Membrane potentials transmembrane potential difference

The involvement of mitochondria in the pro-apoptotic effects of carotenoids has been clearly demonstrated by the fact that P-carotene induces the release of cytochrome c from mitochondria and alters the mitochondrial membrane potential (Aym) in different tumor cells (Palozza et al., 2003a). Moreover, the highly polar xanthophyll neoxanthin has been reported to induce apoptosis in colon cancer cells by a mechanism that involves its accumulation into the mitochondria and a consequent loss of mitochondrial transmembrane potential and releas of cytochrome c and apoptosis-inducing factor (Terasaki et al., 2007). 

Experimentally it is the transmembrane potential difference that is observed by the use of potential-sensitive fluorescent dyes. Two of the components of the transmembrane potential difference are the intra- and extracellular potential differences (Fig. 17.2). If (f)x is the potential in the interior of the cell and 0O the exterior potential, then the transmembrane potential, Em, is given, in the case of a symmetric membrane where D for the two sides cancels, by... 

The transmembrane potential difference can be described by the Goldman equation that relates t tm to the permeabilities of the membrane to specific ions and the concentrations of such major ions on either side of the PM ... 

As outlined in Chapter 3, cell excitability can in part be determined by the maintenance of gradients of Na+, K+ and CP ions. Differential plasma membrane (PM) permeabilities to such ions and the gradients of ion concentration contribute to the transmembrane potential difference (t tm), which is typically about —0.1 volt (V) (inside with respect to the outside). In addition, the cytosolic free concentration of Ca2+ is extremely low (0.1 pM in resting cells and about 10 pM in excited cells) as compared to concentrations of Na+, CP and K+ of about 10, 10 and 100 mM, respectively, in the cytosol and about 100, 100 and 10 mM, respectively, in the extracellular milieu. These huge ion gradients are maintained through the operation of ion pumps such as the adenosine 5 -triphosphate (ATP)-energized Ca2+ pump (Ca2+-ATPase) and the Na+ and K+ pump (Na+, K+-ATPase). 

Finally, its also worth emphasising that related to the transmembrane potential difference, a transmembrane gradient of electric charge known as the Donnan potential is also known to exist. The Donnan potential arises from the inability of larger charged macromolecules or other hxed charges to move across a membrane and for counterions to compensate this gradient. 

These involve the use of techniques to suppress the fast inward Na+ current. Either a voltage can be applied so that the transmembrane potential difference is clamped above the level at which the Na+ current is activated. (36) or tetrodotoxin (37) -which specifically inhibits the Na+ current, can be added, or the membrane can be depolarized by raising the external K+ (38,39). Under each of these conditions the slow inward current can be activated by adding aminophylline or isoproterenol, (39) and by electrical stimulation. 

The formation of protons was detected experimentally during the peroxidation of lipids of bilayer lipid membranes.68,69 It was shown by direct measurements of the aqueous solution pH that X-ray irradiation of the aqueous solution on one side of the bilayer leads to ejection of protons into the solution. As a result, a transmembrane potential difference is generated on BLM. Since after X-ray irradiation peroxide anions are formed in the solution, peroxidation of lipids can be initiated by the anion radical O. 70... 

FIGURE 22.3 Circuit representation of membrane current. The conductance can be nonlinear as indicated by the powers p and q on the state variables m and hi, respectively. These state variables are typically time-varying functions of the transmembrane potential difference, V. The battery, , represents the electrochemical gradient of the ionic species responsible for the current. 

Transmembrane potential difference The potential difference between the inside and the outside of a cell manifests itself very close to membrane. In many cases the potential of the external fluid is... 

Solution—Diffusion Model. In the solution—diffusion model, it is assumed that (/) the RO membrane has a homogeneous, nonporous surface layer (2) both the solute and solvent dissolve in this layer and then each diffuses across it (J) solute and solvent diffusion is uncoupled and each is the result of the particular material s chemical potential gradient across the membrane and (4) the gradients are the result of concentration and pressure differences across the membrane (26,30). The driving force for water transport is primarily a result of the net transmembrane pressure difference and can be represented by equation 5 ... 

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.

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