Potassium-selective membrane system

Without question, sodium and potassium have been the analytes receiving the most attention in conjunction with the development of new analyzers. Almost all instruments on the market utilize the potassium-selective membrane system based on the antibiotic valinomycin in a PVC membrane matrix. For blood measurements, such a membrane is quite adequate. However, in undiluted urine samples, a negative error in the measurement of potassium has been reported (KIO). Apparently, this interference comes from a negatively charged lipophilic component of the urine which can partition into the PVC membrane, reducing the membrane potential (i.e., the membrane is not permselective). Fortunately, this problem can be overcome by incorporating the valinomycin in a silicone rubber-based membrane matrix (A4) into which the unknown anionic component apparently has a less favorable partition coefficient. 

The crowns as model carriers. Many studies involving crown ethers and related ligands have been performed which mimic the ion-transport behaviour of the natural antibiotic carriers (Lamb, Izatt Christensen, 1981). This is not surprising, since clearly the alkali metal chemistry of the cyclic antibiotic molecules parallels in many respects that of the crown ethers towards these metals. As discussed in Chapter 4, complexation of an ion such as sodium or potassium with a crown polyether results in an increase in its lipophilicity (and a concomitant increase in its solubility in non-polar organic solvents). However, even though a ring such as 18-crown-6 binds potassium selectively, this crown is expected to be a less effective ionophore for potassium than the natural systems since the two sides of the crown complex are not as well-protected from the hydro-phobic environment existing in the membrane. 

The phenomenological description of the excitability phenomenon given in Section 1.3 cannot claim to contain a final solution to the problem of the nature of transport systems of biological membranes responsible for nervous impuse generation. Where we stand, we can only conclude that the membrane as a whole is a nonlinear ion conductor whose properties are largely dependent upon the electrice field. For all that, the fact that the use of certain specific blocking compounds—tetrodotoxin and tetraethylammonium—allows the sodium and potassium ionic currents to be separated is alone sufficient to support the conception of selective transport systems located in the lipid matrix... 

The overall response of an ion-selective membrane electrode, such as the BME-44 ionophore -PVC-DOS system for potassium ions, described earlier is the result of several consecutive and parallel processes of which the following four main ones are most frequently considered/... 

Cellular metal ion transport is biologically important because our muscular and nervous systems are regulated by charged species. Cells use membrane channels to extract potassium ions selectively from environments containing both K+ and Na+. Because the K+ ion is /argerthan the Na+ ion, this process cannot be accomplished by simply restricting the channel diameter. Dr. Roderick MacKinnon showed that potassium selectivity arises from a preferential interaction between the potassium cation and the atoms of the protein amino acids composing the channel walls. 

The dissolved oxygen content of a solution can be determined by measuring the diffusion current that results at a selected voltage. The Clark electrode was developed for this purpose and various modifications have subsequently been introduced. It consists basically of a platinum electrode separated from the sample by a membrane which is permeable to oxygen, e.g. Teflon or polyethylene. A reference electrode of silver/silver chloride in potassium chloride is used to complete the system (Figure 4.21). When a voltage that is sufficient to give the... 

Mechanism of Action A proton pump inhibitor that selectively Inhibits the parietal cell membrane enzyme system (hydrogen-potassium adenosine triphosphatase) or proton pump. Therapeutic Effect Suppresses gastric acid secretion. Pharmacokinetics ... 

TiOz coated with potassium ferrocyanide proved to be an effective catalyst for the reduction of C02 to formic acid and formaldehyde.169 A very stable and reproducible catalytic system was prepared by immobilizing Ni2+ and Ru2+ complexes into Nation membrane, which was used for the selective reduction of C02 to formic acid.170 Formic acid was again formed when Zn and Co phthalocyanines were adsorbed onto a Nation membrane on irradiation with visible light in acidic aqueous solution containing triethanolamine as a hole scavenger. Cobalt comns (B i2) acting as homogeneous catalysts in acetonitrile-methanol solutions induced the formation of formic acid and CO.172... 

Other potentiometric electrode systems are ion-selective electrodes such as fluoride, calcium, magnesium, sodium, potassium and chloride, selective gas electrodes based on membranes such as 02, C02, CO, NO, N02 and S02, and enzyme electrodes. These electrodes fall beyond the scope of this book and are not discussed further. 

OUABAIN A chemical of botanic origin that inhibits sodium-potassium-activated ATPase in cell membranes, thereby being toxic to the cell the selection of mutants that are resistant to the toxic effects of ouabain provides the basis of a mutation-detection system in mammalian cells. 

In the myocardium, automaticity is the ability of the cardiac muscle to depolarize spontaneously (i.e., without external electrical stimulation from the autonomic nervous system). This spontaneous depolarization is due to the plasma membrane within the heart that has reduced permeability to potassium (K+) but still allows passive transfer of calcium ions, allowing a net charge to build. Automaticity is most often demonstrated in the sinoatrial (SA) node, the so-called pacemaker cells. Abnormalities in automaticity result in rhythm changes. The mechanism of automaticity involves the pacemaker channels of the HCN (Hyperpolarization-activated, Cyclic Nucleotide-gated) family14 (e.g., If, "funny" current). These poorly selective cation channels conduct more current as the membrane potential becomes more negative, or hyperpolarized. They conduct both potassium and sodium ions. The activity of these channels in the SA node cells causes the membrane potential to slowly become more positive (depolarized) until, eventually, calcium channels are activated and an action potential is initiated. 

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