CATION MODULATION OF SYNAPTOSOMAL RESPIRATION

Abstract— Synaptosomes were prepared from the cerebral cortex of the adult rat by a rapid technique, involving the use of centrifugation in a Ficoll-sucrose discontinuous gradient. Adequate respiratory control ratios were obtained with glutamate and succinate plus rotenone. The addition of Na⁺ to the incubation medium stimulated synaptosomal, State-4 respiration, with a half-maximal response at 15 mM Na⁺. The stimulation by Na⁺ was inhibited by atractylate, oligomycin, ouabain or EDTA. A cooperative interaction between Na⁺ and low concentrations of Mg²⁺ was observed. A significant proportion (39 per cent) of the total Na-K ATPase (EC 3.6.1.4) activity in the discontinuous gradient was localized in the synaptosomal fraction. In the absence of exogenous Mg²⁺, Na⁺ induced a 64 per cent stimulation of the synaptosomal ATPase activity which was sensitive to ouabain. Such stimulation of ATP hydrolysis would account for the formation of increased amounts of ADP, with consequent recycling to ATP through adequately controlled oxidative phosphorylation. These observations demonstrate a significant role for transmembrane cationic gradients in the control of synaptosomal respiration and mitochondrial oxidative phosphorylation. The preparation exhibits moderate respiratory control and should prove useful in studies of integrated mitochondrial oxidative metabolism and neuronal membrane function.