Release and sequestration of Ca2+ by a caffeine- and ryanodine-sensitive store in a sub-population of human SH-SY5Y neuroblastoma cells

We have used single cell fluorescence imaging techniques to examine the role that ryanodine receptors play in the stimulus-induced Ca2+ responses of SH-SY5Y cells. The muscarinic agonist methacholine (1 mM) resulted in a Ca2+ signal in 95% of all cells. Caffeine (30 mM) however stimulated a Ca2+ signal in only 1-7% of N-type (neuroblastic) cells within any given field. The caffeine response was independent of extracellular Ca2+, regenerative in nature, and abolished in a use-dependent fashion by ryanodine. In caffeine-responsive cells, the magnitude of the methacholine-induced Ca2+ signal was inhibited by 75.07 ± 5.51% by pretreatment with caffeine and ryanodine, suggesting that the caffeine-sensitive store may act as a Ca2+ source after muscarinic stimulation. When these data were combined with equivalent data from non-caffeine-responsive cells, the degree of apparent inhibition was significantly reduced. In contrast, after store depletion by caffeine, the Ca2+ signal induced by 55 mM K+ was potentiated 2.5-fold in the presence of ryanodine, suggesting that the store may act a Ca2+ sink after depolarisation. We conclude that a caffeine- and ryanodine-sensitive store can act as a Ca2+ source and sink in SH-SY5Y cells, and that effects of the store can become obscured if data from caffeine-insensitive cells are not excluded.