Voltage-dependent conductance

DA can exert its action on striatal cells via different mechanisms, which include the modulation of different voltage-dependent conductances and indirect effects on excitatory and inhibitory synaptic transmission (reviewed by Nicola et al., 2000). Because DA can diffuse away from its site of release, cellular responses to exogenous DA are likely to be identical or similar to those that follow the synaptic release of DA (Nicola et al., 2000). 

Other neutral peptide sequences in addition to peptaibols are capable of voltage-dependent conductance changes in lipid bilayers. Molle et al. (56)... 

Molle et al. (243) synthesized the 22-residue transmembrane segment of the essential subunit 8 of the Saccharomyces cerevesiae H+ ATP synthetase and observed weakly voltage-dependent conductance levels on different planar lipid bilayers. CD spectra show 60% a-helix for this peptide in low dielectric solvents. The conductance exhibited a second-order concentration dependence, suggestive of antiparallel dimers as the conducting unit (243). 

EIM, prepared from bacterially desugared egg-white, when incorporated in lipid bilayers, produces membranes which show complex voltage-dependent conductance changes, the so-called bilayer action potentials [30]. Furthermore, discrete conductance fluctuations can be observed with appropriate conditions [31]. 

Also, the available data show that the conductivity of the obtained films decreases at low potential values ( " < — 1 V/SCE) as the film thickness increases . In contrast, the polymer conductivity seems to be unchanged at positive potential values. This evokes a voltage-dependent conductivity similar to that described for simple polypyrrole films. Anyhow, the incorporation of the complexes into the polymer films imparts a typical redox conductivity to these materials that allows their use over a large potential range. One can even assert that at the potentials where many of the effects are expected, the electron hopping process between macrocyclic complex sites dominates the global charge transport mechanism. 

Figure 9.10. The voltage-dependent conductance of a zinc oxide varistor. Published with permission from the Journal of Materials Education.

In concluding this section, mention should be made concerning the dipoles of membrane constituents in the BLM. The dipole flip-flop could be important for the voltage-dependent conductance in BLM (e.g. alamethicin-modified BLM). Derzhanski et al. have derived an equation for the current as a function of frequencies. Pastushenko and Chizmadzhev have considered the energetic profile of dipole molecules in biomembranes in general, and in BLM in particular. This topic has been reviewed in detail by Shchipunov and Drachev, and by de Levie [3,15-24]. 

The thickness of PS layer is also one of the factors that influence the contact behavior. As thin PS layer show rectifying characteristics while thick PS layer show an almost symmetric one, Ben-Chorin (Ben-Chorin et al. 1994) proposed that the rectifying barrier is at the interface between PS and the doped substrate. It was also suggested by them (Ben-Chorin et al. 1994) that the usual diode structure formed by a metal contact, a PS layer, and a doped substrate can be visualized as a series combination of a voltage-dependent resistance and a rectifying barrier. Their study shows that the temperature- and voltage-dependent conductivity relationship follows Poole-Frenkel (PF)-t3 e conduction, where transport mechanism in high fields involves field-enhanced thermal excitation from Coulombic traps ... 

In order to derive a mathematical expression which describes quantitatively the voltage-dependent conductance of K+ channels associated with the early inward current in the protoplasmic droplets of Chara corallina, it has been assumed that each channel behaves like a dipole which can exist in two states open and closed . In this case, the K+ conductance is given by... 

Figure 13. Voltage dependent conductance (a) of the early inward K+ current in the absence (0) or presence ( ) of TEA, and (b) of the delayed inward K+ current in the absence of TEA. The continuous line drawn in (a) has been calculated assuming a two state constant dipole model from Reference 48).

The identification of the two voltage dependent conductances remains still very speculative. 

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