This work was supported by National Science Foundation grant MCB-0951199, and in part by the NIH grant EY01910, California Institute of Regenerative Medicine grants RB1-01417 and by the Funda&#231;&#227;o para a Ci&#234;ncia e Tecnologia (FCT) grant SFRH/BD/87256/2012.

1. Ion-selective Self-referencing Microelectrode Preparation
<ol>
	<li>Preparation of ion-selective microelectrode
	<ol>
		<li>Heat pull thin walled borosilicate capillaries without filament (1.5 mm outer diameter, 1.12 mm inner diameter) using a microelectrode puller. 
		Note: This gives tips 3-4 &#181;m in diameter. Smaller tips have higher resistance which makes microelectrodes more susceptible to electronic noise and is also associated with a slower response to a change in ion concentration. Useful information can be found in the paper published by Smith et al.26.</li>
		<li>Silanize the electrodes to render the inner surface hydrophobic to aid retention of the lipophilic ionophore cocktail. Place the microelectrodes in a metal rack and heat O/N in an oven at &#62;100 &#176;C to dry them. The rack is a metal plate with 2 mm diameter holes drilled part of the way through. Place the electrodes in the holes tip upwards with a 250 ml glass beaker over them.</li>
		<li>In the morning, turn the oven off and while wearing insulated gloves, carefully remove the metal rack with the electrodes and beaker in place. Close the oven door to retain the heat.</li>
		<li>Wear latex or nitrile gloves, lab coat and eye protection. With a plastic Pasteur pipette, place a drop of silanization solution I at the base of each electrode (keep the beaker in place; use the pouring lip for pipette access). The silanization solution is vaporized by the hot plate and silanizes the inside of the electrodes. Use a chemical extractor fume hood for this stage. Place the rack/beaker/electrodes back in the hot oven for a few hours to allow any remaining silanization solution to evaporate. 
		Note: For safety reasons, do not turn the oven back on. Place a label on the oven indicating it must not be switched on as it may contain harmful and flammable vapor.</li>
		<li>After cooling, store the microelectrodes in a microelectrode storage jar inside a glass desiccator with 400 g of desiccant. Microelectrodes can be stored thus for many weeks. 
		Note: An alternative silanization method is described in Smith et al.26</li>
		<li>Back-fill the microelectrode with 50 to 100 &#181;l (a length of about 1 cm) of a solution containing 100 mM of the ion to be measured (see Table 1 and Figure 1B). Use a disposable plastic Pasteur pipette heat pulled in a Bunsen burner to a fine filament. Rinse the pipette in dH2O afterward to prevent blockage. 
		Note: Alternatively, adjust the ion concentration of the backfilling solution to match the concentration of ion in the external solution27.</li>
		<li>Observe the microelectrode under a dissecting microscope to ensure the absence of air bubbles.
		<ol>
			<li>If bubbles are present tap the microelectrode lightly with a finger nail while holding the electrode vertically (tip down) and/or push the bubbles out the tip by applying back pressure using a syringe modified with a silicone tube replacing the needle.</li>
		</ol>
		</li>
		<li>Tip-fill the microelectrode with 15 to 20 nl (a length of 30-50 &#181;m) of ion-specific ionophore cocktail (see Table 1). Place a small droplet of ionophore cocktail on the short edge of a microscope slide. Observe the microelectrode tip under a dissecting microscope and move it toward the microscope slide until the microelectrode tip touches the ionophore cocktail for only about half a sec. Draw the ionophore cocktail into the microelectrode by capillary pressure. 
		Note: Avoid a long column of ionophore cocktail as this increases the probe&#8217;s electrical resistance which can make it susceptible to electronic interference (noise) and also slows the response time.</li>
		<li>Mount the microelectrode in a straight microelectrode holder with a gold 1 mm male connector and AgCl(Ag+) wire (Figure 1B).</li>
		<li>Attach the microelectrode holder to the head stage mounted on a three-dimensional computer-controlled electronic micropositioner (Figure 1A).</li>
		<li>Place the microelectrode tip in measuring solution appropriate for the sample to be measured (physiological saline, culture medium, etc.) to allow the microelectrode to stabilize for an hr or two, or even overnight.</li>
	</ol>
	</li>
	<li>Preparation of the reference electrode
	<ol>
		<li>Reference electrodes (Figure 1C) are the same capillaries as above. Cut the capillary with a diamond pencil into 5 cm lengths and fire-polished at each end for 1-2 sec&#160;in a Bunsen flame.</li>
		<li>Fill these electrodes with ~200 &#181;l of a 3 M solution of NaCl, CH3CO2K (potassium acetate) or KCl with 2% agarose. Choose the solution depending on the ion to be measured (the reference electrode should not contain the ion being measured; see Table 1). Mix the agarose and the solution and heat to almost boiling in a microwave. Stir to dissolve the agarose (the solution goes clear).</li>
		<li>Attach the reference electrode to a plastic Pasteur pipette and draw the hot solution into the capillary.</li>
		<li>Drop the electrode into cold 3 M NaCl, CH3CO2K or KCl solution and store in this 3 M solution in sealed tubes prior to use. Discard any reference electrodes with air bubbles.</li>
		<li>Mount the reference electrode in a straight microelectrode holder (pre-filled with 3 M solution) with an AgCl(Ag+) pellet inside and a gold 2 mm male connector (Figure 1C) and attach the electrode and holder onto a manual micro-positioner mounted on a magnetic stand.</li>
	</ol>
	</li>
</ol>
2. Ion-selective Self-referencing Microelectrode Calibration
<ol>
	<li>Preparing calibrating solutions containing the ion of interest as in the reference solution; see Table 1. Bracket the concentration that is in the solution the sample will be in (e.g. culture media, physiological saline). That is, one calibration solution must contain a lower concentration of ion than in the measuring solution, and one higher.
	<ol>
		<li>For example, use saline that contains 1 mM of K+. To bracket this concentration, dissolve KCl powder in deionized water to a concentration of 10, 1 and 0.1 mM in serial dilutions. Use these calibration solutions. Alternatively, use at least two of these solutions.</li>
	</ol>
	</li>
	<li>Immerse the ion-selective microelectrode and the reference electrode in each calibration solution and let the voltage value stabilize for 1 to 3 min&#160;before recording the corresponding voltage using the dedicated software (see Table 1).</li>
	<li>As the software saves the data (amplifier output) as a txt file, copy the data into a spreadsheet file. Plot the microelectrode output (mV) against the logarithm of the molar ion concentration (Figure 2A).</li>
	<li>Apply a linear regression and calculate the Nernst slope, intercept and R2 value. Accept the microelectrode if the Nernst slope is 58 &#177; 11 mV/decade for monovalent ions and 29 &#177; 11 mV/decade for divalent ions (for cations, the Nernst slope is positive, for anions it is negative). Additionally, good microelectrodes should have a strong linear correlation (R2 &#62;0.9; Figure 2B). 
	Note: The mV output of the amplifier used here gives mV reading with a tenfold gain. Values have to be divided by a factor of ten.</li>
	<li>Use the linear regression formula to convert the raw mV output of the microelectrode into actual ion concentration (Figure 2B).</li>
</ol>
3. Validation of the Ion-selective Microelectrode Technique
<ol>
	<li>Preparing an artificial source
	<ol>
		<li>Artificial source capillaries are the same capillaries as above. Heat pull the capillary using a microelectrode puller as in step 1.1.1.</li>
		<li>Backfill these capillaries with 200 &#181;l of a 1 M solution of NaCl, KCl, CaCl22 H2O or pH 4 buffer. Choose the artificial source solution depending on the ion to be measured (see Table 1). 
		Note: Alternatively, pull electrodes with bigger tip diameter (~20 &#181;m) and tip-fill with the same solutions but containing 0.5-1% agarose (agarose will prevent any bulk flow of solution).</li>
		<li>Mount the artificial source capillary on a micromanipulator and immerse it in the solution used to measure the flux of the ion in the samples. Leave the artificial source in the solution for 30 min&#160;to 1 hr to allow stabilization of the gradient.</li>
	</ol>
	</li>
	<li>Validation of the ion-selective microelectrode
	<ol>
		<li>Immerse the ion-selective microelectrode about one centimeter away from the artificial source capillary in the solution used to measure the flux of ion on the samples and close the circuit with the reference electrode as before. Let the voltage value stabilize for 1 to 3 min&#160;before recording the corresponding voltage using the dedicated software for 1 to 2 min. This value corresponds to the buffer value (in literature also referred as reference, background or blank value).</li>
		<li>Move the ion-selective microelectrode to about 5 &#181;m from the artificial source and let the voltage value stabilize for 1 to 3 min&#160;before recording the corresponding voltage using the software for 1 to 2 min.</li>
		<li>Repeat the above procedure by placing the ion selective microelectrode at 10, 20, 40, 80, 160, 320, 640 and 1280 &#181;m away from the artificial source capillary.</li>
		<li>Extract the data as a txt file and copy the values into a spreadsheet file.</li>
	</ol>
	</li>
	<li>Calculate the ion concentration corresponding to the mV values in the same way as for the calibration values. Plot the value. 
	Note: If an ion flux is present, the microelectrode detects a difference in ion concentration between the two positions (Figure 3B). If the artificial source contains more ions of the species measured than the solution, the concentration should be higher close to the source than far away, validating the ability of the ion-selective microelectrode to correctly detect the direction of an ion flux (in this case efflux; for an artificial sink, with lower specific ion concentration than measuring medium, it should be influx).
	<ol>
		<li>Calculate the ion flux using Fick&#8217;s law of diffusion: J = c &#181; (dc/dx) where c is the ion concentration in the solution (mol cm-3), &#181; is the ion mobility (mol cm N-1 sec-1), and dc is the concentration difference over distance dx (cm) (Figure 2C). Ion flux data are usually presented in pmol cm-2 s-1or nmol cm-2 sec-1. 
		Note: An alternative method of ion flux calculation described by Smith et al.26. can be used. Main differences include the use of the diffusion coefficient instead of the ion mobility and the subtraction of the background ion flux (also voltage drift or correction factor) calculated from measurement of ion flux in saline solution without sample.</li>
		<li>Plot the mean of the ion fluxes of each step against the distance from the source (Figure 2D). Moving away from the source, observe an exponential decrease of the flux value validating the ability of the ion-selective microelectrode to sense different magnitude of ion fluxes.</li>
		<li>Do the artificial source validation once for each specific ion intended to be recorded in order to validate its correct direction and magnitude measurements with a large signal-to-noise ratio. 
		Note: Ion flux measurement of the buffer without samples indicates the background level or noise. Typically, buffer measurement shows no clear fluctuation of the ion concentration leading to very small flux that displays variable directions.</li>
	</ol>
	</li>
</ol>
4. Preparation of Measuring Chamber
Note: Before experiments, consider the sample to be measured and how the sample is to be mounted and immobilized for microelectrode measurements.
<ol>
	<li>For Xenopus laevis oocyte measurements cut a 1 cm square of an 800 &#181;m nylon mesh (nitex mesh) and glue it into a plastic Petri dish (Figure 1E).</li>
</ol>
5. Ion Flux Measurement
<ol>
	<li>Measure the ion concentration present in the buffer used to perform the measurements on the sample in the same way as for the calibration solution. X. laevis oocytes require Mark&#8217;s Modified Ringer (MMR). Dissolve NaCl, KCl, CaCl, MgCl and HEPES into deionized water to reach a final concentration of (mM): 100 NaCl, 2 KCl, 2 CaCl, 1 MgCl and 5 HEPES. Adjust the pH of the buffer to 7.5 using NaOH.</li>
	<li>Place the sample into the measuring chamber and bring the ion-selective microelectrode close to the sample (about 10 &#181;m away) using the micropositioner to define the close position of the microelectrode (Figure 3A).</li>
	<li>Start the low frequency (0.3 Hz) excursion (100 &#181;m) of the microelectrode between the close position and a position away from the sample (distant) using the dedicated software. Ensure that the movement of the microelectrode is perpendicular to the surface of the sample. 
	Note: The excursion of the microelectrode can be set on the software. Large excursion increases the gradient read allowing an easier detection of small fluxes during the measurement while lengthens the sampling interval and decreases the temporal resolution. See Figure 3A for an example.</li>
	<li>Start the recording using the software. The microelectrode pauses at each position and the electric potential in mV is recorded on the computer. Obtain measurements for at least 2 min, allowing signal stabilization. For short time-lapse experiments, record potential variations at the position of interest for the whole time course.</li>
	<li>Extract the data as a txt file and copy the values into a spreadsheet file.</li>
	<li>Calculate the ion concentration corresponding to the mV values in the same way as for the calibration values. Plot the value. 
	Note: If an ion flux is present, the microelectrode detects a difference in ion concentration between the two positions (Figure 3B).</li>
	<li>Calculate the ion flux using Fick&#8217;s law of diffusion as before (step 3.3.1).</li>
	<li>Repeat the buffer measurement before measuring a new sample and repeat the procedure of flux measurement and calculation for every new sample.</li>
</ol>
6. Statistical Analysis and Data Presentation
<ol>
	<li>Test the independent effects of the position and/or of the time on ion fluxes under the control condition using an ANCOVA model with mixed effects28. 
	Note: Analysis of covariance (ANCOVA) is a general linear model that mixes regular ANOVA and regression by allowing both categorical and continuous measures to be used as independent variables. In addition, in the presence of correlated errors induced by repeated measures per individual and eventual nested effects, mixed effects models are used to model accurate estimates of both fixed and random effects.</li>
	<li>Calculate pairwise comparisons using Student t-test between group levels with Bonferroni correction for multiple testing28.</li>
	<li>Generate boxplots to summarize ion flux measurements according to position and time. Include p values from the pairwise Student t described above (Figure 3D) and indicate significance levels of p values as follows: *: p &#60;0.05; **: p &#60;0.01; ***: p &#60;0.00129</li>
</ol>

The authors declare that they have no competing financial interests.

The most critical steps for successful measurement of extracellular ion fluxes in vivo are: the reduction of the noise, the correct fabrication of the ion-selective microelectrodes and reference electrode, and the positioning of the sample and both electrodes.
In order to minimize the noise, the recording system should be in an earthed (grounded) Faraday cage preferably with a metal-topped (vibration isolation) table which is also earthed. In addition, the microscope chassis should al...

All animal cells are surrounded by a lipid bilayer membrane which separates the cytoplasm from the outside environment. The cell maintains an electrical membrane potential, negative inside, by active transport of ions1. The membrane potential is a stored energy source which the cell can utilize to operate various molecular devices in the membrane2. Neurons and other excitable cells have large membrane potentials. Rapid opening of sodium channels collapses the membrane potential (depolarization) and produces the action potential which is transported along the length of the neuron2. Aside from these rapid electrical changes, many tissues and organs generate and maintain significant long-term electrical potentials. For example, skin and corneal epithelia generate and maintain trans-epithelial potentials and extracellular electric currents by directional pumping of ions (mainly sodium and chloride)3.
While measurements of endogenous extracellular electric current using the vibrating probe4-6 and measurements of membrane or trans-epithelial potentials using the microelectrode system7-10 allow measurement of the electric parameters of cell membranes and epithelial cell layers, they give no indication of the ion species involved.
Microelectrodes with selective ionophore can measure specific ion concentration in solution. Ion gradients or flux could be measured with two or more electrodes at different positions. However, the intrinsic voltage drift of each probe would be different, causing inaccurate measurements or even detection of a gradient that was not present. A single electrode used in &#8220;self-referencing&#8221; mode whereby it moves at low frequency between two points solves this problem. Now the ion flux can be seen against the background of a relatively slow and stable signal drift (see Figure 3B).
The ion-sensitive measuring system uses ion-selective self-referencing microelectrodes to detect small extracellular fluxes of ions close to tissues or single cells. The system consists of an amplifier which processes the signal from the microelectrode and a micro stepper motor and driver to control the motion of the microelectrode. The ion-selective microelectrode and the reference electrode that close the circuit are connected to the amplifier via a headstage pre-amplifier (Figure 1A). Computer software determines the parameters of the microelectrode movement (frequency, distance) and also records the output of the amplifier. The stepper motor controls the microelectrode movement via a three-dimensional micropositioner. A low frequency vibrating ion-selective microelectrode was first developed in 1990 to measure specific calcium flux11. As well as calcium, commercially accessible ionophore cocktails are now available to make microelectrodes sensitive to sodium, chloride, potassium, hydrogen, magnesium, nitrate, ammonium, fluoride, lithium or mercury.
Basically, the self-referencing ion-selective microelectrode technique converts the activity of a specific ion dissolved in a solution into an electric potential, which can be measured by a voltmeter. The ionophore cocktail is an immiscible liquid (organic, lipophilic) phase with ion-exchange properties. The ionophore selectively complexes (binds) specific ions reversibly and transfers them between the aqueous solution contained in the microelectrode (electrolyte) and the aqueous solution in which the microelectrode is immersed (Figure 1D). This ion transfer leads to an electrochemical equilibrium and a variation of the electric potential between the microelectrode and the reference electrode is measured by the voltmeter. The voltage is proportional to the logarithm of the specific ion activity according to the Nernst equation allowing the calculation of the ion concentration (Figure 2A and B).
At present, several systems allow measurement of ion flux using a similar concept or principle. For example, the Scanning Ion-selective Electrode Technique (SIET)12,13 or the Microelectrode Ion Flux Estimation (MIFE) technique developed by Newman and Shabala14-16 are commercially available and widely used by the research community in order to determine specific ion fluxes occurring at cell membrane and tissue across a variety of animal, plant and single living cell models. Ion-selective microelectrodes have been used to measure hydrogen, potassium and calcium flux across plant roots17, chloride flux in rat cerebral arteries18 and in pollen tubes19, hydrogen flux in skate retinal cells20, calcium flux in mouse bone21, various ion fluxes in fungal hyphae22 and in rat cornea23, and finally calcium flux during single cell wound healing12,24. See also the following review for detailed information on ion-selective self-referencing microelectrodes25.
The following article describes in detail how to prepare and perform measurement of endogenous extracellular ion fluxes using the ion-selective self-referencing microelectrode technique at the single cell level.

<table><tbody><tr><td>IonAmp&amp;nbsp;</td><td>&amp;nbsp;BioCurrents Research Center, Woods Hole, MA, USA</td><td>none</td><td>amplifier created by the BioCurrents Research Center, Woods Hole, MA, USA; Similar system can be purchased from &amp;ldquo;XBL function matters&amp;rdquo; (http://www.xuyue.org/) or from &amp;ldquo;YoungerUSA&amp;rdquo; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>IonAmp32&amp;nbsp;</td><td>&amp;nbsp;BioCurrents Research Center, Woods Hole, MA, USA</td><td>none</td><td>software created by the BioCurrents Research Center, Woods Hole, MA, USA; Similar system can be purchased from &amp;ldquo;XBL function matters&amp;rdquo; (http://www.xuyue.org/) or from &amp;ldquo;YoungerUSA&amp;rdquo; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>Headstage pre-amplifier</td><td>&amp;nbsp;BioCurrents Research Center, Woods Hole, MA, USA</td><td>INA116</td><td>BSR Voltage Follower INA116, designed by the BioCurrents Research Center, Woods Hole, MA, USA; Similar system can be purchased from &amp;ldquo;XBL function matters&amp;rdquo; (http://www.xuyue.org/) or from &amp;ldquo;YoungerUSA&amp;rdquo; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>MicroStep Driver</td><td>&amp;nbsp;BioCurrents Research Center, Woods Hole, MA, USA</td><td>none</td><td>three MicroStep drivers are required for X, Y and Z-positioning; created by the BioCurrents Research Center, Woods Hole, MA, USA; Similar system can be purchased from &amp;ldquo;XBL function matters&amp;rdquo; (http://www.xuyue.org/) or from &amp;ldquo;YoungerUSA&amp;rdquo; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>Manual micropositioner&amp;nbsp;&amp;nbsp;</td><td>World Precision Instruments&amp;nbsp;</td><td>Model KITE-R</td><td>Similar system can be purchased from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>Magnetic stand&amp;nbsp;&amp;nbsp;&amp;nbsp;</td><td>World Precision Instruments</td><td>Model M10</td><td>Similar system can be purchased from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>Vibration isolation table&amp;nbsp;&amp;nbsp;</td><td>Newport Inc.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;</td><td>Model VW-3036-OPT-023040</td><td>Similar system can be purchased from Applicable Electronics(http://www.applicableelectronics.com/)</td></tr><tr><td>Part of three dimentional micropositioner: angle bracket, 90&amp;deg;, slotted faces</td><td>Newport Inc.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;</td><td>Model 360-90</td><td>Assemblage of the three dimantionnal micropositionner requires also Three electric rotary motors for X, Y, Z control, MPH-1 mounting arm with MCA-2 adjustable-angle post and Various Newport connectors and screws to bolt onto vibration table</td></tr><tr><td>Part of three dimentional micropositioner: Peg-Joining Dovetail Stage 0.5 inch X Travel</td><td>Newport Inc.&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;</td><td>460PD-X</td><td>none</td></tr><tr><td>Part of three dimentional micropositioner: Quick-Mount Linear Stage, 0.5 inch XY Travel</td><td>Newport Inc.</td><td>460A-XY</td><td>none</td></tr><tr><td>Kwik-Fil thin walled borosilicate glass capillaries without filament&amp;nbsp;</td><td>World Precision Instruments&amp;nbsp;</td><td>TW150-4</td><td>none</td></tr><tr><td>Electrode puller&amp;nbsp;</td><td>Narishige</td><td>&amp;nbsp;PC-10</td><td>none</td></tr><tr><td>Metal rack</td><td>Made in-house</td><td>none</td><td>Metal electrode holder made in-house by drilling 2 mm wide holes half centimeter spaced in a 10cm by 15cm rectangular base of steel</td></tr><tr><td>Oven</td><td>QL</td><td>Model 10 Lab Oven</td><td>none</td></tr><tr><td>Silanization solution I&amp;nbsp;</td><td>Sigma-Aldrich</td><td>85126</td><td>Hazardous, handle as recommended by provider&amp;nbsp;</td></tr><tr><td>Glass Petri dish; Pyrex</td><td>Fisher Scientific</td><td>316060</td><td>none</td></tr><tr><td>Electrode/micropipette storage jar</td><td>World Precision Instruments&amp;nbsp;</td><td>E215</td><td>none</td></tr><tr><td>Glass dessicator</td><td>Fisher Scientific</td><td>08-595E</td><td>Contains Drierite dessicant (W.A. Hammond Drierite Co. Ltd, Xenia, OH, USA). Place petroleum jelly on the seal to make it airtight.</td></tr><tr><td>Plastic Pasteur pipette&amp;nbsp;</td><td>Fisher Scientific</td><td>11597722</td><td>none</td></tr><tr><td>Bunsen burner</td><td>Fisher Scientific</td><td>S97329</td><td>none</td></tr><tr><td>Microscope slide</td><td>Sigma-Aldrich</td><td>S8902</td><td>none</td></tr><tr><td>Straight microelectrode holder</td><td>Warner Instruments</td><td>QSW-A15P</td><td>with a gold 1 mm male connector and Ag/AgCl wire</td></tr><tr><td>Straight microelectrode holder&amp;nbsp;</td><td>World Precision Instruments</td><td>MEH3S</td><td>with a AgCl(Ag&lt;sup&gt;+&lt;/sup&gt;)pellet inside and a gold 2 mm male connector&amp;nbsp;</td></tr><tr><td>6 cm Petri dish</td><td>VWR</td><td>60872-306</td><td>none</td></tr><tr><td>Nitex mesh</td><td>Dynamic Aqua-Supply Ltd.</td><td>NTX750</td><td>none</td></tr><tr><td>Glue; Loctite epoxy</td><td>VWR</td><td>500043-451</td><td>Mix glue and hardener in equal parts in a plastic weighing boat and mix thoroughly. Sets quickly but leave at RT for 24 h for full curing</td></tr><tr><td>Deionized water&amp;nbsp;</td><td>Sigma-Aldrich</td><td>99053</td><td>none</td></tr><tr><td>Sodium Chloride</td><td>Sigma-Aldrich</td><td>S7653</td><td>none</td></tr><tr><td>Potassium Chloride</td><td>Sigma-Aldrich</td><td>P9333</td><td>none</td></tr><tr><td>Calcium Chloride</td><td>Sigma-Aldrich</td><td>C1016</td><td>none</td></tr><tr><td>Magnesium Chloride</td><td>Sigma-Aldrich</td><td>M8266</td><td>none</td></tr><tr><td>Hepes</td><td>Sigma-Aldrich</td><td>H3375</td><td>none</td></tr><tr><td>Sodium Hydroxyde</td><td>Sigma-Aldrich</td><td>S8045</td><td>none</td></tr><tr><td>Potassium Acetate</td><td>Sigma-Aldrich</td><td>P1190</td><td>none</td></tr><tr><td>Agarose</td><td>Sigma-Aldrich</td><td>A9539</td><td>none</td></tr></tbody></table>

measurement of extracellular ion fluxes using the ion-selective self-referencing microelectrode technique

Cells from animals, plants and single cells are enclosed by a barrier called the cell membrane that separates the cytoplasm from the outside. Cell layers such as epithelia also form a barrier that separates the inside from the outside or different compartments of multicellular organisms. A key feature of these barriers is the differential distribution of ions across cell membranes or cell layers. Two properties allow this distribution: 1) membranes and epithelia display selective permeability to specific ions; 2) ions are transported through pumps across cell membranes and cell layers. These properties play crucial roles in maintaining tissue physiology and act as signaling cues after damage, during repair, or under pathological condition. The ion-selective self-referencing microelectrode allows measurements of specific fluxes of ions such as calcium, potassium or sodium at single cell and tissue levels. The microelectrode contains an ionophore cocktail which is selectively permeable to a specific ion. The internal filling solution contains a set concentration of the ion of interest. The electric potential of the microelectrode is determined by the outside concentration of the ion. As the ion concentration varies, the potential of the microelectrode changes as a function of the log of the ion activity. When moved back and forth near a source or sink of the ion (i.e. in a concentration gradient due to ion flux) the microelectrode potential fluctuates at an amplitude proportional to the ion flux/gradient. The amplifier amplifies the microelectrode signal and the output is recorded on computer. The ion flux can then be calculated by Fick&#8217;s law of diffusion using the electrode potential fluctuation, the excursion of microelectrode, and other parameters such as the specific ion mobility. In this paper, we describe in detail the methodology to measure extracellular ion fluxes using the ion-selective self-referencing microelectrode and present some representative results.

We have previously shown that calcium influx appears after single cell wounding24. We therefore asked whether other ion fluxes occur upon single cell wounding. We used the X. laevis oocyte, a well-established model for single cell wound healing30-34 and electrophysiological recording24,35-39. Interestingly, potassium ions are more concentrated inside X. laevis oocytes (about 110 mM)40 than in the extracellular solution used (in MMR 1x: 1 mM) suggesting an effl...

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Measurement of Extracellular Ion Fluxes Using the Ion-selective Self-referencing Microelectrode Technique

Transporters in cell membranes allow differential segregation of ions across cell membranes or cell layers and play crucial roles during tissue physiology, repair and pathology. We describe the ion-selective self-referencing microelectrode that allows the measurement of specific ion fluxes at single cells and tissues in vivo.

Cells from animals, plants and single cells are enclosed by a barrier called the cell membrane that separates the cytoplasm from the outside. Cell layers ...

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13.9K Views.  University of California, Davis. Transporters in cell membranes allow differential segregation of ions across cell membranes or cell layers and play crucial roles during tissue physiology, repair and pathology. We describe the ion-selective self-referencing microelectrode that allows the measurement of specific ion fluxes at single cells and tissues in vivo.

Video: Measurement of Extracellular Ion Fluxes Using the Ion-selective Self-referencing Microelectrode Technique

Double-barreled and Concentric Microelectrodes for Measurement of Extracellular Ion Signals in Brain Tissue

Electrical activity in the brain is accompanied by significant ion fluxes across membranes, resulting in complex changes in the extracellular concentration of all major ions. As these ion shifts bear significant functional consequences, their quantitative determination is often required to understand the function and dysfunction of neural networks under physiological and pathophysiological conditions. In the present study, we demonstrate the fabrication and calibration of double-barreled ion-selective microelectrodes, which have proven to be excellent tools for such measurements in brain tissue. Moreover, so-called &#8220;concentric&#8221; ion-selective microelectrodes are also described, which, based on their different design, offer a far better temporal resolution of fast ion changes. We then show how these electrodes can be employed in acute brain slice preparations of the mouse hippocampus. Using double-barreled, potassium-selective microelectrodes, changes in the extracellular potassium concentration ([K+]o) in response to exogenous application of glutamate receptor agonists or during epileptiform activity are demonstrated. Furthermore, we illustrate the response characteristics of sodium-sensitive, double-barreled and concentric electrodes and compare their detection of changes in the extracellular sodium concentration ([Na+]o) evoked by bath or pressure application of drugs. These measurements show that while response amplitudes are similar, the concentric sodium microelectrodes display a superior signal-to-noise ratio and response time as compared to the double-barreled design. Generally, the demonstrated procedures will be easily transferable to measurement of other ions species, including pH or calcium, and will also be applicable to other preparations.

We demonstrate the fabrication, calibration and properties of two types of ion-selective microelectrodes (double-barreled and concentric) for measurement of ion concentrations in brain tissue. These are then used in the mouse hippocampal slice preparation to show that excitatory activity changes both extracellular potassium and sodium concentrations.

Electrical activity in the brain is accompanied by significant ion fluxes across membranes, resulting in complex changes in the extracellular ...

Neuroscience

Whole-cell Patch-clamp Recordings for Electrophysiological Determination of Ion Selectivity in Channelrhodopsins

Over the past decade, channelrhodopsins became indispensable in neuroscientific research where they are used as tools to non-invasively manipulate electrical processes in target cells. In this context, ion selectivity of a channelrhodopsin is of particular importance. This article describes the investigation of chloride selectivity for a recently identified anion-conducting channelrhodopsin of Proteomonas sulcata via electrophysiological patch-clamp recordings on HEK293 cells. The experimental procedure for measuring light-gated photocurrents demands a fast switchable &#8211; ideally monochromatic &#8211; light source coupled into the microscope of an otherwise conventional patch-clamp setup. Preparative procedures prior to the experiment are outlined involving preparation of buffered solutions, considerations on liquid junction potentials, seeding and transfection of cells, and pulling of patch pipettes. The actual recording of current-voltage relations to determine the reversal potentials for different chloride concentrations takes place 24 h to 48 h after transfection. Finally, electrophysiological data are analyzed with respect to theoretical considerations of chloride conduction.

This article describes how the ion selectivity of channelrhodopsin is determined with electrophysiological whole-cell patch-clamp recordings using HEK293 cells. Here, the experimental procedure for investigating chloride selectivity of an anion-selective channelrhodopsin is demonstrated. However, the procedure is transferable to other channelrhodopsins of distinct selectivity.

Over the past decade, channelrhodopsins became indispensable in neuroscientific research where they are used as tools to non-invasively manipulate ...

Making, Testing, and Using Potassium Ion Selective Microelectrodes in Tissue Slices of Adult Brain

Potassium ions significantly contribute to the resting membrane potential of cells and, therefore, extracellular K+ concentration is a crucial regulator of cell excitability. Altered concentrations of extracellular K+ affect the resting membrane potential and cellular excitability by shifting the equilibria between closed, open and inactivated states for voltage-dependent ion channels that underlie action potential initiation and conduction. Hence, it is valuable to directly measure extracellular K+ dynamics in health and diseased states. Here, we describe how to make, calibrate and use monopolar K+-selective microelectrodes. We deployed them in adult hippocampal brain slices to measure electrically evoked K+ concentration dynamics. The judicious use of such electrodes is an important part of the tool-kit needed to evaluate cellular and biophysical mechanisms that control extracellular K+ concentrations in the nervous system.

Potassium ions contribute&#160;to the resting membrane potential of cells and extracellular K+ concentration is a crucial regulator of cellular excitability. We describe how to make, calibrate and use monopolar K+-selective microelectrodes. Using such electrodes enables the measurement of electrically evoked K+ concentration dynamics in adult hippocampal slices.

Potassium ions significantly contribute to the resting membrane potential of cells and, therefore, extracellular K+ concentration is a crucial regulator ...

Functional Characterization of Na+/H+ Exchangers of Intracellular Compartments Using Proton-killing Selection to Express Them at the Plasma Membrane

Endosomal acidification is critical for a wide range of processes, such as protein recycling and degradation, receptor desensitization, and neurotransmitter loading in synaptic vesicles. This acidification is described to be mediated by proton ATPases, coupled to ClC chloride transporters. Highly-conserved electroneutral protons transporters, the Na+/H+ exchangers (NHE) 6, 7 and 9 are also expressed in these compartments. Mutations in their genes have been linked with human cognitive and neurodegenerative diseases. Paradoxically, their roles remain elusive, as their intracellular localization has prevented detailed functional characterization. This manuscript shows a method to solve this problem. This consists of the selection of mutant cell lines, capable of surviving acute cytosolic acidification by retaining intracellular NHEs at the plasma membrane. It then depicts two complementary protocols to measure the ion selectivity and activity of these exchangers: (i) one based on intracellular pH measurements using fluorescence video microscopy, and (ii) one based on the fast kinetics of lithium uptake. Such protocols can be extrapolated to measure other non-electrogenic transporters. Furthermore, the selection procedure presented here generates cells with an intracellular retention defective phenotype. Therefore these cells will also express other vesicular membrane proteins at the plasma membrane. The experimental strategy depicted here may therefore constitute a potentially powerful tool to study other intracellular proteins that will be then expressed at the plasma membrane together with the vesicular Na+/H+ exchangers used for the selection.

Functional Characterization of Na+/H+ Exchangers of Intracellular Compartments Using Proton-killing Selection to Express Them at the Plasma Membrane

The first part of this article shows how to select mutant cell lines expressing vesicular Na+/H+ exchangers at their plasma membrane. The second part provides protocols based on intracellular pH measurements and fast ion uptake, which are used to determine the ion selectivity and the kinetic parameters of these exchangers.

Endosomal acidification is critical for a wide range of processes, such as protein recycling and degradation, receptor desensitization, and ...

Use of the Ramsay Assay to Measure Fluid Secretion and Ion Flux Rates in the Drosophila melanogaster Malpighian Tubule

Modulation of renal epithelial ion transport allows organisms to maintain ionic and osmotic homeostasis in the face of varying external conditions. The Drosophila&#160;melanogaster Malpighian (renal) tubule offers an unparalleled opportunity to study the molecular mechanisms of epithelial ion transport, due to the powerful genetics of this organism and the accessibility of its renal tubules to physiological study. Here, we describe the use of the Ramsay assay to measure fluid secretion rates from isolated fly renal tubules, with the use of ion-specific electrodes to measure sodium and potassium concentrations in the secreted fluid. This assay allows study of transepithelial fluid and ion fluxes of ~20 tubules at a time, without the need to transfer the secreted fluid to a separate apparatus to measure ion concentrations. Genetically distinct tubules can be analyzed to assess the role of specific genes in transport processes. Additionally, the bathing saline can be modified to examine the effects of its chemical characteristics, or drugs or hormones added. In summary, this technique allows the molecular characterization of basic mechanisms of epithelial ion transport in the Drosophila tubule, as well as regulation of these transport mechanisms.

Use of the Ramsay Assay to Measure Fluid Secretion and Ion Flux Rates in the Drosophila melanogaster Malpighian Tubule

This protocol describes the use of the Ramsay assay to measure fluid secretion rates from isolated Malpighian (renal) tubules from Drosophila melanogaster. In addition, the use of ion-specific electrodes to measure sodium and potassium concentrations in the secreted fluid, allowing calculation of transepithelial ion flux, is described.

Modulation of renal epithelial ion transport allows organisms to maintain ionic and osmotic homeostasis in the face of varying external conditions. The ...

Application of Electrophysiology Measurement to Study the Activity of Electro-Neutral Transporters

The transport of ions through cell membranes ensures the fine control of ion content within and outside the cell that is indispensable for cell survival. These transport mechanisms are mediated by the activities of specialized transporter proteins. Specifically,pH dynamics are finely controlled by plasma membrane proton (H+) extrusion systems, such as the Na+/H+ exchanger (NHE) protein family. Despite extensive efforts to study the mechanisms underlying NHE regulation, our current understanding of the biophysical and molecular properties of the NHE family is inadequate because of the limited availability of methods to effectively measure NHE activity. In this manuscript, we used H+-selective electrodes during whole-cell patch clamping recording to measure NHE-induced H+ flux. We proposed this approach to overcome some limitations of typically used methods to measure NHE activity, such as radioactive uptake and fluorescent membrane permeants. Measurement of NHE activity using the described method enables high sensitivity and time resolution and more efficient control of intracellular H+ concentrations. H+-selective electrodes are based on the fact that transporter activity creates an ion gradient in close proximity to the cell membrane. An H+-selective electrode moving up to and away from the cell membrane in a repetitive, oscillatory fashion records a voltage difference that is dependent on H+ flux. While H+-selective electrodes are used to detect H+ flux moving out of the cell, the patch clamp method in the whole-cell configuration is used to control the intracellular ion composition. Moreover, application of the giant patch clamp technique allows modification of the intracellular composition of not only ions but also lipids. The transporter activity of NHE isoform 3 (NHE3) was measured using this technical approach to study the molecular basis of NHE3 regulation by phosphoinositides.

This manuscript describes the applications of proton-selective electrodes and patch clamping methods to measure the activity of proton transport systems. These methods overcome some limitations of techniques commonly used to study proton transport activity, such as moderate sensitivity, time resolution and insufficient intracellular milieu control.

The transport of ions through cell membranes ensures the fine control of ion content within and outside the cell that is indispensable for cell survival. ...

Simultaneous Measurements of Intracellular Calcium and Membrane Potential in Freshly Isolated and Intact Mouse Cerebral Endothelium

Cerebral arteries and their respective microcirculation deliver oxygen and nutrients to the brain via blood flow regulation. Endothelial cells line the lumen of blood vessels and command changes in vascular diameter as needed to meet the metabolic demand of neurons. Primary endothelial-dependent signaling pathways of hyperpolarization of membrane potential (Vm) and nitric oxide typically operate in parallel to mediate vasodilation and thereby increase blood flow. Although integral to coordinating vasodilation over several millimeters of vascular length, components of endothelium-derived hyperpolarization (EDH) have been historically difficult to measure. These components of EDH entail intracellular Ca2+ [Ca2+]i increases and subsequent activation of small- and intermediate conductance Ca2+-activated K+ (SKCa/IKCa) channels.
Here, we present a simplified illustration of the isolation of fresh endothelium from mouse cerebral arteries; simultaneous measurements of endothelial [Ca2+]i and Vm using Fura-2 photometry and intracellular sharp electrodes, respectively; and a continuous superfusion of salt solutions and pharmacological agents under physiological conditions (pH 7.4, 37 &#176;C). Posterior cerebral arteries from the Circle of Willis are removed free of the posterior communicating and the basilar arteries. Enzymatic digestion of cleaned posterior cerebral arterial segments and subsequent trituration facilitates removal of adventitia, perivascular nerves, and smooth muscle cells. Resulting posterior cerebral arterial endothelial &#34;tubes&#34; are then secured under a microscope and examined using a camera, photomultiplier tube, and one to two electrometers while under continuous superfusion. Collectively, this method can simultaneously measure changes in endothelial [Ca2+]i and Vm in discrete cellular locations, in addition to the spreading of EDH through gap junctions up to millimeter distances along the intact endothelium. This method is expected to yield a high-throughput analysis of the cerebral endothelial functions underlying mechanisms of blood flow regulation in the normal and diseased brain.

Demonstrated here are protocols for (1) freshly isolating intact cerebral endothelial "tubes" and (2) simultaneous measurements of endothelial calcium and membrane potential during endothelium-derived hyperpolarization. Further, these methods allow for pharmacological tuning of endothelial cell calcium and electrical signaling as individual or interactive experimental variables.

Cerebral arteries and their respective microcirculation deliver oxygen and nutrients to the brain via blood flow regulation. Endothelial cells line the ...

Studying the Activity of Neuropeptides and Other Regulators of the Excretory System in the Adult Mosquito

Studies of insect physiology, particularly in those species that are vectors of pathogens causing disease in humans and other vertebrates, provide the foundation to develop novel strategies for pest control. Here, a series of methods are described that are routinely utilized to determine the functional roles of neuropeptides and other neuronal factors (i.e., biogenic amines) on the excretory system of the mosquito, Aedes aegypti. The Malpighian tubules (MTs), responsible for primary urine formation, can continue functioning for hours when removed from the mosquito, allowing for fluid secretion measurements following hormone treatments. As such, the Ramsay assay is a useful technique to measure secretion rates from isolated MTs. Ion-selective microelectrodes (ISME) can sequentially be used to measure ion concentrations (i.e., Na+ and K+) in the secreted fluid. This assay allows for the measurement of several MTs at a given time, determining the effects of various hormones and drugs. The Scanning Ion-selective Electrode Technique uses ISME to measure voltage representative of ionic activity in the unstirred layer adjacent to the surface of ion transporting organs to determine transepithelial transport of ions in near real time. This method can be used to understand the role of hormones and other regulators on ion absorption or secretion across epithelia. Hindgut contraction assays are also a useful tool to characterize myoactive neuropeptides, that may enhance or reduce the ability of this organ to remove excess fluid and waste. Collectively, these methods provide insight into how the excretory system is regulated in adult mosquitoes. This is important because functional coordination of the excretory organs is crucial in overcoming challenges such as desiccation stress after eclosion and before finding a suitable vertebrate host to obtain a bloodmeal.

This protocol outlines methodologies behind the Ramsay assay, ion-selective microelectrodes, Scanning Ion-selective Electrode Technique (SIET) and in vitro contraction assays, applied to study the adult mosquito excretory system, comprised of the Malpighian tubules and hindgut, to collectively measure ion and fluid secretion rates, contractile activity, and transepithelial ion transport.

Studies of insect physiology, particularly in those species that are vectors of pathogens causing disease in humans and other vertebrates, provide the ...

Extracellular Flux Assay: A Method for Measuring Metabolic Profile of Cells

Source: Hlozkova K. et al. <a href="https://www.jove.com/t/58426">Assessment of the Metabolic Profile of Primary Leukemia Cells.</a> J. Vis. Exp. (2018).
This protocol provides a biochemical approach for measuring extracellular acidification rate (ECAR) and oxygen consumption rate (OCR) of primary leukemic cells with extracellular flux assay. The assessment of the metabolic profile of these cells helps to characterize their demands, leading to personalized therapy.

Source: Hlozkova K. et al. Assessment of the Metabolic Profile of Primary Leukemia Cells. J. Vis. Exp. (2018).
This protocol provides a biochemical ...

JoVE Encyclopedia of Experiments

Cancer Research

Leukemia

In vitro techniques & assays

Optogenetic Electrophysiology Recording of Cells with Light-Dependent Ion Channels

Source: Grimm, C., et al. <a href="https://app.jove.com/v/55497">Whole-cell patch-clamp recordings for electrophysiological determination of ion selectivity in channelrhodopsins.</a> J. Vis. Exp. (2017)
In this video, embryonic kidney cell cultures expressing light-dependent ion channel proteins fused with a fluorescent reporter demonstrate the effect of light on ion movement. Whole-cell patch clamp is used to monitor the movement of ions.

1. Setup Prior to Recordings

	Prepare the recording solutions with ion concentrations as indicated in Table 1.&#8203;
	
		Prepare 15 mL of 2 M stock ...

Measurement of Extracellular Ion Fluxes Using the Ion-selective Self-referencing Microelectrode Technique

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