Recovery from fatigue in fast and slow single intact skeletal muscle fibers from aging mouse

In the present work, we studied the recovery from fatigue (RF) of single intact fast- and slow-twitch muscle fibers from young (age 5--7 months) and old (age 22--24 months) mice. To examine whether differences in RF underlie decreases in muscle strength and endurance with aging, we performed in vitro experiments in manually dissected extensor digitorum longus (EDL) and soleus muscle fibers. We measured the recovery of the maximum force every 5 min for a total period of 30 min after inducing fiber fatigue. Fibers were classified, according to the fatigue index, into the following three groups: 0.75--0.99, 0.5--0.74, and <0.5. Although the tetanic tension of EDL and soleus fibers from young and old mice recovered significantly, no statistically significant difference in tension or recovery time was observed between age groups. These data support the concept that the reported decline in muscle force and endurance with aging is not related to changes in RF of individual muscles fibers.     

Effect of hind-limb suspension on young and adult skeletal muscle. I. Normal mice

The purpose of this study was to determine the effect of hind-limb suspension (HS) on morphometric, histologic, and contractile characteristics of fast extensor digitorum longus (EDL) and slow soleus (SOL) twitch muscles in adult and immature mice. Hind-limb suspension for 2 weeks was used to produce atrophy in two groups of mice, ages 4 and 12 weeks, with nonsuspended animals serving as controls. Young HS mice exhibited marked decreases in SOL weight, length, cross-sectional area (CSA), twitch and tetanic tensions, and rates of tension development and relaxation, with increases in fatigue resistance. HS reduced the diameter of both type I and IIA fibers, increased the percentage of type I fibers, and decreased the percentage of type IIA fibers in both young and adult SOL. Muscle weight, length, CSA, IIA and IIB fiber areas, and maximum rate of tetanic tension development were decreased in EDL of young HS mice; fatigue resistance and EDL half-relaxation times were increased. For most parameters evaluated, slow twitch muscle was more affected than fast twitch. HS affected contractile characteristics less than morphometric or histologic parameters. Rates of tension development and relaxation were the contractile parameters most affected by HS, and the time parameters of contraction were least affected. For all measurements young mice were more affected than adult mice.     

Potassium and caffeine contractures of mouse muscles before and after fatiguing stimulation

To assess the impairment of muscle membrane excitation, excitation–contraction (E–C) coupling, and contractility during muscle fatigue, we monitored the contracture responses of resting and fatigued muscles on exposure to high potassium and caffeine. On exposure to 140 mmol/L potassium, mouse extensor digitorum longus (EDL) developed a contracture which was 15.7% of tetanic tension before fatigue and 31.7% after fatigue, while soleus developed 59.4% contracture before and 68.8% after fatigue. Potassium causes contractures by depolarizing the muscle fiber membrane. Hence, membrane excitation is reduced in fatigued EDL and soleus. On exposure to 32 mmol/L caffeine, the contracture was 7.1% in resting EDL, 8.5% in fatigued EDL, 50.1% in resting soleus, and 43.7% in fatigued soleus. On exposure to 1 mmol/L caffeine followed by rapid cooling, the contracture was 3.0% in resting EDL, 3.2% in fatigued EDL, 21.5% in resting soleus, and 10.3% in fatigued soleus. Caffeine causes contracture by releasing Ca^{+ +} from the sarcoplasmic reticulum. Our results indicate reduced E–C coupling attributable to reduced membrane excitation in fatigued EDL, and reduced contractility in fatigued soleus. © 1994 John Wiley & Sons, Inc.     

Isometric contractile properties of fast- and slow-twitch muscles in normal and spastic mice

Isometric contractile properties of the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of 24 male C57 mice were studied in vitro at 35°C. Ten animals exhibited the hereditary movement disorder known as spasticity. EDL muscles of spastic and normal mice developed similar specific tensions and they had similar twitch tension:tetanic tension ratios. The time-to-peak tension and the time to half relaxation in the twitch of spastic EDL muscles were, on average, slightly longer. SOL muscles of spastic and normal mice also developed similar specific tensions and their twitch contractions had similar times to half-relaxation. The time-to-peak tension in the twitch and the twitch tension: tetanic tension ratio of spastic SOL muscles were, on average, higher than in normal SOL. The significance of our findings remains somewhat uncertain.     

Knee extensor fatigue resistance of young and older men and women performing sustained and brief intermittent isometric contractions

Introduction: Susceptibility to muscle fatigue during aging could depend on muscle activation patterns. Methods: Young (mean age, 22 years) and older (mean age 70 years) men and women completed two fatigue tests of knee extensor muscles using voluntary and electrically stimulated contractions. Results: Older subjects displayed a shift to the left of the torque‐frequency relationship and held a sustained voluntary isometric contraction at 50% maximal strength for significantly longer than young (P < 0.001). Young and old showed similar fatigue during electrically induced, intermittent isometric contractions (1‐s on, 1‐s off for 2 min), but women fatigued less than men (P = 0.001). Stronger muscles fatigued more quickly, and slower contractile properties were associated with longer sustained contractions. Conclusions: The slowing and weakness of older muscle was associated with superior fatigue resistance during sustained isometric contractions. Young and old showed similar fatigue following a series of brief, intermittent contractions, but women fatigued less than men. Muscle Nerve 50 : 393–400, 2014     

Peripheral mechanisms of fatigue in muscles of normal and dystrophic mice.

We evaluated the contribution of different processes to fatigue of normal and dystrophic mouse muscles using an in vitro electromyography chamber. Fatigue was induced by repetitive nerve stimulation at 30 Hz for 0.5 s, every 2.5 s until tension decreased by about 50%. We monitored the compound nerve action potential (AP), compound muscle AP, and isometric tension responses to nerve stimulation, and compound muscle AP and tension responses to direct muscle stimulation. In normal mice, about 50% reduction in nerve-evoked tension occurred by 2.4 min in extensor digitorum longus (EDL), 4.8 min in diaphragm, and 9 min in soleus. Analysis of the responses revealed that the fatigue was caused by failure of more than one process in all muscles, and failure of nerve conduction did not contribute to fatigue in any muscle. Failure of neuromuscular transmission, muscle membrane excitation, and excitation-contraction (E-C) coupling and contractility accounted for 55, 45, and 0%, respectively, of the fatigue in EDL, for 21, 74, and 5% of the fatigue in diaphragm, and for 2, 54, and 44% of the fatigue in soleus. In dystrophic mice, while about 50% reduction in nerve-evoked tension occurred by 8.1 min in EDL and 5.6 min in diaphragm, only 29% reduction in tension occurred by 80 min in soleus. Failure of neuromuscular transmission, muscle membrane excitation, E-C coupling and contractility accounted for 22, 63 and 15% of the fatigue in EDL, for 21, 79, and 0% of the fatigue in diaphragm, and for 15, 59, and 26% of the fatigue in soleus. The proportion of slow-twitch oxidative fibers was more than normal in dystrophic EDL, but the same as normal in dystrophic diaphragm and soleus. The slower onset of fatigue was attributable to lesser failure of neuromuscular transmission in dystrophic EDL, and to lesser failure of E-C coupling and contractility in dystrophic soleus.     

Sustained transmitter output by increased transmitter turnover in limb muscles of old mice

The ability of neuromuscular junctions in old animals to maintain tetanic output was tested in phasic and tonic limb muscles and the physiologic mechanism of maintenance was elucidated by analysis of the turnover of a false transmitter during prolonged tetani. Transmitter release during and after tetani was compared in limb muscles of young (8-9 month) and old (28-30 month) male CBF-1 mice. Amplitudes of end-plate potentials (epp's) in curarized preparations and of spontaneous miniature end-plate potentials (mepp's) were measured in vitro at 30 degrees C in soleus and extensor digitorum longus (edl) muscles. In both young and old soleus muscles, epp amplitude was maintained at about 45% of resting level during the latter part of trains of 1,200 stimuli at 10 Hz but recovered to about 90% control within a few seconds after stimulation ceased. In edl muscles of young mice, epp amplitudes during a 20 Hz train of 1,200 impulses steadily declined to about 20% of control and gradually recovered over 2 min after the tetanus. In old edl muscles, tetanic decay of the epp's was greater and recovery slower than in young muscles, but absolute epp amplitudes were invariably greater. During trains of 6,000 impulses at 10 Hz, plateau epp amplitude decayed to 40-50% in young soleus muscle and 30-40% control in old muscle, but recovery was similar and absolute epp amplitudes were greater in old soleus muscle. A false transmitter precursor, homocholine (HoCh), was used to investigate the mechanism of this prolonged output, and, therefore, the use of HoCh in this system was first validated.(ABSTRACT TRUNCATED AT 250 WORDS)     

Force and stiffness of old dystrophic (mdx) mouse skeletal muscles

It has recently been suggested, based on studies of tissue pathology, that the limb muscles of old mdx mice may be a good model for the muscular changes seen in human Duchenne muscular dystrophy. To test this hypothesis, we measured force and stiffness of soleus and extensor digitorum longus (EDL) muscles of old (20–21 months) mdx mice and age-matched controls. The mdx and control muscles generated similar twitch, tetanic, and eccentric forces. They were also equally stiff. The results show that the mechanics of aged mdx limb muscles differ greatly from Duchenne muscular dystrophy in humans, and disagree with the hypothesis. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:536–539, 1998.     

Contractile properties of slow and fast skeletal muscles from protease activated receptor‐1 null mice

Introduction: Protease‐activated receptors (PARs) may play a role in skeletal muscle development. We compared the contractile properties of slow‐twitch soleus muscles and fast‐twitch extensor digitorum longus (EDL) muscles from PAR‐1 null and littermate control mice. Methods: Contractile function was measured using a force transducer system. Fiber type proportions were determined using immunohistochemistry. Results: Soleus muscles from PAR‐1 null mice exhibited longer contraction times, a leftward shift in the force–stimulation frequency relationship, and decreased fatiguability compared with controls. PAR‐1 null soleus muscles also had increased type 1 and decreased type IIb/x fiber numbers compared with controls. In PAR‐1 null EDL muscles, no differences were found, except for a slower rate of fatigue compared with controls. Conclusions: The absence of PAR‐1 results in a slower skeletal muscle contractile phenotype, likely due to an increase in type I and a decrease in type IIb/x fiber numbers. Muscle Nerve 50: 991–998, 2014     

Carnitine and acyltransferase in experimental neurogenic atrophies: changes with treatment

Summary Carnitine level and carnitine palmityl transferase (CPT) activity were investigated in muscles of patients with infantile and juvenile spinal muscular atrophy and polyneuropathies. A significant decrease of both carnitine and CPT was found in the infantile spinal muscular atrophy, but not in the other neurogenic muscle atrophies. These findings were compared with the experimental effect of denervation and reinnervation upon the lipid metabolism in soleus and extensor digitorum longus (EDL) of adult and newborn rats. Twenty-one days after denervation free and total carnitine decreased significantly in both EDL (P<0.001) and soleus (P<0.05) of adult animals. CPT activity was significantly decreased in the soleus 50 days after denervation (P<0.005). Long-term reinnervation restored the level of carnitine fraction and CPT activity. l-carnitine treatment for 21 days restored the level of free carnitine to normal in the soleus of denervated adult animals. Denervation in newborn rats influenced carnitine concentration in soleus and EDL to a lesser extent; the treatment with l-carnitine raised short-chain acylcarnitines in denervated muscles, while reinnervation restored carnitine level within 50 days.Presented as a preliminary report at the Fifth International Congress on Neuromuscular Diseases, Marseille, France, September 1982     

Effect of muscle creatine content manipulation on contractile properties in mouse muscles

The effects of muscle creatine manipulation on contractile properties in oxidative and glycolytic muscles were evaluated. Whereas control mice (NMRi; n = 12) received normal chow (5 g daily), three experimental groups were created by adding creatine monohydrate (CR group; 5%, 1 week; n = 13); beta-guanidinoproprionic acid, an inhibitor of cellular creatine uptake (beta-GPA group; 1%, 2 weeks; n = 12); or CR following beta-GPA (beta-GPA+CR group; n = 11). Total creatine (TCr) and the contractile properties of incubated soleus and extensor digitorum longus (EDL) muscles were determined. For the soleus, compared with control, TCr increased in the CR group (+25%), decreased in beta-GPA group (-50%), and remained stable in the beta-GPA+CR group, whereas, for the EDL, TCr was similar in the CR, and lower in the beta-GPA (-40%) and beta-GPA+CR (-15%) groups. None of the experimental groups (CR, beta-GPA, or beta-GPA+CR) showed changes in peak tension (P(peak)), time to peak tension, or relaxation in soleus or EDL during twitch or tetanic stimulation. For the soleus, fatigue reduced P(peak) to approximately 60% of initial P(peak); 5 min of recovery restored P(peak) to values approximately 15% higher in CR than in controls. P(peak) recovery was not affected by beta-GPA or beta-GPA+CR in the soleus or any treatment in the EDL. Thus, peak tension recovery is enhanced by creatine intake in oxidative but not glycolytic muscles. This may be implicated in the beneficial action of creatine loading.     

Choline acetyltransferase and acetylcholinesterase activities in muscles of aged mice

The activities of choline acetyltransferase (CAT) and acetylcholinesterase (AChE) were assayed in intact diaphragm, extensor digitorum longus (EDL), and soleus muscles or their homogenates of young (2-6 months) and aged (24-34 months) mice. CAT activity (per mg of protein) was significantly higher in diaphragm and soleus of old mice in comparison with the young but the age change in EDL was negligible. On the other hand, AChE activity (per mg of protein) was significantly higher in EDL of old mice but in diaphragm and soleus muscles the enzyme activity did not show any significant change statistically. The diaphragm muscle was divided into two fractions, one being neuromuscular (NM) fraction and the other the remainder of the muscle (M fraction). No appreciable change in the ratio of the enzyme activities of NM fraction to the one of M fraction was obtained between the young and aged preparations. Thus, it seems likely that there is an age-related change in CAT and AChE activities which might be affected by the degree to which muscle activity is maintained.     

Slowing of fast-twitch muscle in the dystrophic mouse

Isometric twitch tension was measured in fast-twitch and slow-twitch muscles of normal and dystrophic ( ) mice in vivo. In dystrophic mice more than 6 months old the fast-twitch extensor digitorum longus (EDL) showed a prolongation of the time to peak tension as well as the time to relax to one-half peak tension ( ) compared with age-matched controls. In younger dystrophic mice (4 to 6 weeks) the time to peak tension was prolonged but not significantly so. This apparent “slowing” of dystrophic fast-twitch muscle was accompanied by a reduction in both cooling potentiation and post-tetanic potentiation toward values typical of slow-twitch muscle. Slow-twitch soleus muscle (SOL) of old mice was almost unaffected by the dystrophic process with regared to its contractile characteristics. However, there appeared to be a slight, but significant “speeding” of young dystrophic SOL compared with age-matched control muscles. This was apparent in reduced times to peak tension and half-relaxation as well as an enhanced cooling potentiation. We suggest that the altered contractile characteristics result from a change in some intrinsic property of the muscle fibers rather than from extrinsic factors such as the additional perimysial connective tissue seen in these muscles.     

The effect of age and temperature on MDX muscle fatigue

We have studied the in vitro contractile and fatigue characteristics of extensor digitorum longus (EDL) muscles from 8- and 62-week-old dystrophin-deficient (mdx) and control mice at 20°C and 35°C. There were no differences in fatigability at 20°C, but at 35°C the dystrophin-deficient muscles demonstrated increased fatigability compared to controls, with the older mice exhibiting the greatest fatigue. These results suggest a temperature-related mechanism of myofibrillar fatigue in dystrophin-deficient EDL muscles. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:1075–1077, 1998.     

Effect of hind-limb suspension on young and adult skeletal muscle. II. Dystrophic mice

Disuse atrophy induced by limb immobilization reportedly protects dystrophic mouse muscle from histopathological changes. This study was conducted to determine whether disuse atrophy induced by hind-limb suspension (HS) limits the histopathology and contractile abnormalities typically observed in the dystrophic mouse. Two weeks of hind-limb suspension were applied to dystrophic mice (line 129B6F1) at two ages, 4 weeks (6 mice) and 12 weeks (8 mice). Thirty-one untreated dystrophics served as controls. In general, HS exaggerated the dystrophic signs, especially in the younger mice; it reduced animal weight, muscle weight, maximum tetanic and twitch tensions, and rates of tetanic and twitch tension development. HS further slowed the contractile properties of soleus (SOL) and extensor digitorum longus (EDL) muscles, and increased their fatigue resistance. HS reduced the size of type I and IIA fibers in the 6-week SOL and EDL, but not in the 14-week muscles. HS produced a preferential atrophy of SOL type I fibers, with a parallel increase in type IIA fibers. However, it did not alleviate the fiber size variability, degree of necrosis, central nucleation, inflammation, or muscle fibrosis in dystrophic muscles. These data demonstrate that disuse by hind-limb suspension does not prevent the histopathological deterioration or loss of muscle function in 6- and 14-week dystrophic mice.     

Fatiguability and oxidative capacity of forelimb and hind limb muscles of dystrophic mice

Fatigue indices and succinic dehydrogenase (SDH) activities were determined in the extensor digitorum longus (EDL) and soleus muscles of the hind limb and the extensor carpi radialis longus of the forelimb in control and dystrophic mice aged 4 to 26 weeks. A good correlation was found between SDH activities and fatigue indices in muscles from normal mice. In the dystrophic (dy2J) mice, however, this correlation was not present. The EDL muscles from 26-week-old dy2J mice showed a much higher resistance to fatigue than age-matched controls but this was not accompanied by a significant change in SDH. The increased fatigue resistance in dy2J EDL appeared between 8 and 12 weeks of age and was temporally correlated with the onset of fused bursts of spontaneous activity in the hind limb muscles. Nevertheless, there was no conclusive evidence for a link among this spontaneous activity, oxidative enzyme capacity, and fatigue resistance.     

Dystrophic mouse muscles have leaky cell membranes

We examined the spaces occupied by mannitol, sucrose, polyethylene glycol molecular weight 900 (PEG 900), and polyethylene glycol molecular weight 4000 (PEG 4000) in the soleus and extensor digitorum longus (EDL) muscles of normal and dystrophic mice (strain ReG-129 $\text{dy}\text{dy}$). The space occupied by all those tracers was larger in dystrophic than in normal muscles. The enlargement of tracer space was more marked in EDL than in soleus muscles. The smaller tracers (sucrose, mannitol, and PEG 900) equilibrate with nearly all the water in dystrophic EDL whereas there is a water space inaccesible to the larger tracers (inulin and PEG 4000). We suggest that the tracers penetrate into the cell through defects in the cell membrane of the dystrophic muscles.     

Sex differences in time to task failure and blood flow for an intermittent isometric fatiguing contraction

The purpose of this study was to compare the time to task failure, postcontraction hyperemia, and vascular conductance of young men and women for a submaximal intermittent fatiguing contraction performed with the handgrip muscles. Twenty men and 20 women (mean ± SD: 22 ± 4 years) performed an isometric contraction at 50% of maximal voluntary contraction (MVC) (6‐s contraction, 4‐s rest) until task failure. Forearm venous occlusion plethysmography was used to estimate the peak blood flow (after 10‐min occlusion) and blood flow at rest after 6‐s submaximal contractions of varying intensities, and during an intermittent fatiguing contraction at 1‐min intervals and task failure. The time to task failure was longer for the women compared with the men (408 ± 205 s vs. 297 ± 57 s, P < 0.05). Postcontraction hyperemia and vascular conductance were greater for men than for women after nonfatiguing 6‐s submaximal contractions performed at 20%, 40%, 50%, 60%, and 80% of MVC force (P < 0.05). In contrast, hyperemia and vascular conductance were similar for both genders when measured at 50 s into the fatiguing contraction, at each minute thereafter, and at task failure. Regression analysis indicated that the rate of electromyographic activity and perceived exertion were the significant predictors of the time to task failure. The longer time to task failure for women compared with men for an intermittent fatiguing contraction with handgrip muscles was not explained by postcontraction hyperemia or vascular conductance with fatigue. © 2008 Wiley Periodicals, Inc. Muscle Nerve 39: 42–53, 2009     

Influence of temperature on isometric tension development in mouse fast- and slow-twitch skeletal muscles

The influence of temperature (range 35 to 20°C) on the isometric contractile properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of the male C57 mice was studied in vitro, with direct stimulation. Cooling from 35 to 25°C resulted in an average twitch tension potentiation of 42% in EDL and 13% in SOL muscles. Further cooling to 20°C did not produce further potentiation in EDL muscles. The time-to-peak tension and the time to half-relaxation increased 2.4 to 3.2 times for a 10°C cooling in both muscles. The maximum tetanic tension was little changed in cooling from 35 to 30°C, but was depressed 16 to 19% in cooling to 20°C in both muscles. These results from mouse fast and slow muscles were compared with previously published data from muscles of the rat and the cat. The behaviors of the slow SOL muscles in the three species were found to be significantly different, but they could be explained on the basis of their muscle fiber-type composition.