Cell potential model

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 

The basic capacitor model (Chapter 3) is a dielectric between two metal plates with free electrons. In die cell model, the metal plates are replaced by electroljrtes with free ions. 

In the static case with no leakage across the membrane, there is no current flow inside or outside the cell. At the inside membrane, a surface charge density of free ions neutralizes the electric field in the membrane plus the double layer charges, so that the eeU interior has E = 0 and no current flow. There is electroneutrality in the bulk, but surface charges at the interphase. The intracellular bulk volume has a potential, but zero electric field strength. The membrane has a bound surface charge density qsb, which wfll reduce the membrane E-field to (qgf — qsb)/eo- Thus, the E-field in the membrane is less than the potential difference divided by the membrane thickness. 

An action potential can accordingly be measured by nearby electrodes with a unipolar or bipolar technique. 

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The cell potential models of Figures 5.6 and 5.7 are very simplified in order to obtain a basic understanding of the current density and potential fields. They represent a macro view of a system with very small dimensions in particular, the thickness of a human cell membrane is only approximately 7 nm and the channel protein charge positions are accurate to within an angstrom (0.1 nm). In addition, the real cell membrane is covered by a very dynamic system of receptor molecules and other components. Also, electrolytes in contact with proteins have thin charged double layers formed at the interphase. 

The metal and the electroljrte also determine the DC half-cell potential, modeled by the battery B. If there is no electron transfer. Ret is very large and the battery B is decoupled, the electrode is then polarizable with a poorly defined DC potential. But if there is an electrode reaction. Ret has a lower value and connects an additional admittance in parallel with the double layer admittance. This current path is through the faradaic impedance Zf, and the current is the faradaic current if. Faradaic current is related to electrode reactions according to Faraday s law (Section 7.8). The faradaic impedance may dominate the equivalent circuit in the lower Hz and sub-Hz frequency range and at DC. The faradaic impedance is modeled by a complete Cole-like series system. It consists of the resistor Ret... 

Otherwise it has been shown that the accumulation of electrolytes by many cells runs at the expense of cellular energy and is in no sense an equilibrium condition 113) and that the use of equilibrium thermodynamic equations (e.g., the Nemst-equation) is not allowed in systems with appreciable leaks which indicate a kinetic steady-state 114). In addition, a superposition of partial current-voltage curves was used to explain the excitability of biological membranes112 . In interdisciplinary research the adaptation of a successful theory developed in a neighboring discipline may be beneficial, thus an attempt will be made here, to use the mixed potential model for ion-selective membranes also in the context of biomembrane surfaces. 

Fig. 5. Tentative mixed potential model for the sodium-potassium pump in biological membranes the vertical lines symbolyze the surface of the ATP-ase and at the same time the ordinate of the virtual current-voltage curves on either side resulting in different Evans-diagrams. The scale of the absolute potential difference between the ATP-ase and the solution phase is indicated in the upper left comer of the figure. On each side of the enzyme a mixed potential (= circle) between Na+, K+ and also other ions (i.e. Ca2+ ) is established, resulting in a transmembrane potential of around — 60 mV. This number is not essential it is also possible that this value is established by a passive diffusion of mainly K+-ions out of the cell at a different location. This would mean that the electric field across the cell-membranes is not uniformly distributed.

Gregoire, L., Munkarah, A., Rabah, R., Morris, R. T., Lancaster, W. D. (1998). Organotypic culture of human ovarian surface epithelial cells a potential model for ovarian carcinogenesis. In vitro Cell Dev-An 34(8), 636-639. 

To test further this hypothesis, a simulated cell/tissue model system has been devised using quantitatively comparable cell fines, in which the amount of selected antigen (potential reference standard) can be measured accurately on a cell-to-cell basis in fresh and FFPE specimens that are processed under clearly defined but variable conditions, including periods of formalin fixation, delay times of fixation (prefixation time or warm ischemic time), storage conditions, and other technical issues such as thickness of each tissue section, in... 

Cell Culture Models for Evaluation of Sensitizing Potential... 

Cell Culture Models lor Evaluating the Sensitizing Potential ol Xenobiotics... 

Cell potential, 9 607-609 standard, 75 750 Cell Saver, 3 719 Cells, encapsulation of, 76 454 Cell sorting, microfluidics in, 26 971-972 Cell-specific dendritic carriers, 26 797 Cell targeting, dendrimers in, 26 797-798 Cell thermo stating, 73 426 Cellular components inducers, herbicide damage to, 73 297-298 Cellular damage, in aging, 2 810 Cellular diseases, yeast as a model for, 26 496-497... 

Using Models Calculate the percent error of the voltaic cell potential. 

Portero A, Remunan-Lopez C, and Nielsen HM (2002) The potential of chitosan in enhancing peptide and protein absorption across the TR146 cell culture model—An in vitro model of the buccal mucosa. Pharm. Res. 19 169-174. 

There are several cell monolayer models that are frequently used for the evaluation of drug permeability and absorption potential (Table 18.1). For a more detailed discussion, please refer to Chap. 8. Caco-2 cells (adenocarcinoma cells derived from colon) are the most extensively characterized and frequently used of the available cell lines [5-9], A unique feature of Caco-2 cells is that they undergo spontaneous enterocyte differentiation in cell culture. Unlike intestinal enterocytes, Caco-2 cells are immortalized and replicate rapidly into confluent monolayers. When the cells reach confluency during culture on a semi-porous membrane, they start to polarize and form tight junctions, creating an ideal system for permeability and transport studies. During the past decade, use of... 

This publication was initiated on the occasion of the NATO-Advanced Study Institute (ASI) meeting Stem Cells and their potential for clinical application which took place from August 23 - 25, 2006 in Kyiv and from August 26-31, 2006 in Simeiz, Ukraine. The meeting was devoted to hot topics in Stem cell research such as Regulation of Haematopoietic and Non-haematopoietic Stem Cells, Clinical Application of Stem Cells, Preclinical Models and Gene Therapy. 

Cell culture models are routinely used to assess permeability of new potential drug candidates. The simplicity and higher throughput of these models makes them a useful alternative to in vivo studies. These models are used to predict absorption in vivo, rank order compounds and examine absorption mechanism. Transcellular, paracellular, active uptake and efflux mechanisms can be studied with these models. 

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