Caffeine affects Ca uptake and Ca release from intracellular stores: fura-2 measurements in isolated snail neurones.

Using the fluorescent probe fura-2, the average cytoplasmic concentration of free Ca2+ [( Ca]i) was measured in isolated voltage-clamped neurons of the snail Helix pomatia. In normal Ringer solution [Ca]i transients elicited by membrane depolarizations lasting 30-100 s have a voltage dependence similar to that of the calcium current. In the presence of caffeine [Ca]i transients did not depend on the testing voltage, indicating Ca release from intracellular stores. In both cases [Ca]i decayed after the transient increase. The rate of [Ca]i decline was monoexponential and independent of the membrane potential. In caffeine-containing solution the decline was 3 times faster. Steady membrane depolarization in the presence of caffeine induced periodic changes in [Ca]i. A simple model to describe these oscillations on the basis of Ca release from and Ca uptake into intracellular stores predicted that the oscillations could be initiated and modulated by Ca influx into the cytoplasm, which is in line with experimental data.