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Ionomycin induced changes in intra-cellular free calcium in SH-SY5Y human neuroblastoma cells: sources of calcium and effects on [ 3H]-noradrenaline release
In adherent SH-SY5Y human neuroblastoma cells cultured for 14 days to promote uptake and release of [
3H]-noradrenaline, ionomycin induced a biphasic elevation of the intracellular [Ca
2+] ([Ca
2+]
i). This consisted of a rapid transient elevation followed by a marked, persistent secondary elevation. Further study indicated that the peak [Ca
2+]
i elevation was dependent upon intracellular Ca
2+ whilst the secondary elevation was dependent upon extracellular Ca
2+. This profile of response and dependence upon intracellular and extracellular sources of Ca
2+ was similar to that evoked by the muscarinic agonist, methacholine but was independent of inositol 1,4,5-trisphosphate generation. Ionomycin also stimulated the release of [
3H]-noradrenaline from preloaded cells. Both intracellular and extracellular sources of Ca
2+ were needed for the full response and synergised to effect release. Thus, in adherent SH-SY5Y cells, ionomycin elevates [Ca
2+]
i in a complex way in a manner partly analogous to the elevation of [Ca
2+]
i by agonists of phosphoinositidase C-linked receptors. Furthermore the effects of [Ca
2+], elevation on [
3H]-noradrenaline release by these two processes are similar. Such functional consequences may, however, differ under circumstances where the profile and source of Ca
2+ for ionomycin-mediated changes differs to that of receptor agonists.