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.

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.

Hemodynamic responses to neuromuscular electrical stimulation and to metaboreflex evaluation

BACKGROUND

Metabolites produced during muscle exercise can sensitize types III and IV fibers, which account for increasing blood pressure (BP) and vascular resistance in non-exercising limbs, as well as for redistributing the blood flow to active muscles; reflex response is called metaboreflex. Neuromuscular electrical stimulation (NMES) induces greater local muscle metabolic demand than voluntary isometric contractions. Metabolic accumulation is essential to activate muscle metaboreflex; thus, the hypothesis of the current study is that one NMES session can induce metaboreflex with different hemodynamic responses in upper and lower limbs. Objective: investigating whether one acute NMES session could activate metaboreflex by inducing different hemodynamic responses between arms and legs.

METHODS

Twenty (20) healthy subjects (mean age = 47.7 ± 9.4 years, 13 women, mean body mass index = 26±3.4kg/m2) participated in this randomized crossover study. All participants were subjected to two NMES interventions, one in the upper limbs (UPL) and the other in the lower limbs (LL). Mean blood pressure (MBP), blood flow (BF) and vascular resistance (VR) at baseline were used to selectively evaluate metaboreflex responses during NMES interventions and recovery periods with, and without, postexercise circulatory occlusion (PECO+ and PECO-, respectively) through the area under the curve (AUC) in VR.

RESULTS

MBP increased by 13% during UPL interventions and only remained high during PECO+. Changes in MBP were not observed in LL, although BF in the contralateral leg has decreased by 14% during PECO+ protocol. Muscle metaboreflex activation (AUC differences in VR between PECO+ and PECO-) was not different between UPL and LL (p=0.655).

CONCLUSIONS

Acute NMES session has induced similar metaboreflex activation in both arms and legs, although hemodynamic responses differed between interventions.

Blunted blood pressure response to exercise and isolated muscle metaboreflex activation in patients with cirrhosis

We sought to test the hypothesis that the cardiovascular responses to isolated muscle metaboreflex activation would be blunted in patients with cirrhosis. Eleven patients with cirrhosis and 15 healthy controls were evaluated. Blood pressure (BP; oscillometric method), contralateral forearm blood flow (FBF; venous occlusion plethysmography), and heart rate (HR; electrocardiogram) were measured during baseline, isometric handgrip at 30% of maximal voluntary contraction followed by postexercise ischemia (PEI). Forearm vascular conductance (FVC) was calculated as follows: (FBF / mean BP) × 100. Changes in HR during handgrip were similar between groups but tended to be different during PEI (controls: Δ 0.5 ± 1.1 bpm vs. cirrhotic patients: Δ 3.6 ± 1.0 bpm, P = 0.057). Mean BP response to handgrip (controls: Δ 20.9 ± 2.7 mm Hg vs. cirrhotic patients: Δ 10.6 ± 1.5 mm Hg, P = 0.006) and PEI was attenuated in cirrhotic patients (controls: Δ 16.1 ± 1.9 mm Hg vs. cirrhotic patients: Δ 7.2 ± 1.4 mm Hg, P = 0.001). In contrast, FBF and FVC increased during handgrip and decreased during PEI similarly between groups. These results indicate that an abnormal muscle metaboreflex activation explained, at least partially, the blunted pressor response to exercise exhibited by cirrhotic patients. Novelty: Patients with cirrhosis present abnormal muscle metaboreflex activation. BP response was blunted but forearm vascular response was preserved. HR response was slightly elevated.

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.

GABAA receptors modulate sympathetic vasomotor outflow and the pressor response to skeletal muscle metaboreflex activation in humans

KEY POINTS

The activation of the group III/IV skeletal muscle afferents is one of the principal mediators of cardiovascular responses to exercise; however, the neuronal circuitry mechanisms that are involved during the activation of group III/IV muscle afferents in humans remain unknown. Recently, we showed that GABAergic mechanisms are involved in the cardiac vagal withdrawal during the activation of mechanically sensitive (predominantly mediated by group III fibres) skeletal muscle afferents in humans. In the present study, we found that increases in muscle sympathetic nerve activity and mean blood pressure during isometric handgrip exercise and postexercise ischaemia were significantly greater after the oral administration of diazepam, a benzodiazepine that increases GABA_A activity, but not after placebo administration in young healthy subjects. These findings indicate for the first time that GABA_A receptors modulate sympathetic vasomotor outflow and the pressor responses to activation of metabolically sensitive (predominantly mediated by group IV fibres) skeletal muscle afferents in humans.

ABSTRACT

Animal studies have indicated that GABA_A receptors are involved in the neuronal circuitry of the group III/IV skeletal muscle afferent activation-induced neurocardiovascular responses to exercise. In the present study, we aimed to determine whether GABA_A receptors modulate the neurocardiovascular responses to activation of metabolically sensitive (predominantly mediated by group IV fibres) skeletal muscle afferents in humans. In a randomized, double-blinded, placebo-controlled and cross-over design, 17 healthy subjects (eight women) performed 2 min of ischaemic isometric handgrip exercise at 30% of the maximal voluntary contraction followed by 2 min of postexercise ischaemia (PEI). Muscle sympathetic nerve activity (MSNA), blood pressure (BP) and heart rate (HR) were continuously measured and trials were conducted before and 60 min after the oral administration of either placebo or diazepam (10 mg), a benzodiazepine that enhances GABA_A activity. At rest, MSNA was reduced, whereas HR and BP did not change after diazepam administration. During ischaemic isometric handgrip, greater MSNA (pre: ∆13 ± 9 bursts min^-1 vs. post: ∆29 ± 15 bursts min^-1 , P < 0.001), HR (pre: ∆23 ± 11 beats min^-1 vs. post: ∆31 ± 17 beats min^-1 , P < 0.01) and mean BP (pre: ∆33 ± 12 mmHg vs. post: ∆37 ± 12 mmHg, P < 0.01) responses were observed after diazepam. During PEI, MSNA and mean BP remained elevated from baseline before diazepam (∆10 ± 8 bursts min^-1 and ∆25 ± 14 mmHg, respectively) and these elevations were increased after diazepam (∆17 ± 12 bursts min^-1 and ∆28 ± 13 mmHg, respectively) (P ≤ 0.05). Importantly, placebo pill had no effect on neural, cardiac and pressor responses. These findings demonstrate for the first time that GABA_A receptors modulate MSNA and the pressor responses to skeletal muscle metaboreflex activation in humans.

Muscle metaboreflex activation via postexercise ischemia as a tool for teaching cardiovascular physiology for undergraduate students

The cardiovascular responses to exercise are mediated by several interactive neural mechanisms, including central command, arterial baroreflex, and skeletal muscle mechano- and metaboreflex. In humans, muscle metaboreflex activation can be isolated via postexercise ischemia (PEI), which increases sympathetic nerve activity and partially maintains the exercise-induced increase in arterial blood pressure. Here, we describe a practical laboratory class using PEI as a simple and useful technique to teach cardiovascular physiology. In an undergraduate exercise physiology class ( n = 47), a traditional 4-h lecture was conducted discussing the neural control mechanisms of cardiovascular regulation during exercise. Thereafter, eight students (4 men and 4 women) were selected to participate as a volunteer of a practical laboratory class. Each participant performed 90 s of isometric handgrip exercise at 40% of maximal voluntary contraction, followed by 3 min of PEI. Arterial blood pressure and heart rate were measured by digital monitors at rest and during isometric handgrip, PEI, and recovery. In addition, blood samples were collected from the tip of the exercising finger for blood lactate analyses. After the laboratory class, a survey was given to determine the perceptions of the students. The findings demonstrate that this laboratory class has proved to be highly popular with students, who self-reported a significant improvement in their understanding of several aspects of cardiovascular regulation during exercise.

Metaborreflexo inspiratório eleva a pressão arterial em indivíduos obesos e eutróficos

Resumo Introdução: O metaborreflexo, ativado pelo acúmulo de metabólitos durante o exercício, ocasiona vasoconstrição periférica, resultando em elevação da pressão arterial. Indivíduos obesos apresentam redução da endurance muscular inspiratória, sugerindo um acúmulo precoce de metabólitos e, consequentemente, alterações no metaborreflexo inspiratório. Objetivo: Comparar as respostas hemodinâmicas mediadas pelo metaborreflexo inspiratório em indivíduos obesos e em eutróficos. Método: Participaram do estudo vinte indivíduos obesos (31 ± 6 anos, dez homens, 37,5 ± 4,7 kg/m 2 ) e vinte eutróficos (29 ± 8 anos, dez homens, 23,2 ± 1,5 kg/m 2 ) submetidos a avaliação da força muscular respiratória através de manovacuometria. O metaborreflexo inspiratório foi induzido através de exercício resistido a 60% da pressão inspiratória máxima mantido até a exaustão. O protocolo controle consistiu na respiração sem resistência inspiratória (zero cmH 2 O) mantida durante 30 minutos. A pressão arterial e a frequência cardíaca foram mensuradas ao longo dos protocolos, realizados em dias distintos e em ordem randomizada. Resultados: O protocolo de indução do metaborreflexo inspiratório induziu aumento das pressões arteriais sistólica, diastólica e média, bem como da frequência cardíaca semelhante em indivíduos obesos e eutróficos. Conforme esperado, no protocolo controle as variáveis hemodinâmicas permaneceram inalteradas. Conclusão: A força muscular inspiratória não variou (p = 0,814) entre indivíduos obesos e eutróficos. Este estudo sugere que indivíduos obesos apresentam respostas hemodinâmicas, induzidas pelo metaborreflexo inspiratório, semelhantes aos indivíduos eutróficos.

Hemodynamic abnormalities during muscle metaboreflex activation in patients with type 2 diabetes mellitus

Metaboreflex is a reflex triggered during exercise or postexercise muscle ischemia (PEMI) by metaboreceptor stimulation. Typical features of metaboreflex are increased cardiac output (CO) and blood pressure. Patients suffering from metabolic syndrome display hemodynamic abnormalities, with an exaggerated systemic vascular resistance (SVR) and reduced CO response during PEMI-induced metaboreflex. Whether patients with type 2 diabetes mellitus (DM2) have similar hemodynamic abnormalities is unknown. Here we contrast the hemodynamic response to PEMI in 14 patients suffering from DM2 (age 62.7 ± 8.3 yr) and in 15 age-matched controls (CTLs). All participants underwent a control exercise recovery reference test and a PEMI test to obtain the metaboreflex response. Central hemodynamics were evaluated by unbiased operator-independent impedance cardiography. Although the blood pressure response to PEMI was not significantly different between the groups, we found that the SVR and CO responses were reversed in patients with DM2 as compared with the CTLs (SVR: 392.5 ± 549.6 and -14.8 ± 258.9 dyn·s^-1·cm^-5; CO: -0.25 ± 0.63 and 0.46 ± 0.50 l/m, respectively, in DM2 and in CTL groups, respectively; P < 0.05 for both). Of note, stroke volume (SV) increased during PEMI in the CTL group only. Failure to increase SV and CO was the consequence of reduced venous return, impaired cardiac performance, and augmented afterload in patients with DM2. We conclude that patients with DM2 have an exaggerated vasoconstriction in response to metaboreflex activation not accompanied by a concomitant increase in heart performance. Therefore, in these patients, blood pressure response to the metaboreflex relies more on SVR increases rather than on increases in SV and CO. NEW & NOTEWORTHY The main new finding of the present investigation is that subjects with type 2 diabetes mellitus have an exaggerated vasoconstriction in response to metaboreflex activation. In these patients, blood pressure response to the metaboreflex relies more on systemic vascular resistance than on cardiac output increments.

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.

Metaboreflex activation delays heart rate recovery after aerobic exercise in never-treated hypertensive men

KEY POINTS

Recent evidence indicates that metaboreflex regulates heart rate recovery after exercise (HRR). An increased metaboreflex activity during the post-exercise period might help to explain the reduced HRR observed in hypertensive subjects. Using lower limb circulatory occlusion, the present study showed that metaboreflex activation during the post-exercise period delayed HRR in never-treated hypertensive men compared to normotensives. These findings may be relevant for understanding the physiological mechanisms associated with autonomic dysfunction in hypertensive men.

ABSTRACT

Muscle metaboreflex influences heart rate (HR) regulation after aerobic exercise. Therefore, increased metaboreflex sensitivity may help to explain the delayed HR recovery (HRR) reported in hypertension. The present study assessed and compared the effect of metaboreflex activation after exercise on HRR, cardiac baroreflex sensitivity (cBRS) and heart rate variability (HRV) in normotensive (NT) and hypertensive (HT) men. Twenty-three never-treated HT and 25 NT men randomly underwent two-cycle ergometer exercise sessions (30 min, 70% V̇O2 peak ) followed by 5 min of inactive recovery performed with (occlusion) or without (control) leg circulatory occlusion (bilateral thigh cuffs inflated to a suprasystolic pressure). HRR was assessed via HR reduction after 30, 60 and 300 s of recovery (HRR30s, HRR60s and HRR300s), as well as by the analysis of short- and long-term time constants of HRR. cBRS was assessed by sequence technique and HRV by the root mean square residual and the root mean square of successive differences between adjacent RR intervals on subsequent 30 s segments. Data were analysed using two- and three-way ANOVA. HRR60s and cBRS were significant and similarly reduced in both groups in the occlusion compared to the control session (combined values: 20 ± 10 vs. 26 ± 9 beats min-1 and 2.1 ± 1.2 vs. 3.2 ± 2.4 ms mmHg-1 , respectively, P < 0.05). HRR300s and HRV were also reduced in the occlusion session, although these reductions were significantly greater in HT compared to NT (-16 ± 11 vs. -8 ± 15 beats min-1 for HRR300s, P < 0.05). The results support the role of metaboreflex in HRR and suggest that increased metaboreflex sensitivity may partially explain the delayed HRR observed in HT men.

l-Citrulline supplementation attenuates blood pressure, wave reflection and arterial stiffness responses to metaboreflex and cold stress in overweight men

Combined isometric exercise or metaboreflex activation (post-exercise muscle ischaemia (PEMI)) and cold pressor test (CPT) increase cardiac afterload, which may lead to adverse cardiovascular events. l-Citrulline supplementation (l-CIT) reduces systemic arterial stiffness (brachial-ankle pulse wave velocity (baPWV)) at rest and aortic haemodynamic responses to CPT. The aim of this study was to determine the effect of l-CIT on aortic haemodynamic and baPWV responses to PEMI+CPT. In all, sixteen healthy, overweight/obese males (age 24 (sem 6) years; BMI 29·3 (sem 4·0) kg/m2) were randomly assigned to placebo or l-CIT (6 g/d) for 14 d in a cross-over design. Brachial and aortic systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP), aortic augmented pressure (AP), augmentation index (AIx), baPWV, reflection timing (Tr) and heart rate (HR) were evaluated at rest and during isometric handgrip exercise (IHG), PEMI and PEMI+CPT at baseline and after 14 d. No significant effects were evident after l-CIT at rest. l-CIT attenuated the increases in aortic SBP and wave reflection (AP and AIx) during IHG, aortic DBP, MAP and AIx during PEMI, and aortic SBP, DBP, MAP, AP, AIx and baPWV during PEMI+CPT compared with placebo. HR and Tr were unaffected by l-CIT in all conditions. Our findings demonstrate that l-CIT attenuates aortic blood pressure and wave reflection responses to exercise-related metabolites. Moreover, l-CIT attenuates the exaggerated arterial stiffness response to combined metaboreflex activation and cold exposure, suggesting a protective effect against increased cardiac afterload during physical stress.

Muscle metaboreflex and cerebral blood flow regulation in humans: implications for exercise with blood flow restriction

We investigated the effect of activating metabolically sensitive skeletal muscle afferents (muscle metaboreflex) on cerebral blood flow and the potentially confounding influence of concomitant changes in the partial pressure of arterial carbon dioxide. Eleven healthy males (25 ± 4 yr) performed submaximal leg cycling exercise on a semirecumbent cycle ergometer (heart rate: ∼120 beats/min), and assessments were made of the partial pressure of end-tidal carbon dioxide (PetCO2), internal carotid artery blood flow (ICAQ) and conductance (ICACVC), and middle cerebral artery mean blood velocity (MCAvm) and conductance index (MCACVCi).The muscle metaboreflex was activated during cycling with leg blood flow restriction (BFR) or isolated with postexercise ischemia (PEI). In separate trials, PetCO2was either permitted to fluctuate spontaneously (control trial) or was clamped at 1 mmHg above resting levels (PetCO2clamp trial). In the control trial, leg cycling with BFR decreased PetCO2(Δ−4.8 ± 0.9 mmHg vs. leg cycling exercise) secondary to hyperventilation, while ICAQ, ICACVC, and MCAvmwere unchanged and MCACVCidecreased. However, in the PetCO2clamp trial, leg cycling with BFR increased both MCAvm(Δ5.9 ± 1.4 cm/s) and ICAQ(Δ20.0 ± 7.8 ml/min) and attenuated the decrease in MCACVCi, while ICACVCwas unchanged. In the control trial, PEI decreased PetCO2(Δ−7.0 ± 1.3 mmHg vs. rest), MCAvmand MCACVCi, whereas ICAQand ICACVCwere unchanged. In contrast, in the PetCO2clamp trial both ICAQ(Δ18.5 ± 11.9 ml/min) and MCAvm(Δ8.8 ± 2.0 cm/s) were elevated, while ICACVCand MCACVCiwere unchanged. In conclusion, when hyperventilation-related decreases in PetCO2are prevented the activation of metabolically sensitive skeletal muscle afferent fibers increases cerebral blood flow.

Ischemic preconditioning reduces hemodynamic response during metaboreflex activation

Ischemic preconditioning (IP) has been shown to improve exercise performance and to delay fatigue. However, the precise mechanisms through which IP operates remain elusive. It has been hypothesized that IP lowers the sensation of fatigue by reducing the discharge of group III and IV nerve endings, which also regulate hemodynamics during the metaboreflex. We hypothesized that IP reduces the blood pressure response during the metaboreflex. Fourteen healthy males (age between 25 and 48 yr) participated in this study. They underwent the following randomly assigned protocol: postexercise muscle ischemia (PEMI) test, during which the metaboreflex was elicited after dynamic handgrip; control exercise recovery session (CER) test; and PEMI after IP (IP-PEMI) test. IP was obtained by occluding forearm circulation for three cycles of 5 min spaced by 5 min of reperfusion. Hemodynamics were evaluated by echocardiography and impedance cardiography. The main results were that after IP the mean arterial pressure response was reduced compared with the PEMI test (means ± SD +3.37 ± 6.41 vs. +9.16 ± 7.09 mmHg, respectively). This was the consequence of an impaired venous return that impaired the stroke volume during the IP-PEMI more than during the PEMI test (-1.43 ± 15.35 vs. +10.28 ± 10.479 ml, respectively). It was concluded that during the metaboreflex, IP affects hemodynamics mainly because it impairs the capacity to augment venous return and to recruit the cardiac preload reserve. It was hypothesized that this is the consequence of an increased nitric oxide production, which reduces the possibility to constrict venous capacity vessels.

Differences in hemodynamic response to metaboreflex activation between obese patients with metabolic syndrome and healthy subjects with obese phenotype

Patients suffering from obesity and metabolic syndrome (OMS) manifest a dysregulation in hemodynamic response during exercise, with an exaggerated systemic vascular increase. However, it is not clear whether this is the consequence of metabolic syndrome per se or whether it is due to concomitant obesity. The aim of the present investigation was to discover whether OMS and noncomplicated obesity resulted in different hemodynamic responses during the metaboreflex. Twelve metabolically healthy but obese subjects (MHO; 7 women), 13 OMS patients (5 women), and 12 normal age-matched controls (CTL; 6 women) took part in this study. All participants underwent a postexercise muscle ischemia protocol to evaluate the metaboreflex activity. Central hemodynamics were evaluated by impedance cardiography. The main result shows an exaggerated increase in systemic vascular resistance from baseline during the metaboreflex in the OMS patients as compared with the other groups (481.6 ± 180.3, −0.52 ± 177.6, and −60.5 ± 58.6 dynes·s−1·cm−5 for the OMS, the MHO, and the CTL groups, respectively; P < 0.05). Moreover, the MHO subjects and the CTL group showed an increase in cardiac output during the metaboreflex (288.7 ± 325.8 and 703.8 ± 276.2 ml/m increase with respect to baseline), whereas this parameter tended to decrease in the OMS group (−350 ± 236.5 ml/m). However, the blood pressure response, which tended to be higher in the OMS patients, was not statistically different between groups. The results of the present investigation suggest that OMS patients have an exaggerated vasoconstriction in response to metaboreflex activation and that this fact is not due to obesity per se.

Influence of muscle metaboreceptor stimulation on middle cerebral artery blood velocity in humans

Regional anaesthesia to attenuate skeletal muscle afferent feedback abolishes the exercise-induced increase in middle cerebral artery mean blood velocity (MCA Vmean). However, such exercise-related increases in cerebral perfusion are not preserved during post exercise muscle ischaemia (PEMI) where the activation of metabolically sensitive muscle afferents is isolated. We tested the hypothesis that a hyperventilation-mediated decrease in the arterial partial pressure of CO2, hence cerebral vasoconstriction, masks the influence of muscle metaboreceptor stimulation on MCA Vmean during PEMI. Ten healthy men (20 ± 1 years old) performed two trials of fatiguing isometric hand-grip exercise followed by PEMI, in control conditions and with end-tidal CO2 (P ET ,CO2) clamped at ∼1 mmHg above the resting partial pressure. In the control trial, P ET ,CO2 decreased from rest during hand-grip exercise and PEMI, while MCA Vmean was unchanged from rest. By design, P ET ,CO2 remained unchanged from rest throughout the clamp trial, while MCA Vmean increased during hand-grip (+10.6 ±1.8 cm s(-1)) and PEMI (+9.2 ± 1.6 cm s(-1); P < 0.05 versus rest and control trial). Increases in minute ventilation and mean arterial pressure during hand-grip and PEMI were not different in the control and P ET ,CO2 clamp trials (P > 0.05). These findings indicate that metabolically sensitive skeletal muscle afferents play an important role in the regional increase in cerebral perfusion observed in exercise, but that influence can be masked by a decrease in P ET ,CO2 when they are activated in isolation during PEMI.