Voltage gated calcium channels potentiate muscarinic receptor-mediated calcium responses in SH-SY5Y human neuroblastoma cells

Muscarinic receptor-mediated Ca2+ elevations were investigated using the fura-2 method and image analysis. Typical responses consisted of an initial fast and transient phase followed by a sustained phase. Removal of external Ca2+ did not affect the magnitude of the initial phase of the signal whereas the sustained phase was abolished. Hence they may mainly consist of a release from intracellular stores and an influx, respectively. Depolarization of the cells with 30 mM KCl in the presence of external Ca2+ caused a small elevation of [Ca2+]i. In these conditions a considerable potentiation of the fast phase of muscarinic Ca2+ response was seen. The potentiation was dependent on the extracellular Ca2+ as it was abolished in the presence of antagonists of voltage-gated Ca2+ channels or by removal of external Ca2+ prior to KCl. Removal of external Ca2+ after the KCl-induced increase in [Ca2+]i partially inhibited the potentiation. If Ca2+ was readded together with the muscarinic stimulation a maximum potentiation was seen. The results suggest that the muscarinic response is potentiated by a Ca2+-mediated priming of both a fast and transient receptor associated Ca2+ influx pathway and release from the intracellular stores.