Mitochondrial ultrastructural adaptations in fast muscles of mice lacking IL15RA.

The pro-inflammatory cytokine interleukin-15 (IL15) and its receptor α (IL15RA) participate in the regulation of musculoskeletal function and metabolism. Deletion of the Il15ra gene in mice increases spontaneous activity, improves fatigue resistance in the glycolytic extensor digitorum longus (EDL) and protects from diet-induced obesity. In humans, IL15RA single-nucleotide polymorphisms (SNPs) have been linked to muscle strength, metabolism and performance in elite endurance athletes. Taken together, these features suggest a possible role for IL15RA in muscle mitochondrial structure and function. Here, we have investigated the consequences of loss of IL15RA on skeletal muscle fiber-type properties and mitochondrial ultrastructure. Immunostaining of the EDL for myosin heavy chain (MyHC) isoforms revealed no significant changes in fiber type. Electron microscopy (EM) analysis of the EDL indicated an overall higher mitochondria content, and increased cristae density in subsarcolemmal and A-band mitochondrial subpopulations. The higher cristae density in Il15ra-/- mitochondria was associated with higher OPA1 and cardiolipin levels. Overall, these data extend our understanding of the role of IL15RA signaling in muscle oxidative metabolism and adaptation to exercise.

Early accentuated muscle hypertrophy is strongly associated with myonuclear accretion.

The current study explored whether the marked hypertrophic response noted with a short-term unilateral concurrent exercise paradigm was associated with more prominent changes in myonuclei accretion, ribosome biogenesis, and capillarization compared with resistance exercise alone (RE). Ten men (age 25 ± 4 yr) performed aerobic and resistance exercise (AE + RE) for one leg while the other leg did RE. Muscle biopsies were obtained before and after 5 wk of training and subjected to fiber-type specific immunohistochemical analysis, and quantification of total RNA content and mRNA/rRNA transcript abundance. Type II fiber cross-sectional area (CSA) increased with both AE + RE (22%) and RE (16%), while type I fiber CSA increased mainly with AE + RE (16%). The change score tended to differ between legs for type I CSA (P = 0.099), and the increase in smallest fiber diameter was greater in AE + RE than RE (P = 0.029). The number of nuclei per fiber increased after AE + RE in both fiber types, and this increase was greater (P = 0.027) than after RE. A strong correlation was observed between changes in number of nuclei per fiber and fiber CSA in both fiber types, for both AE + RE and RE (r > 0.8, P < 0.004). RNA content increased after AE + RE (24%, P = 0.019), but the change-scores did not differ across legs. The capillary variables generally increased in both fiber types, with no difference across legs. In conclusion, the accentuated hypertrophic response to AE + RE was associated with more pronounced myonuclear accretion, which was strongly correlated with the degree of fiber hypertrophy. This suggests that myonuclear accretion could play a role in facilitating muscle hypertrophy also during very short training periods.

Phenotypic Modulation of Skeletal Muscle Fibers in LPIN1-Deficient Lipodystrophic ( fld) Mice.

Lipin-1 ( Lpin1)-deficient lipodystrophic mice have scant and immature adipocytes and develop transient fatty liver early in life. Unlike normal mice, these mice cannot rely on stored triglycerides to generate adenosine triphosphate (ATP) from the ß-oxidation of fatty acids during periods of fasting. To compensate, these mice store much higher amounts of glycogen in skeletal muscle and liver than wild-type mice in order to support energy needs during periods of fasting. Our studies demonstrated that there are phenotypic changes in skeletal muscle fibers that reflect an adaptation to this unique metabolic situation. The phenotype of skeletal muscle (soleus, gastrocnemius, plantaris, and extensor digitorum longus [EDL]) from Lpin1-/- was evaluated using various methods including immunohistochemistry for myosin heavy chains (Myh) 1, 2, 2a, 2b, and 2x; enzyme histochemistry for myosin ATPase, cytochrome-c oxidase (COX), and succinyl dehydrogenase (SDH); periodic acid-Schiff; and transmission electron microscopy. Fiber-type changes in the soleus muscle of Lpin1-/- mice were prominent and included decreased Myh1 expression with concomitant increases in Myh2 expression and myosin-ATPase activity; this change was associated with an increase in the presence of Myh1/2a or Myh1/2x hybrid fibers. Alterations in mitochondrial enzyme activity (COX and SDH) were apparent in the myofibers in the soleus, gastrocnemius, plantaris, and EDL muscles. Electron microscopy revealed increases in the subsarcolemmal mitochondrial mass in the muscles of Lpin1-/- mice. These data demonstrate that lipin-1 deficiency results in phenotypic fiber-specific modulation of skeletal muscle necessary for compensatory fuel utilization adaptations in lipodystrophy.

THE IMPACT OF THE ACRYLIC MONOMER ON THE MORPHOLOGICAL STRUCTURE OF RAT LINGUAL MUCOSA.

The analysis of publications shows that diverse multiple factors can induce changes in taste sensitivity and the main irritants are the chemicals of different types. However, the study of the effect of the components of dental structural materials on the state of lingual mucosa, in particular, taste sensors, has not been fully elucidated to date. The purpose of the paper was the study of the effect of monomer of the "Ftoraks" base acrylic resin on the state of the rats' lingual mucosa within 2-4 weeks after its impact. The previous paper [5] presents the findings of the study on the impact of the monomer of the "Ftoraks" base acrylic resin on the state of the rats' lingual mucosa in the early period (1 to 7 days) and its subsequent regeneration. The studies have found that the greatest changes in the lingual mucosa occur on day 3 and 7 after the application of monomer, and are of erosive-inflammatory origin. Regeneration of the lingual epithelium is delayed. The studies confirm that the monomer of acrylic resin causes a number of pathological changes in the mucous membrane and muscles of the rat tongue, the nature of which varies depending on the duration of its impact. On day 14 in the lingual mucosa the destructive processes are significantly delayed, substituting for the sclerotic processes in the proper plate and atrophic processes, observed, first of all, in the papillae of the tongue. It is appropriate to assume that such changes in the papillae will lead to violation of the taste reception, first of all, in the areas of lateral surfaces of the body of the tongue and in the root area. At the same time, it should be noted that at the end of the experimental period (on day 28 of the contact of the monomer with the lingual mucosa), in the mucous membrane of the tongue, along with atrophic and sclerotic processes, the destructive changes and inflammatory reaction are evident. We hypothesize that this may indicate about partial recovery of taste sensitivity due to the decrease in the number of gustatory buds, taste papillae of different types and the increase in the period of their regeneration.

Slow- and fast-twitch hindlimb skeletal muscle phenotypes 12 wk after ⠚ nephrectomy in Wistar rats of both sexes.

This study describes fiber-type adaptations in hindlimb muscles, the interaction of sex, and the role of hypoxia on this response in 12-wk ⅚ nephrectomized rats (Nx). Contractile, metabolic, and morphological features of muscle fiber types were assessed in the slow-twitch soleus and the fast-twitch tibialis cranialis muscles of Nx rats, and compared with sham-operated controls. Rats of both sexes were considered in both groups. A slow-to-fast fiber-type transformation occurred in the tibialis cranialis of Nx rats, particularly in males. This adaptation was accomplished by impaired oxidative capacity and capillarity, increased glycolytic capacity, and no changes in size and nuclear density of muscle fiber types. An oxidative-to-glycolytic metabolic transformation was also found in the soleus muscle of Nx rats. However, a modest fast-to-slow fiber-type transformation, fiber hypertrophy, and nuclear proliferation were observed in soleus muscle fibers of male, but not of female, Nx rats. Serum testosterone levels decreased by 50% in male but not in female Nx rats. Hypoxia-inducible factor-1α protein level decreased by 42% in the tibialis cranialis muscle of male Nx rats. These data demonstrate that 12 wk of Nx induces a muscle-specific adaptive response in which myofibers do not change (or enlarge minimally) in size and nuclear density, but acquire markedly different contractile and metabolic characteristics, which are accompanied by capillary rarefaction. Muscle function and sex play relevant roles in these adaptations.

Urethral striated muscle and extracellular matrix morphological characteristics among mildly diabetic pregnant rats: translational approach.

INTRODUCTION AND HYPOTHESIS: Diabetes mellitus (DM) during pregnancy is associated with high levels of urinary incontinence (UI) and pelvic floor muscle dysfunction. Mild DM can lead to changes in urethral striated muscle and extracellular matrix (ECM) in pregnant rats considering both structures as an entire system responsible for urinary continence. METHODS: Ninety-two female Wistar rats were distributed in four experimental groups: virgin, pregnant, diabetic, and diabetic pregnant. In adult life, parental nondiabetic female rats were mated with nondiabetic male rats to obtain newborns. At the first day of birth, newborns received citrate buffer (nondiabetic group) or streptozotocin 100 mg/kg body weight, subcutaneous route (mild DM group). At day 21 of the pregnancy, the rats were lethally anesthetized and the urethra and vagina were extracted as a unit. Urethral and vaginal sections were cut and analyzed by: (a) cytochemical staining for ECM and muscle structural components, (b) immunohistochemistry to identify fast- and slow-muscle fibers, and (c) transmission electron microscopy for ultrastructural analysis of urethral striated muscle. RESULTS: In comparison with the three control groups, variations in the urethral striated muscle and ECM from diabetic pregnant rats were observed including thinning, atrophy, fibrosis, increased area of blood vessels, mitochondria accumulation, increased lipid droplets, glycogen granules associated with colocalization of fast and slow fibers, and a steady decrease in the proportion of fast to slow fibers. CONCLUSIONS: Mild DM and pregnancy can lead to a time-dependent disorder and tissue remodeling in which the urethral striated muscle and ECM has a fundamental function.

ACTN3 R577X genotypes associate with Class II and deepbite malocclusions.

INTRODUCTION: α-Actinins are myofibril anchor proteins that influence the contractile properties of skeletal muscles. ACTN2 is expressed in slow type I and fast type II fibers, whereas ACTN3 is expressed only in fast fibers. ACTN3 homozygosity for the 577X stop codon (ie, changing 577RR to 577XX, the R577X polymorphism) results in the absence of α-actinin-3 in about 18% of Europeans, diminishes fast contractile ability, enhances endurance performance, and reduces bone mass or bone mineral density. We have examined ACTN3 expression and genetic variation in the masseter muscle of orthognathic surgery patients to determine the genotype associations with malocclusion. METHODS: Clinical information, masseter muscle biopsies, and saliva samples were obtained from 60 subjects. Genotyping for ACTN3 single nucleotide polymorphisms, real-time polymerase chain reaction quantitation of muscle gene message, and muscle morphometric fiber type properties were compared to determine statistical differences between genotype and phenotype. RESULTS: Muscle mRNA expression level was significantly different for ACTN3 single nucleotide polymorphism genotypes (P <0.01). The frequency of ACTN3 genotypes was significantly different for the sagittal and vertical classifications of malocclusion, with the clearest association being elevated 577XX genotype in skeletal Class II malocclusion (P = 0.003). This genotype also resulted in significantly smaller diameters of fast type II fibers in masseter muscles (P = 0.002). CONCLUSION: ACTN3 577XX is overrepresented in subjects with skeletal Class II malocclusion, suggesting a biologic influence during bone growth. ACTN3 577XX is underrepresented in subjects with deepbite malocclusion, suggesting that muscle differences contribute to variations in vertical facial dimensions.

[Influence of modeling of gravitational unloading on the postsynaptic acetylcholine receptor organization and acetylcholinesterase activity in neuromuscular synapses of rat fast and slow muscles].

Using immunofluorescent techniques, we have revealed that, after 35 days of rats hindlimb unloading, neuromuscular synapses of fast and slow muscles show enhanced fluorescence intensity and decreased area of fluorescent staining of acetylcholine receptors; increased fluorescent intensity and area of fluorescent staining for acetylcholinesterase. The ratio of the number of postsynaptic acetylcholine receptors and the amount of acetylcholinesterase changed as well as their spatial position in relation to each other. These rearrangements correspond to electrophysiological data on the reduction of the amplitude of the miniature endplate currents in both muscles. Identified synapses restructuring accompanied by a decrease in the volume of muscle fibers. Hindlimb unloading (simulation of hypogravity) leads to an increase in functional activity of acetylcholinesterase on the background of reduced postsynaptic membrane area occupied by acetylcholine receptors. This leads to a decrease in the amplitude of excitatory postsynaptic potentials thereby reducing the nerve-muscle excitation transmission safety factor.

Developmental transition of touch response from slow muscle-mediated coilings to fast muscle-mediated burst swimming in zebrafish.

It is well known that slow and fast muscles are used for long-term sustained movement and short bursts of activity, respectively, in adult animal behaviors. However, the contribution of the slow and fast muscles in early animal movement has not been thoroughly explored. In wild-type zebrafish embryos, tactile stimulation induces coilings consisting of 1-3 alternating contractions of the trunk and tail at 24 hours postfertilization (hpf) and burst swimming at 48 hpf. But, embryos defective in flightless I homolog (flii), which encodes for an actin-regulating protein, exhibit normal coilings at 24 hpf that is followed by significantly slower burst swimming at 48 hpf. Interestingly, actin fibers are disorganized in mutant fast muscle but not in mutant slow muscle, suggesting that slower swimming at 48 hpf is attributable to defects of the fast muscle tissue. In fact, perturbation of the fast muscle contractions by eliminating Ca(2+) release only in fast muscle resulted in normal coilings at 24 hpf and slower burst swimming at 48 hpf, just as flii mutants exhibited. In contrast, specific inactivation of slow muscle by knockdown of the slow muscle myosin genes led to complete loss of coilings at 24 hpf, although normal burst swimming was retained by 48 hpf. These findings indicate that coilings at 24 hpf is mediated by slow muscle only, whereas burst swimming at 48 hpf is executed primarily by fast muscle. It is consistent with the fact that differentiation of fast muscle follows that of slow muscle. This is the first direct demonstration that slow and fast muscles have distinct physiologically relevant contribution in early motor development at different stages.

Genesis of muscle fiber-type diversity during mouse embryogenesis relies on Six1 and Six4 gene expression.

Adult skeletal muscles in vertebrates are composed of different types of myofibers endowed with distinct metabolic and contraction speed properties. Genesis of this fiber-type heterogeneity during development remains poorly known, at least in mammals. Six1 and Six4 homeoproteins of the Six/sine oculis family are expressed throughout muscle development in mice, and Six1 protein is enriched in the nuclei of adult fast-twitch myofibers. Furthermore, Six1/Six4 proteins are known to control the early activation of fast-type muscle genes in myocytes present in the mouse somitic myotome. Using double Six1:Six4 mutants (SixdKO) to dissect in vivo the genesis of muscle fiber-type heterogeneity, we analyzed here the phenotype of the dorsal/epaxial muscles remaining in SixdKO. We show by electron microscopy analysis that the absence of these homeoproteins precludes normal sarcomeric organization of the myofiber leading to a dystrophic aspect, and by immunohistochemistry experiments a deficiency in synaptogenesis. Affymetrix transcriptome analysis of the muscles remaining in E18.5 SixdKO identifies a major role for these homeoproteins in the control of genes that are specifically activated in the adult fast/glycolytic myofibers, particularly those controlling Ca(2+) homeostasis. Absence of Six1 and Six4 leads to the development of dorsal myofibers lacking expression of fast-type muscle genes, and mainly expressing a slow-type muscle program. The absence of restriction of the slow-type program during the fetal period in SixdKO back muscles is associated with a decreased HDAC4 protein level, and subcellular relocalization of the transcription repressor Sox6. Six genes thus behave as essential global regulators of muscle gene expression, as well as a central switch to drive the skeletal muscle fast phenotype during fetal development.

Muscle development and differentiation in the urodele Ambystoma mexicanum.

Muscle differentiation has been widely described in zebrafish and Xenopus, but nothing is known about this process in amphibian urodeles. Both anatomical features and locomotor activity in urodeles are known to show intermediate features between fish and anurans. Therefore, a better understanding of myogenesis in urodeles could be useful to clarify the evolutionary changes that led to the formation of skeletal muscle in the trunk of land vertebrates. We report here a detailed morphological and molecular investigation on several embryonic stages of Ambystoma mexicanum and show that the first differentiating muscle fibers are the slow ones, originating from a myoblast population initially localized close to the notochord that forms a superficial layer on the somitic surface afterwards. Subsequently, fast fibers differentiation ensues. We also identified and cloned A. mexicanum Myf5 as a muscle-specific transcriptional factor likely involved in urodele muscle differentiation.

Human masseter muscle fiber type properties, skeletal malocclusions, and muscle growth factor expression.

PURPOSE: We identified masseter muscle fiber type property differences in subjects with dentofacial deformities. PATIENTS AND METHODS: Samples of masseter muscle were collected from 139 young adults during mandibular osteotomy procedures to assess mean fiber areas and percent tissue occupancies for the 4 fiber types that comprise the muscle. Subjects were classified into 1 of 6 malocclusion groups based on the presence of a skeletal Class II or III sagittal dimension malocclusion and either a skeletal open, deep, or normal bite vertical dimension malocclusion. In a subpopulation, relative quantities of the muscle growth factors IGF-I and GDF-8 gene expression were quantified by real-time polymerase chain reaction. RESULTS: Fiber properties were not different in the sagittal malocclusion groups, but were very different in the vertical malocclusion groups (P ≤ .0004). There were significant mean fiber area differences for type II (P ≤ .0004) and type neonatal-atrial (P = .001) fiber types and for fiber percent occupancy differences for both type I-II hybrid fibers and type II fibers (P ≤ .0004). Growth factor expression differed by gender for IGF-I (P = .02) and GDF-8 (P < .01). The ratio of IGF-I:GDF-8 expression associates with type I and II mean fiber areas. CONCLUSION: Fiber type properties are very closely associated with variations in vertical growth of the face, with statistical significance for overall comparisons at P ≤ .0004. An increase in masseter muscle type II fiber mean fiber areas and percent tissue occupancies is inversely related to increases in vertical facial dimension.

Physiology, structure, and susceptibility to injury of skeletal muscle in mice lacking keratin 19-based and desmin-based intermediate filaments.

Intermediate filaments, composed of desmin and of keratins, play important roles in linking contractile elements to each other and to the sarcolemma in striated muscle. Our previous results show that the tibialis anterior (TA) muscles of mice lacking keratin 19 (K19) lose costameres, accumulate mitochondria under the sarcolemma, and generate lower specific tension than controls. Here we compare the physiology and morphology of TA muscles of mice lacking K19 with muscles lacking desmin or both proteins [double knockout (DKO)]. K19-/- mice and DKO mice showed a threefold increase in the levels of creatine kinase (CK) in the serum. The absence of desmin caused a larger change in specific tension (-40%) than the absence of K19 (-19%) and played the predominant role in contractile function (-40%) and decreased tolerance to exercise in the DKO muscle. By contrast, the absence of both proteins was required to obtain a significantly greater loss of contractile torque after injury (-48%) compared with wild type (-39%), as well as near-complete disruption of costameres. The DKO muscle also showed a significantly greater misalignment of myofibrils than either mutant alone. In contrast, large subsarcolemmal gaps and extensive accumulation of mitochondria were only seen in K19-null TA muscles, and the absence of both K19 and desmin yielded milder phenotypes. Our results suggest that keratin filaments containing K19- and desmin-based intermediate filaments can play independent, complementary, or antagonistic roles in the physiology and morphology of fast-twitch skeletal muscle.

Low intrinsic running capacity is associated with reduced skeletal muscle substrate oxidation and lower mitochondrial content in white skeletal muscle.

Chronic metabolic diseases develop from the complex interaction of environmental and genetic factors, although the extent to which each contributes to these disorders is unknown. Here, we test the hypothesis that artificial selection for low intrinsic aerobic running capacity is associated with reduced skeletal muscle metabolism and impaired metabolic health. Rat models for low- (LCR) and high- (HCR) intrinsic running capacity were derived from genetically heterogeneous N:NIH stock for 20 generations. Artificial selection produced a 530% difference in running capacity between LCR/HCR, which was associated with significant functional differences in glucose and lipid handling by skeletal muscle, as assessed by hindlimb perfusion. LCR had reduced rates of skeletal muscle glucose uptake (∼30%; P = 0.04), glucose oxidation (∼50%; P = 0.04), and lipid oxidation (∼40%; P = 0.02). Artificial selection for low aerobic capacity was also linked with reduced molecular signaling, decreased muscle glycogen, and triglyceride storage, and a lower mitochondrial content in skeletal muscle, with the most profound changes to these parameters evident in white rather than red muscle. We show that a low intrinsic aerobic running capacity confers reduced insulin sensitivity in skeletal muscle and is associated with impaired markers of metabolic health compared with high intrinsic running capacity. Furthermore, selection for high running capacity, in the absence of exercise training, endows increased skeletal muscle insulin sensitivity and oxidative capacity in specifically white muscle rather than red muscle. These data provide evidence that differences in white muscle may have a role in the divergent aerobic capacity observed in this generation of LCR/HCR.

Changes of SM muscles after STG harvest.

Semimembranosus (SM) muscle as well as Semitendinosus-Gracilis (STG) tendon have the same role in knee flexion and tibial internal rotation. Because STG tendons are generally used for anterior cruciate ligament (ACL) reconstruction, some compensational changes in SM muscle might have been induced. The purpose of this study was to investigate the changes of SM muscle affected by harvesting STG tendons. 10 Wistar-strain male rats were divided into control (C) and STG-dissected (STG) groups. Left STG tendons including the distal half of muscle portions were dissected in group STG and only skin incision was performed in group C. 4 weeks after the treatments, fiber types classification and ultrastructural observations were performed. In group STG the decrease of type IIa (fast-twitch fiber with high oxidative capacity) was observed in deep layers of SM muscle (p<0.01). In ultrastructural observations, the increase in lipid droplets and mitochondria and the irregularity of Z disc were observed in deep layers. These morphological changes indicated that the mechanical loading might increase in SM muscle after harvesting of STG. Because of minor injuries in SM muscle, hamstring strength exercise at early stage of rehabilitation program should be carefully performed following ACL reconstruction using STG tendons in clinical practice.

The masticatory system under varying functional load. Part 1: Structural adaptation of rabbit jaw muscles to reduced masticatory load.

Skeletal muscle fibres can change their myosin heavy-chain (MyHC) isoform and cross-sectional area, which determine their contraction velocity and maximum force generation, respectively, to adapt to varying functional loads. In general, reduced muscle activity induces transition towards faster fibres and a decrease in fibre cross-sectional area. In order to investigate the effect of a reduction in masticatory load on three functionally different jaw muscles, the MyHC composition and the corresponding cross-sectional area of fibres were determined in the superficial masseter, superficial temporalis, and digastric muscles of male juvenile New Zealand White rabbits that had been raised on a soft diet (n=8) from 8 to 20 weeks of age and in those of normal diet controls (n=8). Differences between groups were tested for statistical significance using a Mann-Whitney rank sum test. The proportion and cross-sectional area of fibres co-expressing MyHC-I and MyHC-cardiac alpha were significantly smaller in the masseter muscles of the animals that had been fed soft food than in those of the controls. In contrast, the proportions and cross-sectional areas of the various fibre types in the temporalis and digastric muscles did not differ significantly between the groups. The results suggest that reducing the masticatory load during development affects the contraction velocity and maximum force generation of the jaw-closing muscles that are primarily responsible for force generation during chewing. These muscles adapt structurally to the reduced functional load with changes in the MyHC composition and cross-sectional area mainly within their slow fibre compartment.

Satellite cell ablation attenuates short-term fast-to-slow fibre type transformations in rat fast-twitch skeletal muscle.

The purpose of this time-course study was to determine whether satellite cell ablation within rat tibialis anterior (TA) muscles exposed to short-term chronic low-frequency stimulation (CLFS) would limit fast-to-slow fibre type transformations. Satellite cells of the left TA were ablated by exposure to gamma-irradiation before 1, 2, 5 or 10 days of CLFS and 1 week later where required. Control groups received only CLFS or a sham operation. Continuous infusion of 5-bromo-2'-deoxyuridine revealed that CLFS first induced an increase in satellite cell proliferation at 1 day, up to a maximum at 10 days over control (mean +/- SEM, 5.7 +/- 0.7 and 20.4 +/- 1.0 versus 1.5 +/- 0.2 mm(-2), respectively, P < 0.007) that was abolished by gamma-irradiation. Myosin heavy chain mRNA, immunohistochemical and sodium dodecyl sulfate polyacrylamide gel electrophoresis analyses revealed CLFS-induced fast-to-slow fibre type transformation began at 5 days and continued at 10 days; in those muscles that were also exposed to gamma-irradiation, attenuation occurred within the fast fibre population, and the final fast-twitch to slow-twitch adaptation did not occur. These findings indicate satellite cells play active and obligatory roles early on in the time course during skeletal muscle fibre type adaptations to CLFS.

Phenotypical transitions and Ca2+ activation properties in human muscle fibers: effects of a 60-day bed rest and countermeasures.

Muscle biopsies were taken from soleus and vastus lateralis before and after a 60-day bed rest (BR) to examine expression changes in the regulatory proteins of the thin filament and in contractile function. Twenty-four women separated in three groups were submitted to BR or a combined protocol of resistance and aerobic exercises during BR or received a supplementation of amino acids during BR. Ca(2+)-tension relationships were established in single skinned fibers identified by their myosin heavy chain and troponin C isoform expressions. Expression patterns of regulatory proteins were analyzed on muscle pieces. For both muscles, BR produced similar decreases in slow and fast fiber diameters but larger decreases in P(0) maximal forces in slow than in fast fibers. Specific forces were decreased in slow soleus and vastus fibers, which displayed a reduction in Ca(2+) affinity. These changes were accompanied by slow-to-fast transitions in regulatory proteins, with troponins C and T appearing as sensitive markers of unloading. Exercises prevented the changes in fiber diameters and forces and counteracted most of the slow-to-fast transitions. The nutrition program had a morphological beneficial effect on slow fibers. However, these fibers still presented decreases in specific P(0) after BR. Phenotypical transitions due to BR were not prevented by amino acids. Finally, in vastus lateralis muscle, BR induced a decrease in O-glycosylation level that was prevented by exercise and attenuated by nutrition. In conclusion, this study has addressed for the first time in women the respective efficiencies of two countermeasures associated with BR on muscle properties and regulatory protein expression.

Acute effect of drop jumping on throwing performance.

The purpose of the present study was to investigate the acute effect of drop jumping on throwing performance. Eight men and 8 women, moderately trained subjects with basic shot put skills, performed 3 squat underhand front shot throws after a short standard warm-up. Three minutes later they performed 5 maximal consecutive drop jumps from 40 cm. Immediately after the drop jumps, they repeated the squat underhand front shot throws. On another day, their 6 repetition maximum (RM) muscular strength in leg press was assessed. Muscle biopsies were also obtained from vastus lateralis for the determination of fiber-type composition and fiber cross-sectional area. Throwing performance was significantly increased after drop jumping (8.25 +/- 1.1 m vs. 8.63 +/- 1.3 m, p < 0.01). The percentage of type II muscle fiber area was significantly related to the increase in throwing performance after drop jumping (r = 0.76, p < 0.01). The increase in throwing performance was significant in men (8.94 +/- 1 m vs. 9.60 +/- 0.9 m, p < 0.01) but not in women (7.56 +/- 1 m vs. 7.67 +/- 0.9 m, ns). Of note, the percentage of type II fiber area was higher in men than in women (M: 66.4 +/- 13%, F: 50.2 +/- 15%, p < 0.01). Leg press strength (6RM) was moderately related to the increase in throwing performance after drop jumping (r = 0.50, p < 0.05). These results suggest that drop jumping just before a throwing action induces an increase in performance in subjects with a high percentage of type II muscle fiber area and (to a lesser degree) in subjects with enhanced muscular strength.

Effects of ovariectomy on rat genioglossal muscle contractile properties and fiber-type distribution.

OBJECTIVE: To test the hypothesis that ovariectomy has no effects on contractile, histochemical, or biochemical properties of the rat genioglossus (GG). MATERIALS AND METHODS: Eight-week-old female Sprague-Dawley rats were randomly assigned into three groups: normal group (Normal), sham-operated group (Sham), and ovariectomized group (OVX). Four weeks later, genioglossal electromyography activity (EMGgg) and contractile properties were measured, including relative integrated EMG (iEMG), maximal twitch tension, 70%-decay time, and fatigue index (FI). Then rats were sacrificed and paired GG were removed for further analysis. Adenosine-triphosphatase (ATPase) staining was performed to determine the percent fiber-type distribution and to identify cross-sectional area (CSA) of muscle fibers. Myosin heavy chain (MHC) phenotypes were determined by gel electrophoresis. RESULTS: Ovariectomy reduced EMG activity and contractile properties of the GG. Following ovariectomy, the CSA of type IIA and the proportion of MHCIIA decreased significantly. The MHC isoform composition of GG transferred from relative slow-twitch to fast-twitch isoform, following the order MHCIIB --> MHCIIX --> MHCIIA. Sham operation had no effect on any of the parameters. CONCLUSIONS: The hypothesis is rejected. The contractile properties of the GG are sensitive to ovariectomy. These changes were, at least in part, associated with changes in the amount and type of contractile protein expressed.