Abstract
Muscarinic receptor-mediated increases in Ca 2+ in SH-SY5Y neuroblastoma cells consist of an initial fast and transient phase followed by a sustained phase. Activation of voltage-gated Ca 2+ channels prior to muscarinic stimulation resulted in a several-fold potentiation of the fast phase. Unlike the muscarinic response under control conditions, this potentiated elevation of intracellular Ca 2+ was to a large extent dependent on extracellular Ca 2+. In potentiated cells, muscarinic stimulation also activated a rapid Mn 2+ entry. By using known organic and inorganic blockers of cation channels, this influx pathway was easily separated from the known Ca 2+ influx pathways, the store-operated pathway and the voltage-gated Ca 2+ channels. In addition to the Ca 2+ influx, both IP 3 production and Ca 2+ release were also enhanced during the potentiated response. The results suggest that a small increase in intracellular Ca 2+ amplifies the muscarinic Ca 2+ response at several stages, most notably by unravelling an apparently novel receptor-activated influx pathway.
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