The relationship between the free cytoplasmic Ca2+ concentration, [Ca2+](i), and neurotransmitter release was investigated in guinea pig brain synaptosomes and the neurosecretory cell line PC12. Release was induced by α-latrotoxin, which acts in both Ca2+-containing and Ca2+-free incubation media, or by the classical depolarizing agents high K+ and veratridine, which require extracellular Ca2+. Two complementary approaches were used to reveal changes of [Ca2+](i): (i) direct measurement by a fluorescent Ca2+ indicator (quin2) and (ii) study of the Ca2+-dependent phosphorylation of a protein, synapsin I, located at the cytoplasmic surface of synaptic vesicles. Depolarizing agents, when applied in Ca2+-containing medium, induced the [Ca2+](i) to increase promptly 3- to 6-fold, drastically increased synapsin I phosphorylation, and caused stimulation of transmitter release. With α-latrotoxin, the [Ca2+](i) increase was delayed and occurred at a slower rate, the increase of synapsin I phosphorylation was less drastic, and the release response was much more pronounced. In Ca2+-free medium, depolarizing agents released no transmitter and had no effect on [Ca2+](i) or synapsin I phosphorylation, whereas with α-latrotoxin these processes were dissociated: considerable stimulation of the release without apparent change of [Ca2+](i) and synapsin I phosphorylation. We conclude that the relationship between average [Ca2+](i) and transmitter release is not straightforward and, in particular, that the release evoked by α-latrotoxin in Ca2+-free medium is mediated by a factor(s) other than bulk redistribution of Ca2+ from intracellular stores.
|Number of pages||5|
|Journal||Proceedings of the National Academy of Sciences of the United States of America|
|Issue number||2 I|
|Publication status||Published - 1984|
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