Characterization of the exocytotic release of glutamate from guinea-pig cerebral cortical synaptosomes.

A continuous enzyme-linked fluorometric assay was used for determining the characteristics for glutamate exocytosis from guinea-pig cerebrocortical synaptosomes. Ca2+-dependent release can be induced not only by K+, but also by the Na+ channel activator veratridine and the Ca2+ ionophore ionomycin. K+-induced release can be inhibited by the Ca2+ channel inhibitor verapamil. Sr2+ and Ba2+ substitute for Ca2+ in promoting K+-induced release. Agents that would be predicted to transform the transvesicular pH gradient into a membrane potential are without effect on glutamate release. However, the protonophore carbonylcyanide p-trifluoromethoxyphenylhydrazone causes a time-dependent loss of exocytosis that is oligomycin insensitive and may be due to depletion of vesicular glutamate. The Ca2+-independent release of glutamate from the cytosol on depolarization is unchanged or promoted by metabolic inhibitors that lower the ATP/ADP ratio. In contrast. Ca2+-dependent release is ATP dependent and is blocked by the combined inhibition of oxidative phosphorylation and glycolysis.