Type IIx Myosin Heavy Chain Research Articles

Effect of heart transplantation on skeletal muscle metabolic enzyme reserve and fiber type in end‐stage heart failure patients

Skeletal muscle myopathy is a hallmark of chronic heart failure (HF). Phenotypic changes involve shift in myosin heavy chain (MHC) fiber type from oxidative, MHC type I, towards more glycolytic MHC IIx fibers, reductions in oxidative enzyme activity, and increase in glycolytic enzyme activity. However, it is unknown if muscle myopathy is reversed following heart transplantation. The purpose of this study was to determine the effect of heart transplantation on skeletal muscle metabolic enzyme reserve and MHC fiber type in end-stage HF patients. Thirteen HF subjects were prospectively studied before and two months after heart transplantation and a subgroup (n = 6) at eight months after transplantation. Skeletal muscle biopsy of the vastus lateralis was performed and relative MHC composition was determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Lactate dehydrogenase (LDH), citrate synthase (CS), and 3-hydroxyacyl-CoA-dehydrogenase (HACoA) enzyme activity assays were performed to assess glycolytic, oxidative, and beta-oxidative metabolic enzyme reserves, respectively. Lactate dehydrogenase activity (130.5 +/- 13.3 vs. 106.1 +/- 13.2 micromol/g wet wt/min, p < 0.05), CS activity (14.0 +/- 1.2 vs. 9 +/- 0.9 micromol/g wet wt/min, p < 0.05), and HACoA activity (4.5 +/- 0.48 vs. 3.6 +/- 0.3 micromol/g wet wt/min, p < 0.05) decreased two months after heart transplantation. At eight months, LDH activity was restored (139.0 +/- 11 micromol/g wet wt/min), but not CS or HACoA activity compared with before transplantation. There was no significant change in muscle %MHC type I (28.7 +/- 3.5% vs. 25.3 +/- 3.0%, p = NS), %MHC type IIa (33.2 +/- 2.0% vs. 34.6 +/- 1.9%, p = NS), or %MHC type IIx (38.1 +/- 2.8% vs. 40.1 +/- 3.7%, p = NS) fiber type two months after heart transplantation. However, %MHC type I (19.3 +/- 6.6%) was decreased and %MHC type IIx (51.0 +/- 6.5%) was increased at eight months after (p < 0.05) compared with before transplantation. Skeletal muscle glycolytic, oxidative, and beta-oxidative enzymatic reserves are diminished early after heart transplantation, with reduced oxidative capacity persisting late in the first year. The myopathic MHC phenotype present in end-stage HF persists early in the post-operative state and declines further by eight months.

The Transcriptional Coactivator PGC-1β Drives the Formation of Oxidative Type IIX Fibers in Skeletal Muscle

Skeletal muscle must perform different kinds of work, and distinct fiber types have evolved to accommodate these. Previous work had shown that the transcriptional coactivator PGC-1alpha drives the formation of type I and IIA muscle fibers, which are "slow-twitch" and highly oxidative. We show here that transgenic expression of PGC-1beta, a coactivator functionally similar to but distinct from PGC-1alpha, causes a marked induction of IIX fibers, which are oxidative but have "fast-twitch" biophysical properties. PGC-1beta coactivates the MEF2 family of transcription factors to stimulate the type IIX myosin heavy chain (MHC) promoter. PGC-1beta transgenic muscle fibers are rich in mitochondria and are highly oxidative, at least in part due to coactivation by PGC-1beta of ERRalpha and PPARalpha. Consequently, these transgenic animals can run for longer and at higher work loads than wild-type animals. Together, these data indicate that PGC-1beta drives the formation of highly oxidative fibers containing type IIX MHC.

Differential modulation of myosin heavy chain phenotype in an inactive extensor and flexor muscle of adult rats

The effects of chronic neuromuscular inactivity on the phenotype and size of muscle fibres in a fast ankle extensor (medial gastrocnemius, MG) and a fast ankle flexor (tibialis anterior, TA) muscle of the rat hindlimb were determined. Inactivity was produced by spinal cord isolation (SI), i.e. complete spinal cord transections at a mid-thoracic and high sacral level and bilateral deafferentation between the transection sites. After 90 days of SI, the MG and TA muscle weights were 53 and 45% lower than in age-matched controls. Overall mean fibre sizes in the deep (close to the bone) and superficial (away from the bone) regions were approximately 60 and 65% smaller in the MG and approximately 40 and 50% smaller in the TA of SI than control rats, respectively. The myosin heavy chain (MHC) composition shifted towards the faster isoforms after SI: the MG showed an increase in both types IIx (20%) and IIb (23%), whereas the TA showed a marked increase in type IIx (94%) and a decrease in type IIb (18%) MHC. Both muscles in SI rats showed no type IIa and only one MG muscle had approximately 5% type I MHC. These results show that prolonged inactivity has a stronger effect on a fast extensor compared with a fast flexor in the rat hindlimb. The larger decrease in mass and fibre size in the MG than the TA most probably reflects the larger impact of chronic inactivity on the normally more highly recruited extensor than flexor muscle. The primary shift to type IIb MHC in the MG and type IIx MHC in the TA indicate a different default mode for an inactive extensor vs. flexor muscle, and may reflect differing activity-independent neural influences, i.e. neurotrophic factors, on muscle fibre phenotype in extensors vs. flexors.

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.

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.

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).

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.

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.

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.

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.

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.

Effect of oestrogen on myofibre size and myosin expression in growing rats.

This study examined the effect of oestrogen deprivation and replacement on plantaris muscle size and myosin heavy chain (MHC) isoform composition in rats during a period of physiological growth. Seven-week-old female Sprague-Dawley rats were assigned to one of the three treatment groups: (1) control animals (Sham); (2) ovariectomized animals without oestrogen replacement (OVX/CO); and (3) ovariectomized animals with 17beta-oestradiol replacement (OVX/E2). OVX/CO and OVX/E2 animals were pair-fed with Sham animals to rule out the potentially confounding effects of differences in food intake and weight gain. Rats were killed 4 weeks after surgery and the plantaris muscle was removed for analysis. Ovariectomy had no effect on muscle fibre size, but reduced the relative amount of type IIx MHC. This was reversed with oestrogen replacement, suggesting that the reduction in type IIx MHC expression was an oestrogen-mediated effect. Oestrogen replacement reduced type IIb MHC expression and fast muscle fibre size. Changes in fast fibre size and type IIb MHC expression were not seen with ovariectomy, indicating that these changes were not simply due to the presence of oestrogen in the ovariectomized, oestrogen-replaced animals. These results suggest that another ovarian hormone may counteract the effect of oestrogen on fast fibre size and type IIb MHC expression in intact animals.

Exercise training increases oxidative capacity and attenuates exercise-induced ultrastructural damage in skeletal muscle of aged horses

Exercise training improves functional capacity in aged individuals. Whether such training reduces the severity of exercise-induced muscle damage is unknown. The purpose of the present study was to determine the effect of 10 wk of treadmill exercise training on skeletal muscle oxidative capacity and exercise-induced ultrastructural damage in six aged female Quarter horses (>23 yr of age). The magnitude of ultrastructural muscle damage induced by an incremental exercise test before and after training was determined by electron microscopic examination of samples of triceps, semimembranosus, and masseter (control) muscles. Maximal aerobic capacity increased 22% after 10 wk of exercise training. The percentage of type IIa myosin heavy chain increased in semimembranosus muscle, whereas the percentage of type IIx myosin heavy chain decreased in triceps muscle. After training, triceps muscle showed significant increases in activities of both citrate synthase and 3-hydroxyacyl-CoA-dehydrogenase. Attenuation of exercise-induced ultrastructural muscle damage occurred in the semimembranosus muscle at both the same absolute and the same relative workloads after the 10-wk conditioning period. We conclude that aged horses adapt readily to intense aerobic exercise training with improvements in endurance, whole body aerobic capacity, and muscle oxidative capacity, and heightened resistance to exercise-induced ultrastructural muscle cell damage. However, adaptations may be muscle-group specific.

Age-related changes in metabolic properties of equine skeletal muscle associated with muscle plasticity

The purpose of the present study was to determine the age-related changes in myosin heavy chain (MHC) composition and muscle oxidative and glycolytic capacity in 18 horses ranging in age from two to 30 years. Muscle samples were collected by excisional biopsy of the semimebranosus muscle. MHC expression and the key enzymatic activities were measured. There was no significant correlation between horse age and the proportions of type-IIA and type-IIX MHC isoforms. The percentage of type-I MHC isoforms decreased with advancing age. Muscle citrate synthase activity decreased, whereas lactate dehydrogenase activity increased with increasing age. Muscle 3-OH acyl CoA dehydrogenase activity did not change with ageing. The results suggest that, similar to humans, the oxidative capacity of equine skeletal muscle decreases with age. The age-related changes in muscle metabolic properties appear to be consistent with an age-related transition in MHC isoforms of equine skeletal muscle that shifts toward more glycolytic isoforms with age.

Age-related changes in metabolic properties of equine skeletal muscle associated with muscle plasticity

The purpose of the present study was to determine the age-related changes in myosin heavy chain (MHC) composition and muscle oxidative and glycolytic capacity in 18 horses ranging in age from two to 30 years. Muscle samples were collected by excisional biopsy of the semimebranosus muscle. MHC expression and the key enzymatic activities were measured. There was no significant correlation between horse age and the proportions of type-IIA and type-IIX MHC isoforms. The percentage of type-I MHC isoforms decreased with advancing age. Muscle citrate synthase activity decreased, whereas lactate dehydrogenase activity increased with increasing age. Muscle 3-OH acyl CoA dehydrogenase activity did not change with ageing. The results suggest that, similar to humans, the oxidative capacity of equine skeletal muscle decreases with age. The age-related changes in muscle metabolic properties appear to be consistent with an age-related transition in MHC isoforms of equine skeletal muscle that shifts toward more glycolytic isoforms with age.

Calcineurin and heat-shock proteins modulation in clenbuterol-induced hypertrophied rat skeletal muscles.

To examine the changes in heat shock proteins (HSPs) and calcineurin (CaN), a calcium/calmodulin regulated protein phosphatase, in hypertrophied rat skeletal muscles, adult male Wistar rats were administered clenbuterol (CLB, 30 mg l(-1) in drinking water), a beta 2-agonist, for 4 weeks. Compared to controls, CLB-treated rats had significantly larger body (10%) and relative (to body weight) soleus (Sol, 16%), plantaris (Plt, 32%) and gastrocnemius (Gast, 27%) weights. Immunohistochemically classified fast fibers were hypertrophied in the Sol (64%), Plt (62%), and deep (d, 70%) and superficial (s, 44%) regions of the Gast, whereas slow fibers were hypertrophied only in the Plt (47%). The percentage of fast fibers in the Sol increased from 10% to 21%. The myosin heavy chain (MHC) isoform composition shifted from slow to fast in the Sol (increase in the percentage of type IIa MHC and de novo synthesis of type IIx MHC) and Gast-d (de novo synthesis of type IIb MHC) and to the faster isoforms in the Plt (increase in the percentage of type IIb MHC). Hsp72 and Hsp90 levels in CLB-treated rats were 52% and 50% lower in the Sol and 44 and 41% lower in the Gast-d, respectively, than in control rats. In control rats, the relative content of CaN was: Sol>Gast-d>Plt>Gast-s, and CLB treatment enhanced the CaN content by 1.4-, 1.2-, 5.0-, and 3.3-fold, respectively. These results indicate that the down-regulation of HSPs in the Sol and Gast-d was closely related to a decrease in the slow phenotype, and that the relative up-regulation of CaN among the muscles/regions was closely related to the selective hypertrophy of fast fibers in the CLB-treated rats.

Chronic Clenbuterol Administration Alters Myosin Heavy Chain Composition in Standardbred Mares

The purpose of this study was to examine changes in myosin heavy chain (MHC) composition due to chronic clenbuterol administration with or without exercise in mares. Unfit Standardbred mares (aged 10±3 years) were divided into four groups: clenbuterol ( 2.4 μ/ kg BW twice daily) plus exercise (3 days/week for 20 min at 50% VO 2max; CLENEX; n=6), clenbuterol only (CLEN; n=6), exercise only (EX; n=5), and control (CON; n=6). Muscle biopsies were obtained from gluteus medius muscle before and after the eight-week training/administration period. MHC composition was determined via SDS gel electrophoresis and quantified using a scanning and densometric system. CLENEX and CLEN exhibited significant ( P<0.05) MHC changes while EX and CON did not. MHC type IIA decreased (29.8±6.1 to 19.3±4.0%, CLENEX; and 36.8±12.4 to 26.4±7.9%, CLEN) and MHC type IIX increased (59.4±7.2 to 71.8±5.8%, CLENEX; and 50.5±12.5 to 62.0±9.3%, CLEN). Chronic clenbuterol administration with and without exercise resulted in a significant shift in MHC profile in Standardbred mares.

Gender differences in resistance-training-induced myofiber hypertrophy among older adults.

We tested the hypothesis that older men (n = 9, 69 +/- 2 years) would experience greater resistance-training-induced myofiber hypertrophy than older women (n = 5, 66 +/- 1 years) following knee extensor training 3 days per week at 65-80% of one-repetition maximum for 26 weeks. Vastus lateralis biopsies were analyzed for myofiber areas, myosin heavy chain isoform distribution, and levels of mRNA for insulin-like growth factor 1 (IGF-1), IGFR1, and myogenin. Gender x Training interactions (p <.05) indicate greater myofiber hypertrophy for all three primary fiber types (I, IIa, IIx) and enhanced one-repetition maximum strength gain in men compared with women (p <.05). Covarying for serum IGF-1, dehydroepiandrosterone sulfate, or each muscle mRNA did not negate these interactions. In both genders, type IIx myofiber area distribution and myosin heavy chain type IIx distribution decreased with a concomitant increase in type IIa myofiber area distribution (p <.05). In summary, gender differences in load-induced myofiber hypertrophy among older adults cannot be explained by levels of circulating IGF-1 or dehydroepiandrosterone sulfate, or by expression of the myogenic transcripts examined.

Myosin heavy chain composition in human masticatory muscles by immunohistochemistry and gel electrophoresis.

In this study we compared the immunohistochemically quantified fiber type area with the myosin heavy chain (MyHC) contents of a bundle of fibers from a human masticatory muscle. The total cross-sectional areas were determined immunohistochemically for the three major fiber types (I, IIA, and IIX) in bundles of fibers (n=42) taken from the anterior and posterior belly of the human digastric muscle (n=7). The relative MyHC contents of the same fiber bundles were determined electrophoretically (MyHC-I, -IIA, and -IIX; anterior, 32%, 35%, and 33%; posterior, 39%, 42%, and 19%) and compared with the immunohistochemical data (MyHC-I, -IIA, and -IIX; anterior, 32%, 31%, and 37%; posterior, 39%, 45%, and 15%). No significant differences were seen in the mean fiber type distribution between the two techniques; the correlation coefficient ranged from 0.71 to 0.96. The correlation coefficient was higher for MyHC type I and MyHC type IIX than for MyHC type IIA. The MyHC contents of single fibers taken from the posterior belly indicated that many fibers in this belly co-express MyHC-IIA and MyHC-IIX. Despite the presence of these hybrid fibers, the correspondence between both methods was relatively large.