Quantal Ca2+ release from caffeine-sensitive stores in adrenal chromaffin cells.
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.
Cheek
T R
TR
Department of Zoology, University of Cambridge, United Kingdom.
Moreton
R B
RB
Berridge
M J
MJ
Stauderman
K A
KA
Murawsky
M M
MM
Bootman
M D
MD
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
D012433:Ryanodine; D002110:Caffeine; D006632:Histamine; D002118:Calcium