Abstract
A decrease in the osmolarity of incubation medium is accompanied by calcium influx in neuronal presynaptic endings. We studied the influence of Ca 2+ on exocytosis induced by hypotonic shock using the hydrophilic fluorescent dye acridine orange and the hydrophobic fluorescent dye FM2–10. It was shown using acridine orange that lowering of osmolarity to 230 mOsm/l induces exocytosis both in calcium-containing and calcium-free medium. By contrast, we were able to demonstrate calcium-dependence of exocytosis using styryl dye FM2–10. Lowering of osmolarity leads to increase of [ 3H] d-aspartate and [ 3H]GABA release in calcium-free medium. Addition of calcium inhibits hypotonic-induced neurotransmitter release. Decreasing of NaCl concentration to 92 mM in isotonic medium is able to induce d-aspartate and GABA release. Thus, our data suggest that hypotonic swelling induces calcium-independent exocytosis possibly by a “kiss and run” mechanism. Calcium influx mediated by stretch channels is able to provoke full fusion between plasma membrane and synaptic vesicles. [ 3H] d-aspartate and [ 3H]GABA released by hypotonic shock is determined by sodium lowering rather than by osmolarity decreasing itself.
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