IONOMYCIN INDUCED CHANGES IN INTRACELLULAR FREE CALCIUM IN SH-SY5Y HUMAN NEUROBLASTOMA-CELLS - SOURCES OF CALCIUM AND EFFECTS ON [H-3] NORADRENALINE RELEASE

In adherent SH-SY5Y human neuroblastoma cells cultured for 14 days to promote uptake and release of [H-3]-noradrenaline, ionomycin induced a biphasic elevation of the intracellular [Ca2+] ([Ca2+](i)). This cons isted of a rapid transient elevation followed by a marked, persistent secondary elevation. Further study indicated that the peak [Ca2+](i) e levation was dependent upon intracellular Ca2+ whilst the secondary el evation was dependent upon extracellular Ca2+. This profile of respons e and dependence upon intracellular and extracellular sources of Ca2was 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 [H-3]-noradrenaline from preloaded cel ls. Both intracellular and extracellular sources of Ca2+ were needed f or the full response and synergised to effect release. Thus, in adhere nt SH-SY5Y cells, ionomycin elevates [Ca2+](i) in a complex way in a m anner partly analogous to the elevation of [Ca2+](i) by agonists of ph osphoinositidase C-linked receptors. Furthermore the effects of [Ca2+] (i) elevation on [H-3]-noradrenaline release by these two processes ar e similar. Such functional consequences may, however, differ under cir cumstances where the profile and source of Ca2+ for ionomycin-mediated changes differs to that of receptor agonists.