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

In the present study we have investigated cytosolic and mitochondrial Ca(2+) 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 Ca(2+) from intracellular stores and produced cytosolic Ca(2+) signals in form of Ca(2+) waves propagating from the luminal to the basal cell pole. The increase in the cytosolic Ca(2+) concentration was followed by Ca(2+) uptake into mitochondria. Between onset of cytosolic and mitochondrial Ca(2+) signals there was a delay of 10.7 +/- 0.4 s. Ca(2+) 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 Ca(2+) ATPases, did not prevent Ca(2+) accumulation in mitochondria. Carbonyl cyanide m-chlorophenylhydrazone-induced Ca(2+) 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-tert-butylhydroquinone, indicating that under resting conditions mitochondria do not contain releasable Ca(2+) ions. Analysis of the propagation rate of acetylcholine-induced Ca(2+) waves revealed that inhibition of mitochondrial Ca(2+) uptake did not accelerate spreading of cytosolic Ca(2+) signals. Our experiments indicate that in the early phase of secretagogue-induced Ca(2+) signals, mitochondria behave as passive Ca(2+)-buffering elements and do not actively suppress spreading of Ca(2+) signals in pancreatic acinar cells.