Type IIx Research Articles

Composição de fibras musculares esqueléticas de eqüinos jovens da raça Brasileiro de Hipismo

The aim of this study was to typify the skeletal striated fibers of the gluteus medius muscle of young Brasileiro de Hipismo (BH) horses by means of histochemical analysis with m-ATPase and NADH-TR according to the sex and the biopsy depth. It was observed that the frequency (F;%) and the relative cross sectional area (RCSA;%) of the fibers type IIX were greater than the fibers type IIA, which F and RCSA were greater than the fibers type I. The comparison between sex and muscles depht, showed no significant difference in F and RCSA in the three types of fibers. The results of morphometry showed that the gluteus medius muscle has greater glycolitic metabolism and anaerobic capacity because of the presence of large proportion of type IIX fibers. This may be justified by the genetic influence of Thoroughbred in the formation of Brasileiro de Hipismo breed.

Effects of exercise and creatine on myosin heavy chain isoform composition in patients with Charcot–Marie–Tooth disease

It is not known whether myosin heavy chain (MHC) content changes in response to exercise training or creatine supplementation in subjects with Charcot-Marie-Tooth disease (CMT). Based on previous data, we hypothesized that resistance exercise and creatine would increase the percentage of type I MHC composition in the vastus lateralis muscle and that myosin isoform changes would correlate with improved chair rise-time in CMT subjects. To test this hypothesis, 18 CMT subjects were randomly assigned to either a placebo or creatine group. All subjects performed a 12-week, home-based, moderate-intensity resistance training program. Chair rise-time was measured before and after the training program. Muscle biopsies were obtained from the vastus lateralis before and after the 12-week program. Gel electrophoresis showed a significant decrease (approximately 30%) in MHC type I in CMT subjects given creatine supplementation when compared with placebo. There was a nonsignificant increase in both MHC type IIa (approximately 23%) and MHC type IIx (approximately 7%) in CMT subjects given creatine. Reduced MHC type I content and increased MHC type IIa content correlated with faster chair rise-times (i.e., improved muscle performance). The training-induced change in MHC IIa content was inversely correlated with chair rise-time in CMT subjects given creatine. When the two subject groups were combined, there was a linear, negative relationship between the change in MHC type IIa content and chair rise-time after training and a positive relationship between the training-induced change in MHC type I content and chair rise-time. These data suggest that improved function (chair rise-time) was associated with a lower level of MHC type I and increased MHC type IIa composition. Furthermore, the data are consistent with the hypothesis that creatine supplementation alters MHC composition in CMT patients undergoing resistance training and that MHC changes associated with creatine supplementation can improve muscle function.

Changes of muscular fibre types in erector spinae and multifidus muscles in the unstable lumbar spine

In a prospective study conducted between the years 1999 and 2002 we harvested 26 valuable muscle samples from patients with an unstable lumbar spine from the multifidus muscle and erector spinae muscle at the level of the L4/L5/S1 segments and evaluated them using histochemical methods. Control samples were taken from six patients with a acute fracture of the low lumbar spine. A statistical difference was evident between the experimental and control group in type I fibres from the erector muscle. There was a percentage increase in type I fibres and a decrease in type IIA fibres from the erector muscle compared to the control group. The percentage of IIX and IIC fibres in the musculus erector spinae was the same in both groups. The investigated multifidus muscle underwent significant changes in type IIA fibres, where the percentage decreased and the percentage of type IIX fibres was almost double in comparison to the control group. Fibres of types I and IIC were not statistically changed. Type IIX fibres react to the exceeding of normal ranges of movement in the functional spinal unit to prevent an abnormal shift and thus stabilize the segment. These changes in the multifidus muscle are the premise of its mostly local stabilizing function, whereas the erector muscle has predominantly postural, long-term stabilisation features because of the increase of type I fibres.

Differences in the Expression of MHC Isoforms in Single Muscle Fibers After A18 wk Cold Adaptaionin Hamsters

PURPOSE: The purpose of this study was to determine alterations in the expression of MHC isoforms in single muscle fibers after a 18 wk cold exposure in hamsters. METHODS: Ten Golden hamsters (Mesocrietus auratus), aged 15∼17 weeks were randomly assigned to two groups: control group (n=5, CO) and cold exposed group (n=5, CE). The CE group was exposed to cold temperature at 5°C±1 for 18 weeks while the CO group was housed for 18 weeks at 22°C±2. Electrophoretic analyses were performed in single muscle fibers of the vastus intermeidus (VI) muscle (n=50 per muscle) as well as in whole VI muscle. RESULTS: Significant increases in body weight, heart weight, heart rate and body temperature were observed in CE group compared to CO group. Interestingly, blood glucose was significantly lowered in CE as compared to CO (42.2±18.6 vs. 97.4±22.2mg/ dL). MHC type IIA disappeared completely in whole muscle (VI) in CE whereas CO possessed 26% in type IIA. However, CE showed significant increases in MHC type IIX and type IIB as compared to CO. As for single fiber analyses, co-expression of MHC type IIA and IIX was significantly increased in CO, but type IIX and IIB was significantly highly co-expressed in CE, suggesting that type IIX is a dominant isoform in CE. CONCLUSIONS: It is plausible that the transformed MHC isoform type IIX is a muscle adaptation to cold environment, yields metabolic efficiency.

Muscle Architecture of the Rat Hindlimb

The rat is a commonly-used experimental model, however, the architectural properties of its muscular system are poorly documented. PURPOSE: To define the architectural properties of the entire rat hindlimb and to compare the design characteristics among muscle groups. METHODS: Thirty-one muscles from each of six paired hindlimbs were dissected from adult male rats (323 g ± 15 g). Measurements of muscle mass, length, fiber length, sarcomere length, and pennation angle were measured directly and used to calculate normalized fiber length and physiological cross-sectional area (PCSA). Contralateral legs were used to quantify myosin heavy chain (MHC) isoform percentages. Muscles groups were classified according to their joint action. Average fiber length, total muscle group PCSA, and average MHC percentage were used to make among group comparisons. Mann-Whitney U-tests were used for statistical comparisons and reported values are mean ± SE. RESULTS: At the hip joint, extensors had significantly longer fiber lengths than flexors (33.0 mm ±2.2 mm vs. 17.6 mm ±2.2 mm) and larger total PCSAs (1.61 cm2 ± 0.05 cm2 vs. 1.38 cm2 ± 0.05 cm2). Flexors had a greater percentage of type IIa (10 % ± 1 % vs. 8 % ± 1 %) and IIx (25 % ± 2 % vs. 19 % ± 1 %) fibers while extensors had a greater percentage of type IIb (72 % ± 2 % vs. 63 %± 2 %) fibers. At the knee joint, flexors had longer fiber lengths than extensors (35.0 mm ±1.8 mm vs. 14.9 mm ±0.8 mm) and smaller total PCSAs (1.39 cm2 ± 0.04 cm2 vs. 1.69 cm2 ± 0.07 cm2). Knee extensors also had greater type I (9 % ± 2 % vs. 2 % ± 1 %), IIa (14 % ± 1 % vs. 8 % ± 1 %), and IIx (25 % ± 1 % vs. 18 % ± 1 %) percentages while flexors had a greater type IIb (72 % ± 2 % vs. 53 % ± 3 %) percentage. At the ankle joint, plantarflexors had a trend towards longer fiber lengths (16.3 mm ±0.8 mm vs. 13.9 mm ±0.6 mm, P = 0.064) and significantly greater total PCSA (1.45 cm2 ±0.11 cm2vs. 0.49 cm2 ±0.03 cm2). Plantarflexors also had a significantly greater type I percentage (24 % ± 67 % vs. 1 % ± 1%) while dorsiflexors had a significantly greater type IIb percentage (60 % ± 3 % vs. 42 % ± 5 %). CONCLUSIONS: Consistent with other their antigravity function, total extensor muscle group PCSA was larger compared to the total flexor muscle group PCSA at each joint. However, muscle fiber length differences were less distinct, with hip extensors, knee flexors, and ankle plantarflexors, tending to have longer fibers. MHC percentages were also less distinct, with hip extensors, knee flexors, and ankle dorsiflexors having the greater percentage of type IIb fibers. These data will form the foundation for understanding muscle group functional differences in the rat hindlimb.

Temporal Response of Desmin and Dystrophin to Progressive Resistance Exercise

PURPOSE: Our purpose was to investigate the adaptations of the cytoskeletal proteins desmin and dystrophin in relationship to known muscular adaptations of resistance exercise. We measured desmin, dystrophin, and actin protein contents, myosin heavy chain (MHC) isoform distribution, muscle strength, and muscle cross-sectional area (CSA) during 8 weeks of progressive resistance training and following a single bout of unaccustomed resistance exercise. METHODS: Muscle biopsies were taken from the vastus lateralis of 12 untrained men a week prior to exercise (week 0) and at weeks 1,2,4, and 8 following a single bout of exercise (n = 6), or at weeks 1, 2, 4 and 8 of the progressive resistance training program (n = 6). Desmin, dystrophin, and actin protein levels were determined with immunoblotting, and MHC isoform distribution was determined using SDS PAGE at each time point for each group. RESULTS: In the training group, the desmin:actin ratio significantly increased compared to week 0 beginning at week 4 (0.054 to 0.095, P < 0.0001) and continuing through week 8 (0.054 to 0.094, P < 0.0001). The desmin:actin ratio did not change at any time point for the single bout group. Actin and dystrophin protein contents were not changed in either group at any time point. The percentage of MHC type IIa increased and MHC type IIx decreased at week 8 in the training group with no changes occurring in the single bout group. Strength was significantly increased by week 2 (knee extension) and week 4 (leg press), and further increased at week 8 for both these exercises in the training group only. Muscle CSA was significantly increased at week 4 for type II fibers in the training group only (5719+/−382 and 6582+/−640, weeks 0 and 4 respectively, P < 0.05). Finally, a significant negative correlation was observed between the desmin:actin ratio and the percentage of MHC IIx (R = −0.31, P < 0.05, all time points from both groups). CONCLUSION: These data suggest a time course for muscular adaptation to resistance training in which desmin increases shortly after strength gains and in conjunction with hypertrophy, but prior to changes in MHC isoforms, while dystrophin remains unchanged.

Efficacy of 3 days/wk resistance training on myofiber hypertrophy and myogenic mechanisms in young vs. older adults

Resistance training (RT) has shown the most promise in reducing/reversing effects of sarcopenia, although the optimum regime specific for older adults remains unclear. We hypothesized myofiber hypertrophy resulting from frequent (3 days/wk, 16 wk) RT would be impaired in older (O; 60-75 yr; 12 women, 13 men), sarcopenic adults compared with young (Y; 20-35 yr; 11 women, 13 men) due to slowed repair/regeneration processes. Myofiber-type distribution and cross-sectional area (CSA) were determined at 0 and 16 wk. Transcript and protein levels of myogenic regulatory factors (MRFs) were assessed as markers of regeneration at 0 and 24 h postexercise, and after 16 wk. Only Y increased type I CSA 18% (P < 0.001). O showed smaller type IIa (-16%) and type IIx (-24%) myofibers before training (P < 0.05), with differences most notable in women. Both age groups increased type IIa (O, 16%; Y, 25%) and mean type II (O, 23%; Y, 32%) size (P < 0.05). Growth was generally most favorable in young men. Percent change scores on fiber size revealed an age x gender interaction for type I fibers (P < 0.05) as growth among Y (25%) exceeded that of O (4%) men. Myogenin and myogenic differentiation factor D (MyoD) mRNAs increased (P < 0.05) in Y and O, whereas myogenic factor (myf)-5 mRNA increased in Y only (P < 0.05). Myf-6 protein increased (P < 0.05) in both Y and O. The results generally support our hypothesis as 3 days/wk training led to more robust hypertrophy in Y vs. O, particularly among men. However, this differential hypertrophy adaptation was not explained by age variation in MRF expression.

The expression of IGF‐I and myostatin mRNAs in skeletal muscle of hypophysectomized and underfed rats during postnatal growth

To determine the roles of myostatin and insulin-like growth factor-I (IGF-I) during postnatal growth, we examined IGF-I and myostatin mRNA expression in the skeletal muscles of hypophysectomized and underfed rats during postnatal growth. Five-week-old rats were divided into four groups: freely fed control, moderately underfed, severely underfed and hypophysectomized. Four weeks later, blood and muscle samples were gathered to determine serum IGF-I, myosin heavy chain (MHC) isoforms, IGF-I Ea, IGF-I Eb and myostatin mRNA. The weights of soleus, plantaris and masseter muscles were decreased in underfed and hypophysectomized rats. Hypophysectomy resulted in significant increases of type I MHC at the expense of type IIx in plantaris muscle and of neonatal MHC at the expense of types IIx and IIb in masseter muscle. Serum IGF-I was decreased by underfeeding and hypophysectomy. Plantaris muscle IGF-I Ea mRNA in underfed and hypophysectomized rats was significantly lower than in normal controls. Plantaris muscle IGF-I Eb mRNA in underfed rats was significantly lower than in normal controls. Masseter muscle IGF-I Eb mRNA in severely underfed rats was significantly lower than in normal control and hypophysectomized rats. Soleus muscle myostatin mRNA in hypophysectomized rats was significantly higher than in normal and significantly underfed rats. No significant differences in plantaris and masseter muscle myostatin mRNA were observed between groups. Suppressed muscle growth caused by hypophysectomy and underfeeding may be attributed mainly to reduced circulating IGF-I and partially to reduced IGF-I mRNA, rather than to a change in myostatin.

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.

Type IIx myosin heavy chain promoter activity during hindlimb immobilization

Myosin heavy chain (MyHC) is an important skeletal muscle contractile protein that is often regulated at the transcriptional level. During atrophy, fast IIx MyHC mRNA levels are increased 35 fold in young, predominantly slow-twitch soleus muscle. The purpose of this study was to determine if the 5′-flanking region of the rat MyHC IIx gene contains an atrophy-response element. 1007 base pairs of the 5′-flanking region (−1 kb promoter) of the rat MyHC IIx gene were cloned, and deletion mutations were made. Intramuscular injections of plasmid DNA containing the firefly luciferase gene under control of the rat MyHC IIx promoter were made into the soleus of 2-mo-old rats undergoing normal cage activity or 6 days of hindlimb immobilization (HI). Aliquots of soleus muscle homogenates were used for firefly luciferase activity and plasmid DNA isolation for normalization (using real-time PCR with primers and probe designed against the firefly luciferase gene). The −1 and −0.889 kb promoter activities were 1.46-fold and 1.3-fold higher, respectively, in HI soleus compared to control. Deletion to −0.754 and −0.617 kb decreased promoter activity 70% and 30%, respectively, in HI soleus compared to control. In summary, the −1 kb MyHC IIx promoter is responsive to HI, and deletional analyses suggest that the MyHC IIx promoter potentially contains an atrophy-response element between −0.889 and −0.754 kb. Sponsored by the National Institutes of Health: 5F32AR051640-02 (RAS).

Age and sex affect human muscle fibre adaptations to heavy‐resistance strength training

This study assessed age and sex effects on muscle fibre adaptations to heavy-resistance strength training (ST). Twenty-two young men and women (20-30 years old) and 18 older men and women (65-75 years old) completed 9 weeks of heavy-resistance knee extension exercises with the dominant leg 3 days week(-1); the non-dominant leg served as a within-subject, untrained control. Bilateral vastus lateralis muscle biopsies were obtained before and after ST for analysis of type I, IIa and IIx muscle fibre cross-sectional area (CSA) and fibre type distribution. One-repetition maximum (1-RM) strength was also assessed before and after ST. ST resulted in increased CSA of type I, IIa and IIx muscle fibres in the trained leg of young men, type I and IIa fibres in young women, type IIa fibres in older men, and type IIx fibres in older women (all P<0.05). Analysis of fibre type distribution revealed a significant increase in the percentage of type I fibres (P<0.05) along with a decrease in type IIx fibres (P=0.054) after ST only in young women. There were no significant changes in muscle fibre CSA or fibre type distribution in the untrained leg for any group. All groups displayed significant increases in 1-RM (27-39%; all P<0.01). In summary, ST led to significant increases in 1-RM and type II fibre CSA in all groups; however, age and sex influence specific muscle fibre subtype responses to ST.

Temporal response of desmin and dystrophin proteins to progressive resistance exercise in human skeletal muscle

We have investigated the adaptations of the cytoskeletal proteins desmin and dystrophin in relationship to known muscular adaptations of resistance exercise. We measured desmin, dystrophin, and actin protein contents, myosin heavy chain (MHC) isoform distribution, muscle strength, and muscle cross-sectional area (CSA) during 8 wk of progressive resistance training or after a single bout of unaccustomed resistance exercise. Muscle biopsies were taken from the vastus lateralis of 12 untrained men. For the single-bout group (n=6) biopsies were taken 1 wk before the single bout of exercise (week 0) and 1, 2, 4, and 8 wk after this single bout of exercise. For the training group (n=6), biopsies were taken 1 wk before the beginning of the program (week 0) and at weeks 1, 2, 4, and 8 of the progressive resistance training program. Desmin, dystrophin, and actin protein levels were determined with immunoblotting, and MHC isoform distribution was determined using SDS-PAGE at each time point for each group. In the training group, desmin was significantly increased compared with week 0 beginning at week 4 (182% of week 0; P<0.0001) and remained elevated through week 8 (172% of week 0; P<0.0001). Desmin did not change at any time point for the single-bout group. Actin and dystrophin protein contents were not changed in either group at any time point. The percentage of MHC type IIa increased and MHC type IIx decreased at week 8 in the training group with no changes occurring in the single-bout group. Strength was significantly increased by week 2 (knee extension) and week 4 (leg press), and it further increased at week 8 for both these exercises in the training group only. Muscle CSA was significantly increased at week 4 for type II fibers in the training group only (5,719+/-382 and 6,582+/-640 microm2, weeks 0 and 4, respectively; P<0.05). Finally, a significant negative correlation was observed between the desmin-to-actin ratio and the percentage of MHC IIx (R=-0.31; P<0.05, all time points from both groups). These data demonstrate a time course for muscular adaptation to resistance training in which desmin increases shortly after strength gains and in conjunction with hypertrophy, but before changes in MHC isoforms, whereas dystrophin remains unchanged.

Denervation effects on myonuclear domain size of rat diaphragm fibers

Denervation (DNV) of rat diaphragm muscle (DIAm) leads to selective atrophy of type IIx and IIb fibers, whereas the cross-sectional area of type I and IIa fibers remains unchanged or slightly hypertrophied. DIAm DNV also increases satellite cell mitotic activity and myonuclear apoptosis. Similar to other skeletal muscles, DIAm fibers are multinucleated, and each myonucleus regulates the gene products in a finite fiber volume, i.e., myonuclear domain (MND). MND size varies across DIAm fiber types in rank order, I < IIa < IIx < IIb [fiber type based on myosin heavy chain isoform expression]. We hypothesized that, after DNV, the total number of myonuclei per fiber does not change and, accordingly, that MND changes proportionately to the change in fiber size regardless of fiber type. Adult rats underwent unilateral (right side) DIAm DNV, and after 2 wk single fibers were dissected. Fiber cross-sectional area, myonuclear number, and MND were measured by confocal microscopy, and these values in DNV DIAm were compared with those obtained in controls. After DNV, type I fibers hypertrophied, type IIa fiber size was unchanged, and type IIx and IIb fibers atrophied compared with control. The total number of myonuclei per fiber was not affected by DNV. Accordingly, after DNV, type I fiber MND increased by 25%, whereas it decreased in type IIx and IIb fibers by 50 and 70%, respectively. These results suggest that MND is not maintained after DNV-induced DIAm fiber hypertrophy or atrophy. These results are interpreted with respect to consequent effects of DNV on myonuclear transcriptional activity and protein turnover.

トレーニングに対するサラブレッド骨格筋の適応

The thoroughbred horse is a remarkable animal, with both speed and endurance abilities during running. Muscle adaptation associated with training and growth has been examined using various physiological, histochemical and biochemical measurements. We have emphasized the importance of type IIX fiber recruitment in training program for thoroughbred. Because type IIX fiber provides the greatest contribution to total muscle volume in the gluteus medius muscle and increases in oxidative enzymes of the fibers would markedly improve the whole-muscle oxidative capacity. Based on these concepts, we examined the adaptations of type IIX fiber including hybrid fiber to various training programs.It is considered that scientific information about optimal training of these excellent abilities can provide significant insights into exercise physiology and health science not only for thoroughbred horses, but also for human beings.

Intramyocellular lipid and glycogen content are reduced following resistance exercise in untrained healthy males

Resistance exercise has recently been shown to improve whole-body insulin sensitivity in healthy males. Whether this is accompanied by an exercise-induced decline in skeletal muscle glycogen and/or lipid content remains to be established. In the present study, we determined fibre-type-specific changes in skeletal muscle substrate content following a single resistance exercise session. After an overnight fast, eight untrained healthy lean males participated in a approximately 45 min resistance exercise session. Muscle biopsies were collected before, following cessation of exercise, and after 30 and 120 min of post-exercise recovery. Subjects remained fasted throughout the test. Conventional light and (immuno)fluorescence microscopy were applied to assess fibre-type-specific changes in intramyocellular triacylglycerol (IMTG) and glycogen content. A significant 27+/-7% net decline in IMTG content was observed in the type I muscle fibres (P<0.05), with no net changes in the type IIa and IIx fibres. Muscle glycogen content decreased with 23+/-6, 40+/-7 and 44+/-7% in the type I, IIa and IIx muscle fibres, respectively (P<0.05). Fibre-type-specific changes in intramyocellular lipid and/or glycogen content correlated well with muscle fibre-type oxidative capacity. During post-exercise recovery, type I muscle fibre lipid content returned to pre-exercise levels within 120 min. No changes in muscle glycogen content were observed during recovery. We conclude that intramyocellular lipid and glycogen stores are readily used during resistance exercise and this is likely associated with the reported increase in whole-body insulin sensitivity following resistance exercise.

Lateral rectus muscle changes after bilateral neonatal labyrinthectomy in the ferret

The vestibular system is essential to the coordination of eye movements during head movements. Exercise, such as the eye movements mediated by the vestibular system, is a major factor in the development of muscle fiber types and the strength of muscle. In this study, the contents of the inner ear were removed (labyrinthectomy) from (LAB) ferrets at postnatal day 10 (P10) and raised with control (SHAM) animals. At P30, the lateral rectus muscles (LR) were removed to analyze the expression of myosin heavy chain (MHC) isoforms and to measure the least diameter of the developmental, slow and fast type muscle fibers. Another set of animals were sacrificed at P120 to analyze MHC isoform expression and muscle fiber diameters, as well as the contractile characteristics of the LR were measured prior to sacrifice. The average LAB LR was significantly stronger than the SHAM LR and the muscle fibers of the LAB animals were larger in diameter. The LAB animals expressed more type IIx and less slow type MHC. These results support the hypothesis that input from the inner ear influence the development of strength and muscle diameter in the ferret extraocular muscles.

Neonatal bilabyrinthectomy leads to changes in skeletal muscle fiber form and function

Children with hypoactive semicircular canals commonly have delayed motor development. We studied ferrets whose vestibular labyrinths were removed at postnatal day 10 (P10) to evaluate 1) development of motor behavior, 2) soleus contractile characteristics, 3) muscle fiber diameter, and 4) the expression of myosin heavy chain (MHC) isoforms. At P30 labyrinthectomized (LAB) ferrets exhibited a significant delay in motor development. At P120, the LAB ferrets continued to have significantly more difficulty with dynamic balance than the SHAM ferrets. At P30, the muscle fibers were about the same diameter as those in the sham operated ferrets (SHAM) at the same age but significantly more developmental MHC isoforms were expressed in the LAB soleus muscle. At P120, both the LAB soleus and medial gastrocnemius expressed significantly more type IIX MHC. The P120 LAB soleus muscle was significantly weaker and the muscle fibers were significantly smaller. These results support the hypothesis that vestibular receptors are important to the development of upright posture and balance activities, and influence muscle fiber development and MHC expression in postural muscles.

Fasted‐state skeletal muscle protein synthesis after resistance exercise is altered with training

The purpose of the present investigation was to determine how fasted-state protein synthesis was affected, acutely, by resistance training. Eight men (24.8+/-1.7 years, body mass index=23.2+/-1.0 kg m-2; means+/-s.e.m.) undertook an 8 week programme of unilateral resistance exercise training (3 sessions week-1, progression from two to four sets; intensity was 80% of the subjects' single repetition maximum (1RM): knee extension and leg press). Following training, subjects underwent two primed constant infusions of l-[ring-13C6]phenylalanine to determine mixed and myofibrillar muscle protein synthesis (MPS) at rest and 12 h after an acute bout of resistance exercise at the same exercise intensity--each leg 80% of 1RM. Biopsies (vastus lateralis) were taken to measure incorporation of labelled phenylalanine into mixed and myofibrillar skeletal muscle proteins and yield fractional MPS. Training resulted in significant dynamic strength gains that were greater (P<0.001) in the trained leg. Hypertrophy of type IIa and IIx fibres (P<0.05) was observed following training. After training, resting mixed MPS rate was elevated (+48%; P<0.05). Acutely, resistance exercise stimulated mixed MPS only in the untrained leg (P<0.05). Myofibrillar MPS was unchanged at rest following training (P=0.61). Myofibrillar MPS increased after resistance exercise (P<0.05), but was not different between the trained and untrained legs (P=0.36). We observed divergent changes in resting mixed versus myofibrillar protein synthesis with training. In addition, resistance training modified the acute response of MPS to resistance exercise by dampening the increased synthesis of non-myofibrillar proteins while maintaining the synthesis of myofibrillar proteins.

Coupled expression of troponin T and troponin I isoforms in single skeletal muscle fibers correlates with contractility

Striated muscle contraction is powered by actin-activated myosin ATPase. This process is regulated by Ca(2+) via the troponin complex. Slow- and fast-twitch fibers of vertebrate skeletal muscle express type I and type II myosin, respectively, and these myosin isoenzymes confer different ATPase activities, contractile velocities, and force. Skeletal muscle troponin has also diverged into fast and slow isoforms, but their functional significance is not fully understood. To investigate the expression of troponin isoforms in mammalian skeletal muscle and their functional relationship to that of the myosin isoforms, we concomitantly studied myosin, troponin T (TnT), and troponin I (TnI) isoform contents and isometric contractile properties in single fibers of rat skeletal muscle. We characterized a large number of Triton X-100-skinned single fibers from soleus, diaphragm, gastrocnemius, and extensor digitorum longus muscles and selected fibers with combinations of a single myosin isoform and a single class (slow or fast) of the TnT and TnI isoforms to investigate their role in determining contractility. Types IIa, IIx, and IIb myosin fibers produced higher isometric force than that of type I fibers. Despite the polyploidy of adult skeletal muscle fibers, the expression of fast or slow isoforms of TnT and TnI is tightly coupled. Fibers containing slow troponin had higher Ca(2+) sensitivity than that of the fast troponin fibers, whereas fibers containing fast troponin showed a higher cooperativity of Ca(2+) activation than that of the slow troponin fibers. These results demonstrate distinct but coordinated regulation of troponin and myosin isoform expression in skeletal muscle and their contribution to the contractile properties of muscle.

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.