Regulation of ryanodine receptor opening by lumenal Ca(2+) underlies quantal Ca(2+) release in PC12 cells.
Graded or "quantal" Ca(2+) release from intracellular stores has been observed in various cell types following activation of either ryanodine receptors (RyR) or inositol 1,4,5-trisphosphate receptors (InsP(3)R). The mechanism causing the release of Ca(2+) stores in direct proportion to the strength of stimulation is unresolved. We investigated the properties of quantal Ca(2+) release evoked by activation of RyR in PC12 cells, and in particular whether the sensitivity of RyR to the agonist caffeine was altered by lumenal Ca(2+). Quantal Ca(2+) release was observed in cells stimulated with 1 to 40 mM caffeine, a range of caffeine concentrations giving a >10-fold change in lumenal Ca(2+) content. The Ca(2+) load of the caffeine-sensitive stores was modulated by allowing them to refill for varying times after complete discharge with maximal caffeine, or by depolarizing the cells with K(+) to enhance their normal steady-state loading. The threshold for RyR activation was sensitized approximately 10-fold as the Ca(2+) load increased from a minimal to a maximal loading. In addition, the fraction of Ca(2+) released by low caffeine concentrations increased. Our data suggest that RyR are sensitive to lumenal Ca(2+) over the full range of Ca(2+) loads that can be achieved in an intact PC12 cell, and that changes in RyR sensitivity may be responsible for the termination of Ca(2+) release underlying the quantal effect.
Koizumi
S
S
Laboratory of Molecular Signalling, The Babraham Institute, Babraham Hall, Cambridge, CB2 4AT United Kingdom.
Lipp
P
P
Berridge
M J
MJ
Bootman
M D
MD
D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D016716:PC12 Cells; D051381:Rats; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism*
D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D002118:Calcium