[Ca²⁺]_i oscillations from internal stores sustain exocytic secretion from the chromaffin cells of the rat

A large (65%) fraction of in vitro cultured rat chromaffin cells exhibit spontaneous [Ca²⁺]_i oscillations, and the rest can be recruited to oscillate by appropriate stimulations. Based on fura-2 single cell [ca²⁺]_i measurements, evidence is provided that these oscillation originate, via the activation of Ca²⁺-induced Ca²⁺-release, from intracellular Ca²⁺ stores in rapid equilibrium with extracellular Ca²⁺. By combining [Ca²⁺]_i measurements with a specific plaque secretion assay we demonstrate that oscillating cells exhibit a spontaneous exocytic secretory activity whereas the cells with stable [Ca²⁺]_i do not, [Ca²⁺]_i oscillations appear therefore to account for the high unstimulated catecholamine release, an activity typical of the chromaffin cells of the rat.