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

Instituto de Biología y Genética Molecular (IBGM), Universidad de Valladolid y Consejo Superior de Investigaciones Científicas, Departamento de Fisiología y Bioquímica, Facultad de Medicina, E-47005 Valladolid, Spain; and
^* Instituto Teófilo Hernando, Departamento de Farmacología y Terapéutica, Facultad de Medicina, Universidad Autónoma de Madrid, E-28029 Madrid, Spain.

¹Correspondence: IBGM, Departamento de Fisiología y Bioquímica, Facultad de Medicina, E-47005 Valladolid, Spain. E-mail: jgsancho{at}ibgm.uva.es

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.—Villalobos, C., Nuñez, L., Montero, M., García, A. G., Alonso, M. T., Chamero, P., Alvarez, J., García-Sancho, J. Redistribution of Ca²⁺ among cytosol and organella during stimulation of bovine chromaffin cells.

Key Words: calcium • mitochondria • aequorin • nucleus

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