Muscle metaboreflex activation increases ventilation and heart rate during dynamic exercise in humans

Abstract

New Findings What is the central question of this study?Classically, the stimulation of thin‐fibre skeletal muscle afferents, via the application of postexercise circulatory occlusion (PECO) at rest, fails to generate ventilatory responses. We used a new experimental protocol to examine whether the involvement of these metabosensitive afferents in ventilatory control can only be revealed during exercise, when other potentially synergistic inputs that increase central respiratory drive are activated. What is the main finding and its importance?We found that PECO of one leg augmented the ventilatory and heart rate responses to single‐legged exercise of the contralateral leg, suggesting that metaboreceptive muscle afferents contribute to the control of the exercise hyperpnoea. Abstract Inhibition of thin‐fibre skeletal muscle afferent neurotransmission attenuates ventilatory and cardiovascular responses to exercise. However, stimulation of muscle metaboreceptive afferents at rest, via postexercise circulatory occlusion (PECO), classically fails to generate increases in ventilation or heart rate. It is possible that the involvement of muscle afferent feedback in ventilatory control can only be revealed during exercise, when other potentially synergistic inputs that increase central respiratory drive are activated. Therefore, we assessed the cardiorespiratory responses to single‐legged cycling exercise with or without PECO of the contralateral leg. Thirteen healthy participants performed left‐legged cycling exercise (40 or 60 W) followed by either: (i) no PECO (Con trial); or (ii) PECO (PECO trial) of the left leg for 3 min. During this 3 min period, right‐legged cycling exercise was performed at the same workload as the preceding left‐legged exercise (40 or 60 W). During 60 W right‐legged cycling, ventilation relative to baseline was significantly higher in the PECO versus Con trial (22.9 ± 2 versus 18.7 ± 1.8 l min−1; P < 0.05), but there was no difference between the trials performed at 40 W. The change in heart rate was significantly greater during right‐legged cycling in the PECO versus Con trial in the 40 (41.2 ± 4 versus 34.1 ± 3.1 beats min−1; P < 0.05) and 60 W trials (49.7 ± 2.7 versus 43.4 ± 3.7 beats min−1; P < 0.05). There were no differences in oxygen uptake, carbon dioxide production and ratings of perceived exertion between trials. These findings suggest that stimulation of muscle metaboreceptive afferents can drive increases in ventilation and heart rate during dynamic exercise.

Carotid chemoreflex and muscle metaboreflex interact to the regulation of ventilation in patients with heart failure with reduced ejection fraction

“…Of note, during exercise, the carotid chemoreflex and the muscle metaboreflex possibly interact with other operating reflexes like central command (Goodwin et al, ), muscle mechanoreflex (Silva et al, ) and venous distension (Haouzi et al, ). As a result, the effect herein observed could have been greater if we had assessed an interaction between the carotid chemoreflex and the muscle metaboreflex during exercise, rather than recovery (Lam et al, ).…”

Interaction between the muscle metaboreflex and central command – A clearer picture of cardiorespiratory control during exercise

“…The obvious question is, to what extent does the metaboreflex contribute? Also, exaggerated sympathetic responses (not assessed in the study by Lam et al., ) to isolated muscle metaboreflex activation have been documented in patient populations with high prevalence, such as hypertension, heart failure and diabetes. Future studies are needed to determine the extent to which the interaction between the metaboreflex and central command augments the sympathetic response beyond what is observed during isolated metaboreflex activation in these patient populations.…”

“…However, development of experimental protocols that clarify mechanisms and their relative importance has proved difficult because of the likelihood of redundancy and the complexity of central integration. In this issue of Experimental Physiology , Lam, Greenhough, Nazari, White, & Bruce () met this challenge with an elegant and cleverly designed study in humans that improves our understanding of the contribution of metabolically sensitive neural feedback from active skeletal muscle to the cardiorespiratory response to dynamic exercise.…”

“…The study by Lam et al. () provides insight into this mechanism by demonstrating metaboreflex control of exercise ventilation and heart rate only when central command is engaged, i.e. during exercise.…”

“…Advancements in the field often create more questions than answers, and the study by Lam et al. () is no exception. An exaggerated ventilatory response to exercise is characteristic of patients with heart failure, chronic obstructive pulmonary disease and pulmonary hypertension.…”