Name: in vitro assessment of cardiac function using skinned cardiomyocytes
Uploaded: 2020-06-22T13:00:00
Duration: 8 min 19 s
Description: Watch this Scientific Journal Video about In Vitro Assessment of Cardiac Function Using Skinned Cardiomyocytes at JoVE.com

The authors thank Portuguese Foundation for Science and Technology (FCT), European Union, Quadro de Refer&#234;ncia Estrat&#233;gico Nacional (QREN), Fundo Europeu de Desenvolvimento Regional (FEDER) and Programa Operacional Factores de Competitividade (COMPETE) for funding UnIC (UID/IC/00051/2013) research unit. This project is supported by FEDER through COMPETE 2020 - Programa Operacional Competitividade E Internacionaliza&#231;&#227;o (POCI), the project DOCNET (NORTE-01-0145-FEDER-000003), supported by Norte Portugal regional operational programme (NORTE 2020), under the Portugal 2020 partnership agreement, through the European Regional Development Fund (ERDF), the project NETDIAMOND (POCI-01-0145-FEDER-016385), supported by European Structural And Investment Funds, Lisbon&#39;s regional operational program 2020. Patr&#237;cia Rodrigues was funded by FCT (SFRH/BD/96026/2013) and Jo&#227;o Almeida-Coelho was by Universidade do Porto/FMUP and FSE-Fundo Social Europeu, NORTE 2020-Programa Operacional Regional do Norte, (NORTE-08-5369-FSE-000024-Programas Doutorais).

All animal experiments comply with the Guide for the Care and Use of Laboratory Animals (NIH Publication no. 85-23, revised 2011) and the Portuguese law on animal welfare (DL 129/92, DL 197/96; P 1131/97). The competent local authorities approved this experimental protocol (018833).
1. Stock solution preparation (Table 1)
<ol>
	<li>Prepare 1,000 mL of relaxing solution for cardiomyocytes&#39; isolation (RELAX-ISO) by following instructions in Table 2. Dissolve the reagent above in &#8776;500 mL and adjust the pH to 7.0 with KOH. Adjust the final volume to 1000 mL.
	<ol>
		<li>Distribute RELAX-ISO in 50 mL tubes. Store at -20 &#176;C.</li>
	</ol>
	</li>
	<li>Prepare 250 mL of activating solution by following the instructions in Table 3. Dissolve the reagents above in &#8776;100 mL of ultra-pure water. Adjust the pH to 7.1 with 5 M KOH at 15 &#176;C. 
	NOTE: Usually, it is necessary to add a significant amount of KOH to reach the desired pH. Put the volumetric balloon in a box with ice to cool down the solution to 15 &#176;C.
	<ol>
		<li>Adjust the final volume to 250 mL. Agitate this solution continuously with a magnetic stirrer until the moment of mixing it with the relaxing solution.</li>
	</ol>
	</li>
	<li>Prepare 100 mL of relaxing solution by following the instructions in Table 4. Dissolve the reagents above in &#8776;50 mL of ultra-pure water. Adjust the pH to 7.1 with KOH 5 M at 15 &#176;C. 
	NOTE:&#160;Usually, it is necessary to add a significant amount of KOH to reach a&#160;pH of 7.1. Place the volumetric balloon in a box filled with ice to cool down the solution to 15 &#176;C. The ionic strength of the solutions used during the measurements amounted to 180 mM.
	<ol>
		<li>Adjust the final volume to 100 mL. Agitate this solution continuously with a magnetic stirrer until the moment of mixing it with activating solution.</li>
	</ol>
	</li>
	<li>Mix activating and relaxing solutions in the proportions presented in Table 5 to obtain pCa solutions between 5.0 and 6.0.
	<ol>
		<li>Always keep relaxing and activating solutions agitating while mixing both.</li>
		<li>Aliquot each mixture to 2 mL&#160;microtubes.&#160;Store all the microtubes at -20 &#176;C.</li>
	</ol>
	</li>
	<li>Prepare a different batch of pCa solution (4.5 to 6.0) for each protocol.</li>
</ol>
2. Calibration of the force transducer
NOTE: The calibration of the force transducer is a routine procedure that should be performed every couple of months or whenever it is suspected to be uncalibrated. The force transducer is highly sensitive and is easily broken. It should be gently handled in every step of its usage, including calibration, gluing of the cardiomyocyte and cleaning.
<ol>
	<li>Detach the force transducer from the rest of the apparatus.</li>
	<li>With the help of a clamp, place the force transducer horizontally in such a way that the needle points downward in the same orientation that the cardiomyocyte will develop force. This will facilitate hanging a series of masses with known weights (elastic band, suture or pin). 
	NOTE: Check the characteristics of the force transducer before proceeding to this step to check the scale factor [mg/volt] and to avoid excessive weight on the transducer. For the force transducer model, the scale factor is 50 (50 mg correspond to 1 volt) and we use 5 weights between 12.5 and 250 mg.</li>
	<li>Turn the force transducer on and let it warm up for 30 min.</li>
	<li>Start by hanging the lighter mass on the force transducer and registering the corresponding voltage measured at FORCE OUT.
	<ol>
		<li>Repeat this procedure for up to five weights.</li>
	</ol>
	</li>
	<li>Plot force applied to the force transducer (load) versus voltage and check for linearity.</li>
	<li>If there is no linearity, adjust the zero and gain potentiometers in the circuit board of the transducer. Check its specific instruction for further information.
	<ol>
		<li>Turn the zero potentiometer until the output voltage reads 0.0 V.</li>
		<li>Hand a medium weight on the transducer needle and adjust the gain potentiometer to read the corresponding voltage (for instance, 50 mg correspond to 1 V). Remove the weight and re-adjust the zero potentiometer to 0.0.</li>
		<li>Repeat step 2.6.2 until the output with and without the weight are correct.</li>
	</ol>
	</li>
	<li>Mount the force transducer back into the apparatus.</li>
</ol>
3. Setting the experimental apparatus
<ol>
	<li>Thaw one vial of each of the activating, 4.5, 5.0, 5.2, 5.4, 5.6, 5.8, 6.0 and relaxing solutions and maintain them on ice. 
	NOTE: ATP and PCr are labile compounds that should be maintained at cold temperatures.</li>
	<li>Prepare the microscope, testing apparatus and associated computer for use (Figure 1).</li>
	<li>Adjust the temperature so that the in-chamber thermometer reads 15 &#176;C. Perform all experiments at this temperature except for kinase and phosphatase incubations (20 &#176;C).</li>
	<li>Turn on the force-transducer and the motor.</li>
</ol>
4. Extraction and permeabilization of skinned cardiomyocytes
<ol>
	<li>Defrost 50 mL of RELAX-ISO solution.</li>
	<li>Turn on the centrifuge and fast cool it up to 4 &#176;C.</li>
	<li>Thaw 3-5 &#181;g of a myocardial sample in a Petri dish containing&#160;2.5 mL of RELAX-ISO solution.</li>
	<li>Cut the tissue in small pieces with a scalpel blade (Figure 2). Cut the sample in a precise way to avoid unnecessary cells damage.</li>
	<li>Transfer the 2.5 mL of RELAX-ISO solution with the tissue to a Potter-Elvehjem glass using&#160;a cut pipette tip.</li>
	<li>Mechanically disrupt the tissue with a grinder at a rotation speed of 30-40 rpm. Press the tissue 3 times for 2 s each to obtain a good cell suspension.</li>
	<li>Prepare 10% Triton in RELAX-ISO solution (250 &#181;L of Triton with 2.25 mL of RELAX-ISO) in a 15 mL tube and add this solution to the cell suspension.</li>
	<li>Gently mix by inverting the tube 3 times.</li>
	<li>Incubate at room temperature for 1 min and 4 min on ice.</li>
	<li>Wash out the Triton by adding RELAX-ISO up to the top of the 15 mL tube; gently mixing (inverting 3 times the tube) and finally spinning down the cells in an angled centrifuge (1 min at 348 x g). Remove the supernatant up to 3 mL above the cell pellet. 
	NOTE: Remove the supernatant gently to avoid disturbing the cell pellet. Still, some cells in supernatant will be inevitably lost.</li>
	<li>Repeat the step 4.10 at least 4 times or until no more bubbles produced by Triton residues are observed. 
	NOTE: The more wash-out steps are made, the more cells are lost with the discarded supernatant.</li>
	<li>In the last wash-out, remove the supernatant up to a volume of 5-10 mL of cell suspension.</li>
</ol>
5. Selecting and gluing the skinned cardiomyocyte
<ol>
	<li>Put a cell suspension drop on a coverslip on top of a glass slide in the microscope slide holder (Figure 1).</li>
	<li>Select a single rod-shaped cardiomyocyte with a good striation pattern and size (Figure 2).</li>
	<li>Find the needle tips of the force-transducer and the motor using the lowest magnification of the inverted microscope.</li>
	<li>Rotate the coverslip to position the selected cardiomyocyte horizontally so that its ends are aligned with the needle of the force transducer and the motor (Figure 2).</li>
	<li>Place a thin line of glue on the side of the coverslip with the help of a swab tip (Figure 2).</li>
	<li>Immerse the needle tips of the force transducer and the motor into the glue line to create a glue halo around both tips. 
	NOTE: Steps 5.6 - 5.10 are accomplished through careful use of the motorized micropositioners.</li>
	<li>Quickly move the needle tips close to the focal plane of the cardiomyocyte.</li>
	<li>Move the needle tip of the force transducer down so that it glues to one edge of the cardiomyocyte.</li>
	<li>Repeat this procedure with the tip of the motor and the other extremity of the cell. 
	NOTE: This procedure must take less than 2-3 min as the glue starts to cure very fast.</li>
	<li>After 5-8 min, lift the needles &#8776;15 &#181;m to avoid gluing the cell to the coverslip. This is done by moving up both micromanipulators simultaneously.</li>
	<li>Let the glue cure. This procedure can last from 15 to 45 min, depending on the type of glue. In our case, the cardiomyocyte is adequately glued after &#8776;15 min.</li>
</ol>
6. Recording force measurements of active, passive and Ca2+ sensitivity
<ol>
	<li>Fill the first experimental well with the relaxing solution (55-100 &#181;L in the experimental apparatus) and the second experimental well with activating solution.</li>
	<li>Using the camera software, place the region of interest (ROI) in an area of the cardiomyocyte with a clear pattern of striation. 
	NOTE: For cardiac myocytes, the operating sarcomere length varies between 1.8 and 2.2 &#181;m, and the optimal sarcomere length is around 2.15 &#181;m.</li>
	<li>Measure the distance between the two extremes of the cardiomyocyte (from the motor to the transducer glue halo, Figure 3) after the optimal sarcomere length has been set (2.2 &#181;m). Record the value as myocyte length in the software.</li>
	<li>Measure cardiomyocyte width and depth, the latter with the aid of a prism mirror placed perpendicular to the cell. 
	NOTE: A powerful, external light source will be required to visualize the cell through the prism. In case there is no prism, and assuming that cardiac cells have an elliptical shape, cardiomyocyte depth can be inferred as 70% of cardiomyocyte width.</li>
	<li>Calculate cross-sectional area (CSA, mm2) assuming an elliptical shape of the cardiomyocyte. 
	<img alt="Equation 1" src="/files/ftp_upload/60427/60427eq1.jpg" /></li>
	<li>Gently move the microscope stage so that the cell moves from the coverslip to the well containing relaxing solution on the back of the stage. 
	NOTE: This procedure can easily damage the cell. Before moving the cell, gently move the needles up a bit more. Avoid removing the cell out of the solution.</li>
	<li>Select the protocol in software that contains two cell shortening (80% of its initial length), that will occur when the cell is emerged in Ca2+ solution and in relaxing solution, respectively (Figure 1, Supplementary File). 
	NOTE: First &#34;Slack&#34; of the cell will be performed within activating solution and the second &#34;Slack&#34; within relaxing solution. By doing this, the user will be able to calculate the total force (Ftotal) of the cell from the 1st and the passive force (Fpassive) of the cell from the 2nd. Use the formula to calculate active force, Factive = Ftotal - Fpassive. The cell is shortened 80% in order to detach all cross-bridges record force.</li>
	<li>Elicit isometric contraction by moving the microscope stage so that the cardiomyocyte moves from the relaxing to the activating solution (pCa=4.5(1)). 
	NOTE: If the cell is functional, it will immediately contract.</li>
	<li>Upon reaching force plateau, start recording the force data. 
	NOTE: The tests can be done individually. Depending on software there is the possibility to create a sequence of tests that will correspond to the different Ca2+ solutions within a Ca2+-sensitivity protocol (Figure 1, Supplementary File).</li>
	<li>Wait ~10 s and then switch the cell immersed in activating solution. 
	NOTE: It is important to wait 10 s before immersing the cell in relaxing solution. If the cell is moved too early, important data to calculate the redevelopment force of the cell (ktr value) might be lost.</li>
	<li>Quickly move the stage so that the cardiomyocyte immerses in the relaxing solution.</li>
	<li>Wait until the test stops.</li>
	<li>Repeat steps 6.8 - 6.12 so that the cell is activated twice in activating solution (pCa=4.5(2)). 
	NOTE: Typically, after the first activation, cardiomyocyte ends can slightly detach&#160;from the needle tips, changing the cardiomyocyte length, CSA and/or the sarcomere length. Readjust to the desired sarcomere length and introduce the corrected dimensions in the software.
	<ol>
		<li>Continue to step 6.13.2 for Ca2+ sensitivity protocol or save the data, if values of passive and active force of the cell are the only parameters needed and detach the cell from needles and clean them with acetone to remove the glue.</li>
		<li>Adjust the sarcomere length of the cell to 2.2 &#181;m by slightly stretching it again, if necessary.</li>
		<li>Replace the activating solution by the next Ca2+ solution (55-100 &#181;L here). Repeat steps 6.8 - 6.12.</li>
		<li>Replace the activating solution by the next Ca2+ solution (55-100 &#181;L here) and repeat steps 6.8 - 6.12 until all solutions have been tested (5.0, 5.2, 5.4, 5.6, 5.8, 6.0).</li>
		<li>Lastly, re-activate the cell with activating solution (pCa4.5(3)). Repeat steps 6.8- 6.12.</li>
	</ol>
	</li>
</ol>
7. Incubation with kinases and phosphatases
<ol>
	<li>After performing the selected baseline protocol, dilute the kinase/phosphatase in Relaxing solution at the recommended concentration. 
	NOTE: It is recommended to carry out a dose-response curve prior to the experiment.</li>
	<li>Set the temperature of the experimental wells to 20 &#176;C.</li>
	<li>Fill the experimental wells with kinase/phosphatase, relaxing solution and activating solution (55-100 &#181;L).</li>
	<li>Gently move the microscope stage so that the cell becomes immersed in the well containing kinase/phosphatase.</li>
	<li>Incubate the cardiomyocyte with the kinase/phosphatase for at least 30 min or according to the manufacturers&#39; instructions.</li>
	<li>Repeat the selected baseline protocol.</li>
</ol>
8. Finalizing the experiment
<ol>
	<li>Unglue the cardiomyocyte from the tips of the force transducer and motor by stretching the cell.</li>
	<li>Carefully remove the glue halo from the needle tips using a cotton swab soaked in acetone.</li>
	<li>Shut down the equipment.</li>
</ol>
9. Analyzing the data
<ol>
	<li>Collect all files from each cardiomyocyte tested. 
	NOTE: Each test will correspond to one file. This means that for each Ca2+ solution or sarcomere length, there will be a corresponding file.</li>
	<li>Calculate active and passive forces of a single cardiomyocyte.
	<ol>
		<li>Open the file corresponding to first activation (pCa=4.5(1)) using a spreadsheet (Figure 3A, Appendix A in Supplementary File). 
		NOTE: We used a custom-made program to perform the analysis. Please see Appendix A in the Supplementary File.</li>
		<li>Average &#8776;60 values before and average &#8776;60 values after the 1st slack of the cell (when the cell is immersed in Ca2+ solution). These 2 values correspond to a and b, respectively.</li>
		<li>Repeat the same analysis for the 2nd slack of the cell (when the cell is immersed in relaxing solution). These 2 values correspond to c and d, respectively.</li>
		<li>Calculate the difference between a and b (total force, Ftotal).</li>
		<li>Calculate the difference between c and d (passive force, Fpassive).</li>
		<li>Calculate active force, Factive = Ftotal - Fpassive.</li>
		<li>Normalize all force values to CSA (see formula above) to obtain the total tension (Ttotal), passive tension (Tpassive) and active tension (Tactive).</li>
		<li>Repeat step 9.2.1 to 9.2.5 for the 2nd activation (pCa=4.5(2)).</li>
		<li>Consider these values as those representing Ttotal, Tactive and Tpassive of the cardiomyocyte under analysis. 
		NOTE: The first activation of the cell with pCa4.5(1) is usually associated with alterations in cell dimensions. For this reason, the 2nd activation with pCa4.5 is more accurate and is the one to be used.</li>
		<li>Repeat the steps 9.2.1 to 9.2.5 for each file/pCa tested solutions (5.0, 5.2, 5.4, 5.6, 5.8, 6.0, 4.5(3)).</li>
	</ol>
	</li>
	<li>Calculate pCa50 and nHill of a single cardiomyocyte. 
	NOTE: For this analysis, use the non-normalized Factive values from the files 4.5(2), 5.0, 5.2, 5.4, 5.6, 5.8, 6.0 and 4.5(3).
	<ol>
		<li>Place in a spreadsheet file all non-normalized values of Factive for each Ca2+ solution tested (4.5(2),5.0, 5.2, 5.4, 5.6, 5.8, 6.0, 4.5(3)).</li>
		<li>Calculate the correction factor = Factive [4.5(2)] - Factive [4.5(3)] / 7.</li>
		<li>Calculate the corrected values of Factive for each Ca2+ solutions (5.0, 5.2, 5.4, 5.6, 5.8, 6.0) by subtracting Factive - correction factor.</li>
		<li>Calculate the relative force (Frelative) for each Ca2+ solutions by normalizing each Factive values by the corresponding corrected value. 
		NOTE: The Frelative [4.5(3)] should equal 1. Each experimental protocol begins and ends with a control activation at saturating Ca2+ concentration (pCa 4.5(2) and 4.5(3)). This allows force normalization and assessment of the rundown of the preparations through the comparison of changes in maximal Ca2+-activated force (Fmax). If at the end of the experimental protocol, the cardiomyocyte produces less than at least 70% of the maximum force of the first contraction, that cell/measurement should be excluded from the analysis.</li>
		<li>Use the Frelative and the corresponding pCa values to fit to a sigmoidal curve with the following equation F(Ca) = CanHill/ (Ca50nHill + CanHill).</li>
		<li>Extrapolate pCa and nHill values from the equation above mentioned.</li>
	</ol>
	</li>
	<li>Calculate the rate of force redevelopment (ktr) of a single cardiomyocyte.
	<ol>
		<li>Perform a fit to the curve that corresponds to the values immediately after the 1st cell slack.</li>
		<li>Calculate the slope of the curve and this value will correspond to the rate of force redevelopment.</li>
		<li>Repeat step 9.4.1 and 9.4.2 for each Ca2+ solution. 
		NOTE: A poor curve fit will be obtained for the lowest Ca-solutions (R squared&#8804;0.90).</li>
	</ol>
	</li>
</ol>

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Y., Mosca, E., de Tombe, P. P., Cazorla, O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differential+contribution+of+cardiac+sarcomeric+proteins+in+the+myofibrillar+force+response+to+stretch.">Differential contribution of cardiac sarcomeric proteins in the myofibrillar force response to stretch.</a> Pflugers Archiv. 457 (1), 25-36 (2008).</li><li>Falcao-Pires, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diabetes+mellitus+worsens+diastolic+left+ventricular+dysfunction+in+aortic+stenosis+through+altered+myocardial+structure+and+cardiomyocyte+stiffness.">Diabetes mellitus worsens diastolic left ventricular dysfunction in aortic stenosis through altered myocardial structure and cardiomyocyte stiffness.</a> Circulation. 124 (10), 1151-1159 (2011).</li><li>Lim, C. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Anthracyclines+induce+calpain-dependent+titin+proteolysis+and+necrosis+in+cardiomyocytes.">Anthracyclines induce calpain-dependent titin proteolysis and necrosis in cardiomyocytes.</a> Journal Biology Chemistry. 279 (9), 8290-8299 (2004).</li><li>Woulfe, K. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Novel+Method+of+Isolating+Myofibrils+From+Primary+Cardiomyocyte+Culture+Suitable+for+Myofibril+Mechanical+Study.">A Novel Method of Isolating Myofibrils From Primary Cardiomyocyte Culture Suitable for Myofibril Mechanical Study.</a> Frontiers Cardiovascular Medicine. 6, 12 (2019).</li><li>Ait Mou, Y., Bollensdorff, C., Cazorla, O., Magdi, Y., de Tombe, P. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Exploring+cardiac+biophysical+properties.">Exploring cardiac biophysical properties.</a> Global Cardiology Science Practice. 2015, 10 (2015).</li></ol>

The authors have no conflict of interest.

In vitro assessment of cardiac function using skinned cardiomyocytes represents an important technique to clarify the modifications occurring at cardiomyocyte level in physiological (e.g., stretch) and pathological context (e.g., ischemia). This methodology has several advantages such as requiring a minimal amount of myocardium to assess function in cardiomyocytes obtained from defrosted samples; using cardiomyocytes from a wide range of species (mice13, rat1,14,15, rabbit16, pig17, dog18, guinea pig19 and human20) and different cardiac locations, including the atria, left and right ventricles or a specific region of the infarcted heart. Moreover, this technique allows delivering specific concentrations of Ca2+ and energy (ATP) while measuring the function of regulatory and contractile structures in their native configuration.
Despite the simplicity of this technique, there are some critical steps. It is essential to guarantee the quality of each step from the beginning, including sample collection. Myofilament proteins are susceptible to proteases21. Thus it is mandatory to store samples in liquid nitrogen immediately after its collection. Fresh samples, which were not previously frozen, will develop significantly higher forces, so it is not advisable to mix measurement done in fresh and frozen samples in the same protocol. The second most critical step is the cardiomyocytes&#39; extraction. During this procedure, it is crucial to maintain the sample on ice most of the time. A protease inhibitor cocktail can be used to reduce the risk of protein degradation during the extraction/permeabilization22. Thirdly, samples should be cut in smaller pieces using precise scalpel movements since we noted reduced quality cardiomyocytes when this step was disregarded. Another critical step is washing the cardiomyocytes since it is difficult to have the right balance between washing out Triton (permeabilizes the cell but promotes its ungluing) and keeping as&#160;many cells in the supernatant as possible. It is important to first try the extraction and number of washouts for each sample, species or protocol. For instance, in our hands, we noted that ZSF1 obese rat tissue extractions have a &#34;fatty&#34; aspect, which made these cells more slippery during the gluing but not more difficult to measure. The way we circumvent this problem was by performing more experiments to have a reasonable number of cells per animal. Moreover, it is crucial to select a good cell to glue, namely with good striation and reasonable length. If the cardiomyocyte does not have these features, it will mostly detach from the needle tips or develop no/low force. It is also important to use the correct glue for cardiomyocyte attachment, considering the time of gluing and its efficacy to glue the cell to the needle. In our hands, the silicone glue (Table of Materials) cures fast (10-15 min) and strong enough. Finally, the last critical step is related with carefully lifting the cardiomyocyte 5 min after gluing the cell (to avoid gluing the cell to the coverslip) and before moving it to the wells (to avoid the cell to be dragged by the microscope stage). Table 7 summarizes the troubleshooting associated with this technique, its underlying causes and possible solutions to overcome frequent problems.
The major limitation of this method is that it cannot answer all the questions related to the myofilament contractility, such as how fast the myofilaments activate/deactivate. In the in vivo setting, membrane depolarization, intracellular Ca2+ increase and its diffusion to myofilaments need to occur for the myocytes to contract, whereas in skinned cardiomyocytes Ca2+ diffusion to myofilaments occurs immediately when the cell is submerged in the Ca2+ solution. This faster rate of Ca2+ diffusion will bias myofilaments activation/deactivation analysis23.
These experiments are influenced by different factors, including the temperature, solution pH, mechanical perturbation (slack-re-stretch vs. slack) and cell attachment procedures (pin tie vs. glue), all of these variables accounting for literature discrepancies in terms of ktr and the sarcomere length-dependent increase in force4,12.
Future progress of the technique includes performing functional studies in intact rather than permeabilized cardiomyocytes. This technique has the disadvantage of relying on cardiomyocytes freshly isolated (not previously frozen). Another important issue not directly related to this methodology but that may significantly impact it is related to the maximal period of sample frozen storage. Specifically, it is mandatory to establish the degree of myofilament degradation throughout storage time (i.e., for how long frozen samples can be stored in order to assure good quality functional data derived from the extracted cardiomyocytes).

Traditionally, assessment of myocardial mechanical properties has been attempted mostly in multicellular preparations, such as papillary muscles and trabeculae1,2. Multicellular cardiac muscles strips include a heterogeneous population of cells, including contractile cardiomyocytes with an unknown pattern of orientation and force generation, electrical activity and stress/strain distributions as well as a surrounding connective tissue matrix3,4. A preparation without collagen and containing a single cardiomyocyte would allow measurement of sarcomere length and cross-bridge contractile properties in a very precise and controlled manner5,6. Therefore, over the last four decades, several methodologies were developed allowing investigating the mechanical, contractile, and relaxation properties of a single cardiomyocyte6,7. The contractile function of these cells is strongly dependent on sarcomere length and cross-bridge cycling kinetics3. Thus, it is desirable to investigate muscle function directly in single isolated cardiac cells, considering that it allows assessing sarcomere length and performance as well as cross-bridge function and contractile properties. However, isolating and attaching functional cardiomyocytes with a reasonable optical sarcomere resolution while recording force measurement at the &#181;N level is still challenging and evolving3,6. Other challenges are the logistics that need to be installed to isolate cardiomyocytes from freshly collected biopsies. The unpredictability of human biopsies collection, for instance, may jeopardize the feasibility of the experiments.
Moreover, ethical concerns regarding the Replacement, Reduction and Refinement of animal experimentation for scientific procedures (principles of the 3Rs) have promoted study changes at the cellular and tissue level, preferably in human biopsies, or in smaller animal samples. Indeed a progressive refinement of methodologies to assess cardiac function in vitro on a smaller level of complexity allows proper integration of the results to the whole body and translate them to the clinical scenario7. Altogether, using samples stored at -80 &#176;C to extract cardiomyocytes may be an appealing alternative.
The myocardial tissue is cut into small pieces and homogenized with a mortar and a pestle. The result of this homogenization is a suspension of skinned bundled and isolated cells with varying degrees of sarcolemmal damage, wherein the myoplasm is exposed to the bathing medium and all the cellular components are washed out. Structures such as the myofibrils that are further away from the sarcolemma are preserved. Thus, sarcomere shortening and functional properties associated with the myofibrillar apparatus are kept intact and can be recorded8,9.
The cardiomyocyte force measurement system consists of an electromagnetic motor, used to adjust cardiomyocyte length, and a force transducer, that measures isometric cardiomyocyte contraction. A permeabilized, or skinned, cardiomyocyte is placed in an experimental chamber containing a relaxing solution ([Ca2+] &#60; 10 nM) and silicon-glued to 2 thin needles: one attached to the motor and the other to the force transducer. An optical system is used to determine cardiomyocyte morphology and sarcomere length. The experimental protocol often consists of a series of force recordings upon buffer solutions containing different Ca2+ concentrations, the determination of actin-myosin cross-bridge kinetics and the measurement of the passive tension of the mounted cardiomyocytes at pre-defined sarcomere lengths (Figure 1). Isolation of permeabilized cardiomyocytes from myocardial samples frozen in liquid nitrogen (and subsequently stored at -80 &#176;C) is a technique that utilizes cellular mechanics and protein biochemistry for measuring maximal Ca2+-activated (active) force per cross-sectional area (Tactive, kN&#8729;m-2), Ca2+-independent (passive) tension (Tpassive, kN&#8729;m-2), myofilaments Ca2+-sensitivity (pCa50), myofilaments cooperativity (nHill), the rate of force redevelopment (ktr) as well as sarcomere length dependencies of Tactive, Tpassive, pCa50, nHill and ktr.
The goal of this protocol is to illustrate and summarize the potential of the cardiomyocyte force measurement system as a reliable procedure to assess the functional mechanical properties of single skinned cardiomyocytes isolated from frozen samples from different species.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Acetone</td>
			<td>Sigma</td>
			<td>34580</td>
			<td></td>
		</tr>
		<tr>
			<td>Adenosine 5&#8217;-triphosphate disodium salt hydrate (Na2ATP)</td>
			<td>Sigma</td>
			<td>A2383</td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium carbonate (CaCO3)</td>
			<td>Merck</td>
			<td>1.02067.0500</td>
			<td></td>
		</tr>
		<tr>
			<td>Imidazole</td>
			<td>VWR</td>
			<td>24720.157</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium chloride hexahydrate (MgCl2.6H2O)</td>
			<td>Merck</td>
			<td>1.05833.0250</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium chloride solution (MgCl2 1M)</td>
			<td>Sigma</td>
			<td>M1028</td>
			<td></td>
		</tr>
		<tr>
			<td>N,N-Bis(2-hydroxyethyl)taurine (BES)</td>
			<td>Sigma</td>
			<td>B9879</td>
			<td></td>
		</tr>
		<tr>
			<td>Phosphocreatine dissodium salt hydrate (Na2PCr)</td>
			<td>Sigma</td>
			<td>P7936</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium chloride (KCl)</td>
			<td>Merck</td>
			<td>1.04936.1000</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium hydroxide (KOH)</td>
			<td>Merck</td>
			<td>8.14353.1000</td>
			<td></td>
		</tr>
		<tr>
			<td>Propionic acid (C3H6O2)</td>
			<td>Merck</td>
			<td>8.00605.0500</td>
			<td></td>
		</tr>
		<tr>
			<td>Silicone Squeeze Tube</td>
			<td>Marineland</td>
			<td>31003</td>
			<td></td>
		</tr>
		<tr>
			<td>Tritiplex (EGTA)</td>
			<td>Merck</td>
			<td>1.08435.0025</td>
			<td></td>
		</tr>
		<tr>
			<td>Triton&#174; X-100 10%</td>
			<td>Merck</td>
			<td>648463</td>
			<td></td>
		</tr>
		<tr>
			<td>Tissue homogeneizer (GKH GT Motor Control)</td>
			<td>Terre Haute Glas-col</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Length Controller ( Model 315C-I)</td>
			<td>Aurora Scientific</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Force Transducer (Model 403 A)</td>
			<td>Aurora Scientific</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Software ASI 600A</td>
			<td>Aurora Scientific</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Sotware VSL (Model 900B)</td>
			<td>Aurora Scientific</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Inverted Microscope (IX51)</td>
			<td>Olympus</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

in vitro assessment of cardiac function using skinned cardiomyocytes

In this article, we describe the steps required to isolate a single permeabilized (&#34;skinned&#34;) cardiomyocyte and attach it to a force-measuring apparatus and a motor to perform functional studies. These studies will allow measurement of cardiomyocyte stiffness (passive force) and its activation with different calcium (Ca2+)-containing solutions to determine, amongst others: maximum force development, myofilament Ca2+-sensitivity (pCa50), cooperativity (nHill) and the rate of force redevelopment (ktr). This method also enables determination of the effects of drugs acting directly on myofilaments and of the expression of exogenous recombinant proteins on both active and passive properties of cardiomyocytes. Clinically, skinned cardiomyocyte studies highlight the pathophysiology of many myocardial diseases and allow in vitro assessment of the impact of therapeutic interventions targeting the myofilaments. Altogether, this technique enables the clarification of cardiac pathophysiology by investigating correlations between in vitro and in vivo parameters in animal models and human tissue obtained during open heart or transplant surgery.

Watch this Scientific Journal Video about In Vitro Assessment of Cardiac Function Using Skinned Cardiomyocytes at JoVE.com

In Vitro Assessment of Cardiac Function Using Skinned Cardiomyocytes

This protocol aims to describe step-by-step the technique of extraction and assessment of cardiac function using skinned cardiomyocytes. This methodology allows measurement and acutemodulation of myofilament function using small frozen biopsies that can be collected from different cardiac locations, from mice to men.

In this article, we describe the steps required to isolate a single permeabilized (&#34;skinned&#34;) cardiomyocyte and attach it to a force-measuring ...

This technique enables clarification of the pathophysiology of cardiac diseases through investigation of the correlation between in vitro and in vivo parameters in animal models and humans. The main advantage of this technique is that it allows the study of deep myofilament function using very small biopsies or samples that were stored frozen. This technique allows assessing in vitro the impact of therapeutic interventions on myofilaments.Before attempting an experiment, it is advisable to practice the cardiomyocyte extraction a few times to learn how to select, glue, and activate the cardiomyocytes with good size, striation, and shape. Before beginning the procedure, adjust the temperature of the testing apparatus in chamber to 15 degrees Celsius and turn on the force transducer and the motor. Thaw three to five micrograms of myocardial sample in a Petri dish containing 2.5 milliliters of relax ISO solution and use a scalpel to precisely cut the tissue into small pieces without causing unnecessary cell damage.When all of the tissue has been cut, use a cut pipette tip to transfer the entire volume of solution and tissue fragments into a Potter-Elvehjem glass and use a grinder to mechanically disrupt the tissue at 30 to 40 rotations per minute. When a good cell suspension has been obtained, add 250 microliters of Triton to a 15 milliliter tube containing 2.25 milliliters of relax ISO and add the cell suspension to the resulting solution. Gently invert the tube three times to mix and let the tube sit at room temperature for one minute, followed by a four-minute incubation on ice.At the end of the ice incubation, bring the final volume of the tube up to 15 milliliters with additional relax ISO and mix the tubes three times by inversion as demonstrated, then centrifuge the cells, then carefully remove almost all liquid except the last three milliliters of the supernatant. After the last wash out, remove the supernatant up to a volume of five to 10 milliliters of cell suspension. For skinned cardiomyocyte selection, add a drop of cell suspension onto a coverslip placed on top of a glass slide in an inverted microscope slide holder and use the 20X objective to select a single rod-shaped cardiomyocyte with a good striation pattern and size.Use two hands to rotate the coverslip until the selected cardiomyocyte is positioned horizontally with its ends aligned with the needle of the force transducer and the motor and use a pipette tip to place a thin line of glue along the side of the coverslip. To glue the cell into place, immerse the needle tips of the force transducer and the motor into the glue line to create a glue halo around both tips and quickly move the needle tip of the force transducer down so that it glues to one edge of the cardiomyocyte. Repeat this procedure with the tip of the motor and the other extremity of the cell.After five to eight minutes, move both micro manipulators simultaneously to lift the needles about 15 micrometers to avoid gluing the cell to the coverslip. To measure the active and passive forces and the calcium sensitivity, fill the first experimental well with 55 to 100 microliters of relaxing solution and fill the second experimental well with 55 to 100 microliters of activating solution. Using the camera software, set the region of interest to an area of the cardiomyocyte with a clear pattern of striation and set the sarcomere length to 2.2 micrometers.Measure the distance between the two extremes of the cardiomyocyte and record the value as the myocyte length within the software. Next, move the needles slightly higher and gently move the microscope stage so that the cell moves from the coverslip to the well containing the relaxing solution on the back of the stage. Select the protocol that contains two cell shortening to occur when the cell is sequentially emerged in the calcium and relaxing solutions.To elicit an isometric contraction, move the microscope stage so that the cardiomyocyte moves from the relaxing to the activating solution. Upon reaching the force plateau, begin recording the force data. After 10 seconds, switch the cell to the relaxing solution and record the data until the test stops.To determine the calcium sensitivity, replace the activating solution with 55 to 100 microliters of each calcium solution and record the data as just demonstrated. At the end of the measurement, stretch the tips of the force transducer and motor to remove the needles from the cell and use a cotton swab soaked in acetone to carefully remove the glue halo from the needle tips. Although a certain degree of deterioration and force decrease is expected after prolonged experiments, the values of active tension within the functional permeabilized cardiomyocytes should be relatively stable.Here, representative force traces of three out of eight force recordings needed to carry out a protocol of myofilament calcium sensitivity are shown. By transferring the cell to a well containing the activating solution, the cardiomyocytes starts to develop force until it reaches a plateau. After a quick slack test, the baseline values of zero force can be obtained.The slope of the last part of this curve can be used to determine the value of the rate of force redevelopment, which is a measure of the apparent rate of crossbridge attachment and detachment. After transferring the cell back to a well containing the relaxing solution, the cell relaxes and its force drops. In addition to myofilament calcium sensitivity and the length-dependent activation measurements, the sarcomere length dependencies of passive tension and the sarcomere length dependencies of the calcium activated force per cross-sectional area and the calcium independent tension can be calculated.Finally, it is important to note that the way that the cardiomyocytes are isolated significantly impacts the results. Using skinned cardiomyocytes to assess cardiac function in vitro is an important technique to clarify the changes occurring at the cell level as well as to study myofilament mechanisms in different physiological and pathological conditions. This method has the advantage of requiring a minimal amount of myocardial sample and allowing the use of cardiomyocytes from a wide range of species, cardiac locations, and pathologies.

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