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

Giant nerve terminals offer a unique opportunity to learn about dynamic changes in intracellular global Ca(2+) concentration ([Ca(2+)]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 [Ca(2+)]i signalling in the calyx of Held and discuss what these measurements can tell us about endogenous Ca(2+) buffers and Ca(2+) extrusion mechanisms. We conclude that in spite of the favourable experimental conditions, some unresolved questions still remain regarding absolute values for the Ca(2+)-binding ratio, the affinity of the basic fixed buffer and the Ca(2+) affinities of the major endogenous Ca(2+) binding proteins. Uncertainties about some of these presynaptic properties, including the roles of Mg(2+) and ATP (as a Mg(2+) buffer), however, extend to the point that mechanisms controlling the decay of [Ca(2+)]i signals in unperturbed terminals may have to be reconsidered.