Transients in global Ca2+ concentration induced by electrical activity in a giant nerve terminal

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Giant nerve terminals offer a unique opportunity to learn about dynamic changes in intracellular global Ca2+ concentration ([Ca2+]i) because this quantity can be measured precisely with indicator dyes and the composition of the intra-terminal ionic milieu can be controlled. We review here recent literature on [Ca2+]i signalling in the calyx of Held and discuss what these measurements can tell us about endogenous Ca2+ buffers and Ca2+ extrusion mechanisms. We conclude that in spite of the favourable experimental conditions, some unresolved questions still remain regarding absolute values for the Ca2+-binding ratio, the affinity of the basic fixed buffer and the Ca2+ affinities of the major endogenous Ca2+ binding proteins. Uncertainties about some of these presynaptic properties, including the roles of Mg2+ and ATP (as a Mg2+ buffer), however, extend to the point that mechanisms controlling the decay of [Ca2+]i signals in unperturbed terminals may have to be reconsidered.

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