Waves in Rat Cortical Astrocytes

We have used digital fluorescence imaging techniques to explore the interplay between mitochondrial Ca 2 1 uptake and physiological Ca 2 1 signaling in rat cortical astrocytes. A rise in cytosolic Ca 2 1 ([Ca 2 1 ] cyt ), resulting from mobilization of ER Ca 2 1 stores was followed by a rise in mitochondrial Ca 2 1 ([Ca 2 1 ] m , monitored using rhod-2). Whereas [Ca 2 1 ] cyt recovered within z 1 min, the time to recovery for [Ca 2 1 ] m was z 30 min. Dissipating the mitochondrial membrane potential ( Dc m , using the mitochondrial uncoupler carbonyl cyanide p -trifluoromethoxy-phenylhydrazone [FCCP] with oligomycin) prevented mitochondrial Ca 2 1 uptake and slowed the rate of decay of [Ca 2 1 ] cyt transients, suggesting that mitochondrial Ca 2 1 uptake plays a significant role in the clearance of physiological [Ca 2 1 ] cyt loads in astrocytes. Ca 2 1 signals in these cells initiated either by receptor-mediated ER Ca 2 1 release or mechanical stimulation often consisted of propagating waves (measured using fluo-3). In response to either stimulus, the wave traveled at a mean speed of 22.9 6 11.2 m m/s ( n 5 262). This was followed by a wave of mitochondrial depolarization (measured using tetramethylrhodamine ethyl ester [TMRE]), consistent with Ca 2 1 uptake into mitochondria as the Ca 2 1 wave traveled across the cell. Collapse of Dc m to prevent mitochondrial Ca 2 1 uptake significantly increased the rate of propagation of the Ca 2 1 waves by 50%. Taken together, these data suggest that cytosolic Ca 2 1 buffering by mitochondria provides a potent mechanism to regulate the localized spread of astrocytic Ca 2 1