Chromaffin-cell stimulation triggers fast millimolar mitochondrial Ca2+ transients that modulate secretion

Activation of calcium-ion (Ca2+) channels on the plasma membrane and on int racellular Ca2+ stores, such as the endoplasmic reticulum, generates local transient increases in the cytosolic Ca2+ concentration that induce Ca2+ up take by neighbouring mitochondria. Here, by using mitochondrially targeted aequorin proteins with different Ca2+ affinities, we show that half of the chromaffin-cell mitochondria exhibit surprisingly rapid millimolar Ca2+ tra nsients upon stimulation of cells with acetylcholine, caffeine or high conc entrations of potassium ions. Our results show a tight functional coupling of voltage-dependent Ca2+ channels on the plasma membrane, ryanodine recept ors on the endoplasmic reticulum, and mitochondria. Cell stimulation genera tes localized Ca2+ transients, with Ca2+ concentrations above 20-40 mu M, a t these functional units. Protonophores abolish mitochondrial Ca2+ uptake a nd increase stimulated secretion of catecholamines by three- to fivefold. T hese results indicate that mitochondria modulate secretion by controlling t he availability of Ca2+ for exocytosis.