In situ fluorescence imaging of glutamate‐evoked mitochondrial Na + responses in astrocytes

Astrocytes can experience large intracellular Na + changes following the activation of the Na + ‐coupled glutamate transport. The present study investigated whether cytosolic Na + changes are transmitted to mitochondria, which could therefore influence their function and contribute to the overall intracellular Na + regulation. Mitochondrial Na + (Na   + mit ) changes were monitored using the Na + ‐sensitive fluorescent probe CoroNa Red (CR) in intact primary cortical astrocytes, as opposed to the classical isolated mitochondria preparation. The mitochondrial localization and Na + sensitivity of the dye were first verified and indicated that it can be safely used as a selective Na   + mitindicator. We found by simultaneously monitoring cytosolic and mitochondrial Na + using sodium‐binding benzofuran isophthalate and CR, respectively, that glutamate‐evoked cytosolic Na + elevations are transmitted to mitochondria. The resting Na   + mitconcentration was estimated at 19.0 ± 0.8 mM, reaching 30.1 ± 1.2 mM during 200 μM glutamate application. Blockers of conductances potentially mediating Na + entry (calcium uniporter, monovalent cation conductances, K + ATP channels) were not able to prevent the Na   + mitresponse to glutamate. However, Ca 2+ and its exchange with Na + appear to play an important role in mediating mitochondrial Na + entry as chelating intracellular Ca 2+ with BAPTA or inhibiting Na + /Ca 2+ exchanger with CGP‐37157 diminished the Na   + mitresponse. Moreover, intracellular Ca 2+ increase achieved by photoactivation of caged Ca 2+ also induced a Na   + mitelevation. Inhibition of mitochondrial Na/H antiporter using ethylisopropyl‐amiloride caused a steady increase in Na   + mitwithout increasing cytosolic Na + , indicating that Na + extrusion from mitochondria is mediated by these exchangers. Thus, mitochondria in intact astrocytes are equipped to efficiently sense cellular Na + signals and to dynamically regulate their Na + content. © 2006 Wiley‐Liss, Inc.