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The voltage clamp operates by negative feedback. The membrane potentialamplifier measures membranevoltage and sends output to the feedback amplifier; this subtracts the membrane voltage from the command voltage, which it receives from the signal generator. This signal is amplified and output is sent into the axon via the current-passing electrode.
The voltage clamp is an experimental method used by electrophysiologists to measure the ioncurrents through the membranes of excitable cells, such as neurons, while holding the membrane voltage at a set level.[1] A basic voltage clamp will iteratively measure the membrane potential, and then change the membrane potential (voltage) to a desired value by adding the necessary current. This 'clamps' the cell membrane at a desired constant voltage, allowing the voltage clamp to record what currents are delivered. Because the currents applied to the cell must be equal to (and opposite in charge to) the current going across the cell membrane at the set voltage, the recorded currents indicate how the cell reacts to changes in membrane potential.[2] Cell membranes of excitable cells contain many different kinds of ion channels, some of which are voltage-gated. The voltage clamp allows the membrane voltage to be manipulated independently of the ionic currents, allowing the current-voltage relationships of membrane channels to be studied.[3]
History[edit]
The concept of the voltage clamp is attributed to Kenneth Cole[4] and George Marmont[5] in the spring of 1947.[6] They inserted an internal electrode into the giant axon of a squid and began to apply a current. Cole discovered that it was possible to use two electrodes and a feedback circuit to keep the cell'smembrane potential at a level set by the experimenter.
Cole developed the voltage clamp technique before the era of microelectrodes, so his two electrodes consisted of fine wires twisted around an insulating rod. Because this type of electrode could be inserted into only the largest cells, early electrophysiological experiments were conducted almost exclusively on squid axons.
A personal photo of Kenneth Cole, given to Dr. J. Walter Woodbury
Squids squirt jets of water when they need to move quickly, as when escaping a predator. To make this escape as fast as possible, they have an axon that can reach 1 mm in diameter (signals propagate more quickly down large axons). The squid giant axon was the first preparation that could be used to voltage clamp a transmembrane current, and it was the basis of Hodgkin and Huxley's pioneering experiments on the properties of the action potential.[6]
Alan Hodgkin realized that, to understand ion flux across the membrane, it was necessary to eliminate differences in membrane potential.[7] Using experiments with the voltage clamp, Hodgkin and Andrew Huxley published 5 papers in the summer of 1952 describing how ionic currents give rise to the action potential.[8] The final paper proposed the HodgkinâHuxley model which mathematically describes the action potential. The use of voltage clamps in their experiments to study and model the action potential in detail has laid the foundation for electrophysiology; for which they shared the 1963 Nobel Prize in Physiology or Medicine.[7]
Technique[edit]
The voltage clamp is a current generator. Transmembrane voltage is recorded through a 'voltage electrode', relative to ground, and a 'current electrode' passes current into the cell. The experimenter sets a 'holding voltage', or 'command potential', and the voltage clamp uses negative feedback to maintain the cell at this voltage. The electrodes are connected to an amplifier, which measures membrane potential and feeds the signal into a feedback amplifier. This amplifier also gets an input from the signal generator that determines the command potential, and it subtracts the membrane potential from the command potential (Vcommand â Vm), magnifies any difference, and sends an output to the current electrode. Whenever the cell deviates from the holding voltage, the operational amplifier generates an 'error signal', that is the difference between the command potential and the actual voltage of the cell. The feedback circuit passes current into the cell to reduce the error signal to zero. Thus, the clamp circuit produces a current equal and opposite to the ionic current.
Variations of the voltage clamp technique[edit]Two-electrode voltage clamp using microelectrodes[edit]![]()
Two-electrode voltage clamp
The two-electrode voltage clamp (TEVC) technique is used to study properties of membrane proteins, especially ion channels.[9] Researchers use this method most commonly to investigate membrane structures expressed in Xenopusoocytes. The large size of these oocytes allows for easy handling and manipulability.[10]
The TEVC method utilizes two low-resistance pipettes, one sensing voltage and the other injecting current. The microelectrodes are filled with conductive solution and inserted into the cell to artificially control membrane potential. The membrane acts as a dielectric as well as a resistor, while the fluids on either side of the membrane function as capacitors.[10] The microelectrodes compare the membrane potential against a command voltage, giving an accurate reproduction of the currents flowing across the membrane. Current readings can be used to analyze the electrical response of the cell to different applications.
This technique is favored over single-microelectrode clamp or other voltage clamp techniques when conditions call for resolving large currents. The high current-passing capacity of the two-electrode clamp makes it possible to clamp large currents that are impossible to control with single-electrode patch techniques.[11] The two-electrode system is also desirable for its fast clamp settling time and low noise. However, TEVC is limited in use with regard to cell size. It is effective in larger-diameter oocytes, but more difficult to use with small cells. Additionally, TEVC method is limited in that the transmitter of current must be contained in the pipette. It is not possible to manipulate the intracellular fluid while clamping, which is possible using patch clamp techniques.[12] Another disadvantage involves 'space clamp' issues. Cole's voltage clamp used a long wire that clamped the squid axon uniformly along its entire length. TEVC microelectrodes can provide only a spatial point source of current that may not uniformly affect all parts of an irregularly shaped cell.
Dual-cell voltage clamp[edit]
The dual-cell voltage clamp technique is a specialized variation of the two electrode voltage clamp, and is only used in the study of gap junction channels.[13] Gap junctions are pores that directly link two cells through which ions and small metabolites flow freely. When two cells in which gap junction proteins are expressed either endogenously or via injection of mRNA, a junction channel will form between the cells. Since two cells are present in the system, two sets of electrodes are used. A recording electrode and a current injecting electrode are inserted into each cell, and each cell is clamped individually (each set of electrodes is attached to a separate apparatus, and integration of data is performed by computer). To record junctional conductance, the current is varied in the first cell while the recording electrode in the second cell records any changes in Vm for the second cell only. (The process can be reversed with the stimulus occurring in the second cell and recording occurring in the first cell.) Since no variation in current is being induced by the electrode in the recorded cell, any change in voltage must be induced by current crossing into the recorded cell, through the gap junction channels, from the cell in which the current was varied.[13]
Single-electrode voltage clamp[edit]
This category describes a set of techniques in which one electrode is used for voltage clamp. Continuous single-electrode clamp (SEVC-c) technique is often used with patch-clamp recording. Discontinuous single-electrode voltage-clamp (SEVC-d) technique is used with penetrating intracellular recording. This single electrode carries out the functions of both current injection and voltage recording.
Continuous single-electrode clamp (SEVC-c)[edit]
The 'patch-clamp' technique allows the study of individual ion channels. It uses an electrode with a relatively large tip (> 1 micrometer) that has a smooth surface (rather than a sharp tip). This is a 'patch-clamp electrode' (as distinct from a 'sharp electrode' used to impale cells). This electrode is pressed against a cell membrane and suction is applied to pull the cell's membrane inside the electrode tip. The suction causes the cell to form a tight seal with the electrode (a 'gigaohm seal', as the resistance is more than a gigaohm).
SEV-c has the advantage that you can record from small cells that would be impossible to impale with two electrodes. However:
Discontinuous single-electrode voltage-clamp (SEVC-d)[edit]
A single-electrode voltage clamp â discontinuous, or SEVC-d, has some advantages over SEVC-c for whole-cell recording. In this, a different approach is taken for passing current and recording voltage. A SEVC-d amplifier operates on a 'time-sharing' basis, so the electrode regularly and frequently switches between passing current and measuring voltage. In effect, there are two electrodes, but each is in operation for only half of the time it is on. The oscillation between the two functions of the single electrode is termed a duty cycle. During each cycle, the amplifier measures the membrane potential and compares it with the holding potential. An operational amplifier measures the difference, and generates an error signal. This current is a mirror image of the current generated by the cell. The amplifier outputs feature sample and hold circuits, so each briefly sampled voltage is then held on the output until the next measurement in the next cycle. To be specific, the amplifier measures voltage in the first few microseconds of the cycle, generates the error signal, and spends the rest of the cycle passing current to reduce that error. At the start of the next cycle, voltage is measured again, a new error signal generated, current passed etc. The experimenter sets the cycle length, and it is possible to sample with periods as low as about 15 microseconds, corresponding to a 67 kHz switching frequency. Switching frequencies lower than about 10 kHz are not sufficient when working with action potentials that are less than 1 millisecond wide. Note that not all discontinuous voltage-clamp amplifier support switching frequencies higher than 10 kHz.[14]
For this to work, the cell capacitance must be higher than the electrode capacitance by at least an order of magnitude. Capacitance slows the kinetics (the rise and fall times) of currents. If the electrode capacitance is much less than that of the cell, then when current is passed through the electrode, the electrode voltage will change faster than the cell voltage. Thus, when current is injected and then turned off (at the end of a duty cycle), the electrode voltage will decay faster than the cell voltage. As soon as the electrode voltage asymptotes to the cell voltage, the voltage can be sampled (again) and the next amount of charge applied. Thus, the frequency of the duty cycle is limited to the speed at which the electrode voltage rises and decays while passing current. The lower the electrode capacitance the faster one can cycle.
SEVC-d has a major advantage over SEVC-c in allowing the experimenter to measure membrane potential, and, as it obviates passing current and measuring voltage at the same time, there is never a series resistance error. The main disadvantages are that the time resolution is limited and the amplifier is unstable. If it passes too much current, so that the goal voltage is over-shot, it reverses the polarity of the current in the next duty cycle. This causes it to undershoot the target voltage, so the next cycle reverses the polarity of the injected current again. This error can grow with each cycle until the amplifier oscillates out of control (âringingâ); this usually results in the destruction of the cell being recorded. The investigator wants a short duty cycle to improve temporal resolution; the amplifier has adjustable compensators that will make the electrode voltage decay faster, but, if these are set too high the amplifier will ring, so the investigator is always trying to âtuneâ the amplifier as close to the edge of uncontrolled oscillation as possible, in which case small changes in recording conditions can cause ringing. There are two solutions: to âback offâ the amplifier settings into a safe range, or to be alert for signs that the amplifier is about to ring.
References[edit]
Further reading[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Voltage_clamp&oldid=906223791'
A bacterial spheroplast patched with a glass pipette
A patch clamp recording of current reveals transitions between two conductance states of a single ion channel: closed (at top) and open (at bottom).
The patch clamp technique is a laboratory technique in electrophysiology used to study ionic currents in individual isolated living cells, tissue sections, or patches of cell membrane. The technique is especially useful in the study of excitable cells such as neurons, cardiomyocytes, muscle fibers, and pancreaticbeta cells, and can also be applied to the study of bacterial ion channels in specially prepared giant spheroplasts.
Patch clamping can be performed using the voltage clamp technique. In this case, the voltage across the cell membrane is controlled by the experimenter and the resulting currents are recorded. Alternatively, the current clamp technique can be used. In this case the current passing across the membrane is controlled by the experimenter and the resulting changes in voltage are recorded, generally in the form of action potentials.
Erwin Neher and Bert Sakmann developed the patch clamp in the late 1970s and early 1980s. This discovery made it possible to record the currents of single ion channel molecules for the first time, which improved understanding of the involvement of channels in fundamental cell processes such as action potentials and nerve activity. Neher and Sakmann received the Nobel Prize in Physiology or Medicine in 1991 for this work.[1]
Basic technique[edit]Set-up[edit]
Classical patch clamp setup, with microscope, antivibration table, and micromanipulators
During a patch clamp recording, a hollow glass tube known as a micropipette or patch pipette filled with an electrolyte solution and a recording electrode connected to an amplifier is brought into contact with the membrane of an isolated cell. Another electrode is placed in a bath surrounding the cell or tissue as a reference ground electrode. An electrical circuit can be formed between the recording and reference electrode with the cell of interest in between.
Schematic depiction of a pipette puller device used to prepare micropipettes for patch clamp and other recordings
Circuit formed during whole-cell or perforated patch clamp
The solution filling the patch pipette might match the ionic composition of the bath solution, as in the case of cell-attached recording, or match the cytoplasm, for whole-cell recording. The solution in the bath solution may match the physiological extracellular solution, the cytoplasm, or be entirely non-physiological, depending on the experiment to be performed. The researcher can also change the content of the bath solution (or less commonly the pipette solution) by adding ions or drugs to study the ion channels under different conditions.
Depending on what the researcher is trying to measure, the diameter of the pipette tip used may vary, but it is usually in the micrometer range.[2] This small size is used to enclose a membrane surface area or 'patch' that often contains just one or a few ion channel molecules.[3] This type of electrode is distinct from the 'sharp microelectrode' used to puncture cells in traditional intracellular recordings, in that it is sealed onto the surface of the cell membrane, rather than inserted through it.
Typical equipment used during classical patch clamp recording
In some experiments, the micropipette tip is heated in a microforge to produce a smooth surface that assists in forming a high resistance seal with the cell membrane. To obtain this high resistance seal, the micropipette is pressed against a cell membrane and suction is applied. A portion of the cell membrane is suctioned into the pipette, creating an omega-shaped area of membrane which, if formed properly, creates a resistance in the 10â100 gigaohms range, called a 'gigaohm seal' or 'gigaseal'.[3] The high resistance of this seal makes it possible to isolate electronically the currents measured across the membrane patch with little competing noise, as well as providing some mechanical stability to the recording.[4]
Recording[edit]Patch Clamp Method
Patch clamp of a nerve cell within a slice of brain tissue. The pipette in the photograph has been marked with a slight blue color.
Many patch clamp amplifiers do not use true voltage clamp circuitry, but instead are differential amplifiers that use the bath electrode to set the zero current (ground) level. This allows a researcher to keep the voltage constant while observing changes in current. To make these recordings, the patch pipette is compared to the ground electrode. Current is then injected into the system to maintain a constant, set voltage. However much current is needed to clamp the voltage is opposite in sign and equal in magnitude to the current through the membrane.[3]
Alternatively, the cell can be current clamped in whole-cell mode, keeping current constant while observing changes in membrane voltage.[5]
Variations[edit]
Diagram showing variations of the patch clamp technique
Several variations of the basic technique can be applied, depending on what the researcher wants to study. The inside-out and outside-out techniques are called 'excised patch' techniques, because the patch is excised (removed) from the main body of the cell. Cell-attached and both excised patch techniques are used to study the behavior of individual ion channels in the section of membrane attached to the electrode.
Whole-cell patch and perforated patch allow the researcher to study the electrical behavior of the entire cell, instead of single channel currents. The whole-cell patch, which enables low-resistance electrical access to the inside of a cell, has now largely replaced high-resistance microelectrode recording techniques to record currents across the entire cell membrane.
Cell-attached patch[edit]
Cell-attached patch configuration
For this method, the pipette is sealed onto the cell membrane to obtain a gigaseal, while ensuring that the cell membrane remains intact. This allows the recording of currents through single, or a few, ion channels contained in the patch of membrane captured by the pipette. By only attaching to the exterior of the cell membrane, there is very little disturbance of the cell structure.[3] Also, by not disrupting the interior of the cell, any intracellular mechanisms normally influencing the channel will still be able to function as they would physiologically.[6] Using this method it is also relatively easy to obtain the right configuration, and once obtained it is fairly stable.[7]
For ligand-gated ion channels or channels that are modulated by metabotropic receptors, the neurotransmitter or drug being studied is usually included in the pipette solution, where it can interact with what used to be the external surface of the membrane. The resulting channel activity can be attributed to the drug being used, although it is usually not possible to then change the drug concentration inside the pipette. The technique is thus limited to one point in a dose response curve per patch. Therefore, the dose response is accomplished using several cells and patches. However, voltage-gated ion channels can be clamped successively at different membrane potentials in a single patch. This results in channel activation as a function of voltage, and a complete I-V (current-voltage) curve can be established in only one patch. Another potential drawback of this technique is that, just as the intracellular pathways of the cell are not disturbed, they cannot be directly modified either.[7]
Inside-out patch[edit]
Inside-out patch configuration
In the inside-out method, a patch of the membrane is attached to the patch pipette, detached from the rest of the cell, and the cytosolic surface of the membrane is exposed to the external media, or bath.[8] One advantage of this method is that the experimenter has access to the intracellular surface of the membrane via the bath and can change the chemical composition of what the surface of the membrane is exposed to. This is useful when an experimenter wishes to manipulate the environment at the intracellular surface of single ion channels. For example, channels that are activated by intracellular ligands can then be studied through a range of ligand concentrations.
To achieve the inside-out configuration, the pipette is attached to the cell membrane as in the cell-attached mode, forming a gigaseal, and is then retracted to break off a patch of membrane from the rest of the cell. Pulling off a membrane patch often results initially in the formation of a vesicle of membrane in the pipette tip, because the ends of the patch membrane fuse together quickly after excision. The outer face of the vesicle must then be broken open to enter into inside-out mode; this may be done by briefly taking the membrane through the bath solution/air interface, by exposure to a low Ca2+ solution, or by momentarily making contact with a droplet of paraffin or a piece of cured silicone polymer.[9]
Whole-cell recording or whole-cell patch[edit]
Whole-cell patch configuration
Whole-cell recordings involve recording currents through multiple channels simultaneously, over the membrane of the entire cell. The electrode is left in place on the cell, as in cell-attached recordings, but more suction is applied to rupture the membrane patch, thus providing access from the interior of the pipette to the intracellular space of the cell. This provides a means to administer and study how treatments (ex. drugs) can affect cells in real time.[10] Once the pipette is attached to the cell membrane, there are two methods of breaking the patch. The first is by applying more suction. The amount and duration of this suction depends on the type of cell and size of the pipette. The other method requires a large current pulse to be sent through the pipette. How much current is applied and the duration of the pulse also depend on the type of cell.[7] For some types of cells, it is convenient to apply both methods simultaneously to break the patch.
The advantage of whole-cell patch clamp recording over sharp electrode technique recording is that the larger opening at the tip of the patch clamp electrode provides lower resistance and thus better electrical access to the inside of the cell.[11][10] A disadvantage of this technique is that because the volume of the electrode is larger than the volume of the cell, the soluble contents of the cell's interior will slowly be replaced by the contents of the electrode. This is referred to as the electrode 'dialyzing' the cell's contents.[7] After a while, any properties of the cell that depend on soluble intracellular contents will be altered. The pipette solution used usually approximates the high-potassium environment of the interior of the cell to minimize any changes this may cause. There is often a period at the beginning of a whole-cell recording when one can take measurements before the cell has been dialyzed.[7]
Outside-out patch[edit]Patch Clamp Methods
Outside-out patch formation technique. In order: top-left, top-right, bottom-left, bottom-right
The name 'outside-out' emphasizes both this technique's complementarÂity to the inside-out technique, and the fact that it places the external rather than intracellular surface of the cell membrane on the outside of the patch of membrane, in relation to the patch electrode.[6]
The formation of an outside-out patch begins with a whole-cell recording configuration. After the whole-cell configuration is formed, the electrode is slowly withdrawn from the cell, allowing a bulb of membrane to bleb out from the cell. When the electrode is pulled far enough away, this bleb will detach from the cell and reform as a convex membrane on the end of the electrode (like a ball open at the electrode tip), with the original outside of the membrane facing outward from the electrode.[6] As the image at the right shows, this means that the fluid inside the pipette will be simulating the intracellular fluid, while a researcher is free to move the pipette and the bleb with its channels to another bath of solution. While multiple channels can exist in a bleb of membrane, single channel recordings are also possible in this conformation if the bleb of detached membrane is small and only contains one channel.[12]
Outside-out patching gives the experimenter the opportunity to examine the properties of an ion channel when it is isolated from the cell and exposed successively to different solutions on the extracellular surface of the membrane. The experimenter can perfuse the same patch with a variety of solutions in a relatively short amount of time, and if the channel is activated by a neurotransmitter or drug from the extracellular face, a dose-response curve can then be obtained.[13] This ability to measure current through exactly the same piece of membrane in different solutions is the distinct advantage of the outside-out patch relative to the cell-attached method. On the other hand, it is more difficult to accomplish. The longer formation process involves more steps that could fail and results in a lower frequency of usable patches.
Perforated patch[edit]
Perforated patch technique
This variation of the patch clamp method is very similar to the whole-cell configuration. The main difference lies in the fact that when the experimenter forms the gigaohm seal, suction is not used to rupture the patch membrane. Instead, the electrode solution contains small amounts of an antifungal or antibiotic agent, such as amphothericin-B, nystatin, or gramicidin, which diffuses into the membrane patch and forms small pores in the membrane, providing electrical access to the cell interior.[14] When comparing the whole-cell and perforated patch methods, one can think of the whole-cell patch as an open door, in which there is complete exchange between molecules in the pipette solution and the cytoplasm. The perforated patch can be likened to a screen door that only allows the exchange of certain molecules from the pipette solution to the cytoplasm of the cell.
Advantages of the perforated patch method, relative to whole-cell recordings, include the properties of the antibiotic pores, that allow equilibration only of small monovalent ions between the patch pipette and the cytosol, but not of larger molecules that cannot permeate through the pores. This property maintains endogenous levels of divalent ions such as Ca2+ and signaling molecules such as cAMP. Consequently, one can have recordings of the entire cell, as in whole-cell patch clamping, while retaining most intracellular signaling mechanisms, as in cell-attached recordings. As a result, there is reduced current rundown, and stable perforated patch recordings can last longer than one hour.[14] Disadvantages include a higher access resistance, relative to whole-cell, due to the partial membrane occupying the tip of the electrode. This may decrease current resolution and increase recording noise. It can also take a significant amount of time for the antibiotic to perforate the membrane (about 15 minutes for amphothericin-B, and even longer for gramicidin and nystatin). The membrane under the electrode tip is weakened by the perforations formed by the antibiotic and can rupture. If the patch ruptures, the recording is then in whole-cell mode, with antibiotic contaminating the inside of the cell.[14]
Loose patch[edit]
Loose patch clamp technique
Loose patch clamp is different from the other techniques discussed here in that it employs a loose seal (low electrical resistance) rather than the tight gigaseal used in the conventional technique. This technique was used as early as the year 1961, as described in a paper by Strickholm on the impedance of a muscle cell's surface,[15] but received little attention until being brought up again and given a name by Almers, Stanfield, and Stühmer in 1982,[16] after patch clamp had been established as a major tool of electrophysiology.
To achieve a loose patch clamp on a cell membrane, the pipette is moved slowly towards the cell, until the electrical resistance of the contact between the cell and the pipette increases to a few times greater resistance than that of the electrode alone. The closer the pipette gets to the membrane, the greater the resistance of the pipette tip becomes, but if too close a seal is formed, and it could become difficult to remove the pipette without damaging the cell. For the loose patch technique, the pipette does not get close enough to the membrane to form a gigaseal or a permanent connection, nor to pierce the cell membrane.[17] The cell membrane stays intact, and the lack of a tight seal creates a small gap through which ions can pass outside the cell without entering the pipette.
A significant advantage of the loose seal is that the pipette that is used can be repeatedly removed from the membrane after recording, and the membrane will remain intact. This allows repeated measurements in a variety of locations on the same cell without destroying the integrity of the membrane. This flexibility has been especially useful to researchers for studying muscle cells as they contract under real physiological conditions, obtaining recordings quickly, and doing so without resorting to drastic measures to stop the muscle fibers from contracting.[16] A major disadvantage is that the resistance between the pipette and the membrane is greatly reduced, allowing current to leak through the seal, and significantly reducing the resolution of small currents. This leakage can be partially corrected for, however, which offers the opportunity to compare and contrast recordings made from different areas on the cell of interest. Given this, it has been estimated that the loose patch technique can resolve currents smaller than 1 mA/cm2.[17]
Automatic patch clamping[edit]
Automated patch clamp systems have recently been developed, in order to collect large amounts of data inexpensively in a shorter period of time. Such systems typically include a single-use microfluidic device, either an injection molded or a polydimethylsiloxane (PDMS) cast chip, to capture a cell or cells, and an integrated electrode.
In one form of such an automated system, a pressure differential is used to force the cells being studied to be drawn towards the pipette opening until they form a gigaseal. Then, by briefly exposing the pipette tip to the atmosphere, the portion of the membrane protruding from the pipette bursts, and the membrane is now in the inside-out conformation, at the tip of the pipette. In a completely automated system, the pipette and the membrane patch can then be rapidly moved through a series of different test solutions, allowing different test compounds to be applied to the intracellular side of the membrane during recording.[18]
Patch Clamp Technique Pdf ConverterSee also[edit]Patch Clamp ProtocolReferences[edit]
External links[edit]Patch Clamp Technique Pdf Free
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