Ca2+-dependent potentiation of muscarinic receptor-mediated Ca2+ elevation

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.