Transmembrane electric field

Raising the concentration of Ca in the medium pathing, a nerve may relieve conduction block produced by local anesthetics. Relief occurs because Ca alters the surface potential on the membrane, and hence the transmembrane electrical field. This, in turn, reduces the degree of inactivation of the Na channels and the affinity of the latter for the local anaesthetic molecule [25, 27]. 

Although the mechanisms of electroporation, electrofusion, and electroinsertion are not known, biophysical data suggest that the primary field pulse effect is interfacial polarization by ion accumulation at the membrane surfaces. The resulting transmembrane electric field causes rearrangements of the lipids such that pores are formed1718. Electropores anneal slowly (over a period of minutes) when the pulse is switched off. 

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

Photosynthetic reaction centers plug into the chemiosmotic scheme by using light-excited states to create both an oxidant and a reductant. For the purple bacterial reaction centers, these oxidants and reductants are the redox carriers already described, oxidized cytochrome c and reduced ubiquinone QH2. Thus, in combination with Complex III, light drives a relatively straightforward cyclic electron transfer that generates a transmembrane electric field and proton gradient. 

Most membrane proteins are electrically active. If a membrane-spanning protein has a conformational change that involves translocation or displacement of charges, or change in the molar electric moment (A Me), then a transmembrane electric field will shift its chemical equilibrium to favor states that have higher electric moments (Me). The quantitative relationships for the effect of an electric field on the chemical equilibrium represented by eq 5 are given in eqs 6-8 (13, 14) ... 

The optimal field strength for activating both the Na+ and the K+ pumps was 20 V/cm (peak-to-peak). This applied field would generate a Ai im of 12 mV, or an effective transmembrane electric field, Em, of 24 kV/cm. Other field strengths led to reduced activities. 

The important role played by solitons in biological transfer remains to be clarified solitons are said to arise from localized disordered regions on, say, a membrane surface in the presence of the transmembrane electric field the local perturbation then tends to spread and to move leading to changes in the orientation of the lipid membrane molecules. The soliton energy is considerably below the energy band gap of a polypeptide chain but may initiate proton transfer in a hydrogen bonded chain in the presence of an electrostatic field. 

FIGURE 1. Schematic representation of the fluid-mosaic model of a cell membrane showing protein molecules incorporated into a lipid bilayer structure. Lateral diffusion of the proteins and lipid molecules occurs, but the lipids very rarely migrate from one side of the membrane to the other. A transmembrane electrical field arises from the action of vectorial ion pumps (ATPase proteins) in producing ionic concentration differences across the membrane, and from the presence of membrane surface charges. Surface redox reactions may affect this membrane field. 

From these experiments we conclude that the flash-induced P515 bandshift measured under ion limitation is in accordance with the reaction 1 type electrochromic shift as defined by Vredenberg, and can be interpreted as a flash-induced formation of a transmembrane electric field followed by a field dissipation in the dark by a passive flux of ions. The decay rate can be effectively accelerated by the addition of KCl (Fig. 4). Chloride appears to be the effective permeant ion. Acceleration of the dark decay was much less with KAc (data not shown). 

The fast phase (phase a) of the 515 nm electrochromic absorption change, AA515, occurs as a result of the transmembrane electric field set up at the photochemical reaction centres upon flash activation (Witt 1979). 

The redox titrations under high salt conditions show that the contributions to the transmembrane electric field are equally shared between three components one associated with PS2, Em 7.5 associated with... 

The rise of the transmembrane electric field, as revealed by the 515 change, up to the peak value of some 200 mV activates and energizes the ATPases partly. It has to be assumed that the activation persists some time after the cessation of excitation. 

Graber P, Schlodder E and Witt HT (1977) Conformational change of the chloroplast ATPase induced by a transmembrane electric field and its correlation to phosphorylation. Biochim. Biophys. Acta 461, 426-440. 

---