Membrane capacitance, measurement

A variety of methods have been developed to study exocytosis. Neurotransmitter and hormone release can be measured by the electrical effects of released neurotransmitter or hormone on postsynaptic membrane receptors, such as the neuromuscular junction (NMJ see below), and directly by biochemical assay. Another direct measure of exocytosis is the increase in membrane area due to the incorporation of the secretory granule or vesicle membrane into the plasma membrane. This can be measured by increases in membrane capacitance (Cm). Cm is directly proportional to membrane area and is defined as Cm = QAJV, where Cm is the membrane capacitance in farads (F), Q is the charge across the membrane in coulombs (C), V is voltage (V) and Am is the area of the plasma membrane (cm2). The specific capacitance, Q/V, is the amount of charge that must be deposited across 1 cm2 of membrane to change the potential by IV. The specific capacitance, mainly determined by the thickness and dielectric constant of the phospholipid bilayer membrane, is approximately 1 pF/cm2 for intracellular organelles and the plasma membrane. Therefore, the increase in plasma membrane area due to exocytosis is proportional to the increase in Cm. 

The electrophysiological technique used to measure changes in membrane capacitance is the patch clamp [5,6] in the whole-cell recording mode, where the plasma membrane patch in the pipet is ruptured. In another configuration of the patch clamp, the plasma membrane patch is maintained intact. In this case, small currents due to the opening of individual channels can be measured in the membrane patch. The whole-cell patch clamp... 

Rat peritoneal mast-cell exocytosis (as monitored by membrane capacitance measurements) in response to either antigenic stimulation or to the intracellular perfusion with guanine nucleotides (for example, GTP[AS]), occurs after a measurable lag period which has been suggested to be due to the involvement of a GTP-binding regulatory protein [202]. In contrast, stimula-... 

Thinning of the BLM can also be monitored by capacitance measurements across the membrane [390]. Time-dependent capacitance values increase to a plateau beyond which they remain the same (to 0.2%) within the lifetime of the BLM. The measured total capacitance at its plateau value, C together with a knowledge of the surface area of the BLM, Am (determined microscopically), allow an assessment of the thickness of the hydrocarbon layer in the BLM, <5h, from the relationship ... 

Membrane capacitance and conductance were measured using the Wayne-Kerr admittance bridge and measurements were made between 100 Hz and 20 KHz using a PAR lock-in amplifier model 124. One of the results obtained is shown in Figure 1. The dotted curve shown in this figure indicates measured values at high frequencies and the downward slope arises from the presence of electrolyte solutions between the membrane and electrodes. 

Figure 1. Membrane capacitance and conductance of lecithin bilayers. Curve 1 membrane capacitance ((— X —) measured values at high frequencies before correction for series resistance) Curve 2 membrane conductance ((— X —) measured conductances without correction for series resistance).

Giant axons from squid have a large diameter ranging from 300 to 700 ym. Because of this large size, we are able to insert metal electrodes directly into the axon and measure capacitance and conductance across the membrane. This is the most unequivocal method to measure transmembrane capacitance and is far better than the use of external electrodes as done previously by Cole and Curtis (11). In spite of the simplicity and ease of this technique, there are still a few unsolved problems which will be discussed later. Figure 3 shows one of the exemplary results of nerve membrane capacitance and conductance measurements. Comparison of this result with the one shown in Figure 2 readily demonstrates that there are considerable differences between these two sets of curves. 

Table 1 EIS data (membrane capacitance and resistance) and SPR data (thickness measurements) obtained from LGP based bilayers...

Measurements of Capacitance Change. All measurements reported here were made in 10 ml phosphate buffered saline (pH 7.4) and the test device had no selective membrane. All experiments were conducted at room temperature. 

W. M. Arnold and U. Zimmerman, Rotating-freld-indnced rotation and measurement of the membrane capacitance of single mesophyll cells of Avena sativa, Z Naturforsch., 37c, 908-915 (1982). 

Progress was recently made in the construction of an AC admittance modulation system for surface-stabilized lipid membrane biosensors that operated on the basis of the control of the ion permeation by artificial ion channels [40]. A portable admittance modulation measurement device was designed to measure both the in-phase and out-of-phase signal components for determination of the effective ion current and membrane capacitance, respectively [40]. The sensitivity and detection limit of this AC system were tested by studying the interaction of valinomycin with planar BLMs. The electrochemical phenomena were monitored through the in-phase component and measured as conductance changes of the membrane, providing a detection limit of 1 nM for valinomycin. 

Figure 10.2 A universal equivalent circuit for the photoelectric effect. The photochemical event is represented by an RC network including (i) the photoemf ( p(0). (ii) the internal resistance (Rp) of the photocurrent source, (iii) the chemical capacitance (C,), and (iv) the transmembrane resistance Rp). With the exception of a strictly short-circuit measurement, the time course of the photocurrent so generated is further shaped via interaction with the RC network formed by (i) the membrane resistance (/ ,), (ii) the membrane capacitance (C ), and (iii) the access resistance Re). The access resistance (impedance) includes the input impedance of the amplifier, the electrode impedance, and the impedance of the intervening electrolyte solution. (Reproduced from [17].)...

In this simplified form, the membrane capacitance, Cm, is in series with Cox. Thus, Cm = CoxCt/(Ct - Cox), where Ct is the total capacitance of the surface-bound membrane electrode and Cox is the measured capacitance of the electrode before membrane formation. The membrane capacitance, Cm, can thus be estimated at a single frequency. Further, the capacitance of the tightly packed bilayer, Cbl, can be calculated from Cm if the coverage factor and the double-layer capacitance are known Cbl = (Cm — (1 — 0)Cdl)/0. 

Surface-bound membranes formed on PtO electrodes were chemically and mechanically stable. The PtO-OTS-Rh electrodes were monitored by measuring the capacitance while the electrodes were kept in buffer at 4 °C for 11 days. Any dissolution of the surface-bound membrane would result in an increase in capacitance. Little change in capacitance was observed, which indicates that the membranes are stable. 

Here, the membrane capacitance and resistance are chosen as two principle parameters in monitoring the formation of different concentrated unmodified s-BLMs. The time-resolved capatance and resistance are measured following the model described above, based on the recorded s-BLM voltammogram. 

Electrical properties of tissue and cell suspensions were reviewed by Schwan (1957). Membrane capacitance and conductance for pancreatic P-cells were measured by an electrokinetic method (Pethig et al. 2005 Grimnes and Martinsen 2008). The conductivity of blood is dependent on blood velocity this is the Sigman effect (Sigman et al. 1937). 

Also, deoxyribonucleic acid (DNA) molecules and their electrical conduction have been examined the electrical conduction through DNA molecules (Fink and Schonenberger 1999) and resonances in the dielectric absorption of DNA (Foster et al. 1987). Recently, the Pethig group (Chung et al. 2011) has shown that measurement results in a cell suspension up to iO MHz that is dependent on the cytoplasma membrane capacitance and resistance, the ceU diameter, and the suspension conductivity. By using special interdigitated electrodes, the ceU membrane capacitive reactance sort out the resistance above 100 MHz so that the electric field penetrates into the ceU interior and intracellular dielectric properties can be measured. 

Pethig R, Jakubek LM, Sanger RH, Heart E, Corson ED, Smith PJS. 2005. Electrokinetic measurements of membrane capacitance and conductance for pancreatic P-cells. lEE Proc Nanobiotechnol 152(6), 189-193. 

Measurement of Membrane Capacitance. A sufficiently high frequency of light modulation is chosen so that Rm is large compared to the AC impedance (i.e. capacitive reactance) of the membrane. Therefore, in depletion, I can still be described by the relative magnitudes of the internal and external capacitances (Equation 4). As long as Cm Ci (which can be controlled by the size of the illuminated region on the insulated semiconductor), Equation 4 can be transformed into a linear form (Equation 4a). 

When Rm is at least three orders of magnitude larger than Ref the fast transient can be considered as almost instantaneous and that term can be eliminated from the expression. Near I pipr HI responds with the same time constant as Vt to a step in bias potential (Equation 8). lo is the measured alternating photocurrent amplitude immediately after distribution of the step in potential across the capacitances. If is the measured alternating photocurrent amplitude after complete dissipation of the potential across the membrane capacitance. When In(If - II ) is plotted vs. time, the slope is equal to -I/12 (Equation 9). can then be calculated from Equation 6. 

In this way, by measuring Uf and Cm, or //, Co and c, termine experimentally the fiexocoefiicient /. Evaluation of the membrane curvature can be performed electrically from the second harmonic of the membrane capacitance current with a non-zero voltage clamp (the condenser microphone effect) and supposing spherical curvature. The actual curvature c - - C2 of a black lipid membrane (BLM) can be measured interferometrically. " ... 

If suddenly the channels (Figure 5.5) open so that the intracellular potential changes abruptly, the ions must also supply a transient discharge current of the membrane capacitance. At the extracellular side, the current is not with respect to a far-away reference electrode, but concentrated to an interstitial fluid zone near the cell. The current flow can be modeled with local current dipoles and is clearly measurable with unipolar or bipolar pickup electrodes in the interstitial liquid. When the cell is depolarizing, cations... 

The interior has a well-defined electrical capacitance with respect to the outside. Double layers are formed in the electrolyte/membrane interphase both internally and externally. The measured intracellular potential includes the potential of these charged double layers (Section 7.5). A double layer has a thickness on the order of 0.1—10 nm, and the cell membrane is approximately 7 nm. The total measured BLM capacitance, double layers included, is about 1 pF/cm. The charge q necessary to obtain a voltage U across a capacitor is q — CU [coulomb]. With a cell radius of 2 pm, the membrane area is A = 4Tcr = 50 X 10 [m ]. With v = 60 mV, the charge is q = 0.5 pC. Because a... 

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