Calcium-Binding of synaptosomes isolated from rat brain cortex

Abstract

As in our previous report (Kamino, Uyesaka & Inouye,J. Membrane Biol. 17:13, 1974), the absorbance changes of murexide caused by Ca²⁺ and followed up by a dual wavelength spectrophotometer were applied to measure synaptosomal Ca²⁺-binding in the presence of cations such as Rb⁺, Mn²⁺ or La³⁺. All the cations tested showed a significant inhibition of synaptosomal Ca²⁺-binding except Li⁺. The inhibitory effects could be divided into the following three categories: (1) noncompetitive, co-operative K⁺-type, which includes alkali metal ions. The potency of inhibition is K⁺>Rb⁺>Cs⁺>Li⁺, Na⁺=0; (2) competitive Mn²⁺-type which includes many divalent cations. The inhibitory potency was found to be in the following order: Mn²⁺≫Sr²⁺>Cd²⁺, Ba²⁺>Mg²⁺; (3) nonspecific, noncompetitive La³⁺-type: among the cations tested, La³⁺ and Ce³⁺ were found to markedly reduce the Ca-binding capacity of synaptosomal particles, resulting in a noncompetitive inhibition, at least in the range of Ca²⁺ concentration used.