Twitch Fibers Research Articles

Low proteasomal activity in fast skeletal muscle fibers is not associated with increased age-related oxidative damage.

The skeletal muscle is a crucial tissue for maintaining whole body homeostasis. Aging seems to have a disruptive effect on skeletal muscle homeostasis including proteostasis. However, how aging specifically impacts slow and fast twitch fiber types remains elusive. Muscle proteostasis is largely maintained by the proteasomal system. Here we characterized the proteasomal system in two different fiber types, using a non-sarcopenic aging model. By analyzing the proteasomal activity and amount, as well as the polyubiquitinated proteins and the level of protein oxidation in Musculus soleus (Sol) and Musculus extensor digitorum longus (EDL), we found that the slow twitch Sol muscle shows an overall higher respiratory and proteasomal activity in young and old animals. However, especially during aging the fast twitch EDL muscle reduces protein oxidation by an increase of antioxidant capacity. Thus, under adaptive non-sarcopenic conditions, the two fibers types seem to have different strategies to avoid age-related changes.

Physiological comparison of intensity-controlled, isocaloric intermittent and continuous exercise

VO2 fluctuations are argued to be an important mechanism underpinning chronic adaptations following interval training. We compared the effect of exercise modality, continuous vs. intermittent realized at a same intensity, on electrical muscular activity, muscular oxygenation and on whole body oxygen uptake. Twelve participants (24 ± 5 years; VO2peak: 43 ± 6 mL· min−1·kg−1) performed (i) an incremental test to exhaustion to determine peak work rate (WRpeak); two randomized isocaloric exercises at 70%WRpeak; (ii) 1 bout of 30 min; (iii) 30 bouts of 1 min work intercepted with 1 min passive recovery. For electromyography, only the CON exercise showed change for the vastus lateralis root-mean-square (+6.4 ± 5.1%, P < .01, 95%CI 3.2, 8.3) and mean power frequency (−5.2 ± 4.8, P < .01, 95%CI −8.2, −3.5). Metabolic fluctuations (i.e. Oxygen Fluctuation Index and HHb Fluctuation Index) were higher in the intermittent modality, while post-exercise blood lactate concentrations (4.80 ± 1.50 vs. 2.32 ± 1.21 mM, respectively, for the CON and INT, P < .01, 95%CI 1.72, 3.12) and the time spent over 90% of VO2 target (1644 ± 152 vs. 356 ± 301 sec, respectively, for the CON and INT, P < .01, 95%CI 1130, 1446) were higher in the continuous modality. In conclusion, despite a similar energy expenditure and intensity, intermittent and continuous exercises showed two very different physiological responses. The intermittent modality would lead to a larger recruitment of fast twitch fibres that are less mitochondria-equipped and therefore may be more likely respondent to mitochondrial adaptations. In addition, this modality induces greater metabolic variations, a stimulus who could lead to mitochondrial development.

Abundance of ClC-1 chloride channel in human skeletal muscle: fiber type specific differences and effect of training

Cl- channel protein 1 (ClC-1) may be important for excitability and contractility in skeletal muscle, but ClC-1 abundance has not been examined in human muscle. The aim of the present study was to examine ClC-1 abundance in human skeletal muscle, including fiber type specific differences and the effect of exercise training. A commercially available antibody was tested with positive and negative control tissue, and it recognized specifically ClC-1 in the range from 100 to 150 kDa. Abundance of ClC-1 was 38% higher ( P < 0.01) in fast twitch Type IIa muscle fibers than in slow twitch Type I. Muscle ClC-1 abundance did not change with 4 wk of training consisting of 30 min cycling at 85% of maximal heart rate (HRmax) and 3 × 30-s all out sprints or during a 7-wk training period with 10-12 × 30 s uphill cycling and 4-5 × ~4 min cycling at 90%-95% of HRmax. ClC-1 abundance correlated negatively ( P < 0.01) with maximal oxygen consumption ( r = -0.552) and incremental exercise performance ( r = -0.546). In addition, trained cyclists had lower ( P < 0.01) ClC-1 abundance than lesser trained individuals. The present observations indicate that a low abundance of muscle ClC-1 may be beneficial for exercise performance, but the role of abundance and regulation of ClC-1 in skeletal muscle of humans with respect to exercise performance and trainability need to be elucidated. NEW & NOTEWORTHY Abundance of the Cl- channel protein 1 (ClC-1) chloride channel may be important for excitability and contractility in human skeletal muscle and may therefore have implications for fatigue development. In this study, we confirmed ClC-1 specificity for a commercially available antibody, and this study is first to our knowledge to determine ClC-1 protein abundance in human muscle by Western blotting. We observed that abundance of ClC-1 was higher in fast compared with slow twitch fibers and lower in trained individuals than in recreationally active.

P-246 - Distinct proteasomal activity for fast and slow twitch skeletal muscle during aging

Skeletal muscle alterations during aging lead to dysfunctional metabolism, correlating with frailty and early mortality. The loss of proteostasis is a hallmark of aging. Whether proteostasis loss plays a role in muscle aging remains elusive. To address this question we collected muscles, Soleus (SOL, type I) and Extensor digitorum longus (EDL, type II), from young (4 months) and old (25 months) C57BL/6 mice and evaluated the proteasomal system. Initial work showed decreased 26 S activity in old SOL. EDL displayed lower proteasomal activity in both ages compared to any of the SOL ages. Moreover, in order to understand if during aging there is the so-called “fiber switch from fast-to-slow”, we performed western blots against sMHC and fMHC (slow and fast myosin heavy chain, respectively). Preliminary results suggest that young SOL is composed by slow twitch fibers but also contains fast twitch fibers, while young EDL seems to be mostly composed by fast twitch fibers that level down during aging, suggesting the switch. As a conclusion, EDL seems to have less proteasomal activity, however, if this is a contributor or a consequence to the muscle fiber switch during aging still needs further investigation.

A study of the mechanisms of excitation-contraction coupling in frog skeletal muscle based on measurements of [Ca2+] transients inside the sarcoplasmic reticulum.

[Ca2+] transients inside the sarcoplasmic reticulum (SR) were recorded in frog skeletal muscle twitch fibers under voltage clamp using the low affinity indicator Mag Fluo 4 (loaded in its AM form) with the purpose of studying the effect on Ca2+ release of extrinsic Ca2+ buffers (i.e. BAPTA) added at high concentration to the myoplasm. When the extrinsic Ca2+ buffer is added to the myoplasm, part of the released Ca2+ binds to it, reducing the Ca2+ signal reported by a myoplasmic indicator. This, in turn, hinders the quantification of the amount of Ca2+ released. Monitoring release by measuring [Ca2+] inside the SR avoids this problem. The application of extrinsic buffers at high concentration reduced the resting [Ca2+] in the SR ([Ca2+]SR) continuously from a starting value close to 400μM reaching the range of 100μM in about half an hour. The effect of reducing resting [Ca2+]SR on the Ca2+ permeability of the SR activated by voltage clamp depolarization to 0mV was studied in cells where the myoplasmic [Ca2+] ([Ca2+]myo) transients were simultaneously recorded with Rhod2. The Ca2+ release flux was calculated from [Ca2+]myo and divided by [Ca2+]SR to obtain the permeability. Peak permeability was significantly reduced, from 0.026 ± 0.005ms-1 at resting [Ca2+]SR = 372 ± 5μM to 0.021 ± 0.004ms-1 at resting [Ca2+]SR = 120 ± 16μM (n = 4, p = 0.03). The time averaged permeability was not significantly changed (0.009 ± 0.003 and 0.010 ± 0.003ms-1, at the higher and lower [Ca2+]SR respectively). Once the cells were equilibrated with the high buffer intracellular solution, the change in [Ca2+]SR (Δ[Ca2+]SR) in response to voltage clamp depolarization (0mV, 200ms) in 20mM BAPTA was significantly lower (Δ[Ca2+]SR = 30.2 ± 3.5μM from resting [Ca2+]SR = 88.8 ± 13.6μM, n = 5) than in 40mM EGTA (Δ[Ca2+]SR = 72.2 ± 10.4μM from resting [Ca2+]SR = 98.2 ± 15.6μM, n = 4) suggesting that a Ca2+ activated component of release was suppressed by BAPTA.

Muscle Fiber and Performance Changes after Fast Eccentric Complex Training.

The purpose of this study was to examine the effects of a short-term fast eccentric and ballistic complex training program on muscle power, rate of force development (RFD), muscle fiber composition, and cross-sectional area (CSA). Sixteen male physical education students were randomly assigned to either a training group (TG, n = 8) or a control group (n = 8). The TG followed a 6-wk low volume training program, including fast eccentric squat training with an individually optimized load of 74% ± 7% of maximal half-squat strength (1RM) twice per week and a ballistic training session with loaded (30% 1RM) and unloaded jump squats, once per week, all combined with unloaded plyometric jumps. Half squat 1RM was increased in the TG from 1.87 ± 0.28 to 2.14 ± 0.31 kg per kilogram body mass (14.4% ± 9.3%, P = 0.01). The percentage of types I, IIA, and IIX fibers were similar in the two groups at pretesting and did not change after the intervention period (P = 0.53-0.89). Muscle fiber CSA increased in all fiber types by 8.3% to 11.6% (P = 0.02 to 0.001) in TG only. Countermovement jump height and peak power measured at five different external loads (0%-65% of 1RM) only increased in the TG by approximately 20% to 36% (P < 0.01) and approximately 16% to 22% (P < 0.01), respectively. Peak ground reaction force during jump squats remained unchanged in both groups, whereas RFD increased in the TG only (40%-107%, P = 0.001). A combination of low-volume fast eccentric and ballistic jump squat training with plyometric jumps in a strength-power potentiation complex format, induced substantial increases in peak leg muscle power, RFD, and maximal strength, accompanied by gains in CSA of all muscle fiber types, without a reduction in fast twitch fiber composition.

Cardiomyocytes have mosaic patterns of protein expression.

Skeletal myocytes have well-established fast and slow twitch fibers with unique gene and protein specific expression patterns. By immunohistochemical staining, these show a mosaic pattern across myocytes. We hypothesized cardiac myocytes may behave similarly where some proteins are differentially expressed between mature cardiomyocytes. We utilized the tool HPASubC on over 52,000 cardiac images of the Human Protein Atlas to identify differential protein expression patterns by immunohistochemistry across the cardiomyocytes. We matched identified proteins to open chromatin and gene expression data. We identified 143 putative proteins with mosaic patterns of expression across the cardiomyocytes. We validated four of these proteins (MYL3, MYL4, PAM, and MYOM1) and demonstrated unique atrial or ventricular patterns of expression for each. Acetylation of histone H3K27 at the promoters of these four genes were consistent with the atrial/ventricular expression patterns. Despite the generally accepted homogeneity of cardiomyocytes, a small subset of proteins varies between cardiomyocytes in a mosaic pattern. This fundamental process has been previously uncharacterized. These changes may inform on different functional and disease-related activities of proteins in individual cardiomyocytes.

Redox responses are preserved across muscle fibres with differential susceptibility to aging

Age-related loss of muscle mass and function is associated with increased frailty and loss of independence. The mechanisms underlying the susceptibility of different muscle types to age-related atrophy are not fully understood. Reactive oxygen species (ROS) are recognised as important signalling molecules in healthy muscle and redox sensitive proteins can respond to intracellular changes in ROS concentrations modifying reactive thiol groups on Cysteine (Cys) residues. Conserved Cys residues tend to occur in functionally important locations and can have a direct impact on protein function through modifications at the active site or determining protein conformation. The aim of this work was to determine age-related changes in the redox proteome of two metabolically distinct murine skeletal muscles, the quadriceps a predominantly glycolytic muscle and the soleus which contains a higher proportion of mitochondria. To examine the effects of aging on the global proteome and the oxidation state of individual redox sensitive Cys residues, we employed a label free proteomics approach including a differential labelling of reduced and reversibly oxidised Cys residues. Our results indicate the proteomic response to aging is dependent on muscle type but redox changes that occur primarily in metabolic and cytoskeletal proteins are generally preserved between metabolically distinct tissues. Biological significanceSkeletal muscle containing fast twitch glycolytic fibres are more susceptible to age related atrophy compared to muscles with higher proportions of oxidative slow twitch fibres. Contracting skeletal muscle generates reactive oxygen species that are required for correct signalling and adaptation to exercise and it is also known that the intracellular redox environment changes with age. To identify potential mechanisms for the distinct response to age, this article combines a global proteomic approach and a differential labelling of reduced and reversibly oxidised Cysteine residues in two metabolically distinct skeletal muscles, quadriceps and soleus, from adult and old mice. Our results indicate that the global proteomic changes with age in skeletal muscles are dependent on fibre type. However, redox specific changes are preserved across muscle types and accompanied with a reduction in the number of redox sensitive Cysteine residues.

The Super Relaxed State of Myosin in Human Muscle

The Super Relaxed State of myosin (SRX) plays a role in maintaining the low resting metabolic rate of skeletal muscle. Pharmaceuticals that destabilize this state could increase resting muscle and whole body metabolic rate by as much as 1000 Calories/day, providing an effective treatment for obesity and type 2 diabetes. Our previous work on this state in skeletal muscle has all been in animal models. Here we extend this approach to human muscle. Biopsies from the quadriceps lateralis of both lean and obese subjects were used. Measurements of single turnovers showed an SRX with a population of 0.27 ± 0.02 and a lifetime of 150 ± 8 sec in lean subjects and a population of 0.28 ± 0.02 and a lifetime of 158 ± 10 sec in obese subjects. Piperine is a small molecule that has been shown to destabilize the SRX in rabbit fast twitch fibers, with no effect on slow twitch fibers. In human muscle, addition 100μM piperine reduced the population of the SRX by 34% in lean subjects and 25% in obese subjects, with little change in the lifetimes. The human quadriceps lateralis contains ∼50% fast twitch and ∼50% slow twitch fibers. Data obtained with piperine showed two responses with ∼50% of fibers showing a strong inhibition of the SRX and the remaining fibers showing very little, in both lean and obese subjects. Presumably the fibers showing strong inhibition are fast twitch, but this remains to be proven. We conclude that in human muscle the SRX and its response to piperine are similar to those seen previously in rabbit muscle, with little difference between lean and obese subjects. This shows that analogs of piperine that have higher affinity would provide effective treatment for metabolic diseases.

Distinct signal transductions in fast- and slow- twitch muscles upon denervation.

Denervation induces skeletal muscle atrophy, which primarily impairs oxidative slow twitch fibers. The underlying mechanism of this phenomenon, however, remains to be addressed. We hypothesize that denervation‐induced fiber‐specific atrophy may result from the distinct activities of different signaling pathways that are involved in protein synthesis and degradation in fast‐ and slow‐twitch fibers. In this study, 1‐month‐old male mice were subjected to unilateral sciatic denervation for 4 days. Fast‐twitch muscle extensor digitorum longus (EDL) and slow‐twitch muscle soleus were collected from the denervated side and the control side of hind limbs. Total and phosphorylated protein levels of key factors of major signaling pathways in these tissues were determined using western blot assay. Our data showed that total AKT and FoxO3 protein levels were upregulated in denervated muscles as compared with control sides. Phosphorylation of AKT and FoxO3 were proportionally enhanced in denervated EDL but not soleus, indicating AKT activation drives phosphorylation of FoxO3 in EDL but not in soleus upon denervation. As a result, FoxO3‐targeted atrogenes MurF1 and Atrogin1 protein abundances were reduced in denervated EDL but not altered in soleus. In consistent with this change, polyubiquitination were significantly increased in denervated soleus, but only a slight increase in ubiquitination was found in denervated EDL. Autophagy marker LC3 protein level was significantly increased in both muscle types, but in greater extent in EDL after denervation. IRS1 protein level and active ERK were reduced in both muscles upon denervation, which might contribute to the upregulation of total AKT protein level and FoxO3 abundance in EDL and soleus. Total and phosphorylated AMPK protein levels were increased in denervated soleus but not in EDL. Overall, these data reveal that the key signaling pathways that regulate protein synthesis and degradation are more sensitive in soleus than EDL in response to denervation.

Proteome analysis in dystrophic mdx mouse muscle reveals a drastic alteration of key metabolic and contractile proteins after chronic exercise and the potential modulation by anti-oxidant compounds

Weakness and fatigability are typical features of Duchenne muscular dystrophy patients and are aggravated in dystrophic mdx mice by chronic treadmill exercise. In the present study, we describe, the pattern of differentially abundant spots that is associated to the worsening of dystrophy phenotype induced by chronic exercise. Our proteomic analysis pointed out 34 protein spots with different abundance between sedentary and exercised mdx mice. These proteins belong mostly to glucose metabolism, energy production and sarcomere structure categories. Interestingly exercise induced an increase of typical fast twitch fiber proteins (Troponin T fast skeletal muscle, Troponin I fast skeletal muscle and Myozenin-1) combined with an increase of several glycolytic enzymes. Concerning energy transfer, Adenylate kinase, showed a marked decrease when compared with non-exercised mdx. The decline of this enzyme correlates with increased Creatin kinase enzyme, suggesting that a compensatory energy metabolism mechanism could be activated in mdx mouse skeletal muscle following exercise. In addition, we analysed muscles from exercised mdx mice treated with two natural anti-oxidant compounds, apocynin and taurine, that in our previous study, were proved to be beneficial on some pathology related parameters, and we showed that these compounds can counteract exercise-induced changes in the abundance of several proteins. SignificanceMdx mouse model of Duchenne muscular dystrophy shows a phenotype of the disorder milder than in human sufferers. This phenotype can be worsened by a different protocols of chronic exercise. These protocols can mimic the muscle progressive damage observed in humans, can allow studying the effects of inadequate training on dystrophic muscles and have been largely used to assess the ability of a drug to reduce the damage induced by exercise. In this study, we describe for the first time, the pattern of protein variation associated with the worsening of dystrophy phenotype induced by chronic exercise. Our proteomic analysis pointed out 34 protein spots with different amount between sedentary and exercised mdx mice. These proteins belong mostly to glucose metabolism, energy production and sarcomere structure categories and their variation indicates that mdx exercised muscle are not able to carry out the metabolic changes associated to fast-to-slow transition typically observed in aerobically trained muscle.

Age-related reductions in muscle quality influence the relative differences in strength and power

Age-related changes in the relative differences in isokinetic strength and power may reflect fast twitch fiber function. We aimed to examine the influence of muscle quality on the relative differences in strength and power in younger and older men. Twenty younger (20.1±1.5yrs) and 20 older (69.5±3.1yrs) healthy, recreationally active men performed two plantarflexion maximal voluntary isometric contractions (MVCs) and three maximal concentric isokinetic contractions at a slow (0.52rad·s−1) and fast (2.09rad·s−1) velocity. Absolute and normalized (%MVC) isokinetic peak torque (PT), mean power (MP), peak power (PP), the relative differences in PT (%decrease), MP and PP (%increase) from 0.52 to 2.09rad·s−1, and electromyographic (EMG) amplitude were examined. Ultrasonography was used to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality. The younger men exhibited greater absolute isometric PT, isokinetic PT, MP, and PP at 0.52 and 2.09rad·s−1 (P=0.001–0.003); but these differences were no longer present following normalization (P=0.079–0.954). The older men exhibited similar EMG amplitude values but higher EI values (P<0.001), a greater %decrease in PT (43.6% vs. 38.9%; P=0.006), and a lower %increase in MP (167.5% vs. 186.3%; P=0.049) and PP (125.5% vs. 144.5%; P=0.006). Echo intensity was related to the %decrease in PT (r=0.605; P<0.001), %increase in MP (r=−0.419; P=0.009), and %increase in PP (r=−0.605; P<0.001) from 0.52 to 2.09rad·s−1. The absolute age-related reductions in isokinetic strength and power were no longer present following normalization to isometric strength. However, age-related differences in strength and power remained intact when examining the relative differences from slow to fast velocities, which appear to be influenced by the qualitative changes in skeletal muscle.

Copper pyrithione, a booster biocide, induces abnormal muscle and notochord architecture in zebrafish embryogenesis.

The metal pyrithiones, principally zinc (ZnPT) and copper (CuPT), are replacing tributyltin (TBT) as antifouling agents. Zebrafish embryos were exposed within the first hour after fertilization to 12 and 64 µg/L of CuPT for 24 h. Morphological abnormalitiesin notochord and muscle architecture were observed at 96 h post fertilization (hpf). TEM revealed abnormal electron dense deposits in the notochord sheath and muscle fiber degeneration in animals treated with 12 µg/L of CuPT. Embryos that were exposed to 64 µg/L of CuPT displayed severe muscle fiber degeneration including abnormal A and I band patterning and altered z disk arrangement. Abnormalities in the notochord sheath, swelling of the mitochondria and numerous lipid whorls were also noted. Total antioxidant capacity was significantly decreased in embryos exposed to 12 and 64 µg/L of CuPT. Acridine orange staining revealed an increase in apoptosis particularly in the brain, eye, heart and tail regions of both treatment groups. Apoptosis was confirmed with an increase in caspase 3/7 activity in both treatment groups. Severe alternations in primary motor neuron axon extensions, slow tonic muscle fibers and fast twitch fibers were observed in CuPT treated embryos. There was a significant upregulation in sonic hedgehog and myod1 expression at 24 hpf in the 12 µg/L treatment group. Exposed zebrafish embryos showed ultra-structural hallmarks of peroxidative injury and cell death via apoptosis. These changes question the use of copper pyrithione as an antifouling agent.

Met-Activating Genetically Improved Chimeric Factor-1 Promotes Angiogenesis and Hypertrophy in Adult Myogenesis.

Myogenic progenitor cells (activated satellite cells) are able to express both HGF and its receptor cMet. After muscle injury, HGF-Met stimulation promotes activation and primary division of satellite cells. MAGIC-F1 (Met-Activating Genetically Improved Chimeric Factor-1) is an engineered protein that contains two human Met-binding domains that promotes muscle hypertrophy. MAGIC-F1 protects myogenic precursors against apoptosis and increases their fusion ability enhancing muscle differentiation. Hemizygous and homozygous Magic-F1 transgenic mice displayed constitutive muscle hypertrophy. Here we describe microarray analysis on Magic-F1 myogenic progenitor cells showing an altered gene signatures on muscular hypertrophy and angiogenesis compared to wild-type cells. In addition, we performed a functional analysis on Magic-F1+/+ transgenic mice versus controls using treadmill test. We demonstrated that Magic-F1+/+ mice display an increase in muscle mass and cross-sectional area leading to an improvement in running performance. Moreover, the presence of MAGIC-F1 affected positively the vascular network, increasing the vessel number in fast twitch fibers. Finally, the gene expression profile analysis of Magic-F1+/+ satellite cells evidenced transcriptomic changes in genes involved in the control of muscle growth, development and vascularisation. We showed that MAGIC-F1-induced muscle hypertrophy affects positively vascular network, increasing vessel number in fast twitch fibers. This was due to unique features of mammalian skeletal muscle and its remarkable ability to adapt promptly to different physiological demands by modulating the gene expression profile in myogenic progenitors.

Mitochondria Content In Rat Ankle Extensor Muscles Following Chronic Endurance Exercise

Endurance exercise studies involving animals typically measure muscle mitochondria content to verify intensity of training. Most use one of the three calf ankle extensor muscles, which vary in fiber type. The soleus is all slow twitch fibers (SO) while the plantaris and gastrocnemius are a mixture of fast twitch glycolytic (FG) and fast oxidative glycolytic (FOG) fibers. PURPOSE: To compare mitochondria content changes in the soleus, plantaris, and gastrocnemius in response to a widely used moderate intensity exercise program. METHODS: Male, 8-wk-old, Sprague-Dawley rats were divided into two groups: sedentary (S) and exercised (E) on a treadmill 5 d/wk for 6 wks. Exercise duration and intensity were progressively increased to 1 hr at 30 m/min up a 10.5% incline (75-80% VO2max). Whole muscles plus specific FG and FOG regions of gastrocnemius were homogenized (9-10 for each muscle or fiber type) and cytochrome c oxidase activity, a marker of mitochondria content, was determined using a Clark-type oxygen electrode. RESULTS: E significantly increased cytochrome c oxidase compared to the same S muscle (P<0.05) and the amount of increase was similar for all muscles (P>0.05). On a relative basis (% above mean S value), the increases were 60.7 ± 13.3 (mean ± standard error) in soleus, 72.3 ± 6.8 in plantaris, and 78.7 ± 12.4 in gastrocnemius. On an absolute basis (μMol O2 consumed/min/gram muscle), the increases were 28.5 ± 6.3 in soleus, 35.0 ± 3.3 in plantaris, and 25.7 ± 3.8 in gastrocnemius. E was also greater than S in all fiber types considered (P<0.05), but there were considerable differences in the amount of increase. On a relative basis (%), the increase in FOG fibers (115.5 ± 17.0) was significantly greater (P<0.05) than those in SO (60.7 ± 13.3) or FG (42.8 ± 12.0). On an absolute basis, all fibers differed from each other with FOG increasing the most and FG the least (P<0.05). Specifically, FOG increased by 53.4 ± 7.9, SO by 28.5 ± 6.3, and FG by only 5.1 ± 1.4. CONCLUSION: All three whole muscles increase by similar amounts in response to a moderate intensity (75-80% VO2max) exercise program, thus all are appropriate for verifying training status. Caution is advised when using small portions of plantaris and gastrocnemius because of differences in responses of FOG and FG fibers. Supported by UNCG School of HHS Research Excellence Grant.

Age-Related Reductions in Muscle Quality Influence the Relative Differences in Strength and Power

Recent studies have demonstrated that age-related changes in the relative differences in isokinetic strength and power may reflect fast twitch fiber alterations. It is possible skeletal muscle ultrasound (US) imaging may reflect these changes. PURPOSE: The purpose of this study was to examine the influence of muscle quality on the relative differences in strength and power in younger and older adults. METHODS: Twenty young (20.1±1.5 yrs) and 20 older (69.5±3.1 yrs) healthy, recreationally active men performed two plantar flexion maximal voluntary isometric actions (MVCs) and three maximal isokinetic actions on a calibrated isokinetic dynamometer. Panoramic brightness-mode US images of the medial and lateral gastrocnemius were taken to determine subcutaneous fat corrected echo intensity (EI) to represent muscle quality. Isokinetic peak torque (PT), mean power (MP), and peak power (PP) at 0.52 rad·s-1 and 2.09 rad · s-1 were normalized as %MVC. Six separate 2-way mixed factorial analyses of variance were used to evaluate absolute and normalized PT, MP, and PP. Independent samples t-tests were used to compare isometric PT, EI, %decrease in PT and %increase in MP and PP between the younger and older men. Pearson’s correlation coefficients were used to examine the relationship between EI and the %decrease in PT, %increase in MP, and %increase in PP. An alpha of P≤0.05 was used to determine statistical significance. RESULTS: The younger men exhibited greater absolute isometric PT, isokinetic PT, MP, and PP at 0.52 rad·s-1 and 2.09 rad·s-1 (P=0.001-0.003). After normalizing to isometric PT, age-related differences were eliminated. The older men exhibited higher EI (P<0.001), greater %decrease in PT (43.6% vs. 38.9%; P=0.006), and lower %increase in MP (167.5% vs. 186.3%; P=0.049) and PP (125.5% vs. 144.5%; P = 0.006). Echo intensity was significantly related with the %decrease in PT (r=0.605; P<0.001), %increase in MP (r=-0.419; P=0.009), and %increase in PP (r=-0.605; P<0.001) from 0.52 rad·s-1 to 2.09 rad·s-1. CONCLUSIONS: The absolute age-related reductions in isokinetic strength and power are eliminated with normalization. However, the relative differences from slow to fast velocities may reflect the age-related decreases in fast-twitch fiber and subsequent alterations in muscle quality.

Piperine’s mitigation of obesity and diabetes can be explained by its up-regulation of the metabolic rate of resting muscle

We identify a target for treating obesity and type 2 diabetes, the consumption of calories by an increase in the metabolic rate of resting skeletal muscle. The metabolic rate of skeletal muscle can be increased by shifting myosin heads from the super-relaxed state (SRX), with a low ATPase activity, to a disordered relaxed state (DRX), with a higher ATPase activity. The shift of myosin heads was detected by a change in fluorescent intensity of a probe attached to the myosin regulatory light chain in skinned skeletal fibers, allowing us to perform a high-throughput screen of 2,128 compounds. The screen identified one compound, which destabilized the super-relaxed state, piperine (the main alkaloid component of black pepper). Destabilization of the SRX by piperine was confirmed by single-nucleotide turnover measurements. The effect was only observed in fast twitch skeletal fibers and not in slow twitch fibers or cardiac tissues. Piperine increased ATPase activity of skinned relaxed fibers by 66 ± 15%. The Kd was ∼2 µM. Piperine had little effect on the mechanics of either fully active or resting muscle fibers. Previous work has shown that piperine can mitigate both obesity and type 2 diabetes in rodent models of these conditions. We propose that the increase in resting muscle metabolism contributes to these positive effects. The results described here show that up-regulation of resting muscle metabolism could treat obesity and type 2 diabetes and that piperine would provide a useful lead compound for the development of these therapies.

FoxO1: a novel insight into its molecular mechanisms in the regulation of skeletal muscle differentiation and fiber type specification.

FoxO1, a member of the forkhead transcription factor forkhead box protein O (FoxO) family, is predominantly expressed in most muscle types. FoxO1 is a key regulator of muscle growth, metabolism, cell proliferation and differentiation. In the past two decades, many researches have indicated that FoxO1 is a negative regulator of skeletal muscle differentiation while contrasting opinions consider that FoxO1 is crucial for myoblast fusion. FoxO1 is expressed much higher in fast twitch fiber enriched muscles than in slow muscles and is also closely related to muscle fiber type specification. In this review, we summarize the molecular mechanisms of FoxO1 in the regulation of skeletal muscle differentiation and fiber type specification.

Electrically and Hybrid-Induced Muscle Activations: Effects of Muscle Size and Fiber Type.

The effect of three electrical stimulation (ES) frequencies (10, 35, and 50 Hz) on two muscle groups with different proportions of fast and slow twitch fibers (abductor pollicis brevis (APB) and vastus lateralis (VL)) was explored. We evaluated the acute muscles’ responses individually and during hybrid activations (ES superimposed by voluntary activations). Surface electromyography (sEMG) and force measurements were evaluated as outcomes. Ten healthy adults (mean age: 24.4 ± 2.5 years) participated after signing an informed consent form approved by the university Institutional Review Board. Protocols were developed to: 1) compare EMG activities during each frequency for each muscle when generating 25% Maximum Voluntary Contraction (MVC) force, and 2) compare EMG activities during each frequency when additional voluntary activation was superimposed over ES-induced 25% MVC to reach 50% and 75% MVC. Empirical mode decomposition (EMD) was utilized to separate ES artifacts from voluntary muscle activation. For both muscles, higher stimulation frequency (35 and 50Hz) induced higher electrical output detected at 25% of MVC, suggesting more recruitment with higher frequencies. Hybrid activation generated proportionally less electrical activity than ES alone. ES and voluntary activations appear to generate two different modes of muscle recruitment. ES may provoke muscle strength by activating more fatiguing fast acting fibers, but voluntary activation elicits more muscle coordination. Therefore, during the hybrid activation, less electrical activity may be detected due to recruitment of more fatigue-resistant deeper muscle fibers, not reachable by surface EMG.

Effects of conopeptide-containing venom from seven species of Conidae gastropoda on the chick biventer-cervicis nerve-muscle assessed using the ConoServer database.

Conotoxins in the venom of marine gastropods (genus Conus, family Conidae) have been incriminated in fatal human stingings. Conotoxins are peptides (conopeptides) which target specific classes of ion channels and block receptors involved in neuromuscular transmission. Some conopeptides also block receptors involved in neuropathic pain and one such peptide with an analgesic potency greater than that of morphine is marketed for clinical use. To determine the effects of venom from seven species of Conidae, Conus arenatus, Conus coronatus, Conus ebraeus, Conus lividus, Conus miles, Conus rattus, and Conus textile, collected in the inter-tidal zone of the Indian Ocean, East Africa, on the chick biventer-cervicis nervemuscle preparation and to assess the effects using data on conopeptide content in venom of the species examined reported in the literature and the ConoServer database. Only venom extracts from C. arenatus and C. textile, blocked twitch responses and produced depolarization and contracture of slow fibers of the stimulated chick nerve-muscle preparation. This is the first study showing that venom from C. arenatus is a potent inhibitor of neuromuscular transmission. However, in the case of C. textile, a species associated with fatal human stingings, the inhibitor activity was ~ 3-fold greater. These results are consistent with the occurrence of specific α-conopeptides, namely α-4/6-CtxTxID in C. textile and α-CtxArIB in C. arenatus targeting acetylcholine receptors at the neuromuscular junction. Information extractable from the ConoServer database was of limited value for evaluation of our findings since all the species examined contain numerous conopeptides, the majority of which have not been characterized pharmacologically or for which even the gene superfamily is unknown. Venom from C. textile, C. arenatus, C. coronatus, C. ebraeus, and C. rattus produced an initial facilitation of the twitch response similar to that produced by neostigmine. Venom from C. lividus and C. miles had no effect on twitch responses and did not depolarize slow fibers even at high concentrations. Using the chick biventer-cervicis nerve-muscle preparation, which contains both twitch and slow muscle fibers, a neuromuscular blocking and muscle depolarizing action could be demonstrated in venom extracts from C. textile, a Conus species associated with fatal human stingings, and C. arenatus. The results are consistent with the known presence of specific α-conopeptides in these species targeting nAChRs. Venom from C. coronatus, C. ebraeus, C. rattus, C. lividus, and C. miles, although purported to contained numerous conopeptides belonging to a variety of pharmacological classes, were either inactive on the preparation or caused only a minor potentiation of the twitch response. Although the ConoServer database provides valuable global data on conopeptide structure, occurrence and properties, it lacks specific information on receptor targets and affinities.