Cell membranes, electrical

Potassium is the second most abundant cation in the body and is found primarily in the intracellular fluid. Potassium has many important physiologic functions, including regulation of cell membrane electrical action potential (especially in the myocardium), muscular function, cellular metabolism, and glycogen and protein synthesis. Potassium in PN can be provided as chloride, acetate, and phosphate salts. One millimole of potassium phosphate provides 1.47 mEq of elemental potassium. Generally, the concentration of potassium in peripheral PN (PPN) admixtures should not exceed 80 mEq/L (80 mmol/L). While it is safer to also stick to the 80 mEq/L (80 mmol/L) limit for administration through a central vein, the maximum recommended potassium concentration for infusion via a central vein is 150 mEq/L (150 mmol/L).14 Patients with abnormal potassium losses (e.g., loop or thiazide diuretic therapy) may have higher requirements, and patients with renal failure may require potassium restriction. 

Vincristine has been shown to enhance the accumulation of the folate antagonist methotrexate in murine leukemia cells, and the enhancement has been shown to involve inhibition of a specific efflux route for methotrexate (25) the suggestion has been made that the effect of vincristine on methotrexate efflux may be related to alterations of cell membrane electrical activity that appear to occur when cells are treated with vincristine. In this connection, it is worth mentioning that association of tubulin with membrane structures from bovine brain has been described 25a). Both vinblastine and vincristine have been reported to enhance the accumulation of the folate antagonist methotrexate in human leukemic cells (S) there is no evidence, however, to indicate that this interaction has significance in a clinical setting. 

Wang, M. W. Bliss, J. G., unpublished). This extension of the planar model provides a better description of cell membrane electrical behavior during electroporation and provides a more realistic basis for describing molecular transport. 

M.p. 103°C. Noradrenaline is released in the adrenal medulla with adrenaline, and also at the sympathetic nerve endings. Its release from a nerve fibre is followed by binding to a receptor molecule on the next nerve or muscle fibre, probably causing a change in the electrical charge of the receptor-cell membrane. Biosynthetically it normally serves as a precursor for adrenaline. 

Taste-active chemicals react with receptors on the surface of sensory cells in the papillae causing electrical depolarization, ie, drop in the voltage across the sensory cell membrane. The collection of biochemical events that are involved in this process is called transduction (15,16). Not all the chemical steps involved in transduction are known however, it is clear that different transduction mechanisms are involved in different taste quaUties different transduction mechanisms exist for the same chemical in different species (15). Thus the specificity of chemosensory processes, ie, taste and smell, to different chemicals is caused by differences in the sensory cell membrane, the transduction mechanisms, and the central nervous system (14). 

The aperture impedance principle of blood cell counting and sizing, also called the Coulter principle (5), exploits the high electrical resistivity of blood cell membranes. Red blood cells, white blood cells, and blood platelets can all be counted. In the aperture impedance method, blood cells are first diluted and suspended ia an electrolytic medium, then drawn through a narrow orifice (aperture) separating two electrodes (Fig. 1). In the simplest form of the method, a d-c current flows between the electrodes, which are held at different electrical potentials. The resistive cells reduce the current as the cells pass through the aperture, and the current drop is sensed as a change in the aperture resistance. 

Maintenance of electrical potential between the cell membrane exterior and interior is a necessity for the proper functioning of excitable neuronal and muscle cells. Chemical compounds can disturb ion fluxes that are essential for the maintenance of the membrane potentials. Fluxes of ions into the cells or out of the cells can be blocked by ion channel blockers (for example, some marine tox-... 

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.

The net electrochemical driving force is determined by two factors, the electrical potential difference across the cell membrane and the concentration gradient of the permeant ion across the membrane. Changing either one can change the net driving force. The membrane potential of a cell is defined as the inside potential minus the outside, i.e. the potential difference across the cell membrane. It results from the separation of charge across the cell membrane. 

The smooth muscle cell does not respond in an all-or-none manner, but instead its contractile state is a variable compromise between diverse regulatory influences. While a vertebrate skeletal muscle fiber is at complete rest unless activated by a motor nerve, regulation of the contractile activity of a smooth muscle cell is more complex. First, the smooth muscle cell typically receives input from many different kinds of nerve fibers. The various cell membrane receptors in turn activate different intracellular signal-transduction pathways which may affect (a) membrane channels, and hence, electrical activity (b) calcium storage or release or (c) the proteins of the contractile machinery. While each have their own biochemically specific ways, the actual mechanisms are for the most part known only in outline. 

Marine toxins modify the functions of many different types of ion channels in animal cell membranes. These channels may be important for maintaining the cell s resting potential, for generating electrical membrane signals, such as impulses, and for controlling hormonally triggered or metabolic responses. Thus toxins may depolarize membranes, leading to a (sometimes transient) increase in cellular activities, or they may... 

PSB electrolytes are brought close together in the battery cells where they are separated by a polymer membrane that only allows Na ions to go through, producing about 1.5 V across the membrane. Cells are electrically connected in series and parallel to obtain the desired voltage and current levels. The net efficiency of this battery working at room temperature is about 75%. It has been verified in the laboratory and demonstrated at multi-kW scale in the UK [92]. 

The electrical signals are carried by the movement of charged ions across the cell membrane. This makes use of the potential energy stored across the cell membrane in the form of ionic gradients. Concentration gradients for the principal ions across a typical nerve cell membrane are indicated in Fig. 2.1(a). The cell interior has a high concentration of K+ ions and a low concentration of Na+, Cl and Ca + ions relative to the exterior. 

FIGURE 30.2 Electrical equivalent circuit of a cell membrane. 

In the electric organ of fishes, a number of such stacks are connected in parallel and in series. The total voltage attains 500 V in the electric eel. A current pulse of about 0.5 A develops when this voltage appears across an external circuit (in fresh water or seawater). For the electric ray, these numbers are 60 V and 50 A, respectively. The length of such an electric pulse is comparable with the time of cell membrane excitation (i.e., 1 to 2ms, which is quite sufficient to defeat a designated victim). Some species of fish use pulses repeated at certain intervals. 

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