EXO-ENDOCYTOTIC ACTIVITY DURING RECOVERY FROM A BRIEF TETANIC STIMULATION - A ROLE IN CALCIUM EXTRUSION

Synaptic transmission metabolism of calcium and ultrastructural change s were investigated at the nerve-electroplaque synapse of Torpedo marm orata during and after a brief tetanic stimulation. Calcium was found to accumulate in stimulated tissue as a function of the number of stim uli; it was subsequently expelled during the recovery period. This per iod was also accompanied by a marked hydrolysis of energy-rich phospha tes (ATP and creatine phosphate). Histochemical localization combined with electron spectroscopic imaging showed calcium deposits in synapti c vesicles and in other substructures. The number of synaptic vesicles containing a calcium deposit transiently increased at the end of acti vity 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 re sting tissue; it did not rise during the tetanic stimulation. In contr ast, the number of presynaptic pits increased significantly soon after , reaching a maximum value at 1 min after tetanus. These results are d iscussed in the light of current hypotheses. They suggest that synapti c vesicles play an important role in intraterminal calcium homeostasis . The vesicles might sequester calcium ions in synaptic terminals duri ng activity and expel them afterwards by exocytosis.