Ionic permeability measurements

In this section, a brief overview is given of major membrane concepts and materials. Besides membranes made from a mixed ionic-electronic conductor (MIEC), other membranes incorporating an oxygen ion conductor are briefly discussed. Data from oxygen permeability measurements on selected membrane materials are presented. 

Whereas the mechanism of action of insecticides has been studied for many years since the development of synthetic insecticides such as DDT, lindane and parathion during and after World War II, it was not until around the mid-1960 s that their actions on the nervous system were understood at the cellular and membrane level. Since these neuroactive insecticides are known to alter membrane excitation which takes place with a time course of milliseconds, the study of their mechanisms of action can best be performed with the aid of advanced electrophysiological techniques such as voltage clamp which allows us to measure the ionic permeabilities of excitable membranes. Studies along this line unveiled a variety of important features concerning the... 

The possibility of measuring the small fluctuations in / a k occurring at nerve membrane level led to the realisation that such phenomena could provide insight into the microscopic mechanisms of ionic permeability changes. This technique had been successfully employed to examine membrane potential fluctuations and membrane current fluctuations due to the chemically mediated open-closed kinetics of ionic channels [63-68]. 

Ionic permeabilities can be related to salt permeability by [67] Ps = 2 P + P / (P + + P ). Then, by determining P + and P average values for the concentration interval shown in Figure 9.18, the following average value for salt permeability was determined = 1.0 0.3) x lO m/s. This value is rather similar to that obtained from direct salt diffusion measurements [7] (Ps = 3.6x 10 m/s) and shows the possibility of confirming values which have been estimated or determined from two different types of electrical measurements with those directly obtained from completely different (diffusion) experiments. 

Besides the remarks made above, which might notably weaken one s confidence in the electrogenic proton pump theory, let us stress some further complications. First, it is impossible to perform direct proton permeability measurements by the use of tritium in radiotracer methods. Second, several processes might be correlated to the external pH change, like effects caused by CO2 transport, 0H efflux, HCO5 uptake, the permeability of H and ions via pores, and kinetic control of all ionic pumps. 

Seventy years later, this theory largely holds true, although periodically challenged [67, 68]. Observation of transmembrane permeability of ionic species was initially explained by the formation of neutral ion-pair [69, 70]. A comprehensive review of the physicochemical properties influencing permeation has been written by Malkia et al. [5]. The reality is that, despite many studies, the effect of ionization on permeation is still a matter of discussion and active research. In contrast, it became clear that bulk-phase partitioning measurements are not adequate to describe bilayer partitioning [71-73]. 

The formation brine used to establish the core s initial permeability contained 2.7% total dissolved solids, TDS, with a monovalent-divalent (calcium) ratio of 30. Once a core is equilibrated with this brine, any increase in the ratio or drastic decrease in TDS has the potential for decreasing permeability. Obviously fresh water represents a significant decrease in TDS and, hence, the 54% permeability damage. Adding KC1 helps overcome the decreased salinity but, in so doing, increases the ionic ratio resulting in still measureable but usually reduced permeability damage. 

Even very small amounts of calcium provide a desirable decrease in the Na/Ca ratio. Prior studies indicating potassium chloride totally negates permeability reduction may have utilized water that contained some small amount of calcium ion to measure KC1 solution permeability. A second factor, which might explain the lack of KC1 damage reported in prior studies is a low ionic concentration, especially calcium, in the water used to equilibrate the cores prior to the KC1 tests. 

In order to interpret the physicochemical steps of retinal transduction as well as membrane excitability, we analyze macroscopic properties of membranes within biological components. Such membranes separate two aqueous ionic phases the chemical compositions of which are kept constant separately. The total flux through the membrane is directly deduced from the counterbalance quantities in order to maintain the involved thermodynamical affinities constant. From such measurement, we calculate the dynamical membrane permeability. This permeability depends not only on membrane structure but also on internal chemical reactions. 

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