Myosin Heavy Chain Composition Research Articles

Desmin protein content differs between fiber types in human single muscle fibers

Muscle fibers are composed of various proteins that generate and transmit force. These proteins are integral in determining the structural and functional properties of muscle fibers. Differences in force production and recruitment patterns between type I and II muscle fibers may be a factor in determining cytoskeletal protein content. Contractile and cytoskeletal protein concentrations have been shown to differ on the basis of fiber type in whole muscle homogenates. The purpose of this study was to compare the content of the intermediate filament protein desmin between type I and type IIa single muscle fibers from a mixed muscle in human subjects. Biopsies were taken from the vastus lateralis of six recreationally active males. Approximately 150 single muscle fibers were dissected from each sample and analyzed using SDS-PAGE to determine myosin heavy chain (MHC) composition. Following identification, muscle fibers were pooled into two groups (MHC I and MHC IIa). Desmin and actin content within the pooled samples was determined via immunoblotting. On average, muscles samples were composed of 51+\−7 % type I, 2+\−1% type I/IIa, 27+\−5% type IIa, 19+\−4% type IIa/IIx and 1+\−1% type IIx MHC single fibers. Type II fibers had a larger cross-sectional area compared to type I (5574+\−169 vs. 3955+\−94 μm2, respectively) (P<0.05). Desmin and actin contents were 40% and 37% higher in type I fibers compared to type IIa fibers, respectively (P<0.05). However the desmin to actin ratio was similar between pooled type I and IIa single muscle fibers within the vastus lateralis. These data suggest that desmin and actin content is a function of muscle fiber type. These differences in cytoskeletal protein content may have implications for differences in contractile function and eccentric damage characteristics between fiber types.

Myosin heavy chain expression and muscle adaptation to chronic oral breathing in rat

The purpose of this study was to establish if early chronic oral breathing could induce an ultra-structural adaptation of the diaphragm and orofacial muscles related to oral or nasal breathing. Therefore, we performed a bilateral nasal obstruction at day 8 on rat pups and the myosin heavy chain (MHC) composition of the muscles was analyzed at day 21. Nasal obstruction and the related switch to chronic oral breathing were associated with impaired growth, atrophy of olfactory bulbs, hypertrophy of adrenal glands and reduced muscle growth for all muscles studied except the diaphragm. Furthermore, we detected a smaller decrease of MHC 2b compared to MHC 2a and 2x in levator nasolabialis, a muscle involved with nasal breathing. In masseter superficialis and anterior digastric involved with oral breathing, we observed a smaller decrease of MHC 2a compared to MHC 2b or 2x, respectively. No difference was detected in the diaphragm MHC expression of oral breathing animals. Since the relative expression of fatigue resistant MHC fiber types increased in muscles involved with oral breathing, orofacial muscles seem to present a profile in MHC adapted to the transition from nasal to oral breathing, facilitating respiration.

Sex Differences in Single Muscle Fiber Power in Older Adults

This study was conducted to determine whether differences in power at the single muscle fiber level contribute to sex differences in whole muscle power production in the elderly. A total of 16 sedentary older persons (10 women, 6 men), mean age 72 yr, had percutaneous needle biopsy of musculus vastus lateralis. Chemically skinned single muscle fibers were activated with Ca for maximal isometric force measurement (Po). The slack test was performed to determine maximal unloaded shortening velocity (Vo). Force-velocity and power curves were generated via a series of isotonic contractions, allowing measurement of peak power and specific power. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) was used to determine myosin heavy chain composition of single muscle fibers. Whole muscle strength, velocity, and power were measured for knee extension and double leg press. Men had greater whole muscle strength, power, and velocity compared with women. Studied were 274 type I and 33 type IIa single fibers. No significant sex differences were found for fiber size, Po, specific force, Vo, power, or specific power in type I or IIa fibers. Single muscle fiber quality in older women is equivalent to that in older men and can not explain the differences seen in whole muscle strength, power, or function.

Myosin heavy chain and fibre diameter of extrinsic tongue muscles in rhesus monkey

The purpose of this investigation was to identify the myosin heavy chain (MHC) phenotype and fibre diameters of hypoglossal innervated extrinsic tongue muscles in rhesus monkey. Genioglossus, styloglossus and hyoglossus muscle samples obtained from three female rhesus monkeys were analysed for MHC isoforms via gel electrophoresis and stained with MHC antibodies to measure least mean diameters. MHC phenotypes were consistent for all three muscles. Each muscle was predominantly composed of MHC type IIa and I. All three isoforms were significantly different from each other in fibre diameter for styloglossus and genioglossus (IIb>IIa and IIx>I; P<0.001). For hyoglossus, the MHC type II isoforms had larger diameters than the MHC type I isoform (P<0.001). While the extrinsic tongue muscle MHC and/or muscle fibre type composition may be different between mammalian species, there are consistent similarities between the intrinsic and extrinsic tongue muscles. We suggest this is necessary for the highly coordinated activities performed by the tongue such as mastication, respiration and swallowing. The differences in fibre diameters among MHC isoforms suggest a large force gradation, which would be consistent with the coordination of these activities. The similarities among primates in MHC and/or muscle fibre composition as well as similar cortical inputs to the hypoglossal nucleus, suggest that we could expect to see similar MHC phenotype for extrinsic tongue muscles in human.

New insights into skeletal muscle fibre types in the dog with particular focus towards hybrid myosin phenotypes

Electrophoresis, immunoblots, immunohistochemistry and image analysis methods were applied to characterise canine trunk and appendicular muscle fibres according to their myosin heavy chain (MyHC) composition and to determine, on a fibre-to-fibre basis, the correlation between contractile [MyHC (s), myofibrillar ATPase (mATPase) and sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) isoforms], metabolic [succinate dehydrogenase (SDH) and glycerol-3-phosphate dehydrogenase (GPDH) activities and glycogen and phospholamban (PLB) content] and morphological (cross-sectional area and capillary and nuclear densities) features of individual myofibres. An accurate delineation of MyHC-based fibre types was obtained with the developed immunohistochemical method, which showed high sensitivity and objectivity to delineate hybrid fibres with overwhelming dominance of one MyHC isoform. Phenotypic differences in contractile, metabolic and morphological properties seen between fibre types were related to MyHC content. All canine skeletal muscle fibre types had a relatively high histochemical SDH activity but significant differences existed in the order IIA>I>IIX. Mean GPDH was ranked according to fibre type such that I<IIA<IIX. Type IIA fibres were the smallest, type IIX fibres the largest and type I of intermediate size. Capillary and nuclear density decreased in the order IIA>I>IIX. Hybrid fibres, which represented nearly one third of the whole pool of skeletal muscle fibres analysed, had mean values intermediate between their respective pure phenotypes. Slow fibres expressed the slow SERCA isoform and PLB, whereas type II fibres expressed the fast SERCA isoform. Discrimination of myofibres according to their MyHC content was possible on the basis of their contractile, metabolic and morphological features. These intrafibre interrelationships suggest that myofibres of control dogs exhibit a high degree of co-ordination in their physiological, biochemical and morphological characteristics. This study demonstrates that canine skeletal muscle fibres have been misclassified in numerous previous studies and offers useful baseline data and new prospects for future work on muscle-fibre-typing in canine experimental studies.

Intrinsic Properties of the Adult Human Mylohyoid Muscle: Neural Organization, Fiber-Type Distribution, and Myosin Heavy Chain Expression

The mylohyoid (MH) muscle plays a critical role in chewing, swallowing, respiration, and phonation. The present study was designed to test the hypothesis that the functional properties of the MH are reflected by its intrinsic specializations, including the neural organization, fiber-type distribution, and myosin heavy chain (MHC) expression. Adult human MH muscles were investigated to determine the nerve supply pattern using Sihler's stain, banding pattern and types of motor endplates using acetylcholinesterase (AChE) staining and silver impregnation, and muscle fiber type and MHC composition using immunocytochemical and immunoblotting techniques. The adult human MH was found to have the following neuromuscular specializations. First, the muscle was innervated by several branches of the MH nerve derived from the mandibular division of the trigeminal nerve. Each of the nerve branches supplied a distinct region of the muscle, forming a segmental innervation pattern. Second, the MH had a single motor endplate band which was located in the middle of the muscle length. Both en plaque and en grappe types of motor endplates were identified on the MH muscle fibers. Finally, the adult human MH fibers expressed unusual MHC isoforms (i.e., slow-tonic, alpha-cardiac, embryonic, and neonatal) which coexisted with the major MHC isoforms (i.e., slow type I, fast type IIa, and fast type IIx), thus forming various major/unusual (or m/u) MHC hybrid fiber types. The m/u hybrid fibers (84% of the total fiber population) were the predominant fiber types in the adult MH muscle. Determination of the neuromuscular specializations of the MH is helpful for better understanding of the muscle functions and for development of strategies to treat MH-related upper airway disorders.

Effects of 84‐days of bedrest and resistance training on single muscle fibre myosin heavy chain distribution in human vastus lateralis and soleus muscles

This investigation determined the effects of 84 days of bedrest on the composition of myosin heavy chain (MHC) in single skeletal muscle fibres with and without a resistance-training countermeasure programme. Muscle biopsies were obtained from the m. vastus lateralis (VL) and m. soleus (SOL) before and after 84 days of bedrest. While control (BR) subjects (VL n = 9; SOL n = 3) refrained from exercise, BRE subjects (VL n = 8; SOL n = 3) performed knee extensor and plantar flexor resistance exercise every third day. Approximately 110 fibres per sample were analysed for MHC composition using SDS-PAGE. BR-VL had 16 and 14% decreases (P < 0.05) in MHC I and IIa fibres, respectively. There were 10% increases (P < 0.05) in MHC I/IIa, IIa/IIx, I/IIa/IIx, and a approximately 30% increase (P < 0.05) in total hybrid fibres. BRE-VL showed a 15% reduction (P < 0.05) in MHC I fibres, no change in MHC IIa fibres, and a 13% increase (P < 0.05) in total hybrids. BR-SOL had a 19% decrease (P < 0.05) in MHC I fibres with a 22% increase in total hybrids. BRE-SOL showed no change in MHC composition across all fibre types. These data suggest that the exercise countermeasures programme prevented MHC shifts in the SOL and mitigated MHC shifts in the VL. Furthermore, in the VL it appears that the resistance training programme employed in this investigation during bedrest, emphasized the use of MHC IIa phenotype muscle fibres.

Effects of inactivity on fiber size and myonuclear number in rat soleus muscle

The effects of short-term (4 days) and long-term (60 days) neuromuscular inactivity on myonuclear number, size, and myosin heavy chain (MHC) composition of isolated rat soleus fibers were determined using confocal microscopy and gel electrophoresis. Inactivity was produced via spinal cord isolation (SI), i.e., complete spinal cord transections at a midthoracic and a high sacral level and bilateral deafferentation between the transection sites. Compared with control, there was an increase in the percentage of fibers containing the faster MHC isoforms after 60, but not 4, days of SI. The mean sizes of type I and type I+IIa fibers were 41 and 27% and 66 and 56% smaller after 4 and 60 days of SI, respectively. Thus atrophy occurred earlier than the shift in myosin heavy chain (MHC) profile. The number of myonuclei was approximately 30% higher in type I than type I+IIa fibers in control soleus, but after 60 days of SI these values were similar. The number of myonuclei per millimeter in type I fibers was significantly lower than control after 60 days of SI, whereas there was no change in type I+IIa fibers. Thus myonuclei were eliminated from fibers containing only type I MHC. Because the magnitude of the loss of myonuclei was less than the level of atrophy, the myonuclear domains of both type I and type I+IIa fibers were significantly lower than control. Thus chronic (60 days) inactivity results in smaller, faster fibers that contain a higher than normal amount of DNA per unit of cytoplasm. The absence of activation of muscle fibers that are normally the most active (pure type I fibers) resulted in most, but not all, fibers expressing some fast MHC isoforms. The results also indicate that a loss of myonuclei is not a prerequisite for sustained muscle fiber atrophy.

Botulinum neurotoxin type A causes shifts in myosin heavy chain composition in muscle

Botulinum neurotoxin type A has gained widespread use for treatment of a host of neuromuscular conditions. However, the potential effect of this toxin has on the histological and biochemical properties of skeletal muscle remains largely unexplored. The purpose of this study was to characterize the myosin heavy chain (MHC) distribution of adult rat skeletal muscle treated with botulinum neurotoxin type. Varying doses of the toxin were injected into the triceps surae muscle group of one hind limb. Force production was assessed periodically to access the functional deficit incurred. After 10 weeks, animals were sacrificed, muscles removed, and MHC composition determined. Body weight, muscle weight and force of the injected leg were significantly reduced in all groups, while loss of muscle weight and force in the contralateral leg was variable. In the injected plantaris and gastrocnemius muscles, type I MHC increased ∼100%, while type IIa/x decreased ∼50%. In the contralateral gastrocnemius, types I and IIa/x MHC increased ∼100%, while type IIb decreased ∼45%. These data suggest that botulinum neurotoxin causes shifts in MHC composition in injected and contralateral muscles that are contrary to those seen with denervation and similar to those seen with aging.

The expression of neuregulin and erbB receptors in human skeletal muscle: effects of progressive resistance training

The neuregulin/erbB-signaling axis contributes to the development and growth of multiple mammalian tissues including skeletal muscle. In this study, we sought to characterize the native expression of this system in human skeletal muscle and test the hypothesis that a program of progressive resistance training (PRT) would regulate the expression of neuregulin (NRG) and its cognate receptors. Twelve healthy-male subjects underwent 8-weeks of lower-extremity PRT and muscle biopsies were performed at baseline and following 1- and 8-weeks of the intervention. PRT resulted in significant gains in skeletal muscle strength without appreciable changes in fiber size or myosin heavy chain (MHC) composition. At baseline, Western Blot analysis demonstrated expression of erbB2, erbB3 and erbB4 receptors and multiple NRG isoforms. Following 1- and 8-weeks of PRT, no changes erbB2, erbB4 or NRG expression were observed. ErbB3 expression, however, was significantly increased at both time points compared to baseline. Double labeling of muscle cross-sections revealed increased expression of erbB3 following PRT was not exclusive to fibers staining positive for MHC IIa. Thus, erbB2, erbB3, erbB4 and multiple NRG isoforms are natively expressed in human skeletal muscle. Following PRT, a significant increase in erbB3 was observed. The ability to detect basal expression and alterations in response to physiologic stimuli merit further studies examining the role of this system in skeletal muscle.

Effect of Resistance Exercise on Skeletal Muscle Myopathy in Heart Transplant Recipients

The purpose of this study was to determine the efficacy of resistance exercise in reversing skeletal muscle myopathy in heart transplant recipients. Myopathy, engendered by both heart failure and immunosuppression with glucocorticoids, is a post-transplant complication. The sequelae of myopathic disease includes fiber-type shifts and deficits in aerobic metabolic capability. We randomly assigned patients to either 6 months of resistance exercise (training group; n = 8) or a control (control group; n = 7) group. Exercise was initiated at 2 months after transplant. Biopsy of the right vastus lateralis was performed before and after the 6-month intervention. Myosin heavy chain (MHC) composition was assessed using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Biochemical assays were performed to determine citrate synthase, 3-hydroxyacyl-CoA-dehydrogenase, and lactate dehydrogenase activity. There were no group differences (p >or=0.05) in MHC composition and enzymatic reserve at baseline. Improvements in the training group for citrate cynthase (+40%), 3-hydroxyacyl-CoA-dehydrogenase (+10%), and lactate dehydrogenase activity (+48%) were significantly greater (p <or=0.05) than in the control group (+10%, -15%, and +20%, respectively). Oxidative type 1 MHC isoform concentration increased significantly in the training group (+73%, p <or=0.05) but decreased in the control group (-28%; p <or=0.05). Glycolytic type 2x MHC isoform increased significantly (17%; p <or=0.05) in the control group but decreased (-33%; p <or=0.05) in the training group. This is the first study to demonstrate that resistance training elicits myofibrillar shifts from less oxidative type II fibers to more oxidative fatigue-resistant type I fibers in heart transplant recipients. Resistance exercise initiated early in the post-transplant period is efficacious in changing skeletal muscle phenotype through increases in enzymatic reserve and shifts in fiber morphology.

Glucose Metabolism And MHC Isoforms Expression In Skeletal Muscle In Leptin Transgenic Skinny Mice

Leptin is an adipocyte-derived hormone and may play an important role in the regulation of energy homeostasis. Previous studies have suggested that the overexpression of leptin activates AMPK in skeletal muscles to increase glucose and lipid metabolism. PURPOSE To determine whether the overexpression of leptin can change myosin heavy chain (MHC) isoforms coordinatedly with the metabolic enzyme activities. METHODS Male leptin transgenic mice (n=5, Tg) and the control (n=7, Cont) were used. The composition of MHC isoforms (type I, typeIIa/x and type IIb) of gascrocnemius (Gas), tibialis anterior (TA), plantaris (Pl), extensor digitorum longus (EDL), and soleus muscles (Sol) were determined by SDS-PAGE. Citrate synthase (CS), hydroxyacyl-CoA dehydrogenase (HAD) and lactate dehydrogenase (LDH) activities of the deep portion of the TA muscles were also measured. RESULTS There were no significant differences in MHC isoforms expression in any muscles between Tg and Cont, but in the average type IIb MHC to type IIa/x shift was observed. The ratio of CS to LDH activity was significantly higher in Tg group than in Cont (0.262 vs. 0.224, p < 0.013), though there were no significant difference in the CS, HAD and LDH activities between groups (p > 0.05). CONCLUSIONS From these data we concluded that the overexpression of leptin increased glucose metabolism more aerobically, whereas the MHC composition did not change.

Glucose Metabolism And MHC Isoforms Expression In Skeletal Muscle In Leptin Transgenic Skinny Mice

Leptin is an adipocyte-derived hormone and may play an important role in the regulation of energy homeostasis. Previous studies have suggested that the overexpression of leptin activates AMPK in skeletal muscles to increase glucose and lipid metabolism. PURPOSE To determine whether the overexpression of leptin can change myosin heavy chain (MHC) isoforms coordinatedly with the metabolic enzyme activities. METHODS Male leptin transgenic mice (n=5, Tg) and the control (n=7, Cont) were used. The composition of MHC isoforms (type I, typeIIa/x and type IIb) of gascrocnemius (Gas), tibialis anterior (TA), plantaris (Pl), extensor digitorum longus (EDL), and soleus muscles (Sol) were determined by SDS-PAGE. Citrate synthase (CS), hydroxyacyl-CoA dehydrogenase (HAD) and lactate dehydrogenase (LDH) activities of the deep portion of the TA muscles were also measured. RESULTS There were no significant differences in MHC isoforms expression in any muscles between Tg and Cont, but in the average type IIb MHC to type IIa/x shift was observed. The ratio of CS to LDH activity was significantly higher in Tg group than in Cont (0.262 vs. 0.224, p < 0.013), though there were no significant difference in the CS, HAD and LDH activities between groups (p > 0.05). CONCLUSIONS From these data we concluded that the overexpression of leptin increased glucose metabolism more aerobically, whereas the MHC composition did not change.

Voluntary Wheel Running In Rats Receiving Doxorubicin

The chemotherapy drug doxorubicin (DOX) is effective at combating cancerous cell growth and proliferation, but its dosing is limited because chronic usage can result in cardiac dysfunction with eventual heart failure. Previously, we have shown that endurance training prior to DOX administration ameliorates DOX-induced cardiac dysfunction which translates into improved DOX-tolerance in a trained organism. However, the likelihood of training during DOX treatment to prevent cardiac maladaptation has yet to be explored. Additionally, it has been shown that DOX treatment promotes a shift in cardiac myosin heavy chain (MHC) expression from the fast á-isoform to the slow â-isoform. PURPOSE To analyze the effects of voluntary wheel running on cardiac MHC composition in rats receiving DOX. METHODS Female Sprague-Dawley rats were randomly assigned to the voluntary running (n = 9) or the sedentary (n = 8) group. Following a 2-week running wheel acclimation period, animals received weekly IP injections of DOX (2.5 mg/kg) for 6 weeks. The voluntary running group had access to running wheels throughout the treatment period. Following 6 weeks of DOX, left ventricular free walls from the animals were analyzed for MHC composition using SDS-PAGE. RESULTS The week prior to DOX injections, the rats ran a mean of 10.4±5.1 km/ day, and following week 3 of DOX, animals ran a mean of 6.7±3.8 km/day (36% decrease). Following 6 weeks of treatment, animals ran a mean of 4.1±3.0 km/day (61% decrease). The voluntary exercise group expressed a mean á-MHC percentage of 75.5±9.9% which was significantly greater than the mean á-MHC percentage of 56.8±18.5% detected in sedentary animals (p = 0.01). CONCLUSIONS Increased physical activity during DOX treatment preserved contractile MHC isoform distribution in the left ventricle. This suggests that increased physical activity during DOX treatment may help protect the heart from cardiac dysfunction.

Endurance Exercise Does Not Enhance Dystrophic Progression In Heart And Diaphragm Of mdx Mice

Duchenne Muscular Dystrophy (DMD) is a genetic disease that results in deletion of the dystrophin protein, affects approximately 1 in 3,000 male births, and typically leads to death by the late teens to early twenties. Absence of dystrophin leads to muscle wasting and cardiomyopathy due to compromised plasma membrane structure and impaired force transduction. Endurance exercise training involving cardiovascular overload may aid in maintenance of muscle mass and preservation of function. PURPOSE To examine the endurance exercise training-induced responses in diaphragm and cardiac muscles of mdx animals, an accepted model of DMD. METHODS Male C57/BL10 mice served as the healthy control group and age-matched male mdx mice served as the dystrophic group. Four groups of 9 animals were studied: C57 sedentary, C57 swim, mdx sedentary and mdx swim. Mice were progressively trained 5 days/week for 10 weeks, and forelimb grip strength measurements were taken weekly. Following training, diaphragm and cardiac muscles were excised. Myosin heavy chain (MHC) composition, regeneration of myofibers, and collagen content were examined in diaphragm. Cardiac cross sections were examined for evidence of necrosis. Grip strength over time was analyzed using repeated measures ANOVA. All other comparisons between groups were made using ANOVA RESULTS Compared with non-swim trained mice, mdx and C57 swim-trained mice had increased relative forelimb grip strength (4.76±0.07 vs. 5.41±0.09 g/g and 5.09±0.09 vs. 5.36±0.07 g/g respectively, p < 0.05). Diaphragm muscles of swim-trained mdx mice demonstrated increased type IIa (28% vs. 44%, p < 0.05) and decreased type IIx (50% vs. 34%, p < 0.05) MHC proportions. There were no differences between non-swim trained and swim trained mdx mice in proportion of diaphragm fibers displaying centrally located nuclei to normal myofibers (29.9±4.3 vs. 38.7±5.6%), indicative of regeneration, or hydroxyproline content (0.44±0.07 vs. 0.55±0.09 μg/g), indicating collagen accumulation. Furthermore, swimming did not stimulate significant increases in cardiomyocyte necrosis ratings (1.4±0.3 vs. 1.9±0.7) compared with non-swim trained mdx mice. CONCLUSIONS The present swim training protocol led to improved skeletal muscle function in mdx mice as measured by grip strength and adaptations in diaphragm MHC isoform profile. Furthermore, the exercise stimulus did not result in additional damage to diaphragm or cardiac muscles of the dystrophic animals.

Endurance Exercise Does Not Enhance Dystrophic Progression In Heart And Diaphragm Of mdx Mice

Duchenne Muscular Dystrophy (DMD) is a genetic disease that results in deletion of the dystrophin protein, affects approximately 1 in 3,000 male births, and typically leads to death by the late teens to early twenties. Absence of dystrophin leads to muscle wasting and cardiomyopathy due to compromised plasma membrane structure and impaired force transduction. Endurance exercise training involving cardiovascular overload may aid in maintenance of muscle mass and preservation of function. PURPOSE To examine the endurance exercise training-induced responses in diaphragm and cardiac muscles of mdx animals, an accepted model of DMD. METHODS Male C57/BL10 mice served as the healthy control group and age-matched male mdx mice served as the dystrophic group. Four groups of 9 animals were studied: C57 sedentary, C57 swim, mdx sedentary and mdx swim. Mice were progressively trained 5 days/week for 10 weeks, and forelimb grip strength measurements were taken weekly. Following training, diaphragm and cardiac muscles were excised. Myosin heavy chain (MHC) composition, regeneration of myofibers, and collagen content were examined in diaphragm. Cardiac cross sections were examined for evidence of necrosis. Grip strength over time was analyzed using repeated measures ANOVA. All other comparisons between groups were made using ANOVA RESULTS Compared with non-swim trained mice, mdx and C57 swim-trained mice had increased relative forelimb grip strength (4.76±0.07 vs. 5.41±0.09 g/g and 5.09±0.09 vs. 5.36±0.07 g/g respectively, p < 0.05). Diaphragm muscles of swim-trained mdx mice demonstrated increased type IIa (28% vs. 44%, p < 0.05) and decreased type IIx (50% vs. 34%, p < 0.05) MHC proportions. There were no differences between non-swim trained and swim trained mdx mice in proportion of diaphragm fibers displaying centrally located nuclei to normal myofibers (29.9±4.3 vs. 38.7±5.6%), indicative of regeneration, or hydroxyproline content (0.44±0.07 vs. 0.55±0.09 μg/g), indicating collagen accumulation. Furthermore, swimming did not stimulate significant increases in cardiomyocyte necrosis ratings (1.4±0.3 vs. 1.9±0.7) compared with non-swim trained mdx mice. CONCLUSIONS The present swim training protocol led to improved skeletal muscle function in mdx mice as measured by grip strength and adaptations in diaphragm MHC isoform profile. Furthermore, the exercise stimulus did not result in additional damage to diaphragm or cardiac muscles of the dystrophic animals.

Fiber Content and Myosin Heavy Chain Composition of Muscle Spindles in Aged Human Biceps Brachii

The present study investigated potential age-related changes in human muscle spindles with respect to the intrafusal fiber-type content and myosin heavy chain (MyHC) composition in biceps brachii muscle. The total number of intrafusal fibers per spindle decreased significantly with aging, due to a significant reduction in the number of nuclear chain fibers. Nuclear chain fibers in old spindles were short and some showed novel expression of MyHC alpha-cardiac. The expression of MyHC alpha-cardiac in bag1 and bag2 fibers was greatly decreased in the A region. The expression of slow MyHC was increased in nuclear bag1 fibers and that of fetal MyHC decreased in bag2 fibers whereas the patterns of distribution of the remaining MyHC isoforms were generally not affected by aging. We conclude that aging appears to have an important impact on muscle spindle composition. These changes in muscle spindle phenotype may reflect an age-related deterioration in sensory and motor innervation and are likely to have an impact in motor control in the elderly.

Calcineurin and heat shock protein 72 in functionally overloaded rat plantaris muscle

The involvement of calcineurin (CaN) and heat shock protein (Hsp) 72 in the regulation of fiber size and/or phenotype in response to functional overload (FO) was investigated. In one FO group, the plantaris muscle was overloaded by cutting the distal tendons (5–10 mm length) of the soleus and gastrocnemius of 3-week-old male Wistar rats. Cyclosporin A (CsA), a CaN inhibitor, was injected daily (5 mg/kg body weight, i.p.) in a second group of FO rats (FO + CsA group) for a 2-week period. Compared to age-matched controls (Con), the absolute and relative plantaris weights were increased in both FO groups: the hypertrophic response was attenuated in FO + CsA rats. The mean cross-sectional area of each fiber type was increased (∼2.0-fold) in the plantaris of FO rats: CsA treatment attenuated this effect, although the fibers were still larger than in Con rats. The percent composition of myosin heavy chain (MHC) IIb decreased from 54% in Con to 19% in FO rats, whereas types I, IIa, and IIx MHC increased in the FO rats. CsA treatment blunted the shifts in MHC isoforms: the FO + CsA group showed a smaller decrease in type IIb and a smaller increase in type IIx MHC than the FO group. The levels of CaN-A and -B proteins were higher (∼2.5-fold) in FO than Con rats, whereas these values were similar in Con and FO + CsA rats. Hsp72 protein levels were higher in FO (3.6-fold) and FO + CsA (5.2-fold) than Con rats, with the values being significantly higher in the FO + CsA than FO rats. CsA treatment in Con rats had no effects on muscle mass, fiber size, MHC composition, and Hsp72 or CaN levels. Combined, these results suggest that CaN levels are related to changes in both fiber size and phenotype, and that Hsp72 levels are more related to the levels of stress added to the muscle rather than to increases in the slow fiber phenotype in functionally overloaded rat plantaris muscles.

Skeletal muscle microbiopsy: a validation study of a minimally invasive technique

The study of the peripheral skeletal muscle function in patients with chronic obstructive pulmonary disease (COPD) is of growing interest, but often requires biopsies, usually with the Bergström technique. The current study was designed to test the validity of a minimally invasive technique: the microbiopsy. In 17 patients with COPD and four normal subjects, two specimens of the vastus lateralis were taken percutaneously under local anaesthesia, one with a 16-gauge needle (microbiopsy) and the other with the Bergström needle. The enzymatic activity of citrate synthase (CS) and phosphofructokinase (PFK), and the myosin heavy chain (MyoHC) composition were measured for both techniques. The subjects reported no pain or much less with the microbiopsy compared with the Bergström biopsy. The microbiopsy sample weight reached 55+/-17 mg. The two techniques showed excellent agreement for CS activity and MyoHC composition. The PFK activity did not differ statistically between the techniques, but the agreement was moderate. The agreement between both biopsy techniques was stable over time. The median (range) fibre number within the microbiopsy specimens was 144 (38-286). In conclusion, the current study shows the feasibility and validity of a minimally invasive muscle biopsy technique that appears more comfortable for subjects, compared with the Bergström technique.

Myosin heavy chain composition and the economy of contraction in healthy and diseased human myocardium

Changes in myosin heavy chain (MHC) isoform expression and protein composition occur during cardiac disease and it has been suggested that even a minor shift in MHC composition may exert a considerable effect on myocardial energetics and performance. Here an overview is provided of the cellular basis of the energy utilisation in cardiac tissue and novel data are presented concerning the economy of myocardial contraction in diseased atrial and ventricular human myocardium. ATP utilisation and force development were measured at various Ca(2+) concentrations during isometric contraction in chemically skinned atrial trabeculae from patients in sinus rhythm (SR) or with chronic atrial fibrillation (AF) and in ventricular muscle strips from non-failing donor or end-stage failing hearts. Contractile protein composition was analysed by one-dimensional gel electrophoresis. Atrial fibrillation was accompanied by a significant shift from the fast alpha-MHC isoform to the slow beta-MHC isoform, whereas both donor and failing ventricular tissue contained almost exclusively the beta-MHC isoform. Simultaneous measurements of force and ATP utilisation indicated that economy of contraction is preserved in atrial fibrillation and in end-stage human heart failure.