Trans-membrane potentials

The scheme of reactions (5)-(8) is similar in certain aspects to that of electron transfer processes occurring in the membranes of photosynthesiz-ing organisms, where the light absorption also induces a trans-membrane potential difference. The above scheme of processes in multilayer systems explaining the appearance of the photoelectromotive force requires further refinement. But even the data available at present allow these systems to be regarded firstly as convenient models for analyzing the processes occurring... 

Electrical breakdown of the membrane sets an upper limit on the fields that can be applied. Controlled electrical breakdown of the membrane is a fairly well understood process which is used in the techniques of electrofusion and electroporation but it is clearly undesirable in the devices under discussion. Surprisingly, fibroblasts can be cultivated for days with no apparent adverse effects in fields inducing trans-membrane potentials of 80% of the breakdown voltage (unpublished data). 

Both Na and K are involved in various electrophysio-logical functions in higher animals. The [Na ] [K ] ratio is different in intra- and extra-cellular fluids, and the concentration gradients of these ions across cell membranes are the origin of the trans-membrane potential difference that, in nerve and muscle cells, is responsible for the transmission of nerve impulses. A balanced diet therefore includes both Na and salts. Potassium is also an essential plant nutrient, and salts are widely used as fertilizers. Uses of Li and Na in batteries are highlighted in Box 10.3, and the use of KO2 in breathing masks is described in Section 10.6. 

Ehospholipid Bilaver Membranes. A lipid bilayer supported on an aperture between the LAPS and the controlling electrode can be represented by adding the equivalent circuit for a lipid bilayer (5) in series with the LAPS, as in Figure 3. As a thin insulating film, the membrane has a resistance (Rm), and a capacitance (Cm) associated with it. In addition, ion gradients across the membrane may cause trans-membrane potentials. These potentials are represented by the voltage source v in the equivalent circuit. It is assumed that no low resistance pathways around the membrane are present. We would like to be able to measure Rm, Cm, and Vm. 

Measurement of Trans-membrane Potential. For a constant bias potential, negligible current flows through the insulator on the semiconductor chip, so that there is no steady state potential across Rm- The potential across the chip is y - Vm. If in the absence of the membrane an photocurrent inflection point potential was measured at S = pip/ then in the presence of the membrane it will shift positive by the value of the trans-membrane potential as indicated by Equation 3. Measuring Vm is clearly analogous to using the sensor to make pH or redox measurements. 

The methods described in the present paper provide a means to measure phospholipid bilayer conductance, capacitance, and trans-membrane potentials in conjunction with the LAPS. It is quite plausible that the stability of this bilayer system can be significantly improved for many purposes by decreasing the distance between the bilayer and the silicon by micromachining the silicon. Studies in this direction will be reported elsewhere. 

Figure 11.1. Experimental setup for monitoring oscillations in trans-membrane potential across the membrane.

The permeability of the biological membrane is different for different ions and this gives rise to an electric potential difference between the bulk phases at the inner and outer sides of the membrane, respectively. The origin of this potential difference, referred to as the (trans)membrane potential A /,n, is discussed more extensively in Section 19.4. As a rule, the potential at the inner side is lower implying a negative value of In biological membranes A / is typically in the range of a few tens of mV. 

Bramhall, J. S. Morgan, J. I. Perris, A. D. Britten, A. Z. The use of a fluorescent probe to monitor alterations in trans-membrane potential in single cell suspensions. Biochem. Biophys. Res. Commun. 1976, 72, 654-662. 

Schneider, K. Naujok, A. Zimmermann, H. W. Influence of trans-membrane potential and of hydrophobic interactions on dye accumulation in mitochondria of living cells photoaffinity labeling of mitochondrial proteins, action of potential dissipating drugs, and competitive staining. Histochemistry 1994, 101, 455-461. 

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