EPFL | Biomedical Imaging Group

Spontaneous Na+ Transients in Individual Mitochondria of Intact Astrocytes

G. Azarias, D. Van De Ville, M. Unser, J.-Y. Chatton

Proceedings of the Fortieth Annual Meeting of the Union of Swiss Societies for Experimental Biology (USGEB'08), Lausanne VD, Swiss Confederation, February 6-7, 2008, pp. 39.

Mitochondria in intact cells maintain low levels of Na+ despite the large electrochemical gradient favoring cation influx into the matrix. In addition, they display individual spontaneous transient depolarization. We report here that individual mitochondria in living astrocytes exhibit spontaneous increases in their Na+ concentration (Na+mit spiking), as measured using CoroNa Red, a fluorescent probe specific for mitochondrial Na+. We developed an image analysis algorithm to assess the frequency and intensity of Na+mit spiking activity. In a field of view with 30 astrocytes, up to 1400 transients per minute were typically detected under resting conditions, each averaging 12.2±0.8 sec in duration and 35.5±3.2 mM in amplitude. Na+mit spikes coincided with brief mitochondrial depolarizations; they were impaired by mitochondrial depolarization and ruthenium red pointing to the involvement of a cation uniporter. Na+mit spiking activity was significantly inhibited by mitochondrial Na+/H+ exchanger inhibition and sensitive to cellular pH and Na+ concentration. Ca2+ played a permissive role on Na+mit spiking activity. Finally, Na+mit spiking frequency was correlated with cellular ATP level. We observed similar Na+mit spiking behaviour in neurons. This study shows that, under physiological conditions, individual mitochondria exhibit fast Na+ exchange across their inner membrane which reveals a new form of highly dynamic and localized functional regulation.

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