Separation between cytosolic calcium and secretion in chromaffin cells superfused with calcium ramps

This paper describes experiments in which cytosolic Ca2+ concentrations ([Ca2+]i) and catecholamine release were measured in two populations of chromaffin cells stimulated with a solution enriched in K+ (100 mM). Once depolarized, external Ca2+ or Ba2+ ions were offered to cells either as a single 2.5 mM step or as a ramp that linearly increased the concentration from 0 to 2.5 mM over a 10-min period. A clear separation between the changes of the [Ca2+]i and the time course of secretion was observed. Specifically, secretion and [Ca2+]i rose in parallel when a Ca2+ step was used to reach a peak in a few seconds; however, while secretion declined to the basal level, [Ca2+]i remained elevated at a plateau of 400 nM. With a Ca2+ ramp, only a transient small peak of secretion was observed, yet the [Ca2+]i remained elevated throughout the 10-min stimulation period. The separation between secretion and [Ca2+]i was observed even when voltage-dependent Ca2+ channels were expected to remain open (mild depolarization in the presence of 1 microM Bay K 8644). By using Ba2+ steps or ramps, sustained noninactivating secretory responses were obtained. The results suggest that the rate and extent of secretion are not a simple function of the [Ca2+]i at a given time; they are compatible with the following conclusions: (i) A steep extracellular-to-cytosolic Ca2+ gradient is required to produce a sharp increase in the [Ca2+]i at exocytotic sites capable of evoking a fast but transient secretory response. (ii) As a result of Cai(2+)-dependent inactivation of Ca2+ channels, those high [Ca2+]i are possible only at early times after cell depolarization. (iii) The Cai(2+)-dependent supply of storage granules to the secretory machinery cooperates with the supply of Ca2+ through Ca2+ channels to regulate the rate and extent of secretion.