A Lithium Ion-Selective Electrode Based on a Neutral Carrier

Analytical strategies for the measurement of lithium in biological samples

Therapeutic lithium levels in the treatment of manic depressive psychosis must be maintained in the range of 0.5-1.5 mM in the blood, which also contains 140 mM sodium. This paper reviews spectrophotometric, fluorometric, and ion-selective electrode (ISE) reagents and methods for achieving high lithium selectivity over sodium and their use in blood lithium measurement. These include aromatic organic reagents, crown ethers and amide ionophores. Crown ethers and cryptands provide the best lithium selectivity. A chromophoric small-cavity cryptand phenol exhibits greater than 4000:1 selectivity due to rigid configuration of a well preorganized binding site for lithium complexation. It is water soluble, making it easy to apply for blood analysis. Crown ethers with bulky groups inhibit formation of the 2:1 crown:sodium complex, while allowing formation of the 1:1 lithium complex. A PTM 14-crown-4 having a bulky pinane and subunits at the ethano bridge exhibits at least 10,000:1 selectivity for lithium in a flow-through optical sensor probe. Bulky crown ethers used in PVC membrane ion-selective electrodes exhibit lithium selectivities of 1-2000:1. Methods of evaluating selectivities are discussed, along with the correlation of solvent extraction of crown ether complexes and solvent membrane ISE selectivities.

Structure-activity relationships among noncyclic dicarboxamide Li(+)-selective carriers studied in lipid bilayer membranes

The structure-activity relationships among three noncyclic diimide ionophores, designed to be Li+ carriers, were studied in lipid bilayer membranes. These ionophores (ETH1644, ETH1810, and ETH1811) vary in their N-imide substituents, going from two isobutyls to one isobutyl and one cyclohexyl to two cyclohexyls, respectively. ETH1811 was found to form two types of complexes with 1:1 and 1:2 ion-carrier stoichiometries, the former type dominant over most of the ionophore (0.1-10 microM) and salt (0.01-1.0 M) concentration ranges studied. In contrast, ETH1644 and ETH1810 were previously found to form a single type of complex with the 1:2 stoichiometry. The alkali cations selectivity sequence induced by ETH1811 is Li+ (1) greater than Na+ (0.08) greater than K+ (0.02) greater than Cs+ (0.008). These ETH1811-induced ionic selectivites as well as its relative potency in ion transport were found to be inferior to those of ETH1644 and ETH1810 (the latter being the best in this series). The conductance-voltage relationships reported here, for all three ionophores transporting alkali cations, were found to fit with a transport mechanism in which the diffusion of the ion-ionophore complex across the membrane is the single rate-limiting step, with the following exceptions: The addition of the dissociation of the ion-carrier complex as a second rate limiting step, for the Li(+)-ETH1810 and the Li(+)-ETH1811 systems. For ETH1810 the kinetics of the dissociation step is a minor, whereas for ETH1811 case it is a significant, addition (the ratio of the diffusion to dissociation rate constants being 0.08 and 0.2, respectively). The implications of the effects of the continuous structural change of these ionophores on their performance as ion carriers and on the design and synthesis of improved Li(+)-selective ionophores are discussed.

Lithium-selective permeation through lipid bilayer membranes mediated by a di-imide ionophore with nonsymmetrical imide substituents (ETH1810)

The neutral, noncyclic Li(+)-selective ionophore ETH1810, which is a di-imide, differs structurally from previous similar ionophores by removal of the intramolecular symmetry of the N-imide substituents. Properties of this ionophore, as a potential carrier of lithium, were probed through studies of ionophore-induced changes in electrical properties of lipid bilayer membranes. ETH1810 was found capable of transporting lithium and other monovalent cations, across lipid bilayer membranes, forming 2:1 ionophore:ion membrane-permeating species. It was found to be 10-fold more potent than ETH1644, which was the previous best ionophore of this type. The selectivity sequence among alkali cations was found to be: Li+ (1) greater than Na+ (0.009) greater than K+ (0.004) greater than Cs+ (0.0035). Among the physiological alkali cations, it constitutes a 40 (vs. Na+) to 160% (vs. K+) improvement over ETH1644. ETH1810 was also found to be capable of acting as a carrier of biogenic amines and related molecules, with the following selectivity sequence:tryptamine (20) greater than phenylethylamine (7.8) greater than tyramine (4.3) greater than serotonin (2.5) greater than Li+ (1) greater than NH+4 (0.013) greater than dopamine (0.012). It was found that protons, at physiological concentrations, do not interfere with the lithium transport mediated by ETH1810. The relationship between the improvements in ionic selectivity and potency vs. the differences in structural features is discussed.

Selective transport of Li+ across lipid bilayer membranes mediated by an ionophore of novel design (ETH1644)

The neutral noncyclic, lithium-selective ionophore ETH1644, which is structurally different from previously available ionophores of this type, is a selective carrier of Li+ in lipid bilayer membranes of various lipid composition. The ionophore forms a 2:1 carrier/cation complex, and the rate-limiting step in the overall transport process is the diffusion of the carrier/ion complex across the membrane. The selectivity sequence for lithium vs. other ions normally found in biological systems is: Li+ (1) greater than Na+ (0.017) greater than or equal to K+ (0.017) greater than Cl- (0.001), Ca2+ and Mg2+ are impermeant. At neutral pH protons do not interfere with the Li+-carrying ability of this ionophore. On the basis of structural differences and supported by conductance data, it is argued that the improved selectivity of Li+ over the other alkali cations is due more to a decrease in the affinities of the ionophore for the latter cations that to an increase of its affinity to Li+. This ionophore can also act as a carrier of biogenic amines (catecholes, indoles and derivatives), with the structure of the permeant species and mechanism of permeation similar to that observed with the alkali cations. The selectivity sequence is: tryptamine (18.1) greater than phenylethylamine (11.6) greater than tyramine (2.4) greater than Li+ (1) greater than serotonin (0.34) greater than epinephrine (0.09) greater than dopamine (0.05) greater than norepinephrine (0.02), showing the ionophore to be more selective to Li+ than to any of the neurotransmitters studied.

Changes of intracellular sodium and potassium ion concentrations in frog spinal motoneurons induced by repetitive synaptic stimulation

A post-tetanic membrane hyperpolarization following repetitive neuronal activity is a commonly observed phenomenon in the isolated frog spinal cord as well as in neurons of other nervous tissues. We have now used double-barrelled Na+- and K+-ion-sensitive microelectrodes to measure the intracellular Na+- and K+-concentrations and also the extracellular K+-concentration of lumbar spinal motoneurons during and after repetitive stimulation of a dorsal root. The results show that the post-tetanic membrane hyperpolarization occurred at a time when the intracellular [Na+] reached its maximal value, intracellular [K+] had its lowest level and extracellular [K+] was still elevated. The hyperpolarization was blocked by ouabain and reduced by Li+. These data support the previous suggestion that an electrogenic Na+/K+ pump mode may be the mechanism underlying the post-tetanic membrane hyperpolarization.

New Li+-selective ionophores with the potential ability to mediate Li+-transport in vivo. Ionic selectivity and relative potencies, studied in model membranes

A series of structurally-related Li+-selective ionophores were studied in planar lipid bilayer membranes, to assess their potential ability to act as Li+-selective carriers in vivo. The ionophores are acyclic, neutral molecules of similar structure: N,N'-diheptyl-N,N'-diR-5,5-dimethyl-3,7-dioxanonane diamide. The structural differences among them are the N-amide substituents (the R residues) as follows: an aliphatic ether (AS701), tetrahydrofuran (AS706), furan (AS708), an ester (AS702) and an amide (AS704). For each ionophore, the steady-state, single salt, membrane conductances and conductance-voltage behaviors were determined in the presence of LiCl, NaCl and MgCl2. Membrane zero-current potentials were measured for NaCl/LiCl and MgCl2/LiCl mixtures. All five ionophores were found to operate as "equilibrium-domain" carriers of monovalent ions. All select lithium over sodium, but with different magnitudes of selectivity, ranging from PLi/PNa of 13 (for AS701) to PLi/PNa of 2 (for AS708). The ionophores also differ in their ability to mediate Li+ membrane permeation, the order of decreasing potency being: AS701 greater than or equal to AS706 greater than AS702 greater than AS704 greater than or equal to AS708. Of the five molecules studied, the AS701 molecule was found to have the best Li+ over Na+ selectivity and highest potency. These findings indicate that this molecule has the best potential for mediating lithium-selective membrane permeation in vivo, among the group studied.

Lithium distribution across the membrane of motoneurons in the isolated frog spinal cord

Lithium sensitive microelectrodes were used to investigate the transmembrane distribution of lithium ions (Li+) in motoneurons of the isolated frog spinal cord. After addition of 5 mmol.1(-1) LiCl to the bathing solution the extracellular diffusion of Li+ was measured. At a depth of 500 micrometers, about 60 min elapsed before the extracellular Li+ concentration approached that of the bathing solution. Intracellular measurements revealed that Li+ started to enter the cells soon after reaching the motoneuron pool and after up to 120 min superfusion, an intra - to extracellular concentration ratio of about 0.7 was obtained. The resting membrane potential and height of antidromically evoked action potentials were not altered by 5 mmol.1(-1) Li+.

A study of Li+-selective permeation through lipid bilayer membranes mediated by a new ionophore (AS701)

The neutral, noncyclic, imide and ether containing ionophore AS701, has been developed as Li+-selective molecule, to be used potentially as an aid in the Li+-therapy of manic-depressive illness. The present report is a characterization of this molecule in neutral lipid bilayer membranes. This ionophore was found to render the bilayers Li+-selective, acting as a selective carrier of monovalent cations. In addition, this molecule was found to be capable of acting as a selective carrier of monovalent anions. For both types of ions, the rate-limiting step in the process of permeation was found to be the diffusion of the carrier-ion complex through the membrane. The membrane-permeating species were found to be 2 : 1 carrier-ion complexes, carrying either a monovalent cation or a monovalent anion. The selectivity sequence among the ions studied being: Li+(1) greater than ClO4-(0.7) greater than Na+(0.07) greater than K+(0.016) greater than Rb+(0.0095) greater than Cs+(0.0083) greater than Cl-(0.001). Mg2+ and SO42- were found to be impermeant (under present experimental conditions). This sequence shows that the AS701 molecule has low selectivity for ions present in biological media, among those studied (i.e. Na+, K+, Mg2+, Cl- and SO42-). This indicates that these ions will not interfere in the Li+ permeability induced by this carrier in vivo, and that the carrier will not interfere in the normal transport processes of these ions.