Competition between Na(+) and Li(+) for unsealed and cytoskeleton-depleted human red blood cell membrane: a (23)Na multiple quantum filtered and (7)Li NMR relaxation study

Sodium NMR/MRI for anisotropic systems

Sodium ((23)Na) plays a central role in many physiological processes, and its high NMR sensitivity makes it an attractive nucleus for biomedical NMR and MRI research. Many biological tissues contain structures such as fibers and membranes that impose anisotropic translational and rotational motions on the sodium ions. Translational motion can be studied by diffusion measurements. Anisotropic rotational motion results in non-vanishing quadrupolar interaction that it is best studied by exploiting multiple quantum coherences for (23)Na NMR spectroscopy and MRI. The current review covers the application of the various NMR techniques to the study of (23)Na in anisotropic compartments in cartilage, tendon, intervertebral discs, red blood cells, nervous system and muscles.

Selecting ordered environments in NMR of spin 3/2 nuclei via frequency-sweep pulses

We demonstrate that frequency-swept pulses can be used for the selective and enhanced detection of quadrupolar nuclei located in anisotropic environments. The primary driving force for this technique development is the field of sodium-MRI, where sodium signals from locally ordered environments are known to be diagnostic of cartilage defects. We demonstrate here simple one-dimensional images of model systems, in which the signals from free sodium ions are suppressed, while ordered sodium is detected via the narrow central transition signal.

Identification of Li+ binding sites and the effect of Li+ treatment on phospholipid composition in human neuroblastoma cells: a 7Li and 31P NMR study

Li(+) binding in subcellular fractions of human neuroblastoma SH-SY 5 Y cells was investigated using (7)Li NMR spin-lattice (T(1)) and spin-spin (T(2)) relaxation measurements, as the T(1)/T(2) ratio is a sensitive parameter of Li(+) binding. The majority of Li(+) binding occurred in the plasma membrane, microsomes, and nuclear membrane fractions as demonstrated by the Li(+) binding constants and the values of the T(1)/T(2) ratios, which were drastically larger than those observed in the cytosol, nuclei, and mitochondria. We also investigated by (31)P NMR spectroscopy the effects of chronic Li(+) treatment for 4--6 weeks on the phospholipid composition of the plasma membrane and the cell homogenate and found that the levels of phosphatidylinositol and phosphatidylserine were significantly increased and decreased, respectively, in both fractions. From these observations, we propose that Li(+) binding occurs predominantly to membrane domains, and that chronic Li(+) treatment alters the phospholipid composition at these membrane sites. These findings support those from clinical studies that have indicated that Li(+) treatment of bipolar patients results in irregularities in Li(+) binding and phospholipid metabolism. Implications of our observations on putative mechanisms of Li(+) action, including the cell membrane abnormality, the inositol depletion and the G-protein hypotheses, are discussed.

Biomedical applications of 7Li NMR

The biomedical applications of 7Li MRS and MRI have been progressing slowly. The interest derives primarily from the clinical use of Li to treat bipolar disorder. One area of concern is the nature of ionic transport and binding, so as to elucidate the mechanism(s) of therapeutic action and toxicity. Another is the development of a non-invasive, in vivo analytical tool to measure brain Li concentration and environment in humans, both as an adjunct to treatment and as a mechanistic probe. Here we review the most recent progress toward these goals.

Competition between lithium and magnesium ions for the G-protein transducin in the guanosine 5'-diphosphate bound conformation

Li(+) is the most effective drug used to treat bipolar disorder; however, its exact mechanism of action has yet to be elucidated. One hypothesis is that Li(+) competes with Mg2+ for the Mg2+ binding sites on guanine-nucleotide binding proteins (G-proteins). Using 7Li T1 relaxation measurements and fluorescence spectroscopy with the Mg2+ fluorophore furaptra, we detected Li(+)/Mg(2+) competition in three preparations: the purified G-protein transducin (Gt), stripped rod outer segment membranes (SROS), and SROS with purified Gt reattached (ROS-T). When purified ROS-T, SROS or transducin were titrated with Li+ in the presence of fixed amounts of Mg(2+), the apparent Li(+) binding constant decreased due to Li(+)/Mg(2+) competition. Whereas for SROS the competition mechanism was monophasic, for G(t), the competition was biphasic, suggesting that in G(t), Li(+)/Mg(2+) competition occurred with different affinities for Mg(2+) in two types of Mg(2+) binding sites. Moreover, as [Li(+)] increased, the fluorescence excitation spectra of both ROS-T and G(t) were blue shifted, indicating an increase in free [Mg(2+)] compatible with Li(+) displacement of Mg(2+) from two low affinity Mg(2+) binding sites of G(t). G(t) release from ROS-T membrane was also inhibited by Li(+) addition. In summary, we found evidence of Li(+)/Mg(2+) competition in G(t)-containing preparations.

Effects of Li+ transport and Li+ immobilization on Li+/Mg2+ competition in cells: implications for bipolar disorder

Li(+)/Mg(2+) competition has been implicated in the therapeutic action of Li(+) treatment in bipolar illness. We hypothesized that this competition depended on cell-specific properties. To test this hypothesis, we determined the degree of Li(+) transport, immobilization, and Li(+)/Mg(2+) competition in lymphoblastomas, neuroblastomas, and erythrocytes. During a 50 mM/L Li(+)-loading incubation, Li(+) accumulation at 30 min (mmoles Li(+)/L cells) was the greatest in lymphoblastomas (11.1+/-0.3), followed by neuroblastomas (9.3+/-0.5), and then erythrocytes (4.0+/-0.5). Li(+) binding affinities to the plasma membrane in all three cell types were of the same order of magnitude; however, Li(+) immobilization in intact cells was greatest in neuroblastomas and least in erythrocytes. When cells were loaded for 30 min in a 50 mM/L Li(+)-containing medium, the percentage increase in free intracellular [Mg(2+)] in neuroblastoma and lymphoblastoma cells ( approximately 55 and approximately 52%, respectively) was similar, but erythrocytes did not exhibit any substantial increase ( approximately 6%). With the intracellular [Li(+)] at 15 mM/L, the free intracellular [Mg(2+)] increased by the greatest amount in neuroblastomas ( approximately 158%), followed by lymphoblastomas ( approximately 75%), and then erythrocytes ( approximately 50%). We conclude that Li(+) immobilization and transport are related to free intracellular [Mg(2+)] and to the extent of Li(+)/Mg(2+) competition in a cell-specific manner.

Triple-quantum-filtered (23)Na NMR spectroscopy of subcutaneously implanted 9l gliosarcoma in the rat in the presence of TmDOTP(5-1)

The utility of triple-quantum (TQ)-filtered (23)Na NMR spectroscopy for discriminating between intra- and extracellular Na(+)(Na(i)(+) and Na(e)(+), respectively) in a solid tumor in vivo was evaluated using TmDOTP(5-) as a (23)Na shift reagent. Infusion of 80 mM TmDOTP(5-) without added Ca(2+) produced baseline-resolved Na(i)(+) and Na(e)(+) peaks in both single-quantum (SQ) and TQ-filtered (23)Na spectra. The Na(i)(+) signal represented 22+/-4% of the SQ spectrum, but 59+/-10% of the TQ-filtered spectrum. Therefore, the Na(i)(+) contribution in TQ-filtered spectra is much higher than in SQ spectra. Both SQ and TQ-filtered Na(i)(+) signals increased by about 75% 1 h after sacrificing the animal. The TQ-filtered relaxation times did not change during this time, indicating that changes observed in TQ-filtered spectra collected with a preparation time of 3 ms represent changes in the concentration of sodium ions contributing to the TQ-filtered signal. Similar experiments were conducted without TmDOTP(5-) to determine changes in the Na(e)(+) signal in the absence of the shift reagent. The changes in total SQ and TQ-filtered signals 1 h after sacrificing the animal showed that the SQ Na(e)(+) signal decreased by approximately 35%, while the TQ-filtered Na(e)(+) signal did not change significantly. This demonstrates that the TQ-filtered (23)Na signal is relatively insensitive to changes in Na(e)(+) content. To our knowledge, this work represents the first evaluation of multiple-quantum-filtered (23)Na spectroscopy to discriminate between intra- and extracellular Na(+) in a solid tumor in vivo.

Current awareness

In order to keep subscribers up-to-date with the latest developments in their field, John Wiley & Sons are providing a current awareness service in each issue of the journal. The bibliography contains newly published material in the field of NMR in biomedicine. Each bibliography is divided into 9 sections: 1 Books, Reviews ' Symposia; 2 General; 3 Technology; 4 Brain and Nerves; 5 Neuropathology; 6 Cancer; 7 Cardiac, Vascular and Respiratory Systems; 8 Liver, Kidney and Other Organs; 9 Muscle and Orthopaedic. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted.