Unravelling the mysteries of the exercise pressor reflex at the cellular level

Cerebral and muscle tissue oxygenation during exercise in healthy adults: A systematic review

BACKGROUND

Near-infrared spectroscopy (NIRS) technology has allowed for the measurement of cerebral and skeletal muscle oxygenation simultaneously during exercise. Since this technology has been growing and is now successfully used in laboratory and sports settings, this systematic review aimed to synthesize the evidence and enhance an integrative understanding of blood flow adjustments and oxygen (O2) changes (i.e., the balance between O2 delivery and O2 consumption) within the cerebral and muscle systems during exercise.

METHODS

A systematic review was conducted using PubMed, Embase, Scopus, and Web of Science databases to search for relevant studies that simultaneously investigated cerebral and muscle hemodynamic changes using the near-infrared spectroscopy system during exercise. This review considered manuscripts written in English and available before February 9, 2023. Each step of screening involved evaluation by 2 independent authors, with disagreements resolved by a third author. The Joanna Briggs Institute Critical Appraisal Checklist was used to assess the methodological quality of the studies.

RESULTS

Twenty studies were included, of which 80% had good methodological quality, and involved 290 young or middle-aged adults. Different types of exercises were used to assess cerebral and muscle hemodynamic changes, such as cycling (n = 11), treadmill (n = 1), knee extension (n = 5), isometric contraction of biceps brachii (n = 3), and duet swim routines (n = 1). The cerebral hemodynamics analysis was focused on the frontal cortex (n = 20), while in the muscle, the analysis involved vastus lateralis (n = 18), gastrocnemius (n = 3), biceps brachii (n = 5), deltoid (n = 1), and intercostal muscle (n = 1). Overall, muscle deoxygenation increases during exercise, reaching a plateau in voluntary exhaustion, while in the brain, oxyhemoglobin concentration increases with exercise intensity, reaching a plateau or declining at the exhaustion point.

CONCLUSION

Muscle and cerebral oxygenation respond differently to exercise, with muscle increasing O2 utilization and cerebral tissue increasing O2 delivery during exercise. However, at the exhaustion point, both muscle and cerebral oxygenation become compromised. This is characterized by a reduction in blood flow and a decrease in O2 extraction in the muscle, while in the brain, oxygenation reaches a plateau or decline, potentially resulting in motor failure during exercise.

The role of PIEZO ion channels in the musculoskeletal system

PIEZO1 and PIEZO2 are mechanosensitive cation channels that are highly expressed in numerous tissues throughout the body and exhibit diverse, cell-specific functions in multiple organ systems. Within the musculoskeletal system, PIEZO1 functions to maintain muscle and bone mass, sense tendon stretch, and regulate senescence and apoptosis in response to mechanical stimuli within cartilage and the intervertebral disc. PIEZO2 is essential for transducing pain and touch sensations as well as proprioception in the nervous system, which can affect musculoskeletal health. PIEZO1 and PIEZO2 have been shown to act both independently as well as synergistically in different cell types. Conditions that alter PIEZO channel mechanosensitivity, such as inflammation or genetic mutations, can have drastic effects on these functions. For this reason, therapeutic approaches for PIEZO-related disease focus on altering PIEZO1 and/or PIEZO2 activity in a controlled manner, either through inhibition with small molecules, or through dietary control and supplementation to maintain a healthy cell membrane composition. Although many opportunities to better understand PIEZO1 and PIEZO2 remain, the studies summarized in this review highlight how crucial PIEZO channels are to musculoskeletal health and point to promising possible avenues for their modulation as a therapeutic target.

NaV1.9 current in muscle afferent neurons is enhanced by substances released during muscle activity

Skeletal muscle contraction triggers the exercise pressor reflex (EPR) to regulate the cardiovascular system response to exercise. During muscle contraction, substances are released that generate action potential activity in group III and IV afferents that mediate the EPR. Some of these substances increase afferent activity via G-protein-coupled receptor (GPCR) activation, but the mechanisms are incompletely understood. We were interested in determining if tetrodotoxin-resistant (TTX-R) voltage-dependent sodium channels (NaV) were involved and investigated the effect of a mixture of such compounds (bradykinin, prostaglandin, norepinephrine, and ATP, called muscle metabolites). Using whole cell patch-clamp electrophysiology, we show that the muscle metabolites significantly increased TTX-R NaV currents. The rise time of this enhancement averaged ∼2 min, which suggests the involvement of a diffusible second messenger pathway. The effect of muscle metabolites on the current-voltage relationship, channel activation and inactivation kinetics support NaV1.9 channels as the target for this enhancement. When applied individually at the concentration used in the mixture, only prostaglandin and bradykinin significantly enhanced NaV current, but the sum of these enhancements was <1/3 that observed when the muscle metabolites were applied together. This suggests synergism between the activated GPCRs to enhance NaV1.9 current. When applied at a higher concentration, all four substances could enhance the current, which demonstrates that the GPCRs activated by each metabolite can enhance channel activity. The enhancement of NaV1.9 channel activity is a likely mechanism by which GPCR activation increases action potential activity in afferents generating the EPR.NEW & NOTEWORTHY G-protein-coupled receptor (GPCR) activation increases action potential activity in muscle afferents to produce the exercise pressor reflex (EPR), but the mechanisms are incompletely understood. We provide evidence that NaV1.9 current is synergistically enhanced by application of a mixture of metabolites potentially released during muscle contraction. The enhancement of NaV1.9 current is likely one mechanism by which GPCR activation generates the EPR and the inappropriate activation of the EPR in patients with cardiovascular disease.

Physiological role of anticipatory cardiorespiratory responses to exercise

This study aimed to investigate whether anticipatory cardiorespiratory responses vary depending on the intensity of the subsequent exercise bout, and whether anticipatory cardiorespiratory adjustments contribute importantly to enhancing exercise performance during high-intensity exercise. Eleven healthy men were provided advance notice of the exercise intensity and a countdown to generate anticipation during 10 min prior to exercise at 0, 50, 80 or 95% maximal work-rate (Experiment 1). A different group of subjects (n = 15) performed a time to exhaustion trial with or without anticipatory countdown (Experiment 2). In Experiment 1, heart rate (HR), oxygen uptake (V_O2 ) and minute ventilation (V_E ) during pre-exercise resting period increased over time and depended on the subsequent exercise intensity. Specifically, there was already a 7.4% increase in HR from more than 5 min prior to the start of exercise at 95% maximal work-rate, followed by progressively augmented increases of 12.5% between 2 and 3 min before exercise, 24.4% between 0 and 1 min before exercise. In Experiment 2, the initial HR for the first 10 s of exercise in the task with anticipation was 11.4% larger compared to without anticipation (p < 0.01), and the difference in HR between the two conditions decreased in a time-dependent manner. In contrast, the initial increases in V_O2 and V_E were significantly lower in the task with anticipation than that without anticipation. The time to exhaustion during high-intensity exercise was 14.6% longer under anticipation condition compared to no anticipation (135 ± 26 s vs. 119 ± 26 s, p = 0.003). In addition, the enhanced exercise performance correlated positively with increased HR response just before and immediately after exercise onset (p < 0.01). These results showed that anticipatory cardiorespiratory adjustments (feedforward control) via the higher brain that operate before starting exercise may play an important role in minimizing the time delay of circulatory response and enhancing performance after onset of high-intensity exercise in man.

A murine model of the exercise pressor reflex

KEY POINTS

The decerebrate mouse provides a novel working model of the exercise pressor reflex (EPR). The decerebrate mouse model of the EPR is similar to the previously described decerebrate rat model. Studying the EPR in transgenic mouse models can define exact mechanisms of the EPR in health and disease.

ABSTRACT

The exercise pressor reflex (EPR) is defined by a rise in mean arterial pressure (MAP) and heart rate (HR) in response to exercise and is necessary to match metabolic demand and prevent premature fatigue. While this reflex is readily tested in humans, mechanistic studies are largely infeasible. Here, we have developed a novel murine model of the EPR to allow for mechanistic studies in various mouse models. We observed that ventral root stimulation (VRS) in an anaesthetized mouse causes a depressor response and a reduction in HR. In contrast, the same stimulation in a decerebrate mouse causes a rise in MAP and HR which is abolished by dorsal rhizotomy or by neuromuscular blockade. Moreover, we demonstrate a reduced MAP response to VRS using TRPV1 antagonism or in Trpv1 null mice while the response to passive stretch remains intact. Additionally, we demonstrate that intra-arterial infusion of capsaicin results in a dose-related rise in MAP and HR that is significantly reduced by a selective and potent TRPV1 antagonist or is completely abolished in Trpv1 null mice. These data serve to validate the development of a decerebrate mouse model for the study of cardiovascular responses to exercise and further define the role of the TRPV1 receptor in mediating the EPR. This novel model will allow for extensive study of the EPR in unlimited transgenic and mutant mouse lines, and for an unprecedented exploration of the molecular mechanisms that control cardiovascular responses to exercise in health and disease.

Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation

In this review, we focus on significant developments in our understanding of the mechanisms that control the cutaneous vasculature in humans, with emphasis on the literature of the last half-century. To provide a background for subsequent sections, we review methods of measurement and techniques of importance in elucidating control mechanisms for studying skin blood flow. In addition, the anatomy of the skin relevant to its thermoregulatory function is outlined. The mechanisms by which sympathetic nerves mediate cutaneous active vasodilation during whole body heating and cutaneous vasoconstriction during whole body cooling are reviewed, including discussions of mechanisms involving cotransmission, NO, and other effectors. Current concepts for the mechanisms that effect local cutaneous vascular responses to local skin warming and cooling are examined, including the roles of temperature sensitive afferent neurons as well as NO and other mediators. Factors that can modulate control mechanisms of the cutaneous vasculature, such as gender, aging, and clinical conditions, are discussed, as are nonthermoregulatory reflex modifiers of thermoregulatory cutaneous vascular responses.