This work was supported by grants from the Danish Council for Independent Research - Medical Sciences (FSS8020-00288B) and the Novo Nordisk Foundation (NNF160C0023046). This work was also supported by a research grant to Rasmus Kj&#248;bsted from the Danish Diabetes Academy, which is funded by the Novo Nordisk Foundation, grant number NNF17SA0031406. The authors would like to thank Karina Olsen, Betina Bolmgren, and Irene Bech Nielsen (Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen) for their skilled technical assistance.

The authors have nothing to disclose

Intact regulation of glucose uptake in skeletal muscle is important for preserving overall health1. Thus, investigation of muscle glucose uptake often serves as a primary readout when evaluating various health-altering interventions. Here we describe an ex vivo method for measuring glucose uptake in isolated and incubated soleus and EDL muscle from mice in response to insulin and electrically-induced contractions. The method is quick and reliable and allows a precise control of the surrounding mil...

Skeletal muscle possesses the ability to extract large quantities of glucose from the extracellular space in response to insulin and exercise. This helps to maintain whole-body glucose homeostasis and secures glucose supply during times of high energy demand. Since intact regulation of skeletal muscle glucose uptake has been shown to be important for overall health and physical performance1,2, measurements of muscle glucose uptake during various conditions have received much attention. In humans and animals, the hyperinsulinemic-euglycemic clamp has been used as the gold standard technique to assess insulin sensitivity in vivo3,4. In contrast to findings obtained from an oral glucose tolerance test, the hyperinsulinemic-euglycemic clamp technique does not require intact gastrointestinal function or insulin secretion from the pancreas and thus permits insulin responses to be compared between subjects who exhibit variations in gastro-intestinal and/or pancreatic function. Measurements of muscle glucose uptake&#160;in vivo during exercise in humans have been performed frequently since the 1960s5. First by the use of arteriovenous balance techniques6 and later by the use of positron emission tomography (PET) imaging in combination with a positron emitting glucose analogue e.g.&#160;18F-Fluoro-deoxy-glucose7. In rodents, exercise-stimulated muscle glucose uptake in vivo is typically performed by the use of radioactive or stable isotope-labeled glucose analogues8,9,10.
A complementary method to measurements of muscle glucose uptake in vivo, is to isolate and incubate small muscles from rodents and subsequently measure glucose uptake using radioactive or stable isotope-labeled glucose analogues11,12,13. This method allows accurate and reliable quantification of glucose uptake rates in mature skeletal muscle and can be performed in the presence of various insulin concentrations and during contraction elicited by electrical stimulation. More importantly, measurements of glucose uptake in isolated and incubated skeletal muscle are of relevance when investigating the muscle metabolic phenotype of mice that have undergone various interventions (e.g.&#160;nutrition, physical activity, infection, therapeutics). The isolated skeletal muscle model is also a suitable tool for pharmacological compound testing that may affect glucose uptake per se and/or modify insulin sensitivity12,14. In this way, the efficacy of compounds designed to regulate muscle glucose metabolism can be tested and evaluated in a highly controlled milieu before subsequent in vivo testing in pre-clinical animal models.
Under some conditions, metabolic viability may pose a challenge in the isolated and incubated skeletal muscle model system. Indeed, the lack of a circulatory system in the incubated muscles entails that delivery of substrates (e.g.&#160;oxygen and nutrients) fully depends on simple diffusion between the muscle fibers and the surrounding environment. In regards to this, it is of importance that the incubated muscles are small and thin and thus, represent less of a barrier for oxygen diffusion during incubation15. Especially during prolonged incubations for several hours, hypoxic states may develop due to insufficient oxygen supply resulting in muscle energy depletion15. Although various markers of metabolic viability in incubated rat muscle have been reported previously alongside the identification of important variables that help to maintain rat muscle viability15, a comprehensive evaluation of metabolic viability in small incubated mouse muscles is still warranted. Hence, at present, glycogen content has mainly been used as a marker of metabolic viability in incubated mouse skeletal muscle16,17.
Here we describe a detailed protocol to measure basal, insulin- and contraction-stimulated glucose uptake in isolated and incubated soleus and EDL muscle from mice using radiolabeled [3H]2-deoxy-D-glucose and [14C]mannitol as an extracellular marker. In the present study, glucose uptake was measured during a 10-minute period and the method is presented with the use of submaximally and maximally effective insulin concentrations as well as a single contraction protocol. However, the protocols described herein can easily be modified with regards to incubation time, insulin-dosage, and electrical stimulation protocol. Furthermore, we provide a thorough characterization of various markers of metabolic viability in incubated soleus and EDL mouse&#160;muscle. The results indicate that glucose supplementation to the incubation buffer is essential to preserve metabolic viability of muscle incubated for 1 hour.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>[14C]D-mannitol</td>
			<td>American Radiolabeled Chemicals, Inc.</td>
			<td>ARC 0127</td>
			<td></td>
		</tr>
		<tr>
			<td>[3H]2-deoxy-D-glucose&#160;</td>
			<td>American Radiolabeled Chemicals, Inc.</td>
			<td>ART 0103A</td>
			<td></td>
		</tr>
		<tr>
			<td>2-Deoxy-D-glucose</td>
			<td>Sigma</td>
			<td>D8375</td>
			<td></td>
		</tr>
		<tr>
			<td>4-0 USP non-sterile surgical nylon suture</td>
			<td>Harvard Apparatus</td>
			<td>51-7698</td>
			<td></td>
		</tr>
		<tr>
			<td>Streptavidin/HRP (Conjugate)</td>
			<td>DAKO</td>
			<td>P0397</td>
			<td>Used to detect ACC protein</td>
		</tr>
		<tr>
			<td>Akt2 antibody</td>
			<td>Cell Signaling</td>
			<td>3063</td>
			<td></td>
		</tr>
		<tr>
			<td>AMPK&#945;2 antibody</td>
			<td>Santa Cruz</td>
			<td>SC-19131</td>
			<td></td>
		</tr>
		<tr>
			<td>aprotinin</td>
			<td>Sigma</td>
			<td>A1153</td>
			<td></td>
		</tr>
		<tr>
			<td>benzamidine</td>
			<td>Sigma</td>
			<td>B6505</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine serum albumin (BSA)</td>
			<td>Sigma</td>
			<td>A7030</td>
			<td></td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Merck</td>
			<td>1020831000</td>
			<td></td>
		</tr>
		<tr>
			<td>Calibration kit (force)</td>
			<td>Danish Myo Technology A/S</td>
			<td>300041</td>
			<td></td>
		</tr>
		<tr>
			<td>Chemiluminescence</td>
			<td>Millipore</td>
			<td>WBLUF0500</td>
			<td></td>
		</tr>
		<tr>
			<td>D-Glucose</td>
			<td>Merck</td>
			<td>1084180100</td>
			<td></td>
		</tr>
		<tr>
			<td>D-Mannitol</td>
			<td>Sigma</td>
			<td>M4125</td>
			<td></td>
		</tr>
		<tr>
			<td>Data collection program</td>
			<td>National Instruments</td>
			<td>LabVIEW software version 7.1</td>
			<td></td>
		</tr>
		<tr>
			<td>Dialysis tubing</td>
			<td>Visking</td>
			<td>DTV.12000.09 Size No.9</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital imaging system</td>
			<td>BioRad</td>
			<td>ChemiDoc MP</td>
			<td></td>
		</tr>
		<tr>
			<td>EDTA</td>
			<td>Sigma EDS</td>
			<td>E9884</td>
			<td></td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>Sigma</td>
			<td>E4378</td>
			<td></td>
		</tr>
		<tr>
			<td>Electrical Pulse Stimulator</td>
			<td>Digitimer</td>
			<td>D330 MultiStim System</td>
			<td></td>
		</tr>
		<tr>
			<td>Glycerol</td>
			<td>Sigma</td>
			<td>G7757</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Sigma</td>
			<td>H7637</td>
			<td></td>
		</tr>
		<tr>
			<td>IGEPAL CA-630&#160;</td>
			<td>Sigma</td>
			<td>I8896</td>
			<td></td>
		</tr>
		<tr>
			<td>Insulin</td>
			<td>Novo Nordisk</td>
			<td>Actrapid, 100 IE/mL</td>
			<td></td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Merck</td>
			<td>1049361000</td>
			<td></td>
		</tr>
		<tr>
			<td>KH2PO4</td>
			<td>Merck</td>
			<td>104873025</td>
			<td></td>
		</tr>
		<tr>
			<td>leupeptin</td>
			<td>Sigma</td>
			<td>L2884</td>
			<td></td>
		</tr>
		<tr>
			<td>MgSO4</td>
			<td>Merck</td>
			<td>1058860500</td>
			<td></td>
		</tr>
		<tr>
			<td>Muscle Strip Myograph System</td>
			<td>Danish Myo Technology A/S</td>
			<td>Model 820MS</td>
			<td></td>
		</tr>
		<tr>
			<td>Na-Orthovanadate</td>
			<td>Sigma</td>
			<td>S6508</td>
			<td></td>
		</tr>
		<tr>
			<td>Na-Pyrophosphate</td>
			<td>Sigma</td>
			<td>221368</td>
			<td></td>
		</tr>
		<tr>
			<td>Na-Pyruvate</td>
			<td>Sigma</td>
			<td>P2256</td>
			<td></td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Merck</td>
			<td>106041000</td>
			<td></td>
		</tr>
		<tr>
			<td>NaF</td>
			<td>Sigma</td>
			<td>S1504</td>
			<td></td>
		</tr>
		<tr>
			<td>NaHCO3</td>
			<td>VWR</td>
			<td>27778260</td>
			<td></td>
		</tr>
		<tr>
			<td>pACC Ser212 antibody</td>
			<td>Cell Signaling</td>
			<td>3661</td>
			<td></td>
		</tr>
		<tr>
			<td>pAkt Thr308 antibody</td>
			<td>Cell Signaling</td>
			<td>9275</td>
			<td></td>
		</tr>
		<tr>
			<td>pAMPK Thr172 antibody</td>
			<td>Cell Signaling</td>
			<td>2531</td>
			<td></td>
		</tr>
		<tr>
			<td>phenylmethylsulfonylfluoride</td>
			<td>Sigma</td>
			<td>P7626</td>
			<td></td>
		</tr>
		<tr>
			<td>Platinum electrodes</td>
			<td>Danish Myo Technology A/S</td>
			<td>300145</td>
			<td></td>
		</tr>
		<tr>
			<td>pTBC1D4 Ser588 antibody</td>
			<td>Cell Signaling</td>
			<td>8730</td>
			<td></td>
		</tr>
		<tr>
			<td>Scintillation counter</td>
			<td>Perkin Elmer</td>
			<td>Tri-Carb-2910TR</td>
			<td></td>
		</tr>
		<tr>
			<td>Scintillation fluid&#160;</td>
			<td>Perkin Elmer</td>
			<td>6013329</td>
			<td></td>
		</tr>
		<tr>
			<td>Statistical analyses software</td>
			<td>Systat</td>
			<td>SigmaPlot version 14</td>
			<td></td>
		</tr>
		<tr>
			<td>TBC1D4 antibody</td>
			<td>Abcam</td>
			<td>ab189890</td>
			<td></td>
		</tr>
		<tr>
			<td>TissueLyser II&#160;</td>
			<td>Qiagen</td>
			<td>85300</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrapure water</td>
			<td>Merck</td>
			<td>Milli-Q Reference A+ System</td>
			<td></td>
		</tr>
		<tr>
			<td>&#946;-glycerophosphate</td>
			<td>Sigma</td>
			<td>G9422</td>
			<td></td>
		</tr>
	</tbody>
</table>

measurement of insulin- and contraction-stimulated glucose uptake in isolated and incubated mature skeletal muscle from mice

Skeletal muscle is an insulin-responsive tissue and typically takes up most of the glucose that enters the blood after a meal. Moreover, it has been reported that skeletal muscle may increase the extraction of glucose from the blood by up to 50-fold during exercise compared to resting conditions. The increase in muscle glucose uptake during exercise and insulin stimulation is dependent on the translocation of glucose transporter 4 (GLUT4) from intracellular compartments to the muscle cell surface membrane, as well as phosphorylation of glucose to glucose-6-phosphate by hexokinase II. Isolation and incubation of mouse muscles such as m. soleus and m. extensor digitorum longus (EDL) is an appropriate ex vivo model to study the effects of insulin and electrically-induced contraction (a model for exercise) on glucose uptake in mature skeletal muscle. Thus, the ex vivo model permits evaluation of muscle insulin sensitivity and makes it possible to match muscle force production during contraction ensuring uniform recruitment of muscle fibers during measurements of muscle glucose uptake. Moreover, the described model is suitable for pharmacological compound testing that may have an impact on muscle insulin sensitivity or may be of help when trying to delineate the regulatory complexity of skeletal muscle glucose uptake.
Here we describe and provide a detailed protocol on how to measure insulin- and contraction-stimulated glucose uptake in isolated and incubated soleus and EDL muscle preparations from mice using radiolabeled [3H]2-deoxy-D-glucose and [14C]mannitol as an extracellular marker. This allows accurate assessment of glucose uptake in mature skeletal muscle in the absence of confounding factors that may interfere in the intact animal model. In addition, we provide information on metabolic viability of incubated mouse skeletal muscle suggesting that the method&#160;applied possesses some caveats under certain conditions when studying muscle energy metabolism.

As shown in Figure 2 the basal glucose uptake rates were similar between isolated soleus and EDL muscle from female mice. This has also been reported several times before12,13,19,20. Glucose uptake increased by ~0.8 and ~0.6 fold reaching 12 and 9 &#181;mol/g protein/h in soleus and EDL muscle, respectively, in response to a submaxi...

Watch this Scientific Journal Video about Measurement of Insulin- and Contraction-Stimulated Glucose Uptake in Isolated and Incubated Mature Skeletal Muscle from Mice at JoVE.com

Measurement of Insulin- and Contraction-Stimulated Glucose Uptake in Isolated and Incubated Mature Skeletal Muscle from Mice

Intact regulation of muscle glucose uptake is important for maintaining whole body glucose homeostasis. This protocol presents assessment of insulin- and contraction-stimulated glucose uptake in isolated and incubated mature skeletal muscle when delineating the impact of various physiological interventions on whole body glucose metabolism.

Skeletal muscle is an insulin-responsive tissue and typically takes up most of the glucose that enters the blood after a meal. Moreover, it has been ...

measurement-insulin-contraction-stimulated-glucose-uptake-isolated

Research

JoVE Journal

Biology

5.1K Views.  University of Copenhagen. Intact regulation of muscle glucose uptake is important for maintaining whole body glucose homeostasis. This protocol presents assessment of insulin- and contraction-stimulated glucose uptake in isolated and incubated mature skeletal muscle when delineating the impact of various physiological interventions on whole body glucose metabolism. 

Video: Measurement of Insulin- and Contraction-Stimulated Glucose Uptake in Isolated and Incubated Mature Skeletal Muscle from Mice

In vivo Micro-circulation Measurement in Skeletal Muscle by Intra-vital Microscopy

BACKGROUND: Regulatory factors and detailed physiology of in vivo microcirculation have remained not fully clarified after many different modalities of imaging had invented. While many macroscopic parameters of blood flow reflect flow velocity, changes in blood flow velocity and red blood cell (RBC) flux does not hold linear relationship in the microscopic observations. There are reports of discrepancy between RBC velocity and RBC flux, RBC flux and plasma flow volume, and of spatial and temporal heterogeneity of flow regulation in the peripheral tissues in microscopic observations, a scientific basis for the requirement of more detailed studies in microcirculatory regulation using intravital microscopy. METHODS: We modified Jeff Lichtman's method of in vivo microscopic observation of mouse sternomastoid muscles. Mice are anesthetized,
ventilated, and injected with PKH26L-fluorescently labeled RBCs for microscopic observation.RESULT &amp; CONCLUSIONS: Fluorescently labeled RBCs are detected and distinguished well by a wide-field microscope. Muscle contraction evoked by electrical stimulation induced increase in RBC flux. Quantification of other parameters including RBC velocity and capillary density were feasible. Mice tolerated well the surgery, injection of stained RBCs, microscopic observation, and electrical stimulation. No muscle or blood vessel damage was observed, suggesting that our method is relatively less invasive and suited for long-term observations.

A new versatile method for observation of microcirculation is presented. It is considered suitable for long-term observation, and for combination with pharmacophysiological or molecular biological interventions.

BACKGROUND: Regulatory factors and detailed physiology of in vivo microcirculation have remained not fully clarified after many different modalities of ...

Adult and Embryonic Skeletal Muscle Microexplant Culture and Isolation of Skeletal Muscle Stem Cells

Cultured embryonic and adult skeletal muscle cells have a number of different uses. The micro-dissected explants technique described in this chapter is a robust and reliable method for isolating relatively large numbers of proliferative skeletal muscle cells from juvenile, adult or embryonic muscles as a source of skeletal muscle stem cells. The authors have used micro-dissected explant cultures to analyse the growth characteristics of skeletal muscle cells in wild-type and dystrophic muscles. Each of the components of tissue growth, namely cell survival, proliferation, senescence and differentiation can be analysed separately using the methods described here. The net effect of all components of growth can be established by means of measuring explant outgrowth rates. The micro-explant method can be used to establish primary cultures from a wide range of different muscle types and ages and, as described here, has been adapted by the authors to enable the isolation of embryonic skeletal muscle precursors.
Uniquely, micro-explant cultures have been used to derive clonal (single cell origin) skeletal muscle stem cell (SMSc) lines which can be expanded and used for in vivo transplantation. In vivo transplanted SMSc behave as functional, tissue-specific, satellite cells which contribute to skeletal muscle fibre regeneration but which are also retained (in the satellite cell niche) as a small pool of undifferentiated stem cells which can be re-isolated into culture using the micro-explant method.

The micro-dissected explants technique is a robust and reliable method for isolating proliferative skeletal muscle cells from juvenile, adult or embryonic muscles as a source of skeletal muscle stem cells. Uniquely, these cells have been clonally derived to produce skeletal muscle stem cell lines used for in vivo transplantation.

Cultured embryonic and adult skeletal muscle cells have a number of different uses. The micro-dissected explants technique described in this chapter is a ...

Mitochondrial Isolation from Skeletal Muscle

Mitochondria are organelles controlling the life and death of the cell. They participate in key metabolic reactions, synthesize most of the ATP, and regulate a number of signaling cascades2,3. Past and current researchers have isolated mitochondria from rat and mice tissues such as liver, brain and heart4,5. In recent years, many researchers have focused on studying mitochondrial function from skeletal muscles.
Here, we describe a method that we have used successfully for the isolation of mitochondria from skeletal muscles 6. Our procedure requires that all buffers and reagents are made fresh and need about 250-500 mg of skeletal muscle. We studied mitochondria isolated from rat and mouse gastrocnemius and diaphragm, and rat extraocular muscles. Mitochondrial protein concentration is measured with the Bradford assay. It is important that mitochondrial samples be kept ice-cold during preparation and that functional studies be performed within a relatively short time (~1 hr). Mitochondrial respiration is measured using polarography with a Clark-type electrode (Oxygraph system) at 37&deg;C7. Calibration of the oxygen electrode is a key step in this protocol and it must be performed daily. Isolated mitochondria (150 &mu;g) are added to 0.5 ml of experimental buffer (EB). State 2 respiration starts with addition of glutamate (5mM) and malate (2.5 mM). Then, adenosine diphosphate (ADP) (150 &mu;M) is added to start state 3. Oligomycin (1 &mu;M), an ATPase synthase blocker, is used to estimate state 4. Lastly, carbonyl cyanide p-[trifluoromethoxy]-phenyl-hydrazone (FCCP, 0.2 &mu;M) is added to measurestate 5, or uncoupled respiration 6. The respiratory control ratio (RCR), the ratio of state 3 to state 4, is calculated after each experiment. An RCR &ge;4 is considered as evidence of a viable mitochondria preparation.
In summary, we present a method for the isolation of viable mitochondria from skeletal muscles that can be used in biochemical (e.g., enzyme activity, immunodetection, proteomics) and functional studies (mitochondrial respiration).

This protocol describes a procedure to study the respiration of mitochondria isolated from skeletal muscles. This method was adapted from Scorrano et al. (2007). The mitochondrial isolation procedure requires about 2 hours. The mitochondrial respiration can be completed in about 1 hour.

Mitochondria are organelles controlling the life and death of the cell. They participate in key metabolic reactions, synthesize most of the ATP, and ...

Purification of Progenitors from Skeletal Muscle

Skeletal muscle contains multiple progenitor populations of distinct embryonic origins and developmental potential. Myogenic progenitors, usually residing in a "satellite cell position" between the myofiber plasma membrane and the laminin-rich basement membrane that ensheaths it, are self-renewing cells that are solely committed to the myogenic lineage1,2. We have recently described a second class of vessel associated progenitors that can generate myofibroblasts and white adipocytes, which responds to damage by efficiently entering proliferation and provides trophic support to myogenic cells during tissue regeneration3,4. One of the most trusted assays to determine the developmental and regenerative potential of a given cell population relies on their isolation and transplantation5-7. To this end we have optimized protocols for their purification by flow cytometry from enzymatically dissociated muscle, which we will outline in this article. The populations obtained using this method will contain either myogenic or fibro/adipogenic colony forming cells: no other cell types are capable of expanding in vitro or surviving in vivo delivery. However, when these populations are used immediately after the sort for molecular analysis (e.g qRT-PCR) one must keep in mind that the freshly sorted subsets may contain other contaminant cells that lack the ability of forming colonies or engrafting recipients.

Method for the enzymatic dissociation, surface labeling and purification by flow cytometry of fibro/adipogenic and myogenic progenitors from murine skeletal muscle.

Skeletal muscle contains multiple progenitor populations of distinct embryonic origins and developmental potential. Myogenic progenitors, usually residing ...

Fluorescence-based Measurement of Store-operated Calcium Entry in Live Cells: from Cultured Cancer Cell to Skeletal Muscle Fiber

Store operated Ca2+ entry (SOCE), earlier termed capacitative Ca2+ entry, is a tightly regulated mechanism for influx of extracellular Ca2+ into cells to replenish depleted endoplasmic reticulum (ER) or sarcoplasmic reticulum (SR) Ca2+ stores1,2. Since Ca2+ is a ubiquitous second messenger, it is not surprising to see that SOCE plays important roles in a variety of cellular processes, including proliferation, apoptosis, gene transcription and motility. Due to its wide occurrence in nearly all cell types, including epithelial cells and skeletal muscles, this pathway has received great interest3,4. However, the heterogeneity of SOCE characteristics in different cell types and the physiological function are still not clear5-7.
The functional channel properties of SOCE can be revealed by patch-clamp studies, whereas a large body of knowledge about this pathway has been gained by fluorescence-based intracellular Ca2+ measurements because of its convenience and feasibility for high-throughput screening. The objective of this report is to summarize a few fluorescence-based methods to measure the activation of SOCE in monolayer cells, suspended cells and muscle fibers5,8-10. The most commonly used of these fluorescence methods is to directly monitor the dynamics of intracellular Ca2+ using the ratio of F340nm and F380nm (510 nm for emission wavelength) of the ratiometric Ca2+ indicator Fura-2. To isolate the activity of unidirectional SOCE from intracellular Ca2+ release and Ca2+ extrusion, a Mn2+ quenching assay is frequently used. Mn2+ is known to be able to permeate into cells via SOCE while it is impervious to the surface membrane extrusion processes or to ER uptake by Ca2+ pumps due to its very high affinity with Fura-2. As a result, the quenching of Fura-2 fluorescence induced by the entry of extracellular Mn2+ into the cells represents a measurement of activity of SOCE9. Ratiometric measurement and the Mn+2 quenching assays can be performed on a cuvette-based spectrofluorometer in a cell population mode or in a microscope-based system to visualize single cells. The advantage of single cell measurements is that individual cells subjected to gene manipulations can be selected using GFP or RFP reporters, allowing studies in genetically modified or mutated cells. The spatiotemporal characteristics of SOCE in structurally specialized skeletal muscle can be achieved in skinned muscle fibers by simultaneously monitoring the fluorescence of two low affinity Ca2+ indicators targeted to specific compartments of the muscle fiber, such as Fluo-5N in the SR and Rhod-5N in the transverse tubules9,11,12.

The extent of store-operated Ca2+ entry (SOCE) can be monitored using fluorescent Ca2+ indicators. Mn2+ quenching of such indicators assays SOCE in cultured cells and skeletal muscle fibers. A technique allowing spatial and temporal resolution of SOCE by confocal imaging of mechanically skinned muscle fibers is also described.

Store operated Ca2+ entry (SOCE), earlier termed capacitative Ca2+ entry, is a tightly regulated mechanism for influx of extracellular Ca2+ into cells to ...

Ex Vivo Assessment of Contractility, Fatigability and Alternans in Isolated Skeletal Muscles

Described here is a method to measure contractility of isolated skeletal muscles. Parameters such as muscle force, muscle power, contractile kinetics, fatigability, and recovery after fatigue can be obtained to assess specific aspects of the excitation-contraction coupling (ECC) process such as excitability, contractile machinery and Ca2+ handling ability. This method removes the nerve and blood supply and focuses on the isolated skeletal muscle itself. We routinely use this method to identify genetic components that alter the contractile property of skeletal muscle though modulating Ca2+ signaling pathways. Here, we describe a newly identified skeletal muscle phenotype, i.e., mechanic alternans, as an example of the various and rich information that can be obtained using the in vitro muscle contractility assay. Combination of this assay with single cell assays, genetic approaches and biochemistry assays can provide important insights into the mechanisms of ECC in skeletal muscle.

Ex Vivo Assessment of Contractility, Fatigability and Alternans in Isolated Skeletal Muscles

We describe a method to directly measure muscle force, muscle power, contractile kinetics and fatigability of isolated skeletal muscles in an in vitro system using field stimulation. Valuable information on Ca2+ handling properties and contractile machinery of the muscle can be obtained using different stimulating protocols.

 
Described here is a method to measure contractility of isolated skeletal muscles. Parameters such as muscle force, muscle power, contractile kinetics, ...

Isolation and Culture of Individual Myofibers and their Satellite Cells from Adult Skeletal Muscle

Muscle regeneration in the adult is performed by resident stem cells called satellite cells. Satellite cells are defined by their position between the basal lamina and the sarcolemma of each myofiber. Current knowledge of their behavior heavily relies on the use of the single myofiber isolation protocol. In 1985, Bischoff described a protocol to isolate single live fibers from the Flexor Digitorum Brevis (FDB) of adult rats with the goal to create an in vitro system in which the physical association between the myofiber and its stem cells is preserved 1. In 1995, Rosenblattmodified the Bischoff protocol such that myofibers are singly picked and handled separately after collagenase digestion instead of being isolated by gravity sedimentation 2, 3. The Rosenblatt or Bischoff protocol has since been adapted to different muscles, age or conditions 3-6. The single myofiber isolation technique is an indispensable tool due its unique advantages. First, in the single myofiber protocol, satellite cells are maintained beneath the basal lamina. This is a unique feature of the protocol as other techniques such as Fluorescence Activated Cell Sorting require chemical and mechanical tissue dissociation 7. Although the myofiber culture system cannot substitute for in vivo studies, it does offer an excellent platform to address relevant biological properties of muscle stem cells. Single myofibers can be cultured in standard plating conditions or in floating conditions. Satellite cells on floating myofibers are subjected to virtually no other influence than the myofiber environment. Substrate stiffness and coating have been shown to influence satellite cells' ability to regenerate muscles 8, 9 so being able to control each of these factors independently allows discrimination between niche-dependent and -independent responses. Different concentrations of serum have also been shown to have an effect on the transition from quiescence to activation. To preserve the quiescence state of its associated satellite cells, fibers should be kept in low serum medium 1-3. This is particularly useful when studying genes involved in the quiescence state. In serum rich medium, satellite cells quickly activate, proliferate, migrate and differentiate, thus mimicking the in vivo regenerative process 1-3. The system can be used to perform a variety of assays such as the testing of chemical inhibitors; ectopic expression of genes by virus delivery; oligonucleotide based gene knock-down or live imaging. This video article describes the protocol currently used in our laboratory to isolate single myofibers from the Extensor Digitorum Longus (EDL) muscle of adult mice (6-8 weeks old).

Isolation and culture of myofibers is the gold standard in vitro system to study the transition of satellite cells through quiescence, activation and differentiation. Importantly, the single myofiber culture system preserves the myofiber/stem cell association, which is an essential component of the muscle stem cell niche.

Muscle  regeneration in the adult is performed by resident stem cells called satellite  cells. Satellite cells are defined by  their position between the ...

Measurement of Smooth Muscle Function in the Isolated Tissue Bath-applications to Pharmacology Research

Isolated tissue bath assays are a classical pharmacological tool for evaluating concentration-response relationships in a myriad of contractile tissues. While this technique has been implemented for over 100 years, the versatility, simplicity and reproducibility of this assay helps it to remain an indispensable tool for pharmacologists and physiologists alike. Tissue bath systems are available in a wide array of shapes and sizes, allowing a scientist to evaluate samples as small as murine mesenteric arteries and as large as porcine ileum &#8211; if not larger. Central to the isolated tissue bath assay is the ability to measure concentration-dependent changes to isometric contraction, and how the efficacy and potency of contractile agonists can be manipulated by increasing concentrations of antagonists or inhibitors. Even though the general principles remain relatively similar, recent technological advances allow even more versatility to the tissue bath assay by incorporating computer-based data recording and analysis software. This video will demonstrate the function of the isolated tissue bath to measure the isometric contraction of an isolated smooth muscle (in this case rat thoracic aorta rings), and share the types of knowledge that can be created with this technique. Included are detailed descriptions of aortic tissue dissection and preparation, placement of aortic rings in the tissue bath and proper tissue equilibration prior to experimentation, tests of tissue viability, experimental design and implementation, and data quantitation. Aorta will be connected to isometric force transducers, the data from which will be captured using a commercially available analog-to-digital converter and bridge amplifier specifically designed for use in these experiments. The accompanying software to this system will be used to visualize the experiment and analyze captured data.

This protocol describes the measurement of isometric contraction in an isolated smooth muscle preparation, using an isolated tissue bath system and computer-based data acquisition.

Isolated tissue bath assays are a classical pharmacological tool for evaluating concentration-response relationships in a myriad of contractile tissues.  ...

Glucose Uptake Measurement and Response to Insulin Stimulation in In Vitro Cultured Human Primary Myotubes

Skeletal muscle is the largest glucose deposit in mammals and largely contributes to glucose homeostasis. Assessment of insulin sensitivity of muscle cells is of major relevance for all studies dedicated to exploring muscle glucose metabolism and characterizing metabolic alterations. In muscle cells, glucose transporter type 4 (GLUT4) proteins translocate to the plasma membrane in response to insulin, thus allowing massive entry of glucose into the cell. The ability of muscle cells to respond to insulin by increasing the rate of glucose uptake is one of the standard readouts to quantify muscle cell sensitivity to insulin. Human primary myotubes are a suitable in vitro model, as the cells maintain many features of the donor phenotype, including insulin sensitivity. This in vitro model is also suitable for the test of any compounds that could impact insulin responsiveness. Measurements of the glucose uptake rate in differentiated myotubes reflect insulin sensitivity.
In this method, human primary muscle cells are cultured in vitro to obtain differentiated myotubes, and glucose uptake rates with and without insulin stimulation are measured. We provide a detailed protocol to quantify passive and active glucose transport rates using radiolabeled [3H] 2-deoxy-D-Glucose ([3H]2dG). Calculation methods are provided to quantify active basal and insulin-stimulated rates, as well as stimulation fold.

Glucose Uptake Measurement and Response to Insulin Stimulation in In Vitro Cultured Human Primary Myotubes

In this method, human primary muscle cells are cultured in vitro to obtain differentiated myotubes and glucose uptake rates are measured. We provide a detailed protocol to quantify rates in basal and insulin-stimulated states using radiolabeled [3H] 2-deoxy-D-Glucose.

Skeletal muscle is the largest glucose deposit in mammals and largely contributes to glucose homeostasis. Assessment of insulin sensitivity of muscle ...

Isolation and Characterization of Exosomes from Skeletal Muscle Fibroblasts

Exosomes are small extracellular vesicles released by virtually all cells and secreted in all biological fluids. Many methods have been developed for the isolation of these vesicles, including ultracentrifugation, ultrafiltration, and size exclusion chromatography. However, not all are suitable for large scale exosome purification and characterization. Outlined here is a protocol for establishing cultures of primary fibroblasts isolated from adult mouse skeletal muscles, followed by purification and characterization of exosomes from the culture media of these cells. The method is based on the use of sequential centrifugation steps followed by sucrose density gradients. Purity of the exosomal preparations is then validated by western blot analyses using a battery of canonical markers (i.e., Alix, CD9, and CD81). The protocol describes how to isolate and concentrate bioactive exosomes for electron microscopy, mass spectrometry, and uptake experiments for functional studies. It can easily be scaled up or down and adapted for exosome isolation from different cell types, tissues, and biological fluids.

This protocol illustrates the 1) the isolation and culture of primary fibroblasts from the adult mouse gastrocnemius muscle as well as 2) purification and characterization of exosomes using a differential ultracentrifugation method combined with sucrose density gradients followed by western blot analyses.

Exosomes are small extracellular vesicles released by virtually all cells and secreted in all biological fluids. Many methods have been developed for the ...