Quantal Ca2+ release from caffeine-sensitive stores in adrenal chromaffin cells

Cheek, Timothy R.; Moreton, Roger B.; Berridge, Michael J.; Stauderman, Kenneth A.; Murawsky, Michael M. and Bootman, Martin D. (1993). Quantal Ca2+ release from caffeine-sensitive stores in adrenal chromaffin cells. Journal of Biological Chemistry, 268(36) pp. 27076–27083.

URL: http://www.jbc.org/content/268/36/27076.full.pdf+h...


In populations of fura-2-loaded chromaffin cells, caffeine caused a concentration-dependent increase in the intracellular Ca2+ concentration ([Ca2+]i), in the presence or absence of external Ca2+ ([Ca2+]o), that 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 nominally Ca2+-free medium, sustained stimulation of cell populations or single cells with low caffeine concentrations failed to completely empty the caffeine-sensitive stores. In each case, there was a transient [Ca2+]i elevation, but a subsequent challenge with a higher caffeine concentration 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 heterogeneous sensitivity was also demonstrated using ryanodine; pretreatment 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 caffeine 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 increases in direct proportion to the caffeine concentration.

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