Redistribution of Ca2+ among cytosol and organella during stimulation of bovine chromaffin cells

Recent results indicate that Ca2+ transport by organella contributes to shaping Ca2+ signals and exocytosis in adrenal chromaffin cells. Therefore, accurate measurements of [Ca2+] inside cytoplasmic organella are essential for a comprehensive analysis of the Ca2+ redistribution that follows cell stimulation. Here we have studied changes in Ca2+ inside the endoplasmic reticulum, mitochondria, and nucleus by imaging aequorins targeted to these compartments in cells stimulated by brief depolarizing pulses with high K+ solutions. We find that Ca2+ entry through voltage-gated Ca2+ channels generates subplasmalemmal high [Ca2+](c) domains adequate for triggering exocytosis. A smaller increase of [Ca2+](c) is produced in the cell core, which is adequate for recruitment of the reserve pool of secretory vesicles to the plasma membrane. Most of the Ca2+ load is taken up by a mitochondrial pool, M1, closer to the plasma membrane; the increase of [Ca2+](M) stimulates respiration in these mitochondria, providing local support for the exocytotic process. Relaxation of the [Ca2+](c) transient is due to Ca2+ extrusion through the plasma membrane. At this stage, mitochondria release Ca2+ to the cytosol through the Na+/Ca2+ exchanger, thus maintaining [Ca2+](c) discretely increased, especially at core regions of the cell, for periods that outlast the duration of the stimulus.