Mitochondria Exert a Negative Feedback on the Propagation of Intracellular Ca(2+) Waves in Rat Cortical Astrocytes

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