Membrane current noise

Figure 8. Membrane current noise for one-sided application of three different polyene antibiotics of a very close chemical structure shows 1 / -like spectra with intensities proportional to single-channel conductances for these antibiotics (76). Amphotericin B (l), nystatin (2), and mycoheptin (3) spectra refer to the same membrane conductance of 7.1 X 10 8 S at 10 mV.

D. J. M. Poussart, Nerve Membrane Current Noise Direct Measurements Under Voltage Clamp, Proc. Natl Acad. Scl USA 64, 95-99 (1969). 

M.E. Green and M. Yafuso, A study of the noise generated during ion transport across membranes, J. Phys. Chem., 1968, 72, 4072-4078 I. Rubinstein. Mechanism for an electrodiffusional instability in concentration polarization, J. Chem. Soc., Faraday Trans. 2. 1981, 77, 1595-1609 F. Maletzki, H.-W. Rosier and E. Staude, Ion transport across electrodialysis membranes in the overlimiting current density range Stationary voltage current characteristics and current noise power spectra under different conditions of free convection, J. Membr. Sci., 1992, 71, 105-115. 

Nonequilibrium noise generated by carrier-mediated ion transport was studied in lipid bilayers modified by tetranactin (41). As expected, deviations of measured spectral density from the values calculated from the Nyquist formula 1 were found. The instantaneous membrane current was described as the superposition of a steady-state current and a fluctuating current, and for the complex admittance in the Nyquist formula only a small-signal part of the total admittance was taken. The justification of this procedure is occasionally discussed in the literature (see, for example, Tyagai (42) and references cited therein), but is unclear. 

The results of this calculation and comparison to a single-channel recording are presented in Figure 7. The root-mean-square level, which is required to explain 1/f noise in membrane current by transport phenomena through the open channel, is much higher than the actual level. The conclusion is that 1/f noise in an open channel (if it exists) cannot account for 1/f noise in a multichannel membrane. 

Fig. 1. Comparison of the current noise due to a carrier with that due to a pore. (A) Current fluctuations in the presence of valinomycin with lithium (which is poorly transported) and the rubidium (which is well transported) as the cation. Noise increases at high frequencies because the empty carrier must return after carrying one ion across the membrane. (From [8].) (B) Current fluctuations in the presence of alamethicin. Noise decreases at high frequencies because the channels open and close at a limited rate. (From [7].)...

Other phenomena are interesting from the noise point of view. They related to ion transport across membranes/ " equilibrium and nonequilibrium kinetic systems/ nerve membrane noise, " and membrane current fluctuations from ionic channels (Na channels and K channels in axons) in stationary or nonstationary states.Some of these studies have been described in extended reviews. 

L. J. Bruner and J. E. Hall, Autocorrelation Analysis of Hydrophobic Ion Current Noise in Lipid Bilayer Membranes, Biophys. J. 28, 511-514 (1979). 

P. C. Jordan, Current Noise in Transport of Hydrophobic Ions Through Lipid Bilayer Membranes Including Diffusion Polarization in the Aqueous Phase, Biophys. Chem. 12, 1-11 (1980). 

F. Conti, L. J. De Felice, and E. Wanke, Potassium and Sodium Ion Current Noise in the Membrane of Squid Giant Axon, J. Physiol 248, 45-82 (1975). 

The membrane component of the background noise consists mainly of shot noise [8-11], that is the expected electrical noise created by the ions that cross the membrane by leakage or ionic pumps. The spectral density of shot noise is directly proportional to the unidirectional membrane current. Thus, the spectral density of the noise will increase by increasing the surface of the membrane patch, and consequently the total current (leakage and pumps). This implies that the noise conditions will be improved when current is recorded from a small piece of membrane (a patch). 

The best fit of the current noise spectra for membrane potentials around -50 mV was obtained with cj values 3 to 5 times larger than expected according to eqn.(9). The disagreement decreased at more depolarized membrane potentials, vanishing around -30 mV. 

It will be seen that exponential curves of one sort or another crop up everywhere in the study of the rates of reactions. Not only is the change of concentration (or membrane current, etc.) frequently described by an exponential function of time (or by the sum of several such functions), but exponentials also appear in some of the more exotic ways of measuring rates, such as in the study of fluctuations and noise. Furthermore the study of individual molecules, as is possible for some sorts of ion channel, gives rise to probability distributions (e.g. the distribution of the length of time for which an individual channel stays open) that are also described by exponentials (Colquhoun Hawkes, 1983). Unfortunately some of these topics can only be referred to in passing in the present volume. It is, however, important to realize that it is sometimes useful to distinguish between the lifetime of a state (or reaction intermediate) and the rate (or probability) of its decay. 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

Polish the end of the longest wire with fine sandpaper in order to make a flat surface tip end (a sharp end would originate noise current during the measurement). After that, wash the exposed wire with a small amount of HNO3 (1 1) in order to remove any oxide on the surface and finally wipe it well with bidistilled water. Dry it before applying the sensor membrane. 

Patch clamping requires that an electrode, housed within a micropipette, is attached to the cell to make an almost perfect seal with the cell membrane. This generates a very high resistance between the cell and the pipette wall, typically 10 CA2. The resulting transmembrane currents, measured between microelectrodes inside and outside the cell, generate extremely low noise so single channel events can be... 

This technique allows the study of single-ion channels as well as whole-cell ion channel currents. Essentially, the patch-clamp technique is an improved and refined version of the voltage-clamp technique. It requires a low electrical noise borosiUcate glass electrode, also known as a patch electrode or patch pipette, with a relatively large tip (>1 pm) that has a smooth surface rather than a sharp tip as with the conventional microelectrodes. This is a major difference between the patch electrode and the sharp electrode used to impale cells directly through the cell membrane (Figure 16.20). 

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