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...

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 border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>IonAmp&#160;</td>
			<td>&#160;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 &#8220;XBL function matters&#8221; (http://www.xuyue.org/) or from &#8220;YoungerUSA&#8221; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td>
		</tr>
		<tr>
			<td>IonAmp32&#160;</td>
			<td>&#160;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 &#8220;XBL function matters&#8221; (http://www.xuyue.org/) or from &#8220;YoungerUSA&#8221; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td>
		</tr>
		<tr>
			<td>Headstage pre-amplifier</td>
			<td>&#160;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 &#8220;XBL function matters&#8221; (http://www.xuyue.org/) or from &#8220;YoungerUSA&#8221; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td>
		</tr>
		<tr>
			<td>MicroStep Driver</td>
			<td>&#160;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 &#8220;XBL function matters&#8221; (http://www.xuyue.org/) or from &#8220;YoungerUSA&#8221; (http://www.youngerusa.com/) or from Applicable Electronics(http://www.applicableelectronics.com/)</td>
		</tr>
		<tr>
			<td>Manual micropositioner&#160;&#160;</td>
			<td>World Precision Instruments&#160;</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&#160;&#160;&#160;</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&#160;&#160;</td>
			<td>Newport Inc.&#160;&#160;&#160;&#160;&#160;</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&#176;, slotted faces</td>
			<td>Newport Inc.&#160;&#160;&#160;&#160;&#160;</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.&#160;&#160;&#160;&#160;&#160;</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&#160;</td>
			<td>World Precision Instruments&#160;</td>
			<td>TW150-4</td>
			<td>none</td>
		</tr>
		<tr>
			<td>Electrode puller&#160;</td>
			<td>Narishige</td>
			<td>&#160;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&#160;</td>
			<td>Sigma-Aldrich</td>
			<td>85126</td>
			<td>Hazardous, handle as recommended by provider&#160;</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&#160;</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&#160;</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&#160;</td>
			<td>World Precision Instruments</td>
			<td>MEH3S</td>
			<td>with a AgCl(Ag+)pellet inside and a gold 2 mm male connector&#160;</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&#160;</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...

Watch this Scientific Journal Video about Measurement of Extracellular Ion Fluxes Using the Ion-selective Self-referencing Microelectrode Technique at JoVE.com

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 ...

measurement-extracellular-ion-fluxes-using-ion-selective-self

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JoVE Journal

Biology

13.8K vues.  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.