title: Quantal Ca2+ release from caffeine-sensitive stores in adrenal chromaffin cells.
abstract: 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 Ca(2+)-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.
authors.affiliation: Department of Zoology, University of Cambridge, United Kingdom.
authors.forename: T R
authors.initials: TR
authors.lastname: Cheek
authors.forename: R B
authors.initials: RB
authors.lastname: Moreton
authors.forename: M J
authors.initials: MJ
authors.lastname: Berridge
authors.forename: K A
authors.initials: KA
authors.lastname: Stauderman
authors.forename: M M
authors.initials: MM
authors.lastname: Murawsky
authors.forename: M D
authors.initials: MD
authors.lastname: Bootman
mesh_terms: D000313:Adrenal Medulla / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D002478:Cells, Cultured; D002837:Chromaffin Granules; D006632:Histamine / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology
chemical_list: D012433:Ryanodine; D002110:Caffeine; D006632:Histamine; D002118:Calcium