Agonist-evoked mitochondrial Ca2+ signals in mouse pancreatic acinar cells

In the present study we have investigated cytosolic and mitochondrial Ca2+ signals in isolated mouse pancreatic acinar cells double-loaded with the fluorescent probes fluo-3 and rhod-2. Stimulation of pancreatic acinar cells with 500 nM acetylcholine caused release of Ca2+ from intracellular stores and produced cytosolic Ca2+ signals in form of Ca2+ waves propagating from the luminal to the basal cell pole. The increase in the cytosolic Ca2+ concentration was followed by Ca2+ uptake into mitochondria. Between onset of cytosolic and mitochondriaI Ca2+ signals there was a delay of 10.7 +/- 0.4 s. Ca2+ uptake into mitochondria could be inhibited with Ruthenium Red and carbonyl cyanide m-chlorophenylhydrazone, whereas 2,5-di-tert-butylhydroquinone, which inhibits sarco(endo)plasmic reticulum Ca2+ ATPases, did not prevent Ca2+ accumulation in mitochondria. Carbonyl cyanide m-chlorophenylhydrazone-induced Ca2+ release from mitochondria could only be observed after a preceding stimulation of the cell with a physiological agonist or by treatment with 2,5 di-tertbutylhydroquinone, indicating that under resting conditions mitochondria do not contain releasable Ca2+ ions. Analysis of the propagation rate of acetylcholine-induced Ca2+ waves revealed that inhibition of mitochondrial Ca2+ uptake did not accelerate spreading of cytosolic Ca2+ signals. Our experiments indicate that in the early phase of secretagogue-induced Ca2+ signals, mitochondria behave as passive Ca2+-buffering elements and do not actively suppress spreading of Ca2+ signals in pancreatic acinar cells.