Exo-endocytotic activity during recovery from a brief tetanic stimulation: a role in calcium extrusion?

Synaptic transmission, metabolism of calcium and ultrastructural changes were investigated at the nerve-electroplaque synapse of Torpedo marmorata during and after a brief tetanic stimulation. Calcium was found to accumulate in stimulated tissue as a function of the number of stimuli; it was subsequently expelled during the recovery period. This period was also accompanied by a marked hydrolysis of energy-rich phosphates (ATP and creatine phosphate). Histochemical localization combined with electron spectroscopic imaging showed calcium deposits in synaptic vesicles and in other substructures. The number of synaptic vesicles containing a calcium deposit transiently increased at the end of activity and declined later during the recovery phase. Rapid cryofixation of the tissue followed by freeze-fracturing revealed membrane openings (pits) in the presynaptic membrane. The density of pits was low in resting tissue; it did not rise during the tetanic stimulation. In contrast, the number of presynaptic pits increased significantly soon after, reaching a maximum value at 1 min after tetanus. These results are discussed in the light of current hypotheses. They suggest that synaptic vesicles play an important role in intraterminal calcium homeostasis. The vesicles might sequester calcium ions in synaptic terminals during activity and expel them afterwards by exocytosis.