Increased susceptibility of EDL muscles from mdx mice to damage induced by contractions with stretch

Summary Absence of dystrophin in mdx muscles may render the muscle more susceptible to damage when submitted to high stress levels. To test this, typically slow (soleus) and fast (EDL) limb muscles of dystrophic (mdx) and normal (C57BL/10) mice were submitted (in vitro) to a series of isometric contractions, followed by a series of contractions with stretches. Muscle injury was assessed by monitoring the force signal. Membrane damage was evaluated by bathing the muscle in Procion Red, a dye that does not penetrate intact fibres, and subsequent analysis by light microscopy. After isometric contractions, only a very small force drop (<3% of maximal isometric force) was observed which indicated that no injury had occurred in soleus and EDL muscles in either mdx or C57 strains. After contractions with a stretch, a force drop of 10% was observed in soleus muscles from both strains and in EDL muscles from C57 mice. However, in mdx mice EDL muscles displayed an irreversible force drop of 40–60%. Histological analysis of the muscles indicates that force drop is associated with membrane damage. These results show that EDL muscles from mdx mice are more vulnerable than their controls, supporting the structural role hypothesis for dystrophin. Furthermore, they suggest that contractions with stretches may contribute to the muscle damage and degeneration observed in DMD-patients.     

Rapid recovery following contraction-induced injury to in situ skeletal muscles in mdx mice

The muscles of mdx mice lack the subsarcolemmal protein dystrophin, and as a consequence may be more susceptible to damage induced by contractions. The purpose of this study was to characterize the response of muscles in mdx mice to contraction-induced injury in situ. The hypothesis tested was that following a protocol of repeated stretches of maximally activated muscles, the magnitude of the injury is greater for muscles in mdx mice than for muscles in C57BL/10 control mice, and consequently, the muscles in mdx mice recover more slowly. Each stretch was of 20% strain relative to muscle fibre length (Lf) at 0.5 Lf s-1 and was initiated from the force plateau of an isometric contraction. The protocol consisted of a total of ten contractions, with one contraction occurring every ten seconds. The time-course of injury and recovery was determined through measurements of in situ force production at 10, 30, 45 and 60 minutes, and either 12, 24, 48 or 72 hours after the contraction protocol. The initial injury, as assessed by the decrease in force production both immediately and 60 minutes after the contraction protocol, was significantly greater for the muscles in mdx mice compared with those in control mice. Over the next three days, a value for maximum isometric force of sim 80% of the pre-injury value was maintained for muscles in control mice, whereas within three days muscles in mdx mice showed complete recovery of force. For muscles in mdx mice, the greater decrease in force during the contraction protocol and the more rapid recovery indicates an increased susceptibility to contraction-induced injury but an enhanced rate of recovery. This revised version was published online in August 2006 with corrections to the Cover Date.     

Tibialis anterior muscles in mdx mice are highly susceptible to contraction-induced injury

Skeletal muscles of patients with Duchenne muscular dystrophy (DMD) and mdx mice lack dystrophin and are more susceptible to contraction-induced injury than control muscles. Our purpose was to develop an assay based on the high susceptibility to injury of limb muscles in mdx mice for use in evaluating therapeutic interventions. The assay involved two stretches of maximally activated tibialis anterior (TA) muscles in situ. Stretches of 40% strain relative to muscle fiber length were initiated from the plateau of isometric contractions. The magnitude of damage was assessed one minute later by the deficit in isometric force. At all ages (2–19 months), force deficits were four- to seven-fold higher for muscles in mdx compared with control mice. For control muscles, force deficits were unrelated to age, whereas force deficits increased dramatically for muscles in mdx mice after 8 months of age. The increase in susceptibility to injury of muscles from older mdx mice did not parallel similar adverse effects on muscle mass or force production. The in situ stretch protocol of TA muscles provides a valuable assay for investigations of the mechanisms of injury in dystrophic muscle and to test therapeutic interventions for reversing DMD. This revised version was published online in July 2006 with corrections to the Cover Date.     

Ventilation during air breathing and in response to hypercapnia in 5 and 16 month-old <Emphasis Type="Italic">mdx</Emphasis> and C57 mice

Previous studies have shown a blunted ventilatory response to hypercapnia in mdx mice older than 7 months. We test the hypothesis that in the mdx mice ventilatory response changes with age, concomitantly with the increased functional impairment of the respiratory muscles. We thus studied the ventilatory response to CO₂ in 5 and 16 month-old mdx and C57BL10 mice (n = 8 for each group). Respiratory rate (RR), tidal volume (VT), and minute ventilation (VE) were measured, using whole-body plethysmography, during air breathing and in response to hypercapnia (3, 5 and 8% CO₂). The ventilatory protocol was completed by histological analysis of the diaphragm and intercostals muscles. During air breathing, the 16 month-old mdx mice showed higher RR and, during hypercapnia (at 8% CO₂ breathing), significantly lower RR (226 ± 26 vs. 270 ± 21 breaths/min) and VE (1.81 ± 0.35 vs. 3.96 ± 0.59 ml min⁻¹ g⁻¹) (P < 0.001) in comparison to C57BL10 controls. On the other hand, 5 month-old C57BL10 and mdx mice did not present any difference in their ventilatory response to air breathing and to hypercapnia. In conclusion, this study shows similar ventilation during air breathing and in response to hypercapnia in the 5 month-old mdx and control mice, in spite of significant pathological structural changes in the respiratory muscles of the mdx mice. However in the 16 month-old mdx mice we observed altered ventilation under air and blunted ventilation response to hypercapnia compared to age-matched control mice. Ventilatory response to hypercapnia thus changes with age in mdx mice, in line with the increased histological damage of their respiratory muscles. J. Gayraud and S. Matecki contributed equally to this work     

Modulation of insulin-like growth factor (IGF)-I and IGF-binding protein interactions enhances skeletal muscle regeneration and ameliorates the dystrophic pathology in mdx mice

Administration of recombinant human insulin-like growth factor-I (rhIGF-I) has beneficial effects in animal models of muscle injury and muscular dystrophy. However, the results of these studies may have been confounded by interactions of rhIGF-I with endogenous IGF-binding proteins (IGFBPs). To date, no study has examined whether inhibiting IGFBP interactions with endogenous IGF-I can improve muscle fiber regeneration or muscular pathologies. We tested the hypothesis that reducing IGFBP interactions with endogenous IGF-I would enhance muscle regeneration after myotoxic injury and improve the dystrophic pathology in mdx mice. We administered an IGF-I aptamer (NBI-31772; 6 mg/kg per day, continuous infusion) to C57BL/10 mice undergoing regeneration after myotoxic injury or to mdx dystrophic mice. NBI-31772 binds all six IGFBPs with high affinity and releases "free" endogenous IGF-I. NBI-31772 treatment increased the rate of functional repair in fast-twitch tibialis anterior muscles after notexin-induced injury as evidenced by an increase in maximum force producing capacity (P(o)) at 10 days after injury. In contrast, NBI-31772 administration for 28 days did not alter P(o) of extensor digitorum longus (EDL) and soleus muscles or normalized force of diaphragm muscle strips from mdx mice. Although IGFBP inhibition reduced the susceptibility of the fast-twitch EDL and the diaphragm muscle to contraction-mediated damage, it increased muscle fatigability during repeated maximal contractions. Although the results in the myotoxic injury model suggest IGF-I signaling is important in this model, the results in the mdx model are mixed.     

Time course study of the isometric contractile properties of mdx mouse striated muscles

Summary The isometric twitch and tetanic contractions of three hindlimb muscles (soleus, plantaris, extensor digitorum longus) were recorded in situ in groups of mdx and C57BL/10 control mice at young, adult and old ages (3, 4, 6, 8, 13, 26, 39 and 52 weeks). Based on a two-way analysis of variance (age/phenotype) the mdx phenotype did not modify the absolute tension but was associated with a significant decrease in the tetanic tension normalized to muscle weight in all the muscles which became heavier. These results suggest that the contractile material in mdx is not so powerful as in controls. Moreover, significantly faster time to peak and half-relaxation time were observed in mdx soleus and plantaris. Comparison between these contraction characteristics and those of other experimental models suggests that the high percentage of regenerated fibres in mdx muscles could play a role in modifying contractile properties.     

Akt activation prevents the force drop induced by eccentric contractions in dystrophin-deficient skeletal muscle

Skeletal muscles of the mdx mouse, a model of Duchenne Muscular Dystrophy, show an excessive reduction in the maximal tetanic force following eccentric contractions. This specific sign of the susceptibility of dystrophin-deficient muscles to mechanical stress can be used as a quantitative test to measure the efficacy of therapeutic interventions. Using inducible transgenesis in mice, we show that when Akt activity is increased the force drop induced by eccentric contractions in mdx mice becomes similar to that of wild-type mice. This effect is not correlated with muscle hypertrophy and is not blocked by rapamycin treatment. The force drop induced by eccentric contractions is similar in skinned muscle fibers from mdx and Akt-mdx mice when stretch is applied directly to skinned fibers. However, skinned fibers isolated from mdx muscles exposed to eccentric contractions in vivo develop less isometric force than wild-type fibers and this force depression is completely prevented by Akt activation. These experiments indicate that the myofibrillar-cytoskeletal system of dystrophin-deficient muscle is highly susceptible to a damage caused by eccentric contraction when elongation is applied in vivo, and this damage can be prevented by Akt activation. Microarray and PCR analyses indicate that Akt activation induces up-regulation of genes coding for proteins associated with Z-disks and costameres, and for proteins with anti-oxidant or chaperone function. The protein levels of utrophin and dysferlin are also increased by Akt activation.     

Lack of myoblasts migration between transplanted and host muscles of mdx and normal mice

Summary Extensor digitorum longus muscles of normal mice (C57BL/10ScSn hereafter called C57) were orthotopically transplanted into dystrophin-deficient mice (mdx) and reciprocally, mdx Extensor digitorum longus muscles were transplanted into C57 mice. After an initial phase of degeneration, transplanted muscles regenerate nearly completely, as evaluated from the maximum isometric force of muscles isolated 60 days after the surgery. In other similar experiments, instead of isolating the grafted muscles, we excised the antero-external muscles of the leg, including the grafted muscle. Cryostat cross-sections at three levels along the muscles were immunostained with an anti-dystrophin antibody. No muscle cells of dystrophin-deficient muscles grafted into normal mice took the antibody except a few revertant fibres, while all the muscle cells of the normal host were immunostained. Reciprocally, all the muscles cells of normal grafts were stained, whilst no antibody stained the cells of the surrounding muscles of the dystrophin-deficient host. These experiments show that very few if any of the myoblasts or muscle precursor cells, active during the regeneration of grafted muscle, migrate into the adjacent muscles. These results could be explained by the absence, in our work, of injuries of the grafted and adjacent host muscles epimysium and the absence of extensive inflammatory reactions. This lack of myoblast mobility suggest that when myoblast transfer is applied to muscle therapy, it will be necessary to inject myoblasts within each muscle to obtain an efficient treatment.     

Cardiac and skeletal muscle changes associated with rosuvastatin therapy in dystrophic mdx mice

Statins are prescribed to prevent and treat atherosclerotic cardiovascular and metabolic diseases but have controversial effects on skeletal muscles. While statins are a reported cause of myopathy, some studies have suggested that statins could potentially ameliorate dystrophy due to their pleiotropic effects on inflammation, myonecrosis, and autophagy. In the present study, we evaluated the potential benefit of rosuvastatin treatment on heart, limb, and diaphragm muscles in dystrophin-deficient mdx mice at an early stage (45 days of age) of disease. Mdx mice received rosuvastatin (10 mg/kg) by gavage for 30 days beginning at 15 days of age. Normal C57BL/10 mice received rosuvastatin by the same route over the same interval. In the mdx group, rosuvastatin significantly increased IgG-positive fibers (myonecrosis) and the inflammatory areas in the biceps brachii and diaphragm muscles and decreased the anterior limb muscle force (grip strength). Molecular markers of inflammation (TNF-α and NF-kB) and fibrosis (fibronectin) were not altered by rosuvastatin in mdx mice skeletal and cardiac muscles. In normal mice, rosuvastatin increased CK, TNF-α (heart), NF-kB (diaphragm), and fibronectin (heart and diaphragm). Inflammatory areas were seen in all normal muscles of rosuvastatin-treated mice. Rosuvastatin did not benefit dystrophy in the mdx mice and was associated with inflammation in normal cardiac and skeletal muscles.     

Proteomic assessment of the acute phase of dystrophin deficiency in mdx mice

Duchenne muscular dystrophy (DMD) is caused by the absence of a functional dystrophin protein and is modeled by the mdx mouse. The mdx mouse suffers an early necrotic bout in the hind limb muscles lasting from approximately 4 to 7 weeks. The purpose of this investigation was to determine the extent to which dystrophin deficiency changed the proteome very early in the disease process. In order to accomplish this, proteins from gastrocnemius from 6-week-old C57 (n = 6) and mdx (n = 6) mice were labeled with fluorescent dye and subjected to two-dimensional differential in-gel electrophoresis (2D-DIGE). Resulting differentially expressed spots were excised and protein identity determined via MALDI-TOF followed by database searching using MASCOT. Proteins of the immediate energy system and glycolysis were generally down-regulated in mdx mice compared to C57 mice. Conversely, expression of proteins involved in the Kreb’s cycle and electron transport chain were increased in dystrophin-deficient muscle compared to control. Expression of cytoskeletal components, including tubulins, vimentin, and collagen, were increased in mdx mice compared to C57 mice. Importantly, these changes are occurring at only 6 weeks of age and are caused by acute dystrophin deficiency rather than more chronic injury. These data may provide insight regarding early pathologic changes occurring in dystrophin-deficient skeletal muscle.     

Insulin-like growth factor-I analogue protects muscles of dystrophic mdx mice from contraction-mediated damage

Contraction-mediated injury is a major contributing factor to the pathophysiology of muscular dystrophy and therefore therapies that can attenuate this type of injury have clinical relevance. Systemic administration of insulin-like growth factor-I (IGF-I) has been shown to improve muscle function in dystrophic mdx mice, an effect associated with a shift towards a more oxidative muscle phenotype and a reduced susceptibility to contraction-mediated damage. The actions of IGF-I in vivo are modulated by IGF binding proteins (IGFBPs), which generally act to inhibit IGF-I signalling. We tested the hypothesis that an analogue of IGF-I (LR IGF-I), which has significantly reduced binding affinity for IGFBPs, would improve the dystrophic pathology by reducing the susceptibility to muscle injury. Dystrophic mdx and wild-type (C57BL/10) mice were administered LR IGF-I continuously ( approximately 1.5 mg kg(-1) day(-1)) via osmotic mini-pump for 4 weeks. Administration of LR IGF-I reduced the susceptibility of extensor digitorum longus, soleus and diaphragm muscles to contraction damage, as evident from lower force deficits after a protocol of lengthening contractions. In contrast to the mechanism of protection conferred by administration of IGF-I, the protection conferred by LR IGF-I was independent of changes in muscle fatigue and oxidative metabolism. This study further indicates that modulation of IGF-I signalling has therapeutic potential for muscular diseases.     

Sarcoplasmic reticulum function in slow- and fast-twitch skeletal muscles from mdx mice

The aim of the present study was to establish whether alterations in sarcoplasmic reticulum function are involved in the abnormal Ca(2+) homeostasis of skeletal muscle in mice with muscular dystrophy ( mdx). The properties of the sarcoplasmic reticulum and contractile proteins of fast- and slow-twitch muscles were therefore investigated in chemically skinned fibres isolated from the extensor digitorum longus (EDL) and soleus muscles of normal (C57BL/10) and mdx mice at 4 and 11 weeks of development. Sarcoplasmic reticulum Ca(2+) uptake, estimated by the Ca(2+) release following exposure to caffeine, was significantly slower in mdx mice, while the maximal Ca(2+) quantity did not differ in either type of skeletal muscle at either stage of development. In 4-week-old mice spontaneous sarcoplasmic reticulum Ca(2+) leakage was observed in EDL and soleus fibres and this was more pronounced in mdx mice. In addition, the maximal Ca(2+)-activated tension was smaller in mdx than in normal fibres, while the Ca(2+) sensitivity of the contractile apparatus was not significantly different. These results indicate that mdx hindlimb muscles are affected differently by the disease process and suggest that a reduced ability of the Ca(2+)-ATPase to load Ca(2+) and a leaky sarcoplasmic reticulum membrane may be involved in the altered intracellular Ca(2+) homeostasis.     

Making Fast-Twitch Dystrophic Muscles Bigger Protects Them from Contraction Injury and Attenuates the Dystrophic Pathology

The lack of functional dystrophin protein in Duchenne muscular dystrophy (DMD) renders muscle fibers highly fragile and susceptible to damage during contractions. Contraction-mediated injury is a major contributor to the progressive degeneration and etiology of muscle wasting in DMD. The prevailing understanding is that large fibers are highly susceptible to contraction damage and are affected preferentially, whereas smaller fibers are relatively spared in DMD. We tested the hypothesis that a pharmacological treatment that caused myofiber hypertrophy would increase the susceptibility of muscles from dystrophin-deficient mdx mice to contraction-induced injury, and thus aggravate the dystrophic pathology. The β-agonist formoterol (100 μg/kg per day, i.p.) was administered to mdx mice for 28 days. Formoterol increased muscle mass, fiber cross-sectional area, and maximum force producing capacity by 30%, 23%, and 21%, respectively, in fast-twitch tibialis anterior muscles of mdx mice. Myofiber hypertrophy and increased maximum force producing capacity were also observed in the predominantly slow-twitch soleus muscles of mdx mice. Our original hypothesis was rejected since tibialis anterior muscles from formoterol-treated mdx mice had lower cumulative force deficits, indicating that they were less susceptible to contraction-induced injury. Formoterol treatment did not affect injury susceptibility in soleus muscles. These findings indicate that making dystrophic muscles bigger protects them from contraction damage and does not aggravate the dystrophic pathophysiology. These novel results further support the contention that anabolic agents have therapeutic potential for muscle wasting conditions including DMD.Duchenne muscular dystrophy (DMD) is a severe and progressive muscle wasting disorder caused by mutations in the dystrophin gene that result in the complete absence of the membrane-stabilizing protein dystrophin.^1–3 The loss of this integral muscle protein renders dystrophic muscles fragile and highly susceptible to damage from everyday contractions.^4,5 What would normally be considered innocuous contractions in healthy muscle causes tears in muscle fibers and a subsequent influx of Ca²⁺ that activates degenerative pathways in dystrophic muscle.⁶ Repeated injurious events eventually exhaust the regenerative capacity of dystrophic muscles and infiltration of adipose and connective tissue ensues leading to progressive functional impairments in affected patients.⁶Although the most likely cure for DMD will come from gene therapies, either by restoration or replacement of the mutated dystrophin gene, several significant hurdles must be overcome before such treatments become available and accepted.⁷ Until then, alternative therapies are needed that can attenuate the severity and progression of muscle wasting and enhance the quality of life for DMD patients. One of the most widely trialed therapeutic strategies for DMD has been the administration of anabolic agents such as anabolic steroids, myostatin-blocking antibodies/peptides, and β-adrenoceptor agonists (β-agonists).^8–11 These approaches have shown improvements in muscle function in some studies,^9,10,12 but others have shown little or no effect.¹¹Although bigger muscles generally produce more force, it has been suggested that enhancing muscle size in mdx mice would increase their susceptibility to contraction-induced injury.¹³ Previous studies have shown that large, fast type II fibers are damaged selectively after lengthening contractions^14,15 and these fibers are preferentially affected in DMD.^16–19 Conversely, smaller caliber fibers such as those in extraocular and intrinsic laryngeal muscles are spared in DMD patients and animal models of muscular dystrophy.^20–23 Since contraction-induced injury contributes to the dystrophic pathology, it is imperative that potential therapeutic strategies do not adversely affect the susceptibility of dystrophic muscles to self-injury.Using the well-characterized effect of β-agonists to induce skeletal muscle hypertrophy, we tested the hypothesis that making dystrophic muscles bigger would increase their susceptibility to contraction-mediated injury and thus aggravate the dystrophic pathology.     

Expression of Dp260 in muscle tethers the actin cytoskeleton to the dystrophin-glycoprotein complex and partially prevents dystrophy

Dystrophin forms a mechanical link between the actin cytoskeleton and the extracellular matrix in muscle that helps maintain sarcolemmal integrity. Two regions of dystrophin have been shown to bind actin: the N-terminal domain and rod domain repeats 11-17. To better understand the roles of these two domains and whether the rod domain actin-binding domain alone can support a mechanically functional link with actin, we constructed transgenic mice expressing Dp260 in skeletal muscle. Dp260, the retinal isoform of dystrophin, lacks the N-terminal domain and a significant portion of the rod domain, but retains the rod domain actin-binding domain. Our results indicate that Dp260 expression restores a stable association between costameric actin and the sarcolemma, assembles the dystrophin-glycoprotein complex, and significantly slows the progression of the dystrophy in the dystrophin-deficient mdx mouse. We assessed the functional integrity of the mechanical link in Dp260 transgenic mdx mice and found that Dp260 muscles showed normal resistance to contraction-induced injury, but dramatic reductions in force generation similar to those found with mdx muscles. Morphologically, Dp260 muscles displayed reduced amounts of inflammation and fibrosis, but still showed a significant, albeit reduced, amount of degeneration/regeneration. These data demonstrate that protection from contraction-induced injury can dramatically ameliorate, but not completely halt, the dystrophic process. We suggest that a non-mechanical defect, attributed to the loss of the N terminus of dystrophin, is likely responsible for the residual dystrophy observed.     

Degenerative and regenerative features of myofibers differ among skeletal muscles in a murine model of muscular dystrophy

Skeletal muscle myofibers constantly undergo degeneration and regeneration. Histopathological features of 6 skeletal muscles (cranial tibial [CT], gastrocnemius, quadriceps femoris, triceps brachii [TB], lumbar longissimus muscles, and costal part of the diaphragm [CPD]) were compared using C57BL/10ScSn-Dmd ^mdx (mdx) mice, a model for muscular dystrophy versus control, C57BL/10 mice. Body weight and skeletal muscle mass were lower in mdx mice than the control at 4 weeks of age; these results were similar at 6–30 weeks. Additionally, muscular lesions were observed in all examined skeletal muscles in mdx mice after 4 weeks, but none were noted in the controls. Immunohistochemical staining revealed numerous paired box 7-positive satellite cells surrounding the embryonic myosin heavy chain-positive regenerating myofibers, while the number of the former and staining intensity of the latter decreased as myofiber regeneration progressed. Persistent muscular lesions were observed in skeletal muscles of mdx mice between 4 and 14 weeks of age, and normal myofibers decreased with age. Number of muscular lesions was lowest in CPD at all ages examined, while the ratio of normal myofibers was lowest in TB at 6 weeks. In CT, TB, and CPD, Iba1-positive macrophages, the main inflammatory cells in skeletal muscle lesions, showed a significant positive correlation with the appearance of regenerating myofibers. Additionally, B220-positive B-cells showed positive and negative correlation with regenerating and regenerated myofibers, respectively. Our data suggest that degenerative and regenerative features of myofibers differ among skeletal muscles and that inflammatory cells are strongly associated with regenerative features of myofibers in mdx mice.     

Muscle performance following fatigue induced by isotonic and quasi‐isometric contractions of rat extensor digitorum longus and soleus muscles in vitro

Aim: To study the effect of contraction mode on fatigue development. Methods: Muscle fatigue was induced by isotonic and quasi‐isometric contractions in rat soleus (SOL) and extensor digitorum longus (EDL) muscles, using identical stimulation protocol (60 Hz, 400 ms s^?1) for 100 s in SOL and 60 s in EDL. Fatigue was quantified as the decline in peak values of shortening, shortening velocity, relaxation and work during the isotonic contractions, and, correspondingly, of force, rate of force development, relaxation and work during the quasi‐isometric contractions. Maximal test contractions (60 Hz, 1.5 s) performed before and after fatigue were analysed for decline in force development (F_max), rate of force development (dF/dt_max) and relaxation (?dF/dt_max). Results: F _max declined to significantly lower values after isotonic than after quasi‐isometric fatiguing contractions (fatigued in percentage of unfatigued): 58.5 ± 6.4% vs. 64.4 ± 7.0% in SOL, and 30.4 ± 4.1% vs. 33.3 ± 3.6% in EDL, respectively. The same pattern was seen for dF/dt_max which decreased to: 46.3 ± 9.9% vs. 52.3 ± 8.5% in SOL, and 19.1 ± 4.3% vs. 22.3 ± 3.2% in EDL after isotonic and quasi‐isometric contractions, respectively. Similarly, when comparing fatigue development during the two contraction modes, the respective fatigue variables decreased more rapidly and to lower levels during isotonic vs. quasi‐isometric contractions. During maximal test contractions, the dynamic fatigue variables (±dF/dt_max) declined to significantly lower levels than F_max. Conclusions: Fatigue development was significantly larger during isotonic vs. quasi‐isometric contractions. The use of force as the only experimental fatigue variable may underestimate the functional impairment of fatigued muscle, neglecting the fatigue effect on time and length dimensions.     

Modulation of corticospinal excitability during lengthening and shortening contractions in the first dorsal interosseus muscle of humans

Lengthening and shortening contractions are the fundamental patterns of muscle activation underlying various movements. It is still unknown whether or not there is a muscle-specific difference in such a fundamental pattern of muscle activation. The purpose of this study was, therefore, to investigate whether or not the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability in the first dorsal interosseus (FDI) muscle is the same as that of previously tested muscles because the hand muscles are anatomically and functionally different from the other muscles. To this end, we investigated the relationship between the input-output curves of the corticospinal pathway (i.e., the relationship between the stimulus intensities vs. the area of motor-evoked potentials) during lengthening and shortening contractions in 17 healthy subjects. The shape of this relationship was sigmoidal and characterized by a plateau value, maximum slope, and threshold. The plateau value was at the same level between lengthening and shortening contractions. However, the maximum slope (P < 0.01) and threshold (P < 0.01) were significantly higher during lengthening contractions than during shortening contractions. These findings were different from the results of other muscles tested in previous studies (i.e., the soleus muscle and the elbow flexors). That is to say, the plateau value and the maximum slope during lengthening contractions were significantly lower than those during shortening contractions in previous studies. This study provides tentative evidence that the relationship between lengthening and shortening contractions in the modulation of corticospinal excitability differs between muscles, indicating that the underlying neural control is not necessarily the same even though the fundamental patterns of muscle activation are carried out.     

Positive proprioceptive feedback elicited by isometric contractions of ankle flexors on pretibial motoneurons in cats

Pretibial flexor motoneurons were recorded intracellularly in anesthetized cats during unfused isometric contractions of a subpopulation of motor units from either tibialis anterior (TA) or extensor digitorum longus (EDL) muscles. The contractions elicited excitatory postsynaptic potentials in 23 of 28 pretibial flexor motoneurons. No effect was observed in the remaining motoneurons. In control experiments, the effects of electrical stimulation of afferents within the TA nerve were investigated to help identify afferents responsible for the contraction-induced positive feedback. This feedback was ascribed to actions of Ia fibers because the pattern of the contraction-induced excitatory potentials was consistent with the known pattern of Ia discharge; in control experiments, electrical stimulation of group I fibers elicited only monosynaptic excitatory potentials; and the distribution of both the contraction-induced positive feedback among motor nuclei as well as the electrically evoked Ia excitatory monosynaptic potentials were restricted to homonymous and synergic motoneurons. Observation of the Ia contraction-induced positive feedback was facilitated by the absence of Ib autogenic inhibition. This contraction-induced Ia excitatory feedback in ankle flexors might either reinforce Ia-induced reflexes when these muscles are lengthened or help to lift the leg over an obstacle.     

Mechanical and energetic properties of dystrophic (mdx) mouse muscle

The mechanical and energetic properties of extensor digitorum longus (EDL) and soleus muscles of X chromosome-linked muscular dystrophic mutant (mdx) mice aged 4-6 weeks were studied and compared with those of the muscles of normal mice. Maximum tetanic tension, the speed of contraction of relaxation, and the heat production of mdx soleus muscles were not significantly different from those of the normal muscles. However, in mdx EDL muscles, the tension and heat production were significantly reduced, and relaxation was prolonged. To study the cause of these changes in mdx EDL muscles, tension and heat production were measured at various muscle lengths greater than optimum for tension. Both the amount of twitch heat and the heat rate for a tetanus were linearly related to the tension and had non-zero intercepts at zero tension, the activation heat. The twitch activation heat and the tension-related heat in tetani of mdx EDL muscles were not different from those in normal muscles. On the other hand, the tetanus activation heat of mdx EDL muscles was significantly smaller than that of normal muscles. Assuming that the degenerated fibers do not contribute to the active force produced, these results suggest that the amount of Ca2+ released in a contraction is not significantly different between normal and mdx muscles, but the Ca-ATPase activity of the salcoplasmic reticulum is reduced in mdx EDL, which could cause the slowing of relaxation.