impact of Cardiovascular Diseases on Cardiovascular Regulation During Exercise in Humans: Studies on Metaboreflex Activation Elicited by the Post-exercise Muscle Ischemia Method

BACKGROUND

Hemodynamics during dynamic exercise is finely regulated by some neural mechanisms. One of these mechanisms is the metabolic part of the exercise pressor reflex, i.e. the muscle metaboreflex. Hemodynamic response during the metaboreflex is characterised by the recruitment of the reserves in cardiac inotropism, pre-load, after-load and chronotropism. If one of these reserves is exhausted, then the cardiovascular response is achieved by recruiting one of the other reserves, thereby indicating a remarkable plasticity of the control of circulation.

CONCLUSION

In this review, the effects of a number of cardiovascular diseases - such as heart failure, heart failure with preserved ejection fraction, hypertension, type 1 and type 2 diabetes mellitus, obesity and metabolic syndrome - on hemodynamics during the metaboreflex are reviewed.

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.

Role of endothelial nitric oxide in control of peripheral vascular conductance during muscle metaboreflex activation

The muscle metaboreflex is a powerful pressor reflex induced by the activation of chemically sensitive muscle afferents as a result of metabolite accumulation. During submaximal dynamic exercise, the rise in arterial pressure is primarily due to increases in cardiac output, since there is little systemic vasoconstriction. Indeed, in normal animals, we have often shown a small, but significant, peripheral vasodilation during metaboreflex activation, which is mediated, at least in part, by release of epinephrine and activation of vascular β₂-receptors. We tested whether this vasodilation is in part due to increased release of nitric oxide caused by the rise in cardiac output eliciting endothelium-dependent flow-mediated vasodilation. The muscle metaboreflex was activated via graded reductions in hindlimb blood flow during mild exercise with and without nitric oxide synthesis blockade [N^G-nitro-l-arginine methyl ester (l-NAME); 5 mg/kg]. We assessed the role of increased cardiac output in mediating peripheral vasodilation via the slope of the relationship between the rise in nonischemic vascular conductance (conductance of all vascular beds excluding hindlimbs) vs. the rise in cardiac output. l-NAME increased mean arterial pressure at rest and during exercise. The metaboreflex-induced increases in mean arterial pressure were unaltered by l-NAME, whereas the increases in cardiac output and nonischemic vascular conductance were attenuated. However, the slope of the relationship between nonischemic vascular conductance and cardiac output was not affected by l-NAME, indicating that the rise in cardiac output did not elicit vasodilation via increased release of nitric oxide. Thus, although nitric oxide is intrinsic to the vascular tonus, endothelial-dependent flow-mediated vasodilation plays little role in the small peripheral vasodilation observed during muscle metaboreflex activation.

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.

Long-Term Effects of Botulinum Toxin Complex Type A Injection on Mechano- and Metabo-Sensitive Afferent Fibers Originating from Gastrocnemius Muscle

The aim of the present study was to investigate long term effects of motor denervation by botulinum toxin complex type A (BoNT/A) from Clostridium Botulinum, on the afferent fibers originating from the gastrocnemius muscle of rats. Animals were divided in 2 experimental groups: 1) untreated animals acting as control and 2) treated animals in which the toxin was injected in the left muscle, the latter being itself divided into 3 subgroups according to their locomotor recovery with the help of a test based on footprint measurements of walking rats: i) no recovery (B0), ii) 50% recovery (B50) and iii) full recovery (B100). Then, muscle properties, metabosensitive afferent fiber responses to potassium chloride (KCl) and lactic acid injections and Electrically-Induced Fatigue (EIF), and mechanosensitive responses to tendon vibrations were measured. At the end of the experiment, rats were killed and the toxin injected muscles were weighted. After toxin injection, we observed a complete paralysis associated to a loss of force to muscle stimulation and a significant muscle atrophy, and a return to baseline when the animals recover. The response to fatigue was only decreased in the B0 group. The responses to KCl injections were only altered in the B100 groups while responses to lactic acid were altered in the 3 injected groups. Finally, our results indicated that neurotoxin altered the biphasic pattern of response of the mechanosensitive fiber to tendon vibrations in the B0 and B50 groups. These results indicated that neurotoxin injection induces muscle afferent activity alterations that persist and even worsen when the muscle has recovered his motor activity.

Muscle metaboreflex activation during dynamic exercise vasoconstricts ischemic active skeletal muscle

Metabolite accumulation due to ischemia of active skeletal muscle stimulates group III/IV chemosensitive afferents eliciting reflex increases in arterial blood pressure and sympathetic activity, termed the muscle metaboreflex. We and others have previously demonstrated sympathetically mediated vasoconstriction of coronary, renal, and forelimb vasculatures with muscle metaboreflex activation (MMA). Whether MMA elicits vasoconstriction of the ischemic muscle from which it originates is unknown. We hypothesized that the vasodilation in active skeletal muscle with imposed ischemia becomes progressively restrained by the increasing sympathetic vasoconstriction during MMA. We activated the metaboreflex during mild dynamic exercise in chronically instrumented canines via graded reductions in hindlimb blood flow (HLBF) before and after α1-adrenergic blockade [prazosin (50 μg/kg)], β-adrenergic blockade [propranolol (2 mg/kg)], and α1 + β-blockade. Hindlimb resistance was calculated as femoral arterial pressure/HLBF. During mild exercise, HLBF must be reduced below a threshold level before the reflex is activated. With initial reductions in HLBF, vasodilation occurred with the imposed ischemia. Once the muscle metaboreflex was elicited, hindlimb resistance increased. This increase in hindlimb resistance was abolished by α1-adrenergic blockade and exacerbated after β-adrenergic blockade. We conclude that metaboreflex activation during submaximal dynamic exercise causes sympathetically mediated α-adrenergic vasoconstriction in ischemic skeletal muscle. This limits the ability of the reflex to improve blood flow to the muscle.

Combined, but not individual, blockade of ASIC3, P2X, and EP4 receptors attenuates the exercise pressor reflex in rats with freely perfused hindlimb muscles

In healthy humans, tests of the hypothesis that lactic acid, PGE2, or ATP plays a role in evoking the exercise pressor reflex proved controversial. The findings in humans resembled ours in decerebrate rats that individual blockade of the receptors to lactic acid, PGE2, and ATP had only small effects on the exercise pressor reflex provided that the muscles were freely perfused. This similarity between humans and rats prompted us to test the hypothesis that in rats with freely perfused muscles combined receptor blockade is required to attenuate the exercise pressor reflex. We first compared the reflex before and after injecting either PPADS (10 mg/kg), a P2X receptor antagonist, APETx2 (100 μg/kg), an activating acid-sensing ion channel 3 (ASIC) channel antagonist, or L161982 (2 μg/kg), an EP4 receptor antagonist, into the arterial supply of the hindlimb of decerebrated rats. We then examined the effects of combined blockade of P2X receptors, ASIC3 channels, and EP4 receptors on the exercise pressor reflex using the same doses, intra-arterial route, and time course of antagonist injections as those used for individual blockade. We found that neither PPADS (n = 5), APETx2 (n = 6), nor L161982 (n = 6) attenuated the reflex. In contrast, combined blockade of these receptors (n = 7) attenuated the peak (↓27%, P < 0.019) and integrated (↓48%, P < 0.004) pressor components of the reflex. Combined blockade injected intravenously had no effect on the reflex. We conclude that combined blockade of P2X receptors, ASIC3 channels, and EP4 receptors on the endings of thin fiber muscle afferents is required to attenuate the exercise pressor reflex in rats with freely perfused hindlimbs.

Effects of combined exercise on physical fitness and neurotransmitters in children with ADHD: a pilot randomized controlled study

[Purpose] The purpose of the present study was to determine the effect of a jump rope and ball combined exercise program on the physical fitness the neurotransmitter (epinephrine, serotonin) levels of children with attention-deficit hyperactivity disorder. [Subjects and Methods] The subjects were 12 boys attending elementary school, whose grade levels ranged from 1-4. The block randomization method was used to distribute the participants between the combined exercise group (n = 6) and control group (n = 6). The program consisted of a 60-min exercise (10-min warm-up, 40-min main exercise, and 10-min cool down) performed three times a week, for a total of 12 weeks. [Results] The exercise group showed a significant improvement in cardiorespiratory endurance, muscle strength, muscle endurance and flexibility after 12 weeks. A significant increase in the epinephrine level was observed in the exercise group. [Conclusion] The 12-week combined exercise program in the current study (jump rope and ball exercises) had a positive effect on overall fitness level, and neurotransmission in children with attention-deficit hyperactivity disorder.

Neural control of circulation and exercise: a translational approach disclosing interactions between central command, arterial baroreflex, and muscle metaboreflex

The last 100 years witnessed a rapid and progressive development of the body of knowledge concerning the neural control of the cardiovascular system in health and disease. The understanding of the complexity and the relevance of the neuroregulatory system continues to evolve and as a result raises new questions. The purpose of this review is to articulate results from studies involving experimental models in animals as well as in humans concerning the interaction between the neural mechanisms mediating the hemodynamic responses during exercise. The review describes the arterial baroreflex, the pivotal mechanism controlling mean arterial blood pressure and its fluctuations along with the two main activation mechanisms to exercise: central command (parallel activation of central somatomotor and autonomic descending pathways) and the muscle metaboreflex, the metabolic component of exercise pressor reflex (feedback from ergoreceptors within contracting skeletal muscles). In addition, the role of the cardiopulmonary baroreceptors in modulating the resetting of arterial baroreflex is identified, and the mechanisms in the central nervous system involved with the resetting of baroreflex function during dynamic exercise are also described. Approaching a very relevant clinical condition, the review also presents the concept that the impaired arterial baroreflex function is an integral component of the metaboreflex-mediated exaggerated sympathetic tone in subjects with heart failure. This increased sympathetic activity has a major role in causing the depressed ventricular function observed during submaximal dynamic exercise in these patients. The potential contribution of a metaboreflex arising from respiratory muscles is also considered.