Thyroid hormone receptor α in skeletal muscle is essential for T3-mediated increase in energy expenditure.

Thyroid hormones are important for homeostatic control of energy metabolism and body temperature. Although skeletal muscle is considered a key site for thyroid action, the contribution of thyroid hormone receptor signaling in muscle to whole-body energy metabolism and body temperature has not been resolved. Here, we show that T3-induced increase in energy expenditure requires thyroid hormone receptor alpha 1 (TRα1 ) in skeletal muscle, but that T3-mediated elevation in body temperature is achieved in the absence of muscle-TRα1 . In slow-twitch soleus muscle, loss-of-function of TRα1 (TRαHSACre ) alters the fiber-type composition toward a more oxidative phenotype. The change in fiber-type composition, however, does not influence the running capacity or motivation to run. RNA-sequencing of soleus muscle from WT mice and TRαHSACre mice revealed differentiated transcriptional regulation of genes associated with muscle thermogenesis, such as sarcolipin and UCP3, providing molecular clues pertaining to the mechanistic underpinnings of TRα1 -linked control of whole-body metabolic rate. Together, this work establishes a fundamental role for skeletal muscle in T3-stimulated increase in whole-body energy expenditure.

Absence of an aging-related increase in fiber type grouping in athletes and non-athletes.

The aging-related loss of muscle mass is thought to be partly attributable to motor neuron loss and motor unit remodeling that result in fiber type grouping. We examined fiber type grouping in 19- to 85-year-old athletes and non-athletes and evaluated to which extent any observed grouping is explained by the fiber type composition of the muscle. Since regular physical activity may stimulate reinnervation, we hypothesized that fiber groups are larger in master athletes than in age-matched non-athletes. Fiber type grouping was assessed in m. vastus lateralis biopsies from 22 young (19-27 years) and 35 healthy older (66-82 years) non-athletes, and 14 young (20-29 years), 51 middle-aged (38-65 years), and 31 older (66-85 years) athletes. An "enclosed fiber" was any muscle fiber of a particular type surrounded by fibers of the same type only. A fiber type group was defined as a group of fibers with at least one enclosed fiber. Only type II fiber cross-sectional area (FCSA) showed an age-related decline that was greater in athletes (P < .001) than in non-athletes (P = .012). There was no significant age-related effect on fiber group size or fiber group number in athletes or non-athletes, and the observed grouping was similar to that expected from the fiber type composition. At face value, these observations do 1) neither show evidence for an age-related loss and remodeling of motor units nor 2) improved reinnervation with regular physical activity, but 3) histological examination may not reveal the full extent of aging-related motor unit remodeling.

PERK regulates skeletal muscle mass and contractile function in adult mice.

Skeletal muscle mass is regulated by the coordinated activation of several anabolic and catabolic pathways. The endoplasmic reticulum (ER) is a major site of protein folding and a reservoir for calcium ions. Accretion of misfolded proteins or depletion in calcium concentration causes stress in the ER, which leads to the activation of a signaling network known as the unfolded protein response (UPR). In the present study, we investigated the role of the protein kinase R-like endoplasmic reticulum kinase (PERK) arm of the UPR in the regulation of skeletal muscle mass and function in naive conditions and in a mouse model of cancer cachexia. Our results demonstrate that the targeted inducible deletion of PERK reduces skeletal muscle mass, strength, and force production during isometric contractions. Deletion of PERK also causes a slow-to-fast fiber type transition in skeletal muscle. Furthermore, short hairpin RNA-mediated knockdown or pharmacologic inhibition of PERK leads to atrophy in cultured myotubes. While increasing the rate of protein synthesis, the targeted deletion of PERK leads to the increased expression of components of the ubiquitin-proteasome system and autophagy in skeletal muscle. Ablation of PERK also increases the activation of calpains and deregulates the gene expression of the members of the FGF19 subfamily. Furthermore, the targeted deletion of PERK increases muscle wasting in Lewis lung carcinoma tumor-bearing mice. Our findings suggest that the PERK arm of the UPR is essential for the maintenance of skeletal muscle mass and function in adult mice.-Gallot, Y. S., Bohnert, K. R., Straughn, A. R., Xiong, G., Hindi, S. M., Kumar, A. PERK regulates skeletal muscle mass and contractile function in adult mice.

Effect of acute physiological free fatty acid elevation in the context of hyperinsulinemia on fiber type-specific IMCL accumulation.

It is well described that increasing free fatty acids (FFAs) to high physiological levels reduces insulin sensitivity. In sedentary humans, intramyocellular lipid (IMCL) is inversely related to insulin sensitivity. Since muscle fiber composition affects muscle metabolism, whether FFAs induce IMCL accumulation in a fiber type-specific manner remains unknown. We hypothesized that in the setting of acute FFA elevation by lipid infusion within the context of a hyperinsulinemic-euglycemic clamp, IMCL will preferentially accumulate in type 1 fibers. Normal-weight participants (n = 57, mean ± SE: age 24 ± 0.6 yr, BMI 22.2 ± 0.3 kg/m2) who were either endurance trained or sedentary by self-report were recruited from the University of Minnesota (n = 31, n = 15 trained) and University of Pittsburgh (n = 26, n = 14 trained). All participants underwent a hyperinsulinemic-euglycemic clamp in the context of a 6-h infusion of either lipid or glycerol control. A vastus lateralis muscle biopsy was obtained at baseline and end-infusion (6 h). The muscle biopsies were processed and analyzed at the University of Pittsburgh for fiber type-specific IMCL accumulation by Oil-Red-O staining. Regardless of training status, acute elevation of FFAs to high physiological levels (~400-600 meq/l) increased IMCL preferentially in type 1 fibers (+35 ± 11% compared with baseline, +29 ± 11% compared with glycerol control: P < 0.05). The increase in IMCL correlated with a decline in insulin sensitivity as measured by the hyperinsulinemic-euglycemic clamp (r = -0.32, P < 0.01) independent of training status. Regardless of training status, increase of FFAs to a physiological range within the context of hyperinsulinemia shows preferential IMCL accumulation in type 1 fibers.NEW & NOTEWORTHY This novel human study examined the effects of FFA elevation in the setting of hyperinsulinemia on accumulation of fat in specific types of muscle fibers. Within the context of the hyperinsulinemic-euglycemic clamp, we found that an increase of FFAs to a physiological range sufficient to reduce insulin sensitivity is associated with preferential IMCL accumulation in type 1 fibers.

Evaluation of Muscle Fiber Types in German Shepherd Dogs of Different Ages.

The objective of this study was to determine and confirm the percentage of type I and type II muscle fibers that comprise the Gluteus Medius muscle in male and female canines of the German Shepherd breed, with standardized care, in different age groups, using the enzyme histochemical method. Muscle samples were collected from the Gluteus Medius muscles of forty clinically healthy dogs of the German Shepherd breed using the technique of percutaneous needle muscle biopsy. The samples were evaluated using histological and enzyme histochemical methods. The percentages of type I and II fibers and the ratio between the quantity of type I fibers/quantity of type II fibers were evaluated using the parameters of weight, age group, correlation between sex and age group, and between the sexes. It was found that there was no significant difference in relation to the types of fibers for the parameters of weight, age group, and age of the females. The correlation between the ages of the males suggested an increase in the percentage of type I fibers, a decrease in the percentage of type II fibers, or an increase in the ratio during the aging process. It was concluded that there was a decrease in the percentage of type II fibers with advancing age in male dogs, but without significant difference in the percentage of type I and type II fibers in relation to the weight. Anat Rec, 299:1540-1547, 2016. © 2016 Wiley Periodicals, Inc.

Resistance Training Increases Skeletal Muscle Capillarization in Healthy Older Men.

PURPOSE: Skeletal muscle capillarization plays a key role in oxygen and nutrient delivery to muscle. The loss of muscle mass with aging and the concept of anabolic resistance have been, at least partly, attributed to changes in skeletal muscle capillary structure and function. We aimed to compare skeletal muscle capillarization between young and older men and evaluate whether resistance-type exercise training increases muscle capillarization in older men. METHODS: Muscle biopsies were obtained from the vastus lateralis of healthy young (n = 14, 26 ± 2 yr) and older (n = 16, 72 ± 1 yr) adult men, with biopsies before and after 12 wk of resistance-type exercise training in the older subjects. Immunohistochemistry was used to assess skeletal muscle fiber size, capillary contacts (CC) per muscle fiber, and the capillary-to-fiber perimeter exchange (CFPE) index in type I and II muscle fibers. RESULTS: Type II muscle fibers were smaller in old versus young (4507 ± 268 vs 6084 ± 497 µm, respectively, P = 0.007). Type I and type II muscle fiber CC and CFPE index were smaller in old compared with young muscle (CC type I: 3.8 ± 0.2 vs 5.0 ± 0.3; CC type II: 3.2 ± 0.2 vs 4.2 ± 0.2, respectively; both P < 0.001). Resistance-type exercise training increased type II muscle fiber size only. In addition, CC and CFPE index increased in both the type I (26% ± 9% and 27% ± 8%) and type II muscle fibers (33% ± 7% and 24% ± 6%, respectively; all P ≤ 0.001) after 12 wk resistance training in older men. CONCLUSIONS: We conclude that resistance-type exercise training can effectively augment skeletal muscle fiber capillarization in older men. The greater capillary supply may be an important prerequisite to reverse anabolic resistance and support muscle hypertrophy during lifestyle interventions aiming to support healthy aging.

Comparative analysis of rat mesenchymal stem cells derived from slow and fast skeletal muscle in vitro.

PURPOSE: Skeletal muscle comprises different kinds of muscle fibres that can be classified as slow and fast fibres. The purpose of this study was to compare the yield, proliferation, and multi-potentiality of rat mesenchymal stem cells (MSCs) from the tibialis anterior (TA; fast muscle) and soleus (SO; slow muscle) in vitro. METHODS: The TA and SO muscles were harvested, and isolated cells were plated. After two hours, the cells were washed extensively to remove any cell that did not adhere to the cell culture plate. The adherent cells, namely MSCs, were then cultured. Both types of MSCs were differentiated toward the osteogenic, chondrogenic and adipogenic lineages using lineage specific induction factors. RESULTS: The colony-forming unit fibroblast (CFU-F) assay revealed that the SO contained significantly higher quantities of MSCs than the TA. The self-renewal capacity of MSCs derived from the TA was significantly higher at later passages (passage 9-11). Both types of MSCs exhibited similar cell surface antigens to bone marrow (BM)-derived MSCs and were positive for CD29, CD44, and CD90 and negative for CD11b, CD34, and CD45. TA-derived MSCs were superior in terms of osteogenic differentiation capacity, but there was no significant difference in chondrogenic and adipogenic differentiation capacity. CONCLUSION: Our results demonstrated significant differences in the properties of muscle-derived MSCs from different muscle types (i.e. fast or slow muscles). The greater expandability and osteogenic differentiation ability of TA-derived MSCs suggests that fast muscle may be a better source for generating large numbers of MSCs for bone regeneration.

microRNA-151-3p regulates slow muscle gene expression by targeting ATP2a2 in skeletal muscle cells.

MicroRNAs (miRNAs) are a group of small noncoding RNAs that regulate the stability or translation of cognate mRNAs at the post-transcriptional level. Accumulating evidence indicates that miRNAs play important roles in many aspects of muscle function, including muscle growth and development, regeneration, contractility, and muscle fiber type plasticity. In the current study, we examined the function of miR-151-3p in myoblast proliferation and differentiation. Results show that overexpression of miR-151-3p not only upregulates myoblast proliferation, but also decreases slow muscle gene expression (such as MHC-ß/slow and slow muscle troponin I) in both C2C12 myotubes and in primary cultures. Alternatively, inhibition of miR-151-3p by antisense RNA was found to upregulate MHC-ß/slow expression, indicating that miR-151-3p plays a role in muscle fiber type determination. Further investigation into the underlying mechanisms revealed for the first time that miR-151-3p directly targets ATP2a2, a gene encoding for a slow skeletal and cardiac muscle specific Ca(2+) ATPase, SERCA2 thus downregulating slow muscle gene expression. Mechanisms by which the alteration in SERCA2 expression induces changes in other slow muscle gene expression levels needs to be defined in future research.

Effects of concurrent strength and endurance training on genes related to myostatin signaling pathway and muscle fiber responses.

Concurrent training (CT) seems to impair training-induced muscle hypertrophy. This study compared the effects of CT, strength training (ST) and interval training (IT) on the muscle fiber cross-sectional area (CSA) response, and on the expression of selected genes involved in the myostatin (MSTN) signaling mRNA levels. Thirty-seven physically active men were randomly divided into 4 groups: CT (n = 11), ST (n = 11), IT (n = 8), and control group (C) (n = 7) and underwent an 8-week training period. Vastus lateralis biopsy muscle samples were obtained at baseline and 48 hours after the last training session. Muscle fiber CSA, selected genes expression, and maximum dynamic ST (1 repetition maximum) were evaluated before and after training. Type IIa and type I muscle fiber CSA increased from pre- to posttest only in the ST group (17.08 and 17.9%, respectively). The SMAD-7 gene expression significantly increased at the posttest in the ST (53.9%) and CT groups (39.3%). The MSTN and its regulatory genes ActIIb, FLST-3, FOXO-3a, and GASP-1 mRNA levels remained unchanged across time and groups. One repetition maximum increased from pre- to posttest in both the ST and CT groups (ST = 18.5%; CT = 17.6%). Our findings are suggestive that MSTN and their regulatory genes at transcript level cannot differentiate muscle fiber CSA responses between CT and ST regimens in humans.

Satellite cells in human skeletal muscle; from birth to old age.

Changes in satellite cell content play a key role in regulating skeletal muscle growth and atrophy. Yet, there is little information on changes in satellite cell content from birth to old age in humans. The present study defines muscle fiber type-specific satellite cell content in human skeletal muscle tissue over the entire lifespan. Muscle biopsies were collected in 165 subjects, from different muscles of children undergoing surgery (<18 years; n = 13) and from the vastus lateralis muscle of young adult (18­49 years; n = 50), older (50­69 years; n = 53), and senescent subjects (70­86 years; n = 49). In a subgroup of 51 aged subjects (71 ± 6 years), additional biopsies were collected after 12 weeks of supervised resistance-type exercise training. Immunohistochemistry was applied to assess skeletal muscle fiber type-specific composition, size, and satellite cell content. From birth to adulthood, muscle fiber size increased tremendously with no major changes in muscle fiber satellite cell content, and no differences between type I and II muscle fibers. In contrast to type I muscle fibers, type II muscle fiber size was substantially smaller with increasing age in adults (r = −0.56; P < 0.001). This was accompanied by an age-related reduction in type II muscle fiber satellite cell content (r = −0.57; P < 0.001). Twelve weeks of resistance-type exercise training significantly increased type II muscle fiber size and satellite cell content. We conclude that type II muscle fiber atrophy with aging is accompanied by a specific decline in type II muscle fiber satellite cell content. Resistance-type exercise training represents an effective strategy to increase satellite cell content and reverse type II muscle fiber atrophy.

The human tongue slows down to speak: muscle fibers of the human tongue.

Little is known about the specializations of human tongue muscles. In this study, myofibrillar adenosine triphosphatase (mATPase) histochemical staining was used to study the percentage and distribution of slow twitch muscle fibers (slow MFs) within tongue muscles of four neurologically normal human adults and specimens from a 2-year-old human, a newborn human, an adult with idiopathic Parkinson's disease (IPD), and a macaque monkey. The average percentage of slow MFs in adult and the 2-year-old muscle specimens was 54%, the IPD was 45%, while the neonatal human (32%) and macaque monkey (28%) had markedly fewer slow MFs. In contrast, the tongue muscles of the rat and cat have been reported to have no slow MFs. There was a marked spatial gradient in the distribution of slow MFs with the highest percentages found medially and posteriorly. Normal adult tongue muscles were found to have a variety of uniquely specialized features including MF-type grouping (usually found in neuromuscular disorders), large amounts of loose connective tissue, and short branching MFs. In summary, normal adult human tongue muscles have by far the highest proportion of slow MFs of any mammalian tongue studied to date. Moreover, adult human tongue muscles have multiple unique anatomic features. As the tongue shape changes that are seen during speech articulation are unique to humans, we hypothesize that the large proportion of slow MFs and the anatomical specializations observed in the adult human tongue have evolved to perform these movements.

Distribution of myofiber types in the crural musculature of sheep.

In domestic animals, the legs function in both postural maintenance and propulsion. The crural muscles participate in actions of the tarsal and toe joints. Mammalian skeletal muscles consist of myofibers, which are histochemically classified into three myofiber types, slow-twitch/oxidative (SO) or type I, fast-twitch/oxidative/glycolytic (FOG) or type IIA, and fast-twitch/glycolytic (FG) or type IIB myofibers. The histochemical characteristics of myofiber types reflect an aspect of function that myofibers possess. In the present study, we investigated the composition and average diameter of myofiber types of each muscle in crus of sheep and determined their roles in the movement of tarsal and toe joints. The tibialis cranialis muscle was a flat unipennate muscle and not capable to generate a large tension; however, it could function primarily in posture maintenance and play a cooperative role in adjusting standing posture. The flexor hallucis longus and flexor digitorum superficialis muscles were the major muscles that contributed to posture maintenance in leg musculature. These muscles were capable to generate a large tension and participate primarily in standing posture maintenance. The composition and diameter of myofiber types in ovine crural musculature reflected the role of each muscle in posture maintenance and locomotion.

Insulin and fiber type in the offspring of T2DM subjects with resistance training and detraining.

PURPOSE: Effects of resistance training and detraining on glucose and insulin responses to an oral glucose load, muscle fiber type, and muscular performance in the offspring of those with type 2 diabetes (familial insulin resistant (FIR)) were investigated. METHODS: Six FIR participants and 10 controls (C) completed 9 wk of resistance training and 9 wk of detraining. Measures of strength and power, an oral glucose tolerance test, and a muscle biopsy to determine myosin heavy chain (MHC) fiber composition were taken at baseline (T1), after training (T2), and after detraining (T3). RESULTS: Three-repetition maximum increased (P ≤ 0.001) similarly in both groups in all strength measures, e.g., leg press (FIR T1, T2: 121 ± 34 kg, 186 ± 50 kg; C T1, T2: 137 ± 42 kg, 206 ± 64 kg, respectively (means ± SD)). Wingate peak power increased (FIR T1, T2: 505 ± 137 W, 523 ± 143 W; C T1, T2: 636 ± 211 W, 672 ± 223 W, respectively; P ≤ 0.005 (means ± SD)). Training reduced insulin area under the curve more (P = 0.050) in FIR (T1, T2: 1219 ± 734 pmol·L, 837 ± 284 pmol·L, respectively (means ± SD)) than that in C (T1, T2: 647 ± 268 pmol·L, 635 ± 258 pmol·L, respectively (means ± SD)). MHC distribution did not change with training. Strength (three-repetition maximum measures) decreased with detraining (P ≤ 0.001) although Wingate power did not. Detraining increased insulin area under the curve (P = 0.018) in FIR (T2, T3: 837 ± 285 pmol·L, 1040 ± 194 pmol·L, respectively (means ± SD)) but not in C (T2, T3: 635 ± 258 pmol·L, 625 ± 213 pmol·L, respectively (means ± SD)). MHC IIX fibers increased with detraining (P = 0.026). CONCLUSION: FIR appears to have exaggerated responses to resistance training and detraining, with a greater reduction in insulin release with glucose ingestion after training and increase when training ceases. Resistance training has a significant effect on insulin responses and may reduce future risk of type 2 diabetes mellitus among FIR.

Satellite cell count, VO(2max) , and p38 MAPK in inactive to moderately active young men.

Satellite cells (SCs) are responsible for muscle repair following strenuous exercise or injury. SC responses to intervention have been studied, but most studies do not discuss or take into account the substantial variability in SC number among young individuals. We hypothesized that an active lifestyle reflected in higher VO(2max) may be associated with greater SC number. As training alters basal p38-mitogen-activated protein kinase (MAPK) activity, which is associated with SC proliferation, SC count may also correlate with this stress signaling kinase. Muscle biopsies from vastus lateralis of eight male participants were analyzed for fiber type, myogenin, and p38/phospho-p38 MAPK using SDS-PAGE and Western blotting. Immunofluorescence was used to detect Pax7(+) SCs. Two weeks following the biopsy, subjects underwent an incremental treadmill test to determine VO(2max) . A strong positive correlation (P = 0.0087) was found between the number of Pax7(+) nuclei and VO(2max) . Pax7(+) cell number correlated negatively with phospho-p38/p38 MAPK (P = 0.0006), but had no correlation with fiber type or myogenin. SC number is proportional to VO(2max) , and hence it can be postulated that higher levels of physical activity activate SC proliferation but not fusion, underlining the relevance of exercise in stimulating SC pool size even without injury.

Histochemistry and morphometric analysis of muscle fibers from patients with duchenne muscular dystrophy (DMD) / Histoquímica y análisis morfométrico de las fibras musculares de p0acientes con distrofia muscular de duchenne (DMD)

The aim of the study was to analyze the muscle fibers by histochemistry and morphometric methods from patients with Duchenne muscular dystrophy (DMD). Muscle biopsies were taken from the vastus lateralis muscle of five boys between 13 and 15-years of age, with clinical diagnosis of DMD. The histochemistry was performed using myofibrillar ATPases (9.6, 4.6 and 4.3). To morphometrical analysis a computerized semiautomatic system and software Image-Lab was used. ATPase staining showed atrophy of muscle fibers. Fibrosis and adipose deposition occurred in variable degree depending of muscular involvement. The morphometrical analysis showed an increase of size (percentage) to type I fiber than other types in all patients. Furthermore, the type I fiber had a larger cross-sectional area and mean diameter than type IIa and IIb fibers. Both histochemistry and morphometric analysis could be important tools for qualitative and quantitative diagnostics of muscle fibers attacked in this type of disease.

El objetivo del estudio fue analizar las fibras musculares mediante histoquímica y métodos morfométricos en pacientes con distrofia muscular de Duchenne (DMD). Se tomaron biopsias musculares del músculo vasto lateral de cinco niños entre 13 y 15 años de edad, con diagnóstico clínico de DMD. La histoquímica se realizó mediante ATPasa miofibrilar (9.6, 4.6 y 4.3). Para el análisis morfométrico se utilizó un sistema semiautomático computarizado y software de imagen de laboratorio. La tinción de ATPasa mostró una atrofia de las fibras musculares. La fibrosis y depósito adiposo se observó en grado variable dependiendo del compromiso muscular. El análisis morfométrico mostró un aumento de tamaño (porcentaje) de fibras tipo I en todos los pacientes. Además, la fibra tipo I tuvo un área de sección transversal y diámetro medio mayor que las fibras tipos IIa y IIb. Tanto la histoquímica y el análisis morfométrico pueden ser herramientas importantes para el diagnóstico cualitativo y cuantitativo de las fibras musculares comprometidas en este tipo de enfermedad.

Volume regulation in mammalian skeletal muscle: the role of sodium-potassium-chloride cotransporters during exposure to hypertonic solutions.

Controversy exists as to whether mammalian skeletal muscle is capable of volume regulation in response to changes in extracellular osmolarity despite evidence that muscle fibres have the required ion transport mechanisms to transport solute and water in situ. We addressed this issue by studying the ability of skeletal muscle to regulate volume during periods of induced hyperosmotic stress using single, mouse extensor digitorum longus (EDL) muscle fibres and intact muscle (soleus and EDL). Fibres and intact muscles were loaded with the fluorophore, calcein, and the change in muscle fluorescence and width (single fibres only) used as a metric of volume change. We hypothesized that skeletal muscle exposed to increased extracellular osmolarity would elicit initial cellular shrinkage followed by a regulatory volume increase (RVI) with the RVI dependent on the sodium­potassium­chloride cotransporter (NKCC). We found that single fibres exposed to a 35% increase in extracellular osmolarity demonstrated a rapid, initial 27­32% decrease in cell volume followed by a RVI which took 10-20 min and returned cell volume to 90­110% of pre-stimulus values. Within intact muscle, exposure to increased extracellular osmolarity of varying degrees also induced a rapid, initial shrinkage followed by a gradual RVI, with a greater rate of initial cell shrinkage and a longer time for RVI to occur with increasing extracellular tonicities. Furthermore, RVI was significantly faster in slow-twitch soleus than fast-twitch EDL. Pre-treatment of muscle with bumetanide (NKCC inhibitor) or ouabain (Na+,K+-ATPase inhibitor), increased the initial volume loss and impaired the RVI response to increased extracellular osmolarity indicating that the NKCC is a primary contributor to volume regulation in skeletal muscle. It is concluded that mouse skeletal muscle initially loses volume then exhibits a RVI when exposed to increases in extracellular osmolarity. The rate of RVI is dependent on the degree of change in extracellular osmolarity, is muscle specific, and is dependent on the functioning of the NKCC and Na+, K+-ATPase.

The effect of chronically increased intra-abdominal pressure on rectus abdominis muscle histology an experimental study on rabbits.

BACKGROUND: The aim of this study was to specify the histologic response of the rectus abdominis muscle of the rabbit, to the chronically increased intra-abdominal pressure. MATERIALS AND METHODS: Forty-five New Zealand white rabbits were divided into three groups. In all groups, a rubber bag was implanted into the peritoneal cavity. In group A (n=15) the bags were kept empty. In group B (n=15) the bags were filled with normal saline in order to achieve an intra-abdominal pressure of over 12 mmHg. This pressure was kept at this level for 8 wk. In group C (n=15) the intra-abdominal rubber bags were filled with lead covered by silicone, equiponderant to the mean weight of the normal saline insufflated in group B. After 8 wk we took biopsies of the rectus abdominis muscle and counted the proportion of the different types of muscular fibers (type I, IIA, and IIB/X). RESULTS: Significant difference was found in the proportion of the three types of muscle fibers. Intra-abdominal hypertension led to an increase in type I fibers (P=0.008). No difference was noticed between groups A and C. CONCLUSIONS: The histologic response to the increased intra-abdominal pressure was an increase in type I muscle fibers. Charging with lead did not cause any significant change in the proportion of muscular fibers.

Fiber type composition of the sternomastoid and diaphragm muscles of dystrophin-deficient mdx mice.

The muscle fiber phenotype is mainly determined by motoneuron innervation and changes in neuromuscular interaction alter the muscle fiber type. In dystrophin-deficient mdx mice, changes in the molecular assembly of the neuromuscular junction and in nerve terminal sprouting occur in the sternomastoid (STN) muscle during early stages of the disease. In this study, we were interested to see whether early changes in neuromuscular assembly are correlated with alterations in fiber type in dystrophic STN at 2 months of age. A predominance of hybrid fast myofibers (about 52% type IIDB) was observed in control (C57Bl/10) STN. In mdx muscle, the lack of dystrophin did not change this profile (about 54% hybrid type IIDB). Pure fast type IID fibers predominated in normal and dystrophic diaphragm (DIA; about 39% in control and 30% in mdx muscle) and a population of slow Type I fibers was also present (about 10% in control and 13% in mdx muscle). In conclusion, early changes in neuromuscular assembly do not affect the fiber type composition of dystrophic STN. In contrast to the pure fast fibers of the more affected DIA, the hybrid phenotype of the STN may permit dynamic adaptations during progression of the disease.

Comparison of capillary architecture between slow and fast muscles in rats using a confocal laser scanning microscope.

The skeletal muscle is classified into 2 types, slow oxidative or fast glycolytic muscle. For further characterization, we investigated the capillary architecture in slow and fast muscles. The rat soleus and extensor digitorum longus (EDL) muscles were used as representatives of slow and fast muscles, respectively. To investigate capillary density, sections of both types of muscle were stained with alkaline phosphatase; the soleus muscle showed more intense reactivity, indicating that it had a denser capillary structure than the EDL muscle. We then injected fluorescent contrast medium into samples of both muscle types for light and confocal-laser microscopic evaluation. The capillary density and capillary-to-fiber ratio were significantly higher, and the course of the capillaries was more tortuous, in the soleus muscle than in the EDL muscle. Capillary coursed more tortuously in the soleus than in the EDL muscle. Succinate dehydrogenase (SDH) activity, an indicator of mitochondrial oxidative capacity, and vascular endothelial growth factor (VEGF) expression were also significantly higher in the soleus muscle. Thus, we conclude that slow oxidative muscle possess a rich capillary structure to provide demanded oxygen, and VEGF might be involved in the formation and/or maintenance of this highly capillarized architecture.

Deficiency in APOBEC2 leads to a shift in muscle fiber type, diminished body mass, and myopathy.

The apoB RNA-editing enzyme, catalytic polypeptide-like (APOBEC) family of proteins includes APOBEC1, APOBEC3, and activation-induced deaminase, all of which are zinc-dependent cytidine deaminases active on polynucleotides and involved in RNA editing or DNA mutation. In contrast, the biochemical and physiological functions of APOBEC2, a muscle-specific member of the family, are unknown, although it has been speculated, like APOBEC1, to be an RNA-editing enzyme. Here, we show that, although expressed widely in striated muscle (with levels peaking late during myoblast differentiation), APOBEC2 is preferentially associated with slow-twitch muscle, with its abundance being considerably greater in soleus compared with gastrocnemius muscle and, within soleus muscle, in slow as opposed to fast muscle fibers. Its abundance also decreases following muscle denervation. We further show that APOBEC2-deficient mice harbor a markedly increased ratio of slow to fast fibers in soleus muscle and exhibit an approximately 15-20% reduction in body mass from birth onwards, with elderly mutant animals revealing clear histological evidence of a mild myopathy. Thus, APOBEC2 is essential for normal muscle development and maintenance of fiber-type ratios; although its molecular function remains to be identified, biochemical analyses do not especially argue for any role in RNA editing.