Ca2+ UPTAKE BY SYNAPTOSOMES AND ITS EFFECT ON THE INHIBITION OF ACETYLCHOLINE RELEASE BY BOTULINUM TOXIN

Abstract— ⁴⁵Ca²⁺ uptake by cerebral cortex synaptosomes was determined by gel filtration, glass fibre disc filtration under suction and by centrifugation with EGTA present. The filtration methods gave comparable results which were higher than values obtained by the centrifugation method. Uptake was increased by 25mM-K⁺ at all times investigated. The accumulated ⁴⁵Ca²⁺ was bound within the synaptosome. ⁴⁵Ca²⁺-ionophore A23187 stimulated uptake only during the first min; levels of intra-synaptosomal ⁴⁵Ca²⁺ then returned to control values. A23187 also increased intra-synaptosomal Na⁺ and Cl⁻ contents. Botulinum toxin inhibits the K.⁺-stimulated release of [¹⁴C]ACh from synaptosomes but the ionophore released [¹⁴C]ACh from both normal and botulinum-treated preparations in a Ca²⁺-dependent manner. However, it also elicited Ca²⁺-dependent release of [choline. Increased extracellular Ca²⁺ (10 mM and 20 mM) released [¹⁴C]ACh (but not [¹⁴C]choline) from both normal and botulinum-treated synaptosomes. It is concluded that botulinum toxin interferes with the provision of Ca²⁺ essential for the mechanism of ACh release.