Organic anion-sensitive membranes

OATP2B1 (old name OATP-B) is expressed at brush-border membranes of intestinal epithelial cells [32], OATP2B1 exhibited pH-sensitive transport activities for various organic anions such as estrone-3-sulfate, dehydroepiandros-terone sulfate, taurocholic acid, pravastatin, and fexofenadine [33], However, further studies are needed to determine the specific physiological and pharmacokinetic contribution of OATP2B1 for intestinal absorption of these compounds. 

Selective ion electrodes (SIE). Selective ion electrodes are essentially variants of the well-known pH meter. They are membrane indicator types of electrodes in which a potential is developed across a membrane in the presence of the ion the size of the potential is related to the concentration and hence can be used to quantitatively detect and measure the species. However, instead of a glass membrane, as in the pH meter, the membranes consist of organics that are immersible in water. For example, anion-sensitive electrodes use a solution of an anion exchange resin in an organic solvent the liquid can be held in the form of a gel, for example, in polyvinyl chloride. The ion reacts with the organic membrane, setting up an equilibrium between the free ion in solution and the ion bound to the membrane, generating a potential difference, which is measured. 

Table 21-2 lists some liquid-membrane electrodes available from commercial sources. The anion-sensitive electrodes shown make use of a solution containing an anion-exchange resin in an organic solvent. Liquid-membrane electrodes in which the exchange liquid is held in a polyvinyl chloride gel have been developed for Ca-, K", NOj, and BF4. These have the appearance of crystalline electrodes, which are considered in the following section. A homemade liquid-membrane ion-selective electrode is described in Feature 21-1. 

Figure 8.18 Large organic ions and pH-sensitive salts can foul anion-exchange membranes.

A membrane which is less sensitive to traces of detergents is commercially available today from Ionics. Another type of anti-fouling anion-exchange membrane is produced by Tokuyama Soda. The membrane is coated with a thin layer of cation-exchange groups causing electrostatic repulsion of organic molecules. 

Table 23 lists some typical commercially available liquid-membrane electrodes. The anion-sensitive electrodes contain a solution of an anion exchanger in an organic solvent. As mentioned earlier, many of the so-called liquid-membrane electrodes are in fact solids in which the liquid is contained in a polymer (plastic) matrix. The first and most widely used polymer for membrane electrodes is PV C, but other materials have been used as well for compatibility with ionophores and fabrication materials. Polymer-based electrodes are somewhat more convenient to use and more rugged than the older porous disk electrodes. All electrodes listed in Table 23-4 are of the plastic-membrane type. 

In electrodes sensitive to medical compoimds being organic anions, the active membrane components are ion pairs of these anions with hpophilic cations such as tertiary amine salts, quaternary phosphine compounds or metal chelates [43]. 

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