Muscarinic receptor agonist-induced increases in cytosolic Ca²⁺ concentrations in chick ciliary ganglion cells

We used fura-2 microfluorometry to examine the mechanism underlying the muscarinic receptor agonist-induced increase in the cytosolic Ca²⁺ concentration ([Ca]_in) in acutely isolated chick ciliary ganglion neurons. The order of potencies of muscarinic agonists in increasing [Ca]_in was: oxotremorine M > muscarine > methacholine > oxotremorine > bethanechol. The muscarine-induced increase in [Ca]_in persisted after treatment with thapsigargin, which blocked caffeine- and muscarinic agonist-induced Ca²⁺ release. The muscarine-sensitive [Ca]_in increase was inhibited by both L- and N-type Ca²⁺ channel blockers but potentiated by an L-type Ca²⁺ channel agonist. Muscarine was effective in increasing [Ca]_in in the presence of a desensitizing concentration of nicotine, and simultaneous addition of maximal doses of muscarine and nicotine caused an additive increase in [Ca]_in. On the other hand, nicotine-, ATP-, and high K⁺-induced increase in [Ca]_in was markedly potentiated during continuous stimulation with muscarine. These results suggest that muscarinic receptor stimulation increases Ca²⁺ influx passing through voltage-dependent Ca²⁺ channels. However, the muscarine-induced Mn²⁺ influx was observed in only some muscarine-sensitive cells, suggesting that muscarine-induced depolarization is too weak to overcome the inhibitory effect of Mn²⁺ on Ca²⁺ channels.