Increased tetanic calcium in early fatigue of mammalian muscle fibers is accompanied by accelerated force development despite a decreased force.

During the initial phase of fatigue induced by repeated contractions in fast-twitch muscle fibers, tetanic force decreases despite increasing tetanic free cytosolic [Ca ] ([Ca ] ). Here, we hypothesized that the increase in tetanic [Ca ] nevertheless has positive effects on force in early fatigue. Experiments on enzymatically isolated mouse flexor digitorum brevis (FDB) fibers showed that an increase in tetanic [Ca ] during ten 350 ms contractions required trains of electrical pulses to be elicited at short intervals (≤2 s) and at high frequencies (≥70 Hz).

Enzymatically dissociated muscle fibers display rapid dedifferentiation and impaired mitochondrial calcium control.

Cells rapidly lose their physiological phenotype upon disruption of their extracellular matrix (ECM)-intracellular cytoskeleton interactions. By comparing adult mouse skeletal muscle fibers, isolated either by mechanical dissection or by collagenase-induced ECM digestion, we investigated acute effects of ECM disruption on cellular and mitochondrial morphology, transcriptomic signatures, and Ca handling. RNA-sequencing showed striking differences in gene expression patterns between the two isolation methods with enzymatically dissociated fibers resembling myopathic phenotypes.

Sphingomyelinase activity promotes atrophy and attenuates force in human muscle fibres and is elevated in heart failure patients.

Background: Activation of sphingomyelinase (SMase) as a result of a general inflammatory response has been implicated as a mechanism underlying disease-related loss of skeletal muscle mass and function in several clinical conditions including heart failure. Here, for the first time, we characterize the effects of SMase activity on human muscle fibre contractile function and assess skeletal muscle SMase activity in heart failure patients.

Methods: The effects of SMase on force production and intracellular Ca handling were investigated in single intact human muscle fibres.

Impaired sarcoplasmic reticulum Ca release is the major cause of fatigue-induced force loss in intact single fibres from human intercostal muscle.

Key Points: Changes in intramuscular Ca handling contribute to development of fatigue and disease-related loss of muscle mass and function. To date, no data on human intact living muscle fibres have been described. We manually dissected intact single fibres from human intercostal muscle and simultaneously measured force and myoplasmic free [Ca ] at physiological temperature.

SR Ca leak in skeletal muscle fibers acts as an intracellular signal to increase fatigue resistance.

Effective practices to improve skeletal muscle fatigue resistance are crucial for athletes as well as patients with dysfunctional muscles. To this end, it is important to identify the cellular signaling pathway that triggers mitochondrial biogenesis and thereby increases oxidative capacity and fatigue resistance in skeletal muscle fibers. Here, we test the hypothesis that the stress induced in skeletal muscle fibers by endurance exercise causes a reduction in the association of FK506-binding protein 12 (FKBP12) with ryanodine receptor 1 (RYR1).

STIM1 R304W causes muscle degeneration and impaired platelet activation in mice.

STIM1 and ORAI1 regulate store-operated Ca entry (SOCE) in most cell types, and mutations in these proteins have deleterious and diverse effects. We established a mouse line expressing the STIM1 R304 W gain-of-function mutation causing Stormorken syndrome to explore effects on organ and cell physiology. While STIM1 R304 W was lethal in the homozygous state, surviving mice presented with reduced growth, skeletal muscle degeneration, and reduced exercise endurance.

Prolonged force depression after mechanically demanding contractions is largely independent of Ca and reactive oxygen species.

Increased production of reactive oxygen/nitrogen species (ROS) and impaired cellular Ca handling are implicated in the prolonged low-frequency force depression (PLFFD) observed in skeletal muscle after both metabolically and mechanically demanding exercise. Metabolically demanding high-intensity exercise can induce PLFFD accompanied by ROS-dependent fragmentation of the sarcoplasmic reticulum Ca release channels, the ryanodine receptor 1s (RyR1s). We tested whether similar changes occur after mechanically demanding eccentric contractions.

Cyclophilin D, a target for counteracting skeletal muscle dysfunction in mitochondrial myopathy.

Muscle weakness and exercise intolerance are hallmark symptoms in mitochondrial disorders. Little is known about the mechanisms leading to impaired skeletal muscle function and ultimately muscle weakness in these patients. In a mouse model of lethal mitochondrial myopathy, the muscle-specific Tfam knock-out (KO) mouse, we previously demonstrated an excessive mitochondrial Ca(2+) uptake in isolated muscle fibers that could be inhibited by the cyclophilin D (CypD) inhibitor, cyclosporine A (CsA).

Intracellular Ca(2+)-handling differs markedly between intact human muscle fibers and myotubes.

Background: In skeletal muscle, intracellular Ca(2+) is an important regulator of contraction as well as gene expression and metabolic processes. Because of the difficulties to obtain intact human muscle fibers, human myotubes have been extensively employed for studies of Ca(2+)-dependent processes in human adult muscle. Despite this, it is unknown whether the Ca(2+)-handling properties of myotubes adequately represent those of adult muscle fibers.

Muscle dysfunction associated with adjuvant-induced arthritis is prevented by antioxidant treatment.

Background: In addition to the primary symptoms arising from inflamed joints, muscle weakness is prominent and frequent in patients with rheumatoid arthritis (RA). Here, we investigated the mechanisms of arthritis-induced muscle dysfunction in rats with adjuvant-induced arthritis (AIA).

Methods: AIA was induced in the knees of rats by injection of complete Freund's adjuvant and was allowed to develop for 21 days.

Antioxidant treatments do not improve force recovery after fatiguing stimulation of mouse skeletal muscle fibres.

The contractile performance of skeletal muscle declines during intense activities, i.e. fatigue develops.

Effects of N-acetylcysteine on isolated mouse skeletal muscle: contractile properties, temperature dependence, and metabolism.

The effects of the general antioxidant N-acetylcysteine (NAC) on muscle function and metabolism were examined. Isolated paired mouse extensor digitorum longus muscles were studied in the absence or presence of 20 mM NAC. Muscles were electrically stimulated to perform 100 isometric tetanic contractions (300 ms duration) at frequencies resulting in ∼85% of maximal force (70-150 Hz at 25-40 °C).

Doublet discharge stimulation increases sarcoplasmic reticulum Ca2+ release and improves performance during fatiguing contractions in mouse muscle fibres.

Double discharges (doublets) of motor neurones at the onset of contractions increase both force and rate of force development during voluntary submaximal contractions. The purpose of this study was to examine the role of doublet discharges on force and myoplasmic free [Ca(2+)] ([Ca(2+)]i) during repeated fatiguing contractions, using a stimulation protocol mimicking the in vivo activation pattern during running. Individual intact fibres from the flexor digitorum brevis muscle of mice were stimulated at 33°C to undergo 150 constant-frequency (five pulses at 70 Hz) or doublet (an initial, extra pulse at 200 Hz) contractions at 300 ms intervals.

TLR4 as receptor for HMGB1 induced muscle dysfunction in myositis.

Objectives: Polymyositis and dermatomyositis are characterised by muscle weakness and fatigue even in patients with normal muscle histology via unresolved pathogenic mechanisms. In this study, we investigated the mechanisms by which high mobility group box protein 1 (HMGB1) acts to accelerate muscle fatigue development.

Methods: Intact single fibres were dissociated from flexor digitorum brevis (FDB) of wild type, receptor for advanced glycation endproduct (RAGE) knockout and toll like receptor 4 (TLR4) knockout mice and cultured in the absence or presence of recombinant HMGB1.

Impaired mitochondrial respiration and decreased fatigue resistance followed by severe muscle weakness in skeletal muscle of mitochondrial DNA mutator mice.

Mitochondrial dysfunction can drastically impair muscle function, with weakness and exercise intolerance as key symptoms. Here we examine the time course of development of muscle dysfunction in a mouse model of premature ageing induced by defective proofreading function of mitochondrial DNA (mtDNA) polymerase (mtDNA mutator mouse). Isolated fast-twitch muscles and single muscle fibres from young (3-5 months) and end-stage (11 months) mtDNA mutator mice were compared to age-matched control mice.

Dietary nitrate increases tetanic [Ca2+]i and contractile force in mouse fast-twitch muscle.

Dietary inorganic nitrate has profound effects on health and physiological responses to exercise. Here, we examined if nitrate, in doses readily achievable via a normal diet, could improve Ca(2+) handling and contractile function using fast- and slow-twitch skeletal muscles from C57bl/6 male mice given 1 mm sodium nitrate in water for 7 days. Age matched controls were provided water without added nitrate.

Methods to detect Ca(2+) in living cells.

Measurements of free cytosolic Ca(2+) concentration ([Ca(2+)](i)) or free Ca(2+) concentration in cellular organelles have become more routine. The primary reason for this is the availability of membrane permeant forms of Ca(2+) indicators that can easily enter cells. In this chapter, the properties required of an ideal Ca(2+) indicator are identified and the advantages and disadvantages of available Ca(2+) indicators are pointed out.

Increased fatigue resistance linked to Ca2+-stimulated mitochondrial biogenesis in muscle fibres of cold-acclimated mice.

Mammals exposed to a cold environment initially generate heat by repetitive muscle activity (shivering). Shivering is successively replaced by the recruitment of uncoupling protein-1 (UCP1)-dependent heat production in brown adipose tissue. Interestingly, adaptations observed in skeletal muscles of cold-exposed animals are similar to those observed with endurance training.

Skeletal muscle: energy metabolism, fiber types, fatigue and adaptability.

Skeletal muscles cope with a large range of activities, from being able to support the body weight during long periods of upright standing to perform explosive movements in response to an unexpected threat. This requires systems for energy metabolism that can provide energy during long periods of moderately increased energy consumption as well as being able to rapidly increasing the rate of energy production more than 100-fold in response to explosive contractions. In this short review we discuss how muscles can deal with these divergent demands.

Muscle fatigue: from observations in humans to underlying mechanisms studied in intact single muscle fibres.

Prolonged dynamic exercise and sustained isometric contractions induce muscle fatigue, as manifested by decreased performance and a reduction in the maximum voluntary contraction force. Studies with non-invasive measurements in exercising humans show that mechanisms located beyond the sarcolemma are important in the fatigue process. In this review, we describe probable cellular mechanisms underlying fatigue-induced changes in excitation-contraction (E-C) coupling occurring in human muscle fibres during strenuous exercise.

Myogenic skeletal muscle satellite cells communicate by tunnelling nanotubes.

Quiescent satellite cells sit on the surface of the muscle fibres under the basal lamina and are activated by a variety of stimuli to disengage, divide and differentiate into myoblasts that can regenerate or repair muscle fibres. Satellite cells adopt their parent's fibre type and must have some means of communication with the parent fibre. The mechanisms behind this communication are not known.

Impaired myofibrillar function in the soleus muscle of mice with collagen-induced arthritis.

Objective: Progressive muscle weakness is a common feature in patients with rheumatoid arthritis (RA). However, little is known about whether the intrinsic contractile properties of muscle fibers are affected in RA. This study was undertaken to investigate muscle contractility and the myoplasmic free Ca2+ concentration ([Ca2+](i)) in the soleus, a major postural muscle, in mice with collagen-induced arthritis (CIA).

Doxorubicin acts through tumor necrosis factor receptor subtype 1 to cause dysfunction of murine skeletal muscle.

Cancer patients receiving doxorubicin chemotherapy experience both muscle weakness and fatigue. One postulated mediator of the muscle dysfunction is an increase in tumor necrosis factor-alpha (TNF), a proinflammatory cytokine that mediates limb muscle contractile dysfunction through the TNF receptor subtype 1 (TNFR1). Our main hypothesis was that systemic doxorubicin administration would cause muscle weakness and fatigue.

High temperature does not alter fatigability in intact mouse skeletal muscle fibres.

Intense activation of skeletal muscle results in fatigue development, which involves impaired function of the muscle cells resulting in weaker and slower contractions. Intense muscle activity also results in increased heat production and muscle temperature may rise by up to 6 degrees C. Hyperthermia is associated with impaired exercise performance in vivo and recent studies have shown contractile dysfunction and premature fatigue development in easily fatigued muscle fibres stimulated at high temperatures and these defects were attributed to oxidative stress.

Knockdown of TRPC3 with siRNA coupled to carbon nanotubes results in decreased insulin-mediated glucose uptake in adult skeletal muscle cells.

The involvement of Ca(2+) in the insulin-mediated signaling cascade, resulting in glucose uptake in skeletal muscle, is uncertain. Here, we test the hypothesis that Ca(2+) influx through canonical transient receptor potential 3 (TRPC3) channels modulates insulin-mediated glucose uptake in adult skeletal muscle. Experiments were performed on adult skeletal muscle cells of wild-type (WT) and obese, insulin-resistant ob/ob mice.