Because Mg2+ and Li+ ions have similar chemical properties, we have hypothesized that Li+/Mg2+ competition for Mg2+ binding sites is the molecular basis for the therapeutic action of lithium in manic-depressive illness. By fluorescence spectroscopy with furaptra-loaded cells, the free intracellular Mg2+ concentration within the intact neuroblastoma cells was found to increase from 0. 39 +/- 0.04 mM to 0.60 +/- 0.04 mM during a 40-min Li+ incubation in which the total intracellular Li+ concentration increased from 0 to 5.5 mM. Our fluorescence microscopy observations of Li+-free and Li+-loaded cells also indicate an increase in free Mg2+ concentration upon Li+ incubation. By 31P NMR, the free intracellular Mg2+ concentrations for Li+-free cells was 0.35 +/- 0. 03 mM and 0.80 +/- 0.04 mM for Li+-loaded cells (final total intracellular Li+ concentration of 16 mM). If a Li+/Mg2+ competition mechanism is present in neuroblastoma cells, an increase in the total intracellular Li+ concentration is expected to result in an increase in the free intracellular Mg2+ concentration, because Li+ displaces Mg2+ from its binding sites within the nerve cell. The fluorescence spectroscopy, fluorescence microscopy, and 31P NMR spectroscopy studies presented here have shown this to be the case.

Lithium has been used clinically in the treatment of manic depression. However, its pharmacologic mode of action remains unclear. Characteristics of Li ± interactions in red blood cells (RBCs) have been identified. We investigated Li + interactions on human neuroblastoma SH-SY5Y cells by developing a novel 7Li NMR method that provided a clear estimation of the intra-and extracellular amounts of LV in the presence of the shift reagent thulium-1,4,7,10tetrazacyclododecane-N , N', N", N"-tetramethylene phosphonate (HTmDOTP4). The first-order rate constants of Li + influx and efflux for perfused, agarose-embedded SH-SY5Y cells in the presence of 3 mM HTmDOTP4 were 0.055 ±0.006 (n = 4) and -0.025 ±0.006 min~1(n = 3), respectively. Significant increases in the rate constants of Li + influx and efflux in the presence of 0.05 mM veratridine indicated the presence of Nachannel-mediated Litransport in SH-SY5Y cells. 7Li NMR relaxation measurements showed that Li~is immobilized more in human neuroblastoma SH-SY5Y cells than in human RBCs. A 0, starting area of the intracellular resonance of Li * -loaded cells in Li + efflux measurements; A~, intracellular resonance area at time t; AA, atomic absorption; DIDS, 4,4 '-diisothiocyanatostilbene-2,2 '-disulfonic acid; DMEM, Dulbecco's modified Eagle's medium; HTmDOTP 4~, thulium-1,4,7,1 0-tetrazacyclododecane-N, N', N", N"-tetramethylenephosphonate; PCr, phosphocreatine; P,, inorganic phosphorus; ppm, parts per million; RBC, red blood cell; T,, spin-lattice relaxation value; T 2, spin-spin relaxation value; v0, initial rate of Li * influx.

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