Na+/Ca2+ Exchange and Ca2+ Homeostasis in Axon Terminals of Mammalian Central Neurons

doi:10.1196/annals.1387.011

We investigated Ca²⁺ clearance mechanisms (CCMs) at the axonterminals of mammalian central neurons: neurohypophysial (NHP)axon terminals and calyces of Held. Ca²⁺ transients were evokedby applying a short depolarization pulse via a patch pipettecontaining Ca²⁺ indicator dye. Quantitative analysis of theCa²⁺ decay phases revealed that Na⁺/Ca²⁺ exchange (Na/CaX) isa major CCM at both axon terminals. In contrast, no Na/CaX activitywas found in the somata of NHP axon terminals (supraoptic magnocellularneurons), indicating that the distribution of Na⁺/Ca²⁺ exchangersis polarized. Intracellular dialysis of axon terminals witha K⁺-free pipette solution attenuated the Na/CaX activitiesby 90% in the NHP axon terminals and by 60% at the calyx ofHeld, indicating that K⁺-dependent Na⁺/Ca²⁺ exchangers are involved.Studying the effects of specific inhibitors of smooth endoplasmicreticulum Ca²⁺-ATPase (SERCA) and plasma membrane Ca²⁺-ATPase(PMCA) on the Ca²⁺ decay rate revealed that PMCA contributed23% of total Ca²⁺ clearance, but that SERCA made no contributionat the calyx of Held. The contribution of mitochondria was negligiblefor small Ca²⁺ transients, but became apparent at peak Ca²⁺levels higher than 2.5 µM. When mitochondrial functionwas inhibited, the dependence of CCMs on [Ca²⁺]_i at the calyxof Held showed saturation kinetics with K_1/2 = 1.7 µM,suggesting that the Na/CaX activity is saturated at high [Ca²⁺]_i.The presynaptic Na⁺/Ca²⁺ exchanger activity, which competesfor cytosolic Ca²⁺ with mitochondria, may contribute to nonplasticsynaptic transmission at these axon terminals.

Part VIII. NCX and PMCA in Neuronal and Smooth Muscle Function