Cellular/Molecular Separate Ca2 Sources Are Buffered by Distinct Ca2 Handling Systems in

Although the contributionofCa2buffering systems canvarybetweenneuronal types and cellular compartments, it is unknownwhether distinct Ca2 sources within a neuron have different buffers. As individual Ca2 sources can have separate functions, we propose that each is handled by unique systems. Using Aplysia californica bag cell neurons, which initiate reproduction through an afterdischarge involvingmultiple Ca2-dependent processes, we investigated the role of endoplasmic reticulum (ER) andmitochondrial sequestration, as well as extrusion via the plasmamembrane Ca2-ATPase (PMCA) and Na/Ca2 exchanger, to the clearance of voltage-gated Ca2 influx, Ca2-induced Ca2-release (CICR), and store-operated Ca2 influx. Cultured bag cell neurons were filled with the Ca2 indicator, fura-PE3, to image Ca2 under whole-cell voltage clamp. A 5 Hz, 1 min train of depolarizing voltage steps elicited voltage-gated Ca2 influx followedbyEGTA-sensitive CICR from themitochondria. A compartmentmodel of Ca2 indicated the effect of EGTAonCICRwas due to buffering of released mitochondrial Ca2 rather than uptake competition. Removal of voltage-gated Ca2 influx was dominated by the mitochondria and PMCA, with no contribution from the Na/Ca2 exchanger or sarcoplasmic/endoplasmic Ca2-ATPase (SERCA). In contrast, CICR recoverywas slowed by eliminating theNa/Ca2 exchanger and PMCA. Last, store-operated influx, evoked by ER depletion, was removed by the SERCA and depended on the mitochondrial membrane potential. Our results demonstrate that distinct buffering systems are dedicated to particular Ca2 sources. In general, this may represent a means to differentially regulate Ca2-dependent processes, and for Aplysia, influence how reproductive behavior is triggered