Measurement transmembrane potential

In medical practice, methods and instruments relying on electrochemical principles are widely nsed in diagnosing various diseases. The most important ones are electrocardiography, where the transmembrane potential of the muscle cells during contraction of the heart mnscle is measured, and electroencephalography, where impulses from nerve cells of the brain are measured. They also include the numerous instruments nsed to analyze biological fluids by electrochemical methods (see also Section 30.3). 

Klymchenko AS, Stoeckel H, Takeda K et al (2006) Fluorescent probe based on intramolecular proton transfer for fast ratiometric measurement of cellular transmembrane potential. J Phys Chem B 110 13624-13632... 

The voltage difference, sometimes referred to as the transmembrane potential or the membrane electric potential, that is due to the difference of concentrations of ions on either side of the membrane. Typically, such potentials are measured in millivolts (mV) and usually have values between —30 and —70 mV. In most cells, the interior of a cell is negative with respect to the exterior. 

Aizawa et al. developed immunosensors of the first type for syphilis and blood typing [228-230] based on measurements of the transmembrane potential across an immunoresponsive membrane. The potentiometric immunosensor using an antibody against human chorionic gonadotropin (hCG)... 

Electronics to hold transmembrane potential (Vm) constant and measure current flowing across membrane... 

Figure 1. PC2 channels inhibited by Gd3+. The top panel shows currents obtained from PC2 incorporated into a bilayer. This is a control trace showing Cs+ current measured at 15 mV transmembrane potential. The solid and broken lines to the right of the traces indicate the closed and open states, respectively. The second and third traces show currents after the addition of Gd3+. The bottom panel shows the concentration dependence of the block by Gd3+, circles when added to the cytoplasmic side, squares when added to the lumenal side. The Kd is 206 pM Gd3+ only blocks when added to the cytoplasmic side. Modified from (Anyatonwu and Ehrlich 2005)...

Eckert and Renger [153] measured P-680 formation and rereduction at 690 nm with microsecond time resolution and found a phase upon the second of two closely spaced flashes, which was attributed to reduction of an acceptor other than Oa- The flash spacing was such that was expected to be still reduced when the second flash was given. The electron on the putative acceptor, designated recombined with P-680 with a ty2 of 35 ps, and its reduction was not associated with a transmembrane potential. 

Other electrochromic membrane probes of note include, me-rocyanine 540 used to study the transmembrane potential in liver mitochondria although some problems are evident (5, 30, 31). Overall, measurements of transmembrane potential differences in living cells are well established with the methods fairly reliable and robust. Many of these membrane potential measurement strategies are generic and hold for plasma membranes of neurones and other excitable tissues (6). 

It is easy to see that the electrophoretic process of Ca " uptake, if permitted to attain equilibrium against a transmembrane potential of —180 mV, would lead to either an intolerably high concentration of ionized Ca in the mitochondrial matrix, or to the extreme impoverishment of the cytosolic Ca. The latter condition is ruled out by the existence, in the cytoplasm of all cells, of numerous reactions that are obligatorily modulated by in the juM or sub-juM range. In fact, whenever direct measurements have been possible, it has been found that the concentration of free Ca in the cytosol oscillates around 0.1 juM [75-78]. 

It may be noted that for ta = 0, Equation 26.4 gives rise to Equation 26.3. The changes in the transmembrane potential have been experimentally observed and quantitatively measured. 

Fig. 121. Determination of Wassermann antibody in the syphilis test by measurement of the transmembrane potential change (AE). (Redrawn from Aizawa et al., 1977).

D Ammann, P Caroni. Preparation and use of microelectrodes and macroelectrodes for measurement of transmembrane potentials and ion activities. Methods Enzymol 172 136-155, 1989. 

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