Basal Ca2+ and the oscillation of Ca2+ in caffeine‐treated bullfrog sympathetic neurones.

1. Effects of caffeine on the intracellular free Ca2+ concentration ([Ca2+]i) in single bullfrog sympathetic neurones in excised tissue were studied by recording Fura‐2 fluorescence excited at 340, 361 or 380 nm and taking their ratios (R340/380 or R361/380). 2. Caffeine (3‐10 mM) produced oscillation of [Ca2+]i and an ‘apparent’ decrease in the basal level of [Ca2+]i during a period between phasic rises. The mechanism of the latter effect was analysed in relation to the mechanism of the former. 3. Caffeine (3‐10 mM) increased Fura‐2 fluorescence in a range of excitation wavelength from 330 to 390 nm. The ratios of fluorescences, R340/380 and R361/380, however, were not significantly affected by caffeine. These results suggest that the ‘apparent’ reduction in the basal [Ca2+]i seen as a decrease in R340/380 or R361/380 results from a true decrease in [Ca2+]i. 4. Caffeine‐induced decrease in [Ca2+]i persisted for every period between phasic rises of [Ca2+]i during [Ca2+]i oscillation, and after the blockade of [Ca2+]i oscillation by ryanodine. The decrease in the latter condition lasted for more than 20 min. 5. The decrease in the basal [Ca2+]i depended on the external Ca2+ concentration and was not mimicked by the action of cyclic nucleotides. 6. Possible mechanisms underlying the decrease in the basal [Ca2+]i produced by caffeine (effects on Ca2+ transport at the cell or Ca(2+)‐storing organelle membrane) and their significance in relation to the [Ca2+]i oscillation were discussed.