Na~+/Ca~(2+) Exchange and Ca~(2+) Homeostasis in Axon Terminals of Mammalian Central Neurons

摘要

We investigated Ca~(2+) clearance mechanisms (CCMs) at the axon terminals of mammalian central neurons: neurohypophysial (NHP) axon terminals and calyces of Held. Ca~(2+) transients were evoked by applying a short depolarization pulse via a patch pipette containing Ca~(2+) indicator dye. Quantitative analysis of the Ca~(2+) decay phases revealed that Na~+/Ca~(2+) exchange (Na/CaX) is a major CCM at both axon terminals. In contrast, no Na/CaX activity was found in the somata of NHP axon terminals (supraoptic magnocellular neurons), indicating that the distribution of Na~+/Ca~(2+) exchangers is polarized. Intracellular dialysis of axon terminals with a K~+-free pipette solution attenuated the Na/CaX activities by 90% in the NHP axon terminals and by 60% at the calyx of Held, indicating that K~+-dependent Na~+/Ca~(2+) exchangers are involved. Studying the effects of specific inhibitors of smooth endoplasmic reticulum Ca~(2+)-ATPase (SERCA) and plasma membrane Ca~(2+)-ATPase (PMCA) on the Ca~(2+) decay rate revealed that PMCA contributed 23% of total Ca~(2+) clearance, but that SERCA made no contribution at the calyx of Held. The contribution of mitochondria was negligible for small Ca~(2+) transients, but became apparent at peak Ca~(2+) levels higher than 2.5 μM. When mitochondrial function was inhibited, the dependence of CCMs on [Ca~(2+)]_i at the calyx of Held showed saturation kinetics with K_(1/2) = 1.7 μM, suggesting that the Na/CaX activity is saturated at high [Ca~(2+)]_i. The presynaptic Na~+/Ca~(2+) exchanger activity, which competes for cy-tosolic Ca2+ with mitochondria, may contribute to nonplastic synaptic transmission at these axon terminals.