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

摘要

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.