Mechanism underlying the ATP-induced increase in the cytosolic Ca2+ concentration in chick ciliary ganglion neurons.

We examined the mechanism underlying the ATP-induced increase in the cytosolic Ca2+ concentration ([Ca]in) in acutely isolated chick ciliary ganglion neurons, using fura-2 microfluorometry. The ATP-induced increase in [Ca]in was dependent on external Ca2+, was blocked in a dose-dependent manner by reactive blue 2, and was substantially inhibited by both L- and N-type Ca2+ channel blockers. ATP was effective in increasing [Ca]in in the presence of a desensitizing concentration of nicotine (100 microM), and simultaneous addition of maximal doses of ATP and nicotine caused an additive increase in [Ca]in, suggesting that ATP acts on a site distinct from nicotinic acetylcholine receptors. ATP also increased the cytosolic Na+ concentration as determined by sodium-binding benzofuran isophthalate microfluorometry. These results suggest that ATP increases Na+ influx through P2 purinoceptor-associated channels resulting in membrane depolarization, which in turn increases Ca2+ influx through voltage-dependent Ca2+ channels. However, ATP still caused a small increase in [Ca]in under Na+-free conditions, and this [Ca]in increase was little affected by Ca2+ channel blockers. ATP also increased Mn2+ influx under Na+-free conditions, as indicated by quenching of fura-2 fluorescence. These results suggest that nonselective cationic channels activated by ATP are permeable not only to Ca2+ but also to Mn2+, in addition to monovalent cations.