Slow-twitch Oxidative Muscle Research Articles

Caveolin-3 regulates slow oxidative myofiber formation in female mice

Abstract Objectives Caveolin-3 (Cav-3) plays a pivotal role in maintaining skeletal muscle mass and function. Mutations or deletions of Cav-3 can result in the development of various forms of myopathy, which affect the integrity and repair capacity of muscle fiber membranes. However, the potential effect of Cav-3 on myofiber type composition remains unclear. Methods To investigate the effect of Cav-3 on muscle strength and running capacity, we examined the grip force test and the low/high-speed running test. Oxidative and glycolytic myofiber-related genes, proteins, and skeletal muscle fiber composition were measured to determine the role of the Cav-3 in oxidative myofiber formation. Results We report the impact of Cav-3 on enhancing muscle endurance performance in female mice, and the discovery of a new physiological function to increase the proportion of slow-twitch oxidative muscle fiber by analyzing the gastrocnemius and soleus. Skeletal muscle-specific ablation of Cav-3 in female mice increased oxidative myofiber-related gene expression and type I oxidative myofiber composition, with resultant improvements in endurance performance. In male mice, the absence of Cav-3 in skeletal muscle reduced in the expression of glycolytic fiber-related genes and proteins. Conclusions This study identified Cav-3 as a regulator of slow-twitch oxidative muscle fiber formation acting on female mice, which may provide a potential target for improving muscle oxidative function.

Exercise Training Decreases Nitrite Concentration in the Heart and Locomotory Muscles of Rats Without Changing the Muscle Nitrate Content.

Skeletal muscles are postulated to be a potent regulator of systemic nitric oxide homeostasis. In this study, we aimed to evaluate the impact of physical training on the heart and skeletal muscle nitric oxide bioavailability (judged on the basis of intramuscular nitrite and nitrate) in rats. Rats were trained on a treadmill for 8 weeks, performing mainly endurance running sessions with some sprinting runs. Muscle nitrite (NO2-) and nitrate (NO3-) concentrations were measured using a high-performance liquid chromatography-based method, while amino acids, pyruvate, lactate, and reduced and oxidized glutathione were determined using a liquid chromatography coupled with tandem mass spectrometry technique. The content of muscle nitrite reductases (electron transport chain proteins, myoglobin, and xanthine oxidase) was assessed by western immunoblotting. We found that 8 weeks of endurance training decreased basal NO2- in the locomotory muscles and in the heart, without changes in the basal NO3-. In the slow-twitch oxidative soleus muscle, the decrease in NO2- was already present after the first week of training, and the content of nitrite reductases remained unchanged throughout the entire period of training, except for the electron transport chain protein content, which increased no sooner than after 8 weeks of training. Muscle NO2- level, opposed to NO3-, decreases in the time course of training. This effect is rapid and already visible in the slow-oxidative soleus after the first week of training. The underlying mechanisms of training-induced muscle NO2- decrease may involve an increase in the oxidative stress, as well as metabolite changes related to an increased muscle anaerobic glycolytic activity contributing to (1) direct chemical reduction of NO2- or (2) activation of muscle nitrite reductases.

Poor hygiene of housing conditions influences energy metabolism in a muscle type-dependent manner in growing pigs differing in feed efficiency

The ability of pigs to cope with inflammatory challenges may by modified by selection for residual feed intake (RFI), a measure of feed efficiency. In the current study, we evaluated skeletal muscle metabolic responses to degraded hygiene conditions in pigs divergently selected for RFI. At 82 d of age, low RFI and high RFI pigs were housed in either poor or good hygiene conditions. After a 6-week challenge, the poor hygiene conditions induced a decrease in growth performance (P < 0.001) and in plasma IGF-I concentrations (P < 0.003) in both lines. In the slow-twitch oxidative semispinalis muscle, poor hygiene conditions induced a shift towards a more oxidative metabolism and an activation of the AMPK pathway in pigs of both RFI lines. In the fast-twitch glycolytic longississimus muscle, poor hygiene conditions were associated to a less glycolytic metabolism in the HRFI line only. Poor hygiene conditions also increased the protein level of lipidation of microtubule-associated protein 1 light-chain 3β (LC3-II) in both RFI lines, suggesting an activation of the autophagy pathway. Altogether, the data revealed muscle-type specific metabolic adaptations to poor hygiene conditions, which may be related to different strategies to fuel the activated immune system.

Plantar stimulation prevents the decrease in fatigue resistance in rat soleus muscle under one week of hindlimb suspension

Support afferentation in recent years was shown to be a key physiological stimulus controlling postural muscle function, structure and phenotype. Lack of support afferentation under various types of muscle disuse leads to a decline of size and percentage of slow-type fatigue-resistant muscle fibers, which can negatively affect muscle performance and life quality. In this study we simulated support afferentation during rat hindlimb unloading and investigated its effect on postural soleus muscle functional properties and signaling. Plantar mechanical stimulation prevented the unloading-induced muscle fatigue increase, maintained the level of mitochondrial DNA copy number and the percent of slow-type muscle fibers and partially prevented the increase of CpG methylation in pgc 1α promoter region and decline in myonuclear content of several transcriptional activators of slow myosin and PGC1 α expression. So, support afferentation under hindlimb suspension leads to maintaining of a slow-twitch oxidative and fatigue-resistant soleus muscle fibers phenotype.

Strenuous Acute Exercise Induces Slow and Fast Twitch-Dependent NADPH Oxidase Expression in Rat Skeletal Muscle.

The enzymatic complex Nicotinamide Adenine Dinucleotide Phosphate (NADPH) oxidase (NOx) may be the principal source of reactive oxygen species (ROS). The NOX2 and NOX4 isoforms are tissue-dependent and are differentially expressed in slow-twitch fibers (type I fibers) and fast-twitch fibers (type II fibers) of skeletal muscle, making them different markers of ROS metabolism induced by physical exercise. The aim of this study was to investigate NOx signaling, as a non-adaptive and non-cumulative response, in the predominant fiber types of rat skeletal muscles 24 h after one strenuous treadmill exercise session. The levels of mRNA, reduced glycogen, thiol content, NOx, superoxide dismutase, catalase, glutathione peroxidase activity, and PPARGC1α and SLC2A4 gene expression were measured in the white gastrocnemius (WG) portion, the red gastrocnemius (RG) portion, and the soleus muscle (SOL). NOx activity showed higher values in the SOL muscle compared to the RG and WG portions. The same was true of the NOX2 and NOX4 mRNA levels, antioxidant enzymatic activities, glycogen content. Twenty-four hours after the strenuous exercise session, NOx expression increased in slow-twitch oxidative fibers. The acute strenuous exercise condition showed an attenuation of oxidative stress and an upregulation of antioxidant activity through PPARGC1α gene activity, antioxidant defense adaptations, and differential gene expression according to the predominant fiber type. The most prominent location of detoxification (indicated by NOX4 activation) in the slow-twitch oxidative SOL muscle was the mitochondria, while the fast-twitch oxidative RG portion showed a more cytosolic location. Glycolytic metabolism in the WG portion suggested possible NOX2/NOX4 non-regulation, indicating other possible ROS regulation pathways.

Alteration of muscle fiber characteristics and the AMPK-SIRT1-PGC-1α axis in skeletal muscle of growing pigs fed low-protein diets with varying branched-chain amino acid ratios.

There mainly exists four major myosin heavy chains (MyHC) (i.e., I, IIa, IIx, and IIb) in growing pigs. The current study aimed to explore the effects of low-protein diets supplemented with varying branched-chain amino acids (BCAAs) on muscle fiber characteristics and the AMPK-SIRT1-PGC-1α axis in skeletal muscles. Forty growing pigs (9.85 ± 0.35 kg) were allotted to 5 groups and fed with diets supplemented with varying leucine: isoleucine: valine ratios: 1:0.51:0.63 (20% crude protein, CP), 1:1:1 (17% CP), 1:0.75:0.75 (17% CP), 1:0.51:0.63 (17% CP), and 1:0.25:0.25 (17% CP), respectively. The skeletal muscles of different muscle fiber composition, that is, longissimus dorsi muscle (LM, a fast-twitch glycolytic muscle), biceps femoris muscle (BM, a mixed slow- and fast-twitch oxido-glycolytic muscle), and psoas major muscle (PM, a slow-twitch oxidative muscle) were collected and analyzed. Results showed that relative to the control group (1:0.51:0.63, 20% CP), the low-protein diets with the leucine: isoleucine: valine ratio ranging from 1:0.75:0.75 to 1:0.25:0.25 especially augmented the mRNA and protein abundance of MyHC I fibers in BM and lowered the mRNA abundance of MyHC IIb particularly in LM (P < 0.05), with a concurrent increase in the activation of AMPK and the mRNA abundance of SIRT and PGC-1α in BM (P < 0.05). The results reveal that low-protein diets supplemented with optimal BCAA ratio, i.e. 1:0.75:0.75-1:0.25:0.25, induce muscle more oxidative especially in oxido-glycolytic skeletal muscle of growing pigs. These effects are likely associated with the activation of the AMPK-SIRT1-PGC-1α axis.

The relationship of pork longissimus muscle pH to mitochondrial respiratory activities, meat quality and muscle structure

The way the pH falls post-slaughter has an impact on meat quality. Pork longissimus muscles (n=48) were sorted in fast- (FG) (n=15) and normal glycolysing (NG) (n=33) muscles according to the post-slaughter pH 45 min values (FG<6.0; NG>6.0). FG muscles (5.84±0.04) compared with NG muscles (6.27±0.04) were accompanied with higher temperature, electrical conductivity, lightness and yellowness, and reduced grill loss and shear force values (P<0.05), but there were no pH-dependent changes of the drip loss and redness results. FG muscles had higher (P<0.05) percentages of fast-twitch glycolytic and lower proportions of fast-twitch oxidative and slow-twitch oxidative (P<0.05) muscle fibres. The study confirms the relationship of pH value to meat quality and muscle fibre characteristics also showing that pH values have no impact on intrinsic mitochondrial respiratory function.

Hyperpolarized Functional Magnetic Resonance of Murine Skeletal Muscle Enabled by Multiple Tracer-Paradigm Synchronizations

Measuring metabolism's time- and space-dependent responses upon stimulation lies at the core of functional magnetic resonance imaging. While focusing on water's sole resonance, further insight could arise from monitoring the temporal responses arising from the metabolites themselves, in what is known as functional magnetic resonance spectroscopy. Performing these measurements in real time, however, is severely challenged by the short functional timescales and low concentrations of natural metabolites. Dissolution dynamic nuclear polarization is an emerging technique that can potentially alleviate this, as it provides a massive sensitivity enhancement allowing one to probe low-concentration tracers and products in a single-scan. Still, conventional implementations of this hyperpolarization approach are not immediately amenable to the repeated acquisitions needed in real-time functional settings. This work proposes a strategy for functional magnetic resonance of hyperpolarized metabolites that bypasses this limitation, and enables the observation of real-time metabolic changes through the synchronization of stimuli-triggered, multiple-bolus injections of the metabolic tracer 13C1-pyruvate. This new approach is demonstrated with paradigms tailored to reveal in vivo thresholds of murine hind-limb skeletal muscle activation, involving the conversion of 13C1-pyruvate to 13C1-lactate and 13C1-alanine. These functional hind-limb studies revealed that graded skeletal muscle stimulation causes commensurate increases in glycolytic metabolism in a frequency- and amplitude-dependent fashion, that can be monitored on the seconds/minutes timescale using dissolution dynamic nuclear polarization. Spectroscopic imaging further allowed the in vivo visualization of uptake, transformation and distribution of the tracer and products, in fast-twitch glycolytic and in slow-twitch oxidative muscle fiber groups. While these studies open vistas in time and sensitivity for metabolic functional magnetic resonance studies in muscle, the simplicity of our approach makes this technique amenable to a wide range of functional metabolic tracer studies.

Sedentary behavior as a mediator of type 2 diabetes.

Over the past 5 years, the fastest growing new area of physical activity research centered around the concept that the large amount of time people spend sitting inactive may have significant physiological consequences hazardous to human health, including risk for type 2 diabetes and poor metabolism of lipids and glucose. Meta-analysis (10 studies) suggests there is a 112% greater relative risk associated with a large duration of sedentary behavior for type 2 diabetes. Meta-analysis also indicates significantly greater odds for metabolic syndrome. We also summarize results for 7 studies using objective measures of total sedentary time and focusing on cardiometabolic risks in persons at high risk for type 2 diabetes or already diagnosed with type 2 diabetes. The underlying hypothesis introduced in 2004 by the inactivity physiology paradigm has been that frequent and abundant contractile activity by certain types of skeletal muscle can have a potent influence on key physiological processes, even when the intensity is below that achieved through exercise. We explain some of the mechanisms for why the metabolism in slow-twitch oxidative skeletal muscle is key for understanding the healthy responses to low-intensity physical activity (LIPA). Findings from objective measures from inclinometry indicated that the quartile range for weekly sedentary time is ∼29 h/week. The total daily time that people sit, stand, and accumulate nonexercise steps is independent of traditionally recommended moderate-vigorous physical activity. The large amount of sedentary time associated with risk for disease can only be reduced significantly with safe and nonfatiguing LIPA, especially in the most at-risk proportion of the population. Importantly, experimental studies are starting to indicate that it will be especially insightful to understand the acute dose-response effects of LIPA in order to understand why reducing sedentary time can improve lipid and glucose metabolism for the prevention and treatment of chronic disorders related to type 2 diabetes.

Fast-Twitch Glycolytic Skeletal Muscle Is Predisposed to Age-Induced Impairments in Mitochondrial Function

The etiology of mammalian senescence is suggested to involve the progressive impairment of mitochondrial function; however, direct observations of age-induced alterations in actual respiratory chain function are lacking. Accordingly, we assessed mitochondrial function via high-resolution respirometry and mitochondrial protein expression in soleus, quadricep, and lateral gastrocnemius skeletal muscles, which represent type 1 slow-twitch oxidative muscle (soleus) and type 2 fast-twitch glycolytic muscle (quadricep and gastrocnemius), respectively, in young (10-12 weeks) and mature (74-76 weeks) mice. Electron transport through mitochondrial complexes I and III increases with age in quadricep and gastrocnemius, which is not observed in soleus. Mitochondrial coupling efficiency during respiration through complex I also deteriorates with age in gastrocnemius and shows a tendency (p = .085) to worsen in quadricep. These data demonstrate actual alterations in electron transport function that occurs with age and are dependent on skeletal muscle type.

Urocortins Improve Dystrophic Skeletal Muscle Structure and Function through Both PKA- and Epac-Dependent Pathways

In Duchenne muscular dystrophy, the absence of dystrophin causes progressive muscle wasting and premature death. Excessive calcium influx is thought to initiate the pathogenic cascade, resulting in muscle cell death. Urocortins (Ucns) have protected muscle in several experimental paradigms. Herein, we demonstrate that daily s.c. injections of either Ucn 1 or Ucn 2 to 3-week-old dystrophic mdx(5Cv) mice for 2 weeks increased skeletal muscle mass and normalized plasma creatine kinase activity. Histological examination showed that Ucns remarkably reduced necrosis in the diaphragm and slow- and fast-twitch muscles. Ucns improved muscle resistance to mechanical stress provoked by repetitive tetanizations. Ucn 2 treatment resulted in faster kinetics of contraction and relaxation and a rightward shift of the force-frequency curve, suggesting improved calcium homeostasis. Ucn 2 decreased calcium influx into freshly isolated dystrophic muscles. Pharmacological manipulation demonstrated that the mechanism involved the corticotropin-releasing factor type 2 receptor, cAMP elevation, and activation of both protein kinase A and the cAMP-binding protein Epac. Moreover, both STIM1, the calcium sensor that initiates the assembly of store-operated channels, and the calcium-independent phospholipase A(2) that activates these channels were reduced in dystrophic muscle by Ucn 2. Altogether, our results demonstrate the high potency of Ucns for improving dystrophic muscle structure and function, suggesting that these peptides may be considered for treatment of Duchenne muscular dystrophy.

The effects of β-adrenergic stimulation and exercise on NR4A3 protein expression in rat skeletal muscle

β-Adrenergic stimulation and exercise up-regulate the mRNA expression of nuclear receptor NR4A3, which is involved in the regulation of glucose and fatty acid utilization genes in skeletal muscle. The objective of our study was to examine the effects of β-adrenergic stimulation and exercise on the expression of NR4A3 protein in rat skeletal muscle. A single subcutaneous injection of clenbuterol, which is a β2-adrenergic receptor (β2-AR) agonist, increased NR4A3 mRNA and protein expression in the fast-twitch glycolytic triceps muscle. On the other hand, an acute 3-h session of either treadmill running or swimming did not increase the NR4A3 protein level in the exercised muscle, although both treadmill running and swimming increased NR4A3 mRNA. Finally, loss of postural contractile activity because of hindlimb immobilization reduced NR4A3 mRNA and protein in the slow-twitch oxidative soleus muscle. These results suggest that: β-adrenergic stimulation up-regulates not only NR4A3 mRNA but also NR4A3 protein in fast-twitch glycolytic muscle; exercise may increase NR4A3 mRNA but not NR4A3 protein in skeletal muscle; and local postural contractile activity plays a crucial role in maintaining NR4A3 protein expression level in postural muscle.

Transcription of the human uncoupling protein 3 gene is governed by a complex interplay between the promoter and intronic sequences

Uncoupling protein (UCP) 3 is an inner mitochondrial membrane transporter mainly produced in skeletal muscle in humans. UCP3 plays a role in fatty acid metabolism and energy homeostasis and modulates insulin sensitivity. In humans, UCP3 content is higher in fast-twitch glycolytic muscle than in slow-twitch oxidative muscle and is dysregulated in type 2 diabetes. Here, we studied the molecular mechanisms determining human UCP3 levels in skeletal muscle and their regulation by fasting in transgenic mice. We produced a series of transgenic lines with constructs bearing different putative regulatory regions of the human UCP3 gene, including promoter and intron sequences. UCP3 mRNA and reporter gene expression and activity were measured in different skeletal muscles and tissues. The profile of expression and the response to fasting and thyroid hormone of human UCP3 mRNA in transgenic mice with 16 kb of the human UCP3 gene were similar to that of the endogenous human gene. Various parts of the UCP3 promoter did not confer expression in transgenic lines. Inclusion of intron 1 resulted in an expression profile in skeletal muscle that was identical to that of human UCP3 mRNA. Further dissection of intron 1 revealed that distinct regions were involved in skeletal muscle expression, distribution among fibre types and response to fasting. The control of human UCP3 transcription in skeletal muscle is not solely conferred by the promoter, but depends on several cis-acting elements in intron 1, suggesting a complex interplay between the promoter and intronic sequences.

Unravelling the mysteries of the exercise pressor reflex at the cellular level

Unravelling the mysteries of the exercise pressor reflex at the cellular level

The Orphan Nuclear Receptor, NOR-1, a Target of β-Adrenergic Signaling, Regulates Gene Expression that Controls Oxidative Metabolism in Skeletal Muscle

beta 1-3-Adrenoreceptor (AR)-deficient mice are unable to regulate energy expenditure and develop diet-induced obesity on a high-fat diet. We determined previously that beta2-AR agonist treatment activated expression of the mRNA encoding the orphan nuclear receptor, NOR-1, in muscle cells and plantaris muscle. Here we show that beta2-AR agonist treatment significantly and transiently activated the expression of NOR-1 (and the other members of the NR4A subgroup) in slow-twitch oxidative soleus muscle and fast-twitch glycolytic tibialis anterior muscle. The activation induced by beta-adrenergic signaling is consistent with the involvement of protein kinase A, MAPK, and phosphorylation of cAMP response element-binding protein. Stable cell lines transfected with a silent interfering RNA targeting NOR-1 displayed decreased palmitate oxidation and lactate accumulation. In concordance with these observations, ATP production in the NOR-1 silent interfering RNA (but not control)-transfected cells was resistant to (azide-mediated) inhibition of oxidative metabolism and expressed significantly higher levels of hypoxia inducible factor-1alpha. In addition, we observed the repression of genes that promote fatty acid oxidation (peroxisomal proliferator-activated receptor-gamma coactivator-1alpha/beta and lipin-1alpha) and trichloroacetic acid cycle-mediated carbohydrate (pyruvate) oxidation [pyruvate dehydrogenase phosphatase 1 regulatory and catalytic subunits (pyruvate dehydrogenase phosphatases-1r and -c)]. Furthermore, we observed that beta2-AR agonist administration in mouse skeletal muscle induced the expression of genes that activate fatty acid oxidation and modulate pyruvate use, including PGC-1alpha, lipin-1alpha, FOXO1, and PDK4. Finally, we demonstrate that NOR-1 is recruited to the lipin-1alpha and PDK-4 promoters, and this is consistent with NOR-1-mediated regulation of these genes. In conclusion, NOR-1 is necessary for oxidative metabolism in skeletal muscle.

The Expression of MMP-2 Following Immobilization and High-Intensity Running in Plantaris Muscle Fiber in Rats

The effect of 2-week, high-intensity running and a 2-week immobilization on muscle fiber type composition of the plantaris muscle from 18 female, 6-month-old Wistar rats (running, n = 6; immobilization, n = 6; sedentary control, n = 6) was bio- and histochemically investigated. The high-intensity treadmill running began with 20 min (32 m/min, 0% gradient, 75% VO2 max), up to 50 min/day. Right hind limbs were immobilized by an external fixation procedure for 13 days. Muscle mass of the plantaris muscle in the immobilized groups was reduced by 16% in comparison with the sedentary control group. High-intensity running and immobilization increased both mRNA and protein levels of matrix metalloproteinase type 2 (MMP-2) in plantaris. Running and immobilization decreased the percentages of transverse sectional area of fast-twitch glycolytic (FG) type IIb fibers, running increased relative cross-sectional area of fast-twitch oxidative glycolytic (FOG) type IIa muscle fibers, whereas immobilization increased relative cross-sectional area of slow-twitch oxidative (SO) muscle fibers (type I). Our results suggest that both high-intensity running and immobilization are enough to induce overwhelming changes in plantaris.

Effets de la supplémentation en créatine sur la cinétique de régénérescence du muscle squelettique après lésion étendue

Objective. – The aim of the experiment was to study, in rats, the effects of Cr supplementation on the time course of recovery of the soleus, a slow-twitch oxidative muscle, after a notexin-induced injury. Results and Discussion. – Whether or not animals have been supplemented with oral creatine, we observed: 1) soleus muscle mass was recovered at day 35 following injury. 2) MHC profile of regenerated muscles was similar to that of intact muscles at day 28. 3) A full recovery of the CS activity was observed from day 14, while the specific H-LDH activity remained slightly lower than in intact muscles. 4) Myogenin expression peaked at day 3. Conclusion. – In contrast with our hypothesis resulting from in vitro experiments, creatine supplementation failed to show any beneficial effect on the time course of recovery of skeletal muscle assessed in vivo after a myotoxic-induced injury.

Exercise-induced Gene Expression Of Angiogenesis-regulated Transcriptional Factors Differs Between Fiber Types In The Skeletal Muscle

Exercise training promotes many adaptations in skeletal muscle, including the development of new capillaries, known as angiogenesis. Acute exercise induces a greater expression of genes known to promote angiogenesis, such as vascular endothelial growth factor (VEGF). The exercise-induced alteration of VEGF gene expression differs between muscle fiber types (i.e., slow-twitch oxidative muscle fibers and fast-twitch glycolytic muscle fibers). PURPOSE To gain insight into the molecular mechanism underlying the difference of exercise-induced angiogenesis between slow fiber type and fast fiber type, we investigated whether the basal and exercise-induced gene expression of angiogenesis-regulated transcriptional factors in muscle differ between these two fiber types. METHODS We determined a difference of level in gene expression of angiogenesis-regulated transcriptional factors (transforming growth factor [TGF]-beta1, c-Jun and c-Fos) between the muscle of slow fiber type (soleus) and fast fiber type (plantaris) at rest and the end of a single bout of exercise (running on a treadmill for 30-min) using male Sprague-Dawley rats (10 wk old). RESULTS The mRNA expression of TGF-beta1, c-Jun and c-Fos at resting condition was significantly higher in the soleus than the plantaris. On the other hand, the mRNA expressions of TGF-beta1 and c-Jun in the plantaris, but not in the soleus, were significantly increased by exercise. The exercise-induced increase in c-Fos mRNA expression was higher in the plantaris than the soleus, although the mRNA expressions of c-Fos in both muscles were significantly increased by exercise. Thus, slow type fiber constitutively activates transcriptional regulation of angiogenesis at resting condition, whereas the exercise-induced increase in gene expression of angiogenesis-regulated transcriptional factors mainly occurs in fast fiber type of muscle. CONCLUSION The present study suggests that 1) the difference of constitutive gene expression of angiogenesis-regulated transcriptional factors between fast and slow fiber types of muscle may be one of the causal factors for the difference in homeostatic angiogenesis to the muscle capillary and 2) the exercise-induced different manner of gene expression of angiogenesis-regulated transcriptional factors between fast and slow fiber types of muscle may participate in molecular mechanism underlying exercise-enhanced angiogenesis. Supported by grants from the Ministry of Education, Science, Sports and Culture of Japan (15650130, 16500391).

Impact of aging on muscle blood flow in chronic heart failure

Chronic heart failure (CHF) is manifested principally in the elderly population. Therefore, to understand the causes of exercise intolerance in CHF patients, it is imperative to resolve the effects of aging on muscle blood flow (BF) in CHF. To address this issue, we determined the muscle BF response to submaximal treadmill exercise (20 m/min, 5% grade) in young (Y(CHF): 6-8 mo, 412 +/- 11 g, n = 11) and old (O(CHF): 27-29 mo, 494 +/- 10 g, n = 8) Fischer 344 x Brown Norway rats with similar degrees of myocardial infarction-induced left ventricular (LV) dysfunction [resting LV end-diastolic pressure: Y(CHF) = 24 +/- 2, O(CHF) = 22 +/- 2 mmHg; derivative of LV pressure over time: Y(CHF) = 5,168 +/- 285; O(CHF) = 5,050 +/- 165 mmHg/s; lung weight normalized to body weight: Y(CHF) = 9.14 +/- 0.72; O(CHF) = 8.21 +/- 0.29 mg/g (all P > 0.05)]. The exercising heart rate response was blunted in O(CHF) compared with Y(CHF) rats (Y(CHF) = 454 +/- 8, O(CHF) = 395 +/- 9 beats/min; P < 0.05). BF (radiolabeled microspheres) to the total hindlimb musculature and to each of the 28 individual muscles examined was similar between Y(CHF) and O(CHF) rats under resting conditions. During exercise, BF to five of the hindlimb muscles that normally possess a majority of slow-twitch oxidative and fast-twitch oxidative glycolytic muscle fibers increased significantly less (-25 to -42%) for O(CHF) compared with Y(CHF) rats. In contrast, BF to 14 of the hindlimb muscles that normally possess a majority of fast-twitch glycolytic muscle fibers was increased (+22 to +337%) for O(CHF) vs. Y(CHF) rats, which contributed to a greater mass-specific total hindlimb BF response in O(CHF) rats (Y(CHF) = 78 +/- 5, O(CHF) = 100 +/- 11 ml.min(-1).100 g(-1); P < 0.05) and coincided with greater reductions in BF to the kidneys and splanchnic organs during exercise in O(CHF) vs. Y(CHF). In conclusion, there appears to be a profound age-related redistribution of BF from the highly oxidative to the highly glycolytic muscles of the hindlimb during exercise in O(CHF) compared with Y(CHF) rats. This phenomenon is qualitatively similar to that reported previously for healthy young and old rats.

Differential effects of thyroid hormones on energy metabolism of rat slow‐ and fast‐twitch muscles

Thyroid hormone (TH) is an important regulator of mitochondrial content and activity. As mitochondrial content and properties differ depending on muscle-type, we compared mitochondrial regulation and biogenesis by T3 in slow-twitch oxidative (soleus) and fast-twitch mixed muscle (plantaris). Male Wistar rats were treated for 21 to 27 days with T3 (200 microg/kg/day). Oxidative capacity, regulation of mitochondrial respiration by substrates and phosphate acceptors, and transcription factors were studied. In soleus, T3 treatment increased maximal oxygen consumption (Vmax) and the activities of citrate synthase (CS) and cytochrome oxidase (COX) by 100%, 45%, and 71%, respectively (P < 0.001), whereas in plantaris only Vmax increased, by 39% (P < 0.01). ADP-independent respiration rate was increased in soleus muscle by 216% suggesting mitochondrial uncoupling. Mitochondrial substrate utilization in soleus was also influenced by T3, as were mitochondrial enzymes. Lactate dehydrogenase (LDH) activity was elevated in soleus and plantaris by 63% and 11%, respectively (P < 0.01), and soleus creatine kinase was increased by 48% (P < 0.001). T3 increased the mRNA content of the transcriptional co-activator of mitochondrial genes, PGC-1alpha, and the I and IV COX subunits in soleus. The muscle specific response to thyroid hormones could be explained by a lower content of TH receptors in plantaris than soleus. Moreover, TRalpha mRNA level decreased further after T3 treatment. These results demonstrate that TH has a major effect on mitochondrial content, regulation and coupling in slow oxidative muscle, but to a lesser extent in fast muscle, due to the high expression of TH receptors and PGC-1alpha transcription factor.