Regulation of mitochondrial ATP synthesis by calcium: Evidence for a long-term metabolic priming

In recent years, mitochondria have emerged as important targets of agonist- dependent increases in cytosolic Cal-c concentration. Here, we analyzed the significance of Ca2+ signals for the modulation of organelle function by d irectly measuring mitochondrial and cytosolic ATP levels ([ATP](m) and [ATP ](c), respectively) with specifically targeted chimeras of the ATP-dependen t photoprotein luciferase. In both HeLa cells and primary cultures of skele tal myotubes, stimulation with agonists evoking cytosolic and mitochondrial Ca2+ signals caused increases in [ATP], and [ATP], that depended on two pa rameters: (i) the amplitude of the Ca2+ rise in the mitochondrial matrix, a nd (ii) the availability of mitochondrial substrates. Moreover, the Ca2+ el evation induced a long-lasting priming that persisted long after agonist wa shout and caused a major increase in [ATP], upon addition of oxidative subs trates. These results demonstrate a direct role of mitochondrial Ca2+ in dr iving ATP production and unravel a form of cellular memory that allows a pr olonged metabolic activation in stimulated cells.