Spontaneous Ca2+ release from a caffeine and ryanodine-sensitive intracellular Ca2+ store in freshly prepared hepatocytes.

A considerable fraction of freshly prepared hepatocytes loaded with the fluorescent [Ca2+]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 Ca2+. They could no longer be observed when a Ca2+ chelator-(ethylenebis [oxyethylenenitrilo]) tetraacetic acid--was added to medium. Addition of thapsigargin, which is known to release Ca2+ from inositol 1,4,5-trisphosphate-sensitive and -insensitive Ca2+ stores, induced a large transient increase in [Ca2+]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 [Ca2+]i level became much higher and the spikes ceased. These results suggest that spontaneous rhythmic fluctuations in freshly prepared hepatocytes are driven by Ca2+ release from a caffeine- and ryanodine-sensitive calcium-induced calcium release pool.