Mammalian Skeletal Muscle Cells Research Articles

Ryanodine receptor 3 impacts on the expression of aminopeptidase in mammalian skeletal muscle cells during myogenic differentiation

In mammalian skeletal muscle, ryanodine receptor (Ryr) expresses in the membrane of sarcoplasmic reticulum (SR), and is known as Ca2+-releasing channel. Ryr in mammalian skeletal muscle consists of 2 isoforms, Ryr1 and Ryr3. Ryr1 is coupled with dihydropyridine receptors (DHPRs) in transverse tubules, and Ca2+ release is a key step for the excitation-contraction coupling (E-C coupling). However, a physiological role of ryr3 is still unclear. Recently, we confirmed that Ryr3 knockdown induces abnormal myotube formation of C2C12 cells. Further, it is reported that the abnormal shape of C2C12 myotubes were observed by the knockdown of puromycin-sensitive aminopeptidase, a selective hydrolyze of amino acid residue from the N-terminus of peptide and proteins. In the present study, we investigated that the molecular mechanisms for the abnormal myotube formation of C2C12 cells following the knockdown of Ryr3. Aggregation of phalloidin-stained actin fibers were observed in Ryr3-knockdown myotubes. On the other hand, a significantly decrease in the mRNA expression level of puromycin-sensitive aminopeptidase and endoplasmic reticulum aminopeptidase 1 was observed in Ryr3-knockdown C2C12 cells (p<0.05). Therefore, Ryr3 may play a crucial role in the quality control of proteins and peptides during myogenic differentiation via the up-regulation of puromycin-sensitive aminopeptidase and endoplasmic reticulum aminopeptidase 1. This study was partially supported by JSPS KAKENHI (18H03160, K.G.; 19K22825, K.G.; 19KK0254, K.G., S.Y., T.E.; 22H03474, K.G., T.E.; 22K19722, K.G., T.E.; 22H03319, K.G., T.E.; 22K18413, K.G., T.E.), a grant from Graduate School of Health Science, Toyohashi SOZO University (K.G.), and a research grant from Toyohashi SOZO university (K.G.). All authors declare that they have no conflicts of interest. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Poly lactide-co-glycolide encapsulated nano-curcumin promoting antagonistic interactions between HSP 90 and XRCC1 proteins to prevent cypermethrin-induced toxicity: An in silico predicted in vitro and in vivo approach

The present study describes the preparation and characterization of poly-lactide-co-glycolide encapsulated nano-curcumin (NCUR) drug, and its potential efficacy against the pesticide, such as cypermethrin-induced DNA damage and genotoxicity. Cypermethrin, the chosen pesticide, contaminates the aquatic environment after being washed off from the agricultural field to nearby water bodies leading to biomagnification-related perturbation of the ecological balance and overall environmental health by elevating adverse effects on non-target organisms producing toxic metabolites through biotransformation. The physico-chemical properties of NCUR were evaluated by employing the AFM, DLS and UV-Vis techniques. Sustainable release of NCUR, their bio-availability and ability to cross the blood-brain-barrier was assessed in the fish model. The in silico molecular docking study to identify the signalling proteins that interact with phyto-core-compound curcumin (CUR) was undertaken to predict the effectiveness of NCUR to combat pesticide-induced toxicity by modulating p53, PARP, HSP 90 and XRCC1 stress proteins, and other associated parameters in in vivo model using tilapia fish and in vitro model using L6 (mammalian skeletal muscle) cell line. Overall results revealed that negatively charged poly-lactide-co-glycolide (PLGA)-encapsulated NCUR (∼46 nm) showed hyperchromic binding with DNA and modulated the signalling cascades involved in stress and DNA repair mechanisms, corroborating well with the in silico prediction that would pave a new pathway in the arena of chemical and biological sciences to serve mankind.

Time-resolved transcriptional profiling of Trichinella-infected murine myocytes helps to elucidate host\u2013pathogen interactions in the muscle stage

BackgroundParasites of the genus Trichinella are the pathogenic agents of trichinellosis, which is a widespread and severe foodborne parasitic disease. Trichinella spiralis resides primarily in mammalian skeletal muscle cells. After invading the cells of the host organism, T. spiralis must elude or invalidate the host’s innate and adaptive immune responses to survive. It is necessary to characterize the pathogenesis of trichinellosis to help to prevent the occurrence and further progression of this disease. The aims of this study were to elucidate the mechanisms of nurse cell formation, pathogenesis and immune evasion of T. spiralis, to provide valuable information for further research investigating the basic cell biology of Trichinella-infected muscle cells and the interaction between T. spiralis and its host.MethodsWe performed transcriptome profiling by RNA sequencing to identify global changes at 1, 3, 7, 10 and 15 days post-infection (dpi) in gene expression in the diaphragm after the parasite entered and persisted within the murine myocytes; the mice were infected by intravenous injection of newborn larvae. Gene expression analysis was based on the alignment results. Differentially expressed genes (DEGs) were identified based on their expression levels in various samples, and functional annotation and enrichment analysis were performed.ResultsThe most extensive and dynamic gene expression responses in host diaphragms were observed during early infection (1 dpi). The number of DEGs and genes annotated in the Kyoto Encyclopedia of Genes and Genomes and Gene Ontology databases decreased significantly in the infected mice compared to the uninfected mice at 3 and 7 dpi, suddenly increased sharply at 10 dpi, and then decreased to a lower level at 15 dpi, similar to that observed at 3 and 7 dpi. The massive initial reaction of the murine muscle cells to Trichinella infection steadied in the later stages of infection, with little additional changes detected for the remaining duration of the studied process. Although there were hundreds of DEGs at each time point, only 11 genes were consistently up- or downregulated at all 5 time points.ConclusionsThe gene expression patterns identified in this study can be employed to characterize the coordinated response of T. spiralis-infected myocytes in a time-resolved manner. This comprehensive dataset presents a distinct and sensitive picture of the interaction between host and parasite during intracellular infection, which can help to elucidate how pathogens evade host defenses and coordinate the biological functions of host cells to survive in the mammalian environment.Graphical

A Study on the Myoglobin Redox of Pig Skeletal Muscle and Post-Mortem Interval by UV-Vis Spectrophotometric Technique

Myoglobin is a protein widely found in human and mammalian skeletal muscle cells. Studies have shown that the myoglobin measured by western blotting has a good correlation with post-mortem interval (PMI), but the western blotting method is cumbersome and time-consuming, and is not suitable for rapid and accurate analysis of multiple samples. To this end, this study tried to use UV-Vis spectrophotometry (Krzywicki’s method and Viriyarattanasak’s method) to determine the content of myoglobin redox (DeoMb, OxyMb, MetMb) in skeletal muscle at different times after death, and to explore its relationship with PMI and ADH. In this experiment, the left hind leg muscle of the pig was kept in a natural environment. Skeletal muscle were randomly extracted at 2 h, 8 h, 16 h, 24 h, 32 h, 40 h, 48 h, 72 h, 96 h, 120 h, 144 h, 168 h after the pig’s death, weighed, pH measured, and then made into a homogenate dipping solution. The myoglobin redox in the skeletal muscle was measured by UV-Vis spectrophotometry, and its changes with PMI and ADH were explored, which study the correlation with PMI and ADH respectively. The results showed that the skeletal muscle loss, pH, and content of myoglobin redox varied with PMI and ADH; the loss and pH are highly correlated with PMI (or ADH) (R2=0.84-0.91), and the fitting relationship between the two and ADH is better; the correlation between myoglobin redox and PMI (or ADH) under the two UV-Vis spectrophotometry methods is weak (R2=0.03-0.28), and the curve fitting relationship is poor. Unfortunately, we concluded that UV-Vis spectrophotometry determination of myoglobin redox in cadaver muscles cannot appear to be a reliable method for PMI estimation. However, measuring muscle loss and pH may be an potential method to estimate PMI.

Cellular fatty acid level regulates the effect of tolylfluanid on mitochondrial dysfunction and insulin sensitivity in C2C12 skeletal myotubes

Previous research suggests that the endocrine disrupting chemical tolylfluanid (TF) may promote metabolic dysfunction and insulin resistance in humans. The potential impact of TF on skeletal muscle metabolism has yet to be fully investigated. The purpose of this study was to determine whether TF can promote insulin resistance and metabolic dysfunction in mammalian skeletal muscle cells. C2C12 murine skeletal myotubes were exposed to 1 ppm TF for 24 h. To examine the potential effect of cellular fatty acid levels on TF-dependent regulation of mitochondrial metabolism and insulin signaling, we treated skeletal myotubes with 0.25 mM or 1.0 mM oleic acid (OA) during TF exposure trials. Tolylfluanid (1–10 ppm) reduced lipid accumulation by approximately 20% in 0.25 and 1.0 mM OA treated cells. The addition of 0.25 mM OA completely inhibited the TF-dependent reduction in maximal mitochondrial oxygen consumption rate (OCR) while 1.0 mM OA exacerbated the TF-dependent reduction in mitochondrial OCR. Exposing skeletal myotubes to 1 ppm TF promoted an 80% reduction in mitochondrial membrane potential, which was completely inhibited by 0.25 mM OA and partially inhibited by1.0 mM OA. The addition of 0.25 mM OA promoted a TF-dependent increase in insulin-dependent P-Akt (Ser473). In contrast, the addition of 1.0 mM OA promoted a significant reduction in insulin-dependent P-Akt (Ser473). Further, the addition of 1 ppm TF significantly reduced insulin-dependent mTORC1 activity regardless of OA concentration. Finally, TF significantly reduced insulin-dependent protein synthesis in the 1 mM OA treated cells only. Our results demonstrate that the effect of 1 ppm TF on mitochondrial function and insulin-dependent protein synthesis in skeletal myotubes was largely dependent upon cellular fatty acid levels.

Immunoproteomic analysis of the excretory-secretory products of Trichinella pseudospiralis adult worms and newborn larvae

BackgroundThe nematode Trichinella pseudospiralis is an intracellular parasite of mammalian skeletal muscle cells and exists in a non-encapsulated form. Previous studies demonstrated that T. pseudospiralis could induce a lower host inflammatory response. Excretory-secretory (ES) proteins as the most important products of host-parasite interaction may play the main functional role in alleviating host inflammation. However, the ES products of T. pseudospiralis early stage are still unknown. The identification of the ES products of the early stage facilitates the understanding of the molecular mechanisms of the immunomodulation and may help finding early diagnostic markers.ResultsIn this study, we used two-dimensional gel electrophoresis (2-DE)-based western blotting coupled with matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/TOF-MS/MS) to separate and identify the T. pseudospiralis adult worms ES products immunoreaction-positive proteins. In total, 400 protein spots were separated by 2-DE. Twenty-eight protein spots were successfully identified using the sera from infected pigs and were characterized to correlate with 12 different proteins of T. pseudospiralis, including adult-specific DNase II-10, poly-cysteine and histidine-tailed protein isoform 2, serine protease, serine/threonine-protein kinase ULK3, enolase, putative venom allergen 5, chymotrypsin-like elastase family member 1, uncharacterized protein, peptidase inhibitor 16, death-associated protein 1, deoxyribonuclease II superfamily and golgin-45. Bioinformatic analyses showed that the identified proteins have a wide diversity of molecular functions, especially deoxyribonuclease II (DNase II) activity and serine-type endopeptidase activity.ConclusionsEarly candidate antigens from the ES proteins of T. pseudospiralis have been screened and identified. Our results suggest these proteins may play key roles during the T. pseudospiralis infection and suppress the host immune response. Further, they are the most likely antigen for early diagnosis and the development of a vaccine against the parasite.

Centrifugal partition chromatography in the isolation of minor ecdysteroids from Cyanotis arachnoidea

Phytoecdysteroids are known for their various beneficial bioactivities in mammals including a non-hormonal anabolic and adaptogenic effect. Cyanotis arachnoidea extracts are extensively utilized worldwide as ecdysteroid-rich materials for various purposes, e.g. food supplementation, use in agriculture and aquaculture, etc. Preparative chromatography of ecdysteroids requires extensive use of methods of different selectivity, and only a very limited number of papers are available on related application of modern liquid-liquid chromatographic techniques. In this work, a centrifugal partition chromatography (CPC) method was developed for the isolation of two minor ecdysteroids, dacryhainansterone and calonysterone, from a pre-purified commercial extract of Cyanotis arachnoidea. The biphasic solvent system was optimized by HPLC, and was composed of n-hexane - ethyl acetate - methanol - water (1:5:1:5, v/v/v/v). The isolated dacryhainansterone and calonysterone represented 99.1% and 99.7% purity, respectively. Calonysterone exerts a stronger effect on the protein kinase B (Akt) phosphorylation in mammalian skeletal muscle cells than the abundant 20-hydroxyecdysone, while no related data are available on dacryhainansterone. Despite their presence in food supplements, neither compound has appropriately been assessed for safety and efficacy. The reported method allows the gram scale isolation of these compounds, opening ways to their in-depth pharmacological investigation.

Acute high-caffeine exposure increases autophagic flux and reduces protein synthesis in C2C12 skeletal myotubes.

Caffeine is a highly catabolic dietary stimulant. High caffeine concentrations (1-10mM) have previously been shown to inhibit protein synthesis and increase protein degradation in various mammalian cell lines. The purpose of this study was to examine the effect of short-term caffeine exposure on cell signaling pathways that regulate protein metabolism in mammalian skeletal muscle cells. Fully differentiated C2C12 skeletal myotubes either received vehicle (DMSO) or 5mM caffeine for 6h. Our analysis revealed that caffeine promoted a 40% increase in autolysosome formation and a 25% increase in autophagic flux. In contrast, caffeine treatment did not significantly increase the expression of the skeletal muscle specific ubiquitin ligases MAFbx and MuRF1 or 20S proteasome activity. Caffeine treatment significantly reduced mTORC1 signaling, total protein synthesis and myotube diameter in a CaMKKβ/AMPK-dependent manner. Further, caffeine promoted a CaMKII-dependent increase in myostatin mRNA expression that did not significantly contribute to the caffeine-dependent reduction in protein synthesis. Our results indicate that short-term caffeine exposure significantly reduced skeletal myotube diameter by increasing autophagic flux and promoting a CaMKKβ/AMPK-dependent reduction in protein synthesis.

Effects of Insulin-Like Growth Factor-I on the Expression of Atrogin-1/MAFbx in Chick Myotube Cultures.

The expression of atrogin-1/MAFbx, a muscle-specific E3 ubiquitin ligase, is increased in catabolic conditions that result in muscle atrophy. The expression of atrogin-1/MAFbx mRNA is also decreased by the insulin-like growth factor-I (IGF-I) in mammalian skeletal muscle cell cultures. This study investigated the effect of IGF-I on the expression of atrogin-1/MAFbx in chicken skeletal muscle cell cultures. Chick myotubes were incubated with IGF-I for 1, 6, or 24 h. Protein content was increased by IGF-I (100 ng/ml) and incubated for 24 h in chick myotubes. The expression of atrogin-1/MAFbx mRNA decreased in the presence of IGF-I (1, 10, and 100 ng/ml) for 6 h in chick myotubes. The expression of the m-calpain large subunit and cathepsin B mRNA was not decreased by IGF-I. Phosphorylation of Akt and FOXO1 increased in the presence of IGF-I (100 ng/ml) for 1 h in chick myotubes. These results indicate that IGF-I suppresses atrogin-1/MAFbx mRNA expression by phosphorylation of Akt and FOXO1, resulting in an increase in muscle growth in chick myotube cultures.

Ubiquitin-Dependent Modification of Skeletal Muscle by the Parasitic Nematode, Trichinella spiralis.

Trichinella spiralis is a muscle-specific parasitic worm that is uniquely intracellular. T. spiralis reprograms terminally differentiated skeletal muscle cells causing them to de-differentiate and re-enter the cell cycle, a process that cannot occur naturally in mammalian skeletal muscle cells, but one that holds great therapeutic potential. Although the host ubiquitin pathway is a common target for viruses and bacteria during infection, its role in parasite pathogenesis has been largely overlooked. Here we demonstrate that the secreted proteins of T. spiralis contain E2 Ub-conjugating and E3 Ub-ligase activity. The E2 activity is attributed to TsUBE2L3, a novel and conserved T. spiralis enzyme located in the secretory organ of the parasite during the muscle stages of infection. TsUBE2L3 cannot function with any T.spiralis secreted E3, but specifically binds to a panel of human RING E3 ligases, including the RBR E3 ARIH2 with which it interacts with a higher affinity than the mammalian ortholog UbcH7/UBE2L3. Expression of TsUBE2L3 in skeletal muscle cells causes a global downregulation in protein ubiquitination, most predominantly affecting motor, sarcomeric and extracellular matrix proteins, thus mediating their stabilization with regards to proteasomal degradation. This effect is not observed in the presence of the mammalian ortholog, suggesting functional divergence in the evolution of the parasite protein. These findings demonstrate the first example of host-parasite interactions via a parasite-derived Ub conjugating enzyme; an E2 that demonstrates a novel muscle protein stabilization function.

Mammalian Non-CpG Methylation: Stem Cells and Beyond.

Although CpG dinucleotides remain the primary site for DNA methylation in mammals, there is emerging evidence that DNA methylation at non-CpG sites (CpA, CpT and CpC) is not only present in mammalian cells, but may play a unique role in the regulation of gene expression. For some time it has been known that non-CpG methylation is abundant in plants and present in mammalian embryonic stem cells, but non-CpG methylation was thought to be lost upon cell differentiation. However, recent publications have described a role for non-CpG methylation in adult mammalian somatic cells including the adult mammalian brain, skeletal muscle, and hematopoietic cells and new interest in this field has been stimulated by the availability of high throughput sequencing techniques that can accurately measure this epigenetic modification. Genome wide assays indicate that non-CpG methylation is negligible in human fetal brain, but abundant in human adult brain tissue. Genome wide measurement of non-CpG methylation coupled with RNA-Sequencing indicates that in the human adult brain non-CpG methylation levels are inversely proportional to the abundance of mRNA transcript at the associated gene. Additionally specific examples where alterations in non-CpG methylation lead to changes in gene expression have been described; in PGC1α in human skeletal muscle, IFN-γ in human T-cells and SYT11 in human brain, all of which contribute to the development of human disease.

Caffeine promotes autophagy in skeletal muscle cells by increasing the calcium-dependent activation of AMP-activated protein kinase

Caffeine has been shown to promote calcium-dependent activation of AMP-activated protein kinase (AMPK) and AMPK-dependent glucose and fatty acid uptake in mammalian skeletal muscle. Though caffeine has been shown to promote autophagy in various mammalian cell lines it is unclear if caffeine-induced autophagy is related to the calcium-dependent activation of AMPK. The purpose of this study was to examine the role of calcium-dependent AMPK activation in regulating caffeine-induced autophagy in mammalian skeletal muscle cells. We discovered that the addition of the AMPK inhibitor Compound C could significantly reduce the expression of the autophagy marker microtubule-associated protein 1 light chain 3b-II (LC3b-II) and autophagic vesicle accumulation in caffeine treated skeletal muscle cells. Additional experiments using pharmacological inhibitors and RNA interference (RNAi) demonstrated that the calcium/calmodulin-activated protein kinases CaMKKβ and CaMKII contributed to the AMPK-dependent expression of LC3b-II and autophagic vesicle accumulation in a caffeine dose-dependent manner. Our results indicate that in skeletal muscle cells caffeine increases autophagy by promoting the calcium-dependent activation of AMPK.

Epigenetic control of cell identity and plasticity

The DNA centered dogma for genetic information and cell identity is now evolving into a much more complex and flexible dimension provided by the discovery of the Epigenome. This comprises those chromosome structural and topological components that complement DNA information and contribute to genome functional organization. Current concept is that the Epigenome constitutes the dynamic molecular interface allowing the Genome to interact with the Environment. Exploring how the genome interacts with the environment is a key to fully understand cellular and complex organism mechanisms of adaptation and plasticity. Our work focuses on the role of an essential, specialized group or chromatin associated proteins named Polycomb (PcG) that control maintenance of transcription programs during development and in adult life. In particular PcG proteins exert epigenetic “memory” function by modifying chromosome structures at various levels to maintain gene silencing in particular through cell division. While in the past decade substantial progress was made in understanding PcG mechanisms acting in development and partially during cell cycle, very little is known about their role in adult post-mitotic tissues and more in general the role of the epigenome in adaptation. To this, we studied the role of PcG in the context of mammalian skeletal muscle cell differentiation. We previously reported specific dynamics of PRC2 proteins in myoblasts and myotubes, in particular the dynamics of PcG Histone H3 K27 Methyl Transferases (HMT), EZH2 and EZH1, the latter apparently replacing for EZH2 in differentiated myotubes. Ezh1 protein, although almost identical to Ezh2, shows a weak H3K27 HMT activity and its primary function remains elusive. Recent ChIPseq studies performed in differentiating muscle cells revealed that Ezh1 associates with active and not repressed regulatory regions to control RNA pol II elongation. Since H3K27 tri-methylation levels are virtually steady in non-cycling myotubes we asked which could be PRC2-EZH1 function in this context. We postulated that the presence of PRC2-EZH1 complex on active promoters, and not on repressed loci, could be explained by a novel role for PcG to control prompt response to specific to stress conditions and silencing of tissue specific active genes in a global coordinated manner. Data in vitro and in vivo will be presented in support of our hypothesis showing that in terminally differentiated myotubes PRC2-Ezh1 complex H3K27m3 methyltransferase activity is dynamically regulated in response to specific environmental stimuli and that EZH1 function is required in particular to prevent muscle atrophy. Further, we discovered that this dynamics involves a novel pathway controlling PRC2 complex formation, with potential general implications for mechanisms controlling the ability of somatic cells to respond to adverse environmental conditions via chromatin plasticity.

Aciculin interacts with filamin C and Xin and is essential for myofibril assembly, remodeling and maintenance.

Filamin C (FLNc) and Xin actin-binding repeat-containing proteins (XIRPs) are multi-adaptor proteins that are mainly expressed in cardiac and skeletal muscles and which play important roles in the assembly and repair of myofibrils and their attachment to the membrane. We identified the dystrophin-binding protein aciculin (also known as phosphoglucomutase-like protein 5, PGM5) as a new interaction partner of FLNc and Xin. All three proteins colocalized at intercalated discs of cardiac muscle and myotendinous junctions of skeletal muscle, whereas FLNc and aciculin also colocalized in mature Z-discs. Bimolecular fluorescence complementation experiments in developing cultured mammalian skeletal muscle cells demonstrated that Xin and aciculin also interact in FLNc-containing immature myofibrils and areas of myofibrillar remodeling and repair induced by electrical pulse stimulation (EPS). Fluorescence recovery after photobleaching (FRAP) experiments showed that aciculin is a highly dynamic and mobile protein. Aciculin knockdown in myotubes led to failure in myofibril assembly, alignment and membrane attachment, and a massive reduction in myofibril number. A highly similar phenotype was found upon depletion of aciculin in zebrafish embryos. Our results point to a thus far unappreciated, but essential, function of aciculin in myofibril formation, maintenance and remodeling.

Reply from M. I. Lindinger, M. Leung, K. E. Trajcevski and T. J. Hawke

Dr Fraser's letter follows his review on the topic of muscle volume regulation (Usher-Smith et al. 2009) in which the authors take the view that muscle cells do not regulate their volume in response to changes in the tonicity of their extracellular environment. This view is puzzling in view of the fact that most animal cells studied to date display volume regulation characteristics (O'Neill, 1999; Lang, 2007). The fact that mammalian skeletal muscle cells also regulate volume has also been demonstrated (Sitdikov et al. 1989; Urazaev, 1998; Lindinger et al. 2011). To refute the likelihood that mammalian skeletal muscle cells regulate cell volume, Dr Fraser presents some interesting results on amphibian muscle cells, and raises some good points regarding experimental design. He shows that in single Rana pectoris muscle fibres the volume responses occurring after addition of either 100 mm DMSO or 100 mm glycerol result from an initial osmotic loss of volume by muscle, followed by volume recovery. The mechanism by which the cells recover volume is not mentioned, but it may be implied that volume recovery is due to the influx of DMSO or glycerol into the cell interior, and that volume recovery is solely due to achieving complete chemical equilibration of the solute across the sarcolemma. There is no evidence presented that the vacuolation is not an occurrence that occurs with fibres under various non-DMSO conditions in his experimental set-up. It is also unknown if factors other than DMSO, or in combination with DMSO, contributed to the vacuolation or, indeed, if this is a normal response of muscle (Launikonis & Stephenson, 2004). It also appears that some of the methodology and results presented in our paper (Lindinger et al. 2011) were misinterpreted. Regarding methodology, we did not simultaneously add NaCl and remove DMSO as these tasks, perhaps fortunately, cannot be performed simultaneously. In his letter, Fraser referred to procedures performed using single muscle fibres. However, Fig. 4 is in reference to experiments performed on intact muscle. Even when using single fibres, the ‘50 s delay’ is the time after completion of the triple rinsing procedure and the acquisition of new images. The total time from first removal of the bumetanide-containing solution was closer to 5 min. Regardless, when baseline images were obtained prior to imposing the osmotic challenge, no volume changes occurred. When performing experiments in which muscle was incubated with bumetanide with DMSO, the solution containing bumetanide was first removed and flushed, and replaced with solution that was free of bumetanide and DMSO. This required about 5 min; baseline measurements were then obtained over the next 5–10 min – during this period no volume changes occurred. Then extracellular osmolarity was increased, resulting in cell shrinkage, followed by volume recovery. The observed responses cannot be explained by a DMSO-induced phenomenon. We have never observed volume transients during baseline acquisition periods following bumetanide treatments. Furthermore, we have observed the same volume responses reported in these experiments in hundreds of single cells treated with step increases in extracellular osmolarity that have not been exposed to DMSO, i.e. not calcein loaded and not incubated in bumetanide (M. Leung, J Moynes & MI Lindinger, unpublished and presented at several meetings). Dr Fraser's pilot data do raise concerns regarding use of DMSO in physiological experiments and the timing of experiments when using muscle treated with DMSO (also see Santos et al. 2003; Velasco et al. 2003). Further investigation is required to definitively and rigorously demonstrate whether a vacuolation response to DMSO treatment occurs in all such treated frog muscle fibres, as well as cells of other species using the experimental conditions employed by other labs, and then the physiological impact can be determined.

Skeletal muscle regulatory volume response to recovery period affected by extracellular osmolarity change

Mammalian skeletal muscle cells exhibit an initial volume reduction, followed by a regulatory volume increase (RVI) back to baseline status in response to increased extracellular osmolarity. However, the muscle cells may not always reach complete recovery back to starting volume. In the present study, the response of muscle fibres to increased extracellular osmolarity via sucrose, NaCl, and NaLac treatment were investigated to observe RVI. Additionally, a recovery stage was examined to assess fibre response after RVI occurrence, during a period where extracellular osmolarity was replaced with an isosmotic environment. Adult mouse peroneous muscle fibre width measurements were obtained every 5–10 seconds for 30min in response to increased extracellular osmolarity via sucrose, NaCl or NaLactate treatment. Immediately after this period, solution was 3x-rinsed and replaced with plating media. New baseline images were obtained and image collection repeated for 30 minutes. The muscle fibres in the isosmotic environment continued to exhibit an increase in volume throughout the 30min recovery period. Also, the magnitude of peak volume decrease in the preceding treatment period did not negatively impact the degree of recovery. Results suggest that skeletal muscle cells possess an active mechanism to regain volume back towards baseline status irrespective of compensatory changes in extracellular osmolarity. Supported by NSERC of Canada

Quantitative Measurement of Ca 2+ in the Sarcoplasmic Reticulum Lumen of Mammalian Skeletal Muscle

Skeletal muscle stores Ca 2+ in the sarcoplasmic reticulum (SR) and releases it to initiate contraction, but the concentration of luminal Ca 2+ in the SR ([Ca 2+] SR) and the amount that is released by physiological or pharmacological stimulation has been difficult to measure. Here we present a novel, yet simple and direct, method that provides the first quantitative estimates of static content and dynamic changes in [Ca 2+] SR in mammalian skeletal muscle, to our knowledge. The method uses fluo-5N loaded into the SR of single, mammalian skeletal muscle cells (murine flexor digitorum brevis myofibers) and confocal imaging to detect and calibrate the signals. Using this method, we have determined that [Ca 2+] SR, free is 390 μM. 4-Chloro-m-cresol, an activator of the skeletal muscle ryanodine receptor, reduces [Ca 2+] SR, free to ∼8 μM, when values are corrected for background fluorescence from cytoplasmic pools of dye. Prolonged electrical stimulation (10 s) at 50 Hz releases 88% of the SR Ca 2+ content, whereas stimulation at 1 Hz (10 s) releases only 20%. Our results lay the foundation for molecular modeling of the dynamics of luminal SR Ca 2+ and for future studies of the role of SR Ca 2+ in healthy and diseased mammalian muscle.

P3.20 Mouse and human skeletal muscle cells differentiate in a temperature-dependent manner

We have been studying the effects of low temperature on differentiation of mammalian skeletal muscle cells in culture. C2C12 cells and human skeletal muscle cells neither expressed myogenin, one of the myogenic regulatory factors (MRFs), nor fused into multinucleated myotubes at 30°C. However, the cells formed myotubes at the same level as a culture at 38°C, the normal temperature condition, when they were cultured in medium containing both insulin-like growth factor-I (IGF-I) and l-ascorbic acid phosphate, stabilized form of vitamin C (VC).

Skeletal Muscle Regulatory Responses to Increased Extracellular Lactate

Mammalian skeletal muscle cells exhibit cell shrinkage followed by regulatory volume increase (RVI) in response to increases in extracellular osmolarity caused by addition of concentrated NaCl or sucrose. We hypothesized that increased extracellular sodium lactate would similarly result in a transient loss of cell volume, followed by RVI, and that the RVI would be more rapid and pronounced than that seen with NaCl due to entry of lactate into the cell. The present study measured the time course of change single fibre width in response to varied increases in extracellular NaCl or Na‐lactate. Adult mouse EDL single fibres were isolated using collagenase and incubated in DMEM prior to and during experimentation. Fibre images were obtained at 3–6 s intervals for up to 40 min. Fibre images were analyzed for width at 2 sites. Increased osmolarity resulted in a rapid decrease in fibre width, followed by RVI. The volume recovery was faster and more complete when [lactate] ≥20 mmol/L. There was a linear relationship between increases extracellular [lactate], cell shrinkage, and magnitude of RVI. It appears that the inward transport of lactate into skeletal muscle contributes, together with increased NKCC activity, to the RVI in mammalian skeletal muscle.Supported by NSERC of Canada.

Local calcium signals induced by hyper-osmotic stress in mammalian skeletal muscle cells

Strenuous activitiy of skeletal muscle leads to temporary osmotic dysbalance and isolated skeletal muscle fibers exposed to osmotic stress respond with characteristic micro-domain calcium signals. It has been suggested that osmotic stress targets transverse tubular (TT) dihydropyridine receptors (DHPRs) which normally serve as voltage-dependent activators of Ca release via ryanodine receptor (RyR1s) of the sarcoplasmic reticulum (SR). Here, we pursued this hypothesis by imaging the response to hyperosmotic solutions in both mouse skeletal muscle fibers and myotubes. Ca fluctuations in the cell periphery of fibers exposed to osmotic stress were accompanied by a substantial dilation of the peripheral TT. The Ca signals were completely inhibited by a conditioning depolarization that inactivates the DHPR. Dysgenic myotubes, lacking the DHP-receptor-alpha1-subunit, showed strongly reduced, yet not completely inhibited activity when stimulated with solutions of elevated tonicity. The results point to a modulatory, even though not essential, role of the DHP receptor for osmotic stress-induced Ca signals in skeletal muscle.