Effects of Li~+ transport and intracellular binding on Li~+/Mg~(2+) competition in bovine chromaffin cells

摘要

Li~+ transport, intracellular immobilisation and Li~+/Mg~(2+) competition were studied in Li~+-loaded bovine chromaffin cells. Li~+ influx rate constants, k_i, obtained by atomic absorption (AA) spectrophotometry, in control (without and with ouabain) and depolarising (without and with nitrendipine) conditions, showed that L-type voltage-sensitive Ca~(2+) channels have an important role in Li~+ uptake under depolarising conditions. The Li~+ influx apparent rate constant, k_(iapp), determined under control conditions by ~7Li NMR spectroscopy with the cells immobilised and perfused, was much lower than the AA-determined value for the cells in suspension. Loading of cell suspensions with 15 mmol l~(-1) LiCl led, within 90 min, to a AA-measured total intracellular Li~+ concentration, [Li~+]_(iT) = 11.39 ± 0.56 mmol (l cells)~(-1), very close to the steady state value. The intracellular Li~+ T_1/T_2 ratio of ~7Li NMR relaxation times of the Li~+-loaded cells reflected a high degree of Li~+ immobilisation in bovine chromaffin cells, similar to neuroblastoma, but larger than for lymphoblastoma and erythrocyte cells. A 52% increase in the intracellular free Mg~(2+) concentration, Δ[Mg~(2+)]_f = 0.27 ± 0.05 mmol (l cells)~(-1) was measured for chromaffin cells loaded with the Mg~(2+)-specific fluorescent probe furaptra, after 90-min loading with 15 mmol l~(-1) LiCl, using fluorescence spectroscopy, indicating significant displacement of Mg~(2+) by Li~+ from its intracellular binding sites. Comparison with other cell types showed that the extent of intracellular Li~+/Mg~(2+) competition at the same Li~+ loading level depends on intracellular Li~+ transport and immobilisation in a cell-specific manner, being maximal for neuroblastoma cells.