Ryanodine receptor type-1 (RyR1) expression and protein S-nitrosylation pattern in human soleus myofibres following bed rest and exercise countermeasure

The ryanodine receptor type-I (RyR1) is one key player of the excitation-contraction coupling (E-CC) machinery. However, RyR1 expression in human skeletal muscle disuse and plasticity changes are not well documented. We studied the expression and the functional modifications of RyR1 following prolonged bed rest (BR) without and with exercise countermeasure (Resistive Vibration Exercise, RVE). Soleus biopsies were taken from a non-trained control (BR-CTRL) and trained (BR-RVE) group (each n = 10) before and after BR. In BR-CTRL group, a fibre type-specific immunopattern of RyR1 (type-I < type-II) was documented, and RyR1 immunofluorescence intensity and protein expression together with [(3)H]ryanodine binding were decreased after BR. In BR-RVE group, RyR1 immunosignals were increased and fiber type specificity was no longer present. RyR1 protein expression was unchanged, whereas [(3)H]ryanodine binding increased after BR. Confocal and biochemical analysis confirmed subcellular co-localisation and protein-protein interaction of RyR1 with nitric oxide (NO)-synthase type-1 (NOS1). S-nitrosylation of RyR1 was increased in BR-CTRLpost only, suggesting a reduction of RyR1 open channel probability by nitrosylation mechanisms following prolonged disuse. We conclude that following extended body deconditioning in bed rest, RVE countermeasure maintained normal RyR1 expression and nitrosylation patterns required for adequate E-CC in human performance control.     

Long term bed rest with and without vibration exercise countermeasures: effects on human muscle protein dysregulation

The present investigation, the first in the field, was aimed at analyzing differentially, on individual samples, the effects of 55 days of horizontal bed rest, a model for microgravity, on myosin heavy and myosin light chain isoforms distribution (by SDS) and on the proteome (by 2-D DIGE and MS) in the vastus lateralis (VL), a mixed type II/I (～50:50%) head of the quadriceps and in the calf soleus (SOL), a predominantly slow (～35:65%) twitch muscle. Two separate studies were performed on six subjects without (BR) and six with resistive vibration exercise (RVE) countermeasures, respectively. Both VL and SOL underwent in BR decrements of ～15% in cross-sectional area and of ～22% in maximal torque that were prevented by RVE. Myosin heavy chain distribution showed increased type I and decreased type IIA in BR both in VL and in SOL, the opposite with RVE. A substantial downregulation of proteins involved in aerobic metabolism characterized both in SOL and VL in BR. RVE reversed the pattern more in VL than in SOL, whereas proteins involved in anaerobic glycolysis were upregulated. Proteins from the Z-disk region and from costamers were differently dysregulated during bed rest (both BR and RVE), particularly in VL.     

Bed rest decreases mechanically induced myofiber wounding and consequent wound-mediated FGF release

Using a terrestrial model of spaceflight (i.e., bed rest), weinvestigated the amount of myofiber wounding and fibroblast growthfactor (FGF) release that occurs during unloading.Myofiber wounding was determined by serum levels of the creatine kinase MM (CKMM) isoform before and after bed rest. Serum levels of both acidic FGF (aFGF) and basic FGF were also determined. A second group ofsubjects was treated in an identical fashion except that they underwenta resistive exercise program during bed rest. Bed rest alone causedsignificant (P < 0.05;n = 7) reductions in post-bed-restserum levels of both CKMM and aFGF, which were paralleled by asignificant (P < 0.05;n = 7) decrease in myofiber size. Incontrast, bed rest plus resistive exercise resulted in significant (P < 0.05;n = 7) increases in post-bed-restserum levels of both CKMM and aFGF, which were paralleled by inhibitionof the atrophic response. These results suggest that mechanicallyinduced, myofiber wound-mediated FGF release may play an important rolein the etiology of unloading-induced skeletal muscle atrophy.

     

Resistance exercise maintains skeletal muscle protein synthesis during bed rest

Ferrando, Arny A., Kevin D. Tipton, Marcas M. Bamman, andRobert R. Wolfe. Resistance exercise maintains skeletal muscle protein synthesis during bed rest. J. Appl.Physiol. 82(3): 807-810, 1997.Spaceflightresults in a loss of lean body mass and muscular strength. Aground-based model for microgravity, bed rest, results in a loss oflean body mass due to a decrease in muscle protein synthesis (MPS).Resistance training is suggested as a proposed countermeasure forspaceflight-induced atrophy because it is known to increase both MPSand skeletal muscle strength. We therefore hypothesized that scheduledresistance training throughout bed rest would ameliorate the decreasein MPS. Two groups of healthy volunteers were studied during 14 days ofsimulated microgravity. One group adhered to strict bed rest (BR;n = 5), whereas a second group engagedin leg resistance exercise every other day throughout bed rest (BREx;n = 6). MPS was determined directly bythe incorporation of infusedL-[ring-¹³C₆]phenylalanineinto vastus lateralis protein. After 14 days of bed rest, MPS in theBREx group did not change and was significantly greater than in the BRgroup. Thus moderate-resistance exercise can counteract the decrease inMPS during bed rest.

     

Sarco(endo)plasmic reticulum calcium pump isoforms in paralyzed rat slow muscle

To assess the influence of paralysis on the expression of phenotypic protein isoforms related to muscle relaxation, the effects of spinal cord transection (ST) on sarco(endo)plasmic reticulum calcium ATPase (SERCA) pump isoform protein levels in the slow rat soleus were measured. Western blotting using SERCA isoform specific antibodies demonstrated a rapid up-regulation (7 days post ST) of the fast fiber type-specific isoform (SERCA1). In contrast, the slow fiber type-specific isoform, SERCA2, was decreased with a slower time-course. The up-regulation of SERCA1 protein preceded the up-regulation of fast myosin heavy chain (MyHC) (i.e., MyHC-II). Immunohistochemical analyses of single muscle fibers showed that 15 days after ST there was a pronounced increase in the proportion of slow MyHC fibers with SERCA1 confirming that SERCA1 was up-regulated in the slow fibers of the soleus prior to MyHC-II. These data suggest that the expression of the SERCA isoforms (particularly SERCA1) may serve as more sensitive markers of phenotypic adaptation in response to altered levels of contractile activity than the MyHC isoforms. In addition, since the expression of SERCA isoforms was dissociated from MyHC isoforms, regulation of gene expression for these two different protein systems must involve different signaling events and/or synthetic processes.     

The deleterious effects of bed rest on human skeletal muscle fibers are exacerbated by hypercortisolemia and ameliorated by dietary supplementation

Prolonged inactivity associated with bed rest in a clinical setting or spaceflight is frequently associated with hypercortisolemia and inadequate caloric intake. Here, we determined the effect of 28 days of bed rest (BR); bed rest plus hypercortisolemia (BRHC); and bed rest plus essential amino acid (AA) and carbohydrate (CHO) supplement (BRAA) on the size and function of single slow- and fast-twitch muscle fibers. Supplementing meals, the BRAA group consumed 16.5 g essential amino acids and 30 g sucrose at 1100, 1600, and 2100 h, and the BRHC subjects received 5 daily doses of 10–15 mg of oral hydrocortisone sodium succinate throughout bed rest. Bed rest induced atrophy and loss of force (mN) and power (µN·FL·s^–1) in single fibers was exacerbated by hypercortisolemia where soleus peak force declined by 23% in the type I fiber from a prevalue of 0.78 ± 0.02 to 0.60 ± 0.02 mN post bed rest (compared to a 7% decline with bed rest alone) and 27% in the type II fiber (1.10 ± 0.08 vs. 0.81 ± 0.05 mN). In the BRHC group, peak power dropped by 19, 15, and 11% in the soleus type I, and vastus lateralis (VL) type I and II fibers, respectively. The AA/CHO supplement protected against the bed rest-induced loss of peak force in the type I soleus and peak power in the VL type II fibers. These results provide evidence that an AA/CHO supplement might serve as a successful countermeasure to help preserve muscle function during periods of relative inactivity. isotonic contractile properties; peak force and power; calcium sensitivity; essential amino acids     

Impact of resistance exercise during bed rest on skeletal muscle sarcopenia and myosin isoform distribution

Bamman, Marcas M., Mark S. F. Clarke, Daniel L. Feeback,Robert J. Talmadge, Bruce R. Stevens, Steven A. Lieberman, and MichaelC. Greenisen. Impact of resistance exercise during bed rest onskeletal muscle sarcopenia and myosin isoform distribution. J. Appl. Physiol. 84(1): 157-163, 1998.Because resistance exercise (REx) and bed-rest unloading (BRU)are associated with opposing adaptations, our purpose was to test theefficacy of REx against the effects of 14 days of BRU on theknee-extensor muscle group. Sixteen healthy men were randomly assignedto no exercise (NoEx; n = 8) or REx(n = 8). REx performed five sets ofleg press exercise with 80-85% of one repetition maximum (1 RM)every other day during BRU. Muscle samples were removed from the vastuslateralis muscle by percutaneous needle biopsy. Myofiber distributionwas determined immunohistochemically with three monoclonal antibodiesagainst myosin heavy chain (MHC) isoforms (I, IIa, IIx). MHCdistribution was further assessed by quantitative gel electrophoresis.Dynamic 1-RM leg press and unilateral maximum voluntary isometriccontraction (MVC) were determined. Maximal neural activation (root meansquared electromyogram) and rate of torque development (RTD) weremeasured during MVC. Reductions(P < 0.05) in type I (15%) and typeII (17%) myofiber cross-sectional areas were found in NoEx but not inREx. Electrophoresis revealed no changes in MHC isoform distribution. The percentage of type IIx myofibers decreased(P < 0.05) in REx from 9 to 2% anddid not change in NoEx. 1 RM was reduced(P < 0.05) by 9% in NoEx but wasunchanged in REx. MVC fell by 15 and 13% in NoEx and REx,respectively. The agonist-to-antagonist root mean squaredelectromyogram ratio decreased (P < 0.05) 19% in REx. RTD slowed (P < 0.05) by 54% in NoEx only. Results indicate that REx preventedBRU-induced myofiber atrophy and also maintained training-specificstrength. Unlike spaceflight, BRU did not induce shifts in myosinphenotype. The reported benefits of REx may prove useful in prescribingexercise for astronauts in microgravity.

     

Resistance training affects GLUT-4 content in skeletal muscle of humans after 19 days of head-down bed rest.

This study assessed the effects of inactivity on GLUT-4 content of human skeletal muscle and evaluated resistance training as a countermeasure to inactivity-related changes in GLUT-4 content in skeletal muscle. Nine young men participated in the study. For 19 days, four control subjects remained in a -6 degrees head-down tilt at all times throughout bed rest, except for showering every other day. Five training group subjects also remained at bed rest, except during resistance training once in the morning. The resistance training consisted of 30 isometric maximal voluntary contractions for 3 s each; leg-press exercise was used to recruit the extensor muscles of the ankle, knee, and hip. Pauses (3 s) were allowed between bouts of maximal contraction. Muscle biopsy samples were obtained from the lateral aspect of vastus lateralis (VL) muscle before and after the bed rest. GLUT-4 content in VL muscle of the control group was significantly decreased after bed rest (473 +/- 48 vs. 398 +/- 66 counts. min-1. microgram membrane protein-1, before and after bed rest, respectively), whereas GLUT-4 significantly increased in the training group with bed rest (510 +/- 158 vs. 663 +/- 189 counts. min-1. microgram membrane protein-1, before and after bed rest, respectively). The present study demonstrated that GLUT-4 in VL muscle decreased by approximately 16% after 19 days of bed rest, and isometric resistance training during bed rest induced a 30% increase above the value of GLUT-4 before bed rest.     

Human soleus single muscle fiber function with exercise or nutrition countermeasures during 60 days of bed rest

The soleus muscle has been consistently shown to atrophy more than other leg muscles during unloading and is difficult to protect using various exercise countermeasure paradigms. However, the efficacy of aerobic exercise, a known stimulus for oxidative adaptations, has not been tested in combination with resistance exercise (RE), a known hypertrophic stimulus. We hypothesized that a concurrent exercise program (AE + RE) would preserve soleus fiber myosin heavy chain (MHC) I size and function during 60 days of bed rest. A secondary objective was to test the hypothesis that a leucine-enriched high protein diet would partially protect soleus single fiber characteristics. Soleus muscle biopsies were obtained before and after bed rest from a control (BR; n = 7), nutrition (BRN; n = 8), and exercise (BRE; n = 6) group. Single muscle fiber diameter (Dia), peak force (Po), contractile velocity, and power were studied. BR decreased (P < 0.05) MHC I Dia (-14%), Po (-38%), and power (-39%) with no change in contractile velocity. Changes in MHC I size (-13%) and contractile function (approximately 30%) from BRN were similar to BR. BRE decreased (P < 0.05) MHC I Dia (-13%) and Po (-23%), while contractile velocity increased (P < 0.05) 26% and maintained power. These soleus muscle data show 1) the AE + RE exercise program maintained MHC I power but not size and strength, and 2) the nutrition countermeasure did not benefit single fiber size and contractile function. The divergent response in size and functional MHC I soleus properties with the concurrent exercise program was a unique finding further highlighting the challenges of protecting the unloaded soleus.     

Regenerated rat fast muscle transplanted to the slow muscle bed and innervated by the slow nerve,exhibits an identical myosin heavy chain repertoire to that of the slow muscle

 The hypothesis that the limited adaptive range observed in fast rat muscles in regard to expression of the slow myosin is due to intrinsic properties of their myogenic stem cells was tested by examining myosin heavy chain (MHC) expression in regenerated rat extensor digitorum longus (EDL) and soleus (SOL) muscles. The muscles were injured by bupivacaine, transplanted to the SOL muscle bed and innervated by the SOL nerve. Three months later, muscle fibre types were determined. MHC expression in muscle fibres was demonstrated immunohistochemically and analysed by SDS-glycerol gel electrophoresis. Regenerated EDL transplants became very similar to the control SOL muscles and indistinguishable from the SOL transplants. Slow type 1 fibres predominated and the slow MHC-1 isoform was present in more than 90% of all muscle fibres. It contributed more than 80% of total MHC content in the EDL transplants. About 7% of fibres exhibited MHC-2a and about 7% of fibres coexpressed MHC-1 and MHC-2a. MHC-2x/d contributed about 5–10% of the whole MHCs in regenerated EDL and SOL transplants. The restricted adaptive range of adult rat EDL muscle in regard to the synthesis of MHC-1 is not rooted in muscle progenitor cells; it is probably due to an irreversible maturation-related change switching off the gene for the slow MHC isoform. Accepted: 11 June 1996     

Emergence of the mature myosin phenotype in the rat diaphragm muscle

Immunohistochemical analysis of myosin heavy chain (MHC) isoform expression in perinatal and adult rat diaphragm muscles was performed with antibodies which permitted the identification of all known MHC isoforms found in typical rat muscles. Isoform switching, leading to the emergence of the adult phenotype, was more complex than had been previously described. As many as four isoforms could be coexpressed in a single myofiber. Elimination of developmental isoforms did not usually result in the myofiber immediately achieving its adult phenotype. Activation of genes for specific adult isoforms might be delayed to puberty. For example, two of the three fast MHCs, MHC2X and MHC2A appeared perinatally, while MHC2B did not appear until 30 days postnatal. By Day 60 this isoform was present in approximately 27% of the myofibers, but in most myofibers expression of this isoform was transient (i.e., at Day greater than or equal to 115, less than 4% of the myofibers expressed MHC2B). Fibers which contained MHC beta/slow during the late fetal and early neonatal period coexpressed MHCemb. A marked increase in the frequency of fibers containing MHC beta/slow occurred between 4 and 21 days postnatal. These slow fibers arose from a population of myofibers which expressed MHCemb and MHCneo during their development, and they accounted for the majority of slow fibers found in the adult diaphragm. The adult myosin phenotype of the diaphragm myofibers (as determined with immunocytochemistry, and 5% SDS-PAGE) was not achieved until the rat was greater than or equal to 115 days old.     

Macrophage depletion by clodronate liposome attenuates muscle injury and inflammation following exhaustive exercise

Exhaustive exercise promotes muscle injury, including myofiber lesions; however, its exact mechanism has not yet been elucidated. In this study, we tested the hypothesis that macrophage depletion by pretreatment with clodronate liposomes alters muscle injury and inflammation following exhaustive exercise. Male C57BL/6J mice were divided into four groups: rest plus control liposome (n=8), rest plus clodronate liposome (n=8), exhaustive exercise plus control liposome (n=8), and exhaustive exercise plus clodronate liposome (n=8). Mice were treated with clodronate liposome or control liposome for 48 h before undergoing exhaustive exercise on a treadmill. Twenty-four hours after exhaustive exercise, the gastrocnemius muscles were removed for histological and PCR analyses. Exhaustive exercise increased the number of macrophages in the muscle; however, clodronate liposome treatment reduced this infiltration. Although exhaustive exercise resulted in an increase in injured myofibers, clodronate liposome treatment following exhaustive exercise reduced the injured myofibers. Clodronate liposome treatment also decreased the mRNA expression levels of inflammatory cytokines (TNF-α, IL-1β, and IL-6) in the skeletal muscle after exhaustive exercise. These results suggest that macrophages play a critical role in increasing muscle injury by regulating inflammation.     

Mechanisms leading to restoration of muscle size with exercise and transplantation after spinal cord injury

We have shown thatcycling exercise combined with fetal spinal cord transplantationrestored muscle mass reduced as a result of complete transection of thespinal cord. In this study, mechanisms whereby this combinedintervention increased the size of atrophied soleus and plantarismuscles were investigated. Rats were divided into five groups(n = 4, per group): control, nontransected; spinal cordtransected at T10 for 8 wk (Tx); spinal cord transected for 8 wk andexercised for the last 4 wk (TxEx); spinal cord transected for 8 wkwith transplantation of fetal spinal cord tissue into the lesion site 4 wk prior to death (TxTp); and spinal cord transected for 8 wk,exercised for the last 4 wk combined with transplantation 4 wk prior todeath (TxExTp). Tx soleus and plantaris muscles were decreased in sizecompared with control. Exercise and transplantation alone did notrestore muscle size in soleus, but exercise alone minimized atrophy inplantaris. However, the combination of exercise and transplantationresulted in a significant increase in muscle size in soleus andplantaris compared with transection alone. Furthermore, myofibernuclear number of soleus was decreased by 40% in Tx and was notaffected in TxEx or TxTp but was restored in TxExTp. A strongcorrelation (r = 0.85) between myofiber cross-sectional area and myofiber nuclear number was observed in soleus, but not inplantaris muscle, in which myonuclear number did not change with any ofthe experimental manipulations. 5'-Bromo-2'-deoxyuridine-positive nuclei inside the myofiber membrane were observed in TxExTp soleus muscles, indicating that satellite cells had divided and subsequently fused into myofibers, contributing to the increase in myonuclear number. The increase in satellite cell activity did not appear to becontrolled by the insulin-like growth factors (IGF), as IGF-I andIGF-II mRNA abundance was decreased in Tx soleus and plantaris, and wasnot restored with the interventions. These results indicate that,following a relatively long postinjury interval, exercise andtransplantation combined restore muscle size. Satellite cell fusion andrestoration of myofiber nuclear number contributed to increased musclesize in the soleus, but not in plantaris, suggesting that cellularmechanisms regulating muscle size differ between muscles with differentfiber type composition.

     

Maximal and submaximal forces of slow fibers in human soleus after bed rest.

The effects of 2 and 4 mo of bed rest, with or without exercise countermeasures, on the contractile properties of slow fibers in the human soleus muscle were examined. Mean fiber diameters were 8 and 36% smaller after 2 and 4 mo of bed rest, respectively, than the pre-bed rest level. Maximum tetanic force (P(o)), maximum activated force (F(max)) per cross-sectional area (CSA), and the common-logarithm value of free Ca(2+) concentration required for half-maximal activation (pCa(50)) also decreased after 2 and 4 mo of bed rest. In contrast, maximum unloaded shortening velocity (V(o)) was increased after 2 and 4 mo of bed rest. After 1 mo of recovery, fiber diameters, P(o), F(max) per CSA (P > 0.05), and pCa(50) were increased and V(o) decreased toward pre-bed rest levels. Effects of knee extension/flexion exercise by wearing an anti-G Penguin suit for 10 h daily, and the effects of loading or unloading of the plantar flexors with (Penguin-1) or without (Penguin-2) placing the elastic loading elements of the suit, respectively, were investigated during ~2 mo of bed rest. In the Penguin-1 group, mean fiber diameter, P(o), F(max) per CSA, V(o), and pCa(50) were similar before and after bed rest. However, the responses of fiber size and contractile properties to bed rest were not prevented in the Penguin-2 group, although the degree of the changes was less than those induced by bed rest without any countermeasure. These results indicate that long-term bed rest results in reductions of fiber size, force-generation capacity, and Ca(2+) sensitivity, and enhancement of shortening velocity in slow fibers of the soleus. The data indicate that continuous mechanical loading on muscle, such as stretching of muscle, is an effective countermeasure for the prevention of muscular adaptations to gravitational unloading.     

Intense Resistance Exercise Induces Early and Transient Increases in Ryanodine Receptor 1 Phosphorylation in Human Skeletal Muscle

Background

While ryanodine receptor 1 (RyR1) critically contributes to skeletal muscle contraction abilities by mediating Ca²⁺ion oscillation between sarcoplasmatic and myofibrillar compartments, AMP-activated protein kinase (AMPK) senses contraction-induced energetic stress by phosphorylation at Thr¹⁷². Phosphorylation of RyR1 at serine²⁸⁴³ (pRyR1Ser²⁸⁴³) results in leaky RyR1 channels and impaired Ca²⁺homeostasis. Because acute resistance exercise exerts decreased contraction performance in skeletal muscle, preceded by high rates of Ca²⁺-oscillation and energetic stress, intense myofiber contractions may induce increased RyR1 and AMPK phosphorylation. However, no data are available regarding the time-course and magnitude of early RyR1 and AMPK phosphorylation in human myofibers in response to acute resistance exercise.

Purpose

Determine the effects and early time-course of resistance exercise on pRyR1Ser²⁸⁴³ and pAMPKThr¹⁷² in type I and II myofibers.

Methods

7 male subjects (age 23±2 years, height: 185±7 cm, weight: 82±5 kg) performed 3 sets of 8 repetitions of maximum eccentric knee extensions. Muscle biopsies were taken at rest, 15, 30 and 60 min post exercise. pRyR1Ser²⁸⁴³ and pAMPKThr¹⁷² levels were determined by western blot and semi-quantitative immunohistochemistry techniques.

Results

While total RyR1 and total AMPK levels remained unchanged, RyR1 was significantly more abundant in type II than type I myofibers. pRyR1Ser²⁸⁴³ increased 15 min and peaked 30 min (p<0.01) post exercise in both myofiber types. Type I fibers showed relatively higher increases in pRyR1Ser²⁸⁴³ levels than type II myofibers and remained elevated up to 60 min post resistance exercise (p<0.05). pAMPKThr¹⁷² also increased 15 to 30 min post exercise (p<0.01) in type I and II myofibers and in whole skeletal muscle.

Conclusion

Resistance exercise induces acutely increased pRyR1Ser²⁸⁴³ and concomitantly pAMPKThr¹⁷² levels for up to 30 min in resistance exercised myofibers. This provides a time-course by which pRyR1Ser²⁸⁴³ can mechanistically impact Ca²⁺handling properties and consequently induce reduced myofiber contractility beyond immediate fatiguing mechanisms.     

Dynamics of myosin heavy chain isoform transition in the longissimus muscle of domestic and wild pigs during growth: a comparative study

Dynamics of myofiber differentiation/maturation in porcine skeletal muscle is associated with domestication, breeding and rearing conditions. This study was aimed to comparatively elucidate the age-dependent myosin heavy chain (MyHC) isoform expression and transition pattern in domestic and wild pig (WP) skeletal muscle from birth until adulthood. Domestic pigs (DPs) of Large White breed raised in conventional production system were compared with WPs reared in a large hunting enclosure. Muscle samples for immuno/enzyme histochemistry were taken from the longissimus dorsi muscle within 24 h postmortem at 24 to 48 h, 21 to 23 days, 7 months and ~2 years postpartum. Based on the antibody reactivity to MyHCs (NCL-MHCs, A4.74, BF-F3) and succinate dehydrogenase activity, myofibers were classified into I, I/IIa, IIa, IIx and IIb types. In addition, foetal MyHC expression was determined with the use of F158.4C10 antibody. Maturation of the longissimus dorsi muscle in the WP was characterized by an accelerated transformation of the fast to slow MyHC during the first hours postpartum, followed by differentiation towards oxidative myofibers in which type I, IIa and IIx MyHCs predominated. In the DP, the transformation shifted towards glycolytic myofibers that expressed MyHC-IIb. The expression of foetal MyHC was higher in the DP than in the WP at 1 day of age, and the decline in the foetal MyHC during the first 3 weeks was more rapid in the WP than in the DP denoting an accelerated early postnatal muscle maturation in WP than DP piglets. All foetal MyHC-positive myofibers co-expressed IIa isoform, but not vice versa. The intense myofiber hypertrophy was evident from 3 weeks until 7 months of age. In this period, the myofiber cross-sectional area increased up to 10- and 20-fold in the WP and the DP, respectively. In the DP, the hypertrophy of all myofiber types was more pronounced than in the WP, particularly the hypertrophy of IIx and IIb myofibers. To summarize, the comparison between growing DP with wild ancestors showed that genetic selection and rearing conditions lead to substantial changes in the direction and intensity of postnatal MyHC transformation as evidenced by different proportion of individual myofiber types and differences in their hypertrophic potential.