Binding of Lanthanum Ions and Ruthenium Red to Synaptosomes and Its Effects on Neurotransmitter Release

Abstract

A technique for studying the binding of La3+ to synaptosomes in a double-beam spectrophotometer, using mureide as indicator, is described. The binding of La3+ was very rapid and scatchard plots revealed two components, with KD values of 0.6 and 27 .mu.M in a Na+-free medium (sucrose medium) and 2.3 and 63 .mu.M in an ionic medium containing 135 mM Na+. The binding of the cationic dye ruthenium red (RuR) showed only one site, with a KD of 3.7 .mu.M. La3+ binding was partially inhibited by RuR and vice versa, and La3+ was also capable of partially displacing RuR previously bound to the synaptosomes, particularly in the sucrose medium. The release of labeled .gamma.-aminobutyric acid (GABA) stimulated by K+ depolarization was inhibited by La3+ concentrations at or above 1 .mu.M, in the ionic medium, whereas in the sucrose medium 2.5 .mu.M or higher La3+ concentrations notably stimulated the spontaneous release of both GABA and glutamic acid. It is concluded that La3+ and RuR share at least one type of binding site, which is probably the high-affinity La3+ site. Since both La3+ and RuR at low concentrations have been shown to block the depolarization-induced Ca2+ entry in synaptosomes, this site might be related to the voltage-dependent Ca2+ entry involved in neurotransmitter release.