Changes in myosin heavy chain isoform expression of overloaded rat skeletal muscles

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.

A short-term statin treatment changes the contractile properties of fast-twitch skeletal muscles

Background

Cumulative evidence indicates that statins induce myotoxicity. However, the lack of understanding of how statins affect skeletal muscles at the structural, functional, and physiological levels hampers proper healthcare management. The purpose of the present study was to investigate the early after-effects of lovastatin on the slow-twitch soleus (Sol) and fast-twitch extensor digitorum longus (EDL) muscles.

Methods

Adult C57BL/6 mice were orally administrated with placebo or lovastatin [50 mg/kg/d] for 28 days. At the end of the treatment, the isometric ex vivo contractile properties of the Sol and EDL muscles were measured. Subtetanic and tetanic contractions were assessed and contraction kinetics were recorded. The muscles were then frozen for immunohistochemical analyses. Data were analyzed by two-way ANOVA followed by an a posteriori Tukey’s test.

Results

The short-term lovastatin treatment did not induce muscle mass loss, muscle fiber atrophy, or creatine kinase (CK) release. It had no functional impact on slow-twitch Sol muscles. However, subtetanic stimulations at 10 Hz provoked greater force production in fast-twitch EDL muscles. The treatment also decreased the maximal rate of force development (dP/dT) of twitch contractions and prolonged the half relaxation time (1/2RT) of tetanic contractions of EDL muscles.

Conclusions

An early short-term statin treatment induced subtle but significant changes in some parameters of the contractile profile of EDL muscles, providing new insights into the selective initiation of statin-induced myopathy in fast-twitch muscles.

Investigation of the Expression of Myogenic Transcription Factors, microRNAs and Muscle-Specific E3 Ubiquitin Ligases in the Medial Gastrocnemius and Soleus Muscles following Peripheral Nerve Injury

Despite surgical innovation, the sensory and motor outcome after a peripheral nerve injury remains incomplete. One contributing factor to the poor outcome is prolonged denervation of the target organ, leading to apoptosis of both mature myofibres and satellite cells with subsequent replacement of the muscle tissue with fibrotic scar and adipose tissue. In this study, we investigated the expression of myogenic transcription factors, muscle specific microRNAs and muscle-specific E3 ubiquitin ligases at several time points following denervation in two different muscles, the gastrocnemius (containing predominantly fast type fibres) and soleus (slow type) muscles, since these molecules may influence the degree of atrophy following denervation. Both muscles exhibited significant atrophy (compared with the contra-lateral sides) at 7 days following either a nerve transection or crush injury. In the crush model, the soleus muscle showed significantly increased muscle weights at days 14 and 28 which was not the case for the gastrocnemius muscle which continued to atrophy. There was a significantly more pronounced up-regulation of MyoD expression in the denervated soleus muscle compared with the gastrocnemius muscle. Conversely, myogenin was more markedly elevated in the gastrocnemius versus soleus muscles. The muscles also showed significantly contrasting transcriptional regulation of the microRNAs miR-1 and miR-206. MuRF1 and Atrogin-1 showed the highest levels of expression in the denervated gastrocnemius muscle. This study provides further insights regarding the intracellular regulatory molecules that generate and maintain distinct patterns of gene expression in different fibre types following peripheral nerve injury.

Impaired calcium calmodulin kinase signaling and muscle adaptation response in the absence of calpain 3

Mutations in the non-lysosomal, cysteine protease calpain 3 (CAPN3) result in the disease limb girdle muscular dystrophy type 2A (LGMD2A). CAPN3 is localized to several subcellular compartments, including triads, where it plays a structural, rather than a proteolytic, role. In the absence of CAPN3, several triad components are reduced, including the major Ca(2+) release channel, ryanodine receptor (RyR). Furthermore, Ca(2+) release upon excitation is impaired in the absence of CAPN3. In the present study, we show that Ca-calmodulin protein kinase II (CaMKII) signaling is compromised in CAPN3 knockout (C3KO) mice. The CaMK pathway has been previously implicated in promoting the slow skeletal muscle phenotype. As expected, the decrease in CaMKII signaling that was observed in the absence of CAPN3 is associated with a reduction in the slow versus fast muscle fiber phenotype. We show that muscles of WT mice subjected to exercise training activate the CaMKII signaling pathway and increase expression of the slow form of myosin; however, muscles of C3KO mice do not exhibit these adaptive changes to exercise. These data strongly suggest that skeletal muscle's adaptive response to functional demand is compromised in the absence of CAPN3. In agreement with our mouse studies, RyR levels were also decreased in biopsies from LGMD2A patients. Moreover, we observed a preferential pathological involvement of slow fibers in LGMD2A biopsies. Thus, impaired CaMKII signaling and, as a result, a weakened muscle adaptation response identify a novel mechanism that may underlie LGMD2A and suggest a pharmacological target that should be explored for therapy.

Profiling muscle-specific microRNA expression after peripheral denervation and reinnervation in a rat model

MicroRNAs (miRNAs) are a class of highly conserved, non-coding RNAs involved in post-transcriptional gene regulation. The muscle-specific miRNAs, miR-1, miR-133a, and miR-206, are expressed in skeletal muscles and have been shown to contribute to muscle development. To profile their expression after sciatic nerve denervation and reinnervation, the soleus muscles of the rats were analyzed with quantitative real-time PCR at 1 week, 1 month, 2 months, and 4 months after the experiments. In addition, a combined approach using computational prediction by the miRanda website and the Agilent Whole Rat Genome 4 x 44 k oligo microarray experiment was performed to investigate the potential target genes of these three miRNAs in the denervated and reinnervated muscles. The results revealed that with the first downregulation of miR-1 and miR-133a within 1 month in the denervated muscle, the expression of miR-1 and miR-133 increased by approximately 2-fold at 4 months after denervation and reinnervation; on the other hand, the expression of miR-206 was significantly increased to approximately 3-fold 1 month later only following reinnervation but not following denervation, and lasted at least for 4 months. The expression pattern of miR-206 was different from that of miR-1 and miR-133a. Notably, two genes (Hnrpu and Npy) and one gene (Ptprd) were potentially regulated both in the denervated and reinnervated muscle by miR-1 and miR-133a, respectively. There were six potential target genes (Hnrpu, Lsamp, MGC108776, Mef2, Npy, and Ppfibp2) of the upregulated miR-206 in the reinnervated muscle. Among these, three (Hnrpu, Npy, and MGC108776) were potentially regulated by both miR-1 and miR-206. Because the Mef2 transcription factor was reported to promote the transformation of type II fast glycolytic fibers into type I slow oxidative fibers, the upregulation of miR-206 with decreased expression of the Mef2 transcript in the 4 month reinnervated muscle, which presented type II fiber predominance 4 months after nerve microanastomosis, might indicate the role of miR-206 in determining the fiber type after peripheral nerve regeneration.

Ovalbumin-induced sensitization affects non-quantal acetylcholine release from motor nerve terminals and alters contractility of skeletal muscles in mice

Skeletal muscles play key roles in the development of various pathologies, including bronchial asthma and several types of auto-immune disorders, e.g. polymyositis. Since most of these maladies have an immunological/allergic element, this paper is devoted to assessing the impact of immunobiological reorganization on the functional properties of isolated skeletal muscles in mice. A combination of two methods (myography and electrophysiology) was used to evaluate extensor digitorum longus (EDL) and diaphragmatic muscle (DM) in this regard. Conventional myographic technique showed that ovalbumin-induced sensitization (OS) produced different changes in the contractile properties of EDL and DM. The amplitudes of carbachol (CCh)-induced contractions increased in DM but decreased in EDL. Those changes were inversely related to OS-mediated changes of non-quantal acetylcholine (ACh) release intensity within the muscle endplate, as shown by the electrophysiologically measured H-effect. These results clearly show that OS-mediated changes of non-quantal ACh release alter the functional properties of postjunctional ACh receptors and therefore contribute to the disturbance of CCh-induced contractility of skeletal muscles. Other mechanisms of OS-mediated changes of skeletal muscle contractility are also proposed and discussed.

Histone deacetylase degradation and MEF2 activation promote the formation of slow-twitch myofibers

Skeletal muscle is composed of heterogeneous myofibers with distinctive rates of contraction, metabolic properties, and susceptibility to fatigue. We show that class II histone deacetylase (HDAC) proteins, which function as transcriptional repressors of the myocyte enhancer factor 2 (MEF2) transcription factor, fail to accumulate in the soleus, a slow muscle, compared with fast muscles (e.g., white vastus lateralis). Accordingly, pharmacological blockade of proteasome function specifically increases expression of class II HDAC proteins in the soleus in vivo. Using gain- and loss-of-function approaches in mice, we discovered that class II HDAC proteins suppress the formation of slow twitch, oxidative myofibers through the repression of MEF2 activity. Conversely, expression of a hyperactive form of MEF2 in skeletal muscle of transgenic mice promotes the formation of slow fibers and enhances running endurance, enabling mice to run almost twice the distance of WT littermates. Thus, the selective degradation of class II HDACs in slow skeletal muscle provides a mechanism for enhancing physical performance and resistance to fatigue by augmenting the transcriptional activity of MEF2. These findings provide what we believe are new insights into the molecular basis of skeletal muscle function and have important implications for possible therapeutic interventions into muscular diseases.

The effects of dietary energy restriction on overloaded skeletal muscle in rats

We evaluated the effects of three levels of energy intake, 73 % (CON73), 81 % (CON81) and 100 % (CON100) of the ad libitum intake of the control diet, on skeletal muscle growth induced by functional overload in male rats. Unlike most previous studies which have employed chronic or acute food restriction where all nutrients are reduced in the diet, the present study tested the effects of energy deprivation as a single factor without inducing other nutritional deficiencies. Muscular growth of plantaris and soleus muscles was induced by removal of synergist gastrocnemius muscles in one hindlimb; muscles in the other leg were used as sham-operated intra-animal controls. After 30 d, rats on the energy-restricted CON73 and CON81 diets gained less weight and had smaller livers, kidneys, hearts and fat pads (epididymal, retroperitoneal and omental) than CON100 rats (P<0·05). They also had smaller sham-operated plantaris muscles (CON73 -13 %, CON81 -9 %) containing less total protein (CON73 -14 %; CON81 -10 %) than CON100 rats (P<0·05). However, the same measurements in overloaded plantaris muscles were similar among groups. Soleus muscle mass and protein contents were not significantly affected by energy restriction in our study. Percentage distributions of myosin heavy-chain isoforms (types I, IIa, IIx and IIb) were similar among rats in CON100, CON81 and CON73 groups for both plantaris and soleus muscles. We conclude that the growth reduction of plantaris muscle induced by energy restriction at 73 % and 81 % for 30 d was prevented by functional overload in male rats.

Molecular cloning and mRNA expression analysis of carp embryonic, slow and cardiac myosin heavy chain isoforms

Three embryonic class II myosin heavy chains (MYHs) were cloned from the common carp (Cyprinus carpio L.), MYHemb1, MYHemb2 and MYHemb3. MYH DNA clones were also isolated from the slow muscle of adult carp acclimated to 10 degrees C (MYHS10) and 30 degrees C (MYHS30). Phylogenetic analysis demonstrated that MYHemb1 and MYHemb2 belonged to the fast skeletal muscle MYH clade. By contrast, the sequence of MYHemb3 was similar to the adult slow muscle isoforms, MYHS10 and MYHS30. MYHemb1 and MYHemb2 transcripts were first detected by northern blot analysis in embryos 61 h post-fertilization (h.p.f.) at the heartbeat stage, with peak expression occurring in 1-month-old juveniles. MYHemb1 continued to be expressed at low levels in 7-month-old juveniles when MYHemb2 was not detectable. MYHemb3 transcripts appeared at almost the same stage as MYHemb1 transcripts did (61 h.p.f.), and these genes showed a similar pattern of expression. Whole mount in situ hybridization analysis revealed that the transcripts of MYHemb1 and MYHemb2 were expressed in the inner part of myotome, whereas MYHemb3 was expressed in the superficial compartment. MYHS10 and MYHS30 mRNAs were first detected at hatching. In adult stages, the expression of slow muscle MYH mRNAs was dependent on acclimation temperature. MYHS10 mRNA was expressed at an acclimation temperature of 10 and 20 degrees C, but not at 30 degrees C. In contrast, MYHS30 mRNA was strongly expressed at all acclimation temperatures. The predominant MYH transcripts found in adult slow muscle and in embryos at hatching were expressed in adult fast muscle at some acclimation temperatures but not others. A MYH DNA clone was isolated from the cardiac muscle of 10 degrees C-acclimated adult fish (MYHcard). MYHcard mRNA was first detected at 61 h.p.f., but strong signals were only observed in the adult myocardium. The present study has therefore revealed a complex pattern of expression of MYH genes in relation to developmental stage, muscle type and acclimation temperature. None of the skeletal muscle MYHs identified so far was strongly expressed during the late juvenile stage, indicating further developmentally regulated members of the MYH II gene family remain to be discovered.

Effect of changes in food consistency on NADH-ubiquinone oxidoreductase activity and levels of mRNA for ND1, 51kDa, 75kDa and myosin heavy chain isoforms in two different portions of rat masseter muscle

We investigated the effect of a change in food consistency on properties of the masseter muscle in 3-week-old rats fed a soft diet for 9 weeks (Group S) and fed a soft diet for 5 weeks followed by a hard diet for 4 weeks (Group S-H). The NADH-O2 oxidoreductase activity, levels of mRNAs transcribed from genes encoding NADH-ubiquinone oxidoreductase (Complex I: ND1, 51kDa, and 75kDa) and myosin heavy chain (MyHC) isoforms and the phenotype of the muscle fibers were measured in the superficial and deep portions of the muscle. In the period from 8 weeks to 12 weeks of age, NADH-O2 oxidoreductase enzyme activity in both the superficial and deep portions of the muscle showed similar patterns in Group S and Group S-H. In contrast, the ND1, 51kDa and 75kDa mRNA levels in the superficial and deep portions of the masseter muscle in the Group S-H were higher than those of Group S in the 12-week-old rats, except for the 51kDa mRNA in the superficial portion of the masseter muscle. MyHC-IIa and MyHC-IId/x mRNA levels in the superficial portion of the masseter muscle were higher in the Group S-H than in the Group S. These observations suggest that short-term feeding stress such as the transition from a soft diet to a hard diet causes changes in oxidative metabolism, in mRNA levels for the Complex I components ND1 and 75kDa, and the mRNA levels for the MyHC isoforms IIa and IId/x in the superficial portion of rat masseter muscle, but no changes in the composition of muscle fiber types.

Effects of moderate heart failure and functional overload on rat plantaris muscle

It is thought that changes in sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) of skeletal muscle contribute to alterations in skeletal muscle function during congestive heart failure (CHF). It is well established that exercise training can improve muscle function. However, it is unclear whether similar adaptations will result from exercise training in a CHF patient. Therefore, the purpose of this study was to determine whether skeletal muscle during moderate CHF adapts to increased activity, utilizing the functional overload (FO) model. Significant increases in plantaris mass of the CHF-FO and sham-FO groups compared with the CHF and control (sham) groups were observed. Ca(2+) uptake rates were significantly elevated in the CHF group compared with all other groups. No differences were detected in Ca(2+) uptake rates between the CHF-FO, sham, and sham-FO groups. Increases in Ca(2+) uptake rates in moderate-CHF rats were not due to changes in SERCA isoform proportions; however, FO may have attenuated the CHF-induced increases through alterations in SERCA isoform expression. Therefore, changes in skeletal muscle Ca(2+) handling during moderate CHF may be due to alterations in regulatory mechanisms, which exercise may override, by possibly altering SERCA isoform expression.

Role of load bearing in acetylcholinesterase regulation in rat skeletal muscles

Slow antigravity muscles differ from fast muscles with regard to load bearing performed during contraction. We examined the importance of load bearing in regulation of acetylcholinesterase (AChE) expression in slow and fast rat muscles. The levels of AChE mRNA in the slow soleus muscles are about 30% of those in the fast extensor digitorum longus (EDL) muscles. In the soleus muscles unloaded for 8 days by hindlimb suspension, AChE mRNA levels were not significantly different from those in the control soleus muscles. In the suspended animals, AChE transcripts in the EDL muscles decreased to about 80% of control levels. Reduction of the resting muscle tension by joint fixation did not significantly affect the levels of AChE mRNA in the unloaded soleus muscles. Phasic high-frequency electrical stimulation of the unloaded soleus muscles via the sciatic nerve increased their AChE mRNA levels to about 50% of those in the EDL muscles. The levels observed after phasic stimulation were significantly higher than those after low-frequency tonic stimulation, indicating the importance of muscle activation pattern for AChE regulation also in the absence of load bearing. The AChE mRNA levels in the soleus muscles overloaded for 8 days by synergist muscle ablation increased significantly to about 50% of those in the EDL muscle. The load bearing during muscle contraction seems to be a relatively unimportant extrinsic factor in the regulation of the AChE mRNA levels in muscle fibers, except when an increased load induces muscle hypertrophy accompanied by the fusion of satellite cells with the muscle fibers.

Dietary protein does not affect overloaded skeletal muscle in rats

We compared the effects of three levels of dietary protein, i.e., 7% (low protein; LP); 17.5% (adequate protein; CON); or 30% (high protein; HP) on growth of functionally overloaded muscle in Sprague-Dawley male rats. Growth of plantaris and soleus muscles was induced by the surgical removal of gastrocnemius muscles in one hindlimb; muscles in the other leg were used as sham-operated, intra-animal controls. After 4 wk, rats fed the 7% LP diet gained less weight (-29%) and had lighter livers (-20%) and kidneys (-16%) than rats fed the CON diet (P < 0.05). Measurements of rats fed the 30% HP diet were not different from those of CON rats except that their kidneys were larger (+6%) (P < 0.05). The level of dietary protein did not affect the experimentally induced muscular growth in either plantaris or soleus muscles. Gains in overloaded plantaris muscles over sham-operated muscles were not different among rats fed LP, CON and HP diets for muscle mass (+42 to +45%), total protein (+42 to +46%) and myofibrillar protein (+40 to +44%). Soleus muscles also did not differ among diet groups for gains in mass (+20 to +33%), total protein (+20 to +33%) and myofibrillar protein (+21 to +33%). No dietary protein effects were found on myosin heavy chain isoform (I, IIa, IIx, IIb) expression in either plantaris or soleus muscles. We conclude that gains in plantaris and soleus muscle mass, total protein and myofibrillar protein induced by functional overload are not affected by low (7%) or high (30%) protein feeding in young male rats for 4 wk.

Expression of myosin heavy-chain isoforms in the respiratory muscles following inspiratory resistive breathing

We investigated the effect of inspiratory resistive breathing (IRB) on the expression of the genes encoding fast and slow isoforms of myosin heavy chain (MyHC) in respiratory muscles. Eleven mongrel dogs were studied for baseline MyHC messenger RNA (mRNA) expression, seven of which were also used to study the effects of IRB. For this latter objective, awake and spontaneously breathing animals were subjected to 2 h of IRB (80 cm H(2)O/L/s) per day for four consecutive days. mRNA expression was assessed in the diaphragm, external intercostal muscle, and a limb muscle, using both slot- blot and in situ hybridizations with isoform-specific probes. A current semiquantitative scoring method (from 0 to 4) was used to quantify the in situ mRNA expression levels, and slot-blot data were analyzed with densitometry. Prior to IRB, slow- and fast-MyHC mRNA expression was moderate, similar, and homogeneous throughout the different regions of the diaphragm, with scores of 1.50 +/- 0.54 (mean +/- SD) for slow and 2.13 +/- 0.35 for fast mRNAs in the costal region of the diaphragm, and of 1.81 +/- 0.37 for slow and 2. 13 +/- 0.64 for fast mRNAs in the crural region of the diaphragm. Although expression of fast-MyHC mRNA remained unchanged after IRB, the relative expression of the mRNA for the slow isoform increased in costal (+30%), crural (+12%), and external intercostal (+27%) muscles. MyHC mRNA expression did not change in limb muscles. We conclude that breathing with a moderate inspiratory resistance for a short period induces the expression of slow MyHC in respiratory muscles.

Myosin heavy chain profiles in regenerated fast and slow muscles innervated by the same motor nerve become nearly identical

Plasticity of mature muscles exposed to different activation patterns is limited, probably due to restricted adaptive range of their muscle fibres. In this study, we tested whether satellite cells derived from slow muscles can give rise to a normal fast muscle, if transplanted to the fast muscle bed. Marcaine-treated rat soleus and extensor digitorum longus (EDL) muscles were transplanted to the EDL muscle bed and innervated by the 'EDL' nerve. Six months later expression of myosin heavy chain isoforms was analysed by areal densities of fibres, binding specific monoclonal antibodies, and by SDS gel electrophoresis. Both regenerated muscles closely resembled each other. Their myosin heavy chain profiles were similar to those in fast muscles although they were not identical to that in the control EDL muscle. Since not even regenerated EDL was able to reach the myosin heavy chain isoform profile of mature EDL muscle, our experimental model did not permit studying the adaptive capacity of satellite cells in different muscles in its whole extent. However, the results favour the multipotential myoblast stem cell population in rat muscles and underline the importance of the extrinsic regulation of muscle phenotype.

Myogenin, MyoD, and myosin expression after pharmacologically and surgically induced hypertrophy

The relationship between myogenin or MyoD expression and hypertrophy of the rat soleus produced either by clenbuterol and 3,3', 5-triiodo-L-thyronine (CT) treatment or by surgical overload was examined. Mature female rats were subjected to surgical overload of the right soleus with the left soleus serving as a control. Another group received the same surgical treatment but were administered CT. Soleus muscles were harvested 4 wk after surgical overload and weighed. Myosin heavy chain isoforms were separated by using polyacrylamide gel electrophoresis while myogenin and MyoD expression were evaluated by Northern analysis. CT and functional overload increased soleus muscle weight. CT treatment induced the appearance of the fast type IIX myosin heavy chain isoform, depressed myogenin expression, and induced MyoD expression. However, functional overload did not alter myogenin or MyoD expression in CT-treated or non-CT-treated rats. Thus pharmacologically and surgically induced hypertrophy have differing effects on myogenin and MyoD expression, because their levels were associated with changes in myosin heavy chain composition (especially type IIX) rather than changes in muscle mass.

Modulation of MHC isoforms in functionally overloaded and exercised rat plantaris fibers

The effects of 1 and 10 wk of functional overload (FO) of the rat plantaris with (FOTr) and without daily endurance treadmill training on its myosin heavy chain (MHC) composition were studied. After 1 and 10 wk of FO, plantaris mass was 22 and 56% greater in FO and 37 and 94% greater, respectively, in FOTr rats compared with age-matched controls. At 1 wk, pure type I and pure type IIa MHC fibers were hypertrophied in FO (39 and 44%) and FOTr (70 and 87%) rats. By 10 wk all fiber types comprising >5% of the fibers sampled showed a hypertrophic response in both FO groups. One week of FO increased the percentage of hybrid (containing both type I and type IIa MHC) fibers and of fibers containing embryonic MHC. By 10 wk, the percentage of pure type I MHC fibers was approximately 40% in both FO groups compared with 15% in controls, and the percentage of fibers containing embryonic MHC was similar to that in controls. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses showed an increase in type I MHC and a decrease in type IIb MHC in both FO groups at 10 wk, whereas little change was observed at 1 wk. These data are consistent with hypertrophy and transformation from faster to slower MHC isoforms in chronically overloaded muscles. The additional overload imposed by daily endurance treadmill training employed in this study (1.6 km/day; 10% incline) results in a larger hypertrophic response but appears to have a minimal effect on the MHC adaptations.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.