Myosin transitions in chronic stimulation do not involve embryonic isozymes

The adaptive response of skeletal muscle: What is the evidence?

Adult skeletal muscle is capable of adapting its properties in response to changing functional demands. This now sounds like a statement of the obvious, and many people assume it has always been this way. A mere 40 years ago, however, the picture was entirely different. In this Review and personal memoir, I outline the scientific context in which the theory was generated, the objections to it from entrenched opinion, and the way those objections were progressively met. The material should be of some historical interest, but, more importantly, it collects together the full range of evidence on which the current paradigm is based. Muscle Nerve 57: 531-541, 2018.

Adaptive change in electrically stimulated muscle: a framework for the design of clinical protocols

Adult mammalian skeletal muscles have a remarkable capacity for adapting to increased use. Although this behavior is familiar from the changes brought about by endurance exercise, it is seen to a much greater extent in the response to long-term neuromuscular stimulation. The associated phenomena include a markedly increased resistance to fatigue, and this is the key to several clinical applications. However, a more rational basis is needed for designing regimes of stimulation that are conducive to an optimal outcome. In this review I examine relevant factors, such as the amount, frequency, and duty cycle of stimulation, the influence of force generation, and the animal model. From these considerations a framework emerges for the design of protocols that yield an overall functional profile appropriate to the application. Three contrasting examples illustrate the issues that need to be addressed clinically.

Skeletal muscle as a myocardial substitute

Skeletal muscle has long been used in the field of cardiac surgery. Its use has progressed from providing myocardial reinforcement to assisting the heart by actively pumping blood. Early experiments revealed that skeletal muscle assistance could augment pressures and blood flow; however, the results were short-lived due to muscle fatigue. It was later shown that skeletal muscle can be conditioned electrically to be fatigue resistant and therefore may be useful for performing cardiac-type work. Once the details were formed of how to stimulate and manipulate the muscle to assist the heart, several configurations were devised. Cardiomyoplasty and aortomyoplasty refer to wrapping skeletal muscle around the heart or aorta, respectively. These techniques have been applied in humans; however, the effectiveness is controversial. Although most patients improve clinically, the hemodynamic parameters have not shown consistent improvements, and survival data are unknown. Skeletal muscle ventricles offer a promising alternative to both cardiomyoplasty and aortomyoplasty. These are completely separate pumping chambers constructed from skeletal muscle and connected to the circulation in a variety of configurations. Although these have not been tried in humans, the animal data appear quite convincing. The skeletal muscle ventricles have shown the greatest improvements on hemodynamic parameters with great stability over time.

Changes in satellite cell content and myosin isoforms in low-frequency-stimulated fast muscle of hypothyroid rat

Chronic low-frequency stimulation was used to study the effects of enhanced contractile activity on satellite cell content and myosin isoform expression in extensor digitorum longus muscles from hypothyroid rats. As verified by immunohistochemical staining for desmin, vimentin, and myosin heavy chain (MHC) isoforms and by histological analysis, stimulation induced a transformation of existing fast fibers toward slower fibers without signs of fiber deterioration or regeneration. Immunohistochemically detected increases in MHC I and MHC IIa isoforms, as well as reduced numbers of fibers expressing the faster MHC isoforms, mirrored the rearrangement of the thick-filament composition. These changes, especially the upregulation of MHC IIa, were accompanied by an induction of developmental MHC isoforms in the transforming adult fibers. Satellite cell content rose 2.6-, 3.0-, and 3.7-fold over that of corresponding controls (P < 0.05 in all cases) in 5-, 10-, and 20-day-stimulated muscles, respectively. Hypothyroidism alone had no effect on satellite cell content but resulted in a significant reduction in fiber size. The relative satellite cell contents increased (P < 0.05) from 3.8% in euthyroid control muscles to 7.9, 11.5, and 13.8% in the 5-, 10-, and 20-day-stimulated hypothyroid muscles, respectively. In 20-day-stimulated muscles, the relative satellite cell content reached an almost twofold higher level than that of normal slow-twitch soleus muscle. This increase occurred concomitantly with a rise in myonuclear density, most probably because of the fusion of satellite cells with existing fibers.

Effects of pre- and perinatal exposure to hypergravity on muscular structure development in rat

This study evaluated the influence of precocious exposure to hypergravity on the expression of myosin heavy chain (MHC) protein isoforms in nape, masticatory and respiratory developmental rat muscles. Pregnant females were maintained at 1.8 g from the 11th day of gestation to the 7th day after birth. The 7-day-old rats were used for muscle sampling. Hypergravity induced a marked decrease in the weight and protein content of all six muscles. Three MHC isoforms were detected in the young rats' muscles: embryonic (E), perinatal (P) and slow type 1 MHC. In centrifuged nape and masticatory muscles, there was a decrease in MHC E and an increase in P without reduction (indeed, even an increase) in MHC 1, whereas in the respiratory muscle MHC E was increased and MHC 1 decreased. These results indicate that hypergravity produces important changes in the contractile properties not only of antigravity muscles but also masticatory and respiratory muscles. MHC P has a higher shortening velocity than MHC E, which has a higher one than MHC 1. The hypergravity-induced transformations of MHC isoforms would thus lead to increased velocity of all muscles studied. In spite of the observation of a hypergravity-induced muscle hypotrophy, the results of this study reflect the adaptational properties of developing muscles to increased gravitational forces.

Inherited muscular disorder in mutant Japanese quail (Coturnix coturnix japonica): an immunohistochemical study

Cryostat sections of myofibres from the Musculus pectoralis thoracicus of a newly established mutant strain (LWC) of Japanese quail with a myotonic dystrophy-like myopathy were labelled with antibody against myosin heavy chain (MHC) isoforms and neural cell adhesion molecule (N-CAM). The characteristic lesions found in sections of muscle of LWC quail stained with haematoxylin and eosin were type 2B fibre atrophy, sarcoplasmic masses, and ring fibres. Immunohistochemical examination failed to distinguish type 2A and 2B fibres in the LWC quail. Antibody to adult fast MHC, which reacted only with type 2A fibres in normal quail, reacted in LWC quail with type 2B fibres, and to a limited degree with type 2A fibres. Sarcoplasmic masses reacted with both fast and slow MHC antibodies. Some masses also reacted with NCAM antibody, but apparently independently of similar reactions in fibres. These findings suggest that the changes observed in the myofibres of the LWC quail were not neurogenic but represented defects in both the plasma membrane and intermediate filaments.

Skeletal muscle ventricles: another alternative for heart failure

BACKGROUND

Autologous muscle has the potential of generating power for cardiac assistance. Problems with muscle fatigue have been overcome by the development of successful protocols of electrical conditioning and by allowing a recovery interval after the initial harvest.

METHODS

Our laboratory has pursued research with latissimus dorsi muscle pouches, which we term skeletal muscle ventricles (SMVs). By allowing several weeks for a "vascular delay" interval and then electrically conditioning the muscle, these pouches can be connected to the circulation and stimulated to assist the heart.

RESULTS

These pouches have been developed and tested in the canine model in numerous configurations, achieving survival beyond 3 years.

CONCLUSIONS

Although still experimental, SMVs may have the potential of becoming a viable alternative for the future treatment of patients with end-stage heart failure and with infants with certain congenital anomalies.

Morphological changes during fiber type transitions in low-frequency-stimulated rat fast-twitch muscle

This study investigates morphological adaptations of rat extensor digitorum longus muscle to chronic low-frequency stimulation (10 Hz, 10 h/d, up to 61 +/- 7d). During the early stimulation period (2-4 d), increased basophilia and accumulation of RNA were seen predominantly in type-IIB fibers. Putative satellite cell activation, as indicated by 3H-thymidine incorporation, was also evident during this phase. By 12 d, fiber composition remained unaltered, but there was a decrease in the cross-sectional area of the type-IIB fibers. Following 28 d of low-frequency stimulation, the percentage of type-IIB fibers decreased from 43 +/- 3% to 0%, while type-IID fibers increased from 30 +/- 3% to 60 +/- 6%. The fraction of type-IIA fibers tended to increase (controls 19 +/- 3%; stimulated 29 +/- 4%), whereas that of the type-I fibers was unaltered (4 +/- 1%). At this time, the cross-sectional area of type-IID fibers was unaltered, but that of type-IIA and type-I fibers increased. Further stimulation resulted in a return of type-IID fibers to control levels (23 +/- 5%), and a marked increase in type-IIA fibers (45 +/- 8%). The percentage of type-I fibers increased from 4 +/- 1% to 8 +/- 1%. Throughout each stage of chronic stimulation, there was no histological evidence of fiber degeneration and regeneration. These results indicate that, in contrast to the rabbit, chronic low-frequency stimulation-induced fiber conversion in the rat extensor digitorum longus muscle is entirely due to fiber transformation.

Stimulation-induced expression of slow muscle myosin in a fast muscle of the rat. Evidence of an unrestricted adaptive capacity

Fast muscles of the rat hind limb were stimulated continuously at 10 or 20 Hz for periods of 55-61 days by means of an implantable neuromuscular stimulator. Gel electrophoresis clearly demonstrated the presence in stimulated muscles of slow myosin light and heavy chains, although fast isoforms were still present in all cases. Thus, contrary to previous reports, induction of slow myosin isoforms does occur in this, as in other, mammalian species. The time course of the response to stimulation appears to be more extended than that seen in the rabbit.

Quantitative morphology of stimulation-induced damage in rabbit fast-twitch skeletal muscles

The purpose of this study was to examine the contention that stimulation-induced damage, resulting in degeneration with subsequent regeneration, plays a major role in the transformation of fibre type brought about by chronic electrical stimulation. Data from histological and histochemical sections of 9-day-stimulated rabbit fast-twitch muscles were analysed with multivariate statistical techniques. Fibre degeneration and regeneration varied non-systematically between sample areas at any given cross-sectional level. In the extensor digitorum longus muscle, but not in the tibialis anterior, there was more degeneration in proximal than in distal portions of the muscle. The extensor digitorum longus muscle consistently showed more degeneration than the tibialis anterior muscle. Degeneration was less extensive for an intermittent pattern of stimulation that delivered half the aggregate number of impulses of continuous stimulation. Degeneration and regeneration varied markedly between individual rabbits in each of the groups. Sections that revealed the most degeneration and regeneration also had more fibres that reacted positively with an anti-neonatal antibody. Rigorous analysis of different sources of variation has helped to explain apparent conflicts in the literature. The incidence of muscle fibre damage in the stimulated tibialis anterior muscle is low, showing that the contribution of degenerative-regenerative phenomena to fibre type conversion in this muscle is insignificant.

Correlates of fatigue resistance in canine skeletal muscle stimulated electrically for up to one year

In response to patterns of chronic electrical stimulation that increase its overall level of use, mammalian skeletal muscle becomes highly resistant to fatigue. The metabolic basis for this adaptation is well documented in the rabbit, but up to now it has not been possible to identify analogous changes in the dog. In this study, canine latissimus dorsi muscles were stimulated in situ for 2, 6 and 12 mo. Marked increases in fatigue resistance were consistently demonstrated. Citrate synthase and succinic dehydrogenase, conventionally used as markers of oxidative metabolism, did not increase in activity, but enzymes involved in major pathways supplying substrates to the tricarboxylic acid cycle increased up to threefold. Stimulation elevated the volume fraction of mitochondria 1.5-fold and that of lipid droplets 4.5-fold. After 6 mo of stimulation, mean fiber diameter had decreased by 30% and the area occupied by nonmuscle tissue had increased by 11%; these changes showed no further progression at 12 mo. Thus stimulated muscle becomes stably adapted to an increase in use, but the metabolic strategies for achieving increased fatigue resistance vary between species.

Myosin isozyme expression in response to stretch-induced hypertrophy in the Japanese quail

When skeletal muscle is subjected to stretch it undergoes a rapid increase in muscle mass. However, the effect of stretch on the native myosin isozyme content of muscle has received attention only recently. Using the Japanese quail to investigate stretch-induced hypertrophy, we demonstrated an increase in the expression of fast myosin in the predominantly slow anterior latissimus dorsi muscle (ALD). The fast myosin content of the control quail ALD is not sufficient to be quantified on native myosin pyrophosphate gels. After 33 days of stretch, the fast myosin content (N = 10) averaged 16 +/- 11% in the stretched muscles and reached a maximum of 40%. Mean hypertrophy in the stretched muscle, as indicated by muscle weight, was 247 +/- 91% (range, 168-378%). Fast myosin was consistently expressed in muscles with hypertrophy greater than 250%. Muscle fiber size from the stretched muscles contained a greater number of fibers with small cross-sectional areas than was observed in controls. These results indicate that substantial remodeling occurs in the stretched ALD muscle of the Japanese quail.

Coexpression of myosin isoforms in muscle of patients with neurogenic disease

Three well-characterized antimyosin heavy chain monoclonal antibodies (McAbs) were used as immunocytochemical reagents to study myosin isoform expression in relationship to adenosine triphosphatase (ATPase) defined fiber types in human muscle. The biopsy specimens were from patients with neurogenic muscle disease whose muscle exhibited fiber type grouping and group atrophy. The use of McAbs revealed heretofore unrecognized coexpression of multiple myosin isoforms in selected fibers in the pathologic samples which was not apparent with ATPase reactions and not present in normal muscle. The fibers containing multiple myosin isoforms were probably undergoing neurally directed fiber type transformation. Furthermore, a small population of fibers in neurogenic specimens expressed a "prenatal" myosin signifying the presence of regenerating fibers. We also demonstrated immunocytochemical evidence of the persistence of adult slow myosin in denervated mature human skeletal muscle despite the reputed necessity of innervation for maintenance of expression of this myosin isoform proffered by others.

Isomyosin changes after functional electrostimulation of denervated sheep muscle

Isomyosin analyses by biochemical, immunochemical, and histochemical investigations have been carried out in five sheep following unilateral recurrent laryngeal nerve paralysis and direct functional electrostimulation of the denervated cricoarytenoid posterior muscle. Myosin light chains were identified by two-dimensional gel electrophoresis. Myosin heavy chains were analyzed by one-dimensional SDS-polyacrylamide gel electrophoresis. Slow myosin heavy chain was identified by orthogonal peptide mapping and immunochemistry. The stimulation effect at cellular level was determined using adenosine triphosphatase (ATPase) histochemistry. A dramatic increase of the type 1 fiber area (slow, fatigue-resistant fibers) could be seen after many weeks of an increasing regime of low-frequency direct electrical stimulation. Biochemically, the amount of slow myosin was always higher than in normal muscles. Some muscles were transformed almost completely to the slow type. At the time they were studied and with the methods employed, the expression of embryonic isomyosin was not observed. In conclusion, after numerous weeks of maintained functional activity, elicited by direct electrostimulation, the denervated muscle regionally showed areas of hypertrophy or at least lack of atrophy of slow myofibers without major signs of muscle damage.