Gadolinium inhibits group III but not group IV muscle afferent responses to dynamic exercise

The role of clinical neurophysiology in the definition and assessment of fatigue and fatigability

Though a common symptom, fatigue is difficult to define and investigate, occurs in a wide variety of neurological and systemic disorders, with differing pathological causes. It is also often accompanied by a psychological component. As a symptom of long-term COVID-19 it has gained more attention. In this review, we begin by differentiating fatigue, a perception, from fatigability, quantifiable through biomarkers. Central and peripheral nervous system and muscle disorders associated with these are summarised. We provide a comprehensive and objective framework to help identify potential causes of fatigue and fatigability in a given disease condition. It also considers the effectiveness of neurophysiological tests as objective biomarkers for its assessment. Among these, twitch interpolation, motor cortex stimulation, electroencephalography and magnetencephalography, and readiness potentials will be described for the assessment of central fatigability, and surface and needle electromyography (EMG), single fibre EMG and nerve conduction studies for the assessment of peripheral fatigability. The purpose of this review is to guide clinicians in how to approach fatigue, and fatigability, and to suggest that neurophysiological tests may allow an understanding of their origin and interactions. In this way, their differing types and origins, and hence their possible differing treatments, may also be defined more clearly.

Mechanosensitive channels in the mechanical component of the exercise pressor reflex

The cardiovascular response is appropriately regulated during exercise to meet the metabolic demands of the active muscles. The exercise pressor reflex is a neural feedback mechanism through thin-fiber muscle afferents activated by mechanical and metabolic stimuli in the active skeletal muscles. The mechanical component of this reflex is referred to as skeletal muscle mechanoreflex. Its initial step requires mechanotransduction mediated by mechanosensors, which convert mechanical stimuli into biological signals. Recently, various mechanosensors have been identified, and their contributions to muscle mechanoreflex have been actively investigated. Nevertheless, the mechanosensitive channels responsible for this muscular reflex remain largely unknown. This review discusses progress in our understanding of muscle mechanoreflex under healthy conditions, focusing on mechanosensitive channels.

Increased brain 1H-MRS glutamate and lactate signals following maximal aerobic capacity exercise in young healthy males: an exploratory study

Physical exercise involves increased neuronal activity of many brain structures, but 1H-MRS investigations on the effects of human brain glutamate (Glu) concentrations on acute exercise have been sparse. Previous studies consistently found increases in brain lactate (Lac) concentrations following graded exercise up to 85% of the predicted maximal heart rate. However, the reported effects on brain concentrations of glutamine and glutamate were not consistent. This study aimed to determine the effect of acute intense graded maximal exercise on 1H-MRS signals related to concentrations of Glu, glutamate+glutamine (Glx), and Lac. Young adult males were randomly divided into two groups and subjected to 1H-MRS when resting (NE) or shortly after cessation of the intense graded exercise intended to pass the anaerobic threshold (E). 1H-MRS spectra were acquired from the large voxel encompassing the occipito-parietal cortex only once. Estimates of Glu, Glx, and Lac concentrations were calculated in institutional units by normalizing to a spectroscopic signal originating from creatine-containing compounds (Cr). Concentrations of Glu, Glx, and Lac were respectively 11%, 12.6%, and 48.5% higher in E than in NE (p < 0.001). The increased brain Lac signal in the exercising group indicated that in our experiment, vigorous exercise resulted in passing the anaerobic threshold and lactate apparently entered the brain. Concomitantly glutamate-related resonance signals from the vicinity of the occipito-parietal cortex were significantly increased; physiological mechanisms underlying these phenomena require further study. Future studies should evaluate whether the normalization rate of these concentrations is a marker of general physical fitness.

The exercise pressor reflex: An update

The exercise pressor reflex is a feedback mechanism engaged upon stimulation of mechano- and metabosensitive skeletal muscle afferents. Activation of these afferents elicits a reflex increase in heart rate, blood pressure, and ventilation in an intensity-dependent manner. Consequently, the exercise pressor reflex has been postulated to be one of the principal mediators of the cardiorespiratory responses to exercise. In this updated review, we will discuss classical and recent advancements in our understating of the exercise pressor reflex function in both human and animal models. Particular attention will be paid to the afferent mechanisms and pathways involved during its activation, its effects on different target organs, its potential role in the abnormal cardiovascular response to exercise in diseased states, and the impact of age and biological sex on these responses. Finally, we will highlight some unanswered questions in the literature that may inspire future investigations in the field.

Muscle stiffening is associated with muscle mechanoreflex-mediated cardioacceleration

PURPOSE

Although the muscle mechanoreflex is an important mediator to cardiovascular regulation during exercise, its modulation factors remain relatively unknown. Therefore, the purpose of this study was to investigate the effect of muscle stiffness on the muscle mechanoreflex.

METHODS

Participants were divided based on their median muscle stiffness (2.00 Nm/mm) into a low group (n = 15) and a high group (n = 15), and the muscle mechanoreflex was compared between the groups. After a 15-min rest in the supine position, heart rate (HR), blood pressure (BP), stroke volume (SV), and cardiac output (CO) were measured at rest for 3 min and during static passive dorsiflexion (SPD) at 20° for 1 min. Following a 15-min re-rest, muscle stiffness and passive resistive torque were evaluated in the distal end of the muscle belly of the medial gastrocnemius.

RESULTS

Peak relative changes in HR (low group: 6 ± 4% and high group: 12 ± 4%) and CO (low group: 8 ± 10% and high group: 13 ± 9%) were greater in the high group than in the low group (both, P < 0.05). A significant positive correlation was found between resistive torque during SPD and muscle stiffness and peak relative changes in HR (r = 0.51 and 0.61, both P < 0.05). However, there was no correlation between muscle elongation during SPD and peak relative changes in HR (r = - 0.23, P = 0.20).

CONCLUSION

These findings suggest that muscle stiffness may be modulatory factor of muscle mechanoreflex.

GsMTx-4 normalizes the exercise pressor reflex evoked by intermittent muscle contraction in early stage type 1 diabetic rats

Emerging evidence suggests the exercise pressor reflex is exaggerated in early stage type 1 diabetes mellitus (T1DM). Piezo channels may play a role in this exaggeration, as blocking these channels attenuates the exaggerated pressor response to tendon stretch in T1DM rats. However, tendon stretch constitutes a different mechanical and physiological stimuli than that occurring during muscle contraction. Therefore, the purpose of this study was to determine the contribution of Piezo channels in evoking the pressor reflex during an intermittent muscle contraction in T1DM. In unanesthetized decerebrate rats, we compared the pressor and cardioaccelerator responses to intermittent muscle contraction before and after locally injecting grammostola spatulata mechanotoxin 4 (GsMTx-4, 0.25 µM) into the hindlimb vasculature. Although GsMTx-4 has a high potency for Piezo channels, it has also been suggested to block transient receptor potential cation (TRPC) channels. We, therefore, performed additional experiments to control for this possibility by also injecting SKF 96365 (10 µM), a TRPC channel blocker. We found that local injection of GsMTx-4, but not SKF 96365, attenuated the exaggerated peak pressor (ΔMAP before: 33 ± 3 mmHg, after: 22 ± 3 mmHg, P = 0.007) and pressor index (ΔBPi before: 668 ± 91 mmHg·s, after: 418 ± 81 mmHg·s, P = 0.021) response in streptozotocin (STZ) rats (n = 8). GsMTx-4 attenuated the exaggerated early onset pressor and the pressor response over time, which eliminated peak differences as well as those over time between T1DM and healthy controls. These data suggest that Piezo channels are an effective target to normalize the exercise pressor reflex in T1DM.NEW & NOTEWORTHY This is the first study to demonstrate that blocking Piezo channels is effective in ameliorating the exaggerated exercise pressor reflex evoked by intermittent muscle contraction, commonly occurring during physical activity, in T1DM. Thus, these findings suggest Piezo channels may serve as an effective therapeutic target to reduce the acute and prolonged cardiovascular strain that may occur during dynamic exercise in T1DM.

Effects of acute hyperglycemia on the exercise pressor reflex in healthy rats

The exercise pressor reflex is exaggerated in type 2 diabetes mellitus (T2DM). Hyperglycemia, a main characteristic of T2DM, likely contributes to this exaggerated response. However, the isolated effect of acute hyperglycemia, independent of T2DM, on the exercise pressor reflex is not known. Therefore, the purpose of this study was to determine the effect of acute, local exposure to hyperglycemia on the exercise pressor reflex and its two components, namely the mechanoreflex and the metaboreflex, in healthy rats. To accomplish this, we determined the effect of an acute locol intra-arterial glucose infusion (0.25 g/mL) on cardiovascular responses to static contraction (i.e., exercise pressor reflex) and tendon stretch (i.e., mechanoreflex) for 30 s, as well as hindlimb intra-arterial lactic acid (24 mM) injection (i.e., metaboreflex) in fasted unanesthetized, decerebrated Sprague-Dawley rats. We measured and compared changes in mean arterial pressure (MAP) and heart rate (HR) before and after glucose infusion. We found that acute glucose infusion did not affect the pressor response to static contraction (ΔMAP: before: 15 ± 2 mmHg, after: 12 ± 2 mmHg; n = 8, p > 0.05), tendon stretch (ΔMAP: before: 12 ± 1 mmHg, after: 12 ± 3 mmHg; n = 8, p > 0.05), or lactic acid injection (ΔMAP: before: 13 ± 2 mmHg, after: 17 ± 3 mmHg; n = 9, p > 0.05). Likewise, cardioaccelerator responses were unaffected by glucose infusion, p > 0.05 for all. In conclusion, these findings suggest that acute, local exposure to hyperglycemia does not affect the exercise pressor reflex or either of its components.

Recent advances in exercise pressor reflex function in health and disease

Autonomic alterations at the onset of exercise are critical to redistribute cardiac output towards the contracting muscles while preventing a fall in arterial pressure due to excessive vasodilation within the contracting muscles. Neural mechanisms responsible for these adjustments include central command, the exercise pressor reflex, and arterial and cardiopulmonary baroreflexes. The exercise pressor reflex evokes reflex increases in sympathetic activity to the heart and systemic vessels and decreases in parasympathetic activity to the heart, which increases blood pressure (BP), heart rate, and total peripheral resistance through vasoconstriction of systemic vessels. In this review, we discuss recent advancements in our understanding of exercise pressor reflex function in health and disease. Specifically, we discuss emerging evidence suggesting that sympathetic vasoconstrictor drive to the contracting and non-contracting skeletal muscle is differentially controlled by central command and the metaboreflex in healthy conditions. Further, we discuss evidence from animal and human studies showing that cardiovascular diseases, including hypertension, diabetes, and heart failure, lead to an altered exercise pressor reflex function. We also provide an update on the mechanisms thought to underlie this altered exercise pressor reflex function in each of these diseases. Although these mechanisms are complex, multifactorial, and dependent on the etiology of the disease, there is a clear consensus that several mechanisms are involved. Ultimately, approaches targeting these mechanisms are clinically significant as they provide alternative therapeutic strategies to prevent adverse cardiovascular events while also reducing symptoms of exercise intolerance.

Intrathecal injection of brilliant blue G, a P2X7 antagonist, attenuates the exercise pressor reflex in rats

Purinergic 2X (P2X) receptors on the endings of group III and IV afferents play a role in evoking the exercise pressor reflex. Particular attention has been paid to P2X3 receptors because their blockade in the periphery attenuated this reflex. In contrast, nothing is known about the role played by P2X receptors in the spinal cord in evoking the exercise pressor reflex in rats. P2X7 receptors, in particular, may be especially important in this regard because they are found in abundance on spinal glial cells and may communicate with neurons to effect reflexes controlling cardiovascular function. Consequently, we investigated the role played by spinal P2X7 receptors in evoking the exercise pressor reflex in decerebrated rats. We found that intrathecal injection of the P2X7 antagonist brilliant blue G (BBG) attenuated the exercise pressor reflex (blood pressure index: 294 ± 112 mmHg·s before vs. 7 ± 32 mmHg·s after; P < 0.05). Likewise, intrathecal injection of minocycline, which inhibits microglial cell output, attenuated the reflex. In contrast, intrathecal injection of BBG did not attenuate the pressor response evoked by intracarotid injection of sodium cyanide, a maneuver that stimulated carotid chemoreceptors. Moreover, injections of BBG either into the arterial supply of the contracting hindlimb muscles or into the jugular vein did not attenuate the exercise pressor reflex. Our findings support the hypothesis that P2X7 receptors on microglial cells within the spinal cord play a role in evoking the exercise pressor reflex.

Exaggerated exercise pressor reflex in type 2 diabetes: Potential role of oxidative stress

Type 2 diabetes mellitus (T2DM) leads to exaggerated cardiovascular responses to exercise, in part due to an exaggerated exercise pressor reflex. Accumulating data suggest excessive oxidative stress contributes to an exaggerated exercise pressor reflex in cardiovascular-related diseases. Excessive oxidative stress is also a primary underlying mechanism for the development and progression of T2DM. However, whether oxidative stress plays a role in mediating the exaggerated exercise pressor reflex in T2DM is not known. Therefore, this review explores the potential role of oxidative stress leading to increased activation of the afferent arm of the exercise pressor reflex. Several lines of evidence support direct and indirect effects of oxidative stress on the exercise pressor reflex. For example, intramuscular ROS may directly and indirectly (by attenuating contracting muscle blood flow) increase group III and IV afferent activity. Oxidative stress is a primary underlying mechanism for the development of neuropathic pain, which in turn is associated with increased group III and IV afferent activity. These are the same type of afferents that evoke muscle pain and the exercise pressor reflex. Furthermore, oxidative stress-induced release of inflammatory mediators may modulate afferent activity. Collectively, these alterations may result in a positive feedback loop that further amplifies the exercise pressor reflex. An exaggerated reflex increases the risk of adverse cardiovascular events. Thus, identifying the contribution of oxidative stress could provide a potential therapeutic target to reduce this risk in T2DM.

Effect of fatigue-related group III/IV afferent firing on intracortical inhibition and facilitation in hand muscles

Fatiguing exercise causes a reduction in motor drive to the muscle. Group III/IV muscle afferent firing is thought to contribute to this process; however, the effect on corticospinal and intracortical networks is poorly understood. In two experiments, participants performed sustained maximal isometric finger abductions of the first dorsal interosseous (FDI) muscle, with postexercise blood flow occlusion (OCC) to maintain the firing of group III/IV afferents or without occlusion (control; CON). Before and after exercise, single- and paired-pulse transcranial magnetic stimulation (TMS) tested motor evoked potentials (MEPs), intracortical facilitation [ICF (12 ms)], and short-interval intracortical inhibition [SICI₂ (2 ms), SICI₃ (3 ms)]. Ulnar nerve stimulation elicited maximal M waves (M_MAX). For experiment 1 (n = 16 participants), TMS intensities were 70% and 120% of resting motor threshold (RMT) for the conditioning and MEP stimuli, respectively. For experiment 2 (n = 16 participants), the MEP was maintained at 1 mV before and after exercise and the conditioning stimulus individualized. In experiment 1, MEP/M_MAX was reduced after exercise (~48%, P = 0.007) but was not different between conditions. No changes occurred in ICF or SICI. In experiment 2, MEP/M_MAX increased (~27%, P = 0.027) and less inhibition (SICI₂: ~21%, P = 0.021) occurred after exercise for both conditions, whereas ICF decreased for CON only (~28%, P = 0.006). MEPs and SICI₂ were modulated by fatiguing contractions but not by group III/IV afferent firing, whereas sustained afferent firing appeared to counteract postexercise reductions in ICF in FDI. The findings do not support the idea that actions of group III/IV afferents on motor cortical networks contribute to the reduction in voluntary activation observed in other studies.NEW & NOTEWORTHY This is the first study to investigate, in human hand muscles, the action of fatigue-related group III/IV muscle afferent firing on intracortical facilitation and inhibition. In fatigued and nonexercised hand muscles, intracortical inhibition is reduced after exercise but is not modulated differently by the firing of group III/IV afferents. However, facilitation is maintained for the fatigued muscle when group III/IV afferents fire, but these results are unlikely to explain the reduction in voluntary activation observed in other studies.

Pharmacological attenuation of group III/IV muscle afferents improves endurance performance when oxygen delivery to locomotor muscles is preserved

We sought to investigate the role of group III/IV muscle afferents in limiting endurance exercise performance, independently of their role in optimizing locomotor muscle O₂ delivery. While breathing 100% O₂ to ensure a similar arterial O₂ content ([Formula: see text]) in both trials, eight male cyclists performed 5-km time trials under control conditions (H_CTRL) and with lumbar intrathecal fentanyl (H_FENT) impairing neural feedback from the lower limbs. After each time trial, common femoral artery blood flow (FBF) was quantified (Doppler ultrasound) during constant-load cycling performed at the average power of the preceding time trial. The assessment of end-tidal gases, hemoglobin content and saturation, and FBF facilitated the calculation of leg O₂ delivery. Locomotor muscle activation during cycling was estimated from vastus lateralis EMG. With electrical femoral nerve stimulation, peripheral and central fatigue were quantified by pre- to postexercise decreases in quadriceps twitch torque (ΔQ_tw) and voluntary activation (ΔVA), respectively. FBF (~16 mL·min^-1·W^-1; P = 0.6), [Formula: see text] (~24 mL O₂/dL; P = 0.9), and leg O₂ delivery (~0.38 mL O₂·min^-1·W^-1; P = 0.9) were not different during H_CTRL and H_FENT. Mean power output and time to completion were significantly improved by 9% (~310 W vs. ~288 W) and 3% (~479 s vs. ~463 s), respectively, during H_FENT compared with H_CTRL. Quadriceps muscle activation was 9 ± 7% higher during H_FENT compared with H_CTRL (P < 0.05). ΔQ_tw was significantly greater in H_FENT compared with H_CTRL (54 ± 8% vs. 39 ± 9%), whereas ΔVA was not different (~5%; P = 0.3) in both trials. These findings reveal that group III/IV muscle afferent feedback limits whole body endurance exercise performance and peripheral fatigue by restricting neural activation of locomotor muscle.NEW & NOTEWORTHY Group III/IV muscle afferent feedback facilitates endurance performance by optimizing locomotor muscle O₂ delivery but also limits performance by restricting neural drive to locomotor muscle. To isolate the performance-limiting effect of these sensory neurons, we pharmacologically attenuated their central projection during a cycling time trial while controlling for locomotor muscle O₂ delivery. With no difference in leg O₂ delivery, afferent blockade attenuated the centrally mediated restriction in motoneuronal output and improved cycling performance.

Exaggerated mechanoreflex in early-stage type 1 diabetic rats: role of Piezo channels

Recent findings have shown that muscle contraction evokes an exaggerated pressor response in type 1 diabetes mellitus (T1DM) rats; however, it is not known whether the mechanoreflex, which is commonly stimulated by stretching the Achilles tendon, contributes to this abnormal response. Furthermore, the role of mechano-gated Piezo channels, found on thin-fiber afferent endings, in evoking the mechanoreflex in T1DM is also unknown. Therefore, in male and female streptozotocin (STZ, 50 mg/kg)-induced T1DM and healthy control (CTL) rats, we examined the pressor and cardioaccelerator responses to tendon stretch during the early stage of the disease. To determine the role of Piezo channels, GsMTx-4, a selective Piezo channel inhibitor, was injected into the arterial supply of the hindlimb. At 1 wk after STZ injection in unanesthetized, decerebrate rats, we stretched the Achilles tendon for 30 s and measured pressor and cardioaccelerator responses. We then compared pressor and cardioaccelerator responses to tendon stretch before and after GsMTx-4 injection (10 µg/100 ml). We found that the pressor (change in mean arterial pressure) response [41 ± 5 mmHg ( n = 15) for STZ and 18 ± 3 mmHg ( n = 11) for CTL ( P < 0.01)] and cardioaccelerator (change in heart rate) response [18 ± 4 beats/min for STZ ( n = 15) and 8 ± 2 beats/min ( n = 11) for CTL ( P < 0.05)] to tendon stretch were exaggerated in STZ rats. Local injection of GsMTx-4 attenuated the pressor [55 ± 7 mmHg ( n = 6) before and 27 ± 9 mmHg ( n = 6) after GsMTx-4 ( P < 0.01)], but not the cardioaccelerator, response to tendon stretch in STZ rats and had no effect on either response in CTL rats. These data suggest that T1DM exaggerates the mechanoreflex response to tendon stretch and that Piezo channels play a role in this exaggeration.

Stimulation of spinal δ-opioid receptors attenuates the exercise pressor reflex in decerebrate rats

A reflex arising from contracting hindlimb muscle is responsible in part for the increases in arterial pressure and heart rate evoked by exercise. The afferent arm of this reflex comprises group III and IV afferents. δ-Opioid receptors are expressed predominately on the spinal endings of group III afferents, whereas μ-opioid receptors are expressed predominately on the spinal endings of group IV afferents. Using stimuli that activated group III afferents, namely static contraction, calcaneal tendon stretch, and lactic acid injection into the superficial epigastric artery, we tested the hypothesis that, in rats with either patent or ligated femoral arteries, activation of pre- and postsynaptic δ-opioid receptors in the dorsal horn attenuated pressor reflex responses to these stimuli. In rats with patent arteries or ligated femoral arteries, [d-Pen^2,5]enkephalin (DPDPE), a δ-opioid agonist injected intrathecally (10 μg in 10 μl), significantly attenuated the pressor responses to contraction, stretch, and lactic acid (all P < 0.05). Naltrindole, a δ-opioid receptor antagonist, prevented the attenuation. In contrast, DPDPE did not attenuate the pressor response to capsaicin injection into the superficial epigastric artery in either group of rats (both P > 0.05). Intrathecal injection of saline (10 μl), the vehicle for DPDPE, had no effect on the pressor responses in either group of rats. We conclude that activation of spinal δ-opioid receptors attenuates reflexes evoked by group III afferents in both freely perfused and ligated rats.

GsMTx4 reduces the reflex pressor response during dynamic hindlimb skeletal muscle stretch in decerebrate rats

Mechanical signals within contracting skeletal muscles contribute to the generation of the exercise pressor reflex; an important autonomic and cardiovascular control mechanism. In decerebrate rats, the mechanically activated channel inhibitor GsMTx4 was found to reduce the pressor response during static hindlimb muscle stretch; a maneuver used to investigate specifically the mechanical component of the exercise pressor reflex (i.e., the mechanoreflex). However, the effect was found only during the initial phase of the stretch when muscle length was changing and not during the later phase of stretch when muscle length was relatively constant. We tested the hypothesis that in decerebrate, unanesthetized rats, GsMTx4 would reduce the pressor response throughout the duration of a 30 sec, 1 Hz dynamic hindlimb muscle stretch protocol that produced repetitive changes in muscle length. We found that the injection of 10 μg of GsMTx4 into the arterial supply of a hindlimb reduced the peak pressor response (control: 15 ± 4, GsMTx4: 5 ± 2 mmHg, P < 0.05, n = 8) and the pressor response at multiple time points throughout the duration of the stretch. GsMTx4 had no effect on the pressor response to the hindlimb arterial injection of lactic acid which indicates the lack of local off-target effects. Combined with the recent finding that GsMTx4 reduced the pressor response only initially during static stretch in decerebrate rats, the present findings suggest that GsMTx4-sensitive channels respond primarily to mechanical signals associated with changes in muscle length. The findings add to our currently limited understanding of the channels that contribute to the activation of the mechanoreflex.

Physiological responses to interval endurance exercise at different levels of blood flow restriction

PURPOSE

We aimed to identify a blood flow restriction (BFR) endurance exercise protocol that would both maximize cardiopulmonary and metabolic strain, and minimize the perception of effort.

METHODS

Twelve healthy males (23 ± 2 years, 75 ± 7 kg) performed five different exercise protocols in randomized order: HI, high-intensity exercise starting at 105% of the incremental peak power (P _peak); I-BFR30, intermittent BFR at 30% P _peak; C-BFR30, continuous BFR at 30% P _peak; CON30, control exercise without BFR at 30% P _peak; I-BFR0, intermittent BFR during unloaded exercise. Cardiopulmonary, gastrocnemius oxygenation (StO₂), capillary lactate ([La]), and perceived exertion (RPE) were measured.

RESULTS

V̇O₂, ventilation (V̇ _E), heart rate (HR), [La] and RPE were greater in HI than all other protocols. However, muscle StO₂ was not different between HI (set1-57.8 ± 5.8; set2-58.1 ± 7.2%) and I-BRF30 (set1-59.4 ± 4.1; set2-60.5 ± 6.6%, p < 0.05). While physiologic responses were mostly similar between I-BFR30 and C-BFR30, [La] was greater in I-BFR30 (4.2 ± 1.1 vs. 2.6 ± 1.1 mmol L^-1, p = 0.014) and RPE was less (5.6 ± 2.1 and 7.4 ± 2.6; p = 0.014). I-BFR30 showed similar reduced muscle StO₂ compared with HI, and increased blood lactate compared to C-BFR30 exercise.

CONCLUSION

Therefore, this study demonstrate that endurance cycling with intermittent BFR promotes muscle deoxygenation and metabolic strain, which may translate into increased endurance training adaptations while minimizing power output and RPE.

The mechano-gated channel inhibitor GsMTx4 reduces the exercise pressor reflex in rats with ligated femoral arteries

Mechanical and metabolic stimuli arising from contracting muscles evoke the exercise pressor reflex. This reflex is greater in a rat model of simulated peripheral arterial disease in which a femoral artery is chronically ligated than it is in rats with freely perfused femoral arteries. The role played by the mechanically sensitive component of the exaggerated exercise pressor reflex in ligated rats is unknown. We tested the hypothesis that the mechano-gated channel inhibitor GsMTx4, a relatively selective inhibitor of mechano-gated Piezo channels, reduces the exercise pressor reflex in decerebrate rats with ligated femoral arteries. Injection of 10 μg of GsMTx4 into the arterial supply of the hindlimb reduced the pressor response to Achilles tendon stretch (a purely mechanical stimulus) but had no effect on the pressor responses to intra-arterial injection of α,β-methylene ATP or lactic acid (purely metabolic stimuli). Moreover, injection of 10 μg of GsMTx4 into the arterial supply of the hindlimb reduced both the integrated pressor area (control 535 ± 21, GsMTx4 218 ± 24 mmHg·s; P < 0.01), peak pressor (control 29 ± 2, GsMTx4 14 ± 3 mmHg; P < 0.01), and renal sympathetic nerve responses to electrically induced intermittent hindlimb muscle contraction (a mixed mechanical and metabolic stimulus). The reduction of the integrated pressor area during contraction caused by GsMTx4 was greater in rats with ligated femoral arteries than it was in rats with freely perfused femoral arteries. We conclude that the mechanically sensitive component of the reflex contributes to the exaggerated exercise pressor reflex during intermittent hindlimb muscle contractions in rats with ligated femoral arteries.

The mechano-gated channel inhibitor GsMTx4 reduces the exercise pressor reflex in decerebrate rats

KEY POINTS

Mechanical and metabolic stimuli from contracting muscles evoke reflex increases in blood pressure, heart rate and sympathetic nerve activity. Little is known, however, about the nature of the mechano-gated channels on the thin fibre muscle afferents that contribute to evoke this reflex, termed the exercise pressor reflex. We determined the effect of GsMTx4, an inhibitor of mechano-gated Piezo channels, on the exercise pressor reflex evoked by intermittent contraction of the triceps surae muscles in decerebrated, unanaesthetized rats. GsMTx4 reduced the pressor, cardioaccelerator and renal sympathetic nerve responses to intermittent contraction but did not reduce the pressor responses to femoral arterial injection of compounds that stimulate the metabolically-sensitive thin fibre muscle afferents. Expression levels of Piezo2 channels were greater than Piezo1 channels in rat dorsal root ganglia. Our findings suggest that mechanically-sensitive Piezo proteins contribute to the generation of the mechanical component of the exercise pressor reflex in rats. Mechanical and metabolic stimuli within contracting skeletal muscles evoke reflex autonomic and cardiovascular adjustments. In cats and rats, gadolinium has been used to investigate the role played by the mechanical component of this reflex, termed the exercise pressor reflex. Gadolinium, however, has poor selectivity for mechano-gated channels and exerts multiple off-target effects. We tested the hypothesis that GsMTX4, a more selective mechano-gated channel inhibitor than gadolinium and a particularly potent inhibitor of mechano-gated Piezo channels, reduced the exercise pressor reflex in decerebrate rats. Injection of 10 μg of GsMTx4 into the arterial supply of the hindlimb reduced the peak pressor (control: 24 ± 5, GsMTx4: 12 ± 5 mmHg, P < 0.01), cardioaccelerator and renal sympathetic nerve responses to tendon stretch, a purely mechanical stimulus, but had no effect on the pressor responses to intra-arterial injection of α,β-methylene ATP or lactic acid. Moreover, injection of 10 μg of GsMTx4 into the arterial supply of the hindlimb reduced the peak pressor (control: 24 ± 2, GsMTx4: 14 ± 3 mmHg, P < 0.01), cardioaccelerator and renal sympathetic nerve responses to electrically-induced intermittent hindlimb muscle contractions. By contrast, injection of 10 μg of GsMTx4 into the jugular vein had no effect on the pressor, cardioaccelerator, or renal sympathetic nerve responses to contraction. Quantitative RT-PCR and western blot analyses indicated that both Piezo1 and Piezo2 channel isoforms were natively expressed in rat dorsal root ganglia tissue. We conclude that GsMTx4 reduced the exercise pressor reflex in decerebrate rats and that the reduction was attributable, at least in part, to its effect on mechano-gated Piezo channels.

Central and Peripheral Fatigue During Resistance Exercise - A Critical Review

Resistance exercise is a popular form of conditioning for numerous sport disciplines, and recently different modes of strength training are being evaluated for health benefits. Resistance exercise differs significantly in nature, and several variables determine the direction and range of adaptive changes that occur in the muscular and skeletal system of the body. Some modes of resistance training can also be effective in stimulating the cardiovascular system. These variables include exercise selection (general, specific, single or multi joint, dynamic, explosive), type of resistance (free weights, variable resistance, isokinetics), order of exercise (upper and lower body or push and pull exercises), and most of all the training load which includes intensity expressed as % of 1RM, number of repetitions, number of sets and the rest interval between sets. Manipulating these variables allows for specific adaptive changes which may include gains in muscle mass, muscle strength or muscle endurance. It has been well established that during resistance exercise fatigue occurs, regardless of the volume and intensity of work applied. The peripheral mechanisms of fatigue have been studied and explained in more detail than those related to the CNS. This review is an attempt to bring together the latest knowledge regarding fatigue, both peripheral and central, during resistance exercise. The authors of this review concentrated on physiological and biochemical mechanisms underlying fatigue in exercises performed with maximal intensity, as well as those performed to exhaustion with numerous repetitions and submaximal load.

Hindlimb venous distention evokes a pressor reflex in decerebrated rats

The distention of small vessels caused by an increase in blood flow to dynamically exercising muscles has been proposed as a stimulus that activates the thin fiber (groups III and IV) afferents evoking the exercise pressor reflex. This theory has been supported by evidence obtained from both humans and animals. In decerebrated unanesthetized rats with either freely perfused femoral arteries or arteries that were ligated 3 days before the experiment, we attempted to provide evidence in support of this theory by measuring arterial pressure, heart rate, and renal sympathetic nerve discharge while retrogradely injecting Ringer's solution in increasing volumes into the femoral vein just as it excited the triceps surae muscles. We found that the pressor response to injection was directly proportional to the volume injected. Retrograde injection of volumes up to and including 1 mL had no significant effect on either heart rate or renal sympathetic nerve activity. Cyclooxygenase blockade with indomethacin attenuated the reflex pressor response to retrograde injection in both groups of rats. In contrast, gadolinium, which blocks mechanogated channels, attenuated the reflex pressor response to retrograde injection in the “ligated rats,” but had no effect on the response in “freely perfused” rats. Our findings are consistent with the possibility that distension of small vessels within exercising skeletal muscle can serve as a stimulus to the thin fiber afferents evoking the exercise pressor reflex.

In decerebrated unanesthetized rats with either freely perfused femoral arteries or arteries that were ligated 3 days before the experiment, we attempted to provide evidence in support of this theory by measuring arterial pressure, heart rate, and renal sympathetic nerve discharge while retrogradely injecting Ringer's solution in increasing volumes into the femoral vein just as it excited the triceps surae muscles. Our findings are consistent with the possibility that distension of small vessels within exercising skeletal muscle can serve as a stimulus to the thin fiber afferents evoking the exercise pressor reflex.

Fast and slow processes underlie the selection of both step frequency and walking speed

People prefer gaits that minimize their energetic cost. Research focused on step frequency selection suggests that a fast predictive process and a slower optimization process underlie this energy optimization. Our purpose in this study was to test whether the mechanisms controlling step frequency selection are used more generally to select one of the most relevant characteristics of walking - preferred speed. To accomplish this, we contrasted the dynamic adjustments in speed following perturbations to step frequency against the dynamic adjustments in step frequency following perturbations to speed. Despite the use of different perturbations and contexts, we found that the responses were very similar. In both experiments, subjects responded to perturbations by first rapidly changing their speed or step frequency towards their preferred pattern, and then slowly adjusting their gait to converge onto their preferred pattern. We measured similar response times for both the fast processes (1.4±0.3 versus 2.7±0.6 s) and the slow processes (74.2±25.4 versus 79.7±20.2 s). We also found that the fast process, although quite variable in amplitude, dominated the adjustments in both speed and step frequency. These distinct but complementary experiments demonstrate that people appear to rely heavily on prediction to rapidly select the most relevant aspects of their preferred gait and then gradually fine-tune that selection, perhaps using direct optimization of energetic cost.

Purinergic 2X receptors play a role in evoking the exercise pressor reflex in rats with peripheral artery insufficiency

Purinergic 2X (P2X) receptors on the endings of thin fiber afferents have been shown to play a role in evoking the exercise pressor reflex in cats. In this study, we attempted to extend this finding to decerebrated, unanesthetized rats whose femoral arteries were either freely perfused or were ligated 72 h before the start of the experiment. We first established that our dose of pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 10 mg/kg), a P2X receptor antagonist, attenuated the pressor response to α,β-methylene ATP (10 μg/kg), a P2X receptor agonist. We then compared the exercise pressor reflex before and after infusing PPADS into the arterial supply of the hindlimb muscles that were statically contracted. In rats with freely perfused femoral arteries, the peak pressor responses to contraction were not significantly attenuated by PPADS (before PPADS: 19 ± 2 mmHg, 13 min after PPADS: 17 ± 2 mmHg, and 25 min after PPADS: 17 ± 3 mmHg). Likewise, the cardioaccelerator and renal sympathetic nerve responses were not significantly attenuated. In contrast, we found that in rats whose femoral arteries were ligated PPADS significantly attenuated the peak pressor responses to contraction (before PPADS: 37 ± 5 mmHg, 13 min after PPADS: 27 ± 6 mmHg, and 25 min after PPADS: 25 ± 5 mmHg; P < 0.05). Heart rate was not significantly attenuated, but renal SNA was at certain time points over the 30-s contraction period. We conclude that P2X receptors play a substantial role in evoking the exercise pressor reflex in rats whose femoral arteries were ligated but play only a minimal role in evoking the reflex in rats whose femoral arteries were freely perfused.

Blockade of B2 receptors attenuates the responses of group III afferents to static contraction

Recent evidence has been presented demonstrating that group III mechanoreceptors comprise an important part of the sensory arm of the exercise pressor reflex, which in turn functions to increase arterial blood flow to contracting skeletal muscles. Although group III afferents are stimulated by mechanical distortion of their receptive fields, they are also stimulated by bradykinin, which is produced by skeletal muscle when it contracts. Moreover, blockade of B (bradykinin)2 receptors has been shown to decrease the magnitude of the exercise pressor reflex. Nevertheless, the effect of blockade of B2 receptors on responses of group III afferents to contraction is not known. We therefore determined the effect of B2 receptor blockade with HOE 140 (40μg/kg) on the responses to both static and intermittent contraction of group III afferents with endings in the triceps surae muscle of decerebrated unanesthetized cats. We found that HOE 140 significantly attenuated (P=0.04) the responses of 14 group III afferents to static contraction, but did not significantly attenuate (P=0.16) the responses of 16 group III afferents to intermittent contraction. The attenuation induced by HOE 140 was present throughout the static contraction period, and led us to speculate that blockade of B2 receptors on the endings of group III afferents decreased their sensitivity to mechanical events occurring in the working muscles.

Water drinking-related muscle contraction induces the pressor response via mechanoreceptors in conscious rats

Water drinking is known to induce the pressor response. The efferent pathway in this response involves sympathoexcitation, because the pressor response was completely abolished by ganglionic blockade or an α(1)-adrenergic antagonist. However, the afferent pathway in this response has not been identified. In the present study, we hypothesized that water itself stimulates the upper digestive tract to induce the pressor response, and/or drinking-related muscle contraction induces the pressor response via mechanoreceptors. To examine this hypothesis, we evaluated the pressor response induced by spontaneous or passive water drinking in conscious rats. Since the baroreflex modulates and obscures the pressor response, the experiments were conducted using rats with sinoaortic denervation. The pressor response was not suppressed by 1) transient oral surface anesthesia using lidocaine, 2) bilateral denervation of the glossopharyngeal nerve and sensory branch of the superior laryngeal nerve, or 3) denervation of the tunica adventitia in the esophagus. However, the pressor response was significantly suppressed (by -52%) by intravenous gadolinium chloride administration. Electrical stimulation of the hypoglossal nerve induced the pressor response, which was significantly suppressed (by -57%) by intravenous gadolinium chloride administration and completely abolished by severing the distal end of this nerve. These results indicate that afferent signals from mechanoreceptors in drinking-related muscles are involved in the water drinking-induced pressor response.

Blockade of ATP-sensitive potassium channels prevents the attenuation of the exercise pressor reflex by tempol in rats with ligated femoral arteries

We reported previously that tempol attenuated the exercise pressor and muscle mechanoreceptor reflexes in rats whose femoral arteries were ligated, whereas tempol did not attenuate these reflexes in rats whose femoral arteries were freely perfused. Although the mechanism whereby tempol attenuated these reflexes in rats whose femoral artery was ligated was independent of its ability to scavenge reactive oxygen species, its nature remains unclear. An alternative explanation for the tempol-induced attenuation of these reflexes involves ATP-sensitive potassium channels (K(ATP)) and calcium-activated potassium channels (BK(Ca)), both of which are opened by tempol. We tested the likelihood of this explanation by measuring the effects of either glibenclamide (0.1 mg/kg), which blocks K(ATP) channels, or iberiotoxin (20 or 40 μg/kg), which blocks BK(Ca) channels, on the tempol-induced attenuation of the exercise pressor and muscle mechanoreceptor reflexes in decerebrated rats whose femoral arteries were ligated. We found that glibenclamide prevented the tempol-induced attenuation of both reflexes, whereas iberiotoxin did not. We also found that the amount of protein comprising the pore of the K(ATP) channel in the dorsal root ganglia innervating hindlimbs whose femoral artery was ligated was significantly greater than that in the dorsal root ganglia innervating hindlimbs whose femoral arteries were freely perfused. In contrast, the amounts of protein comprising the BK(Ca) channel in the dorsal root ganglia innervating the ligated and freely perfused hindlimbs were not different. We conclude that tempol attenuated both reflexes by opening K(ATP) channels, an effect that hyperpolarized muscle afferents stimulated by static contraction or tendon stretch.

Cardiovascular regulation by skeletal muscle reflexes in health and disease

Heart rate and blood pressure are elevated at the onset and throughout the duration of dynamic or static exercise. These neurally mediated cardiovascular adjustments to physical activity are regulated, in part, by a peripheral reflex originating in contracting skeletal muscle termed the exercise pressor reflex. Mechanically sensitive and metabolically sensitive receptors activating the exercise pressor reflex are located on the unencapsulated nerve terminals of group III and group IV afferent sensory neurons, respectively. Mechanoreceptors are stimulated by the physical distortion of their receptive fields during muscle contraction and can be sensitized by the production of metabolites generated by working skeletal myocytes. The chemical by-products of muscle contraction also stimulate metaboreceptors. Once activated, group III and IV sensory impulses are transmitted to cardiovascular control centers within the brain stem where they are integrated and processed. Activation of the reflex results in an increase in efferent sympathetic nerve activity and a withdrawal of parasympathetic nerve activity. These actions result in the precise alterations in cardiovascular hemodynamics requisite to meet the metabolic demands of working skeletal muscle. Coordinated activity by this reflex is altered after the development of cardiovascular disease, generating exaggerated increases in sympathetic nerve activity, blood pressure, heart rate, and vascular resistance. The basic components and operational characteristics of the reflex, the techniques used in human and animals to study the reflex, and the emerging evidence describing the dysfunction of the reflex with the advent of cardiovascular disease are highlighted in this review.

Effects of leg blood flow restriction during walking on cardiovascular function

INTRODUCTION

Exercise with limb blood flow restriction (BFR) is a very popular exercise modality in Japan and is spreading widely to the rest of the world. The underlying principle of this training modality is that under the conditions of restricted blood flow, even low-intensity exercise can provide significant muscle strength and hypertrophy. One concern, however, is that BFR during exercise may place unnecessary burden on those with compromised cardiac function.

METHODS

We determined the impact of leg BFR during walking on cardiovascular function in 17 young (26 ± 1 yr) healthy volunteers. Each subject underwent five bouts of 2-min treadmill walking at 2 miles·h(-1) with 1-min interval either with or without tourniquet cuffs inflated on both thighs.

RESULTS

Heart rate increased more during the BFR session, whereas stroke volume decreased greater during the BFR session. Blood pressure increased significantly and substantially during the BFR session. Consequently, an increase in double product, an index of myocardial oxygen demand, was more than threefold higher in the BFR condition. Systemic arterial compliance evaluated by stroke volume/pulse pressure ratio significantly increased during the control session by 14% but reduced during the BFR condition by 19%. Popliteal artery flow-mediated vasodilation decreased significantly after the exercise with BFR but not after the control session.

CONCLUSIONS

Even at low intensity, the aerobic exercise with BFR requires a greater cardiac work and decreases endothelial function. Limb BFR during exercise may need to be more cautiously prescribed to those with compromised cardiac conditions.

Distinct fast and slow processes contribute to the selection of preferred step frequency during human walking

Humans spontaneously select a step frequency that minimizes the energy expenditure of walking. This selection might be embedded within the neural circuits that generate gait so that the optimum is pre-programmed for a given walking speed. Or perhaps step frequency is directly optimized, based on sensed feedback of energy expenditure. Direct optimization is expected to be slow due to the compounded effect of delays and iteration, whereas a pre-programmed mechanism presumably allows for faster step frequency selection, albeit dependent on prior experience. To test for both pre-programmed selection and direct optimization, we applied perturbations to treadmill walking to elicit transient changes in step frequency. We found that human step frequency adjustments (n = 7) occurred with two components, the first dominating the response (66 ± 10% of total amplitude change; mean ± SD) and occurring quite quickly (1.44 ± 1.14 s to complete 95% of total change). The other component was of smaller amplitude (35 ± 10% of total change) and took tens of seconds (27.56 ± 16.18 s for 95% completion). The fast process appeared to be too fast for direct optimization and more indicative of a pre-programmed response. It also persisted even with unusual closed-loop perturbations that conflicted with prior experience and rendered the response energetically suboptimal. The slow process was more consistent with the timing expected for direct optimization. Our interpretation of these results is that humans may rely heavily on pre-programmed gaits to rapidly select their preferred step frequency and then gradually fine-tune that selection with direct optimization.

Skeletal muscle reflex-mediated changes in sympathetic nerve activity are abnormal in spontaneously hypertensive rats

In hypertension, the blood pressure response to exercise is exaggerated. We demonstrated previously that this heightened pressor response to physical activity is mediated by an overactive skeletal muscle exercise pressor reflex (EPR), with important contributions from its metaboreflex and mechanoreflex components. However, the mechanisms driving the abnormal blood pressure response to EPR activation are largely unknown. Recent evidence in humans suggests that the muscle metaboreflex partially mediates the enhanced EPR-induced pressor response via abnormally large changes in sympathetic nerve activity (SNA). Whether the muscle mechanoreflex induces similarly exaggerated alterations in SNA in hypertension remains unknown, as does the role of the mechanoreceptors mediating muscle reflex activity. To address these issues, the EPR was selectively activated by electrically inducing hindlimb muscle contraction in decerebrate normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Stimulation of the EPR evoked significantly larger increases in mean arterial pressure (MAP) and renal SNA (RSNA) in SHR compared with WKY (ΔRSNA from baseline: 140 ± 11 vs. 48 ± 8%). The mechanoreflex was stimulated by stretching hindlimb muscle which likewise elicited significantly greater elevations in MAP and RSNA in SHR than WKY (ΔRSNA from baseline: 105 ± 11 vs. 35 ± 7%). Blockade of mechanoreceptors in muscle with gadolinium significantly attenuated the MAP and RSNA responses to contraction and stretch in SHR. These data suggest that 1) the exaggerated pressor response to activation of the EPR and muscle mechanoreflex in hypertension is mediated by abnormally large reflex-induced augmentations in SNA and 2) this accentuated sympathetic responsiveness is evoked, in part, by stimulation of muscle mechanoreceptors.

Transient receptor potential A1 channel contributes to activation of the muscle reflex

This study was undertaken to elucidate the role played by transient receptor potential A1 channels (TRPA1) in activating the muscle reflex, a sympathoexcitatory drive originating in contracting muscle. First, we tested the hypothesis that stimulation of the TRPA1 located on muscle afferents reflexly increases sympathetic nerve activity. In decerebrate rats, allyl isothiocyanate, a TRPA1 agonist, was injected intra-arterially into the hindlimb muscle circulation. This led to a 33% increase in renal sympathetic nerve activity (RSNA). The effect of allyl isothiocyanate was a reflex because the response was prevented by sectioning the sciatic nerve. Second, we tested the hypothesis that blockade of TRPA1 reduces RSNA response to contraction. Thirty-second continuous static contraction of the hindlimb muscles, induced by electrical stimulation of the peripheral cut ends of L(4) and L(5) ventral roots, increased RSNA and blood pressure. The integrated RSNA during contraction was reduced by HC-030031, a TRPA1 antagonist, injected intra-arterially (163 ± 24 vs. 95 ± 21 arbitrary units, before vs. after HC-030031, P < 0.05). Third, we attempted to identify potential endogenous stimulants of TRPA1, responsible for activating the muscle reflex. Increases in RSNA in response to injection into the muscle circulation of arachidonic acid, bradykinin, and diprotonated phosphate, which are metabolic by-products of contraction and stimulants of muscle afferents during contraction, were reduced by HC-030031. These observations suggest that the TRPA1 located on muscle afferents is part of the muscle reflex and further support the notion that arachidonic acid metabolites, bradykinin, and diprotonated phosphate are candidates for endogenous agonists of TRPA1.

Acid-sensing ion channels contribute to the metaboreceptor component of the exercise pressor reflex

The exercise pressor reflex is evoked by both mechanical and metabolic stimuli arising in contracting skeletal muscle. Recently, the blockade of acid-sensing ion channels (ASICs) with amiloride and A-316567 attenuated the reflex. Moreover, amiloride had no effect on the mechanoreceptor component of the reflex, prompting us to determine whether ASICs contributed to the metaboreceptor component of the exercise pressor reflex. The metaboreceptor component can be assessed by measuring mean arterial pressure during postcontraction circulatory occlusion when only the metaboreceptors are stimulated. We examined the effects of amiloride (0.5 microg/kg), A-317567 (10 mM, 0.5 ml), and saline (0.5 ml) on the pressor response to and after static contraction while the circulation was occluded in 30 decerebrated cats. Amiloride (n = 11) and A-317567 (n = 7), injected into the arterial supply of the triceps surae muscles, attenuated the pressor responses both to contraction while the circulation was occluded and to postcontraction circulatory occlusion (all, P < 0.05). Saline (n = 11), however, had no effect on the pressor responses to contraction while the circulation was occluded or to postcontraction circulatory occlusion (both, P > 0.79). Our findings led us to conclude that ASICs contribute to the metaboreceptor component of the exercise pressor reflex.