Membranes potentials for

The membrane potential for a Ag2S pellet develops as the result of a difference in the equilibrium position of the solubility reaction... 

The membrane potential for a E ion-selective electrode results from a difference in the solubility of LaE3 on opposite sides of the membrane, with the potential given by... 

Below a pH of 4 the predominate form of fluoride in solution is HF, which, unlike F , does not contribute to the membrane potential. For this reason, an analysis for total fluoride must be carried out at a pH greater than 4. 

The purpose of this chapter is to describe these experimental approaches for understanding the molecular mechanism of the membrane potentials for ionophore-incorpo-rated liquid membrane ion-selective electrodes. 

The membrane potential reached in a system with only one permeant ion and no perturbing forces is called the equilibrium, or Nernst, potential for that ion thus, the final membrane potential for the system in Figure 6-2 is... 

If the diffusion potential in the membrane is neglected, this equation yields relationships for the membrane potential, for the ISE potential and for the selectivity coefficient. It is apparent that formation of complexes with various numbers of ions in the membrane does not affect the dependence of the ISE potential on the activities of the determinand and interferent according to the Nikolsky equation. 

Cationic response. The equation for the membrane potential for higher concentrations of cation J is obtained in a similar manner ... 

If the permeability of one ion is altered, while all concentrations and other permeabihties remain unchanged, the Goldman equation permits one to predict the changes in the membrane potential. For example, consider the... 

Schleyer M, Schmidt B, Neupert W (1982) Requirement of a membrane potential for the posttranslational transfer of proteins into mitochondria. Eur J Biochem 125 109-116 Schneider A, Behrens M, Scherer P, Pratje E, Michaelis G, Schatz G (1991) Inner membrane protease I, an enzyme mediating intramitochondrial protein sorting in yeast. EMBO J 10 247-254... 

Below we present a well-known calculation of membrane potential based on the classical Teorell-Meyer-Sievers (TMS) membrane model [2], [3]. The essence of this model is in treating the ion-selective membrane as a homogeneous layer of electrolyte solution with constant fixed charge density and with local ionic equilibrium at the membrane/solution interfaces. In spite of the obvious idealization involved in the first assumption the TMS model often yields useful results and represents in fact the main tool for practical membrane calculations. We shall return to TMS once again in 4.4 when discussing the electric current effects upon membrane selectivity. In the case of our present interest, the simplest TMS model of membrane potential for a 1,2 valent electrolyte reads... 

K+-ATPase is the primary source of the membrane potential for most eukaryotic cells and is said to be electrogenic. Because the cell membrane is somewhat permeable to K+, outward diffusion of K+ through the "leaky" membrane along its concentration gradient helps to maintain the membrane potential as does inward leakage of CP. At the same time, Na+ diffuses inward, aided by the membrane potential. Even though the permeability of Na+ is low, a steady state is reached at which the rate of passive inward diffusion of cations just balances the membrane potential set up by the active transport. 

Mitochondrial membrane permeability transition and its role in the mechanism of toxicity of many chemicals is a major area of research focus. The protein thiol status of mitochondria is crucial to the control of the inner-membrane potential for ATP synthesis and the membrane permeability transition as well as cellular integrity. Protein thiols appear to have a major role in signal transduction and are important in the responses to chemical exposure. 

Consider as an example the current of Na+ ion through a sodium channel. As was illustrated in Section 1.7.2, there is an equilibrium membrane potential for which the passive sodium current is zero. This equilibrium potential is given by... 

Once EPPj is formed, it is more likely to be cleaved than to appear as medium PR . If this reaction mechanism is true also for the membrane-bound PRase from R. rubrum the contribution of a membrane potential for the synthesis of PR should be on the release of the product RRj from the enzyme. 

Vospemik M, Pintar A, Bercic G, and Levee J. Experimental verification of ceramic membrane potentials for supporting three phase catalytic reactions. J. Membr. Sci. 2003 223 157-169. 

Interfering ions can also affect the membrane potential for liquid membranes. If it is assumed that solution 2 contains cation N which can exchange with the cation M" " in the liquid membrane, the system becomes... 

When making actual intracellular measurements, the microelectrodes are mounted in micromanipulators for cell penetration. It has been found that beveling the tip of the micropipet ISE aids in the ability to enter the cell and also enables the fabrication of electrodes with smaller tip diameters of O.I (im. Once inside the cell, single-barrel liquid membrane micro-ISEs (as described above) allow for the measurement of only steady-state ion activities. For excitable cells, where ion levels change rapidly, one cannot differentiate the potential changes resulting from variations in the intracellular activity of a specific ion and the living cell membrane potential. For such situations, double-barrel-type liquid membrane micro-ISEs have been developed (K2). 

The purpose of the reference barrel is to allow for the simultaneous measurement of the cell membrane potential while measuring the intracellular ion activities with the ISE portion of the device. Tims, in practice, a second single-barrel reference micropipet electrode is placed in the bathing solution outside the cell so that the potential between the two KCl-filled electrodes can always be monitored to obtain the instantaneous cell membrane potential (E ). The potential of the liquid membrane ISE barrel can also be monitored versus the external reference electrode. In this manner, potential changes due to variations in the cell membrane potential can be taken into account when calculating the intracellular ion activities. Alternatively, only the potential difference between both barrels of the electrode could be monitored. This potential should only be dependent on the intracellular activity of the analyte ion (not affected by the cell membrane potential). For certain ion measurements, e.g., K using a valinomycin based liquid micropipet electrode, leakage of K+ from the reference barrel could present a problem. In such cases, the reference barrel and the outer reference pipet should be filled with a solution other than 3 M KCl. 

Figure 9. a) Current-voltage relationship for GABA currents recorded in the presence (O) and absence ( ) of 0.1 ug/ml 22,23-dihydroavermectin Bla (DHAVM). The DHAVM was applied for 30 min. Both GABA and DHAVM were applied by microperfusion, b) The percentage inhibition of the peak GABA-induced current is plotted as a function of membrane potential for 3 different cells (l,(D,A) each exposed to 0.1 ug/ml DHAVM. 

Verapamil has a high degree of effect on nodal conduction, and therefore affects the action potential. Blockade of calcium channels results in an increase in the time required to reach the membrane potential for calcium, resulting in a dose-dependent lengthening of phase 2. 

ERP includes both the action potential duration and the time for membrane repolarization and stimulus to threshold. Mexiletine, by blocking fast sodium channels, blunts the amplitude of the cardiac action potential (phase 0 is decreased). The decrease in sodium influx and decrease in phase 0 amplitude results in a lower membrane potential as the action potential enters phase 2 and 3. This results in the membrane potential, at the end of phase 0. being closer to the membrane potential for potassium and calcium, so the influx of these ions is of shorter duration, resulting in a decrease in the length of phase 1 and 2 and a steeper slope of phase 3 repolarization. Phase 4, however, is lengthened, and the rate of rise decreased, with the decrease in the rate of sodium influx. Thus, the cardiac membrane remains relatively refractory. even after repolarization, and the ratio of the ERP to the action potential duration increases. 

Diffusion coefficient for solute i in a polymer film (cm /s) Equilibrium membrane potential for single ion (mV)... 

It would appear that changes in the intracellular or extracellular concentration of potassium ions markedly alter the resting membrane potential. For this reason, neurophysiologists treated an excitable cell as if it were an electrochemical, or Nernst, cell. The resting potential for one permeant species could therefore be explained by the familiar Nernst equation ... 

Eisenmann [19 defined the membrane potential for monovalent ions as follows ... 

Louzao, M.C. et al., A fluorimetric method based on changes in membrane potential for screening paralytic shellfish toxins in mussels, Anal Biochem 289, 246, 2001. 

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