Interior positive membrane potentials

Figure 3. Description of systems used to generate and measure interior positive membrane potentials. (A) A schematic diagram for the ascorbate-TCNQ-K3Fe(CN)6 reaction. The interior positive membrane potential formation is catalyzed by the lipophilic electron carrier TCNQ, which mediates the flow of electrons from ascorbate inside the vesicle to K3Fe(CN)6 outside. (B) Membrane potential formation in reconstituted proteoliposomes was followed by the fluorescent probe oxonol V. (Reproduced with permission from reference 17. Copyright 1991 American Society for Biochemistry and Molecular Biology.)...

Equation (4.4.1b) expresses impermeability of the ideally cation-permselective interface under consideration for anions j is the unknown cationic flux (electric current density). Furthermore, (4.4.1d) asserts continuity of the electrochemical potential of cations at the interface, whereas (4.4. lg) states electro-neutrality of the interior of the interface, impenetrable for anions. Here N is a known positive constant, e.g., concentration of the fixed charges in an ion-exchanger (membrane), concentration of metal in an electrode, etc. E in (4.4.1h) is the equilibrium potential jump from the solution to the interior of the interface, given by the expression ... 

The polar lipid headgroup zone contains the phospholipid and steroid phosphorus-nitrogen, carbonyl, hydroxyl and hydration water moieties which combine to establish a substantial dipolar potential. This positive potential is of a magnitude of several hundred millivolts across the membrane headgroup zone. In the membrane hydrocarbon interior, the electrostatic field must be at least 450 to 750 mV (14) and controls ion current across the interior as we 1 1 as possibly influencing selective ion adsorption to the membrane surface. 

In this table, P represents anions of protein and organic phosphate. The membrane is permeable to the group represented by P. The mean values of the charge on P are -6.7 and -1.08 for the interior and the exterior of the cell, respectively. An electrical potential difference of At// = i/t, t// = 90 mV is measured, i and o denote the intracellular and extracellular, respectively. The activity coefficients of components inside and outside the cell are assumed to be the same, and pressure and temperature are 1 atm and 310 K. Assume that the diffusion flows in from the surroundings are positive and the diffusion flows out are negative. Using tracers, the unidirectional flows are determined as follows ... 

Many membrane components such as phospholipid include moieties such as the C + = and 0 -P + exhibit polarisation. The membrane dipole potential ( )d has its origins in the dipole moments of polar groups from the lipidic components of the bilayer, it seems likely that the water molecules at the molecular surface of membrane also make a contribution (1). The organisation of the membrane components that contribute to this potential have been verified from neutron diffraction studies and NMR spectroscopy (23) and quite recently using cryo-EM techniques has also added quantitative estimations of the potential (24). These dipolar groups seem to be oriented in a way such that the potential located towards the hydrophobic interior of the membrane is positive with respect to the pole located towards... 

In addition, ion concentration gradients existing between two sides of a membrane produce an electrical potential difference, ranging between 50 and 100 millivolts or mV (10 volt), the outside being positive with respect to the interior. This is the direct consequence of the distribution of cations, especially potassium and sodium ions. Any stimulation by electrical, mechanical, or chemical means at one point of the membrane will create a change in the potential membrane at that point. The altered potential, also called the active potential, will move as a wave over the membrane surface. This provides a means of rapid communication between different regions of a cell. In the case of an elongated nerve cell, this constitutes a nerve impulse. 

In many, possibly all, biological membranes, the lipids are distributed asymmetrically. The outer half of the bilayer consists mainly of neutral lipids, whereas the inner half contains the negatively charged examples, particularly phosphatidylserine. The interior of such a membrane can be 300 mV more positive than the solution that bathes the outside. Such differences in potential can be measured by the potassium nonactin probe (Latorreand Hall, 1976) (see Section 14.2 for nonactin). Such potential differences indicate the source of some typical membrane properties such as the gating potentials of nerves. 

Two lipophilic neutral molecules are commonly incorporated into membranes with the aim of altering their dipole potential, namely, phloretin, which creates a dipole potential negative toward the interior of the lipid film, and KC, which creates a dipole potential positive toward the interior, because of the presence of a carbonyl... 

Thus, the electrode is primarily responsive to DA that is protonized at pH 7.4 on the side-chain amino-group[171j. The positive effect of Nafion is increased by the low value of dopamine diffusion coeflScient in this membrane (Doa = 1 x 10 m s ). Therefore, the diffusion layer is restricted to the interior of the film and the product of the oxidation, DOQ (a potential neurotoxine) cannot penetrate into the brain liquid. A further favorable consequence is that the diffusion-layer thickness is the same in vivo as in vitro where the calibration of the sensor is provided. 

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