Measurement of intracellular Ca2+ in the bullfrog sympathetic ganglion cells using fura-2 fluorescence.

The intracellular free Ca2+ concentration ([Ca2+]i) of the bullfrog sympathetic ganglion cell was measured with fura-2 fluorescence under various conditions, and compared with changes in membrane potential recorded with an intracellular electrode. The [Ca2+]i was 109 nM on average under the resting condition and increased by raising the extracellular K+, stimulating repetitively the pre- or post-ganglionic nerve, or by applying acetylcholine or muscarine. Since all these procedures depolarized the cell membrane, most of the rise in [Ca2+]i could be the result of opening of voltage-dependent Ca2+ channels. However, Ca2+ entries through nicotinic acetylcholine receptor channels and the channel activated by the muscarinic acetylcholine receptor were also indicated by considering the threshold for the opening of voltage-dependent Ca2+ channels (for both entries) or a limited number of the cells showing the latter response.