Li⁺ Influx and Binding, and Li⁺/Mg²⁺ Competition in Bovine Chromaffin Cell Suspensions as Studied by ⁷Li NMR and Fluorescence Spectroscopy

Abstract

Li⁺ influx by bovine chromaffin cells, obtained from bovine adrenal medulla, was studied in intact cell suspensions using ⁷Li NMR spectroscopy with the shift reagent [Tm(HDOTP)]^4-. The influx rate constants, k_i, were determined in the absence and in the presence of two Na⁺ membrane transport inhibitors. The values obtained indicate that both voltage sensitive Na⁺ channels and (Na⁺/K⁺)-ATPase play an important role in Li⁺ uptake by these cells. ⁷Li NMR T₁ and T₂ relaxation times for intracellular Li⁺ in bovine chromaffin cells provided a T₁/T₂ ratio of 305, showing that Li⁺ is highly, immobilized due to strong binding to intracellular structures. Using fluorescence spectroscopy and the Mg²⁺ fluorescent probe, furaptra, the free intracellular Mg²⁺ concentration in the bovine chromaffin cells incubated with 15 mM LiCl was found to increase by about mM after the intracellular Li⁺ concentration reached a steady state. Therefore, once inside the cell, Li⁺ is able to displace Mg²⁺ from its binding sites.