Fiber types in canine muscles: myosin isoform expression and functional characterization

This study was aimed to achieve a definitive and unambiguous identification of fiber types in canine skeletal muscles and of myosin isoforms that are expressed therein. Correspondence of canine myosin isoforms with orthologs in other species as assessed by base sequence comparison was the basis for primer preparation and for expression analysis with RT-PCR. Expression was confirmed at protein level with histochemistry, immunohistochemistry, and SDS-PAGE combined together and showed that limb and trunk muscles of the dog express myosin heavy chain (MHC) type 1, 2A, and 2X isoforms and the so-called “type 2dog” fibers express the MHC-2X isoform. MHC-2A was found to be the most abundant isoform in the trunk and limb muscle. MHC-2X was expressed in most but not all muscles and more frequently in hybrid 2A-2X fibers than in pure 2X fibers. MHC-2B was restricted to specialized extraocular and laryngeal muscles, although 2B mRNA, but not 2B protein, was occasionally detected in the semimembranosus muscle. Isometric tension (Po) and maximum shortening velocity ( Vo) were measured in single fibers classified on the basis of their MHC isoform composition. Purified myosin isoforms were extracted from single muscle fibers and characterized by the speed ( Vf) of actin filament sliding on myosin in an in vitro motility assay. A close proportionality between Vo and Vf indicated that the diversity in Vo was due to the different myosin isoform composition. Vo increased progressively in the order 1/slow < 2A < 2X < 2B, thus confirming the identification of the myosin isoforms and providing their first functional characterization of canine muscle fibers.

Short-term electrical stimulation alters tongue muscle fibre type composition

OBJECTIVE

To examine whether short-term exogenous activation of a tongue muscle induced a phenotypic shift from a fast to a slow fibre-type, and thus assess a potential therapeutic avenue to protect against obstructive sleep apnoea (OSA).

METHODS

New Zealand White rabbit genioglossus (GG) muscle, characteristically a fast muscle, was continuously stimulated at a frequency attributed to slow muscle (10Hz, 3V DC pulses) using an implanted micro-circuit for 7 days. Changes in muscle fibre types and aerobic capacity were assessed between stimulated and un-stimulated (control) groups using immunohistochemistry and electrophoresis for myosin heavy chain (MHC) and assayed for citrate synthase.

RESULTS

Compared to the un-stimulated control group, stimulated GG muscles had more (approximately 13%) type I MHC (slow-twitch) content; a proportional decrease in type II MHC (fast-twitch) isoform also occurred in the stimulated GG muscle (P<0.05). Electrophoresis analysis on whole muscle and single fibre MHC showed an increased type I expression in the stimulated GG muscle (P<0.01). A commensurate rise in citrate synthase activity, indicating a change in aerobic capacity, was also observed in the stimulated GG muscles.

CONCLUSION

Together, these results demonstrate a successful alteration in tongue muscle characteristics using exogenous electrical stimulation and perhaps a potential therapeutic application for OSA.

Immunohistochemical characterization of slow and fast myosin heavy chain composition of muscle fibres in the styloglossus muscle of the human and macaque (Macaca rhesus)

OBJECTIVE

Muscle fibre contractile diversity is thought to be increased by the hybridization of multiple myosin heavy chain (MHC) isoforms in single muscle fibres. Reports of hybrid fibres composed of MHCI and MHCII isoforms in human, but not macaque, tongue muscles, suggest a human adaptation for increased tongue muscle contractile diversity. Here we test whether hybrid fibres composed of MHCI and MHCII are unique to human tongue muscles or are present as well in the macaque.

METHODS

MHC composition of the macaque and human styloglossus was characterized with antibodies that allowed identification of three muscle fibre phenotypes, a slow phenotype composed of MHCI, a fast phenotype composed of MHCII and a hybrid phenotype composed of MHCI and MHCII.

RESULTS

The fast phenotype constitutes 68.5% of fibres in the macaque and 43.4% of fibres in the human (P<0.0001). The slow phenotype constitutes 20.2% of fibres in the macaque and 39.3% of fibres in the human (P<0.0001). The hybrid phenotype constitutes 11.2% of fibres in the macaque and 17.3% of fibres in the human (P=0.0002). Macaques and humans do not differ in fiber size (cross-sectional area, diameter). However, measures of fibre size differ by phenotype such that fast>hybrid>slow (P<0.05).

CONCLUSION

These data demonstrate differences in the relative percent of muscle fibre phenotypes in the macaque and human styloglossus but also demonstrate that all three phenotypes are present in both species. These data suggest a similar range of mechanical properties in styloglossus muscle fibres of the macaque and human.

Assist–Control Mechanical Ventilation Attenuates Ventilator-induced Diaphragmatic Dysfunction

Controlled mechanical ventilation induced a profound diaphragm muscle dysfunction and atrophy. The effects of diaphragmatic contractions with assisted mechanical ventilation on diaphragmatic isometric, isotonic contractile properties, or the expression of muscle atrophy factor-box (MAF-box), the gene responsible for muscle atrophy, are unknown. We hypothesize that assisted mechanical ventilation will preserve diaphragmatic force and prevent overexpression of MAF-box. Studying sedated rabbits randomized equally into control animals, those with 3 days of assisted ventilation, and those with controlled ventilation, we assessed in vitro diaphragmatic isometric and isotonic contractile function. The concentrations of contractile proteins, myosin heavy chain isoform, and MAF-box mRNA were measured. Tetanic force decreased by 14% with assisted ventilation and 48% with controlled ventilation. Maximum shortening velocity tended to increase with controlled compared with assisted ventilation and control. Peak power output decreased 20% with assisted ventilation and 41% with controlled ventilation. Contractile proteins were unchanged with either modes of ventilation; myosin heavy chain 2X mRNA tended to increase and that of 2A to decrease with controlled ventilation. MAF-box gene was overexpressed with controlled ventilation. We conclude that preserving diaphragmatic contractions during mechanical ventilation attenuates the force loss induced by complete inactivity and maintains MAF-box gene expression in control.

Muscular adaptations to computer-guided strength training with eccentric overload

AIMS

In order to investigate the muscular adaptations to a novel form of strength training, 18 male untrained subjects performed 4 weeks of low resistance-high repetition knee extension exercise.

METHODS

Nine of them trained on a conventional weight resistance device (Leg curler, CON/ECC group), with loads equivalent to 30% of the concentric one-repetition maximum (1RM) for both the concentric and eccentric phase of movement. The other nine trained on a newly developed computer-driven device (CON/ECC-OVERLOAD group) with the concentric load equivalent to 30% of the concentric 1RM and the eccentric load equivalent to 30% of the eccentric 1RM.

RESULTS

Training resulted in significantly (P < or = 0.05) increased peak torque and a tendency (P=0.092) to increased muscle cross-sectional area for the CON/ECC-OVERLOAD but not the CON/ECC group, while strength endurance capacity was significantly (P < or = 0.05) increased in the CON/ECC group only. RT-PCR revealed significantly increased myosin heavy chain (MHC) IIa and lactate dehydrogenase (LDH) A mRNAs, a tendency for increased MHC IIx mRNA (P = 0.056) and high correlations between the changes in MHC IIx and LDH A mRNAs (r=0.97, P=0.001) in the CON/ECC-OVERLOAD group.

CONCLUSIONS

These results indicate a shift towards a more type II dominated gene expression pattern in the vasti laterales muscles of the CON/ECC-OVERLOAD group in response to training. We suggest that the increased eccentric load in the CON/ECC-OVERLOAD training leads to distinct adaptations towards a stronger, faster muscle.

Maximum shortening velocity and myosin heavy-chain isoform expression in human masseter muscle fibers

While human masseter muscle is known to have unusual co-expression of myosin heavy-chain proteins, cellular kinetics of individual fibers has not yet been tested. Here we examine if myosin heavy-chain protein content is closely correlated to fiber-shortening speed, as previously reported in other human muscles, or if these proteins do not correlate well to shortening speeds, as has been demonstrated previously in rat muscle. Slack-test recordings of single, skinned human masseter fibers at 15 degrees C revealed maximum shortening velocities generally slower and much more variable than those recorded in human limb muscle. The slowest fiber recorded had a maximum shortening velocity (V0) value of 0.027 muscle lengths x s(-1), several times slower than the slowest type I fibers previously measured in humans. By contrast, human limb muscle controls produced V0 measurements comparable with previously published results. Analysis by gel electrophoresis found 63% of masseter fibers to contain pure type I MyHC and the remainder to co-express mostly type I in various combinations with IIA and IIX isoforms. V0 in masseter fibers forms a continuum in which no clear relationship to MyHC isoform content is apparent.