ATP concentrations and muscle tension increase linearly with muscle contraction

Anchoring Au nanoclusters into coordination polymers: A novel approach toward ATP detection and its application

Adenosine triphosphate (ATP) is the main source of energy required for all life activities and is used as a biomarker for diseases such as cancer. It is of great significance to design a novel fluorescent probe with favorable performance for monitoring the changes of ATP concentration. Herein, a fluorescence probe named ZnCPs@AuNCs for ATP sensing was designed and fabricated by integrating AuNCs into ZnCPs. The emission intensity of AuNCs was greatly enhanced upon the formation of the ZnCPs@AuNCs nanocomposites, which may be attributed to ZnCPs restricting the molecular motion of AuNCs. Upon the introduction of ATP, the fluorescence intensity at 564 nm of ZnCPs@AuNCs is quenched. According to this phenomenon, a sensitive and reliable ATP sensing platform was established. Moreover, ZnCPs@AuNCs were incorporated into a poly (vinyl alcohol) matrix for the fabrication of fluorescent film, which exhibited solid-state fluorescence. Inspired by the remarkable fluorescent properties of ZnCPs@AuNCs, the fluorescent hydrogel was prepared by mixing ZnCPs@AuNCs with κ-carrageenan, which demonstrated a response to ATP and favorable self-healing ability. This work presents a perspective of ZnCPs@AuNCs in multiple applications such as biosensing, fluorescent film, and hydrogel construction.

Disruption of P2Y2 signaling promotes breast tumor cell dissemination by reducing ATP-dependent calcium elevation and actin localization to cell junctions

The tumor microenvironment and wound healing after injury both contain extremely high concentrations of the extracellular signaling molecule, adenosine triphosphate (ATP) compared to normal tissue. P2Y2 receptor, an ATP-activated purinergic receptor, is typically associated with pulmonary, endothelial, and neurological cell signaling. Here we report its role and importance in breast epithelial cell signaling and how it is altered in metastatic breast cancer. In response to ATP activation, P2Y2 receptor signaling causes an increase of intracellular Ca 2+ in non-tumorigenic breast epithelial cells, while their tumorigenic and metastatic counterparts have significantly reduced Ca 2+ responses. The non-tumorigenic cells respond to increased Ca 2+ with actin polymerization and localization to cell edges, while the metastatic cells remained unaffected. The increase in intracellular Ca 2+ after ATP stimulation was blunted using a P2Y2 antagonist, which also prevented actin mobilization and caused cell dissemination from spheroids in non-tumorigenic breast epithelial cells. Furthermore, the lack of Ca 2+ concentration changes and actin mobilization in the metastatic breast cancer cells could be due to reduced P2Y2 expression, which correlates with poorer overall survival in breast cancer patients. This study elucidates rapid changes that occur after elevated intracellular Ca 2+ in breast epithelial cells and how metastatic cancer cells have adapted to evade this cellular response.

Research Progress on the Experimental Model and Underlying Mechanistic Studies of Tension-Type Headaches

PURPOSE OF REVIEW

Tension-type headaches (TTH) significantly diminish patients' quality of life and increase absenteeism, thereby imposing a substantial economic burden. Animal models are essential tools for studying disease mechanisms and drug development. However, until now, little focus has been placed on summarizing the animal models of TTH and associated mechanistic studies. This narrative review discusses the current animal models of TTH and related mechanistic studies to provide insights into the pathophysiological mechanisms of and treatments for TTH.

RECENT FINDINGS

The primary method for constructing an animal model of TTH involves injecting a solution of pain relievers, such as adenosine triphosphate, nerve growth factor, or a high concentration of salt solution, into the neck to initiate harmful cervical muscle responses. This model enables the examination of the interaction between peripheral muscles and central sensitization, which is crucial for understanding the pathophysiology of TTH. Mechanistic studies based on this model have investigated the effect of the P2X receptor antagonist, P2X7 receptor blockade, the P2Y1 receptor agonist 2-MESADP, P2Y1 receptor antagonist MRS2179, nitric oxide synthase inhibitors, and acetylsalicylic acid. Despite notable advancements, the current model of TTH has limitations, including surgical complexity and the inability to replicate chronic tension-type headache (CTTH). To gain a more comprehensive understanding and develop more effective treatment methods, future studies should focus on simplifying surgical procedures, examining other predisposing factors, and establishing a model for chronic TTH. This will offer a deeper insight into the pathophysiological mechanism of TTH and pave the way for improved treatment approaches.

Lactate and hydrogen ions play a predominant role in evoking the exercise pressor reflex during ischaemic contractions but not during freely perfused contractions

We investigated the role played by lactate and hydrogen in evoking the exercise pressor reflex (EPR) in decerebrated rats whose hindlimb muscles were either freely perfused or ischaemic. Production of lactate and hydrogen by the contracting hindlimb muscles was manipulated by knocking out the myophosphorylase gene (pygm). In knockout rats (pygm-/-; n = 13) or wild-type rats (pygm+/+; n = 13), the EPR was evoked by isometrically contracting the triceps surae muscles. Blood pressure, tension, blood flow, renal sympathetic nerve activity and blood lactate concentrations were measured. Intramuscular metabolites and pH changes induced by the contractions were quantified by 31P-magnetic resonance spectroscopy (n = 5). In a subset of pygm-/- rats (n = 5), contractions were evoked with prior infusion of lactate (pH 6.0) in an attempt to restore the effect of lactate and hydrogen ions. Contraction of freely perfused muscles increased blood lactate and decreased muscle pH in pygm+/+ rats only. Despite these differences, the reflex pressor and sympathetic responses to freely perfused contraction did not differ between groups (P = 0.992). During ischaemia, contraction increased muscle lactate and hydrogen ion production in pygm+/+ rats (P < 0.0134), whereas it had no effect in pygm-/- rats (P > 0.783). Likewise, ischaemia exaggerated the reflex pressor, and sympathetic responses to contraction in pygm+/+ but not in pygm-/- rats. This exaggeration was restored when a solution of lactate (pH 6.0) was infused prior to the contraction in pygm-/- rats. We conclude that lactate and hydrogen accumulation in contracting myocytes play a key role in evoking the metabolic component of the EPR during ischaemic but not during freely perfused contractions. KEY POINTS: Conflicting results exist about the role played by lactate and hydrogen ions in evoking the exercise pressor reflex. Using CRISP-Cas9, we rendered the myophosphorylase gene non-functional to block the production of lactate and hydrogen ions. The exercise pressor reflex was evoked in decerebrated rats by statically contracting the triceps surae muscles with or without muscle ischaemia. Static contraction elevated the concentration of lactate and hydrogen ions in pygm+/+ but not in pygm-/- rats. Despite these differences, the exercise pressor reflex was not different between groups. Acute muscle ischaemia exaggerated the concentration of lactate and hydrogen ions in pygm+/+ but not in pygm-/- rats. Likewise, acute muscle ischaemia exaggerated the exercise pressor reflex in pygm+/+ but not in pygm-/- rats. We conclude that lactate and hydrogen play a key role in evoking the exercise pressor reflex during ischaemic but not during freely perfused contractions.

Amplified P2X3 pathway activity in muscle afferent dorsal root ganglion neurons and exercise pressor reflex regulation in hindlimb ischaemia-reperfusion

Hindlimb ischaemia-reperfusion (IR) is among the most prominent pathophysiological conditions observed in peripheral artery disease (PAD). An exaggerated arterial blood pressure (BP) response during exercise is associated with an elevated risk of cardiovascular events in individuals with PAD. However, the precise mechanisms leading to this exaggerated BP response are poorly elucidated. The P2X3 signalling pathway, which plays a key role in modifying the exercise pressor reflex (EPR), is the focus of the present study. We determined the regulatory role of P2X3 on the EPR in a rat model of hindlimb IR. In vivo and in vitro approaches were used to determine the expression and functions of P2X3 in muscle afferent nerves and EPR in IR rats. We found that in IR rats there was (1) upregulation of P2X3 protein expression in the L4-6 dorsal root ganglia (DRG); (2) amplified P2X currents in isolated isolectin B4 (IB4)-positive muscle DRG neurons; and (3) amplification of the P2X-mediated BP response. We further verified that both A-317491 and siRNA knockdown of P2X3 significantly decreased the activity of P2X currents in isolated muscle DRG neurons. Moreover, inhibition of muscle afferents' P2X3 receptor using A-317491 was observed to alleviate the exaggerated BP response induced by static muscle contraction and P2X-induced BP response by α,β-methylene ATP injection. P2X3 signalling pathway activity is amplified in muscle afferent DRG neurons in regulating the EPR following hindlimb IR.

Neurochemical mechanism of muscular pain: Insight from the study on delayed onset muscle soreness

We reviewed fundamental studies on muscular pain, encompassing the characteristics of primary afferent fibers and neurons, spinal and thalamic projections, several muscular pain models, and possible neurochemical mechanisms of muscle pain. Most parts of this review were based on data obtained from animal experiments, and some researches on humans were also introduced. We focused on delayed-onset muscle soreness (DOMS) induced by lengthening contractions (LC), suitable for studying myofascial pain syndromes. The muscular mechanical withdrawal threshold (MMWT) decreased 1-3 days after LC in rats. Changing the speed and range of stretching showed that muscle injury seldom occurred, except in extreme conditions, and that DOMS occurred in parameters without muscle damage. The B2 bradykinin receptor-nerve growth factor (NGF) route and COX-2-glial cell line-derived neurotrophic factor (GDNF) route were involved in the development of DOMS. The interactions between these routes occurred at two levels. A repeated-bout effect was observed in MMWT and NGF upregulation, and this study showed that adaptation possibly occurred before B2 bradykinin receptor activation. We have also briefly discussed the prevention and treatment of DOMS.

ASIC3 plays a protective role in delayed-onset muscle soreness (DOMS) through muscle acid sensation during exercise

Immediate exercise-induced pain (IEIP) and DOMS are two types of exercise-induced muscle pain and can act as barriers to exercise. The burning sensation of IEIP occurs during and immediately after intensive exercise, whereas the soreness of DOMS occurs later. Acid-sensing ion channels (ASICs) within muscle afferents are activated by H+ and other chemicals and have been shown to play a role in various chronic muscle pain conditions. Here, we further defined the role of ASICs in IEIP, and also tested if ASIC3 is required for DOMS. After undergoing exhaustive treadmill exercise, exercise-induced muscle pain was assessed in wild-type (WT) and ASIC3-/- mice at baseline via muscle withdrawal threshold (MWT), immediately, and 24 h after exercise. Locomotor movement, grip strength, and repeat exercise performance were tested at baseline and 24 h after exercise to evaluate DOMS. We found that ASIC3-/- had similar baseline muscle pain, locomotor activity, grip strength, and exercise performance as WT mice. WT showed diminished MWT immediately after exercise indicating they developed IEIP, but ASIC3-/- mice did not. At 24 h after baseline exercise, both ASIC3-/- and WT had similarly lower MWT and grip strength, however, ASIC3-/- displayed significantly lower locomotor activity and repeat exercise performance at 24 h time points compared to WT. In addition, ASIC3-/- mice had higher muscle injury as measured by serum lactate dehydrogenase and creatine kinase levels at 24 h after exercise. These results show that ASIC3 is required for IEIP, but not DOMS, and in fact might play a protective role to prevent muscle injury associated with strenuous exercise.

Adipose-derived mesenchymal stem cells preserve cardiac function via ANT-1 in dilated cardiomyopathy hamster model

Introduction

Idiopathic dilated cardiomyopathy (DCM) is associated with abnormalities in cytoskeletal proteins, mitochondrial ATP transporter, microvasculature, and fibrosis. Mesenchymal stem cells (MSCs) can ameliorate distressed mitochondrial and structural proteins, as well as fibrosis, via the paracrine effect of cytokines. This study aimed to investigate whether the transplantation of adipose tissue-derived MSCs (ADSCs) reverses histological and functional abnormalities in the distressed myocardium of DCM-like hamsters by modulating the expression of adenine nucleotide translocase 1 (ANT-1).

Methods

Eighteen weeks after birth, ADSCs were implanted onto the cardiac surface of δ-sarcoglycan (SG)-deficient hamsters or sham surgery was performed.

Results

Left ventricular ejection fraction and end-systolic diameter were maintained in ADSC-treated animals for four weeks, ATP concentration was considerably elevated in the cardiomyocytes of these animals, and ANT-1 expression was significantly upregulated as well. The expression of extracellular matrix and myocardial cytoskeletal proteins, such as collagen, SG, and α-dystroglycan, did not differ between groups. However, significant improvements in myosin and Smad4 expression, cardiomyocyte hypertrophy, and capillary density occurred in the ADSC-treated group.

Conclusions

We demonstrated that ADSCs might maintain cardiac function in the DCM hamster model by enhancing ATP concentration, as well as mitochondrial transporter and myosin expression, indicating their potential for DCM treatment.

Myostatin Inhibition-Induced Increase in Muscle Mass and Strength Was Amplified by Resistance Exercise Training, and Dietary Essential Amino Acids Improved Muscle Quality in Mice

It has been frequently reported that myostatin inhibition increases muscle mass, but decreases muscle quality (i.e., strength/muscle mass). Resistance exercise training (RT) and essential amino acids (EAAs) are potent anabolic stimuli that synergistically increase muscle mass through changes in muscle protein turnover. In addition, EAAs are known to stimulate mitochondrial biogenesis. We have investigated if RT amplifies the anabolic potential of myostatin inhibition while EAAs enhance muscle quality through stimulations of mitochondrial biogenesis and/or muscle protein turnover. Mice were assigned into ACV (myostatin inhibitor), ACV+EAA, ACV+RT, ACV+EAA +RT, or control (CON) over 4 weeks. RT, but not EAA, increased muscle mass above ACV. Despite differences in muscle mass gain, myofibrillar protein synthesis was stimulated similarly in all vs. CON, suggesting a role for changes in protein breakdown in muscle mass gains. There were increases in MyoD expression but decreases in Atrogin-1/MAFbx expression in ACV+EAA, ACV+RT, and ACV+EAA+RT vs. CON. EAA increased muscle quality (e.g., grip strength and maximal carrying load) without corresponding changes in markers of mitochondrial biogenesis and neuromuscular junction stability. In conclusion, RT amplifies muscle mass and strength through changes in muscle protein turnover in conjunction with changes in implicated signaling, while EAAs enhance muscle quality through unknown mechanisms.

Crosstalk Between Skeletal Muscle and Immune System: Which Roles Do IL-6 and Glutamine Play?

The skeletal muscle was always seen from biomechanical and biochemical views. It is well-established that an active muscle brings many benefits for different body organs and tissues, including the immune system. Since the 1970s, many studies have shown the importance of regular exercise and physical activity in increasing the body's ability to fight opportunist infections, as well as a strategy to fight established diseases. This interaction was mainly attributed to the glutamine, a non-essential amino acid produced by the active skeletal muscle and primarily consumed by rapidly dividing cells, including lymphocytes and monocytes/macrophages, as their main source of energy. Therefore, these cells' function would be significantly improved by the presence of a bigger glutamine pool, facilitating phagocytosis, antigen-presentation, proliferative capacity, cytokine synthesis and release, among other functions. Despite its importance, glutamine is not the only molecule to connect these two tissues. The presence of cytokines is crucial for a proper immune system function. Many of them have well-established pro-inflammatory properties, while others are known for their anti-inflammatory role. Interleukin-6 (IL-6), however, has been in the center of many scientific discussions since it can act as pro- and anti-inflammatory cytokine depending on the tissue that releases it. Skeletal muscle is an essential source of IL-6 with anti-inflammatory properties, regulating the function of the immune cells after tissue injury and the healing process. Therefore, this review aims to discuss further the role of these four components (glutamine, and interleukin-6, and its interface with monocytes/macrophages, and lymphocytes) on the communication between the skeletal muscle and the immune system.

ATP-Responsive and ATP-Fueled Self-Assembling Systems and Materials

Adenosine triphosphate (ATP) is a central metabolite that plays an indispensable role in various cellular processes, from energy supply to cell-to-cell signaling. Nature has developed sophisticated strategies to use the energy stored in ATP for many metabolic and non-equilibrium processes, and to sense and bind ATP for biological signaling. The variations in the ATP concentrations from one organelle to another, from extracellular to intracellular environments, and from normal cells to cancer cells are one motivation for designing ATP-triggered and ATP-fueled systems and materials, because they show great potential for applications in biological systems by using ATP as a trigger or chemical fuel. Over the last decade, ATP has been emerging as an attractive co-assembling component for man-made stimuli-responsive as well as for fuel-driven active systems and materials. Herein, current advances and emerging concepts for ATP-triggered and ATP-fueled self-assemblies and materials are discussed, shedding light on applications and highlighting future developments. By bringing together concepts of different domains, that is from supramolecular chemistry to DNA nanoscience, from equilibrium to non-equilibrium self-assembly, and from fundamental sciences to applications, the aim is to cross-fertilize current approaches with the ultimate aim to bring synthetic ATP-dependent systems closer to living systems.

Bradykinin stimulates prostaglandin E2 release in human skeletal muscular fibroblasts

Local mediator prostaglandins and bradykinin are involved in inflammation and pain. We explored bradykinin effects on prostaglandin E2 (PGE2) release from fibroblasts derived from human skeletal muscular biopsies. Bradykinin induced PGE₂ release through bradykinin B2 receptors, since PGE₂ release was blocked by the bradykinin B2 receptor selective antagonist FR173657 and B2 receptor agonist (Hyp³)-bradykinin showed effects comparable to bradykinin. Consistently, bradykinin induced both mRNA cyclooxygenase 2 (COX-2) and protein. Bradykinin also induced ERK1/2 and p38 phosphorylation and provoked the translocation from the cytosol to the nucleus of p65/NF-kB. The release of PGE₂ by bradykinin could be blocked inhibiting COX-2 and p65/NF-kB, ERK1/2 or p38 activation. Both ERK1/2 and p38 were upstream to NF-kB inasmuch siRNAs significantly blocked the p65/NF-kB activation induced by bradykinin. Thus, bradykinin, acting via B2 receptors, induced PGE₂ release through ERK1/2 and p38-dependent pathways and consequent p65/NF-kB translocation to nucleus. p65/NF-kB induced COX-2 transcription. The release of PGE₂ provide a possible explanation for the role of bradykinin in inflammatory diseases.

P2X4 Receptors on Muscle Macrophages Are Required for Development of Hyperalgesia in an Animal Model of Activity-Induced Muscle Pain

Activity-induced pain is common in those with chronic musculoskeletal pain and limits participation in daily activities and exercise. Our laboratory developed a model of activity-induced pain and shows that depletion of muscle macrophages prevents development of hyperalgesia. Adenosine triphosphate (ATP) is released from fatiguing muscle and activates purinergic receptors (P2X), and P2X4 receptors are expressed on macrophages. We hypothesized that exercise releases ATP to activate P2X4 receptors on muscle macrophages, which subsequently release interleukin-1β (IL-1β) to produce hyperalgesia. In an animal model of activity-induced pain, using male and female C57BL6/J mice, we show increased expression of P2X4 on muscle macrophages, and blockade of P2X4 receptors in muscle prevented development of hyperalgesia. Using a lentivirus expressing an artificial micro-RNA to P2X4 under the control of a CD68 promoter, we decreased expression of P2X4 mRNA in cultured macrophages, decreased expression of P2X4 protein in muscle macrophages in vivo, and prevented development of activity-induced hyperalgesia. We further show that macrophages primed with LPS differentially released IL-1β when treated with ATP in neutral or acidic pH. Lastly, blockade of IL-1β in muscle prevented development of hyperalgesia in this model. Thus, our data suggest that P2X4 receptors could be a valid pharmacological target to control activity-induced muscle pain experienced by patients with chronic musculoskeletal pain.

Acid-sensing ion channels blockade attenuates pressor and sympathetic responses to skeletal muscle metaboreflex activation in humans

In animals, the blockade of acid-sensing ion channels (ASICs), cation pore-forming membrane proteins located in the free nerve endings of group IV afferent fibers, attenuates increases in arterial pressure (AP) and sympathetic nerve activity (SNA) during muscle contraction. Therefore, ASICs play a role in mediating the metabolic component (skeletal muscle metaboreflex) of the exercise pressor reflex in animal models. Here we tested the hypothesis that ASICs also play a role in evoking the skeletal muscle metaboreflex in humans, quantifying beat-by-beat mean AP (MAP; finger photoplethysmography) and muscle SNA (MSNA; microneurography) in 11 men at rest and during static handgrip exercise (SHG; 35% of the maximal voluntary contraction) and postexercise muscle ischemia (PEMI) before (B) and after (A) local venous infusion of either saline or amiloride (AM), an ASIC antagonist, via the Bier block technique. MAP (BAM +30 ± 6 vs. AAM +25 ± 7 mmHg, P = 0.001) and MSNA (BAM +14 ± 9 vs. AAM +10 ± 6 bursts/min, P = 0.004) responses to SHG were attenuated under ASIC blockade. Amiloride also attenuated the PEMI-induced increases in MAP (BAM +25 ± 6 vs. AAM +16 ± 6 mmHg, P = 0.0001) and MSNA (BAM +16 ± 9 vs. AAM +8 ± 8 bursts/min, P = 0.0001). MAP and MSNA responses to SHG and PEMI were similar before and after saline infusion. We conclude that ASICs play a role in evoking pressor and sympathetic responses to SHG and the isolated activation of the skeletal muscle metaboreflex in humans. NEW & NOTEWORTHY We showed that regional blockade of the acid-sensing ion channels (ASICs), induced by venous infusion of the antagonist amiloride via the Bier block anesthetic technique, attenuated increases in arterial pressure and muscle sympathetic nerve activity during both static handgrip exercise and postexercise muscle ischemia. These findings indicate that ASICs contribute to both pressor and sympathetic responses to the activation of the skeletal muscle metaboreflex in humans.

Muscle-on-a-chip with an on-site multiplexed biosensing system for in situ monitoring of secreted IL-6 and TNF-α

Despite the increasing number of organs-on-a-chip that have been developed in the past decade, limited efforts have been made to integrate a sensing system for in situ continual measurements of biomarkers from three-dimensional (3D) tissues. Here, we present a custom-made integrated platform for muscle cell stimulation under fluidic conditions connected with a multiplexed high-sensitivity electrochemical sensing system for in situ monitoring. To demonstrate this, we use our system to measure the release levels and release time of interleukin 6 and tumor necrosis factor alpha in vitro by 3D muscle microtissue under electrical and biological stimulations. Our experimental design has enabled us to perform multiple time point measurements using functionalized screen-printed gold electrodes with sensitivity in the ng mL^-1 range. This affordable setup is uniquely suited for monitoring factors released by 3D single cell types upon external stimulation for metabolic studies.

Thin-fibre receptors expressing acid-sensing ion channel 3 contribute to muscular mechanical hypersensitivity after exercise

BACKGROUND

Delayed onset muscle soreness (DOMS) is characterized by mechanical hyperalgesia after lengthening contractions (LC). It is relatively common and causes disturbance for many people who require continuous exercise, yet its molecular and peripheral neural mechanisms are poorly understood.

METHODS

We examined whether muscular myelinated Aδ-fibres, in addition to unmyelinated C-fibres, are involved in LC-induced mechanical hypersensitivity, and whether acid-sensing ion channel (ASIC)-3 expressed in thin-fibre afferents contributes to this type of pain using a rat model of DOMS. The peripheral contribution of ASIC3 was investigated using single-fibre electrophysiological recordings in extensor digitorum longus muscle-peroneal nerve preparations in vitro.

RESULTS

Behavioural tests demonstrated a significant decrease of the muscular mechanical withdrawal threshold following LC to ankle extensor muscles, and it was improved by intramuscular injection of APETx2 (2.2 μM), a selective blocker of ASIC3. The lower concentration of APETx2 (0.22 µM) and its vehicle had no effect on the threshold. Intramuscular injection of APETx2 (2.2 μM) in naïve rats without LC did not affect the withdrawal threshold. In the ankle extensor muscles that underwent LC one day before the electrophysiological recordings, the mechanical response of Aδ- and C-fibres was significantly facilitated (i.e. decreased response threshold and increased magnitude of the response). The facilitated mechanical response of the Aδ- and C-fibres was significantly suppressed by selective blockade of ASIC3 with APETx2, but not by its vehicle.

CONCLUSIONS

These results clearly indicate that ASIC3 contributes to the augmented mechanical response of muscle thin-fibre receptors in delayed onset muscular mechanical hypersensitivity after LC.

SIGNIFICANCE

Here, we show that not only C- but also Aδ-fibre nociceptors in the muscle are involved in mechanical hypersensitivity after lengthening contractions, and that acid-sensing ion channel (ASIC)-3 expressed in the thin-fibre nociceptors is responsible for the mechanical hypersensitivity. ASIC3 might be a novel pharmacological target for pain after exercise.

A "noisy" electrical stimulation protocol favors muscle regeneration in vitro through release of endogenous ATP

An in vitro system of electrical stimulation was used to explore whether an innovative "noisy" stimulation protocol derived from human electromyographic recordings (EMGstim) could promote muscle regeneration. EMGstim was delivered to cultured mouse myofibers isolated from Flexor Digitorum Brevis, preserving their satellite cells. In response to EMGstim, immunostaining for the myogenic regulatory factor myogenin, revealed an increased percentage of elongated myogenin-positive cells surrounding the myofibers. Conditioned medium collected from EMGstim-treated cell cultures, promoted satellite cells differentiation in unstimulated myofiber cell cultures, suggesting that extracellular soluble factors could mediate the process. Interestingly, the myogenic effect of EMGstim was mimicked by exogenously applied ATP (0.1 μM), reduced by the ATP diphosphohydrolase apyrase and prevented by blocking endogenous ATP release with carbenoxolone. In conclusion, our results show that "noisy" electrical stimulations favor muscle progenitor cell differentiation most likely via the release of endogenous ATP from contracting myofibres. Our data also suggest that "noisy" stimulation protocols could be potentially more efficient than regular stimulations to promote in vivo muscle regeneration after traumatic injury or in neuropathological diseases.

ATP-Induced Increase in Intracellular Calcium Levels and Subsequent Activation of mTOR as Regulators of Skeletal Muscle Hypertrophy

Intracellular signaling pathways, including the mammalian target of rapamycin (mTOR) and the mitogen-activated protein kinase (MAPK) pathway, are activated by exercise, and promote skeletal muscle hypertrophy. However, the mechanisms by which these pathways are activated by physiological stimulation are not fully understood. Here we show that extracellular ATP activates these pathways by increasing intracellular Ca²⁺ levels ([Ca²⁺]_i), and promotes muscle hypertrophy. [Ca²⁺]_i in skeletal muscle was transiently increased after exercise. Treatment with ATP induced the increase in [Ca²⁺]_i through the P2Y₂ receptor/inositol 1,4,5-trisphosphate receptor pathway, and subsequent activation of mTOR in vitro. In addition, the ATP-induced increase in [Ca²⁺]_i coordinately activated Erk1/2, p38 MAPK and mTOR that upregulated translation of JunB and interleukin-6. ATP also induced an increase in [Ca²⁺]_i in isolated soleus muscle fibers, but not in extensor digitorum longus muscle fibers. Furthermore, administration of ATP led to muscle hypertrophy in an mTOR- and Ca²⁺-dependent manner in soleus, but not in plantaris muscle, suggesting that ATP specifically regulated [Ca²⁺]_i in slow muscles. These findings suggest that ATP and [Ca²⁺]_i are important mediators that convert mechanical stimulation into the activation of intracellular signaling pathways, and point to the P2Y receptor as a therapeutic target for treating muscle atrophy.

Adenosine Promotes Endplate nAChR Channel Activity in Adult Mouse Skeletal Muscle Fibers via Low Affinity P1 Receptors

Adenosine is a powerful modulator of skeletal neuromuscular transmission, operating via inhibitory or facilitatory purinergic-type P1 receptors. To date, studies have been focused mainly on the effect of adenosine on presynaptic P1 receptors controlling transmitter release. In this study, using two-microelectrode voltage-clamp and single-channel patch-clamp recording techniques, we have explored potential postsynaptic targets of adenosine and their modulatory effect on nicotinic acetylcholine receptor (nAChR)-mediated synaptic responses in adult mouse skeletal muscle fibers in vitro. In the whole-mount neuromuscular junction (NMJ) preparation, adenosine (100 μM) significantly reduced the frequency of the miniature endplate currents (MEPCs) and slowed their rising and decay time. Consistent with a postsynaptic site of action, adenosine and the potent P1 receptor agonist NECA significantly increased the open probability, the frequency and the open time of single nAChR channels, recorded at the endplate region. Using specific ligands for the P1 receptor subtypes, we found that the low-affinity P1 receptor subtype A_2B was responsible for mediating the effects of adenosine on the nAChR channel openings. Our data suggest that at the adult mammalian NMJ, adenosine acts not only presynaptically to modulate acetylcholine transmitter release, but also at the postsynaptic level, to enhance the activity of nAChRs. Our findings open a new scenario in understanding of purinergic regulation of nAChR activity at the mammalian endplate region.

The effects of adenine nucleotide perfusion on interstitial adenosine production in rat skeletal muscle

The purpose of the present study was to utilize the microdialysis technique in rat skeletal muscle to perfuse varying concentrations of AMP, ADP, and ATP into the interstitium to examine the effects that these adenine nucleotides have on the production of adenosine in the interstitial space. Interstitial adenosine production appears to be related to the type (ATP, ADP, or AMP) and concentration (2-60 μmol/L) of the adenine nucleotide perfused. Interstitial adenosine levels increased (P < 0.05) from baseline (0.18 ± 0.02 and 0.22 ± 0.02 μmol/L) to 0.23 ± 0.02 and 0.41 ± 0.05 μmol/L following 5 and 30 μmol/L AMP perfusion, respectively. Similarly, perfusion with 30 μmol/L ADP and 30, 40, and 60 μmol/L ATP resulted in an increase (P < 0.05) in interstitial adenosine concentration from baseline (0.25 ± 0.02, 0.26 ± 0.02, 0.19 ± 0.03, and 0.14 ± 0.02 μmol/L) to 0.30 ± 0.02, 0.32 ± 0.02, 0.36 ± 0.04, and 0.33 ± 0.04 μmol/L, respectively. Interestingly, the most prominent increase in interstitial adenosine production occurred during the perfusion of 60 μmol/L ATP (126% increase from baseline). These data strongly suggest that interstitial ATP may play a more potent role in stimulating interstitial adenosine production as compared with ADP or AMP. In addition, interstitial adenosine production can occur independent of muscle contraction (voluntary or involuntary) or hypoxia when adequate concentrations of adenine nucleotides are available.

Sex differences in primary muscle afferent sensitization following ischemia and reperfusion injury

BACKGROUND

Chronic pain conditions are more prevalent in women, but most preclinical studies into mechanisms of pain generation are performed using male animals. Furthermore, whereas group III and IV nociceptive muscle afferents provoke central sensitization more effectively than their cutaneous counterparts, less is known about this critical population of muscle nociceptors. Here, we compare the physiology of individual muscle afferents in uninjured males and females. We then characterize the molecular, physiological, and behavioral effects of transient ischemia and reperfusion injury (I/R), a model we have extensively studied in males and in females.

METHODS

Response properties and phenotypes to mechanical, thermal, and chemical stimulation were compared using an ex vivo muscle/nerve/dorsal root ganglia (DRG)/spinal cord recording preparation. Analyses of injury-related changes were also performed by assaying evoked and spontaneous pain-related behaviors, as well as mRNA expression of the affected muscle and DRGs. The appropriate analyses of variance and post hoc tests (with false discovery rate corrections when needed) were performed for each measure.

RESULTS

Females have more mechanically sensitive muscle afferents and show greater mechanical and thermal responsiveness than what is found in males. With I/R, both sexes show fewer cells responsive to an innocuous metabolite solution (ATP, lactic acid, and protons), and lower mechanical thresholds in individual afferents; however, females also possess altered thermal responsiveness, which may be related to sex-dependent changes in gene expression within the affected DRGs. Regardless, both sexes show similar increases in I/R-induced pain-like behaviors.

CONCLUSIONS

Here, we illustrate a unique phenomenon wherein discrete, sex-dependent mechanisms of primary muscle afferent sensitization after ischemic injury to the periphery may underlie similar behavioral changes between the sexes. Furthermore, although the group III and IV muscle afferents are fully developed functionally, the differential mechanisms of sensitization manifest prior to sexual maturity. Hence, this study illustrates the pressing need for further exploration of sex differences in afferent function throughout the lifespan for use in developing appropriately targeted pain therapies.

The Role Played by Adenosine in Modulating Reflex Sympathetic and Pressor Responses Evoked by Stimulation of TRPV1 in Muscle Afferents

BACKGROUND/AIMS

Activation of metabolite-sensitive transient receptor potential vanilloid type 1 (TRPV1) receptors (capsaicin receptors) in afferent nerves of the hindlimb muscles of rats increases renal sympathetic nerve activity (RSNA) and blood pressure (BP) via a reflex mechanism. The purpose of this study was to examine the role of adenosine in modulating the reflex RSNA and BP responses to stimulation of TRPV1.

METHODS

RSNA and BP responses were recorded in rats. Immunofluorescence and patch-clamp methods were employed to examine the receptor mechanisms responsible for the effects of adenosine.

RESULTS

Adenosine, in the concentration of 100 µM, injected into the femoral artery had an inhibitory effect on the reflex RSNA and BP responses induced by capsaicin. Likewise, arterial injection of adenosine analogue CGS21680 (A2A subtype receptor agonist, 10 µM and100 µM) also attenuated the reflex responses. In addition, co-existence of A2A and TRPV1 was observed in the dorsal root ganglion neurons. The prior application of adenosine or CGS21680 inhibited the magnitude of capsaicin-induced currents in muscle sensory neurons.

CONCLUSION

Adenosine contributes to muscle afferent TRPV1-engaged reflex sympathetic and pressor responses. It is likely that TRPV1 response is impaired as the levels of adenosine are increased in the hindlimb muscles under diseased conditions.

The 'sensory tolerance limit': A hypothetical construct determining exercise performance?

Neuromuscular fatigue compromises exercise performance and is determined by central and peripheral mechanisms. Interactions between the two components of fatigue can occur via neural pathways, including feedback and feedforward processes. This brief review discusses the influence of feedback and feedforward mechanisms on exercise limitation. In terms of feedback mechanisms, particular attention is given to group III/IV sensory neurons which link limb muscle with the central nervous system. Central corollary discharge, a copy of the neural drive from the brain to the working muscles, provides a signal from the motor system to sensory systems and is considered a feedforward mechanism that might influence fatigue and consequently exercise performance. We highlight findings from studies supporting the existence of a 'critical threshold of peripheral fatigue', a previously proposed hypothesis based on the idea that a negative feedback loop operates to protect the exercising limb muscle from severe threats to homeostasis during whole-body exercise. While the threshold theory remains to be disproven within a given task, it is not generalisable across different exercise modalities. The 'sensory tolerance limit', a more theoretical concept, may address this issue and explain exercise tolerance in more global terms and across exercise modalities. The 'sensory tolerance limit' can be viewed as a negative feedback loop which accounts for the sum of all feedback (locomotor muscles, respiratory muscles, organs, and muscles not directly involved in exercise) and feedforward signals processed within the central nervous system with the purpose of regulating the intensity of exercise to ensure that voluntary activity remains tolerable.

Bradykinin Contributes to Sympathetic and Pressor Responses Evoked by Activation of Skeletal Muscle Afferents P2X in Heart Failure

BACKGROUND/AIMS

Published data suggest that purinergic P2X receptors of muscle afferent nerves contribute to the enhanced sympathetic nervous activity (SNA) and blood pressure (BP) responses during static exercise in heart failure (HF). In this study, we examined engagement of bradykinin (BK) in regulating responses of SNA and BP evoked by P2X stimulation in rats with HF. We further examined cellular mechanisms responsible for BK. We hypothesized that BK potentiates P2X currents of muscle dorsal root ganglion (DRG) neurons, and this effect is greater in HF due to upregulation of BK kinin B2 and P2X3 receptor. As a result, BK amplifies muscle afferents P2X-mediated SNA and BP responses.

METHODS

Renal SNA and BP responses were recorded in control rats and rats with HF. Western Blot analysis and patch-clamp methods were employed to examine the receptor expression and function of DRG neurons involved in the effects of BK.

RESULTS

BK injected into the arterial blood supply of the hindlimb muscles heightened the reflex SNA and BP responses induced by P2X activation with α,β-methylene ATP to a greater degree in HF rats. In addition, HF upregulated the protein expression of kinin B2 and P2X3 in DRG and the prior application of BK increased the magnitude of α,β-methylene ATP-induced currents in muscle DRG neurons from HF rats.

CONCLUSION

BK plays a facilitating role in modulating muscle afferent P2X-engaged reflex sympathetic and pressor responses. In HF, P2X responsivness is augmented due to increases in expression of kinin B2 and P2X3 receptors and P2X current activity.

Neural Contributions to Muscle Fatigue: From the Brain to the Muscle and Back Again

: During exercise, there is a progressive reduction in the ability to produce muscle force. Processes within the nervous system as well as within the muscles contribute to this fatigue. In addition to impaired function of the motor system, sensations associated with fatigue and impairment of homeostasis can contribute to the impairment of performance during exercise. This review discusses some of the neural changes that accompany exercise and the development of fatigue. The role of brain monoaminergic neurotransmitter systems in whole-body endurance performance is discussed, particularly with regard to exercise in hot environments. Next, fatigue-related alterations in the neuromuscular pathway are discussed in terms of changes in motor unit firing, motoneuron excitability, and motor cortical excitability. These changes have mostly been investigated during single-limb isometric contractions. Finally, the small-diameter muscle afferents that increase firing with exercise and fatigue are discussed. These afferents have roles in cardiovascular and respiratory responses to exercise, and in the impairment of exercise performance through interaction with the motor pathway, as well as in providing sensations of muscle discomfort. Thus, changes at all levels of the nervous system, including the brain, spinal cord, motor output, sensory input, and autonomic function, occur during exercise and fatigue. The mix of influences and the importance of their contribution vary with the type of exercise being performed.

Purinergic P2X Receptors and Heightened Exercise Pressor Reflex in Peripheral Artery Disease

Arterial blood pressure (BP) and vasoconstriction regulated by sympathetic nerve activity (SNA) are heightened during exercise in patients with peripheral artery disease (PAD). The exercise pressor reflex is considered as a neural mechanism responsible for the exaggerated autonomic responses to exercise in PAD. A series of studies have employed a rat model of PAD to examine signal pathways at receptor and cellular levels by which the exercise pressor reflex is amplified. This review will summarize results obtained from recent human and animal studies with respect to contribution of muscle afferents to augmented SNA and BP responses in PAD. The role played by adenosine triphosphate (ATP) and ATP sensitive purinergic P2X receptors will be emphasized.

ATP-sensitive muscle afferents activate spinal trigeminal neurons with meningeal afferent input in rat - pathophysiological implications for tension-type headache

BACKGROUND

Tension-type headache and other primary headaches may be triggered or aggravated by disorders of pericranial muscles, which is possibly due to convergent or collateral afferent input from meningeal and muscular receptive areas. In rodent models high extracellular concentrations of ATP caused muscle nociception and central sensitization of second order neurons. In a rat model of meningeal nociception we asked if spinal trigeminal activity induced by ATP can be modulated by local anaesthesia of distinct muscles.

METHODS

Ongoing activity was recorded from spinal trigeminal neurons with afferent input from the cranial dura mater, the temporal muscle and neck muscles. The stable ATP analogue α,β-methylene adenosine 5'-triphosphate (α,β-meATP, 10 mM) was injected into the ipsilateral temporal muscle, 30 min later followed by injection of local anaesthetics (lidocaine, 2 %) into the ipsilateral neck muscles and/or the temporal muscle.

RESULTS

Injection of α,β-meATP into the temporal muscle caused progressive increase in ongoing activity of most of the spinal trigeminal neurons within 30 min. Injection of lidocaine into the neck muscles and/or the temporal muscle reduced this activation to previous levels within 10 min.

CONCLUSIONS

Distinct spinal trigeminal neurons processing meningeal nociceptive information are under the control of convergent afferent input from several pericranial muscles. Blockade of at least one of these inputs can normalize central trigeminal activity. This may explain why therapeutic manipulations of head muscles can be beneficial in primary headaches.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Regular physical activity prevents chronic pain by altering resident muscle macrophage phenotype and increasing interleukin-10 in mice

Regular physical activity in healthy individuals prevents development of chronic musculoskeletal pain; however, the mechanisms underlying this exercise-induced analgesia are not well understood. Interleukin-10 (IL-10), an antiinflammatory cytokine that can reduce nociceptor sensitization, increases during regular physical activity. Since macrophages play a major role in cytokine production and are present in muscle tissue, we propose that physical activity alters macrophage phenotype to increase IL-10 and prevent chronic pain. Physical activity was induced by allowing C57BL/6J mice free access to running wheels for 8 weeks and compared to sedentary mice with no running wheels. Using immunohistochemical staining of the gastrocnemius muscle to label regulatory (M2, secretes antiinflammatory cytokines) and classical (M1, secretes proinflammatory cytokines) macrophages, the percentage of M2-macrophages increased significantly in physically active mice (68.5% ± 4.6% of total) compared with sedentary mice (45.8% ± 7.1% of total). Repeated acid injections into the muscle enhanced mechanical sensitivity of the muscle and paw in sedentary animals, which does not occur in physically active mice; no sex differences occur in either sedentary or physically active mice. Blockade of IL-10 systemically or locally prevented the analgesia in physically active mice, ie, mice developed hyperalgesia. Conversely, sedentary mice pretreated systemically or locally with IL-10 had reduced hyperalgesia after repeated acid injections. Thus, these results suggest that regular physical activity increases the percentage of regulatory macrophages in muscle and that IL-10 is an essential mediator in the analgesia produced by regular physical activity.

Cardiovascular Adaptations to Exercise Training

Aerobic exercise training leads to cardiovascular changes that markedly increase aerobic power and lead to improved endurance performance. The functionally most important adaptation is the improvement in maximal cardiac output which is the result of an enlargement in cardiac dimension, improved contractility, and an increase in blood volume, allowing for greater filling of the ventricles and a consequent larger stroke volume. In parallel with the greater maximal cardiac output, the perfusion capacity of the muscle is increased, permitting for greater oxygen delivery. To accommodate the higher aerobic demands and perfusion levels, arteries, arterioles, and capillaries adapt in structure and number. The diameters of the larger conduit and resistance arteries are increased minimizing resistance to flow as the cardiac output is distributed in the body and the wall thickness of the conduit and resistance arteries is reduced, a factor contributing to increased arterial compliance. Endurance training may also induce alterations in the vasodilator capacity, although such adaptations are more pronounced in individuals with reduced vascular function. The microvascular net increases in size within the muscle allowing for an improved capacity for oxygen extraction by the muscle through a greater area for diffusion, a shorter diffusion distance, and a longer mean transit time for the erythrocyte to pass through the smallest blood vessels. The present article addresses the effect of endurance training on systemic and peripheral cardiovascular adaptations with a focus on humans, but also covers animal data.

Combined, but not individual, blockade of ASIC3, P2X, and EP4 receptors attenuates the exercise pressor reflex in rats with freely perfused hindlimb muscles

In healthy humans, tests of the hypothesis that lactic acid, PGE2, or ATP plays a role in evoking the exercise pressor reflex proved controversial. The findings in humans resembled ours in decerebrate rats that individual blockade of the receptors to lactic acid, PGE2, and ATP had only small effects on the exercise pressor reflex provided that the muscles were freely perfused. This similarity between humans and rats prompted us to test the hypothesis that in rats with freely perfused muscles combined receptor blockade is required to attenuate the exercise pressor reflex. We first compared the reflex before and after injecting either PPADS (10 mg/kg), a P2X receptor antagonist, APETx2 (100 μg/kg), an activating acid-sensing ion channel 3 (ASIC) channel antagonist, or L161982 (2 μg/kg), an EP4 receptor antagonist, into the arterial supply of the hindlimb of decerebrated rats. We then examined the effects of combined blockade of P2X receptors, ASIC3 channels, and EP4 receptors on the exercise pressor reflex using the same doses, intra-arterial route, and time course of antagonist injections as those used for individual blockade. We found that neither PPADS (n = 5), APETx2 (n = 6), nor L161982 (n = 6) attenuated the reflex. In contrast, combined blockade of these receptors (n = 7) attenuated the peak (↓27%, P < 0.019) and integrated (↓48%, P < 0.004) pressor components of the reflex. Combined blockade injected intravenously had no effect on the reflex. We conclude that combined blockade of P2X receptors, ASIC3 channels, and EP4 receptors on the endings of thin fiber muscle afferents is required to attenuate the exercise pressor reflex in rats with freely perfused hindlimbs.

Skeletal Muscle an Active Compartment in the Sequestering and Metabolism of Doxorubicin Chemotherapy

Doxorubicin remains one of the most widely used chemotherapeutic agents however its effect on healthy tissue, such as skeletal muscle, remains poorly understood. The purpose of the current study was to examine the accumulation of doxorubicin (DOX) and its metabolite doxorubicinol (DOXol) in skeletal muscle of the rat up to 8 days after the administration of a 1.5 or 4.5 mg kg-1 i.p. dose. Subsequent to either dose, DOX and DOXol were observed in skeletal muscle throughout the length of the experiment. Interestingly an efflux of DOX was examined after 96 hours, followed by an apparent re-uptake of the drug which coincided with a spike and rapid decrease of plasma DOX concentrations. The interstitial space within the muscle did not appear to play a significant rate limiting compartment for the uptake or release of DOX or DOXol from the tissue to the circulation. Furthermore, there was no evidence that DOX preferentially accumulated in a specific muscle group with either dose. It appears that the sequestering of drug in skeletal muscle plays an acute and important role in the systemic availability and metabolism of DOX which may have a greater impact on the clinical outcome than previously considered.

Adenosine enhances acetylcholine receptor channel openings and intracellular calcium 'spiking' in mouse skeletal myotubes

AIMS

The autocrine activity of the embryonic isoform of the nicotinic acetylcholine receptor is crucial for the correct differentiation and trophism of skeletal muscle cells before innervation. The functional activity of extracellular adenosine and adenosine receptor subtypes expressed in differentiating myotubes is still unknown. In this study, we performed a detailed analysis of the role of adenosine receptor-mediated effects on the autocrine-mediated nicotinic acetylcholine receptor channel openings and the associated spontaneous intracellular calcium 'spikes' generated in differentiating mouse myotubes in vitro.

METHODS

Cell-attached patch-clamp recordings and intracellular calcium imaging experiments were performed in contracting myotubes derived from mouse satellite cells.

RESULTS

The endogenous extracellular adenosine and the adenosine receptor-mediated activity modulated the properties of the embryonic isoform of the nicotinic acetylcholine receptor in myotubes in vitro, by increasing the mean open time and the open probability of the ion channel, and sustaining nicotinic acetylcholine receptor-driven intracellular [Ca(2+) ]i 'spikes'. The pharmacological characterization of the adenosine receptor-mediated effects suggested a prevalent involvement of the A2B adenosine receptor subtype.

CONCLUSION

We propose that the interplay between endogenous adenosine and nicotinic acetylcholine receptors represents a potential novel strategy to improve differentiation/regeneration of skeletal muscle.

ATP decreases mechanical sensitivity of muscle thin-fiber afferents in rats

ATP is an energy rich substance contained in cells in the order of mM. It is released when cells are damaged and when muscle is compressed or contracted. Subcutaneous injection of ATP induces pain-related behavior and hyperalgesia to mechanical and heat stimulation in rats. However, the effects of ATP in muscle have not been fully studied. In the present study we examined the effects of ATP on muscle C-fiber afferent activities using single fiber recordings, and on nociceptive behavior. Muscle C-fiber activities were recorded in vitro using extensor digitorum longus muscle-common peroneal nerve preparations excised from rats deeply anesthetized with pentobarbital. ATP (100 μM and 1 mM, but not 1 μM) superfused for 5 min before the mechanical stimulation suppressed the mechanical responses of muscle thin fibers irrespective of whether they excited the fiber. This suppressive effect was reversed by P2X receptor antagonists PPADS (100 μM) and suramin (300 μM). We also found that subcutaneous injection of ATP (10 mM) induced nociceptive behavior, whereas intramuscular injection had no effect. These findings showed that effects of ATP on muscle afferents differ from those on cutaneous afferents.

The Warburg effect: evolving interpretations of an established concept

Metabolic reprogramming and altered bioenergetics have emerged as hallmarks of cancer and an area of active basic and translational cancer research. Drastically upregulated glucose transport and metabolism in most cancers regardless of the oxygen supply, a phenomenon called the Warburg effect, is a major focuses of the research. Warburg speculated that cancer cells, due to defective mitochondrial oxidative phosphorylation (OXPHOS), switch to glycolysis for ATP synthesis, even in the presence of oxygen. Studies in the recent decade indicated that while glycolysis is indeed drastically upregulated in almost all cancer cells, mitochondrial respiration continues to operate normally at rates proportional to oxygen supply. There is no OXPHOS-to-glycolysis switch but rather upregulation of glycolysis. Furthermore, upregulated glycolysis appears to be for synthesis of biomass and reducing equivalents in addition to ATP production. The new finding that a significant amount of glycolytic intermediates is diverted to the pentose phosphate pathway (PPP) for production of NADPH has profound implications in how cancer cells use the Warburg effect to cope with reactive oxygen species (ROS) generation and oxidative stress, opening the door for anticancer interventions taking advantage of this. Recent findings in the Warburg effect and its relationship with ROS and oxidative stress controls will be reviewed. Cancer treatment strategies based on these new findings will be presented and discussed.

Muscle mechanoreflex overactivity in hypertension: a role for centrally-derived nitric oxide

The cardiovascular response to exercise is abnormally large in hypertension. Over the past decade, it has become clear that the exercise pressor reflex (a peripheral feed-back mechanism originating in skeletal muscle) contributes significantly to the generation of this hyper-responsiveness. Further, it has been determined that overactivity of the mechanically (muscle mechanoreflex) and chemically (muscle metaboreflex) sensitive components of the exercise pressor reflex underpin its dysfunction. Given the recent attention in the literature, this review focuses upon the aberrant function of the muscle mechanoreflex in this disease. Evidence supporting a role for the mechanoreflex in the pathogenesis of the exaggerated cardiovascular response to physical activity is highlighted. The peripheral and central mechanisms that may be responsible for mechanoreflex overactivity in hypertension are likewise discussed. Particular attention is given to emerging evidence implicating a role for centrally-derived nitric oxide in this process.

Exaggerated increases in blood pressure during isometric muscle contraction in hypertension: role for purinergic receptors

Physical activity is a cornerstone therapy for the primary prevention and treatment of hypertension, which is becoming increasingly prevalent in modern societies. During exercise, heart rate and blood pressure (BP) increase in order to acutely meet the metabolic demands of the working skeletal muscle. In hypertensive adults, isometric exercise-induced increases in BP are excessive, potentially increasing the risk of an acute cardiovascular event during or after physical activity. Recently, the skeletal muscle metaboreflex has emerged as a significant contributor to the development of aberrant cardiovascular control during isometric exercise in this clinical population. Our laboratory has conducted a series of studies characterizing the skeletal muscle metaboreflex in hypertensive humans. We and others have demonstrated that hypertension is characterized by greater increases in muscle sympathetic nerve activity and BP during selective activation of the metaboreflex during post-exercise muscle ischemia compared to the increases noted in healthy age-matched normotensive adults, suggesting that the skeletal muscle metaboreflex is exaggerated in human hypertension. The focus of this review is the skeletal muscle metaboreflex (i.e., the metabolic component of the exercise pressor reflex) in hypertension, with particular emphasis on the potential role of purinergic receptors in mediating the exaggerated responses to muscle metaboreflex activation.

Sensitization of group III and IV muscle afferents in the mouse after ischemia and reperfusion injury

Ischemic myalgia is a unique type of muscle pain in the patient population. The role that discrete muscle afferent subpopulations play in the generation of pain during ischemic events, however, has yet to be determined. Using 2 brachial artery occlusion models to compare prolonged ischemia or transient ischemia with reperfusion of the muscles, we found that both injuries caused behavioral decrements in grip strength, as well as increased spontaneous pain behaviors. Using our ex vivo forepaw muscles, median and ulnar nerves, dorsal root ganglion, and spinal cord recording preparation, we found after both prolonged and transient ischemia that there was a significant increase in the number of afferents that responded to both noxious and non-noxious chemical (lactate, adenosine triphosphate, varying pH) stimulation of the muscles compared to uninjured controls. However, we found an increase in firing to heat stimuli specifically in muscle afferents during prolonged ischemia, but a distinct increase in afferent firing to non-noxious chemicals and decreased mechanical thresholds after transient ischemia. The unique changes in afferent function observed also corresponded with distinct patterns of gene expression in the dorsal root ganglia. Thus, the development of ischemic myalgia may be generated by unique afferent-based mechanisms during prolonged and transient ischemia.

PERSPECTIVE

This study analyzed the response properties of thinly myelinated group III and unmyelinated group IV muscle afferents during prolonged and transient ischemia in addition to pain behaviors and alterations in DRG gene expression in the mouse. Results suggest that mechanisms of pain generation during prolonged ischemia may be different from ischemia/reperfusion.

Muscle hyperalgesia induced by peripheral P2X3 receptors is modulated by inflammatory mediators

ATP, via activation of P2X3 receptors, has been highlighted as a key target in inflammatory hyperalgesia. Therefore, the aim of this study was to confirm whether the activation of P2X3 receptors in the gastrocnemius muscle of rats induces mechanical muscle hyperalgesia and, if so, to analyze the involvement of the classical inflammatory mediators (bradykinin, prostaglandins, sympathetic amines, pro-inflammatory cytokines and neutrophil migration) in this response. Intramuscular administration of the non-selective P2X3 receptor agonist α,β-meATP in the gastrocnemius muscle of rats induced mechanical muscle hyperalgesia, which, in turn, was prevented by the selective P2X3 and P2X2/3 receptors antagonist A-317491, the selective bradykinin B1-receptor antagonist Des-Arg9-[Leu8]-BK (DALBK), the cyclooxygenase inhibitor indomethacin, the β1- or β2-adrenoceptor antagonist atenolol and ICI 118,551, respectively. Also, the nonspecific selectin inhibitor fucoidan. α,β-meATP induced increases in the local concentration of the pro-inflammatory cytokines tumor necrosis factor-α (TNF-α) and interleukin 1β (IL-1β), which were reduced by bradykinin antagonist. Finally, α,β-meATP also induced neutrophil migration. Together, these findings suggest that α,β-meATP induced mechanical hyperalgesia in the gastrocnemius muscle of rats via activation of peripheral P2X3 receptors, which involves bradykinin, prostaglandins, sympathetic amines, pro-inflammatory cytokines release and neutrophil migration. It is also indicated that bradykinin is the key modulator of the mechanical muscle hyperalgesia induced by P2X3 receptors. Therefore, we suggest that P2X3 receptors are important targets to control muscle inflammatory pain.

Autonomic responses to exercise: group III/IV muscle afferents and fatigue

Group III and IV muscle afferents originating in exercising limb muscle play a significant role in the development of fatigue during exercise in humans. Feedback from these sensory neurons to the central nervous system (CNS) reflexively increases ventilation and central (cardiac output) and peripheral (limb blood flow) hemodynamic responses during exercise and thereby assures adequate muscle blood flow and O2 delivery. This response depicts a key factor in minimizing the rate of development of peripheral fatigue and in optimizing aerobic exercise capacity. On the other hand, the central projection of group III/IV muscle afferents impairs performance and limits the exercising human via its diminishing effect on the output from spinal motoneurons which decreases voluntary muscle activation (i.e. facilitates central fatigue). Accumulating evidence from recent animal studies suggests the existence of two subtypes of group III/IV muscle afferents. While one subtype only responds to physiological and innocuous levels of endogenous intramuscular metabolites (lactate, ATP, protons) associated with 'normal', predominantly aerobic exercise, the other subtype only responds to higher and concurrently noxious levels of metabolites present in muscle during ischemic contractions or following, for example, hypertonic saline infusions. This review discusses the mechanisms through which group III/IV muscle afferent feedback mediates both central and peripheral fatigue in exercising humans. We also briefly summarize the accumulating evidence from recent animal and human studies documenting the existence of two subtypes of group III/IV muscle afferents and the relevance of this discovery to the interpretation of previous work and the design of future studies.

Electrical stimulation induces IL-6 in skeletal muscle through extracellular ATP by activating Ca(2+) signals and an IL-6 autocrine loop

Interleukin-6 (IL-6) is an important myokine that is highly expressed in skeletal muscle cells upon exercise. We assessed IL-6 expression in response to electrical stimulation (ES) or extracellular ATP as a known mediator of the excitation-transcription mechanism in skeletal muscle. We examined whether the canonical signaling cascade downstream of IL-6 (IL-6/JAK2/STAT3) also responds to muscle cell excitation, concluding that IL-6 influences its own expression through a positive loop. Either ES or exogenous ATP (100 μM) increased both IL-6 expression and p-STAT3 levels in rat myotubes, a process inhibited by 100 μM suramin and 2 U/ml apyrase. ATP also evoked IL-6 expression in both isolated skeletal fibers and extracts derived from whole FDB muscles. ATP increased IL-6 release up to 10-fold. STAT3 activation evoked by ATP was abolished by the JAK2 inhibitor HBC. Blockade of secreted IL-6 with a neutralizing antibody or preincubation with the STAT3 inhibitor VIII reduced STAT3 activation evoked by extracellular ATP by 70%. Inhibitor VIII also reduced by 70% IL-6 expression evoked by ATP, suggesting a positive IL-6 loop. In addition, ATP increased up to 60% the protein levels of SOCS3, a negative regulator of the IL-6 signaling pathway. On the other hand, intracellular calcium chelation or blockade of IP3-dependent calcium signals abolished STAT3 phosphorylation evoked by either extracellular ATP or ES. These results suggest that expression of IL-6 in stimulated skeletal muscle cells is mediated by extracellular ATP and nucleotide receptors, involving IP3-dependent calcium signals as an early step that triggers a positive IL-6 autocrine loop.

Anatomical and physiological factors contributing to chronic muscle pain

Chronic muscle pain remains a significant source of suffering and disability despite the adoption of pharmacologic and physical therapies. Muscle pain is mediated by free nerve endings distributed through the muscle along arteries. These nerves project to the superficial dorsal horn and are transmitted primarily through the spinothalamic tract to several cortical and subcortical structures, some of which are more active during the processing of muscle pain than other painful conditions. Mechanical forces, ischemia, and inflammation are the primary stimuli for muscle pain, which is reflected in the array of peripheral receptors contributing to muscle pain-ASIC, P2X, and TRP channels. Sensitization of peripheral receptors and of central pain processing structures are both critical for the development and maintenance of chronic muscle pain. Further, variations in peripheral receptors and central structures contribute to the significantly greater prevalence of chronic muscle pain in females.

Exaggerated exercise pressor reflex in adults with moderately elevated systolic blood pressure: role of purinergic receptors

The neurocirculatory responses to exercise are exaggerated in hypertension, increasing cardiovascular risk, yet the mechanisms remain incompletely understood. The aim of this study was to examine the in vitro effectiveness of pyridoxal-5-phosphate as a purinergic (P2) receptor antagonist in isolated murine dorsal root ganglia (DRG) neurons and the in vivo contribution of P2 receptors to the neurocirculatory responses to exercise in older adults with moderately elevated systolic blood pressure (BP). In vitro, pyridoxal-5-phosphate attenuated the ATP-induced increases in [Ca2+]i (73 ± 15 vs. 11 ± 3 nM; P < 0.05). In vivo, muscle sympathetic nerve activity (MSNA; peroneal microneurography) and arterial BP (Finometer) were assessed during exercise pressor reflex activation (static handgrip followed by postexercise ischemia; PEI) during a control trial (normal saline) and localized P2 receptor blockade (pyridoxal-5-phosphate). Compared with normotensive adults (63 ± 2 yr, 117 ± 2/70 ± 2 mmHg), adults with moderately elevated systolic BP (65 ± 1 yr, 138 ± 5/79 ± 3 mmHg) demonstrated greater increases in MSNA and BP during handgrip and PEI. Compared with the control trial, local antagonism of P2 receptors during PEI partially attenuated MSNA (39 ± 4 vs. 34 ± 5 bursts/min; P < 0.05) in adults with moderately elevated systolic BP. In conclusion, these data demonstrate pyridoxal-5-phosphate is an effective P2 receptor antagonist in isolated DRG neurons, which are of particular relevance to the exercise pressor reflex. Furthermore, these findings indicate that exercise pressor reflex function is exaggerated in older adults with moderately elevated systolic BP and further suggest a modest role of purinergic receptors in evoking the abnormally large reflex-mediated increases in sympathetic activity during exercise in this clinical population.

Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb

With fatiguing exercise, firing of group III/IV muscle afferents reduces voluntary activation and force of the exercised muscles. These afferents can also act across agonist/antagonist pairs, reducing voluntary activation and force in nonfatigued muscles. We hypothesized that maintained firing of group III/IV muscle afferents after a fatiguing adductor pollicis (AP) contraction would decrease voluntary activation and force of AP and ipsilateral elbow flexors. In two experiments (n = 10) we examined voluntary activation of AP and elbow flexors by measuring changes in superimposed twitches evoked by ulnar nerve stimulation and transcranial magnetic stimulation of the motor cortex, respectively. Inflation of a sphygmomanometer cuff after a 2-min AP maximal voluntary contraction (MVC) blocked circulation of the hand for 2 min and maintained firing of group III/IV muscle afferents. After a 2-min AP MVC, maximal AP voluntary activation was lower with than without ischemia (56.2 ± 17.7% vs. 76.3 ± 14.6%; mean ± SD; P < 0.05) as was force (40.3 ± 12.8% vs. 57.1 ± 13.8% peak MVC; P < 0.05). Likewise, after a 2-min AP MVC, elbow flexion voluntary activation was lower with than without ischemia (88.3 ± 7.5% vs. 93.6 ± 3.9%; P < 0.05) as was torque (80.2 ± 4.6% vs. 86.6 ± 1.0% peak MVC; P < 0.05). Pain during ischemia was reported as Moderate to Very Strong. Postfatigue firing of group III/IV muscle afferents from the hand decreased voluntary drive and force of AP. Moreover, this effect decreased voluntary drive and torque of proximal unfatigued muscles, the elbow flexors. Fatigue-sensitive group III/IV muscle nociceptors act to limit voluntary drive not only to fatigued muscles but also to unfatigued muscles within the same limb.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

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.