Caffeine- and muscarinic receptor agonist-sensitive Ca2+ stores in chick ciliary ganglion cells.

To investigate the presence and the role of intracellular Ca2+ stores in chick ciliary ganglion cells, the concentration of cytosolic free Ca2+ ([Ca]in) was measured in acutely isolated neurons, using fura-2 microfluorometry. Caffeine caused a substantial increase in [Ca]in following or during high K+ depolarization; this response was inhibited by treatment of the cells with thapsigargin or with caffeine plus ryanodine. The peak value and the rate of the depolarization-induced [Ca]in increase were not much altered by either of these treatments, which deplete caffeine-sensitive Ca2+ stores. The muscarinic receptor agonists muscarine, oxotremorine M, and methacholine, caused substantial increases in [Ca]in, in a manner that was partially dependent on Ca2+. These agonists also caused a rise in [Ca]in during K+ depolarization, which rise was inhibited by treatment with thapsigargin or with caffeine plus ryanodine. The response to oxotremorine M during depolarization was strongly inhibited by 10 nM 4-DAMP, but was not inhibited by 1 microM pirenzepine or by 1 microM AF-DX 116. These results indicate that chick ciliary ganglion cells possess Ca2+ stores that are activated by both caffeine and a second messenger generated by the activation of the M3 muscarinic receptor subtype.