Force deficit during the onset of muscle hypertrophy

The purpose was to study selected structural changes associated with the deficit in maximum specific force (N/cm2) during the early development of skeletal muscle hypertrophy. Ablation of gastrocnemius and plantaris muscles was performed bilaterally in 35-day-old rats (n = 41), and the soleus muscle was studied from days 1 to 30 thereafter. Compared with control muscles from age-matched unoperated rats (n = 48), muscle mass and cross-sectional area increased in parallel from 28 to 52% over the 30-day postoperative period. Specific force of hypertrophied muscle was depressed 38% at days 1 and 3, and by 28% from days 5 to 30 after synergistic muscle ablation compared with age-matched control values. Interstitial space was 38% greater than the control value of 20.4 +/- 1 microliters/100 mg at day 1 only. Protein concentration was depressed 15% for 7 days after the ablation operation, and connective tissue protein concentration was unchanged. The relative magnitude of increased mean fiber cross-sectional area was less than that of muscle mass until day 7 after ablation. Mononuclear cell infiltration in interfascicular spaces occurred from days 3 to 30 without light microscopic evidence of muscle fiber injury. Initial functional deficits are explained in part by an enlarged interstitial space and decreased protein concentration; later deficits are likely accounted for by intracellular changes.     

Mechanical loading stimulates hypertrophy in tissue-engineered skeletal muscle: Molecular and phenotypic responses

Mechanical loading of skeletal muscle results in molecular and phenotypic adaptations typified by enhanced muscle size. Studies on humans are limited by the need for repeated sampling, and studies on animals have methodological and ethical limitations. In this investigation, three-dimensional skeletal muscle was tissue-engineered utilizing the murine cell line C2C12, which bears resemblance to native tissue and benefits from the advantages of conventional in vitro experiments. The work aimed to determine if mechanical loading induced an anabolic hypertrophic response, akin to that described in vivo after mechanical loading in the form of resistance exercise. Specifically, we temporally investigated candidate gene expression and Akt-mechanistic target of rapamycin 1 signalling along with myotube growth and tissue function. Mechanical loading (construct length increase of 15%) significantly increased insulin-like growth factor-1 and MMP-2 messenger RNA expression 21 hr after overload, and the levels of the atrophic gene MAFbx were significantly downregulated 45 hr after mechanical overload. In addition, p70S6 kinase and 4EBP-1 phosphorylation were upregulated immediately after mechanical overload. Maximal contractile force was augmented 45 hr after load with a 265% increase in force, alongside significant hypertrophy of the myotubes within the engineered muscle. Overall, mechanical loading of tissue-engineered skeletal muscle induced hypertrophy and improved force production.     

Mechanisms of nascent fiber formation during avian skeletal muscle hypertrophy

This study examined two putative mechanisms of new fiber formation in postnatal skeletal muscle, namely longitudinal fragmentation of existing fibers and de novo formation. The relative contributions of these two mechanisms to fiber formation in hypertrophying anterior latissimus dorsi (ALD) muscle were assessed by quantitative analysis of their nuclear populations. Muscle hypertrophy was induced by wing-weighting for 1 week. All nuclei formed during the weighting period were labeled by continuous infusion of 5-bromo-2'-deoxyuridine (BrdU), a thymidine analog, and embryonic-like fibers were identified using an antibody to ventricular-like embryonic (V-EMB) myosin. The number of BrdU-labeled and unlabeled nuclei in V-EMB-positive fibers were counted. Wing-weighting resulted in significant muscle enlargement and the appearance of many V-EMB+ fibers. The majority of V-EMB+ fibers were completely independent of mature fibers and had a nuclear density characteristics of developing fibers. Furthermore, nearly 100% of the nuclei in independent V-EMB+ fibers were labeled. These findings strongly suggest that most V-EMB+ fibers were nascent fibers formed de novo during the weighting period by satellite cell activation and fusion. Nascent fibers were found primarily in the space between fascicles where they formed a complex anastomosing network of fibers running at angles to one another. Although wing-weighting induced an increase in the number of branched fibers, there was no evidence that V-EMB+ fibers were formed by longitudinal fragmentation. The location of newly formed fibers in wing-weighted and regenerating ALD muscle was compared to determine whether satellite cells in the ALD muscle were unusual in that, if stimulated to divide, they would form fibers in the inter- and intrafascicular space. In contrast to wing-weighted muscle, nascent fibers were always found closely associated with necrotic fibers. These results suggest that wing-weighting is not simply another model of regeneration, but rather produces a unique environment which induces satellite cell migration and subsequent fiber formation in the interfascicular space. De novo fiber formation is apparently the principal mechanism for the hyperplasia reported to occur in the ALD muscle undergoing hypertrophy induced by wing-weighting.     

Somatosensory evoked potentials during long-term isometric muscle contraction

The effect of sustained isometric contraction on somatosensory evoked potentials (SEPs) was studied. An attenuation of early SEP components N20--P30, P30--N35) was observed during the last minute of the endurance time. The late components (P45--N55, N55--P100) showed a significant increase of the amplitude during the first minute of the contraction. The amplitude of N35--P45 did not change during voluntary contraction, although it was decreased immediately after the contraction. An increase of the integrated EMG of forearm flexors was observed in the last minute of the endurance time. The maximal voluntary contraction was decreased immediately after the sustained contraction. The observed changes in SEPs could be attributed to possible changes in sensory information and motor command due to motor task.     

Expression of fibroblast growth factor family during postnatal skeletal muscle hypertrophy

The potential role of the fibroblast growth factor (FGF) familyduring stretch-induced postnatal skeletal muscle hypertrophy wasanalyzed by using an avian wing-weighting model. After 2 or 11 days ofweighted stretch, anterior latissimus dorsi (ALD) muscles were, onaverage, 34 (P < 0.01) and 85%(P < 0.01) larger, respectively, than unweighted ALD control muscles. By using quantitative RT-PCR, FGF-1 mRNA expression was found to be significantly decreased in ALDmuscles stretched for 2 or 11 days. In contrast, FGF-4 and FGF-10 mRNAexpression was significantly increased 2 days after initiation ofstretch. FGF-2, FGF-10, fibroblast growth factor receptor 1, andFREK mRNA expression was significantly increased at 11 days poststretch. Increases in FGF-2 and FGF-4 protein could bedetected throughout the myofiber periphery after 11 days of stretch. Ona cellular level, FGF-2 and FGF-4 proteins were differentiallylocalized. This differential expression pattern and proteinlocalization of the FGF family in response to stretch-induced hypertrophy suggest distinct roles for individual FGFs during thepostnatal hypertrophy process.

     

Protein synthesis persists during necrotic cell death

Cell death is an intrinsic part of metazoan development and mammalian immune regulation. Whereas the molecular events orchestrating apoptosis have been characterized extensively, little is known about the biochemistry of necrotic cell death. Here, we show that, in contrast to apoptosis, the induction of necrosis does not lead to the shut down of protein synthesis. The rapid drop in protein synthesis observed in apoptosis correlates with caspase-dependent breakdown of eukaryotic translation initiation factor (eIF) 4G, activation of the double-stranded RNA-activated protein kinase PKR, and phosphorylation of its substrate eIF2-alpha. In necrosis induced by tumor necrosis factor, double-stranded RNA, or viral infection, de novo protein synthesis persists and 28S ribosomal RNA fragmentation, eIF2-alpha phosphorylation, and proteolytic activation of PKR are absent. Collectively, these results show that, in contrast to apoptotic cells, necrotic dying cells retain the opportunity to synthesize proteins.     

Thermal adaptation of microbial respiration persists throughout long-term soil carbon decomposition

Soil microbial respiration is expected to show adaptations to changing temperatures, greatly weakening the magnitude of feedback over time, as shown in labile carbon substrates. However, whether such thermal adaptation persists during long-term soil carbon decomposition as carbon substrates decrease in decomposability remains unknown. Here, we conducted a 6-year incubation experiment in natural and arable soils with distinct properties under three temperatures (10, 20 and 30°C). Mass-specific microbial respiration was consistently lower under higher long-term incubation temperatures, suggesting the occurrence and persistence of microbial thermal adaptation in long-term soil carbon decomposition. Furthermore, changes in microbial community composition and function largely explained the persistence of microbial respiratory thermal adaptation. If such thermal adaptation generally occurs in large low-decomposability carbon pools, warming-induced soil carbon losses may be lower than previously predicted and thus may not contribute as much as expected to greenhouse warming.     

Development of rat muscle during short- and long-term hindlimb suspension

Histochemical and contractile properties of developing rat soleus (Sol) and plantaris (P) muscles were studied after hindlimb suspension to determine the effects of reduced activity levels on muscle development. Suspension (S) began at age 18 days and lasted for 14, 28, and 206 days, and results were compared with age-matched controls. Body weights were normal until 14 days and Sol growth was inhibited more than P, weighing 38 and 47% of controls at 46 and 224 days compared with 68 and 59% in P. The Sol did not develop into a slow-twitch (ST) muscle as evidenced by faster times to peak tension and half-relaxation times, faster times to develop 50% of maximum tetanic tension (Po) and a mean of 33% fewer ST fibers. Twitch tension and Po were lower in S-Sol and S-P, but force/cross-sectional area was unchanged. Fiber areas were smaller, but no structural changes characteristic of disuse atrophy were found. Fiber type populations were unchanged in P, and contractile properties were only minimally affected, demonstrating the greater importance of activity for ST muscles during development.     

Heart C-protein is transiently expressed during skeletal muscle development in the embryo, but persists in cultured myogenic cells

The expression of cardiac and white skeletal C-protein isoforms was analyzed in developing chicken embryos and in primary skeletal muscle cell cultures by immunoblot and immunofluorescence staining using polyclonal antibodies specific for both of the two different proteins. In the embryo, cardiac C-protein was detected in the developing heart from very early stages through adulthood. In skeletal muscle, cardiac C-protein is shown to be transiently expressed between Days 3 and 15 during development. In contrast, the expression of white skeletal C-protein is gradual and progressive starting approximately from Day 15 on in development. In primary cell cultures of skeletal muscle, however, cardiac C-protein remained expressed throughout prolonged culture time, this in conjunction with white skeletal C-protein. Thus the down regulation of cardiac C-protein and the transition from cardiac C-protein to adult skeletal (white) C-protein which was observed during skeletal muscle development in vivo, does not seem to go to completion in the in vitro system.