The ventilatory component of the muscle metaboreflex: catch me if you can!

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Altered locomotor muscle metaboreflex control of ventilation in patients with COPD

We investigated the locomotor muscle metaboreflex control of ventilation, circulation, and dyspnea in patients with chronic obstructive pulmonary disease (COPD). Ten patients [forced expiratory volume in 1 second (FEV1; means ± SD) = 43 ± 17% predicted] and nine age- and sex-matched controls underwent 1) cycling exercise followed by postexercise circulatory occlusion (PECO) to activate the metaboreflex or free circulatory flow to inactivate it, 2) cold pressor test to interpret whether any altered reflex response was specific to the metaboreflex arc, and 3) muscle biopsy to explore the metaboreflex arc afferent side. We measured airflow, dyspnea, heart rate, arterial pressure, muscle blood flow, and vascular conductance during reflexes activation. In addition, we measured fiber types, glutathione redox balance, and metaboreceptor-related mRNAs in the vastus lateralis. Metaboreflex activation increased ventilation versus free flow in patients (∼15%, P < 0.020) but not in controls (P > 0.450). In contrast, metaboreflex activation did not change dyspnea in patients (P = 1.000) but increased it in controls (∼100%, P < 0.001). Other metaboreflex-induced responses were similar between groups. Cold receptor activation increased ventilation similarly in both groups (P = 0.46). Patients had greater type II skeletal myocyte percentage (14%, P = 0.010), lower glutathione ratio (-34%, P = 0.015), and lower nerve growth factor (NGF) mRNA expression (-60%, P = 0.031) than controls. Therefore, COPD altered the locomotor muscle metaboreflex control of ventilation. It increased type II myocyte percentage and elicited redox imbalance, potentially producing more muscle metaboreceptor stimuli. Moreover, it decreased NGF expression, suggesting a downregulation of metabolically sensitive muscle afferents.NEW & NOTEWORTHY This study's integrative physiology approach provides evidence for a specific alteration in locomotor muscle metaboreflex control of ventilation in patients with COPD. Furthermore, molecular analyses of a skeletal muscle biopsy suggest that the amount of muscle metaboreceptor stimuli derived from type II skeletal myocytes and redox imbalance overcame a downregulation of metabolically sensitive muscle afferents.