Li+ as substrate of the synaptosomal Na+/H+ antiporter

The influence of replacing external Na+ by choline+ on Li+ uptake into rat cortical synaptosomes was studied. Tetraphenylphosphonium+ and methylamine distribution techniques were used to estimate the plasma membrane potential and the transmembrane H+ gradients, respectively. H+ efflux was monitored by automatic titration in the pH-stat mode. In the Na+- and K+-free medium, synaptosomes concentrated Li+ about 10-fold at 1 mM Li+o in the presence of ouabain. Varying external free Ca2+ between 13 and 300 microM, or addition of MgCl2 had no effect on Li+ uptake. Ouabain-insensitive Li+ transport was separated into two components: 1) non-saturable Li+ influx with a rate constant of 0.6/min; 2) saturable uptake, which obeyed Michaelis-Menten kinetics (Km, 2.0 mM Li+; Vmax, 7.3 mmol of Li+/liter and min). Li+ uptake was competitively inhibited by amiloride (Ki, 3.2 microM; Hill coefficient, 1.0) and external Na+ (Ki, 5.8 mM). External Li+ scarcely accelerated Na+ efflux and phloretin failed to inhibit Li+ uptake, indicating that Li+ uptake was not directly coupled to Na+ gradient. Because of a reversal of the H+ transport by the pHi-regulating system, synaptosomes accumulated acid in the Na+-free medium. Li+ influx was electroneutral, but impaired H+ gradients and was coupled to the simultaneous release of stoichiometric amounts of H+ at less than 3 mM Li+o. Uptake of Li+ was linearly related to H+ gradients imposed onto the plasma membrane by varying external pH. In the steady state, internal Li+ was close to the value predicted for passive distribution. It is concluded that in Na+-free media Li+ uptake at low external Li+ is predominantly driven by transmembrane H+ gradients. The stoichiometric exchange of Li+ for H+ is mediated by the Na+/H+ antiporter. The Li+ distribution ratio is close to the electrochemical activity coefficient since protons are passively distributed across the synaptosomal plasma membrane in the absence of external Na+.