Ionic dependence of the velocity of release of ATP from permeabilized cholinergic synaptic vesicles

Evidence is provided to show that synaptic vesicles have an internal matrix. Suspensions of cholinergic synaptic vesicles isolated from the electric organ of Torpedo marmorata fish were permeabilized in solutions containing low concentrations of Na+ or Ca2+. The release of ATP from the vesicular matrix was 10 times more effective with Ca2+ than with Na+. We ascertained whether these two cations induced a different velocity of release of ATP from the matrix. The release of ATP was monitored with the chemiluminescent reaction of luciferin-luciferase. The light signal generated was the result of the kinetics of ATP release of the enzymatic reaction. To overcome the kinetics of the enzymatic reaction, the light records were deconvoluted. The actual kinetics of ATP release of vesicles containing Na+ or Ca2+ were coincident. To validate this result, comparison was made with ATP release from intact nerve terminals which were already deconvoluted. The results show that the real time course of release is longer than that obtained from synaptic vesicles. This was as expected given that the release of neurotransmitters is due to successive molecular steps of synaptic vesicle exocytosis.