This work was supported in part by the US National Institutes of Health (R01 HL 111600) and the American Heart Association (12SDG9010015).

All procedures involving animals were approved by the Animal Care and Use Committee of the University of California, Davis, and adhered to the Guide for the Care and Use of Laboratory Animals published by the National Institutes of Health.
1. Preparation
<ol>
	<li>Prepare two concentrated (25X) stocks of modified Tyrode&#8217;s solution in advance and store at 4 &#176;C: (1) Stock I (in mM: NaCl 3,205, CaCl2 32.5, KCl 117.5, NaH2PO4 29.75, MgCl2 26.25) and (2) Stock II (in mM: NaHCO3 500).
	<ol>
		<li>Freshly prepare 2 L Tyrode&#39;s solution of the following composition (in mM): NaCl 128.2, CaCl2 1.3, KCl 4.7, MgCl2 1.05, NaH2PO4 1.19, NaHCO3 20 and glucose 11.1 by combining 1840 ml&#160;of deionized (DI) water, 80 ml&#160;Stock I, 80 ml&#160;of Stock II, and 4 g of glucose. Do not directly mix Stock I and Stock II; add them to 1,840 ml&#160;DI to avoid precipitation.</li>
	</ol>
	</li>
	<li>Prepare stock solution of the excitation-contraction uncoupler blebbistatin at 10 mg/ml in anhydrous di-methylsulfoxide (DMSO) in advance and store the stock solution at 4 &#176;C.</li>
	<li>Prepare stock solutions of fluorescent dyes: (1) Voltage-sensitive dye RH237 (1 mg/ml&#160;in DMSO) and (2) Low-affinity calcium indicator Fluo-5N AM (2 mg/ml in DMSO) according to manufacturer&#8217;s recommendations. Use the preparations immediately to avoid decomposition with subsequent loss of loading capacity.</li>
	<li>Prime the recirculating Langendorff perfusion system with oxygenated (95% O2, 5% CO2) Tyrode&#8217;s solution. Adjust the flow of O2/CO2 to keep the pH of the Tyrode&#8217;s solution at 7.4 &#177; 0.05. Use an in-line nylon woven net filter (pore size: 11 &#181;m) to continuously filter the perfusate.
	<ol>
		<li>Prime the perfusion system at RT, as subsequent loading of Fluo-5N AM is performed at RT. For Fluo-5N AM loading, use a small volume of recirculating perfusate (~ 150 ml). After dye loading, use a larger volume of perfusate (1 - 2 L, see Figure 1C).</li>
	</ol>
	</li>
	<li>Turn on data acquisition system and prepare for continuous monitoring of ECG and perfusion pressure. Prepare pressure-monitoring system: Connect a pressure transducer to the perfusion line and monitor the perfusion pressure with a transbridge amplifier. Adjust the baseline perfusion pressure to 0 mmHg when the heart is not attached to the perfusion system.</li>
	<li>Align the optical mapping cameras to ensure spatial alignment between the Vm and SR Ca2+ signals.
	<ol>
		<li>First, focus the cameras by placing a ruler or an object with text into the perfusion dish. Turn the mapping cameras into a live view mode and adjust the focus until the text is crisp and clear. Acquire a single frame image from each camera.</li>
		<li>Overlay these images and adjust the transparency of the top image so that both images are visible. The text in the images should overlap exactly. If not perfectly aligned, adjust the angle of the dichroic mirror or the position of each camera until the two images are aligned.</li>
	</ol>
	</li>
</ol>
2. Harvesting, Perfusion, and Dye-loading of Rabbit Heart
<ol>
	<li>Secure the rabbit in an approved rabbit restrainer. Deeply anesthetize via an intravenous injection (marginal ear vein) of sodium pentobarbital (50 mg/kg) and heparin (1,000 IU).
	<ol>
		<li>When the rabbit displays lack of nociceptive reflexes, make a midline skin incision to expose the sternum and ribs. Cut through the sternum with blunt-tip surgical scissors from the xyphoid to the manubrium. Take care not to damage the heart while opening the sternum. Spread the ribs to expose the heart.</li>
		<li>Quickly excise the entire heart-lung block by rapidly cutting all vessels and connective tissue. Immediately place the heart-lung block into ice-cold Tyrode&#8217;s solution.</li>
	</ol>
	</li>
	<li>Locate the aorta for retrograde perfusion. Cut the ascending aorta just proximal to the three branches of the aortic arch. Cannulate the aorta to an 8 G cannula connected to the perfusion system. Use a piece of USP 0 silk suture to secure the aorta onto the cannula.</li>
	<li>Carefully dissect the trachea, lungs, and epicardial fat from the heart. With a sharp forceps and dissection scissors, locate the mitral valve and carefully damage or remove one leaflet to prevent solution congestion in the left ventricle.</li>
	<li>Submerge the heart in the glass-jacketed perfusion chamber horizontally with the anterior surface of the heart facing up for imaging. Secure the cannula to a piece of Sylgard with U-shaped pins onto the silicon bottom of the dish to prevent movement of the heart during mapping (Figure 1D). If desired, insert a small insect pin at the apex of the heart for stabilization.</li>
	<li>Position electrocardiogram (ECG) electrodes in the bath on the right and left side of the heart. Fully submerge the electrodes in the bath and ensure they are not in contact with the heart surface to provide a volume-conducted ECG analogous to a Lead I configuration. Verify a normal Lead I ECG morphology. Adjust the flow (25 - 35 ml/min) to ensure that the aortic pressure is maintained at 60 - 70 mmHg.</li>
	<li>Turn off the room lights and add 0.3 - 0.6 ml&#160;of blebbistatin stock solution (step 1.2) to the perfusate (final concentration 10 - 20 &#181;M).
	<ol>
		<li>Turn off room lights to avoid photoinactivation of blebbistatin. If necessary, a small spotlight or headlamp can be used to provide task lighting. 
		Note: Blebbistatin is a myosin ATPase inhibitor and an excitation-contraction uncoupler. Because Fluo-5N loading requires extended (60 min) RT (hypothermia) conditions (see steps 2.8 &#8211; 2.10), cardiac energy production may be compromised. Therefore, addition of blebbistatin into the perfusate prior to dye loading may reduce the energy demand16 and eliminate motion artifacts during subsequent optical recordings17.</li>
	</ol>
	</li>
	<li>When contraction of the heart has ceased (10 - 15 min, verified on the aortic pressure recording), switch to the small-volume recirculating perfusion system (see step 1.4.1).</li>
	<li>Prepare and load Fluo-5N AM loading solution: Add 0.25 ml&#160;of Fluo-5N AM stock solution (step 1.3) to 0.25 ml&#160;warm 20% Pluronic F127. Add 0.5 ml&#160;warm Tyrode&#8217;s solution to the mixture, mix well, and add to the small-volume recirculating perfusion system.</li>
	<li>Continuously monitor perfusion pressure and ECG and adjust the flow if necessary. Dye loading takes approximately 1 hr at RT. 45 min after loading has begun, turn on the circulating water bath to begin warming the perfusion system and perfusate to 37 &#176;C.</li>
	<li>After 60 min of Fluo-5N AM loading, switch the perfusion to the larger volume recirculating perfusion system (see step 1.4).</li>
	<li>Dilute 50 &#181;l&#160;of voltage sensitive dye RH237 stock solution (step 1.3) in 1 ml&#160;of warm Tyrode&#39;s solution and add slowly (over ~ 5 min) into an injection port proximal to the cannula.</li>
</ol>
3. Optical Mapping
<ol>
	<li>During the final moments of dye loading, position the heart and focus the optical mapping cameras to ensure the appropriate field of view for the experiment.</li>
	<li>If desired, place a bipolar pacing electrode on the epicardial surface of the heart for pacing.</li>
	<li>Place a plastic or glass cover slip on the surface of the perfusion chamber to reduce motion artifact on the liquid surface that may be present due to recirculation of the perfusate. 
	Note: As an alternative to a cover slip, use a clean plastic 50 mm Petri dish cover.</li>
	<li>Focus the light guides to uniformly illuminate the surface of the heart with excitation light. Use blue light emitting diode (LED) light sources and further filter the light with a 475 - 495 nm bandpass filter (Figure 1A).</li>
	<li>Collect emitted fluorescence with a macroscope and two complementary metal-oxide-semiconductor (CMOS) optical mapping cameras. Split the emitted light with a dichroic mirror at 545 nm.
	<ol>
		<li>Long-pass filter the longer wavelength moiety, containing the Vm signal, at 700 nm. Band-pass filter the shorter wavelength moiety, containing the SR Ca2+ signal, from 502 - 534 nm (Figure 1A). Set the frequency of optical data acquisition to 0.5 - 1 kHz.</li>
	</ol>
	</li>
	<li>For each optical recording, first initiate the desired pacing protocol, turn on the excitation light, and collect 1 - 4 sec of data. If desired, synchronize the pacing protocols and data acquisition. 
	Note: It is not possible to re-load the Fluo-5N AM, therefore the signal-to-noise ratio (SNR) will decrease throughout the experiment due to dye leakage from the SR. Limit experimental protocol to 1 - 2 hr&#160;to ensure high SNR values.</li>
	<li>Following the experiment, wash the perfusion system in the sequence of DI water, 70% reagent alcohol, and again with DI water.</li>
	<li>Filter each dataset with a spatial Gaussian filter (radius 3 pixels) using commercially available software packages according to the manufacturer&#8217;s protocol.</li>
	<li>If necessary, spatially align the Vm and SR Ca2+ data sets. As in Step 1.6, overlay images from each camera and verify that the anatomical structures of the heart (i.e., coronary vasculature, edges of the atria or ventricles) or other items in the field of view (i.e., cannula, pacing electrode) exactly overlap. If not, spatial alignment of the datasets is necessary.</li>
	<li>Shift the top transparent image in the x- and y-direction until exact overlap is achieved, making note of the number of pixels the image must be shifted in each direction. The entire dataset corresponding to the top image (either Vm or SR Ca2+) must then be shifted by the determined number of pixels in each direction to assure precise alignment between the Vm and SR Ca2+ datasets.</li>
</ol>

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The authors declare that they have no competing financial interests.

The keys to successful Fluo-5N dye loading are the small-volume recirculating perfusion setup, which allows for a high Fluo-5N concentration without the need for large amounts of dye, the length of loading time (1 hr), and performing the loading at RT. If loading is performed at physiological temperatures, cellular enzymatic activity quickly cleaves the &#8211;AM tag when the dye crosses the cell membrane, trapping the dye molecules in the cytosol and not allowing them to cross the SR membrane. At RT, however, enzymatic activity is slowed and a sufficient amount of dye can cross both the cell membrane and SR membrane before the &#8211;AM tag is cleaved, trapping the dye in the SR.
Even with optimal loading conditions, however, signal amplitudes and fractional fluorescence changes (&#916;F/F) with this approach will be lower than with traditional optical mapping of Vm and intracellular Ca2+ with high-affinity dyes. For example, with the optical setup used here that consists of LED excitation light, macroscopic objectives, large filters and dichroic mirror (5 cm x 5 cm), and CMOS cameras, fractional fluorescence changes (&#916;F/F) are on the order of 1 - 2% for the Fluo-5N signal and 2 - 3% for the RH237 signal15. The fractional fluorescence will, of course, depend on the specific optical configuration, light source, and cameras of a particular setup, but even under optimal conditions the Fluo-5N signal is significantly lower than what is typically observed for recordings of intracellular Ca2+ with high-affinity dyes. For example, on the same setup, &#916;F/F is on the order of 20 - 30% for Rhod-2 when green (545 nm) excitation light is used18. The green excitation light is also closer to the excitation peak of RH237, thus the &#916;F/F of RH237 is also larger in this case, around 4 - 5%. Thus, for those labs already performing dual optical mapping of Vm and intracellular Ca2+, care should be taken to optimize all components of the optical setup to ensure maximal &#916;F/F for the Fluo-5N signal.
With this approach, investigators can precisely probe SR Ca2+ dynamics and its impact on Vm in ways that are not possible with existing approaches. For example, quantitative measurements of SR Ca2+ leak (as in Figure 3A) and the precise time constant of SR Ca2+ recovery (tau &#8211;a measure of SR Ca2+-ATPase [SERCA] activity) can be performed. Using existing approaches for optical mapping of intracellular Ca2+ with high-affinity dyes, these measurements may be &#8216;contaminated&#8217; with transmembrane Ca2+ flux (i.e., contributions from L-type Ca2+ channels and the Na+-Ca2+ exchanger). Furthermore, high-affinity Ca2+ indicators have inherently slower kinetics than low-affinity indicators5, thus Fluo-5N signals are expected to report on precise SR Ca2+ movements with high fidelity and essentially no time delay between actual Ca2+ changes and corresponding changes in fluorescence.
The ability to investigate the detailed mechanisms by which SR Ca2+ release and reuptake contribute to arrhythmogenic phenomena is another advantage of this approach. For example, using this methodology, we recently reported how refractoriness of SR Ca2+ release contributes to the onset of Ca2+ alternans and that during ventricular fibrillation, SR Ca2+ release remains nearly continuously refractory15. This was the first report of SR Ca2+ dynamics during fibrillation and highlights the key advantage of SR Ca2+ imaging in the intact heart: spatially distinct and complex phenomena such as spatially discordant alternans and fibrillation can be studied. Unfortunately, this type of information cannot be gleaned from isolated cells, where fibrillation is not possible, or from methods that only record from a single location on the intact heart12.
Furthermore, simultaneous optical mapping of Vm and free SR Ca2+ in the intact heart may provide a novel tool for assessing abnormal Ca2+ handling in pathological conditions, including heart failure. The abnormalities in SR Ca2+ regulation, including activation and termination of SR Ca2+ release, diastolic SR Ca2+ leak, and SR Ca2+ uptake - all major steps in Ca2+ cycling and all potentially altered in heart failure - can be directly studied. In addition, this methodology could also be a useful tool for assessing the effects of novel therapeutic interventions, such as RyR stabilization19 and SERCA modulation20.
This approach is not without limitations, however, including the low &#916;F/F as discussed above and the inability to precisely calibrate fluorescence values to exact [Ca2+]SR concentrations. Unfortunately, variability in dye loading, non-uniform excitation and emission light due to the curved surface of the heart, and photo-bleaching and dye leak from the SR throughout the course of the experiment make it extremely difficult to calibrate Fluo-5N signals to exact [Ca2+]SR in the intact heart. Previous in vitro studies in physiological buffer solutions and permeabilized isolated myocytes have reported such calibrations and have indicated that changes in Fluo-5N fluorescence are proportional to changes in [Ca2+]SR and the fluorescence is not expected to saturate at physiological [Ca2+]SR levels11. Thus, investigators should choose the method of Ca2+ mapping (low- or high-affinity dye) based on their specific experimental objectives and physiological investigations.

Dual optical mapping of intracellular Ca2+ and transmembrane potential (Vm) in the intact Langendorff-perfused heart has become a mainstay of investigations in cardiac electrophysiology, including mechanisms of arrhythmia and excitation-contraction coupling1-4. This approach has provided unprecedented knowledge into normal and abnormal electrophysiology and, importantly, into the bidirectional relationship between Vm and intracellular Ca2+. However, optical mapping of intracellular Ca2+ with high-affinity fluorescent indicators (such as Rhod-2 and Fluo-4) only reports on bulk changes in intracellular Ca2+ and is unable to distinguish whether these changes are due to transmembrane Ca2+ flux, release and reuptake into intracellular stores, or in most instances, some combination of both. Furthermore, high-affinity Ca2+ indicators have slow on-off kinetics and may not accurately report rapid changes in Ca2+ concentration5.
Each action potential triggers a rise in intracellular Ca2+, known as the intracellular Ca2+ transient (CaT). In the mammalian heart, approximately 70 &#8211; 90% of the total CaT is due to release of Ca2+ from the sarcoplasmic reticulum (SR) via opening of ryanodine receptors (RyRs)6. Within the SR, approximately half of the total Ca2+ is bound to calsequestrin (CSQ) and other intra-SR buffers7, which play an important role in SR Ca2+ homeostasis8,9. The amount of free SR Ca2+ dictates the driving force for SR Ca2+ release as well as gating of RyR, and therefore has a significant impact on the intracellular CaT. Furthermore, alterations in SR Ca2+ release or reuptake can, in turn, impact Vm via the electrogenic Na+-Ca2+ exchange, which may have arrhythmogenic consequences. Therefore, in addition to the CaT, monitoring of free SR Ca2+ can provide important insights into contractile and electrophysiological dysfunction.
Over the past several years, investigators have made significant advances in the monitoring of SR Ca2+ in isolated cardiac myocytes and from a single location on the intact heart. One such method requires rapid pulses of caffeine to open RyRs and the SR Ca2+ content is then inferred or calculated from the immediate rise in intracellular Ca2+10. Another intriguing approach uses low-affinity Ca2+ indicators, such as Fluo-5N11 or Mag-Fluo412, which bind to free SR Ca2+. These indicators have dissociation constants (Kd) in the range of 10 &#8211; 400 &#956;M and therefore exhibit minimal fluorescence in the cytosol compared to the SR lumen, where the Ca2+ concentration ([Ca2+]SR) is in the millimolar range. Using low-affinity Ca2+ indicators, several aspects of SR Ca2+ cycling have been investigated at the level of the isolated myocyte, including fractional SR Ca2+ release and the mechanisms of Ca2+ alternans13,14. However, in order to fully understand the heterogeneous nature of SR Ca2+ cycling in the intact heart and the role of SR Ca2+ in spatially distinct arrhythmic phenomena, methods for imaging SR Ca2+ across the epicardial surface of the intact heart are required15.
This article describes methodology for dual optical mapping of free SR Ca2+ and Vm in the intact Langendorff-perfused rabbit heart with the low-affinity Ca2+ indicator Fluo-5N. In addition to revealing SR Ca2+ characteristics spatially across the epicardial surface of the heart, this approach has the advantage of simultaneous monitoring of Vm, allowing for investigations into the bidirectional relationship between Vm and SR Ca2+.

<table border="0" cellpadding="0" cellspacing="0" width="956">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">NaCl</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">S271-1</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">CaCl2 (2H2O)</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">C79-500</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">KCl</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">S217-500</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">MgCl2 (6H2O)</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">M33-500</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">NaH2PO4 (H2O)</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">S369-500</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">NaHCO3</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">S233-3</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">D-Glucose</td>
			<td style="width:167px;">Fisher Scientific</td>
			<td style="width:151px;">D16-1</td>
			<td style="width:423px;">Component of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">95% O2 5% CO2</td>
			<td style="width:167px;">AirGas</td>
			<td style="width:151px;">carbogen</td>
			<td style="width:423px;">For oxygenation and pH of Tyrode&#39;s solution</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Blebbistatin</td>
			<td style="width:167px;">Tocris Bioscience</td>
			<td style="width:151px;">1760</td>
			<td style="width:423px;">Excitation-contraction uncoupler</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">RH237</td>
			<td style="width:167px;">Biotium</td>
			<td style="width:151px;">61018</td>
			<td style="width:423px;">Voltage-sensitive dye</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Fluo-5N AM</td>
			<td style="width:167px;">Invitrogen</td>
			<td style="width:151px;">F-26915</td>
			<td style="width:423px;">Low-affinity Ca2+ indicator; Alternative: Invitrogen F-14204; Loading must be performed at room temperature</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Pluronic F127</td>
			<td style="width:167px;">Biotium</td>
			<td style="width:151px;">59004</td>
			<td style="width:423px;">For Ca2+ indicator loading; Warm until the solutiion is clear before use</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Dimethyl sulphoxide (DMSO)</td>
			<td style="width:167px;">Sigma-Aldrich</td>
			<td style="width:151px;">D2650</td>
			<td style="width:423px;">For dissolving blebbistatin and dyes</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Filter</td>
			<td style="width:167px;">EMD Millipore</td>
			<td style="width:151px;">NY1104700</td>
			<td style="width:423px;">11um in-line filter</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Pressure Transducer</td>
			<td style="width:167px;">WPI</td>
			<td style="width:151px;">BLPR2</td>
			<td style="width:423px;">For measuring perfusion pressure</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Transbridge Transducer Amplifier</td>
			<td style="width:167px;">WPI</td>
			<td style="width:151px;">SYS-TBM4M</td>
			<td style="width:423px;">For transducing/amplifing pressure signal; PowerLab may also be used with appropriate BioAmp</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">PowerLab 26T</td>
			<td style="width:167px;">ADInstruments</td>
			<td style="width:151px;"></td>
			<td style="width:423px;">For continuous recording of pressure and ECG signals</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">THT Macroscope</td>
			<td style="width:167px;">SciMedia</td>
			<td style="width:151px;"></td>
			<td style="width:423px;">Macroscopic optical setup. Details: 0.63x objective (NA=0.31), 2x condensing objective, resultant field of view = 3.1x3.1 cm, depth of focus = ~1.5mm</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">MiCam Ultima-L CMOS</td>
			<td style="width:167px;">SciMedia</td>
			<td style="width:151px;"></td>
			<td style="width:423px;">Optical mapping cameras</td>
		</tr>
		<tr height="57">
			<td height="57" style="height:57px;width:216px;">Precision LED Spot Light</td>
			<td style="width:167px;">Mightex</td>
			<td style="width:151px;">PLS-0470-030-15-S</td>
			<td style="width:423px;">LED light source</td>
		</tr>
	</tbody>
</table>

optical mapping of intra-sarcoplasmic reticulum ca2+ and transmembrane potential in the langendorff-perfused rabbit heart

Sarcoplasmic reticulum (SR) Ca2+ handling plays a key role in normal excitation-contraction coupling and aberrant SR Ca2+ handling is known to play a significant role in certain types of arrhythmia. Because arrhythmias are spatially distinct, emergent phenomena, they must be investigated at the tissue level. However, methods for directly probing SR Ca2+ in the intact heart remain limited. This article describes the protocol for dual optical mapping of transmembrane potential (Vm) and free intra-SR [Ca2+] ([Ca2+]SR) in the Langendorff-perfused rabbit heart. This approach takes advantage of the low-affinity Ca2+ indicator Fluo-5N, which has minimal fluorescence in the cytosol where intracellular [Ca2+] ([Ca2+]i) is relatively low but exhibits significant fluorescence in the SR lumen where [Ca2+]SR is in the millimolar range. In addition to revealing SR Ca2+ characteristics spatially across the epicardial surface of the heart, this approach has the distinct advantage of simultaneous monitoring of Vm, allowing for investigations into the bidirectional relationship between Vm and SR Ca2+ and the role of SR Ca2+ in arrhythmogenic phenomena.

Figure 1A shows a schematic diagram of the optical configuration for dual Vm and SR Ca2+ mapping. With this setup, there is complete spectral separation of the Vm and SR Ca2+ signals (Figure 1B). A diagram of the dual-loop perfusion system used for Fluo-5N dye loading is illustrated in Figure 1C. Figure 1D shows the horizontal orientation of the heart in the perfusion dish. Representative Vm and SR Ca2+ optical traces during continuous pacing from different locations on the epicardial surface of the heart are shown in Figure 2A. The optical field of view with this setup is 31 mm x 31 mm and the orientation of the heart within the field of view is shown in Figure 2B. Activation maps are also shown in Figure 2B and are constructed by selecting the activation time of each pixel and plotting this value on an isochronal map (note the typical delay of ~ 8 - 10 msec between the Vm and SR Ca2+ activation maps). Importantly, key action potential (AP) properties such as AP rise time (from 10% to 90% of the amplitude [Trise]), and AP duration at 80% repolarization (APD80), are not statistically different between hearts loaded with RH237 and Fluo-5N compared to those loaded with RH237 and Rhod-2 (Trise: 14.9 msec &#177; 1.9 msec vs. 14.2 msec &#177; 1.2 msec and APD80: 155.3 msec &#177; 5.8 msec vs. 156.9 msec &#177; 5.7 msec for Fluo-5N and Rhod-2, respectively; p &#62;0.05 for both; n = 8/group), indicating minimal effect of Fluo-5N or dye loading on the AP. Figure 2C demonstrates the ability to observe and quantify relative changes in diastolic SR Ca2+ load and systolic SR Ca2+ release amplitudes at various pacing cycle lengths. Only relative changes in these parameters can be quantified, as absolute calibration ([Ca2+]SR) of the signal in the intact heart is not possible.
This approach is particularly useful for characterizing and quantifying pathological SR Ca2+ handling, as is shown in Figure 3. Following application of the &#946;-adrenergic receptor agonist isoproterenol (100 nM), clear diastolic Ca2+ leak from the SR is observed (red arrows, Figure 3A). If the leak is large enough, Ca2+-mediated triggered activity may occur (Figure 3B).
<img alt="Figure 1" src="/files/ftp_upload/53166/53166fig1.jpg" /> 
Figure 1. System Setup for Dual Mapping of Vm and SR Ca2+. (A) Diagram of the optical setup, including necessary filters and dichroic mirror for proper spectral separation. (B) Imaging of hearts loaded with either Fluo-5N only (i) or RH237 only (ii) indicates complete spectral separation of the Vm and SR Ca2+ fluorescent signals. Ex: excitation, Em: emission. (C) Diagram of the dual-loop perfusion system for normal perfusion and Fluo-5N dye loading. (D) Image of a cannulated heart and ECG electrodes in the perfusion dish. Panels A and B reproduced with permission from Wang et al.15. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53166/53166fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/53166/53166fig2.jpg" /> 
Figure 2. Vm and SR Ca2+&#160;Optical Traces during Continuous and Non-continuous Pacing. (A) Representative Vm and SR Ca2+ optical traces during continuous pacing at a cycle length&#160;of 300 msec. Signals are shown from the individual locations indicated in (B). Note the delay of ~ 8 - 10 msec between the Vm and SR Ca2+ activation times as is typical for normal excitation-contraction coupling. (B) Image of a heart within the mapping field of view (white arrowhead indicates pacing electrodes) and activation maps of Vm and SR Ca2+. (C) SR Ca2+ optical traces demonstrating changes in diastolic SR Ca2+ load and the amplitude of SR Ca2+ release following abrupt changes in pacing rate. All traces start with pacing at CL = 300 msec. Pacing rate is then abruptly changed to CL = 500 msec (i), CL = 400 msec (ii), or CL = 250 msec (iii). When longer CLs are initiated (i - ii), a larger SR Ca2+ release first occurs (due to the longer diastolic interval and recovery of RyRs) and the diastolic SR Ca2+ load slightly decreases to a new steady-state (horizontal dashed line). When a shorter CL is initiated, however, a smaller SR Ca2+ release amplitude occurs (due to the shorter diastolic interval and incomplete recovery of RyRs) and diastolic SR Ca2+ load increases to a new steady-state (horizontal dashed line). Alternation of SR Ca2+ release amplitude also occurs at CL = 250 msec. S: small release amplitude, L: large release amplitude. (CL: cycle length) <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53166/53166fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/53166/53166fig3.jpg" /> 
Figure 3. Diastolic SR Ca2+ Leak and Subsequent Triggered Activity.&#160;(A) ECG, Vm, and SR Ca2+ traces during baseline (left) and following treatment with 100 nM isoproterenol (Iso, right). With Iso, significant diastolic SR Ca2+ leak is observed (red arrows). SR Ca2+ signal amplitudes have been normalized before and after Iso. The sino-atrial node was ablated to allow for pacing CL = 300 msec in both cases. (B) If SR Ca2+ leak is large enough, it may trigger focal activity (premature ventricular complexes, PVCs). In this example, PVCs are interrupting the normal sinus rhythm. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53166/53166fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Optical Mapping of Intra-Sarcoplasmic Reticulum Ca2+ and Transmembrane Potential in the Langendorff-perfused Rabbit Heart at JoVE.com

Optical Mapping of Intra-Sarcoplasmic Reticulum Ca2+ and Transmembrane Potential in the Langendorff-perfused Rabbit Heart

This article describes the detailed protocol and equipment necessary for dual optical mapping of transmembrane potential (Vm) and free intra-sarcoplasmic reticulum (SR) Ca2+ in the Langendorff-perfused rabbit heart. This method allows for direct observation and quantification of Vm and SR Ca2+ dynamics in the intact heart.

Optical Mapping of Intra-Sarcoplasmic Reticulum Ca2+ and Transmembrane Potential in the Langendorff-perfused Rabbit Heart

Sarcoplasmic reticulum (SR) Ca2+ handling plays a key role in normal excitation-contraction coupling and aberrant SR Ca2+ handling is known to play a ...

optical-mapping-intra-sarcoplasmic-reticulum-ca2-transmembrane

Nanyang Technological University

Research

JoVE Journal

Biology

9.2K Views.  University of California Davis. This article describes the detailed protocol and equipment necessary for dual optical mapping of transmembrane potential (Vm) and free intra-sarcoplasmic reticulum (SR) Ca2+ in the Langendorff-perfused rabbit heart. This method allows for direct observation and quantification of Vm and SR Ca2+ dynamics in the intact heart.

Video: Optical Mapping of Intra-Sarcoplasmic Reticulum Ca2+ and Transmembrane Potential in the Langendorff-perfused Rabbit Heart

Proper Care and Cleaning of the Microscope

Keeping the microscope optics clean is important for high-quality imaging. Dust, fingerprints, excess immersion oil, or mounting medium on or in a microscope causes reduction in contrast and resolution. DIC is especially sensitive to contamination and scratches on the lens surfaces. This protocol details the procedure for keeping the microscope clean.

Keeping the microscope optics clean is important for high-quality imaging. Dust, fingerprints, excess immersion oil, or mounting medium on or in a microscope causes reduction in contrast and resolution. DIC is especially sensitive to contamination and scratches on the lens surfaces. This protocol details the procedure for keeping the microscope clean.

Keeping the microscope optics clean is important for high-quality imaging. Dust, fingerprints, excess immersion oil, or mounting medium on or in a ...

Optical Mapping of Langendorff-perfused Rat Hearts

Optical mapping of the cardiac surface with voltage-sensitive fluorescent dyes has become an important tool to investigate electrical excitation in experimental models that range in scale from cell cultures to whole-organs[1, 2]. Using state-of-the-art optical imaging systems, generation and propagation of action potentials during normal cardiac rhythm or throughout initiation and maintenance of arrhythmias can be visualized almost instantly[1]. The latest commercially-available systems can provide information at exceedingly high spatiotemporal resolutions and were based on custom-built equipment initially developed to overcome the obstacles imposed by more conventional electrophysiological methods[1]. Advancements in high-resolution and high-speed complementary metal-oxide-semiconductor (CMOS) cameras and intensely-bright, light-emitting diodes (LEDs) as well as voltage-sensitive dyes, optics, and filters have begun to make electrical signal acquisition practical for cardiovascular cell biologists who are more accustomed to working with microscopes. Although the newest generation of CMOS cameras can acquire 10,000 frames per second on a 16,384 pixel array, depending on the type of sample preparation, long-established fluorescence acquisition technologies such as photodiode arrays, laser scanning systems, and cooled charged-coupled device (CCD) cameras still have some distinct advantages with respect to dynamic range, signal-to-noise ratio, and quantum efficiency[1, 3]. In the present study, Lewis rat hearts were perfused ex vivo with a crystalloid perfusate (Krebs-Henseleit solution) at 37&deg;C on a modified Langendorff apparatus. After a 20 minute stabilization period, the hearts were intermittently perfused with 11 mMol/L 2,3-butanedione monoxime to eliminate contraction-associated motion during image acquisition. For optical mapping, we loaded hearts with the fast-response potentiometric probe di-8-ANEPPS[4] (5 &mu;Mol/L) and briefly illuminated the preparation with 475&plusmn;15 nm excitation light. During a typical 2 second period of illumination, &gt;605 nm light emitted from the cardiac preparation was imaged with a high-speed CMOS camera connected to a horizontal macroscope. For this demonstration, hearts were paced at 300 beats per minute with a coaxial electrode connected to an isolated electrical stimulation unit. Simultaneous bipolar electrographic recordings were acquired and analyzed along with the voltage signals using readily-available software. In this manner, action potentials on the surface of Langendorff-perfused rat hearts can be visualized and registered with electrographic signals.

This article describes a high temporal and spatial resolution technique to optically image action potential movement on the surface of Langendorff-perfused rat hearts using a potentiometric dye (di-8-ANEPPS).

Optical mapping of the cardiac surface with voltage-sensitive fluorescent dyes has become an important tool to investigate electrical excitation in ...

Semi-automated Optical Heartbeat Analysis of Small Hearts

We have developed a method for analyzing high speed optical recordings from Drosophila, zebrafish and embryonic mouse hearts (Fink, et. al., 2009). Our Semi-automatic Optical Heartbeat Analysis (SOHA) uses a novel movement detection algorithm that is able to detect cardiac movements associated with individual contractile and relaxation events. The program provides a host of physiologically relevant readouts including systolic and diastolic intervals, heart rate, as well as qualitative and quantitative measures of heartbeat arrhythmicity. The program also calculates heart diameter measurements during both diastole and systole from which fractional shortening and fractional area changes are calculated. Output is provided as a digital file compatible with most spreadsheet programs. Measurements are made for every heartbeat in a record increasing the statistical power of the output. We demonstrate each of the steps where user input is required and show the application of our methodology to the analysis of heart function in all three genetically tractable heart models.

We have developed a Semi-automated Optical Heartbeat Analysis method (SOHA) for analyzing high speed optical recordings from Drosophila, zebrafish and embryonic mouse hearts. We demonstrate the application of our methodology to the analysis of heart function in fruit fly and embryonic mouse hearts.

We have developed a method for analyzing high speed optical recordings from Drosophila, zebrafish and embryonic mouse hearts (Fink, et. al., 2009). Our ...

Preparation of embryos for Electron Microscopy of the Drosophila embryonic heart tube

The morphogenesis of the Drosophila embryonic heart tube has emerged as a valuable model system for studying cell migration, cell-cell adhesion and cell shape changes during embryonic development. One of the challenges faced in studying this structure is that the lumen of the heart tube, as well as the membrane features that are crucial to heart tube formation, are difficult to visualize in whole mount embryos, due to the small size of the heart tube and intra-lumenal space relative to the embryo. The use of transmission electron microscopy allows for higher magnification of these structures and gives the advantage of examining the embryos in cross section, which easily reveals the size and shape of the lumen. In this video, we detail the process for reliable fixation, embedding, and sectioning of late stage Drosophila embryos in order to visualize the heart tube lumen as well as important cellular structures including cell-cell junctions and the basement membrane.

Preparation of embryos for Electron Microscopy of the Drosophila embryonic heart tube

We describe a process for fixation, embedding, sectioning, and imaging of late stage Drosophila embryos for Trasmission Electron Microscopy of the embryonic heart tube. This technique allows for the visualization of the heart tube lumen as well as the basement membrane, which lines the lumen of the heart.

The morphogenesis of the Drosophila  embryonic heart tube has emerged as a valuable model system for studying cell migration, cell-cell adhesion and cell ...

Quantification of &gamma;H2AX Foci in Response to Ionising Radiation

DNA double-strand breaks (DSBs), which are induced by either endogenous metabolic processes or by exogenous sources, are one of the most critical DNA lesions with respect to survival and preservation of genomic integrity. An early response to the induction of DSBs is phosphorylation of the H2A histone variant, H2AX, at the serine-139 residue, in the highly conserved C-terminal SQEY motif, forming &gamma;H2AX1. Following induction of DSBs, H2AX is rapidly phosphorylated by the phosphatidyl-inosito 3-kinase (PIKK) family of proteins, ataxia telangiectasia mutated (ATM), DNA-protein kinase catalytic subunit and ATM and RAD3-related (ATR)2. Typically, only a few base-pairs (bp) are implicated in a DSB, however, there is significant signal amplification, given the importance of chromatin modifications in DNA damage signalling and repair. Phosphorylation of H2AX mediated predominantly by ATM spreads to adjacent areas of chromatin, affecting approximately 0.03% of total cellular H2AX per DSB2,3. This corresponds to phosphorylation of approximately 2000 H2AX molecules spanning ~2 Mbp regions of chromatin surrounding the site of the DSB and results in the formation of discrete &gamma;H2AX foci which can be easily visualized and quantitated by immunofluorescence microscopy2. The loss of &gamma;H2AX at DSB reflects repair, however, there is some controversy as to what defines complete repair of DSBs; it has been proposed that rejoining of both strands of DNA is adequate however, it has also been suggested that re-instatement of the original chromatin state of compaction is necessary4-8. The disappearence of &gamma;H2AX involves at least in part, dephosphorylation by phosphatases, phosphatase 2A and phosphatase 4C5,6. Further, removal of &gamma;H2AX by redistribution involving histone exchange with H2A.Z has been implicated7,8. Importantly, the quantitative analysis of &gamma;H2AX foci has led to a wide range of applications in medical and nuclear research. Here, we demonstrate the most commonly used immunofluorescence method for evaluation of initial DNA damage by detection and quantitation of &gamma;H2AX foci in &gamma;-irradiated adherent human keratinocytes9.

Quantification of &#947;H2AX Foci in Response to Ionising Radiation

Quantification of DNA double-strand streaks using &gamma;H2AX formation as a molecular marker has become an invaluable tool in radiation biology. Here we demonstrate the use of an immunofluorescence assay for quantification of &gamma;H2AX foci after exposure of cells to radiation.

DNA double-strand breaks (DSBs), which are induced by either endogenous metabolic processes or by exogenous sources, are one of the most critical DNA ...

Quantitation of &gamma;H2AX Foci in Tissue Samples

DNA double-strand breaks (DSBs) are particularly lethal and genotoxic lesions, that can arise either by endogenous (physiological or pathological) processes or by exogenous factors, particularly ionizing radiation and radiomimetic compounds. Phosphorylation of the H2A histone variant, H2AX, at the serine-139 residue, in the highly conserved C-terminal SQEY motif, forming &gamma;H2AX, is an early response to DNA double-strand breaks1. This phosphorylation event is mediated by the phosphatidyl-inosito 3-kinase (PI3K) family of proteins, ataxia telangiectasia mutated (ATM), DNA-protein kinase catalytic subunit and ATM and RAD3-related (ATR)2. Overall, DSB induction results in the formation of discrete nuclear &gamma;H2AX foci which can be easily detected and quantitated by immunofluorescence microscopy2. Given the unique specificity and sensitivity of this marker, analysis of &gamma;H2AX foci has led to a wide range of applications in biomedical research, particularly in radiation biology and nuclear medicine. The quantitation of &gamma;H2AX foci has been most widely investigated in cell culture systems in the context of ionizing radiation-induced DSBs. Apart from cellular radiosensitivity, immunofluorescence based assays have also been used to evaluate the efficacy of radiation-modifying compounds. In addition, &gamma;H2AX has been used as a molecular marker to examine the efficacy of various DSB-inducing compounds and is recently being heralded as important marker of ageing and disease, particularly cancer3. Further, immunofluorescence-based methods have been adapted to suit detection and quantitation of &gamma;H2AX foci ex vivo and in vivo4,5. Here, we demonstrate a typical immunofluorescence method for detection and quantitation of &gamma;H2AX foci in mouse tissues.

Quantitation of &#947;H2AX Foci in Tissue Samples

Quantitation of DNA double-strand breaks on the basis of &gamma;H2AX foci has become an invaluable tool, particularly in radiation biology, for the evaluation of tissue radiosensitivity and effects of radiation modifying compounds. Here we demonstrate the use of an immunofluorescence assay for quantitation of &gamma;H2AX foci in tissue samples.

DNA double-strand breaks (DSBs) are particularly lethal and genotoxic lesions, that can arise either by endogenous (physiological or pathological) ...

Evaluation of the Spatial Distribution of &gamma;H2AX following Ionizing Radiation

An early molecular response to DNA double-strand breaks (DSBs) is phosphorylation of the Ser-139 residue within the terminal SQEY motif of the histone H2AX1,2. This phosphorylation of H2AX is mediated by the phosphatidyl-inosito 3-kinase (PI3K) family of proteins, ataxia telangiectasia mutated (ATM), DNA-protein kinase catalytic subunit and ATM and RAD3-related (ATR)3. The phosphorylated form of H2AX, referred to as &gamma;H2AX, spreads to adjacent regions of chromatin from the site of the DSB, forming discrete foci, which are easily visualized by immunofluorecence microscopy3. Analysis and quantitation of &gamma;H2AX foci has been widely used to evaluate DSB formation and repair, particularly in response to ionizing radiation and for evaluating the efficacy of various radiation modifying compounds and cytotoxic compounds4.
Given the exquisite specificity and sensitivity of this de novo marker of DSBs, it has provided new insights into the processes of DNA damage and repair in the context of chromatin. For example, in radiation biology the central paradigm is that the nuclear DNA is the critical target with respect to radiation sensitivity. Indeed, the general consensus in the field has largely been to view chromatin as a homogeneous template for DNA damage and repair. However, with the use of &gamma;H2AX as molecular marker of DSBs, a disparity in &gamma;-irradiation-induced &gamma;H2AX foci formation in euchromatin and heterochromatin has been observed5-7. Recently, we used a panel of antibodies to either mono-, di- or tri- methylated histone H3 at lysine 9 (H3K9me1, H3K9me2, H3K9me3) which are epigenetic imprints of constitutive heterochromatin and transcriptional silencing and lysine 4 (H3K4me1, H3K4me2, H3K4me3), which are tightly correlated actively transcribing euchromatic regions, to investigate the spatial distribution of &gamma;H2AX following ionizing radiation8. In accordance with the prevailing ideas regarding chromatin biology, our findings indicated a close correlation between &gamma;H2AX formation and active transcription9. Here we demonstrate our immunofluorescence method for detection and quantitation of &gamma;H2AX foci in non-adherent cells, with a particular focus on co-localization with other epigenetic markers, image analysis and 3D-modeling.

Evaluation of the Spatial Distribution of &#947;H2AX following Ionizing Radiation

Microscopic analysis of &gamma;H2AX foci, which form following the phosphorylation of H2AX at Ser-139 in response to DNA double-strand breaks, has become an invaluable tool in radiation biology. Here we used an antibody to mono-methylated histone H3 at lysine 4 as an epigenetic marker of actively transcribing euchromatin, to evaluate the spatial distribution of radiation-induced &gamma;H2AX formation within the nucleus.

An early molecular response to DNA double-strand breaks (DSBs) is phosphorylation of the Ser-139 residue within the terminal SQEY motif of the histone ...

Clonogenic Assay: Adherent Cells

The clonogenic (or colony forming) assay has been established for more than 50 years; the original paper describing the technique was published in 19561. Apart from documenting the method, the initial landmark study generated the first radiation-dose response curve for X-ray irradiated mammalian (HeLa) cells in culture1. Basically, the clonogenic assay enables an assessment of the differences in reproductive viability (capacity of cells to produce progeny; i.e. a single cell to form a colony of 50 or more cells) between control untreated cells and cells that have undergone various treatments such as exposure to ionising radiation, various chemical compounds (e.g. cytotoxic agents) or in other cases genetic manipulation. The assay has become the most widely accepted technique in radiation biology and has been widely used for evaluating the radiation sensitivity of different cell lines. Further, the clonogenic assay is commonly used for monitoring the efficacy of radiation modifying compounds and for determining the effects of cytotoxic agents and other anti-cancer therapeutics on colony forming ability, in different cell lines. A typical clonogenic survival experiment using adherent cells lines involves three distinct components, 1) treatment of the cell monolayer in tissue culture flasks, 2) preparation of single cell suspensions and plating an appropriate number of cells in petri dishes and 3) fixing and staining colonies following a relevant incubation period, which could range from 1-3 weeks, depending on the cell line. Here we demonstrate the general procedure for performing the clonogenic assay with adherent cell lines with the use of an immortalized human keratinocyte cell line (FEP-1811)2. Also, our aims are to describe common features of clonogenic assays including calculation of the plating efficiency and survival fractions after exposure of cells to radiation, and to exemplify modification of radiation-response with the use of a natural antioxidant formulation.

The applicability of the clonogenic assay for evaluating reproductive viability has been established for more than 50 years. Here we demonstrate the general procedure for performing the clonogenic assay with adherent cells.

The clonogenic (or colony forming) assay has been established for more than 50 years; the original paper describing the technique was published in 19561. ...

Assessment of Calcium Sparks in Intact Skeletal Muscle Fibers

Maintaining homeostatic Ca2+ signaling is a fundamental physiological process in living cells. Ca2+ sparks are the elementary units of Ca2+ signaling in the striated muscle fibers that appear as highly localized Ca2+ release events mediated by ryanodine receptor (RyR) Ca2+ release channels on the sarcoplasmic reticulum (SR) membrane. Proper assessment of muscle Ca2+ sparks could provide information on the intracellular Ca2+&#160;handling properties of healthy and diseased striated muscles. Although Ca2+ sparks events are commonly seen in resting cardiomyocytes, they are rarely observed in resting skeletal muscle fibers; thus there is a need for methods to generate and analyze sparks in skeletal muscle fibers.
Detailed here is an experimental protocol for measuring Ca2+ sparks in isolated flexor digitorm brevis (FDB) muscle fibers using fluorescent Ca2+ indictors and laser scanning confocal microscopy. In this approach, isolated FDB fibers are exposed to transient hypoosmotic stress followed by a return to isotonic physiological solution. Under these conditions, a robust Ca2+ sparks response is detected adjacent to the sarcolemmal membrane in young healthy FDB muscle fibers. Altered Ca2+ sparks response is detected in dystrophic or aged skeletal muscle fibers. This approach has recently demonstrated that membrane-delimited signaling involving cross-talk between inositol (1,4,5)-triphosphate receptor (IP3R) and RyR contributes to Ca2+ spark activation in skeletal muscle. In summary, our studies using osmotic stress induced Ca2+ sparks showed that this intracellular response reflects a muscle signaling mechanism in physiology and aging/disease states, including mouse models of muscle dystrophy (mdx mice) or amyotrophic lateral sclerosis (ALS model).

Described here is a method to directly measure calcium sparks, the elementary units of Ca2+ release from sarcoplasmic reticulum&#160;in intact skeletal muscle fibers. This method utilizes osmotic-stress-mediated triggering of Ca2+ release from ryanodine receptor in isolated muscle fibers. The dynamics and homeostatic capacity of intracellular Ca2+ signaling can be employed to assess muscle function in health and disease.

Maintaining homeostatic Ca2+ signaling is a fundamental physiological process in living cells. Ca2+ sparks are the elementary units of Ca2+ signaling in ...

Voltage and Calcium Dual Channel Optical Mapping of Cultured HL-1 Atrial Myocyte Monolayer

Optical mapping has proven to be a valuable technique to detect cardiac electrical activity on both intact ex vivo hearts and in cultured myocyte monolayers. HL-1 cells have been widely used as a 2-Dimensional cellular model for studying diverse aspects of cardiac physiology. However, it has been a great challenge to optically map calcium (Ca) transients and action potentials simultaneously from the same field of view in a cultured HL-1 atrial cell monolayer. This is because special handling and care is required to prepare healthy cells that can be electrically captured and optically mapped. Therefore, we have developed an optimal working protocol for dual channel optical mapping. In this manuscript, we have described in detail how to perform the dual channel optical mapping experiment. This protocol is a useful tool to enhance the understanding of action potential propagation and Ca kinetics in arrhythmia development.

This article describes the technique used to perform dual channel optical mapping in cultured HL-1 atrial cell monolayers. This unique protocol allows the simultaneous visualization of both calcium (Ca) and voltage (Vm) activity in the same area for the detailed detection and analysis of electrophysiological properties of culture monolayers.