Spontaneous Ca 2+ release from a caffeine and ryanodine–sensitive intracellular Ca 2+ store in freshly prepared hepatocytes

A considerable fraction of freshly prepared hepatocytes loaded with the fluorescent [Ca 2+ ] i indicator fura‐2 exhibited spontaneous rhythmic fluctuations that tended to decrease with increasing length of incubation after isolation. These oscillations were dependent on the external Ca 2+ . They could no longer be observed when a Ca 2+ chelator–(ethylene bis [oxyethylenenitrilo]) tetraacetic acid–was added to medium. Addition of thapsigargin, which is known to release Ca 2+ from inositol 1,4,5‐trisphosphate–sensitive and ‐insensitive Ca 2+ stores, induced a large transient increase in [Ca 2+ ] i and abolished the fluctuations. When the cells were treated with 2 mmol/L caffeine, frequency was increased, whereas 10 mmol/L caffeine induced a single large peak followed by a persistent plateau. Moreover, addition of dibutyryl cAMP led to decreased frequency of fluctuations. Ryanodine caused larger fluctuations; thereafter the [Ca 2+ ] i level became much higher and the spikes ceased. These results suggest that spontaneous rhythmic fluctuations in freshly prepared hepatocytes are driven by Ca 2+ release from a caffeine‐ and ryanodine‐sensitive calcium‐induced calcium release pool. (Hepatology 1994;19:514–517).