QUANTAL CA2-SENSITIVE STORES IN ADRENAL CHROMAFFIN CELLS( RELEASE FROM CAFFEINE)

In populations of fura-2-loaded chromaffin cells, caffeine caused a co ncentration-dependent increase in the intracellular Ca2+ concentration ([Ca2+]i), in the presence or absence of external Ca2+ ([Ca2+]o), tha t was saturable, reversible, and inhibited in a use-dependent fashion by ryanodine. These data confirm that caffeine mobilizes Ca2+ from the ryanodine-sensitive intracellular stores in chromaffin cells. In nomi nally Ca2+-free medium, sustained stimulation of cell populations or s ingle cells with low caffeine concentrations failed to completely empt y the caffeine-sensitive stores. In each case, there was a transient [ Ca2+]i elevation, but a subsequent challenge with a higher caffeine co ncentration evoked a further [Ca2+]i rise, indicating that Ca2+ stores within individual cells were heterogeneous in their sensitivities to caffeine and that caffeine-induced Ca2+ release was quantal. The heter ogeneous sensitivity was also demonstrated using ryanodine; pretreatme nt of cell populations with increasing caffeine concentrations with a constant ryanodine concentration, caused a dose-dependent irreversible inhibition of the response to the subsequent addition of a maximal ca ffeine concentration. We conclude that, within single chromaffin cells , intracellular Ca2+ stores are heterogeneous in their sensitivity to caffeine and the fraction of Ca2+ stores mobilized by caffeine increas es in direct proportion to the caffeine concentration.